Endogenous RGS proteins regulate presynaptic and postsynaptic function: functional expression of RGS-insensitive Galpha subunits in central nervous system neurons

HDAC4 governs a transcriptional program essential for synaptic plasticity and memory

Neuronal activity influences genes involved in circuit development and information processing. However, the molecular basis of this process remains poorly understood. We found that HDAC4, a histone deacetylase that shuttles between the nucleus and cytoplasm, controls a transcriptional program essential for synaptic plasticity and memory. The nuclear import of HDAC4 and its association with chromatin is negatively regulated by NMDA receptors. In the nucleus, HDAC4 represses genes encoding constituents of central synapses, thereby affecting synaptic architecture and strength. Furthermore, we show that a truncated form of HDAC4 encoded by an allele associated with mental retardation is a gain-of-function nuclear repressor that abolishes transcription and synaptic transmission despite the loss of the deacetylase domain. Accordingly, mice carrying a mutant that mimics this allele exhibit deficits in neurotransmission, spatial learning, and memory. These studies elucidate a mechanism of experience-dependent plasticity and define the biological role of HDAC4 in the brain.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

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.

N-terminal residues control proteasomal degradation of RGS2, RGS4, and RGS5 in human embryonic kidney 293 cells

Regulator of G protein signaling (RGS) proteins modulate G protein-coupled receptor (GPCR) signaling. The N termini of some RGS4-family proteins provide receptor specificity and also contain an N-end rule determinant that results in ubiquitylation and decreased protein expression. The relevance of these mechanisms to other RGS proteins is not fully understood. Thus we examined function, receptor specificity, and expression of R4 subfamily RGS proteins (RGS2, -3, -4, -5, and -8). Although the N terminus plays a key role in protein stability in human embryonic kidney (HEK) 293 cells, we were unable to demonstrate specificity of RGS2, -3, -4, -5, or -8 for muscarinic receptors (M(1), M(3), and M(5)). However, cellular RGS activity (8 = 3 > 2) was strongly correlated with expression; RGS4 and -5 had minimal expression and activity. Stabilizing mutations of RGS4 and -5 (C2S) enhanced expression and function with a greater influence on RGS4 than on RGS5. We were surprised to find that a predicted destabilizing mutation in RGS8 (A2C) did not markedly affect expression and had no effect on function. In contrast, a destabilizing mutation in RGS2 (RGS2-Q2L) recently identified as a rare N-terminal genetic variant in a Japanese hypertensive cohort (J Hypertens 23:1497-1505, 2005) showed significantly reduced expression and inhibition of angiotensin II (AT(1)) receptor-stimulated accumulation of inositol phosphates. We were surprised to find that RGS2-Q2R, also predicted to be destabilizing, showed nearly normal expression and function. Thus, proteasomal regulation of RGS expression in HEK293 cells strongly controls RGS function and a novel RGS2 mutation with decreased protein expression could be relevant to the pathophysiology of hypertension in humans.

Rapid activation of inwardly rectifying potassium channels by immobile G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) mediate slow synaptic transmission and many other effects of small molecule and peptide neurotransmitters. In the standard model of GPCR signaling, receptors and G-proteins diffuse laterally within the plane of the plasma membrane and encounter each other by random collision. This model predicts that signaling will be most efficient if both GPCRs and G-proteins are free to diffuse, thus maximizing collision frequency. However, neuronal GPCRs are often recruited to and enriched at specific synaptic locations, suggesting receptor mobility is restricted in these cells. Here, we test the hypothesis that restricting GPCR mobility impairs signaling in neurons by limiting the frequency of collisions between receptors and G-proteins. Mu-opioid receptors (MORs) were immobilized on the surface of cerebellar granule neurons by avidin-mediated cross-linking, and inwardly rectifying potassium (GIRK) channels were used as rapid indicators of G-protein activation. Mobile and immobile MORs activated GIRK channels with the same onset kinetics and agonist sensitivity in these neurons. In a heterologous expression system, GFP (green fluorescent protein)-tagged G alpha(oA) subunits remained mobile after cross-linking, but their mobility was reduced in the presence of immobile MORs, suggesting that these receptors and subunits were transiently precoupled. In addition, channel activation could be reconstituted with immobile GPCRs, G-protein heterotrimers, and GIRK channels. These results show that collision frequency is not rate-limiting for G-protein activation in CNS neurons, and are consistent with the idea that signaling components are compartmentalized or preassembled.

Acute amphetamine down-regulates RGS4 mRNA and protein expression in rat forebrain: distinct roles of D1and D2dopamine receptors

Administration of psychostimulants modulates mRNA of several regulators of guanine nucleotide-binding protein signaling (RGSs) proteins in the brain. In the present study, the regulation of amphetamine-induced decrease of RGS4 expression in the rat forebrain was evaluated. RGS4 mRNA was reduced by amphetamine in an inverse, dose-dependent manner. The lowest dose (2.5 mg/kg) decreased RGS4 mRNA in caudate putamen for up to 6 h after injection whereas the decrease in several frontal cortical areas was detected at 3 h only. Analysis of RGS4 immunoreactivity by western blotting revealed a decrease 3 h after amphetamine solely in the caudate putamen. Systemic administration of D(1) (SCH23390) or D(2) (eticlopride) receptor antagonists blocked amphetamine-induced locomotion but amphetamine augmented both the SCH23390-induced increase and the eticlopride-induced decrease in RGS4 mRNA in the caudate putamen. Further, the down-regulation of RGS4 immunoreactivity by eticlopride was robust whereas the effect of SCH23390 was blunted as compared with its effect on mRNA. These data suggest that, by decreasing RGS4 expression in the caudate putamen via D(1) receptors, acute amphetamine could disinhibit RGS4-sensitive guanine nucleotide-binding protein alpha-subunit i- and/or q-coupled signaling pathways and favor mechanisms that counterbalance D(1) receptor stimulation.

Endogenous Regulator of G-Protein Signaling Proteins Regulate the Kinetics of Gαq/11-Mediated Modulation of Ion Channels in Central Nervous System Neurons

Slow synaptic potentials are generated when metabotropic G-protein-coupled receptors activate heterotrimeric G-proteins, which in turn modulate ion channels. Many neurons generate excitatory postsynaptic potentials mediated by G-proteins of the Galphaq/11 family, which in turn activate phospholipase C-beta. Accessory GTPase-activating proteins (GAPs) are thought to be required to accelerate GTP hydrolysis and rapidly turn off G-proteins, but the involvement of GAPs in neuronal Galphaq/11 signaling has not been examined. Here, we show that regulator of G-protein signaling (RGS) proteins provide necessary GAP activity at neuronal Galphaq/11 subunits. We reconstituted inhibition of native 2-pore domain potassium channels in cerebellar granule neurons by expressing chimeric Galpha subunits that are activated by Galphai/o-coupled receptors, thus bypassing endogenous Galphaq/11 subunits. RGS-insensitive variants of these chimeras mediated inhibition of potassium channels that developed and recovered more slowly than inhibition mediated by RGS-sensitive (wild-type) chimeras or native Galphaq/11 subunits. These changes were not accompanied by a change in agonist sensitivity, as might be expected if RGS proteins acted primarily as effector antagonists. The slowed recovery from potassium channel inhibition was largely reversed by an additional mutation that mimics the RGS-bound state. These results suggest that endogenous RGS proteins regulate the kinetics of rapid Galphaq/11-mediated signals in central nervous system neurons by providing GAP activity.

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

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

GoLoco motif peptides as probes of Galpha subunit specificity in coupling of G-protein-coupled receptors to ion channels

Biochemical and structural studies of signaling proteins have revealed critical features of peptide motifs at the interaction surfaces between proteins. Such information can be used to design small peptides that can be used as functional probes of specific interactions in signaling cascades. This article describes the use of a novel domain (the GoLoco motif) found in several members of the regulators of G-protein signaling (RGS) protein family to probe the specificity of Galpha subunit involvement in the coupling of dopamine and somatostatin receptors to ion channels in the AtT20 neuroendocrine cell line. Peptides encoding the GoLoco motifs of RGS12 and AGS3 were perfused into single cells during electrical recording of agonist-induced current responses by whole cell patch clamp methods. The particular sequences chosen have been demonstrated to bind selectively to the GDP-bound form of Galphai, but not Galphao, and preclude association of Gbetagamma and Galphai subunits. A functional manifestation of this property is observed in the progressive uncoupling of D2 dopamine receptors and Kir3.1/3.2 channels with repeated agonist application. Similar uncoupling is not observed with somatostatin receptors nor with D2 receptors coupling to calcium channels, suggesting Galpha subunit specificity in these signaling pathways. Motifs found in other proteins in the GPCR signaling machinery may also prove useful in assessing G-protein signaling specificity and complexity in single cells in the future.