Oreochromis mossambicus (tilapia) corticotropin-releasing hormone: cDNA sequence and bioactivity

Although hypothalamic corticotropin-releasing hormone (CRH) is involved in the stress response in all vertebrate groups, only a limited number of stud ies on this neuroendocrine peptide deals with non-mammalian neuroendocrine systems. We determined the cDNA sequence of the CRH precursor of the teleos t Oreochromis mossambicus (tilapia) and studied the biological potency of t he CRH peptide in a homologous teleost bioassay. Polymerase chain reaction (PCR) with degenerate and specific primers yielded fragments of tilapia CRH cDNA. Full-length CRH cDNA (988 nucleotides) was obtained by screening a t ilapia hypothalamus cDNA library with the tilapia CRH PCR products. The pre cursor sequence (167 amino acids) contains a signal peptide, the CRH peptid e and a motif conserved among all vertebrate CRH precursors. Tilapia CRH (4 1 aa) displays between 63% and 80% amino acid sequence identity to CRH from other vertebrates, whereas the degree of identity to members of the uroten sin I/urocortin lineage is considerably lower. In a phylogenetic tree, base d on alignment of all full CRH peptide precursors presently known, the thre e teleost CRH precursors (tilapia; sockeye salmon, Oncorhynchus nerka; whit e sucker, Catostomus commersoni) form a monophyletic group distinct from am phibian and mammalian precursors. Despite the differences between the prima ry structures of tilapia and rat CRH, maximally effective concentrations of tilapia and rat CRH were equally potent in stimulating adrenocorticotropic hormone (ACTH) and I-MSH release by tilapia pituitaries in vitro. The tila pia and salmon CRH sequences show that more variation exists between orthol ogous vertebrate CRH structures, and teleost CRHs in particular than previo usly recognized. Whether the structural differences reflect different mecha nisms of action of this peptide in the stress response remains to be invest igated.