On the Relative Stability of Dimeric Interfaces of the Mu-Opioid Receptor Inferred from Recent Crystallographic Studies

Abstract

The mu-opioid (MOP) receptor is the predominant target for most clinically used analgesics. In response to demonstrated correlations between opioid dependence and the MOP receptor, researchers have focused on alternative targets. Opioid receptor oligomers are among those that have been suggested to mediate analgesia without the common opioid-related adverse effects. Thus, obtaining a molecular-level understanding of the nature of receptor-receptor interactions in the membrane, either within, or between receptor subtypes, can both create new opportunities for drug discovery, and address the role of oligomerization in receptor function. The recent MOP receptor crystal structure has inspired hypotheses of dimerization contacts, specifically: a closely packed interface involving transmembrane (TM) helices TM5 and TM6, and a less compact one involving TM1, TM2, and helix 8 (H8). These interfaces exhibit similar arrangements to those found in crystals of the chemokine receptor CXCR4 and the kappa-opioid receptor, respectively. While it is tempting to speculate that the tighter TM5/TM6 arrangement has physiological relevance, additional studies are necessary to a) understand the relative dimer stability at TM5/TM6, TM1/TM2/H8, and other interfaces in the membrane, b) evaluate the contribution of the engineered T4 lysozyme (T4L) to the TM5/TM6 association, and c) investigate possible variability across different receptors. To begin to address these questions we have performed umbrella sampling molecular dynamics simulations of coarse-grained representations of the MOP receptor interacting at the TM5/TM6 (with and without T4L) or TM1/TM2/H8 interfaces in an explicit lipid-water environment. We have derived relative estimates of the dimerization free energy at the two specific interfaces and from these we suggest relative dimer lifetimes and dimeric fractions in a lipid mimetic environment. This information can help design future experiments aimed at understanding the role of dimerization in receptor function.