Molecular biology of channel catfish gonadotropin receptors: 1. Cloning ofa functional luteinizing hormone receptor and preovulatory induction of gene expression

There is little known about the molecular biology of piscine gonadotropin r eceptors, and information about gene expression during reproductive develop ment is particularly lacking. We have cloned the LH receptor (LHR) in the c hannel catfish (cc), and examined its gene expression throughout a reproduc tive cycle. A cDNA encoding the receptor was isolated from the testis using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplifi cation of cDNA ends procedures. It encoded a 696-amino acid protein that sh owed the greatest homology (46-50% identity) With the known LHRs and lesser similarity with FSH receptors and thyroid-stimulating hormone receptors (4 4-47% and 42-44% identity, respectively). In addition, two characteristics unique to the LHRs were conserved in the cloned receptor and the encoding g ene: presence of an intron corresponding to intron 10 in mammals and turkey and occurrence of a double cysteine residue in the cytoplasmic tail for po tential palmitoylation. The ccLHR gene was well expressed in the gonads and kidney and merely detectable in the gills, muscle, and spleen. The isolate d cDNA encoded an active ccLHR protein,as the recombinant receptor expresse d in COS7 cells activated a cAMP response element-driven reporter gene (luc iferase) upon exposure to hCG in a dose-dependent manner. Seasonal changes in the ovarian expression of the ccLHR gene, as examined by measuring the t ranscript abundance by quantitative real-time RT-PCR, remained rather low d uring most of the reproductive cycle but was acutely induced around the tim e of spawning. This pattern of expression correlates well with the reported expression of its ligand (LH) in fishes and concurs with the notion that L H is a key regulator of the periovulatory maturational events.