Zebrafish as a model for vertebrate reproduction: characterization of the first functional zebrafish (Danio rerio) gonadotropin receptor

Vertebrate reproduction is tightly regulated by conserved glycoprotein hormones produced by the pituitary gland. Follicle-stimulating hormone (FSH) in tetrapods and gonadotropic hormone I (GTH-I) in fishes are orthologous glycoprotein hormones that control the timing of egg production and the number of eggs produced. Zebrafish, a well-established genetic model for developmental biology, also offers potential advantages for studies of reproductive toxicology, especially for modeling the impact of pollutants on fish reproductive processes. To facilitate these studies we have identified, expressed, and characterized the zebrafish GTH-I receptor. This receptor (zfGTHR-I)exhibits strong sequence similarity to the tetrapod FSH receptors and to GTHR-I from salmon and catfish. Human 293 cells transfected with zfGTHR-I exhibit increased cAMP levels after treatment with carp pituitary extracts or human FSH, but not when treated with a ligand to a related receptor (human chorionic gonadotropin). Northern blotting and RT-PCR analyses indicate that zfGTHR is expressed in ovaries from sexually mature fish, but not in immature fish. Several alternative splice variants of the receptor affecting putative exons 2-4 that encode dramatically shortened receptor fragments lacking the transmembrane domain as well as regions previously implicated in ligand binding were identified by RT-PCR. The zfGTHR-I sequence opens the way to study effects of genetic mutations or chemicals on ovarian zfGTHR-I expression and function in zebrafish.