Abstract
Von Willebrand factor (VWF) is a plasma glycoprotein that mediates platelet adhesion at sites of vessel injury. It is synthesized in megakaryocytes and endothelial cells and is assembled in the endoplasmic reticulum and Golgi into an array of multimers. Upon secretion from microvascular endothelium, VWF multimers can further self-associate under shear stress and form surface-bound fibers of potentially enormous sizes capable of spanning the lumens of vessels up to 300 mm in diameter (Zheng et al. Nature Communications 2015 In press). These structures are normally removed by the plasma metalloprotease ADAMTS13. However, when ADAMTS13 is inactivated or when massive VWF secretion overwhelms the capacity of ADAMTS13 to process VWF, these structures persist in the microcirculation and bind platelets avidly to form occlusive thrombi, a process characteristic of the devastating disease thrombotic thrombocytopenic purpura (TTP). These microvascular VWF-platelet thrombi have also been implicated in the microvascular dysfunction that accompanies malaria, sickle cell disease, and sepsis. We recently identified high density lipoprotein particles (HDL) as being able to prevent VWF self-association into thick strands (Chung et al. Blood 2015 in revision). In these studies, we also studied VWF self-association in citrated human plasma under shear stress in a test tube in the presence of EDTA (to inhibit ADAMTS13). VWF self-associated and adsorbed to the tube surface, a phenomenon prevented by addition of HDL at concentrations above those already present in plasma. When EDTA was not added to the plasma, the majority of the VWF was not cleaved but was nevertheless stabilized in solution. This result suggests that when ADAMTS13 has been progressively inactivated by citrate at 37°C, it is able to prevent VWF self-association. It is not clear why EDTA-inhibited ADAMTS13 did not stabilize VWF to the same extent as citrate-inhibited ADAMTS13. It is possible that EDTA and citrate have different effects on the stabilization function of ADAMTS13. Further, addition of recombinant ADAMTS13 to citrated plasma (final ratio VWF monomer:ADAMTS13 = 1.6:1) did not enhance VWF cleavage under shear, but completely stabilized the VWF multimers. These results demonstrate a new function for ADAMTS13: it regulates VWF adhesive activity by preventing VWF self-association through direct binding instead of cleavage. Therefore, we hypothesize that the relative levels of VWF, HDL, and ADAMTS13 in plasma regulate the propensity of VWF multimers to self-associate under shear stress. While high VWF levels and high shear stress favor VWF self-association, high HDL and ADAMTS13 levels prevent self-association. We tested the hypothesis with plasma from wild-type or knockout mice on the C57BL6 background. In comparison to humans, wild-type C57BL6 mice have low VWF levels, high HDL levels (calculated from HDL-cholesterol levels), and express a truncated version of ADAMTS13. Further, ADAMTS13-deficient C57BL6 mice do not spontaneously develop microvascular occlusion. Unlike human citrated plasma, when citrated plasma from wild-type mice was sheared in the presence of EDTA, the VWF multimers did not self-associate. We attributed this difference from human plasma to the low VWF:HDL ratio in this mouse strain. When the plasma from apolipoprotein (Apo) A-I knockout mice was sheared in the presence of EDTA, the VWF multimers also did not self-associate, which we attributed to the low VWF level and the ability of EDTA-inhibited truncated ADAMTS13 to stabilize VWF. When the plasma of a double knockout of ApoA-I and ADAMTS13 was sheared, the VWF self-associated and adsorbed to the tube surface. Addition of HDL to this double knockout plasma stabilized the VWF. The VWF antigen levels in wild-type, single and double knockout mouse plasma were comparable. Double knockout mice challenged with a bolus injection of VWF developed more severe thrombocytopenia than did mice with either single ApoA-I or ADAMTS13 deficiency. Together, these results suggest that ADAMTS13 synergizes with HDL in stabilizing VWF and dampening its self-association into hyperadhesive forms under shear stress, and that interplay between concentrations of VWF, ADAMTS13, and HDL particles can determine the propensity for developing TTP and its severity once developed.
Disclosures
No relevant conflicts of interest to declare.
Activation of von Willebrand factor via mechanical unfolding of its discontinuous autoinhibitory module
