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PubMed2003

The circulatory half-lives of alpha-profibrin and alpha-fibrin monomer, and comparisons with other fibrin(ogen) derivatives.

Previous studies showed that alpha-fibrin monomer (lacking both A-fibrinopeptides, FPA) is normally cleared from the circulation before it assembles into a clot. Recent studies indicate that substantial quantities of an intermediate, alpha-profibrin lacking only one of the two FPA are produced in the course of conversion of human fibrinogen to fibrin. Since clearance of the alpha-fibrin monomer is saturable and receptor mediated, the extent to which alpha-profibrin or other fibrin(ogen) derivatives might compete for monomer uptake was deemed important. We compared plasma decay of injected human alpha-fibrin, fibrinogen, and alpha-profibrin in rabbits using rabbit anti-human fibrinogen for assays. The circulatory half-life of human alpha-fibrin monomer was short (t(1/2) = 2.3 h) and followed a simple exponential decay curve, as anticipated from clearance of rabbit alpha-fibrin. It was absorbed as fast as it permeated the extravascular space with no redistribution. Human fibrinogen had a long half-life (t(1/2) = 39.5 h), calculated from the double exponential plasma decay curves (redistribution + catabolism) observed over 28 h. The alpha-profibrin had an intermediary half-life (t(1/2) = 11 h) determined from double exponential decay curves. Since redistribution accompanied the slow clearance of alpha-profibrin, its binding by the fibrin receptor(s) must be weak, probably too weak to compete with the clearance of alpha-fibrin monomer. The initial production of alpha-fibrin monomer is only partially dependent on prior formation of alpha-profibrin, as recently shown. Thus, it is the slow clearance and the weak competition from alpha-profibrin that underlie the occurrence of substantial levels of alpha-profibrin unaccompanied by detectable levels of alpha-fibrin monomer in many subjects with vascular disease.

Animals↗
PubMed2005

Biochemical characterization of autologous fibrin sealants produced by CryoSeal and Vivostat in comparison to the homologous fibrin sealant product Tissucol/Tisseel.

Different principles for production of "autologous fibrin sealant" have been established, and commercial devices employing these methods are nowadays available and used in clinical routine. Users might anticipate for these autologous fibrin sealants features comparable to commercial homologous fibrin sealants, used in surgical routine for many years. However, only little is known about biochemical properties, formation, cross-linking and stability of fibrin sealant clots produced for autologous use with the aid of commercially available devices. We have investigated protein composition, formation and stability of clots obtained from autologuous fibrin sealants produced with commercially available devices (CryoSeal and Vivostat) and compared these parameters to those of the industrially produced homologous fibrin sealant Tissucol/Tisseel. The CryoSeal product is a mixture of many plasma proteins; the Vivostat product and Tissucol/Tisseel appear as comparatively pure plasma derivatives. The products differ in their protein composition and concentrations, including their concentration in fibrin. Significant fibrin alpha and gamma-chain cross-linking by FXIIIa occurs only in Tissucol/Tisseel clots. In test tubes CryoSeal and Vivostat (tranexamic acid-free formulation) fibrin clots liquefy within 1-2 days, but Vivostat (tranexamic acid containing formulation) clots were stable for 4 days and showed partial liquefaction after 5 days. Tissucol/Tisseel clots, containing the protease inhibitor aprotinin, appeared unchanged over the observation period of 5 days. In an in vitro model mimicking in vivo conditions (diffusion of protease inhibitors and proteolytic digestion) clot liquefaction occurs at day 1 for all autologous fibrin sealants clots, with an observable delay for the tranexamic acid containing Vivostat, and day 5 for Tissucol/Tisseel clots. Characterization of the CryoSeal and Vivostat fibrin sealants and Tissucol/Tisseel and their performance show a clear difference in biochemical properties.

Blood Coagulation↗
PubMed2000

Smooth muscle cell outgrowth stimulated by fibrin degradation products. The potential role of fibrin fragment E in restenosis and atherogenesis.

This study is based on the observation that deposition of thrombus within the arterial wall and on its surface is a consistent response to the vascular injury of angioplasty and of angioplasty lesions at risk of rapid restenosis. Mitogenic activity is stimulated by fibrin degradation products in extracts of human atherosclerotic plaques and plasmin digests of fibrin, and this has been attributed to products that include fibrin fragment E. The effect of human fibrin degradation products on smooth muscle outgrowth from rabbit aortic medial explants now has been explored in culture. Every batch of fibrin degradation products was first tested on the in vivo chick chorioallantoic membrane model for the ability to stimulate cell proliferation, including angiogenesis as shown previously. Increasing concentrations of fibrin degradation products were stimulated significantly earlier and with greater outgrowth of smooth muscle cells than controls, up to an optimum at 92 microg/mL fibrin degradation products. The effect of fibrin degradation products was blocked by the prior admixture of a specific antifragment E antiserum, but not by an antifragment D antiserum. Purified commercial fibrinogen E is inactive, but when treated with thrombin to resemble fibrin E it stimulated smooth muscle cell outgrowth, and this was not seen with comparable dosages of fragment D. We propose that fibrin degradation products, in particular fibrin fragment E, provide an abundant in situ early initiator of smooth muscle cell migration and proliferation in restenosis and atherogenesis.

Angioplasty↗
PubMed1980

Studies on circulating soluble fibrin: separation of 125I-Des-AB fibrin and 131I-fibrinogen by gel filtration.

The behaviour of labelled des-AB fibrin in plasma was studied by gel filtration after it had been injected into rabbits. Purified rabbit [125I]des-AB fibrin was prepared by clotting of [125I]fibrinogen by thrombin and solubilizing the formed clot in buffered 3 M urea. Gel filtration of this material on urea-equilibrated columns showed a single peak identical to the elution profile of fibrinogen. This indicated the existence of monomeric des-AB fibrin. When plasma from rabbits injected with monomeric [125I]des-AB fibrin and [131I]fibrinogen was gel-filtered through plasma-equibrated columns, two separate peaks of radioactivities were obtained. The first peak eluted mainly with the void volume and contained [125I]des-AB fibrin whereas the second peak eluting within the fractionation range contained [131I]fibrinogen. Identical elution profiles were obtained in in vitro studies when monomeric [125I]des-AB fibrin was mixed with plasma containing [131I]fibrinogen and gel-filtered through plasma-equilibrated columns. We conclude from these studies that monomeric des-AB fibrin formed high-molecular weight aggregates or changed its conformation posing as a larger molecule than fibrinogen when injected into rabbits. No complex formation between des-AB fibrin and fibrinogen was observed as [131I]fibrinogen was not incorporated into des-AB fibrin aggregates. Thus, soluble des-AB fibrin can circulate in the blood without forming fibrin-fibrinogen complexes.

Animals↗
PubMed1990

[125I]fibrin deposition occurs at both early and late intervals of IgE-dependent or contact sensitivity reactions elicited in mouse skin. Mast cell-dependent augmentation of fibrin deposition at early intervals in combined IgE-dependent and contact sensitivity reactions.

When elicited in the skin of mice, either IgE-dependent immediate hypersensitivity reactions or T cell-dependent contact sensitivity (CS) reactions result in local extravasation of [125I]fibrinogen and deposition of [125I]fibrin. However, these two types of reaction differ in kinetics and in requirement for IgE, mast cells, or T cells. In the present study, we investigated the kinetics and magnitude of [125I]fibrin deposition in combined IgE-dependent and CS reactions elicited simultaneously at the same site and compared the results with those obtained when the two reactions were elicited at separate sites. We found that [125I]fibrin deposition in pure IgE-dependent reactions was greater at 2 or 6 h after challenge than at 24 h, but that significant fibrin deposition persisted at those sites 24 h after challenge. In CS reactions, [125I]fibrin deposition was detected as early as 2 h after challenge, indicating that fibrin deposition accompanies the "early component" of CS detected by Van Loveren et al. with the use of measurements of tissue swelling. But much more [125I]fibrin deposition was present in CS reactions at 24 h than at 2 or 6 h after Ag challenge. When IgE-dependent and CS reactions were elicited at the same site, [125I]fibrin deposition at early intervals (2 to 6 h) after challenge was increased three- to 25-fold compared with that seen in isolated CS reactions, but at 24 h the results in the combined reactions were virtually identical to those in CS responses. Studies in genetically mast cell-deficient and congenic normal mice indicated that mast cells were required for expression of the IgE-dependent augmentation of [125I]fibrin deposition observed at early intervals in combined IgE-dependent and CS reactions, but not for the [125I]fibrin deposition associated with "pure" CS reactions. These findings indicate that the net effect of IgE-dependent mast cell activation on CS responses is to increase the fibrin deposition associated with these responses, but this effect is appreciated only at early intervals after elicitation of the reaction.

Animals↗
PubMed1980

Fibrin in peritonitis. I. Beneficial and adverse effects of fibrin in experimental E. coli peritonitis.

Fibrin has classically been considered a defense mechanism of the peritoneal cavity. We have studied the role of purified fibrin in the pathogenesis of intraperitoneal infection. Implantation of 0.5% bovine fibrin clots containing 2 X 10(8) E. coli into the rat peritoneal cavity reduces the 24-hour mortality rate from 100% to 0% compared to bacteria in a similar volume of saline solution. However, the 10-day mortality rate with fibrin is 90%; 100% develop intraperitoneal abscesses. Animals receiving sterile clots lyse than over 1 to 2 weeks without abscess formation. As few as 10(2) E. coli per fibrin clot produce abscesses, but 10(7) or more are required to produce death; without fibrin less than 10(7) E. coli neither kill nor produce intraperitoneal infections. Both late death and abscess size with 2 X 10(8) E. coli are directly proportional to the fibrin clot size but not the concentration of fibrin in the clot. Operative debridement of the fibrin at 4 or 24 hours completely eliminates abscess formation in surviving animals. In vitro growth of E. coli is neither stimulated nor inhibited by fibrin or fibrinogen. Fibrin delays systemic sepsis, but the entrapped bacteria cannot be easily eliminated by normal intraperitoneal bactericidal mechanisms and abscess formation occurs. Thus radical peritoneal debridement or anticoagulation may reduce the septic complications of peritonitis.

Abscess↗
PubMed1994

Effect of an eccentric severe stenosis on fibrin(ogen) deposition on severely damaged vessel wall in arterial thrombosis. Relative contribution of fibrin(ogen) and platelets.

BACKGROUND: Coronary thrombosis is a dynamic process dependent on the pathological substrate, the local shear forces, and blood factors. METHODS AND RESULTS: We investigated the effect of a severe (80%) eccentric stenosis on fibrin(ogen) interaction with a deeply damaged vessel wall, its relation to platelet deposition in thrombus formation, and the influence of time on thrombus growth. Porcine 125I-fibrinogen and autologous 111In-platelets were injected into pigs instrumented for extracorporeal circulation and treated with low-dose heparin (aPTT ratio < 1.5) that has been previously shown and herein confirmed not to affect platelet and/or fibrin(ogen) attachment. Tunica media, as a model of severely injured vessel wall, was mounted in a tubular perfusion chamber containing an eccentric axisymmetric sinusoidal stenosis obstructing the lumen and exposed for 1, 5, and 10 minutes to perfusing blood. A shear rate of 424 s-1 at the laminar, parallel parabolic local flow perfused segments one to two orders of magnitude greater at the apex of the stenosis. Fibrin(ogen) deposition, its axial distribution with respect to the apex, and its relation to platelet deposition were determined by an ex vivo analysis of the test substrates. Fibrin(ogen) and platelet deposition were both significantly higher at the apex of the stenosis than at either the prestenotic or poststenotic area at all the studied perfusion times (P < .02). However, fibrin(ogen) deposition demonstrated a significantly smaller degree of increase from the prestenotic area to the apex as well as a smaller degree of decrease from the latter to the poststenotic region, compared with platelet deposition (P < .05). Although both fibrin(ogen) and platelet deposition increased over time, the ratio of fibrin(ogen) to platelets showed a progressive decrease that became significant from 5 to 10 minutes (P < .03) at either low or high shear rate. The rate of platelet deposition was relatively constant; however, fibrin(ogen) deposition progressively decreased, especially at the apex. CONCLUSIONS: On severely damaged vessel wall, fibrin(ogen) and platelet deposition is maximal at the apex of the stenosis where shear rate is extremely high and parallel streamlines are deformed. Nevertheless, fibrin(ogen) deposition is significantly less dependent on high shear rate than is platelet deposition, and the pattern is not influenced by time. Finally, fibrin(ogen) deposition appears to be predominant in the thrombus layers adjacent to a severely damaged vessel wall regardless of the local shear stress levels and flow conditions.

Animals↗
PubMedDate not supplied

Identification and distribution of fibrinogen, fibrin, and fibrin(ogen) degradation products in atherosclerosis. Use of monoclonal antibodies.

Samples of normal and atherosclerotic vessels obtained from vascular and cardiothoracic surgery were examined for the distribution of fibrinogen/fibrin I, fibrin II, and fibrin(ogen) degradation products (Fragment D/DD) by using recently characterized monoclonal antibodies that recognize and distinguish the three molecular forms (MAbs 18C6, T2G1, and GC4, respectively) with the ABC-immunoperoxidase technique. In normal aortas, little fibrinogen/fibrin I or fibrin II was present and no fibrin(ogen) degradation products could be detected. In early lesions and in fibrous plaques, fibrinogen/fibrin I and fibrin II were distributed in long threads and surrounding vessel wall cells and macrophages. Fibrin(ogen) degradation products were not seen in early lesions. In fibrous and advanced plaques, fibrinogen/fibrin I, fibrin II, and fibrin(ogen) degradation products were detected in areas of loose connective tissue, in thrombus, and around cholesterol crystals. The results of this study suggest that increased fibrin formation and degradation may be associated with progression of atherosclerotic disease. The observed distribution of the different molecular forms of fibrinogen also suggests the possibility that the cells present in the lesions actively participate in the fibrinogen-to-fibrin transition within the vessel wall.

Antibodies, Monoclonal↗
PubMed1993

GPR-phoresis, a novel approach to determining fibrin monomer and other macromolecular derivatives of fibrinogen and fibrin in blood.

An electrophoretic method for determining (i) cross-linked fibrin-complexes, (ii) fibrin-monomer, (iii) fibrinogen-dimers, (iv) normal fibrinogen and (v) degradation products in plasma, has been devised. The technique is based on differences in their migration characteristics in the presence and absence of Gly-Pro-Arg (GPR), a specific inhibitor of fibrin aggregation. In buffer containing 2.5 mM GPR, fibrin monomer and fibrinogen co-migrate anodally, but, unlike fibrinogen which does not depend on GPR for solubility, the fibrin monomers precipitate when they traverse a boundary between buffer containing and buffer lacking GPR. By limiting the GPR to a 2 cm zone of buffer under the conditions employed, the precipitation of fibrin monomer occurs in a sharp band 4 mm anodally to the sample application point. Cross-linked fibrin complexes have slower mobility than fibrin monomer and precipitate in a broad band behind the monomer. Dimeric fibrinogen, like fibrinogen itself but unlike the fibrin complexes, is not constrained to migration within the GPR boundary and passes through it, but behind the band for normal fibrinogen due to sieving by the gel. Fibrinogen and all but low molecular weight degradation products can be specifically precipitated within electrophoregrams by heat denaturation at 47 degrees C. After washing unrelated protein away, the fibrin(ogen) derivatives can be measured by staining with Coomassie blue. Since the method does not depend on immunoprobing for specific staining, it provides an inexpensive and rapid means for differential assessment of the prevalence of the fibrinogen derivatives in disease states and in models of disease, regardless of animal species.

Electrophoresis↗
PubMed1986

Involvement of finger domain and kringle 2 domain of tissue-type plasminogen activator in fibrin binding and stimulation of activity by fibrin.

Human tissue-type plasminogen activator (t-PA) catalyses the conversion of inactive plasminogen into active plasmin, the main fibrinolytic enzyme. This process is confined to the fibrin surface by specific binding of t-PA to fibrin and stimulation of its activity by fibrin. Tissue-type plasminogen activator contains five domains designated finger, growth factor, kringle 1, kringle 2 and protease. The involvement of the domains in fibrin specificity was investigated with a set of variant proteins lacking one or more domains. Variant proteins were produced by expression in Chinese hamster ovary cells of plasmids containing part of the coding sequence for the activator. It was found that kringle 2 domain only is involved in stimulation of activity by fibrin. In the absence of plasminogen and at low concentration of fibrin, binding of t-PA is mainly due to the finger domain, while at high fibrin concentrations also kringle 2 is involved in fibrin binding. In the presence of plasminogen, fibrin binding of the kringle 2 region of t-PA also becomes important at low fibrin concentrations.

Animals↗
PubMed1987

Pulmonary fibrin deposition and increased microvascular permeability to protein following fibrin microembolism in dogs: a structure-function relationship.

The effects of fibrin microembolism were examined using an infusion of a prothrombin activator (Echis carinatus venom, ECV; 30 min, 0.5 NIH thrombin equivalent units/kg) in acute mongrel dogs prepared with a pulmonary lymph cannula (n = 6, 12.3-21.5 kg). Lymph flow increased approximately 2.5-fold after 1-1.5 hr of elevated left atrial pressure (Pla = 20 cm H2O; 26 +/- 7 to 63 +/- 16 microliter/min, P less than 0.01) and the plasma to lymph protein concentration ratio (CP/CL) declined from 0.66 +/- .04 to 0.54 +/- .16 (P less than 0.01, x +/- SE). After Pla was reduced to control levels, the initiation of fibrin microembolism was associated with an approximate 2.7-fold elevation of lymph flow (62 +/- 8 microliters/min, P less than 0.01) and the CP/CL was not changed (0.56 +/- 0.04, P = ns). When Pla was increased following microembolism, lymph flow more than doubled to 117 +/- 24 microliter/min (P less than 0.01) and the CP/CL remained unaltered (0.56 +/- 0.03, P = ns). These changes were associated with afibrinogenemia and the appearance of fibrin degradation products (FDP) in plasma (150 +/- 50 micrograms/ml) and lymph (80 micrograms/ml) in three of the animals tested. No consistent pattern was seen in the CL/CP of separate endogenous plasma proteins after each intervention. These data support the view that pulmonary fibrin microembolism without inhibition of the fibrinolytic system was associated with an early increased pulmonary microvascular permeability to protein. In a separate group of similarly prepared animals (n = 8, 13-21.5 kg) without a lymph catheter, scanning electron microscopic observations showed branching fibrin microemboli to partially occlude some pulmonary arterioles. Mixed thrombus formations in larger precapillary blood vessels were also seen. Ultrastructural observations revealed the deposition of fibrin strands (periodicity = 220-230 A) within the pulmonary capillaries. Some of these deposits were overlaid by lamellar pseudopodia from endothelial cells and the fibrin appeared to be within these cells. Although plasmalemmal vesicles seemed to be more numerous in the endothelial cells with adjacent fibrin deposits, no gaps or breaks were seen in the densely stained interendothelial cell junctions and/or the endothelial cell membrane of the affected lung capillaries. Activated neutrophils and platelets were more numerous in the pulmonary capillaries following EVC. These data suggest that the presence of FDP and/or fibrin deposits within the pulmonary microvasculature may influence the early functional integrity of pulmonary endothelial cells at sites of fibrin accumulation.

Animals↗
PubMed1990

Effects of intact fibrin and partially plasmin-degraded fibrin on kinetic properties of one-chain tissue-type plasminogen activator.

A comparative kinetic analysis of the enzymatic activities of one-chain and two-chain tissue-type plasminogen activator (t-PA) demonstrates that two-chain t-PA catalyzes the hydrolysis of the peptide substrate D-Val-Leu-Arg-pNA about 4-fold more effectively than one-chain t-PA. The difference is accounted for almost entirely by a corresponding difference is the kcat values of the enzymes, whereas the Km values are similar. The amidolytic activity of two-chain t-PA is not enhanced by intact or partially plasmin-degraded fibrin. In contrast, the activity of one-chain t-PA is stimulated up to 3.7-fold by intact fibrin and up to 4.7-fold by plasmin-degraded fibrin (fibrin X-fragment). The stimulatory effects are realized via increases in the kcat values. It appears thus that in the presence of fibrin the intrinsically inferior catalytic properties of one-chain t-PA become similar to the properties of two-chain t-PA. The dependency of the activity of one-chain t-PA on the concentration of fibrin monomer is consistent with a single association site of both proteins and an association constant of Kass = 6.25 x 10(6) l/mol. Stimulation of one-chain t-PA by plasmin-degraded fibrin is more complex and appears to involve two different binding sites with association constants of Kass = 0.67 x 10(9) l/mol and Kass = 3.85 x 10(6) l/mol, respectively. The stimulatory effects of fibrin and partially plasmin-degraded fibrin on one-chain t-PA are suppressed by epsilon-aminocaproic acid and by a monoclonal antibody directed against the lysine binding site of t-PA. The latter findings support the notion that fibrin activation of one-chain t-PA is mediated by the lysine binding site on kringel domains of the enzyme.

Amino Acid Sequence↗
PubMed1997

Covalent cross-linking of fibronectin to fibrin is required for maximal cell adhesion to a fibronectin-fibrin matrix.

In a blood clot, fibrin and plasma fibronectin (pFN) are covalently cross-linked by activated factor XIII (factor XIIIa) to form pFN-fibrin multimers. To determine the functional significance of covalent pFN-fibrin interactions, we have developed an in vitro model which allows the incorporation of recombinant FN (recFN) molecules into a covalently cross-linked recFN-fibrin matrix. Using the baculovirus expression system, we have expressed recFN monomers composed of the amino-terminal 70-kDa region and the first 11 type III repeats (WT) with mutations in the glutamines at positions 3 and 4 (Q2) or at 3, 4, and 16 (Q3). Examination of the covalent incorporation of these recFNs into fibrin clots confirms that glutamines 3 and 4 are major participants in FN-fibrin cross-linking as the mutation of these sites reduces cross-linking efficiency by 65%. Additional mutation of the glutamine at position 16, however, eliminates >99% of cross-linking suggesting that it also may be factor XIIIa reactive. When the Q3 recFN-fibrin clots were used as substrates for cell adhesion, there was a decrease in both cell attachment and spreading when compared with the WT recFN-fibrin clots. These data demonstrate that for maximal cell attachment to a FN-fibrin clot, FN must be cross-linked to fibrin by factor XIIIa.

3T3 Cells↗
PubMed1971

The effect of fibrin-stabilizing factor on the subunit structure of human fibrin.

The formation of human fibrin from fibrinogen has been examined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate, a method which separates a mixture of proteins on the basis of differences in molecular weight. It has been found that the plasma from a patient with a congenital deficiency of fibrin-stabilizing factor forms clots lacking the cross links among the alpha- and gammachains found in normal, cross-linked human fibrin. The addition of purified fibrin-stabilizing factor or normal plasma to the deficient plasma results in extensive cross-linking of the chains. Thus, the fibrinogen in the fibrin-stabilizing factor deficient plasma appears to be normal and forms fibrin which contains dimeric, cross-linked gamma-chains and polymeric, high molecular weight forms of alpha-chains. By the use of these electrophoretic methods, it has also been possible to develop a highly sensitive method for measuring the content of fibrin-stabilizing factor in plasma. This method depends upon the use of urea-treated fibrinogen, which is completely devoid of fibrin-stabilizing factor, but which forms the usual cross-linked subunits after conversion to fibrin by thrombin in the presence of fibrin-stabilizing factor.

Acetates↗
PubMed1989

Interaction between plasminogen activator inhibitor type 1 (PAI-1) bound to fibrin and either tissue-type plasminogen activator (t-PA) or urokinase-type plasminogen activator (u-PA). Binding of t-PA/PAI-1 complexes to fibrin mediated by both the finger and the kringle-2 domain of t-PA.

Plasminogen activation is catalyzed both by tissue-type-(t-PA) and by urokinase-type plasminogen activator (u-PA). This reaction is controlled by plasminogen activator inhibitor type 1 (PAI-1) that is either present in plasma or bound to fibrin, present in a thrombus. We studied the mechanism of in vitro inhibition of both t-PA and u-PA activity by PAI-1 bound to fibrin. It is shown that activation of latent PAI-1 unmasks a specific fibrin-binding site that is distinct from its reactive site. This reactive site of activated PAI-1 bound to fibrin is fully exposed to form complexes with t-PA and u-PA, that are unable to activate plasminogen. Upon complex formation with either one of the plasminogen activators, PAI-1 apparently undergoes a conformational change and loses its affinity for fibrin. Consequently, complexes of u-PA and PAI-1 dissociate from the fibrin matrix and are encountered in the fluid phase. In contrast, t-PA/PAI-1 complexes remain bound to fibrin. By employing recombinant t-PA deletion-mutant proteins, that precisely lack domains involved in fibrin binding, we demonstrate that binding of t-PA/PAI-1 complexes is mediated by both the "finger" (F) and the "kringle-2" (K2) domain of t-PA. A model is proposed that explains inhibition of the fibrinolytic process, at the level of plasminogen activation by t-PA, directed by PAI-1 bound to fibrin. An implication of the proposed model is that t-PA/PAI-1 complexes and free t-PA compete for the same binding sites on fibrin.

Animals↗
PubMed1984

The endoendothelial fibrin lining as the crucial barrier and the role of fibrin(ogenin) gels in controlling transcapillary transport.

The interface between the two portions of the 'vessel-blood organ', viz., the vessel wall and the circulating blood, is considered by the author to be the endoendothelial fibrin lining (EEFL). The view that the endothelium, consisting of the endothelial cells and the interendothelial cement substance, is the primary filtration barrier in capillary permeability (CP) is no longer tenable. There is considerable evidence that the primary barrier is an endocapillary protein layer, originally postulated by Danielli in 1940. Copley considered this layer to be identical with the EEFL formed in the more or less immobile portion of the plasmatic zone in close proximity to the vessel wall. Processes of fibrin formation and fibrinolysis can occur there homeostatically, undisturbed by the flow of blood. The fibrinopeptides and plasminopeptides, freed at this site by the conversion of fibrinogen to fibrin and of plasminogen to plasmin, respectively, were reported by Copley et al in 1966 to augment CP. These peptides thus take part in the steadily occurring normal physiological CP. This is facilitated by the porosity of the EEFL due to the network or gel structure of fibrin strands. The author's concept that the EEFL acts as the primary barrier, controlling transendothelial transport and transport across the basement membrane (BM), is discussed on the basis of older and recent findings by several investigators. In particular, the BM is dealt with in some detail as a barrier. Emphasis is placed on the existence of fibrin as a main constituent of the BM, hitherto not generally known. This was demonstrated by direct evidence in the production of (non-thrombocytopenic) vascular purpura with fibrin antiserum. Numerous tiny foci of fibrin(ogenin) gels are expected to stud the BM. Augmented capillary fragility (CF) due to increased fibrinolysis of many of these focal fibrin gels result in petechial hemorrhages. CF and CP are physical properties of the blood capillary wall which behave antagonistically and are controlled by fibrin formation and fibrinolysis, steadily occurring in the vascular layers including the BM. This barrier secures the integrity of the capillary wall by preventing extravasation of blood or hemorrhages. New experimental approaches to verify the detection of fibrin in the microstructure of the capillary wall are proposed. Moreover, hemorheological experimentation, models and treatments are needed to establish whether or not the EEFL is the crucial, critical barrier in CP, as proposed.

Animals↗
PubMed1985

Tissue plasminogen activator and urokinase mediate the binding of Glu-plasminogen to plasma fibrin I. Evidence for new binding sites in plasmin-degraded fibrin I.

The effect of tissue plasminogen activator (TPA) or urokinase on the specific binding of human Glu-plasminogen to fibrin I formed in plasma by clotting with Reptilase was studied using 125I-plasminogen and 131I-fibrinogen. In the absence of TPA, small amounts of plasminogen were bound to fibrin I. TPA induced binding of plasminogen to plasma fibrin I that was dependent upon the concentrations of TPA and plasminogen as well as upon the time of incubation. Plasminogen binding occurred in association with fibrin clot lysis and the formation in the clot supernatant of alpha 2-plasmin inhibitor-plasmin complexes. Urokinase also induced binding of plasminogen to plasma fibrin I that was concentration- and time-dependent. The molecular form of plasminogen bound to the fibrin I plasma clot was identified as Glu-plasminogen by dodecyl sulfate-polyacrylamide gel electrophoresis and by fast performance liquid chromatography. Further studies demonstrated that fibrin I formed from fibrinogen that had been progressively degraded by plasmin-bound Glu-plasminogen. The mole ratio of plasminogen bound increased with the time of plasmin digestion. Glu-plasminogen did not bind to fibrin I formed from fibrinogen progressively digested by human leukocyte elastase, thereby demonstrating the specificity of plasmin. These studies demonstrate that plasminogen activators regulate the binding of Glu-plasminogen to fibrin I by catalyzing plasmin-mediated modifications in the fibrin substrate.

Densitometry↗
PubMed1994

Uncoupling fibrin from integrin receptors hastens fibrinolysis at the platelet-fibrin interface.

A well-characterized in vitro model system composed of thrombin-stimulated gel-filtered human platelets, fibrin-(ogen), plasminogen, and recombinant tissue plasminogen activator (rt-PA) was used to examine the relationship between platelet-fibrin adhesive interactions and the lytic resistance of a platelet-rich thrombus. Laser light scattering kinetic experiments demonstrated that the ligand-mimetic peptide D-RGDW and an anti-alpha IIb beta 3 monoclonal antibody both inhibited clot retraction, but neither integrin-targeted reagent affected the overall delay in lysis of "bulk" fibrin caused by thrombin-stimulated platelets. However, lysis of the model platelet-rich thrombus did proceed some 30% more quickly when treated with a plasminogen activator inhibitor (PAI)-resistant t-PA variant. Taken together, these results confirm that platelet-released PAI-1 is a major determinant of global lytic resistance. Next events occurring during fibrinolysis in the unique microenvironment near the platelet surface were monitored by scanning electron microscopy and quantitative fluorescence microscopy. Scanning electron micrographs of the partially lysed model thrombus in the presence of 200 mumol/L of D-RGDW showed no platelet aggregates, and fibrin was attached by fewer strands to the platelets. Quantitative fluorescence microscopy, using fluorescein-labeled fibrin, showed that fibrin adherent to the surface of thrombin-stimulated platelets lysed 20% to 50% more slowly than bulk fibrin (monitored in parallel by laser light scattering). Furthermore, this microspectroscopic technique showed that D-RGDW reduced the quantity of platelet-bound fibrin, and accelerated lysis near the platelet surface with both native rt-PA and the PAI-resistant variant. These observations suggest that the dense network of fibrin bound to the platelet surface is protected from fibrinolysis by tissue-type plasminogen activators. Further, uncoupling fibrin from its platelet receptors uniquely hastens fibrinolysis at the cell/fibrin interface.

Adult↗