Plasma Fibronectin in Hemostasis and Thrombosis: Evidence for Fibrin-dependent and Independent Pathways

Hemostasis is the physiological process to stop bleeding, while thrombosis is the pathological vessel occlusive process leading to heart attack, stroke, and deep vein thrombosis, the leading cause of mortality and morbidity worldwide. Plasma fibronectin (pFn), the plasma form of fibronectin (Fn), exists in circulation in high abundance. However, the exact role of pFn in hemostasis and thrombosis is unclear. pFn has long been suspected to play a role in hemostasis but direct evidence has been lacking. Using our unique Fg-/-/pFn-/-, VWF-/-/pFn-/-, and Fg-/-/VWF-/-/pFn-/- mouse models, we demonstrated that pFn is vital for the control of bleeding in fibrinogen deficient mice and in wild-type mice treated with anticoagulants. We further found that, at the site of vessel injury, pFn rapidly deposits and initiates hemostasis even before platelet accumulation (the first wave of hemostasis). This pFn deposition is independent of fibrinogen, von Willebrand factor, β3 integrin, and platelets. Our perfusion chamber assay indicates that subendothelial collagen can mediate the rapid pFn depostion. Using confocal and scanning electron microscopy, we reveal that pFn actively integrates into fibrin, which increases fibrin fiber diameter and enhances the mechanical strength of clots as determined by thromboelastography. Using an inferior vena cava thrombosis model, we found that pFn depletion attenuated venous thrombosis, suggesting the involvement of pFn in deep vein thrombosis. Interestingly, pFn promotes platelet aggregation when linked with fibrin but inhibits this process when fibrin is absent. Therefore, pFn may gradually switch from supporting hemostasis to inhibiting thrombosis and vessel occlusion following the fibrin gradient that decreases farther from the injured endothelium. In summary, our data established that pFn is a supportive factor in hemostasis, which is vital under coagulation deficient (both genetic and therapeutic) conditions. By interacting with fibrin and platelet β3 integrin, pFn plays a unique self-limiting regulatory role in thrombosis, suggesting pFn transfusion may be a potential therapy for bleeding disorders, particularly in association with anticoagulant therapy.Ph.D.2017-03-22 00:00:0