The evolution of the relaxin peptide family and their receptors

The relaxin peptide family in humans consists of relaxin-1, 2 and 3 and the insulin-like peptides (INSL)-3, 4, 5 and 6. The evolution of this family has been controversial; points of contention include the existence of an invertebrate relaxin and the absence of a ruminant relaxin. Over the past four years we have performed a comprehensive analysis of the relaxin peptide family using all available vertebrate and invertebrate genomes. Contrary to previous reports an invertebrate relaxin was not found; sequence similarity searches indicate the family emerged during early vertebrate evolution. Phylogenetic analyses revealed the presence ofpotential relaxin-3, relaxin and INSL5 homologs in fish; dating their emergence far earlier than previously believed. There are four known relaxin peptide family receptors; the relaxin and INSL3 receptors, the leucine rich repeat containing G protein-coupled receptors (LGR), LGR7 and LGR8 respectively; and the two relaxin-3 receptors, GPCR135 and GPCR142. Database searching identified several invertebrate ancestors of LGR7 and LGR8; the absence of an invertebrate relaxin suggests the presence of an unidentified invertebrate ligand for these receptors. No invertebrate ancestors of GPCR135 or GPCR142 were found. Based on the theory that interacting proteins co-evolve together, phylogenetic analyses of the relaxin peptide family receptors were performed to provide insight into interactions within the relaxin system. Co-evolution between INSL5 and GPCR142, as evidenced by the loss of both genes in the rat and dog and their similar expression profiles, predicted GPCR142 to be the endogeneous INSL5 receptor. This interaction has since been confirmed experimentally. The emergence and presence of multiple GPCR135 homologs in fish reflected similar findings for relaxin-3. It seems likely the ancestral relaxin system was relaxin-3 acting through GPCR135, before LGR7 was "acquired" as a relaxin receptor early in vertebrate development.