Identification of three putative GnRH receptor subtypes in vertebrates

The majority of vertebrates have two or three forms of gonadotropin-releasi ng hormone (GnRH), which appear to have arisen by successive gene duplicati on events. This suggests the possibility of concordant gene duplications of the GnRH. receptor to produce two or more cognate receptors. Since the ext racellular loop 3 (EC3) domain of mammalian GnRH receptors plays a role in distinguishing the different forms of GnRH, we have contemplated that the s equence of this domain will differ significantly in the putative cognate re ceptors. Degenerate oliognucleotides encoding the sequences of the transmem brane domains preceding and following EC3 were used for PCR amplification o f genomic DNA from zebrafish (Brachydanio rerio), goldfish (Carassius aurat us), African clawed frog (Xenopus laevis), chicken (Gallus domesticus), and lizard (Agama atra). Isolation and sequencing of specific clones revealed that they fell into three groups. Two of these were most similar to the mam malian pituitary GnRH receptor and were therefore designated Type IA and Ty pe IB. The third form (designated Type II) was most different from the othe rs and was identified in Xenopus, lizard, and human DNA. These findings sup port the concept of the existence of three distinct GnRH receptors, which h ave evolved in conjunction with three distinct GnRH ligand classes present in many vertebrates. (C) 1998 Academic Press.