Upstream eventT4 in serum, Decreased
Reduced, Anterior swim bladder inflation
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Thyroperoxidase inhibition leading to reduced young of year survival via anterior swim bladder inflation||adjacent|
|fathead minnow||Pimephales promelas||NCBI|
Life Stage Applicability
Key Event Relationship Description
Reduced T4 levels in serum prohibit local production of active T3 hormone by deiodinases expressed in the target tissues. There is evidence suggesting that anterior swim bladder inflation relies on increased thyroid hormone levels at this specific developmental time point.
While evidence supports a link between reduced serum T4 levels and reduced anterior swim bladder inflation, experimental evidence suggests that inhibition of T4 synthesis does not result in reduced posterior swim bladder inflation. The absence of effects on posterior chamber inflation is possibly due to maternal transfer of T4 into the eggs. These maternally derived THs are depleted at 21 dpf and cannot offset TPO inhibition, resulting in impaired anterior chamber inflation. This has been previously suggested by Reider and Connaughton (2014) with specific reference to eye development.
Maternal thyroid hormone levels in embryos have been demonstrated in zebrafish, fathead minnow, brown trout, striped bass, tilapia, rabbitfish, conger eel, sea bream and different species of salmon (Walpita et al., 2007; Chang et al., 2012; Power et al., 2001; Brown et al., 1987, 1988). Alt et al. (2006) found a first differentiated thyroid follicle in zebrafish at 55 hours post fertilization. Elsalini et al. (2003) used immunohistochemistry to show the development of the first thyroid follicles producing thyroid hormone at 72 hours post fertilization in zebrafish. During further larval development, the number of follicles increases. Therefore early developmental processes (before thyroid activation) that are dependent on T4, such as posterior swim bladder inflation, might not be affected by chemicals reducing T4 synthesis. Nelson et al. (2016) and Stinckens et al. (2016) indeed found that MBT (a thyroperoxidase inhibitor) decreased T4 levels in both zebrafish (5 days post fertilization) and fathead minnow (6 days post fertilization), which is after activation of the thyroid gland for both species, while it did not affect posterior inflation. Stinckens et al. (unpublished results) also found that exposure to the thyroperoxidase inhibitor benzophenone-2 did not affect posterior swim bladder inflation.
Evidence Supporting this KER
Thyroid hormones are known to be involved in development, especially in metamorphosis in amphibians and in embryonic-to-larval transition (Liu and Chan, 2002) and larval-to-juvenile transition (Brown et al., 1997) in fish. The formation of the anterior chamber coincides with the second transition phase (Winata et al., 2009) and with a peak in T4 synthesis (Chang et al., 2012) suggesting that anterior inflation is under thyroid hormone regulation.
- Chang et al. (2012) observed an increase of whole body T4 concentrations in zebrafish larvae at 21 days post fertilization, corresponding to the timing of anterior swim bladder inflation.
- Nelson et al. (2016) showed reduced whole body T4 levels at 6 days post fertilization together with delayed anterior inflation after exposure of fathead minnow embryos to 2-mercaptobenzothiazole. All anterior chambers eventually inflated but their size was reduced and morphology deviated.
- Stinckens et al. (2016) showed reduced whole body T4 levels both at 5 (before anterior inflation) and 32 days post fertilization (after anterior inflation) when exposed to 2-mercaptobenzothiazole (a thyroperoxidase inhibitor) from 0 to 32 days post fertilization. A large percentage of MBT-exposed fish had an uninflated anterior swim bladder, although some recovery was observed over time.
- Stinckens et al. (2016) further showed a significant correlation between whole body T4 levels and anterior chamber volume, with reduced T4 levels leading to smaller anterior chambers.
- Zebrafish exposed to methimazole, a commonly used reference TPO inhibitor, did not inflate their anterior swim bladder until 32 dpf, and did not show any sign of recovery (Stinckens et al., unpublished data).
Uncertainties and Inconsistencies
The mechanism through which reduced T4 hormone concentrations in serum result in anterior chamber inflation impairment is not yet understood. The anterior chamber is formed by evagination from the cranial end of the posterior chamber (Robertson et al., 2007, Winata et al., 2009). Several hypotheses could explain effects on anterior chamber inflation due to reduced T4 levels:
- Evagination from the posterior chamber could be impaired. Villeneuve et al. (unpublished results) showed that although the anterior bud was present after exposure to a deiodinase 2 inhibitor, the anterior chamber did not inflate.
- The formation of the tissue layers of the anterior swim bladder could be affected, although Villeneuve et al. (unpublished results) observed intact tissue layers of the anterior swim bladder after exposure to a deiodinase 2 inhibitor.
- The anterior chamber inflates is inflated with gas from the posterior chamber through the communicating duct. Impaired gas exchange between the two chambers could be at the basis of impaired anterior inflation. Both Nelson et al. (2016) and Stinckens et al. (2016) found that posterior chambers were larger when anterior chambers were smaller or not inflated at all. The sum of the areas of the posterior and anterior chambers remained constant independent of inflation of the anterior chamber (Stinkens et al., 2016). These results suggest retention of the gas in the posterior chamber.
- Since gas exchange relies on a functional communicating duct between the posterior and anterior chamber, and the communicating duct is known to progressively narrow and eventually close during development, a dysfunctional communicating duct or a closure prior to anterior inflation could inhibit inflation. However, Villeneuve et al. (unpublished results) showed that the communicating duct was anatomically intact and open after exposure to iopanoic acid (a deiodinase 2 inhibitor), still leading to impaired anterior inflation.
- Lactic acid production which is essential for producing gas to fill the swim bladder could be affected, although the observation that the total amount of gas in both chambers is not affected when anterior inflation is impaired seems to contradict this (Stinckens et al., 2016).
- Possibly there is an effect on the production of surfactant, which is crucial to maintain the surface tension necessary for swim bladder inflation.
Quantitative Understanding of the Linkage
Stinckens et al. (2016) showed a significant linear relationship between whole body T4 levels and anterior chamber volume (measured as surface in 2D images), with reduced T4 levels leading to smaller anterior chambers.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The evidence for a relationship between circulating T4 levels and inflation of the anterior chamber of the swim bladder currently comes from work on zebrafish and fathead minnow.
- Alt, B., Reibe, S., Feitosa, N.M., Elsalini, O.A., Wendl, T., Rohr, K.B., 2006. Analysis of origin and growth of the thyroid gland in zebrafish. Dev. Dyn. 235, 1872–1883, http://dx.doi.org/10.1002/dvdy.20831.
- Brown, C.L., Doroshov, S.I., Nunez, J.M., Hadley, C., Vaneenennaam, J., Nishioka, R.S. and Bern, H.A. 1988. Maternal triiodothyronine injections cause increases in swimbladder inflation and survival rates in larval striped bass, Morone saxatilis. J. Exp. Zool. 248: 168–176.
- Brown, C.L., Sullivan, C.V., Bern, H.A. and Dickhoff, W.W. 1987. Occurrence of thyroid hormones in early developmental stages of teleost fish. Trans. Am. Fish. Soc. Symp. 2: 144–150.
- Brown, D.D., 1997. The role of thyroid hormone in zebrafish and axolotl development. Proc. Natl. Acad. Sci. U. S. A. 94, 13011–13016, http://dx.doi.org/ 10.1073/pnas.94.24.13011.
- Chang, J., Wang, M., Gui, W., Zhao, Y., Yu, L., Zhu, G., 2012. Changes in thyroid hormone levels during zebrafish development. Zool. Sci. 29, 181–184, http:// dx.doi.org/10.2108/zsj.29.181.
- Elsalini, O.A., Rohr, K.B., 2003. Phenylthiourea disrupts thyroid function in developing zebrafish. Dev. Genes Evol. 212, 593–598, http://dx.doi.org/10. 1007/s00427-002-0279-3.
- Liu, Y.W., Chan, W.K., 2002. Thyroid hormones are important for embryonic to larval transitory phase in zebrafish. Differentiation 70, 36–45, http://dx.doi. org/10.1046/j.1432-0436.2002.700104.x.
- Nelson KR, Schroeder AL, Ankley GT, Blackwell BR, Blanksma C, Degitz SJ, Flynn KM, Jensen KM, Johnson RD, Kahl MD, Knapen D, Kosian PA, Milsk RY, Randolph EC, Saari T, Stinckens E, Vergauwen L, Villeneuve DL. 2016. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole – Part I: fathead minnow. Aquatic Toxicology 173: 192-203.
- Power DM, Llewellyn L, Faustino M, Nowell MA, Björnsson BT, Einarsdottir IE, Canario AV, Sweeney GE. Thyroid hormones in growth and development of fish. Comp Biochem Physiol C Toxicol Pharmacol. 2001 Dec; 130(4):447-59.
- Reider, M., Connaughton, V.P., 2014. Effects of Low-Dose Embryonic Thyroid Disruption and Rearing Temperature on the Development of the Eye and Retina in Zebrafish. Birth Defects Res. Part B Dev. Reprod. Toxicol. 101, 347–354, http://dx.doi.org/10.1002/bdrb.21118.
- Roberston, G.N., McGee, C.A.S., Dumbarton, T.C., Croll, R.P., Smith, F.M., 2007. Development of the swim bladder and its innervation in the zebrafish, Danio rerio. J. Morphol. 268, 967–985, http://dx.doi.org/10.1002/jmor.
- Stinckens E, Vergauwen L, Schroeder AL, Maho W, Blackwell BR, Witters H, Blust R, Ankley GT, Covaci A, Villeneuve DL, Knapen D. 2016. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole – Part II: zebrafish. Aquatic Toxicology 173:204-217.
- Walpita, C.N., Van der Geyten, S., Rurangwa, E., Darras, V.M., 2007. The effect of 3,5,3′-triiodothyronine supplementation on zebrafish (Danio rerio) embryonic development and expression of iodothyronine deiodinases and thyroid hormone receptors. Gen. Comp. Endocrinol. 152, 206–214, http://dx.doi.org/ 10.1016/j.ygcen.2007.02.020.
- Winata, C.L., Korzh, S., Kondrychyn, I., Zheng, W., Korzh, V., Gong, Z., 2009. Development of zebrafish swimbladder: the requirement of Hedgehog signaling in specification and organization of the three tissue layers. Dev. Biol. 331, 222–236, http://dx.doi.org/10.1016/j.ydbio.2009.04.035.