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 studies on this neuroendocrine peptide deals with non-mammalian neuroendocrine systems. We determined the cDNA sequence of the CRH precursor of the teleost Oreochromis mossambicus (tilapia) and studied the biological potency of the 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 tilapia hypothalamus cDNA library with the tilapia CRH PCR products. The precursor sequence (167 amino acids) contains a signal peptide, the CRH peptide and a motif conserved among all vertebrate CRH precursors. Tilapia CRH (41 aa) displays between 63% and 80% amino acid sequence identity to CRH from other vertebrates, whereas the degree of identity to members of the urotensin I/urocortin lineage is considerably lower. In a phylogenetic tree, based on alignment of all full CRH peptide precursors presently known, the three teleost CRH precursors (tilapia; sockeye salmon, Oncorhynchus nerka; white sucker, Catostomus commersoni) form a monophyletic group distinct from amphibian and mammalian precursors. Despite the differences between the primary 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 tilapia and salmon CRH sequences show that more variation exists between orthologous vertebrate CRH structures, and teleost CRHs in particular than previously recognized. Whether the structural differences reflect different mechanisms of action of this peptide in the stress response remains to be investigated.