Recent progress in relaxin-3-related research.

Relaxin-3 (R3) is the latest member of the insulin (INSL) superfamily, which is composed of peptides with diverse sequences held together by characteristic disulfide links connecting A and B peptide chains. R3 has nearly exclusive expression in the brainstem and was demonstrated to be an additional ligand for LGR7. We recently identified R3 as a ligand for two orphan G-protein coupled receptors, GPCR135 and GPCR142. The predominant brain expression for both R3 and GPCR135, coupled with their high affinity interaction, strongly suggests that R3 is the endogenous ligand for GPCR135. Both R3 and GPCR135 from different species are highly conserved from genetic sequences to in vitro pharmacology. By contrast, GPCR142 is a pseudogene in the rat, and the mouse gene is less conserved with human GPCR142, suggesting that GPCR142 may have a diminished role as a receptor for R3 in the rodent. In addition, the tissue expression pattern of GPCR142, which is primarily in peripheral tissue, is drastically different from that of R3, suggesting that GPCR142 may have an endogenous ligand other than R3. Sequence analysis among INSL/relaxin family members shows that INSL5 is the closest member of R3. We were able to demonstrate that INSL-5 is an agonist for GPCR142 but not for GPCR135. We also showed that the mRNA expression pattern of INSL-5 overlaps with that of GPCR142. By substituting the A chain of R3 with the A chain of INSL-5, we devised a chimeric peptide (R3/I5) that is about 1000-fold selective for GPCR135 and GPCR142 over LGR7. Autoradiographic distribution of GPCR135 binding sites using [125I]R3/I5 in rat brain shows that GPCR135 receptor is most prominent in areas known for the processing of sensory signals.