Molecular biology of the channel catfish gonadotropin receptors: 2. Complementary DNA cloning, functional expression, and seasonal gene expression ofthe follicle-stimulating hormone receptor

Molecular cloning of the channel catfish FSH receptor is reported together with temporal changes in the gene expression throughout a reproductive cycl e. A cDNA encoding the receptor was isolated from the testis using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) procedures. The cDNA coded for a 662-amino acid protein t hat was most identical (51%-59%) to salmon gonadotropin receptor I and the FSH receptors of higher vertebrates, and less identical to LH receptors and thyrotropin receptors (45%-49% and 46%-47%, respectively). In addition, PC R analysis of the genomic DNA showed the absence of the LH receptor-specifi c intron. Expression of the channel catfish FSH receptor gene was highly re stricted to the testis and ovary, except for a low-level expression in the spleen. Transfected COS cells expressed an active recombinant receptor as d etermined by the ligand-specific activation of a cAMP-responsive reporter g ene (luciferase). The recombinant receptor was activated by human FSH and, to a small extent, hCG. Seasonal changes in the ovarian expression of the F SH receptor gene, examined by measuring the transcript abundance by quantit ative real-time RT-PCR, showed a rise around the time of onset of ovarian r ecrudescence and a decrease prior to spawning. This pattern of seasonal exp ression of FSH receptor differs significantly from that of the LH receptor, which we reported recently. The differential expression of the two gonadot ropin receptor genes, in addition to the differential secretion of the gona dotropic hormones, seem to be critical for the regulation of steroidogenesi s and other gonadal physiological processes.