SNAPSHOT

Event: 279: Thyroperoxidase, Inhibition

Short Name: Thyroperoxidase, Inhibition

AOPs Including This Key Event

Stressors

Biological Context

Cell term

Organ term

TPO inhibition is a MIE conserved across taxa, with supporting data from experimental models and human clinical testing. This conservation is likely a function of the high degree of protein sequence similarity in the catalytic domain of mammalian peroxidases (Taurog, 1999). Ample data available for human, rat, and porcine TPO inhibition demonstrate qualitative concordance across these species (Schmultzer et al., 2007; Paul et al., 2013; Hornung et al., 2010). A comparison of rat TPO and pig TPO, bovine lactoperoxidase, and human TPO inhibition by genistein demonstrated good qualitative and quantitative (40–66%) inhibition across species, as indicated by quantification of MIT and DIT production (Doerge and Chang, 2002). Ealey et al. (1984) demonstrated peroxidase activity in guinea pig thyroid tissue using 3,3'-diaminobenzidine tetrahydrochloride (DAB) as a substrate that is oxidized by the peroxidase to form a brown insoluble reaction product. Formation of this reaction product was inhibited by 3-amino-1,2,4-triazole and the TPO inhibitor, methimazole (MMI). A comparative analysis of this action of MMI between rat- and human-derived TPO indicates concordance of qualitative response. Data also suggest an increased quantitative sensitivity to MMI in rat compared to human (Vickers et al., 2012). Paul et al. (2013) tested 12 chemicals using the guaiacol assay using both porcine and rat thyroid microsomes. The authors concluded that there was an excellent qualitative concordance between rat and porcine TPO inhibition, as all chemicals that inhibited TPO in porcine thyroid microsomes also inhibited TPO in rat thyroid microsomes when tested within the same concentration range. In addition, these authors noted a qualitative concordance that ranged from 1.5 to 50-fold differences estimated by relative potency. Similary, Takayama et al. (1986) found a very large species difference in potency for sulfamonomethoxine between cynomologus monkeys and rats.

Event: 277: Thyroid hormone synthesis, Decreased

Short Name: TH synthesis, Decreased

AOPs Including This Key Event

Stressors

Biological Context

Cell term

Organ term

Decreased TH synthesis resulting from TPO or NIS inhibition is conserved across taxa, with in vivo evidence from humans, rats, amphibians, some fish specis, and birds, and in vitro evidence from rat and porcine microsomes. Indeed, TPO and NIS mutations result in congenital hypothyroidism in humans (Bakker et al., 2000; Spitzweg and Morris, 2010), demonstrating the essentiality of TPO and NIS function toward maintaining euthyroid status. Though decreased serum T4 is used as a surrogate measure to indicate chemical-mediated decreases in TH synthesis, clinical and veterinary management of hyperthyroidism and Grave's disease using propylthiouracil and methimazole, known to decrease TH synthesis, indicates strong medical evidence for chemical inhibition of TPO (Zoeller and Crofton, 2005).

Event: 281: Thyroxine (T4) in serum, Decreased

Short Name: T4 in serum, Decreased

AOPs Including This Key Event

Stressors

Biological Context

Organ term

The overall evidence supporting taxonomic applicability is strong. THs are evolutionarily conserved molecules present in all vertebrate species (Hulbert, 2000; Yen, 2001). Moreover, their crucial role in zebra fish (Thienpont et al., 2011), amphibian and lamprey metamorphoses is well established (Manzon and Youson, 1997; Yaoita and Brown, 1990; Furlow and Neff, 2006). Their existence and importance has also been described in many different animal and plant kingdoms (Eales, 1997; Heyland and Moroz, 2005), while their role as environmental messenger via exogenous routes in echinoderms confirms the hypothesis that these molecules are widely distributed among the living organisms (Heyland and Hodin, 2004). However, the role of TH in the different species depends on the expression and function of specific proteins (e.g receptors or enzymes) under TH control and may vary across species and tissues. As such extrapolation regarding TH action across species should be done with caution.

With few exceptions, vertebrate species have circulating T4 (and T3) that are bound to transport proteins in blood. Clear species differences exist in serum transport proteins (Dohler et al., 1979; Yamauchi and Isihara, 2009). There are three major transport proteins in mammals; thyroid binding globulin (TBG), transthyretin (TTR), and albumin. In adult humans, the percent bound to these proteins is about 75, 15 and 10 percent, respectively (Schussler 2000).  In contrast, in adult rats the majority of THs are bound to TTR. Thyroid binding proteins are developmentally regulated in rats. TBG is expressed in rats until approximately postnatal day (PND) 60, with peak expression occurring during weaning (Savu et al., 1989). However, low levels of TBG persist into adult ages in rats and can be experimentally induced by hypothyroidism, malnutrition, or caloric restriction (Rouaze-Romet et al., 1992). While these species differences impact TH half-life (Capen, 1997) and possibly regulatory feedback mechanisms, there is little information on quantitative dose-response relationships of binding proteins and serum hormones during development across different species. Serum THs are still regarded as the most robust measurable key event causally linked to downstream adverse outcomes.

Event: 1007: Reduced, Anterior swim bladder inflation

Short Name: Reduced, Anterior swim bladder inflation

AOPs Including This Key Event

Biological Context

Organ term

The evidence for impaired inflation of the anterior chamber of the swim bladder currently comes from work on zebrafish and fathead minnow.

Event: 1008: Reduced, Hearing

Short Name: Reduced, Hearing

AOPs Including This Key Event

Biological Context

Organ term

• A sense of hearing is known to exist in a wide range of vertebrates and invertebrates, although the organs and structures involved vary widely.

Event: 1005: Reduced, Swimming performance

Short Name: Reduced, Swimming performance

AOPs Including This Key Event

Biological Context

Importance of swimming performance for natural behaviour is generally applicable to fish.

Event: 1006: Reduced, Young of year survival

Short Name: Reduced, Young of year survival

AOPs Including This Key Event

Biological Context

Survival is important for all species.

Event: 360: Decrease, Population trajectory

Short Name: Decrease, Population trajectory

AOPs Including This Key Event

Biological Context

Consideration of population size and changes in population size over time is potentially relevant to all living organisms.

Relationship: 1032: Reduced, Anterior swim bladder inflation leads to Reduced, Hearing

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Within fish we can distinguish between hearing generalists (non-specialists) such as cichlids, salmonids, sunfishes and toadfishes and hearing specialists which have accessory hearing structures (specializations) such as the Weberian apparatus in otophysines, supbrabranchial chambers in labyrinth fish and auditory bullae in mormyrids (Ladich and Wysocki, 2003; Ladich and Fay, 2013). In fish that do not possess an anterior chamber with a function in hearing this KER is not applicable.

Key Event Relationship Description

Apart from a role in buoyancy that is not completely understood with regard to the relation to the function of the posterior chamber, the anterior chamber of the swim bladder of many fish species has an additional role in the production and/or detection of sound (Popper et al., 1974; Bang et al., 2002). Several fish families have Weberian ossicles (tiny bones, also called the Weberian apparatus), connecting the anterior chamber to the inner ear resulting in an amplification of sound waves. Therefore it is plausible to assume that if the anterior chamber does not inflate or inflates to a reduced size, the connection to the Weberian ossicles is lost and hearing is impaired.

Evidence Supporting this KER

It is plausible to assume that if the anterior chamber does not inflate or inflates to a reduced size, the connection to the Weberian ossicles is lost and hearing is impaired.

• Bang et al. (2002) developed a behavioural screening method for detecting hearing defects in zebrafish. In this method they measure a rapid escape reflex in response to a loud sound. They tested 6500 wildtype fish and found that 1% of them had a hearing deficit. When investigating the morphology of the auditory system of these non-responders, they found that nearly all of them showed abnormalities in the swim bladder or Weberian ossicles. Specifically, in 36% of the cases there was only one swim bladder chamber and it was clear that the swim bladder did not touch the first Weberian ossicle (the tripus). Another 36% showed abnormalities in the vertebrae associated with the Weberian ossicles. Fish with normal acoustically mediated startle responses showed no obvious malformations of the swim bladder or Weberian ossicles.

• Ladich and Wysocki (2003) removed the Weberian ossicle directly associated with the anterior chamber (the tripod) in goldfish and showed a frequency-dependent increase of the threshold for perceiving sound.

• Different families of catfish have large variation in the morphology of the swim bladder as well as in the number and size of Weberian ossicles. Lechner and Ladich (2008) showed that over a large range of catfish families larger swim bladders and larger as well as higher numbers of ossicles were related to better hearing abilities.

• Yan et al. (2000) experimentally deflated the swim bladder of goldfish and found that this resulted in a frequency-dependent increase of the threshold for perceiving sound.

Include consideration of temporal concordance here

References

Bang, P.I., Yelick, P.C., Malicki, J.J., Sewell, W.F., 2002. High-throughput behavioral screening method for detecting auditory response defects in zebrafish. Journal of Neuroscience Methods 118, 177-187.

Ladich, F., Fay, R.R., 2013. Auditory evoked potential audiometry in fish. Reviews in Fish Biology and Fisheries 23, 317-364.

Ladich, F., Wysocki, L.E., 2003. How does tripus extirpation affect auditory sensitivity in goldfish? Hearing Research 182, 119-129.

Lechner, W., Ladich, F., 2008. Size matters: Diversity in swimbladders and Weberian ossicles affects hearing in catfishes. Journal of Experimental Biology 211, 1681-1689.

Popper, A.N., 1974. Response of swim bladder of goldfish (Carassius auratus) to acoustic stimuli. Journal of Experimental Biology 60, 295-304.

Yan, H.Y., Fine, M.L., Horn, N.S., Colon, W.E., 2000. Variability in the role of the gasbladder in fish audition. Journal of Comparative Physiology a-Sensory Neural and Behavioral Physiology 186, 435-445.

Relationship: 1033: Reduced, Hearing leads to Reduced, Young of year survival

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

Impaired hearing could result in an impact on ecologically relevant endpoint, such as predator avoidance and prey capture. Therefore, it can be assumed that an affect on hearing could reduce young of year survival.

Evidence Supporting this KER

• In birds, acoustic signals play key roles in territory defense and mate attraction (Slabbekoorn and Ripmeester, 2008).

Roles of Acoustic signaling in fish (reviewed by Kasumayan 2009):

• Reproductive isolation - among fish capable of generating sound, sound emission during spawning is the most prominent life stage during which acoustic signaling occurs. Includes mate attraction, courtship, establishment of territory.
• Defensive sounds - fright and stress, alert conspecifics to potential threats.
• Organization of group/aggregative behaviors
• Feeding behaviors - in many fish conditioned reflex to the sounds of conspecifics feeding can be formed and cause orientation or attraction of fish toward their source, particularly in combination with corresponding visual stimuli and odors.

References

• Kasumayan AO. 2009. Acoustic signaling in fish. J. Ichthyology. 49:963-1020.
• SLABBEKOORN, H. and RIPMEESTER, E. A. P. (2008), Birdsong and anthropogenic noise: implications and applications for conservation. Molecular Ecology, 17: 72–83. doi:10.1111/j.1365-294X.2007.03487.x

Relationship: 1039: T4 in serum, Decreased leads to Reduced, Anterior swim bladder inflation

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

The evidence for a relationship between circulating T4 levels and inflation of the anterior chamber of the swim bladder currently comes from work on zebrafish and fathead minnow.

Key Event Relationship Description

Reduced T4 levels in serum prohibit local production of active T3 hormone by deiodinases expressed in the target tissues. There is evidence suggesting that anterior swim bladder inflation relies on increased thyroid hormone levels at this specific developmental time point.

While evidence supports a link between reduced serum T4 levels and reduced anterior swim bladder inflation, experimental evidence suggests that inhibition of T4 synthesis does not result in reduced posterior swim bladder inflation. The absence of effects on posterior chamber inflation is possibly due to maternal transfer of T4 into the eggs. These maternally derived THs are depleted at 21 dpf and cannot offset TPO inhibition, resulting in impaired anterior chamber inflation. This has been previously suggested by Reider and Connaughton (2014) with specific reference to eye development.

Maternal thyroid hormone levels in embryos have been demonstrated in zebrafish, fathead minnow, brown trout, striped bass, tilapia, rabbitfish, conger eel, sea bream and different species of salmon (Walpita et al., 2007; Chang et al., 2012; Power et al., 2001; Brown et al., 1987, 1988). Alt et al. (2006) found a first differentiated thyroid follicle in zebrafish at 55 hours post fertilization. Elsalini et al. (2003) used immunohistochemistry to show the development of the first thyroid follicles producing thyroid hormone at 72 hours post fertilization in zebrafish. During further larval development, the number of follicles increases. Therefore early developmental processes (before thyroid activation) that are dependent on T4, such as posterior swim bladder inflation, might not be affected by chemicals reducing T4 synthesis. Nelson et al. (2016) and Stinckens et al. (2016) indeed found that MBT (a thyroperoxidase inhibitor) decreased T4 levels in both zebrafish (5 days post fertilization) and fathead minnow (6 days post fertilization), which is after activation of the thyroid gland for both species, while it did not affect posterior inflation. Stinckens et al. (unpublished results) also found that exposure to the thyroperoxidase inhibitor benzophenone-2 did not affect posterior swim bladder inflation.

Evidence Supporting this KER

Thyroid hormones are known to be involved in development, especially in metamorphosis in amphibians and in embryonic-to-larval transition (Liu and Chan, 2002) and larval-to-juvenile transition (Brown et al., 1997) in fish. The formation of the anterior chamber coincides with the second transition phase (Winata et al., 2009) and with a peak in T4 synthesis (Chang et al., 2012) suggesting that anterior inflation is under thyroid hormone regulation.

• Chang et al. (2012) observed an increase of whole body T4 concentrations in zebrafish larvae at 21 days post fertilization, corresponding to the timing of anterior swim bladder inflation.
• Nelson et al. (2016) showed reduced whole body T4 levels at 6 days post fertilization together with delayed anterior inflation after exposure of fathead minnow embryos to 2-mercaptobenzothiazole. All anterior chambers eventually inflated but their size was reduced and morphology deviated.
• Stinckens et al. (2016) showed reduced whole body T4 levels both at 5 (before anterior inflation) and 32 days post fertilization (after anterior inflation) when exposed to 2-mercaptobenzothiazole (a thyroperoxidase inhibitor) from 0 to 32 days post fertilization. A large percentage of MBT-exposed fish had an uninflated anterior swim bladder, although some recovery was observed over time.
• Stinckens et al. (2016) further showed a significant correlation between whole body T4 levels and anterior chamber volume, with reduced T4 levels leading to smaller anterior chambers.
• Zebrafish exposed to methimazole, a commonly used reference TPO inhibitor, did not inflate their anterior swim bladder until 32 dpf, and did not show any sign of recovery (Stinckens et al., unpublished data).

The mechanism through which reduced T4 hormone concentrations in serum result in anterior chamber inflation impairment is not yet understood. The anterior chamber is formed by evagination from the cranial end of the posterior chamber (Robertson et al., 2007, Winata et al., 2009). Several hypotheses could explain effects on anterior chamber inflation due to reduced T4 levels:

• Evagination from the posterior chamber could be impaired. Villeneuve et al. (unpublished results) showed that although the anterior bud was present after exposure to a deiodinase 2 inhibitor, the anterior chamber did not inflate.
• The formation of the tissue layers of the anterior swim bladder could be affected, although Villeneuve et al. (unpublished results) observed intact tissue layers of the anterior swim bladder after exposure to a deiodinase 2 inhibitor.
• The anterior chamber inflates is inflated with gas from the posterior chamber through the communicating duct. Impaired gas exchange between the two chambers could be at the basis of impaired anterior inflation. Both Nelson et al. (2016) and Stinckens et al. (2016) found that posterior chambers were larger when anterior chambers were smaller or not inflated at all. The sum of the areas of the posterior and anterior chambers remained constant independent of inflation of the anterior chamber (Stinkens et al., 2016). These results suggest retention of the gas in the posterior chamber.
• Since gas exchange relies on a functional communicating duct between the posterior and anterior chamber, and the communicating duct is known to progressively narrow and eventually close during development, a dysfunctional communicating duct or a closure prior to anterior inflation could inhibit inflation. However, Villeneuve et al. (unpublished results) showed that the communicating duct was anatomically intact and open after exposure to iopanoic acid (a deiodinase 2 inhibitor), still leading to impaired anterior inflation.
• Lactic acid production which is essential for producing gas to fill the swim bladder could be affected, although the observation that the total amount of gas in both chambers is not affected when anterior inflation is impaired seems to contradict this (Stinckens et al., 2016).
• Possibly there is an effect on the production of surfactant, which is crucial to maintain the surface tension necessary for swim bladder inflation.

References

• Alt, B., Reibe, S., Feitosa, N.M., Elsalini, O.A., Wendl, T., Rohr, K.B., 2006. Analysis of origin and growth of the thyroid gland in zebrafish. Dev. Dyn. 235, 1872–1883, http://dx.doi.org/10.1002/dvdy.20831.
• Brown, C.L., Doroshov, S.I., Nunez, J.M., Hadley, C., Vaneenennaam, J., Nishioka, R.S. and Bern, H.A. 1988. Maternal triiodothyronine injections cause increases in swimbladder inflation and survival rates in larval striped bass, Morone saxatilis. J. Exp. Zool. 248: 168–176.
• Brown, C.L., Sullivan, C.V., Bern, H.A. and Dickhoff, W.W. 1987. Occurrence of thyroid hormones in early developmental stages of teleost fish. Trans. Am. Fish. Soc. Symp. 2: 144–150.
• Brown, D.D., 1997. The role of thyroid hormone in zebrafish and axolotl development. Proc. Natl. Acad. Sci. U. S. A. 94, 13011–13016, http://dx.doi.org/ 10.1073/pnas.94.24.13011.
• Chang, J., Wang, M., Gui, W., Zhao, Y., Yu, L., Zhu, G., 2012. Changes in thyroid hormone levels during zebrafish development. Zool. Sci. 29, 181–184, http:// dx.doi.org/10.2108/zsj.29.181.
• Elsalini, O.A., Rohr, K.B., 2003. Phenylthiourea disrupts thyroid function in developing zebrafish. Dev. Genes Evol. 212, 593–598, http://dx.doi.org/10. 1007/s00427-002-0279-3.
• Liu, Y.W., Chan, W.K., 2002. Thyroid hormones are important for embryonic to larval transitory phase in zebrafish. Differentiation 70, 36–45, http://dx.doi. org/10.1046/j.1432-0436.2002.700104.x.
• Nelson KR, Schroeder AL, Ankley GT, Blackwell BR, Blanksma C, Degitz SJ, Flynn KM, Jensen KM, Johnson RD, Kahl MD, Knapen D, Kosian PA, Milsk RY, Randolph EC, Saari T, Stinckens E, Vergauwen L, Villeneuve DL. 2016. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole – Part I: fathead minnow. Aquatic Toxicology 173: 192-203.
• Power DM, Llewellyn L, Faustino M, Nowell MA, Björnsson BT, Einarsdottir IE, Canario AV, Sweeney GE. Thyroid hormones in growth and development of fish. Comp Biochem Physiol C Toxicol Pharmacol. 2001 Dec; 130(4):447-59.
• Reider, M., Connaughton, V.P., 2014. Effects of Low-Dose Embryonic Thyroid Disruption and Rearing Temperature on the Development of the Eye and Retina in Zebrafish. Birth Defects Res. Part B Dev. Reprod. Toxicol. 101, 347–354, http://dx.doi.org/10.1002/bdrb.21118.
• Roberston, G.N., McGee, C.A.S., Dumbarton, T.C., Croll, R.P., Smith, F.M., 2007. Development of the swim bladder and its innervation in the zebrafish, Danio rerio. J. Morphol. 268, 967–985, http://dx.doi.org/10.1002/jmor.
• Stinckens E, Vergauwen L, Schroeder AL, Maho W, Blackwell BR, Witters H, Blust R, Ankley GT, Covaci A, Villeneuve DL, Knapen D. 2016. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole – Part II: zebrafish. Aquatic Toxicology 173:204-217.
• Walpita, C.N., Van der Geyten, S., Rurangwa, E., Darras, V.M., 2007. The effect of 3,5,3′-triiodothyronine supplementation on zebrafish (Danio rerio) embryonic development and expression of iodothyronine deiodinases and thyroid hormone receptors. Gen. Comp. Endocrinol. 152, 206–214, http://dx.doi.org/ 10.1016/j.ygcen.2007.02.020.
• Winata, C.L., Korzh, S., Kondrychyn, I., Zheng, W., Korzh, V., Gong, Z., 2009. Development of zebrafish swimbladder: the requirement of Hedgehog signaling in specification and organization of the three tissue layers. Dev. Biol. 331, 222–236, http://dx.doi.org/10.1016/j.ydbio.2009.04.035.

Relationship: 1030: Reduced, Young of year survival leads to Decrease, Population trajectory

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Key Event Relationship Description

If young of year survival is reduced, ultimately the population trajectory will decrease.

Evidence Supporting this KER

It is widely accepted that if young of year survival is reduced, the population trajectory will eventually decrease.

Relationship: 1034: Reduced, Anterior swim bladder inflation leads to Reduced, Swimming performance

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Importance of swimming performance for natural behaviour is generally applicable to fish.

Key Event Relationship Description

Effects on swim bladder inflation can alter swimming performance and buoyancy of fish, which is essential for predator avoidance, energy sparing, migration, reproduction and feeding behaviour, resulting in lower young-of-year survival.

Evidence Supporting this KER

The weight of evidence supporting a direct linkage between these two KEs, i.e. reduced anterior swim bladder inflation and reduced swimming performance, is weak.

The anterior chamber of the swim bladder has a function in regulating the buoyancy of fish, by altering the volume of the swim bladder (Roberston et al., 2007). Fish rely on the lipid and gas content in their body to regulate their position within the water column, with the latter being more efficient at increasing body buoyancy. Therefore, fish with functional swim bladders have no problem supporting their body (Brix 2002), while it is highly likely that impaired inflation severely impacts swimming performance. Fish with no functional swim bladder can survive, but are severely disadvantaged., making the likelihood of surviving smaller.

Buoyancy is one of the primary mechanisms of fish to regulate behaviour, swimming performance and energy expenditure. Robertson et al., (2007) suggested that the swim bladder only starts regulating buoyancy actively from 32 dpf onward in zebrafish, indicating that impaired swim bladder inflation possibly affects swimming activity during late development. The swim bladder only becomes capable of regulating buoyancy when it has fully developed into a double-chambered. However, during an MBT exposure, a TPO inhibitor, for 32 days post fertilization (dpf) in zebrafish, the swimming activity of fish was impacted starting at 26 dpf if the inflation of the anterior chamber of the swim bladder was impaired or had no normal structure/size (Stinckens et al., 2016). This effect was also observed after a 32 dpf exposure to MMI, however only for the highest tested concentration (Stinckens et al., unpublished data).

It has also been reported that larvae that fail to inflate their swim bladder use additional energy to maintain buoyancy (Lindsey et al., 2010, Goodsell et al. 1996), possibly contributing to reduced swimming activity. Furthermore, Chatain (1994) associated larvae with non-inflated swim bladders with numerous complications, such as spinal deformities and lordosis and reduced growth rates, adding to the impact on swimming behaviour.

An increasing incidence of swim bladder non-inflation has also been reported in Atlantic salmon (Poppe et al. 1997). Affected fish had severely altered balance and buoyancy, observed through a specific swimming behaviour, as the affected fish were swimming upside down in an almost vertical position (Poppe et al. 1997).

During an MMI exposure, a TPO inhibitor, for 32 dpf in zebrafish, the swimming activity of fish was impacted starting at 26 dpf if the inflation of the anterior chamber of the swim bladder was impaired (Stinckens et al., unpublished). However, this effect was only observed for the highest tested concentration. For the lowest tested concentration, during which the anterior swim bladder was severly impacted as well, no effect on swimming capacity could be observed. As Robertson et al., (2007) reported, the swim bladder only starts regulating buoyancy actively from 32 dpf onward in zebrafish, possibly explaining the lack of effect on swimming capacity for lower MMI concentrations.

The function of the posterior chamber has been clearly linked to buoyancy control and survival (Czesny et al., 2005; Woolley and Qin, 2010; Kurata et al., 2014). The link between anterior chamber inflation and impaired swimming capacity however is less clear. The most important function of the anterior chamber is producing and transducing sound through the Weberian Apparatus (Popper, 1974; Lechner and Ladich, 2008), with only a slight contribution in bouncy control. It is highly plausible that impaired inflation or size of the anterior swim bladder could lead to a reduction in young-of-year survival as hearing loss would affect their ability to respond to their surrounding environment, thus impacting ecological relevant endpoints such as predator avoidance or prey seeking (Wisenden et al., 2008; Fay2009).

References

Roberston, G.N., McGee, C.A.S., Dumbarton, T.C., Croll, R.P., Smith, F.M., 2007.Development of the swim bladder and its innervation in the zebrafish, Danio rerio. J. Morphol. 268, 967–985, http://dx.doi.org/10.1002/jmor.

Brix O (2002) The physiology of living in water. In: Hart PJ, Reynolds J (eds) Handbook of Fish Biology and Fisheries, Vol. 1, pp. 70–96. Blackwell Publishing, Malden, USA.

Stinckens, E., Vergauwen, L., Schroeder, A.L., Maho, W., Blackwell, B., Witter, H.,Blust, R., Ankley, G.T., Covaci, A., Villenueve, D.L., Knapen, D., 2016. Disruption of thyroid hormone balance after 2-mercaptobenzothiazole exposure causes swim bladder inflation impairment—part II: zebrafish. Aquat. Toxicol. 173:204-17.

Lindsey, B.W., Smith, F.M., Croll, R.P., 2010. From inflation to flotation: contributionof the swimbladder to whole-body density and swimming depth duringdevelopment of the zebrafish (Danio rerio). Zebrafish 7, 85–96, http://dx.doi.org/10.1089/zeb.2009.0616.

Goodsell, D.S., Morris, G.M., Olsen, A.J. 1996. Automated docking of fleixble ligands. Applications of Autodock. J. Mol. Recogonition, 9:1-5.

Chatain, B., 1994. Abnormal swimbladder development and lordosis in sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus). Aquaculture 119:371–379.

Poppe, T.T., Hellberg, H., Griffiths, D., Mendal, H. 1977. Swim bladder abnormality in farmed Atlantic salmon, Salmo salar. Diseases of aquatic organisms 30:73-76.

Czesny, S.J., Graeb, B.D.S., Dettmersn, J.M., 2005. Ecological consequences of swimbladder noninflation for larval yellow perch. Trans. Am. Fish. Soc. 134,1011–1020, http://dx.doi.org/10.1577/T04-016.1.

Woolley, L.D., Qin, J.G., 2010. Swimbladder inflation and its implication to theculture of marine finfish larvae. Rev. Aquac. 2, 181–190, http://dx.doi.org/10.1111/j.1753-5131.2010.01035.x.

Kurata, M., Ishibashi, Y., Takii, K., Kumai, H., Miyashita, S., Sawada, Y., 2014.Influence of initial swimbladder inflation failure on survival of Pacific bluefintuna, Thuunus orientalis (Temminck and Schlegl) larvae. Aquacult. Res. 45,882–892.

Lechner, W., Ladich, F., 2008. Size matters: diversity in swimbladders andWeberian ossicles affects hearing in catfishes. J. Exp. Biol. 211, 1681–1689.

Wisenden, B.D., Pogatschnik, J., Gibson, D., Bonacci, L., Schumacher, A., Willet, A.,2008. Sound the alarm: learned association of predation risk with novelauditory stimuli by fathead minnows (Pimephales promelas) and glowlighttetras (Hemigrammus erythrozonus) after single simultaneous pairings withconspecific chemical alarm cues. Environ. Biol. Fish 81, 141–147.

Fay, R., 2009. Soundscapes and the sense of hearing of fishes. Integrative Zool. 4,26–32.

Relationship: 1029: Reduced, Swimming performance leads to Reduced, Young of year survival

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Importance of swimming performance on young of year survival is generally applicable to fish.

Evidence Supporting this KER

Reduced swimming performance is likely to affect essential endpoints such as predator avoidance, feeding behaviour and reproduction. These parameters are biologicaly plausible to affect young-of-year survival, especially in a non-laboratory environment where food is scarce and predators are abundant.

Relationship: 309: Thyroperoxidase, Inhibition leads to TH synthesis, Decreased

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Inhibition of TPO activity is widely accepted to directly impact TH synthesis. This is true for both rats and humans, as well as some fishes, frogs and birds. Most of the data supporting a causative relationship between TPO inhibition and altered TH synthesis is derived from animal studies, in vitro thyroid microsomes from rats or pigs, and a limited number of human ex vivo (Nagasaka and Hidaka, 1976; Vickers et al., 2012) and clinical studies. There are data to support that gene mutations in TPO result in congenital hypothyroidism, underscoring the essential role of TPO in human thyroid hormone synthesis.

Key Event Relationship Description

Thyroperoxidase (TPO) is a heme-containing apical membrane protein within the follicular lumen of thyrocytes that acts as the enzymatic catalyst for thyroid hormone (TH) synthesis (Taurog, 2005). Two commonly used reference chemicals, propylthiouracil (PTU) and methimazole (MMI), are drugs that inhibit the ability of TPO to: a) activate iodine and transfer it to thyroglobulin (Tg) (Davidson et al., 1978); and, b) couple thyroglobulin (Tg)-bound iodotyrosyls to produce Tg-bound thyroxine (T4) and triiodothyronine (T3) (Taurog, 2005).

Evidence Supporting this KER

The weight of evidence supporting a direct linkage between the MIE, TPO inhibition, and the KE of decreased TH synthesis, is strong and supported by more than three decades of research in animals, including humans (Cooper et al., 1982; Cooper et al.,1983; Divi and Doerge, 1994).

The biological plausibility for this KER is rated Strong. TPO is the only enzyme capable of de novo systhesis of TH. TPO catalyzes several reactions, including the oxidation of iodide, nonspecific iodination of tyrosyl residues of thyroglobulin (Tg) to form monoiodotyrosyl (MIT) or diiodotyrosyl (DIT) residues, and the coupling of these Tg-bound iodotyrosyls to produce Tg-bound T3 and T4 (Divi and Doerge, 1994; Kessler et al., 2008; Ruf et al., 2006; Taurog et al., 1996, 2005). Therefore, inhibition of TPO activity is widely accepted to directly impact TH synthesis.

Empirical support for this KER is strong. There are several papers that have measured alterations in TPO and subsequent effects on TH synthesisTaurog et al. (1996) showed decreased guicaol activity, decreased bound I¹²⁵, and subsequent decreases in newly formed T3 and T4 per molecule of Tg, following exposure to PTU, MMI and some antibiotics.  Following in vivo exposure to PTU in rats (Cooper et al., 1982; 1983), there are concentration and time-dependent decreases in thyroid protein bound iodine and serum T4 and T3 that recovered one month after cessation of PTU exposure.  In addition, measures of thyroidal iodine content were highly correlated with intra-thyroidal PTU concentration. Vickers et al. (2012) demonstrated dose- and time- dependent inhibition of TPO activity in both human and rat thyroid homogenates exposed to MMI.  Tietge et al (2010) recently showed decreases in thyroidal T4 following MMI exposure in Xenopus.  Doerge et al (1998) showed that a tryphenylmethane dye, malachite green, inhibited TPO and lowered thyroxine production. A recent paper used a series of benzothiazoles and showed TPO inhibition (guicaol assay) and inhibition of TSH stimulated thyroxine release from Xenopus thyroid gland explant cultures (Hornung et al., 2015).

Temporal Evidence: The temporal nature of this KER is applicable to all life stages, including development (Seed et al., 2005). The impact of decreased TPO activity on thyroidal hormone synthesis is similar across all ages. Good evidence for the temporal relationship of the KER comes from thyroid system modeling (e.g., Degon et al., 2008; Fisher et al., 2013) using data using data from studies of iodine deficiency and chemicals that inhibit NIS. In addition, there is ample evidence of the temporal impacts of TPO inhibition on TH synthesis, using ex vivo and in vitro measures that demonstrate the time course of inhibition following chemical exposures, including some data from human thyroid microsomes and ex vivo thyroid slices (Vickers et al., 2012). Future work is needed that measures both TPO inhibition and TH production during development. 

Dose-Response Evidence: Dose-response data is available from a number of studies that correlate TPO inhibition with decreased TH production measured using a variety of endpoints including iodine organification (e.g., Taurog et al., 1996), inhibition of guicaol oxidation in thyroid microsomes (e.g., Doerge and Chang, 2002), and direct measure of thyroid gland T4 concentrations (e.g., Hornung et al., 2015). However, there is a lack of dose-response data from developmental studies showing direct linkages from TPO inhibition to thyroidal TH synthesis.

While it is clear that TPO inhibition will lead to altered hormone synthesis, there is a need for data that will inform quantitative modeling of the relationship between TPO inhibition and the magnitude of effects on thyroid hormone synthesis.

It is important to note that data from studies on genistein highlight this uncertainty. Doerge and colleagues have demonstrated that for this compound up to 80% TPO inhibition did not result in decreased serum T4 in rats (Doerge and Chang, 2002). This is not consistent with other prototypical TPO inhibitors (e.g., PTU, MMI). It remains to be determined, if for some presently unknown reason, that genistein is an outlier or not. This again points to the need for quantitative modeling of the relationship between TPO inhibtion and downstream KEs.

Quantitative Understanding of the Linkage

There are only a limited number of studies where both TPO inhibition and iodine organification have been measured in vivo, and there are not enough data available to make any definitive quantitative correlations. One in vivo study in rats exposed to the TPO inhibitor genistein found no in vivo impact on serum thyroid hormone concentrations, even when TPO was inhibited up to 80% (Chang and Doerge, 2000).

References

Chang HC, Doerge DR. Dietary genistein inactivates rat thyroid peroxidase in vivo without an apparent hypothyroid effect. Toxicol Appl Pharmacol 168:244–252 (2000).

Cooper DS, Kieffer JD, Halpern R, Saxe V, Mover H, Maloof F, Ridgway EC (1983) Propylthiouracil (PTU) pharmacology in the rat. II. Effects of PTU on thyroid function. Endocrinology 113:921-928.

Cooper DS, Saxe VC, Meskell M, Maloof F, Ridgway EC. Acute effects of propylthiouracil (PTU) on thyroidal iodide organification and peripheral iodothyronine deiodination: correlation with serum PTU levels measured by radioimmunoassay. J Clin Endocrinol Metab. 1982 54(1):101-7.

Davidson, B., Soodak, M., Neary, J.T., Strout, H.V., and Kieffer, J.D. (1978). The irreversible inactivation of thyroid peroxidase by methylmercaptoimidazole, thiouracil, and propylthiouracil in vitro and its relationship to in vivo findings. Endocrinology 103:871–882.

Divi, R. L., and Doerge, D. R. (1994). Mechanism-based inactivation of lactoperoxidase and thyroid peroxidase by resorcinol derivatives. Biochemistry 33(32), 9668-74.

Doerge DR, Chang HC, Divi RL, Churchwell Mechanism for inhibition of thyroid peroxidase by leucomalachite green. Chem Res Toxicol. 1998 11(9):1098-104.

Doerge DR, Chang HC.  Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo.  J Chromatogr B Analyt Technol Biomed Life Sci. 2002 Sep 25;777(1-2):269-79

Hornung MW, Kosian PA, Haselman JT, Korte JJ, Challis K, Macherla C, Nevalainen E, Degitz SJ.  In Vitro, Ex Vivo, and In Vivo Determination of Thyroid Hormone Modulating Activity of Benzothiazoles.Toxicol Sci. 2015 146(2):254-64.

Kessler, J., Obinger, C., and Eales, G. (2008). Factors influencing the study of peroxidase-generated iodine species and implications for thyroglobulin synthesis. Thyroid 18(7), 769-74, 10.1089/thy.2007.0310.

Nagasaka, A., and Hidaka, H. (1976). Effect of antithyroid agents 6-propyl-2-thiouracil and 1-mehtyl-2-mercaptoimidazole on human thyroid iodine peroxidase. J. Clin. Endocrinol. Metab. 43:152–158.

Ruf, J., and Carayon, P. (2006). Structural and functional aspects of thyroid peroxidase. Archives of biochemistry and biophysics 445(2), 269-77, 10.1016/j.abb.2005.06.023.

Taurog, A., Dorris, M. L., and Doerge, D. R. (1996). Mechanism of simultaneous iodination and coupling catalyzed by thyroid peroxidase. Archives of biochemistry and biophysics 330(1), 24-32,

Taurog A. 2005. Hormone synthesis. In: Werner and Ingbar’s The Thyroid: A Fundamental and Clinical Text (Braverman LE, Utiger RD, eds). Philadelphia:Lippincott, Williams and Wilkins, 47–81.

Tietge JE, Butterworth BC, Haselman JT, Holcombe GW, Hornung MW, Korte JJ, Kosian PA, Wolfe M, Degitz SJ.   Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis.  Aquat Toxicol. 2010 98(1):44-50

Vickers AE, Heale J, Sinclair JR, Morris S, Rowe JM, Fisher RL. Thyroid organotypic rat and human cultures used to investigate drug effects on thyroid function, hormone synthesis and release pathways. Toxicol Appl Pharmacol. 2012 260(1):81-8.

Relationship: 305: TH synthesis, Decreased leads to T4 in serum, Decreased

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

While a majority of the empirical evidence comes from work with laboratory rodents, there is a large amount of supporting data from humans (with anti-hyperthyroidism drugs including propylthiouracil and methimazole), some amphibian species (e.g., frog), and some avian species (e.g, chicken).  The following ares samples from a large literature that supports this concept: Cooper et al. (1982; 1983); Hornung et al. (2010); Van Herck et al. (2013); Paul et al. (2013); Alexander et al. (2017).

Key Event Relationship Description

Thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) are synthesized by NIS and TPO in the thyroid gland as iodinated thyroglobulin (Tg) and stored in the colloid of thyroid follicles. Secretion from the follicle into serum is a multi-step process. The first involves thyroid stimulating hormone (TSH) stimulation of the separation of the peptide linkage between Tg and TH. The next steps involve endocytosis of colloid, fusion of the endosome with the basolateral membrane of the thyrocyte, and finally release of TH into blood. More detailed descriptions of this process can be found in reviews by Braverman and Utiger (2012) and Zoeller et al. (2007).

Evidence Supporting this KER

The weight of evidence linking these two KEs of decreased TH synthesis and decreased T4 in serum is strong. It is commonly accepted dogma that decreased synthesis in the thyroid gland will result in decreased circulating TH (serum T4).

The biological relationship between two KEs in this KER is well understood and documented fact within the scientific community.

It is widely accepted that TPO inhibition leads to declines in serum T4 levels in adult mammals. This is due to the fact that the sole source for circulating T4 derives from hormone synthesis in the thyroid gland. Indeed, it has been known for decades that insufficient dietary iodine will lead to decreased serum TH concentrations due to inadequate synthesis. Strong qualitative and quantitative relationships exist between reduced TH synthesis and reduced serum T4 (Ekerot et al., 2013; Degon et al., 2008; Cooper et al., 1982; 1983; Leonard et al., 2016; Zoeller and Tan, 2007).  There is more limited evidence supporting the relationship between decreased TH synthesis and lowered circulating hormone levels during development.  Lu and Anderson (1994) followed the time course of TH synthesis, measured as thyroxine secretion rate, in non-treated pregnant rats and correlated it with serum T4 levels More recently, modeling of TH in the rat fetus demonstrates the quantitative relationship between TH synthesis and serum T4 concentrations (Hassan et al., 2017). Furthermore, a wide variety of drugs and chemicals that inhibit TPO are known to result in decreased release of TH from the thyroid gland, as well as decreased circulating TH concentrations. This is evidenced by a very large number of studies that employed a wide variety of techniques, including thyroid gland explant cultures, tracing organification of 131-I and in vivo treatment of a variety of animal species with known TPO inhibitors (King and May,1984; Atterwill et al., 1990; Brown et al., 1986; Brucker-Davis, 1998; Hornung et al., 2010; Hurley et al., 1998; Kohrle, 2008).

Temporal Evidence: The temporal nature of this KER is applicable to all life stages, including development (Seed et al., 2005).  There are currently no studies that measured both TPO synthesis and TH production during development. However, the impact decreased TH synthesis on serum hormones is similar across all ages. Good evidence for the temporal relationship comes from thyroid system modeling of the impacts of iodine deficiency and NIS inhibition (e.g., Degon et al., 2008; Fisher et al., 2013). In addition, recovery experiments have demonstrated that serum thyroid hormones recovered in athyroid mice following grafting of in-vitro derived follicles (Antonica et al., 2012).   

Dose-response Evidence: Dose-response data is lacking from studies that include concurrent measures of both TH synthesis and serum TH concentrations. However, data is available demonstrating correlations between thyroidal TH and serum TH concentrations during gestation and lactation during development (Gilbert et al., 2013).  This data was used to develop a rat quantitative biologically-based dose-response model for iodine deficiency (Fisher et al., 2013).

There are no inconsistencies in this KER, but there are some uncertainties. The first uncertainty stems from the paucity of data for quantitative modeling of the relationship between the degree of synthesis decrease and resulting changes in circulating T4 concentrations. In addition, most of the data supporting this KER comes from inhibition of TPO, and there are a number of other processes (e.g., endocytosis, lysosomal fusion, basolateral fusion and release) that are not as well studied.

Quantitative Understanding of the Linkage

  Fisher et al. (2013) published a quantitative biologically-based dose-response model for iodine deficiency in the rat. This model provides quantitative relationships for thyroidal T4 synthesis (iodine organification) and predictions of serum T4 concentrations in developing rats. There are other computational models that include thyroid hormone synthesis. Ekerot et al. (2012) modeled TPO, T3, T4 and TSH in dogs and humans based on exposure to myeloperoxidase inhibitors that also inhibit TPO.  This model was recently adapted for rat (Leonard et al., 2016) and Hassan et al (2017) have extended it to include the pregnant rat dam in response to TPO inhibition induced by PTU. While the original model predicted serum TH and TSH levels as a function of oral dose, it was not used to explicitly predict the relationship between serum hormones and TPO inhibition, or thyroidal hormone synthesis. Leonard et al. (2016) recently incorporated TPO inhibition into the model. Degon et al (2008) developed a human thyroid model that includes TPO, but does not make quantitative prediction of organification changes due to inhibition of the TPO enzyme.

References

Alexander EK, Pearce EN, Brent GA, Brown RS, Chen H, Dosiou C, Grobman WA, Laurberg P, Lazarus JH, Mandel SJ, Peeters RP, Sullivan S.  2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017 Mar;27(3):315-389.

Antonica F, Kasprzyk DF, Opitz R, Iacovino M, Liao XH, Dumitrescu AM, Refetoff S, Peremans K, Manto M, Kyba M, Costagliola S.  Generation of functional thyroid from embryonic stem cells.  Nature. 2012 491(7422):66-71.

Atterwill CK, Fowler KF. A comparison of cultured rat FRTL-5 and porcine thyroid cells for predicting the thyroid toxicity of xenobiotics. Toxicol In Vitro. 1990. 4(4-5):369-74.

Braverman, L.E. and Utiger, R.D. (2012). Werner and Ingbar's The Thyroid: A Fundamental and Clinical Text (10 ed.). Philadelphia, PA: Lippincott Williams & Wilkins. pp. 775-786. ISBN 978-1451120639.

Brown CG, Fowler KL, Nicholls PJ, Atterwill C. Assessment of thyrotoxicity using in vitro cell culture systems. Food Chem Toxicol. 1986 24(6-7):557-62.

Brucker-Davis F. Effects of environmental synthetic chemicals on thyroid function. Thyroid. 1998 8(9):827-56.

Cooper DS, Kieffer JD, Halpern R, Saxe V, Mover H, Maloof F, Ridgway EC (1983) Propylthiouracil (PTU) pharmacology in the rat. II. Effects of PTU on thyroid function. Endocrinology 113:921-928.

Cooper DS, Saxe VC, Meskell M, Maloof F, Ridgway EC.Acute effects of propylthiouracil (PTU) on thyroidal iodide organification and peripheral iodothyronine deiodination: correlation with serum PTU levels measured by radioimmunoassay. J Clin Endocrinol Metab. 1982 54(1):101-7.

Degon, M., Chipkin, S.R., Hollot, C.V., Zoeller, R.T., and Chait, Y. (2008). A computational model of the human thyroid. Mathematical Biosciences 212, 22–53

Ekerot P, Ferguson D, Glämsta EL, Nilsson LB, Andersson H, Rosqvist S, Visser SA. Systems pharmacology modeling of drug-induced modulation of thyroid hormones in dogs and translation to human. Pharm Res. 2013 30(6):1513-24.

Fisher JW, Li S, Crofton K, Zoeller RT, McLanahan ED, Lumen A, Gilbert ME.  Evaluation of iodide deficiency in the lactating rat and pup using a biologically based dose-response model. Toxicol Sci. 2013 132(1):75-86.

Gilbert ME, Hedge JM, Valentín-Blasini L, Blount BC, Kannan K, Tietge J, Zoeller RT, Crofton KM, Jarrett JM, Fisher JW.  An animal model of marginal iodine deficiency during development: the thyroid axis and neurodevelopmental outcome.  Toxicol Sci. 2013 132(1):177-95.

Hassan, I, El-Masri, H., Kosian, PA, Ford, J, Degitz, SJ and Gilbert, ME. Quantitative Adverse Outcome Pathway for Neurodevelopmental Effects of Thyroid Peroxidase-Induced Thyroid Hormone Synthesis Inhibition. Toxicol Sci. 2017 Nov 1;160(1):57-73

Hornung MW, Degitz SJ, Korte LM, Olson JM, Kosian PA, Linnum AL, Tietge JE. Inhibition of thyroid hormone release from cultured amphibian thyroid glands by methimazole, 6-propylthiouracil, and perchlorate. Toxicol Sci. 2010 118(1):42-51.

Hurley PM. Mode of carcinogenic action of pesticides inducing thyroid follicular cell tumors in rodents. Environ Health Perspect. 1998 106(8):437-45.

King DB, May JD. Thyroidal influence on body growth. J Exp Zool. 1984 Dec;232(3):453-60.

Köhrle J. Environment and endocrinology: the case of thyroidology. Ann Endocrinol (Paris). 2008 69(2):116-22.

Leonard JA, Tan YM, Gilbert M, Isaacs K, El-Masri H. Estimating margin of exposure to thyroid peroxidase inhibitors using high-throughput in vitro data, high-throughput exposure modeling, and physiologically based pharmacokinetic/pharmacodynamic modeling. Toxicol Sci. 2016 151(1):57-70.

Lu, M-H, and Anderson, RR. Thyroxine secretion rats during pregnancy in the rat.  Endo Res. 1994. 20(4):343-364.

Paul KB, Hedge JM, Macherla C, Filer DL, Burgess E, Simmons SO, Crofton KM, Hornung MW. Cross-species analysis of thyroperoxidase inhibition by xenobiotics demonstrates conservation of response between pig and rat. Toxicology. 2013. 312:97-107.

Van Herck SL, Geysens S, Delbaere J, Darras VM.  Regulators of thyroid hormone availability and action in embryonic chicken brain development. Gen Comp Endocrinol. 2013. 190:96-104.

Zoeller, R. T., Tan, S. W., and Tyl, R. W. (2007). General background on the hypothalamic-pituitary-thyroid (HPT) axis. Critical reviews in toxicology 37(1-2), 11-53.

Relationship: 366: Thyroperoxidase, Inhibition leads to T4 in serum, Decreased

AOPs Referencing Relationship

Evidence Supporting Applicability of this Relationship

Use of TPO inhibitors as anti-hyperthyroidism drugs, in humans and pets (Emiliano et al., 2010; Trepanier, 2006), in amphibian and avian species (Coady et al., 2010; Grommen et al., 2011; Rosebrough et al., 2006; Tietge et al., 2012), demonstrate decreased serum TH concentrations in vivo in rats (US EPA, 2005) and strongly supports a causative linkage between inhibition of TPO and decreased serum T4 across species.

Key Event Relationship Description

Thyroperoxidase (TPO) is the enzyme that catalyzes iodine organification of thyroglobulin to produce thyroglobulin (Tg)-bound T3 and T4 in the lumen of thyroid follicles. Tg-bound THs are endocytosed across the apical lumen-follicular cell membrane, undergo thyroglobulin proteolysis, followed by hormone section into the blood stream (see Taurog, 2005 for review). This indirect KER describes the relationship of TPO inhibition to reduced circulating levels of thyroid hormone (TH) in the serum.

Evidence Supporting this KER

The weight of evidence linking thyroperoxidase inhibition to reductions in circulating serum TH is strong. Many studies support this basic linkage. There is no inconsistent data.

It is a well-accepted fact that inhibition of the only enzyme capable of synthesizing THs, TPO, results in subsequent decrease in serum TH concentrations.  A large amount of evidence from clinical and animal studies clearly support the commonly accepted dogma that inhibition of TPO leads to decreased serum THs. 

The majority of research in support of this KER involve exposure to known TPO inhibitors and measurement of serum hormones.  There are a many in vivo studies that link decreases in serum TH concentrations with exposure to xenobiotics that inhibit thyroperoxidase (TPO) (Brucker-Davis, 1998; Hurley, 1998; Boas et al., 2006; Crofton, 2008; Kohrle, 2008; Pearce and Braverman, 2009; Murk et al., 2013). 

While these studies support the connection between exposure to a known TPO inhibitor and decreased TH, many of these studies do not empirically measure TPO inhibition or decreased TH synthesis. Thus, many studies support the indirect linkage between TPO inhibition (for chemicals identified as TPO inhibitors in in vivo or ex vivo studies) and decreased TH, with the well accepted theory that these proceed via decreased TH synthesis. That exposure to TPO inhibitors leads to decreased serum TH concentrations, via decreased TH synthesis is strongly supported by decades of mechanistic research in a variety of species.

This indirect relationship is also evidenced by the use of clinically-relevant anti-hyperthyroidism drugs, MMI and PTU (Laurberg & Anderson, 2014; Sundaresh et al., 2013). These drugs are both recognized TPO inhibitors and are part of a standard drug-based regimen of care for clinically hyperthyroid patients including those with Grave's disease. Serum THs are measured as the bioindicator of successful treatment with anti-hyperthyroidism drugs; the actual decrease in TH synthesis in the thyroid gland is implied in the efficacious use of these drugs (Trepanier, 2006).

In rats, MMI and PTU are often used as control chemicals to decrease serum THs to study biological phenomena related to disruption of TH homeostasis (many examples, including Zoeller and Crofton, 2005; Morreale de Escobar et al, 2004; Schwartz et al., 1997; Herwig et al., 2014; Wu et al., 2013; Pathak et al., 2011). Further, MMI is recommended as a positive control for use in the Amphibian Metamorphosis (Frog) Assay within Tier 1 of the U.S. EPA Endocrine Disruptor Screening Program (US EPA, 2009; Coady et al., 2010), an assay used to evaluate the potential for chemicals to disrupt TH homeostasis. PTU has been suggested a positive control chemical in the guidance for the Comparative Developmental Thyroid Assay (US EPA, 2005), a non-guideline assay used to evaluate the potential for chemicals to disrupt TH homeostasis during gestation and early neonatal development.

Thus, an indirect key event relationship between TPO inhibition and decreased serum THs is strongly supported by a large database of clinical medicine and investigative research with whole animals (with a great deal of supporting evidence in rats and frogs).

Temporal Evidence: The temporal nature of this KER is applicable to all life stages, including development (Seed et al., 2005).  The qualitative impact of thyroperoxidase inhibition on serum hormones is similar across all ages. The temporal nature of the impact on serum THs by TPO inhibitors in developmental exposure studies is evidenced by the duration of exposure and developmental age (Goldey et al., 1995; Ahmed et al., 2010; Tietge et al., 2010), as well as recovery after cessation of exposure (Cooke et al., 1993; Goldey et al., 1995; Sawin et al., 1998; Axelstad et al., 2008; Shibutani et al., 2009; Lasley and Gilbert, 2011).  The temporal relationship between TPO inhibitor exposure duration and serum hormone decreases in adult organisms has been widely demonstrated (e.g., Hood et al., 1999; Mannisto et al., 1979). In addition, MMI and PTU induced decreases in serum T4 are alleviated by thyroid hormone replacement in both fetal and postnatal age rats (Calvo et al., 1990; Sack et al., 1995; Goldey and Crofton, 1998). Computational modeling of the thyroid also provides evidence for the indirect temporal relationship between these two KEs (e.g., Degon et al., 2008; Fisher et a., 2013).

Dose-Response Evidence: Empirical data is available from enough studies in animals treated with TPO inhibitors during development to make it readily accepted dogma that a dose-response relationship exists between TPO inhibition and serum TH concentrations.  Again, these studies do not empirically measure TPO inhibition or decreased TH synthesis, but rely on the strong support of decades of mechanistic research in a variety of species of the causative relationship between these KEs.  Examples of dose-responsive changes in TH concentrations following developmental exposure to TPO inhibitors include studies a variety of species, including: rodents (Blake and Henning, 1985; Goldey et al., 1995; Sawin et al., 1998); frogs (Tietge et al., 2013); fish tissue levels (Elsalini and Rohr, 2003.); and, chickens (Wishe et al., 1979). Computational modeling of the thyroid also provides evidence for the indirect dose-response relationship between these two KEs (e.g., Leonard et al., 2016; Fisher et a., 2013).

There are no inconsistencies in this KER, but there are some uncertainties. The predominant uncertainty regarding the indirect key event relationship between inhibition of TPO activity and decreased serum T4 is the quantitative nature of this relationship, i.e., to what degree must TPO be inhibited in order to decrease serum T4 by a certain magnitude. Many animal (rat) studies typically employ relatively high exposures of TPO-inhibiting chemicals that result in hypothyroidism (severe decrements in T4 and T3). Thus, a dose-response relationship between TPO inhibition and decreased serum T4 is not typically defined. However, there are numerous publications demonstrating clear dose- and duration- dependent relationships between TPO inhibitors dose and reduced serum T3 and T4 in rodent models (see for example: Cooper et al., 1983; Hood et al., 1999; Goldey et al., 2005; Gilbert, 2011). The relationship between maternal and fetal levels of hormone following chemically-induced TPO inhibiton has not been well characterized and may differ based on kinetics. Reductions in serum TH in the fetus, in rat and human is derived a chemical’s effect on the maternal thyroid gland as well as the fetal thyroid gland.

Quantitative Understanding of the Linkage

The indirect linkage between exposure to known TPO inhibitors and decreased serum TH has not been defined quantitatively. The two key event relationships that mediate this relationship (TPO inhibition leading to decreased TH synthesis, and decreased TH synthesis leading to decreased serum TH) have been incorporated into some quantitative models. A quantitative biologically-based dose-response model for iodine deficiency in the rat includes relationships between thyroidal T4 synthesis and serum T4 concentrations in developing rats Fisher et al. (2013). Ekerot et al. (2012) modeled TPO, T3, T4 and TSH in dogs and humans based on exposure to myeloperoxidase inhibitors that also inhibit TPO and was has recently adapted for rat (Leonard et al., 2016). While the original model predicted serum TH and TSH levels as a function of oral dose, it was not used to explicitly predict the relationship between serum hormones and TPO inhibition, or thyroidal hormone synthesis. Leonard et al. (2016) recently incorporated TPO inhibition into the model. Degon et al (2008) developed a human thyroid model that includes TPO but does not make quantitative prediction of organification changes due to inhibition of the TPO enzyme.

References

Ahmed OM, Abd El-Tawab SM, Ahmed RG.  Effects of experimentally induced maternal hypothyroidism and hyperthyroidism on the development of rat offspring: I. The development of the thyroid hormones-neurotransmitters and adenosinergic system interactions.  Int J Dev Neurosci. 2010 28(6):437-54

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

Blake HH, Henning SJ. Effect of propylthiouracil dose on serum thyroxine, growth, and weaning in young rats. Am J Physiol. 1985 248(5 Pt 2):R524-30.

Boas M, Feldt-Rasmussen U, Skakkebaek NE, Main KM.  Environmental chemicals and thyroid function. Eur J Endocrinol. 2006 154:599-611.

Brucker-Davis F. Effects of environmental synthetic chemicals on thyroid function. Thyroid. 1998 8:827-56.

Calvo R, Obregón MJ, Ruiz de Oña C, Escobar del Rey F, Morreale de Escobar G.  Congenital hypothyroidism, as studied in rats. Crucial role of maternal thyroxine but not of 3,5,3'-triiodothyronine in the protection of the fetal brain.  J Clin Invest. 1990 Sep;86(3):889-99.

Coady K, Marino T, Thomas J, Currie R, Hancock G, Crofoot J, McNalley L, McFadden L, Geter D, Klecka G. 2010. Evaluation of the amphibian metamorphosis assay: exposure to the goitrogen methimazole and the endogenous thyroid hormone L-thyroxine. Environmental toxicology and chemistry / SETAC. Apr;29:869-880.

Cooke PS, Kirby JD, Porcelli J.  Increased testis growth and sperm production in adult rats following transient neonatal goitrogen treatment: optimization of the propylthiouracil dose and effects of methimazole.  J Reprod Fertil. 1993 97(2):493-9

Cooper DS, Kieffer JD, Halpern R, Saxe V, Mover H, Maloof F, Ridgway EC (1983) Propylthiouracil (PTU) pharmacology in the rat. II. Effects of PTU on thyroid function. Endocrinology 113:921-928.

Crofton KM Thyroid disrupting chemicals: mechanisms and mixtures.  Int J Androl. 2008 31:209-23

Degon, M., Chipkin, S.R., Hollot, C.V., Zoeller, R.T., and Chait, Y. (2008). A computational model of the human thyroid. Mathematical Biosciences 212: 22–53.

Ekerot P, Ferguson D, Glämsta EL, Nilsson LB, Andersson H, Rosqvist S, Visser SA. Systems pharmacology modeling of drug-induced modulation of thyroid hormones in dogs and translation to human. Pharm Res. 2013 Jun;30(6):1513-24.

Fisher JW, Li S, Crofton K, Zoeller RT, McLanahan ED, Lumen A, Gilbert ME.  Evaluation of iodide deficiency in the lactating rat and pup using a biologically based dose-response model. Toxicol Sci. 2013 132(1):75-86.

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Emiliano, A.B., Governale, L., Parks, M., Cooper, D.S., 2010. Shifts in propylthiouracil and methimazole prescribing practices: antithyroid drug use in the United States from 1991 to 2008. J. Clin. Endocrinol. Metab. 95, 2227–2233.

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Hood A, Liu YP, Gattone VH, 2nd, Klaassen CD (1999) Sensitivity of thyroid gland growth to thyroid stimulating hormone (TSH) in rats treated with antithyroid drugs. Toxicol Sci 49:263-271.

Hurley PM.  Mode of carcinogenic action of pesticides inducing thyroid follicular cell tumors in rodents. Environ Health Perspect. 1998 106:437-45.

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