Control of Platelet Adhesion by Rigidity Sensing at the Surface of Fibrin Clot.

Thrombus formation at sites of vascular injury must occur quickly to reduce blood loss, but is carefully controlled to limit vessel occlusion. Arrest of bleeding is mediated by adhesion and aggregation of platelets and the formation of the fibrin clot. While the interactions responsible for platelet adhesion and thrombus growth have been extensively researched, the mechanisms that limit platelet adhesion are not clear. We have previously demonstrated that plasma fibrinogen is a potent inhibitor of integrin-mediated leukocyte adhesion to fibrin clots and surface-bound fibrinogen, and have provided evidence that fibrinogen reduces cell adhesion by binding to the surface of fibrin rather than blocking leukocyte integrins. Accordingly, cells that engage fibrinogen molecules loosely bound to fibrin (soft substrate) are not able to consolidate their grip on the surface; subsequently, cells detach. Conversely, cells that adhere to the naked fibrin clot (rigid substrate) adhere firmly. Since fibrin and immobilized fibrinogen support platelet adhesion, we examined the effect of soluble fibrinogen on integrin α_IIbβ₃-mediated adhesion. We show that the anti-adhesive fibrinogen layer formed on the surface of fibrin inhibits platelet adhesion. We also demonstrate that fibrinogen immobilized on plastic at high densities (>20 μg/ml) supports weak platelet adhesion whereas at low concentrations (∼2 μg/ml) it is highly adhesive. An investigation of the mechanism underlying differential platelet adhesion indicates that platelet adhesion to rigid substrates (low-density fibrinogen and naked fibrin gel) induces much stronger phosphorylation of FAK and Syk kinases than that to soft substrates (high-density fibrinogen and fibrin exposed to soluble fibrinogen). Furthermore, the rigid, but not the soft substrates induce recruitment of signaling molecules talin and skelemin to α_IIbβ₃-containing focal adhesions. Consistent with their limited ability to induce sufficient signaling, soft substrates do not support platelet spreading. These data suggest that circulating fibrinogen prevents stable platelet adhesion by modifying the mechanical properties of the fibrin clot’s surface which results in reduced force generation and insufficient signaling.