von Willebrand Factor (vWF) is a large plasma glycoprotein which is required for platelets to recognize and bind to damaged endothelial surfaces. Bovine vWF binds directly to human platelets in a manner which is a model for the interaction of human vWF with platelets on subendothelial surfaces. The goals of the project are: (a) to identify the regions of the vWf molecule which are responsible for its binding to platelets, to collagen, and to glycosaminoglycans, (b) to identify the platelet receptor for vWF and determine the mechanism by which vWF binds to platelets, and (c) to describe the effects of vWF binding on the metabolic responses of the platelets, and, conversely, the effects of other agonists on platelet responsiveness to vWF. The functional domains on the vWF molecule will be identified and characterized by proteolytic fragmentation of the vWF, by chemical and enzymatic modification of vWF, and by use of monoclonal antibodies to vWF. The platelet receptor for vWF will be isolated by affinity chromatography and will also be identified in situ by photoaffinity labeling. Platelet responsiveness will be studied by measuring vWF binding, aggregation responses, shape change and the platelet release reaction. Alterations in platelet protein labeling will be monitored and the distribution of receptors on activated and non-activated platelets will be visualized by electron microscopy with gold-conjugated specific probes for the receptors. These studies should provide a better understanding of the basic mechanisms underlying platelet recognition of sites of damage in blood vessels. They may suggest new ways to deal with the thrombotic complications associated with diseased or damaged blood vessels or with artificial surfaces such as prosthetic heart valves, dialysis membranes, or vascular prostheses.
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