Role for phosphoinositide 3-kinase in Fc{gamma}RIIA-induced platelet shape change

1 Division of Hematology, Brigham and Women's Hospital, Department of Medicine, Harvard Medical School, Boston 02115; and 2 Beth Israel Deaconess Medical Center, Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115; and 3 Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104 Submitted 25 April 2003 ; accepted in final form 29 May 2003 Platelets transform from disks to irregular spheres, grow filopodia, form ruffles, and spread on surfaces coated with anti-Fc RIIA antibody. Fc RIIA 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 Fc RIIA 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 Fc RIIA and clearly distinguish the Fc RIIA signaling pathway to actin filament assembly from the thrombin receptor protease-activated receptor (PAR)-1 pathway. actin assembly; CD32A Address for reprint requests and other correspondence: H. Falet, Division of Hematology, Brigham and Women's Hospital, 221 Long-wood Ave., EBRC 301, Boston, MA 02115 (E-mail: hfalet{at}rics.bwh.harvard.edu ).