The Platelet Integrin IIb3 Differentially Interacts with Fibrin Versus Fibrinogen

Fibrinogen binding to the integrin IIb3 mediates platelet aggregation and spreading on fibrinogen-coated surfaces. However, in vivo IIb3 activation and fibrinogen conversion to fibrin occur simultaneously, although the relative contributions of fibrinogen versus fibrin to IIb3-mediated platelet functions are unknown. Here, we compared the interaction of IIb3 with fibrin and fibrinogen to explore their differential effects. A microscopic bead coated with fibrinogen or monomeric fibrin produced by treating the immobilized fibrinogen with thrombin was captured by a laser beam and repeatedly brought into contact with surface-attached purified IIb3. When IIb3-ligand complexes were detected, the rupture forces were measured and displayed as force histograms. Monomeric fibrin displayed a higher probability of interacting with IIb3 and a greater binding strength. IIb3-fibrin interactions were also less sensitive to inhibition by abciximab and eptifibatide. Both fibrinogen- and fibrin-IIb3 interactions were partially inhibited by RGD peptides, suggesting the existence of common RGD-containing binding motifs. This assumption was supported using the fibrin variants D97E or D574E with mutated RGD motifs. Fibrin made from a fibrinogen / variant lacking the C IIb3-binding motif was more reactive with IIb3 than the parent fibrinogen. These results demonstrate that fibrin is more reactive with IIb3 than fibrinogen. Fibrin is also less sensitive to IIb3 inhibitors, suggesting that fibrin and fibrinogen have distinct binding requirements. In particular, the maintenance of IIb3 binding activity in the absence of the C-dodecapeptide and the -chain RGD sequences suggests that the IIb3-binding sites in fibrin are not confined to its known -chain and RGD motifs.