Use of monoclonal antibody and colloidal gold in E.M. localization of von Willebrand factor in megakaryocytes and platelets

Inositol hexakisphosphate increases the size of platelet aggregates

The inositol phosphates, InsP₅ and InsP₆, have recently been identified as binding partners of fibrinogen, which is critically involved in hemostasis by crosslinking activated platelets at sites of vascular injury. Here, we investigated the putative physiological role of this interaction and found that platelets increase their InsP₆ concentration upon stimulation with the PLC-activating agonists thrombin, collagen I and ADP and present a fraction of it at the outer plasma membrane.

Cone and plate analysis in whole blood revealed that InsP₆ specifically increases platelet aggregate size. This effect is fibrinogen-dependent, since it is inhibited by an antibody that blocks fibrinogen binding to platelets. Furthermore, InsP₆ has only an effect on aggregate size of washed platelets when fibrinogen is present, while it has no influence in presence of von Willebrand factor or collagen. By employing blind docking studies we predicted the binding site for InsP₆ at the bundle between the γ and β helical subunit of fibrinogen.

Since InsP₆ is unable to directly activate platelets and it did not exhibit an effect on thrombin formation or fibrin structure, our data indicate that InsP₆ might be a hemostatic agent that is produced by platelets upon stimulation with PLC-activating agonists to promote platelet aggregation by supporting crosslinking of fibrinogen and activated platelets.

Angiodysplasia in von Willebrand Disease: Understanding the Clinical and Basic Science

Severe and intractable gastrointestinal bleeding caused by angiodysplasia is a debilitating problem for up to 20% of patients with von Willebrand disease (VWD). Currently, the lack of an optimal treatment for this recurrent problem presents an ongoing challenge for many physicians in their management of affected patients. Over the past few years, studies have pointed to a regulatory role for the hemostatic protein, von Willebrand factor (VWF), in angiogenesis, providing a novel target for the modulation of vessel development. This article will review the clinical implications and molecular pathology of angiodysplasia in VWD.

Biophysical approaches promote advances in the understanding of von Willebrand factor processing and function

The large multimeric plasma glycoprotein von Willebrand factor (VWF) is essential for primary hemostasis by recruiting platelets to sites of vascular injury. VWF multimers respond to elevated hydrodynamic forces by elongation, thereby increasing their adhesiveness to platelets. Thus, the activation of VWF is force-induced, as is its inactivation. Due to these attributes, VWF is a highly interesting system from a biophysical point of view, and is well suited for investigation using biophysical approaches. Here, we give an overview on recent studies that predominantly employed biophysical methods to gain novel insights into multiple aspects of VWF: Electron microscopy was used to shed light on the domain structure of VWF and the mechanism of VWF secretion. High-resolution stochastic optical reconstruction microscopy, atomic force microscopy (AFM), microscale thermophoresis and fluorescence correlation spectroscopy allowed identification of protein disulfide isomerase isoform A1 as the VWF dimerizing enzyme and, together with molecular dynamics simulations, postulation of the dimerization mechanism. Advanced mass spectrometry led to detailed identification of the glycan structures carried by VWF. Microfluidics was used to illustrate the interplay of force and VWF function. Results from optical tweezers measurements explained mechanisms of the force-dependent functions of VWF's domains A1 and A2 and, together with thermodynamic approaches, increased our understanding of mutation-induced dysfunctions of platelet-binding. AFM-based force measurements and AFM imaging enabled exploration of intermonomer interactions and their dependence on pH and divalent cations. These advances would not have been possible by the use of biochemical methods alone and show the benefit of interdisciplinary research approaches.

Analysis of the storage and secretion of von Willebrand factor in blood outgrowth endothelial cells derived from patients with von Willebrand disease

Patients with von Willebrand disease (VWD) are often heterozygous for a missense mutation in the von Willebrand factor (VWF) gene. Investigating the pathogenic features of VWF mutations in cells directly derived from patients has been challenging. Here, we have used blood outgrowth endothelial cells (BOECs) isolated from human peripheral blood to analyze the storage and secretion of VWF. BOECs showed full endothelial characteristics and responded to Weibel-Palade body (WPB) secretagogues except desmopressin. We examined BOECs derived from a single subject heterozygous for a type 2N mutation (p.Arg854Gln) and from 4 patients with type 1 VWD who were, respectively, heterozygous for p.Ser1285Pro, p.Leu1307Pro, p.Tyr1584Cys, and p.Cys2693Tyr. Compared with normal BOECs, BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr showed morphologically abnormal WPB and retention of VWF in the endoplasmic reticulum, whereas BOECs heterozygous for p.Arg854Gln or p.Tyr1584Cys showed normal WPB. The agonist-induced exocytosis of WPB from BOECs and formation of VWF strings on BOECs heterozygous for p.Ser1285Pro, p.Leu1307Pro, or p.Cys2693Tyr, but not for p.Arg854Gln or p.Tyr1584Cys, were reduced. In conclusion, VWD phenotype can be recapitulated in BOECs, and thus BOECs provide a feasible bona fide cell model to study the pathogenic effects of VWF mutations.

Absence of functional activity of tissue plasminogen activator in patients with severe forms of von Willebrand's disease

Some patients with von Willebrand's disease do not respond to stimuli such as venous occlusion and infusion of a vasopressin analogue DDAVP. In these patients, fibrinolytic activity is not enhanced and von Willebrand's factor is not released into the blood. Skin biopsies and cryostat sections were used to study the fibrinolytic activity of skin vessels and localization of tissue plasminogen activator (t-PA) in three patients with severe form of von Willebrand's disease. On fibrin films, no fibrinolysis developed around the skin vessels of the patients; however, using specific polyclonal and monoclonal antibodies to t-PA, and peroxidase coupled specific IgG, presence of t-PA antigen was demonstrated in endothelial cells (EC) of all of them. In plasma no t-PA activity was detected either before or after venous occlusion although t-PA inhibitor activity was in a normal range. Small amounts of t-PA antigen was measured in blood by ELISA. From these results, it is concluded that in patients with severe forms of von Willebrand's disease, t-PA present in EC is not functional and can not transform plasminogen into plasmin.

von Willebrand factor antigen in urine

Human urine was analyzed using a sensitive enzyme linked immunosorbent assay (ELISA) for von Willebrand factor (VWF) antigen. Urine of healthy persons contained VWF immunoreactivity. In the urine of a patient with severe von Willebrand disease, the VWF antigen was not detectable before but after intravenous infusion of von Willebrand factor-Factor VIII (VWF-FVIII) concentrate. The VWF antigen in normal urine was analyzed by gel permeation high performance liquid chromatography (HPLC). Three immunoreactive components of Mr 350 KDa, 60 KDa, and 20 KDa, respectively, were observed. Chromatography on concanavalin A-Sepharose revealed heterogeneity of the major 60 KDa component since only part of the material had affinity for the matrix. The 350 KDa material displayed affinity for Con A-Sepharose but not the 20 KDa. Gel electrophoresis combined with immunoblotting of normal urine gave similar results to those obtained by HPLC analysis. Immunoreactive components with apparent molecular weights similar to the largest urinary antigens were also observed in normal plasma but they were, nevertheless, not identical to the urinary antigens.

Application of an enzyme-linked immunosorbent assay (ELISA) to von Willebrand factor (vWF) and its derivatives

Sandwich ELISA systems were developed to measure human von Willebrand factor (vWF). In one system the microtitre plates were coated with goat F(ab')2 to vWF. The F(ab')2 different molecular forms in the soluble phase. VWF antigen bound by the F(ab')2 antibody was subsequently determined by using horse-radish peroxidase labeled goat Fab' to vWF. In plasma 3 X 10(-4) units of vWF per ml could be detected with this system. In a different approach the antigen bound by the F(ab')2 antibody was probed by monoclonal antibodies to multimeric as well as to the reduced and carboxymethylated (RCM) form of vWF. Using these monoclonals and RCM-as well as native plasma as antigen, the total antigen and the relative proportion of multimeric forms in the sample were estimated.