Abstract 5885: Biomechanical Properties of the Glycoprotein Ib-von Willebrand Factor Interaction Under Flow
Binding of von Willebrand Factor (VWF) to platelet glycoprotein Ib (GPIb) is essential for arterial thrombus formation. Establishing whether the VWF-GPIb interaction is supported by a catch bond (increasing adhesive strength with increasing tensile stress) or a slip bond (decreasing adhesive strength with increasing tensile stress) is a key to understanding the biomechanical regulation of platelet adhesion. To address this question, we perfused plasma-free blood cell suspensions over immobilized VWF A1 domain (VWFA1) at shear rates between 100 and 15,000 s−1. Platelet adhesion was monitored by epifluorescence microscopy, using mepacrine-labeled platelets, or reflection interference contrast microscopy, which resolves the contact area between platelet membrane and adhesive surface without requiring labeling. The velocity of platelets rolling on the surface was measured as a reflection of the lifetime (off-rate) of the bonds supporting adhesion. We found a non-significant decrease in velocity at shear rate between 100 and 500 s−1 (from 6.48±0.29 to 6.31±0.25 μm/s; p=0.27) followed by a gradual increase to 11.25±0.69 μm/s that leveled off between 2000 and 3000 s−1. The same results were obtained with VWFA1-coated polystyrene beads perfused with erythrocytes over immobilized GPIb. The initial drop in velocity was replaced by a significant increase (from 3.54±1.39 μm/s at 100 s−1 to 6.62±0.66 μm/s at 500 s−1; p<0.001) when platelets were perfused in the absence of erythrocytes. The difference was due entirely to the higher translocation velocity at the lower shear rate in the presence than absence of erythrocytes, suggesting that the latter interfere with platelet adhesion in slow flow. Of note, platelet velocity progressively decreased as a function of exposure to light, from a median of 2.94 to 0.98 μm/s (p<0.001) in 2 min at a shear rate of 100 s−1. This may help explain previous data indicating a catch-bond behavior at lower shear rates (Yago et al, J Clin Invest, 118: 3195–3207, 2008). Instead, we conclude that VWF binding to GPIb transition from a slip to a catch bond, with enhancing adhesive strength, at shear rate above 3000 s−1 (B. de Laat is a Visiting Investigator from Department of Plasma Proteins, Sanquin Research, Amsterdam, The Netherlands).