Rhes, the Ras homolog enriched in striatum, is reduced under conditions of dopamine supersensitivity

Involvement of the Protein Ras Homolog Enriched in the Striatum, Rhes, in Dopaminergic Neurons' Degeneration: Link to Parkinson's Disease

Rhes is one of the most interesting genes regulated by thyroid hormones that, through the inhibition of the striatal cAMP/PKA pathway, acts as a modulator of dopamine neurotransmission. Rhes mRNA is expressed at high levels in the dorsal striatum, with a medial-to-lateral expression gradient reflecting that of both dopamine D₂ and adenosine A_2A receptors. Rhes transcript is also present in the hippocampus, cerebral cortex, olfactory tubercle and bulb, substantia nigra pars compacta (SNc) and ventral tegmental area of the rodent brain. In line with Rhes-dependent regulation of dopaminergic transmission, data showed that lack of Rhes enhanced cocaine- and amphetamine-induced motor stimulation in mice. Previous studies showed that pharmacological depletion of dopamine significantly reduces Rhes mRNA levels in rodents, non-human primates and Parkinson's disease (PD) patients, suggesting a link between dopaminergic innervation and physiological Rhes mRNA expression. Rhes protein binds to and activates striatal mTORC1, and modulates L-DOPA-induced dyskinesia in PD rodent models. Finally, Rhes is involved in the survival of mouse midbrain dopaminergic neurons of SNc, thus pointing towards a Rhes-dependent modulation of autophagy and mitophagy processes, and encouraging further investigations about mechanisms underlying dysfunctions of the nigrostriatal system.

MicroRNA-101 inhibits the expression of Rhes, a striatal-enriched small G-protein, at the post-transcriptional level in vitro

Objective

Ras homolog enriched in striatum (Rhes) is a small GTP-binding protein that is predominantly localized in the striatal region of the brain. Rhes affects various signaling pathways and plays important roles in Huntington’s disease development caused by striatal anomalies. However, the mechanism underlying the regulation of Rhes expression is not fully understood. We hypothesized that Rhes expression might be regulated by microRNAs (miRNAs), which are small noncoding RNAs that regulate gene expression by interacting with the 3′-untranslated region (3′UTR) of mRNA. This study therefore investigated the interaction between miRNAs and the Rhes mRNA 3′UTR.

Results

The results of luciferase assay showed that miR-101, the miRNA determined to have the highest possibility of interacting with the Rhes mRNA 3′UTR using DIANA-microT, significantly inhibits luciferase activity, suggesting that miR-101 directly targets the Rhes mRNA 3′UTR. Additionally, Rhes protein levels in cultured cells co-transfected with a plasmid containing the complete Rhes cDNA and miR-101 were significantly downregulated by miR-101 as demonstrated by western blot analysis. These results support our hypothesis that Rhes expression is regulated by miRNA and indicate that miR-101 may be a potent modulator of Rhes expression in striatal neurons.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3654-5) contains supplementary material, which is available to authorized users.

The Thyroid Hormone-target Gene Rhes a Novel Crossroad for Neurological and Psychiatric Disorders: New Insights from Animal Models

Ras homolog enriched in striatum (Rhes) is predominantly expressed in the corpus striatum. Rhes mRNA is localized in virtually all dopamine D1 and D2 receptor-bearing medium-sized spiny neurons (MSNs), and cholinergic interneurons of striatum. Early studies in rodents showed that Rhes is developmentally regulated by thyroid hormone, as well as by dopamine innervation in adult rat, monkey and human brains. At cellular level, Rhes interferes with adenosine A2A- and dopamine D1 receptor-dependent cAMP/PKA pathway, upstream of the activation of the heterotrimeric G protein complex. Besides its involvement in GPCR-mediated signaling, Rhes modulates Akt pathway activation, acts as E3-ligase of mutant huntingtin, whose sumoylation accounts for neurotoxicity in Huntington's disease, and physically interacts with Beclin-1, suggesting its potential involvement in autophagy-related cellular events. In addition, this protein can also bind to and activate striatal mTORC1, one of the key players in l-DOPA-induced dyskinesia in rodent models of Parkinson's disease. Accordingly, lack of Rhes attenuated such motor disturbances in 6-OHDA-lesioned Rhes knockout mice. In support of its role in MSN-dependent functions, several studies documented that mutant animals displayed alterations in striatum-related phenotypes reminiscent of psychiatric illness in humans, including deficits in prepulse inhibition of startle reflex and, most interestingly, a striking enhancement of behavioral responses elicited by caffeine, phencyclidine or amphetamine. Overall, these data suggest that Rhes modulates molecular and biochemical events underlying striatal functioning, both in physiological and pathological conditions.

Decreased Rhes mRNA levels in the brain of patients with Parkinson's disease and MPTP-treated macaques

In rodent and human brains, the small GTP-binding protein Rhes is highly expressed in virtually all dopaminoceptive striatal GABAergic medium spiny neurons, as well as in large aspiny cholinergic interneurons, where it is thought to modulate dopamine-dependent signaling. Consistent with this knowledge, and considering that dopaminergic neurotransmission is altered in neurological and psychiatric disorders, here we sought to investigate whether Rhes mRNA expression is altered in brain regions of patients with Parkinson’s disease (PD), Schizophrenia (SCZ), and Bipolar Disorder (BD), when compared to healthy controls (about 200 post-mortem samples). Moreover, we performed the same analysis in the putamen of non-human primate Macaca Mulatta, lesioned with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Overall, our data indicated comparable Rhes mRNA levels in the brain of patients with SCZ and BD, and their respective healthy controls. In sharp contrast, the putamen of patients suffering from PD showed a significant 35% reduction of this transcript, compared to healthy subjects. Interestingly, in line with observations obtained in humans, we found 27% decrease in Rhes mRNA levels in the putamen of MPTP-treated primates. Based on the established inhibitory influence of Rhes on dopamine-related responses, we hypothesize that its striatal downregulation in PD patients and animal models of PD might represent an adaptive event of the dopaminergic system to functionally counteract the reduced nigrostriatal innervation.

The Small GTP-Binding Protein Rhes Influences Nigrostriatal-Dependent Motor Behavior During Aging

BACKGROUND

Here we aimed to evaluate: (1) Rhes mRNA expression in mouse midbrain, (2) the effect of Rhes deletion on the number of dopamine neurons, (3) nigrostriatal-sensitive behavior during aging in knockout mice.

METHODS

Radioactive in situ hybridization was assessed in adult mice. The beam-walking test was executed in 3-, 6- and 12-month-old mice. Immunohistochemistry of midbrain tyrosine hydroxylase (TH)-positive neurons was performed in 6- and 12-month-old mice.

RESULTS

Rhes mRNA is expressed in TH-positive neurons of SNpc and the ventral tegmental area. Moreover, lack of Rhes leads to roughly a 20% loss of nigral TH-positive neurons in both 6- and 12-month-old mutants, when compared with their age-matched controls. Finally, lack of Rhes triggers subtle alterations in motor performance and coordination during aging.

CONCLUSIONS

Our findings indicate a fine-tuning role of Rhes in regulating the number of TH-positive neurons of the substantia nigra and nigrostriatal-sensitive motor behavior during aging.

Rasd2 Modulates Prefronto-Striatal Phenotypes in Humans and 'Schizophrenia-Like Behaviors' in Mice

Rasd2 is a thyroid hormone target gene, which encodes for a GTP-binding protein enriched in the striatum where, among other functions, it modulates dopaminergic neurotransmission. Here we report that human RASD2 mRNA is abundant in putamen, but it also occurs in the cerebral cortex, with a distinctive expression pattern that differs from that present in rodents. Consistent with its localization, we found that a genetic variation in RASD2 (rs6518956) affects postmortem prefrontal mRNA expression in healthy humans and is associated with phenotypes of relevance to schizophrenia, including prefrontal and striatal grey matter volume and physiology during working memory, as measured with magnetic resonance imaging. Interestingly, quantitative real-time PCR analysis indicated that RASD2 mRNA is slightly reduced in postmortem prefrontal cortex of patients with schizophrenia. In the attempt to uncover the neurobiological substrates associated with Rasd2 activity, we used knockout mice to analyze the in vivo influence of this G-protein on the prepulse inhibition of the startle response and psychotomimetic drug-related behavioral response. Data showed that Rasd2 mutants display deficits in basal prepulse inhibition that, in turn, exacerbate gating disruption under psychotomimetic drug challenge. Furthermore, we documented that lack of Rasd2 strikingly enhances the behavioral sensitivity to motor stimulation elicited by amphetamine and phencyclidine. Based on animal model data, along with the finding that RASD2 influences prefronto-striatal phenotypes in healthy humans, we suggest that genetic mutation or reduced levels of this G-protein might have a role in cerebral circuitry dysfunction underpinning exaggerated psychotomimetic drugs responses and development of specific biological phenotypes linked to schizophrenia.

Genetic deletion of Rhes or pharmacological blockade of mTORC1 prevent striato-nigral neurons activation in levodopa-induced dyskinesia

Ras homolog enriched in striatum (Rhes) is a small GTP-binding protein that modulates signal transduction at dopamine receptors, and also activates mammalian target of rapamycin complex 1 (mTORC1). Rhes binding to mTORC1 is hypothesized to play a role in motor disorders such as levodopa-induced dyskinesia. Here, we investigate the behavioral and in vivo neurocircuitry changes associated with genetic deletion of Rhes or inhibition of mTORC1 signaling in the mouse model of levodopa-induced dyskinesia. 6-Hydroxydopamine-hemilesioned Rhes knockout mice and wild-type littermates were chronically treated with levodopa. In parallel, 6-hydroxydopamine-hemilesioned naïve mice were chronically treated with levodopa or levodopa plus rapamycin, to block mTORC1 pathway activation. Dyskinetic movements were monitored during levodopa treatment along with motor activity on the rotarod. Finally, dyskinetic mice underwent microdialysis probe implantation in the dopamine-depleted striatum and ipsilateral substantia nigra reticulata, and GABA and glutamate levels were monitored upon acute challenge with levodopa. Both Rhes knockouts and rapamycin-treated mice developed less dyskinesia than controls, although only rapamycin-treated mice fully preserved rotarod performance on levodopa. Levodopa elevated nigral GABA and glutamate in controls but not in Rhes knockouts or rapamycin-treated mice. Levodopa also stimulated striatal glutamate in controls and Rhes knockouts but not in rapamycin-treated mice. We conclude that both genetic deletion of Rhes and pharmacological blockade of mTORC1 significantly attenuate dyskinesia development by reducing the sensitization of striato-nigral medium-sized spiny neurons to levodopa. However, mTORC1 blockade seems to provide a more favorable behavioral outcome and a wider effect on neurochemical correlates of dyskinesia.

Rhes influences striatal cAMP/PKA-dependent signaling and synaptic plasticity in a gender-sensitive fashion

Mechanisms of gender-specific synaptic plasticity in the striatum, a brain region that controls motor, cognitive and psychiatric functions, remain unclear. Here we report that Rhes, a GTPase enriched in medium spiny neurons (MSNs) of striatum, alters the striatal cAMP/PKA signaling cascade in a gender-specific manner. While Rhes knockout (KO) male mice, compared to wild-type (WT) mice, had a significant basal increase of cAMP/PKA signaling pathway, the Rhes KO females exhibited a much stronger response of this pathway, selectively under the conditions of dopamine/adenosine-related drug challenge. Corticostriatal LTP defects are exclusively found in A2AR/D2R-expressing MSNs of KO females, compared to KO males, an effect that is abolished by PKA inhibitors but not by the removal of circulating estrogens. This suggests that the synaptic alterations found in KO females could be triggered by an aberrant A2AR/cAMP/PKA activity, but not due to estrogen-mediated effect. Consistent with increased cAMP signaling, D1R-mediated motor stimulation, haloperidol-induced catalepsy and caffeine-evoked hyper-activity are robustly enhanced in Rhes KO females compared to mutant males. Thus Rhes, a thyroid hormone-target gene, plays a relevant role in gender-specific synaptic and behavioral responses.

Rhes regulates dopamine D2 receptor transmission in striatal cholinergic interneurons

Ras homolog enriched in striatum (Rhes) is highly expressed in striatal medium spiny neurons (MSNs) of rodents. In the present study, we characterized the expression of Rhes mRNA across species, as well as its functional role in other striatal neuron subtypes. Double in situ hybridization analysis showed that Rhes transcript is selectively localized in striatal cholinergic interneurons (ChIs), but not in GABAergic parvalbumin- or in neuropeptide Y-positive cell populations. Rhes is closely linked to dopamine-dependent signaling. Therefore, we recorded ChIs activity in basal condition and following dopamine receptor activation. Surprisingly, instead of an expected dopamine D2 receptor (D2R)-mediated inhibition, we observed an aberrant excitatory response in ChIs from Rhes knockout mice. Conversely, the effect of D1R agonist on ChIs was less robust in Rhes mutants than in controls. Although Rhes deletion in mutants occurs throughout the striatum, we demonstrate that the D2R response is altered specifically in ChIs, since it was recorded in pharmacological isolation, and prevented either by intrapipette BAPTA or by GDP-β-S. Moreover, we show that blockade of Cav2.2 calcium channels prevented the abnormal D2R response. Finally, we found that the abnormal D2R activation in ChIs was rescued by selective PI3K inhibition thus suggesting that Rhes functionally modulates PI3K/Akt signaling pathway in these neurons. Our findings reveal that, besides its expression in MSNs, Rhes is localized also in striatal ChIs and, most importantly, lack of this G-protein, significantly alters D2R modulation of striatal cholinergic excitability.

PKA modulates iron trafficking in the striatum via small GTPase, Rhes

Ras homolog enriched in striatum (Rhes), is a highly conserved small guanosine-5'-triphosphate (GTP) binding protein belonging to the Ras superfamily. Rhes is involved in the dopamine receptor-mediated signaling and behavior though adenylyl cyclase. The striatum-specific GTPase share a close homology with Dexras1, which regulates iron trafficking in the neurons when activated though the post-translational modification called s-nitrosylation by nitric oxide (NO). We report that Rhes physiologically interacted with Peripheral benzodiazepine receptor-associated protein7 and participated in iron uptake via divalent metal transporter 1 similar to Dexras1. Interestingly, Rhes is not S-nitrosylated by NO-treatment, however phosphorylated by protein kinase A at the site of serine-239. Two Rhes mutants - the phosphomimetic form (serine 239 to aspartic acid) and constitutively active form (alanine 173 to valine) - displayed an increase in iron uptake compared to the wild-type Rhes. These findings suggest that Rhes may play a crucial role in striatal iron homeostasis.

The role of Rhes, Ras homolog enriched in striatum, in neurodegenerative processes

Rhes is a small GTPase whose expression is highly enriched in striatum. It shares homology with Ras proteins, but also contains a C-terminal extension, thus suggesting additional functions. Signaling by 7 transmembrane receptors through heterotrimeric G proteins is inhibited by Rhes. However, perhaps the most remarkable feature of this small GTPase described thus far is that it can account for the selective vulnerability of the striatum in Huntington's Disease (HD). HD is an autosomal dominant neurodegenerative disease caused by a poly-glutamine expansion in the protein huntingtin. Despite the presence of huntingtin throughout the brain and the rest of the body, the striatum is selectively degenerated. Recent work shows that Rhes acts as an E3 ligase for attachment of SUMO (small ubiquitin-like modifier). As this post-translational modification decreases the formation of huntingtin aggregates and promotes cell death, this property of Rhes offers an explanation for selective striatal vulnerability in HD. In addition, the sequestering of Rhes through its binding to mutant huntingtin may decrease the ability of Rhes to perform vital physiological functions in the neuron. Thus, as Rhes is an attractive candidate for HD therapy, a thorough understanding of its physiological functions will allow for specific targeting of its pathological functions.

The DAO gene is associated with schizophrenia and interacts with other genes in the Taiwan Han Chinese population

BACKGROUND

Schizophrenia is a highly heritable disease with a polygenic mode of inheritance. Many studies have contributed to our understanding of the genetic underpinnings of schizophrenia, but little is known about how interactions among genes affect the risk of schizophrenia. This study aimed to assess the associations and interactions among genes that confer vulnerability to schizophrenia and to examine the moderating effect of neuropsychological impairment.

METHODS

We analyzed 99 SNPs from 10 candidate genes in 1,512 subject samples. The permutation-based single-locus, multi-locus association tests, and a gene-based multifactorial dimension reduction procedure were used to examine genetic associations and interactions to schizophrenia.

RESULTS

We found that no single SNP was significantly associated with schizophrenia. However, a risk haplotype, namely A-T-C of the SNP triplet rsDAO7-rsDAO8-rsDAO13 of the DAO gene, was strongly associated with schizophrenia. Interaction analyses identified multiple between-gene and within-gene interactions. Between-gene interactions including DAO*DISC1 , DAO*NRG1 and DAO*RASD2 and a within-gene interaction for CACNG2 were found among schizophrenia subjects with severe sustained attention deficits, suggesting a modifying effect of impaired neuropsychological functioning. Other interactions such as the within-gene interaction of DAO and the between-gene interaction of DAO and PTK2B were consistently identified regardless of stratification by neuropsychological dysfunction. Importantly, except for the within-gene interaction of CACNG2, all of the identified risk haplotypes and interactions involved SNPs from DAO.

CONCLUSIONS

These results suggest that DAO, which is involved in the N-methyl-d-aspartate receptor regulation, signaling and glutamate metabolism, is the master gene of the genetic associations and interactions underlying schizophrenia. Besides, the interaction between DAO and RASD2 has provided an insight in integrating the glutamate and dopamine hypotheses of schizophrenia.

Effects of the Ras homolog Rhes on Akt/protein kinase B and glycogen synthase kinase 3 phosphorylation in striatum

G protein-coupled receptors (GPCR) signal not only through heterotrimeric G proteins, but also through alternate pathways. Thus, dopamine D2 receptors in the striatum signal through Gαi/o and also by promoting formation of a multi-protein complex containing β-arrestin2, protein phosphatase 2A (PP2A), and Akt in order to dephosphorylate Akt. Lithium, on the other hand, disrupts this complex to increase Akt phosphorylation. Rhes is a striatally enriched GTP-binding protein that has been shown to inhibit dopamine receptor-mediated behavior and signaling through heterotrimeric G proteins. Therefore, our objective was to test whether Rhes similarly affects signaling through the Akt/GSK3 pathway in the striatum. Rhes(-/-) mice showed basally increased Akt and GSK3β phosphorylation relative to rhes(+/+) mice that was not further enhanced by lithium treatment. Furthermore, they responded to the D1/D2 agonist apomorphine with increased Akt and GSK3 phosphorylation. Co-immunoprecipitation experiments revealed that apomorphine treatment recruits PP 2A-C to Akt in both rhes(+/+) and rhes(-/-) mice. Lithium did not disrupt their interaction in rhes(-/-) mice as there was little basal interaction. Rhes co-immunoprecipitated with β-arrestins, suggesting that it is integral to the multi-protein complex. Thus, Rhes is necessary for Akt dephosphorylation by the striatal multi-protein complex, and in its absence, a lithium-treated phenotype results.

Attenuation of Rhes activity significantly delays the appearance of behavioral symptoms in a mouse model of Huntington's disease

Huntington's disease (HD) is a neuropsychiatric disorder characterized by choreiform movement of the limbs, cognitive disability, psychosis and dementia. It is invariably associated with an abnormally long CAG expansion within the IT15 gene on human chromosome 4. Although the mutant huntingtin protein is ubiquitously expressed in HD patients, cellular degeneration occurs predominantly in neurons within the corpus striatum and cerebral cortex. The Ras homolog Rhes is expressed very selectively in the precise brain areas affected by HD. Recent in vitro work suggests that Rhes may be a co-factor with mutant huntingtin in cell death. The objective of the present study was to examine whether the inhibition of Rhes would attenuate or delay the symptoms of HD in vivo. We used a transgenic mouse model of HD crossed with Rhes knockout mice to show that the behavioral symptoms of HD are regulated by Rhes. HD⁺/Rhes^−/− mice showed significantly delayed expression of HD-like symptoms in this in vivo model. Drugs that block or inhibit the actions of Rhes may be useful as the first treatments for HD.

Dopamine receptor inactivation in the caudate-putamen differentially affects the behavior of preweanling and adult rats

The irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) has been used to study the ontogeny of dopamine (DA) receptor functioning in young and adult rats. Most notably, systemic administration of EEDQ blocks the DA agonist-induced behaviors of adult rats, while leaving the behavior of preweanling rats unaffected. The purpose of the present study was to: (a) determine whether the age-dependent actions of EEDQ involve receptors located in the dorsal caudate-putamen (CPu) and (b) confirm that EEDQ's behavioral effects result from the inactivation of DA receptors rather than some other receptor type. In Experiment 1, EEDQ or DMSO was bilaterally infused into the CPu on PD 17 or PD 84. After 24h, rats were given bilateral microinjections of the full DA agonist R(-)-propylnorapomorphine (NPA) or vehicle into the dorsal CPu and behavior was assessed for 40 min. In Experiment 2, preweanling rats were treated as just described, except that DA receptors were protected from EEDQ-induced alkylation by administering systemic injections of D1 (SCH23390) and D2 (sulpiride) receptor antagonists. As predicted, microinjecting EEDQ into the dorsal CPu attenuated the NPA-induced locomotor activity and stereotypy of adult rats. In contrast, rats given bilateral EEDQ infusions on PD 17 exhibited a potentiated locomotor response when treated with NPA. Experiment 2 showed that DA receptor inactivation was responsible for NPA's actions. A likely explanation for these results is that EEDQ inactivates a sizable percentage of DA receptors on PD 17, but leaves the remaining receptors in a supersensitive state. This receptor supersensitivity, which probably involves alterations in G protein coupling, could account for NPA-induced locomotor potentiation. It is likely that adult rats to not show a similar EEDQ-induced change in receptor dynamics or DA receptor inactivation was more complete in older animals and effectively eliminated the expression of DA agonist-induced behaviors.

Striatum specific protein, Rhes regulates AKT pathway

The Rhes/RASD2 GTPase complex is involved in dopamine D1/D2 receptor-mediated signaling and behavior. This GTP binding protein belongs to the RAS superfamily, along with Dexras1/RASD1, and is primarily expressed in the striatum. RASDs differ from typical small GTPases as they have an extended C-terminal tail of roughly 7 kDa. Previously, it has been shown that dopamine depletion reduces Rhes mRNA expression in the brain. Here we show that Rhes interacts with p85, the regulatory subunit of PI3K. Specifically, the C-terminal unique tail region of Rhes is responsible for this interaction. The interaction between p85 and the C-terminal region of Rhes is enhanced upon growth factor treatment in vitro, while AKT translocation to the membrane is facilitated in the presence of Rhes or the Rhes-p85 complex. These findings suggest that Rhes is a novel striatal regulator of the AKT-mediated pathway in the striatum.

"Liking" and "wanting" linked to Reward Deficiency Syndrome (RDS): hypothesizing differential responsivity in brain reward circuitry

In an attempt to resolve controversy regarding the causal contributions of mesolimbic dopamine (DA) systems to reward, we evaluate the three main competing explanatory categories: "liking,""learning," and "wanting" [1]. That is, DA may mediate (a) the hedonic impact of reward (liking), (b) learned predictions about rewarding effects (learning), or (c) the pursuit of rewards by attributing incentive salience to reward-related stimuli (wanting). We evaluate these hypotheses, especially as they relate to the Reward Deficiency Syndrome (RDS), and we find that the incentive salience or "wanting" hypothesis of DA function is supported by a majority of the evidence. Neuroimaging studies have shown that drugs of abuse, palatable foods, and anticipated behaviors such as sex and gaming affect brain regions involving reward circuitry, and may not be unidirectional. Drugs of abuse enhance DA signaling and sensitize mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Addictive drugs have in common that they are voluntarily selfadministered, they enhance (directly or indirectly) dopaminergic synaptic function in the nucleus accumbens (NAC), and they stimulate the functioning of brain reward circuitry (producing the "high" that drug users seek). Although originally believed simply to encode the set point of hedonic tone, these circuits now are believed to be functionally more complex, also encoding attention, reward expectancy, disconfirmation of reward expectancy, and incentive motivation. Elevated stress levels, together with polymorphisms of dopaminergic genes and other neurotransmitter genetic variants, may have a cumulative effect on vulnerability to addiction. The RDS model of etiology holds very well for a variety of chemical and behavioral addictions.

Rhes: a GTP-binding protein integral to striatal physiology and pathology

Rhes, the Ras Homolog Enriched in Striatum, is a GTP-binding protein whose gene was discovered during a screen for mRNAs preferentially expressed in rodent striatum. This 266 amino acid protein is intermediate in size between small Ras-like GTP-binding proteins and α-subunits of heterotrimeric G proteins. It is most closely related to another Ras-like GTP-binding protein termed Dexras1 or AGS1. Although subsequent studies have shown that the rhes gene is expressed in other brain areas in addition to striatum, the striatal expression level is relatively high, and Rhes protein is likely to play a vital role in striatal physiology and pathology. Indeed, it has recently been shown to interact with the Huntingtin protein and play a pivotal role in the selective vulnerability of striatum in Huntington's disease (HD). Not surprisingly, Rhes can interact with multiple proteins to affect striatal physiology at multiple levels. Functional studies have indicated that Rhes plays a role in signaling by striatal G protein-coupled receptors (GPCR), although the details of the mechanism remain to be determined. Rhes has been shown to bind to both α- and β-subunits of heterotrimeric G proteins and to affect signaling by both Gi/o- and Gs/olf-coupled receptors. In this context, Rhes can be classified as a member of the family of accessory proteins to GPCR signaling. With documented effects in dopamine- and opioid-mediated behaviors, an interaction with thyroid hormone systems and a role in HD pathology, Rhes is emerging as an important protein in striatal physiology and pathology.

Post-translational modification biology of glutamate receptors and drug addiction

Post-translational covalent modifications of glutamate receptors remain a hot topic. Early studies have established that this family of receptors, including almost all ionotropic and metabotropic glutamate receptor subtypes, undergoes active phosphorylation at serine, threonine, or tyrosine residues in their intracellular domains. Recent evidence identifies several glutamate receptor subtypes to be direct substrates for palmitoylation at cysteine residues. Other modifications such as ubiquitination and sumoylation at lysine residues also occur to certain glutamate receptors. These modifications are dynamic and reversible in nature and are regulatable by changing synaptic inputs. The regulated modifications significantly impact the receptor in many ways, including interrelated changes in biochemistry (synthesis, subunit assembling, and protein–protein interactions), subcellular redistribution (trafficking, endocytosis, synaptic delivery, and clustering), and physiology, usually associated with changes in synaptic plasticity. Glutamate receptors are enriched in the striatum and cooperate closely with dopamine to regulate striatal signaling. Emerging evidence shows that modification processes of striatal glutamate receptors are sensitive to addictive drugs, such as psychostimulants (cocaine and amphetamine). Altered modifications are believed to be directly linked to enduring receptor/synaptic plasticity and drug-seeking. This review summarizes several major types of modifications of glutamate receptors and analyzes the role of these modifications in striatal signaling and in the pathogenesis of psychostimulant addiction.

Rhes and AGS1/Dexras1 affect signaling by dopamine D1 receptors through adenylyl cyclase

The GTP binding proteins Rhes and AGS1/Dexras1 define a subfamily of the Ras superfamily and have been shown to affect signaling by G-protein-coupled receptors. We tested the effects of both proteins at an early stage of signaling by dopamine receptors, activation of adenylyl cyclase. Rhes decreased dopamine D1 receptor agonist-stimulated cAMP accumulation in a pertussis toxin-sensitive manner. It had no effect on cAMP accumulation in the absence of agonist. AGS1/Dexras1, on the other hand, decreased cAMP accumulation in both vehicle-treated and agonist-treated cells, resulting in a higher percentage of stimulation by agonist or a higher signal-to-noise ratio. The effects of AGS1/Dexras1 on cAMP accumulation were not blocked by pertussis toxin, suggesting that it may produce these effects through interaction with a G(α) i monomer. Both Rhes and AGS1/Dexras1 were associated with GTP-bound G(α) i in pull-down assays. However, Rhes had no effect on the ability of activated D2 receptor to inhibit cAMP. Neither Rhes nor AGS1/Dexras1 interacted with the D1 receptor in pull-down assays. These findings show that, in addition to its well-known effects on signaling through Gi-coupled receptors, AGS1/Dexras1 can affect signaling through a Gs/olf-coupled receptor. Furthermore, the results suggest that Rhes exerts some of its effects by interacting with G(α) i.

Exploration of sex differences in Rhes effects in dopamine mediated behaviors

Studies have shown that Ras homolog enriched in striatum (Rhes) proteins are highly expressed in areas of the central nervous system that have high dopaminergic innervation. In this study, we used Rhes mutant mice (Wild type, Rhes KO, Rhes Heterozygous) of both sexes to explore differences in the effects of Rhes protein levels in basal levels of activity, anxiety, and stereotypy, in relation to sex. Adult male and female mice were evaluated in an open field test for measuring basal levels of activity and anxiety for 5 consecutive days, and they were tested in the apomorphine-induced stereotypy paradigm. Rhes protein levels affected basal levels of activity but it was not found to be related to sex differences. Moreover, a decrease in Rhes protein levels was linked to a nonsignificant anxiolytic effect, mainly in female mice. Finally, a decrease in Rhes protein levels does not affect dopamine D(1) and D(2) receptor (D(1)/D(2)) synergism in female or male mice. Together, these results suggest that Rhes protein levels affect locomotion activity, and have an influence in anxiety depending on sex; Rhes protein levels do not affect D(1)/D(2) synergism in both sexes.

Mice lacking rhes show altered morphine analgesia, tolerance, and dependence

Rhes, the Ras Homolog Enriched in Striatum, is an intermediate-size GTP binding protein. Although its full functions are not yet known, it has been shown to affect signaling and behaviors mediated by G protein-coupled receptors. Here we have tested whether Rhes affects behaviors mediated by opioid receptors. Wild type and rhes-deficient mice were administered morphine and tested for analgesia in formalin and tail flick tests. Rhes⁻/⁻ mice showed significantly enhanced analgesia in both tests relative to rhes+/+ mice. Furthermore, rhes⁻/⁻ mice did not display tolerance to repeated morphine administration and displayed significantly less withdrawal than rhes+/+ mice. These findings indicate that Rhes is involved in behaviors mediated by mu opioid receptors and in the adaptive response to repeated morphine administration.

Rhes, a physiologic regulator of sumoylation, enhances cross-sumoylation between the basic sumoylation enzymes E1 and Ubc9

We recently reported that the small G-protein Rhes has the properties of a SUMO-E3 ligase and mediates mutant huntingtin (mHtt) cytotoxicity. We now demonstrate that Rhes is a physiologic regulator of sumoylation, which is markedly reduced in the corpus striatum of Rhes-deleted mice. Sumoylation involves activation and transfer of small ubiquitin-like modifier (SUMO) from the thioester of E1 to the thioester of Ubc9 (E2) and final transfer to lysines on target proteins, which is enhanced by E3s. We show that E1 transfers SUMO from its thioester directly to lysine residues on Ubc9, forming isopeptide linkages. Conversely, sumoylation on E1 requires transfer of SUMO from the thioester of Ubc9. Thus, the process regarded as "autosumoylation" reflects intermolecular transfer between E1 and Ubc9, which we designate "cross-sumoylation." Rhes binds directly to both E1 and Ubc9, enhancing cross-sumoylation as well as thioester transfer from E1 to Ubc9.

Maladaptive striatal plasticity in L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with l-DOPA remains the most effective treatment for Parkinson's disease, but causes dyskinesia (abnormal involuntary movements) in the vast majority of the patients. The basic mechanisms of l-DOPA-induced dyskinesia (LID) have become the object of intense research focusing on neurochemical and molecular adaptations in the striatum. Here we review this vast literature and highlight trends that converge into a unifying pathophysiological interpretation. We propose that the core molecular alteration of striatal neurons in LID consists in an inability to turn down supersensitive signaling responses downstream of DA D1 receptors (where supersensitivity is primarily caused by DA denervation). The sustained activation of intracellular signaling pathways induced by each dose of l-DOPA leads to abnormal cellular plasticity and high bioenergetic expenditure. The over-exploitation of signaling pathways and energy reserves during treatment impairs the ability of striatal neurons to dynamically gate cortically driven motor commands. LID thus exemplifies a disorder where 'too much' molecular plasticity leads to plasticity failure in the striatum.

Rhes, a protein with selective expression in the striatum, plays a major role in Huntington’s disease pathogenesis

Evaluation of: Subramaniam S, Sixt KM, Barrow R, Snyder SH: Rhes, a striatal specific protein, mediates mutant-huntingtin cytotoxicity. Science 324, 1327–1330 (2009). Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder characterized by choreiform movements, cognitive deficits and psychiatric disturbances. The disease is caused by an abnormal expansion of a CAG repeat located in exon 1 of the gene encoding the huntingtin protein (Htt). The genetic defect encodes a polyglutamine tract in the N-terminal part of Htt that confers a toxic function to the protein. The most striking neuropathological hallmark in HD patients is the selective atrophy of the striatum. The mechanisms underlying the particular vulnerability of the striatum are unknown. Subramaniam and collaborators demonstrate that the cytotoxicity of mutant Htt is greatly enhanced in the presence of the small GTPase, Rhes, a protein of unclear function that has a preferential expression in the striatum. The study demonstrates that Rhes is an E3 ligase, interacts with mutant Htt and modifies it through SUMOylation, a post-transcriptional process that consists of the addition of the protein SUMO1 to mutant Htt. By contrast, the GTPase activity of Rhes does not seem to be involved in the toxicity of mutant Htt. The Rhes-mediated sumoylation of mutant Htt eventually leads to reduced levels of neuroprotective insoluble aggregates, and increased levels of the toxic soluble form of mutant Htt. These completely novel results shed new light on HD pathogenesis. The selective expression of Rhes in the striatum and its role in mutant Htt toxicity could explain why the striatum is so vulnerable in HD. This work may lead to new therapeutic strategies targeting Rhes.

Thyroid hormone receptor alpha plays an essential role in the normalisation of adult-onset hypothyroidism-related hypoexpression of synaptic plasticity target genes in striatum

Thyroid hormone (TH) deficiency leads to molecular changes resulting in behavioural deficits. TH action is mediated by two types of nuclear receptors (TRs), TRalpha and TRbeta, which control target gene transcription. The relative contributions of the two TR products in mediating adult TH responses are poorly understood. As TRalpha1 transcripts are widely distributed in the brain, they presumably mediate most of the TH effects. This report examines the role and specific functions of T3 receptor isoforms on regulation of striatal synaptic plasticity indicators using adult hypothyroid mutant mice that fail to express single or multiple TR gene products. We then evaluated the effect of this hypothyroidism, with or without subsequent administration of T3, on T3 nuclear receptor (TRalpha1, TRbeta) and synaptic plasticity gene expression in TRalpha(0/0), TRbeta(-/-) and wild-type 129/SV mice. Hypothyroid wild-type mice exhibited reduced TRbeta, RC3, CaMKII and Rhes expression. The mRNA levels of Rhes and CaMKII were the same in all three hypothyroid substrains. By contrast, hypothyroid TRbeta(-/-) mice had higher RC3 mRNA levels than wild-type. T3 administration restored TRbeta, RC3 and CaMKII levels in hypothyroid wild-type mice, without significant Rhes upregulation. T3 administration normalised expression of all genes studied in hypothyroid TRbeta(-/-) but not TRalpha(0/0) mice. Thus, TRalpha apparently plays an essential role in restoring the expression of the TH-regulated genes potentially involved in striatal synaptic plasticity.

Ontogeny and dopaminergic regulation in brain of Ras homolog enriched in striatum (Rhes)

Rhes is one of several signaling molecules preferentially expressed in the striatum. This GTP-binding protein affects dopamine-mediated signaling and behavior. Denervating the striatum of its dopaminergic inputs in adulthood reduces rhes mRNA expression. Here we show that dopamine depletion in adult rats by 6-hydroxydopamine caused a significant decrease in striatal Rhes protein levels as measured by Western blotting. The role of dopamine input on rhes mRNA induction during ontogeny was also examined. Rhes mRNA was measured on postnatal days 4, 6, 8, 10, 15, and 24 with in situ hybridization to determine its normal ontogeny. Signal in striatum was detectable, but very low, on postnatal day 4 and increased gradually to peak levels at days 15 and 24. Outside of the striatum, rhes mRNA was expressed at high levels in hippocampus and cerebellum during the postnatal period. Hippocampal signal was initially highest in CA3 and dentate gyrus, but shifted to higher expression in CA1 by the late postnatal period. Several other nuclei showed low levels of rhes mRNA during ontogeny. Depletion of dopamine by 6-hydroxydopamine injection on postnatal day 4 did not affect the ontogenetic development of rhes mRNA, such that expression did not differ statistically in lesioned versus vehicle-treated animals tested in adulthood. These findings suggest that although dopamine input is not necessary for the ontogenetic development of rhes mRNA expression, changes in both rhes mRNA and Rhes protein are integral components of the response of the adult striatum to dopamine depletion.

The Ras homolog Rhes affects dopamine D1 and D2 receptor-mediated behavior in mice

Dopamine activates five different receptor subtypes and a complex array of intracellular signaling pathways. Rhes is a striatally expressed guanidine triphosphate-binding protein involved in dopamine signaling. Here we have used mutant mice to test whether Rhes (Ras homolog enriched in striatum) is involved in D1 and D2 dopamine receptor-mediated behaviors. Rhes was not necessary for the expression of normal D1/D2 receptor synergism, as measured by apomorphine-induced stereotypy. The stereotypic responses to D1/D2 costimulation and to D2 stimulation alone were significantly increased in mice lacking Rhes, but D1 receptor-mediated grooming was reduced in these mice. These results suggest that Rhes is normally inhibitory to behaviors induced by D1/D2 receptor costimulation and by D2 receptor stimulation alone. Rhes, however, seems to facilitate the D1-specific behavior of grooming.

The monomeric G proteins AGS1 and Rhes selectively influence Galphai-dependent signaling to modulate N-type (CaV2.2) calcium channels

Activator of G protein Signaling 1 (AGS1) and Ras homologue enriched in striatum (Rhes) define a new group of Ras-like monomeric G proteins whose signaling properties and physiological roles are just beginning to be understood. Previous results suggest that AGS1 and Rhes exhibit distinct preferences for heterotrimeric G proteins, with AGS1 selectively influencing Galphai and Rhes selectively influencing Galphas. Here, we demonstrate that AGS1 and Rhes trigger nearly identical modulation of N-type Ca(2+) channels (Ca(V)2.2) by selectively altering Galphai-dependent signaling. Whole-cell currents were recorded from HEK293 cells expressing Ca(V)2.2 and Galphai- or Galphas-coupled receptors. AGS1 and Rhes reduced basal current densities and triggered tonic voltage-dependent (VD) inhibition of Ca(V)2.2. Additionally, each protein attenuated agonist-initiated channel inhibition through Galphai-coupled receptors without reducing channel inhibition through a Galphas-coupled receptor. The above effects of AGS1 and Rhes were blocked by pertussis toxin (PTX) or by expression of a Gbetagamma-sequestering peptide (masGRK3ct). Transfection with HRas, KRas2, Rap1A-G12V, Rap2B, Rheb2, or Gem failed to duplicate the effects of AGS1 and Rhes on Ca(V)2.2. Our data provide the first demonstration that AGS1 and Rhes exhibit similar if not identical signaling properties since both trigger tonic Gbetagamma signaling and both attenuate receptor-initiated signaling by the Gbetagamma subunits of PTX-sensitive G proteins. These results are consistent with the possibility that AGS1 and Rhes modulate Ca(2+) influx through Ca(V)2.2 channels under more physiological conditions and thereby influence Ca(2+)-dependent events such as neurosecretion.

T3 administration in adult hypothyroid mice modulates expression of proteins involved in striatal synaptic plasticity and improves motor behavior

Adult-onset hypothyroidism is associated with neurological changes such as cognitive dysfunction and impaired learning, which may be related to alterations of synaptic plasticity. We investigate the consequence of adult-onset hypothyroidism on thyroid-mediated transcription events in striatal synaptic plasticity, and the effect of triiodothyronine (T3) replacement. We used hypothyroid mice, treated with propylthiouracil (PTU) and methimazole (MMI), with or without subsequent administration of T3. We evaluated the amount of T3 nuclear receptors (TRalpha1, TRbeta) and striatal plasticity indicators: neurogranin (RC3), Ras homolog enriched in striatum (Rhes), Ca2+/calmodulin-dependent protein kinase (CaMKII), and dopamine- and cAMP-regulated phosphoprotein (DARPP-32). In addition, we assessed hypothyroid mice motor behavior as related to striatum synaptic functions. Hypothyroid mice exhibited significantly reduced TRbeta, RC3 and Rhes expression. T3 administration reversed the expression of TRbeta, RC3, and up-regulated CaMKII levels as well as motor behavior, and decreased DARPP-32 protein phosphorylation. We suggest that thyroid hormone modulation had a major impact on striatal synaptic plasticity of adult mice which produced in turn motor behavior modifications.

The GTP-binding protein Rhes modulates dopamine signalling in striatal medium spiny neurons

Rhes is a small GTP-binding protein prominently localized in the striatum. Previous findings obtained in cell culture systems demonstrated an involvement of Rhes in cAMP/PKA signalling pathway, at a level proximal to the activation of heterotrimeric G-protein complex. However, its role in the striatum has been, so far, only supposed. Here we studied the involvement of Rhes in dopaminergic signalling, by employing mice with a null mutation in the Rhes gene. We demonstrated that the absence of Rhes modulates cAMP/PKA signalling in both striatopallidal and striatonigral projection neurons by increasing Golf protein levels and, in turn, influencing motor responses challenged by dopaminergic agonist/antagonist. Interestingly, we also show that Rhes is required for a correct dopamine-mediated GTP binding, a function mainly associated to stimulation of dopamine D2 receptors. Altogether, our results indicate that Rhes is an important modulator of dopaminergic transmission in the striatum.

Reserpine-induced model of stress suppresses mucosal immunity

Stress contributes significantly to the development of many diseases. In clinical studies, a strong correlation between depression and immune dysfunction has been shown. Our previous studies indicated that sympathetic innervation can regulate intestinal mucosal immunity through sympathetic synapses, but the mechanism in stress/depression-induced intestinal immune deficiency was unclear. Using a mouse model in which behavioural stress/depression is chemically induced by reserpine, it is found that there is a substantial deficiency of intestinal local humoral and particularly specific antibody response to the antigen stimulation in reserpine-treated group. No significant difference of CD4+, CD8+ or Mac1+ cells between reserpine-treated and control groups was detected in the intestine. This deficiency is closely correlated with stress/depression. A possible correlation between stress, cytokine secretion and humoral immunity in vivo is postulated.

Mechanistic pathways and biological roles for receptor-independent activators of G-protein signaling

Signal processing via heterotrimeric G-proteins in response to cell surface receptors is a central and much investigated aspect of how cells integrate cellular stimuli to produce coordinated biological responses. The system is a target of numerous therapeutic agents and plays an important role in adaptive processes of organs; aberrant processing of signals through these transducing systems is a component of various disease states. In addition to G-protein coupled receptor (GPCR)-mediated activation of G-protein signaling, nature has evolved creative ways to manipulate and utilize the Galphabetagamma heterotrimer or Galpha and Gbetagamma subunits independent of the cell surface receptor stimuli. In such situations, the G-protein subunits (Galpha and Gbetagamma) may actually be complexed with alternative binding partners independent of the typical heterotrimeric Galphabetagamma. Such regulatory accessory proteins include the family of regulator of G-protein signaling (RGS) proteins that accelerate the GTPase activity of Galpha and various entities that influence nucleotide binding properties and/or subunit interaction. The latter group of proteins includes receptor-independent activators of G-protein signaling (AGS) proteins that play surprising roles in signal processing. This review provides an overview of our current knowledge regarding AGS proteins. AGS proteins are indicative of a growing number of accessory proteins that influence signal propagation, facilitate cross talk between various types of signaling pathways, and provide a platform for diverse functions of both the heterotrimeric Galphabetagamma and the individual Galpha and Gbetagamma subunits.