von Willebrand factor mutation promotes thrombocytopathy by inhibiting integrin αIIbβ3

Abstract

von Willebrand disease type 2B (vWD-type 2B) is characterized by gain-of-function mutations in von Willebrand factor (vWF) that enhance its binding to the glycoprotein Ib-IX-V complex on platelets. Patients with vWD-type 2B have a bleeding tendency that is linked to loss of vWF multimers and/or thrombocytopenia. In this study, we uncovered evidence that platelet dysfunction is a third possible mechanism for bleeding tendency. We found that platelet aggregation, secretion, and spreading were diminished due to inhibition of integrin αIIbβ3 in platelets from mice expressing a vWD-type 2B-associated vWF (vWF/p.V1316M), platelets from a patient with the same mutation, and control platelets pretreated with recombinant vWF/p.V1316M. Impaired platelet function coincided with reduced thrombus growth. Further, αIIbβ3 activation and activation of the small GTPase Rap1 were impaired by vWF/p.V1316M following exposure to platelet agonists (thrombin, ADP, or convulxin). Conversely, thrombin-or ADP-induced Ca 2+ store release, which is required for αIIbβ3 activation, was normal, indicating that vWF/p.V1316M acts downstream of Ca 2+ release and upstream of Rap1. We found normal Syk phosphorylation and PLCγ2 activation following collagen receptor signaling, further implying that vWF/p.V1316M acts directly on or downstream of Ca 2+ release. These data indicate that the vWD-type 2B mutation p.V1316M is associated with severe thrombocytopathy, which likely contributes to the bleeding tendency in vWD-type 2B.

LIM kinase/cofilin dysregulation promotes macrothrombocytopenia in severe von Willebrand disease-type 2B

“…Of note, it was reported that the Rho signaling pathway could be controlled by the small GTPase Rap1 (40). Interestingly, we have recently demonstrated in VWD-type 2B p.V1316M that the thrombopathy originates from an impaired activation of Rap1 (29). It is therefore tempting to speculate that a similar lack of Rap1 activation occurs in 2B MKs, thereby dysregulating the RhoA/ROCK/LIMK/cofilin pathway, which subsequently provokes destabilization of the actin network.…”

“…An example hereof is the bleeding disorder in VWD-type 2B, in which thrombopathy (29) and macrothrombocytopenia are part of complex clinical manifestations observed in patients (13). Long considered a paradoxical bleeding disorder because gain-of-function mutations of plasma vWF should Figure 9.…”

N-linked glycans within the A2 domain of von Willebrand factor modulate macrophage-mediated clearance

“…15,27,28 Intriguingly, the majority of VWF amino acid substitutions associated with enhanced clearance are clustered within the A1 domain. 14 Emerging evidence suggests that at least some of these VWF mutations result in enhanced macrophage-mediated clearance in vivo.…”

Thrombocytopathy and type 2B von Willebrand disease

“…While thrombocytopenia and the absence of high-molecular-weight vWF are reasonable explanations for type 2B bleeding, the study by Casari et al (8) in this issue of the JCI adds an additional explanation. Their current work suggests a third mechanism for type 2B bleeding; thrombocytopathy as a result of dysregulated platelet signaling is a consequence of variant vWF binding to platelets ( Figure 1B).…”

“…How the interaction of a mutant ligand or receptor in the vWF/GP-Ib-IX axis leads to signaling dysregulation awaits more detailed investigation. decreased activation of the platelet fibrinogen integrin receptor, αIIbβ3, as a consequence of type 2B vWF binding to platelet GP-Ib-IX (8). Indeed, decreased activation of αIIbβ3 would significantly impact hemostasis.…”