Evolutionary origin of the structure and function of gonadotropin- inhibitory hormone: Insights from lampreys

Gonadotropin (GTH)-inhibitory hormone (Gn IH) is a novel hypothalamic neuropeptide that inhibits GTH secretion in mammals and birds by acting on gonadotropes and Gn RH neurons within the hypothalamic-pituitary-gonadal axis. Gn IH and its orthologs that have an LPXRFamide (X = L orQ) motif at the Cterminus (LPXRFamide peptides) have been identified in representative species of gnathostomes. However, the identity of an LPXRFamide peptide had yet to be identified in agnathans, the most ancient lineage of vertebrates, leaving open the question of the evolutionary origin of Gn IH and its ancestral function(s). In this study, we identified an LPXRFamide peptide gene encoding three peptides (LPXRFa-1a, LPXRFa-1b, and LPXRFa-2) from the brain of sea lamprey by synteny analysis and cDNA cloning, and the mature peptides by immunoaffinity purification and mass spectrometry. The expression of lamprey LPXRFamide peptide precursor mRNA was localized in the brain and gonad by RT-PCR and in the hypothalamus by in situ hybridization. Immunohistochemistry showed appositions of lamprey LPXRFamide peptide immunoreactive fibers in close proximity to Gn RH-III neurons, suggesting that lamprey LPXRFamide peptides act on Gn RH-III neurons. In addition, lamprey LPXRFa-2 stimulated the expression of lamprey Gn RH-III protein in the hypothalamus and GTHβ mRNA expression in the pituitary. Synteny and phylogenetic analyses suggest that the LPXRFamide peptide gene diverged from a common ancestral gene likely through gene duplication in the basal vertebrates. These results suggest that one ancestral function of LPXRFamide peptides may be stimulatory compared with the inhibitory function seen in laterevolved vertebrates (birds and mammals).