To investigate the possibility that thrombin and/or other platelet activators change the platelet surface expression of glycoprotein IV (GPIV, CD36), we used a panel of five GPIV-specific monoclonal antibodies (OKM5, 5F1, FA6-152, 8A6, and F13) directed against different epitopes. All these antibodies bound to resting platelets in a concentration-dependent and saturable manner, as determined by flow cytometry of washed platelets. Thrombin (1 U/mL) induced an approximately twofold increase in the platelet surface binding of each of these monoclonal antibodies. Immunofluorescence microscopy demonstrated an internal pool of GPIV that, after thrombin stimulation, redistributed to the platelet surface. In a whole-blood fiow-cytometric assay, a-thrombin and the G lycoprotein (GP) IV, also known as GPIIIb and CD36, is present on the surface of human platelets and other cells, including endothelial cells, monocytes, erythrocytes, and megakaryocytes. 1 GPIV is composed of a single highly glycosylated polypeptide chain with an apparent molecular mass of 88 kD.2 - 3 The structure of GPIV has several homologies with a recently described lysosomal integral membrane protein (LIMP II), suggesting that GPIV and LIMP II belong to a new gene family. 4 There are between 12 000 and 25 000 copies of GPIV on the platelet surface.
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6GPIV is very resistant to protease digestion. 78 GPIV may play a number of important roles in platelet function and pathophysiology. First, GPIV has been reported to be a collagen receptor involved in the initial phase of platelet-collagen adhesion. 910 Second, GPIV has been reported to be a receptor for thrombospondin 3 ' 11 and may therefore have a role in thrombospondin-dependent platelet aggregation 1213 and thrombospondin-dependent platelet-monocyte adhesion. 14 Third, platelet GPIV is physically associated with the fyn, lyn, and yes protein tyrosine kinases and may be involved in platelet signal transduction.1 -15 Fourth, GPIV is a receptor for the adhesion of Plasmodium falciparumAniected erythrocytes. 1617 Fifth, GPIV may have a role in the pathophysiology of thrombotic thrombocytopenic purpura (TTP), because the plasma of a Received November 5, 1993; revision accepted April 26, 1994 The platelet-specific antigen Nak* is present on GPIV.
23- 24 The Nak a -negative phenotype occurs with a high incidence (=3%) in Japanese blood donors 25 but occurs in only 0.2% of US blood donors.1 Because they lack detectable GPIV, 2324 the platelets of Nak'-negative individuals have been used to study the role of GPIV in platelet function. Nak'-negative individuals show reduced adhesion to collagen in flowing whole blood.
10The lack of evidence of a hemostatic defect in Nak'-negative individuals suggests that binding of collagen to other proteins (eg, the GPIa-IIa complex 26 and GPVI 27 ) may be sufficient to avoid an overt bleeding diathesis. Thrombospondin binding to the platelets of Nak'-negative individuals appears to be normal, 2829 which calls into question the role of GPIV as a thrombospondin...