Rgs6 is required for adult maintenance of dopaminergic neurons in the ventral substantia nigra

Regulator of G protein signaling 6 (RGS6) in dopamine neurons promotes EtOH seeking, behavioral reward, and susceptibility to relapse

Mesolimbic dopamine (DA) transmission is believed to play a critical role in mediating reward responses to drugs of abuse, including alcohol (EtOH). The neurobiological mechanisms underlying EtOH-seeking behavior and dependence are not fully understood, and abstinence remains the only effective way to prevent alcohol use disorders (AUDs). Here, we developed novel RGS6fl/fl; DAT-iCreER mice to determine the role of RGS6 in DA neurons on EtOH consumption, reward, and relapse behaviors. We found that RGS6 is expressed in DA neurons in both human and mouse ventral tegmental area (VTA), and that RGS6 loss in mice upregulates DA transporter (DAT) expression in VTA DA neuron synaptic terminals. Remarkably, loss of RGS6 in DA neurons significantly reduced EtOH consumption, preference, and reward in a manner indistinguishable from that seen in RGS6-/- mice. Strikingly, RGS6 loss from DA neurons before or after EtOH behavioral reward is established significantly reduced (~ 50%) re-instatement of reward following extinguishment, demonstrating distinct roles of RGS6 in promoting reward and relapse susceptibility to EtOH. These studies identify DA neurons as the locus of RGS6 action in promoting EtOH consumption, preference, reward, and relapse. RGS6 is unique among R7 RGS proteins in promoting rather than suppressing behavioral responses to drugs of abuse and to modulate EtOH behavioral reward. This is a result of RGS6's pre-synaptic actions that we hypothesize promote VTA DA transmission by suppressing GPCR-Gαi/o-DAT signaling in VTA DA neurons. These studies identify RGS6 as a potential therapeutic target for behavioral reward and relapse to EtOH.

A splice acceptor variant in RGS6 associated with intellectual disability, microcephaly, and cataracts disproportionately promotes expression of a subset of RGS6 isoforms

Intellectual disability (ID) is associated with an increased risk of developing psychiatric disorders, suggesting a common underlying genetic factor. Importantly, altered signaling and/or expression of regulator of G protein signaling 6 (RGS6) is associated with ID and numerous psychiatric disorders. RGS6 is highly conserved and undergoes complex alternative mRNA splicing producing ~36 protein isoforms with high sequence similarity historically necessitating a global approach in functional studies. However, our recent analysis in mice revealed RGS6 is most highly expressed in CNS with RGS6L(+GGL) isoforms predominating. A previously reported genetic variant in intron 17 of RGS6 (c.1369-1G>C), associated with ID, may provide further clues into RGS6L(+GGL) isoform functional delineation. This variant was predicted to alter a highly conserved canonical 3' acceptor site creating an alternative branch point within exon 18 (included in a subset of RGS6L(+GGL) transcripts) and a frameshift forming an early stop codon. We previously identified this alternative splice site and demonstrated its use generates RGS6Lζ(+GGL) isoforms. Here, we show that the c.1369-1G>C variant disrupts the canonical, preferred (>90%) intron 17 splice site and leads to the exclusive use of the alternate exon 18 splice site, inducing disproportionate expression of a subset of isoforms, particularly RGS6Lζ(+GGL). Furthermore, RGS6 global knockout mice do not exhibit ID. Thus, ID caused by the c.1369-1G>C variant likely results from altered RGS6 isoform expression, rather than RGS6 isoform loss. In summary, these studies highlight the importance of proper RGS6 splicing and identify a previously unrecognized role of G protein signaling in ID.

Regulator of G protein signaling protein 6 alleviates acute lung injury by inhibiting inflammation and promoting cell self-renewal in mice

BACKGROUND

Acute respiratory distress syndrome (ARDS) is a disease with high mortality and morbidity. Regulator of G protein signaling protein 6 (RGS6), identified as a tumor suppressor gene, has received increasing attention owing to its close relationship with oxidative stress and inflammation. However, the association between ARDS and RGS6 has not been reported.

METHODS

Congruously regulated G protein-coupled receptor (GPCR)-related genes and differentially expressed genes (DEGs) in an acute lung injury (ALI) model were identified, and functional enrichment analysis was conducted. In an in vivo study, the effects of RGS6 knockout were studied in a mouse model of ALI induced by lipopolysaccharide (LPS). HE staining, ELISA, and immunohistochemistry were used to evaluate pathological changes and the degree of inflammation. In vitro, qRT‒PCR, immunofluorescence staining, and western blotting were used to determine the dynamic changes in RGS6 expression in cells. The RGS6 overexpression plasmid was constructed for transfection. qRT‒PCR was used to assess proinflammatory factors transcription. Western blotting and flow cytometry were used to evaluate apoptosis and reactive oxygen species (ROS) production. Organoid culture was used to assess the stemness and self-renewal capacity of alveolar epithelial type II cells (AEC2s).

RESULTS

A total of 110 congruously regulated genes (61 congruously upregulated and 49 congruously downregulated genes) were identified among GPCR-related genes and DEGs in the ALI model. RGS6 was downregulated in vivo and in vitro in the ALI model. RGS6 was expressed in the cytoplasm and accumulated in the nucleus after LPS stimulation. Compared with the control group, we found higher mortality, more pronounced body weight changes, more serious pulmonary edema and pathological damage, and more neutrophil infiltration in the RGS6 knockout group upon LPS stimulation in vivo. Moreover, AEC2s loss was significantly increased upon RGS6 knockout. Organoid culture assays showed slower alveolar organoid formation, fewer alveolar organoids, and impaired development of new structures after passaging upon RGS6 knockout. In addition, RGS6 overexpression decreased ROS production as well as proinflammatory factor transcription in macrophages and decreased apoptosis in epithelial cells.

CONCLUSIONS

RGS6 plays a protective role in ALI not only in early inflammatory responses but also in endogenous lung stem cell regeneration.

Genes positively regulated by Mef2c in cortical neurons are enriched for common genetic variation associated with IQ and educational attainment

The myocyte enhancer factor 2 C (MEF2C) gene encodes a transcription factor important for neurogenesis and synapse development and contains common variants associated with intelligence (IQ) and educational attainment (EA). Here, we took gene expression data from the mouse cortex of a Mef2c mouse model with a heterozygous DNA binding-deficient mutation of Mef2c (Mef2c-het) and combined these data with MEF2C ChIP-seq data from cortical neurons and single-cell data from the mouse brain. This enabled us to create a set of genes that were differentially regulated in Mef2c-het mice, represented direct target genes of MEF2C and had elevated in expression in cortical neurons. We found this gene-set to be enriched for genes containing common genetic variation associated with IQ and EA. Genes within this gene-set that were down-regulated, i.e. have reduced expression in Mef2c-het mice versus controls, were specifically significantly enriched for both EA and IQ associated genes. These down-regulated genes were enriched for functionality in the adenylyl cyclase signalling system, which is known to positively regulate synaptic transmission and has been linked to learning and memory. Within the adenylyl cyclase signalling system, three genes regulated by MEF2C, CRHR1, RGS6, and GABRG3, are associated at genome-wide significant levels with IQ and/or EA. Our results indicate that genetic variation in MEF2C and its direct target genes within cortical neurons contribute to variance in cognition within the general population, and the molecular mechanisms involved include the adenylyl cyclase signalling system's role in synaptic function.

Regulator of G protein signaling 6 (RGS6) in ventral tegmental area (VTA) dopamine neurons promotes EtOH seeking, behavioral reward and susceptibility to relapse

Mesolimbic dopamine (DA) transmission is believed to play a critical role in mediating reward responses to drugs of abuse, including alcohol (EtOH). EtOH is the most abused substance worldwide with chronic consumption often leading to the development of dependence and abuse. Unfortunately, the neurobiological mechanisms underlying EtOH-seeking behavior and dependence are not fully understood, and abstinence remains the only effective way to prevent alcohol use disorders (AUDs). Here, we developed novel RGS6 fl/fl ; DAT-iCreER mice to determine the role of RGS6 in VTA DA neurons on EtOH consumption and reward behaviors. We found that RGS6 is expressed in DA neurons in both human and mouse VTA, and that RGS6 loss in mice upregulates DA transporter (DAT) expression in VTA DA neuron synaptic terminals. Remarkably, loss of RGS6 in VTA DA neurons significantly reduced EtOH consumption, preference, and reward in a manner indistinguishable from that seen in RGS6 -/- mice. Strikingly, RGS6 loss from VTA DA neurons before or after EtOH behavioral reward is established significantly reduced (∼50%) re-instatement of reward following extinguishment, demonstrating distinct roles of RGS6 in promoting reward and relapse susceptibility to EtOH. These studies illuminate a critical role of RGS6 in the mesolimbic circuit in promoting EtOH seeking, reward, and reinstatement. We propose that RGS6 functions to promote DA transmission through its function as a negative modulator of GPCR-Gα i/o -DAT signaling in VTA DA neurons. These studies identify RGS6 as a potential therapeutic target for behavioral reward and relapse to EtOH.

In utero and post-natal opioid exposure followed by mild traumatic brain injury contributes to cortical neuroinflammation, mitochondrial dysfunction, and behavioral deficits in juvenile rats

Maternal opioid use poses a significant health concern not just to the expectant mother but also to the fetus. Notably, increasing numbers of children born suffering from neonatal opioid withdrawal syndrome (NOWS) further compounds the crisis. While epidemiological research has shown the heightened risk factors associated with NOWS, little research has investigated what molecular mechanisms underly the vulnerabilities these children carry throughout development and into later life. To understand the implications of in utero and post-natal opioid exposure on the developing brain, we sought to assess the response to one of the most common pediatric injuries: minor traumatic brain injury (mTBI). Using a rat model of in utero and post-natal oxycodone (IUO) exposure and a low force weight drop model of mTBI, we show that not only neonatal opioid exposure significantly affects neuroinflammation, brain metabolites, synaptic proteome, mitochondrial function, and altered behavior in juvenile rats, but also, in conjunction with mTBI these aberrations are further exacerbated. Specifically, we observed long term metabolic dysregulation, neuroinflammation, alterations in synaptic mitochondria, and impaired behavior were impacted severely by mTBI. Our research highlights the specific vulnerability caused by IUO exposure to a secondary stressor such as later life brain injury. In summary, we present a comprehensive study to highlight the damaging effects of prenatal opioid abuse in conjunction with mild brain injury on the developing brain.

RGS6 negatively regulates inhibitory G protein signaling in dopamine neurons and positively regulates binge-like alcohol consumption in mice

BACKGROUND AND PURPOSE

Drugs of abuse, including alcohol, increase dopamine in the mesocorticolimbic system via actions on dopamine neurons in the ventral tegmental area (VTA). Increased dopamine transmission can activate inhibitory G protein signalling pathways in VTA dopamine neurons, including those controlled by GABAB and D2 receptors. Members of the R7 subfamily of regulator of G protein signalling (RGS) proteins can regulate inhibitory G protein signalling, but their influence on VTA dopamine neurons is unclear. Here, we investigated the influence of RGS6, an R7 RGS family memberthat has been implicated in the regulation of alcohol consumption in mice, on inhibitory G protein signalling in VTA dopamine neurons.

EXPERIMENTAL APPROACH

We used molecular, electrophysiological and genetic approaches to probe the impact of RGS6 on inhibitory G protein signalling in VTA dopamine neurons and on binge-like alcohol consumption in mice.

KEY RESULTS

RGS6 is expressed in adult mouse VTA dopamine neurons and it modulates inhibitory G protein signalling in a receptor-dependent manner, tempering D2 receptor-induced somatodendritic currents and accelerating deactivation of synaptically evoked GABAB receptor-dependent responses. RGS6-/- mice exhibit diminished binge-like alcohol consumption, a phenotype replicated in female (but not male) mice lacking RGS6 selectively in VTA dopamine neurons.

CONCLUSIONS AND IMPLICATIONS

RGS6 negatively regulates GABAB - and D2 receptor-dependent inhibitory G protein signalling pathways in mouse VTA dopamine neurons and exerts a sex-dependent positive influence on binge-like alcohol consumption in adult mice. As such, RGS6 may represent a new diagnostic and/or therapeutic target for alcohol use disorder.

Aging, Parkinson's Disease, and Models: What Are the Challenges?

Parkinson's disease (PD) is a chronic, neurodegenerative condition characterized by motor symptoms such as bradykinesia, rigidity, and tremor, alongside multiple nonmotor symptoms. The appearance of motor symptoms is linked to progressive dopaminergic neuron loss within the substantia nigra. PD incidence increases sharply with age, suggesting a strong association between mechanisms driving biological aging and the development and progression of PD. However, the role of aging in the pathogenesis of PD remains understudied. Numerous models of PD, including cell models, toxin-induced models, and genetic models in rodents and nonhuman primates (NHPs), reproduce different aspects of PD, but preclinical studies of PD rarely incorporate age as a factor. Studies using patient neurons derived from stem cells via reprogramming methods retain some aging features, but their characterization, particularly of aging markers and reproducibility of neuron type, is suboptimal. Investigation of age-related changes in PD using animal models indicates an association, but this is likely in conjunction with other disease drivers. The biggest barrier to drawing firm conclusions is that each model lacks full characterization and appropriate time-course assessments. There is a need to systematically investigate whether aging increases the susceptibility of mouse, rat, and NHP models to develop PD and understand the role of cell models. We propose that a significant investment in time and resources, together with the coordination and sharing of resources, knowledge, and data, is required to accelerate progress in understanding the role of biological aging in PD development and improve the reliability of models to test interventions.

Exploring the Genomic Patterns in Human and Mouse Cerebellums Via Single-Cell Sequencing and Machine Learning Method

In mammals, the cerebellum plays an important role in movement control. Cellular research reveals that the cerebellum involves a variety of sub-cell types, including Golgi, granule, interneuron, and unipolar brush cells. The functional characteristics of cerebellar cells exhibit considerable differences among diverse mammalian species, reflecting a potential development and evolution of nervous system. In this study, we aimed to recognize the transcriptional differences between human and mouse cerebellum in four cerebellar sub-cell types by using single-cell sequencing data and machine learning methods. A total of 321,387 single-cell sequencing data were used. The 321,387 cells included 4 cell types, i.e., Golgi (5,048, 1.57%), granule (250,307, 77.88%), interneuron (60,526, 18.83%), and unipolar brush (5,506, 1.72%) cells. Our results showed that by using gene expression profiles as features, the optimal classification model could achieve very high even perfect performance for Golgi, granule, interneuron, and unipolar brush cells, respectively, suggesting a remarkable difference between the genomic profiles of human and mouse. Furthermore, a group of related genes and rules contributing to the classification was identified, which might provide helpful information for deepening the understanding of cerebellar cell heterogeneity and evolution.

Spatial RNA Sequencing Identifies Robust Markers of Vulnerable and Resistant Human Midbrain Dopamine Neurons and Their Expression in Parkinson's Disease

Defining transcriptional profiles of substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) dopamine neurons is critical to understanding their differential vulnerability in Parkinson’s Disease (PD). Here, we determine transcriptomes of human SNc and VTA dopamine neurons using LCM-seq on a large sample cohort. We apply a bootstrapping strategy as sample input to DESeq2 and identify 33 stably differentially expressed genes (DEGs) between these two subpopulations. We also compute a minimal sample size for identification of stable DEGs, which highlights why previous reported profiles from small sample sizes display extensive variability. Network analysis reveal gene interactions unique to each subpopulation and highlight differences in regulation of mitochondrial stability, apoptosis, neuronal survival, cytoskeleton regulation, extracellular matrix modulation as well as synapse integrity, which could explain the relative resilience of VTA dopamine neurons. Analysis of PD tissues showed that while identified stable DEGs can distinguish the subpopulations also in disease, the SNc markers SLIT1 and ATP2A3 were down-regulated and thus appears to be biomarkers of disease. In summary, our study identifies human SNc and VTA marker profiles, which will be instrumental for studies aiming to modulate dopamine neuron resilience and to validate cell identity of stem cell-derived dopamine neurons.

Proteomic Profiling of the Substantia Nigra to Identify Determinants of Lewy Body Pathology and Dopaminergic Neuronal Loss

Proteinaceous aggregates containing α-synuclein protein called Lewy bodies in the substantia nigra is a hallmark of Parkinson's disease. The molecular mechanisms of Lewy body formation and associated neuronal loss remain largely unknown. To gain insights into proteins and pathways associated with Lewy body pathology, we performed quantitative profiling of the proteome. We analyzed substantia nigra tissue from 51 subjects arranged into three groups: cases with Lewy body pathology, Lewy body-negative controls with matching neuronal loss, and controls with no neuronal loss. Using a label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach, we characterized the proteome both in terms of protein abundances and peptide modifications. Statistical testing for differential abundance of the most abundant 2963 proteins, followed by pathway enrichment and Bayesian learning of the causal network structure, was performed to identify likely drivers of Lewy body formation and dopaminergic neuronal loss. The identified pathways include (1) Arp2/3 complex-mediated actin nucleation; (2) synaptic function; (3) poly(A) RNA binding; (4) basement membrane and endothelium; and (5) hydrogen peroxide metabolic process. According to the data, the endothelial/basement membrane pathway is tightly connected with both pathologies and likely to be one of the drivers of neuronal loss. The poly(A) RNA-binding proteins, including the ones relevant to other neurodegenerative disorders (e.g., TDP-43 and FUS), have a strong inverse correlation with Lewy bodies and may reflect an alternative mechanism of nigral neurodegeneration.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Midbrain Dopaminergic Neuron Development at the Single Cell Level: In vivo and in Stem Cells

Parkinson's disease (PD) is a progressive neurodegenerative disorder that predominantly affects dopaminergic (DA) neurons of the substantia nigra. Current treatment options for PD are symptomatic and typically involve the replacement of DA neurotransmission by DA drugs, which relieve the patients of some of their motor symptoms. However, by the time of diagnosis, patients have already lost about 70% of their substantia nigra DA neurons and these drugs offer only temporary relief. Therefore, cell replacement therapy has garnered much interest as a potential treatment option for PD. Early studies using human fetal tissue for transplantation in PD patients provided proof of principle for cell replacement therapy, but they also highlighted the ethical and practical difficulties associated with using human fetal tissue as a cell source. In recent years, advancements in stem cell research have made human pluripotent stem cells (hPSCs) an attractive source of material for cell replacement therapy. Studies on how DA neurons are specified and differentiated in the developing mouse midbrain have allowed us to recapitulate many of the positional and temporal cues needed to generate DA neurons in vitro. However, little is known about the developmental programs that govern human DA neuron development. With the advent of single-cell RNA sequencing (scRNA-seq) and bioinformatics, it has become possible to analyze precious human samples with unprecedented detail and extract valuable high-quality information from large data sets. This technology has allowed the systematic classification of cell types present in the human developing midbrain along with their gene expression patterns. By studying human development in such an unbiased manner, we can begin to elucidate human DA neuron development and determine how much it differs from our knowledge of the rodent brain. Importantly, this molecular description of the function of human cells has become and will increasingly be a reference to define, evaluate, and engineer cell types for PD cell replacement therapy and disease modeling.

RGS Proteins as Critical Regulators of Motor Function and Their Implications in Parkinson's Disease

Parkinson disease (PD) is a devastating, largely nonfamilial, age-related disorder caused by the progressive loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Release of DA from these neurons into the dorsal striatum is crucial for regulating movement and their loss causes PD. Unfortunately, the mechanisms underlying SNc neurodegeneration remain unclear, and currently there is no cure for PD, only symptomatic treatments. Recently, several regulator of G protein signaling (RGS) proteins have emerged as critical modulators of PD pathogenesis and/or motor dysfunction and dyskinesia: RGSs 4, 6, 9, and 10. Striatal RGS4 has been shown to exacerbate motor symptoms of DA loss by suppressing M₄-autoreceptor-Gα _i/o signaling in striatal cholinergic interneurons. RGS6 and RGS9 are key regulators of D₂R-Gα _i/o signaling in SNc DA neurons and striatal medium spiny neurons, respectively. RGS6, expressed in human and mouse SNc DA neurons, suppresses characteristic PD hallmarks in aged mice, including SNc DA neuron loss, motor deficits, and α-synuclein accumulation. After DA depletion, RGS9 (through its inhibition of medium spiny neuron D₂R signaling) suppresses motor dysfunction induced by L-DOPA or D₂R-selective agonists. RGS10 is highly expressed in microglia, the brain's resident immune cells. Within the SNc, RGS10 may promote DA neuron survival through the upregulation of prosurvival genes and inhibition of microglial inflammatory factor expression. Thus, RGSs 4, 6, 9, and 10 are critical modulators of cell signaling pathways that promote SNc DA neuron survival and/or proper motor control. Accordingly, these RGS proteins represent novel therapeutic targets for the treatment of PD pathology. SIGNIFICANCE STATEMENT: Parkinson disease (PD), the most common movement disorder, is a progressive neurodegenerative disease characterized by substantia nigra pars compacta (SNc) dopamine (DA) neuron loss and subsequent motor deficits. Current PD therapies only target disease motor symptomology and are fraught with side effects. Therefore, researchers have begun to explore alternative therapeutic options. Regulator of G protein signaling (RGS) proteins, whether primarily expressed in SNc DA neurons (RGS6), striatal neurons (RGSs 4 and 9), or microglia (RGS10), modulate key signaling pathways important for SNc DA neuron survival and/or proper motor control. As such, RGS proteins represent novel therapeutic targets in PD.

Regulator of G-protein signaling (RGS) proteins as drug targets: Progress and future potentials

G protein-coupled receptors (GPCRs) play critical roles in regulating processes such as cellular homeostasis, responses to stimuli, and cell signaling. Accordingly, GPCRs have long served as extraordinarily successful drug targets. It is therefore not surprising that the discovery in the mid-1990s of a family of proteins that regulate processes downstream of GPCRs generated great excitement in the field. This finding enhanced the understanding of these critical signaling pathways and provided potentially new targets for pharmacological intervention. These regulators of G-protein signaling (RGS) proteins were viewed by many as nodes downstream of GPCRs that could be targeted with small molecules to tune signaling processes. In this review, we provide a brief overview of the discovery of RGS proteins and of the gradual and continuing discovery of their roles in disease states, focusing particularly on cancer and neurological disorders. We also discuss high-throughput screening efforts that have led to the discovery first of peptide-based and then of small-molecule inhibitors targeting a subset of the RGS proteins. We explore the unique mechanisms of RGS inhibition these chemical tools have revealed and highlight the most up-to-date studies using these tools in animal experiments. Finally, we discuss the future opportunities in the field, as there are clearly more avenues left to be explored and potentials to be realized.

RGS6 and RGS7 Discriminate between the Highly Similar Gαi and Gαo Proteins Using a Two-Tiered Specificity Strategy

RGS6 and RGS7 are regulators of G protein signaling (RGS) proteins that inactivate heterotrimeric (αβγ) G proteins and mediate diverse biological functions, such as cardiac and neuronal signaling. Uniquely, both RGS6 and RGS7 can discriminate between Gα_o and Gα_i1-two similar Gα subunits that belong to the same G_i sub-family. Here, we show that the isolated RGS domains of RGS6 and RGS7 are sufficient to achieve this specificity. We identified three specific RGS6/7 "disruptor residues" that can attenuate RGS interactions toward Gα subunits and demonstrated that their insertion into a representative high-activity RGS causes a significant, yet non-specific, reduction in activity. We further identified a unique "modulatory" residue that bypasses this negative effect, specifically toward Gα_o. Hence, the exquisite specificity of RGS6 and RGS7 toward closely related Gα subunits is achieved via a two-tier specificity system, whereby a Gα-specific modulatory motif overrides the inhibitory effect of non-specific disruptor residues. Our findings expand the understanding of the molecular toolkit used by the RGS family to achieve specific interactions with selected Gα subunits-emphasizing the functional importance of the RGS domain in determining the activity and selectivity of RGS R7 sub-family members toward particular Gα subunits.

Age-dependent nigral dopaminergic neurodegeneration and α-synuclein accumulation in RGS6-deficient mice

Parkinson's is primarily a non-familial, age-related disorder caused by α-synuclein accumulation and the progressive loss of dopamine neurons in the substantia nigra pars compacta (SNc). G protein-coupled receptor (GPCR)-cAMP signaling has been linked to a reduction in human Parkinson's incidence and α-synuclein expression. Neuronal cAMP levels are controlled by GPCRs coupled to Gs or Gi/o, which increase or decrease cAMP, respectively. Regulator of G protein signaling 6 (RGS6) powerfully inhibits Gi/o signaling. Therefore, we hypothesized that RGS6 suppresses D2 autoreceptor- Gi/o signaling in SNc dopamine neurons promoting neuronal survival and reducing α-synuclein expression. Here we provide novel evidence that RGS6 critically suppresses late-age-onset SNc dopamine neuron loss and α-synuclein accumulation. RGS6 is restrictively expressed in human SNc dopamine neurons and, despite their loss in Parkinson's, all surviving neurons express RGS6. RGS6-/- mice exhibit hyperactive D2 autoreceptors with reduced cAMP signaling in SNc dopamine neurons. Importantly, RGS6-/- mice recapitulate key sporadic Parkinson's hallmarks, including: SNc dopamine neuron loss, reduced nigrostriatal dopamine, motor deficits, and α-synuclein accumulation. To our knowledge, Rgs6 is the only gene whose loss phenocopies these features of human Parkinson's. Therefore, RGS6 is a key regulator of D2R-Gi/o signaling in SNc dopamine neurons, protecting against Parkinson's neurodegeneration and α-synuclein accumulation.

Genetic Analysis of Rare Human Variants of Regulators of G Protein Signaling Proteins and Their Role in Human Physiology and Disease

Regulators of G protein signaling (RGS) proteins modulate the physiologic actions of many neurotransmitters, hormones, and other signaling molecules. Human RGS proteins comprise a family of 20 canonical proteins that bind directly to G protein-coupled receptors/G protein complexes to limit the lifetime of their signaling events, which regulate all aspects of cell and organ physiology. Genetic variations account for diverse human traits and individual predispositions to disease. RGS proteins contribute to many complex polygenic human traits and pathologies such as hypertension, atherosclerosis, schizophrenia, depression, addiction, cancers, and many others. Recent analysis indicates that most human diseases are due to extremely rare genetic variants. In this study, we summarize physiologic roles for RGS proteins and links to human diseases/traits and report rare variants found within each human RGS protein exome sequence derived from global population studies. Each RGS sequence is analyzed using recently described bioinformatics and proteomic tools for measures of missense tolerance ratio paired with combined annotation-dependent depletion scores, and protein post-translational modification (PTM) alignment cluster analysis. We highlight selected variants within the well-studied RGS domain that likely disrupt RGS protein functions and provide comprehensive variant and PTM data for each RGS protein for future study. We propose that rare variants in functionally sensitive regions of RGS proteins confer profound change-of-function phenotypes that may contribute, in newly appreciated ways, to complex human diseases and/or traits. This information provides investigators with a valuable database to explore variation in RGS protein function, and for targeting RGS proteins as future therapeutic targets.

Striatal Tyrosine Hydroxylase Is Stimulated via TAAR1 by 3-Iodothyronamine, But Not by Tyramine or β-Phenylethylamine

The trace amine-associated receptor 1 (TAAR1) is expressed by dopaminergic neurons, but the precise influence of trace amines upon their functional activity remains to be fully characterized. Here, we examined the regulation of tyrosine hydroxylase (TH) by tyramine and beta-phenylethylamine (β-PEA) compared to 3-iodothyronamine (T₁AM). Immunoblotting and amperometry were performed in dorsal striatal slices from wild-type (WT) and TAAR1 knockout (KO) mice. T₁AM increased TH phosphorylation at both Ser¹⁹ and Ser⁴⁰, actions that should promote functional activity of TH. Indeed, HPLC data revealed higher rates of L-dihydroxyphenylalanine (DOPA) accumulation in WT animals treated with T₁AM after the administration of a DOPA decarboxylase inhibitor. These effects were abolished both in TAAR1 KO mice and by the TAAR1 antagonist, EPPTB. Further, they were specific inasmuch as Ser⁸⁴⁵ phosphorylation of the post-synaptic GluA1 AMPAR subunit was unaffected. The effects of T₁AM on TH phosphorylation at both Ser¹⁹ (CamKII-targeted), and Ser⁴⁰ (PKA-phosphorylated) were inhibited by KN-92 and H-89, inhibitors of CamKII and PKA respectively. Conversely, there was no effect of an EPAC analog, 8-CPT-2Me-cAMP, on TH phosphorylation. In line with these data, T₁AM increased evoked striatal dopamine release in TAAR1 WT mice, an action blunted in TAAR1 KO mice and by EPPTB. Mass spectrometry imaging revealed no endogenous T₁AM in the brain, but detected T₁AM in several brain areas upon systemic administration in both WT and TAAR1 KO mice. In contrast to T₁AM, tyramine decreased the phosphorylation of Ser⁴⁰-TH, while increasing Ser⁸⁴⁵-GluA1 phosphorylation, actions that were not blocked in TAAR1 KO mice. Likewise, β-PEA reduced Ser⁴⁰-TH and tended to promote Ser⁸⁴⁵-GluA1 phosphorylation. The D₁ receptor antagonist SCH23390 blocked tyramine-induced Ser⁸⁴⁵-GluA1 phosphorylation, but had no effect on tyramine- or β-PEA-induced Ser⁴⁰-TH phosphorylation. In conclusion, by intracellular cascades involving CaMKII and PKA, T₁AM, but not tyramine and β-PEA, acts via TAAR1 to promote the phosphorylation and functional activity of TH in the dorsal striatum, supporting a modulatory influence on dopamine transmission.

Regulating the regulators: Epigenetic, transcriptional, and post-translational regulation of RGS proteins

Regulators of G protein signaling (RGS) are a family of proteins classically known to accelerate the intrinsic GTPase activity of G proteins, which results in accelerated inactivation of heterotrimeric G proteins and inhibition of G protein coupled receptor signaling. RGS proteins play major roles in essential cellular processes, and dysregulation of RGS protein expression is implicated in multiple diseases, including cancer, cardiovascular and neurodegenerative diseases. The expression of RGS proteins is highly dynamic and is regulated by epigenetic, transcriptional and post-translational mechanisms. This review summarizes studies that report dysregulation of RGS protein expression in disease states, and presents examples of drugs that regulate RGS protein expression. Additionally, this review discusses, in detail, the transcriptional and post-transcriptional mechanisms regulating RGS protein expression, and further assesses the therapeutic potential of targeting these mechanisms. Understanding the molecular mechanisms controlling the expression of RGS proteins is essential for the development of therapeutics that indirectly modulate G protein signaling by regulating expression of RGS proteins.

Regulation of G-protein coupled receptor signalling underpinning neurobiology of mood disorders and depression

G-protein coupled receptors (GPCRs) have long been at the center of investigations of the neurobiology of depression and mood disorders. Different facets of GPCR signalling pathways, including those controlling monoaminergic and neuropeptidergic hormonal systems are believed to be dysregulated in major depressive and bipolar disorders. Although these receptors are key molecular targets for a variety of therapeutic agents and continue to be the focus of intense pharmaceutical development, the molecular mechanisms activated by these GPCRs and underpin the pathological basis of mood disorders remain poorly understood. This review will discuss some of the emerging regulatory mechanisms of GPCR signaling in the central nervous system (CNS) involving protein-protein interactions, downstream effectors and cross-talk with other signaling molecules and their potential involvement in the neurobiology of psychiatric disease.

The evolution of regulators of G protein signalling proteins as drug targets - 20 years in the making: IUPHAR Review 21

Regulators of G protein signalling (RGS) proteins are celebrating the 20th anniversary of their discovery. The unveiling of this new family of negative regulators of G protein signalling in the mid-1990s solved a persistent conundrum in the G protein signalling field, in which the rate of deactivation of signalling cascades in vivo could not be replicated in exogenous systems. Since then, there has been tremendous advancement in the knowledge of RGS protein structure, function, regulation and their role as novel drug targets. RGS proteins play an important modulatory role through their GTPase-activating protein (GAP) activity at active, GTP-bound Gα subunits of heterotrimeric G proteins. They also possess many non-canonical functions not related to G protein signalling. Here, an update on the status of RGS proteins as drug targets is provided, highlighting advances that have led to the inclusion of RGS proteins in the IUPHAR/BPS Guide to PHARMACOLOGY database of drug targets.

Specificity of Pitx3-Dependent Gene Regulatory Networks in Subsets of Midbrain Dopamine Neurons

Dysfunction of midbrain dopaminergic (mDA) neurons is involved in Parkinson's disease (PD) and neuropsychiatric disorders. Pitx3 is expressed in mDA neuron subsets of the substantia nigra compacta (SNc) and of the ventral tegmental area (VTA) that are degeneration-sensitive in PD. The genetic network(s) and mode(s) of action of Pitx3 in these mDA neurons remain poorly characterized. We hypothesized that, given their distinct neuronal identities, Pitx3-expressing neurons of SNc and VTA should differ in their Pitx3-controlled gene expression networks and this may involve subset-specific co-regulators. Expression profiling of purified mDA neuronal subsets indicates that Pitx3 regulates different sets of genes in SNc and VTA, such as activating the expression of primary cilium gene products specifically in VTA neurons. Interaction network analysis pointed to the participation of differentially expressed Lhx/Lmo family members in the modulation of Pitx3 action in SNc and VTA mDA neurons. Conversely, global binding patterns of Pitx3 on genomic DNA of human dopaminergic cells revealed that Pitx3 is often co-recruited to regions that foster the formation of GATA-bHLH-BRN complexes, which usually involve Lmo co-regulatory proteins. We focused on Lmo3 for its preferential expression in SNc neurons and demonstrated that it functions as a transcriptional co-activator of Pitx3 by enhancing its activity on genomic regulatory elements. In summary, we defined the SN and VTA-specific programs of Pitx3-dependent gene expression and identified Lmo3 as a SN-specific co-regulator of Pitx3-dependent transcription.

RGS6 as a Novel Therapeutic Target in CNS Diseases and Cancer

Regulator of G protein signaling (RGS) proteins are gatekeepers regulating the cellular responses induced by G protein-coupled receptor (GPCR)-mediated activation of heterotrimeric G proteins. Specifically, RGS proteins determine the magnitude and duration of GPCR signaling by acting as a GTPase-activating protein for Gα subunits, an activity facilitated by their semiconserved RGS domain. The R7 subfamily of RGS proteins is distinguished by two unique domains, DEP/DHEX and GGL, which mediate membrane targeting and stability of these proteins. RGS6, a member of the R7 subfamily, has been shown to specifically modulate Gαi/o protein activity which is critically important in the central nervous system (CNS) for neuronal responses to a wide array of neurotransmitters. As such, RGS6 has been implicated in several CNS pathologies associated with altered neurotransmission, including the following: alcoholism, anxiety/depression, and Parkinson's disease. In addition, unlike other members of the R7 subfamily, RGS6 has been shown to regulate G protein-independent signaling mechanisms which appear to promote both apoptotic and growth-suppressive pathways that are important in its tumor suppressor function in breast and possibly other tissues. Further highlighting the importance of RGS6 as a target in cancer, RGS6 mediates the chemotherapeutic actions of doxorubicin and blocks reticular activating system (Ras)-induced cellular transformation by promoting degradation of DNA (cytosine-5)-methyltransferase 1 (DNMT1) to prevent its silencing of pro-apoptotic and tumor suppressor genes. Together, these findings demonstrate the critical role of RGS6 in regulating both G protein-dependent CNS pathology and G protein-independent cancer pathology implicating RGS6 as a novel therapeutic target.

Microglia-Derived Cytokines/Chemokines Are Involved in the Enhancement of LPS-Induced Loss of Nigrostriatal Dopaminergic Neurons in DJ-1 Knockout Mice

Mutation of DJ-1 (PARK7) has been linked to the development of early-onset Parkinson's disease (PD). However, the underlying molecular mechanism is still unclear. This study is aimed to compare the sensitivity of nigrostriatal dopaminergic neurons to lipopolysaccharide (LPS) challenge between DJ-1 knockout (KO) and wild-type (WT) mice, and explore the underlying cellular and molecular mechanisms. Our results found that the basal levels of interferon (IFN)-γ (the hub cytokine) and interferon-inducible T-cell alpha chemoattractant (I-TAC) (a downstream mediator) were elevated in the substantia nigra of DJ-1 KO mice and in microglia cells with DJ-1 deficiency, and the release of cytokine/chemokine was greatly enhanced following LPS administration in the DJ-1 deficient conditions. In addition, direct intranigral LPS challenge caused a greater loss of nigrostriatal dopaminergic neurons and striatal dopamine content in DJ-1 KO mice than in WT mice. Furthermore, the sensitization of microglia cells to LPS challenge to release IFN-γ and I-TAC was via the enhancement of NF-κB signaling, which was antagonized by NF-κB inhibitors. LPS-induced increase in neuronal death in the neuron-glia co-culture was enhanced by DJ-1 deficiency in microglia, which was antagonized by the neutralizing antibodies against IFN-γ or I-TAC. These results indicate that DJ-1 deficiency sensitizes microglia cells to release IFN-γ and I-TAC and causes inflammatory damage to dopaminergic neurons. The interaction between the genetic defect (i.e. DJ-1) and inflammatory factors (e.g. LPS) may contribute to the development of PD.

Roles for Regulator of G Protein Signaling Proteins in Synaptic Signaling and Plasticity

The regulator of G protein signaling (RGS) family of proteins serves critical roles in G protein-coupled receptor (GPCR) and heterotrimeric G protein signal transduction. RGS proteins are best understood as negative regulators of GPCR/G protein signaling. They achieve this by acting as GTPase activating proteins (GAPs) for Gα subunits and accelerating the turnoff of G protein signaling. Many RGS proteins also bind additional signaling partners that either regulate their functions or enable them to regulate other important signaling events. At neuronal synapses, GPCRs, G proteins, and RGS proteins work in coordination to regulate key aspects of neurotransmitter release, synaptic transmission, and synaptic plasticity, which are necessary for central nervous system physiology and behavior. Accumulating evidence has revealed key roles for specific RGS proteins in multiple signaling pathways at neuronal synapses, regulating both pre- and postsynaptic signaling events and synaptic plasticity. Here, we review and highlight the current knowledge of specific RGS proteins (RGS2, RGS4, RGS7, RGS9-2, and RGS14) that have been clearly demonstrated to serve critical roles in modulating synaptic signaling and plasticity throughout the brain, and we consider their potential as future therapeutic targets.

No apparent transmission of transgenic α-synuclein into nigrostriatal dopaminergic neurons in multiple mouse models

Background

α–synuclein (α–syn) is the main component of intracytoplasmic inclusions deposited in the brains of patients with Parkinson’s disease (PD) and certain other neurodegenerative disorders. Recent studies have explored the ability of α–syn to propagate between or across neighboring neurons and supposedly “infect” them with a prion–like mechanism. However, much of this research has used stereotaxic injections of heterologous α–syn fibrils to induce the spreading of inclusions in the rodent brains. Whether α–syn is able to transmit from the host cells to their neighboring cells in vivo is unclear.

Methods

Using immunestaining, we examined the potential propagation of α–syn into nigrostriatal dopaminergic (DA) neurons in three lines of transgenic mice that overexpress human wild–type α–syn (hα–syn) in different neuron populations.

Results

After testing for three different routes by which hα–syn propagation might occur, we were unable to find any evidence that hα–syn behaved like a prion and could be transmitted overtime into the DA neurons initially lack of hα–syn expression.

Conclusions

In transgenic mice hα–syn does not have the ability to propagate at pathologically significant levels between or across neurons. It must be noted that these observations do not disprove the studies that show its prion–like qualities, but rather that propagation is not detectable in transgenic models that do not use any injections of heterologous proteins or viral vectors to induce a spreading state.

Molecular heterogeneity of midbrain dopaminergic neurons--Moving toward single cell resolution

Since their discovery, midbrain dopamine (DA) neurons have been researched extensively, in part because of their diverse functions and involvement in various neuropsychiatric disorders. Over the last few decades, reports have emerged that midbrain DA neurons were not a homogeneous group, but that DA neurons located in distinct anatomical locations within the midbrain had distinctive properties in terms of physiology, function, and vulnerability. Accordingly, several studies focused on identifying heterogeneous gene expression across DA neuron clusters. Here we review the importance of understanding DA neuron heterogeneity at the molecular level, previous studies detailing heterogeneous gene expression in DA neurons, and finally recent work which brings together previous heterogeneous gene expression profiles in a coordinated manner, at single cell resolution.

Peptide TFP5/TP5 derived from Cdk5 activator P35 provides neuroprotection in the MPTP model of Parkinson's disease

TFP5/TP5 rescues dopaminergic neurodegeneration induced by MPTP in a mouse model of Parkinson’s disease (PD). The neuroprotective effect of TFP5/TP5 peptide is also associated with marked reduction in neuroinflammation and apoptosis. Selective inhibition of Cdk5/p25 by TFP5/TP5 peptide identifies the kinase as a potential target to reduce neurodegeneration in PD.

Parkinson’s disease (PD) is a chronic neurodegenerative disorder characterized by the loss of dopamine neurons in the substantia nigra, decreased striatal dopamine levels, and consequent extrapyramidal motor dysfunction. Recent evidence indicates that cyclin-dependent kinase 5 (Cdk5) is inappropriately activated in several neurodegenerative conditions, including PD. To date, strategies to specifically inhibit Cdk5 hyperactivity have not been successful without affecting normal Cdk5 activity. Previously we reported that TFP5 peptide has neuroprotective effects in animal models of Alzheimer’s disease. Here we show that TFP5/TP5 selective inhibition of Cdk5/p25 hyperactivation in vivo and in vitro rescues nigrostriatal dopaminergic neurodegeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP/MPP+) in a mouse model of PD. TP5 peptide treatment also blocked dopamine depletion in the striatum and improved gait dysfunction after MPTP administration. The neuroprotective effect of TFP5/TP5 peptide is also associated with marked reduction in neuroinflammation and apoptosis. Here we show selective inhibition of Cdk5/p25 ­hyperactivation by TFP5/TP5 peptide, which identifies the kinase as a potential therapeutic target to reduce neurodegeneration in Parkinson’s disease.

Identification of neurodegenerative factors using translatome-regulatory network analysis

For degenerative disorders of the CNS, the main obstacle to therapeutic advancement has been the challenge of identifying the key molecular mechanisms underlying neuronal loss. We developed a combinatorial approach including translational profiling and brain regulatory network analysis to search for key determinants of neuronal survival or death. Following the generation of transgenic mice for cell type-specific profiling of midbrain dopaminergic neurons, we established and compared translatome libraries reflecting the molecular signature of these cells at baseline or under degenerative stress. Analysis of these libraries by interrogating a context-specific brain regulatory network led to the identification of a repertoire of intrinsic upstream regulators that drive the dopaminergic stress response. The altered activity of these regulators was not associated with changes in their expression levels. This strategy can be generalized for the identification of molecular determinants involved in the degeneration of other classes of neurons.

Cre-driven optogenetics in the heterogeneous genetic panorama of the VTA

The selectivity of optogenetics commonly relies on genetic promoters to manipulate specific populations of neurons through the use of Cre-driver lines. All studies performed in the ventral tegmental area (VTA) so far have utilized promoters present in groups of cells that release dopamine (DA), GABA, or glutamate. However, neurons that co-release neurotransmitters and variabilities within groups of neurons that release the same neurotransmitter present challenges when evaluating the results. Further complexity is introduced by ectopic expression patterns often occurring in transgenic Cre-drivers. New perspectives could be unfolded by identifying and selecting different types of promoter for driving the Cre recombinase. Here, we discuss some promising candidates and highlight the advantages or disadvantages of different methods for creating novel transgenic lines.