Cloning, functional characterization, and expression of thyrotropin receptors in the thyroid of amago salmon (Oncorhynchus rhodurus).

Two thyrotropin receptor cDNAs (sTSH-Ra and sTSH-Rb) were cloned from thyroid tissue of the amago salmon, Oncorhynchus rhodurus. sTSH-Ra and sTSH-Rb showed the highest degrees of sequence homology to mammalian TSH receptors. Functional characterization in COS-7 cells transiently transfected with sTSH-Ra or sTSH-Rb showed the largest increase in cAMP when exposed to bovine TSH. RT-PCR analysis demonstrated that sTSH-Ra and sTSH-Rb were expressed in the basibranchial region, but not in the ovary, testis, liver, kidney or brain. In situ hybridization revealed that sTSH-Ra and sTSH-Rb were exclusively expressed in thyroid follicular epithelial cells of amago salmon undergoing smoltification. These results indicated that the cloned cDNAs encode functional TSH receptor proteins. This is the first report of isolation of TSH receptor molecules from nonmammalian vertebrates.