title: Molecular evolution of GPCRs: Secretin/secretin receptors.
abstract: In mammals, secretin is a 27-amino acid peptide that was first studied in 1902 by Bayliss and Starling from the extracts of the jejunal mucosa for its ability to stimulate pancreatic secretion. To date, secretin has only been identified in tetrapods, with the earliest diverged secretin found in frogs. Despite being the first hormone discovered, secretin's evolutionary origin remains enigmatic, it shows moderate sequence identity in nonmammalian tetrapods but is highly conserved in mammals. Current hypotheses suggest that although secretin has already emerged before the divergence of osteichthyans, it was lost in fish and retained only in land vertebrates. Nevertheless, the cognate receptor of secretin has been identified in both actinopterygian fish (zebrafish) and sarcopterygian fish (lungfish). However, the zebrafish secretin receptor was shown to be nonbioactive. Based on the present information that the earliest diverged bioactive secretin receptor was found in lungfish, and its ability to interact with both vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide potently suggested that secretin receptor was descended from a VPAC-like receptor gene before the Actinopterygii-Sarcopterygii split in the vertebrate lineage. Hence, secretin and secretin receptor have gone through independent evolutionary trajectories despite their concurrent emergence post-2R. A functional secretin-secretin receptor axis has probably emerged in the amphibians. Although the pleiotropic actions of secretin are well documented in the literature, only limited information of its physiological functions in nonmammalian tetrapods have been reported. To decipher the structural and functional divergence of secretin and secretin receptor, functional characterization of the ligand-receptor pair in nonmammals would be the next perspective for investigation.
authors.affiliation: School of Biological SciencesThe University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong.
authors.forename: Janice K V
authors.initials: JK
authors.lastname: Tam
authors.affiliation: School of Biological SciencesThe University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong.
authors.forename: Leo T O
authors.initials: LT
authors.lastname: Lee
authors.affiliation: School of Biological SciencesThe University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong.
authors.forename: Jun
authors.initials: J
authors.lastname: Jin
authors.affiliation: School of Biological SciencesThe University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong bkcc@hku.hk.
authors.forename: Billy K C
authors.initials: BK
authors.lastname: Chow
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D019143:Evolution, Molecular; D005399:Fishes; D006801:Humans; D007583:Jejunum / Q000201:enzymology; D008969:Molecular Sequence Data; D010179:Pancreas / Q000378:metabolism; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000235:genetics* / Q000378:metabolism*; D011964:Receptors, Gastrointestinal Hormone / Q000235:genetics* / Q000378:metabolism*; D012633:Secretin / Q000235:genetics* / Q000378:metabolism; D016415:Sequence Alignment; D015398:Signal Transduction; D014660:Vasoactive Intestinal Peptide / Q000378:metabolism
chemical_list: D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D043562:Receptors, G-Protein-Coupled; D011964:Receptors, Gastrointestinal Hormone; C029937:secretin receptor; D012633:Secretin; D014660:Vasoactive Intestinal Peptide
keywords: divergence; evolution; origin; secretin; secretin receptor