Selective interactions of spinophilin with the C-terminal domains of the δ- and μ-opioid receptors and G proteins differentially modulate opioid receptor signaling

Historical perspectives and recent advances in small molecule ligands of selective/biased/multi-targeted μ/δ/κ opioid receptor (2019-2022)

The opioids have been used for more than a thousand years and are not only the most widely prescribed drugs for moderate to severe pain and acute pain, but also the preferred drugs. However, their non-analgesic effects, especially respiratory depression and potential addiction, are important factors that plague the safety of clinical use and are an urgent problem for pharmacological researchers to address. Current research on analgesic drugs has evolved into different directions: de-opioidization; application of pharmacogenomics to individualize the use of opioids; development of new opioids with less adverse effects. The development of new opioid drugs remains a hot research topic, and with the in-depth study of opioid receptors and intracellular signal transduction mechanisms, new research ideas have been provided for the development of new opioid analgesics with less side effects and stronger analgesic effects. The development of novel opioid drugs in turn includes selective opioid receptor ligands, biased opioid receptor ligands, and multi-target opioid receptor ligands and positive allosteric modulators (PAMs) or antagonists and the single compound as multi-targeted agnoists/antagonists for different receptors. PAMs strategies are also getting newer and are the current research hotspots, including the BMS series of compounds and others, which are extensive and beyond the scope of this review. This review mainly focuses on the selective/biased/multi-targeted MOR/DOR/KOR (mu opioid receptor/delta opioid receptor/kappa opioid receptor) small molecule ligands and involves some cryo-electron microscopy (cryoEM) and structure-based approaches as well as the single compound as multi-targeted agnoists/antagonists for different receptors from 2019 to 2022, including discovery history, activities in vitro and vivo, and clinical studies, in an attempt to provide ideas for the development of novel opioid analgesics with fewer side effects.

The κ-opioid receptor-induced autophagy is implicated in stress-driven synaptic alterations

Recent evidence has shown that G protein-coupled receptors (GPCRs) are direct sensors of the autophagic machinery and opioid receptors regulate neuronal plasticity and neurotransmission with an as yet unclarified mechanism. Using in vitro and in vivo experimental approaches, this study aims to clarify the potential role of autophagy and κ-opioid receptor (κ-OR) signaling in synaptic alterations. We hereby demonstrate that the selective κ-OR agonist U50,488H, induces autophagy in a time-and dose-dependent manner in Neuro-2A cells stably expressing the human κ-OR by upregulating microtubule-associated protein Light Chain 3-II (LC3-II), Beclin 1 and Autophagy Related Gene 5 (ATG5). Pretreatment of neuronal cells with pertussis toxin blocked the above κ-OR-mediated cellular responses. Our molecular analysis also revealed a κ-OR-driven upregulation of becn1 gene through ERK1,2-dependent activation of the transcription factor CREB in Neuro-2A cells. Moreover, our studies demonstrated that sub-chronic U50,488H administration in mice causes profound increases of specific autophagic markers in the hippocampus with a concomitant decrease of several pre-and post-synaptic proteins, such as spinophilin, postsynaptic density protein 95 (PSD-95) and synaptosomal associated protein 25 (SNAP25). Finally, using acute stress, a stimulus known to increase the levels of the endogenous κ-OR ligand dynorphin, we are demonstrating that administration of the κ-ΟR selective antagonist, nor-binaltorphimine (norBNI), blocks the induction of autophagy and the stress-evoked reduction of synaptic proteins in the hippocampus. These findings provide novel insights about the essential role of autophagic machinery into the mechanisms through which κ-OR signaling regulates brain plasticity.

Novel N-Substituted Benzomorphan-Based Compounds: From MOR-Agonist/DOR-Antagonist to Biased/Unbiased MOR Agonists

Modifications at the basic nitrogen of the benzomorphan scaffold allowed the development of compounds able to segregate physiological responses downstream of the receptor signaling, opening new possibilities in opioid drug development. Alkylation of the phenyl ring in the N-substituent of the MOR-agonist/DOR-antagonist LP1 resulted in retention of MOR affinity. Moreover, derivatives 7a, 7c, and 7d were biased MOR agonists toward ERK1,2 activity stimulation, whereas derivative 7e was a low potency MOR agonist on adenylate cyclase inhibition. They were further screened in the mouse tail flick test and PGE2-induced hyperalgesia and drug-induced gastrointestinal transit.

The Delta-Opioid Receptor; a Target for the Treatment of Pain

Nowadays, pain represents one of the most important societal burdens. Current treatments are, however, too often ineffective and/or accompanied by debilitating unwanted effects for patients dealing with chronic pain. Indeed, the prototypical opioid morphine, as many other strong analgesics, shows harmful unwanted effects including respiratory depression and constipation, and also produces tolerance, physical dependence, and addiction. The urgency to develop novel treatments against pain while minimizing adverse effects is therefore crucial. Over the years, the delta-opioid receptor (DOP) has emerged as a promising target for the development of new pain therapies. Indeed, targeting DOP to treat chronic pain represents a timely alternative to existing drugs, given the weak unwanted effects spectrum of DOP agonists. Here, we review the current knowledge supporting a role for DOP and its agonists for the treatment of pain. More specifically, we will focus on the cellular and subcellular localization of DOP in the nervous system. We will also discuss in further detail the molecular and cellular mechanisms involved in controlling the cellular trafficking of DOP, known to differ significantly from most G protein-coupled receptors. This review article will allow a better understanding of how DOP represents a promising target to develop new treatments for pain management as well as where we stand as of our ability to control its cellular trafficking and cell surface expression.

A highly conserved δ-opioid receptor region determines RGS4 interaction

The δ-opioid receptor (δ-OR) couples to Gi/Go proteins to modulate a variety of responses in the nervous system. Τhe regulator of G protein signalling 4 (RGS4) was previously shown to directly interact within the C-terminal region of δ-OR using its N-terminal domain to negatively modulate opioid receptor signalling. Herein, using molecular dynamics simulations and in vitro pull-down experiments we delimit this interaction to 12 helix 8 residues of δ-ΟR and to the first 17 N-terminal residues (NT) of RGS4. Monitoring the complex arrangement and stabilization between RGS4 and δ-OR by molecular dynamics simulations combined with mutagenesis studies, we defined that two critical interactions are formed: one between Phe329 of helix8 of δ-ΟR and Pro9 of the NT of RGS4 and the other a salt bridge between Glu323 of δ-ΟR and Lys17 of RGS4. Our observations allow drafting for the first time a structural model of a ternary complex including the δ-opioid receptor, a G protein and a RGS protein. Furthermore, the high degree of conservation among opioid receptors of the RGS4-binding region, points to a conserved interaction mode between opioid receptors and this important regulatory protein.

Synthesis and Structure-Activity Relationships of LP1 Derivatives: N-Methyl-N-phenylethylamino Analogues as Novel MOR Agonists

The opioid pharmacological profile of cis-(−)-N-normetazocine derivatives is deeply affected by the nature of their N-substituents. Here, our efforts were focused on the synthesis and pharmacological evaluation of novel derivatives of the lead LP1, a multitarget opioid analgesic compound featuring an N-phenylpropanamido substituent. LP1 derivatives 5a–d and 6a–d were characterized by flexible groups at the N-substituent that allow them to reposition themselves relative to cis-(−)-N-normetazocine nucleus, thus producing different pharmacological profiles at the mu, delta and kappa opioid receptors (MOR, DOR and KOR) in in vitro and in vivo assays. Among the series, compound 5c, with the best in vitro and in vivo profile, resulted a MOR agonist which displays a K_i^MOR of 6.1 nM in a competitive binding assay, and an IC₅₀ value of 11.5 nM and an I_max of 72% in measurement of cAMP accumulation in HEK293 cells stably expressing MOR, with a slight lower efficacy than LP1. Moreover, in a mouse model of acute thermal nociception, compound 5c, intraperitoneally administered, exhibits naloxone-reversed antinociceptive properties with an ED₅₀ of 4.33 mg/kg. These results expand our understanding of the importance of N-substituent structural variations in the opioid receptor profile of cis-(−)-N-normetazocine derivatives and identify a new MOR agonist useful for the development of novel opioid analgesics for pain treatment.

Characterisation of spinophilin immunoreactivity in postmortem human brain homogenates

Spinophilin is a multifunctional scaffold protein that regulates the formation and function of dendritic spines and plays a role in neuronal migration. The distinct roles of spinophilin depend on its localization and the direct interaction with other proteins, which may target spinophilin to specific locations within the cell. Several studies suggest a role of spinophilin in the pathophysiology of neurological or psychiatric diseases. However, the majority have been performed in animals or cultured cells. Thus, the aim of the present study was to characterise the regional and subcellular expression of spinophilin immunoreactivity by western blot in postmortem human brain. Two specific immunoreactive bands for spinophilin were observed: an intense band migrating at around 120kDa, which seems to correspond to the apparent molecular weight of spinophilin described by other authors, and a less intense band of around 95kDa. This second form seems to be a proteolysis or cleavage product of the ~120kDa spinophilin. Interestingly, the subcellular distribution of both bands was different. In membrane fraction, the ~120kDa spinophilin band was the most abundant, whereas in cytosol it was the ~95kDa form. Furthermore, a different regional distribution for ~120kDa spinophilin band was observed, with the highest expression in prefrontal cortex, followed by hippocampus and cerebellum, and the lowest in caudate nucleus. Altogether, these results constitute a useful reference for future studies of spinophilin in pathological and non-pathological human brain tissues.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

A novel regulatory role of RGS4 in STAT5B activation, neurite outgrowth and neuronal differentiation

The Regulator of G protein Signalling 4 (RGS4) is a multitask protein that interacts with and negatively modulates opioid receptor signalling. Previously, we showed that the δ-opioid receptor (δ-OR) forms a multiprotein signalling complex consisting of Gi/Go proteins and the Signal Transducer and Activator of Transcription 5B (STAT5B) that leads to neuronal differentiation and neurite outgrowth upon δ-ΟR activation. Here, we investigated whether RGS4 could participate in signalling pathways to regulate neurotropic events. We demonstrate that RGS4 interacts directly with STAT5B independently of δ-ΟR presence both in vitro and in living cells. This interaction involves the N-terminal portion of RGS4 and the DNA-binding SH3 domain of STAT5B. Expression of RGS4 in HEK293 cells expressing δ-OR and/or erythropoietin receptor results in inhibition of [D-Ser², Leu⁵, Thr⁶]-enkephalin (DSLET)-and erythropoietin-dependent STAT5B phosphorylation and subsequent transcriptional activation. DSLET-dependent neurite outgrowth of neuroblastoma cells is also blocked by RGS4 expression, whereas primary cortical cultures of RGS4 knockout mice (RGS4^-/-) exhibit enhanced neuronal sprouting after δ-OR activation. Additional studies in adult brain extracts from RGS4^-/- mice revealed increased levels of p-STAT5B. Finally, neuronal progenitor cultures from RGS4^-/- mice exhibit enhanced proliferation with concomitant increases in the mRNA levels of the anti-apoptotic STAT5B target genes bcl2 and bcl-xl. These observations suggest that RGS4 is implicated in opioid dependent neuronal differentiation and neurite outgrowth via a "non-canonical" signaling pathway regulating STAT5B-directed responses.

Role of Spinophilin in Group I Metabotropic Glutamate Receptor Endocytosis, Signaling, and Synaptic Plasticity

Activation of Group I metabotropic glutamate receptors (mGluRs) activates signaling cascades, resulting in calcium release from intracellular stores, ERK1/2 activation, and long term changes in synaptic activity that are implicated in learning, memory, and neurodegenerative diseases. As such, elucidating the molecular mechanisms underlying Group I mGluR signaling is important for understanding physiological responses initiated by the activation of these receptors. In the current study, we identify the multifunctional scaffolding protein spinophilin as a novel Group I mGluR-interacting protein. We demonstrate that spinophilin interacts with the C-terminal tail and second intracellular loop of Group I mGluRs. Furthermore, we show that interaction of spinophilin with Group I mGluRs attenuates receptor endocytosis and phosphorylation of ERK1/2, an effect that is dependent upon the interaction of spinophilin with the C-terminal PDZ binding motif encoded by Group I mGluRs. Spinophilin knock-out results in enhanced mGluR5 endocytosis as well as increased ERK1/2, AKT, and Ca(2+) signaling in primary cortical neurons. In addition, the loss of spinophilin expression results in impaired mGluR5-stimulated LTD. Our results indicate that spinophilin plays an important role in regulating the activity of Group I mGluRs as well as their influence on synaptic activity.

Mice Lacking GPR88 Show Motor Deficit, Improved Spatial Learning, and Low Anxiety Reversed by Delta Opioid Antagonist

BACKGROUND

GPR88 is an orphan G protein coupled receptor highly enriched in the striatum, and previous studies have focused on GPR88 function in striatal physiology. The receptor is also expressed in other brain areas, and here we examined whether GPR88 function extends beyond striatal-mediated responses.

METHODS

We created Gpr88 knockout mice and examined both striatal and extrastriatal regions at molecular and cellular levels. We also tested striatum-, hippocampus-, and amygdala-dependent behaviors in Gpr88(-/-) mice using extensive behavioral testing.

RESULTS

We found increased G protein coupling for delta opioid receptor (DOR) and mu opioid, but not other Gi/o coupled receptors, in the striatum of Gpr88 knockout mice. We also found modifications in gene transcription, dopamine and serotonin contents, and dendritic morphology inside and outside the striatum. Behavioral testing confirmed striatal deficits (hyperactivity, stereotypies, motor impairment in rotarod). In addition, mutant mice performed better in spatial tasks dependent on hippocampus (Y-maze, novel object recognition, dual solution cross-maze) and also showed markedly reduced levels of anxiety (elevated plus maze, marble burying, novelty suppressed feeding). Strikingly, chronic blockade of DOR using naltrindole partially improved motor coordination and normalized spatial navigation and anxiety of Gpr88(-/-) mice.

CONCLUSIONS

We demonstrate that GPR88 is implicated in a large repertoire of behavioral responses that engage motor activity, spatial learning, and emotional processing. Our data also reveal functional antagonism between GPR88 and DOR activities in vivo. The therapeutic potential of GPR88 therefore extends to cognitive and anxiety disorders, possibly in interaction with other receptor systems.

Postendocytic Sorting of Adrenergic and Opioid Receptors: New Mechanisms and Functions

The endocytic pathway tightly regulates the activity of G protein-coupled receptors (GPCRs). Much of our understanding of this relationship between GPCR endocytic trafficking and signaling comes from studies done on catecholamine and opioid receptors. After ligand-induced endocytosis, a key sorting step in the endosome determines whether receptors are recycled back to the cell surface, leading to recovery of signaling, or are degraded in the lysosome, leading to desensitization. Recycling of GPCRs, unlike that of many other proteins, is an active process driven by specific sequences on the receptor and proteins that interact with this sequence. Recent data suggest that sequence-dependent recycling plays complex roles in regulating both the timing and location of GPCR signaling. This chapter will describe our current understanding of the mechanisms regulating GPCR sorting in the endosome and discuss emerging ideas on their role in GPCR signaling, focusing on adrenergic and opioid receptors as prototypes.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

RGS2 and RGS4 proteins: New modulators of the κ-opioid receptor signaling

Previous studies have shown that RGS4 associates with the C-termini of μ- and δ-opioid receptors in living cells and plays a key role in Gi/Go protein coupling selectivity and signalling of these receptors [12,20]. To deduce whether similar effects also occur for the κ-opioid receptor (κ-ΟR) and define the ability of members of the Regulators of G protein Signaling (RGS) of the B/R4 subfamily to interact with κ-ΟR subdomains we generated glutathione S-transferase fusion peptides encompassing the carboxyl-termini of κ-OR (κ-CT). Results from pull down experiments indicated that RGS2 and RGS4 directly interact within different domains of the κ-CT. Co-precipitation studies in living cells indicated that RGS2 and RGS4 associate with κ-ΟR constitutively and upon receptor activation and confer selectivity for coupling with a specific subset of G proteins. Expression of both members, RGS2 and/or RGS4, in 293F cells attenuated κ-agonist mediated-adenylyl cyclase inhibition and extracellular signal regulated kinase (ERK1,2) phosphorylation with a different amplitude in their modulatory effect in κ-ΟR signaling. Our findings demonstrate that RGS2 and RGS4 are new interacting partners that play key roles in G protein coupling to negatively regulate κ-ΟR signaling.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

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.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).