Molecular Mechanisms of Platelet Adhesion and Platelet Aggregation
Certain primitive cell systems, such as cellular slime molds, exist either in unicellular vegetative forms or in a differentiated state in which they become adhesive and aggregate into a multicellular structure. An example of this mechanism of cellular behavior is shown in Figure 1, which illustrates the life cycle of Acrasis rosea, an acrasid cellular slime mold. The uninucleate ameboid cells move, divide by binary fission, deplete the local environment of the food supply, aggregate, and form fruiting bodies. The mechanisms associated with the development of the adhesive state have been studied most extensively in the dictyostelid cellular slime molds. When the cells of this species become adhesive, they synthesize surface polyvalent carbohydrate-binding proteins or lectins that mediate cell to cell adhesion (Frasier and Glaser, 1979; Barondes, 1981). This stage of the cell cycle is associated with activation of a large number of new genes (Blumberg et al., 1982), presumably coding for cell-surface proteins that mediate the conversion to the adhesive state. Cell adhesion culminating in aggregation takes place via the interaction of carbohydrate-binding sites of a cell-surface lectin with oligosaccharides on membrane glycoprotein receptors. The initial interaction of one lectin molecule with one receptor oligosaccharide followed by binding at multiple sites leads to rapid and stable cohesion (aggregation). Discoidin I and II, two closely related galactose-binding lectins synthesized by Dictyostelium dis coideum during the conversion from the noncohesive to the aggregating cohesive stage, may function at different stages of the aggregation process by binding to different oligosaccharide-containing receptors at the cell surface (Berger and Armant, 1982). Thus, at least in primitive systems, the aggregation process appears to involve the sequential exposure of a family of cell-surface recognition molecules, some of which serve as lectins. It is probable that human platelets may recapitulate some of these primitive cellular responses during the process of agonist-induced aggregation.
KeywordsPlatelet Aggregation Human Platelet Platelet Adhesion Platelet Membrane Fibrinogen Binding
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