The molecular adapter SLP-76 relays signals from platelet integrin αIIbβ3 to the actin cytoskeleton

Platelet adhesion to fibrinogen through integrin alpha (IIb)beta (3) triggers actin rearrangements and cell spreading. Mice deficient in the SLP-76 adapter molecule bleed excessively, and their platelets spread poorly on fibrinogen. Here me used human platelets and a Chinese hamster ovary (CHO) cell expression system to better define the role of SLP-76 in alpha (IIb)beta (3) signaling. CHO cell adhesion to fibrinogen required cu,,P, and stimulated tyrosine phosphorylation of SLP-76, SLP-76 phosphorylation required coexpression of Syk tyrosine kinase and stimulated association of SLP-76 with the adapter, Nck, and with the Rac exchange factor, Vav1. SLP-76 expression increased lamellipodia formation induced by Syk and Vav1 in adherent CHO cells (p < 0.001). Although lamellipodia formation requires Rac, SLP-76 functioned downstream of Rac by potentiating adhesion-dependent activation of PAK kinase (p < 0.001), a Rac effector that associates with Nck, In platelets, adhesion to fibrinogen stimulated the association of SLP-76 with the SLAP-130 adapter and with VASP, a SLAP-130 binding partner implicated in actin reorganization. Furthermore, SLAP-130 colocalized with VASP at the periphery of spread platelets. Thus, SLP-76 functions to relay signals from cr,P, to effecters of cytoskeletal reorganization. Therefore, deficient recruitment of specific adapters and effecters to sites of adhesion may explain the integrin phenotype of SLP-76(-/-) platelets.