You are here:
Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole - Part II: zebrafish
Citation:
Stinckens, E., L. Vergauwen, A. Schroeder, W. Maho, B. Blackwell, H. Witters, R. Blust, G. Ankley, A. Covaci, Dan Villeneuve, AND D. Knapen. Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole - Part II: zebrafish. AQUATIC TOXICOLOGY. Elsevier Science Ltd, New York, NY, 173:204-217, (2016).
Impact/Purpose:
The fish early life stage (FELS) toxicity test (OECD 210; OCSPP 850.1400) is the most widely used chronic ecotoxicity test. There is strong interest in development of alternatives to reduce costs, increase efficiency, and minimize animal use. Development of adverse outcome pathways that link pathway perturbations that could be measured either using in vitro systems or in short-term high throughput fish embryo tests to potential adverse effects on fish growth or survival have been viewed as a means to support reduction, refinement, and/or replacement of FELS testing. The present study contributes to the overall goal, while at the same time addressing the topic of thyroid axis disruption, a critical concern relative to implementation of the endocrine disruptor screening program (EDSP). Specifically, experiments were conducted to evaluate the potential linkage between thyroid peroxidase (TPO) inhibition and swim bladder inflation using zebrafish as a model organism. Consistent with results from experiments with fathead minnows, these results with zebrafish show the anterior, but not posterior, swim bladder inflation was impacted by exposure to the environmentally relevant TPO inhibitor 2-mercaptobenzothiazole. The studies show a clear relationship between T4 levels and the surface of the anterior swim bladder chamber. While surface area of the anterior chamber has not yet been directly linked to impaired growth or survival, the results demonstrate an important life stage dependence on the sensitivity to this particular thyroid-disrupting molecular initiating event (MIE). Together with results from analogous studies in the fathead minnow, these results provide important evidence which supports the development of formal AOP descriptions linking specific thyroid-disrupting MIEs to significant phenotypic outcomes. Once described, these AOPs can support the using of in vitro TPO inhibition assays to predict phenotypic outcomes with potential ecological relevance. Further, they suggest alternative short-term in vivo tests with larval fish that could be used to screen chemicals for thyroid disrupting activity and possibly distinguish various thyroid disrupting modes of action. Results of these studies directly support CSS project 12.01, Task 1.3a, Thyroid-axis related formal AOP development. This paper is the product of an international collaboration between the US EPA Mid-Continent Ecology Division and the University of Antwerp
Description:
Disruption of the thyroid hormone (TH) system is increasingly being recognized as an important mode of action that can lead to ecologically relevant adverse outcomes, especially during embryonic development. The present study was designed to further characterize the effects of disruption of TH metabolism on swim bladder inflation during zebrafish early-life stages using 2-mercaptobenzothiazole (MBT) as an environmentally relevant thyroid peroxidase (TPO) inhibitor. Zebrafish were exposed to different MBT concentrations until 120/168 hours post fertilisation (hpf) and 32 days post fertilisation (dpf), in two sets of experiments, to document the effects of TPO inhibition on posterior and anterior swim bladder inflation respectively. Whole body T3 and T4 levels were measured at 120 hpf and 32 dpf. At 120 hpf, MBT did not directly impair posterior chamber inflation, nor did it affect the surface of the posterior chamber. At 32 dpf, MBT exposure impaired anterior chamber inflation and size. Whole-body T4 decreased after MBT exposure at both time points, while T3 levels were unaltered. There was a clear relationship between T4 levels and the surface of the anterior swim bladder chamber at 32 dpf. The absence of effects on posterior inflation can possibly be explained by maternal transfer of T4 into the eggs. These maternally derived THs are depleted at 32 dpf and cannot offset TPO inhibition, resulting in impaired anterior chamber inflation. Therefore, we hypothesize that TPO inhibition only becomes relevant during late development with respect to swim bladder development, after depletion of maternally derived T4. In contrast, iodothyronine deiodinase (ID) inhibition, as shown by our previous knockdown studies, already impairs posterior chamber inflation during early development, thereby also preventing the anterior chamber to inflate at later age. Our findings suggest that thyroid disruptors impact swim bladder inflation, with an important distinction among specific subtypes of TH disrupting compounds. These results can be helpful for delineating AOPs leading from TPO inhibition, ID inhibition and other TH related molecular initiating events to impaired swim bladder inflation in fish during early life stages. Once described, such AOPs can support the use of in vitro enzyme inhibition assays for predicting reduced survival due to impaired posterior chamber inflation, as well as due to anterior chamber inflation, the latter occurring in a FELS timeframe and therefore not observed in a ZFET test.
