RGSZ1 and GAIP regulate mu- but not delta-opioid receptors in mouse CNS: role in tachyphylaxis and acute tolerance

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.

Homeostatic cAMP regulation by the RGS7 complex controls depression-related behaviors

Affective disorders arise from abnormal responses of the brain to prolonged exposure to challenging environmental stimuli. Recent work identified the orphan receptor GPR158 as a molecular link between chronic stress and depression. Here we reveal a non-canonical mechanism by which GPR158 exerts its effects on stress-induced depression by the complex formation with Regulator of G protein Signaling 7 (RGS7). Chronic stress promotes membrane recruitment of RGS7 via GPR158 in the medial prefrontal cortex (mPFC). The resultant complex suppresses homeostatic regulation of cAMP by inhibitory GPCRs in the region. Accordingly, RGS7 loss in mice induces an antidepressant-like phenotype and resiliency to stress, whereas its restoration within the mPFC is sufficient to rescue this phenotype in a GPR158-dependent way. These findings mechanistically link the unusual orphan receptor-RGS complex to a major stress mediator, the cAMP system and suggest new avenues for pharmacological interventions in affective disorders.

Genetic Analysis of Rare Human Variants of Regulators of G Protein Signaling Proteins and Their Role in Human Physiology and Disease

Regulators of G protein signaling (RGS) proteins modulate the physiologic actions of many neurotransmitters, hormones, and other signaling molecules. Human RGS proteins comprise a family of 20 canonical proteins that bind directly to G protein-coupled receptors/G protein complexes to limit the lifetime of their signaling events, which regulate all aspects of cell and organ physiology. Genetic variations account for diverse human traits and individual predispositions to disease. RGS proteins contribute to many complex polygenic human traits and pathologies such as hypertension, atherosclerosis, schizophrenia, depression, addiction, cancers, and many others. Recent analysis indicates that most human diseases are due to extremely rare genetic variants. In this study, we summarize physiologic roles for RGS proteins and links to human diseases/traits and report rare variants found within each human RGS protein exome sequence derived from global population studies. Each RGS sequence is analyzed using recently described bioinformatics and proteomic tools for measures of missense tolerance ratio paired with combined annotation-dependent depletion scores, and protein post-translational modification (PTM) alignment cluster analysis. We highlight selected variants within the well-studied RGS domain that likely disrupt RGS protein functions and provide comprehensive variant and PTM data for each RGS protein for future study. We propose that rare variants in functionally sensitive regions of RGS proteins confer profound change-of-function phenotypes that may contribute, in newly appreciated ways, to complex human diseases and/or traits. This information provides investigators with a valuable database to explore variation in RGS protein function, and for targeting RGS proteins as future therapeutic targets.

Suppression of RGSz1 function optimizes the actions of opioid analgesics by mechanisms that involve the Wnt/β-catenin pathway

Regulator of G protein signaling z1 (RGSz1), a member of the RGS family of proteins, is present in several networks expressing mu opioid receptors (MOPRs). By using genetic mouse models for global or brain region-targeted manipulations of RGSz1 expression, we demonstrated that the suppression of RGSz1 function increases the analgesic efficacy of MOPR agonists in male and female mice and delays the development of morphine tolerance while decreasing the sensitivity to rewarding and locomotor activating effects. Using biochemical assays and next-generation RNA sequencing, we identified a key role of RGSz1 in the periaqueductal gray (PAG) in morphine tolerance. Chronic morphine administration promotes RGSz1 activity in the PAG, which in turn modulates transcription mediated by the Wnt/β-catenin signaling pathway to promote analgesic tolerance to morphine. Conversely, the suppression of RGSz1 function stabilizes Axin2-Gαz complexes near the membrane and promotes β-catenin activation, thereby delaying the development of analgesic tolerance. These data show that the regulation of RGS complexes, particularly those involving RGSz1-Gαz, represents a promising target for optimizing the analgesic actions of opioids without increasing the risk of dependence or addiction.

The Benefits of Genetic Addiction Risk Score (GARS™) Testing in Substance Use Disorder (SUD)

Following 25 years of extensive research by many scientists worldwide, a panel of ten reward gene risk variants, called the Genetic Addiction Risk Score (GARS), has been developed. In unpublished work, when GARS was compared to the Addiction Severity Index (ASI), which has been used in many clinical settings, GARS significantly predicted the severity of both alcohol and drug dependency. In support of early testing for addiction and other RDS subtypes, parents caught up in the current demographic of 127 people, both young and old, dying daily from opiate/opioid overdose, need help. In the past, families would have never guessed that their loved ones would die or could be in real danger due to opiate addiction. Author, Bill Moyers, in Parade Magazine, reported that as he traveled around the United States, he found many children with ADHD and other spectrum disorders like Autism, and noted that many of these children had related conditions like substance abuse. He called for better ways to identify these children and treat them with approaches other than addictive pharmaceuticals. To our knowledge, GARS is the only panel of genes with established polymorphisms reflecting the Brain Reward Cascade (BRC), which has been correlated with the ASI-MV alcohol and drug risk severity score. While other studies are required to confirm and extend the GARS test to include other genes and polymorphisms that associate with an hypodopaminergic trait, these results provide clinicians with a non-invasive genetic test. Genomic testing, such as GARS, can improve clinical interactions and decision-making. Knowledge of precise polymorphic associations can help in the attenuation of guilt and denial, corroboration of family gene-o-grams; assistance in risk-severity-based decisions about appropriate therapies, including pain medications and risk for addiction; choice of the appropriate level of care placement (i.e., inpatient, outpatient, intensive outpatient, residential); determination of the length of stay in treatment; determination of genetic severity-based relapse and recovery liability and vulnerability; determination of pharmacogenetic medical monitoring for better clinical outcomes (e.g., the A1 allele of the DRD2 gene reduces the binding to opioid delta receptors in the brain, thus, reducing Naltrexone's clinical effectiveness); and supporting medical necessity for insurance scrutiny.

Regulator of G protein signaling 20 enhances cancer cell aggregation, migration, invasion and adhesion

Several RGS (regulator of G protein signaling) proteins are known to be upregulated in a variety of tumors but their roles in modulating tumorigenesis remain undefined. Since the expression of RGS20 is elevated in metastatic melanoma and breast tumors, we examined the effects of RGS20 overexpression and knockdown on the cell mobility and adhesive properties of different human cancer cell lines, including cervical cancer HeLa, breast adenocarcinoma MDA-MB-231, and non-small cell lung carcinoma H1299 and A549 cells. Expression of RGS20 enhanced cell aggregation, migration, invasion and adhesion as determined by hanging drop aggregation, wound healing, transwell chamber migration and invasion assays. Conversely, shRNA-mediated knockdown of endogenous RGS20 impaired these responses. In addition, RGS20 elevated the expression of vimentin (a mesenchymal cell marker) but down-regulated the expression of E-cadherin, two indicators commonly associated with metastasis. These results suggest that the expression of RGS20 may promote metastasis of tumor cells.

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.

GPER1 stimulation alters posttranslational modification of RGSz1 and induces desensitization of 5-HT1A receptor signaling in the rat hypothalamus

Hyperactivity of the hypothalamic-pituitary-adrenal axis is a consistent biological characteristic of depression, and response normalization coincides with clinical responsiveness to antidepressant medications. Desensitization of serotonin 1A receptor (5-HT1AR) signaling in the hypothalamic paraventricular nucleus of the hypothalamus (PVN) follows selective serotonin reuptake inhibitor (SSRI) antidepressant treatment and contributes to the antidepressant response. Estradiol alone produces a partial desensitization of 5-HT1AR signaling and synergizes with SSRIs to result in a complete and more rapid desensitization than with SSRIs alone as measured by a decrease in the oxytocin and adrenocorticotrophic hormone (ACTH) responses to 5-HT1AR stimulation. G protein-coupled estrogen receptor 1 (GPER1) is necessary for estradiol-induced desensitization of 5-HT1AR signaling, although the underlying mechanisms are still unclear. We now find that stimulation of GPER1 with the selective agonist G-1 and nonselective stimulation of estrogen receptors dramatically alter isoform expression of a key component of the 5-HT1AR signaling pathway, RGSz1, a GTPase-activating protein selective for Gαz, the Gα subunit necessary for 5-HT1AR-mediated hormone release. RGSz1 isoforms are differentially glycosylated, SUMOylated, and phosphorylated, and differentially distributed in subcellular organelles. High-molecular-weight RGSz1 is SUMOylated and glycosylated, localized to the detergent-resistant microdomain (DRM) of the cell membrane, and increased by estradiol and G-1 treatment. Because activated Gαz also localizes to the DRM, increased DRM-localized RGSz1 by estradiol and G-1 could reduce Gαz activity, functionally uncoupling 5-HT1AR signaling. Peripheral G-1 treatment produced a partial reduction in oxytocin and ACTH responses to 5-HT1AR stimulation similar to direct injections into the PVN. Together, these results identify GPER1 and RGSz1 as novel targets for the treatment of depression.

A physiologically required G protein-coupled receptor (GPCR)-regulator of G protein signaling (RGS) interaction that compartmentalizes RGS activity

G protein-coupled receptors (GPCRs) can interact with regulator of G protein signaling (RGS) proteins. However, the effects of such interactions on signal transduction and their physiological relevance have been largely undetermined. Ligand-bound GPCRs initiate by promoting exchange of GDP for GTP on the Gα subunit of heterotrimeric G proteins. Signaling is terminated by hydrolysis of GTP to GDP through intrinsic GTPase activity of the Gα subunit, a reaction catalyzed by RGS proteins. Using yeast as a tool to study GPCR signaling in isolation, we define an interaction between the cognate GPCR (Mam2) and RGS (Rgs1), mapping the interaction domains. This reaction tethers Rgs1 at the plasma membrane and is essential for physiological signaling response. In vivo quantitative data inform the development of a kinetic model of the GTPase cycle, which extends previous attempts by including GPCR-RGS interactions. In vivo and in silico data confirm that GPCR-RGS interactions can impose an additional layer of regulation through mediating RGS subcellular localization to compartmentalize RGS activity within a cell, thus highlighting their importance as potential targets to modulate GPCR signaling pathways.

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.

Modulation of μ-opioid receptor signaling by RGS19 in SH-SY5Y cells

Regulator of G-protein signaling protein 19 (RGS19), also known as Gα-interacting protein (GAIP), acts as a GTPase accelerating protein for Gαz as well as Gαi/o subunits. Interactions with GAIP-interacting protein N-terminus and GAIP-interacting protein C-terminus (GIPC) link RGS19 to a variety of intracellular proteins. Here we show that RGS19 is abundantly expressed in human neuroblastoma SH-SY5Y cells that also express µ- and δ- opioid receptors (MORs and DORs, respectively) and nociceptin receptors (NOPRs). Lentiviral delivery of short hairpin RNA specifically targeted to RGS19 reduced RGS19 protein levels by 69%, with a similar reduction in GIPC. In RGS19-depleted cells, there was an increase in the ability of MOR (morphine) but not of DOR [(4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80)] or NOPR (nociceptin) agonists to inhibit forskolin-stimulated adenylyl cyclase and increase mitogen-activated protein kinase (MAPK) activity. Overnight treatment with either MOR [D-Ala, N-Me-Phe, Gly-ol(5)-enkephalin (DAMGO) or morphine] or DOR (D-Pen(5)-enkephalin or SNC80) agonists increased RGS19 and GIPC protein levels in a time- and concentration-dependent manner. The MOR-induced increase in RGS19 protein was prevented by pretreatment with pertussis toxin or the opioid antagonist naloxone. Protein kinase C (PKC) activation alone increased the level of RGS19 and inhibitors of PKC 5,6,7,13-tetrahydro-13-methyl-5-oxo-12H-indolo[2,3-a]pyrrolo[3,4-c]carbazole-12-propanenitrile and mitogen-activated protein kinase kinase 1 2-(2-amino-3-methoxyphenyl)-4H-chromen-4-one, but not protein kinase A (H89), completely blocked DAMGO-induced RGS19 protein accumulation. The findings show that RGS19 and GIPC are jointly regulated, that RGS19 is a GTPase accelerating protein for MOR with selectivity over DOR and NOPR, and that chronic MOR or DOR agonist treatment increases RGS19 levels by a PKC and the MAPK pathway-dependent mechanism.

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.

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.

Estradiol induces partial desensitization of serotonin 1A receptor signaling in the paraventricular nucleus of the hypothalamus and alters expression and interaction of RGSZ1 and Gαz

Hyperactivity of hypothalamic-pituitary mediated hormone responses, such as to stimulation with a serotonin 1A (5-HT(1A)) receptor agonist, are a feature of depression which are normalized with clinical improvement during drug therapy. We previously reported that SSRIs induce desensitization of 5-HT(1A) receptor signaling in the paraventricular nucleus of the hypothalamus (PVN) while estradiol benzoate (EB) produces a more rapid, partial desensitization. In the current study, time course and dose-response experiments demonstrated that two once daily doses of EB is the minimum needed to induce the desensitization response as indicated by 5-HT(1A) receptor-stimulated release of oxytocin and that 10 μg/kg/day EB produces the maximal response, a partial desensitization of approximately 40%. The effects of two once daily injections of 10 μg/kg/day EB on Gαz and RGSZ1 proteins were examined as components of the 5-HT(1A) receptor signaling system, which mediates the release of oxytocin and adrenocorticotropic hormone. RGSZ1 appears to be a major target for EB-mediated responses in the 5-HT(1A) receptor signaling system. A 55 kD membrane-associate RGSZ1 protein was greatly increased in the PVN and rest of the hypothalamus and moderately increased in the dorsal hippocampus and amygdala after EB treatment as well as after an acute dose of a 5-HT(1A) receptor agonist. These results suggest that EB is a candidate for adjuvant therapy with SSRIs to hasten the therapeutic response and that RGSZ1 is a major target of EB therapy which could be explored as a target for novel therapeutic approaches for the treatment of depression.

Ultrastructural relationship between the AMPA-GluR2 receptor subunit and the mu-opioid receptor in the mouse central nucleus of the amygdala

Activation of GluR2-expressing non-calcium-permeable AMPA-type glutamate receptors in the central nucleus of the amygdala (CeA) may play an important role in integrating emotion and memory with goal-directed behaviors involved in opioid addiction. The location of non-calcium-permeable AMPA receptors within distinct neuronal compartments (i.e., soma, dendrite, or axon) is an important functional feature of these proteins; however, their ultrastructural location and subcellular relationship with mu-opioid receptors (μOR) in the CeA are unknown. Immunocytochemical electron microscopy was used to characterize the ultrastructural distribution of GluR2 and its association with μOR in the mouse CeA. A single-labeling analysis of GluR2 distribution employing immunoperoxidase or immunogold markers revealed that this protein was frequently affiliated with intracellular vesicular organelles, as well as the plasma membrane of CeA neuronal profiles. Among all GluR2-labeled neuronal structures, over 85% were dendrites or somata. Unlabeled axon terminals frequently formed asymmetric excitatory-type synaptic junctions with GluR2-labeled dendritic profiles. Dual-labeling immunocytochemical analysis showed that GluR2 and μOR were co-localized in neuronal compartments. Among all dual-labeled structures, approximately 80% were dendritic. Synaptic inputs to these dual-labeled dendrites were frequently from unlabeled axon terminals forming asymmetric excitatory-type synapses. The presence of GluR2 in dendritic profiles receiving asymmetric synapses suggests that activation of the non-calcium-permeable AMPA receptor plays a role in the postsynaptic modulation of excitatory signaling involving CeA neuronal circuits that coordinate sensory, affective, and behavioral processes involved in drug addiction. Given the critical role of non-calcium-permeable AMPA receptor function in neural and behavioral adaptability, their dendritic association with μOR in CeA dendrites provides a neuronal substrate for opioid-mediated plasticity.

RGS19 enhances cell proliferation through its C-terminal PDZ motif

Regulator of G protein signaling 19 (RGS19), also known as Galpha-interacting protein (GAIP), is a GTPase activating protein (GAP) for Galpha(i) subunits. Apart from its GAP function, RGS19 has been implicated in growth factor signaling through binding to GAIP-interacting protein C-terminus (GIPC) via its C-terminal PDZ-binding motif. To gain additional insight on its function, we have stably expressed RGS19 in a number of mammalian cell lines and examined its effect on cell proliferation. Interestingly, overexpression of RGS19 stimulated the growth of HEK293, PC12, Caco2, and NIH3T3 cells. This growth promoting effect was not shared by other RGS proteins including RGS4, RGS10 and RGS20. Despite its ability to stimulate cell proliferation, RGS19 failed to induce neoplastic transformation in NIH3T3 cells as determined by focus formation and soft-agar assays, and it did not induce tumor growth in athymic nude mice. Deletion mutants of RGS19 lacking the PDZ-binding motif failed to complex with GIPC and did not exhibit any growth promoting effect. Overexpression of GIPC alone in HEK293 cells stimulated cell proliferation whereas its knockdown in H1299 non-small cell lung carcinomas suppressed cell proliferation. This study demonstrates that RGS19, in addition to acting as a GAP, is able to stimulate cell proliferation in a GIPC-dependent manner.

Regulator of G protein-signaling proteins and addictive drugs

Regulator of G protein-signaling (RGS) proteins are a family of more than 30 intracellular proteins that negatively modulate intracellular signaling of receptors in the G protein-coupled receptor family. This family includes receptors for opioids, cannabinoids, and dopamine that mediate the acute effects of addictive drugs or behaviors and chronic effects leading to the development of addictive disease. Members of the RGS protein family, by negatively modulating receptor signaling, influence the intracellular processes that lead to addiction. In turn, addictive drugs control the expression levels of several RGS proteins. This review will consider the distribution and mechanisms of action of RGS proteins, particularly the R4 and R7 families that have been implicated in the actions of addictive drugs, how knowledge of these proteins is contributing to an understanding of addictive processes, and whether specific RGS proteins could provide targets for the development of medications to manage and/or treat addiction.

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.

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.

Small molecule protein-protein interaction inhibitors as CNS therapeutic agents: current progress and future hurdles

Protein-protein interactions are a crucial element in cellular function. The wealth of information currently available on intracellular-signaling pathways has led many to appreciate the untapped pool of potential drug targets that reside downstream of the commonly targeted receptors. Over the last two decades, there has been significant interest in developing therapeutics and chemical probes that inhibit specific protein-protein interactions. Although it has been a challenge to develop small molecules that are capable of occluding the large, often relatively featureless protein-protein interaction interface, there are increasing numbers of examples of small molecules that function in this manner with reasonable potency. This article will highlight the current progress in the development of small molecule protein-protein interaction inhibitors that have applications in the treatment or study of central nervous system function and disease. In particular, we will focus upon recent work towards developing small molecule inhibitors of amyloid-beta and alpha-synuclein aggregation, inhibitors of critical components of G-protein-signaling pathways, and PDZ domain inhibitors.

Association analysis of genes encoding the nociceptin receptor (OPRL1) and its endogenous ligand (PNOC) with alcohol or illicit drug dependence

Recent studies in animal models have shown that the nociceptin system, comprising nociceptin (or OFQ/N, encoded by PNOC) and the nociceptin receptor (an opioid receptor-like protein encoded by OPRL1), may be involved in alcohol and other drug reward pathways. To determine whether the nociceptin system is associated with alcohol or illicit drug dependence in humans, we analyzed 10 single nucleotide polymorphisms (SNPs) in OPRL1 and 15 SNPs in PNOC in a sample of 1923 European Americans from 219 multiplex alcohol dependent families ascertained by the Collaborative Study on the Genetics of Alcoholism. The SNPs spanned both genes and several kb of their flanking sequences, and were in high linkage disequilibrium. Neither gene was associated with alcohol or illicit drug dependence, although two SNPs in PNOC showed marginal association with alcoholism and one with illicit drug dependence (P = 0.04-0.05). Secondary analyses suggested that two adjacent SNPs in intron 1 of OPRL1 were marginally associated with opioid dependence (P = 0.05); none of the SNPs in PNOC were associated with opioid dependence.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

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.

Regulator of G protein signaling proteins differentially modulate signaling of mu and delta opioid receptors

Effects of regulator of G protein signaling (RGS) proteins on mu and delta opioid receptors were investigated in HEK293 cells. Co-expression of RGS1, RGS2, RGS4, RGS9, RGS10 or RGS19 (Galpha-interacting protein (GAIP)) significantly reduced [Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol]-Enkephalin (DAMGO)-induced inhibition of adenylyl cyclase (AC) mediated by mu opioid receptor, but only RGS9 decreased the effects of [Tyr-D-Pen-Gly-p-Chloro-Phe-D-Pen]-Enkephalin (DPDPE) mediated by delta opioid receptor. When C-tails of the receptors were exchanged (mu/deltaC and delta/muC chimeras), RGS proteins decreased delta/muC-mediated AC inhibition, but none had significant effects on that via mu/deltaC receptor. Thus, the C-terminal domains of the receptors are critical for the differential effects of RGS proteins, which may be due to differences in receptor-G protein-RGS protein interactions in signaling complexes.

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.

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.

Cellular mechanisms that determine selective RGS protein regulation of G protein-coupled receptor signaling

Regulators of G protein signaling (RGS proteins) bind directly to activated Galpha subunits to inhibit their signaling. However, recent findings show that RGS proteins selectively regulate signaling by certain G protein-coupled receptors (GPCRs) in cells, irrespective of the coupled G protein. New studies support an emerging model that suggests RGS proteins utilize both direct and indirect mechanisms to form stable functional pairs with preferred GPCRs to selectively modulate the signaling functions of those receptors and linked G proteins. Here, we discuss these findings and their implications for established models of GPCR signaling.

RGS17/RGSZ2 and the RZ/A family of regulators of G-protein signaling

Regulators of G-protein signaling (RGS proteins) comprise over 20 different proteins that have been classified into subfamilies on the basis of structural homology. The RZ/A family includes RGSZ2/RGS17 (the most recently discovered member of this family), GAIP/RGS19, RGSZ1/RGS20, and the RGSZ1 variant Ret-RGS. The RGS proteins are GTPase activating proteins (GAPs) that turn off G-proteins and thus negatively regulate the signaling of G-protein coupled receptors (GPCRs). In addition, some RZ/A family RGS proteins are able to modify signaling through interactions with adapter proteins (such as GIPC and GIPN). The RZ/A proteins have a simple structure that includes a conserved amino-terminal cysteine string motif, RGS box and short carboxyl-terminal, which confer GAP activity (RGS box) and the ability to undergo covalent modification and interact with other proteins (amino-terminal). This review focuses on RGS17 and its RZ/A sibling proteins and discusses the similarities and differences among these proteins in terms of their palmitoylation, phosphorylation, intracellular localization and interactions with GPCRs and adapter proteins. The specificity of these RGS protein for different Galpha proteins and receptors, and the consequences for signaling are discussed. The tissue and brain distribution, and the evolving understanding of the roles of this family of RGS proteins in receptor signaling and brain function are highlighted.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

N-terminally truncated variant of the mouse GAIP/RGS19 lacks selectivity of full-length GAIP/RGS19 protein in regulating ORL1 receptor signaling

The regulators of G protein signaling (RGS) are a family of proteins with conserved RGS domains and play essential roles in regulating G protein-mediated signal transduction and physiological events. GAIP/RGS19 (G alpha interacting protein, also classified as RGS19), a member of the RGS family, has been shown to negatively regulate the signaling of many G protein-coupled receptors, including the opioid receptors. Two GAIP/RGS19 mRNA variants, resulted from an alternative splicing of exon 2 of the GAIP/RGS19 gene, were identified in multiple mouse tissues. One of the transcripts consists of a complete set of exons and encodes a full-length GAIP/RGS19 protein, and the other does not have exon 2 and therefore encodes an N-terminal 22 residue truncated short GAIP/RGS19 protein. When co-expressed with either the opioid-receptor-like (ORL1) receptor or one of the mu, delta, and kappa opioid receptors, by transfecting dual-expression plasmids into COS-7 cells, the full-length GAIP/RGS19 was more effective than the N-terminally truncated variant and was more selective in regulating the ORL1 receptor signaling than in regulating the mu, delta, and kappa opioid receptors, as measured by the effectiveness to increase the agonist-stimulated GTPase activity and to reverse the agonist-induced inhibition of cyclic AMP accumulation. In the same assays, the N-terminally truncated GAIP/RGS19 did not distinguish ORL1 from the mu, delta, and kappa opioid receptors. In contrast, co-expression of RGS4 with either ORL1 or opioid receptors showed the selectivity of RGS4 for regulating opioid receptors was mu > kappa > delta > ORL1, an order completely different from that of GAIP/RGS19. The results suggest that GAIP/RGS19 prefers regulating ORL1 receptor signaling over other opioid receptors, and that the N-terminal domain of GAIP/RGS19 plays a crucial role in its receptor preference.

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

The regulator of G-protein signaling RGS17(Z2) is a member of the RGS-Rz subfamily of GTPase-activating proteins (GAP) that efficiently deactivate GalphazGTP subunits. We have found that in the central nervous system (CNS), the levels of RGSZ2 mRNA and protein are elevated in the hypothalamus, midbrain, and pons-medulla, and that RGSZ2 is glycosylated in synaptosomal membranes isolated from CNS tissue. In analyzing the function of RGSZ2 in the CNS, we found that when the expression of RGSZ2 was impaired, the antinociceptive response to morphine and [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin (DAMGO) augmented. This potentiation involved mu-opioid receptors and increased tolerance to further doses of these agonists administered 24 h later. High doses of morphine promoted agonist desensitization even within the analgesia time-course, a phenomenon that appears to be related to the great capacity of morphine to activate Gz proteins. In contrast, the knockdown of RGSZ2 proteins did not affect the activity of delta receptor agonists, [D-Pen2,5]-enkephalin (DPDPE), and [D-Ala2] deltorphin II. In membranes from periaqueductal gray matter (PAG), both RGSZ2 and the related RGS20(Z1) co-precipitated with mu-opioid receptors. While a morphine challenge reduced the association of Gi/o/z with mu receptors, it increased their association with the RGSZ2 and RGSZ1 proteins. However, only Galphaz subunits co-precipitated with RGSZ2. Doses of morphine that produced acute tolerance maintained the association of Galpha subunits with RGSZ proteins even after the analgesic effects had ceased. These results indicate that both RGSZ1 and RGSZ2 proteins influence mu receptor signaling by sequestering Galpha subunits, therefore behaving as effector antagonists.

Genetic tests of biologic systems in affective disorders

To liberate candidate gene analyses from criticisms of inexhaustiveness of examination of specific candidate genes, or incompleteness in the choice of candidate genes to study for specific neurobiological pathways, study of sizeable sets of genes pertinent to each putative pathophysiological pathway is required. For many years, genes have been tested in a 'one by one' manner for association with major affective disorders, primarily bipolar illness. However, it is conceivable that not individual genes but abnormalities in several genes within a system or in several neuronal, neural, or hormonal systems are implicated in the functional hypotheses for etiology of affective disorders. Compilation of candidate genes for entire pathways is a challenge, but can reasonably be carried out for the major affective disorders as discussed here. We present here five groupings of genes implicated by neuropharmacological and other evidence, which suggest 252 candidate genes worth examining. Inexhaustiveness of gene interrogation would apply to many studies in which only one polymorphism per gene is analyzed. In contrast to whole-genome association studies, a study of a limited number of candidate genes can readily exploit information on genomic sequence variations obtained from databases and/or resequencing, and has an advantage of not having the complication of an extremely stringent statistical criterion for association.

Social and environmental influences on opioid sensitivity in rats: importance of an opioid's relative efficacy at the mu-receptor

RATIONALE

Evidence indicates that social and environmental enrichment can influence the functional maturation of the central nervous system and may affect an organism's sensitivity to centrally acting drugs.

OBJECTIVE

The purpose of the present study was to examine the effects of social and environmental enrichment on sensitivity to mu-opioids possessing a range of relative efficacies at the mu-receptor.

METHODS

Rats were obtained at weaning (21 days) and divided into two groups immediately upon arrival. Isolated rats were housed individually in opaque laboratory cages with no visual or tactile contact with other rats; enriched rats were housed socially in groups of four in large cages and given various novel objects on a daily basis. After 6 weeks under these conditions, the effects of morphine, levorphanol, buprenorphine, butorphanol, and nalbuphine were examined in the warm-water, tail-withdrawal procedure and the place-conditioning procedure.

RESULTS

In the tail-withdrawal procedure, isolated and enriched rats did not differ in sensitivity to morphine (1.0-30 mg/kg) and levorphanol (0.3-10 mg/kg), but enriched rats were more sensitive to buprenorphine (0.03-3.0 mg/kg), butorphanol (0.3-30 mg/kg), and nalbuphine (0.3-30 mg/kg). In drug combination tests, butorphanol and nalbuphine antagonized the effects of morphine in isolated rats under conditions in which they produced high levels of antinociception in enriched rats. In the place-conditioning procedure, doses of 10 morphine and 3.0 levorphanol established a place preference in both groups of rats, whereas doses of 0.3 buprenorphine, 3.0 butorphanol, and 10 nalbuphine established a place preference only in enriched rats.

CONCLUSIONS

These findings may be taken as evidence that enriched rats are more sensitive than isolated rats to the effects of lower-efficacy mu-opioids and that social and environmental enrichment leads to functional alterations in opioid receptor populations.

Look before leaping: combined opioids may not be the rave

The use of combinations of potent opioids is a common clinical practice. The addition of one potent opioid to another has been recommended to reduce opioid side effects, improve pain control, and limit dose escalation of the first opioid. The advantages of using combined opioids have been reported to be relative to differences in receptor activation versus endocytosis (RAVE). However, the advantages and detriment to combining opioids are related to naturally occurring opioid receptor dimers. Dimers and oligomers result in a unique opioid pharmacodynamics which influence opioid binding, G protein interactions, desensitization, receptor trafficking, and endocytosis. The pharmacodynamics of dimers may lead to positive or negative cooperativity when two opioids are combined. The use of multiple opioids in practice can lead to increased risk for dosing errors, reduced patient compliance, increased drug interactions and cost. Opioid combinations should not be used until prospective randomized trials clarify the benefits and safety.

GLUT1CBP(TIP2/GIPC1) interactions with GLUT1 and myosin VI: evidence supporting an adapter function for GLUT1CBP

We identified a novel interaction between myosin VI and the GLUT1 transporter binding protein GLUT1CBP(GIPC1) and first proposed that as an adapter molecule it might function to couple vesicle-bound proteins to myosin VI movement. This study refines the model by identifying two myosin VI binding domains in the GIPC1 C terminus, assigning respective oligomerization and myosin VI binding functions to separate N- and C-terminal domains, and defining a central region in the myosin VI tail that binds GIPC1. Data further supporting the model demonstrate that 1) myosin VI and GIPC1 interactions do not require a mediating protein; 2) the myosin VI binding domain in GIPC1 is necessary for intracellular interactions of GIPC1 with myosin VI and recruitment of overexpressed myosin VI to membrane structures, but not for the association of GIPC1 with such structures; 3) GIPC1/myosin VI complexes coordinately move within cellular extensions of the cell in an actin-dependent and microtubule-independent manner; and 4) blocking either GIPC1 interactions with myosin VI or GLUT1 interactions with GIPC1 disrupts normal GLUT1 trafficking in polarized epithelial cells, leading to a reduction in the level of GLUT1 in the plasma membrane and concomitant accumulation in internal membrane structures.

Effector antagonism by the regulators of G protein signalling (RGS) proteins causes desensitization of mu-opioid receptors in the CNS

RATIONALE

In cell culture systems, agonists can promote the phosphorylation and internalization of receptors coupled to G proteins (GPCR), leading to their desensitization. However, in the CNS opioid agonists promote a profound desensitization of their analgesic effects without diminishing the presence of their receptors in the neuronal membrane. Recent studies have indicated that CNS proteins of the RGS family, specific regulators of G protein signalling, may be involved in mu-opioid receptor desensitization in vivo.

OBJECTIVE

In this work we review the role played by RGS proteins in the intensity and duration of the effects of mu-opioid receptor agonists, and how they influence the delayed tolerance that develops in response to specific doses of opioids.

RESULTS

RGS proteins are GTPase-activating proteins (GAP) that accelerate the hydrolysis of GalphaGTP to terminate signalling at effectors. The GAP activity of RGS-R4 and RGS-Rz proteins restricts the amplitude of opioid analgesia, and the efficient deactivation of GalphazGTP subunits by RGS-Rz proteins prevents mu receptor desensitization. However, RGS-R7 proteins antagonize effectors by binding to and sequestering mu receptor-activated Galphai/o/z subunits. Thus, they reduce the pool of receptor-regulated G proteins and hence, the effects of agonists. The delayed tolerance observed following morphine administration correlates with the transfer of Galpha subunits from mu receptors to RGS-R7 proteins and the subsequent stabilization of this association.

CONCLUSION

In the CNS, the RGS proteins control the activity of mu opioid receptors through GAP-dependent (RGS-R4 and RGS-Rz) as well as by GAP-independent mechanisms (RGS-R7). As a result, they can both antagonize effectors and desensitize receptors under certain circumstances.

Morphine alters the selective association between mu-opioid receptors and specific RGS proteins in mouse periaqueductal gray matter

In the CNS, several regulators of G-protein signalling (RGS) modulate the activity of mu-opioid receptors. In pull-down assays performed on membranes from mouse periaqueductal gray matter (PAG), mu-opioid receptors co-precipitated with delta-opioid receptors, Gi/o/z/q proteins, and the regulators of G-protein signalling RGS4, RGS9-2, RGS14, RGSZ1 and RGSZ2. No RGS2, RGS7, RGS10 and RGS11 proteins were associated with the mu receptors in these PAG membranes. In mice, an intracerebroventricular dose of 10 nmol morphine produced acute tolerance at mu receptors but did not disrupt the co-precipitation of mu-delta receptor complexes. However, this opioid reduced by more than 50% the co-precipitation of G alpha i/o/z subunits with mu receptors, and altered their association with some of the RGS proteins at 30 min, 3 h and 24 h after its administration. The association of RGS9-2 with mu receptors diminished by 30-40% 24 h after the administration of morphine, while that of RGSZ2 and of RGSZ1 increased. Morphine treatment recruited RGS4 to the PAG membranes, and 30 min and 3 h after the opioid challenge its association with mu receptors had increased. However, 24 h after morphine administration, the co-precipitation of RGS4 had decreased by about 30%. The opioid produced no change in the membrane levels of RGS9-2, RGS14, RGSZ1 and RGSZ2. Thus, in PAG synaptosomal membranes, a dynamic and selective link exists between, mu-opioid receptors, Gi/o/z proteins and certain RGS proteins.

RGS-Rz and RGS9-2 proteins control mu-opioid receptor desensitisation in CNS: the role of activated Galphaz subunits

Two consecutive i.c.v. administrations of analgesic doses of mu-opioid receptor agonists lead to a profound desensitisation of the latter receptors; a third dose produced less than 20% of the effect obtained with the first administration. Desensitisation was still effective 24h later. Impairing the activity of Galphaz but not Galphai2 subunits prevented tolerance developing after the administration of three consecutive doses of morphine. Further, the i.c.v. injection of Galphai2 subunits potentiated morphine analgesia and abolished acute tolerance, whereas i.c.v.-administered Galphaz subunits produced a rapid and robust loss of the response to morphine. The RGSZ1 and RGSZ2 proteins selectively deactivate GalphazGTP subunits, and their knockdown increased the effects produced by the first dose of morphine. However, impairing their activity also accelerated tachyphylaxis following successive doses of morphine, and facilitated the development of acute morphine tolerance. In contrast, inhibiting the RGS9-2 proteins, which bind to GalphaoGTP and GalphaiGTP but only weakly deactivates them, preserved the effects of consecutive morphine doses and abolished the generation of acute tolerance. Therefore, desensitisation of mu-opioid receptors can be achieved by reducing the responsiveness of post-receptor elements (via the possible action of activated Galphaz subunits) and/or by depleting the pool of receptor-regulated G proteins that agonists need to propagate their effects, e.g., through the activity of RGS9-2 proteins.

RGS Proteins: New Players in the Field of Opioid Signaling and Tolerance Mechanisms

In this article we review recent advances in our understanding of the crucial role of the Regulator of G protein Signaling (RGS) proteins in opioid signaling mechanisms and opioid tolerance development. Opioids exert their physiologic effects via complex G protein-coupled receptor-signaling mechanisms, and RGS proteins are now known to tightly regulate the G protein signaling cycle. RGS proteins contain GTPase-accelerating protein activity within their characteristic RGS domain and various other receptor signaling-related properties of their other functional domains. There have been more than 20 RGS proteins reported in the literature, and multiple RGS proteins have been shown to negatively regulate G protein-mediated opioid signaling, facilitate opioid receptor desensitization and internalization, and affect the rate at which opioid tolerance develops. Using RGS proteins as targets for future drug therapy aimed at modulating opioid effectiveness in both acute and chronic pain settings may be an important advance in the treatment of pain.

Expression of neural RGS-R7 and Gbeta5 Proteins in Response to Acute and Chronic Morphine

The R7 subfamily of regulators of G-protein signaling (RGS) proteins (RGS6, RGS7, RGS9-2, and RGS11), and its binding protein Gbeta5, are found in neural structures of mouse brain. A single intracerebroventricular priming dose of 10 nmol morphine gave rise to acute tolerance to the analgesic effects of successive identical test doses of the opioid. At 2 h after administering the acute opioid, RGS7 mRNA levels in the striatum plus those of RGS9-2 in the striatum and thalamus were increased, whereas RGS9-2 and RGS11 mRNA were reduced in the cortex. Similar but attenuated RGS-R7 mRNA changes persisted 24 h after acute morphine administration. No changes in Gbeta5 mRNA levels were observed. At 2 days after commencing sustained morphine treatment, the levels of mRNA for RGS7, RGS9-2, RGS11, and Gbeta5 increased in most of the brain structures studied (striatum, thalamus, periaqueductal gray matter (PAG), and cortex). In these morphine tolerant-dependent mice, the greater changes were found for RGS9-2 in the thalamus (>500%) and PAG (>200%). In post-dependent mice, the increases in RGS-R7 and Gbeta5 mRNA still persisted in the PAG and striatum at 8 and 16 days after starting the chronic opioid treatment. The raised mRNA levels promoted by chronic, but not by acute, morphine, were accompanied by increases in the encoded proteins. This is probably a result of the costabilization of the RGS-R7 and Gbeta5 proteins forming heterodimers. Opioid-induced adaptations of RGS-R7 and Gbeta5 genes may regulate the severity of morphine-induced tolerance/dependence and the duration of the post-dependent period, helping to recover the normal response.

GIPC recruits GAIP (RGS19) to attenuate dopamine D2 receptor signaling

Pleiotropic G proteins are essential for the action of hormones and neurotransmitters and are activated by stimulation of G protein-coupled receptors (GPCR), which initiates heterotrimer dissociation of the G protein, exchange of GDP for GTP on its Galpha subunit and activation of effector proteins. Regulator of G protein signaling (RGS) proteins regulate this cascade and can be recruited to the membrane upon GPCR activation. Direct functional interaction between RGS and GPCR has been hypothesized. We show that recruitment of GAIP (RGS19) by the dopamine D2 receptor (D2R), a GPCR, required the scaffold protein GIPC (GAIP-interacting protein, C terminus) and that all three were coexpressed in neurons and neuroendocrine cells. Dynamic translocation of GAIP to the plasma membrane and coassembly in a protein complex in which GIPC was a required component was dictated by D2R activation and physical interactions. In addition, two different D2R-mediated responses were regulated by the GTPase activity of GAIP at the level of the G protein coupling in a GIPC-dependent manner. Since GIPC exclusively interacted with GAIP and selectively with subsets of GPCR, this mechanism may serve to sort GPCR signaling in cells that usually express a large repertoire of GPCRs, G proteins, and RGS.