Gbeta5 prevents the RGS7-Galphao interaction through binding to a distinct Ggamma-like domain found in RGS7 and other RGS proteins

Role of GPCR Signaling in Anthracycline-Induced Cardiotoxicity

Anthracyclines are a class of chemotherapeutics commonly used to treat a range of cancers. Despite success in improving cancer survival rates, anthracyclines have dose-limiting cardiotoxicity that prevents more widespread clinical utility. Currently, the therapeutic options for these patients are limited to the iron-chelating agent dexrazoxane, the only FDA-approved drug for anthracycline cardiotoxicity. However, the clinical use of dexrazoxane has failed to replicate expectations from preclinical studies. A limited list of GPCRs have been identified as pathogenic in anthracycline-induced cardiotoxicity, including receptors (frizzled, adrenoreceptors, angiotensin II receptors) previously implicated in cardiac remodeling in other pathologies. The RNA sequencing of iPSC-derived cardiac myocytes from patients has increased our understanding of the pathogenic mechanisms driving cardiotoxicity. These data identified changes in the expression of novel GPCRs, heterotrimeric G proteins, and the regulatory pathways that govern downstream signaling. This review will capitalize on insights from these experiments to explain aspects of disease pathogenesis and cardiac remodeling. These data provide a cornucopia of possible unexplored potential pathways by which we can reduce the cardiotoxic side effects, without compromising the anti-cancer effects, of doxorubicin and provide new therapeutic options to improve the recovery and quality of life for patients undergoing chemotherapy.

The roles of RGS proteins in cardiometabolic disease

G protein-coupled receptors (GPCRs) are the most prominent receptors on the surface of the cell and play a central role in the regulation of cardiac and metabolic functions. GPCRs transmit extracellular stimuli to the interior of the cells by activating one or more heterotrimeric G proteins. The duration and intensity of G protein-mediated signalling are tightly controlled by a large array of intracellular mediators, including the regulator of G protein signalling (RGS) proteins. RGS proteins selectively promote the GTPase activity of a subset of Gα subunits, thus serving as negative regulators in a pathway-dependent manner. In the current review, we summarise the involvement of RGS proteins in cardiometabolic function with a focus on their tissue distribution, mechanisms of action and dysregulation under various disease conditions. We also discuss the potential therapeutic applications for targeting RGS proteins in treating cardiometabolic conditions and current progress in developing RGS modulators. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

A Global Map of G Protein Signaling Regulation by RGS Proteins

The control over the extent and timing of G protein signaling is provided by the regulator of G protein signaling (RGS) proteins that deactivate G protein α subunits (Gα). Mammalian genomes encode 20 canonical RGS and 16 Gα genes with key roles in physiology and disease. To understand the principles governing the selectivity of Gα regulation by RGS, we examine the catalytic activity of all canonical human RGS proteins and their selectivity for a complete set of Gα substrates using real-time kinetic measurements in living cells. The data reveal rules governing RGS-Gα recognition, the structural basis of its selectivity, and provide principles for engineering RGS proteins with defined selectivity. The study also explores the evolution of RGS-Gα selectivity through ancestral reconstruction and demonstrates how naturally occurring non-synonymous variants in RGS alter signaling. These results provide a blueprint for decoding signaling selectivity and advance our understanding of molecular recognition principles.

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Systematic analysis reveals G protein selectivity of all canonical RGS proteins

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RGS proteins rely on selectivity bar codes for selective G protein recognition

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Transplantation of bar codes across RGS proteins switches their G protein preferences

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Natural variants, mutations, and evolution shape RGS selectivity

Structural organization of a major neuronal G protein regulator, the RGS7-Gβ5-R7BP complex

Signaling by the G-protein-coupled receptors (GPCRs) plays fundamental role in a vast number of essential physiological functions. Precise control of GPCR signaling requires action of regulators of G protein signaling (RGS) proteins that deactivate heterotrimeric G proteins. RGS proteins are elaborately regulated and comprise multiple domains and subunits, yet structural organization of these assemblies is poorly understood. Here, we report a crystal structure and dynamics analyses of the multisubunit complex of RGS7, a major regulator of neuronal signaling with key roles in controlling a number of drug target GPCRs and links to neuropsychiatric disease, metabolism, and cancer. The crystal structure in combination with molecular dynamics and mass spectrometry analyses reveals unique organizational features of the complex and long-range conformational changes imposed by its constituent subunits during allosteric modulation. Notably, several intermolecular interfaces in the complex work in synergy to provide coordinated modulation of this key GPCR regulator.

Regulation of G Protein βγ Signaling

Heterotrimeric guanine nucleotide-binding proteins (G proteins) deliver external signals to the cell interior, upon activation by the external signal stimulated G protein-coupled receptors (GPCRs).While the activated GPCRs control several pathways independently, activated G proteins control the vast majority of cellular and physiological functions, ranging from vision to cardiovascular homeostasis. Activated GPCRs dissociate GαGDPβγ heterotrimer into GαGTP and free Gβγ. Earlier, GαGTP was recognized as the primary signal transducer of the pathway and Gβγ as a passive signaling modality that facilitates the activity of Gα. However, Gβγ later found to regulate more number of pathways than GαGTP does. Once liberated from the heterotrimer, free Gβγ interacts and activates a diverse range of signaling regulators including kinases, lipases, GTPases, and ion channels, and it does not require any posttranslation modifications. Gβγ family consists of 48 members, which show cell- and tissue-specific expressions, and recent reports show that cells employ the subtype diversity in Gβγ to achieve desired signaling outcomes. In addition to activated GPCRs, which induce free Gβγ generation and the rate of GTP hydrolysis in Gα, which sequester Gβγ in the heterotrimer, terminating Gβγ signaling, additional regulatory mechanisms exist to regulate Gβγ activity. In this chapter, we discuss structure and function, subtype diversity and its significance in signaling regulation, effector activation, regulatory mechanisms as well as the disease relevance of Gβγ in eukaryotes.

Regulator of G-protein signaling Gβ5-R7 is a crucial activator of muscarinic M3 receptor-stimulated insulin secretion

In pancreatic β cells, muscarinic cholinergic receptor M3 (M3R) stimulates glucose-induced secretion of insulin. Regulator of G-protein signaling (RGS) proteins are critical modulators of GPCR activity, yet their role in β cells remains largely unknown. R7 subfamily RGS proteins are stabilized by the G-protein subunit Gβ5, such that the knockout of the Gnb5 gene results in degradation of all R7 subunits. We found that Gnb5 knockout in mice or in the insulin-secreting MIN6 cell line almost completely eliminates insulinotropic activity of M3R. Moreover, overexpression of Gβ5-RGS7 strongly promotes M3R-stimulated insulin secretion. Examination of this noncanonical mechanism in Gnb5^-/- MIN6 cells showed that cAMP, diacylglycerol, or Ca²⁺ levels were not significantly affected. There was no reduction in the amplitude of free Ca²⁺ responses in islets from the Gnb5^-/- mice, but the frequency of Ca²⁺ oscillations induced by cholinergic agonist was lowered by more than 30%. Ablation of Gnb5 impaired M3R-stimulated phosphorylation of ERK1/2. Stimulation of the ERK pathway in Gnb5^-/- cells by epidermal growth factor restored M3R-stimulated insulin release to near normal levels. Identification of the novel role of Gβ5-R7 in insulin secretion may lead to a new therapeutic approach for improving pancreatic β-cell function.-Wang, Q., Pronin, A. N., Levay, K., Almaca, J., Fornoni, A., Caicedo, A., Slepak, V. Z. Regulator of G-protein signaling Gβ5-R7 is a crucial activator of muscarinic M3 receptor-stimulated insulin secretion.

Intermolecular Interaction between Anchoring Subunits Specify Subcellular Targeting and Function of RGS Proteins in Retina ON-Bipolar Neurons

In vertebrate retina, light responses generated by the rod photoreceptors are transmitted to the second-order neurons, the ON-bipolar cells (ON-BC), and this communication is indispensible for vision in dim light. In ON-BCs, synaptic transmission is initiated by the metabotropic glutamate receptor, mGluR6, that signals via the G-protein Go to control opening of the effector ion channel, TRPM1. A key role in this process belongs to the GTPase Activating Protein (GAP) complex that catalyzes Go inactivation upon light-induced suppression of glutamate release in rod photoreceptors, thereby driving ON-BC depolarization to changes in synaptic input. The GAP complex has a striking molecular complexity. It contains two Regulator of G-protein Signaling (RGS) proteins RGS7 and RGS11 that directly act on Go and two adaptor subunits: RGS Anchor Protein (R9AP) and the orphan receptor, GPR179. Here we examined the organizational principles of the GAP complex in ON-BCs. Biochemical experiments revealed that RGS7 binds to a conserved site in GPR179 and that RGS11 in vivo forms a complex only with R9AP. R9AP and GPR179 are further integrated via direct protein-protein interactions involving their cytoplasmic domains. Elimination of GPR179 prevents postsynaptic accumulation of R9AP. Furthermore, concurrent knock-out of both R9AP and RGS7 does not reconfigure the GAP complex and completely abolishes synaptic transmission, resulting in a novel mouse model of night blindness. Based on these results, we propose a model of hierarchical assembly and function of the GAP complex that supports ON-BCs visual signaling.

Heterotrimeric G protein signaling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a signalopathy of renal tubular epithelial cells caused by naturally occurring mutations in two distinct genes, polycystic kidney disease 1 (PKD1) and 2 (PKD2). Genetic variants in PKD1, which encodes the polycystin-1 (PC-1) protein, remain the predominant factor associated with the pathogenesis of nearly two-thirds of all patients diagnosed with PKD. Although the relationship between defective PC-1 with renal cystic disease initiation and progression remains to be fully elucidated, there are numerous clinical studies that have focused upon the control of effector systems involving heterotrimeric G protein regulation. A major regulator in the activation state of heterotrimeric G proteins are G protein-coupled receptors (GPCRs), which are defined by their seven transmembrane-spanning regions. PC-1 has been considered to function as an unconventional GPCR, but the mechanisms by which PC-1 controls signal processing, magnitude, or trafficking through heterotrimeric G proteins remains to be fully known. The diversity of heterotrimeric G protein signaling in PKD is further complicated by the presence of non-GPCR proteins in the membrane or cytoplasm that also modulate the functional state of heterotrimeric G proteins within the cell. Moreover, PC-1 abnormalities promote changes in hormonal systems that ultimately interact with distinct GPCRs in the kidney to potentially amplify or antagonize signaling output from PC-1. This review will focus upon the canonical and noncanonical signaling pathways that have been described in PKD with specific emphasis on which heterotrimeric G proteins are involved in the pathological reorganization of the tubular epithelial cell architecture to exacerbate renal cystogenic pathways.

Regulator of G Protein Signaling 7 (RGS7) Can Exist in a Homo-oligomeric Form That Is Regulated by Gαo and R7-binding Protein

RGS (regulator of G protein signaling) proteins of the R7 subfamily (RGS6, -7, -9, and -11) are highly expressed in neurons where they regulate many physiological processes. R7 RGS proteins contain several distinct domains and form obligatory dimers with the atypical Gβ subunit, Gβ5 They also interact with other proteins such as R7-binding protein, R9-anchoring protein, and the orphan receptors GPR158 and GPR179. These interactions facilitate plasma membrane targeting and stability of R7 proteins and modulate their activity. Here, we investigated RGS7 complexes using in situ chemical cross-linking. We found that in mouse brain and transfected cells cross-linking causes formation of distinct RGS7 complexes. One of the products had the apparent molecular mass of ∼150 kDa on SDS-PAGE and did not contain Gβ5 Mass spectrometry analysis showed no other proteins to be present within the 150-kDa complex in the amount close to stoichiometric with RGS7. This finding suggested that RGS7 could form a homo-oligomer. Indeed, co-immunoprecipitation of differentially tagged RGS7 constructs, with or without chemical cross-linking, demonstrated RGS7 self-association. RGS7-RGS7 interaction required the DEP domain but not the RGS and DHEX domains or the Gβ5 subunit. Using transfected cells and knock-out mice, we demonstrated that R7-binding protein had a strong inhibitory effect on homo-oligomerization of RGS7. In contrast, our data indicated that GPR158 could bind to the RGS7 homo-oligomer without causing its dissociation. Co-expression of constitutively active Gαo prevented the RGS7-RGS7 interaction. These results reveal the existence of RGS protein homo-oligomers and show regulation of their assembly by R7 RGS-binding partners.

Differential effects of the Gβ5-RGS7 complex on muscarinic M3 receptor-induced Ca2+ influx and release

The G protein β subunit Gβ5 uniquely forms heterodimers with R7 family regulators of G protein signaling (RGS) proteins (RGS6, RGS7, RGS9, and RGS11) instead of Gγ. Although the Gβ5-RGS7 complex attenuates Ca(2+) signaling mediated by the muscarinic M3 receptor (M3R), the route of Ca(2+) entry (i.e., release from intracellular stores and/or influx across the plasma membrane) is unknown. Here, we show that, in addition to suppressing carbachol-stimulated Ca(2+) release, Gβ5-RGS7 enhanced Ca(2+) influx. This novel effect of Gβ5-RGS7 was blocked by nifedipine and 2-aminoethoxydiphenyl borate. Experiments with pertussis toxin, an RGS domain-deficient mutant of RGS7, and UBO-QIC {L-threonine,(3R)-N-acetyl-3-hydroxy-L-leucyl-(aR)-a-hydroxybenzenepropanoyl-2,3-idehydro-N-methylalanyl-L-alanyl-N-methyl-L-alanyl-(3R)-3-[[(2S,3R)-3-hydroxy-4- methyl-1-oxo-2-[(1-oxopropyl)amino]pentyl]oxy]-L-leucyl-N,O-dimethyl-,(7→1)-lactone (9CI)}, a novel inhibitor of Gq, showed that Gβ5-RGS7 modulated a Gq-mediated pathway. These studies indicate that Gβ5-RGS7, independent of RGS7 GTPase-accelerating protein activity, couples M3R to a nifedipine-sensitive Ca(2+) channel. We also compared the action of Gβ5-RGS7 on M3R-induced Ca(2+) influx and release elicited by different muscarinic agonists. Responses to Oxo-M [oxotremorine methiodide N,N,N,-trimethyl-4-(2-oxo-1-pyrrolidinyl)-2-butyn-1-ammonium iodide] were insensitive to Gβ5-RGS7. Pilocarpine responses consisted of a large release and modest influx components, of which the former was strongly inhibited whereas the latter was insensitive to Gβ5-RGS7. McN-A-343 [(4-hydroxy-2-butynyl)-1-trimethylammonium-3-chlorocarbanilate chloride] was the only compound whose total Ca(2+) response was enhanced by Gβ5-RGS7, attributed to, in part, by the relatively small Ca(2+) release this partial agonist stimulated. Together, these results show that distinct agonists not only have differential M3R functional selectivity, but also confer specific sensitivity to the Gβ5-RGS7 complex.

Timing is everything: GTPase regulation in phototransduction

As the molecular mechanisms of vertebrate phototransduction became increasingly clear in the 1980s, a persistent problem was the discrepancy between the slow GTP hydrolysis catalyzed by the phototransduction G protein, transducin, and the much more rapid physiological recovery of photoreceptor cells from light stimuli. Beginning with a report published in 1989, a series of studies revealed that transducin GTPase activity could approach the rate needed to explain physiological recovery kinetics in the presence of one or more factors present in rod outer segment membranes. One by one, these factors were identified, beginning with PDEγ, the inhibitory subunit of the cGMP phosphodiesterase activated by transducin. There followed the discovery of the crucial role played by the regulator of G protein signaling, RGS9, a member of a ubiquitous family of GTPase-accelerating proteins, or GAPs, for heterotrimeric G proteins. Soon after, the G protein β isoform Gβ5 was identified as an obligate partner subunit, followed by the discovery or R9AP, a transmembrane protein that anchors the RGS9 GAP complex to the disk membrane, and is essential for the localization, stability, and activity of this complex in vivo. The physiological importance of all of the members of this complex was made clear first by knockout mouse models, and then by the discovery of a human visual defect, bradyopsia, caused by an inherited deficiency in one of the GAP components. Further insights have been gained by high-resolution crystal structures of subcomplexes, and by extensive mechanistic studies both in vitro and in animal models.

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.

Subcellular localization of regulator of G protein signaling RGS7 complex in neurons and transfected cells

The R7 family of regulators of G protein signaling (RGS) is involved in many functions of the nervous system. This family includes RGS6, RGS7, RGS9, and RGS11 gene products and is defined by the presence of the characteristic first found in Disheveled, Egl-10, Pleckstrin (DEP), DEP helical extension (DHEX), Gγ-like, and RGS domains. Herein, we examined the subcellular localization of RGS7, the most broadly expressed R7 member. Our immunofluorescence studies of retinal and dorsal root ganglion neurons showed that RGS7 concentrated at the plasma membrane of cell bodies, in structures resembling lamellipodia or filopodia along the processes, and at the dendritic tips. At the plasma membrane of dorsal root ganglia neurons, RGS7 co-localized with its known binding partners R7 RGS binding protein (R7BP), Gαo, and Gαq. More than 50% of total RGS7-specific immunofluorescence was present in the cytoplasm, primarily within numerous small puncta that did not co-localize with R7BP. No specific RGS7 or R7BP immunoreactivity was detected in the nuclei. In transfected cell lines, ectopic RGS7 had both diffuse cytosolic and punctate localization patterns. RGS7 also localized in centrosomes. Structure-function analysis showed that the punctate localization was mediated by the DEP/DHEX domains, and centrosomal localization was dependent on the DHEX domain.

Molecular organization of the complex between the muscarinic M3 receptor and the regulator of G protein signaling, Gbeta(5)-RGS7

The complex of the regulator of G protein signaling (RGS), Gbeta(5)-RGS7, can inhibit signal transduction via the M3 muscarinic acetylcholine receptor (M3R). RGS7 consists of three distinct structural entities: the DEP domain and its extension DHEX, the Ggamma-like (GGL) domain, which is permanently bound to Gbeta subunit Gbeta(5), and the RGS domain responsible for the interaction with Galpha subunits. Inhibition of the M3R by Gbeta(5)-RGS7 is independent of the RGS domain but requires binding of the DEP domain to the third intracellular loop of the receptor. Recent studies identified the dynamic intramolecular interaction between the Gbeta(5) and DEP domains, which suggested that the Gbeta(5)-RGS7 dimer could alternate between the "open" and "closed" conformations. Here, we identified point mutations that weaken DEP-Gbeta(5) binding, presumably stabilizing the open state, and tested their effects on the interaction of Gbeta(5)-RGS7 with the M3R. We found that these mutations facilitated binding of Gbeta(5)-RGS7 to the recombinant third intracellular loop of the M3R but did not enhance its ability to inhibit M3R-mediated Ca(2+) mobilization. This led us to the idea that the M3R can effectively induce the Gbeta(5)-RGS7 dimer to open; such a mechanism would require a region of the receptor distinct from the third loop. Indeed, we found that the C-terminus of M3R interacts with Gbeta(5)-RGS7. Truncation of the C-terminus rendered the M3R insensitive to inhibition by wild-type Gbeta(5)-RGS7; however, the open mutant of Gbeta(5)-RGS7 was able to inhibit signaling by the truncated M3R. The GST fusion of the M3R C-tail could not bind to wild-type Gbeta(5)-RGS7 but could associate with its open mutant as well as with the separated recombinant DEP domain or Gbeta(5). Taken together, our data are consistent with the following model: interaction of the M3R with Gbeta(5)-RGS7 causes the DEP domain and Gbeta(5) to dissociate from each other and bind to the C-tail, and the DEP domain also binds to the third loop, thereby inhibiting M3R-mediated signaling.

Structure, function, and localization of Gβ5-RGS complexes

Members of the R7 subfamily of regulator of G protein signaling (RGS) proteins (RGS6, 7, 9, and 11) exist as heterodimers with the G protein beta subunit Gβ5. These protein complexes are only found in neurons and are defined by the presence of three domains: DEP/DHEX, Gβ5/GGL, and RGS. This article summarizes published work in the following areas: (1) the functional significance of structural organization of Gβ5-R7 complexes, (2) regional distribution of Gβ5-R7 in the nervous system and regulation of R7 family expression, (3) subcellular localization of Gβ5-R7 complexes, and (4) novel binding partners of Gβ5-R7 proteins. The review points out some contradictions between observations made by different research groups and highlights the importance of using alternative experimental approaches to obtain conclusive information about Gβ5-R7 function in vivo.

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.

The Gbeta5-RGS7 complex selectively inhibits muscarinic M3 receptor signaling via the interaction between the third intracellular loop of the receptor and the DEP domain of RGS7

Regulators of G protein signaling (RGS) make up a diverse family primarily known as GTPase-activating proteins (GAPs) for heterotrimeric G proteins. In addition to the RGS domain, which is responsible for GAP activity, most RGS proteins contain other distinct structural motifs. For example, members of the R7 family of RGS proteins contain a DEP, GGL, and novel DHEX domain and are obligatory dimers with G protein beta subunit Gbeta5. Here we show that the Gbeta5-RGS7 complex can inhibit Ca2+ mobilization elicited by muscarinic acetylcholine receptor type 3 (M3R), but not by other Gq-coupled receptors such as M1, M5, histamine H1, and GNRH receptors. The isolated DEP domain of RGS7 is sufficient for the inhibition of M3R signaling, whereas the deletion of the DEP domain renders the Gbeta5-RGS7 complex ineffective. Deletion of a portion of the third intracellular loop allowed the receptor (M3R-short) to signal but rendered it insensitive to the effect of the Gbeta5-RGS7 complex. Accordingly, the recombinant DEP domain bound in vitro to the GST-fused i3 loop of the M3R. These results identify a novel molecular mechanism that can impart receptor subtype selectivity on signal transduction via Gq-coupled muscarinic receptors.

Postnatal induction and localization of R7BP, a membrane-anchoring protein for regulator of G protein signaling 7 family-Gbeta5 complexes in brain

Members of the regulator of G protein signaling 7 (RGS7) (R7) family and Gbeta5 form obligate heterodimers that are expressed predominantly in the nervous system. R7-Gbeta5 heterodimers are GTPase-activating proteins (GAPs) specific for Gi/o-class Galpha subunits, which mediate phototransduction in retina and the action of many modulatory G protein-coupled receptors (GPCRs) in brain. Here we have focused on the R7-family binding protein (R7BP), a recently identified palmitoylated protein that can bind R7-Gbeta5 complexes and is hypothesized to control the intracellular localization and function of the resultant heterotrimeric complexes. We show that: 1) R7-Gbeta5 complexes are obligate binding partners for R7BP in brain because they co-immunoprecipitate and exhibit similar expression patterns. Furthermore, R7BP and R7 protein accumulation in vivo requires Gbeta5. 2) Expression of R7BP in Neuro2A cells at levels approximating those in brain recruits endogenous RGS7-Gbeta5 complexes to the plasma membrane. 3) R7BP immunoreactivity in brain concentrates in neuronal soma, dendrites, spines or unmyelinated axons, and is absent or low in glia, myelinated axons, or axon terminals. 4) RGS7-Gbeta5-R7BP complexes in brain extracts associate inefficiently with detergent-resistant lipid raft fractions with or without G protein activation. 5) R7BP and Gbeta5 protein levels are upregulated strikingly during the first 2-3 weeks of postnatal brain development. Accordingly, we suggest that R7-Gbeta5-R7BP complexes in the mouse or rat could regulate signaling by modulatory Gi/o-coupled GPCRs in the developing and adult nervous systems.

R7-binding protein targets the G protein beta 5/R7-regulator of G protein signaling complex to lipid rafts in neuronal cells and brain

Background

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins), composed of Gα, Gβ, and Gγ subunits, are positioned at the inner face of the plasma membrane and relay signals from activated G protein-coupled cell surface receptors to various signaling pathways. Gβ5 is the most structurally divergent Gβ isoform and forms tight heterodimers with regulator of G protein signalling (RGS) proteins of the R7 subfamily (R7-RGS). The subcellular localization of Gβ 5/R7-RGS protein complexes is regulated by the palmitoylation status of the associated R7-binding protein (R7BP), a recently discovered SNARE-like protein. We investigate here whether R7BP controls the targeting of Gβ5/R7-RGS complexes to lipid rafts, cholesterol-rich membrane microdomains where conventional heterotrimeric G proteins and some effector proteins are concentrated in neurons and brain.

Results

We show that endogenous Gβ5/R7-RGS/R7BP protein complexes are present in native neuron-like PC12 cells and that a fraction is targeted to low-density, detergent-resistant membrane lipid rafts. The buoyant density of endogenous raft-associated Gβ5/R7-RGS protein complexes in PC12 cells was similar to that of lipid rafts containing the palmitoylated marker proteins PSD-95 and LAT, but distinct from that of the membrane microdomain where flotillin was localized. Overexpression of wild-type R7BP, but not its palmitoylation-deficient mutant, greatly enriched the fraction of endogenous Gβ5/R7-RGS protein complexes in the lipid rafts. In HEK-293 cells the palmitoylation status of R7BP also regulated the lipid raft targeting of co-expressed Gβ5/R7-RGS/R7BP proteins. A fraction of endogenous Gβ5/R7-RGS/R7BP complexes was also present in lipid rafts in mouse brain.

Conclusion

A fraction of Gβ5/R7-RGS/R7BP protein complexes is targeted to low-density, detergent-resistant membrane lipid rafts in PC12 cells and brain. In cultured cells, the palmitoylation status of R7BP regulated the lipid raft targeting of endogenous or co-expressed Gβ5/R7-RGS proteins. Taken together with recent evidence that the kinetic effects of the Gβ5 complex on GPCR signaling are greatly enhanced by R7BP palmitoylation through a membrane-anchoring mechanism, our data suggest the targeting of the Gβ5/R7-RGS/R7BP complex to lipid rafts in neurons and brain, where G proteins and their effectors are concentrated, may be central to the G protein regulatory function of the complex.

Intramolecular interaction between the DEP domain of RGS7 and the Gbeta5 subunit

The R7 family of RGS proteins (RGS6, -7, -9, -11) is characterized by the presence of three domains: DEP, GGL, and RGS. The RGS domain interacts with Galpha subunits and exhibits GAP activity. The GGL domain permanently associates with Gbeta5. The DEP domain interacts with the membrane anchoring protein, R7BP. Here we provide evidence for a novel interaction within this complex: between the DEP domain and Gbeta5. GST fusion of the RGS7 DEP domain (GST-R7DEP) binds to both native and recombinant Gbeta5-RGS7, recombinant Gbetagamma complexes, and monomeric Gbeta5 and Gbeta1 subunits. Co-immunoprecipitation and FRET assays supported the GST pull-down experiments. GST-R7DEP reduced FRET between CFP-Gbeta5 and YFP-RGS7, indicating that the DEP-Gbeta5 interaction is dynamic. In transfected cells, R7BP had no effect on the Gbeta5/RGS7 pull down by GST-R7DEP. The DEP domain of RGS9 did not bind to Gbeta5. Substitution of RGS7 Glu-73 and Asp-74 for the corresponding Ser and Gly residues (ED/SG mutation) of RGS9 diminished the DEP-Gbeta5 interaction. In the absence of R7BP both the wild-type RGS7 and the ED/SG mutant attenuated muscarinic M3 receptor-mediated Ca2+ mobilization. In the presence of R7BP, wild-type RGS7 lost this inhibitory activity, whereas the ED/SG mutant remained active. Taken together, our results are consistent with the following model. The Gbeta5-RGS7 molecule can exist in two conformations: "closed" and "open", when the DEP domain and Gbeta5 subunit either do or do not interact. The closed conformation appears to be less active with respect to its effect on Gq-mediated signaling than the open conformation.

Multi-tasking RGS proteins in the heart: the next therapeutic target?

Regulator of G-protein-signaling (RGS) proteins play a key role in the regulation of G-protein-coupled receptor (GPCR) signaling. The characteristic hallmark of RGS proteins is a conserved approximately 120-aa RGS region that confers on these proteins the ability to serve as GTPase-activating proteins (GAPs) for G(alpha) proteins. Most RGS proteins can serve as GAPs for multiple isoforms of G(alpha) and therefore have the potential to influence many cellular signaling pathways. However, RGS proteins can be highly regulated and can demonstrate extreme specificity for a particular signaling pathway. RGS proteins can be regulated by altering their GAP activity or subcellular localization; such regulation is achieved by phosphorylation, palmitoylation, and interaction with protein and lipid-binding partners. Many RGS proteins have GAP-independent functions that influence GPCR and non-GPCR-mediated signaling, such as effector regulation or action as an effector. Hence, RGS proteins should be considered multifunctional signaling regulators. GPCR-mediated signaling is critical for normal function in the cardiovascular system and is currently the primary target for the pharmacological treatment of disease. Alterations in RGS protein levels, in particular RGS2 and RGS4, produce cardiovascular phenotypes. Thus, because of the importance of GPCR-signaling pathways and the profound influence of RGS proteins on these pathways, RGS proteins are regulators of cardiovascular physiology and potentially novel drug targets as well.

The GAPs, GEFs, and GDIs of heterotrimeric G-protein alpha subunits

The heterotrimeric G-protein alpha subunit has long been considered a bimodal, GTP-hydrolyzing switch controlling the duration of signal transduction by seven-transmembrane domain (7TM) cell-surface receptors. In 1996, we and others identified a superfamily of “regulator of G-protein signaling” (RGS) proteins that accelerate the rate of GTP hydrolysis by Gα subunits (dubbed GTPase-accelerating protein or “GAP” activity). This discovery resolved the paradox between the rapid physiological timing seen for 7TM receptor signal transduction in vivo and the slow rates of GTP hydrolysis exhibited by purified Gα subunits in vitro. Here, we review more recent discoveries that have highlighted newly-appreciated roles for RGS proteins beyond mere negative regulators of 7TM signaling. These new roles include the RGS-box-containing, RhoA-specific guanine nucleotide exchange factors (RGS-RhoGEFs) that serve as Gα effectors to couple 7TM and semaphorin receptor signaling to RhoA activation, the potential for RGS12 to serve as a nexus for signaling from tyrosine kinases and G-proteins of both the Gα and Ras-superfamilies, the potential for R7-subfamily RGS proteins to couple Gα subunits to 7TM receptors in the absence of conventional Gβγ dimers, and the potential for the conjoint 7TM/RGS-box Arabidopsis protein AtRGS1 to serve as a ligand-operated GAP for the plant Gα AtGPA1. Moreover, we review the discovery of novel biochemical activities that also impinge on the guanine nucleotide binding and hydrolysis cycle of Gα subunits: namely, the guanine nucleotide dissociation inhibitor (GDI) activity of the GoLoco motif-containing proteins and the 7TM receptor-independent guanine nucleotide exchange factor (GEF) activity of Ric‑8/synembryn. Discovery of these novel GAP, GDI, and GEF activities have helped to illuminate a new role for Gα subunit GDP/GTP cycling required for microtubule force generation and mitotic spindle function in chromosomal segregation.

G-protein signaling: back to the future

Heterotrimeric G-proteins are intracellular partners of G-protein-coupled receptors (GPCRs). GPCRs act on inactive Galpha.GDP/Gbetagamma heterotrimers to promote GDP release and GTP binding, resulting in liberation of Galpha from Gbetagamma. Galpha.GTP and Gbetagamma target effectors including adenylyl cyclases, phospholipases and ion channels. Signaling is terminated by intrinsic GTPase activity of Galpha and heterotrimer reformation - a cycle accelerated by 'regulators of G-protein signaling' (RGS proteins). Recent studies have identified several unconventional G-protein signaling pathways that diverge from this standard model. Whereas phospholipase C (PLC) beta is activated by Galpha(q) and Gbetagamma, novel PLC isoforms are regulated by both heterotrimeric and Ras-superfamily G-proteins. An Arabidopsis protein has been discovered containing both GPCR and RGS domains within the same protein. Most surprisingly, a receptor-independent Galpha nucleotide cycle that regulates cell division has been delineated in both Caenorhabditis elegans and Drosophila melanogaster. Here, we revisit classical heterotrimeric G-protein signaling and explore these new, non-canonical G-protein signaling pathways.

Biochemical purification and functional analysis of complexes between the G-protein subunit Gbeta5 and RGS proteins

Regulator of G-protein signaling (RGS) proteins of the R7 subfamily (RGS6, 7, 9, and 11) contain a unique Ggamma-like (GGL) domain that enables their association with the G-protein beta subunit Gbeta5. The existence of these complexes was demonstrated by their purification from native tissues as well as by reconstitution in vitro. According to pulse-chase analysis, Gbeta5 and RGS7 monomers undergo rapid proteolytic degradation in cells, whereas the dimer is stable. Studies of the functional role of Gbeta5-RGS dimers using GTPase activity, ion channel, and calcium mobilization assays showed that, similarly to other RGS proteins, they can negatively regulate G-protein-mediated signal transduction. Protein-protein interactions involving the Gbeta5-RGS7 complex can be studied in cells using fluorescence resonance energy transfer utilizing Gbeta5, RGS, and Galpha subunits fused to the cyan and yellow versions of green fluorescent protein.

Enhancement of pheromone response by RGS9 and Gβ5 in yeast

The G-protein gamma-subunit-like (GGL) domain present within a subfamily of RGS proteins binds specifically to Gbeta5. This interaction and resulting biological effect impacts the standard model of heterotrimeric G-protein signaling. It has been hypothesized that the RGS/Gbeta5 may potentially substitute for Gbetagamma in the heterotrimeric complex. Saccharomyces cerevisiae pheromone responsive mating signaling pathway is primarily driven by Gbetagamma. We evaluated GGL containing RGS9 and RGS7 for functional complementation in a RGS (sst2Delta) knockout yeast strain. The potential of Gbeta5 to augment the function of these RGS proteins was also evaluated. While Gbeta5 had no effect on RGS7, coexpression of Gbeta5 with RGS9 enhanced cell cycle arrest, suggesting that under certain conditions, RGS9 and Gbeta5 may possibly function as betagamma dimer. Furthermore, we demonstrate that Gbeta5 can complement a ste4Delta, the yeast beta-subunit, thus providing the first evidence of functional complementation of a mammalian Gbeta.

Characterization of liver-cirrhosis nodules by analysis of gene-expression profiles and patterns of allelic loss

To disclose genetic mechanisms involved in development or progression of hepatocellular carcinoma (HCC), we used a genome-wide cDNA microarray consisting of 8,448 genes to compare gene-expression profiles among 12 liver-cirrhosis nodules (LCNs) and five specimens of HCC excised from a single patient and carefully prepared by laser-capture microdissection (LCM). The expression patterns enabled us to identify 72 genes that were frequently upregulated and 57 that were downregulated specifically in the LCN specimens as compared to the HCCs. We also documented upregulation of 31 genes and downregulation of seven others in both HCC and LCN tissues. Several types of intracellular kinase, including receptor-type kinase, were upregulated in LCNs. Expression patterns of HCCs and LCNs generally represented two genetically distinct groups when subjected to a hierarchical clustering analysis, although expression profiles of two of the LCNs resembled the HCC pattern. Analysis of allelic losses at microsatellite loci revealed that LCNs showed frequent loss of heterozygosity (LOH) (33%) in chromosomal regions 6q and 22q; over half of the LCNs had lost an allele for at least one of the 28 loci examined. The presence of early genetic changes among LCNs, with additional genetic changes occurring during formation of HCCs, suggests that hepatocellular carcinogenesis follows the multistep model established for colon cancers and that some LCNs may be precancerous lesions.

Selective expression of regulators of G-protein signaling (RGS) in the human central nervous system

The human tissue distribution of the nineteen known human regulators of G-protein signaling (RGS) is described. Measurement of RGS mRNA levels in human brain and in nine peripheral tissues revealed striking tissue preferences in gene expression. Five RGS members were identified with enriched expression in brain. RGS4, RGS7, RGS8, RGS11 and RGS17 were all significantly expressed in striatal regions including the nucleus accumbens and putamen. RGS4 had the highest measured levels of mRNA expression and was highly enriched in the gyrus of the cortex and in the parahippocampus. RGS7 and RGS17 had overlapping distribution profiles and were both noticeably enriched in the cerebellum. Several RGS family members showed high expression in peripheral tissues. RGS5 was preferentially expressed in heart, and RGS1, RGS13, RGS18 and GAIP were predominately expressed in lymphocytes. RGS1 was also highly enriched in the lung, as was RGS2 and RGS16. Five family members, RGS3, RGS9, RGS10, RGS 12 and RGS14 had a broad and overlapping mRNA distribution. These results suggest roles of the individual RGS members in a diversity of functions in humans and support a role of several RGS members in the regulation of central nervous system function via modulation of signaling by G-protein coupled receptors.

Human RGS6 gene structure, complex alternative splicing, and role of N terminus and G protein gamma-subunit-like (GGL) domain in subcellular localization of RGS6 splice variants

RGS proteins are defined by the presence of a semiconserved RGS domain that confers the GTPase-activating activity of these proteins toward certain G alpha subunits. RGS6 is a member of a subfamily of RGS proteins distinguished by the presence of DEP and GGL domains, the latter a G beta 5-interacting domain. Here we report identification of 36 distinct transcripts of human RGS6 that arise by unusually complex processing of the RGS6 gene, which spans 630 kilobase pairs of genomic DNA in human chromosome 14 and is interrupted by 19 introns. These transcripts arise by use of two alternative transcription sites and complex alternative splicing mechanisms and encode proteins with long or short N-terminal domains, complete or incomplete GGL domains, 7 distinct C-terminal domains and a common internal domain where the RGS domain is found. The role of structural diversity in the N-terminal and GGL domains of RGS6 splice variants in their interaction with G beta 5 and subcellular localization and of G beta 5 on RGS6 protein localization was examined in COS-7 cells expressing various RGS6 splice variant proteins. RGS6 splice variants with complete GGL domains interacted with G beta 5, irrespective of the type of N-terminal domain, while those lacking a complete GGL domain did not. RGS6 protein variants displayed subcellular distribution patterns ranging from an exclusive cytoplasmic to exclusive nuclear/nucleolar localization, and co-expression of G beta 5 promoted nuclear localization of RGS6 proteins. Analysis of our results show that the long N-terminal and GGL domain sequences of RGS6 proteins function as cytoplasmic retention sequences to prevent their nuclear/nucleolar accumulation. These findings provide the first evidence for G beta 5-independent functions of the GGL domain and for a role of G beta 5 in RGS protein localization. This study reveals extraordinary complexity in processing of the human RGS6 gene and provides new insights into how structural diversity in the RGS6 protein family is involved in their localization and likely function(s) in cells.

RGS3 mediates a calcium-dependent termination of G protein signaling in sensory neurons

G proteins modulate synaptic transmission. Regulators of G protein signaling (RGS) proteins accelerate the intrinsic GTPase activity of Galpha subunits, and thus terminate G protein activation. Whether RGS proteins themselves are under cellular control is not well defined, particularly in native cells. In dorsal root ganglion neurons overexpressing RGS3, we find that G protein signaling is rapidly terminated (or "desensitized") by calcium influx through voltage-gated channels. This rapid desensitization is most likely mediated by direct binding of calcium to RGS3, as deletion of an EF-hand domain in RGS3 abolishes both the desensitization (observed physiologically) and a calcium-RGS3 interaction (observed in a gel-shift assay). A naturally occurring variant of RGS3 that lacks the EF hand neither binds calcium nor produces rapid desensitization, giving rise instead to a slower calcium-dependent desensitization that is attenuated by a calmodulin antagonist. Thus, activity-evoked calcium entry in sensory neurons may provide differential control of G protein signaling, depending on the isoform of RGS3 expressed in the cells. In complex neural circuits subjected to abundant synaptic inhibition by G proteins (as occurs in dorsal spinal cord), rapid termination of inhibition by electrical activity by EF hand-containing RGS3 may ensure the faithful transmission of information from the most active sensory inputs.

G beta 5.RGS7 inhibits G alpha q-mediated signaling via a direct protein-protein interaction

A subfamily of regulators of G protein signaling (RGS) proteins consisting of RGS6, -7, -9, and -11 is characterized by the presence of a unique Ggamma-like domain through which they form obligatory dimers with the G protein subunit Gbeta5 in vivo. In Caenorhabditis elegans, orthologs of Gbeta5.RGS dimers are implicated in regulating both Galphai and Galphaq signaling, and in cell-based assays these dimers regulate Galphai/o- and Galphaq/11-mediated pathways. However, initial studies with purified Gbeta5.RGS6 or Gbeta5.RGS7 showed that they only serve as GTPase activating proteins for Galphao. Pull-down assays and co-immunoprecipitation with these dimers failed to detect their binding to either Galphao or Galphaq, indicating that the interaction might require additional factors present in vivo. Here, we asked if the RGS7.Gbeta5 complex binds to Galphaq using fluorescence resonance energy transfer (FRET) in transiently transfected mammalian cells. RGS7, Gbeta5, and Galpha subunits were tagged with yellow variants of green fluorescent protein. First we confirmed the functional activity of the fusion proteins by co-immunoprecipitation and also their effect on signaling. Second, we again demonstrate the interaction between RGS7 and Gbeta5 using FRET. Finally, using both FRET spectroscopy on cell suspensions and microscopy of individual cells, we showed FRET between the yellow fluorescence protein-tagged RGS7.Gbeta5 complex and cyan fluorescence protein-tagged Galphaq, indicating a direct interaction between these molecules.

Instability of GGL domain-containing RGS proteins in mice lacking the G protein beta-subunit Gbeta5

RGS (regulator of G protein signaling) proteins containing the G protein gamma-like (GGL) domain (RGS6, RGS7, RGS9, and RGS11) interact with the fifth member of the G protein beta-subunit family, Gbeta5. This interaction is necessary for the stability of both the RGS protein and for Gbeta5. Consistent with this notion, we have found that elevation of RGS9-1 mRNA levels by transgene expression does not increase RGS9-1 protein level in the retina, suggesting that Gbeta5 levels may be limiting. To examine further the interactions of Gbeta5 and the GGL domain-containing RGS proteins, we inactivated the Gbeta5 gene. We found that the levels of GGL domain-containing RGS proteins in retinas and in striatum are eliminated or reduced drastically, whereas the levels of Ggamma2 and RGS4 proteins remain normal in the absence of Gbeta5. The homozygous Gbeta5 knockout (Gbeta5-/-) mice derived from heterozygous knockout mating are runty and exhibit a high preweaning mortality rate. We concluded that complex formation between GGL domain-containing RGS proteins and the Gbeta5 protein is necessary to maintain their mutual stability in vivo. Furthermore, in the absence of Gbeta5 and all four RGS proteins that form protein complexes with Gbeta5, the animals that survive into adulthood are viable and have no gross defects in brain or retinal morphology.

Ggamma subunit-selective G protein beta 5 mutant defines regulators of G protein signaling protein binding requirement for nuclear localization

The signal transducing function of Gbeta(5) in brain is unknown. When studied in vitro Gbeta(5) is the only heterotrimeric Gbeta subunit known to interact with both Ggamma subunits and regulators of G protein signaling (RGS) proteins. When tested with Ggamma, Gbeta(5) interacts with other classical components of heterotrimeric G protein signaling pathways such as Galpha and phospholipase C-beta. We recently demonstrated nuclear expression of Gbeta(5) in neurons and brain (Zhang, J. H., Barr, V. A., Mo, Y., Rojkova, A. M., Liu, S., and Simonds, W. F. (2001) J. Biol. Chem. 276, 10284-10289). To gain further insight into the mechanism of Gbeta(5) nuclear localization, we generated a Gbeta(5) mutant deficient in its ability to interact with RGS7 while retaining its ability to bind Ggamma, and we compared its properties to the wild-type Gbeta(5). In HEK-293 cells co-transfection of RGS7 but not Ggamma(2) supported expression in the nuclear fraction of transfected wild-type Gbeta(5). In contrast the Ggamma-preferring Gbeta(5) mutant was not expressed in the HEK-293 cell nuclear fraction with either co-transfectant. The Ggamma-selective Gbeta(5) mutant was also excluded from the cell nucleus of transfected PC12 cells analyzed by laser confocal microscopy. These results define a requirement for RGS protein binding for Gbeta(5) nuclear expression.

Identification of protein kinase C isozymes responsible for the phosphorylation of photoreceptor-specific RGS9-1 at Ser475

Inactivation of the visual G-protein transducin by GTP hydrolysis is regulated by the GTPase-accelerating protein (GAP) RGS9-1. Regulation of RGS9-1 itself is poorly understood, but we found previously that it is subject to a light- and Ca(2+)-sensitive phosphorylation on Ser(475). Because there are much higher RGS9-1 levels in cones than in rods, we investigated whether Ser(475) is phosphorylated in rods using Coneless mice and found that both the phosphorylation and its regulation by light occur in rods. Therefore, we used rod outer segments as the starting material for the purification of RGS9-1 kinase activity. Two major peaks of activity corresponded to protein kinase C (PKC) isozymes, PKCalpha and PKCtheta. A synthetic peptide corresponding to the Ser(475) RGS9-1 sequence and RGS9-1 were substrates for recombinant PKCalpha and PKCtheta. This phosphorylation was removed efficiently by protein phosphatase 2A, an endogenous phosphatase in rod outer segments, but not by PP1 or PP2B. Phosphorylation of RGS9-1 by PKC had little effect on its activity in solution but significantly decreased its affinity for its membrane anchor protein and GAP enhancer, RGS9-1 anchor protein (R9AP). PKCtheta immunostaining was at higher levels in cone outer segments than in rod outer segments, as was found for the components of the RGS9-1 GAP complex. Thus, PKC-mediated phosphorylation of RGS9-1 represents a potential mechanism for feedback control of the kinetics of photoresponse recovery in both rods and cones, with this mechanism probably especially important in cones.

Regulators of G-protein signalling: multifunctional proteins with impact on signalling in the cardiovascular system

Regulator of G-protein signalling (RGS) proteins form a superfamily of at least 25 proteins, which are highly diverse in structure, expression patterns, and function. They share a 120 amino acid homology domain (RGS domain), which exhibits GTPase accelerating activity for alpha-subunits of heterotrimeric G-proteins, and thus, are negative regulators of G-protein-mediated signalling. Based on the organisation of the Rgs genes, structural similarities, and differences in functions, they can be divided into at least six subfamilies of RGS proteins and three more families of RGS-like proteins. Many of these proteins regulate signalling processes within cells, not only via interaction with G-protein alpha-subunits, but are G-protein-regulated effectors, Gbetagamma scavenger, or scaffolding proteins in signal transduction complexes as well. The expression of at least 16 different RGS proteins in the mammalian or human myocardium have been described. A subgroup of at least eight was detected in a single atrial myocyte. The exact functions of these proteins remain mostly elusive, but RGS proteins such as RGS4 are involved in the regulation of G(i)-protein betagamma-subunit-gated K(+) channels. An up-regulation of RGS4 expression has been consistently found in human heart failure and some animal models. Evidence is increasing that the enhanced RGS4 expression counter-regulates the G(q/11)-induced signalling caused by hypertrophic stimuli. In the vascular system, RGS5 seems to be an important signalling regulator. It is expressed in vascular endothelial cells, but not in cultured smooth muscle cells. Its down-regulation, both in a model of capillary morphogenesis and in an animal model of stroke, render it a candidate gene, which may be involved in the regulation of capillary growth, angiogenesis, and in the pathophysiology of stroke.

Gating properties of GIRK channels activated by Galpha(o)- and Galpha(i)-coupled muscarinic m2 receptors in Xenopus oocytes: the role of receptor precoupling in RGS modulation

'Regulators of G protein Signalling' (RGSs) accelerate the activation and deactivation kinetics of G protein-gated inwardly rectifying K(+) (GIRK) channels. In an apparent paradox, RGSs do not reduce steady-state GIRK current amplitudes as expected from the accelerated rate of deactivation when reconstituted in Xenopus oocytes. We present evidence here that this kinetic anomaly is dependent on the degree of G protein-coupled receptor (GPCR) precoupling, which varies with different Galpha(i/o)-RGS complexes. The gating properties of GIRK channels (Kir3.1/Kir3.2a) activated by muscarinic m2 receptors at varying levels of G protein expression were examined with or without the co-expression of either RGS4 or RGS7 in Xenopus oocytes. Different levels of specific m2 receptor-Galpha coupling were established by uncoupling endogenous pertussis toxin (PTX)-sensitive Galpha(i/o) subunits with PTX, while expressing varying amounts of a single PTX-insensitive subunit (Galpha(i1(C351G)), Galpha(i2(C352G)), Galpha(i3(C351G)), Galpha(oA(C351G)), or Galpha(oB(C351G))). Co-expression of each of the PTX-insensitive Galpha(i/o) subunits rescued acetylcholine (ACh)-elicited GIRK currents (I(K,ACh)) in a concentration-dependent manner, with Galpha(o) isoforms being more effective than Galpha(i) isoforms. Receptor-independent 'basal' GIRK currents (I(K,basal)) were reduced with increasing expression of PTX-insensitive Galpha subunits and were accompanied by a parallel rise in I(K,ACh). These effects together are indicative of increased Gbetagamma scavenging by the expressed Galpha subunit and the subsequent formation of functionally coupled m2 receptor-G protein heterotrimers (Galpha((GDP))betagamma). Co-expression of RGS4 accelerated all the PTX-insensitive Galpha(i/o)-coupled GIRK currents to a similar extent, yet reduced I(K,ACh) amplitudes 60-90 % under conditions of low Galpha(i/o) coupling. Kinetic analysis indicated the RGS4-dependent reduction in steady-state GIRK current was fully explained by the accelerated deactivation rate. Thus kinetic inconsistencies associated with RGS4-accelerated GIRK currents occur at a critical threshold of G protein coupling. In contrast to RGS4, RGS7 selectively accelerated Galpha(o)-coupled GIRK currents. Co-expression of Gbeta5, in addition to enhancing the kinetic effects of RGS7, caused a significant reduction (70-85 %) in steady-state GIRK currents indicating RGS7-Gbeta5 complexes disrupt Galpha(o) coupling. Altogether these results provide further evidence for a GPCR-Galphabetagamma-GIRK signalling complex that is revealed by the modulatory affects of RGS proteins on GIRK channel gating. Our functional experiments demonstrate that the formation of this signalling complex is markedly dependent on the concentration and composition of G protein-RGS complexes.

RGS6 interacts with SCG10 and promotes neuronal differentiation. Role of the G gamma subunit-like (GGL) domain of RGS6

RGS proteins comprise a large family of proteins named for their ability to negatively regulate heterotrimeric G protein signaling. RGS6 is a member of the R7 RGS protein subfamily endowed with DEP (disheveled, Egl-10, pleckstrin) and GGL (G protein gamma subunit-like) domains in addition to the RGS domain present in all RGS proteins. RGS6 exists in multiple splice variant forms with identical RGS domains but possessing complete or incomplete GGL domains and distinct N- and C-terminal domains. Here we report that RGS6 interacts with SCG10, a neuronal growth-associated protein. Using yeast two-hybrid analysis to map protein interaction domains, we identified the GGL domain of RGS6 as the SCG10-interacting region and the stathmin domain of SCG10 as the RGS6-interacting region. Pull-down studies in COS-7 cells expressing SCG10 and RGS6 splice variants revealed that SCG10 co-precipitated RGS6 proteins with complete GGL domains but not those with incomplete GGL domains, and vice versa. Expression of SCG10-interacting forms of RGS6 with SCG10 in PC12 or COS-7 cells resulted in co-localization of both proteins. RGS6 potentiated the ability of SCG10 to disrupt microtubule organization in PC12 and COS-7 cells. Furthermore, expression of SCG10 and RGS6 each enhanced NGF-induced PC12 cell differentiation, and co-expression of SCG10 with RGS6 produced synergistic effects on NGF-induced PC12 differentiation. These effects of RGS6 on microtubules and neuronal differentiation were observed only with RGS6 proteins with complete GGL domains. Mutation of a critical residue required for interaction of RGS proteins with G proteins did not affect the ability of RGS6 to induce neuronal differentiation. These findings identify SCG10 as a binding partner for the GGL domain of RGS6 and provide the first evidence for regulatory effects of an RGS protein on neuronal differentiation. Our results suggest that RGS6 induces neuronal differentiation by a novel mechanism involving interaction of SCG10 with its GGL domain and independent of RGS6 interactions with heterotrimeric G proteins.

G proteins and phototransduction

Phototransduction is the process by which a photon of light captured by a molecule of visual pigment generates an electrical response in a photoreceptor cell. Vertebrate rod phototransduction is one of the best-studied G protein signaling pathways. In this pathway the photoreceptor-specific G protein, transducin, mediates between the visual pigment, rhodopsin, and the effector enzyme, cGMP phosphodiesterase. This review focuses on two quantitative features of G protein signaling in phototransduction: signal amplification and response timing. We examine how the interplay between the mechanisms that contribute to amplification and those that govern termination of G protein activity determine the speed and the sensitivity of the cellular response to light.

Spinal cord injury induces expression of RGS7 in microglia/macrophages in rats

RGS proteins regulate G protein-mediated signalling pathways through direct interaction with the Galpha subunits and facilitation of GTP hydrolysis. An RGS subfamily consisting of RGS 6, 7, 9, and 11 also interacts with the G protein beta subunit Gbeta5 via a characteristic Ggamma-like domain. Thus far, these complexes were found only in neurons, with RGS7 being the most widely distributed in the brain. Here we confirm the expression of RGS7 in spinal neurons and show as a novel finding that following an experimental spinal cord injury in rats, expression of RGS7 is induced in a subpopulation of other cells. Immunofluorescent confocal microscopy using a series of cell specific antibodies identified these RGS7 positive cells as activated microglia and/or invading peripheral macrophages. To rule out interference from the adjacent neurons and confirm the presence of RGS7-Gbeta5 complex in inflammatory cells, we performed immunocytochemistry, RT-PCR, Western blot, and immunoprecipitation using microglial (BV2) and peripheral macrophage (RAW) cell lines. Expression of RGS7 mRNA and protein are nearly undetectable in non-stimulated BV2 and RAW cells, but remarkably increased after stimulation with LPS or TNF-alpha In addition, RGS7-positive cells were also found in the perinodular rim in the rat spleen. Our findings show that RGS7-Gbeta5 complex is expressed in immunocompetent cells such as resident microglia and peripheral macrophages following spinal cord injury. This expression might contribute to the post-traumatic inflammatory responses in the central nervous system.

Inhibition of somatostatin receptor 5-signaling by mammalian regulators of G-protein signaling (RGS) in yeast

Regulators of G-protein signaling (RGSs) are negative regulators of G-protein coupled receptor (GPCR)-mediated signaling that function to limit the lifetime of receptor-activated G(alpha)-proteins. Here we show that four mammalian RGSs differentially inhibit the activation of a FUS1--LacZ reporter gene by the STE2 encoded GPCR in yeast. In order to examine the role of the GPCR in modulating RGS function, we functionally expressed the human somatostatin receptor 5 (SST(5)) in yeast. In the absence of RGSs, FUS1--LacZ activation in response to somatostatin increased in a dose-dependent manner in cells expressing SST(5). In contrast to the results obtained with Ste2p, all RGSs completely inhibited SST(5)-mediated signaling even at concentrations of agonist as high as 10(minus sign5) M. The ability of RGSs to inhibit SST(5) signaling was further assessed in cells expressing modified Gpa1 proteins. Even though SST(5)-mediated FUS1--LacZ activation was 5-fold more efficient with a Gpa1p/G(i3alpha) chimera, response to somatostatin was completely abolished by all four RGSs. Furthermore, we demonstrate that RGS1, RGS2 and RGS5 have reduced ability to inhibit SST(5)-mediated activation of the RGS-resistant Gpa1p(Gly302Ser) mutant suggesting that the ability to interact with the G(alpha)-protein is required for the inhibition of signaling. Taken together, our results indicate that RGSs serve as better GAPs for Gpa1p when activated by SST(5) than when this G-protein is activated by Ste2p.

A novel role of RGS9: inhibition of retinal guanylyl cyclase

Cyclic GMP plays a key role in retinal phototransduction and its photoreceptor concentration is precisely controlled by the cooperative action of cGMP phosphodiesterase (PDE) and retinal guanylyl cyclase (retGC). However, studies of the relationship between these two systems have focused only on a Ca2+-mediated, indirect connection. This article summarizes our studies strongly suggesting that RGS9-1 is directly involved in the cooperative action of PDE and retGC, and that this ingenious mechanism plays an important role in tuning of cGMP concentration in photoreceptors.

RGS9-G beta 5 substrate selectivity in photoreceptors. Opposing effects of constituent domains yield high affinity of RGS interaction with the G protein-effector complex

RGS proteins regulate the duration of G protein signaling by increasing the rate of GTP hydrolysis on G protein alpha subunits. The complex of RGS9 with type 5 G protein beta subunit (G beta 5) is abundant in photoreceptors, where it stimulates the GTPase activity of transducin. An important functional feature of RGS9-G beta 5 is its ability to activate transducin GTPase much more efficiently after transducin binds to its effector, cGMP phosphodiesterase. Here we show that different domains of RGS9-G beta 5 make opposite contributions toward this selectivity. G beta 5 bound to the G protein gamma subunit-like domain of RGS9 acts to reduce RGS9 affinity for transducin, whereas other structures restore this affinity specifically for the transducin-phosphodiesterase complex. We suggest that this mechanism may serve as a general principle conferring specificity of RGS protein action.

Phosphorylation and nuclear translocation of a regulator of G protein signaling (RGS10)

Heterotrimeric G proteins are involved in the transduction of hormonal and sensory signals across plasma membranes of eukaryotic cells. Hence, they are a critical point of control for a variety of agents that modulate cellular function. Activation of these proteins is dependent on GTP binding to their alpha (Galpha) subunits. Regulators of G protein signaling (RGS) bind specifically to activated Galpha proteins, potentiating the intrinsic GTPase activity of the Galpha proteins and thus expediting the termination of Galpha signaling. Although there are several points in most G protein controlled signaling pathways that are affected by reversible covalent modification, little evidence has been shown addressing whether or not the functions of RGS proteins are themselves regulated by such modifications. We report in this study the acute functional regulation of RGS10 thru the specific and inducible phosphorylation of RGS10 protein at serine 168 by cAMP-dependent kinase A. This phosphorylation nullifies the RGS10 activity at the plasma membrane, which controls the G protein-dependent activation of the inwardly rectifying potassium channel. Surprisingly, the phosphorylation-mediated attenuation of RGS10 activity was not manifested in an alteration of its ability to accelerate GTPase activity of Galpha. Rather, the phosphorylation event correlates with translocation of RGS10 from the plasma membrane and cytosol into the nucleus.

Inhibition of retinal guanylyl cyclase by the RGS9-1 N-terminus

Cyclic GMP plays a key role in retinal phototransduction and its photoreceptor concentration is precisely controlled by the cooperative action of cGMP phosphodiesterase (PDE) and retinal guanylyl cyclase (retGC). However, studies of the relationship between these two systems have focused only on a Ca(2+)-mediated, indirect connection. Using a retinal "regulator of G-protein signaling" (RGS9-1) and its fragments, we show that the N-terminus of RGS9-1 inhibits retGC activity. We also indicate that the GGL domain and/or the RGS domain function as an internal suppressor against the N-terminus, suggesting that proteins bound to these domains regulate the inhibitory activity of the N-terminus. Direct interaction of retGC with RGS9-1 and its N-terminus is also proved by immunoprecipitation and an overlay technique. Since RGS9-1 also controls the lifetime of transducin-activated PDE through regulating GTPase activity of transducin, this study strongly suggests that RGS9-1 mediates the direct interaction between PDE and retGC systems, and that this ingenious mechanism plays an important role in tuning of cGMP concentration in photoreceptors.

Suppression of cellular invasion by activated G-protein subunits Galphao, Galphai1, Galphai2, and Galphai3 and sequestration of Gbetagamma

It was shown previously that platelet-activating factor receptors (PAF-Rs) inhibit invasiveness of colonic and kidney epithelial cells induced by the src and Met oncogenes via a pertussis toxin-sensitive mechanism. Therefore, Madin-Darby canine kidney (MDCKts.src) cells were stably transfected with constitutively activated forms of Galphao, Galphai1, Galphai2, Galphai3 (AGalphao/i), two Gbetagamma sequestering proteins [C-terminal end of beta-adrenergic receptor kinase (ct-betaARK) and the Galphat subunit of retinal G-protein transducin], and Gbeta1-Ggamma2 subunits alone or in combination. Cellular invasion induced by src, Met, and leptin was abrogated by the AGalphao/i, ct-betaARK, and Galphat-positive clones, but was induced by coexpression of Gbeta1gamma2. In contrast, invasion stimulated by the trefoil factors (TFFs) pS2 and intestinal trefoil factor in MDCKts.src cells or human colonic epithelial cells PCmsrc and HCT8/S11 was insensitive to PAF, AGalphao, AGalphai1, and AGalphai2, but was abolished by AGalphai3 and the protease-activated receptor-1 (PAR-1) agonist thrombin receptor-activating peptide. Depletion of free Gbetagamma heterodimers by ct-betaARK resulted in a remarkable decrease of cellular adhesion and spreading on collagen matrix. Our data demonstrate the following: 1) PAF-Rs impair cellular invasion induced by src, Met, and leptin via the activation of Galphao and Galphai1 to -3; 2) invasion induced by TFFs is selectively inhibited by PAR-1 receptors and Galphai3 activation; and 3) Gbetagamma dimers are required as positive effectors of invasion pathways induced by oncogenes and epigenetic factors. Thus, redistribution of Galphao/Galphai and Gbeta/gamma heterotrimeric G-proteins by PAF-R and PAR-1 exert differential functions on positive and negative signaling pathways involved in cellular invasion and may serve as potential targets for anticancer therapy.

Different regulation of RGS2 mRNA by haloperidol and clozapine

Expression of RGS2 mRNA was transiently up-regulated in rat striatum (25% in the medial part and 50% in the lateral part), in contrast to cingulate cortex and lateral septum, 30 min after acute treatment with haloperidol (2 mg/kg, i.p.). This effect disappeared 24 hours post-drug treatment, similar to the acute and strong up-regulation (700% at 30 min) of c-fos mRNA. RGS3, 5, 6, 8 or 9 mRNAs were not affected. Clozapine (20 mg/kg, i.p.) at an approximately equivalent dose of D2 receptor occupancy in the striatum did not significantly affect RGS and c-fos mRNAs levels. We suggest that RGS2 mRNA expression may be differently up-regulated in a region-specific manner by antipsychotics.