von Willebrand factor: the old, the new and the unknown

Platelet adhesion involves a novel interaction between vimentin and von Willebrand factor under high shear stress

The interaction between platelet receptor glycoprotein Ibα and the A1 domain of von Willebrand factor (VWF) mediates tethering/translocation of platelets to sites of vascular injury. Unexpectedly, we observed platelets translocating over A1A2A3 domains protein slower than on A1 domain at high shear stress. This observation suggests an additional interaction between A domains and an adhesive receptor. We investigated vimentin because we have data showing the interaction of vimentin with the A2 domain of VWF. Moreover, vimentin is expressed on the platelet surface. This novel interaction was analyzed by using purified VWF, recombinant proteins, anti-vimentin antibodies, parallel flow chamber adhesion assays, flow cytometry, and vimentin-deficient murine platelets. The active form of VWF bound to vimentin, and the purified A2 domain blocked that binding. The interaction of a gain-of-function A1A2A3 mutant with platelet was reduced using anti-vimentin antibody. Platelet adhesion to wild-type (WT) A1A2A3 protein, collagen, and fibrin(ogen) was inhibited (32-75%) by anti-vimentin antibody under high shear stress. Compared with WT mice, platelets from vimentin-deficient mice had a reduced flow-dependent adhesion to both collagen and purified murine VWF. Last, the vimentin knockout mice had a prolonged tail bleeding time. The results describe that platelet vimentin engages VWF during platelet adhesion under high shear stress.

Diagnostic challenges in patients with bleeding phenotype and von Willebrand exon 28 polymorphism p.D1472H

Exon 28 polymorphism p.D1472H is associated with significantly lower von Willebrand Ristocetin cofactor activity (VWF:RCoF) to von Willebrand antigen (VWF:Ag) ratio compared to normal, but has been reported as not conferring haemorrhagic risk. The impact of this polymorphism while assessing symptomatic patients for von Willebrand disease (VWD) has not been previously analysed. We retrospectively reviewed charts of children with clinically significant bleeding and abnormal VW panel who underwent VW exon 28 analysis. Twenty-three of 63 patients studied had p.D1472H. Of these 23 patients, 6 with borderline low VWF:RCo were given provisional diagnosis of VWD type 1 by treating physicians, which could be alternatively explained as due to the effect of p.D1472H. None of the patients with low VWF:RCo, decreased VWF:RCo/VWF:Ag ratio and p.D1472H had VWD type 2M mutations identified. This study illustrates the challenge in diagnosing VWD using ristocetin-based VW assay in symptomatic patients with p.D1472H.

New development in von Willebrand disease

PURPOSE OF REVIEW

Von Willebrand disease (VWD) is an autosomally inherited bleeding disorder caused by a deficiency or abnormality of von Willebrand factor (VWF). VWF is a multimeric adhesive protein produced mainly by the endothelial cells. VWF is crucial in primary hemostasis because it promotes platelet adhesion to the subendothelium at the sites of vascular injury and in coagulation because VWF is the carrier of factor VIII. VWD is highly heterogeneous because the molecular mechanisms underlying the different clinical and laboratory phenotypes may be complex. VWD is classified into quantitative deficiencies of VWF (type 1 and type 3 VWD) and qualitative variants (type 2 VWD), because of a dysfunctional VWF. Whereas inheritance is autosomal dominant and bleeding tendency is heterogeneous in type 1 and 2, type 3 patients present moderate-to-severe bleeding diathesis and display a recessive pattern of inheritance.

RECENT FINDINGS

Although the responsible genetic background has been extensively clarified over the recent years, providing insights on the structure-function relationship of the protein, the cellular basis of the disorder is being investigated for a few mutations only recently. In several cases, increased clearance of the mutant VWF may be responsible for the disease. Standardized criteria for the definition of bleeding history and appropriate history collection are now available, but estimates of bleeding risk are largely lacking.

SUMMARY

VWD, the most frequent inherited bleeding disorder, has been the subject of extensive pathophysiological and clinical studies. The novel evidences provide accurate insights on the mechanisms of the disease and the bleeding risk associated with VWF deficiency or abnormality.

Antithrombotics in Heart Failure With Reduced Ejection Fraction and Normal Sinus Rhythm

Objective: To review the thromboembolic risk, pathophysiology associated with the risk, and literature investigating the use of antithrombotics in patients with heart failure with reduced ejection fraction and normal sinus rhythm (HFrEF-NSR). Data Sources: An English language literature search was performed with MEDLINE/PubMed and Embase from January 1950 to October 2013 using the search terms heart failure, HFrEF, systolic heart failure, cardiomyopathy, left ventricular dysfunction, sinus rhythm, thromboembolism, deep vein thrombosis, pulmonary embolism, myocardial infarction, acute coronary syndrome, acute coronary events, coronary artery disease, stroke, and cerebrovascular events to identify relevant articles. References in the retrieved articles were also assessed to identify other important articles. Study Selection and Data Abstraction: All pertinent original studies, reviews, consensus documents, and guidelines were evaluated for inclusion. Data Synthesis: Patients with HFrEF-NSR may be predisposed to developing thromboembolic events. Studies that have examined the role of antithrombotics (warfarin and/or antiplatelet therapy) for reducing thromboembolic risk have been inconclusive. The WASH and HELAS pilot trials—the only studies with a no-antithrombotics or placebo comparator group—did not find a benefit with antithrombotic therapy but found an increased risk of bleeding with warfarin and of hospitalizations with aspirin. Although the clinical outcome studies (WATCH and WARCEF) suggested that warfarin may reduce stroke risk compared with antiplatelet therapy, the lack of a placebo group and lower-than-projected enrollment prevents definitive conclusions from being made. Conclusions: Current evidence does not support the routine use of antithrombotics for preventing thromboembolic events in patients with HFrEF-NSR without compelling indications.

G protein-dependent basal and evoked endothelial cell vWF secretion

von Willebrand factor (vWF) secretion by endothelial cells (ECs) is essential for hemostasis and thrombosis; however, the molecular mechanisms are poorly understood. Interestingly, we observed increased bleeding in EC-Gα13(-/-);Gα12(-/-) mice that could be normalized by infusion of human vWF. Blood from Gα12(-/-) mice exhibited significantly reduced vWF levels but normal vWF multimers and impaired laser-induced thrombus formation, indicating that Gα12 plays a prominent role in EC vWF secretion required for hemostasis and thrombosis. In isolated buffer-perfused mouse lungs, basal vWF levels were significantly reduced in Gα12(-/-), whereas thrombin-induced vWF secretion was defective in both EC-Gαq(-/-);Gα11(-/-) and Gα12(-/-) mice. Using siRNA in cultured human umbilical vein ECs and human pulmonary artery ECs, depletion of Gα12 and soluble N-ethylmaleimide-sensitive-fusion factor attachment protein α (α-SNAP), but not Gα13, inhibited both basal and thrombin-induced vWF secretion, whereas overexpression of activated Gα12 promoted vWF secretion. In Gαq, p115 RhoGEF, and RhoA-depleted human umbilical vein ECs, thrombin-induced vWF secretion was reduced by 40%, whereas basal secretion was unchanged. Finally, in vitro binding assays revealed that Gα12 N-terminal residues 10-15 mediated the binding of Gα12 to α-SNAP, and an engineered α-SNAP binding-domain minigene peptide blocked basal and evoked vWF secretion. Discovery of obligatory and complementary roles of Gα12 and Gαq/11 in basal vs evoked EC vWF secretion may provide promising new therapeutic strategies for treatment of thrombotic disease.

Von Willebrand factor, angiodysplasia and angiogenesis

The large multimeric glycoprotein Von Willebrand factor (VWF) is best known for its role in haemostasis; however in recent years other functions of VWF have been identified, indicating that this protein is involved in multiple vascular processes. We recently described a new role for VWF in controlling angiogenesis, which may have significant clinical implications for patients with Von Willebrand disease (VWD), a genetic or acquired condition caused by the deficiency or dysfunction of VWF. VWD can be associated with angiodysplasia, a condition of degenerative blood vessels often present in the gastrointestinal tract, linked to dysregulated angiogenesis. Angiodysplasia can cause severe intractable bleeding, often refractory to conventional VWD treatments. In this review we summarise the evidence showing that VWF controls angiogenesis, and review the angiogenic pathways which have been implicated in this process. We discuss the possible mechanisms though which VWF regulates angiopoietin-2 (Ang-2) and integrin αvβ3, leading to signalling through vascular endothelial growth factor receptor-2 (VEGFR2), one of the most potent activators of angiogenesis. We also review the evidence that links VWF with angiodysplasia, and how the newly identified function of VWF in controlling angiogenesis may pave the way for the development of novel therapies for the treatment of angiodysplasia in congenital VWD and in acquired conditions such as Heyde syndrome.

Pharmacokinetics and safety of a novel recombinant human von Willebrand factor manufactured with a plasma-free method: a prospective clinical trial

Safety and pharmacokinetics (PK) of recombinant von Willebrand factor (rVWF) combined at a fixed ratio with recombinant factor VIII (rFVIII) were investigated in 32 subjects with type 3 or severe type 1 von Willebrand disease (VWD) in a prospective phase 1, multicenter, randomized clinical trial. rVWF was well tolerated and no thrombotic events, inhibitors, or serious adverse events were observed. The PK of rVWF ristocetin cofactor activity, VWF antigen, and collagen-binding activity were similar to those of the comparator plasma-derived (pd) VWF-pdFVIII. In vivo cleavage of ultra-large molecular-weight rVWF multimers by ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13; the endogenous VWF protease) and generation of characteristic satellite bands were demonstrated. In 2 subjects with specific nonneutralizing anti-VWF-binding antibodies already detectable before rVWF infusion, a reduction in VWF multimers and VWF activity was observed. Stabilization of endogenous FVIII was enhanced following post-rVWF-rFVIII infusion as shown by the difference in area under the plasma concentration curve compared with pdVWF-pdFVIII (AUC0-∞) (P < .01). These data support the concept of administering rVWF alone once a therapeutic level of endogenous FVIII is achieved.

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.

Clearance of von Willebrand factor

Quantitative deficiencies in von Willebrand factor (VWF) are associated with abnormal hemostasis that can manifest in bleeding or thrombotic complications. Consequently, many studies have endeavored to elucidate the mechanisms underlying the regulation of VWF plasma levels. This review focuses on the role of VWF clearance pathways. A summary of recent developments are provided, including results from genetic studies, the relationship between glycosylation and VWF clearance, the contribution of increased VWF clearance to the pathogenesis of von Willebrand disease and the identification of VWF clearance receptors. These different studies converge in their conclusion that VWF clearance is a complex phenomenon that involves multiple mechanisms. Deciphering how such different mechanisms coordinate their role in this process is but one of the remaining challenges. Nevertheless, a better insight into the complex clearance pathways of VWF may help us to better understand the clinical implications of aberrant clearance in the pathogenesis of von Willebrand disease and perhaps other disorders as well as aid in developing alternative therapeutic approaches.

Translational medicine advances in von Willebrand disease

Following the recognition of von Willebrand disease (VWD) in 1926 and the cloning of the gene for von Willebrand factor (VWF) in 1985, significant advances have been made in our fundamental knowledge of both the disease and the protein. Some of this new knowledge has also begun to impact the clinical management of VWD. First, the progressive increase in our understanding of the molecular genetic basis of VWD has resulted in rational applications of molecular testing to complement the current range of phenotypic tests for VWD. These molecular genetic strategies are most effectively directed at the prenatal diagnosis of type 3 VWD and confirmatory testing for types 2B and 2N disease. In contrast, the use of molecular testing to clarify the diagnosis of type 1 VWD is of marginal benefit, at best. In terms of VWD therapies, a new recombinant VWF concentrate has recently completed successful clinical trials and is now awaiting more widespread application. There have even been some preclinical successes with VWF gene transfer although the clinical rationale for this therapeutic strategy needs careful consideration. Much more remains to be learnt about the biology of VWF and further translational advances for the enhancement of VWD care will inevitably be realized.

Genomic approach to identify factors that drive the formation of three-dimensional structures by EA.hy926 endothelial cells

Understanding the mechanisms responsible for tube formation by endothelial cells (ECs) is of major interest and importance in medicine and tissue engineering. Endothelial cells of the human cell line EA.hy926 behave ambivalently when cultured on a random positioning machine (RPM) simulating microgravity. Some cells form tube-like three-dimensional (3D) aggregates, while other cells (AD) continue to grow adherently. Between the fifth and seventh day of culturing, the two types of cell growth achieve the greatest balance. We harvested ECs that grew either adherently or as 3D aggregates separately after five and seven days of incubation on the RPM, and applied gene array analysis and PCR techniques to investigate their gene expression profiles in comparison to ECs growing adherently under normal static 1 g laboratory conditions for equal periods of time. Using gene arrays, 1,625 differentially expressed genes were identified. A strong overrepresentation of transient expression differences was found in the five-day, RPM-treated samples, where the number of genes being differentially expressed in comparison to 1 g cells was highest as well as the degree of alteration regarding distinct genes. We found 27 genes whose levels of expression were changed at least 4-fold in RPM-treated cells as compared to 1 g controls. These genes code for signal transduction and angiogenic factors, cell adhesion, membrane transport proteins or enzymes involved in serine biosynthesis. Fifteen of them, with IL8 (interleukin 8) and VWF (von Willebrand factor) the most prominently affected, showed linkages to genes of another 20 proteins that are important in cell structure maintenance and angiogenesis and extended their network of interaction. Thus, the study reveals numerous genes, which mutually influence each other during initiation of 3D growth of endothelial cells.

An ELISA for the quantitation of von Willebrand factor: osteoprotegerin complexes in plasma

BACKGROUND

Von Willebrand factor (VWF) is pivotal in arterial thrombosis, and osteoprotegerin (OPG) is besides being a bone protein also related to cardiovascular diseases. OPG can bind VWF, but the significance of this interaction is not known.

OBJECTIVES

The aim was to develop an assay for measurement of von Willebrand factor-osteoprotegerin complex (VWF:OPG) in human plasma. Furthermore, the significance of VWF:OPG complex as a marker of cardiovascular disease (CVD) was evaluated.

PATIENTS/METHODS

A sandwich ELISA for quantification of VWF:OPG was developed using a polyclonal rabbit anti-human VWF capturing antibody and a monoclonal anti-human OPG detecting antibody. Samples were quantified relative to a standard curve obtained from dilutions of a plasma pool from healthy individuals. The assay was evaluated in two groups of patients with CVD and two groups of asymptomatic individuals with and without documented coronary calcification (total n=118).

RESULTS AND CONCLUSIONS

The assay detected VWF:OPG complexes in human plasma, while no significant signal was observed when testing solutions containing VWF or recombinant OPG alone. Importantly, the ELISA assay was able to detect in vitro formed complexes between human VWF and recombinant OPG in a dose-dependent manner. There was a large inter-individual variation in plasma VWF:OPG levels, but we found no significant differences in the level of VWF:OPG complexes between the four groups. Thus, we conclude that increasing OPG plasma levels in atherosclerotic CVD are not derived from increased levels of complexed form of VWF and OPG, but are more likely due to increased amounts of OPG secreted into the circulation.

Von Willebrand disease-associated angiodysplasia: a few answers, still many questions

The association between angiodysplasia and von Willebrand disease (VWD) has been known for more than 40 years. Bleeding in the gastrointestinal tract associated with angiodysplasia worsens the clinical course of this inherited haemorrhagic disorder and management may become difficult and challenging. Angiodysplasia associated with acquired defects or dysfunctions of von Willebrand factor (VWF) has also been reported in a variety of conditions such as monoclonal gammopathies, Heyde syndrome and in carriers of ventricular assist devices. The most recent advances concerning the mechanistic, clinical and therapeutic aspects of VWD-associated angiodysplasia are summarized in this review, together with the limitations of our knowledge that warrant further research in the frame of international cooperation.

von Willebrand factor and cancer: a renewed interest

Von Willebrand factor (VWF), the largest human plasma protein, is an adhesive multimeric glycoprotein that mediates platelet adhesion to both the subendothelial matrix and endothelial surfaces and acts as a carrier for coagulation factor VIII in the circulation. Besides its essential role in hemostasis, there is growing evidence from the literature that VWF has an additional antitumor effect, mainly by exerting negative modulation on angiogenesis and apoptosis. Current knowledge on the link between VWF and cancer is summarized in this review, based on an analysis of the most important experimental and clinical studies.