The RGSZ2 protein exists in a complex with mu-opioid receptors and regulates the desensitizing capacity of Gz proteins

αN-Acetyl β-Endorphin Is an Endogenous Ligand of σ1Rs That Regulates Mu-Opioid Receptor Signaling by Exchanging G Proteins for σ2Rs in σ1R Oligomers

The opioid peptide β-endorphin coexists in the pituitary and brain in its ^αN-acetylated form, which does not bind to opioid receptors. We now report that these neuropeptides exhibited opposite effects in in vivo paradigms, in which ligands of the sigma type 1 receptor (σ1R) displayed positive effects. Thus, ^αN-acetyl β-Endorphin reduced vascular infarct caused by permanent unilateral middle cerebral artery occlusion and diminished the incidence of N-methyl-D-aspartate acid-promoted convulsive syndrome and mechanical allodynia caused by unilateral chronic constriction of the sciatic nerve. Moreover, ^αN-acetyl β-Endorphin reduced the analgesia of morphine, β-Endorphin and clonidine but enhanced that of DAMGO. All these effects were counteracted by β-Endorphin and absent in σ1R^-/- mice. We observed that σ1Rs negatively regulate mu-opioid receptor (MOR)-mediated morphine analgesia by binding and sequestering G proteins. In this scenario, β-Endorphin promoted the exchange of σ2Rs by G proteins at σ1R oligomers and increased the regulation of G proteins by MORs. The opposite was observed for the ^αN-acetyl derivative, as σ1R oligomerization decreased and σ2R binding was favored, which displaced G proteins; thus, MOR-regulated transduction was reduced. Our findings suggest that the pharmacological β-Endorphin-specific epsilon receptor is a σ1R-regulated MOR and that β-Endorphin and ^αN-acetyl β-Endorphin are endogenous ligands of σ1R.

Cellular and circuit diversity determines the impact of endogenous opioids in the descending pain modulatory pathway

The descending pain modulatory pathway exerts important bidirectional control of nociceptive inputs to dampen and/or facilitate the perception of pain. The ventrolateral periaqueductal gray (vlPAG) integrates inputs from many regions associated with the processing of nociceptive, cognitive, and affective components of pain perception, and is a key brain area for opioid action. Opioid receptors are expressed on a subset of vlPAG neurons, as well as on both GABAergic and glutamatergic presynaptic terminals that impinge on vlPAG neurons. Microinjection of opioids into the vlPAG produces analgesia and microinjection of the opioid receptor antagonist naloxone blocks stimulation-mediated analgesia, highlighting the role of endogenous opioid release within this region in the modulation of nociception. Endogenous opioid effects within the vlPAG are complex and likely dependent on specific neuronal circuits activated by acute and chronic pain stimuli. This review is focused on the cellular heterogeneity within vlPAG circuits and highlights gaps in our understanding of endogenous opioid regulation of the descending pain modulatory circuits.

A Novel Morphine Drinking Model of Opioid Dependence in Rats

An animal model of voluntary oral morphine consumption would allow for a pre-clinical evaluation of new treatments aimed at reducing opioid intake in humans. However, the main limitation of oral morphine consumption in rodents is its bitter taste, which is strongly aversive. Taste aversion is often overcome by the use of adulterants, such as sweeteners, to conceal morphine taste or bitterants in the alternative bottle to equalize aversion. However, the adulterants’ presence is the cause for consumption choice and, upon removal, the preference for morphine is not preserved. Thus, current animal models are not suitable to study treatments aimed at reducing consumption elicited by morphine itself. Since taste preference is a learned behavior, just-weaned rats were trained to accept a bitter taste, adding the bitterant quinine to their drinking water for one week. The latter was followed by allowing the choice of quinine or morphine (0.15 mg/mL) solutions for two weeks. Then, quinine was removed, and the preference for morphine against water was evaluated. Using this paradigm, we show that rats highly preferred the consumption of morphine over water, reaching a voluntary morphine intake of 15 mg/kg/day. Morphine consumption led to significant analgesia and hyperlocomotion, and to a marked deprivation syndrome following the administration of the opioid antagonist naloxone. Voluntary morphine consumption was also shown to generate brain oxidative stress and neuroinflammation, signs associated with opioid dependence development. We present a robust two-bottle choice animal model of oral morphine self-administration for the evaluation of therapeutic interventions for the treatment of morphine dependence.

Molecular rhythm alterations in prefrontal cortex and nucleus accumbens associated with opioid use disorder

Severe and persistent disruptions to sleep and circadian rhythms are common in people with opioid use disorder (OUD). Preclinical evidence suggests altered molecular rhythms in the brain modulate opioid reward and relapse. However, whether molecular rhythms are disrupted in the brains of people with OUD remained an open question, critical to understanding the role of circadian rhythms in opioid addiction. Using subjects' times of death as a marker of time of day, we investigated transcriptional rhythms in the brains of subjects with OUD compared to unaffected comparison subjects. We discovered rhythmic transcripts in both the dorsolateral prefrontal cortex (DLPFC) and nucleus accumbens (NAc), key brain areas involved in OUD, that were largely distinct between OUD and unaffected subjects. Fewer rhythmic transcripts were identified in DLPFC of subjects with OUD compared to unaffected subjects, whereas in the NAc, nearly double the number of rhythmic transcripts was identified in subjects with OUD. In NAc of subjects with OUD, rhythmic transcripts peaked either in the evening or near sunrise, and were associated with an opioid, dopamine, and GABAergic neurotransmission. Associations with altered neurotransmission in NAc were further supported by co-expression network analysis which identified OUD-specific modules enriched for transcripts involved in dopamine, GABA, and glutamatergic synaptic functions. Additionally, rhythmic transcripts in DLPFC and NAc of subjects with OUD were enriched for genomic loci associated with sleep-related GWAS traits, including sleep duration and insomnia. Collectively, our findings connect transcriptional rhythm changes in opioidergic, dopaminergic, GABAergic signaling in the human brain to sleep-related traits in opioid addiction.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Mice Expressing Regulators of G protein Signaling-insensitive Gαo Define Roles of μ Opioid Receptor Gαo and Gαi Subunit Coupling in Inhibition of Presynaptic GABA Release

Regulators of G protein signaling (RGS) proteins modulate signaling by G protein-coupled receptors. Using a knock-in transgenic mouse model with a mutation in Gαo that does not bind RGS proteins (RGS-insensitive), we determined the effect of RGS proteins on presynaptic μ opioid receptor (MOR)-mediated inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). The MOR agonists [d-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) and met-enkephalin (ME) inhibited evoked inhibitory postsynaptic currents (eIPSCs) in the RGS-insensitive mice compared with wild-type (WT) littermates, respectively. Fentanyl inhibited eIPSCs similarly in both WT and RGS-insensitive mice. There were no differences in opioid agonist inhibition of spontaneous GABA release between the genotypes. To further probe the mechanism underlying these differences between opioid inhibition of evoked and spontaneous GABA release, specific myristoylated Gα peptide inhibitors for Gαo₁ and Gαi_1-3 that block receptor-G protein interactions were used to test the preference of agonists for MOR-Gα complexes. The Gαo₁ inhibitor reduced DAMGO inhibition of eIPSCs, but Gαi_1-3 inhibitors had no effect. Both Gαo₁ and Gαi_1-3 inhibitors separately reduced fentanyl inhibition of eIPSCs but had no effects on ME inhibition. Gαi_1-3 inhibitors blocked the inhibitory effects of ME and fentanyl on miniature postsynaptic current (mIPSC) frequency, but both Gαo₁ and Gαi_1-3 inhibitors were needed to block the effects of DAMGO. Finally, baclofen-mediated inhibition of GABA release is unaffected in the RGS-insensitive mice and in the presence of Gαo₁ and Gαi_1-3 inhibitor peptides, suggesting that GABA_B receptor coupling to G proteins in vlPAG presynaptic terminals is different than MOR coupling. SIGNIFICANCE STATEMENT: Presynaptic μ opioid receptors (MORs) in the ventrolateral periaqueductal gray are critical for opioid analgesia and are negatively regulated by RGS proteins. These data in RGS-insensitive mice provide evidence that MOR agonists differ in preference for Gαo versus Gαi and regulation by RGS proteins in presynaptic terminals, providing a mechanism for functional selectivity between agonists. The results further define important differences in MOR and GABA_B receptor coupling to G proteins that could be exploited for new pain therapies.

Calmodulin Supports TRPA1 Channel Association with Opioid Receptors and Glutamate NMDA Receptors in the Nervous Tissue

Transient receptor potential ankyrin member 1 (TRPA1) belongs to the family of thermo TRP cation channels that detect harmful temperatures, acids and numerous chemical pollutants. TRPA1 is expressed in nervous tissue, where it participates in the genesis of nociceptive signals in response to noxious stimuli and mediates mechanical hyperalgesia and allodynia associated with different neuropathies. The glutamate N-methyl-d-aspartate receptor (NMDAR), which plays a relevant role in allodynia to mechanical stimuli, is connected via histidine triad nucleotide-binding protein 1 (HINT1) and type 1 sigma receptor (σ1R) to mu-opioid receptors (MORs), which mediate the most potent pain relief. Notably, neuropathic pain causes a reduction in MOR antinociceptive efficacy, which can be reversed by blocking spinal NMDARs and TRPA1 channels. Thus, we studied whether TRPA1 channels form complexes with MORs and NMDARs that may be implicated in the aforementioned nociceptive signals. Our data suggest that TRPA1 channels functionally associate with MORs, delta opioid receptors and NMDARs in the dorsal root ganglia, the spinal cord and brain areas. These associations were altered in response to pharmacological interventions and the induction of inflammatory and also neuropathic pain. The MOR-TRPA1 and NMDAR-TRPA1 associations do not require HINT1 or σ1R but appear to be mediated by calcium-activated calmodulin. Thus, TRPA1 channels may associate with NMDARs to promote ascending acute and chronic pain signals and to control MOR antinociception.

The Sigma 2 receptor promotes and the Sigma 1 receptor inhibits mu-opioid receptor-mediated antinociception

The Sigma-1 receptor (σ1R) has emerged as an interesting pharmacological target because it inhibits analgesia mediated by mu-opioid receptors (MOR), and also facilitates the development of neuropathic pain. Based on these findings, the recent cloning of the Sigma-2 receptor (σ2R) led us to investigate its potential role as a regulator of opioid analgesia and of pain hypersensitivity in σ2R knockout mice. In contrast to σ1R deficient mice, σ2R knockout mice developed mechanical allodynia following establishment of chronic constriction injury-induced neuropathic pain, which was alleviated by the σ1R antagonist S1RA. The analgesic effects of morphine, [D-Ala, N-MePhe, Gly-ol]-encephalin (DAMGO) and β-endorphin increased in σ1R-/- mice and diminished in σ2R-/- mice. The analgesic effect of morphine was increased in σ2R-/- mice by treatment with S1RA. However, σ2R-/- mice and wild-type mice exhibited comparable antinociceptive responses to the delta receptor agonist [D-Pen2,5]-encephalin (DPDPE), the cannabinoid type 1 receptor agonist WIN55,212-2 and the α2-adrenergic receptor agonist clonidine. Therefore, while σR1 inhibits and σ2R facilitates MOR-mediated analgesia these receptors exchange their roles when regulating neuropathic pain perception. Our study may help identify new pharmacological targets for diminishing pain perception and improving opioid detoxification therapies.

Unique Pharmacological Properties of the Kappa Opioid Receptor Signaling Through Gαz as Shown with Bioluminescence Resonance Energy Tranfer

Opioid receptors (ORs) convert extracellular messages to signaling events by coupling to the heterotrimeric G proteins, Gα•βγ Classic pharmacological methods, such as [³⁵S]GTPγS binding and inhibition of cyclic AMP production, allow for general opioid characterization, but they are subject to the varying endogenous Gα proteins in a given cell type. Bioluminescence resonance energy transfer (BRET) technology offers new insight by allowing the direct observation of Gα subunit-specific effects on opioid pharmacology. Using a Venus-tagged Gβγ and nanoluciferase-tagged truncated G protein receptor kinase 3, an increase in BRET signal correlated with OR activation mediated by a specific Gα protein. The magnitude of the BRET signal was normalized to the maximum response obtained with 10 µM 2-(3,4-dichlorophenyl)-N-methyl-N-[(1R,2R)-2-pyrrolidin-1-ylcyclohexyl]acetamide (U50,488) for the kappa OR (KOR). Opioids reached equilibrium with the KOR, and concentration-response curves were generated. Although the full agonists U50,488, salvinorin A, nalfurafine, and dynorphin peptides were equally efficacious regardless of the Gα subunit present, the concentration-response curves were leftward shifted when the KOR was signaling through Gαz compared with other Gαi/o subunits. In contrast, the Gα subunit distinctly affected both the efficacy and potency of partial kappa agonists, such as the benzomorphans, and the classic mu opioid antagonists, naloxone, naltrexone, and nalmefene. For example, (-)pentazocine had EC₅₀ values of 7.3 and 110 nM and maximal stimulation values of 79% and 35% when the KOR signaled through Gαz and Gαi1, respectively. Together, these observations suggest KOR pharmacology varies based on the specific Gα subunit coupled to the KOR. SIGNIFICANCE STATEMENT: Opioid receptors couple to various heterotrimeric Gαβγ proteins to convert extracellular cues to precise intracellular events. This paper focuses on how the various inhibitory Gα subunits influence the pharmacology of full and partial agonists at the kappa opioid receptor. Using a bioluminescent assay, the efficacy and potency of kappa opioids was determined. Opioid signaling was more potent through Gαz compared with other Gα proteins. These observations suggest that Gαz may impact opioid pharmacology and cellular physiology more than previously thought.

Regulator of G-Protein Signaling (RGS) Protein Modulation of Opioid Receptor Signaling as a Potential Target for Pain Management

Opioid drugs are the gold standard for the management of pain, but their use is severely limited by dangerous and unpleasant side effects. All clinically available opioid analgesics bind to and activate the mu-opioid receptor (MOR), a heterotrimeric G-protein-coupled receptor, to produce analgesia. The activity of these receptors is modulated by a family of intracellular RGS proteins or regulators of G-protein signaling proteins, characterized by the presence of a conserved RGS Homology (RH) domain. These proteins act as negative regulators of G-protein signaling by serving as GTPase accelerating proteins or GAPS to switch off signaling by both the Gα and βγ subunits of heterotrimeric G-proteins. Consequently, knockdown or knockout of RGS protein activity enhances signaling downstream of MOR. In this review we discuss current knowledge of how this activity, across the different families of RGS proteins, modulates MOR activity, as well as activity of other members of the opioid receptor family, and so pain and analgesia in animal models, with particular emphasis on RGS4 and RGS9 families. We discuss inhibition of RGS proteins with small molecule inhibitors that bind to sensitive cysteine moieties in the RH domain and the potential for targeting this family of intracellular proteins as adjuncts to provide an opioid sparing effect or as standalone analgesics by promoting the activity of endogenous opioid peptides. Overall, we conclude that RGS proteins may be a novel drug target to provide analgesia with reduced opioid-like side effects, but that much basic work is needed to define the roles for specific RGS proteins, particularly in chronic pain, as well as a need to develop newer inhibitors.

Regulators of G-Protein Signaling (RGS) Proteins Promote Receptor Coupling to G-Protein-Coupled Inwardly Rectifying Potassium (GIRK) Channels

Regulators of G-protein signaling (RGS) proteins negatively modulate presynaptic μ-opioid receptor inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). Paradoxically, we find that G-protein-coupled receptor (GPCR) activation of G-protein-gated inwardly rectifying K⁺ channels (GIRKs) in the vlPAG is reduced in an agonist- and receptor-dependent manner in transgenic knock-in mice of either sex expressing mutant RGS-insensitive Gαo proteins. μ-Opioid receptor agonist activation of GIRK currents was reduced for DAMGO and fentanyl but not for [Met⁵]-enkephalin acetate salt hydrate (ME) in the RGS-insensitive heterozygous (Het) mice compared with wild-type mice. The GABA_B agonist baclofen-induced GIRK currents were also reduced in the Het mice. We confirmed the role of Gαo proteins in μ-opioid receptor and GABA_B receptor signaling pathways in wild-type mice using myristoylated peptide inhibitors of Gαo₁ and Gαi_1-3 The results using these inhibitors indicate that receptor activation of GIRK channels is dependent on the preference of the agonist-stimulated receptor for Gαo versus that for Gαi. DAMGO and fentanyl-mediated GIRK currents were reduced in the presence of the Gαo₁ inhibitor, but not the Gαi_1-3 inhibitors. In contrast, the Gαo₁ peptide inhibitor did not affect ME activation of GIRK currents, which is consistent with results in the Het mice, but the Gαi_1-3 inhibitors significantly reduced ME-mediated GIRK currents. Finally, the reduction in GIRK activation in the Het mice plays a role in opioid- and baclofen-mediated spinal antinociception, but not supraspinal antinociception. Thus, our studies indicate that RGS proteins have multiple mechanisms of modulating GPCR signaling that produce negative and positive regulation of signaling depending on the effector.SIGNIFICANCE STATEMENT Regulators of G-protein signaling (RGS) proteins positively modulate GPCR coupling to GIRKs, and this coupling is critical for opioid- and baclofen-mediated spinal antinociception, whereas μ-opioid receptor-mediated supraspinal antinociception depends on presynaptic inhibition that is negatively regulated by RGS proteins. The identification of these opposite roles for RGS proteins has implications for signaling via other GPCRs.

RGS proteins as targets in the treatment of intestinal inflammation and visceral pain: New insights and future perspectives

Regulators of G protein signaling (RGS) proteins provide timely termination of G protein-coupled receptor (GPCR) responses. Serving as a central control point in GPCR signaling cascades, RGS proteins are promising targets for drug development. In this review, we discuss the involvement of RGS proteins in the pathophysiology of the gastrointestinal inflammation and their potential to become a target for anti-inflammatory drugs. Specifically, we evaluate the emerging evidence for modulation of selected receptor families: opioid, cannabinoid and serotonin by RGS proteins. We discuss how the regulation of RGS protein level and activity may modulate immunological pathways involved in the development of intestinal inflammation. Finally, we propose that RGS proteins may serve as a prognostic factor for survival rate in colorectal cancer. The ideas introduced in this review set a novel conceptual framework for the utilization of RGS proteins in the treatment of gastrointestinal inflammation, a growing major concern worldwide.

Regulator of G Protein Signaling 17 as a Negative Modulator of GPCR Signaling in Multiple Human Cancers

Regulators of G protein signaling (RGS) proteins modulate G protein-coupled receptor (GPCR) signaling networks by terminating signals produced by active Gα subunits. RGS17, a member of the RZ subfamily of RGS proteins, is typically only expressed in appreciable amounts in the human central nervous system, but previous works have shown that RGS17 expression is selectively upregulated in a number of malignancies, including lung, breast, prostate, and hepatocellular carcinoma. In addition, this upregulation of RGS17 is associated with a more aggressive cancer phenotype, as increased proliferation, migration, and invasion are observed. Conversely, decreased RGS17 expression diminishes the response of ovarian cancer cells to agents commonly used during chemotherapy. These somewhat contradictory roles of RGS17 in cancer highlight the need for selective, high-affinity inhibitors of RGS17 to use as chemical probes to further the understanding of RGS17 biology. Based on current evidence, these compounds could potentially have clinical utility as novel chemotherapeutics in the treatment of lung, prostate, breast, and liver cancers. Recent advances in screening technologies to identify potential inhibitors coupled with increasing knowledge of the structural requirements of RGS-Gα protein-protein interaction inhibitors make the future of drug discovery efforts targeting RGS17 promising. This review highlights recent findings related to RGS17 as both a canonical and atypical RGS protein, its role in various human disease states, and offers insights on small molecule inhibition of RGS17.

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.

Opioid receptor interacting proteins and the control of opioid signaling

Opioid receptors are seven-transmembrane domain receptors that couple to intracellular signaling molecules by activating heterotrimeric G proteins. However, the receptor and G protein do not function in isolation but their activities are modulated by several accessory and scaffolding proteins. Examples include arrestins, kinases, and regulators of G protein signaling proteins. Accessory proteins contribute to the observed potency and efficacy of agonists, but also to the direction of signaling and the phenomenon of biased agonism. This review will present current knowledge of such proteins and how they may provide targets for future drug design.

Analysis of candidate genes for morphine preference quantitative trait locus Mop2

Compared to DBA/2J (D2), C57BL/6J (B6) inbred mice exhibit strong morphine preference when tested using a two-bottle choice drinking paradigm. A morphine preference quantitative trait locus (QTL), Mop2, was originally mapped to proximal chromosome (Chr) 10 using a B6xD2 F2 intercross population, confirmed with reciprocal congenic strains and fine mapped with recombinant congenic strains. These efforts identified a ∼ 10-Million base pair (Mbp) interval, underlying Mop2, containing 35 genes. To further reduce the interval, mice from the D2.B6-Mop2-P1 congenic strain were backcrossed to parental D2 mice and two new recombinant strains of interest were generated: D2.B6-Mop2-P1.pD.dB and D2.B6-Mop2-P1.pD.dD. Results obtained from testing these strains in the two-bottle choice drinking paradigm suggest that the gene(s) responsible for the Mop2 QTL is one or more of 22 remaining within the newly defined interval (∼ 7.6 Mbp) which includes Oprm1 and several other genes related to opioid pharmacology. Real-time qRT-PCR analysis of Oprm1 and opioid-related genes Rgs17, Ppp1r14c, Vip, and Iyd revealed both between-strain and within-strain expression differences in comparisons of saline- and morphine-treated B6 and D2 mice. Analysis of Rgs17 protein levels also revealed both between-strain and within-strain differences in comparisons of saline- and morphine-treated B6 and D2 mice. Results suggest that the Mop2 QTL represents the combined influence of multiple genetic variants on morphine preference in these two strains. Relative contributions of each variant remain to be determined.

RGS17: an emerging therapeutic target for lung and prostate cancers

Ligands for G-protein-coupled receptors (GPCRs) represent approximately 50% of currently marketed drugs. RGS proteins modulate heterotrimeric G proteins and, thus, GPCR signaling, by accelerating the intrinsic GTPase activity of the Gα subunit. Given the prevalence of GPCR targeted therapeutics and the role RGS proteins play in G protein signaling, some RGS proteins are emerging as targets in their own right. One such RGS protein is RGS17. Increased RGS17 expression in some prostate and lung cancers has been demonstrated to support cancer progression, while reduced expression of RGS17 can lead to development of chemotherapeutic resistance in ovarian cancer. High-throughput screening is a powerful tool for lead compound identification, and utilization of high-throughput technologies has led to the discovery of several RGS inhibitors, thus far. As screening technologies advance, the identification of novel lead compounds the subsequent development of targeted therapeutics appears promising.

Differential control of opioid antinociception to thermal stimuli in a knock-in mouse expressing regulator of G-protein signaling-insensitive Gαo protein

Regulator of G-protein signaling (RGS) proteins classically function as negative modulators of G-protein-coupled receptor signaling. In vitro, RGS proteins have been shown to inhibit signaling by agonists at the μ-opioid receptor, including morphine. The goal of the present study was to evaluate the contribution of endogenous RGS proteins to the antinociceptive effects of morphine and other opioid agonists. To do this, a knock-in mouse that expresses an RGS-insensitive (RGSi) mutant Gαo protein, Gαo(G184S) (Gαo RGSi), was evaluated for morphine or methadone antinociception in response to noxious thermal stimuli. Mice expressing Gαo RGSi subunits exhibited a naltrexone-sensitive enhancement of baseline latency in both the hot-plate and warm-water tail-withdrawal tests. In the hot-plate test, a measure of supraspinal nociception, morphine antinociception was increased, and this was associated with an increased ability of opioids to inhibit presynaptic GABA neurotransmission in the periaqueductal gray. In contrast, antinociception produced by either morphine or methadone was reduced in the tail-withdrawal test, a measure of spinal nociception. In whole-brain and spinal cord homogenates from mice expressing Gαo RGSi subunits, there was a small loss of Gαo expression and an accompanying decrease in basal G-protein activity. Our results strongly support a role for RGS proteins as negative regulators of opioid supraspinal antinociception and also reveal a potential novel function of RGS proteins as positive regulators of opioid spinal antinociceptive pathways.

GPCRs promote the release of zinc ions mediated by nNOS/NO and the redox transducer RGSZ2 protein

AIMS

Morphine signaling via the μ-opioid receptor (MOR) is coupled to redox-dependent zinc release from endogenous stores. Thus, MOR activation stimulates the complex formed by RGSZ2 (a regulator of G protein signaling) and neural nitric oxide synthase (nNOS) to produce NO, and to recruit PKCγ and Raf-1 in a zinc-dependent manner. Accordingly, we investigated whether redox regulation of zinc metabolism was unique to the MOR, or if it is a signaling mechanism shared by G-protein coupled receptors (GPCRs).

RESULTS

A physical interaction with the RGSZ2-nNOS complex was detected for the following GPCRs: neuropeptides, MOR and δ-opioid (DOR); biogenic amines, 5HT1A, 5HT2A, α2A, D1 and D2; acetylcholine, muscarinic M2 and M4; excitatory amino acid glutamate, mGlu2 and mGlu5; and derivatives of arachidonic acid (anandamide), CB1. Agonist activation of these receptors induced the release of zinc ions from the RGSZ2 zinc finger via a nNOS/NO-dependent mechanism, recruiting PKCγ and Raf-1 to the C terminus or the third internal loop of the GPCR.

INNOVATION

A series of GPCRs share an unexpected mechanistic feature, the nNOS/NO-dependent regulation of zinc ion signaling via a redox mechanism. The RGSZ2 protein emerges as a potential redox zinc switch that converts NO signals into zinc signals, thereby able to modulate the function of redox sensor proteins like PKCγ or Raf-1.

CONCLUSION

Redox mechanisms are crucial for the successful propagation of GPCR signals in neurons. Thus, dysfunctions of GPCR-regulated NO/zinc signaling may contribute to neurodegenerative and mood disorders such as Alzheimer's disease and depression.

Sensitivity to μ-opioid receptor-mediated anti-nociception is determined by cross-regulation between μ- and δ-opioid receptors at supraspinal level

BACKGROUND AND PURPOSE

The perception of pain and its inhibition varies considerably between individuals, and this variability is still unexplained. The aim of the present study is to determine whether functional interactions between opioid receptors are involved in the inter-individual variability in the sensitivity to μ-opioid receptor agonists.

EXPERIMENTAL APPROACH

Anti-nociceptive tests, radioligand binding, stimulation of [(35) S]GTP-γ-S binding, inhibition of cAMP production and co-immunoprecipitation experiments were performed in two strains of rat (Sprague-Dawley bred at our university - SDU - and Wistar) that differ in their sensitivity to opioids.

KEY RESULTS

The increased anti-nociceptive potency of µ-opioid receptor agonists in SDU rats was reversed by the δ-opioid receptor antagonist, naltrindole. Inhibition of the binding of [(3) H] naltrindole by µ-opioid receptor agonists was different in brain membranes from SDU and Wistar rats. Differences were also evident in the effect of δ-opioid receptor ligands on the binding of [(35) S]GTP-γ-S stimulated by µ-opioid receptors agonists. No strain-related differences were detected in spinal cord membranes. The potency of morphine to inhibit cAMP production in brain membranes varied between the strains, in the presence of deltorphin II and naltrindole. Co-immunoprecipitation experiments demonstrated that δ-opioid receptors were associated with μ-opioid receptors to a higher extent in brain synaptosomal fractions from SDU than in those from Wistar rats.

CONCLUSIONS AND IMPLICATIONS

There was increased supraspinal cross-talk between μ and δ-opioid receptors in SDU, as compared with Wistar rats. This was related to an enhanced sensitivity to anti-nociception induced by µ-opioid receptor agonists.

Variation in regulator of G-protein signaling 17 gene (RGS17) is associated with multiple substance dependence diagnoses

Background

RGS17 and RGS20 encode two members of the regulator of G-protein signaling RGS-Rz subfamily. Variation in these genes may alter their transcription and thereby influence the function of G protein-coupled receptors, including opioid receptors, and modify risk for substance dependence.

Methods

The association of 13 RGS17 and eight RGS20 tag single nucleotide polymorphisms (SNPs) was examined with four substance dependence diagnoses (alcohol (AD), cocaine (CD), opioid (OD) or marijuana (MjD)] in 1,905 African Americans (AAs: 1,562 cases and 343 controls) and 1,332 European Americans (EAs: 981 cases and 351 controls). Analyses were performed using both χ² tests and logistic regression analyses that covaried sex, age, and ancestry proportion. Correlation of genotypes and mRNA expression levels was assessed by linear regression analyses.

Results

Seven RGS17 SNPs showed a significant association with at least one of the four dependence traits after a permutation-based correction for multiple testing (0.003≤P_empirical≤0.037). The G allele of SNP rs596359, in the RGS17 promoter region, was associated with AD, CD, OD, or MjD in both populations (0.005≤P_empirical≤0.019). This allele was also associated with significantly lower mRNA expression levels of RGS17 in YRI subjects (P = 0.002) and non-significantly lower mRNA expression levels of RGS17 in CEU subjects (P = 0.185). No RGS20 SNPs were associated with any of the four dependence traits in either population.

Conclusions

This study demonstrated that variation in RGS17 was associated with risk for substance dependence diagnoses in both AA and EA populations.

The function of alpha-2-adrenoceptors in the rat locus coeruleus is preserved in the chronic constriction injury model of neuropathic pain

RATIONALE

Peripheral neuropathic pain is a chronic condition that may produce plastic changes in several brain regions. The noradrenergic locus coeruleus (LC) is a crucial component of ascending and descending pain pathways, both of which are frequently compromised after nerve injury.

OBJECTIVES

The objective of the study was to examine whether chronic constriction injury (CCI), a model of neuropathic pain, alters noradrenergic activity in the rat LC.

METHODS

Activity in the LC was assessed by electrophysiology and microdialysis, while protein expression was monitored in western blots and by immunohistochemistry.

RESULTS

The pain threshold had dropped in injured rats 7 days after inducing neuropathy. While alpha-2-adrenoceptors mediate activity in the LC and in its terminal areas, no alterations in either spontaneous neuronal activity or extracellular noradrenaline levels were observed following CCI. Moreover, alpha-2-adrenoceptor activity in the LC of CCI rats remained unchanged after systemic administration of UK14,304, RX821002 or desipramine. Accordingly, extracellular noradrenaline levels in the LC were similar in CCI and control animals following local administration of clonidine or RX821002. In addition, there were no changes in the expression of the alpha-2-adrenoceptors, Gαi/z subunits or the regulators of G-protein signaling. However, pERK1/2 (phosphorylated extracellular signal-regulated kinases 1/2) expression augmented in the spinal cord, paragigantocellularis nucleus (PGi) and dorsal raphe nucleus (DRN) following CCI.

CONCLUSIONS

Neuropathic pain is not accompanied by modifications in tonic LC activity after the onset of pain. This may indicate that the signals from the PGi and DRN, the excitatory and inhibitory afferents of the LC, cancel one another out.

μ-Opioid receptors and regulators of G protein signaling (RGS) proteins: from a symposium on new concepts in mu-opioid pharmacology

Mu-opioid receptors (MOR) are the therapeutic target for opiate analgesic drugs and also mediate many of the side-effects and addiction liability of these compounds. MOR is a seven-transmembrane domain receptor that couples to intracellular signaling molecules by activating heterotrimeric G proteins. However, the receptor and G protein do not function in isolation but their activities are moderated by several accessory and scaffolding proteins. One important group of accessory proteins is the regulator of G protein signaling (RGS) protein family, a large family of more than thirty members which bind to the activated Gα subunit of the heterotrimeric G protein and serve to accelerate signal termination. This action negatively modulates receptor signaling and subsequent behavior. Several members of this family, in particular RGS4 and RGS9-2 have been demonstrated to influence MOR signaling and morphine-induced behaviors, including reward. Moreover, this interaction is not unidirectional since morphine has been demonstrated to modulate expression levels of RGS proteins, especially RGS4 and RGS9-2, in a tissue and time dependent manner. In this article, I will discuss our work on the regulation of MOR signaling by RGS protein activity in cultured cell systems in the context of other in vitro and behavioral studies. In addition I will consider implications of the bi-directional interaction between MOR receptor activation and RGS protein activity and whether RGS proteins might provide a suitable and novel target for medications to manage addictive behaviors.

SUMO-SIM interactions regulate the activity of RGSZ2 proteins

The RGSZ2 gene, a regulator of G protein signaling, has been implicated in cognition, Alzheimer's disease, panic disorder, schizophrenia and several human cancers. This 210 amino acid protein is a GTPase accelerating protein (GAP) on Gαi/o/z subunits, binds to the N terminal of neural nitric oxide synthase (nNOS) negatively regulating the production of nitric oxide, and binds to the histidine triad nucleotide-binding protein 1 at the C terminus of different G protein-coupled receptors (GPCRs). We now describe a novel regulatory mechanism of RGS GAP function through the covalent incorporation of Small Ubiquitin-like MOdifiers (SUMO) into RGSZ2 RGS box (RH) and the SUMO non covalent binding with SUMO-interacting motifs (SIM): one upstream of the RH and a second within this region. The covalent attachment of SUMO does not affect RGSZ2 binding to GPCR-activated GαGTP subunits but abolishes its GAP activity. By contrast, non-covalent binding of SUMO with RH SIM impedes RGSZ2 from interacting with GαGTP subunits. Binding of SUMO to the RGSZ2 SIM that lies outside the RH does not affect GαGTP binding or GAP activity, but it could lead to regulatory interactions with sumoylated proteins. Thus, sumoylation and SUMO-SIM interactions constitute a new regulatory mechanism of RGS GAP function and therefore of GPCR cell signaling as well.

The histidine triad nucleotide-binding protein 1 supports mu-opioid receptor-glutamate NMDA receptor cross-regulation

A series of pharmacological and physiological studies have demonstrated the functional cross-regulation between MOR and NMDAR. These receptors coexist at postsynaptic sites in midbrain periaqueductal grey (PAG) neurons, an area implicated in the analgesic effects of opioids like morphine. In this study, we found that the MOR-associated histidine triad nucleotide-binding protein 1 (HINT1) is essential for maintaining the connection between the NMDAR and MOR. Morphine-induced analgesic tolerance is prevented and even rescued by inhibiting PKC or by antagonizing NMDAR. However, in the absence of HINT1, the MOR becomes supersensitive to morphine before suffering a profound and lasting desensitization that is refractory to PKC inhibition or NMDAR antagonism. Thus, HINT1 emerges as a key protein that is critical for sustaining NMDAR-mediated regulation of MOR signaling strength. Thus, HINT1 deficiency may contribute to opioid-intractable pain syndromes by causing long-term MOR desensitization via mechanisms independent of NMDAR.

RGSZ2 binds to the neural nitric oxide synthase PDZ domain to regulate mu-opioid receptor-mediated potentiation of the N-methyl-D-aspartate receptor-calmodulin-dependent protein kinase II pathway

Morphine increases the production of nitric oxide (NO) via the phosphoinositide 3-kinase/Akt/neural nitric oxide synthase (nNOS) pathway. Subsequently, NO enhances N-methyl-D-aspartate receptor (NMDAR)/calmodulin-dependent protein kinase II (CaMKII) cascade, diminishing the strength of morphine-activated Mu-opioid receptor (MOR) signaling. During this process, NO signaling is restricted by the association of nNOS to the MOR.

AIMS

Here, we examined how nNOS/NO signaling is downregulated by the morphine-activated MOR and how this regulation affects antinociception.

RESULTS

Accordingly, we show that the MOR-NMDAR regulatory loop relies on the negative control of nNOS activity exerted by RGSZ2, a protein physically coupled to the MOR. This regulation requires binding of the nNOS N terminal PDZ domain to the RGSZ2 PDZ binding motifs that lie upstream of the RGS box.

INNOVATION

Indeed, in RGSZ2-deficient mice morphine over-stimulates the nNOS/NO/NMDAR/CaMKII pathway, causing analgesic tolerance to develop rapidly. Recovery of RGSZ2 levels or inhibition of nNOS, protein kinase C, NMDAR, or CaMKII function restores MOR signaling and morphine recovers its full analgesic potency.

CONCLUSION

This RGSZ2-dependent regulation of NMDAR activity is relevant to persistent pain disorders associated with heightened NMDAR-mediated glutamate responses and the reduced antinociceptive capacity of opioids.

Control of striatal signaling by g protein regulators

Signaling via heterotrimeric G proteins plays a crucial role in modulating the responses of striatal neurons that ultimately shape core behaviors mediated by the basal ganglia circuitry, such as reward valuation, habit formation, and movement coordination. Activation of G protein-coupled receptors (GPCRs) by extracellular signals activates heterotrimeric G proteins by promoting the binding of GTP to their α subunits. G proteins exert their effects by influencing the activity of key effector proteins in this region, including ion channels, second messenger enzymes, and protein kinases. Striatal neurons express a staggering number of GPCRs whose activation results in the engagement of downstream signaling pathways and cellular responses with unique profiles but common molecular mechanisms. Studies over the last decade have revealed that the extent and duration of GPCR signaling are controlled by a conserved protein family named regulator of G protein signaling (RGS). RGS proteins accelerate GTP hydrolysis by the α subunits of G proteins, thus promoting deactivation of GPCR signaling. In this review, we discuss the progress made in understanding the roles of RGS proteins in controlling striatal G protein signaling and providing integration and selectivity of signal transmission. We review evidence on the formation of a macromolecular complex between RGS proteins and other components of striatal signaling pathways, their molecular regulatory mechanisms and impacts on GPCR signaling in the striatum obtained from biochemical studies and experiments involving genetic mouse models. Special emphasis is placed on RGS9-2, a member of the RGS family that is highly enriched in the striatum and plays critical roles in drug addiction and motor control.

Mu-opioid receptors transiently activate the Akt-nNOS pathway to produce sustained potentiation of PKC-mediated NMDAR-CaMKII signaling

BACKGROUND

In periaqueductal grey (PAG) matter, cross-talk between the Mu-opioid receptor (MOR) and the glutamate N-methyl-D-Aspartate receptor (NMDAR)-CaMKII pathway supports the development of analgesic tolerance to morphine. In neurons, histidine triad nucleotide binding protein 1 (HINT1) connects the regulators of G protein signaling RGSZ1 and RGSZ2 to the C terminus of the MOR. In response to morphine, this HINT1-RGSZ complex binds PKCgamma, and afterwards, the interplay between PKCgamma, Src and Gz/Gi proteins leads to sustained potentiation of NMDAR-mediated glutamate responses.

METHODOLOGY/PRINCIPAL FINDINGS

Following an intracerebroventricular (icv) injection of 10 nmol morphine, Akt was recruited to the synaptosomal membrane and activated by Thr308 and Ser473 phosphorylation. The Akt activation was immediately transferred to neural Nitric Oxide Synthase (nNOS) Ser1417. Afterwards, nitric oxide (NO)-released zinc ions recruited PKCgamma to the MOR to promote the Src-mediated phosphorylation of the Tyr1325 NMDAR2A subunit. This action increased NMDAR calcium flux and CaMKII was activated in a calcium-calmodulin dependent manner. CaMKII then acted on nNOS Ser847 to produce a sustained reduction in NO levels. The activation of the Akt-nNOS pathway was also reduced by the binding of these proteins to the MOR-HINT1 complex where they remained inactive. Tolerance to acute morphine developed as a result of phosphorylation of MOR cytosolic residues, uncoupling from the regulated G proteins which are transferred to RGSZ2 proteins. The diminished effect of morphine was prevented by LNNA, an inhibitor of nNOS function, and naltrindole, a delta-opioid receptor antagonist that also inhibits Akt.

CONCLUSIONS/SIGNIFICANCE

Analysis of the regulatory phosphorylation of the proteins included in the study indicated that morphine produces a transient activation of the Akt/PKB-nNOS pathway. This activation occurs upstream of PKCgamma and Src mediated potentiation of NMDAR activity, ultimately leading to morphine tolerance. In summary, the Akt-nNOS pathway acts as a primer for morphine-triggered events which leads to the sustained potentiation of the NMDAR-CaMKII pathway and MOR inhibition.

Regulators of G protein signaling proteins as central components of G protein-coupled receptor signaling complexes

The regulators of G protein signaling (RGS) proteins bind directly to G protein alpha (Gα) subunits to regulate the signaling functions of Gα and their linked G protein-coupled receptors (GPCRs). Recent studies indicate that RGS proteins also interact with GPCRs, not just G proteins, to form preferred functional pairs. Interactions between GPCRs and RGS proteins may be direct or indirect (via a linker protein) and are dictated by the receptors, rather than the linked G proteins. Emerging models suggest that GPCRs serve as platforms for assembling an overlapping and distinct constellation of signaling proteins that perform receptor-specific signaling tasks. Compelling evidence now indicates that RGS proteins are central components of these GPCR signaling complexes. This review will outline recent discoveries of GPCR/RGS pairs as well as new data in support of the idea that GPCRs serve as platforms for the formation of multiprotein signaling complexes.

Membrane functional organisation and dynamic of mu-opioid receptors

The activation and signalling activity of the membrane mu-opioid receptor (MOP-R) involve interactions among the receptor, G-proteins, effectors and many other membrane or cytosolic proteins. Decades of investigation have led to identification of the main biochemical processes, but the mechanisms governing the successive protein-protein interactions have yet to be established. We will need to unravel the dynamic membrane organisation of this complex and multifaceted molecular machinery if we are to understand these mechanisms. Here, we review and discuss advances in our understanding of the signalling mechanism of MOP-R resulting from biochemical or biophysical studies of the organisation of this receptor in the plasma membrane.

Differential modulation of mu- and delta-opioid receptor agonists by endogenous RGS4 protein in SH-SY5Y cells

Regulator of G-protein signaling (RGS) proteins are a family of molecules that control the duration of G protein signaling. A variety of RGS proteins have been reported to modulate opioid receptor signaling. Here we show that RGS4 is abundantly expressed in human neuroblastoma SH-SY5Y cells that endogenously express mu- and delta-opioid receptors and test the hypothesis that the activity of opioids in these cells is modulated by RGS4. Endogenous RGS4 protein was reduced by approximately 90% in SH-SY5Y cells stably expressing short hairpin RNA specifically targeted to RGS4. In these cells, the potency and maximal effect of delta-opioid receptor agonist (SNC80)-mediated inhibition of forskolin-stimulated cAMP accumulation was increased compared with control cells. This effect was reversed by transient transfection of a stable RGS4 mutant (HA-RGS4C2S). Furthermore, MAPK activation by SNC80 was increased in cells with knockdown of RGS4. In contrast, there was no change in the mu-opioid (morphine) response at adenylyl cyclase or MAPK. FLAG-tagged opioid receptors and HA-RGS4C2S were transiently expressed in HEK293T cells, and co-immunoprecipitation experiments showed that the delta-opioid receptor but not the mu-opioid receptor could be precipitated together with the stable RGS4. Using chimeras of the delta- and mu-opioid receptors, the C-tail and third intracellular domain of the delta-opioid receptor were suggested to be the sites of interaction with RGS4. The findings demonstrate a role for endogenous RGS4 protein in modulating delta-opioid receptor signaling in SH-SY5Y cells and provide evidence for a receptor-specific effect of RGS4.

Gz mediates the long-lasting desensitization of brain CB1 receptors and is essential for cross-tolerance with morphine

Background

Although the systemic administration of cannabinoids produces antinociception, their chronic use leads to analgesic tolerance as well as cross-tolerance to morphine. These effects are mediated by cannabinoids binding to peripheral, spinal and supraspinal CB1 and CB2 receptors, making it difficult to determine the relevance of each receptor type to these phenomena. However, in the brain, the CB1 receptors (CB1Rs) are expressed at high levels in neurons, whereas the expression of CB2Rs is marginal. Thus, CB1Rs mediate the effects of smoked cannabis and are also implicated in emotional behaviors. We have analyzed the production of supraspinal analgesia and the development of tolerance at CB1Rs by the direct injection of a series of cannabinoids into the brain. The influence of the activation of CB1Rs on supraspinal analgesia evoked by morphine was also evaluated.

Results

Intracerebroventricular (icv) administration of cannabinoid receptor agonists, WIN55,212-2, ACEA or methanandamide, generated a dose-dependent analgesia. Notably, a single administration of these compounds brought about profound analgesic tolerance that lasted for more than 14 days. This decrease in the effect of cannabinoid receptor agonists was not mediated by depletion of CB1Rs or the loss of regulated G proteins, but, nevertheless, it was accompanied by reduced morphine analgesia. On the other hand, acute morphine administration produced tolerance that lasted only 3 days and did not affect the CB1R. We found that both neural mu-opioid receptors (MORs) and CB1Rs interact with the HINT1-RGSZ module, thereby regulating pertussis toxin-insensitive Gz proteins. In mice with reduced levels of these Gz proteins, the CB1R agonists produced no such desensitization or morphine cross-tolerance. On the other hand, experimental enhancement of Gz signaling enabled an acute icv administration of morphine to produce a long-lasting tolerance at MORs that persisted for more than 2 weeks, and it also impaired the analgesic effects of cannabinoids.

Conclusion

In the brain, cannabinoids can produce analgesic tolerance that is not associated with the loss of surface CB1Rs or their uncoupling from regulated transduction. Neural specific Gz proteins are essential mediators of the analgesic effects of supraspinal CB1R agonists and morphine. These Gz proteins are also responsible for the long-term analgesic tolerance produced by single doses of these agonists, as well as for the cross-tolerance between CB1Rs and MORs.

RGS17, an overexpressed gene in human lung and prostate cancer, induces tumor cell proliferation through the cyclic AMP-PKA-CREB pathway

We have identified RGS17 as a commonly induced gene in lung and prostate tumors. Through microarray and gene expression analysis, we show that expression of RGS17 is up-regulated in 80% of lung tumors, and also up-regulated in prostate tumors. Through knockdown and overexpression of RGS17 in tumor cells, we show that RGS17 confers a proliferative phenotype and is required for the maintenance of the proliferative potential of tumor cells. We show through exon microarray, transcript analysis, and functional assays that RGS17 promotes cyclic AMP (cAMP)-responsive element binding protein (CREB)-responsive gene expression, increases cAMP levels, and enhances forskolin-mediated cAMP production. Furthermore, inhibition of cAMP-dependent kinase prevents tumor cell proliferation, and proliferation is partially rescued by RGS17 overexpression. In the present study, we show a role for RGS17 in the maintenance of tumor cell proliferation through induction of cAMP signaling and CREB phosphorylation. The prevalence of the induction of RGS17 in tumor tissues of various types further implicates its importance in the maintenance of tumor growth.

Modulation of opioid receptor function by protein-protein interactions

Opioid receptors, MORP, DORP and KORP, belong to the family A of G protein coupled receptors (GPCR), and have been found to modulate a large number of physiological functions, including mood, stress, appetite, nociception and immune responses. Exogenously applied opioid alkaloids produce analgesia, hedonia and addiction. Addiction is linked to alterations in function and responsiveness of all three opioid receptors in the brain. Over the last few years, a large number of studies identified protein-protein interactions that play an essential role in opioid receptor function and responsiveness. Here, we summarize interactions shown to affect receptor biogenesis and trafficking, as well as those affecting signal transduction events following receptor activation. This article also examines protein interactions modulating the rate of receptor endocytosis and degradation, events that play a major role in opiate analgesia. Like several other GPCRs, opioid receptors may form homo or heterodimers. The last part of this review summarizes recent knowledge on proteins known to affect opioid receptor dimerization.

Fine mapping of a major QTL influencing morphine preference in C57BL/6 and DBA/2 mice using congenic strains

C57BL/6J (B6) and DBA/2J (D2) mice differ in behaviors related to substance abuse, including voluntary morphine consumption and preference in a two-bottle choice paradigm. Two major quantitative trait loci (QTL) for morphine consumption and preference exist between these strains on chromosomes (Chrs.) 6 and 10 when the two-bottle choice involves morphine in saccharin vs quinine in saccharin. Here, we report the refinement of the Chr. 10 QTL in subcongenic strains of D2.B6-Mop2 congenic mice described previously. With these subcongenic mouse strains, we have divided the introgressed region of Chr. 10 containing the QTL gene(s) into two segments, one between the acromere and Stxbp5 (in D2.B6-Mop2-P1 mice) and the other between marker D10Mit211 and marker D10Mit51 (in D2.B6-Mop2-D1 mice). We find that, similar to B6 mice, the D2.B6-Mop2-P1 congenic mice exhibit a strong preference for morphine over quinine, whereas D2.B6-Mop2-D1 congenic mice avoid morphine (similar to D2 mice). We have also created a line of double congenic mice, B6.D2-Mop2.Qui, which contains both Chr. 10 and Chr. 6 QTL. We find that they are intermediate in their morphine preference scores when compared with B6 and D2 animals. Overall, these data suggest that the gene(s) involved in morphine preference in the morphine-quinine two-bottle choice paradigm are contained within the proximal region of Chr. 10 (which harbors Oprm1) between the acromere and Stxbp5, as well as on distal Chr. 6 between marker D6Mit10 and the telomere.

Lack of receptor-selective effects of either RGS2, RGS3 or RGS4 on muscarinic M3- and gonadotropin-releasing hormone receptor-mediated signalling through G alpha q/11

Termination of signalling by G-protein-coupled receptors requires inactivation of the G alpha-subunits of heterotrimeric G-proteins and the re-association of G alpha- and G betagamma-subunits. Inactivation of G alpha-subunits is achieved by the hydrolysis of bound GTP by an intrinsic GTPase activity, which is considerably enhanced by GTPase activating proteins. Regulators of G-protein signalling (RGS) proteins are a large family of GTPase activating proteins, many of which have structures indicating roles beyond GTPase activating protein activity and suggesting that the identity of the RGS protein recruited may also be critical to other aspects of signalling. There is some evidence of selective effects of RGS proteins against different G-protein-coupled receptors coupling to the same signalling pathways and growing evidence of physical interactions between RGS proteins and G-protein-coupled receptors. However, it is unclear as to how common such interactions are and the circumstances under which they are functionally relevant. Here we have examined potential selectivity of RGS2, 3 and 4 against signalling mediated by G alpha q/11-coupled muscarinic M3 receptors and gonadotropin-releasing hormone in an immortalised mouse pituitary cell line. Despite major structural differences between these two receptor types and agonist-dependent phosphorylation of the muscarinic M3- but not gonadotropin-releasing hormone receptor, signalling by both receptors was similarly inhibited by expression of either RGS2 or RGS3, whereas RGS4 has little effect. Thus, at least in these circumstances, RGS protein-dependent inhibition of signalling is not influenced by the nature of the G-protein-coupled receptor through which the signalling is mediated.

Emerging extranuclear roles of protein SUMOylation in neuronal function and dysfunction

Post-translational protein modifications are integral components of signalling cascades that enable cells to efficiently, rapidly and reversibly respond to extracellular stimuli. These modifications have crucial roles in the CNS, where the communication between neurons is particularly complex. SUMOylation is a post-translational modification in which a member of the small ubiquitin-like modifier (SUMO) family of proteins is conjugated to lysine residues in target proteins. It is well established that SUMOylation controls many aspects of nuclear function, but it is now clear that it is also a key determinant in many extranuclear neuronal processes, and it has also been implicated in a wide range of neuropathological conditions.

Calcium/calmodulin-dependent protein kinase II supports morphine antinociceptive tolerance by phosphorylation of glycosylated phosducin-like protein

The long isoform of the phosducin-like protein (PhLPl) is widely expressed in the brain and it is thought to influence G-protein signalling by regulating the activity of Gbetagamma dimers. We show that in the mature nervous system, PhLPl exists as both a 38kDa non-glycosylated isoform and as glycosylated isoforms of about 45, 100 and 150kDa. Additionally, neural PhLPl is subject to serine phosphorylation, which augments upon the activation of Mu-opioid receptors (MORs), as does its association with Gbetagamma subunits and 14-3-3 proteins. While the intracerebroventricular (icv) administration of morphine to mice rapidly reduced the association of MORs with G proteins, it increased the serine phosphorylation of these receptors. Moreover, activated Ca2+/calmodulin-dependent protein kinase II (CaMKII) accumulated in the MOR environment and phosphorylated PhLPl was seen to co-precipitate with these opioid receptors. Opioid-induced phosphorylation of PhLPl was impaired by inhibiting the activity of CaMKII and, in these circumstances, the association of PhLPl with Gbetagamma dimers and 14-3-3 proteins was diminished. Furthermore, these events were coupled with the recovery of G protein regulation by the MORs, while there was a decrease in serine phosphorylation of these receptors and morphine antinociceptive tolerance diminished. It seems that CaMKII phosphorylation of PhLPl stabilizes the PhLPl.Gbetagamma complex by promoting its binding to 14-3-3 proteins. When this complex fails to bind to 14-3-3 proteins, the association of PhLPl with Gbetagamma is probably disrupted by GalphaGDP subunits and the MORs recover control on G proteins.

RGS14 prevents morphine from internalizing Mu-opioid receptors in periaqueductal gray neurons

Opioid agonists display different capacities to stimulate mu-opioid receptor (MOR) endocytosis, which is related to their ability to provoke the phosphorylation of specific cytosolic residues in the MORs. Generally, opioids that efficiently promote MOR endocytosis and recycling produce little tolerance, as is the case for [D-Ala(2), N-MePhe(4),Gly-ol(5)] encephalin (DAMGO). However, morphine produces rapid and profound antinociceptive desensitization in the adult mouse brain associated with little MOR internalization. The regulator of G-protein signaling, the RGS14 protein, associates with MORs in periaqueductal gray matter (PAG) neurons, and when RGS14 is silenced morphine increased the serine 375 phosphorylation in the C terminus of the MOR, a GRK substrate. Subsequently, these receptors were internalized and recycled back to the membrane where they accumulated on cessation of antinociception. These mice now exhibited a resensitized response to morphine and little tolerance developed. Thus, in morphine-activated MORs the RGS14 prevents GRKs from phosphorylating those residues required for beta-arresting-mediated endocytosis. Moreover morphine but not DAMGO triggered a process involving calcium/calmodulin-dependent kinase II (CaMKII) in naïve mice, which contributes to MOR desensitization in the plasma membrane. In RGS14 knockdown mice morphine failed to activate this kinase. It therefore appears that phosphorylation and internalization of MORs disrupts the CaMKII-mediated negative regulation of these opioid receptors.

Sumoylated RGS-Rz proteins act as scaffolds for Mu-opioid receptors and G-protein complexes in mouse brain

The RGSZ1 and RGSZ2 proteins, members of the RGS-Rz subfamily of GTPase-activating proteins (GAP), are involved in Mu-opioid receptor desensitization. The expression of these proteins, as well as of their main target the Gz protein, is virtually restricted to the nervous tissue. In synaptosomal membranes, these Rz proteins undergo post-translational modifications such as glycosylation and phosphorylation, and they may covalently attach to small ubiquitin-like modifier (SUMO) proteins. While RGSZ1 exists in conjugated and non-conjugated forms, RGSZ2 is mostly conjugated to SUMO-1, SUMO-2 and SUMO-3 proteins. These sumoylated forms of the GAPs readily associated with Mu-opioid receptors but they associated only poorly with Delta receptors. Furthermore, G alpha i2 and G alpha z subunits co-precipitated with the sumoylated forms of RGSZ1/Z2 proteins, but to a lesser extent with the Ser phosphorylated SUMO-free form of RGSZ1. Upon Mu-opioid receptor activation, there is a strong increase in the association of G alpha proteins with RGSZ2 proteins that persists for intervals longer than 24 h. This effect probably accounts for their role in Mu-opioid receptor desensitization. Only a moderate increase was observed with RGSZ1, the non-sumoylated form of which probably acts as an efficient GAP for these G alpha subunits. Therefore, sumoylation regulates the biological activity of RGS-Rz proteins and it is likely that it serves to switch their behavior, from that of a GAP for activated G alpha subunits to that of a scaffold protein for specific signaling proteins.

Phosphorylation-independent attenuation of GPCR signalling

The uncoupling of G-protein-coupled receptors (GPCRs) from their cognate heterotrimeric G proteins provides an essential physiological 'feedback' mechanism that protects against both acute and chronic overstimulation of receptors. The primary mechanism by which GPCR activity is regulated is the feedback phosphorylation of activated GPCRs by kinases that are dependent on second messengers, GPCR kinases (GRKs) and arrestins. It has recently become apparent, however, that GRK2-mediated regulation of GPCR responsiveness also involves a phosphorylation-independent component that requires both heterotrimeric G-protein alpha-subunit interactions and GPCR binding. Moreover, in addition to GRK2, a growing number of GPCR-interacting proteins might contribute to the phosphorylation-independent G-protein uncoupling of GPCRs. Here, new information about the mechanisms underlying this phosphorylation-independent regulation of receptor and G-protein coupling is reviewed.

Ligand-specific receptor states: Implications for opiate receptor signalling and regulation

Opiate drugs produce their effects by acting upon G protein coupled receptors (GPCRs) and although they are among the most effective analgesics available, their clinical use is restricted by unwanted side effects such as tolerance, physical dependence, respiratory depression, nausea and constipation. As a class, opiates share a common profile of unwanted effects but there are also significant differences in ligand liability for producing these actions. A growing number of studies show that GPCRs may exist in multiple active states that differ in their signalling and regulatory properties and which may distinctively bind different agonists. In this review we summarize evidence supporting the existence of multiple active conformations for MORs and DORs, analyze information favouring the existence of ligand-specific receptor states and assess how ligand-selective efficacy may contribute to the production of longer lasting, better tolerated opiate analgesics.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

How regulators of G protein signaling achieve selective regulation

The regulators of G protein signaling (RGS) are a family of cellular proteins that play an essential regulatory role in G protein-mediated signal transduction. There are multiple RGS subfamilies consisting of over 20 different RGS proteins. They are basically the guanosine triphosphatase (GTPase)-accelerating proteins that specifically interact with G protein alpha subunits. RGS proteins display remarkable selectivity and specificity in their regulation of receptors, ion channels, and other G protein-mediated physiological events. The molecular and cellular mechanisms underlying such selectivity are complex and cooperate at many different levels. Recent research data have provided strong evidence that the spatiotemporal-specific expression of RGS proteins and their target components, as well as the specific protein-protein recognition and interaction through their characteristic structural domains and functional motifs, are determinants for RGS selectivity and specificity. Other molecular mechanisms, such as alternative splicing and scaffold proteins, also significantly contribute to RGS selectivity. To pursue a thorough understanding of the mechanisms of RGS selective regulation will be of great significance for the advancement of our knowledge of molecular and cellular signal transduction.

RGSZ1 interacts with protein kinase C interacting protein PKCI-1 and modulates mu opioid receptor signaling

Protein kinase C interacting protein (PKCI-1) was identified among the potential interactors from a yeast two hybrid screen of human brain library using N terminal of RGSZ1 as a bait. The cysteine string region, unique to the RZ subfamily, contributes to the observed interaction because PKCI-1 interacted with N-terminus of RGS17 and GAIP, but not with that of RGS2 or RGS7 where cysteine string motif is absent. The interaction between RGSZ1 and PKCI-1 was confirmed by coimmunoprecipitation and immunofluorescence. PKCI-1 and RGSZ1 could be detected by coimmunoprecipitation using 14-3-3 antibody in cells transfected with PKCI-1 or RGSZ1 respectively, but when transfected with PKCI-1 and RGSZ1 together, only RGSZ1 could be detected. Phosphorylation of Galphaz by protein kinase C (PKC) reduces the ability of the RGS to effectively function as GTPase accelerating protein for Galphaz, and interferes with ability of Galphaz to interact with betagamma complex. We investigated the roles of 14-3-3 and PKCI-1 in phosphorylation of Galphaz. Phosphorylation of Galphaz by PKC was inhibited by 14-3-3 and the presence of PKCI-1 did not provide any further inhibition. PKCI-1 interacts with mu opioid receptor and suppresses receptor desensitization and PKC related mu opioid receptor phosphorylation [W. Guang, H. Wang, T. Su, I.B. Weinstein, J.B. Wang, Mol. Pharmacol. 66 (2004) 1285.]. Previous studies have also shown that mu opioid receptor co-precipitates with RGSZ1 and influence mu receptor signaling by acting as effector antagonists [J. Garzon, M. Rodriguez-Munoz, P. Sanchez-Blazquez, Neuropharmacology 48 (2005) 853., J. Garzon, M. Rodriguez-Munoz, A. Lopez-Fando, P. Sanchez-Blazquez Neuropsychopharmacology 30 (2005) 1632.]. Inhibition of cAMP by mu opioid receptor was significantly reduced by RGSZ1 and this effect was enhanced in combination with PKCI-1. Our studies thus provide a link between the previous observations mentioned above and indicate that the major function of PKCI-1 is to modulate mu opioid receptor signaling pathway along with RGSZ1, rather than directly mediating the Galphaz RGSZ1 interaction.

Genetic or pharmacological inactivation of the dopamine D 1 receptor differentially alters the expression of regulator of G‐protein signalling (Rgs) transcripts

Dysregulation of dopamine (DA) receptor signalling induces specific changes in behaviour, neuronal circuitry and gene expression in the mammalian forebrain. In order to better understand signalling adaptations at the molecular level, we used high-density oligonucleotide microarrays (Codelink Mouse 20K) to define alterations in the expression of transcripts encoding regulator of G-protein coupled receptor signalling in dopamine D1 receptor knockout mice (Drd1a-KO). Regulator of G-protein signalling (Rgs) 2, Rgs4, and Rgs9 were significantly decreased in the striatum (STR) of Drd1a-KO mice. These changes were confirmed by in situ hybridization, and were also observed in the nucleus accumbens (NAc). In contrast, analysis of the medial frontal cortex (MFC) revealed a significant decrease in Rgs17 expression exclusively, and a modest up-regulation of Rgs5 transcript. The expression of these gene products were not significantly altered in the dopamine-poor visual cortex (VC). The Drd1a-KO mouse, and a rabbit model of in utero cocaine exposure, in which D1R signalling is permanently reduced, possess analogous morphological and functional alterations in dopamine-modulated brain circuits; thus we also examined long-lasting changes in RGS transcript expression following prenatal exposure to cocaine. In sharp contrast to the Drd1a-KO, Rgs2 and Rgs4 were unchanged, and Rgs9 and Rgs17 transcripts were increased in prenatal cocaine-exposed progeny. These data suggest that an absolute absence of D1R signalling (Drd1a-KO) and hypomorphic D1R signalling (prenatal cocaine) produce common alterations in neuronal morphology, but distinct outcomes in molecular neuroadaptations.