Mechanism of μ-opioid receptor inhibition by orphan GPR88

From the approximately 800 members of the G protein-coupled receptor (GPCR) family, more than 100 remain orphans (oGPCRs). There is evidence indicating that some oGPCRs may carry out a physiological role independently from endogenous ligands; this includes forming heteromers with other GPCRs and altering their functional and pharmacological properties via allosteric interactions. Recent studies have shown that some of these oGPCRs, e.g. GPR88 and GPR139, allosterically inhibit opioid activity by interacting with the μ-opioid receptor (μOR). Here, we have focused on the characterization of the interaction between GPR88 and µOR and the allosteric mechanism of inhibition. We confirmed that GPR88 inhibits µOR function in striatal neuronal primary cultures. Moreover, using a peptide-interfering approach combined with biophysical and biochemical techniques, we identified that GPR88 and µOR interact via transmembrane helix 6. A combination of molecular dynamic simulations and site-directed mutagenesis have allowed to propose that the negative regulatory role of GPR88 on µOR is due to the Q298 6.49 side chain of GPR88.