BPS2026 - Novel dimerization inhibitors for the orphan class C GPCR GPRC5B

G protein-coupled receptors (GPCRs) are grouped in different classes depending on their sequence homology, structure, and signaling features. Class C GPCRs are characterized by an elongated N terminus into the so-called cysteine-rich domain (CRD), and the bilobate venus flytrap module (VFT), which contains the orthosteric binding site for most of class C GPCRs, while the transmembrane domain (7TMD) hosts multiple allosteric binding sites. Interestingly, both the extracellular and the transmembrane domains are involved in homo- and heterodimerization patterns. However, these “canonical” features are not present for the members of the family of GPRC5 orphan class C GPCR, including the GPRC5B receptor. For this receptor, neither the endogenous ligands nor the coupling partner is known. GPRC5B has been previously shown to undergo dimerization with prostacyclin IP receptor in smooth muscle cells, and with the PGE 2 receptor EP2 in macrophages, where inactivation of GPRC5B diminishes EP2-dependent signaling. In this study, combining standard homology modeling and AlphaFold, we were able to model the structure of the GPCR5B both as a monomer and as a dimer with EP2, thus identifying the dimerization interface: It is located in an area corresponding to the allosteric site KS2 identified in class A GPCRs. Furthermore, by screening the identified site with molecular docking, we were able to identify three small-molecule ligands able to inhibit GPRC5B-EP2 dimerization. Among the identified compounds, K303MP20 inhibits the interaction of GPRC5B with both EP2 and IP receptors. These findings show a successful application of molecular modeling and virtual screening for the identification of novel potential therapeutic agents, particularly in cases where critical information, such as the experimental structure, is unavailable.