Sequence-Specific Regulation of Endocytic Lifetimes Modulates Arrestin-Mediated Signaling at the μ Opioid Receptor

Functional selectivity at the mu opioid receptor ( mu R), a prototypical G-protein-coupled receptor that is a physiologically relevant target for endogenous opioid neurotransmitters and analgesics, has been a major focus for drug discovery in the recent past. Functional selectivity is a cumulative effect of the magnitudes of individual signaling pathways, e.g., the G alpha(i)-mediated and the arrestin-mediated pathways for mu R. The present work tested the hypothesis that lifetimes of agonist-induced receptor-arrestin clusters at the cell surface control the magnitude of arrestin signaling, and therefore functional selectivity, at mu R. We show that endomorphin-2 (EM2),an arrestin-biased ligand for mu R, lengthens surface lifetimes of receptor-arrestin clusters significantly compared with morphine. The lengthening of lifetimes required two specific leucines on the C-terminal tail of mu R. Mutation of these leucines to alanines decreased the magnitude of arrestin-mediated signaling by EM2 without affecting G-protein signaling, suggesting that lengthened endocytic lifetimes were required for arrestin-biased signaling by EM2. Lengthening surface lifetimes by pharmacologically slowing endocytosis was sufficient to increase arrestin-mediated signaling by both EM2 and the clinically relevant agonist morphine. Our findings show that distinct ligands can leverage specific sequence elements on mu R to regulate receptor endocytic lifetimes and the magnitude of arrestin-mediated signaling, and implicate these sequences as important determinants of functional selectivity in the opioid system.