Effect of slip bonds due to type 2B von Willebrand Disease on platelet adhesion, aggregation, and contractile forces

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PROJECT SUMMARY/ABSTRACT Von Willebrand Disease (VWD) is characterized by a defect in the blood protein von Willebrand Factor (VWF) and leads to uncontrolled bleeding. VWF is necessary for initial platelet tethering and subsequent platelet adhesion to the injured vasculature. Glycoprotein Ib (GPIb) is a receptor found on the platelet surface that recognizes the A1 domain of VWF. When the vascular endothelium is damaged, VWF binds to exposed fibrillar collagen and to GPIb, tethering the platelet to the surface. Then, cytoskeletal forces acting through integrin αIIBβ3 allow platelets to adhere and aggregate, forming a strong and stable platelet-rich plug. The GPIb-A1 bond forms under conditions of high shear and is characterized as a “catch” bond, meaning it has a longer lifetime under conditions of increased tensile force. In type 2B VWD, a gain-of-function mutation to VWF causes an increased affinity for platelet binding at low shear. The type 2B VWF mutation leads to spontaneous platelet binding and subsequently uncontrolled bleeding. Previous work has demonstrated that with type 2B VWD, VWF and GPIb no longer form a catch bond but instead a “slip” bond, which is characterized by a shorter bond lifetime under conditions of increased tensile force. Additionally, platelet cytoskeletal forces can act independently of integrin αIIBβ3 through the GPIb-A1 bond. However, prior work has failed to fully explain how slip bonds in type 2B VWD affect the mechanics of platelet adhesion and platelet plug formation. I hypothesize that changes in the force- lifetime relationship of GPIb-A1 bonds due to type 2B VWD will affect platelet adhesion and aggregation due to an inability to withstand cytoskeletal forces. Platelet adhesion and contractile force will be measured at the single platelet level using flexible, cantilever-like nanoposts, while the contractile force generated by a platelet aggregate will be quantified using a microfluidic device. Using these methods, I can characterize how the biomechanics of platelet plug formation are affected by type 2B VWD, further explaining the bleeding phenotype. In the future, this knowledge relating to hemostasis initiation can be utilized to diagnose or treat type 2B VWD.