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||non-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.
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. Since most of the more biologically active T3 originates from the conversion of T4, decreased circulatory T4 levels are plausibly linked to reduced anterior chamber inflation.
- 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 and delayed anterior inflation (lower proportion of inflation at 14 dpf compared to controls) 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 zebrafish were 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.
- Stinckens et al. (2020) established a significant correlation between reduced whole body T4 levels and reduced anterior chamber volume in 32 day old zebrafish across two compound exposures. This includes methimazole and propylthiouracil, two inhibitors of TH synthesis. Godfrey et al. (2017) also observed impaored anterior chamber inflation after exposure of zebrafish to Methimazole. Stinckens et al. (2020) continued the follow-up until 32 dpf and observed no recovery.
- Chopra et al. (2019) found that a nonsense mutation of the duox gene, coding for the enzyme dual oxidase involved in thyroid hormone synthesis, resulted in decreased intrafollicular T4 levels and impaired anterior chamber inflation until at least 54 dpf in zebrafish.
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 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 quantitative relationship between whole body T4 levels and anterior chamber volume (measured as surface in 2D images) in 32 day old juvenile zebrafish, with reduced T4 levels leading to smaller anterior chambers after continuous exposure to 2-mercaptobenzothiazole, a thyroid hormone synthesis inhibitor.
Stinckens et al. (2020, supplementary information) established a significant linear quantitative relationship between reduced T4 levels and reduced anterior chamber volume (measured as surface in 2D images) in 32 day old juvenile zebrafish across two compound exposures. This includes methimazole and propylthiouracil, two inhibitors of TH synthesis.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Reduced anterior chamber inflation upon disruption of the thyroid hormone system is consistently accompanied by reduced whole body T4 levels when fish are exposed to thyroid hormone synthesis inhibitors. however, when fish are exposed to deiodinase inhibitors, in the absence of feedback processes, stable T4 levels would be expected. Stable T4 levels were indeed observed in 14, 21 and 32 day old zebrafish exposed to iopanoic acid, a deiodinase inhibitor. While Cavallin et al. (2017) found a consistent relationship between reduced whole body T3 levels, and reduced anterior chamber inflation after exposure of fathead minnows to iopanoic acid, they observed increased T4 levels. Possibly, the inhibition of the conversion of T4 to T3 resulted in a compensatory mechanism that increased T4 levels. This was accompanied by increases of deiodinase 2 and 3 mRNA levels, which also indicate a compensatory response to deiodinase inhibition.
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.
This KER is probably not sex-dependent since both females and males rely on synthesis of THs for regulation of vital processes. Additionally, zebrafish are undifferentiated gonochorists, and gonad differentiation starts only around 23-25 dpf (Uchida et al., 2002), after the time point of anterior chamber inflation (around 21 dpf).
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