Essential role of glycoprotein Ibα in platelet activation.

Glycoprotein (GP) Ib, the ligand-binding subunit of platelet GPIb-IX complex, interacts with von Willebrand factor (VWF) exposed at the injured vessel wall, initiating platelet adhesion, activation, hemostasis, and thrombus formation. The cytoplasmic tail of GPIb interacts with 14-3-3, regulate ng the VWF-GPIb-elicited signal transduction and VWF binding function of GPIb. However, we unexpectedly found that the GPIb-14-3-3 association, beyond VWF-dependent function, is essential for general platelet activation. We found that the GPIb cytoplasmic tail peptide MPC, a potential GPIb inhibitor, by itself induced platelet aggregation, integrin αIIbβ3 activation, granule secretion, and phosphatidylserine (PS) exposure. Conversely, the deletion of the cytoplasmic tail of GPIb in mouse platelets (10aa-/-) decreased platelet aggregation, integrin IIb3 activation, granule secretion, and PS exposure induced by various physiological agonists. Phosphoproteome-based kinase activity profiling revealed significantly upregulated protein kinase C (PKC) activity in MPC-treated platelets. MPC-induced platelet activation was abolished by the pan-PKC inhibitor and PKC deletion. Decreased PKC activity was observed in both resting and agonist-stimulated 10aa-/- platelets. GPIb regulates PKC activity by sequestering 14-3-3 from PKC. In vivo, the deletion of the GPIb cytoplasmic tail impaired mouse hemostasis and thrombus formation and protected against platelet-dependent pulmonary thromboembolism. Therefore, our findings demonstrate an essential role for the GPIb cytoplasmic tail in regulating platelet general activation and thrombus formation beyond the VWF-GPIb axis.

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