The molecular adapter SLP-76 relays signals from platelet integrin alpha(IIb)beta(3) to the actin cytoskeleton

Platelet adhesion to fibrinogen through integrin alpha (IIb)beta (3) trigge rs actin rearrangements and cell spreading. Mice deficient in the SLP-76 ad apter molecule bleed excessively, and their platelets spread poorly on fibr inogen. 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 stimulate d tyrosine phosphorylation of SLP-76, SLP-76 phosphorylation required coexp ression of Syk tyrosine kinase and stimulated association of SLP-76 with th e adapter, Nck, and with the Rac exchange factor, Vav1. SLP-76 expression i ncreased lamellipodia formation induced by Syk and Vav1 in adherent CHO cel ls (p < 0.001). Although lamellipodia formation requires Rac, SLP-76 functi oned 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 r eorganization. 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 in tegrin phenotype of SLP-76(-/-) platelets.