Roles and interactions among protease-activated receptors and P2ry12 in hemostasis and thrombosis

Reversible Platelet Integrin αIIbβ3 Activation and Thrombus Instability

Integrin αIIbβ3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbβ3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbβ3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbβ3. Our analysis points to the (autocrine) ADP P2Y₁ and P2Y₁₂ receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

Protease-activated receptor antagonists prevent thrombosis when dual antiplatelet therapy is insufficient in an occlusive thrombosis microfluidic model

Background

Platelet activation and arterial thrombosis on a ruptured atherosclerotic plaque is a major cause of myocardial infarction. Dual antiplatelet therapy (DAPT), the combination of platelet aggregation inhibitors, aspirin and a P2Y12 antagonist, is used to prevent arterial thrombosis. However, many people continue to have arterial thrombosis and myocardial infarction despite DAPT, indicating that additional therapies are required where DAPT is insufficient.

Objectives

To determine whether antagonists of protease-activated receptors (PARs) can prevent occlusive thrombosis under conditions where DAPT is insufficient.

Methods

We used human whole blood in a microfluidic model of occlusive thrombosis to compare conditions under which DAPT is effective to those under which DAPT was not. Cangrelor (a P2Y12 antagonist) and aspirin were used to mimic DAPT. We then investigated whether the PAR1 antagonist vorapaxar or the PAR4 antagonist BMS 986120, alone or in combination with DAPT, prevented occlusive thrombosis.

Results and Conclusions

A ruptured plaque exposes collagen fibers and is often rich in tissue factor, triggering activation of platelets and coagulation. Occlusive thrombi formed on type I collagen in the presence or absence of tissue factor (TF). However, although DAPT prevented occlusive thrombosis in the absence of TF, DAPT had little effect when TF was also present. Under these conditions, PAR antagonism was also ineffective. However, occlusive thrombosis was prevented by combining DAPT with PAR antagonism. These data demonstrate that PAR antagonists may be a useful addition to DAPT in some patients and further demonstrate the utility of in vitro models of occlusive thrombosis.

Genetic deletion of platelet PAR4 results in reduced thrombosis and impaired hemostatic plug stability

BACKGROUND

Protease-activated receptor 4 (PAR4) is expressed by a wide variety of cells, including megakaryocytes/platelets, immune cells, cardiomyocytes, and lung epithelial cells. It is the only functional thrombin receptor on murine platelets. A global deficiency of PAR4 is associated with impaired hemostasis and reduced thrombosis.

OBJECTIVE

We aimed to generate a mouse line with a megakaryocyte/platelet-specific deletion of PAR4 (PAR4fl/fl ;PF4Cre+ ) and use the mouse line to investigate the role of platelet PAR4 in hemostasis and thrombosis in mice.

METHODS

Platelets from PAR4fl/fl ;PF4Cre+ were characterized in vitro. Arterial and venous thrombosis was analyzed. Hemostatic plug formation was analyzed using a saphenous vein laser injury model in mice with global or megakaryocyte/platelet-specific deletion of PAR4 or wild-type mice treated with thrombin or glycoprotein VI (GPVI) inhibitors.

RESULTS

PAR4fl/fl ;PF4Cre+ platelets were unresponsive to thrombin or specific PAR4 stimulation but not to other agonists. PAR4-/- and PAR4fl/fl ;PF4Cre+ mice both exhibited a similar reduction in arterial thrombosis compared to their respective controls. More importantly, we show for the first time that platelet PAR4 is critical for venous thrombosis in mice. In addition, PAR4-/- mice and PAR4fl/fl ;PF4Cre+ mice exhibited a similar impairment in hemostatic plug stability in a saphenous vein laser injury model. Inhibition of thrombin in wild-type mice gave a similar phenotype. Combined PAR4 deficiency on platelets with GPVI inhibition did not impair hemostatic plug formation but further reduced plug stability.

CONCLUSION

We generated a novel PAR4fl/fl ;PF4Cre+ mouse line. We used this mouse line to show that PAR4 signaling in platelets is critical for arterial and venous thrombosis and hemostatic plug stability.

Updated Understanding of Platelets in Thrombosis and Hemostasis: The Roles of Integrin PSI Domains and their Potential as Therapeutic Targets

Platelets are small blood cells known primarily for their ability to adhere and aggregate at injured vessels to arrest bleeding. However, when triggered under pathological conditions, the same adaptive mechanism of platelet adhesion and aggregation may cause thrombosis, a primary cause of heart attack and stroke. Over recent decades, research has made considerable progress in uncovering the intricate and dynamic interactions that regulate these processes. Integrins are heterodimeric cell surface receptors expressed on all metazoan cells that facilitate cell adhesion, movement, and signaling, to drive biological and pathological processes such as thrombosis and hemostasis. Recently, our group discovered that the plexin-semaphorin-integrin (PSI) domains of the integrin β subunits exert endogenous thiol isomerase activity derived from their two highly conserved CXXC active site motifs. Given the importance of redox reactions in integrin activation and its location in the knee region, this PSI domain activity may be critically involved in facilitating the interconversions between integrin conformations. Our monoclonal antibodies against the β3 PSI domain inhibited its thiol isomerase activity and proportionally attenuated fibrinogen binding and platelet aggregation. Notably, these antibodies inhibited thrombosis without significantly impairing hemostasis or causing platelet clearance. In this review, we will update mechanisms of thrombosis and hemostasis, including platelet versatilities and immune-mediated thrombocytopenia, discuss critical contributions of the newly discovered PSI domain thiol isomerase activity, and its potential as a novel target for anti-thrombotic therapies and beyond.

The endothelial protein C receptor plays an essential role in the maintenance of pregnancy

Placenta-mediated pregnancy complications are a major challenge in the management of maternal-fetal health. Maternal thrombophilia is a suspected risk factor, but the role of thrombotic processes in these complications has remained unclear. Endothelial protein C receptor (EPCR) is an anticoagulant protein highly expressed in the placenta. EPCR autoantibodies and gene variants are associated with poor pregnancy outcomes. In mice, fetal EPCR deficiency results in placental failure and in utero death. We show that inhibition of molecules involved in thrombin generation or in the activation of maternal platelets allows placental development and embryonic survival. Nonetheless, placentae exhibit venous thrombosis in uteroplacental circulation associated with neonatal death. In contrast, maternal EPCR deficiency results in clinical and histological features of placental abruption and is ameliorated with concomitant Par4 deficiency. Our findings unveil a causal link between maternal thrombophilia, uterine hemorrhage, and placental abruption and identify Par4 as a potential target of therapeutic intervention.

F2R Polymorphisms and Clopidogrel Efficacy and Safety in Patients With Minor Stroke or TIA

OBJECTIVE

To investigate the association between protease-activated receptor-1 (PAR-1) gene F2R polymorphisms and efficacy of clopidogrel for minor stroke or TIA.

METHODS

Three single nucleotide polymorphisms (CYP2C19*2 [681G>A, rs4244285], CYP2C19*3 [636G>A, rs4986893], and F2R [IVSn-14 A/T, rs168753]) were genotyped among 2,924 patients randomized to clopidogrel plus aspirin (n = 1,461) or aspirin alone (n = 1,463). The primary efficacy outcome was new stroke (ischemic or hemorrhagic) and the safety outcome was any bleeding.

RESULTS

Overall, 859 (29.4%) were AA homozygotes, 1,479 (50.6%) were AT heterozygotes, and 586 (20.0%) were TT homozygotes for F2R IVSn-14 polymorphisms; 1,716 (58.7%) were carriers of at least 1 CYP2C19 loss-of-function allele (*2 or *3). Compared with aspirin alone, patients with clopidogrel-aspirin treatment had a low risk of new stroke in patients with AT genotype (7.6% vs 11.3%; hazard ratio [HR], 0.63; 95% confidence interval [CI], 0.44-0.89) and TT genotype (5.8% vs 11.6%; HR, 0.46; 95% CI, 0.25-0.82) but not in carriers of the AA genotype (10.8% vs 11.6%; HR, 0.95; 95% CI, 0.63-1.44) (p = 0.03 for interaction). The association between F2R IVSn-14 A/T polymorphism and clopidogrel response was present regardless of the carrier status of the CYP2C19 loss-of-function alleles. The F2R IVSn-14 genotypes were not associated with the risk of any bleeding for clopidogrel-aspirin treatment (p = 0.66 for interaction).

CONCLUSIONS

Among patients with minor ischemic stroke or TIA who were receiving clopidogrel and aspirin, those carrying the F2R IVSn-14 T allele had a lower rate of recurrent stroke than those who were not.

CLINICALTRIALSGOV IDENTIFIER

NCT00979589.

The role of P2Y6R in cardiovascular diseases and recent development of P2Y6R antagonists

As a member of the P2Y receptor family with a typical 7-transmembrane structure, P2Y₆ purinergic receptor (P2Y₆R) belongs to the G-protein-coupled nucleotide receptor activating the phospholipase-C signaling pathway. P2Y₆R is widely involved in a range of human diseases, including atherosclerosis and other cardiovascular diseases, gradually attracting attention owing to its inappropriate or excessive activation. In addition, it was reported that P2Y₆R might regulate inflammatory responses by governing the maturation and secretion of proinflammatory cytokines. Hence, several P2Y₆R antagonists have been subjected to evaluation as new therapeutic strategies in recent years. This review was aimed at summarizing the role of P2Y₆R in the pathogenesis of cardiovascular diseases, with an insight into the recent progress on discovery of P2Y₆R antagonists.

Factor XIa-specific IgG and a reversal agent to probe factor XI function in thrombosis and hemostasis

Thrombosis is a major cause of morbidity and mortality. Current antithrombotic drugs are not ideal in that they must balance prevention of thrombosis against bleeding risk. Inhibition of coagulation factor XI (FXI) may offer an improvement over existing antithrombotic strategies by preventing some forms of thrombosis with lower bleeding risk. To permit exploration of this hypothesis in humans, we generated and characterized a series of human immunoglobulin Gs (IgGs) that blocked FXIa active-site function but did not bind FXI zymogen or other coagulation proteases. The most potent of these IgGs, C24 and DEF, inhibited clotting in whole human blood and prevented FeCl3-induced carotid artery occlusion in FXI-deficient mice reconstituted with human FXI and in thread-induced venous thrombosis in rabbits at clinically relevant doses. At doses substantially higher than those required for inhibition of intravascular thrombus formation in these models, DEF did not increase cuticle bleeding in rabbits or cause spontaneous bleeding in macaques over a 2-week study. Anticipating the desirability of a reversal agent, we also generated a human IgG that rapidly reversed DEF activity ex vivo in human plasma and in vivo in rabbits. Thus, an active site-directed FXIa-specific antibody can block thrombosis in animal models and, together with the reversal agent, may facilitate exploration of the roles of FXIa in human disease.

The contribution of thrombin-induced platelet activation to thrombus growth is diminished under pathological blood shear conditions

Developing novel anti-platelet therapies is an important clinical strategy for the prevention of arterial thromboses which cause heart attacks and most strokes. Thrombin activates platelets via protease-activated receptors (PARs), and PAR antagonists are currently under investigation as antithrombotics. Yet despite these clinical advances, the importance of PARs to platelet activation during thromboses formed under pathological conditions has not been investigated. To this end, we examined the role of PAR-dependent platelet activation in thrombus formation in the presence of elevated blood shear rates. We used two in vivo thrombosis models and an ex vivo whole blood flow approach in PAR4-/-mice, whose platelets are unresponsive to thrombin, to show that the contribution of PAR-mediated platelet activation to thrombosis is diminished at pathological blood shear rates as a direct result of decreased incorporation of thrombin-activated platelets into growing thrombi. Our ex vivo observations were replicated in human whole blood treated with a PAR1 antagonist. These results define a novel, shear-regulated role for thrombin/PAR-dependent platelet activation during thrombosis and provide important insights into the conditions under which PAR antagonists may best be used for the prevention of acute coronary syndromes.

Animal Models of Thrombosis From Zebrafish to Nonhuman Primates: Use in the Elucidation of New Pathologic Pathways and the Development of Antithrombotic Drugs

Thrombosis is a leading cause of morbidity and mortality worldwide. Animal models are used to understand the pathological pathways involved in thrombosis and to test the efficacy and safety of new antithrombotic drugs. In this review, we will first describe the central role a variety of animal models of thrombosis and hemostasis has played in the development of new antiplatelet and anticoagulant drugs. These include the widely used P2Y12 antagonists and the recently developed orally available anticoagulants that directly target factor Xa or thrombin. Next, we will describe the new players, such as polyphosphate, neutrophil extracellular traps, and microparticles, which have been shown to contribute to thrombosis in mouse models, particularly venous thrombosis models. Other mouse studies have demonstrated roles for the factor XIIa and factor XIa in thrombosis. This has spurred the development of strategies to reduce their levels or activities as a new approach for preventing thrombosis. Finally, we will discuss the emergence of zebrafish as a model to study thrombosis and its potential use in the discovery of novel factors involved in thrombosis and hemostasis. Animal models of thrombosis from zebrafish to nonhuman primates are vital in identifying pathological pathways of thrombosis that can be safely targeted with a minimal effect on hemostasis. Future studies should focus on understanding the different triggers of thrombosis and the best drugs to prevent each type of thrombotic event.

The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors

Platelets are critical for haemostasis, thrombosis, and inflammatory responses, but the events that lead to mature platelet production remain incompletely understood. The bone marrow has been proposed to be a major site of platelet production, although there is indirect evidence that the lungs might also contribute to platelet biogenesis. Here, by directly imaging the lung microcirculation in mice, we show that a large number of megakaryocytes circulate through the lungs, where they dynamically release platelets. Megakaryocytes that release platelets in the lungs originate from extrapulmonary sites such as the bone marrow; we observed large megakaryocytes migrating out of the bone marrow space. The contribution of the lungs to platelet biogenesis is substantial, accounting for approximately 50% of total platelet production or 10 million platelets per hour. Furthermore, we identified populations of mature and immature megakaryocytes along with haematopoietic progenitors in the extravascular spaces of the lungs. Under conditions of thrombocytopenia and relative stem cell deficiency in the bone marrow, these progenitors can migrate out of the lungs, repopulate the bone marrow, completely reconstitute blood platelet counts, and contribute to multiple haematopoietic lineages. These results identify the lungs as a primary site of terminal platelet production and an organ with considerable haematopoietic potential.

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.

Protease-Activated Receptor-1 Antagonists Post-Percutaneous Coronary Intervention

Thrombin is a potent platelet agonist, and protease-activated receptor-1 (PAR-1) is the main thrombin receptor in human platelets and thrombin. PAR-1 antagonism has attracted interest as a potential therapeutic target to reduce atherothrombotic events in patients with atherosclerotic disease, especially coronary artery disease. In this review, the author describes the rationale of PAR-1 antagonism for the reduction of atherothrombotic events and reviews the key phase 3 trial results, with special attention to analyses in percutaneous coronary intervention patients.

Systems-Pharmacology Dissection of Traditional Chinese Medicine Compound Saffron Formula Reveals Multi-scale Treatment Strategy for Cardiovascular Diseases

Cardiovascular diseases (CVDs) have been regarding as “the world’s first killer” of human beings in recent years owing to the striking morbidity and mortality, the involved molecular mechanisms are extremely complex and remain unclear. Traditional Chinese medicine (TCM) adheres to the aim of combating complex diseases from an integrative and holistic point of view, which has shown effectiveness in CVDs therapy. However, system-level understanding of such a mechanism of multi-scale treatment strategy for CVDs is still difficult. Here, we developed a system pharmacology approach with the purpose of revealing the underlying molecular mechanisms exemplified by a famous compound saffron formula (CSF) in treating CVDs. First, by systems ADME analysis combined with drug targeting process, 103 potential active components and their corresponding 219 direct targets were retrieved and some key interactions were further experimentally validated. Based on this, the network relationships among active components, targets and diseases were further built to uncover the pharmacological actions of the drug. Finally, a “CVDs pathway” consisted of several regulatory modules was incorporated to dissect the therapeutic effects of CSF in different pathological features-relevant biological processes. All this demonstrates CSF has multi-scale curative activity in regulating CVD-related biological processes, which provides a new potential way for modern medicine in the treatment of complex diseases.

Leukemia-associated Rho guanine-nucleotide exchange factor is not critical for RhoA regulation, yet is important for platelet activation and thrombosis in mice

BACKGROUND

RhoA is an important regulator of platelet responses downstream of Gα13 , yet we still know little about its regulation in platelets. Leukemia-associated Rho guanine-nucleotide exchange factor (GEF [LARG]), a RhoA GEF, is highly expressed in platelets and may constitute a major upstream activator of RhoA. To this end, it is important to determine the role of LARG in platelet function and thrombosis.

METHODS AND RESULTS

Using a platelet-specific gene knockout, we show that the absence of LARG results in a marked reduction in aggregation and dense-granule secretion in response to the thromboxane mimetic U46619 and proteinase-activated receptor 4-activating peptide, AYPGKF, but not to adenosine diphosphate. In a ferric chloride thrombosis model in vivo, this translated into a defect, under mild injury conditions. Importantly, agonist-induced RhoA activation was not affected by the absence of LARG, although basal activity was reduced, suggesting that LARG may play a housekeeper role in regulating constitutive RhoA activity.

CONCLUSIONS

LARG plays an important role in platelet function and thrombosis in vivo. However, although LARG may have a role in regulating the resting activation state of RhoA, its role in regulating platelet function may principally be through RhoA-independent pathways, possibly through other Rho family members.

Cardiovascular and pharmacological implications of haem-deficient NO-unresponsive soluble guanylate cyclase knock-in mice

Oxidative stress, a central mediator of cardiovascular disease, results in loss of the prosthetic haem group of soluble guanylate cyclase (sGC), preventing its activation by nitric oxide (NO). Here we introduce Apo-sGC mice expressing haem-free sGC. Apo-sGC mice are viable and develop hypertension. The haemodynamic effects of NO are abolished, but those of the sGC activator cinaciguat are enhanced in apo-sGC mice, suggesting that the effects of NO on smooth muscle relaxation, blood pressure regulation and inhibition of platelet aggregation require sGC activation by NO. Tumour necrosis factor (TNF)-induced hypotension and mortality are preserved in apo-sGC mice, indicating that pathways other than sGC signalling mediate the cardiovascular collapse in shock. Apo-sGC mice allow for differentiation between sGC-dependent and -independent NO effects and between haem-dependent and -independent sGC effects. Apo-sGC mice represent a unique experimental platform to study the in vivo consequences of sGC oxidation and the therapeutic potential of sGC activators.

Haem-free, NO-insensitive soluble guanylate cyclase (apo-sGC) generated during oxidative stress contributes to cardiovascular pathology. By generating and characterizing apo-sGC knock-in mice, Thoonen et al. provide a scientific ground for the therapeutic concept of sGC activators, and dissect the relevance of the NO-sGC axis.

Systems analysis of gene ontology and biological pathways involved in post-myocardial infarction responses

Background

Pathway analysis has been widely used to gain insight into essential mechanisms of the response to myocardial infarction (MI). Currently, there exist multiple pathway databases that organize molecular datasets and manually curate pathway maps for biological interpretation at varying forms of organization. However, inconsistencies among different databases in pathway descriptions, frequently due to conflicting results in the literature, can generate incorrect interpretations. Furthermore, although pathway analysis software provides detailed images of interactions among molecules, it does not exhibit how pathways interact with one another or with other biological processes under specific conditions.

Methods

We propose a novel method to standardize descriptions of enriched pathways for a set of genes/proteins using Gene Ontology terms. We used this method to examine the relationships among pathways and biological processes for a set of condition-specific genes/proteins, represented as a functional biological pathway-process network. We applied this algorithm to a set of 613 MI-specific proteins we previously identified.

Results

A total of 96 pathways from Biocarta, KEGG, and Reactome, and 448 Gene Ontology Biological Processes were enriched with these 613 proteins. The pathways were represented as Boolean functions of biological processes, delivering an interactive scheme to organize enriched information with an emphasis on involvement of biological processes in pathways. We extracted a network focusing on MI to demonstrate that tyrosine phosphorylation of Signal Transducer and Activator of Transcription (STAT) protein, positive regulation of collagen metabolic process, coagulation, and positive/negative regulation of blood coagulation have immediate impacts on the MI response.

Conclusions

Our method organized biological processes and pathways in an unbiased approach to provide an intuitive way to identify biological properties of pathways under specific conditions. Pathways from different databases have similar descriptions yet diverse biological processes, indicating variation in their ability to share similar functional characteristics. The coverages of pathways can be expanded with the incorporation of more biological processes, predicting involvement of protein members in pathways. Further, detailed analyses of the functional biological pathway-process network will allow researchers and scientists to explore critical routes in biological systems in the progression of disease.

Approval of the first protease-activated receptor antagonist: Rationale, development, significance, and considerations of a novel anti-platelet agent

Twenty-three years after the discovery of the first thrombin receptor, now known as protease-activated receptor 1 (PAR1), the first drug targeting this receptor is available for human use. The PAR1 inhibitor, vorapaxar (Zontivity, MSD), was recently approved by the FDA for use in the USA for the prevention of thrombotic cardiovascular events in patients with a history of myocardial infarction or peripheral artery disease. In this review, we detail the rationale, development, as well as the clinical significance and considerations of vorapaxar, the original PAR antagonist and the latest anti-platelet agent in the pharmaco-armoury against arterial thrombosis.

Individualising dual antiplatelet therapy after percutaneous coronary intervention: the IDEAL-PCI registry

OBJECTIVE

To evaluate the clinical utility of individualising dual antiplatelet therapy (DAPT) after percutaneous coronary intervention (PCI) in an all-comers population, including ST-elevation myocardial infarction (STEMI) patients.

SETTING

Tertiary care single centre registry.

PARTICIPANTS

1008 consecutive PCI patients with stent implantation, without exclusion criteria.

INTERVENTION

Peri-interventional individualisation of DAPT, guided by multiple electrode aggregometry (MEA), to overcome high on-treatment platelet reactivity (HPR) to ADP-induced (≥50 U) and arachidonic acid (AA)-induced aggregation (>35 U).

OUTCOME MEASURES

The primary efficacy end point was definite stent thrombosis (ST) at 30 days. The primary safety end point was thrombolysis in myocardial infarction (TIMI) major and minor bleeding. Secondary end points were probable ST, myocardial infarction, cardiovascular death and the combined end point: major cardiac adverse event (MACE).

RESULTS

53% of patients presented with acute coronary syndrome (9% STEMI, 44% non-ST-elevation). HPR to ADP after 600 mg clopidogrel loading occurred in 30% of patients (73±19 U vs 28±11 U; p<0.001) and was treated by prasugrel or ticagrelor (73%), or clopidogrel (27%) reloading (22±12 U; p<0.001). HPR to ADP after prasugrel loading occurred in 2% of patients (82±26 U vs 19±10 U; p<0.001) and was treated with ticagrelor (34±15 U; p=0.02). HPR to AA occurred in 9% of patients with a significant higher proportion in patients with HPR to ADP (22% vs 4%, p<0.001) and was treated with aspirin reloading. Definite ST occurred in 0.09% of patients (n=1); probable ST, myocardial infarction, cardiovascular death and MACE occurred in 0.19% (n=2), 0.09% (n=1) and 1.8% (n=18) of patients. TIMI major and minor bleeding did not differ between patients without HPR and individualised patients (2.6% for both).

CONCLUSIONS

Individualisation of DAPT with MEA minimises early thrombotic events in an all-comers PCI population to an unreported degree without increasing bleeding. A randomised multicentre trial utilising MEA seems warranted.

TRIAL REGISTRATION NUMBER

http://www.clinicaltrials.gov; NCT01515345.

Redundancy and interaction of thrombin- and collagen-mediated platelet activation in tail bleeding and carotid thrombosis in mice

OBJECTIVE

Current antiplatelet strategies to prevent myocardial infarction and stroke are limited by bleeding risk. A better understanding of the roles of distinct platelet-activating pathways is needed. We determined whether platelet activation by 2 key primary activators, thrombin and collagen, plays distinct, redundant, or interacting roles in tail bleeding and carotid thrombosis in mice.

APPROACH AND RESULTS

Platelets from mice deficient for the thrombin receptor protease-activated receptor-4 (Par4) and the collagen receptor glycoprotein VI protein (GPVI) lack responses to thrombin and collagen, respectively. We examined tail bleeding and FeCl3-induced carotid artery occlusion in mice lacking Par4, GPVI, or both. We also examined a series of Par mutants with increasing impairment of thrombin signaling in platelets. Ablation of thrombin signaling alone by Par4 deficiency increased blood loss in the tail bleeding assay and impaired occlusive thrombus formation in the carotid occlusion assay. GPVI deficiency alone had no effect. Superimposing GPVI deficiency on Par4 deficiency markedly increased effect size in both assays. In contrast to complete ablation of thrombin signaling, 9- and 19-fold increases in EC50 for thrombin-induced platelet activation had only modest effects.

CONCLUSIONS

The observation that loss of Par4 uncovered large effects of GPVI deficiency implies that Par4 and GPVI made independent, partially redundant contributions to occlusive thrombus formation in the carotid and to hemostatic clot formation in the tail under the experimental conditions examined. At face value, these results suggest that thrombin- and collagen-induced platelet activation can play partially redundant roles, despite important differences in how these agonists are made available to platelets.

Substituted indoles as selective protease activated receptor 4 (PAR-4) antagonists: Discovery and SAR of ML354

Herein we report the discovery and SAR of an indole-based protease activated receptor-4 (PAR-4) antagonist scaffold derived from a similarity search of the Vanderbilt HTS collection, leading to MLPCN probe ML354 (VU0099704). Using a novel PAC-1 fluorescent αIIbβ3 activation assay this probe molecule antagonist was found to have an IC50 of 140nM for PAR-4 with 71-fold selectivity versus PAR-1 (PAR-1IC50=10μM).

Can the antiplatelet effects of cangrelor be reliably studied in mice under in vivo and in vitro conditions using flow cytometry?

BACKGROUND

The effects of blood platelet inhibitors are often not quite equivalent under in vivo and in vitro conditions. Amongst various models of human pathology using laboratory animals, mice offer several benefits that make them convenient tools for studying the putative therapeutic value of various compounds. However, despite its advantages, the mouse model has methodological limitations concerning the small amount of blood available and technical difficulties with its collection. Among the variety of available methods used to study blood platelet activation and/or reactivity, flow cytometry seems an attractive technique that largely minimizes the constraints of using small rodents and enables outcomes of laboratory research to be transferred successfully to clinical practice. In this study we aimed at a critical evaluation of the optimal discriminative flow cytometric protocol, useful for reliable studies of the effect of cangrelor, a P2Y12 receptor antagonist, on mouse platelets under in vitro and in vivo conditions.

METHODS

Blood samples were drawn from two-month-old female BALB/c mice. Protocols differing in methods of anesthesia, blood withdrawal, anticoagulation, gating antibodies, blood preparation and fixation were tested to optimize the one best suited to discrimination between resting and activated platelets. The antiplatelet capabilities of cangrelor were tested in vitro (140 μM in whole blood) and in vivo (7.8 mg/kg b.w. administered once, directly into the bloodstream through the vena cava of the anesthetized animal, 15 min prior to blood withdrawal). Expressions of P-selectin, activated α(IIb)β3 complex and GPIba were monitored using two-color flow cytometry.

RESULTS

"Washed blood" anticoagulated with low molecular weight heparin demonstrated the best discrimination between circulating (resting) platelets and upon their in vitro response to thrombin, collagen or ADP in freshly-stained unfixed cell suspensions. Cangrelor inhibited the expression of the active form of the integrin a(IIb)β3 to approximately the same extent under in vitro and in vivo conditions (84.5 ± 7.7% vs. 75.4 ± 19.5% for the in vitro and in vivo approaches, respectively, n.s.).

CONCLUSIONS

The agreement between the in vivo and in vitro approaches with respect to cangrelor-inhibited hallmarks of blood platelet activation and reactivity supports our proposal that flow cytometry is useful and reliable for determining the effects of antiplatelet agents on the activation of circulating platelets in the mouse model, as well as the in vitro response of platelets to agonists.

Cross-modulatory effects of clopidogrel and heparin on platelet and fibrin incorporation in thrombosis

Pharmacologic inhibition of platelet activation and aggregation is a mainstay for reducing the incidence of arterial thrombosis, whereas anticoagulation is the primary approach for preventing the development of venous thrombosis. The effect of standard pharmacologic agents on their reciprocal vessel - anticoagulants on arterial thrombosis and platelet inhibitor on venous thrombosis - is relatively understudied. This study was designed to evaluate murine large-vessel arterial or venous thrombosis under conditions of either fibrin or platelet inhibition. In this study, heparin and clopidogrel were used as standard anticoagulant and platelet inhibitor, respectively, evaluating both large artery and vein thrombosis in mice, using in-vivo fluorescence imaging to simultaneously measure fibrin and platelet levels at the thrombus induction site. Heparin reduced both fibrin and platelet development in both arteries and veins, with stronger influences on fibrin accrual. Clopidogrel had a stronger effect in arteries, reducing both platelet and fibrin accumulation. Clopidogrel also reduced platelet accumulation with venous thrombosis, but the reductions in fibrin formation did not reach statistical significance. These findings illustrate the interactive role of platelet activity and coagulation in the development of large-vessel thrombosis, with inhibition of one thrombotic component showing profound effects on the other component in both arterial and venous thrombosis.

Platelet immunoreceptor tyrosine-based activation motif (ITAM) signaling and vascular integrity

Platelets are well-known for their critical role in hemostasis, that is, the prevention of blood loss at sites of mechanical vessel injury. Inappropriate platelet activation and adhesion, however, can lead to thrombotic complications, such as myocardial infarction and stroke. To fulfill its role in hemostasis, the platelet is equipped with various G protein-coupled receptors that mediate the response to soluble agonists such as thrombin, ADP, and thromboxane A2. In addition to G protein-coupled receptors, platelets express 3 glycoproteins that belong to the family of immunoreceptor tyrosine-based activation motif receptors: Fc receptor γ chain, which is noncovalently associated with the glycoprotein VI collagen receptor, C-type lectin 2, the receptor for podoplanin, and Fc receptor γII A, a low-affinity receptor for immune complexes. Although both genetic and chemical approaches have documented a critical role for platelet G protein-coupled receptors in hemostasis, the contribution of immunoreceptor tyrosine-based activation motif receptors to this process is less defined. Studies performed during the past decade, however, have identified new roles for platelet immunoreceptor tyrosine-based activation motif signaling in vascular integrity in utero and at sites of inflammation. The purpose of this review is to summarize recent findings on how platelet immunoreceptor tyrosine-based activation motif signaling controls vascular integrity, both in the presence and absence of mechanical injury.

Targeting platelet thrombin receptor signaling to prevent thrombosis

Platelets contribute fundamentally to ischemic heart disease, and antiplatelet therapy has been critical to reducing acute thrombotic complications of atherosclerotic disease. Thrombin, by acting on protease activated receptors (PAR), is one of the most potent platelet activators. PAR-1 antagonists may therefore provide more comprehensive antithrombotic effects. We review the pathophysiology of atherothrombosis, platelet activation by thrombin, the role of platelet protease activated receptors (PAR), and the clinical data supporting their use.

Antiplatelet and anticoagulant therapy for atherothrombotic disease: the role of current and emerging agents

Coronary atherothrombotic disease, including chronic stable angina and acute coronary syndromes (ACS), is associated with significant global burden. The acute clinical manifestations of atherothrombotic disease are mediated by occlusive arterial thrombi that impair tissue perfusion and are composed of a core of aggregated platelets, generated by platelet activation, and a superimposed fibrin mesh produced by the coagulation cascade. Long-term antithrombotic therapies, namely oral antiplatelet agents and anticoagulants, have demonstrated variable clinical effects. Aspirin and P2Y12 adenosine diphosphate (ADP) receptor antagonists have been shown to reduce the risk for thrombosis and ischaemic events by blocking the thromboxane (Tx) A2 and platelet P2Y12 activation pathways, respectively, whereas the benefits of oral anticoagulants have not been consistently documented. However, even in the presence of aspirin and a P2Y12 receptor antagonist, the risk for ischaemic events remains substantial because platelet activation continues via pathways independent of TxA2 and ADP, most notably the protease-activated receptor (PAR)-1 platelet activation pathway stimulated by thrombin. Emerging antithrombotic therapies include those targeting the platelet, such as the new P2Y12 antagonists and a novel class of oral PAR-1 antagonists, and those inhibiting the coagulation cascade, such as the new direct factor Xa antagonists, the direct thrombin inhibitors, and a novel class of factor IX inhibitors. The role of emerging antiplatelet agents and anticoagulants in the long-term management of patients with atherothrombotic disease will be determined by the balance of efficacy and safety in large ongoing clinical trials.

Platelet ITAM signaling is critical for vascular integrity in inflammation

Platelets play a critical role in maintaining vascular integrity during inflammation, but little is known about the underlying molecular mechanisms. Here we report that platelet immunoreceptor tyrosine activation motif (ITAM) signaling, but not GPCR signaling, is critical for the prevention of inflammation-induced hemorrhage. To generate mice with partial or complete defects in these signaling pathways, we developed a protocol for adoptive transfer of genetically and/or chemically inhibited platelets into thrombocytopenic (TP) mice. Unexpectedly, platelets with impaired GPCR signaling, a crucial component of platelet plug formation and hemostasis, were indistinguishable from WT platelets in their ability to prevent hemorrhage at sites of inflammation. In contrast, inhibition of GPVI or genetic deletion of Clec2, the only ITAM receptors expressed on mouse platelets, significantly reduced the ability of platelets to prevent inflammation-induced hemorrhage. Moreover, transfusion of platelets without ITAM receptor function or platelets lacking the adapter protein SLP-76 into TP mice had no significant effect on vascular integrity during inflammation. These results indicate that the control of vascular integrity is a major function of immune-type receptors in platelets, highlighting a potential clinical complication of novel antithrombotic agents directed toward the ITAM signaling pathway.

Heparin rescues factor V Leiden-associated placental failure independent of anticoagulation in a murine high-risk pregnancy model

Low molecular weight heparin (LMWH) is being tested as an experimental drug for improving pregnancy outcome in women with inherited thrombophilia and placenta-mediated pregnancy complications, such as recurrent pregnancy loss. The role of thrombotic processes in these disorders remains unproven, and the issue of antithrombotic prophylaxis is intensely debated. Using a murine model of factor V Leiden-associated placental failure, we show that treatment of the mother with LMWH allows placental development to proceed and affords significant protection from fetal loss. Nonetheless, the therapeutic effect of LMWH is not replicated by anticoagulation; fondaparinux and a direct Xa inhibitor, C921-78, achieve anticoagulation similar to LMWH but produce little or no improvement in pregnancy outcome. Genetic attenuation of maternal platelet aggregation is similarly ineffective. In contrast, even a partial loss of thrombin sensitivity of maternal platelets protects pregnancies. Neonates born from these pregnancies are growth retarded, suggesting that placental function is only partially restored. The placentae are smaller but do not reveal any evidence of thrombosis. Our data demonstrate an anticoagulation-independent role of LMWH in protecting pregnancies and provide evidence against the involvement of thrombotic processes in thrombophilia-associated placental failure. Importantly, thrombin-mediated maternal platelet activation remains central in the mechanism of placental failure.

High-resolution crystal structure of human protease-activated receptor 1

Protease-Activated Receptor-1 (PAR1) is the prototypical member of a family of G protein-coupled receptors that mediate cellular responses to thrombin and related proteases. Thrombin irreversibly activates PAR1 by cleaving the N-terminal exodomain of the receptor, which exposes a tethered peptide ligand that binds the receptor’s heptahelical bundle to effect G protein-activation. Here we report a 2.2Å resolution crystal structure of human PAR1 bound to vorapaxar, a PAR1 antagonist. The structure reveals an unusual mode of drug binding that explains how a small molecule binds virtually irreversibly to inhibit receptor activation by PAR1’s tethered ligand. In contrast to deep, solvent-exposed binding pockets observed in other peptide-activated GPCRs, the vorapaxar-binding pocket is superficial but has little surface exposed to the aqueous solvent. PARs are important targets for drug development. The structure reported here will aid development of improved PAR1 antagonists and discovery of antagonists to other members of this receptor family.

Safety and efficacy of targeting platelet proteinase-activated receptors in combination with existing anti-platelet drugs as antithrombotics in mice

BACKGROUND AND PURPOSE

Developing novel anti-platelet strategies is fundamental to reducing the impact of thrombotic diseases. Thrombin activates platelets via proteinase-activated receptors (PARs), and PAR antagonists are being evaluated in clinical trials for prevention of arterial thrombosis. However, one such trial was recently suspended due to increased bleeding in patients receiving a PAR₁ antagonist in addition to anti-platelet drugs that most often included both aspirin and clopidogrel. Therefore, it remains unclear how to best manipulate PARs for safe antithrombotic activity. To address this, we have examined potential interactions between existing anti-platelet drugs and strategies that target PARs.

EXPERIMENTAL APPROACH

We used in vivo mouse models in which interactions between various anti-platelet strategies could be evaluated. We examined the effects on thrombosis and haemostasis in PAR₄ -/- mice (platelets unresponsive to thrombin) treated with therapeutic doses of either aspirin or clopidogrel.

KEY RESULTS

Using a model in which occlusive thrombosis occurred in PAR₄ -/- mice or wild-type mice treated with aspirin or clopidogrel, PAR₄ -/- mice treated with either anti-platelet agent showed marked protection against thrombosis. This antithrombotic effect occurred without any effect on haemostasis with aspirin, but not clopidogrel. Furthermore, specifically targeting thrombin-induced platelet activation (via PARs) improved the therapeutic window of non-specifically inhibiting thrombin functions (via anticoagulants).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that PAR antagonists used in combination with aspirin provide a potent yet safe antithrombotic strategy in mice and provide insights into the safety and efficacy of using PAR antagonists for the prevention of acute coronary syndromes in humans.

Protease activated receptor-1 (PAR-1) mediated platelet aggregation is dependent on clopidogrel response

INTRODUCTION

Clopidogrel inhibits ADP mediated platelet aggregation through inhibition of the P2Y12 receptor by its active metabolite. Thrombin induces platelet aggregation by binding to protease activated receptor-1 (PAR-1), and inhibition of PAR-1 has been evaluated in patients treated with clopidogrel to reduce ischemic events after acute coronary syndromes. Residual PAR-1 mediated platelet aggregation may be dependent on extent of clopidogrel response.

MATERIAL AND METHODS

Platelet aggregation was measured in 55 patients undergoing elective PCI at 16-24 hours after 600 mg clopidogrel loading dose by light transmittance aggregometry using ADP 20 μM and thrombin receptor agonist peptide (TRAP) at 15 μM and 25 μM as agonists. Genomic DNA was genotyped for common CYP2C19 variants.

RESULTS

Increasing quartiles of 20 μM ADP induced platelet aggregation after clopidogrel loading were associated with increasing levels of TRAP mediated platelet aggregation. Patients in the highest quartile (clopidogrel non-responders) of post treatment ADP aggregation had significantly higher TRAP mediated aggregation than the patients in the lowest quartile (clopidogrel responders) [TRAP 15 μM: 79.6 ± 5% vs. 69.5 ± 8%, p<0.001].

CONCLUSIONS

Non-responders to clopidogrel show increased residual platelet aggregation induced by TRAP, whereas clopidogrel responders exhibit attenuated response to TRAP. Addition of PAR-1 antiplatelet drugs may be most effective in patients with reduced clopidogrel response and high residual TRAP mediated platelet aggregation.

Targeting proteinase-activated receptors: therapeutic potential and challenges

Proteinase-activated receptors (PARs), a family of four seven-transmembrane G protein-coupled receptors, act as targets for signalling by various proteolytic enzymes. PARs are characterized by a unique activation mechanism involving the proteolytic unmasking of a tethered ligand that stimulates the receptor. Given the emerging roles of these receptors in cancer as well as in disorders of the cardiovascular, musculoskeletal, gastrointestinal, respiratory and central nervous system, PARs have become attractive targets for the development of novel therapeutics. In this Review we summarize the mechanisms by which PARs modulate cell function and the roles they can have in physiology and diseases. Furthermore, we provide an overview of possible strategies for developing PAR antagonists.