AOP-Wiki

AOP ID and Title:

AOP 159: Thyroperoxidase inhibition leading to increased mortality via reduced anterior swim bladder inflation
Short Title: TPOi anterior swim bladder

Graphical Representation

Authors

Dries Knapen [1], [dries.knapen (at)uantwerpen.be]

Lucia Vergauwen [1], [lucia.vergauwen(at)uantwerpen.be]

Evelyn Stinckens [1], [evelyn.stinckens(at)uantwerpen.be]

Dan Villeneuve [2], [villeneuve.dan*(at)epa.gov]

[1] Zebrafishlab, Veterinary Physiology and Biochemistry, Department of Veterinary Sciences, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium

[2] United States Environmental Protection Agency, Mid-Continent Ecology Division, 6201 Congdon Blvd, Duluth, MN, USA.

Status

Author status OECD status OECD project SAAOP status
Open for adoption Under Development 1.35 Included in OECD Work Plan

Abstract

This AOP describes the sequence of events leading from thyroperoxidase inhibition to increased mortality via reduced anterior swim bladder inflation. The enzyme thyroperoxidase (TPO) is essential for the synthesis of thyroxine (T4) and triiodothyronine (T3) in the thyroid follicles. Inhibition of TPO reduces thyroid hormone (TH) levels. Thyroid hormones are critical in regulating developmental processes and thyroid hormone disruption can interfere with normal development. Swim bladder inflation is known to be under TH control (Brown et al., 1988; Liu and Chan, 2002). Many fish species have a swim bladder which is a gas-filled organ that typically consists of two chambers (Robertson et al., 2007). The posterior chamber inflates during early development in the embryonic phase, while the anterior chamber inflates during late development in the larval phase. Both the posterior and the anterior chamber have an important role in regulating buoyancy, and the anterior chamber has an additional role in hearing (Robertson et al., 2017). This AOP describes how inhibition of TPO results in decreased synthesis of T4 in the thyroid follicles. This reduces the availability of T4 for conversion to the more biologically active T3. Reduced T3 levels prohibit normal inflation of the anterior swim bladder chamber. Due to its role in regulating buoyancy, this results in reduced swimming performance. Since reduced swimming performance resuls in a decreased ability to forage and avoid predators, this reduces chances of survival. The final adverse outcome is a decrease of the population trajectory. Since many AOPs eventually lead to this more general adverse outcome at the population level, the more specific and informative adverse outcome at the organismal level, increased mortality, is used in the AOP title. Support for this AOP is mainly based on chemical exposures in zebrafish and fathead minnows (Nelson et al., 2016; Godfrey et al., 2017; Stinckens et al., 2016, 2020). Additional evidence of a link between reduced TH synthesis and reduced anterior chamber inflation is available from a study where a mutation was inserted in the gene coding for dual oxidase, an other enzyme that is important for TH synthesis (Chopra et al., 2019). This AOP is part of a larger AOP network describing how decreased synthesis and/or decreased biological activation of THs leads to incomplete or improper inflation of the swim bladder, leading to reduced swimming performance, increased mortality and decreased population trajectory (Knapen et al., 2018; Knapen et al., 2020; Villeneuve et al., 2018). An AOP initiated by TPO inhibition and leading to posterior chamber inflation has not been developed so far, since available evidence shows that TPO inhibition is less likely to cause effects on inflation of the posterior chamber during early embryonic development. This important aspect of life stage specificity is explained in more detail below (see Domain of applicability).

Background

The larger AOP network describing the effect of deiodinase and thyroperoxidase inhibition on swim bladder inflation consists of 5 AOPs:

Summary of the AOP

Events

Molecular Initiating Events (MIE), Key Events (KE), Adverse Outcomes (AO)

Sequence Type Event ID Title Short name
1 MIE 279 Thyroperoxidase, Inhibition Thyroperoxidase, Inhibition
2 KE 277 Thyroid hormone synthesis, Decreased TH synthesis, Decreased
3 KE 281 Thyroxine (T4) in serum, Decreased T4 in serum, Decreased
4 KE 1003 Decreased, Triiodothyronine (T3) in serum Decreased, Triiodothyronine (T3) in serum
5 KE 1007 Reduced, Anterior swim bladder inflation Reduced, Anterior swim bladder inflation
6 KE 1005 Reduced, Swimming performance Reduced, Swimming performance
7 AO 351 Increased Mortality Increased Mortality
8 AO 360 Decrease, Population trajectory Decrease, Population trajectory

Key Event Relationships

Upstream Event Relationship Type Downstream Event Evidence Quantitative Understanding
Thyroperoxidase, Inhibition adjacent Thyroid hormone synthesis, Decreased Moderate Low
Thyroid hormone synthesis, Decreased adjacent Thyroxine (T4) in serum, Decreased Moderate Low
Thyroxine (T4) in serum, Decreased adjacent Decreased, Triiodothyronine (T3) in serum Moderate Moderate
Decreased, Triiodothyronine (T3) in serum adjacent Reduced, Anterior swim bladder inflation Moderate Moderate
Reduced, Anterior swim bladder inflation adjacent Reduced, Swimming performance Moderate Low
Reduced, Swimming performance adjacent Increased Mortality Moderate Low
Increased Mortality adjacent Decrease, Population trajectory High Moderate
Thyroperoxidase, Inhibition non-adjacent Thyroxine (T4) in serum, Decreased High Low
Thyroxine (T4) in serum, Decreased non-adjacent Reduced, Anterior swim bladder inflation Moderate Moderate

Stressors

Name Evidence
Methimazole High
Mercaptobenzothiazole High
Propylthiouracil High

Overall Assessment of the AOP

The attached document includes:

  • Support for biological plausibility of KERs
  • Support for essentiality of KEs
  • Empirical support for KERs
  • Dose and temporal concordance table covering the larger AOP network

Overall, the weight of evidence for the sequence of key events laid out in the AOP is moderate to high. Nonetheless, the exact underlying mechanism of TH disruption leading to impaired swim bladder inflation is not understood.

Domain of Applicability

Life Stage Applicability
Life Stage Evidence
larvae High
Taxonomic Applicability
Term Scientific Term Evidence Links
zebrafish Danio rerio High NCBI
fathead minnow Pimephales promelas High NCBI
Sex Applicability
Sex Evidence
Unspecific High

Taxonomic: Organogenesis of the swim bladder begins with an evagination from the gut. In physostomous fish, a connection between the swim bladder and the gut is retained. In physoclystous fish, once initial inflation by gulping atmospheric air at the water surface has occurred, the swim bladder is closed off from the digestive tract and swim bladder volume is regulated by gas secretion into the swim bladder (Wooley and Qin, 2010). This AOP is currently mainly based on experimental evidence from studies on zebrafish and fathead minnows, physostomous fish with a two-chambered swim bladder. This AOP is not applicable to fish that do not have a second swim bladder chamber that inflates during larval development, e.g., the Japanese rice fish (Oryzias latipes).

Life stage: The current AOP is applicable to the larval life stage, the period in which the anterior chamber of the swim bladder inflates (21 days post fertilization in zebrafish). While evidence supports a link between reduced serum T4 levels and reduced anterior swim bladder inflation in late larvae, experimental evidence suggests that inhibition of T4 synthesis is less likely to result in reduced posterior swim bladder inflation in embryos. The reduced likelihood of effects on posterior chamber inflation is probably due to maternal transfer of T4 into the eggs limiting the importance of TH synthesis by TPO in the earliest embryonic stages. These maternally derived THs are depleted at 21 days post fertilization (dpf) and this is probably why 21 day old larvae are sensitive to TPO inhibtion, resulting in impaired anterior chamber inflation.

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 (Ruuskanen and Hsu, 2018; Walpita et al., 2007; Chang et al., 2012; Hsu et al., 2014; Power et al., 2001; Brown et al., 1987, 1988). Campinho et al. (2014) confirmed that maternal thyroid hormones are essential for normal brain development in zebrafish by knocking down MCT8, responsable for transporting thyroid hormones into the cells. 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 (hpf) in zebrafish. Walter et al. (2019) showed decreased levels of T4 in 72 hpf embryos exposed to propylthiouracil (PTU), a TPO inhibitor, confirming embryonic TH synthesis at 72 hpf. At 24 hpf PTU had no effect on TH levels, confirming that at least at the whole body level, there is no detectable TH synthesis at 24 hpf. Time points between 24 and 72 hpf have not been investigated. Therefore, it is still uncertain when exactly embryonic TH synthesis is activated. Therefore early developmental processes (before thyroid activation) that are dependent on T4, such as posterior swim bladder inflation, may be less sensitive to chemicals reducing T4 synthesis. Nelson et al. (2016) and Stinckens et al. (2016) indeed found that 2-mercaptobenzothiazole (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 chamber inflation. Godfrey et al. (2017) also reported normal posterior chamber inflation after exposure of zebrafish embryos to methimazole, a TPO inhibitor. In the latter study, only perfluorooctanoic acid (a deiodinase inhibitor in pig, Stinckens et al., 2018) affected posterior chamber inflation. Stinckens et al. (2018) also showed that sodium perchlorate, an inhibitor of the sodium iodide symporter (NIS), did not impact posterior chamber inflation. Since this symporter is essential for providing iodide for TH synthesis, this is indirect evidence for this hypothesis of life stage specificity. It is important to note that posterior chamber inflation should be observed until at least 6 dpf in zebrafish to exclude delayed swim bladder inflation (which may be due to a more general developmental delay, rather than a specific effect on the swim bladder). 

Sex: Sex differences are typically not investigated in tests using early life stages of fish and it is currently unclear whether sex-related differences are important in this AOP. Zebrafish are undifferentiated gonochorists since both sexes initially develop an immature ovary (Maack and Segner, 2003). Immature ovary development progresses until approximately the onset of the third week. Later, in female fish immature ovaries continue to develop further, while male fish undergo transformation of ovaries into testes. Final transformation into testes varies among male individuals, however finishes usually around 6 weeks post fertilization. Since the anterior chamber inflates around 21 days post fertilization, when sex differentiation is still in its early stages, sex differences are expected to play a minor role in the current AOP.

Essentiality of the Key Events

Overall, the confidence in the supporting data for essentiality of KEs within the AOP is moderate. There is indirect evidence that reduced thyroid hormone synthesis causes reduced anterior swim bladder inflation from a study where a similar MIE was targeted: Chopra et al. (2019) showed that knockdown of dual oxidase, important for thyroid hormone synthesis, reduced anterior swim bladder inflation. Additionally, there is indirect evidence from deiodinase knockdowns supporting the downstream part of the AOP linking decreased T3 levels to reduced swim bladder inflation (targeted at posterior chamber inflation, not specifically at anterior chamber inflation).

 

Weight of Evidence Summary

Biological plausibility: see Table. Overall, the weight of evidence for the biological plausibility of the KERs in the AOP is moderate since there is empirical support for an association between the sets of KEs and the KERs are plausible based on analogy to accepted biological relationships, but scientific understanding is not completely established.

Empirical support: see Table. Overall, the empirical support for the KERs in the AOP is moderate since dependent changes in sets of KEs following exposure to several specific stressors has been demonstrated, with limited evidence for dose and temporal concordance and some uncertainties.

Quantitative Consideration

There is some level of quantitative understanding that can form the basis for development of a quantitative AOP. Quantitative relationships between reduced T4 and reduced T3, and between reduced T3 and reduced anterior chamber inflation were established. The latter is particularly critical for linking impaired swim bladder inflation to TH disruption.

Considerations for Potential Applications of the AOP (optional)

A growing number of environmental pollutants are known to adversely affect the thyroid hormone system, and major gaps have been identified in the tools available for the identification, and the hazard and risk assessment of these thyroid hormone disrupting chemicals. Villeneuve et al. (2014) discussed the relevance of swim bladder inflation as a potential key event and endpoint of interest in fish tests. Knapen et al. (2020) provide an example of how the adverse outcome pathway (AOP) framework and associated data generation can address current testing challenges in the context of fish early-life stage tests, and fish tests in general. A suite of assays covering all the essential biological processes involved in the underlying toxicological pathways can be implemented in a tiered screening and testing approach for thyroid hormone disruption, using the levels of assessment of the OECD’s Conceptual Framework for the Testing and Assessment of Endocrine Disrupting Chemicals as a guide. Specifically, for this AOP, thyroperoxidase inhibition can be assessed using an in chemico assay, measurements of T4 and T3 levels could be added to the Fish Embryo Acute Toxicity
(FET) test (OECD TG 236) and to the Fish Early Life Stage Toxicity (FELS) Test (OECD TG210), and assessments of anterior chamber inflation and swimming performance could be added to the FELS Test.

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., 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, C.L., Doroshov, S.I., Nunez, J.M., Hadley, C., Vaneenennaam, J., Nishioka, R.S., Bern, H.A., 1988. MATERNAL TRIIODOTHYRONINE INJECTIONS CAUSE INCREASES IN SWIMBLADDER INFLATION AND SURVIVAL RATES IN LARVAL STRIPED BASS, MORONE-SAXATILIS. Journal of Experimental Zoology 248, 168-176.

Campinho, M.A., Saraiva, J., Florindo, C., Power, D.M., 2014. Maternal Thyroid Hormones Are Essential for Neural Development in Zebrafish. Molecular Endocrinology 28, 1136-1149.

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.

Chopra, K., Ishibashi, S., Amaya, E., 2019. Zebrafish duox mutations provide a model for human congenital hypothyroidism. Biology Open 8.

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.

Godfrey, A., Hooser, B., Abdelmoneim, A., Horzmann, K.A., Freemanc, J.L., Sepulveda, M.S., 2017. Thyroid disrupting effects of halogenated and next generation chemicals on the swim bladder development of zebrafish. Aquatic Toxicology 193, 228-235.

Hsu, C.W., Tsai, S.C., Shen, S.C., Wu, S.M., 2014. Profiles of thyrotropin, thyroid hormones, follicular cells and type I deiodinase gene expression during ontogenetic development of tilapia larvae and juveniles. Fish Physiology and Biochemistry 40, 1587-1599.

Knapen, D., Angrish, M.M., Fortin, M.C., Katsiadaki, I., Leonard, M., Margiotta-Casaluci, L., Munn, S., O'Brien, J.M., Pollesch, N., Smith, L.C., Zhang, X.W., Villeneuve, D.L., 2018. Adverse outcome pathway networks I: Development and applications. Environmental Toxicology and Chemistry 37, 1723-1733.

Knapen, D., Stinckens, E., Cavallin, J.E., Ankley, G.T., Holbech, H., Villeneuve, D.L., Vergauwen, L., 2020. Toward an AOP Network-Based Tiered Testing Strategy for the Assessment of Thyroid Hormone Disruption. Environmental Science & Technology 54, 8491-8499.

Liu, Y.W., Chan, W.K., 2002. Thyroid hormones are important for embryonic to larval transitory phase in zebrafish. Differentiation 70, 36-45.

Maack, G., Segner, H., 2003. Morphological development of the gonads in zebrafish. Journal of Fish Biology 62, 895-906.

Nelson, K., Schroeder, A., Ankley, G., Blackwell, B., Blanksma, C., Degitz, S., Flynn, K., Jensen, K., Johnson, R., Kahl, M., Knapen, D., Kosian, P., Milsk, R., Randolph, E., Saari, T., Stinckens, E., Vergauwen, L., Villeneuve, D., 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, D.M., Llewellyn, L., Faustino, M., Nowell, M.A., Bjornsson, B.T., Einarsdottir, I.E., Canario, A.V., Sweeney, G.E., 2001. Thyroid hormones in growth and development of fish. Comp Biochem Physiol C Toxicol Pharmacol 130, 447-459.

Robertson, G.N., McGee, C.A.S., Dumbarton, T.C., Croll, R.P., Smith, F.M., 2007. Development of the swimbladder and its innervation in the zebrafish, Danio rerio. Journal of Morphology 268, 967-985.

Ruuskanen, S., Hsu, B.Y., 2018. Maternal Thyroid Hormones: An Unexplored Mechanism Underlying Maternal Effects in an Ecological Framework. Physiological and Biochemical Zoology 91, 904-916.

Stinckens, E., Vergauwen, L., Ankley, G.T., Blust, R., Darras, V.M., Villeneuve, D.L., Witters, H., Volz, D.C., Knapen, D., 2018. An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish. Aquatic Toxicology 200, 1-12.

Stinckens, E., Vergauwen, L., Blackwell, B.R., Anldey, G.T., Villeneuve, D.L., Knapen, D., 2020. Effect of Thyroperoxidase and Deiodinase Inhibition on Anterior Swim Bladder Inflation in the Zebrafish. Environmental Science & Technology 54, 6213-6223.

Stinckens, E., Vergauwen, L., Schroeder, A., Maho, W., Blackwell, B., Witters, H., Blust, R., Ankley, G., Covaci, A., Villeneuve, D., 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.

Villeneuve, D., Angrish, M., Fortin, M., Katsiadaki, I., Leonard, M., Margiotta-Casaluci, L., Munn, S., O'Brien, J., Pollesch, N., Smith, L., Zhang, X., Knapen, D., 2018. Adverse Outcome Pathway Networks II: Network Analytics. Environ Toxicol Chem doi: 10.1002/etc.4124.

Villeneuve, D., Volz, D.C., Embry, M.R., Ankley, G.T., Belanger, S.E., Leonard, M., Schirmer, K., Tanguay, R., Truong, L., Wehmas, L., 2014. Investigating alternatives to the fish early-life stage test: a strategy for discovering and annotating adverse outcome pathways for early fish development. Environmental Toxicology and Chemistry 33, 158-169.

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.

Walter, K.M., Miller, G.W., Chen, X.P., Yaghoobi, B., Puschner, B., Lein, P.J., 2019. Effects of thyroid hormone disruption on the ontogenetic expression of thyroid hormone signaling genes in developing zebrafish (Danio rerio). General and Comparative Endocrinology 272, 20-32.

Woolley, L.D., Qin, J.G., 2010. Swimbladder inflation and its implication to the culture of marine finfish larvae. Reviews in Aquaculture 2, 181-190.

Appendix 1

List of MIEs in this AOP

Event: 279: Thyroperoxidase, Inhibition

Short Name: Thyroperoxidase, Inhibition

Key Event Component

Process Object Action
iodide peroxidase activity thyroid peroxidase decreased

AOPs Including This Key Event

Stressors

Name
2(3H)-Benzothiazolethione
2-mercaptobenzothiazole
Ethylene thiourea
Mercaptobenzothiazole
Methimazole
Propylthiouracil
Resorcinol
Thiouracil
Ethylenethiourea
Amitrole
131-55-5
2,2',4,4'-Tetrahydroxybenzophenone
Daidzein
Genistein
4-Nonylphenol
4-propoxyphenol
Sulfamethazine

Biological Context

Level of Biological Organization
Molecular

Cell term

Cell term
thyroid follicular cell

Organ term

Organ term
thyroid follicle

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

There is a wealth of information on the inhibition of TPO by drugs such as MMI and PTU, as well as environmental xenobiotics. In the landmark paper on thyroid disruption by environmental chemicals, Brucker-Davis (1998) identified environmental chemicals that depressed TH synthesis by inhibiting TPO. Hurley (1998) listed TPO as a major target for thyroid tumor inducing pesticides. More recent work has tested over 1000 chemicals using a high-throughput screening assay (Paul-Friedman et al., 2016).

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
rat Rattus norvegicus High NCBI
humans Homo sapiens High NCBI
pigs Sus scrofa High NCBI
Xenopus laevis Xenopus laevis High NCBI
chicken Gallus gallus High NCBI
zebrafish Danio rerio High NCBI
fathead minnow Pimephales promelas High NCBI
mouse Mus musculus NCBI
Life Stage Applicability
Life Stage Evidence
All life stages High
Sex Applicability
Sex Evidence
Female High
Male High

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.

Key Event 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. TPO catalyzes several reactions in the thyroid gland, including: the oxidation of iodide; nonspecific iodination of tyrosyl residues of thyroglobulin (Tg); and, the coupling of iodotyrosyls to produce Tg-bound monoiodotyrosine (MIT) and diiodotyrosine (DIT) (Divi et al., 1997; Kessler et al., 2008; Ruf et al., 2006; Taurog et al., 1996). The outcome of TPO inhibition is decreased synthesis of thyroxine (T4) and triiodothyronine (T3), a decrease in release of these hormones from the gland into circulation, and unless compensated, a consequent decrease in systemic concentrations of T4, and possibly T3. The primary product of TPO-catalyzed TH synthesis is T4 (Taurog et al., 1996; Zoeller et al., 2007) that would be peripherally or centrally deiodinated to T3.

It is important to note that TPO is a complex enzyme and that has two catalytic cycles and is capable of iodinating multiple species (Divi et al., 1997). Alterations in all of these events are not covered by some of the commonly used assays that measure “TPO inhibition” (e.g., guicacol and AmplexUltraRed, see below).  Therefore, in the context of this AOP we are using TPO inhibiton not in the classical sense, but instead to refer to the empirical data derived from the assays commonly used assays to investigate environmental chemicals.

Figure 1 below illustrates the enzymatic and nonenzymatic reactions mediated by TPO that result in the synthesis of thyroxine (T4) .

Inhibition of TPO can be reversible, with transient interaction between the enzyme and the chemical, or irreversible, whereby suicide substrates permanently inactivate the enzyme. Reversible and irreversible TPO inhibition may be determined by the chemical structure, may be concentration dependent, or may be influenced by other conditions, including the availability of iodine (Doerge and Chang, 2002).

The ontogeny of TPO has been determined using both direct and indirect evidence.  Available evidence suggests the 11th to 12th fetal week as the beginning of functional TPO in humans. In rodents, TPO function begins late in the second fetal week, with the first evidence of T4 secretion on gestational day 17 (Remy et al., 1980). Thyroid-specific genes appear in the thyroid gland according to a specific temporal pattern; thyroglobulin (Tg), TPO (Tpo), and TSH receptor (Tshr) genes are expressed by gestational day 14 in rats, and the sodium iodide symporter, NIS (Nis), is expressed by gestational day 16 in rats. Maturation to adult function is thought to occur within a few weeks after parturition in rats and mice, and within the first few months in neonatal humans (Santisteban and Bernal, 2005).  Tg is first detected in human fetuses starting at 5th week of gestation and rises throughout gestation (Thorpe-Beeston et al., 1992), but iodine trapping and T4 production does not occur until around 10-12 weeks.  Also, the dimerization of Tg, a characteristic of adult TH storage, is not found until much later in human gestation (Pintar, 2000). In rats, Tg immunoreactivity does not appear until day 15 of gestation (Fukiishi et al., 1982; Brown et al., 2000). The vast majority of research and knowledge on Tg is from mammals, although genomic orthologs are known for a variety of other species (Holzer et al., 2016).  It is important to note that prior to the onset of fetal thyroid function, TH are still required by the developing fetus which until that time relies solely on maternal sources. Chemical-induced TPO inhibition can affect synthesis in the maternal gland and in the fetal gland.

How it is Measured or Detected

There are no approved OECD or EPA guideline study protocols for measurement of TPO inhibition.. However, there is an OECD scoping document on identification of chemicals that modulate TH signaling that provides details on a TPO assay (OECD, 2017). 

From the early 1960's, microsomal fractions prepared from porcine thyroid glands and isolated porcine follicles were used as a source of TPO for inhibition experiments (Taurog, 2005). Limited information has been published using microsomes from human goiter samples (Vickers et al., 2012) and rat thyroid glands (Paul et al., 2013; 2014; Paul-Friedman et al., 2016).

TPO activity has been measured for decades via indirect assessment by kinetic measurement of the oxidation of guaiacol (Chang & Doerge 2000; Hornung et al., 2010; Schmutzler et al., 2007).  This method is a low-throughput assay due to the very rapid kinetics of the guaiacol oxidation reaction. More recently, higher-throughput methods using commercial fluorescent and luminescent substrates with rodent, porcine, and human microsomal TPO have been developed (Vickers et al., 2012; Paul et al., 2013; 2014; Kaczur et al., 1997). This assay substitutes a pre-fluorescent substrate (Amplex UltraRed) for guaiacol, that when incubated with a source of peroxidase and excess hydrogen peroxidase, results in a stable fluorescent product proportional to TPO activity (Vickers et al., 2012).  The stability of the fluorescent reaction product allows this assay to be used in a higher throughput format (Paul-Friedman et al., 2016). This approach is appropriate for high-throughput screening but does not elucidate the specific mechanism by which a chemical may inhibit TPO (Paul-Friedman et al., 2016), and as with most in vitro assays, is subject to various sources of assay interference (Thorne et al., 2010).

HPLC has been used to measure of the activity of TPO via formation of the precursors monoiodotyrosine (MIT), diiodotyrosine (DIT), and both T3 and T4, in a reaction mixture containing TPO, or a surrogate enzyme such as lactoperoxidase (Divi & Doerge 1994). The tools and reagents for this method are all available. However, HPLC or other analytical chemistry techniques make this a low throughput assay, depending on the level of automation. A primary advantage of this in vitro method is that it directly informs hypotheses regarding the specific mechanism by which a chemical may impact thyroid hormone synthesis in vitro.  

References

Brown RS, Shalhoub V, Coulter S, Alex S, Joris I, De Vito W, Lian J, Stein GS.  Developmental regulation of thyrotropin receptor gene expression in the fetal and neonatal rat thyroid: relation to thyroid morphology and to thyroid-specific gene expression.  Endocrinology. 2000 Jan;141(1):340-5.

Brucker-Davis F. 1998. Effects of environmental synthetic chemicals on thyroid function. Thyroid 8:827-856.

Chang, H. C. and D. R. Doerge (2000) Dietary genistein inactivates rat thyroid peroxidase in vivo without an apparent hypothyroid effect. Toxicol Appl Pharmacol. 168:244-252.

Divi, R. L., & Doerge, D. R. (1994). Mechanism-based inactivation of lactoperoxidase and thyroid peroxidase by resorcinol derivatives. Biochemistry 33(32), 9668–9674.

Divi, R. L., Chang, H. C., & Doerge, D. R. (1997). Anti-Thyroid Isoflavones from Soybean. Biochem. Pharmacol.  54(10), 1087–1096.

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

Ealey PA, Henderson B, Loveridge N.A quantitative study of peroxidase activity in unfixed tissue sections of the guinea-pig thyroid gland. Histochem J. 1984 Feb;16(2):111-22.

Fukiishi Y, Harauchi T, Yoshizaki T, Hasegawa Y, Eguchi Y.  Ontogeny of thyroid peroxidase activity in perinatal rats. Acta Endocrinol (Copenh). 1982 101(3):397-402.

Holzer G, Morishita Y, Fini JB, Lorin T, Gillet B, Hughes S, Tohmé M, Deléage G, Demeneix B, Arvan P, Laudet V. Thyroglobulin Represents a Novel Molecular Architecture of Vertebrates. J Biol Chem. 2016 Jun 16.

Hornung, M. W., Degitz, S. J., Korte, L. M., Olson, J. M., Kosian, P. a, Linnum, A. L., & Tietge, J. E. (2010). Inhibition of thyroid hormone release from cultured amphibian thyroid glands by methimazole, 6-propylthiouracil, and perchlorate. Toxicol Sci 118(1), 42–51.

Hurley PM. 1998. Mode of carcinogenic action of pesticides inducing thyroid follicular cell tumors in rodents. Environ Health Perspect 106:437-445.

Kaczur, V., Vereb, G., Molnár, I., Krajczár, G., Kiss, E., Farid, N. R., & Balázs, C. (1997). Effect of anti-thyroid peroxidase (TPO) antibodies on TPO activity measured by chemiluminescence assay. Clin. Chem 43(8 Pt 1), 1392–6.

Kessler, J., Obinger, C., Eales, G., 2008. Factors influencing the study of peroxidase- generated iodine species and implications for thyroglobulin synthesis. Thyroid 18, 769–774.

OECD (2017) New Scoping Document on in vitro and ex vivo Assays for the Identification of Modulators of Thyroid Hormone Signalling. Series on Testing and Assessment. No. 207.  ISSN: 20777876 (online) http://dx.doi.org/10.1787/20777876

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

Paul, K.B., Hedge, J.M., Rotroff, D.M., Hornung, M.W., Crofton, K.M., Simmons, S.O. 2014. Development of a thyroperoxidase inhibition assay for high-throughput screening. Chem.  Res. Toxicol. 27(3), 387-399.

Paul-Friedman K, Watt ED, Hornung MW, Hedge JM, Judson RS, Crofton KM, Houck KA, Simmons SO. 2016. Tiered High-Throughput Screening Approach to Identify Thyroperoxidase Inhibitors Within the ToxCast Phase I and II Chemical Libraries.  Toxicol Sci. 151:160-80.

Pintar, J.E. (2000) Normal development of the hypothalamic-pituitary-thyroid axis. In. Werner & Ingbar’s The Thyroid. (8th ed), Braverman. L.E. and Utiger, R.D. (eds) Lippincott Williams and Wilkins, Philadelphia.

Remy L, Michel-Bechet M, Athouel-Haon AM, Magre S. Critical study of endogenous peroxidase activity: its role in the morphofunctional setting of the thyroid follicle in the rat fetus. Acta Histochem. 1980;67(2):159-72.

Ruf, J., & Carayon, P. (2006). Structural and functional aspects of thyroid peroxidase. Archives of Biochemistry and Biophysics, 445(2), 269–77.

Santisteban P, Bernal J. Thyroid development and effect on the nervous system. Rev Endocr Metab Disord. 2005 Aug;6(3):217-28.

Schmutzler, C., Bacinski, A., Gotthardt, I., Huhne, K., Ambrugger, P., Klammer, H., Schlecht, C., Hoang-Vu, C., Gruters, A., Wuttke, W., Jarry, H., Kohrle, J., 2007a. The ultraviolet filter benzophenone 2 interferes with the thyroid hormone axis in rats and is a potent in vitro inhibitor of human recombinant thyroid peroxidase. Endocrinology 148, 2835–2844.

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

Taurog, a, Dorris, M. L., & Doerge, D. R. (1996). Mechanism of simultaneous iodination and coupling catalyzed by thyroid peroxidase. Archives of Biochemistry and Biophysics, Taurog A. Molecular evolution of thyroid peroxidase. Biochimie. 1999 May;81(5):557-62

Takayama S, Aihara K, Onodera T, Akimoto T. Antithyroid effects of propylthiouracil and sulfamonomethoxine in rats and monkeys. Toxicol Appl Pharmacol. 1986 Feb;82(2):191-9.

Thorne N, Auld DS, Inglese J.  Apparent activity in high-throughput screening: origins of compound-dependent assay interference. Curr Opin Chem Biol. 2010 Jun;14(3):315-24.

Thorpe-Beeston JG, Nicolaides KH, McGregor AM. Fetal thyroid function. Thyroid. 1992 Fall;2(3):207-17. Review.

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 Apr 1;260(1):81-8.

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

List of Key Events in the AOP

Event: 277: Thyroid hormone synthesis, Decreased

Short Name: TH synthesis, Decreased

Key Event Component

Process Object Action
thyroid hormone generation thyroid hormone decreased

AOPs Including This Key Event

Stressors

Name
Propylthiouracil
Methimazole

Biological Context

Level of Biological Organization
Cellular

Cell term

Cell term
thyroid follicular cell

Organ term

Organ term
thyroid gland

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

not applicable as this KE is not an MIE

Propylthiouracil

6-n-proylthiouracil is a common positive control

Methimazole

Methimazole is a very common positve control

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
rat Rattus norvegicus High NCBI
human Homo sapiens High NCBI
Pig Pig High NCBI
Xenopus laevis Xenopus laevis Moderate NCBI
zebrafish Danio rerio High NCBI
fathead minnow Pimephales promelas Moderate NCBI
Life Stage Applicability
Life Stage Evidence
All life stages High
Sex Applicability
Sex Evidence
Male High
Female High

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).

 

Key Event Description

The thyroid hormones (TH), triiodothyronine (T3) and thyroxine (T4) are thyrosine based hormones. Synthesis of TH is regulated by thyroid-stimulating hormone (TSH) binding to its receptor and thyroidal availability of iodine via the sodium iodide symporter (NIS). Other proteins contributing to TH production in the thyroid gland, including thyroperoxidase (TPO), dual oxidase enzymes (DUOX), and pendrin are also necessary for iodothyronine production (Zoeller et al., 2007).

The production of THs in the thyroid gland and resulting serum concentrations are controlled by a negatively regulated feedback mechanism.  Decreased T4 and T3 serum concentrations activates the hypothalamus-pituitary-thyroid (HPT) axis which upregulates thyroid-stimulating hormone (TSH) that acts to increase production of additional THs (Zoeller and Tan, 2007). This regulatory system includes: 1) the hypothalamic secretion of the thyrotropin-releasing hormone (TRH); 2) the thyroid-stimulating hormone (TSH) secretion from the anterior pituitary; 3) hormonal transport by the plasma binding proteins; 4) cellular uptake mechanisms at the tissue level; 5) intracellular control of TH concentration by deiodinating mechanisms; 6) transcriptional function of the nuclear TH receptor; and 7) in the fetus, the transplacental passage of T4 and T3 (Zoeller et al., 2007).

TRH and the TSH primarily regulate the production of T4, often considered a “pro-hormone,” and to a lesser extent of T3, the transcriptionally active TH. Most of the hormone released from the thyroid gland into circulation is in the form of T4, while peripheral deiodination of T4 is responsible for the majority of circulating T3. Outer ring deiodination of T4 to T3 is catalyzed by the deiodinases 1 and 2 (DIO1 and DIO2), with DIO1 expressed mainly in liver and kidney, and DIO2 expressed in several tissues including the brain (Bianco et al., 2006). Conversion of T4 to T3 takes place mainly in liver and kidney, but also in other target organs such as in the brain, the anterior pituitary, brown adipose tissue, thyroid and skeletal muscle (Gereben et al., 2008; Larsen, 2009). 

Most evidence for the ontogeny of TH synthesis comes from measurements of serum hormone concentrations.  And, importantly, the impact of xenobiotics on fetal hormones must include the influence of the maternal compartment since a majority of fetal THs are derived from maternal blood early in fetal life, with a transition during mid-late gestation to fetal production of THs that is still supplemented by maternal THs. In humans, THs can be found in the fetus as early as gestational weeks 10-12, and concentations rise continuously until birth. At term, fetal T4 is similar to maternal levels, but T3 remains 2-3 fold lower than maternal levels.  In rats, THs can be detected in the fetus as early as the second gestational week, but fetal synthesis does not start until gestational day 17 with birth at gestational day 22-23. Maternal THs continue to supplement fetal production until parturition. (see Howdeshell, 2002; Santisteban and Bernal, 2005 for review). The ontogeny of TPO inhibition during development by environmental chemicals is a data gap.

Decreased TH synthesis in the thyroid gland may result from several possible molecular-initiating events (MIEs) including: 1) Disruption of key catalytic enzymes or cofactors needed for TH synthesis, including TPO, NIS, or dietary iodine insufficiency. Theoretically, decreased synthesis of Tg could also affect TH production (Kessler et al., 2008; Yi et al., 1997). Mutations in genes that encode requisite proteins in the thyroid may also lead to impaired TH synthesis, including mutations in pendrin associated with Pendred Syndrome (Dossena et al., 2011), mutations in TPO and Tg (Huang and Jap 2015), and mutations in NIS (Spitzweg and Morris, 2010). 2) Decreased TH synthesis in cases of clinical hypothyroidism may be due to Hashimoto's thyroiditis or other forms of thyroiditis, or physical destruction of the thyroid gland as in radioablation or surgical treatment of thyroid lymphoma. 3) It is possible that TH synthesis may also be reduced subsequent to disruption of the negative feedback mechanism governing TH homeostasis, e.g. pituitary gland dysfunction may result in a decreased TSH signal with concomitant T3 and T4 decreases. 4) More rarely, hypothalamic dysfunction can result in decreased TH synthesis. 

Increased fetal thyroid levels are also possible. Maternal Graves disease, which results in fetal thyrotoxicosis (hyperthyroidism and increased serum T4 levels), has been successfully treated by maternal administration of TPO inhibitors (c.f., Sato et al., 2014).  

It should be noted that different species and different lifestages store different amounts of TH precursor and iodine within the thyroid gland. Thus, decreased TH synthesis via transient iodine insufficiency or inhibition of TPO may not affect TH release from the thyroid gland until depletion of stored iodinated Tg. Adult humans may store sufficient Tg-DIT residues to serve for several months to a year of TH demand (Greer et al., 2002; Zoeller, 2004). Neonates and infants have a much more limited supply of less than a week.

How it is Measured or Detected

Decreased TH synthesis is often implied by measurement of TPO and NIS inhibition measured clinically and in laboratory models as these enzymes are essential for TH synthesis. Rarely is decreased TH synthesis measured directly, but rather the impact of chemicals on the quantity of T4 produced in the thyroid gland, or the amount of T4 present in serum is used as a marker of decreased T4 release from the thyroid gland (e.g., Romaldini et al., 1988). Methods used to assess TH synthesis include, incorporation of radiolabel tracer compounds, radioimmunoassay, ELISA, and analytical detection.   

Recently, amphibian thyroid explant cultures have been used to demonstrate direct effects of chemicals on TH synthesis, as this model contains all necessary synthesis enzymes including TPO and NIS (Hornung et al., 2010). For this work THs was measured by HPLC/ICP-mass spectometry. Decreased TH synthesis and release, using T4 release as the endpoint, has been shown for thiouracil antihyperthyroidism drugs including MMI, PTU, and the NIS inhibitor perchlorate (Hornung et al., 2010).

TIQDT (Thyroxine-immunofluorescence quantitative disruption test) is a method that provides an immunofluorescent based estimate of thyroxine in the gland of zebrafish (Thienpont et al 2011).  This method has been used for ~25 xenobiotics (e.g., amitrole, perchlorate, methimazole, PTU, DDT, PCBs).  The method detected changes for all chemicals known to directly impact TH synthesis in the thyroid gland (e.g., NIS and TPO inbibitors), but not those that upregulate hepatic catabolism of T4.

References

Bakker B, Bikker H, Vulsma T, de Randamie JS, Wiedijk BM, De Vijlder JJ. 2000. Two decades of screening for congenital hypothyroidism in The Netherlands: TPO gene mutations in total iodide organification defects (an update). The Journal of clinical endocrinology and metabolism.  85:3708-3712.

Bianco AC, Kim BW. (2006). Deiodinases: implications of the local control of thyroid hormone action. J Clin Invest. 116: 2571–2579.

Dossena S, Nofziger C, Brownstein Z, Kanaan M, Avraham KB, Paulmichl M. (2011). Functional characterization of pendrin mutations found in the Israeli and Palestinian populations. Cell Physiol Biochem. 28: 477-484.Gereben B, Zavacki AM, Ribich S, Kim BW, Huang SA, Simonides WS, Zeöld A, Bianco AC. (2008). Cellular and molecular basis of deiodinase-regulated thyroid hormone signalling. Endocr Rev. 29:898–938.

Gereben B, Zeöld A, Dentice M, Salvatore D, Bianco AC.  Activation and inactivation of thyroid hormone by deiodinases: local action with general consequences.  Cell Mol Life Sci. 2008 Feb;65(4):570-90

Greer MA, Goodman G, Pleus RC, Greer SE. Health effects assessment for environmental perchlorate contamination: the dose response for inhibition of thyroidal radioiodine uptake in humans. Environ Health Perspect. 2002. 110:927-937.

Howdeshell KL. 2002. A model of the development of the brain as a construct of the thyroid system. Environ Health Perspect. 110 Suppl 3:337-48.

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

Huang CJ and Jap TS. 2015. A systematic review of genetic studies of thyroid disorders in Taiwan. J Chin Med Assoc. 78: 145-153.

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

Larsen PR. (2009). Type 2 iodothyronine deiodinase in human skeletal muscle: new insights into its physiological role and regulation. J Clin Endocrinol Metab. 94:1893-1895.

Romaldini JH, Farah CS, Werner RS, Dall'Antonia Júnior RP, Camargo RS. 1988.  "In vitro" study on release of cyclic AMP and thyroid hormone in autonomously functioning thyroid nodules.  Horm Metab Res.20:510-2.

Santisteban P, Bernal J. Thyroid development and effect on the nervous system. Rev Endocr Metab Disord. 2005 Aug;6(3):217-28.

Spitzweg C, Morris JC. 2010. Genetics and phenomics of hypothyroidism and goiter due to NIS mutations. Molecular and cellular endocrinology. 322:56-63.

Thienpont B, Tingaud-Sequeira A, Prats E, Barata C, Babin PJ, Raldúa D.  Zebrafish eleutheroembryos provide a suitable vertebrate model for screening chemicals that impair thyroid hormone synthesis.  Environ Sci Technol. 2011. 45(17):7525-32.

Yi X, Yamamoto K, Shu L, Katoh R, Kawaoi A. Effects of Propyithiouracil (PTU) Administration on the Synthesis and Secretion of Thyroglobulin in the Rat Thyroid Gland: A Quantitative Immuno-electron Microscopic Study Using Immunogold Technique. Endocr Pathol. 1997 Winter;8(4):315-325.

Zoeller RT.  Interspecies differences in susceptibility to perturbation of thyroid hormone homeostasis requires a definition of "sensitivity" that is informative for risk analysis. Regul Toxicol Pharmacol. 2004 Dec;40(3):380.

Zoeller RT, Crofton KM. 2005.  Mode of action: developmental thyroid hormone insufficiency--neurological abnormalities resulting from exposure to propylthiouracil. Crit Rev Toxicol. 35:771-81

Zoeller RT, Tan SW, Tyl RW. 2007. General background on the hypothalamic-pituitary-thyroid (HPT) axis. Critical reviews in toxicology.  37:11-53.

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

Short Name: T4 in serum, Decreased

Key Event Component

Process Object Action
abnormal circulating thyroxine level thyroxine decreased

AOPs Including This Key Event

AOP ID and Name Event Type
Aop:42 - Inhibition of Thyroperoxidase and Subsequent Adverse Neurodevelopmental Outcomes in Mammals KeyEvent
Aop:54 - Inhibition of Na+/I- symporter (NIS) leads to learning and memory impairment KeyEvent
Aop:8 - Upregulation of Thyroid Hormone Catabolism via Activation of Hepatic Nuclear Receptors, and Subsequent Adverse Neurodevelopmental Outcomes in Mammals KeyEvent
Aop:65 - XX Inhibition of Sodium Iodide Symporter and Subsequent Adverse Neurodevelopmental Outcomes in Mammals KeyEvent
Aop:134 - Sodium Iodide Symporter (NIS) Inhibition and Subsequent Adverse Neurodevelopmental Outcomes in Mammals KeyEvent
Aop:152 - Interference with thyroid serum binding protein transthyretin and subsequent adverse human neurodevelopmental toxicity KeyEvent
Aop:159 - Thyroperoxidase inhibition leading to increased mortality via reduced anterior swim bladder inflation KeyEvent
Aop:175 - Thyroperoxidase inhibition leading to altered amphibian metamorphosis KeyEvent
Aop:176 - Sodium Iodide Symporter (NIS) Inhibition leading to altered amphibian metamorphosis KeyEvent
Aop:194 - Hepatic nuclear receptor activation leading to altered amphibian metamorphosis KeyEvent
Aop:366 - Competitive binding to thyroid hormone carrier protein transthyretin (TTR) leading to altered amphibian metamorphosis KeyEvent
Aop:367 - Competitive binding to thyroid hormone carrier protein thyroid binding globulin (TBG) leading to altered amphibian metamorphosis KeyEvent
Aop:363 - Thyroperoxidase inhibition leading to increased mortality via altered eye structure KeyEvent

Stressors

Name
Propylthiouracil
Methimazole

Biological Context

Level of Biological Organization
Tissue

Organ term

Organ term
serum

Evidence for Perturbation by Stressor

Propylthiouracil

6-n-propylthouracil is a classic positive control for inhibition of TPO

Perchlorate

Perchlorate ion (ClO− ₄) is a classic positive control for inhibition of NIS

Methimazole

Classic positive control

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
human Homo sapiens High NCBI
rat Rattus norvegicus High NCBI
mouse Mus musculus High NCBI
chicken Gallus gallus Moderate NCBI
Xenopus laevis Xenopus laevis Moderate NCBI
Pig Pig High NCBI
zebrafish Danio rerio High NCBI
fathead minnow Pimephales promelas High NCBI
Life Stage Applicability
Life Stage Evidence
All life stages High
Sex Applicability
Sex Evidence
Female High
Male High

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 zebrafish (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.

Key Event Description

All iodothyronines are derived from the modification of tyrosine molecules (Taurog, 2000). There are two biologically active thyroid hormones (THs) in serum, triiodothyronine (T3) and T4, and a few inactive iodothyronines (rT3, 3,5-T2).  T4 is the predominant TH in circulation, comprising approximately 80% of the TH excreted from the thyroid gland and is the pool from which the majority of T3 in serum is generated (Zoeller et al., 2007). As such, serum T4 changes usually precede changes in other serum THs.  Decreased thyroxine (T4) in serum results result from one or more MIEs upstream and is considered a key biomarker of altered TH homeostasis (DeVito et al., 1999). 

Serum T4 is used as a biomarker of TH status because the circulatory system serves as the major transport and delivery system for TH delivery to tissues. The majority of THs in the blood are bound to transport proteins (Bartalena and Robbins, 1993). In serum, it is the unbound, or ‘free’ form of the hormone that is thought to be available for transport into tissues. Free hormones are approximately 0.03 and 0.3 percent for T4 and T3, respectively. There are major species differences in the predominant binding proteins and their affinities for THs (see below). However, there is broad agreement that changes in serum concentrations of THs is diagnostic of thyroid disease or chemical-induced disruption of thyroid homeostasis (DeVito et al., 1999; Miller et al., 2009; Zoeller et al., 2007).

Normal serum T4 reference ranges can be species and lifestage specific. In rodents, serum THs are low in the fetal circulation, increasing as the fetal thyroid gland becomes functional on gestational day 17, just a few days prior to birth. After birth serum hormones increase steadily, peaking at two weeks, and falling slightly to adult levels by postnatal day 21 (Walker et al., 1980; Harris et al., 1978; Goldey et al., 1995; Lau et al., 2003). Similarly, in humans, adult reference ranges for THs do not reflect the normal ranges for children at different developmental stages, with TH concentrations highest in infants, still increased in childhood, prior to a decline to adult levels coincident with pubertal development (Corcoran et al. 1977; Kapelari et al., 2008). In some frog species, there is an analogous peak in thyroid hormones in tadpoles that starts around embryonic NF stage 56, peaks at Stage 62 and the declines to lower levels by Stage 56 (Sternberg et al., 2011; Leloup and Buscaglia, 1977). 

How it is Measured or Detected

Serum T3 and T4 can be measured as free (unbound) or total (bound + unbound). Free hormone concentrations are clinically considered more direct indicators of T4 and T3 activities in the body, but in animal studies, total T3 and T4 are typically measured. Historically, the most widely used method in toxicology is the radioimmunoassay (RIA). The method is routinely used in rodent endocrine and toxicity studies. The ELISA method is a commonly used as a human clinical test method. Analytical determination of iodothyronines (T3, T4, rT3, T2) and their conjugates, though methods employing HPLC, liquid chromatography, immuno luminescence, and mass spectrometry are less common, but are becoming increasingly available (Hornung et al., 2015; DeVito et al., 1999; Baret and Fert, 1989; Spencer, 2013; Samanidou V.F et al., 2000; Rathmann D. et al., 2015 ).  It is important to note that thyroid hormones concentrations can be influenced by a number of intrinsic and extrinsic factors (e.g., circadian rhythms, stress, food intake, housing, noise) (see for example, Döhler et al., 1979).

Any of these measurements should be evaluated for the relationship to the actual endpoint of interest, repeatability, reproducibility, and lower limits of quantification using a fit-for-purpose approach (i.e., different regulatory needs will require different levels of confidence in the AOP). This is of particular significance when assessing the very low levels of TH present in fetal serum. Detection limits of the assay must be compatible with the levels in the biological sample.  All three of the methods summarized above would be fit-for-purpose, depending on the number of samples to be evaluated and the associated costs of each method. Both RIA and ELISA measure THs by an indirect methodology, whereas analytical determination is the most direct measurement available. All these methods, particularly RIA, are repeatable and reproducible.

 

References

Axelrad DA, Baetcke K, Dockins C, Griffiths CW, Hill RN, Murphy PA, Owens N, Simon NB, Teuschler LK. Risk assessment for benefits analysis: framework for analysis of a thyroid-disrupting chemical. J Toxicol Environ Health A. 2005 68(11-12):837-55.

Baret A. and Fert V.  T4 and ultrasensitive TSH immunoassays using luminescent enhanced xanthine oxidase assay. J Biolumin Chemilumin. 1989. 4(1):149-153

Bartalena L, Robbins J. Thyroid hormone transport proteins. Clin Lab Med. 1993 Sep;13(3):583-98. Bassett JH, Harvey CB, Williams GR. (2003). Mechanisms of thyroid hormone receptor-specific nuclear and extra nuclear actions. Mol Cell Endocrinol. 213:1-11.

Capen CC. Mechanistic data and risk assessment of selected toxic end points of the thyroid gland. Toxicol Pathol. 1997 25(1):39-48.

Cope RB, Kacew S, Dourson M. A reproductive, developmental and neurobehavioral study following oral exposure of tetrabromobisphenol A on Sprague-Dawley rats. Toxicology. 2015 329:49-59.

Corcoran JM, Eastman CJ, Carter JN, Lazarus L. (1977). Circulating thyroid hormone levels in children. Arch Dis Child. 52: 716-720.

Crofton KM. Developmental disruption of thyroid hormone: correlations with hearing dysfunction in rats. Risk Anal. 2004 Dec;24(6):1665-71.

DeVito M, Biegel L, Brouwer A, Brown S, Brucker-Davis F, Cheek AO, Christensen R, Colborn T, Cooke P, Crissman J, Crofton K, Doerge D, Gray E, Hauser P, Hurley P, Kohn M, Lazar J, McMaster S, McClain M, McConnell E, Meier C, Miller R, Tietge J, Tyl R. (1999). Screening methods for thyroid hormone disruptors. Environ Health Perspect. 107:407-415.

Döhler KD, Wong CC, von zur Mühlen A (1979).   The rat as model for the study of drug effects on thyroid function: consideration of methodological problems.  Pharmacol Ther B. 5:305-18.

Eales JG. (1997). Iodine metabolism and thyroid related functions in organisms lacking thyroid follicles: Are thyroid hormones also vitaminsProc Soc Exp Biol Med. 214:302-317.

Furlow JD, Neff ES. (2006). A developmental switch induced by thyroid hormone: Xenopus laevis metamorphosis. Trends Endocrinol Metab. 17:40–47.

Goldey ES, Crofton KM. Thyroxine replacement attenuates hypothyroxinemia, hearing loss, and motor deficits following developmental exposure to Aroclor 1254 in rats. Toxicol Sci. 1998 45(1):94-10

Goldey ES, Kehn LS, Lau C, Rehnberg GL, Crofton KM.  Developmental exposure to polychlorinated biphenyls (Aroclor 1254) reduces circulating thyroid hormone concentrations and causes hearing deficits in rats. Tox Appl Pharmacol. 1995 135(1):77-88.

Harris AR, Fang SL, Prosky J, Braverman LE, Vagenakis AG.  Decreased outer ring monodeiodination of thyroxine and reverse triiodothyronine in the fetal and neonatal rat.  Endocrinology. 1978 Dec;103(6):2216-22

Heyland A, Hodin J. (2004). Heterochronic developmental shift caused by thyroid hormone in larval sand dollars and its implications for phenotypic plasticity and the evolution of non-feeding development. Evolution. 58: 524-538.

Heyland A, Moroz LL. (2005). Cross-kingdom hormonal signaling: an insight from thyroid hormone functions in marine larvae. J Exp Biol. 208:4355-4361.

 Hill RN, Crisp TM, Hurley PM, Rosenthal SL, Singh DV. Risk assessment of thyroid follicular cell tumors.  Environ Health Perspect. 1998 Aug;106(8):447-57.

Hornung MW, Kosian P, Haselman J, Korte J, Challis K, Macherla C, Nevalainen E, Degitz S (2015) In vitro, ex vivo and in vivo determination of thyroid hormone modulating activity of benzothiazoles. Toxicol Sci 146:254-264.

Hulbert AJ. Thyroid hormones and their effects: a new perspective. Biol Rev Camb Philos Soc. 2000 Nov;75(4):519-631. Review.

Kapelari K, Kirchlechner C, Högler W, Schweitzer K, Virgolini I, Moncayo R. 2008. Pediatric reference intervals for thyroid hormone levels from birth to adulthood: a retrospective study. BMC Endocr Disord. 8: 15.

Lau C, Thibodeaux JR, Hanson RG, Rogers JM, Grey BE, Stanton ME, Butenhoff JL, Stevenson LA.  Exposure to perfluorooctane sulfonate during pregnancy in rat and mouse. II: postnatal evaluation.  Toxicol Sci. 2003 Aug;74(2):382-92.

Leloup, J., and M. Buscaglia. La triiodothyronine: hormone de la métamorphose des amphibiens. CR Acad Sci 284 (1977): 2261-2263.

Liu J, Liu Y, Barter RA, Klaassen CD.: Alteration of thyroid homeostasis by UDP-glucuronosyltransferase inducers in rats: a dose-response study. J Pharmacol Exp Ther 273, 977-85, 1994

Manzon RG, Youson JH. (1997). The effects of exogenous thyroxine (T4) or triiodothyronine (T3), in the presence and absence of potassium perchlorate, on the incidence of metamorphosis and on serum T4 and T3 concentrations in larval sea lampreys (Petromyzon marinus L.). Gen Comp Endocrinol. 106:211-220. 

McClain RM. Mechanistic considerations for the relevance of animal data on thyroid neoplasia to human risk assessment. Mutat Res. 1995 Dec;333(1-2):131-42

Miller MD, Crofton KM, Rice DC, Zoeller RT.  Thyroid-disrupting chemicals: interpreting upstream biomarkers of adverse outcomes. Environ Health Perspect. 2009 117(7):1033-41

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Zoeller, R. T., R. Bansal, et al. (2005). "Bisphenol-A, an environmental contaminant that acts as a thyroid hormone receptor antagonist in vitro, increases serum thyroxine, and alters RC3/neurogranin expression in the developing rat brain." Endocrinology 146(2): 607-612.

Zoeller RT, Tan SW, Tyl RW. General background on the hypothalamic-pituitary-thyroid (HPT) axis. Crit Rev Toxicol. 2007 Jan-Feb;37(1-2):11-53

Event: 1003: Decreased, Triiodothyronine (T3) in serum

Short Name: Decreased, Triiodothyronine (T3) in serum

Key Event Component

Process Object Action
abnormal circulating hormone level 3,3',5'-triiodothyronine decreased

AOPs Including This Key Event

Biological Context

Level of Biological Organization
Tissue

Organ term

Organ term
serum

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
zebrafish Danio rerio High NCBI
fathead minnow Pimephales promelas High NCBI
African clawed frog Xenopus laevis High NCBI
Life Stage Applicability
Life Stage Evidence
All life stages High
Sex Applicability
Sex Evidence
Unspecific High

Taxonomic: The overall evidence supporting taxonomic applicability is strong. With few exceptions vertebrate species have circulating T3 and T4 that are bound to transport proteins in blood. Clear species differences exist in transport proteins (Yamauchi and Isihara, 2009). Specifically, the majority of supporting data for TH decreases in serum come from rat studies, and the predominant iodothyronine binding protein in rat serum is transthyretin (TT4). TT4 demonstrates a reduced binding affinity for T4 when compared with thyroxine binding globulin (TBG), the predominant serum binding protein for T4 in humans. This difference in serum binding protein affinity for THs is thought to modulate serum half-life for T4; the half-life of T4 in rats is 12-24 hr, wherease the half-life in humans is 5-9 days (Capen, 1997). While these species differences impact hormone half-life, possibly regulatory feedback mechanisms, and quantitative dose-response relationships, measurement of serum THs is still regarded as a measurable key event causatively linked to downstream adverse outcomes.

THs are evolutionarily conserved molecules present in all vertebrate species (Hulbert, 2000; Yen, 2001). Moreover, their crucial role in amphibian and larbean metamorphoses is well established (Manzon and Youson, 1997; Yaoita and Brown, 1990). Their existence and importance has been also described in many differrent 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 may differ depending on the expression or function of specific proteins (e.g receptors or enzymes) that are related to TH function, and therefore extrapolation between species should be done with cautious.

Life stage: Thyroid hormones are essential in all life stages, but elevations of circulating levels are associated with specific developmental events.

Sex: Thyroid hormones are essential in both sexes.

 

Key Event Description

There are two biologically active thyroid hormones (THs), triiodothyronine (T3) and thyroxine (T4), and a few less active iodothyronines (rT3, 3,5-T2), which are all derived from the modification of tyrosine molecules (Hulbert, 2000). However, the plasma concentrations of the other iodothyronines are significantly lower than those of T3 and T4. The different iodothyronines are formed by the sequential outer or inner ring monodeiodination of T4 by the deiodinating enzymes, Dio1, Dio2, and Dio3 (Gereben et al., 2008). Deiodinase structure is considered to be unique, as THs are the only molecules in the body that incorporate iodide.

The circulatory system serves as the major transport and delivery system for THs from synthesis in the gland to delivery to tissues. The majority of THs in the blood are bound to transport proteins (Bartalena and Robbins, 1993). In humans, the major transport proteins are TBG (thyroxine binding globulin), TTR (transthyretin) and albumin. The percent bound to these proteins in adult humans is about 75, 15 and 10 percent, respectively (Schussler 2000). Unbound (free) hormones are approximately 0.03 and 0.3 percent for T4 and T3, respectively. In serum, it is the free form of the hormone that is active.

There are major species differences in the predominant binding proteins and their affinities for THs (see section below on Taxonomic applicability). However, there is broad agreement that changes in serum concentrations of THs is diagnostic of thyroid disease or chemical-induced disruption of thyroid homeostasis (Zoeller et al., 2007).

It is notable that the changes measured in the TH concentration reflect mainly the changes in the serum transport proteins rather than changes in the thyroid status. These thyroid-binding proteins serve as hormonal store which ensure their even and constant distribution in the different tissues, while they protect the most sensitive ones in the case of severe changes in thyroid availability, like in thyroidectomies (Obregon et al., 1981). Until recently, it was believed that all of the effects of TH were mediated by the binding of T3 to the thyroid nuclear receptors (TRa and TRb), a notion which is now questionable due to the increasing evidence that support the non-genomic action of TH (Davis et al., 2010, Moeller et al., 2006). Many non-nuclear TH binding sites have been identified to date and they usually lead to rapid cellular response in TH-effects (Bassett et al., 2003), but the specific pathways that are activated in this regard need to be elucidated.

The production of THs in the thyroid gland and the circulation levels in the bloodstream are self-controlled by an efficiently regulated feedback mechanism across the Hypothalamus-Pituitary-Thyroid (HPT) axis. One of the most unique characteristics of TH is their ability to regulate their own concentration, not only in the plasma level, but also in the individual cell level, to maintain their homeostasis. This is succeed by the efficient regulatory mechanism of the thyroid hormone axis which consists of the following: (1) the hypothalamic secretion of the thyrotropin-releasing hormone (TRH), (2) the thyroid-stimulating hormone (TSH) secretion from the anterior pituitary, (3) hormonal transport by the plasma binding proteins, (4) cellular uptake mechanisms in the cell level, (5) intracellular control of TH concentration by the deiodinating mechanism (6) transcriptional function of the nuclear thyroid hormone receptor and (7) in the fetus, the transplacental passage of T4 and T3 (Cheng et al., 2010).

In regards to the brain, the TH concentration involves also an additional level of regulation, namely the hormonal transport through the Blood Brain Barrier (BBB) (Williams, 2008). The TRH and the TSH are actually regulating the production of pro-hormone T4 and in a lesser extent of T3, which is the biologically active TH. The rest of the required amount of T3 is produced by outer ring deiodination of T4 by the deiodinating enzymes D1 and D2 (Bianco et al., 2006), a process which takes place mainly in liver and kidneys but also in other target organs such as in the brain, the anterior pituitary, brown adipose tissue, thyroid and skeletal muscle (Gereben et al., 2008; Larsen, 2009). Both hormones exert their action in almost all tissues of mammals and they are acting intracellularly, and thus the uptake of T3 and T4 by the target cells is a crucial step of the overall pathway. The trans-membrane transport of TH is performed mainly through transporters that differ depending on the cell type (Hennemann et al., 2001; Friesema et al., 2005; Visser et al., 2008). Many transporter proteins have been identified up to date but the monocarboxylate transporters (Mct8, Mct10) and the anion-transporting polypeptide (OATP1c1) show the highest degree of affinity towards TH (Jansen et al., 2005).

T3 and T4 have significant effects on normal development, neural differentiation, growth rate and metabolism (Yen, 2001; Brent, 2012; Williams, 2008), with the most prominent ones to occur during the fetal development and early childhood. The clinical features of hypothyroidism and hyperthyroidism emphasize the pleiotropic effects of these hormones on many different pathways and target organs. The thyroidal actions though are not only restricted to mammals, as their high significance has been identified also for other vertebrates, with the most well-studied to be the amphibian metamorphosis (Furlow and Neff, 2006). The importance of the thyroid-regulated pathways becomes more apparent in iodine deficient areas of the world, where a higher rate of cretinism and growth retardation has been observed and linked to decreased TH levels (Gilbert et al., 2012). Another very common cause of severe hypothyroidism in human is the congenital hypothyroidism, but the manifestation of these effects is only detectable in the lack of adequate treatment and is mainly related to neurological impairment and growth retardation (Glinoer, 2001), emphasizing the role of TH in neurodevelopment in all above cases. In adults, the thyroid-related effects are mainly linked to metabolic activities, such as deficiencies in oxygen consumption, and in the metabolism of the vitamin, proteins, lipids and carbohydrates, but these defects are subtle and reversible (Oetting and Yen, 2007). Blood tests to detect the amount of thyroid hormone (T4) and thyroid stimulating hormone (TSH) are routinely done for newborn babies for the diagnosis of congenital hypothyroidism at the earliest stage possible.

How it is Measured or Detected

T3 and T4 can be measured as free (unbound) or total (bound + unbound). Free hormone are considered more direct indicators of T4 and T3 activities in the body. The majority of T3 and T4 measurements are made using either RIA or ELISA kits. In animal studies, total T3 and T4 are typically measured as the concentrations of free hormone are very low and difficult to detect. Historically, the most widely used method in toxicology is RIA. The method is routinely used in rodent endocrine and toxicity studies. The ELISA method has become more routine in rodent studies. The ELISA method is a commonly used as a human clinical test method.

Recently, analytical determination of iodothyronines (T3, T4, rT3, T2) and their conjugates through methods employing HLPC and mass spectrometry have become more common (DeVito et al., 1999; Miller et al., 2009; Hornung et al., 2015; Nelson et al., 2016; Stinckens et al., 2016).

Any of these measurements should be evaluated for fit-for-purpose, relationship to the actual endpoint of interest, repeatability, and reproducibility. All three of the methods summarized above would be fit-for-purpose, depending on the number of samples to be evaluated and the associated costs of each method. Both RIA and ELISA measure THs by a an indirect methodology, whereas analytical determination is the most direct measurement available. All of these methods, particularly RIA, are repeatable and reproducible.

References

  • Bartalena L, Robbins J.Thyroid hormone transport proteins.Clin Lab Med. 1993 Sep;13(3):583-98.
  • Bassett JH, Harvey CB, Williams GR. (2003). Mechanisms of thyroid hormone receptor-specific nuclear and extra nuclear actions. Mol Cell Endocrinol. 213:1-11.
  • Bianco AC, Kim BW. (2006). Deiodinases: implications of the local control of thyroid hormone action. J Clin Invest. 116: 2571–2579.
  • Brent GA. (2012). Mechanisms of thyroid hormone action. J Clin Invest. 122: 3035-3043.
  • Cheng SY, Leonard JL, Davis PJ. (2010).Molecular aspects of thyroid hormone actions. Endocr Rev. 31:139–170.
  • Davis PJ, Zhou M, Davis FB, Lansing L, Mousa SA, Lin HY. (2010). Mini-review: Cell surface receptor for thyroid hormone and nongenomic regulation of ion fluxes in excitable cells. Physiol Behav. 99:237–239.
  • DeVito M, Biegel L, Brouwer A, Brown S, Brucker-Davis F, Cheek AO, Christensen R, Colborn T, Cooke P, Crissman J, Crofton K, Doerge D, Gray E, Hauser P, Hurley P, Kohn M, Lazar J, McMaster S, McClain M, McConnell E, *Meier C, Miller R, Tietge J, Tyl R. (1999). Screening methods for thyroid hormone disruptors. Environ Health Perspect. 107:407-415.
  • Eales JG. (1997). Iodine metabolism and thyroid related functions in organisms lacking thyroid follicles: Are thyroid hormones also vitamins? Proc Soc Exp Biol Med. 214:302-317.
  • Friesema EC, Jansen J, Milici C, Visser TJ. (2005). Thyroid hormone transporters. Vitam Horm. 70: 137–167.
  • Furlow JD, Neff ES. (2006). A developmental switch induced by thyroid hormone: Xenopus laevis metamorphosis. Trends Endocrinol Metab. 17:40–47.
  • Gereben B, Zavacki AM, Ribich S, Kim BW, Huang SA, Simonides WS, Zeöld A, Bianco AC. (2008). Cellular and molecular basis of deiodinase-regulated thyroid hormone signalling. Endocr Rev. 29:898–938.
  • Gilbert ME, Rovet J, Chen Z, Koibuchi N. (2012).Developmental thyroid hormone disruption: prevalence, environmental contaminants and neurodevelopmental consequences. Neurotoxicology. 33: 842-852.
  • Glinoer D. (2001).Potential consequences of maternal hypothyroidism on the offspring: evidence and implications. Horm Res. 55:109-114.
  • Hennemann G, Docter R, Friesema EC, de Jong M, Krenning EP, Visser TJ. (2001). Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability. Endocr Rev. 22:451-476.
  • Heyland A, Hodin J. (2004). Heterochronic developmental shift caused by thyroid hormone in larval sand dollars and its implications for phenotypic plasticity and the evolution of non-feeding development. Evolution. 58: 524-538.
  • Heyland A, Moroz LL. (2005). Cross-kingdom hormonal signaling: an insight from thyroid hormone functions in marine larvae. J Exp Biol. 208:4355-4361.
  • Hornung, M.W., Kosian, P.A., Haselman, J.T., Korte, J.J., Challis, K., Macherla, C., Nevalainen, E., Degitz, S.J., 2015. In Vitro, Ex Vivo, and In Vivo Determination of Thyroid Hormone Modulating Activity of Benzothiazoles. Toxicological Sciences 146, 254-264.
  • Hulbert A J. (2000). Thyroid hormones and their effects: A new perspective. Biol Rev. 75: 519-631.
  • Jansen J, Friesema EC, Milici C, Visser TJ. (2005). Thyroid hormone transporters in health and disease. Thyroid. 15: 757-768.
  • Larsen PR. (2009).Type 2 iodothyronine deiodinase in human skeletal muscle: new insights into its physiological role and regulation. J Clin Endocrinol Metab. 94:1893-1895.
  • Manzon RG, Youson JH. (1997). The effects of exogenous thyroxine (T4) or triiodothyronine (T3), in the presence and absence of potassium perchlorate, on the incidence of metamorphosis and on serum T4 and T3 concentrations in larval sea lampreys (Petromyzon marinus L.). Gen Comp Endocrinol. 106:211-220.
  • Miller MD, Crofton KM, Rice DC, Zoeller RT. (2009).Thyroid-disrupting chemicals: interpreting upstream biomarkers of adverse outcomes. Environ Health Perspect. 117:1033-1041.
  • Moeller LC, Dumitrescu AM, Seo H, Refetoff S. (2006). Thyroid hormone mediated changes in gene expression can be initiated by cytosolic action of the thyroid hormone receptor β through the phosphatidylinositol 3-kinase pathway. NRS. 4:1-4.Nelson, K., Schroeder, A., Ankley, G., Blackwell, B., Blanksma, C., Degitz, S., Flynn, K., Jensen, K., Johnson, R., Kahl, M., Knapen, D., Kosian, P., Milsk, R., Randolph, E., Saari, T., Stinckens, E., Vergauwen, L., Villeneuve, D., 2016. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow. Aquatic Toxicology 173, 192-203.Obregon MJ, Mallol J, Escobar del Rey F, Morreale de Escobar G. (1981). Presence of l-thyroxine and 3,5,3-triiodo-l-thyronine in tissues from thyroidectomised rats. Endocrinology 109:908-913.
  • Oetting A, Yen PM. (2007). New insights into thyroid hormone action. Best Pract Res Clin Endocrinol Metab. 21:193–208.
  • Schussler, G.C. (2000). The thyroxine-binding proteins. Thyroid 10:141–149.
  • Stinckens, E., Vergauwen, L., Schroeder, A., Maho, W., Blackwell, B., Witters, H., Blust, R., Ankley, G., Covaci, A., Villeneuve, D., 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.
  • Visser WE, Friesema EC, Jansen J, Visser TJ. (2008). Thyroid hormone transport in and out of cells. Trends Endocrinol Metab. 19:50-56.
  • Williams GR. (2008). Neurodevelopmental and neurophysiological actions of thyroid hormone. J Neuroendocrinol. 20:784–794.
  • Yamauchi K1, Ishihara A. Evolutionary changes to transthyretin: developmentally regulated and tissue-specific gene expression.FEBS J. 2009 Oct;276(19):5357-66.
  • Yaoita Y, Brown DD. (1990). A correlation of thyroid hormone receptor gene expression with amphibian metamorphosis. Genes Dev. 4:1917-1924.
  • Yen PM. (2001). Physiological and molecular basis of thyroid hormone action. Physiol Rev. 81:1097-1142.
  • Zoeller RT, Tan SW, Tyl RW. General background on the hypothalamic-pituitary-thyroid (HPT) axis. Crit Rev Toxicol. 2007 Jan-Feb;37(1-2):11-53

 

Event: 1007: Reduced, Anterior swim bladder inflation

Short Name: Reduced, Anterior swim bladder inflation

Key Event Component

Process Object Action
swim bladder inflation anterior chamber swim bladder decreased

AOPs Including This Key Event

Biological Context

Level of Biological Organization
Organ

Organ term

Organ term
swim bladder

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
zebrafish Danio rerio High NCBI
fathead minnow Pimephales promelas High NCBI
Life Stage Applicability
Life Stage Evidence
larvae High
Sex Applicability
Sex Evidence
Unspecific High

Taxonomic: Teleost fish can be divided in two groups according to swim bladder morphology: physoclistous (e.g., yellow perch) and physostomus (e.g., zebrafish and fathead minnow). Physostomus fish retain a duct between the digestive tract and the swim bladder during adulthood allowing them to gulp air at the surface to fill the swim bladder. In contrast, in physoclistous fish, once initial inflation by gulping atmospheric air at the water surface has occurred, the swim bladder is closed off from the digestive tract and swim bladder volume is regulated by gas secretion into the swim bladder (Woolley and Qin, 2010). The evidence for impaired inflation of the anterior chamber of the swim bladder currently comes from work on zebrafish and fathead minnow (Stinckens et al., 2016; Nelson et al., 2016; Cavallin et al., 2017; Godfrey et al., 2017; Stinckens et al., 2020). While zebrafish and fathead minnows are physostomous fish with a two-chambered swim bladder, the Japanese rice fish (Oryzias latipes) is a physoclistous fish with a single chambered swim bladder that inflates during early development. The key event 'reduced anterior chamber inflation' is not applicable to such fish species.

Life stage: The anterior chamber inflates during a specific developmental time frame. In zebrafish, the anterior chamber inflates around 21 days post fertilization (dpf) which is during the larval stage. In the fathead minnow, the anterior chamber inflates around 14 dpf, also during the larval stage. Therefore this KE is only applicable to the larval life stage.

Sex: Zebrafish are undifferentiated gonochorists since both sexes initially develop an immature ovary (Maack and Segner, 2003). Immature ovary development progresses until approximately the onset of the third week. Later, in female fish immature ovaries continue to develop further, while male fish undergo transformation of ovaries into testes. Final transformation into testes varies among male individuals, however finishes usually around 6 weeks post fertilization. Since the anterior chamber inflates around 21 days post fertilization, sex differences are expected to play a minor role.

Key Event Description

The swim bladder of bony fish is evolutionary homologous to the lung (Zheng et al., 2011). The teleost swim bladder is a gas-filled structure that consists of two chambers, the posterior and anterior chamber. In zebrafish, the posterior chamber inflates around 96 h post fertilization (hpf) which is 2 days post hatch, and the anterior chamber inflates around 21 dpf. In fathead minnow, the posterior and anterior chamber inflate around 6 and 14 dpf respectively. Inflation of the anterior swim bladder chamber is part of the larval-to-juvenile transition in fish, together with the development of adult fins and fin rays, ossification of the axial skeleton, formation of an adult pigmentation pattern, scale formation, maturation and remodeling of organs including the lateral line, nervous system, gut and kidneys (McMenamin and Parichy, 2013).

The anterior chamber is formed by evagination from the cranial end of the posterior chamber (Robertson et al., 2007). Dumbarton et al. (2010) showed that the anterior chamber of zebrafish has particularly closely packed and highly organized bundles of muscle fibres, suggesting that contraction of these muscles would reduce swim bladder volume. While it had previously been suggested that the posterior chamber had a more important role as a hydrostatic organ, this implies high importance of the anterior chamber for buoyancy. The anterior chamber has an additional role in hearing (Bang et al., 2002). Weberian ossicles (the Weberian apparatus) connect the anterior chamber to the inner ear resulting in an amplification of sound waves. Reduced inflation of the anterior chamber may manifest itself as either a complete failure to inflate the chamber or reduced size of the chamber. Reduced size is often associated with a deviating morphology.

How it is Measured or Detected

In several fish species, inflation of the anterior chamber can be observed using a stereomicroscope because the larvae are still transparent during the larval stage. This is for example true for zebrafish and fathead minnow. Anterior chamber size can then be measured based on photographs with a calibrator.

References

  • Bang, P.I., Yelick, P.C., Malicko, 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.
  • Cavallin, J.E., Ankley, G.T., Blackwell, B.R., Blanksma, C.A., Fay, K.A., Jensen, K.M., Kahl, M.D., Knapen, D., Kosian, P.A., Poole, S.T., Randolph, E.C., Schroeder, A.L., Vergauwen, L., Villeneuve, D.L., 2017. Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid. Environmental Toxicology and Chemistry 36, 2942-2952.
  • Dumbarton, T.C., Stoyek, M., Croll, R.P., Smith, F.M., 2010. Adrenergic control of swimbladder deflation in the zebrafish (Danio rerio). J. Exp. Biol. 213,2536–2546, http://dx.doi.org/10.1242/jeb.039792.
  • Godfrey, A., Hooser, B., Abdelmoneim, A., Horzmann, K.A., Freemanc, J.L., Sepulveda, M.S., 2017. Thyroid disrupting effects of halogenated and next generation chemicals on the swim bladder development of zebrafish. Aquatic Toxicology 193, 228-235.
  • McMenamin, S.K., Parichy, D.M., 2013. Metamorphosis in Teleosts. Animal Metamorphosis 103, 127-165.
  • 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.
  • 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, A., Maho, W., Blackwell, B., Witters, H., Blust, R., Ankley, G., Covaci, A., Villeneuve, D., 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.
  • Stinckens, E., Vergauwen, L., Blackwell, B.R., Anldey, G.T., Villeneuve, D.L., Knapen, D., 2020. Effect of Thyroperoxidase and Deiodinase Inhibition on Anterior Swim Bladder Inflation in the Zebrafish. Environmental Science & Technology 54, 6213-6223.
  • Woolley, L.D., Qin, J.G., 2010. Swimbladder inflation and its implication to the culture of marine finfish larvae. Reviews in Aquaculture 2, 181-190.
  • Zheng, W., Wang, Z., Collins, J.E., Andrews, R.M., Stemple, D., Gong, Z. 2011. Comparative transcriptome analyses indicate molecular homology of zebrafish swim bladder and mammalian lung. PLoS One 6, http://dx.doi.org/10.1371/

Event: 1005: Reduced, Swimming performance

Short Name: Reduced, Swimming performance

Key Event Component

Process Object Action
aquatic locomotion decreased

AOPs Including This Key Event

Biological Context

Level of Biological Organization
Individual

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
zebrafish Danio rerio High NCBI
teleost fish teleost fish High NCBI
fathead minnow Pimephales promelas High NCBI
Life Stage Applicability
Life Stage Evidence
All life stages High
Sex Applicability
Sex Evidence
Unspecific High

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

Life stage: Importance of swimming performance for natural behaviour is generally applicable across all life stages.

Sex: Importance of swimming performance for natural behaviour is generally applicable across sexes.

Key Event Description

Adequate swimming performance in fish is essential for behaviour such as foraging, predator avoidance and reproduction.

How it is Measured or Detected

For fish larvae, automated observation and tracking systems are commercially available and increasingly used for measuring swimming performance including distance travelled, duration of movements, swimming speed, etc. This kind of measurements is often included in publications describing effects of chemicals in zebrafish larvae (Hagenaars et al., 2014; Stinckens et al., 2016; Vergauwen et al., 2015).

For juvenile and adult fish, measurements of swim performance vary. However, in some circumstances, a swim tunnel has been used to measure various data (Fu et al., 2013).

References

Fu C, Cao ZD, Fu SJ. 2013. The effects of caudal fin loss and regeneration on the swimming performance of three cyprinid fish species with different swimming capactities. The Journal of Experimental Biology 216:3164-3174. doi:10.1242/jeb.084244

Hagenaars, A., Stinckens, E., Vergauwen, L., Bervoets, L., Knapen, D., 2014. PFOSaffects posterior swim bladder chamber inflation and swimming performanceof zebrafish larvae. Aquat. Toxicol. 157, 225–235.

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.

Vergauwen, Lucia; Nørgaard Schmidt, Stine; Maho, Walid; Stickens, Evelyn; Hagenaars, An; Blust, Ronny; Mayer, Philipp; Covaci, Adrian; Knapen, Dries. 2014. A high throughput passive dosing format for the Fish Embryo Acute Toxicity test. Chemosphere. 139: 9-17.

List of Adverse Outcomes in this AOP

Event: 351: Increased Mortality

Short Name: Increased Mortality

Key Event Component

Process Object Action
mortality increased

AOPs Including This Key Event

AOP ID and Name Event Type
Aop:16 - Acetylcholinesterase inhibition leading to acute mortality AdverseOutcome
Aop:96 - Axonal sodium channel modulation leading to acute mortality AdverseOutcome
Aop:104 - Altered ion channel activity leading impaired heart function AdverseOutcome
Aop:113 - Glutamate-gated chloride channel activation leading to acute mortality AdverseOutcome
Aop:160 - Ionotropic gamma-aminobutyric acid receptor activation mediated neurotransmission inhibition leading to mortality AdverseOutcome
Aop:161 - Glutamate-gated chloride channel activation leading to neurotransmission inhibition associated mortality AdverseOutcome
Aop:138 - Organic anion transporter (OAT1) inhibition leading to renal failure and mortality AdverseOutcome
Aop:177 - Cyclooxygenase 1 (COX1) inhibition leading to renal failure and mortality AdverseOutcome
Aop:186 - unknown MIE leading to renal failure and mortality AdverseOutcome
Aop:312 - Acetylcholinesterase Inhibition leading to Acute Mortality via Impaired Coordination & Movement​ AdverseOutcome
Aop:320 - Binding of viral S-glycoprotein to ACE2 receptor leading to acute respiratory distress associated mortality AdverseOutcome
Aop:155 - Deiodinase 2 inhibition leading to increased mortality via reduced posterior swim bladder inflation AdverseOutcome
Aop:156 - Deiodinase 2 inhibition leading to increased mortality via reduced anterior swim bladder inflation AdverseOutcome
Aop:157 - Deiodinase 1 inhibition leading to increased mortality via reduced posterior swim bladder inflation AdverseOutcome
Aop:158 - Deiodinase 1 inhibition leading to increased mortality via reduced anterior swim bladder inflation AdverseOutcome
Aop:159 - Thyroperoxidase inhibition leading to increased mortality via reduced anterior swim bladder inflation AdverseOutcome
Aop:363 - Thyroperoxidase inhibition leading to increased mortality via altered eye structure AdverseOutcome

Biological Context

Level of Biological Organization
Population

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
zebrafish Danio rerio High NCBI
Gallus gallus Gallus gallus High NCBI
fathead minnow Pimephales promelas High NCBI
Life Stage Applicability
Life Stage Evidence
All life stages High
Sex Applicability
Sex Evidence
Unspecific High

All living things are susceptible to mortality.

Key Event Description

Increased mortality refers to an increase in the number of individuals dying in an experimental replicate group or in a population over a specific period of time.

How it is Measured or Detected

Mortality is typically measured by observation. Lack of any heart beat, gill movement, and body movement are typical signs of death used in the evaluation of mortality of animals.

Mortality can be measured:

  • in the lab by recording mortality during prolonged exposure experiments
  • in dedicated mesocosms, or in drainable ponds
  • in the field, for example by determining age structure after one capture, or by capture-tag-recapture efforts

Regulatory Significance of the AO

Increased mortality is one of the most common regulatory assessment endpoints, along with reduced growth and reduced reproduction.

Event: 360: Decrease, Population trajectory

Short Name: Decrease, Population trajectory

Key Event Component

Process Object Action
population growth rate population of organisms decreased

AOPs Including This Key Event

AOP ID and Name Event Type
Aop:23 - Androgen receptor agonism leading to reproductive dysfunction (in repeat-spawning fish) AdverseOutcome
Aop:25 - Aromatase inhibition leading to reproductive dysfunction AdverseOutcome
Aop:29 - Estrogen receptor agonism leading to reproductive dysfunction AdverseOutcome
Aop:30 - Estrogen receptor antagonism leading to reproductive dysfunction AdverseOutcome
Aop:100 - Cyclooxygenase inhibition leading to reproductive dysfunction via inhibition of female spawning behavior AdverseOutcome
Aop:122 - Prolyl hydroxylase inhibition leading to reproductive dysfunction via increased HIF1 heterodimer formation AdverseOutcome
Aop:123 - Unknown MIE leading to reproductive dysfunction via increased HIF-1alpha transcription AdverseOutcome
Aop:155 - Deiodinase 2 inhibition leading to increased mortality via reduced posterior swim bladder inflation AdverseOutcome
Aop:156 - Deiodinase 2 inhibition leading to increased mortality via reduced anterior swim bladder inflation AdverseOutcome
Aop:157 - Deiodinase 1 inhibition leading to increased mortality via reduced posterior swim bladder inflation AdverseOutcome
Aop:158 - Deiodinase 1 inhibition leading to increased mortality via reduced anterior swim bladder inflation AdverseOutcome
Aop:159 - Thyroperoxidase inhibition leading to increased mortality via reduced anterior swim bladder inflation AdverseOutcome
Aop:101 - Cyclooxygenase inhibition leading to reproductive dysfunction via inhibition of pheromone release AdverseOutcome
Aop:102 - Cyclooxygenase inhibition leading to reproductive dysfunction via interference with meiotic prophase I /metaphase I transition AdverseOutcome
Aop:63 - Cyclooxygenase inhibition leading to reproductive dysfunction AdverseOutcome
Aop:103 - Cyclooxygenase inhibition leading to reproductive dysfunction via interference with spindle assembly checkpoint AdverseOutcome
Aop:292 - Inhibition of tyrosinase leads to decreased population in fish AdverseOutcome
Aop:310 - Embryonic Activation of the AHR leading to Reproductive failure, via epigenetic down-regulation of GnRHR AdverseOutcome
Aop:16 - Acetylcholinesterase inhibition leading to acute mortality AdverseOutcome
Aop:312 - Acetylcholinesterase Inhibition leading to Acute Mortality via Impaired Coordination & Movement​ AdverseOutcome
Aop:334 - Glucocorticoid Receptor Agonism Leading to Impaired Fin Regeneration AdverseOutcome
Aop:336 - DNA methyltransferase inhibition leading to population decline (1) AdverseOutcome
Aop:337 - DNA methyltransferase inhibition leading to population decline (2) AdverseOutcome
Aop:338 - DNA methyltransferase inhibition leading to population decline (3) AdverseOutcome
Aop:339 - DNA methyltransferase inhibition leading to population decline (4) AdverseOutcome
Aop:340 - DNA methyltransferase inhibition leading to transgenerational effects (1) AdverseOutcome
Aop:341 - DNA methyltransferase inhibition leading to transgenerational effects (2) AdverseOutcome
Aop:289 - Inhibition of 5α-reductase leading to impaired fecundity in female fish AdverseOutcome
Aop:297 - Inhibition of retinaldehyde dehydrogenase leads to population decline AdverseOutcome
Aop:346 - Aromatase inhibition leads to male-biased sex ratio via impacts on gonad differentiation AdverseOutcome
Aop:299 - Excessive reactive oxygen species production leading to population decline via reduced fatty acid beta-oxidation AdverseOutcome
Aop:311 - Excessive reactive oxygen species production leading to population decline via mitochondrial dysfunction AdverseOutcome
Aop:216 - Excessive reactive oxygen species production leading to population decline via follicular atresia AdverseOutcome
Aop:238 - Excessive reactive oxygen species production leading to population decline via lipid peroxidation AdverseOutcome
Aop:326 - Thermal stress leading to population decline (3) AdverseOutcome
Aop:325 - Thermal stress leading to population decline (2) AdverseOutcome
Aop:324 - Thermal stress leading to population decline (1) AdverseOutcome
Aop:363 - Thyroperoxidase inhibition leading to increased mortality via altered eye structure AdverseOutcome

Biological Context

Level of Biological Organization
Population

Domain of Applicability

Taxonomic Applicability
Term Scientific Term Evidence Links
all species all species High NCBI
Life Stage Applicability
Life Stage Evidence
All life stages Not Specified
Sex Applicability
Sex Evidence
Unspecific Not Specified

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

Key Event Description

Maintenance of sustainable fish and wildlife populations (i.e., adequate to ensure long-term delivery of valued ecosystem services) is an accepted regulatory goal upon which risk assessments and risk management decisions are based.

How it is Measured or Detected

Population trajectories, either hypothetical or site specific, can be estimated via population modeling based on measurements of vital rates or reasonable surrogates measured in laboratory studies. As an example, Miller and Ankley 2004 used measures of cumulative fecundity from laboratory studies with repeat spawning fish species to predict population-level consequences of continuous exposure.

Regulatory Significance of the AO

Maintenance of sustainable fish and wildlife populations (i.e., adequate to ensure long-term delivery of valued ecosystem services) is a widely accepted regulatory goal upon which risk assessments and risk management decisions are based.

References

  • Miller DH, Ankley GT. 2004. Modeling impacts on populations: fathead minnow (Pimephales promelas) exposure to the endocrine disruptor 17ß-trenbolone as a case study. Ecotoxicology and Environmental Safety 59: 1-9.

Appendix 2

List of Key Event Relationships in the AOP