Identification and characterisation of monoclonal antibodies that impair the activation of human thrombin activatable fibrinolysis inhibitor through different mechanisms

Identification of heparin interaction sites on thrombin-activatable fibrinolysis inhibitor that modulate plasmin-mediated activation, thermal stability, and antifibrinolytic potential

Background

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma zymogen that provides a molecular link between coagulation and fibrinolysis. Studies have shown that the presence of glycosaminoglycans accelerates TAFI activation by plasmin and stabilizes activated TAFI (TAFIa).

Objectives

We aimed to define the elements of TAFI structure that allow these effects.

Methods

Based on crystallographic studies and homology to heparin-binding proteins, we performed mutagenesis of surface-exposed charged residues on TAFI that putatively constitute heparin-binding sites. We determined heparin binding, kinetics of activation by plasmin in the presence or absence of heparin, thermal stability, and antifibrinolytic potential of each variant.

Results

Mutagenesis of Lys211 and Lys212 did not impair heparin binding but affected the ability of TAFI to be activated by plasmin. Mutagenesis of Lys306 and His308 did not impair heparin binding, but mutation of His308 had a severe negative effect on TAFI/TAFIa function. Mutation of Arg320 and Lys324 in combination markedly decreased heparin binding but had no effect on heparin-mediated acceleration of TAFI activation by plasmin while somewhat decreasing TAFIa stabilization by heparin. Mutagenesis of Lys327 and Arg330 decreased (but did not eliminate) heparin binding while decreasing the ability of heparin to accelerate plasmin-mediated TAFI activation, stabilize TAFIa, and increase the antifibrinolytic ability of TAFIa. A quadruple mutant of Arg320, Lys324, Lys327, and Arg330 completely lost heparin-binding ability and stabilization of the enzyme by heparin.

Conclusion

Basic residues in the dynamic flap of TAFIa define a functionally relevant heparin-binding site, but additional heparin-binding sites may be present on TAFI.

Fibrin(ogen) as a Therapeutic Target: Opportunities and Challenges

Fibrinogen is one of the key molecular players in haemostasis. Thrombin-mediated release of fibrinopeptides from fibrinogen converts this soluble protein into a network of fibrin fibres that form a building block for blood clots. Thrombin-activated factor XIII further crosslinks the fibrin fibres and incorporates antifibrinolytic proteins into the network, thus stabilising the clot. The conversion of fibrinogen to fibrin also exposes binding sites for fibrinolytic proteins to limit clot formation and avoid unwanted extension of the fibrin fibres. Altered clot structure and/or incorporation of antifibrinolytic proteins into fibrin networks disturbs the delicate equilibrium between clot formation and lysis, resulting in either unstable clots (predisposing to bleeding events) or persistent clots that are resistant to lysis (increasing risk of thrombosis). In this review, we discuss the factors responsible for alterations in fibrin(ogen) that can modulate clot stability, in turn predisposing to abnormal haemostasis. We also explore the mechanistic pathways that may allow the use of fibrinogen as a potential therapeutic target to treat vascular thrombosis or bleeding disorders. Better understanding of fibrinogen function will help to devise future effective and safe therapies to modulate thrombosis and bleeding risk, while maintaining the fine balance between clot formation and lysis.

Thrombin Activatable Fibrinolysis Inhibitor (TAFI): An Updated Narrative Review

Thrombin activatable fibrinolysis inhibitor (TAFI), a proenzyme, is converted to a potent attenuator of the fibrinolytic system upon activation by thrombin, plasmin, or the thrombin/thrombomodulin complex. Since TAFI forms a molecular link between coagulation and fibrinolysis and plays a potential role in venous and arterial thrombotic diseases, much interest has been tied to the development of molecules that antagonize its function. This review aims at providing a general overview on the biochemical properties of TAFI, its (patho)physiologic function, and various strategies to stimulate the fibrinolytic system by interfering with (activated) TAFI functionality.

Carboxypeptidase U (CPU, TAFIa, CPB2) in Thromboembolic Disease: What Do We Know Three Decades after Its Discovery?

Procarboxypeptidase U (proCPU, TAFI, proCPB2) is a basic carboxypeptidase zymogen that is converted by thrombin(-thrombomodulin) or plasmin into the active carboxypeptidase U (CPU, TAFIa, CPB2), a potent attenuator of fibrinolysis. As CPU forms a molecular link between coagulation and fibrinolysis, the development of CPU inhibitors as profibrinolytic agents constitutes an attractive new concept to improve endogenous fibrinolysis or to increase the efficacy of thrombolytic therapy in thromboembolic diseases. Furthermore, extensive research has been conducted on the in vivo role of CPU in (the acute phase of) thromboembolic disease, as well as on the hypothesis that high proCPU levels and the Thr/Ile325 polymorphism may cause a thrombotic predisposition. In this paper, an overview is given of the methods available for measuring proCPU, CPU, and inactivated CPU (CPUi), together with a summary of the clinical data generated so far, ranging from the current knowledge on proCPU concentrations and polymorphisms as potential thromboembolic risk factors to the positioning of different CPU forms (proCPU, CPU, and CPUi) as diagnostic markers for thromboembolic disease, and the potential benefit of pharmacological inhibition of the CPU pathway.

Elucidation of the molecular mechanisms of two nanobodies that inhibit thrombin-activatable fibrinolysis inhibitor activation and activated thrombin-activatable fibrinolysis inhibitor activity

Essentials Thrombin-activatable fibrinolysis inhibitor (TAFI) is a risk factor for cardiovascular disorders. TAFI inhibitory nanobodies represent a promising step in developing profibrinolytic therapeutics. We have solved three crystal structures of TAFI in complex with inhibitory nanobodies. Nanobodies inhibit TAFI through distinct mechanisms and represent novel profibrinolytic leads.

SUMMARY

Background Thrombin-activatable fibrinolysis inhibitor (TAFI) is converted to activated TAFI (TAFIa) by thrombin, plasmin, or the thrombin-thrombomodulin complex (T/TM). TAFIa is antifibrinolytic, and high levels of TAFIa are associated with an increased risk for cardiovascular disorders. TAFI-inhibitory nanobodies represent a promising approach for developing profibrinolytic therapeutics. Objective To elucidate the molecular mechanisms of inhibition of TAFI activation and TAFIa activity by nanobodies with the use of X-ray crystallography and biochemical characterization. Methods and results We selected two nanobodies for cocrystallization with TAFI. VHH-a204 interferes with all TAFI activation modes, whereas VHH-i83 interferes with T/TM-mediated activation and also inhibits TAFIa activity. The 3.05-Å-resolution crystal structure of TAFI-VHH-a204 reveals that the VHH-a204 epitope is localized to the catalytic moiety (CM) in close proximity to the TAFI activation site at Arg92, indicating that VHH-a204 inhibits TAFI activation by steric hindrance. The 2.85-Å-resolution crystal structure of TAFI-VHH-i83 reveals that the VHH-i83 epitope is located close to the presumptive thrombomodulin-binding site in the activation peptide (AP). The structure and supporting biochemical assays suggest that VHH-i83 inhibits TAFIa by bridging the AP to the CM following TAFI activation. In addition, the 3.00-Å-resolution crystal structure of the triple TAFI-VHH-a204-VHH-i83 complex demonstrates that the two nanobodies can simultaneously bind to TAFI. Conclusions This study provides detailed insights into the molecular mechanisms of TAFI inhibition, and reveals a novel mode of TAFIa inhibition. VHH-a204 and VHH-i83 merit further evaluation as potential profibrinolytic therapeutics.

Structure-function relationships in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is an important regulator in the balance of coagulation and fibrinolysis. TAFI is a metallocarboxypeptidase that circulates in plasma as zymogen. Activated TAFI (TAFIa) cleaves C-terminal lysine or arginine residues from peptide substrates. The removal of C-terminal lysine residues from partially degraded fibrin leads to reduced plasmin formation and thus attenuation of fibrinolysis. TAFI also plays a role in inflammatory processes via the removal of C-terminal arginine or lysine residues from bradykinin, thrombin-cleaved osteopontin, C3a, C5a and chemerin. TAFI has been studied extensively over the past three decades and recent publications provide a wealth of information, including crystal structures, mutants and structural data obtained with antibodies and peptides. In this review, we combined and compared available data on structure/function relationships of TAFI.

Generation of a stable thrombin-activatable fibrinolysis inhibitor deletion mutant exerting full carboxypeptidase activity without activation

BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a zymogen that can be activated to form activated TAFI (TAFIa) (Ala93-Val401) through removal of the N-terminal activation peptide (Phe1-Arg92). TAFIa is thermally unstable, and the role of the activation peptide in the activity and stability of TAFI zymogen remains unclear.

OBJECTIVES

To better understand the role of the activation peptide in the activity and stability of TAFI.

METHODS

We constructed a deletion mutant, TAFI-CIIYQ-∆1-73 , in which the first 73 amino acids of the activation peptide are absent. The intrinsic activity and functional stability were determined with a chromogenic assay. The activation of TAFI-CIIYQ-∆1-73 by TAFI activators was evaluated with western blot analysis.

RESULTS

In comparison with TAFI-CIIYQ, the deletion mutant exerted high intrinsic activity ('full' apparent TAFIa activity) without cleavage by TAFI activators. TAFI-CIIYQ-∆1-73 was cleavable by thrombin. However, in the presence of thrombomodulin, the thrombin-mediated cleavage of TAFI-CIIYQ-∆1-73 was not accelerated. TAFI-CIIYQ-∆1-73 showed a similar functional stability profile to that of TAFI-CIIYQ. Full cleavage by thrombin did not affect the apparent carboxypeptidase activity of TAFI-CIIYQ-∆1-73 , but resulted in a significant loss of functional stability.

CONCLUSIONS

A stable deletion mutant of TAFI with full carboxypeptidase activity without activation is described. The segment Ala74-Arg92 in the activation peptide contributes significantly to the role of the activation peptide in stabilization of the catalytic moiety in TAFI zymogen.

Novel or expanding current targets in fibrinolysis

Globally the leading cause of long-term disability and mortality stems from cardiovascular diseases, which creates an enormous economic burden. Currently available treatments for intravascular thrombosis consist of a large repertoire of antithrombotic agents targeting coagulation and platelet function. However, the only agents available to enhance fibrinolysis are recombinant or modified forms of plasminogen activators. Their clinical use is limited by low efficacy, life-threatening side-effects (primarily caused by the high systemic dose required) and the inapplicability for prophylactic use. This review provides an update on the latest advances in targeting the antifibrinolytic proteins, plasminogen activator inhibitor-1 and thrombin-activatable fibrinolysis inhibitor, and will highlight novel therapeutic avenues to enhance fibrinolysis.

Monoclonal antibodies targeting the antifibrinolytic activity of activated thrombin-activatable fibrinolysis inhibitor but not the anti-inflammatory activity on osteopontin and C5a

BACKGROUND

Recently, anti-thrombin-activatable fibrinolysis inhibitor (TAFI) mAbs selectively inhibiting plasmin-mediated TAFI activation were shown to stimulate fibrinolysis in vitro and in vivo, suggesting, in contrast to other findings, that plasmin-mediated TAFI activation plays an important role in fibrinolysis regulation.

OBJECTIVE

To further characterize the effects of two plasmin-specific anti-TAFI mAbs (MA-TCK11A9 and MA-TCK26D6) on TAFI-dependent inhibition of fibrinolysis.

METHODS AND RESULTS

Both mAbs inhibited plasmin-mediated but not thrombin/thrombomodulin-mediated TAFI activation, whereas neither inhibited the cleavage of hippuryl-arginine by activated TAFI (TAFIa). They stimulated tissue-type plasminogen activator-induced fibrinolysis in different clot lysis models through a TAFI-dependent mechanism, especially in the presence of thrombomodulin (TM), a condition in which TAFI is largely activated by the thrombin-TM complex. In a fibrinolysis-based TAFIa activity assay, both mAbs inhibited TAFIa, whereas other mAbs targeting thrombin-TM-mediated TAFI activation did not. The inhibition of TAFIa activity, however, was substrate-specific, because neither mAb inhibited the cleavage of thrombin-activated osteopontin and C5a by TAFIa, thus sparing the anti-inflammatory activity of TAFIa.

CONCLUSIONS

Our anti-TAFI mAbs, by selectively inhibiting TAFIa activity on fibrin, may represent the prototype of a new class of TAFI inhibitors with improved pharmacologic activity.

Thrombomodulin and the vascular endothelium: insights into functional, regulatory, and therapeutic aspects

Thrombomodulin (TM) is a 557-amino acid protein with a broad cell and tissue distribution consistent with its wide-ranging physiological roles. When expressed on the lumenal surface of vascular endothelial cells in both large vessels and capillaries, its primary function is to mediate endothelial thromboresistance. The complete integral membrane-bound protein form displays five distinct functional domains, although shorter soluble (functional) variants comprising the extracellular domains have also been reported in fluids such as serum and urine. TM-mediated binding of thrombin is known to enhance the specificity of the latter serine protease toward both protein C and thrombin activatable fibrinolysis inhibitor (TAFI), increasing their proteolytic activation rate by almost three orders of magnitude with concomitant anticoagulant, antifibrinolytic, and anti-inflammatory benefits to the vascular wall. Recent years have seen an abundance of research into the cellular mechanisms governing endothelial TM production, processing, and regulation (including flow-mediated mechanoregulation)--from transcriptional and posttranscriptional (miRNA) regulation of TM gene expression, to posttranslational processing and release of the expressed protein--facilitating greater exploitation of its therapeutic potential. The goal of the present paper is to comprehensively review the endothelial/TM system from these regulatory perspectives and draw some fresh conclusions. This paper will conclude with a timely examination of the current status of TM's growing therapeutic appeal, from novel strategies to improve the clinical efficacy of recombinant TM analogs for resolution of vascular disorders such as disseminated intravascular coagulation (DIC), to an examination of the complex pleiotropic relationship between statin treatment and TM expression.

Insights into thrombin activatable fibrinolysis inhibitor function and regulation

Fibrinolysis is initiated when the zymogen plasminogen is converted to plasmin via the action of plasminogen activators. Proteolytic cleavage of fibrin by plasmin generates C-terminal lysine residues capable of binding both plasminogen and the plasminogen activator, thereby stimulating plasminogen activator-mediated plasminogen activation and propagating fibrinolysis. This positive feedback mechanism is regulated by activated thrombin activatable fibrinolysis inhibitor (TAFIa), which cleaves C-terminal lysine residues from the fibrin surface, thereby decreasing its cofactor activity. TAFI can be activated by thrombin alone, but the rate of activation is accelerated when in complex with thrombomodulin. Plasmin is also known to activate TAFI. TAFIa has no known physiologic inhibitors and consequently, its primary regulatory mechanism involves its intrinsic thermal instability. The rate of TAFI activation and stability of the active form, TAFIa, function in maintaining its concentration above the threshold value required to down-regulate fibrinolysis. Although all methods to quantify TAFI or TAFIa have their limitations, epidemiologic studies have indicated that elevated TAFI levels are correlated with an increased risk of venous thrombosis. Major efforts have been made to develop TAFI inhibitors that can either directly interfere with TAFIa activity or impair its activation. However, the anti-inflammatory properties of TAFIa might complicate the development and application of a TAFIa inhibitor that aims to increase the efficiency of thrombolytic therapy.

Intravenous and oral administrations of DD2 [7-Amino-2-(sulfanylmethyl)heptanoic acid] produce thrombolysis through inhibition of plasma TAFIa in rats with tissue factor-induced microthrombosis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma zymogen that is activated by thrombin in plasma. In fibrinolytic processes, carboxy-terminal lysine (Lys) residues in partially degraded fibrin are important sites for plasminogen binding and activation, and an active form of TAFI (TAFIa) inhibits fibrinolysis by eliminating these residues proteolytically. We synthesized DD2 [7-Amino-2-(sulfanylmethyl)heptanoic acid], a Lys analogue containing sulfur, as an inhibitor of TAFIa and investigated its pharmacological profile and pathophysiological role in thrombolysis via in vitro and in vivo studies. DD2 specifically inhibited plasma TAFIa activity with an apparent IC(50) (50% inhibitory concentration) value of 3.4×10(-8)M under the present experimental condition and enhanced tissue plasminogen activator-mediated clot lysis in a concentration-dependent manner. In order to study tissue factor (TF)-induced microthrombosis in an animal model, rats were given intravenous injection (2.5mg/kg and higher) or oral administration (10mg/kg and higher) of DD2. This attenuated TF-induced glomerular fibrin deposition and increased the plasma levels of fibrin degradation products and D-dimer in a dose-dependent manner. A DD2 dose approximately 4X higher than the dose used in intravenous injections was required to achieve an equivalent thrombolytic effect to that seen following oral administration. Moreover, the oral absorption efficiency of DD2 into the vasculature was 29.8%. These results indicate that both intravenous and oral administration of DD2 enhanced endogenous fibrinolysis and reduced thrombi in a TF-induced microthrombosis model.

Increased zymogen activity of thrombin-activatable fibrinolysis inhibitor prolongs clot lysis

BACKGROUND AND OBJECTIVES

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a zymogen that can be activated by proteolytic cleavage into the active enzyme TAFIa. Hydrolysis of the C-terminal lysines on fibrin by TAFIa results in a down-regulation of fibrinolysis. Recent studies demonstrated that the zymogen also exerts an intrinsic enzymatic activity. Our objective was to identify and characterize zymogen-stimulatory nanobodies.

METHODS AND RESULTS

The screening of 24 nanobodies against TAFI revealed that two nanobodies (i.e. Vhh-TAFI-a51 and Vhh-TAFI-i103) were able to stimulate the zymogen activity 10- to 21-fold compared with the baseline zymogen activity of TAFI. The increase in catalytic efficiency can be attributed mainly to an increased catalytic rate, as no change in the K(M) -value was observed. The stability, the susceptibility towards PTCI and GEMSA and the kinetics of the stimulated zymogen activity differ significantly from those of TAFIa activity. Epitope mapping revealed that both Asp(75) and Thr(301) are major determinants in the binding of these nanobodies to TAFI. Localization of the epitope strongly suggests that this instability is as a result of a disruption of the stabilizing interactions between the activation peptide and the dynamic flap region (residues 296-350). In TAFI-depleted plasma reconstituted with a non-activatable variant of TAFI (TAFI-R92A), clot lysis could be prolonged by nanobody-induced stimulation of its zymogen activity as well as by increasing its concentration.

CONCLUSIONS

Increasing the zymogen activity of TAFI results in an antifibrinolytic effect.

Fibrinolytic status in acute coronary syndromes: evidence of differences in relation to clinical features and pathophysiological pathways

Limited data are available on the role of innate fibrinolysis in acute coronary syndromes (ACS). In the present study we evaluated the dynamic alterations of fibrinolytic markers in patients presenting with ACS. Tissue-type-(tPA) and urokinase type-(uPA) plasminogen activators, plasminogen activator inhibitor (PAI-1) antigen and activity and thrombin activatable fibrinolysis inhibitor (TAFI) were analysed in 50 patients with ST elevation myocardial infarction (STEMI), 47 non-STEMI patients (NSTEMI), 40 patients with stable coronary artery disease (CAD) and 39 controls. The parameters were measured on day 1 and days 3, 7 and 30. Counts of monocyte subsets, monocyte-platelet aggregates and plasma inflammatory cytokines were assessed on admission. On day 1, TAFI was higher in NSTEMI vs. STEMI (p<0.001) while PAI-1 activity was higher in STEMI (p<0.001). In STEMI, uPA activity levels was low on day 1 but significantly increased on day 30 (p<0.001). TAFI levels were increased in NSTEMI on day 1 and gradually reduced by day 30 (p<0.05). In STEMI, TAFI levels peaked at day 7 (p<0.05) and dropped significantly by day 30 (p<0.05). CD14++CD16+ monocytes were independently associated with PAI-1 activity in ACS (p=0.03). Monocyte-platelet aggregates rather than platelet-free monocytes were an independent determinant of tPA, PAI-1 antigen and TAFI on a multivariate analysis (p<0.05). There are significant differences in fibrinolytic activity between patients with STEMI and NSTEMI. These changes could reflect the role of these factors in post-MI myocardial healing. Monocyte-platelet interactions are independently associated with the regulation of the fibrinolytic status in ACS.

Thrombin activatable fibrinolysis inhibitor

Thrombin activatable fibrinolysis inhibitor (TAFI) was discovered two decades ago as a consequence of the identification of an unstable carboxypeptidase (CPU), which was formed upon thrombin activation of the respective pro-enzyme (proCPU). The antifibrinolytic function of the activated form (TAFIa, CPU) is directly linked to its capacity to remove C-terminal lysines from the surface of the fibrin clot. No endogenous inhibitors have been identified, but TAFIa activity is regulated by its intrinsic temperature-dependent instability with a half-life of 8 to 15 min at 37 °C. A variety of studies have demonstrated a role for TAFI/TAFIa in venous and arterial diseases. In addition, a role in inflammation and cell migration has been shown. Since an elevated level of TAFIa it is a potential risk factor for thrombotic disorders, many inhibitors, both at the level of activation or at the level of activity, have been developed and were proven to exhibit a profibrinolytic effect in animal models. Pharmacologically active inhibitors of the TAFI/TAFIa system may open new ways for the prevention of thrombotic diseases or for the establishment of adjunctive treatments during thrombolytic therapy.