| c-Cbl protein functions as an E3 ligase and scaffolding protein, where three residues, Y700, Y731, and Y774, upon phosphorylation, have been shown to initiate several signaling cascades. In this study, we investigated the role of these phospho-tyrosine residues in the platelet functional responses upon integrin engagement. We observed that c-Cbl Y700, Y731 and Y774 undergo phosphorylation upon platelet adhesion to immobilized fibrinogen, which was inhibited in the presence of PP2, a pan-src family kinase (SFK) inhibitor, suggesting that c-Cbl is phosphorylated downstream of SFKs. However, OXSI-2, a Syk inhibitor, significantly reduced c-Cbl phosphorylation at residues Y774 and Y700, without affecting Y731 phosphorylation. Interestingly, PP2 inhibited both platelet spreading on fibrinogen as well as clot retraction, whereas OXSI-2 blocked only platelet spreading, suggesting a differential role of these tyrosine residues. The physiological role of c-Cbl and Y731 was studied using platelets from c-Cbl KO and c-CblYF/YF knock-in mice. c-Cbl KO and c-CblYF/YF platelets had a significantly reduced spreading over immobilized fibrinogen. Furthermore, clot retraction with c-Cbl KO and c-Cbl YF/YF platelets was drastically delayed. These results indicate that c-Cbl and particularly its phosphorylated residue Y731 plays an important role in platelet outside-in signaling contributing to platelet spreading and clot retraction. |
