Understanding the Regulation of the Shear-Enhanced Binding of the Platelet Protein GPIbalpha and the A1 Domain of von Willebrand Factor

Platelets are an integral part of the early steps of thrombus formation, particularly in areas of high shear. Thrombus formation in areas of high shear is mediated by binding between glycoprotein (GP) Ibalpha expressed on the surface of platelets as a member of the GPIb-IX-V complex and the A1 domain of von Willebrand Factor (VWF). Platelets and VWF must however be able to co-exist in the blood without interaction except in the case of injury when they muct be able to quickly and efficiently form a clot limiting blood loss. This requirement suggests the presence of a sophisticated regulatory mechanism. The GPIbalpha-A1 bond is regulated by a combination intradomain and interdomain interactions in VWF as well as by signaling events in platelets. Using protein constructs which have had different domains outside of the A1 domain deleted, shear-based assays and single force spectroscopy experiments suggest that intradomain interaction is part of an important regulatory mechanism, in which neighboring domains shield the GPIbalpha binding site on the A1 domain. In an effort to further refine our understanding of interdomain interaction and to determine the role of force induced intradomain interactions, a reliable, well characterized system for studying a large number of protein constructs was established. Implementing this system, it has been demonstrated that both application of force across the Al domain of VWF and the presence of the N-terminal linker chain, a region of VWF which links the A1 domain to the D'D3 domain are crucially responsible for GPlbalpha-A1 bond regulation. Lastly, it has been demonstrated that signaling induced by interactions with the GPIbalpha cytoplasmic tail are crucial in binding of platelets from whole blood to VWF coated surfaces, but have little influence on the behavior of washed platelets binding to A1 coated surfaces in our assays. Our increased knowledge about the regulation of platelet binding to VWF may aid in the design of therapies to treat a wide range of diseases which are caused by both unwanted increases and unwanted decreases in clot formation.