Characterization and comparison of RGS2 and RGS4 as GTPase-activating proteins for m2 muscarinic receptor-stimulated G(i)

Selective optogenetic inhibition of Gαq or Gαi signaling by minimal RGS domains disrupts circuit functionality and circuit formation

Optogenetic techniques provide genetically targeted, spatially and temporally precise approaches to correlate cellular activities and physiological outcomes. In the nervous system, G protein-coupled receptors (GPCRs) have essential neuromodulatory functions through binding extracellular ligands to induce intracellular signaling cascades. In this work, we develop and validate an optogenetic tool that disrupts Gαq signaling through membrane recruitment of a minimal regulator of G protein signaling (RGS) domain. This approach, Photo-induced Gα Modulator-Inhibition of Gαq (PiGM-Iq), exhibited potent and selective inhibition of Gαq signaling. Using PiGM-Iq we alter the behavior of Caenorhabditis elegans and Drosophila with outcomes consistent with GPCR-Gαq disruption. PiGM-Iq changes axon guidance in cultured dorsal root ganglia neurons in response to serotonin. PiGM-Iq activation leads to developmental deficits in zebrafish embryos and larvae resulting in altered neuronal wiring and behavior. Furthermore, by altering the minimal RGS domain, we show that this approach is amenable to Gαi signaling. Our unique and robust optogenetic Gα inhibiting approaches complement existing neurobiological tools and can be used to investigate the functional effects neuromodulators that signal through GPCR and trimeric G proteins.

Residue-level determinants of RGS R4 subfamily GAP activity and specificity towards the Gi subfamily

The structural basis for the GTPase-accelerating activity of regulators of G protein signaling (RGS) proteins, as well as the mechanistic basis for their specificity in interacting with the heterotrimeric (αβγ) G proteins they inactivate, is not sufficiently understood at the family level. Here, we used biochemical assays to compare RGS domains across the RGS family and map those individual residues that favorably contribute to GTPase-accelerating activity, and those residues responsible for attenuating RGS domain interactions with Gα subunits. We show that conserved interactions of RGS residues with both the Gα switch I and II regions are crucial for RGS activity, while the reciprocal effects of "modulatory" and "disruptor" residues selectively modulate RGS activity. Our results quantify how specific interactions between RGS domains and Gα subunits are set by a balance between favorable RGS residue interactions with particular Gα switch regions, and unfavorable interactions with the Gα helical domain.

Emerging Roles for Regulator of G Protein Signaling 2 in (Patho)physiology

Since their discovery in the mid-1990s, regulator of G protein signaling (RGS) proteins have emerged as key regulators of signaling through G protein-coupled receptors. Among the over 20 known RGS proteins, RGS2 has received increasing interest as a potential therapeutic drug target with broad clinical implications. RGS2 is a member of the R4 subfamily of RGS proteins and is unique in that it is selective for Gα _q Despite only having an RGS domain, responsible for the canonical GTPase activating protein activity, RGS2 can regulate additional processes, such as protein synthesis and adenylate cyclase activity, through protein-protein interactions. Here we provide an update of the current knowledge of RGS2 function as it relates to molecular mechanisms of regulation as well as its potential role in regulating a number of physiologic systems and pathologies, including cardiovascular disease and central nervous system disorders, as well as various forms of cancer. SIGNIFICANCE STATEMENT: Regulator of G protein signaling (RGS) proteins represent an exciting class of novel drug targets. RGS2, in particular, could have broad clinical importance. As more details are emerging on the regulation of RGS2 in various physiological systems, the potential utility of this small protein in therapeutic development is increasing.

Regulator of G protein signaling 2 (Rgs2) regulates neural crest development through Pparδ-Sox10 cascade

Neural crest cells are multipotent progenitors that migrate extensively and differentiate into numerous derivatives. The developmental plasticity and migratory ability of neural crest cells render them an attractive model for studying numerous aspects of cell progression. We observed that zebrafish rgs2 was expressed in neural crest cells. Disrupting Rgs2 expression by using a dominant negative rgs2 construct or rgs2 morpholinos reduced GTPase-activating protein activity, induced the formation of neural crest progenitors, increased the proliferation of nonectomesenchymal neural crest cells, and inhibited the formation of ectomesenchymal neural crest derivatives. The transcription of pparda (which encodes Pparδ, a Wnt-activated transcription factor) was upregulated in Rgs2-deficient embryos, and Pparδ inhibition using a selective antagonist in the Rgs2-deficient embryos repaired neural crest defects. Our results clarify the mechanism through which the Rgs2-Pparδ cascade regulates neural crest development; specifically, Pparδ directly binds to the promoter and upregulates the transcription of the neural crest specifier sox10. This study reveals a unique regulatory mechanism, the Rgs2-Pparδ-Sox10 signaling cascade, and defines a key molecular regulator, Rgs2, in neural crest development.

Cholinergic activity as a new target in diseases of the heart

The autonomic nervous system is an important modulator of cardiac signaling in both health and disease. In fact, the significance of altered parasympathetic tone in cardiac disease has recently come to the forefront. Both neuronal and nonneuronal cholinergic signaling likely play a physiological role, since modulating acetylcholine (ACh) signaling from neurons or cardiomyocytes appears to have significant consequences in both health and disease. Notably, many of these effects are solely due to changes in cholinergic signaling, without altered sympathetic drive, which is known to have significant adverse effects in disease states. As such, it is likely that enhanced ACh-mediated signaling not only has direct positive effects on cardiomyocytes, but it also offsets the negative effects of hyperadrenergic tone. In this review, we discuss recent studies that implicate ACh as a major regulator of cardiac remodeling and provide support for the notion that enhancing cholinergic signaling in human patients with cardiac disease can reduce morbidity and mortality. These recent results support the idea of developing large clinical trials of strategies to increase cholinergic tone, either by stimulating the vagus or by increased availability of Ach, in heart failure.

The Ras-binding domain region of RGS14 regulates its functional interactions with heterotrimeric G proteins

RGS14 is a 60 kDa protein that contains a regulator of G protein signaling (RGS) domain near its N-terminus, a central region containing a pair of tandem Ras-binding domains (RBD), and a GPSM (G protein signaling modulator) domain (a.k.a. Gi/o-Loco binding [GoLoco] motif) near its C-terminus. The RGS domain of RGS14 exhibits GTPase accelerating protein (GAP) activity toward Gαi/o proteins, while its GPSM domain acts as a guanine nucleotide dissociation inhibitor (GDI) on Gαi1 and Gαi3. In the current study, we investigate the contribution of different domains of RGS14 to its biochemical functions. Here we show that the full-length protein has a greater GTPase activating activity but a weaker inhibition of nucleotide dissociation relative to its isolated RGS and GPSM regions, respectively. Our data suggest that these differences may be attributable to an inter-domain interaction within RGS14 that promotes the activity of the RGS domain, but simultaneously inhibits the activity of the GPSM domain. The RBD region seems to play an essential role in this regulatory activity. Moreover, this region of RGS14 is also able to bind to members of the B/R4 subfamily of RGS proteins and enhance their effects on GPCR-activated Gi/o proteins. Overall, our results suggest a mechanism wherein the RBD region associates with the RGS domain region, producing an intramolecular interaction within RGS14 that enhances the GTPase activating function of its RGS domain while disfavoring the negative effect of its GPSM domain on nucleotide dissociation.

The eIF2B-interacting domain of RGS2 protects against GPCR agonist-induced hypertrophy in neonatal rat cardiomyocytes

The protective effect of Regulator of G protein Signaling 2 (RGS2) in cardiac hypertrophy is thought to occur through its ability to inhibit the chronic GPCR signaling that promotes pathogenic growth both in vivo and in cultured cardiomyocytes. However, RGS2 is known to have additional functions beyond its activity as a GTPase accelerating protein, such as the ability to bind to eukaryotic initiation factor, eIF2B, and inhibit protein synthesis. The RGS2 eIF2B-interacting domain (RGS2(eb)) was examined for its ability to regulate hypertrophy in neonatal ventricular myocytes. Both full-length RGS2 and RGS2(eb) were able to inhibit agonist-induced cardiomyocyte hypertrophy, but RGS2(eb) had no effect on receptor-mediated inositol phosphate production, cAMP production, or ERK 1/2 activation. These results suggest that the protective effects of RGS2 in cardiac hypertrophy may derive at least in part from its ability to govern protein synthesis.

A "new twist" on RGS protein selectivity

G protein-coupled receptors mediate a wide array of physiologic stimuli and, together with their regulators such as RGS2, are essential components of cellular signaling and function. RGS2 is a selective inhibitor of the Gαq class of α subunits. In this issue of Structure, Nance and colleagues provide structural insight into the features of RGS2 that mediate its potent and selective regulation of Gαq function.

Structural and functional analysis of the regulator of G protein signaling 2-gαq complex

The heterotrimeric G protein Gαq is a key regulator of blood pressure, and excess Gαq signaling leads to hypertension. A specific inhibitor of Gαq is the GTPase activating protein (GAP) known as regulator of G protein signaling 2 (RGS2). The molecular basis for how Gαq/11 subunits serve as substrates for RGS proteins and how RGS2 mandates its selectivity for Gαq is poorly understood. In crystal structures of the RGS2-Gαq complex, RGS2 docks to Gαq in a different orientation from that observed in RGS-Gαi/o complexes. Despite its unique pose, RGS2 maintains canonical interactions with the switch regions of Gαq in part because its α6 helix adopts a distinct conformation. We show that RGS2 forms extensive interactions with the α-helical domain of Gαq that contribute to binding affinity and GAP potency. RGS subfamilies that do not serve as GAPs for Gαq are unlikely to form analogous stabilizing interactions.

Fine-tuning of GPCR signals by intracellular G protein modulators

Heterotrimeric G proteins convey receptor signals to intracellular effectors. Superimposed over the basic GPCR-G protein-effector scheme are three types of auxiliary proteins that also modulate Gα. Regulator of G protein signaling proteins and G protein signaling modifier proteins respectively promote GTPase activity and hinder GTP/GDP exchange to limit Gα activation. There are also diverse proteins that, like GPCRs, can promote nucleotide exchange and thus activation. Here we review the impact of these auxiliary proteins on GPCR signaling. Although their precise physiological functions are not yet clear, all of them can produce significant effects in experimental systems. These signaling changes are generally consistent with established effects on isolated Gα; however, the activation state of Gα is seldom verified and many such changes appear also to reflect the physical disruption of or indirect effects on interactions between Gα and its associated GPCR, Gβγ, and/or effector.

Role of Cholinergic Innervation and RGS2 in Atrial Arrhythmia

The heart receives sympathetic and parasympathetic efferent innervation as well as the ability to process information internally via an intrinsic cardiac autonomic nervous system (ICANS). For over a century, the role of the parasympathetics via vagal acetylcholine release was related to controlling primarily heart rate. Although in the late 1800s shown to play a role in atrial arrhythmia, the myocardium took precedence from the mid-1950s until in the last decade a resurgence of interest in the autonomics along with signaling cascades, regulators, and ion channels. Originally ignored as being benign and thus untreated, recent emphasis has focused on atrial arrhythmia as atrial fibrillation (AF) is the most common arrhythmia seen by the general practitioner. It is now recognized to have significant mortality and morbidity due to resultant stroke and heart failure. With the aging population, there will be an unprecedented increased burden on health care resources. Although it has been known for more than half a century that cholinergic stimulation can initiate AF, the classical concept focused on the M2 receptor and its signaling cascade including RGS4, as these had been shown to have predominant effects on nodal function (heart rate and conduction block) as well as contractility. However, recent evidence suggests that the M3 receptor may also playa role in initiation and perpetuation of AF and thus RGS2, a putative regulator of the M3 receptor, may be a target for therapeutic intervention. Mice lacking RGS2 (RGS2^−/−), were found to have significantly altered electrophysiological atrial responses and were more susceptible to electrically induced AF. Vagally induced or programmed stimulation-induced AF could be blocked by the selective M3R antagonist, darifenacin. These results suggest a potential surgical target (ICANS) and pharmacological targets (M3R, RGS2) for the management of AF.

Regulators of G-protein signaling in the heart and their potential as therapeutic targets

Signal transduction through G-protein-coupled receptors (GPCRs) is central for the regulation of virtually all cellular functions and has been widely implicated in human disease. Regulators of G-protein signaling (RGS proteins) belong to a diverse protein family that was originally discovered for their ability to accelerate signal termination in response to GPCR stimulation, thereby reducing the amplitude and duration of GPCR effects. All RGS proteins share a common RGS domain that interacts with G protein α subunits and mediates their biological regulation of GPCR signaling. However, RGS proteins differ widely in size and the organization of their sequences flanking the RGS domain, which contain several additional functional domains that facilitate protein-protein (or protein-lipid) interactions. RGS proteins are subject to posttranslational modifications, and, in addition, their expression, activity, and subcellular localization can be dynamically regulated. Thus, there exists a wide array of mechanisms that facilitate their proper function as modulators and integrators of G-protein signaling. Several RGS proteins have been implicated in the cardiac remodeling response and heart rate regulation, and changes in RGS protein expression and/or function are believed to participate in the pathophysiology of cardiac hypertrophy, failure and arrhythmias as well as hypertension. This review is based on recent advances in our understanding of the expression pattern, regulation, and functional role of canonical RGS proteins, with a special focus on the healthy heart and the diseased heart. In addition, we discuss their potential and promise as therapeutic targets as well as strategies to modulate their expression and function.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

The proline-rich N-terminal domain of G18 exhibits a novel G protein regulatory function

The protein G18 (also known as AGS4 or GPSM3) contains three conserved GoLoco/GPR domains in its central and C-terminal regions that bind to inactive Galpha(i), whereas the N-terminal region has not been previously characterized. We investigated whether this domain might itself regulate G protein activity by assessing the abilities of G18 and mutants thereof to modulate the nucleotide binding and hydrolytic properties of Galpha(i1) and Galpha(o). Surprisingly, in the presence of fluoroaluminate (AlF(4)(-)) both G proteins bound strongly to full-length G18 (G18wt) and to its isolated N-terminal domain (G18DeltaC) but not to its GoLoco region (DeltaNG18). Thus, it appears that its N-terminal domain promotes G18 binding to fluoroaluminate-activated Galpha(i/o). Neither G18wt nor any G18 mutant affected the GTPase activity of Galpha(i1) or Galpha(o). In contrast, complex effects were noted with respect to nucleotide binding. As inferred by the binding of [(35)S]GTPgammaS (guanosine 5'-O-[gamma-thio]triphosphate) to Galpha(i1), the isolated GoLoco region as expected acted as a guanine nucleotide dissociation inhibitor, whereas the N-terminal region exhibited a previously unknown guanine nucleotide exchange factor effect on this G protein. On the other hand, the N terminus inhibited [(35)S]GTPgammaS binding to Galpha(o), albeit to a lesser extent than the effect of the GoLoco region on Galpha(i1). Taken together, our results identify the N-terminal region of G18 as a novel G protein-interacting domain that may have distinct regulatory effects within the G(i/o) subfamily, and thus, it could potentially play a role in differentiating signals between these related G proteins.

Evidence for enhanced M3 muscarinic receptor function and sensitivity to atrial arrhythmia in the RGS2-deficient mouse

Atrial fibrillation (AF) is the most common arrhythmia seen in general practice. Muscarinic ACh receptors (M2R, M3R) are involved in vagally induced AF. M2R and M3R activate the heterotrimeric G proteins, G(i) and G(q), respectively, by promoting GTP binding, and these in turn activate distinct K(+) channels. Signaling is terminated by GTP hydrolysis, a process accelerated by regulator of G protein signaling (RGS) proteins. RGS2 is selective for G(q) and thus may regulate atrial M3R signaling. We hypothesized that knockout of RGS2 (RGS2(-/-)) would render the atria more susceptible to electrically induced AF. One-month-old male RGS2(-/-) and C57BL/6 wild-type (WT) mice were instrumented for intracardiac electrophysiology. Atrial effective refractory periods (AERPs) were also determined in the absence and presence of carbachol, atropine, and/or the selective M3R antagonist darifenacin. Susceptibility to electrically induced AF used burst pacing and programmed electrical stimulation with one extrastimulus. Real-time RT-PCR measured atrial and ventricular content of RGS2, RGS4, M2R, M3R, and M4R mRNA. AERP was lower in RGS2(-/-) compared with WT mice in both the high right atrium (HRA) (30 +/- 1 vs. 34 +/- 1 ms, P < 0.05) and mid right atrium (MRA) (21 +/- 1 vs. 24 +/- 1 ms, P < 0.05). Darifenacin eliminated this difference (HRA: 37 +/- 2 vs. 39 +/- 2 ms, and MRA: 30 +/- 2 vs. 30 +/- 1, P > 0.4). RGS2(-/-) were more susceptible than WT mice to atrial tachycardia/fibrillation (AT/F) induction (11/22 vs. 1/25, respectively, P < 0.05). Muscarinic receptor expression did not differ between strains, whereas M2R expression was 70-fold higher than M3R (P < 0.01). These results suggest that RGS2 is an important cholinergic regulator in the atrium and that RGS2(-/-) mice have enhanced susceptibility to AT/F via enhanced M3 muscarinic receptor activity.

Structural determinants of G-protein alpha subunit selectivity by regulator of G-protein signaling 2 (RGS2)

"Regulator of G-protein signaling" (RGS) proteins facilitate the termination of G protein-coupled receptor (GPCR) signaling via their ability to increase the intrinsic GTP hydrolysis rate of Galpha subunits (known as GTPase-accelerating protein or "GAP" activity). RGS2 is unique in its in vitro potency and selectivity as a GAP for Galpha(q) subunits. As many vasoconstrictive hormones signal via G(q) heterotrimer-coupled receptors, it is perhaps not surprising that RGS2-deficient mice exhibit constitutive hypertension. However, to date the particular structural features within RGS2 determining its selectivity for Galpha(q) over Galpha(i/o) substrates have not been completely characterized. Here, we examine a trio of point mutations to RGS2 that elicits Galpha(i)-directed binding and GAP activities without perturbing its association with Galpha(q). Using x-ray crystallography, we determined a model of the triple mutant RGS2 in complex with a transition state mimetic form of Galpha(i) at 2.8-A resolution. Structural comparison with unliganded, wild type RGS2 and of other RGS domain/Galpha complexes highlighted the roles of these residues in wild type RGS2 that weaken Galpha(i) subunit association. Moreover, these three amino acids are seen to be evolutionarily conserved among organisms with modern cardiovascular systems, suggesting that RGS2 arose from the R4-subfamily of RGS proteins to have specialized activity as a potent and selective Galpha(q) GAP that modulates cardiovascular function.

Structural diversity in the RGS domain and its interaction with heterotrimeric G protein alpha-subunits

Regulator of G protein signaling (RGS) proteins accelerate GTP hydrolysis by Galpha subunits and thus facilitate termination of signaling initiated by G protein-coupled receptors (GPCRs). RGS proteins hold great promise as disease intervention points, given their signature role as negative regulators of GPCRs-receptors to which the largest fraction of approved medications are currently directed. RGS proteins share a hallmark RGS domain that interacts most avidly with Galpha when in its transition state for GTP hydrolysis; by binding and stabilizing switch regions I and II of Galpha, RGS domain binding consequently accelerates Galpha-mediated GTP hydrolysis. The human genome encodes more than three dozen RGS domain-containing proteins with varied Galpha substrate specificities. To facilitate their exploitation as drug-discovery targets, we have taken a systematic structural biology approach toward cataloging the structural diversity present among RGS domains and identifying molecular determinants of their differential Galpha selectivities. Here, we determined 14 structures derived from NMR and x-ray crystallography of members of the R4, R7, R12, and RZ subfamilies of RGS proteins, including 10 uncomplexed RGS domains and 4 RGS domain/Galpha complexes. Heterogeneity observed in the structural architecture of the RGS domain, as well as in engagement of switch III and the all-helical domain of the Galpha substrate, suggests that unique structural determinants specific to particular RGS protein/Galpha pairings exist and could be used to achieve selective inhibition by small molecules.

RGS protein specificity towards Gq- and Gi/o-mediated ERK 1/2 and Akt activation, in vitro

Extracellular Regulated Kinases (ERK) and Protein Kinase B (Akt) are intermediaries in relaying extracellular growth signals to intracellular targets. Each pathway can become activated upon stimulation of G protein-coupled receptors mediated by G(q) and G(i/o) proteins subjected to regulation by RGS proteins. The goal of the study was to delineate the specificity in which cardiac RGS proteins modulate G(q)and G(i/o)-induced ERK and Akt phosphorylation. To isolate G(q)- and G(i/o)-mediated effects, we exclusively expressed muscarinic M(2) or M(3) receptors in COS-7 cells. Western blot analyses demonstrated increase of phosphorylation of ERK 1.7-/3.3-fold and Akt 2.4-/6-fold in M(2)-/M(3)- expressing cells through carbachol stimulation. In co-expressions, M(3)/G(q)-induced activation of Akt was exclusively blunted through RGS3s/RGS3, whereas activation of ERK was inhibited additionally through RGS2/RGS5. M(2)/G(i/o) induced Akt activation was inhibited by all RGS proteins tested. RGS2 had no effect on M(2)/G(i/o)-induced ERK activation. The high degree of specificity in RGS proteins-depending modulation of G(q)- and G(i/o)-mediated ERK and Akt activation in the muscarinic network cannot merely be attributed exclusively to RGS protein selectivity towards G(q) or G(i/o) proteins. Counter-regulatory mechanisms and inter-signaling cross-talk may alter the sensitivity of GPCR-induced ERK and Akt activation to RGS protein regulation.

Up-regulation of endogenous RGS2 mediates cross-desensitization between Gs and Gq signaling in osteoblasts

Regulator of G protein signaling (RGS) proteins limit G protein signals. In this study, we investigated the role of RGS2 in the control of G protein signaling cascades in osteoblasts, the cells responsible for bone formation. Expression of RGS2 was up-regulated in primary cultures of mouse calvarial osteoblasts by parathyroid hormone-related peptide (PTHrP)-(1-34), which stimulates G(s) signaling. RGS2 was also up-regulated by extracellular ATP, which selectively activates G(q), as well as by forskolin and phorbol myristate acetate, which activate targets downstream of G(s) and G(q), respectively. To assess the role of endogenous RGS2, we characterized G(s) and G(q) signaling in osteoblasts derived from wild type and rgs2(-/-) mice. Under control conditions, nucleotide-stimulated calcium release, endothelin-stimulated accumulation of inositol phosphates, and PTHrP-stimulated cAMP accumulation were equivalent in osteoblasts isolated from wild type and rgs2(-/-) mice. Thus, basal levels of endogenous RGS2 do not appear to regulate G(s) or G(q) signaling in osteoblasts. Interestingly, forskolin treatment of wild type but not rgs2(-/-) osteoblasts suppressed both endothelin-stimulated accumulation of inositol phosphates and nucleotide-stimulated calcium release, indicating that up-regulation of RGS2 by G(s) signaling desensitizes G(q) signals. Furthermore, pretreatment with ATP suppressed PTHrP-dependent cAMP accumulation in wild type but not rgs2(-/-) osteoblasts, implying that up-regulation of RGS2 by G(q) signaling desensitizes G(s) signals. Our findings demonstrate that endogenously expressed RGS2 can limit G(s) signaling. Moreover, up-regulation of RGS2 contributes to cross-desensitization of G(s)- and G(q)-coupled signals.

Regulator of G-protein signaling (RGS) proteins differentially control chondrocyte differentiation

Control of chondrocyte differentiation is attained, in part, through G-protein signaling, but the functions of the RGS family of genes, well known to control G-protein signaling at the Galpha subunit, have not been studied extensively in chondrogenesis. Recently, we have identified the Rgs2 gene as a regulator of chondrocyte differentiation. Here we extend these studies to additional Rgs genes. We demonstrate that the Rgs4, Rgs5, Rgs7, and Rgs10 genes are differentially regulated during chondrogenic differentiation in vitro and in vivo. To investigate the roles of RGS proteins during cartilage development, we overexpressed RGS4, RGS5, RGS7, and RGS10 in the chondrogenic cell line ATDC5. We found unique and overlapping effects of individual Rgs genes on numerous parameters of chondrocyte differentiation. In particular, RGS5, RGS7, and RGS10 promote and RGS4 inhibits chondrogenic differentiation. The identification of Rgs genes as novel regulators of chondrogenesis will contribute to a better understanding of both normal cartilage development and the etiology of chondrodysplasias and osteoarthritis.

Modulation of subfamily B/R4 RGS protein function by 14-3-3 proteins

Regulator of G protein signalling (RGS) proteins are primarily known for their ability to act as GTPase activating proteins (GAPs) and thus attenuate G protein function within G protein-coupled receptor (GPCR) signalling pathways. However, RGS proteins have been found to interact with additional binding partners, and this has introduced more complexity to our understanding of their potential role in vivo. Here, we identify a novel interaction between RGS proteins (RGS4, RGS5, RGS16) and the multifunctional protein 14-3-3. Two isoforms, 14-3-3beta and 14-3-3epsilon, directly interact with all three purified RGS proteins and data from in vitro steady state GTP hydrolysis assays show that 14-3-3 inhibits the GTPase activity of RGS4 and RGS16, but has limited effects on RGS5 under comparable conditions. Moreover in a competitive pull-down experiment, 14-3-3epsilon competes with Galphao for RGS4, but not for RGS5. This mechanism is further reinforced in living cells, where 14-3-3epsilon sequesters RGS4 in the cytoplasm and impedes its recruitment to the plasma membrane by Galpha protein. Thus, 14-3-3 might act as a molecular chelator, preventing RGS proteins from interacting with Galpha, and ultimately prolonging the signal transduction pathway. In conclusion, our findings suggest that 14-3-3 proteins may indirectly promote GPCR signalling via their inhibitory effects on RGS GAP function.

Psychostimulants, madness, memory... and RGS proteins?

The ingestion of psychostimulant drugs by humans imparts a profound sense of alertness and well-being. However, repeated use of these drugs in some individuals will induce a physiological state of dependence, characterized by compulsive behavior directed toward the acquisition and ingestion of the drug, at the expense of customary social obligations. Drugs of abuse and many other types of experiences share the ability to alter the morphology and density of neuronal dendrites and spines. Dopaminergic modulation of corticostriatal synaptic plasticity is necessary for these morphological changes. Changes in the density of dendritic spines on striatal neurons may underlie the development of this pathological pattern of drug-seeking behavior. Identifying proteins that regulate dopaminergic signaling are of value. A family of proteins, the regulators of G protein signaling (RGS) proteins, which regulate signaling from G protein-coupled receptors, such as dopamine and glutamate, may be important in this regard. By regulating corticostriatal synaptic plasticity, RGS proteins can influence presynaptic activity, neurotransmitter release, and postsynaptic depolarization and thereby play a key role in the development of this plasticity. Pharmacological agents that modify RGS activity in humans could be efficacious in ameliorating the dependence on psychostimulant drugs.

A Key Serine for the GTPase-Activating Protein Function of Regulator of G Protein Signaling Proteins Is Not a General Target for 14-3-3 Interactions

Mammalian regulator of G protein signaling (RGS) proteins are highly conserved within the RGS domain. Of amino acids that are universal, a serine residue at the C terminus of this domain has been described as the binding site in RGS7 for 14-3-3 proteins. However, studies with the related RGS3 indicate that the site of interaction is not within the RGS domain. We confirm that the interaction of RGS3 with 14-3-3tau and 14-3-3zeta requires Ser264 and not the RGS domain and show both that mutation of the conserved RGS domain serine, Ser496 in RGS3, to either alanine or aspartate does not prevent binding of 14-3-3 proteins and that 14-3-3 proteins do not inhibit GTPase-activating protein (GAP) activity against receptor-activated Galpha(o1). However, mutation of Ser496 does directly impair the action of RGS3 as a GAP against receptor-activated Galpha(o1). We mutated the equivalent serine residue in the family B/R4 RGS proteins RGS1 and RGS16. Using two distinct assay formats, conversion to aspartate virtually abolished GAP activity, whereas conversion to alanine decreased potency 20-fold. Neither alteration modulated interactions with 14-3-3tau or 14-3-3zeta, but the 14-3-3 proteins did not modulate the GAP activity of the wild-type or mutant RGS proteins. Although interactions between 14-3-3 proteins and many RGS proteins can be observed, this does not involve this conserved serine and does not inherently modify GAP function.

Increased baclofen-stimulated G protein coupling and deactivation in rat brain cortex during development

The number and affinity of GABA(B) receptors (assayed by the specific antagonist [(3)H]CGP54626A) was unchanged when compared in carefully washed cerebrocortical membranes from young (12-day-old) and adult (90-day-old) rats. In contrast, high-affinity GTPase activity, both basal and baclofen-stimulated was significantly higher (by 45% and 56%, respectively) in adult than in young rats. Similar results were obtained by concomitant determination of agonist (baclofen)-stimulated GTP gamma S binding. Under standard conditions, baclofen-stimulated GTPase activity was further considerably enhanced by exogenously added regulator of G protein function, RGS1, but not by RGS16. RGS16 was able to affect agonist-stimulated GTPase activity only in the presence of markedly increase substrate (GTP) concentrations. RGS1 alone slightly increased GTPase activity in adult rats, but neither RGS1 nor RGS16 influenced GTPase activity in membrane preparations isolated from young animals. These findings indicate increasing functional activity of trimeric G protein(s) involved in GABAergic transmission in the developing rat brain cortex and suggest a high potential of RGS1 in regulation of high-affinity GTPase activity.

RGS17/RGSZ2, a Novel Regulator of Gi/o, Gz, and Gq Signaling

To identify novel regulators of Galpha(o), the most abundant G-protein in brain, we used yeast two-hybrid screening with constitutively active Galpha(o) as bait and identified a new regulator of G-protein signaling (RGS) protein, RGS17 (RGSZ2), as a novel human member of the RZ (or A) subfamily of RGS proteins. RGS17 contains an amino-terminal cysteine-rich motif and a carboxyl-terminal RGS domain with highest homology to hRGSZ1- and hRGS-Galpha-interacting protein. RGS17 RNA was strongly expressed as multiple species in cerebellum and other brain regions. The interactions between hRGS17 and active forms of Galpha(i1-3), Galpha(o), Galpha(z), or Galpha(q) but not Galpha(s) were detected by yeast two-hybrid assay, in vitro pull-down assay, and co-immunoprecipitation studies. Recombinant RGS17 acted as a GTPase-activating protein (GAP) on free Galpha(i2) and Galpha(o) under pre-steady-state conditions, and on M2-muscarinic receptor-activated Galpha(i1), Galpha(i2), Galpha(i3), Galpha(z), and Galpha(o) in steady-state GTPase assays in vitro. Unlike RGSZ1, which is highly selective for G(z), RGS17 exhibited limited selectivity for G(o) among G(i)/G(o) proteins. All RZ family members reduced dopamine-D2/Galpha(i)-mediated inhibition of cAMP formation and abolished thyrotropin-releasing hormone receptor/Galpha(q)-mediated calcium mobilization. RGS17 is a new RZ member that preferentially inhibits receptor signaling via G(i/o), G(z), and G(q) over G(s) to enhance cAMP-dependent signaling and inhibit calcium signaling. Differences observed between in vitro GAP assays and whole-cell signaling suggest additional determinants of the G-protein specificity of RGS GAP effects that could include receptors and effectors.

Regional expression of RGS4 mRNA in human brain+

Regulators of G-protein signalling (RGS) proteins are a recently discovered class of proteins that modulate G-protein activity. More than 20 RGS proteins have been identified and are expressed throughout the body and brain. In particular, RGS4 appears to regulate dopamine receptor function and has been implicated in several dopamine related diseases, including schizophrenia. This study presents an extensive examination of the regional distribution of RGS4 mRNA in postmortem human brain. Using in situ hybridization, the expression levels of RGS4 mRNA were determined in human hemicoronal (Talairach sections +8 and -20) brain sections. In the rostral slice (Talairach +8) highest levels were found in the inferior frontal cortex, the superior frontal, and the cingulate cortex. Slightly lower levels were found in the insular cortex and inferior temporal cortex. The caudate, putamen and nucleus accumbens had lower levels. In the caudal slice (-20), the cortical layers showed the highest levels, with moderate levels observed in the parahippocampal gyrus, low levels in the CA-pyramidal region, and almost undetectable levels in the thalamus. In the frontal cortex a dense band was apparent near one of the inner layers of the cortex. In conclusion, RGS4 mRNA distribution in human postmortem tissue from normal persons was very dense in most cortical layers examined, with lower density in the basal ganglia and thalamus.

RGS2 binds directly and selectively to the M1 muscarinic acetylcholine receptor third intracellular loop to modulate Gq/11alpha signaling

RGS proteins serve as GTPase-activating proteins and/or effector antagonists to modulate Galpha signaling events. In live cells, members of the B/R4 subfamily of RGS proteins selectively modulate G protein signaling depending on the associated receptor (GPCR). Here we examine whether GPCRs selectively recruit RGS proteins to modulate linked G protein signaling. We report the novel finding that RGS2 binds directly to the third intracellular (i3) loop of the G(q/11)-coupled M1 muscarinic cholinergic receptor (M1 mAChR; M1i3). This interaction is selective because closely related RGS16 does not bind M1i3, and neither RGS2 nor RGS16 binds to the G(i/o)-coupled M2i3 loop. When expressed in cells, RGS2 and M1 mAChR co-localize to the plasma membrane whereas RGS16 does not. The N-terminal region of RGS2 is both necessary and sufficient for binding to M1i3, and RGS2 forms a stable heterotrimeric complex with both activated G(q)alpha and M1i3. RGS2 potently inhibits M1 mAChR-mediated phosphoinositide hydrolysis in cell membranes by acting as an effector antagonist. Deletion of the N terminus abolishes this effector antagonist activity of RGS2 but not its GTPase-activating protein activity toward G(11)alpha in membranes. These findings predict a model where the i3 loops of GPCRs selectively recruit specific RGS protein(s) via their N termini to regulate the linked G protein. Consistent with this model, we find that the i3 loops of the mAChR subtypes (M1-M5) exhibit differential profiles for binding distinct B/R4 RGS family members, indicating that this novel mechanism for GPCR modulation of RGS signaling may generally extend to other receptors and RGS proteins.

Recruitment of RGS2 and RGS4 to the plasma membrane by G proteins and receptors reflects functional interactions

N-terminally green fluorescent protein (GFP)-tagged regulator of G protein signaling (RGS) 2 and RGS4 fusion proteins expressed in human embryonic kidney 293 cells localized to the nucleus and cytosol, respectively. They were selectively recruited to the plasma membrane by G proteins and correspondingly by receptors that activate those G proteins: GFP-RGS2 when coexpressed with Galphas, beta2-adrenergic receptor, Galphaq, or AT1A angiotensin II receptor, and GFP-RGS4 when coexpressed with Galphai2 or M2 muscarinic receptor. G protein mutants with reduced RGS affinity did not produce this effect, implying that the recruitment involves direct binding to G proteins and is independent of downstream signaling events. Neither agonists nor inverse agonists altered receptor-promoted RGS association with the plasma membrane, and expressing either constitutively activated or poorly activated G protein mutants produced effects similar to those of their wild-type counterparts. Thus, intracellular interactions between these proteins seem to be relatively stable and insensitive to the activation state of the G protein, in contrast to the transient increases in RGS-G protein association known to be caused by G protein activation in solution-based assays. G protein effects on RGS localization were mirrored by RGS effects on G protein function. RGS4 was more potent than RGS2 in promoting steady-state Gi GTPase activity, whereas RGS2 inhibited Gs-dependent increases in intracellular cAMP, suggesting that G protein signaling in cells is regulated by the selective recruitment of RGS proteins to the plasma membrane.

Predicting therapeutic value in the lead optimization phase of drug discovery

Recombinant and natural cellular assays for human G-protein-coupled receptors are used to optimize initial lead molecules obtained from screening. Although the activity of these molecules can be assessed on human genotype receptors, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds. This opens the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans. This review discusses the mechanisms by which cells can impose phenotypic selectivity on molecules and approaches to reduce this practical problem for drug discovery.