Structural studies of fibrinolysis: how to disassemble a clot

Fibrin is degraded by the fibrinolytic system wich a plasminogen activator converts plasminogen to plasmin, a serine protease that cleaves specific bonds in fibrin leading to solubilization. To elucidate the biophysical processes involved in conversion of insoluble fibers to soluble fragments, fibrin was treared with either plasmin or the combination of plasminogen and its activator, and morphologic changes were observed using scanning electron microscopy. Initial changes in the fibrin matrix included creation of many free fiber ends and gaps in the continuity of fibers. With more extensive digestion, free fiber segments associated laterally, resulting in formation of thick fiber handles. Supernatants of digesting clots, containing soluble derivatives, were negatively contrasted and examined by transmission electron microscopy large complex fragments containing portions of multiple fibers were observed, as were piaces of individual fibers and smaller fragments. Some large fragments had sharply defined ends, indicating that they had been cleaved perpendicularly to the fiber direction. Other fibers showed splayed ends or a lacy meshwork of surrounding protofibrils. Fibrinolysis was also followed by confocal microscopy of plasma clots labeled with colloidal gold, so that changes at the lysis front of hydrated clots could be examined in real time. Clots made up of thin fibers were cleaved more slowly than clots made up of thick fibers, even though individual thin fibers were cleaved more rapidly. These results indicate that fibrinolytic degradation results in larger pieces than previously identified, and that plasmin digestion proceeds locally by transverse cutting across fibers rather than by progressive cleavage uniformly around the fiber. A model is proposed for the crawling of plasmin across fibrin fibers.