A proposed reference method for plasminogen activators that enables calculation of enzyme activities in SI units

A novel ex vivo approach for measuring plasminogen activation upon established plasma clots

Background

The fibrinolytic system plays a critical role in maintaining hemostasis. Central to fibrinolysis is the degradation of fibrin by plasmin, produced in the circulation following the activation of plasminogen by plasminogen activators (PAs). Accurately measuring the plasminogen activation rate is vital for the understanding of fibrinolytic processes, particularly in the context of thrombolysis. Yet, due to the insoluble nature of fibrin, in vitro and ex vivo investigations of PA-mediated plasminogen activation have proven challenging. As researchers frequently adopt soluble fibrinogen fragments and/or alter the experimental system beyond what is physiologically relevant, they limit the validation and interpretation of their findings. Here, we present a novel, high-throughput assay for measuring plasminogen activation rates on natural, plasma-derived fibrin that optimally simulates in vivo conditions.

Method

Human plasma was used as the source of plasmin(ogen) and fibrin(ogen). "Halo-shaped" plasma clots were produced in a 96-well plate using a thrombin-containing clotting mixture, ensuring that an optically compatible and plasma-free center is maintained in each well. Subsequent additions of a plasmin chromogenic substrate and different PAs were followed by absorbance measurements over time to extract the corresponding enzyme kinetics information.

Results and Discussion

Validation experiments demonstrated the capability of our approach to accurately model fibrin-dependent and -independent plasminogen activation as well as sensitively detect variations in plasminogen and fibrinogen plasma levels.

Conclusion

This assay allows a straightforward, yet powerful, measurement of plasminogen activation rates on established plasma clots, with the capability of properly assessing fibrin- and non-fibrin-dependent plasminogen activation by various therapeutic PAs.

Measuring Fibrinolysis

Physiological fibrinolysis under normal conditions progresses slowly, in contrast to coagulation which is triggered rapidly to stop bleeding and defend against microbial invasion. Methods to detect fibrinolysis abnormalities are less simple and poorly standardized compared with common coagulation tests. Fibrinolysis can be accelerated by preparing euglobulin from plasma to reduce endogenous inhibitors, or by adding plasminogen activators to normal plasma. However, these manipulations complicate interpretation of results and diagnosis of a "fibrinolysis deficit." Many observational studies on antigen levels of fibrinolysis inhibitors, plasminogen activator inhibitor 1 or thrombin-activatable fibrinolysis inhibitor, zymogen or active enzyme have been published. However, conclusions are mixed and there are clear problems with harmonization of results. Viscoelastic methods have the advantage of being rapid and are used as point-of-care tests. They also work with whole blood, allowing the contribution of platelets to be explored. However, there are no agreed protocols for applying viscoelastic methods in acute care for the diagnosis of hyperfibrinolysis or to direct therapy. The emergence of SARS-CoV-2 and the dangers of associated coagulopathy provide new challenges. A common finding in hospitalized patients is high levels of D-dimer fibrin breakdown products, indicative of ongoing fibrinolysis. Well-established problems with D-dimer testing standardization signal that we should be cautious in using results from such tests as prognostic indicators or to target therapies.

Development and Testing of Thrombolytics in Stroke

Despite recent advances in recanalization therapy, mechanical thrombectomy will never be a treatment for every ischemic stroke because access to mechanical thrombectomy is still limited in many countries. Moreover, many ischemic strokes are caused by occlusion of cerebral arteries that cannot be reached by intra-arterial catheters. Reperfusion using thrombolytic agents will therefore remain an important therapy for hyperacute ischemic stroke. However, thrombolytic drugs have shown limited efficacy and notable hemorrhagic complication rates, leaving room for improvement. A comprehensive understanding of basic and clinical research pipelines as well as the current status of thrombolytic therapy will help facilitate the development of new thrombolytics. Compared with alteplase, an ideal thrombolytic agent is expected to provide faster reperfusion in more patients; prevent re-occlusions; have higher fibrin specificity for selective activation of clot-bound plasminogen to decrease bleeding complications; be retained in the blood for a longer time to minimize dosage and allow administration as a single bolus; be more resistant to inhibitors; and be less antigenic for repetitive usage. Here, we review the currently available thrombolytics, strategies for the development of new clot-dissolving substances, and the assessment of thrombolytic efficacies in vitro and in vivo.

Development of an updated assay for prekallikrein activator in albumin and immunoglobulin therapeutics

Background

Prekallikrein activator (PKA) is a contaminating enzyme found in therapeutic albumin and immunoglobulin products. The level is commonly measured using methods such as that defined by the European Pharmacopoeia (Ph Eur) with traceability to the WHO International Standard for PKA. This method generally works well, but problems are sometimes observed.

Materials and methods

A simplified one‐step method has been developed to replace the existing Ph Eur two‐step method which consists of kallikrein generation followed by kallikrein measurement using a chromogenic substrate. Analysis of data from the one‐stage method is simplified by the use of a dedicated online app.

Results

The one‐stage method was validated against the current Ph Eur method using batches of albumin and immunoglobulins. Problem batches of immunoglobulins were investigated using the one‐stage method. Improved methodology using true initial rate determinations and use of acid‐treated prekallikrein substrate (PKS) helped understand and reduce artefactual results.

Conclusions

The one‐stage method and associated app streamline real‐time determination of PKA and promote good principles of enzyme assays to limit substrate depletion, while also conserving expensive PKS. Blanking steps and reproducibility are simplified.

Pediatric Musculoskeletal Infection: Hijacking the Acute-Phase Response

Tissue injury activates the acute-phase response mediated by the liver, which promotes coagulation, immunity, and tissue regeneration. To survive and disseminate, musculoskeletal pathogens express virulence factors that modulate and hijack this response. As the acute-phase reactants required by these pathogens are most abundant in damaged tissue, these infections are predisposed to occur in tissues following traumatic or surgical injury. Staphylococcus aureus expresses the virulence factors coagulase and von Willebrand binding protein to stimulate coagulation and to form a fibrin abscess that protects it from host immune-cell phagocytosis. After the staphylococcal abscess community reaches quorum, which is the colony density that enables cell-to-cell communication and coordinated gene expression, subsequent expression of staphylokinase stimulates activation of fibrinolysis, which ruptures the abscess wall and results in bacterial dissemination. Unlike Staphylococcus aureus, Streptococcus pyogenes expresses streptokinase and other virulence factors to activate fibrinolysis and to rapidly disseminate throughout the body, causing diseases such as necrotizing fasciitis. Understanding the virulence strategies of musculoskeletal pathogens will help to guide clinical diagnosis and decision-making through monitoring of acute-phase markers such as C-reactive protein, erythrocyte sedimentation rate, and fibrinogen.

Measuring fibrinolysis: from research to routine diagnostic assays

Development and standardization of fibrinolysis methods have progressed more slowly than coagulation testing and routine high-throughput screening tests for fibrinolysis are still lacking. In laboratory research, a variety of approaches are available and are applied to understand the regulation of fibrinolysis and its contribution to the hemostatic balance. Fibrinolysis in normal blood is slow to develop. For practical purposes plasminogen activators can be added to clotting plasma, or euglobulin prepared to reduce endogenous inhibitors, but results are complicated by these manipulations. Observational studies to identify a 'fibrinolysis deficit' have concluded that excess fibrinolysis inhibitors, plasminogen activator inhibitor 1 (PAI-1) or thrombin-activatable fibrinolysis inhibitor (TAFI), zymogen or active enzyme, may be associated with an increased risk of thrombosis. However, results are not always consistent and problems of adequate standardization are evident with these inhibitors and also for measurement of fibrin degradation products (D-dimer). Few methods are available to investigate fibrinolysis under flow, or in whole blood, but viscoelastic methods (VMs) such as ROTEM and TEG do permit the contribution of cells, and importantly platelets, to be explored. VMs are used to diagnose clinical hyperfibrinolysis, which is associated with high mortality. There is a debate on the usefulness of VMs as a point-of-care test method, particularly in trauma. Despite the difficulties of many fibrinolysis methods, research on the fibrinolysis system, taking in wider interactions with hemostasis proteins, is progressing so that in future we may have more complete models and better diagnostic methods and therapeutics.

Purification, biochemical, and thermal properties of fibrinolytic enzyme secreted by Bacillus cereus SRM-001

The discovery of microbial fibrinolytic enzymes is essential to treat cardiovascular diseases. This study reports the discovery of a fibrinolytic enzyme secreted by Bacillus cereus SRM-001, a microorganism isolated from the soil of a chicken waste-dump yard. The B. cereus SRM-001 was cultured and the secreted fibrinolytic enzyme purified to show that it is a ∼28 kDa protein. The purified enzyme was characterized for its kinetics, biochemical and thermal properties to show that it possesses properties similar to plasmin. A HPLC-MS/MS analysis of trypsin digested protein indicated that the fibrinolytic enzyme shared close sequence homology with serine proteases reported for other Bacillus sp. The results show that the B. cereus SRM-001 secreted enzyme is a ∼28 kDa serine protease that possesses fibrinolytic potential.

Activity Regulation by Fibrinogen and Fibrin of Streptokinase from Streptococcus Pyogenes

Streptokinase is a virulence factor of streptococci and acts as a plasminogen activator to generate the serine protease plasmin which promotes bacterial metastasis. Streptokinase isolated from group C streptococci has been used therapeutically as a thrombolytic agent for many years and its mechanism of action has been extensively studied. However, group A streptococci are associated with invasive and potentially fatal infections, but less detail is available on the mechanism of action of streptokinase from these bacteria. We have expressed recombinant streptokinase from a group C strain to investigate the therapeutic molecule (here termed rSK-H46A) and a molecule isolated from a cluster 2a strain from group A (rSK-M1GAS) which is known to produce the fibrinogen binding, M1 protein, and is associated with life-threatening disease. Detailed enzyme kinetic models have been prepared which show how fibrinogen-streptokinase-plasminogen complexes regulate plasmin generation, and also the effect of fibrin interactions. As is the case with rSK-H46A our data with rSK-M1GAS support a "trigger and bullet" mechanism requiring the initial formation of SK•plasminogen complexes which are replaced by more active SK•plasmin as plasmin becomes available. This model includes the important fibrinogen interactions that stimulate plasmin generation. In a fibrin matrix rSK-M1GAS has a 24 fold higher specific activity than the fibrin-specific thrombolytic agent, tissue plasminogen activator, and 15 fold higher specific activity than rSK-H46A. However, in vivo fibrin specificity would be undermined by fibrinogen stimulation. Given the observed importance of M1 surface receptors or released M1 protein to virulence of cluster 2a strain streptococci, studies on streptokinase activity regulation by fibrin and fibrinogen may provide additional routes to addressing bacterial invasion and infectious diseases.

Biosimilars: the process is the product. The example of recombinant streptokinase

BACKGROUND

Worldwide, streptokinase remains the most used thrombolytic agent for the treatment of myocardial infarction. Recombinant streptokinase, from E. coli, is increasingly used in developing countries as a biosimilar of native streptokinase; however, potency assignments relative to the WHO International Standard (IS) are highly variable with potentially dangerous consequences. A proportion of recombinant streptokinase appears to be incompletely processed, retaining the amino-terminal methionine engineered for intracellular expression.

OBJECTIVES

To investigate and quantify the impact of an amino-terminal methionine on streptokinase activity.

METHODS

Mature native streptokinase (rSK) was cloned and a novel variant constructed to include an amino-terminal methionine (rSK-Met) that is not susceptible to processing during expression. Potencies of rSK and rSK-Met were determined relative to the WHO IS using a chromogenic solution (European Pharmacopoeia) assay, and fibrin-based assays.

RESULTS

In the chromogenic solution assay there was no measurable difference between rSK and rSK-Met activities. In the fibrin-based methods, however, potency estimates for rSK-Met were greatly reduced compared with rSK, and fibrinolytic activity for rSK-Met was shown to increase over time with methionine aminopeptidase treatment. This apparent difference in activity and fibrin selectivity was consistent with potency estimates for several different batches of commercial recombinant streptokinase products also tested; consequently, different potencies would be assigned to therapeutic recombinant streptokinase products depending on the degree of amino-terminal methionine processing, and on the pharmacopoeial assay method used, affecting the dosage patients receive. This has serious health implications and provides an example of the danger in the unregulated clinical use of biosimilars.

Constitutive optimized production of streptokinase in Saccharomyces cerevisiae utilizing glyceraldehyde 3-phosphate dehydrogenase promoter of Pichia pastoris

A novel expression vector constructed from genes of Pichia pastoris was applied for heterologous gene expression in Saccharomyces cerevisiae. Recombinant streptokinase (SK) was synthesized by cloning the region encoding mature SK under the control of glyceraldehyde 3-phosphate dehydrogenase (GAP) promoter of Pichia pastoris in Saccharomyces cerevisiae. SK was intracellularly expressed constitutively, as evidenced by lyticase-nitroanilide and caseinolytic assays. The functional activity was confirmed by plasminogen activation assay and in vitro clot lysis assay. Stability and absence of toxicity to the host with the recombinant expression vector as evidenced by southern analysis and growth profile indicate the application of this expression system for large-scale production of SK. Two-stage statistical approach, Plackett-Burman (PB) design and response surface methodology (RSM) was used for SK production medium optimization. In the first stage, carbon and organic nitrogen sources were qualitatively screened by PB design and in the second stage there was quantitative optimization of four process variables, yeast extract, dextrose, pH, and temperature, by RSM. PB design resulted in dextrose and peptone as best carbon and nitrogen sources for SK production. RSM method, proved as an efficient technique for optimizing process conditions which resulted in 110% increase in SK production, 2352 IU/mL, than for unoptimized conditions.

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.

Lytic and mechanical stability of clots composed of fibrin and blood vessel wall components

BACKGROUND

Proteases expressed in atherosclerotic plaque lesions generate collagen fragments, release glycosaminoglycans (chondroitin sulfate [CS] and dermatan sulfate [DS]) and expose extracellular matrix (ECM) proteins (e.g. decorin) at sites of fibrin formation.

OBJECTIVE

Here we address the effect of these vessel wall components on the lysis of fibrin by the tissue plasminogen activator (tPA)/plasminogen system and on the mechanical stability of clots.

METHODS AND RESULTS

MMP-8-digested collagen fragments, isolated CS, DS, glycosylated decorin and its core protein were used to prepare mixed matrices with fibrin (additives present at a 50-fold lower mass concentration than fibrinogen). Scanning electron microscopy (SEM) showed that the presence of ECM components resulted in a coarse fibrin structure, most pronounced for glycosylated decorin causing an increase in the median fiber diameter from 85 to 187 nm. Rheological measurements indicated that these structural alterations were coupled to decreased shear resistance (1.8-fold lower shear stress needed for gel/fluid transition of the clots containing glycosylated decorin) and rigidity (reduction of the storage modulus from 54.3 to 33.2 Pa). The lytic susceptibility of the modified fibrin structures was increased. The time to 50% lysis by plasmin was reduced approximately 2-fold for all investigated ECM components (apart from the core protein of decorin which produced a moderate reduction of the lysis time by 25%), whereas fibrin-dependent plasminogen activation by tPA was inhibited by up to 30%.

CONCLUSION

ECM components compromise the chemical and mechanical stability of fibrin as a result of changes in its ultrastructure.

Mechanical stability and fibrinolytic resistance of clots containing fibrin, DNA, and histones

Neutrophil extracellular traps are networks of DNA and associated proteins produced by nucleosome release from activated neutrophils in response to infection stimuli and have recently been identified as key mediators between innate immunity, inflammation, and hemostasis. The interaction of DNA and histones with a number of hemostatic factors has been shown to promote clotting and is associated with increased thrombosis, but little is known about the effects of DNA and histones on the regulation of fibrin stability and fibrinolysis. Here we demonstrate that the addition of histone-DNA complexes to fibrin results in thicker fibers (increase in median diameter from 84 to 123 nm according to scanning electron microscopy data) accompanied by improved stability and rigidity (the critical shear stress causing loss of fibrin viscosity increases from 150 to 376 Pa whereas the storage modulus of the gel increases from 62 to 82 pascals according to oscillation rheometric data). The effects of DNA and histones alone are subtle and suggest that histones affect clot structure whereas DNA changes the way clots are lysed. The combination of histones + DNA significantly prolongs clot lysis. Isothermal titration and confocal microscopy studies suggest that histones and DNA bind large fibrin degradation products with 191 and 136 nm dissociation constants, respectively, interactions that inhibit clot lysis. Heparin, which is known to interfere with the formation of neutrophil extracellular traps, appears to prolong lysis time at a concentration favoring ternary histone-DNA-heparin complex formation, and DNase effectively promotes clot lysis in combination with tissue plasminogen activator.

Regulation of fibrinolysis by C-terminal lysines operates through plasminogen and plasmin but not tissue-type plasminogen activator

BACKGROUND

Binding of tissue-type plasminogen (Pgn) activator (t-PA) and Pgn to fibrin regulates plasmin generation, but there is no consistent, quantitative understanding of the individual contribution of t-PA finger and kringle 2 domains to the regulation of fibrinolysis. Kringle domains bind to lysines in fibrin, and this interaction can be studied by competition with lysine analogs and removal of C-terminal lysines by carboxypeptidase B (CPB).

METHODS

High-throughput, precise clot lysis assays incorporating the lysine analog tranexamic acid (TA) or CPB and genetically engineered variants of t-PA were performed. In particular, wild-type (WT) t-PA (F-G-K1-K2-P) and a domain-switched variant K1K1t-PA (F-G-K1-K1-P) that lacks kringle 2 but retains normal t-PA structure were compared to probe the importance of fibrin lysine binding by t-PA kringle 2.

RESULTS

WT t-PA showed higher rates of fibrinolysis than K1K1t-PA, but the inhibitory effects of TA or CPB were very similar for WT t-PA and the variant t-PA (< 10% difference). Urokinase plasminogen activator (u-PA)-catalyzed fibrinolysis was also inhibited by TA, even though Pgn activation could be stimulated. Fibrin treated with factor XIIIa (FXIIIa) generates crosslinked degradation products, but these did not affect the results obtained with WT t-PA and K1K1t-PA.

CONCLUSIONS

t-PA kringle 2 has a minor role in the initial interaction of t-PA and fibrin, but stimulation of fibrinolysis by C-terminal lysines (or inhibition by carboxypeptidases or TA) operates through Pgn and plasmin binding, not through t-PA. This is also true when fibrin is crosslinked by treatment with FXIIIa.

Defibrotide blunts the prothrombotic effect of thalidomide on endothelial cells

Patients with multiple myeloma (MM) are at relatively high risk of developing thromboembolic events such deep venous thrombosis (DVT) where thalidomide therapy has been identified to increase this risk. Defibrotide (DF), a polydisperse oligonucleotide, showed previously to counteract the alterations in endothelial cells (ECs) induced by lipopolysaccharide. It prompts us to investigate the impact of thalidomide on ECs and whether DF modulates changes in fibrinolysis induced by thalidomide. In this in vitro study, MM by itself alters the profibrinolytic potential of ECs decreasing the tissue plasminogen activator (t-PA) and increasing the plasminogen activator inhibitor 1 (PAI-1) levels which is potentiated by thalidomide. Defibrotide was able to counteract these effects. Additionally, DF upregulated the t-PA and downregulated PAI-1 gene expression modulated by thalidomide. Defibrotide also protects ECs from thalidomide-mediated cell death without interfering with its antitumor effects. These findings support DF clinical use for the prevention of DVT induced by immunomodulatory drugs.

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.

The fibrinolytic mechanism of defibrotide: effect of defibrotide on plasmin activity

Fibrinolytic activity has been shown to be reduced in many vascular diseases, including hepatic veno-occlusive disease after stem cell transplantation, a microangiopathy characterized by sinusoidal endothelial cell injury. Defibrotide is a polydisperse oligonucleotide with antithrombotic, profibrinolytic, anti-ischemic, and antiadhesive properties. Numerous clinical studies have shown promising activity of defibrotide in the treatment and prevention of veno-occlusive disease, with minimal toxicity. In corollary laboratory studies, defibrotide has been shown to decrease plasminogen activator inhibitor-1, increase tissue plasminogen activator levels, and increase overall plasma fibrinolytic activity in patients. Plasmin, a potent and nonspecific serine protease, plays a pivotal role in fibrinolysis by virtue of its ability to effectively degrade fibrin clots. In this study, defibrotide increases the activity of plasmin in hydrolyzing its substrate in a dose-dependent and length-dependent manner. Similar concentration-dependent effects of defibrotide were observed when plasmin was generated by tissue plasminogen activator or urokinase activation of plasminogen. In contrast, defibrotide had no direct effect on the activation of plasminogen to plasmin. Defibrotide was also able to enhance the activity of plasmin in degrading fibrin clot formed from fibrinogen, plasminogen, and thrombin. This effect was also concentration-dependent and directly correlated with the enzymatic activity of plasmin. This study therefore demonstrates that defibrotide is capable of enhancing the activity of plasmin and so contributes to its fibrinolytic activity. Taken together, these results support the effect of defibrotide in restoring the fibrinolytic vascular phenotype, in microangiopathic conditions such as veno-occlusive disease.

Activation of single-chain urokinase-type plasminogen activator by platelet-associated plasminogen: a mechanism for stimulation of fibrinolysis by platelets

BACKGROUND AND OBJECTIVE

Platelets are essential for hemostasis, and they cause resistance to fibrinolysis by tissue-type plasminogen activator. In contrast, platelets enhance fibrinolysis mediated by single-chain urokinase-type plasminogen activator (scu-PA). This study investigated the mechanism behind this profibrinolytic role of platelets.

METHODS AND RESULTS

Platelets enhanced scu-PA activity, but not urokinase-type plasminogen activator (u-PA) activity, in plasma clot lysis and chromogenic assays. We established, using the non-cleavable scu-PA mutant (Lys158-->Glu) and protease inhibitors, that platelets increased activation to u-PA by a serine protease. Activation of scu-PA was platelet-dependent, even in plasma. It occurred in platelet-rich but not in platelet-poor plasma, as assessed by sodium dodecylsulfate polyacrylamide gel electrophoresis and zymography after addition of plasminogen activator inhibitor-1. Candidate proteases that are known to activate scu-PA and are present in platelet preparations were investigated. Factor VII activating protease was detected in platelet preparations by western blotting, but its inhibition by antibodies did not inhibit activation of scu-PA by platelets. Plasmin and plasma kallikrein both mimicked the platelet effect, but were distinguished by their responses to a range of inhibitors. Analysis of platelet-associated protease activity and the time course of scu-PA activation pointed towards plasminogen, and the data were consistent with a mechanism of reciprocal activation. The essential role of plasminogen was revealed using platelets from plasminogen-deficient mice, which could not activate scu-PA. Local plasminogen on platelet membranes was markedly more effective than solution-phase plasminogen in activation of scu-PA.

CONCLUSIONS

Platelets enhance fibrinolysis by scu-PA through reciprocal activation of scu-PA and platelet-associated plasminogen, a system that is potentially important in the lysis of platelet-rich thrombi.

A nonfibrin macromolecular cofactor for tPA-mediated plasmin generation following cellular injury

Tissue-type plasminogen activator (tPA) is an extracellular protease that converts plasminogen into plasmin. For tPA to generate plasmin under biologic conditions, a cofactor must first bring tPA and plasminogen into physical proximity. Fibrin provides this cofactor for tPA-mediated plasmin generation in blood. Despite being naturally devoid of fibrin(ogen), tPA-mediated plasmin formation also occurs in the brain. The fibrin-like cofactor(s) that facilitates plasmin formation in the injured brain has remained unknown. Here we show that protein aggregates formed during neuronal injury provide a macromolecular, nonfibrin cofactor that promotes tPA-mediated plasmin formation and subsequent cell breakdown. The binding of plasminogen and tPA to these protein aggregates occurs via distinct mechanisms. Importantly, nonneuronal cell types also exhibit this cofactor effect upon injury, indicating a general phenomenon. This novel cofactor identified in nonviable cells has ramifications for ischemic stroke where tPA is used clinically and where plasmin activity within the injured brain is unwanted. A means of selectively inhibiting the binding of tPA to nonviable cells while preserving its association with fibrin may be of benefit for the treatment of ischemic stroke.

Acoustic determination of performance and equivalence of plasminogen activators

A reliable method for the measurement of different plasminogen activators is of great interest for both manufacturing and clinical medicine. A one-step assay based on a thickness shear mode acoustic sensor has been developed for this purpose. Two separate mixtures of substrates (fibrinogen and plasminogen) and enzymes (thrombin and the plasminogen activator) were mixed, and placed on the acoustic sensor surface. During the assay, the resonant frequency of a quartz crystal oscillating in the thickness shear mode was measured and used to find a characteristic clot dissolution time, from the sample addition to the time at the maximum dissolution rate. Calibrations of the acoustic assay were done for tissue-type plasminogen activator (t-PA) as well as for the other plasminogen activators: urokinase (u-PA); streptokinase (SK) and staphylokinase (SAK). All gave relative standard deviations of about 12%. Since the same method was used for all of the activators, their activities were compared, resolving the differences between their unit definitions. Linear relationships were found between urokinase and streptokinase which activate plasminogen directly and between t-PA and staphylokinase which require fibrin as a cofactor. The relationship between the groups was found to curve, indicating the difference between the two mechanisms. The acoustic method, therefore, may be used as a rapid and cost-effective reference method for the standardization and comparison of different plasminogen activators.

Fibrinolysis in a lipid environment: modulation through release of free fatty acids

BACKGROUND

Thrombolysis is conventionally regarded as dissolution of the fibrin matrix of thrombi by plasmin, but the structure of clots in vivo includes additional constituents (proteins, phospholipids) that modulate their solubilization.

OBJECTIVE

We examined the presence of free fatty acids in thrombi and their effects on distinct stages of fibrinolysis (plasminogen activation, plasmin activity).

METHODS AND RESULTS

Using the fluorescent probe acrylodated intestinal fatty acid-binding protein, variable quantities (up to millimolar concentrations) of free fatty acids were demonstrated in surgically removed human thrombi. Oleic acid at relevant concentrations reversibly inhibits more than 90% of the amidolytic activity of plasmin on a synthetic substrate (Spectrozyme PL), but only partially inhibits its fibrinolytic activity measured using turbidimetry. Chromogenic assays detecting the generated plasmin activity show that plasminogen activation by tissue-type plasminogen activator (t-PA) is completely blocked by oleic acid in the fluid phase, but is accelerated on a fibrin matrix. A recombinant derivative of t-PA (reteplase) develops higher fibrin specificity in the presence of oleic acid, because both the inhibition of plasminogen activation in free solution and its enhancement on fibrin template are stronger than with wild-type t-PA.

CONCLUSION

Through the stimulation of plasminogen activation on a fibrin template and the inhibition of plasminogen activators and plasmin in the fluid phase, free fatty acids confine the action of fibrinolytic proteases to the site of clotting, where they partially oppose the thrombolytic barrier function of phospholipids.

The regulation by fibrinogen and fibrin of tissue plasminogen activator kinetics and inhibition by plasminogen activator inhibitor 1

BACKGROUND

Tissue plasminogen activator (tPA) is unusual in the coagulation and fibrinolysis cascades in that it is produced as an active single-chain enzyme (sctPA) rather than a zymogen. Two chain tPA (tctPA) is produced by plasmin but there are conflicting reports in the literature on the behaviour of sc- and tctPA and little work on inhibition by the specific inhibitor plasminogen activator inhibitor-1 (PAI-1) under physiological conditions.

OBJECTIVES

To perform a systematic study on the kinetics of sctPA and tctPA as plasminogen activators and targets for PAI-1.

METHODS

Detailed kinetic studies were performed in solution and in the presence of template stimulators, fibrinogen and fibrin, including native fibrin and partially digested fibrin. Numerical simulation techniques were utilized to cope with the challenges of investigating kinetics of activation and inhibition in the presence of fibrin(ogen).

RESULTS

Enzyme efficiency (k(cat)/K(m)) was higher for tctPA than sctPA in solution with chromogenic substrate (3-fold) and plasminogen (7-fold) but in the presence of templates, such as fibrinogen and native or cleaved fibrin, the difference disappeared. sctPA was more susceptible to PAI-1 in buffer solution and in the presence of fibrinogen; however, in the presence of fibrin, PAI-1 inhibited more slowly and there was no difference between sc and tctPA.

CONCLUSIONS

Fibrinogen and fibrin modulate the activity of tPA differently in regard to their activation of plasminogen and inhibition by PAI-1. Fibrinogen and fibrin stimulate tPA activity against plasminogen but fibrin protects tPA from PAI-1 to promote fibrinolysis.

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.

Phospholipid barrier to fibrinolysis: role for the anionic polar head charge and the gel phase crystalline structure

The massive presence of phospholipids is demonstrated in frozen sections of human arterial thrombi. Purified platelet phospholipids and synthetic phospholipids retard in vitro tissue-type plasminogen activator (tPA)-induced fibrinolysis through effects on plasminogen activation and plasmin function. The inhibition of plasminogen activation on the surface of fibrin correlates with the fraction of anionic phospholipid. The phospholipids decrease the amount of tPA penetrating into the clot by 75% and the depth of the reactive surface layer occupied by the activator by up to 30%, whereas for plasmin both of these parameters decrease by approximately 50%. The phospholipids are not only a diffusion barrier, they also bind the components of the fibrinolytic system. Isothermal titration calorimetry shows binding characterized with dissociation constants in the range 0.35-7.64 microm for plasmin and tPA (lower values with more negative phospholipids). The interactions are endothermic and thermodynamically driven by an increase in entropy, probably caused by the rearrangements in the ordered gel structure of the phospholipids (in line with the stronger inhibition at gel phase temperatures compared with liquid crystalline phase temperatures). These findings show a phospholipid barrier, which should be overcome during lysis of arterial thrombi.