Equilin does not affect thyroid hormone signaling in the developing Xenopus laevis tadpole brain

Toxcast/Tox21 is a massive federally run research effort dedicated to better understanding the potential toxicity of thousands of compounds in a high throughput manner. Among this list of compounds is equilin, an estrogen-like compound that was flagged as a potential thyroid hormone agonist. Here we examine if equilin acts like a thyroid hormone agonist on cellular and molecular mechanisms of brain development in Xenopus laevis tadpoles. To examine the effect of equilin, tadpoles were divided into eight groups and received 4 days of exposure. The experimental groups were as follows: 1 μL, 10 μL, and 100 μL of equilin, 1 μL, 10μM, and 100 μM of 17-β estradiol as an estrogen control, 15 μg/mL thyroxine (T 4 ) as a thyroid hormone control, and a no-exposure control. After 4 days of treatment, animals were treated with CldU to label dividing cells for 2hr and then euthanized in MS-222. After fixation, body length was measured and the brains dissected out. IHC was performed on brains for CldU to label proliferating neural progenitor cells. Brains were then whole-mounted and analyzed using confocal microscopy. We found that equilin did not increase the number of dividing progenitor cells in a T 4 -like manner. Instead, equilin decreased proliferation in a dose-dependent manner, as did estradiol. The same paradigm was performed separately staining for caspase-3 and h2ax, finding that equilin increased cell death in contrast to CNTL and T 4 . In another experiment, RNA was extracted from tadpole brains in each group and qPCR was performed to assess change in expression of thyroid hormone-sensitive genes, Equilin did not affect gene expression in a thyroid hormone-like manner. Our data indicate that equilin does not act as a thyroid hormone agonist in the Xenopus laevis nervous system but instead acts similarly to estradiol. Our data strongly suggest that equilin is not a TH disruptor, contrary to the findings of the ToxCast/Tox21 dataset.