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Pathological consequences of von Willebrand factor-platelet glycoprotein Ib interaction: type 2B von Willebrand disease as a study model – PlaWillPath

Von Willebrand factor-platelet interaction: from beneficial to deleterious

Von Willebrand disease type 2B: a model to study unwanted interaction between von Willebrand factor (VWF) and platelet glycoprotein Ib (GPIb) <br /> <br />When VWF-GPIb interaction occurs in a spontaneous manner, a situation arising in various pathologies, it induces thrombocytopenia of unknown origin. This thrombocytopenia represents a bad prognosis factor. We will engineer a murine model to unravel the molecular bases of this process. <br />

Pathological consequences associated with an unwanted VWF-GPIba interaction

The goal is to investigate the role of the von VWF-GPIb axis in an array of biological processes. Although this interaction is crucial for formation of the platelet plug, there are a number of pathological situations where it can exert negative effects. A mouse model displaying spontaneous VWF-GPIb interaction would allow testing these aspects. We will establish a murine model of type 2B von Willebrand disease (VWD) as the prototype disease model for pathologies with presence of “active” VWF.

A type 2B mouse model will be generated by homologous recombination, replacing the endogenous VWF gene by a gene carrying a type 2B mutation. The phenotype of the mice will then be analyzed on a hemostatic and thrombotic level. To analyze the thrombocytopenia, megakaryopoiesis and thrombopoiesis will be studied in these mice. Platelet apoptosis and platelet clearance will also be investigated. Finally the mice will be useful to test new potential treatments for type 2B VWD.

The generation of the type 2B VWD murine model is ongoing and the mice should be available by the end of the year. Platelet function was studied using the transient murine model and we could observe that results differ according to the mutation under investigation. A severe mutation induces platelet desensitisation, leading to an inhibition of platelet aggregation with various agonists.

Altogether with the type 2B VWD murine model, this project will shed light on the consequences of unwanted VWF-GPIba interaction, an interaction which is increasingly described in a number of pathologies and whose deleterious effects start to be increasingly recognized.

No publication yet for this project.

In case of vascular injury, the formation of the platelet thrombus or primary hemostasis represents one important step leading to arrest of bleeding. This step involves several molecular players such as the platelet receptor GPIba and von Willebrand factor (VWF), a multimeric plasma protein which binds to vascular collagens exposed in the subendothelium. This immobilization of VWF on collagens leads to change of conformation of the molecule, allowing it to adopt an «active» conformation or platelet binding conformation. Indeed the VWF-GPIba interaction should not occur in the absence of vascular injury to avoid formation of intravascular thrombi.
However there are a number of pathological situations where the VWF-GPIba interaction occurs when unwanted. This involves diseases where «active» VWF was detected such as thrombocytopenic thrombotic purpura, malaria or HELLP syndrome. Mutations on VWF can also lead to this active conformation such as in type 2B von Willebrand disease (VWD). Surprisingly, this VWD type induces a bleeding phenotype due to the accompanying thrombocytopenia for which the underlying causes remain elusive.
In the last few years however, several studies have reported negative biological effects resulting from VWF-GPIba interaction not provoked by hemostasis necessity. These effects could also contribute to the thrombocytopenia in patients. Indeed it appears that some patients with type 2B VWD display impaired megakaryopoiesis. Also, the VWF-GPIba interaction can potentially lead to platelet apoptosis. An animal model displaying spontaneous VWF-GPIba interaction would allow testing of these different mechanisms in vivo. We have thus decided to establish a murine model of type 2 VWD as the prototype disease model for pathologies with presence of “active” VWF. Recently we have validated a transient type 2B VWD mouse model using hydrodynamic injection of murine VWF cDNA carrying type 2B mutations in the VWF-deficient mice that have been developed by the project coordinator. Our results show that the mice develop a phenotype closely mimicking that of patients with type 2B VWD, with thrombocytopenia, presence of circulating platelet aggregates and enlarged platelets visible on blood smears. The mice also appear to have defective hemostasis. In this project we also propose to engineer a knock-in mouse in which the endogeneous VWF gene will be replaced by a gene carrying a type 2B VWD mutation. This first and unique model will have the advantage of expressing VWF in all the compartments where the protein is usually expressed: endothelial cells, platelets, plasma and subendothelium in contrast to the transient model where VWF is expressed only in plasma.
Our goal is to use both these models to better understand the pathological consequences of unwanted VWF-GPIba interaction and to elucidate the mechanisms leading to thrombocytopenia. Three hypotheses will be tested: 1) Abnormal megakaryopoiesis and thrombopoiesis; 2) Platelet apoptosis; 3) Accelerated clearance of platelets.
The type 2B VWD murine models will also be useful to test new potential treatments, more particularly two negative regulators of the VWF-GPIba interaction, a natural one, beta(2) glycoprotein I and an aptamer inhibiting VWF binding to GPIba.
The project associates two teams with a long-standing experience on VWF and VWD pathophysiology. Team 1 is widely recognized for its expertise on VWF-related mouse models, including the VWF-deficient mice. Team 2 has expertise on shear models and has recently shown a shear effect on platelet production in type 2B VWD from megakaryocytes.
Altogether with the type 2B VWD murine model, this project will shed light on the consequences of unwanted VWF-GPIba interaction, an interaction which is increasingly described in a number of pathologies and whose deleterious effects start to be increasingly recognized.

Project coordinator

Madame Cecile DENIS (INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE - DELEGATION PARIS XI) – cecile.denis@inserm.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UMRS 765 UNIVERSITE DE PARIS V - RENE DESCARTES
U770 INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE - DELEGATION PARIS XI

Help of the ANR 532,867 euros
Beginning and duration of the scientific project: October 2011 - 36 Months

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