Circulating but not immobilized N-deglycosylated von Willebrand factor increases platelet adhesion under flow conditions.

The role of von Willebrand factor (VWF) as a shear stress activated platelet adhesive has been related to a coiled-elongated shape conformation. The forces dominating this transition have been suggested to be controlled by the proteins polymeric architecture. However, the fact that 20% of VWF molecular weight originates from glycan moieties has so far been neglected in these calculations. In this study, we present a systematic experimental investigation on the role of N-glycosylation for VWF mediated platelet adhesion under flow. A microfluidic flow chamber with a stenotic compartment that allows one to mimic various physiological flow conditions was designed for the efficient analysis of the adhesion spectrum. Surprisingly, we found an increase in platelet adhesion with elevated shear rate, both qualitatively and quantitatively fully conserved when N-deglycosylated VWF (N-deg-VWF) instead of VWF was immobilized in the microfluidic channel. This has been demonstrated consistently over four orders of magnitude in shear rate. In contrast, when N-deg-VWF was added to the supernatant, an increase in adhesion rate by a factor of two was detected compared to the addition of wild-type VWF. It appears that once immobilized, the role of glycans is at least modified if not-as found here for the case of adhesion-negated. These findings strengthen the physical impact of the circulating polymer on shear dependent platelet adhesion events. At present, there is no theoretical explanation for an increase in platelet adhesion to VWF in the absence of its N-glycans. However, our data indicate that the effective solubility of the protein and hence its shape or conformation may be altered by the degree of glycosylation and is therefore a good candidate for modifying the forces required to uncoil this biopolymer.

Relaxation of ultralarge VWF bundles in a microfluidic-AFM hybrid reactor.

The crucial role of the biopolymer "Von Willebrand factor" (VWF) in blood platelet binding is tightly regulated by the shear forces to which the protein is exposed in the blood flow. Under high-shear conditions, VWFs ability to immobilize blood platelets is strongly increased due to a change in conformation which at sufficient concentration is accompanied by the formation of ultra large VWF bundles (ULVWF). However, little is known about the dynamic and mechanical properties of such bundles. Combining a surface acoustic wave (SAW) based microfluidic reactor with an atomic force microscope (AFM) we were able to study the relaxation of stretched VWF bundles formed by hydrodynamic stress. We found that the dynamical response of the network is well characterized by stretched exponentials, indicating that the relaxation process proceeds through hopping events between a multitude of minima. This finding is in accordance with current ideas of VWF self-association. The longest relaxation time does not show a clear dependence on the length of the bundle, and is dominated by the internal conformations and effective friction within the bundle.