Dynamics of membrane-cytoskeleton interactions in activated blood platelets

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Abstract
The dynamics of actin polymerization, cytoskeleton formation and interaction with membrane and cytoplasmic proteins as a result of [human] platelet activation by temperature ADP or thrombin were studied. The polymerization of .apprx. 30% of platelet DNase I available actin to a nonavailable state was rapid and complete within 10 s after platelet activation with ADP and thrombin. This polymerization might be related to shape change rather than to aggregation or secretion. A similar value of actin polymerization was obtained when platelets were induced to change shape by cooling. This polymerization was partially reversible upon deactivation of the platelets by apyrase, hirudin or rewarming. Cycles of temperature-mediated activation and deactivation showed a cyclic variation in the state of actin, with a tendency to refractivity to further changes after a couple of cycles. No correlation was observed between microtubule integrity and actin polymerization when studies were performed with platelets pretreated with colchicine. Analysis of the Triton residue composition showed that the cytoskeleton of resting platelets was composed mainly of actin and myosin in a 4.5:1 ratio. Activation with ADP and thrombin lead to the association and incorporation of several other proteins (actin binding protein, 95,000 daltons, 3-4 proteins in the 35,000-dalton region and 2 proteins in the 17,000-dalton region with the cytoskeleton). The incorporation of these proteins had a dynamic nature that depended on both the state of aggregation and the reversibility of the activation. Activation led to a significant increase in the total cytoskeletal proteins, and although low temperature also induced such an increase, the cytoskeletal pattern of cooled platelets was not different from that of resting platelets. A complete reversibility in morphology and amount of protein was observed with temperature cycling. The dynamic nature of the state of actin in platelets is discussed.