Thyroid disruption in developing zebrafish (Danio rerio) - consequences on morphology, physiology and behavior
Sensory and Behavioural Biology
Developmental Biology
Final Report Abstract
A novel development in ecotoxicology is the establishment of „adverse outcome pathways“, which aim to link specific molecular events to a toxicological and/or ecological effect. According to that concept, we asked whether different molecular changes of the fish thyroid axis can translate into the same disruption of eye development, which is known to be regulated by thyroid hormones. Moreover, we wanted to elucidate the exact affected pathways and the functional outcomes for the organism, which have not been clearly described yet. We aimed to address these open questions by comparing the impact of two different thyroid disruptors on the eye development of zebrafish embryos. On the one hand we hypothesized that the disruption of eye morphology is a converging event that is independent of the molecular MoA of the thyroid disruptor. Therefore, we chose two different thyroid disruptors with known MoA: PTU as an inhibitor of thyroid hormone synthesis and TBBPA as disruptor of the thyroid receptors. On the other hand, we hypothesized that the chemically-induced dysfunctional eye development will result in disrupted visual physiology and vision-related behavior of the individual. As a first step and proof-of-principle, we investigated the effects of both compounds on expression of thyroid-related genes to demonstrate the influence on different key players of the complex thyroid system. Both substances had significant impact on gene expression levels of thyroid-related genes in exposed larvae, but the reaction pattern differed clearly, which shows that the molecular changes affected different pathways within the thyroid hormone system of the developing fish. To investigate the affected pathways in detail, we performed a whole genome microarray analyze with RNA isolated from eye tissue of the exposed larvae. This revealed that specific eye-related genes were differentially expressed due to the thyroid disruptor treatment. Moreover, we could show that these changes were only partially reversible after a recovery phase in clean water. Thus, an important outcome of this project is that it goes beyond previous descriptive reports on thyroid disruptor effects on eye development in that the project findings promote the understanding of the molecular pathways through which thyroid hormones regulate eye development and where endocrine disruptors might interfere. Moreover, the project advanced the knowledge to what extent early life effects of thyroid disruptors are persistent or reversible. Even though the molecular response obviously differed between both compounds used in our study, the associated cellular changes in the retina of exposed larvae were very similar in their response pattern, but not regarding the threshold: while PTU had a strong impact on the development of the retinal pigment epithelium (decreased size and pigmentation), the effects of TBBPA were similar but not as pronounced. Analyses of body size and eye size and the resulting ratio revealed that both substances significantly decreased the relative eye size of the developing larvae. The next evident step in our study was to demonstrate whether the molecular and morphological changes have impact on the visual performance and vision-based behavioral capacities of the exposed larvae. We used the optokinetic response, swimming activity, light-preference and light response as fitness-related endpoints to investigate how general and vision-related behaviors of exposed larvae were altered by the treatment with the two different thyroid disruptors. All investigated endpoints showed clear impairment due to the treatment with thyroid disruptors, which proves that the molecular and morphological changes translate into impaired physiology and behavior of exposed fish. The results of the present study provide evidence that thyroid disruptors which induce different molecular changes in the thyroid system can lead to uniform alterations in eye development of zebrafish larvae. A second major finding from this study is that the impaired eye development, as it arises under exposure to the thyroid-disrupting compounds, indeed implies an impaired visual performance, as evidenced from physiological and behavioral tests. These findings are especially relevant, as development and functionality of the eyes are essential for the survival of fish larvae in the environment, which is steadily contaminated with different endocrine disruptors.
Publications
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(2015) Thyroid disruption in zebrafish (Danio rerio) larvae: different molecular response patterns lead to impaired eye development and visual functions. Aquatic Toxicology
Baumann, L., Ros, A., Rehberger, K., Neuhauss, S., Segner, H.
