Platelet activation induces metalloproteinase-dependent GP VI cleavage to down-regulate platelet reactivity to collagen

Plasma Soluble Glycoprotein VI, Platelet Function, Bleeding, and Ischemic Events in Patients Undergoing Elective Percutaneous Coronary Intervention

BACKGROUND AND AIMS

 Glycoprotein VI (GPVI) is the major platelet-specific collagen receptor. GPVI shedding with generation of soluble GPVI (sGPVI) is an endogenous feedback mechanism preventing platelet overstimulation. sGPVI has not been investigated in patients with chronic coronary syndrome (CCS) undergoing percutaneous coronary intervention (PCI), especially regarding its potential value as a predictor of ischemic and bleeding risk.

METHODS

 Baseline plasma sGPVI levels were available in 318 patients with CCS undergoing PCI. Platelet function was assessed by measuring both adenosine diphosphate (ADP) and collagen-induced platelet aggregation. Co-primary endpoints were a composite of death or myocardial injury at 48 hours after PCI, and Bleeding Academic Research Consortium (BARC) type 1 to 5 bleeding at 30 days.

RESULTS

 There was no significant correlation between sGPVI and platelet function at baseline or at 48 hours after PCI and loading with antiplatelet drugs. Baseline plasma sGPVI levels were not associated with the ischemic risk: the incidence of the ischemic endpoint was 25.0% in the lower, 22.9% in the middle, and 26.7% in the upper sGPVI tertile (p = 0.82). There was a significant nonlinear relationship between sGPVI and the risk of bleeding: the incidence of the bleeding endpoint was 11.8% in the lower, 12.6% in the middle, and 26.4% in the upper sGPVI tertile (p = 0.006).

CONCLUSION

 In patients with CCS undergoing PCI, plasma levels of sGPVI did not correlate with ADP- or collagen-induced platelet aggregation. Patients with higher baseline levels of sGPVI may carry an increased risk of bleeding at 30 days after PCI but no excess risk of ischemic events.

The subtilisin-like protease furin regulates hemin-induced CD63 surface expression on platelets

Accumulation of free heme B in the plasma can be the result of severe hemolytic events, when the scavenger system for free hemoglobin and heme B is overwhelmed. Free heme B can be oxidized into toxic hemin, which has been proven to activate platelet degranulation and aggregation and promote thrombosis. In the present study we analyzed the effect of hemin on the activation-mediated lysosomal degranulation and CD63 surface expression on platelets using classic flow cytometry and fluorescence microscopy techniques. Classical platelet activators were used as control to distinguish the novel effects of hemin from known activation pathways. CD63 is a tetraspanin protein, also known as lysosomal-associated membrane protein 3 or LAMP-3. In resting platelets CD63 is located within the membrane of delta granules and lysosomes of platelet, from where it is integrated into the platelet outer membrane upon stimulation. We were able to show that hemin like the endogenous platelet activators ADP, collagen or thrombin does provoke CD63 re-localization. Interestingly, only hemin-induced CD63 externalization is dependent on the subtilisin-like pro-protein convertase furin as shown by inhibitor experiments. Furthermore, we were able to demonstrate that hemin induces lysosome secretion, a source of the hemin-mediated CD63 presentation. Again, only the hemin-induced lysosome degranulation is furin dependent. In summary we have shown that the pro-protein convertase furin plays an important role in hemin-mediated lysosomal degranulation and CD63 externalization.

The Subtilisin-Like Protease Furin Regulates Hemin-Dependent Ectodomain Shedding of Glycoprotein VI

INTRODUCTION

 Hemolysis results in release of free hemoglobin and hemin liberation from erythrocytes. Hemin has been described to induce platelet activation and to trigger thrombosis.

METHODS

 We evaluated the effect of hemin on platelet function and surface expression of the platelet collagen receptor glycoprotein VI (GPVI). Isolated platelets were stimulated with increasing concentrations of hemin.

RESULTS

 We found that hemin strongly enhanced platelet activation, aggregation, and aggregate formation on immobilized collagen under flow. In contrast, we found that surface expression of GPVI was significantly reduced upon hemin stimulation with high hemin concentrations indicating that hemin-induced loss of surface GPVI does not hinder platelet aggregation. Loss of hemin-induced surface expression of GPVI was caused by shedding of the ectodomain of GPVI as verified by immunoblotting and is independent of the GPVI or CLEC-2 mediated ITAM (immunoreceptor-tyrosine-based-activation-motif) signaling pathway as inhibitor studies revealed. Hemin-induced GPVI shedding was independent of metalloproteinases such as ADAM10 or ADAM17, which were previously described to regulate GPVI degradation. Similarly, concentration-dependent shedding of CD62P was also induced by hemin. Unexpectedly, we found that the subtilisin-like proprotein convertase furin controls hemin-dependent GPVI shedding as shown by inhibitor studies using the specific furin inhibitors SSM3 and Hexa-D-arginine. In the presence of SSM3 and Hexa-D-arginine, hemin-associated GPVI degradation was substantially reduced. Further, SSM3 inhibited hemin-induced but not CRP-XL-induced platelet aggregation and thrombus formation, indicating that furin controls specifically hemin-associated platelet functions.

CONCLUSION

 In summary, we describe a novel mechanism of hemin-dependent GPVI shedding and platelet function mediated by furin.

Elevated Soluble Podoplanin Associates with Hypercoagulability in Patients with Nephrotic Syndrome

Podoplanin (PDPN) promotes platelet aggregation and activation by interacting with C-type lectin-like receptor 2(CLEC-2) on platelets. The interaction between the upregulated PDPN and platelet CLEC-2 stimulates venous thrombosis. PDPN was identified as a risk factor for coagulation and thrombosis in inflammatory processes. Hypercoagulability is defined as the tendency to develop thrombosis according to fibrinogen and/or D dimer levels. Nephrotic syndrome is also considered to be a hypercoagulable state. The aim of this study is to investigate the association of soluble PDPN/CLEC-2 with hypercoagulability in nephrotic syndrome. Thirty-five patients with nephrotic syndrome and twenty-seven healthy volunteers were enrolled. PDPN, CLEC-2 and GPVI concentrations were tested by enzyme-linked immunosorbent assay (ELISA). Patients with nephrotic syndrome showed higher serum levels of PDPN and GPVI in comparison to healthy controls (P < .001, P = .001). PDPN levels in patients with nephrotic syndrome were significantly correlated with GPVI (r = 0.311; P = .025), hypoalbuminemia (r = −0.735; P < .001), hypercholesterolemia (r = 0.665; P < .001), hypertriglyceridemia (r = 0.618; P < .001), fibrinogen (r = 0.606; P < .001) and D-dimer (r = 0.524; P < .001). Area under the curve (AUC) for the prediction of hypercoagulability in nephrotic syndrome using PDPN was 0.886 (95% CI 0.804-0.967, P < .001). Cut-off value for the risk probability was 5.88 ng/ml. The sensitivity of PDPN in predicting hypercoagulability was 0.806, and the specificity was 0.846. When serum PDPN was >5.88 ng/ml, the risk of hypercoagulability was significantly increased in nephrotic syndrome (OR = 22.79, 95% CI 5.92-87.69, P < .001). In conclusion, soluble PDPN levels were correlated with hypercoagulability in nephrotic syndrome. PDPN has the better predictive value of hypercoagulability in nephrotic syndrome as well as was a reliable indicator of hypercoagulable state.

Platelet GPVI (Glycoprotein VI) and Thrombotic Complications in the Venous System

The immunoglobulin receptor GPVI (glycoprotein VI) is selectively expressed on megakaryocytes and platelets and is currently recognized as a receptor for not only collagen but also a variety of plasma and vascular proteins, including fibrin, fibrinogen, laminin, fibronectin, and galectin-3. Deficiency of GPVI is protective in mouse models of experimental thrombosis, pulmonary thromboembolism as well as in thromboinflammation, suggesting a role of GPVI in arterial and venous thrombus formation. In humans, platelet GPVI deficiency is associated with a mild bleeding phenotype, whereas a common variant rs1613662 in the GP6 gene is considered a risk factor for venous thromboembolism. However, preclinical studies on the inhibition of GPVI-ligand interactions are focused on arterial thrombotic complications. In this review we discuss the emerging evidence for GPVI in venous thrombus formation and leukocyte-dependent thromboinflammation, extending to venous thromboembolism, pulmonary thromboembolism, and cancer metastasis. We also recapitulate indications for circulating soluble GPVI as a biomarker of thrombosis-related complications. Collectively, we conclude that the current evidence suggests that platelet GPVI is also a suitable cotarget in the prevention of venous thrombosis due to its role in thrombus consolidation and platelet-leukocyte complex formation.

Immobilized collagen prevents shedding and induces sustained GPVI clustering and signaling in platelets

Collagen, the most thrombogenic constituent of blood vessel walls, activates platelets through glycoprotein VI (GPVI). In suspension, following platelet activation by collagen, GPVI is cleaved by A Disintegrin And Metalloproteinase (ADAM)10 and ADAM17. In this study, we use single-molecule localization microscopy and a 2-level DBSCAN-based clustering tool to show that GPVI remains clustered along immobilized collagen fibers for at least 3 hours in the absence of significant shedding. Tyrosine phosphorylation of spleen tyrosine kinase (Syk) and Linker of Activated T cells (LAT), and elevation of intracellular Ca²⁺, are sustained over this period. Syk, but not Src kinase-dependent signaling is required to maintain clustering of the collagen integrin α2β1, whilst neither is required for GPVI. We propose that clustering of GPVI on immobilized collagen protects GPVI from shedding in order to maintain sustained Src and Syk-kinases dependent signaling, activation of integrin α2β1, and continued adhesion.

Platelet glycoprotein VI-dependent thrombus stabilization is essential for the intraportal engraftment of pancreatic islets

Platelet activation and thrombus formation have been implicated to be detrimental for intraportal pancreatic islet transplants. The platelet-specific collagen receptor glycoprotein VI (GPVI) plays a key role in thrombosis through cellular activation and the subsequent release of secondary mediators. In aggregometry and in a microfluidic dynamic assay system modeling flow in the portal vein, pancreatic islets promoted platelet aggregation and triggered thrombus formation, respectively. While platelet GPVI deficiency did not affect the initiation of these events, it was found to destabilize platelet aggregates and thrombi in this process. Interestingly, while no major difference was detected in early thrombus formation after intraportal islet transplantation, genetic GPVI deficiency or acute anti-GPVI treatment led to an inferior graft survival and function in both syngeneic mouse islet transplantation and xenogeneic human islet transplantation models. These results demonstrate that platelet GPVI signaling is indispensable in stable thrombus formation induced by pancreatic islets. GPVI deficiency resulted in thrombus destabilization and inferior islet engraftment indicating that thrombus formation is necessary for a successful intraportal islet transplantation in which platelets are active modulators.

Loss of GPVI and GPIbα contributes to trauma-induced platelet dysfunction in severely injured patients

Trauma-induced coagulopathy (TIC) is a complex, multifactorial failure of hemostasis that occurs in 25% of severely injured patients and results in a fourfold higher mortality. However, the role of platelets in this state remains poorly understood. We set out to identify molecular changes that may underpin platelet dysfunction after major injury and to determine how they relate to coagulopathy and outcome. We performed a range of hemostatic and platelet-specific studies in blood samples obtained from critically injured patients within 2 hours of injury and collected prospective data on patient characteristics and clinical outcomes. We observed that, although platelet counts were preserved above critical levels, circulating platelets sampled from trauma patients exhibited a profoundly reduced response to both collagen and the selective glycoprotein VI (GPVI) agonist collagen-related peptide, compared with those from healthy volunteers. These responses correlated closely with overall clot strength and mortality. Surface expression of the collagen receptors GPIbα and GPVI was reduced on circulating platelets in trauma patients, with increased levels of the shed ectodomain fragment of GPVI detectable in plasma. Levels of shed GPVI were highest in patients with more severe injuries and TIC. Collectively, these observations demonstrate that platelets experience a loss of GPVI and GPIbα after severe injury and translate into a reduction in the responsiveness of platelets during active hemorrhage. In turn, they are associated with reduced hemostatic competence and increased mortality. Targeting proteolytic shedding of platelet receptors is a potential therapeutic strategy for maintaining hemostatic competence in bleeding and improving the efficacy of platelet transfusions.

Device-Induced Hemostatic Disorders in Mechanically Assisted Circulation

Mechanically assisted circulation (MAC) sustains the blood circulation in the body of a patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) or on ventricular assistance with a ventricular assist device (VAD) or on extracorporeal membrane oxygenation (ECMO) with a pump-oxygenator system. While MAC provides short-term (days to weeks) support and long-term (months to years) for the heart and/or lungs, the blood is inevitably exposed to non-physiological shear stress (NPSS) due to mechanical pumping action and in contact with artificial surfaces. NPSS is well known to cause blood damage and functional alterations of blood cells. In this review, we discussed shear-induced platelet adhesion, platelet aggregation, platelet receptor shedding, and platelet apoptosis, shear-induced acquired von Willebrand syndrome (AVWS), shear-induced hemolysis and microparticle formation during MAC. These alterations are associated with perioperative bleeding and thrombotic events, morbidity and mortality, and quality of life in MCS patients. Understanding the mechanism of shear-induce hemostatic disorders will help us develop low-shear-stress devices and select more effective treatments for better clinical outcomes.

Calcium chelation: a novel approach to reduce cryopreservation-induced damage to frozen platelets

BACKGROUND

Cryopreserved platelets are phenotypically and functionally different to conventionally stored platelets. Calcium may be released from internal stores during the freeze-thaw process, initiating signaling events which lead to these alterations. It was hypothesized that the addition of a calcium chelator prior to cryopreservation may mitigate some of these changes.

METHODS

Buffy coat-derived platelets that had been pooled and split were tested fresh and following cryopreservation (n = 8 per group). Platelets were cryopreserved using 5%-6% dimethylsulfoxide (DMSO) or were supplemented with increasing concentrations of the internal calcium chelator, BAPTA-AM (100 μM, 200 μM, or 400 μM), prior to storage at -80°C.

RESULTS

Supplementation of platelets with BAPTA-AM prior to freezing improved platelet recovery in a dose response manner (400 μM: 84 ± 2%) compared to standard DMSO cryopreserved platelets (70 ± 4%). There was a loss of GPIbα, GPVI, and GPIIb/IIIa receptors on platelets following cryopreservation, which was rescued when platelets were supplemented with BAPTA-AM (400 μM: p < 0.0001 for all). Platelet activation markers, such as phosphatidylserine and P-selectin, were externalized on platelets following cryopreservation. However, the addition of BAPTA-AM significantly reduced the increase of these activation markers on cryopreserved platelets (400 μM: p < 0.0001 for both). Both cryopreserved platelet groups exhibited similar functionality as assessed by thromboelastography, forming clots at a faster rate than fresh platelets.

CONCLUSIONS

This study demonstrates that calcium plays a crucial role in mediating cryopreservation-induced damage to frozen platelets. The addition of the calcium chelator, BAPTA-AM, prior to cryopreservation reduces this damage.

Platelets: emerging facilitators of cellular crosstalk in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease in which a variety of circulating pro-inflammatory cells and dysregulated molecules are involved in disease aetiology and progression. Platelets are an important cellular element in the circulation that can bind several dysregulated molecules (such as collagen, thrombin and fibrinogen) that are present both in the synovium and the circulation of patients with RA. Platelets not only respond to dysregulated molecules in their environment but also transport and express their own inflammatory mediators, and serve as regulators at the boundary between haemostasis and immunity. Activated platelets also produce microparticles, which further convey signalling molecules and receptors to the synovium and circulation, thereby positioning these platelet-derived particles as strategic regulators of inflammation. These diverse functions come together to make platelets facilitators of cellular crosstalk in RA. Thus, the receptor functions, ligand binding potential and dysregulated signalling pathways in platelets are becoming increasingly important for treatment in RA. This Review aims to highlight the role of platelets in RA and the need to closely examine platelets as health indicators when designing effective pharmaceutical targets in this disease.

Soluble GPVI is elevated in injured patients: shedding is mediated by fibrin activation of GPVI

Soluble glycoprotein VI (sGPVI) is shed from the platelet surface and is a marker of platelet activation in thrombotic conditions. We assessed sGPVI levels together with patient and clinical parameters in acute and chronic inflammatory conditions, including patients with thermal injury and inflammatory bowel disease and patients admitted to the intensive care unit (ICU) for elective cardiac surgery, trauma, acute brain injury, or prolonged ventilation. Plasma sGPVI was measured by enzyme-linked immunosorbent assay and was elevated on day 14 after thermal injury, and was higher in patients who developed sepsis. sGPVI levels were associated with sepsis, and the value for predicting sepsis was increased in combination with platelet count and Abbreviated Burn Severity Index. sGPVI levels positively correlated with levels of D-dimer (a fibrin degradation product) in ICU patients and patients with thermal injury. sGPVI levels in ICU patients at admission were significantly associated with 28- and 90-day mortality independent of platelet count. sGPVI levels in patients with thermal injury were associated with 28-day mortality at days 1, 14, and 21 when adjusting for platelet count. In both cohorts, sGPVI associations with mortality were stronger than D-dimer levels. Mechanistically, release of GPVI was triggered by exposure of platelets to polymerized fibrin, but not by engagement of G protein-coupled receptors by thrombin, adenosine 5'-diphosphate, or thromboxane mimetics. Enhanced fibrin production in these patients may therefore contribute to the observed elevated sGPVI levels. sGPVI is an important platelet-specific marker for platelet activation that predicts sepsis progression and mortality in injured patients.

Platelet Function During Extracorporeal Membrane Oxygenation in Adult Patients: A Systematic Review

Background: Hemorrhagic and thromboembolic complications are common during treatment with extracorporeal membrane oxygenation (ECMO), resulting in considerable morbidity and mortality. This emphasizes the clinical relevance of understanding hemostatic changes occurring during ECMO treatment. As platelets are key players in hemostasis, detailed knowledge on how ECMO treatment affects platelet function is of great importance. We therefore aimed to systematically summarize and discuss existing knowledge on platelet function during ECMO treatment in adult patients. Methods: Systematic review complying with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Objectives and methods were specified in a PROSPERO protocol (ID no CRD42018084059). The MEDLINE/PubMed, EMBASE, and Web of Science databases were systematically searched on September 10, 2018. A standardized quality assessment tool was used to assess the risk of bias in included studies. Primary outcome was platelet function during ECMO treatment, measured as platelet adhesion, activation or aggregation. Secondary outcomes were thrombosis, bleeding, and mortality during ECMO treatment. Results: A total of 591 studies were identified, of which seven were eligible for inclusion in the qualitative synthesis. Of these, one study investigated expression of platelet adhesion receptors and found them to be reduced during ECMO treatment; two studies reported a decrease in platelet activation markers during ECMO treatment; and five studies demonstrated reduced platelet aggregation during ECMO treatment. Three studies reported on thrombosis, mortality and/or bleeding during ECMO treatment; no thromboembolic events were reported; all three studies reported frequent bleeding episodes defined on basis of transfusion requirements. An in-hospital mortality of 35-40% and a 30-day mortality of roughly 30% were reported in three different studies. Conclusions: The present systematic review reveals a substantial knowledge gap regarding platelet function during ECMO treatment in adult patients and underscores the demand for more and well-designed studies on this topic. There is suggested evidence of reduced platelet adhesion, decreased platelet activation, and reduced platelet aggregation in adult patients during ECMO treatment. Importantly, platelet aggregation results need to be interpreted in the light of low platelet counts. The associations of platelet function and bleeding and/or thromboembolic complications during ECMO treatment remain to be fully elucidated.

Soluble fibrin causes an acquired platelet glycoprotein VI signaling defect: implications for coagulopathy

Essentials Collagen and thrombin when used simultaneously generate highly activated platelets. The effect of thrombin stimulation on subsequent glycoprotein VI (GPVI) function was observed. Soluble fibrin, but not protease activated receptor (PAR) activation, prevented GPVI activation. Circulating soluble fibrin in coagulopathic blood may cause an acquired GPVI signaling defect.

SUMMARY

Background In coagulopathic blood, circulating thrombin may drive platelet dysfunction. Methods/Results Using calcium dye-loaded platelets, the effect of thrombin exposure and soluble fibrin generation on subsequent platelet GPVI function was investigated. Exposure of apixaban-treated platelet-rich plasma (12% PRP) to thrombin (1-10 nm), but not ADP or thromboxane mimetic U46619 exposure, dramatically blocked subsequent GPVI activation by convulxin, collagen-related peptide or fibrillar collagen. Consistent with soluble fibrin multimerizing and binding GPVI, the onset of convulxin insensitivity required 200-500 s of thrombin exposure, was not mimicked by exposure to PAR-1/4 activating peptides, was not observed with washed platelets, and was blocked by fibrin polymerization inhibitor (GPRP) or factor XIIIa inhibitor (T101). PAR-1 signaling through Gαq was not required because vorapaxar blocked thrombin-induced calcium mobilization but had no effect on the ability of thrombin to impair GPVI-signaling. Convulxin insensitivity was unaffected by the metalloprotease inhibitor GM6001 or the αIIb β3 antagonist GR144053, indicating negligible roles for GPVI shedding or αIIb β3 binding of fibrin. Thrombin treatment of washed platelets resuspended in purified fibrinogen also produced convulxin insensitivity that was prevented by GPRP. Exposure of apixaban/PPACK-treated whole blood to thrombin-treated fibrinogen resulted in > 50% decrease in platelet deposition in a collagen microfluidic assay that required soluble fibrin assembly. Conclusions Conversion of only 1% plasma fibrinogen in coagulopathic blood would generate 90 nm soluble fibrin, far exceeding ~1 nmGPVI in blood. Soluble fibrin, rather than thrombin-induced platelet activation throuh PAR-1 and PAR-4, downregulated GPVI-signaling in response to stimuli, and may lead to subsequent hypofunction of endogenous or transfused platelets.

Focusing on plasma glycoprotein VI

New methods for analysing both platelet and plasma forms of the platelet-specific collagen receptor, glycoprotein VI (GPVI) in experimental models or human clinical samples, and the development of the first therapeutic compounds based on dimeric soluble GPVI-Fc or anti-GPVI antibody-based constructs, coincide with increased understanding of the potential pathophysiological role of GPVI ligand binding and shedding. Platelet GPVI not only mediates platelet activation at the site of vascular injury where collagen is exposed, but is also implicated in the pathogenesis of other diseases, such as atherosclerosis and coagulopathy, rheumatoid arthritis and tumour metastasis. Here, we describe some of the critical mechanisms for generating soluble GPVI from platelets, and future avenues for exploiting this unique platelet-specific receptor for diagnosis and/or disease prevention.

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.

Clinical significance of receptor shedding-platelet GPVI as an emerging diagnostic and therapeutic tool

Platelet membrane bedecked with a wide array of receptors offers a platform to regulate platelet responsiveness, thrombotic propensity, inflammatory disposition, and immune reactivity under diverse pathophysiological conditions. Ectopic proteolytic cleavage of such receptors irreversibly inactivates receptor-mediated intracellular signaling governing cellular functions, further releases soluble fragments into circulation which might modulate functions of target cells. Glycoprotein VI-(GPVI) is a membrane glycoprotein expressed in platelets and megakaryocytes. Platelet GPVI surface expression is enhanced following acute ischemic events like myocardial infarction and cerebral stroke, serves as an imminent diagnostic tool independent of markers of tissue necrosis, and is associated with poor prognosis. Platelets undergo GPVI shedding and thereby contribute to soluble plasma levels of sGPVI, with distinct diagnostic and prognostic attributes. This review summarizes the functional significance and mechanistic basis whereby GPVI surface availability is up- or downregulated on platelets and the impact of GPVI in diagnostic, prognostic, and therapeutic strategies in diseases where platelets play a regulatory role. Further, we also highlight how novel non-invasive platelet-based diagnostic and therapeutic strategies have evolved utilizing GPVI for lesion-directed antithrombotic therapy or to counteract atherosclerotic disposition to ameliorate care of patients particularly in the context of cardio-cerebro-vascular medicine.

Characterization of the platelet transcriptome by RNA sequencing in patients with acute myocardial infarction

Transcripts in platelets are largely produced in precursor megakaryocytes but remain physiologically active as platelets translate RNAs and regulate protein/RNA levels. Recent studies using transcriptome sequencing (RNA-seq) characterized the platelet transcriptome in limited number of non-diseased individuals. Here, we expand upon these RNA-seq studies by completing RNA-seq in platelets from 32 patients with acute myocardial infarction (MI). Our goals were to characterize the platelet transcriptome using a population of patients with acute MI and relate gene expression to platelet aggregation measures and ST-segment elevation MI (STEMI) (n = 16) vs. non-STEMI (NSTEMI) (n = 16) subtypes. Similar to other studies, we detected 9565 expressed transcripts, including several known platelet-enriched markers (e.g. PPBP, OST4). Our RNA-seq data strongly correlated with independently ascertained platelet expression data and showed enrichment for platelet-related pathways (e.g. wound response, hemostasis, and platelet activation), as well as actin-related and post-transcriptional processes. Several transcripts displayed suggestively higher (FBXL4, ECHDC3, KCNE1, TAOK2, AURKB, ERG, and FKBP5) and lower (MIAT, PVRL3, and PZP) expression in STEMI platelets compared to NSTEMI. We also identified transcripts correlated with platelet aggregation to TRAP (ATP6V1G2, SLC2A3), collagen (CEACAM1, ITGA2), and ADP (PDGFB, PDGFC, ST3GAL6). Our study adds to current platelet gene expression resources by providing transcriptome-wide analyses in platelets isolated from patients with acute MI. In concert with prior studies, we identify various genes for further study in regards to platelet function and acute MI. Future platelet RNA-seq studies examining more diverse sets of healthy and diseased samples will add to our understanding of platelet thrombotic and non-thrombotic functions.

Baseline Platelet Activation and Reactivity in Patients with Critical Limb Ischemia

Background

Patients with critical limb ischemia (CLI) have a high risk to develop cardiovascular events (CVE). We hypothesized that in CLI patients platelets would display increased baseline activation and reactivity.

Objectives

We investigated baseline platelet activation and platelet reactivity in patients with CLI.

Patients/Methods

In this study baseline platelet activation and platelet reactivity in response to stimulation of all major platelet activation pathways were determined in 20 CLI patients (11 using aspirin and 9 using vitamin K-antagonists) included in the Juventas-trial (clinicaltrials.gov NCT00371371) and in 17 healthy controls. Platelet activation was quantified with flow cytometric measurement of platelet P-selectin expression and fibrinogen binding.

Results

CLI patients not using aspirin showed higher baseline platelet activation compared to healthy controls. Maximal reactivity to stimulation of the collagen and thrombin activation pathway was decreased in CLI patients compared to healthy controls. In line, attenuated platelet reactivity to stimulation of multiple activation pathways was associated with several traditional risk factors for cardiovascular disease.

Conclusions

Baseline platelet activation was increased in CLI patients, whereas the reactivity of circulating platelets to several stimulatory agents is decreased. Reactivity of platelets was inversely correlated with cardiovascular risk factors.

Circulating levels of platelet α-granule cytokines in trauma patients

OBJECTIVE AND DESIGN

To elucidate whether platelets differentiate cytokine release following trauma, we prospectively measured three major platelet-derived cytokines in 213 trauma patients on hospital arrival.

METHODS

We measured plasma levels of the anti-inflammatory β-thromboglobulins (βTGs), transforming growth factor-β1 (TGFβ1) and the pro-inflammatory platelet factor 4 (PF4) cytokines. We also measured soluble glycoprotein VI (sGPVI), procoagulant platelet microparticles (PMPs) and white blood cell (WBC) counts, and evaluated in vitro platelet function in primary and secondary haemostasis by aggregometry and thromboelastometry, respectively. We evaluated associations of each cytokine by multivariate regression including injury severity score (ISS), WBC counts, sGPVI and platelet counts as explanatory variables.

RESULTS

Severely injured patients (ISS > 15) had higher levels of βTGs and TGFβ1 (both p < 0.01) but lower levels of PF4 (p = 0.02). GPVI and PMPs levels correlated with TGFβ1 and PF4 whereas we found no significant association between cytokine levels and measures of haemostasis. By multivariate regression, a high WBC count was associated with high levels of TGFβ1 (p = 0.01) and βTGs (p < 0.01) but with low levels of PF4 (p = 0.03).

CONCLUSION

Severely injured patients had higher levels of βTGs and TGFβ1 but lower levels of the PF4; a high WBC count predicted this anti-inflammatory profile of platelet cytokines.

Establishment of immunoassay for platelet-derived soluble glycoprotein VI, a novel platelet marker

Soluble Glycoprotein VI (GPVI) is an attractive marker for disorders marked by platelet activation, such as thrombotic microangiopathy, myocardial infarction, and stroke. Several groups have already developed an immunoassay for soluble GPVI; however, there are several discrepancies between the groups' assays. In this study, we prepared the two types of recombinant soluble GPVI, the monomeric form GPVI (GPVI-His) and the dimeric form of GPVI (GPVI-Fc), moreover, we generated four anti-GPVI antibodies, F1232-7-1 (7S1), F1232-10-2 (10S2), F1232-19-1 (19D1), and F1232-21-1 (21D1). The former 2 antibodies (7S1 and 10S2) had a high affinity for both GPVI-His and GPVI-Fc, while the latter 2 antibodies (19D1 and 21D1) showed a high affinity for GPVI-Fc but low affinity for GPVI-His. All of the antibodies comparably recognized surface GPVI on resting platelets. Furthermore, we established two immunoassays for soluble GPVI, 7S1/10S2-HRP and 19D1/21D1-HRP (capture antibody/detection antibody). 7S1/10S2-HRP showed equivalent reactivity with GPVI-His and GPVI-Fc, whereas 19D1/21D1-HRP had high affinity for GPVI-Fc but low reactivity with GPVI-His. In terms of reactivity with platelet-derived soluble GPVI, 7S1/10S2-HRP demonstrated sensitive detection whereas 19D1/21D1-HRP was nonreactive. Taken together, 7S1/10S2-HRP is a suitable candidate for a reliable soluble GPVI immunoassay as it has a high affinity for monomeric GPVI.

Properties of soluble glycoprotein VI, a potential platelet activation biomarker

Glycoprotein VI (GPVI) plays a critical role in the platelet response to collagen. Clinical studies suggest that the plasma level of soluble GPVI (sGPVI) is a highly specific and useful platelet activation marker. However, many properties of sGPVI have not been fully characterized, such as its sensitivity in detecting platelet activation and its elimination rate from the blood. In this study we established a sandwich enzyme-linked immunosorbent assay for human sGPVI, which cross-reacts to cynomolgus monkey sGPVI, and evaluated the time course of sGPVI production in a cynomolgus monkey model of lipopolysaccharide (LPS)-induced thrombocytopenia. The sGPVI levels in this model were dramatically elevated and returned to baseline by 24 hours after LPS injection, the change was more pronounced than the existing platelet activation biomarker, soluble P-selectin (sP-selectin) levels. The elimination half-life of recombinant human sGPVI was about 2.5 hours following intravenous administration to monkeys. These results suggest that plasma sGPVI closely reflects platelet activation in the bloodstream and has a short half-life. sGPVI would be a useful biomarker for disorders marked by platelet activation and for monitoring anti-platelet therapy.

New advances in treating thrombotic diseases: GPVI as a platelet drug target

The recent introduction of highly effective antiplatelet drugs has contributed to the significant improvement in the treatment of acute coronary syndromes. However, limitations remain. Recurrence of ischaemic vascular events results in poor prognosis. Drugs of high antithrombotic efficacy are associated with an increased risk of bleeding, which is important in patients at risk of stroke. An attractive target for the development of new antithrombotics is platelet glycoprotein VI (GPVI) because its blockade seems to combine ideally efficiency and safety. In this review, we summarise current knowledge on the physiological role of GPVI in haemostasis and thrombosis. We also discuss evidence regarding the effectiveness and safety of strategies to inhibit GPVI.

Hinokitiol is a novel glycoprotein VI antagonist on human platelets

Hinokitiol (4-isopropyl-tropolone) is a bioactive compound with various pharmacological activities that is found in the wood of cupressaceous plants. Platelet activation plays an important role in thrombogenesis. In our previous study, hinokitiol specifically inhibited collagen-induced platelet aggregation ex vivo and prolonged thrombogenesis in vivo. The glycoprotein (GP) VI and integrin α2β1 are major collagen receptors that mediate platelet adhesion and aggregation. In our current study, we investigated which of these collagen receptors is involved in the hinokitiol-mediated inhibition of platelet activation. Treatment with 2-100 µM hinokitiol caused a dose-dependent right, parallel shift in the collagen concentration-response curve (0.5-10 µg/ml), with no change in the maximal responses. Furthermore, hinokitiol inhibited platelet aggregation and relative [Ca(2+)]i mobilization stimulated by convulxin, an agonist of GP VI, but not by aggretin, an agonist of integrin α2β1, indicating that hinokitiol mediates the inhibition of platelet activation through GP VI, rather than through integrin α2β1. Hinokitiol also specifically inhibited the convulxin-mediated activation of protein kinase C, phospholipase Cγ2, Akt, mitogen-activated protein kinases, and Lyn. Hinokitiol markedly diminished the co-immunoprecipitation of GP VI-bound Lyn after convulxin stimulation. In conclusion, hinokitiol, an antagonist of collagen GP VI may represent a novel antiplatelet drug for the prevention of thrombi associated with coronary and cerebral artery diseases.

The future of glycoprotein VI as an antithrombotic target

The treatment of acute coronary syndromes has been considerably improved in recent years with the introduction of highly efficient antiplatelet drugs. However, there are still significant limitations: the recurrence of adverse vascular events remains a problem, and the improvement in efficacy is counterbalanced by an increased risk of bleeding, which is of particular importance in patients at risk of stroke. One of the most attractive targets for the development of new molecules with potential antithrombotic activity is platelet glycoprotein (GP)VI, because its blockade appears to ideally combine efficacy and safety. This review summarizes current knowledge on GPVI regarding its structure, its function, and its role in physiologic hemostasis and thrombosis. Strategies for inhibiting GPVI are presented, and evidence of the antithrombotic efficacy and safety of GPVI antagonists is provided.

Platelet receptor shedding

Receptor shedding is a mechanism for irreversible removal of transmembrane cell surface receptors by proteolysis of the receptor at a position near the extracellular surface of the plasma membrane. This process generates a soluble ectodomain fragment and a membrane-associated remnant fragment, and is distinct from loss of receptor surface expression by internalization or microparticle release or secretion of alternatively spliced soluble forms of receptors lacking a transmembrane domain. There has been an increased focus on new methods for analyzing shedding of platelet glycoprotein (GP)Ib-IX-V and GPVI because these receptors are platelet specific and are critical for the initiation of platelet adhesion and activation in thrombus formation at arterial shear rates. Platelet receptor shedding provides a mechanism for downregulating surface expression resulting in loss of ligand binding, decreasing the surface density affecting receptor cross linking and signalling and generation of proteolytic fragments that may be functional and/or provide platelet-specific biomarkers.

Design and reshaping of an scFv directed against human platelet glycoprotein VI with diagnostic potential

Blood platelets play a key role in physiological hemostasis and in thrombosis. As a consequence, platelet functional analysis is widely used in the diagnosis of hemorrhagic disorders as well as in the evaluation of thrombosis risks and of the efficacy of antithrombotics. Glycoprotein (GP) VI is a platelet-specific collagen-signaling receptor. Clinical studies suggest that increased GPVI expression is associated with a risk of arterial thrombosis. Conversely, GPVI deficiencies have been identified in patients with defective platelet responses to collagen. Currently, there is no standard test available for measuring GPVI expression, essentially because antibodies usually cross-link GPVI upon binding, leading to platelet activation and consecutive changes in GPVI expression. Here, we designed a recombinant monovalent antibody fragment (scFv) derived from an anti-GPVI monoclonal IgG, 3J24, with the characteristics required to analyze GPVI expression. Guided by in silico modeling and V-KAPPA chain analysis, a Protein L (PpL) recognition pattern was engineered in the scFv, making possible its purification and detection using PpL conjugates. The PpL affinity-purified scFv is functional. It retains GPVI-binding specificity and allows detection of platelet surface-expressed GPVI without inducing platelet activation. In conclusion, the reshaped scFv may be very useful in the development of diagnostic approaches.

Sandwich Immunoassay for Soluble Glycoprotein VI in Patients with Symptomatic Coronary Artery Disease

BACKGROUND

Platelet glycoprotein VI (pGPVI) expression is increased in acute coronary syndrome (ACS), reflecting platelet activation. There is no reliable method available to measure pGPVI. Our aim was to develop a bead-based sandwich immunoassay to measure soluble GPVI (sGPVI).

METHODS

Based on antibodies for sGPVI developed earlier, we established and validated a bead-based sandwich immunoassay in 2438 consecutive patients with stable angina pectoris (SAP; n = 1371), non–ST-elevation myocardial infarction (NSTEMI; n = 724), and ST-elevation MI (STEMI; n = 343). In a subgroup (n = 1011), we measured surface expression of pGPVI using flow cytometry.

RESULTS

The assay revealed a working range of 8–500 ng/L. Intra- and interassay imprecision was &lt;7% and &lt;14%, respectively. Patients with NSTEMI and STEMI showed significantly lower mean sGPVI concentrations than patients with SAP [mean (SD), 8.4 (3.6) μg/L and 8.6 (4.1) μg/L vs 9.8 (4.8) μg/L; P = 0.002], whereas subgroup analysis revealed significantly enhanced pGPVI in NSTEMI (n = 276) and STEMI (n = 80) patients compared with SAP (n = 655) [mean fluorescence intensity (SD), 21.2 (8.1) and 19.8 (6.8) vs 18.5 (7.7); P = 0.002 and P = 0.018]. pGPVI and sGPVI were inversely correlated (r = −0.076; P = 0.023). Area under the ROC curve was 0.716, 95% CI 0.681–0.751, for sGPVI, distinguishing patients with SAP from those with ACS, and was superior (P = 0.044) to the curve of subgroup analysis for pGPVI (0.624, 95% CI 0.586–0.662). sGPVI (P = 0.023) and pGPVI (P = 0.028) had better association with the development of ACS than troponin I (P = 0.055) in the very early stage of disease, based on logistic regression analysis.

CONCLUSIONS

This sandwich immunoassay reliably measures sGPVI and may help to identify patients with ACS earlier than other laboratory markers.

Coagulation-induced shedding of platelet glycoprotein VI mediated by factor Xa

This study evaluated shedding of the platelet collagen receptor, glycoprotein VI (GPVI) in human plasma. Collagen or other ligands induce metalloproteinase-mediated GPVI ectodomain shedding, generating approximately 55-kDa soluble GPVI (sGPVI) and approximately 10-kDa platelet-associated fragments. In the absence of GPVI ligands, coagulation of platelet-rich plasma from healthy persons induced GPVI shedding, independent of added tissue factor, but inhibitable by metalloproteinase inhibitor, GM6001. Factor Xa (FXa) common to intrinsic and tissue factor-mediated coagulation pathways was critical for sGPVI release because (1) shedding was strongly blocked by the FXa-selective inhibitor rivaroxaban but not FIIa (thrombin) inhibitors dabigatran or hirudin; (2) Russell viper venom that directly activates FX generated sGPVI, with complete inhibition by enoxaparin (inhibits FXa and FIIa) but not hirudin; (3) impaired GPVI shedding during coagulation of washed platelets resuspended in FX-depleted plasma was restored by adding purified FX; and (4) purified FXa induced GM6001-inhibitable GPVI shedding from washed platelets. In 29 patients with disseminated intravascular coagulation, mean plasma sGPVI was 53.9 ng/mL (95% confidence interval, 39.9-72.8 ng/mL) compared with 12.5 ng/mL (95% confidence interval, 9.0-17.3 ng/mL) in thrombocytopenic controls (n = 36, P < .0001), and 14.6 ng/mL (95% confidence interval, 7.9-27.1 ng/mL) in healthy subjects (n = 25, P = .002). In conclusion, coagulation-induced GPVI shedding via FXa down-regulates GPVI under procoagulant conditions. FXa inhibitors have an unexpected role in preventing GPVI down-regulation.

Proteolysis of platelet receptors in humans and other species

In the past 5 years, metalloproteinase-mediated ectodomain shedding of platelet receptors has emerged as a new mechanism for modulating platelet function. By regulating surface expression of the platelet-specific receptors, glycoprotein (GP)VI that binds collagen, and GPIbalpha (the major ligand-binding subunit of the GPIb-IX-V complex) that binds von Willebrand factor (VWF) and other procoagulant and proinflammatory ligands, shedding not only irreversibly downregulates GPVI/GPIbalpha function, but generates proteolytic fragments that might be unique biomarkers or modulators in plasma. This is potentially significant because GPVI and GPIbalpha are involved in initiating thrombotic diseases such as heart attack and stroke, as well as autoimmune diseases where anti-platelet antibodies result in thrombocytopenia. Altered expression levels of GPIbalpha/GPVI are associated with both thrombotic propensity and platelet aging, suggesting an additional role in platelet clearance. Although emerging data are elucidating molecular mechanisms underlying GPIbalpha/GPVI shedding, evidence for the functional consequences of shedding in vivo, either clinically or in animal models, is far more limited. Here we consider recent published evidence for GPVI or GPIbalpha shedding in humans, nonhuman primates and mice, and whether conservation of sheddase cleavage sites across species points to a functional role for metalloproteolytic shedding in vivo.

GPVI and the not so eager cleaver

Platelets control their responsiveness, in part, by shedding adhesion and signaling receptors from their surface. The molecular mechanism by which this occurs, however,is incompletely understood. In this issue of Blood, Bender and colleagues make judicious use of mice genetically deficient in selected candidate proteases to shed new light on the unexpected complexity of ectodomain shedding.

Glycoprotein Ibα receptor instability is associated with loss of quality in platelets produced in culture

The development of culture processes for hematopoietic progenitors could lead to the development of a complementary source of platelets for therapeutic purposes. However, functional characterization of culture-derived platelets remains limited, which raises some uncertainties about the quality of platelets produced in vitro. The aim of this study was to define the proportion of functional platelets produced in cord blood CD34+ cell cultures. Toward this, the morphological and functional properties of culture-derived platelet-like particles (PLPs) were critically compared to that of blood platelets. Flow cytometry combined with transmission electron microscopy analyses revealed that PLPs formed a more heterogeneous population of platelets at a different stage of maturation than blood platelets. The majority of PLPs harbored the fibrinogen receptor αIIbβ3, but a significant proportion failed to maintain glycoprotein (GP)Ibα surface expression, a component of the vWF receptor essential for platelet functions. Importantly, GPIbα extracellular expression correlated closely with platelet function, as the GPIIb+ GPIbα+ PLP subfraction responded normally to agonist stimulation as evidenced by α-granule release, adhesion, spreading, and aggregation. In contrast, the GPIIb+ GPIbα⁻ subfraction was unresponsive in most functional assays and appeared to be metabolically inactive. The present study confirms that functional platelets can be generated in cord blood CD34+ cell cultures, though these are highly susceptible to ectodomain shedding of receptors associated with loss of function. Optimization of culture conditions to prevent these deleterious effects and to homogenize PLPs is necessary to improve the quality and yields of culture-derived platelets before they can be recognized as a suitable complementary source for therapeutic purposes.

Differentially regulated GPVI ectodomain shedding by multiple platelet-expressed proteinases

Glycoprotein VI (GPVI) mediates platelet activation on exposed subendothelial collagens at sites of vascular injury and thereby contributes to normal hemostasis, but also to the occlusion of diseased vessels in the setting of myocardial infarction or stroke. GPVI is an attractive target for antithrombotic therapy, particularly because previous studies have shown that anti-GPVI antibodies induce irreversible down-regulation of the receptor in circulating platelets by internalization and/or ectodomain shedding. Metalloproteinases of the a disintegrin and metalloproteinase (ADAM) family have been proposed to mediate this ectodomain shedding, but direct evidence for this is lacking. Here, we studied GPVI shedding in vitro and in vivo in newly generated mice with a megakaryocyte-specific ADAM10 deficiency and in Adam17(ex/ex) mice, which lack functional ADAM17. We demonstrate that GPVI cleavage in vitro can occur independently through either ADAM10 or ADAM17 in response to distinct stimuli. In contrast, antibody (JAQ1)-induced GPVI shedding in vivo occurred in mice lacking both ADAM10/ADAM17 in their platelets, suggesting the existence of a third GPVI cleaving platelet enzyme. This was supported by in vitro studies on ADAM10/ADAM17 double-deficient platelets. These results reveal that ectodomain shedding of GPVI can be mediated through multiple differentially regulated platelet-expressed proteinases with obvious therapeutic implications.

Ectodomain shedding of Fcalpha receptor is mediated by ADAM10 and ADAM17

FcalphaR (CD89) plays important roles in immunoglobulin A (IgA)-mediated immune responses. Soluble forms of FcalphaR (sFcalphaR) are found in the culture supernatants of FcalphaR-expressing cells, in human serum and in the serum of FcalphaR transgenic mice, and have been suggested to be produced through a proteolytic process. However, little is known about the mechanism involved in the proteolytic release of sFcalphaR. In this study, we investigated the shedding mechanism of FcalphaR and determined the nature of the proteinase involved in FcalphaR shedding. In chemical inhibitor assays, shedding of FcalphaR was dramatically inhibited by EDTA, EGTA and a broad-spectrum metalloproteinase inhibitor, GM6001, suggesting that a metalloproteinase was responsible for FcalphaR shedding. Overexpression of dominant-negative mutants of ADAM (a disintegrin and metalloproteinase) 10 and ADAM17 markedly inhibited the production of sFcalphaR. Finally, knockdown of both endogenous ADAM10 and endogenous ADAM17 inhibited FcalphaR shedding, demonstrating that ADAM10 and ADAM17 were involved in the shedding of FcalphaR. The characterization of ADAM10 and ADAM17 as sFcalphaR-releasing enzymes provides a novel insight into the molecular mechanism of sFcalphaR production and will help in further elucidation of the physiological and pathological roles of sFcalphaR.

Measuring soluble platelet glycoprotein VI in human plasma by ELISA

Recent experimental evidence demonstrates that the platelet-specific collagen receptor, glycoprotein (GP)VI is essentially all uncleaved on normal circulating platelets, but is shed from the platelet surface in a metalloproteinase-dependent manner in response to GPVI ligands (including collagen), anti-GPVI antibodies or activation at the platelet Fc receptor, FcgammaRIIa. This raises the question of whether shed ectodomain fragment in plasma could be a useful biomarker of thrombotic risk and/or autoimmune thrombocytopenia. In this study, we developed a sandwich enzyme-linked immunosorbent assay (ELISA) for measuring soluble GPVI in human plasma, using rabbit anti-GPVI polyclonal antibody in the solid-phase, murine anti-GPVI monoclonal antibody (1A12) in the fluid-phase and horseradish peroxidase (HRP)-coupled anti-mouse antibody and enhanced chemiluminescence (ECL) for detection. The ELISA was optimized for sensitivity, reproducibility, inter- and intra-assay precision, addition and recovery and detected GPVI in plasma with a lower detection limit of approximately 1 ng/mL. Effects of different anti-coagulants (trisodium citrate, acid-citrate-dextrose or EDTA) were negligible. In ten healthy donors, soluble plasma GPVI levels were 18.9 +/- 4.1 ng/mL. Treating normal platelet-rich plasma with a GPVI ligand (collagen-related peptide, CRP), calmodulin inhibitor W7 (that induces GPVI shedding without platelet activation) or N-ethylmaleimide (that directly activates platelet sheddases), under conditions previously shown to induce GPVI shedding, also increased plasma GPVI levels by up to approximately 7-fold, compared to previously reported autoimmune (anti-GPVI) patient plasma where soluble GPVI was approximately 10-fold higher than normal. Characterization of this sensitive ELISA should facilitate analysis of functional/diagnostic role(s) for soluble GPVI in human plasma associated with thrombotic/immune dysfunction.

Platelet matrix metalloprotease-1 mediates thrombogenesis by activating PAR1 at a cryptic ligand site

Matrix metalloproteases (MMPs) play important roles in normal and pathological remodeling processes including atherothrombotic disease, inflammation, angiogenesis, and cancer. MMPs have been viewed as matrix-degrading enzymes, but recent studies have shown that they possess direct signaling capabilities. Platelets harbor several MMPs that modulate hemostatic function and platelet survival; however their mode of action remains unknown. We show that platelet MMP-1 activates protease-activated receptor-1 (PAR1) on the surface of platelets. Exposure of platelets to fibrillar collagen converts the surface-bound proMMP-1 zymogen to active MMP-1, which promotes aggregation through PAR1. Unexpectedly, MMP-1 cleaves PAR1 at a distinct site that strongly activates Rho-GTP pathways, cell shape change and motility, and MAPK signaling. Blockade of MMP1-PAR1 curtails thrombogenesis under arterial flow conditions and inhibits thrombosis in animals. These studies provide a link between matrix-dependent activation of metalloproteases and platelet-G protein signaling and identify MMP1-PAR1 as a potential target for the prevention of arterial thrombosis.

Regulation of platelet glycoprotein VI (GPVI) surface expression and of soluble GPVI in patients with atrial fibrillation (AF) and acute coronary syndrome (ACS)

BACKGROUND

The platelet collagen receptor glycoprotein VI (GPVI) mediates platelet adhesion to subendothelial matrix and thrombus formation in acute coronary syndrome (ACS). This study examined patients with both ACS and stable coronary artery disease (CAD), which presented with atrial fibrillation (AF) and sinus rhythm (SR).

METHODS AND RESULTS

We evaluated 992 patients with acute or stable CAD, and determined platelet surface expression of GPVI using flow cytometry. Seventy-eight patients presented with nonvalvular persistent AF. After 1:1 propensity score matching 156 matched cases with 78 pairs were obtained. Patients with AF and ACS showed a significantly decreased GPVI expression compared to patients with ACS and SR, whereas patients with stable angina pectoris (SA) presented with low level activation and no significant difference between SR and AF [mean fluorescence intensity (MFI) for ACS (SR Vs. AF): 20 +/- 6.3 Vs. 17.7 +/- 4.4; P = 0.023; SA (SR Vs. AF): 18.8 +/- 9.4 Vs. 18.1 +/- 6.1; P = 0.649]. In contrast, soluble GPVI was increased in ACS and AF accordingly [plasma GPVI (ng/ml) for ACS (SR Vs. AF): 1.4 +/- 0.8 Vs. 1.9 +/- 1.1; P = 0.038; SA (SR Vs. AF): 0.9 +/- 0.4 Vs. 1.1 +/- 0.5; P = 0.127].

CONCLUSION

Platelet GPVI surface expression is decreased in patients with AF and ACS compared to patients with SR and ACS. Nonvalvular AF is related to indices of chronic platelet activation and might be responsible for a down-regulation of GPVI receptor density on platelets, while soluble GPVI was increased in ACS and AF accordingly.

Metalloproteinase regulation improves in vitro generation of efficacious platelets from mouse embryonic stem cells

Embryonic stem cells (ESCs) could potentially compensate for the lack of blood platelets available for use in transfusions. Here, we describe a new method for generating mouse ESC-derived platelets (ESPs) that can contribute to hemostasis in vivo. Flow cytometric sorting of cells from embryoid bodies on day 6 demonstrated that c-Kit⁺ integrin αIIb (αIIb)⁺ cells, but not CD31⁺ cells or vascular endothelial cadherin⁺ cells, are capable of megakaryopoiesis and the release of platelet-like structures by day 12. αIIbβ3-expressing ESPs exhibited ectodomain shedding of glycoprotein (GP)Ibα, GPV, and GPVI, but not αIIbβ3 or GPIbβ. ESPs showed impaired αIIbβ3 activation and integrin-mediated actin reorganization, critical events for normal platelet function. However, the administration of metalloproteinase inhibitors GM6001 or TAPI-1 during differentiation increased the expression of GPIbα, improving both thrombogenesis in vitro and posttransfusion recovery in vivo. Thus, the regulation of metalloproteinases in culture could be useful for obtaining high-quality, efficacious ESPs as an alternative platelet source for transfusions.

A novel antiplatelet antibody therapy that induces cAMP-dependent endocytosis of the GPVI/Fc receptor gamma-chain complex

Platelet adhesion to vascular subendothelium, mediated in part by interactions between collagen and glycoprotein VI (GPVI) complexed with Fc receptor gamma-chain, is crucial for thrombus formation. Antiplatelet therapy benefits patients with various thrombotic and ischemic diseases, but the safety and efficacy of existing treatments are limited. Recent data suggest GPVI as a promising target for a novel antiplatelet therapy, for example, GPVI-specific Abs that deplete GPVI from the surface of platelets. Here, we characterized GPVI-specific auto-Abs (YA-Abs) from the first reported patient with ongoing platelet GPVI deficiency caused by the YA-Abs. To obtain experimentally useful human GPVI-specific mAbs with characteristics similar to YA-Abs, we generated human GPVI-specific mouse mAbs and selected 2 representative mAbs, mF1201 and mF1232, whose binding to GPVI was inhibited by YA-Abs. In vitro, mF1201, but not mF1232, induced human platelet activation and GPVI shedding, and mF1232 inhibited collagen-induced human platelet aggregation. Administration of mF1201 and mF1232 to monkeys caused GPVI immunodepletion with and without both significant thrombocytopenia and GPVI shedding, respectively. When a human/mouse chimeric form of mF1232 (cF1232) was labeled with a fluorescent endocytosis probe and administered to monkeys, fluorescence increased in circulating platelets and surface GPVI was lost. Loss of platelet surface GPVI mediated by cF1232 was successfully reproduced in vitro in the presence of a cAMP-elevating agent. Thus, we have characterized cAMP-dependent endocytosis of GPVI mediated by a human GPVI-specific mAb as what we believe to be a novel antiplatelet therapy.

Compromised ITAM-based platelet receptor function in a patient with immune thrombocytopenic purpura

BACKGROUND

Receptors on platelets that contain immunoreceptor tyrosine-based activation motifs (ITAMs) include collagen receptor glycoprotein (GP) VI, and FcgammaRIIa, a low affinity receptor for immunoglobulin (Ig) G.

OBJECTIVES

We examined the function of GPVI and FcgammaRIIa in a patient diagnosed with immune thrombocytopenic purpura (ITP) who had unexplained pathological bruising despite normalization of the platelet count with treatment.

METHODS AND RESULTS

Patient platelets aggregated normally in response to ADP, arachadonic acid and epinephrine, but not to GPVI agonists, collagen or collagen-related peptide, or to FcgammaRII-activating monoclonal antibody (mAb) 8.26, suggesting ITAM receptor dysfunction. Plasma contained an anti-GPVI antibody by MAIPA and aggregated normal platelets. Aggregating activity was partially (approximately 60%) blocked by FcgammaRIIa-blocking antibody, IV.3, and completely blocked by soluble GPVI ectodomain. Full-length GPVI on the patient platelet surface was reduced to approximately 10% of normal levels, and a approximately 10-kDa GPVI cytoplasmic tail remnant and cleaved FcgammaRIIa were detectable by western blot, indicating platelet receptor proteolysis. Plasma from the patient contained approximately 150 ng mL(-1) soluble GPVI by ELISA (normal plasma, approximately 15 ng mL(-1)) and IgG purified from patient plasma caused FcgammaRIIa-mediated, EDTA-sensitive cleavage of both GPVI and FcgammaRIIa on normal platelets.

CONCLUSIONS

In ITP patients, platelet autoantibodies can curtail platelet receptor function. Platelet ITAM receptor dysfunction may contribute to the increased bleeding phenotype observed in some patients with ITP.

Ex vivo inhibition of thrombus formation by an anti-glycoprotein VI Fab fragment in non-human primates without modification of glycoprotein VI expression

OBJECTIVES

Glycoprotein (GP)VI is an attractive target for the development of new antithrombotic drugs. Its deficiency protects animals in several models of thrombosis, arterial stenosis and ischemia--reperfusion while inducing no major bleeding tendency. The Fab fragment of one anti-GPVI monoclonal antibody (9O12.2) inhibits all GPVI functions in vitro. The aim of this study was to determine the ex vivo effects of 9O12.2 Fab on hemostasis, coagulation and thrombosis in non-human primates.

METHODS AND RESULTS

Blood samples were collected from cynomolgus monkeys before and after (30, 90 and 150 min, 1 and 7 days) a bolus injection of 9O12.2 Fab (4 mg kg(-1)) or vehicle. Platelet counts and coagulation tests (prothrombin time, activated partial thromboplastin time) were not modified following Fab injection. The PFA-100 closure time increased during the first hours and returned to initial values on day + 1. Platelet-bound Fab was detected from 30 min to 24 h after Fab injection without GPVI depletion at any time. Collagen-induced platelet aggregation was selectively and fully inhibited at 30 min. Thrombus formation on collagen in flowing whole blood (1500 s(-1)) was delayed and decreased, and collagen-induced or tissue factor-induced thrombin generation in platelet-rich plasma was profoundly inhibited.

CONCLUSION

The anti-GPVI 9O12.2 Fab inhibits thrombus formation ex vivo in non-human primates with a composite effect on platelet activation and thrombin generation in the absence of GPVI depletion.

The highly specific platelet glycoprotein (GP) VI agonist trowaglerix impaired collagen-induced platelet aggregation ex vivo through matrix metalloproteinase-dependent GPVI shedding

BACKGROUND

C-type lectin proteins (CLPs) have diverse targets including platelet GPIb, GPVI and integrin alpha(2)beta(1), and affect platelet function in a various way. In this study, we characterized a huge, heterodimeric venom protein, trowaglerix, which belongs to the CLP family.

METHODS

We purified a potent platelet-aggregation inducer, trowaglerix, from the crude venom of Tropidolaemus wagleri. Biotinylated trowaglerix was used for binding assays, and immunoblotting was used to investigate the signal transduction involved.

RESULTS

Two distinct subunits of trowaglerix with similar masses of around 16 kDa were eluted by high-performance liquid chromatography after reduction and alkylation. Trowaglerix induced platelet aggregation of washed human platelets and platelet-rich plasma (PRP) in a concentration-dependent manner. Biotinylated trowaglerix specifically bound to platelet membrane GPVI, but not to GPIb or alpha(2) integrin. Treatment with trowaglerix induced GPVI loss in human platelets in vitro and impaired the platelet aggregation of mouse PRP ex vivo in response to collagen but not in response to adenosine diphosphate (ADP). However, GM6001, a matrix metalloproteinase (MMP) inhibitor, inhibited trowaglerix-induced GPVI cleavage and restored the platelet responsiveness of PRP to collagen.

CONCLUSIONS

Trowaglerix activates platelets through specific binding to GPVI, leading to kinases-dependent exposure of functional alpha(IIb)beta(3) and platelet aggregation, and also induces MMP-dependent GPVI shedding from platelets.

Novel therapeutic targets at the platelet vascular interface

Platelet activation in vivo can be part of the hemostatic response to injury or a pathological response to disease. In either setting, platelets adhere to the vessel wall and to each other, forming a closely packed mass interspersed with fibrin. Recent studies have identified new molecules on the platelet surface and within platelets that support and regulate thrombus growth and stability, ensuring that platelet accumulation after injury is sufficient to stop bleeding, but not so exuberant that vascular occlusion occurs. An understanding of how this balance is achieved helps to illuminate the events of platelet activation and, at the same time, provides potential targets for new classes of antiplatelet agents.