Thyroid Hormone Signaling Is Required for Dynamic Variation in Opsins in the Retina during Metamorphosis of the Japanese Flounder (Paralichthys olivaceus)

SIMPLE SUMMARY: Fish adapt to changes in their external light environment by adjusting their visual system. Fish that live at different depths can perceive different spectral ranges, leading to differences in the type and expression of their optic proteins. The Japanese flounder (Paralichthys olivaceus) is a valuable cold temperate benthic marine fish that undergoes a series of metamorphoses during its juvenile stage. The vision of flounder larvae is reshaped to adapt to changes in their light environment. Previous studies have shown that thyroid hormone (TH) signaling is critical for flounder metamorphosis. In the present study, we investigated the tissue composition and the metamorphosis temporal expression profile of five opsin genes in flounder larvae. We also investigated the effect of TH on the opsin genes’ expression levels by adding TH and thiourea (a TH inhibitor, TU) to the culture water. In addition, we investigated the thyroid hormone receptor (TR)-mediated targeting regulatory relationship between TH and the opsin genes to further explore the function of TH in visual remodeling during flounder metamorphosis. ABSTRACT: In the present study, we investigated the function of thyroid hormones (TH) in visual remodeling during Japanese flounder (Paralichthys olivaceus) metamorphosis through cellular molecular biology experiments. Our results showed that the expression of the five opsin genes of the flounder were highest in eye tissue and varied with the metamorphosis process. The expression of rh1, sws2aβ and lws was positively regulated by exogenous TH, but inhibited by thiourea (TU) compared to the control group. In addition, there was a significant increase in sws2aβ and lws in the rescue experiments performed with TU-treated larvae (p < 0.05). Meanwhile, T3 levels in flounder larvae were increased by TH and decreased by TU. Based on the differences in the expression of the three isoforms of the thyroid hormone receptor (TR) (Trαa, Trαb and Trβ), we further hypothesized that T3 may directly or indirectly regulate the expression of sws2aβ through Trαa. This study demonstrates the regulatory role of TH in opsins during flounder metamorphosis and provides a basis for further investigation on the molecular mechanisms underlying the development of the retinal photoreceptor system in flounders.