Localization in the fibrinogen gamma-chain of a new site that is involved in the acceleration of the tissue-type plasminogen activator-catalysed activation of plasminogen

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.

Novel variant fibrinogen γp.C352R produced hypodysfibrinogenemia leading to a bleeding episode and failure of infertility treatment

INTRODUCTION

We identified a patient with a novel heterozygous variant fibrinogen, γp.C352R (Niigata II; N-II), who had a bleeding episode and failed infertility treatment and was suspected to have hypodysfibrinogenemia based on low and discordant fibrinogen levels (functional assay 0.33 g/L, immunological assay 0.91 g/L). We analyzed the mechanism of this rare phenotype of a congenital fibrinogen disorder.

MATERIALS AND METHODS

Patient plasma fibrinogen was purified and protein characterization and thrombin-catalyzed fibrin polymerization performed. Recombinant fibrinogen-producing Chinese hamster ovary (CHO) cells were established and the assembly and secretion of variant fibrinogen analyzed by ELISA and western blotting.

RESULTS

Purified N-II plasma fibrinogen had a small lower molecular weight band below the normal γ-chain and slightly reduced fibrin polymerization. A limited proportion of p.C352R fibrinogen was secreted into the culture medium of established CHO cell lines, but the γ-chain of p.C352R was synthesized and variant fibrinogen was assembled inside the cells.

CONCLUSION

We demonstrated that fibrinogen N-II, γp.C352R was associated with markedly reduced secretion of variant fibrinogen from CHO cells, that fibrin polymerization of purified plasma fibrinogen was only slightly affected, and that fibrinogen N-II produces hypodysfibrinogenemia in plasma.

c.259A>C in the fibrinogen gene of alpha chain (FGA) is a fibrinogen with thrombotic phenotype

Introduction

Dysfibrinogenemia is a rare inherited disease that results from mutation in one of the three fibrinogen genes. Diagnosis can be misleading since it may present as a bleeding tendency or thrombosis and a specific coagulation test for diagnosis is not routinely available

Aim

To search for a new candidate gene of thrombophilia in a family with three generations of arterial and venous thrombosis.

Methods

Whole exome sequencing followed by Sanger validation and segregation analysis was carried out. In addition, structural modeling was performed. Screening for thrombophilia along with blood counts, prothrombin time, activated partial thromboplastin, thrombin, reptilase time, and fibrinogen was done in each patient.

Results and discussion

A missense c.259A>C, p.K87Q (g.chr4: 155510050A-C) (rs764281241) in FGA gene was found in all three siblings without any other known thrombophilia marker to explain thrombosis in all three siblings. It is expected to be damaging by six out of seven prediction programs and is very rare in the entire population with Exac=0.000008.

Conclusion

The occurrence of the c.259A>C mutation in FGA may well explain the thrombosis phenotype of the affected family and is suggested as a new marker for thrombophilia phenotype.

Affimer proteins as a tool to modulate fibrinolysis, stabilize the blood clot, and reduce bleeding complications

Bleeding complications secondary to surgery, trauma, or coagulation disorders are important causes of morbidity and mortality. Although fibrin sealants are considered to minimize blood loss, this is not widely adopted because of its high cost and/or risk for infection. We present a novel methodology employing nonantibody fibrinogen-binding proteins, termed Affimers, to stabilize fibrin networks with the potential to control excessive bleeding. Two fibrinogen-specific Affimer proteins, F5 and G2, were identified and characterized for their effects on clot structure/fibrinolysis, using turbidimetric and permeation analyses and confocal and electron microscopy. Binding studies and molecular modeling identified interaction sites, whereas plasmin generation assays determined effects on plasminogen activation. In human plasma, F5 and G2 prolonged clot lysis time from 9.8 ± 1.1 minutes in the absence of Affimers to 172.6 ± 7.4 and more than 180 minutes (P < .0001), respectively, and from 7.6 ± 0.2 to 28.7 ± 5.8 (P < .05) and 149.3 ± 9.7 (P < .0001) minutes in clots made from purified fibrinogen. Prolongation in fibrinolysis was consistent across plasma samples from healthy control patients and individuals at high bleeding risk. F5 and G2 had a differential effect on clot structure and G2 profoundly altered fibrin fiber arrangement, whereas F5 maintained physiological clot structure. Affimer F5 reduced fibrin-dependent plasmin generation and was predicted to bind fibrinogen D fragment close to tissue plasminogen activator (tPA; residues γ312-324) and plasminogen (α148-160) binding sites, thus interfering with tPA-plasminogen interaction and representing 1 potential mechanism for modulation of fibrinolysis. Our Affimer proteins provide a novel methodology for stabilizing fibrin networks with potential future clinical implications to reduce bleeding risk.

Hepatic fibrinogen storage disease: identification of two novel mutations (p.Asp316Asn, fibrinogen Pisa and p.Gly366Ser, fibrinogen Beograd) impacting on the fibrinogen γ-module

BACKGROUND

Quantitative fibrinogen deficiencies (hypofibrinogenemia and afibrinogenemia) are rare congenital disorders characterized by low/unmeasurable plasma fibrinogen antigen levels. Their genetic basis is invariably represented by mutations within the fibrinogen genes (FGA, FGB and FGG coding for the Aα, Bβ and γ chains). Currently, only four mutations (p.Gly284Arg, p.Arg375Trp, delGVYYQ 346-350, p.Thr314Pro), all affecting the fibrinogen γ chain, have been reported to cause fibrinogen storage disease (FSD), a disorder characterized by protein aggregation, endoplasmic reticulum retention and hypofibrinogenemia.

OBJECTIVES

To investigate the genetic basis of FSD in two hypofibrinogenemic patients.

METHODS

The mutational screening of the fibrinogen genes was performed by direct DNA sequencing. The impact of identified mutations on fibrinogen structure was investigated by in-silico molecular modeling. Liver histology was evaluated by light microscopy, electron microscopy and immunocytochemistry.

RESULTS

Here, we describe two hypofibrinogenemic children with persistent abnormal liver function parameters. Direct sequencing of the coding portion of fibrinogen genes disclosed two novel FGG missense variants (p.Asp316Asn, fibrinogen Pisa; p.Gly366Ser, fibrinogen Beograd), both present in the heterozygous state and affecting residues located in the fibrinogen C-terminal γ-module. Liver sections derived from biopsies of the two patients were examined by immunocytochemical analyses, revealing hepatocyte cytoplasmic inclusions immunoreactive to anti-fibrinogen antibodies.

CONCLUSIONS

Our work strongly confirms the clustering of mutations causing FSD in the fibrinogen γ chain between residues 284 and 375. Based on an in-depth structural analysis of all FSD-causing mutations and on their resemblance to mutations leading to serpinopathies, we also comment on a possible mechanism explaining fibrinogen polymerization within hepatocytes.

Fibrinogen Melbourne: a novel congenital hypodysfibrinogenemia caused by γ326Cys-Phe in the fibrinogen γ chain, presenting as massive splanchnic venous thrombosis

Congenital dysfibrinogenemias are characterized by structural abnormalities in fibrinogen, which may lead to abnormal function. Fibrinogen has critical roles in coagulation, platelet aggregation and fibrinolysis; accordingly, abnormal fibrinogen function can result in a clinical phenotype, which varies from asymptomatic (around 55%) to bleeding (25%) and/or thrombosis (20%). We describe a novel γ326Cys→Phe mutation in the fibrinogen γ gene causing hypodysfibrinogenemia associated with life-threatening thrombosis in a family from Melbourne, Australia.

Enhanced fibrinolysis by proteolysed coagulation factor Xa

We previously showed that coagulation factor Xa (FXa) enhances activation of the fibrinolysis zymogen plasminogen to plasmin by tissue plasminogen activator (tPA). Implying that proteolytic modulation occurs in situ, intact FXa (FXaalpha) must be sequentially cleaved by plasmin or autoproteolysis, producing FXabeta and Xa33/13, which acquire necessary plasminogen binding sites. The implicit function of Xa33/13 in plasmin generation has not been demonstrated, nor has FXaalpha/beta or Xa33/13 been studied in clot lysis experiments. We now report that purified Xa33/13 increases tPA-dependent plasmin generation by at least 10-fold. Western blots confirmed that in situ conversion of FXaalpha/beta to Xa33/13 correlated to enhanced plasmin generation. Chemical modification of the FXaalpha active site resulted in the proteolytic generation of a product distinct from Xa33/13 and inhibited the enhancement of plasminogen activation. Identical modification of Xa33/13 had no effect on tPA cofactor function. Due to its overwhelming concentration in the clot, fibrin is the accepted tPA cofactor. Nevertheless, at the functional level of tPA that circulates in plasma, FXaalpha/beta or Xa33/13 greatly reduced purified fibrin lysis times by as much as 7-fold. This effect was attenuated at high levels of tPA, suggesting a role when intrinsic plasmin generation is relatively low. FXaalpha/beta or Xa33/13 did not alter the apparent size of fibrin degradation products, but accelerated the initial cleavage of fibrin to fragment X, which is known to optimize the tPA cofactor activity of fibrin. Thus, coagulation FXaalpha undergoes proteolytic modulation to enhance fibrinolysis, possibly by priming the tPA cofactor function of fibrin.

Fibrin targeting of echogenic liposomes with inactivated tissue plasminogen activator

Fibrin-specific molecular targeting strategies are desirable for site-specific imaging and treatment of late stage atheroma, but fibrin-specific antibodies are difficult to produce and present immunogenicity problems. Tissue plasminogen activator (tPA) is an endogenous protein that has been shown to bind fibrin with high affinity and may circumvent antibody difficulties. Use of tPA-derived proteins or peptides, however, requires that the plasminogen-activating proteolytic activity be neutralized or removed. As an initial step in determining the feasibility of this targeting strategy, human recombinant tPA (Activase) was irreversibly inhibited with D-phe-L-pro-L-arg-chloromethyl ketone (PPACK) and conjugated to intrinsically echogenic liposomes (ELIP) by a thioether coupling protocol. Fibrin-binding affinities were assessed with a novel two-stage fibrin pad ELISA. We achieved 95-99% inactivation, while retaining both tPA fibrin-binding activities of K(D) approximately 2 nM and 33 nM. Thermodynamic analysis of the PPACK-inactivated tPA (tPA(P)) revealed highly exothermic interactions, indicative of ionic associations, especially for the higher affinity. The conjugation efficiency of tPA(P) to ELIP was within the range of that previously achieved for IgG and exhibited satisfactory fibrin targeting, characterized by striking increases of enthalpy and entropy increments. Evidence for coupling of noncovalent association energetics with the phosphatidylethanolamine major phase transition, observed in previous IgG antibody conjugations, was also evident in this case, but the nature of the transduction mechanism was different. These results demonstrate that tPA-derived components lacking proteolytic activity can be employed as fibrin-targeting agents for delivery of therapeutic and diagnostic formulations.

Decorin Modulates Fibrin Assembly and Structure

Emerging evidence indicates that fibrin clotting is regulated by different external factors. We demonstrated recently that decorin, a regulator of collagen fibrillogenesis and transforming growth factor-beta activity, binds to the D regions of fibrinogen (Dugan, T.A., Yang, V. W.-C., McQuillan, D.J., and Höök, M. (2003) J. Biol. Chem. 278, 13655-13662). We now report that the decorin-fibrinogen interaction alters the assembly, structure, and clearance of fibrin fibers. Relative to fibrinogen, substoichiometric amounts of decorin core protein modulated clotting, whereas an excess of an active decorin peptide was necessary for similar activity. These concentration-dependent effects suggest that decorin bound to the D regions sterically modulates fibrin assembly. Scanning electron microscopy images of fibrin clotted in the presence of increasing concentrations of decorin core protein showed progressively decreasing fiber diameter. The sequestration of Zn(2+) ions from the N-terminal fibrinogen-binding region abrogated decorin incorporation into the fibrin network. Compared with linear thicker fibrin fibers, the curving thin fibers formed with decorin underwent accelerated tissue-type plasminogen activator-dependent fibrinolysis. Collectively, these data demonstrate that decorin can regulate fibrin organization and reveal a novel mechanism by which extracellular matrix components can participate in hemostasis, thrombosis, and wound repair.

Fibrinogen Seoul (FGG Ala341Asp): a novel mutation associated with hypodysfibrinogenemia

Dysfibrinogenemia is a coagulation disorder caused by a variety of structural abnormalities in the fibrinogen molecule that result in fibrinogen function. The molecular basis of hypodysfibrinogenemia was investigated in a 66-year-old woman with peripheral artery obstructive disease and in her family members. Plasma level of functional fibrinogen determined using the Clauss method was lower (75 mg/dL; normal, 140-460 mg/dL) than that measured with immunologic nephelometric assay (137 mg/dL; normal, 180-400 mg/dL). Similar results were also observed in two family members through two generations. DNA was extracted from whole blood, and the coding regions and intron/exon boundaries of gamma chain gene (FGG) were amplified. A novel (Fibrinogen Seoul) heterozygous FGG mutation (GCT->GAT, Ala341Asp) was identified in all three affected family members. Thrombin-catalyzed polymerization was found to be defective on the analysis of purified fibinogen from the propositus. Molecular modeling also showed a conformational change of fibrinogen structure.

Structural basis of the cofactor function of denatured albumin in plasminogen activation by tissue-type plasminogen activator

Certain denatured proteins function as cofactors in the activation of plasminogen by tissue-type plasminogen activator. The present study approached the structural requirements for the cofactor activity of a model protein (human serum albumin). Heat denaturation of 100-230 microM albumin (80 degrees C and 60-90 min) reproducibly yielded aggregates with radius in the range of 10-150 nm. The major determinant of the cofactor potency was the size of the aggregates. The increase of particle size correlated with the cofactor activity, and there was a minimal requirement for the size of the cofactor (about 10 nm radius). Similar to other proteins, the molecular aggregates with cofactor function contained a significant amount of antiparallel intermolecular beta-sheets. Plasmin pre-digestion increased the cofactor efficiency (related to C-terminal lysine exposure) and did not affect profoundly the structure of the aggregates, suggesting a long-lasting and even a self-augmenting cofactor function of the denatured protein.

In vitro fibrin clot formation and fibrinolysis using heterozygous plasma fibrinogen from γAsn319, Asp320 deletion dysfibrinogen, Otsu I

INTRODUCTION

We have reported a heterozygous dysfibrinogenemia, fibrinogen Otsu I, caused by the deletion of gammaAsn319 and gammaAsp320, which was originally identified in the dysfibrinogen Vlissingen/Frankfurt IV (V/FIV) associated with thrombosis. Unlike the V/FIV family, the Otsu propositus showed no thrombotic tendencies. To analyze the relationship between thrombosis and the heterozygous plasma variant fibrinogen, we used purified plasma fibrinogen from the Otsu patient and compared it with a normal control.

MATERIALS AND METHODS

Thrombin-induced fibrin clot formation and clot structure were observed by fibrin polymerization and scanning electron microscopy, respectively. For in vitro observation of fibrinolysis, plasmin generation and clot lysis assays were performed by the addition of tissue type plasminogen activation (tPA) and plasminogen.

RESULTS AND CONCLUSIONS

Polymerization of Otsu was markedly impaired, while fibrin fibers were much thicker and the density of the bundles of fibrin fibers was less and porous compared with normal. Lysis of the Otsu clot was not significantly different from normal when a tPA and plasminogen mixture was overlaid onto the clots. For Otsu, the penetration of the tPA/plasminogen mixture into the clot was much faster than normal and the protection against plasmin cleavage was impaired; however, tPA-induced plasmin activation of the Otsu fibrin was slower than that of normal fibrin, resulting in a clot lysis of Otsu similar to normal.

Fibrinogen and fibrin structure and functions

Fibrinogen molecules are comprised of two sets of disulfide-bridged Aalpha-, Bbeta-, and gamma-chains. Each molecule contains two outer D domains connected to a central E domain by a coiled-coil segment. Fibrin is formed after thrombin cleavage of fibrinopeptide A (FPA) from fibrinogen Aalpha-chains, thus initiating fibrin polymerization. Double-stranded fibrils form through end-to-middle domain (D:E) associations, and concomitant lateral fibril associations and branching create a clot network. Fibrin assembly facilitates intermolecular antiparallel C-terminal alignment of gamma-chain pairs, which are then covalently 'cross-linked' by factor XIII ('plasma protransglutaminase') or XIIIa to form 'gamma-dimers'. In addition to its primary role of providing scaffolding for the intravascular thrombus and also accounting for important clot viscoelastic properties, fibrin(ogen) participates in other biologic functions involving unique binding sites, some of which become exposed as a consequence of fibrin formation. This review provides details about fibrinogen and fibrin structure, and correlates this information with biological functions that include: (i) suppression of plasma factor XIII-mediated cross-linking activity in blood by binding the factor XIII A2B2 complex. (ii) Non-substrate thrombin binding to fibrin, termed antithrombin I (AT-I), which down-regulates thrombin generation in clotting blood. (iii) Tissue-type plasminogen activator (tPA)-stimulated plasminogen activation by fibrin that results from formation of a ternary tPA-plasminogen-fibrin complex. Binding of inhibitors such as alpha2-antiplasmin, plasminogen activator inhibitor-2, lipoprotein(a), or histidine-rich glycoprotein, impairs plasminogen activation. (iv) Enhanced interactions with the extracellular matrix by binding of fibronectin to fibrin(ogen). (v) Molecular and cellular interactions of fibrin beta15-42. This sequence binds to heparin and mediates platelet and endothelial cell spreading, fibroblast proliferation, and capillary tube formation. Interactions between beta15-42 and vascular endothelial (VE)-cadherin, an endothelial cell receptor, also promote capillary tube formation and angiogenesis. These activities are enhanced by binding of growth factors like fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF), and cytokines like interleukin (IL)-1. (vi) Fibrinogen binding to the platelet alpha(IIb)beta3 receptor, which is important for incorporating platelets into a developing thrombus. (vii) Leukocyte binding to fibrin(ogen) via integrin alpha(M)beta2 (Mac-1), which is a high affinity receptor on stimulated monocytes and neutrophils.

Investigation of residues in the fibrin(ogen) γ chain involved in tissue plasminogen activator binding and plasminogen activation

In order to characterize tissue plasminogen activator (t-PA) binding to gamma-chain residues in fibrinogen, we generated variant fibrinogens substituting alanine for gamma D316, gamma D318, gamma D320, and gamma K321. We measured thrombin-catalyzed polymerization and found normal polymerization with gamma K321A, no polymerization with gamma D316A, and, as reported by Lounes et al. in 2002, impaired polymerization with gamma D318A and gamma D320A. We measured t-PA binding in a solid-phase assay, and t-PA activity by the generation of plasmin. Comparing normal fibrin with fibrinogen, we found a seven-fold increase in binding and a two-fold increase in activity. Binding to all variant fibrinogens was the same as normal. In contrast, t-PA binding to all variant fibrins was weaker than binding to normal fibrin, 2.5-fold for gamma K321A, seven-fold for gamma D320A and 10-fold for gamma D316A and gamma D318A. Plasmin generation in the presence of variant fibrinogens was similar, although not identical, to normal, and plasmin generation in the presence of variant fibrins was impaired for the Asp to Ala variants. As the three variants with the weakest t-PA binding and least activity also showed impaired polymerization, our results support previous findings demonstrating the DD:E complex, found in the normal fibrin polymer, is necessary for the fibrin enhanced binding of t-PA and activation of plasminogen.

Thrombophilic dysfibrinogen Tokyo V with the amino acid substitution of γ Ala327Thr: formation of fragile but fibrinolysis-resistant fibrin clots and its relevance to arterial thromboembolism

Thrombophilic dysfibrinogen Tokyo V was identified in a 43-year-old man with recurrent thromboembolism. Based on analyses of the patient fibrinogen genes, the amino acid sequence of the aberrant fibrinogen peptide, and deglycosylation experiments, fibrinogen Tokyo V was shown to have an amino acid substitution of γ Ala327Thr and possibly extra glycosylation at γ Asn325 because the mutation confers the N-linked glycosylation consensus sequence Asn-X-Thr. The mutation resulted in impaired function and hypofibrinogenemia (hypodysfibrinogen). Polymerization of fibrin monomers derived from patient fibrinogen was severely impaired with a partial correction in the presence of calcium, resulting in very low clottability. Additionally, a large amount of soluble cross-linked fibrin was formed upon thrombin treatment in the presence of factor XIII and calcium. However, Tokyo V–derived fibrin was resistant to degradation by tissue plasminogen activator (tPA)–catalyzed plasmin digestion. The structure of Tokyo V fibrin appeared severely perturbed, since there are large pores inside the tangled fibrin networks and fiber ends at the boundaries. Taken together, these data suggest that Tokyo V fibrin clots are fragile, so that fibrinolysis-resistant insoluble fibrin and soluble fibrin polymers may be released to the circulation, partly accounting for the recurrent embolic episodes in the patient. (Blood. 2004;103:3045-3050)

Fibrinogen gamma chain functions

This review covers the functional features of the fibrinogen gamma chains including their participation in fibrin polymerization and cross-linking, their role in the initiation of fibrinolysis, their binding and regulation of factor XIII activity, their interactions with platelets and other cells, and their role in mediating thrombin binding to fibrin, a thrombin inhibitory function termed 'antithrombin I'.

Fibrin-mediated plasminogen activation

Fibrin, but not fibrinogen, enhances the rate of activation of plasminogen by tissue type plasminogen activator (t-PA). Studies with enzymatic and chemical fragments of fibrinogen showed that several sites in fibrinogen are involved in this rate enhancement; these are, A alpha 148-160 (located in CNBr fragment FCB-2), and FCB-5 (a CNBr fragment comprising gamma 312-324), and recently discovered sites in the fibrinogen alpha C domains. All these sites are buried in fibrinogen, but exposed in fibrin and some fibrinogen fragments. For the first two of these, located in the D-domains, this was shown by the fact that monoclonal antibodies against A alpha 148-160 and gamma 312-324 bind to fibrin and rate enhancing fibrin(ogen) fragments, but not to fibrinogen. Direct binding studies indicate that at physiological concentrations plasminogen binds to FCB-2, and t-PA to FCB-5. More detailed studies have demonstrated the importance of residues A alpha-157 and A alpha-152, and that the minimum stretch with rate enhancing properties is A alpha 154-159. The sites in the alpha C domains await further identification. With the recently reported three-dimensional structure of fragments D and D-dimer it is now possible to explain these findings at the molecular level. Molecular calculations and experimental data show that the site A alpha 148-160 in fibrinogen is covered among others by a part of the A alpha chain (A alpha 166-195) that forms an alpha-helix, and by a globular domain formed by the beta-chain. On fibrin formation, the last two may move away, and give access to A alpha 148-160. It is conceivable that in the alpha C domain sites are involved in the early phases of fibrinolysis. The site A alpha 148-160 and that in FCB-5 may be more important at later stages. It is also clear that fibrin polymerization is important. This polymerization has probably several effects: exposure of the rate enhancing sites; mutual positioning of the t-PA and plasminogen binding sites; a concentrating effect of t-PA and plasminogen on the fibrin surface; effects on the kinetic properties of t-PA and plasminogen. These effects together explain the rate enhancement.

Hereditary disorders of fibrinogen

Fibrinogen, a 340-kDa plasma protein, is composed of two identical molecular halves each consisting of three non-identical A alpha-, B beta- and gamma-chain subunits held together by multiple disulfide bonds. Fibrinogen is shown to have a trinodular structure; that is, one central nodule, the E domain, and two identical outer nodules, the D-domains, linked by two coiled-coil regions. After activation with thrombin, a pair of binding sites comprising Gly-Pro-Arg is exposed in the central nodule and combines with its complementary binding site a in the outer nodule of another molecules. By using crystallographic analysis, the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between gamma Asp-364 and gamma Asp-330, and guanidino group of alpha Arg-3 between the carboxyl group of gamma Asp-364 and gamma Gln-329 in the a site. Half molecule-staggered, double-stranded protofibrils are thus formed. 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. Thereafter, carboxyl-terminal regions of the alpha-chains are untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and networks. Although many enigmas still remain concerning the exact mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In this review, these molecular interactions of fibrinogen and fibrin are discussed on the basis of the data provided by hereditary dysfibrinogens on introducing representative molecules at each step of fibrin clot formation.

Identification and characterization of novel tPA- and plasminogen-binding sites within fibrin(ogen) alpha C-domains

Molecular defects in the alphaC-domains of some abnormal fibrinogens have been associated with impaired fibrin-mediated activation of plasminogen (Pg) by its activator tPA, suggesting the involvement of these domains in fibrinolysis. To test this suggestion, we expressed in E. coli the alphaC-fragment (residues Aalpha221-610) corresponding to the entire alphaC-domain as well as its NH(2)- and COOH-terminal halves (residues Aalpha221-391 and Aalpha392-610) and tested their effects on activation of Pg and their interaction with Pg and tPA. When the activation was monitored by cleavage of a chromogenic substrate with newly formed plasmin, the reaction was much more efficient in the presence of the alphaC-fragment. This stimulation was abolished upon digestion of the alphaC-fragment with plasmin. In surface plasmon resonance experiments, both tPA and Pg bound to the immobilized alphaC-fragment with K(d)s of 33 and 32 nM, respectively. Similar results were obtained by ELISA. This binding occurred via independent sites since saturating amounts of Pg did not prevent binding of tPA and vice versa. Both sites were localized in the COOH-terminal half of the alphaC-domain since the Aalpha392-610 fragment bound both tPA and Pg and was an effective stimulator whereas Aalpha221-391 was inactive. These results indicate that the fibrinogen alphaC-domains contain novel high-affinity tPA- and Pg-binding sites that play an important role in the regulation of fibrinolysis.

Conversion of fibrinogen to fibrin: mechanism of exposure of tPA- and plasminogen-binding sites

Conversion of fibrinogen into fibrin results in the exposure of cryptic interaction sites and modulation of various activities. To elucidate the mechanism of this exposure, we tested the accessibility of the Aalpha148-160 and gamma312-324 fibrin-specific epitopes that are involved in binding of plasminogen and its activator tPA, in several fragments derived from fibrinogen (fragment D and its subfragments) and fibrin (cross-linked D-D fragment and its noncovalent complex with the E(1) fragment, D-D. E(1)). Neither D nor D-D bound tPA, plasminogen, or anti-Aalpha148-160 and anti-gamma312-324 monoclonal antibodies, indicating that their fibrin-specific epitopes were inaccessible. The Aalpha148-160 epitope became exposed only upon proteolytic removal of the beta- and gamma-modules from D. At the same time, both epitopes were accessible in the D-D.E(1) complex, indicating that the DD.E interaction resulted in their exposure. This exposure was reversible since the dissociation of the D-D.E(1) complex made the sites unavailable, while reconstitution of the complex made them exposed. The results indicate that upon fibrin assembly, driven primarily by the interaction between complementary sites of the D and E regions, the D regions undergo conformational changes that cause the exposure of their plasminogen- and tPA-binding sites. These changes may be involved in the regulation of fibrin assembly and fibrinolysis.

Role of the beta-strand insert in the central domain of the fibrinogen gamma-module

The crystal structure of the fibrinogen gamma-module (residues gamma143-411) [Yee, V. C., et al. (1997) Structure 5, 125-138] revealed an unusual feature. Namely, residues gamma381-390 in the functionally important COOH-terminal region form a beta-strand that is inserted into an antiparallel beta-sheet of the central domain (gamma192-286), while the rest (gamma393-411) seems to be flexible. To clarify the structural and functional importance of this beta-strand insert, we analyzed the folding status of the plasmin-derived fibrinogen fragment D(3) and several truncated variants of the gamma-module expressed in Escherichia coli. It was found that D(3), in which most of the COOH-terminal domain of the gamma-module (gamma287-379) is removed proteolytically, retains a gamma374-405 peptide that seems to be associated noncovalently with the bulk of the molecule via its beta-strand insert region. A study of the denaturation-renaturation process of D(3) suggested that without this peptide its truncated gamma-module remains folded but is destabilized. This was confirmed directly with the truncated recombinant variants of the gamma-module, including residues gamma148-392, gamma148-373, and gamma148-286. They all were folded, but those devoid of the beta-strand insert were destabilized. The results indicate that although the beta-strand insert contributes to the stabilization of the gamma-module, it can be removed without destroying the compact structure of the latter. On the basis of this finding and some other observations, we propose a mechanism for the function-related conformational changes in the fibrin(ogen) gamma-modules.

Alterations in fibrin detected in coronary sinus blood after heparin and coronary angiography with a nonionic contrast agent (iohexol)

Although heparin and some radiographic contrast agents inhibit coagulation, thrombi can still form in their presence. The chemical environment in which a thrombus forms affects fibrin structure that may alter the ability of the thrombus to be lysed. Therefore, we assessed changes in fibrin structure in 13 patients referred for coronary angiography. Blood was obtained from the femoral vein, femoral artery, ascending aorta, left main coronary artery (LMCA), and coronary sinus (CS) before, during, and after coronary angiography was performed with iohexol. The number of fibrin monomers per fiber cross section was determined by turbidity measurements of fibrin gels formed from plasma samples. At baseline there was no difference in the number of fibrin monomers per fiber cross section in plasma gels generated from the different sampling sites. After iohexol administration, there was a significant decrease in the number of fibrin monomers per fiber cross section at the sampling sites ranging from - 13% to -25% compared with the respective baseline values with the largest change in the LMCA CS (51+/-16 to 38+/-15, p <0.025). Transcardiac (LM - CS value) changes in the number of fibrin monomers per fiber cross section were dependent on the timing of the sample collection in the CS. In 7 patients, the CS sample was collected approximately 2 minutes after injection of contrast material and there was no transcardiac difference. When the CS sample was obtained during contrast injection (n=6) a large transcardiac change occurred (44+/-10 to 32+/-14, p=0.01). These data show transient changes in fibrin structure during coronary angiography with iohexol. The thinner fibers formed in the presence of iohexol were more resistant to fibrinolysis.

Binding of Tissue-Plasminogen Activator to Fibrin: Effect of Ultrasound

Ultrasound reversibly alters the structure of polymerized fibrin, an effect that could influence tissue-plasminogen activator (t-PA) binding. We have, therefore, characterized the effects of ultrasound on binding of t-PA to fibrin using a novel system in which radiolabeled, active-site blocked, single chain tissue-plasminogen activator flowed through a fibrin gel at constant rate, and specific binding was determined by monitoring incorporation of radiolabel. Results using polymerized fibrin were compared with those using a surface of fibrin immobilized on Sepharose beads in a similar system. Interaction of t-PA with surface-immobilized fibrin involved two classes of binding sites (Kd = 31 nmol/L and 244 nmol/L) and a maximum binding ratio of 3.8 mol t-PA/mol fibrin. Ultrasound increased Kd for the high affinity site to 46 nmol/L (P < .0001), but it had no significant effects on the Kd 244 nmol/L site nor on Bmax. Tissue-plasminogen activator binding to noncrosslinked fibrin involved two sites with Kds of 267 nmol/L and 952 nmol/L, while a single Kd 405 nmol/L site was identified for crosslinked fibrin. Ultrasound had no significant effect on the binding affinity for noncrosslinked fibrin, but Bmaxwas increased in the presence of ultrasound, from 31 μmol/L to 43 μmol/L (P < .0001). Ultrasound decreased the Kd for crosslinked fibrin to 343 nmol/L (P = .026) and also increased Bmax from 22 μmol/L to 25 μmol/L (P = .015). Ultrasound also affected the kinetics of t-PA binding to fibrin, significantly accelerating the rate of dissociation by 77% ± 5% for noncrosslinked fibrin and by 69% ± 3% for crosslinked fibrin (P < .001 for each). These results indicate that ultrasound exposure accelerates t-PA binding, alters binding affinity, and increases maximum binding to polymerized fibrin, effects that may result from ultrasound-induced changes in fibrin structure.

Binding of Tissue-Plasminogen Activator to Fibrin: Effect of Ultrasound

Ultrasound reversibly alters the structure of polymerized fibrin, an effect that could influence tissue-plasminogen activator (t-PA) binding. We have, therefore, characterized the effects of ultrasound on binding of t-PA to fibrin using a novel system in which radiolabeled, active-site blocked, single chain tissue-plasminogen activator flowed through a fibrin gel at constant rate, and specific binding was determined by monitoring incorporation of radiolabel. Results using polymerized fibrin were compared with those using a surface of fibrin immobilized on Sepharose beads in a similar system. Interaction of t-PA with surface-immobilized fibrin involved two classes of binding sites (Kd = 31 nmol/L and 244 nmol/L) and a maximum binding ratio of 3.8 mol t-PA/mol fibrin. Ultrasound increased Kd for the high affinity site to 46 nmol/L (P < .0001), but it had no significant effects on the Kd 244 nmol/L site nor on Bmax. Tissue-plasminogen activator binding to noncrosslinked fibrin involved two sites with Kds of 267 nmol/L and 952 nmol/L, while a single Kd 405 nmol/L site was identified for crosslinked fibrin. Ultrasound had no significant effect on the binding affinity for noncrosslinked fibrin, but Bmaxwas increased in the presence of ultrasound, from 31 μmol/L to 43 μmol/L (P < .0001). Ultrasound decreased the Kd for crosslinked fibrin to 343 nmol/L (P = .026) and also increased Bmax from 22 μmol/L to 25 μmol/L (P = .015). Ultrasound also affected the kinetics of t-PA binding to fibrin, significantly accelerating the rate of dissociation by 77% ± 5% for noncrosslinked fibrin and by 69% ± 3% for crosslinked fibrin (P < .001 for each). These results indicate that ultrasound exposure accelerates t-PA binding, alters binding affinity, and increases maximum binding to polymerized fibrin, effects that may result from ultrasound-induced changes in fibrin structure.

Domain structure and functional activity of the recombinant human fibrinogen gamma-module (gamma148-411)

Human fibrinogen gamma-module comprising residues gamma148-411 was expressed in Escherichia coli and refolded in vitro. Differential scanning calorimetry revealed that in addition to the two previously identified independently folded thermolabile domains, one in each half of the module, the gamma-module also contains one or two thermostable domains that melt above 65 degrees C. To localize the latter, an NH2-terminal 6-kDa fragment was prepared by limited proteolysis of the recombinant gamma-module. It melted at high temperature, indicating that this portion is folded into a compact structure that represents a thermostable domain, also identified in the proteolytic fibrinogen fragment D1 which contains the natural gamma-module. Thus the NH2-terminal half of the gamma-module forms two domains, a thermostable one and a thermolabile one, leaving the rest of the module to be responsible for the formation of the other one or two domains. The thermal stability of some domains was lower in the recombinant gamma-module than in its natural counterpart in D1, reflecting most probably the loss of interactions with neighboring domains; however, the major functional sites were essentially preserved. The module bound Ca2+ and was stabilized by it against denaturation and proteolysis. It inhibited fibrin polymerization and was efficiently cross-linked by factor XIIIa. The gamma-module supported adhesion of platelets via their GP IIbIIIa (alpha(IIb)beta3) receptor in the same manner as D1 fragment. It also supported the adhesion of alpha(M)beta2- (Mac-1-) transfected cells and in the fluid phase was more effective than D1 as an inhibitor of that adhesion, suggesting that the Mac-1 binding site is better exposed.

Analysis of tissue plasminogen activator specificity using peptidyl fluorogenic substrates

A series of 54 fluorogenic substrates have been synthesized and evaluated for tissue-type plasminogen activator (tPA) hydrolysis in an attempt to create efficient sensitive substrates for tPA and to investigate substrate structure-efficiency correlations. All substrates contain the 6-amino-1-naphthalenesulfonamide (ANSN) leaving group, Arg in the P1 position, various amino acids in the P2 and P3 positions, and various substituents in the sulfonamide moiety of the leaving group (P' position). The majority of substrates have relatively low K(M) values (< 100 microM), reaching as low as 2.6 microM, and reasonably high k(cat) values (up to 3.6 s(-1)). These substrates have higher affinity, higher hydrolysis rates, and higher efficiency for two-chain tPA than for the single-chain form of this enzyme. Analysis of the P3 structure influence on substrate efficiency demonstrates that compounds which contain D-isomers of N-blocked bulky amino acids, such as Phe, Leu, and Val, in this position are more efficient for tPA than substrates with N-unblocked small amino acids (Ser or Pro) in the P3 position. The second-order rate constants and k(cat) values for substrate hydrolysis increase with decreases in the P2 amino acid hydrophobicity in the following manner: Leu < Val and Gly < Ser < Pro. Substrates which contain an ANSN leaving group had a higher affinity for tPA than substrates with p-nitroaniline or 7-amino-4-methylcoumarin leaving groups. Analyses of substrate hydrolysis dependence on the substrate P' structure show that the k(cat) and the second-order rate constants increased with an increase in the size of monoalkyl substituent in the sulfonamide moiety, whereas substrates which contain either glycine methyl ester or a dialkyl group displayed the lowest efficiency for tPA. The substrate Boc-(p-F)Phe-Pro-Arg-ANSNHC2H5 allowed quantitation of tPA at a concentration as low as 1 pM, a concentration significantly lower than the plasma concentration of this protein. Evaluation of the activation of single-chain tPA by factor Xa demonstrates that prothrombinase is approximately 3-fold more efficient in activating sc-tPA than factor Xa alone, increasing the initial rate of activation from 0.0055 nM/s per 1 nM of factor Xa to 0.017 nM/s per 1 nM.

Bovine erythrocyte haemolysates enhance plasminogen activation by tissue-type plasminogen activator

An active fraction that accelerates plasminogen activation by tissue-type plasminogen activator (t-PA) was purified from a haemolysate of bovine erythrocytes. When the haemolysate was mixed with t-PA, it produced a 2- to 3-fold increase in plasminogen activation as measured by an insoluble fibrinolytic assay system and a soluble amidolytic assay system with the chromogenic substrate S-2251. Zymographic analysis showed that, while the haemolysate increased t-PA activity, it did not alter the electrophoretic characteristics of the t-PA nor did it induce any fibrinolysis in the absence of t-PA or plasminogen. The haemolysate was devoid of plasmin and plasminogen activator activity but was most effective in accelerating plasminogen activation by t-PA in the presence of substrate. Based on the purification characteristics of the active fraction in the haemolysate, it appears to have a molecular weight of less than 10 kDa.

Tissue plasminogen activator (tPA) inhibits plasmin degradation of fibrin. A mechanism that slows tPA-mediated fibrinolysis but does not require alpha 2-antiplasmin or leakage of intrinsic plasminogen

Thrombolysis is dramatically slower when high concentrations of lytic agent are used. This paradoxical observation, first described as "plasminogen steal," was originally believed to be due to depletion of extrinsic plasminogen and consequent leaching of clot-bound plasminogen. We report that administration of increasing concentrations of recombinant human tissue plasminogen activator (tPA) to fibrin gels resulted in lysis rates that displayed a maximum, with significantly slower rates found at higher tPA, regardless of whether plasminogen was supplied extrinsically or intrinsically. A similar maximum in lysis rates was observed in a system lacking an extrinsic phase when plasminogen was added to fibrin suspensions preincubated with increasing tPA. Thus, intrinsic plasminogen leakage and alpha 2-antiplasmin were not required for the decreased lysis at high tPA. No maximum was observed for increasing concentrations of urokinase. Using fibrin suspensions or gels preincubated with tPA before addition of plasmin, we report that tPA, but not urokinase, caused a dose-dependent inhibition of the fibronolytic action of plasmin. With respect to optimal dosage schemes and the design of novel lytic agents, these findings indicate that (a) there exists a biochemical mechanism against minimizing reperfusion time with increasing tPA dosages and (b) the fibrin affinity of tPA may cause reduced fibrinolysis by plasmin.

Stimulating effect on tissue-type plasminogen activator--a new and sensitive indicator of denatured fibrinogen

A high clottability and a short thrombin clotting time have routinely been considered as evidence of genuineness of the fibrinogen molecule. Since denatured fibrinogen stimulates the t-PA-catalysed conversion of plasminogen to plasmin, it was of interest to study the sensitivity of t-PA-stimulation as evidence of fibrinogen denaturation. Therefore, fibrinogen was intentionally exposed to various denaturating conditions (freeze-drying, heating, EDTA, alkali), and the clottability, the thrombin clotting time and the t-PA-stimulating effect were recorded. We found that the clottability was a poor indicator of fibrinogen denaturation, whereas the t-PA-stimulating effect could detect even mild fibrinogen denaturation. The thrombin clotting time was shortened after freeze-drying or heating at 47 degrees C, in spite of what might have been expected. Thus, denaturation is not necessarily accompanied by a prolonged clotting time. In some instances therefore, the t-PA-stimulation is an even more sensitive and reliable indicator of fibrinogen denaturation than is the thrombin clotting time. Consequently, this parameter should be combined with the thrombin clotting time to characterise preparations of fibrinogen.

Fibrin structures during tissue-type plasminogen activator-mediated fibrinolysis studied by laser light scattering: relation to fibrin enhancement of plasminogen activation

The aim was to relate fibrin structure and the stimulatory effect of fibrin on plasminogen activation during t-PA-mediated fibrinolysis using Lys78-plasminogen as activator substrate. Structural studies were undertaken by static and dynamic laser light scattering, cryo transmission electron microscopy and by the measurement of conversion of fibrin to X-, Y- and D-fragments. The kinetics of plasmin formation were monitored by measurement of the rate of pNA-release from Val-Leu-Lys-pNA. The process of fibrin formation and degradation comprised three phases. In the first phase, protofibrils with an average length of about 10 times that of fibrinogen were formed. The duration of this phase decreased with increasing t-PA concentration. The second phase was characterized by a sudden elongation and lateral aggregation of fibrin fibers, most pronounced at low levels of t-PA, and by formation of fragment X-polymer. The third phase was dominated by fragmentation of fibers and by formation of Y- and D-fragments. Plasmin degraded the fibers from within, resulting in the formation of long loose bundles, which subsequently disintegrated into thin filaments with a length of less than 10 and a mass per length close to one relative to fibrinogen. Plasmin generation at high t-PA concentrations sets in just prior to (and at low t-PA concentrations shortly after) the onset of the rapid second phase of elongation and lateral aggregation of fibrin fibers. The maximal rate of plasmin formation per mol t-PA was the same at all concentrations of activator and was achieved close to the time of the peak level of fragment X-polymer. Plasmin formation ceased after formation of substantial amounts of Y- and D-fragments. At this stage the length was between 300 and 3 and the mass per length close to 1, both relative to fibrinogen. In conclusion our results indicate that (1) formation of short fibrin protofibrils is the minimal requirement for the onset of the stimulatory effect of fibrin on plasminogen activation by t-PA, (2) formation of fragment X protofibrils is sufficient to induce optimal stimulation of plasminogen activation, and (3) plasmin degrades laterally aggregated fibrin fibers from within, resulting in the conversion of the fibers into long loose bundles, which later disintegrate into thin filaments.

Degradation of the alpha-chain of fibrin by human neutrophil elastase reduces the stimulating effect of fibrin on plasminogen activation

The degradation of fibrin by human neutrophil elastase (HNE) and the interference of such degradation on the stimulating effect of fibrin on plasminogen activation by tissue plasminogen activator (t-PA) was studied. By using SDS electrophoresis and Western blotting with subsequent immunostaining with monoclonal antibodies, degradation of the fibrin molecule was monitored. This degradation was related to the stimulating effect on plasminogen activation. Degradation of the alpha-chain was seen to occur before degradation of the beta- and gamma-chains. On the alpha-chain it was found that C-terminal degradation occurred prior to visible degradation of the N-terminal end. This C-terminal degradation was associated with a fall in the stimulation of plasminogen activation, coinciding with a corresponding reduction in the polymerization of fibrin. With further degradation, including N-terminal proteolysis of the alpha-chain, the stimulating effect of fibrin was reduced to that of fibrinogen.

CONCLUSIONS

Our results indicate that HNE degradation of the alpha-chain of fibrin occurs initially from the C-terminal end, affecting the polymerization of fibrin. This impaired polymerization may be important for the observed reduction in the t-PA mediated plasminogen activation.

Sites in fibrin involved in the acceleration of plasminogen activation by t-PA. Possible role of fibrin polymerisation

Polymeric fibrin is a strong enhancer of the activation of plasminogen by t-PA. At least two types of sites are involved in this enhancement i.e. a site within A alpha-(148-160), and a site within gamma-(311-379). These sites are not accessible in fibrinogen, but are exposed upon conversion of fibrinogen to fibrin. This explains why fibrinogen has no rate-enhancing properties, and helps to explain the effects of fibrin. Fibrin with its ordered structure appears to exert its rate-enhancing effect by presenting the above sites for interaction with t-PA and plasminogen; thus concentrating and correctly orienting these two reactants on its surface and inducing conformational changes which lead to higher catalytic efficiencies.

Study of tissue-type plasminogen activator binding sites on fibrin using distinct fragments of fibrinogen

It is well established that tissue-type plasminogen activator (t-PA) binds to the D region of fibrin(ogen) and that two distinct CNBr fragments of fibrinogen (FCB), FCB-2 and FCB-5, comprising parts of this region, stimulate plasminogen activation by t-PA. In the present work, ligand-binding studies were performed to characterize the interactions between t-PA and the corresponding fibrin regions using a well defined model of a fibrin surface and both FCB-2 and FCB-5 in liquid and solid phase. Binding isotherms showed a characteristic Langmuir adsorption saturation profile. The dissociation constants determined for the binding of t-PA to immobilized FCB-2 (Kd = 0.70 +/- 0.10 nM) and FCB-5 (Kd = 0.47 +/- 0.08 nM) were of the same order of magnitude as the Kd for fibrin binding (Kd = 1 +/- 0.2 nM). The specificity of the binding was demonstrated by the ability of soluble FCB-2 and FCB-5 to inhibit t-PA binding to solid-phase fibrin (Ki = 3.3 microM and 6.4 microM, respectively). The binding of t-PA to fibrin and to immobilized FCB-2 was partially inhibited by the lysine analogue 6-aminohexanoic acid (Ki = 123 +/- 47 microM and 364 microM, respectively) but was not modified by carboxypeptidase B, thus indicating involvement of internal lysine residues. Removal of lysine residues by treatment with, successively, plasmin and carboxypeptidase B, produced only a partial inhibition of t-PA binding, thus confirming the existence of both a lysine-dependent and a lysine-independent mechanism of binding of t-PA to both fibrin and FCB-2. In contrast, the binding of t-PA to FCB-5 was not significantly affected by 6-aminohexanoic acid. Altogether, these data indicate that the mechanism of binding of t-PA to fibrin involves mainly a lysine-independent interaction with the D region which is contributed by sequences present in FCB-5 and FCB-2; contribution to binding by a lysine-dependent interaction was detected only in FCB-2 and is probably of minor relevance as suggested by the limited effect of 6-aminohexanoic acid.

Molecular assembly of plasminogen and tissue-type plasminogen activator on an evolving fibrin surface

A well characterized model of an intact and a degraded surface of fibrin that represents the states of fibrin during the initiation and the progression of fibrinolysis was used to quantitatively characterize the molecular interplay between tissue-type plasminogen activator (t-PA), plasminogen and fibrin. The molecular assembly of t-PA and plasminogen on these surfaces was investigated using combinations of proteins that preclude complications due to side reactions caused by generated plasmin: native plasminogen with di-isopropylphosphofluoridate-inactivated t-PA, and a recombinant human plasminogen with the active-site Ser741 mutagenized to Ala which renders the catalytic site inactive. Under these conditions, neither the affinity nor the maximal number of binding sites for plasminogen were modified by the presence of t-PA, indicating that binding sites for plasminogen pre-exist in intact fibrin and are not dependent on the presence of t-PA. In contrast, when plasminogen activation is allowed, increasing binding of plasminogen to the progressively degraded fibrin surface is directly correlated (r = 0.98) to the appearance of the fibrin E-fragment as shown using a monoclonal antibody (FDP-14) that has its epitope in the E domain of fibrin. t-PA was shown to bind with a high affinity to both the intact (Kd = 3.3 +/- 0.6 nM) and the degraded surface of fibrin (Kd = 1.2 +/- 0.4 nM). Binding of t-PA to carboxy-terminal lysine residues of degraded fibrin was shown to be efficiently competed by physiological concentrations of plasminogen (2 microM), indicating that the affinity of t-PA for these residues was lower than that of plasminogen (Kd = 0.66 +/- 0.22 microM) and unrelated to the high affinity of t-PA for specific binding sites on intact fibrin. These data confirm and establish that the generation of carboxy-terminal lysine residues on fibrin during ongoing fibrinolysis, and the binding of plasminogen to these sites, is an important pathway in the acceleration of clot dissolution.

Endothelial cell proteases: physiological role and regulation

Endothelial cell-derived proteases can be classified according to their physiological role. The proteases involved in extracellular matrix degradation are important in endothelial cell migration and thereby in angiogenesis. They include the urokinase-type plasminogen activator (uPA) and the metalloproteases, collagenases, gelatinases and stromelysin. uPA secreted from endothelial cells remains associated with the cell membrane, on specific receptors localized in the vicinity of the receptors for plasminogen. This favours the local activation of plasminogen into plasmin. Plasmin, generated on the cell surface, is fully active as it is not inhibited by alpha 2-antiplasmin. Plasmin acts directly by degrading some components of the extracellular matrix and indirectly by activating the prometalloproteases. Secretion of PAI by migrating cells is generally stimulated by the same factors that induce uPA secretion, limiting the degradation of the matrix to the pericellular path. The degradation of the fibrin clot involves the tissue-type plasminogen activator tPA, which like the uPA activates plasminogen to plasmin. This system is also regulated by two different mechanisms. On the one hand, fibrin itself favours its own degradation by formation of a ternary complex, fibrin-plasminogen-tPA, in which the affinity of tPA for plasminogen is markedly increased, as compared to the affinity of unbound tPA. In addition, plasmin generated on the clot is protected from inhibition by alpha 2-antiplasmin. On the other hand, as for uPA, tPA is inhibited by PAI-1. The importance of the regulation of this system is illustrated by the thrombotic risk observed when there is either a decrease in tPA or an increase in PAI-1, and inversely by haemorrhages in the case of increase in tPA.