The protease factor Xla (fXIa) contributes to hemostasis by sustaining thrombin generation after initiation of blood coagulation. In addition to contributing to fibrin formation, fXIa promotes clot resistance to fibrinolysis in a process involving the proteinase TAFI. Several lines of evidence indicate fXIa contributes to pathologic coagulation. Elevated plasma levels of the fXIa precursor fXI are associated with increased risks for arterial and venous thrombosis in humans. We observed that fXI deficient mice are resistant to vascular occlusion in arterial injury models, indicating fXIa contributes to platelet rich thrombus formation. FXI binds to platelets in vitro, and is converted to fXIa by thrombin or factor XI la (fXlla). The importance of these interactions in vivo or in complex in vitro systems is not known, and the relative importance of thrombin and fXlla to fXI activation is not clear. FXI I deficiency does not cause a bleeding abnormality, indicating fXI is activated in vivo by other proteases. However, fXII deficient mice are resistant to arterial occlusion, raising the possibility that formation of a pathologic occlusive thrombus involves processes distinct from those involved in normal hemostasis.
In Aim 1 of this proposal, we will use mice with deficiencies of fXI, fXII, and TAFI to investigate the importance of these proteins in models of pathologic coagulation. Using a panel of novel recombinant fXI variants to reconstitute fXI deficient mice, we will determine the importance of fXI interactions with platelets, thrombin and fXlla to these models, and look for alternative functions for fXI and fXII unrelated to classical coagulation pathways.
In Aim 2, a murine fXI-dependent bleeding model will be used to determine if fXI interactions with platelets, thrombin and fXlla are required for hemostasis.
In Aim 3 we will use human and murine platelet rich plasma systems to examine the importance of fXI and fXII to thrombin generation and platelet activation in vitro, with the primary goal of determining if there are differences between the two species. These studies will allow us to compare the importance of fXI, fXII and TAFI to normal hemostasis and to pathologic coagulation, and will provide valuable information regarding the mechanism(s) for fXI activation in complex systems. Ultimately, the work may demonstrate that fXI and/or fXII contribute in a substantive way to blood vessel occlusion, and are therefore, legitimate targets for novel investigative therapies to treat or prevent vascular thromboembolism in humans. Relevance: Anticoagulant drugs have been very beneficial for treating abnormal blood clots in a variety of diseases, but have the disadvantage of leaving the patient prone to serious bleeding. We are looking for new targets in blood for anticoagulation therapy that will not be associated with high bleeding risk.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Research Project (R01)
Project #
Application #
Study Section
Hemostasis and Thrombosis Study Section (HT)
Program Officer
Link, Rebecca P
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
Vanderbilt University Medical Center
Schools of Medicine
United States
Zip Code
Mohammed, Bassem M; Ivanov, Ivan; Matafonov, Anton et al. (2018) Activity of Factor XII-Locarno. Res Pract Thromb Haemost 2:168-173
Mohammed, Bassem M; Matafonov, Anton; Ivanov, Ivan et al. (2018) An update on factor XI structure and function. Thromb Res 161:94-105
Mohammed, B M; Cheng, Q; Matafonov, A et al. (2018) Factor XI promotes hemostasis in factor IX-deficient mice. J Thromb Haemost 16:2044-2049
Tillman, Benjamin; Gailani, David (2018) Inhibition of Factors XI and XII for Prevention of Thrombosis Induced by Artificial Surfaces. Semin Thromb Hemost 44:60-69
Gailani, D; Mohammed, B M; Cheng, Q (2018) Factor XI and pulmonary infections. Haemophilia 24:519-521
Tillman, Benjamin F; Gruber, Andras; McCarty, Owen J T et al. (2018) Plasma contact factors as therapeutic targets. Blood Rev 32:433-448
Ivanov, Ivan; Matafonov, Anton; Gailani, David (2017) Single-chain factor XII: a new form of activated factor XII. Curr Opin Hematol 24:411-418
Ivanov, Ivan; Matafonov, Anton; Sun, Mao-Fu et al. (2017) Proteolytic properties of single-chain factor XII: a mechanism for triggering contact activation. Blood 129:1527-1537
Ivanov, Ivan; Shakhawat, Ruhama; Sun, Mao-Fu et al. (2017) Nucleic acids as cofactors for factor XI and prekallikrein activation: Different roles for high-molecular-weight kininogen. Thromb Haemost 117:671-681
Woodruff, R S; Ivanov, I; Verhamme, I M et al. (2017) Generation and characterization of aptamers targeting factor XIa. Thromb Res 156:134-141

Showing the most recent 10 out of 58 publications