A heterodimer-selective agonist shows in vivo relevance of G protein-coupled receptor dimers

Opioid receptors: toward separation of analgesic from undesirable effects

The use of opioid analgesics for pain has always been hampered by their many side effects; in particular, the addictive liability associated with chronic use. Recently, attempts to develop analgesic agents with reduced side effects have targeted either the putative opioid receptor splice variants or the receptor hetero-oligomers. This review discusses the potential for receptor splice variant- and the hetero-oligomer-based discovery of new opioid analgesics. We also examine an alternative approach of using receptor mutants for pain management. Finally, we discuss the role of the biased agonism observed and the recently reported opioid receptor crystal structures in guiding the future development of opioid analgesics.

Homodimerization of adenosine A₁ receptors in brain cortex explains the biphasic effects of caffeine

Using bioluminescence resonance energy transfer and proximity ligation assays, we obtained the first direct evidence that adenosine A₁ receptors (A₁Rs) form homomers not only in cell cultures but also in brain cortex. By radioligand binding experiments in the absence or in the presence of the A₁Rs allosteric modulator, adenosine deaminase, and by using the two-state dimer receptor model to fit binding data, we demonstrated that the protomer-protomer interactions in the A₁R homomers account for some of the pharmacological characteristics of agonist and antagonist binding to A₁Rs. These pharmacological properties include the appearance of cooperativity in agonist binding, the change from a biphasic saturation curve to a monophasic curve in self-competition experiments and the molecular cross-talk detected when two different specific molecules bind to the receptor. In this last case, we discovered that caffeine binding to one protomer increases the agonist affinity for the other protomer in the A₁R homomer, a pharmacological characteristic that correlates with the low caffeine concentrations-induced activation of agonist-promoted A₁R signaling. This pharmacological property can explain the biphasic effects reported at low and high concentration of caffeine on locomotor activity.

Anti-analgesic effect of the mu/delta opioid receptor heteromer revealed by ligand-biased antagonism

Delta (DOR) and mu opioid receptors (MOR) can complex as heteromers, conferring functional properties in agonist binding, signaling and trafficking that can differ markedly from their homomeric counterparts. Because of these differences, DOR/MOR heteromers may be a novel therapeutic target in the treatment of pain. However, there are currently no ligands selective for DOR/MOR heteromers, and, consequently, their role in nociception remains unknown. In this study, we used a pharmacological opioid cocktail that selectively activates and stabilizes the DOR/MOR heteromer at the cell surface by blocking its endocytosis to assess its role in antinociception. We found that mice treated chronically with this drug cocktail showed a significant right shift in the ED50 for opioid-mediated analgesia, while mice treated with a drug that promotes degradation of the heteromer did not. Furthermore, promoting degradation of the DOR/MOR heteromer after the right shift in the ED50 had occurred, or blocking signal transduction from the stabilized DOR/MOR heteromer, shifted the ED50 for analgesia back to the left. Taken together, these data suggest an anti-analgesic role for the DOR/MOR heteromer in pain. In conclusion, antagonists selective for DOR/MOR heteromer could provide an avenue for alleviating reduced analgesic response during chronic pain treatment.

Synergism between a serotonin 5-HT2A receptor (5-HT2AR) antagonist and 5-HT2CR agonist suggests new pharmacotherapeutics for cocaine addiction

Relapse to cocaine dependence, even after extended abstinence, involves a number of liability factors including impulsivity (predisposition toward rapid, unplanned reactions to stimuli without regard to negative consequences) and cue reactivity (sensitivity to cues associated with cocaine-taking which can promote cocaine-seeking). These factors have been mechanistically linked to serotonin (5-hydroxytryptamine, 5-HT) signaling through the 5-HT(2A) receptor (5-HT(2A)R) and 5-HT(2C)R; either a selective 5-HT(2A)R antagonist or a 5-HT(2C)R agonist suppresses impulsivity and cocaine-seeking in preclinical models. We conducted proof-of-concept analyses to evaluate whether a combination of 5-HT(2A)R antagonist plus 5-HT(2C)R agonist would have synergistic effects over these liability factors for relapse as measured in a 1-choice serial reaction time task and cocaine self-administration/reinstatement assay. Combined administration of a dose of the selective 5-HT(2A)R antagonist M100907 plus the 5-HT(2C)R agonist WAY163909, each ineffective alone, synergistically suppressed cocaine-induced hyperactivity, inherent and cocaine-evoked impulsive action, as well as cue- and cocaine-primed reinstatement of cocaine-seeking behavior. The identification of synergism between a 5-HT(2A)R antagonist plus a 5-HT(2C)R agonist to attenuate these factors important in relapse indicates the promise of a bifunctional ligand as an anti-addiction pharmacotherapeutic, setting the stage to develop new ligands with improved efficacy, potency, selectivity, and in vivo profiles over the individual molecules.

GPCR Heterodimerization in the Reproductive System: Functional Regulation and Implication for Biodiversity

A G protein-coupled receptor (GPCR) functions not only as a monomer or homodimer but also as a heterodimer with another GPCR. GPCR heterodimerization results in the modulation of the molecular functions of the GPCR protomer, including ligand binding affinity, signal transduction, and internalization. There has been a growing body of reports on heterodimerization of multiple GPCRs expressed in the reproductive system and the resultant functional modulation, suggesting that GPCR heterodimerization is closely associated with reproduction including the secretion of hormones and the growth and maturation of follicles and oocytes. Moreover, studies on heterodimerization among paralogs of gonadotropin-releasing hormone (GnRH) receptors of a protochordate, Ciona intestinalis, verified the species-specific regulation of the functions of GPCRs via multiple GnRH receptor pairs. These findings indicate that GPCR heterodimerization is also involved in creating biodiversity. In this review, we provide basic and current knowledge regarding GPCR heterodimers and their functional modulation, and explore the biological significance of GPCR heterodimerization.

A FLIPR assay for evaluating agonists and antagonists of GPCR heterodimers

Calcium signaling plays a major role in the function of cells. Measurement of intracellular calcium mobilization is a robust assay that can be performed in a high-throughput manner to study the effect of compounds on potential drug targets. Pharmaceutical companies frequently use calcium signaling assays to screen compound libraries on G-protein-coupled receptors (GPCRs). In this chapter we describe the application of FLIPR technology to the evaluation of GPCR-induced calcium mobilization. We also include the implications of GPCR hetero-oligomerization and the identification of heteromeric receptors as novel drug targets on high-throughput calcium screening.

The cannabinoid receptor CB1 modulates the signaling properties of the lysophosphatidylinositol receptor GPR55

The G protein-coupled receptor (GPCR) 55 (GPR55) and the cannabinoid receptor 1 (CB1R) are co-expressed in many tissues, predominantly in the central nervous system. Seven transmembrane spanning (7TM) receptors/GPCRs can form homo- and heteromers and initiate distinct signaling pathways. Recently, several synthetic CB1 receptor inverse agonists/antagonists, such as SR141716A, AM251, and AM281, were reported to activate GPR55. Of these, SR141716A was marketed as a promising anti-obesity drug, but was withdrawn from the market because of severe side effects. Here, we tested whether GPR55 and CB1 receptors are capable of (i) forming heteromers and (ii) whether such heteromers could exhibit novel signaling patterns. We show that GPR55 and CB1 receptors alter each others signaling properties in human embryonic kidney (HEK293) cells. We demonstrate that the co-expression of FLAG-CB1 receptors in cells stably expressing HA-GPR55 specifically inhibits GPR55-mediated transcription factor activation, such as nuclear factor of activated T-cells and serum response element, as well as extracellular signal-regulated kinases (ERK1/2) activation. GPR55 and CB1 receptors can form heteromers, but the internalization of both receptors is not affected. In addition, we observe that the presence of GPR55 enhances CB1R-mediated ERK1/2 and nuclear factor of activated T-cell activation. Our data provide the first evidence that GPR55 can form heteromers with another 7TM/GPCR and that this interaction with the CB1 receptor has functional consequences in vitro. The GPR55-CB1R heteromer may play an important physiological and/or pathophysiological role in tissues endogenously co-expressing both receptors.

14-Alkoxy- and 14-acyloxypyridomorphinans: μ agonist/δ antagonist opioid analgesics with diminished tolerance and dependence side effects

In the search for opioid ligands with mixed functional activity, a series of 5'-(4-chlorophenyl)-4,5α-epoxypyridomorphinans possessing alkoxy or acyloxy groups at C-14 was synthesized and evaluated. In this series, the affinity and functional activity of the ligands were found to be influenced by the nature of the substituent at C-14 as well as by the substituent at N-17. Whereas the incorporation of a 3-phenylpropoxy group at C-14 on N-methylpyridomorhinan gave a dual MOR agonist/DOR agonist 17h, its incorporation on N-cyclopropylmethylpyridomorphinan gave a MOR agonist/DOR antagonist 17d. Interestingly, 17d, in contrast to 17h, did not produce tolerance or dependence effects upon prolonged treatment in cells expressing MOR and DOR. Moreover, 17d displayed greatly diminished analgesic tolerance as compared to morphine upon repeated administration, thus supporting the hypothesis that ligands with MOR agonist/DOR antagonist functional activity could emerge as novel analgesics devoid of tolerance, dependence, and related side effects.

Clinically employed opioid analgesics produce antinociception via μ-δ opioid receptor heteromers in Rhesus monkeys

Morphine and related drugs are widely employed as analgesics despite the side effects associated with their use. Although morphine is thought to mediate analgesia through mu opioid receptors, delta opioid receptors have been implicated in mediating some side effects such as tolerance and dependence. Here we present evidence in rhesus monkeys that morphine, fentanyl, and possibly methadone selectively activate mu-delta heteromers to produce antinociception that is potently antagonized by the delta opioid receptor antagonist, naltrindole (NTI). Studies with HEK293 cells expressing mu-delta heteromeric opioid receptors exhibit a similar antagonism profile of receptor activation in the presence of NTI. In mice, morphine was potently inhibited by naltrindole when administered intrathecally, but not intracerebroventricularly, suggesting the possible involvement of mu-delta heteromers in the spinal cord of rodents. Taken together, these results strongly suggest that, in primates, mu-delta heteromers are allosterically coupled and mediate the antinociceptive effects of three clinically employed opioid analgesics that have been traditionally viewed as mu-selective. Given the known involvement of delta receptors in morphine tolerance and dependence, our results implicate mu-delta heteromers in mediating both antinociception and these side effects in primates. These results open the door for further investigation in humans.

Tuned-Affinity Bivalent Ligands for the Characterization of Opioid Receptor Heteromers

Opioid receptors, including the mu and delta opioid receptors (MOR and DOR) are important targets for the treatment of pain. Although there is mounting evidence that these receptors form heteromers, the functional role of the MOR/DOR heteromer remains unresolved. We have designed and synthesized bivalent ligands as tools to elucidate the functional role of the MOR/DOR heteromer. Our ligands (L2 and L4) are comprised of a compound with low affinity at the DOR tethered to a compound with high affinity at the MOR, with the goal of producing ligands with "tuned affinity" at MOR/DOR heteromers compared to DOR homomers. Here we show that both L2 and L4 demonstrate enhanced affinity at MOR/DOR heteromers compared to DOR homomers, thereby providing unique pharmacological tools to dissect the role of the MOR/DOR heteromer in pain.

The δ opioid receptor agonist SNC80 selectively activates heteromeric μ-δ opioid receptors

Coexpressed and colocalized μ- and δ-opioid receptors have been established to exist as heteromers in cultured cells and in vivo. However the biological significance of opioid receptor heteromer activation is less clear. To explore this significance, the efficacy of selective activation of opioid receptors by SNC80 was assessed in vitro in cells singly and coexpressing opioid receptors using a chimeric G-protein-mediated calcium fluorescence assay, SNC80 produced a substantially more robust response in cells expressing μ-δ heteromers than in all other cell lines. Intrathecal SNC80 administration in μ- and δ-opioid receptor knockout mice produced diminished antinociceptive activity compared with wild type. The combined in vivo and in vitro results suggest that SNC80 selectively activates μ-δ heteromers to produce maximal antinociception. These data contrast with the current view that SNC80 selectively activates δ-opioid receptor homomers to produce antinociception. Thus, the data suggest that heteromeric μ-δ receptors should be considered as a target when SNC80 is employed as a pharmacological tool in vivo.

Pharmacological modulation of chemokine receptor function

G protein-coupled chemokine receptors and their peptidergic ligands are interesting therapeutic targets due to their involvement in various immune-related diseases, including rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, chronic obstructive pulmonary disease, HIV-1 infection and cancer. To tackle these diseases, a lot of effort has been focused on discovery and development of small-molecule chemokine receptor antagonists. This has been rewarded by the market approval of two novel chemokine receptor inhibitors, AMD3100 (CXCR4) and Maraviroc (CCR5) for stem cell mobilization and treatment of HIV-1 infection respectively. The recent GPCR crystal structures together with mutagenesis and pharmacological studies have aided in understanding how small-molecule ligands interact with chemokine receptors. Many of these ligands display behaviour deviating from simple competition and do not interact with the chemokine binding site, providing evidence for an allosteric mode of action. This review aims to give an overview of the evidence supporting modulation of this intriguing receptor family by a range of ligands, including small molecules, peptides and antibodies. Moreover, the computer-assisted modelling of chemokine receptor-ligand interactions is discussed in view of GPCR crystal structures. Finally, the implications of concepts such as functional selectivity and chemokine receptor dimerization are considered.

The significance of G protein-coupled receptor crystallography for drug discovery

Crucial as molecular sensors for many vital physiological processes, seven-transmembrane domain G protein-coupled receptors (GPCRs) comprise the largest family of proteins targeted by drug discovery. Together with structures of the prototypical GPCR rhodopsin, solved structures of other liganded GPCRs promise to provide insights into the structural basis of the superfamily's biochemical functions and assist in the development of new therapeutic modalities and drugs. One of the greatest technical and theoretical challenges to elucidating and exploiting structure-function relationships in these systems is the emerging concept of GPCR conformational flexibility and its cause-effect relationship for receptor-receptor and receptor-effector interactions. Such conformational changes can be subtle and triggered by relatively small binding energy effects, leading to full or partial efficacy in the activation or inactivation of the receptor system at large. Pharmacological dogma generally dictates that these changes manifest themselves through kinetic modulation of the receptor's G protein partners. Atomic resolution information derived from increasingly available receptor structures provides an entrée to the understanding of these events and practically applying it to drug design. Supported by structure-activity relationship information arising from empirical screening, a unified structural model of GPCR activation/inactivation promises to both accelerate drug discovery in this field and improve our fundamental understanding of structure-based drug design in general. This review discusses fundamental problems that persist in drug design and GPCR structural determination.

6'-Guanidinonaltrindole (6'-GNTI) is a G protein-biased κ-opioid receptor agonist that inhibits arrestin recruitment

κ-Opioid receptor (KOR) agonists do not activate the reward pathway stimulated by morphine-like μ-opioid receptor (MOR) agonists and thus have been considered to be promising nonaddictive analgesics. However, KOR agonists produce other adverse effects, including dysphoria, diuresis, and constipation. The therapeutic promise of KOR agonists has nonetheless recently been revived by studies showing that their dysphoric effects require arrestin recruitment, whereas their analgesic effects do not. Moreover, KOR agonist-induced antinociceptive tolerance observed in vivo has also been proposed to be correlated to the ability to induce arrestin-dependent phosphorylation, desensitization, and internalization of the receptor. The discovery of functionally selective drugs that are therapeutically effective without the adverse effects triggered by the arrestin pathway is thus an important goal. We have identified such an extreme G protein-biased KOR compound, 6'-guanidinonaltrindole (6'-GNTI), a potent partial agonist at the KOR receptor for the G protein activation pathway that does not recruit arrestin. Indeed, 6'-GNTI functions as an antagonist to block the arrestin recruitment and KOR internalization induced by other nonbiased agonists. As an extremely G protein-biased KOR agonist, 6'-GNTI represents a promising lead compound in the search for nonaddictive opioid analgesic as its signaling profile suggests that it will be without the dysphoria and other adverse effects promoted by arrestin recruitment and its downstream signaling.

Circadian-related heteromerization of adrenergic and dopamine D₄ receptors modulates melatonin synthesis and release in the pineal gland

Dopamine and adrenergic receptor complexes form under a circadian-regulated cycle and directly modulate melatonin synthesis and release from the pineal gland.

The role of the pineal gland is to translate the rhythmic cycles of night and day encoded by the retina into hormonal signals that are transmitted to the rest of the neuronal system in the form of serotonin and melatonin synthesis and release. Here we describe that the production of both melatonin and serotonin by the pineal gland is regulated by a circadian-related heteromerization of adrenergic and dopamine D₄ receptors. Through α_1B-D₄ and β₁-D₄ receptor heteromers dopamine inhibits adrenergic receptor signaling and blocks the synthesis of melatonin induced by adrenergic receptor ligands. This inhibition was not observed at hours of the day when D₄ was not expressed. These data provide a new perspective on dopamine function and constitute the first example of a circadian-controlled receptor heteromer. The unanticipated heteromerization between adrenergic and dopamine D₄ receptors provides a feedback mechanism for the neuronal hormone system in the form of dopamine to control circadian inputs.

Animals respond to cycles of light and dark with patterns in sleeping, feeding, body temperature alterations, and other biological functions. The pineal gland translates these light signals received from the retina into a language understandable to the rest of the body through the rhythmic synthesis and release of melatonin in response to the light and dark cycle. This process is controlled by adrenergic receptors. One impressive and mysterious aspect of the system is the rapid ability of rhythmic melatonin production and/or degradation to respond to changes in the cycle. In this study, we demonstrate that part of this response is due to the formation of receptor-receptor complexes (heteromers) between the adrenergic receptors α_1B or β₁ and the D₄ dopamine receptor. Using both biochemical and biophysical methods in transfected cells and in ex vivo tissue we show that dopamine, a neurotransmitter, inhibits adrenergic receptor signaling through these heteromers. This inhibition causes a dramatic decrease in melatonin production of the pineal gland. We postulate that these heteromers provide a rapid feedback mechanism for the neuronal hormone system to modulate circadian-controlled outputs.

Computational and experimental analysis of the transmembrane domain 4/5 dimerization interface of the serotonin 5-HT(1A) receptor

Experimental evidence suggests that most members of class A G-protein coupled receptors (GPCRs) can form homomers and heteromers in addition to functioning as single monomers. In particular, serotonin (5-HT) receptors were shown to homodimerize and heterodimerize with other GPCRs, although the details and the physiological role of the oligomerization has not yet been fully elucidated. Here we used computational modeling of the 5-HT(1A) receptor monomer and dimer to predict residues important for dimerization. Based on these results, we carried out rationally designed site-directed mutagenesis. The ability of the mutants to dimerize was evaluated using different FRET-based approaches. The reduced levels of acceptor photobleaching-Förster resonance energy transfer (FRET) and the lower number of monomers participating in oligomers, as assessed by lux-FRET, confirmed the decreased ability of the mutants to dimerize and the involvement of the predicted contacts (Trp175(4.64), Tyr198(5.41), Arg151(4.40), and Arg152(4.41)) at the interface. This information was reintroduced as constraints for computational protein-protein docking to obtain a high-quality dimer model. Analysis of the refined model as well as molecular dynamics simulations of wild-type (WT) and mutant dimers revealed compensating interactions in dimers composed of WT and W175A mutant. This provides an explanation for the requirement of mutations of Trp175(4.64) in both homomers for disrupting dimerization. Our iterative computational-experimental study demonstrates that transmembrane domains TM4/TM5 can form an interaction interface in 5-HT(1A) receptor dimers and indicates that specific amino acid interactions maintain this interface. The mutants and the optimized model of the dimer structure may be used in functional studies of serotonin dimers.

Octreotide long-acting repeatable for acromegaly

Acromegaly remains a therapeutic challenge for the endocrinologist. Among the available therapeutic options, octreotide long-acting repeatable (Sandostatin(®) LAR(®), Novartis) plays a chief role, both as a primary therapy and as an adjuvant treatment after unsuccessful surgery. A plethora of papers and a meta-analysis have demonstrated its efficacy in: control of clinical picture; achievement of safe growth hormone and normal age-matched IGF-I levels (both factors associated with restoration of normal life expectancy) in 60-70% of patients; control of tumor volume (with real shrinkage in over half of cases); and halt or reversal of most acromegaly-associated comorbidities. Treatment is well tolerated in most patients and can be safely prolonged for many years if required.

Receptor and channel heteromers as pain targets

Recent discoveries indicate that many G-protein coupled receptors (GPCRs) and channels involved in pain modulation are able to form receptor heteromers. Receptor and channel heteromers often display distinct signaling characteristics, pharmacological properties and physiological function in comparison to monomer/homomer receptor or ion channel counterparts. It may be possible to capitalize on such unique properties to augment therapeutic efficacy while minimizing side effects. For example, drugs specifically targeting heteromers may have greater tissue specificity and analgesic efficacy. This review will focus on current progress in our understanding of roles of heteromeric GPCRs and channels in pain pathways as well as strategies for controlling pain pathways via targeting heteromeric receptors and channels. This approach may be instrumental in the discovery of novel classes of drugs and expand our repertoire of targets for pain pharmacotherapy.

Molecular mechanisms of opioid receptor-dependent signaling and behavior

Opioid receptors have been targeted for the treatment of pain and related disorders for thousands of years and remain the most widely used analgesics in the clinic. Mu (μ), kappa (κ), and delta (δ) opioid receptors represent the originally classified receptor subtypes, with opioid receptor like-1 (ORL1) being the least characterized. All four receptors are G-protein coupled and activate inhibitory G proteins. These receptors form homo- and heterodimeric complexes and signal to kinase cascades and scaffold a variety of proteins.The authors discuss classic mechanisms and developments in understanding opioid tolerance and opioid receptor signaling and highlight advances in opioid molecular pharmacology, behavioral pharmacology, and human genetics. The authors put into context how opioid receptor signaling leads to the modulation of behavior with the potential for therapeutic intervention. Finally, the authors conclude there is a continued need for more translational work on opioid receptors in vivo.

History and biological significance of GPCR heteromerization in the neuroendocrine system

G protein-coupled receptors (GPCRs) constitute a large family of seven transmembrane proteins that regulate major cellular functions. The important role of GPCRs in the neuroendocrine system is outlined by the great interest of pharmaceutical companies in developing new drugs targeting these receptors. GPCRs exist as monomers, but can also be organized in oligomeric structures composed of either homo- or heteromers. GPCR heteromerization may play an important role in modulating and fine-tuning GPCR function and signaling. The literature reports many examples of GPCR heteromers in vitro raising the question of the physiological relevance of these complexes in tissues. Considerable efforts are currently being directed towards conclusive evidence for the existence of GPCRs heteromers in vivo, a crucial step for the validation of the concept of GPCR heteromerization and future drug development. The present review will give a brief history of GPCR oligomerization and emphasize the importance and physiological relevance of GPCR heteromerization by discussing key examples of GPCR couples.

Heterodimerization of serotonin receptors 5-HT1A and 5-HT7 differentially regulates receptor signalling and trafficking

Serotonin receptors 5-HT1A and 5-HT7 are highly co-expressed in brain regions implicated in depression. However, their functional interaction has not been established. In the present study we show that 5-HT1A and 5-HT7 receptors form heterodimers both in vitro and in vivo. Foerster resonance energy transfer-based assays revealed that, in addition to heterodimers, homodimers composed either by 5-HT1A or 5-HT7 receptors together with monomers co-exist in cells. The highest affinity to form the complex was obtained for the 5-HT7-5-HT7 homodimers, followed by the 5-HT7-5-HT1A heterodimers and 5-HT1A-5-HT1A homodimers. Functionally, heterodimerization decreases 5-HT1A receptor-mediated activation of Gi-protein without affecting 5-HT7 receptor-mediated signalling. Moreover, heterodimerization markedly decreases the ability of the 5-HT1A receptor to activate G-protein gated inwardly rectifying potassium channels in a heterologous system. The inhibitory effect on such channels was also preserved in hippocampal neurons, demonstrating a physiological relevance of heteromerization in vivo. In addition, heterodimerization is critically involved in initiation of the serotonin-mediated 5-HT1A receptor internalization and also enhances the ability of the 5-HT1A receptor to activate the mitogen-activated protein kinases. Finally, we found that production of 5-HT7 receptors in hippocampus continuously decreases during postnatal development, indicating that the relative concentration of 5-HT1A-5-HT7 heterodimers and, consequently, their functional importance undergoes pronounced developmental changes.

Cys-27 variant of human δ-opioid receptor modulates maturation and cell surface delivery of Phe-27 variant via heteromerization

The important role of G protein-coupled receptor homo/heteromerization in receptor folding, maturation, trafficking, and cell surface expression has become increasingly evident. Here we investigated whether the human δ-opioid receptor (hδOR) Cys-27 variant that shows inherent compromised maturation has an effect on the behavior of the more common Phe-27 variant in the early secretory pathway. We demonstrate that hδOR-Cys-27 acts in a dominant negative manner and impairs cell surface delivery of the co-expressed hδOR-Phe-27 and impairs conversion of precursors to the mature form. This was demonstrated by metabolic labeling, Western blotting, flow cytometry, and confocal microscopy in HEK293 and human SH-SY5Y neuroblastoma cells using differentially epitope-tagged variants. The hδOR-Phe-27 precursors that were redirected to the endoplasmic reticulum-associated degradation were, however, rescued by a pharmacological chaperone, the opioid antagonist naltrexone. Co-immunoprecipitation of metabolically labeled variants revealed that both endoplasmic reticulum-localized precursors and mature receptors exist as homo/heteromers. The existence of homo/heteromers was confirmed in living cells by bioluminescence resonance energy transfer measurements, showing that the variants have a similar propensity to form homo/heteromers. By forming both homomers and heteromers, the hδOR-Cys-27 variant may thus regulate the levels of receptors at the cell surface, possibly leading to altered responsiveness to opioid ligands in individuals carrying the Cys-27 variant.

Contribution of mu and delta opioid receptors to the pharmacological profile of kappa opioid receptor subtypes

Molecular cloning has identified three opioid receptors: mu (MOR), delta (DOR) and kappa (KOR). Yet, cloning of these receptor types has offered little clarification to the diverse pharmacological profiles seen within the growing number of novel opioid ligands, which has led to the proposal of multiple subtypes. In the present study, utilizing in vitro and in vivo methods including the use of opioid receptor knockout mice, we find that certain antinociceptive effects of the KOR-1 and KOR-2 subtype-selective ligands (+)-(5α,7α,8β)-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzene-acetamide (U69, 593) and 4-[(3,4-Dichlorophenyl)acetyl]-3-(1-pyrrolidinylmethyl)-1-piperazine-carboxylic acid methyl ester fumarate (GR89, 696), respectively, are potentiated by antagonism of MOR and DOR receptors. We believe that our findings can be best explained by the existence of KOR-DOR and KOR-MOR heteromers. We only find evidence for the existence of these heteromers in neurons mediating mechanical nociception, but not thermal nociception. These findings have important clinical ramifications as they reveal new drug targets that may provide avenues for more effective pain therapies.

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.

Allosteric interactions between δ and κ opioid receptors in peripheral sensory neurons

The peripheral δ opioid receptor (DOR) is an attractive target for analgesic drug development. There is evidence that DOR can form heteromers with the κ-opioid receptor (KOR). As drug targets, heteromeric receptors offer an additional level of selectivity and, because of allosteric interactions between protomers, functionality. Here we report that selective KOR antagonists differentially altered the potency and/or efficacy of DOR agonists in primary cultures of adult rat peripheral sensory neurons and in a rat behavioral model of thermal allodynia. In vitro, the KOR antagonist nor-binaltorphimine (nor-BNI) enhanced the potency of [D-Pen(2,5)]-enkephalin (DPDPE), decreased the potency of [D-Ala(2),D-Leu(5)]-enkephalin (DADLE), and decreased the potency and efficacy of 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80) to inhibit prostaglandin E(2) (PGE(2))-stimulated adenylyl cyclase activity. In vivo, nor-BNI enhanced the effect of DPDPE and decreased the effect of SNC80 to inhibit PGE(2)-stimulated thermal allodynia. In contrast to nor-BNI, the KOR antagonist 5'-guanidinonaltrindole (5'-GNTI) reduced the response of DPDPE both in cultured neurons and in vivo. Evidence for DOR-KOR heteromers in peripheral sensory neurons included coimmunoprecipitation of DOR with KOR, a DOR-KOR heteromer selective antibody augmented the antinociceptive effect of DPDPE in vivo, and the DOR-KOR heteromer agonist 6'-GNTI inhibited adenylyl cyclase activity in vitro as well as PGE(2)-stimulated thermal allodynia in vivo. Taken together, these data suggest that DOR-KOR heteromers exist in rat primary sensory neurons and that KOR antagonists can act as modulators of DOR agonist responses most likely through allosteric interactions between the protomers of the DOR-KOR heteromer.

Effects of neuropeptide FF system on CB₁ and CB₂ receptors mediated antinociception in mice

It has been demonstrated that opioid and cannabinoid receptor systems can produce similar signal transduction and behavioural effects. Neuropeptide FF (NPFF) belongs to an opioid-modulating peptide family. NPFF has been reported to play important roles in control of pain and analgesia through interactions with the opioid system. We were interested in whether the central and peripheral antinociception of cannabinoids could be influenced by supraspinal NPFF system. The present study examined the effects of NPFF and related peptides on the antinociceptive activities induced by the non-selective cannabinoid receptors agonist WIN55,212-2, given by supraspinal and intraplantar routes. In mice, the central and peripheral antinociception of WIN55,212-2 are mediated by cannabinoid CB(1) and CB(2) receptors, respectively. Interestingly, central administration of NPFF significantly reduced central and peripheral analgesia of cannabinoids in dose-dependent manners. In contrast, dNPA and NPVF (i.c.v.), two highly selective agonists for NPFF(2) and NPFF(1) receptors, dose-dependently augmented the antinociception caused by intracerebroventricular and intraplantar injection of WIN55,212-2. Additionally, pretreatment with the NPFF receptors selective antagonist RF9 (i.c.v.) markedly reduced the cannabinoid-modulating activities of NPFF and related peptides in nociceptive assays. These data provide the first evidence for a functional interaction between NPFF and cannabinoid systems, indicating that activation of central NPFF receptors interferes with cannabinoid-mediated central and peripheral antinociception. Intriguingly, the present work may pave the way for a new strategy of using combination treatment of cannabinoid and NPFF agonists for pain management. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

D-type prostanoid receptor enhances the signaling of chemoattractant receptor-homologous molecule expressed on T(H)2 cells

BACKGROUND

Prostaglandin (PG) D(2) is substantially involved in allergic responses and signals through the 7 transmembrane-spanning/G protein-coupled receptors, chemoattractant receptor-homologous molecule expressed on T(H)2 cells (CRTH2), and D-type prostanoid (DP) receptor.

OBJECTIVE

Although the proinflammatory function of CRTH2 is well recognized and CRTH2 is hence considered an important emerging pharmacotherapeutic target, the role of the DP receptor in mediating the biological effects of PGD(2) in patients with allergic inflammation has remained unclear.

METHODS

The cross-talk of CRTH2 and DP receptors was investigated by using both a recombinant HEK293 cell model and human eosinophils in Ca(2+) mobilization assays, coimmunoprecipitation, Western blotting, radioligand binding, and immunofluorescence.

RESULTS

We show that CRTH2 and DP receptors modulate one another's signaling properties and form CRTH2/DP heteromers without altering their ligand-binding capacities. We find that the DP receptor amplifies the CRTH2-induced Ca(2+) release from intracellular stores and coincidentally forfeits its own signaling potency. Moreover, desensitization or pharmacologic blockade of the DP receptor hinders CRTH2-mediated signal transduction. However, CRTH2 internalization occurs independently of the DP receptor. In cells that express both receptors, pharmacologic blockade of Gα(q/11) proteins abolishes the Ca(2+) response to both CRTH2 and DP agonists, whereas inhibition of Gα(i) proteins selectively attenuates the CRTH2-mediated response but not the DP signal.

CONCLUSION

Our data demonstrate the capacity of DP receptors to amplify the biological response to CRTH2 activation. Therefore the CRTH2/DP heteromer might not only represent a functional signaling unit for PGD(2) but also a potential target for the development of heteromer-directed therapies to treat allergic diseases.

The best of both worlds? Bitopic orthosteric/allosteric ligands of g protein-coupled receptors

It is now acknowledged that G protein-coupled receptors, the largest class of drug targets, adopt multiple active states that can be preferentially stabilized by orthosteric ligands or allosteric modulators, thus giving rise to the phenomenon of pathway-biased signaling. In the past few years, researchers have begun to explore the potential of linking orthosteric and allosteric pharmacophores to yield bitopic hybrid ligands. This approach is an extension of the more traditional bivalent ligand concept and shares some of the same challenges, including the choice and role of the linker between the two pharmacophores and the validation of mechanism of action. Nonetheless, the promise of bitopic ligands is the generation of novel chemical tools that have improved affinity and/or selectivity profiles. Previously identified functionally selective compounds (and medicines) also may act via a bitopic mechanism, suggesting that the phenomenon is more widespread than currently appreciated.

G-protein-coupled receptor heteromer dynamics

G-protein-coupled receptors (GPCRs) represent the largest family of cell surface receptors, and have evolved to detect and transmit a large palette of extracellular chemical and sensory signals into cells. Activated receptors catalyze the activation of heterotrimeric G proteins, which modulate the propagation of second messenger molecules and the activity of ion channels. Classically thought to signal as monomers, different GPCRs often pair up with each other as homo- and heterodimers, which have been shown to modulate signaling to G proteins. Here, we discuss recent advances in GPCR heteromer systems involving the kinetics of the early steps in GPCR signal transduction, the dynamic property of receptor-receptor interactions, and how the formation of receptor heteromers modulate the kinetics of G-protein signaling.

N-naphthoyl-beta-naltrexamine (NNTA), a highly selective and potent activator of μ/kappa-opioid heteromers

Numerous G protein-coupled receptors (GPCRs) have been shown to form heteromeric receptors in cell-based assays. Among the many heteromers reported in the opioid receptor family are μ/κ, κ/δ, and μ/δ. However, the in vivo physiological and behavioral relevance for the proposed heteromers have not yet been established. Here we report a unique example of a ligand, N-naphthoyl-β-naltrexamine (NNTA) that selectively activates heteromeric μ/κ-opioid receptors in HEK-293 cells and induces potent antinociception in mice. NNTA was an exceptionally potent agonist in cells expressing μ/κ-opioid receptors. Intriguingly, it was found to be a potent antagonist in cells expressing only μ-receptors. In the mouse tail-flick assay, intrathecal (i.t.) NNTA produced antinociception that was ~100-fold greater than by intracerebroventricular (i.c.v.) administration. The κ-antagonist, norBNI, decreased the i.t. potency, and the activity was virtually abolished in μ-opioid receptor knockout mice. No tolerance was induced i.t., but marginal tolerance (3-fold) was observed via the i.c.v. route. Moreover, NNTA produced neither significant physical dependence nor place preference in the ED50 dose range. Taken together, this work provides an important pharmacologic tool for investigating the in vivo functional relevance of heteromeric μ/κ-opioid receptors and suggests an approach to potent analgesics with fewer deleterious side effects.

Heteromerization of the μ- and δ-opioid receptors produces ligand-biased antagonism and alters μ-receptor trafficking

Heteromerization of opioid receptors has been shown to alter opioid receptor pharmacology. However, how receptor heteromerization affects the processes of endocytosis and postendocytic sorting has not been closely examined. This question is of particular relevance for heteromers of the μ-opioid receptor (MOR) and δ-opioid receptor (DOR), because the MOR is recycled primarily after endocytosis and the DOR is degraded in the lysosome. Here, we examined the endocytic and postendocytic fate of MORs, DORs, and DOR/MOR heteromers in human embryonic kidney 293 cells stably expressing each receptor alone or coexpressing both receptors. We found that the clinically relevant MOR agonist methadone promotes endocytosis of MOR but also the DOR/MOR heteromer. Furthermore, we show that DOR/MOR heteromers that are endocytosed in response to methadone are targeted for degradation, whereas MORs in the same cell are significantly more stable. It is noteworthy that we found that the DOR-selective antagonist naltriben mesylate could block both methadone- and [D-Ala2,NMe-Phe4,Gly-ol5]-enkephalin-induced endocytosis of the DOR/MOR heteromers but did not block signaling from this heteromer. Together, our results suggest that the MOR adopts novel trafficking properties in the context of the DOR/MOR heteromer. In addition, they suggest that the heteromer shows "biased antagonism," whereby DOR antagonist can inhibit trafficking but not signaling of the DOR/MOR heteromer.

Allostery at G protein-coupled receptor homo- and heteromers: uncharted pharmacological landscapes

For many years seven transmembrane domain G protein-coupled receptors (GPCRs) were thought to exist and function exclusively as monomeric units. However, evidence both from native cells and heterologous expression systems has demonstrated that GPCRs can both traffic and signal within higher-order complexes. As for other protein-protein interactions, conformational changes in one polypeptide, including those resulting from binding of pharmacological ligands, have the capacity to alter the conformation and therefore the response of the interacting protein(s), a process known as allosterism. For GPCRs, allosterism across homo- or heteromers, whether dimers or higher-order oligomers, represents an additional topographical landscape that must now be considered pharmacologically. Such effects may offer the opportunity for novel therapeutic approaches. Allosterism at GPCR heteromers is particularly exciting in that it offers additional scope to provide receptor subtype selectivity and tissue specificity as well as fine-tuning of receptor signal strength. Herein, we introduce the concept of allosterism at both GPCR homomers and heteromers and discuss the various questions that must be addressed before significant advances can be made in drug discovery at these GPCR complexes.

Opioid-induced down-regulation of RGS4: role of ubiquitination and implications for receptor cross-talk

Regulator of G protein signaling protein 4 (RGS4) acts as a GTPase accelerating protein to modulate μ- and δ- opioid receptor (MOR and DOR, respectively) signaling. In turn, exposure to MOR agonists leads to changes in RGS4 at the mRNA and/or protein level. Here we have used human neuroblastoma SH-SY5Y cells that endogenously express MOR, DOR, and RGS4 to study opioid-mediated down-regulation of RGS4. Overnight treatment of SH-SY5Y cells with the MOR agonist DAMGO or the DOR agonist DPDPE decreased RGS4 protein by ∼60% accompanied by a profound loss of opioid receptors but with no change in RGS4 mRNA. The decrease in RGS4 protein was prevented by the pretreatment with pertussis toxin or the opioid antagonist naloxone. The agonist-induced down-regulation of RGS4 proteins was completely blocked by treatment with the proteasome inhibitors MG132 or lactacystin or high concentrations of leupeptin, indicating involvement of ubiquitin-proteasome and lysosomal degradation. Polyubiquitinated RGS4 protein was observed in the presence of MG132 or the specific proteasome inhibitor lactacystin and promoted by opioid agonist. The loss of opioid receptors was not prevented by MG132, demonstrating a different degradation pathway. RGS4 is a GTPase accelerating protein for both Gα(i/o) and Gα(q) proteins. After overnight treatment with DAMGO to reduce RGS4 protein, signaling at the Gα(i/o)-coupled DOR and the Gα(q)-coupled M(3) muscarinic receptor (M(3)R) was increased but not signaling of the α(2) adrenergic receptor or bradykinin BK(2) receptor, suggesting the development of cross-talk between the DOR and M(3)R involving RGS4.

Heteromerization of G protein-coupled receptors: relevance to neurological disorders and neurotherapeutics

Because G protein-coupled receptors (GPCRs) are numerous, widely expressed and involved in major physiological responses, they represent a relevant therapeutic target for drug discovery, particularly regarding pharmacological treatments of neurological disorders. Among the biological phenomena regulating receptor function, GPCR heteromerization is an important emerging area of interest and investigation. There is increasing evidence showing that heteromerization contributes to the pharmacological heterogeneity of GPCRs by modulating receptor ontogeny, activation and recycling. Although in many cases the physiological relevance of receptor heteromerization has not been fully established, the unique pharmacological and functional properties of heteromers are likely to lead to new strategies in clinical medicine. This review describes the main GPCR heteromers and their implications for major neurological disorders such as Parkinson's disease, schizophrenia and addiction. A better understanding of molecular mechanisms underlying drug interactions related to the targeting of receptor heteromers could provide more specific and efficient therapeutic agents for the treatment of brain diseases.

Exploring a role for heteromerization in GPCR signalling specificity

The critical involvement of GPCRs (G-protein-coupled receptors) in nearly all physiological processes, and the presence of these receptors at the interface between the extracellular and the intracellular milieu, has positioned these receptors as pivotal therapeutic targets. Although a large number of drugs targeting GPCRs are currently available, significant efforts have been directed towards understanding receptor properties, with the goal of identifying and designing improved receptor ligands. Recent advances in GPCR pharmacology have demonstrated that different ligands binding to the same receptor can activate discrete sets of downstream effectors, a phenomenon known as 'ligand-directed signal specificity', which is currently being explored for drug development due to its potential therapeutic advantage. Emerging studies suggest that GPCR responses can also be modulated by contextual factors, such as interactions with other GPCRs. Association between different GPCR types leads to the formation of complexes, or GPCR heteromers, with distinct and unique signalling properties. Some of these heteromers activate discrete sets of signalling effectors upon activation by the same ligand, a phenomenon termed 'heteromer-directed signalling specificity'. This has been shown to be involved in the physiological role of receptors and, in some cases, in disease-specific dysregulation of a receptor effect. Hence targeting GPCR heteromers constitutes an emerging strategy to select receptor-specific responses and is likely to be useful in achieving specific beneficial therapeutic effects.

G protein activation by serotonin type 4 receptor dimers: evidence that turning on two protomers is more efficient

The discovery that class C G protein-coupled receptors (GPCRs) function as obligatory dimeric entities has generated major interest in GPCR oligomerization. Oligomerization now appears to be a common feature among all GPCR classes. However, the functional significance of this process remains unclear because, in vitro, some monomeric GPCRs, such as rhodopsin and β(2)-adrenergic receptors, activate G proteins. By using wild type and mutant serotonin type 4 receptors (5-HT(4)Rs) (including a 5-HT(4)-RASSL) expressed in COS-7 cells as models of class A GPCRs, we show that activation of one protomer in a dimer was sufficient to stimulate G proteins. However, coupling efficiency was 2 times higher when both protomers were activated. Expression of combinations of 5-HT(4), in which both protomers were able to bind to agonists but only one could couple to G proteins, suggested that upon agonist occupancy, protomers did not independently couple to G proteins but rather that only one G protein was activated. Coupling of a single heterotrimeric G(s) protein to a receptor dimer was further confirmed in vitro, using the purified recombinant WT RASSL 5-HT(4)R obligatory heterodimer. These results, together with previous findings, demonstrate that, differently from class C GPCR dimers, class A GPCR dimers have pleiotropic activation mechanisms.

A κ Opioid Pharmacophore Becomes a Spinally Selective κ-δ Agonist When Modified with a Basic Extender Arm

We have explored the concept of a molecular extender arm attached to a κ opioid agonist pharmacophore 3 (ICI-199,441) in an effort to potentially interact with a complementary group on a neighboring opioid receptor. The molecular arm containing a terminal amine group was lengthened incrementally from 11 up to 18 atoms. Increasing the number of atoms in the arm produced virtually no change in the mouse intracerebroventricular (i.c.v.) antinociceptive potency. In contrast, the intrathecal (i.t.) potency of 6 (KDA-16) with a 16-atom arm was dramatically increased, as reflected by its antinociceptive i.c.v./i.t. ED50 ratio of ∼130. Further lengthening led to a decreased ED50 ratio. In vivo selective antagonist studies of KDA-16 revealed that κ and δ opioid receptors were responsible for the greatly enhanced i.t. potency. Calcium release experiments in HEK-293 cells suggested that KDA-16 selectively activate κ-δ heteromers. These data are consistent with the reported possible presence of heteromeric κ-δ opioid receptors in mouse spinal cord but not in the brain. The use of a molecular extender arm may be useful for developing spinally selective analgesics.

G protein-coupled receptor oligomerization for what?

Although the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last decade, under some premises the existence of a supramolecular organization of these receptors begins now to be widely accepted by the scientific community. Indeed, GPCR oligomers may enhance the diversity and performance by which extracellular signals are transferred to the G proteins in the process of receptor transduction, although the mechanism that underlie this phenomenon remains still unexplained. Recently, a trans-conformational switching model has been proposed as a mechanism allowing direct inhibition of receptor activation. Thus, heterotropic receptor-receptor allosteric regulations are behind the GPCR oligomeric function. Accordingly, we revise here how GPCR oligomerization impinge in several important receptor functions like biosynthesis, plasma membrane diffusion or velocity, pharmacology and signaling. Overall, the rationale of receptor oligomerization might lie in the cellular need of sensing complex extracellular signals and to translate into a simple computational mode.