Force Sensing by the Vascular Protein Von Willebrand Factor is Tuned by a Strong Intermonomer Interaction

Abstract

The large plasma glycoprotein von Willebrand factor (VWF) senses hydrodynamic forces in the blood stream and responds to elevated forces with abrupt elongation, thereby increasing its adhesiveness to platelets and collagen. Remarkably, forces on VWF are elevated at sites of vascular injury, where VWF's hemostatic potential is important to mediate platelet aggregation and to recruit platelets to the subendothelial layer. Adversely, elevated forces in stenosed vessels lead to an increased risk of VWF-mediated thrombosis. To dissect the remarkable force sensing ability of VWF, we have performed atomic force microscopy (AFM)-based single-molecule force measurements on dimers, the smallest repeating subunits of VWF multimers. We have identified a strong intermonomer interaction that involves the D4 domain and critically depends on the presence of divalent ions. Dissociation of this strong interaction occurred at forces above ∼50 pN and provided ∼80 nm of additional length to the elongation of dimers. Corroborated by the static conformation of VWF, visualized by AFM imaging, we estimate that under physiological conditions approximately half of VWF's constituent dimers are firmly closed via the strong intermonomer interaction. As firmly closed dimers drastically shorten VWF's effective length contributing to force sensing, they can be expected to markedly tune VWF's sensitivity to hydrodynamic flow in the blood and to thereby significantly affect VWF's function in hemostasis and thrombosis.