Molecular characterization, gene expression and transcriptional regulation of thyroid hormone receptors in Senegalese sole

Thyroid hormones (THs) play a key role in larval development, growth and metamorphosis in flatfish. Their genomic effects are mediated by thyroid hormone receptors (TRs). In this study, cDNAs encoding for TR alpha A TR alpha B, and TR beta have been sequenced in Senegalese sole (Solea senegalensis). Main domains and conserved motifs were identified. Also, a truncated TR alpha B isoform (referred to as TR alpha Btr) and a spliced TR beta variant (referred to as TR beta v) were detected. A phylogenetic analysis grouped both TR alpha and TR beta genes into two separate clusters with their fish and mammalian Counterparts. Expression proflies during larval development and in juvenile tissues were analyzed using a real-time PCR approach. In juvenile fish, TR alpha A, TR alpha B, TR beta v, and TR beta showed distinct transcript levels in tissues. During metamorphosis, only TR beta v and TR beta modified their mRNA levels in a similar way to the T4 contents. To evaluate the possible regulation of TRs by their cognate ligand T4 during sole metamorphosis, larvae were exposed to the goitrogen thiourea (TU). TR beta transcripts decreased significantly at 11 and 15 days after treatment. Moreover, adding exogenous T4 hormone to TU-treated larvae restored the steady-state levels or even increased TR beta and TR beta v mRNAs with respect to the Untreated control. Overall, these results demonstrate that TR beta transcription is up-regulated by THs playing a major role during metamorphosis in Senegalese sole. (C) 2008 Elsevier Inc. All rights reserved.