Biomechanical Properties of Bonds Mediated by the von Willebrand Factor A1 Domain during Platelet Adhesion under Flow Conditions.

The initial attachment of platelets to vascular lesions exposed to a high wall shear rate (γw) depends on the interaction between the membrane glycoprotein (GP) Ibα, a component of the GP Ib-IX-V receptor complex, and the A1 domain of surface-immobilized von Willebrand factor (VWFA1). We performed perfusion experiments under different flow conditions to measure transient platelet contacts onto immobilized recombinant VWFA1 and determine the probability of bond formation (capturing) and resistance to tensile stress (bond lifetime) of VWFA1-GP Ibα interactions. To define how molecular conformations influence the biomechanical properties of the bonds, we compared fragments exhibiting the native dimeric assembly of A1 domain with monomeric fragments obtained by selective purification of recombinant protein expressed in stable D. melanogaster cell lines. The minimum coating concentration of dimeric VWFA1 at which platelet adhesion events were statistically significantly different from nonspecific interactions on uncoated glass was 1 μg/ml. In the range of γw between 30 and 30,000 s⁻¹, there was no threshold value for the initiation of adhesion, as seen for selectins. The number of adhering platelets first increased and then decreased with monotonically increasing γw, indicating the effect of transport phenomena as well as hydrodynamic forces on the VWFA1-GP Ibα interaction. Maximum event number was at 5,000 s⁻¹ for dimeric and 1,500 s⁻¹ for monomeric VWFA1. The platelet count had no statistically relevant influence on the efficiency of capturing and duration of adhesive contacts. As γw increased, a higher coating concentration of the VWF A1 domain was required to initiate platelet adhesion. The coating concentration determined the number of individual adhesion events that could occur over a defined period of time but did not affect the residence time, which is a measure of the strength of the bond between the receptor and the ligand. Hydrodynamic forces generated by blood flow shortened the duration of VWF-GP Ibα interactions. The percentage of platelets that had a residence time of less than 0.1 s increased almost linearly with increasing γw. Dimeric A1 domain was more efficient than the monomeric counterpart in promoting platelet adhesion as it displayed activity at lower coating concentrations. At permissible γw, a 10-fold higher monomer than dimer coating concentration (2 vs. 20 μg/ml) was required to obtain a similar capturing efficiency. Moreover, the upper limit of γw compatible with the initiation of adhesion was significantly higher for dimeric as compared to monomeric A1 domain. Doubling the dimer coating concentration resulted in a 5-fold increase in the γw limit for adhesion, but the same increase in the monomer coating concentration did not enhance the probability of bond formation at higher γw. In spite of the substantial difference in capturing efficiency, monomeric and dimeric VWFA1 supported platelet adhesion events of similar duration at any given γw, indicating that a different molecular conformation did not affect the lifetime of the interaction with GP Ibα. These results indicate that the dimeric assembly of A1 domains in VWF multimers may be crucial to support the initiation of platelet adhesion at high shear rates, but the duration of each adhesion event is limited by intrinsic properties of the individual VWFA1-GP Ibα bond.