Yeast-Based Screening for Inhibitors of RGS Proteins

Developmental Approaches to Chronic Pain: A Narrative Review

Chronic pain, which can potentially develop from acute pain, subacute pain, or breakthrough pain, is generally defined as pain persisting for greater than three months with minimal relief. Chronic pain can be associated with a myriad of medical conditions. It is also one of the most common causes of disability, physical suffering, depression, and reduced quality of life. Treatment can vary depending on the underlying pathophysiology and can involve physical therapy, non-pharmaceutical approaches, pharmaceutical drugs, and invasive procedures. Currently available pharmaceutical agents have been effective for short-term management of chronic pain conditions, but few options address chronic pain with long-term efficacy. First-line pharmaceutical agents can potentially include over-the-counter (OTC) or prescription-strength non-steroidal anti-inflammatory drugs (NSAIDs), which have been linked to numerous side effects. If chronic pain persists, steroids are frequently used to provide longer relief. For more progressive or resistant chronic pain and/or in conjunction with invasive procedures, opioids have been utilized for acute treatment and for long-term maintenance. While these agents have proven to be effective for both acute and long-term use due to their modulation at various peripheral and central opioid receptors, they can be associated with numerous side effects and tied to the risk of addiction. As such, an unmet need exists to identify treatment modalities that provide opioid-like pain relief without opioid-induced adverse effects and the potential for addiction. This narrative review will provide an overview of the currently available treatment modalities for chronic pain and their adverse event profiles, as well as a review of therapies that are currently in development and/or preclinical trials for the management and treatment of chronic pain.

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.

Genetic study of early-onset Graves' disease in the Chinese Han population

Graves' disease (GD) is a complex autoimmune disorder in which genetic and environmental factors are both involved in the pathogenesis. Early-onset patients have a shorter exposure time to environmental factors and are, therefore, good models to help understand the genetic architecture of GD. Based on previous studies of early-onset GD, 11 single nucleotide polymorphisms (SNPs) and their related SNPs (R²  > .6), SNPs located within a ±1-Mb region of the FOXP3 gene, and 20 validated GD-risk SNPs were selected and screened for genotyping in 3735 GD and 4893 control patients to investigate whether early-onset GD is a subtype of GD with distinct susceptibility genes. Ultimately, we did not confirm the reported genetic markers of early-onset GD in our Chinese Han population but found that a GD-risk SNP located in the human leukocyte antigen class I region-rs4947296-was more strongly correlated with early-onset GD than non-early-onset GD. In addition, heterogeneity analysis of GD patients suggests that it may be more reasonable to define early-onset GD as an onset age ≤20 years.

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.

Modification and functional inhibition of regulator of G-protein signaling 4 (RGS4) by 4-hydroxy-2-nonenal

Oxidative stress has been implicated as a component of various pathologies including ischemia/reperfusion injury (IRI) and neurodegenerative diseases such as Parkinson's disease (PD) and schizophrenia. Similarly, regulator of G-protein signaling 4 (RGS4) has been implicated as an important player in each of these pathologies. RGS4, like other RGS proteins, is responsible for temporally regulating G-protein coupled receptor signaling by increasing the intrinsic GTPase activity of Gα subunit of the heterotrimeric signaling complex. In this study we evaluated whether modification by 4-hydroxy-2-nonenal (4HNE), a common lipid peroxidation product, inhibits RGS4. Using immunoprecipitation, we first determined RGS4 modification was occurring in cells at concentrations of 4HNE within reported physiological conditions. Following this determination, we evaluated modification of RGS4 by 4HNE by both Western blot and mass spectrometry (MS). Once it was established that covalent modification occurred only on cysteine containing constructs, tryptic digest followed by mass spectrometry analysis revealed modification occurs at cysteine residues 71, 148, and 183. In order to determine the effect 4HNE had on RGS4 activity, a steady-state colorimetric assay was used to analyze the GAP activity of Δ51-RGS4 as well as the cysteine null mutant. From the data, we determined that RGS4 activity can be modulated by 4HNE through modification at cysteine residues similar to previously reported small molecule inhibition of RGS4.

RGS17: an emerging therapeutic target for lung and prostate cancers

Ligands for G-protein-coupled receptors (GPCRs) represent approximately 50% of currently marketed drugs. RGS proteins modulate heterotrimeric G proteins and, thus, GPCR signaling, by accelerating the intrinsic GTPase activity of the Gα subunit. Given the prevalence of GPCR targeted therapeutics and the role RGS proteins play in G protein signaling, some RGS proteins are emerging as targets in their own right. One such RGS protein is RGS17. Increased RGS17 expression in some prostate and lung cancers has been demonstrated to support cancer progression, while reduced expression of RGS17 can lead to development of chemotherapeutic resistance in ovarian cancer. High-throughput screening is a powerful tool for lead compound identification, and utilization of high-throughput technologies has led to the discovery of several RGS inhibitors, thus far. As screening technologies advance, the identification of novel lead compounds the subsequent development of targeted therapeutics appears promising.

A high throughput screen for RGS proteins using steady state monitoring of free phosphate formation

G-protein coupled receptors are a diverse group that are the target of over 50% of marketed drugs. Activation of these receptors results in the exchange of bound GDP for GTP in the Gα subunit of the heterotrimeric G-protein. The Gα subunit dissociates from the β/γ subunits and both proceed to affect downstream signaling targets. The signal terminates by the hydrolysis of GTP to GDP and is temporally regulated by Regulators of G-protein Signaling (RGS) proteins that act as GTPase Activating Proteins (GAPs). This makes RGS proteins potentially desirable targets for “tuning” the effects of current therapies as well as developing novel pharmacotherapies. Current methods for evaluating RGS activity depend on laborious and/or expensive techniques. In this study we developed a simple and inexpensive assay for the steady state analysis of RGS protein GAP activity, using RGS4, RGS8 and RGS17 as models. Additionally, we report the use of RGS4 as a model for high throughput assay development. After initial setup, this assay can be conducted in a highly parallel fashion with a read time of less than 8 minutes for a 1536-well plate. The assay exhibited a robust Z-factor of 0.6 in a 1536-well plate. We conducted a pilot screen for inhibitors using a small, 2320 compound library. From this screen, 13 compounds were identified as compounds for further analysis. The successful development of this assay for high-throughput screening provides a low cost, high speed, simple method for assessing RGS protein activity.

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.

Development of a novel high-throughput screen and identification of small-molecule inhibitors of the Gα-RGS17 protein-protein interaction using AlphaScreen

In this study, the authors used AlphaScreen technology to develop a high-throughput screening method for interrogating small-molecule libraries for inhibitors of the Gα(o)-RGS17 interaction. RGS17 is implicated in the growth, proliferation, metastasis, and the migration of prostate and lung cancers. RGS17 is upregulated in lung and prostate tumors up to a 13-fold increase over patient-matched normal tissues. Studies show RGS17 knockdown inhibits colony formation and decreases tumorigenesis in nude mice. The screen in this study uses a measurement of the Gα(o)-RGS17 protein-protein interaction, with an excellent Z score exceeding 0.73, a signal-to-noise ratio >70, and a screening time of 1100 compounds per hour. The authors screened the NCI Diversity Set II and determined 35 initial hits, of which 16 were confirmed after screening against controls. The 16 compounds exhibited IC(50) <10 µM in dose-response experiments. Four exhibited IC(50) values <6 µM while inhibiting the Gα(o)-RGS17 interaction >50% when compared to a biotinylated glutathione-S-transferase control. This report describes the first high-throughput screen for RGS17 inhibitors, as well as a novel paradigm adaptable to many other RGS proteins, which are emerging as attractive drug targets for modulating G-protein-coupled receptor signaling.

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.

Allosteric inhibition of the regulator of G protein signaling-Galpha protein-protein interaction by CCG-4986

Regulator of G protein signaling (RGS) proteins act to temporally modulate the activity of G protein subunits after G protein-coupled receptor activation. RGS proteins exert their effect by directly binding to the activated Galpha subunit of the G protein, catalyzing the accelerated hydrolysis of GTP and returning the G protein to its inactive, heterotrimeric form. In previous studies, we have sought to inhibit this GTPase-accelerating protein activity of the RGS protein by using small molecules. In this study, we investigated the mechanism of CCG-4986 [methyl-N-[(4-chlorophenyl)sulfonyl]-4-nitro-benzenesulfinimidoate], a previously reported small-molecule RGS inhibitor. Here, we find that CCG-4986 inhibits RGS4 function through the covalent modification of two spatially distinct cysteine residues on RGS4. We confirm that modification of Cys132, located near the RGS/Galpha interaction surface, modestly inhibits Galpha binding and GTPase acceleration. In addition, we report that modification of Cys148, a residue located on the opposite face of RGS4, can disrupt RGS/Galpha interaction through an allosteric mechanism that almost completely inhibits the Galpha-RGS protein-protein interaction. These findings demonstrate three important points: 1) the modification of the Cys148 allosteric site results in significant changes to the RGS interaction surface with Galpha; 2) this identifies a "hot spot" on RGS4 for binding of small molecules and triggering an allosteric change that may be significantly more effective than targeting the actual protein-protein interaction surface; and 3) because of the modification of a positional equivalent of Cys148 in RGS8 by CCG-4986, lack of inhibition indicates that RGS proteins exhibit fundamental differences in their responses to small-molecule ligands.

Regulator of G protein-signaling proteins and addictive drugs

Regulator of G protein-signaling (RGS) proteins are a family of more than 30 intracellular proteins that negatively modulate intracellular signaling of receptors in the G protein-coupled receptor family. This family includes receptors for opioids, cannabinoids, and dopamine that mediate the acute effects of addictive drugs or behaviors and chronic effects leading to the development of addictive disease. Members of the RGS protein family, by negatively modulating receptor signaling, influence the intracellular processes that lead to addiction. In turn, addictive drugs control the expression levels of several RGS proteins. This review will consider the distribution and mechanisms of action of RGS proteins, particularly the R4 and R7 families that have been implicated in the actions of addictive drugs, how knowledge of these proteins is contributing to an understanding of addictive processes, and whether specific RGS proteins could provide targets for the development of medications to manage and/or treat addiction.

Polyplexed flow cytometry protein interaction assay: a novel high-throughput screening paradigm for RGS protein inhibitors

Intracellular signaling cascades are a series of regulated protein-protein interactions that may provide a number of targets for potential drug discovery. Here, the authors examine the interaction of regulators of G-protein signaling (RGS) proteins with the G-protein Galphao, using a flow cytometry protein interaction assay (FCPIA). FCPIA accurately measures nanomolar binding constants of this protein-protein interaction and has been used in high-throughput screening. This report focuses on 5 RGS proteins (4, 6, 7, 8, and 16). To increase the content of screens, the authors assessed high-throughput screening of these RGS proteins in multiplex, by establishing binding constants of each RGS with Galphao in isolation, and then in a multiplex format with 5 RGS proteins present. To use this methodology as a higher-content multiplex protein-protein interaction screen, they established Z-factor values for RGS proteins in multiplex of 0.73 to 0.92, indicating this method is suitable for screening using FCPIA. To increase throughput, they also compressed a set of 8000 compounds by combining 4 compounds in a single assay well. Subsequent deconvolution of the compounds mixtures verified the identification of active compounds at specific RGS targets in their mixtures using the polyplexed FCPIA method.

Regulator of G protein signaling protein suppression of Galphao protein-mediated alpha2A adrenergic receptor inhibition of mouse hippocampal CA3 epileptiform activity

Activation of G protein-coupled alpha(2) adrenergic receptors (ARs) inhibits epileptiform activity in the hippocampal CA3 region. The specific mechanism underlying this action is unclear. This study investigated which subtype(s) of alpha(2)ARs and G proteins (Galpha(o) or Galpha(i)) are involved in this response using recordings of mouse hippocampal CA3 epileptiform bursts. Application of epinephrine (EPI) or norepinephrine (NE) reduced the frequency of bursts in a concentration-dependent manner: (-)EPI > (-)NE >>> (+)NE. To identify the alpha(2)AR subtype involved, equilibrium dissociation constants (pK(b)) were determined for the selective alphaAR antagonists atipamezole (8.79), rauwolscine (7.75), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101; 6.87), and prazosin (5.71). Calculated pK(b) values correlated best with affinities determined previously for the mouse alpha(2A)AR subtype (r = 0.98, slope = 1.07). Furthermore, the inhibitory effects of EPI were lost in hippocampal slices from alpha(2A)AR-but not alpha(2C)AR-knockout mice. Pretreatment with pertussis toxin also reduced the EPI-mediated inhibition of epileptiform bursts. Finally, using knock-in mice with point mutations that disrupt regulator of G protein signaling (RGS) binding to Galpha subunits to enhance signaling by that G protein, the EPI-mediated inhibition of bursts was significantly more potent in slices from RGS-insensitive Galpha(o)(G184S) heterozygous (Galpha(o)+/GS) mice compared with either Galpha(i2)(G184S) heterozygous (Galpha(i2)+/GS) or control mice (EC(50) = 2.5 versus 19 and 23 nM, respectively). Together, these findings indicate that the inhibitory effect of EPI on hippocampal CA3 epileptiform activity uses an alpha(2A)AR/Galpha(o) protein-mediated pathway under strong inhibitory control by RGS proteins. This suggests a possible role for RGS inhibitors or selective alpha(2A)AR agonists as a novel antiepileptic drug therapy.

Novel peptide ligands of RGS4 from a focused one-bead, one-compound library

Regulators of G protein signaling accelerate GTP hydrolysis by G alpha subunits and profoundly inhibit signaling by G protein-coupled receptors. The distinct expression patterns and pathophysiologic regulation of regulators of G protein signaling proteins suggest that inhibitors may have therapeutic potential. We previously reported the design, mechanistic evaluation, and structure-activity relationships of a disulfide-containing cyclic peptide inhibitor of RGS4, YJ34 (Ac-Val-Lys-c[Cys-Thr-Gly-Ile-Cys]-Glu-NH(2), S-S) (Roof et al., Chem Biol Drug Des, 67, 2006, 266). Using a focused one-bead, one-compound peptide library that contains features known to be necessary for the activity of YJ34, we now identify peptides that bind to RGS4. Six peptides showed confirmed binding to RGS4 by flow cytometry. Two analogs of peptide 2 (Gly-Thr-c[Cys-Phe-Gly-Thr-Cys]-Trp-NH(2), S-S with a free or acetylated N-terminus) inhibited RGS4-stimulated G alpha(o) GTPase activity at 25-50 microM. They selectively inhibit RGS4 but not RGS7, RGS16, and RGS19. Their inhibition of RGS4 does not depend on cysteine-modification of RGS4, as they do not lose activity when all cysteines are removed from RGS4. Peptide 2 has been modeled to fit in the same binding pocket predicted for YJ34 but in the reverse orientation.

Isolation and characterization of a novel human RGS mutant displaying gain-of-function activity

Regulator of G protein signaling (RGS) proteins play a crucial role in the adaptation of cells to stimulation by G protein-coupled receptors via heterotrimeric G proteins. Alterations in RGS function have been implicated in a wide range of disease states, leading to many researchers focusing on controlling the action of these regulatory proteins. Previous studies have centered on reducing or inhibiting the action of RGS proteins, utilizing inactive mutants or small molecular RGS inhibitors. Here we describe the isolation and characterization of a novel human RGS4 mutant which displays enhanced or gain-of-function (GOF) activity. RGS4(S30C) demonstrates GOF activity both in an in vivo yeast-based signalling pathway and in vitro against the Galpha(o1) subunit contained in an alpha(2A)-adrenoreceptor-Galpha(o1)(C351I) fusion protein. Mutational analysis of serine 30 identified a number of alternative substitutions that result in GOF activity. GOF activity was retained upon transposition of the serine 30-cysteine mutation to the equivalent serine residue in human RGS16. As with previously identified GOF mutants, RGS4(S30C/S30F/S30K) demonstrate increased steady state protein levels, however these mutants also demonstrate enhanced GAP activity through an additional mechanism distinct from the increased protein content. The identification of human RGS mutants with GOF activity may provide novel therapeutic agents for the treatment of signaling-based diseases and the ability to transpose these mutations to other human RGS proteins extends their application to multiple pathways.

Advances and perspectives in Leishmania cell based drug-screening procedures

Efforts for the development of new therapeutics, essential for the control of leishmaniasis rely mainly on screening of potentially effective compounds in pathogen growth/multiplication assays, both in vitro and in vivo. Screenings designed to closely reflect the situation in vivo are currently labor-intensive and expensive, since they require intracellular amastigotes and animal models. Screenings designed to facilitate rapid testing of a large number of drugs are not performed on the clinically relevant parasite stage, but the promastigotes. The ability to select transgenic Leishmania expressing reporter proteins, such as the green fluorescent protein (GFP) or the luciferase, opened up new possibilities for the development of drug screening tests. In this review we will focus on available methodologies for direct drug screening purposes against the mammalian stage of the parasite, with emphasis on the future developments that could improve sensitivity, reliability, versatility and the throughput of the intracellular model screening.

Leishmania amastigotes as targets for drug screening

Direct drug screening against the mammalian stage of Leishmania has been hampered by cost and the time consuming effort required to accomplish it. The ability to derive transgenic Leishmania expressing reporter genes opened up new possibilities for the development of drug screening tests. Further developments to standardize and gather multiple informations could now be envisionned. We will discuss on such available methodologies that could improve sensitivity, reliability, versatility and the rapidity, of the screen based on intracellular model.

Identification of small-molecule inhibitors of RGS4 using a high-throughput flow cytometry protein interaction assay

Regulators of G-protein signaling (RGS) proteins are important components of signal transduction pathways initiated through G-protein-coupled receptors (GPCRs). RGS proteins accelerate the intrinsic GTPase activity of G-protein alpha-subunits (Galpha) and thus shorten the time course and reduce the magnitude of G-protein alpha- and betagamma-subunit signaling. Inhibiting RGS action has been proposed as a means to enhance the activity and specificity of GPCR agonist drugs, but pharmacological targeting of protein-protein interactions has typically been difficult. The aim of this project was to identify inhibitors of RGS4. Using a Luminex 96-well plate bead analyzer and a novel flow-cytometric protein interaction assay to assess Galpha-RGS interactions in a high-throughput screen, we identified the first small-molecule inhibitor of an RGS protein. Of 3028 compounds screened, 1, methyl N-[(4-chlorophenyl)sulfonyl]-4-nitrobenzenesulfinimidoate (CCG-4986), inhibited RGS4/Galpha(o) binding with 3 to 5 muM potency. It binds to RGS4, inhibits RGS4 stimulation of Galpha(o) GTPase activity in vitro, and prevents RGS4 regulation of mu-opioid-inhibited adenylyl cyclase activity in permeabilized cells. Furthermore, CCG-4986 is selective for RGS4 and does not inhibit RGS8. Thus, we demonstrate the feasibility of targeting RGS/Galpha protein-protein interactions with small molecules as a novel means to modulate GPCR-mediated signaling processes.

JX401, A p38α Inhibitor Containing a 4-Benzylpiperidine Motif, Identified via a Novel Screening System in Yeast

In vivo screening of compounds for potential pharmacological activity is more advantageous than in vitro screening. In vivo screens eliminate the isolation of compounds that cannot cross biological membranes, are cytotoxic, or are not specific to the target. However, animal-based or even cell-based systems are usually expensive, time-consuming, and laborious. Here we describe the identification of inhibitors of the mitogen-activated protein kinase p38alpha via a high throughput screen using yeast cells. p38alpha is hyperactive in inflammatory diseases, and various indications suggest that its inhibition would reverse inflammation. However, there are currently no p38alpha inhibitors in clinical use. Because the human p38alpha imposes severe growth retardation when expressed in yeast, we screened a library of 40,000 randomly selected small molecules for compounds that would restore a normal growth rate. We identified two compounds; both share a structural motif of 4-benzylpiperidine, and both were shown to be efficient and selective p38alpha inhibitors in vitro. They were also active in mammalian cells, as manifested by their ability to reversibly inhibit myoblast differentiation. Thus, the yeast screen identified efficient and specific p38alpha inhibitors that are capable of crossing biological membranes, are not toxic, and function in mammalian cells. The rapid and cost-efficient high-throughput screening used here could be applied for isolation of inhibitors of various targets.

Investigating RGS proteins in yeast

Regulator of G protein signalling (RGS) proteins are vital in the adaptation of cells to stimulation via G protein-coupled receptors. Yeast have been integral in elucidating the important role that RGS proteins play within cellular processes. In addition to extensive characterisation of the endogenous RGS proteins, these organisms have enabled the identification and analysis of numerous mammalian homologues. The simplicity and plasticity of the yeast pheromone-response pathway has facilitated studies which would have been impossible in mammalian systems and it is certain that yeast will continue to have a great impact on this field of research in the future.

Functional analysis of heterologous GPCR signalling pathways in yeast

G protein-coupled receptors (GPCRs) regulate diverse biological processes in eukaryotes and such conservation allows an almost unrestricted interchange of signalling components between different cell types. Yeasts are attractive hosts in which to study GPCRs--they are amenable to both genetic and biochemical manipulation and their robustness, low cost and our ability to create strains that lack endogenous GPCRs make them ideal starting points for the development of assays suitable for high-throughput screening. Here we introduce readers to the possibilities of using yeast to analyse GPCRs describing the endogenous signalling pathways, the development of assays for heterologous GPCRs and the technology to elucidate GPCR structure and activity, focusing on the budding yeast Saccharomyces cerevisiae and recent developments using the fission yeast Schizosaccharomyces pombe.

A direct fluorescence-based assay for RGS domain GTPase accelerating activity

Diverse extracellular signals regulate seven transmembrane-spanning receptors to modulate cellular physiology. These receptors signal primarily through activation of heterotrimeric guanine nucleotide binding proteins (G proteins). A major determinant of heterotrimeric G protein signaling in vivo and in vitro is the intrinsic GTPase activity of the Galpha subunit. RGS (regulator of G protein signaling) domain-containing proteins are GTPase accelerating proteins specific for Galpha subunits. In this article, we describe the use of the ribose-conjugated fluorescent guanine nucleotide analog BODIPYFL-GTP as a spectroscopic probe to measure intrinsic and RGS protein-catalyzed nucleotide hydrolysis by Galphao. BODIPYFL-GTP bound to Galphao exhibits a 200% increase in fluorescence quantum yield. Hydrolysis of BODIPYFL-GTP to BODIPYFL-GDP reduces the quantum yield to 27% above its unbound value. We demonstrate that BODIPYFL-GTP can be used as a rapid real-time probe for measuring RGS domain-catalyzed GTP hydrolysis by Galphao. We demonstrate the effectiveness of this assay in the analysis of loss-of-function point mutants of both Galphao and RGS12. This assay should be useful in screening for and analyzing RGS protein inhibitory compounds.

Analysis of RGSZ1 protein interaction with Galphai subunits

RGSZ1 has been reported to interact with G-protein subunits of the Galphai family and function as a GTPase-accelerating protein on intrinsic Galphai GTPase activity. This article describes several experimental approaches and assays used to investigate the effect of RGSZ1 on Galphai subunits. The formats described here include physical and functional interaction assays by which the association of RGSZ1 with Galphai is explored both in vitro and in vivo. The methods analyzing physical interaction include pull-down and coimmunoprecipitation assays. We also apply yeast two-hybrid techniques to detect RGSZ1 protein interaction with Galpha subunits. Additionally, we developed several functional assay systems to identify the functional relationship between RGSZ1 and Galphai, such as the single turnover GTPase assay, yeast pheromone response assay, mitogen-activated protein kinase assay, and serum response element reporter assay.

Enhancement of pheromone response by RGS9 and Gβ5 in yeast

The G-protein gamma-subunit-like (GGL) domain present within a subfamily of RGS proteins binds specifically to Gbeta5. This interaction and resulting biological effect impacts the standard model of heterotrimeric G-protein signaling. It has been hypothesized that the RGS/Gbeta5 may potentially substitute for Gbetagamma in the heterotrimeric complex. Saccharomyces cerevisiae pheromone responsive mating signaling pathway is primarily driven by Gbetagamma. We evaluated GGL containing RGS9 and RGS7 for functional complementation in a RGS (sst2Delta) knockout yeast strain. The potential of Gbeta5 to augment the function of these RGS proteins was also evaluated. While Gbeta5 had no effect on RGS7, coexpression of Gbeta5 with RGS9 enhanced cell cycle arrest, suggesting that under certain conditions, RGS9 and Gbeta5 may possibly function as betagamma dimer. Furthermore, we demonstrate that Gbeta5 can complement a ste4Delta, the yeast beta-subunit, thus providing the first evidence of functional complementation of a mammalian Gbeta.