Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow

Abstract

In the present study, a hypothesized adverse outcome pathway linking inhibition of thyroid peroxidase (TPO) activity to impaired swim bladder inflation was investigated in two experiments in which fathead minnows (Pimephales promelas) were exposed to 2-mercaptobenzothiazole (MBT). Continuous exposure to 1mg MBT/L for up to 22 days had no effect on inflation of the posterior chamber of the swim bladder, which typically inflates around 6 days post fertilization (dpf), a period during which maternally-derived thyroid hormone is presumed to be present. In contrast, inflation of the anterior swim bladder, which occurs around 14dpf, was impacted. Specifically, at 14dpf, approximately 50% of fish exposed to 1mg MBT/L did not have an inflated anterior swim bladder. In fish exposed to MBT through 21 or 22dpf, the anterior swim bladder was able to inflate, but the ratio of the anterior/posterior chamber length was significantly reduced compared to controls. Both abundance of thyroid peroxidase mRNA and thyroid follicle histology suggest that fathead minnows mounted a compensatory response to the presumed inhibition of TPO activity by MBT. Time-course characterization showed that fish exposed to MBT for at least 4 days prior to normal anterior swim bladder inflation had significant reductions in anterior swim bladder size, relative to the posterior chamber, compared to controls. These results, along with similar results observed in zebrafish (see part II, this issue) are consistent with the hypothesis that thyroid hormone signaling plays a significant role in mediating anterior swim bladder inflation and development in cyprinids, and that role can be disrupted by exposure to thyroid hormone synthesis inhibitors. Nonetheless, possible thyroid-independent actions of MBT on anterior swim bladder inflation cannot be ruled out based on the present results. Overall, although anterior swim bladder inflation has not been directly linked to survival as posterior swim bladder inflation has, potential links to adverse ecological outcomes are plausible given involvement of the anterior chamber in sound production and detection.

Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid

“…Consistent with the results of MBT exposure in fathead minnows and zebrafish [6,7], posterior swim bladder length was significantly increased in all of the IOP treatments at 14 and 18 dpf, with the exception of the 6 mg IOP/L treatment group at 18 dpf (Figure 7). One possible explanation of the increased posterior swim bladder length at 14 and 18 dpf is delayed evagination of the anterior swim bladder from the posterior chamber, leaving excess gas in the posterior chamber and thereby increasing the size of the posterior chamber [8].…”

“…In a previous study, exposure to a model TPO inhibitor, 2‐mercaptobenzothiazole (MBT), resulted in impaired anterior swim bladder inflation (14 dpf) in larval fathead minnows. Posterior swim bladder inflation, which occurs earlier in development (6 dpf) was not affected by MBT exposure, possibly because of the availability of maternally deposited thyroid hormone in the yolk . Similar responses were observed in zebrafish, where exposure to MBT did not impair inflation of the posterior chamber at 5 dpf, but anterior chamber inflation and size were impaired at 32 dpf .…”

“…The results broadly align with AOPs 155–158 by providing further evidence that perturbation of deiodinase activity in early life stage fish can result in critical morphological effects (e.g., swim bladder inflation) via perturbation of key events associated with the HPT axis (e.g., thyroid hormone production); however, the increased T3 abundance in the presence of decreased posterior chamber inflation in the embryo study, along with the potentially more prominent roles of dio2 and dio3 in mediating the effects, suggests that some refinements to the existing AOPs may be needed. While further gene knockdown experiments and/or chemical exposure experiments have the potential to strengthen the hypothesized causal relationship between HPT perturbation and swim bladder effects in fathead minnows and other fish species, the present study examining the effects of deiodinase inhibition, along with previous studies investigating the effects of TPO inhibition , demonstrates that swim bladder inflation can be expected as a common effect of short‐term early life stage fish exposure to thyroid‐disrupting chemicals.…”

Zebrafishduoxmutations provide a model for human congenital hypothyroidism

“…MBT is a potent TPO inhibitor and its role was recently examined in swim bladder inflation in the fathead minnow (Pimephales promelas) (Nelson et al, 2016) and zebrafish (Stinckens et al, 2016). Among minnows, larvae continuously exposed to MBT showed a concentration-dependent decrease in anterior lobe size (Nelson et al, 2016). Meanwhile, MBT-treated zebrafish larvae were reported to fare worse than minnows, where 22% larvae exposed to the highest concentration failed to inflate the anterior chamber (Stinckens et al, 2016).…”

Adverse outcome pathway networks I: Development and applications

“…Currently, many AOPs are being developed in this networkguided fashion (see Angrish et al 2016;Nelson et al 2016;Stinckens et al 2016;Cavallin et al 2017;LaLone et al 2017b). However, it is expected that as the AOP knowledgebase matures, AOP development will increasingly focus on filling data and knowledge gaps in the AOP-Wiki.…”