Resumen de Thyroid hormone receptor expression in the obligatory paedomorphic salamander "Necturus maculosus"

Virginie Vlaeminck-Guillem, Rachid Safi, Philippe Guillem, Emmanuelle Leteurtre, Martine Duterque-Coquillaud, Vincent Laudet

• Amphibian metamorphosis is under the strict control of thyroid hormones (TH). These hormones induce metamorphosis by controlling gene expression through binding to thyroid hormone receptors (TRs). "Necturus maculosus" is considered to be an obligatory paedomorphic Amphibian since metamorphosis never occurs spontaneously and cannot be induced by pharmacological means. Since metamorphosis depends on the acquisition of response of tadpole tissues to thyroid hormone, we aimed to determine TR gene expression patterns in "Necturus maculosus" as well as the expression of two TH-related genes: "Cytosolic Thyroid Hormone-Binding Protein (CTHBP)-M2-pyruvate kinase", a gene encoding a cytosolic TH binding protein and stromelysin 3, a direct TH target gene in "Xenopus laevis". Tissue samples were obtained from specimens of "Necturus maculosus". We performed in situ hybridization using non-cross-hybridizing RNA probes obtained from the cloned "Necturus" TRalpha and TRbeta genes. We found clear expression of "Necturus" TRalpha gene in several tissues including the central nervous system, epithelial cells of digestive and urinary organs, as well as myocardium and skeletal muscle. TRbeta was also expressed in the brain. In other tissues, hybridization signals were too low to draw reliable conclusions about their precise distribution. In addition, we observed that the expression of CTHBP and ST3 is largely distinct from that of TRs. The fact that we observed a clear expression of TRalpha and TRbeta which are evolutionary conserved, suggests that "Necturus" tissues express TRs. Our results thus indicate that, in contrast to previously held hypotheses, "Necturus" tissues are TH responsive.