Platelet receptors activated via mulitmerization: glycoprotein VI, GPIb-IX-V, and CLEC-2

Angiopoietin-like protein 2 inhibits thrombus formation

Acute myocardial infarction is mainly caused by a lack of blood flood in the coronary artery. Angiopoietin-like protein 2 (ANGPTL2) induces platelet activation and thrombus formation in vitro through binding with immunoglobulin-like receptor B, an immunoglobulin superfamily receptor. However, the mechanism by which it regulates platelet function in vivo remains unclear. In this study, we investigated the role of ANGPTL2 during thrombosis in relationship with ST-segment elevation myocardial infarction (STEMI) with spontaneous recanalization (SR). In a cohort of 276 male and female patients, we measured plasma ANGPTL2 protein levels. Using male Angptl2-knockout and wild-type mice, we examined the inhibitory effect of Angptl2 on thrombosis and platelet activation both in vivo and ex vivo. We found that plasma and platelet ANGPTL2 levels were elevated in patients with STEMI with SR compared to those in non-SR (NSR) patients, and was an independent predictor of SR. Angptl2 deficiency accelerated mesenteric artery thrombosis induced by FeCl3 in Angptl2-/- compared to WT animals, promoted platelet granule secretion and aggregation induced by thrombin and collogen while purified ANGPTL2 protein supplementation reversed collagen-induced platelet aggregation. Angptl2 deficiency also increased platelet spreading on immobilized fibrinogen and clot contraction. In collagen-stimulated Angptl2-/- platelets, Src homology region 2 domain-containing phosphatase (Shp)1-Y564 and Shp2-Y580 phosphorylation were attenuated while Src, Syk, and Phospholipase Cγ2 (PLCγ2) phosphorylation increased. Our results demonstrate that ANGPTL2 negatively regulated thrombus formation by activating ITIM which can suppress ITAM signaling pathway. This new knowledge provides a new perspective for designing future antiplatelet aggregation therapies.

A change of rhodocytin's suprastructure turns the agonist into an antagonist of tumor cell induced platelet aggregation

Cancer cells circulating in the blood attach to platelets by direct cell-cell interactions via several receptor-counterreceptor contacts and indirectly by fibrin bridges which connect the two cell types by distinct integrin receptors. In the microenvironment of these tumor cell platelet aggregates (TCPAs), the tumor cells are shielded from the shear stress of the blood flow and from attack by the immune system. This supports hematogenous metastasis and tumor cell induced thrombosis. Platelet activation is triggered by binding of podoplanin on cancer cells to the platelet receptor Clec-2. Therefore, we hypothesize that targeting this initial step will prevent the entire cascade leading to the formation of TCPAs. Rhodocytin, a heterodimeric (αβ)2 C-type lectin from the Malayan pit viper Calloselasma rhodostoma, binds to Clec-2 and thereby induces TCPA formation. Remarkably, mutations in rhodocytin that disturbed formation of oligomers, blocked the podoplanin-Clec-2 axis and prevented platelet activation. Therefore, we used lysine reactive chemicals to modify rhodocytin isolated from the crude snake venom. Blue native gel electrophoresis and far western blotting showed a change of rhodocytin's suprastructure triggered by acetylation and PEGylation. Mass spectrometry analysis of altered lysines suggested that their modifications interfered with the formation of rhodocytin tetramers. When tested in assays for tumor cell induced platelet aggregation, we found that derivatization turned rhodocytin from an agonist into an antagonist. This observation indicates that Clec-2 is a valid target receptor molecule to curb TCPA formation and to prevent hematogenous metastasis and tumor cell induced thrombosis in cancer patients.

What do we know about platelets in myocardial ischemia-reperfusion injury and why is it important?

Myocardial ischemia-reperfusion injury (MIRI), the joint result of ischemic injury and reperfusion injury, is associated with poor outcomes in patients with acute myocardial infarction undergoing primary percutaneous coronary intervention. Accumulating evidence demonstrates that activated platelets directly contribute to the pathogenesis of MIRI through participating in the formation of microthrombi, interaction with leukocytes, secretion of active substances, constriction of microvasculature, and activation of spinal afferent nerves. The molecular mechanisms underlying the above detrimental effects of activated platelets include the homotypic and heterotypic interactions through surface receptors, transduction of intracellular signals, and secretion of active substances. Revealing the roles of platelet activation in MIRI and the associated mechanisms would provide potential targets/strategies for the clinical evaluation and treatment of MIRI. Further studies are needed to characterize the temporal (ischemia phase vs. reperfusion phase) and spatial (systemic vs. local) distributions of platelet activation in MIRI by multi-omics strategies. To improve the likelihood of translating novel cardioprotective interventions into clinical practice, basic researches maximally replicating the complexity of clinical scenarios would be necessary.

S100A8/A9 drives the formation of procoagulant platelets through GPIbα

S100A8/A9, also known as "calprotectin" or "MRP8/14," is an alarmin primarily secreted by activated myeloid cells with antimicrobial, proinflammatory, and prothrombotic properties. Increased plasma levels of S100A8/A9 in thrombo-inflammatory diseases are associated with thrombotic complications. We assessed the presence of S100A8/A9 in the plasma and lung autopsies from patients with COVID-19 and investigated the molecular mechanism by which S100A8/A9 affects platelet function and thrombosis. S100A8/A9 plasma levels were increased in patients with COVID-19 and sustained high levels during hospitalization correlated with poor outcomes. Heterodimeric S100A8/A9 was mainly detected in neutrophils and deposited on the vessel wall in COVID-19 lung autopsies. Immobilization of S100A8/A9 with collagen accelerated the formation of a fibrin-rich network after perfusion of recalcified blood at venous shear. In vitro, platelets adhered and partially spread on S100A8/A9, leading to the formation of distinct populations of either P-selectin or phosphatidylserine (PS)-positive platelets. By using washed platelets, soluble S100A8/A9 induced PS exposure but failed to induce platelet aggregation, despite GPIIb/IIIa activation and alpha-granule secretion. We identified GPIbα as the receptor for S100A8/A9 on platelets inducing the formation of procoagulant platelets with a supporting role for CD36. The effect of S100A8/A9 on platelets was abolished by recombinant GPIbα ectodomain, platelets from a patient with Bernard-Soulier syndrome with GPIb-IX-V deficiency, and platelets from mice deficient in the extracellular domain of GPIbα. We identified the S100A8/A9-GPIbα axis as a novel targetable prothrombotic pathway inducing procoagulant platelets and fibrin formation, in particular in diseases associated with high levels of S100A8/A9, such as COVID-19.

Doxorubicin-Induced Platelet Activation and Clearance Relieved by Salvianolic Acid Compound: Novel Mechanism and Potential Therapy for Chemotherapy-Associated Thrombosis and Thrombocytopenia

Doxorubicin (Dox) is a widely utilized chemotherapeutic; however, it carries side effects, including drug-induced immune thrombocytopenia (DITP) and increased risk of venous thromboembolism (VTE). Currently, the mechanisms for Dox-associated DITP and VTE are poorly understood, and an effective inhibitor to relieve these complications remains to be developed. In this study, we found that Dox significantly induced platelet activation and enhanced platelet phagocytosis by macrophages and accelerated platelet clearance. Importantly, we determined that salvianolic acid C (SAC), a water-soluble compound derived from Danshen root traditionally used to treat cardiovascular diseases, inhibited Dox-induced platelet activation more effectively than current standard-of-care anti-platelet drugs aspirin and ticagrelor. Mechanism studies with tyrosine kinase inhibitors indicate contributions of phospholipase C, spleen tyrosine kinase, and protein kinase C signaling pathways in Dox-induced platelet activation. We further demonstrated that Dox enhanced platelet-cancer cell interaction, which was ameliorated by SAC. Taken together, these findings suggest SAC may be a promising therapy to reduce the risk of Dox-induced DITP, VTE, and the repercussions of amplified platelet-cancer interaction in the tumor microenvironment.

Anti-GPVI nanobody blocks collagen- and atherosclerotic plaque-induced GPVI clustering, signaling, and thrombus formation

BACKGROUND

The collagen receptor glycoprotein VI (GPVI) is an attractive antiplatelet target due to its critical role in thrombosis but minor involvement in hemostasis.

OBJECTIVE

To investigate GPVI receptor involvement in platelet activation by collagen-I and atherosclerotic plaque using novel blocking and non-blocking anti-GPVI nanobodies (Nbs).

METHODS

Nb effects on GPVI-mediated signaling and function were assessed by western blot and whole blood thrombus formation under flow. GPVI clustering was visualized in thrombi using fluorescently labeled Nb28.

RESULTS

Under arterial shear, inhibitory Nb2 blocks thrombus formation and platelet activation on collagen and plaque, but only reduces adhesion on plaque. In contrast, adhesion on collagen, but not plaque, is decreased by blocking integrin α2β1. Adhesion on plaque is maintained despite inhibition of integrins αvβ3, α5β1, α6β1, and αIIbβ3. Only combined αIIbβ3 and α2β1 blockade inhibits adhesion and thrombus formation to the same extent as Nb2 alone. Nb2 prevents GPVI signaling, with loss of Syk, Lat, and PLCɣ2 phosphorylation, especially to plaque stimulation. Non-blocking fluorescently labeled Nb28 reveals distinct GPVI distribution patterns on collagen and plaque, with GPVI clustering clearly apparent on collagen fibers and less frequent on plaque. Clustering on collagen fibers is lost in the presence of Nb2.

CONCLUSIONS

This work emphasizes the critical difference in GPVI-mediated platelet activation by plaque and collagen; it highlights the importance of GPVI clustering for downstream signaling and thrombus formation. Labeled Nb28 is a novel tool for providing mechanistic insight into this process and the data suggest Nb2 warrants further investigation as a potential anti-thrombotic agent.

Of vascular defense, hemostasis, cancer, and platelet biology: an evolutionary perspective

We have established considerable expertise in studying the role of platelets in cancer biology. From this expertise, we were keen to recognize the numerous venous-, arterial-, microvascular-, and macrovascular thrombotic events and immunologic disorders are caused by severe, acute-respiratory-syndrome coronavirus 2 (SARS-CoV-2) infections. With this offering, we explore the evolutionary connections that place platelets at the center of hemostasis, immunity, and adaptive phylogeny. Coevolutionary changes have also occurred in vertebrate viruses and their vertebrate hosts that reflect their respective evolutionary interactions. As mammals adapted from aquatic to terrestrial life and the heavy blood loss associated with placentalization-based live birth, platelets evolved phylogenetically from thrombocytes toward higher megakaryocyte-blebbing-based production rates and the lack of nuclei. With no nuclei and robust RNA synthesis, this adaptation may have influenced viral replication to become less efficient after virus particles are engulfed. Human platelets express numerous receptors that bind viral particles, which developed from archetypal origins to initiate aggregation and exocytic-release of thrombo-, immuno-, angiogenic-, growth-, and repair-stimulatory granule contents. Whether by direct, evolutionary, selective pressure, or not, these responses may help to contain virus spread, attract immune cells for eradication, and stimulate angiogenesis, growth, and wound repair after viral damage. Because mammalian and marsupial platelets became smaller and more plate-like their biophysical properties improved in function, which facilitated distribution near vessel walls in fluid-shear fields. This adaptation increased the probability that platelets could then interact with and engulf shedding virus particles. Platelets also generate circulating microvesicles that increase membrane surface-area encounters and mark viral targets. In order to match virus-production rates, billions of platelets are generated and turned over per day to continually provide active defenses and adaptation to suppress the spectrum of evolving threats like SARS-CoV-2.

Platelet-Released Factors: Their Role in Viral Disease and Applications for Extracellular Vesicle (EV) Therapy

Platelets, which are small anuclear cell fragments, play important roles in thrombosis and hemostasis, but also actively release factors that can both suppress and induce viral infections. Platelet-released factors include sCD40L, microvesicles (MVs), and alpha granules that have the capacity to exert either pro-inflammatory or anti-inflammatory effects depending on the virus. These factors are prime targets for use in extracellular vesicle (EV)-based therapy due to their ability to reduce viral infections and exert anti-inflammatory effects. While there are some studies regarding platelet microvesicle-based (PMV-based) therapy, there is still much to learn about PMVs before such therapy can be used. This review provides the background necessary to understand the roles of platelet-released factors, how these factors might be useful in PMV-based therapy, and a critical discussion of current knowledge of platelets and their role in viral diseases.

Platelets in COVID-19 disease: friend, foe, or both?

Immuno-thrombosis of COVID-19 results in the activation of platelets and coagulopathy. Antiplatelet therapy has been widely used in COVID-19 patients to prevent thrombotic events. However, recent analysis of clinical trials does not support the major effects of antiplatelet therapy on mortality in hospitalized COVID-19 patients, despite the indisputable evidence for an increased risk of thrombotic complications in COVID-19 disease. This apparent paradox calls for an explanation. Platelets have an important role in sensing and orchestrating host response to infection, and several platelet functions related to host defense response not directly related to their well-known hemostatic function are emerging. In this paper, we aim to review the evidence supporting the notion that platelets have protective properties in maintaining endothelial barrier integrity in the course of an inflammatory response, and this role seems to be of particular importance in the lung. It might, thus, well be that the inhibition of platelet function, if affecting the protective aspect of platelet activity, might diminish clinical benefits resulting from the inhibition of the pro-thrombotic phenotype of platelets in immuno-thrombosis of COVID-19. A better understanding of the platelet-dependent mechanisms involved in the preservation of the endothelial barrier is necessary to design the antiplatelet therapeutic strategies that inhibit the pro-thrombotic activity of platelets without effects on the vaso-protective function of platelets safeguarding the pulmonary endothelial barrier during multicellular host defense in pulmonary circulation.

Molecular study of sticky platelet syndrome using exome sequencing

Introduction: Sticky platelet syndrome (SPS) is a prothrombotic condition characterized by increased platelet aggregation that causes arterial and venous thrombosis. Its diagnosis is reached by identifying increased aggregation using low concentrations of adenosine diphosphate and epinephrine in platelet aggregation tests. Objectives: To identify common mutations through exome sequencing in two patients from the same family diagnosed with SPS and, thus, contribute to the molecular study of this disease. Materials and methods: Descriptive study. In January 2018, exome sequencing was performed in a 10-year-old patient treated at Fundación HOMI (Bogotá D.C., Colombia), index case, and in one of his adult first-degree relatives, both with a history of thrombotic disease and diagnosed with SPS. Exome sequencing was performed at the Complexo Hospitalario Universitario de Santiago de Compostela (Spain) using the SureSelect Clinical Research Exome V2 software by Agilent. Results: Exome sequencing led to detect genetic variants in both cases when compared with the reference sequence. The following variant was identified in the two samples: a cytosine to thymine transition at position c.236 (NM_000174.4) of the glycoprotein (GP)Ib-IX-V complex platelet membrane receptor, which causes a heterozygous transition of the amino acid threonine to isoleucine (i.e., a transition from hydrophilic amino acid to a hydrophobic amino acid) at position p. 79 of the extracellular leucine-rich repeat domain of GPIbα subunit of the (GP)Ib–IX complex, involving a conformational change of the main receptor of ligands IB alpha, which might result in platelet hyperaggregation and thrombosis. This variant has not been described in patients with SPS to date. Conclusion: The mutation identified in both samples could be related to SPS considering the importance of glycoprotein IX in platelet function.

Platelets, Constant and Cooperative Companions of Sessile and Disseminating Tumor Cells, Crucially Contribute to the Tumor Microenvironment

Although platelets and the coagulation factors are components of the blood system, they become part of and contribute to the tumor microenvironment (TME) not only within a solid tumor mass, but also within a hematogenous micrometastasis on its way through the blood stream to the metastatic niche. The latter basically consists of blood-borne cancer cells which are in close association with platelets. At the site of the primary tumor, the blood components reach the TME via leaky blood vessels, whose permeability is increased by tumor-secreted growth factors, by incomplete angiogenic sprouts or by vasculogenic mimicry (VM) vessels. As a consequence, platelets reach the primary tumor via several cell adhesion molecules (CAMs). Moreover, clotting factor VII from the blood associates with tissue factor (TF) that is abundantly expressed on cancer cells. This extrinsic tenase complex turns on the coagulation cascade, which encompasses the activation of thrombin and conversion of soluble fibrinogen into insoluble fibrin. The presence of platelets and their release of growth factors, as well as fibrin deposition changes the TME of a solid tumor mass substantially, thereby promoting tumor progression. Disseminating cancer cells that circulate in the blood stream also recruit platelets, primarily by direct cell-cell interactions via different receptor-counterreceptor pairs and indirectly by fibrin, which bridges the two cell types via different integrin receptors. These tumor cell-platelet aggregates are hematogenous micrometastases, in which platelets and fibrin constitute a particular TME in favor of the cancer cells. Even at the distant site of settlement, the accompanying platelets help the tumor cell to attach and to grow into metastases. Understanding the close liaison of cancer cells with platelets and coagulation factors that change the TME during tumor progression and spreading will help to curb different steps of the metastatic cascade and may help to reduce tumor-induced thrombosis.

Thrombocytopenia: Perioperative Considerations for Patients Undergoing Cardiac Surgery

The etiologies of thrombocytopenia in patients presenting for cardiac surgery are extensive, but clinically relevant conditions generally can be categorized by those related to decreased platelet production or increased platelet destruction. Many causes require mere acknowledgment and availability of allogeneic platelet transfusion; others have unique considerations for which providers should be familiar. The purpose of this review is to provide an overview of the common causes of thrombocytopenia, summarize the literature, and discuss perioperative considerations for patients undergoing cardiac surgery.

Novel antiplatelet strategies targeting GPVI, CLEC-2 and tyrosine kinases

Antiplatelet medications comprise the cornerstone of treatment for diseases that involve arterial thrombosis, including acute coronary syndromes (ACS), stroke and peripheral arterial disease. However, antiplatelet medications may cause bleeding and, furthermore, thrombotic events may still recur despite treatment. The interaction of collagen with GPVI receptors on the surface of platelets has been identified as one of the major players in the pathophysiology of arterial thrombosis that occurs following atherosclerotic plaque rupture. Promisingly, GPVI deficiency in humans appears to have a minimal impact on bleeding. These findings together suggest that targeting platelet GPVI may provide a novel treatment strategy that provides additional antithrombotic efficacy with minimal disruption of normal hemostasis compared to conventional antiplatelet medications. CLEC-2 is gaining interest as a therapeutic target for a variety of thrombo-inflammatory disorders including deep vein thrombosis (DVT) with treatment also predicted to cause minimal disruption to hemostasis. GPVI and CLEC-2 signal through Src, Syk and Tec family tyrosine kinases, providing additional strategies for inhibiting both receptors. In this review, we summarize the evidence regarding GPVI and CLEC-2 and strategies for inhibiting these receptors to inhibit platelet recruitment and activation in thrombotic diseases.

The Serine/Threonine Protein Phosphatase 2A (PP2A) Regulates Syk Activity in Human Platelets

Distinct membrane receptors activate platelets by Src-family-kinase (SFK)-, immunoreceptor-tyrosine-based-activation-motif (ITAM)-dependent stimulation of spleen tyrosine kinase (Syk). Recently, we reported that platelet activation via glycoprotein (GP) VI or GPIbα stimulated the well-established Syk tyrosine (Y)-phosphorylation, but also stoichiometric, transient protein kinase C (PKC)-mediated Syk serine(S)297 phosphorylation in the regulatory interdomain-B, suggesting possible feedback inhibition. The transient nature of Syk S297 phosphorylation indicated the presence of an unknown Syk pS297 protein phosphatase. In this study, we hypothesize that the S-protein phosphatase 2A (PP2A) is responsible for Syk pS297 dephosphorylation, thereby affecting Syk Y-phosphorylation and activity in human washed platelets. Using immunoblotting, we show that specific inhibition of PP2A by okadaic acid (OA) alone leads to stoichiometric Syk S297 phosphorylation, as analyzed by Zn²⁺-Phos-tag gels, without affecting Syk Y-phosphorylation. Pharmacological inhibition of Syk by PRT060318 or PKC by GF109203X only minimally reduced OA-induced Syk S297 phosphorylation. PP2A inhibition by OA preceding GPVI-mediated platelet activation induced by convulxin extended Syk S297 phosphorylation but inhibited Syk Y-phosphorylation. Our data demonstrate a novel biochemical and functional link between the S-protein phosphatase PP2A and the Y-protein kinase Syk in human platelets, and suggest that PP2A represents a potential enhancer of GPVI-mediated Syk activity caused by Syk pS297 dephosphorylation.

GSK669, a NOD2 receptor antagonist, inhibits thrombosis and oxidative stress via targeting platelet GPVI

BACKGROUND AND PURPOSE

Previously, we discovered that the activation of nucleotide-binding oligomerization domain 2 (NOD2) enhances platelet activation. We here investigated the antiplatelet and antithrombotic potential of GSK669, a NOD2 antagonist.

EXPERIMENTAL APPROACH

Effects of GSK669 on platelet functions, reactive oxygen species (ROS) and proinflammatory cytokine generation were detected. NOD2-/- platelets were used to confirm GSK669 target. The interaction between GSK669 and glycoprotein VI (GPVI) was detected using surface plasmon resonance (SPR) spectroscopy. GPVI downstream signaling was examined by Western blot. The antithrombotic and antioxidative effects were investigated using mouse mesenteric arteriole thrombosis model and pulmonary embolism model.

KEY RESULTS

GSK669 significantly inhibits platelet proinflammatory cytokine release induced by muramyl dipeptide, platelet aggregation, ATP release, and ROS generation induced by collagen and collagen related peptide (CRP). Platelet spreading and clot retraction are also inhibited. GSK669 also decreases collagen-induced phosphorylation of Src, Syk, PLCγ2, and Akt. The antiplatelet effect of GSK669 is NOD2-independent and mediated by GPVI antagonism. Consistent with its antiplatelet activity as a GPVI antagonist, GSK669 inhibits platelet adhesion on collagen in flow condition. Notably, GSK669 inhibits mouse mesenteric arteriole thrombosis similarly to aspirin without bleeding. The antithrombotic effect of GSK669 is further confirmed in the pulmonary embolism model; decreased malonaldehyde (MDA) and increased superoxide dismutase (SOD) levels in mouse plasma reveal a significant antioxidant effect of GSK669.

CONCLUSION AND IMPLICATIONS

Beyond its anti-inflammatory effect as a NOD2 antagonist, GSK669 is also an efficient and safe antiplatelet agent combined with antioxidant effect by targeting GPVI. An antiplatelet agent bearing antioxidative and anti-inflammatory activities without bleeding risk may have therapeutic advantage over current antiplatelet drugs for atherothrombosis.

Molecular Drivers of Platelet Activation: Unraveling Novel Targets for Anti-Thrombotic and Anti-Thrombo-Inflammatory Therapy

Cardiovascular diseases (CVDs) are the leading cause of death globally-partly a consequence of increased population size and ageing-and are major contributors to reduced quality of life. Platelets play a major role in hemostasis and thrombosis. While platelet activation and aggregation are essential for hemostasis at sites of vascular injury, uncontrolled platelet activation leads to pathological thrombus formation and provokes thrombosis leading to myocardial infarction or stroke. Platelet activation and thrombus formation is a multistage process with different signaling pathways involved to trigger platelet shape change, integrin activation, stable platelet adhesion, aggregation, and degranulation. Apart from thrombotic events, thrombo-inflammation contributes to organ damage and dysfunction in CVDs and is mediated by platelets and inflammatory cells. Therefore, in the past, many efforts have been made to investigate specific signaling pathways in platelets to identify innovative and promising approaches for novel antithrombotic and anti-thrombo-inflammatory strategies that do not interfere with hemostasis. In this review, we focus on some of the most recent data reported on different platelet receptors, including GPIb-vWF interactions, GPVI activation, platelet chemokine receptors, regulation of integrin signaling, and channel homeostasis of NMDAR and PANX1.

Covid-19: The Rollercoaster of Fibrin(Ogen), D-Dimer, Von Willebrand Factor, P-Selectin and Their Interactions with Endothelial Cells, Platelets and Erythrocytes

Severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2), also known as coronavirus disease 2019 (COVID-19)-induced infection, is strongly associated with various coagulopathies that may result in either bleeding and thrombocytopenia or hypercoagulation and thrombosis. Thrombotic and bleeding or thrombotic pathologies are significant accompaniments to acute respiratory syndrome and lung complications in COVID-19. Thrombotic events and bleeding often occur in subjects with weak constitutions, multiple risk factors and comorbidities. Of particular interest are the various circulating inflammatory coagulation biomarkers involved directly in clotting, with specific focus on fibrin(ogen), D-dimer, P-selectin and von Willebrand Factor (VWF). Central to the activity of these biomarkers are their receptors and signalling pathways on endothelial cells, platelets and erythrocytes. In this review, we discuss vascular implications of COVID-19 and relate this to circulating biomarker, endothelial, erythrocyte and platelet dysfunction. During the progression of the disease, these markers may either be within healthy levels, upregulated or eventually depleted. Most significant is that patients need to be treated early in the disease progression, when high levels of VWF, P-selectin and fibrinogen are present, with normal or slightly increased levels of D-dimer (however, D-dimer levels will rapidly increase as the disease progresses). Progression to VWF and fibrinogen depletion with high D-dimer levels and even higher P-selectin levels, followed by the cytokine storm, will be indicative of a poor prognosis. We conclude by looking at point-of-care devices and methodologies in COVID-19 management and suggest that a personalized medicine approach should be considered in the treatment of patients.

Monocyte upregulation of podoplanin during early sepsis induces complement inhibitor release to protect liver function

Multiple organ failure in sepsis is a progressive failure of several interdependent organ systems. Liver dysfunction occurs early during sepsis and is directly associated with patient death; however, the underlying mechanism of liver dysfunction is unclear. Platelet transfusion benefits patients with sepsis, and inhibition of complement activation protects liver function in septic animals. Herein, we explored the potential link between platelets, complement activation, and liver dysfunction in sepsis. We found that deletion of platelet C-type lectin-like receptor 2 (CLEC-2) exacerbated liver dysfunction in early sepsis. Platelet CLEC-2-deficient mice exhibited higher complement activation, more severe complement attack in the liver, and lower plasma levels of complement inhibitors at early time points after E. coli infection. Circulating monocytes expressed the CLEC-2 ligand podoplanin in early sepsis, and podoplanin binding induced release of complement inhibitors from platelets. Injection of complement inhibitors released from platelets reduced complement attack and attenuated liver dysfunction in septic mice. These findings indicate a new function of platelets in the regulation of complement activation during sepsis.

Platelets in Healthy and Disease States: From Biomarkers Discovery to Drug Targets Identification by Proteomics

Platelets are a heterogeneous small anucleate blood cell population with a central role both in physiological haemostasis and in pathological states, spanning from thrombosis to inflammation, and cancer. Recent advances in proteomic studies provided additional important information concerning the platelet biology and the response of platelets to several pathophysiological pathways. Platelets circulate systemically and can be easily isolated from human samples, making proteomic application very interesting for characterizing the complexity of platelet functions in health and disease as well as for identifying and quantifying potential platelet proteins as biomarkers and novel antiplatelet therapeutic targets. To date, the highly dynamic protein content of platelets has been studied in resting and activated platelets, and several subproteomes have been characterized including platelet-derived microparticles, platelet granules, platelet releasates, platelet membrane proteins, and specific platelet post-translational modifications. In this review, a critical overview is provided on principal platelet proteomic studies focused on platelet biology from signaling to granules content, platelet proteome changes in several diseases, and the impact of drugs on platelet functions. Moreover, recent advances in quantitative platelet proteomics are discussed, emphasizing the importance of targeted quantification methods for more precise, robust and accurate quantification of selected proteins, which might be used as biomarkers for disease diagnosis, prognosis and therapy, and their strong clinical impact in the near future.

Differential dynamics of early stages of platelet adhesion and spreading on collagen IV- and fibrinogen-coated surfaces

Background: Upon wound formation, platelets adhere to the neighboring extracellular matrix and spread on it, a process which is critical for physiological wound healing. Multiple external factors, such as the molecular composition of the environment and its mechanical properties, play a key role in this process and direct its speed and outcome. Methods: We combined live cell imaging, quantitative interference reflection microscopy and cryo-electron tomography to characterize, at a single platelet level, the differential spatiotemporal dynamics of the adhesion process to fibrinogen- and collagen IV-functionalized surfaces. Results: Initially, platelets sense both substrates by transient rapid extensions of filopodia. On collagen IV, a short-term phase of filopodial extension is followed by lamellipodia-based spreading. This transition is preceded by the extension of a single or couple of microtubules into the platelet's periphery and their apparent insertion into the core of the filopodia. On fibrinogen surfaces, the filopodia-to-lamellipodia transition was partial and microtubule extension was not observed leading to limited spreading, which could be restored by manganese or thrombin. Conclusions: Based on these results, we propose that interaction with collagen IV stimulate platelets to extend microtubules to peripheral filopodia, which in turn, enhances filopodial-to-lamellipodial transition and overall lamellipodia-based spreading. Fibrinogen, on the other hand, fails to induce these early microtubule extensions, leading to full lamellipodia spreading in only a fraction of the seeded platelets. We further suggest that activation of integrin αIIbβ3 is essential for filopodial-to-lamellipodial transition, based on the capacity of integrin activators to enhance lamellipodia spreading on fibrinogen.

Feedback Regulation of Syk by Protein Kinase C in Human Platelets

The spleen tyrosine kinase (Syk) is essential for immunoreceptor tyrosine-based activation motif (ITAM)-dependent platelet activation, and it is stimulated by Src-family kinase (SFK)-/Syk-mediated phosphorylation of Y352 (interdomain-B) and Y525/526 (kinase domain). Additional sites for Syk phosphorylation and protein interactions are known but remain elusive. Since Syk S297 phosphorylation (interdomain-B) was detected in platelets, we hypothesized that this phosphorylation site regulates Syk activity via protein kinase C (PKC)-and cyclic adenosine monophosphate (cAMP)-dependent pathways. ADP, the GPVI-agonist convulxin, and the GPIbα-agonist echicetin beads (EB) were used to stimulate human platelets with/without effectors. Platelet aggregation and intracellular messengers were analyzed, along with phosphoproteins, by immunoblotting using phosphosite-specific antibodies or phos-tags. ADP, convulxin, and EB upregulated Syk S297 phosphorylation, which was inhibited by iloprost (cAMP pathway). Convulxin-stimulated Syk S297 phosphorylation was stoichiometric, transient, abolished by the PKC inhibitor GF109203X, and mimicked by the PKC activator PDBu. Convulxin/EB stimulated Syk S297, Y352, and Y525/526 phosphorylation, which was inhibited by SFK and Syk inhibitors. GFX and iloprost inhibited convulxin/EB-induced Syk S297 phosphorylation but enhanced Syk tyrosine (Y352/Y525/526) and substrate (linker adaptor for T cells (LAT), phospholipase γ2 (PLC γ2)) phosphorylation. GFX enhanced convulxin/EB-increases of inositol monophosphate/Ca²⁺. ITAM-activated Syk stimulates PKC-dependent Syk S297 phosphorylation, which is reduced by SFK/Syk/PKC inhibition and cAMP. Inhibition of Syk S297 phosphorylation coincides with enhanced Syk activation, suggesting that S297 phosphorylation represents a mechanism for feedback inhibition in human platelets.

Platelet Carcinoembryonic Antigen Cell Adhesion Molecule 5 (CEACAM5) as a Possible Novel Diagnostic Tool for Evaluation of Acute Coronary Syndrome

Background

Acute coronary syndrome (ACS) occurs approximately every 40 seconds, and was an underlying cause of death in 1 out of every 7 deaths. More accurate indicators are needed to distinguish patients with ACS from patients manifesting negative changes in electrocardiogram (ECG) and myocardial enzymes. This study aimed to investigate whether the expression of platelet carcinoembryonic antigen cell adhesion molecule-5 (CEACAM5/CEA/CD66e) could help predict ACS.

Material/Methods

We enrolled 82 participants (mean age 60 years, 33 females and 49 males). The expression of CEA on washed human platelets was assessed using two-color flow cytometry. The CEA levels on platelets and in serum of these 82 consecutive patients were detected using two-color whole-blood flow cytometry analysis and a custom-made Luminex multiplex assay, respectively.

Results

CEA was expressed on the surface of human platelets. The expression of platelet CEA (P<0.01), but not serum CEA (P=0.30), was significantly higher in patients with ACS compared to patients with normal coronary artery. Increased platelet CEA levels could serve as a new independent indicator for ACS (P=0.0003). Platelet CEA testing (P=0.000002), as well as cardiac troponin I (cTnI) (P=0.0005), can diagnose ACS with high sensitivity and specificity, and, combined with cTnI (P<0.0001), can improve the diagnostic value.

Conclusions

Platelet CEA expression was higher in individuals presenting with ACS. Hence, platelet CEA might be a novel and reliable biomarker for ACS. Large-scale studies are needed to confirm this hypothesis.

CLEC2 and CLEC5A: Pathogenic Host Factors in Acute Viral Infections

The protective roles of endosomal toll-like receptors (TLRs) and cytosolic nucleic acid sensors are well elucidated, but the pathogenic host factors during viral infections remain unclear. Spleen tyrosine kinase (Syk)-coupled C-type lectins (CLECs) CLEC2 and CLEC5A are highly expressed on platelets and myeloid cells, respectively. CLEC2 has been shown to recognize snake venom aggretin and the endogenous ligand podoplanin and acts as a critical regulator in the development and immunothrombosis. Although CLEC2 has been reported to interact with type I immunodeficiency virus (HIV-1), its role in viral infections is still unclear. CLEC5A binds to fucose and mannose moieties of dengue virus membrane glycans, as well as to N-acetylglucosamine (GlcNAc)/N-acetylmuramic acid (MurNAc) disaccharides that form the backbone of L. monocytogenes peptidoglycans. Recently, we demonstrated that both CLEC2 and CLEC5A are critical in microbe-induced “neutrophil extracellular trap” (NET) formation and proinflammatory cytokine production. Moreover, activation of CLEC2 by dengue virus (DV) and H5N1 influenza virus (IAV) induces the release of extracellular vesicles (EVs), which further enhance NETosis and proinflammatory cytokine production via CLEC5A and Toll-like receptor 2 (TLR2). These findings not only illustrate the immunomodulatory effects of EVs during platelet-leukocyte interactions, but also demonstrate the critical roles of CLEC2 and CLEC5A in acute viral infections.

Platelet-derived alpha-granules are associated with inflammation in patients with NK/T-cell lymphoma-associated hemophagocytic syndrome

Due to the variable overlap of multiple symptoms, accurate early diagnosis of NK/T-cell lymphoma-associated hemophagocytic syndrome (NK/T-LAHS) is difficult, making the prognosis extremely poor. Hemophagocytic syndrome (HPS) is now diagnosed primarily based on the hemophagocytic lymphohistiocytosis (HLH)-2004 diagnostic criteria, and platelet count is one of the baseline evaluations. However, in our study, the data showed that decreased platelets were not only a clinical feature of HPS but also the key cells that regulate inflammation by releasing α-granules containing upregulated platelet factor 4 (PF4) and downregulated platelet-derived growth factors (PDGFs). Furthermore, we found that angiopoietin-4 (ANG-4), which has significant differential expression, has been less reported, that may affect hematopoiesis and proinflammatory responses and can be used as diagnostic biomarkers together with PF4 and PDGFs.

cAMP- and cGMP-elevating agents inhibit GPIbα-mediated aggregation but not GPIbα-stimulated Syk activation in human platelets

Background

The glycoprotein (GP) Ib-IX-V complex is a unique platelet plasma membrane receptor, which is essential for platelet adhesion and thrombus formation. GPIbα, part of the GPIb-IX-V complex, has several physiological ligands such as von Willebrand factor (vWF), thrombospondin and distinct coagulation factors, which trigger platelet activation. Despite having an important role, intracellular GPIb-IX-V signaling and its regulation by other pathways are not well defined. Our aim was to establish the intracellular signaling response of selective GPIbα activation in human platelets, in particular the role of the tyrosine kinase Syk and its regulation by cAMP/PKA and cGMP/PKG pathways, respectively. We addressed this using echicetin beads (EB), which selectively bind to GPIbα and induce platelet aggregation.

Methods

Purified echicetin from snake Echis carinatus venom was validated by mass spectrometry. Washed human platelets were incubated with EB, in the presence or absence of echicetin monomers (EM), Src family kinase (SFK) inhibitors, Syk inhibitors and the cAMP- and cGMP-elevating agents iloprost and riociguat, respectively. Platelet aggregation was analyzed by light transmission aggregometry, protein phosphorylation by immunoblotting. Intracellular messengers inositolmonophosphate (InsP1) and Ca²⁺_i were measured by ELISA and Fluo-3 AM/FACS, respectively.

Results

EB-induced platelet aggregation was dependent on integrin α_IIbβ₃ and secondary mediators ADP and TxA₂, and was antagonized by EM. EB stimulated Syk tyrosine phosphorylation at Y352, which was SFK-dependent and Syk-independent, whereas Y525/526 phosphorylation was SFK-dependent and partially Syk-dependent. Furthermore, phosphorylation of both Syk Y352 and Y525/526 was completely integrin α_IIbβ₃-independent but, in the case of Y525/526, was partially ADP/TxA₂-dependent. Syk activation, observed as Y352/ Y525/Y526 phosphorylation, led to the phosphorylation of direct substrates (LAT Y191, PLCγ2 Y759) and additional targets (Akt S473). PKA/PKG pathways inhibited EB-induced platelet aggregation and Akt phosphorylation but, surprisingly, enhanced Syk and LAT/PLCγ2 tyrosine phosphorylation. A similar PKA/PKG effect was confirmed with convulxin−/GPVI-stimulated platelets. EB-induced InsP1 accumulation/InsP3 production and Ca²⁺-release were Syk-dependent, but only partially inhibited by PKA/PKG pathways.

Conclusion

EB and EM are specific agonists and antagonists, respectively, of GPIbα-mediated Syk activation leading to platelet aggregation. The cAMP/PKA and cGMP/PKG pathways do not inhibit but enhance GPIbα−/GPVI-initiated, SFK-dependent Syk activation, but strongly inhibit further downstream responses including aggregation. These data establish an important intracellular regulatory network induced by GPIbα.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0428-1) contains supplementary material, which is available to authorized users.

Structurally Robust and Functionally Highly Versatile-C-Type Lectin (-Related) Proteins in Snake Venoms

Snake venoms contain an astounding variety of different proteins. Among them are numerous C-type lectin family members, which are grouped into classical Ca²⁺- and sugar-binding lectins and the non-sugar-binding snake venom C-type lectin-related proteins (SV-CLRPs), also called snaclecs. Both groups share the robust C-type lectin domain (CTLD) fold but differ in a long loop, which either contributes to a sugar-binding site or is expanded into a loop-swapping heterodimerization domain between two CLRP subunits. Most C-type lectin (-related) proteins assemble in ordered supramolecular complexes with a high versatility of subunit numbers and geometric arrays. Similarly versatile is their ability to inhibit or block their target molecules as well as to agonistically stimulate or antagonistically blunt a cellular reaction triggered by their target receptor. By utilizing distinct interaction sites differentially, SV-CLRPs target a plethora of molecules, such as distinct coagulation factors and receptors of platelets and endothelial cells that are involved in hemostasis, thrombus formation, inflammation and hematogenous metastasis. Because of their robust structure and their high affinity towards their clinically relevant targets, SV-CLRPs are and will potentially be valuable prototypes to develop new diagnostic and therapeutic tools in medicine, provided that the molecular mechanisms underlying their versatility are disclosed.

Platelet membrane lipid rafts protein composition varies following GPVI and CLEC-2 receptors activation

Lipid rafts are membrane microdomains that have been proposed to play an important role in several platelet-signalling cascades, including those mediated by the receptors Glycoprotein VI (GPVI), and C-type lectin domain family 1 member B (CLEC-2), both involved in thrombus formation. We have performed a LC-MS/MS proteomic analysis of lipid rafts isolated from platelets activated through GPVI and CLEC-2 as well as from resting platelets. Our aim was to determine the magnitude of changes in lipid rafts protein composition and to elucidate the relevance of these alterations in platelet function. A number of relevant signalling proteins were found enriched in lipid rafts following platelet activation (such as the tyrosine protein kinases Fyn, Lyn and Yes; the G proteins G(i) and G(z); and cAMP protein kinase). Interestingly, our results indicate that the relative enrichment of lipid rafts in these signalling proteins may not be a consequence of protein translocation to these domains upon platelet stimulation, but the result of a massive loss in cytoskeletal proteins after platelet activation. Thus, this study may help to better understand the effects of platelet activation in the reorganization of lipid rafts and set the basis for further proteomic studies of these membrane microdomains in platelets. SIGNIFICANCE: We performed the first proteomic comparative analysis of lipid rafts- protein composition in platelets activated through GPVI and CLEC-2 receptors and in resting state. We identified a number of signalling proteins essential for platelet activation relatively enriched in platelets activated through both receptors, and we show that lipid rafts reorganization upon platelet activation leads to a loss in cytoskeletal proteins, highly associated to these domains in resting platelets.

Fc-independent immune thrombocytopenia via mechanomolecular signaling in platelets

Immune thrombocytopenia (ITP) is a prevalent autoimmune disease characterized by autoantibody-induced platelet clearance. Some ITP patients are refractory to standard immunosuppressive treatments such as intravenous immunoglobulin (IVIg). These patients often have autoantibodies that target the ligand-binding domain (LBD) of glycoprotein Ibα (GPIbα), a major subunit of the platelet mechanoreceptor complex GPIb-IX. However, the molecular mechanism of this Fc-independent platelet clearance is not clear. Here, we report that many anti-LBD monoclonal antibodies such as 6B4, but not AK2, activated GPIb-IX in a shear-dependent manner and induced IVIg-resistant platelet clearance in mice. Single-molecule optical tweezer measurements of antibodies pulling on full-length GPIb-IX demonstrated that the unbinding force needed to dissociate 6B4 from the LBD far exceeds the force required to unfold the juxtamembrane mechanosensory domain (MSD) in GPIbα, unlike the AK2-LBD unbinding force. Binding of 6B4, not AK2, induced shear-dependent unfolding of the MSD on the platelet, as evidenced by increased exposure of a linear sequence therein. Imaging flow cytometry and aggregometry measurements of platelets and LBD-coated platelet-mimetic beads revealed that 6B4 can sustain crosslinking of platelets under shear, whereas 6B4 Fab and AK2 cannot. These results suggest a novel mechanism by which anti-LBD antibodies can exert a pulling force on GPIb-IX via platelet crosslinking, activating GPIb-IX by unfolding its MSD and inducing Fc-independent platelet clearance.

Loss of Reelin protects mice against arterial thrombosis by impairing integrin activation and thrombus formation under high shear conditions

Reelin is a secreted glycoprotein and essential for brain development and plasticity. Recent studies provide evidence that Reelin modifies platelet actin cytoskeletal dynamics. In this study we sought to dissect the contribution of Reelin in arterial thrombus formation. Here we analyzed the impact of Reelin in arterial thrombosis ex vivo and in vivo using Reelin deficient (reeler) and wildtype mice. We found that Reelin is secreted upon platelet activation and mediates signaling via glycoprotein (GP)Ib, the amyloid precursor protein (APP) and apolipoprotein E receptor 2 (ApoER2) to induce activation of Akt, extracellular signal-regulated kinase (Erk), SYK and Phospholipase Cγ2. Moreover, our data identifies Reelin as first physiological ligand for platelet APP. Platelets from reeler mice displayed attenuated platelet adhesion and significantly reduced thrombus formation under high shear conditions indicating an important role for Reelin in GPIb-dependent integrin α_IIbβ₃ activation. Accordingly, adhesion to immobilized vWF as well as integrin activation and the phosphorylation of Erk and Akt after GPIb engagement was reduced in Reelin deficient platelets. Defective Reelin signaling translated into protection from arterial thrombosis and cerebral ischemia/reperfusion injury beside normal hemostasis. Furthermore, treatment with an antagonistic antibody specific for Reelin protects wildtype mice from occlusive thrombus formation. Mechanistically, GPIb co-localizes to the major Reelin receptor APP in platelets suggesting that Reelin-induced effects on GPIb signaling are mediated by APP-GPIb interaction. These results indicate that Reelin is an important regulator of GPIb-mediated platelet activation and may represent a new therapeutic target for the prevention and treatment of cardio- and cerebrovascular diseases.

Extracellular signal-regulated kinase 5 associates with casein kinase II to regulate GPIb-IX-mediated platelet activation via the PTEN/PI3K/Akt pathway

Essentials The mechanisms of extracellular signal-regulated kinase 5 (ERK5) in GPIb-IX signaling are unclear. Function of ERK5 in GPIb-IX was tested using aggregation, western blotting, and mass spectrometry. The protein interacting with ERK5 in human platelets was identified as casein kinase II (CKII). ERK5 associates with CKII to regulate the activation of the PI3K/Akt pathway in GPIb-IX signaling.

SUMMARY

Background The platelet glycoprotein (GP) Ib-IX complex plays essential roles in thrombosis and hemostasis. The mitogen-activated protein kinases (MAPKs) ERK1/2 and p38 have been shown to be important in the GPIb-IX-mediated signaling leading to integrin activation. However, the roles of the MAPK extracellular signal-regulated kinase 5 (ERK5) in GPIb-IX-mediated platelet activation are unknown. Objective To reveal the function and mechanisms of ERK5 in GPIb-IX-mediated platelet activation. Methods The functions of ERK5 in GPIb-IX-mediated human platelet activation were assessed using botrocetin/VWF, ristocetin/VWF, or platelet adhesion to von Willebrand factor (VWF) under shear stress in the presence of a specific inhibitor of ERK5. ERK5-associated proteins were pulled down from Chinese hamster ovary (CHO) cells transfected with HA-tagged-ERK5, identified by mass spectrometry, and confirmed in human platelets. Roles of ERK5-associated proteins in GPIb-IX-mediated platelet activation were clarified using specific inhibitors. Results The phosphorylation levels of ERK5 were significantly enhanced in human platelets stimulated with botrocetin/VWF or ristocetin/VWF. The ERK5 inhibitor XMD8-92 suppressed the second wave of human platelet aggregation induced by botrocetin/VWF or ristocetin/VWF and inhibited human platelet adhesion on immobilized VWF under shear stress. Casein kinase II (CKII) was identified as an ERK5-associated protein in human platelets. The CKII inhibitor TBB, similar to the ERK5 inhibitor XMD8-92, specifically restrained PTEN phosphorylation, therefore suppressing Akt phosphorylation in human platelets treated with botrocetin/VWF. Conclusion ERK5 associates with CKII to play essential roles in GPIb-IX-mediated platelet activation via the PTEN/PI3K/Akt pathway.

Effects of preoperative aspirin on perioperative platelet activation and dysfunction in patients undergoing off-pump coronary artery bypass graft surgery: A prospective randomized study

The benefit of aspirin use after coronary artery bypass graft surgery has been well proven. However, the effect of preoperative aspirin use in patients undergoing off-pump coronary artery bypass graft surgery (OPCAB) has not been evaluated sufficiently. To evaluate platelet function changes during OPCAB due to preoperative aspirin use, we conducted a randomized controlled trial using flow cytometry and the Multiplate^® analyzer. Forty-eight patients scheduled for elective OPCAB were randomized to the aspirin continuation (100 mg/day until operative day) and discontinuation (4 days before the operative day) groups. Platelet function was measured using the platelet activation markers CD62P, CD63, and PAC-1 by flow cytometry, and platelet aggregation was measured using the Multiplate^® analyzer, after the induction of anesthesia (baseline), at the end of the operation, and 24 and 48 h postoperatively. Findings of conventional coagulation assays, thromboelastography by ROTEM^® assays, and postoperative bleeding—related clinical outcomes were compared between groups. No significant change in CD62P, CD63, or PAC-1 was observed at the end of the operation or 24 or 48 h postoperatively compared with baseline in either group. The area under the curve for arachidonic acid—stimulated platelet aggregation, measured by the Multiplate^® analyzer, was significantly smaller in the aspirin continuation group (P < 0.01). However, chest tube drainage and intraoperative and postoperative transfusion requirements did not differ between groups. Our study showed that preoperative use of aspirin for OPCAB did not affect perioperative platelet activation, but it impaired platelet aggregation, which did not affect postoperative bleeding, by arachidonic acid.

Phospholipase D1 is a regulator of platelet-mediated inflammation

Glycoprotein (GP)Ib is not only required for stable thrombus formation but for platelet-mediated inflammatory responses. Phospholipase (PL)D1 is essential for GPIb-dependent aggregate formation under high shear conditions while nothing is known about PLD1-induced regulation of GPIb in platelet-mediated inflammation and the underlying mechanisms. This study aimed to investigate the relevance of PLD1 for platelet-mediated endothelial and leukocyte recruitment and activation in vitro and in vivo. Pld1^-/- platelets showed strongly reduced adhesion to TNFα stimulated endothelial cells (ECs) under high shear conditions ex vivo. Normal cytoskeletal reorganization of Pld1^-/- platelets but reduced integrin activation after adhesion to inflamed ECs confirmed that defective integrin activation is responsible for reduced platelet adhesion to ECs. This, together with significantly reduced CD40L expression on platelets led to reduced chemotactic and adhesive properties of ECs in vitro. Under flow conditions, recruitment of leukocytes to collagen-adherent platelets was reduced. Under inflammatory conditions in vivo, reduced platelet and leukocyte recruitment and arrest to the injured carotid artery was observed in Pld1^-/- mice. In a second in vivo model of venous thrombosis, platelet adhesion to activated endothelial cells was reduced while leukocyte recruitment was attenuated in PLD1 deficient mice. Mechanistically, PLD1 modulates PLCγ2 phosphorylation and integrin activation via Src kinases without affecting vWF binding to GPIb. Thus, PLD1 is important for GPIb-induced inflammatory processes of platelets and might be a promising target to reduce platelet-mediated inflammation.

Podoplanin enhances lung cancer cell growth in vivo by inducing platelet aggregation

Podoplanin/Aggrus, known as a platelet aggregation-inducing factor, is frequently overexpressed in lung squamous cell carcinomas (LSCC) and glioblastomas among other tumours, and its expression has been reported to be correlated with poor prognosis. However, the contribution of podoplanin to malignant progression has been elusive. Here we demonstrate that in podoplanin-positive LSCC cells, their growth was abrogated by podoplanin knockout in vivo but not in vitro. Conversely, ectopic expression of podoplanin promoted cell growth in vivo and facilitated intratumoral platelet activation. Consistently, LSCC cells evoked podoplanin-mediated platelet aggregation (PMPA), and the releasates from platelets during PMPA promoted the growth of LSCC cells in vitro. Phospho-receptor-tyrosine-kinase array analysis revealed that epidermal growth factor receptor (EGFR) phosphorylation of LSCC cells was responsible for the growth promotion induced by platelet releasates. Treatment with an antiplatelet agent or podoplanin-neutralizing antibody depressed the growth of an LSCC tumour xenograft via suppression of EGFR phosphorylation. These results suggested that podoplanin in LSCC enhanced cell growth by inducing PMPA in vivo and contributed to malignant progression.

Platelet "first responders" in wound response, cancer, and metastasis

Platelets serve as "first responders" during normal wounding and homeostasis. Arising from bone marrow stem cell lineage megakaryocytes, anucleate platelets can influence inflammation and immune regulation. Biophysically, platelets are optimized due to size and discoid morphology to distribute near vessel walls, monitor vascular integrity, and initiate quick responses to vascular lesions. Adhesion receptors linked to a highly reactive filopodia-generating cytoskeleton maximizes their vascular surface contact allowing rapid response capabilities. Functionally, platelets normally initiate rapid clotting, vasoconstriction, inflammation, and wound biology that leads to sterilization, tissue repair, and resolution. Platelets also are among the first to sense, phagocytize, decorate, or react to pathogens in the circulation. These platelet first responder properties are commandeered during chronic inflammation, cancer progression, and metastasis. Leaky or inflammatory reaction blood vessel genesis during carcinogenesis provides opportunities for platelet invasion into tumors. Cancer is thought of as a non-healing or chronic wound that can be actively aided by platelet mitogenic properties to stimulate tumor growth. This growth ultimately outstrips circulatory support leads to angiogenesis and intravasation of tumor cells into the blood stream. Circulating tumor cells reengage additional platelets, which facilitates tumor cell adhesion, arrest and extravasation, and metastasis. This process, along with the hypercoagulable states associated with malignancy, is amplified by IL6 production in tumors that stimulate liver thrombopoietin production and elevates circulating platelet numbers by thrombopoiesis in the bone marrow. These complex interactions and the "first responder" role of platelets during diverse physiologic stresses provide a useful therapeutic target that deserves further exploration.

Basic mechanisms of platelet receptor shedding

Proteolytic shedding of the extracellular ectodomain of platelet receptors provides a key mechanism for irreversible loss of ligand-binding capacity, and for regulating platelet function in health and disease. Platelets derived from megakaryocytes are small anucleate cells in peripheral blood, with the ability to rapidly adhere, become activated, and secrete an array of procoagulant and proinflammatory factors at sites of vascular injury or disease, and to form a platelet aggregate (thrombus) which is not only critical in normal hemostasis and wound healing, but in atherothrombotic diseases including myocardial infarction and ischemic stroke. Basic mechanisms of receptor shedding on platelets have important distinctions from how receptors on other cell types might be shed, in that shedding is rapidly initiated (within seconds to minutes) and occurs under altered shear conditions encountered in flowing blood or experimentally ex vivo. This review will consider the key components of platelet receptor shedding, that is, the receptor with relevant cleavage site, the (metallo)proteinase or sheddase and how its activity is regulated, and the range of known regulatory factors that control platelet receptor shedding including receptor-associated molecules such as calmodulin, factors controlling sheddase surface expression and activity, and other elements such as shear stress, plasma membrane properties, cellular activation status or age. Understanding these basic mechanisms of platelet receptor shedding is significant in terms of utilizing receptor surface expression or soluble proteolytic fragments as platelet-specific biomarkers and/or ultimately therapeutic targeting of these mechanisms to control platelet reactivity and function.

Platelet Activation: The Mechanisms and Potential Biomarkers

Beyond hemostasis and thrombosis, an increasing number of studies indicate that platelets play an integral role in intercellular communication, mediating inflammatory and immunomodulatory activities. Our knowledge about how platelets modulate inflammatory and immunity has greatly improved in recent years. In this review, we discuss recent advances in the pathways of platelet activation and potential application of platelet activation biomarkers to diagnosis and prediction of disease states.

Antiplatelet drugs: which targets for which treatments?

The current standard care for acute coronary syndromes is dual antiplatelet therapy combining the COX1 inhibitor aspirin with a drug targeting the P2Y12 receptor, together with anticoagulation during and after early revascularization by percutaneous intervention. In very high-risk patients, glycoprotein (GP) IIb/IIIa antagonists may also be used. Secondary prevention of ischemic events requires dual antiplatelet therapy for several months followed by lifelong low-dose aspirin. The duration of treatment and the drugs to combine nevertheless remain matters of debate and the focus of ongoing research. Despite great progress, there is still room for improved efficacy and this could involve new targets for both antiplatelet drugs (like the thrombin receptor PAR1) and anticoagulants. However, improved efficacy is offset by an increased risk of bleeding. Stroke patients are still waiting for better treatment, their bleeding risk being particularly high. New targets including the collagen receptor, glycoprotein VI (GPVI), and the GPIb-von Willebrand factor axis, governing platelet interaction with the diseased vessel wall, should enable us to complete the armamentarium of antiplatelet drugs.

Activation of phosphatidylinositol 3-kinase β by the platelet collagen receptors integrin α2β1 and GPVI: The role of Pyk2 and c-Cbl

Phosphatidylinositol 3-kinaseβ (PI3Kβ) plays a predominant role in integrin outside-in signaling and in platelet activation by GPVI engagement. We have shown that the tyrosine kinase Pyk2 mediates PI3Kβ activation downstream of integrin αIIbβ3, and promotes the phosphorylation of the PI3K-associated adaptor protein c-Cbl. In this study, we compared the functional correlation between Pyk2 and PI3Kβ upon recruitment of the two main platelet collagen receptors, integrin α2β1 and GPVI. PI3Kβ-mediated phosphorylation of Akt was inhibited in Pyk2-deficient platelets adherent to monomeric collagen through integrin α2β1, but occurred normally upon GPVI ligation. Integrin α2β1 engagement led to Pyk2-independent association of c-Cbl with PI3K. However, c-Cbl was not phosphorylated in adherent platelets, and phosphorylation of Akt occurred normally in c-Cbl-deficient platelets, indicating that the c-Cbl is dispensable for Pyk2-mediated PI3Kβ activation. Stimulation of platelets with CRP, a selective GPVI ligand, induced c-Cbl phosphorylation in the absence of Pyk2, but failed to promote its association with PI3K. Pyk2 activation was completely abrogated in PI3KβKD, but not in PI3KγKD platelets, and was strongly inhibited by Src kinases and phospholipase C inhibitors, and by BAPTA-AM. The absence of PI3Kβ activity also hampered GPVI-induced tyrosine-phosphorylation and activation of PLCγ2, preventing intracellular Ca2+ increase and phosphorylation of pleckstrin. Moreover, GPVI-induced intracellular Ca2+ increase and pleckstrin phosphorylation were also strongly inhibited in human platelets treated with the PI3Kβ inhibitor TGX-221. These results outline important differences in the regulation of PI3Kβ by GPVI and integrin α2β1 and suggest that inhibition of Pyk2 may target PI3Kβ activation in a selective context of platelet stimulation.

Current state and novel approaches of antiplatelet therapy

An unresolved problem with clinical use of antiplatelet therapy is that a significant number of individuals either still get thrombosis or run the risk of life-threatening bleeding. Antiplatelet drugs are widely used clinically, either chronically for people at risk of athero/thrombotic disease or to prevent thrombus formation during surgery. However, a subpopulation may be resistant to standard doses, while the platelet targets of these drugs are also critical for the normal hemostatic function of platelets. In this review, we will briefly examine current antiplatelet therapy and existing targets while focusing on new potential approaches for antiplatelet therapy and improved monitoring of effects on platelet reactivity in individuals, ultimately to improve antithrombosis with minimal bleeding. Primary platelet adhesion-signaling receptors, glycoprotein (GP)Ib-IX-V and GPVI, that bind von Willebrand factor/collagen and other prothrombotic factors are not targeted by drugs in clinical use, but they are of particular interest because of their key role in thrombus formation at pathological shear.

Sp1/Sp3 transcription factors regulate hallmarks of megakaryocyte maturation and platelet formation and function

Sp1 and Sp3 belong to the specificity proteins (Sp)/Krüppel-like transcription factor family. They are closely related, ubiquitously expressed, and recognize G-rich DNA motifs. They are thought to regulate generic processes such as cell-cycle and growth control, metabolic pathways, and apoptosis. Ablation of Sp1 or Sp3 in mice is lethal, and combined haploinsufficiency results in hematopoietic defects during the fetal stages. Here, we show that in adult mice, conditional pan-hematopoietic (Mx1-Cre) ablation of either Sp1 or Sp3 has minimal impact on hematopoiesis, whereas the simultaneous loss of Sp1 and Sp3 results in severe macrothrombocytopenia. This occurs in a cell-autonomous manner as shown by megakaryocyte-specific (Pf4-Cre) double-knockout mice. We employed flow cytometry, cell culture, and electron microscopy and show that although megakaryocyte numbers are normal in bone marrow and spleen, they display a less compact demarcation membrane system and a striking inability to form proplatelets. Through megakaryocyte transcriptomics and platelet proteomics, we identified several cytoskeleton-related proteins and downstream effector kinases, including Mylk, that were downregulated upon Sp1/Sp3 depletion, providing an explanation for the observed defects in megakaryopoiesis. Supporting this notion, selective Mylk inhibition by ML7 affected proplatelet formation and stabilization and resulted in defective ITAM receptor-mediated platelet aggregation.

Spleen tyrosine kinase contributes to acute renal allograft rejection in the rat

Kidney allografts induce strong T-cell and antibody responses which mediate acute rejection. Spleen tyrosine kinase (Syk) is expressed by most leucocytes, except mature T cells, and is involved in intracellular signalling following activation of the Fcγ-receptor, B-cell receptor and some integrins. A role for Syk signalling has been established in antibody-dependent native kidney disease, but little is known of Syk in acute renal allograft rejection. Sprague-Dawley rats underwent bilateral nephrectomy and received an orthotopic Wistar renal allograft. Recipient rats were treated with a Syk inhibitor (CC0482417, 30 mg/kg/bid), or vehicle, from 1 h before surgery until being killed 5 days later. Vehicle-treated recipients developed severe allograft failure with marked histologic damage in association with dense leucocyte infiltration (T cells, macrophages, neutrophils and NK cells) and deposition of IgM, IgG and C3. Immunostaining identified Syk expression by many infiltrating leucocytes. CC0482417 treatment significantly improved allograft function and reduced histologic damage, although allograft injury was still clearly evident. CC0482417 failed to prevent T-cell infiltration and activation within the allograft. However, CC0482417 significantly attenuated acute tubular necrosis, infiltration of macrophages and neutrophils and thrombosis of peritubular capillaries. In conclusion, this study identifies a role for Syk in acute renal allograft rejection. Syk inhibition may be a useful addition to T-cell-based immunotherapy in renal transplantation.

Primary haemostasis: newer insights

At the same time as biophysical and omics approaches are drilling deeper into the molecular details of platelets and other blood cells, as well as their receptors and mechanisms of regulation, there is also an increasing awareness of the functional overlap between human vascular systems. Together, these studies are redefining the intricate networks linking haemostasis and thrombosis with inflammation, infectious disease, cancer/metastasis and other vascular pathophysiology. The focus of this state-of-the-art review is some of the newer advances relevant to primary haemostasis. Of particular interest, platelet-specific primary adhesion-signalling receptors and associated activation pathways control platelet function in flowing blood and provide molecular links to other systems. Platelet glycoprotein (GP)Ibα of the GPIb-IX-V complex and GPVI not only initiate platelet aggregation and thrombus formation by primary interactions with von Willebrand factor and collagen, respectively, but are also involved in coagulation, leucocyte engagement, bacterial or viral interactions, and are relevant as potential risk markers in a range of human diseases. Understanding these systems in unprecedented detail promises significant advances in evaluation of individual risk, in new diagnostic or therapeutic possibilities and in monitoring the response to drugs or other treatment.

Analysis of platelet function and dysfunction

Although platelets act as central players of haemostasis only their cross-talk with other blood cells, plasma factors and the vascular compartment enables the formation of a stable thrombus. Multiple activation processes and complex signalling networks are responsible for appropriate platelet function. Thus, a variety of platelet function tests are available for platelet research and diagnosis of platelet dysfunction. However, universal platelet function tests that are sensitive to all platelet function defects do not exist and therefore diagnostic algorithms for suspected platelet function disorders are still recommended in clinical practice. Based on the current knowledge of human platelet activation this review evaluates point-of-care related screening tests in comparison with specific platelet function assays and focuses on their diagnostic utility in relation to severity of platelet dysfunction. Further, systems biology-based platelet function methods that integrate global and specific analysis of platelet vessel wall interaction (advanced flow chamber devices) and post-translational modifications (platelet proteomics) are presented and their diagnostic potential is addressed.

New mechanisms of antiplatelet activity of nifedipine, an L-type calcium channel blocker

Platelet hyperactivity often occursd in hypertensive patients and is a key factor in the development of cardiovascular diseases including thrombosis and atherosclerosis. Nifedipine, an L-type calcium channel blocker, is widely used for hypertension and coronary heart disease therapy. In addition, nifedipine is known to exhibit an antiplatelet activity, but the underlying mechanisms involved remain unclear. Several transcription factors such as peroxisome proliferator-activated receptors (PPARs) and nuclear factor kappa B (NF-κB) exist in platelets and have an ability to regulate platelet aggregation through a non-genomic mechanism. The present article focuses on describing the mechanisms of the antiplatelet activity of nifedipine via PPAR activation. It has been demonstrated that nifedipine treatment increases the activity and intracellular amount of PPAR-β/-γ in activated platelets. Moreover, the antiplatelet activity of nifedipine is mediated by PPAR-β/-γ-dependent upon the up-regulation of the PI3K/AKT/NO/cyclic GMP/PKG pathway, and inhibition of protein kinase Cα (PKCα) activity via an interaction between PPAR-β/-γ and PKCα. Furthermore, suppressing NF-κB activation by nifedipine through enhanced association of PPAR-β/-γ with NF-κB has also been observed in collagen-stimulated platelets. Blocking PPAR-β/-γ activity or increasing NF-κB activation greatly reverses the antiplatelet activity and inhibition of intracellular Ca²⁺ mobilization, PKCα activity, and surface glycoprotein IIb/IIIa expression caused by nifedipine. Thus, PPAR-β/-γ- dependent suppression of NF-κB activation also contributes to the antiplatelet activity of nifedipine. Consistently, administration of nifedipine markedly reduces fluorescein sodium-induced vessel thrombus formation in mice, which is considerably inhibited when the PPAR-β/-γ antagonists are administrated simultaneously. Collectively, these results provide important information regarding the mechanism by which nifedipine inhibits platelet aggregation and thrombus formation through activation of PPAR-β/-γ- mediated signaling pathways. These findings highlight that PPARs are novel therapeutic targets for preventing and treating platelet-hyperactivity-related vascular diseases.

Identification of a juxtamembrane mechanosensitive domain in the platelet mechanosensor glycoprotein Ib-IX complex

How glycoprotein (GP)Ib-IX complex on the platelet surface senses the blood flow through its binding to the plasma protein von Willebrand factor (VWF) and transmits a signal into the platelet remains unclear. Here we show that optical tweezer-controlled pulling of the A1 domain of VWF (VWF-A1) on GPIb-IX captured by its cytoplasmic domain induced unfolding of a hitherto unidentified structural domain before the dissociation of VWF-A1 from GPIb-IX. Additional studies using recombinant proteins and mutant complexes confirmed its existence in GPIb-IX and enabled localization of this quasi-stable mechanosensitive domain of ∼60 residues between the macroglycopeptide region and the transmembrane helix of the GPIbα subunit. These results suggest that VWF-mediated pulling under fluid shear induces unfolding of the mechanosensitive domain in GPIb-IX, which may possibly contribute to platelet mechanosensing and/or shear resistance of VWF-platelet interaction. The identification of the mechanosensitive domain in GPIb-IX has significant implications for the pathogenesis and treatment of related blood diseases.

TAILS N-terminomics of human platelets reveals pervasive metalloproteinase-dependent proteolytic processing in storage

Proteases, and specifically metalloproteinases, have been linked to the loss of platelet function during storage before transfusion, but the underlying mechanisms remain unknown. We used a dedicated N-terminomics technique, iTRAQ terminal amine isotopic labeling of substrates (TAILS), to characterize the human platelet N-terminome, proteome, and posttranslational modifications throughout platelet storage over 9 days under blood-banking conditions. From the identified 2938 proteins and 7503 unique peptides, we characterized N-terminal methionine excision, co- and posttranslational Nα acetylation, protein maturation, and proteolytic processing of proteins in human platelets. We also identified for the first time 10 proteins previously classified by the Human Proteome Organization as "missing" in the human proteome. Most N termini (77%) were internal neo-N termini (105 were novel potential alternative translation start sites, and 2180 represented stable proteolytic products), thus highlighting a prominent yet previously uncharacterized role of proteolytic processing during platelet storage. Protease inhibitor studies revealed metalloproteinases as being primarily responsible for proteolytic processing (as opposed to degradation) during storage. System-wide identification of metalloproteinase and other proteinase substrates and their respective cleavage sites suggests novel mechanisms of the effect of proteases on protein activity and platelet function during storage. All data sets and metadata are available through ProteomeXchange with the data set identifier PXD000906.

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.

Platelet receptor expression and shedding: glycoprotein Ib-IX-V and glycoprotein VI

Quantity, quality, and lifespan are 3 important factors in the physiology, pathology, and transfusion of human blood platelets. The aim of this review is to discuss the proteolytic regulation of key platelet-specific receptors, glycoprotein(GP)Ib and GPVI, involved in the function of platelets in hemostasis and thrombosis, and nonimmune or immune thrombocytopenia. The scope of the review encompasses the basic science of platelet receptor shedding, practical aspects related to laboratory analysis of platelet receptor expression/shedding, and clinical implications of using the proteolytic fragments as platelet-specific biomarkers in vivo in terms of platelet function and clearance. These topics can be relevant to platelet transfusion regarding both changes in platelet receptor expression occurring ex vivo during platelet storage and/or clinical use of platelets for transfusion. In this regard, quantitative analysis of platelet receptor profiles on blood samples from individuals could ultimately enable stratification of bleeding risk, discrimination between causes of thrombocytopenia due to impaired production vs enhanced clearance, and monitoring of response to treatment prior to change in platelet count.