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Crafting a gene expression timeline for the thyroid in the early-life stages of fathead minnows (Pimephales promelas)
Randolph, E., G. Ankley, A. Schroeder, K. Nelson, S. Robinson, AND Dan Villeneuve. Crafting a gene expression timeline for the thyroid in the early-life stages of fathead minnows (Pimephales promelas). SETAC, North America, Salt Lake City, UT, November 01 - 05, 2015.
The hypothalamic-pituitary-thyroid (HPT) axis plays a number of critical roles in chordate physiology including regulation of metabolism, growth, and development. While the role of the HPT axis and thyroid hormone signaling in mammalian and amphibian development is well established there is a gap in knowledge surrounding fish species. The construction of an ontogeny of HPT-related gene expression can help toxicologists to predict when an organism is most susceptible to thyroid disrupting chemicals. In this study a timeline documenting expression of key gene expression associated with the HPT axis over the course of early-life stages of the fathead minnows (Pimephales promelas) was constructed. Fathead minnows were raised under optimal laboratory conditions and sampled every other day throughout their early life, starting from 15 hours post-fertilization continuing up to 30 days post-hatch. Total RNA was extracted from sampled fish and expression of genes important to the HPT axis including thyroid peroxidase (TPO), thyroid stimulating hormone (TSH), sodium-iodide symporter, thyrotropin-releasing hormone, and deiodinases 1, 2, and 3 (Dio 1, 2, 3), and several sulfotransferases was measured using Real-Time Polymerase Chain Reaction. TSH and TPO mRNA levels remained very low throughout embryonic development, but showed a significant increase immediately after hatching. Peaks in TSH and TPO expression corresponded closely with known peaks in thyroid hormone concentrations during larval development and also corresponded with periods of posterior and anterior swimbladder inflation in the species. Dio 1 and 3 mRNA expression was observed during embryonic and larval development. Dio 2 mRNA expression was low throughout embryonic development, but increased significantly following hatching and throughout larval development. These results suggest that fathead minnows begin synthesizing endogenous thyroid hormone following hatching, while maternally-derived thyroid hormone appears important for embryonic development through the action of Dio 1 and 3. This basic understanding will aid development adverse outcome pathways detailing impacts of thyroid axis disruption during fish early life stages.