Pleiotropic phenotype of a genomic knock-in of an RGS-insensitive G184S Gnai2 allele

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.

Biologic therapy for rheumatoid arthritis: clinical efficacy and predictors of response

We present evidence of a link between interferonβ-1b (IFN-β) and G-protein signaling by demonstrating that IFN-β can induce the expression of the negative regulator of G-protein signaling 1 (RGS1). RGS1 reduces G-protein activation and immune cell migration by interacting with heterotrimeric G-proteins and enhancing their intrinsic GTPase activity. In this study, IFN-β treatment resulted in the induction of RGS1 in peripheral blood mononuclear cells (PBMCs), monocytes, T cells, and B cells. Induction of RGS1 by IFN-β was concentration dependent and observed at both the RNA and protein level. Other members of the RGS family were not induced by IFN-β, and induction of RGS1 required the activation of the IFN receptor. In addition, RGS1 induction was observed in PBMCs obtained from IFN-β-treated multiple sclerosis patients suggesting a possible, as yet unexplored, involvement of G-protein regulation in disease treatment. The upregulation of RGS1 by IFN-β has not been previously reported.

Polyplexed flow cytometry protein interaction assay: a novel high-throughput screening paradigm for RGS protein inhibitors

Intracellular signaling cascades are a series of regulated protein-protein interactions that may provide a number of targets for potential drug discovery. Here, the authors examine the interaction of regulators of G-protein signaling (RGS) proteins with the G-protein Galphao, using a flow cytometry protein interaction assay (FCPIA). FCPIA accurately measures nanomolar binding constants of this protein-protein interaction and has been used in high-throughput screening. This report focuses on 5 RGS proteins (4, 6, 7, 8, and 16). To increase the content of screens, the authors assessed high-throughput screening of these RGS proteins in multiplex, by establishing binding constants of each RGS with Galphao in isolation, and then in a multiplex format with 5 RGS proteins present. To use this methodology as a higher-content multiplex protein-protein interaction screen, they established Z-factor values for RGS proteins in multiplex of 0.73 to 0.92, indicating this method is suitable for screening using FCPIA. To increase throughput, they also compressed a set of 8000 compounds by combining 4 compounds in a single assay well. Subsequent deconvolution of the compounds mixtures verified the identification of active compounds at specific RGS targets in their mixtures using the polyplexed FCPIA method.

The R7 RGS protein family: multi-subunit regulators of neuronal G protein signaling

G protein-coupled receptor signaling pathways mediate the transmission of signals from the extracellular environment to the generation of cellular responses, a process that is critically important for neurons and neurotransmitter action. The ability to promptly respond to rapidly changing stimulation requires timely inactivation of G proteins, a process controlled by a family of specialized proteins known as regulators of G protein signaling (RGS). The R7 group of RGS proteins (R7 RGS) has received special attention due to their pivotal roles in the regulation of a range of crucial neuronal processes such as vision, motor control, reward behavior, and nociception in mammals. Four proteins in this group, RGS6, RGS7, RGS9, and RGS11, share a common molecular organization of three modules: (i) the catalytic RGS domain, (ii) a GGL domain that recruits G beta(5), an outlying member of the G protein beta subunit family, and (iii) a DEP/DHEX domain that mediates interactions with the membrane anchor proteins R7BP and R9AP. As heterotrimeric complexes, R7 RGS proteins not only associate with and regulate a number of G protein signaling pathway components, but have also been found to form complexes with proteins that are not traditionally associated with G protein signaling. This review summarizes our current understanding of the biology of the R7 RGS complexes including their structure/functional organization, protein-protein interactions, and physiological roles.

Regulation of G-protein-coupled signaling pathways in allergic inflammation

Allergic diseases such as asthma are elicited by maladaptive activation of immune cells such as mast cells and lymphocytes by otherwise innocuous allergens. The numerous mediators secreted by such cells promote both acute inflammation and, in many instances, chronic tissue remodeling. Most of these compounds exert their effects on end-organ targets such as epithelial and endothelial cells and airway smooth muscle by activating G-protein-coupled receptors (GPCRs), which are by far the most abundant type of cell surface receptor. Since GPCRs are also the most common target of allergy therapeutics, a better understanding of their intracellular signaling mechanisms is vital to improve the efficacy of such drugs or to develop new targets. In this review, we focus on some of the new regulatory elements that control the duration and amplitude of GPCR signal transduction pathways in immune effector cells and end-organ structural cells affected by allergic inflammation.

GNAI2 and regulators of G protein signaling as a potential Noonan syndrome mechanism

Noonan syndrome (NS OMIM 163950) is a relatively common autosomal dominant developmental disorder characterized by short stature, specific facial features, and congenital cardiac anomalies. Approximately 50-66% of cases have defined mutations in the K-ras/Raf/MEK/ERK pathway that lead to constitutive signaling, but a significant number remain unexplained. We hypothesize that enhanced signaling through Galpha(i2) (from the GNAI2 gene) may also produce a NS-like phenotype. This is based on a recently described mouse model in which RGS-mediated inhibition of Galpha(i2) is prevented by a knock-in mutation (G184S) that blocks RGS binding [Huang et al., Mol. Cell. Biol. 2006;26:6870-9]. The mice have short body length, cardiac hypertrophy, a triangular face with wide-set eyes and ears, and hematologic alterations. There is a slight increase in ERK activation and a pronounced enhancement of PI3K/Akt phosphorylation in MEFs from these mice suggesting that abnormal increases in Galpha(i2) signaling could represent a novel upstream mechanism for NS. This suggests a novel set of candidate genes for NS (GNAI2 and RGS proteins) and if validated could have important implications for therapy as well.

Regulator of G protein signaling protein suppression of Galphao protein-mediated alpha2A adrenergic receptor inhibition of mouse hippocampal CA3 epileptiform activity

Activation of G protein-coupled alpha(2) adrenergic receptors (ARs) inhibits epileptiform activity in the hippocampal CA3 region. The specific mechanism underlying this action is unclear. This study investigated which subtype(s) of alpha(2)ARs and G proteins (Galpha(o) or Galpha(i)) are involved in this response using recordings of mouse hippocampal CA3 epileptiform bursts. Application of epinephrine (EPI) or norepinephrine (NE) reduced the frequency of bursts in a concentration-dependent manner: (-)EPI > (-)NE >>> (+)NE. To identify the alpha(2)AR subtype involved, equilibrium dissociation constants (pK(b)) were determined for the selective alphaAR antagonists atipamezole (8.79), rauwolscine (7.75), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101; 6.87), and prazosin (5.71). Calculated pK(b) values correlated best with affinities determined previously for the mouse alpha(2A)AR subtype (r = 0.98, slope = 1.07). Furthermore, the inhibitory effects of EPI were lost in hippocampal slices from alpha(2A)AR-but not alpha(2C)AR-knockout mice. Pretreatment with pertussis toxin also reduced the EPI-mediated inhibition of epileptiform bursts. Finally, using knock-in mice with point mutations that disrupt regulator of G protein signaling (RGS) binding to Galpha subunits to enhance signaling by that G protein, the EPI-mediated inhibition of bursts was significantly more potent in slices from RGS-insensitive Galpha(o)(G184S) heterozygous (Galpha(o)+/GS) mice compared with either Galpha(i2)(G184S) heterozygous (Galpha(i2)+/GS) or control mice (EC(50) = 2.5 versus 19 and 23 nM, respectively). Together, these findings indicate that the inhibitory effect of EPI on hippocampal CA3 epileptiform activity uses an alpha(2A)AR/Galpha(o) protein-mediated pathway under strong inhibitory control by RGS proteins. This suggests a possible role for RGS inhibitors or selective alpha(2A)AR agonists as a novel antiepileptic drug therapy.

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.

R9AP and R7BP: traffic cops for the RGS7 family in phototransduction and neuronal GPCR signaling

RGS (regulator of G protein signaling) proteins have emerged as crucial regulators, effectors and integrators in G-protein-coupled receptor (GPCR) signaling networks. Many RGS proteins accelerate GTP hydrolysis by Galpha subunits, thereby regulating G protein activity, whereas certain RGS proteins also transduce Galpha signals to downstream targets. Particularly intriguing are members of the RGS7 (R7) family (RGS6, RGS7, RGS9 and RGS11), which heterodimerize with Gbeta5. In Caenorhabditis elegans, R7-Gbeta5 heterodimers regulate synaptic transmission, anesthetic action and behavior. In vertebrates, they regulate vision, postnatal development, working memory and the action of psychostimulants or morphine. Here we highlight R9AP and R7BP, a related pair of recently identified SNARE-like R7-family binding proteins, which regulate intracellular trafficking, expression and function of R7-Gbeta5 heterodimers in retina and brain. Emerging understanding of R7BP and R9AP promises to provide new insights into neuronal GPCR signaling mechanisms relevant to the causes and treatment of neurological disorders.

Endogenous regulators of G protein signaling differentially modulate full and partial mu-opioid agonists at adenylyl cyclase as predicted by a collision coupling model

Regulator of G protein signaling (RGS) proteins accelerate the endogenous GTPase activity of Galpha(i/o) proteins to increase the rate of deactivation of active Galpha-GTP and Gbetagamma signaling molecules. Previous studies have suggested that RGS proteins are more effective on less efficiently coupled systems such as with partial agonist responses. To determine the role of endogenous RGS proteins in functional responses to mu-opioid agonists of different intrinsic efficacy, Galpha(i/o) subunits with a mutation at the pertussis toxin (PTX)-sensitive cysteine (C351I) and with or without a mutation at the RGS binding site (G184S) were stably expressed in C6 glioma cells expressing a mu-opioid receptor. Cells were treated overnight with PTX to inactivate endogenous G proteins. Maximal inhibition of forskolin-stimulated adenylyl cyclase by the low-efficacy partial agonists buprenorphine and nalbuphine was increased in cells expressing RGS-insensitive Galpha(o)(CIGS), Galpha(i2)(CIGS), or Galpha(i3)(CIGS) compared with their Galpha(CI) counterparts, but the RGS-insensitive mutation had little or no effect on the maximal inhibition by the higher efficacy agonists DAMGO and morphine. The potency of all the agonists to inhibit forskolin-stimulated adenylyl cyclase was increased in cells expressing RGS-insensitive Galpha(o)(CIGS), Galpha(i2)(CIGS), or Galpha(i3)(CIGS), regardless of efficacy. These data are comparable with predictions based on a collision coupling model. In this model, the rate of G protein inactivation, which is modulated by RGS proteins, and the rate of G protein activation, which is affected by agonist intrinsic efficacy, determine the maximal agonist response and potency at adenylyl cyclase under steady state conditions.

Resistance to diet-induced obesity and improved insulin sensitivity in mice with a regulator of G protein signaling-insensitive G184S Gnai2 allele

OBJECTIVE

Guanine nucleotide binding protein (G protein)-mediated signaling plays major roles in endocrine/metabolic function. Regulators of G protein signaling (RGSs, or RGS proteins) are responsible for the subsecond turn off of G protein signaling and are inhibitors of signal transduction in vitro, but the physiological function of RGS proteins remains poorly defined in part because of functional redundancy.

RESEARCH DESIGN AND METHODS

We explore the role of RGS proteins and G alpha(i2) in the physiologic regulation of body weight and glucose homeostasis by studying genomic "knock-in" mice expressing RGS-insensitive G alpha(i2) with a G184S mutation that blocks RGS protein binding and GTPase acceleration.

RESULTS

Homozygous G alpha(i2)(G184S) knock-in mice show slightly reduced adiposity. On a high-fat diet, male G alpha(i2)(G184S) mice are resistant to weight gain, have decreased body fat, and are protected from insulin resistance. This appears to be a result of increased energy expenditure. Both male and female G alpha(i2)(G184S) mice on a high-fat diet also exhibit enhanced insulin sensitivity and increased glucose tolerance despite females having similar weight gain and adiposity compared with wild-type female mice.

CONCLUSIONS

RGS proteins and G alpha(i2) signaling play important roles in the control of insulin sensitivity and glucose metabolism. Identification of the specific RGS proteins involved might permit their consideration as potential therapeutic targets for obesity-related insulin resistance and type 2 diabetes.

Novel aspects of G-protein-coupled receptor signalling--different ways to achieve specificity

Our understanding of signal transduction via G-protein-coupled receptors (GPCR) has developed dramatically during the last decades. The initial idea of linear signalling pathways transferring information from the cell membrane to the nucleus has evolved into a complicated network of signalling pathways offering the possibility of crosstalk, fine tuning and specific regulation at multiple levels. During the recent meeting on GPCRs at the Karolinska Institutet, Stockholm novel aspects of GPCR signalling were presented and discussed. Here, we will discuss several possibilities for GPCRs to achieve specificity in signal transduction, such as the phenomenon of biased agonism, receptor multimerization, the role of co-receptors, the regulation of heterotrimeric G proteins as well as multiple G(s)-dependent pathways to extracellular single-regulated protein kinases.

Endogenous RGS proteins modulate SA and AV nodal functions in isolated heart: implications for sick sinus syndrome and AV block

G protein-coupled receptors play a pivotal role in regulating cardiac automaticity. Their function is controlled by regulator of G protein signaling (RGS) proteins acting as GTPase-activating proteins for Galpha subunits to suppress Galpha(i) and Galpha(q) signaling. Using knock-in mice in which Galpha(i2)-RGS binding and negative regulation are disrupted by a genomic Galpha(i2)G184S (GS) point mutation, we recently (Fu Y, Huang X, Zhong H, Mortensen RM, D'Alecy LG, Neubig RR. Circ Res 98: 659-666, 2006) showed that endogenous RGS proteins suppress muscarinic receptor-mediated bradycardia. To determine whether this was due to direct regulation of cardiac pacemakers or to alterations in the central nervous system or vascular responses, we examined isolated, perfused hearts. Isoproterenol-stimulated beating rates of heterozygote (+/GS) and homozygote (GS/GS) hearts were significantly more sensitive to inhibition by carbachol than were those of wild type (+/+). Even greater effects were seen in the absence of isoproterenol; the potency of muscarinic-mediated bradycardia was enhanced fivefold in GS/GS and twofold in +/GS hearts compared with +/+. A(1)-adenosine receptor-mediated bradycardia was unaffected. In addition to effects on the sinoatrial node, +/GS and GS/GS hearts show significantly increased carbachol-induced third-degree atrioventricular (AV) block. Atrial pacing studies demonstrated an increased PR interval and AV effective refractory period in GS/GS hearts compared with +/+. Thus loss of the inhibitory action of endogenous RGS proteins on Galpha(i2) potentiates muscarinic inhibition of cardiac automaticity and conduction. The severe carbachol-induced sinus bradycardia in Galpha(i2)G184S mice suggests a possible role for alterations of Galpha(i2) or RGS proteins in sick sinus syndrome and pathological AV block.

The membrane anchor R7BP controls the proteolytic stability of the striatal specific RGS protein, RGS9-2

A member of the RGS (regulators of G protein signaling) family, RGS9-2 is a critical regulator of G protein signaling pathways that control locomotion and reward signaling in the brain. RGS9-2 is specifically expressed in striatal neurons where it forms complexes with its newly discovered partner, R7BP (R7 family binding protein). Interaction with R7BP is important for the subcellular targeting of RGS9-2, which in native neurons is found in plasma membrane and its specializations, postsynaptic densities. Here we report that R7BP plays an additional important role in determining proteolytic stability of RGS9-2. We have found that co-expression with R7BP dramatically elevates the levels of RGS9-2 and its constitutive subunit, Gbeta5. Measurement of the RGS9-2 degradation kinetics in cells indicates that R7BP markedly reduces the rate of RGS9-2.Gbeta5 proteolysis. Lentivirus-mediated RNA interference knockdown of the R7BP expression in native striatal neurons results in the corresponding decrease in RGS9-2 protein levels. Analysis of the molecular determinants that mediate R7BP/RGS9-2 binding to result in proteolytic protection have identified that the binding site for R7BP in RGS proteins is formed by pairing of the DEP (Disheveled, EGL-10, Pleckstrin) domain with the R7H (R7 homology), a domain of previously unknown function that interacts with four putative alpha-helices of the R7BP core. These findings provide a mechanism for the regulation of the RGS9 protein stability in the striatal neurons.

Identification of small-molecule inhibitors of RGS4 using a high-throughput flow cytometry protein interaction assay

Regulators of G-protein signaling (RGS) proteins are important components of signal transduction pathways initiated through G-protein-coupled receptors (GPCRs). RGS proteins accelerate the intrinsic GTPase activity of G-protein alpha-subunits (Galpha) and thus shorten the time course and reduce the magnitude of G-protein alpha- and betagamma-subunit signaling. Inhibiting RGS action has been proposed as a means to enhance the activity and specificity of GPCR agonist drugs, but pharmacological targeting of protein-protein interactions has typically been difficult. The aim of this project was to identify inhibitors of RGS4. Using a Luminex 96-well plate bead analyzer and a novel flow-cytometric protein interaction assay to assess Galpha-RGS interactions in a high-throughput screen, we identified the first small-molecule inhibitor of an RGS protein. Of 3028 compounds screened, 1, methyl N-[(4-chlorophenyl)sulfonyl]-4-nitrobenzenesulfinimidoate (CCG-4986), inhibited RGS4/Galpha(o) binding with 3 to 5 muM potency. It binds to RGS4, inhibits RGS4 stimulation of Galpha(o) GTPase activity in vitro, and prevents RGS4 regulation of mu-opioid-inhibited adenylyl cyclase activity in permeabilized cells. Furthermore, CCG-4986 is selective for RGS4 and does not inhibit RGS8. Thus, we demonstrate the feasibility of targeting RGS/Galpha protein-protein interactions with small molecules as a novel means to modulate GPCR-mediated signaling processes.