RasGRP1 promotes amphetamine-induced motor behavior through a Rhes interaction network ("Rhesactome") in the striatum

Rhes, a striatal enriched protein, regulates post-translational small-ubiquitin-like-modifier (SUMO) modification of nuclear proteins and alters gene expression

Rhes (Ras homolog enriched in the striatum), a multifunctional protein that regulates striatal functions associated with motor behaviors and neurological diseases, can shuttle from cell to cell via the formation of tunneling-like nanotubes (TNTs). However, the mechanisms by which Rhes mediates diverse functions remain unclear. Rhes is a small GTPase family member which contains a unique C-terminal Small Ubiquitin-like Modifier (SUMO) E3-like domain that promotes SUMO post-translational modification of proteins (SUMOylation) by promoting "cross-SUMOylation" of the SUMO enzyme SUMO E1 (Aos1/Uba2) and SUMO E2 ligase (Ubc-9). Nevertheless, the identity of the SUMO substrates of Rhes remains largely unknown. Here, by combining high throughput interactome and SUMO proteomics, we report that Rhes regulates the SUMOylation of nuclear proteins that are involved in the regulation of gene expression. Rhes increased the SUMOylation of histone deacetylase 1 (HDAC1) and histone 2B, while decreasing SUMOylation of heterogeneous nuclear ribonucleoprotein M (HNRNPM), protein polybromo-1 (PBRM1) and E3 SUMO-protein ligase (PIASy). We also found that Rhes itself is SUMOylated at 6 different lysine residues (K32, K110, K114, K120, K124, and K245). Furthermore, Rhes regulated the expression of genes involved in cellular morphogenesis and differentiation in the striatum, in a SUMO-dependent manner. Our findings thus provide evidence for a previously undescribed role for Rhes in regulating the SUMOylation of nuclear targets and in orchestrating striatal gene expression via SUMOylation.

SUMO modifies GβL and mediates mTOR signaling

The mechanistic target of rapamycin (mTOR) signaling is influenced by multiple regulatory proteins and post-translational modifications; however, underlying mechanisms remain unclear. Here, we report a novel role of small ubiquitin-like modifier (SUMO) in mTOR complex assembly and activity. By investigating the SUMOylation status of core mTOR components, we observed that the regulatory subunit, GβL (G protein β-subunit-like protein, also known as mLST8), is modified by SUMO1, 2, and 3 isoforms. Using mutagenesis and mass spectrometry, we identified that GβL is SUMOylated at lysine sites K86, K215, K245, K261, and K305. We found that SUMO depletion reduces mTOR-Raptor (regulatory protein associated with mTOR) and mTOR-Rictor (rapamycin-insensitive companion of mTOR) complex formation and diminishes nutrient-induced mTOR signaling. Reconstitution with WT GβL but not SUMOylation-defective KR mutant GβL promotes mTOR signaling in GβL-depleted cells. Taken together, we report for the very first time that SUMO modifies GβL, influences the assembly of mTOR protein complexes, and regulates mTOR activity.

Curbing Rhes Actions: Mechanism-based Molecular Target for Huntington's Disease and Tauopathies

A highly interconnected network of diverse brain regions is necessary for the precise execution of human behaviors, including cognitive, psychiatric, and motor functions. Unfortunately, degeneration of specific brain regions causes several neurodegenerative disorders, but the mechanisms that elicit selective neuronal vulnerability remain unclear. This knowledge gap greatly hinders the development of effective mechanism-based therapies, despite the desperate need for new treatments. Here, we emphasize the importance of the Rhes (Ras homolog-enriched in the striatum) protein as an emerging therapeutic target. Rhes, an atypical small GTPase with a SUMO (small ubiquitin-like modifier) E3-ligase activity, modulates biological processes such as dopaminergic transmission, alters gene expression, and acts as an inhibitor of motor stimuli in the brain striatum. Mutations in the Rhes gene have also been identified in selected patients with autism and schizophrenia. Moreover, Rhes SUMOylates pathogenic form of mutant huntingtin (mHTT) and tau, enhancing their solubility and cell toxicity in Huntington's disease and tauopathy models. Notably, Rhes uses membrane projections resembling tunneling nanotubes to transport mHTT between cells and Rhes deletion diminishes mHTT spread in the brain. Thus, we predict that effective strategies aimed at diminishing brain Rhes levels will prevent or minimize the abnormalities that occur in HD and tauopathies and potentially in other brain disorders. We review the emerging technologies that enable specific targeting of Rhes in the brain to develop effective disease-modifying therapeutics.

Rhes depletion promotes striatal accumulation and aggregation of mutant huntingtin in a presymptomatic HD mouse model

Introduction

Huntington's disease (HD) is caused by CAG trinucleotide repeats in the HTT gene. Selective neurodegeneration in the striatum is prominent in HD, despite widespread expression of mutant HTT (mHTT). Ras homolog enriched in the striatum (Rhes) is a GTP-binding protein enriched in the striatum, involved in dopamine-related behaviors and autophagy regulation. Growing evidence suggests Rhes plays a critical role in the selective striatal degeneration in HD, but its specific function in this context remains complex and controversial.

Methods

In this study, we utilized CRISPR/Cas9 to knockdown Rhes at different disease stages through adeno-associated virus (AAV) transduction in HD knock-in (KI) mice. Immunoblotting and immunofluorescence were employed to assess the impact of Rhes depletion on mHTT levels, neuronal loss, astrogliosis and autophagy activity.

Results

Rhes depletion in 22-week-old HD KI mice (representing the presymptomatic stage) led to mHTT accumulation, reduced neuronal cell staining, and increased astrogliosis. However, no such effects were observed in 36-week-old HD KI mice (representing the symptomatic stage). Additionally, Rhes deletion in 22-week-old HD KI mice resulted in increased P62 levels, reduced LC3-II levels, and unchanged phosphorylation of mTOR and beclin-1, unchanged mTOR protein level, except for a decrease in beclin-1.

Discussion

Our findings suggest that knockdown Rhes promotes striatal aggregation of mutant huntingtin by reducing autophagy activity in a mTOR-independent manner. Rhes plays a protective role during the presymptomatic stage of HD KI mice.

Functional diversity in the RAS subfamily of small GTPases

RAS small GTPases regulate important signalling pathways and are notorious drivers of cancer development and progression. While most research to date has focused on understanding and addressing the oncogenic potential of three RAS oncogenes: HRAS, KRAS, and NRAS; the full RAS subfamily is composed of 35 related GTPases with diverse cellular functions. Most remain deeply understudied despite strong evolutionary conservation. Here, we highlight a group of 17 poorly characterized RAS GTPases that are frequently down-regulated in cancer and evidence suggests may function not as oncogenes, but as tumour suppressors. These GTPases remain largely enigmatic in terms of their cellular function, regulation, and interaction with effector proteins. They cluster within two families we designate as 'distal-RAS' (D-RAS; comprised of DIRAS, RASD, and RASL10) and 'CaaX-Less RAS' (CL-RAS; comprised of RGK, NKIRAS, RERG, and RASL11/12 GTPases). Evidence of a tumour suppressive role for many of these GTPases supports the premise that RAS subfamily proteins may collectively regulate cellular proliferation.

Rhes protein transits from neuron to neuron and facilitates mutant huntingtin spreading in the brain

Rhes (RASD2) is a thyroid hormone-induced gene that regulates striatal motor activity and promotes neurodegeneration in Huntington disease (HD) and tauopathy. Rhes moves and transports the HD protein, polyglutamine-expanded huntingtin (mHTT), via tunneling nanotube (TNT)-like membranous protrusions between cultured neurons. However, similar intercellular Rhes transportation in the intact brain was unknown. Here, we report that Rhes induces TNT-like protrusions in the striatal medium spiny neurons (MSNs) and transported between dopamine-1 receptor (D1R)-MSNs and D2R-MSNs of intact striatum and organotypic brain slices. Notably, mHTT is robustly transported within the striatum and from the striatum to the cortical areas in the brain, and Rhes deletion diminishes such transport. Moreover, Rhes moves to the cortical regions following restricted expression in the MSNs of the striatum. Thus, Rhes is a first striatum-enriched protein demonstrated to move and transport mHTT between neurons and brain regions, providing new insights into interneuronal protein transport in the brain.

Regulation of GTPase function by autophosphorylation

A unifying feature of the RAS superfamily is a conserved GTPase cycle by which these proteins transition between active and inactive states. We demonstrate that autophosphorylation of some GTPases is an intrinsic regulatory mechanism that reduces nucleotide hydrolysis and enhances nucleotide exchange, altering the on/off switch that forms the basis for their signaling functions. Using X-ray crystallography, nuclear magnetic resonance spectroscopy, binding assays, and molecular dynamics on autophosphorylated mutants of H-RAS and K-RAS, we show that phosphoryl transfer from GTP requires dynamic movement of the switch II region and that autophosphorylation promotes nucleotide exchange by opening the active site and extracting the stabilizing Mg2+. Finally, we demonstrate that autophosphorylated K-RAS exhibits altered effector interactions, including a reduced affinity for RAF proteins in mammalian cells. Thus, autophosphorylation leads to altered active site dynamics and effector interaction properties, creating a pool of GTPases that are functionally distinct from their non-phosphorylated counterparts.

Abnormal RasGRP1 Expression in the Post-Mortem Brain and Blood Serum of Schizophrenia Patients

Schizophrenia (SCZ) is a polygenic severe mental illness. Genome-wide association studies (GWAS) have detected genomic variants associated with this psychiatric disorder and pathway analyses have indicated immune system and dopamine signaling as core components of risk in dorsolateral-prefrontal cortex (DLPFC) and hippocampus, but the mechanistic links remain unknown. The RasGRP1 gene, encoding for a guanine nucleotide exchange factor, is implicated in dopamine signaling and immune response. RasGRP1 has been identified as a candidate risk gene for SCZ and autoimmune disease, therefore representing a possible point of convergence between mechanisms involving the nervous and the immune system. Here, we investigated RasGRP1 mRNA and protein expression in post-mortem DLPFC and hippocampus of SCZ patients and healthy controls, along with RasGRP1 protein content in the serum of an independent cohort of SCZ patients and control subjects. Differences in RasGRP1 expression between SCZ patients and controls were detected both in DLPFC and peripheral blood of samples analyzed. Our results indicate RasGRP1 may mediate risk for SCZ by involving DLPFC and peripheral blood, thus encouraging further studies to explore its possible role as a biomarker of the disease and/or a target for new medication.

Global Rhes knockout in the Q175 Huntington's disease mouse model

Huntington’s disease (HD) results from an expansion mutation in the polyglutamine tract in huntingtin. Although huntingtin is ubiquitously expressed in the body, the striatum suffers the most severe pathology. Rhes is a Ras-related small GTP-binding protein highly expressed in the striatum that has been reported to modulate mTOR and sumoylation of mutant huntingtin to alter HD mouse model pathogenesis. Reports have varied on whether Rhes reduction is desirable for HD. Here we characterize multiple behavioral and molecular endpoints in the Q175 HD mouse model with genetic Rhes knockout (KO). Genetic RhesKO in the Q175 female mouse resulted in both subtle attenuation of Q175 phenotypic features, and detrimental effects on other kinematic features. The Q175 females exhibited measurable pathogenic deficits, as measured by MRI, MRS and DARPP32, however, RhesKO had no effect on these readouts. Additionally, RhesKO in Q175 mixed gender mice deficits did not affect mTOR signaling, autophagy or mutant huntingtin levels. We conclude that global RhesKO does not substantially ameliorate or exacerbate HD mouse phenotypes in Q175 mice.

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.

Impaired inhibitory GABAergic synaptic transmission and transcription studied in single neurons by Patch-seq in Huntington's disease

Transcriptional dysregulation in Huntington's disease (HD) causes functional deficits in striatal neurons. Here, we performed Patch-sequencing (Patch-seq) in an in vitro HD model to investigate the effects of mutant Huntingtin (Htt) on synaptic transmission and gene transcription in single striatal neurons. We found that expression of mutant Htt decreased the synaptic output of striatal neurons in a cell autonomous fashion and identified a number of genes whose dysregulation was correlated with physiological deficiencies in mutant Htt neurons. In support of a pivotal role for epigenetic mechanisms in HD pathophysiology, we found that inhibiting histone deacetylase 1/3 activities rectified several functional and morphological deficits and alleviated the aberrant transcriptional profiles in mutant Htt neurons. With this study, we demonstrate that Patch-seq technology can be applied both to better understand molecular mechanisms underlying a complex neurological disease at the single-cell level and to provide a platform for screening for therapeutics for the disease.

Mutant Huntingtin stalls ribosomes and represses protein synthesis in a cellular model of Huntington disease

The polyglutamine expansion of huntingtin (mHTT) causes Huntington disease (HD) and neurodegeneration, but the mechanisms remain unclear. Here, we found that mHtt promotes ribosome stalling and suppresses protein synthesis in mouse HD striatal neuronal cells. Depletion of mHtt enhances protein synthesis and increases the speed of ribosomal translocation, while mHtt directly inhibits protein synthesis in vitro. Fmrp, a known regulator of ribosome stalling, is upregulated in HD, but its depletion has no discernible effect on protein synthesis or ribosome stalling in HD cells. We found interactions of ribosomal proteins and translating ribosomes with mHtt. High-resolution global ribosome footprint profiling (Ribo-Seq) and mRNA-Seq indicates a widespread shift in ribosome occupancy toward the 5' and 3' end and unique single-codon pauses on selected mRNA targets in HD cells, compared to controls. Thus, mHtt impedes ribosomal translocation during translation elongation, a mechanistic defect that can be exploited for HD therapeutics.

The mammalian target of rapamycin (mTOR) kinase mediates haloperidol-induced cataleptic behavior

The mammalian target of rapamycin (mTOR) is a ubiquitously expressed serine/threonine kinase protein complex (mTORC1 or mTORC2) that orchestrates diverse functions ranging from embryonic development to aging. However, its brain tissue-specific roles remain less explored. Here, we have identified that the depletion of the mTOR gene in the mice striatum completely prevented the extrapyramidal motor side effects (catalepsy) induced by the dopamine 2 receptor (D2R) antagonist haloperidol, which is the most widely used typical antipsychotic drug. Conversely, a lack of striatal mTOR in mice did not affect catalepsy triggered by the dopamine 1 receptor (D1R) antagonist SCH23390. Along with the lack of cataleptic effects, the administration of haloperidol in mTOR mutants failed to increase striatal phosphorylation levels of ribosomal protein pS6 (S235/236) as seen in control animals. To confirm the observations of the genetic approach, we used a pharmacological method and determined that the mTORC1 inhibitor rapamycin has a profound influence upon post-synaptic D2R-dependent functions. We consistently found that pretreatment with rapamycin entirely prevented (in a time-dependent manner) the haloperidol-induced catalepsy, and pS6K (T389) and pS6 (S235/236) signaling upregulation, in wild-type mice. Collectively, our data indicate that striatal mTORC1 blockade may offer therapeutic benefits with regard to the prevention of D2R-dependent extrapyramidal motor side effects of haloperidol in psychiatric illness.

Nurr1 performs its anti-inflammatory function by regulating RasGRP1 expression in neuro-inflammation

Nurr1, a transcription factor belonging to the orphan nuclear receptor, has an essential role in the generation and maintenance of dopaminergic neurons and is important in the pathogenesis of Parkinson’ disease (PD). In addition, Nurr1 has a non-neuronal function, and it is especially well known that Nurr1 has an anti-inflammatory function in the Parkinson’s disease model. However, the molecular mechanisms of Nurr1 have not been elucidated. In this study, we describe a novel mechanism of Nurr1 function. To provide new insights into the molecular mechanisms of Nurr1 in the inflammatory response, we performed Chromatin immunoprecipitation sequencing (ChIP-Seq) on LPS-induced inflammation in BV2 cells and finally identified the RasGRP1 gene as a novel target of Nurr1. Here, we show that Nurr1 directly binds to the RasGRP1 intron to regulate its expression. Moreover, we also identified that RasGRP1 regulates the Ras-Raf-MEK-ERK signaling cascade in LPS-induced inflammation signaling. Finally, we conclude that RasGRP1 is a novel regulator of Nurr1’s mediated inflammation signaling.

Cyclic GMP-AMP synthase promotes the inflammatory and autophagy responses in Huntington disease

Huntington disease (HD) is a genetic disorder caused by glutamine-expansion in the huntingtin (mHTT) protein, which affects motor, psychiatric, and cognitive function, but the mechanisms remain unclear. mHTT is known to induce DNA damage and affect autophagy, both associated with inflammatory responses, but what mediates all these were unknown. Here we report that cGAS, a DNA damage sensor, is highly upregulated in the striatum of a mouse model and HD human patient’s tissue. We found ribosomes, which make proteins, are robustly accumulated on the cGAS mRNA in HD cells. cGAS depletion decreases—and cGAS expression increases—both inflammatory and autophagy responses in HD striatal cells. Thus, cGAS is a therapeutic target for HD. Blocking cGAS will prevent/slow down HD symptoms.

Huntington disease (HD) is caused by an expansion mutation of the N-terminal polyglutamine of huntingtin (mHTT). mHTT is ubiquitously present, but it induces noticeable damage to the brain’s striatum, thereby affecting motor, psychiatric, and cognitive functions. The striatal damage and progression of HD are associated with the inflammatory response; however, the underlying molecular mechanisms remain unclear. Here, we report that cGMP-AMP synthase (cGAS), a DNA sensor, is a critical regulator of inflammatory and autophagy responses in HD. Ribosome profiling revealed that the cGAS mRNA has high ribosome occupancy at exon 1 and codon-specific pauses at positions 171 (CCG) and 172 (CGT) in HD striatal cells. Moreover, the protein levels and activity of cGAS (based on the phosphorylated STING and phosphorylated TBK1 levels), and the expression and ribosome occupancy of cGAS-dependent inflammatory genes (Ccl5 and Cxcl10) are increased in HD striatum. Depletion of cGAS diminishes cGAS activity and decreases the expression of inflammatory genes while suppressing the up-regulation of autophagy in HD cells. In contrast, reinstating cGAS in cGAS-depleted HD cells activates cGAS activity and promotes inflammatory and autophagy responses. Ribosome profiling also revealed that LC3A and LC3B, the two major autophagy initiators, show altered ribosome occupancy in HD cells. We also detected the presence of numerous micronuclei, which are known to induce cGAS, in the cytoplasm of neurons derived from human HD embryonic stem cells. Collectively, our results indicate that cGAS is up-regulated in HD and mediates inflammatory and autophagy responses. Thus, targeting the cGAS pathway may offer therapeutic benefits in HD.

Rhes Tunnels: A Radical New Way of Communication in the Brain's Striatum?

Ras homolog enriched in the striatum (Rhes) is a striatal enriched protein that promotes the formation of thin membranous tubes resembling tunneling nanotubes (TNT)-"Rhes tunnels"-that connect neighboring cell and transport cargoes: vesicles and proteins between the neuronal cells. Here the literature on TNT-like structures is reviewed, and the implications of Rhes-mediated TNT, the mechanisms of its formation, and its potential in novel cell-to-cell communication in regulating striatal biology and disease are emphasized. Thought-provoking ideas regarding how Rhes-mediated TNT, if it exists, in vivo, would radically change the way neurons communicate in the brain are discussed.

RasGRP1 is a causal factor in the development of l-DOPA-induced dyskinesia in Parkinson's disease

The therapeutic effects of l-3,4-dihydroxyphenylalanine (l-DOPA) in patients with Parkinson's disease (PD) severely diminishes with the onset of abnormal involuntary movement, l-DOPA-induced dyskinesia (LID). However, the molecular mechanisms that promote LID remain unclear. Here, we demonstrated that RasGRP1 [(guanine nucleotide exchange factor (GEF)] controls the development of LID. l-DOPA treatment rapidly up-regulated RasGRP1 in the striatum of mouse and macaque model of PD. The lack of RasGRP1 in mice (RasGRP1^-/- ) dramatically diminished LID without interfering with the therapeutic effects of l-DOPA. Besides acting as a GEF for Ras homolog enriched in the brain (Rheb), the activator of the mammalian target of rapamycin kinase (mTOR), RasGRP1 promotes l-DOPA-induced extracellular signal-regulated kinase (ERK) and the mTOR signaling in the striatum. High-resolution tandem mass spectrometry analysis revealed multiple RasGRP1 downstream targets linked to LID vulnerability. Collectively, the study demonstrated that RasGRP1 is a critical striatal regulator of LID.

Exaggerated mitophagy: a weapon of striatal destruction in the brain?

Mechanisms responsible for neuronal vulnerability in the brain remain unclear. Striatal neurons are preferentially damaged by 3-nitropropionic acid (3-NP), a mitochondrial complex-II inhibitor, causing striatal damage reminiscent of Huntington's disease (HD), but the mechanisms of the selectivity are not as well understood. We have discovered that Rhes, a protein enriched in the striatum, removes mitochondria via the mitophagy process. The process becomes intensified in the presence of 3-NP, thereby eliminating most of the mitochondria from the striatum. We put forward the hypothesis that Rhes acts as a 'mitophagy ligand' in the brain and promotes mitophagy via NIX, a mitophagy receptor. Since Rhes interacts and promotes toxicity in association with mutant huntingtin (mHTT), the genetic cause of HD, it is tempting to speculate on whether the exaggerated mitophagy may be a contributing factor to the striatal lesion found in HD. Thus, Rhes-mediated exaggerated mitophagy may act as a weapon of striatal destruction in the brain.

Rhes, a striatal-enriched protein, promotes mitophagy via Nix

Elimination of dysfunctional mitochondria via mitophagy is essential for cell survival and neuronal functions. But, how impaired mitophagy participates in tissue-specific vulnerability in the brain remains unclear. Here, we find that striatal-enriched protein, Rhes, is a critical regulator of mitophagy and striatal vulnerability in brain. In vivo interactome and density fractionation reveal that Rhes coimmunoprecipitates and cosediments with mitochondrial and lysosomal proteins. Live-cell imaging of cultured striatal neuronal cell line shows Rhes surrounds globular mitochondria, recruits lysosomes, and ultimately degrades mitochondria. In the presence of 3-nitropropionic acid (3-NP), an inhibitor of succinate dehydrogenase, Rhes disrupts mitochondrial membrane potential (ΔΨ _m ) and promotes excessive mitophagy and cell death. Ultrastructural analysis reveals that systemic injection of 3-NP in mice promotes globular mitochondria, accumulation of mitophagosomes, and striatal lesion only in the wild-type (WT), but not in the Rhes knockout (KO), striatum, suggesting that Rhes is critical for mitophagy and neuronal death in vivo. Mechanistically, Rhes requires Nix (BNIP3L), a known receptor of mitophagy, to disrupt ΔΨ _m and promote mitophagy and cell death. Rhes interacts with Nix via SUMO E3-ligase domain, and Nix depletion totally abrogates Rhes-mediated mitophagy and cell death in the cultured striatal neuronal cell line. Finally, we find that Rhes, which travels from cell to cell via tunneling nanotube (TNT)-like cellular protrusions, interacts with dysfunctional mitochondria in the neighboring cell in a Nix-dependent manner. Collectively, Rhes is a major regulator of mitophagy via Nix, which may determine striatal vulnerability in the brain.

The striatal-enriched protein Rhes is a critical modulator of cocaine-induced molecular and behavioral responses

Previous evidence pointed out a role for the striatal-enriched protein Rhes in modulating dopaminergic transmission. Based on the knowledge that cocaine induces both addiction and motor stimulation, through its ability to enhance dopaminergic signaling in the corpus striatum, we have now explored the involvement of Rhes in the effects associated with this psychostimulant. Our behavioral data showed that a lack of Rhes in knockout animals caused profound alterations in motor stimulation following cocaine exposure, eliciting a significant leftward shift in the dose-response curve and triggering a dramatic hyperactivity. We also found that Rhes modulated either short- or long-term motor sensitization induced by cocaine, since lack of this protein prevents both of them in mutants. Consistent with this in vivo observation, we found that lack of Rhes in mice caused a greater increase in striatal cocaine-dependent D1R/cAMP/PKA signaling, along with considerable enhancement of Arc, zif268, and Homer1 mRNA expression. We also documented that lack of Rhes in mice produced cocaine-related striatal alterations in proteomic profiling, with a differential expression of proteins clustering in calcium homeostasis and cytoskeletal protein binding categories. Despite dramatic striatal alterations associated to cocaine exposure, our data did not reveal any significant changes in midbrain dopaminergic neurons as a lack of Rhes did not affect: (i) DAT activity; (ii) D2R-dependent regulation of GIRK; and (iii) D2R-dependent regulation of dopamine release. Collectively, our results strengthen the view that Rhes acts as a pivotal physiological “molecular brake” for striatal dopaminergic system overactivation induced by psychostimulants, thus making this protein of interest in regulating the molecular mechanism underpinning cocaine-dependent motor stimulatory effects.

Bioinformatics analysis of Ras homologue enriched in the striatum, a potential target for Huntington's disease therapy

Huntington's disease (HD) is a lethal neurodegenerative disorder for which no cure is available yet. It is caused by abnormal expansion of a CAG triplet in the gene encoding the huntingtin protein (Htt), with consequent expansion of a polyglutamine repeat in mutated Htt (mHtt). This makes mHtt highly unstable and aggregation prone. Soluble mHtt is linked to cytotoxicity and neurotoxicity, whereas mHtt aggregates are thought to be neuroprotective. While Htt and mHtt are ubiquitously expressed throughout the brain and peripheral tissues, HD is characterized by selective degradation of the corpus striatum, without notable alterations in peripheral tissues. Screening for mRNAs preferentially expressed in rodent striatum led to the discovery of a GTP binding protein homologous to Ras family members. Due to these features, the newly discovered protein was termed Ras Homolog Enriched in Striatum (RHES). The aetiological role of RHES in HD has been ascribed to its small ubiquitin-like modifier (SUMO)-E3 ligase function. RHES sumoylates mHtt with higher efficiency than wild-type Htt, thereby protecting mHtt from degradation and increasing the amounts of the soluble form. Although RHES is an attractive target for HD treatment, essential information about protein structure and function are still missing. With the aim of investigating RHES 3D structure and function, bioinformatic analyses and molecular modelling have been performed in the present study, based on which, RHES regions predicted to be involved in the interaction with mHtt or the SUMO-E2 ligase Ubc9 have been identified. These regions have been used to design peptides aimed at inhibiting RHES interactions and, therefore, mHtt sumoylation; in turn, these peptides will be used to develop small molecule inhibitors by both rational design and virtual screening of large compound libraries. Once identified, RHES sumoylation inhibitors may open the road to the development of therapeutic agents against the severe, and currently untreatable, HD.

Rhes travels from cell to cell and transports Huntington disease protein via TNT-like protrusion

Tunneling nanotubes (TNT) are membranous tubes that connect two cells, but their functional roles and mechanism of biogenesis remain obscure. Sharma and Subramaniam demonstrate that Rhes, a brain-enriched protein, increases biogenesis of TNT-like cellular protrusions or “Rhes tunnels” through which Rhes travels from cell to cell and transports Huntington disease (HD) protein.

Tunneling nanotubes (TNT) are thin, membranous, tunnel-like cell-to-cell connections, but the mechanisms underlying their biogenesis or functional role remains obscure. Here, we report, Rhes, a brain-enriched GTPase/SUMO E3-like protein, induces the biogenesis of TNT-like cellular protrusions, “Rhes tunnels,” through which Rhes moves from cell to cell and transports Huntington disease (HD) protein, the poly-Q expanded mutant Huntingtin (mHTT). The formation of TNT-like Rhes tunnels requires the Rhes’s serine 33, C-terminal CAAX, and a SUMO E3-like domain. Electron microscopy analysis revealed that TNT-like Rhes tunnels appear continuous, cell–cell connections, and <200 nm in diameter. Live-cell imaging shows that Rhes tunnels establish contact with the neighboring cell and deliver Rhes-positive cargoes, which travel across the plasma membrane of the neighboring cell before entering it. The Rhes tunnels carry Rab5a/Lyso 20-positive vesicles and transport mHTT, but not normal HTT, mTOR, or wtTau proteins. SUMOylation-defective mHTT, Rhes C263S (cannot SUMOylate mHTT), or CRISPR/Cas9-mediated depletion of three isoforms of SUMO diminishes Rhes-mediated mHTT transport. Thus, Rhes promotes the biogenesis of TNT-like cellular protrusions and facilitates the cell–cell transport of mHTT involving SUMO-mediated mechanisms.

Lack of Rhes Increases MDMA-Induced Neuroinflammation and Dopamine Neuron Degeneration: Role of Gender and Age

Ras homolog enriched in striatum (Rhes) is a protein that exerts important physiological functions and modulates psychostimulant drug effects. On this basis, the object of this study was to assess 3,4-methylenedioxymethamphetamine (MDMA) effects on microglial (CD11b) and astroglial (GFAP) activation and on dopamine neuron degeneration (TH) in wild-type (WT) and Rhes knockout (KO) male and female mice of different ages. Motor activity was also evaluated. Adult (3 months) MDMA-treated mice displayed an increase in GFAP-positive cells in striatum (STR), whereas the substantia nigra pars compacta (SNc) was affected only in male mice. In these mice, the increase of CD11b was more extensive including STR, SNc, motor cortex (CTX), ventral tegmental area (VTA), and nucleus accumbens (NAc). MDMA administration also affected TH immunoreactivity in both STR and SNc of male but not female WT and Rhes KO mice. In middle-aged mice (12 months), MDMA administration further increased GFAP and CD11b and decreased TH immunoreactivity in STR and SNc of all mice. Finally, MDMA induced a higher increase of motor activity in adult Rhes KO male, but not female mice. The results show that Rhes protein plays an important role on MDMA-mediated neuroinflammation and neurodegeneration dependent on gender and age, and confirm the important role of Rhes protein in neuroinflammatory and neurodegenerative processes.

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.

Striatal Transcriptome and Interactome Analysis of Shank3-overexpressing Mice Reveals the Connectivity between Shank3 and mTORC1 Signaling

Mania causes symptoms of hyperactivity, impulsivity, elevated mood, reduced anxiety and decreased need for sleep, which suggests that the dysfunction of the striatum, a critical component of the brain motor and reward system, can be causally associated with mania. However, detailed molecular pathophysiology underlying the striatal dysfunction in mania remains largely unknown. In this study, we aimed to identify the molecular pathways showing alterations in the striatum of SH3 and multiple ankyrin repeat domains 3 (Shank3)-overexpressing transgenic (TG) mice that display manic-like behaviors. The results of transcriptome analysis suggested that mammalian target of rapamycin complex 1 (mTORC1) signaling may be the primary molecular signature altered in the Shank3 TG striatum. Indeed, we found that striatal mTORC1 activity, as measured by mTOR S2448 phosphorylation, was significantly decreased in the Shank3 TG mice compared to wild-type (WT) mice. To elucidate the potential underlying mechanism, we re-analyzed previously reported protein interactomes, and detected a high connectivity between Shank3 and several upstream regulators of mTORC1, such as tuberous sclerosis 1 (TSC1), TSC2 and Ras homolog enriched in striatum (Rhes), via 94 common interactors that we denominated “Shank3-mTORC1 interactome”. We noticed that, among the 94 common interactors, 11 proteins were related to actin filaments, the level of which was increased in the dorsal striatum of Shank3 TG mice. Furthermore, we could co-immunoprecipitate Shank3, Rhes and Wiskott-Aldrich syndrome protein family verprolin-homologous protein 1 (WAVE1) proteins from the striatal lysate of Shank3 TG mice. By comparing with the gene sets of psychiatric disorders, we also observed that the 94 proteins of Shank3-mTORC1 interactome were significantly associated with bipolar disorder (BD). Altogether, our results suggest a protein interaction-mediated connectivity between Shank3 and certain upstream regulators of mTORC1 that might contribute to the abnormal striatal mTORC1 activity and to the manic-like behaviors of Shank3 TG mice.