Summary
The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these ...key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.
The classic intrinsic pathway of coagulation is triggered by contact activation of the plasma protease factor (F)XII, followed by sequential proteolytic activation of FX1 and FIX. While a key ...mechanism for initiating coagulation in some clinically useful in vitro assays, the absence of abnormal bleeding associated with congenital FXII deficiency indicates that the intrinsic pathway is not important for normal blood coagulation in vivo. However, recent work with mice lacking FXII or FXI suggest that these proteases make important contributions to formation of pathologic intravascular thrombi. In models of arterial injury, FXII or FXI null mice are protected from formation of platelet rich occlusive thrombi to a degree similar to that seen in FIX deficient mice (a model for the severe bleeding disorder hemophilia B) or to wild type mice treated with high dose heparin. FXII or FXI deficiency does not appear to prevent the initiation of thrombus formation in these models, but instead causes significant thrombus instability that prevents occlusion of the vessel. These findings raise the possibility that a pathway similar or identical to the intrinsic pathway may operate in vivo under some circumstances. Furthermore, the disproportionate importance of FXII and FXI to occlusive thrombus formation compared to normal hemostasis makes these proteases attractive candidates for therapeutic inhibitors to treat or prevent thromboembolic disorders.
Abstract Background The plasma protease factor XIa (FXIa) has become a target of interest for therapeutics designed to prevent or treat thrombotic disorders. Methods We used a solution-based, ...directed evolution approach called systematic evolution of ligands by exponential enrichment (SELEX) to isolate RNA aptamers that target the FXIa catalytic domain. Results Two aptamers, designated 11.16 and 12.7, were identified that bound to previously identified anion binding and serpin bindings sites on the FXIa catalytic domain. The aptamers were non-competitive inhibitors of FXIa cleavage of a tripeptide chromogenic substrate and of FXIa activation of factor IX. In normal human plasma, aptamer 12.7 significantly prolonged the aPTT clotting time. Conclusions The results show that novel inhibitors of FXIa can be prepared using SELEX techniques. RNA aptamers can bind to distinct sites on the FXIa catalytic domain and noncompetitively inhibit FXIa activity toward its primary macromolecular substrate factor IX with different levels of potency. Such compounds can be developed for use as therapeutic inhibitors.
Background: Previous reports have noted that factor (F) XI and FXII and prekallikrein (the contact phase proteases) are absent in fish. Objectives: A broad survey of recently completed genomes was ...undertaken to find where during the course of vertebrate evolution these coagulation factors appeared. Methods: BLAST searches were conducted for the various factors on genomes of lamprey, puffer fish, zebra fish, frog, chicken, platypus, and opossum. Results: It was confirmed that FXII is absent from fish; it is present in frog, platypus, and opossum, but is absent in chicken, an apparent example of gene loss. A single gene corresponding to the evolutionary predecessor of FXI and prekallikrein occurs in frog, chicken, and platypus. The opossum (a marsupial) has both prekallikrein and FXI, completing the full complement of these genes that occurs in eutherian mammals. Conclusions: The step‐by‐step accrual of genes for these factors by a series of timely gene duplications has been confirmed by phylogenetic analysis and other considerations.
Factor XI (FXI) has structural and mechanistic features that distinguish it from other coagulation proteases. A relatively recent addition to vertebrate plasma coagulation, FXI is a homodimer, with ...each subunit containing four apple domains and a protease domain. The apple domains form a disk structure with binding sites for platelets, high molecular weight kininogen, and the substrate factor IX (FIX). FXI is converted to the active protease FXIa by cleavage of the Arg369−Ile370 bond on each subunit. This converts the catalytic domains to the active forms, and unmasks exosites on the apple domains required for FIX binding. FXI activation by factor XIIa or thrombin proceeds through an intermediate with only one activated submit (1/2‐FXIa). 1/2‐FXIa activates FIX in a similar manner to FXIa. While the importance of the homodimeric structure of FXI is not certain, it may represent a strategy for binding to FIX and a platelet surface simultaneously.
Summary
Background
Inorganic polyphosphates (polyP), which are secreted by activated platelets (short‐chain polyP) and accumulate in some bacteria (long‐chain polyP), support the contact activation ...of factor XII (FXII) and accelerate the activation of FXI.
Objectives
The aim of the present study was to evaluate the role of FXI in polyP‐mediated coagulation activation and experimental thrombus formation.
Methods and Results
Pretreatment of plasma with antibodies that selectively inhibit FXI activation by activated FXII (FXIIa) or FIX) activation by activated FXI (FXIa) were not able to inhibit the procoagulant effect of long or short‐chain polyP in plasma. In contrast, the FXIIa inhibitor, corn trypsin inhibitor, blocked the procoagulant effect of long and short polyP in plasma. In a purified system, long polyP significantly enhanced the rate of FXII and prekallikrein activation and the activation of FXI by thrombin but not by FXIIa. In FXI‐deficient plasma, long polyP promoted clotting of plasma in an FIX‐dependent manner. In a purified system, the activation of FXII and prekallikrein by long polyP promoted FIX activation and prothombin activation. In an ex vivo model of occlusive thrombus formation, inhibition of FXIIa with corn trypsin inhibitor but not of FXI with a neutralizing antibodies abolished the prothrombotic effect of long polyP.
Conclusions
We propose that long polyP promotes FXII‐mediated blood coagulation bypassing FXI. Accordingly, some polyp‐containing pathogens may have evolved strategies to exploit polyP‐initiated FXII activation for virulence, and selective inhibition of FXII may improve the host response to pathogens.
Essentials
Mice lacking factor IX (FIX) or factor XI (FXI) were tested in a saphenous vein bleeding model.
FIX‐deficient mice displayed a hemostatic defect and FXI‐deficient mice were similar to wild ...type mice.
Infusion of FXI or over‐expression of FXI in FIX‐deficient mice improved hemostasis.
FXI may affect the phenotype of FIX‐deficiency (hemophilia B).
Summary
Background
In humans, deficiency of coagulation factor XI may be associated with a bleeding disorder, but, until recently, FXI‐deficient mice did not appear to have a hemostatic abnormality. A recent study, however, indicated that FXI‐deficient mice show a moderate hemostatic defect in a saphenous vein bleeding (SVB) model.
Objectives
To study the effect of FXI on bleeding in mice with normal levels of the FXI substrate FIX and in mice lacking FIX (a murine model of hemophilia B).
Methods
Wild‐type mice and mice lacking either FIX (F9−) or FXI (F11−/−) were tested in the SVB model. The plasma levels of FXI in F11−/− mice were manipulated by infusion of FXI or its active form FXIa, or by overexpressing FXI by the use of hydrodynamic tail vein injection.
Results
F9− mice showed a significant defect in the SVB model, whereas F11−/− mice and wild‐type mice were indistinguishable. Intravenous infusion of FXI or FXIa into, or overexpression of FXI in, F9− mice improved hemostasis in the SVB model. Overexpression of a FXI variant lacking a FIX‐binding site also improved hemostasis in F9− mice.
Conclusions
Although we were unable to demonstrate a hemostatic defect in F11−/− mice in the SVB model, our results support the premise that supraphysiological levels of FXI improve hemostasis in F9− mice through FIX‐independent pathways.
Factor XI and pulmonary infections Gailani, D.; Mohammed, B. M.; Cheng, Q.
Haemophilia : the official journal of the World Federation of Hemophilia,
July 2018, Letnik:
24, Številka:
4
Journal Article
Factor XI (FXI) and factor IX (FIX) are zymogens of plasma serine proteases required for normal hemostasis. The purpose of this work was to evaluate FXI and FIX as potential therapeutic targets by ...means of a refined ferric chloride (FeCl3)‐induced arterial injury model in factor‐deficient mice. Various concentrations of FeCl3 were used to establish the arterial thrombosis model in C57BL/6 mice. Carotid artery blood flow was completely blocked within 10 min in C57BL/6 mice by application of 3.5% FeCl3. In contrast, FXI‐ and FIX‐deficient mice were fully protected from occlusion induced by 5% FeCl3, and were partially protected against the effect of 7.5% FeCl3. The protective effect was comparable to very high doses of heparin (1000 units kg−1) and substantially more effective than aspirin. While FXI and FIX deficiencies were indistinguishable in the carotid artery injury model, there was a marked difference in a tail‐bleeding‐time assay. FXI‐deficient and wild‐type mice have similar bleeding times, while FIX deficiency was associated with severely prolonged bleeding times (>5.8‐fold increase, P < 0.01). Given the relatively mild bleeding diathesis associated with FXI deficiency, therapeutic inhibition of FXI may be a reasonable strategy for treating or preventing thrombus formation.
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