Role for phosphoinositide 3-kinase in FcγRIIA-induced platelet shape change

Platelets transform from disks to irregular spheres, grow filopodia, form ruffles, and spread on surfaces coated with anti-FcgammaRIIA antibody. FcgammaRIIA cross-linking leads to a tenfold increase in actin filament barbed end exposure and robust actin assembly. Activation of the small GTPases Rac and Cdc42 follows FcgammaRIIA cross-linking. Shape change, actin filament barbed end exposure, and quantifiable actin assembly require phosphoinositide 3-kinase (PI3-kinase) activity and a rise in intracellular calcium. PI3-kinase inhibition blocks activation of Rac, but not of Cdc42, and diminishes the association of Arp2/3 complex and CapZ with polymerized actin. Furthermore, addition of constitutively active D-3 phosphorylated polyphosphoinositides or recombinant PI3-kinase subunits to octylglucoside-permeabilized platelets elicits actin filament barbed end exposure by releasing gelsolin and CapZ from the cytoskeleton. Our findings place PI3-kinase activity upstream of Rac, gelsolin, and Arp2/3 complex activation induced by FcgammaRIIA and clearly distinguish the FcgammaRIIA signaling pathway to actin filament assembly from the thrombin receptor protease-activated receptor (PAR)-1 pathway.