Regulation of RGS-RhoGEFs by Galpha12 and Galpha13 proteins

Genome-wide identification of Gα family in grass carp (Ctenopharyngodon idella) and reproductive regulation functional characteristics of Cignaq

BACKGROUND

The Gα family plays a crucial role in the complex reproductive regulatory network of teleosts. However, the characterization and function of Gα family members, especially Gαq, remain poorly understood in teleosts. To analyze the characterization, expression, and function of grass carp (Ctenopharyngodon idella) Gαq, we identified the Gα family members in grass carp genome, and analyzed the expression, distribution, and signal transduction of Gαq/gnaq. We also explored the role of Gαq in the reproductive regulation of grass carp.

RESULTS

Our results showed that the grass carp genome contains 27 Gα genes with 46 isoforms, which are divided into four subfamilies: Gαs, Gαi/o, Gαq/11, and Gα12/13. The expression level of Cignaq in the testis was the highest and significantly higher than in other tissues, followed by the hypothalamus and brain. The luteinizing hormone receptor (LHR) was mainly localized to the nucleus in grass carp oocytes, with signals also present in follicular cells. In contrast, Gαq signal was mainly found in the cytoplasm of oocytes, with no signal in follicular cells. In the testis, Gαq and LHR were co-localized in the cytoplasm. Furthermore, the grass carp Gαq recombinant protein significantly promoted Cipgr expression.

CONCLUSIONS

These results provided preliminary evidence for understanding the role of Gαq in the reproductive regulation of teleosts.

Untangling the role of RhoA in the heart: protective effect and mechanism

RhoA (ras homolog family member A) is a small G-protein that transduces intracellular signaling to regulate a broad range of cellular functions such as cell growth, proliferation, migration, and survival. RhoA serves as a proximal downstream effector of numerous G protein-coupled receptors (GPCRs) and is also responsive to various stresses in the heart. Upon its activation, RhoA engages multiple downstream signaling pathways. Rho-associated coiled-coil-containing protein kinase (ROCK) is the first discovered and best characterized effector or RhoA, playing a major role in cytoskeletal arrangement. Many other RhoA effectors have been identified, including myocardin-related transcription factor A (MRTF-A), Yes-associated Protein (YAP) and phospholipase Cε (PLCε) to regulate transcriptional and post-transcriptional processes. The role of RhoA signaling in the heart has been increasingly studied in last decades. It was initially suggested that RhoA signaling pathway is maladaptive in the heart, but more recent studies using cardiac-specific expression or deletion of RhoA have revealed that RhoA activation provides cardioprotection against stress through various mechanisms including the novel role of RhoA in mitochondrial quality control. This review summarizes recent advances in understanding the role of RhoA in the heart and its signaling pathways to prevent progression of heart disease.

A neurodevelopmental disorder mutation locks G proteins in the transitory pre-activated state

Many neurotransmitter receptors activate G proteins through exchange of GDP for GTP. The intermediate nucleotide-free state has eluded characterization, due largely to its inherent instability. Here we characterize a G protein variant associated with a rare neurological disorder in humans. GαoK46E has a charge reversal that clashes with the phosphate groups of GDP and GTP. As anticipated, the purified protein binds poorly to guanine nucleotides yet retains wild-type affinity for G protein βγ subunits. In cells with physiological concentrations of nucleotide, GαoK46E forms a stable complex with receptors and Gβγ, impeding effector activation. Further, we demonstrate that the mutant can be easily purified in complex with dopamine-bound D2 receptors, and use cryo-electron microscopy to determine the structure, including both domains of Gαo, without nucleotide or stabilizing nanobodies. These findings reveal the molecular basis for the first committed step of G protein activation, establish a mechanistic basis for a neurological disorder, provide a simplified strategy to determine receptor-G protein structures, and a method to detect high affinity agonist binding in cells.

Integrin β3 directly inhibits the Gα13-p115RhoGEF interaction to regulate G protein signaling and platelet exocytosis

The integrins and G protein-coupled receptors are both fundamental in cell biology. The cross talk between these two, however, is unclear. Here we show that β3 integrins negatively regulate G protein-coupled signaling by directly inhibiting the Gα13-p115RhoGEF interaction. Furthermore, whereas β3 deficiency or integrin antagonists inhibit integrin-dependent platelet aggregation and exocytosis (granule secretion), they enhance G protein-coupled RhoA activation and integrin-independent secretion. In contrast, a β3-derived Gα13-binding peptide or Gα13 knockout inhibits G protein-coupled RhoA activation and both integrin-independent and dependent platelet secretion without affecting primary platelet aggregation. In a mouse model of myocardial ischemia/reperfusion injury in vivo, the β3-derived Gα13-binding peptide inhibits platelet secretion of granule constituents, which exacerbates inflammation and ischemia/reperfusion injury. These data establish crucial integrin-G protein crosstalk, providing a rationale for therapeutic approaches that inhibit exocytosis in platelets and possibly other cells without adverse effects associated with loss of cell adhesion.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

A current view of G protein-coupled receptor - mediated signaling in pulmonary hypertension: finding opportunities for therapeutic intervention

Pathological vascular remodeling is observed in various cardiovascular diseases including pulmonary hypertension (PH), a disease of unknown etiology that has been characterized by pulmonary artery vasoconstriction, right ventricular hypertrophy, vascular inflammation, and abnormal angiogenesis in pulmonary circulation. G protein-coupled receptors (GPCRs) are the largest family in the genome and widely expressed in cardiovascular system. They regulate all aspects of PH pathophysiology and represent therapeutic targets. We overview GPCRs function in vasoconstriction, vasodilation, vascular inflammation-driven remodeling and describe signaling cross talk between GPCR, inflammatory cytokines, and growth factors. Overall, the goal of this review is to emphasize the importance of GPCRs as critical signal transducers and targets for drug development in PH.

Selective coupling of the S1P3 receptor subtype to S1P-mediated RhoA activation and cardioprotection

Sphingosine-1-phosphate (S1P), a bioactive lysophospholipid, is generated and released at sites of tissue injury in the heart and can act on S1P1, S1P2, and S1P3 receptor subtypes to affect cardiovascular responses. We established that S1P causes little phosphoinositide hydrolysis and does not induce hypertrophy indicating that it does not cause receptor coupling to Gq. We previously demonstrated that S1P confers cardioprotection against ischemia/reperfusion by activating RhoA and its downstream effector PKD. The S1P receptor subtypes and G proteins that regulate RhoA activation and downstream responses in the heart have not been determined. Using siRNA or pertussis toxin to inhibit different G proteins in NRVMs we established that S1P regulates RhoA activation through Gα13 but not Gα12, Gαq, or Gαi. Knockdown of the three major S1P receptors using siRNA demonstrated a requirement for S1P3 in RhoA activation and subsequent phosphorylation of PKD, and this was confirmed in studies using isolated hearts from S1P3 knockout (KO) mice. S1P treatment reduced infarct size induced by ischemia/reperfusion in Langendorff perfused wild-type (WT) hearts and this protection was abolished in the S1P3 KO mouse heart. CYM-51736, an S1P3-specific agonist, also decreased infarct size after ischemia/reperfusion to a degree similar to that achieved by S1P. The finding that S1P3 receptor- and Gα13-mediated RhoA activation is responsible for protection against ischemia/reperfusion suggests that selective targeting of S1P3 receptors could provide therapeutic benefits in ischemic heart disease.

G protein α12 (Gα12) is a negative regulator of kidney injury molecule-1-mediated efferocytosis

Kidney injury molecule-1 (KIM-1) is a receptor for the "eat me" signal, phosphatidylserine, on apoptotic cells. The specific upregulation of KIM-1 by injured tubular epithelial cells (TECs) enables them to clear apoptotic cells (also known as efferocytosis), thereby protecting from acute kidney injury. Recently, we uncovered that KIM-1 binds directly to the α-subunit of heterotrimeric G₁₂ protein (Gα₁₂) and inhibits its activation by reactive oxygen species during renal ischemia-reperfusion injury (Ismail OZ, Zhang X, Wei J, Haig A, Denker BM, Suri RS, Sener A, Gunaratnam L. Am J Pathol 185: 1207-1215, 2015). Here, we investigated the role that Gα₁₂ plays in KIM-1-mediated efferocytosis by TECs. We showed that KIM-1 remains bound to Gα₁₂ and suppresses its activity during phagocytosis. When we silenced Gα₁₂ expression using small interefering RNA, KIM-1-mediated engulfment of apoptotic cells was increased significantly; in contrast overexpression of constitutively active Gα₁₂ (QLGα₁₂) resulted in inhibition of efferocytosis. Inhibition of RhoA, a key effector of Gα₁₂, using a chemical inhibitor or expression of dominant-negative RhoA, had the same effect as inhibition of Gα₁₂ on efferocytosis. Consistent with this, silencing Gα₁₂ suppressed active RhoA in KIM-1-expressing cells. Finally, using primary TECs from Kim-1^+/+ and Kim-1^-/- mice, we confirmed that engulfment of apoptotic cells requires KIM-1 expression and that silencing Gα₁₂ enhanced efferocytosis by primary TECs. Our data reveal a previously unknown role for Gα₁₂ in regulating efferocytosis and that renal TECs require KIM-1 to mediate this process. These results may have therapeutic implications given the known harmful role of Gα₁₂ in acute kidney injury.

G Protein-Coupled Receptor and RhoA-Stimulated Transcriptional Responses: Links to Inflammation, Differentiation, and Cell Proliferation

The low molecular weight G protein RhoA (rat sarcoma virus homolog family member A) serves as a node for transducing signals through G protein-coupled receptors (GPCRs). Activation of RhoA occurs through coupling of G proteins, most prominently, G12/13, to Rho guanine nucleotide exchange factors. The GPCR ligands that are most efficacious for RhoA activation include thrombin, lysophosphatidic acid, sphingosine-1-phosphate, and thromboxane A2. These ligands also stimulate proliferation, differentiation, and inflammation in a variety of cell and tissues types. The molecular events underlying these responses are the activation of transcription factors, transcriptional coactivators, and downstream gene programs. This review describes the pathways leading from GPCRs and RhoA to the regulation of activator protein-1, NFκB (nuclear factor κ-light-chain-enhancer of activated B cells), myocardin-related transcription factor A, and Yes-associated protein. We also focus on the importance of two prominent downstream transcriptional gene targets, the inflammatory mediator cyclooxygenase 2, and the matricellular protein cysteine-rich angiogenic inducer 61 (CCN1). Finally, we describe the importance of GPCR-induced activation of these pathways in the pathophysiology of cancer, fibrosis, and cardiovascular disease.

G protein-coupled receptor 35: an emerging target in inflammatory and cardiovascular disease

G protein-coupled receptor 35 (GPR35) is an orphan receptor, discovered in 1998, that has garnered interest as a potential therapeutic target through its association with a range of diseases. However, a lack of pharmacological tools and the absence of convincingly defined endogenous ligands have hampered the understanding of function necessary to exploit it therapeutically. Although several endogenous molecules can activate GPR35 none has yet been confirmed as the key endogenous ligand due to reasons that include lack of biological specificity, non-physiologically relevant potency and species ortholog selectivity. Recent advances have identified several highly potent synthetic agonists and antagonists, as well as agonists with equivalent potency at rodent and human orthologs, which will be useful as tool compounds. Homology modeling and mutagenesis studies have provided insight into the mode of ligand binding and possible reasons for the species selectivity of some ligands. Advances have also been made in determining the role of the receptor in disease. In the past, genome-wide association studies have associated GPR35 with diseases such as inflammatory bowel disease, type 2 diabetes, and coronary artery disease. More recent functional studies have implicated it in processes as diverse as heart failure and hypoxia, inflammation, pain transduction and synaptic transmission. In this review, we summarize the progress made in understanding the molecular pharmacology, downstream signaling and physiological function of GPR35, and discuss its emerging potential applications as a therapeutic target.

Gα13/PDZ-RhoGEF/RhoA signaling is essential for gastrin-releasing peptide receptor-mediated colon cancer cell migration

Gastrin-releasing peptide receptor (GRPR) is ectopically expressed in over 60% of colon cancers. GRPR expression has been correlated with increased colon cancer cell migration. However, the signaling pathway by which GRPR activation leads to increased cancer cell migration is not well understood. We set out to molecularly dissect the GRPR signaling pathways that control colon cancer cell migration through regulation of small GTPase RhoA. Our results show that GRP stimulation activates RhoA predominantly through G13 heterotrimeric G-protein signaling. We also demonstrate that postsynaptic density 95/disk-large/ZO-1 (PDZ)-RhoGEF (PRG), a member of regulator of G-protein signaling (RGS)-homology domain (RH) containing guanine nucleotide exchange factors (RH-RhoGEFs), is the predominant activator of RhoA downstream of GRPR. We found that PRG is required for GRP-stimulated colon cancer cell migration, through activation of RhoA-Rho-associated kinase (ROCK) signaling axis. In addition, PRG-RhoA-ROCK pathway also contributes to cyclo-oxygenase isoform 2 (Cox-2) expression. Increased Cox-2 expression is correlated with increased production of prostaglandin-E2 (PGE2), and Cox-2-PGE2 signaling contributes to total GRPR-mediated cancer cell migration. Our analysis reveals that PRG is overexpressed in colon cancer cell lines. Overall, our results have uncovered a key mechanism for GRPR-regulated colon cancer cell migration through the Gα13-PRG-RhoA-ROCK pathway.

G protein-coupled receptor accessory proteins and signaling: pharmacogenomic insights

The identification and characterization of the genes encoding G protein-coupled receptors (GPCRs) and the proteins necessary for the processes of ligand binding, GPCR activation, inactivation, and receptor trafficking to the membrane are discussed in the context of human genetic disease. In addition to functional GPCR variants, the identification of genetic disruptions affecting proteins necessary to GPCR functions have provided insights into the function of these pathways. Gsα and Gβ subunit polymorphisms have been found to result in complex phenotypes. Disruptions in accessory proteins that normally modify or organize heterotrimeric G-protein coupling may also result in disease states. These include the contribution of variants of the regulator of G protein signaling (RGS) protein to hypertension; the role variants of the activator of G protein signaling (AGS) proteins to phenotypes (such as the type III AGS8 variant to hypoxia); the contribution of G protein-coupled receptor kinase (GRK) proteins, such as GRK4, in disorders such as hypertension. The role of accessory proteins in GPCR structure and function is discussed in the context of genetic disorders associated with disruption of the genes that encode them. An understanding of the pharmacogenomics of GPCR and accessory protein signaling provides the basis for examining both GPCR pharmacogenetics and the genetics of monogenic disorders that result from disruption of given receptor systems.

The C-terminus of the long AKAP13 isoform (AKAP-Lbc) is critical for development of compensatory cardiac hypertrophy

The objective of this study was to determine the role of A-Kinase Anchoring Protein (AKAP)-Lbc in the development of heart failure, by investigating AKAP-Lbc-protein kinase D1 (PKD1) signaling in vivo in cardiac hypertrophy. Using a gene-trap mouse expressing a truncated version of AKAP-Lbc (due to disruption of the endogenous AKAP-Lbc gene), that abolishes PKD1 interaction with AKAP-Lbc (AKAP-Lbc-ΔPKD), we studied two mouse models of pathological hypertrophy: i) angiotensin (AT-II) and phenylephrine (PE) infusion and ii) transverse aortic constriction (TAC)-induced pressure overload. Our results indicate that AKAP-Lbc-ΔPKD mice exhibit an accelerated progression to cardiac dysfunction in response to AT-II/PE treatment and TAC. AKAP-Lbc-ΔPKD mice display attenuated compensatory cardiac hypertrophy, increased collagen deposition and apoptosis, compared to wild-type (WT) control littermates. Mechanistically, reduced levels of PKD1 activation are observed in AKAP-Lbc-ΔPKD mice compared to WT mice, resulting in diminished phosphorylation of histone deacetylase 5 (HDAC5) and decreased hypertrophic gene expression. This is consistent with a reduced compensatory hypertrophy phenotype leading to progression of heart failure in AKAP-Lbc-ΔPKD mice. Overall, our data demonstrates a critical in vivo role for AKAP-Lbc-PKD1 signaling in the development of compensatory hypertrophy to enhance cardiac performance in response to TAC-induced pressure overload and neurohumoral stimulation by AT-II/PE treatment.

Interaction of Protease-Activated Receptor 2 with G Proteins and β-Arrestin 1 Studied by Bioluminescence Resonance Energy Transfer

G protein-coupled receptors are well recognized as being able to activate several signaling pathways through the activation of different G proteins as well as other signaling proteins such as β-arrestins. Therefore, understanding how such multiple GPCR-mediated signaling can be integrated constitute an important aspect. Here, we applied bioluminescence resonance energy transfer (BRET) to shed more light on the G protein coupling profile of trypsin receptor, or protease-activated receptor 2 (PAR2), and its interaction with β-arrestin1. Using YFP and Rluc fusion constructs expressed in COS-7 cells, BRET data revealed a pre-assembly of PAR2 with both Gαi1 and Gαo and a rapid and transient activation of these G proteins upon receptor activation. In contrast, no pre-assembly of PAR2 with Gα12 could be detected and their physical association can be measured with a very slow and sustained kinetics similar to that of β-arrestin1 recruitment. These data demonstrate the coupling of PAR2 with Gαi1, Gαo, and Gα12 in COS-7 cells with differences in the kinetics of GPCR-G protein coupling, a parameter that very likely influences the cellular response. Moreover, this further illustrates that pre-assembly or agonist-induced G protein interaction depends on receptor-G protein pairs indicating another level of complexity and regulation of the signaling of GPCR-G protein complexes and its multiplicity.

Modification of p115RhoGEF Ser(330) regulates its RhoGEF activity

p115RhoGEF is a member of a family of Rho-specific guanine nucleotide exchange factors that also contains a regulator of G protein signaling homology domain (RH-RhoGEFs) that serves as a link between Gα13 signaling and RhoA activation. While the mechanism of regulation of p115RhoGEF by Gα13 is becoming well-known, the role of other regulatory mechanisms, such as post-translational modification or autoinhibition, in mediating p115RhoGEF activity is less well-characterized. Here, putative phosphorylation sites on p115RhoGEF are identified and characterized. Mutation of Ser(330) leads to a decrease in serum response element-mediated transcription as well as decreased activation by Gα13 in vitro. Additionally, this study provides the first report of the binding kinetics between full-length p115RhoGEF and RhoA in its various nucleotide states and examines the binding kinetics of phospho-mutant p115RhoGEF to RhoA. These data, together with other recent reports on regulatory mechanisms of p115RhoGEF, suggest that this putative phosphorylation site serves as a means for initiation or relief of autoinhibition of p115RhoGEF, providing further insight into the regulation of its activity.

AKAP13 Rho-GEF and PKD-binding domain deficient mice develop normally but have an abnormal response to β-adrenergic-induced cardiac hypertrophy

BACKGROUND

A-kinase anchoring proteins (AKAPs) are scaffolding molecules that coordinate and integrate G-protein signaling events to regulate development, physiology, and disease. One family member, AKAP13, encodes for multiple protein isoforms that contain binding sites for protein kinase A (PKA) and D (PKD) and an active Rho-guanine nucleotide exchange factor (Rho-GEF) domain. In mice, AKAP13 is required for development as null embryos die by embryonic day 10.5 with cardiovascular phenotypes. Additionally, the AKAP13 Rho-GEF and PKD-binding domains mediate cardiomyocyte hypertrophy in cell culture. However, the requirements for the Rho-GEF and PKD-binding domains during development and cardiac hypertrophy are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To determine if these AKAP13 protein domains are required for development, we used gene-trap events to create mutant mice that lacked the Rho-GEF and/or the protein kinase D-binding domains. Surprisingly, heterozygous matings produced mutant mice at Mendelian ratios that had normal viability and fertility. The adult mutant mice also had normal cardiac structure and electrocardiograms. To determine the role of these domains during β-adrenergic-induced cardiac hypertrophy, we stressed the mice with isoproterenol. We found that heart size was increased similarly in mice lacking the Rho-GEF and PKD-binding domains and wild-type controls. However, the mutant hearts had abnormal cardiac contractility as measured by fractional shortening and ejection fraction.

CONCLUSIONS

These results indicate that the Rho-GEF and PKD-binding domains of AKAP13 are not required for mouse development, normal cardiac architecture, or β-adrenergic-induced cardiac hypertrophic remodeling. However, these domains regulate aspects of β-adrenergic-induced cardiac hypertrophy.

G12/13-dependent signaling of G-protein-coupled receptors: disease context and impact on drug discovery

G-protein-coupled receptors (GPCRs) transmit extracellular signals across the plasma membrane via intracellular activation of heterotrimeric G proteins. The signal transduction pathways of Gs, Gi and Gq protein families are widely studied, whereas signaling properties of G12 proteins are only emerging. Many GPCRs were found to couple to G12/13 proteins in addition to coupling to one or more other types of G proteins. G12/13 proteins couple GPCRs to activation of the small monomeric GTPase RhoA. Activation of RhoA modulates various downstream effector systems relevant to diseases such as hypertension, artherosclerosis, asthma and cancer. GPCR screening assays exist for Gs-, Gi- and Gq-linked pathways, whereas a drug-screening assay for the G12-Rho pathway was developed only recently. The review gives an overview of the present understanding of the G12/13-related biology of GPCRs.

The Pasteurella multocida toxin: a new paradigm for the link between bacterial infection and cancer

The concept that bacterial infection could cause cancer has only recently become accepted because of the strong epidemiological and molecular evidence for a major carcinogenic role played by Helicobacter pylori. However, information on other potential bacterial carcinogens is very limited and thereby unconvincing. A different approach is to assess bacteria for potentially pro-carcinogenic properties. The Pasteurella multocida toxin (PMT) has many properties that mark it out as a potential carcinogen. PMT is a highly potent mitogen and has been demonstrated to block apoptosis. PMT modifies and activates members of three of the four families of heterotrimeric G-proteins, all of which have potential roles in carcinogenesis. Many signalling components downstream of these G-proteins are known proto-oncogenes and have been shown to be activated by PMT. These include, amongst others, the Rho GTPase, focal adhesion kinase, cyclooxygenase-2, β-catenin signalling and calcium signalling. PMT action potentially influences many of the acquired Hanahan/Weinberg capabilities necessary for oncogenic transformation. Although there is little evidence that PMT might have a role in human cancer, it serves as an important and novel paradigm for a bacterial link to cancer.

Receptor-G protein interaction studied by bioluminescence resonance energy transfer: lessons from protease-activated receptor 1

Since its development, the bioluminescence resonance energy transfer (BRET) approach has been extensively applied to study G protein-coupled receptors (GPCRs) in real-time and in live cells. One of the major aspects of GPCRs investigated in considerable details is their physical coupling to the heterotrimeric G proteins. As a result, new concepts have emerged, but few questions are still a matter of debate illustrating the complexity of GPCR-G protein interactions and coupling. Here, we summarized the recent advances on our understanding of GPCR-G protein coupling based on BRET approaches and supported by other FRET-based studies. We essentially focused on our recent studies in which we addressed the concept of preassembly vs. the agonist-dependent interaction between the protease-activated receptor 1 (PAR1) and its cognate G proteins. We discussed the concept of agonist-induced conformational changes within the preassembled PAR1-G protein complexes as well as the critical question how the multiple coupling of PAR1 with two different G proteins, Gαi1 and Gα12, but also β-arrestin 1, can be regulated.

Cellular and molecular action of the mitogenic protein-deamidating toxin from Pasteurella multocida

The mitogenic toxin from Pasteurella multocida (PMT) is a member of the dermonecrotic toxin family, which includes toxins from Bordetella, Escherichia coli and Yersinia. Members of the dermonecrotic toxin family modulate G-protein targets in host cells through selective deamidation and/or transglutamination of a critical active site Gln residue in the G-protein target, which results in the activation of intrinsic GTPase activity. Structural and biochemical data point to the uniqueness of PMT among these toxins in its structure and action. Whereas the other dermonecrotic toxins act on small Rho GTPases, PMT acts on the α subunits of heterotrimeric G(q) -, G(i) - and G(12/13) -protein families. To date, experimental evidence supports a model in which PMT potently stimulates various mitogenic and survival pathways through the activation of G(q) and G(12/13) signaling, ultimately leading to cellular proliferation, whilst strongly inhibiting pathways involved in cellular differentiation through the activation of G(i) signaling. The resulting cellular outcomes account for the global physiological effects observed during infection with toxinogenic P. multocida, and hint at potential long-term sequelae that may result from PMT exposure.

Identification of critical residues in G(alpha)13 for stimulation of p115RhoGEF activity and the structure of the G(alpha)13-p115RhoGEF regulator of G protein signaling homology (RH) domain complex

RH-RhoGEFs are a family of guanine nucleotide exchange factors that contain a regulator of G protein signaling homology (RH) domain. The heterotrimeric G protein Gα(13) stimulates the guanine nucleotide exchange factor (GEF) activity of RH-RhoGEFs, leading to activation of RhoA. The mechanism by which Gα(13) stimulates the GEF activity of RH-RhoGEFs, such as p115RhoGEF, has not yet been fully elucidated. Here, specific residues in Gα(13) that mediate activation of p115RhoGEF are identified. Mutation of these residues significantly impairs binding of Gα(13) to p115RhoGEF as well as stimulation of GEF activity. These data suggest that the exchange activity of p115RhoGEF is stimulated allosterically by Gα(13) and not through its interaction with a secondary binding site. A crystal structure of Gα(13) bound to the RH domain of p115RhoGEF is also presented, which differs from a previously crystallized complex with a Gα(13)-Gα(i1) chimera. Taken together, these data provide new insight into the mechanism by which p115RhoGEF is activated by Gα(13).

Recent insights into Pasteurella multocida toxin and other G-protein-modulating bacterial toxins

Over the past few decades, our understanding of the bacterial protein toxins that modulate G proteins has advanced tremendously through extensive biochemical and structural analyses. This article provides an updated survey of the various toxins that target G proteins, ending with a focus on recent mechanistic insights in our understanding of the deamidating toxin family. The dermonecrotic toxin from Pasteurella multocida (PMT) was recently added to the list of toxins that disrupt G-protein signal transduction through selective deamidation of their targets. The C3 deamidase domain of PMT has no sequence similarity to the deamidase domains of the dermonecrotic toxins from Escherichia coli (cytotoxic necrotizing factor [CNF]1-3), Yersinia (CNFY) and Bordetella (dermonecrotic toxin). The structure of PMT-C3 belongs to a family of transglutaminase-like proteins, with active site Cys-His-Asp catalytic triads distinct from E. coli CNF1.

G protein subunit Galpha13 binds to integrin alphaIIbbeta3 and mediates integrin "outside-in" signaling

Integrins mediate cell adhesion to the extracellular matrix and transmit signals within the cell that stimulate cell spreading, retraction, migration, and proliferation. The mechanism of integrin outside-in signaling has been unclear. We found that the heterotrimeric guanine nucleotide-binding protein (G protein) Galpha13 directly bound to the integrin beta3 cytoplasmic domain and that Galpha13-integrin interaction was promoted by ligand binding to the integrin alphaIIbbeta3 and by guanosine triphosphate (GTP) loading of Galpha13. Interference of Galpha13 expression or a myristoylated fragment of Galpha13 that inhibited interaction of alphaIIbbeta3 with Galpha13 diminished activation of protein kinase c-Src and stimulated the small guanosine triphosphatase RhoA, consequently inhibiting cell spreading and accelerating cell retraction. We conclude that integrins are noncanonical Galpha13-coupled receptors that provide a mechanism for dynamic regulation of RhoA.

Coordinate Gα13 and Wnt6-β-catenin signaling in F9 embryonal carcinoma cells is required for primitive endoderm differentiation

The mouse F9 embryonal carcinoma cell line is ideally suited to study the epithelial-to-mesenchymal transition accompanying the differentiation of primitive to parietal extraembryonic endoderm. In F9 cells, the application of exogenous agents including retinoic acid or activation of signal transduction cascades downstream of G-proteins triggers widespread changes in gene expression and leads to the formation of primitive endoderm. The epithelial-to-mesenchymal transition is completed and parietal endoderm develops as of result of increasing PKA activity in primitive endoderm cells. Expression of a constitutively active form of Gα13(Q226L) is sufficient to induce F9 cells into parietal endoderm and a model is emerging that a signaling axis linking G-protein signaling to RhoA and the ERM protein moesin is required for differentiation. In this study, we found that expression of either p115RhoGEF or a constitutively active, GTPase-deficient form of RhoA(L63) promoted primitive, but not parietal, endoderm formation. The overexpression of Gα13(Q226L) or p115RhoGEF, but not Rho(L63), caused β-catenin to translocate to the nucleus. Surprisingly, the stimulation of the Wnt-β-catenin pathway was accompanied by nuclear β-catenin and primitive endoderm formation, even when a dominant negative was used to block the signaling axis at the level of p115RhoGEF or when ROCK activity was inhibited using the pharmacological agent Y-27632. Together, results indicate that the coordinate signaling by two independent pathways, one involving canonical Wnt-β-catenin activation of target genes and the other with Gα13 signaling to ERM proteins to modulate cytoarchitectural changes, is required during the retinoic acid induced differentiation of F9 cells to primitive endoderm.

Regulation of RhoGEF proteins by G12/13-coupled receptors

G protein-coupled receptors (GPCRs) represent a large family of seven transmembrane receptors, which communicate extracellular signals into the cellular lumen. The human genome contains 720-800 GPCRs, and their diverse signal characteristics are determined by their specific tissue and subcellular expression profiles, as well as their coupling profile to the various G protein families (G(s), G(i), G(q), G(12)). The G protein coupling pattern links GPCR activation to the specific downstream effector pathways. G(12/13) signalling of GPCRs has been studied only recently in more detail, and involves activation of RhoGTPase nucleotide exchange factors (RhoGEFs). Four mammalian RhoGEFs regulated by G(12/13) proteins are known: p115-RhoGEF, PSD-95/Disc-large/ZO-1 homology-RhoGEF, leukemia-associated RhoGEF and lymphoid blast crisis-RhoGEF. These link GPCRs to activation of the small monomeric GTPase RhoA, and other downstream effectors. Misregulated G(12/13) signalling is involved in multiple pathophysiological conditions such as cancer, cardiovascular diseases, arterial and pulmonary hypertension, and bronchial asthma. Specific targeting of G(12/13) signalling-related diseases of GPCRs hence provides novel therapeutic approaches. Assays to quantitatively measure GPCR-mediated activation of G(12/13) are only emerging, and are required to understand the G(12/13)-linked pharmacology. The review gives an overview of G(12/13) signalling of GPCRs with a focus on RhoGEF proteins as the immediate mediators of G(12/13) activation.

Activation of leukemia-associated RhoGEF by Galpha13 with significant conformational rearrangements in the interface

The transient protein-protein interactions induced by guanine nucleotide-dependent conformational changes of G proteins play central roles in G protein-coupled receptor-mediated signaling systems. Leukemia-associated RhoGEF (LARG), a guanine nucleotide exchange factor for Rho, contains an RGS homology (RH) domain and Dbl homology/pleckstrin homology (DH/PH) domains and acts both as a GTPase-activating protein (GAP) and an effector for Galpha(13). However, the molecular mechanism of LARG activation upon Galpha(13) binding is not yet well understood. In this study, we analyzed the Galpha(13)-LARG interaction using cellular and biochemical methods, including a surface plasmon resonance (SPR) analysis. The results obtained using various LARG fragments demonstrated that active Galpha(13) interacts with LARG through the RH domain, DH/PH domains, and C-terminal region. However, an alanine substitution at the RH domain contact position in Galpha(13) resulted in a large decrease in affinity. Thermodynamic analysis revealed that binding of Galpha(13) proceeds with a large negative heat capacity change (DeltaCp degrees ), accompanied by a positive entropy change (DeltaS degrees ). These results likely indicate that the binding of Galpha(13) with the RH domain triggers conformational rearrangements between Galpha(13) and LARG burying an exposed hydrophobic surface to create a large complementary interface, which facilitates complex formation through both GAP and effector interfaces, and activates the RhoGEF. We propose that LARG activation is regulated by an induced-fit mechanism through the GAP interface of Galpha(13).

Expression of RHOGTPase regulators in human myometrium

BACKGROUND

RHOGTPases play a significant role in modulating myometrial contractility in uterine smooth muscle. They are regulated by at least three families of proteins, RHO guanine nucleotide exchange factors (RHOGEFs), RHOGTPase-activating proteins (RHOGAPs) and RHO guanine nucleotide inhibitors (RHOGDIs). RHOGEFs activate RHOGTPases from the inactive GDP-bound to the active GTP-bound form. RHOGAPs deactivate RHOGTPases by accelerating the intrinsic GTPase activity of the RHOGTPases, converting them from the active to the inactive form. RHOGDIs bind to GDP-bound RHOGTPases and sequester them in the cytosol, thereby inhibiting their activity. Ezrin-Radixin-Moesin (ERM) proteins regulate the cortical actin cytoskeleton, and an ERM protein, moesin (MSN), is activated by and can also activate RHOGTPases.

METHODS

We therefore investigated the expression of various RHOGEFs, RHOGAPs, a RHOGDI and MSN in human myometrium, by semi-quantitative reverse transcription PCR, real-time fluorescence RT-PCR, western blotting and immunofluorescence microscopy. Expression of these molecules was also examined in myometrial smooth muscle cells.

RESULTS

ARHGEF1, ARHGEF11, ARHGEF12, ARHGAP5, ARHGAP24, ARHGDIA and MSN mRNA and protein expression was confirmed in human myometrium at term pregnancy, at labour and in the non-pregnant state. Furthermore, their expression was detected in myometrial smooth muscle cells. It was determined that ARHGAP24 mRNA expression significantly increased at labour in comparison to the non-labour state.

CONCLUSION

This study demonstrated for the first time the expression of the RHOGTPase regulators ARHGEF1, ARHGEF11, ARHGEF12, ARHGAP5, ARHGAP24, ARHGDIA and MSN in human myometrium, at term pregnancy, at labour, in the non-pregnant state and also in myometrial smooth muscle cells. ARHGAP24 mRNA expression significantly increased at labour in comparison to the non-labouring state. Further investigation of these molecules may enable us to further our knowledge of RHOGTPase regulation in human myometrium during pregnancy and labour.

Pharmacogenomics of G protein-coupled receptor signaling: insights from health and disease

The identification and characterization of the processes of G protein-coupled receptor (GPCR) activation and inactivation have refined not only the study of the GPCRs but also the genomics of many accessory proteins necessary for these processes. This has accelerated progress in understanding the fundamental mechanisms involved in GPCR structure and function, including receptor transport to the membrane, ligand binding, activation and inactivation by GRK-mediated (and other) phosphorylation. The catalog of G(s)alpha and Gbeta subunit polymorphisms that result in complex phenotypes has complemented the effort to catalog the GPCRs and their variants. The study of the genomics of GPCR accessory proteins has also provided insight into pathways of disease, such as the contributions of regulator of G protein signaling (RGS) protein to hypertension and activator of G protein signaling (AGS) proteins to the response to hypoxia. In the case of the G protein-coupled receptor kinases (GRKs), identified originally in the retinal tissues that converge on rhodopsin, proteins such as GRK4 have been identified that have been subsequently associated with hypertension. Here, we review the structure and function of GPCR and associated proteins in the context of the gene families that encode them and the genetic disorders associated with their altered function. An understanding of the pharmacogenomics of GPCR signaling provides the basis for examining the GPCRs disrupted in monogenic disease and the pharmacogenetics of a given receptor system.

A lysophosphatidic acid receptor lacking the PDZ-binding domain is constitutively active and stimulates cell proliferation

Lysophosphatidic acid (LPA) is an extracellular signaling lipid that regulates cell proliferation, survival, and motility of normal and cancer cells. These effects are produced through G protein-coupled LPA receptors, LPA(1) to LPA(5). We generated an LPA(1) mutant lacking the SerValVal sequence of the C-terminal PDZ-binding domain to examine the role of this domain in intracellular signaling and other cellular functions. B103 neuroblastoma cells expressing the mutant LPA(1) showed rapid cell proliferation and tended to form colonies under serum-free conditions. The enhanced cell proliferation of the mutant cells was inhibited by exogenous expression of the plasmids inhibiting G proteins including G(betagamma), G(alphai) and G(alphaq) or G(alpha12/13), or treatment with pertussis toxin, phosphoinositide 3-kinase (PI3K) inhibitors or a Rho inhibitor. We confirmed that the PI3K-Akt and Rho pathways were intrinsically activated in mutant cells by detecting increases in phosphorylated Akt in western blot analyses or by directly measuring Rho activity. Interestingly, expression of the mutant LPA(1) in non-tumor mouse fibroblasts induced colony formation in a clonogenic soft agar assay, indicating that oncogenic pathways were activated. Taken together, these observations suggest that the mutant LPA(1) constitutively activates the G protein signaling leading to PI3K-Akt and Rho pathways, resulting in enhanced cell proliferation.

LARG and mDia1 link Galpha12/13 to cell polarity and microtubule dynamics

Regulation of cell polarity is a process observed in all cells. During directed migration, cells orientate their microtubule cytoskeleton and the microtubule-organizing-center (MTOC), which involves integrins and downstream Cdc42 and glycogen synthase kinase-3beta activity. However, the contribution of G protein-coupled receptor signal transduction for MTOC polarity is less well understood. Here, we report that the heterotrimeric Galpha(12) and Galpha(13) proteins are necessary for MTOC polarity and microtubule dynamics based on studies using Galpha(12/13)-deficient mouse embryonic fibroblasts. Cell polarization involves the Galpha(12/13)-interacting leukemia-associated RhoGEF (LARG) and the actin-nucleating diaphanous formin mDia1. Interestingly, LARG associates with pericentrin and localizes to the MTOC and along microtubule tracks. We propose that Galpha(12/13) proteins exert essential functions linking extracellular signals to microtubule dynamics and cell polarity via RhoGEF and formin activity.

Positive feedback between Dia1, LARG, and RhoA regulates cell morphology and invasion

The RhoA-effector Dia1 controls actin-dependent processes such as cytokinesis, SRF transcriptional activity, and cell motility. Dia1 polymerizes actin through its formin homology (FH) 2 domain. Here we show that Dia1 acts upstream of RhoA independently of its effects on actin assembly. Dia1 binds to the leukemia-associated Rho-GEF (LARG) through RhoA-dependent release of Dia1 autoinhibition. The FH2 domain stimulates the guanine nucleotide exchange activity of LARG in vitro. Our results reveal that Dia1 is necessary for LPA-stimulated Rho/ROCK signaling and bleb-associated cancer cell invasion. Thus, Dia1-dependent RhoA activation constitutes a positive feedback mechanism to modulate cell behavior.

Rho GTPase/Rho kinase inhibition as a novel target for the treatment of glaucoma

Rho kinase (ROCK1 and ROCK2) is a serine/threonine kinase that serves as an important downstream effector of Rho GTPase, and plays a critical role in regulating the contractile tone of smooth muscle tissues in a calcium-independent manner. Several lines of experimental evidence indicate that modulating ROCK activity within the aqueous humor outflow pathway using selective inhibitors could achieve very significant benefits for the treatment of increased intraocular pressure in patients with glaucoma. The rationale for such an approach stems from experimental data suggesting that both ROCK and Rho GTPase inhibitors can increase aqueous humor drainage through the trabecular meshwork, leading to a decrease in intraocular pressure. In addition to their ocular hypotensive properties, inhibitors of both ROCK and Rho GTPase have been shown to enhance ocular blood flow, retinal ganglion cell survival and axon regeneration. These properties of the ROCK and Rho GTPase inhibitors indicate that targeting the Rho GTPase/ROCK pathway with selective inhibitors represents a novel therapeutic approach aimed at lowering increased intraocular pressure in glaucoma patients.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

A new approach to producing functional G alpha subunits yields the activated and deactivated structures of G alpha(12/13) proteins

The oncogenic G(12/13) subfamily of heterotrimeric G proteins transduces extracellular signals that regulate the actin cytoskeleton, cell cycle progression, and gene transcription. Previously, structural analyses of fully functional G alpha(12/13) subunits have been hindered by insufficient amounts of homogeneous, functional protein. Herein, we report that substitution of the N-terminal helix of G alpha(i1) for the corresponding region of G alpha12 or G alpha13 generated soluble chimeric subunits (G alpha(i/12) and G alpha(i/13)) that could be purified in sufficient amounts for crystallographic studies. Each chimera bound guanine nucleotides, G betagamma subunits, and effector proteins and exhibited GAP responses to p115RhoGEF and leukemia-associated RhoGEF. Like their wild-type counterparts, G alpha(i/13), but not G alpha(i/12), stimulated the activity of p115RhoGEF. Crystal structures of the G alpha(i/12) x GDP x AlF4(-) and G alpha(i/13) x GDP complexes were determined using diffraction data extending to 2.9 and 2.0 A, respectively. These structures reveal not only the native structural features of G alpha12 and G alpha13 subunits, which are expected to be important for their interactions with GPCRs and effectors such as G alpha-regulated RhoGEFs, but also novel conformational changes that are likely coupled to GTP hydrolysis in the G alpha(12/13) class of heterotrimeric G proteins.

Angiotensin II up-regulates the leukemia-associated Rho guanine nucleotide exchange factor (RhoGEF), a regulator of G protein signaling domain-containing RhoGEF, in vascular smooth muscle cells

In vascular smooth muscle, stimulation of heterotrimeric G protein-coupled receptors (GPCRs) by various contractile agonists activates intracellular signaling molecules to result in an increase in cytosolic Ca2+ and the subsequent phosphorylation of myosin light chain (MLC) by Ca2+/calmodulin-dependent MLC kinase. In addition, a portion of agonist-induced contraction is partially mediated by the Ca2+-independent activation of the small G protein RhoA and a downstream target, Rho-kinase. The activation of RhoA is controlled by several regulatory proteins, including guanine nucleotide exchange factors (GEFs). GEFs activate RhoA by promoting the release of GDP and then facilitating the binding of GTP. There are three Rho-specific GEFs (RhoGEFs) in vascular smooth muscle that contain a binding domain [regulator of G protein signaling (RGS) domain] capable of linking GPCRs to RhoA activation: PDZ-RhoGEF, leukemia-associated RhoGEF (LARG), and p115RhoGEF. We hypothesized that RGS domain-containing RhoGEFs, especially LARG, participate in linking GPCR to RhoA activation in vascular smooth muscle. We observed that angiotensin II up-regulates LARG via the AT1 receptor, and this up-regulation is signaled via the phosphatidylinositol 3-kinase pathway. Furthermore, angiotensin II treatment caused a small, but significant, increase in the component of contractile responses sensitive to Rho-kinase antagonism. These observations support the hypothesis that RhoGEFs, particularly LARG, participate in linking GPCR to RhoA activation in vascular smooth muscle.

The G protein-coupled receptors: pharmacogenetics and disease

Genetic variation in G-protein coupled receptors (GPCRs) is associated with a wide spectrum of disease phenotypes and predispositions that are of special significance because they are the targets of therapeutic agents. Each variant provides an opportunity to understand receptor function that complements a plethora of available in vitro data elucidating the pharmacology of the GPCRs. For example, discrete portions of the proximal tail of the dopamine D1 receptor have been discovered, in vitro, that may be involved in desensitization, recycling and trafficking. Similar in vitro strategies have been used to elucidate naturally occurring GPCR mutations. Inactive, over-active or constitutively active receptors have been identified by changes in ligand binding, G-protein coupling, receptor desensitization and receptor recycling. Selected examples reviewed include those disorders resulting from mutations in rhodopsin, thyrotropin, luteinizing hormone, vasopressin and angiotensin receptors. By comparison, the recurrent pharmacogenetic variants are more likely to result in an altered predisposition to complex disease in the population. These common variants may affect receptor sequence without intrinsic phenotype change or spontaneous induction of disease and yet result in significant alteration in drug efficacy. These pharmacogenetic phenomena will be reviewed with respect to a limited sampling of GPCR systems including the orexin/hypocretin system, the beta2 adrenergic receptors, the cysteinyl leukotriene receptors and the calcium-sensing receptor. These developments will be discussed with respect to strategies for drug discovery that take into account the potential for the development of drugs targeted at mutated and wild-type proteins.

Prostaglandin E2 promotes colon cancer cell growth through a Gs-axin-beta-catenin signaling axis

How cyclooxygenase-2 (COX-2) and its proinflammatory metabolite prostaglandin E2 (PGE2) enhance colon cancer progression remains poorly understood. We show that PGE2 stimulates colon cancer cell growth through its heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor, EP2, by a signaling route that involves the activation of phosphoinositide 3-kinase and the protein kinase Akt by free G protein betagamma subunits and the direct association of the G protein alphas subunit with the regulator of G protein signaling (RGS) domain of axin. This leads to the inactivation and release of glycogen synthase kinase 3beta from its complex with axin, thereby relieving the inhibitory phosphorylation of beta-catenin and activating its signaling pathway. These findings may provide a molecular framework for the future evaluation of chemopreventive strategies for colorectal cancer.