Platelet aggregation at sites of vascular injury is essential for hemostasis and arterial thrombosis. It has long been assumed that platelet aggregation and thrombus growth are initiated by soluble agonists generated at sites of vascular injury. By using high-resolution intravital imaging techniques and hydrodynamic analyses, we show that platelet aggregation is primarily driven by changes in blood flow parameters (rheology), with soluble agonists having a secondary role, stabilizing formed aggregates. We find that in response to vascular injury, thrombi initially develop through the progressive stabilization of discoid platelet aggregates. Analysis of blood flow dynamics revealed that discoid platelets preferentially adhere in low-shear zones at the downstream face of forming thrombi, with stabilization of aggregates dependent on the dynamic restructuring of membrane tethers. These findings provide insight into the prothrombotic effects of disturbed blood flow parameters and suggest a fundamental reinterpretation of the mechanisms driving platelet aggregation and thrombus growth.

Rupture of a vulnerable atherosclerotic plaque causes thrombus formation and precipitates cardiovascular diseases. In addition to the thrombogenic content of a plaque, also the hemodynamic microenvironment plays a major role in thrombus formation. How the altered hemodynamics around a plaque promote pathological thrombus formation is not well understood. In this study, we provide evidence that plaque geometries result in fluid mechanical conditions that promote platelet aggregation and thrombus formation by increased accumulation and activity of von Willebrand factor (vWF) at poststenotic sites. Resonant-scanning multiphoton microscopy revealed that in vivo arterial stenosis of a damaged carotid artery markedly increased platelet aggregate formation in the stenotic outlet region. Complementary in vitro studies using microfluidic stenotic chambers, designed to mimic the flow conditions in a stenotic artery, showed enhanced platelet aggregation in the stenotic outlet region at 60-80% channel occlusion over a range of input wall shear rates. The poststenotic thrombus formation was critically dependent on bloodborne vWF and autocrine platelet stimulation. In stenotic chambers containing endothelial cells, flow provoked increased endothelial vWF secretion in the stenotic outlet region, contributing to exacerbated platelet aggregation. Taken together, this study identifies a role for the shear-sensitive protein vWF in transducing hemodynamic forces that are present around a stenosis to a prothrombogenic microenvironment resulting in spatially confined and exacerbated platelet aggregation in the stenosis outlet region. The developed stenotic microfluidic chamber offers a realistic platform for in vitro evaluation of shear-dependent thrombus formation in the setting of atherosclerosis.

To explore the molecular basis of von Willebrand factor (VWF) clearance, an experimental model employing VWF-deficient mice was developed. Biodistribution was examined by the injection of radiolabeled VWF, which was primarily directed to the liver with minor amounts in other organs. Disappearance of VWF from plasma was characterized by a rapid initial phase (t1 ⁄2 ␣ ‫؍‬ 13 min) and a slow secondary phase (t1 ⁄2 ␤ ‫؍‬ 3 h), with a mean residence time (MRT) of 2.8 h. A similar clearance was observed for VWF consisting of only high or low molecular weight multimers, indicating that, in our experimental model, clearance is independent of multimeric distribution. This allowed us to compare the survival of fulllength VWF to truncated variants. Deletion of both the amino-terminal D -D3 and carboxyl-terminal D4-CK domains resulted in a fragment with a similar clearance to wild-type VWF. Deletion of only the D -D3 region was associated with an almost 2-fold lower recovery and increased clearance (MRT ‫؍‬ 1.6 h), whereas deletion of only the D4-CK region resulted in a significantly reduced clearance (MRT ‫؍‬ 4.5 h, p < 0.02). These results point to a role of the D -D3 region in preventing clearance of VWF. Furthermore, replacement of D3 domain residue Arg-1205 by His resulted in a markedly increased clearance (MRT ‫؍‬ 0.3 h; p ‫؍‬ 0.004). Therefore, this mutation seems to abrogate the protective effect of the D -D3 region. In vitro analysis of this mutant also revealed a 2-fold reduced affinity for VWF propeptide at low pH, showing that mutation of Arg-1205 results not only in an increased clearance rate but is also associated with an impaired pH-dependent interaction with VWF propeptide.von Willebrand factor (VWF) 1 is a multimeric plasma protein that participates in the hemostatic process. The absence of functional VWF is associated with an abnormal bleeding tendency known as von Willebrand Disease (VWD) (1, 2). VWF contributes to hemostasis in a dual manner. First, it promotes the adhesion of platelets at sites of vascular injury by acting as a molecular bridge between the sub-endothelial collagen matrix and the platelet-surface receptor complex glycoprotein (Gp) Ib␣/IX/V (3). In addition, VWF serves as a carrier protein for coagulation factor VIII (FVIII) in the circulation. This chaperone function results in stabilization of the FVIII heterodimeric structure (4) and protection of FVIII from premature clearance by the low density lipoprotein receptor-related protein (5, 6).VWF is produced and stored in endothelial cells and megakaryocytes. It is synthesized as pre-pro-VWF, a single chain polypeptide with the domain structure