Distinct Role for Fyn-Kinase in αIIbß3-Mediated Signaling.

Recent studies have shown that Src-family kinases (SFKs) play an important role in mediating integrin signaling. αIIbβ3 has been shown to interact with multiple SFK members. We analyzed the role of Fyn in αIIbβ3-signaling and hemostasis. Fyn associated with αIIbβ3 in resting as well as thrombin aggregated platelets. In contrast, the interaction between αIIbβ3 and another SFK member, cSrc, was seen predominantly in resting but not in thrombin aggregated platelets. Based on this difference, we explored if the Fyn−/− mice exhibit any abnormalities in hemostasis and platelet function. Fyn−/− mice had significantly prolonged second bleeding times compared to wild-type (WT) controls (3.61 min for Fyn−/− compared to 1.04 min for WT mice, n=11). Platelets from Fyn−/− mice also exhibited delayed spreading on fibrinogen coated surfaces. Fyn, but not cSrc, localized to focal adhesions in CHO cells adherent to fibrinogen via αIIbβ3, and association of Fyn and cSrc depends on different segments of the cytoplasmic tail of the β3 subunit in this model cell system In the CHO cell system, Fyn mediated αIIbβ3-dependent cell spreading in the absence of Rho-GTPase recruitment. Using mutant forms of Fyn, we found that its kinase activity is required for its localization to focal adhesions and to mediate αIIbβ3-dependent cell spreading. These results show previously unidentified roles for Fyn in αIIbβ3-signaling and hemostasis.