Coagulation Factor XII Recruitment and Activation During Thrombus Formation

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PROJECT SUMMARY An antithrombotic medication not associated with increased hemorrhage would be a transformative advance in the treatment of coagulation disorders. Inhibiting coagulation factor XII (FXII) has emerged as a therapeutic strategy that could “decouple” hemostasis from thrombosis to achieve antithrombotic efficacy without bleeding complications. Enthusiasm for FXII as a therapeutic target stems from the observation that severe congenital FXII deficiency is not associated with a bleeding diathesis while blockade or deletion of FXII in preclinical models consistently protects against thrombosis. However, despite its potential clinical importance, the mechanisms underlying platelet-dependent FXII activation in vivo remain unclear. Using a mass spectrometry-based proteomic screen, we have identified integrin αIIbβ3 as the putative platelet receptor for FXII. These results have been followed up with a number of additional studies reproducibly demonstrating the FXII-αIIbβ3 interaction and localizing the integrin binding region to the kringle domain (KD) of the FXII heavy chain. Our central hypothesis is that binding of FXII zymogen to platelet integrin αIIbβ3 potentiates FXII activation and is necessary and sufficient for FXII-mediated coagulation. In Aim 1 of this proposal, we will map the region(s) of the FXII KD responsible for binding to αIIbβ3 via competitive inhibition assays using recombinantly-generated fragments of the KD. We will also evaluate the role of the FXII KD in a mouse model of thrombosis to determine if loss of the KD can prevent thrombus formation in vivo. In Aim 2, we will work closely with our collaborators to conduct precision structural studies of the FXII-αIIbβ3 complex. In Aim 3, we will focus on the mechanisms by which FXII binding to αIIbβ3 leads to activation of FXII and downstream coagulation. The proposed work will provide important new insights into the molecular mechanism of FXII recruitment, activation, and propagation at the platelet surface and inform efforts to develop novel therapeutics based on inhibition of the FXII-αIIbβ3 complex.