SNAPSHOT

Event: 1487: Binding, SH/seleno proteins

Short Name: Binding, SH/seleno proteins

AOPs Including This Key Event

Stressors

Biological Context

Event: 1392: Oxidative Stress

Short Name: Oxidative Stress

AOPs Including This Key Event

Biological Context

Oxidative stress is produced in, and can occur in, any species from bacteria through to humans.

Event: 1488: Glutamate dyshomeostasis

Short Name: Glutamate dyshomeostasis

AOPs Including This Key Event

Biological Context

Cell term

Organ term

Event: 55: N/A, Cell injury/death

Short Name: N/A, Cell injury/death

AOPs Including This Key Event

Biological Context

Cell term

Cell death is an universal event occurring in cells of any species. ^[11]

Event: 188: N/A, Neuroinflammation

Short Name: N/A, Neuroinflammation

AOPs Including This Key Event

Biological Context

Organ term

Neuroinflammation is observed in human, monkey, rat, mouse, and zebrafish, in association with neurodegeneration or following toxicant exposure. Some references (non-exhaustive list) are given below for illustration:

In human: Vennetti et al., 2006

In monkey (Macaca fascicularis): Charleston et al., 1994, 1996

Iin rat: Little et al., 2012; Zurich et al., 2002; Eskes et al., 2002

In mouse: Liu et al., 2012

In zebrafish: Xu et al., 2014.

Event: 1492: Tissue resident cell activation

Short Name: Tissue resident cell activation

AOPs Including This Key Event

Biological Context

Extend to at least invertebrates

Not to plants and not to single-celled organisms

BRAIN:

Neuroinflammation is observed in human, monkey, rat, mouse, and zebrafish, in association with neurodegeneration or following toxicant exposure. Some references (non-exhaustive list) are given below for illustration:

In human: Vennetti et al., 2006

In monkey (Macaca fascicularis): Charleston et al., 1994, 1996

In rat: Little et al., 2012; Zurich et al., 2002; Eskes et al., 2002

In mouse: Liu et al., 2012

In zebrafish: Xu et al., 2014.

Event: 1493: Increased Pro-inflammatory mediators

Short Name: Increasaed pro-inflammatory mediators

AOPs Including This Key Event

Biological Context

Event: 386: Decrease of neuronal network function

Short Name: Neuronal network function, Decreased

AOPs Including This Key Event

Biological Context

Organ term

In vitro studies in brain slices applying electrophysiological techniques showed significant variability among species (immature rats, rabbits and kittens) related to synaptic latency, duration, amplitude and efficacy in spike initiation (reviewed in Erecinska et al., 2004).

Event: 341: Impairment, Learning and memory

Short Name: Impairment, Learning and memory

AOPs Including This Key Event

Biological Context

Basic forms of learning behavior such as habituation have been found in many taxa from worms to humans (Alexander, 1990). More complex cognitive processes such as executive function likely reside only in higher mammalian species such as non-human primates and humans. Recently, larval zebrafish has also been suggested as a model for the study of learning and memory (Roberts et al., 2013).

Relationship: 1689: Binding, SH/seleno proteins leads to Oxidative Stress

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Mechanistic support for the link between interference of SH/SeH groups of proteins and induction of oxidative stress can be found in Zebrafish, rodents (mouse and rat) and to some extent in man (see Table 2).

Key Event Relationship Description

Proteins with cysteine amino acid residues contain thiol (SH) groups, and proteins with selenocysteine amino acid residues contain selenol (SeH) are characterized as cysteine-/selenoprotein family. Thiol and selenol groups exhibit reactivity toward electrophiles and oxidants and have high binding affinities for metals (Higdon, 2012; Nagy, 2013; Winterbourn, 2008; Winther, 2014).

Figure 1. (Poole, 2015) Structures of cysteinyl and selenocysteinyl residues within proteins. The aminoacyl groups are shown to the left, with dotted lines representing peptide bonds to the next residue on either side. Both protonated (left) and deprotonated (right) forms of these amino acids are depicted with average pKa values.

The selenoprotein family composes of proteins with diverse functionality, however, several are classified as antioxidant enzymes (Reeves, 2009) and this function is of particular importance for this KER. Relevant for this KER there are two well-studied functional selenoprotein families which are described to be expressed in the brain; (i) the Glutathione Peroxidase (GPx) family, involved in detoxification of peroxidases; (ii) the Thioredoxin Reductase (TrxR) family, which is involved in the regeneration of reduced thioredoxin (Pillai, 2014). However, there is also a number of other selenoproteins with diverse functions, from selenium transport (SelP), to ER stress response (SelK, M, N, S, T and Sep15, as well as DIO2) (Pisoschi, 2015; Reeves, 2009). Due to their described functionalities (summarized in table below) an increased oxidative stress as a consequence of interference with selenoprotein function, through binding to active-site thiol-/selenol groups will primarily concern the interference with proteins of the GPx- and TrxR families, as well as SelH, K, S, R, W, and P selenoproteins.

Table1

https://aopwiki.org/system/dragonfly/production/2018/01/26/9o3c62z5ej_AOP17_KER1_Table1.pdf

Binding to thiol/sulfhydyryl groups of these proteins can firstly result in structural modifications of these proteins, which in turn negatively effects the catalytic capacity and thereby reducing or blocking the metabolic capacity to neutralize reactive oxygen species (Fernandes, 1996; Rajanna, 1995), secondly, SH/SeH binding would also the instrinsic primary antioxidant functionalities of selenoproteins (Kohen, 2002; Pisoschi, 2015).

Evidence Supporting this KER

Primary antioxidants are mainly chain breakers, able to scavenge radical species by hydrogen donation. Secondary antioxidants are singlet oxygen quenchers, peroxide decomposers, metal chelators, oxidative enzyme inhibitors (Pisosci and Pop 2015).

Thiol- and selenol containing proteins have a high affinity for binding soft metals which contributes to the target site – brain – distribution of such toxicants (Farina, 2011).

GPx family

GPxs are tetrameric enzymes where their thiol groups can either act directly act as a reductant, or they catalyze reduction of hydrogen peroxide and/or phospholipid hydroperoxides through glutathione co-factors (Hanschmann, 2013; Labunskyy, 2014)

TrxR family

The thioredoxin reductase (TxRs) family of selenoproteins are homodimeric flavoenzymes, which mediate the reduction of oxidized Txn at the expense of NADPH (Birben et al., 2012). Inhibition of TrxR enzymes have been shown to lead to oxidative stress (Carvalho, 2008).

SelP

Downregulation of intracellular SelP by use of small interfering RNA (siRNA) impaired the viability of human astrocytes and made them more susceptible to hydroperoxide-induced oxidative stress, pointing to a direct contribution of SeP to ROS clearance (Steinbrenner, 2006)

Mercury

The selenol group (-SeH) of selenocysteines is generally more reactive than thiols (-SH) towards mercury (Sugiura 1976, Khan, 2009). Methyl mercury (MeHg) can target both the GPx and TrxR proteins thereby causing induction of oxidative stress and neurotoxicity (Branco, 2017; Carvalho, 2008; Farina, 2011).

Table 2

https://aopwiki.org/system/dragonfly/production/2018/01/26/94pzx39utv_AOP17_KER1_Table2.pdf

Acrylamide (acrylamide is a common food contaminant generated by heat processing)

No literature supporting the link “SH/she binding leads to oxidative stress” for acrylamide as stressor in brain/neural tissue can be found.

Acrolein

No literature supporting the link “SH/she binding leads to oxidative stress” for acrolein as stressor in brain/neural tissue can be found.

Another important group of thiol-containing proteins are the metal-binging detoxifying metallothioneins. This protein family bind mercury and lead, and this binding thus serves as a protective mechanism and also protects against metal toxicity and oxidative stress (Aschner, 2006).

Lactational exposure to methylmercury (10 mg/L in drinking water) significantly increased cerebellar GSH level and GR activity. Possibly a compensatory response to mercury-induced oxidative stress (Franco et al., 2006)

Methylmercury cytotoxicity in PC12 cells is mediated by primary glutathione depletion independent of excess reactive oxygen species generation (Gatti et al., 2004).

References

Relationship: 1685: Oxidative Stress leads to Glutamate dyshomeostasis

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

In the central nervous system (CNS), glutamate (Glu) is rapidly taken up at the synaptic cleft to mitigate potential excitotoxicity (Meldrum, 2000). Reuptake is carried out by the electrochemical gradient of Glu across the plasma membrane and is accomplished by Glu transporter proteins, referred to as excitatory amino acid transporters (EAATs). These transporter proteins are predominantly expressed in astrocytes, but they are also be found in other neural cells, such as oligodendrocyte, neuron, and microglia membranes (Danbolt, 2001). Functional Glu transporters are located on cell surface membranes. The activities of these transporters are regulated by a redistribution of these proteins to or from the plasma membrane (Robinson 2002), under the control of several signaling pathways. Five different families of EAATs have been recognized (EAAT1–EAAT5). They vary in Na⁺ and/or K⁺ coupling abilities. Their names differ based on the presence of the transporter in human or in other mammals (see Table 1).

Table 1: Glu transporters in human and mammals and their occurrence in CNS cells. From Rajda et al., 2017

These transporters co-localize with, form physical (co-immunoprecipitable) interactions with, and functionally couple to various 'energy-generating' systems, including the Na(+)/K(+)-ATPase, the Na⁺/Ca²⁺ exchanger, glycogen metabolizing enzymes, glycolytic enzymes, and mitochondria/mitochondrial proteins. This functional coupling is bi-directional with many of these systems both being regulated by glutamate transport and providing the 'fuel' to support glutamate uptake (Robinson and Jackson, 2016). The Na⁺ gradient, which depends on Na/K ATPase pump and consequently of ATP production and intracellular levels, provides the energy to move Glu from the outside into the cells, accompanied by two Na⁺ and an H⁺ ; at the same time, K⁺ moves in the opposite direction (Boron and Boulpaep, 2003). Mitochondrial dysfunction leads to a decrease in ATP synthesis, impaired Ca²⁺ content, and concomitant increase in the levels of ROS and RNS (Beal, 2005). Free radicals, which are electrically unstable, have a central role in several physiological and pathological processes. Both ROS and RNS originate from endogenous and exogenous sources. Mitochondria, endoplasmic reticulum, peroxisomes, phagocytic cells, and others serve as endogenous sources, and environmental factors, such as alcohol, tobacco, pollution, industrial solvents, pesticides, heavy metals, specified medicines, etc. make up the prepondarance of exogenous factors. Significant amounts of reactive oxygen species (ROS) and reactive nitrogen species (RNS,) are formed during oxidative phosphorylation, when the greatest amount of ATP is produced. Cellular antioxidants production serves as a countermeasure against this process (Su et al., 2013; Szalardi et al., 2015). Most cells, including astrocytes, have protective mechanisms against ROS, predominantly in the form of the tripeptide thiol, glutathione (GSH) (Hsie et al., 1996). This process stays in a highly sensitive balance. In the specific case when ROS and RNS synthesis exceeds antioxidant synthesis it results in oxidative stress (Reddy, 2006; Ghafouribar et al., 2008; Su et al., 2013; Szalardi et al., 2015; Valko et al., 2007; Yankovskaya et al., 2003; Senoo-Matsuda et al., 2003;  Schon and Manfredi, 2003).

Evidence Supporting this KER

Due to the tight coupling of Glu transporters with energy production, and to the important role of Glu transporters in Glu homeostasis, perturbations of energy metabolism such as mitochondrial dysfunction and increased production of ROS lead to Glu dyshomeostasis  (Boron and Boulpaep, 2003). In particular, it was shown that ROS inhibit glutamate uptake by astrocytes (Sorg et al., 1997), and that  glutamate release is mediated by ROS-activated volume-sensitive outwardly rectifying anion channels (Liu et al., 2009).

Porciuncula et al., (2003). Methylmercury (2-10 mM) inhibits glutamate uptake in synaptic vesicles isolated from rat brain in a concentration-dependent manner (with LD₅₀ at 50 mM). It also inhibits the H⁺-ATPase activity in a concentration-dependent manner with similar LD₅₀. This suggests that the vesicular glutamate uptake is impaired by methylmercury and that this effect involves the H⁺-ATPase.

Roos et al., (2009). Methylmercury induced ROS production in rat brain cortical slices after 2h exposure at 100 and 200 mM and after 5h exposure at 50 mM. Guanosine (0.5 - 5 mM), ebselen (1-5 mM) and diphenyl diselenide (1-5 mM) blocked the methylmercury-induced ROS production. The inhibitor of NMDA receptors, MK801 (50 mM) equally blocked the methylmercury-induced ROS production by two potential mechanisms of action: (i) mercury by affecting mitochondria iincreased ROS formation, which decrease glutamate uptake and consequently increased extracellular glutamate acting on NMDA receptors; (ii) The ROS formation is secondary to overstimulation of NMDA receptors, due to mercury-induced decrease in glutamate uptake.

Roos et al., (2011). Experiments performed in isolated mitochondria from rat liver slices showed that methylmercury (25 mM) increased ROS production (measured by dichlorofluoroscein). Methionine treatment (50-250 mM) was effective in reducing ROS formation.

Juarez et al., (2002) Microdialysis probe in adult Wistar rats showed that acute exposure to methylmercury (10, 100 mM) induced an increase release of extracellular glutamate (9.8 fold at 10 mM and 2.4 fold at 100 mM). This extracellular glutamate level remained elevated at least 90 min following methylmercury exposure.

Allen et al., (2001). Cerebral cortical astrocytes were treated with methylmercury (1 mM for 24h or 10 mM for 30 min) and loaded with [U-¹³C] glutamate. In the methylmercruy-treated group, a decrease of  [U-¹³C] lactate was observed. This lactate can only be derived from mitochondrial metabolism, via the tricarboxylic acid, showing a link between mitochondrial dysfunction and glutamate metabolism. In addition, the decreased lactate production might be detrimental to surrounding cells, since lactate has been shown to be an important substrate for neurons.

The astrocytic enzyme glutamine synthetase (GS), transforming glutamate in glutamine, which is taken up by neurons, is also a SH-containing protein, which is inhibited by mercury binding (Kwon and Park, 2003). This participate to glutamate dyshomeostasis  linking this KE directly to the MIE.

References

Allen, J. W., H. El-Oqayli, M. Aschner, T. Syversen and U. Sonnewald (2001). "Methylmercury has a selective effect on mitochondria in cultured astrocytes in the presence of [U-(13)C]glutamate." Brain Res 908(2): 149-154.

Bergles DE, Diamond JS, Jahr CE. Clearance of glutamate inside the synapse and beyond. Curr Opin Neurobiol. 1999;9:293–298.

Beal M.F. Mitochondria take center stage in aging and neurodegeneration. Ann. Neurol. 2005;58:495–505.

Berry JD, Shefner JM, Conwit R, Schoenfeld D, Keroack M, Felsenstein D, Krivickas L, David WS, Vriesendorp F, Pestronk A, et al. Design and initial results of a multi-phase randomized trial of ceftriaxone in amyotrophic lateral sclerosis. PLoS One. 2013;8:e61177.

Boron WF, Boulpaep EL. 2003. A cellular and molecular approach. Medical Physiology. Updated Edition. Elsevier, Saunders.

Casado M, Bendahan A, Zafra F, Danbolt NC, Aragón C, Giménez C, Kanner BI. Phosphorylation and modulation of brain glutamate transporters by protein kinase C. J Biol Chem. 1993;268:27313–27317.

Colton CK, Kong Q, Lai L, Zhu MX, Seyb KI, Cuny GD, Xian J, Glicksman MA, Lin C-LG. Identification of translational activators of glial glutamate transporter EAAT2 through cell-based high-throughput screening: an approach to prevent excitotoxicity. J Biomol Screen. 2010;15:653–662.

Danbolt NC. Glutamate uptake. Prog Neurobiol. 2001;65:1–105.

Fontana AC, Beleboni RO, Wojewodzic MW, Dos SWF, Coutinho-Netto J, Grutle NJ, Watts SD, Danbolt NC, Amara SG. Enhancing glutamate transport: mechanism of action of Parawixin1, a neuroprotective compound from Parawixia bistriata spider venom. Mol Pharmacol. 2007;72:1228–1237.

Ghafourifar P., Mousavizadeh K., Parihar M.S., Nazarewicz R.R., Parihar A., Zenebe W.J. 2008. Mitochondria in multiple sclerosis. Front. Biosci. 13:3116–3126. doi: 10.2741/2913.

Gonzalez MI, Robinson MB. Neurotransmitter transporters: why dance with so many partners? Curr Opin Pharmacol. 2004;4:30–35.

Hediger MA. Glutamate transporters in kidney and brain. Am J Physiol. 1999;277:F487–F492.

 Hsie AW, Recio L, Katz DS, Lee CQ, Wagner M, et al. (1986) Evidence for reactive oxygen species inducing mutations in mammalian cells. Proc Natl Acad Sci U S A 83: 9616–9620

Juarez, B. I., M. L. Martinez, M. Montante, L. Dufour, E. Garcia and M. E. Jimenez-Capdeville (2002). "Methylmercury increases glutamate extracellular levels in frontal cortex of awake rats." Neurotoxicol Teratol 24(6): 767-771.

Kwon, O.-S., Park, Y.-J. In vitro and in vivo dose-dependent inhibition of methylmercury on glutamine synthetase in the brain of different species (2003) Environmental Toxicology and Pharmacology, 14 (1-2), pp. 17-24.

Liu HT, Akita T, Shimizu T, Sabirov RZ, Okada Y. 2009. Bradykinin-induced astrocyte–neuron signalling: glutamate release is mediated by ROS-activated volume-sensitive outwardly rectifying anion channels. J Physiol. 587(Pt 10): 2197–2209.

Meldrum BS. 2000. Glutamate as a neurotransmitter in the brain: review of physiology and pathology. J. Nutr. 130(4S Suppl):1007S–1015S.

Mitrovic AD, Plesko F, Vandenberg RJ. Zn²⁺inhibits the anion conductance of the glutamate transporter EAAT4. J Biol Chem. 2001;276:26071–26076.

Porciuncula, L. O., J. B. Rocha, R. G. Tavares, G. Ghisleni, M. Reis and D. O. Souza (2003). "Methylmercury inhibits glutamate uptake by synaptic vesicles from rat brain." Neuroreport 14(4): 577-580.

Rajda C, Pukoli D, Bende Z, Majalath Z, Vécsei L. 2017. Excitotoxins, Mitochondrial and Redox Disturbances in Multiple Sclerosis. Int J Mol Sci. 2017 Feb; 18(2): 353. doi:  10.3390/ijms18020353.

Reddy P.H. 2006. Mitochondrial oxidative damage in aging and Alzheimer’s disease: Implications for mitochondrially targeted antioxidant therapeutics. J. Biomed. Biotechnol.  doi: 10.1155/JBB/2006/31372.

Robinson MB. 2002. Regulated trafficking of neurotransmitter transporters: common notes but different melodies. J Neurochem. 80(1):1-11.

Robinson, M. B. and J. G. Jackson (2016). "Astroglial glutamate transporters coordinate excitatory signaling and brain energetics." Neurochem Int 98: 56-71.

Roos, D. H., R. L. Puntel, M. M. Santos, D. O. Souza, M. Farina, C. W. Nogueira, M. Aschner, M. E. Burger, N. B. Barbosa and J. B. Rocha (2009). "Guanosine and synthetic organoselenium compounds modulate methylmercury-induced oxidative stress in rat brain cortical slices: involvement of oxidative stress and glutamatergic system." Toxicol In Vitro 23(2): 302-307.

Roos, D. H., R. L. Puntel, M. Farina, M. Aschner, D. Bohrer, J. B. Rocha and N. B. de Vargas Barbosa (2011). "Modulation of methylmercury uptake by methionine: prevention of mitochondrial dysfunction in rat liver slices by a mimicry mechanism." Toxicol Appl Pharmacol 252(1): 28-35.

Rothstein JD, Patel S, Regan MR, Haenggeli C, Huang YH, Bergles DE, Jin L, Dykes Hoberg M, Vidensky S, Chung DS, Toan SV, Bruijn LI, Su ZZ, Gupta P, Fisher PB. Beta-lactam antibiotics offer neuroprotection by increasing glutamate transporter expression. Nature. 2005;433:73–77.

Sattler R, Rothstein JD. Regulation and dysregulation of glutamate transporters. Handb Exp Pharmacol. 2006;175:277–303

Seal RP, Amara SG. Excitatory amino acid transporters: a family in flux. Annu Rev Pharmacol Toxicol. 1999;39:431–456.

Senoo-Matsuda N., Hartman P.S., Akatsuka A., Yoshimura S., Ishii N. 2003. A complex II defect affects mitochondrial structure, leading to ced-3- and ced-4-dependent apoptosis and aging. J. Biol. Chem. 278:22031–22036. doi: 10.1074/jbc.M211377200.

Schon E.A., Manfredi G. 2003. Neuronal degeneration and mitochondrial dysfunction. J. Clin Investig. 111:303–312. doi: 10.1172/JCI200317741.

Sorg O, Horn TF, Yu N, Gruol DL, Bloom FE. 1997.Inhibition of astrocyte glutamate uptake by reactive oxygen species: role of antioxidant enzymes.Mol Med. 3(7): 431–440.

Su K., Bourdette D., Forte M. 2013. Mitochondrial dysfunction and neurodegeneration in multiple sclerosis. Front. Physiol. 4:169. doi: 10.3389/fphys.2013.00169. 

Szalárdy L., Zádori D., Klivényi P., Toldi J., Vécsei L. 2015. Electron transport disturbances and neurodegeneration: From albert Szent-Györgyi’s concept (Szeged) till novel approaches to boost mitochondrial bioenergetics. Oxid. Med. Cell. Longev. 2015:498401. doi: 10.1155/2015/498401.

Valko M., Leibfritz D., Moncol J., Cronin M.T., Mazur M., Telser J. 2007. Free radicals and antioxidants in normal physiological functions and human disease. Int. J. Biochem. Cell Biol. 39:44–84. doi: 10.1016/j.biocel.2006.07.001.

Vandenberg RJ, Mitrovic AD, Johnston GA. Molecular basis for differential inhibition of glutamate transporter subtypes by zinc ions. Mol Pharmacol. 1998;54:189–196.

Vandenberg RJ, Ryan RM. Mechanisms of glutamate transport. Physiol Rev. 2013;93:1621–1657.

Xing X, Chang L-C, Kong Q, Colton CK, Lai L, Glicksman MA, Lin C-LG, Cuny GD. Structure-activity relationship study of pyridazine derivatives as glutamate transporter EAAT2 activators. Bioorg Med Chem Lett. 2011;21:5774–5777.

Yankovskaya V., Horsefield R., Törnroth S., Luna-Chavez C., Miyoshi H., Léger C., Byrne B., Cecchini G., Iwata S. 2003. Architecture of succinate dehydrogenase and reactive oxygen species generation. Science.299:700–704. doi: 10.1126/science.1079605.

Relationship: 1686: Glutamate dyshomeostasis leads to N/A, Cell injury/death

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Support for the link between glutamate dyshomeostasis and cell injury /death can be found in rats, and mouse. However, as the neurotransmitter glutamate is already found in insects, it is plausible that this KER is valid throughout taxa (Harris, 2014).

Key Event Relationship Description

Glutamate is the major excitatory neurotransmitter in the mammalian CNS, where it plays key roles in development, learning, memory and response to injury. However, glutamate at high concentrations at the synaptic cleft acts as a toxin, inducing neuronal injury and death (Meldrum, 2000; Ozawa et al., 1998). Glutamate-mediated neurotoxicity has been dubbed as “excitotoxicity”, referring to the consequence of the overactivation of the N-methyl D-aspartate (NMDA)–type glutamate receptors (cf AOP 48), leading to increased Na⁺ and Ca²⁺ influx into neurons (Choi, 1992; Pivovarova & Andrews, 2010). Increased intracellular Ca²⁺ levels are associated with the generation of oxidative stress and neurotoxicity (Lafon-Cazal, 1993). Accordingly, the control of extracellular levels of glutamate dictates its physiological/pathological actions and this equilibrium is maintained primarily by the action of several glutamate transporters (such as GLAST, GLT1, and EAAC1) located on astrocytic cell membranes, which remove the excitatory neurotransmitter from the synaptic cleft, keeping its extracellular concentrations below toxic levels (Anderson and Swanson, 2000; Maragakis and Rothstein, 2001; Szydlowska & Tymianski, 2010).

In addition to synaptic transmission, physiological stimulation of glutamate receptors can mediate trophic effects and promote neuronal plasticity. During development, NMDA receptors initiate a cascade of signal transduction events and gene expression changes primarily involving Ca²⁺-mediated signaling, induced by activation of either Ca²⁺- permeable receptor channels or voltage-sensitive Ca²⁺ channels. The consecutive activation of major protein kinase signaling pathways, such as Ras-MAPK/ERK and PI3-K-Akt, contributes to regulation of gene expression through the activation of key transcription factors, such as CREB, SRF, MEF-2, NF-kappaB. Metabotropic glutamate receptors can also engage these signaling pathways, in part by transactivating receptor tyrosine kinases. Indirect effects of glutamate receptor stimulation are due to the release of neurotrophic factors, such as brain derived neurotrophic factor through glutamate-induced release of trophic factors from glia. The trophic effect of glutamate receptor activation is developmental stage-dependent and may play an important role in determining the selective survival of neurons that made proper connections. During this sensitive developmental period, interference with glutamate receptor function may lead to widespread neuronal loss (for review, Balazs, 2006).

Evidence Supporting this KER

Glutamate dyshomeostasis and in particular excess of glutamate in the synaptic cleft will lead to overactivation of ionotropic glutamate receptors and cause cell injury/death, as described in AOP 48. The excess of glutamate can result from decreased uptake in astrocytes (Brookes and Kristt, 1989; Aschner et al., 2000), or neurons (Porciuncula et al., 2003; Moretto et al., 2005). But also from the increased release (Reynolds and Raecz, 1987). This neurotoxic cascade involves calcium overload and ROS production leading to oxidative stress (Meldrum, 2000; Ozawa et al., 1998;Lafon-Cazal, 1993; Ceccatelli et al., 2010). Chemicals binding to sulfhydryl (SH)-/seleno-proteins cause a direct oxidative stress by perturbing mitochondrial respiratory chain proteins and by decreasing anti-oxidant defense mechanism (see KER : MIE to KEdown oxidative stress) and an indirect oxidative stress via perturbation of glutamate homeostasis/excitotoxicity. Thus, there may be some redundancy in the empirical support between this KER and the KER linking KEup oxidative stress and KEdown cell injury/death.

Glutamate has been shown to regulate BDNF production (Tao, 1998, 2002). Accordingly, glutamate may also indirectly contribute to cell injury/death by inducing modifications in the brain levels of trophic factors, since it is known that changes in trophic support can lead to cell injury/death, as well as to perturbation in the physiological establishment of neuronal network (for review, Zhao, 2017).

No uncertainty or inconsistency reported yet.

References

Albrecht, J., Talbot, M., Kimelberg, H.K., Aschner, M., 1993. The role of sulfhydryl groups and calcium in the mercuric chloride-induced inhibition of glutamate uptake in rat primary astrocyte cultures. Brain Res 607, 249-254.

Anderson, C.M., Swanson, R.A., 2000. Astrocyte glutamate transport: review of properties, regulation, and physiological functions. Glia 32, 1-14.

Aschner, M., Yao, C.P., Allen, J.W., Tan, K.H., 2000. Methylmercury alters glutamate transport in astrocytes. Neurochem Int 37, 199-206.

Balazs, R., 2006. Trophic effect of glutamate. Curr Top Med Chem 6, 961-968.

Brookes, N., Kristt, D.A., 1989. Inhibition of amino acid transport and protein synthesis by HgCl2 and methylmercury in astrocytes: selectivity and reversibility. J Neurochem 53, 1228-1237.

Ceccatelli, S., Dare, E., Moors, M., 2010. Methylmercury-induced neurotoxicity and apoptosis. Chem Biol Interact 188, 301-308.

Choi, D.W., 1992. Excitotoxic cell death. J Neurobiol 23, 1261-1276.

Feng, S., Xu, Z., Liu, W., Li, Y., Deng, Y., Xu, B., 2014. Preventive effects of dextromethorphan on methylmercury-induced glutamate dyshomeostasis and oxidative damage in rat cerebral cortex. Biol Trace Elem Res 159, 332-345.

Fonfria, E., Vilaro, M.T., Babot, Z., Rodriguez-Farre, E., Sunol, C., 2005. Mercury compounds disrupt neuronal glutamate transport in cultured mouse cerebellar granule cells. J Neurosci Res 79, 545-553.

Harris, K.D., Weiss, M., Zahavi, A., 2014. Why are neurotransmitters neurotoxic? An evolutionary perspective. F1000Res 3, 179.

Juarez, B.I., Martinez, M.L., Montante, M., Dufour, L., Garcia, E., Jimenez-Capdeville, M.E., 2002. Methylmercury increases glutamate extracellular levels in frontal cortex of awake rats. Neurotoxicol Teratol 24, 767-771.

Lafon-Cazal, M., Pietri, S., Culcasi, M., Bockaert, J., 1993. NMDA-dependent superoxide production and neurotoxicity. Nature 364, 535-537.

Liu, W., Xu, Z., Deng, Y., Xu, B., Wei, Y., Yang, T., 2013. Protective effects of memantine against methylmercury-induced glutamate dyshomeostasis and oxidative stress in rat cerebral cortex. Neurotox Res 24, 320-337.

LoPachin, R.M., Schwarcz, A.I., Gaughan, C.L., Mansukhani, S., Das, S., 2004. In vivo and in vitro effects of acrylamide on synaptosomal neurotransmitter uptake and release. Neurotoxicology 25, 349-363.

Lovell, M.A., Xie, C., Markesbery, W.R., 2000. Acrolein, a product of lipid peroxidation, inhibits glucose and glutamate uptake in primary neuronal cultures. Free Radic Biol Med 29, 714-720.

Maragakis, N.J., Rothstein, J.D., 2001. Glutamate transporters in neurologic disease. Arch Neurol 58, 365-370

Meldrum, B.S., 2000. Glutamate as a neurotransmitter in the brain: review of physiology and pathology. J Nutr 130, 1007S-1015S.

Moretto, M.B., Funchal, C., Santos, A.Q., Gottfried, C., Boff, B., Zeni, G., Pureur, R.P., Souza, D.O., Wofchuk, S., Rocha, J.B., 2005. Ebselen protects glutamate uptake inhibition caused by methyl mercury but does not by Hg2+. Toxicology 214, 57-66.

Morken, T.S., Sonnewald, U., Aschner, M., Syversen, T., 2005. Effects of methylmercury on primary brain cells in mono- and co-culture. Toxicol Sci 87, 169-175.

Ozawa, S., Kamiya, H., Tsuzuki, K., 1998. Glutamate receptors in the mammalian central nervous system. Prog Neurobiol 54, 581-618.

Pivovarova, N.B., Andrews, S.B., 2010. Calcium-dependent mitochondrial function and dysfunction in neurons. FEBS J 277, 3622-3636.

Porciuncula, L.O., Rocha, J.B., Tavares, R.G., Ghisleni, G., Reis, M., Souza, D.O., 2003. Methylmercury inhibits glutamate uptake by synaptic vesicles from rat brain. Neuroreport 14, 577-580.

Reynolds, J.N., Racz, W.J., 1987. Effects of methylmercury on the spontaneous and potassium-evoked release of endogenous amino acids from mouse cerebellar slices. Can J Physiol Pharmacol 65, 791-798.

Szydlowska, K., Tymianski, M., 2010. Calcium, ischemia and excitotoxicity. Cell Calcium 47, 122-129.

Tao, X., West, A.E., Chen, W.G., Corfas, G., Greenberg, M.E., 2002. A calcium-responsive transcription factor, CaRF, that regulates neuronal activity-dependent expression of BDNF. Neuron 33, 383-395.

Tian, S.M., Ma, Y.X., Shi, J., Lou, T.Y., Liu, S.S., Li, G.Y., 2015. Acrylamide neurotoxicity on the cerebrum of weaning rats. Neural Regen Res 10, 938-943.

Xu, B., Xu, Z.F., Deng, Y., Liu, W., Yang, H.B., Wei, Y.G., 2012. Protective effects of MK-801 on methylmercury-induced neuronal injury in rat cerebral cortex: involvement of oxidative stress and glutamate metabolism dysfunction. Toxicology 300, 112-120.

Yin, Z., Milatovic, D., Aschner, J.L., Syversen, T., Rocha, J.B., Souza, D.O., Sidoryk, M., Albrecht, J., Aschner, M., 2007. Methylmercury induces oxidative injury, alterations in permeability and glutamine transport in cultured astrocytes. Brain Res 1131, 1-10.

Zhao, H., Alam, A., San, C.Y., Eguchi, S., Chen, Q., Lian, Q., Ma, D., 2017. Molecular mechanisms of brain-derived neurotrophic factor in neuro-protection: Recent developments. Brain Res 1665, 1-21.

Relationship: 365: N/A, Cell injury/death leads to N/A, Neuroinflammation

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

California sea lions that have been exposed to the marine biotoxin DomA developed an acute or chronic toxicosis marked by seizures, whereas histopathological analysis revealed neuroinflammation characterised by gliosis (Kirkley et al., 2014).

Key Event Relationship Description

The pioneering work of Kreutzberg and coworkers (1995, 1996) has shown that neuronal injury leads to neuroinflammation, with microglia and astrocyte reactivities. Several chemokines and chemokines receptors (fraktalkine, CD200) control the neuron-microglia interactions, and a loss of this control can trigger microglial reactivity (Blank and Prinz, 2013; Chapman et al., 2000; Streit et al., 2001). Upon injury causing neuronal death (mainly necrotic), signals termed Damage-Associated Molecular Patterns (DAMPs) are released by damaged neurons and promote microglial reactivity (Marin-Teva et al., 2011; Katsumoto et al., 2014). Toll-like receptors (TLRs) are pattern-recognition receptors that recognize specific pathogen- and danger-associated molecular signatures (PAMPs and DAMPs) and subsequently initiate inflammatory and immune responses. Microglial cells express TLRs, mainly TLR-2, which can detect neuronal cell death (for review, see Hayward and Lee, 2014). TLR-2 functions as a master sentry receptor to detect neuronal death and tissue damage in many different neurological conditions including nerve trans-section injury, traumatic brain injury and hippocampal excitotoxicity (Hayward and Lee, 2014). Astrocytes, the other cellular mediator of neuroinflammation (Ranshoff and Brown, 2012) are also able to sense tissue injury via TLR-3 (Farina et al., 2007; Rossi, 2015).

Evidence Supporting this KER

It is widely accepted that cell/neuronal injury and death lead to neuroinflammation (microglial and astrocyte reactivities) in adult brain. In the developing brain, neuroinflammation was observed after neurodegeneration induced by excitotoxic lesions (Acarin et al., 1997; Dommergues et al., 2003) or after ethanol exposure (Tiwari et al., 2012; Ahmad et al., 2016). It is important to note that physiological activation of microglial cells is observed during normal brain development for removal of apoptotic debris (Ashwell 1990, 1991). But exposure to toxicant (ethanol), excitotoxic insults (kainic acid) or traumatic brain injury during development can also induce apoptosis in hippocampus and cerebral cortex, as measured either by TUNEL, BID or caspase 3 upregulation associated to an inflammatory response, as evidenced by increased level of pro- inflammatory cytokines IL-1b, TNF-a, of NO, of p65 NF-kB or of the marker of astrogliosis, glial fibrillary acidic protein (GFAP), suggesting that, during brain development, neuroinflammation can also be triggerred by apoptosis induced by several types of insult (Tiwari and Chopra, 2012; Baratz et al., 2015; Mesuret et al., 2014).

Include consideration of temporal concordance here

Pb

In 3D cultures prepared from fetal rat brain cells exposed to Pb (10^-6 - 10^-4 M for 10 days), Pb-induced neuronal death was evidenced by a decrease of cholinergic and GABAergic markers associated to a decrease in protein content, and was accompanied by microglial and astrocyte reactivities (Zurich et al., 2002). These effects were more pronounced in immature than in differentiated cultures (Zurich et al., 2002). In adult rats, exposure to 100 ppm of Pb for 8 weeks caused neuronal death, evidenced by an increase in apoptosis (TUNEL) that was associated with microglial reactivity and an increase in IL-1b, TNF-a and i-NOS expression (Liu et al., 2012). Acute exposure to Pb (25 mg/kg, ip, for 3 days) increased GFAP and glutamate synthetase expression with impaiment of glutamate uptake and probable neuronal injury (Struzunska, 2000; Struzunska et al., 2001).

It is interesting to note that glial cells and in particular astrocytes are able to accumulate lead, suggesting that thes cells may be also a primary target of lead neurotoxic effects (Zurich et al., 1998; Lindhal et al., 1999). 

Domoic acid

• Astrogliosis is one of the histopathological findings revealed by the assessment of brains derived from patients diagnosed with Amnesic Shellfish Poisoning (ASP) (reviewed in Pulido, 2008). In a reference study, where the brain of a patient after acute DomA intoxication has been examined in great detail gliosis has been detected in the overlying cortex, dorsal and ventral septal nuclei, the secondary olfactory areas and the nucleus accumbens (Cendes et al., 1995). Reactive astrogliosis has also been confirmed in the sixth cortical layer and subjacent white matter in the orbital and lateral basal areas, the first and second temporal gyri, the fusiform gyrus, the parietal parasagittal cortex, and the insula (Cendes et al., 1995).

• Adult rats have been assessed seven days after the administration of DomA (2.25 mg/kg i.p.) and revealed astrocytosis identified by glial fibrillary acidic protein (GFAP)-immunostaining and activation of microglia by GSI-B4 histochemistry (Appel et al., 1997). More investigators have suggested that DomA can activate microglia (Ananth et al., 2001; Chandrasekaran et al., 2004).

• DomA treatment (2 mg/kg once a day for 3 weeks) in mice significantly stimulates the expression of inflammatory mediators, including IL-1β (1.7 fold increase), TNF-α (2 fold increase), GFAP (1.4 fold increase), Cox-2 (3 fold increase), and iNOS (1.6 fold increase) compared to controls (Lu et al, 2013).

• Adult female and male mice have been injected i.p. with 4mg/kg (LD50) of DomA and Real-time PCR has been performed in the brain derived at 30, 60 and 240 min post-injection. The inflammatory response element cyclooxygenase 2 (COX-2) has been found to be 8 fold increased at the 30 and 60 min time points and then showed a descent back toward basal expression levels by 240 min (Ryan et al., 2005).

• Adult male rats treated with 2 mg/kg DomA i.p. have been sacrificed after 3 or 7 d and shown that GFAP and lectin staining could identify regions of reactive gliosis within areas of neurodegeneration but at higher magnifications compared to the ones used for neurodegeneration (Appel et al., 1997; Scallet et al., 2005).

• At 5 days and 3 months following DomA administration of male Wistar rats, a large number of OX-42 positive microglial cells exhibiting intense immunoreactivity in CA1 and CA3 regions of the hippocampus have been detected. With an antibody against GFAP, immunoreactive astrocytes have been found to be sparsely distributed in the hippocampus derived from DomA treated rats after 3 months' time interval (Ananth et al., 2003). At 5 days after the administration of DomA, GFAP positive astrocytes have been found increased in the hippocampus (Ananth et al., 2003).

Mercury

Young mice receiving a fish diet (MeHgCl)  for 3 months exhibited in cortex a decrease of the chemokine Ccl₂ and neuronal death, as measured by a decrease in cell density, as well as microglial reactivity (increase in Iba1-labelled cells) (Godefroy et al., 2012)

Perinatal exposure to MeHgCl (GD7-PD21, 0.5 mg/kg bw/day in drinking water) lead to a delayed decrease (PD 36) of cholinergic muscarinic receptors in cerebellum accompanied by astrogliosis (Roda et al., 2008).

Immature rat brain cell cultures maintained in 3D conditions were exposed to either MeHgCl or HgCl₂ (10^-9 – 10^-6 M, for 10 days). This treatment caused microglial and astrocyte activation without neuronal death, but a reversible decrease of the expression of the neuronal marker MAP2 (Monnet-Tschudi et al., 1996 ; Eskes et al., 2002).

Adult marmoset exposed acutely to 5 mg Hg/kg/day p.o. exhibited apoptosis in occipital cortex, as well as glial reactivity (GFAP and Iba1 increased). Mercury content in occipital cortex was 31 mg/g (Yamamoto et al., 2012).

Monkeys exposed to MeHgCl (50 mg/bw for 6,12,18 months) showed microglial and astrocyte activation without any change in neuronal number. Both astrocyte and microglia accumulated elevated levels of inorganic mercury, suggesting a direct effect of mercury on glial cells (Charleston et al., 1996).

Human LUHMES cells as model of dopaminergic neurons and  the human astrocyte cell line CFF-STTG1 were exposed to MeHgCl (0.25 -5 mM), thiomersal (0.25 – 5 mM) or HgCl2 (5-35 mM), what affected their cell viability. Neurons were much more sensitive than astrocytes (Lohner et al., 2015).

A direct activation of rat primary microglial cells and astrocytes was observed after exposure to MeHgCl (10^-10-10^-6 M, for 5 days). (Eskes et al., 2002).

Astrocyte + microglia in co-cultures exposed to mercury (1-5 mM for 30 min to 6 days) showed lower levels of GSH in microglia than in astrocytes (Ni et al., 2011 ; 2012).

Human primary astrocyte cell line exposed to MeHgCl (1.125 mM) for 24h and 72h did not exhibit an increase of GFAP, but of NfkB after the 72h (Malfa et al., 2014).

Human mast cells (leukemic LAD2, derived from umbilical cord blood) showed an increase of IL-6 release when exposed to HgCl₂ (0.1-10 mM, for 10 min to 24h). It is hypothesized that mast cell activation could lead to BBB disruption and to neuroinflammation. (Kempurai et al., 2010).

Sex dependency

In prairie voles 10 weeks exposure to 600 ppm HgCl₂ in drinking water lead to an increase of TNF-a in hippocampus of male, but not in female (Curtis et al., 2011).

Acrylamide (acrylamide is a common food contaminant generated by heat processing)

Adult mice received 10, 20, 30 mg/kg bw for 4 weeks. The dose of 20 mg/kg bw caused neurological symptoms (ex. cognitive impairment) associated to an increased oxidative stress, a decrease of GSH and glial reactivity (GFAP and Iba1 increased) in cortex, hippocampus and striatum. An increase in TNF-a, IL-1b and i-NOS expression in all 3 brain regions was also observed. (Santhanasabepathy et al., 2015)

Isolated and/or co-cultures of microglial cells or astrocytes treated with acrylamide 0-1mM for 24-96h exhibited an increased release of TNF-a, IL-1b, IL-6 and G-CSF, suggesting a direct effect of acrylamide on glial cells (Zhao et al., 2017a,b).

Neonatal rat astrocytes treated with acrylamide (0.1-1mM) for 7, 11, 15, or 20 days increased their proliferation rate as measured by PCNA staining. Astrocyte proliferation is also a sign of reactivity. (Aschner et al., 2005).

Acrolein

Adult rat received an infusion of acrolein (15, 50, 150 nmoles/0.5 ml) directly in substantia nigra which caused a decrease of Tyrosine hydroxylase immunostaining, an increase in caspase 1 and an activation of microglial cells and astrocytes (Wang et al., 2017).

Similar treatment as above induced an increase in lipid peroxidation, of hsp32 and of caspase 1 with an increase in GFAP and in ED1 (marker of macrophagic microglial cells) as well as of IL-1b (Zhao et al., 2017).

Pb

Sobin and coworkers (2013) described a Pb-induced decrease in dentate gyrus volume associated with microglial reactivity at low dose of Pb (30 ppm), but not at high doses (330 ppm), plausibly due to the death of microglial cells at the high dose of Pb.

Pb decreased IL-6 secretion by isolated astrocytes (Quian et al., 2007). Such a decrease was also observed in isolated astrocytes treated with methylmercury, and was reverted in microglia astrocyte co-cultures, suggesting that cell-cell interactions can modify the response to a toxicant (Eskes et al., 2002). It is interesting to note that glial cells and in particular astrocytes are able to accumulate lead, suggesting that thes cells may be also a primary target of lead neurotoxic effects (Zurich et al., 1998; Lindhal et al., 1999).

Domoic acid

Adult male and female Sprague Dawley rats have received a single intraperitoneal (i.p.) injection of DomA (0, 1.0, 1.8 mg/kg) and have been sacrificed 3 h after the treatment. Histopathological analysis of these animals has shown no alterations for GFAP immunostaining in the dorsal hippocampus and olfactory bulb, indicating absence of reactive gliosis (Baron et al., 2013).

The exposed zebrafish from the 36-week treatment with DomA showed no neuroinflammation in brain (Hiolski et al., 2014). At the same time, microarray analysis revealed no significant changes in gfap gene expression, a marker of neuroinflammation and astrocyte activation (Hiolski et al., 2014).

Mercury

Mouse developmental exposure to 50 mM of HgCl₂ in maternal drinking water from GD8 to PD21 did not induce any change in GM-CSF, IFN-g, IL-1b, IL-2, IL-4, IL-5, IL-6, IL-10, IL-12p70. IL-13, IL-17, MCP1, MIP2 and TNF-a measured by Luminex in brain slices of PD21 and PD70. No sex differences, but brain increase of IgG and increased sociability in females (Zhang et al., 2012).

3D rat brain cell cultures treated for 10 days with HgCl2 or MeHgCl (10-10 - 10-6 M) exhibited increased apotosis measured by TUNEL, but exclusively in immature cultures. The proportion of cells undergoing apoptotis  was highest for astrocytes than for neurons. But the apoptotic nuclei were not associated with reactive microglial cells as evidenced by double staining (Monnet-Tschudi, 1998).

Acrylamide

A 2 weeks exposure to acrylamide in drinking water (44mg/kg/day) induced behavioral effects, such a decreased in locomotor activity, but with no effect at gene level on neuronal and inflammatory markers analyzed in somatosensory and motor cortex (Bowyer et al., 2009).

References

Acarin L, González B, Castellano B, Castro AJ. 1997. Quantitative analysis of microglial reaction to a cortical excitotoxic lesion in the early postnatal brain. ExpNeurol 147: 410-417.

Ahmad A, Shah SA, Badshah H, Kim MJ, Ali T, Yoon GH, et al. 2016. Neuroprotection by Vitamin C Against Ethanol-Induced Neuroinflammation Associated Neurodegeneration in the Developing Rat Brain. CNS Neurol Disord Drug Targets 15(3): 360-370.

Ananth C, Thameem DS, Gopalakrishnakone P, Kaur C. Domoic acid-induced neuronal damage in the rat hippocampus: changes in apoptosis related genes (bcl-2, bax, caspase-3) and microglial response. J Neurosci Res., 2001, 66: 177-190.

Ananth C, Gopalakrishnakone P, Kaur C. Induction of inducible nitric oxide synthase expression in activated microglia following domoic acid (DA)-induced neurotoxicity in the rat hippocampus. Neurosci Lett., 2003, 338: 49-52.

Appel NM, Rapoport SI, O’Callaghan JP, Bell JM, Freed LM. Sequelae of parenteral domoic acid administration in rats: comparison of effects on different metabolic markers in brain. Brain Res., 1997, 754: 55-64.

Aschner, M., Wu, Q., Friedman, M.A., 2005. Effects of acrylamide on primary neonatal rat astrocyte functions. Ann N Y Acad Sci. 1053, 444-54.

Ashwell K. 1990. Microglia and cell death in the developing mouse cerebellum. DevBrain Res 55: 219-230.

Ashwell K. 1991. The distribution of microglia and cell death in the fetal rat forebrain. DevBrain Res 58: 1-12.

Baratz R, Tweedie D, Wang JY, Rubovitch V, Luo W, Hoffer BJ, et al. 2015. Transiently lowering tumor necrosis factor-alpha synthesis ameliorates neuronal cell loss and cognitive impairments induced by minimal traumatic brain injury in mice. J Neuroinflammation 12: 45.

Baron AW, Rushton SP, Rens N, Morris CM, Blain PG, Judge SJ. Sex differences in effects of low level domoic acid exposure. Neurotoxicology, 2013, 34: 1-8.

Blank T, Prinz M. Microglia as modulators of cognition and neuropsychiatric disorders. Glia, 2013, 61: 62-70.

Bowyer, J.F., et al., 2009. The mRNA expression and histological integrity in rat forebrain motor and sensory regions are minimally affected by acrylamide exposure through drinking water. Toxicol Appl Pharmacol. 240, 401-11.

Chandrasekaran A, Ponnambalam G, Kaur C. Domoic acid-induced neurotoxicity in the hippocampus of adult rats. Neurotox Res., 2004, 6:1 05-117.

Chapman GA, Moores K, Harrison D, Campbell CA, Stewart BR, Strijbos PJLM. Fractalkine Cleavage from Neuronal Membrans Represents an Acute Event in Inflammatory Response to Excitotoxic Brain Damage. J Neurosc., 2000, 20 RC87: 1-5.

Charleston JS, Body RL, Bolender RP, Mottet NK, Vahter ME, Burbacher TM: Changes in the number of astrocytes and microglia in the thalamus of the monkey Macaca fascicularis following long-term subclinical methylmercury exposure. NeuroToxicology 1996, 17:127-138.

Cendes F, Andermann F, Carpenter S, Zatorre RJ, Cashman NR. Temporal lobe epilepsy caused by domoic acid intoxication: evidence for glutamate receptor-mediated excitotoxicity in humans. Ann Neurol., 1995, 37: 123-126.

Thomas Curtis, J., et al., 2011. Chronic inorganic mercury exposure induces sex-specific changes in central TNFalpha expression: importance in autism? Neurosci Lett. 504, 40-4.

Dommergues MA, Plaisant F, Verney C, Gressens P. 2003. Early microglial activation following neonatal excitotoxic brain damage in mice: a potential target for neuroprotection. Neuroscience 121(3): 619-628.

Eskes C, Honegger P, Juillerat-Jeanneret L, Monnet-Tschudi F. 2002. Microglial reaction induced by noncytotoxic methylmercury treatment leads to neuroprotection via interactions with astrocytes and IL-6 release. Glia 37(1): 43-52.

Farina C, Aloisi F, Meinl E. Astrocytes are active players in cerebral innate immunity. Trends Immunol, 2007, 28(3): 138-145.

Godefroy, D., et al., 2012. The chemokine CCL2 protects against methylmercury neurotoxicity. Toxicol Sci. 125, 209-18.

Hayward JH, Lee SJ. A Decade of Research on TLR2 Discovering Its Pivotal Role in Glial Activation and Neuroinflammation in Neurodegenerative Diseases. Experimental Neurobiology, 2014, 23(2): 138-147.

Hiolski EM, Kendrick PS, Frame ER, Myers MS, Bammler TK, Beyer RP, Farin FM, Wilkerson HW, Smith DR, Marcinek DJ, Lefebvre KA., Chronic low-level domoic acid exposure alters gene transcription and impairs mitochondrial function in the CNS. Aquat Toxicol., 2014, 155: 151-159.

Katsumoto A, Lu H, Miranda AS, Ransohoff RM. Ontogeny and functions of central nervous system macrophages. J Immunol., 2014, 193(6): 2615-2621.

Kempuraj, D., et al., 2010. Mercury induces inflammatory mediator release from human mast cells. J Neuroinflammation. 7, 20.

Kirkley KS, Madl JE, Duncan C, Gulland FM, Tjalkens RB. Domoic acid-induced seizures in California sea lions (Zalophus californianus) are associated with neuroinflammatory brain injury. Aquat Toxicol., 2014, 156C: 259-268.

Kreutzberg GW. Microglia, the first line of defence in brain pathologies. Arzneimttelforsch, 1995, 45: 357-360.

Kreutzberg GW. Microglia : a sensor for pathological events in the CNS. Trends Neurosci., 2006, 19: 312-318.

Lindhal LS, Bird L, Legare ME, Mikeska G, Bratton GR, Tiffany-Castiglioni E. 1999. Differential ability of astroglia and neuronal cells to accumulate lead: Dependence on cell type and on degree of differentiation. ToxSci 50: 236-243.

Liu MC, Liu XQ, Wang W, Shen XF, Che HL, Guo YY, et al., Involvement of microglia activation in the lead induced long-term potentiation impairment. PLoS One, 2012, 7(8): e43924.

Lohren, H., et al., 2015. Toxicity of organic and inorganic mercury species in differentiated human neurons and human astrocytes. J Trace Elem Med Biol. 32, 200-8.

Lu J, Wu DM, Zheng YL, Hu B, Cheng W, Zhang ZF, Li MQ. Troxerutin counteracts domoic acid-induced memory deficits in mice by inhibiting CCAAT/enhancer binding protein β-mediated inflammatory response and oxidative stress. J Immunol., 2013, 190: 3466-3479.

Malfa, G.A., et al., 2014. "Reactive" response evaluation of primary human astrocytes after methylmercury exposure. J Neurosci Res. 92, 95-103.

Marin-Teva JL, Cuadros MA, Martin-Oliva D, Navascues J., Microglia and neuronal cell death. Neuron glia biology, 2011, 7(1): 25-40.

Mesuret G, Engel T, Hessel EV, Sanz-Rodriguez A, Jimenez-Pacheco A, Miras-Portugal MT, et al. 2014. P2X7 receptor inhibition interrupts the progression of seizures in immature rats and reduces hippocampal damage. CNS neuroscience & therapeutics 20(6): 556-564.

Ni, M., et al., 2011. Comparative study on the response of rat primary astrocytes and microglia to methylmercury toxicity. Glia. 59, 810-20.

Ni, M., et al., 2012. Glia and methylmercury neurotoxicity. J Toxicol Environ Health A. 75, 1091-101.

Pulido OM. Domoic acid toxicologic pathology: a review. Mar Drugs, 2008, 6: 180-219.

Qian Y, Zheng Y, Weber D, Tiffany-Castiglioni E. 2007. A 78-kDa glucose-regulated protein is involved in the decrease of interleukin-6 secretion by lead treatment from astrocytes. American journal of physiology Cell physiology 293(3): C897-905.

Ransohoff RM, Brown MA. Innate immunity in the central nervous system. J Clin Invest., 2012, 122(4): 1164-1171.

Roda, E., et al., 2008. Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat. J Chem Neuroanat. 35, 285-94.

Rossi D. Astrocyte physiopathology: At the crossroads of intercellular networking, inflammation and cell death. Prog Neurobiol., 2015, 130: 86-120.

Ryan JC, Morey JS, Ramsdell JS, Van Dolah FM. Acute phase gene expression in mice exposed to the marine neurotoxin domoic acid. Neuroscience, 2005, 136: 1121-1132.

Santhanasabapathy, R., et al., 2015. Farnesol quells oxidative stress, reactive gliosis and inflammation during acrylamide-induced neurotoxicity: Behavioral and biochemical evidence. Neuroscience. 308, 212-27.

Scallet AC, Schmued LC, Johannessen JN. Neurohistochemical biomarkers of the marine neurotoxicant, domoic acid. Neurotoxicol Teratol., 2005, 27: 745-752.

Sobin C, Montoya MG, Parisi N, Schaub T, Cervantes M, Armijos RX. 2013. Microglial disruption in young mice with early chronic lead exposure. Toxicol Lett 220(1): 44-52.

Streit WJ, Conde J, Harrison JK. Chemokines and Alzheimer's disease. Neurobiol Aging., 2001, 22: 909-913.

Struzynska L. 2000. The protective role of astroglia in the early period of experimental lead toxicity in the rat. Acta Neurobiol Exp (Wars) 60(2): 167-173.

Struzynska L, Bubko I, Walski M, Rafalowska U. 2001. Astroglial reaction during the early phase of acute lead toxicity in the adult rat brain. Toxicology 165: 121-131.

Tiwari V, Chopra K. 2012. Attenuation of oxidative stress, neuroinflammation, and apoptosis by curcumin prevents cognitive deficits in rats postnatally exposed to ethanol. Psychopharmacology (Berl) 224(4): 519-535.

Wang, Y.T., et al., 2017. Acrolein acts as a neurotoxin in the nigrostriatal dopaminergic system of rat: involvement of alpha-synuclein aggregation and programmed cell death. Sci Rep. 7, 45741.

Yamamoto, M., et al., 2012. Increased expression of aquaporin-4 with methylmercury exposure in the brain of the common marmoset. J Toxicol Sci. 37, 749-63.

Zhang, Y., Bolivar, V.J., Lawrence, D.A., 2012. Developmental exposure to mercury chloride does not impair social behavior of C57BL/6 x BTBR F(1) mice. J Immunotoxicol. 9, 401-10.

Zhao, M., et al., 2017. Effect of acrylamide-induced neurotoxicity in a primary astrocytes/microglial co-culture model. Toxicol In Vitro. 39, 119-125.

Zhao, M., et al., 2017. Acrylamide-induced neurotoxicity in primary astrocytes and microglia: Roles of the Nrf2-ARE and NF-kappaB pathways. Food Chem Toxicol. 106, 25-35.

Zhao, W.Z., et al., 2017. Neuroprotective Effects of Baicalein on Acrolein-induced Neurotoxicity in the Nigrostriatal Dopaminergic System of Rat Brain. Mol Neurobiol.

Zurich MG, Monnet-Tschudi F, Berode M, Honegger P. 1998. Lead acetate toxicity in vitro: Dependence on the cell composition of the cultures. Toxicol In Vitro 12(2): 191-196.

Zurich M-G, Eskes C, Honegger P, Bérode M, Monnet-Tschudi F. 2002. Maturation-dependent neurotoxicity of lead aceate in vitro: Implication of glial reactions. J Neurosc Res 70: 108-116.

Relationship: 1718: N/A, Cell injury/death leads to Tissue resident cell activation

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

The pioneering work of Kreutzberg and coworkers (1995, 1996) has shown that neuronal injury leads to neuroinflammation, with microglia and astrocyte reactivity. Several chemokines and chemokines receptors (fraktalkine, CD200) control the neuron-microglia interactions, and a loss of this control can trigger microglial reactivity (Blank and Prinz, 2013; Chapman et al., 2000; Streit et al., 2001). Upon injury causing neuronal death (mainly necrotic), signals termed Damage-Associated Molecular Patterns (DAMPs) are released by damaged neurons and promote microglial reactivity (Marin-Teva et al., 2011; Katsumoto et al., 2014). Toll-like receptors (TLRs) are pattern-recognition receptors that recognize specific pathogen- and danger-associated molecular signatures (PAMPs and DAMPs) and subsequently initiate inflammatory and immune responses. Microglial cells express TLRs, mainly TLR-2, which can detect neuronal cell death (for review, see Hayward and Lee, 2014). TLR-2 functions as a master sentry receptor to detect neuronal death and tissue damage in many different neurological conditions including nerve trans-section injury, traumatic brain injury and hippocampal excitotoxicity (Hayward and Lee, 2014). Astrocytes, the other cellular mediator of neuroinflammation (Ranshoff and Brown, 2012) are also able to sense tissue injury via TLR-3 (Farina et al., 2007; Rossi, 2015).

Evidence Supporting this KER

It is widely accepted that cell/neuronal injury and death lead to neuroinflammation (microglial and astrocyte reactivities) in adult brain. In the developing brain, neuroinflammation was observed after neurodegeneration induced by excitotoxic lesions (Acarin et al., 1997; Dommergues et al., 2003) or after ethanol exposure (Tiwari et al., 2012; Ahmad et al., 2016). It is important to note that physiological activation of microglial cells is observed during normal brain development for removal of apoptotic debris (Ashwell 1990, 1991). But exposure to toxicant (ethanol), excitotoxic insults (kainic acid) or traumatic brain injury during development can also induce apoptosis in hippocampus and cerebral cortex, as measured either by TUNEL, BID or caspase 3 upregulation associated to an inflammatory response, as evidenced by increased level of pro- inflammatory cytokines IL-1b, TNF-a, of NO, of p65 NF-kB or of the marker of astrogliosis, glial fibrillary acidic protein (GFAP), suggesting that, during brain development, neuroinflammation can also be triggerred by apoptosis induced by several types of insult (Tiwari and Chopra, 2012; Baratz et al., 2015; Mesuret et al., 2014).

Mercury

Young mice receiving a fish diet (MeHgCl)  for 3 months exhibited in cortex a decrease of the chemokine Ccl₂ and neuronal death, as measured by a decrease in cell density, as well as microglial reactivity (increase in Iba1-labelled cells) (Godefroy et al., 2012)

Perinatal exposure to MeHgCl (GD7-PD21, 0.5 mg/kg bw/day in drinking water) lead to a delayed decrease (PD 36) of cholinergic muscarinic receptors in cerebellum accompanied by astrogliosis (Roda et al., 2008).

Immature rat brain cell cultures maintained in 3D conditions were exposed to either MeHgCl or HgCl₂ (10^-9 – 10^-6 M, for 10 days). This treatment caused microglial and astrocyte activation without neuronal death, but a reversible decrease of the expression of the neuronal marker MAP2 (Monnet-Tschudi et al., 1996 ; Eskes et al., 2002).

Adult marmoset exposed acutely to 5 mg Hg/kg/day p.o. exhibited apoptosis in occipital cortex, as well as glial reactivity (GFAP and Iba1 increased). Mercury content in occipital cortex was 31 mg/g (Yamamoto et al., 2012).

Monkeys exposed to MeHgCl (50 mg/bw for 6,12,18 months) showed microglial and astrocyte activation without any change in neuronal number. Both astrocyte and microglia accumulated elevated levels of inorganic mercury, suggesting a direct effect of mercury on glial cells (Charleston et al., 1996).

Human LUHMES cells as model of dopaminergic neurons and  the human astrocyte cell line CFF-STTG1 were exposed to MeHgCl (0.25 -5 mM), thiomersal (0.25 – 5 mM) or HgCl2 (5-35 mM), what affected their cell viability. Neurons were much more sensitive than astrocytes (Lohner et al., 2015).

A direct activation of rat primary microglial cells and astrocytes was observed after exposure to MeHgCl (10^-10-10^-6 M, for 5 days). (Eskes et aé., 2002).

Astrocyte + microglia in co-cultures exposed to mercury (1-5 mM for 30 min to 6 days) showed lower levels of GSH in microglia than in astrocytes (Ni et al., 2011 ; 2012).

Human primary astrocyte cell line exposed to MeHgCl (1.125 mM) for 24h and 72h did not exhibit an increase of GFAP, but of NfkB after the 72h (Malfa et al., 2014).

Human mast cells (leukemic LAD2, derived from umbilical cord blood) showed an increase of IL-6 release when exposed to HgCl₂ (0.1-10 mM, for 10 min to 24h). It is hypothesized that mast cell activation could lead to BBB disruption and to neuroinflammation. (Kempurai et al., 2010).

Sex dependency

In prairie voles 10 weeks exposure to 600 ppm HgCl₂ in drinking water lead to an increase of TNF-a in hippocampus of male, but not in female (Curtis et al., 2011).

Acrylamide (acrylamide is a common food contaminant generated by heat processing)

Adult mice received 10, 20, 30 mg/kg bw for 4 weeks. The dose of 20 mg/kg bw caused neurological symptoms (ex. cognitive impairment) associated to an increased oxidative stress, a decrease of GSH and glial reactivity (GFAP and Iba1 increased) in cortex, hippocampus and striatum. An increase in TNF-a, IL-1b and i-NOS expression in all 3 brain regions was also observed. (Santhanasabepathy et al., 2015)

Isolated and/or co-cultures of microglial cells or astrocytes treated with acrylamide 0-1mM for 24-96h exhibited an increased release of TNF-a, IL-1b, IL-6 and G-CSF, suggesting a direct effect of acrylamide on glial cells (Zhao et al., 2017a,b).

Neonatal rat astrocytes treated with acrylamide (0.1-1mM) for 7, 11, 15, or 20 days increased their proliferation rate as measured by PCNA staining. Astrocyte proliferation is also a sign of reactivity. (Aschner et al., 2005).

Acrolein

Adult rat received an infusion of acrolein (15, 50, 150 nmoles/0.5 ml) directly in substantia nigra which caused a decrease of Tyrosine hydroxylase immunostaining, an increase in caspase 1 and an activation of microglial cells and astrocytes (Wang et al., 2017).

Similar treatment as above induced an increase in lipid peroxidation, of hsp32 and of caspase 1 with an increase in GFAP and in ED1 (marker of macrophagic microglial cells) as well as of IL-1b (Zhao et al., 2017).

Mercury

Mouse developmental exposure to 50 mM of HgCl₂ in maternal drinking water from GD8 to PD21 did not induce any change in GM-CSF, IFN-g, IL-1b, IL-2, IL-4, IL-5, IL-6, IL-10, IL-12p70. IL-13, IL-17, MCP1, MIP2 and TNF-a measured by Luminex in brain slices of PD21 and PD70. No sex differences, but brain increase of IgG and increased sociability in females (Zhang et al., 2012).

3D rat brain cell cultures treated for 10 days with HgCl2 or MeHgCl (10-10 - 10-6 M) exhibited increased apotosis measured by TUNEL, but exclusively in immature cultures. The proportion of cells undergoing apoptotis  was highest for astrocytes than for neurons. But the apoptotic nuclei were not associated with reactive microglial cells as evidenced by double staining (Monnet-Tschudi, 1998).

Acrylamide

A 2 weeks exposure to acrylamide in drinking water (44mg/kg/day) induced behavioral effects, such a decreased in locomotor activity, but with no effect at gene level on neuronal and inflammatory markers analyzed in somatosensory and motor cortex (Bowyer et al., 2009).

References

Acarin L, González B, Castellano B, Castro AJ. 1997. Quantitative analysis of microglial reaction to a cortical excitotoxic lesion in the early postnatal brain. ExpNeurol 147: 410-417.

Ahmad A, Shah SA, Badshah H, Kim MJ, Ali T, Yoon GH, et al. 2016. Neuroprotection by Vitamin C Against Ethanol-Induced Neuroinflammation Associated Neurodegeneration in the Developing Rat Brain. CNS Neurol Disord Drug Targets 15(3): 360-370.

Aschner, M., Wu, Q., Friedman, M.A., 2005. Effects of acrylamide on primary neonatal rat astrocyte functions. Ann N Y Acad Sci. 1053, 444-54.

Ashwell K. 1990. Microglia and cell death in the developing mouse cerebellum. DevBrain Res 55: 219-230.

Ashwell K. 1991. The distribution of microglia and cell death in the fetal rat forebrain. DevBrain Res 58: 1-12.

Baratz R, Tweedie D, Wang JY, Rubovitch V, Luo W, Hoffer BJ, et al. 2015. Transiently lowering tumor necrosis factor-alpha synthesis ameliorates neuronal cell loss and cognitive impairments induced by minimal traumatic brain injury in mice. J Neuroinflammation 12: 45.

Blank T, Prinz M. Microglia as modulators of cognition and neuropsychiatric disorders. Glia, 2013, 61: 62-70.

Bowyer, J.F., et al., 2009. The mRNA expression and histological integrity in rat forebrain motor and sensory regions are minimally affected by acrylamide exposure through drinking water. Toxicol Appl Pharmacol. 240, 401-11.

Chapman GA, Moores K, Harrison D, Campbell CA, Stewart BR, Strijbos PJLM. Fractalkine Cleavage from Neuronal Membrans Represents an Acute Event in Inflammatory Response to Excitotoxic Brain Damage. J Neurosc., 2000, 20 RC87: 1-5.

Charleston JS, Body RL, Bolender RP, Mottet NK, Vahter ME, Burbacher TM: Changes in the number of astrocytes and microglia in the thalamus of the monkey Macaca fascicularis following long-term subclinical methylmercury exposure. NeuroToxicology 1996, 17:127-138.

Thomas Curtis, J., et al., 2011. Chronic inorganic mercury exposure induces sex-specific changes in central TNFalpha expression: importance in autism? Neurosci Lett. 504, 40-4.

Dommergues MA, Plaisant F, Verney C, Gressens P. 2003. Early microglial activation following neonatal excitotoxic brain damage in mice: a potential target for neuroprotection. Neuroscience 121(3): 619-628.

Eskes C, Honegger P, Juillerat-Jeanneret L, Monnet-Tschudi F. 2002. Microglial reaction induced by noncytotoxic methylmercury treatment leads to neuroprotection via interactions with astrocytes and IL-6 release. Glia 37(1): 43-52.

Farina C, Aloisi F, Meinl E. Astrocytes are active players in cerebral innate immunity. Trends Immunol, 2007, 28(3): 138-145.

Godefroy, D., et al., 2012. The chemokine CCL2 protects against methylmercury neurotoxicity. Toxicol Sci. 125, 209-18.

Hayward JH, Lee SJ. A Decade of Research on TLR2 Discovering Its Pivotal Role in Glial Activation and Neuroinflammation in Neurodegenerative Diseases. Experimental Neurobiology, 2014, 23(2): 138-147.

Katsumoto A, Lu H, Miranda AS, Ransohoff RM. Ontogeny and functions of central nervous system macrophages. J Immunol., 2014, 193(6): 2615-2621.

Kempuraj, D., et al., 2010. Mercury induces inflammatory mediator release from human mast cells. J Neuroinflammation. 7, 20.

Kreutzberg GW. Microglia, the first line of defence in brain pathologies. Arzneimttelforsch, 1995, 45: 357-360.

Kreutzberg GW. Microglia : a sensor for pathological events in the CNS. Trends Neurosci., 2006, 19: 312-318.

Lohren, H., et al., 2015. Toxicity of organic and inorganic mercury species in differentiated human neurons and human astrocytes. J Trace Elem Med Biol. 32, 200-8.

Malfa, G.A., et al., 2014. "Reactive" response evaluation of primary human astrocytes after methylmercury exposure. J Neurosci Res. 92, 95-103.

Marin-Teva JL, Cuadros MA, Martin-Oliva D, Navascues J., Microglia and neuronal cell death. Neuron glia biology, 2011, 7(1): 25-40.

Mesuret G, Engel T, Hessel EV, Sanz-Rodriguez A, Jimenez-Pacheco A, Miras-Portugal MT, et al. 2014. P2X7 receptor inhibition interrupts the progression of seizures in immature rats and reduces hippocampal damage. CNS neuroscience & therapeutics 20(6): 556-564.

Ni, M., et al., 2011. Comparative study on the response of rat primary astrocytes and microglia to methylmercury toxicity. Glia. 59, 810-20.

Ni, M., et al., 2012. Glia and methylmercury neurotoxicity. J Toxicol Environ Health A. 75, 1091-101.

Ransohoff RM, Brown MA. Innate immunity in the central nervous system. J Clin Invest., 2012, 122(4): 1164-1171.

Roda, E., et al., 2008. Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat. J Chem Neuroanat. 35, 285-94.

Rossi D. Astrocyte physiopathology: At the crossroads of intercellular networking, inflammation and cell death. Prog Neurobiol., 2015, 130: 86-120.

Santhanasabapathy, R., et al., 2015. Farnesol quells oxidative stress, reactive gliosis and inflammation during acrylamide-induced neurotoxicity: Behavioral and biochemical evidence. Neuroscience. 308, 212-27.

Streit WJ, Conde J, Harrison JK. Chemokines and Alzheimer's disease. Neurobiol Aging., 2001, 22: 909-913.

Tiwari V, Chopra K. 2012. Attenuation of oxidative stress, neuroinflammation, and apoptosis by curcumin prevents cognitive deficits in rats postnatally exposed to ethanol. Psychopharmacology (Berl) 224(4): 519-535

Wang, Y.T., et al., 2017. Acrolein acts as a neurotoxin in the nigrostriatal dopaminergic system of rat: involvement of alpha-synuclein aggregation and programmed cell death. Sci Rep. 7, 45741.

Yamamoto, M., et al., 2012. Increased expression of aquaporin-4 with methylmercury exposure in the brain of the common marmoset. J Toxicol Sci. 37, 749-63.

Zhang, Y., Bolivar, V.J., Lawrence, D.A., 2012. Developmental exposure to mercury chloride does not impair social behavior of C57BL/6 x BTBR F(1) mice. J Immunotoxicol. 9, 401-10.

Zhao, M., et al., 2017. Effect of acrylamide-induced neurotoxicity in a primary astrocytes/microglial co-culture model. Toxicol In Vitro. 39, 119-125.

Zhao, M., et al., 2017. Acrylamide-induced neurotoxicity in primary astrocytes and microglia: Roles of the Nrf2-ARE and NF-kappaB pathways. Food Chem Toxicol. 106, 25-35.

Zhao, W.Z., et al., 2017. Neuroprotective Effects of Baicalein on Acrolein-induced Neurotoxicity in the Nigrostriatal Dopaminergic System of Rat Brain. Mol Neurobiol.

Relationship: 1687: N/A, Neuroinflammation leads to N/A, Cell injury/death

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

Cells of the innate (microglia and astrocytes) and of the adaptive (infiltrating monocytes and lymphocytes) immune system of the brain have various ways to kill neighboring cells. This is in part due to evolutionary-conserved mechanisms evolved to kill virus-infected cells or tumor cells; in part it is a bystander phenomenon due to the release of mediators that should activate other cells and contribute to the killing of invading micro-organisms. An exaggerated or unbalanced activation of immune cells can thus lead to parenchymal (neuronal) cell death (Gehrmann et al., 1995). Mediators known to have such effects comprise components of the complement system and cytolkines/death receptor ligands triggering programmed cell death (Dong and Benveniste, 2001). Various secreted proteases (e.g. matrix metalloproteases), lipid mediators (e.g. ceramide or gangliosides) or reactive oxygen species can contribute to bystander death of neurons (Chao et al., 1995; Nakajima et al., 2002; Brown and Bal-Price, 2003; Ktaft and Harry, 2011; Taetsch and Block, 2013). The equimolar production of superoxide and NO from glial cells can lead to high steady levels of peoxynitrite, which is a very potent cytotoxicant (Yuste et al., 2015). Already stressed neurons, with an impaired anti-oxidant defence system, are more sensitive to such mediators (Xu et al., 2015). Healthy cells continuously display anti "eat-me" signals, while damaged and stressed neurons/neurites display "eat-me" signals that may be recognized by microglia as signals to start phagocytosis (Neher et al., 2012). Reactive astrocytes are also able to release neurotoxic molecules (Mena and Garcia de Ybenes, 2008; Niranjan, 2014). However, astrocytes may also be protective due to their capacity to quench free radicals and secrete neurotrophic factors. The activation of astrocytes may reduce neurotrophic support to neurons (for review, Mena and Garcia de Ybenes, 2008).

Evidence Supporting this KER

In vitro co-culture experiments have demonstrated that reactive glial cells (microglia and astrocytes) can kill neurons (Chao et al., 1995; Brown and Bal-Price, 2003; Kraft and Harry, 2011; Taetzsch and Block, 2013) and that interventions with e.g. i-NOS inhibition can rescue the neurons (Yadav et al., 2012; Brzozowski et al., 2015). Drugs that block Toll like receptor pathways, which are expressed by glial cells have been proven to be protective by decreasing ROS and RNS production (Lucas et al., 2013).

Reactive microglia can remove synapses, a process known as synapse stripping (Banati et al., 1993; Kettenmann et al., 2013). Reactive astrocytes were also associated with neurite and synapse reduction (Calvo-Ochoa et al., 2014). Microglia can modulate synapse plasticity, an effect mediated by cytokines. During development, microglia can promote synaptogenesis or engulf synapses, a process known as synaptic pruning (for review, Jebelli et al., 2015). It is hypothesized that alterations in microglia functioning during synapse formation and maturation of the brain can have significant long-term effects on the final established neural circuits (for review, Harry and Kraft, 2012). The fact that astrocytes can receive and respond to the synaptic information produced by neuronal activity, owing to their expression of a wide range of neurotransmitter receptors, has given rise to the concept of tripartite synapse (for review, Perez-Alvarez and Araque, 2013; Bezzi and Volterra, 2001). Pro-inflammatory cytokines, such as TNF-a, IL-1b and IL-6, which are produced by reactive astrocytes, are on one side implicated in synapse formation and scaling, long-term potentiation and neurogenesis (for review, Bilbo and Schwartz, 2009) and on the other side can kill neurons (Chao et al., 1995; Kraft and Harry, 2011). Taken together, this suggests that neuron-glia interactions are tightly regulated and that an imbalance, such as increased or long-term release of these inflammatory mediators may lead to deleterious effects on neurons.

Mercury

Mercury accumulates in the brain particularly in astrocytes and induce astrocyte swelling, excitatory amino acid release and decreased anti-oxidant protections (Shanker et al., 2003; Allen et al., 2001), features that are also observed in reactive astrocytes. Due to the central role of astrocytes for neuronal function (control of water transport, production of trophic factors, of anti-oxidants, tri-partite synapse,… (Ximeres da Silva, 2016; Bezzi and Volterra, 2001; Hertz and Zielke, 2004; Sidoryk-Wegrzynowicz et al., 2011), it is thought that neuronal dysfunction may be secondary to disturbance in astrocytes (Aschner et al., 2007).

Perinatal exposure (GD7-PD21) of rat to MeHgCl (0.5 mg/kg bw/day) in drinking water lead to gliosis in cerebellum of immature rats (PD21) without affecting the cholinergic system. In contrast, at PD36, astrogliosis was accompanied by an increase of muscarinic M2-immunopositive Bergman cells and a lack of M3 muscarinic receptors in the molecular layer. These results suggest that astrogliosis which is observed first at PD21 may be responsible of the delayed effects of mercury on neurons (Roda et al., 2008).

Developmental exposure of mice from GD8 to PD21 to 50 mM HgCl₂ in maternal drinking water: Female offsprings exhibited higher neuroinflammation which is associated with altered social behavior (Zhang et al., 2013).

MG17, a novel triazole derivative, was able to reduce mercury-induced upregulation of IL-1b, IL-6 and TNF-a (measured by RT-PCR) and proved to be protective against mercury-induced neurodegeneration (Matharasala et al., 2017).

Adult rats exposed to MeHg (5mg/kg bw) for 12 consecutive days exhibited piknotic nuclei in cerebellar granule cells, what was reverted by a co-administration  of CA074 an inhibitor of cathepsin released by activated microglia. These observations strongly suggest that the mercury–induced neuronal pathological changes are secondary to microglial activation (Sakamoto et al., 2008).

Acrylamide

Rats exposed to acrylamide (20 mg/kg bw for 4 weeks) together with farmesol (sequiterpene) showed a downregulation of astrogliosis (i.e. decreased GFAP) and of microgliosis (i.e. decreased Iba1) and of TNF-a, Il-1b and i-NOS in cortex, hippocampus and striatum. This was associated with a marked improvement in motor coordination and a decrease in markers of oxidative stress, as compared to rats exposed to acrylamide alone (Santhanasabapathy et al., 2015).

In 3D rat brain cell-cultures, co-administration of the pro-inflammatory cytokine IL-6 (10 ng/ml) together with non-cytotoxic concentrations of MeHgCl (3 x 10^-7 M) for 10 days protected from the mercury-induced decreased in MAP2 immunostaining, suggesting a positive effect of IL-6, in accord with its descibed trophic activity (Eskes et al., 2002).

References

Allen, J.W., Shanker, G., Aschner, M., 2001. Methylmercury inhibits the in vitro uptake of the glutathione precursor, cystine, in astrocytes, but not in neurons. Brain Res. 894, 131-40.

Aschner, M., et al., 2007. Involvement of glutamate and reactive oxygen species in methylmercury neurotoxicity. Braz J Med Biol Res. 40, 285-91.

Banati, R.B., et al., 1993. Cytotoxicity of microglia. Glia. 7, 111-8.

Bezzi, P., Volterra, A., 2001. A neuron-glia signalling network in the active brain. Curr Opin Neurobiol. 11, 387-94.

Bilbo, S.D., Schwarz, J.M., 2009. Early-life programming of later-life brain and behavior: a critical role for the immune system. Front Behav Neurosci. 3, 14.

Brown GC, Bal-Price A. 2003. Inflammatory neurodegeneration mediated by nitric oxide, glutamate, and mitochondria. Mol Neurobiol 27(3): 325-355.

Brzozowski MJ, Jenner P, Rose S. 2015. Inhibition of i-NOS but not n-NOS protects rat primary cell cultures against MPP(+)-induced neuronal toxicity. J Neural Transm 122(6): 779-788.

Calvo-Ochoa, E., et al., 2014. Short-term high-fat-and-fructose feeding produces insulin signaling alterations accompanied by neurite and synaptic reduction and astroglial activation in the rat hippocampus. J Cereb Blood Flow Metab. 34, 1001-8.

Chao CC, Hu S, Peterson PK. 1995. Glia, cytokines, and neurotoxicity. CritRevNeurobiol 9: 189-205.

Dong Y, Benveniste EN. 2001. Immune Function of Astrocytes. Glia 36: 180-190.

Eskes C, Honegger P, Juillerat-Jeanneret L, Monnet-Tschudi F. 2002. Microglial reaction induced by noncytotoxic methylmercury treatment leads to neuroprotection via interactions with astrocytes and IL-6 release. Glia 37(1): 43-52.

Gehrmann J, Banati RB, Wiessnert C, Hossmann KA, Kreutzberg GW. 1995. Reactive microglia in cerebral ischaemia: An early mediator of tissue damage? NeuropatholApplNeurobiol 21: 277-289.

Harry, G.J., Kraft, A.D., 2012. Microglia in the developing brain: a potential target with lifetime effects. Neurotoxicology. 33, 191-206.

Hertz, L., Zielke, H.R., 2004. Astrocytic control of glutamatergic activity: astrocytes as stars of the show. Trends Neurosci. 27, 735-43.

Jebelli, J., et al., 2015. Glia: guardians, gluttons, or guides for the maintenance of neuronal connectivity? Ann N Y Acad Sci. 1351, 1-10.

Kettenmann, H., Kirchhoff, F., Verkhratsky, A., 2013. Microglia: new roles for the synaptic stripper. Neuron. 77, 10-8.

Kraft AD, Harry GJ. 2011. Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity. International journal of environmental research and public health 8(7): 2980-3018.

Lucas, K., Maes, M., 2013. Role of the Toll Like receptor (TLR) radical cycle in chronic inflammation: possible treatments targeting the TLR4 pathway. Mol Neurobiol. 48, 190-204.

Matharasala, G., Samala, G., Perumal, Y., 2017. MG17, a novel triazole derivative abrogated neuroinflammation and related neurodegenerative symptoms in rodents. Curr Mol Pharmacol.

Mena MA, Garcia de Yebenes J. 2008. Glial cells as players in parkinsonism: the "good," the "bad," and the "mysterious" glia. Neuroscientist 14(6): 544-560.

Nakajima K, Tohyama Y, Kohsaka S, Kurihara T. 2002. Ceramide activates microglia to enhance the production/secretion of brain-derived neurotrophic factor (BDNF) without induction of deleterious factors in vitro. J Neurochem 80: 697-705.

Niranjan R. 2014. The role of inflammatory and oxidative stress mechanisms in the pathogenesis of Parkinson's disease: focus on astrocytes. Mol Neurobiol 49(1): 28-38.

Neher JJ, Neniskyte U, Brown GC. 2012. Primary phagocytosis of neurons by inflamed microglia: potential roles in neurodegeneration. Frontiers in pharmacology 3: 27.

Perez-Alvarez, A., Araque, A., 2013. Astrocyte-neuron interaction at tripartite synapses. Curr Drug Targets. 14, 1220-4.

Roda, E., et al., 2008. Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat. J Chem Neuroanat. 35, 285-94.

Sakamoto, M., et al., 2008. Possible involvement of cathepsin B released by microglia in methylmercury-induced cerebellar pathological changes in the adult rat. Neurosci Lett. 442, 292-6.

Shanker, G., Syversen, T., Aschner, M., 2003. Astrocyte-mediated methylmercury neurotoxicity. Biol Trace Elem Res. 95, 1-10.

Santhanasabapathy, R., et al., 2015. Farnesol quells oxidative stress, reactive gliosis and inflammation during acrylamide-induced neurotoxicity: Behavioral and biochemical evidence. Neuroscience. 308, 212-27.

Sidoryk-Wegrzynowicz, M., et al., 2011. Role of astrocytes in brain function and disease. Toxicol Pathol. 39, 115-23.

Taetzsch T, Block ML. 2013. Pesticides, microglial NOX2, and Parkinson's disease. J Biochem Mol Toxicol 27(2): 137-149.

Ximenes-da-Silva, A., 2016. Metal Ion Toxins and Brain Aquaporin-4 Expression: An Overview. Front Neurosci. 10, 233.

Xu, S., et al., 2015. Wogonin prevents rat dorsal root ganglion neurons death via inhibiting tunicamycin-induced ER stress in vitro. Cell Mol Neurobiol. 35, 389-398.

Yadav, S., et al., 2012. Role of secondary mediators in caffeine-mediated neuroprotection in maneb- and paraquat-induced Parkinson's disease phenotype in the mouse. Neurochem Res. 37, 875-84.

Yuste, J.E., et al., 2015. Implications of glial nitric oxide in neurodegenerative diseases. Front Cell Neurosci. 9, 322.

Zhang, Y., Bolivar, V.J., Lawrence, D.A., 2013. Maternal exposure to mercury chloride during pregnancy and lactation affects the immunity and social behavior of offspring. Toxicol Sci. 133, 101-11.

Relationship: 1719: Increasaed pro-inflammatory mediators leads to N/A, Cell injury/death

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

Cells of the innate (microglia and astrocytes) and of the adaptive (infiltrating monocytes and lymphocytes) immune system of the brain have various ways to kill neighboring cells. This is in part due to evolutionary-conserved mechanisms evolved to kill virus-infected cells or tumor cells; in part it is a bystander phenomenon due to the release of mediators that should activate other cells and contribute to the killing of invading micro-organisms. An exaggerated or unbalanced activation of immune cells can thus lead to parenchymal (neuronal) cell death (Gehrmann et al., 1995). Mediators known to have such effects comprise components of the complement system and cytolkines/death receptor ligands triggering programmed cell death (Dong and Benveniste, 2001). Various secreted proteases (e.g. matrix metalloproteases), lipid mediators (e.g. ceramide or gangliosides) or reactive oxygen species can contribute to bystander death of neurons (Chao et al., 1995; Nakajima et al., 2002; Brown and Bal-Price, 2003; Kraft and Harry, 2011; Taetsch and Block, 2013). The equimolar production of superoxide and NO from glial cells can lead to high steady levels of peoxynitrite, which is a very potent cytotoxicant (Yuste et al., 2015). Already stressed neurons, with an impaired anti-oxidant defence system, are more sensitive to such mediators (Xu et al., 2015). Healthy cells continuously display anti "eat-me" signals, while damaged and stressed neurons/neurites display "eat-me" signals that may be recognized by microglia as signals to start phagocytosis (Neher et al., 2012). Reactive astrocytes are also able to release neurotoxic molecules (Mena and Garcia de Ybenes, 2008; Niranjan, 2014). However, astrocytes may also be protective due to their capacity to quench free radicals and secrete neurotrophic factors. The activation of astrocytes may reduce neurotrophic support to neurons (for review, Mena and Garcia de Ybenes, 2008).

Evidence Supporting this KER

In vitro co-culture experiments have demonstrated that reactive glial cells (microglia and astrocytes) can kill neurons (Chao et al., 1995; Brown and Bal-Price, 2003; Kraft and Harry, 2011; Taetzsch and Block, 2013) and that interventions with e.g. i-NOS inhibition can rescue the neurons (Yadav et al., 2012; Brzozowski et al., 2015). Drugs that block Toll like receptor pathways, which are expressed by glial cells have been proven to be protective by decreasing ROS and RNS production (Lucas et al., 2013).

Reactive microglia can remove synapses, a process known as synapse stripping (Banati et al., 1993; Kettenmann et al., 2013). Reactive astrocytes were also associated with neurite and synapse reduction (Calvo-Ochoa et al., 2014). Microglia can modulate synapse plasticity, an effect mediated by cytokines. During development, microglia can promote synaptogenesis or engulf synapses, a process known as synaptic pruning (for review, Jebelli et al., 2015). It is hypothesized that alterations in microglia functioning during synapse formation and maturation of the brain can have significant long-term effects on the final established neural circuits (for review, Harry and Kraft, 2012). The fact that astrocytes can receive and respond to the synaptic information produced by neuronal activity, owing to their expression of a wide range of neurotransmitter receptors, has given rise to the concept of tripartite synapse (for review, Perez-Alvarez and Araque, 2013; Bezzi and Volterra, 2001). Pro-inflammatory cytokines, such as TNF-a, IL-1b and IL-6, which are produced by reactive astrocytes, are on one side implicated in synapse formation and scaling, long-term potentiation and neurogenesis (for review, Bilbo and Schwartz, 2009) and on the other side can kill neurons (Chao et al., 1995; Kraft and Harry, 2011). Taken together, this suggests that neuron-glia interactions are tightly regulated and that an imbalance, such as increased or long-term release of these inflammatory mediators may lead to deleterious effects on neurons.

Mercury

Mercury accumulates in the brain particularly in astrocytes and induce astrocyte swelling, excitatory amino acid release and decreased anti-oxidant protections (Shanker et al., 2003; Allen et al., 2001), features that are also observed in reactive astrocytes. Due to the central role of astrocytes for neuronal function (control of water transport, production of trophic factors, of anti-oxidants, tri-partite synapse,… (Ximeres da Silva, 2016; Bezzi and Volterra, 2001; Hertz and Zielke, 2004; Sidoryk-Wegrzynowicz et al., 2011), it is thought that neuronal dysfunction may be secondary to disturbance in astrocytes (Aschner et al., 2007).

Perinatal exposure (GD7-PD21) of rat to MeHgCl (0.5 mg/kg bw/day) in drinking water lead to gliosis in cerebellum of immature rats (PD21) without affecting the cholinergic system. In contrast, at PD36, astrogliosis was accompanied by an increase of muscarinic M2-immunopositive Bergman cells and a lack of M3 muscarinic receptors in the molecular layer. These results suggest that astrogliosis which is observed first at PD21 may be responsible of the delayed effects of mercury on neurons (Roda et al., 2008).

Developmental exposure of mice from GD8 to PD21 to 50 mM HgCl₂ in maternal drinking water: Female offsprings exhibited higher neuroinflammation which is associated with altered social behavior (Zhang et al., 2013).

MG17, a novel triazole derivative, was able to reduce mercury-induced upregulation of IL-1b, IL-6 and TNF-a (measured by RT-PCR) and proved to be protective against mercury-induced neurodegeneration (Matharasala et al., 2017).

Adult rats exposed to MeHg (5mg/kg bw) for 12 consecutive days exhibited piknotic nuclei in cerebellar granule cells, what was reverted by a co-administration  of CA074 an inhibitor of cathepsin released by activated microglia. These observations strongly suggest that the mercury–induced neuronal pathological changes are secondary to microglial activation (Sakamoto et al., 2008).

Acrylamide

Rats exposed to acrylamide (20 mg/kg bw for 4 weeks) together with farmesol (sequiterpene) showed a downregulation of astrogliosis (i.e. decreased GFAP) and of microgliosis (i.e. decreased Iba1) and of TNF-a, Il-1b and i-NOS in cortex, hippocampus and striatum. This was associated with a marked improvement in motor coordination and a decrease in markers of oxidative stress, as compared to rats exposed to acrylamide alone (Santhanasabapathy et al., 2015).

In 3D rat brain cell-cultures, co-administration of the pro-inflammatory cytokine IL-6 (10 ng/ml) together with non-cytotoxic concentrations of MeHgCl (3 x 10^-7 M) for 10 days protected from the mercury-induced decreased in MAP2 immunostaining, suggesting a positive effect of IL-6, in accord with its descibed trophic activity (Eskes et al., 2002).

References

Allen, J.W., Shanker, G., Aschner, M., 2001. Methylmercury inhibits the in vitro uptake of the glutathione precursor, cystine, in astrocytes, but not in neurons. Brain Res. 894, 131-40.

Aschner, M., et al., 2007. Involvement of glutamate and reactive oxygen species in methylmercury neurotoxicity. Braz J Med Biol Res. 40, 285-91.

Banati, R.B., et al., 1993. Cytotoxicity of microglia. Glia. 7, 111-8.

Bezzi, P., Volterra, A., 2001. A neuron-glia signalling network in the active brain. Curr Opin Neurobiol. 11, 387-94.

Bilbo, S.D., Schwarz, J.M., 2009. Early-life programming of later-life brain and behavior: a critical role for the immune system. Front Behav Neurosci. 3, 14.

Brown GC, Bal-Price A. 2003. Inflammatory neurodegeneration mediated by nitric oxide, glutamate, and mitochondria. Mol Neurobiol 27(3): 325-355.

Brzozowski MJ, Jenner P, Rose S. 2015. Inhibition of i-NOS but not n-NOS protects rat primary cell cultures against MPP(+)-induced neuronal toxicity. J Neural Transm 122(6): 779-788.

Calvo-Ochoa, E., et al., 2014. Short-term high-fat-and-fructose feeding produces insulin signaling alterations accompanied by neurite and synaptic reduction and astroglial activation in the rat hippocampus. J Cereb Blood Flow Metab. 34, 1001-8.

Chao CC, Hu S, Peterson PK. 1995. Glia, cytokines, and neurotoxicity. CritRevNeurobiol 9: 189-205.

Dong Y, Benveniste EN. 2001. Immune Function of Astrocytes. Glia 36: 180-190.

Eskes C, Honegger P, Juillerat-Jeanneret L, Monnet-Tschudi F. 2002. Microglial reaction induced by noncytotoxic methylmercury treatment leads to neuroprotection via interactions with astrocytes and IL-6 release. Glia 37(1): 43-52.

Gehrmann J, Banati RB, Wiessnert C, Hossmann KA, Kreutzberg GW. 1995. Reactive microglia in cerebral ischaemia: An early mediator of tissue damage? NeuropatholApplNeurobiol 21: 277-289.

Harry, G.J., Kraft, A.D., 2012. Microglia in the developing brain: a potential target with lifetime effects. Neurotoxicology. 33, 191-206.

Hertz, L., Zielke, H.R., 2004. Astrocytic control of glutamatergic activity: astrocytes as stars of the show. Trends Neurosci. 27, 735-43.

Jebelli, J., et al., 2015. Glia: guardians, gluttons, or guides for the maintenance of neuronal connectivity? Ann N Y Acad Sci. 1351, 1-10.

Kettenmann, H., Kirchhoff, F., Verkhratsky, A., 2013. Microglia: new roles for the synaptic stripper. Neuron. 77, 10-8.

Kraft AD, Harry GJ. 2011. Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity. International journal of environmental research and public health 8(7): 2980-3018.

Lucas, K., Maes, M., 2013. Role of the Toll Like receptor (TLR) radical cycle in chronic inflammation: possible treatments targeting the TLR4 pathway. Mol Neurobiol. 48, 190-204.

Matharasala, G., Samala, G., Perumal, Y., 2017. MG17, a novel triazole derivative abrogated neuroinflammation and related neurodegenerative symptoms in rodents. Curr Mol Pharmacol.

Mena MA, Garcia de Yebenes J. 2008. Glial cells as players in parkinsonism: the "good," the "bad," and the "mysterious" glia. Neuroscientist 14(6): 544-560.

Nakajima K, Tohyama Y, Kohsaka S, Kurihara T. 2002. Ceramide activates microglia to enhance the production/secretion of brain-derived neurotrophic factor (BDNF) without induction of deleterious factors in vitro. J Neurochem 80: 697-705.

Niranjan R. 2014. The role of inflammatory and oxidative stress mechanisms in the pathogenesis of Parkinson's disease: focus on astrocytes. Mol Neurobiol 49(1): 28-38.

Neher JJ, Neniskyte U, Brown GC. 2012. Primary phagocytosis of neurons by inflamed microglia: potential roles in neurodegeneration. Frontiers in pharmacology 3: 27.

Perez-Alvarez, A., Araque, A., 2013. Astrocyte-neuron interaction at tripartite synapses. Curr Drug Targets. 14, 1220-4.

Roda, E., et al., 2008. Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat. J Chem Neuroanat. 35, 285-94.

Sakamoto, M., et al., 2008. Possible involvement of cathepsin B released by microglia in methylmercury-induced cerebellar pathological changes in the adult rat. Neurosci Lett. 442, 292-6.

Shanker, G., Syversen, T., Aschner, M., 2003. Astrocyte-mediated methylmercury neurotoxicity. Biol Trace Elem Res. 95, 1-10.

Santhanasabapathy, R., et al., 2015. Farnesol quells oxidative stress, reactive gliosis and inflammation during acrylamide-induced neurotoxicity: Behavioral and biochemical evidence. Neuroscience. 308, 212-27.

Sidoryk-Wegrzynowicz, M., et al., 2011. Role of astrocytes in brain function and disease. Toxicol Pathol. 39, 115-23.

Taetzsch T, Block ML. 2013. Pesticides, microglial NOX2, and Parkinson's disease. J Biochem Mol Toxicol 27(2): 137-149.

Ximenes-da-Silva, A., 2016. Metal Ion Toxins and Brain Aquaporin-4 Expression: An Overview. Front Neurosci. 10, 233.

Xu, S., et al., 2015. Wogonin prevents rat dorsal root ganglion neurons death via inhibiting tunicamycin-induced ER stress in vitro. Cell Mol Neurobiol. 35, 389-398.

Yadav, S., et al., 2012. Role of secondary mediators in caffeine-mediated neuroprotection in maneb- and paraquat-induced Parkinson's disease phenotype in the mouse. Neurochem Res. 37, 875-84.

Yuste, J.E., et al., 2015. Implications of glial nitric oxide in neurodegenerative diseases. Front Cell Neurosci. 9, 322.

Zhang, Y., Bolivar, V.J., Lawrence, D.A., 2013. Maternal exposure to mercury chloride during pregnancy and lactation affects the immunity and social behavior of offspring. Toxicol Sci. 133, 101-11.

Relationship: 1688: N/A, Cell injury/death leads to Neuronal network function, Decreased

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

Under physiological conditions, in the developing nervous system, apoptosis occurs during the process of synaptogenesis, where competition leads to the loss of excess neurons and to the connection of the appropriate neurons (Buss et al., 2006; Mennerick and Zorumski, 2000; Oppenheim, 1991). When a stressor increases the number of apoptotic cells this KE has a negative effect on synaptogenesis as the reduced number of neurons (besides the ones that have been already eliminated through the physiological process of apoptosis) provides limited dendritic fields for receiving synaptic inputs from incoming axons. At the same time the loss of neurons also means that there are less axons to establish synaptic contacts (Olney, 2014), leading to reduced synaptogenesis. The ability of a neuron to communicate is based on neural network formation that relies on functional synapse establishment (Colón-Ramos, 2009). The main roles of synapses are the regulation of intercellular communication in the nervous system, and the information flow within neural networks. The connectivity and functionality of neural networks depends on where and when synapses are formed. Therefore, the decreased synapse formation due to cell death during the process of synaptogenesis is critical and leads to decrease of neural network formation and function in the adult brain.

Synaptic transmission and plasticity require the integrity of the anatomical substrate. The connectivity of axons emanating from one set of cells to post-synaptic side of synapse on the dendrites of the receiving cells must be intact for effective communication between neurons. Changes in the placement of cells within the network due to delays in neuronal migration, the absence of a full formation of dendritic arbors and spine upon which synaptic contacts are made, and the lagging of transmission of electrical impulses due to insufficient myelination will individually and cumulatively impair synaptic function.

Therefore, chemicals inducing neuronal cell death by apoptosis or necrosis, or interfering with a particular system of neurotransmitters, will alter network structure and function.

Evidence Supporting this KER

Recently, Dekkers et al. 2013 have reviewed how under physiological conditions components of the apoptotic machinery in developing brain regulate synapse formation and neuronal connectivity. For example, caspase activation is known to be required for axon pruning during development to generate neuronal network (reviewed in Dekkers et al., 2013). Experimental work carried out in Drosophila melanogaster and in mammalian neurons shows that components of apoptotic machinery are involved in axonal degeneration that can consequently interfere with synapse formation (reviewed in Dekkers et al., 2013). Furthermore, Bax mutant mice studies indicate that the lack of this pro-apoptotic protein BAX leads to disruption of intrinsically photosensitive retinal ganglion cells spacing and dendritic stratification that affects synapse localization and function (Chen et al., 2013).

Neuronal network formation and function are established via the process of synaptogenesis. The developmental period of synaptogenesis is critical for the formation of the basic circuitry of the nervous system, although neurons are able to form new synapses throughout life (Rodier, 1995). The brain electrical activity dependence on synapse formation is critical for proper neuronal communication.

Alterations in synaptic connectivity lead to refinement of neuronal networks during development (Cline and Haas, 2008). Indeed, knockdown of PSD-95 arrests the functional and morphological development of glutamatergic synapses (Ehrlich et al., 2007).

Studies of the last 30 years demonstrated that astrocytes possess functional receptors for neurotransmitters and respond to their stimulation via release of gliotransmitters, including glutamate. These findings have led to a new concept of neuron–glia intercommunication where astrocytes play an unsuspected dynamic role by integrating neuronal inputs and modulating synaptic activity (Rossi and Volterra, 2009). According to the concept termed "tripartite synapse", the emerging view is that brain function actually arises from the coordinated activity of a network comprising both neurons and astrocytes. Furthermore, myelinating glial cells are well-known to insulate axons and to speed up action potential propagation. Be it motor skill learning or social behaviors in higher vertebrates, proper myelination is critical in shaping brain functions. Neurons rely on their myelinating partners not only for setting conduction speed, but also for regulating the ionic environment and fueling their energy demands with metabolites. Also, long-term axonal integrity and neuronal survival are maintained by oligodendrocytes and loss of this well-coordinated axon-glial interplay contributes to brain diseases (Saab and Nave, 2017). Therefore, reduction in glial cell number and/or reduction in myelination of axons, will very much impact the neural network function.

Mercury

Acrylamide

No publications found to support this KE

Ogawa et al. reported decreased apoptosis and an increase in the number of Gabaergic interneurons.

References

Bridges, K., Venables, B., Roberts, A., 2017. Effects of dietary methylmercury on the dopaminergic system of adult fathead minnows and their offspring. Environ Toxicol Chem 36, 1077-1084.

Buss, R.R., Sun, W., Oppenheim, R.W., 2006. Adaptive roles of programmed cell death during nervous system development. Annu Rev Neurosci 29, 1-35.

Chen, S.K., Chew, K.S., McNeill, D.S., Keeley, P.W., Ecker, J.L., Mao, B.Q., Pahlberg, J., Kim, B., Lee, S.C., Fox, M.A., Guido, W., Wong, K.Y., Sampath, A.P., Reese, B.E., Kuruvilla, R., Hattar, S., 2013. Apoptosis regulates ipRGC spacing necessary for rods and cones to drive circadian photoentrainment. Neuron 77, 503-515.

Cline, H., Haas, K., 2008. The regulation of dendritic arbor development and plasticity by glutamatergic synaptic input: a review of the synaptotrophic hypothesis. J Physiol 586, 1509-1517.

Colon-Ramos, D.A., 2009. Synapse formation in developing neural circuits. Curr Top Dev Biol 87, 53-79.

Dekkers, M.P., Nikoletopoulou, V., Barde, Y.A., 2013. Cell biology in neuroscience: Death of developing neurons: new insights and implications for connectivity. J Cell Biol 203, 385-393.

Ehrlich, I., Klein, M., Rumpel, S., Malinow, R., 2007. PSD-95 is required for activity-driven synapse stabilization. Proc Natl Acad Sci U S A 104, 4176-4181.

Falluel-Morel, A., Sokolowski, K., Sisti, H.M., Zhou, X., Shors, T.J., Dicicco-Bloom, E., 2007. Developmental mercury exposure elicits acute hippocampal cell death, reductions in neurogenesis, and severe learning deficits during puberty. J Neurochem 103, 1968-1981.

Ferraro, L., Tomasini, M.C., Tanganelli, S., Mazza, R., Coluccia, A., Carratu, M.R., Gaetani, S., Cuomo, V., Antonelli, T., 2009. Developmental exposure to methylmercury elicits early cell death in the cerebral cortex and long-term memory deficits in the rat. Int J Dev Neurosci 27, 165-174.

Jacob, S., Thangarajan, S., 2017. Effect of Gestational Intake of Fisetin (3,3',4',7-Tetrahydroxyflavone) on Developmental Methyl Mercury Neurotoxicity in F1 Generation Rats. Biol Trace Elem Res 177, 297-315.

Mennerick, S., Zorumski, C.F., 2000. Neural activity and survival in the developing nervous system. Mol Neurobiol 22, 41-54.

Nagashima, K., 1997. A review of experimental methylmercury toxicity in rats: neuropathology and evidence for apoptosis. Toxicol Pathol 25, 624-631.

Nagashima, K., Fujii, Y., Tsukamoto, T., Nukuzuma, S., Satoh, M., Fujita, M., Fujioka, Y., Akagi, H., 1996. Apoptotic process of cerebellar degeneration in experimental methylmercury intoxication of rats. Acta Neuropathol 91, 72-77.

Ogawa, B., Ohishi, T., Wang, L., Takahashi, M., Taniai, E., Hayashi, H., Mitsumori, K., Shibutani, M., 2011. Disruptive neuronal development by acrylamide in the hippocampal dentate hilus after developmental exposure in rats. Arch Toxicol 85, 987-994.

Olney, J.W., 2014. Focus on apoptosis to decipher how alcohol and many other drugs disrupt brain development. Front Pediatr 2, 81.

Onishchenko, N., Tamm, C., Vahter, M., Hokfelt, T., Johnson, J.A., Johnson, D.A., Ceccatelli, S., 2007. Developmental exposure to methylmercury alters learning and induces depression-like behavior in male mice. Toxicol Sci 97, 428-437.

Oppenheim, R.W., 1991. Cell death during development of the nervous system. Annu Rev Neurosci 14, 453-501.

Perea, G., Navarrete, M., Araque, A., 2009. Tripartite synapses: astrocytes process and control synaptic information. Trends Neurosci 32, 421-431.

Rodier, P.M., 1995. Developing brain as a target of toxicity. Environ Health Perspect 103 Suppl 6, 73-76.

Rossi, D., Volterra, A., 2009. Astrocytic dysfunction: insights on the role in neurodegeneration. Brain Res Bull 80, 224-232.

Saab, A.S., Nave, K.A., 2017. Myelin dynamics: protecting and shaping neuronal functions. Curr Opin Neurobiol 47, 104-112.

Teixeira, F.B., Fernandes, R.M., Farias-Junior, P.M., Costa, N.M., Fernandes, L.M., Santana, L.N., Silva-Junior, A.F., Silva, M.C., Maia, C.S., Lima, R.R., 2014. Evaluation of the effects of chronic intoxication with inorganic mercury on memory and motor control in rats. Int J Environ Res Public Health 11, 9171-9185.

Wang, Y.T., Lin, H.C., Zhao, W.Z., Huang, H.J., Lo, Y.L., Wang, H.T., Lin, A.M., 2017. Acrolein acts as a neurotoxin in the nigrostriatal dopaminergic system of rat: involvement of alpha-synuclein aggregation and programmed cell death. Sci Rep 7, 45741.

Relationship: 359: Neuronal network function, Decreased leads to Impairment, Learning and memory

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Synaptic transmission and plasticity are achieved via mechanisms common across taxonomies. LTP has been recorded in aplysia, lizards, turtles, birds, mice, guinea pigs, rabbits and rats. Deficiencies in hippocampally based learning and memory following developmental hypothyroidism have been documented mainly in rodents and humans.

Key Event Relationship Description

Learning and memory is one of the outcomes of the functional expression of neurons and neural networks from mammalian to invertebrates. Damage or destruction of neurons by chemical compounds during development when they are in the process of synapses formation, integration and formation of neural networks, will derange the organization and function of these networks, thereby setting the stage for subsequent impairment of learning and memory. Exposure to the potential developmental toxicants during neuronal differentiation and synaptogenesis will increase risk of functional neuronal network damage leading to learning and memory impairment.

Impairments in learning and memory are measured using behavioral techniques. It is well accepted that these alterations in behavior are the result of structural or functional changes in neurocircuitry. Functional impairments are often measured using field potentials of critical synaptic circuits in hippocampus and cortex. A number of studies have been performed in rodent models that reveal deficits in both excitatory and inhibitory synaptic transmission in the hippocampus as a result of developmental thyroid insufficiency (Wang et al., 2012; Oerbeck et al., 2003; Wheeler et al., 2011; Wheeler et al., 2015; Willoughby et al., 2014; Davenport and Dorcey, 1972; Tamasy et al., 1986; Akaike, 1991; Axelstad et al., 2008; Gilbert and Sui, 2006; Gilbert et al., 2016; Gilbert, 2011; Gilbert et al., 2016). A well-established functional readout of memory at the synaptic level is known as long-term potentiation (LTP) (i.e., a persistent strengthening of synapses based on recent patterns of activity). Deficiencies in LTP are generally regarded as potential substrates of learning and memory impairments. In rodent models where synaptic function is impaired by TH deficiencies, deficits in hippocampus-mediated memory are also prevalent (Gilbert and Sui, 2006; Gilbert et al., 2016; Gilbert, 2011; Gilbert et al., 2016).

Evidence Supporting this KER

A number of studies have consistently reported alterations in synaptic transmission resulting from developmental TH disruption, and leading to decreased cognition.

Long-term potentiation (LTP) is a long-lasting increase in synaptic efficacy (not always and not always high frequency stimulation leads to LTP), and its discovery suggested that changes in synaptic strength could provide the substrate for learning and memory (reviewed in Lynch, 2004). Moreover, LTP is intimately related to the theta rhythm, an oscillation long associated with learning. Learning-induced enhancement in neuronal excitability, a measurement of neural network function, has also been shown in hippocampal neurons following classical conditioning in several experimental approaches (reviewed in Saar and Barkai, 2003).

On the other hand, memory requires the increase in magnitude of EPSCs to be developed quickly and to be persistent for few weeks at least without disturbing already potentiated contacts. Once again, a substantial body of evidence has demonstrated that tight connection between LTP and diverse instances of memory exist (reviewed in Lynch, 2004).

A review on Morris water maze (MWM) as a tool to investigate spatial learning and memory in laboratory rats also pointed out that the disconnection between neuronal networks rather than the brain damage of certain regions is responsible for the impairment of MWM performance. Functional integrated neural networks that involve the coordination action of different brain regions are consequently important for spatial learning and MWM performance (D'Hooge and De Deyn, 2001).

Moreover, it is well accepted that alterations in synaptic transmission and plasticity contribute to deficits in cognitive function. There are a number of studies that have linked exposure to TPO inhibitors (e.g., PTU, MMI), as well as iodine deficient diets, to changes in serum TH levels, which result in alterations in both synaptic function and cognitive behaviors (Akaike et al., 1991; Vara et al., 2002; Gilbert and Sui, 2006; Axelstad et al., 2008; Taylor et al., 2008; Gilbert, 2011; Gilbert et al., 2016), described in the indirect KER "Decrease of TH synthesis leads to learning and memory deficits".

Developmental hypothyroidism reduces the functional integrity in brain regions critical for learning and memory. Neurophysiological indices of synaptic transmission of excitatory and inhibitory circuitry are impaired in the hippocampus of hypothyroid animals. Both hippocampal regions (area CA1 and dentate gyrus) exhibit alterations in excitatory and inhibitory synaptic transmission following reductions in serum TH in the pre and early postnatal period (Vara et al., 2002; Sui and Gilbert, 2003; Sui et al., 2005; Gilbert and Sui, 2006; Taylor et al., 2008; Gilbert, 2011; Gilbert et al., 2016). These alterations persist into adulthood despite a recovery to euthyroid conditions in blood. The latter observation indicates that these alterations represent permanent changes in brain function caused by transient hormones insufficiencies induced during critical window of development.  

Because the adult hippocampus is involved in learning and memory, it is a brain region of remarkable plasticity. Use-dependent synaptic plasticity is critical during brain development for synaptogenesis and fine tuning of synaptic connectivity. In the adult brain, similar plasticity mechanisms underlie use-dependency that underlies learning and memory, as exhibited in LTP model of synaptic memory. Hypothyroidism during development reduces the capacity for synaptic plasticity in juvenile and adult offspring (Vara et al., 2002; Sui and Gilbert, 2003; Dong et al., 2005; Sui et al., 2005; Gilbert and Sui, 2006; Taylor et al., 2008; Gilbert, 2011; Gilbert et al., 2016). Decrease of neuronal network function and plasticity are observed coincident with deficits in learning tasks that require the hippocampus.

- Wang et al., 2012: This study showed that maternal subclinical hypothyroidism impairs spatial learning in the offspring, as well as the efficacy and optimal time of T4 treatment in pregnancy. Female adult Wistar rats were randomly divided into six groups: control, hypothyroid (H), subclinical hypothyroid (SCH) and SCH treated with T4, starting from GD10, GD13 and GD17, respectively, to restore normal TH levels. Results indicate that progenies of SCH and H groups demonstrated significantly longer mean latency in the water maze test (on the 2^nd training day, latency was ~83% higher in H group, and ~50% higher in SCH), and a lower amplification percentage of the amplitude (~15% lower in H group, and 12% lower in SCH), and slope of the field excitatory postsynaptic potential (fEPSP) recording (~20% lower in H group, and 17% lower in SCH), compared to control group. T4 treatment at GD10 and GD13 significantly shortened mean latency and increased the amplification percentage of the amplitude and slope of the fEPSPs of the progeny of rats with subclinical hypothyroidism. However, T4 treatment at GD17 showed only minimal effects on spatial learning in the offspring. Altogether these data indicate direct correlation between decrease of neural network function and learning and memory deficits.

- Liu et al., 2010 This study assessed the effects of hypothyroidism in 60 female rats who were divided into three groups: (i) maternal subclinical hypothyroidism (total thyroidectomy with T4 infusion), (ii) maternal hypothyroidism (total thyroidectomy without T4 infusion), and (iii) control (sham operated). The Morris water maze tests revealed that pups from the subclinical hypothyroidism group showed long-term memory deficits, and a trend toward short-term memory deficits.

- Gilbert and Sui, 2006 Administration of 3 or 10 ppm PTU to pregnant and lactating dams via the drinking water from GD6 until PND30 caused a 47% and 65% reduction in serum T4, in the dams of the low and high-dose groups, respectively. Baseline synaptic transmission was impaired in PTU-exposed animals: mean EPSP slope (by ~60% with 10 ppm PTU) and population spike amplitudes (by ~70% with 10 ppm PTU) in the dentate gyrus were reduced in a dose-dependent manner in adult offspring of PTU-treated dams. High-dose animals (10 ppm) demonstrated very little evidence of learning despite 16 consecutive days of training (~5-fold higher mean latency to find the hidden platform, used as an index of learning).

- Gilbert et al., 2016 Exposure to PTU during development produced dose-dependent reductions in mRNA expression of nerve growth factor (Ngf) in whole hippocampus of neonates. These changes in basal expression persisted to adulthood despite the return to euthyroid conditions in blood. Developmental PTU treatment dramatically reduced the activity-dependent expression of neurotrophins and related genes in neonate hippocampus and was accompanied by deficits in hippocampal-based learning (e.g., mean latency to find a hidden platform, at 2^nd trial resulted ~60% higher in rats treated with 10 ppm PTU).

- Gilbert, 2011 Trace fear conditioning deficits to context and to cue reported in animals treated with PTU and who also displayed synaptic transmission and LTP deficits in hippocampus. Baseline synaptic transmission was impaired in PTU-exposed animals (by ~50% in animal treated with 3 ppm PTU). EPSP slope amplitudes in the dentate gyrus were reduced in a dose-dependent manner in adult offspring of PTU-treated dams.

BPA, an environmental toxicant known to inhibit NIS-mediated iodide uptake (Wu Y et al., 2016) has been found to cause learning and memory deficits in rodents as described below:

- Jang et al., 2012 In this study, pregnant female C57BL/6 mice (F0) were exposed to BPA (0.1-10 mg/kg) from gestation day 6 to 17, and female offspring (F2) from F1 generation mice were analysed. Exposure of F0 mice to BPA (10 mg/kg) decreased hippocampal neurogenesis (~ 30% decrease of hippocampal BrdU⁺ cells vs control) in F2 female mice. High-dose BPA (10 mg/kg) caused neurocognitive deficit (i.e., reduced memory retention) as shown by passive avoidance testing (~ 33% decrease vs control) in F2 mice. Furthermore, 10 mg/kg BPA decreased the hippocampal levels of BDNF (~ 35% lower vs control) in F2 mice. These results suggest that BPA exposure (NIS inhibitor) in pregnant mothers could decrease hippocampal neurogenesis (decreased number of neurons) and cognitive function in future generations.

In humans, the data linking these two specific KE are much more limited, but certainly clear reductions in IQ, with specific impairments in hippocampus-mediated functions have been observed.

- Wheeler et al., 2015 This study assessed hippocampal functioning in adolescents with congenital hypothyroidism (CH), using functional magnetic resonance imaging (fMRI). 14 adolescents with CH and 14 typically developing controls (TDC) were studied. Hippocampal activation was greater for pairs than items in both groups, but this difference was only significant in TDC. When the groups were directly compared, the right anterior hippocampus was the primary region in which the TDC and CH groups differed for this pair memory effect. Results signify that adolescents with CH show abnormal hippocampal functioning during verbal memory processing, in order to compensate for the effects induced by TH deficit in the brain.

- Wheeler et al., 2012 In this study hippocampal neuronal network function was measured based on synaptic performance using fMRI and was altered while subjects engaged in a memory task. Data showed paired word recognition deficits in adolescents with congenital hypothyroidism (N = 14; age range, 11.5-14.7 years) compared with controls (N = 15; age range, 11.2-15.5 years), with no impairment on simple word lists. Analysis of functional magnetic resonance imaging showed that adolescents with congenital hypothyroidism had both increased magnitude of hippocampal activation relative to controls and bilateral hippocampal activation when only the left was observed in controls. Furthermore, the increased activation in the congenital hypothyroidism group was correlated with the severity of the hypothyroidism experienced early in life.

- Willoughby et al., 2013 Analogously, in this study, fMRI revealed increased hippocampus activation with word pair recognition task in CH and children born to women with hypothyroxinemia during midgestation. These differences in functional activation were not seen with single word recognition, but were revealed when retention of word pair associations was probed. The latter is a task requiring engagement of the hippocampus.

A series of important findings suggest that the biochemical changes that happen after induction of LTP also occur during memory acquisition, showing temporality between the two KEs (reviewed in Lynch, 2004).

- Morris et al., 1986 This study found that blocking the NMDA receptor of the neuronal network with AP5 inhibits spatial learning in rats. Most importantly, in the same study they measured brain electrical activity and recorded that this agent also inhibits LTP, however, they have not proven that spatial learning and LTP inhibition are causally related.

Since then a number of NMDA receptor antagonists have been studied towards their ability to induce impairment of learning and memory. It is worth mentioning that similar findings have been found in human subjects:

- Grunwald et al., 1999 By combining behavioural and electrophysiological data from patients with temporal lobe epilepsy exposed to ketamine, involvement of NMDA receptors in human memory processes was demonstrated.

The last KE preceding the AO (learning and memory deficits), i.e. "Decreased Neural Network Function", is also common to the AOP 13, entitled "Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities" (https://aopwiki.org/aops/13). In this AOP 13, data on lead (Pb) exposure as reference chemical are reported. While these studies do not refer to TH disruption, they provide empirical support for the same KER described in the present AOP.

Pb2+: Exposure to low levels of Pb2+, during early development, has been implicated in long-lasting behavioural abnormalities and cognitive deficits in children (Needleman et al., 1975; Needleman and Gatsonis, 1990; Bellinger et al., 1991; 1992; Baghurst et al., 1992; Leviton et al., 1993; Needleman et al., 1996; Finkelstein et al., 1998; Lanphear et al., 2000; 2005; Canfield et al., 2003; Bellinger 2004; Lanphear et al., 2005; Surkan et al., 2007; Jusko et al., 2008; Neal and Guilarte, 2010) and experimental animals (Brockel and Cory-Slechta, 1998; Murphy and Regan, 1999; Moreira et al., 2001). Multiple lines of evidence suggest that Pb2+ can impair hippocampus-mediated learning in animal models (reviewed in Toscano and Guilarte, 2005).

- Jett et al., 1997 Female rats exposed to Pb²⁺ through gestation and lactation have shown more severe impairment of memory than male rats with similar Pb²⁺ exposures.

- De Souza Lisboa et al., 2005 This study reported that exposure to Pb²⁺ during both pregnancy and lactation caused depressive-like behaviour (detected in the forced swimming test) in female but not male rats.

- Anderson et al., 2012 This study investigated the neurobehavioral outcomes in Pb²⁺-exposed rats (250, 750 and 1500 ppm Pb²⁺ acetate in food) during gestation and through weaning and demonstrated that these outcomes are very much influenced by sex and rearing environment. In females, Pb²⁺ exposure lessened some of the benefits of enriched environment on learning, whereas, in males, enrichment does help to overcome detrimental effects of Pb²⁺ on learning. Regarding reference memory, environmental enrichment has not been beneficial in females when exposure to Pb²⁺ occurs, in contrast to males.

- Jaako-Movits et al., 2005 Wistar rat pups were exposed to 0.2% Pb²⁺ via their dams' drinking water from PND 1 to PND 21 and directly via drinking water from weaning until PND 30. At PND 60 and 80, the neurobehavioural assessment has revealed that developmental Pb²⁺ exposure induces persistent increase in the level of anxiety and inhibition of contextual fear conditioning. The same behavioural syndrome in rats has been described in Salinas and Huff, 2002.

- Finkelstein et al., 1998 These observations are in agreement with observations on humans, as children exposed to low levels of Pb²⁺ displayed attention deficit, increased emotional reactivity and impaired memory and learning.

- Kumar and Desiraju, 1992 In Wistar rats fed with lead acetate (400 µg/g body weight/day) from PND 2 until PND 60, EEG findings showed statistically significant reduction in the delta, theta, alpha and beta band EEG spectral power in motor cortex and hippocampus, but not in delta and beta bands power of motor cortex in wakeful state. After 40 days of recovery, animals were assessed for their neurobehaviour, and revealed that Pb²⁺ treated animals showed more time and sessions in attaining criterion of learning than controls.

Further data obtained using animal behavioral techniques demonstrate that NMDA mediated synaptic transmission is decreased by Pb²⁺ exposure (Cory-Slechta, 1995; Cohn and Cory-Slechta, 1993 and 1994).

- Xiao et al., 2014 Rat pups from parents exposed to 2 mM PbCl₂ three weeks before mating until their weaning (pre-weaning Pb²⁺) and weaned pups exposed to 2 mM PbCl₂ for nine weeks (post-weaning Pb²⁺) were assessed for their spatial learning and memory by MWM on PND 85-90. The study revealed that both rat pups in pre-weaning Pb²⁺ and post-weaning Pb²⁺ groups performed significantly worse than those in the control group. The number of synapses in pre-weaning Pb²⁺ group increased significantly, but it was still less than that of control group. The number of synapses in post-weaning Pb²⁺ group was also less than that of control group, although the number of synapses had no differences between post-weaning Pb²⁺ and control groups before MWM. In both pre-weaning Pb²⁺ and post-weaning Pb²⁺ groups, synaptic structural parameters such as thickness of postsynaptic density (PSD), length of synaptic active zone and synaptic curvature increased, whereas width of synaptic cleft decreased compared to controls.

The last KE preceding the AO (learning and memory deficits), i.e. "Decreased Neural Network Function", is also common to the AOP 17, entitled " Binding of electrophilic chemicals to SH(thiol)-group of proteins and /or to seleno-proteins during brain development leads to impairment of learning and memory" (https://aopwiki.org/aops/17). In this AOP 17, data on mercury exposure as reference chemical are reported. While these studies do not refer to TH disruption, they provide empirical support for the same KER described in the present AOP.

Sokolowski et al. 2013. Rats at postnatal day 7 received a single injection of methylmercury (0.6 microgr/g, that caused caspase activation in the hilus of granule cell layer in hippocampus. At PD 21, a decrease in cell number or 22% in hilus and of 27% in granule cell layer, as well as a decreased proliferation of neural precursor cells of 25% were observed. This was associated with a decrease of spatial memory as assessed by Morris water maze.

Eddins et al., 2008. Mice exposed during postnatal week 1-3 to 2-5 mg/kg mercury chloride in 0.01 ml/g of NaCl injectd s.c. The behavioral tests at 3 months of age revealed learning deficits (radial maze), which was associated with increased levels of monoamines in frontal cortex.

Zanoli et al., 1994. Single injection of methylmercury (8 mg/kg by gavage) at gestational day 15. Offsprings analyzed at 14, 21, and 60 days of age exhibited a decrease in the number of muscarinic receptors at 14 and 21 days and a decrease in avoidance latency at 60 days, indicating learning and memory deficits.

Zanoli et al., 2001. Single injection of methylmercury (8 mg/kg) at gestational day 8. Brain was removed at PD 21 and 60. An  increase in tryptophan level in hippocampus was detected at both days. At PD 21, a decrease in anthranilic acid and an increase in quinolinic acid was found. No change in glutamic acid nor in aspartic acid were detected.

Montgomery et al., 2008. C57/B6 mice exposed during pregnancy (GD 8-18) with food containing methylmercury (0.01 mg/kg body wheight). Tested when adult, they showed deficits in motor function, coordination, overall activity and impairment in reference memory.

Glover et al., 2009. Balb mice exposed to methylmercury in diet (low dose: 1.5 mg/kg; high dose: 4.5 mg/kg) during 11 weeks (6 weeks prior mating, 3 weeks during gestation and 2 weeks post-partum). Offsprings tested at PD 15 showed an accumulation of Hg in brain (0.08 mg/kg for low dose and 0.25 mg/kg for the high dose). At hte cellular level, there was alterations in gene expression for cytoskeleton, cell processes, cell adhesion, cell differentiation, development), which could be all involved in cellular network formation. This was associated with behavioral impairment, i.e. a decrease in exploratory activity measured in open field.

Onishchenko et al., 2007. Pregnant mice received 0.5 mg methylmercury/kg/day in drinking water from gestational dy 7 until day 7 after delivery. Offspring behavior was monitored at 5-15 and 26-36 weeks of age. Mercury-induced alterations in reference memory were detected.

Cagiano et al., 1990. Pregnant rat received at GD 15 8mg/kg of methylmercury by gavage. Offsprings were tested at day 16, 21 and 60. A reduced functional activity of glutamatergic system associated with disturbances in learning and memory were observed.

Rice, 1992. Female monkeys exposed to 10, 25 and 50 microg/kg/day to methylmercury. Male unexposed. Infants separated from mother at birth and exposed to similar doses did not show gross intellectual impairment, but interferences with temporal discrimination.

Sahin et al., 2016. Exposure of rat pups for 5 weeks or 5 months with mercury chloride (4.6 microg/kg as first injection, followed each day by 0.07 microg/kg/day). Learning and memory impairment measured by passive avoidance and Morris-water-maze was found in 5-weeks group, but not in the 5-month group. This was accompanied by hearing loss.

In humans:

Orenstein et al., 2014. Maternal peripartum hair mercury level was measured to assess prenatal mercury exposure. The concentrations of mercury was found in the range of 0.3-5.1 microg/g, similar to fish eating population in US. However, statistical analyses revealed that each microg/g increase in hair Hg was associated with a decrement in visula memory, learning and verbal memory.

Yorifuji et al., 2011. A survey of the Minamata exposed population made in 1971 to assess pre- and post-natal exposure revealed a methylmercury-induced impairment of intelligence as well as behavioral dysfunction.

One of the most difficult issues for neuroscientists is to link neuronal network function to cognition, including learning and memory. It is still unclear what modifications of neuronal circuits need to happen in order to alter motor behaviour as it is recorded in a learning and memory test (Mayford et al., 2012), meaning that there is no clear understanding about the how these two KEs are connected.

Several epidemiological studies where Pb2+ exposure levels have been studied in relation to neurobehavioural alterations in children have been reviewed in Koller et al. 2004. This review has concluded that in some occasions there is negative correlation between Pb2+ dose and cognitive deficits of the subjects due to high influence of social and parenting factors in cognitive ability like learning and memory (Koller et al. 2004), meaning that not always Pb2+ exposure is positively associated with learning and memory impairment in children.

The direct relationship of alterations in neural network function and specific cognitive deficits is difficult to ascertain given the many forms that learning and memory can take and the complexity of synaptic interactions in even the simplest brain circuit. Linking of neurophysiological assessments to learning and memory processes have, by necessity, been made across simple monosynaptic connections and largely focused on the hippocampus. Alterations in synaptic function have been found in the absence of behavioral impairments. This may result from measuring only one component in the complex brain circuitry that underlies 'cognition', behavioral tests that are not sufficiently sensitive for the detection of subtle cognitive impairments, and behavioral plasticity whereby tasks are solved by the animal via different strategies developed as a consequence of developmental insult.

Finally, in order to provide empirical support for this KER, data on the effects of lead (Pb) exposure are reported. However, Pb exposure is not always associated with learning and memory impairment in children. In this regard, Koller's review has commented that in some occasions, low-level Pb dose and cognitive deficits of the subjects are negatively correlated, and this may be due to the high influence of social and parenting factors in cognitive ability, like learning and memory (Koller et al., 2004).

Mercury

Olczak et al., 2001. Postnatal exposure of rats to Thimerosal (4 injections with 12, 240, 1440 and 3000 microgHg/kg per injection). Effects were measured in adult, which exhibited alterations in dopaminergic system with decline in the density of striatal D2 receptors, with a higher sensitivity for males. No alterations in spatial learning and memory was observed, but impairments of motor activity, increased anxiety (open fiel measurment), which are other symptoms of autism spectrum disorder.

Franco et al., 2006. Lactational exposure of mice to methylmercury in drinking water (10 mg/L). Analysis at weaning revealed only impairment in motor performances.

Franco et al., 2007. Lactational exposure of mice with mercury chloride (0.5 and 1.5 mg/kg,  i.p. injection once a day).. At weaning , animals exhibited an increased level of mercury in cerebellum associated with motor deficit.

References

Akaike M, Kato, N., Ohno, H., Kobayashi, T. (1991). Hyperactivity and spatial maze learning impairment of adult rats with temporary neonatal hypothyroidism. Neurotoxicol Teratol 13:317-322.

Anderson DW, Pothakos K, Schneider JS. (2012). Sex and rearing condition modify the effects of perinatal lead exposure on learning and memory. Neurotoxicology 33: 985-995.

Axelstad M, Hansen PR, Boberg J, Bonnichsen M, Nellemann C, Lund SP, Hougaard KS, U H. (2008). Developmental neurotoxicity of Propylthiouracil (PTU) in rats: relationship between transient hypothyroxinemia during development and long-lasting behavioural and functional changes. Toxicol Appl Pharmacol 232:1-13.

Baghurst PA, Tong S, Sawyer MG, Burns J, McMichael AJ. (1992). Environmental exposure to lead and children’s intelligence at the age of seven years. The Port Pirie Cohort Study. N Engl J Med. 327: 1279-1284.

Bellinger D, Sloman J, Leviton A, Rabinowitz M, Needleman HL, Waternaux C. (1991). Low-level lead exposure and children's cognitive function in the preschool years. Pediatrics. 87: 219-227.

Bellinger DC, Stiles KM, Needleman HL. (1992). Low-level lead exposure, intelligence and academic achievement: a long-term follow-up study. Pediatrics. 90: 855-861.

Bellinger DC. (2004). Lead. Pediatrics 113: 1016-1022.

Brockel BJ, Cory-Slechta DA. (1998). Lead, attention, and impulsive behavior: changes in a fixed-ratio waiting-for-reward paradigm. Pharmacol Biochem Behav. 60: 545-552.

Cagiano, R., et al. (1990). "Evidence that exposure to methyl mercury during gestation induces behavioral and neurochemical changes in offspring of rats." Neurotoxicol Teratol 12(1): 23-28.

Canfield RL, Henderson CR Jr, Cory-Slechta D, Cox C, Jusko TA, Lanphear BP. (2003). Intellectual impairment in children with blood lead concentrations below 10 μg per deciliter. N Engl J Med. 348: 1517-1526.

Cao X, Huang S, Ruan D. (2008). Enriched environment restores impaired hippocampal long-term potentiation and water maze performance induced by developmental lead exposure in rats. Dev Psychobiol. 50: 307-313.

Cohn J, Cory-Slechta DA. (1993). Subsensitivity of lead-exposed rats to the accuracy-impairing and rate-altering effects of MK-801 on a multiple schedule of repeated learning and performance. Brain Res. 600: 208-218.

Cohn J, Cory-Slechta DA. (1994). Lead exposure potentiates the effects of N-methyl-D-asparate on repeated learning. Neurotoxicol Teratol. 16: 455-465.

Cory-Slechta DA. (1995). MK-801 subsensitivity following postweaning lead exposure. Neurotoxicology 16: 83-95.

de Souza Lisboa S F, Gonzalves G, Komatsu F, Salci Queiroz CA, Aparecido Almeida A, Gastaldello Moreira EN. (2005). Developmental lead exposure induces depressive-like behavior in female rats. Drug Chem Toxicol. 28: 67-77.

D'Hooge R, De Deyn PP. (2001). Applications of the Morris water maze in the study of learning and memory. Brain Res Brain Res Rev. 36: 60-90.

Dong J, Yin H, Liu W, Wang P, Jiang Y, Chen J. (2005). Congenital iodine deficiency and hypothyroidism impair LTP and decrease C-fos and C-jun expression in rat hippocampus. Neurotoxicology 26:417-426.

Eddins, D., et al. (2008). "Mercury-induced cognitive impairment in metallothionein-1/2 null mice." Neurotoxicol Teratol 30(2): 88-95.

Finkelstein Y, Markowitz ME, Rosen JF. (1998). Low-level lead-induced neurotoxicity in children: an update on central nervous system effects. Brain Res Rev. 27: 168-176.

Franco, J. L., et al. (2006). "Cerebellar thiol status and motor deficit after lactational exposure to methylmercury." Environ Res 102(1): 22-28.

Franco, J. L., et al. (2007). "Lactational exposure to inorganic mercury: evidence of neurotoxic effects." Neurotoxicol Teratol 29(3): 360-367.

Gilbert ME, Kelly ME, Samsam TE, Goodman JH. (2005). Chronic developmental lead exposure reduces neurogenesis in adult rat hippocampus but does not impair spatial learning. Toxicol Sci. 86: 365-374.

Gilbert ME. (2011). Impact of low-level thyroid hormone disruption induced by propylthiouracil on brain development and function. Toxicol Sci 124:432-445.

Gilbert ME, Sanchez-Huerta K, Wood C. (2016). Mild Thyroid Hormone Insufficiency During Development Compromises Activity-Dependent Neuroplasticity in the Hippocampus of Adult Male Rats. Endocrinology 157:774-787.

Gilbert ME, Sui L. (2006). Dose-dependent reductions in spatial learning and synaptic function in the dentate gyrus of adult rats following developmental thyroid hormone insufficiency. Brain Res 1069:10-22.

Glover, C. N., et al. (2009). "Methylmercury speciation influences brain gene expression and behavior in gestationally-exposed mice pups." Toxicol Sci 110(2): 389-400.

Grunwald T, Beck H, Lehnertz K, Blümcke I, Pezer N, Kurthen M, Fernández G, Van Roost D, Heinze HJ, Kutas M, Elger CE. (1999). Evidence relating human verbal memory to hippocampal N-methyl-D-aspartate receptors. Proc Natl Acad Sci U S A. 96: 12085-12089.

Jaako-Movits K, Zharkovsky T, Romantchik O, Jurgenson M, Merisalu E, Heidmets LT, Zharkovsky A. (2005). Developmental lead exposure impairs contextual fear conditioning and reduces adult hippocampal neurogenesis in the rat brain. Int J Dev Neurosci. 23: 627-635.

Jang YJ, Park HR, Kim TH, Yang WJ, Lee JJ, Choi SY, Oh SB, Lee E, Park JH, Kim HP, Kim HS, Lee J. (2012). High dose bisphenol A impairs hippocampal neurogenesis in female mice across generations. Toxicology. Jun 14;296(1-3):73-82.

Jett DA, Kuhlmann AC, Farmer SJ, Guilarte TR. (1997). Age-dependent effects of developmental lead exposure on performance in the Morris water maze. Pharmacol Biochem Behav. 57: 271-279.

Jusko TA, Henderson CR, Lanphear BP, Cory-Slechta DA, Parsons PJ, Canfield RL. (2008). Blood lead concentrations < 10 microg/dL and child intelligence at 6 years of age. Environ Health Perspect 116:243-248.

Koller K, Brown T, Spurgeon A, Levy L. (2004). Recent developments in low-level lead exposure and intellectual impairment in children. Environ Health Perspect. 112: 987-994.

Kumar MV, Desiraju T. (1992). EEG spectral power reduction and learning disability in rats exposed to lead through postnatal developing age. Indian J Physiol Pharmacol. 36: 15-20.

Lanphear BP, Dietrich K, Auinger P, Cox C. (2000). Cognitive deficits associated with blood lead concentrations <10 microg/dL in US children and adolescents. Public Health Rep. 115: 521-529.

Lanphear BP, Hornung R, Khoury J, Yolton K, Baghurst P, Bellinger DC, Canfield RL, Dietrich KN, Bornschein R, Greene T, Rothenberg SJ, Needleman HL, Schnaas L, Wasserman G, Graziano J, Roberts R. (2005). Low-level environmental lead exposure and children’s intellectual function: an international pooled analysis. Environ Health Perspect. 113: 894-899.

Leviton A, Bellinger D, Allred EH, Rabinowitz M, Needleman H, Schoenbaum S. (1993). Pre- and postnatal low-level lead exposure and children's dysfunction in school. Environ Res 60:30-43.

Liu D, Teng W, Shan Z, Yu X, Gao Y, Wang S, Fan C, Wang H, Zhang H. (2010). The effect of maternal subclinical hypothyroidism during pregnancy on brain development in rat offspring. Thyroid 20:909–915.

Lynch MA. (2004). Long-term potentiation and memory. Physiol Rev. 84: 87-136.

Mayford M, Siegelbaum SA, Kandel ER. (2012). Synapses and memory storage. Cold Spring Harb Perspect Biol. 4. pii: a005751.

Montgomery, K. S., et al. (2008). "Chronic, low-dose prenatal exposure to methylmercury impairs motor and mnemonic function in adult C57/B6 mice." Behav Brain Res 191(1): 55-61.

Moreira EG, Vassilieff I, Vassilieff VS. (2001). Developmental lead exposure: behavioral alterations in the short and long term. Neurotoxicol Teratol. 23: 489-495.

Morris RG, Anderson E, Lynch GS, Baudry M. (1986). Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5. Nature 319: 774-776.

Murphy KJ, Regan CM. (1999). Low level lead exposure in the early postnatal period results in persisting neuroplastic deficits associated with memory consolidation. J Neurochem. 72: 2099-2104.

Neal AP, Guilarte TR. (2010). Molecular Neurobiology of Lead (Pb2+): Effects on Synaptic Function. Mol Neurobiol. 42: 151-160.

Needleman HL, Epstein S, Carnow B, Scanlon J, Parkinson D, Samuels S, Mazzochi A, David O. (1975). Letter: Blood-lead levels, behaviour and intelligence. Lancet 1: 751-752.

Needleman HL, Gatsonis CA. (1990). Low-level lead exposure and the IQ of children. A meta-analysis of modern studies. Jama 263: 673-678.

Needleman HL, Riess JA, Tobin MJ, Biesecker GE, Greenhouse JB. (1996). Bone Lead Levels and Delinquent Behavior. Jama 275: 363-369.

Olczak, M., et al. (2011). "Persistent behavioral impairments and alterations of brain dopamine system after early postnatal administration of thimerosal in rats." Behav Brain Res 223(1): 107-118.

Onishchenko, N., et al. (2007). "Developmental exposure to methylmercury alters learning and induces depression-like behavior in male mice." Toxicol Sci 97(2): 428-437.

Orenstein, S. T., et al. (2014). "Prenatal organochlorine and methylmercury exposure and memory and learning in school-age children in communities near the New Bedford Harbor Superfund site, Massachusetts." Environ Health Perspect 122(11): 1253-1259.

Rice, D. C. (1992). "Effects of pre- plus postnatal exposure to methylmercury in the monkey on fixed interval and discrimination reversal performance." Neurotoxicology 13(2): 443-452.

Saar D, Barkai E. (2003). Long-term modifications in intrinsic neuronal properties and rule learning in rats. Eur J Neurosci. 17: 2727-2734.

Sahin, D., et al. (2016). "Effects of gestational and lactational exposure to low dose mercury chloride (HgCl2) on behaviour, learning and hearing thresholds in WAG/Rij rats." EXCLI J 15: 391-402.

Salinas JA, Huff NC. (2002). Lead and conditioned fear to contextual and discrete cues. Neurotoxicol Teratol. 24: 541-550.

Sokolowski, K., et al. (2013). "Neural stem cell apoptosis after low-methylmercury exposures in postnatal hippocampus produce persistent cell loss and adolescent memory deficits." Dev Neurobiol 73(12): 936-949.

Surkan PJ, Zhang A, Trachtenberg F, Daniel DB, McKinlay S, Bellinger DC. (2007). Neuropsychological function in children with blood lead levels <10 microg/dL. Neurotoxicology. 28: 1170-1177.

Sui L, Anderson WL, Gilbert ME. (2005). Impairment in short-term but enhanced long-term synaptic potentiation and ERK activation in adult hippocampal area CA1 following developmental thyroid hormone insufficiency. Toxicol Sci 85:647-656.

Sui L, Gilbert ME. (2003). Pre- and postnatal propylthiouracil-induced hypothyroidism impairs synaptic transmission and plasticity in area CA1 of the neonatal rat hippocampus. Endocrinology 144:4195-4203.

Taylor MA, Swant J, Wagner JJ, Fisher JW, Ferguson DC. (2008). Lower thyroid compensatory reserve of rat pups after maternal hypothyroidism: correlation of thyroid, hepatic, and cerebrocortical biomarkers with hippocampal neurophysiology. Endocrinology 149:3521-3530.

Toscano CD, Guilarte TR. (2005). Lead neurotoxicity: From exposure to molecular effects. Brain Res Rev. 49: 529-554.

Vara H, Martinez B, Santos A, Colino A. (2002). Thyroid hormone regulates neurotransmitter release in neonatal rat hippocampus. Neuroscience 110:19-28.

Wang S, Teng W, Gao Y, Fan C, Zhang H, Shan Z. (2012). Early levothyroxine treatment on maternal subclinical hypothyroidism improves spatial learning of offspring in rats. J Neuroendocrinol 24:841–848.

Wheeler SM, McAndrews MP, Sheard ED, Rovet J. (2012). Visuospatial associative memory and hippocampal functioning in congenital hypothyroidism. J Int Neuropsychol Soc 18:49-56.

Wheeler SM, McLelland VC, Sheard E, McAndrews MP, Rovet JF. (2015). Hippocampal Functioning and Verbal Associative Memory in Adolescents with Congenital Hypothyroidism. Front Endocrinol (Lausanne) 6:163.

Willoughby KA, McAndrews MP, Rovet J. (2013). Effects of early thyroid hormone deficiency on children's autobiographical memory performance. J Int Neuropsychol Soc 19:419-429.

Willoughby KA, McAndrews MP, Rovet JF. (2014). Effects of maternal hypothyroidism on offspring hippocampus and memory. Thyroid 24:576-584.

Wu Y, Beland FA1, Fang JL. (2016). Effect of triclosan, triclocarban, 2,2',4,4'-tetrabromodiphenyl ether, and bisphenol A on the iodide uptake, thyroid peroxidase activity, and expression of genes involved in thyroid hormone synthesis. Toxicol In Vitro. Apr;32:310-9.

Xiao Y, Fu H, Han X, Hu X, Gu H, Chen Y, Wei Q, Hu Q. (2014). Role of synaptic structural plasticity in impairments of spatial learning and memory induced by developmental lead exposure in Wistar rats. PLoS One. 23;9(12):e115556.

Xu J, Yan HC, Yang B, Tong LS, Zou YX, Tian Y. (2009). Effects of lead exposure on hippocampal metabotropic glutamate receptor subtype 3 and 7 in developmental rats. J Negat Results Biomed. 8: 5.

Yorifuji, T., et al. (2011). "Long-term exposure to methylmercury and psychiatric symptoms in residents of Minamata, Japan." Environ Int 37(5): 907-913.

Zanoli, P., et al. (1994). "Methyl mercury during late gestation affects temporarily the development of cortical muscarinic receptors in rat offspring." Pharmacol Toxicol 75(5): 261-264.

Zanoli, P., et al. (2001). "Prenatal exposure to methyl mercury in rats: focus on changes in kynurenine pathway." Brain Res Bull 55(2): 235-238.

Relationship: 1690: Oxidative Stress leads to N/A, Cell injury/death

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Rat, Mouse: (Sarafian et al., 1994; Castoldi et al., 2000; Kaur, Aschner and Syversen, 2006; Franco et al., 2007; Lu et al., 2011; Polunas et al., 2011)

(Richetti et al., 2011) - Adult and healthy zebrafish of both sexes (12 animals and housed in 3 L) mercury chloride final concentration of 20 mg/L. Mercury chloride promoted a significant decrease in acetylcholinesterase activity and the antioxidant competence was also decreased.

(Berntssen, Aatland and Handy, 2003) - Atlantic salmon (Salmo salar L.) were supplemented with mercuric chloride (0, 10, or 100 mg Hg per kg) or methylmercury chloride (0, 5, or 10 mg Hg per kg) for 4 months.

Methylmercury chloride

• accumulated significantly in the brain of fish fed 5 or 10 mg/kg
• No mortality or growth reduction
• - 2-fold increase in the antioxidant enzyme super oxide dismutase (SOD) in the brain
• 10 mg/kg - 7-fold increase of lipid peroxidative products (thiobarbituric acid reactive substances, TBARS) and a subsequently 1.5-fold decrease in anti oxidant enzyme activity (SOD and glutathione peroxidase, GSH-Px). Fish also had pathological damage (vacoulation and necrosis), significantly reduced neural enzyme activity (5-fold reduced monoamine oxidase, MAO, activity), and reduced overall post-feeding activity behaviour.

Mercuric chloride

• accumulated significantly in the brain only at 100 mg/kg
• No mortality or growth reduction
• 100 mg/kg -  significant reduced neural MAO activity and pathological changes (astrocyte proliferation) in the brain, however, neural SOD and GSH-Px enzyme activity, lipid peroxidative products (TBARS), and post feeding behaviour did not differ from controls.

Key Event Relationship Description

Oxidative stress (OS) as a concept in redox biology and medicine has been formulated in 1985 (Sies, 2015). OS is intimately linked to cellular energy balance and comes from the imbalance between the generation and detoxification of reactive oxygen and nitrogen species (ROS/RNS) or from a decay of the antioxidant protective ability. OS is characterized by the reduced capacity of endogenous systems to fight against the oxidative attack directed towards target biomolecules (Wang and Michaelis, 2010; Pisoschi and Pop, 2015).  Glutathione, the most important redox buffer in cells (antioxidant), cycles between reduced glutathione (GSH) and oxidized glutathione disulfide (GSSG), and serves as a vital sink for control of ROS levels in cells (Reynolds et al., 2007).  Several case-control studies have reported the link between lower concentrations of GSH, higher levels of GSSG and the development of diseases (Rossignol and Frye, 2014). OS can cause cellular damage and subsequent cell death because the ROS oxidize vital cellular components such as lipids, proteins, and nucleic acids (Gilgun-Sherki, Melamed and Offen, 2001; Wang and Michaelis, 2010).

The central nervous system is especially vulnerable to free radical damage since it has a high oxygen consumption rate, an abundant lipid content and reduced levels of antioxidant enzymes (Coyle and Puttfarcken, 1993; Markesbery, 1997). It has been show that the developing brain is particularly vulnerable to neurotoxicants and OS due to differentiation processes, changes in morphology, lack of physiological barriers and less intrinsic capacity to cope with cellular stress (Grandjean and Landrigan, 2014; Sandström et al., 2017). OS has been link to brain aging, neurodegenerative diseases, and other related adverse conditions.  There is evidence that free radicals play a role in cerebral ischemia-reperfusion, head injury, Parkinson’s disease, amyotrophic lateral sclerosis, Down’s syndrome, and Alzheimer’s disease due to cellular damage (Markesbery, 1997; Gilgun-Sherki, Melamed and Offen, 2001; Wang and Michaelis, 2010). OS has also been linked to neurodevelopmental diseases and deficits like autism spectrum disorder and postnatal motor coordination deficits (Wells et al., 2009; Rossignol and Frye, 2014; Bhandari and Kuhad, 2015)

Evidence Supporting this KER

A noteworthy insight, early on, was the perception that oxidation-reduction (redox) reactions in living cells are utilized in fundamental processes of redox regulation, collectively termed ‘redox signaling’ and ‘redox control’ (Sies, 2015).

Free radical-induced damage in OS has been confirmed as a contributor to the pathogenesis and patho-physiology of many chronic diseases, such as Alzheimer, atherosclerosis, Parkinson, but also in traumatic brain injury, sepsis, stroke, myocardial infraction, inflammatory diseases, cataracts and cancer (Bar-Or et al., 2015; Pisoschi and Pop, 2015). It has been assessed that oxidative stress is correlated with over 100 diseases, either as source or outcome (Pisoschi and Pop, 2015).

Therefore, the fact that ROS over-production can kill neurons is well accepted (Brown and Bal-Price, 2003; Taetzsch and Block, 2013). This ROS over-production can occur in the neurons themselves or can also have a glial origin (Yuste et al., 2015).

Mercury

Oxidative stress has been implicated in the pathogenesis of methylmercury (MeHg) neurotoxicity. Studies of mature neurons suggest that the mitochondrion may be a major source of MeHg-induced reactive oxygen species and a critical mediator of MeHg-induced neuronal death, likely by activation of apoptotic pathways. (Polunas et al., 2011)

(Yoshida et al., 2011) – WT and metallothionein (MT)-I/II null mice exposed to low-levels of mercury vapor (Hg⁰) during postnatal development. Repeatedly exposed to Hg⁰ at 0.030 mg/m3 (range: 0.023-0.043 mg/m3 ), for 6 hr per day until the 20th day postpartum.

• 12 weeks of age - Hg⁰ -exposed MT-I/II null mice showed a significant decrease in total locomotor activity in females, though learning ability and spatial learning ability were not affected.
•  
• Metallothionein I/II are more susceptible to mercury exposure

(Lu et al., 2011) - MeHg in the mouse cerebrum (in vivo) and in cultured Neuro-2a cells (in vitro).

• In vivo - 50µg/kg/day MeHg for 7 consecutive weeks - increased levels of lipid peroxidation in the plasma and cerebral cortex. Decreased GSH level and increase the expressions of caspase-3, -7, and -9, accompanied by Bcl-2 down-regulation and up-regulation of Bax, Bak, and p53.
• In vitro – 3 and 5 µM MeHg - reduced cell viability, increased oxidative stress damage, and induced several features of mitochondria-dependent apoptotic signals, including increased sub-G1 hypodiploids, mitochondrial dysfunctions, and the activation of PARP, and caspase cascades.  
• These MeHg-induced apoptotic-related signals could be remarkably reversed by antioxidant NAC.

(Sarafian et al., 1994) - Hypothalamic  mouse neural cell line GT1-7 without and with expression construct for the anti-apoptotic proto-oncogene, bcl-2.

• 3h exposure, 10 µM MeHg - increased formation of reactive ROS, and decreased levels of GSH, associated with 20% cell death. Cells transfected with an expression construct bcl-2, displayed attenuated ROS induction and negligible cell death.
• 24h exposure, 5 µM MeHg - killed 56% of control cells, but only 19% of bcl-2-transfected cells.
• By using diethyl maleate to deplete cells of GSH, we demonstrate that the differential sensitivity to MeHg was not due solely to intrinsically different GSH levels. The data suggest that MeHg-mediated cell killing correlates more closely with ROS generation than with GSH levels and that bcl-2 protects MeHg-treated cells by suppressing ROS generation.

(Castoldi et al., 2000) - In vitro exposure of primary cultures of rat CGCs to MeHg resulted in a time- and concentration-dependent cell death.

• 1 hr exposure, 5–10 µM MeHg - impairment of mitochondrial activity, de-energization of mitochondria and plasma membrane lysis, resulting in necrotic cell death.
• 1hr exposure, 0.5–1 µM MeHg - did not compromise cell viability, mitochondrial membrane potential and function at early time points.
• 1hr exposure, 1 µM MeHg - only a small population of neurons (+-20%) dies by necrosis. The surviving neurons show network damage, but maintain membrane integrity, mitochondrial membrane potential and function at early time points. Later, however, the cells progressively display the morphological signs of apoptosis.
• 18hr exposure, 0.5–1 µM MeHg – cells progressively underwent apoptosis reaching the 100% cell death
• insulin-like growth factor-I partially rescued CGCs from MeHg-triggered apoptosis.

(Kaur,et al., 2006) - primary cell cultures of cerebellar neurons and astrocytes from 7-day-old NMRI mice. 5 mM MeHg for 30 min.

• Twenty-one days post-astrocyte isolation - 250mM N-acetyl cysteine (NAC) or 3mM di-ethyl maleate (DEM) added to the wells 12 h prior to MeHg exposure
• 7 days post-neurons isolation - 200mM of NAC or 1.8mM of DEM added to the wells 12 h prior to MeHg exposure
• The intracellular GSH content was modified by pretreatment with NAC or DEM for 12 h.
• Treatment with 5 mM Me Hg for 30 min led to significant (p < 0.05) increase in ROS and reduction (p < 0.001) in GSH content.
• Depletion of intracellular GSH by DEM further increased the generation of MeHg-induced ROS in both cell cultures.
• NAC supplementation increased intracellular GSH and provided protection against MeHg-induced oxidative stress in both cell cultures.

(Franco et al., 2007) – Mitochondrial enriched fractions from adult (2 months old) Swiss Albino male mice.

• MeHg and HgCl2 (10–100 µM) significantly decreased mitochondrial viability; this phenomenon was positively correlated to mercurial-induced glutathione oxidation.
• Both mercurials induced a significant reduction of GSH in a dose-dependent manner.
• Correlation analyses showed significant positive correlations between mitochondrial viability and glutathione content for MeHg (Pearson coefficient) 0.933; P < 0.01) and or HgCl2 (Pearson coefficient ) 0.854; P < 0.01).
• Quercetin (100–300 µM) prevented mercurial-induced disruption of mitochondrial viability. Moreover, quercetin, which did not display any chelating effect on MeHg or HgCl2, prevented mercurial-induced glutathione oxidation.

(Polunas et al., 2011) - Murine embryonal carcinoma (EC) cells, which differentiate into neurons following exposure to retinoic acid.

• 4h exposure, 1.5 mM MeHg - earlier and significantly higher levels of ROS production and more extensive mitochondrial depolarization in neurons than in undifferentiated EC cells. cyclosporin A (CsA) completely inhibited mitochondrial depolarization by MeHg in EC cells but only delayed this response in the neurons. In contrast, CsA significantly inhibited MeHg-induced neuronal ROS production. Cyt c release was also more extensive in neurons, with less protection afforded by CsA.

(Sandström et al., 2016) - in vitro 3D human neural tissues from neural progenitor cells derived from human embryonic stem cells. Single MeHg exposure at day 42 of 3D culturing (week 6) and material was collected 72 h after.

• 1-10 μM - LDH activity increased, confirming induced cell death.
• 5 and 10 μM - increased HMOX1 gene expression as indirect marker of oxidative stress.

Acrylamide

(Allam et al., 2011) - sixty albino Rattus norvegicus, 45 virgin females and 15 mature males. This study examined its effects on the development of external features in cubs.

• prenatal intoxicated group - newborns from mothers treated with ACR from day 7 (GD 7) of gestation till birth
• perinatal intoxicated group - newborns from mothers treated with ACR from GD7 of gestation till D28 after birth

ACR administered either prenatally or perinatally has been shown to induce significant retardation in the new- borns’ body weights development, increase of thiobarbituric acid- reactive substances (TBARS) and oxidative stress (significant reductions in glutathione         reduced, total thiols, superoxide dismutase and peroxidase activities) in the developing cerebellum. ACR treatment delayed the proliferation in the granular layer and delayed both cell migration and differentiation. Purkinje cell loss was also seen in acrylamide-treated  animals. Ultrastructural studies of Purkinje cells in the perinatal group showed microvacuolations and cell loss.

(Lakshmi et al., 2012) - Wistar male albino rats, four groups (n = 6 per group)

• II – (Acrylamide) ACR - 30 mg/kg ACR for 30 days: increase in the lipid peroxidative (LPO), protein carbonyl, hydroxyl radical and hydroperoxide levels with subsequent decrease in the activities of enzymic antioxidants and level of GSH. Cortex showed condensed nuclei along with damaged cells. Decrease in the expression of Bcl2 along with simultaneous increase in the expressions of Bax and Bad as compared to control.
• II rats – ACR + Fish oil -0.5 ml/kg b.w.fish oil orally 10 min before ACR induction with 30 mg/kg for 30 days – reversed significantly all the OS markers.

Mercury-induced upregulation of GSH level and GR activity as an adaptive mechanism following lactational exposure to methylmercury (10 mg/L in drinking water) associated with motor deficit, suggesting neuronal impairment (Franco et al., 2006).

References

Allam,  a et al. (2011) ‘Prenatal and perinatal acrylamide disrupts the development of cerebellum in rat: Biochemical and morphological studies.’, Toxicology and industrial health, 27, pp. 291–306. doi: 10.1177/0748233710386412.

Bar-Or, D. et al. (2015) ‘Oxidative stress in severe acute illness’, Redox Biology. Elsevier, 4, pp. 340–345. doi: 10.1016/j.redox.2015.01.006.

Berntssen, M. H. G., Aatland, A. and Handy, R. D. (2003) ‘Chronic dietary mercury exposure causes oxidative stress, brain lesions, and altered behaviour in Atlantic salmon (Salmo salar) parr’, Aquatic Toxicology, 65(1), pp. 55–72. doi: 10.1016/S0166-445X(03)00104-8.

Bhandari, R. and Kuhad, A. (2015) ‘Neuropsychopharmacotherapeutic efficacy of curcumin in experimental paradigm of autism spectrum disorders’, Life Sciences. Elsevier Inc., 141, pp. 156–169. doi: 10.1016/j.lfs.2015.09.012.

Castoldi, A. F. et al. (2000) ‘Early acute necrosis, delayed apoptosis and cytoskeletal breakdown in cultured cerebellar granule neurons exposed to methylmercury’, Journal of Neuroscience Research, 59(6), pp. 775–787. doi: 10.1002/(SICI)1097-4547(20000315)59:6<775::AID-JNR10>3.0.CO;2-T.

Coyle, J. and Puttfarcken, P. (1993) ‘Glutamate Toxicity’, Science, 262, pp. 689–95.

Franco, J. L. et al. (2006) ‘Cerebellar thiol status and motor deficit after lactational exposure to methylmercury’, Environmental Research, 102(1), pp. 22–28. doi: 10.1016/j.envres.2006.02.003.

Franco, J. L. et al. (2007) ‘Mercurial-induced hydrogen peroxide generation in mouse brain mitochondria: Protective effects of quercetin’, Chemical Research in Toxicology, 20(12), pp. 1919–1926. doi: 10.1021/tx7002323.

Gilgun-Sherki, Y., Melamed, E. and Offen, D. (2001) ‘Oxidative stress induced-neurodegenerative diseases: The need for antioxidants that penetrate the blood brain barrier’, Neuropharmacology, 40(8), pp. 959–975. doi: 10.1016/S0028-3908(01)00019-3.

Grandjean, P. and Landrigan, P. J. (2014) ‘Neurobehavioural effects of developmental toxicity’, The Lancet Neurology, 13(3), pp. 330–338. doi: 10.1016/S1474-4422(13)70278-3.

Kaur, P., Aschner, M. and Syversen, T. (2006) ‘Glutathione modulation influences methyl mercury induced neurotoxicity in primary cell cultures of neurons and astrocytes’, NeuroToxicology, 27(4), pp. 492–500. doi: 10.1016/j.neuro.2006.01.010.

Lakshmi, D. et al. (2012) ‘Ameliorating effect of fish oil on acrylamide induced oxidative stress and neuronal apoptosis in cerebral cortex’, Neurochemical Research, 37(9), pp. 1859–1867. doi: 10.1007/s11064-012-0794-1.

Lu, T. H. et al. (2011) ‘Involvement of oxidative stress-mediated ERK1/2 and p38 activation regulated mitochondria-dependent apoptotic signals in methylmercury-induced neuronal cell injury’, Toxicology Letters. Elsevier Ireland Ltd, 204(1), pp. 71–80. doi: 10.1016/j.toxlet.2011.04.013.

Markesbery, W. R. (1997) ‘Oxidative stress hypothesis in Alzheimer’s disease’, Free Radical Biology and Medicine, 23(1), pp. 134–147. doi: 10.1016/S0891-5849(96)00629-6.

Pisoschi, A. M. and Pop, A. (2015) ‘The role of antioxidants in the chemistry of oxidative stress: A review’, European Journal of Medicinal Chemistry. Elsevier Masson SAS, 97, pp. 55–74. doi: 10.1016/j.ejmech.2015.04.040.

Polunas, M. et al. (2011) ‘Role of oxidative stress and the mitochondrial permeability transition in methylmercury cytotoxicity’, NeuroToxicology. Elsevier B.V., 32(5), pp. 526–534. doi: 10.1016/j.neuro.2011.07.006.

Reynolds, A. et al. (2007) ‘Oxidative Stress and the Pathogenesis of Neurodegenerative Disorders’, International Review of Neurobiology, 82(7), pp. 297–325. doi: 10.1016/S0074-7742(07)82016-2.

Richetti, S. K. et al. (2011) ‘Acetylcholinesterase activity and antioxidant capacity of zebrafish brain is altered by heavy metal exposure’, NeuroToxicology. Elsevier B.V., 32(1), pp. 116–122. doi: 10.1016/j.neuro.2010.11.001.

Rossignol, D. A. and Frye, R. E. (2014) ‘Evidence linking oxidative stress, mitochondrial dysfunction, and inflammation in the brain of individuals with autism’, Frontiers in Physiology, 5 APR(April), pp. 1–15. doi: 10.3389/fphys.2014.00150.

Sandström, J. et al. (2016) ‘Toxicology in Vitro Development and characterization of a human embryonic stem cell-derived 3D neural tissue model for neurotoxicity testing’, Tiv, pp. 1–12. doi: 10.1016/j.tiv.2016.10.001.

Sandström, J. et al. (2017) ‘Potential mechanisms of development-dependent adverse effects of the herbicide paraquat in 3D rat brain cell cultures’, NeuroToxicology, 60, pp. 116–124. doi: 10.1016/j.neuro.2017.04.010.

Sarafian, T. A. et al. (1994) ‘Bcl-2 Expression Decreases Methyle Mercury-Induced Free-Radical Generation and Cel Killing in a Neural Cell Line’, Toxicol. Lett., 74(2), pp. 149–155.

Sies, H. (2015) ‘Oxidative stress: A concept in redox biology and medicine’, Redox Biology. Elsevier, 4, pp. 180–183. doi: 10.1016/j.redox.2015.01.002.

Wang, X. and Michaelis, E. K. (2010) ‘Selective neuronal vulnerability to oxidative stress in the brain’, Frontiers in Aging Neuroscience, 2(MAR), pp. 1–13. doi: 10.3389/fnagi.2010.00012.

Wells, P. G. et al. (2009) ‘Oxidative stress in developmental origins of disease: Teratogenesis, neurodevelopmental deficits, and cancer’, Toxicological Sciences, 108(1), pp. 4–18. doi: 10.1093/toxsci/kfn263.

Yoshida, M. et al. (2011) ‘Neurobehavioral changes and alteration of gene expression in the brains of metallothionein-I/II null mice exposed to low levels of mercury vapor during postnatal development’, The Journal of Toxicological Sciences, 36(5), pp. 539–547. doi: 10.2131/jts.36.539.

Yuste, J.E., et al., 2015. Implications of glial nitric oxide in neurodegenerative diseases. Front Cell Neurosci. 9, 322.