Pharmacological Inhibition of Myocardin-related Transcription Factor Pathway Blocks Lung Metastases of RhoC-Overexpressing Melanoma

Melanoma is the most dangerous form of skin cancer with the majority of deaths arising from metastatic disease. Evidence implicates Rho-activated gene transcription in melanoma metastasis mediated by the nuclear localization of the transcriptional coactivator, myocardin-related transcription factor (MRTF). Here, we highlight a role for Rho and MRTF signaling and its reversal by pharmacologic inhibition using in vitro and in vivo models of human melanoma growth and metastasis. Using two cellular models of melanoma, we clearly show that one cell type, SK-Mel-147, is highly metastatic, has high RhoC expression, and MRTF nuclear localization and activity. Conversely, SK-Mel-19 melanoma cells have low RhoC expression, and decreased levels of MRTF-regulated genes. To probe the dependence of melanoma aggressiveness to MRTF transcription, we use a previously developed small-molecule inhibitor, CCG-203971, which at low micromolar concentrations blocks nuclear localization and activity of MRTF-A. In SK-Mel-147 cells, CCG-203971 inhibits cellular migration and invasion, and decreases MRTF target gene expression. In addition, CCG-203971-mediated inhibition of the Rho/MRTF pathway significantly reduces cell growth and clonogenicity and causes G₁ cell-cycle arrest. In an experimental model of melanoma lung metastasis, the RhoC-overexpressing melanoma cells (SK-Mel-147) exhibited pronounced lung colonization compared with the low RhoC-expressing SK-Mel-19. Furthermore, pharmacologic inhibition of the MRTF pathway reduced both the number and size of lung metastasis resulting in a marked reduction of total lung tumor burden. These data link Rho and MRTF-mediated signaling with aggressive phenotypes and support targeting the MRTF transcriptional pathway as a novel approach to melanoma therapeutics. Mol Cancer Ther; 16(1); 193-204. ©2016 AACR.

Abstract P182: The Novel Perivascular Adipose Tissue Adipokine, Chemerin, Signals Through G i - and Calcium-Dependent Mechanisms

Chemerin is an adipokine associated with inflammation, increased blood pressure, and may be a link between the pathologies of obesity and hypertension. We tested the hypothesis that chemerin-induced contraction of the vasculature occurs via the chemerin receptor and calcium flux in smooth muscle cells. Known mediators of the amplified arterial responsiveness seen in hypertension (L-type Ca 2+ channels, Src, and Rho kinase) were interrogated by isometric contraction of rat aortic rings in parallel with calcium kinetics of rat aortic smooth muscle cells. Western blots were also used to observe phosphorylation of Erk/MAPK. Chemerin-9 (nonapeptide of the chemerin S 157 isoform) caused a concentration-dependent contraction of isolated aorta (EC 50 100 nM) and elicited a concentration-dependent intracellular calcium response (EC 50 10 nM). Both calcium influx and isometric contraction, respectively, were reduced (units of “% of vehicle response”) by Pertussis toxin (G i inhibitor; 0±3% and 23±9%), verapamil (L-type Ca 2+ channel inhibitor; 38±20% and 23±4%), PP1 (Src inhibitor; 43±23% and 15±4%), and Y27632 (Rho Kinase inhibitor; 58±23% and 22±4%) but U73122 (PLC inhibitor) had little to no effect (71±31% and 71±12%). PD098059 (Erk/MAPK inhibitor) did not inhibit chemerin-9 induced contraction (117±19%) and phosphorylation did not change after chemerin-9 stimulation [1.12±0.14 (44 kDa) and 1.11±0.29 (42 kDa) fold-increase with chemerin-9 contraction compared to vehicle, p>0.05]. The chemerin receptor-selective antagonist CCX832 inhibited chemerin-9-induced calcium flux and aortic contraction and calcium flux (0.1±10.3% and 10±7%). These data support a chemerin-induced contractile mechanism in vascular smooth muscle that functions through the G i -linked chemerin receptor to activate L-type Ca 2+ channels, Src, and Rho kinase. There is mounting evidence linking chemerin to hypertension and this mechanism brings us one step closer to targeting chemerin as a unique form of therapy.

Abstract 68: Neuroblastoma drug response profiles are associated with gene expression profiles

Background: High risk neuroblastoma patients have a poor response to initial chemotherapy, with 20% unable to achieve a complete response. Genomic studies have identified 6 targeted agents whose pathways have been identified in some high risk neuroblastoma tumors. The low frequency of established driver mutations, along with the variety of molecular mechanisms for cancer pathway dysregulation, suggest development of gene expression-based predictive biomarkers in addition to sequence-based biomarkers. Our platform for developing gene expression-based biomarkers includes primary neuroblastoma cell lines, high throughput quantification of cell survival, exome sequencing, and genome-wide expression profiling.

Methods: Cell survival was quantified for each of thirty primary neuroblastoma cell lines in the presence of each of six drugs (bortezomib, crizotinib, dasatinib, lapatinib, sorafenib, and vorinostat) using high throughput cell survival measurements. Titration curves were based on treatment with vehicle as well as sixteen concentrations of each drug (replicated within and between plates). Summary descriptions of the response of each cell line to each drug were obtained using the four parameter logistic model as well as the area under the curve. Exome sequences were obtained using Illumina HiSeq technology. Genome-wide expression profiles were obtained using Affymetrix GeneChip technology. Inference was based on parametric univariate and non-parametric multivariate tests.

Results: For three of the drugs (bortezomib, dasatinib, and vorinostat) the variation in response between cell lines was statistically significant (p < 0.05; F ≥3) and assay results were reproducible in the context of the differences between cell lines (for the other three drugs the estimated standard deviation of log10 EC50 ≤ 0.2). Genome-wide expression profiles were associated with variation in drug response between cell lines, for example, high PDGFRB expression indicates sensitivity to dasatinib (p < 0.001).

Conclusion: High throughput cell survival measurements provided for identification of predictive biomarkers of neuroblastoma drug sensitivity. These biomarkers are currently being studied in the PEDS-PLAN (Pediatric Precision Laboratory Advanced Neuroblastoma Therapy) clinical trial.

Citation Format: Jeffrey P. Bond, Elizabeth VanSickle, Thomas S. Dexhemier, Richard R. Neubig, Giselle L. Sholler. Neuroblastoma drug response profiles are associated with gene expression profiles. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 68.

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.

M4 Muscarinic Receptor Signaling Ameliorates Striatal Plasticity Deficits in Models of L-DOPA-Induced Dyskinesia

A balanced interaction between dopaminergic and cholinergic signaling in the striatum is critical to goal-directed behavior. But how this interaction modulates corticostriatal synaptic plasticity underlying learned actions remains unclear--particularly in direct-pathway spiny projection neurons (dSPNs). Our studies show that in dSPNs, endogenous cholinergic signaling through M4 muscarinic receptors (M4Rs) promoted long-term depression of corticostriatal glutamatergic synapses, by suppressing regulator of G protein signaling type 4 (RGS4) activity, and blocked D1 dopamine receptor dependent long-term potentiation (LTP). Furthermore, in a mouse model of L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID) in Parkinson's disease (PD), boosting M4R signaling with positive allosteric modulator (PAM) blocked aberrant LTP in dSPNs, enabled LTP reversal, and attenuated dyskinetic behaviors. An M4R PAM also was effective in a primate LID model. Taken together, these studies identify an important signaling pathway controlling striatal synaptic plasticity and point to a novel pharmacological strategy for alleviating LID in PD patients.

RGS Proteins and Gαi2 Modulate Sleep, Wakefulness, and Disruption of Sleep/ Wake States after Isoflurane and Sevoflurane Anesthesia

STUDY OBJECTIVES

This study tested the hypothesis that Regulators of G protein Signaling (RGS) proteins contribute to the regulation of wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep, and to sleep disruption caused by volatile anesthetics.

METHODS

The three groups used in this study included wild-type (WT; n = 7) mice and knock-in mice that were heterozygous (+/GS; n = 7) or homozygous (GS/GS; n = 7) for an RGS-insensitive allele that causes prolonged Gαi2 signaling. Mice were implanted with electrodes for recording sleep and conditioned for 1 week or more to sleep in the laboratory. Using within and between groups designs, 24-h recordings of wakefulness, NREM sleep, and REM sleep were compared across three interventions: (1) baseline (control) and after 3 h of being anesthetized with (2) isoflurane or (3) sevoflurane.

RESULTS

Baseline recordings during the light phase revealed that relative to WT mice, homozygous RGS-insensitive (GS/GS) mice exhibit significantly increased wakefulness and decreased NREM and REM sleep. During the dark phase, these state-specific differences remained significant but reversed direction of change. After cessation of isoflurane and sevoflurane anesthesia there was a long-lasting and significant disruption of sleep and wakefulness. The durations of average episodes of wakefulness, NREM sleep, and REM sleep were significantly altered as a function of genotype and isoflurane and sevoflurane anesthesia.

CONCLUSIONS

RGS proteins and Gαi2 play a significant role in regulating states of wakefulness, NREM sleep, and REM sleep. Genotype-specific differences demonstrate that RGS proteins modulate sleep disruption caused by isoflurane and sevoflurane anesthesia. The results also support the conclusion that isoflurane and sevoflurane anesthesia do not satisfy the homeostatic drive for sleep.

Band-pass processing in a GPCR signaling pathway selects for NFAT transcription factor activation

Many biological processes are rhythmic and proper timing is increasingly appreciated as being critical for development and maintenance of physiological functions. To understand how temporal modulation of an input signal influences downstream responses, we employ microfluidic pulsatile stimulation of a G-protein coupled receptor, the muscarinic M3 receptor, in single cells with simultaneous real-time imaging of both intracellular calcium and NFAT nuclear localization. Interestingly, we find that reduced stimulation with pulses of ligand can give more efficient transcription factor activation, if stimuli are timed appropriately. Our experiments and computational analyses show that M3 receptor-induced calcium oscillations form a low pass filter while calcium-induced NFAT translocation forms a high pass filter. The combination acts as a band-pass filter optimized for intermediate frequencies of stimulation. We demonstrate that receptor desensitization and NFAT translocation rates determine critical features of the band-pass filter and that the band-pass may be shifted for different receptors or NFAT dynamics. As an example, we show that the two NFAT isoforms (NFAT4 and NFAT1) have shifted band-pass windows for the same receptor. While we focus specifically on the M3 muscarinic receptor and NFAT translocation, band-pass processing is expected to be a general theme that applies to multiple signaling pathways.

Selectivity and anti-Parkinson's potential of thiadiazolidinone RGS4 inhibitors

Many current therapies target G protein coupled receptors (GPCR), transporters, or ion channels. In addition to directly targeting these proteins, disrupting the protein-protein interactions that localize or regulate their function could enhance selectivity and provide unique pharmacologic actions. Regulators of G protein signaling (RGS) proteins, especially RGS4, play significant roles in epilepsy and Parkinson's disease. Thiadiazolidinone (TDZD) inhibitors of RGS4 are nanomolar potency blockers of the biochemical actions of RGS4 in vitro. Here, we demonstrate the substantial selectivity (8- to >5000-fold) of CCG-203769 for RGS4 over other RGS proteins. It is also 300-fold selective for RGS4 over GSK-3β, another target of this class of chemical scaffolds. It does not inhibit the cysteine protease papain at 100 μM. CCG-203769 enhances Gαq-dependent cellular Ca(2+) signaling in an RGS4-dependent manner. TDZD inhibitors also enhance Gαi-dependent δ-OR inhibition of cAMP production in SH-SY-5Y cells, which express endogenous receptors and RGS4. Importantly, CCG-203769 potentiates the known RGS4 mechanism of Gαi-dependent muscarinic bradycardia in vivo. Furthermore, it reverses raclopride-induced akinesia and bradykinesia in mice, a model of some aspects of the movement disorder in Parkinson's disease. A broad assessment of compound effects revealed minimal off-target effects at concentrations necessary for cellular RGS4 inhibition. These results expand our understanding of the mechanism and specificity of TDZD RGS inhibitors and support the potential for therapeutic targeting of RGS proteins in Parkinson's disease and other neural disorders.

RGS-Insensitive G Proteins as In Vivo Probes of RGS Function

Guanine nucleotide-binding proteins of the inhibitory (Gi/o) class play critical physiological roles and the receptors that activate them are important therapeutic targets (e.g., mu opioid, serotonin 5HT1a, etc.). Gi/o proteins are negatively regulated by regulator of G protein signaling (RGS) proteins. The redundant actions of the 20 different RGS family members have made it difficult to establish their overall physiological role. A unique G protein mutation (G184S in Gαi/o) prevents RGS binding to the Gα subunit and blocks all RGS action at that particular Gα subunit. The robust phenotypes of mice expressing these RGS-insensitive (RGSi) mutant G proteins illustrate the profound action of RGS proteins in cardiovascular, metabolic, and central nervous system functions. Specifically, the enhanced Gαi2 signaling through the RGSi Gαi2(G184S) mutant knock-in mice shows protection against cardiac ischemia/reperfusion injury and potentiation of serotonin-mediated antidepressant actions. In contrast, the RGSi Gαo mutant knock-in produces enhanced mu-opioid receptor-mediated analgesia but also a seizure phenotype. These genetic models provide novel insights into potential therapeutic strategies related to RGS protein inhibitors and/or G protein subtype-biased agonists at particular GPCRs.

FBXO44-Mediated Degradation of RGS2 Protein Uniquely Depends on a Cullin 4B/DDB1 Complex

The ubiquitin-proteasome system for protein degradation plays a major role in regulating cell function and many signaling proteins are tightly controlled by this mechanism. Among these, Regulator of G Protein Signaling 2 (RGS2) is a target for rapid proteasomal degradation, however, the specific enzymes involved are not known. Using a genomic siRNA screening approach, we identified a novel E3 ligase complex containing cullin 4B (CUL4B), DNA damage binding protein 1 (DDB1) and F-box protein 44 (FBXO44) that mediates RGS2 protein degradation. While the more typical F-box partners CUL1 and Skp1 can bind FBXO44, that E3 ligase complex does not bind RGS2 and is not involved in RGS2 degradation. These observations define an unexpected DDB1/CUL4B-containing FBXO44 E3 ligase complex. Pharmacological targeting of this mechanism provides a novel therapeutic approach to hypertension, anxiety, and other diseases associated with RGS2 dysregulation.

Inhibition of myocardin-related transcription factor/serum response factor signaling decreases lung fibrosis and promotes mesenchymal cell apoptosis

Myofibroblasts are crucial to the pathogenesis of tissue fibrosis. Their formation of stress fibers results in the release of myocardin-related transcription factor (MRTF), a transcriptional coactivator of serum response factor (SRF). MRTF-A (Mkl1)-deficient mice are protected from lung fibrosis. We hypothesized that the SRF/MRTF pathway inhibitor CCG-203971 would modulate myofibroblast function in vitro and limit lung fibrosis in vivo. Normal and idiopathic pulmonary fibrosis lung fibroblasts were treated with/without CCG-203971 (N-[4-chlorophenyl]-1-[3-(2-furanyl)benzoyl]-3-piperidine carboxamide) and/or Fas-activating antibody in the presence/absence of transforming growth factor (TGF)-β1, and apoptosis was assessed. In vivo studies examined the effect of therapeutically administered CCG-203971 on lung fibrosis in two distinct murine models of fibrosis induced by bleomycin or targeted type II alveolar epithelial injury. In vitro, CCG-203971 prevented nuclear localization of MRTF-A; increased the apoptotic susceptibility of normal and idiopathic pulmonary fibrosis fibroblasts; blocked TGF-β1-induced myofibroblast differentiation; and inhibited TGF-β1-induced expression of fibronectin, X-linked inhibitor of apoptosis, and plasminogen activator inhibitor-1. TGF-β1 did not protect fibroblasts or myofibroblasts from apoptosis in the presence of CCG-203971. In vivo, CCG-203971 significantly reduced lung collagen content in both murine models while decreasing alveolar plasminogen activator inhibitor-1 and promoting myofibroblast apoptosis. These data support a central role of the SRF/MRTF pathway in the pathobiology of lung fibrosis and suggest that its inhibition can help resolve lung fibrosis by promoting fibroblast apoptosis.

The Concise Guide to PHARMACOLOGY 2013/14: overview

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties from the IUPHAR database. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. This compilation of the major pharmacological targets is divided into seven areas of focus: G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors & Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and GRAC and provides a permanent, citable, point-in-time record that will survive database updates.

International Union of Basic and Clinical Pharmacology. XC. multisite pharmacology: recommendations for the nomenclature of receptor allosterism and allosteric ligands

Allosteric interactions play vital roles in metabolic processes and signal transduction and, more recently, have become the focus of numerous pharmacological studies because of the potential for discovering more target-selective chemical probes and therapeutic agents. In addition to classic early studies on enzymes, there are now examples of small molecule allosteric modulators for all superfamilies of receptors encoded by the genome, including ligand- and voltage-gated ion channels, G protein-coupled receptors, nuclear hormone receptors, and receptor tyrosine kinases. As a consequence, a vast array of pharmacologic behaviors has been ascribed to allosteric ligands that can vary in a target-, ligand-, and cell-/tissue-dependent manner. The current article presents an overview of allostery as applied to receptor families and approaches for detecting and validating allosteric interactions and gives recommendations for the nomenclature of allosteric ligands and their properties.

Induction of the matricellular protein CCN1 through RhoA and MRTF-A contributes to ischemic cardioprotection

Activation of RhoA, a low molecular-weight G-protein, plays an important role in protecting the heart against ischemic stress. Studies using non-cardiac cells demonstrate that the expression and subsequent secretion of the matricellular protein CCN1 is induced by GPCR agonists that activate RhoA. In this study we determined whether and how CCN1 is induced by GPCR agonists in cardiomyocytes and examined the role of CCN1 in ischemic cardioprotection in cardiomyocytes and the isolated perfused heart. In neonatal rat ventricular myocytes (NRVMs), sphingosine 1-phosphate (S1P), lysophosphatidic acid (LPA) and endothelin-1 induced robust increases in CCN1 expression while phenylephrine, isoproterenol and carbachol had little or no effect. The ability of agonists to activate the small G-protein RhoA correlated with their ability to induce CCN1. CCN1 induction by S1P was blocked when RhoA function was inhibited with C3 exoenzyme or a pharmacological RhoA inhibitor. Conversely overexpression of RhoA was sufficient to induce CCN1 expression. To delineate the signals downstream of RhoA we tested the role of MRTF-A (MKL1), a co-activator of SRF, in S1P-mediated CCN1 expression. S1P increased the nuclear accumulation of MRTF-A and this was inhibited by the functional inactivation of RhoA. In addition, pharmacological inhibitors of MRTF-A or knockdown of MRTF-A significantly diminished S1P-mediated CCN1 expression, indicating a requirement for RhoA/MRTF-A signaling. We also present data indicating that CCN1 is secreted following agonist treatment and RhoA activation, and binds to cells where it can serve an autocrine function. To determine the functional significance of CCN1 expression and signaling, simulated ischemia/reperfusion (sI/R)-induced apoptosis was assessed in NRVMs. The ability of S1P to protect against sI/R was significantly reduced by the inhibition of RhoA, ROCK or MRTF-A or by CCN1 knockdown. We also demonstrate that ischemia/reperfusion induces CCN1 expression in the isolated perfused heart and that this functions as a cardioprotective mechanism, evidenced by the significant increase in infarct development in response to I/R in the cardiac specific CCN1 KO relative to control mice. Our findings implicate CCN1 as a mediator of cardioprotection induced by GPCR agonists that activate RhoA/MRTF-A signaling.

Identification of protein kinase C activation as a novel mechanism for RGS2 protein upregulation through phenotypic screening of natural product extracts

Biochemical high-throughput screening is widely used in drug discovery, using a variety of small molecule libraries. However, broader screening strategies may be more beneficial to identify novel biologic mechanisms. In the current study we used a β-galactosidase complementation method to screen a selection of microbial-derived pre-fractionated natural product extracts for those that increase regulator of G protein signaling 2 (RGS2) protein levels. RGS2 is a member of a large family of proteins that all regulate signaling through G protein-coupled receptors (GPCRs) by accelerating GTPase activity on active Gα as well as through other mechanisms. RGS2(-/-) mice are hypertensive, show increased anxiety, and are prone to heart failure. RGS2 has a very short protein half-life due to rapid proteasomal degradation, and we propose that enhancement of RGS2 protein levels could be a beneficial therapeutic strategy. Bioassay-guided fractionation of one of the hit strains yielded a pure compound, Indolactam V, a known protein kinase C (PKC) activator, which selectively increased RGS2 protein levels in a time- and concentration-dependent manner. Similar results were obtained with phorbol 12-myristate 13-acetate as well as activation of the Gq-coupled muscarinic M3 receptor. The effect on RGS2 protein levels was blocked by the nonselective PKC inhibitor Gö6983 (3-[1-[3-(dimethylamino)propyl]-5-methoxy-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione), the PKCβ-selective inhibitor Ruboxastaurin, as well as small interfering RNA-mediated knockdown of PKCβ. Indolactam V-mediated increases in RGS2 protein levels also had functional effects on GPCR signaling. This study provides important proof-of-concept for our screening strategy and could define a negative feedback mechanism in Gq/Phospholipase C signaling through RGS2 protein upregulation.

Conditional disruption of interactions between Gαi2 and regulator of G protein signaling (RGS) proteins protects the heart from ischemic injury

BACKGROUND

Regulator of G protein signaling (RGS) proteins suppress G protein coupled receptor signaling by catalyzing the hydrolysis of Gα-bound guanine nucleotide triphosphate. Transgenic mice in which RGS-mediated regulation of Gαi2 is lost (RGS insensitive Gαi2G184S) exhibit beneficial (protection against ischemic injury) and detrimental (enhanced fibrosis) cardiac phenotypes. This mouse model has revealed the physiological significance of RGS/Gαi2 interactions. Previous studies of the Gαi2G184S mutation used mice that express this mutant protein throughout their lives. Thus, it is unclear whether these phenotypes result from chronic or acute Gαi2G184S expression. We addressed this issue by developing mice that conditionally express Gαi2G184S.

METHODS

Mice that conditionally express RGS insensitive Gαi2G184S were generated using a floxed minigene strategy. Conditional expression of Gαi2G184S was characterized by reverse transcription polymerase chain reaction and by enhancement of agonist-induced inhibition of cAMP production in isolated cardiac fibroblasts. The impact of conditional RGS insensitive Gαi2G184S expression on ischemic injury was assessed by measuring contractile recovery and infarct sizes in isolated hearts subjected to 30 min ischemia and 2 hours reperfusion.

RESULTS

We demonstrate tamoxifen-dependent expression of Gαi2G184S, enhanced inhibition of cAMP production, and cardioprotection from ischemic injury in hearts conditionally expressing Gαi2G184S. Thus the cardioprotective phenotype previously reported in mice expressing Gαi2G184S does not require embryonic or chronic Gαi2G184S expression. Rather, cardioprotection occurs following acute (days rather than months) expression of Gαi2G184S.

CONCLUSIONS

These data suggest that RGS proteins might provide new therapeutic targets to protect the heart from ischemic injury. We anticipate that this model will be valuable for understanding the time course (chronic versus acute) and mechanisms of other phenotypic changes that occur following disruption of interactions between Gαi2 and RGS proteins.

Gain-of-function mutation in Gnao1: a murine model of epileptiform encephalopathy (EIEE17)?

G protein-coupled receptors strongly modulate neuronal excitability but there has been little evidence for G protein mechanisms in genetic epilepsies. Recently, four patients with epileptic encephalopathy (EIEE17) were found to have mutations in GNAO1, the most abundant G protein in brain, but the mechanism of this effect is not known. The GNAO1 gene product, Gαo, negatively regulates neurotransmitter release. Here, we report a dominant murine model of Gnao1-related seizures and sudden death. We introduced a genomic gain-of-function knock-in mutation (Gnao1 (+/G184S)) that prevents Go turnoff by Regulators of G protein signaling proteins. This results in rare seizures, strain-dependent death between 15 and 40 weeks of age, and a markedly increased frequency of interictal epileptiform discharges. Mutants on a C57BL/6J background also have faster sensitization to pentylenetetrazol (PTZ) kindling. Both premature lethality and PTZ kindling effects are suppressed in the 129SvJ mouse strain. We have mapped a 129S-derived modifier locus on Chromosome 17 (within the region 41-70 MB) as a Modifer of G protein Seizures (Mogs1). Our mouse model suggests a novel gain-of-function mechanism for the newly defined subset of epileptic encephalopathy (EIEE17). Furthermore, it reveals a new epilepsy susceptibility modifier Mogs1 with implications for the complex genetics of human epilepsy as well as sudden death in epilepsy.

Cellular mechanisms of tissue fibrosis. 8. Current and future drug targets in fibrosis: focus on Rho GTPase-regulated gene transcription

Tissue fibrosis occurs with excessive extracellular matrix deposition from myofibroblasts, resulting in tissue scarring and inflammation. It is driven by multiple mediators, such as the G protein-coupled receptor ligands lysophosphatidic acid and endothelin, as well as signaling by transforming growth factor-β, connective tissue growth factor, and integrins. Fibrosis contributes to 45% of deaths in the developed world. As current therapeutic options for tissue fibrosis are limited and organ transplantation is the only effective treatment for end-stage disease, there is an imminent need for efficacious antifibrotic therapies. This review discusses the various molecular pathways involved in fibrosis. It highlights the Rho GTPase signaling pathway and its downstream gene transcription output through myocardin-related transcription factor and serum response factor as a convergence point for targeting this complex set of diseases.

Targeting the myofibroblast genetic switch: inhibitors of myocardin-related transcription factor/serum response factor-regulated gene transcription prevent fibrosis in a murine model of skin injury

Systemic sclerosis (SSc), or scleroderma, similar to many fibrotic disorders, lacks effective therapies. Current trials focus on anti-inflammatory drugs or targeted approaches aimed at one of the many receptor mechanisms initiating fibrosis. In light of evidence that a myocardin-related transcription factor (MRTF)-and serum response factor (SRF)-regulated gene transcriptional program induced by Rho GTPases is essential for myofibroblast activation, we explored the hypothesis that inhibitors of this pathway may represent novel antifibrotics. MRTF/SRF-regulated genes show spontaneously increased expression in primary dermal fibroblasts from patients with diffuse cutaneous SSc. A novel small-molecule inhibitor of MRTF/SRF-regulated transcription (CCG-203971) inhibits expression of connective tissue growth factor (CTGF), α-smooth muscle actin (α-SMA), and collagen 1 (COL1A2) in both SSc fibroblasts and in lysophosphatidic acid (LPA)-and transforming growth factor β (TGFβ)-stimulated fibroblasts. In vivo treatment with CCG-203971 also prevented bleomycin-induced skin thickening and collagen deposition. Thus, targeting the MRTF/SRF gene transcription pathway could provide an efficacious new approach to therapy for SSc and other fibrotic disorders.

Redox modification of nuclear actin by MICAL-2 regulates SRF signaling

The serum response factor (SRF) binds to coactivators, such as myocardin-related transcription factor-A (MRTF-A), and mediates gene transcription elicited by diverse signaling pathways. SRF/MRTF-A-dependent gene transcription is activated when nuclear MRTF-A levels increase, enabling the formation of transcriptionally active SRF/MRTF-A complexes. The level of nuclear MRTF-A is regulated by nuclear G-actin, which binds to MRTF-A and promotes its nuclear export. However, pathways that regulate nuclear actin levels are poorly understood. Here, we show that MICAL-2, an atypical actin-regulatory protein, mediates SRF/MRTF-A-dependent gene transcription elicited by nerve growth factor and serum. MICAL-2 induces redox-dependent depolymerization of nuclear actin, which decreases nuclear G-actin and increases MRTF-A in the nucleus. Furthermore, we show that MICAL-2 is a target of CCG-1423, a small molecule inhibitor of SRF/MRTF-A-dependent transcription that exhibits efficacy in various preclinical disease models. These data identify redox modification of nuclear actin as a regulatory switch that mediates SRF/MRTF-A-dependent gene transcription.

Conformational dynamics of a regulator of G-protein signaling protein reveals a mechanism of allosteric inhibition by a small molecule

Regulators of G protein signaling (RGS) proteins are key players in regulating signaling via G protein-coupled receptors. RGS proteins directly bind to the Gα-subunits of activated heterotrimeric G-proteins, and accelerate the rate of GTP hydrolysis, thereby rapidly deactivating G-proteins. Using atomistic simulations and NMR spectroscopy, we have studied in molecular detail the mechanism of action of CCG-50014, a potent small molecule inhibitor of RGS4 that covalently binds to cysteine residues on RGS4. We apply temperature-accelerated molecular dynamics (TAMD) to carry out enhanced conformational sampling of apo RGS4 structures, and consistently find that the α5-α6 helix pair of RGS4 can spontaneously span open-like conformations, allowing binding of CCG-50014 to the buried side-chain of Cys95. Both NMR experiments and MD simulations reveal chemical shift perturbations in residues in the vicinity of inhibitor binding site as well as in the RGS4-Gα binding interface. Consistent with a loss of G-protein binding, GAP activity, and allosteric mechanism of action of CCG-50014, our simulations of the RGS4-Gα complex in the presence of inhibitor suggest a relatively unstable protein-protein interaction. These results have potential implications for understanding how the conformational dynamics among RGS proteins may play a key role in the sensitivity of inhibitors.

Regulation of protease-activated receptor 1 signaling by the adaptor protein complex 2 and R4 subfamily of regulator of G protein signaling proteins

The G protein-coupled protease-activated receptor 1 (PAR1) is irreversibly proteolytically activated by thrombin. Hence, the precise regulation of PAR1 signaling is important for proper cellular responses. In addition to desensitization, internalization and lysosomal sorting of activated PAR1 are critical for the termination of signaling. Unlike most G protein-coupled receptors, PAR1 internalization is mediated by the clathrin adaptor protein complex 2 (AP-2) and epsin-1, rather than β-arrestins. However, the function of AP-2 and epsin-1 in the regulation of PAR1 signaling is not known. Here, we report that AP-2, and not epsin-1, regulates activated PAR1-stimulated phosphoinositide hydrolysis via two different mechanisms that involve, in part, a subset of R4 subfamily of "regulator of G protein signaling" (RGS) proteins. A significantly greater increase in activated PAR1 signaling was observed in cells depleted of AP-2 using siRNA or in cells expressing a PAR1 (420)AKKAA(424) mutant with defective AP-2 binding. This effect was attributed to AP-2 modulation of PAR1 surface expression and efficiency of G protein coupling. We further found that ectopic expression of R4 subfamily members RGS2, RGS3, RGS4, and RGS5 reduced activated PAR1 wild-type signaling, whereas signaling by the PAR1 AKKAA mutant was minimally affected. Intriguingly, siRNA-mediated depletion analysis revealed a function for RGS5 in the regulation of signaling by the PAR1 wild type but not the AKKAA mutant. Moreover, activation of the PAR1 wild type, and not the AKKAA mutant, induced Gαq association with RGS3 via an AP-2-dependent mechanism. Thus, AP-2 regulates activated PAR1 signaling by altering receptor surface expression and through recruitment of RGS proteins.

Reversible inhibitors of regulators of G-protein signaling identified in a high-throughput cell-based calcium signaling assay

Regulator of G-protein signaling (RGS) proteins potently suppress G-protein coupled receptor (GPCR) signal transduction by accelerating GTP hydrolysis on activated heterotrimeric G-protein α subunits. RGS4 is enriched in the CNS and is proposed as a therapeutic target for treatment of neuropathological states including epilepsy and Parkinson's disease. Therefore, identification of novel RGS4 inhibitors is of interest. An HEK293-FlpIn cell-line stably expressing M3-muscarinic receptor with doxycycline-regulated RGS4 expression was employed to identify compounds that inhibit RGS4-mediated suppression of M3-muscarinic receptor signaling. Over 300,000 compounds were screened for an ability to enhance Gαq-mediated calcium signaling in the presence of RGS4. Compounds that modulated the calcium response in a counter-screen in the absence of RGS4 were not pursued. Of the 1365 RGS4-dependent primary screen hits, thirteen compounds directly target the RGS-G-protein interaction in purified systems. All thirteen compounds lose activity against an RGS4 mutant lacking cysteines, indicating that covalent modification of free thiol groups on RGS4 is a common mechanism. Four compounds produce >85% inhibition of RGS4-G-protein binding at 100μM, yet are >50% reversible within a ten-minute time frame. The four reversible compounds significantly alter the thermal melting temperature of RGS4, but not G-protein, indicating that inhibition is occurring through interaction with the RGS protein. The HEK cell-line employed for this study provides a powerful tool for efficiently identifying RGS-specific modulators within the context of a GPCR signaling pathway. As a result, several new reversible, cell-active RGS4 inhibitors have been identified for use in future biological studies.