scholarly journals Decoding Gq Signaling in Platelets

Blood ◽  
2018 ◽  
Vol 132 (Supplement 1) ◽  
pp. 866-866
Author(s):  
Xi Chen ◽  
Shuchi Gupta ◽  
Matthew Cooper ◽  
Daniel Dehelian ◽  
Lawrence F. Brass ◽  
...  
Keyword(s):  
G Protein ◽  
G Proteins ◽  
Platelet Activation ◽  
Conflicts Of Interest ◽  
Shape Change ◽  
Rgs Proteins ◽  
Functional Responses ◽  
Binding Interface ◽  
Computational Alanine Scanning ◽  
Binding Interfaces

Abstract Most platelet agonists work through G protein coupled receptors (GPCRs), activating pathways that involve members of the Gq, Gi, and G12 families of heterotrimeric G proteins. Gq is critical for most functional responses during platelet activation. A defect in Gq expression in patients leads to impaired platelet secretion and aggregation, which is associated with bleeding diathesis. We have previously shown that the Regulators of G protein Signaling (RGS) proteins function as negative regulators for platelet activation by limiting G protein-dependent signaling, but much remains to be learned about the mechanisms by which G proteins can be regulated during platelet activation. Here we took advantage of an RGS-insensitive Gq(G188S) mutant mouse line to decipher the regulatory complex of Gq-dependent signaling in platelets. The G188S mutation disrupts RGS18 and Gq interaction in a way similar to what we have observed in an analogous Gi2(G184S) mutation. However, in contrast to increased platelet activation in Gi2(G184S) expressing platelets, G188S expressing platelets have decreased activation in response to thrombin, TxA2 and ADP, but not to collagen. The hemostatic thrombi formed in G188S mice are significantly reduced with constant embolization as compared with WT controls. Underlying these changes is a decrease in Gq-dependent signaling events, whereas shape change, which is G13-mediated, is unaffected. Our data further reveal that PLCβ3 and GRK2 complex with activated Gq, but not to Gi2. The G188S mutation in Gq not only prevents the binding of RGS proteins to Gq but also impairs the Gq/PLCβ3 interaction, though it does not affect the GRK2 binding to Gq. Instead, there an increase of GRK2 binding to Gq. Finally, the structural analysis shows that PLCβ3 shares a large overlapping binding area with RGS18 to Gq. Computational alanine scanning to predict binding interfaces indicates that the G188S mutation resides in a region important for the binding of both PLC and RGS. The PLC binding interface also overlaps with that of GRK2, which suggests that when PLC is in complex with Gq, it precludes the binding of both RGS proteins and GRK2. In the G188S mutant, however, PLC binding is reduced, which may reduce competition and allow more GRK2 to bind. The binding interfaces of RGS and GRK2 are not predicted to be overlapping, raising the possibility that the two proteins could bind simultaneously to activated WT Gq. Collectively, these observations 1) indicate that the feedback mechanism of Gq inactivation in platelets is different from that of Gi2, 2) reveal, for the first time, that a Gq/PLCβ/RGS/GRK2 signaling node is present in platelets and plays an important role in Gq-dependent signaling during platelet activation, and 3) show that the G188S mutation affect Gq-mediated signaling by disrupting Gq/PLCβ/RGS interaction. Disclosures No relevant conflicts of interest to declare.

scholarly journals Probing the mutational landscape of regulators of G protein signaling proteins in cancer

2020 ◽  
Vol 13 (617) ◽  
pp. eaax8620 ◽  
Author(s):  
Vincent DiGiacomo ◽  
Marcin Maziarz ◽  
Alex Luebbers ◽  
Jillian M. Norris ◽  
Pandu Laksono ◽  
...  
Keyword(s):  
G Protein ◽  
G Proteins ◽  
Mammalian Cells ◽  
G Protein Signaling ◽  
Rgs Proteins ◽  
Protein Signaling ◽  
Loss Of Function ◽  
Gpcr Signaling ◽  
Mutational Landscape ◽  
Binding Interface

The advent of deep-sequencing techniques has revealed that mutations in G protein–coupled receptor (GPCR) signaling pathways in cancer are more prominent than was previously appreciated. An emergent theme is that cancer-associated mutations tend to cause enhanced GPCR pathway activation to favor oncogenicity. Regulators of G protein signaling (RGS) proteins are critical modulators of GPCR signaling that dampen the activity of heterotrimeric G proteins through their GTPase-accelerating protein (GAP) activity, which is conferred by a conserved domain dubbed the “RGS-box.” Here, we developed an experimental pipeline to systematically assess the mutational landscape of RGS GAPs in cancer. A pan-cancer bioinformatics analysis of the 20 RGS domains with GAP activity revealed hundreds of low-frequency mutations spread throughout the conserved RGS domain structure with a slight enrichment at positions that interface with G proteins. We empirically tested multiple mutations representing all RGS GAP subfamilies and sampling both G protein interface and noninterface positions with a scalable, yeast-based assay. Last, a subset of mutants was validated using G protein activity biosensors in mammalian cells. Our findings reveal that a sizable fraction of RGS protein mutations leads to a loss of function through various mechanisms, including disruption of the G protein–binding interface, loss of protein stability, or allosteric effects on G protein coupling. Moreover, our results also validate a scalable pipeline for the rapid characterization of cancer-associated mutations in RGS proteins.

Effects of Enhanced P2X1 Receptor Ca2+ Influx on Functional Responses in Human Platelets

2002 ◽  
Vol 88 (09) ◽  
pp. 495-502 ◽  
Author(s):  
Michael Rolf ◽  
Martyn Mahaut-Smith
Keyword(s):  
Morphological Change ◽  
G Protein ◽  
Platelet Activation ◽  
Shape Change ◽  
Human Platelets ◽  
Functional Responses ◽  
P2x1 Receptor ◽  
G Protein Coupled

SummaryG-protein-coupled P2Y1 and P2Y12 receptors play key roles in platelet activation, however the importance of ionotropic P2X1 receptors remains unclear. Platelet P2X1 responses are highly labile in vitro, but were greatly enhanced by increasing [Ca2+]o in the range 1–10 mM. The P2X1 agonist α,β-MeATP stimulated a shape change which saturated at peak [Ca2+]i of ≥ 400 nM, without evidence for aggregation. The maximal P2X1-evoked transmission decrease was 82% of that obtained via P2Y1 receptors. α., β-MeATP caused a disc to sphere transformation in virtually all platelets, but lacked the long processes produced by ADP. Following block of P2Y1 receptors with A3P5PS, co-stimulation with α., β-MeATP and ADP failed to induce aggregation despite the generation of peak [Ca2+]i responses similar to those stimulated via P2Y1 receptors. Therefore early, transient Ca2+ influx via P2X1 receptors can contribute to platelet activation by stimulating a significant morphological change, but does not readily synergise with P2Y12 receptors to support aggregation.

scholarly journals RGS10 and RGS18 differentially limit platelet activation, promote platelet production, and prolong platelet survival

Blood ◽  
2020 ◽  
Vol 136 (15) ◽  
pp. 1773-1782 ◽  
Author(s):  
Daniel DeHelian ◽  
Shuchi Gupta ◽  
Jie Wu ◽  
Chelsea Thorsheim ◽  
Brian Estevez ◽  
...  
Keyword(s):  
Platelet Count ◽  
G Protein ◽  
Platelet Activation ◽  
Platelet Reactivity ◽  
Rgs Proteins ◽  
Platelet Recovery ◽  
Platelet Production ◽  
Platelet Survival

Abstract G protein–coupled receptors are critical mediators of platelet activation whose signaling can be modulated by members of the regulator of G protein signaling (RGS) family. The 2 most abundant RGS proteins in human and mouse platelets are RGS10 and RGS18. While each has been studied individually, critical questions remain about the overall impact of this mode of regulation in platelets. Here, we report that mice missing both proteins show reduced platelet survival and a 40% decrease in platelet count that can be partially reversed with aspirin and a P2Y12 antagonist. Their platelets have increased basal (TREM)-like transcript-1 expression, a leftward shift in the dose/response for a thrombin receptor–activating peptide, an increased maximum response to adenosine 5′-diphosphate and TxA2, and a greatly exaggerated response to penetrating injuries in vivo. Neither of the individual knockouts displays this constellation of findings. RGS10−/− platelets have an enhanced response to agonists in vitro, but platelet count and survival are normal. RGS18−/− mice have a 15% reduction in platelet count that is not affected by antiplatelet agents, nearly normal responses to platelet agonists, and normal platelet survival. Megakaryocyte number and ploidy are normal in all 3 mouse lines, but platelet recovery from severe acute thrombocytopenia is slower in RGS18−/− and RGS10−/−18−/− mice. Collectively, these results show that RGS10 and RGS18 have complementary roles in platelets. Removing both at the same time discloses the extent to which this regulatory mechanism normally controls platelet reactivity in vivo, modulates the hemostatic response to injury, promotes platelet production, and prolongs platelet survival.

Genetic and Pharmacologic Evidence Shows That Cdc42 GTPase Plays a Central Role in the Regulation of Both GPVI- and Non-GPVI-Dependent Activation of Platelets.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 2997-2997 ◽  
Author(s):  
Huzoor Akbar ◽  
Kevin Funk ◽  
Mark Berryman ◽  
Joshua Raines ◽  
Rehana Perveen ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
G Protein ◽  
Conflicts Of Interest ◽  
Therapeutic Potential ◽  
Shape Change ◽  
Specific Inhibitor ◽  
Small Gtpases ◽  
Clot Retraction ◽  
Coated Surfaces ◽  
Induced Aggregation

Abstract Abstract 2997 Poster Board II-975 Cdc42 and Rac1, members of the Rho family of small GTPases, play critical roles in reorganization of actin cytoskeleton in platelets. Previously we have shown that Rac1 GTPase is involved in regulation of platelet secretion and aggregation by diverse signaling pathways (J Thromb Haemost 2007; 5: 1747-55). Others have reported that Rac1 is essential for GPVI-, but not G protein-dependent platelet aggregation (Pflugers Arch. 2009; 457:1173-85). Cdc42 was recently reported to be involved in collagen, but not collagen related peptide (CRP), a GPVI specific agonist, induced platelet aggregation (Platelets 2008; 19: 199-210). In this study we have investigated the role of Cdc42 in regulation of platelet function by using complementary approaches of (a) mouse gene targeting of Cdc42, and (b) specific inhibition of Cdc42 activity by a newly identified chemical inhibitor of Cdc42, CASIN (Cdc42 activity-specific inhibitor). Platelets from Cdc42−/− mice exhibited a complete lack of filopodia formation and spreading on collagen coated surfaces. Threshold concentrations of collagen, CRP or thrombin failed to induce shape change or aggregation in platelets from Cdc42−/− mice compared with induction of shape change and maximal aggregation in platelets from Cdc42+/+ mice. Platelets from Cdc42−/− mice, as compared to Cdc42+/+ mice, exhibited a significant inhibition of CRP- or thrombin-induced secretion of ATP and release of P-selectin from the dense- and alpha-granules respectively. Increasing concentrations of the agonists only partially corrected the defective aggregation and secretion responses in Cdc42−/− platelets. These data provide the genetic evidence that Cdc42 is required for collagen, CRP and thrombin mediated platelet signaling and activation. Treatment of platelets with CASIN, but not a pharmacologically inactive analog, blocked collagen induced activation of Cdc42 without detectably affecting the Rac1 activity. Human platelets pre-incubated with CASIN (10 micro-M) exhibited a complete lack of filopodia formation and spreading on collagen coated surfaces. Further, treatment of platelets with CASIN (1-10 micro-M) inhibited: (a) aggregation induced by collagen, CRP, thrombin, ADP or U46619; (b) release of P-selectin and secretion of ATP induced by U46619; and (c) collagen induced phosphorylation of Akt. Addition of CASIN to platelets also blocked collagen or CRP induced aggregation in aspirinated platelets in the presence of apyrase. In other experiments, addition of CASIN to citrated platelet-rich plasma inhibited thrombin induced clot retraction. Significantly, removal of CASIN from the platelet samples by washing reversed inhibition of aggregation as well as clot retraction, reflecting a reversible suppression of Cdc42 activity by CASIN. Administration of CASIN into C57Bl/6 mice inhibited ex vivo platelet aggregation induced by collagen or ADP as well as significantly prolonged tail bleeding times. These data suggest that: (a) Cdc42 plays an essential, non-redundant role in platelet filopodia formation, spreading, secretion, aggregation and clot retraction; (b) Cdc42 is involved in GPVI, non-GPVI- and G protein-dependent signaling in platelets; (c) the pharmacologic inhibitor CASIN is capable of specifically and reversibly inhibiting Cdc42 activity in platelets, mimicking Cdc42 genetic knockout in mice. Altogether, our studies strongly implicate Cdc42 as a novel anti-platelet target, and present evidence that the Cdc42 specific small molecule inhibitor, CASIN, may have therapeutic potential. Disclosures: No relevant conflicts of interest to declare.

Gβ1 a Component Of The Heterotrimeric G Protein Is a New Protein Phosphatase 1c Interacting Protein That Regulates Platelet Activation

Blood ◽  
2013 ◽  
Vol 122 (21) ◽  
pp. 3508-3508
Author(s):  
Subhashree Pradhan ◽  
Tanvir Khatlani ◽  
Satya P. Kunapuli ◽  
K. Vinod Vijayan
Keyword(s):  
Signal Transduction ◽  
G Protein ◽  
Platelet Activation ◽  
Protein Interactions ◽  
Protein Phosphatase ◽  
Conflicts Of Interest ◽  
Thrombus Formation ◽  
Adaptor Protein ◽  
Gpcr Signaling

Abstract Platelet activation at the site of injury is tied to signal transduction events that are mediated by protein kinases and phosphatases. Reversible tyrosine, serine/threonine (Ser/Thr) phosphorylation-dependent assembly and/or disassembly of effector (cytoskeletal, signaling and adaptor) protein complexes propagate signaling downstream of G protein coupled receptors (GPCRs). Compared to kinases, the contribution of Ser/Thr phosphatases and its effectors in GPCR signaling studies is not well explored. Our previous studies had revealed that the catalytic subunit of protein phosphatase 1γ (PP1cγ) support GPCR signaling and thrombus formation. Since cell signaling networks are dependent on protein-protein interactions, we sought to identify the potential effectors of PP1cγ. We employed yeast two-hybrid interaction studies with the full length PP1cγ fused to GAL4 activating domain as bait and screened human bone marrow library. A novel interaction of PP1cγ with a protein called Gβ1 (GNB1) was identified. Gβ1 is a component of the heterotrimeric G proteins like the Gα and couple to GPCR. However, unlike Gα subunits, Gβ1 is unexplored in platelets. Co-immunoprecipitation (co-IP) studies validated PP1cγ-Gβ1 interaction in 293 cells expressing PP1cγ-HA and Gβ1-FLAG. Importantly, Gβ1 interacted with all the PP1c isoforms, suggesting that Gβ1 could target all PP1c isoforms to the GPCR complex. Purified PP1c bound to recombinant Gβ1-GST protein but not to GST protein, indicating that the in vitro interaction of PP1c with Gβ1 was direct and independent of Gα and Gγ subunits. A small molecule inhibitor of G protein βγ, gallein decreased thrombin-induced human platelet aggregation and adhesion to immobilized fibrinogen. There is a paucity of Gβ1-/- platelets because Gβ1-/- mice die within 2 days of birth due to microencephaly. siRNA mediated depletion of Gβ1 in murine megakaryocytes reduced PAR4-activating peptide induced soluble fibrinogen binding to αIIbβ3. These studies suggest a functional role for Gβ1 in GPCR signaling. PP1c co-immunoprecipitated with Gβ1 in resting platelets and agonist (thrombin and ADP) treatment under non-stirring conditions induced dissociation of PP1c from Gβ1. These studies indicate that Gβ1-PP1c complex in platelets is responsive to agonist. Furthermore, PP1c and Gβ1 associated with P2Y12 receptor in resting but not agonist activated platelets in a co-IP assay, suggesting a role for this complex in G protein signaling. Finally, agonist induced dissociation of PP1c from Gβ1 correlated with the association of PP1c with the downstream GPCR effector phospholipase C β3 (PLCβ3) with a concomitant dephosphorylation of PLCβ3 at Ser1105. Since previous studies have revealed that PLCβ3 activity is inhibited by Ser1105 phosphorylation, our observation suggest that agonist-induced association of PP1c with PLCβ3 facilitates dephosphorylation and activation of PLCβ3. These studies highlight a coupling of GPCR signaling with the phosphatase driven signal transduction during platelet activation. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Regulators of G Protein Signaling Attenuate the G Protein–mediated Inhibition of N-Type Ca Channels

1999 ◽  
Vol 113 (1) ◽  
pp. 97-110 ◽  
Author(s):  
Karim Melliti ◽  
Ulises Meza ◽  
Rory Fisher ◽  
Brett Adams
Keyword(s):  
G Protein ◽  
G Proteins ◽  
G Protein Signaling ◽  
Rgs Proteins ◽  
Ca Channel ◽  
Protein Signaling ◽  
Ca Channels ◽  
Channel Inhibition ◽  
Kinetics Of ◽  
In Cells

Regulators of G protein signaling (RGS) proteins bind to the α subunits of certain heterotrimeric G proteins and greatly enhance their rate of GTP hydrolysis, thereby determining the time course of interactions among Gα, Gβγ, and their effectors. Voltage-gated N-type Ca channels mediate neurosecretion, and these Ca channels are powerfully inhibited by G proteins. To determine whether RGS proteins could influence Ca channel function, we recorded the activity of N-type Ca channels coexpressed in human embryonic kidney (HEK293) cells with G protein–coupled muscarinic (m2) receptors and various RGS proteins. Coexpression of full-length RGS3T, RGS3, or RGS8 significantly attenuated the magnitude of receptor-mediated Ca channel inhibition. In control cells expressing α1B, α2, and β3 Ca channel subunits and m2 receptors, carbachol (1 μM) inhibited whole-cell currents by ∼80% compared with only ∼55% inhibition in cells also expressing exogenous RGS protein. A similar effect was produced by expression of the conserved core domain of RGS8. The attenuation of Ca current inhibition resulted primarily from a shift in the steady state dose–response relationship to higher agonist concentrations, with the EC50 for carbachol inhibition being ∼18 nM in control cells vs. ∼150 nM in RGS-expressing cells. The kinetics of Ca channel inhibition were also modified by RGS. Thus, in cells expressing RGS3T, the decay of prepulse facilitation was slower, and recovery of Ca channels from inhibition after agonist removal was faster than in control cells. The effects of RGS proteins on Ca channel modulation can be explained by their ability to act as GTPase-accelerating proteins for some Gα subunits. These results suggest that RGS proteins may play important roles in shaping the magnitude and kinetics of physiological events, such as neurosecretion, that involve G protein–modulated Ca channels.

scholarly journals ASK1, a Novel Regulator of Thrombosis, Is Activated Downstream of G-Protein Coupledreceptors in Human Platelets

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 4163-4163
Author(s):  
Randall Derstine ◽  
Meghna Ulhas Naik ◽  
Ramya Turaga ◽  
Ulhas P Naik
Keyword(s):  
Map Kinase ◽  
G Protein ◽  
Platelet Activation ◽  
Molecular Mechanisms ◽  
Conflicts Of Interest ◽  
Human Platelets ◽  
P2y12 Receptor ◽  
Kinase Signaling ◽  
Dense Granules ◽  
Map Kinase Signaling

Abstract In the event of vascular injury, platelets rapidly adhere to sub-endothelial matrix proteins such as collagen and Von Willebrand factor and activate to form a stable hemostatic platelet plug. Defects in the molecular mechanisms dictating platelet plug formation are responsible for numerous thrombotic disorders. Elucidating the signaling pathways and molecular mechanisms of platelet activation is paramount to the development of safer and more effective anti-thrombotic drugs. While it is known that MAP-Kinase signaling participates in platelet activation, it is unknown how MAP-Kinase signaling specifically mediates platelet activation. Our laboratory has identified the presence and activation of a MAP-Kinase Kinase Kinase known as Apoptosis Signal Regulating Kinase 1 (ASK1). We have demonstrated using an ASK1 knockout mouse model that ablation of ASK1 leads to a significantly increased (p = .0003) time of vessel occlusion associated with unstable thrombus formation following a carotid artery injury induced by 10% FeCl3. Furthermore, ASK1 knockout mice display protection from pulmonary thromboembolism induced by an intravenous injection of collagen and epinephrine. In order to determine the kinetics of ASK1 activation by physiological agonists, washed human platelets (4 x 108 platelets/mL) were treated with 0.1 U/mL of thrombin for 30”, 1’, 3’, 5’, and 8’. Robust activation of ASK1 by thrombin occurred as early as 30 seconds up until 5 min, after which ASK1 activation decreased sharply. Platelets treated with 100 µM of PAR1 (SFLLRN) or PAR4 (AYPGKF) peptides resulted in strong ASK1 activation, suggesting that both the PAR1 and PAR4 receptors lead to ASK1 activation. Inhibition of Src family kinases by PP2 or PI3K by wortmannin or Rho kinase by Y-27632 had no effect on thrombin-induced ASK1 activation. However, inhibition of PLC-β2, a mediator of platelet activation downstream of the PAR1/4 receptors, strongly inhibited ASK1 activation by thrombin. We next determined whether TxA2 generation was responsible for ASK1 activation by thrombin. Washed platelets were pre-treated with 1 mM aspirin to block TxA2 generation, followed by treatment with 0.1 U/mL of thrombin. It was found that blocking TxA2 generation eliminated ASK1 activation by thrombin at 30” and 1’, but not at a later time point, suggesting there may be an additional pathway contributing to ASK1 activation. The observation that TxA2 generation contributes to ASK1 activation by thrombin seemed to correlate with the finding that treatment of platelets with 1 µM of the TxA2 mimetic U46619, which activates the TP-α receptor, could also activate ASK1. We also determined whether ADP released from dense granules, which would activate the P2Y1 and P2Y12 receptors, leads to ASK1 activation. To test this, washed platelets were pre-treated with 1 U/mL of apyrase to hydrolyze secreted ADP. It was found that apyrase treatment completely eliminates ASK1 activation by thrombin, suggesting a strong dependency of thrombin-induced ASK1 activation on ADP release from dense granules. To further investigate this possibility, washed platelets were pre-treated with 50 µM of the P2Y1 antagonist MRS2179 or P2Y12 antagonist 2-MeSAMP, followed by treatment with 0.1 U/mL of thrombin. Antagonism of the P2Y12 receptor and not P2Y1 receptor severely diminished ASK1 activation by thrombin. This indicates that ASK1 activation by thrombin is also dependent on ADP released from dense granules and subsequent activation of the P2Y12 receptor. Surprisingly, collagen, a strong activator of platelets, was unable to activate ASK1 in washed platelets at a concentration of 2 µg/mL. Similarly, 2 µM epinephrine treatment also had no effect. However, when washed platelets were treated with 2 µg/mL collagen and 2 µM epinephrine together, a strong ASK1 activation was observed (p=.0012). This suggests the existence of a novel mechanism for ASK1 activation by simultaneous stimulation of the collagen receptors GPVI/α2β1 and epinephrine receptor α2A. The finding that ASK1 activation occurs downstream of TP-α, P2Y12, and possibly α2A receptors highlights the importance of ASK1 in regulation of these G-Protein Coupled Receptors in platelet activation. In conclusion, our data indicates ASK1 to be a key mediator in platelet activation and represents a novel target for anti-thrombotic drug therapy. Disclosures No relevant conflicts of interest to declare.

Coagulation Factor XIIa Activates Platelets and Is the Physiological Agonist of Protease-Activated Receptor 3.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 770-770 ◽  
Author(s):  
Yingying Mao ◽  
Todd M Getz ◽  
Jianguo Jin ◽  
Satya P. Kunapuli
Keyword(s):  
Intracellular Calcium ◽  
Platelet Activation ◽  
Conflicts Of Interest ◽  
Thrombus Formation ◽  
Shape Change ◽  
Coagulation Factor ◽  
Calcium Mobilization ◽  
Dependent Manner ◽  
Concentration Dependent Manner ◽  
Factor Xiia

Abstract Abstract 770 Protease-activated receptors (PARs) are G-protein coupled receptors that are activated by proteases. Thrombin is the major agonist for PAR1 and PAR4, whereas tryptase and coagulation factor Xa are the agonists for PAR2. In addition to these major agonists, PARs can be activated by other coagulation proteases. The physiological agonist of PAR3 has not been identified to date; as a result, the molecular pharmacology and physiology of PAR3 remain poorly understood. The purpose of this study is to identify a physiological agonist to PAR3. We used PAR4 null murine platelets, which are known to express only PAR3. In this study, we tested the effect of several coagulation proteases and found that only coagulation factor XIIa (FXIIa) activated PAR4-/- murine platelets, in a concentration-dependent manner. FXIIa caused murine platelet shape change, aggregation, secretion and thromboxane A2 generation and this activation was abolished by C1 esterase inhibitor, a FXIIa inhibitor. FXIIa-induced murine platelet activation was completely abolished by BMS200261, a PAR1 antagonist, without affecting the catalytic activity of FXIIa. As murine platelets do not express PAR1, these data indicate that BMS200261 acts as an antagonist of PAR3 and hence inhibits FXIIa-induced platelet activation. FXIIa also caused mobilization of intracellular calcium from murine platelets and this calcium increase is abolished by BMS200261 in the presence or absence of the PAR4. PAR1 and PAR4 couple to Gq to cause intracellular calcium increases. YM-254890, a Gq inhibitor, abrogates PAR1- or PAR4-mediated calcium mobilization. However, YM-254890 did not affect FXIIa –induced platelet calcium mobilization in murine platelets. FXIIa caused activation of Gq-/- mice platelets similar to wild -type platelets, suggesting that FXIIa -induced calcium mobilization in platelets is independent of Gq pathways. Furthermore, FXIIa-induced platelet activation was completely abolished by BAPTA-AM, which indicates that calcium is required for FXIIa-induced platelet activation. Furthermore, FXIIa caused phosphorylation of Erk and Akt in PAR4 null murine platelets and this phosphorylation was abolished by BMS200261, but not by YM-254890. These observations may explain previous reports that demonstrated lack of stable thrombus formation in FXII null mice. We conclude that FXIIa activates platelets through PAR3 independently of Gq pathways leading to calcium mobilization and activation of Erk and Akt. Disclosures: No relevant conflicts of interest to declare.

An Allosteric Modulator of PAR1 Demonstrates Selective Inhibition of G Protein Coupling and Impairs Thrombus Formation In Vivo

Blood ◽  
2011 ◽  
Vol 118 (21) ◽  
pp. 1138-1138
Author(s):  
Omozuanvbo R Aisiku ◽  
Chris Dockendorff ◽  
Suzanne F Gunnick ◽  
Daniel A. Smith ◽  
James R. Dilks ◽  
...  
Keyword(s):  
G Protein ◽  
Platelet Activation ◽  
Mdck Cells ◽  
Thrombus Formation ◽  
Shape Change ◽  
Rho Kinase ◽  
Hek293 Cells ◽  
Allosteric Modulator ◽  
Induced Aggregation ◽  
Palmitoylation Site

Abstract Abstract 1138 Protease-activated receptor-1 (PAR1) is a widely expressed G protein-coupled receptor (GPCR) that functions in thrombus formation, inflammation, and mitogenesis. Like other GPCRs, PAR1 can assume multiple conformations and possess multiple binding sites. We hypothesized that the binding of small molecules to sites outside of the tethered ligand binding pocket can stabilize these alternative conformations, resulting in novel signaling properties. To identify allosteric modulators of PAR1, we screened >300,000 compounds for the ability to inhibit PAR1-mediated dense granule release. Following the identification of potent compounds with a 1,3-diaminophenyl scaffold, 81 analogs were synthesized or procured and tested for inhibition of PAR1-mediated granule secretion. Structural features that were required for optimal inhibition included a 4-carbon chain at the “western end” and a 2'-substituted benzamide at the “eastern end” of the molecule. The most potent compound, ML161, inhibited PAR1-mediated platelet activation with an IC50 of 300 nM, >10-fold more potently than JF5, a previously identified allosteric modulator of PAR1. ML161 inhibited platelet activation induced by the PAR1 agonists SFLLRN and thrombin, but not other platelet agonists including the PAR4 agonist AYPGKF, PMA, ionophore, collagen, or ADP. ML161 inhibited SFLLRN-induced Ca2+ flux in platelets and HEK293 cells overexpressing PAR1, confirming activity at PAR1. Modeling of multiple Ca2+ flux curves at different ML161 concentrations over a range of SFLLRN doses indicated an allosteric mode of inhibition. A similar analysis of the effect of ML161 on PAR1-mediated P-selectin expression confirmed an allosteric mechanism. In platelet aggregation studies, ML161 inhibited SFLLRN-induced aggregation, but not shape change, raising the possibility that it inhibits Gαq-mediated, but not Gα12/13-mediated pathways. Shape change in the presence of ML161 was sensitive to the Rho kinase inhibitor Y27632, indicating involvement of Rho kinase in ML161-resistant signaling. ML161 failed to inhibit PAR1-mediated decreases in transepithelial resistance (TER) in MDCK cells overexpressing Gα12. In contrast, orthosteric inhibitors of PAR1, 3,5-difluoro aminoisoxazole and SCH79797, blocked PAR1-mediated shape change in platelets and PAR1-mediated decrease in TER in MDCK cells. These results confirmed that PAR1 modified by ML161 couples to Gα12/13, but loses coupling to Gαq. Although ML161 did not inhibit activation through human PAR4, whose 8th helix lacks a palmitoylation site and contains amino acids that disrupt helices, it inhibited activation through mouse PAR4, which like human PAR1 possesses a constrained 8th helix with a C-terminal palmitoylation site. Consistent with its ability to inhibit mouse PAR4-mediated activation of mouse platelets, 5 mg/kg ML161 inhibited by >90% platelet accumulation during thrombus formation following laser-induced injury of the cremaster arteriole in mice. These results demonstrate that ML161 acts as an allosteric modulator of PAR1 that blocks coupling to Gαq, but not Gα12/13. ML161 demonstrates saturable inhibition as well as selective blockade of Gα subunits coupling and could provide improved control of PAR1 function in the setting of thrombotic disease. Disclosures: No relevant conflicts of interest to declare.

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