PADGEM: An Adhesion Receptor for Leukocytes on Stimulated Platelets and Endothelial Cells

Abstract

Platelets are anucleate cells that circulate in the blood in a resting form. At the site of tissue injury, platelets undergo a variety of morphological and biochemical modifications. These include a shape change with the formation of pseudopodia (1); the secretion of the content of storage granules, namely the α-granules, the dense granules, and the lysosomes (2); and the expression of new receptors (3) and of procoagulant activity (4) on the cell surface. In vitro, this activation process can be triggered by numerous agonists, such as thrombin, collagen, epinephrine, ADP, and the calcium ionophore A23187 (5). Upon activation, platelets also shed membrane microparticles rich in procoagulant activity (6).

Using a monoclonal antibody, KC4, raised against activated platelets that does not bind to unstimulated platelets, we identified a novel protein that is expressed on the surface of platelets only after their activation (7). This protein was designated platelet activation-dependent granule to external membrane protein (PADGEM) (8).

Using a similar approach and a monoclonal antibody called S12, McEver and Martin (9) identified an antigen expressed on the surface of platelets only after their activation; although originally thought to be glycoprotein (GP) IIa, this antigen was subsequently proven to be a novel protein, since termed granule membrane protein 140 (10). Cooperative studies led to the conclusion that PADGEM and GMP 140 are the same protein. This protein has also been given the designations LECAM 3 (11) and CD62 (12); recently we and our colleagues have agreed to a new nomenclature in which this protein has been renamed P-selectin.

Subcellular Localization and Cell Distribution of PADGEM

Immunoflourescence staining of resting platelets with KC4 is negative, but permeabilized platelets show a punctate pattern of fluorescence in the cell interior. Thrombin-stimulated platelets that are not permeabilized stain with a rim pattern, demonstrating the translocation of PADGEM from an internal location to the cell surface upon activation (8).