Changes in the distribution and organization of platelet actin induced by diamide and its functional consequences

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Abstract
Exposure of blood platelets to diamide (azodicarboxylic acid-bis-dimethylamide) results in oxidation of sulphydryl groups present in the cytoskeleton and other proteins. This results in dramatic changes in functional behaviour of the cells. The distribution and level of organization of the major cytoskeletal protein actin has been studied analytically by the DNase-I inhibition assay and morphologically by electron microscopy (EM) of Triton X-100 treated platelets adherent to EM grids. Exposure to diamide results in a redistribution of actin within the cells reflected in an increase in cytoskeletal F-actin and a concomitant decrease in cytosolic actin. The magnitude of these changes depends upon the concentration of diamide and the time of exposure. Diamide also alters platelet aggregatory functions in response to certain stimuli. Treatment of normal human platelets with 0.1 mM diamide proceeds via disaggregation (5 min exposure to diamide), inhibition of aggregation (30 min exposure), to finally a normalization of the aggregation response after 60-120 min incubation with diamide. In parallel with the return to full functional response the distribution of F-actin between the cytoskeleton and cytoplasmic compartments returns to the control pattern. Incubation of the platelets with 0.5 mM diamide for 60 or more minutes leads to total inhibition of the aggregatory ability. In these cells the cytoskeleton associated F-actin remains significantly elevated and the structural organization of the cytoskeleton is markedly altered. In contrast to the network of filaments subadjacent to the surface membrane seen in unstimulated platelets, the cytoskeleton now shows electron dense zones in the more central parts of the cytoplasm. This diamide-induced structural reorganization of platelet cytoskeletal elements, associated with the inhibition of functional responses, emphasizes the dynamic nature of the membrane-cytoskeletal axis and its importance in the expression of shape changes and aggregatory phenomena in response to surface stimuli.