Characterization of the interactions of plasminogen and tissue and vampire bat plasminogen activators with fibrinogen, fibrin, and the complex of D-dimer noncovalently linked to fragment E

4D intravital imaging studies identify platelets as the predominant cellular procoagulant surface in a mouse model of hemostasis

Interplay between platelets, coagulation/fibrinolytic factors, and endothelial cells (ECs) is necessary for effective hemostatic plug formation. This study describes a novel four-dimensional (4D) imaging platform to visualize and quantify hemostatic plug components with high spatiotemporal resolution. Fibrin accumulation following laser-induced endothelial ablation was observed at the EC-platelet plug interface, controlled by the antagonistic balance between fibrin generation and breakdown. Phosphatidylserine (PS) was first detected in close physical proximity to the fibrin ring, followed by exposure across the endothelium. Impaired PS exposure in cyclophilinD -/- mice resulted in a significant reduction in fibrin accumulation. Adoptive transfer and inhibitor studies demonstrated a key role for platelets, but not ECs, in fibrin generation during hemostatic plug formation. Inhibition of fibrinolysis with tranexamic acid (TXA) led to increased fibrin accumulation in WT mice, but not in cyclophilinD -/- mice or WT mice treated with antiplatelet drugs. These studies implicate platelets as the functionally dominant procoagulant surface during hemostatic plug formation. In addition, they suggest that impaired fibrin formation due to reduced platelet procoagulant activity is not reversed by TXA treatment.

Identification, purification, and pharmacological activity analysis of Desmodus rotundus salivary plasminogen activator alpha1 (DSPAα1) expressed in transgenic rabbit mammary glands

Desmodus rotundus plasminogen activator alpha 1(DSPAα1) is a thrombolytic protein with advantages, such as a long half-life, high accuracy and specificity for thrombolysis, wide therapeutic window, and no neurotoxicity. To date, DSPAα1 has only been expressed in the Chinese hamster ovary, insect cells, transgenic tobacco plants, and Pichia pastoris. To the best of our knowledge, we are the first to report the expression of DSPAα1 in transgenic rabbit mammary glands, extract the product, and analyze its pharmacology activity. An efficient mammary gland-specific expression vector pCL25/DSPAα1 was transferred to prokaryotic zygotes in rabbits by microinjection to generate six DSPAα1 transgenic rabbits. The recombinant DSPAα1 (rDSPAα1) expression in transgenic rabbit milk was 1.19 ± 0.26 mg/mL. The rDSPAα1 purification protocol included pretreatment, ammonium sulfate precipitation, benzamidine affinity chromatography, cation exchange chromatography, and Cibacron blue affinity chromatography; approximately 98% purity was achieved using gel electrophoresis. According to sequencing results, the primary structure of rDSPAα1 was consistent with the theoretical design sequence, and its molecular weight was consistent with that of the natural protein. N-terminal sequencing results indicated rDSPAα1 to be a mature protein, as the goat signal peptide sequence of the expression vector was no longer detected. The fibrinolytic activity of rDSPAα1 was estimated to be 773,333 IU/mg. Fibrin-agarose plate assay and in vitro rat blood clot degradation assay showed that rDSPAα1 had strong thrombolytic activity. In conclusion, we report recombinant DSPAα1 with high thrombolytic activity expressed in transgenic rabbit mammary glands.

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.

Structural Biology and Protein Engineering of Thrombolytics

Myocardial infarction and ischemic stroke are the most frequent causes of death or disability worldwide. Due to their ability to dissolve blood clots, the thrombolytics are frequently used for their treatment. Improving the effectiveness of thrombolytics for clinical uses is of great interest. The knowledge of the multiple roles of the endogenous thrombolytics and the fibrinolytic system grows continuously. The effects of thrombolytics on the alteration of the nervous system and the regulation of the cell migration offer promising novel uses for treating neurodegenerative disorders or targeting cancer metastasis. However, secondary activities of thrombolytics may lead to life-threatening side-effects such as intracranial bleeding and neurotoxicity. Here we provide a structural biology perspective on various thrombolytic enzymes and their key properties: (i) effectiveness of clot lysis, (ii) affinity and specificity towards fibrin, (iii) biological half-life, (iv) mechanisms of activation/inhibition, and (v) risks of side effects. This information needs to be carefully considered while establishing protein engineering strategies aiming at the development of novel thrombolytics. Current trends and perspectives are discussed, including the screening for novel enzymes and small molecules, the enhancement of fibrin specificity by protein engineering, the suppression of interactions with native receptors, liposomal encapsulation and targeted release, the application of adjuvants, and the development of improved production systems.

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.

Clot Formation in the Presence of Acetylsalicylic Acid Leads to Increased Lysis Rates Regardless of the Chosen Thrombolysis Strategy

BACKGROUND

Patients with acute ischemic strokes frequently take an acetylsalicylic acid (ASA) premedication. We determined the impact of ASA on different thrombolysis strategies in vitro.

METHODS

For two clot types made from platelet-rich plasma (one with and one without ASA) lysis rates were measured by weight loss after 1 h for five different groups: in control group A clots were solely placed in plasma; in groups B and C clots were treated with rt-PA (60 kU/ml), and in groups D and E clots were treated with desmoteplase (DSPA; 2 µg/ml). Ultrasound (2 MHz, 0.179 W/cm2) was included in groups C and E. The fibrin mesh structures of the clots were investigated by electron microscopy.

RESULTS

For both clot types lysis rates increased significantly for all treatment strategies compared to their control group (each p < 0.001). The addition of ASA significantly increased the lysis rate in all 5 groups (each p < 0.001) and led to a ceiling effect concerning the treatment. A semiquantitative analysis of transmission electron micrographs revealed a decreased fibrin density for clots with ASA. For both clot types DSPA and ultrasound led to a significant dissolution of the fibrin mesh (both p = 0.029).

CONCLUSIONS

In vitro ASA pretreatment leads to significantly increased lysis rates due to a weaker fibrin mesh in platelet-rich plasma clots.

Effect of fibrin degradation products on fibrinolytic process

Fibrin clot lysis by plasminogen/plasmin system results in fibrin degradation products formation with subsequent release into bloodstream. The fragments contain specific binding sites for fibrinolytic system components and can interact with them. In this study, we investigated the way in which fibrin fragments effect fibrinolytic process. We have shown that high molecular weight products of fibrin degradation and fibrin fragments of DDE-complex and DD, but not end product Е3, stimulate plasmin formation. Additionally, components of DDE-complex mixture of fragments Е1 and Е2 have potentiation ability. The intermediate fibrin fragments hmFDPs and DDE attenuate clot lysis by plasmin and hmFDPs protect plasmin from α2-antiplasmin inhibition but under further fragmentation to endpoint fibrin fragments loose this ability. The plasma inhibitors reduce fibrinolytic system activity generated by the degradation products. Thus, fibrin fragments formed during the clot lysis can bind and move out fibrinolytic system components from clot volume and in this way result in clot resistance to hydrolysis.

In Vitro Examination of the Thrombolytic Efficacy of Desmoteplase and Therapeutic Ultrasound Compared with rt-PA

The aim of the study described here was to evaluate the thrombolytic efficacy of combined treatment with the fibrin-selective plasminogen activator desmoteplase (DSPA) and therapeutic ultrasound (sonothrombolysis [STL]) compared with conventional rt-PA (recombinant tissue plasminogen activator) treatment in vitro. Lysis rates were determined by the weight loss of platelet-rich plasma (PRP) clots treated with rt-PA (60 kU/mL) or DSPA (2 μg/mL) combined with pulsed wave ultrasound (2 MHz, 0.179 W/cm(2)). To reveal the individual effects of medication and ultrasound, lysis rates were also determined for DSPA monotherapy and for combined treatment with rt-PA and ultrasound. Clots solely placed in plasma served as the control group. Lysis increased significantly with rt-PA (26.5 ± 7.8%) and DSPA (30.5 ± 6%) compared with the control group (18.2 ± 5.9%) (each p < 0.001). DSPA lysis was more effective than rt-PA lysis (without STL: p = 0.015, with STL: p = 0.01). Combined treatment with DSPA and 2-MHz STL significantly exceeded rt-PA lysis (32.8% vs. 26.5%, p < 0.001).

Basic mechanisms and regulation of fibrinolysis

Fibrinolysis appears in many diverse physiological situations, and the components of the system are well established, along with mechanistic details for the individual reactions and some high-resolution structures. Key questions in understanding the regulation of fibrinolysis surround mechanisms of initiation and propagation, the localization of fibrinolysis reactions to the fibrin clot, and the influence of fibrin structure and clot composition on thrombolysis. This review covers these key areas with a focus on recent developments on fibrin structure and binding, the effects of a variety of cell types, the consequences of histones and DNA released by neutrophils, and the influence of flow. A complete understanding of the regulation of fibrinolysis will come from the building of detailed mathematical models. Suitable models are at an early stage of development, but may improve as model clots increase in complexity to incorporate the components and interactions listed above.

Zinc delays clot lysis by attenuating plasminogen activation and plasmin-mediated fibrin degradation

Zinc circulates free in plasma at a concentration of 0.1-2 µM, but its levels increase locally when it is released from activated platelets. Although zinc influences many processes in haemostasis, its effect on fibrinolysis has not been thoroughly investigated. Using a fluorescent zinc-binding probe, we demonstrated that zinc binds tissue-type plasminogen activator (tPA) and plasmin with high affinity (Kd values of 0.2 µM), and surface plasmon resonance studies revealed that zinc binds fibrin with a Kd of 12.8 µM. Zinc had no effect on the affinity of plasminogen or plasmin for fibrin, but increased the affinity of tPA by two-fold. In the presence of 5 µM zinc, the catalytic efficiency of plasminogen activation by tPA was reduced by approximately two-fold, both in the absence or presence of fibrin. Zinc attenuated plasmin-mediated degradation of the fibrinogen alpha-chain by 43 %, but had no effect on trypsin degradation. tPA-mediated fibrin clot lysis was prolonged 2.5-fold by zinc in a concentration-dependent fashion, and tPA-mediated plasma clot lysis was attenuated by 1.5-fold. Therefore, our data indicate that zinc modulates fibrinolysis by attenuating tPA-mediated plasminogen activation and plasmin-induced fibrin degradation. These findings suggest that local release of zinc by platelets attenuates fibrinolysis.

Design of a novel chimeric tissue plasminogen activator with favorable Vampire bat plasminogen activator properties

Fibrinolytic agents are widely used in treatment of the thromboembolic disorders. The new generations like recombinant tissue plasminogen activator (t-PA, alteplase) are not showing promising results in clinical practice in spite of displaying specific binding to fibrin in vitro. Vampire bat plasminogen activator (b-PA) is a plasminogen activator with higher fibrin affinity and specificity in comparison to t-PA resulting in reduced probability of hemorrhage. b-PA is also resistant to plasminogen activator inhibitor-1 (PAI-1) showing higher half-life compared to other variants of t-PA. However, its non-human origin was a driving force to design a human t-PA with favorable properties of b-PA. In the present study, we designed a chimeric t-PA with desirable b-PA properties and this new molecule was called as CT-b. The construct was prepared through kringle 2 domain removal and replacement of t-PA finger domain with b-PA one. In addition, the KHRR sequence at the initial part of protease domain was replaced by four alanine residues. The novel construct was integrated in Pichia pastoris genome by electroporation. Catalytic activity was investigated in the presence and absence of fibrin. The purified protein was analyzed by western blot. Fibrin binding and PAI resistance assays were also conducted. The activity of the recombinant protein in the presence of fibrin was 1560 times more than its activity in the absence of fibrin, showing its higher specificity to fibrin. The fibrin binding of CT-b was 1.2 fold more than t-PA. In addition, it was inhibited by PAI enzyme 44% less than t-PA. Although the presented data demonstrate a promising in vitro activity, more in vivo studies are needed to confirm the therapeutic advantage of this novel plasminogen activator.

Reduced plasminogen binding and delayed activation render γ'-fibrin more resistant to lysis than γA-fibrin

Fibrin (Fn) clots formed from γ'-fibrinogen (γ'-Fg), a variant with an elongated γ-chain, are resistant to lysis when compared with clots formed from the predominant γA-Fg, a finding previously attributed to differences in clot structure due to delayed thrombin-mediated fibrinopeptide (FP) B release or impaired cross-linking by factor XIIIa. We investigated whether slower lysis of γ'-Fn reflects delayed plasminogen (Pg) binding and/or activation by tissue plasminogen activator (tPA), reduced plasmin-mediated proteolysis of γ'-Fn, and/or altered cross-linking. Clots formed from γ'-Fg lysed more slowly than those formed from γA-Fg when lysis was initiated with tPA/Pg when FPA and FPB were both released, but not when lysis was initiated with plasmin, or when only FPA was released. Pg bound to γ'-Fn with an association rate constant 22% lower than that to γA-Fn, and the lag time for initiation of Pg activation by tPA was longer with γ'-Fn than with γA-Fn. Once initiated, however, Pg activation kinetics were similar. Factor XIIIa had similar effects on clots formed from both Fg isoforms. Therefore, slower lysis of γ'-Fn clots reflects delayed FPB release, which results in delayed binding and activation of Pg. When clots were formed from Fg mixtures containing more than 20% γ'-Fg, the upper limit of the normal level, the delay in lysis was magnified. These data suggest that circulating levels of γ'-Fg modulate the susceptibility of clots to lysis by slowing Pg activation by tPA and provide another example of the intimate connections between coagulation and fibrinolysis.

New antithrombotic drugs: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

This article focuses on new antithrombotic drugs that are in or are entering phase 3 clinical testing. Development of these new agents was prompted by the limitations of existing antiplatelet, anticoagulant, or fibrinolytic drugs. Addressing these unmet needs, this article (1) outlines the rationale for development of new antithrombotic agents; (2) describes the new antiplatelet, anticoagulant, and fibrinolytic drugs; and (3) provides clinical perspectives on the opportunities and challenges faced by these novel agents.

Venomous mammals: a review

The occurrence of venom in mammals has long been considered of minor importance, but recent fossil discoveries and advances in experimental techniques have cast new light into this subject. Mammalian venoms form a heterogeneous group having different compositions and modes of action and are present in three classes of mammals, Insectivora, Monotremata, and Chiroptera. A fourth order, Primates, is proposed to have venomous representatives. In this review we highlight recent advances in the field while summarizing biochemical characteristics of these secretions and their effects upon humans and other animals. Historical aspects of venom discovery and evolutionary hypothesis regarding their origin are also discussed.

A high affinity interaction of plasminogen with fibrin is not essential for efficient activation by tissue-type plasminogen activator

Fibrin (Fn) enhances plasminogen (Pg) activation by tissue-type plasminogen activator (tPA) by serving as a template onto which Pg and tPA assemble. To explore the contribution of the Pg/Fn interaction to Fn cofactor activity, Pg variants were generated and their affinities for Fn were determined using surface plasmon resonance (SPR). Glu-Pg, Lys-Pg (des(1-77)), and Mini-Pg (lacking kringles 1-4) bound Fn with K(d) values of 3.1, 0.21, and 24.5 μm, respectively, whereas Micro-Pg (lacking all kringles) did not bind. The kinetics of activation of the Pg variants by tPA were then examined in the absence or presence of Fn. Whereas Fn had no effect on Micro-Pg activation, the catalytic efficiencies of Glu-Pg, Lys-Pg, and Mini-Pg activation in the presence of Fn were 300- to 600-fold higher than in its absence. The retention of Fn cofactor activity with Mini-Pg, which has low affinity for Fn, suggests that Mini-Pg binds the tPA-Fn complex more tightly than tPA alone. To explore this possibility, SPR was used to examine the interaction of Mini-Pg with Fn in the absence or presence of tPA. There was 50% more Mini-Pg binding to Fn in the presence of tPA than in its absence, suggesting that formation of the tPA-Fn complex exposes a cryptic site that binds Mini-Pg. Thus, our data (a) indicate that high affinity binding of Pg to Fn is not essential for Fn cofactor activity, and (b) suggest that kringle 5 localizes and stabilizes Pg within the tPA-Fn complex and contributes to its efficient activation.

Desmoteplase

The high fibrin specificity of Desmodus rotundus salivary plasminogen activator alpha1 (desmoteplase) renders it a promising candidate for the treatment of acute ischemic stroke. In the DIAS (Desmoteplase in Acute Ischemic Stroke) and DEDAS (Dose Escalation study of Desmoteplase in Acute ischemic Stroke) Phase II studies, doses of 90 microg/kg and 125 microg/kg desmoteplase were reported to have acceptable safety profiles, leading to potentially superior reperfusion compared with placebo, with possible clinical efficacy for up to 9 h after the onset of symptoms in patients with a significant ischemic penumbra selected from magnetic resonance perfusion-diffusion weighted mismatches imaging. However, a Phase III clinical trial (DIAS-2) was unable to detect any benefit from desmoteplase when given 3 - 9 h after stroke onset. In this study with a modest sample size, certain methodological factors may have reduced its potential to detect a desmoteplase effect, as only 30% of these patients had a visible occlusion at presentation, with only small core and mismatched lesion volumes. Indeed, it is surprising that a study testing an occluded vessel 'reopener' was conducted in a cohort of stroke patients, the majority of whom was known not to have a detected vessel occlusion. It has also been claimed that the DIAS-2 patients selection using core/penumbra mismatch calculation may not have followed an appropriate mismatch threshold. However, the corrective value of changing the mismatch threshold remains unclear, because the relative mismatch volumes were in fact higher in the 'negative' DIAS-2 than in the 'positive' DIAS and DEDAS. Two Phase II randomized trials with tPA, Diffusion-weighted imaging Evaluation For Understanding Stroke Evolution (DEFUSE) and Echoplanar Imaging Thrombolytic Evaluation Trial (EPITHET) provided strong biological support for the relation between infarct growth, reperfusion and clinical outcome in the 3 - 6 h time window after onset of stroke using penumbral imaging. In this frame, why exactly desmoteplase should have specific advantages over tPA, is not clear. Taken together, these findings may also lead to the disappointing conclusion that vessel recanalization after 4.5 - 5 h from stroke onset may generally be inefficacious for tissue salvage. Nevertheless, other randomized Phase III clinical trials (DIAS-3 and DIAS-4) are currently under way with a planned sample size of 320 patients having vessel occlusion or high-grade stenosis on MRI or CT-angiography in the proximal cerebral arteries.

New antithrombotic drugs: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This chapter focuses on new antithrombotic drugs that are in phase II or III clinical testing. Development of these new agents was prompted by limitations of existing antiplatelet, anticoagulant, or fibrinolytic drugs. Addressing these unmet needs, this chapter (1) outlines the rationale for development of new antithrombotic agents, (2) describes the new antiplatelet, anticoagulant, and fibrinolytic drugs, and (3) provides clinical perspectives on the opportunities and challenges faced by these novel agents.

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.

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

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

Prion protein stimulates tissue-type plasminogen activator-mediated plasmin generation via a lysine-binding site on kringle 2

Recombinant human prion-protein (PrP23-231) stimulates plasminogen activation by tissue-type plasminogen activator (t-PA). The stimulatory activity is conserved in the N-terminal fragment (PrP23-110). It has further been shown by others that PrP(c) binds to kringle-domains of plasminogen. We compared the stimulatory activity of recombinant PrP23-231 and PrP23-110 on plasminogen activation catalyzed by t-PA, urokinase (u-PA), streptokinase and Desmodus salivary plasminogen activator (DSPAalpha1). As these plasminogen activators are distinct, with respect to their kringle domains we studied their binding to immobilized PrP23-110. Plasminogen activation was measured in a chromogenic assay in vitro and binding studies were carried out using surface plasmon resonance technology. We found that recombinant full-length prion protein, PrP23-231, and PrP23-110 specifically stimulate t-PA mediated plasminogen activation. Two hundred nanomoles per liter of PrP23-110 stimulated 1.8 nmol L(-1) t-PA 48-fold, 180 nmol L(-1) DSPA(alpha1) 2.5-fold, 1.8 nmol L(-1) u-PA 1.1-fold, and 1.8 nmol L(-1) streptokinase 1.8-fold. Our data show no specific binding for streptokinase. In contrast all plasminogen activators carrying a kringle domain bound to PrP23-110. We further studied the effect of lysine on binding to PrP23-110 and on plasminogen activation by DSPA(alpha1) or t-PA. Lysine decreased both the binding of t-PA to PrP23-110 and the stimulation of plasmin generation by t-PA. Both binding and plasminogen activation of DSPA(alpha1) were not influenced by the presence of lysine. All plasminogen activators tested bearing kringle domains bind to PrP23-110. Binding to PrP23-110 is not sufficient for stimulation of plasmin generation. Thus the lysine-binding site of kringle 2 that is unique to t-PA appears to mediate the specific stimulation of plasminogen activation by the cellular prion protein.

Do antiangiogenic protein fragments have amyloid properties?

Tumor growth requires proteolytic activity. As a consequence, protein breakdown products are present in the circulation of patients with cancer. Within the past decade a large number of proteolytic fragments have been identified that inhibit angiogenesis and tumor growth. The mechanism of action of these inhibitors is still poorly understood. We recently found that the effects of the angiogenesis inhibitor endostatin on endothelial cells is critically dependent on the presence of cross-beta structure, a structure also present in amyloidogenic polypeptides in plaques of patients with amyloidosis, such as Alzheimer disease. We also showed that cross-beta structure containing endostatin is a ligand for tissue-type plasminogen activator (tPA). We noted that many angiogenesis inhibitors stimulate tPA-mediated plasminogen activation. Because the presence of cross-beta structure is the common denominator in tPA-binding ligands, we hypothesize that these endogenous antiangiogenic proteolytic fragments share features with amyloidogenic polypeptides. We postulate that the cross-beta structural fold is present in these antiangiogenic polypeptide fragments and that this structure mediates the inhibitory effects. The hypothesis provides new insights in the potential mechanisms of these angiogenesis inhibitors and offers opportunities to improve their use.

alpha Domain deletion converts streptokinase into a fibrin-dependent plasminogen activator through mechanisms akin to staphylokinase and tissue plasminogen activator

The mechanism of action of plasminogen (Pg) activators may affect their therapeutic properties in humans. Streptokinase (SK) is a robust Pg activator in physiologic fluids in the absence of fibrin. Deletion of a "catalytic switch" (SK residues 1-59), alters the conformation of the SK alpha domain and converts SKDelta59 into a fibrin-dependent Pg activator through unknown mechanisms. We show that the SK alpha domain binds avidly to the Pg kringle domains that maintain Glu-Pg in a tightly folded conformation. By virtue of deletion of SK residues 1-59, SKDelta59 loses the ability to unfold Glu-Pg during complex formation and becomes incapable of nonproteolytic active site formation. In this manner, SKDelta59 behaves more like staphylokinase than like SK; it requires plasmin to form a functional activator complex, and in this complex SKDelta59 does not protect plasmin from inhibition by alpha(2)-antiplasmin. At the same time, SKDelta59 is unlike staphylokinase or SK and is more like tissue Pg activator, because it is a poor activator of the tightly folded form of Glu-Pg in physiologic solutions. SKDelta59 can only activate Glu-Pg when it was unfolded by fibrin interactions or by Cl(-)-deficient buffers. Taken together, these studies indicate that an intact alpha domain confers on SK the ability to nonproteolytically activate Glu-Pg, to unfold and process Glu-Pg substrate in physiologic solutions, and to alter the substrate-inhibitor interactions of plasmin in the activator complex. The loss of an intact alpha domain makes SKDelta59 activate Pg through classical "fibrin-dependent mechanisms" (akin to both staphylokinase and tissue Pg activator) that include: 1) a marked preference for a fibrin-bound or unfolded Glu-Pg substrate, 2) a requirement for plasmin in the activator complex, and 3) the creation of an activator complex with plasmin that is readily inhibited by alpha(2)-antiplasmin.

Catheter-directed Thrombolytic Therapy for Limb Ischemia: Current Status and Controversies

Absence of urokinase from the United States market for the past 4 years has resulted in increasing experience with other plasminogen activators in catheter-directed thrombolytic therapy. The differences in the pharmacologic properties and biologic behavior of these agents may translate into clinical outcomes that are distinct. Some of these manifestations can be predicted based on the existing large clinical trials in the acute myocardial infarction literature. However, because of the fundamental differences in techniques and thrombolytic regimens, extrapolation of the coronary data may not always predict the performance of these agents in peripheral catheter-directed fibrinolysis. In this article, the current status of the available lytic agents in the treatment of limb ischemia is reviewed.

Catheter-directed Thrombolytic Therapy for Limb Ischemia: Current Status and Controversies

Absence of urokinase from the United States market for the past 4 years has resulted in increasing experience with other plasminogen activators in catheter-directed thrombolytic therapy. The differences in the pharmacologic properties and biologic behavior of these agents may translate into clinical outcomes that are distinct. Some of these manifestations can be predicted based on the existing large clinical trials in the acute myocardial infarction literature. However, because of the fundamental differences in techniques and thrombolytic regimens, extrapolation of the coronary data may not always predict the performance of these agents in peripheral catheter-directed fibrinolysis. In this article, the current status of the available lytic agents in the treatment of limb ischemia is reviewed.

A kinetic analysis of the tissue plasminogen activator and DSPAalpha1 cofactor activities of untreated and TAFIa-treated soluble fibrin degradation products of varying size

The kinetics of tissue plasminogen activator (t-PA) and DSPAalpha1-catalyzed plasminogen activation using untreated and TAFIa-treated fibrin degradation products (FDPs), ranging in weight average molecular weight (M(w)) from 0.48 x 10(6) to 4.94 x 10(6) g/mol, were modeled according to the steady-state template model. The FDPs served as effective cofactors for both activators. The intrinsic catalytic efficiencies of both t-PA (17.4 x 10(5) m(-1) s(-1)) and DSPAalpha1 (6.0 x 10(5) m(-1) s(-1)) were independent of FDP M(w). The intrinsic catalytic efficiency of t-PA was 12-fold higher than that measured under identical conditions with intact fibrin as the cofactor. At sub-saturating levels of cofactor and substrate, rates were strongly dependent on FDP M(w) with DSPAalpha1 but not t-PA. Loss of activity with decreasing FDP M(w) correlated with loss of finger-dependent binding of the activators to the FDPs. TAFIa treatment of the FDPs resulted in 90- and 215-fold decreases in the catalytic efficiencies of t-PA (0.20 x 10(5) m(-)(1) s(-1)) and DSPAalpha1 (0.028 x 10(5) m(-1) s(-1)), yielding cofactors that were still 30- and 50-fold better than fibrinogen with t-PA and DSPAalpha1, respectively. Our results show that for both activators the products released during fibrinolysis are very effective cofactors for plasminogen activation, and both t-PA and DSPAalpha1 cofactor activity are strongly down-regulated by TAFIa.

Thrombin-activable fibrinolysis inhibitor attenuates (DD)E-mediated stimulation of plasminogen activation by reducing the affinity of (DD)E for tissue plasminogen activator. A potential mechanism for enhancing the fibrin specificity of tissue plasminogen activator

A complex of d-dimer noncovalently associated with fragment E ((DD)E), a degradation product of cross-linked fibrin that binds tissue plasminogen activator (t-PA) and plasminogen (Pg) with affinities similar to those of fibrin, compromises the fibrin specificity of t-PA by stimulating systemic Pg activation. In this study, we examined the effect of thrombin-activable fibrinolysis inhibitor (TAFI), a latent carboxypeptidase B (CPB)-like enzyme, on the stimulatory activity of (DD)E. Incubation of (DD)E with activated TAFI (TAFIa) or CPB (a) produces a 96% reduction in the capacity of (DD)E to stimulate t-PA-mediated activation of Glu- or Lys-Pg by reducing k(cat) and increasing K(m) for the reaction; (b) induces the release of 8 mol of lysine/mol of (DD)E, although most of the stimulatory activity is lost after release of only 4 mol of lysine/mol (DD)E; and (c) reduces the affinity of (DD)E for Glu-Pg, Lys-Pg, and t-PA by 2-, 4-, and 160-fold, respectively. Because TAFIa- or CPB-exposed (DD)E produces little stimulation of Glu-Pg activation by t-PA, (DD)E is not degraded into fragment E and d-dimer, the latter of which has been reported to impair fibrin polymerization. These data suggest a novel role for TAFIa. By attenuating systemic Pg activation by (DD)E, TAFIa renders t-PA more fibrin-specific.

Identification of the mechanism responsible for the increased fibrin specificity of TNK-tissue plasminogen activator relative to tissue plasminogen activator

TNK-tissue plasminogen activator (TNK-t-PA), a bioengineered variant of tissue-type plasminogen activator (t-PA), has a longer half-life than t-PA because the glycosylation site at amino acid 117 (N117Q, abbreviated N) has been shifted to amino acid 103 (T103N, abbreviated T) and is resistant to inactivation by plasminogen activator inhibitor 1 because of a tetra-alanine substitution in the protease domain (K296A/H297A/R298A/R299A, abbreviated K). TNK-t-PA is more fibrin-specific than t-PA for reasons that are poorly understood. Previously, we demonstrated that the fibrin specificity of t-PA is compromised because t-PA binds to (DD)E, the major degradation product of cross-linked fibrin, with an affinity similar to that for fibrin. To investigate the enhanced fibrin specificity of TNK-t-PA, we compared the kinetics of plasminogen activation for t-PA, TNK-, T-, K-, TK-, and NK-t-PA in the presence of fibrin, (DD)E or fibrinogen. Although the activators have similar catalytic efficiencies in the presence of fibrin, the catalytic efficiency of TNK-t-PA is 15-fold lower than that for t-PA in the presence of (DD)E or fibrinogen. The T and K mutations combine to produce this reduction via distinct mechanisms because T-containing variants have a higher K(M), whereas K-containing variants have a lower k(cat) than t-PA. These results are supported by data indicating that T-containing variants bind (DD)E and fibrinogen with lower affinities than t-PA, whereas the K and N mutations have no effect on binding. Reduced efficiency of plasminogen activation in the presence of (DD)E and fibrinogen but equivalent efficiency in the presence of fibrin explain why TNK-t-PA is more fibrin-specific than t-PA.