The regulator of G protein signaling RGS4 selectively enhances alpha 2A-adreoreceptor stimulation of the GTPase activity of Go1alpha and Gi2alpha

Host-induced gene silencing of a regulator of G protein signalling gene (VdRGS1) confers resistance to Verticillium wilt in cotton

Verticillium wilt (VW), caused by soil-borne fungi of the genus Verticillium, is a serious disease affecting a wide range of plants and leading to a constant and major challenge to agriculture worldwide. Cotton (Gossypium hirsutum) is the world's most important natural textile fibre and oil crop. VW of cotton is a highly devastating vascular disease; however, few resistant germplasms have been reported in cotton. An increasing number of studies have shown that RNA interference (RNAi)-based host-induced gene silencing (HIGS) is an effective strategy for improving plant resistance to pathogens by silencing genes essential for the pathogenicity of these pathogens. Here, we have identified and characterized multifunctional regulators of G protein signalling (RGS) in the Verticillium dahliae virulence strain, Vd8. Of eight VdRGS genes, VdRGS1 showed the most significant increase in expression in V. dahliae after treating with the roots of cotton seedlings. Based on the phenotype detection of VdRGS1 deletion and complementation mutants, we found that VdRGS1 played crucial roles in spore production, hyphal development, microsclerotia formation and pathogenicity. Tobacco rattle virus-mediated HIGS in cotton plants silenced VdRGS1 transcripts in invaded V. dahliae strains and enhanced broad-spectrum resistance to cotton VW. Our data demonstrate that VdRGS1 is a conserved and essential gene for V. dahliae virulence. HIGS of VdRGS1 provides effective control against V. dahliae infection and could obtain the durable disease resistance in cotton and in other VW-susceptible host crops by developing the stable transformants.

Intrathecal RGS4 inhibitor, CCG50014, reduces nociceptive responses and enhances opioid-mediated analgesic effects in the mouse formalin test

BACKGROUND

The regulator of G-protein signaling protein type 4 (RGS4) accelerates the guanosine triphosphatase activity of G(αi) and G(αo), resulting in the inactivation of G-protein-coupled receptor signaling. An opioid receptor (OR), a G(αi)-coupled receptor, plays an important role in pain modulation in the central nervous system. In this study, we examined whether (1) spinal RGS4 affected nociceptive responses in the formalin pain test, (2) this RGS4-mediated effect was involved in OR activation, and (3) the µ-OR agonist-induced antinociceptive effect was modified by RGS4 modulation.

METHODS

Formalin (1%, 20 µL) was injected subcutaneously into the right hindpaws of male 129S4/SvJae×C57BL/6J (RGS4(+/+) or RGS4(-/-)) mice, and the licking responses were counted for 40 minutes. The time periods (seconds) spent licking the injected paw during 0 to 10 minutes (early phase) and 10 to 40 minutes (late phase) were measured as indicators of acute nociception and inflammatory pain response, respectively. An RGS4 inhibitor, CCG50014, and/or a µ-OR agonist, [D-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO), were intrathecally injected 5 minutes before the formalin injection. A nonselective OR antagonist, naloxone, was intraperitoneally injected 30 minutes before the CCG50014 injection.

RESULTS

Mice that received the formalin injection exhibited typical biphasic nociceptive behaviors. The nociceptive responses in RGS4-knockout mice were significantly decreased during the late phase but not during the early phase. Similarly, intrathecally administered CCG50014 (10, 30, or 100 nmol) attenuated the nociceptive responses during the late phase in a dose-dependent manner. The antinociceptive effect of the RGS4 inhibitor was totally blocked by naloxone (5 mg/kg). In contrast, intrathecal injection of DAMGO achieved a dose-dependent reduction of the nociceptive responses at the early and late phases. This analgesic effect of DAMGO was significantly enhanced by the genetic depletion of RGS4 or by coadministration of CCG50014 (10 nmol).

CONCLUSIONS

These findings demonstrated that spinal RGS4 inhibited the endogenous or exogenous OR-mediated antinociceptive effect in the formalin pain test. Thus, the inhibition of RGS4 activity can enhance OR agonist-induced analgesia. The enhancement of OR agonist-induced analgesia by coadministration of the RGS4 inhibitor suggests a new therapeutic strategy for the management of inflammatory pain.

The GABAB receptor associates with regulators of G-protein signaling 4 protein in the mouse prefrontal cortex and hypothalamus

Regulators of G-protein signaling (RGS) proteins regulate certain G-protein-coupled receptor (GPCR)-mediated signaling pathways. The GABA_B receptor (GABA_BR) is a GPCR that plays a role in the stress response. Previous studies indicate that acute immobilization stress (AIS) decreases RGS4 in the prefrontal cortex (PFC) and hypothalamus (HY) and suggest the possibility of a signal complex composed of RGS4 and GABA_BR. Therefore, in the present study, we tested whether RGS4 associates with GABA_BR in these brain regions. We found the co-localization of RGS4 and GABA_BR subtypes in the PFC and HY using double immunohistochemistry and confirmed a direct association between GABA_B2R and RGS4 proteins using co-immunoprecipitation. Furthermore, we found that AIS decreased the amount of RGS4 bound to GABA_B2R and the number of double-positive cells. These results indicate that GABA_BR forms a signal complex with RGS4 and suggests that RGS4 is a regulator of GABA_BR. [BMB Reports 2014; 47(6): 324-329]

Regulator of G protein signaling 4 [corrected] is a crucial modulator of antidepressant drug action in depression and neuropathic pain models

Regulator of G protein signaling 4 (Rgs4) is a signal transduction protein that controls the function of monoamine, opiate, muscarinic, and other G protein-coupled receptors via interactions with Gα subunits. Rgs4 is expressed in several brain regions involved in mood, movement, cognition, and addiction and is regulated by psychotropic drugs, stress, and corticosteroids. In this study, we use genetic mouse models and viral-mediated gene transfer to examine the role of Rgs4 in the actions of antidepressant medications. We first analyzed human postmortem brain tissue and found robust up-regulation of RGS4 expression in the nucleus accumbens (NAc) of subjects receiving standard antidepressant medications that target monoamine systems. Behavioral studies of mice lacking Rgs4, including specific knockdowns in NAc, demonstrate that Rgs4 in this brain region acts as a positive modulator of the antidepressant-like and antiallodynic-like actions of several monoamine-directed antidepressant drugs, including tricyclic antidepressants, selective serotonin reuptake inhibitors, and norepinephrine reuptake inhibitors. Studies using viral-mediated increases in Rgs4 activity in NAc further support this hypothesis. Interestingly, in prefrontal cortex, Rgs4 acts as a negative modulator of the actions of nonmonoamine-directed drugs that are purported to act as antidepressants: the N-methyl-D-aspartate glutamate receptor antagonist ketamine and the delta opioid agonist (+)-4-[(αR)-α-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide. Together, these data reveal a unique modulatory role of Rgs4 in the brain region-specific actions of a wide range of antidepressant drugs and indicate that pharmacological interventions at the level of RGS4 activity may enhance the actions of such drugs used for the treatment of depression and neuropathic pain.

Impact of fusion to Gα(i2) and co-expression with RGS proteins on pharmacological properties of human cannabinoid receptors CB₁R and CB₂R

OBJECTIVES

G protein coupled receptor (GPCR)-Gα fusion proteins are often employed to investigate receptor/G protein interaction. In this study, the impact of Gα fusion proteins on pharmacology of CBRs, both mediating signals through Gα(i) proteins, were investigated. Gα(i2) was fused to the C-terminus of the CBRs or co-expressed with non-fused Gα(i2) in Sf9 cells, always together with Gβ₁γ₂. Furthermore, the impact of RGS proteins on CBR signaling in combination with the CBR fusion approach was examined, using RGS4 and RGS19 as paradigms.

METHODS

CBR ligands were characterized in the steady-state GTPase assay and pharmacological properties of ligands in the different test systems were correlated.

KEY FINDINGS

Fusion of CBRs to Gα(i2) enhanced the maximal stimulatory effects of ligands compared to the co-expression system, especially for CB₂R. RGS4, but not RGS19, behaved as a GTPase-activating protein at CBRs in the Gα(i2) co-expression and fusion system. Fusion of GPCR, most prominently CB₂R, to Gα(i2) , and co-expression with RGS4 altered the pharmacological properties of ligands.

CONCLUSIONS

Our data suggest that fusion of CB₂R to Gα(i2) and co-expression with RGS4 impedes with conformational changes. Moreover, our results support the concept of ligand-specific receptor conformations. Finally, this paper describes the most sensitive CBR test system currently available.

Fusion proteins as model systems for the analysis of constitutive GPCR activity

In many cases, the coexpression of GPCRs with G-proteins and/or regulators of G-protein signaling (RGS-proteins) allows a successful reconstitution of high-affinity agonist binding and functional responses. However, in some cases, coexpressed GPCRs and G-proteins interact inefficiently, resulting in weak [³⁵S]GTPγS- and steady-state GTPase assay signals. This may be, for example, caused by a rapid dissociation of the G-protein from the plasma membrane, as has been reported for Gα(s). Moreover, for a detailed characterization of GPCR/G-protein interactions, it may be required to work with a defined GPCR/G-protein stoichiometry and to avoid cross-interaction with endogenous G-proteins. Cross-talk to endogenous G-proteins has been shown to play a role in some mammalian expression systems. These problems can be addressed by the generation of GPCR-Gα fusion proteins and their expression in Sf9 insect cells. When the C-terminus of the receptor is fused to the N-terminus of the G-protein, a 1:1 stoichiometry of both proteins is achieved. In addition, the close proximity of GPCR and G-protein in fusion proteins leads to enhanced interaction efficiency, resulting in increased functional signals. This approach can also be extended to fusion proteins of GPCRs with RGS-proteins, specifically when steady-state GTP hydrolysis is used as read-out. GPCR-RGS fusion proteins optimize the interaction of RGS-proteins with coexpressed Gα subunits, since the location of the RGS-protein is close to the site of receptor-mediated G-protein activation. Moreover, in contrast to coexpression systems, GPCR-Gα and GPCR-RGS fusion proteins provide a possibility to imitate physiologically occurring interactions, for example, the precoupling of receptors and G-proteins or the formation of complexes between GPCRs, G-proteins and RGS-proteins (transducisomes). In this chapter, we describe the technique for the generation of fusion proteins and show the application of this approach for the characterization of constitutively active receptors.

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.

A structural determinant that renders G alpha(i) sensitive to activation by GIV/girdin is required to promote cell migration

Although several non-receptor activators of heterotrimeric G proteins have been identified, the structural features of G proteins that determine their interaction with such activators and the subsequent biological effects are poorly understood. Here we investigated the structural determinants in G alpha(i3) necessary for its regulation by GIV/girdin, a guanine-nucleotide exchange factor (GEF) that activates G alpha(i) subunits. Using G protein activity and in vitro pulldown assays we demonstrate that G alpha(i3) is a better substrate for GIV than the highly homologous G alpha(o). We identified Trp-258 in the G alpha(i) subunit as a novel structural determinant for GIV binding by comparing GIV binding to G alpha(i3)/G alpha(o) chimeras. Mutation of Trp-258 to the corresponding Phe in G alpha(o) decreased GIV binding in vitro and in cultured cells but did not perturb interaction with other G alpha-binding partners, i.e. G betagamma, AGS3 (a guanine nucleotide dissociation inhibitor), GAIP/RGS19 (a GTPase-activating protein), and LPAR1 (a G protein-coupled receptor). Activation of G alpha(i3) by GIV was also dramatically reduced when Trp-258 was replaced with Tyr, Leu, Ser, His, Asp, or Ala, highlighting that Trp is required for maximal activation. Moreover, when mutant G alpha(i3) W258F was expressed in HeLa cells they failed to undergo cell migration and to enhance Akt signaling after growth factor or G protein-coupled receptor stimulation. Thus activation of G alpha(i3) by GIV is essential for biological functions associated with G alpha(i3) activation. In conclusion, we have discovered a novel structural determinant on G alpha(i) that plays a key role in defining the selectivity and efficiency of the GEF activity of GIV on G alpha(i) and that represents an attractive target site for designing small molecules to disrupt the G alpha(i)-GIV interface for therapeutic purposes.

RGS16 is a marker for prognosis in colorectal cancer

BACKGROUND

The RGS family, comprising 22 homologues of proteins, plays a role in cellular proliferation, differentiation, membrane trafficking, and embryonic development through the involvement of the mitogen-activated protein kinase signaling pathway.

METHODS

In order to demonstrate the importance of RGS16 expression for the prediction of prognosis of colorectal cancer (CRC), we analyzed RGS16 gene expression in 22 human gastrointestinal cell lines and 124 paired cases of CRC and noncancerous regions.

RESULTS

RGS16 was expressed in 17 human gastrointestinal cancer cell lines examined in this study. RGS16 expression was higher in colorectal cancer tissue than in corresponding normal tissue (P < 0.001) in messenger RNA (mRNA) and protein levels. Patients in the RGS16 high-expression group showed a poorer overall survival rate than those in the low-expression group (P < 0.001), indicating that high RGS16 expression was an independent prognostic factor.

CONCLUSION

The present study suggests that RGS16 is useful as a predictive marker for patient prognosis of CRC.

Histamine H(4) receptor-RGS fusion proteins expressed in Sf9 insect cells: a sensitive and reliable approach for the functional characterization of histamine H(4) receptor ligands

The human histamine H(4) receptor (hH(4)R), co-expressed with Galpha(i2) and Gbeta(1)gamma(2) in Sf9 cells, is highly constitutively active. In the steady-state GTPase assay, the full agonist histamine (HA) induces only a relatively small signal (approximately 20-30%), resulting in a low signal-to background ratio. In order to improve this system for ligand screening purposes, the effects of the regulators of G-protein signaling (RGS) RGS4 and RGS19 (GAIP) were investigated. RGS4 and GAIP were fused to the C-terminus of hH(4)R or co-expressed with non-fused hH(4)R, always combined with Galpha(i2) and Gbeta(1)gamma(2). The non-fused RGS proteins did not significantly increase the relative effect of HA. With the hH(4)R-RGS4 fusion protein the absolute GTPase activities, but not the relative HA-induced signal were increased. Fusion of hH(4)R with GAIP caused a selective increase of the HA signal, resulting in an enhanced signal-to-noise ratio. A detailed characterization of the hH(4)R-GAIP fusion protein (co-expressed with Galpha(i2) and Gbeta(1)gamma(2)) and a comparison with the data obtained for the non-fused hH(4)R (co-expressed with Galpha(i2) and Gbeta(1)gamma(2)) led to the following results: (i) the relative agonist- and inverse agonist-induced signals at hH(4)R-GAIP are markedly increased. (ii) Compared to the wild-type hH(4)R, standard ligands show unaltered potencies and efficacies at hH(4)R-GAIP. (iii) Like hH(4)R, hH(4)R-GAIP shows high and NaCl-resistant constitutive activity. (iv) hH(4)R-GAIP shows the same G-protein selectivity profile as the non-fused hH(4)R. Collectively, hH(4)R-GAIP provides a sensitive test system for the characterization of hH(4)R ligands and can replace the non-fused hH(4)R in steady-state GTPase assays.

Regulator of G protein signaling 4 confers selectivity to specific G proteins to modulate mu- and delta-opioid receptor signaling

In vitro studies have shown that the Regulator of G protein Signaling 4 (RGS4) interacts with the C-termini of mu- and delta-opioid receptors (mu-OR, delta-OR) (Georgoussi et al., 2006, Cell. Signal.18, 771-782). Herein we demonstrate that RGS4 associates with these receptors in living cells and forms selective complexes with Gi/Go proteins in a receptor dependent manner. This interaction occurs within the predicted fourth intracellular loop of mu, delta-ORs as part of a signaling complex consisting of the opioid receptor, activated Galpha and RGS4. RGS4 is recruited to the plasma membrane upon opioid receptor stimulation. Expression of RGS4 in HEK293 cells attenuated agonist-mediated extracellular signal regulated kinase (ERK1,2) phosphorylation for both receptors and accelerated agonist-induced internalization of the delta-OR. RGS4 lacking its N-terminal domain failed to interact with both opioid receptors and to modulate opioid receptor signaling. Our findings demonstrate that RGS4 plays a key role in G protein coupling selectivity and signaling of the mu- and delta-OmicronRs.

Regulator of G protein signaling 2 mediates cardiac compensation to pressure overload and antihypertrophic effects of PDE5 inhibition in mice

The heart initially compensates for hypertension-mediated pressure overload by enhancing its contractile force and developing hypertrophy without dilation. Gq protein-coupled receptor pathways become activated and can depress function, leading to cardiac failure. Initial adaptation mechanisms to reduce cardiac damage during such stimulation remain largely unknown. Here we have shown that this initial adaptation requires regulator of G protein signaling 2 (RGS2). Mice lacking RGS2 had a normal basal cardiac phenotype, yet responded rapidly to pressure overload, with increased myocardial Gq signaling, marked cardiac hypertrophy and failure, and early mortality. Swimming exercise, which is not accompanied by Gq activation, induced a normal cardiac response, while Rgs2 deletion in Galphaq-overexpressing hearts exacerbated hypertrophy and dilation. In vascular smooth muscle, RGS2 is activated by cGMP-dependent protein kinase (PKG), suppressing Gq-stimulated vascular contraction. In normal mice, but not Rgs2-/- mice, PKG activation by the chronic inhibition of cGMP-selective phosphodiesterase 5 (PDE5) suppressed maladaptive cardiac hypertrophy, inhibiting Gq-coupled stimuli. Importantly, PKG was similarly activated by PDE5 inhibition in myocardium from both genotypes, but PKG plasma membrane translocation was more transient in Rgs2-/- myocytes than in controls and was unaffected by PDE5 inhibition. Thus, RGS2 is required for early myocardial compensation to pressure overload and mediates the initial antihypertrophic and cardioprotective effects of PDE5 inhibitors.

Src phosphorylation of micro-receptor is responsible for the receptor switching from an inhibitory to a stimulatory signal

Recent studies have revealed that in G protein-coupled receptor signalings switching between G protein- and beta-arrestin (betaArr)-dependent pathways occurs. In the case of opioid receptors, the signal is switched from the initial inhibition of adenylyl cyclase (AC) to an increase in AC activity (AC activation) during prolonged agonist treatment. The mechanism of such AC activation has been suggested to involve the switching of G proteins activated by the receptor, phosphorylation of signaling molecules, or receptor-dependent recruitment of cellular proteins. Using protein kinase inhibitors, dominant negative mutant studies and mouse embryonic fibroblast cells isolated from Src kinase knock-out mice, we demonstrated that mu-opioid receptor (OPRM1)-mediated AC activation requires direct association and activation of Src kinase by lipid raft-located OPRM1. Such Src activation was independent of betaArr as indicated by the ability of OPRM1 to activate Src and AC after prolonged agonist treatment in mouse embryonic fibroblast cells lacking both betaArr-1 and -2. Instead the switching of OPRM1 signals was dependent on the heterotrimeric G protein, specifically Gi2 alpha-subunit. Among the Src kinase substrates, OPRM1 was phosphorylated at Tyr336 within NPXXY motif by Src during AC activation. Mutation of this Tyr residue, together with mutation of Tyr166 within the DRY motif to Phe, resulted in the complete blunting of AC activation. Thus, the recruitment and activation of Src kinase by OPRM1 during chronic agonist treatment, which eventually results in the receptor tyrosine phosphorylation, is the key for switching the opioid receptor signals from its initial AC inhibition to subsequent AC activation.

Chronic cold exposure increases RGS7 expression and decreases alpha(2)-autoreceptor-mediated inhibition of noradrenergic locus coeruleus neurons

Chronic stress exposure alters the central noradrenergic neurons originating from the locus coeruleus (LC). Previously, we demonstrated that evoked increases in the firing rate of LC neurons and their release of norepinephrine are enhanced following chronic cold exposure. In the present studies, we tested the hypothesis that reduced feedback inhibition of LC neurons might underlie these alterations in LC activity by examining the effect of alpha(2)-autoreceptor stimulation on LC activity in chronically stressed rats using in vivo and in vitro single unit recordings. Given that regulators of G-protein signaling (RGS) proteins can impact the coupling of alpha(2)-autoreceptors to downstream signaling cascades, we also explored the expression of several RGS proteins following chronic stress exposure. We observed that the alpha(2)-autoreceptor-evoked inhibition of LC neurons was reduced and that the expression of RGS7 was increased following chronic stress exposure. Finally, we demonstrated that intracellular administration of RGS7 via patch clamp electrodes mimicked the stress-induced decrease in clonidine-evoked autoreceptor-mediated inhibition. These novel data provide a mechanism to explain how chronic stress-induced alterations in receptor coupling can result in changes in alpha(2)-autoreceptor control of noradrenergic function throughout the central nervous system, potentially leading to alterations in anxiety-related behaviors, and may suggest novel therapeutic targets for the treatment of mood and anxiety disorders.

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

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

Endogenous RGS proteins attenuate Galpha(i)-mediated lysophosphatidic acid signaling pathways in ovarian cancer cells

Lysophosphatidic acid is a bioactive phospholipid that is produced by and stimulates ovarian cancer cells, promoting proliferation, migration, invasion, and survival. Effects of LPA are mediated by cell surface G-protein coupled receptors (GPCRs) that activate multiple heterotrimeric G-proteins. G-proteins are deactivated by Regulator of G-protein Signaling (RGS) proteins. This led us to hypothesize that RGS proteins may regulate G-protein signaling pathways initiated by LPA in ovarian cancer cells. To determine the effect of endogenous RGS proteins on LPA signaling in ovarian cancer cells, we compared LPA activity in SKOV-3 ovarian cancer cells expressing G(i) subunit constructs that are either insensitive to RGS protein regulation (RGSi) or their RGS wild-type (RGSwt) counterparts. Both forms of the G-protein contained a point mutation rendering them insensitive to inhibition with pertussis toxin, and cells were treated with pertussis toxin prior to experiments to eliminate endogenous G(i/o) signaling. The potency and efficacy of LPA-mediated inhibition of forskolin-stimulated adenylyl cyclase activity was enhanced in cells expressing RGSi G(i) proteins as compared to RGSwt G(i). We further showed that LPA signaling that is subject to RGS regulation terminates much faster than signaling thru RGS insensitive G-proteins. Finally, LPA-stimulated SKOV-3 cell migration, as measured in a wound-induced migration assay, was enhanced in cells expressing Galpha(i2) RGSi as compared to cells expressing Galpha(i2) RGSwt, suggesting that endogenous RGS proteins in ovarian cancer cells normally attenuate this LPA effect. These data establish RGS proteins as novel regulators of LPA signaling in ovarian cancer cells.

Isolation and characterization of a novel human RGS mutant displaying gain-of-function activity

Regulator of G protein signaling (RGS) proteins play a crucial role in the adaptation of cells to stimulation by G protein-coupled receptors via heterotrimeric G proteins. Alterations in RGS function have been implicated in a wide range of disease states, leading to many researchers focusing on controlling the action of these regulatory proteins. Previous studies have centered on reducing or inhibiting the action of RGS proteins, utilizing inactive mutants or small molecular RGS inhibitors. Here we describe the isolation and characterization of a novel human RGS4 mutant which displays enhanced or gain-of-function (GOF) activity. RGS4(S30C) demonstrates GOF activity both in an in vivo yeast-based signalling pathway and in vitro against the Galpha(o1) subunit contained in an alpha(2A)-adrenoreceptor-Galpha(o1)(C351I) fusion protein. Mutational analysis of serine 30 identified a number of alternative substitutions that result in GOF activity. GOF activity was retained upon transposition of the serine 30-cysteine mutation to the equivalent serine residue in human RGS16. As with previously identified GOF mutants, RGS4(S30C/S30F/S30K) demonstrate increased steady state protein levels, however these mutants also demonstrate enhanced GAP activity through an additional mechanism distinct from the increased protein content. The identification of human RGS mutants with GOF activity may provide novel therapeutic agents for the treatment of signaling-based diseases and the ability to transpose these mutations to other human RGS proteins extends their application to multiple pathways.

Protean agonism at the dopamine D2 receptor: (S)-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of Go1 but an antagonist/inverse agonist for Gi1,Gi2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D(2) receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins could be expressed in an inducible fashion. Varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [(35)S]GTPgammaS onto each of Galpha(i1),Galpha(i2),Galpha(i3), and Galpha(o1). By contrast, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when Galpha(o1) was the target G protein but an antagonist/inverse agonist at Galpha(i1),Galpha(i2), and Galpha(i3). In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D(2) receptor-G protein fusion proteins, and the high-affinity state was eliminated by guanine nucleotide. (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine bound to an apparent single state of the constructs in which the D(2) receptor was fused to Galpha(i1),Galpha(i2), or Galpha(i3). However, it bound to distinct high- and low-affinity states of the D(2) receptor-Galpha(o1) fusion, with the high-affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D(2) receptor when expression of pertussis toxin-resistant forms of each of Galpha(i1), Galpha(i2), Galpha(i3), and Galpha(o1) was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of Galpha(o1). p-Tyramine displayed a protean ligand profile similar to that of (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine but with lower potency. These results demonstrate (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine to be a protean agonist at the D(2) receptor and may explain in vivo actions of this ligand.

Short- and long-term regulation of adenylyl cyclase activity by delta-opioid receptor are mediated by Galphai2 in neuroblastoma N2A cells

Activation of the opioid receptor results in short-term inhibition of intracellular cAMP levels followed by receptor desensitization and subsequent increase of cAMP above the control level (adenylyl cyclase superactivation). Using adenovirus to deliver pertussis toxin-insensitive mutants of the alpha-subunits of G(i/o) that are expressed in neuroblastoma Neuro2A cells (Galpha(i2), Galpha(i3), and Galpha(o)), we examined the identities of the G proteins involved in the short- and long-term action of the delta-opioid receptor (DOR). Pertussis toxin pretreatment completely abolished the ability of [d-Pen(2), d-Pen(5)]-enkephalin (DPDPE) to inhibit forskolin-stimulated intracellular cAMP production. Expression of the C352L mutant of Galpha(i2), and not the C351L mutants of Galpha(i3) or Galpha(o), rescued the short-term effect of DPDPE after pertussis toxin treatment. The ability of Galpha(i2) in mediating DOR inhibition of adenylyl cyclase activity was also reflected in the ability of Galpha(i2), not Galpha(i3) or Galpha(o), to coimmunoprecipitate with DOR. Coincidently, after long-term DPDPE treatment, pertussis toxin treatment eliminated the antagonist naloxone-induced superactivation of adenylyl cyclase activity. Again, only the C352L mutant of Galpha(i2) restored the adenylyl cyclase superactivation after pertussis toxin treatment. More importantly, the C352L mutant of Galpha(i2) remained associated with DOR after long-term agonist and pertussis toxin treatment whereas the wild-type Galpha(i2) did not. These data suggest that Galpha(i2) serves as the signaling molecule in both DOR-mediated short- and long-term regulation of adenylyl cyclase activity.

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.

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.

RGS16 is a negative regulator of SDF-1-CXCR4 signaling in megakaryocytes

Regulators of G-protein signaling (RGS) constitute a family of proteins involved in the negative regulation of signaling through heterotrimeric G protein-coupled receptors (GPCRs). Several RGS proteins have been implicated in the down-regulation of chemokine signaling in hematopoietic cells. The chemokine stromal-cell-derived factor 1 (SDF-1) activates migration of hematopoietic progenitors cells but fails to activate mature megakaryocytes despite high levels of CXC chemokine receptor 4 (CXCR4) receptor expression in these cells. This prompted us to analyze RGS expression and function during megakaryocyte differentiation. We found that RGS16 and RGS18 mRNA expression was up-regulated during this process. Overexpressing RGS16 mRNA in the megakaryocytic MO7e cell line inhibited SDF-1-induced migration, mitogen-activated protein kinase (MAPK) and protein kinase B (AKT) activation, whereas RGS18 overexpression had no effect on CXCR4 signaling. Knocking down RGS16 mRNA via lentiviral-mediated RNA interference increased CXCR4 signaling in MO7e cells and in primary megakaryocytes. Thus, our data reveal that RGS16 is a negative regulator of CXCR4 signaling in megakaryocytes. We postulate that RGS16 regulation is a mechanism that controls megakaryocyte maturation by regulating signals from the microenvironment.

Pertussis toxin-sensitive Gi/o proteins are involved in nerve growth factor-induced pro-survival Akt signaling cascade in PC12 cells

In Galpha(z)-deficient mice, survival of sympathetic neurons is significantly attenuated in the presence of pertussis toxin (PTX). This suggests that G(i/o) proteins may have distinct roles in neuronal survival. Here, we investigated the possible involvement of G(i/o) proteins in nerve growth factor (NGF)-induced pro-survival phosphatidylinositol-3-kinase (PI3K)/Akt signaling in rat pheochromocytoma PC12 cells. Treatment of PC12 cells with NGF increased the Akt phosphorylation level in a time- and dose-dependent manner. The NGF-dependent Akt activation was partially attenuated by PTX or overexpression of regulators of G protein signaling Z1 (RGSZ1) and Galpha-interacting protein (GAIP)), indicating the participation of G(i/o) proteins. In contrast, epidermal growth factor (EGF)-mediated Akt phosphorylation was unaffected by PTX or RGSZ1 and GAIP. Expression of PTX-resistant mutants of Galpha(i1), Galpha(i3), Galpha(oA), and Galpha(oB), but not Galpha(i2), abolished the inhibitory effect of PTX on NGF-induced Akt activation. The use of transducin as a Gbetagamma scavenger further revealed that Gbetagamma subunits rather than Galpha(i/o) acted as the signal transducer. The activation profiles of Akt-regulated downstream effectors such as Bad, IKK, and nuclear factor-kappaB (NFkappaB) were also examined. NGF-stimulated phosphorylation of Bad and IKK and transcriptional activity of NFkappaB were indeed sensitive to treatments with PTX. This is the first study that demonstrates the involvement of G(i/o) proteins in NGF-induced Akt signaling.

Activation of an alpha2A-adrenoceptor-Galphao1 fusion protein dynamically regulates the palmitoylation status of the G protein but not of the receptor

Post-translational thio-acylation of a fusion protein between the alpha2A-adrenoceptor and the alpha subunit of the G protein G(o1) is both dynamic and regulated by agonist binding. Incorporation of [3H]palmitate into the fusion protein was reduced substantially in the presence of the agonist adrenaline. This was dependent on the concentration of adrenaline and correlated with occupancy of the ligand binding site. Both the receptor and G-protein elements of the fusion construct incorporated [3H]palmitate but this occurred more rapidly for the G-protein element and regulation of acylation by the agonist occurred only for the G protein. The kinetics of de-palmitoylation of the alpha2A-adrenoceptor-Galpha(o1) fusion were accelerated markedly by agonist. Again, this reflected modulation of the G protein but not of the receptor. Agonist-induced regulation of the kinetics of thio-acylation of the G protein was abolished, however, in a mutant unable to bind guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTP[S]) in response to adrenaline. Despite the dynamic nature of the post-translational acylation and its regulation by agonist, the ability of adrenaline to activate the G protein, monitored by stimulation of the binding of [35S]GTP[S] to such fusion constructs, was unaffected by the palmitoylation potential of either the receptor or G-protein element.

Analysis of RGSZ1 protein interaction with Galphai subunits

RGSZ1 has been reported to interact with G-protein subunits of the Galphai family and function as a GTPase-accelerating protein on intrinsic Galphai GTPase activity. This article describes several experimental approaches and assays used to investigate the effect of RGSZ1 on Galphai subunits. The formats described here include physical and functional interaction assays by which the association of RGSZ1 with Galphai is explored both in vitro and in vivo. The methods analyzing physical interaction include pull-down and coimmunoprecipitation assays. We also apply yeast two-hybrid techniques to detect RGSZ1 protein interaction with Galpha subunits. Additionally, we developed several functional assay systems to identify the functional relationship between RGSZ1 and Galphai, such as the single turnover GTPase assay, yeast pheromone response assay, mitogen-activated protein kinase assay, and serum response element reporter assay.

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.

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channel activation kinetics via Galphai and Galphao-coupled receptors are determined by Galpha-specific interdomain interactions that affect GDP release rates

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channels have distinct gating properties when activated by receptors coupled specifically to Galpha(o) versus Galpha(i) subunit isoforms, with Galpha(o)-coupled currents having approximately 3-fold faster agonist-evoked activation kinetics. To identify the molecular determinants in Galpha subunits mediating these kinetic differences, chimeras were constructed using pertussis toxin (PTX)-insensitive Galpha(oA) and Galpha(i2) mutant subunits (Galpha(oA(C351G)) and Galpha(i2(C352G))) and examined in PTX-treated Xenopus oocytes expressing muscarinic m2 receptors and Kir3.1/3.2a channels. These experiments revealed that the alpha-helical N-terminal region (amino acids 1-161) and the switch regions of Galpha(i2) (amino acids 162-262) both partially contribute to slowing the GIRK activation time course when compared with the Galpha(oA(C351G))-coupled response. When present together, they fully reproduce Galpha(i2(C352G))-coupled GIRK kinetics. The Galpha(i2) C-terminal region (amino acids 263-355) had no significant effect on GIRK kinetics. Complementary responses were observed with chimeras substituting the Galpha(o) switch regions into the Galpha(i2(C352G)) subunit, which partially accelerated the GIRK activation rate. The Galpha(oA)/Galpha(i2) chimera results led us to examine an interaction between the alpha-helical domain and the Ras-like domain previously implicated in mediating a 4-fold slower in vitro basal GDP release rate in Galpha(i1) compared with Galpha(o). Mutations disrupting the interdomain contact in Galpha(i2(C352G)) at either the alphaD-alphaE loop (R145A) or the switch III loop (L233Q/A236H/E240T/M241T), significantly accelerated the GIRK activation kinetics consistent with the Galpha(i2) interdomain interface regulating receptor-catalyzed GDP release rates in vivo. We propose that differences in Galpha(i) versus Galpha(o)-coupled GIRK activation kinetics are due to intrinsic differences in receptor-catalyzed GDP release that rate-limit Gbetagamma production and is attributed to heterogeneity in Galpha(i) and Galpha(o) interdomain contacts.

Structure-Based Design, Synthesis, and Activity of Peptide Inhibitors of RGS4 GAP Activity

One of the principal roles of the multifunctional regulator of G-protein signaling (RGS) proteins is to terminate G-protein-coupled receptor (GPCR) signaling by binding to the G-protein Galpha subunit, thus acting as GTPase-activating proteins (GAPs). In principle, then, selective inhibitors of this GAP function would have potential as therapeutic agents, as they could be used to augment the effects of endogenous or exogenous GPCR agonists. Using the published RGS4-G(ialpha1) X-ray structure, we have designed and synthesized a series of cyclic peptides, modeled on the G(ialpha) switch I region, that inhibit RGS4 GAP activity, presumably by blocking the interaction between RGS4 and G(ialpha1). These compounds should prove useful for elucidating RGS-mediated activity and serve as a starting point for the development of a novel class of therapeutic agent.

Differential interaction of GRK2 with members of the G alpha q family

Regulators of G protein signaling (RGS) proteins bind to active G alpha subunits and accelerate the rate of GTP hydrolysis and/or block interaction with effector molecules, thereby decreasing signal duration and strength. RGS proteins are defined by the presence of a conserved 120-residue region termed the RGS domain. Recently, it was shown that the G protein-coupled receptor kinase 2 (GRK2) contains an RGS domain that binds to the active form of G alpha(q). Here, the ability of GRK2 to interact with other members of the G alpha(q) family, G alpha(11), G alpha(14), and G alpha(16), was tested. The signaling of all members of the G alpha(q) family, with the exception of G alpha(16), was inhibited by GRK2. Immunoprecipitation of full-length GRK2 or pull down of GST-GRK2-(45-178) resulted in the detection of G alpha(q), but not G alpha(16), in an activation-dependent manner. Moreover, activated G alpha(16) failed to promote plasma membrane (PM) recruitment of a GRK2-(45-178)-GFP fusion protein. Assays with chimeric G alpha(q)(-)(16) subunits indicated that the C-terminus of G alpha(q) mediates binding to GRK2. Despite showing no interaction with GRK2, G alpha(16) does interact with RGS2, in both inositol phosphate and PM recruitment assays. Thus, GRK2 is the first identified RGS protein that discriminates between members of the G alpha(q) family, while another RGS protein, RGS2, binds to both G alpha(q) and G alpha(16).

Concerted stimulation and deactivation of pertussis toxin-sensitive G proteins by chimeric G protein-coupled receptor-regulator of G protein signaling 4 fusion proteins: analysis of the contribution of palmitoylated cysteine residues to the GAP activity of RGS4

Agonists stimulated high-affinity GTPase activity in membranes of HEK293 cells following coexpression of the alpha 2A-adrenoceptor and a pertussis toxin-resistant mutant of Go1 alpha. Enzyme kinetic analysis of Vmax and Km failed to detect regulation of the effect of agonist by a GTPase activating protein. This did occur, however, when cells were also transfected to express RGS4. Both elements of a fusion protein in which the N-terminus of RGS4 was linked to the C-terminal tail of the alpha 2A-adrenoceptor were functional, as it was able to provide concerted stimulation and deactivation of the G protein. By contrast, the alpha 2A-adrenoceptor-RGS4 fusion protein stimulated but did not enhance deactivation of a form of Go1 alpha that is resistant to the effects of regulator of G protein signaling (RGS) proteins. Employing this model system, mutation of Asn128 but not Asn88 eliminated detectable GTPase activating protein activity of RGS4 against Go1 alpha. Mutation of all three cysteine residues that are sites of post-translational acylation in RGS4 also eliminated GTPase activating protein activity but this was not achieved by less concerted mutation of these sites. These studies demonstrate that a fusion protein between a G protein-coupled receptor and an RGS protein is fully functional in providing both enhanced guanine nucleotide exchange and GTP hydrolysis of a coexpressed G protein. They also provide a direct means to assess, in mammalian cells, the effects of mutation of the RGS protein on function in circumstances in which the spatial relationship and orientation of the RGS to its target G protein is defined and maintained.

Src-mediated RGS16 tyrosine phosphorylation promotes RGS16 stability

The amplitude of signaling evoked by stimulation of G protein-coupled receptors may be controlled in part by the GTPase accelerating activity of the regulator of G protein signaling (RGS) proteins. In turn, subcellular targeting, protein-protein interactions, or post-translational modifications such as phosphorylation may shape RGS activity and specificity. We found previously that RGS16 undergoes tyrosine phosphorylation on conserved tyrosine residues in the RGS box. Phosphorylation on Tyr(168) was mediated by the epidermal growth factor receptor (EGFR). We show here that endogenous RGS16 is phosphorylated after epidermal growth factor stimulation of MCF-7 cells. In addition, p60-Src or Lyn kinase phosphorylated recombinant RGS16 in vitro, and RGS16 underwent phosphorylation in the presence of constitutively active Src (Y529F) in EGFR(-) CHO-K1 cells. Blockade of endogenous Src activity by selective inhibitors attenuated RGS16 phosphorylation induced by pervanadate or receptor stimulation. Furthermore, the rate of RGS16 degradation was reduced in cells expressing active Src or treated with pervanadate or a G protein-coupled receptor ligand (CXCL12). Induction of RGS16 tyrosine phosphorylation was associated with increased RGS16 protein levels and enhanced GAP activity in cell membranes. These results suggest that Src mediates RGS16 tyrosine phosphorylation, which may promote RGS16 stability.

Molecular basis of opioid dependence: role of signal regulation by G-proteins

1. Morphine and opiate narcotics are potent analgesics that have a high propensity to induce tolerance and physical dependence following their repeated administration. 2. The molecular basis of opiate dependence has not been completely elucidated, although the participation of opioid receptors is a prerequisite. Cellular dependence on opioids is believed to result from the chronic stimulation of opioid-regulated signalling networks. 3. As G-protein-coupled receptors, the opioid receptors must rely on heterotrimeric G-proteins for signal transduction. Recent advances in our understanding of G-protein signalling have unveiled novel signalling molecules and mechanisms, some of which may be intricately involved in the manifestation of opiate dependence. 4. In the present review, we will attempt to trace chronic opioid signals along elaborate G-protein-regulated pathways.

Mechanism of action of Gq to inhibit G beta gamma modulation of CaV2.2 calcium channels: probed by the use of receptor-G alpha tandems

The stable interaction of a G-protein coupled receptor and a particular partner G-protein was made possible by creating tandems between the alpha(2A) adrenergic receptor (alpha(2A)-R) and pertussis toxin-resistant mutants of different G alpha subunits of heterotrimeric G-proteins. Both alpha(2A)-R-G alpha(o) and alpha(2A)-R-G alpha(i) proved able to reconstitute agonist-induced voltage-dependent inhibition of N-type calcium channels (Ca(V)2.2) similar to the wild-type alpha(2A)-R when expressed in COS-7 cells. The interaction of G(q) with the G(i/o) signaling pathways was studied by expressing either G alpha(q) or a chimeric construct based on G alpha(q) containing the last five amino acids of G alpha(z), which is activated by alpha(2A)-R. It was found that G alpha(qz5) activated by the wild-type alpha(2A)-R inhibited Ca(V)2.2 currents in a voltage-independent fashion. Furthermore, G alpha(qz5) counteracted the voltage-dependent inhibition resulting from alpha(2A)-R-G alpha(o) activation. We subsequently investigated the basis for the behavior of G alpha(qz5). Our evidence suggests that this occurs as a result of a downstream effect of activation of G alpha(qz5) because it was blocked by C-terminal construct of phospholipase C beta 1. Furthermore it is likely to occur in part via protein kinase C (PKC) activation, because the PKC activator phorbol dibutyrate mimicked the effects of G alpha(qz5) in alpha(2A)-R-G alpha(o)-transfected cells. Conversely, cells expressing both alpha(2A)-R-G alpha(o) and G alpha(qz5) exhibited a partial restoration of voltage-dependent inhibition in the presence of the PKC inhibitor bisindolylmaleimide I (GF 109203X). The potential sites of phosphorylation are discussed.

A spatial focusing model for G protein signals. Regulator of G protein signaling (RGS) protien-mediated kinetic scaffolding

Regulators of G protein signaling (RGS) are GTPase-accelerating proteins (GAPs), which can inhibit heterotrimeric G protein pathways. In this study, we provide experimental and theoretical evidence that high concentrations of receptors (as at a synapse) can lead to saturation of GDP-GTP exchange making GTP hydrolysis rate-limiting. This results in local depletion of inactive heterotrimeric G-GDP, which is reversed by RGS GAP activity. Thus, RGS enhances receptor-mediated G protein activation even as it deactivates the G protein. Evidence supporting this model includes a GTP-dependent enhancement of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding to G(i) by RGS. The RGS domain of RGS4 is sufficient for this, not requiring the NH(2)- or COOH-terminal extensions. Furthermore, a kinetic model including only the GAP activity of RGS replicates the GTP-dependent enhancement of GTPgammaS binding observed experimentally. Finally in a Monte Carlo model, this mechanism results in a dramatic "spatial focusing" of active G protein. Near the receptor, G protein activity is maintained even with RGS due to the ability of RGS to reduce depletion of local Galpha-GDP levels permitting rapid recoupling to receptor and maintained G protein activation near the receptor. In contrast, distant signals are suppressed by the RGS, since Galpha-GDP is not depleted there. Thus, a novel RGS-mediated "kinetic scaffolding" mechanism is proposed which narrows the spatial range of active G protein around a cluster of receptors limiting the spill-over of G protein signals to more distant effector molecules, thus enhancing the specificity of G(i) protein signals.

Activation of Gz attenuates Rap1-mediated differentiation of PC12 cells

We previously identified a specific activation-dependent interaction between the alpha subunit of the heterotrimeric G protein, G(z), and a regulator of Rap1 signaling, Rap1GAP (Meng, J., Glick, J. L., Polakis, P., and Casey, P. J. (1999) J. Biol. Chem. 274, 36663-36669). We now demonstrate that activated forms of Galpha(z) are able to recruit Rap1GAP from a cytosolic location to the membrane. Using PC12 cells as a model for neuronal differentiation, the influence of G(z) activation on Rap1-mediated cell differentiation was examined. Introduction of constitutively-activated Galpha(z) into PC12 cells markedly attenuated the differentiation process of these cells induced by a cAMP analogue. Treatment of PC12 cells expressing wild type Galpha(z) with a specific agonist to the alpha(2A)-adrenergic receptor also attenuated cAMP-induced PC12 cell differentiation, demonstrating that receptor-mediated activation of G(z) was also effective in this regard. Furthermore, activation of G(z) decreased the ability of the cAMP analogue to trigger both Rap1 and extracellular-regulated kinase (ERK) activation. Differentiation of PC12 cells induced by nerve growth factor (NGF) is also thought to be a Rap1-mediated process, and G(z) activation was found to attenuate this process as well. Rap1 activation, ERK phosphorylation, and PC12 cell differentation induced by NGF treatment were all significantly attenuated by either transfection of constitutively activated Galpha(z) or receptor-mediated G(z) activation. Based on these findings, a model is proposed in which activation of G(z) results in recruitment of Rap1GAP to the membrane where it can effectively down-regulate Rap1 signaling. The implications of these findings in regard to a possible role for G(z) in neuronal development are discussed.

Regulation of the avidity of ternary complexes containing the human 5-HT(1A) receptor by mutation of a receptor contact site on the interacting G protein alpha subunit

1 Fusion proteins were constructed between the human 5-HT(1A) receptor and pertussis toxin-resistant forms of both G(i1)alpha and G(o1)alpha mutated at residue(351) from cysteine to either glycine or isoleucine. Each of these was expressed stably in HEK293 cells. 2 Increasing concentrations of GDP inhibited binding of the agonist [(3)H]-8-OH-DPAT but not the antagonist [(3)H]-MPPF to each construct. 3 The IC(50) for GDP was greater for constructs containing isoleucine at residue(351) of the G proteins compared to those with glycine at this position. 4 The G protein antagonist suramin had similar effects to GDP on the binding of [(3)H]-8-OH-DPAT. 5 The proportion of 5-HT(1A) receptor binding sites detected by [(3)H]-MPPF that displayed high affinity for 8-OH-DPAT was significantly greater when the interacting G protein contained isoleucine rather than glycine at residue(351). 6 The 5-HT(1A) receptor displayed similar avidity of interaction with G(i1)alpha and G(o1)alpha. 7 These results indicate that a higher avidity ternary complex is formed between 8-OH-DPAT, the 5-HT(1A) receptor and G proteins when isoleucine rather than glycine is located at residue(351) of the interacting G protein.

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

RGS2 and RGS4 were studied for their effects as GTPase activating proteins (GAPs) on receptor-activated G(i) in a novel steady-state assay using membranes from Sf9 cells quadruply infected with baculoviruses encoding the m2 muscarinic receptor, G(alphai2), G(beta1), and G(gamma2). In the presence of the muscarinic agonist carbachol, regulator of G protein signaling 2 (RGS2) and RGS4 each produced up to a 10-fold increase in agonist-dependent GTPase activity. The observed K(m) for GTP in this system was increased in the presence of RGS4. NaCl and KCl inhibited the GAP activities of both RGS2 and RGS4, although they had no effect on GTPase activity in the absence of RGS proteins. MgCl(2) had a complex effect on GTPase activity, with optimal RGS2 and RGS4 GAP activities occurring, respectively, at high micromolar and low millimolar concentrations of free Mg(2+). The concentration dependence of RGS GAP activity was assessed, and RGS4 was found to be more potent than RGS2 by up to an order of magnitude. This direct observation confirms a similar difference in potency found when these two RGS proteins were compared for their ability to inhibit signaling downstream of G(i). RGS2 yielded Hill coefficients greater than 2.0, suggesting that it may bind in a positively cooperative manner to oligomeric structures containing more than one G protein. Furthermore, RGS4 yielded a bell-shaped dose-dependence under low magnesium (0.5 mM) conditions, which is also consistent with the idea of RGS cooperativity.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

Enhanced detection of receptor constitutive activity in the presence of regulators of G protein signaling: applications to the detection and analysis of inverse agonists and low-efficacy partial agonists

Fusion proteins between the human 5-hydroxytryptamine (5-HT)(1A) receptor and either wild type or certain pertussis toxin-resistant forms of G(o1)alpha and G(i1)alpha display constitutive GTPase activity that can be inhibited by the inverse agonist spiperone. Addition of recombinant regulator of G protein signaling (RGS) 1 or RGS16 to membranes expressing these fusion proteins resulted in elevation of this constitutive GTPase activity without significantly altering the binding affinity of antagonist/inverse agonist ligands. For a 5-HT(1A) receptor-(Cys(351)Ile)G(o1)alpha fusion protein the increase in basal GTPase activity was greater than 4-fold. Enzyme kinetic analysis demonstrated that the effect of RGS1 was as a GTPase-activating protein for the fusion construct. In the presence of the RGS proteins, both agonists and inverse agonists produced much more robust regulation of high-affinity GTPase activity than in their absence. This allowed detection of the partial agonist nature of WAY100635, which has been described previously as a neutral antagonist at the 5-HT(1A) receptor. Of a range of ligands studied, only haloperidol functioned as a neutral ligand in the presence of RGS1. These studies show that addition of a recombinant RGS protein provides a simple and novel means to elevate the fraction of basal membrane GTPase activity contributed by the constitutive activity of a receptor. By so doing, it also greatly enhances the ability to detect and analyze the effects of inverse agonists and to discriminate between neutral ligands and those with low levels of positive intrinsic efficacy.

Reciprocal mutations of highly conserved residues in transmembrane helices 2 and 7 of the alpha(2A)-adrenoceptor restore agonist activation of G(i1)alpha

Fusion proteins were constructed between the alpha(2A)-adrenoceptor and the alpha-subunit of the G-protein G(i1). Mutation of the highly conserved Asp(79) in transmembrane (TM) helix 2 of the receptor to Asn reduced the capacity of agonists to activate G(i1)alpha by 95% without altering [3H]antagonist or agonist ligand-binding affinity. A reciprocal mutation in TM helix 7 (Asn(422)Asp) was without effect on signalling effectiveness. Combination of these two mutations overcame the effect of the Asp(79)Asp mutation. By examining alterations in this helix 2-helix 7 microdomain, we further demonstrate the utility of receptor-G-protein fusion proteins to quantitate mutational effects on receptor-G-protein interactions and information transfer.

Engineering a V(2) vasopressin receptor agonist- and regulator of G-protein-signaling-sensitive G protein

It is extremely difficult to detect guanine nucleotide exchange or hydrolysis stimulated by receptors which couple to G(s)alpha. Furthermore, G(s)alpha is largely resistant to the GTPase-activating properties of RGS proteins. Coexpression of the vasopressin V(2) receptor with a series of chimeric G protein alpha subunits in which the C-terminal 6-12 amino acids of G(i1)alpha were replaced with the equivalent sequence of G(s)alpha allowed robust vasopressin-stimulated [(35)S]GTPgammaS binding. Vasopressin did not stimulate the GTPase activity of fusion proteins between the V(2) receptor and either G(s)alpha or G(i1)alpha. However, it produced a concentration-dependent stimulation of V(max) for a V(2) receptor-G(i1)alpha/Gs6alpha fusion protein. This construct bound [(3)H]vasopressin with high affinity and this was competed by other ligands with rank order anticipated for the V(2) receptor. RGS1 enhanced vasopressin stimulation of V(2) receptor-G(i1)alpha/G(s)6alpha in a concentration-dependent manner. RGS-GAIP was substantially less potent. Enzyme kinetic analysis demonstrated that RGS1 increased both V(max) of the GTPase activity and the observed K(m) for GTP, consistent with RGS1 accelerating the rate of GTP hydrolysis of the chimeric G protein, whereas the agonist vasopressin accelerates guanine nucleotide exchange. This approach provides a sensitive assay for V(2) receptor agonist ligands and may be amenable to many other G(s)alpha-coupled receptors.

Bridging with GAPs: receptor communication through RGS proteins

The regulators of heterotrimeric guanosine triphosphate (GTP)-binding protein (G protein) signaling (RGS proteins) were named for their ability to act as GTP-activating proteins (GAPs) for G proteins and, thus, limit the signal generated by G protein-coupled receptors (GPCRs). In addition to this characteristic biochemical trait, RGS proteins constitute a large family of structurally diverse proteins with variable sequence motifs that permit additional specific interactions. RGS proteins may also serve as a bridge from GPCRs to receptor tyrosine kinases or transmembrane channels, allowing signals from GPCRs to regulate signaling through other types of receptors, and vice versa.

Differential regulation of G protein-gated inwardly rectifying K(+) channel kinetics by distinct domains of RGS8

1. The contribution of endogenous regulators of G protein signalling (RGS) proteins to G protein modulated inwardly rectifying K(+) channel (GIRK) activation/deactivation was examined by expressing mutants of Galpha(oA) insensitive to both pertussis toxin (PTX) and RGS proteins in rat sympathetic neurons. 2. GIRK channel modulation was reconstituted in PTX-treated rat sympathetic neurons following heterologous expression of G protein subunits. Under these conditions, noradrenaline-evoked GIRK channel currents displayed: (1) a prominent lag phase preceding activation, (2) retarded activation and deactivation kinetics, and (3) a lack of acute desensitization. 3. Unexpectedly, heterologous expression of RGS8 in neurons expressing PTX-i-RGS-insensitive Galpha(oA) shortened the lag phase and restored rapid activation, but retarded the deactivation phase further. These effects were found to arise from the N-terminus, but not the core domain, of RGS8 thus suggesting actions on channel modulation independently of GTPase acceleration. 4. These findings indicate that different domains of RGS8 make distinct contributions to the temporal regulation of GIRK channels. The RGS8 core domain accelerates termination of the G-protein cycle presumably by increasing Galpha GTPase activity. In contrast, the N-terminal domain of RGS8 appears to promote entry into the G protein cycle, possibly by enhancing coupling of receptors to the G protein heterotrimer. Together, these opposing effects should allow for an increase in temporal fidelity without a dramatic decrease in signal strength.

Multiple phosphorylation sites in RGS16 differentially modulate its GAP activity

Regulators of G-protein signaling (RGS) are GTPase-activating proteins (GAP) for activated Galpha subunits. We found that mouse RGS16, when expressed in HEK293T cells, is phosphorylated constitutively at serine 194 based on in vivo orthophosphate labeling experiments, while serine 53 is phosphorylated in a ligand-dependent manner upon stimulation by epinephrine in cells expressing the alpha2A adrenergic receptor. Phosphorylation on both sites impairs its GAP activity and subsequent attenuation on heterotrimeric G-protein-stimulated extracellular signal-regulated protein kinase activity. This is the first report of RGS functional downregulation by phosphorylation via a G-protein-coupled receptor.

Differential capacities of the RGS1, RGS16 and RGS-GAIP regulators of G protein signaling to enhance alpha2A-adrenoreceptor agonist-stimulated GTPase activity of G(o1)alpha

Recombinant RGS1, RGS16 and RGS-GAIP, but not RGS2, were able to substantially further stimulate the maximal GTPase activity of G(o1)alpha promoted by agonists at the alpha2A-adrenoreceptor in a concentration-dependent manner. Kinetic analysis of the regulation of an alpha2A-adrenoreceptor-G(o1)alpha fusion protein by all three RGS proteins revealed that they had similar affinities for the receptor-G protein fusion. However, their maximal effects on GTP hydrolysis varied over threefold with RGS16 > RGS1 > RGS-GAIP. Both RGS1 and RGS16 reduced the potency of the alpha2A-adrenoreceptor agonist adrenaline by some 10-fold. A lower potency shift was observed for the partial agonist UK14304 and the effect was absent for the weak partial agonist oxymetazoline. Each of these RGS proteins altered the intrinsic activity of both UK14304 and oxymetazoline relative to adrenaline. Such results require the RGS interaction with G(o1)alpha to alter the conformation of the alpha2A-adrenoreceptor and are thus consistent with models invoking direct interactions between RGS proteins and receptors. These studies demonstrate that RGS1, RGS16 and RGS-GAIP show a high degree of selectivity to regulate alpha2A-adrenoreceptor-activated G(o1)alpha rather than G(i1)alpha, G(i2)alpha or G(i3)alpha and different capacities to inactivate this G protein.

Disease-specific changes in regulator of G-protein signaling 4 (RGS4) expression in schizophrenia

Complex defects in neuronal signaling may underlie the dysfunctions that characterize schizophrenia. Using cDNA microarrays, we discovered that the transcript encoding regulator of G-protein signaling 4 (RGS4) was the most consistently and significantly decreased in the prefrontal cortex of all schizophrenic subjects examined. The expression levels of ten other RGS family members represented on the microarrays were unchanged and hierarchical data analysis revealed that as a group, 274 genes associated with G-protein signaling were unchanged. Quantitative in situ hybridization verified the microarray RGS4 data, and demonstrated highly correlated decreases in RGS4 expression across three cortical areas of ten subjects with schizophrenia. RGS4 expression was not altered in the prefrontal cortex of subjects with major depressive disorder or in monkeys treated chronically with haloperidol. Interestingly, targets for 70 genes mapped to the major schizophrenia susceptibility locus 1q21--22 were present on the microarrays, of which only RGS4 gene expression was consistently altered. The combined data indicate that a decrease in RGS4 expression may be a common and specific feature of schizophrenia, which could be due either to genetic factors or a disease- specific adaptation, both of which could affect neuronal signaling.

Agonist trafficking of G(i/o)-mediated alpha(2A)-adrenoceptor responses in HEL 92.1.7 cells

1. The ability of 19 agonists to elevate Ca(2+) and inhibit forskolin-induced cyclic AMP elevation through alpha(2A)-adrenoceptors in HEL 92.1.7 cells was investigated. Ligands of catecholamine-like- (five), imidazoline- (nine) and non-catecholamine-non-imidazoline-type (five) were included. 2. The relative maximum responses were similar in both assays. Five ligands were full or nearly full agonists, six produced 20 - 70% of the response to a full agonist and the remaining eight gave lower responses (< 20%) so that their potencies were difficult to evaluate. 3. Marked differences in the potencies of the agonists with respect to the two measured responses were seen. The catecholamines were several times less potent in decreasing cyclic AMP than in increasing Ca(2+), whereas the other, both imidazoline and ox-/thiazoloazepine ligands, were several times more potent with respect to the former than the latter response. For instance, UK14,304 was more potent than adrenaline with respect to the cyclic AMP response but less potent than adrenaline with respect to the Ca(2+) response. 4. All the responses were sensitive to pertussis toxin-pretreatment. Also the possible role of PLA(2), beta-adrenoceptors or ligand transport or metabolism as a source of error could be excluded. The results suggest that the active receptor states produced by catecholamines and the other agonists are markedly different and therefore have different abilities to activate different signalling pathways.

The human delta opioid receptor activates G(i1)alpha more efficiently than G(o1)alpha

To assess the relative capacity of the human delta opioid receptor to activate closely related G proteins, fusion proteins were constructed in which the alpha-subunits of either G(i1) or G(o1), containing point mutations to render them insensitive to the actions of pertussis toxin, were linked in-frame with the C-terminus of the receptor. Following transient and stable expression in HEK 293 cells, both constructs bound the antagonist [(3)H]naltrindole with high affinity. D-ala(2),D-leu(5) Enkephalin effectively inhibited forskolin-stimulated adenylyl cyclase activity in intact cells in a concentration-dependent, but pertussis toxin-insensitive, manner. The high-affinity GTPase activity of both constructs was also stimulated by D-ala(2),D-leu(5) enkephalin with similar potency. However, enzyme kinetic analysis of agonist stimulation of GTPase activity demonstrated that the GTP turnover number produced in response to D-ala(2),D-leu(5) enkephalin was more than three times greater for G(i1)alpha than for G(o1)alpha. As the effect of agonist in both cases was to increase V:(max) without increasing the observed K:(m) for GTP, this is consistent with receptor promoting greater guanine nucleotide exchange, and thus activation, of G(i1)alpha compared with G(o1)alpha. An equivalent fusion protein between the human mu opioid receptor-1 and G(i1)alpha produced a similar D-ala(2),D-leu(5) enkephalin-induced GTP turnover number as the delta opioid receptor-G(i1)alpha fusion construct, consistent with agonist occupation of these two opioid receptor subtypes being equally efficiently coupled to activation of G(i1)alpha.