Inhibition of platelet activation by peptide analogs of the β₃-intracellular domain of platelet integrin α_IIbβ₃ conjugated to the cell-penetrating peptide Tat(48–60)

Abstract

Activation of the platelet integrin-receptor α_IIbβ₃ is the final pathway of platelet aggregation, regardless of the initiating stimulus. Many studies suggest that there are several cytoplasmic proteins such as talin and β₃-endonexin that bind to N⁷⁴⁴PLY⁷⁴⁷ and N⁷⁵⁶ITY⁷⁵⁹ motif of the β₃ cytoplasmic tail and play the major role in the receptor activation. In this study, we investigated the role of the membrane distal region of human β₃ cytoplasmic tail and specifically the N⁷⁴³NPLYKEA⁷⁵⁰ and T⁷⁵⁵NITYRGT⁷⁶² sequence that contains an NXXY motif, in platelet aggregation, secretion, α_IIbβ₃ activation (PAC-1 binding) and fibrinogen binding. We synthesized two peptides corresponding to the above sequences as well as their conjugates with the Tat(48–60) cell-penetrating peptide. The capability of conjugates to penetrate the platelet membrane was investigated with confocal laser scanning microscopy using carboxyfluorescein (CF)-labeled peptides. Our results showed that the conjugated with the Tat(48–60) sequence peptides penetrate the platelet membrane and inhibit platelet aggregation in both PRP and washed platelets in a dose-dependent manner. The Tat-β₃743–750 conjugate exhibited similar inhibitory activity in PRP and in washed platelets whereas the Tat-β₃755–762 conjugate was more potent inhibitor of aggregation in washed platelets than in PRP. Both conjugated peptides were also able to inhibit P-selectin membrane expression as well as PAC-1 and fibrinogen binding to the platelets, the Tat-β₃755–762 conjugate being more potent than Tat-β₃743–750. The Tat(48–60) peptide and the peptides β₃743–750 and β₃755–762, which were not conjugated to the Tat(48–60) sequence, did not exhibit any inhibitory effect on the above parameters. In conclusion, the present study shows for the first time that the peptide analogs of the intracellular domain of the β₃ subunit β₃743–750 and β₃755–762 conjugated to the cell-penetrating peptide Tat(48–60) are capable of penetrating the platelet membrane and expressing biological activity by inhibiting the activation of α_IIbβ₃, the fibrinogen binding to the activated receptor as well as platelet aggregation. Further studies are necessary to support whether such conjugated peptides may be useful tools for the development of potent antiplatelet agents acting intracellularly through the platelet integrin α_IIbβ₃.

• α_IIbβ₃ receptor
• cell-penetrating conjugates
• platelet activation inhibitors
• Tat-conjugated peptides
• β₃ cytoplasmic domain