Agonist-selective patterns of µ-opioid receptor phosphorylation revealed by phosphosite-specific antibodies

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Identification of phosphorylation sites in the COOH-terminal tail of the μ-opioid receptor

Phosphorylation is considered a key event in the signalling and regulation of the μ opioid receptor (MOPr). Here, we used mass spectroscopy to determine the phosphorylation status of the C-terminal tail of the rat MOPr expressed in human embryonic kidney 293 (HEK-293) cells. Under basal conditions, MOPr is phosphorylated on Ser(363) and Thr(370), while in the presence of morphine or [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO), the COOH terminus is phosphorylated at three additional residues, Ser(356) , Thr(357) and Ser(375). Using N-terminal glutathione S transferase (GST) fusion proteins of the cytoplasmic, C-terminal tail of MOPr and point mutations of the same, we show that, in vitro, purified G protein-coupled receptor kinase 2 (GRK2) phosphorylates Ser(375), protein kinase C (PKC) phosphorylates Ser(363), while CaMKII phosphorylates Thr(370). Phosphorylation of the GST fusion protein of the C-terminal tail of MOPr enhanced its ability to bind arrestin-2 and -3. Hence, our study identifies both the basal and agonist-stimulated phospho-acceptor sites in the C-terminal tail of MOPr, and suggests that the receptor is subject to phosphorylation and hence regulation by multiple protein kinases.

Morphine desensitization and cellular tolerance are distinguished in rat locus ceruleus neurons

μ-Opioid receptor desensitization is considered an initial step in the development of tolerance. Curiously, the commonly used opioid morphine produces robust tolerance but minimal acute desensitization. This study was designed to test the hypothesis that desensitization is indeed present in morphine-treated animals and is distinguished from cellular tolerance by time course of recovery and mechanism. To induce tolerance, rats were treated with continuously released morphine for 1 week. Morphine-mediated activation of G protein-coupled inwardly rectifying potassium conductance was measured using voltage-clamp recordings from locus ceruleus neurons in brain slices from naive or morphine-treated rats. Cellular tolerance was observed as a decrease in morphine efficacy in slices from morphine-treated rats. This tolerance persisted for at least 6 h. An additional reduction in morphine-mediated current was observed when slices from morphine-treated rats were continuously maintained in morphine at approximately the circulating plasma concentration. This additional reduction recovered within 1 h after removal of morphine from the slice and represents desensitization that developed in the tolerant animal. Recovery from desensitization, but not long-lasting tolerance, was facilitated by protein phosphatase 1 (PP1) activity. Furthermore, desensitization, but not tolerance, was reversed by protein kinase C (PKC) inhibitor but not by an inhibitor of c-Jun N-terminal kinase. Therefore, morphine treatment leads to both long-lasting cellular tolerance and readily reversible desensitization, which are differentially dependent on PP1 and PKC activity and combine to result in a substantial decrease in morphine effectiveness. This PKC-mediated desensitization may contribute to the previously reported PKC-dependent reversal of behavioral tolerance.

Casting a wider net: whole-cell assays to capture varied and biased signaling

The observation of complex receptor behaviors has shown how ligands can have multiple efficacies and can also differentially stimulate certain cellular signaling pathways over others (i.e., biased signaling). Conventional pharmacological assays (usually proximal to the receptor) will detect ligands that produce the signal defined by the assay (i.e., Ca²⁺, cAMP, and others) but otherwise may miss biased ligands that produce little activation of pathways not measured by the assay. In theory, this is less of a hazard for generic whole-cell assays, which may be sensitive to multiple signaling inputs. Whole-cell assays have the advantage of detecting effects induced by a variety of receptor interactions with cytosolic proteins, including those that may be previously unknown. These ideas are discussed within the context of the high-throughput flow cytometry measurement of receptor internalization described by Wu et al. in the current issue of the journal. The internalization of receptors can be a useful therapeutic modality and the article by Wu et al. illustrates how this new assay, targeted to downstream cellular effects, can uncover unique ligand efficacies linked to receptor internalization.

Mechanisms of rapid opioid receptor desensitization, resensitization and tolerance in brain neurons

Agonists acting on µ-opioid receptors (MOR) are very effective analgesics but cause tolerance during long-term or repeated exposure. Intensive efforts have been made to find novel opioid agonists that are efficacious analgesics but can elude the signalling events that cause tolerance. µ-Opioid agonists differentially couple to downstream signalling mechanisms. Some agonists, such as enkephalins, D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), methadone and sufentanyl are efficacious at mediating G-protein and effector coupling, as well as triggering MOR regulatory events that include MOR phosphorylation, β-arrestin binding, receptor endocytosis and recycling. By contrast, morphine and closely related alkaloids can mediate efficacious MOR-effector coupling but poorly trigger receptor regulation. Several models have been proposed to relate differential MOR regulation by different opioids with their propensity to cause tolerance. Most are based on dogma that β-arrestin-2 (βarr-2) binding causes MOR desensitization and/or that MOR endocytosis and recycling are required for receptor resensitization. This review will examine some of these notions in light of recent evidence establishing that MOR dephosphorylation and resensitization do not require endocytosis. Recent evidence from opioid-treated animals also suggests that impaired MOR-effector coupling is driven, at least in part, by enhanced desensitization, as well as impaired resensitization that appears to be βarr-2 dependent. Better understanding of how chronic exposure to opioids alters receptor regulatory mechanisms may facilitate the development of effective analgesics that produce limited tolerance.

Endomorphin-2: a biased agonist at the μ-opioid receptor

Previously we correlated the efficacy for G protein activation with that for arrestin recruitment for a number of agonists at the μ-opioid receptor (MOPr) stably expressed in HEK293 cells. We suggested that the endomorphins (endomorphin-1 and -2) might be biased toward arrestin recruitment. In the present study, we investigated this phenomenon in more detail for endomorphin-2, using endogenous MOPr in rat brain as well as MOPr stably expressed in HEK293 cells. For MOPr in neurons in brainstem locus ceruleus slices, the peptide agonists [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) and endomorphin-2 activated inwardly rectifying K(+) current in a concentration-dependent manner. Analysis of these responses with the operational model of pharmacological agonism confirmed that endomorphin-2 had a much lower operational efficacy for G protein-mediated responses than did DAMGO at native MOPr in mature neurons. However, endomorphin-2 induced faster desensitization of the K(+) current than did DAMGO. In addition, in HEK293 cells stably expressing MOPr, the ability of endomorphin-2 to induce phosphorylation of Ser375 in the COOH terminus of the receptor, to induce association of arrestin with the receptor, and to induce cell surface loss of receptors was much more efficient than would be predicted from its efficacy for G protein-mediated signaling. Together, these results indicate that endomorphin-2 is an arrestin-biased agonist at MOPr and the reason for this is likely to be the ability of endomorphin-2 to induce greater phosphorylation of MOPr than would be expected from its ability to activate MOPr and to induce activation of G proteins.

The subtleties of µ-opioid receptor phosphorylation

The link between µ-opioid receptor phosphorylation and function is of critical importance to our understanding of the mechanisms underlying tolerance to opioid drugs. Increasingly sophisticated techniques are needed to assess the phosphorylation status of GPCRs, such as the use of phosphosite-specific antibodies that can monitor the kinetics of phosphorylation and dephosphorylation of individual residues in a receptor. Here the use of phosphosite-specific antibodies, raised against phosphorylated residues in the COOH-terminus of the µ-opioid receptor is discussed, along with some of the important findings that this approach has so far revealed. These include the finding that the µ-opioid receptor is constitutively phosphorylated, and that upon agonist removal it undergoes dephosphorylation equally well whether it is at the cell surface or internalized in endosomes. Thus already these phosphosite-specific antibodies are providing important new information about µ-opioid receptor function and the actions of opioid drugs.

LINKED ARTICLE

This article is a commentary on Doll et al., pp. 298-307 of this issue. To view this paper visit http://dx.doi.org/10.1111/j.1476-5381.2011.01382.x.

GRK2 protein-mediated transphosphorylation contributes to loss of function of μ-opioid receptors induced by neuropeptide FF (NPFF2) receptors

Neuropeptide FF (NPFF) interacts with specific receptors to modulate opioid functions in the central nervous system. On dissociated neurons and neuroblastoma cells (SH-SY5Y) transfected with NPFF receptors, NPFF acts as a functional antagonist of μ-opioid (MOP) receptors by attenuating the opioid-induced inhibition of calcium conductance. In the SH-SY5Y model, MOP and NPFF(2) receptors have been shown to heteromerize. To understand the molecular mechanism involved in the anti-opioid activity of NPFF, we have investigated the phosphorylation status of the MOP receptor using phospho-specific antibody and mass spectrometry. Similarly to direct opioid receptor stimulation, activation of the NPFF(2) receptor by [D-Tyr-1-(NMe)Phe-3]NPFF (1DMe), an analog of NPFF, induced the phosphorylation of Ser-377 of the human MOP receptor. This heterologous phosphorylation was unaffected by inhibition of second messenger-dependent kinases and, contrarily to homologous phosphorylation, was prevented by inactivation of G(i/o) proteins by pertussis toxin. Using siRNA knockdown we could demonstrate that 1DMe-induced Ser-377 cross-phosphorylation and MOP receptor loss of function were mediated by the G protein receptor kinase GRK2. In addition, mass spectrometric analysis revealed that the phosphorylation pattern of MOP receptors was qualitatively similar after treatment with the MOP agonist Tyr-D-Ala-Gly (NMe)-Phe-Gly-ol (DAMGO) or after treatment with the NPFF agonist 1DMe, but the level of multiple phosphorylation was more intense after DAMGO. Finally, NPFF(2) receptor activation was sufficient to recruit β-arrestin2 to the MOP receptor but not to induce its internalization. These data show that NPFF-induced heterologous desensitization of MOP receptor signaling is mediated by GRK2 and could involve transphosphorylation within the heteromeric receptor complex.

Quantitative encoding of the effect of a partial agonist on individual opioid receptors by multisite phosphorylation and threshold detection

In comparison to endogenous ligands of seven-transmembrane receptors, which typically act as full agonists, many drugs act as partial agonists. Partial agonism is best described as a "macroscopic" property that is manifest at the level of physiological systems or cell populations; however, whether partial agonists also encode discrete regulatory information at the "microscopic" level of individual receptors is not known. Here, we addressed this question by focusing on morphine, a partial agonist drug for μ-type opioid peptide receptors (MORs), and by combining quantitative mass spectrometry with cell biological analysis to investigate the reduced efficacy of morphine, compared to that of a peptide full agonist, in promoting receptor endocytosis. We showed that these chemically distinct ligands produced a complex and qualitatively similar mixture of phosphorylated opioid receptor forms in intact cells. Quantitatively, however, the different agonists promoted disproportionate multisite phosphorylation of a specific serine and threonine motif, and we found that modification at more than one residue was essential for the efficient recruitment of the adaptor protein β-arrestin that mediated subsequent endocytosis of MORs. Thus, quantitative encoding of agonist-selective endocytosis at the level of individual opioid receptors was based on the conserved biochemical principles of multisite phosphorylation and threshold detection.

Desensitization and trafficking of μ-opioid receptors in locus ceruleus neurons: modulation by kinases

The phosphorylation of μ-opioid receptors (MOPRs) by G protein-coupled receptor kinases (GRKs), followed by arrestin binding, is thought to be a key pathway leading to desensitization and internalization. The present study used the combination of intracellular and whole-cell recordings from rats and mice, as well as live cell imaging of Flag-tagged MOPRs from mouse locus ceruleus neurons, to examine the role of protein kinases in acute desensitization and receptor trafficking. Inhibition of GRKs by using heparin or GRK2-mutant mice did not block desensitization or alter the rate of recovery from desensitization. The nonselective kinase inhibitor staurosporine did not reduce the extent of [Met(5)]enkephalin (ME)-induced desensitization but increased the rate of recovery from desensitization. In the presence of staurosporine, ME-activated FlagMOPRs were internalized but did not traffic away from the plasma membrane. The increased rate of recovery from desensitization correlated with the enhancement in the recycling of receptors to the plasma membrane. ME-induced MOPR desensitization persisted and the trafficking of receptors was modified after inhibition of protein kinases. The results suggest that desensitization of MOPRs may be an early step after agonist binding that is modulated by but is not dependent on kinase activity.

Analgesic tolerance to high-efficacy agonists but not to morphine is diminished in phosphorylation-deficient S375A μ-opioid receptor knock-in mice

Morphine is one of the most potent analgesic drugs. However, the utility of morphine in the management of chronic pain is limited by its rapid development of tolerance. Morphine exerts all of its pharmacological effects via the μ-opioid receptor. In many systems, tolerance is associated with phosphorylation and desensitization of G-protein-coupled receptors (GPCRs). In case of the μ-opioid receptor, phosphorylation occurs in an agonist-selective manner. High-efficacy agonists such as [d-Ala(2)-MePhe(4)-Gly-ol]enkephalin (DAMGO), fentanyl, or etonitazene stimulate the phosphorylation of both C-terminal threonine 370 (T370) and serine 375 (S375). In contrast, morphine promotes the phosphorylation of S375 but fails to stimulate T370 phosphorylation. Here, we have assessed the contribution of S375 phosphorylation to the development of antinociceptive tolerance to high- and low-efficacy μ agonists in vivo. We show that S375 phosphorylation of the μ-opioid receptor occurs in intact mouse brain in a dose-dependent manner after administration of morphine, fentanyl, or etonitazene. In knock-in mice expressing the phosphorylation-deficient S375A mutant of the μ-opioid receptor, morphine and fentanyl exhibited greater dose-dependent antinociceptive responses than in wild-type mice. However, acute and chronic tolerance to morphine was retained in S375A mutant mice. In contrast, antinociceptive tolerance after repeated subcutaneous application of etonitazene or repeated intracerebroventricular application of DAMGO was diminished. Thus, tolerance to μ agonists with different efficacies develops through distinct pathways. Whereas tolerance induced by DAMGO or etonitazene requires agonist-driven phosphorylation of S375, the development and maintenance of antinociceptive tolerance to morphine occurs independent of S375 phosphorylation.