Effect of synthetic peptides corresponding to residues 313-332 of the α_IIb subunit on platelet activation and fibrinogen binding to α_IIbβ₃

The platelet integrin receptor α_IIbβ₃ plays a critical role in thrombosis and haemostasis by mediating interactions between platelets and several ligands but primarily fibrinogen. It has been shown previously that the YMESRADR KLAEVGRVYLFL (313-332) sequence of the α_IIb subunit plays an important role in platelet activation, fibrinogen binding and α_IIbβ₃-mediated outside-in signalling. Furthermore, we recently showed that the 20-residue peptide (20-mer) α_IIb 313-332, is a potent inhibitor of platelet aggregation and fibrinogen binding to α_IIbβ ₃, interacting with fibrinogen rather than the receptor. In an effort to determine the sequence and the minimum length required for the biological activity of the above 20-mer, we synthesized seven octapeptides, each overlapping by six residues, covering the entire sequence and studied their effect on platelet activation as well as fibrinogen binding to activated platelets. We show for the first time that octapeptides containing the RAD sequence are capable of inhibiting platelet aggregation and secretion as well as fibrinogen binding to the activated α_IIbβ₃, possibly interacting with the ligand rather than the receptor. This suggests that the RAD sequence, common to all the inhibitory peptides, is critical for their biological activity. However, the presence of the YMES sequence, adjacent to RAD, significantly increases the peptide's biological potency. The development of such inhibitors derived from the 313-332 region of the α_IIb subunit may be advantageous against the RGD-like antagonists as they could inhibit platelet activation without interacting with α_IIbβ₃, thus failing to further induce α_IIbβ₃-mediated outside-in signalling.