Von Willebrand Factor In Thrombosis Research Articles

Thrombosis and Von Willebrand Factor in Chuvash Polycythemia

Background: In Chuvash polycythemia (CP), homozygous germline VHLR200W results in augmented hypoxia-sensing, elevated erythropoietin and hemoglobin, and increased morbidity and mortality from venous and arterial thromboses.1 In CP patients and controls from Russia's Chuvash Republic, the probability of new thrombosis in CP increased with age, smoking, and higher expression of THBS1, the gene encoding thrombospondin-1.2 In mice, hypoxia leads to upregulation of THBS1 through HIF-23 and to upregulation of VWF .4 THBS1 reportedly regulates the size of circulating von Willebrand Factor (VWF) multimers5 through inhibiting proteolytic cleavage of VWF by ADAMTS13.6 We hypothesized that upregulation of THBS1 in CP may be a risk for thrombosis through an effect on ADAMTS13 and VWF.Methods: We enrolled into an observational study 155 patients with CP and 154 controls matched by age, sex and place of residence. Written informed consent was obtained from all subjects according to the Declaration of Helsinki. We genotyped for VHLR200W, measured baseline plasma THBS-1 concentrations2 and followed the subjects for a median of 9.4 years. Here, we report results for determining baseline circulating levels of ADAMTS13 antigen, activity and specific activity and of VWF antigen and differential multimers in up to 57 CP patients and 30 controls.Results: CP subjects had higher hemoglobin and erythropoietin concentrations and lower white blood cell and platelet counts (Table 1). They were also more likely to be current smokers. There was a trend to higher THBS-1 concentrations in the CP patients (p=0.084), which was significant after adjusting for platelet count (P=0.016). ADAMTS13 and VWF antigen levels did not differ between CP patients and controls but ADAMTS13 activity and especially specific activity were lower in CP. Plasma THBS-1 concentration correlated positively with ADAMTS13 antigen concentration (r=0.358, P=0.001) but negatively with specific activity (r=-0.363, P=0.001) (Figure 1). Eight (14.0%) CP patients (aged 27-76) years had a history of thrombosis (Table 1). The mean serum VWF antigen concentration (P=0.001) and differential VWF multimer size (P=0.090) were higher in CP patients with a history of thrombosis (Figure 2). During the prospective follow-up period, 10 CP patients (17.5%) experienced a new thrombosis at a median age of 46 (range 26-70) years. Three patients also experienced an additional thrombosis for a total of 13 new thromboses: myocardial infarction (n=5), stroke (n=3), splanchnic thrombosis (n=2), deep vein thrombosis (n=2), pulmonary embolism (n=1). Baseline serum VWF antigen levels (P=0.019) and VWF multimer size (P=0.099) were higher in subjects who developed a new thrombosis during follow-up (Figure 3), and these relationships persisted after adjusting for smoking history.Discussion: THBS1 is an adhesive glycoprotein that mediates cell-to-cell and cell-to-matrix interactions and facilitates platelet aggregation.7 In mice, platelet-derived THBS1 modulated arterial thrombosis only in the presence of VWF.8 Our present findings suggest that CP subjects have lower ADAMTS13 specific activity than controls and that this lower activity may be related to higher plasma THBS1 concentrations. Lower ADAMTS13 specific activity suggests the presence of an inhibitor or partial inactivation of the protease. Furthermore, higher VWF antigen and multimer size in CP patients is associated with a past history of thrombosis and a greater risk for new thrombosis in the future. More broadly, our findings suggest a possible implication of VWF in thrombosis of conditions associated with an up-regulated hypoxic response. Further studies may identify new pathways of prevention and treatment with broad applicability.

Autoregulation of von Willebrand factor function by a disulfide bond switch.

Force-dependent binding of platelet glycoprotein Ib (GPIb) receptors to plasma von Willebrand factor (VWF) plays a key role in hemostasis and thrombosis. Previous studies have suggested that VWF activation requires force-induced exposure of the GPIb binding site in the A1 domain that is autoinhibited by the neighboring A2 domain. However, the biochemical basis of this "mechanopresentation" remains elusive. From a combination of protein chemical, biophysical, and functional studies, we find that the autoinhibition is controlled by the redox state of an unusual disulfide bond near the carboxyl terminus of the A2 domain that links adjacent cysteine residues to form an eight-membered ring. Only when the bond is cleaved does the A2 domain bind to the A1 domain and block platelet GPIb binding. Molecular dynamics simulations indicate that cleavage of the disulfide bond modifies the structure and molecular stresses of the A2 domain in a long-range allosteric manner, which provides a structural explanation for redox control of the autoinhibition. Significantly, the A2 disulfide bond is cleaved in ~75% of VWF subunits in healthy human donor plasma but in just ~25% of plasma VWF subunits from heart failure patients who have received extracorporeal membrane oxygenation support. This suggests that the majority of plasma VWF binding sites for platelet GPIb are autoinhibited in healthy donors but are mostly available in heart failure patients. These findings demonstrate that a disulfide bond switch regulates mechanopresentation of VWF.

FAM20c-Mediated Serine 1613 Phosphorylation in the A2 Domain of Von Willebrand Factor Regulates ADAMTS13 Activity and Thrombus Formation

Protein phosphorylation represents a common mechanism to regulate the structure and function of proteins. Although vast amount of extracellular proteins including secreted plasma proteins are phosphorylated, historically, phosphorylation has been intensively investigated for intracellular proteins. The plasma and subendothelial protein von Willebrand factor (VWF) undergoes post translational modifications such as glycosylation and sulphation to reach the mature protein product. However, phosphorylation of VWF has not been described. We have used mass spectrometry to analyze purified plasma VWF, and identified that serine 1613 within the A2 domain was phosphorylated. A natural occurring mutation on this residue (S1613P) causes von Willebrand disease Type 2A by increasing the susceptibility of VWF to be cleaved by ADAMTS13. Notably, S1613 overlapped with the S-X-E/pS motif, which is the consensus site for phosphorylation by an atypical kinase, FAM20c (family with sequence similarity 20, member C). Localized to the inner lumen of the golgi/endoplasmic reticulum, FAM20c is secreted and likely responsible for the phosphorylation of several secreted proteins bearing the S-X-E/ps motif. Therefore, we further investigated whether VWF can undergo phosphorylation by FAM20c and how such modification impacts the function of VWF, particularly on the activity of ADAMTS13. In vitro, recombinant FAM20c directly phosphorylated recombinant VWF-A1A2A3 domain protein and purified plasma VWF. Further analysis revealed that the isolated A2 domain but not A1 or A3 domain was phosphorylated by FAM20c. Phosphorylation was assessed employing 32P labeling of proteins, protein shift in phospho tag gel and mass spectrometry. Treatment with λ phosphatase diminished phosphorylation and a defective FAM20c kinase mutant failed to phosphorylate A2 and VWF proteins, confirming the phosphorylation event. In addition, FAM20c-mediated phosphorylation was markedly reduced in a non-phosphorylatable A2 S1613A mutant. Thus, all these outcomes indicate that the secreted kinase FAM20c can phosphorylate S1613 in the A2 domain of VWF. To explore the functional effect of S1613 phosphorylation, we compared the plasma-mediated cleavage of wild type (WT)A2, phosphomimetic S1613D mutant and the nonphosphorylatable A2 S1613A mutant. Unexpectedly, and in sharp contrast to the WT and S1613A variants, the S1613D mutant was effectively cleaved in the presence of the enzyme inhibitor, EDTA. In addition, cleavage of the S1613D mutant was robust and slightly faster than that of the WT and S1613A. These studies suggest that phosphorylation of S1613 in VWF may facilitate the cleavage of VWF multimers. To further explore the physiological relevance of phosphorylated VWF in thrombosis, we generated phospho VWF S1613 and nonphosphorylated S1613 VWF antibodies and studied their effect on thrombus formation. In a microfluidic perfusion system, whole blood supplemented with 50 μg/ml of phosphoVWF antibody but not the nonphosphoVWF antibody, markedly potentiated thrombus formation on a collagen-coated surface. Collectively, these studies suggest that S1613 phosphorylation of VWF suppress thrombus formation, in part by facilitating cleavage of the VWF multimers. These studies identify for the first time that VWF can undergo phosphorylation and opens new avenues for regulation of VWF function by phosphorylation. DisclosuresNo relevant conflicts of interest to declare.

Neutrophil Extracellular Traps and von Willebrand Factor in Thrombosis

For many years, my lab has been studying the interplay between inflammation and thrombosis. These processes occur together, stimulate each other and share cellular and molecular components. The latest example of a common functional component is neutrophil extracellular traps (NETs). NETs are chromatin released together with toxic granular components from highly stimulated neutrophils. Originally found to trap/sequester invading pathogens, they were soon also seen to be part of sterile inflammatory and thrombotic processes. They interact with von Willebrand factor (VWF), which is also involved in platelet and leukocyte recruitment and is crucial for venous thrombus development after inferior vena cava stenosis. It is possible that NETs and VWF work together and cleavage of either by DNase 1 or ADAMTS13 is beneficial in ischemia/reperfusion injury. This will be discussed together with the role of NETs and the enzyme that generates them (PAD4) in animal models of deep vein thrombosis, myocardial infarction and in physiological wound healing. Interestingly, we observed that various cancers in mice prime neutrophils for NETosis. This causes cancer-associated thrombosis, and the production of NETs may affect tumor biology. Disclosures No relevant conflicts of interest to declare.

Von Willebrand Factor Directly Interacts With DNA From Neutrophil Extracellular Traps

Inflammatory conditions provoke essential processes in the human vascular system. It leads to the formation of ultralarge von Willebrand factor (VWF) fibers, which are immobilized on the endothelial cell surface and transform to highly adhesive strings under shear conditions. Furthermore, leukocytes release a meshwork of DNA (neutrophil extracellular traps) during the process of the recently discovered cell death program NETosis. In the present study, we characterized the interaction between VWF and DNA and possible binding sites to underline the role of VWF in thrombosis and inflammation besides its function in platelet adhesion. Both functionalized surfaces and intact cell layers of human umbilical vein endothelial cells were perfused with isolated, protein-free DNA or leukocytes from whole blood at distinct shear rates. DNA-VWF interaction was monitored using fluorescence microscopy, ELISA-based assays, molecular dynamics simulations, and electrostatic potential calculations. Isolated DNA, as well as DNA released by stimulated leukocytes, was able to bind to shear-activated, but not inactivated, VWF. However, DNA-VWF binding does not alter VWF degradation by a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13. Moreover, DNA-VWF interaction can be blocked using unfractionated and low-molecular-weight heparin, and DNA-VWF complexes attenuate platelet binding to VWF. These findings were supported using molecular dynamics simulations and electrostatic calculations of the A1- and A2-domains. Our findings suggest that VWF directly binds and immobilizes extracellular DNA released from leukocytes. Therefore, we hypothesize that VWF might act as a linker for leukocyte adhesion to endothelial cells, supporting leukocyte extravasation and inflammation.

Von Willebrand disease biology

*Department of Molecular Medicine and Surgery, Karolinska Institutet, Karolinska University Hospital, Stockholm, Sweden; Department of Thrombosis and Hemostasis, C2-R Einthoven Laboratory for Experimental Vascular Medicine, LeidenUniversity Medical Center, Leiden, The Netherlands; Centre for Haematology, Imperial College London, HammersmithHospital Campus, London;§INSERM U770, Cedex, France; and –Department of Pathology and Molecular Medicine, QueensUniversity, Kingston, ON, Canada

Structure and function of snake venom toxins interacting with human von Willebrand factor

Hemostatic plug formation is a complex event mediated by platelets, subendothelial matrices and von Willebrand factor (VWF) at the vascular injury. Snake venom proteins have an excellent potency to regulate the interaction between VWF and platelet membrane receptors in vitro. Two protein families, C-type lectin-like proteins and Zn 2+-metalloproteinases, have been found to affect platelet–VWF interaction. Botrocetin and bitiscetin from viper venom are disulfide-linked heterodimers with C-type lectin-like motif, and modulate VWF to elicit platelet glycoprotein Ib (GPIb)-binding activity via the A1 domain of VWF leading to the platelet agglutination. The crystal structures of botrocetin and bitiscetin together with complex from the VWF A1 domain indicate the following: (1) a central concave domain formed by two subunits of botrocetin or bitiscetin provides the binding site for VWF, (2) these modulators directly bind to the A1 domain of VWF in close proximity to the GPIb binding site, (3) both modulators induce no significant conformational change on the GPIb-binding site of the A1 domain but could provide a supplemental platform fitting for GPIb. These results suggest that the modulating mechanisms of these venoms are different from those performed by either antibiotic ristocetin in vitro or extremely high shear stress in vivo. Other modulator toxins include kaouthiagin and jararhagin, chimeric proteins composed of metalloproteinase, disintegrin-like and Cys-rich domains. These toxins cleave VWF and reduce its platelet agglutinating or collagen-binding activity. Kaouthiagin from cobra venom specifically cleaves between Pro708 and Asp709 in the C-terminal VWF A1 domain resulting in the decrease of the multimer structure of VWF. Recently a plasma proteinase, which specifically cleaves VWF into a smaller multimer, has been elucidated to be a reprolysin-like metalloproteinase with thrombospondin motif family (ADAMTS). This endogenous metalloproteinase (ADAMTS-13) specifically cleaves between Tyr842 and Met843 in the A2 domain of VWF regulating its physiological hemostatic activity. These VWF-binding snake venom proteins are suitable probes for basic research on platelet plug formation mediated by VWF, for subsidiary diagnostic use for von Willebrand disease or platelet disorder, and might be potently applicable to the regulation of VWF in thrombosis and hemostasis. Structural information of these venom proteins together with recombinant technology might strongly promote the construction of a new antihemostatic drug in the near future.

Involvement of von Willebrand factor in thrombosis following asparaginase-prednisone-vincristine therapy for leukemia.

To determine if factor VIII-von Willebrand factor (vWF) complex is involved in the thrombosis associated with asparaginase-prednisone-vincristine induction therapy for acute lymphoblastic leukemia, plasma vWF was analyzed by sodium dodecyl sulfate-agarose gel electrophoresis and crossed immunoelectrophoresis. Five patients with cerebral thrombosis were studied; all had a decreased platelet count following the complication. Sequential studies of three patients disclosed changes in plasma vWF multimer pattern. One patient who was studied serially from 2 days before to 1 day after the event, had an increase in unusually large plasma vWF multimers that disappeared after the complication. The other two patients who were studied at presentation subsequently showed a decrease in plasma large vWF multimers, especially remarkable in the patient having the sharpest decrease in platelet count. No appreciable difference in vWF multimer pattern, when compared to normal pooled plasma, was found in the remaining two patients who were only studied at presentation, or in the seven controls who received the same treatment but did not develop thrombosis. Crossed immunoelectrophoretic analyses of two patients tested disclosed a right shift of immunoprecipitin line in one and a left shift in the other. Our findings suggest that the thrombotic complications resulted from platelet agglutination by plasma vWF.