Heteromerization of Endogenous Mu and Delta Opioid Receptors Induces Ligand-Selective Co-Targeting to Lysosomes

Bivalent and bitopic ligands of the opioid receptors: The prospects of a dual approach

Opioid receptors belonging to the class A G-protein coupled receptors (GPCRs) are the targets of choice in the treatment of acute and chronic pain. However, their on-target side effects such as respiratory depression, tolerance and addiction have led to the advent of the 'opioid crisis'. In the search for safer analgesics, bivalent and more recently, bitopic ligands have emerged as valuable tool compounds to probe these receptors. The activity of bivalent and bitopic ligands rely greatly on the allosteric nature of the GPCRs. Bivalent ligands consist of two pharmacophores, each binding to the individual orthosteric binding site (OBS) of the monomers within a dimer. Bitopic or dualsteric ligands bridge the gap between the OBS and the spatially distinct, less conserved allosteric binding site (ABS) through the simultaneous occupation of these two sites. Bivalent and bitopic ligands stabilize distinct conformations of the receptors which ultimately translates into unique signalling and pharmacological profiles. Some of the interesting properties shown by these ligands include improved affinity and/or efficacy, subtype and/or functional selectivity and reduced side effects. This review aims at providing an overview of some of the bivalent and bitopic ligands of the opioid receptors and, their pharmacology in the hope of inspiring the design and discovery of the next generation of opioid analgesics.

Therapeutic potential of opioid receptor heteromers in chronic pain and associated comorbidities

Chronic pain affects 20% to 45% of the global population and is often associated with the development of anxio-depressive disorders. Treatment of this debilitating condition remains particularly challenging with opioids prescribed to alleviate moderate to severe pain. However, despite strong antinociceptive properties, numerous adverse effects limit opioid use in the clinic. Moreover, opioid misuse and abuse have become a major health concern worldwide. This prompted efforts to design original strategies that would efficiently and safely relieve pain. Targeting of opioid receptor heteromers is one of these. This review summarizes our current knowledge on the role of heteromers involving opioid receptors in the context of chronic pain and anxio-depressive comorbidities. It also examines how heteromerization in native tissue affects ligand binding, receptor signalling and trafficking properties. Finally, the therapeutic potential of ligands designed to specifically target opioid receptor heteromers is considered. 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.

Mu opioid receptor-mediated release of endolysosome iron increases levels of mitochondrial iron, reactive oxygen species, and cell death

Objectives

Opioids including morphine and DAMGO activate mu-opioid receptors (MOR), increase intracellular reactive oxygen species (ROS) levels, and induce cell death. Ferrous iron (Fe2+) through Fenton-like chemistry increases ROS levels and endolysosomes are "master regulators of iron metabolism" and contain readily-releasable Fe2+ stores. However, mechanisms underlying opioid-induced changes in endolysosome iron homeostasis and downstream-signaling events remain unclear.

Methods

We used SH-SY5Y neuroblastoma cells, flow cytometry, and confocal microscopy to measure Fe2+ and ROS levels and cell death.

Results

Morphine and DAMGO de-acidified endolysosomes, decreased endolysosome Fe2+ levels, increased cytosol and mitochondria Fe2+ and ROS levels, depolarized mitochondrial membrane potential, and induced cell death; effects blocked by the nonselective MOR antagonist naloxone and the selective MOR antagonist β-funaltrexamine (β-FNA). Deferoxamine, an endolysosome-iron chelator, inhibited opioid agonist-induced increases in cytosolic and mitochondrial Fe2+ and ROS. Opioid-induced efflux of endolysosome Fe2+ and subsequent Fe2+ accumulation in mitochondria were blocked by the endolysosome-resident two-pore channel inhibitor NED-19 and the mitochondrial permeability transition pore inhibitor TRO.

Conclusions

Opioid agonist-induced increases in cytosolic and mitochondrial Fe2+ and ROS as well as cell death appear downstream of endolysosome de-acidification and Fe2+ efflux from the endolysosome iron pool that is sufficient to affect other organelles.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 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 (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Knock-In Mouse Models to Investigate the Functions of Opioid Receptors in vivo

Due to their low expression levels, complex multi-pass transmembrane structure, and the current lack of highly specific antibodies, the assessment of endogenous G protein-coupled receptors (GPCRs) remains challenging. While most of the research regarding their functions was performed in heterologous systems overexpressing the receptor, recent advances in genetic engineering methods have allowed the generation of several unique mouse models. These animals proved to be useful to investigate numerous aspects underlying the physiological functions of GPCRs, including their endogenous expression, distribution, interactome, and trafficking processes. Given their significant pharmacological importance and central roles in the nervous system, opioid peptide receptors (OPr) are often referred to as prototypical receptors for the study of GPCR regulatory mechanisms. Although only a few GPCR knock-in mouse lines have thus far been generated, OPr are strikingly well represented with over 20 different knock-in models, more than half of which were developed within the last 5 years. In this review, we describe the arsenal of OPr (mu-, delta-, and kappa-opioid), as well as the opioid-related nociceptin/orphanin FQ (NOP) receptor knock-in mouse models that have been generated over the past years. We further highlight the invaluable contribution of such models to our understanding of the in vivo mechanisms underlying the regulation of OPr, which could be conceivably transposed to any other GPCR, as well as the limitations, future perspectives, and possibilities enabled by such tools.

Central Nervous System Distribution of an Opioid Agonist Combination with Synergistic Activity

Novel combinations of specific opioid agonists like loperamide and oxymorphindole targeting the µ- and δ-opioid receptors, respectively, have shown increased potency with minimized opioid-associated risks. However, whether their interaction is pharmacokinetic or pharmacodynamic in nature has not been determined. This study quantitatively determined whether these drugs have a pharmacokinetic interaction that alters systemic disposition or central nervous system (CNS) distribution. We performed intravenous and oral in vivo pharmacokinetic assessments of both drugs after discrete dosing and administration in combination to determine whether the combination had any effect on systemic pharmacokinetic parameters or CNS exposure. Drugs were administered at 5 or 10 mg/kg i.v. or 30 mg/kg orally to institute for cancer research (ICR) mice and 5 mg/kg i.v. to Friend leukemia virus strain B mice of the following genotypes: wild-type, breast cancer resistance protein (Bcrp^-/- ) (Bcrp knockout), Mdr1a/b^-/- [P-glycoprotein (P-gp) knockout], and Bcrp^-/- Mdr1a/b^-/- (triple knockout). In the combination, clearance of oxymorphindole (OMI) was reduced by approximately half, and the plasma area under the concentration-time curve (AUC) increased. Consequently, brain and spinal cord AUCs for OMI in the combination also increased proportionately. Both loperamide and OMI are P-gp substrates, but administration of the two drugs in combination does not alter efflux transport at the CNS barriers. Because OMI alone shows appreciable brain penetration but little therapeutic efficacy on its own, and because loperamide's CNS distribution is unchanged in the combination, the mechanism of action for the increased potency of the combination is most likely pharmacodynamic and most likely occurs at receptors in the peripheral nervous system. This combination has favorable characteristics for future development. SIGNIFICANCE STATEMENT: Opioids have yet to be replaced as the most effective treatments for moderate-to-severe pain and chronic pain, but their side effects are dangerous. Combinations of opioids with peripheral activity, such as loperamide and oxymorphindole, would be valuable in that they are effective at much lower doses and have reduced risks for dangerous side effects because the µ-opioid receptor agonist is largely excluded from the CNS.

Convergent, functionally independent signaling by mu and delta opioid receptors in hippocampal parvalbumin interneurons

Functional interactions between G protein-coupled receptors are poised to enhance neuronal sensitivity to neuromodulators and therapeutic drugs. Mu and Delta opioid receptors (MORs and DORs) can interact when overexpressed in the same cells, but whether co-expression of endogenous MORs and DORs in neurons leads to functional interactions is unclear. Here, in mice, we show that both MORs and DORs inhibit parvalbumin-expressing basket cells (PV-BCs) in hippocampal CA1 through partially occlusive signaling pathways that terminate on somato-dendritic potassium channels and presynaptic calcium channels. Using photoactivatable opioid neuropeptides, we find that DORs dominate the response to enkephalin in terms of both ligand-sensitivity and kinetics, which may be due to relatively low expression levels of MOR. Opioid-activated potassium channels do not show heterologous desensitization, indicating that MORs and DORs signal independently. In a direct test for heteromeric functional interactions, the DOR antagonist TIPP-Psi does not alter the kinetics or potency of either the potassium channel or synaptic responses to photorelease of the MOR agonist DAMGO. Thus, aside from largely redundant and convergent signaling, MORs and DORs do not functionally interact in PV-BCs in a way that impacts somato-dendritic potassium currents or synaptic transmission. These findings imply that crosstalk between MORs and DORs, either in the form of physical interactions or synergistic intracellular signaling, is not a preordained outcome of co-expression in neurons.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Ultrastructural evidence for mu and delta opioid receptors at noradrenergic dendrites and glial profiles in the cat locus coeruleus

The Locus Coeruleus (LC) is a pontine nucleus involved in many physiological processes, including the control of the sleep/wake cycle (SWC). At cellular level, the LC displays a high density of opioid receptors whose activation decreases the activity of LC noradrenergic neurons. Also, microinjections of morphine administered locally in the LC of the cat produce sleep associated with synchronized brain activity in the electroencephalogram (EEG). Even though much of the research on sleep has been done in the cat, the subcellular location of opioid receptors in the LC and their relationship with LC noradrenergic neurons is not known yet in this species. Therefore, we conducted a study to describe the ultrastructural localization of mu-opioid receptors (MOR), delta-opioid receptors (DOR) and tyrosine hydroxylase (TH) in the cat LC using high resolution electron microscopy double-immunocytochemical detection. MOR and DOR were localized mainly in dendrites (45% and 46% of the total number of profiles respectively), many of which were noradrenergic (35% and 53% for MOR and DOR, respectively). TH immunoreactivity was more frequent in dendrites (65% of the total number of profiles), which mostly also expressed opioid receptors (58% and 73% for MOR and DOR, respectively). Because the distribution of MORs and DORs are similar, it is possible that a substantial sub-population of neurons co-express both receptors, which may facilitate the formation of MOR-DOR heterodimers. Moreover, we found differences in the cat subcellular DOR distribution compared with the rat. This opens the possibility to the existence of diverse mechanisms for opioid modulation of LC activity.

Opioids and Their Receptors: Present and Emerging Concepts in Opioid Drug Discovery

The interest in opioids such as morphine, the prototypical opioid ligand, has been maintained throughout the years [...].