RGS9-2 modulates D2 dopamine receptor-mediated Ca2+ channel inhibition in rat striatal cholinergic interneurons

Amphetamine up-regulates activator of G-protein signaling 1 mRNA and protein levels in rat frontal cortex: the role of dopamine and glucocorticoid receptors

Acute and chronic exposure to psychostimulants results in altered function of G-protein-coupled receptors in the forebrain. It is believed that neuroadaptations in G-protein signaling contribute to behavioral sensitivity to psychostimulants that persists over a prolonged drug-free period. Proteins termed activators of G-protein signaling (AGS) have been characterized as potent modulators of both receptor-dependent and receptor-independent G-protein signaling. Nevertheless, the regulation of AGS gene and protein expression by psychostimulants remains poorly understood. In the present study, we investigated amphetamine (AMPH)-induced changes in expression patterns of several forebrain-enriched AGS proteins. A single exposure to AMPH (2.5 mg/kg i.p.) selectively induced gene expression of AGS1, but not Rhes or AGS3 proteins, in the rat prefrontal cortex (PFC) as measured 3 h later. Induction of AGS1 mRNA in the PFC by acute AMPH was transient and dose-dependent. Even repeated treatment with AMPH for 5 days did not produce lasting changes in AGS1 mRNA and protein levels in the PFC as measured 3 weeks post treatment. However, at this time point, a low dose AMPH challenge (1 mg/kg i.p.) induced a robust behavioral response and upregulated AGS1 expression in the PFC selectively in animals with an AMPH history. The effects of AMPH on AGS1 expression in the PFC were blocked by a D2, but not D1, dopamine receptor antagonist and partially by a glucocorticoid receptor antagonist. Collectively, the present study suggests that (1) AGS1 represents a regulator of G-protein signaling that is rapidly inducible by AMPH in the frontal cortex, (2) AGS1 regulation in the PFC parallels behavioral activation by acute AMPH in drug-naive animals and hypersensitivity to AMPH challenge in sensitized animals, and (3) D2 dopamine and glucocorticoid receptors regulate AMPH effects on AGS1 in the PFC. Changes in AGS1 levels in the PFC may result in abnormal receptor-to-G-protein coupling that alters cortical sensitivity to psychostimulants.

R7BP complexes with RGS9-2 and RGS7 in the striatum differentially control motor learning and locomotor responses to cocaine

In the striatum, signaling through G protein-coupled dopamine receptors mediates motor and reward behavior, and underlies the effects of addictive drugs. The extent of receptor responses is determined by RGS9-2/Gbeta5 complexes, a striatally enriched regulator that limits the lifetime of activated G proteins. Recent studies suggest that the function of RGS9-2/Gbeta5 is controlled by the association with an additional subunit, R7BP, making elucidation of its contribution to striatal signaling essential for understanding molecular mechanisms of behaviors mediated by the striatum. In this study, we report that elimination of R7BP in mice results in motor coordination deficits and greater locomotor response to morphine administration, consistent with the essential role of R7BP in maintaining RGS9-2 expression in the striatum. However, in contrast to previously reported observations with RGS9-2 knockouts, mice lacking R7BP do not show higher sensitivity to locomotor-stimulating effects of cocaine. Using a striatum-specific knockdown approach, we show that the sensitivity of motor stimulation to cocaine is instead dependent on RGS7, whose complex formation with R7BP is dictated by RGS9-2 expression. These results indicate that dopamine signaling in the striatum is controlled by concerted interplay between two RGS proteins, RGS7 and RGS9-2, which are balanced by a common subunit, R7BP.

Distribution of RGS9-2 in neurons of the mouse striatum

Regulators of G protein signaling (RGS) proteins negatively modulate G protein-coupled receptor (GPCR) signaling activity by accelerating G protein hydrolysis of GTP, hastening pathway shutoff. A wealth of data from cell culture experiments using exogenously expressed proteins indicates that RGS9 and other RGS proteins have the potential to down-regulate a significant number of pathways. We have used an array of biochemical and tissue staining techniques to examine the subcellular localization and membrane binding characteristics of endogenous RGS9-2 and known binding partners in rodent striatum and tissue homogenates. A small fraction of RGS9-2 is present in the soluble cytoplasmic fraction, whereas the majority is present primarily associated with the plasma membrane and structures insoluble in non-ionic detergents that efficiently extract the vast majority of its binding partners, R7BP and G(beta5). It is specifically excluded from the cell nucleus in mouse striatal tissue. In cultured striatal neurons, RGS9-2 is found at extrasynaptic sites primarily along the dendritic shaft near the spine neck. Heterogeneity in RGS9-2 detergent solubility along with its unique subcellular localization suggests that its mechanism of membrane anchoring and localization is complex and likely involves additional proteins beside R7BP. An important nuclear function for RGS9-2 seems unlikely.

Regulators of G protein signaling in neuropsychiatric disorders

Regulators of G protein signaling (RGS) comprise a diverse group of about 40 proteins which determine signaling amplitude and duration via modulation of receptor/G protein or receptor/effector coupling. Several members of the RGS family are expressed in the brain, where they have precise roles in regulation of important physiological processes. The unique functions of each RGS can be attributed to its structure, distinct pattern of expression, and regulation, and its preferential interactions with receptors, Galpha subunits and other signaling proteins. Evidence suggests dysfunction of RGS proteins is related to several neuropathological conditions. Moreover, clinical and preclinical work reveals that the efficacy and/or side effects of treatments are highly influenced by RGS activity. This article summarizes findings on RGS proteins in vulnerability to several neuropsychiatric disorders, the mechanism via which RGS proteins control neuronal responses and their potential use as drug targets.

Nonequilibrium calcium dynamics regulate the autonomous firing pattern of rat striatal cholinergic interneurons

Striatal cholinergic interneurons discharge rhythmically in two patterns associated with different afterhyperpolarization timescales, each dictated by a different calcium-dependent potassium current. Single spiking depends on a medium-duration afterhyperpolarization (mAHP) generated by rapid SK currents that are associated with N-type calcium channels. Periodic bursting is driven by a delayed and slowly decaying afterhyperpolarization (sAHP) current associated with L-type channels. Using calcium imaging we show that the calcium transients underlying these currents exhibit two corresponding timescales throughout the somatodendritic tree. This result is not consistent with spatial compartmentalization of calcium entering through the two calcium channels and acting on the two potassium currents, or with differences in channel gating kinetics of the calcium dependent potassium currents. Instead, we show that nonequilibrium dynamics of calcium redistribution among cytoplasmic binding sites with different calcium binding kinetics can give rise to multiple timescales within the same cytoplasmic volume. The resulting independence of mAHP and sAHP currents allows cytoplasmic calcium to control two different and incompatible firing patterns (single spiking or bursting and pausing), depending on whether calcium influx is pulsatile or sustained. During irregular firing, calcium entry at both timescales can be detected, suggesting that an interaction between the medium and slow calcium-dependent afterhyperpolarizations may underlie this firing pattern.

Two R7 regulator of G-protein signaling proteins shape retinal bipolar cell signaling

RGS7, RGS11, and their binding partner Gbeta5 are localized to the dendritic tips of retinal ON bipolar cells (ON-BPC), where mGluR6 responds to glutamate released from photoreceptor terminals by activation of the RGS7/RGS11 substrate, Galphao. To determine their functions in retinal signaling, we investigated cell-specific expression patterns of RGS7 and RGS11 by immunostaining, and measured light responses by electroretinography in mice with targeted disruptions of the genes encoding them. RGS7 staining is present in dendritic tips of all rod ON-BPC, but missing in those for subsets of cone ON-BPC, whereas the converse was true for RGS11 staining. Genetic disruption of either RGS7 or RGS11 produced delays in the ON-BPC-derived electroretinogram b-wave, but no changes in the photoreceptor-derived a-wave. Homozygous RGS7 mutant mice had delays in rod-driven b-waves, whereas RGS11 mutant mice had delays in rod-driven, and especially in cone-driven b-waves. The b-wave delays were further enhanced in mice homozygous for both RGS7 and RGS11 gene disruptions. Thus, RGS7 and RGS11 act in parallel to regulate the kinetics of ON bipolar cell responses, with differential impacts on the rod and cone pathways.

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.

Direct spatial control of Epac1 by cyclic AMP

Epac1 is a guanine nucleotide exchange factor (GEF) for the small G protein Rap and is directly activated by cyclic AMP (cAMP). Upon cAMP binding, Epac1 undergoes a conformational change that allows the interaction of its GEF domain with Rap, resulting in Rap activation and subsequent downstream effects, including integrin-mediated cell adhesion and cell-cell junction formation. Here, we report that cAMP also induces the translocation of Epac1 toward the plasma membrane. Combining high-resolution confocal fluorescence microscopy with total internal reflection fluorescence and fluorescent resonance energy transfer assays, we observed that Epac1 translocation is a rapid and reversible process. This dynamic redistribution of Epac1 requires both the cAMP-induced conformational change as well as the DEP domain. In line with its translocation, Epac1 activation induces Rap activation predominantly at the plasma membrane. We further show that the translocation of Epac1 enhances its ability to induce Rap-mediated cell adhesion. Thus, the regulation of Epac1-Rap signaling by cAMP includes both the release of Epac1 from autoinhibition and its recruitment to the plasma membrane.

RGS9-2: probing an intracellular modulator of behavior as a drug target

Regulators of G-protein signaling (RGS proteins) comprise a large family of signal transduction molecules that modulate G-protein-coupled-receptor (GPCR) function. Among the RGS proteins expressed in the brain, RGS9-2 is very abundant in the striatum, a brain region involved in movement, motivation, mood and addiction. This protein negatively modulates signal transduction thus playing a key part in striatal function and resultant behavioral responses. In particular, there is evidence of important interactions with mu-opioid- and dopamine D(2)-receptor signaling pathways. Several studies indicate that manipulations of RGS9-2 levels in the striatum might greatly affect pharmacological responses. These findings indicate that treatment strategies targeting RGS9-2 levels or activity might be used to enhance responses to drugs acting at GPCRs and/or prevent undesired drug actions.

Functional comparison of RGS9 splice isoforms in a living cell

Two isoforms of the GTPase-activating protein, regulator of G protein signaling 9 (RGS9), control such fundamental functions as vision and behavior. RGS9-1 regulates phototransduction in rods and cones, and RGS9-2 regulates dopamine and opioid signaling in the basal ganglia. To determine their functional differences in the same intact cell, we replaced RGS9-1 with RGS9-2 in mouse rods. Surprisingly, RGS9-2 not only supported normal photoresponse recovery under moderate light conditions but also outperformed RGS9-1 in bright light. This versatility of RGS9-2 results from its ability to inactivate the G protein, transducin, regardless of its effector interactions, whereas RGS9-1 prefers the G protein-effector complex. Such versatility makes RGS9-2 an isoform advantageous for timely signal inactivation across a wide range of stimulus strengths and may explain its predominant representation throughout the nervous system.

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

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

Dopamine induced switch in the subthreshold dynamics of the striatal cholinergic interneurons: a numerical study

The striatum is a part of the basal ganglia, which are a group of nuclei in the brain associated with motor control, cognition and learning. Striatal cholinergic interneurons (AchNs) play a crucial role in these functions. AchNs are tonically active in vivo and in vitro, and are able to fire in the absence of synaptic inputs. AchNs respond to sensory stimuli and sensorimotor learning by transiently suppressing their firing activity. This pause is dopamine signal sensitive, but the neurophysiological mechanism of the dopaminergic influence is under debate. Both the regular spiking response as well as the pause response are influenced by the inwardly rectifying outward G(kir), a slow hyperpolarization activated noninactivating G(h), and calcium and calcium-dependent potassium conductances [Wilson, C., Goldberg, J., 2006. Origin of the slow afterhyperpolarization and slow rhythmic bursting in striatal cholinergic interneurons. J. Neurophysiol. 95(1), 196-204; Wilson, C., 2005. The mechanism of intrinsic amplification of hyperpolarizations and spontaneous bursting in striatal cholinergic interneurons. Neuron 45(4), 575-585]. Recent experimental evidence has shown that dopaminergic modulations on G(h), G(kir) and calcium conductances influence the AchN's excitability [Deng, P., Zhang, Y., Xu, Z., 2007. Involvement of I(h) in dopamine modulation of tonic firing in striatal cholinergic interneurons. J. Neurosci. 27(12), 3148-3156; Aosaki, T., Kiuchi, K., Kawaguchi, Y., 1998. Dopamine D(1)-like receptor activation excites rat striatal large aspiny neurons in vitro. J. Neurosci. 18(14), 5180-5190]. We employed computational models of the AchN to analyze the conductance based dopaminergic changes. We analyzed the robustness of these subthreshold oscillations and how they are affected by dopaminergic modulation. Our results predict that these conductances allow the dopamine to switch the AchN between stable oscillatory and fixed-point behaviors. The present approach and results show that dopamine receptors (D(1) and D(2)) mediate opposing effects on this switch and therefore on the suprathreshold excitability as well. The switching effect of the dopaminergic signal is the major qualitative feature that can serve as a building block for higher network-level descriptions. To our knowledge this is the first paper that synthesizes the growing body of experimental literature about the dopaminergic modulation of the AchNs into a modelling framework.

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.

Lack of receptor-selective effects of either RGS2, RGS3 or RGS4 on muscarinic M3- and gonadotropin-releasing hormone receptor-mediated signalling through G alpha q/11

Termination of signalling by G-protein-coupled receptors requires inactivation of the G alpha-subunits of heterotrimeric G-proteins and the re-association of G alpha- and G betagamma-subunits. Inactivation of G alpha-subunits is achieved by the hydrolysis of bound GTP by an intrinsic GTPase activity, which is considerably enhanced by GTPase activating proteins. Regulators of G-protein signalling (RGS) proteins are a large family of GTPase activating proteins, many of which have structures indicating roles beyond GTPase activating protein activity and suggesting that the identity of the RGS protein recruited may also be critical to other aspects of signalling. There is some evidence of selective effects of RGS proteins against different G-protein-coupled receptors coupling to the same signalling pathways and growing evidence of physical interactions between RGS proteins and G-protein-coupled receptors. However, it is unclear as to how common such interactions are and the circumstances under which they are functionally relevant. Here we have examined potential selectivity of RGS2, 3 and 4 against signalling mediated by G alpha q/11-coupled muscarinic M3 receptors and gonadotropin-releasing hormone in an immortalised mouse pituitary cell line. Despite major structural differences between these two receptor types and agonist-dependent phosphorylation of the muscarinic M3- but not gonadotropin-releasing hormone receptor, signalling by both receptors was similarly inhibited by expression of either RGS2 or RGS3, whereas RGS4 has little effect. Thus, at least in these circumstances, RGS protein-dependent inhibition of signalling is not influenced by the nature of the G-protein-coupled receptor through which the signalling is mediated.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Re-emergence of striatal cholinergic interneurons in movement disorders

Twenty years ago, striatal cholinergic neurons were central figures in models of basal ganglia function. But since then, they have receded in importance. Recent studies are likely to lead to their re-emergence in our thinking. Cholinergic interneurons have been implicated as key players in the induction of synaptic plasticity and motor learning, as well as in motor dysfunction. In Parkinson's disease and dystonia, diminished striatal dopaminergic signalling leads to increased release of acetylcholine by interneurons, distorting network function and inducing structural changes that undoubtedly contribute to the symptoms. By contrast, in Huntington's disease and progressive supranuclear palsy, there is a fall in striatal cholinergic markers. This review gives an overview of these recent experimental and clinical studies, placing them within the context of the pathogenesis of movement disorders.

A role of RGS proteins in drug addiction

The diverse family of Regulators of G protein signaling (RGS) proteins are widely distributed proteins with multiple functions, including GAP activity for heterotrimeric G protein alpha subunits. Three members of the RGS family, RGS9-2, RGS4 and RGSz, have been shown to play an essential modulatory role in psychostimulant and opiate drug actions. Interestingly, these proteins show distinct structure, distribution pattern and cellular localization. In addition, each of these proteins is differentially regulated by drugs of abuse in particular brain networks and appears to modulate distinct signal transduction events. The striatal enriched RGS9 plays a prominent role in opiate and psychostimulant drug reward; RGS4 appears to modulate opiate dependence via actions in the locus coeruleus, whereas RGSz modulates analgesia via activation of the PKC pathway.

Direct G protein modulation of Cav2 calcium channels

The regulation of presynaptic, voltage-gated calcium channels by activation of heptahelical G protein-coupled receptors exerts a crucial influence on presynaptic calcium entry and hence on neurotransmitter release. Receptor activation subjects presynaptic N- and P/Q-type calcium channels to a rapid, membrane-delimited inhibition-mediated by direct, voltage-dependent interactions between G protein betagamma subunits and the channels-and to a slower, voltage-independent modulation involving soluble second messenger molecules. In turn, the direct inhibition of the channels is regulated as a function of many factors, including channel subtype, ancillary calcium channel subunits, and the types of G proteins and G protein regulatory factors involved. Twenty-five years after this mode of physiological regulation was first described, we review the investigations that have led to our current understanding of its molecular mechanisms.

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

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

How regulators of G protein signaling achieve selective regulation

The regulators of G protein signaling (RGS) are a family of cellular proteins that play an essential regulatory role in G protein-mediated signal transduction. There are multiple RGS subfamilies consisting of over 20 different RGS proteins. They are basically the guanosine triphosphatase (GTPase)-accelerating proteins that specifically interact with G protein alpha subunits. RGS proteins display remarkable selectivity and specificity in their regulation of receptors, ion channels, and other G protein-mediated physiological events. The molecular and cellular mechanisms underlying such selectivity are complex and cooperate at many different levels. Recent research data have provided strong evidence that the spatiotemporal-specific expression of RGS proteins and their target components, as well as the specific protein-protein recognition and interaction through their characteristic structural domains and functional motifs, are determinants for RGS selectivity and specificity. Other molecular mechanisms, such as alternative splicing and scaffold proteins, also significantly contribute to RGS selectivity. To pursue a thorough understanding of the mechanisms of RGS selective regulation will be of great significance for the advancement of our knowledge of molecular and cellular signal transduction.

Psychosis pathways converge via D2high dopamine receptors

The objective of this review is to identify a target or biomarker of altered neurochemical sensitivity that is common to the many animal models of human psychoses associated with street drugs, brain injury, steroid use, birth injury, and gene alterations. Psychosis in humans can be caused by amphetamine, phencyclidine, steroids, ethanol, and brain lesions such as hippocampal, cortical, and entorhinal lesions. Strikingly, all of these drugs and lesions in rats lead to dopamine supersensitivity and increase the high-affinity states of dopamine D2 receptors, or D2High, by 200-400% in striata. Similar supersensitivity and D2High elevations occur in rats born by Caesarian section and in rats treated with corticosterone or antipsychotics such as reserpine, risperidone, haloperidol, olanzapine, quetiapine, and clozapine, with the latter two inducing elevated D2High states less than that caused by haloperidol or olanzapine. Mice born with gene knockouts of some possible schizophrenia susceptibility genes are dopamine supersensitive, and their striata reveal markedly elevated D2High states; suchgenes include dopamine-beta-hydroxylase, dopamine D4 receptors, G protein receptor kinase 6, tyrosine hydroxylase, catechol-O-methyltransferase, the trace amine-1 receptor, regulator of G protein signaling RGS9, and the RIIbeta form of cAMP-dependent protein kinase (PKA). Striata from mice that are not dopamine supersensitive did not reveal elevated D2High states; these include mice with knockouts of adenosine A2A receptors, glycogen synthase kinase GSK3beta, metabotropic glutamate receptor 5, dopamine D1 or D3 receptors, histamine H1, H2, or H3 receptors, and rats treated with ketanserin or aD1 antagonist. The evidence suggests that there are multiple pathways that convergetoelevate the D2High state in brain regions and that this elevation may elicit psychosis. This proposition is supported by the dopamine supersensitivity that is a common feature of schizophrenia and that also occurs in many types of genetically altered, drug-altered, and lesion-altered animals. Dopamine supersensitivity, in turn, correlates with D2High states. The finding that all antipsychotics, traditional and recent ones, act on D2High dopamine receptors further supports the proposition.

Bacterial artificial chromosome transgenic analysis of dynamic expression patterns of regulator of G-protein signaling 4 during development. II. Subcortical regions

A large family of regulator of G protein signaling (RGS) proteins modulates signaling through G-protein-coupled receptors. Previous studies have implicated RGS4 as a vulnerability gene in schizophrenia. To begin to understand structure-function relationships, we have utilized bacterial artificial chromosome (BAC) methods to create transgenic mice that express green fluorescent protein (GFP) under the control of endogenous RGS4 enhancer elements, circumventing the lack of suitable antibodies for analysis of dynamic patterns of expression. This report follows from the accompanying mapping paper in cerebral cortex, with a focus on developmental and mature expression patterns in subcortical telencephalic, diencephalic and brainstem areas. Based on reporter distribution, the data suggest that alterations in RGS4 function will engender a complex phenotype of increased and decreased neuronal output, with developmental, regional, and cellular specificity.

Making the case for a candidate vulnerability gene in schizophrenia: Convergent evidence for regulator of G-protein signaling 4 (RGS4)

Both genetic and environmental factors have been associated with an increased risk for schizophrenia. These factors are not mutually exclusive; a single gene can be a genetic factor (due to a mutation in the gene sequence) and a target of a physiological response to an environmental stimulus, both with the common endpoint of altered expression of the gene. Regulator of G-protein signaling 4 (RGS4) has been implicated as such a gene from three lines of evidence. First, a subset of genetic studies revealed an association between schizophrenia and non-functional polymorphisms in the RGS4 gene. Second, across the cortical mantle the expression of RGS4 mRNA is decreased in a diagnosis-specific manner in subjects with schizophrenia. Third, neurobiological studies demonstrate that RGS4 is highly responsive to environmental stimuli and capable of modulating the function of G-protein coupled neurotransmitter receptors implicated in schizophrenia. RGS4 is an example of a molecule that may underlie increased vulnerability through either genetic or non-genetic mechanisms, which we suggest may be typical of other genes in a complex, polygenic disorder such as schizophrenia.

R7BP augments the function of RGS7*Gbeta5 complexes by a plasma membrane-targeting mechanism

The RGS7 (R7) family of G protein regulators, Gbeta5, and R7BP form heterotrimeric complexes that potently regulate the kinetics of G protein-coupled receptor signaling. Reversible palmitoylation of R7BP regulates plasma membrane/nuclear shuttling of R7*Gbeta5*R7BP heterotrimers. Here we have investigated mechanisms whereby R7BP controls the function of the R7 family. We show that unpalmitoylated R7BP undergoes nuclear/cytoplasmic shuttling and that a C-terminal polybasic motif proximal to the palmitoylation acceptor sites of R7BP mediates nuclear localization, palmitoylation, and plasma membrane targeting. These results suggest a novel mechanism whereby palmitoyltransferases and nuclear import receptors both utilize the C-terminal domain of R7BP to determine the trafficking fate of R7*Gbeta5*R7BP heterotrimers. Analogous mechanisms may regulate other signaling proteins whose distribution between the plasma membrane and nucleus is controlled by palmitoylation. Lastly, we show that cytoplasmic RGS7*Gbeta5*R7BP heterotrimers and RGS7*Gbeta5 heterodimers are equivalently inefficient regulators of G protein-coupled receptor signaling relative to plasma membrane-bound heterotrimers bearing palmitoylated R7BP. Therefore, R7BP augments the function of the complex by a palmitoylation-regulated plasma membrane-targeting mechanism.

Cellular mechanisms that determine selective RGS protein regulation of G protein-coupled receptor signaling

Regulators of G protein signaling (RGS proteins) bind directly to activated Galpha subunits to inhibit their signaling. However, recent findings show that RGS proteins selectively regulate signaling by certain G protein-coupled receptors (GPCRs) in cells, irrespective of the coupled G protein. New studies support an emerging model that suggests RGS proteins utilize both direct and indirect mechanisms to form stable functional pairs with preferred GPCRs to selectively modulate the signaling functions of those receptors and linked G proteins. Here, we discuss these findings and their implications for established models of GPCR signaling.

Mammalian RGS proteins: Multifunctional regulators of cellular signalling

Regulators of G-protein signalling (RGS) proteins are a large and diverse family initially identified as GTPase activating proteins (GAPs) of heterotrimeric G-protein Galpha-subunits. At least some can also influence Galpha activity through either effector antagonism or by acting as guanine nucleotide dissociation inhibitors (GDIs). As our understanding of RGS protein structure and function has developed, so has the realisation that they play roles beyond G-protein regulation. Such diversity of function is enabled by the variety of RGS protein structure and their ability to interact with other cellular molecules including phospholipids, receptors, effectors and scaffolds. The activity, sub-cellular distribution and expression levels of RGS proteins are dynamically regulated, providing a layer of complexity that has yet to be fully elucidated.

RGS4-dependent attenuation of M4 autoreceptor function in striatal cholinergic interneurons following dopamine depletion

Parkinson disease is a neurodegenerative disorder whose symptoms are caused by the loss of dopaminergic neurons innervating the striatum. As striatal dopamine levels fall, striatal acetylcholine release rises, exacerbating motor symptoms. This adaptation is commonly attributed to the loss of interneuronal regulation by inhibitory D(2) dopamine receptors. Our results point to a completely different, new mechanism. After striatal dopamine depletion, D(2) dopamine receptor modulation of calcium (Ca(2+)) channels controlling vesicular acetylcholine release in interneurons was unchanged, but M(4) muscarinic autoreceptor coupling to these same channels was markedly attenuated. This adaptation was attributable to the upregulation of RGS4-an autoreceptor-associated, GTPase-accelerating protein. This specific signaling adaptation extended to a broader loss of autoreceptor control of interneuron spiking. These observations suggest that RGS4-dependent attenuation of interneuronal autoreceptor signaling is a major factor in the elevation of striatal acetylcholine release in Parkinson disease.

Selection of neural differentiation-specific genes by comparing profiles of random differentiation

Differentiation of embryonic stem cells (ESCs) into neurons requires a high level of transcriptional regulation. To further understand the transcriptional regulation of neural differentiation of ESCs, we used oligonucleotide microarray to examine the gene expressions of the guided differentiation (GD) model for dopaminergic (DA) neurons from mouse ESCs. We also determined the gene expression profiles of the random differentiation (RD) model of mouse ESCs into embryoid bodies. From K-means clustering analysis using the expression patterns of the two models, most of the genes (1,282 of 1,884 genes [68.0%]) overlapped in their expression patterns. Six hundred twenty-two differentially expressed genes (DEGs) from the GD model by random variance F-test were classified by their critical molecular functions in neurogenesis and DNA replication (Gene Ontology analysis). However, 400 genes among GD-DEGs (64.3%) showed a high correlation with RD in Spearman's correlation analysis (Spearman's coefficient p(s) >or= .6). The genes showing marginal correlation (-.4 < p(s) < .6) were present in the early stages of differentiation of both GD and RD, which were non-specific to brain development. Finally, we distinguished 66 GD-specific genes based on p(s) <or= -.4, the molecular functions of which were related mainly to vesicle formation, neurogenesis, and transcription factors. From among these GD-specific genes, we confirmed the expression of Serpini1 and Rab33a in P19 differentiation models and adult brains. From these results, we identified the specific genes required for neural differentiation by comparing gene expressions of GD with RD; these would potentially be the highly specific candidate genes necessary for differentiation of DA neurons.

Subcellular targeting of RGS9-2 is controlled by multiple molecular determinants on its membrane anchor, R7BP

RGS9-2, a member of the R7 regulators of G protein signaling (RGS) protein family of neuronal RGS, is a critical regulator of G protein signaling. In striatal neurons, RGS9-2 is tightly associated with a novel palmitoylated protein, R7BP (R7 family binding protein). Here we report that R7BP acts to target the localization of RGS9-2 to the plasma membrane. Examination of the subcellular distribution in native striatal neurons revealed that both R7BP and RGS9-2 are almost entirely associated with the neuronal membranes. In addition to the plasma membrane, a large portion of RGS9-2 was found in the neuronal specializations, the postsynaptic densities, where it forms complexes with R7BP and its constitutive partner Gbeta5. Using site-directed mutagenesis we found that the molecular determinants that specify the subcellular targeting of RGS9-2.Gbeta5.R7BP complex are contained within the 21 C-terminal amino acids of R7BP. This function of the C terminus was found to require the synergistic contributions of its two distinct elements, a polybasic motif and palmitoylated cysteines, which when combined are sufficient for directing the intracellular localization of the constituent protein. In differentiated neurons, the C-terminal targeting motif of R7BP was found to be essential for mediating its postsynaptic localization. In addition to the plasma membrane targeting elements, we identified two functional nuclear localization sequences that can mediate the import of R7BP into the nucleus upon depalmitoylation. These findings provide a mechanism for the subcellular targeting of RGS9-2 in neurons.

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.

D2 dopamine receptors colocalize regulator of G-protein signaling 9-2 (RGS9-2) via the RGS9 DEP domain, and RGS9 knock-out mice develop dyskinesias associated with dopamine pathways

Regulator of G-protein signaling 9-2 (RGS9-2), a member of the RGS family of G GTPase accelerating proteins, is expressed specifically in the striatum, which participates in antipsychotic-induced tardive dyskinesia and in levodopa-induced dyskinesia. We report that RGS9 knock-out mice develop abnormal involuntary movements when inhibition of dopaminergic transmission is followed by activation of D2-like dopamine receptors (DRs). These abnormal movements resemble drug-induced dyskinesia more closely than other rodent models. Recordings from striatal neurons of these mice establish that activation of D2-like DRs abnormally inhibits glutamate-elicited currents. We show that RGS9-2, via its DEP domain (for Disheveled, EGL-10, Pleckstrin homology), colocalizes with D2DRs when coexpressed in mammalian cells. Recordings from oocytes coexpressing D2DR or the m2 muscarinic receptor and G-protein-gated inward rectifier potassium channels show that RGS9-2, via its DEP domain, preferentially accelerates the termination of D2DR signals. Thus, alterations in RGS9-2 may be a key factor in the pathway leading from D2DRs to the side effects associated with the treatment both of psychoses and Parkinson's disease.

RACK1 binds to a signal transfer region of G betagamma and inhibits phospholipase C beta2 activation

Receptor for Activated C Kinase 1 (RACK1), a novel G betagamma-interacting protein, selectively inhibits the activation of a subclass of G betagamma effectors such as phospholipase C beta2 (PLCbeta2) and adenylyl cyclase II by direct binding to G betagamma (Chen, S., Dell, E. J., Lin, F., Sai, J., and Hamm, H. E. (2004) J. Biol. Chem. 279, 17861-17868). Here we have mapped the RACK1 binding sites on G betagamma. We found that RACK1 interacts with several different G betagamma isoforms, including G beta1gamma1, Gbeta1gamma2, and Gbeta5gamma2, with similar affinities, suggesting that the conserved residues between G beta1 and G beta5 may be involved in their binding to RACK1. We have confirmed this hypothesis and shown that several synthetic peptides corresponding to the conserved residues can inhibit the RACK1/G betagamma interaction as monitored by fluorescence spectroscopy. Interestingly, these peptides are located at one side of G beta1 and have little overlap with the G alpha subunit binding interface. Additional experiments indicate that the G betagamma contact residues for RACK1, in particular the positively charged amino acids within residues 44-54 of G beta1, are also involved in the interaction with PLCbeta2 and play a critical role in G betagamma-mediated PLCbeta2 activation. These data thus demonstrate that RACK1 can regulate the activity of a G betagamma effector by competing for its binding to the signal transfer region of G betagamma.

Selective inhibition of alpha1A-adrenergic receptor signaling by RGS2 association with the receptor third intracellular loop

Regulators of G-protein signaling (RGS) proteins act directly on Galpha subunits to increase the rate of GTP hydrolysis and to terminate signaling. However, the mechanisms involved in determining their specificities of action in cells remain unclear. Recent evidence has raised the possibility that RGS proteins may interact directly with G-protein-coupled receptors to modulate their activity. By using biochemical, fluorescent imaging, and functional approaches, we found that RGS2 binds directly and selectively to the third intracellular loop of the alpha1A-adrenergic receptor (AR) in vitro, and is recruited by the unstimulated alpha1A-AR to the plasma membrane in cells to inhibit receptor and Gq/11 signaling. This interaction was specific, because RGS2 did not interact with the highly homologous alpha1B- or alpha1D-ARs, and the closely related RGS16 did not interact with any alpha1-ARs. The N terminus of RGS2 was required for association with alpha1A-ARs and inhibition of signaling, and amino acids Lys219, Ser220, and Arg238 within the alpha1A-AR i3 loop were found to be essential for this interaction. These findings demonstrate that certain RGS proteins can directly interact with preferred G-protein-coupled receptors to modulate their signaling with a high degree of specificity.

Palmitoylation regulates plasma membrane-nuclear shuttling of R7BP, a novel membrane anchor for the RGS7 family

The RGS7 (R7) family of RGS proteins bound to the divergent Gβ subunit Gβ5 is a crucial regulator of G protein–coupled receptor (GPCR) signaling in the visual and nervous systems. Here, we identify R7BP, a novel neuronally expressed protein that binds R7–Gβ5 complexes and shuttles them between the plasma membrane and nucleus. Regional expression of R7BP, Gβ5, and R7 isoforms in brain is highly coincident. R7BP is palmitoylated near its COOH terminus, which targets the protein to the plasma membrane. Depalmitoylation of R7BP translocates R7BP–R7–Gβ5 complexes from the plasma membrane to the nucleus. Compared with nonpalmitoylated R7BP, palmitoylated R7BP greatly augments the ability of RGS7 to attenuate GPCR-mediated G protein–regulated inward rectifying potassium channel activation. Thus, by controlling plasma membrane nuclear–shuttling of R7BP–R7–Gβ5 complexes, reversible palmitoylation of R7BP provides a novel mechanism that regulates GPCR signaling and potentially transduces signals directly from the plasma membrane to the nucleus.