Coevolution of the spexin/galanin/kisspeptin family: Spexin activates galanin receptor type II and III.

The novel neuropeptide spexin (SPX) was discovered using bioinformatics. The function of this peptide is currently under investigation. Here, we identified SPX along with a second SPX gene (SPX2) in vertebrate genomes. Syntenic analysis and relocating SPXs and their neighbor genes on reconstructed vertebrate ancestral chromosomes revealed that SPXs reside in the near vicinity of the kisspeptin (KISS) and galanin (GAL) family genes on the chromosomes. Alignment of mature peptide sequences showed some extent of sequence similarity among the 3 peptide groups. Gene structure analysis indicated that SPX is more closely related to GAL than KISS. These results suggest that the SPX, GAL, and KISS genes arose through local duplications before 2 rounds (2R) of whole-genome duplication. Receptors of KISS and GAL (GAL receptor [GALR]) are phylogenetically closest among rhodopsin-like G protein-coupled receptors, and synteny revealed the presence of 3 distinct receptor families KISS receptor, GALR1, and GALR2/3 before 2R. A ligand-receptor interaction study showed that SPXs activate human, Xenopus, and zebrafish GALR2/3 family receptors but not GALR1, suggesting that SPXs are natural ligands for GALR2/3. Particularly, SPXs exhibited much higher potency toward GALR3 than GAL. Together, these results identify the coevolution of SPX/GAL/KISS ligand genes with their receptor genes. This study demonstrates the advantage of evolutionary genomics to explore the evolutionary relationship of a peptide gene family that arose before 2R by local duplications.