Translational control by RGS2

The roles of RGS proteins in cardiometabolic disease

G protein-coupled receptors (GPCRs) are the most prominent receptors on the surface of the cell and play a central role in the regulation of cardiac and metabolic functions. GPCRs transmit extracellular stimuli to the interior of the cells by activating one or more heterotrimeric G proteins. The duration and intensity of G protein-mediated signalling are tightly controlled by a large array of intracellular mediators, including the regulator of G protein signalling (RGS) proteins. RGS proteins selectively promote the GTPase activity of a subset of Gα subunits, thus serving as negative regulators in a pathway-dependent manner. In the current review, we summarise the involvement of RGS proteins in cardiometabolic function with a focus on their tissue distribution, mechanisms of action and dysregulation under various disease conditions. We also discuss the potential therapeutic applications for targeting RGS proteins in treating cardiometabolic conditions and current progress in developing RGS modulators. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

KSHV promotes oncogenic FOS to inhibit nuclease AEN and transactivate RGS2 for AKT phosphorylation

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 is a lytic RNA-binding protein. We applied BCBL-1 cells in lytic KSHV infection and performed UV cross-linking immunoprecipitation (CLIP) followed by RNA-seq of the CLIPed RNA fragments (CLIP-seq). We identified ORF57-bound transcripts from 544 host protein-coding genes. By comparing with the RNA-seq profiles from BCBL-1 cells with latent and lytic KSHV infection and from HEK293T cells with and without ORF57 expression, we identified FOS and CITED2 RNAs being two common ORF57-specific RNA targets. FOS dimerizes with JUN as a transcription factor AP-1 involved in cell proliferation, differentiation, and transformation. Knockout of the ORF57 gene from the KSHV genome led BAC16-iSLK cells incapable of FOS expression in KSHV lytic infection. The dysfunctional KSHV genome in FOS expression could be rescued by Lenti-ORF57 virus infection. ORF57 protein does not regulate FOS translation but binds to the 13-nt RNA motif near the FOS RNA 5' end and prolongs FOS mRNA half-life 7.7 times longer than it is in the absence of ORF57. This binding of ORF57 to FOS RNA is competitive to the binding of a host nuclease AEN (also referred to as ISG20L1). KSHV infection inhibits the expression of AEN, but not exosomal RNA helicase MTR4. FOS expression mediated by ORF57 inhibits AEN transcription, but transactivates RGS2, a regulator of G-protein coupled receptors. FOS binds a conserved AP-1 site in the RGS2 promoter and enhances RGS2 expression to phosphorylate AKT. Altogether, we have discovered that KSHV ORF57 specifically binds and stabilizes FOS RNA to increase FOS expression, thereby disturbing host gene expression and inducing pathogenesis during KSHV lytic infection.

Elucidating the role of Rgs2 expression in the PVN for metabolic homeostasis in mice

OBJECTIVE

RGS2 is a GTPase activating protein that modulates GPCR-Gα signaling and mice lacking RGS2 globally exhibit metabolic alterations. While RGS2 is known to be broadly expressed throughout the body including the brain, the relative contribution of brain RGS2 to metabolic homeostasis remains unknown. The purpose of this study was to characterize RGS2 expression in the paraventricular nucleus of hypothalamus (PVN) and test its role in metabolic homeostasis.

METHODS

We used a combination of RNAscope in situ hybridization (ISH), immunohistochemistry, and bioinformatic analyses to characterize the pattern of Rgs2 expression in the PVN. We then created mice lacking Rgs2 either prenatally or postnatally in the PVN and evaluated their metabolic consequences.

RESULTS

RNAscope ISH analysis revealed a broad but regionally enriched Rgs2 mRNA expression throughout the mouse brain, with the highest expression being observed in the PVN along with several other brain regions, such as the arcuate nucleus of hypothalamus and the dorsal raphe nucleus. Within the PVN, we found that Rgs2 is specifically enriched in CRH+ endocrine neurons and is further increased by calorie restriction. Functionally, although Sim1-Cre-mediated prenatal deletion of Rgs2 in PVN neurons had no major effects on metabolic homeostasis, AAV-mediated adult deletion of Rgs2 in the PVN led to significantly increased food intake, body weight (both fat and fat-free masses), body length, and blood glucose levels in both male and female mice. Strikingly, we found that prolonged postnatal loss of Rgs2 leads to neuronal cell death in the PVN, while rapid body weight gain in the early phase of viral-mediated PVN Rgs2 deletion is independent of PVN neuronal loss.

CONCLUSIONS

Our results provide the first evidence to show that PVN Rgs2 expression is not only sensitive to metabolic challenge but also critically required for PVN endocrine neurons to function and maintain metabolic homeostasis.

Autophagy induction promoted by m6A reader YTHDF3 through translation upregulation of FOXO3 mRNA

Autophagy is crucial for maintaining cellular energy homeostasis and for cells to adapt to nutrient deficiency, and nutrient sensors regulating autophagy have been reported previously. However, the role of eiptranscriptomic modifications such as m⁶A in the regulation of starvation-induced autophagy is unclear. Here, we show that the m⁶A reader YTHDF3 is essential for autophagy induction. m⁶A modification is up-regulated to promote autophagosome formation and lysosomal degradation upon nutrient deficiency. METTL3 depletion leads to a loss of functional m⁶A modification and inhibits YTHDF3-mediated autophagy flux. YTHDF3 promotes autophagy by recognizing m⁶A modification sites around the stop codon of FOXO3 mRNA. YTHDF3 also recruits eIF3a and eIF4B to facilitate FOXO3 translation, subsequently initiating autophagy. Overall, our study demonstrates that the epitranscriptome regulator YTHDF3 functions as a nutrient responder, providing a glimpse into the post-transcriptional RNA modifications that regulate metabolic homeostasis.

Low Expression of RGS2 Promotes Poor Prognosis in High-Grade Serous Ovarian Cancer

Simple Summary

Recent advances in molecular medicine have indicated G-protein coupled receptors (GPCRs) as possible therapeutic targets in ovarian cancer. The cellular effects of GPCRs are determined by regulator of G protein signaling (RGS) proteins. Especially RGS2 has currently moved into focus of cancer therapy. Therefore, we retrospectively analyzed RGS2 and its association with the prognosis of high-grade serous ovarian cancer (HGSOC). Here, we provide in situ and in silico analyses regarding the expression patterns and prognostic value of RGS2. In silico we found that RGS2 is barely detectable in tumor cells on the mRNA level in bulk and single-cell data. Applying immunohistochemistry in 519 HGSOC patients, we detected moderate to strong protein expression of RGS2 in situ in approximately half of the cohort, suggesting regulation by post translational modification. Furthermore, low protein expression of RGS2 was associated with an inferior overall- and progression-free survival. These results warrant further research of its role and related new therapeutic implications in HGSOC.

Abstract

RGS2 regulates G-protein signaling by accelerating hydrolysis of GTP and has been identified as a potentially druggable target in carcinomas. Since the prognosis of patients with high-grade serous ovarian carcinoma (HGSOC) remains utterly poor, new therapeutic options are urgently needed. Previous in vitro studies have linked RGS2 suppression to chemoresistance in HGSOC, but in situ data are still missing. In this study, we characterized the expression of RGS2 and its relation to prognosis in HGSOC on the protein level by immunohistochemistry in 519 patients treated at Charité, on the mRNA level in 299 cases from TCGA and on the single-cell level in 19 cases from publicly available datasets. We found that RGS2 is barely detectable on the mRNA level in both bulk tissue (median 8.2. normalized mRNA reads) and single-cell data (median 0 normalized counts), but variably present on the protein level (median 34.5% positive tumor cells, moderate/strong expression in approximately 50% of samples). Interestingly, low expression of RGS2 had a negative impact on overall survival (p = 0.037) and progression-free survival (p = 0.058) on the protein level in lower FIGO stages and in the absence of residual tumor burden. A similar trend was detected on the mRNA level. Our results indicated a significant prognostic impact of RGS2 protein suppression in HGSOC. Due to diverging expression patterns of RGS2 on mRNA and protein levels, posttranslational modification of RGS2 is likely. Our findings warrant further research to unravel the functional role of RGS2 in HGSOC, especially in the light of new drug discovery.

R4 RGS proteins as fine tuners of immature and mature hematopoietic cell trafficking

G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors. They are involved in almost every physiologic process and consequently have a pivotal role in an extensive number of pathologies, including genetic, neurologic, and immune system disorders. Indeed, the vast array of GPCRs mechanisms have led to the development of a tremendous number of drug therapies and already account for about a third of marketed drugs. These receptors mediate their downstream signals primarily via G proteins. The regulators of G-protein signaling (RGS) proteins are now in the spotlight as the critical modulatory factors of active GTP-bound Gα subunits of heterotrimeric G proteins to fine-tune the biologic responses driven by the GPCRs. Also, they possess noncanonical functions by multiple mechanisms, such as protein-protein interactions. Essential roles and impacts of these RGS proteins have been revealed in physiology, including hematopoiesis and immunity, and pathologies, including asthma, cancers, and neurologic disorders. This review focuses on the largest subfamily of R4 RGS proteins and provides a brief overview of their structures and G-proteins selectivity. With particular interest, we explore and highlight, their expression in the hematopoietic system and the regulation in the engraftment of hematopoietic stem/progenitor cells (HSPCs). Distinct expression patterns of R4 RGS proteins in the hematopoietic system and their pivotal roles in stem cell trafficking pave the way for realizing new strategies for enhancing the clinical performance of hematopoietic stem cell transplantation. Finally, we discuss the exciting future trends in drug development by targeting RGS activity and expression with small molecules inhibitors and miRNA approaches.

Regulator of G protein signaling 2 inhibits Gαq-dependent uveal melanoma cell growth

Activating mutations in Gα_q/11 are a major driver of uveal melanoma (UM), the most common intraocular cancer in adults. While progress has recently been made in targeting Gα_q/11 for UM therapy, the crucial role for these proteins in normal physiology and their high structural similarity with many other important GTPase proteins renders this approach challenging. The aim of the current study was to validate whether a key regulator of Gq signaling, regulator of G protein signaling 2 (RGS2), can inhibit Gα_q-mediated UM cell growth. We used two UM cell lines, 92.1 and Mel-202, which both contain the most common activating mutation Gα_q^Q209L and developed stable cell lines with doxycycline-inducible RGS2 protein expression. Using cell viability assays, we showed that RGS2 could inhibit cell growth in both of these UM cell lines. We also found that this effect was independent of the canonical GTPase-activating protein activity of RGS2 but was dependent on the association between RGS2 and Gα_q. Furthermore, RGS2 induction resulted in only partial reduction in cell growth as compared to siRNA-mediated Gαq knockdown, perhaps because RGS2 was only able to reduce mitogen-activated protein kinase signaling downstream of phospholipase Cβ, while leaving activation of the Hippo signaling mediators yes-associated protein 1/TAZ, the other major pathway downstream of Gα_q, unaffected. Taken together, our data indicate that RGS2 can inhibit UM cancer cell growth by associating with Gα_q^Q209L as a partial effector antagonist.

RGS2-mediated translational control mediates cancer cell dormancy and tumor relapse

Slow-cycling/dormant cancer cells (SCCs) have pivotal roles in driving cancer relapse and drug resistance. A mechanistic explanation for cancer cell dormancy and therapeutic strategies targeting SCCs are necessary to improve patient prognosis, but are limited because of technical challenges to obtaining SCCs. Here, by applying proliferation-sensitive dyes and chemotherapeutics to non-small cell lung cancer (NSCLC) cell lines and patient-derived xenografts, we identified a distinct SCC subpopulation that resembled SCCs in patient tumors. These SCCs displayed major dormancy-like phenotypes and high survival capacity under hostile microenvironments through transcriptional upregulation of regulator of G protein signaling 2 (RGS2). Database analysis revealed RGS2 as a biomarker of retarded proliferation and poor prognosis in NSCLC. We showed that RGS2 caused prolonged translational arrest in SCCs through persistent eukaryotic initiation factor 2 (eIF2α) phosphorylation via proteasome-mediated degradation of activating transcription factor 4 (ATF4). Translational activation through RGS2 antagonism or the use of phosphodiesterase 5 inhibitors, including sildenafil (Viagra), promoted ER stress-induced apoptosis in SCCs in vitro and in vivo under stressed conditions, such as those induced by chemotherapy. Our results suggest that a low-dose chemotherapy and translation-instigating pharmacological intervention in combination is an effective strategy to prevent tumor progression in NSCLC patients after rigorous chemotherapy.

Emerging Roles for Regulator of G Protein Signaling 2 in (Patho)physiology

Since their discovery in the mid-1990s, regulator of G protein signaling (RGS) proteins have emerged as key regulators of signaling through G protein-coupled receptors. Among the over 20 known RGS proteins, RGS2 has received increasing interest as a potential therapeutic drug target with broad clinical implications. RGS2 is a member of the R4 subfamily of RGS proteins and is unique in that it is selective for Gα _q Despite only having an RGS domain, responsible for the canonical GTPase activating protein activity, RGS2 can regulate additional processes, such as protein synthesis and adenylate cyclase activity, through protein-protein interactions. Here we provide an update of the current knowledge of RGS2 function as it relates to molecular mechanisms of regulation as well as its potential role in regulating a number of physiologic systems and pathologies, including cardiovascular disease and central nervous system disorders, as well as various forms of cancer. SIGNIFICANCE STATEMENT: Regulator of G protein signaling (RGS) proteins represent an exciting class of novel drug targets. RGS2, in particular, could have broad clinical importance. As more details are emerging on the regulation of RGS2 in various physiological systems, the potential utility of this small protein in therapeutic development is increasing.

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.

RGS2 promotes the translation of stress-associated proteins ATF4 and CHOP via its eIF2B-inhibitory domain

Regulator of G protein signaling 2 (RGS2) is upregulated by multiple forms of stress and can augment translational attenuation associated with the phosphorylation of the initiation factor eIF2, a hallmark of several stress-induced coping mechanisms. Under stress-induced translational inhibition, key factors, such as ATF4, are selectively expressed via alternative translation mechanisms. These factors are known to regulate molecular switches that control cell fate by regulating pro-survival and pro-apoptotic signals. The molecular mechanisms that balance these opposing responses to stresses are unclear. The present results suggest that RGS2 may be an important regulatory component in the cellular stress response through its translational control abilities. Previously, we have shown that RGS2 can interact with the translation initiation factor, eIF2B, and inhibit de novo protein synthesis. Here, we demonstrate that the expression of either full length RGS2 or its eIF2B-interacting domain (RGS2^eb) significantly increases levels of ATF4 and CHOP, both of which are linked to stress-induced apoptosis. Furthermore, we show that these effects are translationally regulated and independent of eIF2 phosphorylation. The present results thus point to a novel function of RGS2 in the stress response directly related to its ability to reduce global protein synthesis.

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.

RGS10 Regulates the Expression of Cyclooxygenase-2 and Tumor Necrosis Factor Alpha through a G Protein-Independent Mechanism

The small regulator of G protein signaling protein RGS10 is a key regulator of neuroinflammation and ovarian cancer cell survival; however, the mechanism for RGS10 function in these cells is unknown and has not been linked to specific G protein pathways. RGS10 is highly enriched in microglia, and loss of RGS10 expression in microglia amplifies production of the inflammatory cytokine tumor necrosis factor α (TNFα) and enhances microglia-induced neurotoxicity. RGS10 also regulates cell survival and chemoresistance of ovarian cancer cells. Cyclooxygenase-2 (COX-2)-mediated production of prostaglandins such as prostaglandin E₂ (PGE₂) is a key factor in both neuroinflammation and cancer chemoresistance, suggesting it may be involved in RGS10 function in both cell types, but a connection between RGS10 and COX-2 has not been reported. To address these questions, we completed a mechanistic study to characterize RGS10 regulation of TNFα and COX-2 and to determine if these effects are mediated through a G protein-dependent mechanism. Our data show for the first time that loss of RGS10 expression significantly elevates stimulated COX-2 expression and PGE₂ production in microglia. Furthermore, the elevated inflammatory signaling resulting from RGS10 loss was not affected by Gα_i inhibition, and a RGS10 mutant that is unable to bind activated G proteins was as effective as wild type in inhibiting TNFα expression. Similarly, suppression of RGS10 in ovarian cancer cells enhanced TNFα and COX-2 expression, and this effect did not require G_i activity. Together, our data strongly indicate that RGS10 inhibits COX-2 expression by a G protein-independent mechanism to regulate inflammatory signaling in microglia and ovarian cancer cells.

Downregulation of regulator of G protein signaling 2 expression in breast invasive carcinoma of no special type: Clinicopathological associations and prognostic relevance

Changes in the expression of regulator of G protein signaling 2 (RGS2) are involved in the genesis and development of a number of malignancies. However, the association between changes in the expression of RGS2 and breast invasive carcinoma of no special type (BIC-NST) remains unknown. The present study found that, in comparison to normal breast tissue, BIC-NST exhibited low expression of RGS2 mRNA and protein, as detected using data mining and immunohistochemical analysis. The low expression of RGS2 was associated with the positive status of hormone receptor expression in BIC-NST. Kaplan-Meier analysis revealed that patients with low RGS2 expression had a significantly poorer overall survival rate. Furthermore, multivariate Cox regression analysis demonstrated that the RGS2 low expression was an independent high-risk factor. Gene set enrichment analysis using data from The Cancer Genome Atlas supported these results. In summary, the results of the current study indicate that RGS2 acts as a suppressor gene in the progression of BIC-NST. To the best of our knowledge, the present study is the first concerning the association between RGS2 and hormone receptors in BIC-NST, as well as that between RGS2 expression and the prognosis of patients with BIC-NST. However, the effect of RGS2 in breast cancer requires further investigation.

Human Missense Mutations in Regulator of G Protein Signaling 2 Affect the Protein Function Through Multiple Mechanisms

Regulator of G protein signaling 2 (RGS2) plays a significant role in alleviating vascular contraction and promoting vascular relaxation due to its GTPase accelerating protein activity toward Gαq. Mice lacking RGS2 display a hypertensive phenotype, and several RGS2 missense mutations have been found predominantly in hypertensive human subjects. However, the mechanisms whereby these mutations could impact blood pressure is unknown. Here, we selected 16 rare, missense mutations in RGS2 identified in various human exome sequencing projects and evaluated their ability to inhibit intracellular calcium release mediated by angiotensin II receptor type 1 (AT1R). Four of them had reduced function and were further investigated to elucidate underlying mechanisms. Low protein expression, protein mislocalization, and reduced G protein binding were identified as likely mechanisms of the malfunctioning mutants. The Q2L mutant had 50% lower RGS2 than wild-type (WT) protein detected by Western blot. Confocal microscopy demonstrated that R44H and D40Y had impaired plasma membrane targeting; only 46% and 35% of those proteins translocated to the plasma membrane when coexpressed with Gα_q Q209L compared with 67% for WT RGS2. The R188H mutant had a significant reduction in Gα_q binding affinity (10-fold increase in K_i compared with WT RGS2 in a flow cytometry competition binding assay). This study provides functional data for 16 human RGS2 missense variants on their effects on AT1R-mediated calcium mobilization and provides molecular understanding of those variants with functional loss in vitro. These molecular behaviors can provide insight to inform antihypertensive therapeutics in individuals with variants having reduced function.

Cardiomyocyte specific overexpression of a 37 amino acid domain of regulator of G protein signalling 2 inhibits cardiac hypertrophy and improves function in response to pressure overload in mice

Regulator of G protein signalling 2 (RGS2) is known to play a protective role in maladaptive cardiac hypertrophy and heart failure via its ability to inhibit G_q- and G_s- mediated GPCR signalling. We previously demonstrated that RGS2 can also inhibit protein translation and can thereby attenuate cell growth. This G protein-independent inhibitory effect has been mapped to a 37 amino acid domain (RGS2^eb) within RGS2 that binds to eukaryotic initiation factor 2B (eIF2B). When expressed in neonatal rat cardiomyocytes, RGS2^eb attenuates both protein synthesis and hypertrophy induced by G_q- and G_s- activating agents. In the current study, we investigated the potential cardioprotective role of RGS2^eb by determining whether RGS2^eb transgenic (RGS2^eb TG) mice with cardiomyocyte specific overexpression of RGS2^eb show resistance to the development of hypertrophy in comparison to wild-type (WT) controls. Using transverse aortic constriction (TAC) in a pressure-overload hypertrophy model, we demonstrated that cardiac hypertrophy was inhibited in RGS2^eb TG mice compared to WT controls following four weeks of TAC. Expression of the hypertrophic markers atrial natriuretic peptide (ANP) and β-myosin heavy chain (MHC-β) was also reduced in RGS2^eb TG compared to WT TAC animals. Furthermore, cardiac function in RGS2^eb TG TAC mice was significantly improved compared to WT TAC mice. Notably, cardiomyocyte cell size was significantly decreased in TG compared to WT TAC mice. These results suggest that RGS2 may limit pathological cardiac hypertrophy at least in part via the function of its eIF2B-binding domain.

Regulator of G protein signaling 2 is a key regulator of pancreatic β-cell mass and function

Pancreatic β-cell death and dysfunction contributes to the pathogenesis of both type 1 and type 2 diabetes. We aimed to examine whether the regulator of G protein signaling protein 2 (RGS2), a multifunctional inhibitor of G protein-coupled receptor (GPCR) signaling, impacts β-cell death and function. Metabolic phenotypes, β-cell secretory function, and glucose and insulin tolerance were measured in RGS2 knockout (RGS2^−/−) mice and their wild-type (RGS2^+/+) littermate controls. β-Cell death was evaluated in RGS2-knockdown and -overexpressing β cells and RGS2^−/− islets by flow cytometry, western blot, ELISA, TUNEL staining, and apoptosis RT² profiler PCR array analysis. β-Cell mass was evaluated in pancreases from RGS2^−/− and RGS2^+/+ mice at 1 day, 4 weeks, and 25 weeks of age. Our data show that RGS2^−/− islets secreted more insulin than RGS2^+/+ islets when challenged with glucose or exendin-4. RGS2-knockdown cells are susceptible to hypoxia induced cell death while RGS2-overexpressing cells are protected from cell death. Depletion of RGS2 in islets alters expression of apoptosis-related genes and RGS2^−/− islets are prone to apoptosis compared with RGS2^+/+ islets. Ultimately, excessive insulin secretion and increased β-cell apoptosis contributed to a 70% reduction in pancreatic β-cell mass in RGS2^−/− mice compared with RGS2^+/+ mice at 25 weeks of age. RGS2 has critical roles in maintaining pancreatic β-cell mass via modulating β-cell function and apoptosis. It may serve as a druggable target to help prevent pancreatic β-cell loss in the treatment of diabetes.

Knockdown of regulator of G-protein signalling 2 (Rgs2) leads to abnormal early mouse embryo development in vitro

Regulator of G-protein signalling 2 (Rgs2) is involved in G-protein-mediated signalling by negatively regulating the activity of the G-protein α-subunit. In the present study, the expression patterns of Rgs2 in mouse ovarian tissues and early embryos were determined by semiquantitative reverse transcription-polymerase chain reaction, immunohistochemistry and immunofluorescent analyses. Rgs2 expression was observed in the ovarian tissues of adult female mice, with an almost equal expression levels during different stages of the oestrous cycle. Rgs2 was abundant in the cytoplasm, membrane, nuclei and spindles of intact polar bodies in mouse early embryos at different developmental stages from the zygote to blastocyst. The effect of Rgs2 knockdown on early embryonic development in vitro was examined by microinjecting Rgs2-specific short interfering (si) RNAs into mouse zygotes. Knockdown of endogenous Rgs2 expression led to abnormal embryonic development in vitro, with a considerable number of early embryos arrested at the 2- or 4-cell stage. Moreover, mRNA expression of three zygotic gene activation-related genes (i.e. Zscan4, Tcstv1 and MuERV-L) was decreased significantly in 2-cell arrested embryos. These results suggest that Rgs2 plays a critical role in early embryo development.

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.

RGS2 expression predicts amyloid-β sensitivity, MCI and Alzheimer's disease: genome-wide transcriptomic profiling and bioinformatics data mining

Alzheimer's disease (AD) is the most frequent cause of dementia. Misfolded protein pathological hallmarks of AD are brain deposits of amyloid-β (Aβ) plaques and phosphorylated tau neurofibrillary tangles. However, doubts about the role of Aβ in AD pathology have been raised as Aβ is a common component of extracellular brain deposits found, also by in vivo imaging, in non-demented aged individuals. It has been suggested that some individuals are more prone to Aβ neurotoxicity and hence more likely to develop AD when aging brains start accumulating Aβ plaques. Here, we applied genome-wide transcriptomic profiling of lymphoblastoid cells lines (LCLs) from healthy individuals and AD patients for identifying genes that predict sensitivity to Aβ. Real-time PCR validation identified 3.78-fold lower expression of RGS2 (regulator of G-protein signaling 2; P=0.0085) in LCLs from healthy individuals exhibiting high vs low Aβ sensitivity. Furthermore, RGS2 showed 3.3-fold lower expression (P=0.0008) in AD LCLs compared with controls. Notably, RGS2 expression in AD LCLs correlated with the patients' cognitive function. Lower RGS2 expression levels were also discovered in published expression data sets from postmortem AD brain tissues as well as in mild cognitive impairment and AD blood samples compared with controls. In conclusion, Aβ sensitivity phenotyping followed by transcriptomic profiling and published patient data mining identified reduced peripheral and brain expression levels of RGS2, a key regulator of G-protein-coupled receptor signaling and neuronal plasticity. RGS2 is suggested as a novel AD biomarker (alongside other genes) toward early AD detection and future disease modifying therapeutics.

RGS2 squelches vascular Gi/o and Gq signaling to modulate myogenic tone and promote uterine blood flow

Uterine artery blood flow (UABF) is critical to maintaining uterine perfusion in nonpregnant states and for uteroplacental delivery of nutrients and oxygen to the fetus during pregnancy. Impaired UABF is implicated in infertility and several pregnancy complications including fetal growth restriction, small for gestational age, and preeclampsia. The etiology of abnormal UABF is not known. Here, we determined whether deficiency or loss of RGS2, a GTPase-activating protein for Gq/11 and Gi/o class G proteins, affects UABF in nonpregnant mice. We used Doppler ultrasonography to assess UABF in wild type (WT), Rgs2 heterozygous (Rgs2+/-), and homozygous knockout (Rgs2-/-) mice. Video microscopy was used for ex vivo examination of uterine artery myogenic tone and fura-2 imaging for in vitro assessment of internal stores Ca(2+) release. We found that baseline UABF velocity was markedly decreased while impedance measured as resistive index (WT = 0.58 ± 0.04 vs. Rgs2-/- = 0.71 ± 0.03, P < 0.01) and pulsatile index (WT = 0.90 ± 0.06 vs. Rgs2-/- = 1.25 ± 0.11, P < 0.01) was increased in Rgs2-/- mice. Uterine artery tone was augmented in Rgs2+/- and Rgs2-/- mice, which was normalized to WT levels following Gi/o and Gq inactivation. Conversely, blockade of ryanodine receptors increased WT myogenic tone to RGS2 mutant levels. The data together indicate that RGS2 deficiency decreases UABF by increasing myogenic tone at least partly through prolonged G protein activation. Mutations that decrease vascular RGS2 expression may be a predisposition to decreased uterine blood flow. Targeting G protein signaling therefore might improve uterine and uteroplacental underperfusion disorders.

Digoxin-Mediated Upregulation of RGS2 Protein Protects against Cardiac Injury

Regulator of G protein signaling (RGS) proteins have emerged as novel drug targets since their discovery almost two decades ago. RGS2 has received particular interest in cardiovascular research due to its role in regulating Gqsignaling in the heart and vascular smooth muscle. RGS2(-/-)mice are hypertensive, prone to heart failure, and display accelerated kidney fibrosis. RGS2 is rapidly degraded through the proteasome, and human mutations leading to accelerated RGS2 protein degradation correlate with hypertension. Hence, stabilizing RGS2 protein expression could be a novel route in treating cardiovascular disease. We previously identified cardiotonic steroids, including digoxin, as selective stabilizers of RGS2 protein in vitro. In the current study we investigated the functional effects of digoxin-mediated RGS2 protein stabilization in vivo. Using freshly isolated myocytes from wild-type and RGS2(-/-)mice treated with vehicle or low-dose digoxin (2µg/kg/day for 7 days) we demonstrated that agonist-induced cAMP levels and cardiomyocyte contractility was inhibited by digoxin in wild-type but not in RGS2(-/-)mice. This inhibition was accompanied by an increase in RGS2 protein levels in cardiomyocytes as well as in whole heart tissue. Furthermore, digoxin had protective effects in a model of cardiac injury in wild-type mice and this protection was lost in RGS2(-/-)mice. Digoxin is the oldest known therapy for heart failure; however, beyond its activity at the Na(+)/K(+)-ATPase, the exact mechanism of action is not known. The current study adds a novel mechanism, whereby through stabilizing RGS2 protein levels digoxin could exert its protective effects in the failing heart.

Regulator of G Protein Signaling 2: A Versatile Regulator of Vascular Function

Regulators of G protein signaling (RGS) proteins of the B/R4 family are widely expressed in the cardiovascular system where their role in fine-tuning G protein signaling is critical to maintaining homeostasis. Among members of this family, RGS2 and RGS5 have been shown to play key roles in cardiac and smooth muscle function by tightly regulating signaling pathways that are activated through Gq/11 and Gi/o classes of heterotrimeric G proteins. This chapter reviews accumulating evidence supporting a key role for RGS2 in vascular function and the implication of changes in RGS2 function and/or expression in the pathogenesis of blood pressure disorders, particularly hypertension. With such understanding, RGS2 and the signaling pathways it controls may emerge as novel targets for developing next-generation antihypertensive drugs/agents.

Expression and regulation of regulator of G-protein signaling protein-2 (RGS2) in equine and bovine follicles prior to ovulation: molecular characterization of RGS2 transactivation in bovine granulosa cells

The luteinizing hormone preovulatory surge stimulates several signal pathways essential for ovulation, and the regulator of G-protein signaling protein-2 (RGS2) is thought to be involved in this process. The objectives of this study were to characterize the regulation of RGS2 transcripts in equine and bovine follicles prior to ovulation and to determine its transcriptional control in bovine granulosa cells. To assess the regulation of equine RGS2 prior to ovulation, RT-PCR was performed using total RNA extracted from equine follicles collected at various times after human chorionic gonadotropin (hCG) injection. Results showed that RGS2 mRNA levels were very low at 0 h but markedly increased 12-39 h post-hCG (P < 0.05). In the bovine species, results revealed that RGS2 mRNA levels were low in small and dominant follicles and in ovulatory follicles obtained at 0 h, but markedly increased in ovulatory follicles 6-24 h post-hCG (P < 0.05). To study the molecular control of RGS2 expression, primary cultures of bovine granulosa cells were used. Stimulation with forskolin induced an up-regulation of RGS2 mRNA in vitro. Studies using 5'-deletion mutants identified a minimal region containing full-length basal and forskolin-inducible RGS2 promoter activities. Site-directed mutagenesis indicated that these activities were dependent on CRE and ETS1 cis-elements. Electrophoretic mobility shift assays confirmed the involvement of these elements and revealed their interactions with CREB1 and ETS1 proteins. Chromatin immunoprecipitation assays confirmed endogenous interactions of these proteins with the RGS2 promoter in granulosa cells. Forskolin-inducible RGS2 promoter activity and mRNA expression were markedly decreased by PKA and ERK1/2 inhibitors, and treatment with an antagonist of PGR (RU486) and inhibitors of PTGS2 (NS398) and EGFR (PD153035) blocked the forskolin-dependent RGS2 transcript expression, suggesting the importance of RGS2 in ovulation. Collectively, this study reports for the first time the gonadotropin-dependent up-regulation of RGS2 in equine and bovine preovulatory follicles and presents some of the regulatory controls involved in RGS2 gene expression in granulosa cells.

The eIF2B-interacting domain of RGS2 protects against GPCR agonist-induced hypertrophy in neonatal rat cardiomyocytes

The protective effect of Regulator of G protein Signaling 2 (RGS2) in cardiac hypertrophy is thought to occur through its ability to inhibit the chronic GPCR signaling that promotes pathogenic growth both in vivo and in cultured cardiomyocytes. However, RGS2 is known to have additional functions beyond its activity as a GTPase accelerating protein, such as the ability to bind to eukaryotic initiation factor, eIF2B, and inhibit protein synthesis. The RGS2 eIF2B-interacting domain (RGS2(eb)) was examined for its ability to regulate hypertrophy in neonatal ventricular myocytes. Both full-length RGS2 and RGS2(eb) were able to inhibit agonist-induced cardiomyocyte hypertrophy, but RGS2(eb) had no effect on receptor-mediated inositol phosphate production, cAMP production, or ERK 1/2 activation. These results suggest that the protective effects of RGS2 in cardiac hypertrophy may derive at least in part from its ability to govern protein synthesis.

Activation of Gαq subunits up-regulates the expression of the tumor suppressor Fhit

The tumor suppressor Fhit protein is defective or absent in many tumor cells due to methylation, mutation or deletion of the FHIT gene. Despite numerous attempts to unravel the functions of Fhit, the mechanisms by which the function and expression of Fhit are regulated remain poorly understood. We have recently shown that activated Gαq subunits interact directly with Fhit and enhance its inhibitory effect on cell growth. Here we investigated the regulation of Fhit expression by Gq. Our results showed that Fhit was up-regulated specifically by activating Gα subunits of the Gq subfamily but not by those of the other G protein subfamilies. This up-regulation effect was mediated by a PKC/MEK pathway independent of Src-mediated Fhit Tyr(114) phosphorylation. We further demonstrated that elevated Fhit expression was due to the specific regulation of Fhit protein synthesis in the ribosome by activated Gαq, where the regulations of cap-dependent protein synthesis were apparently not required. Moreover, we showed that activated Gαq could increase cell-cell adhesion through Fhit. These findings provide a possible handle to modulate the level of the Fhit tumor suppressor by manipulating the activity of Gq-coupled receptors.

Functional role, mechanisms of regulation, and therapeutic potential of regulator of G protein signaling 2 in the heart

G protein-mediated signal transduction is essential for the regulation of cardiovascular function, including heart rate, growth, contraction, and vascular tone. Regulators of G protein Signaling (RGS proteins) fine-tune G protein-coupled receptor-induced signaling by regulating its magnitude and duration through direct interaction with the α subunits of heterotrimeric G proteins. Changes in the RGS protein expression and/or function in the heart often lead to pathophysiological changes and are associated with cardiac disease in animals and humans, including hypertrophy, fibrosis development, heart failure, and arrhythmias. This article focuses on Regulator of G protein Signaling 2 (RGS2), which is widely expressed in many tissues and is highly regulated in its expression and function. Most information to date has been obtained in biochemical, cellular, and animal studies, but data from humans is emerging. We review recent advances on the functional role of cardiovascular RGS2 and the mechanisms that determine its signaling selectivity, expression, and functionality. We highlight key unanswered questions and discuss the potential of RGS2 as a therapeutic target.

Stress-induced transcription of regulator of G protein signaling 2 (RGS2) by heat shock transcription factor HSF1

Expression of the RGS2 gene modulates RGS2 activity toward G protein-coupled signaling in diverse cellular processes. In this study, RGS2 transcription was induced in HeLa and rat aorta smooth muscle cells by exposure to febrile temperatures or proteotoxic stress. The promoter region of RGS2 contained a binding sequence of HSF1, which is an activator of the heat shock protein gene, and was inducibly bound by stress-activated HSF1. A single nucleotide change identified in the RGS2 promoter of hypertensive patients abolished HSF1-regulated expression of RGS2, suggesting that activated HSF1 is involved in blood pressure regulation via modulation of RGS2 expression.

RGS2 is a component of the cellular stress response

Regulator of G protein signaling (RGS) proteins are GTPase accelerating proteins for heterotrimeric G protein α-subunits. RGS2 has recently been shown to have additional G protein-independent functions including control of ion channel currents, microtubule polymerization, and protein synthesis. Cellular levels of RGS2 mRNA and protein are upregulated in response to various forms of stress suggesting that it may be a stress-adaptive protein; however, direct evidence to support this notion has remained elusive. In this report, we show that thermal stress upregulates RGS2 expression and this serves to arrest de novo protein synthesis. The latter is an established cellular response to stress. Inhibiting the stress-induced RGS2 upregulation by way of siRNA knockdown diminished the repression of global protein synthesis. The collective results of our study implicate RGS2 upregulation as a cellular mechanism of controlling de novo protein synthesis in response to stress. This work provides greater insight into the stress proteome and the role of RGS2.

Cardiotonic steroids stabilize regulator of G protein signaling 2 protein levels

Regulator of G protein signaling 2 (RGS2), a G(q)-specific GTPase-activating protein, is strongly implicated in cardiovascular function. RGS2(-/-) mice are hypertensive and prone to heart failure, and several rare human mutations that accelerate RGS2 degradation have been identified among patients with hypertension. Therefore, pharmacological up-regulation of RGS2 protein levels might be beneficial. We used a β-galactosidase complementation method to screen several thousand compounds with known pharmacological functions for those that increased RGS2 protein levels. Several cardiotonic steroids (CTSs), including ouabain and digoxin, increased RGS2 but not RGS4 protein levels. CTSs increased RGS2 protein levels through a post-transcriptional mechanism, by slowing protein degradation. RGS2 mRNA levels in primary vascular smooth muscle cells were unaffected by CTS treatment, whereas protein levels were increased 2- to 3-fold. Na(+)/K(+)-ATPase was required for the increase in RGS2 protein levels, because the effect was lost in Na(+)/K(+)-ATPase-knockdown cells. Furthermore, we demonstrated that CTS-induced increases in RGS2 levels were functional and reduced receptor-stimulated, G(q)-dependent, extracellular signal-regulated kinase phosphorylation. Finally, we showed that in vivo treatment with digoxin led to increased RGS2 protein levels in heart and kidney. CTS-induced increases in RGS2 protein levels and function might modify several deleterious mechanisms in hypertension and heart failure. This novel CTS mechanism might contribute to the beneficial actions of low-dose digoxin treatment in heart failure. Our results support the concept of small-molecule modulation of RGS2 protein levels as a new strategy for cardiovascular therapy.

Alteration in RGS2 expression level is associated with changes in haloperidol induced extrapyramidal features in a mutant mouse model

Antipsychotic induced Parkinsonism (AIP) is a common adverse effect of antipsychotic drug treatment among schizophrenia patients. Two previous studies showed association of the rs4606 SNP in the 3' untranslated region of the regulator of G protein signaling 2 gene (RGS2) with susceptibility to AIP. Since rs4606 reportedly influences expression of RGS2, we applied a translational approach and studied the effect of chronic (24 days) exposure to haloperidol on AIP-like features in mice carrying a mutation that causes lower Rgs2 gene expression. Haloperidol and vehicle treated male mice heterozygous (HET) or homozygous (HOM) for the mutation, or wild type (WT), were evaluated for open field locomotion, catalepsy duration, pole test performance and rota-rod latency to fall. We showed that in haloperidol treated mice lower Rgs2 expression is associated with better performance on the open field, catalepsy and rota-rod tests but not the pole test. Results were most consistent for the 0.2 mg/kg/d haloperidol dose. These observations support the possible involvement of RGS2 in mechanisms underlying susceptibility to AIP.

Regulators of G-protein signaling in the heart and their potential as therapeutic targets

Signal transduction through G-protein-coupled receptors (GPCRs) is central for the regulation of virtually all cellular functions and has been widely implicated in human disease. Regulators of G-protein signaling (RGS proteins) belong to a diverse protein family that was originally discovered for their ability to accelerate signal termination in response to GPCR stimulation, thereby reducing the amplitude and duration of GPCR effects. All RGS proteins share a common RGS domain that interacts with G protein α subunits and mediates their biological regulation of GPCR signaling. However, RGS proteins differ widely in size and the organization of their sequences flanking the RGS domain, which contain several additional functional domains that facilitate protein-protein (or protein-lipid) interactions. RGS proteins are subject to posttranslational modifications, and, in addition, their expression, activity, and subcellular localization can be dynamically regulated. Thus, there exists a wide array of mechanisms that facilitate their proper function as modulators and integrators of G-protein signaling. Several RGS proteins have been implicated in the cardiac remodeling response and heart rate regulation, and changes in RGS protein expression and/or function are believed to participate in the pathophysiology of cardiac hypertrophy, failure and arrhythmias as well as hypertension. This review is based on recent advances in our understanding of the expression pattern, regulation, and functional role of canonical RGS proteins, with a special focus on the healthy heart and the diseased heart. In addition, we discuss their potential and promise as therapeutic targets as well as strategies to modulate their expression and function.

Resistance to age-related, normal body weight gain in RGS2 deficient mice

RGS2 (regulator of G protein signaling 2) is known to limit signals mediated via Gq- and Gs-coupled GPCRs (G protein coupled receptors), and it has been implicated in the differentiation of several cells types. The physiology of RGS2 knockout mice (rgs2(-/-)) has been studied in some detail, however, a metabolic phenotype has not previously been reported. We observed that old (21-24month) rgs2(-/-) mice weigh much less than wild-type C57BL/6 controls, and exhibit greatly reduced fat deposits, decreased serum lipids, and low leptin levels. Lower weight was evident as early as four weeks and continued throughout life. Younger adult male rgs2(-/-) mice (4-8months) were found to show similar strain-related differences as the aged animals, as well improved glucose clearance and insulin sensitivity, and enhanced beta-adrenergic and glucagon signaling in isolated hepatocytes. In addition, rgs2(-/-) pre-adipocytes had reduced levels of differentiation markers (Peroxisome proliferator-activated receptor γ (PPARγ); lipoprotein lipase (Lpl); CCAAT/enhancer binding protein α (CEBPα)) and also rgs2(-/-) white adipocytes were small relative to controls, suggesting altered adipogenesis. In wild-type animals, RGS2 mRNA was decreased in brown adipose tissue after cold exposure (7 h at 4 °C) but increased in white adipose tissue in response to a high fat diet, also suggesting a role in lipid storage. No differences between strains were detected with respect to food intake, energy expenditure, GPCR-stimulated lipolysis, or adaptive thermogenesis. In conclusion this study points to RGS2 as being an important regulatory factor in controlling body weight and adipose function.

Genetic deletion of regulator of G-protein signaling 4 (RGS4) rescues a subset of fragile X related phenotypes in the FMR1 knockout mouse

Fragile X syndrome (FXS), the most common cause of inherited mental retardation, is caused by the loss of the mRNA binding protein, FMRP. Persons with FXS also display epileptic seizures, social anxiety, hyperactivity, and autistic behaviors. The metabotropic glutamate receptor theory of FXS postulates that in the absence of FMRP, enhanced signaling though G-protein coupled group I metabotropic glutamate receptors in the brain contributes to many of the abnormalities observed in the disorder. However, recent evidence suggests that alterations in cellular signaling through additional G-protein coupled receptors may also be involved in the pathogenesis of FXS, thus providing impetus for examining downstream molecules. One group of signaling molecules situated downstream of the receptors is the regulator of G-protein signaling (RGS) proteins. Notably, RGS4 is highly expressed in brain and has been shown to negatively regulate signaling through Group I mGluRs and GABA(B) receptors. To examine the potential role for RGS4 in the pathogenesis of FXS, we generated FXS/RGS4 double knockout mice. Characterization of these mice revealed that a subset of FXS related phenotypes, including increased body weight, altered synaptic protein expression, and abnormal social behaviors, were rescued in the double knockout mice. Other phenotypes, such as hyperactivity and macroorchidism, were not affected by the loss of RGS4. These findings suggest that tissue and cell-type specific differences in GPCR signaling and RGS function may contribute to the spectrum of phenotypic differences observed in FXS.

RGS-insensitive Gα subunits: probes of Gα subtype-selective signaling and physiological functions of RGS proteins

The Regulator of G protein Signaling (RGS) proteins were identified as a family in 1996 and humans have more than 30 such proteins. Their best known function is to suppress G Protein-Coupled Receptors (GPCR) signaling by increasing the rate of Gα turnoff through stimulation of GTPase activity (i.e., GTPase acceleration protein or GAP activity). The GAP activity of RGS proteins on the Gαi and Gαq family of G proteins can terminate signals initiated by both α and βγ subunits. RGS proteins also serve as scaffolds, assembling signal-regulating modules. Understanding the physiological roles of RGS proteins is of great importance, as GPCRs are major targets for drug development. The traditional method of using RGS knockout mice has provided some information about the role of RGS proteins but in many cases effects are modest, perhaps because of redundancy in RGS protein function. As an alternative approach, we have utilized a glycine-to-serine mutation in the switch 1 region of Gα subunits that prevents RGS binding. The mutation has no known effects on Gα binding to receptor, Gβγ, or effectors. Alterations in function resulting from the G>S mutation imply a role for both the specific mutated Gα subunit and its regulation by RGS protein activity. Mutant rodents expressing these G>S mutant Gα subunits have strong phenotypes and provide important information about specific physiological functions of Gαi2 and Gαo and their control by RGS. The conceptual framework behind this approach and a summary of recent results is presented in this chapter.

Regulator of G protein signaling proteins as drug targets: current state and future possibilities

Regulators of G protein signaling (RGS) proteins have emerged in the past two decades as novel drug targets in many areas of research. Their importance in regulating signaling via G protein-coupled receptors has become evident as numerous studies have been published on the structure and function of RGS proteins. A number of genetic models have also been developed, demonstrating the potential clinical importance of RGS proteins in various disease states, including central nervous system disorders, cardiovascular disease, diabetes, and several types of cancer. Apart from their classical mechanism of action as GTPase-activating proteins (GAPs), RGS proteins can also serve other noncanonical functions. This opens up a new approach to targeting RGS proteins in drug discovery as the view on the function of these proteins is constantly evolving. This chapter summarizes the latest development in RGS protein drug discovery with special emphasis on noncanonical functions and regulatory mechanisms of RGS protein expression. As more reports are being published on this group of proteins, it is becoming clear that modulation of GAP activity might not be the only way to therapeutically target RGS proteins.

Thinking outside of the "RGS box": new approaches to therapeutic targeting of regulators of G protein signaling

Regulators of G protein signaling (RGS) proteins are emerging as potentially important drug targets. The mammalian RGS protein family has more than 20 members and they share a common ∼120-residue RGS homology domain or "RGS box." RGS proteins regulate signaling via G protein-coupled receptors by accelerating GTPase activity at active α subunits of G proteins of the G(q) and G(i/o) families. Most studies searching for modulators of RGS protein function have been focused on inhibiting the GTPase accelerating protein activity. However, many RGS proteins contain additional domains that serve other functions, such as interactions with proteins or subcellular targeting. Here, we discuss a rationale for therapeutic targeting of RGS proteins by regulation of expression or allosteric modulation to permit either increases or decreases in RGS function. Several RGS proteins have reduced expression or function in pathophysiological states, so strategies to increase RGS function would be useful. Because several RGS proteins are rapidly degraded by the N-end rule pathway, finding ways to stabilize them may prove to be an effective way to enhance RGS protein function.

Non-canonical functions of RGS proteins

Regulators of G protein signalling (RGS) proteins are united into a family by the presence of the RGS domain which serves as a GTPase-activating protein (GAP) for various Galpha subunits of heterotrimeric G proteins. Through this mechanism, RGS proteins regulate signalling of numerous G protein-coupled receptors. In addition to the RGS domains, RGS proteins contain diverse regions of various lengths that regulate intracellular localization, GAP activity or receptor selectivity of RGS proteins, often through interaction with other partners. However, it is becoming increasingly appreciated that through these non-RGS regions, RGS proteins can serve non-canonical functions distinct from inactivation of Galpha subunits. This review summarizes the data implicating RGS proteins in the (i) regulation of G protein signalling by non-canonical mechanisms, (ii) regulation of non-G protein signalling, (iii) signal transduction from receptors not coupled to G proteins, (iv) activation of mitogen-activated protein kinases, and (v) non-canonical functions in the nucleus.

Ectopic expression of eIF2Bepsilon in rat skeletal muscle rescues the sepsis-induced reduction in guanine nucleotide exchange activity and protein synthesis

Eukaryotic initiation factor 2B (eIF2B) is a guanine nucleotide exchange factor (GEF) whose activity is both tightly regulated and rate-controlling with regard to global rates of protein synthesis. Skeletal muscle eIF2B activity and expression of its catalytic epsilon-subunit (eIF2Bepsilon) have been implicated as potential contributors to the altered rates of protein synthesis in a number of physiological conditions and experimental models. The objective of this study was to directly examine the effects of exogenously expressed eIF2Bepsilon in vivo on GEF activity and protein synthetic rates in rat skeletal muscle. A plasmid encoding FLAG-eIF2Bepsilon was transfected into the tibialis anterior (TA) of one leg, while the contralateral TA received a control plasmid. Ectopic expression of eIF2Bepsilon resulted in increased GEF activity in TA homogenates of healthy rats, demonstrating that the expressed protein was catalytically active. In an effort to restore a deficit in eIF2B activity, we utilized an established model of chronic sepsis in which skeletal muscle eIF2B activity is known to be impaired. Ectopic expression of eIF2Bepsilon in the TA rescued the sepsis-induced deficit in GEF activity and muscle protein synthesis. The results demonstrate that modulation of eIF2Bepsilon expression may be sufficient to correct deficits in skeletal muscle protein synthesis associated with sepsis and other muscle-wasting conditions.

RGS2 inhibits beta-adrenergic receptor-induced cardiomyocyte hypertrophy

The chronic stimulation of certain G protein-coupled receptors promotes cardiomyocyte hypertrophy and thus plays a pivotal role in the development of human heart failure. The beta-adrenergic receptors (beta-AR) are unique among these in that they signal via Gs, whereas others, such as the alpha1-adrenergic (alpha1-AR) and endothelin-1 (ET-1) receptors, predominantly act through Gq. In this study, we investigated the potential role of regulator of G protein signalling 2 (RGS2) in modulating the hypertrophic effects of the beta-AR agonist isoproterenol (ISO) in rat neonatal ventricular cardiomyocytes. We found that ISO-induced hypertrophy in rat neonatal ventricular myocytes was accompanied by the selective upregulation of RGS2 mRNA, with little or no change in RGS1, RGS3, RGS4 or RGS5. The adenylyl cyclase activator forskolin had a similar effect suggesting that it was mediated through cAMP production. To study the role of RGS2 upregulation in beta-AR-dependent hypertrophy, cardiomyocytes were infected with adenovirus encoding RGS2 and assayed for cell growth, markers of hypertrophy, and beta-AR signalling. ISO-induced increases in cell surface area were virtually eliminated by the overexpression of RGS2, as were increases in alpha-skeletal actin and atrial natriuretic peptide. RGS2 overexpression also significantly attenuated ISO-induced extracellular signal-regulated kinases 1 and 2 (ERK1/2) and Akt activation, which may account for, or contribute to, its observed antihypertrophic effects. In contrast, RGS2 overexpression significantly activated JNK MAP kinase, while decreasing the potency but not the maximal effect of ISO on cAMP accumulation. In conclusion, the present results suggest that RGS2 negatively regulates hypertrophy induced by beta-AR activation and thus may play a protective role in cardiac hypertrophy.