Subunit Structure of Human Fibrinogen, Soluble Fibrin, and Cross-Linked Insoluble Fibrin

The three unique polypeptide chains of human fibrinogen differ significantly in molecular weight. Cross-linkage of fibrin by fibrin-stabilizing factor results in the rapid formation of cross-links between gamma-chains and a slower formation of cross-links between alpha-chains. beta-Chains are not involved directly in the cross-linking of fibrin. Reduced, cross-linked fibrin contains uncross-linked beta-chains, dimers of gamma-chain, and higher polymers of alpha-chain. Although it is uncertain whether the gamma-gamma dimers are formed by chains in different molecules of fibrin, the polymers of alpha-chain in fibrin can only be accounted for by cross-linkage of alpha-chains in different molecules. The nature of cross-linkage among the subunits in fibrin can account well for the three-dimensional, covalent structure of cross-linked, insoluble fibrin.     

Early cross-linked fibrin in human plasma contains alpha-polymers with intact fibrinopeptide A.

alpha-polymer formation, as opposed to gamma-chain dimerization has been considered a relatively late event in factor XIII-induced fibrin stabilization. Recently it has been shown, however, that plasma from healthy individuals and from patients with fibrinaemia contains small amounts of soluble fibrin/fibrinogen oligomers interlinked through dimerized gamma-chains as well as cross-linked alpha-chains. The present work was carried out to see if these early alpha-chain polymers also arise during coagulation of plasma in vitro. Plasma samples from healthy individuals, prepared by immediate centrifugation of blood collected without anticoagulant, were allowed to clot spontaneously for varying periods. The plasma clots were solubilized in SDS-urea-mercaptoethanol and samples were subjected to SDS-PAGE and Western blotting using polyclonal antibodies to human fibrinogen, or monoclonal antibodies specific either for A alpha/alpha-chains, for fibrinopeptide A-containing chains, for the N-terminus of the fibrin beta-chain or for the gamma-chains. Fibrin/fibrinogen oligomers were seen to form long before visible gelation of plasma. These oligomers were cross-linked through gamma-chain dimerization, but also through A alpha- or alpha-chain polymerization. The number and amount of alpha-polymers containing A alpha-chains increased immediately after clot formation, but these disappeared about 20 min later, due to complete removal of fibrinopeptide A (FPA) by thrombin. It is concluded that alpha-polymer formation is a very early event during plasma coagulation in vitro, and that both A alpha- and alpha-chains are involved.     

Elimination of fibrin γ-chain cross-linking by FXIIIa increases pulmonary embolism arising from murine inferior vena cava thrombi

The onset of venous thromboembolism, including pulmonary embolism, represents a significant health burden affecting more than 1 million people annually worldwide. Current treatment options are based on anticoagulation, which is suboptimal for preventing further embolic events. In order to develop better treatments for thromboembolism, we sought to understand the structural and mechanical properties of blood clots and how this influences embolism in vivo. We developed a murine model in which fibrin γ-chain cross-linking by activated Factor XIII is eliminated (FGG3X) and applied methods to study thromboembolism at whole-body and organ levels. We show that FGG3X mice have a normal phenotype, with overall coagulation parameters and platelet aggregation and function largely unaffected, except for total inhibition of fibrin γ-chain cross-linking. Elimination of fibrin γ-chain cross-linking resulted in thrombi with reduced strength that were prone to fragmentation. Analysis of embolism in vivo using Xtreme optical imaging and light sheet microscopy demonstrated that the elimination of fibrin γ-chain cross-linking resulted in increased embolization without affecting clot size or lysis. Our findings point to a central previously unrecognized role for fibrin γ-chain cross-linking in clot stability. They also indirectly indicate mechanistic targets for the prevention of thrombosis through selective modulation of fibrin α-chain but not γ-chain cross-linking by activated Factor XIII to reduce thrombus size and burden, while maintaining clot stability and preventing embolism.

Thrombosis is complicated by life-threatening embolic events, caused by parts of an intravascular blood clot breaking off and traveling downstream to block other blood vessels supplying critical organs. Thromboembolism occurs in both the venous and arterial circulation and is associated with life-threatening pulmonary embolism (PE) (1) and ischemic stroke (2). PE occurs when thrombi in the deep veins of the limb embolise and passage with the flowing blood through the inferior vena cava, the right atrium, and ventricle of the heart to the lungs (3), causing pulmonary hypertension and respiratory failure (4). Venous thromboembolism (VTE), comprising deep vein thrombosis (DVT) and PE, which globally affects over 1 million people each year (1), results in substantial healthcare costs (5) and is a major cause of death worldwide (1, 6). Thromboembolism is clinically challenging to treat. Anticoagulation with vitamin K antagonists or direct oral anticoagulants are used to treat VTE and prophylactically to prevent VTE recurrence (7). In PE, localized thrombolysis with plasminogen activators is challenging and often only used as a last resort to help remove emboli resistant to anticoagulation (8). Improvements in treatment and prevention of thromboembolic disorders are therefore urgently needed.Recent studies indicate that structural and functional properties of the clot could be critical in thromboembolism, and these parameters may offer novel areas for therapeutic intervention. Hypofibrinolysis was reported to increase the risk of a first DVT (9), while changes in clot properties (increased clot formation rate and fiber density, reduced fibrinolysis) promoted the recurrence of DVT (10). Abnormalities in the establishment of clot viscoelastic properties have been shown to increase risk of PE (11), and reduced clot elastic modulus has been associated with VTE recurrence (12). However, the exact mechanisms linking altered clot properties to increased thromboembolic risk are unclear, and therefore, treatment options remain limited and rely on dissolution of fibrin networks and prevention of future clot formation, which carry significant risk of bleeding events.A key regulator of clot mechanical properties is coagulation Factor XIII (FXIII), a protransglutaminase that is converted into the active transglutaminase (FXIIIa) by thrombin (13). FXIIIa catalyzes the formation of γ-glutamyl-ε-lysine isopeptide bonds between adjacent molecules within the fibrin fibers to substantially increase elastic moduli and reduce storage moduli of both individual fibrin fibers and fibrin networks (14–17), therefore making the clot more elastic and less viscous. FXIIIa cross-links fibrin γ-chain residues Q398 and Q399 with K406 (18, 19), and α-chain residues Q221, Q237, Q328, and Q366 with numerous lysine residues (20–22). We previously demonstrated a critical role for γ-chain cross-linking by FXIIIa in generating clot viscoelastic properties, in particular by increasing the elastic or Young’s modulus, using a human recombinant fibrinogen γ-3X (γ-Q398N/Q399N/K406R) mutant of the essential γ-chain cross-linking sites (23–25).The fibrin γ-chain cross-linking sites for FXIIIa are highly conserved, and based on our previous in vitro data (23–25), we have now generated a genetically modified mouse in which the fibrin γ-chain cross-linking sites are mutated (FGG3X) to understand the role of fibrin fiber cross-linking in predisposition to embolic disease. We confirm the importance of γ-chain cross-linking in enhancing clot mechanical properties in vivo. Furthermore, using two protocols to study VTE, we demonstrate that lack of γ-chain cross-linking by FXIIIa increases thromboembolism using advanced whole-body and whole-organ imaging. We further show that fibrin fibers lacking γ-chain cross-linking are more prone to rupture at lower stress. These data indicate that fibrin γ-chain cross-linking enhances the resistance of fibrin fibers to rupture, consequently reducing clot fragmentation and thromboembolism.     

Multicolour immuno-staining of fibrinogen polypeptide chains for identification of their derivatives in electrophoregrams.

A novel electrophoretic procedure enabling multiple, direct immunoprobing of electrophoregrams without depending on Western blotting is described, and applied to the identification of the derivatives formed in the early stages of clot stabilization. Multicolour immunostainings for positive identification of cross-linked chains in partially stabilized fibrin clots indicated that the early products of alpha-chain cross-linking by factor XIII are largely hybrids of co-cross-linking of alpha- and gamma-chains rather than alpha-chain polymers suggested from previous studies employing non-specific staining of electrophoregrams. Furthermore, plasma-fibrinogen dimers were found to contain cross-linked alpha-chains with an electrophoretic mobility very near that of gamma-gamma-dyads. A similar product is produced by tissue-transglutaminase, but not by factor XIII.     

Isolation, Characterization, and Location of a Donor-Acceptor Unit from Cross-Linked Fibrin

The cross-linking systems of bovine and human fibrins were studied by the introduction of a radioactive substitute donor as an inhibitor of fibrin cross-linking, separation of the constituent polypeptide chains after sulfitolysis, and tryptic digestion of the labeled gamma-chains. The information gathered from this approach enabled us to isolate and characterize the complete donor-acceptor unit in tryptic digests of fibrin gamma-gamma cross-linked systems. In both bovine and human fibrin, this kind of cross-linking is accomplished by reciprocal bridging between overlapping carboxy-terminal segments of neighboring gamma-chains. The amino acid sequence of the carboxy-terminal heptadecapeptide of the bovine gamma-chain was determined and an alignment of the corresponding region of the human gamma-chain established.     

Alpha-Chain cross-linking in fibrin(ogen) Marburg

The fibrinogen structural variant, Marburg (A alpha 1-460B beta gamma)2, is comprised of normal B beta and gamma chains but contains severely truncated A alpha chains that are missing approximately one half of their factor XIIIa cross-linking domain. Immunochemical studies of fibrin(ogen) Marburg were conducted to characterize the degree to which deletion of a defined A alpha-chain segment, A alpha 461-610, can affect the process of fibrin stabilization, ie, the factor XIIIa- mediated covalent interaction that occurs between alpha chains of neighboring fibrin molecules and between alpha chains and alpha 2 antiplasmin (alpha 2PI). The ability of Marburg (and control) alpha chains to serve as a substrate for factor XIIIa and undergo cross- linking was examined in an in vitro plasma clotting system. The capacity for alpha-chain cross-linking was evaluated both as the covalent incorporation of the small synthetic peptide, NQEQVSPLTLLK (which represents the first 12 amino acids of alpha 2PI and includes the factor XIIIa-sensitive glutamine residue responsible for the cross- linking of alpha 2PI to fibrin), and as the appearance of native (ie, natural), high-molecular-weight, cross-linked alpha-chain species. Antibodies specific for the (A)alpha and gamma/gamma-gamma chains of fibrin(ogen) and for the peptide and its parent protein, alpha 2PI (68 kD), were used as immunoblotting probes to visualize the various cross- linked products formed during in vitro clotting. Recalcification of Marburg plasma in the presence of increasing concentrations of peptide resulted in the formation of peptide-decorated Marburg alpha-chain monomers. Their size at the highest peptide concentration examined indicated the incorporation of a maximum of 3 to 4 mol of peptide per mole of alpha-chain. In the absence of alpha 2PI 1-12 peptide, the alpha chains of Marburg fibrin cross-linked to form oligomers and polymers, as well as heterodimers that included alpha 2PI. Both the peptide-decorated monomers and the native cross-linked alpha-chain species of Marburg fibrin were smaller than their control plasma counterparts, consistent with the truncated structure of the parent Marburg A alpha chain. Collectively, the findings indicate that, although deletion of the A alpha chain region no. 461-610 in fibrinogen Marburg prevents formation of an extensive alpha polymer network (presumably due to the absence of critical COOH-terminal lysine residues), it does not interfere with initial events in the fibrin stabilization process, namely, factor XIII binding and the ability of alpha chains to undergo limited cross-linking to one another and to alpha 2PI.     

Fibrinogen A alpha and gamma-chain dimers as potential differential indicators of atherosclerotic and thrombotic vascular disease

Cross-linked fibrin(ogen) dimers are known to be elevated in the plasma of subjects with occlusive vascular disease, and are thought to be fibrin dimers. Immunoelectrophoretic analyses of the dimers, however, indicate that (1) they are predominantly fibrinogen rather than fibrin dimers, and (2) they contain cross-linked A alpha-chains (A alpha-dyads) instead of the gamma-chain dyads that are rapidly formed by factor XIII during blood coagulation. Furthermore, the mobilities of the A alpha-dyads differ from the cross-linked alpha-chain products that accompany the gamma-chain cross-linking by factor XIII. Instead, the mobilities coincide with the distinct A alpha-dyads that are produced by tissue transglutaminase, an intracellular enzyme not normally present in plasma. The intimal fibrinogen deposits in atherosclerotic aortas also possess fibrinopeptide A and cross-linked A alpha-chains. Thus, both the plasma fibrinogen dimers and the intimal fibrinogen deposits appear to derive from the action of released tissue transglutaminase more so than factor XIII. It is proposed that, in the absence of other indications of cytolytic processes, the levels of A alpha-dyads in plasma reflect ongoing cellular injury accompanying atherogenesis. The extent to which gamma-dyads accompany the A alpha-dyads may signal progression of the disease to advanced stages in which ulcerations and occlusive lesions trigger thrombotic complications.     

A novel assay for factor XIII based on cross-linking of synthetic peptides: analysis of different substrates.

A novel assay for factor XIII is described that utilizes exclusively small synthetic peptides as substrates for the cross-linking reaction catalyzed by activated factor XIII (FXIIIa). The acyl donor substrate (selection peptide) is immobilized on a microplate via biotin while the acyl acceptor substrate (detection peptide) is labeled with the fluorochrome Oregon green to allow sensitive detection without the need for secondary enzyme systems for signal amplification. Starting with an amino acid sequence from the fibrin gamma-chain (GQQHHLGGAKQAGDV) as a prototype peptide, the influence of amino acid exchanges were investigated with respect to their impact on the FXIIIa-catalyzed reaction. It was found that FXIIIa readily accepts a broad range of substrate peptides, with a proline neighboring the essential lysine having the most detrimental effect. The assay appears to be valuable for the molecular characterization of factor XIII and may be used for a deeper investigation into the substrate requirements of this final enzyme of wound repair, and eventually also for the characterization of other transglutaminases.     

Electron microscopy of fibrin Paris I

Fibrinogen Paris I, a congenital fibrinogen abnormality, is characterized by delayed fibrin aggregation and poor clot retraction owing to the replacement of normal gamma-chains by mutant gamma-chains, which are termed gamma-Paris I. Available evidence indicates that the structural abnormality involves the amino acid sequence near the COOH- terminus of the mutant chain and probably includes the region containing the normal gamma-chain crosslinking site. Electron microscopy was carried out on Paris I fibrin. In place of the normally interwoven network of branching cross-striated fibers, negatively or positively contrasted Paris I fibrin was characterized by nonfibrous clumps of material connected by distince fibrous strands tending to be thinner and more irregular in width than normal fibrin. Most Paris I fibrin fibers tended to the aperiodic, although cross-striations were observed occasionally in negatively contrasted specimens and rarely in positively contrasted specimens. In addition, Paris I fibrin frequently showed relatively short, abruptly terminating fibers. The gross ultrastructural differences between normal and Paris I fibrin suggest that for fibrin assembly to take place normally, a region(s) in the fibrin molecule near to or possibly overlapping the COOH-terminal gamma- chain crosslinking site must be preserved or at least not sterically hindered.     

Structure and function of human fibrinogen inferred from dysfibrinogens

Fibrinogen is a 340-kDa plasma protein that is composed of two identical molecular halves, each consisting of three non-identical subunit polypeptides designated as A alpha, B beta- and gamma-chains held together by multiple disulfide bonds. Fibrinogen has a trinodular structure, i.e., one central E domain comprizing the amino-terminal regions of paired individual three polypeptides, and two identical outer D domains. These three nodules are linked by two coiled-coil regions [1,2]. After activation with thrombin, a tripeptide segment consisting of Gly-Pro-Arg is exposed at the amino-terminus of each alpha-chain residing at the center of the E domain and combines with its complementary binding site, called the 'a' site, residing in the carboxyl-terminal region of the gamma-chain in the outer D domain of another molecule. By crystallographic analysis [3], the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between the carboxyl group of gamma Asp-364 and the carboxyamide of Gln-329 in the 'a' site. Half molecule-staggered, double-stranded fibrin protofibrils are thus formed [4,5]. Upon abutment of two adjacent D domains on the same strand, D-D self association takes place involving Arg-275, Tyr-280 and Ser-300 of the gamma-chain on the surface of the abutting two D domains [3]. Thereafter, carboxyl-terminal regions of the fibrin a-chains are thought to be untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and interwoven networks after appropriate branching [6-9]. Although many enigmas still remain regarding the mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In my talk, I would like to discuss these molecular interactions of fibrinogen and fibrin based on the up-date data provided by analyses of normal as well as hereditary dysfibrinogens, particularly in the latter by introducing representative molecules at each step of fibrin clot formation.     

Localization of a fibrin gamma-chain polymerization site within segment Thr-374 to Glu-396 of human fibrinogen.

Fibrinogen fragment D1 was converted to fragment D3 by plasmic digestion. This conversion eliminates the ability of the fragment to interact with thrombin-exposed sites on fibrin monomer. Peptides released during this plasmic digestion were assayed for the presence of a polymerization site by affinity chromatography on fibrin monomer-Sepharose. We found that a 33-residue peptide, corresponding to gamma-chain Thr-374 to Lys-406, binds to immobilized fibrin monomer. This peptide is a shorter variant of a previously isolated 38-residue peptide (gamma-chain Thr-374 to Val-411) that contains a polymerization site [Olexa, S. A. & Budzynski, A. Z. (1981) J. Biol. Chem. 256, 3544-3549]. The peptide mixture derived from fragment D1 was digested further with Staphylococcus aureus protease V8, and a 23-residue peptide, gamma-chain Thr-374 to Glu-396, carrying a polymerization site, was isolated by affinity chromatography. This 23-residue peptide inhibits the polymerization of desA-fibrinogen. We conclude that a polymerization site complementary to the site exposed by removal of fibrinopeptide A is present in this segment. The localization of the polymerization site within the gamma-chain segment 374-396 implies that the polymerization site does not overlap with segments of the gamma-chain that are responsible for platelet aggregation and for Staphylococcus clumping (residues 400-411 and 397-411, respectively) or with the residues involved in factor XIIIa-catalyzed fibrin crosslinking (Gln-398 and Lys-406).     

Factor XIII and venous thromboembolism

Plasma factor XIII (FXIII) is a tetrameric zymogen consisting of two potentially active A subunits (FXIII-A) and two carrier/inhibitory B subunits (FXIII-B). In the final phase of the coagulation cascade, FXIII is converted into an active transglutaminase (FXIIIa) by thrombin and Ca (2 + ). FXIIIa strengthens fibrin clot mechanically by cross-linking fibrin chains. In addition, FXIIIa is a key regulator of fibrinolysis, protecting newly formed fibrin from the fibrinolytic machinery by binding α (2)-plasmin inhibitor to the fibrin meshwork. FXIII is essential for maintaining hemostasis; its severe deficiency causes a life-threatening bleeding diathesis. The involvement of FXIII in thrombotic diseases and its association with the risk of these disorders is less clear. The role of FXIII in atherothrombotic diseases has been recently reviewed. This article offers a general overview of the relationship between FXIII and venous thromboembolism (VTE), to collect individual publications on this topic, present conclusions, and examine limitations of published studies. Special attention is given to the association of FXIII-A polymorphism with the risk of VTE, which has provoked considerable interest over the last decade.     

Acceleration of fibrinolysis by the N-terminal peptide of alpha 2- plasmin inhibitor

When blood plasma containing the NH2-terminal 12-residue peptide (N- peptide) of alpha 2-plasmin inhibitor (alpha 2PI; alpha 2-antiplasmin) was clotted in the presence of calcium ions, the N-peptide and alpha 2PI were cross-linked to fibrin by activated coagulation factor XIII. The amount of N-peptide cross-linked to fibrin was proportional to the concentration of N-peptide present in plasma. On the other hand, the amount of alpha 2PI cross-linked to fibrin was decreased by the presence of N-peptide, and the decrease was in reverse relationship to the increase of cross-linking of N-peptide. Spontaneous fibrinolysis or fibrinolysis induced by tissue plasminogen activator was accelerated by the presence of N-peptide, and the acceleration was dependent on the concentrations of N-peptide and directly proportional to inhibition of alpha 2PI cross-linking exerted by N-peptide. The acceleration was more pronounced when the clot was compacted by platelet-mediated clot retraction or by a squeeze. Fibrinolysis of an alpha 2PI-deficient or a factor XIII-deficient plasma clot was not accelerated by N-peptide. These findings were substantiated in a purified system and support the previous proposal that alpha 2PI is cross-linked to fibrin at the glutamine residue that is next to the NH2-terminus of alpha 2PI, and this factor XIII-mediated cross-linking of alpha 2PI is significant in inhibition of physiologically occurring endogenous fibrinolysis.     

Isolation and Characterization of the S-Carboxymethyl Derivatives of Crosslinked and Noncrosslinked Human Fibrin

The S-carboxymethyl derivative chains of crosslinked and noncrosslinked fibrins were prepared from purified human fibrinogen. For crosslinked fibrin, fibrinogen was clotted with thrombin in the presence of calcium and purified human factor XIII. For noncrosslinked fibrin, ethylenediaminetetraacetate was substituted for factor XIII and calcium. After reduction with dithiothrcitol and alkylation with tritiated iodoacetic acid, the derivative chains were separated on carboxymethyl cellulose in a sodium acetate-pH gradient that contained 8 M urea. Purity of the separated chains was determined by polyacrylamide gel electrophoresis at acid and at neutral pH. The derivative chains of noncrosslinked fibrin were eluted from carboxymethyl cellulose in the order: gamma-chain, beta-chain, and alpha-chain. Each of the purified derivative chains was characterized and identified by amino-terminal aminoacid analysis, aminoacid composition, tryptic peptide mapping, and molecular weight estimation by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. In like manner, the derivative components of crosslinked fibrin were eluted from carboxymethyl cellulose in the order: gamma-gamma-dimer, beta-chain, and alpha-polymer. Application of the same analytical criteria and comparision with the derivatives of noncrosslinked fibrin confirmed the identity of these components. These data provide conclusive evidence that crosslinking of human fibrin involves formation of peptide bonds between two gamma-chains to form gamma-gamma-dimer and between multiple alpha-chains to form high molecular weight polymers of alpha-chains.     

The factor XIII V34L polymorphism accelerates thrombin activation of factor XIII and affects cross-linked fibrin structure

Factor XIII on activation by thrombin cross-links fibrin. A common polymorphism Val to Leu at position 34 in the FXIII A subunit is under investigation as a risk determinant of thrombosis. Because Val34Leu is close to the thrombin cleavage site, the hypothesis that it would alter the function of FXIII was tested. Analysis of FXIII subunit proteolysis by thrombin using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography showed that FXIII 34Leu was cleaved by thrombin more rapidly and by lower doses than 34Val. Mass spectrometry of isolated activation peptides confirmed the predicted single methyl group difference and demonstrated that the thrombin cleavage site is unaltered by Val34Leu. Kinetic analysis of activation peptide release demonstrated that the catalytic efficiency (k(cat)/K(m)) of thrombin was 0.5 for FXIII 34Leu and 0.2 (micromol/L)(-1) x sec(-1) for 34Val. Presence of fibrin increased the catalytic efficiency to 4.8 and 2.2 (micromol/L)(-1) x sec(-1), respectively. Although the 34Leu peptide was released at a similar rate as fibrinopeptide A, the 34Val peptide was released more slowly than fibrinopeptide A but more quickly than fibrinopeptide B generation. Cross-linking of gamma- and alpha-chains appeared earlier when fibrin was incubated with FXIII 34Leu than with 34Val. Fully activated 34Leu and 34Val FXIII showed similar cross-linking activity. Analysis of fibrin clots prepared using plasma from FXIII 34Leu subjects by turbidity and permeability measurements showed reduced fiber mass/length ratio and porosity compared to 34Val. The structural differences were confirmed by electron microscopy. These results demonstrate that Val34Leu accelerates activation of FXIII by thrombin and consequently affects the structure of the cross-linked fibrin clot.     

Characterization of wild-type and mutant alpha2-antiplasmins: fibrinolysis enhancement by reactive site mutant.

During human blood clotting, alpha2-antiplasmin (alpha2AP) becomes covalently linked to fibrin when activated blood clotting factor XIII (FXIIIa) catalyzes the formation of an isopeptide bond between glutamine at position two in alpha2AP and a specific epsilon-lysyl group in each of the alpha-chains of fibrin. This causes fibrin to become resistant to plasmin-mediated lysis. We found that chemically Arg-modified alpha2AP, which lacked plasmin-inhibitory activity, competed effectively with native alpha2AP for becoming cross-linked to fibrin and as a consequence, enhanced fibrinolysis. Recombinant alpha2AP reported to date by other groups either lacked or possessed a low level of FXIIIa substrate activity. As a first step in the development of an engineered protein that might have potential as a localized fibrin-specific fibrinolytic enhancer, we expressed recombinant alpha2AP in Pichia pastoris yeast. Two forms of nonglycosylated recombinant alpha2AP were expressed, isolated and characterized: (1) wild-type, which was analogous to native alpha2AP, and (2) a mutant form, which had Ala substituted for the reactive-site Arg364. Both the wild-type and mutant forms of alpha2AP functioned as FXIIIa substrates with affinities and kinetic efficiencies comparable to those of native alpha2AP, despite each having an additional acetylated Met blocking group at their respective amino-termini. Wild-type recombinant alpha2AP displayed full plasmin inhibitory activity, while mutant alpha2AP had none. Neither the absence of glycosylation nor blockage of the amino-terminus affected plasmin-inhibitory or FXIIIa substrate activities of wild-type alpha2AP. When our mutant alpha2AP, which lacked plasmin-inhibitory function, was added to human plasma or whole blood clots, urokinase (UK)-induced clot lysis was enhanced in a dose-dependent manner, indicating that mutant alpha2AP augmented lysis by competing with native alpha2AP for FXIIIa-catalyzed incorporation into fibrin.     

Incorporation of fibrin molecules containing fibrinopeptide A alters clot ultrastructure and decreases permeability

Previous studies have shown that a heterozygous mutation in the fibrinogen Aalpha chain gene, which results in an Aalpha R16C substitution, causes fibrinolytic resistance in the fibrin clot. This mutation prevents thrombin cleavage of fibrinopeptide A from mutant Aalpha R16C chains, but not from wild-type Aalpha chains. However, the mechanism underlying the fibrinolytic resistance is unclear. Therefore, this study investigated the biophysical properties of the mutant fibrin that contribute to fibrinolytic resistance. Fibrin clots made from the mutant fibrinogen incorporated molecules containing fibrinopeptide A into the polymerised clot, which resulted in a 'spiky' clot ultrastructure with barbed fibrin strands. The clots were less stiff than normal fibrin and were cross-linked slower by activated FXIII, but had an increased average fiber diameter, were more dense, had smaller pores and were less permeable. Protein sequencing showed that unclottable fibrinogen remaining in the supernatant consisted entirely of homodimeric Aalpha R16C fibrinogen, whereas both cleaved wild-type alpha chains and uncleaved Aalpha R16C chains were in the fibrin clot. Therefore, fibrinolytic resistance of the mutant clots is probably a result of altered clot ultrastructure caused by the incorporation of fibrin molecules containing fibrinopeptide A, resulting in larger diameter fibers and decreased permeability to fibrinolytic enzymes.     

Increased plasma concentration of cross-linked fibrin polymers in acute myocardial infarction

Thrombin cleaves fibrinopeptides from fibrinogen, converting it to fibrin monomer, and activates factor XIII, which catalyzes the formation of intermolecular epsilon-(gamma-glutamyl)-lysine bonds to stabilize the fibrin polymer. The formation of factor XIIIa-catalyzed fibrin polymers during clotting of plasma and purified fibrinogen in vivo was followed by a sodium dodecyl sulfate agarose gel technique, and an increase in both amount and size of gamma-chain cross-linked polymers was demonstrated before visible clot formation. Plasma from patients presenting with acute myocardial infarction showed increases in the plasma concentration of fibrin polymer and in the proportion of total fibrinogen present as polymer, as determined by a quantitative adaptation of the electrophoretic technique. The plasma concentration in patients with subendocardial or transmural myocardial infarction showed significant (p less than .005) increases to 4.0 +/- 1.0% and 3.6 +/- .8%, respectively, as compared with the concentration in normal plasma (0.8 +/- 0.1%). There was no difference in plasma concentration in samples from patients with transmural compared with those with subendocardial myocardial infarction. This study provides the first demonstration of factor XIIIa cross-linked fibrin polymers in thrombotic disease and indicates the presence of increased activity of both thrombin and factor XIIIa in patients with acute myocardial infarction.     

Accessibility of epitopes on fibrin clots and fibrinogen gels.

Radiolabeled antibodies were perfused into fibrin clots and fibrinogen gels formed in vitro to assess the reactivity of selected epitopes. An antifibrinogen monoclonal antibody (MoAb) (antibody 1D4/xl-f), directed against an epitope in the A alpha-chain C-terminal region (A alpha 241-476), bound to 35% of the epitope in crosslinked fibrin clots and 37% of the same epitope in factor XIII-induced fibrinogen gel networks. A different MoAb (4-2/xl-f, anti gamma 392-406) bound to only 7% of the epitope in both fibrin and fibrinogen gels. As expected, an antifibrin MoAb (antibody T2G1, antiB beta 15-21) did not bind to fibrinogen gels, but bound to fibrin, although to only 14% of the available T2G1-reactive epitopes. An antibody that does not recognize fibrin (antibody 1-8C6, antiB beta 1-21) predictably did not bind to fibrin clots and bound to 35% of the 1-8C6 epitopes present in fibrinogen gels, a level of binding also observed with antibody T2G1 and fibrinogen gels only after the latter were treated with thrombin. T2G1 epitope expression was affected much more than 1D4/xl-f epitope expression in clots formed in buffers of high or low ionic strength, conditions known to influence clot structure. Studies on the availability, in quantitative terms, of the T2G1-reactive epitope in fibrin clots is of particular importance because this antibody is currently being used in clinical trials as a clot imaging agent.     

A faster-acting and more potent form of tissue plasminogen activator.

Current treatment with tissue plasminogen activator (tPA) requires an intravenous infusion (1.5-3 h) because the clearance of tPA from the circulation is rapid (t 1/2 approximately 6 min). We have developed a tPA variant, T103N,N117Q, KHRR(296-299)AAAA (TNK-tPA) that has substantially slower in vivo clearance (1.9 vs. 16.1 ml per min per kg for tPA in rabbits) and near-normal fibrin binding and plasma clot lysis activity (87% and 82% compared with wild-type tPA). TNK-tPA exhibits 80-fold higher resistance to plasminogen activator inhibitor 1 than tPA and 14-fold enhanced relative fibrin specificity. In vitro, TNK-tPA is 10-fold more effective at conserving fibrinogen in plasma compared to tPA. Arterial venous shunt models of fibrinolysis in rabbits indicate that TNK-tPA (by bolus) induces 50% lysis in one-third the time required by tPA (by infusion). TNK-tPA is 8- and 13-fold more potent in rabbits than tPA toward whole blood clots and platelet-enriched clots, respectively. TNK-tPA conserves fibrinogen and, because of its slower clearance and normal clot lysis activity, is effective as a thrombolytic agent when given as a bolus at a relatively low dose.