Altered thrombus formation in von Willebrand factor-deficient mice expressing von Willebrand factor variants with defective binding to collagen or GPIIbIIIa

Neutrophil extracellular traps formation may be involved in the association of propranolol with the development of portal vein thrombosis

BACKGROUND & AIMS

Nonselective β blockers (NSBBs) facilitate the development of portal vein thrombosis (PVT) in liver cirrhosis. Considering the potential effect of NSBBs on neutrophils and neutrophil extracellular traps (NETs), we speculated that NSBBs might promote the development of PVT by stimulating neutrophils to release NETs.

MATERIALS AND METHODS

Serum NETs biomarkers were measured, use of NSBBs was recorded, and PVT was evaluated in cirrhotic patients. Carbon tetrachloride and ferric chloride (FeCl3) were used to induce liver fibrosis and PVT in mice, respectively. After treatment with propranolol and DNase I, neutrophils in peripheral blood, colocalization and expression of NETs in PVT specimens, and NETs biomarkers in serum were measured. Ex vivo clots lysis analysis was performed and portal vein velocity and coagulation parameters were tested.

RESULTS

Serum MPO-DNA level was significantly higher in cirrhotic patients treated with NSBBs, and serum H3Cit and MPO-DNA levels were significantly higher in those with PVT. In fibrotic mice, following treatment with propranolol, DNase I significantly shortened the time of FeCl3-induced PVT formation, lowered the peripheral blood neutrophils labelled by CD11b/Ly6G, inhibited the positive staining of H3Cit and the expression of H3Cit and MPO proteins in PVT tissues, and reduced serum nucleosome level. Furthermore, the addition of DNase I to tissue plasminogen activator (tPA) significantly accelerated clots lysis as compared with tPA alone. Propranolol reduced portal vein velocity in fibrotic mice, but did not influence coagulation parameters.

CONCLUSION

Our study provides a clue to the potential impact of NETs formation on the association of NSBBs with the development of PVT.

Garcinol acts as a novel integrin αIIbβ3 inhibitor in human platelets

AIMS

Platelet activation plays a central role in arterial thrombosis. Platelets are activated by adhesive proteins (i.e., collagen) or soluble agonists (i.e., thrombin), the respective receptor-specific signaling cause inside-out signaling, leading to the binding of fibrinogen to integrin α_IIbβ₃. This binding triggers outside-in signaling, resulting in platelet aggregation. Garcinol, a polyisoprenylated benzophenone, is extracted from the fruit rind of Garcinia indica. Although garcinol exhibits considerable bioactivities, few studies have investigated the effect of garcinol on platelet activation.

MAIN METHODS

Aggregometry, immunoblotting, flow cytometer, confocal microscopic analysis, fibrin clot retraction, animal studies such as fluorescein-induced platelet plug formation in mesenteric microvessels, acute pulmonary thromboembolism, and tail bleeding time were performed in this study.

KEY FINDINGS

This study indicates that garcinol inhibited platelet aggregation stimulated by collagen, thrombin, arachidonic acid, and U46619. Garcinol reduced integrin α_IIbβ₃ inside-out signaling, including ATP release; cytosolic Ca²⁺ mobilization; P-selectin expression; and Syk, PLCγ2/PKC, PI3K/Akt/GSK3β, MAPKs, and NF-κB activation stimulated by collagen. Garcinol directly inhibited integrin α_IIbβ₃ activation by interfering with FITC-PAC-1 and FITC-triflavin by collagen. Additionally, garcinol affected integrin α_IIbβ₃-mediated outside-in signaling, such as decreasing platelet adhesion and the single-platelet spreading area; suppressing integrin β₃, Src, FAK, and Syk phosphorylation on immobilized fibrinogen; and inhibiting thrombin-stimulated fibrin clot retraction. Garcinol substantially reduced mortality caused by pulmonary thromboembolism and prolonged the occlusion time of thrombotic platelet plug formation without extending bleeding time in mice.

SIGNIFICANCE

This study identified that garcinol, a novel antithrombotic agent, acts as a naturally occurring integrin α_IIbβ₃ inhibitor.

Characterization of Ferric Chloride-Induced Arterial Thrombosis Model of Mice and the Role of Red Blood Cells in Thrombosis Acceleration

PURPOSE

The ferric chloride (FeCl₃)-induced thrombosis model is widely used for thrombosis research. However, it lacks standardization with uncertainty in the exact mechanism of thrombosis. This study aimed to characterize thrombus formation in a mouse model.

MATERIALS AND METHODS

We investigated thrombus formation and stability using various FeCl₃ concentrations (10%, 20%, 30%, 40%, and 50%, w/v) in carotid arteries of the Institute of Cancer Research (ICR) and C57BL/6N mice using the FeCl₃-induced thrombosis model. We also investigated thrombus histopathology using immunohistochemistry and electron microscopy.

RESULTS

Higher FeCl₃ concentrations induced dose-dependent, faster, larger, and more stable thrombus formation in both strains of mice. However, the ICR mice showed better dose-responses in thrombus formation and stability compared to the C57BL/6N mice. Thrombi were fibrin- and platelet-rich without significant changes across FeCl₃ concentrations. However, the content of red blood cells (RBCs) increased with increasing FeCl₃ concentrations (p for trend <0.001) and inversely correlated with time to occlusion (r=-0.65, p<0.001). While platelets and fibrin were evenly distributed over the thrombus, RBCs were predominantly located near the FeCl₃ treatment area. Transmission electron microscopy showed that RBCs attached to and were surrounded by aggregates of degranulated platelets, suggesting their potential role in platelet activation.

CONCLUSION

Faster and larger thrombus formation is induced in a dose-dependent manner by a wide range of FeCl₃ concentrations, but the stable thrombus formation requires higher FeCl₃ concentrations. Mouse strain affects thrombus formation and stability. RBCs and their interaction with platelets play a key role in the acceleration of FeCl₃-induced thrombosis.

von Willebrand disease: what does the future hold?

von Willebrand disease (VWD) is characterized by its heterogeneous clinical manifestation, which complicates its diagnosis and management. The clinical management of VWD has remained essentially unchanged over the last 30 years or so, using von Willebrand factor (VWF) concentrates, desmopressin, and anti-fibrinolytic agents as main tools to control bleeding. This is in contrast to hemophilia A, for which a continuous innovative path has led to novel treatment modalities. Despite current VWD management being considered effective, quality-of-life studies consistently reveal a higher than anticipated burden of VWD on patients, which is particularly true for women. Apparently, despite our perceived notion of current therapeutic efficiency, there is space for innovation with the goal of reaching superior efficacy. Developing innovative treatments for VWD is complex, especially given the heterogeneity of the disease and the multifunctional nature of VWF. In this perspective article, we describe several potential strategies that could provide the basis for future VWD treatments. These include genetic approaches, such as gene therapy using dual-vector adenoassociated virus and transcriptional silencing of mutant alleles. Furthermore, protein-based approaches to increase factor FVIII levels in VWD-type 3 or 2N patients are discussed. Finally, antibody-based options to interfere with VWF degradation (for congenital VWD-type 2A or acquired von Willebrand syndrome-type 2A) or increase endogenous VWF levels (for VWD-type 1) are presented. By highlighting these potential strategies, we hope to initiate an innovative path, which ultimately would allow us to better serve VWD patients and their specific needs.

In vivo modulation of a dominant-negative variant in mouse models of von Willebrand disease type 2A

Essentials Treatment options for von Willebrand disease (VWD) patients are limited. The p.P1127_C1948delinsR deletion/variant is a useful model to study VWD in vitro and in vivo. Counteracting dominant-negative effects restores von Willebrand factor multimerization in mice. This is the first siRNA-based treatment applied to a mouse model of VWD-type 2A. ABSTRACT: Background Treatment options for patients suffering from von Willebrand disease (VWD) are limited. Von Willebrand factor (VWF) is a polymeric protein that undergoes regulated dimerization and subsequent multimerization during its biosynthesis. Numerous heterozygous variants within the VWF gene display a dominant-negative effect and result in severe VWD. Previous studies have suggested that preventing the assembly of wild-type and mutant heteropolymers using siRNAs may have beneficial effects on VWF phenotypes in vitro. Objectives To study heterozygous dominant-negative variants in vivo, we developed a mouse model of VWD-type 2A and tested two independent strategies to modulate its detrimental effect. Methods The p.P1127_C1948delinsR deletion/variant, causing defective VWF multimerization, was expressed in mice as a model of VWD-type 2A variant. Two corrective strategies were applied. For the first time in a mouse model of VWD, we applied siRNAs selectively inhibiting translation of the mutant transcripts and we combined the VWD-type 2A deletion with the Cys to Arg substitution at position 2773, which is known to prevent dimerization. Results The RNA silencing approach induced a modest but consistent improvement of the VWF multimer profile. However, due to incomplete efficiency, the dominant-negative effect of the original variant could not be completely prevented. In contrast, the DNA approach resulted in increased antigen levels and restoration of a normal multimer profile. Conclusions Our data showed that preventing the detrimental impact of dominant-negative VWF variants by independent molecular mechanisms has beneficial consequences in vivo, in mouse models of dominant VWD.

Time-dependent interactions of blood platelets and cancer cells, accompanied by extramedullary hematopoiesis, lead to increased platelet activation and reactivity in a mouse orthotopic model of breast cancer - implications for pulmonary and liver metastasis

Aging has become a significant risk factor for several diseases, including breast cancer.

Platelet activation and platelet-cancer cell aggregate fractions were found to increase with tumor progression in a mouse model of breast cancer. At advanced stages of tumor development, platelets from mice with breast cancer were hyperreactive to low agonist concentrations and hyporeactive to high ones. Platelet activation and reactivity were strongly associated with breast cancer metastasis in the lungs and extramedullary hematopoiesis in the liver. A greater fraction of platelet aggregates was observed in 4T1-injected mice at the advanced stages of breast cancer. In vitro, platelet activation was elevated after incubation with 4T1 cells, and thrombin-stimulated platelets formed aggregates with 4T1 cells. Neither GPIbα, nor GPIIb/IIIa blocking antibodies, were able to affect platelet-cancer cell aggregation in vitro.

The primed circulating platelets became more sensitive to subthreshold stimuli at advanced stages of tumor development, and the formation of platelet-cancer cell aggregates increased with cancer progression. Our findings demonstrate that the age-associated progression of breast cancer cells is connected with increased platelet functioning, and that it can be manifested by the increased number of metastases and extramedullary hematopoiesis in a time-dependent-manner.

Von Willebrand Disease: From In Vivo to In Vitro Disease Models

Von Willebrand factor (VWF) plays an essential role in primary hemostasis and is exclusively synthesized and stored in endothelial cells and megakaryocytes. Upon vascular injury, VWF is released into the circulation where this multimeric protein is required for platelet adhesion. Defects of VWF lead to the most common inherited bleeding disorder von Willebrand disease (VWD). Three different types of VWD exist, presenting with varying degrees of bleeding tendencies. The pathophysiology of VWD can be investigated by examining the synthesis, storage and secretion in VWF producing cells. These cells can either be primary VWF producing cells or transfected heterologous cell models. For many years transfected heterologous cells have been used successfully to elucidate many aspects of VWF synthesis. However, those cells do not fully reflect the characteristics of primary cells. Obtaining primary endothelial cells or megakaryocytes with a VWD phenotype, requires invasive procedures, such as vessel collection or a bone marrow biopsy. A more recent and promising development is the isolation of endothelial colony forming cells (ECFCs) from peripheral blood as a true-to-nature cell model. Alternatively, various animal models are available but limiting, therefore, new approaches are needed to study VWD and other bleeding disorders. A potential versatile source of endothelial cells and megakaryocytes could be induced pluripotent stem cells (iPSCs). This review gives an overview of models that are available to study VWD and VWF and will discuss novel approaches that can be considered to improve the understanding of the structural and functional mechanisms underlying this disease.

Physiological Roles of the von Willebrand Factor-Factor VIII Interaction

Von Willebrand factor (VWF) and coagulation factor VIII (FVIII) circulate as a complex in plasma and have a major role in the hemostatic system. VWF has a dual role in hemostasis. It promotes platelet adhesion by anchoring the platelets to the subendothelial matrix of damaged vessels and it protects FVIII from proteolytic degradation. Moreover, VWF is an acute phase protein that has multiple roles in vascular inflammation and is massively secreted from Weibel-Palade bodies upon endothelial cell activation. Activated FVIII on the other hand, together with coagulation factor IX forms the tenase complex, an essential feature of the propagation phase of coagulation on the surface of activated platelets. VWF deficiency, either quantitative or qualitative, results in von Willebrand disease (VWD), the most common bleeding disorder. The deficiency of FVIII is responsible for Hemophilia A, an X-linked bleeding disorder. Here, we provide an overview on the role of the VWF-FVIII interaction in vascular physiology.

How useful are ferric chloride models of arterial thrombosis?

The ferric chloride models of arterial thrombosis are useful tools with which to investigate the cellular and molecular mechanisms that contribute to arterial thrombosis. Recent insights have, however, revealed the complex and multifaceted mechanism by which ferric chloride induces thrombus formation. Here, we discuss the strengths and weaknesses of the ferric chloride models of arterial thrombosis. Particular focus is given to the phenotypes of different knockout mice in the ferric chloride models and how these compare to other models with independent modes of initiation. Further, we discuss the relevance of the ferric chloride models to the human pathology of atherothrombotic disease.

Characterization of hemostasis in mice lacking the novel thrombosis susceptibility gene Slc44a2

INTRODUCTION

Recent genome wide association studies (GWAS) identified a novel susceptibility locus for thrombosis, harbouring the SLC44A2 gene which encodes the Solute Carrier Family 44 Member 2 protein (SLC44A2). Thus far, SLC44A2 has not been studied in the context of thrombosis, and may be a unique contributor to thrombotic disease. Here we utilize mice lacking SLC44A2 (Slc44a2^-/-) to evaluate a possible role of SLC44A2 in hemostasis.

METHODS

Slc44a2^-/- mice were evaluated in key aspects of normal hemostasis including a challenge of vascular damage by applying laser induced injury to the cremaster muscle arteriole.

RESULTS

Slc44a2^-/- mice had comparable levels of thrombin generation and gene expression of coagulation related genes, as compared to littermate wild type controls. Lower levels of circulating plasma Von Willebrand factor (VWF) were measured in Slc44a2^-/- mice, while no difference in VWF multimerization or vascular localization was detected. Upon in vivo laser injury of the cremaster arterioles, we detected an impairment of clot formation for Slc44a2^-/- mice.

CONCLUSIONS

Although mice lacking SLC44A2 are normal for several hemostasis parameters, we do observe a reduction of plasma VWF levels and an altered response upon vascular damage, which suggests that SLC44A2 contributes to hemostasis upon injury. These findings are in line with the reported GWAS data and support further research on SLC44A2 in thrombosis.

Impact of PI3Kα (Phosphoinositide 3-Kinase Alpha) Inhibition on Hemostasis and Thrombosis

Objective- PI3Kα (phosphoinositide 3-kinase alpha) is a therapeutic target in oncology, but its role in platelets and thrombosis remains ill characterized. In this study, we have analyzed the role of PI3Kα in vitro, ex vivo, and in vivo in 2 models of arterial thrombosis. Approach and Results- Using mice selectively deficient in p110α in the megakaryocyte lineage and isoform-selective inhibitors, we confirm that PI3Kα is not mandatory but participates to thrombus growth over a collagen matrix at arterial shear rate. Our data uncover a role for PI3Kα in low-level activation of the GP (glycoprotein) VI-collagen receptor by contributing to ADP secretion and in turn full activation of PI3Kβ and Akt/PKB (protein kinase B). This effect was no longer observed at high level of GP VI agonist concentration. Our study also reveals that over a vWF (von Willebrand factor) matrix, PI3Kα regulates platelet stationary adhesion contacts under arterial flow through its involvement in the outside-in signaling of vWF-engaged αIIbβ3 integrin. In vivo, absence or inhibition of PI3Kα resulted in a modest but significant decrease in thrombus size after superficial injuries of mouse mesenteric arteries and an increased time to arterial occlusion after carotid lesion, without modification in the tail bleeding time. Considering the more discrete and nonredundant role of PI3Kα compared with PI3Kβ, selective PI3Kα inhibitors are unlikely to increase the bleeding risk at least in the absence of combination with antiplatelet drugs or thrombopenia. Conclusions- This study provides mechanistic insight into the role of PI3Kα in platelet activation and arterial thrombosis.

Targeting von Willebrand Factor in Ischaemic Stroke: Focus on Clinical Evidence

Despite great efforts in stroke research, disability and recurrence rates in ischaemic stroke remain unacceptably high. To address this issue, one potential target for novel therapeutics is the glycoprotein von Willebrand factor (vWF), which increases in thrombogenicity especially under high shear rates as it bridges between vascular sub-endothelial collagen and platelets. The rationale for vWF as a potential target in stroke comes from four bodies of evidence. (1) Animal models which recapitulate the pathogenesis of stroke and validate the concept of targeting vWF for stroke prevention and the use of the vWF cleavage enzyme ADAMTS13 in acute stroke treatment. (2) Extensive epidemiologic data establishing the prognostic role of vWF in the clinical setting showing that high vWF levels are associated with an increased risk of first stroke, stroke recurrence or stroke-associated mortality. As such, vWF levels may be a suitable marker for further risk stratification to potentially fine-tune current risk prediction models which are mainly based on clinical and imaging data. (3) Genetic studies showing an association between vWF levels and stroke risk on genomic levels. Finally, (4) studies of patients with primary disorders of excess or deficiency of function in the vWF axis (e.g. thrombotic thrombocytopenic purpura and von Willebrand disease, respectively) which demonstrate the crucial role of vWF in atherothrombosis. Therapeutic inhibition of VWF by novel agents appears particularly promising for secondary prevention of stroke recurrence in specific sub-groups of patients such as those suffering from large artery atherosclerosis, as designated according to the TOAST classification.

High and long-term von Willebrand factor expression after Sleeping Beauty transposon-mediated gene therapy in a mouse model of severe von Willebrand disease

Essentials von Willebrand disease (VWD) is the most common inherited bleeding disorder. Gene therapy for VWD offers long-term therapy for VWD patients. Transposons efficiently integrate the large von Willebrand factor (VWF) cDNA in mice. Liver-directed transposons support sustained VWF expression with suboptimal multimerization.

SUMMARY

Background Type 3 von Willebrand disease (VWD) is characterized by complete absence of von Willebrand factor (VWF). Current therapy is limited to treatment with exogenous VWF/FVIII products, which only provide a short-term solution. Gene therapy offers the potential for a long-term treatment for VWD. Objectives To develop an integrative Sleeping Beauty (SB) transposon-mediated VWF gene transfer approach in a preclinical mouse model of severe VWD. Methods We established a robust platform for sustained transgene murine VWF (mVWF) expression in the liver of Vwf-/- mice by combining a liver-specific promoter with a sandwich transposon design and the SB100X transposase via hydrodynamic gene delivery. Results The sandwich SB transposon was suitable to deliver the full-length mVWF cDNA (8.4 kb) and supported supra-physiological expression that remained stable for up to 1.5 years after gene transfer. The sandwich vector stayed episomal (~60 weeks) or integrated in the host genome, respectively, in the absence or presence of the transposase. Transgene integration was confirmed using carbon tetrachloride-induced liver regeneration. Analysis of integration sites by high-throughput analysis revealed random integration of the sandwich vector. Although the SB vector supported long-term expression of supra-physiological VWF levels, the bleeding phenotype was not corrected in all mice. Long-term expression of VWF by hepatocytes resulted in relatively reduced amounts of high-molecular-weight multimers, potentially limiting its hemostatic efficacy. Conclusions Although this integrative platform for VWF gene transfer is an important milestone of VWD gene therapy, cell type-specific targeting is yet to be achieved.

The physical spacing between the von Willebrand factor D'D3 and A1 domains regulates platelet adhesion in vitro and in vivo

Essentials The role of von Willebrand factor (VWF) domains in regulating platelet adhesion was studied in vivo. Multimeric VWF with spacers at the N- and C-terminus of VWF-A1 were systematically tested. N-terminal modified VWF avidly bound platelet GpIbα, causing VWD Type2B like phenotype in mice. Novel anti-D'D3 mAbs suggest that changes at the D'D3-A1 interface may be biologically relevant.

SUMMARY

Background Previous ex vivo studies using truncated VWF (von Willebrand factor) suggest that domain-level molecular architecture may control platelet-GpIbα binding function. Objective We determined if this is the case with multimeric VWF in vivo. Methods Full-length human VWF ('hV') was modified with a 22-amino acid mucinous stretch at either the N-terminus of VWF-A1 to create 'hNV' or C-terminus to yield 'hCV'. This extends the physical distance between VWF-A1 and the adjacent domains by ~6 nm. Similar mucin inserts were also introduced into a human-murine chimera ('h[mA1]V') where murine-A1 replaced human-A1 in hV. This yielded 'h[mA1]NV' and 'h[mA1]CV', with N- and C-terminal inserts. The constructs were tested ex vivo and in vivo. Results Mucin insertion at the N-terminus, but not C-terminus, in both types of constructs resulted in >50-fold increase in binding to immobilized GpIbα. N-terminal insertion also resulted in greater shear-induced platelet activation, more thrombus formation on collagen, enhanced platelet accumulation and slower platelet translocation on immobilized VWF in microfluidics assays. Hydrodynamic injection-based expression of h[mA1]NV, but not h[mA1]V or h[mA1]CV, in VWF-/- mice caused profound thrombocytopenia, reduced plasma VWF concentrations, lower multimer distribution, and incessant tail bleeding that is reminiscent of von Willebrand disease type 2B. Platelet plugs were noted in the portal veins and hepatic arteries. An anti-D'D3 mAb DD3.3 that displays enhanced binding to VWF containing the N-terminal mucin insert also exhibited increased binding to wild-type VWF under shear and upon ristocetin addition. Conclusion Conformation changes at the VWF D'D3-A1 interface may be a key regulator of thrombosis in vivo. Structural features at the A1-A2 interface are likely of less significance.

Macrophage scavenger receptor SR-AI contributes to the clearance of von Willebrand factor

Previously, we found that LDL-receptor related protein-1 on macrophages mediated shear stress-dependent clearance of von Willebrand factor. In control experiments, however, we observed that von Willebrand factor also binds to macrophages independently of this receptor under static conditions, suggesting the existence of additional clearance-receptors. In search for such receptors, we focused on the macrophage-specific scavenger-receptor SR-AI. von Willebrand factor displays efficient binding to SR-AI (half-maximum binding 14±5 nM). Binding is calcium-dependent and is inhibited by 72±4% in the combined presence of antibodies against the A1- and D4-domains. Association with SR-AI was confirmed in cell-binding experiments. In addition, binding to bone marrow-derived murine SR-AI-deficient macrophages was strongly reduced compared to binding to wild-type murine macrophages. Following expression via hydrodynamic gene transfer, we determined ratios for von Willebrand factor-propeptide over von Willebrand factor-antigen, a marker of von Willebrand factor clearance. Propeptide/antigen ratios were significantly reduced in SR-AI-deficient mice compared to wild-type mice (0.6±0.2 versus 1.3±0.3; P<0.0001), compatible with a slower clearance of von Willebrand factor in SR-AI-deficient mice. Interestingly, mutants associated with increased clearance (von Willebrand factor/p.R1205H and von Willebrand factor/p.S2179F) had significantly increased binding to purified SR-AI and SR-AI expressed on macrophages. Accordingly, propeptide/antigen ratios for these mutants were reduced in SR-AI-deficient mice. In conclusion, we have identified SR-AI as a novel macrophage-specific receptor for von Willebrand factor. Enhanced binding of von Willebrand factor mutants to SR-AI may contribute to the increased clearance of these mutants.

TRAF3 negatively regulates platelet activation and thrombosis

CD40 ligand (CD40L), a member of the tumor necrosis factor (TNF) superfamily, binds to CD40, leading to many effects depending on target cell type. Platelets express CD40L and are a major source of soluble CD40L. CD40L has been shown to potentiate platelet activation and thrombus formation, involving both CD40-dependent and -independent mechanisms. A family of proteins called TNF receptor associated factors (TRAFs) plays key roles in mediating CD40L-CD40 signaling. Platelets express several TRAFs. It has been shown that TRAF2 plays a role in CD40L-mediated platelet activation. Here we show that platelet also express TRAF3, which plays a negative role in regulating platelet activation. Thrombin- or collagen-induced platelet aggregation and secretion are increased in TRAF3 knockout mice. The expression levels of collagen receptor GPVI and integrin αIIbβ3 in platelets were not affected by deletion of TRAF3, suggesting that increased platelet activation in the TRAF3 knockout mice was not due to increased expression platelet receptors. Time to formation of thrombi in a FeCl₃-induced thrombosis model was significantly shortened in the TRAF3 knockout mice. However, mouse tail-bleeding times were not affected by deletion of TRAF3. Thus, TRAF3 plays a negative role in platelet activation and in thrombus formation in vivo.

Platelet receptors as therapeutic targets: Past, present and future

Anti-platelet drugs reduce arterial thrombosis after plaque rupture and erosion, prevent stent thrombosis and are used to prevent and treat myocardial infarction and ischaemic stroke. Some of them may also be helpful in treating less frequent diseases such as thrombotic thrombocytopenic purpura. The present concise review aims to cover current and future developments of anti-platelet drugs interfering with the interaction of von Willebrand factor (VWF) with glycoprotein (GP) Ibα, and directed against GPVI, GPIIb/IIIa (integrin αIIbβ3), the thrombin receptor PAR-1, and the ADP receptor P2Y12. The high expectations of having novel antiplatelet drugs which selectively inhibit arterial thrombosis without interfering with normal haemostasis could possibly be met in the near future.

Abnormal von Willebrand factor secretion, factor VIII stabilization and thrombus dynamics in type 2N von Willebrand disease mice

Essentials Type 2N von Willebrand disease involves impaired von Willebrand factor to factor VIII binding. Type 2N von Willebrand disease mutations exhibit qualitative and mild quantitative deficiencies. Type 2N von Willebrand disease mice exhibit unstable venous hemostatic thrombi. The factor VIII-binding ability of von Willebrand factor regulates arteriole thrombosis dynamics.

SUMMARY

Background von Willebrand factor (VWF) and factor VIII (FVIII) circulate as a non-covalent complex, with VWF serving as the carrier for FVIII. VWF indirectly influences secondary hemostasis by stabilizing FVIII and transporting it to the site of primary hemostasis. Type 2N von Willebrand disease involves impaired binding of VWF to FVIII, resulting in decreased plasma levels of FVIII. Objectives In these studies, we characterize the impact of three type 2N VWD variants (R763A, R854Q, R816W) on VWF secretion, FVIII stabilization and thrombus formation in a murine model. Methods Type 2N VWD mice were generated by hydrodynamic injections of mutant murine VWF cDNAs and the influence of these variants on VWF secretion and FVIII binding was evaluated. In vivo hemostasis and the dynamics of thrombus formation and embolization were assessed using a murine tail vein transection hemostasis model and an intravital thrombosis model in the cremaster arterioles. Results Type 2N VWD variants were associated with decreased VWF secretion using cell and animal-based models. FVIII-binding to type 2N variants was impaired in vitro and was variably stabilized in vivo by expressed or infused 2N variant VWF protein. Both transgenic type 2N VWD and FVIII knockout (KO) mice demonstrated impaired thrombus formation associated with decreased thrombus stability. Conclusions The type 2N VWD phenotype can be recapitulated in a murine model and is associated with both quantitative and qualitative VWF deficiencies and impaired thrombus formation. Patients with type 2N VWD may have normal primary hemostasis formation but decreased thrombus stability related to ineffective secondary hemostasis.

Feasibility of the functional expression of the human organic anion transporting polypeptide 1B1 (OATP1B1) and its genetic variant 521T/C in the mouse liver

The objective of this study was to examine the feasibility of functional expression of the human organic anion transporting polypeptide 1B1 (hOATP1B1) forms in the liver of the mouse. After the mouse received the gene of interest (i.e., luciferase as the reporter or hOATP1B1) via hydrodynamic gene delivery (HGD) method, the expression was found to be liver-specific while alterations in the serum biochemistry and hepatocyte histology were apparently transient and reversible. The reporter activity was also detected in the plasma, but not in the blood cell in mice that received HGD, suggesting that the protein is probably released due to transiently increased permeability in hepatocytes by HGD. Using this delivery condition, the expression of hOATP1B1 was readily detected in the liver, but not in other tissues, of the mice receiving HGD for the transporter gene. Compared with the sham control mice, the uptake of pravastatin into the liver increased significantly in mice receiving hOATP1B1 wild type; the uptake parameters decreased consistently in mice expressing the 521T>C variant compared with that of the wild type control. These observations suggest that the functional expression of human transporter gene in mice is feasible, further suggesting that this treatment is practically useful in the pharmacokinetic studies for hOATP1B1 substrates.

Hyperfibrinolysis increases blood–brain barrier permeability by a plasmin- and bradykinin-dependent mechanism

Hydrodynamic transfection of plasmids encoding for plasminogen activators leads to a hyperfibrinolytic state in mice. Hyperfibrinolysis increases BBB permeability via a plasmin- and bradykinin-dependent mechanism.

A genetically-engineered von Willebrand disease type 2B mouse model displays defects in hemostasis and inflammation

von Willebrand disease (VWD)-type 2B is characterized by gain-of-function mutations in the von Willebrand factor (VWF) A1-domain, leading to increased affinity for its platelet-receptor, glycoprotein Ibα. We engineered the first knock-in (KI) murine model for VWD-type 2B by introducing the p.V1316M mutation in murine VWF. Homozygous KI-mice replicated human VWD-type 2B with macrothrombocytopenia (platelet counts reduced by 55%, platelet volume increased by 44%), circulating platelet-aggregates and a severe bleeding tendency. Also, vessel occlusion was deficient in the FeCl3-induced thrombosis model. Platelet aggregation induced by thrombin or collagen was defective for KI-mice at all doses. KI-mice manifested a loss of high molecular weight multimers and increased multimer degradation. In a model of VWF-string formation, the number of platelets/string and string-lifetime were surprisingly enhanced in KI-mice, suggesting that proteolysis of VWF/p.V1316M is differentially regulated in the circulation versus the endothelial surface. Furthermore, we observed increased leukocyte recruitment during an inflammatory response induced by the reverse passive Arthus reaction. This points to an active role of VWF/p.V1316M in the exfiltration of leukocytes under inflammatory conditions. In conclusion, our genetically-engineered VWD-type 2B mice represent an original model to study the consequences of spontaneous VWF-platelet interactions and the physiopathology of this human disease.

Platelet-derived VWF in the stroke spotlight

In this issue of Blood, Verhenne et al present data showing the role of platelet-derived von Willebrand factor (VWF) in mediating ischemic stroke injury using a murine model. They created mice with either endothelial VWF or platelet-derived VWF and examined each phenotype for bleeding and thrombosis. Their intriguing findings were that mice lacking platelet-derived VWF, but with adequate endothelial VWF stores, demonstrated normal hemostasis in a tail bleeding model and normal carotid artery thrombosis. Mice with only platelet-derived VWF had defective hemostasis and defective carotid artery thrombosis, but experienced significant cerebral infarction using a stroke model with middle cerebral artery occlusion (see figure). In contrast, minimal infarcts were seen for VWF-deficient mice. These data suggest that platelet-derived VWF plays a specific role in stroke pathology.

Shear stress-independent binding of von Willebrand factor-type 2B mutants p.R1306Q & p.V1316M to LRP1 explains their increased clearance

BACKGROUND

von Willebrand factor (VWF) is cleared in a shear stress- and macrophage-dependent manner by LRP1. von Willebrand disease (VWD)-type 2B mutants are endocytosed more efficiently than wild-type (wt)-VWF by macrophages.

OBJECTIVE

To investigate if VWD-type 2B mutations in the VWF A1-domain affect LRP1 binding and LRP1-dependent clearance.

METHODS

Recombinant Fc-tagged A1 domain (A1-Fc, A2-Fc, A3-Fc) and full-length VWF (wt or mutants thereof) were tested for binding to LRP1 or a recombinant fragment thereof in a static immunosorbent assay. Mutant and wt-VWF were also compared for clearance in mice lacking macrophage LRP1 (macLRP1(-) ) and control mice (macLRP1(+) ).

RESULTS

We found that A1-Fc but not A2-Fc or A3-Fc binds dose-dependently to LRP1. Binding of A1-Fc to LRP1 was markedly enhanced by the VWD-type 2B mutation p.V1316M. As expected, full-length wt-VWF was unable to bind LRP1 under static conditions unless ristocetin was added. In contrast, the presence of the p.V1316M or p.R1306Q mutation induced spontaneous binding to LRP1 without the need for ristocetin or shear stress. Both mutants were cleared more rapidly than wt-VWF in control macLRP1(+) mice. Surprisingly, deletion of macrophage LRP1 abrogated the increased clearance of the VWF/p.R1306Q and VWF/p.V1316M mutant.

CONCLUSION

The VWF A1-domain contains a binding site for LRP1. Certain VWD-type 2B mutations relieve the need for shear stress to induce LRP1 binding. Enhanced LRP1 binding coincides with a reduced survival of VWF/p.R1306Q and VWF/p.V1316M. Our data provide a rationale for reduced VWF levels in at least some VWD-type 2B patients.

Emerging understanding of roles for arterioles in inflammation

Arterioles, capillaries, and venules all actively change their cellular functions and phenotypes during inflammation in ways that are essential for maintenance of homeostasis and self-defense, and are also associated with many inflammatory disorders. ECs, together with pericytes and ECM proteins, can regulate blood flow, the coagulation cascade, fluid and solute exchange, and leukocyte trafficking. While capillary and venular functions in inflammation are well characterized, the arteriolar contribution to inflammation has only recently come into focus. Arterioles differ from venules in structure, EC morphology, shear environment, expression, and distribution of surface ligands; hence, regulation and function of arteriolar wall cells during inflammation may also be distinct from venules. Recent work indicates that in response to proinflammatory stimuli, arterioles alter barrier function, and support leukocyte and platelet interactions through upregulation of adhesion molecules. This suggests that in addition to their role in blood flow regulation, arterioles may also participate in inflammatory responses. In this review, we will discuss mechanisms that characterize arteriolar responses to proinflammatory stimuli. We will detail how distinct arteriolar features contribute to regulation of barrier function and leukocyte-EC interactions in inflammation, and further highlight the potential priming effects of arteriolar responses on venular function and progression of inflammatory responses.

Analysis of the role of von Willebrand factor, platelet glycoprotein VI-, and α2β1-mediated collagen binding in thrombus formation

Rare missense mutations in the von Willebrand factor (VWF) A3 domain that disrupt collagen binding have been found in patients with a mild bleeding phenotype. However, the analysis of these aberrant VWF-collagen interactions has been limited. Here, we have developed mouse models of collagen-binding mutants and analyzed the function of the A3 domain using comprehensive in vitro and in vivo approaches. Five loss-of-function (p.S1731T, p.W1745C, p.S1783A, p.H1786D, A3 deletion) and 1 gain-of-function (p.L1757A) variants were generated in the mouse VWF complementary DNA. The results of these various assays were consistent, although the magnitude of the effects were different: the gain-of-function (p.L1757A) variant showed consistent enhanced collagen binding whereas the loss-of-function mutants showed variable degrees of functional deficit. We further analyzed the impact of direct platelet-collagen binding by blocking glycoprotein VI (GPVI) and integrin α2β1 in our ferric chloride murine thrombosis model. The inhibition of GPVI demonstrated a comparable functional defect in thrombosis formation to the VWF(-/-) mice whereas α2β1 inhibition demonstrated a milder bleeding phenotype. Furthermore, a delayed and markedly reduced thrombogenic response was still evident in VWF(-/-), GPVI, and α2β1 blocked animals, suggesting that alternative primary hemostatic mechanisms can partially rescue the bleeding phenotype associated with these defects.

Systemic inhibition and liver-specific over-expression of PAI-1 failed to improve survival in all-inclusive populations or homogenous cohorts of CLP mice

BACKGROUND

The role of plasminogen activator inhibitor type-1 (PAI-1) in abdominal sepsis remains elusive.

OBJECTIVES

To study the influence of inhibition and over-expression of PAI-1 upon survival in cecal ligation and puncture (CLP) sepsis.

METHODS

(i) Mice underwent moderate CLP and received 10 mg kg(-1) of either monoclonal anti-PAI-1 (MA-MP6H6) or control (MA-Control) antibody intravenously at 0, 18 or 30 h post-CLP. The 30-h treatment group was additionally stratified into mice predicted to survive (P-SUR) or die (P-DIE) based on IL 6 measured at 24 h post-CLP. (ii) PAI-1 expression was induced with pLIVE.PAI-1 plasmid administered 72 h pre-CLP. Blood was sampled for 5 days and survival was monitored for 28 days.

RESULTS

MA-MP6H6 effectively neutralized active PAI-1 and fully restored fibrinolysis while PAI-1 over-expression was liver-specific and correlated with PAI-1 increase in the blood. Without stratification, MA-MP6H6 co-/post-treatment conferred no survival benefit. Prospective stratification (IL-6 cut-off: 14 ng mL(-1) ) suggested increased mortality by MA-MP6H6 treatment in P-SUR that reached 30% difference (vs. MA-Control; P < 0.05) after a retrospective cut-off readjustment to 3.3 ng mL(-1) for better P-SUR homogeneity. Subsequent prospective anti-PAI-1 treatment in P-SUR mice with 3.3 ng mL(-1) cut-off demonstrated a negative but statistically insignificant effect: mortality was higher by 17% after MA-MP6H6 vs. MA-Control. Over-expression of PAI 1 did not alter post-CLP survival. Neither PAI-1 inhibition nor over-expression meaningfully modified inflammatory response and/or organ function.

CONCLUSIONS

Restoration of fibrinolysis in early abdominal sepsis was not beneficial and it may prove detrimental in subjects with the lowest risk of death, while preemptive PAI-1 up-regulation at the current magnitude was not protective.

A von Willebrand factor fragment containing the D'D3 domains is sufficient to stabilize coagulation factor VIII in mice

Plasma factor VIII (FVIII) and von Willebrand factor (VWF) circulate together as a complex. We identify VWF fragments sufficient for FVIII stabilization in vivo and show that hepatic expression of the VWF D'D3 domains (S764-P1247), either as a monomer or a dimer, is sufficient to raise FVIII levels in Vwf(-/-) mice from a baseline of ∼5% to 10%, to ∼50% to 100%. These results demonstrate that a fragment containing only ∼20% of the VWF sequence is sufficient to support FVIII stability in vivo. Expression of the VWF D'D3 fragment fused at its C terminus to the Fc segment of immunoglobulin G1 results in markedly enhanced survival in the circulation (t1/2 > 7 days), concomitant with elevated plasma FVIII levels (>25% at 7 days) in Vwf(-/-) mice. Although the VWF D'D3-Fc chimera also exhibits markedly prolonged survival when transfused into FVIII-deficient mice, the cotransfused FVIII is rapidly cleared. Kinetic binding studies show that VWF propeptide processing of VWF D'D3 fragments is required for optimal FVIII affinity. The reduced affinity of VWF D'D3 and VWF D'D3-Fc for FVIII suggests that the shortened FVIII survival in FVIII-deficient mice transfused with FVIII and VWF D'D3/D'D3-Fc is due to ineffective competition of these fragments with endogenous VWF for FVIII binding.

Expression of a structurally constrained von Willebrand factor variant triggers acute thrombotic thrombocytopenic purpura in mice

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease that presents with thrombocytopenia, disseminated thrombosis, hemolytic anemia, and organ dysfunction. The etiology of TTP has revealed that patients share a deficiency in plasma protease a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13), the enzyme responsible for cleaving ultra-large von Willebrand factor (VWF) multimers into nonthrombogenic fragments. Therefore, existing TTP mouse models were developed by targeted disruption of the ADAMTS13 gene. ADAMTS13(-/-) mice are mostly asymptomatic in the absence of a trigger, as redundant proteases appear to take on VWF processing. As an alternative approach to creating one such model, we devised a strategy based on the expression of a cleavage-resistant VWF mutant in mice. The creation of a disulfide bond within the A2 domain of VWF was found to render VWF multimers resistant to proteolysis by plasma proteases under flow. Furthermore, mice expressing the murine VWF/p.S1494C-p.A1534C mutant present with symptoms characteristics of acute TTP such as thrombocytopenia, red cell shredding, accumulation of VWF-rich thrombi in the microvasculature, and advanced TTP symptoms such as renal dysfunction and splenomegaly. Because this model appears to faithfully emulate the pathophysiology of TTP, it should prove most useful in the study of microangiopathic diseases and their treatment.

Accelerated uptake of VWF/platelet complexes in macrophages contributes to VWD type 2B-associated thrombocytopenia

Von Willebrand disease (VWD) type 2B is characterized by mutations causing enhanced binding of von Willebrand factor (VWF) to platelets. Bleeding tendency is associated with heterogeneous clinical manifestations, including moderate to severe thrombocytopenia. The underlying mechanism of the thrombocytopenia has remained unclear. Here, a mouse model of VWD type 2B was used to investigate pathways contributing to thrombocytopenia. Immunohistochemical analysis of blood smears revealed that mutant VWF was exclusively detected on platelets of thrombocytopenic VWD type 2B mice, suggesting that thrombocytopenic VWD type 2B mice were elevated two- to threefold upon chemical macrophage depletion. Colocalization of platelets with CD68-positive Kupffer cells and CD168-positive marginal macrophages in liver and spleen, respectively, confirmed the involvement of macrophages in the removal of VWF/platelet complexes. Significantly more platelets were found in liver and spleen of VWD type 2B mice compared with control mice. Finally, platelet survival was significantly shorter in VWD type 2B mice compared with control mice, providing a rationale for lower platelet counts in VWD type 2B mice. In conclusion, our data indicate that VWF type 2B binds to platelets and that this is a signal for clearance by macrophages, which could contribute to the thrombocytopenia in patients with VWD type 2B.

Von Willebrand Factor Abnormalities Studied in the Mouse Model: What We Learned about VWF Functions

Up until recently, von Willebrand Factor (VWF) structure-function relationships have only been studied through in vitro approaches. A powerful technique known as hydrodynamic gene transfer, which allows transient expression of a transgene by mouse hepatocytes, has led to an important shift in VWF research. Indeed this approach has now enabled us to transiently express a number of VWF mutants in VWF-deficient mice in order to test the relative importance of specific residues in different aspects of VWF biology and functions in an in vivo setting. As a result, mice reproducing various types of von Willebrand disease have been generated, models that will be useful to test new therapies. This approach also allowed a more precise identification of the importance of VWF interaction with subendothelial collagens and with platelets receptors in hemostasis and thrombosis. The recent advances gathered from these studies as well as the pros and cons of the technique will be reviewed here.

On the versatility of von Willebrand factor

Von Willebrand factor (VWF) is a large multimeric protein, the function of which has been demonstrated to be pivotal to the haemostatic system. Indeed, quantitative and/or qualitative abnormalities of VWF are associated with the bleeding disorder Von Willebrand disease (VWD). Moreover, increased plasma concentrations of VWF have been linked to an increased risk for thrombotic complications. In the previous decades, many studies have contributed to our understanding of how VWF is connected to the haemostatic system, particularly with regard to structure-function relationships. Interactive sites for important ligands of VWF (such as factor VIII, collagen, glycoprotein Ibα, integrin αIIbβ3 and protease ADAMTS13) have been identified, and mutagenesis studies have confirmed the physiological relevance of the interactions between VWF and these ligands. However, we have also become aware that VWF has a more versatile character than previously thought, given its potential role in various non-hemostatic processes, like intimal thickening, tumor cell apoptosis and inflammatory processes. In the presence review, a summary of our knowledge on VWF structure-function relationships is provided in the context of the “classical” haemostatic task of VWF and in perspective of pathological processes beyond haemostasis.

Terminal platelet production is regulated by von Willebrand factor

It is established that proplatelets are formed from mature megakaryocytes (MK) as intermediates before platelet production. Recently, the presence of proplatelets was described in blood incubated in static conditions. We have previously demonstrated that platelet and proplatelet formation is upregulated by MK exposure to high shear rates (1800 s⁻¹) on immobilized von Willebrand factor (VWF). The purpose of the present study was to investigate whether VWF is involved in the regulation of terminal platelet production in blood. To this end, Vwf ^−/− mice, a model of severe von Willebrand disease, were used to create a situation in which blood cells circulate in a vascular tree that is completely devoid of VWF. Murine platelets were isolated from Vwf ^−/− and Vwf ^+/+ blood, exposed to VWF at 1800 s⁻¹ in a microfluidic platform, and examined by means of videomicroscopy, as well as fluorescence and activation studies. Proplatelets became visible within 5 minutes, representing 38% of all platelets after 12 minutes and 46% after 28 min. The proportion of proplatelets was 1.8-fold higher in blood from Vwf^−/− mice than from Vwf^+/+ mice, suggesting a role of VWF in vivo. Fragmentation of these proplatelets into smaller discoid platelets was also observed in real-time. Platelets remained fully activatable by thrombin. Compensation of plasmatic VWF following hydrodynamic gene transfer in Vwf^−/− mice reduced the percentage of proplatelets to wild-type levels. A thrombocytopenic mouse model was studied in the flow system, 7 days after a single 5-FU injection. Compared to untreated mouse blood, a 2-fold increase in the percentage of proplatelets was detected following exposure to 1800 s⁻¹ on VWF of samples from mice treated with 5-FU. In conclusion, VWF and shear stress together appear to upregulate proplatelet reorganization and platelet formation. This suggests a new function for VWF in vivo as regulator of bloodstream thrombopoiesis.

Liver sinusoidal endothelial cell lectin inhibits CTL-dependent virus clearance in mouse models of viral hepatitis

Liver sinusoidal endothelial cell lectin (LSECtin) was recently reported to suppress intrahepatic T cell immunity and to limit immune-mediated liver injury. However, its role in the outcome and pathogenesis of viral infection has not yet been elucidated. Using a mouse model infected with a hepatotropic adenovirus, we found that the absence of LSECtin led to a higher frequency of intrahepatic effector CTLs. These cells produced higher levels of antiviral cytokines and cytotoxic factors and exhibited an increased expression of the transcription factors T-bet and Runx3. This phenotype observed in the LSECtin-knockout cells mediated a more efficient virus-specific cytotoxicity compared with that of wild-type cells. As a consequence, LSECtin deficiency significantly accelerated liver adenovirus clearance. In contrast, LSECtin upregulation in the liver delayed viral clearance; this delayed clearance was accompanied by the downregulation of the antiviral activity of CTLs. We further constructed an immunocompetent mouse model of acute hepatitis B viral infection to demonstrate that LSECtin significantly delayed the clearance of hepatitis B virus from blood and infected hepatocytes by limiting the frequency of hepatitis B virus-specific IFN-γ-producing cells. Consistent with this function, LSECtin was upregulated in the liver of mouse models of viral hepatitis. Taken together, our results suggest that LSECtin may facilitate the reduction of liver inflammation at the cost of delaying virus clearance and that this effect might be hijacked by the virus as an escape mechanism.

GDF-15 prevents platelet integrin activation and thrombus formation

BACKGROUND

Integrin-mediated platelet function plays an important role in primary hemostasis. Growth-differentiation factor 15 (GDF-15) has been shown to inhibit β(2) -integrin activation in leukocytes.

METHODS

We investigated the effect of GDF-15 on platelet integrin activation in vitro and in different in vivo models of thrombus formation.

RESULTS

GDF-15(-/-) mice showed an accelerated thrombus formation and a reduced survival rate after collagen-induced pulmonary thromboembolism. In reconstitution experiments, recombinant GDF-15 decelerated thrombus formation and prolonged the bleeding time. In vitro experiments demonstrated that GDF-15 pretreated, agonist-stimulated platelets showed decreased binding to fibrinogen in flow chamber assays and reduced activation of β(1) - and β(3) -integrins in flow cytometry experiments. Pretreating human and mouse platelets with GDF-15 reduced platelet aggregation. Mechanistically, GDF-15 prevents agonist-induced Rap1- dependent α(II) (b) β(3) activation by activating PKA. Platelet P-selectin expression and dense granule secretion after stimulation were unaffected by GDF-15, indicating a specific effect of GDF-15 on integrin activation.

CONCLUSION

GDF-15 specifically inhibits platelet integrin activation. These findings may have profound clinical implications for the treatment of hemostatic conditions involving platelets.

Mutations in the A3 domain of von Willebrand factor inducing combined qualitative and quantitative defects in the protein

Two unrelated families were recruited in the French Reference Center for von Willebrand Disease with moderate bleeding symptoms associated with low von Willebrand factor (VWF) antigen levels, decreased collagen binding assay, and no or partial response to desmopressin. Genetic analysis showed the presence of heterozygous mutations in the A3 domain away from the collagen-binding surface: 1 never reported previously (p.L1696R) and another (p.P1824H) described in a Spanish family. The mutations were reproduced by site-directed mutagenesis and mutant VWF was expressed in different expression systems, COS-7 cells, baby hamster kidney cells, and in VWF-deficient mice through hydrodynamic injection. p.L1696R and p.P1824H were associated with very low expression levels both in vitro and in vivo, with intracellular retention for p.P1824H. Both homozygous mutants displayed decreased binding to collagen types I and III but also decreased binding to platelet glycoproteins Ib and IIbIIIa. Co-transfections with wild-type VWF partially corrected these defects, except that collagen binding remained abnormal. The in vivo thrombosis response was severely reduced for both heterozygous mutants. In conclusion, we report 2 VWF A3 domain mutations that induce a combined qualitative and quantitative defect.

A murine model to characterize the antithrombotic effect of molecules targeting human von Willebrand factor

von Willebrand factor (VWF) is a promising target for developing antithrombotic drugs. The absence of accessible animal models impedes the study of specific human VWF (huVWF) targeting molecules in thrombosis. huVWF is not functional in the mouse because of a lack of interaction between huVWF and murine glycoprotein Ib. Using site-directed mutagenesis, we have replaced single or multiple amino acids in huVWF with their murine counterparts to eliminate species incompatibility. Using hydrodynamic injection, we have expressed the different chimeric VWF constructs into VWF(-/-) mice. Only huVWF with a complete murine A1 domain insertion was able to correct bleeding in vivo and form occlusive thrombi in mesenteric vessels after FeCl(3) treatment. Using this model, we tested the antithrombotic effect of monoclonal antibodies against huVWF, blocking its interaction with collagens (mAbs 203 and 505) or with glycoprotein IIbIIIa (mAb 9). The 3 mAbs inhibited the thrombotic process in arterioles of VWF(-/-) mice expressing huVWFmuA1. Inhibiting VWF-interaction with collagens was more potent, emphasizing the potential of such a target as an antithrombotic tool. Our results validate our murine model as a simple in vivo tool to evaluate anti-huVWF agents.

In vivo analysis of the role of O-glycosylations of von Willebrand factor

The objective of this project was to study the function of O-glycosylations in von Willebrand factor (VWF) life cycle. In total, 14 different murine Vwf cDNAs mutated on one or several O-glycosylations sites were generated: 9 individual mutants, 2 doublets, 2 clusters and 1 mutant with all 9 murine glycosylation sites mutated (Del-O-Gly). We expressed each mutated cDNA in VWF deficient-mice by hydrodynamic injection. An immunosorbent assay with Peanut Agglutinin (PNA) was used to verify the O-glycosylation status. Wild-type (WT) VWF expressed by hepatocytes after hydrodynamic injection was able to bind PNA with slightly higher affinity than endothelial-derived VWF. In contrast, the Del-O-Gly VWF mutant did not bind PNA, demonstrating removal of O-linked glycans. All mutants displayed a normal multimeric pattern. Two mutants, Del-O-Gly and T1255A/T1256A, led to expression levels 50% lower than those induced by WT VWF and their half-life in vivo was significantly reduced. When testing the capacity of each mutant to correct the bleeding time of VWF-deficient mice, we found that S1486A, T1255A, T1256A and the doublet T1255A/T1256A were unable to do so. In conclusion we have shown that O-glycosylations are dispensable for normal VWF multimerization and biosynthesis. It also appears that some O-glycosylation sites, particularly the T1255 and T1256 residues, are involved in the maintenance of VWF plasma levels and are essential for normal haemostasis. As for the S1486 residue, it seems to be important for platelet binding as demonstrated in vitro using perfusion experiments.

Use of a mouse model to elucidate the phenotypic effects of the von Willebrand factor cleavage mutants, Y1605A/M1606A and R1597W

BACKGROUND

von Willebrand Factor (VWF) is tightly regulated by the metalloproteinase ADAMTS13, which cleaves VWF to reduce VWF multimer size and binding affinity for collagen and platelets.

OBJECTIVE

This study examines two VWF mutations, R1597W (enhanced cleavage) and Y1605A-M1606A (decreased cleavage), to determine their impact on VWF, in addition to ADAMTS13-mediated cleavage.

METHODS

In vitro mouse ADAMTS13 digestions were performed on recombinant proteins. VWF knockout mice received hydrodynamic injections of mouse Vwf cDNA, following which VWF antigen, multimer profile and VWF propeptide levels were determined. A ferric chloride injury model of thrombosis was also evaluated.

RESULTS

In vitro ADAMTS13 digestion of full-length mouse VWF required > 97-fold higher ADAMTS13 levels for Y1605A/M1606A, and 68% lower ADAMTS13 levels for R1597W compared with wild type. In vivo, R1597W had reduced VWF:Ag and both mutations exhibited increased VWF propeptide/VWF:Ag ratios. R1597W multimers show a lower molecular weight profile compared with wild type and Y1605A/M1606A mice. When co-injected with Adamts13 cDNA, Y1605A/M1606A multimers were larger compared with wild type, and R1597W showed only a single multimer band and decreased clearance via VWFpp/VWF:Ag ratio. R1597W was associated with reduced thrombus formation but normal platelet accumulation in a ferric chloride injury model while Y1605A/M1606A had a loss of occlusive thrombi but increased platelet accumulation compared with wild type.

CONCLUSIONS

This study demonstrates that mutations that alter ADAMTS13 cleavage also can affect VWF clearance, VWF antigen level, multimer structure and thrombotic potential in the VWF knockout hydrodynamic injection model.

von Willebrand factor: at the crossroads of bleeding and thrombosis

Hemostasis and thrombosis represent two sides of the same coin. Hemostasis maintains blood fluidity in the vascular system while allowing for rapid thrombus formation to prevent excessive hemorrhage after blood vessel injury. Thrombosis is a pathologic extension of the normal hemostatic mechanism, occurring when unwanted clot formation develops in certain pathological situations. The molecular mechanisms underlying both phenomena are fundamentally identical. One of the key players in both processes is the plasma glycoprotein von Willebrand factor, which perfectly illustrates this duality between hemostatic and thrombotic mechanisms. The purpose of this review is to discuss novel findings on the role of von Willebrand factor at this interface, and how some of these findings may help develop new therapeutic strategies.

Identification of galectin-1 and galectin-3 as novel partners for von Willebrand factor

OBJECTIVE

Although von Willebrand factor (VWF) is a heavily glycosylated protein, its potential to associate with glycan-binding proteins is poorly investigated. Here, we explored its interaction with the glycan-binding proteins galectin-1 and galectin-3.

METHODS AND RESULTS

Immunofluorescence analysis using Duolink proximity ligation assays revealed that VWF colocalizes with galectin-1 and galectin-3 in endothelial cells, both before and after stimulation of endothelial cells. Moreover, galectin-1 was found along the typical VWF bundles that are released by endothelial cells. Galectin-1 and galectin-3 could be coprecipitated with VWF from plasma in immunoprecipitation assays, whereas plasma levels of galectin-1 and galectin-3 were significantly reduced in VWF-deficient mice. Binding studies using purified proteins confirmed that VWF could directly interact with both galectins, predominantly via its N-linked glycans. In search of the physiological relevance of the VWF-galectin interaction, we found that inhibition of galectins in in vitro perfusion assays was associated with increased VWF-platelet string formation, a phenomenon that was reproduced in galectin-1/galectin-3 double-deficient mice. These mice were also characterized by a more rapid formation of initial thrombi following ferric chloride-induced injury.

CONCLUSIONS

We have identified galectin-1 and galectin-3 as novel partners for VWF, and these proteins may modulate VWF-mediated thrombus formation.

Macrophage LRP1 contributes to the clearance of von Willebrand factor

The relationship between low-density lipoprotein receptor-related protein-1 (LRP1) and von Willebrand factor (VWF) has remained elusive for years. Indeed, despite a reported absence of interaction between both proteins, liver-specific deletion of LRP1 results in increased VWF levels. To investigate this discrepancy, we used mice with a macrophage-specific deficiency of LRP1 (macLRP1(-)) because we previously found that macrophages dominate VWF clearance. Basal VWF levels were increased in macLRP1(-) mice compared with control mice (1.6 ± 0.4 vs 1.0 ± 0.4 U/mL). Clearance experiments revealed that half-life of human VWF was significantly increased in macLRP1(-) mice. Ubiquitous blocking of LRP1 or additional lipoprotein receptors by overexpressing receptor-associated protein in macLRP1(-) mice did not result in further rise of VWF levels (0.1 ± 0.2 U/mL), in contrast to macLRP1(+) mice (rise in VWF, 0.8 ± 0.4 U/mL). This points to macLRP1 being the only lipoprotein receptor regulating VWF levels. When testing the mechanism(s) involved, we observed that VWF-coated beads adhered efficiently to LRP1 but only when exposed to shear forces exceeding 2.5 dyne/cm(2), implying the existence of shear stress-dependent interactions. Furthermore, a mechanism involving β2-integrins that binds both VWF and LRP1 also is implicated because inhibition of β2-integrins led to increased VWF levels in control (rise, 0.19 ± 0.16 U/mL) but not in macLRP1(-) mice (0.08 ± 0.15 U/mL).

Targeting von Willebrand factor and platelet glycoprotein Ib receptor

Atherothrombotic events, such as acute coronary syndrome or stroke, are the result of platelet activation. Von Willebrand factor (vWF), a multimeric glycoprotein, plays a key role in aggregation of platelets, especially under high-shear conditions. Acting as bridging element or ligand between damaged endothelial sites and the glycoprotein Ib (GPIb) receptor on platelets, vWF is responsible for platelet adhesion and aggregation. This vWF activation and further platelet aggregation mainly occurs under high shear stress present in small arterioles or during deficiency of the vWF-cleaving protease ADAMTS13. There are several substances targeting vWF itself or its binding receptor GPIb on platelets. Two antibodies are directed against vWF: AJW200, an IgG4 humanized monoclonal antibody, and 82D6A3, a monoclonal antibody of the collagen-binding A-3 domain of vWF. ALX-0081 and ALX-0681 are bivalent humanized nanobodies targeting the GPIb binding site of vWF. Aptamers are oligonucleotides with drug-like properties that share some of the attributes of monoclonal antibodies. ARC1779 is a second-generation, nuclease-resistant aptamer, binding to the activated vWF A1 domain and ARC15105 is a chemically advanced follower with an assumed higher affinity to vWF. Antibodies targeting GPIbα are h6B4-Fab, a murine monoclonal antibody; GPG-290, a recombinant, chimeric protein containing the amino-terminal 290 amino acids of GPIbα linked to human IgG1 Fc; and the monoclonal antibody SZ2. There are a number of promising preclinical results and development of some agents (AJW 200, ARC1779 and ALX-0081) has already reached Phase II trials.

Pathologic mechanisms of type 1 VWD mutations R1205H and Y1584C through in vitro and in vivo mouse models

Type 1 VWD is the mild to moderate reduction of VWF levels. This study examined the mechanisms underlying 2 common type 1 VWD mutations, the severe R1205H and more moderate Y1584C. In vitro biosynthesis was reduced for both mutations in human and mouse VWF, with the effect being more severe in R1205H. VWF knockout mice received hydrodynamic injections of mouse Vwf cDNA. Lower VWF antigen levels were demonstrated in both homozygous and heterozygous forms for both type 1 mutations from days 14-42. Recombinant protein infusions and hydrodynamic-expressed VWF propeptide to antigen ratios demonstrate that R1205H mouse VWF has an increased clearance rate, while Y1584C is normal. Recombinant ADAMTS13 digestions of Y1584C demonstrated enhanced cleavage of both human and mouse VWF115 substrates. Hydrodynamic-expressed VWF shows a loss of high molecular weight multimers for Y1584C compared with wild-type and R1205H. At normal physiologic levels of VWF, Y1584C showed reduced thrombus formation in a ferric chloride injury model while R1205H demonstrated similar thrombogenic activity to wild-type VWF. This study has elucidated several novel mechanisms for these mutations and highlights that the type 1 VWD phenotype can be recapitulated in the VWF knockout hydrodynamic injection model.

Combating innate inflammation: a new paradigm for acute treatment of stroke?

Interference with early steps of platelet adhesion/activation by inhibition of the von Willebrand factor (vWF) receptor glycoprotein (GP)Ib, its ligand vWF, or the collagen receptor GPVI, profoundly limits infarction in the mouse stroke model of transient middle cerebral artery occlusion (tMCAO). A similar pathogenic role was revealed for coagulation factor XII (FXII). Although these findings strongly suggest that microvascular thrombus formation is the leading pathophysiological event in acute stroke, recent studies have shown that these molecules have the additional capacity to guide inflammatory processes, thereby providing an intriguing alternative mechanistic explanation for these observations. Surprisingly, mice lacking T cells are also protected from acute stroke, and these T cell effects are antigen independent. Thus, acute ischemic stroke can be redefined as a thrombo-inflammatory disorder, and multifunctional molecules such as GPIb, GPVI, and FXII may provide new therapeutic targets linking inflammation and thrombus formation.

Binding of von Willebrand Factor to Collagen and Glycoprotein Ibα, But Not to Glycoprotein IIb/IIIa, Contributes to Ischemic Stroke in Mice—Brief Report

Objective— To unravel crucial von Willebrand factor (VWF) interactions that are detrimental in stroke development.

Methods and Results— VWF −/− mice received gene transfer to express mutants of VWF defective either in binding to fibrillar collagen, glycoprotein (GP)Ibα or GPIIb/IIIa, and underwent 60 minutes of transient middle cerebral artery occlusion. In VWF −/− mice reconstituted with VWF mutants defective in binding to collagen or GPIbα, protection against stroke was sustained, whereas VWF lacking the GPIIb/IIIa binding site restored full susceptibility similar to normal VWF.

Conclusion— VWF-collagen and VWF-GPIbα (but not VWF-GPIIb/IIIa) interactions are instrumental in thrombus formation after transient middle cerebral artery occlusion, and their inhibition could be a promising target for stroke treatment.

Mutation and ADAMTS13-dependent modulation of disease severity in a mouse model for von Willebrand disease type 2B

Von Willebrand disease (VWD)–type 2B originates from a gain-of-function mutation in von Willebrand factor (VWF), resulting in enhanced platelet binding. Clinical manifestations include increased bleeding tendency, loss of large multimers, thrombocytopenia, and circulating platelet aggregates. We developed a mouse model to study phenotypic consequences of VWD-type 2B mutations in murine VWF: mVWF/R1306Q and mVWF/V1316M. Both mutations allow normal multimerization but are associated with enhanced ristocetin-induced platelet aggregation, typical for VWD-type 2B. In vivo expression resulted in thrombocytopenia and circulating aggregates, both of which were more pronounced for mVWF/V1316M. Furthermore, both mutants did not support correction of bleeding time or arterial vessel occlusion in a thrombosis model. They further displayed a 2- to 3-fold reduced half-life and induced a 3- to 6-fold increase in number of giant platelets compared with wild-type VWF. Loss of large multimers was observed in 50% of the mice. The role of ADAMTS13 was investigated by expressing both mutants in VWF/ADAMTS13 double-deficient mice. ADAMTS13 deficiency resulted in more and larger circulating platelet aggregates for both mutants, whereas the full multimer range remained present in all mice. Thus, we established a mouse model for VWD-type 2B and found that phenotype depends on mutation and ADAMTS13.

Arterial thrombosis: relevance of a model with two levels of severity assessed by histologic, ultrastructural and functional characterization

BACKGROUND

We previously described a model of laser-induced thrombosis in mesenteric arterioles with superficial and deep levels of injury producing a transient thrombus resolving within 2 min and a larger almost occlusive thrombus, respectively. Both types of lesion were sensitive to platelet GPIIb-IIIa and P2Y(12) inhibition, whereas only deep injuries were sensitive to thrombin blockade.

OBJECTIVE

The aim of the present study was to use histologic methods and electron and intravital microscopy to characterize the lesions and thrombi and to extend our knowledge of the sensitivity of this model to genetic and pharmacologic inhibition.

RESULTS

A superficial injury was found to detach the endothelial cells and expose a collagen III- and IV-rich subendothelium where platelets could adhere. Tissue factor and fibrin were not detected. Deeper penetration of the external elastic lamina occurred in deep injuries, with exposure of collagen I, III and IV. Here the thrombus was composed of platelets exhibiting a decreasing gradient of degranulation from the deepest lesion area to the surface. Fibrin was found close to the most activated platelets. Consistently, glycoprotein VI (GPVI)-collagen and GPIb-von Willebrand factor (VWF) interactions were found to be critical in superficial injuries. After deep lesion, thrombus formation was modestly reduced in GPVI-immunodepleted mice and still strongly inhibited in VWF(-/-) mice. Combined hirudin infusion and GPVI depletion further inhibited thrombosis after deep injury.

CONCLUSIONS

This study confirms the feasibility of inducing arterial thrombosis with distinct levels of severity and establishes the central roles of collagen and VWF in thrombus formation after superficial injury. Collagen, VWF and thrombin all appear to contribute to thrombosis after deep arterial lesion.