Determinants of von Willebrand factor activity elicited by ristocetin and botrocetin: studies on a human von Willebrand factor-binding antibody

High shear-dependent loss of membrane integrity and defective platelet adhesion following disruption of the GPIbα-filamin interaction

Platelets have evolved a highly specialized membrane skeleton that provides stability to the plasma membrane and facilitates adhesion under high shear stress. The cytoskeletal anchorage of glycoprotein (GP) Ibα plays an important role in regulating the membrane skeleton. However, its role in regulating membrane stability remains unknown. To investigate this role, we have developed a new mouse model that expresses wild-type human GPIbα (hGPIbα(WT)), or a mutant form of human GPIbα that has a selective defect in its ability to bind filamin A and anchor to the membrane skeleton (hGPIbα(FW)-Phe568Ala and Trp570Ala substitutions). Our study demonstrates that the link between platelet GPIb and the cytoskeleton does not alter the intrinsic ligand binding function of GPIbα or the ability of the receptor to stimulate integrin α(IIb)β(3)-dependent spreading. However, exposure of hGPIbα(FW) platelets to pathologic shear rate levels (5000 to 40,000 s(-1)) leads to the development of unstable membrane tethers, defective platelet adhesion, and loss of membrane integrity, leading to complete disintegration of the platelet cell body. These outcomes suggest that the GPIbα-filamin A interaction not only regulates the architecture of the membrane skeleton, but also maintains the mechanical stability of the plasma membrane under conditions of high shear.

Structure and function of snake venom proteins affecting platelet plug formation

Many snake venom proteins have been isolated that affect platelet plug formation by interacting either with platelet integrins, membrane glycoprotein Ib (GPIb), or plasma von Willebrand factor (VWF). Among them, disintegrins purified from various snake venoms are strong inhibitors of platelet aggregation. Botrocetin and bitiscetin derived from Bothrops jararaca and Bitis arietans venom, respectively, induce VWF-dependent platelet agglutination in vitro. Several GPIb-binding proteins have also been isolated from snake venoms. In this review, we focus on the structure and function of those snake venom proteins that influence platelet plug formation. These proteins are potentially useful as reagents for the sub-diagnosis of platelet disorder or von Willebrand disease, as well as for clinical and basic research of thrombosis and hemostasis.

Development of a collagen-binding activity assay as a screening test for type II von Willebrand disease in dogs

OBJECTIVE

To develop an assay to measure canine von Willebrand factor (vWF):collagen-binding activity (CBA) to screen for type 2 von Willebrand disease (vWD) in dogs.

SAMPLE POPULATION

293 plasma samples submitted for analysis of canine vWF antigen (vWF:Ag) and 12 control plasma samples from dogs with inherited type 2 or 3 vWD.

PROCEDURE

Bovine collagens were evaluated for suitability as binding substrate for vWF. Assay sensitivity to depletion, proteolytic degradation, or a genetic deficiency of high-molecular-weight vWF were determined. Amounts of vWF:Ag and vWF:CBA were measured. The ratio of vWF:Ag to vWF:CBA was used to discriminate between type 1 and type 2 vWD.

RESULTS

An assay for canine vWF activity was developed by use of mixed collagen (types I and III). When vWF:Ag was used to subtype vWD, 48% of the dogs were classified as clinically normal, 9% as indeterminate, and 43% as type 1 vWD. Inclusion of vWF activity resulted in reclassification of 5% of those identified as type 1 to type 2 vWD. However, vWF:CBA of the reclassified dogs was not persistently abnormal, a finding compatible with acquired type 2 vWD. Some Doberman Pinschers had lower antigen-to-activity ratios than other breeds with type 1 vWD, suggesting that Doberman Pinschers have more functional circulating vWF.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of canine vWF activity should be included among the vWF-specific assays used to confirm type 2 vWD. The prevalence of inherited forms of type 2 vWD in screened dogs is lower than acquired forms that can result secondary to underlying disease.

von Willebrand's disease in the dog and cat

Von Willebrand's disease (vWD) is a bleeding disorder that is recognized commonly in dogs but rarely in cats. This article presents the pathophysiology and clinical features of vWD, including a discussion of inherited and acquired types of vWD. Laboratory evaluation and treatment of vWD are reviewed.

Thrombotic thrombocytopenia induced in dogs and pigs. The role of plasma and platelet vWF in animal models of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenia with severe depletion of plasma von Willebrand factor (vWF) was induced in normal large animals (5 dogs and 2 pigs) by botrocetin, a Bothrops factor requiring vWF for platelet agglutination. Botrocetin (90 to 100 U/kg, 2.14 to 2.38 mg/kg, in a single i.v. injection) reduced plasma vWF activity to < 0.1 U/mL for 24 hours. During this period, multimeric analysis of plasma vWF antigen (Ag) revealed the loss of intermediate- and high-molecular-weight forms with a concomitant increase in lower molecular weight forms. A moderate reduction in factor VIII (FVIII) activity was observed. The vWF reduction was accompanied by transient thrombocytopenia and prolonged bleeding times during the deficiency state. Occlusive platelet thrombi were detected by transmission electron microscopy in the microcirculation of lung and spleen but not kidney or brain 30 minutes after the botrocetin injection. Recovery of plasma vWF and platelet count occurred within 48 hours and was associated with the appearance in the plasma of unusually large forms of vWF:Ag multimers. The vWF:Ag multimer distribution was normal at 72 hours. The ultrastructural distribution of vWF in unstimulated normal porcine and canine platelets was examined by using immunogold staining. VWF was detected in the alpha-granules of normal pig platelets but was not observed in platelets from normal dogs. However, both animals developed thrombotic thrombocytopenia when injected with botrocetin. A second group of animals (2 dogs and 3 pigs) with von Willebrand disease (vWD) was given a single botrocetin injection (90 to 100 U/kg). No thrombocytopenia occurred.(ABSTRACT TRUNCATED AT 250 WORDS)