The Kunitz 1 and Kunitz 3 domains of tissue factor pathway inhibitor are required for efficient inhibition of factor Xa

The TFPIα C-terminal tail is essential for TFPIα-FV-short-protein S complex formation and synergistic enhancement of TFPIα

BACKGROUND

For maximal TFPIα functionality, 2 synergistic cofactors, protein S and FV-short, are required. Both interact with TFPIα, protein S through Kunitz 3 residues Arg199/Glu226 and FV-short with the C-terminus. How these interactions impact the synergistic enhancement remains unclear.

OBJECTIVES

To determine the importance of the TFPIα-protein S and TFPIα-FV-short interactions for TFPIα enhancement.

METHODS

TFPIα variants unable to bind protein S (K3m [R199Q/E226Q]) or FV-short (ΔCT [aa 1-249]) were generated. TFPIα-FV-short binding was studied by plate-binding and co-immunoprecipitation assays; functional TFPIα enhancement by FXa inhibition and prothrombin activation.

RESULTS

While WT TFPIα and TFPIα K3m bound FV-short with high affinity (Kd∼2nM), TFPIα ΔCT did not. K3m, in contrast to WT, did not incorporate protein S in a TFPIα-FV-short-protein S complex while TFPIα ΔCT bound neither FV-short nor protein S. Protein S enhanced WT TFPIα-mediated FXa inhibition, but not K3m, in the absence of FV-short. However, once FV-short was present, protein S efficiently enhanced TFPIα K3m (EC50: 4.7nM vs 2.0nM for WT). FXa inhibition by ΔCT was not enhanced by protein S alone or combined with FV-short. In FXa-catalyzed prothrombin activation assays, FV-short enhanced TFPIα K3m function in the presence of protein S (5.5 vs 10.4-fold enhancement of WT) whereas ΔCT showed reduced or lack of enhancement by FV-short and protein S, respectively.

CONCLUSION

Full TFPIα function requires the presence of both cofactors. While synergistic enhancement can be achieved in the absence of TFPIα-protein S interaction, only TFPIα with an intact C-terminus can be synergistically enhanced by protein S and FV-short.

Protein arginine deiminase 4 inactivates tissue factor pathway inhibitor-alpha by enzymatic modification of functional arginine residues

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an important regulator of coagulation and a link between inflammation and thrombosis. During thrombotic events, TFPI is proteolytically inactivated by neutrophil elastase while bound to neutrophil extracellular traps (NETs). Protein arginine deiminase 4 (PAD4) catalyzes the conversion of arginine to citrulline and is crucial for NET formation.

OBJECTIVES

Here, we show that PAD4 inactivates full-length TFPIα by citrullination of its functional arginines.

METHODS

Citrullination of TFPIα and of TFPI-constructs by PAD4 was studied using western blotting and mass spectrometry. Binding of TFPIα to PAD4 was investigated using a solid-phase assay. Functional consequences were investigated by factor Xa inhibition and thrombin generation assays.

RESULTS

Nanomolar PAD4 amounts eliminated factor Xa inhibition by TFPIα. A citrullinated mutant Kunitz 2 domain did not inhibit factor Xa. Citrullination of TFPIα was found to be time- and concentration-dependent. Immunoprecipitation of citrullinated proteins from whole blood after neutrophil activation suggested the presence of TFPIα. Negatively charged phospholipids inhibited citrullination and truncated variants K1K2 and TFPI 1-161, and the isolated K2 domain were less efficiently citrullinated by PAD4. TFPIα bound to PAD4 with nanomolar affinity and involved the basic C-terminus. Thrombin generation in TFPI-deficient plasma demonstrated reduced anticoagulant activity of citrullinated TFPI. Mass spectrometry demonstrated citrullination of surface-exposed arginine residues in TFPIα after incubation with PAD4.

CONCLUSION

Full-length TFPIα is sensitive to citrullination by PAD4, which causes loss of factor Xa inhibition. This process may play a role in the increased thrombosis risk associated with inflammation.

The functional role of the autolysis loop in the regulation of factor X upon hemostatic response

BACKGROUND

The regulation of factor X (FX) is critical to maintain the balance between blood coagulation and fluidity.

OBJECTIVES

To functionally characterize the role of the FX autolysis loop in the regulation of the zymogen and active form of FX.

METHODS

We introduced novel N-linked glycosylations on the surface-exposed loop spanning residues 143-150 (chymotrypsin numbering) of FX. The activity and inhibition of recombinant FX variants was quantified in pure component assays. The in vitro thrombin generation potential of the FX variants was evaluated in FX-depleted plasma.

RESULTS

The factor VIIa (FVIIa)-mediated activation and prothrombin activation was reduced, presumably through steric hinderance. Prothrombin activation was, however, recovered in presence of cofactor factor Va (FVa) despite a reduced prothrombinase assembly. The introduced N-glycans exhibited position-specific effects on the interaction with two FXa inhibitors: tissue factor pathway inhibitor (TFPI) and antithrombin (ATIII). Ki for the inhibition by full-length TFPI of these FXa variants was increased by 7- to 1150-fold, whereas ATIII inhibition in the presence of the heparin-analog Fondaparinux was modestly increased by 2- to 15-fold compared with wild-type. When supplemented in zymogen form, the FX variants exhibited reduced thrombin generation activity relative to wild-type FX, whereas enhanced procoagulant activity was measured for activated FXa variants.

CONCLUSION

The autolysis loop participates in all aspects of FX regulation. In plasma-based assays, a modest decrease in FX activation rate appeared to knock down the procoagulant response even when down regulation of FXa activity by inhibitors was reduced.

The Composition and Physical Properties of Clots in COVID-19 Pathology

Hemostasis is a finely tuned process of which dysregulation can lead either to bleeding or thrombotic complications. The latter is often caused by the hypercoagulable state as it is also seen in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, i.e., in COVID-19 patients. COVID-19 patients requiring hospitalization often suffer from thrombotic events that could not be predicted using routine coagulation assays. Recently, several studies have reported ROtational ThromboElastoMetry (ROTEM) as a promising tool to predict outcomes in COVID-19 patients. In this review we give an overview of ROTEM with a particular focus on the interpretation of the symmetrical clot formation curve in relation to coagulopathy in COVID-19 patients. Furthermore, we have introduced new parameters that might help to better distinguish between COVID-19 patients and outcomes.

The contribution of TFPIα to the hemostatic response to injury in mice

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an essential regulator of coagulation, limiting thrombin generation and preventing thrombosis. In humans and mice, TFPIα is the sole isoform present in platelets.

OBJECTIVE

Here, we asked whether TFPIα, because of its release from platelets at sites of injury, has a unique role in limiting the hemostatic response.

METHODS

TFPIα-mutant (TfpiΔα/Δα ) mice were generated by introducing a stop codon in the C-terminus. Platelet accumulation, platelet activation, and fibrin accumulation were measured following penetrating injuries in the jugular vein and cremaster muscle arterioles, and imaged by fluorescence and scanning electron microscopy. Time to bleeding cessation was recorded in the jugular vein studies.

RESULTS

TfpiΔα/Δα mice were viable and fertile. Plasma TFPI levels were normal in the TfpiΔα/Δα mice, no TFPI protein or activity was present in their platelets and thrombin-antithrombin complex levels were indistinguishable from Tfpi+/+ littermates. There was a small, but statistically significant reduction in the time to bleeding cessation following jugular vein puncture injury in the TfpiΔα/Δα mice, but no measurable changes in platelet or fibrin accumulation or in hemostatic plug architecture following injury of the micro- or macrovasculature.

CONCLUSION

Loss of TFPIα expression does not produce a global prothrombotic state in mice. Platelet TFPIα is expected to be released or displayed in a focal manner at the site of injury, potentially accumulating to high concentrations in the narrow gaps between platelets. If so, the data from the vascular injury models studied here indicate this is not essential for a normal hemostatic response in mice.

Characterization of the enzymatic activity of the serine protease domain of Factor VII activating protease (FSAP)

Factor VII (FVII) activating protease (FSAP) is a circulating serine protease. Human genetic studies, based on the Marburg I (MI) (Gly221Glu, chymotrypsin numbering system) polymorphism, implicate FSAP in the pathogenesis of many diseases. Here, we describe the molecular and functional changes caused by the Gly221Glu substitution in the 220 loop using recombinant proteins expressed in E. coli. The serine protease domain (SPD) of wild type (WT) FSAP displayed auto-catalytic activation whereas the MI isoform displayed very low autocatalytic activation and low proteolytic activity against the chromogenic substrate S-2288, Factor VII, tissue factor pathway inhibitor as well as pro-urokinase. Introduction of a thermolysin cleavage site in the activation position (Arg15Gln) led to cleavage of both WT- and MI-SPD and the resulting WT-SPD, but not the MI-SPD, was active. Mutating the Gly221 position to Asp, Gln and Leu led to a loss of activity whereas the Ala substitution was partially active. These results suggest a disturbance of the active site, or non-accessibility of the substrate to the active site in MI-SPD. With respect to regulation with metal ions, calcium, more than sodium, increased the enzymatic activity of WT-SPD. Thus, we describe a novel method for the production of recombinant FSAP-SPD to understand the role of the MI-single nucleotide polymorphism (SNP) in the regulation of its activity.

NMR structure determination of Ixolaris and factor X(a) interaction reveals a noncanonical mechanism of Kunitz inhibition

Ixolaris is a potent tick salivary anticoagulant that binds coagulation factor Xa (FXa) and zymogen FX, with formation of a quaternary tissue factor (TF)/FVIIa/ FX(a)/Ixolaris inhibitory complex. Ixolaris blocks TF-induced coagulation and PAR2 signaling and prevents thrombosis, tumor growth, and immune activation. We present a high-resolution structure and dynamics of Ixolaris and describe the structural basis for recognition of FX. Ixolaris consists of 2 Kunitz domains (K1 and K2) in which K2 is strikingly dynamic and encompasses several residues involved in FX binding. This indicates that the backbone plasticity of K2 is critical for Ixolaris biological activity. Notably, a nuclear magnetic resonance-derived model reveals a mechanism for an electrostatically guided, high-affinity interaction between Ixolaris and FX heparin-binding (pro)exosite, resulting in an allosteric switch in the catalytic site. This is the first report revealing the structure-function relationship of an anticoagulant targeting a zymogen serving as a scaffold for TF inhibition.

Re-evaluation of mouse tissue factor pathway inhibitor and comparison of mouse and human tissue factor pathway inhibitor physiology

Essentials Mouse models are often used to define roles of tissue factor pathway inhibitor (TFPI) in man. TFPI isoform-specific KOs reveal unexpected differences between mouse and human TFPI physiology. Mouse plasma contains 20 times more TFPI than man, derived from TFPIγ, a form not found in man. TFPIγ null mice, expressing only TFPI isoforms α and β, may better reflect the human situation. SUMMARY: Background Mouse models can provide insight into the pathophysiology of human thrombosis and hemostasis. Tissue factor pathway inhibitor (TFPI) regulates coagulation through protein S (PS)-enhanced factor (F) Xa inhibition and FXa-dependent inhibition of FVIIa/tissue factor (TF) activity. TFPI is expressed as isoforms α and β in man, and α, β and γ in the mouse. Objective Assess the reliability of extending TFPI-related studies in mice to humans. Method Compare mouse and human TFPI physiology using a variety of methods. Results Mouse TFPI and human TFPI are similar in regard to: (i) the mechanisms for FVIIa/TF and FXa inhibition; (ii) TFPIα is a soluble form and TFPIβ is glycosyl phosphatidyl inositol (GPI) membrane anchored; (iii) the predominant circulating form of TFPI in plasma is lipoprotein-associated; (iv) low levels of TFPIα circulate in plasma and increase following heparin treatment; and (v) TFPIα is the isoform in platelets. They differ in that: (i) mouse TFPI circulates at a ~20-fold higher concentration; (ii) mouse lines with isolated isoform deletions show this circulating mouse TFPI is derived from TFPIγ; (iii) sequences homologous to the mouse TFPIγ exon are present in many species, including man, but in primates are unfavorable for splicing; and (iv) tandem mass spectrometry (MS/MS) detects sequences for TFPI isoforms α and β in human plasma and α and γ in mouse plasma. Conclusion To dissect the pathophysiological roles of human TFPIα and TFPIβ, studies in TFPIγ null mice, expressing only α and β, only α or only β should better reflect the human situation.

Factor Xa and VIIa inhibition by tissue factor pathway inhibitor is prevented by a monoclonal antibody to its Kunitz-1 domain

Essentials Activated FVII (FVIIa) and FX (FXa) are inhibited by tissue factor pathway inhibitor (TFPI). A monoclonal antibody, mAb2F22, was raised against the N-terminal fragment of TFPI (1-79). mAb2F22 bound exclusively to the K1 domain of TFPI (KD ∼1 nm) and not to the K2 domain. mAb2F22 interfered with inhibition of both FVIIa and FXa activities and restored clot formation.

SUMMARY

Background Initiation of coagulation is induced by binding of activated factor VII (FVIIa) to tissue factor (TF) and activation of factor X (FX) in a process regulated by tissue factor pathway inhibitor (TFPI). TFPI contains three Kunitz-type protease inhibitor domains (K1-K3), of which K1 and K2 block the active sites of FVIIa and FXa, respectively. Objective To produce a monoclonal antibody (mAb) directed towards K1, to characterize the binding epitope, and to study its effect on TFPI inhibition. Methods A monoclonal antibody, mAb2F22, was raised against the N-terminal TFPI(1-79) fragment. Binding data were obtained by surface plasmon resonance analysis. The Fab-fragment of mAb2F22, Fab2F22, was expressed and the structure of its complex with TFPI(1-79) determined by X-ray crystallography. Effects of mAb2F22 on TFPI inhibition were measured in buffer- and plasma-based systems. Results mAb2F22 bound exclusively to K1 of TFPI (KD ~1 nm) and not to K2. The crystal structure of Fab2F22/TFPI (1-79) mapped an epitope on K1 including seven residues upstream of the domain. TFPI inhibition of TF/FVIIa amidolytic activity was neutralized by mAb2F22, although the binding epitope on K1 did not include the P1 residue. Binding of mAb2F22 to K1 blocked TFPI inhibition of the FXa amidolytic activity and normalized hemostasis in hemophilia human A-like plasma and whole blood. Conclusion mAb2F22 blocked TFPI inhibition of both FVIIa and FXa activities and mapped a FXa exosite for binding to K1. It reversed TFPI feedback inhibition of TF/FVIIa-induced coagulation and restored clot formation in FVIII-neutralized human plasma and blood.

Role of exosite binding modulators in the inhibition of Fxa by TFPI

Tissue factor pathway inhibitor (TFPI) down-regulates the extrinsic coagulation pathway by inhibiting FXa and FVIIa. Both TFPI and FXa interact with several plasma proteins (e. g. prothrombin, FV/FVa, protein S) and non-proteinaceous compounds (e. g. phospholipids, heparin). It was our aim to investigate effects of ligands that bind to FXa and TFPI on FXa inhibition by full-length TFPI (designated TFPI) and truncated TFPI (TFPI1-150). Inhibition of FXa by TFPI and TFPI1-150 and effects of phospholipids, heparin, prothrombin, FV, FVa, and protein S thereon was quantified from progress curves of conversion of the FXa-specific chromogenic substrate CS11-(65). Low concentrations negatively charged phospholipids (~10 μM) already maximally stimulated (up to 5- to 6-fold) FXa inhibition by TFPI. Unfractionated heparin at concentrations (0.2–1 U/ml) enhanced FXa inhibition by TFPI ~8-fold, but impaired inhibition at concentrations > 1 U/ml. Physiological protein S and FV concentrations both enhanced FXa inhibition by TFPI 2- to 3-fold. In contrast, thrombin-activated FV (FVa) impaired the ability of TFPI to inhibit FXa. FXa inhibition by TFPI1–150 was not affected by FV, FVa, protein S, phospholipids and heparin. TFPI potently inhibited FXa-catalysed prothrombin activation in the absence of FVa, but hardly inhibited prothrombin activation in the presence of thrombin-activated FVa. In conclusion, physiological concentrations TFPI (0.25–0.5 nM TFPI) inhibit FXa with a t1/2 between 3–15 minutes. Direct FXa inhibition by TFPI is modulated by physiological concentrations prothrombin, FV, FVa, protein S, phospholipids and heparin indicating the importance of these modulators for the in vivo anticoagulant activity of TFPI.

Suppressive Role of Tissue Factor Pathway Inhibitor-α in Platelet-Dependent Fibrin Formation under Flow Is Restricted to Low Procoagulant Strength

Tissue factor pathway inhibitor-alpha (TFPI-α) is a Kunitz-type serine protease inhibitor, which suppresses coagulation by inhibiting the tissue factor (TF)/factor VIIa complex as well as factor Xa. In static plasma-phospholipid systems, TFPI-α thus suppresses both factor Xa and thrombin generation. In this article, we used a microfluidics approach to investigate how TFPI-α regulates fibrin clot formation in platelet thrombi at low wall shear rate. We therefore hypothesized that the anticoagulant effect of TFPI-α in plasma is a function of the local procoagulant strength-defined as the magnitude of thrombin generation under flow, due to local activities of TF/factor VIIa and factor Xa. To test this hypothesis, we modulated local coagulation by microspot coating of flow channels with 0 to 100 pM TF/collagen, or by using blood from patients with haemophilia A or B. For blood or plasma from healthy subjects, blocking of TFPI-α enhanced fibrin formation, extending from a platelet thrombus, under flow only at <2 pM coated TF. This enhancement was paralleled by an increased thrombin generation. For mouse plasma, genetic deficiency in TFPI enhanced fibrin formation under flow also at 0 pM TF microspots. On the other hand, using blood from haemophilia A or B patients, TFPI-α antagonism markedly enhanced fibrin formation at microspots with up to 100 pM coated TF. We conclude that, under flow, TFPI-α is capable to antagonize fibrin formation in a manner dependent on and restricted by local TF/factor VIIa and factor Xa activities.

Platelet full length TFPI-α in healthy volunteers is not affected by sex or hormonal use

BACKGROUND

Only 10% of plasma TFPIα (TFPI) exists in the full length form, the rest circulates as a C-terminally truncated form. However, blood platelets exclusively contain full length TFPI, which is released at the site of injury upon platelet activation, and which could play an important local regulatory role in thrombin generation and prevention of thrombosis.

METHODS

The anticoagulant activities of full length and truncated TFPI were investigated using thrombin generation assays. Blood samples were obtained from 30 healthy volunteers (10 male subjects, 10 female subjects, and 10 females using oral contraceptives). Platelet TFPI was released in platelet rich plasma and in platelet isolates using convulxin or thrombin, and measured by free TFPI ELISA and thrombin generation assays.

RESULTS

Full length TFPI and platelet TFPI were much more potent inhibitors of thrombin generation than truncated TFPI, which was virtually inactive. Although mean plasma TFPI antigen levels decreased from men (0.30 nM) to women (0.20 nM) to women using oral contraceptives (0.11 nM), no relevant differences were found in platelet TFPI among those subgroups.

CONCLUSIONS

Platelets release similar amounts of TFPI regardless of plasma TFPI concentrations and is unaffected by sex or oral contraceptive use. We speculate that platelet TFPI is important to prevent systemic coagulation and thrombosis and restrict thrombus formation to the site of the growing platelet plug. The stable contribution of platelet TFPI to the anticoagulant potential in plasma is likely to become particularly relevant under conditions of low plasma TFPI levels in combination of oral contraceptives use.

The C-terminus of tissue factor pathway inhibitor-α inhibits factor V activation by protecting the Arg1545 cleavage site

Essentials The C-terminus of tissue factor pathway inhibitor (TFPIα) binds to the B-domain of factor V (FV). The functional consequences of this interaction were investigated in plasma and model systems. The TFPIα C-terminus inhibited thrombin generation in plasma, but not in the presence of FVa. The TFPIα C-terminus inhibited FV activation by preventing cleavage at Arg1545 .

SUMMARY

Background Factor V (FV) is a carrier and a cofactor of the anticoagulant protein tissue factor pathway inhibitor-α (TFPIα), whose basic C-terminus binds to an acidic region in the B-domain of FV. Proteolysis of FV at Arg709 , Arg1018 and Arg1545 by activated FX (FXa) or thrombin removes the B-domain, and converts FV into a procoagulant cofactor (activated FV [FVa]) of FXa in the prothrombinase complex. However, retention of the acidic region in partially activated FV makes prothrombinase activity susceptible to inhibition by TFPIα. Objective/Methods To investigate the effect of the TFPIα C-terminal peptide (TFPIα C-term) on thrombin generation in plasma and on FV activation in model systems. Results TFPIα C-term inhibited tissue factor-initiated and FXa-initiated thrombin generation in a dose-dependent manner. Failure to inhibit thrombin generation in FV-depleted plasma reconstituted with FVa indicated that the peptide effect was mediated by the acidic region of FV, and was localized at the level of FV activation and/or prothrombinase. In model systems, TFPIα C-term inhibited both FV activation and prothrombinase activity. Western blot analysis showed that the peptide impaired cleavage at Arg1545 by both thrombin and FXa. The inhibition was stronger for FV-short, which binds TFPIα with higher affinity. Similar results were obtained with full-length TFPIα. Conclusions Cleavage of FV at Arg1545 , which abolishes the anticoagulant properties of FV and commits FV to the procoagulant pathway, is inhibited by binding of the TFPIα C-terminus to the FV acidic region. Possible targets of this new anticoagulant function of TFPIα are low-abundance FV(a) species retaining the acidic region.

Amblyomin-X having a Kunitz-type homologous domain, is a noncompetitive inhibitor of FXa and induces anticoagulation in vitro and in vivo

BACKGROUND

Cancer has long been associated with thrombosis and many of the standard chemotherapeutics used to treat cancer are pro-thrombotic. Thus, the identification of novel selective anticancer drugs that also have antithrombotic properties is of enormous significance. Amblyomin-X is an anticancer protein derived from the salivary glands of the Amblyomma cajennense tick.

METHODS

In this work, we determined the inhibition profile of Amblyomin-X and its effect on activated partial thromboplastin time (aPTT) and prothrombin time (PT), using various approaches such as, kinetic analyses, amidolytic assays, SDS-PAGE, and mass spectrometry.

RESULTS

Amblyomin-X inhibited factor Xa, prothrombinase and tenase activities. It was hydrolyzed by trypsin and plasmin. MS/MS data of tryptic hydrolysate of Amblyomin-X suggested the presence of Cys(8)-Cys(59) and Cys(19)-Cys(42) but not Cys(34)-Cys(55) disulfide bond. Instead of Cys(34)-Cys(55), two noncanonical Cys(34)-Cys(74) and Cys(55)-Cys(74) disulfide bonds were identified. Furthermore, when Amblyomin-X (1mg/kg) injected in rabbits, it prolonged aPTT and PT.

CONCLUSION

Amblyomin-X is a noncompetitive inhibitor (Ki=3.9μM) of factor Xa. It is a substrate for plasmin and trypsin, but not for factor Xa and thrombin. The disulfide Cys(34)-Cys(55) bond probably scrambles with interchain seventh free cysteine residues (Cys(74)) of Amblyomin-X. The prolongation of PT and aPTT is reversible.

GENERAL SIGNIFICANCE

In term of anticoagulant property, this is structural and functional characterization of Amblyomin-X. All together, these results and previous findings suggest that Amblyomin-X has a potential to become an anticancer drug with antithrombotic property.

Inhibition of tissue factor:factor VIIa-catalyzed factor IX and factor X activation by TFPI and TFPI constructs

BACKGROUND

TFPI is a Kunitz-type protease inhibitor that downregulates the extrinsic coagulation pathway by inhibiting factor Xa (FXa) and FVIIa. All three Kunitz domains (KD1, KD2, and KD3) and protein S are required for optimal inhibition of FXa and FVIIa. There is limited information on Kunitz domain requirements of the inhibition of TF:FVIIa-catalyzed FIX and FX activation by TFPI.

AIM

To investigate the role of the Kunitz domains of TFPI and protein S in the inhibition of FX and FIX activation.

METHODS

Inhibition of TF:FVIIa-catalyzed FX and FIX activation by full-length TFPI (TFPIFL ) and TFPI constructs was quantified from progress curves of FXa and FIXa generation measured with chromogenic substrates.

RESULTS AND CONCLUSIONS

TFPIFL inhibited TF:FVIIa-catalyzed FIX activation with a Ki of 16.7 nmol L(-1) . Protein S reduced the Ki to 1.0 nmol L(-1) . TFPI1-150 and KD1-KD2 had 10-fold higher Ki values and were not stimulated by protein S. Single Kunitz domains were poor inhibitors of TF:FVIIa-catalyzed FIX activation (Ki >800 nm). FX activation was measured at limiting FVIIa and excess TF or vice versa. At both conditions, TFPIFL , TFPI1-150 , and KD1-KD2 showed similar inhibition of FX activation. However, at low phospholipid concentrations, TFPIFL was ~ 15-fold more active than TFPI1-150 or KD1-KD2. Apparently, excess phospholipids act as a kind of sink for TFPIFL , limiting its availability for TF:FVIIa inhibition. Preformed FXa:TFPIFL/1-150 complexes rapidly and stoichiometrically inhibited FIX and FX activation by TF:FVIIa, indicating that binary TFPI:FXa complex formation is the limiting step in TF:FVIIa inhibition. Protein S also enhanced inhibition of TF:FVIIa-catalyzed FX activation by TFPI.

Biology of tissue factor pathway inhibitor

Recent studies of the anticoagulant activities of the tissue factor (TF) pathway inhibitor (TFPI) isoforms, TFPIα and TFPIβ, have provided new insight into the biochemical and physiological mechanisms that underlie bleeding and clotting disorders. TFPIα and TFPIβ have tissue-specific expression patterns and anticoagulant activities. An alternative splicing event in the 5' untranslated region allows for translational regulation of TFPIβ expression. TFPIα has 3 Kunitz-type inhibitor domains (K1, K2, K3) and a basic C terminus, whereas TFPIβ has the K1 and K2 domains attached to a glycosylphosphatidyl inositol-anchored C terminus. TFPIα is the only isoform present in platelets, whereas endothelial cells produce both isoforms, secreting TFPIα and expressing TFPIβ on the cell surface. TFPIα and TFPIβ inhibit both TF-factor VIIa-dependent factor Xa (FXa) generation and free FXa. Protein S enhances FXa inhibition by TFPIα. TFPIα produces isoform-specific inhibition of prothrombinase during the initiation of coagulation, an anticoagulant activity that requires an exosite interaction between its basic C terminus and an acidic region in the factor Va B domain. Platelet TFPIα may be optimally localized to dampen initial thrombin generation. Similarly, endothelial TFPIβ may be optimally localized to inhibit processes that occur when endothelial TF is present, such as during the inflammatory response.

Small peptides blocking inhibition of factor Xa and tissue factor-factor VIIa by tissue factor pathway inhibitor (TFPI)

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor that inhibits activated factor X (FXa) via a slow-tight binding mechanism and tissue factor-activated FVII (TF-FVIIa) via formation of a quaternary FXa-TFPI-TF-FVIIa complex. Inhibition of TFPI enhances coagulation in hemophilia models. Using a library approach, we selected and subsequently optimized peptides that bind TFPI and block its anticoagulant activity. One peptide (termed compound 3), bound with high affinity to the Kunitz-1 (K1) domain of TFPI (Kd ∼1 nM). We solved the crystal structure of this peptide in complex with the K1 of TFPI at 2.55-Å resolution. The structure of compound 3 can be segmented into a N-terminal anchor; an Ω-shaped loop; an intermediate segment; a tight glycine-loop; and a C-terminal α-helix that is anchored to K1 at its reactive center loop and two-stranded β-sheet. The contact surface has an overall hydrophobic character with some charged hot spots. In a model system, compound 3 blocked FXa inhibition by TFPI (EC50 = 11 nM) and inhibition of TF-FVIIa-catalyzed FX activation by TFPI (EC50 = 2 nM). The peptide prevented transition from the loose to the tight FXa-TFPI complex, but did not affect formation of the loose FXa-TFPI complex. The K1 domain of TFPI binds and inhibits FVIIa and the K2 domain similarly inhibits FXa. Because compound 3 binds to K1, our data show that K1 is not only important for FVIIa inhibition but also for FXa inhibition, i.e. for the transition of the loose to the tight FXa-TFPI complex. This mode of action translates into normalization of coagulation of hemophilia plasmas. Compound 3 thus bears potential to prevent bleeding in hemophilia patients.

Aptamer BAX 499 mediates inhibition of tissue factor pathway inhibitor via interaction with multiple domains of the protein

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is a multidomain protein that negatively regulates the coagulation cascade. TFPI inhibits the tissue factor (TF)-activated factor VII-activated FX (FXa) complex during TF-mediated coagulation initiation. The aptamer BAX 499 binds specifically to TFPI and inhibits its function, mediating a procoagulant effect in both in vitro and in vivo models of hemophilia.

OBJECTIVES

This study sought to identify the regions of TFPI that are critical for BAX 499 binding, and to determine how binding mediates aptamer inhibition of TFPI.

METHODS AND RESULTS

In vitro biochemical methods were used to evaluate the BAX 499 interaction with and inhibition of TFPI. Binding experiments indicated that the full-length TFPI protein is required for tight aptamer binding. Binding-competition experiments implicated the Kunitz 1, Kunitz 3 and C-terminal domains of TFPI in aptamer binding, a finding that is supported by hydrogen-deuterium exchange experiments, and indicated that aptamer and FXa can bind simultaneously to TFPI. In enzymatic assays, BAX 499 inhibited TFPI in a manner that is distinct from domain-specific antibodies, and aptamer inhibitory activity is reduced in the presence of the TFPI cofactor protein S.

CONCLUSIONS

These studies demonstrate that BAX 499 binds to TFPI via multiple domains of the protein in a manner that is distinct from other TFPI inhibitors, mediating a mechanism of inhibition that does not involve direct competition with FXa. With this unique inhibitory mechanism, BAX 499 provides a useful tool for studying TFPI biology in health and disease.

Tissue factor pathway inhibitor and liver diseases

Tissue factor pathway inhibitor (TFPI) is a proteinase inhibitor that is synthesized by microvascular endothelial cells and can primarily exert anticoagulant and anti-inflammatory effects. In chronic liver disease and early liver cirrhosis, tissue factors are excessively expressed due to continuous stress, infections and inflammatory stimulation in liver microvascular endothelial cells, which may lead to the rise of TFPI concentration. In severe liver disease and liver cirrhosis complicated with portal vein thrombosis (PVT), the level of TFPI may decline for massive consumption. Recombinant TFPI (rTFPI) can effectively protect against PVT, reduce the mortality of disseminated or diffuse intravascular coagulation and improve inflammation. Understanding of the role of TFPI in liver diseases may provide potential strategy for the prevention and treatment of liver cirrhosis and its complications.

Direct inhibition of factor VIIa by TFPI and TFPI constructs

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is a multi-Kunitz domain protease inhibitor that down-regulates the extrinsic coagulation pathway by inhibiting FXa and FVIIa.

OBJECTIVES

To investigate the role of the three Kunitz domains (KDs) of TFPI in FVIIa inhibition using full-length TFPI (TFPIfl ) and truncated TFPI constructs.

METHODS

Inhibition of FVIIa with/without relipidated tissue factor (TF) or soluble TF (sTF) by TFPIfl /TFPI constructs was quantified with a FVIIa-specific chromogenic substrate.

RESULTS AND CONCLUSIONS

TFPIfl inhibited TF-FVIIa via a monophasic reaction, which is rather slow at low TFPIfl concentrations (t½  ≈ 5 min at 2 nm TFPI) and has a Ki of 4.6 nm. In the presence of sTF and without TF, TFPIfl was a poor FVIIa inhibitor, with Ki values of 122 nm and 1118 nm, respectively. This indicates that phospholipids and TF significantly contribute to FVIIa inhibition by TFPIfl . TFPI constructs without the KD3-c-terminus (TFPI1-150 and KD1-KD2) were 7-10-fold less effective than TFPIfl in inhibiting TF-FVIIa and sTF-FVIIa, indicating that the KD3-C-terminus significantly contributes to direct inhibition of FVIIa by TFPI. Compared with KD1-KD2, KD1 was a poor TF-FVIIa inhibitor (Ki =434 nm), which shows that the KD2 domain of TFPI also contributes to FVIIa inhibition. Protein S stimulated TF-FVIIa inhibition by TFPIfl (Ki =0.7 nm). In the presence of FXa, a tight quaternary TF-FVIIa-TFPI-FXa complex is formed with TFPIfl , TFPI1-150 and KD1-KD2, with Ki values of < 0.15 nm, 0.5 nm and 0.8 nm, respectively, indicating the KD3-C-terminus is not a prerequisite for quaternary complex formation. Phospholipids and the Gla-domain of FXa are required for quaternary complex formation.