The interaction of
plasminogen with platelets and their localization during
thrombus formation and fibrinolysis under flow are not defined. Using a novel model of whole blood thrombi, formed under flow, we examine dose-dependent fibrinolysis using fluorescence microscopy. Fibrinolysis was dependent upon flow and the balance between
fibrin formation and
plasminogen activation, with
tissue plasminogen activator-mediated lysis being more efficient than
urokinase plasminogen activator-mediated lysis. Fluorescently labeled
plasminogen radiates from platelet aggregates at the base of thrombi, primarily in association with
fibrin.
Hirudin attenuates, but does not abolish
plasminogen binding, denoting the importance of
fibrin. Flow cytometry revealed that stimulation of platelets with
thrombin/
convulxin significantly increased the
plasminogen signal associated with
phosphatidylserine (PS)-exposing platelets. Binding was attenuated by
tirofiban and
Gly-Pro-Arg-Pro amide, confirming a role for
fibrin in amplifying
plasminogen binding to PS-exposing platelets. Confocal microscopy revealed direct binding of
plasminogen and
fibrinogen to different platelet subpopulations. Binding of
plasminogen and
fibrinogen co-localized with PAC-1 in the center of spread platelets. In contrast, PS-exposing platelets were PAC-1 negative, and bound
plasminogen and
fibrinogen in a protruding "cap." These data show that different subpopulations of platelets harbor
plasminogen by diverse mechanisms and provide an essential scaffold for the accumulation of fibrinolytic
proteins that mediate fibrinolysis under flow.