Flow-induced elongation of von Willebrand factor precedes tension-dependent activation

Abstract

Von Willebrand factor, an ultralarge concatemeric blood protein, must bind to platelet GPIbα during bleeding to mediate hemostasis, but not in the normal circulation to avoid thrombosis. Von Willebrand factor is proposed to be mechanically activated by flow, but the mechanism remains unclear. Using microfluidics with single-molecule imaging, we simultaneously monitored reversible Von Willebrand factor extension and binding to GPIbα under flow. We show that Von Willebrand factor is activated through a two-step conformational transition: first, elongation from compact to linear form, and subsequently, a tension-dependent local transition to a state with high affinity for GPIbα. High-affinity sites develop only in upstream regions of VWF where tension exceeds ~21 pN and depend upon electrostatic interactions. Re-compaction of Von Willebrand factor is accelerated by intramolecular interactions and increases GPIbα dissociation rate. This mechanism enables VWF to be locally activated by hydrodynamic force in hemorrhage and rapidly deactivated downstream, providing a paradigm for hierarchical mechano-regulation of receptor–ligand binding.

Platelet‐type von Willebrand disease: Local disorder of the platelet GPIbα β‐switch drives high‐affinity binding to von Willebrand factor

“…A rheologically malleable bond between the von Willebrand factor (VWF) and platelets is essential to primary hemostasis and necessary for subsequent coagulation and the arrest of bleeding. This bond between the VWF A1 domain and the platelet VWF receptor, GPIbα, occurs under the rheological shear of blood flow, which unravels large multimeric species of VWF exposing the A1 domains to platelet GPIbα . It must be strong enough to recruit platelets to sites of vascular injury, weak enough so as to not be prothrombotic, but not too weak as to obviate the interaction altogether.…”

Accumulations of von Willebrand factor within ECMO oxygenators: Potential indicator of coagulation abnormalities in critically ill patients?

“…ADAMTS13 deficiency (due to consumption and/or decreased production) results in high levels of plasmatic HMWvWF multimers, in turn inducing platelet aggregation or thrombus formation under high shear stress, followed by thromboembolic complications and thrombocytopenia. This is simply due to the increased number of presented binding domains accompanied by stronger mechanical forces applied to the domains resulting from larger molecules . Low concentrations of ADAMTS13 are also associated with an increased risk of arterial thrombosis including myocardial infarction and cerebrovascular disease .…”

“…Due to the size of HMWvWF multimers, even in a (re‐)coiled state, the flow around the gas exchange fibers is able to exert forces high enough to elongate the HMWvWF multimers (>50 Pa) and thus, expose the binding domains permanently. This shear‐induced extension of vWF multimers is a function of the applied forces . Originating from the initial depositions in low flow/low shear areas (eg, the contact region of two crossing gas exchange fibers), the attached HMWvWF multimers protrude into the high flow/high shear regions in the narrow gap between two neighboring gas exchange fibers (Figure ).…”

“…14 The additional localization of Pselectin-positive regions close to the vWF-structures indicates platelet accumulations due to pathologically high shear rates (>5000 s −1 ). 14,32,33 Shear stress induced a conformational change from a compacted, globular state to a stretched, elongated conformation of the vWF multimers. 34 The formation of long vWF-fibers (>20 µm) or -nets in platelet-free plasma under pathologically high shear rates was demonstrated under in vitro conditions using collagen-coated as well as in collagen-free micropost-impingement microfluidic devices.…”

“…This is simply due to the increased number of presented binding domains accompanied by stronger mechanical forces applied to the domains resulting from larger molecules. 33 Low concentrations of ADAMTS13 are also associated with an increased risk of arterial thrombosis 40 including myocardial infarction 41 and cerebrovascular disease. 42,43 In the present study, due to the lack of laboratory measurements of vWF and ADAMTS13 levels/activities in plasma no correlations with the extent of vWF deposits on the gas exchange fibers can be drawn.…”