Functional evaluation of the structural features of proteases and their substrate in fibrin surface degradation

Fibrinolysis: an illustrated review

In response to vessel injury (or other pathological conditions), the hemostatic process is activated, resulting in a fibrous, cellular-rich structure commonly referred to as a blood clot. Succeeding the clot's function in wound healing, it must be resolved. This illustrated review focuses on fibrinolysis-the degradation of blood clots or thrombi. Fibrin is the main mechanical and structural component of a blood clot, which encases the cellular components of the clot, including platelets and red blood cells. Fibrinolysis is the proteolytic degradation of the fibrin network that results in the release of the cellular components into the bloodstream. In the case of thrombosis, fibrinolysis is required for restoration of blood flow, which is accomplished clinically through exogenously delivered lytic factors in a process called external lysis. Fibrinolysis is regulated by plasminogen activators (tissue-type and urokinase-type) that convert plasminogen into plasmin to initiate fiber lysis and lytic inhibitors that impede this lysis (plasminogen activator inhibitors, alpha 2-antiplasmin, and thrombin activatable fibrinolysis inhibitor). Furthermore, the network structure has been shown to regulate lysis: thinner fibers and coarser clots lyse faster than thicker fibers and finer clots. Clot contraction, a result of platelets pulling on fibers, results in densely packed red blood cells (polyhedrocytes), reduced permeability to fibrinolytic factors, and increased fiber tension. Extensive research in the field has allowed for critical advancements leading to improved thrombolytic agents. In this review, we summarize the state of the field, highlight gaps in knowledge, and propose future research questions.

Fibrin structure, viscoelasticity and lysis face the interplay of biorelevant polyions

PURPOSE OF REVIEW

In the past 5 decades, heparins have been widely used as anticoagulants in the prevention and treatment of thrombosis. Subsequent development of heparin variants of various size and charge facilitated the discovery of their multiple biological actions and nonanticoagulant benefits. Platelet-derived or microbial polyphosphates, as well as DNA released in the course of neutrophil extracellular trap-formation are additional polyanions, which can modulate the development and stability of thrombi associated with cancer or inflammation. In this review, we focus on the size-dependent and electric charge-dependent modulatory effects of the three polyanions of different chemical structure.

RECENT FINDINGS

The polycationic histones have been recognized as potential biomarkers and therapeutic targets in several diseases related to inflammation and thrombosis. Since combating histones with activated protein C or heparin could cause unwanted bleeding, the quest for nonanticoagulant histone-neutralizing agents is ongoing. Polyanions may neutralize or exaggerate certain histone-mediated effects depending on their electric charge, size and histone effects under investigation. Several prothrombotic effects of polyphosphates and DNA are also size-dependent.

SUMMARY

The efficiency of future therapeutics targeting prothrombotic polyanions or histones is not a simple matter of electric charge, but may rely on a delicate combination of size, charge and chemical composition.

Thrombosis-associated hypofibrinogenemia: novel abnormal fibrinogen variant FGG c.8G>A with oxidative posttranslational modifications

Here, we present the first case of fibrinogen variant FGG c.8G>A. We investigated the behaviour of this mutated fibrinogen in blood coagulation using fibrin polymerization, fibrinolysis, fibrinopeptides release measurement, mass spectrometry (MS), and scanning electron microscopy (SEM). The case was identified by routine coagulation testing of a 34-year-old man diagnosed with thrombosis. Initial genetic analysis revealed a heterozygous mutation in exon 1 of the FGG gene encoding gamma chain signal peptide. Fibrin polymerization by thrombin and reptilase showed the normal formation of the fibrin clot. However, maximal absorbance within polymerization was lower and fibrinolysis had a longer degradation phase than healthy control. SEM revealed a significant difference in clot structure of the patient, and interestingly, MS detected several posttranslational oxidations of fibrinogen. The data suggest that the mutation FGG c.8G>A with the combination of the effect of posttranslational modifications causes a novel case of hypofibrinogenemia associated with thrombosis.

Engineered Molecular Therapeutics Targeting Fibrin and the Coagulation System: a Biophysical Perspective

The coagulation cascade represents a sophisticated and highly choreographed series of molecular events taking place in the blood with important clinical implications. One key player in coagulation is fibrinogen, a highly abundant soluble blood protein that is processed by thrombin proteases at wound sites, triggering self-assembly of an insoluble protein hydrogel known as a fibrin clot. By forming the key protein component of blood clots, fibrin acts as a structural biomaterial with biophysical properties well suited to its role inhibiting fluid flow and maintaining hemostasis. Based on its clinical importance, fibrin is being investigated as a potentially valuable molecular target in the development of coagulation therapies. In this topical review, we summarize our current understanding of the coagulation cascade from a molecular, structural and biophysical perspective. We highlight single-molecule studies on proteins involved in blood coagulation and report on the current state of the art in directed evolution and molecular engineering of fibrin-targeted proteins and polymers for modulating coagulation. This biophysical overview will help acclimatize newcomers to the field and catalyze interdisciplinary work in biomolecular engineering toward the development of new therapies targeting fibrin and the coagulation system.

The Utility and Potential of Mathematical Models in Predicting Fibrinolytic Outcomes

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

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

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

Inherent fibrin fiber tension propels mechanisms of network clearance during fibrinolysis

Proper wound healing necessitates both coagulation (the formation of a blood clot) and fibrinolysis (the dissolution of a blood clot). A thrombus resistant to clot dissolution can obstruct blood flow, leading to vascular pathologies. This study seeks to understand the mechanisms by which individual fibrin fibers, the main structural component of blood clots, are cleared from a local volume during fibrinolysis. We observed 2-D fibrin networks during lysis by plasmin, recording the clearance of each individual fiber. We found that, in addition to transverse cleavage of fibers, there were multiple other pathways by which clot dissolution occurred, including fiber bundling, buckling, and collapsing. These processes are all influenced by the concentration of plasmin utilized in lysis. The network fiber density influenced the kinetics and distribution of these pathways. Individual cleavage events often resulted in large morphological changes in network structure, suggesting that the inherent tension in fibers played a role in fiber clearance. Using images before and after a cleavage event to measure fiber lengths, we estimated that fibers are strained ~23% beyond their equilibrium length during polymerization. To understand the role of fiber tension in fibrinolysis we modeled network clearance under differing amounts of fiber polymerized strain (prestrain). The comparison of experimental and model data indicated that fibrin tension enables 35% more network clearance due to network rearrangements after individual cleavage events than would occur if fibers polymerized in a non-tensed state. Our results highlight many characteristics and mechanisms of fibrin breakdown, which have implications on future fibrin studies, our understanding of the fibrinolytic process, and the development of thrombolytic therapies. STATEMENT OF SIGNIFICANCE: Fibrin fibers serve as the main structural element of blood clots. They polymerize under tension and have remarkable extensibility and elasticity. After the cessation of wound healing, fibrin must be cleared from the vasculature by the enzyme plasmin in order to resume normal blood flow: a process called fibrinolysis. In this study we investigate the mechanisms that regulate the clearance of individual fibrin fibers during fibrinolysis. We show that the inherent tension in fibers enhances the action of plasmin because every fiber cleavage event results in a redistribution of the network tension. This network re-equilibration causes fibers to buckle, bundle, and even collapse, leading to a more rapid fiber clearance than plasmin alone could provide.

Direct interaction of fibrinogen with lipid microparticles modulates clotting kinetics and clot structure

Extensive studies revealed the role of blood lipid microparticles (liposomes, microvesicles) in activation of coagulation cascade. The direct interaction of fibrinogen/fibrin with lipid surfaces and its consequence for hemostasis received much less attention. We observed pronounced changes in both clot morphology and kinetics of fibrin clotting in the presence of artificial liposomes. The evidence was obtained that lipid microparticles per se present a diffusion barrier to the three-dimensional fibril assembling and pose spatial restrictions for fiber elongation. On the other hand, fibrinogen adsorption results in its high local concentration on liposome surface that accelerates fibrin polymerization. Adsorption induces Fg secondary structure alterations which may contribute to the abnormal clot morphology. In dependence on lipid composition and size of microparticles, the interplay of all the outlined mechanisms determines functionally important changes of clot morphology. The obtained results contribute to the knowledge of clotting mechanisms in the presence of artificial and natural lipid microparticles.

A Mathematical Model of Bivalent Binding Suggests Physical Trapping of Thrombin within Fibrin Fibers

Thrombin is an enzyme that plays many important roles in the blood clotting process; it activates platelets, cleaves coagulation proteins within feedback loops, and cleaves fibrinogen into fibrin, which polymerizes into fibers to form a stabilizing gel matrix in and around growing clots. Thrombin also binds to the formed fibrin matrix, but this interaction is not well understood. Thrombin-fibrin binding is often described as two independent, single-step binding events, one high-affinity and one low-affinity. However, kinetic schemes describing these single-step binding events do not explain experimentally-observed residency times of fibrin-bound thrombin. In this work, we study a bivalent, sequential-step binding scheme as an alternative to the high-affinity event and, in addition to the low-affinity one. We developed mathematical models for the single- and sequential-step schemes consisting of reaction-diffusion equations to compare to each other and to experimental data. We then used Bayesian inference, in the form of Markov chain Monte Carlo, to learn model parameter distributions from previously published experimental data. For the model to best fit the data, we made an additional assumption that thrombin was irreversibly sequestered; we hypothesized that this could be due to thrombin becoming physically trapped within fibrin fibers as they formed. We further estimated that ∼30% of thrombin in the experiments to which we compare our model output became physically trapped. The notion of physically trapped thrombin may provide new insights into conflicting observations regarding the speed of fibrinolysis. Finally, we show that our new model can be used to further probe scenarios dealing with thrombin allostery.

Lytic Susceptibility, Structure, and Mechanical Properties of Fibrin in Systemic Lupus Erythematosus

Among complications of systemic lupus erythematosus (SLE), thrombotic events are relatively common and contribute significantly to the morbidity and mortality rates. An increased risk of thrombosis in various diseases has been shown to be associated with the lytic stability and mechanical stiffness of the fibrin clot determined by its structure. Here we studied alterations of the fibrin clot properties in relation to disease severity in SLE patients. Plasma clots from 28 SLE patients were characterized by the kinetics of formation and fibrinolytic dissolution (using dynamic turbidimetry), the network and fiber ultrastructure (scanning electron microscopy), viscoelasticity (shear rheometry), and the rate and degree of crosslinking (Western blotting) correlated with the disease activity, blood composition, and compared to clotting of pooled normal human plasma. Clots made from plasma of SLE patients were lysed faster with exogenous t-PA than control clots from normal plasma without a significant difference between those from active (SLEDAI>4) and inactive (SLEDAI<4) SLE patients. Clots from the blood of patients with active SLE were characterized by significantly slower onset, but faster rate of fibrin polymerization and a higher optical density due to thicker fibers compared to those from inactive SLE and control pooled normal plasma. The rheological parameters of the clots (storage and loss moduli) were significantly increased in the active SLE patients along with enhanced fibrin crosslinking and hyperfibrinogenemia. The structural and rheological alterations displayed a strong positive correlation with high fibrinogen levels and other laboratory markers of immune inflammation. In conclusion, changes in the blood composition associated with active systemic inflammation in SLE cause significant alterations in the lytic resistance of fibrin clots associated with changes in polymerization kinetics, viscoelastic properties, and structure. The formation of more rigid prothrombotic fibrin clots in the plasma of SLE patients is likely due to the inflammatory hyperfibrinogenemia and greater extent of crosslinking. However, the higher susceptibility of the SLE clots to fibrinolysis may be a protective and/or compensatory mechanism that reduces the risk of thrombotic complications and improves patient outcomes.

Molecular and Physical Mechanisms of Fibrinolysis and Thrombolysis from Mathematical Modeling and Experiments

Despite the common use of thrombolytic drugs, especially in stroke treatment, there are many conflicting studies on factors affecting fibrinolysis. Because of the complexity of the fibrinolytic system, mathematical models closely tied with experiments can be used to understand relationships within the system. When tPA is introduced at the clot or thrombus edge, lysis proceeds as a front. We developed a multiscale model of fibrinolysis that includes the main chemical reactions: the microscale model represents a single fiber cross-section; the macroscale model represents a three-dimensional fibrin clot. The model successfully simulates the spatial and temporal locations of all components and elucidates how lysis rates are determined by the interplay between the number of tPA molecules in the system and clot structure. We used the model to identify kinetic conditions necessary for fibrinolysis to proceed as a front. We found that plasmin regulates the local concentration of tPA through forced unbinding via degradation of fibrin and tPA release. The mechanism of action of tPA is affected by the number of molecules present with respect to fibrin fibers. The physical mechanism of plasmin action (crawling) and avoidance of inhibition is defined. Many of these new findings have significant implications for thrombolytic treatment.

Biophysical Mechanisms Mediating Fibrin Fiber Lysis

The formation and dissolution of blood clots is both a biochemical and a biomechanical process. While much of the chemistry has been worked out for both processes, the influence of biophysical properties is less well understood. This review considers the impact of several structural and mechanical parameters on lytic rates of fibrin fibers. The influences of fiber and network architecture, fiber strain, FXIIIa cross-linking, and particle transport phenomena will be assessed. The importance of the mechanical aspects of fibrinolysis is emphasized, and future research avenues are discussed.

Diffusion-Reaction Models of Genipin Incorporation into Fibrin Networks

Genipin is a naturally derived small molecule that crosslinks compounds containing primary amines including many natural biopolymers. A diffusion-reaction model to predict the rates of delivery and incorporation of genipin into fibrin networks is presented. Genipin crosslink formation within fibrin hydrogels is a multi-step process that requires genipin diffusion and reaction with primary amines in hydrated networks. The reaction rate of genipin into fibrin gels was measured via spectroscopy while the rate of marginal crosslink formation was measured by rheology. Covalent coupling between genipin and primary amines in fibrin gels obeys second-order kinetics in genipin concentration with an effective activation energy of -71.9 ± 3.2 kJ-mol^-1. Genipin diffusion-reaction within fibrin gels exhibits Thiele moduli between 0.02-0.28, which suggests that the systems studied herein are reaction-limited. Genipin-crosslinked fibrin clots are resistant to fibrinolytic degradation as measured by rheology. Finally, active genipin can be delivered from poly(D,L-lactide-co-glycolide) matrices to gels at rates that are comparable to the characteristic rate of incorporation in fibrin networks. Taken together, this work establishes a quantitative framework to engineer controlled release systems for genipin delivery into protein-based hydrogel networks.

DNA, histones and neutrophil extracellular traps exert anti-fibrinolytic effects in a plasma environment

In response to various inflammatory stimuli, neutrophils secrete neutrophil extracellular traps (NETs), web-like meshworks of DNA, histones and granular components forming supplementary scaffolds in venous and arterial thrombi. Isolated DNA and histones are known to promote thrombus formation and render fibrin clots more resistant to mechanical forces and tissue-type plasminogen activator (tPA)-induced enzymatic digestion. The present study extends our earlier observations to a physiologically more relevant environment including plasma clots and NET-forming neutrophils. A range of techniques was employed including imaging (scanning electron microscopy (SEM), confocal laser microscopy, and photoscanning of macroscopic lysis fronts), clot permeability measurements, turbidimetric lysis and enzyme inactivation assays. Addition of DNA and histones increased the median fibre diameter of plasma clots formed with 16 nM thrombin from 108 to 121 and 119 nm, respectively, and decreased their permeability constant from 6.4 to 3.1 and 3.7×10(-9) cm(2). Histones effectively protected thrombin from antithrombin-induced inactivation, while DNA inhibited plasminogen activation on the surface of plasma clots and their plasmin-induced resolution by 20 and 40 %, respectively. DNA and histones, as well as NETs secreted by phorbol-myristate-acetate-activated neutrophils, slowed down the tPA-driven lysis of plasma clots and the latter effect could be reversed by the addition of DNase (streptodornase). SEM images taken after complete digestion of fibrin in NET-containing plasma clots evidenced retained NET scaffold that was absent in DNase-treated clots. Our results show that DNA and histones alter the fibrin architecture in plasma clots, while NETs contribute to a decreased lytic susceptibility that can be overcome by DNase.

Ambivalent roles of carboxypeptidase B in the lytic susceptibility of fibrin

BACKGROUND

Removal of C-terminal lysine residues that are continuously exposed in lysing fibrin is an established anti-fibrinolytic mechanism dependent on the plasma carboxypeptidase TAFIa, which also removes arginines that are exposed at the time of fibrinogen clotting by thrombin.

OBJECTIVE

To evaluate the impact of alterations in fibrin structure mediated by constitutive carboxypeptidase activity on the function of fibrin as a template for tissue plasminogen activator-(tPA) induced plasminogen activation and its susceptibility to digestion by plasmin.

METHODS AND RESULTS

We used the stable carboxypeptidase B (CPB), which shows the same substrate specificity as TAFIa. If 1.5 - 6μM fibrinogen was clotted in the presence of 8U/mL CPB, a denser fibrin network was formed with thinner fibers (the median fiber diameter decreased from 138 - 144nm to 89 - 109nm as established with scanning electron microscopy). If clotting was initiated in the presence of 5 - 10μM arginine, a similar decrease in fiber diameter (82 -95nm) was measured. The fine structure of arginine-treated fibrin enhanced plasminogen activation by tPA, but slowed down lysis monitored using fluorescent tPA and confocal laser microscopy. However, if lysis was initiated with plasmin in CPB-treated fibrin, the rate of dissolution increased to a degree corresponding to doubling of the plasmin concentration.

CONCLUSION

The present data evidence that CPB activity generates fine-mesh fibrin which is more difficult to lyse by tPA, but conversely, CPB and plasmin together can stimulate fibrinolysis, possibly by enhancing plasmin diffusion.

A safe and efficient method to retrieve mesenchymal stem cells from three-dimensional fibrin gels

Mesenchymal stem cells (MSCs) display multipotent characteristics that make them ideal for potential therapeutic applications. MSCs are typically cultured as monolayers on tissue culture plastic, but there is increasing evidence suggesting that they may lose their multipotency over time in vitro and eventually cease to retain any resemblance to in vivo resident MSCs. Three-dimensional (3D) culture systems that more closely recapitulate the physiological environment of MSCs and other cell types are increasingly explored for their capacity to support and maintain the cell phenotypes. In much of our own work, we have utilized fibrin, a natural protein-based material that serves as the provisional extracellular matrix during wound healing. Fibrin has proven to be useful in numerous tissue engineering applications and has been used clinically as a hemostatic material. Its rapid self-assembly driven by thrombin-mediated alteration of fibrinogen makes fibrin an attractive 3D substrate, in which cells can adhere, spread, proliferate, and undergo complex morphogenetic programs. However, there is a significant need for simple cost-effective methods to safely retrieve cells encapsulated within fibrin hydrogels to perform additional analyses or use the cells for therapy. Here, we present a safe and efficient protocol for the isolation of MSCs from 3D fibrin gels. The key ingredient of our successful extraction method is nattokinase, a serine protease of the subtilisin family that has a strong fibrinolytic activity. Our data show that MSCs recovered from 3D fibrin gels using nattokinase are not only viable but also retain their proliferative and multilineage potentials. Demonstrated for MSCs, this method can be readily adapted to retrieve any other cell type from 3D fibrin gel constructs for various applications, including expansion, bioassays, and in vivo implantation.

Aβ delays fibrin clot lysis by altering fibrin structure and attenuating plasminogen binding to fibrin

Alzheimer disease is characterized by the presence of increased levels of the β-amyloid peptide (Aβ) in the brain parenchyma and cerebral blood vessels. This accumulated Aβ can bind to fibrin(ogen) and render fibrin clots more resistant to degradation. Here, we demonstrate that Aβ(42) specifically binds to fibrin and induces a tighter fibrin network characterized by thinner fibers and increased resistance to lysis. However, Aβ(42)-induced structural changes cannot be the sole mechanism of delayed lysis because Aβ overlaid on normal preformed clots also binds to fibrin and delays lysis without altering clot structure. In this regard, we show that Aβ interferes with the binding of plasminogen to fibrin, which could impair plasmin generation and fibrin degradation. Indeed, plasmin generation by tissue plasminogen activator (tPA), but not streptokinase, is slowed in fibrin clots containing Aβ(42), and clot lysis by plasmin, but not trypsin, is delayed. Notably, plasmin and tPA activities, as well as tPA-dependent generation of plasmin in solution, are not decreased in the presence of Aβ(42). Our results indicate the existence of 2 mechanisms of Aβ(42) involvement in delayed fibrinolysis: (1) through the induction of a tighter fibrin network composed of thinner fibers, and (2) through inhibition of plasmin(ogen)-fibrin binding.

Factor XIII, clot structure, thrombosis

Blood coagulation factor XIII (FXIII) is a tetrameric protein consisting of two catalytic A (FXIII-A) and two carrier/inhibitory B (FXIII-B) subunits. It is a zymogen, which becomes transformed into an active transglutaminase (FXIIIa) in the final phase of coagulation cascade by thrombin and Ca(2+). FXIII is essential for hemostasis, its deficiency results in severe bleeding diathesis. FXIIIa mechanically stabilizes fibrin by cross-linking its α-, and γ-chains. It also protects newly formed fibrin from fibrinolysis, primarily by cross-linking α(2)-plasmin inhibitor to fibrin. Beside the above prothrombotic effects, it is involved in limiting thrombus growth by down-regulating platelet adhesion to fibrin. Elevated FXIII level seems to be a gender-specific risk factor of both coronary artery disease and peripheral arterial disease, it represents an increased risk only in females. The association of FXIII level with the risk of ischemic stroke and venous thromboembolism was investigated only in a few studies from which no clear conclusion could be drawn. Among the FXIII subunit polymorphisms, concerning their effect on the risk of thrombotic diseases, only FXIII-A p.Val34Leu was investigated extensively. Meta-analyses of reported data suggest that this polymorphism provides a moderate protection against coronary artery disease and venous thromboembolism, but not against ischemic stroke. Gene-gene and gene-environmental interactions might modify its effect. Further studies are required to explore the effect of other FXIII subunit polymorphism on the risk of thrombotic diseases.

Hindered dissolution of fibrin formed under mechanical stress

BACKGROUND

Recent data indicate that stretching forces cause a dramatic decrease in clot volume accompanied by gross conformational changes of fibrin structure.

OBJECTIVE

The present study attempts to characterize the lytic susceptibility of fibrin exposed to mechanical stress as a model for fibrin structures observed in vivo.

METHODS AND RESULTS

The relevance of stretched fibrin models was substantiated by scanning electron microscopic (SEM) evaluation of human thrombi removed during surgery, where surface fibrin fibers were observed to be oriented in the direction of shear forces, whereas interior fibers formed a random spatial meshwork. These structural variations were modeled in vitro with fibrin exposed to adjustable mechanical stress. After two- and three-fold longitudinal stretching (2 × S, 3 × S) the median fiber diameter and pore area in SEM images of fibrin decreased two- to three-fold. Application of tissue plasminogen activator (tPA) to the surface of model clots, which contained plasminogen, resulted in plasmin generation which was measured in the fluid phase. After 30-min activation 12.6 ± 0.46 pmol mm(-2) plasmin was released from the non-stretched clot (NS), 5.5 ± 1.11 pmol mm(-2) from 2 × S and 2.3 ± 0.36 pmol mm(-2) from 3 × S clot and this hampered plasmin generation was accompanied by decreased release of fibrin degradation products from stretched fibrins. Confocal microscopic images showed that a green fluorescent protein-fusion variant of tPA accumulated in the superficial layer of NS, but not in stretched fibrin.

CONCLUSION

Mechanical stress confers proteolytic resistance to fibrin, which is a result of impaired plasminogen activation coupled to lower plasmin sensitivity of the denser fibrin network.

The interplay between tissue plasminogen activator domains and fibrin structures in the regulation of fibrinolysis: kinetic and microscopic studies

Regulation of tissue-type plasminogen activator (tPA) depends on fibrin binding and fibrin structure. tPA structure/function relationships were investigated in fibrin formed by high or low thrombin concentrations to produce a fine mesh and small pores, or thick fibers and coarse structure, respectively. Kinetics studies were performed to investigate plasminogen activation and fibrinolysis in the 2 types of fibrin, using wild-type tPA (F-G-K1-K2-P, F and K2 binding), K1K1-tPA (F-G-K1-K1-P, F binding), and delF-tPA (G-K1-K2-P, K2 binding). There was a trend of enzyme potency of tPA > K1K1-tPA > delF-tPA, highlighting the importance of the finger domain in regulating activity, but the differences were less apparent in fine fibrin. Fine fibrin was a better surface for plasminogen activation but more resistant to lysis. Scanning electron and confocal microscopy using orange fluorescent fibrin with green fluorescent protein-labeled tPA variants showed that tPA was strongly associated with agglomerates in coarse but not in fine fibrin. In later lytic stages, delF-tPA-green fluorescent protein diffused more rapidly through fibrin in contrast to full-length tPA, highlighting the importance of finger domain-agglomerate interactions. Thus, the regulation of fibrinolysis depends on the starting nature of fibrin fibers and complex dynamic interaction between tPA and fibrin structures that vary over time.

Proteolytic resistance conferred to fibrinogen by von Willebrand factor

The formation of platelet-rich thrombi under high shear rates requires both fibrinogen and von Willebrand factor (VWF) as molecular adhesives between platelets. We attempted to describe the role of VWF as a potential substrate and modulator of the fibrinolytic system using binding assays, as well as kinetic measurements on the cleavage of fibrin(ogen) and a synthetic plasmin substrate (Spectrozyme-PL). The similar dissociation constants for the binding of plasminogen, plasmin, and active site-blocked plasmin onto immobilised VWF suggest that the primary binding site in plasmin(ogen) is not the active site. The progressive loss of clottability and generation of degradation products during fibrinogen digestion with plasmin were delayed in the presence of VWF at physiological concentrations, while VWF cleavage was not detectable. Determination of kinetic parameters for fibrinogen degradation by plasmin, miniplasmin and microplasmin showed that VWF did not modify the Km, whereas kcat values decreased with increasing VWF concentrations following the kinetic model of non-competitive inhibition. Inhibitory constants calculated for VWF were in the range of its physiological plasma concentration (5.4 mg/ml, 5.7 mg/ml and 10.0 mg/ml for plasmin, miniplasmin and microplasmin, respectively) and their values suggested a modulating role of the kringle 5 domain in the interaction between VWF and (mini)plasmin. VWF had no effect on the amidolytic activity of plasmin on Spectrozyme-PL, or on fibrin dissolution by (mini)plasmin. Our data suggest that VWF, while a poor plasmin substrate relative to fibrinogen, protects fibrinogen against degradation by plasmin preserving its clottability in plasma and its adhesive role in platelet-rich thrombi.

Understanding the enzymology of fibrinolysis and improving thrombolytic therapy

Cardiovascular disease is responsible for 17 million deaths per year but acute myocardial infarction and stroke can be treated with thrombolytics ("clot busters"), which are plasminogen activators. However, despite many years of study and huge investment from the pharmaceutical industry, clinical trials of new drugs have often been disappointing. Part of the problem may be our incomplete understanding of the regulation of plasminogen activation in vivo. We have developed precise in vitro methods and with the application of computer simulations, we hope to improve our understanding of plasminogen activation to facilitate improvements in thrombolytic therapy.

Impaired fibrinolytic potential related to elevated ?1-proteinase inhibitor levels in patients with pulmonary thromboembolism

The contribution of neutrophil leukocyte elastase (NE) to in vivo thrombolysis is still an open question. The present study examines the impact of variable levels of alpha1-proteinase inhibitor (alpha1-PI) (the major plasma inhibitor of NE) on fibrinolysis within the setting of thromboembolic diseases. Blood samples were taken from 56 patients with pulmonary thromboembolism prior to treatment. alpha1-PI and alpha1-PI-NE complex were measured in the serum and plasma with immunoturbidimetric and enzyme-linked immunosorbent assay (ELISA) methods, respectively. The fibrinolytic potential [spontaneous, tissue-type plasminogen activator (tPA) induced, and plasmin induced] of the plasma was evaluated in vitro with turbidimetric clot lysis assay. Correlation analysis (Pearson product-moment correlation coefficient, r) of the turbidimetric lysis parameters and the blood levels of alpha1-PI and alpha1-PI-NE complex was carried out. Fibrinolysis is slower in clots prepared from plasma containing elevated levels of alpha1-PI and alpha1-PI-NE complex. The maximal turbidity of the plasma clots shows significant correlation with the alpha1-PI level (r=0.39, p=0.003) and the correlation of the maximal turbidity and the tPA-induced lysis time is also significant (r=0.77, p<0.001). The lysis time correlates with the plasma level of alpha1-PI-NE complex, if fibrinolysis is induced with tPA (r=0.37, p=0.02), but not with plasmin (r=0.19, p=0.4). Our study shows that in pulmonary thromboembolism elevated levels of alpha1-PI are associated with suppressed plasma fibrinolytic potential. This effect can be at least partially explained by the coarse fibrin network structure and retarded plasminogen activator-dependent fibrinolysis.

Dynamic changes of fibrin architecture during fibrin formation and intrinsic fibrinolysis of fibrin-rich clots

Clotting and fibrinolysis are initiated simultaneously in vivo, and fibrinolysis usually occurs without any individualized lysis front (intrinsic fibrinolysis). We have developed a novel model to assess whether morphological changes resulting from intrinsic fibrinolysis are similar to those previously reported at the lysis front using externally applied lytic agents. Fibrin assembly and fibrinolysis were followed in real-time by confocal microscopy using gold-labeled fibrinogen molecules. An increase in fiber absorbance (30%, p < 0.01) and a decrease in fiber diameter (60%, p < 0.01) due to the ongoing accumulation and packing of fibrin molecules were the most significant detectable features occurring during fibrin assembly. Similar features with a similar magnitude were observed during fibrin dissolution, but in the reverse order and with a 3-fold increase in duration. Then, lysing fibers were progressively transected laterally, and thinner fibers were cleaved at a 2.5-fold faster rate than thicker fibers (p < 0.001). Frayed lysing fibers were seen to interact progressively with adjoining fibers (agglomeration), leading to a 76 and 88% increase in the network pore diameter (p < 0.05) and fiber diameter (p < 0.01), respectively. At the maximum decrease in fiber absorbance (46%, p < 0.05), the network suddenly collapsed with the release of large fragments that gradually vanished. Morphological changes of fibrin that occur during intrinsic fibrinolysis are similar as those observed next to the lysis front, although they are not restricted spatially to the clot/surrounding milieu interface but are observed through the entire clot.

Myosin: a noncovalent stabilizer of fibrin in the process of clot dissolution

Myosin modulates the fibrinolytic process as a cofactor of the tissue plasminogen activator and as a substrate of plasmin. We report now that myosin is present in arterial thrombi and it forms reversible noncovalent complexes with fibrinogen and fibrin with equilibrium dissociation constants in the micromolar range (1.70 and 0.94 microM, respectively). Competition studies using a peptide inhibitor of fibrin polymerization (glycl-prolyl-arginyl-proline [GPRP]) indicate that myosin interacts with domains common in fibrinogen and fibrin and this interaction is independent of the GPRP-binding polymerization site in the fibrinogen molecule. An association rate constant of 1.81 x 10(2) M(-1) x s(-1) and a dissociation rate constant of 3.07 x 10(-4) s(-1) are determined for the fibrinogen-myosin interaction. Surface plasmon resonance studies indicate that fibrin serves as a matrix core for myosin aggregation. The fibrin clots equilibrated with myosin are stabilized against dissolution initiated by plasminogen and tissue-type plasminogen activator (tPA) or urokinase (at fibrin monomer-myosin molar ratio as high as 30) and by plasmin under static and flow conditions (at fibrin monomer-myosin molar ratio lower than 15). Myosin exerts similar effects on the tPA-induced dissolution of blood plasma clots. Covalent modification involving factor XIIIa does not contribute to this stabilizing effect; myosin is not covalently attached to the clot by the time of complete cross-linking of fibrin. Thus, our in vitro data suggest that myosin detected in arterial thrombi binds to the polymerized fibrin, in the bound form its tPA-cofactor properties are masked, and the myosin fibrin clot is relatively resistant to plasmin.

Influence of soluble fibrin on reaction kinetics of plasmin type 1 and type 2 with alpha2-antiplasmin

This study investigates, by slow binding kinetics methods, reaction kinetics of both plasmin types 1 and 2 with alpha -antiplasmin in the presence of increasing concentrations of either epsilon-amino-caproic acid (EACA) or soluble fibrin. All curves of plasmin-alpha -antiplasmin interaction followed the same pattern, indicating reversible slow binding inhibition with an initial loose complex and a following tight complex. Without soluble fibrin or EACA, differences between plasmin types 1 and 2 could be seen in the initial loose complex formation. The presence of increasing concentrations of EACA slowed down the first step of the reaction (without any effect on the second step), resulting in increasing values for K. Plasmin type 1 demonstrated a steep slope of K at an EACA concentration of 1 mmol/l. In plasmin type 2, the increase of K started at higher EACA concentrations. The value for K at a high EACA concentration (100 mmol/l) was the same for both plasmin types. In contrast to EACA, increasing concentrations of soluble fibrin slowed down both reaction steps. At high concentrations of soluble fibrin, the inhibitory effect of alpha -antiplasmin was almost completely abolished. Our data demonstrate that the effect of soluble fibrin and the lysine analogue EACA on plasmin-antiplasmin reactions are different and that the use of lysine analogues does not mimic fibrin in laboratory analyses of plasmin inhibition. In addition, our data indicate theoretical differences between plasmin type 1 and plasmin type 2, when used for local thrombolytic therapy.(2) (2) (i initial) (i initial) (i initial) (i initial) (2)

Engineering design of optimal strategies for blood clot dissolution

Blood clots form under hemodynamic conditions and can obstruct flow during angina, acute myocardial infarction, stroke, deep vein thrombosis, pulmonary embolism, peripheral thrombosis, or dialysis access graft thrombosis. Therapies to remove these clots through enzymatic and/or mechanical approaches require consideration of the biochemistry and structure of blood clots in conjunction with local transport phenomena. Because blood clots are porous objects exposed to local hemodynamic forces, pressure-driven interstitial permeation often controls drug penetration and the overall lysis rate of an occlusive thrombus. Reaction engineering and transport phenomena provide a framework to relate dosage of a given agent to potential outcomes. The design and testing of thrombolytic agents and the design of therapies must account for (a) the binding, catalytic, and systemic clearance properties of the therapeutic enzyme; (b) the dose and delivery regimen; (c) the biochemical and structural aspects of the thrombotic occlusion; (d) the prevailing hemodynamics and anatomical location of the thrombus; and (e) therapeutic constraints and risks of side effects. These principles also impact the design and analysis of local delivery devices.

Gel-sol transition can describe the proteolysis of extracellular matrix gels

We monitored the cell-free solubilization of extracellular matrix and fibronectin gels, catalyzed by exogenous proteinases. The corresponding measurements could not be interpreted according to usual proteinase kinetics. The observation that this experimental system did not consist in surface but in bulk degradation and appeared specific to gel substrates, incited us to use gelation-related approaches to describe these kinetics. We show that the gel-sol transition theory adequately describes the enzyme reactions. This allowed formulation and experimental confirmation of a power law relating macroscopic changes of the gel to enzyme kinetics. This approach could also be used for other power laws predicted by the gel-sol transition theory, allowing a better understanding of macroscopic modification of the extracellular matrix during proteolysis, which is implied in many biological situations, especially tumor dissemination.

Fibrinogen bellingham: a gamma-chain R275C substitution and a beta-promoter polymorphism in a thrombotic member of an asymptomatic family

Congenital dysfibrinogenemia is a rare cause of unexplained thrombosis. However, most individuals with dysfibrinogenemia are asymptomatic, suggesting that co-morbid factors contribute to thrombo-embolic events. The potential roles of additional genetic or acquired prothrombotic risk factors are poorly understood because detailed family studies are lacking. Herein, we describe a family whose propositus was a young Caucasian man with recurrent venous thrombo-emboli and dysfibrinogenemia due to heterozygosity for an Arg-->Cys substitution at residue 275 in the gamma-chain. The only additional thrombophilic abnormality found in the proband was heterozygosity for a G/A transition at position -455 in the fibrinogen beta-chain promoter; a genotype associated with high acute phase levels of fibrinogen. The proband's father, who died of a cerebral artery thrombosis, carried the gammaR275C substitution but not the beta-promoter -455 variant. Among 14 living relatives, eight were heterozygous for one or the other mutation and only one, a 21-year-old niece, was dually affected. None had suffered bleeding or thrombosis. In vitro studies of the proband's purified fibrinogen revealed markedly abnormal thrombin-catalyzed polymerization and delayed fibrin clot lysis by tPA-activated plasmin. We hypothesize that the gammaR275C substitution predisposes to thrombosis by generating clots that are relatively resistant to fibrinolysis. The clinical risk is low, however, in the absence of an additional thrombophilic mutation. The beta-promoter variant could, theoretically, contribute to this risk by augmenting expression of the dysfibrinogen under conditions of stress. Like the common hereditary thrombophilias, heterozygous familial dysfibrinogenemia induces thrombosis in the setting of multiple prothrombotic influences.

Fibrinolysis: an unfinished agenda

There has been a recent decline in interest in fibrinolysis, suggesting that its physiological basis is sufficiently understood and that therapeutic thrombolysis has reached its limit. The importance of the subject has not diminished since cardiovascular disease is now a leading health problem even in developing countries. Certain highlights and inconsistencies are reviewed. The clinical trials of tissue plasminogen activator (t-PA) revealed a major discrepancy between its fibrinolytic efficacy and its clinical benefit (the 't-PA paradox') that is unexplained. Dose-finding studies also showed that the fibrinolytic efficacy of t-PA required significant nonspecific plasminogen activation. Furthermore, the longstanding belief that t-PA is responsible for physiological fibrinolysis and urokinase-type PA (u-PA) for pericellular plasminogen activation is belied by extensive experimental animal data, but these findings have had little impact on traditional thinking. As a result, the mechanisms responsible for the u-PA paradigm of fibrinolysis have received little attention. Clinical experience with pro-u-PA remains limited and most clinical trials have used infusion rates at which pro-u-PA is largely converted systemically to urokinase. This is due to the unanticipated instability of pro-u-PA in plasma at pharmacological concentrations. Insufficient understanding of basic mechanisms of fibrinolysis has handicapped the design of chimeric or mutant activators. It is submitted that physiological fibrinolysis remains to be better defined, and that it is premature to conclude that therapeutic thrombolysis will be inevitably accompanied by side effects that undermine this method of inducing reperfusion.

Influence of fibrin network conformation and fibrin fiber diameter on fibrinolysis speed: dynamic and structural approaches by confocal microscopy

Abnormal fibrin architecture is thought to be a determinant factor of hypofibrinolysis. However, because of the lack of structural knowledge of the process of fibrin digestion, relationships between fibrin architecture and hypofibrinolysis remain controversial. To elucidate further structural and dynamic changes occurring during fibrinolysis, cross-linked plasma fibrin was labeled with colloidal gold particles, and fibrinolysis was followed by confocal microscopy. Morphological changes were characterized at fibrin network and fiber levels. The observation of a progressive disaggregation of the fibrin fibers emphasizes that fibrinolysis proceeds by transverse cutting rather than by progressive cleavage uniformly around the fiber. Plasma fibrin clots with a tight fibrin conformation made of thin fibers were dissolved at a slower rate than those with a loose fibrin conformation made of thicker (coarse) fibers, although the overall fibrin content remained constant. Unexpectedly, thin fibers were cleaved at a faster rate than thick ones. A dynamic study of FITC-recombinant tissue plasminogen activator distribution within the fibrin matrix during the course of fibrinolysis showed that the binding front was broader in coarse fibrin clots and moved more rapidly than that of fine plasma fibrin clots. These dynamic and structural approaches to fibrin digestion at the network and the fiber levels reveal aspects of the physical process of clot lysis. Furthermore, these results provide a clear explanation for the hypofibrinolysis related to a defective fibrin architecture as described in venous thromboembolism and in premature coronary artery disease.

Thrombolytic properties of leukocytes from peripheral blood in healthy subjects and in patients with acute cerebral ischemia

Polymorphonuclear leukocytes are activated in acute ischemic stroke. Activated polymorphonuclear leukocytes may contribute to thrombolysis by proteolytic degradation of fibrin and by modification of the plasminogen system. We used an in vitro thrombolysis model to investigate (1) thrombolytic properties of leukocytes in young and healthy subjects, (2) to test the hypothesis of increased polymorphonuclear leukocyte-associated thrombolysis in patients with acute cerebral ischemia, and (3) to assess plasminogen-dependent and -independent thrombolytic properties of polymorphonuclear leukocyte elastase. Coincubation of polymorphonuclear leukocytes with fibrin clots led to increased thrombolysis, a process reaching statistical significance after 8 hours [1x10(7) polymorphonuclear leukocytes/mL; 12.8+/-1.9% (mean+/-SEM), spontaneous clot lysis: 7.3+/-0.7%]. Polymorphonuclear leukocytes inside clots caused more efficient thrombolysis than polymorphonuclear leukocytes in the incubation medium. Spontaneous and polymorphonuclear leukocyte-associated lysis tended to be lower in patients with acute cerebral ischemia (n=9, 24 hours, 9.5+/-1.8% and 12.9+/-2.2%) than in age- and sex-matched control subjects (n=8; 12.2+/-2.0% and 17.4+/-1.9%). In the presence of alpha(2)-antiplasmin, thrombolysis tended to be faster with elastase-digested plasminogen (miniplasminogen) than with native plasminogen. Purified polymorphonuclear leukocyte elastase itself had no thrombolytic effect. We conclude that the thrombolytic capacity of polymorphonuclear leukocytes from peripheral blood is small and slow and may have been overestimated in previous reports. Polymorphonuclear leukocyte thrombolytic activity may not be increased in acute cerebral ischemia. Miniplasminogen may be an interesting adjunct to plasminogen activators in acute stroke models.

Structural origins of fibrin clot rheology

The origins of clot rheological behavior associated with network morphology and factor XIIIa-induced cross-linking were studied in fibrin clots. Network morphology was manipulated by varying the concentrations of fibrinogen, thrombin, and calcium ion, and cross-linking was controlled by a synthetic, active-center inhibitor of FXIIIa. Quantitative measurements of network features (fiber lengths, fiber diameters, and fiber and branching densities) were made by analyzing computerized three-dimensional models constructed from stereo pairs of scanning electron micrographs. Large fiber diameters and lengths were established only when branching was minimal, and increases in fiber length were generally associated with increases in fiber diameter. Junctions at which three fibers joined were the dominant branchpoint type. Viscoelastic properties of the clots were measured with a rheometer and were correlated with structural features of the networks. At constant fibrinogen but varying thrombin and calcium concentrations, maximal rigidities were established in samples (both cross-linked and noncross-linked) which displayed a balance between large fiber sizes and great branching. Clot rigidity was also enhanced by increasing fiber and branchpoint densities at greater fibrinogen concentrations. Network morphology is only minimally altered by the FXIIIa-catalyzed cross-linking reaction, which seems to augment clot rigidity most likely by the stiffening of existing fibers.

Structural Studies of Fibrinolysis by Electron Microscopy

Fibrin is degraded by the fibrinolytic system in which a plasminogen activator converts plasminogen to plasmin, a serine protease that cleaves specific bonds in fibrin leading to solubilization. To elucidate further the biophysical processes involved in conversion of insoluble fibers to soluble fragments, fibrin was treated with either plasmin or the combination of plasminogen and plasminogen activator, and morphologic changes were observed using scanning electron microscopy. These changes were correlated with biochemical analysis and with characterization of released, soluble fragments by transmission electron microscopy. Initial changes in the fibrin matrix included creation of many free fiber ends and gaps in the continuity of fibers. With more extensive digestion, free fiber segments associated laterally, resulting in formation of thick fiber bundles. Supernatants of digesting clots, containing soluble derivatives, were negatively contrasted and examined by transmission electron microscopy. Large, complex fragments containing portions of multiple fibers were observed, as were pieces of individual fibers and smaller fragments previously identified. Some large fragments had sharply defined ends, indicating that they had been cleaved perpendicularly to the fiber direction. Other fibers showed splayed ends or a lacy meshwork of surrounding protofibrils. Longer times generated more small fragments whose molecular composition could be inferred from their appearance. These results indicate that fibrinolytic degradation results in larger pieces than previously identified and that plasmin digestion proceeds locally by transverse cutting across fibers rather than by progressive cleavage uniformly around the fiber.

Structural Studies of Fibrinolysis by Electron Microscopy

Fibrin is degraded by the fibrinolytic system in which a plasminogen activator converts plasminogen to plasmin, a serine protease that cleaves specific bonds in fibrin leading to solubilization. To elucidate further the biophysical processes involved in conversion of insoluble fibers to soluble fragments, fibrin was treated with either plasmin or the combination of plasminogen and plasminogen activator, and morphologic changes were observed using scanning electron microscopy. These changes were correlated with biochemical analysis and with characterization of released, soluble fragments by transmission electron microscopy. Initial changes in the fibrin matrix included creation of many free fiber ends and gaps in the continuity of fibers. With more extensive digestion, free fiber segments associated laterally, resulting in formation of thick fiber bundles. Supernatants of digesting clots, containing soluble derivatives, were negatively contrasted and examined by transmission electron microscopy. Large, complex fragments containing portions of multiple fibers were observed, as were pieces of individual fibers and smaller fragments previously identified. Some large fragments had sharply defined ends, indicating that they had been cleaved perpendicularly to the fiber direction. Other fibers showed splayed ends or a lacy meshwork of surrounding protofibrils. Longer times generated more small fragments whose molecular composition could be inferred from their appearance. These results indicate that fibrinolytic degradation results in larger pieces than previously identified and that plasmin digestion proceeds locally by transverse cutting across fibers rather than by progressive cleavage uniformly around the fiber.

Fibrinolysis with des-kringle derivatives of plasmin and its modulation by plasma protease inhibitors

Quantitative characterization of the interaction of des-kringle1-5-plasmin (microplasmin) with fibrin(ogen) and plasma protease inhibitors may serve as a tool for further evaluation of the role of kringle domains in the regulation of fibrinolysis. Comparison of fibrin(ogen) degradation products yielded by plasmin, miniplasmin (des-kringle1-4-plasmin), microplasmin, and trypsin on SDS gel electrophoresis indicates that the differences in the enzyme structure result in different rates of product formation, whereas the products of the four proteases are very similar in molecular weight. Kinetic parameters show that plasmin is the most efficient enzyme in fibrinogen degradation, and the kcat/KM ratio decreases in parallel with the loss of the kringle domains. The catalytic sites of the four proteases have similar affinities for fibrin (KM values between 0.12 and 0.21 microM). Trypsin has the highest catalytic constant for fibrin digestion (kcat = 0.47 s-1), and among plasmins with different kringle structures, the loss of kringle5 results in a markedly lower catalytic rate constant (kcat = 0.0076 s-1 for microplasmin vs 0.048 s-1 for miniplasmin and 0.064 s-1 for plasmin). In addition, microplasmin is inactivated by plasmin inhibitor (k" = 3.9 x 10(5) M-1 s-1) and antithrombin (k" = 1.4 x 10(3) M-1 s-1) and the rate of inactivation decreases in the presence of fibrin(ogen). Heparin (250 nM) accelerates the inactivation of microplasmin by antithrombin (k" = 10.5 x 10(3) M-1 s-1 ), whereas that by plasmin inhibitor is not affected (k" = 4.2 x 10(5) M-1 s-1).

Flow rate-modulated dissolution of fibrin with clot-embedded and circulating proteases

The efficiency of plasmin, miniplasmin, and neutrophil leukocyte elastase in fibrin digestion is well characterized in static systems. Since in vivo the components of the fibrinolytic system are permanently exposed to flow, we have developed two in vitro models and studied the effect of shear forces on fibrin dissolution with these proteases. Cylindrical nonocclusive fibrin clots are perfused at various flow rates through their preformed axial channel, and dissolution of fibrin is followed by measuring the absorbance of degradation products released into the circulating fluid phase. In one experimental setting, fibrin surface is degraded with enzymes applied in the recirculating fluid phase; in another setting, clots containing gel-embedded proteases are perfused with enzyme-free buffer. As shear rate at fibrin surface is changed from 25 to 500 s(-1), the rate of product release by recirculated enzymes increases 2.8-, 2.9-, and 4-fold for plasmin, miniplasmin, and porcine pancreatic elastase, respectively. Buffer-perfused fibrin containing gel-embedded plasmin or miniplasmin is disintegrated by shear forces at a relatively early stage of dissolution, and this disassembly is related to the formation of fragment Y (150 kDa) and fragment D (100 kDa) fibrin degradation products. Fibrin clots degraded by incorporated polymorphonuclear leukocyte elastase, which yields different degradation products, do not disassemble abruptly, even at the highest shear rate (500 s(-1)). Our results suggest that fibrin surface degradation is accelerated with increasing shear rate and that plasmin or miniplasmin embedded in the clot promotes the release of particular clot remnants into the circulating phase, whereas polymorphonuclear leukocyte elastase does not.