Platelet ADP receptors contribute to the initiation of intravascular coagulation

Role and recent progress of P2Y12 receptor in cancer development

P2Y12 receptor (P2Y12R) is an adenosine-activated G protein-coupled receptor (GPCR) that plays a central role in platelet function, hemostasis, and thrombosis. P2Y12R activation can promote platelet aggregation and adhesion to cancer cells, promote tumor angiogenesis, and affect the tumor immune microenvironment (TIME) and tumor drug resistance, which is conducive to the progression of cancers. Meanwhile, P2Y12R inhibitors can inhibit this effect, suggesting that P2Y12R may be a potential therapeutic target for cancer. P2Y12R is involved in cancer development and metastasis, while P2Y12R inhibitors are effective in inhibiting cancer. However, a new study suggests that long-term use of P2Y12R inhibitors may increase the risk of cancer and the mechanism remains to be explored. In this paper, we reviewed the structural and functional characteristics of P2Y12R and its role in cancer. We explored the role of P2Y12R inhibitors in different tumors and the latest advances by summarizing the basic and clinical studies on the effects of P2Y12R inhibitors on tumors.

The Impact of ADP Inhibition on Traumatic Brain Injury Outcomes

Platelet inhibition correlates with severity of traumatic brain injury and may be associated with mortality. Adenosine diphosphate participates in platelet aggregation via the activation of the ADP receptors, P2Y1 and P2Y12. Prior work suggests this ADP pathway is significant in managing patients with head injuries. This study aimed to measure the influence of ADP inhibition on outcomes after a traumatic brain injury (TBI), as measured by thromboelastography with platelet mapping (TEG-PM). Outcomes were defined as (a) hospital length of stay; (b) ICU length of stay, (c) mortality, and (d) progression of hemorrhage on CT. The resulting cohort was split into quartiles to compare the effect of increasingly inhibited ADP values on the identified outcomes. Comparisons of 2 groups of patients were also conducted, one defined by ADP inhibition less than or equal to 60% and the other group by ADP inhibition of greater than 60%. 98 patients were included in final analysis, with 72.4% having ADP inhibition less than 60%. These patients were significantly older and had lower global injury severity scores (ISSs), although their head-specific ISS was equivalent. Compared to the group with ADP inhibition over 60%, there was no significant difference in mortality, hospital or ICU length of stay, or progression of lesion on CT. Patients with ADP less than 60% inhibited had smaller ISS and higher GCS, indicating they were less injured than those with greater ADP inhibition, consistent with prior literature. The equivalent ICU and hospital length of stay and mortality suggests that ADP inhibition plays a smaller role in outcomes. Additional study with a larger sample size and guideline-based assessments is necessary to further define the impact of ADP inhibition and to determine the role of platelet transfusion in this population.

Cell Surface Platelet Tissue Factor Expression: Regulation by P2Y12 and Link to Residual Platelet Reactivity

BACKGROUND

ADP-induced platelet activation leads to cell surface expression of several proteins, including TF (tissue factor). The role of ADP receptors in platelet TF modulation is still unknown. We aimed to assess the (1) involvement of P2Y1 and P2Y12 receptors in ADP-induced TF exposure; (2) modulation of TFpos-platelets in anti-P2Y12-treated patients with coronary artery disease. Based on the obtained results, we revisited the intracellular localization of TF in platelets.

METHODS

The effects of P2Y1 or P2Y12 antagonists on ADP-induced TF expression and activity were analyzed in vitro by flow cytometry and thrombin generation assay in blood from healthy subjects, P2Y12-/-, and patients with gray platelet syndrome. Ex vivo, P2Y12 inhibition of TF expression by clopidogrel/prasugrel/ticagrelor, assessed by VASP (vasodilator-stimulated phosphoprotein) platelet reactivity index, was investigated in coronary artery disease (n=238). Inhibition of open canalicular system externalization and electron microscopy (TEM) were used for TF localization.

RESULTS

In blood from healthy subjects, stimulated in vitro by ADP, the percentage of TFpos-platelets (17.3±5.5%) was significantly reduced in a concentration-dependent manner by P2Y12 inhibition only (-81.7±9.5% with 100 nM AR-C69931MX). In coronary artery disease, inhibition of P2Y12 is paralleled by reduction of ADP-induced platelet TF expression (VASP platelet reactivity index: 17.9±11%, 20.9±11.3%, 40.3±13%; TFpos-platelets: 10.5±4.8%, 9.8±5.9%, 13.6±6.3%, in prasugrel/ticagrelor/clopidogrel-treated patients, respectively). Despite this, 15% of clopidogrel good responders had a level of TFpos-platelets similar to the poor-responder group. Indeed, a stronger P2Y12 inhibition (130-fold) is required to inhibit TF than VASP. Thus, a VASP platelet reactivity index <20% (as in prasugrel/ticagrelor-treated patients) identifies patients with TFpos-platelets <20% (92% sensitivity). Finally, colchicine impaired in vitro ADP-induced TF expression but not α-granule release, suggesting that TF is open canalicular system stored as confirmed by TEM and platelet analysis of patients with gray platelet syndrome.

CONCLUSIONS

Data show that TF expression is regulated by P2Y12 and not P2Y1; P2Y12 antagonists downregulate the percentage of TFpos-platelets. In clopidogrel good-responder patients, assessment of TFpos-platelets highlights those with residual platelet reactivity. TF is stored in open canalicular system, and its membrane exposure upon activation is prevented by colchicine.

The Function and Regulation of Platelet P2Y12 Receptor

In addition to the key role in hemostasis and thrombosis, platelets have also been wildly acknowledged as immune regulatory cells and involving in the pathogenesis of inflammation-related diseases. Since purine receptor P2Y12 plays a crucial role in platelet activation, P2Y12 antagonists such as clopidogrel, prasugrel, and ticagrelor have been widely used in cardiovascular diseases worldwide in recent decades due to their potent antiplatelet and antithrombotic effects. Meanwhile, the role of P2Y12 in inflammatory diseases has also been extensively studied. Relatively, there are few studies on the regulation of P2Y12. This review first summarizes the various roles of P2Y12 in the process of platelet activation, as well as downstream effects and signaling pathways; then introduces the effects of P2Y12 in inflammatory diseases such as sepsis, atherosclerosis, cancer, autoimmune diseases, and asthma; and finally reviews the current researches on P2Y12 regulation.

Effects of Guideline-Based Correction of Platelet Inhibition on Outcomes in Moderate to Severe Isolated Blunt Traumatic Brain Injury

Platelet dysfunction has been demonstrated after traumatic brain injury (TBI) regardless of the use of platelet inhibitors. The purpose of this study was to determine the efficacy of a platelet-mapping thromboelastography (PM-TEG) in predicting TBI patients who would benefit from platelet transfusion. We hypothesized that adenosine diphosphate (ADP) and arachadonic acid (AA) inhibition in patients with TBI is associated with increased mortality and can be corrected with platelet transfusion. This is a retrospective review of patients admitted to a level 1 trauma center from January 2016 through September 2017 with moderate to severe blunt TBI (msTBI), defined by an initial Glasgow Coma Scale (GCS) ≤12 with intracranial hemorrhage. Patients received PM-TEG. Those with platelet dysfunction (ADP or AA inhibition ≥60%) received one unit of platelets followed by repeat PM-TEG, until inhibition <60% or three units of platelets. Cohorts were defined as patients initially without (NPI) and with (PI) inhibition and subdivided into those whose inhibition corrected (PI-C) versus those whose did not correct (PI-NC). From 69 patients with isolated blunt TBI, 40 (58%) presented with NPI, 29 (42%) with PI. Of those with PI, 16 (55%) were with PI-C and 13 (45%) with PI-NC. Platelet inhibition in msTBI patients undergoing guideline-based transfusion is associated with age and GCS and an increase in mortality. Platelet inhibition seems to have a more adverse effect on patients >55 years of age or with GCS <8. Correction of platelet inhibition normalized mortality to that of NPI.

Ticagrelor versus clopidogrel in reducing inflammatory cell infiltration of thrombus aspirated in patients with ST-elevation myocardial infarction

BACKGROUND

Ticagrelor provides more rapid, potent, and consistent anti-platelet efficacy than clopidogrel. This randomized trial aimed to evaluate the anti-inflammation effects of ticagrelor versus clopidogrel on thrombus aspirated from the ST-elevation myocardial infarction (STEMI) patients.

METHOD

A total of 98 patients with STEMI and intended percutaneous coronary intervention (PCI) were randomly assigned to receive clopidogrel (600-mg loading dose) or ticagrelor (180-mg loading dose), of whom 55 with large thrombus burden underwent thrombus aspiration during PCI. Thrombus specimens were successfully aspirated from 49 patients. Finally, 24 patients in the clopidogrel group and 23 in the ticagrelor group completed the study. Inflammatory cells within thrombi were assessed by hematoxylin-eosin and immunohistochemistry stainings.

RESULTS

Compared with the clopidogrel group, the number of total inflammatory cells per mm² thrombus area in the ticagrelor group was decreased by 28% (P = 0.009). The numbers of neutrophils and myeloperoxidase-positive cells per mm² thrombus area in the ticagrelor group were respectively decreased by 35% (P = 0.016) and 28% (P = 0.047), as compared with those in the clopidogrel group. Moreover, ticagrelor treatment reduced the ratio of monocytes number higher than 250 per mm² thrombus area compared with clopidogrel treatment (4% versus 29%, P = 0.048).

CONCLUSION

In patients with undergoing PCI for STEMI, the loading dose ticagrelor regimen was associated with a reduction in inflammatory cell infiltration within thrombus compared with the loading dose clopidogrel regimen.

Purinergic signaling elements are correlated with coagulation players in peripheral blood and leukocyte samples from COVID-19 patients

For over a year, the coronavirus disease 2019 has been affecting the world population by causing severe tissue injuries and death in infected people. Adenosine triphosphate (ATP) and the nicotinamide adenine dinucleotide (NAD +) are two molecules that are released into the extracellular microenvironment after direct virus infection or cell death caused by hyper inflammation and coagulopathy. Also, these molecules are well known to participate in multiple pathways and have a pivotal role in the purinergic signaling pathway. Thus, using public datasets available on the Gene Expression Omnibus (GEO), we analyzed raw proteomics data acquired using mass spectrometry (the gold standard method) and raw genomics data from COVID-19 patient samples obtained by microarray. The data was analyzed using bioinformatics and statistical methods according to our objectives. Here, we compared the purinergic profile of the total leukocyte population and evaluated the levels of these soluble biomolecules in the blood, and their correlation with coagulation components in COVID-19 patients, in comparison to healthy people or non-COVID-19 patients. The blood metabolite analysis showed a stage-dependent inosine increase in COVID-19 patients, while the nucleotides ATP and ADP had positive correlations with fibrinogen and other coagulation proteins. Also, ATP, ADP, inosine, and hypoxanthine had positive and negative correlations with clinical features. Regarding leukocyte gene expression, COVID-19 patients showed an upregulation of the P2RX1, P2RX4, P2RX5, P2RX7, P2RY1, P2RY12, PANX1, ADORA2B, NLPR3, and F3 genes. Yet, the ectoenzymes of the canonical and non-canonical adenosinergic pathway (ENTPD1 and CD38) are upregulated, suggesting that adenosine is produced by both active adenosinergic pathways. Hence, approaches targeting these biomolecules or their specific purinoreceptors and ectoenzymes may attenuate the high inflammatory state and the coagulopathy seen in COVID-19 patients. KEY MESSAGES : Adenosinergic pathways are modulated on leukocytes from COVID-19 patients. Plasmatic inosine levels are increased in COVID-19 patients. ATP, ADP, AMP, hypoxanthine, and inosine are correlated with coagulation players. The nucleotides and nucleosides are correlated with patients' clinical features. The P2 receptors and ectoenzymes are correlated with Tissue factor in COVID-19.

Mechanisms of thrombosis and research progress on targeted antithrombotic drugs

Globally, the incidence of thromboembolic diseases has increased in recent years, accompanied by an increase in patient mortality. Currently, several targeting delivery strategies have been developed to treat thromboembolic diseases. In this review, we discuss the mechanisms of thrombolysis and current anticoagulant drugs, particularly those with targeting capability, highlighting advances in the accurate treatment of thrombolysis with fewer adverse effects. Such approaches include magnetic drug-loading systems combined with molecular imaging to recanalize blood vessels and systems based on chimeric Arg-Gly-Asp (RGD) sequences that can target platelet glycoprotein receptor. With such progress in targeted antithrombotic drugs, targeted thrombolysis treatment shows significant potential benefit for patients.

Rhinoplasty with Fillers and Fat Grafting

Nonsurgical rhinoplasty is one choice for cases in which open surgery may be harmful, the deformity is not indicated to correct with open surgery, or in patients who have phobia of general anesthesia or any type of surgery. Autologous fat injection or fillers are most common materials currently available in the market. In this article, we explain the indications, contraindications, methods, and complications of this treatment.

Essential thrombocythemia: a hemostatic view of thrombogenic risk factors and prognosis

Essential thrombocythemia (ET) is a classical myeloproliferative neoplasm that is susceptible to hypercoagulable state due to impaired hemostatic system, so that thrombotic complications are the leading cause of mortality in ET patients. The content used in this article has been obtained by the PubMed database and Google Scholar search engine from English-language articles (2000-2019) using the following keywords: "Essential thrombocythemia," "Thrombosis," "Risk factors" and "Hemostasis. In this neoplasm, the count and activity of cells such as platelets, leukocytes, endothelial cells, as well as erythrocytes are increased, which can increase the risk of thrombosis through rising intercellular interactions, expression of surface markers, and stimulation of platelet aggregation. In addition to these factors, genetic polymorphisms in hematopoietic stem cells (HSCs), including mutations in JAK2, CALR, MPL, or genetic abnormalities in other genes associated with the hemostatic system may be associated with increased risk of thrombotic events. Moreover, disruption of coagulant factors can pave the way for thrombogeneration. Therefore, the identification of markers related to cell activation, genetic abnormalities, or alternation in the coagulant system can be used together as diagnostic and prognostic markers for the occurrence of thrombosis among ET patients. Thus, because thrombotic complications are the main factors of mortality in ET patients, a hemostatic viewpoint and risk assessment of cellular, genetic, and coagulation factors can have prognostic value and contribute to the choice of effective treatment and prevention of thrombosis.

P2Y12 Inhibition beyond Thrombosis: Effects on Inflammation

The P2Y₁₂ receptor is a key player in platelet activation and a major target for antithrombotic drugs. The beneficial effects of P2Y₁₂ receptor antagonists might, however, not be restricted to the primary and secondary prevention of arterial thrombosis. Indeed, it has been established that platelet activation also has an essential role in inflammation. Additionally, nonplatelet P2Y₁₂ receptors present in immune cells and vascular smooth muscle cells might be effective players in the inflammatory response. This review will investigate the biological and clinical impact of P2Y₁₂ receptor inhibition beyond its platelet-driven antithrombotic effects, focusing on its anti-inflammatory role. We will discuss the potential molecular and cellular mechanisms of P2Y₁₂-mediated inflammation, including cytokine release, platelet–leukocyte interactions and neutrophil extracellular trap formation. Then we will summarize the current evidence on the beneficial effects of P2Y₁₂ antagonists during various clinical inflammatory diseases, especially during sepsis, acute lung injury, asthma, atherosclerosis, and cancer.

Combination Antiplatelet and Oral Anticoagulant Therapy in Patients With Coronary and Peripheral Artery Disease

Antiplatelet therapy is the mainstay for the treatment of acute and chronic arterial disease involving the coronary and peripheral beds. However, questions remain about optimal antithrombotic therapy for long-term treatment of chronic vascular disease. The observation that dual antiplatelet therapy with acetylsalicylic acid and clopidogrel was associated with lower thrombotic event rates than acetylsalicylic acid monotherapy in patients with acute coronary syndromes and those undergoing percutaneous coronary intervention changed the treatment paradigm. Moreover, the demonstration that more pharmacodynamically potent P2Y₁₂ inhibitors than clopidogrel were associated with fewer thrombotic event occurrences further solidified the dual antiplatelet therapy approach. However, recurrent thrombotic events occur in ≈1 in 10 patients in the first year following an acute coronary syndrome event, despite treatment with the most potent P2Y₁₂ inhibitors, a limitation that has stimulated interest in exploring the efficacy and safety of approaches using anticoagulants on top of antiplatelet therapy. These investigations have included treatment with very-low-dose oral anticoagulation, and even its replacement of acetylsalicylic acid in the presence of a P2Y₁₂ inhibitor, in patients stabilized after an acute coronary syndrome event. Recent basic and translational studies have suggested noncanonical effects of coagulation factor inhibition that may further modulate clinical benefits. This in-depth review will discuss developments in our understanding of the roles that platelets and coagulation factors play in atherothrombosis and review the rationale and clinical evidence for combining antiplatelet and oral anticoagulant therapy in patients with coronary and peripheral artery disease.

Characterisation of the Ral GTPase inhibitor RBC8 in human and mouse platelets

The Ral GTPases, RalA and RalB, have been implicated in numerous cellular processes, but are most widely known for having regulatory roles in exocytosis. Recently, we demonstrated that deletion of both Ral genes in a platelet-specific mouse gene knockout caused a substantial defect in surface exposure of P-selectin, with only a relatively weak defect in platelet dense granule secretion that did not alter platelet functional responses such as aggregation or thrombus formation. We sought to investigate the function of Rals in human platelets using the recently described Ral inhibitor, RBC8. Initial studies in human platelets confirmed that RBC8 could effectively inhibit Ral GTPase activation, with an IC50 of 2.2 μM and 2.3 μM for RalA and RalB, respectively. Functional studies using RBC8 revealed significant, dose-dependent inhibition of platelet aggregation, secretion (α- and dense granule), integrin activation and thrombus formation, while α-granule release of platelet factor 4, Ca2+ signalling or phosphatidylserine exposure were unaltered. Subsequent studies in RalAB-null mouse platelets pretreated with RBC8 showed dose-dependent decreases in integrin activation and dense granule secretion, with significant inhibition of platelet aggregation and P-selectin exposure at 10 μM RBC8. This study strongly suggests therefore that although RBC8 is useful as a Ral inhibitor in platelets, it is likely also to have off-target effects in the same concentration range as for Ral inhibition. So, whilst clearly useful as a Ral inhibitor, interpretation of data needs to take this into account when assessing roles for Rals using RBC8.

Platelets directly enhance neutrophil transmigration in response to oxidised low-density lipoprotein

Beyond their primary role in haemostasis and tissue repair, platelets are causally involved in the onset of inflammatory reactions, cell proliferation and immune response. Platelet activation and platelet binding to the endothelium result in release of chemokines and increased expression of adhesion molecules, which promote the recruitment of leukocytes that will eventually migrate across the endothelium into the tissue. Here, we provide the first evidence that platelets stimulated with oxidised low-density lipoprotein (oxLDL) directly enhance recruitment and transmigration of neutrophils, via cell-cell interaction. OxLDL immediately activates platelets, which then rapidly bind to neutrophils, foster their activation and facilitate transmigration through an endothelial monolayer. The observed effects of oxLDL on platelet-neutrophil aggregate (PNA) formation depend on incubation time, lipoprotein concentration and the degree of oxidative modification of LDL. PNA form within minutes following stimulation by oxLDL and remain for up to 1 h post stimulation, while native LDL is unable to induce platelet-neutrophil interactions. In the presence of acetylsalicylic acid the formation of PNA in response to oxLDL is virtually absent, and platelets fail to further enhance oxLDL-induced neutrophil transmigration. P2Y1 and P2Y12 inhibitors have less pronounced effects on PNA formation in response to oxLDL. Furthermore, we demonstrate that the PI3K pathway is essential for efficient neutrophil transmigration induced by oxLDL. Consequently, platelets enhance neutrophil transmigration in response to oxLDL and might thereby contribute essentially to the amplification of inflammatory processes within the vessel wall, which fosters the development of atherosclerosis.

Platelet, monocyte and neutrophil activation and glucose tolerance in South African Mixed Ancestry individuals

Platelet activation has been described in patients with chronic inflammation, however in type 2 diabetes mellitus it remains controversial. We compared levels of platelet leucocyte aggregates, monocyte and granulocyte activation across glucose tolerance statuses in mixed ancestry South Africans. Individuals (206) were recruited from Bellville-South, Cape Town, and included 66% with normal glucose tolerance, 18.7% pre-diabetes, 8.7% screen-detected diabetes and 6.3% known diabetes. Monocyte and neutrophil activation were measured by calculating the percentage of cells expressing CD142 and CD69 while platelet monocyte aggregates were defined as CD14⁺⁺ CD42b⁺ events and platelet neutrophil aggregates as CD16⁺⁺ CD42b⁺ events. The percentage of monocytes and neutrophils expressing CD69 and CD142 was significantly higher in known diabetes and prediabetes, but, lowest in screen-detected diabetes (both p ≤ 0.016). The pattern was similar for platelet monocyte and neutrophil aggregates (both p ≤ 0.003). In robust linear regressions adjusted for age and gender, known diabetes was significantly and positively associated with the percentage of monocytes expressing CD69 [beta 11.06 (p = 0.016)] and CD42b (PMAs) [19.51 (0.003)] as well as the percentage of neutrophils expressing CD69 [14.19 (<0.0001)] and CD42b [17.7 (0.001)]. We conclude that monitoring platelet activation in diagnosed diabetic patients may have a role in the management and risk stratification.

Propagation of thrombosis by neutrophils and extracellular nucleosome networks

Neutrophils, early mediators of the innate immune defense, are recruited to developing thrombi in different types of thrombosis. They amplify intravascular coagulation by stimulating the tissue factor-dependent extrinsic pathway via inactivation of endogenous anticoagulants, enhancing factor XII activation or decreasing plasmin generation. Neutrophil-dependent prothrombotic mechanisms are supported by the externalization of decondensed nucleosomes and granule proteins that together form neutrophil extracellular traps. These traps, either in intact or fragmented form, are causally involved in various forms of experimental thrombosis as first indicated by their role in the enhancement of both microvascular thrombosis during bacterial infection and carotid artery thrombosis. Neutrophil extracellular traps can be induced by interactions of neutrophils with activated platelets; vice versa, these traps enhance adhesion of platelets via von Willebrand factor. Neutrophil-induced microvascular thrombus formation can restrict the dissemination and survival of blood-borne bacteria and thereby sustain intravascular immunity. Dysregulation of this innate immune pathway may support sepsis-associated coagulopathies. Notably, neutrophils and extracellular nucleosomes, together with platelets, critically promote fibrin formation during flow restriction-induced deep vein thrombosis. Neutrophil extracellular traps/extracellular nucleosomes are increased in thrombi and in the blood of patients with different vaso-occlusive pathologies and could be therapeutically targeted for the prevention of thrombosis. Thus, during infections and in response to blood vessel damage, neutrophils and externalized nucleosomes are major promoters of intravascular blood coagulation and thrombosis.

Biological Properties of Some Volatile Phenylpropanoids

Plants and their extracts are the new field of interest for many scientists and also of some pharmaceutical industries. In order to provide more information for their usage in the prevention and treatment of diseases many clinical trials and researches are being carried out. In this review the biological activities and the mechanism of action of volatile phenylpropanoids (PPs) found in essential oils (EOs) are presented. The aim of this overview is to show that volatile PPs found in EOs can exert many of the biological activities which are generally attributed to EOs. Almost all of the PPs possess antimicrobial, anti-inflammatory and anticancer activities. These are related to the different substitution of the phenylpropane molecule. For each isolated group not only one, but more pharmacological activities can be credited.

Anti-platelet activity of panaxatriol saponins is mediated by suppression of intracellular calcium mobilization and ERK2/p38 activation

BACKGROUND

Increased platelet aggregation is implicated in the pathogenesis of ischemic stroke and anti-platelet strategy may contribute to its therapy. Panaxatriol saponin (PTS), the main components extracted from Panax notoginseng, has been shown to be efficacious in the prevention and treatment of ischemic stroke in China. The aim of this study is to determine the anti-platelet activity and explore the underlying mechanisms.

METHODS

Inhibitory effect of PTS and its main ginsenosides on agonists-induced platelet aggregation was determined using rabbit or human platelets. Intracellular Ca(2+) concentration ([Ca(2+)]i) mobilization was detected with fura-2/AM probe. MAPKs phosphorylation was determined by Western blotting.

RESULTS

Our results showed PTS inhibited the rabbit platelet aggregation induced by various agonists (collagen, thrombin and ADP). The three main ginsenosides (Rg1, Re and R1) existing in PTS also showed anti-platelet activity, while their combination exhibited no synergistic effect on rabbit platelet aggregation. Further study demonstrated that PTS and its main ginsenosides also exhibited inhibitory effect on human platelet aggregation. Mechanism study demonstrated that pre-treatment with PTS inhibited the agonists-induced intracellular calcium mobilization. Moreover, PTS significantly suppressed the activation of both ERK2 and p38 by the agonists via reducing the phosphorylation of ERK2 and p38.

CONCLUSION

We proved that PTS is effective in anti-platelet aggregation, which may, at least in part, be related to the suppression of intracellular calcium mobilization and ERK2/p38 activation. This study may provide one reasonable explanation for the efficacy of PTS on the prevention and treatment of ischemic stroke.

Adenosine diphosphate platelet dysfunction on thromboelastogram is independently associated with increased morality in traumatic brain injury

PURPOSE

The purpose of this study is to determine if adenosine diphosphate (ADP) platelet dysfunction on thromboelastogram (TEG) is associated with increased in-hospital mortality in patients with head trauma. The hypothesis is that ADP dysfunction is associated with increased mortality.

METHODS

This retrospective review evaluated trauma patients admitted to a level 1 trauma center from February 2011 to October 2013 who received a TEG. Patients were included if the TEG was drawn within the first 24 h of admission and the head abbreviated injury score was greater than or equal to three. Patients were categorized as severe ADP dysfunction if the degree of ADP inhibition on TEG exceeded 60 %.

RESULTS

A total of 90 patients were included (no ADP dysfunction n = 37; ADP dysfunction n = 53). Initial Glasgow Coma Scale [GCS (12 ± 4 vs. 11 ± 5; p = 0.26)] and use of pre-injury antiplatelet agents (30 vs. 28 %; p = 0.88) were similar. Patients with ADP dysfunction on TEG had a higher in-hospital mortality rate (8 vs. 32 %; p < 0.01). ADP dysfunction was independently associated with in-hospital mortality upon fixed logistic regression (OR 6.2; 95 % CI 1.2-33) while controlling for age, gender, hypotension, pre-injury antiplatelet agents, GCS and Injury Severity Score.

CONCLUSION

ADP dysfunction on TEG is associated with increased mortality in patients with traumatic brain injury.

[Anti-platelets without a bleeding risk: novel targets and strategies]

Anti-platelet agents such as aspirin, clopidogrel and antagonists of integrin αIIbβ3 allowed to efficiently reduce morbidity and mortality associated with arterial thrombosis. A major limit of these drugs is that they increase the risk of bleeding. During the last few years, several innovative anti-thrombotic strategies with a potentially low bleeding risk were proposed. These approaches target the collagen receptor glycoprotein (GP) VI, the GPIb/von Willebrand factor axis, the thrombin receptor PAR-1, the activated form of integrin αIIbβ3 or the ADP receptor P2Y1. While an antagonist of PAR-1 was recently marketed, the clinical proofs of the efficiency and safety of the other agents remain to be established. This review evaluates these new anti-platelet approaches toward safer anti-thrombotic therapies.

Biological basis and pathological relevance of microvascular thrombosis

Microvascular thrombosis indicates a pathological occlusion of microvessels by fibrin- and/or platelet-rich thrombi. It is observed during systemic infections, cancer, myocardial infarction, stroke, neurodegenerative diseases and in thrombotic microangiopathies. Microvessel thrombosis can cause greatly differing symptoms that range from limited changes in plasma coagulation markers to severe multi-organ failure. Because microvessel thrombi are difficult to detect and often occur only transiently, their importance for disease development and host biology is likely markedly under-appreciated. Recently, clear indications for a biological basis of microvascular thrombosis have been obtained. During systemic infections microvessel thrombosis can mediate an intravascular innate immune response (immunothrombosis). This biological form of thrombosis is based on the generation of fibrin inside blood vessels and is critically triggered by neutrophils and their interactions with platelets which result in the release of neutrophil extracellular traps (extracellular nucleosomes). Immunothrombosis is critically supported by neutrophil elastase and the activator molecules of blood coagulation tissue factor and factor XII. Identification of the biological driving forces of microvascular thrombosis should help to elucidate the mechanisms promoting pathological vessel occlusions in both microvessels and large vessels.

Complications of injectable fillers, part 2: vascular complications

Accidental intra-arterial filler injection may cause significant tissue injury and necrosis. Hyaluronic acid (HA) fillers, currently the most popular, are the focus of this article, which highlights complications and their symptoms, risk factors, and possible treatment strategies. Although ischemic events do happen and are therefore important to discuss, they seem to be exceptionally rare and represent a small percentage of complications in individual clinical practices. However, the true incidence of this complication is unknown because of underreporting by clinicians. Typical clinical findings include skin blanching, livedo reticularis, slow capillary refill, and dusky blue-red discoloration, followed a few days later by blister formation and finally tissue slough. Mainstays of treatment (apart from avoidance by meticulous technique) are prompt recognition, immediate treatment with hyaluronidase, topical nitropaste under occlusion, oral acetylsalicylic acid (aspirin), warm compresses, and vigorous massage. Secondary lines of treatment may involve intra-arterial hyaluronidase, hyperbaric oxygen therapy, and ancillary vasodilating agents such as prostaglandin E1. Emergency preparedness (a "filler crash cart") is emphasized, since early intervention is likely to significantly reduce morbidity. A clinical summary chart is provided, organized by complication presentation.

Unmet needs in the management of acute myocardial infarction: role of novel protease-activated receptor-1 antagonist vorapaxar

Platelet activation with subsequent aggregation is a complex process leading to thrombus formation, which remains a key component for atherothrombotic manifestations, in particular myocardial infarction. Therefore, antiplatelet therapies are pivotal for the treatment of these patients. Current oral antiplatelet therapies used for secondary prevention of ischemic recurrences include aspirin and adenosine diphosphate P2Y12 platelet-receptor antagonists. However, despite these therapies, patients who have experienced a myocardial infarction remain at risk for ischemic recurrences. Therefore, more aggressive secondary prevention measures have been an area of research, including identifying additional targets modulating platelet-activation and -aggregation processes. Among these, thrombin-mediated platelet activation via protease-activated receptors (PARs) has been subject to extensive clinical investigation. Several PAR-1 receptor antagonists have been developed. However, vorapaxar is the only one that has completed large-scale clinical investigation. The present manuscript will provide an overview on the role of thrombin-mediated signaling, the impact of PAR-1 blockade with vorapaxar on ischemic and bleeding outcomes, and the potential role for vorapaxar in clinical practice.

Ticagrelor reduces neutrophil recruitment and lung damage in abdominal sepsis

Abstract Platelets play an important role in abdominal sepsis and P2Y12 receptor antagonists have been reported to exert anti-inflammatory effects. Herein, we assessed the impact of platelet inhibition with the P2Y12 receptor antagonist ticagrelor on pulmonary neutrophil recruitment and tissue damage in a model of abdominal sepsis. Wild-type C57BL/6 mice were subjected to cecal ligation and puncture (CLP). Animals were treated with ticagrelor (100 mg/kg) or vehicle prior to CLP induction. Edema formation and bronchoalveolar neutrophils as well as lung damage were quantified. Flow cytometry was used to determine expression of platelet-neutrophil aggregates, neutrophil activation and CD40L expression on platelets. CLP-induced pulmonary infiltration of neutrophils at 24 hours was reduced by 50% in ticagrelor-treated animals. Moreover, ticagrelor abolished CLP-provoked lung edema and decreased lung damage score by 41%. Notably, ticagrelor completely inhibited formation of platelet-neutrophil aggregates and markedly reduced thrombocytopenia in CLP animals. In addition, ticagrelor reduced platelet shedding of CD40L in septic mice. Our data indicate that ticagrelor can reduce CLP-induced pulmonary neutrophil recruitment and lung damage suggesting a potential role for platelet antagonists, such as ticagrelor, in the management of patients with abdominal sepsis.

ADP-induced platelet aggregation and thrombin generation are increased in Essential Thrombocythemia and Polycythemia Vera

INTRODUCTION

Essential Thrombocythemia (ET) and Polycythemia Vera (PV) patients are characterized by an increased rate of thrombotic complications and by several abnormalities of platelets, more pronounced in JAK2V617F positive patients. The aim of this study was to characterize the platelet aggregation as well as the platelet procoagulant potential induced by several different agonists in ET and PV patients.

MATERIALS AND METHODS

Venous blood samples were obtained from 65 ET and 51 PV patients. Whole blood impedance aggregometry was utilized to characterize platelet aggregation induced by collagen, ADP, thrombin receptor activating peptide and arachidonic acid, while the Calibrated Automated Thrombogram (CAT) assay was used to determine the thrombin generation (TG) potential induced by ADP in platelet-rich plasma. CAT assay was also performed in the presence of annexin V to evaluate the contribution of platelet phospholipids to TG.

RESULTS AND CONCLUSIONS

ADP-induced platelet aggregation and TG were significantly increased in ET and PV patients compared to controls. The highest values were observed in JAK2V617F positive patients and in patients on aspirin. In these subjects, annexin V was less effective in inhibiting both basal and ADP-induced TG. This study demonstrates for first time that platelets from ET and PV patients are more responsive to the ADP stimulus, in terms of both increased platelet aggregation, and enhanced TG, particularly in the JAK2V617F positive patients. Our data support the hypothesis that the use of ADP receptor inhibitors, in addition to aspirin, might be considered in the prevention of thrombosis in these conditions, by allowing a more complete inhibition of platelet functions.

A critical role of thrombin/PAR-1 in ADP-induced platelet secretion and the second wave of aggregation

BACKGROUND

The stable or second wave of platelet aggregation often observed in ADP-stimulated platelet-rich plasma (PRP) with an artificially lowered extracellular calcium level has been attributed to enhanced thromboxane A2 (TXA2 ) generation and inhibition of ectonucleotidase activity. However, the role of thrombin in ADP-induced platelet secretion and the second wave of aggregation is unknown.

OBJECTIVES AND METHODS

We employed aggregometry, flow cytometry, immunoblotting and ELISA to determine whether and how thrombin participates in ADP-induced platelet secretion and the second wave of aggregation.

RESULTS

ADP induces a phosphoinositide 3-kinase (PI3K) pathway-dependent thrombin generation, presumably resulting from the cleavage of αII b β3 -associated prothrombin. Generated thrombin subsequently activates protease-activated receptor-1 (PAR-1) and mediates dense granule secretion and the second wave of platelet aggregation in ADP-stimulated citrated PRP. Thus, ADP-induced dense granule secretion and the second wave of platelet aggregation in PRP were similarly and non-additively blocked by thrombin inhibitor hirudin, PAR-1 antagonist SCH-79797 or PI3K inhibitor wortmannin. Moreover, ADP stimulation caused the dissociation of prothrombin from αII b β3 and an increased plasma thrombin level; both were prevented by wortmannin. Furthermore, the wortmannin-inhibited second wave of platelet aggregation by ADP was restored by a subaggregation concentration of PAR-1 activating peptide SFLLRN. Blocking TXA2 production with indomethacin or restoring extracellular calcium to physiological concentration did not influence this thrombin/PAR-1 dependence.

CONCLUSIONS

A PI3K-dependent thrombin generation and the resultant PAR-1 activation serve as an indispensable mechanism to relay the platelet activation process induced by ADP.

Platelet-based coagulation: different populations, different functions

Platelets in a thrombus interact with (anti)coagulation factors and support blood coagulation. In the concept of cell-based control of coagulation, three different roles of platelets can be distinguished: control of thrombin generation, support of fibrin formation, and regulation of fibrin clot retraction. Here, we postulate that different populations of platelets with distinct surface properties are involved in these coagulant functions. Platelets with elevated Ca(2+) and exposed phosphatidylserine control thrombin and fibrin generation, while platelets with activated α(IIb) β(3) regulate clot retraction. We review how coagulation factor binding depends on the platelet activation state. Furthermore, we discuss the ligands, platelet receptors and downstream intracellular signaling pathways implicated in these coagulant functions. These insights lead to an adapted model of platelet-based coagulation.

A platelet target for venous thrombosis? P2Y1 deletion or antagonism protects mice from vena cava thrombosis

A role for platelets in the pathogenesis of venous thrombosis was suggested by clinical and preclinical studies. However, examination of the platelet receptor, P2Y1, in this area has been limited. The goal of the current study was to examine effects of P2Y1 deletion, or selective antagonism with MRS2500, in oxidative venous thrombosis in mice. The P2Y12 antagonist, clopidogrel, was included as a reference agent. Anesthetized C57BL/6 or genetically modified mice underwent 3.5 or 5 % FeCl(3)-induced vena cava thrombosis. Pharmacokinetic properties of MRS2500 were defined for dose selection. Platelet aggregation and renal or tail bleeding times (BT) were measured to put antithrombotic effects into perspective. P2Y1 deletion significantly reduced (p < 0.001) venous thrombus weight by 74 % in 3.5 % FeCl(3) injury compared to P2Y1(+/+) littermates. MRS2500 (2 mg/kg, i.v.) significantly decreased (p < 0.001) thrombus weight 64 % in C57BL/6 mice. In the more severe 5 % FeCl(3)-induced injury model, thrombus weight significantly (p < 0.001) decreased 68 % in P2Y1(-/-) mice versus P2Y1(+/+) mice, and MRS2500 (2 mg/kg) was also beneficial (54 % decrease, p < 0.01). Renal BT doubled in P2Y1(-/-) versus P2Y1(+/+) mice, and increased threefold with MRS2500 compared to vehicle. Tail BT was markedly prolonged in P2Y1(-/-) mice (7.9X) and in C57BL/6 mice given MRS2500. The current study demonstrates that P2Y1 deletion or antagonism significantly reduced venous thrombosis in mice, suggesting that P2Y1 receptors play a role in the pathogenesis of venous thrombosis, at least in this species. However as with many antithrombotic agents the benefit comes at the potential price of an increase in provoked bleeding times.

Safety and efficacy of protease-activated receptor-1 antagonists in patients with coronary artery disease: a meta-analysis of randomized clinical trials

BACKGROUND

Thrombin receptor antagonists blocking protease-activated receptor-1 (PAR-1) on platelets represent a new class of oral antiplatelet agents for patients with atherothrombotic disease manifestations.

OBJECTIVES

We investigated the safety and efficacy of PAR-1 antagonists in patients with coronary artery disease (CAD).

PATIENTS/METHODS

Randomized, placebo-controlled trials of the PAR-1 antagonists atopaxar or vorapaxar in CAD patients were identified. The primary safety endpoint was the composite of Thrombolysis In Myocardial Infarction (TIMI) clinically significant bleeding. The primary efficacy endpoint was the composite of death, myocardial infarction (MI) or stroke.

RESULTS

A total of 41 647 patients from eight trials were included. PAR-1 antagonists were associated with higher risks of TIMI clinically significant (odds ratio [OR] 1.48, 95% confidence interval [CI] 1.39-1.57, P < 0.001), major (OR 1.46, 95% CI 1.28-1.67, P < 0.001) and minor (OR 1.67, 95% CI 1.40-2.00, P < 0.001) bleeding than placebo in the fixed-effects model. PAR-1 antagonists reduced the composite of death, MI or stroke as compared with placebo (OR 0.87, 95% CI 0.81-0.92, P < 0.001), driven by a lower risk of MI (OR 0.85, 95% CI 0.78-0.92, P < 0.001). Conversely, PAR-1 antagonists and placebo did not differ in terms of risk of death (OR 0.99, 95% CI 0.90-1.09, P = 0.81) or stroke (OR 0.96, 95% CI 0.84-1.10, P = 0.59).

CONCLUSIONS

PAR-1 antagonists decrease ischemic events in patients with CAD as compared with placebo, mainly driven by a reduction in MI, at the cost of an increased risk of clinically significant bleeding.

Markers of hypercoagulability in CAD patients. Effects of single aspirin and clopidogrel treatment

BACKGROUND

Cardiovascular disease with disturbances in the haemostatic system, might lead to thrombotic complications with clinical manifestations like acute myocardial infarction (AMI) and stroke. Activation of the coagulation cascade with subsequent increased thrombin generation, characterizes a prothrombotic phenotype. In the present study we investigated whether prothrombotic markers were associated with risk factors and clinical subgroups in a cohort of patients with angiographically verified coronary artery disease (CAD). The patients were randomized to long-term treatment with the antiplatelet drugs aspirin or clopidogrel, and we further investigated the effect on hypercoagulability of such treatment for 1 year, of which limited data exists.

METHODS

Venous blood samples were collected in fasting condition between 08:00 and 10:30 am, at baseline when all patients were on aspirin therapy (n = 1001) and in 276 patients after 1 year follow-up on aspirin or clopidogrel. In vivo thrombin generation was assessed by prothrombin fragment 1 + 2 (F1+2) and D-dimer, and the endogenous thrombin potentiale (ETP) in the calibrated automated thrombogram (CAT) assay, representing ex vivo thrombin generation. In addition soluble tissue factor (sTF) and free- and total tissue factor pathway inhibitor (TFPI) were measured.

RESULTS

We found age to be significantly associated with F1+2 and D-dimer (β = 0.229 and β =0.417 respectively, p <0.001, both). Otherwise, only weak associations were found. F1+2 and D-dimer were higher in women compared to men (p <0.001 and p = 0.033, respectively). Smokers had elevated levels of ETP compared to non-smokers (p = 0.014). Additionally, patients on renin-angiotensin system (RAS) inhibition showed significantly higher levels of F1+2, compared to non-users (p = 0.013). Both aspirin and clopidogrel reduced levels of ETP after 12 months intervention (p = 0.003 and p <0.001, respectively) and the levels of F1+2 were significantly more reduced on aspirin compared to clopidogrel (p = 0.023).

CONCLUSIONS

In the present population of stable CAD, we could demonstrate a more hypercoagulable profile among women, smokers and patients on RAS medication, assessed by the prothrombotic markers F1+2, D-dimer and ETP. Long-term antiplatelet treatment with aspirin alone seems to attenuate thrombin generation to a greater extent than with clopidogrel alone. The study is registered at http://www.clinicaltrials.gov: NCT00222261.

P2Y(12) receptors in platelets and other hematopoietic and non-hematopoietic cells

The P2Y(12) receptor is a Gi-coupled ADP receptor first described in blood platelets where it plays a central role in the complex processes of activation and aggregation. Platelet granules store important amounts of ADP which are released upon stimulation by interaction of platelets with the damaged vessel wall. Therefore, the P2Y(12) receptor is a key player in primary hemostasis and in arterial thrombosis and is an established target of antithrombotic drugs like the thienopyridine compounds ticlopidine, clopidogrel, and prasugrel or the direct, reversible antagonists ticagrelor and cangrelor. Beyond the platelet physiology and pharmacology, recent studies have revealed the expression of the P2Y(12) receptor in other hematopoietic cells including leukocyte subtypes and microglia in the central nervous system as well as in vascular smooth muscle cells. These studies indicate putative roles of the P2Y(12) receptor in inflammatory states and diseases of the brain, lung, and blood vessels. The selective role of P2Y(12) among other P2 receptors as well as the possible impact of P2Y(12) targeting drugs in these processes remain to be evaluated.

Prasugrel inhibits platelet-leukocyte interaction and reduces inflammatory markers in a model of endotoxic shock in the mouse

Prasugrel, through its active metabolite, reduces atherothrombosis and its clinical manifestations by inhibiting platelet activation and aggregation. Platelets also contribute to inflammation through interaction with different classes of leukocytes. We investigated whether the inhibitory effect of prasugrel on platelets also counteract inflammatory responses. The effect of prasugrel active metabolite, R-138727, was investigated on platelet P-selectin expression, platelet adhesion to polymorphonuclear leukocytes (PMN) and monocytes (MN) and Mac-1 expression in PMN and MN, in vitro, in human cells. The ex vivo effect of prasugrel administration on P-selectin, thromboxane (TXB)2 formation, platelet-PMN conjugates and Mac-1 expression in PMN triggered by PAR-4 agonist peptide was examined in whole blood from healthy mice as well as from mice in which an acute inflammatory reaction was induced by treatment with endotoxin. The effect of prasugrel on inflammatory markers in endotoxin-treated animals was also tested in vivo. R-138727 inhibited agonist-stimulated expression of platelet P-selectin, platelet-PMN and platelet-MN adhesion and platelet-dependent Mac-1 expression in leukocytes. Addition of aspirin did not modify the inhibitory effect elicited by R-138727. Treatment of mice with prasugrel resulted in a profound inhibition of platelet P-selectin expression, TXB2 production, platelet-PMN adhesion and Mac-1 expression in PMN induced by ex vivo stimulation with PAR-4 agonist peptide of whole blood from healthy or endotoxin-treated mice. Measurement of markers revealed that prasugrel reduced TXB2 and tumour necrosis factor-α synthesis and increased nitric oxide metabolites in endotoxin-treated mice in vivo. In conclusion, prasugrel reduces platelet interactions with PMN and MN. Through these effects prasugrel may curb platelet-mediated inflammatory responses.

Current concepts of platelet activation: possibilities for therapeutic modulation of heterotypic vs. homotypic aggregation

Thrombogenic and inflammatory activity are two distinct aspects of platelet biology, which are sustained by the ability of activated platelets to interact with each other (homotypic aggregation) and to adhere to circulating leucocytes (heterotypic aggregation). These two events are regulated by distinct biomolecular mechanisms that are selectively activated in different pathophysiological settings. They can occur simultaneously, for example, as part of a pro-thrombotic/pro-inflammatory response induced by vascular damage, or independently, as in certain clinical conditions in which abnormal heterotypic aggregation has been observed in the absence of intravascular thrombosis. Current antiplatelet drugs have been developed to target specific molecular signalling pathways mainly implicated in thrombus formation, and their ever increasing clinical use has resulted in clear benefits in the treatment and prevention of arterial thrombotic events. However, the efficacy of currently available antiplatelet drugs remains suboptimal, most likely because their therapeutic action is limited to only few of the signalling pathways involved in platelet homotypic aggregation. In this context, modulation of heterotypic aggregation, which is believed to contribute importantly to acute thrombotic events, as well to the pathophysiology of atherosclerosis itself, may offer benefits over and above the classical antiplatelet approach. This review will focus on the distinct biomolecular pathways that, following platelet activation, underlie homotypic and heterotypic aggregation, aiming potentially to identify novel therapeutic targets.

Pharmacological modulation of the inflammatory actions of platelets

Patients with inflammatory diseases often exhibit a change in platelet function, with these alterations being clearly distinct from the well-characterized role of platelets in haemostasis and thrombosis. It has recently been revealed that platelets can behave as innate inflammatory cells in immune responses with roles in leukocyte recruitment, migration into tissues, release of cytotoxic mediators, and in tissue remodelling following injury.Platelets exhibit a wide range of receptors for mediators involved in the inflammatory pathway and the immune response (Fig. 1). These include purinergic receptors, selectins, integrins, toll-like receptors, immunoglobulins, and chemokine receptors, but the precise role platelets play in the inflammatory process is still under investigation. Nevertheless, given that many of these receptors are distinct from those involved in thrombosis and haemostasis, this raises the real possibility of targeting these receptors to regulate inflammatory diseases without compromising haemostasis.

Clopidogrel, a P2Y12 receptor antagonist, potentiates the inflammatory response in a rat model of peptidoglycan polysaccharide-induced arthritis

The P2Y12 receptor plays a crucial role in the regulation of platelet activation by several agonists, which is irreversibly antagonized by the active metabolite of clopidogrel, a widely used anti-thrombotic drug. In this study, we investigated whether reduction of platelet reactivity leads to reduced inflammatory responses using a rat model of erosive arthritis. We evaluated the effect of clopidogrel on inflammation in Lewis rats in a peptidoglycan polysaccharide (PG-PS)-induced arthritis model with four groups of rats: 1) untreated, 2) clopidogrel-treated, 3) PG-PS-induced, and 4) PG-PS-induced and clopidogrel-treated. There were significant differences between the PG-PS+clopidogrel group when compared to the PG-PS group including: increased joint diameter and clinical manifestations of inflammation, elevated plasma levels of pro-inflammatory cytokines (IL-1 beta, interferon (IFN) gamma, and IL-6), an elevated neutrophil blood count and an increased circulating platelet count. Plasma levels of IL-10 were significantly lower in the PG-PS+clopidogrel group compared to the PG-PS group. Plasma levels of platelet factor 4 (PF4) were elevated in both the PG-PS and the PG-PS+clopidogrel groups, however PF4 levels showed no difference upon clopidogrel treatment, suggesting that the pro- inflammatory effect of clopidogrel may be due to its action on cells other than platelets. Histology indicated an increase in leukocyte infiltration at the inflammatory area of the joint, increased pannus formation, blood vessel proliferation, subsynovial fibrosis and cartilage erosion upon treatment with clopidogrel in PG-PS-induced arthritis animals. In summary, animals treated with clopidogrel showed a pro-inflammatory effect in the PG-PS-induced arthritis animal model, which might not be mediated by platelets. Elucidation of the mechanism of clopidogrel-induced cell responses is important to understand the role of the P2Y12 receptor in inflammation.

P2 receptors and platelet function

Following vessel wall injury, platelets adhere to the exposed subendothelium, become activated and release mediators such as TXA(2) and nucleotides stored at very high concentration in the so-called dense granules. Released nucleotides and other soluble agents act in a positive feedback mechanism to cause further platelet activation and amplify platelet responses induced by agents such as thrombin or collagen. Adenine nucleotides act on platelets through three distinct P2 receptors: two are G protein-coupled ADP receptors, namely the P2Y(1) and P2Y(12) receptor subtypes, while the P2X(1) receptor ligand-gated cation channel is activated by ATP. The P2Y(1) receptor initiates platelet aggregation but is not sufficient for a full platelet aggregation in response to ADP, while the P2Y(12) receptor is responsible for completion of the aggregation to ADP. The latter receptor, the molecular target of the antithrombotic drugs clopidogrel, prasugrel and ticagrelor, is responsible for most of the potentiating effects of ADP when platelets are stimulated by agents such as thrombin, collagen or immune complexes. The P2X(1) receptor is involved in platelet shape change and in activation by collagen under shear conditions. Each of these receptors is coupled to specific signal transduction pathways in response to ADP or ATP and is differentially involved in all the sequential events involved in platelet function and haemostasis. As such, they represent potential targets for antithrombotic drugs.

Bleeding manifestations of congenital and drug-induced defects of the platelet P2Y12 receptor for adenosine diphosphate

P2Y12, one of the two platelet receptors for adenosine diphosphate (ADP), plays a central role in platelet function. Defects of P2Y12 should be suspected when ADP, even at high concentrations (≥10 µM), is unable to induce full, irreversible platelet aggregation. Patients with congenital P2Y12 defects display a mild-to-moderate bleeding diathesis of variable severity, characterised by mucocutaneous bleeding and excessive post-surgical and post-traumatic blood loss. Drugs that inhibit P2Y12 are potent antithrombotic drugs, attesting the central role played by P2Y12 in platelet thrombus formation. Clopidogrel, the most widely used drug that inhibits P2Y12, is effective both in monotherapy and in combination with acetylsalicylic acid (ASA). Its most important drawback is the inability to inhibit adequately P2Y12-dependent platelet function in about 1/3 of patients, at the recommended therapeutic doses. The incidence of bleeding events is similar in ASA-treated and clopidogrel-treated patients; however, the combination of ASA and clopidogrel causes more bleeding than each drug in monotherapy. Compared to clopidogrel, new drugs inhibiting P2Y12, such as prasugrel and ticagrelor, decrease the risk of cardiovascular events and increase the risk of bleeding complications, because they adequately inhibit P2Y12-dependent platelet function in the vast majority of treated patients.

Lack of P-selectin glycoprotein ligand-1 protects mice from thrombosis after collagen/epinephrine challenge

INTRODUCTION

In thrombotic processes, during the association of leukocytes with platelets and endothelial cells, P-selectin glycoprotein ligand-1 (PSGL-1) binds to P-selectin, expressed on activated platelets and endothelial cells. Our aim was to establish the role of PSGL-1 in thrombus formation by evaluating the response to thrombotic stimuli in wild type and PSGL-1 knockout mice.

MATERIALS AND METHODS

Mice were challenged by tail vein injection of (i) 15 μg collagen plus 3 μg epinephrine (coll/epi) (ii) 7.5 μg collagen plus 1.5 μg epinephrine or (iii) saline. Retro-orbital blood samples were collected in ACD anticoagulaed tubes and platelet and leukocyte counts were measured. In addition, kidneys, liver, spleen and lungs were investigated for fibrin deposition by immunohistochemistry and Western-blotting. Frozen sections were analysed for double labeling for platelet and leukocyte presence.

RESULTS

After coll/epi challenge, the number of platelets and leukocytes decreased significantly in both genotypes. Lower agonist concentration resulted in an attenuated platelet decrease in PSGL-1 knockout mice compared to the controls, however changes in leukocyte and neutrophil counts were not significantly different in the two strains. In knockout mice considerably less fibrin deposition has been observed in the lungs by Western-blotting and immunohistochemistry. After coll/epi challenge the lungs of the PSGL-1 knockout animals contained both platelets and leukocytes but less thrombi has been detected than in wild-type mice.

CONCLUSIONS

Our results indicate that the deficiency of PSGL-1 results in milder thrombocytopenia, less fibrin deposition and lower number of thrombosed blood vessels, suggesting that this molecule is essential for multicellular interactions during thrombus formation.

Increased thrombosis susceptibility and altered fibrin formation in STAT5-deficient mice

To explore the effect(s) of growth hormone signaling on thrombosis, we studied signal transduction and transcription factor 5 (STAT5)-deficient mice and found markedly reduced survival in an in vivo thrombosis model. These findings were not explained by a compensatory increase in growth hormone secretion. There was a modest increase in the activity of several procoagulant factors, but there was no difference in the rate or magnitude of thrombin generation in STAT5-deficient mice relative to control. However, thrombin-triggered clot times were markedly shorter, and fibrin polymerization occurred more rapidly in plasma from STAT5-deficient mice. Fibrinogen depletion and mixing studies indicated that the effect on fibrin polymerization was not due to intrinsic changes in fibrinogen, but resulted from changes in the concentration of a circulating plasma inhibitor. While thrombin-triggered clot times were significantly shorter in STAT5-deficient animals, reptilase-triggered clot times were unchanged. Accordingly, while the rate of thrombin-catalyzed release of fibrinopeptide A was similar, the release of fibrinopeptide B was accelerated in STAT5-deficient plasma versus control. Taken together, these studies demonstrated that the loss of STAT5 resulted in a decrease in the concentration of a plasma inhibitor affecting thrombin-triggered cleavage of fibrinopeptide B. This ultimately resulted in accelerated fibrin polymerization and greater thrombosis susceptibility in STAT5-deficient animals.

Spatial distribution of factor Xa, thrombin, and fibrin(ogen) on thrombi at venous shear

BACKGROUND

The generation of thrombin is a critical process in the formation of venous thrombi. In isolated plasma under static conditions, phosphatidylserine (PS)-exposing platelets support coagulation factor activation and thrombin generation; however, their role in supporting coagulation factor binding under shear conditions remains unclear. We sought to determine where activated factor X (FXa), (pro)thrombin, and fibrin(ogen) are localized in thrombi formed under venous shear.

METHODOLOGY/PRINCIPAL FINDINGS

Fluorescence microscopy was used to study the accumulation of platelets, FXa, (pro)thrombin, and fibrin(ogen) in thrombi formed in vitro and in vivo. Co-perfusion of human blood with tissue factor resulted in formation of visible fibrin at low, but not at high shear rate. At low shear, platelets demonstrated increased Ca(2+) signaling and PS exposure, and supported binding of FXa and prothrombin. However, once cleaved, (pro)thrombin was observed on fibrin fibers, covering the whole thrombus. In vivo, wild-type mice were injected with fluorescently labeled coagulation factors and venous thrombus formation was monitored in mesenteric veins treated with FeCl(3). Thrombi formed in vivo consisted of platelet aggregates, focal spots of platelets binding FXa, and large areas binding (pro)thrombin and fibrin(ogen).

CONCLUSIONS/SIGNIFICANCE

FXa bound in a punctate manner to thrombi under shear, while thrombin and fibrin(ogen) distributed ubiquitously over platelet-fibrin thrombi. During thrombus formation under venous shear, thrombin may relocate from focal sites of formation (on FXa-binding platelets) to dispersed sites of action (on fibrin fibers).

Platelet thrombin receptor antagonism and atherothrombosis

Clinical manifestations of atherothrombotic disease, such as acute coronary syndromes, cerebrovascular events, and peripheral arterial disease, are major causes of mortality and morbidity worldwide. Platelet activation and aggregation are ultimately responsible for the progression and clinical presentations of atherothrombotic disease. The current standard of care, dual oral antiplatelet therapy with aspirin and the P2Y(12) adenosine diphosphate (ADP) receptor inhibitor clopidogrel, has been shown to improve outcomes in patients with atherothrombotic disease. However, aspirin and P2Y(12) inhibitors target the thromboxane A(2) and the ADP P2Y(12) platelet activation pathways and minimally affect other pathways, while agonists such as thrombin, considered to be the most potent platelet activator, continue to stimulate platelet activation and thrombosis. This may help explain why patients continue to experience recurrent ischaemic events despite receiving such therapy. Furthermore, aspirin and P2Y(12) receptor antagonists are associated with bleeding risk, as the pathways they inhibit are critical for haemostasis. The challenge remains to develop therapies that more effectively inhibit platelet activation without increasing bleeding complications. The inhibition of the protease-activated receptor-1 (PAR-1) for thrombin has been shown to inhibit thrombin-mediated platelet activation without increasing bleeding in pre-clinical models and small-scale clinical trials. PAR-1 inhibition in fact does not interfere with thrombin-dependent fibrin generation and coagulation, which are essential for haemostasis. Thus PAR-1 antagonism coupled with existing dual oral antiplatelet therapy may potentially offer more comprehensive platelet inhibition without the liability of increased bleeding.