Truncated μ-Opioid Receptors With 6 Transmembrane Domains Are Essential for Opioid Analgesia

Exoticin as a selective agonist of 6TM μ opioid receptors identifies endogenous chaperones essential for its activity

BACKGROUND

Classical opioids are effective analgesics but carry various side effects, necessitating safer alternatives. Truncated six-transmembrane mu opioid receptors (6TM-μORs) mediate potent analgesia with fewer side effects and are a promising therapeutic target. However, few ligands known selectively target 6TM-μORs. Moreover, endogenous chaperones are believed essential for 6TM-μOR ligand binding and function.

PURPOSE

To identify a 6TM-μOR selective agonist and elucidate requisite endogenous chaperones.

METHODS

Virtual screening was used to identify promising selective 6TM-μOR agonists from traditional Chinese medicines. The role of 6TM-μOR in Exoticin analgesia was validated in loss- and gain-of-function models. APEX2 proteomics profiled proximal proteins under Exoticin or IBNtxA. Interactions were further characterized in vivo and in vitro.

RESULTS

Exoticin was shortlisted for its selective binding to 6TM-μOR and ability to induce 6TM-μOR-dependent signal transduction. Exoticin analgesia was sensitive to β-FNA and absent in E11 KO mice, but restored in mice infected with AAV-μOR1G. Slc3a2, Lrrc59, and Ppp1cb co-interacted with 6TM-μOR1G and were equally essential for Exoticin binding and 6TM-μOR1G activity.

CONCLUSION

Exoticin is a promising selective agonist of 6TM μ opioid receptors with broad-spectrum analgesic efficacy but few side effects. Slc3a2, Lrrc59, Ppp1cb are endogenous chaperones essential for 6TM-μOR ligand binding and function.

Strategies towards safer opioid analgesics-A review of old and upcoming targets

Opioids continue to be of use for the treatment of pain. Most clinically used analgesics target the μ opioid receptor whose activation results in adverse effects like respiratory depression, addiction and abuse liability. Various approaches have been used by the field to separate receptor-mediated analgesic actions from adverse effects. These include biased agonism, opioids targeting multiple receptors, allosteric modulators, heteromers and splice variants of the μ receptor. This review will focus on the current status of the field and some upcoming targets of interest that may lead to a safer next generation of analgesics. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

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.

Dysregulated expression of the alternatively spliced variant mRNAs of the mu opioid receptor gene, OPRM1, in the medial prefrontal cortex of male human heroin abusers and heroin self-administering male rats

Heroin, a mu agonist, acts through the mu opioid receptor. The mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating an array of splice variants that are conserved from rodent to humans. Increasing evidence suggests that these OPRM1 splice variants are pharmacologically important in mediating various actions of mu opioids, including analgesia, tolerance, physical dependence, rewarding behavior, as well as addiction. In the present study, we examine expression of the OPRM1 splice variant mRNAs in the medial prefrontal cortex (mPFC), one of the major brain regions involved in decision-making and drug-seeking behaviors, of male human heroin abusers and male rats that developed stable heroin-seeking behavior using an intravenous heroin self-administration (SA) model. The results show similar expression profiles among multiple OPRM1 splice variants in both human control subjects and saline control rats, illustrating conservation of OPRM1 alternative splicing from rodent to humans. Moreover, the expressions of several OPRM1 splice variant mRNAs were dysregulated in the postmortem mPFCs from heroin abusers compared to the control subjects. Similar patterns were observed in the rat heroin SA model. These findings suggest potential roles of the OPRM1 splice variants in heroin addiction that could be mechanistically explored using the rat heroin SA model.

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.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

From Pharmacology to Physiology: Endocrine Functions of μ-Opioid Receptor Networks

The steady rise in opioid users and abusers has uncovered multiple detrimental health consequences of perturbed opioid receptor signaling, thereby creating the need to better understand the biology of these systems. Among endogenous opioid networks, μ-receptors have received special attention due to their unprecedented biological complexity and broad implications in homeostatic functions. Here, we review the origin, molecular biology, and physiology of endogenous opioids with a special focus on μ-opioid receptor networks within the endocrine system. Moreover, we summarize the current evidence supporting an involvement of the latter in regulating distinct endocrine functions. Finally, we combine these insights to present an integrated perspective on μ-opioid receptor biology and provide an outlook on future studies and unresolved questions in this field.

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.

Peripherally acting opioid analgesics and peripherally-induced analgesia

The management of pain, particularly chronic pain, is still an area of medical need. In this context, opioids remain a gold standard for the treatment of pain. However, significant side effects, mainly of central origin, limit their clinical use. Here, we review recent progress to improve the therapeutic and safety profiles of opioids for pain management. Characterization of peripheral opioid-mediated pain mechanisms have been a key component of this process. Several studies identified peripheral µ, δ, and κ opioid receptors (MOR, DOR, and KOR, respectively) and nociceptin/orphanin FQ (NOP) receptors as significant players of opioid-mediated antinociception, able to achieve clinically significant effects independently of any central action. Following this, particularly from a medicinal chemistry point of view, main efforts have been directed towards the peripheralization of opioid receptor agonists with the objective of optimizing receptor activity and minimizing central exposure and the associated undesired effects. These activities have allowed the characterization of a great variety of compounds and investigational drugs that show low central nervous system (CNS) penetration (and therefore a reduced side effect profile) yet maintaining the desired opioid-related peripheral antinociceptive activity. These include highly hydrophilic/amphiphilic and massive molecules unable to easily cross lipid membranes, substrates of glycoprotein P (a extrusion pump that avoids CNS penetration), nanocarriers that release the analgesic agent at the site of inflammation and pain, and pH-sensitive opioid agonists that selectively activate at those sites (and represent a new pharmacodynamic paradigm). Hopefully, patients with pain will benefit soon from the incorporation of these new entities.

Recent advances in the opioid mu receptor based pharmacotherapy for rheumatoid arthritis

INTRODUCTION

Opioids are used for severe forms of acute and cancer pain. Over the last years, their potential use in patients with noncancer pain such as those with rheumatoid arthritis (RA) has been postulated. A recent population-based comparative study showed that chronic opioid use was 12% vs. 4% among RA and non-RA patients, respectively. Another study showed an increase from 7.4% to 16.9% (2002 to 2015). In general, there has been an increasing tendency to use opioids in recent years.

AREAS COVERED

The authors have performed an extensive literature search using PubMed for articles including noncancer pain and the use of the mu opioid receptor (MOR) agonists in patients with RA.

EXPERT OPINION

Data is not sufficient to support opioid use for the treatment of chronic pain in patients with RA. Data is scarce and inconclusive. Rheumatologists should think and ponder the question: Why is this patient in pain? Differential diagnosis should include a disease flare, degenerative changes of the musculoskeletal system, and fibromyalgia. And while there are new strategies for opioid administration currently being researched, unfortunately, they are far from being applied to human subjects in the everyday clinical setting, and are still being evaluated at an experimental level. CNS: Central nervous system; DORs: delta opioid receptor agonists; GI: Gastrointestinal; GPCRs: G protein-coupled receptors; IL: Interleukin; JAK: Janus kinase; KORs: kappa opioid receptor agonists; MCPs: Metacarpophalangeal joints; MORs: Mu opioid receptor agonists; MTPs: Metatarsophalangeal joints; NSAIDs: Non-steroidal anti-inflammatory drugsOA: Osteoarthritis; ORs: Opioid receptors; PD: Pharmacodynamic; PIPs: Proximal interphalangeal joints; PK: Pharmacokinetic; PNS: Peripheral nervous system; RA: Rheumatoid arthritis; RGS: Regulator of G protein signaling; SSRIs: Selective serotonin reuptake inhibitors; TNF: Tumor necrosis factor.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

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.

Exploring μ-Opioid Receptor Splice Variants as a Specific Molecular Target for New Analgesics

Since a μ-opioid receptor gene containing multiple exons has been identified, the variety of splice variants for μ-opioid receptors have been reported in various species. Amidino-TAPA and IBNtxA have been discovered as new analgesics with different pharmacological profiles from morphine. These new analgesics show a very potent analgesic effect but do not have dependence liability. Interestingly, these analgesics show the selectivity to the morphine-insensitive μ-opioid receptor splice variants. The splice variants, sensitive to these new analgesics but insensitive to morphine, may be a better molecular target to develop the analgesics without side effects.

Molecular Basis of Opioid Action: From Structures to New Leads

Since the isolation of morphine from the opium poppy over 200 years ago, the molecular basis of opioid action has remained the subject of intense inquiry. The identification of specific receptors responsible for opioid function and the discovery of many chemically diverse molecules with unique opioid-like efficacies have provided glimpses into the molecular logic of opioid action. Recent revolutions in the structural biology of transmembrane proteins have, for the first time, yielded high-resolution views into the 3-dimensional shapes of all 4 opioid receptors. These studies have begun to decode the chemical logic that enables opioids to specifically bind and activate their receptor targets. A combination of spectroscopic experiments and computational simulations has provided a view into the molecular movements of the opioid receptors, which itself gives rise to the complex opioid pharmacology observed at the cellular and behavioral levels. Further diversity in opioid receptor structure is driven by both genetic variation and receptor oligomerization. These insights have enabled computational drug discovery efforts, with some evidence of success in the design of completely novel opioids with unique efficacies. The combined progress over the past few years provides hope for new, efficacious opioids devoid of the side effects that have made them the scourge of humanity for millennia.

Alternative splicing events during adipogenesis from hMSCs

Adipogenesis, the developmental process of progenitor-cell differentiating into adipocytes, leads to fat metabolic disorders. Alternative splicing (AS), a ubiquitous regulatory mechanism of gene expression, allows the generation of more than one unique messenger RNA (mRNA) species from a single gene. Till now, alternative splicing events during adipogenesis from human mesenchymal stem cells (hMSCs) are not yet fully elucidated. We performed RNA-Seq coupled with bioinformatics analysis to identify the differentially expressed AS genes and events during adipogenesis from hMSCs. A global survey separately identified 1262, 1181, 1167, and 1227 ASE involved in the most common types of AS including cassette exon, alt3, and alt5, especially with cassette exon the most prevalent, at 7, 14, 21, and 28 days during adipogenesis. Interestingly, 122 differentially expressed ASE referred to 118 genes, and the three genes including ACTN1 (alt3 and cassette), LRP1 (alt3 and alt5), and LTBP4 (cassette, cassette_multi, and unknown), appeared in multiple AS types of ASE during adipogenesis. Except for all the identified ASE of LRP1 occurred in the extracellular topological domain, alt3 (84) in transmembrane domain significantly differentially expressed was the potential key event during adipogenesis. Overall, we have, for the first time, conducted the global transcriptional profiling during adipogenesis of hMSCs to identify differentially expressed ASE and ASE-related genes. This finding would provide extensive ASE as the regulator of adipogenesis and the potential targets for future molecular research into adipogenesis-related metabolic disorders.

Emerging Insights into Mu Opioid Pharmacology

Opioid analgesics, most of which act through mu opioid receptors, have long represented valuable therapeutic agents to treat severe pain. Concerted drug development efforts for over a 100 years have resulted in a large variety of opioid analgesics used in the clinic, but all of them continue to exhibit the side effects, especially respiratory depression, that have long plagued the use of morphine. The recent explosion in fatalities resulting from overdose of prescription and synthetic opioids has dramatically increased the need for safer analgesics, but recent developments in mu receptor research have provided new strategies to develop such drugs. This chapter reviews recent advances in developing novel opioid analgesics from an understanding of mu receptor structure and function. This includes a summary of the mechanism of agonist binding deduced from the crystal structure of mu receptors. It will also highlight the development of novel agonist mechanisms, including biased agonists, bivalent ligands, and allosteric modulators of mu receptor function, and describe how receptor phosphorylation modulates these pathways. Finally, it will summarize research on the alternative pre-mRNA splicing mechanisms that produces a multiplicity of mu receptor isoforms. Many of these isoforms exhibit different pharmacological specificities and brain circuitry localization, thus providing an opportunity to develop novel drugs with increased therapeutic windows.

Untangling the complexity of opioid receptor function

Mu opioid receptor agonists are among the most powerful analgesic medications but also among the most addictive. The current opioid crisis has energized a quest to develop opioid analgesics that are devoid of untoward effects. Since their discovery in the 1970's, there have been major advances in our understanding of the endogenous opioid systems that these drugs target. Yet many questions remain and the development of non-addictive opioid analgesics has not been achieved. However, access to new molecular, genetic and computational tools have begun to elucidate the structural dynamics of opioid receptors, the scaffolding that links them to intracellular signaling cascades, their cellular trafficking and the distinct ways that various opioid drugs modify them. This mini-review highlights some of the chemical and pharmacological findings and new perspectives that have arisen from studies using these tools. They reveal multiple layers of complexity of opioid receptor function, including a spatiotemporal specificity in opioid receptor-induced cellular signaling, ligand-directed biased signaling, allosteric modulation of ligand interactions, heterodimerization of different opioid receptors, and the existence of slice variants with different ligand specificity. By untangling these layers, basic research into the chemistry and pharmacology of opioid receptors is guiding the way towards deciphering the mysteries of tolerance and physical dependence that have plagued the field and is providing a platform for the development of more effective and safer opioids.

Advances in Achieving Opioid Analgesia Without Side Effects

Opioids are the most effective drugs for the treatment of severe pain, but they also cause addiction and overdose deaths, which have led to a worldwide opioid crisis. Therefore, the development of safer opioids is urgently needed. In this article, we provide a critical overview of emerging opioid-based strategies aimed at effective pain relief and improved side effect profiles. These approaches comprise biased agonism, the targeting of (i) opioid receptors in peripheral inflamed tissue (by reducing agonist access to the brain, the use of nanocarriers, or low pH-sensitive agonists); (ii) heteromers or multiple receptors (by monovalent, bivalent, and multifunctional ligands); (iii) receptor splice variants; and (iv) endogenous opioid peptides (by preventing their degradation or enhancing their production by gene transfer). Substantial advancements are underscored by pharmaceutical development of new opioids such as peripheral κ-receptor agonists, and by treatments augmenting the action of endogenous opioids, which have entered clinical trials. Additionally, there are several promising novel opioids comprehensively examined in preclinical studies, but also strategies such as biased agonism, which might require careful rethinking.

Advances in Achieving Opioid Analgesia Without Side Effects

Opioids are the most effective drugs for the treatment of severe pain, but they also cause addiction and overdose deaths, which have led to a worldwide opioid crisis. Therefore, the development of safer opioids is urgently needed. In this article, we provide a critical overview of emerging opioid-based strategies aimed at effective pain relief and improved side effect profiles. These approaches comprise biased agonism, the targeting of (i) opioid receptors in peripheral inflamed tissue (by reducing agonist access to the brain, the use of nanocarriers, or low pH-sensitive agonists); (ii) heteromers or multiple receptors (by monovalent, bivalent, and multifunctional ligands); (iii) receptor splice variants; and (iv) endogenous opioid peptides (by preventing their degradation or enhancing their production by gene transfer). Substantial advancements are underscored by pharmaceutical development of new opioids such as peripheral κ-receptor agonists, and by treatments augmenting the action of endogenous opioids, which have entered clinical trials. Additionally, there are several promising novel opioids comprehensively examined in preclinical studies, but also strategies such as biased agonism, which might require careful rethinking.