Protein Kinase C Signaling Dysfunction in Von Willebrand Disease (p.V1316M) Type 2B Platelets

von Willebrand disease (VWD) type 2B is characterized by gain-of-function mutations in von Willebrand factor (VWF), enhancing its binding affinity for the platelet receptor glycoprotein (GP)Ibα. VWD type 2B patients display a bleeding tendency associated with loss of high-molecular-weight VWF multimers and variable thrombocytopenia. We recently demonstrated that a marked defect in agonist-induced activation of the small GTPase, Rap1, and integrin αIIbβ3 in VWD (p.V1316M) type 2B platelets also contributes to the bleeding tendency. Here, we investigated the molecular mechanisms underlying impaired platelet Rap1 signaling in this disease. Two distinct pathways contribute to Rap1 activation in platelets: rapid activation mediated by the calcium-sensing guanine nucleotide exchange factor CalDAG-GEF-I (CDGI) and sustained activation that is dependent on signaling by protein kinase C (PKC) and the adenosine 5'-diphosphate receptor P2Y12. To investigate which Rap1 signaling pathway is affected, we expressed VWF/p.V1316M by hydrodynamic gene transfer in wild-type and mice. Using αIIbβ3 integrin activation as a read-out, we demonstrate that platelet dysfunction in VWD (p.V1316M) type 2B affects PKC-mediated, but not CDGI-mediated, activation of Rap1. Consistently, we observed decreased PKC substrate phosphorylation and impaired granule release in stimulated VWD type 2B platelets. Interestingly, the defect in PKC signaling was caused by a significant increase in baseline PKC substrate phosphorylation in circulating VWD (p.V1316M) type 2B platelets, suggesting that the VWF-GPIbα interaction leads to preactivation and exhaustion of the PKC pathway. Consistent with PKC preactivation, VWD (p.V1316M) type 2B mice also exhibited marked shedding of platelet GPIbα. In summary, our studies identify altered PKC signaling as the underlying cause of platelet hypofunction in p.V1316M-associated VWD type 2B.