Cross-linking of wild-type and mutant alpha 2-antiplasmins to fibrin by activated factor XIII and by a tissue transglutaminase

Detection of factor XIII inhibitors in 33 patients with autoimmune factor XIII deficiency in Japan

Autoimmune factor XIII (FXIII) deficiency (AiF13D) is a rare hemorrhagic disease. The anti-FXIII autoantibodies that cause this disease are classified into three types: type Aa inhibits the heterotetramer assembly and activation of FXIII, type Ab inhibits the enzymatic activity of activated FXIII, and type B enhances the elimination of FXIII from the blood. The former two are FXIII inhibitors and may be lethal if overlooked by conventional functional assays. To reliably detect both types of FXIII inhibitors, a new assay was developed by incorporating 5-(biotinamido)pentylamine (BAPA) into α2-plasmin inhibitor (PI-BAPA assay). This assay was tested on plasma samples from 128 participants, including 60 healthy controls, 35 patients with non-immune acquired FXIII deficiency, and 33 patients with AiF13D (29 with type Aa inhibitors and 4 with type Ab inhibitors). The PI-BAPA assay successfully detected type Aa and Ab inhibitors in 5-step dilution cross-mixing tests between patient and normal plasma. This assay also showed comparable or superior inhibition rates in the 1:1 mixing test compared to conventional ammonia release and amine incorporation assays. Receiver operating characteristic curve analysis confirmed the excellent specificity and sensitivity of this assay for determining inhibition rates, and the assay has already been used for AiF13D diagnosis.

Transglutaminase Activities of Blood Coagulant Factor XIII Are Dependent on the Activation Pathways and on the Substrates

Factor XIII (FXIII) catalyzes formation of γ-glutamyl-ε-lysyl crosslinks between reactive glutamines (Q) and lysines (K). In plasma, FXIII is activated proteolytically (FXIII-A*) by the concerted action of thrombin and Ca2+. Cellular FXIII is activated nonproteolytically (FXIII-A°) by elevation of physiological Ca2+ concentrations. FXIII-A targets plasmatic and cellular substrates, but questions remain on correlating FXIII activation, resultant conformational changes, and crosslinking function to different physiological substrates. To address these issues, the characteristics of FXIII-A* versus FXIII-A° that contribute to transglutaminase activity and substrate specificities were investigated. Crosslinking of lysine mimics into a series of Q-containing substrates were measured using in-gel fluorescence, mass spectrometry, and UV-Vis spectroscopy. Covalent incorporation of fluorescent monodansylcadaverine revealed that FXIII-A* exhibits greater activity than FXIII-A° toward Q residues within Fbg αC (233-425 WT, Q328P Seoul II, and Q328PQ366N) and actin. FXIII-A* and FXIII-A° displayed similar activities toward α2-antiplasmin (α2AP), fibronectin, and Fbg αC (233-388, missing FXIII-binding site αC 389-402). Furthermore, the N-terminal α2AP peptide (1-15) exhibited similar kinetic properties for FXIII-A* and FXIII-A°. MALDI-TOF mass spectrometry assays with glycine ethyl ester and Fbg αC (233-425 WT, αC E396A, and truncated αC (233-388) further documented that FXIII-A* exerts greater benefit from the αC 389-402 binding site than FXIII-A°. Conformational properties of FXIII-A* versus A° are proposed to help promote transglutaminase function toward different substrates. A combination of protein substrate disorder and secondary FXIII-binding site exposure are utilized to control activity and specificity. From these studies, greater understandings of how FXIII-A targets different substrates are achieved.

Plasmin Inhibitor in Health and Diabetes: Role of the Protein as a Therapeutic Target

The vascular obstructive thrombus is composed of a mesh of fibrin fibers with blood cells trapped in these networks. Enhanced fibrin clot formation and/or suppression of fibrinolysis are associated with an increased risk of vascular occlusive events. Inhibitors of coagulation factors and activators of plasminogen have been clinically used to limit fibrin network formation and enhance lysis. While these agents are effective at reducing vascular occlusion, they carry a significant risk of bleeding complications. Fibrin clot lysis, essential for normal hemostasis, is controlled by several factors including the incorporation of antifibrinolytic proteins into the clot. Plasmin inhibitor (PI), a key antifibrinolytic protein, is cross-linked into fibrin networks with higher concentrations of PI documented in fibrin clots and plasma from high vascular risk individuals. This review is focused on exploring PI as a target for the prevention and treatment of vascular occlusive disease. We first discuss the relationship between the PI structure and antifibrinolytic activity, followed by describing the function of the protein in normal physiology and its role in pathological vascular thrombosis. Subsequently, we describe in detail the potential use of PI as a therapeutic target, including the array of methods employed for the modulation of protein activity. Effective and safe inhibition of PI may prove to be an alternative and specific way to reduce vascular thrombotic events while keeping bleeding risk to a minimum. Key Points Plasmin inhibitor (PI) is a key protein that inhibits fibrinolysis and stabilizes the fibrin network.This review is focused on discussing mechanistic pathways for PI action, role of the molecule in disease states, and potential use as a therapeutic target.

Ethacrynic acid is an inhibitor of human factor XIIIa

Background

Ethacrynic acid (EA) is a loop diuretic that is approved orally and parenterally to manage edema-associated diseases. Nevertheless, it was earlier reported that it is also associated with bleeding upon its parenteral administration. In this report, we investigated the effects of EA on human factor XIIIa (FXIIIa) of the coagulation process using a variety of techniques.

Methods

A series of biochemical and computational methods have been used in this study. The potency and efficacy of human FXIIIa inhibition by EA was evaluated using a bisubstrate-based fluorescence trans-glutamination assay under near physiological conditions. To establish the physiological relevance of FXIIIa inhibition by EA, the effect on FXIIIa-mediated polymerization of fibrin(ogen) as well as the formation of fibrin(ogen) – α2-antiplasmin complex was evaluated using SDS-PAGE experiments. The selectivity profile of EA against other coagulation proteins was assessed by evaluating EA’s effect on human clotting times in the activated partial thromboplastin time (APTT) and the prothrombin time (PT) assays. We also used molecular modeling studies to put forward a putative binding mode for EA in the active site of FXIIIa. Results involving EA were the average of at least three experiments and the standard error ± 1 was provided. In determining the inhibition parameters, we used non-linear regression analysis.

Results

FXIIIa is a transglutaminase that works at the end of the coagulation process to form an insoluble, rigid, and cross-linked fibrin rich blood clot. In fact, inhibition of FXIIIa-mediated biological processes has been reported to result in a bleeding diathesis. Inhibition of FXIIIa by EA was investigated given the nucleophilic nature of the thiol-containing active site of the enzyme and the Michael acceptor-based electrophilicity of EA. In a bisubstrate-based fluorescence trans-glutamination assay, EA inhibited FXIIIa with a moderate potency (IC50 ~ 105 µM) and efficacy (∆Y ~ 66%). In SDS-PAGE experiments, EA appears to significantly inhibit the FXIIIa-mediated polymerization of fibrin(ogen) as well as the formation of fibrin(ogen) – α2-antiplasmin complex which indicates that EA affects the physiological functions of FXIIIa. Interestingly, EA did not affect the clotting times of human plasma in the APTT and the PT assays at the highest concentration tested of 2.5 mM suggesting the lack of effects on the coagulation serine proteases and potentially the functional selectivity of EA with respect to the clotting process. Molecular modeling studies demonstrated that the Michael acceptor of EA forms a covalent bond with catalytic residue of Cys314 in the active site of FXIIIa.

Conclusions

Overall, our studies indicate that EA inhibits the physiological function of human FXIIIa in vitro which may potentially contribute to the bleeding complications that were reported with the association of the parenteral administration of EA.

The Utility and Potential of Mathematical Models in Predicting Fibrinolytic Outcomes

The enzymatic degradation of blood clots, fibrinolysis, is an important part of a healthy hemostatic system. If intrinsic fibrinolysis is ineffective, thrombolysis - the clinically-induced enzymatic degradation of blood clots - may be necessary to treat life-threatening conditions. In this review we discuss recent models of fibrinolysis and thrombolysis, and open questions that could be resolved through modeling and modeling-experimental collaboration. In particular, we focus on 2- and 3-dimensional models that can be used to study effects of fibrin network structure and realistic blood vessel geometries on the phenomena underlying lytic outcomes. Significant open questions such as the role of clot contraction, network and inherent fiber tension, and fibrinolytic inhibitors in lysis could benefit from mathematical models aimed at understanding the underlying biological mechanisms.

Factor XIII-A: An Indispensable "Factor" in Haemostasis and Wound Healing

Factor XIII (FXIII) is a transglutaminase enzyme that catalyses the formation of ε-(γ-glutamyl)lysyl isopeptide bonds into protein substrates. The plasma form, FXIIIA₂B₂, has an established function in haemostasis, with fibrin being its principal substrate. A deficiency in FXIII manifests as a severe bleeding diathesis emphasising its crucial role in this pathway. The FXIII-A gene (F13A1) is expressed in cells of bone marrow and mesenchymal lineage. The cellular form, a homodimer of the A subunits denoted FXIII-A, was perceived to remain intracellular, due to the lack of a classical signal peptide for its release. It is now apparent that FXIII-A can be externalised from cells, by an as yet unknown mechanism. Thus, three pools of FXIII-A exist within the circulation: plasma where it circulates in complex with the inhibitory FXIII-B subunits, and the cellular form encased within platelets and monocytes/macrophages. The abundance of this transglutaminase in different forms and locations in the vasculature reflect the complex and crucial roles of this enzyme in physiological processes. Herein, we examine the significance of these pools of FXIII-A in different settings and the evidence to date to support their function in haemostasis and wound healing.

Termination of bleeding by a specific, anticatalytic antibody against plasmin

BACKGROUND

Excessive, plasmin-mediated fibrinolysis augments bleeding and contributes to death in some patients. Current therapies for fibrinolytic bleeding are limited by modest efficacy, low potency, and off-target effects.

OBJECTIVES

To determine whether an antibody directed against unique loop structures of the plasmin protease domain may have enhanced specificity and potency for blocking plasmin activity, fibrinolysis, and experimental hemorrhage.

METHODS

The binding specificity, affinity, protease cross-reactivity and antifibrinolytic properties of a monoclonal plasmin inhibitor antibody (Pi) were examined and compared with those of epsilon aminocaproic acid (EACA), which is a clinically used fibrinolysis inhibitor.

RESULTS

Pi specifically recognized loop 5 of the protease domain, and did not bind to other serine proteases or nine other non-primate plasminogens. Pi was ~7 logs more potent in neutralizing plasmin cleavage of small-molecule substrates and >3 logs more potent in quenching fibrinolysis than EACA. Pi was similarly effective in blocking catalysis of a small-molecule substrate as α2 -antiplasmin, which is the most potent covalent inhibitor of plasmin, and was a more potent fibrinolysis inhibitor. Fab or chimerized Fab fragments of Pi were equivalently effective. In vivo, in a humanized model of fibrinolytic surgical bleeding, Pi significantly reduced bleeding to a greater extent than a clinical dose of EACA.

CONCLUSIONS

A mAb directed against unique loop sequences in the protease domain is a highly specific, potent, competitive plasmin inhibitor that significantly reduces experimental surgical bleeding in vivo.

Evaluating the Effects of Fibrinogen αC Mutations on the Ability of Factor XIII to Crosslink the Reactive αC Glutamines (Q237, Q328, Q366)

Fibrinogen (Fbg) levels and extent of fibrin polymerization have been associated with various pathological conditions such as cardiovascular disease, arteriosclerosis, and coagulation disorders. Activated factor XIII (FXIIIa) introduces γ-glutamyl-ε-lysinyl isopeptide bonds between reactive glutamines and lysines in the fibrin network to form a blood clot resistant to fibrinolysis. FXIIIa crosslinks the γ-chains and at multiple sites in the αC region of Fbg. Fbg αC contains a FXIII binding site involving αC (389-402) that is located near three glutamines whose reactivities rank Q237 >> Q366 ≈ Q328. Mass spectrometry and two-dimensional heteronuclear single-quantum correlation nuclear magnetic resonance assays were used to probe the anchoring role that αC E396 may play in controlling FXIII function and characterize the effects of Q237 on the reactivities of Q328 and Q366. Studies with αC (233-425) revealed that the E396A mutation does not prevent the transglutaminase function of FXIII A₂ or A₂B₂. Other residues must play a compensatory role in targeting FXIII to αC. Unlike full Fbg, Fbg αC (233-425) did not promote thrombin cleavage of FXIII, an event contributing to activation. With the αC (233-425) E396A mutant, Q237 exhibited slower reactivities compared with αC wild-type (WT) consistent with difficulties in directing this N-terminal segment toward an anchored FXIII interacting at a weaker binding region. Q328 and Q366 became less reactive when Q237 was replaced with inactive N237. Q237 crosslinking is proposed to promote targeting of Q328 and Q366 to the FXIII active site. FXIII thus uses Fbg αC anchoring sites and distinct Q environments to regulate substrate specificity.

Mass spectrometry-based molecular mapping of native FXIIIa cross-links in insoluble fibrin clots

The roles of factor XIIIa-specific cross-links in thrombus formation, regression, or probability for embolization are largely unknown. A molecular understanding of fibrin architecture at the level of these cross-links could inform the development of therapeutic strategies to prevent the sequelae of thromboembolism. Here, we present an MS-based method to map native factor XIIIa cross-links in the insoluble matrix component of whole-blood or plasma-fibrin clots and in in vivo thrombi. Using a chaotrope-insoluble digestion method and quantitative cross-linking MS, we identified the previously mapped fibrinogen peptides that are responsible for covalent D-dimer association, as well as dozens of novel cross-links in the αC region of fibrinogen α. Our findings expand the known native cross-linked species from one to over 100 and suggest distinct antiparallel registries for interprotofibril association and covalent attachment of serpins that regulate clot dissolution.

Inhibition of Fibrinolysis by Coagulation Factor XIII

The inhibitory effect of coagulation factor XIII (FXIII) on fibrinolysis has been studied for at least 50 years. Our insight into the underlying mechanisms has improved considerably, aided in particular by the discovery that activated FXIII cross-links α2-antiplasmin (α2AP) to fibrin. In this review, the most important effects of different cross-linking reactions on fibrinolysis are summarized. A distinction is made between fibrin-fibrin cross-links studied in purified systems and fibrin-α2AP cross-links studied in plasma or whole blood systems. While the formation of γ chain dimers in fibrin does not affect clot lysis, the formation of α chain polymers has a weak inhibitory effect. Only strong cross-linking of fibrin, associated with high molecular weight α chain polymers and/or γ chain multimers, results in a moderate inhibition fibrinolysis. The formation of fibrin-α2AP cross-links has only a weak effect on clot lysis, but this effect becomes strong when clot retraction occurs. Under these conditions, FXIII prevents α2AP being expelled from the clot and makes the clot relatively resistant to degradation by plasmin.

Differences in the inhibition of coagulation factor XIII-A from animal species revealed by Michael Acceptor- and thioimidazol based blockers

INTRODUCTION

The A-subunit of blood coagulation factor XIII is a pro-transglutaminase, which cross-links α- and γ-fibrin-chains in its activated form. Selective inhibitors against FXIII-A may be desirable drugs to prevent the development of thromboses. Animal models are generally used for proof of principle and for toxicological studies in drug development. The aim of the study was to investigate the specificity of a set of FXIII-A-blockers against FXIII-A from different species, i.e. human, dog, mouse, rat and pig. Thus the usefulness of different animal species for FXIII-A-blocker drug development should be evaluated.

MATERIALS AND METHODS

FXIII-A proteins were recombinantly produced in insect cells and purified to homogeneity. They were characterized by SDS- and native PAGE, a transamidase assay and isopeptidase assay. The inhibition second-order rate constants of different irreversible inhibitors were determined using the isopeptidase assay.

RESULTS

All FXIII-A species were able to assemble with recombinant human FXIII-B into a heterotetrameric complex. Kinetic parameters of FXIII-A species were determined. Second-order rate constants for FXIII-A inhibition by two irreversible inhibitors were determined and differed considerably. FXIII-A species of dog, mouse and rat were inhibited in a manner similar to human FXIII-A. Pig FXIII-A however was resistant to a previously described non-peptidic inhibitor. Furthermore, the results showed considerably better inhibition with the novel peptide-based inhibitor compared to the non-peptidic compound.

CONCLUSIONS

Our data shows that biochemical interspecies comparison studies are a prerequisite for animal studies. Peptide-derived inhibitors carrying a Michael Acceptor Pharmacophore (MAP) are a promising new class of FXIII-A-inhibitors.

Substrates of Factor XIII-A: roles in thrombosis and wound healing

FXIII (Factor XIII) is a Ca2+-dependent enzyme which forms covalent ϵ-(γ-glutamyl)lysine cross-links between the γ-carboxy-amine group of a glutamine residue and the ϵ-amino group of a lysine residue. FXIII was originally identified as a protein involved in fibrin clot stabilization; however, additional extracellular and intracellular roles for FXIII have been identified which influence thrombus resolution and tissue repair. The present review discusses the substrates of FXIIIa (activated FXIII) involved in thrombosis and wound healing with a particular focus on: (i) the influence of plasma FXIIIa on the formation of stable fibrin clots able to withstand mechanical and enzymatic breakdown through fibrin–fibrin cross-linking and cross-linking of fibrinolysis inhibitors, in particular α2-antiplasmin; (ii) the role of intracellular FXIIIa in clot retraction through cross-linking of platelet cytoskeleton proteins, including actin, myosin, filamin and vinculin; (iii) the role of intracellular FXIIIa in cross-linking the cytoplasmic tails of monocyte AT1Rs (angiotensin type 1 receptors) and potential effects on the development of atherosclerosis; and (iv) the role of FXIIIa on matrix deposition and tissue repair, including cross-linking of extracellular matrix proteins, such as fibronectin, collagen and von Willebrand factor, and the effects on matrix deposition and cell–matrix interactions. The review highlights the central role of FXIIIa in the regulation of thrombus stability, thrombus regulation, cell–matrix interactions and wound healing, which is supported by observations in FXIII-deficient humans and animals.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Reactivity of the N-terminal region of fibronectin protein to transglutaminase 2 and factor XIIIA

Transglutaminase 2 (TG2) is secreted by a non-classical pathway into the extracellular space, where it has several activities pertinent to fibronectin (FN), including binding to the gelatin-binding domain of FN and acting as an integrin co-receptor. Glutamines in the N-terminal tail of FN are known to be susceptible to transamidation by both TG2 and activated blood coagulation factor XIII (FXIIIa). We used immunoblotting, limited proteolysis, and mass spectrometry to localize glutamines within FN that are subject to TG2-catalyzed incorporation of dansylcadaverine in comparison to residues modified by FXIIIa. Such analysis of plasma FN indicated that Gln-3, Gln-7, and Gln-9 in the N-terminal tail and Gln-246 of the linker between fifth and sixth type I modules ((5)F1 and (6)F1) are transamidated by both enzymes. Only minor incorporation of dansylcadaverine was detected elsewhere. Labeling of C-terminally truncated FN constructs revealed efficient TG2- or FXIIIa-catalyzed dansylcadaverine incorporation into the N-terminal residues of constructs as small as the 29-kDa fragment that includes (1-5)F1 and lacks modules from the adjacent gelatin-binding domain. However, when only (1-3)F1 were present, dansylcadaverine incorporation into the N-terminal residues of FN was lost and instead was in the enzymes, near the active site of TG2 and terminal domains of FXIIIa. Thus, these results demonstrate that FXIIIa and TG2 act similarly on glutamines at either end of (1-5)F1 and transamidation specificity of both enzymes is achieved through interactions with the intact 29K fragment.

Serpin Inhibition Mechanism: A Delicate Balance between Native Metastable State and Polymerization

The serpins (serine proteinase inhibitors) are structurally similar but functionally diverse proteins that fold into a conserved structure and employ a unique suicide substrate-like inhibitory mechanism. Serpins play absolutely critical role in the control of proteases involved in the inflammatory, complement, coagulation and fibrinolytic pathways and are associated with many conformational diseases. Serpin's native state is a metastable state which transforms to a more stable state during its inhibitory mechanism. Serpin in the native form is in the stressed (S) conformation that undergoes a transition to a relaxed (R) conformation for the protease inhibition. During this transition the region called as reactive center loop which interacts with target proteases, inserts itself into the center of β-sheet A to form an extra strand. Serpin is delicately balanced to perform its function with many critical residues involved in maintaining metastability. However due to its typical mechanism of inhibition, naturally occurring serpin variants produces conformational instability that allows insertion of RCL of one molecule into the β-sheet A of another to form a loop-sheet linkage leading to its polymerization and aggregation. Thus understanding the molecular basis and amino acid involved in serpin polymerization mechanism is critical to devising strategies for its cure.

Contribution of conserved lysine residues in the alpha2-antiplasmin C terminus to plasmin binding and inhibition

α(2)-Antiplasmin is the physiological inhibitor of plasmin and is unique in the serpin family due to N- and C-terminal extensions beyond its core domain. The C-terminal extension comprises 55 amino acids from Asn-410 to Lys-464, and the lysine residues (Lys-418, Lys-427, Lys-434, Lys-441, Lys-448, and Lys-464) within this region are important in mediating the initial interaction with kringle domains of plasmin. To understand the role of lysine residues within the C terminus of α(2)-antiplasmin, we systematically and sequentially mutated the C-terminal lysines, studied the effects on the rate of plasmin inhibition, and measured the binding affinity for plasmin via surface plasmon resonance. We determined that the C-terminal lysine (Lys-464) is individually most important in initiating binding to plasmin. Using two independent methods, we also showed that the conserved internal lysine residues play a major role mediating binding of the C terminus of α(2)-antiplasmin to kringle domains of plasmin and in accelerating the rate of interaction between α(2)-antiplasmin and plasmin. When the C terminus of α(2)-antiplasmin was removed, the binding affinity for active site-blocked plasmin remained high, suggesting additional exosite interactions between the serpin core and plasmin.

Molecular mechanism of the interaction between activated factor XIII and its glutamine donor peptide substrate

BACKGROUND

Activated factor XIII (FXIII), a dimer of truncated A-subunits (FXIII-A2*), is a transglutaminase that crosslinks primary amines to peptide-bound glutamine residues. Because in the few natural substrates of FXIII-A2* no consensus sequence could be identified around the reactive glutamine, studying the interaction between individual substrates and FXIII-A2* is of primary importance. Most of the alpha2-plasmin inhibitor (alpha2PI) molecules become truncated by a plasma protease, and the truncated isoform (N1-alpha2PI) is an important substrate of FXIII-A2*. The crosslinking of N1-alpha2PI to fibrin plays a major role in protecting fibrin from fibrinolysis.

METHODS

We studied the interaction of FXIII-A2* with its dodecapeptide glutamine donor substrate, N1-alpha2PI(1-12), the sequence of which corresponds to the N-terminal sequence of N1-alpha2PI. Kinetic parameters for N1-alpha2PI(1-12) and for its truncated or synthetic mutants were determined by a spectrophotometric assay. The interaction of N1-alpha2PI(1-12) with FXIII-A2* was investigated by proton nuclear magnetic resonance (NMR) and saturating transfer difference (STD) NMR.

RESULTS AND CONCLUSIONS

Kinetic experiments with peptides in which the Asn1 residue was either truncated or replaced by alanine and proton NMR analysis of the FXIII-A2*-N1-alpha2PI(1-12) complex demonstrated that Asn1 is essential for effective enzyme-substrate interaction. Experiments with C-terminally truncated peptides proved that amino acids 7-12 are essential for the interaction of N1-alpha2PI(1-12) with the enzyme, and suggested the existence of a secondary binding site on FXIII-A2*. Hydrophobic residues, particularly Leu10 and the C-terminal Lys12, seemed to be especially important in this respect, and direct interaction between hydrophobic C-terminal residues and FXIII-A2* was demonstrated by STD NMR.

Addition of a sequence from alpha2-antiplasmin transforms human serum albumin into a blood clot component that speeds clot lysis

Background

The plasma protein α₂-antiplasmin (α₂AP) is cross-linked to fibrin in blood clots by the transglutaminase factor XIIIa, and in that location retards clot lysis. Competition for this effect could be clinically useful in patients with thrombosis. We hypothesized that fusion of N-terminal portions of α₂-antiplasmin to human serum albumin (HSA) and production of the chimeric proteins in Pichia pastoris yeast would produce a stable and effective competitor protein.

Results

Fusion protein α₂AP(13-42)-HSA was efficiently secreted from transformed yeast and purified preparations contained within a mixed population the full-length intact form, while fusions with longer α₂AP moieties were inefficiently secreted and/or degraded. The α₂AP(13-42)-HSA protein, but not recombinant HSA, was cross-linked to both chemical lysine donors and fibrin or fibrinogen by factor XIIIa, although with less rapid kinetics than native α₂AP. Excess α₂AP(13-42)-HSA competed with α₂AP for cross-linking to chemical lysine donors more effectively than a synthetic α₂AP(13-42) peptide, and reduced the α₂AP-dependent resistance to fibrinolysis of plasma clots equally effectively as the peptide. Native α₂AP was found in in vivo clots in rabbits to a greater extent than α₂AP(13-42), however.

Conclusion

In this first report of transfer of transglutamination substrate status from one plasma protein to another, fusion protein α₂AP(13-42)-HSA was shown to satisfy initial requirements for a long-lasting, well-tolerated competitive inhibitor of α₂-antiplasmin predicted to act in a clot-localized manner.

Evidence that alpha2-antiplasmin becomes covalently ligated to plasma fibrinogen in the circulation: a new role for plasma factor XIII in fibrinolysis regulation

BACKGROUND

Plasma alpha2-antiplasmin (alpha2AP) is a rapid and effective inhibitor of the fibrinolytic enzyme plasmin. Congenital alpha2AP deficiency results in a severe hemorrhagic disorder due to accelerated fibrinolysis. It is well established that in the presence of thrombin-activated factor XIII (FXIIIa), alpha2AP becomes covalently ligated to the distal alpha chains of fibrin or fibrinogen at lysine 303 (two potential sites per molecule). Some time ago we showed that alpha2AP is covalently linked to plasma fibrinogen . That singular observation led to our hypothesis that native plasma factor XIII (FXIII), which is known to catalyze covalent cross-linking of fibrinogen in the presence of calcium ions, can also incorporate alpha2AP into fibrinogen in the circulation.

RESULTS AND CONCLUSIONS

We now provide evidence that FXIII incorporates I 125-labelled alpha2AP into the Aalpha-chain sites on fibrinogen or fibrin. We also measured the content of alpha2AP in isolated plasma fibrinogen fractions by ELISA and found that substantial amounts were present (1.2-1.8 moles per mole fibrinogen). We propose that alpha2AP becomes ligated to fibrinogen while in the circulation through the action of FXIII, and that its immediate presence in plasma fibrinogen contributes to regulation of in vivo fibrinolysis.

Molecular cloning and functional expression of rabbit alpha2-antiplasmin

The rabbit is frequently employed in small animal models of in-vivo coagulation and fibrinolysis, but the degree to which its plasma proteins resemble their human counterparts is incompletely known. Our aims were: to determine the nucleotide sequence of a full-length rabbit liver alpha(2)-antiplasmin (alpha(2)AP) cDNA; compare it with its human, murine, and bovine counterparts; and express it in functional form. Partial cDNAs encoding 5' and 3' portions of the alpha(2)AP mRNA were obtained by reverse transcriptase (RT)-polymerase chain reaction (PCR), using rabbit liver RNA and 'guessmer' oligonucleotides based on known sequences. This information was employed to design additional oligonucleotides for RT-PCR and rapid amplification of cDNA ends (RACE) procedures yielding overlapping clones corresponding to the entire rabbit alpha(2)AP mRNA sequence. Mature alpha(2)AP was expressed as a hexahistidine-tagged recombinant protein in Escherichia coli, purified by nickel-chelate affinity and ion exchange chromatography, and its reaction with plasmin and soluble fibrin assessed electrophoretically and compared with an analogous recombinant human alpha(2)AP. The consensus rabbit alpha(2)AP cDNA is 2,157 bp long, and encodes a 464-residue mature sequence and a 27-amino-acid presequence. The mature protein is 82% identical to its human counterpart, and its recombinant form produced denaturation-resistant complexes with both plasmin and fibrin. The region of greatest sequence divergence lies within the C-terminal extension of alpha(2)AP, unique in the serpin family of proteins to which alpha(2)AP belongs. The highly conserved nature of rabbit alpha(2)AP reinforces its role as a vital antifibrinolytic protein and supports fibrinolytic investigations in models employing this small animal.

Why alpha-antiplasmin must be converted to a derivative form for optimal function

BACKGROUND

Human alpha(2)-antiplasmin (alpha(2)AP), the primary inhibitor of fibrinolysis, is secreted from the liver into plasma as a 464-residue protein with Met as the N-terminus. An R6W polymorphism has been suggested to affect fibrinolytic rate. Within circulating blood, antiplasmin-cleaving enzyme (APCE) cleaves Met-alpha(2)AP(R6) faster than Met-alpha(2)AP(W6) at the Pro12-Asn13 bond to yield Asn-alpha(2)AP.

OBJECTIVES

To compare Met-alpha(2)AP(R6), Met-alpha(2)AP(W6) and Asn-alpha(2)AP for crosslinking with fibrin and the ability to protect fibrin from digestion by plasmin.

METHODS AND RESULTS

Asn-alpha(2)AP utilizes Gln2 (Gln14 in Met-alpha(2)AP) to become crosslinked to fibrin approximately twelvefold faster than Met-alpha(2)AP(R6) or Met-alpha(2)AP(W6), and this enhances the resistance of fibrin to plasmin. All three forms of alpha(2)AP inhibit plasmin at identical rates. The N-terminal 12-residue peptide of Met-alpha(2)AP slows crosslinking of Met-alpha(2)AP(R6) or Met-alpha(2)AP(W6) by limiting access of factor XIIIa to Gln14 rather than shifting crosslinking to other Gln residues. Edman sequencing and mass analyses of tryptic peptides from each alpha(2)AP crosslinked with 5-(biotinamido)pentylamine showed Gln14 as the only major crosslinking site. Residues 5-8, GRQL in Met-alpha(2)AP(R6), and residues 1-8, MEPLGWQL in Met-alpha(2)AP(W6), slow fibrin crosslinking.

CONCLUSION

Gln14 in both Met-alpha(2)AP(R6) and Met-alpha(2)AP(W6) is sheltered by the N-terminal 12-residue peptide, which, when cleaved, yields Asn-alpha(2)AP, which is rapidly crosslinked to fibrin and maximally protects it from plasmin. The R6 W polymorphism in Met-alpha(2)AP does not affect its crosslinking to fibrin, but it does slow cleavage by APCE and reduces the amount of Asn-alpha(2)AP available for rapid crosslinking to fibrin.

Fibrinolysis is amplified by converting alpha-antiplasmin from a plasmin inhibitor to a substrate

alpha(2)-Antiplasmin (alpha(2)-AP) is the fast serpin inhibitor of plasmin and appears to limit the success of treatment for thrombosis. We examined the mechanisms through which monoclonal antibodies (mAbs) against alpha(2)-AP amplify fibrinolysis. The mAbs RWR, 49 and 77 interfered with the ability of alpha(2)-AP to inhibit plasmin, microplasmin and trypsin. In solution, mAbs 49 and 77 bound to alpha(2)-AP with 5-fold to 10-fold higher relative affinity than mAb-RWR, while mAb-RWR bound with greater avidity to immobilized or denatured alpha(2)-AP. Binding studies with chimeric alpha(2)-APs revealed that none of the mAbs bound to sites in alpha(2)-AP that form putative contacts with plasmin, namely the carboxy terminal lysines of alpha(2)-AP, or the reactive center loop in the serpin domain of alpha(2)-AP. Rather, mAb-RWR recognized an epitope in the amino-terminus of alpha(2)-AP (L(13)GNQEPGGQTALKSPPGVCS(32)) near the site at which alpha(2)-AP cross-links to fibrin. mAbs 49 and 77 bound to another conformational epitope in the serpin domain of alpha(2)-AP. mAbs 49 and 77 markedly increased the stoichiometry of plasmin inhibition by alpha(2)-AP (from 1.1 +/- 0.1 to 51 +/- 4 and 67 +/- 7) indicating that they convert alpha(2)-AP from an inhibitor to a substrate of plasmin. This was confirmed by sodium dodecylsulfate polyacrylamide gel electrophoresis analysis showing cleavage of alpha(2)-AP by plasmin in the presence of these mAbs. In summary, these mAbs appear to act at sites distinct from known alpha(2)-AP-plasmin contacts to enhance fibrinolysis by converting alpha(2)-AP from an inhibitor to a plasmin substrate.

Characterizing the specificity of activated Factor XIII for glutamine-containing substrate peptides

Activated Factor XIII (FXIIIa) is a transglutaminase that catalyzes the formation of gamma-glutamyl-epsilon-lysine crosslinks in the fibrin network. To better understand the source of FXIIIa substrate specificity, Q-containing substrates based on beta-casein, K9-peptide, and alpha(2)-antiplasmin were characterized. alpha(2)AP (1-15, Q2, Q4) and alpha(2)AP (1-15, Q2, Q4N, K12R) are highly promising peptide models since they exhibited k(cat)/K(m) values comparable to intact beta-casein. In the absence of a lysine-like donor, FXIIIa could promote deamidation of a reactive Q to an E and solution NMR served as an effective strategy for monitoring this reaction. A tendency toward deamidation allowed greater investigations of the alpha(2)-antiplasmin based peptides. FXIIIa preferentially selects the Q2 residue for carrying out crosslinking processes. The E3 and Q4 provide supporting roles in binding. When a crosslinking reaction occurs at Q2, the Q4 position is sterically blocked from reactivity. By contrast, deamidation of Q2 to E2 allows, for the first time, observation of reactivity at Q4. The K12 position provides an additional favorable site of interaction with the FXIIIa surface. The sensitivity of alpha(2)AP (1-15, Q2, Q4) to amino acid changes at Q2, Q4, and K12 suggests the importance of individual FXIIIa subsites that are controlled by chemical environment and sterics.

The fibrinolysis inhibitor alpha2-antiplasmin in the human cornea

PURPOSE

To determine whether the cornea contains and expresses, at the gene level, the major plasmin inhibitor alpha2-antiplasmin.

METHODS

Corneal sections were immunostained for alpha2-antiplasmin. Extracts of human corneal stroma, epithelium, and endothelium were subjected to immunodot blot and Western blot analysis. Total RNA and alpha2-antiplasmin specific primers were used for RT-PCR. The cDNA was sequenced.

RESULTS

Alpha2-antiplasmin was observed in all three corneal layers by immunolocalization and Western blots. The major alpha2-antiplasmin form observed in most extracts was the 70-kDa form. Total alpha2-antiplasmin was present at 0.119 +/- 0.014 microg/epithelium (n = 10) and 1.45 +/- 0.47 microg/stroma (n = 10). Alpha2-antiplasmin mRNA was detected in epithelial and stromal extracts and cultured human corneal stromal cells. The sequences of the PCR products were identical to that for human alpha2-antiplasmin.

CONCLUSIONS

Alpha2-antiplasmin and its mRNA are present in the cornea and may serve to regulate corneal plasmin activity.

Antiplasmin-cleaving enzyme is a soluble form of fibroblast activation protein

Circulating antiplasmin-cleaving enzyme (APCE) has a role in fibrinolysis and appears structurally similar to fibroblast activation protein (FAP), a cell-surface proteinase that promotes invasiveness of certain epithelial cancers. To explore this potential relationship, we performed comparative structure/function analyses of the 2 enzymes. APCE from human plasma and recombinant FAP (rFAP) exhibited identical pH optima of 7.5, extinction coefficients (in(280 nm)(1%)) of 20.2 and 20.5, common sequences of tryptic peptides, and cross-reactivity with FAP antibody. APCE and rFAP are homodimers with monomeric subunits of 97 and 93 kDa. Only homodimers appear to have enzymatic activity, with essentially identical kinetics toward Met-alpha2-antiplasmin (Met-alpha2AP) and peptide substrates. APCE and rFAP cleave both Pro3-Leu4 and Pro12-Asn13 bonds of Met-alpha2AP, but relative kcat/Km values for Pro12-Asn13 are about 16-fold higher than for Pro3-Leu4. APCE and rFAP demonstrate higher kcat/Km values toward a peptide modeled on P4-P4' sequence surrounding the Pro12-Asn13 primary cleavage site than for Z-Gly-Pro-AMC and Ala-Pro-AFC substrates. These data support APCE as a soluble derivative of FAP and Met-alpha2AP as its physiologic substrate. Conversion of Met-alpha2AP by membrane or soluble FAP to the more easily fibrin-incorporable form, Asn-alpha2AP, may increase plasmin inhibition within fibrin surrounding certain neoplasms and have an impact on growth and therapeutic susceptibility.

Antiplasmin

Much of the basic biochemistry of antiplasmin was described more than 20 years ago and yet it remains an enigmatic member of the serine protease inhibitor (serpin) family. It possesses all of the characteristics of other inhibitory serpins but in addition it has unique N- and C-terminal extensions which significantly modify its activities. The N-terminus serves as a substrate for Factor XIIIa leading to crosslinking and incorporation of antiplasmin into a clot as it is formed. Although free antiplasmin is an excellent inhibitor of plasmin, the fibrin bound form of the serpin appears to be the major regulator of clot lysis. The C-terminal portion of antiplasmin is highly conserved between species and contains several charged amino acids including four lysines with one of these at the C-terminus. This portion of the molecule mediates the initial interaction with plasmin and is a key component of antiplasmin's rapid and efficient inhibitory mechanism. Studies of mice with targeted deletion of antiplasmin have confirmed its importance as a major regulator of fibrinolysis and re-emphasized its value as a potential therapeutic target.

Role of FcRgamma and factor XIIIA in coated platelet formation

Platelet activation in response to dual stimulation with collagen and thrombin results in the formation of a subpopulation of activated platelets known as coated platelets. Coated platelets are characterized by high surface levels of alpha-granule proteins and phosphatidylserine, which support the assembly of procoagulant protein complexes. Using murine models, we tested the hypothesis that the collagen receptor-associated molecule FcRgamma and the transglutaminase factor XIIIA are required for the formation of coated platelets. Following dual stimulation with the collagen receptor agonist convulxin and thrombin, 68% of platelets from C57BL/6 mice acquired the coated platelet phenotype, defined by high surface levels of fibrinogen and von Willebrand factor and decreased binding of the alphaIIbbeta3 activation-dependent antibody PE-JON/A. In FcRgamma-/- mice, only 10% of platelets became "coated" after dual stimulation with convulxin plus thrombin (P < .05 vs C57BL/6 platelets). Decreased coated platelet formation in FcRgamma-/- platelets was accompanied by decreased annexin V binding (P < .01) and decreased platelet procoagulant activity (P < .05). Platelets from FXIIIA-/- mice did not differ from control platelets in coated platelet formation or annexin V binding. We conclude that FcRgamma, but not factor XIIIA, is essential for formation of highly procoagulant coated platelets following dual stimulation with collagen and thrombin.

A novel plasma proteinase potentiates alpha2-antiplasmin inhibition of fibrin digestion

Human alpha2-antiplasmin (alpha2AP), also known as alpha2-plasmin inhibitor, is the major inhibitor of the proteolytic enzyme plasmin that digests fibrin. There are 2 N-terminal forms of alpha2AP that circulate in human plasma: a 464-residue protein with Met as the N-terminus, Met-alpha2AP, and a 452-residue version with Asn as the N-terminus, Asn-alpha2AP. We have discovered and purified a proteinase from human plasma that cleaves the Pro12-Asn13 bond of Met-alpha2AP to yield Asn-alpha2AP and have named it antiplasmin-cleaving enzyme (APCE). APCE is similar in primary structure and catalytic properties to membrane-bound fibroblast activation protein/seprase for which a physiologic substrate has not been clearly defined. We found that Asn-alpha2AP becomes cross-linked to fibrin by activated factor XIII approximately 13 times faster than native Met-alpha2AP during clot formation and that clot lysis rates are slowed in direct proportion to the ratio of Asn-alpha2AP to Met-alpha2AP in human plasma. We conclude that APCE cleaves Met-alpha2AP to the derivative Asn-alpha2AP, which is more efficiently incorporated into fibrin and consequently makes it strikingly resistant to plasmin digestion. APCE may represent a new target for pharmacologic inhibition, since less generation and incorporation of Asn-alpha2AP could result in a more rapid removal of fibrin by plasmin during atherogenesis, thrombosis, and inflammatory states.

Effect of a synthetic carboxy-terminal peptide of alpha(2)-antiplasmin on urokinase-induced fibrinolysis

alpha(2)-Antiplasmin (alpha(2)AP) interferes with the binding of plasminogen to fibrin because lysine residues in its carboxy-terminal region compete with those in fibrin, presumably the same way that free lysine or epsilon-aminocaproic acid (EACA) inhibits plasminogen binding to fibrin. While this overall process causes an inhibition of fibrinolysis, the converse was observed with a 26-residue synthetic peptide (AP26) corresponding to the carboxy-terminal region of alpha(2)AP. The AP26 peptide, in fact, accelerated urokinase-induced lysis of (1) fully crosslinked fibrin with complete gamma-dimer and alpha-polymer formation; (2) partially crosslinked fibrin that had undergone only gamma-dimerization; and (3) noncrosslinked fibrin. The AP26 peptide also inhibited factor XIIIa-catalyzed crosslinking of fibrin alpha-chains, and this also accelerated lysis of fibrin. EACA had no effect. In the presence of noncrosslinked fibrin, AP26 promoted plasminogen activation by urokinase and fibrinolysis. EACA only slightly increased the rate of plasminogen activation, and as expected, it inhibited fibrinolysis. Since AP26 peptide enhanced the lysis of partially crosslinked and noncrosslinked fibrin, our results indicate that inhibition of factor XIIIa-catalyzed alpha-polymer formation by AP26, although associated with accelerated fibrinolysis, is not the primary mechanism. Instead, our data support the conclusion that AP26 enhances the conversion of plasminogen to plasmin approximately 5-fold, probably by inducing a conformational change in plasminogen structure just as occurs with low concentrations of lysine or EACA. At higher concentrations, however, AP26 apparently does not approach the avidity or affinity of lysine or EACA for the kringle structures of plasminogen or plasmin so that their binding to fibrin is blocked. Whether AP26 alone, or as part of another molecule, could have potential for enhancing thrombolysis will require further study.

Characterization of crosslinking sites in fibrinogen for plasminogen activator inhibitor 2 (PAI-2)

PAI-2 is a serpin that can be crosslinked to fibrin(ogen) via the Gln-Gln-Ile-Gln sequence (residues 83-86). We have characterized the lysine residues in fibrinogen to which PAI-2 is crosslinked by tissue transglutaminase and factor XIIIa. There was no competition with the crosslinking of alpha 2-antiplasmin, another inhibitor of fibrinolysis, which was specific for Lys 303 in the A alpha chain. PAI-2 was crosslinked to several lysine residues, all in the A alpha chain, 148, 176, 183, 230, 413, and 457, but not to Lys 303. The contrast with alpha 2-antiplasmin was clear from studies with truncated fibrinogens and competition by peptides. This was confirmed and extended by mass spectrometry of peptides after protease digestion of crosslinked products, which identified the lysine residues to which the inhibitors were crosslinked. PAI-2 remained active after cross-linking and inhibited fibrin breakdown, even by two-chain t-PA. Thus, a second inhibitor of fibrinolysis, in addition to alpha 2-antiplasmin, is crosslinked to fibrin and protects it from lysis.