A monoclonal antibody specific for Lys-plasminogen. Application to the study of the activation pathways of plasminogen in vivo

Proteoform-Resolved Profiling of Plasminogen Activation Reveals Novel Abundant Phosphorylation Site and Primary N-Terminal Cleavage Site

Plasminogen (Plg), the zymogen of plasmin (Plm), is a glycoprotein involved in fibrinolysis and a wide variety of other physiological processes. Plg dysregulation has been implicated in a range of diseases. Classically, human Plg is categorized into two types, supposedly having different functional features, based on the presence (type I) or absence (type II) of a single N-linked glycan. Using high-resolution native mass spectrometry, we uncovered that the proteoform profiles of human Plg (and Plm) are substantially more extensive than this simple binary classification. In samples derived from human plasma, we identified up to 14 distinct proteoforms of Plg, including a novel highly stoichiometric phosphorylation site at Ser339. To elucidate the potential functional effects of these post-translational modifications, we performed proteoform-resolved kinetic analyses of the Plg-to-Plm conversion using several canonical activators. This conversion is thought to involve at least two independent cleavage events: one to remove the N-terminal peptide and another to release the active catalytic site. Our analyses reveal that these processes are not independent but are instead tightly regulated and occur in a step-wise manner. Notably, N-terminal cleavage at the canonical site (Lys77) does not occur directly from intact Plg. Instead, an activation intermediate corresponding to cleavage at Arg68 is initially produced, which only then is further processed to the canonical Lys77 product. Based on our results, we propose a refined categorization for human Plg proteoforms. In addition, we reveal that the proteoform profile of human Plg is more extensive than that of rat Plg, which lacks, for instance, the here-described phosphorylation at Ser339.

Plasminogen: an enigmatic zymogen

Plasminogen is an abundant plasma protein that exists in various zymogenic forms. Plasmin, the proteolytically active form of plasminogen, is known for its essential role in fibrinolysis. To date, therapeutic targeting of the fibrinolytic system has been for 2 purposes: to promote plasmin generation for thromboembolic conditions or to stop plasmin to reduce bleeding. However, plasmin and plasminogen serve other important functions, some of which are unrelated to fibrin removal. Indeed, for >40 years, the antifibrinolytic agent tranexamic acid has been administered for its serendipitously discovered skin-whitening properties. Plasmin also plays an important role in the removal of misfolded/aggregated proteins and can trigger other enzymatic cascades, including complement. In addition, plasminogen, via binding to one of its dozen cell surface receptors, can modulate cell behavior and further influence immune and inflammatory processes. Plasminogen administration itself has been reported to improve thrombolysis and to accelerate wound repair. Although many of these more recent findings have been derived from in vitro or animal studies, the use of antifibrinolytic agents to reduce bleeding in humans has revealed additional clinically relevant consequences, particularly in relation to reducing infection risk that is independent of its hemostatic effects. The finding that many viruses harness the host plasminogen to aid infectivity has suggested that antifibrinolytic agents may have antiviral benefits. Here, we review the broadening role of the plasminogen-activating system in physiology and pathophysiology and how manipulation of this system may be harnessed for benefits unrelated to its conventional application in thrombosis and hemostasis.

Assessing Plasmin Generation in Health and Disease

Fibrinolysis is an important process in hemostasis responsible for dissolving the clot during wound healing. Plasmin is a central enzyme in this process via its capacity to cleave fibrin. The kinetics of plasmin generation (PG) and inhibition during fibrinolysis have been poorly understood until the recent development of assays to quantify these metrics. The assessment of plasmin kinetics allows for the identification of fibrinolytic dysfunction and better understanding of the relationships between abnormal fibrin dissolution and disease pathogenesis. Additionally, direct measurement of the inhibition of PG by antifibrinolytic medications, such as tranexamic acid, can be a useful tool to assess the risks and effectiveness of antifibrinolytic therapy in hemorrhagic diseases. This review provides an overview of available PG assays to directly measure the kinetics of plasmin formation and inhibition in human and mouse plasmas and focuses on their applications in defining the role of plasmin in diseases, including angioedema, hemophilia, rare bleeding disorders, COVID-19, or diet-induced obesity. Moreover, this review introduces the PG assay as a promising clinical and research method to monitor antifibrinolytic medications and screen for genetic or acquired fibrinolytic disorders.

DNA-bound elastase of neutrophil extracellular traps degrades plasminogen, reduces plasmin formation, and decreases fibrinolysis: proof of concept in septic shock plasma

Activation of platelets and neutrophils in septic shock results in the formation of microvascular clots containing an intricate scaffold of fibrin with neutrophil extracellular traps (NETs) DNA. NETs contain multiple components that might impact endogenous fibrinolysis, resulting in failure to lyse clots in the microcirculation and residual systemic microthrombosis. We propose herein that the reservoir of human neutrophil elastase (HNE) on NETs may directly interfere with the fibrinolytic mechanism via a plasminogen proteolytic pathway. To investigate this mechanism, we constructed fibrin-NETs matrices by seeding and activating neutrophils onto a fibrin surface and monitored plasminogen activation or degradation. We demonstrate that the elastase activity of HNE-DNA complexes is protected from inhibition by plasma antiproteases and sustains its ability to degrade plasminogen. Using mass spectrometry proteomic analysis, we identified plasminogen fragments composed of kringle (K) domains (K₁₊₂₊₃, k_1+2+3+4) and the serine protease (SP) region (K₅-SP). We further demonstrate that patients with septic shock with disseminated intravascular coagulation have circulating HNE-DNA complexes, HNE-derived plasminogen fragments, a low plasminogen concentration, and a reduced capacity to generate plasmin onto fibrin. In conclusion, we show that NETs bearing active HNE-DNA complexes reduce plasminogen into fragments, thus impairing fibrinolysis by decreasing the local plasminogen concentration, plasminogen binding to fibrin, and localized plasmin formation.-Barbosa da Cruz, D., Helms, J., Aquino, L. R., Stiel, L., Cougourdan, L., Broussard, C., Chafey, P., Riès-Kautt, M., Meziani, F., Toti, F., Gaussem, P., Anglés-Cano, E. DNA-bound elastase of neutrophil extracellular traps degrades plasminogen, reduces plasmin formation, and decreases fibrinolysis: proof of concept in septic shock plasma.

Amplified endogenous plasmin activity resolves acute thrombotic thrombocytopenic purpura in mice

Essentials Plasmin is able to proteolyse von Willebrand factor. It was unclear if plasmin influences acute thrombotic thrombocytopenic purpura (TTP). Plasmin levels are increased during acute TTP though suppressed via plasmin(ogen) inhibitors. Allowing amplified endogenous plasmin activity in mice results in resolution of TTP signs.

SUMMARY

Background Thrombotic thrombocytopenic purpura (TTP) is an acute life-threatening pathology, caused by occlusive von Willebrand factor (VWF)-rich microthrombi that accumulate in the absence of ADAMTS-13. We previously demonstrated that plasmin can cleave VWF and that plasmin is generated in patients during acute TTP. However, the exact role of plasmin in TTP remains unclear. Objectives Investigate if endogenous plasmin-mediated proteolysis of VWF can influence acute TTP episodes. Results In mice with an acquired ADAMTS-13 deficiency, plasmin is generated during TTP as reflected by increased plasmin-α2-antiplasmin (PAP)-complex levels. However, mice still developed TTP, suggesting that this increase is not sufficient to control the pathology. As mice with TTP also had increased plasminogen activator inhibitor 1 (PAI-1) levels, we investigated whether blocking the plasmin(ogen) inhibitors would result in the generation of sufficient plasmin to influence TTP outcome in mice. Interestingly, when amplified plasmin activity was allowed (α2-antiplasmin-/- mice with inhibited PAI-1) in mice with an acquired ADAMTS-13 deficiency, a resolution of TTP signs was observed as a result of an increased proteolysis of VWF. In line with this, in patients with acute TTP, increased PAP-complex and PAI-1 levels were also observed. However, neither PAP-complex levels nor PAI-1 levels were related to TTP signs and outcome. Conclusions In conclusion, endogenous plasmin levels are increased during acute TTP, although limited via suppression through α2-antiplasmin and PAI-1. Only when amplified plasmin activity is allowed, plasmin can function as a back-up for ADAMTS-13 in mice and resolve TTP signs as a result of an increased proteolysis of VWF.

Label-Free Kinetic Studies of Hemostasis-Related Biomarkers Including D-Dimer Using Autologous Serum Transfusion

The objective of this study was to evaluate the elimination kinetics of hemostasis-related biomarkers including the prothrombin activation fragment F1+2, thrombin-antithrombin complex (TAT), plasmin-α₂-antiplasmin complex (PAP), and D-dimer in humans. Autologous serum was used as a biomarker source and infused into 15 healthy volunteers. Serum was prepared from whole blood in the presence of recombinant tissue-type plasminogen activator (final concentration 20 μg/mL) to induce plasmin generation required for PAP and D-dimer formation. Serum transfusions (50 mL/30 min) were well tolerated by all subjects. Endogenous thrombin formation was not induced by serum infusions as measured using a highly sensitive oligonucleotide-based enzyme capture assay. Median peak levels (x-fold increase over baseline) of F1+2, TAT, PAP, and D-dimer of 3.7 nmol/L (28.9), 393 ng/mL (189.6), 3,829 ng/mL (7.0), and 13.4 mg/L (34.2) were achieved at the end of serum infusions. During a 48 h lasting follow-up period all biomarkers showed elimination kinetics of a two-compartment model. Median (interquartile range) terminal half-lives were 1.9 (1.3–3.6) h for F1+2, 0.7 (0.7–2.6) h for TAT, and 10.8 (8.8–11.4) h for PAP. With 15.8 (13.1–23.1) h the D-dimer half-life was about twice as long as previously estimated from radiolabeling studies in animals and small numbers of human subjects. The serum approach presented here allows label-free and simultaneous analysis of the elimination kinetics of various hemostasis-related biomarkers. Based on these data changes in biomarker levels could more precisely used to estimate the activity level of the hemostatic system.

Crystal structure of the native plasminogen reveals an activation-resistant compact conformation

BACKGROUND

Plasminogen is the zymogen form of plasmin and the precursor of angiostatin. It has been implicated in a variety of disease states, including thrombosis, bleeding and cancers. The native plasminogen, known as Glu-plasminogen, contains seven domains comprising the N-terminal peptide domain (NTP), five kringle domains (K1-K5) and the C-terminal serine protease domain (SP). Previous studies have established that the lysine binding site (LBS) of the conserved kringle domains plays a crucial role in mediating the regulation of plasminogen function. However, details of the related conformational mechanism are unknown.

OBJECTIVES

We aim to understand in more detail the conformational mechanism of plasminogen activation involving the kringles.

METHODS

We crystallized the native plasminogen under physiologically relevant conditions and determined the structure at 3.5 Å resolution. We performed structural analyses and related these to the literature data to gain critical understanding of the plasminogen activation.

RESULTS AND CONCLUSIONS

The structure reveals the precise architecture of the quaternary complex. It shows that the Glu-plasminogen renders its compact form as an activation-resistant conformation for the proteolytic activation. The LBSs of all kringles, except K1, are engaged in intra-molecular interactions while only K1-LBS is readily available for ligand binding or receptor anchorage. The structure also provides insights into the interactions between plasminogen and α2-antiplasmin, the primary physiological inhibitor of plasmin. Furthermore, the data presented explain why a conformational transition to the open form is necessary for plasminogen activation as well as angiostatin generation, and provide a rationale for the functional hierarchy of the different kringles.

Serine-proteases as plasminogen activators in terms of fibrinolysis

OBJECTIVES

This review should give an overview about the natural human plasminogen activators and their various modified variants as well as similar substances isolated from animals, microorganisms and plants. When a blood clot is formed in a blood vessel, it avoids the oxygen supply of the surrounding tissue. A fast fibrinolytic therapy should redissolve the blood vessel and reduce the degradation of the tissue. All proteases that are part of the human blood coagulation and fibrinolytic system belong to the serine protease family. t-PA (tissue plasminogen activator) and u-PA (urokinase plasminogen activator) are the naturally occurring fibrinolytic agents that are also used in therapy.

KEY FINDINGS

Despite many years of research, t-PA is still the gold standard in fibrinolytic therapy. But it has to be given as an infusion, which needs time. Modified fibrinolytic substances are, were, or perhaps will be in the market. They have different advantages over t-PA, but often the disadvantages predominate.

CONCLUSION

Many substances have been developed but an optimal fibrinolytic agent combined with a simple administration is not in therapeutic use to date.

Inactivation of ADAMTS13 by plasmin as a potential cause of thrombotic thrombocytopenic purpura

BACKGROUND

ADAMTS13 deficiency causes accumulation of unusually large von Willebrand factor molecules, which cross-link platelets in the circulation or on the endothelial surface. This process of intravascular agglutination leads to the microangiopathy thrombotic thrombocytopenic purpura (TTP). Most TTP patients have acquired anti-ADAMTS13 autoantibodies that inhibit enzyme function and/or clear it from the circulation. However, the reason for ADAMTS13 deficiency is not always easily identified in a subset of patients.

OBJECTIVES

To determine the origin of ADAMTS13 deficiency in a case of acquired TTP.

METHODS

Western blotting of ADAMTS13 in plasmas from acute and remission phases was used.

RESULTS

The ADAMTS13 deficiency was not caused by mutations or (detectable) autoantibodies; however, an abnormal ADAMTS13 truncated fragment (100 kDa) was found in acute-phase but not remission-phase plasma. This fragment resulted from enzymatic proteolysis, as recombinant ADAMTS13 was also cleaved when in the presence of acute-phase but not remission-phase plasma. Inhibitor screening showed that ADAMTS13 was cleaved by a serine protease that could be dose-dependently inhibited by addition of exogenous α₂ -antiplasmin. Examination of the endogenous α₂-antiplasmin antigen and activity confirmed deficiency of α₂ -antiplasmin function in acute-phase but not remission-phase plasma. To investigate the possibility of ADAMTS13 cleavage by plasmin in plasma, urokinase-type plasminogen activator was added to an (unrelated) congenital α₂ -antiplasmin-deficient plasma sample to activate plasminogen. This experiment confirmed cleavage of endogenous ADAMTS13 similar to that observed in our TTP patient.

CONCLUSION

We report the first acquired TTP patient with cleaved ADAMTS13 and show that plasmin is involved.

Insight into the profibrinolytic activity of dermatan sulfate: effects on the activation of plasminogen mediated by tissue and urinary plasminogen activators

INTRODUCTION

Dermatan sulfate (DS) is well-known for its anticoagulant activity through binding to heparin cofactor II to enhance antithrombin action. It has also been suggested that DS has a profibrinolytic effect, although the exact molecular mechanism is as yet unknown.

MATERIALS AND METHODS

An in vitro amidolytic method was used to study the effect of high and low molecular weight-DS on the activation of Glu and Lys-plasminogen by tissue and urinary plasminogen activators (t-PA and u-PA).

RESULTS

Both high and low molecular weight-DS exhibited a stimulating effect on the activation of plasminogen by PAs. Interestingly, high molecular weight-DS stimulated Glu and Lys-plasminogen activation by t-PA and u-PA in a way and to an extent similar to that in which fibrin(ogen) degradation products (PDF) increased the t-PA assay. Meanwhile low molecular weight-DS had a lower effect. No DS had any effect on plasmin or u-PA amidolytic activity. The facilitation of the conversion of Glu-plasminogen to plasmin in the presence of DS was confirmed by SDS-PAGE; high molecular weight-DS effect was greater than low molecular weight-DS in accordance with the chromogenic assays. Moreover, the combination of PDF and high and low molecular weight-DS, respectively, did not further stimulate t-PA activation of either Glu or Lys-plasminogen suggesting that both substances may compete for the same binding sites.

CONCLUSIONS

Through in vitro assays we demonstrated that high and low molecular weight-DS enhance plasminogen activation by u-PA and t-PA, suggesting that the profibrinolytic activity of DS might be via potentiation of plasminogen conversion to plasmin.

Functional hierarchy of plasminogen kringles 1 and 4 in fibrinolysis and plasmin-induced cell detachment and apoptosis

Plasmin(ogen) kringles 1 and 4 are involved in anchorage of plasmin(ogen) to fibrin and cells, an essential step in fibrinolysis and pericellular proteolysis. Their contribution to these processes was investigated by selective neutralization of their lysine-binding function. Blocking the kringle 1 lysine-binding site with monoclonal antibody 34D3 fully abolished binding and activation of Glu-plasminogen and prevented both fibrinolysis and plasmin-induced cell detachment-induced apoptosis. In contrast, blocking the kringle 4 lysine-binding site with monoclonal antibody A10.2 did not impair its activation although it partially inhibited plasmin(ogen) binding, fibrinolysis and cell detachment. This remarkable, biologically relevant, distinctive response was not observed for plasmin or Lys-plasminogen; each antibody inhibited their binding and activation of Lys-plasminogen to a limited extent, and full inhibition of fibrinolysis required simultaneous neutralization of both kringles. Thus, in Lys-plasminogen and plasmin, kringles 1 and 4 act as independent and complementary domains, both able to support binding and activation. We conclude that Glu-/Lys-plasminogen and plasmin conformations are associated with transitions in the lysine-binding function of kringles 1 and 4 that modulate fibrinolysis and pericellular proteolysis and may be of biological relevance during athero-thrombosis and inflammatory states. These findings constitute the first biological link between plasmin(ogen) transitions and functions.

Molecular mechanisms of fibrinolysis

The molecular mechanisms that finely co-ordinate fibrin formation and fibrinolysis are now well defined. The structure and function of all major fibrinolytic proteins, which include serine proteases, their inhibitors, activators and receptors, have been characterized. Measurements of real time, dynamic molecular interactions during fibrinolysis of whole blood clots can now be carried out in vitro. The development of gene-targeted mice deficient in one or more fibrinolytic protein(s) has demonstrated expected and unexpected roles for these proteins in both intravascular and extravascular settings. In addition, genetic analysis of human deficiency syndromes has revealed specific mutations that result in human disorders that are reflective of either fibrinolytic deficiency or excess. Elucidation of the fine control of fibrinolysis under different physiological and pathological haemostatic states will undoubtedly lead to novel therapeutic interventions. Here, we review the fundamental features of intravascular plasmin generation, and consider the major clinical syndromes resulting from abnormalities in fibrinolysis.

Quantification of carbamylated LDL in human sera by a new sandwich ELISA

BACKGROUND

We previously suggested that increased carbamylated LDL (cLDL), a product of nonenzymatic modification of LDL in human serum by urea-derived cyanate, may cause cardiovascular complications in patients with chronic renal insufficiency. An assay for precise measurement of cLDL in serum was not previously available.

METHODS

Polyclonal antibodies against human cLDL and nonmodified, native LDL (nLDL) were raised in rabbits and extensively purified by affinity chromatography. New sandwich ELISAs to measure cLDL and nLDL with use of these antibodies were developed. Serum concentrations of cLDL and nLDL were measured by the sandwich ELISAs in 41 patients with end-stage renal disease (ESRD) and 40 healthy controls.

RESULTS

Both assays showed satisfactory reproducibility, linearity, and recovery. The assays could detect 2.7 mg/L cLDL with a linear detection range of 5-1000 mg/L and 5 mg/L nLDL with a linear detection range of 50-1000 mg/L. These measurements showed that patients with ESRD have significantly increased serum cLDL [281.5 (46.9) mg/L compared with 86.1 (29.7) mg/L in a control group; P <0.001]. There was no significant difference in nLDL concentrations between the groups.

CONCLUSIONS

These assays are a potentially valuable tool for cardiovascular research in renal patients and healthy individuals. The cLDL concentration appears to be the highest among all previously described modified LDL isoforms in both controls and ESRD patients.

What the structure of angiostatin may tell us about its mechanism of action

Originally discovered in 1994 by Folkman and coworkers, angiostatin was identified through its antitumor effects in mice and later shown to be a potent inhibitor of angiogenesis. An internal fragment of plasminogen, angiostatin consists of kringle domains that are known to be lysine-binding. The crystal structure of angiostatin was the first multikringle domain-containing structure to be published. This review will focus on what is known about the structure of angiostatin and its implications in function from the current literature.

Tumor necrosis factor-alpha and angiostatin are mediators of endothelial cytotoxicity in bronchoalveolar lavages of patients with acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is characterized by an extensive alveolar capillary leak, permitting contact between intra-alveolar factors and the endothelium. To investigate whether factors contained in the alveolar milieu induce cell death in human lung microvascular endothelial cells, we exposed these cells in vitro to bronchoalveolar lavage fluid (BALF) supernatants from control patients, patients at risk of developing ARDS, and patients with early- and late-phase ARDS. In contrast to BALF from control patients, a significant cytotoxicity was found in BALF from patients at risk of developing ARDS, with late-phase ARDS, and especially from patients with early-phase ARDS. Subsequently, we determined the levels of factors known to exert cytotoxicity in endothelial cells, i.e., tumor necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta1, and angiostatin. BALF from patients at risk of developing ARDS, with early-phase ARDS, and with late-phase ARDS, contained increased levels of TNF-alpha and angiostatin, but not of TGF-beta1, as compared with BALF from control patients. Whereas inhibition of TGF-beta1 had no effect in this setting, neutralization of TNF-alpha or angiostatin inhibited the cytotoxic activity on endothelial cells of part of the early-phase ARDS BALF. These results indicate that TNF-alpha and angiostatin may contribute to ARDS-related endothelial injury.

Enzyme-linked immunosorbent assay for the specific detection of angiostatin-like plasminogen moieties in biological samples

An enzyme-linked immunosorbent assay (ELISA) was developed for the specific detection of human angiostatin-like plasminogen moieties (comprising kringles 1-4) in biological samples. The assay involves prior removal of all other plasminogen moieties by immunoadsorption of diluted samples (to about 10 ng/ml plasminogen) with a mixture of insolubilized MA-42B12 (directed against kringle 5) and MA-31E9 (directed against the proteinase domain). The recovery of angiostatin during this procedure is > or = 95%. Subsequently, angiostatin-like fragments are detected in an ELISA, based on two monoclonal antibodies reacting with nonoverlapping epitopes in the kringle 1-3 domain: MA-36E6 for capture and MA-34D3 for tagging. The assay has a lower detection limit of about 0.1 ng/ml and is performed with intra- and interassay coefficient of variation of 2.4% and 15%. In tumor fluids obtained from cancer patients (n = 10), angiostatin levels ranged between 0.24 and 6.7 microg/ml (1.62+/-0.60 microg/ml; mean+/-S.E.M.) The identity of angiostatin was confirmed by immunoblotting using specific monoclonal antibodies. A weak correlation (r = .66) was observed with the total plasminogen concentration in these samples. This ELISA thus appears suitable for the specific quantitation of angiostatin-like plasminogen moieties in biological samples, and may be useful to study its (patho)physiological relevance.

Epsilon amino caproic acid inhibits streptokinase-plasminogen activator complex formation and substrate binding through kringle-dependent mechanisms

Lysine side chains induce conformational changes in plasminogen (Pg) that regulate the process of fibrinolysis or blood clot dissolution. A lysine side-chain mimic, epsilon amino caproic acid (EACA), enhances the activation of Pg by urinary-type and tissue-type Pg activators but inhibits Pg activation induced by streptokinase (SK). Our studies of the mechanism of this inhibition revealed that EACA (IC(50) 10 microM) also potently blocked amidolytic activity by SK and Pg at doses nearly 10000-fold lower than that required to inhibit the amidolytic activity of plasmin. Different Pg fragments were used to assess the role of the kringles in mediating the inhibitory effects of EACA: mini-Pg which lacks kringles 1-4 of Glu-Pg and micro-Pg which lacks all kringles and contains only the catalytic domain. SK bound with similar affinities to Glu-Pg (K(A) = 2.3 x 10(9) M(-1)) and to mini-Pg (K(A) = 3.8 x 10(9) M(-)(1)) but with significantly lower affinity to micro-Pg (K(A) = 6 x 10(7) M(-)(1)). EACA potently inhibited the binding of Glu-Pg to SK (K(i) = 5.7 microM), but was less potent (K(i) = 81.1 microM) for inhibiting the binding of mini-Pg to SK and had no significant inhibitory effects on the binding of micro-Pg and SK. In assays simulating substrate binding, EACA also potently inhibited the binding of Glu-Pg to the SK-Glu-Pg activator complex, but had negligible effects on micro-Pg binding. Taken together, these studies indicate that EACA inhibits Pg activation by blocking activator complex formation and substrate binding, through a kringle-dependent mechanism. Thus, in addition to interactions between SK and the protease domain, interactions between SK and the kringle domain(s) play a key role in Pg activation.

Circulating Activated Platelets Assist THP-1 Monocytoid/Endothelial Cell Interaction Under Shear Stress

Circulating complexes of leukocytes and activated platelets are markers for atherosclerosis, but their interaction with the arterial endothelial lining has not been studied. Therefore, the effect of activated platelets on rolling and adhesion of labeled human THP-1 monocytoid cells to human umbilical vein endothelial cell (HUVEC) monolayers was studied by epifluorescence microscopy in a parallel plate flow chamber. In the absence of activated platelets, THP-1 rolling on resting HUVEC was negligible at shear rates greater than 300 s−1. Activation of HUVEC with 100 nmol/L phorbol myristate acetate (PMA) increased THP-1 cell adhesion at shear rates less than 400 s−1. Therefore, a shear rate of 400 s−1 was identified as a threshold for THP-1 adhesion. THP-1 rolling on activated HUVEC was reduced by 64% after L-selectin inhibition but was not affected by P-selectin inhibition. The addition of 1 to 50 thrombin receptor-activating peptide (TRAP)-activated platelets per THP-1 cell enhanced interactions between THP-1 cells and HUVEC, resulting in a steep bell-shaped dose-response curve, with a peak of 10 ± 3 rolling cells/50 seconds at 3 platelets per THP-1 cell (P &lt; .01v control) with a concomitant 2- to 3-fold increase of firmly adhering cells (P &lt; .01 v control). In reconstituted blood, low numbers of activated platelets had the same effect on THP-1 rolling and adhesion. P-selectin inhibition reduced platelet/THP-1 cell interaction in suspension and deposition of the complexes on the endothelial monolayer. Inhibition of both P- and L-selectin reduced THP-1/HUVEC interactions to 14% (P &lt; .01, n = 4). Sialidase digestion and removal of terminal sialic acid residues from HUVEC or THP-1 cells but not from platelets abolished the platelet mediated augmentation of THP-1 cell adhesion. Thus, THP-1 rolling on HUVEC is shear-dependent and largely mediated by L-selectin. P-selectin expressed on activated platelets increases monocytoid cell adhesion to endothelial cells at shear rates found in coronary arteries through interactions with both endothelial and monocytoid cells and may facilitate macrophage accumulation in the vessel wall.

A catalytic switch and the conversion of streptokinase to a fibrin-targeted plasminogen activator

Plasminogen (Pg) activators such as streptokinase (SK) save lives by generating plasmin to dissolve blood clots. Some believe that the unique ability of SK to activate Pg in the absence of fibrin limits its therapeutic utility. We have found that SK contains an unusual NH(2)-terminal "catalytic switch" that allows Pg activation through both fibrin-independent and fibrin-dependent mechanisms. Unlike SK, a mutant (rSKDelta59) fusion protein lacking the 59 NH(2)-terminal residues was no longer capable of fibrin-independent Pg activation (k(cat)/K(m) decreased by >600-fold). This activity was restored by coincubation with equimolar amounts of the NH(2)-terminal peptide rSK1-59. Deletion of the NH(2) terminus made rSKDelta59 a Pg activator that requires fibrin, but not fibrinogen, for efficient catalytic function. The fibrin-dependence of the rSKDelta59 activator complex apparently resulted from selective catalytic processing of fibrin-bound Pg substrates in preference to other Pg forms. Consistent with these observations, the presence (rSK) or absence (rSKDelta59) of the SK NH(2)-terminal peptide markedly altered fibrinolysis of human clots suspended in plasma. Like native SK, rSK produced incomplete clot lysis and complete destruction of plasma fibrinogen; in contrast, rSKDelta59 produced total clot lysis and minimal fibrinogen degradation. These studies indicate that structural elements in the NH(2) terminus are responsible for SK's unique mechanism of fibrin-independent Pg activation. Because deletion of the NH(2) terminus alters SK's mechanism of action and targets Pg activation to fibrin, there is the potential to improve SK's therapeutic efficacy.

Generation of an angiostatin-like fragment from plasminogen by stromelysin-1 (MMP-3)

Matrix metalloproteinase-3 (MP-3 or stromelysin-1) specifically hydrolyzes the Glu59-Asn60, Pro447-Val448, and Pro544-Ser545 peptide bonds in plasminogen, yielding a 55 kDa NH2-terminal angiostatin-like domain (comprising kringles 1-4), a 14 kDa domain comprising kringle 5, and a 30 kDa domain comprising the serine proteinases domain. The conversion is completely abolished in the presence of the MMP inhibitors EDTA or 1,10-phenanthroline. Biospecific interactions analysis indicates that binding of proMMP-3 and MMP-3 to plasminogen occurs with comparable affinity (KA of 4.7 x 10(6) and 4.1 x 10(6) M-1, respectively) and is mediated via the miniplasminogen moiety (kringle 5 plus the proteinase domain) and via the catalytic domain of MMP-3. Thus, proteolytic cleavage of plasminogen by MMP-3 generates angiostatin-like fragments.

The kringle domains of human plasminogen

The mature form of the zymogen, human plasminogen (HPlg), contains 791 amino acids present in a single polypeptide chain. The fibrinolytic enzyme, human plasmin (HPlm), is formed from HPlg as a result of activator-catalysed cleavage of the Arg561-Val562 peptide bond in HPlg. The resulting HPlm contains a heavy chain of 561 amino acid residues, originating from the N-terminus of HPlg, doubly disulfide-linked to a light chain of 230 amino acid residues. This latter region, containing the C-terminus of HPlg, is homologous to serine proteases such as trypsin and elastase. The heavy chain of HPlm consists of five repeating triple-disulfide-linked peptide regions, c. 80 amino acid residues in length, termed kringles (K), that are responsible for interactions of HPlg and HPlm with substrates, inhibitors and regulators of HPlg activation. Important among the ligands of the kringles are positive activation effectors, typified by lysine and its analogues, and negative activation effectors, such as Cl-. The kringle domains of HPlg that participate in these binding interactions are K1, K4 and K5, and perhaps K2. These modules appear to function as independent domains. The amino acid residues important in these kringle/ligand binding interactions have been proposed by structural determinations, and their relative importance quantified by site-directed mutagenesis experimentation.

Screening panels of monoclonal antibodies using phage-displayed antigen

A procedure is described to screen panels of hybridomas or purified monoclonal antibodies using antigen displayed on the surface of filamentous bacteriophage. In this system, samples containing murine monoclonal antibodies are incubated with phage-displayed antigen in microtiter plates coated with rabbit anti-mouse IgG, and bound antibody-phage complex is detected with horseradish peroxidase-sheep anti-phage M13 conjugate. The assay has been validated with a panel of 16 monoclonal antibodies directed against human plasminogen, using phage-displayed miniplasmin-(ogen) (amino acids Ala444 through Asn791 comprising kringle 5 and the proteinase domain of plasminogen) or microplasminogen (amino acids Ala543 through Asn791 comprising the proteinase domain). Six monoclonal antibodies were identified directed against miniplasminogen and miniplasmin; this was confirmed using a microtiter plate coated with antigens. One of these monoclonal antibodies (MA-42B12) did not react with microplasminogen, suggesting that its epitope is comprised within the kringle 5 domain. This test is rapid and sensitive (detecting 10-20 ng/ml of monoclonal antibody), and screening can be performed using phage-displayed zymogens or active enzymes or selected domains thereof. The procedure eliminates the need for large amounts of purified antigen for screening. Furthermore, immunization can be performed with partially purified antigen because only antibodies raised against the antigen of interest will be identified with the use of phage-displayed antigen. Therefore, this test may offer distinct advantages over the classical one-site enzyme-linked immunosorbent assay using antigen-coated microtiter plates.

Mechanisms of physiological fibrinolysis

The fibrinolytic system comprises an inactive proenzyme, plasminogen, that is converted by plasminogen activators to the active enzyme, plasmin, which degrades fibrin. Two immunologically distinct plasminogen activators (PA) have been identified: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). t-PA mediated plasminogen activation is mainly involved in the dissolution of fibrin in the circulation, whereas u-PA mediated plasminogen activation mainly plays a role in pericellular proteolysis. Plasminogen activation is regulated by specific molecular interactions between its main components, such as binding of plasminogen and t-PA to fibrin, or to specific cellular receptors resulting in enhanced plasminogen activation, inhibition of t-PA and u-PA by plasminogen activator inhibitors (PAI) and inhibition of plasmin by alpha 2-antiplasmin. Controlled synthesis and release of PAs and PAIs primarily from endothelial cells also contributes to the regulation of physiological fibrinolysis. The lysine binding sites situated in the kringle structures of plasminogen play a crucial role in the regulation of fibrinolysis by modulating its binding to fibrin and to cell surfaces, and by controlling the inhibition rate of plasmin by alpha 2-antiplasmin.

Recombinant lys-plasminogen, but not glu-plasminogen, improves recombinant tissue-type plasminogen activator-induced coronary thrombolysis in dogs

OBJECTIVES

This study examined the modification of recombinant tissue-type plasminogen activator (rt-PA)-induced thrombolysis by recombinant lys-plasminogen.

BACKGROUND

Recombinant tissue-type plasminogen activator restores flow in the thrombosed coronary artery, but the artery often reoccludes. The rt-PA-induced thrombolysis is a result of activation of plasminogen bound to fibrin in the thrombus and results in generation of the fibrinolytic enzyme plasmin. Small amounts of lys-plasminogen are formed when rt-PA is used. Lys-plasminogen binds to fibrin with a 10-fold greater affinity than the predominant native glu-plasminogen, leading to a loose fibrin structure.

METHODS

Dogs with electrically induced occlusive intracoronary thrombus were treated with saline solution (n = 9), glu-plasminogen (2 mg/kg body weight, n = 5) or lys-plasminogen (2 mg/kg, n = 5), followed by infusion of rt-PA (1 mg/kg over 20 min) 10 min later.

RESULTS

Reperfusion rates were similar in all groups of dogs, but the time to reflow was lowest in dogs given lys-plasminogen compared with those given saline solution or glu-plasminogen before rt-PA (mean [+/- SE] 14 +/- 2 vs. 22 +/- 2 and 23 +/- 3 min, respectively, p < 0.05). None of the reperfused coronary arteries reoccluded in the lys-plasminogen plus rt-PA group, whereas 75% reoccluded in dogs given saline solution plus rt-PA, and 50% reoccluded in those given glu-plasminogen plus rt-PA. Accordingly, duration of reflow was greater in the lys-plasminogen plus rt-PA group (> 120 vs. 39 +/- 7 and 82 +/- 21 min, respectively, p < 0.05). Plasminogen activator inhibitor-1 activity decreased during rt-PA infusion and thereafter increased in all dogs, but less so in dogs given lys-plasminogen (p < 0.05 vs. those given saline solution before rt-PA).

CONCLUSIONS

Treatment with recombinant lys-plasminogen before rt-PA reduces time to reflow and sustains reflow after thrombolysis, whereas glu-plasminogen has no such effect.

Overview on fibrinolysis: plasminogen activation pathways on fibrin and cell surfaces

Plasminogen activation at the surface of fibrin or of cell membranes is a sophisticated specialized system for localized extracellular proteolysis implicated in a large variety of biological functions (fibrinolysis, cell migration and extracellular matrix degradation). Assembly of plasminogen and/or activators at specific binding sites induces conformational changes that make accessible the scissile peptide bond of plasminogen and exposes the active centre of the tissue-type plasminogen activator. The mechanism of activation by pro-urokinase, a second type of activator that binds to cell membrane but not to fibrin, is far from being understood. It may be able, however, in contrast to urokinase, to specifically activate plasminogen bound to partially degraded fibrin. An extremely low Km and high catalytic rate are characteristic of the process of activation at surfaces. In contrast, activation in liquid phase by tissue-type plasminogen activator proceeds at an extremely low catalytic rate. The initiation and amplification of plasminogen activation depend on specific interactions between the modular constitutive units of these proteins and binding sites present on cell or fibrin surfaces. Thus, the most important mechanism for the acceleration of fibrinolysis and pericellular proteolysis is the unveiling of carboxy-terminal lysine residues on these surfaces, to which plasminogen may bind. Since plasminogen bound to carboxy-terminal lysines of progressively degraded fibrin or membranes is readily transformed into plasmin by fibrin-bound t-PA, this mechanism represents the most important pathway for the acceleration and amplification of fibrinolysis. Alpha-2-antiplasmin, by inhibiting plasmin release from surfaces, regulates the extent and rate of this process but has no effect on fibrin-bound or membrane-bound plasmin. Lipoprotein(a), a particle possessing a plasminogen-like apolipoprotein, apo(a), may interfere with this mechanism by inhibiting the specific binding of plasminogen to lysine residues in membrane or fibrin surfaces.

Mechanisms of activation of mammalian plasma fibrinolytic systems with streptokinase and with recombinant staphylokinase

The molecular basis of the marked interspecies variability in the response of plasma fibrinolytic systems to activation by streptokinase (SK) or recombinant staphylokinase (STAR) was studied using highly purified plasminogens and alpha 2-antiplasmins from five representative species (man, baboon, rabbit, dog and cow). Human plasminogen reacted rapidly and stoichiometrically with both SK and STAR to yield potent plasminogen activators (catalytic efficiencies, kcat/Km, of 1.0 microM-1 x s-1 and 0.3 microM-1 x s-1, respectively). The complex with SK was insensitive to alpha 2-antiplasmin, which, however, rapidly inhibited the complex with STAR (second-order rate constant, k1,app of 8 x 10(6) M-1 x s-1). In a system composed of a 0.06-ml 125I-fibrin-labeled plasma clot submerged in 0.30 ml plasma, both SK and STAR had potent fibrinolytic properties, causing 50% clot lysis in 2 h (EC50), with 120 nM and 13 nM, respectively. Clot lysis with SK was non-fibrin specific (residual fibrinogen < 10%), whereas lysis with STAR was highly fibrin specific (residual fibrinogen 76%). Canine plasminogen reacted avidly with SK, but SK was rapidly degraded; it reacted rapidly and quantitatively with STAR to form a potent plasminogen-activating complex (kcat/Km of 0.4 microM-1 x s-1) which was sensitive to neutralization by alpha 2-antiplasmin (k1,app of 6 x 10(5) M-1 x s-1). In a canine plasma milieu, SK was relatively potent (EC50 200 nM) and fibrin specific, whereas STAR was very potent (EC50 1.3 nM) but poorly fibrin specific. Baboon and rabbit plasminogen did not form stable stoichiometric complexes with SK, but reacted stoichiometrically and quantitatively with STAR. The complexes with STAR, however, had low catalytic efficiencies for the activation of their autologous plasminogens (kcat/Km 0.02 microM-1 x s-1) and reacted more slowly with alpha 2-antiplasmin (k1,app 5-10 x 10(5) M-1 x s-1). Bovine plasminogen was virtually unreactive towards both SK and STAR as well as to their complexes with human plasminogen, as monitored by measurement of the initial activation rates. The resistance to fibrinogen degradation with STAR observed in the human system could be transferred to the canine system by reconstituting canine plasma, depleted of plasminogen and alpha 2-antiplasmin, with the human proteins. Conversely, the sensitivity to fibrinogen degradation of the canine system could be transferred to the human system by reconstituting depleted plasma with canine plasminogen and alpha 2-antiplasmin. It is concluded that the variability in the response of mammalian plasma fibrinolytic systems to activation with SK or STAR is determined mainly by the extent of complex formation of these compounds with plasminogen, by the catalytic efficiencies of the complexes for the activation of autologous plasminogen and by the rate of inhibition of these complexes by alpha 2-antiplasmin.

Interaction of staphylokinase with different molecular forms of plasminogen

In order to obtain more information on the mechanism of plasminogen activation by staphylokinase (STA), we have studied the interaction between recombinant STA (STAR) and different molecular forms of human plasminogen, including Glu-plasminogen (native moiety), Lys-plasminogen (partially degraded moiety) and low-molecular-mass (LMM) plasminogen (moiety lacking kringles 1-4). Addition of 2 microM STAR to 0.4 microM Glu-plasminogen, Lys-plasminogen or LMM plasminogen resulted in the generation of proteolytic activity towards the chromogenic substrate D-Val-Leu-Lys-NH-PhNO2 (S-2251) corresponding to the exposure of 1 active center/plasminogen molecule. Complex formation was associated with conversion of the one-chain plasminogen moieties to two-chain plasmin, and with quantitative conversion of Glu-plasminogen to Lys-plasmin. The stoichiometry of the plasminogen-STAR complex, determined by binding of the complex to Lys-Sepharose and measurement of residual STAR, was found to be equimolar. The plasminogen-STAR complexes were inhibited by alpha 2-antiplasmin with second-order rate constants of 2.4 +/- 0.17 x 10(6) M-1 s-1 for Glu-plasminogen, 2.4 +/- 0.21 x 10(6) M-1 s-1 for Lys-plasminogen and 9.4 +/- 1.5 x 10(4) M-1 s-1 for LMM plasminogen. Glu-plasmin-STAR, Lys-plasmin-STAR and LMM plasmin-STAR had comparable catalytic efficiencies (kcat/Km) for the activation of Glu-plasminogen (0.24-0.29 microM-1 s-1), Lys-plasminogen (0.57-0.79 microM-1 s-1) or LMM plasminogen (0.11-0.16 microM-1 s-1). In a human plasma milieu in vitro STAR, Glu-plasmin-STAR, Lys-plasmin-STAR and LMM-plasmin-STAR were equally effective for the lysis of 125I-fibrin-labeled human plasma clots [50% clot lysis in 2 h (EC50) with 11-13 nM test compound] and equally fibrin-selective (residual fibrinogen levels of 72-84% after 2 h at EC50). Our results thus confirm that plasminogen and STAR form a 1:1 stoichiometric complex in which plasminogen is converted to plasmin and Glu-plasminogen to Lys-plasmin. The lysine-binding sites in kringles 1-4 of plasminogen are not required for the complex formation with STAR, nor for the enzyme activity of the complex with STAR in purified systems and in a human plasma milieu. The lysine-binding sites are, however, important for the rate of the inhibition of the complexes by alpha 2-antiplasmin.

Mapping of the human plasmin domain recognized by the unique plasmin receptor of group A streptococci

A high-affinity surface receptor for human plasmin has been reported on certain group A streptococci. To map the region of the plasmin molecule that binds to the bacterial receptor, isolated domains of plasmin were tested for their ability to inhibit the binding of intact radiolabeled plasmin to receptor-positive bacteria. Complete inhibition of binding of labeled plasmin to bacteria by isolated heavy chains was achieved, but this inhibition was not as efficient on a molar basis when compared with that of unlabeled plasmin. By contrast, a conformationally altered form of native plasminogen was found to bind to bacteria and was as efficient a competitive inhibitor as intact plasmin was. The results of this study indicate that the selective binding of human plasmin to a group A streptococcus is dependent on structures present in the conformationally altered form of native plasminogen or plasmin that are not found on the native zymogen, the plasminogen with NH2-terminal glutamic acid.

An anti-plasminogen antibody preparation that inhibits the activation of human plasminogen but enhances the activator activity of the B-chain-streptokinase complex

An anti-Glu-plasminogen (GLU-PLG) polyclonal antibody antiserum was prepared in the goat, and specific IgG anti-GLU-PLG antibodies were purified by affinity chromatography using a sepharose-GLU-PLG column. In chromogenic assay studies, the anti-GLU-PLG antibody preparation was found to be an effective inhibitor of the activation of PLG, and it produced different inhibition curves with four different PLG activators. 80% inhibition of streptokinase (SK) activation of GLU-PLG occurred with an anti-GLU-PLG antibody/GLU-PLG molar ratio of 1:1, whereas at this ratio only 28% and 36% inhibition of the plasmin-streptokinase (PLN-SK) and the B-chain-streptokinase (B-SK) complexes occurred. At a 1:1 molar ratio of antibody to PLN, no inhibition of PLN activity occurred. When the anti-GLU-PLG antibody preparation was incubated with each PLG activator, an enhancement in the activator activity of the B-SK complex, but not the other activators was observed with mini-plasminogen (MINI-PLG). Enhancement occurred at a molar ratio of 1:1 and reached a peak of 97% enhancement at a molar ratio of 10:1. Enhanced activator activity of the B-SK complex of 49% occurred at a molar ratio of 1:1 when GLU-PLG was the substrate. At higher molar ratios inhibition of activator activity on GLU-PLG was observed, but not on MINI-PLG. These results indicate that there are multiple activator binding sites on PLG, that these four activators all bind differently to GLU-PLG and MINI-PLG, that some antibody populations that are specific for the PLN B-chain can stimulate activator activity, while other antibody populations that are specific for either the PLN A-chain or the B-chain are inhibitory.

Comparative kinetic analysis of the activation of human plasminogen by natural and recombinant single-chain urokinase-type plasminogen activator

Single-chain urokinase-type plasminogen activator (scu-PA) may be obtained from conditioned cell culture media (natural scu-PA) or by expression of the cDNA encoding human scu-PA in Escherichia coli (recombinant scu-PA). The activation of Glu-plasminogen by natural and recombinant scu-PA can be described by a sequence of three reactions, each of which obeys Michaelis-Menten kinetics. Initial activation of plasminogen to plasmin by scu-PA (reaction I) occurs with a high affinity (Km below 0.8 microM) for both scu-PAs, while the catalytic rate constant (k2) is 0.017 s-1 for recombinant scu-PA but only 0.0009 s-1 for natural scu-PA. Subsequent conversion of scu-PA to urokinase (two-chain urokinase-type plasminogen activator, tcu-PA) by generated plasmin (reaction II) occurs with a comparable affinity (Km about 5 microM) for natural and recombinant scu-PA and with a k2 of 0.23 s-1 for natural and 1.2 s-1 for recombinant scu-PA. Finally, activation of plasminogen by tcu-PA (reaction III) occurs with low affinity (Km 30-50 microM) but with a high catalytic rate constant (k2 about 5 s-1) for both natural and recombinant tcu-PA. The differences in the kinetic parameters of the activation of plasminogen by natural or recombinant scu-PA are thus mainly due to differences in turnover rate in the first reaction. Indeed, the catalytic rate constant of the first reaction is about 20-times higher for recombinant scu-PA than for natural scu-PA. Thus, surprisingly, the artificial, unglycosylated recombinant scu-PA molecule has a better catalytic efficiency than its natural glycosylated counterpart.

A monoclonal antibody directed against the high-affinity lysine-binding site (LBS) of human plasminogen. Role of LBS in the regulation of fibrinolysis

One of thirty murine monoclonal antibodies, raised by immunization with human plasmin-alpha 2-antiplasmin complex, was found to be directed against the high-affinity lysine-binding site in plasminogen. Indeed, this antibody (MA-HAL) reacted with plasminogen and with a fragment of plasminogen composed of the first three triple-loop structures (LBS I) and was displaced by 6-aminohexanoic acid (50% displacement at 25 microM). In competitive radioimmunoassays the binding of radiolabeled plasminogen to MA-HAL was reduced to 50% with 2.3 microM alpha 2-antiplasmin or 1.3 microM histidine-rich glycoprotein, which corresponds to the known dissociation constants between these ligands and the high-affinity lysine-binding site of plasminogen. MA-HAL did not influence the activation of plasminogen by tissue-type plasminogen activator in the absence of CNBr-digested fibrinogen, but abolished the effect of CNBr-digested fibrinogen on the Michaelis constant of the reaction. MA-HAL reduced the reaction rate between plasmin and alpha 2-antiplasmin by a factor 20 and abolished the binding of plasminogen to fibrin. These results indicate that MA-HAL specifically binds to and masks the high-affinity lysine-binding site of plasminogen. It therefore is a useful tool for the investigation of the role of this structure in the regulation of fibrinolysis, both at the level of fibrin-stimulated activation of plasminogen and of the inhibition of generated plasmin.