Wu, S., Roch, G., Cervini, L., Rivier, J., and Sherwood, N. (2008) Newly-identified receptors for PHI and GHRH-like peptide in zebrafish help to elucidate the mammalian secretin superfamily. Journal of molecular endocrinology. 41(5):343-366.
A group of ten hormones in humans are structurally related and known as the secretin superfamily. These hormones bind to G-protein-coupled receptors that activate the cAMP pathway and are clustered as the secretin or B family. We used molecular techniques to clone two full length receptor cDNAs in zebrafish, which were analyzed for amino acid sequence and ligand-binding motifs, phylogenetic position, synteny, tissue expression, functional response, and signaling pathway. Evidence is provided that the two cDNAs are the peptide histidine-isoleucine (PHI) receptor and PRP receptor, which is known as growth hormone-releasing hormone-like peptide (GHRH-LP) receptor in non-mammals. Further we cloned a zebrafish cDNA encoding the peptides PHI and vasoactive intestinal peptide (VIP). The PHI-R cDNA, transfected into COS7 cells, responded to zebrafish PHI in a sensitive and dose-dependent manner (EC50 = 1.8 X 10-9M) but not to PACAP and VIP. The GHRH-LP receptor responded to both zebrafish GHRH-LP1 and GHRH with a 3.5 fold greater response to the former. For comparison, two zebrafish receptors (PAC1 & VPAC1) and two human receptors (VPAC2 & GHRH) were tested with human and/or zebrafish peptides. Unexpectedly, zebrafish VIP activated its PAC1 receptor suggesting that in evolution, PAC1 is not always a specific receptor for PACAP. We conclude that zebrafish, like goldfish, have a specific receptor for PHI and GHRH-LP. Our evidence that zebrafish PHI is more potent than human PHM in activating the human VPAC2 receptor (EC50 = 7.4 X 10-9M) supports our suggestion that the VPAC2R and PHI-R shared a common ancestral receptor.