Protease-activated receptors 1 and 4 do not stimulate G(i) signaling pathways in the absence of secreted ADP and cause human platelet aggregation independently of G(i) signaling

Phosphorylation of (Ser 291) in the linker insert of Syk negatively regulates ITAM signaling in platelets

Receptor-induced tyrosine phosphorylation of spleen tyrosine kinase (Syk) has been studied extensively in hematopoietic cells. Metabolic mapping and high-resolution mass spectrometry, however, indicate that one of the most frequently detected phosphorylation sites encompassed S297 (S291 in mice) located within the linker B region of Syk. It has been reported that Protein kinase C (PKC) phosphorylates Syk S297, thus influencing Syk activity. However, conflicting studies suggest that this phosphorylation enhances as well as reduces Syk activity. To clarify the function of this site, we generated Syk S291A knock-in mice. We used platelets as a model system as they possess Glycoprotein VI (GPVI), a receptor containing an immunoreceptor tyrosine-based activation motif (ITAM) which transduces signals through Syk. Our analysis of the homozygous mice indicated that the knock-in platelets express only one isoform of Syk, while the wild-type expresses two isoforms at 69 and 66 kDa. When the GPVI receptor was activated with collagen-related peptide (CRP), we observed an increase in functional responses and phosphorylations in Syk S291A platelets. This potentiation did not occur with AYPGKF or 2-MeSADP, although they also activate PKC isoforms. Although there was potentiation of platelet functional responses, there was no difference in tail bleeding times. However, the time to occlusion in the FeCl3 injury model was enhanced. These data indicate that the effects of Syk S291 phosphorylation represent a significant outcome on platelet activation and signaling in vitro but also reveals its multifaceted nature demonstrated by the differential effects on physiological responses in vivo.

Role of Prednisolone in Platelet Activation by Inhibiting TxA2 Generation through the Regulation of cPLA2 Phosphorylation

Glucocorticoids have been commonly used in the treatment of inflammation and immune-mediated diseases in human beings and small animals such as cats and dogs. However, excessive use can lead to Cushing's syndrome along with several thrombotic and cardiovascular diseases. Although it is well-known that glucocorticoids exert a significant effect on coagulation, the effect of cortisol on platelet function is much less clear. Thus, we aimed to study the effects of prednisolone, one of the commonly used glucocorticoids, on the regulation of platelet function using murine platelets. We first evaluated the concentration-dependent effect of prednisolone on 2-MeSADP-induced platelet function and found that the 2-MeSADP-induced secondary wave of aggregation and dense granule secretion were completely inhibited from 500 nM prednisolone. Since 2-MeSADP-induced secretion and the resultant secondary wave of aggregation are mediated by TxA₂ generation, this result suggested a role of prednisolone in platelet TxA₂ generation. Consistently, prednisolone did not affect the 2-MeSADP-induced aggregation in aspirinated platelets, where the secondary wave of aggregation and secretion were blocked by eliminating the contribution of TxA₂ generation by aspirin. In addition, thrombin-induced platelet aggregation and secretion were inhibited in the presence of prednisolone by inhibiting the positive-feedback effect of TxA₂ generation on platelet function. Furthermore, prednisolone completely inhibited 2-MeSADP-induced TxA₂ generation, confirming the role of prednisolone in TxA₂ generation. Finally, Western blot analysis revealed that prednisolone significantly inhibited 2-MeSADP-induced cytosolic phospholipase A₂ (cPLA₂) and ERK phosphorylation in non-aspirinated platelets, while only cPLA₂ phosphorylation, but not ERK phosphorylation, was significantly inhibited by prednisolone in aspirinated platelets. In conclusion, prednisolone affects platelet function by the inhibition of TxA₂ generation through the regulation of cPLA₂ phosphorylation, thereby shedding light on its clinical characterization and treatment efficacy in dogs with hypercortisolism in the future.

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

Platelet Redox Imbalance in Hypercholesterolemia: A Big Problem for a Small Cell

The imbalance between reactive oxygen species (ROS) synthesis and their scavenging by anti-oxidant defences is the common soil of many disorders, including hypercholesterolemia. Platelets, the smallest blood cells, are deeply involved in the pathophysiology of occlusive arterial thrombi associated with myocardial infarction and stroke. A great deal of evidence shows that both increased intraplatelet ROS synthesis and impaired ROS neutralization are implicated in the thrombotic process. Hypercholesterolemia is recognized as cause of atherosclerosis, cerebro- and cardiovascular disease, and, closely related to this, is the widespread acceptance that it strongly contributes to platelet hyperreactivity via direct oxidized LDL (oxLDL)-platelet membrane interaction via scavenger receptors such as CD36 and signaling pathways including Src family kinases (SFK), mitogen-activated protein kinases (MAPK), and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. In turn, activated platelets contribute to oxLDL generation, which ends up propagating platelet activation and thrombus formation through a mechanism mediated by oxidative stress. When evaluating the effect of lipid-lowering therapies on thrombogenesis, a large body of evidence shows that the effects of statins and proprotein convertase subtilisin/kexin type 9 inhibitors are not limited to the reduction of LDL-C but also to the down-regulation of platelet reactivity mainly by mechanisms sensitive to intracellular redox balance. In this review, we will focus on the role of oxidative stress-related mechanisms as a cause of platelet hyperreactivity and the pathophysiological link of the pleiotropism of lipid-lowering agents to the beneficial effects on platelet function.

Inhibitors of protease activated receptor 4 (PAR4): a review of recent patents (2013-2021)

INTRODUCTION

Protease-activated receptor 4 (PAR4), belonging to a subfamily of G-protein-coupled receptors (GPCR), is expressed on the surface of Human platelets, and the activation of it can lead to platelets aggregation. Studies demonstrated that PAR4 inhibition protect mice from arterial/arteriolar thrombosis, pulmonary embolism and cerebral infarct, while do not affect the haemostatic responses integrity. Therefore, PAR4 has been a promising target for the development of anti-thrombotic agents.

AREAS COVERED

This review covers recent patents and literature on PAR4 and their application published between 2013 and 2021.

EXPERT OPINION

PAR4 is a promising anti-thrombotic target and PAR4 inhibitors are important biologically active compounds for the treatment of thrombosis. Most the recent patents and literature focus on PAR4 selective inhibitors, and BMS-986120 and BMS-986141, which were developed by BMS, have entered clinical trials. With the deep understanding of the crystal structures and biological functions of PAR4, we believe that many other novel types of molecules targeting PAR4 would enter the clinical studies or the market.

α-Ketoglutarate Inhibits Thrombosis and Inflammation by Prolyl Hydroxylase-2 Mediated Inactivation of Phospho-Akt

Background

Phospho-Akt1 (pAkt1) undergoes prolyl hydroxylation at Pro125 and Pro313 by the prolyl hydroxylase-2 (PHD2) in a reaction decarboxylating α-ketoglutarate (αKG). We investigated whether the αKG supplementation could inhibit Akt-mediated activation of platelets and monocytes, in vitro as well as in vivo, by augmenting PHD2 activity.

Methods

We treated platelets or monocytes isolated from healthy individuals with αKG in presence of agonists in vitro and assessed the signalling molecules including pAkt1. We supplemented mice with dietary αKG and estimated the functional responses of platelets and monocytes ex vivo. Further, we investigated the impact of dietary αKG on inflammation and thrombosis in lungs of mice either treated with thrombosis-inducing agent carrageenan or infected with SARS-CoV-2.

Findings

Octyl αKG supplementation to platelets promoted PHD2 activity through elevated intracellular αKG to succinate ratio, and reduced aggregation in vitro by suppressing pAkt1(Thr308). Augmented PHD2 activity was confirmed by increased hydroxylated-proline and enhanced binding of PHD2 to pAkt in αKG-treated platelets. Contrastingly, inhibitors of PHD2 significantly increased pAkt1 in platelets. Octyl-αKG followed similar mechanism in monocytes to inhibit cytokine secretion in vitro. Our data also describe a suppressed pAkt1 and reduced activation of platelets and leukocytes ex vivo from mice supplemented with dietary αKG, unaccompanied by alteration in their number. Dietary αKG significantly reduced clot formation and leukocyte accumulation in various organs including lungs of mice treated with thrombosis-inducing agent carrageenan. Importantly, in SARS-CoV-2 infected hamsters, we observed a significant rescue effect of dietary αKG on inflamed lungs with significantly reduced leukocyte accumulation, clot formation and viral load alongside down-modulation of pAkt in the lung of the infected animals.

Interpretation

Our study suggests that dietary αKG supplementation prevents Akt-driven maladies such as thrombosis and inflammation and rescues pathology of COVID19-infected lungs.

Funding

Study was funded by the Department of Biotechnology (DBT), Govt. of India (grants: BT/PR22881 and BT/PR22985); and the Science and Engineering Research Board, Govt. of India (CRG/000092).

Molecular Proteomics and Signalling of Human Platelets in Health and Disease

Platelets are small anucleate blood cells that play vital roles in haemostasis and thrombosis, besides other physiological and pathophysiological processes. These roles are tightly regulated by a complex network of signalling pathways. Mass spectrometry-based proteomic techniques are contributing not only to the identification and quantification of new platelet proteins, but also reveal post-translational modifications of these molecules, such as acetylation, glycosylation and phosphorylation. Moreover, target proteomic analysis of platelets can provide molecular biomarkers for genetic aberrations with established or non-established links to platelet dysfunctions. In this report, we review 67 reports regarding platelet proteomic analysis and signalling on a molecular base. Collectively, these provide detailed insight into the: (i) technical developments and limitations of the assessment of platelet (sub)proteomes; (ii) molecular protein changes upon ageing of platelets; (iii) complexity of platelet signalling pathways and functions in response to collagen, rhodocytin, thrombin, thromboxane A₂ and ADP; (iv) proteomic effects of endothelial-derived mediators such as prostacyclin and the anti-platelet drug aspirin; and (v) molecular protein changes in platelets from patients with congenital disorders or cardiovascular disease. However, sample sizes are still low and the roles of differentially expressed proteins are often unknown. Based on the practical and technical possibilities and limitations, we provide a perspective for further improvements of the platelet proteomic field.

The Molecular Aspects of Disturbed Platelet Activation through ADP/P2Y12 Pathway in Multiple Sclerosis

Epidemiological studies confirm a high risk of ischemic events in secondary-progressive multiple sclerosis (SP MS) patients, directly associated with an increased level of pro-thrombotic activity of platelets. Our work aimed to verify potential molecular abnormalities of the platelet P2Y₁₂ receptor expression and functionality as a cause of an increased risk of thromboembolism observed in the course of MS. We have demonstrated an enhanced platelet reactivity in response to adenosine diphosphate (ADP) in SP MS relative to controls. We have also shown an increased mRNA expression for the P2RY12 gene in both platelets and megakaryocytes, as well as enhanced density of these receptors on the platelet surface. We postulate that one of the reasons for the elevated risk of ischemic events observed in MS may be a genetically or phenotypically reinforced expression of the platelet P2Y₁₂ receptor. In order to analyze the effect of the PAR1 (protease activated receptor type 1) signaling pathway on the expression level of P2Y₁₂, we also analyzed the correlation parameters between P2Y₁₂ expression and the markers of platelet activation in MS induced by selective PAR1 agonist (thrombin receptor activating peptide-6, TRAP-6). Identifying the molecular base responsible for the enlarged pro-thrombotic activity of platelets in SP MS could contribute to the implementation of prevention and targeted treatment, reducing the development of cardiovascular disorders in the course of the disease.

PCTP contributes to human platelet activation by enhancing dense granule secretion

PCTP (phosphatidylcholine transfer protein) was discovered recently to regulate aggregation of human platelets stimulated with PAR4 activating peptide (PAR4AP). However, the role of PCTP following thrombin stimulation, the mechanisms by which PCTP contributes to platelet activation, and the role of PCTP with other receptors remained unknown. As mouse platelets do not express PCTP, we treated human platelets with various agonists in the presence of the specific PCTP inhibitor A1. We observed that PCTP inhibition significantly reduced dense granule secretion in response to thrombin, PAR1AP, PAR4AP, convulxin (GPVI agonist) and FcγRIIA crosslinking. In contrast, among these agonists, PCTP inhibition reduced aggregation only to low dose thrombin and PAR4AP. Unlike its effects on dense granule secretion, PCTP inhibition did not reduce alpha granule secretion in response to thrombin or PAR4AP. PCTP inhibition reduced both the increase in cytoplasmic Ca²⁺ as well as PKC activity downstream of thrombin. These data are consistent with PCTP contributing to secretion of dense granules, and to being particularly important to human PAR4 early signaling events. Future studies will address further these molecular mechanisms and consequences for hemostasis and thrombosis.

PAK Membrane Translocation and Phosphorylation Regulate Platelet Aggregation Downstream of Gi and G12/13 Pathways

Platelet activation plays a pivotal role in physiological hemostasis and pathological thrombosis causing heart attack and stroke. Previous studies conclude that simultaneous activation of Gi and G_12/13 signaling pathways is sufficient to cause platelet aggregation. However, using Gq knockout mice and Gq-specific inhibitors, we here demonstrated that platelet aggregation downstream of coactivation of Gi and G_12/13 depends on agonist concentrations; coactivation of Gi and G_12/13 pathways only induces platelet aggregation under higher agonist concentrations. We confirmed Gi and G_12/13 pathway activation by showing cAMP (cyclic adenosine monophosphate) decrease and RhoA activation in platelets stimulated at both low and high agonist concentrations. Interestingly, we found that though Akt and PAK (p21-activated kinase) translocate to the platelet membrane upon both low and high agonist stimulation, membrane-translocated Akt and PAK only phosphorylate at high agonist concentrations, correlating well with platelet aggregation downstream of concomitant Gi and G_12/13 pathway activation. PAK inhibitor abolishes Akt phosphorylation, inhibits platelet aggregation in vitro and arterial thrombus formation in vivo. We propose that the PAK-PI3K/Akt pathway mediates platelet aggregation downstream of Gi and G_12/13, and PAK may represent a potential antiplatelet and antithrombotic target.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

Regulators of G-protein signaling, RGS2 and RGS4, inhibit protease-activated receptor 4-mediated signaling by forming a complex with the receptor and Gα in live cells

BACKGROUND

Protease-activated receptor 4 (PAR4) is a seven transmembrane G-protein coupled receptor (GPCR) activated by endogenous proteases, such as thrombin. PAR4 is involved in various pathophysiologies including cancer, inflammation, pain, and thrombosis. Although regulators of G-protein signaling (RGS) are known to modulate GPCR/Gα-mediated pathways, their specific effects on PAR4 are not fully understood at present. We previously reported that RGS proteins attenuate PAR1- and PAR2-mediated signaling through interactions with these receptors in conjunction with distinct Gα subunits.

METHODS

We employed a bioluminescence resonance energy transfer technique and confocal microscopy to examine potential interactions among PAR4, RGS, and Gα subunits. The inhibitory effects of RGS proteins on PAR4-mediated downstream signaling and cancer progression were additionally investigated by using several assays including ERK phosphorylation, calcium mobilization, RhoA activity, cancer cell proliferation, and related gene expression.

RESULTS

In live cells, RGS2 interacts with PAR4 in the presence of Gα_q while RGS4 binding to PAR4 occurs in the presence of Gα_q and Gα_12/13. Co-expression of PAR4 and Gα_q induced a shift in the subcellular localization of RGS2 and RGS4 from the cytoplasm to plasma membrane. Combined PAR4 and Gα_12/13 expression additionally promoted translocation of RGS4 from the cytoplasm to the membrane. Both RGS2 and RGS4 abolished PAR4-activated ERK phosphorylation, calcium mobilization and RhoA activity, as well as PAR4-mediated colon cancer cell proliferation and related gene expression.

CONCLUSIONS

RGS2 and RGS4 forms ternary complex with PAR4 in Gα-dependent manner and inhibits its downstream signaling. Our findings support a novel physiological function of RGS2 and RGS4 as inhibitors of PAR4-mediated signaling through selective PAR4/RGS/Gα coupling. Video Abstract.

Role of GRK6 in the Regulation of Platelet Activation through Selective G Protein-Coupled Receptor (GPCR) Desensitization

Platelet G protein-coupled receptors (GPCRs) regulate platelet function by mediating the response to various agonists, including adenosine diphosphate (ADP), thromboxane A₂, and thrombin. Although GPCR kinases (GRKs) are considered to have the crucial roles in most GPCR functions, little is known regarding the regulation of GPCR signaling and mechanisms of GPCR desensitization by GRKs in platelets. In this study, we investigated the functional role of GRK6 and the molecular basis for regulation of specific GPCR desensitization by GRK6 in platelets. We used GRK6 knockout mice to evaluate the functional role of GRK6 in platelet activation. Platelet aggregation, dense- and α-granule secretion, and fibrinogen receptor activation induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in GRK6^−/− platelets compared to the wild-type (WT) platelets. However, collagen-related peptide (CRP)-induced platelet aggregation and secretion were not affected in GRK6^−/− platelets. Interestingly, platelet aggregation induced by co-stimulation of serotonin and epinephrine which activate G_q-coupled 5HT_2A and G_z-coupled α_2A adrenergic receptors, respectively, was not affected in GRK6^−/− platelets, suggesting that GRK6 was involved in specific GPCR regulation. In addition, platelet aggregation in response to the second challenge of ADP and AYPGKF was restored in GRK6^−/− platelets whereas re-stimulation of the agonist failed to induce aggregation in WT platelets, indicating that GRK6 contributed to P2Y₁, P2Y₁₂, and PAR4 receptor desensitization. Furthermore, 2-MeSADP-induced Akt phosphorylation and AYPGKF-induced Akt, extracellular signal-related kinase (ERK), and protein kinase Cδ (PKCδ) phosphorylation were significantly potentiated in GRK6^−/− platelets. Finally, GRK6^−/− mice exhibited an enhanced and stable thrombus formation after FeCl₃ injury to the carotid artery and shorter tail bleeding times, indicating that GRK6^−/− mice were more susceptible to thrombosis and hemostasis. We conclude that GRK6 plays an important role in regulating platelet functional responses and thrombus formation through selective GPCR desensitization.

Vorapaxar in the treatment of cardiovascular diseases

Vorapaxar specifically and effectively inhibits protease activated receptor-1 and may reduce thrombin-mediated ischemic events without interfering primary hemostasis. In the TRA-2P-TIMI 50 trial, vorapaxar reduced the risk of primary ischemic outcome but with increased bleeding risk. In the post hoc analysis, in patients with a history of myocardial infarction, peripheral artery disease, the net clinical outcome favored vorapaxar therapy with 10% reduction in cardiovascular death, myocardial infarction, stroke, urgent coronary revascularization and moderate or severe bleeding. Based on these favorable results, vorapaxar was approved for the reduction of thrombotic cardiovascular events in patients with prior myocardial infarction or with peripheral artery disease on top of standard antiplatelet therapy. A careful patient selection is needed to balance efficacy versus safety.

The Marine-Derived Triterpenoid Frondoside A Inhibits Thrombus Formation

Background: The marine-derived triterpenoid frondoside A inhibits the phosphatidylinositol-3-kinase (PI3K) pathway in cancer cells. Because this pathway is also crucially involved in platelet activation, we studied the effect of frondoside A on thrombus formation. Methods: Frondoside A effects on platelet viability, surface adhesion molecule expression, and intracellular signaling were analyzed by flow cytometry and Western blot. The effect of frondoside A was analyzed by photochemically induced thrombus formation in the mouse dorsal skinfold chamber model and by tail vein bleeding. Results: Concentrations of up to 15 µM frondoside A did not affect the viability of platelets, but reduced their surface expression of P-selectin (CD62P) and the activation of glycoprotein (GP)IIb/IIIa after agonist stimulation. Additional mechanistic analyses revealed that this was mediated by downregulation of PI3K-dependent Akt and extracellular-stimuli-responsive kinase (ERK) phosphorylation. Frondoside A significantly prolonged the complete vessel occlusion time in the mouse dorsal skinfold chamber model of photochemically induced thrombus formation and also the tail vein bleeding time when compared to vehicle-treated controls. Conclusion: Our findings demonstrated that frondoside A inhibits agonist-induced CD62P expression and activation of GPIIb/IIIa. Moreover, frondoside A suppresses thrombus formation. Therefore, this marine-derived triterpenoid may serve as a lead compound for the development of novel antithrombotic drugs.

Heterotrimeric G Proteins as Therapeutic Targets in Drug Discovery

Heterotrimeric G proteins are molecular switches in GPCR signaling pathways and regulate a plethora of physiological and pathological processes. GPCRs are efficient drug targets, and more than 30% of the drugs in use target them. However, selectively targeting an individual GPCR may be undesirable in various multifactorial diseases in which multiple receptors are involved. In addition, abnormal activation or expression of G proteins is frequently associated with diseases. Furthermore, G proteins harboring mutations often result in malignant diseases. Thus, targeting G proteins instead of GPCRs might provide alternative approaches for combating these diseases. In this review, we discuss the biochemistry of heterotrimeric G proteins, describe the G protein-associated diseases, and summarize the currently known modulators that can regulate the activities of G proteins. The outlook for targeting G proteins to treat diverse diseases is also included in this manuscript.

Enhanced potency of prasugrel on protease-activated receptors following bivalirudin treatment for PCI as compared to clopidogrel

ACS patients undergoing percutaneous coronary intervention (PCI) when treated with bivalirudin and clopidogrel had increased frequency of early stent thrombosis. 24 patients referred for intervention with planned bivalirudin therapy, not previously treated with a P2Y₁₂ inhibitor and not receiving heparins or αIIbβ3 inhibitors were randomized to treatment with either clopidogrel (600 mg) or prasugrel (60 mg). Platelet aggregation (PA) was measured by light transmission aggregometry (LTA) of platelet-rich plasma in response to ADP, PAR1/PAR4 thrombin receptor agonists and collagen at baseline and at 1, 2, 4 and 16 h following the cessation of bivalirudin infusion. Prasugrel-mediated inhibition of PA was significantly greater than that of clopidogrel at all time points for ADP as well as PAR1. There was an unanticipated, significantly greater protection of PAR4-mediated platelet aggregation only detected with prasugrel and not observed with clopidogrel. We further examined the effect of the hyperreactive PAR4 Thr120 variant in the protease-activated receptor 4 (PAR4), single nucleotide polymorphism (SNP) rs773902 on aggregation protection. The PAR4 protective effect with prasugrel was lost in individuals carrying the PAR4 Thr120 variant, and not in Ala120 homozygote. PAR1, ADP and collagen inhibition was not significantly affected in the hyperreactive PAR4 Thr120 variant. We documented that the P2Y₁₂ ADP receptor-mediated regulation of the strength of the high-affinity conformation of αIIbβ3 as detected by PAC-1 ab, and in control of platelet adhesiveness through Rap1 GTPase protein activation. Importantly, the PAR4 Thr120 variant resulted in the increased rate and magnitude of Rap1 activation. Human platelet PAR4 mediated-activation of αIIbβ3 was phospholipase C beta (PLCβ)-dependent and unlike mouse platelet PI3K-independent. These data identify a PAR4-dependent inhibitory mechanism for the prasugrel-mediated platelet inhibition, not seen with clopidogrel that could explain the reduction in stent thrombosis documented in clinical trials with prasugrel.

Characterization of the distinct mechanism of agonist-induced canine platelet activation

Platelet activation has a major role in hemostasis and thrombosis. Various agonists including adenosine diphosphate (ADP) and thrombin interact with G protein-coupled receptors (GPCRs) which transduce signals through various G proteins. Recent studies have elucidated the role of GPCRs and their corresponding G proteins in the regulation of events involved in platelet activation. However, agonist-induced platelet activation in companion animals has not been elucidated. This study was designed to characterize the platelet response to various agonists in dog platelets. We found that 2-methylthio-ADP-induced dog platelet aggregation was blocked in the presence of either P2Y₁ receptor antagonist MRS2179 or P2Y₁₂ receptor antagonist AR-C69931MX, suggesting that co-activation of both the P2Y₁ and P2Y₁₂ receptors is required for ADP-induced platelet aggregation. Thrombin-induced dog platelet aggregation was inhibited in the presence of either AR-C69931MX or the PKC inhibitor GF109203X, suggesting that thrombin requires secreted ADP to induce platelet aggregation in dog platelets. In addition, thrombin-mediated Akt phosphorylation was inhibited in the presence of GF109203X or AR-C69931MX, indicating that thrombin causes Gi stimulation through the P2Y₁₂ receptor by secreted ADP in dog platelets. Unlike human and murine platelets, protease-activated receptor 4 (PAR4)-activating peptide AYPGKF failed to cause dog platelet aggregation. Moreover, PAR1-activating peptide SFLLRN or co-stimulation of SFLLRN and AYPGKF failed to induce dog platelet aggregation. We conclude that ADP induces platelet aggregation through the P2Y₁ and P2Y₁₂ receptors in dogs. Unlike human and murine platelets, selective activation of the PAR4 receptor may be insufficient to cause platelet aggregation in dog platelets.

Signaling Pathways of Receptors Involved in Platelet Activation and Shedding of These Receptors in Stored Platelets

All cells encounter various signals coming from the surrounding environment and they need to receive and respond to these signals in order to perform their functions. Cell surface receptors are responsible for signal transduction .Platelets are blood cells which perform several functions using diverse receptors. Platelet concentrate is one of the most consumed blood products. However, due to the short lifespan of the platelets and platelets damage during storage, we face shortage of platelet products. One of the damages that platelets undergo during storage is the loss of surface receptors. Since cell surface receptors are responsible for all cell functions, the loss of platelet receptors reduces the quality of platelet products. In this study, we reviewed the important receptors involved in platelet activation and their associated signaling pathways. We also looked at the platelet receptors that shed during storage and the causes of this incident. We found that GPIbα, P-selectin, CD40 and GPVI are platelet receptors that fall during platelet storage at room temperature. Considering that GPVI and GPIbα are the most important receptors which involved in platelet activation, their shedding can cause decrease in platelet activation after transfusion and decrease thrombus consistence. Shear stress and platelet contact with the container wall are among the mechanisms discussed in this process, but studies in this area have to be continued.

The protease-activated receptor 4 Ala120Thr variant alters platelet responsiveness to low-dose thrombin and protease-activated receptor 4 desensitization, and is blocked by non-competitive P2Y12 inhibition

Essentials The rs773902 SNP results in differences in platelet protease-activated receptor (PAR4) function. The functional consequences of rs773902 were analyzed in human platelets and stroke patients. rs773902 affects thrombin-induced platelet function, PAR4 desensitization, stroke association. Enhanced PAR4 Thr120 effects on platelet function are blocked by ticagrelor. SUMMARY: Background F2RL3 encodes protease-activated receptor (PAR) 4 and harbors an A/G single-nucleotide polymorphism (SNP) (rs773902) with racially dimorphic allelic frequencies. This SNP mediates an alanine to threonine substitution at residue 120 that alters platelet PAR4 activation by the artificial PAR4-activation peptide (PAR4-AP) AYPGKF. Objectives To determine the functional effects of rs773902 on stimulation by a physiological agonist, thrombin, and on antiplatelet antagonist activity. Methods Healthy human donors were screened and genotyped for rs773902. Platelet function in response to thrombin was assessed without and with antiplatelet antagonists. The association of rs773902 alleles with stroke was assessed in the Stroke Genetics Network study. Results As compared with rs773902 GG donors, platelets from rs773902 AA donors had increased aggregation in response to subnanomolar concentrations of thrombin, increased granule secretion, and decreased sensitivity to PAR4 desensitization. In the presence of PAR1 blockade, this genotype effect was abolished by higher concentrations of or longer exposure to thrombin. We were unable to detect a genotype effect on thrombin-induced PAR4 cleavage, dimerization, and lipid raft localization; however, rs773902 AA platelets required a three-fold higher level of PAR4-AP for receptor desensitization. Ticagrelor, but not vorapaxar, abolished the PAR4 variant effect on thrombin-induced platelet aggregation. A significant association of modest effect was detected between the rs773902 A allele and stroke. Conclusion The F2RL3 rs773902 SNP alters platelet reactivity to thrombin; the allelic effect requires P2Y12 , and is not affected by gender. Ticagrelor blocks the enhanced reactivity of rs773902 A platelets. PAR4 encoded by the rs773902 A allele is relatively resistant to desensitization and may contribute to stroke risk.

Thrombus Aspiration for ST-Segment-Elevation Myocardial Infarction in Modern Era: Still an Issue of Debate?

The role of manual thrombus aspiration (TA) during primary percutaneous coronary intervention (PPCI) for acute ST-segment-elevation myocardial infarction has been a matter of intense research and debate now. Although recent randomized controlled clinical trials (notably TASTE [Thrombus Aspiration in ST-Elevation Myocardial Infarction in Scandinavia] and TOTAL [Trial of Routine Aspiration Thrombectomy With PCI Versus PCI Alone in Patients With STEMI]) do not supply evidence supporting the routine use of TA in patients with ST-segment-elevation myocardial infarction, manual TA remains a therapeutic option for interventional cardiologists when treating patients with substantial thrombus burden during PPCI. It remains unknown whether patients might actually benefit from TA applied in a more selective manner depending on the thrombus burden during PPCI, instead of routine application. In this review, we summarize current knowledge on the instruments used in the TA procedure, positive as well as negative clinical effects of TA during PPCI, and analyze the potential reasons for observed effects, in an effort to help the clinical decision making by physicians for the use of TA in individual ST-segment-elevation myocardial infarction patients during PPCI.

Protease-Activated Receptor 4 (PAR4): A Promising Target for Antiplatelet Therapy

Cardiovascular diseases (CVDs) are currently among the leading causes of death worldwide. Platelet aggregation is a key cellular component of arterial thrombi and major cause of CVDs. Protease-activated receptors (PARs), including PAR1, PAR2, PAR3 and PAR4, fall within a subfamily of seven-transmembrane G-protein-coupled receptors (GPCR). Human platelets express PAR1 and PAR4, which contribute to the signaling transduction processes. In association with CVDs, PAR4 not only contributes to platelet activation but also is a modulator of cellular responses that serve as hallmarks of inflammation. Although several antiplatelet drugs are available on the market, they have many side effects that limit their use. Emerging evidence shows that PAR4 targeting is a safer strategy for preventing thrombosis and consequently may improve the overall cardiac safety profile. Our present review summarizes the PAR4 structural characteristics, activation mechanism, role in the pathophysiology of diseases and understanding the association of PAR4 targeting for improved cardiac protection. Conclusively, this review highlights the importance of PAR4 antagonists and its potential utility in different CVDs.

Coordination of platelet agonist signaling during the hemostatic response in vivo

The local microenvironment within an evolving hemostatic plug shapes the distribution of soluble platelet agonists, resulting in a gradient of platelet activation. We previously showed that thrombin activity at a site of vascular injury is spatially restricted, resulting in robust activation of a subpopulation of platelets in the hemostatic plug core. In contrast, adenosine 5'-diphosphate (ADP)/P2Y₁₂ signaling contributes to the accumulation of partially activated, loosely packed platelets in a shell overlying the core. The contribution of the additional platelet agonists thromboxane A₂ (TxA₂) and epinephrine to this hierarchical organization was not previously shown. Using a combination of genetic and pharmacologic approaches coupled with real-time intravital imaging, we show that TxA₂ signaling is critical and nonredundant with ADP/P2Y₁₂ for platelet accumulation in the shell region but not required for full platelet activation in the hemostatic plug core, where thrombin activity is highest. In contrast, epinephrine signaling is dispensable even in the presence of a P2Y₁₂ antagonist. Finally, dual P2Y₁₂ and thrombin inhibition does not substantially inhibit hemostatic plug core formation any more than thrombin inhibition alone, providing further evidence that thrombin is the primary driver of platelet activation in this region. Taken together, these studies show for the first time how thrombin, P2Y₁₂, and TxA₂ signaling are coordinated during development of a hierarchical organization of hemostatic plugs in vivo and provide novel insights into the impact of dual antiplatelet therapy on hemostasis and thrombosis.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

Role for Thrombin Receptor Antagonism With Vorapaxar in Secondary Prevention of Atherothrombotic Events: From Bench to Bedside

In spite of treatment with the current standard of care antiplatelet regimens including dual antiplatelet therapy, recurrence rates of ischemic events remain elevated for high-risk patients with atherosclerotic disease. This may be in part attributed to the fact that other key platelet activation pathways remain uninhibited and can thus continue to trigger platelet activation and lead to thrombotic complications. Thrombin is a powerful inducer of platelet activation and mediates its effects directly on platelets through protease activator receptors (PARs), particularly the PAR-1 subtype, making PAR-1 inhibition an attractive approach for reducing atherothrombotic events. These observations have led to the development of several PAR-1 antagonists. Vorapaxar is a direct inhibitor of PAR-1 and the only agent of this class approved for the prevention of recurrent ischemic events in patients with prior myocardial infarction or peripheral artery disease. In the present manuscript, we present a review of the pathophysiologic role of thrombin on thrombotic complications, the impact of vorapaxar on outcomes, including the most recent updates deriving from clinical trials, as well as future perspectives in the field.

Targeting a Proteinase-Activated Receptor 4 (PAR4) Carboxyl Terminal Motif to Regulate Platelet Function

Thrombin initiates human platelet aggregation by coordinately activating proteinase-activated receptors (PARs) 1 and 4. However, targeting PAR1 with an orthosteric-tethered ligand binding-site antagonist results in bleeding, possibly owing to the important role of PAR1 activation on cells other than platelets. Because of its more restricted tissue expression profile, we have therefore turned to PAR4 as an antiplatelet target. We have identified an intracellular PAR4 C-terminal motif that regulates calcium signaling and β-arrestin interactions. By disrupting this PAR4 calcium/β-arrestin signaling process with a novel cell-penetrating peptide, we were able to inhibit both thrombin-triggered platelet aggregation in vitro and clot consolidation in vivo. We suggest that targeting PAR4 represents an attractive alternative to blocking PAR1 for antiplatelet therapy in humans.

A novel mechanism regulating human platelet activation by MMP-2-mediated PAR1 biased signaling

Platelets contain and release several matrix metalloproteinases (MMPs). Among these, active MMP-2 enhances platelet aggregation by favoring the activation of phosphatidylinositol 3- kinase (PI3K) and contributes to arterial thrombosis. The platelet surface target of MMP-2 and the mechanism through which it primes platelets to respond to subsequent stimuli are still unknown. We show that active MMP-2 enhances platelet activation induced by weak stimuli by cleaving PAR1 at a noncanonical extracellular site different from the thrombin-cleavage site and thus initiates biased receptor signaling, triggering only some of the signaling pathways normally activated by full PAR1 agonism. The novel PAR1-tethered ligand exposed by MMP-2 stimulates PAR1-dependent G_q and G_12/13 pathway activation, triggering p38-MAPK phosphorylation, Ca⁺² fluxes, and PI3K activation, but not G_i signaling; this is insufficient to cause platelet aggregation, but it is enough to predispose platelets to fully respond to G_i-activating stimuli. Integrin α_IIbβ₃ is a necessary cofactor for PAR1 cleavage by MMP-2 by binding the MMP-2 hemopexin domain, thus favoring the interaction of the enzyme with PAR1. Our studies unravel a novel mechanism regulating platelet activation that involves the binding of MMP-2 to integrin α_IIbβ₃ and the subsequent cleavage of PAR1 by active MMP-2 at a noncanonical site, exposing a previously undescribed tethered ligand that triggers biased G-protein agonism and thus predisposes platelets to full activation by other stimuli. These results identify the MMP-2-α_IIbβ₃-PAR1 interaction as a potential target for the prevention of arterial thrombosis.

The proteolytic profile of human cancer procoagulant suggests that it promotes cancer metastasis at the level of activation rather than degradation

Proteases are essential for tumour progression and many are over-expressed during this time. The main focus of research was the role of these proteases in degradation of the basement membrane and extracellular matrix (ECM), thereby enabling metastasis to occur. Cancer procoagulant (CP), a protease present in malignant tumours, but not normal tissue, is a known activator of coagulation factor X (FX). The present study investigated the function of CP in cancer progression by focussing on its enzymatic specificity. FX cleavage was confirmed using SDS-PAGE and MALDI-TOF MS and compared to the proteolytic action of CP on ECM proteins, including collagen type IV, laminin and fibronectin. Contrary to previous reports, CP cleaved FX at the conventional activation site (between Arg-52 and Ile-53). Additionally, degradation of FX by CP occurred at a much slower rate than degradation by conventional activators. Complete degradation of the heavy chain of FX was only visible after 24 h, while degradation by RVV was complete after 30 min, supporting postulations that the procoagulant function of CP may be of secondary importance to its role in cancer progression. Of the ECM proteins tested, only fibronectin was cleaved. The substrate specificity of CP was further investigated by screening synthetic peptide substrates using a novel direct CP assay. The results indicate that CP is not essential for either cancer-associated blood coagulation or the degradation of ECM proteins. Rather, they suggest that this protease may be required for the proteolytic activation of membrane receptors.

Protease-activated receptor 4: a critical participator in inflammatory response

Protease-activated receptors (PARs) are G protein-coupled receptors of which four members PAR1, PAR2, PAR3, and PAR4 have been identified, characterized by a typical mechanism of activation involving various related proteases. The amino-terminal sequence of PARs is cleaved by a broad array of proteases, leading to specific proteolytic cleavage which forms endogenous tethered ligands to induce agonist-biased PAR activation. The biological effect of PARs activated by coagulation proteases to regulate hemostasis and thrombosis plays an enormous role in the cardiovascular system, while PAR4 can also be activated by trypsin, cathepsin G, the activated factor X of the coagulation cascade, and trypsin IV. Irrespective of its role in thrombin-induced platelet aggregation, PAR4 activation is believed to be involved in inflammatory lesions, as show by investigations that have unmasked the effects of PAR4 on neutrophil recruitment, the regulation of edema, and plasma extravasation. This review summarizes the roles of PAR4 in coagulation and other extracellular protease pathways, which activate PAR4 to participate in normal regulation and disease.

Signaling-mediated cooperativity between glycoprotein Ib-IX and protease-activated receptors in thrombin-induced platelet activation

Thrombin-induced cellular response in platelets not only requires protease-activated receptors (PARs), but also involves another thrombin receptor, the glycoprotein Ib-IX complex (GPIb-IX). It remains controversial how thrombin binding to GPIb-IX stimulates platelet responses. It was proposed that GPIb-IX serves as a dock that facilitates thrombin cleavage of protease-activated receptors, but there are also reports suggesting that thrombin binding to GPIb-IX induces platelet activation independent of PARs. Here we show that GPIb is neither a passive thrombin dock nor a PAR-independent signaling receptor. We demonstrate a novel signaling-mediated cooperativity between PARs and GPIb-IX. Low-dose thrombin-induced PAR-dependent cell responses require the cooperativity of GPIb-IX signaling, and conversely, thrombin-induced GPIb-IX signaling requires cooperativity of PARs. This mutually dependent cooperativity requires a GPIb-IX-specific 14-3-3-Rac1-LIMK1 signaling pathway, and activation of this pathway also requires PAR signaling. The cooperativity between GPIb-IX signaling and PAR signaling thus drives platelet activation at low concentrations of thrombin, which are important for in vivo thrombosis.

Platelet thrombin receptor antagonism with vorapaxar: pharmacology and clinical trial development

Oral antiplatelet therapies for secondary prevention of ischemic recurrences in patients with atherosclerotic disease manifestations include aspirin and P2Y12 receptor antagonists. Despite the use of these therapies, patients remain at risk for recurrent ischemic events, which may be attributed to other platelet signaling pathways which continue to be activated. More intense antithrombotic strategies have been investigated, including identifying additional targets to modulate platelet activation. Among these, thrombin-mediated platelet activation through PAR-1 has been subject to broad clinical investigation. Vorapaxar is the only PAR-1 receptor antagonists that completed large-scale clinical investigations and is approved for clinical use. This manuscript provides an overview of the pharmacology and clinical trial development of vorapaxar as well as its role in clinical practice.

Circulating primers enhance platelet function and induce resistance to antiplatelet therapy

BACKGROUND

Aspirin and P2Y12 antagonists are antiplatelet compounds that are used clinically in patients with thrombosis. However, some patients are 'resistant' to antiplatelet therapy, which increases their risk of developing acute coronary syndromes. These patients often present with an underlying condition that is associated with altered levels of circulating platelet primers and platelet hyperactivity. Platelet primers cannot stimulate platelet activation, but, in combination with physiologic stimuli, significantly enhance platelet function.

OBJECTIVES

To explore the role of platelet primers in resistance to antiplatelet therapy, and to evaluate whether phosphoinositide 3-kinase (PI3K) contributes to this process.

METHODS AND RESULTS

We used platelet aggregation, thromboxane A2 production and ex vivo thrombus formation as functional readouts of platelet activity. Platelets were treated with the potent P2Y12 inhibitor AR-C66096, aspirin, or a combination of both, in the presence or absence of the platelet primers insulin-like growth factor-1 (IGF-1) and thrombopoietin (TPO), or the Gz-coupled receptor ligand epinephrine. We found that platelet primers largely overcame the inhibitory effects of antiplatelet compounds on platelet functional responses. IGF-1-mediated and TPO-mediated, but not epinephrine-mediated, enhancements in the presence of antiplatelet drugs were blocked by the PI3K inhibitors wortmannin and LY294002.

CONCLUSIONS

These results demonstrate that platelet primers can contribute to antiplatelet resistance. Furthermore, our data demonstrate that there are PI3K-dependent and PI3K-independent mechanisms driving primer-mediated resistance to antiplatelet therapy.

Thrombin-induced lysosomal exocytosis in human platelets is dependent on secondary activation by ADP and regulated by endothelial-derived substances

Exocytosis of lysosomal contents from platelets has been speculated to participate in clearance of thrombi and vessel wall remodelling. The mechanisms that regulate lysosomal exocytosis in platelets are, however, still unclear. The aim of this study was to identify the pathways underlying platelet lysosomal secretion and elucidate how this process is controlled by platelet inhibitors. We found that high concentrations of thrombin induced partial lysosomal exocytosis as assessed by analysis of the activity of released N-acetyl-β-glucosaminidase (NAG) and by identifying the fraction of platelets exposing the lysosomal-associated membrane protein (LAMP)-1 on the cell surface by flow cytometry. Stimulation of thrombin receptors PAR1 or PAR4 with specific peptides was equally effective in inducing LAMP-1 surface expression. Notably, lysosomal exocytosis in response to thrombin was significantly reduced if the secondary activation by ADP was inhibited by the P2Y12 antagonist cangrelor, while inhibition of thromboxane A2 formation by treatment with acetylsalicylic acid was of minor importance in this regard. Moreover, the NO-releasing drug S-nitroso-N-acetyl penicillamine (SNAP) or the cyclic AMP-elevating eicosanoid prostaglandin I2 (PGI2) significantly suppressed lysosomal exocytosis. We conclude that platelet inhibitors that mimic functional endothelium such as PGI2 or NO efficiently counteract lysosomal exocytosis. Furthermore, we suggest that secondary release of ADP and concomitant signaling via PAR1/4- and P2Y12 receptors is important for efficient platelet lysosomal exocytosis by thrombin.

Development and characterization of monoclonal antibodies against Protease Activated Receptor 4 (PAR4)

BACKGROUND

Protease activated receptor 4 (PAR4) is a G protein coupled receptor (GPCR) which is activated by proteolytic cleavage of its N-terminal exodomain. This generates a tethered ligand that activates the receptor and triggers downstream signaling events. With the current focus in the development of anti-platelet therapies shifted towards PARs, new reagents are needed for expanding the field's knowledge on PAR4. Currently, there are no PAR4 reagents which are able to detect activation of the receptor.

METHODS

Monoclonal PAR4 antibodies were purified from hybridomas producing antibody that were generated by fusing splenocytes with NS-1 cells. Immunoblotting, immunofluorescence, and flow cytometry were utilized to detect the epitope for each antibody and to evaluate the interaction of the antibodies with cells.

RESULTS

Here, we report the successful generation of three monoclonal antibodies to the N-terminal extracellular domain of PAR4: 14H6, 5F10, and 2D6. We mapped the epitope on PAR4 of 14H6, 5F10, and 2D6 antibodies to residues (48-53), (41-47), and (73-78), respectively. Two of the antibodies (14H6 and 5F10) interacted close to the thrombin cleavage and were sensitive to α-thrombin cleavage of PAR4. In addition, 5F10 was able to partially inhibit the cleavage of PAR4 expressed in HEK293 cells by α-thrombin.

CONCLUSIONS

These new antibodies provide a means to monitor endogenous PAR4 expression and activation by proteases on cells.

Phosphatidylinositol-3,4,5-trisphosphate stimulates Ca(2+) elevation and Akt phosphorylation to constitute a major mechanism of thromboxane A2 formation in human platelets

Phosphatidylinositol trisphosphate (PIP3) has been implicated in many platelet functions however many of the mechanisms need clarification. We have used cell permeable analogues of PIP3,1-O-(1,2-di-palmitoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC16-PIP3) or 1-O-(1,2-di-octanoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC8-PIP3) to study their effects on activation on washed human platelets. Addition of either DiC8- or DiC16-PIP3 to human platelets induced aggregation in the presence of extracellular Ca(2+). This was reduced by the presence of indomethacin, the phospholipase C inhibitor U73122 and apyrase. DiC8-PIP3 induced the phosphorylation of Akt-Ser(473) which was reduced by the Akt inhibitor IV, wortmannin and EGTA (suggesting a dependence on Ca(2+) entry). In Fura2 loaded platelets DiC8-PIP3 was effective at increasing intracellular Ca(2+) in a distinct and transient manner that was reduced in the presence of indomethacin, U73122 and 2-aminoethyl diphenylborinate (2APB). Ca(2+) elevation was reduced by the non-SOCE inhibitor LOE908 and also by the SOCE inhibitor BTP2. DiC8-PIP3 induced the release of Ca(2+) from stores which was not affected by the proton dissipating agent bafilomycin A1 and was more potent than the two-pore channel agonist DiC8-PI[3,5]P2 suggesting release from an endoplasmic reticulum type store. DiC8-PIP3 weakly induced the tyrosine phosphorylation of Syk but not of PLCγ2. Finally like thrombin DiC8-PIP3 induced the formation of thromboxane B2 that was inhibited by the Akt inhibitor IV. These studies suggest that PIP3 via Ca(2+) elevation and Akt phosphorylation forms a central role in thromboxane A2 formation and the amplification of platelet activation.

Impact of the PI3-kinase/Akt pathway on ITAM and hemITAM receptors: haemostasis, platelet activation and antithrombotic therapy

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are activated in response to various stimulants, and they regulate many processes including inflammation; the stress response; gene transcription; and cell proliferation, differentiation, and death. Increasing reports have shown that the PI3Ks and their downstream effector Akt are activated by several platelet receptors that regulate platelet activation and haemostasis. Platelets express two immunoreceptor tyrosine based activation motif (ITAM) receptors, collagen receptor glycoprotein VI (GPVI) and Fcγ receptor IIA (FcγRIIA), which are characterized by two YxxL sequences separated by 6-12 amino acids. Activation of an ITAM receptor initiates a reaction cascade via its YxxL sequence in which signaling molecules such as spleen tyrosine kinase (Syk), linker for activation of T cells (LAT) and phospholipase C γ2 (PLCγ2) become activated, leading to platelet activation. Platelets also express another receptor, C-type lectin 2 (CLEC-2), which has a single YxxL sequence, so it is appropriately called a hemITAM receptor. ITAM receptors and the hemITAM receptor share many signaling features. Here we will summarize our current knowledge about how the PI3K/Akt pathway regulates (hem)ITAM receptor-mediated platelet activation and haemostasis and discuss the possible benefits of targeting PI3K/Akt as an antithrombotic therapy.

G-protein-coupled receptors signaling pathways in new antiplatelet drug development

Platelet G-protein-coupled receptors influence platelet function by mediating the response to various agonists, including ADP, thromboxane A2, and thrombin. Blockade of the ADP receptor, P2Y12, in combination with cyclooxygenase-1 inhibition by aspirin has been among the most widely used pharmacological strategies to reduce cardiovascular event occurrence in high-risk patients. The latter dual pathway blockade strategy is one of the greatest advances in the field of cardiovascular medicine. In addition to P2Y12, the platelet thrombin receptor, protease activated receptor-1, has also been recently targeted for inhibition. Blockade of protease activated receptor-1 has been associated with reduced thrombotic event occurrence when added to a strategy using P2Y12 and cyclooxygenase-1 inhibition. At this time, the relative contributions of these G-protein-coupled receptor signaling pathways to in vivo thrombosis remain incompletely defined. The observation of treatment failure in ≈10% of high-risk patients treated with aspirin and potent P2Y12 inhibitors provides the rationale for targeting novel pathways mediating platelet function. Targeting intracellular signaling downstream from G-protein-coupled receptor receptors with phosphotidylionisitol 3-kinase and Gq inhibitors are among the novel strategies under investigation to prevent arterial ischemic event occurrence. Greater understanding of the mechanisms of G-protein-coupled receptor-mediated signaling may allow the tailoring of antiplatelet therapy.

Gq-mediated Akt translocation to the membrane: a novel PIP3-independent mechanism in platelets

Akt is an important signaling molecule regulating platelet aggregation. Akt is phosphorylated after translocation to the membrane through Gi signaling pathways by a phosphatidylinositol-3,4,5-trisphosphate (PIP3)-dependent mechanism. However, Akt is more robustly phosphorylated by thrombin compared with adenosine 5'-diphosphate in platelets. This study investigated the mechanisms of Akt translocation as a possible explanation for this difference. Stimulation of washed human platelets with protease-activated receptor agonists caused translocation of Akt to the membrane rapidly, whereas phosphorylation occurred later. The translocation of Akt was abolished in the presence of a Gq-selective inhibitor or in Gq-deficient murine platelets, indicating that Akt translocation is regulated downstream of Gq pathways. Interestingly, phosphatidylinositol 3-kinase (PI3K) inhibitors or P2Y12 antagonist abolished Akt phosphorylation without affecting Akt translocation to the membrane, suggesting that Akt translocation occurs through a PI3K/PIP3/Gi-independent mechanism. An Akt scaffolding protein, p21-activated kinase (PAK), translocates to the membrane after stimulation with protease-activated receptor agonists in a Gq-dependent manner, with the kinetics of translocation similar to that of Akt. Coimmunoprecipitation studies showed constitutive association of PAK and Akt, suggesting a possible role of PAK in Akt translocation. These results show, for the first time, an important role of the Gq pathway in mediating Akt translocation to the membrane in a novel Gi/PI3K/PIP3-independent mechanism.

Targeting the anionic region of human protease-activated receptor 4 inhibits platelet aggregation and thrombosis without interfering with hemostasis

BACKGROUND

Human platelet activation and aggregation is a complex process. To date, many therapies have been developed targeting proteins that mediate this process to prevent unwanted activation. However, the current standard of care for acute coronary syndromes still has limitations, including bleeding risk.

OBJECTIVE

To evaluate the protease-activated receptor 4 (PAR4) anionic cluster as a viable antiplatelet target by using a polyclonal antibody (CAN12).

METHODS

We used western blotting, aggregation and secretion ex vivo to evaluate the ability of CAN12 to interact with PAR4 and inhibit platelet activation. The effects of CAN12 in vivo were evaluated with the Rose Bengal arterial thrombosis model and two models of hemostasis.

RESULTS

CAN12 was able to interact with human PAR4 and delay PAR4 cleavage. In addition, CAN12 inhibited thrombin-induced human platelet aggregation and secretion in a dose-dependent manner. The specificity of CAN12 was agonist-dependent. In vivo, we determined that CAN12 was able to inhibit arterial thrombosis, and, using two independent methods, we found that CAN12 did not influence hemostasis.

CONCLUSION

Targeting the extracellular anionic cluster on PAR4 is a viable novel strategy as an antiplatelet therapy.

Functional role of protease activated receptors in vascular biology

Protease activated receptors (PARs) are a small family of G protein-coupled receptors (GPCR) mediating the cellular effects of some proteases of the coagulation system, such as thrombin, or other proteases, such as trypsin or metalloproteinase 1. As the prototype of PARs, PAR1 is a seven transmembrane GPCR that, upon cleavage by thrombin, unmasks a new amino-terminus able to bind intramolecularly to PAR1 itself thus inducing signaling. In the vascular system, thrombin and other proteases of the coagulation-fibrinolysis system, such as plasmin, factor VIIa and factor Xa, activated protein C, are considered physiologically relevant agonists, and PARs appear to largely account for the cellular effects of these enzymes. In the vasculature, PARs are expressed on platelets, endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). In the vessel wall, under physiological conditions, PARs are mainly expressed in ECs and participate in the regulation of vascular tone, by inducing endothelium-dependent relaxation. PAR activation on ECs promotes conversion of these cells into a proinflammatory phenotype, causes increase of vascular permeability, and the exposure/secretion of proteins and cytokines mediating the local accumulation of platelets and leukocytes. These effects contribute to the vascular consequences of sepsis and of diseases such as acute lung injury and acute respiratory distress syndrome. In normal arteries PARs are to a much lesser amount expressed on VSMCs. However, in conditions associated with endothelial dysfunction, PARs mediate contraction, proliferation, migration, hypertrophy of VSMCs and their production of extracellular matrix, thereby contributing to the pathophysiology of atherosclerosis and hypertension. Inhibition of protease-PAR interaction might thus become a potential therapeutic target in various vascular diseases.

Increased responsiveness to thrombin through protease-activated receptors (PAR)-1 and -4 in active Crohn's disease

BACKGROUND AND AIMS

Platelets are essential in hemostasis and inflammation, thereby linking coagulation with inflammation. Abundant thrombin generation in association with inflammation is considered a major reason for the increased risk for thromboembolic events. We therefore investigated platelet responsiveness to thrombin.

METHODS

In this case-control study 85 patients with Crohn's disease (active CD 42, remission 43) and 30 sex- and age-matched controls were enrolled. Clinical disease activity (Harvey-Bradshaw-Index) was assessed and CD-related data were determined by chart review. Platelets' response to protease activated receptor-1 and -4 (PAR-1, -4) was assessed by whole blood platelet aggregometry (MEA), levels of platelets adhering to monocytes (PMA), and platelet surface P-selectin.

RESULTS

Platelets' aggregation after activation with the specific PAR-1 agonist (SFLLRN) and PAR-4 agonist (AYPGKF) was higher in patients with active CD compared to patients in remission and controls (p=0.0068 and p=0.0023 for SFLLRN, p=0.0019 and 0.0003 for AYPGKF). Likewise, levels of PMA after activation with PAR-1 and PAR-4 receptor agonists were higher in patients with active CD compared to patients in remission and controls (p=0.0001 and p<0.0001 for SFLLRN, p=0.0329 and p=0.0125 for AYPGKF). However, P-selectin expression on human platelets showed heterogeneous results. Only PAR-1 activation of platelets resulted in significant differences between CD patients and controls (p=0.0001 and p=0.0022 for active and inactive CD versus controls, respectively).

CONCLUSIONS

Our data suggest a new mechanism of platelet activation which has the potential to increase risk for thromboembolism in patients with active CD which might be due to platelets poised for thrombin-inducible activation.

Differential signaling by protease-activated receptors: implications for therapeutic targeting

Protease-activated receptors (PARs) are a family of four G protein-coupled receptors that exhibit increasingly appreciated differences in signaling and regulation both within and between the receptor class. By nature of their proteolytic self-activation mechanism, PARs have unique processes of receptor activation, "ligand" binding, and desensitization/resensitization. These distinctive aspects have presented both challenges and opportunities in the targeting of PARs for therapeutic benefit-the most notable example of which is inhibition of PAR1 on platelets for the prevention of arterial thrombosis. However, more recent studies have uncovered further distinguishing features of PAR-mediated signaling, revealing mechanisms by which identical proteases elicit distinct effects in the same cell, as well as how distinct proteases produce different cellular consequences via the same receptor. Here we review this differential signaling by PARs, highlight how important distinctions between PAR1 and PAR4 are impacting on the progress of a new class of anti-thrombotic drugs, and discuss how these more recent insights into PAR signaling may present further opportunities for manipulating PAR activation and signaling in the development of novel therapies.

Akt signaling in platelets and thrombosis

Akt is a Ser-Thr kinase with pleiotropic effects on cell survival, growth and metabolism. Recent evidence from gene-deletion studies in mice, and analysis of human platelets treated with Akt inhibitors, suggest that Akt regulates platelet activation, with potential consequences for thrombosis. Akt activation is regulated by the level of phosphoinositide 3-phosphates, and proteins that regulate concentrations of this lipid also regulate Akt activation and platelet function. Although the effectors through which Akt contributes to platelet activation are not definitively known, several candidates are discussed, including endothelial nitric oxide synthase, glycogen synthase kinase 3β, phosphodiesterase 3A and the integrin β(3) tail. Selective inhibitors of Akt isoforms or of proteins that contribute to its activation, such as individual PI3K isoforms, may make attractive targets for antithrombotic therapy. This review summarizes the current literature describing Akt activity and its regulation in platelets, including speculation regarding the future of Akt or its regulatory pathways as targets for the development of antithrombotic therapies.