THE PLATELET CYTOSKELETON

The platelet cytoskeleton contains two actin filament-based components . One is the cytoplasmic actin filaments which fill the cytoplasm and mediate contractile events. The other is the membrane skeleton, which coats the plasma membrane and regulates properties of the membrane suc h as its contours and stability. In the unstimulated platelet, only 30 -40% of the actin is polymerized into filaments; the rest is thought t o be prevented from polymerizing by the association of thymosin beta(4 ) with monomeric actin and by the association of gelsolin with the bar bed ends of pre-existing actin filaments. When platelets are activated , there is a rapid increase in actin polymerization; new filaments fil l the extending filopodia and form a network at the periphery of the p latelet. As a result of activation, myosin binds to cytoplasmic actin filaments, causing them to move towards the center of the platelet. As platelets aggregate, additional cytoskeletal reorganizations occur: G P IIb-IIIa associates with adhesive ligand in a platelet aggregate; th is results in the association of GP IIb-IIIa, membrane skeleton protei ns, and signaling molecules with cytoplasmic actin. Future studies sho uld help to elucidate the significance of the cytoskeleton in regulati ng signal transduction events in platelets.