Role of the membrane skeleton in preventing the shedding of procoagulant-rich microvesicles from the platelet plasma membrane.

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Abstract
The platelet plasma membrane is lined by a membrane skeleton appears to contain short actin filaments cross-linked by actin-binding protein. Actin-binding protein is in turn associated with specific plasma membrane glycoproteins. The aim of this study was to determine whether the membrane skeleton regulates properties of the plasma membrane. Platelets were incubated with agents that disrupted the association of the membrane skeleton with membrane glycoproteins. The consequences of this change on plasma membrane properties were examined. The agents that were used were ionophore A23187 and dibucaine. Both agents activated calpain (the Ca2+-dependent protease), resulting in the hydrolysis of actin-binding protein and decreased association of actin with membrane glycoproteins. Disruption of actin-membrane interactions was accompanied by the shedding of procoagulant-rich microvesicles from the plasma membrane. The shedding of microvesicles correlated with the hydrolysis of actin-binding protein and the disruption of actin-membrane interactions. When the calpain-induced disruption of actin-membrane interactions was inhibited, the shedding of microvesicles was inhibited. These data are consistent with the hypothesis that association of the membrane skeleton with the plasma membrane maintains the integrity of the plasma membrane, preventing the shedding of procoagulant-rich microvesicles from the membrane of unstimulated platelets. They raise the possibility that the procoagulant-rich microvesicles that are released under a variety of physiological and pathological conditions may result from the dissociation of the platelet membrane skeleton from its membrane attachment sites.