Direct evidence for two distinct G proteins coupling with thrombin receptors in human neuroblastoma SH-EP cells

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

PAR1 and PAR2 couple to overlapping and distinct sets of G proteins and linked signaling pathways to differentially regulate cell physiology

The protease-activated receptors (PAR1 and PAR2) are unusual G protein-coupled receptors that are activated by distinct serine proteases and are coexpressed in many different cell types. Limited recent evidence suggests these closely related receptors regulate different physiological outputs in the same cell, although little is known about the comparative signaling pathways used by these receptors. Here we report that PAR1 and PAR2 couple to overlapping and distinct sets of G proteins to regulate receptor-specific signaling pathways involved in cell migration. In functionally PAR-null COS-7 cells, ectopically expressed PAR1 and PAR2 both form stable complexes with G alpha(q), G alpha(11), G alpha(14), G alpha(12), and G alpha(13). It is surprising that PAR1 but not PAR2 coupled to G alpha(o), G alpha(i1), and G alpha(i2). Consistent with these observations, PAR1 and PAR2 stimulation of inositol phosphate production and RhoA activation was blocked by specific inhibitors of G(q/11) and G(12/13) signaling, respectively. Both receptors stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, but only PAR1 inhibited adenylyl cyclase activity, and pertussis toxin blocked PAR1 effects on both adenylyl cyclase and ERK1/2 signaling. Neu7 astrocytes express native PAR1 and PAR2 receptors that activate inositol phosphate, RhoA, and ERK1/2 signaling. However, only PAR1 inhibited adenylyl cyclase activity. PAR1 and PAR2 also stimulate Neu7 cell migration. PAR1 effects on ERK1/2 phosphorylation and cell migration were blocked both by pertussis toxin and by the mitogen-activated protein kinase kinase/ERK inhibitor [1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126)], whereas PAR2 effects were only blocked by U0126. These studies demonstrate that PAR1 and PAR2 physically and functionally link to overlapping and distinct profiles of G proteins to differentially regulate downstream signaling pathways and cell physiology.

Proteinases and signalling: pathophysiological and therapeutic implications via PARs and more

Proteinases like thrombin, trypsin and tissue kallikreins are now known to regulate cell signaling by cleaving and activating a novel family of G-protein-coupled proteinase-activated receptors (PARs 1-4) via exposure of a tethered receptor-triggering ligand. On their own, short synthetic PAR-selective PAR-activating peptides (PAR-APs) mimicking the tethered ligand sequences can activate PARs 1, 2 and 4 and cause physiological responses both in vitro and in vivo. Using the PAR-APs as sentinel probes in vivo, it has been found that PAR activation can affect the vascular, renal, respiratory, gastrointestinal, musculoskeletal and nervous systems (both central and peripheral nervous system) and can promote cancer metastasis and invasion. In general, responses triggered by PARs 1, 2 and 4 are in keeping with an innate immune inflammatory response, ranging from vasodilatation to intestinal inflammation, increased cytokine production and increased or decreased nociception. Further, PARs have been implicated in a number of disease states, including cancer and inflammation of the cardiovascular, respiratory, musculoskeletal, gastrointestinal and nervous systems. In addition to activating PARs, proteinases can cause hormone-like effects by other signalling mechanisms, like growth factor receptor activation, that may be as important as the activation of PARs. We, therefore, propose that the PARs themselves, their activating serine proteinases and their associated signalling pathways can be considered as attractive targets for therapeutic drug development. Thus, proteinases in general must now be considered as 'hormone-like' messengers that can signal either via PARs or other mechanisms.

Astrocytic control of synaptic NMDA receptors

Astrocytes express a wide range of G-protein coupled receptors that trigger release of intracellular Ca2+, including P2Y, bradykinin and protease activated receptors (PARs). By using the highly sensitive sniffer-patch technique, we demonstrate that the activation of P2Y receptors, bradykinin receptors and protease activated receptors all stimulate glutamate release from cultured or acutely dissociated astrocytes. Of these receptors, we have utilized PAR1 as a model system because of favourable pharmacological and molecular tools, its prominent expression in astrocytes and its high relevance to neuropathological processes. Astrocytic PAR1-mediated glutamate release in vitro is Ca2+ dependent and activates NMDA receptors on adjacent neurones in culture. Activation of astrocytic PAR1 in hippocampal slices induces an APV-sensitive inward current in CA1 neurones and causes APV-sensitive neuronal depolarization in CA1 neurones, consistent with release of glutamate from astrocytes. PAR1 activation enhances the NMDA receptor-mediated component of synaptic miniature EPSCs, evoked EPSCs and evoked EPSPs in a Mg2+-dependent manner, which may reflect spine head depolarization and consequent reduction of NMDA receptor Mg2+ block during subsequent synaptic currents. The release of glutamate from astrocytes following PAR1 activation may also lead to glutamate occupancy of some perisynaptic NMDA receptors, which pass current following relief of tonic Mg2+ block during synaptic depolarization. These results suggest that astrocytic G-protein coupled receptors that increase intracellular Ca2+ can tune synaptic NMDA receptor responses.

Role of protease-activated receptors in the vascular system

Thrombin is one of the key molecules involved in the development of vascular diseases. Thrombin does not only serve as a coagulation factor, but it also exerts cellular effects by activating protease (proteinase)-activated receptors (PARs), a family of seven-transmembrane G protein-coupled receptors. This study focused on the role of PARs in the vascular system. Among the four members so far identified, PAR-1 and PAR-2 were found to play an important role in the vascular system, while the functional roles of PAR-3 and PAR-4 appear to be mostly limited to platelets. The endothelial cells play a primary role in mediating the vascular effects of PARs under physiological conditions, while PARs of the smooth muscle cells can be induced under pathological conditions, and therefore play a more pathophysiological role. PAR-1 and PAR-2 mediate various vascular effects including regulation of vascular tone, proliferation and hypertrophy of smooth muscle and angiogenesis. Since proteases are activated under pathological conditions such as hemorrhage, tissue damage, and inflammation, PARs are suggested to play a critical role in the development of functional and structural abnormality in the vascular lesion. Understanding the functional role of PARs in the vascular system can thus help in the development of new strategies for the prevention and therapy of vascular diseases.

Alpha-thrombin-mediated phosphatidylinositol 3-kinase activation through release of Gbetagamma dimers from Galphaq and Galphai2

Chinese hamster embryonic fibroblasts (IIC9 cells) express the Galpha subunits Galphas, Galphai2, Galphai3, Galphao, Galpha(q/11), and Galpha13. Consistent with reports in other cell types, alpha-thrombin stimulates a subset of the expressed G proteins in IIC9 cells, namely Gi2, G13, and Gq as measured by an in vitro membrane [35S]guanosine 5'-O-(3-thio)triphosphate binding assay. Using specific Galpha peptides, which block coupling of G-protein receptors to selective G proteins, as well as dominant negative xanthine nucleotide-binding Galpha mutants, we show that activation of the phosphatidylinositol 3-kinase/Akt pathway is dependent on Gq and Gi2. To examine the role of the two G proteins, we examined the events upstream of PI 3-kinase. The activation of the PI 3-kinase/Akt pathway by alpha-thrombin in IIC9 cells is blocked by the expression of dominant negative Ras and beta-arrestin1 (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053, and Goel, R., Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2002) J. Biol. Chem. 277, 18640-18648), indicating a role for Ras and beta-arrestin1. Interestingly, inhibition of Gi2 and Gq activation blocks Ras activation and beta-arrestin1 membrane translocation, respectively. Furthermore, expression of the Gbetagamma sequestrant, alpha-transducin, inhibits both Ras activation and membrane translocation of beta-arrestin1, suggesting that Gbetagamma dimers from Galphai2 and Galphaq activate different effectors to coordinately regulate the PI 3-kinase/Akt pathway.

Common Signaling Pathways Link Activation of Murine PAR-1, LPA, and S1P Receptors to Proliferation of Astrocytes

Receptors for the serine protease thrombin and for lysophospholipids are coupled to G proteins and control a wide range of cellular functions, including mitogenesis. Activators of these receptors are present in blood, and can enter the brain during central nervous system (CNS) injury. Reactive astrogliosis, a prominent component of CNS injury with potentially harmful consequences, may involve proliferation of astrocytes. In this study, we have examined the expression and activation of protease activated receptors (PARs), lysophosphatidic acid (LPA) receptors, and sphingosine-1-phosphate (S1P) receptors on murine astrocytes. We show that activation of these three receptor classes can lead to astrogliosis in vivo and proliferation of astrocytes in vitro. Cultured murine cortical astrocytes express mRNA for multiple receptor subtypes of PAR (PAR-1-4), LPA (LPA-1-3) and S1P (S1P-1, -3, -4, and -5) receptors. Comparison of the intracellular signaling pathways of glial PAR-1, LPA, and S1P receptors indicates that each receptor class activates multiple downstream signaling pathways, including Gq/11-directed inositol lipid/Ca2+ signaling, Gi/o activation of mitogen-activated protein kinases (MAPK) (extracellular signal-regulated kinase 1/2 and stress activated protein kinase/c-jun N-terminal kinase, but not p38), and activation of Rho pathways. Furthermore, activation of these different receptor classes can differentially regulate two transcription factor pathways, serum response element and nuclear factor of activated T cells. Blockade of Gi/o signaling with pertussis toxin, MAPK activation with 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene (U0126), or Rho kinase signaling with R-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane carboxamide (Y27632) can markedly reduce the proliferative response of glial cells to PAR-1, LPA, or S1P receptor activation, suggesting that each of these pathways is important in coupling of receptor activation to glial proliferation.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

A novel role for Gq alpha in alpha-thrombin-mediated mitogenic signalling pathways

alpha-Thrombin activates several G-proteins including members of the Gq, Gi, and G12/13 families, although the physiological importance of these proteins is still not completely understood. We specifically investigated the role of Gq alpha in modulating alpha-thrombin-induced mitogenesis. In Gqa1 cells, a stable cell line expressing reduced amounts of Gq alpha, concentrations of alpha-thrombin (1 NIH unit/ml), which induce cell cycle reentry and progression into S phase in wild-type IIC9 cells, do not stimulate phosphatidylinositol (PI) hydrolysis, the rapid early phase of ERK activity, and transit through G1 into S phase as quantified by cyclin-dependent kinase (CDK)4-cyclin D activity and [3H]thymidine incorporation. Interestingly, high concentrations of alpha-thrombin restore these activities and cell cycle progression into S phase. While, it is well documented that alpha-thrombin-induced sustained ERK activity mediates important responses for transit through G1 into S phase, the importance of the rapid, Gq-dependent phase as a prerequisite for alpha-thrombin-mediated mitogenesis has not been appreciated.

alpha-Thrombin induces rapid and sustained Akt phosphorylation by beta-arrestin1-dependent and -independent mechanisms, and only the sustained Akt phosphorylation is essential for G1 phase progression

In Chinese hamster embryonic fibroblasts (IIC9 cells) alpha-thrombin activates the MAPK(ERK) and phosphatidylinositol 3-OH-kinase (PI 3-kinase)/Akt pathways, and both are essential for progression through the G(1) phase of the cell cycle. We investigated in IIC9 cells, the role of beta-arrestin1 in alpha-thrombin signaling to these pathways. alpha-Thrombin stimulates rapid and sustained PI 3-kinase and Akt activities. Expression of a dominant negative beta-arrestin1 (beta-arrestin1(V53D)) inhibits rapid but not sustained PI 3-kinase and Akt activities. Surprisingly, expression of beta-arrestin1(V53D) does not block activation of the MAPK(ERK) pathway. PI 3-kinase and Akt activities are also inhibited by expression of a beta-arrestin1 mutant, which impairs binding to c-Src (beta-arrestin1(P91G-P121E)), indicating the involvement of c-Src in the rapid stimulation of the PI 3-kinase/Akt pathway. Consistent with these results, PP1, a selective inhibitor of c-Src family kinases, prevents alpha-thrombin-stimulated Akt phosphorylation. Expression of beta- arrestin1(V53D) does not prevent G(1) progression, as its expression has no effect on [(3)H]thymidine incorporation into DNA. In agreement with the ineffectiveness of beta-arrestin1(V53D) to block G(1) progression, cyclin D1 protein amounts and CDK4-cyclin D1 activity is unaffected by expression of beta-arrestin1(V53D). Thus in IIC9 cells, alpha-thrombin activates rapid beta-arrestin1-dependent and sustained beta-arrestin1-independent Akt activity, suggesting that two mechanisms are involved. Furthermore, although blocking the beta-arrestin1-independent PI 3-kinase/Akt pathway prevents G(1) progression, inhibition of the beta-arrestin1-dependent pathway does not, indicating different roles for the rapid and sustained activities.

Phospholipase C-delta1 is activated by capacitative calcium entry that follows phospholipase C-beta activation upon bradykinin stimulation

To characterize the regulatory mechanism of phospholipase C-delta1 (PLC-delta1) in the bradykinin (BK) receptor-mediated signaling pathway, we used a clone of PC12 cells, which stably overexpress PLC-delta1 (PC12-D1). Stimulation with BK induced a significantly higher Ca(2+) elevation and inositol 1,4,5-trisphosphate (IP(3)) production with a much lower half-maximal effective concentration (EC(50)) of BK in PC12-D1 cells than in wild type (PC12-W) or vector-transfected (PC12-V) cells. However, BK-induced intracellular Ca(2+) release and IP(3) generation was similar between PC12-V and PC12-D1 cells in the absence of extracellular Ca(2+), suggesting that the availability of extracellular Ca(2+) is essential to the activation of PLC-delta1. When PC12-D1 cells were treated with agents that induce Ca(2+) influx, more IP(3) was produced, suggesting that the Ca(2+) entry induces IP(3) production in PC12-D1 cells. Furthermore, the additional IP(3) production after BK-induced capacitative calcium entry was detected in PC12-D1 cells, suggesting that PLC-delta1 is mainly activated by capacitative calcium entry. When cells were stimulated with BK in the presence of extracellular Ca(2+), [(3)H]norepinephrine secretion was much greater from PC12-D1 cells than from PC12-V cells. Our results suggest that PLC-delta1 is activated by capacitative calcium entry following the activation of PLC-beta, additively inducing IP(3) production and Ca(2+) rise in BK-stimulated PC12 cells.

Phospholipase C-delta1 and oxytocin receptor signalling: evidence of its role as an effector

Although the oxytocin receptor modulates intracellular Ca2+ ion levels in myometrium, the identities of signal molecules have not been clearly clarified. Our previous studies on oxytocin receptor signalling demonstrated that 80 kDa Ghalpha is a signal mediator [Baek, Kwon, Lee, Kim, Muralidhar and Im (1996) Biochem. J. 315, 739-744]. To elucidate the effector in the oxytocin receptor signalling pathway, we evaluated the oxytocin-mediated activation of phospholipase C (PLC) by using solubilized membranes from human myometrium and a three-component preparation containing the oxytocin receptor-Ghalpha-PLC-delta1 complex. PLC-delta1 activity in the three-component preparation, as well as PLC activity in solubilized membranes, was increased by oxytocin in the presence of Ca2+ and activated Ghalpha (GTP-bound Ghalpha). Furthermore the stimulated PLC-delta1 activity resulting from activation of Ghalpha via the oxytocin receptor was significantly attenuated by the selective oxytocin antagonist desGly-NH2d(CH2)5[Tyr(Me)2,Thr4]ornithine vasotocin or GDP. Consistent with these observations, co-immunoprecipitation and co-immunoadsorption of PLC-delta1 in the three-component preparation by anti-Gh7alpha antibody resulted in the PLC-delta1 being tightly coupled to activated Ghalpha on stimulation of the oxytocin receptor. These results indicate that PLC-delta1 is the effector for Ghalpha-mediated oxytocin receptor signalling.