Disruption of the Na ⁺ ion binding site as a mechanism for positive allosteric modulation of the mu-opioid receptor

Abstract

Positive allosteric modulation of the mu-opioid receptor (MOPr), the site of action of all clinically used opioids, represents a potential approach for the management of pain. We recently reported on positive allosteric modulators of MOPr (mu-PAMs), a class A G protein coupled receptor (GPCR). This study was designed to examine the mechanism of allostery by comparing the degree to which opioid ligand structure governs modulation. To do this we examined the interaction of the mu-PAM, BMS-986122, with a chemically diverse range of MOPr orthosteric ligands. Generally, for full agonists BMS-986122 enhanced the binding affinity and potency to activate G protein with no alteration in the maximal effect. In contrast, lower efficacy agonists including morphine were insensitive to alterations in binding affinity and showed little to no change in potency to stimulate G protein. Instead, there was an increase in maximal G protein stimulation. Antagonists were unresponsive to the modulatory effects of BMS-986122. Sodium is a known endogenous allosteric modulator of MOPr and alters orthosteric agonist affinity and efficacy. The sensitivity of an orthosteric ligand to BMS-986122 was strongly correlated with its sensitivity to NaCl. In addition, BMS-986122 decreased the ability of NaCl to modulate agonist binding in an allosteric fashion. Overall, BMS-986122 displayed marked probe dependence that was based upon the efficacy of the orthosteric ligand and can be explained using the Monod-Wyman-Changeux two-state model of allostery. Furthermore, disruption of the Na + ion binding site may represent a common mechanism for allosteric modulation of class A GPCRs.endogenous opioids | opiates | GTPgammaS | ligand binding | receptor states T he mu-opioid receptor (MOPr) is the site of action of all clinically used opioid drugs. MOPr is a class A G proteincoupled receptor (GPCR) that activates heterotrimeric Gi/o proteins. Clinically used opioid agonists bind to the orthosteric site on MOPr and although they are efficacious at causing pain relief, have a number of unwanted side effects resulting from direct MOPr activation. We have recently discovered and presented a preliminary characterization of positive allosteric modulators of MOPr (mu-PAMs) and are currently pursuing the idea that mu-PAMs could be a viable way to manage pain (1, 2). The ligand BMS-986122 (Fig. S1) represents the most active mu-PAM currently identified. It was discovered in a high-throughput screen for its ability to enhance the recruitment of β-arrestin to MOPr by the agonist endomorphin-1. Although having little agonist activity on its own, this modulator has the ability to enhance the affinity, potency, and/or maximal response of MOPr agonists. In the same systems, BMS-986122 has no activity when the delta opioid receptor (DOPr) is expressed, indicating the importance of MOPr for BMS-986122 activity.The study of allosteric modulation of GPCRs has recently gained momentum (3) and represents a relatively unexplored avenue for drug development (4, 5). Allosteric mo...

Ligand-Based Discovery of a New Scaffold for Allosteric Modulation of the μ-Opioid Receptor

“…The prototype μ -PAM BMS-986122 also showed the highest levels of cooperativity with methadone and its isomers. 19 Because MS1 is a new scaffold, this similar probe dependence may be reflective of a similar mechanism of action and/or mode of binding. In addition, MS1 enhanced the maximal activation of the partial agonist morphine to activate G-protein.…”

“…This again fits with the probe dependence of BMS-986122 in which the efficacy of partial agonists was increased. The mechanism of BMS-986122 action was found to be through allosteric disruption of sodium ion binding 19 and it would be interesting to determine if this new chemotype also functions in a similar manner.…”

Discovery, Synthesis, and Molecular Pharmacology of Selective Positive Allosteric Modulators of the δ-Opioid Receptor

“…21 The μ receptor PAM 2-(3-bromo-4-methoxyphenyl)-3-((4-chlorophenyl)sulfonyl)thiazolidine ( 16 , BMS-986122) has been found to differentially increase the affinity of various orthosteric agonists, and the magnitude of the affinity increase ( α value) produced by 16 correlates with the intrinsic activity of the orthosteric ligand used. 22 The mechanism by which 16 induces this affinity modulation is suggested to be via reducing the affinity of Na + for its binding site on the μ receptor. The precise binding site for 16 on the μ receptor has not been clearly established, and it is unknown whether the δ receptor PAMs described here bind to an analogous binding site on the δ receptor or act via a similar mechanism.…”

Endogenous Allosteric Modulators of G Protein–Coupled Receptors

“…This sodium site appears to collapse in active-state structures, suggesting that it plays a key role in constraining GPCRs in an inactive state (Wootten et al, 2013b;Katritch et al, 2014). Interestingly, the sodium site has also been recently implicated in the regulation of biased agonism by the d-opioid receptor and in the actions of synthetic small-molecule allosteric ligands for the m-opioid receptor (Livingston and Traynor, 2014). It should be noted, however, that a subset of class A GPCRs does not possess the requisite acidic residue at the 2.50 position.…”