Tissue factor (TF)-dependent blood coagulation plays a primary role in hemostasis after tissue injury and also in pathogenesis. Binding of factor VII to TF is the initial step in TF-dependent blood coagulation. A key reaction must then follow: activation of the bound factor VII to give rise to catalytically active VIIa/TF complexes. Little is known about how the activation of factor VII and the subsequent VIIa/TF-catalytic activity are regulated in vivo and to what extent anti-coagulantly active cell surface heparin sulfate proteoglycans play a role in the regulation of these critical steps in TF-induced coagulation. Further, which specific amino acid side chains in VIIa and TF are involved in the recognition of tissue factor pathway inhibitor (TFPI) and the cell biology of VIIa/TF remain unknown. The primary focus of the present proposal is to obtain important information on these unanswered questions. The specific goals are: (1) Test a hypothesis, both in vitro and in vivo, that cell surface heparin sulfate proteoglycans limit activation of factor VII bound to TF expressed on cell surfaces and thereby regulated TF-induced coagulation (2) Test a hypothesis that recognition structures for substrate factor X presents in VIIa/TF complex contribute to the enhancing effect of Xa during the quaternary complex formation of VIIa/TF/TFPI/Xa (3) Test a hypothesis that cell surface TF redistributes upon complex formation with VIIa, Xa and TFPI and define the role of TFPI in this process (4) Test a hypothesis that the binding of TFPI/Xa to cell surface VIIa/TF affects internalization of cell surface VIIa/TF complexes and elucidate the intracellular trafficking of internalized VIIa and TF. In the proposed studies, various well established techniques, such as radioligand binding, coagulation assays, electron microscopy, and a rabbit intravascular coagulation model system will be used. Information obtained from these studies will aid in understanding important clinical issues, such as why infused factor VIIa is effective in controlling the bleeding in hemophiliacs. The above studies will further our understanding of the structural mechanism of macromolecular assembly of VIIa/TF/TFPI/Xa. Such an understanding may lead to the design and application of therapeutic measures aimed at the treatment of hemorrhagic disease and thrombotic complications.
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