Binding of adhesive proteins to cellular receptors plays a role in wound healing, hemostasis and bacterial clearance. A platelet adhesive protein receptor contains Glycoprotein (GP) IIb-IIIa and is functionally, structurally, and antigenically related to membrane proteins of fibroblasts, endothelial cells, and mononuclear cells. GPIIb-IIIa function is regulated by divalent cations and is abnormal in variant thrombasthenmia. Changes in the functional state of this receptor seem to correlate with the surface availability of a region of GPIIb identified by the PMI-1 monoclonal antibody. The PMI-1 antibody also inhibits platelet adhesion to collagen. We will investigate the basis of these alterations in PMI-1 epitope expression and adhesive protein receptor dysfunction with a long range goal of evaluation of such dysfunction in trauma patients. We will utilize radioligand binding assays to analyze divalent cation effects on PMI-1 epitope expression in comparison to adhesive protein receptor function and the state of assembly of the GPIIb-IIIa heterodimer in platelets, their membranes, and in detergent solution. We will utilize chemical or enzymatic fragmentation of GPIIb to prepare digests which contain the PMI-1 epitope. Fragments bearing that epitope will be isolated and characterized and utilized for production of additional PMI-1 like mono and polyclonal antibodies. The electrophoretic mobilities of the PMI-1 bearing fragments will be compared to those from patients with variant thrombasthenmia characterized by abnormal receptor function and PMI-1 regulation. The capacity of these antibodies to inhibit platelet adhesion to collagen and to detect adhesion receptor dysfunction in other cell types will be evaluated. In vitro assays will be used to determine the substrate specificity PMI-1 inhibition of adhesion. To test the hypothesis that the epitope is regulated during contact between cells and surfaces, we will quantitate PMI-1 binding during cell attachment in the presence or absence of cell spreading. Finally, we will employ immunofluorescence and immunoelectron microscopy of vertical frozen sections to analyze the topography of PMI-1 epitope exposure during cell attachment.
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