Cytoplasmic regions of the β3 subunit of integrin αIIbβ3 involved in platelet adhesion on fibrinogen under flow conditions

Summary.  Platelet adhesion to surface-bound fibrinogen depends on integrin α_IIbβ₃. In the present study, we investigated the role of the regions ⁷⁴⁹EATSTFT⁷⁵⁶N and ⁷⁵⁵TNITYRG⁷⁶²T of the β₃ cytoplasmic tail in the regulation of platelet adhesion under flow conditions, by introducing peptide mimetics in platelets. Introduction of peptide EATSTFTN (E–N) increased surface coverage by 35%, an effect caused by 25% more adhesion. In contrast, peptide TNITYRGT (T–T) decreased surface coverage by 16%, as a result of 25% less adhesion. An S→P substitution in the E–N peptide, thereby mimicking a mutation in Glanzmann's thrombasthenia, abolished the effect of E–N. A suboptimal concentration of cytochalasin D is known to enhance ligand binding to α_IIbβ₃ in platelet suspensions. Under flow, cytochalasin D (1 µmol L^?1) induced 50% more platelet adhesion, with a strong reduction in platelet spreading. Both peptides opposed the increase in adhesion by cytochalasin D and partly (E–N) and completely (T–T) restored platelet spreading. Thus, the ⁷⁴⁹EATSTFT⁷⁵⁶N and ⁷⁵⁵TNITYRG⁷⁶²T regions of β₃ contribute to the regulation of αIIbβ3 anchorage to the cytoskeleton and platelet spreading to an adhesive surface.