Wiskott-Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT) are
caused by mutations of the WAS protein (WASP) gene. All hematopoietic
stem cell-derived lineages, including platelets, express WASP. Platel
ets from WAS patients are smaller than their normal counterparts and d
efects in platelet aggregation and actin polymerization have been repo
rted. To determine if WASP is important for normal platelet function,
we examined its role in signal transduction. We found that collagen bu
t not thrombopoietin or thrombin induces a rapid and robust increase i
n tyrosine phosphorylation of platelet-associated WASP. Collagen-induc
ed tyrosine phosphorylation of WASP was inhibited by cytochalasin D an
d wortmannin. respectively, suggesting that actin polymerization and p
hosphatidylinositol 3-kinase (Pla-kinase) play a role in the induction
of tyrosine phosphorylation of WASP. Binding of glutathion S-transfer
ase (GST)Grb2 to WASP was seen in the lysate of resting platelets. The
binding was reduced when lysates from collagen-stimulated platelets w
ere incubated with GST-Grb2, suggesting that tyrosine phosphorylation
of WASP may directly or indirectly modulate the adapter function of WA
SP. Although thrombin- and thrombopoietin-induced increase in tyrosine
phosphorylation of WASP is negligible or marginal, WASP from thrombin
-activated platelets became incorporated into the Triton X-100-insolub
le 10,000g sedimentable residue in an aggregation-dependent manner, su
ggesting that it may have a regulatory role in platelet cytoskeletal p
rocesses during aggregation. Lastly. we found that WASP is cleaved in
response to activation of calpain, a protease that may have a role in
postaggregation signaling processes. Our data suggest that collagen sp
ecifically induces an increase in tyrosine phosphorylation of WASP and
that WASP is involved in signaling during thrombin-induced aggregatio
n by its redistribution to the cytoskeleton and its cleavage during ag
gregation. (C) 1998 by The American Society of Hematology.