Scanning Electron Microscopy Study of Endothelial Injury and Thrombus Formation in WT and VWF Deficient Mice.

Abstract 3062

Poster Board II-1038

VWF is a large plasma glycoprotein required for normal hemostasis, and performs its function through binding to coagulation Factor VIII, and via interactions with both platelet surface glycoproteins and the activated and/or damaged vascular surface. We have developed a scanning electron microscopy (SEM) protocol to visualize endothelial damage and thrombus formation in wild-type and VWF-deficient mice. Thrombus formation is initiated by ferric chloride, and subsequently at defined time points, the circulation is rapidly flushed and aldehyde fixed. The carotid artery is removed, externally fixed, sectioned (both longitudinally and in cross-section), processed for SEM, and visualized. With this protocol we have obtained high-quality images (exceeding 100,000x) of FeCl3-induced endothelial damage and thrombus formation in C57BL/6 and VWF-deficient mice at baseline, and at 30, 60, 90, 120, 240, and 300 seconds post-injury (please access http://sites.google.com/site/mottolab/ to view images). Interestingly, we find that FeCl3 induces little, if any, endothelial denudation and collagen exposure at these time points, with the endothelium clearly appearing changed from baseline, but not damaged. Thus, initial platelet adhesion seems to be occurring in the absence of collagen exposure in this model. In wild-type mice, platelets adhere rapidly to the endothelial surface and assume a cross-linked appearance by 90 seconds, with continual inward growth of the thrombus through the 300 second time point. In VWF-deficient mice, platelets also adhere rapidly to the endothelial surface, but in contrast, remain recognizable longer without assuming a highly-activated phenotype. Compared with wild-type, at all time points examined the VWF-deficient thrombus appears smaller with considerably less cross-linking and platelet activation. Interestingly, during the course of these experiments we also have identified what appears to be red blood cells (RBCs) participating in thrombus formation. Similar to platelets, RBCs interact directly with the endothelial surface, and subsequently become elongated in the direction of blood flow. These elongated RBCs are often observed to cluster and bind platelets, with the subsequent formation of large platelet-erythrocyte complexes. Further characterization of these complexes and the role they may play in thrombus formation is currently in progress. Additionally, similar SEM studies are underway with both ADAMTS13-deficient and GPIb alpha-deficient mice, and with mice transiently expressing in vivo biotinylated VWF for visualization of this molecule at high magnification and resolution. These studies should help better define the mechanisms of endothelial activation and thrombus formation as they occur in situ.