Homologous Regulation of Mu Opioid Receptor Recycling by G βγ , Protein Kinase C, and Receptor Phosphorylation

The μ-opioid receptor-mediated Gi/o protein and β-arrestin2 signaling pathways both contribute to morphine-induced side effects

The μ-opioid receptor-biased agonist theory holds that Gio protein signaling mediates the analgesic effect of opioids and the related side effects via the β-arrestin2 signaling pathway. A series of μ-opioid-biased agonists have been developed in accordance with this theory, and the FDA has approved TRV130 (as a representative of biased agonists) for marketing. However, several reports have raised the issue of opioid side effects associated with the use of agonists. In this study, five permeable peptides were designed to emulate 11 S/T phosphorylation sites at the μ-opioid receptor (MOR) carboxyl-terminal. In vitro experiments were performed to detect the activation level of G proteins from the cAMP inhibition assay and the β-arrestin2 recruitment by the BRET assay. Designed peptides might effectively interfere with the activation of the Gio and β-arrestin2 pathways when combined with morphine. The resulting morphine-induced tolerance, respiratory inhibition, and constipation in mice showed that the β-arrestin2 pathway was responsible for morphine tolerance while the Gio signaling pathway was involved with respiratory depression and constipation and that these side effects were significantly related to phosphorylation sites S363 and T370. This study may provide new directions for the development of safer and more effective opioid analgesics, and the designed peptides may be an effective tool for exploring the mechanism by which μ-opioid receptors function, with the potential of reducing the side effects that are associated with clinical opioid treatment.

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.

Characterization of the real-time internalization of nine GPCRs reveals distinct dependence on arrestins and G proteins

G protein-coupled receptors (GPCRs) are seven transmembrane receptors that respond to external stimuli and undergo conformational changes to activate G proteins and modulate cellular processes leading to biological outcomes. To prevent overstimulation and prolonged exposure to stimuli, GPCRs are regulated by internalization. While the canonical GPCR internalization mechanism in mammalian cells is arrestin-dependent, clathrin-mediated endocytosis, more diverse GPCR internalization mechanisms have been described over the years. However, there is a lack of consistent methods used in the literature making it complicated to determine a receptor's internalization pathway. Here, we utilized a highly efficient time-resolved Förster resonance energy transfer (TR-FRET) internalization assay to determine the internalization profile of nine distinct GPCRs representing the GPCR classes A, B and C and with different G protein coupling profiles. This technique, coupled with clustered regularly interspaced palindromic repeats (CRISPR) engineered knockout cells allows us to effectively study the involvement of heterotrimeric G proteins and non-visual arrestins. We found that all the nine receptors internalized upon agonist stimulation in a concentration-dependent manner and six receptors showed basal internalization. Yet, there is no correlation between the receptor class and primary G protein coupling to the arrestin and G protein dependence for GPCR internalization. Overall, this study presents a platform for studying internalization that is applicable to most GPCRs and may even be extended to other membrane proteins. This method can be easily applicable to other endocytic machinery of interest and ultimately will lend itself towards the construction of comprehensive receptor internalization profiles.

Vesicle-associated membrane protein 2 is a cargo-selective v-SNARE for a subset of GPCRs

Vesicle fusion at the plasma membrane is critical for releasing hormones and neurotransmitters and for delivering the cognate G protein-coupled receptors (GPCRs) to the cell surface. The SNARE fusion machinery that releases neurotransmitters has been well characterized. In contrast, the fusion machinery that delivers GPCRs is still unknown. Here, using high-speed multichannel imaging to simultaneously visualize receptors and v-SNAREs in real time in individual fusion events, we identify VAMP2 as a selective v-SNARE for GPCR delivery. VAMP2 was preferentially enriched in vesicles that mediate the surface delivery of μ opioid receptor (MOR), but not other cargos, and was required selectively for MOR recycling. Interestingly, VAMP2 did not show preferential localization on MOR-containing endosomes, suggesting that v-SNAREs are copackaged with specific cargo into separate vesicles from the same endosomes. Together, our results identify VAMP2 as a cargo-selective v-SNARE and suggest that surface delivery of specific GPCRs is mediated by distinct fusion events driven by distinct SNARE complexes.

Fluorescence resonance energy transfer (FRET) spatiotemporal mapping of atypical P38 reveals an endosomal and cytosolic spatial bias

Mitogen-activated protein kinase (MAPK) p38 is a central regulator of intracellular signaling, driving physiological and pathological pathways. With over 150 downstream targets, it is predicted that spatial positioning and the availability of cofactors and substrates determines kinase signaling specificity. The subcellular localization of p38 is highly dynamic to facilitate the selective activation of spatially restricted substrates. However, the spatial dynamics of atypical p38 inflammatory signaling are understudied. We utilized subcellular targeted fluorescence resonance energy transfer (FRET) p38 activity biosensors to map the spatial profile of kinase activity. Through comparative analysis of plasma membrane, cytosolic, nuclear, and endosomal compartments, we confirm a characteristic profile of nuclear bias for mitogen-activated kinase kinase 3/6 (MKK3/6) dependent p38 activation. Conversely, atypical p38 activation via thrombin-mediated protease-activated receptor 1 (PAR1) activity led to enhanced p38 activity at the endosome and cytosol, limiting nuclear p38 activity, a profile conserved for prostaglandin E2 activation of p38. Conversely, perturbation of receptor endocytosis led to spatiotemporal switching of thrombin signaling, reducing endosomal and cytosolic p38 activity and increasing nuclear activity. The data presented reveal the spatiotemporal dynamics of p38 activity and provide critical insight into how atypical p38 signaling drives differential signaling responses through spatial sequestration of kinase activity.

Roles of receptor phosphorylation and Rab proteins in G protein-coupled receptor function and trafficking

The G Protein-Coupled Receptors form the most abundant family of membrane proteins and are crucial physiological players in the homeostatic equilibrium, which we define as health. They also participate in the pathogenesis of many diseases and are frequent targets of therapeutic intervention. Considering their importance, it is not surprising that different mechanisms regulate their function, including desensitization, resensitization, internalization, recycling to the plasma membrane, and degradation. These processes are modulated in a highly coordinated and specific way by protein kinases and phosphatases, ubiquitin ligases, protein adaptors, interaction with multifunctional complexes, molecular motors, phospholipid metabolism, and membrane distribution. This review describes significant advances in the study of the regulation of these receptors by phosphorylation and endosomal traffic (where signaling can take place); we revisited the bar code hypothesis and include two additional observations: a) that different phosphorylation patterns seem to be associated with internalization and endosome sorting for recycling or degradation, and b) that, surprisingly, phosphorylation of some G protein-coupled receptors appears to be required for proper receptor insertion into the plasma membrane. Significance Statement G protein-coupled receptor phosphorylation is an early event in desensitization/ signaling switching, endosomal traffic, and internalization. These events seem crucial for receptor responsiveness, cellular localization, and fate (recycling/ degradation) with important pharmacological/ therapeutic implications. Phosphorylation sites vary depending on the cells in which they are expressed and on the stimulus that leads to such covalent modification. Surprisingly, evidence suggests that phosphorylation also seems to be required for proper insertion into the plasma membrane for some receptors.

Functional independence of endogenous µ- and δ-opioid receptors co-expressed in cholinergic interneurons

Class A G-protein-coupled receptors (GPCRs) normally function as monomers, although evidence from heterologous expression systems suggests that they may sometimes form homodimers and/or heterodimers. This study aims to evaluate possible functional interplay of endogenous µ- and δ-opioid receptors (MORs and DORs) in mouse neurons. Detecting GPCR dimers in native tissues, however, has been challenging. Previously, MORs and DORs co-expressed in transfected cells have been reported to form heterodimers, and their possible co-localization in neurons has been studied in knock-in mice expressing genetically engineered receptors fused to fluorescent proteins. Here, we find that single cholinergic neurons in the mouse striatum endogenously express both MORs and DORs. The receptors on neurons from live brain slices were fluorescently labeled with a ligand-directed labeling reagent, NAI-A594. The selective activation of MORs and DORs, with DAMGO (µ-agonist) and deltorphin (δ-agonist) inhibited spontaneous firing in all cells examined. In the continued presence of agonist, the firing rate returned to baseline as the result of receptor desensitization with the application of deltorphin but was less observed with the application of DAMGO. In addition, agonist-induced internalization of DORs but not MORs was detected. When MORs and DORs were activated simultaneously with [Met⁵]-enkephalin, desensitization of MORs was facilitated but internalization was not increased. Together, these results indicate that while MORs and DORs are expressed in single striatal cholinergic interneurons, the two receptors function independently.

Mechanisms of selective G protein-coupled receptor localization and trafficking

The trafficking of G protein-coupled receptors (GPCRs) to different membrane compartments has recently emerged as being a critical determinant of the signaling profiles of activation. GPCRs, which share many structural and functional similarities, also share many mechanisms that traffic them between compartments. This sharing raises the question of how the trafficking of individual GPCRs is selectively regulated. Here, we will discuss recent studies addressing the mechanisms that contribute to selectivity in endocytic and biosynthetic trafficking of GPCRs.

Compartment-specific opioid receptor signaling is selectively modulated by different dynorphin peptides

Many signal transduction systems have an apparent redundancy built into them, where multiple physiological agonists activate the same receptors. Whether this is true redundancy, or whether this provides an as-yet unrecognized specificity in downstream signaling, is not well understood. We address this question using the kappa opioid receptor (KOR), a physiologically relevant G protein-coupled receptor (GPCR) that is activated by multiple members of the Dynorphin family of opioid peptides. We show that two related peptides, Dynorphin A and Dynorphin B, bind and activate KOR to similar extents in mammalian neuroendocrine cells and rat striatal neurons, but localize KOR to distinct intracellular compartments and drive different post-endocytic fates of the receptor. Strikingly, localization of KOR to the degradative pathway by Dynorphin A induces sustained KOR signaling from these compartments. Our results suggest that seemingly redundant endogenous peptides can fine-tune signaling by regulating the spatiotemporal profile of KOR signaling.

The Life Cycle of the Mu-Opioid Receptor

Opioid receptors (ORs) are undisputed targets for the treatment of pain. Unfortunately, targeting these receptors therapeutically poses significant challenges including addiction, dependence, tolerance, and the appearance of side effects, such as respiratory depression and constipation. Moreover, misuse of prescription and illicit narcotics has resulted in the current opioid crisis. The mu-opioid receptor (MOR) is the cellular mediator of the effects of most commonly used opioids, and is a prototypical G protein-coupled receptor (GPCR) where new pharmacological, signalling and cell biology concepts have been coined. This review summarises the knowledge of the life cycle of this therapeutic target, including its biogenesis, trafficking to and from the plasma membrane, and how the regulation of these processes impacts its function and is related to pathophysiological conditions.

High-throughput screening and validation of antibodies against synaptic proteins to explore opioid signaling dynamics

Antibodies represent powerful tools to examine signal transduction pathways. Here, we present a strategy integrating multiple state-of-the-art methods to produce, validate, and utilize antibodies. Focusing on understudied synaptic proteins, we generated 137 recombinant antibodies. We used yeast display antibody libraries from the B cells of immunized rabbits, followed by FACS sorting under stringent conditions to identify high affinity antibodies. The antibodies were validated by high-throughput functional screening, and genome editing. Next, we explored the temporal dynamics of signaling in single cells. A subset of antibodies targeting opioid receptors were used to examine the effect of treatment with opiates that have played central roles in the worsening of the 'opioid epidemic.' We show that morphine and fentanyl exhibit differential temporal dynamics of receptor phosphorylation. In summary, high-throughput approaches can lead to the identification of antibody-based tools required for an in-depth understanding of the temporal dynamics of opioid signaling.

Post-translational Modifications of Opioid Receptors

Post-translational modifications (PTMs) are key events in signal transduction since they affect protein function by regulating their abundance and/or activity. PTMs involve the covalent attachment of functional groups to specific amino acids. Since they tend to be generally reversible, PTMs serve as regulators of signal transduction pathways. G-protein-coupled receptors (GPCRs) are major signaling proteins that undergo multiple types of PTMs. In this Review, we focus on the opioid receptors, members of GPCR family A, and highlight recent advances in the field that have underscored the importance of PTMs in the functional regulation of these receptors. Since opioid receptor activity plays a central role in the development of tolerance and addiction to morphine and other drugs of abuse, understanding the molecular mechanisms regulating receptor activity is of fundamental importance.

Opioid Pharmacology under the Microscope

The powerful analgesic effects of opioid drugs have captivated the interest of physicians and scientists for millennia, and the ability of opioid drugs to produce serious undesired effects has been recognized for a similar period of time (Kieffer and Evans, 2009). Many of these develop progressively with prolonged or repeated drug use and then persist, motivating particular interest in understanding how opioid drugs initiate adaptive or maladaptive modifications in neural function or regulation. Exciting advances have been made over the past several years in elucidating drug-induced changes at molecular, cellular, and physiologic scales of analysis. The present review will highlight some recent cellular studies that we believe bridge across scales and will focus on optical imaging approaches that put opioid drug action "under the microscope." SIGNIFICANCE STATEMENT: Opioid receptors are major pharmacological targets, but their signaling at the cellular level results from a complex interplay between pharmacology, regulation, subcellular localization, and membrane trafficking. This minireview discusses recent advances in understanding the cellular biology of opioid receptors, emphasizing particular topics discussed at the 50th anniversary of the International Narcotics Research Conference. Our goal is to highlight distinct signaling and regulatory properties emerging from the cellular biology of opioid receptors and discuss potential relevance to therapeutics.