Morphometric analysis of the effects of αArp2 on filopodia and lamellipodia and on morphology during spreading on glass.

(A-D) Effect of intact antibody on platelet spreading. Nl indicates untreated (n = 259 platelets); Tx, permeabilized only (n = 391); Pre, permeabilized and loaded with 1.0 μg/mL antibodies from preimmune serum from the same rabbit (n = 444); and 0.1, 0.2, 0.3, and 1.0, permeabilized and loaded with different concentrations of αArp2, in μg/mL, final concentration (n = 423, n = 385, n = 512, n = 609, respectively). (E) Effect of Fab fragments and rArp2 on platelet spreading. Platelets were permeabilized as in Figure3 and treated as follows: Pre indicates preimmune antibodies from the same rabbit (1μg/mL); rArp2, αArp2 and recombinant Arp2 protein (1 μg/mL and 5 μg/mL, respectively); Fab, Fab fragments of αArp2 (1 μg/mL); and αArp2, intact αArp2 antibody (1 μg/mL). Note that Fab fragments freeze 60% of the platelets in the rounded stage, whereas pretreatment of the antibody with rArp2 protein eliminates this effect. Blebbing was seen with intact antibody and with Fab fragments, though less frequently with Fab fragments.