Real time imaging analysis of platelets aggregation and thrombus formation under shear stress

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[Background] After secretion from Weibel-Palade body in vascular endothelial cells (VECs), von Willebrand factor (vWF) circulates as ultra-large multimer forms (ULM-vWF) and adheres to subendothelial collagen, to which platelets subsequently adheres to form platelet thrombi. Lacking of vWF leads to bleeding tendency (vWF disease) and excess amounts of ULM-vWF leads to thrombotic disease (thrombotic thrombocytopenic purpura). [Aim] Employing gene deficiency mouse of vWF cleaving enzyme (ADAMTS13) hybridized with green fluorescent protein (GFP) expressing transgenic mouse (KO mouse), we analyzed the dynamics of vWF exocytosis as well as platelet adhesion and its aggregation under shear stress by intra-vital confocal microscopy. [Results] (1) DDAVP, known to stimulate vWF exocytosis, increased ULM-vWF which was visualized as long strings using fluorescent labeled anti-vWF antibody. (2) The strings were longest at 5th minute, and longer in KO mice (5.28±4.28 vs 2.89±2.08 μm). (3) GFP expressing platelets adhered to being secreted ULM-vWF. (4) After topical infusion of 2.5% FeC12, longer (26.34±8.28 vs 10.73±3.11 μm) and more stable (2.35±1.23 vs 0.63±0.21 s) strings were visualized in KO mice. [Conclusion] (1) Platelets adhesion and aggregation were successfully monitored by intra-vital confocal microscopy. (2) ADAMTS13 appeared to cleave ULM-vWF under shear stress when it is being secreted. (3) The enhanced secretion itself, however, did not develop microthrombus even in KO mice. (4) Longer and more stable ULM-vWF adhered to injured VECs by ferric chloride treatment in KO mice. (5) Larger thrombus was formed in KO-mice at the initial phase after laser-irradiated injury of vascular wall, whereas there was no difference in the size of stabilized thrombus. (6) ADAMTS13 was shown to be responsible for the cleavage of ULM-vWF under shear stress, and lack of its activity appeared to be thrombogenic.