Developmental Profiles and Thyroid Hormone Regulation of Brain Transcripts in Frogs: A Species Comparison with Emphasis on <i>Physalaemus</i> <i>pustulosus</i>

Abstract

In amphibians, thyroid hormones (THs) are considered key regulators of brain remodeling during metamorphosis, while sex steroids (estrogens and androgens) control sexual differentiation and gonadal development. However, these two endocrine axes can interact during tadpole brain development. Previously, we demonstrated that THs affect sex steroid-related gene expression in the developing brain of Silurana tropicalis and Rana pipiens; however, the gene expression changes differed between species. We chose to study a third anuran species, Physalaemuspustulosus, to test new hypotheses about the role of THs in the regulation of brain gene expression. We first established developmental transcript profiles of TH- and sex steroid-related genes in the brain of P. pustulosus. Then, following the same protocols as in our previous studies, we investigated triiodothyronine (T3) regulation of brain transcripts in premetamorphic P.pustulosus and then compared the results with our previous two studies. In the case of TH-related genes, TH receptor beta (trbeta) and deiodinase type 3 (dio3), mRNA developmental profiles were similar in the three species and with respect to other species in the published literature. However, the profiles of TH receptor alpha (tralpha) and deiodinase type 2 (dio2) mRNA revealed differences between anuran species. Among the three anurans we have studied, the direction of the T3 regulation of TH-related genes was overall similar, but the magnitude of gene expression change differed depending on the rate of metamorphosis in a given species. For the sex steroid-related genes, each species exhibited similar developmental profiles but differed in their response to T3. In P. pustulosus, T3 reduced the expression of aromatase (cyp19) while increasing mRNA levels of androgen and estrogen receptors. These results are similar to previous research in R. pipiens but differ from data for S. tropicalis, for which we found an increase in androgen synthesis enzymes but no effect on cyp19. Together, we propose that T3 has the potential to induce the brain androgen system in anurans. This could be achieved by increasing androgen synthesis enzymes (S. tropicalis) or by decreasing estrogen synthesis (due to a decrease in cyp19 in P. pustulosus and R. pipiens). In conclusion, we demonstrated that mechanisms of hormone interactions differ between anuran species, but in all cases T3 appears to affect the balance of sex steroids in the brain, stimulating the androgen system. We have shown that the regulation of sex steroid-related genes by T3 is more similar among closely related species than species with similar reproductive and developmental characteristics.

Identification of thyroid hormone receptors α and β genes and their expression profiles during metamorphosis in Rana chensinensis*

“…Our results showed that rcTRα expression maintained a high level and changed only moderately in the brain throughout metamorphosis, whereas rcTRβ expression changed markedly around climax . These expression profiles of TRα and TRβ in the brain are similar to variations reported for L. sylvaticus and X. laevis, X. tropicalis, R. pipiens, and P. pustulosus (Duarte-Guterman et al, 2012a). These findings suggest that the expression profiles of TRα and TRβ in developing brains have common characteristics for most species of tailless amphibians (Furlow and Neff, 2006;Hogan et al, 2007;Denver et al, 2009).…”

“…In the brain of X. laevis tadpoles, TRβ mRNA was detected earliest at Nieuwkoop stage 58 and maximal expression occurred at Nieuwkoop stage 60 (which corresponds to Gosner stage 41; Gosner, 1960), and in the juvenile frog, TRβ levels remained at a late prometamorphic level (Krain and Denver, 2004). Recently researchers compared the developmental profiles between TRα and TRβ in the brain of X. tropicalis, R. pipiens, and Physalaemus pustulosus and concluded that TRα does not significantly change during development but TRβ dramatically increases at climax (Duarte-Guterman et al, 2012a). Our results showed that rcTRα expression maintained a high level and changed only moderately in the brain throughout metamorphosis, whereas rcTRβ expression changed markedly around climax .…”

“…Thyroid hormones exert their functions by binding to their receptors acting as DNA-binding transcription factors. In the metamorphosis of amphibians, the levels of thyroid hormones may be sufficient, but different levels of TRs in various tissues and organs contribute to their diverse effects (Forrest et al, 1990;Ogielska and Kotusz, 2004;Wang et al, 2008;Duarte-Guterman and Trudeau, 2010;Duarte-Guterman et al, 2012a, 2012bNavarro-Martin et al, 2012;Lou et al, 2014). Therefore, understanding the profiles of TR expression in different organs is important for studying the roles of this hormone in regulating metamorphosis.…”

“…Previous studies also showed that TRα and TRβ are differentially expressed in various tissues of different species. Some studies, especially with transgenic animals, emphasized the importance of differential expression of TRs (Buchholz et al, 2004(Buchholz et al, , 2006Hogan et al, 2007;Duarte-Guterman and Trudeau, 2010;Duarte-Guterman et al, 2012a). Recent reports show that in X. tropicalis, knockdown of Trα could enhance tadpole growth while delaying natural metamorphosis (Wen and Shi, 2015).…”

Aromatase, estrogen receptors and brain development in fish and amphibians

“…In Xenopus tropicalis, ERα and ERß genes are expressed at low level in the brain at larval stage 51 (before gonadal sex differentiation which occur between NF54 and NF59 stages), but become extremely abundant in the larva at stage 60 (metamorphic climax). They remain expressed at relatively high levels after metamorphosis, i.e at juvenile stages [137,139]. The precise distribution of estrogen receptors mRNA or proteins have not been studied during development in amphibian species.…”