Novel pro-hemostatic and anti-thrombotic therapies
Prinicipal investigator: Prof Joost Meijers PhD
Blood coagulation is an important host defense mechanism to prevent bleeding after injury. However, this system is also involved in obstruction of vessels with a clot: thrombosis. In our research, we investigate the blood coagulation mechanism to better understand the events that lead to bleeding or thrombosis, and to identify (novel) agents that can be used for pro-hemostatic or anti-thrombotic therapy.
Coagulation occurs when the plasma protease activated factor VII comes into contact, and subsequently forms a complex, with tissue factor (TF). The TF/factor VIIa-complex can activate factor X and activated factor X can convert prothrombin into thrombin. Thrombin, a key contributor in coagulation, in turn converts fibrinogen into fibrin. In addition to this direct factor Xa generation, the TF/factor VIIa-complex can also indirectly activate factor X. The indirect route of factor X activation goes via the activation of factor IX. Factor IXa in the presence of cofactor factor VIIIa can activate factor X, thereby forming an amplification loop. For this sequence of events, TF must come into contact with blood, for instance upon injury or inflammation. Additionally, a TF independent pathway has evolved in vertebrates. Coagulation factor XII can be activated on charged surfaces (for instance polyphosphate, RNA) by a process called contact activation. Following autoactivation, factor XIIa can activate factor XI, eventually leading to the formation of thrombin, via factor IX, with its cofactor factor VIIIa, and factor X as described above. Deficiency in one of the factors involved in this pathway results in a variety of bleeding disorders. For example Hemophilia A (factor VIII) or Hemophilia B (factor IX) result in severe and often spontaneous bleeding. In contrast to factor VIII or factor IX deficiencies congenital factor XI deficiency, also known as Hemophilia C, typically causes only mild and injury-induced bleeding, where factor XII deficient patients do not have a bleeding tendency at all. These observations suggest that contact activation is not essential for normal haemostasis in vivo.
Two other proteins are important to mention regarding contact activation, namely high molecular weight kininogen (HK) and prekallikrein (PK). Patients deficient in either protein do not exhibit a bleeding phenotype, despite a prolonged aPTT clotting time. HK forms a noncovalent complex with factor XI, which is necessary for the binding of factor XI to negatively charged surfaces and for its activation to factor XIa by factor XIIa; HK serves as a nonenzymatic cofactor in this reaction. Prekallikrein also circulates in complex with HK and is the precursor of kallikrein (Kal), a serine protease that can liberate kinins, but can also cleave factor XII to generate additional factor XIIa.
Fig.1 The contact system is a direct link between inflammation and coagulation. Activation of factor XII induces factor XI activation and this will ultimately lead to thrombin formation. Factor XII can be activated via negatively charged substances like polyphosphates, RNA and neutrophil extracellular traps (NETs). Activated factor XII can also cleave plasma prekallikrein (PK) into kallikrein (Kal), which on its turn releases several vasoactive substances like bradykinin. At the same time, plasma kallikrein produces additional factor XIIa. Prekallikrein and factor XI are both bound to high molecular weight kininogen (HK) in plasma. Indicated alongside of the coagulation factors are the various types of inhibitor (antisense oligonucleotides, antibodies, small molecule inhibitors and naturally occurring inhibitors).
In our research, we have identified the components of the intrinsic pathway and especially the contact system as optimal targets for antithrombotic therapy. With the use of antibodies and antisense technology, we could show effective anticoagulation without the bleeding risk that accompanies currently used anticoagulants. These studies are currently expanded to determine if this strategy is also effective and safe in thrombosis on atherosclerotic vessels.
- Vergouwen MDI, Knaup VL, Roelofs JJTH, de Boer OJ, Meijers JCM. Effect of recombinant ADAMTS13 on microthrombosis and brain injury after experimental subarachnoid hemorrhage. J Thromb Haemostas 2014 Mar 28. [Epub ahead of print].
- Meijers JCM. No contact, no thrombosis? Blood 2014; 123(11):1629
- Van Montfoort ML, Knaup VL, Marquart JA, Bakhtiari K, Castellino FJ, Hack CE, Meijers JCM. Two novel inhibitory anti-human factor XI antibodies prevent cessation of blood flow in a murine venous thrombosis model. Thromb Haemost 2013; 110(5):1065-73.
- Bakhtiari K, Kamphuisen PW, Mancuso ME, Hamulyak K, Schutgens REG, Santagostino E, Meijers JCM. Clot lysis phenotype and response to recombinant factor VIIa in plasma of haemophilia A inhibitor patients. Br J Haematol 2013; 162(6):827-35Maas C, Meijers JCM, Marquart JA, Bakhtiari K, Weeterings C, de Groot PhG, Urbanus RT. Activated factor V is a cofactor for the activation of factor XI by thrombin in plasma. Proc Natl Acad Sci USA 2010; 107(20):9083-7.
- Schuijt TJ, Bakhtiari K, Daffre S, DePonte K, Wielders SJH, Marquart JA, Hovius JW, van der Poll T, Fikrig E, Bunce MW, Camire RM, Nicolaes GAF, Meijers JCM, van ‘t Veer C. Factor Xa activation of factor V is of paramount importance in initiating the coagulation system. Lessons from a tick salivary protein. Circulation 2013; 128(3):254-66.
- Van Montfoort ML, Meijers JCM. Anticoagulation beyond direct thrombin and factor Xa inhibitors: indications for targeting the intrinsic pathway? Thromb Haemost 2013; 110(2):223-32.
- Van Montfoort ML, Stephan F, Lauw MN, Hutten BA, van Mierlo GJ, Solati S, Middeldorp S, Meijers JCM, Zeerleder S. Circulating nucleosomes and neutrophil activation as risk factors for deep vein thrombosis. Arterioscler Thromb Vasc Biol 2013; 33(1):147-51.