Research ArticleGPCR SIGNALING

Multisite phosphorylation is required for sustained interaction with GRKs and arrestins during rapid μ-opioid receptor desensitization

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Sci. Signal.  17 Jul 2018:
Vol. 11, Issue 539, eaas9609
DOI: 10.1126/scisignal.aas9609

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Controlling opioid receptor desensitization

Chronic opioid use leads to tolerance and the need for increasing doses to achieve pain relief, euphoria, and other effects. At the molecular level, tolerance occurs because of μ-opioid receptor desensitization: Opioid activation of its receptors triggers their phosphorylation and internalization, ultimately reducing the cellular response to opioids. Miess et al. investigated why high-efficacy opioids induce greater receptor internalization than lower-efficacy opioids such as morphine. The kinase GRK2 and the scaffolding protein β-arrestin regulate the desensitization of opioid receptors and other G protein–coupled receptors, and the authors assessed the recruitment of GRK2 and β-arrestin to phosphorylation site receptor mutants. GRK2 was more rapidly recruited to μ-opioid receptors by high-efficacy opioids than by morphine, and GRK2 promoted receptor desensitization to high-efficacy opioids through phosphorylation-dependent and phosphorylation-independent means. These results provide molecular insight into μ-opioid receptor desensitization, which may aid in the development of synthetic opioids that do not induce tolerance and thus have reduced potential for addiction.

Abstract

G protein receptor kinases (GRKs) and β-arrestins are key regulators of μ-opioid receptor (MOR) signaling and trafficking. We have previously shown that high-efficacy opioids such as DAMGO stimulate a GRK2/3-mediated multisite phosphorylation of conserved C-terminal tail serine and threonine residues, which facilitates internalization of the receptor. In contrast, morphine-induced phosphorylation of MOR is limited to Ser375 and is not sufficient to drive substantial receptor internalization. We report how specific multisite phosphorylation controlled the dynamics of GRK and β-arrestin interactions with MOR and show how such phosphorylation mediated receptor desensitization. We showed that GRK2/3 was recruited more quickly than was β-arrestin to a DAMGO-activated MOR. β-Arrestin recruitment required GRK2 activity and MOR phosphorylation, but GRK recruitment also depended on the phosphorylation sites in the C-terminal tail, specifically four serine and threonine residues within the 370TREHPSTANT379 motif. Our results also suggested that other residues outside this motif participated in the initial and transient recruitment of GRK and β-arrestins. We identified two components of high-efficacy agonist desensitization of MOR: a sustained component, which required GRK2-mediated phosphorylation and a potential soluble factor, and a rapid component, which was likely mediated by GRK2 but independent of receptor phosphorylation. Elucidating these complex receptor-effector interactions represents an important step toward a mechanistic understanding of MOR desensitization that leads to the development of tolerance and dependence.

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