The Platelet Integrin αIIbβ3 Differentially Interacts with Fibrin Versus Fibrinogen

Abstract

Fibrinogen binding to the integrin ␣IIb␤3 mediates platelet aggregation and spreading on fibrinogen-coated surfaces. However, in vivo ␣IIb␤3 activation and fibrinogen conversion to fibrin occur simultaneously, although the relative contributions of fibrinogen versus fibrin to ␣IIb␤3-mediated platelet functions are unknown. Here, we compared the interaction of ␣IIb␤3 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 ␣IIb␤3. When ␣IIb␤3-ligand complexes were detected, the rupture forces were measured and displayed as force histograms. Monomeric fibrin displayed a higher probability of interacting with ␣IIb␤3 and a greater binding strength. ␣IIb␤3-fibrin interactions were also less sensitive to inhibition by abciximab and eptifibatide. Both fibrinogen-and fibrin-␣IIb␤3 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 ␣IIb␤3-binding motif was more reactive with ␣IIb␤3 than the parent fibrinogen. These results demonstrate that fibrin is more reactive with ␣IIb␤3 than fibrinogen. Fibrin is also less sensitive to ␣IIb␤3 inhibitors, suggesting that fibrin and fibrinogen have distinct binding requirements. In particular, the maintenance of ␣IIb␤3 binding activity in the absence of the ␥C-dodecapeptide and the ␣-chain RGD sequences suggests that the ␣IIb␤3-binding sites in fibrin are not confined to its known ␥-chain and RGD motifs.

An unmet clinical need: The history of thrombus imaging

“…Thrombin, the major hemostatic coagulation enzyme, activates protease-activated receptors (PAR-receptors) highly expressed in platelets and endothelial cells, myocytes and neurons, which amplify the inflammatory pathway activity well beyond the acute coagulation process. 3 Fibrin, the mortar of clots, recruits and activates platelets through the platelet receptor α IIb β 3 , 4 as well as attracts leukocytes through interaction with the α M β 2 (mac-1) integrin receptor. 5 Leukocyte binding to fibrin increases phagocytosis, NF κ B-mediated transcription, production of chemokines and cytokines, and degranulation.…”

αIIbβ3 binding to a fibrinogen fragment lacking the γ-chain dodecapeptide is activation dependent and EDTA inducible

“…38 (4) Binding of fibrin to aIIbb3 has different physicochemical properties than binding to fibrinogen. 39 Identifying ancillary binding sites for fibrinogen and/or fibrin on aIIbb3 would provide a more comprehensive understanding of fibrinogen binding to platelets. Such sites may furnish novel therapeutic targets to prevent platelet thrombus formation.…”

The Emerging Roles of CIB1 in Cancer

“…Base on the relationship between CIB1 and αIIbβ3, CIB1 is also involved in cell migration [61]. WASP binding to CIB1 affects the affinity of αIIbβ3 for fibrinogen (also called factor I), a soluble plasma glycoprotein is converted by thrombin into fibrin during blood coagulation [62-64]. CIB1 can bind to N terminus of the WASP and recruit it to αIIbβ3, increasing αIIbβ3-mediated cell adhesion in stimulated platelets [65].…”