Molecular insights into the μ-opioid receptor selective signaling

GPCR functional selectivity whereby a ligand discriminates specific signaling pathways has opened new opportunities for the design of safer drugs. Ligands orchestrate GPCR signaling cascades by modulating the receptor conformational landscape. Our study provides insights into the dynamic mechanism enabling opioid ligands to selectively activate the G protein over the β-arrestin pathways through the μ-opioid receptor (μOR). We combined functional assays in living cells, solution NMR spectroscopy and enhanced-sampling molecular dynamic simulations to identify the specific μOR conformations induced by G protein-selective agonists. In particular, we describe the dynamic and allosteric communications between the ligand-binding pocket and the receptor intracellular domains, through conserved motifs in class A GPCRs. Most strikingly, the selective agonists triggered μOR conformational changes in the intracellular loop 1 and helix 8 domains, which may impair β-arrestin binding or signaling. The findings may apply to other GPCR families and provide key molecular information that could facilitate the design of selective ligands.