The platelet glycoprotein (GP) Ib/IX/V receptor complex controls crucial steps in haemostasis and thrombosis by mediating adhesion to von Willebrand factor (VWF). Nevertheless, the downstream consequences of VWF binding to GPIb remain unclear. The GPIb/IX/V complex consists of four polypeptides GPIbα, GPIbβ, GPIX and GPV. We have shown that a palmitylated (Pal-) peptide based on the sequence of the GPIbβ cytoplasmic domain between R151 and A161 (Pal-RRLRARARARA abbreviated to RARA) increases the velocity of platelets rolling on VWF, and inhibits VWF-dependent adhesion and aggregation.

The peptide RARA is delivered into platelets and does not cause any reduction in the surface expression of GPIb-IX-V. We examined the effect of RARA and control peptides (Pal-RAAAARARARA and Pal-RRLRADADADA) on GPIb/IX/V-mediated tyrosine kinase signalling induced by VWF/GPIb interactions under shear conditions using a cone and plate viscometer. Whole platelet lysate phosphotyrosine immunoblots were characterised in the presence and absence of RARA and the control peptides before and after shearing at 120 dynes/cm2 for 5 mins. We then further investigated the effect of RARA on PI-3 kinase signalling, the tyrosine kinase Src and calcium influx under the conditions described above.

The platelet permeable peptide RARA alone specifically caused protein tyrosine phosphorylation under resting conditions and caused platelet microaggregate formation with a 35% reduction in single platelets as assessed by flow cytometry. In addition RARA significantly inhibited shear-induced aggregation. When tyrosine phosphorylation was examined in platelets sheared for 5 minutes at 120 dynes/cm2, we observed RARA failed to inhibit tyrosine phosphorylation despite inhibiting shear-induced aggregation.

RARA caused Src kinase activation as determined by site-specific phosphorylation of Y416 on Src. Furthermore the Src inhibitor PP1 inhibited RARA induced tyrosine phosphorylation. RARA caused a Src-dependent increase in intracellular [Ca2+] and inhibited any further increase in intracellular calcium induced by thrombin. Pretreatment of platelets with the PI-3 kinase inhibitor wortmannin demonstrated that tyrosine phosphorylation induced by RARA was not PI-3 kinase dependent.

These results indicate that a platelet permeable peptide corresponding to specific residues of the cytoplasmic tail of GPIbβ directly stimulates Src, and suggest that the mechanism of its inhibitory effect on VWF-mediated platelet adhesion and aggregation involves the tyrosine kinase-mediated desensitisation of a GPIb/IX/V-triggered signalling pathway. This study demonstrates a novel role for the GPIbβ cytoplasmic tail in mediating Src-dependent shear induced signalling through GPIb/VWF.

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