PB0807 - Essential role of glycoprotein Ibα in von Willebrand factor-independent platelet activation

Background: Glycoprotein (GP) Ibα, 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 GPIbα interacts with 14-3-3ζ, regulating the VWF-GPIbα-elicited signal transduction and VWF binding function of GPIbα. However, the role of this region in VWF-independent platelet activation remains unclear.

Aims: To investigate the role of the GPIbα cytoplasmic tail in VWF-independent platelet activation and the underlying mechanism.

Methods: The GPIbα C-terminal peptide MPαC and the GPIbα C-terminal 10 amino acid-deleted mouse model (10aa-/-) were used to study the effect of the elevation and deletion of the GPIbα cytoplasmic tail on platelet function, respectively.

Results: The GPIbα cytoplasmic tail peptide MPαC by itself induced platelet aggregation, integrin αIIbβ3 activation, granule secretion, and phosphatidylserine (PS) exposure. Conversely, the deletion of the cytoplasmic tail of GPIbα in mouse platelets (10aa-/-) decreased platelet aggregation, integrin αIIβ3 activation, granule secretion, and PS exposure induced by U46619, ADP, PAR4-AP, and collagen. Phosphoproteome-based kinase activity profiling revealed significantly upregulated protein kinase C (PKC) activity in MPαC-treated platelets. MPαC-induced platelet activation was abolished by the pan-PKC inhibitor and PKCα deletion. Decreased PKC activity was observed in both resting and agonist-stimulated 10aa-/- platelets. Furthermore, impaired and enhanced association of 14-3-3ζ with PKCα was observed in MPαC-treated platelets and the GPIbα cytoplasmic tail-deleted platelets (10aa-/-), respectively. In vivo, the deletion of the GPIbα cytoplasmic tail prolonged mouse tail bleeding and arteriole occlusion time and protected against PAR4-AP and U46619-induced pulmonary thromboembolism.

Conclusion(s): The GPIbα cytoplasmic tail regulates platelet general activation beyond the GPIbα-VWF axis and thrombosis formation. The enhancement or deletion of the GPIbα cytoplasmic tail disrupts 14-3-3 homeostasis, activating or inhibiting PKC and platelet function. Our study provides a conceptual advance in understanding the physiological relevance of platelet GPIbα.