Molecular cloning, characterization and expression of thyroid-stimulating hormone receptor in channel catfish

Thyroid-stimulating hormone receptors (TSHRs) are primarily expressed in the thyroid of vertebrates, however recently, transcripts encoding TSHR have been found abundantly in the gonads in a variety of fish species. The purpose of this study is to characterize the channel catfish TSHR and to examine whether the transcript are translated into protein in the gonad or store the transcript as maternal RNA for later use. The cDNA encoding the TSHR was isolated from the channel catfish thyroid but the transcript was determined to be expressed in a number of tissues, including the gonads. In fact, the ovarian expression of TSHR changed significantly during the reproductive season and peaked after the vitellogenic growth phase. Furthermore, the TSHR transcript was also detected in unfertilized eggs but not in fertilized egg of catfish. LM-PAT analysis demonstrated that catfish TSHR transcripts were fully polyadenylated in thyroidal follicles, gonads and unfertilized eggs suggesting that they were translated into protein opposed to being "stored mRNA". Western blot analysis using polyclonal antibodies against the catfish TSHR verified this assumption by visualizing immunoreactive protein in the thyroid, testis, and the post-vitellogenic ovary in abundance. A functional assay clearly showed that the recombinant catfish TSHR was specifically activated by bovine TSH but not by recombinant catfish follicle-stimulating hormone (FSH) and luteinizing hormone (LH). As in other species, the heterologous gonadotropin, hCG, partially activated the receptor. These results suggested that TSHR plays important roles for gametogenesis rather than embryogenesis.