C-terminal Splice Variants of the Mouse µ-Opioid Receptor Differ in Morphine-induced Internalization and Receptor Resensitization

OPRM1 rs2075572 has potential to affect plasma buprenorphine level in opioid users, but not OPRM1 rs562859

OPRM1 gene encoding mu-opioid receptor (MOR) is the primary candidate gene for buprenorphine (BUP) pharmacogenetics. OPRM1 undergoes alternative splicing leading to multiple MOR subtypes. Thus, in the current study 2 SNPs (rs1799972 and rs562859) were selected due to evidence for their contribution to alternative splicing of OPRM1. The effects of 2 SNPs of OPRM1 gene on plasma buprenorphine and norbuprenorphine levels in a sample of 233 OUD patients receiving BUP/naloxone were examined. Polymorphisms were analyzed by PCR and RFLP. BUP and norbuprenorphine concentrations in plasma were measured by LC-MS/MS. OPRM1 rs2075572 GC + CC (0.12 ng/ml) had significantly higher plasma BUP level compared to GG (0.084 ng/ml) (p = 0.043). Although there was not a statistically significant difference between OPRM1 rs562859 genotypes (p = 0.46), patients with OPRM1 rs562859 CT + TT had higher plasma BUP and BUP-related values as compared to those with CC. In conclusion, the effect of OPRM1 rs2075572 on BUP levels in opioid users' plasma was shown in a Caucasian population for the first time. On the other hand, OPRM1 rs562859 seems not to influence the BUP pharmacology.

A Balancing Act: Learning from the Past to Build a Future-Focused Opioid Strategy

The harmful side effects of opioid drugs such as respiratory depression, tolerance, dependence, and abuse potential have limited the therapeutic utility of opioids for their entire clinical history. However, no previous attempt to develop effective pain drugs that substantially ameliorate these effects has succeeded, and the current opioid epidemic affirms that they are a greater hindrance to the field of pain management than ever. Recent attempts at new opioid development have sought to reduce these side effects by minimizing engagement of the regulatory protein arrestin-3 at the mu-opioid receptor, but there is significant controversy around this approach. Here, we discuss the ongoing effort to develop safer opioids and its relevant historical context. We propose a new model that reconciles results previously assumed to be in direct conflict to explain how different signaling profiles at the mu-opioid receptor contribute to opioid tolerance and dependence. Our goal is for this framework to inform the search for a new generation of lower liability opioid analgesics.

How do opioids control pain circuits in the brainstem during opioid-induced disorders and in chronic pain? Implications for the treatment of chronic pain

ABSTRACT

Brainstem areas involved in descending pain modulation are crucial for the analgesic actions of opioids. However, the role of opioids in these areas during tolerance, opioid-induced hyperalgesia (OIH), and in chronic pain settings remains underappreciated. We conducted a revision of the recent studies performed in the main brainstem areas devoted to descending pain modulation with a special focus on the medullary dorsal reticular nucleus (DRt), as a distinctive pain facilitatory area and a key player in the diffuse noxious inhibitory control paradigm. We show that maladaptive processes within the signaling of the µ-opioid receptor (MOR), which entail desensitization and a switch to excitatory signaling, occur in the brainstem, contributing to tolerance and OIH. In the context of chronic pain, the alterations found are complex and depend on the area and model of chronic pain. For example, the downregulation of MOR and δ-opioid receptor (DOR) in some areas, including the DRt, during neuropathic pain likely contributes to the inefficacy of opioids. However, the upregulation of MOR and DOR, at the rostral ventromedial medulla, in inflammatory pain models, suggests therapeutic avenues to explore. Mechanistically, the rationale for the diversity and complexity of alterations in the brainstem is likely provided by the alternative splicing of opioid receptors and the heteromerization of MOR. In conclusion, this review emphasizes how important it is to consider the effects of opioids at these circuits when using opioids for the treatment of chronic pain and for the development of safer and effective opioids.

Structural and Molecular Determinants for Isoform Bias at Human Histamine H3 Receptor Isoforms

The human histamine H₃ receptor (hH₃R) is predominantly expressed in the CNS, where it regulates the synthesis and release of histamine and other neurotransmitters. Due to its neuromodulatory role, the hH₃R has been associated with various CNS disorders, including Alzheimer's and Parkinson's disease. Markedly, the hH₃R gene undergoes extensive splicing, resulting in 20 isoforms, of which 7TM isoforms exhibit variations in the intracellular loop 3 (IL3) and/or C-terminal tail. Particularly, hH₃R isoforms that display variations in IL3 (e.g., hH₃R-365) are shown to differentially signal via Gα_i-dependent pathways upon binding of biased agonists (e.g., immepip, proxifan, imetit). Nevertheless, the mechanisms underlying biased agonism at hH₃R isoforms remain unknown. Using a structure-function relationship study with a broad range of H₃R agonists, we thereby explored determinants underlying isoform bias at hH₃R isoforms that exhibit variations in IL3 (i.e., hH₃R-445, -415, -365, and -329) in a Gα_i-dependent pathway (cAMP inhibition). Hence, we systematically characterized hH₃R isoforms on isoform bias by comparing various ligand properties (i.e., structural and molecular) to the degree of isoform bias. Importantly, our study provides novel insights into the structural and molecular basis of receptor isoform bias, highlighting the importance to study GPCRs with multiple isoforms to better tailor drugs.

Design and development of novel, short, stable dynorphin-based opioid agonists for safer analgesic therapy

Kappa opioid receptors have exceptional potential as an analgesic target, seemingly devoid of many problematic Mu receptor side-effects. Kappa-selective, small molecule pharmaceutical agents have been developed, but centrally mediated side-effects limit clinical translation. We modify endogenous dynorphin peptides to improve drug-likeness and develop safer KOP receptor agonists for clinical use. Using rational, iterative design, we developed a series of potent, selective, and metabolically stable peptides from dynorphin 1-7. Peptides were assessed for in vitro cAMP-modulation against three opioid receptors, metabolic stability, KOP receptor selectivity, desensitisation and pERK-signalling capability. Lead peptides were evaluated for in vivo efficacy in a rat model of inflammatory nociception. A library of peptides was synthesised and assessed for pharmacological and metabolic stability. Promising peptide candidates showed low nanomolar KOP receptor selectivity in cAMP assay, and improved plasma and trypsin stability. Selected peptides showed bias towards cAMP signalling over pERK activity, also demonstrating reduced desensitisation. In vivo, two peptides showed significant opioid-like antinociception comparable to morphine and U50844H. These highly potent and metabolically stable peptides are promising opioid analgesic leads for clinical translation. Since they are somewhat biased peptide Kappa agonists they may lack many significant side-effects, such as tolerance, addiction, sedation, and euphoria/dysphoria, common to opioid analgesics.

Cellular Tolerance Induced by Chronic Opioids in the Central Nervous System

Opioids are powerful analgesics that elicit acute antinociceptive effects through their action the mu opioid receptor (MOR). However opioids are ineffective for chronic pain management, in part because continuous activation of MORs induces adaptive changes at the receptor level and downstream signaling molecules. These adaptations include a decrease in receptor-effector coupling and changes to second messenger systems that can counteract the persistent activation of MORs by opioid agonists. Homeostatic regulation of MORs and downstream signaling cascades are viewed as precursors to developing tolerance. However, despite numerous studies identifying crucial mechanisms that contribute to opioid tolerance, no single regulatory mechanism that governs tolerance in at the cellular and systems level has been identified. Opioid tolerance is a multifaceted process that involves both individual neurons that contain MORs and neuronal circuits that undergo adaptations following continuous MOR activation. The most proximal event is the agonist/receptor interaction leading to acute cellular actions. This review discusses our understanding of mechanisms that mediate cellular tolerance after chronic opioid treatment that, in part, is mediated by agonist/receptor interaction acutely.

Modulation of OPRM1 Alternative Splicing by Morphine and HIV-1 Nef

Clinically used opioids, such as morphine, activate the mu opioid receptor (MOR) encoded by Opioid Receptor Mu 1 (OPRM1) gene. Examination of the opioid receptor genes showed that the human OPRM1 pre-mRNA undergoes extensive alternative splicing events and capable of expressing 21 isoforms. However, characterization of OPRM1 signaling is generalized, and only one isoform (MOR-1) has been extensively studied. Compounding this issue is the increasing significance of intravenous drug abuse in HIV neuropathogenesis. Here, we investigated the molecular impact of morphine and HIV-1 on regulation of OPRM1 pre-mRNA splicing in in vitro and in vivo models. Our results suggested that morphine treatment specifically induces the alternative splicing of MOR-1X isoform among the other isoforms analyzed in neuronal cells. Interestingly, alternative splicing and expression of MOR-1X isoform was also induced in postmortem brain tissues obtained from people with HIV (PWH). Additionally, treatment of control rats with morphine induced alternative splicing of MOR-1X in the brain regions involved in the reward pathways. More interestingly, HIV-1 transgenic (HIV-1Tg) rats, showed an additive induction of MOR-1X isoform with the exposure to morphine. To further assess the possible role of HIV secretory proteins in alternative splicing of OPRM1 gene, we analyzed the impact of HIV-1 Tat, gp120 and Nef proteins on alternative splicing of MOR-1X isoform. While the Tat and gp120 had no visible effects, treatment of neurons with Nef induced MOR-1X alternative splicing that was comparable to treatment with morphine. Altogether, our results suggest that HIV-1 may alter MOR isoform expression with Nef protein by amplifying the rate of MOR-1X alternative splicing induced by morphine.

Attenuated G protein signaling and minimal receptor phosphorylation as a biochemical signature of low side-effect opioid analgesics

Multi-receptor targeting has been proposed as a promising strategy for the development of opioid analgesics with fewer side effects. Cebranopadol and AT-121 are prototypical bifunctional ligands targeting the nociceptin/orphanin FQ peptide receptor (NOP) and µ-opioid receptor (MOP) that elicit potent analgesia in humans and nonhuman primates, respectively. Cebranopadol was reported to produce typical MOP-related side effects such as respiratory depression and reward, whereas AT-121 appeared to be devoid of these liabilities. However, the molecular basis underlying different side effect profiles in opioid analgesics remains unknown. Here, we examine agonist-induced receptor phosphorylation and G protein signaling profiles of a series of chemically diverse mixed MOP/NOP agonists, including cebranopadol and AT-121. We found that these compounds produce strikingly different MOP phosphorylation profiles. Cebranopadol, AT-034 and AT-324 stimulated extensive MOP phosphorylation, whereas AT-201 induced selective phosphorylation at S375 only. AT-121, on the other hand, did not promote any detectable MOP phosphorylation. Conversely, none of these compounds was able to elicit strong NOP phosphorylation and low NOP receptor phosphorylation correlated with partial agonism in a GIRK-channel assay. Our results suggest a close correlation between MOP receptor phosphorylation and side effect profile. Thus, bifunctional MOP/NOP opioid ligands combining low efficacy G protein signaling at both NOP and MOP with no detectable receptor phosphorylation appear to be devoid of side-effects such as respiratory depression, abuse liability or tolerance development, as with AT-121.

Exploring Pharmacological Functions of Alternatively Spliced Variants of the Mu Opioid Receptor Gene, Oprm1, via Gene-Targeted Animal Models

The mu opioid receptor has a distinct place in the opioid receptor family, since it mediates the actions of most opioids used clinically (e.g., morphine and fentanyl), as well as drugs of abuse (e.g., heroin). The single-copy mu opioid receptor gene, OPRM1, goes through extensive alternative pre-mRNA splicing to generate numerous splice variants that are conserved from rodents to humans. These OPRM1 splice variants can be classified into three structurally distinct types: (1) full-length 7 transmembrane (TM) carboxyl (C)-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Distinct pharmacological functions of these splice variants have been demonstrated by both in vitro and in vivo studies, particularly by using several unique gene-targeted mouse models. These studies provide new insights into our understanding of the complex actions of mu opioids with regard to OPRM1 alternative splicing. This review provides an overview of the studies that used these gene-targeted mouse models for exploring the functional importance of Oprm1 splice variants.

Alternative Pre-mRNA Splicing of the Mu Opioid Receptor Gene, OPRM1: Insight into Complex Mu Opioid Actions

Most opioid analgesics used clinically, including morphine and fentanyl, as well as the recreational drug heroin, act primarily through the mu opioid receptor, a class A Rhodopsin-like G protein-coupled receptor (GPCR). The single-copy mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating multiple splice variants or isoforms via a variety of alternative splicing events. These OPRM1 splice variants can be categorized into three major types based on the receptor structure: (1) full-length 7 transmembrane (TM) C-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Increasing evidence suggests that these OPRM1 splice variants are pharmacologically important in mediating the distinct actions of various mu opioids. More importantly, the OPRM1 variants can be targeted for development of novel opioid analgesics that are potent against multiple types of pain, but devoid of many side-effects associated with traditional opiates. In this review, we provide an overview of OPRM1 alternative splicing and its functional relevance in opioid pharmacology.

HA-MOP knockin mice express the canonical µ-opioid receptor but lack detectable splice variants

G protein-coupled receptors (GPCRs) are notoriously difficult to detect in native tissues. In an effort to resolve this problem, we have developed a novel mouse model by fusing the hemagglutinin (HA)-epitope tag sequence to the amino-terminus of the µ-opioid receptor (MOP). Although HA-MOP knock-in mice exhibit reduced receptor expression, we found that this approach allowed for highly efficient immunodetection of low abundant GPCR targets. We also show that the HA-tag facilitates both high-resolution imaging and immunoisolation of MOP. Mass spectrometry (MS) confirmed post-translational modifications, most notably agonist-selective phosphorylation of carboxyl-terminal serine and threonine residues. MS also unequivocally identified the carboxyl-terminal ³⁸⁷LENLEAETAPLP³⁹⁸ motif, which is part of the canonical MOP sequence. Unexpectedly, MS analysis of brain lysates failed to detect any of the 15 MOP isoforms that have been proposed to arise from alternative splicing of the MOP carboxyl-terminus. For quantitative analysis, we performed multiple successive rounds of immunodepletion using the well-characterized rabbit monoclonal antibody UMB-3 that selectively detects the ³⁸⁷LENLEAETAPLP³⁹⁸ motif. We found that >98% of HA-tagged MOP contain the UMB-3 epitope indicating that virtually all MOP expressed in the mouse brain exhibit the canonical amino acid sequence.

Fritzwanker et al. develop a knock-in transgenic mouse line in which the hemagglutinin epitope tag sequence is fused with the amino-terminus of the µ-opioid receptor. Their model enables more efficient immunodetection of G protein-coupled receptors.

Relevance of Mu-Opioid Receptor Splice Variants and Plasticity of Their Signaling Sequelae to Opioid Analgesic Tolerance

Opioid dose escalation to effectively control pain is often linked to the current prescription opioid abuse epidemic. This creates social as well as medical imperatives to better understand the mechanistic underpinnings of opioid tolerance to develop interventions that minimize it, thereby maximizing the analgesic effectiveness of opioids. Profound opioid analgesic tolerance can be observed in the absence of mu-opioid receptor (MOR) downregulation, aggregate MOR G protein uncoupling, and MOR desensitization, in the absence of impaired G protein coupled receptor kinase phosphorylation, arrestin binding, or endocytosis. Thus, we have explored alternative biochemical sequelae that might better account for opioid analgesic tolerance. Our findings indicate that substantial plasticity among upstream and downstream components of opioid receptor signaling and the emergence of alternative signaling pathways are major contributors to opioid analgesic tolerance. An exemplar of this plasticity is our findings that chronic morphine upregulates the MOR variants MOR-1B2 and MOR-1C1 and phosphorylation of their C-terminal sites not present in MOR-1, events causally associated with the chronic morphine-induced shift in MOR G protein coupling from predominantly Gi/Go inhibitory to Gs-stimulatory adenylyl cyclase signaling. The unique feature(s) of these variants that underlies their susceptibility to adapting to chronic morphine by altering the nature of their G protein coupling reveals the richness and pliability of MOR signaling that is enabled by generating a wide diversity of MOR variants. Furthermore, given differential anatomical expression patterns of MOR variants, MOR splice variant-dependent adaptations to chronic morphine could enable mechanistic underpinnings of tolerance and dependence that are CNS region- and cell-specific.

Rapid assessment of G protein signaling of four opioid receptors using a real-time fluorescence-based membrane potential assay

Opioids are the most powerful analgesics used clinically; however, severe side effects limit their long-term use. Various concepts involving biased intracellular signaling, partial agonism or multi-receptor targeting have been proposed to identify novel opioids with increased analgesic efficacy but reduced side effects. The search for such 'better opioids' implies screening of huge compound libraries and requires highly reliable, easy to perform and high throughput screening (HTS) assays. Here, we utilize an established membrane potential assay to monitor activation of G protein-coupled inwardly rectifying potassium (GIRK) channels, one of the main effectors of opioid receptor signaling, as readout to determine pharmacological profiles of opioids in a non-invasive manner. Specifically, in this study, we optimize assay conditions and extend the application of this assay to screen all four members of the opioid receptor family, stably expressed in AtT-20 and HEK293 cells. This ultra-sensitive system yielded EC₅₀ values in the nano-molar range. We further validate this system for screening cells stably co-expressing two opioid receptors, which could be a valuable tool for investigating bi-functional ligands and studying interactions between receptors. Additionally, we demonstrate the utility of this assay to study antagonists as well as ligands with varying efficacies. Our results suggest that this assay could easily be up-scaled to HTS assay in order to efficiently study receptor activation and screen for novel opioids.

Mu Opioids Induce Biased Signaling at the Full-Length Seven Transmembrane C-Terminal Splice Variants of the mu Opioid Receptor Gene, Oprm1

The biased signaling has been extensively studied in the original mu opioid receptor (MOR-1), particularly through G protein and β-arrestin2 signaling pathways. The concept that the G protein pathway is often linked to the therapeutic effect of the drug, while the β-arrestin pathway is associated to the side effects has been proposed to develop biased analgesic compounds with limited side-effects associated with traditional opiates. The mu opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing, generating multiple splice variants or isoforms that are conserved from rodent to human. One type of the Oprm1 splice variants are the full-length 7 transmembrane (7TM) C-terminal splice variants, which have identical receptor structures including entire binding pocket, but contain a different intracellular C-terminal tail resulted from 3' alternative splicing. Increasing evidence suggest that these full-length 7TM C-terminal variants play important roles in mu opioid pharmacology, raising questions regarding biased signaling at these multiple C-terminal variants. In the present study, we investigated the effect of different C-terminal variants on mu agonist-induced G protein coupling, β-arrestin2 recruitment, and ultimately, signaling bias. We found that mu agonists produced marked differences in G protein activation and β-arrestin2 recruitment among various C-terminal variants, leading to biased signaling at various level. Particularly, MOR-1O, an exon 7-associated variant, showed greater β-arrestin2 bias for most mu agonists than MOR-1, an exon 4-associated variant. Biased signaling of G protein-coupled receptors has been defined by evidences that different agonists can produce divergent signaling transduction pathways through a single receptor. Our findings that a single mu agonist can induce differential signaling through multiple 7TM splice variants provide a new perspective on biased signaling at least for Oprm1, which perhaps is important for our understanding of the complex mu opioid actions in vivo where all the 7TM splice variants co-exist.

A Truncated Six Transmembrane Splice Variant MOR-1G Enhances Expression of the Full-Length Seven Transmembrane μ-Opioid Receptor through Heterodimerization

The μ-opioid receptor gene undergoes extensive alternative splicing to generate an array of splice variants. One group of splice variants excludes the first transmembrane (TM) domain and contains six TM domains. These 6TM variants are essential for the action of a novel class of analgesic drugs, including 3-iodobenzoyl-6β-naltrexamide, which is potent against a spectrum of pain models without exhibiting the adverse side effects of traditional opiates. The 6TM variants are also involved in analgesic action through other drug classes, including δ-opioid and κ-opioids and α ₂-adrenergic drugs. Of the five 6TM variants in mouse, mouse μ-opioid receptor (mMOR)-1G is abundant and conserved from rodent to human. In the present study, we demonstrate a new function of mMOR-1G in enhancing expression of the full-length 7TM μ-opioid receptor, mMOR-1. When coexpressed with mMOR-1 in a Tet-Off inducible CHO cell line, mMOR-1G has no effect on mMOR-1 mRNA expression but greatly increases mMOR-1 protein expression in a dose-dependent manner determined by opioid receptor binding and [³⁵S] guanosine 5'-3-O-(thio)triphosphate binding. Subcellular fractionation analysis using OptiPrep density gradient centrifugation shows an increase of functional mMOR-1 receptor in plasma membrane-enriched fractions. Using a coimmunoprecipitation approach, we further demonstrate that mMOR-1G physically associates with mMOR-1 starting at the endoplasmic reticulum, suggesting a chaperone-like function. These data provide a molecular mechanism for how mMOR-1G regulates expression and function of the full-length 7TM μ-opioid receptor. SIGNIFICANCE STATEMENT: The current study establishes a novel function of mouse μ-opioid receptor (mMOR)-1G, a truncated splice variant with six transmembrane (TM) domains of the mouse μ-opioid receptor gene, in enhancing expression of the full-length 7TM mMOR-1 through a chaperone-like function.

Members of the same pharmacological family are not alike: Different opioids, different consequences, hope for the opioid crisis?

Pain management is the specialized medical practice of modulating pain perception and thus easing the suffering and improving the life quality of individuals suffering from painful conditions. Since this requires the modulation of the activity of endogenous systems involved in pain perception, and given the large role that the opioidergic system plays in pain perception, opioids are currently the most effective pain treatment available and are likely to remain relevant for the foreseeable future. This contributes to the rise in opioid use, misuse, and overdose death, which is currently characterized by public health officials in the United States as an epidemic. Historically, the majority of preclinical rodent studies were focused on morphine. This has resulted in our understanding of opioids in general being highly biased by our knowledge of morphine specifically. However, recent in vitro studies suggest that direct extrapolation of research findings from morphine to other opioids is likely to be flawed. Notably, these studies suggest that different opioid analgesics (opioid agonists) engage different downstream signaling effects within the cell, despite binding to and activating the same receptors. This recognition implies that, in contrast to the historical status quo, different opioids cannot be made equivalent by merely dose adjustment. Notably, even at equianalgesic doses, different opioids could result in different beneficial and risk outcomes. In order to foster further translational research regarding drug-specific differences among opioids, here we review basic research elucidating differences among opioids in pharmacokinetics, pharmacodynamics, their capacity for second messenger pathway activation, and their interactions with the immune system and the dopamine D2 receptors.

Agonist-selective NOP receptor phosphorylation correlates in vitro and in vivo and reveals differential post-activation signaling by chemically diverse agonists

Agonists of the nociceptin/orphanin FQ opioid peptide (NOP) receptor, a member of the opioid receptor family, are under active investigation as novel analgesics, but their modes of signaling are less well characterized than those of other members of the opioid receptor family. Therefore, we investigated whether different NOP receptor ligands showed differential signaling or functional selectivity at the NOP receptor. Using newly developed phosphosite-specific antibodies to the NOP receptor, we found that agonist-induced NOP receptor phosphorylation occurred primarily at four carboxyl-terminal serine (Ser) and threonine (Thr) residues, namely, Ser³⁴⁶, Ser³⁵¹, Thr³⁶², and Ser³⁶³, and proceeded with a temporal hierarchy, with Ser³⁴⁶ as the first site of phosphorylation. G protein-coupled receptor kinases 2 and 3 (GRK2/3) cooperated during agonist-induced phosphorylation, which, in turn, facilitated NOP receptor desensitization and internalization. A comparison of structurally distinct NOP receptor agonists revealed dissociation in functional efficacies between G protein-dependent signaling and receptor phosphorylation. Furthermore, in NOP-eGFP and NOP-eYFP mice, NOP receptor agonists induced multisite phosphorylation and internalization in a dose-dependent and agonist-selective manner that could be blocked by specific antagonists. Our study provides new tools to study ligand-activated NOP receptor signaling in vitro and in vivo. Differential agonist-selective NOP receptor phosphorylation by chemically diverse NOP receptor agonists suggests that differential signaling by NOP receptor agonists may play a role in NOP receptor ligand pharmacology.

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

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.

Spinal CX3CL1/CX3CR1 May Not Directly Participate in the Development of Morphine Tolerance in Rats

CX3CL1 (fractalkine), the sole member of chemokine CX3C family, is implicated in inflammatory and neuropathic pain via activating its receptor CX3CR1 on neural cells in spinal cord. However, it has not been fully elucidated whether CX3CL1 or CX3CR1 contributes to the development of morphine tolerance. In this study, we found that chronic morphine exposure did not alter the expressions of CX3CL1 and CX3CR1 in spinal cord. And neither exogenous CX3CL1 nor CX3CR1 inhibitor could affect the development of morphine tolerance. The cellular localizations of spinal CX3CL1 and CX3CR1 changed from neuron and microglia, respectively, to all the neural cells during the development of morphine tolerance. A microarray profiling revealed that 15 members of chemokine family excluding CX3CL1 and CX3CR1 were up-regulated in morphine-treated rats. Our study provides evidence that spinal CX3CL1 and CX3CR1 may not be involved in the development of morphine tolerance directly.

Research Resource: Real-Time Analysis of Somatostatin and Dopamine Receptor Signaling in Pituitary Cells Using a Fluorescence-Based Membrane Potential Assay

Stable somatostatin analogues and dopamine receptor agonists are the mainstay for the pharmacological treatment of functional pituitary adenomas; however, only a few cellular assays have been developed to detect receptor activation of novel compounds without disrupting cells to obtain the second messenger content. Here, we adapted a novel fluorescence-based membrane potential assay to characterize receptor signaling in a time-dependent manner. This minimally invasive technique provides a robust and reliable read-out for ligand-induced receptor activation in permanent and primary pituitary cells. The mouse corticotropic cell line AtT-20 endogenously expresses both the somatostatin receptors 2 (sst2) and 5 (sst5). Exposure of wild-type AtT-20 cells to the sst2- and sst5-selective agonists BIM-23120 and BIM-23268, respectively, promoted a pertussis toxin- and tertiapin-Q-sensitive reduction in fluorescent signal intensity, which is indicative of activation of G protein-coupled inwardly rectifying potassium (GIRK) channels. After heterologous expression, sst1, sst3, and sst4 receptors also coupled to GIRK channels in AtT-20 cells. Similar activation of GIRK channels by dopamine required overexpression of dopamine D2 receptors (D2Rs). Interestingly, the presence of D2Rs in AtT-20 cells strongly facilitated GIRK channel activation elicited by the sst2-D2 chimeric ligand BIM-23A760, suggesting a synergistic action of sst2 and D2Rs. Furthermore, stable somatostatin analogues produced strong responses in primary pituitary cultures from wild-type mice; however, in cultures from sst2 receptor-deficient mice, only pasireotide and somatoprim, but not octreotide, induced a reduction in fluorescent signal intensity, suggesting that octreotide mediates its pharmacological action primarily via the sst2 receptor.

Receptor Reserve Moderates Mesolimbic Responses to Opioids in a Humanized Mouse Model of the OPRM1 A118G Polymorphism

The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function.

Role of Phosphorylation Sites in Desensitization of µ-Opioid Receptor

Phosphorylation of residues in the C-terminal tail of the µ-opioid receptor (MOPr) is thought to be a key step in desensitization and internalization. Phosphorylation of C-terminal S/T residues is required for internalization (Just et al., 2013), but its role in desensitization is unknown. This study examined the influence of C-terminal phosphorylation sites on rapid desensitization of MOPr. Wild-type MOPr, a 3S/T-A mutant (S363A, T370A, S375A) that maintains internalization, 6S/T-A (S363A, T370A, S375A, T376A, T379A, T383A) and 11S/T-A (all C-terminal S/T residues mutated) mutants not internalized by MOPr agonists were stably expressed in AtT20 cells. Perforated patch-clamp recordings of MOPr-mediated activation of G-protein-activated inwardly rectifying potassium channel (Kir3.X) (GIRK) conductance by submaximal concentrations of Met(5)-enkephalin (ME) and somatostatin (SST; coupling to native SST receptor [SSTR]) were used to examine desensitization induced by exposure to ME and morphine for 5 minutes at 37°C. The rates of ME- and morphine-induced desensitization did not correlate with phosphorylation using phosphorylation site-specific antibodies. ME-induced MOPr desensitization and resensitization did not differ from wild-type for 3S/T-A and 6S/T-A but was abolished in 11S/T-A. Morphine-induced desensitization was unaffected in all three mutants, as was heterologous desensitization of SSTR. Morphine-induced desensitization (but not ME) was reduced by protein kinase C inhibition in wild-type MOPr and abolished in the 11S/T-A mutant, as was heterologous desensitization. These findings establish that MOPr desensitization can occur independently of S/T phosphorylation and internalization; however, C-terminal phosphorylation is necessary for some forms of desensitization because mutation of all C-terminal sites (11S/T-A) abolishes desensitization induced by ME.

VPAC2 receptor expression in human normal and neoplastic tissues: evaluation of the novel MAB SP235

The vasoactive intestinal peptide receptor 2 (VPAC2) is widely distributed throughout the body and is also overexpressed in a variety of human neoplastic tissues. However, little is known about its precise tissue distribution, regulation and function, which is in part be due to the lack of specific monoclonal anti-VPAC2 antibodies. In this study, we extensively characterised the novel rabbit monoclonal anti-VPAC2 antibody (clone SP235) using transfected cells and mouse, rat and human tissues. SP235 was then subjected to a comparative immunohistochemical study on a series of 167 histological specimens from formalin-fixed, paraffin-embedded human tumours and adjacent normal tissues. SP235 detected a broad band migrating at a molecular weight of 50-70 kDa in western blotting analyses of various mouse tissues as well as VPAC2- but not VPAC1-transfected human embryonic kidney 293 cells. SP235 yielded an efficient immunostaining of distinct cell populations in human tissue samples with a predominance of plasma membrane staining, which was completely abolished by preadsorption with its immunising peptide. SP235 immunohistochemistry detected VPAC2 receptors in lymphocytes present in spleen, tonsils, lymph nodes and Peyer's patches, chief cells of gastric mucosa, exocrine and endocrine pancreas, kidney tubules and blood vessels. In addition, VPAC2 was observed in thyroid, gastric and lung carcinomas, pancreatic adenocarcinomas, sarcomas and neuroendocrine tumours. SP235 may prove of great value in the identification of VPAC2 receptors during routine histopathological examination. VPAC2 visualisation with this simple and rapid immunohistochemical method will facilitate identification of candidate tumours for vasoactive intestinal peptide (VIP)-based diagnostics or therapeutic interventions.

Opioid receptor desensitization: mechanisms and its link to tolerance

Opioid receptors (OR) are part of the class A of G-protein coupled receptors and the target of the opiates, the most powerful analgesic molecules used in clinic. During a protracted use, a tolerance to analgesic effect develops resulting in a reduction of the effectiveness. So understanding mechanisms of tolerance is a great challenge and may help to find new strategies to tackle this side effect. This review will summarize receptor-related mechanisms that could underlie tolerance especially receptor desensitization. We will focus on the latest data obtained on molecular mechanisms involved in opioid receptor desensitization: phosphorylation, receptor uncoupling, internalization, and post-endocytic fate of the receptor.

Heterologous regulation of agonist-independent μ-opioid receptor phosphorylation by protein kinase C

BACKGROUND AND PURPOSE

Homologous agonist-induced phosphorylation of the μ-opioid receptor (MOR) is initiated at the carboxyl-terminal S375, followed by phosphorylation of T370, T376 and T379. In HEK293 cells, this sequential and hierarchical multi-site phosphorylation is specifically mediated by G-protein coupled receptor kinases 2 and 3. In the present study, we provide evidence for a selective and dose-dependent phosphorylation of T370 after activation of PKC by phorbol esters.

EXPERIMENTAL APPROACH

We used a combination of phospho site-specific antibodies, kinase inhibitors and siRNA knockdown screening to identify kinases that mediate agonist-independent phosphorylation of the MOR in HEK293 cells. In addition, we show with phospho site-specific antibodies were also used to study constitutive phosphorylation at S363 of MORs in mouse brain in vivo.

KEY RESULTS

Activation of PKC by phorbol esters or heterologous activation of substance P receptors co-expressed with MORs in the same cell induced a selective and dose-dependent phosphorylation of T370 that specifically requires the PKCα isoform. Inhibition of PKC activity did not compromise homologous agonist-driven T370 phosphorylation. In addition, S363 was constitutively phosphorylated in both HEK293 cells and mouse brain in vivo. Constitutive S363 phosphorylation required ongoing PKC activity. When basal PKC activity was decreased, S363 was also a substrate for homologous agonist-stimulated phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Our results have disclosed novel mechanisms of heterologous regulation of MOR phosphorylation by PKC. These findings represent a useful starting point for definitive experiments elucidating the exact contribution of PKC-driven MOR phosphorylation to diminished MOR responsiveness in morphine tolerance and pathological pain.

Challenges for opioid receptor nomenclature: IUPHAR Review 9

Recent developments in the study of the structure and function of opioid receptors raise significant challenges for the definition of individual receptor types and the development of a nomenclature that precisely describes isoforms that may subserve different functions in vivo. Presentations at the 2013 meeting of the International Narcotics Research Conference in Cairns, Australia, considered some of the new discoveries that are now unravelling the complexities of opioid receptor signalling. Variable processing of opioid receptor messenger RNAs may lead to the presence of several isoforms of the μ receptor. Each opioid receptor type can function either as a monomer or as part of a homo- or heterodimer or higher multimer. Additionally, recent evidence points to the existence of agonist bias in the signal transduction pathways activated through μ receptors, and to the presence of regulatory allosteric sites on the receptors. This brief review summarizes the recent discoveries that raise challenges for receptor definition and the characterization of signal transduction pathways activated by specific receptor forms.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Functional selectivity and time-dependence of μ-opioid receptor desensitization at nerve terminals in the mouse ventral tegmental area

BACKGROUND AND PURPOSE

The majority of studies examining desensitization of the μ-opioid receptor (MOR) have examined those located at cell bodies. However, MORs are extensively expressed at nerve terminals throughout the mammalian nervous system. This study is designed to investigate agonist-induced MOR desensitization at nerve terminals in the mouse ventral tegmental area (VTA).

EXPERIMENTAL APPROACH

MOR function was measured in mature mouse brain slices containing the VTA using whole-cell patch-clamp electrophysiology. Presynaptic MOR function was isolated from postsynaptic function and the functional selectivity, time-dependence and mechanisms of agonist-induced MOR desensitization were examined.

KEY RESULTS

MORs located at GABAergic nerve terminals in the VTA were completely resistant to rapid desensitization induced by the high-efficacy agonists DAMGO and Met-enkephalin. MORs located postsynaptically on GABAergic cell bodies readily underwent rapid desensitization in response to DAMGO. However, after prolonged (>7 h) treatment with Met-enkephalin, profound homologous MOR desensitization was observed. Morphine could induce rapid MOR desensitization at nerve terminals when PKC was activated.

CONCLUSIONS AND IMPLICATIONS

Agonist-induced MOR desensitization in GABAergic neurons in the VTA is compartment-selective as well as agonist-selective. When MORs are located at cell bodies, higher-efficacy agonists induce greater levels of rapid desensitization than lower-efficacy agonists. However, the converse is true at nerve terminals where agonists that induce MOR desensitization via PKC are capable of rapid agonist-induced desensitization while higher-efficacy agonists are not. MOR desensitization induced by higher-efficacy agonists at nerve terminals only takes place after prolonged receptor activation.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2

Mu-opioid receptor splice variants: sex-dependent regulation by chronic morphine

The gene encoding the mu-opioid receptor (MOR) generates a remarkable diversity of subtypes, the functional significance of which remains largely unknown. The structure of MOR could be a critical determinant of MOR functionality and its adaptations to chronic morphine exposure. As MOR antinociception has sexually dimorphic dimensions, we determined the influence of sex, stage of estrus cycle, and chronic systemic morphine on levels of MOR splice variant mRNA in rat spinal cord. Chronic systemic morphine influenced the spinal expression of mRNA encoding rMOR-1B2 and rMOR-1C1 in a profoundly sex-dependent fashion. In males, chronic morphine resulted in a twofold increase in expression levels of rMOR-1B2 and rMOR-1C1 mRNA. This effect of chronic morphine was completely absent in females. Increased density of MOR protein in spinal cord of males accompanied the chronic morphine-induced increase in MOR variant mRNA, suggesting that it reflected an increase in corresponding receptor protein. These results suggest that tolerance/dependence results, at least in part, from different adaptational strategies in males and females. The signaling consequences of the unique composition of the C-terminus tip of rMOR-1C1 and rMOR-1B2 could point the way to defining the molecular components of sex-dependent tolerance and withdrawal mechanisms. Chronic systemic morphine increases levels of mRNA encoding two splice variants of mu-opioid receptor (MOR), MOR-1B2 and MOR-1C1, variants differing from rMOR-1 in their C-terminal (and phosphorylation sites therein) and thus possibly signaling sequelae. This adaptation is sex-specific. It occurs in the spinal cord of males, but not females, indicating the importance of sex-specific mechanisms for and treatments of tolerance and addiction.

Identification of a splice variant of mouse TRPA1 that regulates TRPA1 activity

Transient receptor potential ankyrin 1 (TRPA1) protein is a nonselective cation channel. Although many studies suggest that TRPA1 is involved in inflammatory and neuropathic pain, its mechanism remains unclear. Here we identify an alternative splice variant of the mouse Trpa1 gene. TRPA1a (full-length) and TRPA1b (splice variant) physically interact with each other and TRPA1b increases the expression of TRPA1a in the plasma membrane. TRPA1a and TRPA1b co-expression significantly increases current density in response to different agonists without affecting their single-channel conductance. Exogenous overexpression of Trpa1b gene in wild-type and TRPA1KO DRG neurons also increases TRPA1a-mediated AITC responses. Moreover, expression levels of Trpa1a and Trpa1b mRNAs change dynamically in two pain models (complete Freund’s adjuvant-induced inflammatory pain and partial sciatic nerve ligation-induced neuropathic pain models). These results suggest that TRPA1 may be regulated through alternative splicing under these pathological conditions.

TRPA1 is a transient receptor potential channel family member and is involved in the detection of nociceptive stimuli. Zhou et al. identify an alternative splice variant of TRPA1, which increases TRPA1 plasma expression and channel function, and enhances pain-like behaviour in mice.

Isolating and characterizing three alternatively spliced mu opioid receptor variants: mMOR-1A, mMOR-1O, and mMOR-1P

Extensive alternative pre-mRNA splicing of the mu opioid receptor gene, OPRM1, has demonstrated an array of splice variants in mice, rats and humans. Three classes of splice variants have been identified: full-length seven transmembrane (TM) domain variants with C-terminal splicing, truncated 6TM variants and single TM variants. The current studies isolates and characterizes an additional three full-length C-terminal splice variants generated from the mouse OPRM1 gene: mMOR-1A, mMOR-1O, and mMOR-1P. Using RT-qPCR, we demonstrated differential expression of these variants' mRNAs among selected brain regions, supporting region-specific alternative splicing. When expressed in Chinese Hamster Ovary cells, all the variants displayed high mu binding affinity and selectivity with subtle differences in the affinities toward some agonists. [³⁵S]γGTP binding assays revealed marked differences in agonist-induced G protein activation in both potency and efficacy among the variants. Together with the previous studies of mu agonist-induced phosphorylation and internalization in several carboxyl terminal splice variants, the current studies further suggest the existence of biased signaling of various agonists within each individual variant and/or among different variants.

Morphine regulates expression of μ-opioid receptor MOR-1A, an intron-retention carboxyl terminal splice variant of the μ-opioid receptor (OPRM1) gene via miR-103/miR-107

The μ-opioid receptor (MOR-1) gene OPRM1 undergoes extensive alternative splicing, generating an array of splice variants. Of these variants, MOR-1A, an intron-retention carboxyl terminal splice variant identical to MOR-1 except for the terminal intracellular tail encoded by exon 3b, is quite abundant and conserved from rodent to humans. Increasing evidence indicates that miroRNAs (miRNAs) regulate MOR-1 expression and that μ agonists such as morphine modulate miRNA expression. However, little is known about miRNA regulation of the OPRM1 splice variants. Using 3'-rapid amplification cDNA end and Northern blot analyses, we identified the complete 3'-untranslated region (3'-UTR) for both mouse and human MOR-1A and their conserved polyadenylation site, and defined the role the 3'-UTR in mRNA stability using a luciferase reporter assay. Computer models predicted a conserved miR-103/107 targeting site in the 3'-UTR of both mouse and human MOR-1A. The functional relevance of miR-103/107 in regulating expression of MOR-1A protein through the consensus miR-103/107 binding sites in the 3'-UTR was established by using mutagenesis and a miR-107 inhibitor in transfected human embryonic kidney 293 cells and Be(2)C cells that endogenously express human MOR-1A. Chronic morphine treatment significantly upregulated miR-103 and miR-107 levels, leading to downregulation of polyribosome-associated MOR-1A in both Be(2)C cells and the striatum of a morphine-tolerant mouse, providing a new perspective on understanding the roles of miRNAs and OPRM1 splice variants in modulating the complex actions of morphine in animals and humans.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Deciphering µ-opioid receptor phosphorylation and dephosphorylation in HEK293 cells

BACKGROUND AND PURPOSE

The molecular basis of agonist-selective signalling at the µ-opioid receptor is poorly understood. We have recently shown that full agonists such as [D-Ala(2)-MePhe(4)-Gly-ol]enkephalin (DAMGO) stimulate the phosphorylation of a number of carboxyl-terminal phosphate acceptor sites including threonine 370 (Thr(370)) and serine 375 (Ser(375)), and that is followed by a robust receptor internalization. In contrast, morphine promotes a selective phosphorylation of Ser(375) without causing rapid receptor internalization.

EXPERIMENTAL APPROACH

Here, we identify kinases and phosphatases that mediate agonist-dependent phosphorylation and dephosphorylation of the µ-opioid receptor using a combination of phosphosite-specific antibodies and siRNA knock-down screening in HEK293 cells.

KEY RESULTS

We found that DAMGO-driven phosphorylation of Thr(370) and Ser(375) was preferentially catalysed by G-protein-coupled receptor kinases (GRKs) 2 and 3, whereas morphine-driven Ser(375) phosphorylation was preferentially catalysed by GRK5. On the functional level, inhibition of GRK expression resulted in enhanced µ-opioid receptor signalling and reduced receptor internalization. Analysis of GRK5-deficient mice revealed that GRK5 selectively contributes to morphine-induced Ser(375) phosphorylation in brain tissue. We also identified protein phosphatase 1γ as a µ-opioid receptor phosphatase that catalysed Thr(370) and Ser(375) dephosphorylation at or near the plasma membrane within minutes after agonist removal, which in turn facilitates receptor recycling.

CONCLUSIONS AND IMPLICATIONS

Together, the morphine-activated µ-opioid receptor is a good substrate for phosphorylation by GRK5 but a poor substrate for GRK2/3. GRK5 phosphorylates µ-opioid receptors selectively on Ser(375), which is not sufficient to drive significant receptor internalization.

Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain. A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists. Considerable effort has been expended on developing an understanding of the molecular and cellular processes that underlie acute MOR signaling, short-term receptor regulation, and the progression of events that lead to tolerance for different MOR agonists. Although great progress has been made in the past decade, many points of contention and controversy cloud the realization of this progress. This review attempts to clarify some confusion by clearly defining terms, such as desensitization and tolerance, and addressing optimal pharmacological analyses for discerning relative importance of these cellular mechanisms. Cellular and molecular mechanisms regulating MOR function by phosphorylation relative to receptor desensitization and endocytosis are comprehensively reviewed, with an emphasis on agonist-biased regulation and areas where knowledge is lacking or controversial. The implications of these mechanisms for understanding the substantial contribution of MOR signaling to opioid tolerance are then considered in detail. While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids. Further elucidation of the cellular mechanisms that are regulated by opioids will be necessary for the successful development of MOR-based approaches to new pain therapeutics that limit the development of tolerance.

The N-terminally truncated µ3 and µ3-like opioid receptors are transcribed from a novel promoter upstream of exon 2 in the human OPRM1 gene

The human µ opioid receptor gene, OPRM1, produces a multitude of alternatively spliced transcripts encoding full-length or truncated receptor variants with distinct pharmacological properties. The majority of these transcripts are transcribed from the main promoter upstream of exon 1, or from alternate promoters associated with exons 11 and 13. Two distinct transcripts encoding six transmembrane domain (6TM) hMOR receptors, µ3 and µ3-like, have been reported, both starting with the first nucleotide in exon 2. However, no mechanism explaining their initiation at exon 2 has been presented. Here we have used RT-PCR with RNA from human brain tissues to demonstrate that the µ3 and µ3-like transcripts contain nucleotide sequences from the intron 1-exon 2 boundary and are transcribed from a novel promoter located upstream of exon 2. Reporter gene assays confirmed the ability of the novel promoter to drive transcription in human cells, albeit at low levels. We also report the identification of a "full-length" seven transmembrane domain (7TM) version of µ3, hMOR-1A2, which also contains exon 1, and a novel transcript, hMOR-1Y2, with the potential to encode the previously reported hMOR-1Y receptor, but with exon Y spliced to exon 4 instead of exon 5 as in hMOR-1Y. Heterologous expression of GFP-tagged hMOR variants in HEK 293 cells showed that both 6TM receptors were retained in the intracellular compartment and were unresponsive to exogenous opioid exposure as assessed by their ability to redistribute or affect cellular cAMP production, or to promote intracellular Ca(2+) release. Co-staining with an antibody specific for endoplasmic reticulum (ER) indicated that the µ3-like receptor was retained at the ER after synthesis. 7TM receptors hMOR-1A2 and hMOR-1Y2 resided in the plasma membrane, and were responsive to opioids. Notably, hMOR-1A2 exhibits novel functional properties in that it did not internalize in response to the opioid peptide [D-Ala2, N-Me-Phe4, Gly-ol5]enkephalin (DAMGO).

Differentiation of opioid drug effects by hierarchical multi-site phosphorylation

Differences in the ability of opioid drugs to promote regulated endocytosis of μ-opioid receptors are related to their tendency to produce drug tolerance and dependence. Here we show that drug-specific differences in receptor internalization are determined by a conserved, 10-residue sequence in the receptor's carboxyl-terminal cytoplasmic tail. Diverse opioids induce receptor phosphorylation at serine (S)375, present in the middle of this sequence, but opioids differ markedly in their ability to drive higher-order phosphorylation on flanking residues [threonine (T)370, T376, and T379]. Multi-phosphorylation is required for the endocytosis-promoting activity of this sequence and occurs both sequentially and hierarchically, with S375 representing the initiating site. Higher-order phosphorylation involving T370, T376, and T379 specifically requires GRK2/3 isoforms, and the same sequence controls opioid receptor internalization in neurons. These results reveal a biochemical mechanism differentiating the endocytic activity of opioid drugs.

Epigenetics of µ-opioid receptors: intersection with HIV-1 infection of the central nervous system

The abuse of intravenous drugs, such as heroin, has become a major public health concern due to the increased risk of HIV-1 infection. Opioids such as heroin were originally identified and subsequently abused for their analgesic effects. However, many investigations have found additional effects of opioids, including regulation of the immune system. As such, chronic opioid abuse has been shown to promote HIV-1 pathogenesis and facilitate HIV-1-associated neurocognitive dysfunction. Clinical opioids, such as morphine and methadone, as well as illicit opioids, such as heroin, exert their effects primarily through interactions with the µ-opioid receptor (MOR). However, the mechanisms by which opioids enhance neurocognitive dysfunction through MOR-mediated signaling pathways are not completely understood. New findings in the regulation of MOR expression, particularly epigenetic and transcriptional regulation as well as alternative splicing, sheds new insights into possible mechanisms of HIV-1 and opiate synergy. In this review, we identify mechanisms regulating MOR expression and propose novel mechanisms by which opioids and HIV-1 may modulate this regulation. Additionally, we suggest that differential regulation of newly identified MOR isoforms by opioids and HIV-1 has functional consequence in enhancing HIV-1 neurocognitive dysfunction.

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.

Non-Coding RNAs Regulating Morphine Function: With Emphasis on the In vivo and In vitro Functions of miR-190

Non-coding RNAs (ncRNAs), especially microRNAs, are reported to be involved in a variety of biological processes, including several processes related to drug addiction. It has been suggested that the biological functions of opioids, one typical type of addictive drugs, are regulated by ncRNAs. In the current review, we examine a variety of mechanisms through which ncRNAs could regulate μ-opioid receptor (OPRM1) activities and thereby contribute to the development of opioid addiction. Using miR-23b as an example, we present the possible ways in which ncRNA-mediated regulation of OPRM1 expression could impact opioid addiction. Using miR-190 as an example, we demonstrate the critical roles played by ncRNAs in the signal cascade from receptor to systemic responses, including the possible modulation of adult neurogenesis and in vivo contextual memory. After discussing the possible targets of ncRNAs involved in the development of opioid addiction, we summarize the mechanisms underlying the interaction between ncRNAs and opioid addiction and present suggestions for further study.

Differential expression and HIV-1 regulation of μ-opioid receptor splice variants across human central nervous system cell types

The μ-opioid receptor (MOR) is known to undergo extensive alternative splicing as numerous splice variants of MOR have been identified. However, the functional significance of MOR variants, as well as how splice variants other than MOR-1 might differentially regulate human immunodeficiency virus type-1 (HIV-1) pathogenesis in the central nervous system (CNS), or elsewhere, has largely been ignored. Our findings suggest that there are specific differences in the MOR variant expression profile among CNS cell types, and that the expression levels of these variants are differentially regulated by HIV-1. While MOR-1A mRNA was detected in astroglia, microglia, and neurons, MOR-1 and MOR-1X were only found in astroglia. Expression of the various forms of MOR along with the chimeric G protein qi5 in HEK-293T cells resulted in differences in calcium/NFAT signaling with morphine treatment, suggesting that MOR variant expression might underlie functional differences in MOR-effector coupling and intracellular signaling across different cell types. Furthermore, the data suggest that the expression of MOR-1 and other MOR variants may also be differentially regulated in the brains of HIV-infected subjects with varying levels of neurocognitive impairment. Overall, the results reveal an unexpected finding that MOR-1 may not be the predominant form of MOR expressed by some CNS cell types and that other splice variants of MOR-1, with possible differing functions, may contribute to the diversity of MOR-related processes in the CNS.

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

BACKGROUND AND PURPOSE

Morphine activates the µ-opioid receptor without causing its rapid endocytosis. In contrast, full agonists such as [d-Ala(2) -MePhe(4) -Gly-ol]enkephalin (DAMGO) or etonitazene stimulate a rapid and profound internalization. However, the detailed molecular events underlying the differential regulation of receptor trafficking by distinct opioid agonists remain incompletely understood.

EXPERIMENTAL APPROACH

Here, we have generated phosphosite-specific antibodies for the carboxyl-terminal residues serine 363 (Ser363), threonine 370 (Thr370) and serine 375 (Ser375), which enabled us to selectively detect either the Ser363-, Thr370- or Ser375-phosphorylated form of the receptor.

KEY RESULTS

We showed that agonist-induced phosphorylation occurs at Thr370 and Ser375, whereas Ser363 is constitutively phosphorylated in the absence of agonist. We further demonstated that DAMGO and etonitazene stimulated the phosphorylation of both Thr370 and Ser375. In contrast, morphine promoted the phosphorylation of Ser375, but failed to stimulate Thr370 phosphorylation. In the presence of DAMGO, Ser375 phosphorylation occurred at a faster rate than phosphorylation of Thr370, indicating that Ser375 is the primary site of agonist-dependent phosphorylation. Activation of PKC by phorbol 12-myristate 13-acetate increased receptor phosphorylation only on Thr370, but not on Ser375, indicating that Thr370 can also undergo heterologous PKC-mediated phosphorylation. We also showed that µ receptor dephosphorylation can occur within minutes at or near the plasma membrane, and that agonist removal is a major prerequisite for Thr370 and Ser375 dephosphorylation.

CONCLUSIONS AND IMPLICATIONS

Together, we showed for the first time that distinct agonists stimulate site-specific patterns of phosphorylation, which are intimately related to their ability to elicit µ-opioid receptor sequestration.

LINKED ARTICLE

This article is commented on by Kelly, pp. 294-297 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2011.01387.x.

Unidirectional cross-activation of GRPR by MOR1D uncouples itch and analgesia induced by opioids

Spinal opioid-induced itch, a prevalent side effect of pain management, has been proposed to result from pain inhibition. We now report that the μ-opioid receptor (MOR) isoform MOR1D is essential for morphine-induced scratching (MIS), whereas the isoform MOR1 is required only for morphine-induced analgesia (MIA). MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, relaying itch information. We show that morphine triggers internalization of both GRPR and MOR1D, whereas GRP specifically triggers GRPR internalization and morphine-independent scratching. Providing potential insight into opioid-induced itch prevention, we demonstrate that molecular and pharmacologic inhibition of PLCβ3 and IP3R3, downstream effectors of GRPR, specifically block MIS but not MIA. In addition, blocking MOR1D-GRPR association attenuates MIS but not MIA. Together, these data suggest that opioid-induced itch is an active process concomitant with but independent of opioid analgesia, occurring via the unidirectional cross-activation of GRPR signaling by MOR1D heterodimerization.

Quantitative Detection of µ Opioid Receptor: Western Blot Analyses Using µ Opioid Receptor Knockout Mice

Increasing evidence suggests that µ opioid receptor (MOP) expression is altered during the development of and withdrawal from substance dependence. Although anti-MOP antibodies have been hypothesized to be useful for estimating MOP expression levels, inconsistent MOP molecular weights (MWs) have been reported in studies using anti-MOP antibodies. In the present study, we generated a new anti-MOP antibody (N38) against the 1-38 amino acid sequence of the mouse MOP N-terminus and conducted Western blot analysis with wildtype and MOP knockout brain lysates to determine the MWs of intrinsic MOP. The N38 antibody detected migrating bands with relative MWs of 60-67 kDa in the plasma membrane fraction isolated from wildtype brain, but not from the MOP knockout brain. These migrating bands exhibited semi-linear density in the range of 3-30 µg membrane proteins/lane. The N38 antibody may be useful for quantitatively detecting MOP.

Ubiquitination in the first cytoplasmic loop of μ-opioid receptors reveals a hierarchical mechanism of lysosomal down-regulation

μ-Type opioid receptors (MORs) are members of the large seven-transmembrane receptor family which transduce the effects of both endogenous neuropeptides and clinically important opioid drugs. Prolonged activation of MORs promotes their proteolytic degradation by endocytic trafficking to lysosomes. This down-regulation process is known to contribute to homeostatic regulation of cellular opioid responsiveness, but mechanisms that mediate and control MOR down-regulation have not been defined. We show here that lysosomal down-regulation of MORs is ESCRT (endosomal sorting complex required for transport)-dependent and involves ubiquitin-promoted transfer of internalized MORs from the limiting endosome membrane to lumen. We also show that MOR down-regulation measured by conventional radioligand binding assay is determined specifically by ubiquitination in the first cytoplasmic loop. Surprisingly, we were unable to find any role of ubiquitination in determining whether internalized receptors recycle or are delivered to lysosomes. Instead, this decision is dictated specifically by the MOR C-tail and occurs irrespectively of the presence or absence of receptor ubiquitination. Our results support a hierarchical organization of discrete ubiquitin-independent and -dependent sorting operations, which function non-redundantly in the conserved down-regulation pathway to mediate precise endocytic control. Furthermore, they show that this hierarchical mechanism discriminates the endocytic regulation of naturally occurring MOR isoforms. Moreover, they are the first to reveal, we believe, for any seven-transmembrane receptor, a functional role of ubiquitination in the first cytoplasmic loop.

Functional selectivity at the μ-opioid receptor: implications for understanding opioid analgesia and tolerance

Opioids are the most effective analgesic drugs for the management of moderate or severe pain, yet their clinical use is often limited because of the onset of adverse side effects. Drugs in this class produce most of their physiological effects through activation of the μ opioid receptor; however, an increasing number of studies demonstrate that different opioids, while presumably acting at this single receptor, can activate distinct downstream responses, a phenomenon termed functional selectivity. Functional selectivity of receptor-mediated events can manifest as a function of the drug used, the cellular or neuronal environment examined, or the signaling or behavioral measure recorded. This review summarizes both in vitro and in vivo work demonstrating functional selectivity at the μ opioid receptor in terms of G protein coupling, receptor phosphorylation, interactions with β-arrestins, receptor desensitization, internalization and signaling, and details on how these differences may relate to the progression of analgesic tolerance after their extended use.

Cellular morphine tolerance produced by βarrestin-2-dependent impairment of μ-opioid receptor resensitization

Chronic morphine treatment produces behavioral and cellular opioid tolerance that has been proposed to be caused by attenuated μ-opioid receptor (MOR) recovery from desensitization (resensitization). The process of MOR resensitization is thought to require βarrestin-2 (βarr-2)-dependent trafficking of desensitized receptors to endosomal compartments, followed by recycling of resensitized receptors back to the plasma membrane. However, there is little direct evidence for this, particularly in native neurons. This study used whole-cell patch-clamp recording in locus ceruleus (LC) neurons from wild-type (w.t.) and βarr-2 knock-out (k.o.) mice to examine whether βarr-2/dynamin-dependent trafficking is required for MOR resensitization in neurons from opioid-naive and morphine-treated mice. Surprisingly, recovery of MOR from acute desensitization in LC neurons does not require βarr-2- or dynamin-dependent trafficking. To the contrary, MOR resensitization was accelerated by disruption of either βarr-2 or dynamin function. Chronic morphine treatment caused cellular MOR tolerance and concurrently impaired MOR resensitization in neurons from w.t. mice, as expected from previous studies, but neither occurred in neurons from βarr-2 k.o. mice. Moreover, the impairment of MOR resensitization caused by chronic morphine was reversed in w.t. neurons when G-protein-coupled receptor kinase-2 (GRK2) or dynamin function was disrupted. Together, these results establish that βarr-2/dynamin-dependent receptor regulation is not required for MOR resensitization in LC neurons. Furthermore, chronic morphine treatment modifies GRK2-βarr-2-dynamin-dependent MOR trafficking to impair receptor resensitization, thereby contributing to opioid tolerance in LC neurons by reducing the number of functional receptors on the surface membrane.

Regulation of opioid receptor signalling: implications for the development of analgesic tolerance

Opiate drugs are the most effective analgesics available but their clinical use is restricted by severe side effects. Some of these undesired actions appear after repeated administration and are related to adaptive changes directed at counteracting the consequences of sustained opioid receptor activation. Here we will discuss adaptations that contribute to the development of tolerance. The focus of the first part of the review is set on molecular mechanisms involved in the regulation of opioid receptor signalling in heterologous expression systems and neurons. In the second part we assess how adaptations that take place in vivo may contribute to analgesic tolerance developed during repeated opioid administration.

Parecoxib and its metabolite valdecoxib directly interact with cannabinoid binding sites in CB1-expressing HEK 293 cells and rat brain tissue

Cyclooxygenase 2 inhibitors (COX 2) such as parecoxib (par) and valdecoxib (val) are used in the treatment of neuropathic pain. Using the radioligand binding assay it was demonstrated that both the prodrug par as well as its active metabolite val have a specific affinity to the cannabinoid (CB) receptor measured in CB1-expressing HEK 293 cells and rat brain tissue. Agonist activity was detected by GTPγS assays, cAMP formation experiments and ex vivo modulation of glutamate and GABA release of the rat brain tissue. In comparison to the specific cannabinoid agonist, WIN 55,212-2, the two COX 2 inhibitors are about 2 orders of magnitude less potent. The data suggest that the analgesic effects of par and its metabolite val in Wistar rats may be at least partially mediated by a direct interaction with the CB1 receptors. The COX 2 inhibitors appear to be a hypothetically useful tool for add-on therapy of neuropathic pain.

Ultrastructural relationship between the mu opioid receptor and its interacting protein, GPR177, in striatal neurons

GPR177, the mammalian ortholog of Drosophila Wntless/Evi/Sprinter, was recently identified as a novel mu-opioid receptor (MOR) interacting protein. GPR177 is a trans-membrane protein pivotal to mediating the secretion of Wnt signaling proteins. Wnt proteins, in turn, are essential in regulating neuronal development, a phenomenon inhibited upon chronic exposure to MOR agonists such as morphine and heroin. We previously showed that GPR177 and MOR are co-localized in the mouse dorsolateral striatum; however, the nature of this interaction was not fully elucidated. Therefore, in the present study, we examined cellular substrates for interactions between GPR177 and MOR using a combined immunogold-silver and peroxidase detection approach in coronal sections in the dorsolateral segment of the striatum. Semi-quantitative analysis of the ultrastructural distribution of GPR177 and MOR in striatal somata and in dendritic processes showed that, of the somata and dendritic processes exhibiting GPR177, 32% contained MOR immunolabeling while for profiles exhibiting MOR, 37% also contained GPR177 immunoreactivity. GPR177-labeled particles were localized predominantly along both the plasma membrane and within the cytoplasm of MOR-labeled dendrites. Somata and dendritic processes that contained both GPR177 and MOR more often received symmetric (inhibitory-type) synapses from unlabeled axon terminals. To further define the phenotype of GPR177 and MOR-containing cellular profiles, triple immunofluorescence detection showed that GPR177 and MOR are localized in neurons containing the opioid peptide, enkephalin, within the dorsolateral striatum. The results provide an anatomical substrate for interactions between MOR and its interacting protein, GPR177, in striatal opioid-containing neurons that may underlie the morphological alterations produced in neurons by chronic opiate use.