Human insulin-like peptide 5 (INSL5) is a gut hormone exclusively produced by colonic L-cells, and its biological functions are mediated by relaxin family peptide receptor 4 (RXFP4). Its structure features two peptide chains (A- and B-) cross-linked by one intra-A-chain and two inter-chain disulfide bonds. The chemical or recombinant assembly of INSL5 has been found to be very challenging and time-consuming because of the aggregative nature of both chains and the multi-step purifications. Therefore, developing structurally and synthetically simplified INSL5 analogues is of foremost priority in order to facilitate the elucidation of its physiological roles.
We have previously generated a novel analogue (INSL5-A13) by shortening the INSL5 A-chain and deleting the intra-A-chain disulfide bond. The overall synthetic yield of INSL5-A13 was 17.4-fold higher than that of endogenous INSL5. We showed that the analogue INSL5-A13 exhibited agonistic activity at RXFP4 in vitro and was capable of stimulating colorectal propulsion in wild-type mice, but not in RXFP4 knockout mice. Recently, we have modified the INSL5-A13 analogue and the resulting analogue INSL5-A13:B23NR was shown to block agonist-induced activity in cells and animal models. We showed that INSL5-A13:B23NR was able to block the agonist-induced increased colon motility in mice of both genders that express the receptor, RXFP4. Our results suggest that our RXFP4 agonist and antagonist peptides are not only valuable tools for pharmacological research to elucidate the physiology of the INSL5-RXFP4 system, but also potential lead compounds in developing therapeutics for colon motility disorders (e.g. constipation and diarrhoea).