High factor VIII concentrations interfere with glycoprotein VI-mediated platelet activation in vitro

BACKGROUND

The recruitment of activated factor VIII (FVIII) at the surface of activated platelets is a key step toward the burst of thrombin and fibrin generation during thrombus formation at the site of vascular injury. It involves binding to phosphatidylserine and, possibly, to fibrin-bound αIIbβ3. Seminal work had shown the binding of FVIII to resting platelets, yet without a clear understanding of a putative physiological relevance.

OBJECTIVES

To characterize the effects of FVIII-platelet interaction and its potential modulation of platelet function.

METHODS

FVIII was incubated with washed platelets. The effects on platelet activation (spontaneously or triggered by collagen and thrombin) were studied by flow cytometry and light transmission aggregometry. We explored the involvement of downstream pathways by studying phosphorylation profiles (Western blot). The FVIII-glycoprotein (GP) VI interaction was investigated by ELISA, confocal microscopy, and proximity ligation assay.

RESULTS

FVIII bound to the surface of resting and activated platelets in a dose-dependent manner. FVIII at supraphysiological concentrations did not induce platelet activation but rather specifically inhibited collagen-induced platelet aggregation and altered glycoprotein VI (GPVI)-dependent phosphorylation. FVIII, freed of its chaperone protein von Willebrand factor (VWF), interacted in close proximity with GPVI at the platelet surface.

CONCLUSION

We showed that VWF-free FVIII binding to, or close to, GPVI modulates platelet activation in vitro. This may represent an uncharacterized negative feedback loop to control overt platelet activation. Whether locally activated FVIII concentrations achieved during platelet accumulation and thrombus formation at the site of vascular injury in vivo are compatible with such a function remains to be determined.

The localization, origin, and impact of platelets in the tumor microenvironment are tumor type-dependent

BACKGROUND

How platelets interact with and influence the tumor microenvironment (TME) remains poorly characterized.

METHODS

We compared the presence and participation of platelets in the TME of two tumors characterized by highly different TME, PyMT AT-3 mammary tumors and B16F1 melanoma.

RESULTS

We show that whereas firmly adherent platelets continuously line tumor vessels of both AT-3 and B16F1 tumors, abundant extravascular stromal clusters of platelets from thrombopoietin-independent origin were present only in AT-3 mammary tumors. We further show that platelets influence the angiogenic and inflammatory profiles of AT-3 and B16F1 tumors, though with very different outcomes according to tumor type. Whereas thrombocytopenia increased bleeding in both tumor types, it further caused severe endothelial degeneration associated with massive vascular leakage, tumor swelling, and increased infiltration of cytotoxic cells, only in AT-3 tumors.

CONCLUSIONS

These results indicate that while platelets are integral components of solid tumors, their localization and origin in the TME, as well as their impact on its shaping, are tumor type-dependent.

Platelets and neutrophils cooperate to induce increased neutrophil extracellular trap formation in JAK2V617F myeloproliferative neoplasms

BACKGROUND

Neutrophils participate in the pathogenesis of thrombosis through the formation of neutrophil extracellular traps (NETs). Thrombosis is the main cause of morbidity and mortality in patients with myeloproliferative neoplasms (MPNs). Recent studies have shown an increase in NET formation (NETosis) both in patients with JAK2V617F neutrophils and in mouse models, and reported the participation of NETosis in the pathophysiology of thrombosis in mice.

OBJECTIVES

This study investigated whether JAK2V617F neutrophils are sufficient to promote thrombosis or whether their cooperation with other blood cell types is necessary.

METHODS

NETosis was studied in PF4iCre;Jak2V617F/WT mice expressing JAK2V617F in all hematopoietic lineages, as occurs in MPNs, and in MRP8Cre;Jak2V617F/WT mice in which JAK2V617F is expressed only in leukocytes.

RESULTS

In PF4iCre;Jak2V617F/WT mice, an increase in NETosis and spontaneous lung thrombosis abrogated by DNAse administration were observed. The absence of spontaneous NETosis or lung thrombosis in MRP8Cre;Jak2V617F/WT mice suggested that mutated neutrophils alone are not sufficient to induce thrombosis. Ex vivo experiments demonstrated that JAK2V617F-mutated platelets trigger NETosis by JAK2V617F-mutated neutrophils. Aspirin treatment in PF4iCre;Jak2V617F/WT mice reduced NETosis and reduced lung thrombosis. In cytoreductive-therapy-free patients with MPN treated with aspirin, plasma NET marker concentrations were lower than that in patients with MPN not treated with aspirin.

CONCLUSION

Our study demonstrates that JAK2V617F neutrophils alone are not sufficient to promote thrombosis; rather, platelets cooperate with neutrophils to promote NETosis in vivo. A new role for aspirin in thrombosis prevention in MPNs was also identified.

Neutrophil, NETs and Behçet's disease: A review

Behçet's disease (BD) is a chronic systemic vasculitis characterized by recurrent oral and genital ulcers, skin lesions, articular, neurological, vascular and sight-threatening ocular inflammation. BD is thought to share both autoimmune and autoinflammatory disease features. BD is triggered by environmental factors such as infectious agents in genetically predisposed subjects. Neutrophils seem to play an instrumental role in BD and recent works regarding the role of neutrophils extracellular traps (NETs) provides new insight in the pathophysiology of BD and the mechanisms involved in immune thrombosis. This review provides a recent overview on the role of neutrophils and NETs in the pathogenesis of BD.

Imaging Cerebral Arteries Tortuosity and Velocities by Transcranial Doppler Ultrasound Is a Reliable Assessment of Brain Aneurysm in Mouse Models

Background

During the past few decades, several pathophysiological processes contributing to intracranial aneurysm (IA) rupture have been identified, including irregular IA shape, altered hemodynamic stress within the IA, and vessel wall inflammation. The use of preclinical models of IA and imaging tools is paramount to better understand the underlying disease mechanisms.

Methods

We used 2 established mouse models of IA, and we analyzed the progression of the IA by magnetic resonance imaging, transcranial Doppler, and histology.

Results

In both models of IA, we observed, by transcranial Doppler, a significant decrease of the blood velocities and wall shear stress of the internal carotid arteries. We also observed the formation of tortuous arteries in both models that were correlated with the presence of an aneurysm as confirmed by magnetic resonance imaging and histology. A high grade of tortuosity is associated with a significant decrease of the mean blood flow velocities and a greater artery dilation.

Conclusions

Transcranial Doppler is a robust and convenient imaging method to evaluate the progression of IA. Detection of decreased blood flow velocities and increased tortuosity can be used as reliable indicators of IA.

Novel ELISA for the specific detection of protease NEXIN-1 in human biological samples

Introduction

Serpin E2 or protease nexin‐1 (PN‐1) is a glycoprotein belonging to the serpin superfamily, whose function is closely linked to its ability to inhibit thrombin and proteases of the plasminergic system.

Objectives

In the absence of specific quantitative methods, an ELISA for the quantification of human PN‐1 was characterized and used in biological fluids.

Methods

The ELISA for human PN‐1 was developed using two monoclonal antibodies raised against human recombinant PN‐1. PN‐1 was quantified in plasma, serum, platelet secretion from controls and patients with hemophilia A and in conditioned medium of aortic tissue.

Results

A linear dose–response curve was observed between 2 and 35 ng/mL human PN‐1. Intra‐ and interassay coefficients of variation were 6.2% and 11.1%, respectively. Assay recoveries of PN‐1 added to biological samples were ≈95% in plasma, ≈97% in platelet reaction buffer, and ≈93% in RPMI cell culture medium. Levels of PN‐1 secreted from activated human platelets from controls was similar to that of patients with hemophilia A. PN‐1 could be detected in conditioned media of aneurysmal aorta but not in that of control aorta.

Conclusion

This is the first fully characterized ELISA for human serpin E2 level in biological fluids. It may constitute a relevant novel tool for further investigations on the pathophysiological role of serpin E2 in a variety of clinical studies.

Syndecan-1 Is Overexpressed in Human Thoracic Aneurysm but Is Dispensable for the Disease Progression in a Mouse Model

Glycosaminoglycans (GAGs) pooling has long been considered as one of the histopathological characteristics defining thoracic aortic aneurysm (TAA) together with smooth muscle cells (SMCs) apoptosis and elastin fibers degradation. However, little information is known about GAGs composition or their potential implication in TAA pathology. Syndecan-1 (SDC-1) is a heparan sulfate proteoglycan that is implicated in extracellular matrix (ECM) interaction and assembly, regulation of SMCs phenotype, and various aspects of inflammation in the vascular wall. Therefore, the aim of this study was to determine whether SDC-1 expression was regulated in human TAA and to analyze its role in a mouse model of this disease. In the current work, the regulation of SDC-1 was examined in human biopsies by RT-qPCR, ELISA, and immunohistochemistry. In addition, the role of SDC-1 was evaluated in descending TAA in vivo using a mouse model combining both aortic wall weakening and hypertension. Our results showed that both SDC-1 mRNA and protein are overexpressed in the media layer of human TAA specimens. RT-qPCR experiments revealed a 3.6-fold overexpression of SDC-1 mRNA (p = 0.0024) and ELISA assays showed that SDC-1 protein was increased 2.3 times in TAA samples compared with healthy counterparts (221 ± 24 vs. 96 ± 33 pg/mg of tissue, respectively, p = 0.0012). Immunofluorescence imaging provided evidence that SMCs are the major cell type expressing SDC-1 in TAA media. Similarly, in the mouse model used, SDC-1 expression was increased in TAA specimens compared to healthy samples. Although its protective role against abdominal aneurysm has been reported, we observed that SDC-1 was dispensable for TAA prevalence or rupture. In addition, SDC-1 deficiency did not alter the extent of aortic wall dilatation, elastin degradation, collagen deposition, or leukocyte recruitment in our TAA model. These findings suggest that SDC-1 could be a biomarker revealing TAA pathology. Future investigations could uncover the underlying mechanisms leading to regulation of SDC-1 expression in TAA.

Imaging Modalities for Intracranial Aneurysm: More Than Meets the Eye

Intracranial aneurysms (IA) are often asymptomatic and have a prevalence of 3 to 5% in the adult population. The risk of IA rupture is low, however when it occurs half of the patients dies from subarachnoid hemorrhage (SAH). To avoid this fatal evolution, the main treatment is an invasive surgical procedure, which is considered to be at high risk of rupture. This risk score of IA rupture is evaluated mainly according to its size and location. Therefore, angiography and anatomic imaging of the intracranial aneurysm are crucial for its diagnosis. Moreover, it has become obvious in recent years that several other factors are implied in this complication, such as the blood flow complexity or inflammation. These recent findings lead to the development of new IA imaging tools such as vessel wall imaging, 4D-MRI, or molecular MRI to visualize inflammation at the site of IA in human and animal models. In this review, we will summarize IA imaging techniques used for the patients and those currently in development.

Serpins, New Therapeutic Targets for Hemophilia

Hemostasis is a tightly regulated process characterized by a finely tuned balance between procoagulant and anticoagulant systems. Among inherited hemostatic conditions, hemophilia is one of the most well-known bleeding disorders. Hemophilia A (HA) and B (HB) are due to deficiencies in coagulation factor VIII (FVIII) or FIX, respectively, leading to unwanted bleeding. Until recently, hemophilia treatment has consisted of prophylactic replacement therapy using plasma-derived or recombinant FVIII in cases of HA or FIX in cases of HB. Because FVIII and FIX deficiencies lead to an imbalance between procoagulant and anticoagulant systems, a recent upcoming strategy implies blocking of endogenous anticoagulant proteins to compensate for the procoagulant factor deficit, thus restoring hemostatic equilibrium. Important physiological proteins of the anticoagulant pathways belong to the serpin (serine protease inhibitor) family and, recently, different experimental and clinical studies have demonstrated that targeting natural serpins could decrease bleeding in hemophilia. Here, we aim to review the different, recent studies demonstrating that blocking serpins such as antithrombin, protein Z-dependent protease inhibitor, and protease nexin-1 or modifying a serpin like α1-antitrypsin could rebalance coagulation in hemophilia. Furthermore, we underline the potential therapeutic use of serpins for the treatment of hemophilia.

Development and characterization of single-domain antibodies neutralizing protease nexin-1 as tools to increase thrombin generation

BACKGROUND

Protease nexin-1 (PN-1) is a member of the serine protease inhibitor (Serpin)-family, with thrombin as its main target. Current polyclonal and monoclonal antibodies against PN-1 frequently cross-react with plasminogen activator inhibitor-1 (PAI-1), a structurally and functionally homologous Serpin.

OBJECTIVES

Here, we aimed to develop inhibitory single-domain antibodies (VHHs) that show specific binding to both human (hPN-1) and murine (mPN-1) PN-1.

METHODS

PN-1-binding VHHs were isolated via phage-display using llama-derived or synthetic VHH-libraries. Following bacterial expression, purified VHHs were analyzed in binding and activity assays.

RESULTS AND CONCLUSIONS

By using a llama-derived library, 2 PN-1 specific VHHs were obtained (KB-PN1-01 and KB-PN1-02). Despite their specificity, none displayed inhibitory activity toward hPN-1 or mPN-1. From the synthetic library, 4 VHHs (H12, B11, F06, A08) could be isolated that combined efficient binding to both hPN-1 and mPN-1 with negligible binding to PAI-1. Of these, B11, F06, and A08 were able to fully restore thrombin activity by blocking PN-1. As monovalent VHH, half-maximal inhibitory concentration values for hPN-1 were 50 ± 10, 290 ± 30, and 960 ± 390 nmol/L, for B11, F06, and A08, respectively, and 1580 ± 240, 560 ± 130, and 2880 ± 770 nmol/L for mPN-1. The inhibitory potential was improved 4- to 7-fold when bivalent VHHs were engineered. Importantly, all VHHs could block PN-1 activity in plasma as well as PN-1 released from activated platelets, one of the main sources of PN-1 during hemostasis. In conclusion, we report the generation of inhibitory anti-PN-1 antibodies using a specific approach to avoid cross-reactivity with the homologous Serpin PAI-1.

Interferon Alpha Therapy Increases Pro-Thrombotic Biomarkers in Patients with Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPN) are associated with an increased risk of arterial and venous thrombosis. Pegylated-interferon alpha (IFN) and hydroxyurea (HU) are commonly used to treat MPN, but their effect on hemostasis has not yet been studied. The aim of our study was to determine whether IFN and HU impact the biological hemostatic profile of MPN patients by studying markers of endothelial, platelet, and coagulation activation. A total of 85 patients (50 polycythemia vera and 35 essential thrombocythemia) were included: 28 treated with IFN, 35 with HU, and 22 with no cytoreductive drug (non-treated, NT). Von Willebrand factor, shear-induced platelet aggregation, factor VIII coagulant activity (FVIII:C), fibrinogen, and thrombin generation with and without exogenous thrombomodulin were significantly higher in IFN-treated patients compared to NT patients, while protein S anticoagulant activity was lower. In 10 patients in whom IFN therapy was discontinued, these hemostatic biomarkers returned to the values observed in NT patients, strongly suggesting an impact of IFN therapy on endothelial and coagulation activation. Overall, our study shows that treatment with IFN is associated with significant and reversible effects on the biological hemostatic profile of MPN patients. Whether they could be associated with an increased thrombotic risk remains to be determined in further randomized clinical studies.

Platelets trigger perivascular mast cell degranulation to cause inflammatory responses and tissue injury

Platelet responses have been associated with end-organ injury and mortality following complex insults such as cardiac surgery, but how platelets contribute to these pathologies remains unclear. Our studies originated from the observation of microvascular platelet retention in a rat cardiac surgery model. Ensuing work supported the proximity of platelet aggregates with perivascular mast cells (MCs) and demonstrated that platelet activation triggered systemic MC activation. We then identified platelet activating factor (PAF) as the platelet-derived mediator stimulating MCs and, using chimeric animals with platelets defective in PAF generation or MCs lacking PAF receptor, defined the role of this platelet-MC interaction for vascular leakage, shock, and tissue inflammation. In application of these findings, we demonstrated that inhibition of platelet activation in modeled cardiac surgery blunted MC-dependent inflammation and tissue injury. Together, our work identifies a previously undefined mechanism of inflammatory augmentation, in which platelets trigger local and systemic responses through activation of perivascular MCs.

The reversed passive Arthus reaction as a model for investigating the mechanisms of inflammation-associated hemostasis

In recent years, accumulating evidence has indicated that platelets continuously repair vascular damage at sites of inflammation and/or infection. Studies in mouse models of inflammation have highlighted the fact that the mechanisms underlying bleeding prevention by platelets in inflamed organs can substantially differ from those supporting primary hemostasis following tail tip transection or thrombus formation in models of thrombosis. As a consequence, exploration of the hemostatic function of platelets in inflammation, as well as assessment of the risk of inflammation-induced bleeding associated with a platelet deficit and/or the use of anti-thrombotic drugs, require the use of dedicated experimental models. In the present review, we present the pros and cons of the cutaneous reversed passive Arthus reaction, a model of inflammation which has been instrumental in studying how inflammation causes vascular injury and how platelets continuously intervene to repair it. The limitations and common issues encountered when working with mouse models of inflammation for investigating platelet functions in inflammation are also discussed.

Platelets Are at the Nexus of Vascular Diseases

Platelets are important actors of cardiovascular diseases (CVD). Current antiplatelet drugs that inhibit platelet aggregation have been shown to be effective in CVD treatment. However, the management of bleeding complications is still an issue in vascular diseases. While platelets can act individually, they interact with vascular cells and leukocytes at sites of vascular injury and inflammation. The main goal remains to better understand platelet mechanisms in thrombo-inflammatory diseases and provide new lines of safe treatments. Beyond their role in hemostasis and thrombosis, recent studies have reported the role of several aspects of platelet functions in CVD progression. In this review, we will provide a comprehensive overview of platelet mechanisms involved in several vascular diseases.

Microfluidic Modeling of Thrombolysis

Objective- Despite the high clinical relevance of thrombolysis, models for its study in human flowing blood are lacking. Our objective was to develop a microfluidic model for comparative evaluation of thrombolytic therapeutic strategies. Approach and Results- Citrated human blood was supplemented with 3,3'-dihexyloxacarbocyanine iodide and Alexa Fluor 647 fibrinogen conjugate, recalcified, and perfused for 3 to 4 minutes at venous or arterial wall shear rate in microfluidic flow chambers coated with collagen and tissue factor to generate nonocclusive fluorescent thrombi. A second perfusion was performed for 10 minutes with rhodamine-6G-labeled citrated whole blood, supplemented or not with r-tPA (recombinant tissue-type plasminogen activator), fluorescein isothiocyanate-conjugated r-tPA, and Alexa Fluor 568 plasminogen conjugate. Plasminogen and r-tPA bound to preformed thrombi and r-tPA caused a concentration-dependent decrease in thrombus fibrin content (up to 50% reduction at 15 µg/mL r-tPA) as assessed by fluorescence microscopy. Fibrinolysis was confirmed by measurement of D-dimers in the output flow. Remarkably, despite ongoing fibrinolysis, new platelets continued to be recruited to the thrombus under lysis. Under the arterial condition, combining r-tPA with hirudin enhanced fibrinolysis but did not prevent the recruitment of new platelets, which was, however, prevented by antiplatelet agents (ticagrelor or the GPVI [glycoprotein VI]-blocking antigen-binding fragment 9O12). Conclusions- Our microfluidic thrombolysis model is suitable for studying thrombolysis and testing the efficacy of drugs used in combination with r-tPA. Real-time analysis of fibrin and platelets during r-tPA-mediated fibrinolysis at arterial or venous flow conditions showed that platelets continue to accumulate during fibrinolysis. Such platelet accumulation may impair r-tPA-mediated recanalization.

Critical role of neutrophil extracellular traps (NETs) in patients with Behcet's disease

OBJECTIVES

Behçet's disease (BD) is a chronic systemic vasculitis. Thrombosis is a frequent and life-threatening complication. The pathogenesis of BD is poorly understood and evidence supporting a role for primed neutrophils in BD-associated thrombotic risk is scant. To respond to inflammatory insults, neutrophils release web-like structures, known as neutrophil extracellular traps (NETs), which are prothrombotic. We evaluated the role of NETs and markers of NETs in BD.

METHODS

Blood samples were collected from patients with BD, according to the International Study Group Criteria for Behçet's disease, and healthy donors (HD). NET components, including cell-free DNA (CfDNA) and neutrophil enzymes myeloperoxidase (MPO), were assessed in serum or in purified neutrophils from patients with BD and HD.

RESULTS

Patients with active BD had elevated serum cfDNA levels and MPO-DNA complexes compared with patients with inactive BD and to HD. In addition, levels of cfDNA and MPO-DNA complexes were significantly higher in patients with BD with vascular involvement compared with those without vascular symptoms. Purified neutrophils from patients with BD exhibited spontaneous NETosis compared with HD. Thrombin generation in BD plasma was significantly increased and positively correlated with the levels of MPO-DNA complexes and cfDNA. Importantly, DNAse treatment significantly decreased thrombin generation in BD plasma but not in HD plasma. In addition, biopsy materials obtained from patients with BD showed NETs production in areas of vasculitic inflammation and thrombosis.

CONCLUSIONS

Our data show that NETs and markers of NETS levels are elevated in patients with BD and contribute to the procoagulant state. Targeting NETs may represent a potential therapeutic target for the reduction or prevention of BD-associated thrombotic risk.

Vascular endothelial cell expression of JAK2V617F is sufficient to promote a pro-thrombotic state due to increased P-selectin expression

Thrombosis is the main cause of morbidity and mortality in patients with JAK2^V617F myeloproliferative neoplasms. Recent studies have reported the presence of JAK2^V617F in endothelial cells of some patients with myeloproliferative neoplasms. We investigated the role of endothelial cells that express JAK2^V617F in thrombus formation using an in vitro model of human endothelial cells overexpressing JAK2^V617F and an in vivo model of mice with endothelial-specific JAK2^V617F expression. Interestingly, these mice displayed a higher propensity for thrombus. When deciphering the mechanisms by which JAK2^V617F-expressing endothelial cells promote thrombosis, we observed that they have a pro-adhesive phenotype associated with increased endothelial P-selectin exposure, secondary to degranulation of Weibel-Palade bodies. We demonstrated that P-selectin blockade was sufficient to reduce the increased propensity of thrombosis. Moreover, treatment with hydroxyurea also reduced thrombosis and decreased the pathological interaction between leukocytes and JAK2^V617F-expressing endothelial cells through direct reduction of endothelial P-selectin expression. Taken together, our data provide evidence that JAK2^V617F-expressing endothelial cells promote thrombosis through induction of endothelial P-selectin expression, which can be reversed by hydroxyurea. Our findings increase our understanding of thrombosis in patients with myeloproliferative neoplasms, at least those with JAK2^V617F-positive endothelial cells, and highlight a new role for hydroxyurea. This novel finding provides the proof of concept that an acquired genetic mutation can affect the pro-thrombotic nature of endothelial cells, suggesting that other mutations in endothelial cells could be causal in thrombotic disorders of unknown cause, which account for 50% of recurrent venous thromboses.

Glycoprotein VI in securing vascular integrity in inflamed vessels

Glycoprotein VI (GPVI), the main platelet receptor for collagen, has been shown to play a central role in various models of thrombosis, and to be a minor actor of hemostasis at sites of trauma. These observations have made of GPVI a novel target for antithrombotic therapy, as its inhibition would ideally combine efficacy with safety. Nevertheless, recent studies have indicated that GPVI could play an important role in preventing bleeding caused by neutrophils in the inflamed skin and lungs. Remarkably, there is evidence that the GPVI-dependent hemostatic function of platelets at the acute phase of inflammation in these organs does not involve aggregation. From a therapeutic perspective, the vasculoprotective action of GPVI in inflammation suggests that blocking of GPVI might bear some risks of bleeding at sites of neutrophil infiltration. In this review, we summarize recent findings on GPVI functions in inflammation and discuss their possible clinical implications and applications.

Uptake of Plasmin-PN-1 Complexes in Early Human Atheroma

Zymogens are delivered to the arterial wall by radial transmural convection. Plasminogen can be activated within the arterial wall to produce plasmin, which is involved in evolution of the atherosclerotic plaque. Vascular smooth muscle cells (vSMCs) protect the vessels from proteolytic injury due to atherosclerosis development by highly expressing endocytic LDL receptor-related protein-1 (LRP-1), and by producing anti-proteases, such as Protease Nexin-1 (PN-1). PN-1 is able to form covalent complexes with plasmin. We hypothesized that plasmin-PN-1 complexes could be internalized via LRP-1 by vSMCs during the early stages of human atheroma. LRP-1 is also responsible for the capture of aggregated LDL in human atheroma. Plasmin activity and immunohistochemical analyses of early human atheroma showed that the plasminergic system is activated within the arterial wall, where intimal foam cells, including vSMCs and platelets, are the major sites of PN-1 accumulation. Both PN-1 and LRP-1 are overexpressed in early atheroma at both messenger and protein levels. Cell biology studies demonstrated an increased expression of PN-1 and tissue plasminogen activator by vSMCs in response to LDL. Plasmin-PN-1 complexes are internalized via LRP-1 in vSMCs, whereas plasmin alone is not. Tissue PN-1 interacts with plasmin in early human atheroma via two complementary mechanisms: plasmin inhibition and tissue uptake of plasmin-PN-1 complexes via LRP-1 in vSMCs. Despite this potential protective effect, plasminogen activation by vSMCs remains abnormally elevated in the intima in early stages of human atheroma.

CalDAG-GEFI Deficiency Reduces Atherosclerotic Lesion Development in Mice

OBJECTIVE

Platelets are important for the development and progression of atherosclerotic lesions. However, relatively little is known about the contribution of platelet signaling to this pathological process. Our recent work identified 2 independent, yet synergistic, signaling pathways that lead to the activation of the small GTPase Rap1; one mediated by the guanine nucleotide exchange factor, CalDAG-GEFI (CDGI), the other by P2Y12, a platelet receptor for adenosine diphosphate and the target of antiplatelet drugs. In this study, we evaluated lesion formation in atherosclerosis-prone low-density lipoprotein receptor deficient (Ldlr(-/-)) mice lacking CDGI or P2Y12 in hematopoietic cells.

APPROACH AND RESULTS

Lethally irradiated Ldlr(-/-) mice were reconstituted with bone marrow from wild-type (WT), Caldaggef1(-/-) (cdgI(-/-)), p2y12(-/-), or cdgI(-/-)p2y12(-/-) (double knockout [DKO]) mice and fed a high-fat diet for 12 weeks. Ldlr(-/-) chimeras deficient for CDGI or P2Y12 developed significantly smaller atherosclerotic lesions in the aortic sinus and in aortas when compared with the Ldlr(-/-)/WT controls. We also observed a significant reduction in platelet-leukocyte aggregates in blood from hypercholesterolemic Ldlr(-/-)/cdgI(-/-) and Ldlr(-/-)/p2y12(-/-) chimeras. Consistently, fewer macrophages and neutrophils were detected in the aortic sinus of Ldlr(-/-)/cdgI(-/-) and Ldlr(-/-)/ p2y12(-/-) chimeras. Compared with controls, the plaque collagen content was significantly higher in Ldlr(-/-) chimeras lacking CDGI. Interestingly, no statistically significant additive effects were seen in Ldlr(-/-)/DKO chimeras when compared with chimeras lacking only CDGI.

CONCLUSIONS

Our findings suggest that CDGI is critical for atherosclerotic plaque development in hypercholesterolemic Ldlr(-/-) mice because of its contribution to platelet-leukocyte aggregate formation and leukocyte recruitment to the lesion area.

Tissue factor-positive tumor microvesicles activate platelets and enhance thrombosis in mice

ESSENTIALS: Cancer patients have a high rate of venous thrombosis (VT) but the underlying mechanisms are unknown. Tumor-derived, tissue factor-positive microvesicles in platelet activation in vitro and in vivo were studied. Tumor-derived, tissue factor-positive microvesicles enhanced VT in mice. Platelets may contribute to VT in some cancer patients, and this could be prevented with antiplatelet drugs.

BACKGROUND

Cancer patients have an approximately 4-fold increased risk of venous thromboembolism (VTE) compared with the general population, and cancer patients with VTE have reduced survival. Tumor cells constitutively release small membrane vesicles called microvesicles (MVs) that may contribute to thrombosis in cancer patients. Clinical studies have shown that levels of circulating tumor-derived, tissue factor-positive (TF(+) ) MVs in pancreatic cancer patients are associated with VTE. Objectives We tested the hypothesis that TF(+) tumor-derived MVs (TMVs) activate platelets in vitro and in mice.

MATERIALS AND METHODS

We selected two human pancreatic adenocarcinoma cell lines expressing high (BxPc-3) and low (L3.6pl) levels of TF as models to study the effect of TF(+) TMVs on platelets and thrombosis.

RESULTS AND CONCLUSIONS

We found that both types of TF(+) TMVs activated human platelets and induced aggregation in vitro in a TF and thrombin-dependent manner. Further, injection of BxPc-3 TF(+) TMVs triggered platelet activation in vivo and enhanced thrombosis in two mouse models of venous thrombosis in a TF-dependent manner. Importantly, BxPc-3 TF(+) TMV-enhanced thrombosis was reduced in Par4-deficient mice and in wild-type mice treated with clopidogrel, suggesting that platelet activation was required for enhanced thrombosis. These studies suggest that TF(+) TMV-induced platelet activation contributes to thrombosis in cancer patients.

Platelet Inhibitors Reduce Rupture in a Mouse Model of Established Abdominal Aortic Aneurysm

OBJECTIVE

Rupture of abdominal aortic aneurysms causes a high morbidity and mortality in the elderly population. Platelet-rich thrombi form on the surface of aneurysms and may contribute to disease progression. In this study, we used a pharmacological approach to examine a role of platelets in established aneurysms induced by angiotensin II infusion into hypercholesterolemic mice.

APPROACH AND RESULTS

Administration of the platelet inhibitors aspirin or clopidogrel bisulfate to established abdominal aortic aneurysms dramatically reduced rupture. These platelet inhibitors reduced abdominal aortic platelet and macrophage recruitment resulting in decreased active matrix metalloproteinase-2 and matrix metalloproteinase-9. Platelet inhibitors also resulted in reduced plasma concentrations of platelet factor 4, cytokines, and components of the plasminogen activation system in mice. To determine the validity of these findings in human subjects, a cohort of aneurysm patients were retrospectively analyzed using developed and validated algorithms in the electronic medical record database at Vanderbilt University. Similar to mice, administration of aspirin or P2Y12 inhibitors was associated with reduced death among patients with abdominal aortic aneurysm.

CONCLUSIONS

These results suggest that platelets contribute to abdominal aortic aneurysm progression and rupture.

Protease nexin-1 regulates retinal vascular development

We recently identified protease nexin-1 (PN-1) or serpinE2, as a possibly underestimated player in maintaining angiogenic balance. Here, we used the well-characterized postnatal vascular development of newborn mouse retina to further investigate the role and the mechanism of action of PN-1 in physiological angiogenesis. The development of retinal vasculature was analysed by endothelial cell staining with isolectin B4. PN-1-deficient (PN-1(-/-)) retina displayed increased vascularization in the postnatal period, with elevated capillary thickness and density, compared to their wild-type littermate (WT). Moreover, PN-1(-/-) retina presented more veins/arteries than WT retina. The kinetics of retinal vasculature development, retinal VEGF expression and overall retinal structure were similar in WT and PN-1(-/-) mice, but we observed a hyperproliferation of vascular cells in PN-1(-/-) retina. Expression of PN-1 was analysed by immunoblotting and X-Gal staining of retinas from mice expressing beta-galactosidase under a PN-1 promoter. PN-1 was highly expressed in the first week following birth and then progressively decreased to a low level in adult retina where it localized on the retinal arteries. PCR arrays performed on mouse retinal RNA identified two angiogenesis-related factors, midkine and Smad5, that were overexpressed in PN-1(-/-) newborn mice and this was confirmed by RT-PCR. Both the higher vascularization and the overexpression of midkine and Smad5 mRNA were also observed in gastrocnemius muscle of PN-1(-/-) mice, suggesting that PN-1 interferes with these pathways. Together, our results demonstrate that PN-1 strongly limits physiological angiogenesis and suggest that modulation of PN-1 expression could represent a new way to regulate angiogenesis.

RASA3 is a critical inhibitor of RAP1-dependent platelet activation

The small GTPase RAP1 is critical for platelet activation and thrombus formation. RAP1 activity in platelets is controlled by the GEF CalDAG-GEFI and an unknown regulator that operates downstream of the adenosine diphosphate (ADP) receptor, P2Y12, a target of antithrombotic therapy. Here, we provide evidence that the GAP, RASA3, inhibits platelet activation and provides a link between P2Y12 and activation of the RAP1 signaling pathway. In mice, reduced expression of RASA3 led to premature platelet activation and markedly reduced the life span of circulating platelets. The increased platelet turnover and the resulting thrombocytopenia were reversed by concomitant deletion of the gene encoding CalDAG-GEFI. Rasa3 mutant platelets were hyperresponsive to agonist stimulation, both in vitro and in vivo. Moreover, activation of Rasa3 mutant platelets occurred independently of ADP feedback signaling and was insensitive to inhibitors of P2Y12 or PI3 kinase. Together, our results indicate that RASA3 ensures that circulating platelets remain quiescent by restraining CalDAG-GEFI/RAP1 signaling and suggest that P2Y12 signaling is required to inhibit RASA3 and enable sustained RAP1-dependent platelet activation and thrombus formation at sites of vascular injury. These findings provide insight into the antithrombotic effect of P2Y12 inhibitors and may lead to improved diagnosis and treatment of platelet-related disorders.

CalDAG-GEFI deficiency protects mice from FcγRIIa-mediated thrombotic thrombocytopenia induced by CD40L and β2GPI immune complexes

INTRODUCTION

Platelet activation via the Fcγ receptor IIa (FcγRIIa) is implicated in the pathogenesis of immune complex (IC)-mediated thrombocytopenia and thrombosis (ITT). We previously showed that ICs composed of antigen and antibodies targeting CD40 ligand (CD40L) or β2 Glycoprotein I (β2GPI) induce ITT in mice transgenic for human FcγRIIa (hFcR) but not wild-type controls (which lack FcγRIIa). Here we evaluated the contribution of the guanine nucleotide exchange factor, CalDAG-GEFI, and P2Y12, key regulators of Rap1 signaling in platelets, to ITT induced by these clinically relevant ICs.

METHODS

Pre-formed anti-CD40L or anti-β2GPI ICs were injected into hFcR/Caldaggef1(+/+) or hFcR/Caldaggef1(-/-) mice, with or without clopidogrel pretreatment. Animals were observed for symptoms of shock for 30 min, during which time core body temperature was monitored. Platelet counts were obtained before and 30 min after IC injection. Lungs were assessed for thrombosis by histology or near-infrared imaging.

RESULTS

Both CD40L and β2GPI ICs rapidly induced severe thrombocytopenia, shock and a reduction in body temperature in hFcR/Caldaggef1(+/+) mice. hFcR/Caldaggef1(-/-) mice were protected from CD40L and β2GPI IC-induced thrombocytopenia and shock, whereas P2Y12 inhibition had only a modest effect on IC-induced ITT. Consistent with these findings, IC-induced integrin activation in vitro and the accumulation of activated platelets in the lungs of IC-challenged mice was strongly dependent on CalDAG-GEFI.

CONCLUSIONS

Our studies demonstrate that CalDAG-GEFI plays a critical role in platelet activation, thrombocytopenia and thrombosis induced by clinically relevant ICs in mice. Thus, CalDAG-GEFI may be a promising target for the intervention of IC-associated, FcγRIIa-mediated thrombotic conditions.

Abstract 394: Differential Effects of Platelet Inhibition on Angiotensin II-Induced Abdominal Aortic Aneurysm Rupture in Mice

Objective: Currently, there is no proven medical approach to attenuate the expansion or rupture of abdominal aortic aneurysms (AAAs). It is estimated that >90% of AAA patients are on anti-platelet regiments without documented benefit or clinical evaluation. Therefore, we examined the effects of pharmaceutical blockade of platelet activation in either early or late-stage angiotensin II (AngII)-induced AAA formation.

Methods and Results: Male Ldlr -/- mice (8 weeks) fed a saturated fat and cholesterol-enriched diet were infused with AngII (1,000 ng/kg/min) via mini-osmotic pumps for 28 days. In one experiment, groups of mice received either aspirin (30 mg/L via drinking water [ASA]) or diet supplemented with the P2Y 12 inhibitor clopidogrel (50 mg/kg/day [Plavix]) 1 week prior to and throughout infusion (n > 15 all groups). All treatments were associated with a significant increase in aortic rupture-induced death versus placebo groups (P < 0.05; Plavix [58% vs. 0%]; and ASA [64% vs. 10%]). In a second experiment, Ldlr -/- mice were similarly fed and infused for 28 days and then stratified, based on aortic lumen diameters, utilizing noninvasive ultrasonography into 4 different groups: ASA (30 mg/L via drinking water), ASA placebo control, Plavix (50 mg/kg/day via food), and Plavix placebo control. Mice were continuously infused with AngII for an additional 42 days. Ex vivo platelet reactivity to adenosine diphosphate and arachidonic acid was conducted throughout the experiment to confirm systemic drug delivery. All treatments were associated with a significant decrease in aortic rupture-induced death versus placebo groups (P < 0.05; [66% placebo vs. 8% ASA] and [58% placebo vs. 0% Plavix]. None of the treatments affected plasma cholesterol, lipoprotein distributions, or increases in systolic blood pressure in either study. In vivo imaging and histopathology demonstrated these anti-platelet agents reduced abdominal macrophage infiltration and matrix metalloproteinase activity in the intervention study.

Conclusion: This study demonstrates different roles of platelets in the initiation and progression of AAA and warrants further investigation.

Abstract 21: The Calcium-Binding Protein S100A1 Negatively Regulates Collagen-Dependent Platelet Activation and Thrombosis in Mice

Introduction: S100A1 is a member of the S100 family of calcium-binding proteins. S100A1 controls Ca2+ dynamics in cardiomyocytes and plays an important role in heart failure. S100A1 is also strongly expressed in mouse platelets, but its role in platelet biology has not been investigated.

Goal: To determine the role of S100A1 in platelet activation and thrombosis.

Methods and Results: Platelet activation in response to threshold levels of convulxin, a specific agonist for the collagen receptor GPVI, showed significantly increased activation of αIIbβ3 integrin and α-granule release in S100A1-deficient (SKO) platelets compared with wild-type (WT) platelets. Consistently, SKO platelets also showed a more robust aggregation response to convulxin and collagen. In contrast, SKO platelets responded normally to stimulation with PAR4 receptor-activating peptide or ADP. Adhesion of SKO platelets to collagen under flow conditions was not significantly different to that of WT platelets. However, we observed a ~3-fold increase in phosphatidylserine (PS)-positive SKO platelets bound to the collagen surface (p<0.001, n=9 mice/group), suggesting that S100A1 also regulates the procoagulant response in platelets. Consistent with this hypothesis, we observed increased coated platelet formation and more sustained calcium transients in SKO platelets compared to controls. The increased reactivity of SKO platelets to GPVI agonists is explained by a ~1.5-fold increase in GPVI receptors expressed on the surface of these cells (p<0.001, n=20 mice/group). A similar increase in GPVI expression was also found in bone marrow-derived megakaryocytes. When subjected to the FeCl3 carotid artery thrombosis model, the time to vessel occlusion was significantly shorter in SKO mice compared to WT controls (4.55 ± 0.27 min vs 7.00 ± 0.62 min, respectively; p<0.01; n=8 mice/group). Furthermore, by using a collagen-rich adventitial surface inserted intralumenally in the carotid artery, we observed that thrombus formation in SKO mice is significantly more stable compared to WT mice (p=0.01, n=7 WT, n=8 SKO)

Conclusions: We here identify S100A1 as a negative regulator of GPVI expression and collagen-dependent platelet activation and thrombosis in mice.

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.

Chemoproteomic discovery of AADACL1 as a regulator of human platelet activation

A comprehensive knowledge of the platelet proteome is necessary for understanding thrombosis and for envisioning antiplatelet therapies. To discover other biochemical pathways in human platelets, we screened platelets with a carbamate library designed to interrogate the serine hydrolase subproteome and used competitive activity-based protein profiling to map the targets of active carbamates. We identified an inhibitor that targets arylacetamide deacetylase-like 1 (AADACL1), a lipid deacetylase originally identified in invasive cancers. Using this compound, along with highly selective second-generation inhibitors of AADACL1, metabolomics, and RNA interference, we show that AADACL1 regulates platelet aggregation, thrombus growth, RAP1 and PKC activation, lipid metabolism, and fibrinogen binding to platelets and megakaryocytes. These data provide evidence that AADACL1 regulates platelet and megakaryocyte activation and highlight the value of this chemoproteomic strategy for target discovery in platelets.

Endothelial protease nexin-1 is a novel regulator of A disintegrin and metalloproteinase 17 maturation and endothelial protein C receptor shedding via furin inhibition

OBJECTIVE

Human protein C is a plasma serine protease that plays a key role in hemostasis, and activated protein C (aPC) is known to elicit protective responses in vascular endothelial cells. This cytoprotective activity requires the interaction of the protease with its cell membrane receptor, endothelial protein C receptor. However, the mechanisms regulating the beneficial cellular effects of aPC are not well known. We aimed to determine whether a serine protease inhibitor called protease nexin-1 (PN-1) or serpinE2, expressed by vascular cells, can modulate the effect of aPC on endothelial cells.

APPROACH AND RESULTS

We found that vascular barrier protective and antiapoptotic activities of aPC were reduced both in endothelial cells underexpressing PN-1 and in endothelial cells whose PN-1 function was blocked by a neutralizing antibody. Our in vitro data were further confirmed in vivo. Indeed, we found that vascular endothelial growth factor-mediated hyperpermeability in the skin of mice was markedly reduced by local intradermal injection of aPC in wild-type mice but not in PN-1-deficient mice. Furthermore, we demonstrated a previously unknown protective role of endothelial PN-1 on endothelial protein C receptor shedding. We provided evidence that PN-1 inhibits furin, a serine protease that activates a disintegrin and metalloproteinase 17 involved in the shedding of endothelial protein C receptor. We indeed evidenced a direct interaction between PN-1 and furin in endothelial cells.

CONCLUSIONS

Our results thus demonstrate an original role of PN-1 as a furin convertase inhibitor, providing new insights for understanding the regulation of endothelial protein C receptor-dependent aPC endothelial protective effects.

Abstract 486: Platelet ITAM Signaling is Critical for Maintenance of Vascular Integrity in Inflammation

Blood platelets, long recognized for their importance in primary hemostasis, have recently been identified as critical regulators of vascular integrity during inflammation and cancer. Interestingly, the contribution of platelets to the maintenance of vascular integrity in in[[Unable to Display Character: &#64258;]]ammation seems to be independent of their ability to form a clot. However, the underlying molecular mechanism remains unknown. Here we report that platelet immunoreceptor tyrosine activation motif (ITAM) signaling but not G protein-coupled receptor (GPCR) signaling is critical for the prevention of inflammation-induced hemorrhage. To generate mice with partial or complete defects in these signaling pathways, we developed a novel protocol for the adoptive transfer of genetically and/or chemically inhibited platelets into thrombocytopenic mice. Unexpectedly, platelets with impaired GPCR signaling, a crucial component of platelet plug formation and hemostasis, were indistinguishable from wild-type platelets in their ability to prevent hemorrhage at sites of inflammation. In contrast, inhibition of GPVI or genetic deletion of CLEC-2, the only ITAM receptors expressed on mouse platelets, significantly reduced the ability of platelets to prevent inflammation-induced hemorrhage. Moreover, transfusion of platelets without ITAM receptor function or platelets lacking the adapter protein SLP-76 into thrombocytopenic mice had no significant effect on vascular integrity during inflammation. These results indicate that the control of vascular integrity is a major function of immune-type receptors in platelets, highlighting a potential clinical complication of novel anti-thrombotic agents directed towards the ITAM signaling pathway.

Abstract 130: Platelet Rap1 Signaling, Mediated by CalDAG-GEFI and P2Y12, Contributes to Atherosclerotic Lesion Development in Mice

Background and Objective

The development and progression of atherosclerotic lesions is mediated by various blood cell types, including platelets and neutrophils. We recently identified CalDAG-GEFI (CDGI) as a critical component of integrin-mediated adhesion of platelets and neutrophils at sites of vascular injury. In platelets, signaling by CDGI mediates the early activation of Rap1, while receptor for ADP, P2Y12, the target of the clinically used drug Plavix, is required for sustained Rap1 activation and thrombus stability. In this study, we evaluated lesion formation in atherosclerosis-prone low-density lipoprotein receptor deficient (Ldlr-/-) mice lacking CDGI and/or P2Y12 in hematopoietic cells.

Methods and Results

Ldlr-/- mice were irradiated and reconstituted with bone-marrow from wild-type (WT), CalDAG-GEFI-/- (CdgI), P2y12-/-, or CdgI-/- P2y12-/- (DKO) mice. Reconstituted mice were fed a fat and cholesterol enriched diet (21% fat, 0.2% cholesterol) for 12 weeks, ad libitum. Atherosclerotic lesions in the aortic sinus of Ldlr-/-;CdgI-/- chimeras were 42% smaller than those in Ldlr-/-;WT controls (0.18 ± 0.02 mm2 vs 0.31 ± 0.05 mm2, respectively; n = 13 each group, p < 0.001). Lesions in Ldlr-/-;P2y12-/- chimeras were also significantly smaller compared to controls (0.22 ± 0.10 mm2; n = 13, p < 0.05). Lesions in Ldlr-/-;DKO were reduced by 48% compared to WT controls (0.16 ± 0.02 mm2; n = 13, p < 0.001 ), but they were not statistically different from Ldlr-/-;CdgI-/- chimeras. Platelet adhesion and activation on collagen under flow was markedly impaired in Ldlr-/-;CdgI-/- and Ldlr-/-;DKO blood, but only partially impaired in Ldlr-/-;P2y12-/- blood. Importantly, firm neutrophil adhesion to collagen-bound platelets was significantly reduced in Ldlr-/-;CdgI-/- and Ldlr-/-;DKO blood, but not in Ldlr-/-;P2y12-/- blood. Total cholesterol and triglyceride levels were similar among groups.

Conclusion

Our findings reveal a critical role for CDGI and P2Y12, and Rap1-dependent platelet activation, in promoting atherosclerotic lesion development in hypercholesterolemic mice. Further studies are required to determine if the observed protection in Ldlr-/-;CdgI-/- chimeras is due to impaired neutrophil function.

Abstract 283: Platelet Inhibition Increases Angiotensin II-induced Abdominal Aortic Aneurysm Incidence and Rupture in Mice

Objective

Platelets play a central role in both hemostasis and thrombosis. The coagulation protease thrombin activates platelets by cleavage of protease-activated receptors (PAR1 and PAR4 in humans, and PAR3 and PAR4 in mice). Circulating thrombin is increased in patients with abdominal aortic aneurysms (AAAs). We recently demonstrated that PAR4 deficiency in mice increased the incidence of aneurysm (P = 0.001) and rupture-induced death (P = 0.003) in an angiotensin II (AngII) infusion model of AAA. Furthermore, platelet depletion significantly increased rupture in this model (P = 0.048). The purpose of this study was to examine clinically used anti-platelet drugs in this mouse model of AAA.

Methods and Results

Male Ldlr -/- mice (8-12 weeks in age) were fed a fat and cholesterol-enriched diet (21% milk fat, 0.2% cholesterol). Groups of mice received either aspirin (30 mg/L via drinking water [ASA]), or diet supplemented with the direct thrombin inhibitor dabigatran etexilate (10 g/kg chow [DE]) or the P2Y 12 inhibitor clopidogrel (50 mg/kg/day [Plavix]) 1 week prior to and throughout AngII (1,000 ng/kg/min) infusion for 28 days. Drug bioavailability was confirmed with all treatments. Medial diameters in the suprarenal aortic region were increased significantly from baseline to day 28 in all groups infused with AngII, as measured by in vivo ultrasound. Medial diameters were not different in any of the treatment groups compared with placebo controls. However, DE (87% vs. 47%) and Plavix (82% vs. 40%) significantly increased the incidence of AAA versus placebo groups (P < 0.05). ASA also increased the incidence of AAA (93% vs. 70% P = NS). Importantly, all treatments had a significant increase in aortic rupture-induced death versus placebo groups (P < 0.05; DE [67% vs. 7%]; Plavix [41% vs. 0%]; and ASA [64% vs. 10%]). None of the treatments affected total plasma cholesterol, lipoprotein-cholesterol distributions, or AngII-induced increases in systolic blood pressure.

Conclusion

This study indicates that the presence of functional platelets reduces the formation and rupture of AAA in this mouse model. This suggests that inhibition of platelet function may be detrimental to patients with existing AAAs, a conclusion that will be addressed in future mouse studies.

Platelet ITAM signaling is critical for vascular integrity in inflammation

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

Abstract 53: Immunoreceptor Tyrosine Activation Motif Signaling in Platelets Is Critical for the Maintenance of Vascular Integrity During Inflammation

Introduction: Blood platelets, long recognized for their importance in primary hemostasis, have recently been identified as critical regulators of vascular integrity during inflammation and cancer. Interestingly, the contribution of platelets to the maintenance of vascular integrity in inflammation seems to be independent of their ability to form a clot.

Goal: To identify signaling molecules critical for the contribution of platelets to vascular integrity in inflammation.

Methods: Transgenic mice with platelet-specific expression of the interleukin (IL)4 receptor were rendered thrombocytopenic by infusion of antibodies to IL4R. Platelets from various knockout mice or inhibitor-treated wild-type (WT) platelets were transfused into thrombocytopenic (TP) mice and tested for their ability to support vascular integrity in a reverse passive Arthus reaction (rpA) model. Tissue hemoglobin (Hb) was used to quantify red blood cells in the inflamed tissue, a marker of disrupted vascular integrity.

Results: Impaired vascular integrity at the site of inflammation was markedly higher in TP mice compared to controls (Hb: 1448±176 vs. 100±8 μg/cm2). Transfusion of WT platelets into TP mice strongly reduced tissue Hb (150±46 μg/ cm2). Transfused platelets from mice lacking the thrombin receptor PAR4 and/or functional P2Y12 (ADP receptor) also prevented disruption of vascular integrity (Hb: 210±66, 136±16 μg and 360±120 μg/cm2), suggesting that signaling by G protein-coupled receptors is not important for this response. Furthermore, infusion of heparin into WT mice did not lead to increased tissue Hb. Immunoreceptor tyrosine activation motif (ITAM) signaling, however, was critical for platelet-dependent maintenance of vascular integrity, as tissue Hb was markedly increased in TP mice transfused with platelets lacking the signal trandsucing adapter protein SLP-76 (1212±160 μg/cm2) or platelets lacking functional ITAM signaling receptors GPVI (852±196 μg/cm2), CLEC-2 (1062±188 μg/cm2), or both (1498±128 μg/cm2). Data are mean±SEM, n=4-5 mice per group.

Conclusions: We have established a novel mouse model for the adoptive transfer of platelets into TP mice. Using this model we can show that the ITAM but not GPCR signaling in platelets is critical for the maintenance of vascular integrity during inflammation. Our studies highlight a potential clinical complication of future anti-platelet agents targeted towards ITAM signaling receptors such as GPVI.

The mouse dorsal skinfold chamber as a model for the study of thrombolysis by intravital microscopy

Although intravital microscopy models of thrombosis in mice have contributed to dissect the mechanisms of thrombus formation and stability, they have not been well adapted to study long-term evolution of occlusive thrombi. Here, we assessed the suitability of the dorsal skinfold chamber (DSC) for the study of thrombolysis and testing of thrombolytic agents by intravital microscopy. We show that induction of FeCl3-induced occlusive thrombosis is achievable in microvessels of DSCs, and that thrombi formed in DSCs can be visualised by intravital microscopy using brightfield transmitted light, or fluorescent staining of thrombus components such as fibrinogen, platelets, leukocytes, and von Willebrand factor. Direct application of control saline or recombinant tissue-plasminogen activator (rtPA) to FeCl3-produced thrombi in DSCs did not affect thrombus size or induce recanalisation. However, in the presence of hirudin, rtPA treatment caused a rapid dose-dependent lysis of occlusive thrombi, resulting in recanalisation within 1 hour after treatment. Skin haemorrhage originating from vessels located inside and outside the FeCl3-injured area was also observed in DSCs of rtPA-treated mice. We further show that rtPA-induced thrombolysis was enhanced in plasminogen activator inhibitor-1-deficient (PAI-1-/-) mice, and dropped considerably as the time between occlusion and treatment application increased. Together, our results show that by allowing visualization and measurement of thrombus lysis and potential bleeding complications of thrombolytic treatments, the DSC provides a model for studying endogenous fibrinolysis and for first-line screening of thrombolytic agents. Furthermore, using this system, we found that thrombin and clot aging impair the thrombolytic action of rtPA towards FeCl3-produced thrombi.

Platelet glycoprotein VI dimerization, an active process inducing receptor competence, is an indicator of platelet reactivity

OBJECTIVE

The immune receptor homologue glycoprotein VI (GPVI)/FcR receptor γ chain complex is primarily responsible for platelet activation by collagen. There is growing evidence that optimal binding of GPVI to collagen depends on the assembly of GPVI dimers. The valence of GPVI on resting platelets needs to be clearly established because platelet avidity for collagen would be greater if GPVI is constitutively expressed as a dimer than as a monomer.

METHODS AND RESULTS

Using a monoclonal antibody (9E18) that preferentially binds to GPVI dimers, we found that GPVI was maintained in a monomeric form on human resting platelets under the control of intraplatelet cAMP concentration. Activation by soluble agonists or von Willebrand factor induced a shift toward GPVI dimerization related to increased platelet adhesion to collagen. A correlation between platelet binding of 9E18 and P-selectin exposure was observed in patients experiencing coronary artery disease, and antagonists of the ADP receptor P2Y12 limited ADP-induced GPVI dimerization.

CONCLUSION

The rapid assembly of highly competent dimers of GPVI at sites of vascular lesion represents an important step in the sequence of events leading to platelet activation by collagen. GPVI dimers could represent a new marker to analyze platelet reactivity.

Rap1-Rac1 circuits potentiate platelet activation

OBJECTIVE

The goal of this study was to investigate the potential crosstalk between Rap1 and Rac1, 2 small GTPases central to platelet activation, particularly downstream of the collagen receptor GPVI.

METHODS AND RESULTS

We compared the activation response of platelets with impaired Rap signaling (double knock-out; deficient in both the guanine nucleotide exchange factor, CalDAG-GEFI, and the Gi-coupled receptor for ADP, P2Y12), to that of wild-type platelets treated with a small-molecule Rac inhibitor, EHT 1864 (wild-type /EHT). We found that Rac1 is sequentially activated downstream of Rap1 on stimulation via GPVI. In return, Rac1 provides important feedback for both CalDAG-GEFI- and P2Y12-dependent activation of Rap1. When analyzing platelet responses controlled by Rac1, we observed (1) impaired lamellipodia formation, clot retraction, and granule release in both double knock-out and EHT 1864-treated wild-type platelets; and (2) reduced calcium store release in EHT 1864-treated wild-type but not double knock-out platelets. Consistent with the latter finding, we identified 2 pools of Rac1, one activated immediately downstream of GPVI and 1 activated downstream of Rap1.

CONCLUSIONS

We demonstrate important crosstalk between Rap1 and Rac1 downstream of GPVI. Whereas Rap1 signaling directly controls sustained Rac1 activation, Rac1 affects CalDAG-GEFI- and P2Y12-dependent Rap1 activation via its role in calcium mobilization and granule/ADP release, respectively.

Relative contributions of stromal interaction molecule 1 and CalDAG-GEFI to calcium-dependent platelet activation and thrombosis

BACKGROUND

Stromal interaction molecule 1 (STIM1) was recently identified as a critical component of store-operated calcium entry (SOCE) in platelets. We previously reported the Ca(2+) -sensing guanine nucleotide exchange factor CalDAG-GEFI as a critical molecule in Ca(2+) signaling in platelets.

OBJECTIVE

To evaluate the contribution of STIM1/SOCE to Ca(2+) -dependent platelet activation and thrombosis, we here compared the activation responses of platelets lacking STIM1 and platelets lacking CalDAG-GEFI.

METHODS

The murine Stim1 gene was conditionally deleted in the megakaryocyte/platelet lineage. CalDAG-GEFI(-/-) and Stim1(fl/fl) PF4-Cre mice, along with littermate control mice, were used for in vitro and in vivo experiments under flow as well as static conditions.

RESULTS

Integrin α(IIb) β(3) -mediated aggregation was markedly impaired in CalDAG-GEFI-deficient but not STIM1-deficient platelets, under both static and flow conditions. In contrast, deficiency in either STIM1 or CalDAG-GEFI significantly impaired the ability of platelets to express phosphatidylserine on the cell surface. When subjected to a laser injury thrombosis model, mice lacking STIM1 in platelets were characterized by the formation of unstable platelet-rich thrombi and delayed and reduced fibrin generation in injured arterioles. In CalDAG-GEFI(-/-) mice, fibrin generation was also delayed and reduced, but platelet accumulation was almost abolished.

CONCLUSIONS

Our studies suggest that: (i) STIM1/SOCE is critical for the procoagulant activity but not the proadhesive function of platelets; and (ii) at the site of vascular injury, STIM1 and CalDAG-GEFI are critical for the first wave of thrombin generation mediated by procoagulant platelets.

Platelet protease nexin-1, a serpin that strongly influences fibrinolysis and thrombolysis

BACKGROUND

Protease nexin-1 (PN-1) is a serpin that inhibits plasminogen activators, plasmin, and thrombin. PN-1 is barely detectable in plasma, but we have shown recently that PN-1 is present within the α-granules of platelets.

METHODS AND RESULTS

In this study, the role of platelet PN-1 in fibrinolysis was investigated with the use of human platelets incubated with a blocking antibody and platelets from PN-1-deficient mice. We showed by using fibrin-agar zymography and fibrin matrix that platelet PN-1 inhibited both the generation of plasmin by fibrin-bound tissue plasminogen activator and the activity of fibrin-bound plasmin itself. Rotational thromboelastometry and laser scanning confocal microscopy were used to demonstrate that PN-1 blockade or deficiency resulted in increased clot lysis and in an acceleration of the lysis front. Protease nexin-1 is thus a major determinant of the lysis resistance of platelet-rich clots. Moreover, in an original murine model in which thrombolysis induced by tissue plasminogen activator can be measured directly in situ, we observed that vascular recanalization was significantly increased in PN-1-deficient mice. Surprisingly, general physical health, after tissue plasminogen activator-induced thrombolysis, was much better in PN-1-deficient than in wild-type mice.

CONCLUSIONS

Our results reveal that platelet PN-1 can be considered as a new important regulator of thrombolysis in vivo. Inhibition of PN-1 is thus predicted to promote endogenous and exogenous tissue plasminogen activator-mediated fibrinolysis and may enhance the therapeutic efficacy of thrombolytic agents.

Macrophages and platelets are the major source of protease nexin-1 in human atherosclerotic plaque

OBJECTIVE

Protease nexin-1 (PN-1), a serpin constitutively expressed by vascular smooth muscle cells and endothelial cells, inhibits thrombin, plasminogen activators, and plasmin and can thus be expected to play a role in vascular biology. The present study addressed the question of PN-1 expression in human atherothrombosis.

METHODS AND RESULTS

Immunohistochemistry and biochemical studies confirmed that PN-1 was expressed at a moderate level in the medial layer of normal human arteries and showed that PN-1 expression was increased in atherothrombotic lesions. In early noncomplicated plaques, PN-1 was associated with infiltrating mononuclear cells. A strong PN-1 signal was observed in advanced lesions, principally in intraplaque hemorrhage-related structures. Monocytes/macrophages and platelets were identified as the main sources of PN-1 within atherothrombotic material. Isolated human monocytes and platelets both expressed high levels of active PN-1, and monocyte PN-1 expression was upregulated, at both messenger and protein levels, in response to stimulation by lipopolysaccharides. In contrast, PN-1 expression was downregulated during their differentiation into macrophages which were shown to produce degraded forms of PN-1.

CONCLUSIONS

Platelets and monocytes/macrophages are a major source of PN-1 in human atherothrombotic plaques. PN-1 could thus represent a new actor in the evolution of atherosclerotic lesions.