Expression and T3 regulation of thyroid hormone- and sex steroid-related genes during Silurana (Xenopus) tropicalis early development

Abstract In amphibians, thyroid hormones (THs) are the primary regulators of metamorphosis; however, their physiological role during embryogenesis remains unclear. First, we established complete developmental profiles for TH receptors ( tralpha and trbeta ), deiodinases ( dio ; types 1, 2, 3), estrogen receptors ( eralpha and erbeta ) and androgen receptor ( ar ) mRNA levels during embryogenesis and early larval stages in Silurana ( Xenopus ) tropicalis (from Nieuwkoop and Faber (NF) stage 2 until NF 46). Real-time RT-PCR analyses in whole embryos and larvae revealed that all transcripts except tralpha were detected throughout development; tralpha only appears after gastrulation. The first significant increase in the expression of tralpha and trbeta was observed before hatching, between NF 21 and NF 27 (2.5- and 11-fold, respectively). In order to test if these genes could be regulated by THs during early larval development, embryos were exposed to triiodothyronine (T3; 0.5, 5.0, 50 nM) from NF 27 to NF 46. T3 exposure caused a dose-dependent increase relative to control in the expression of tralpha , trbeta , dio (types 2 and 3), ar , and 5α-reductase type 1 in whole larvae. These results indicate that in S. tropicalis , tr and dio can be induced by T3 as early as NF 46, a response that had only been characterized later during frog metamorphosis. In addition, T3 also affected androgen-related gene expression, supporting our hypothesis that THs are involved in male development in frogs.