Superficial accumulation of plasminogen during plasma clot lysis

Catheter-directed thrombolysis for acute limb ischemia

Acute limb ischemia is a potentially life-threatening clinical event. Thrombosis in situ, bypass graft thrombosis, and embolic occlusion are the three major precipitating events leading to acute limb ischemia. Management of acute ischemia depends on the clinical status of the affected limb and patient comorbidities. Catheter-directed thrombolysis (CDT) is the treatment of choice for patients with relatively mild acute limb ischemia (Rutherford categories I and IIa) with no contraindications to thrombolytic therapy. Patients with severe acute limb ischemia (Rutherford category IIb) need emergent revascularization. CDT should be considered, nonetheless, if the relative risks compared with primary operation are favorable. CDT is a life- and limb-saving treatment for many patients despite limitations of efficacy and associated complications. This article is a review of the etiology of acute arterial occlusion; clinical triage of patients presenting with acute limb ischemia; catheter guide wire techniques, pharmacological agents, and devices in current use for CDT; as well as the outcomes of CDT.

Human complement receptor type 1-directed loading of tissue plasminogen activator on circulating erythrocytes for prophylactic fibrinolysis

Plasminogen activators (PAs) are not used for thromboprophylaxis due to rapid clearance, bleeding, and extravascular toxicity. We describe a novel strategy that overcomes these limitations. We conjugated tissue-type PA (tPA) to a monoclonal antibody (mAb) against complement receptor type 1 (CR1) expressed primarily on human RBCs. Anti-CR1/tPA conjugate, but not control conjugate (mIgG/tPA), bound to human RBCs (1.2 x 10(3) tPA molecules/cell at saturation), endowing them with fibrinolytic activity. In vitro, RBC-bound anti-CR1/tPA caused 90% clot lysis versus 20% by naive RBCs. In vivo, more than 40% of anti-CR1/(125)I-tPA remained within the circulation ( approximately 90% bound to RBCs) 3 hours after injection in transgenic mice expressing human CR1 (TgN-hCR1) versus less than 10% in wild-type (WT) mice, without RBC damage; approximately 90% of mIgG/(125)I-tPA was cleared from the circulation within 30 minutes in both WT and TgN-hCR1 mice. Anti-CR1/tPA accelerated lysis of pulmonary emboli and prevented stable occlusive carotid arterial thrombi from forming after injection in TgN-hCR1 mice, but not in WT mice, whereas soluble tPA and mIgG/tPA were ineffective. Anti-CR1/tPA caused 20-fold less rebleeding in TgN-hCR1 mice than the same dose of tPA. CR1-directed immunotargeting of PAs to circulating RBCs provides a safe and practical means to deliver fibrinolytics for thromboprophylaxis in settings characterized by a high imminent risk of thrombosis.

Thrombin activatable fibrinolysis inhibitor (TAFI)--how does thrombin regulate fibrinolysis?

The thrombin-catalysed conversion of plasma fibrinogen into fibrin and the development of an insoluble fibrin clot are the final steps of the coagulation cascade during haemostasis. A delicate balance between coagulation and fibrinolysis determines the stability of the fibrin clot. Thrombin plays a central role in this process, it not only forms the clot but it is also involved in stabilizing the clot by activating thrombin activatable fibrinolysis inhibitor (TAFI). Activated TAFI protects the fibrin clot against lysis. Here we will discuss the mechanisms for regulation of fibrinolysis by thrombin. The role of the coagulation system for the generation of thrombin and for the activation of TAFI implies that defects in thrombin generation will directly affect the protection of clots against lysis. Thus, defects in activation of TAFI might contribute to the severity of bleeding disorders. Vice versa an increased activation of TAFI due to an increased rate of thrombin generation might lead to thrombotic disorders. Specific inhibitors of activated TAFI or inhibitors that interfere with the generation of thrombin might provide novel therapeutic strategies for thrombolytic therapy. Besides having a role in the regulation of fibrinolysis, TAFI may also have an important function in the regulation of inflammation, wound healing and blood pressure.

Fibrin Affinity of Erythrocyte-Coupled Tissue-Type Plasminogen Activators Endures Hemodynamic Forces and Enhances Fibrinolysis in Vivo

Plasminogen activators (PAs; e.g., tissue-type, tPA) coupled to red blood cells (RBCs) display in vivo features useful for thromboprophylaxis: prolonged circulation, minimal extravasation, and preferential lysis of nascent versus preexisting clots. Yet, factors controlling the activity of RBC-bound PAs in vivo are not defined and may not mirror the profile of soluble PAs. We tested the role of RBC/PA binding to fibrin in fibrinolysis. RBC/tPA and RBC/tPA variant with low fibrin affinity (rPA) bound to and lysed plasminogen-containing fibrin clots in vitro comparably. In contrast, when coinjected in mice with fibrin emboli lodging in pulmonary vasculature, only RBC/tPA accumulated in lungs, which resulted in a more extensive fibrinolysis versus RBC/rPA (p < 0.01). Reconciling this apparent divergence between in vitro and in vivo behaviors, RBC/tPA, but not RBC/rPA perfused over fibrin in vitro at physiological shear stress bound to fibrin clots and caused greater fibrinolysis versus RBC/rPA (p < 0.001). These results indicate that because of high fibrin affinity, RBC/tPA binding to clots endures hemodynamic stress, which enhances fibrinolysis. Behavior of RBC/PAs under hemodynamic pressure is an important predictor of their performance in vivo.

Endothelial targeting of a recombinant construct fusing a PECAM-1 single-chain variable antibody fragment (scFv) with prourokinase facilitates prophylactic thrombolysis in the pulmonary vasculature

Means to prevent thrombus extension and local recurrence remain suboptimal, in part because of the limited effectiveness of existing thrombolytics. In theory, plasminogen activators could be used for this purpose if they could be anchored to the vascular lumen by targeting stably expressed, noninternalized determinants such as platelet-endothelial-cell adhesion molecule 1 (PECAM-1). We designed a recombinant molecule fusing low-molecular-weight single-chain prourokinase plasminogen activator (lmw-scuPA) with a single-chain variable fragment (scFv) of a PECAM-1 antibody to generate the prodrug scFv/lmw-scuPA. Cleavage by plasmin generated fibrinolytically active 2-chain lmw-uPA. This fusion protein (1) bound specifically to PECAM-1-expressing cells; (2) was rapidly cleared from blood after intravenous injection; (3) accumulated in the lungs of wild-type C57BL6/J, but not PECAM-1 null mice; and (4) lysed pulmonary emboli formed subsequently more effectively than lmw-scuPA, thereby providing support for the concept of thromboprophylaxis using recombinant scFv-fibrinolytic fusion proteins that target endothelium.

Understanding the enzymology of fibrinolysis and improving thrombolytic therapy

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

Thrombin activatable fibrinolysis inhibitor (TAFI) at the interface between coagulation and fibrinolysis

The thrombin-catalysed conversion of plasma fibrinogen into fibrin and the development of an insoluble fibrin clot are the final steps of the coagulation cascade during haemostasis. A delicate balance between coagulation and fibrinolysis determines the stability of the fibrin clot. Thrombin Activatable Fibrinolysis Inhibitor (TAFI) plays an important role in this process. TAFI is activated by thrombin and protects the fibrin clot against lysis. The role of TAFI in bleeding and thrombotic disorders is discussed as well as its novel emerging role in wound healing and inflammation.

Fibrinogen and fibrin

Fibrinogen is a large, complex, fibrous glycoprotein with three pairs of polypeptide chains linked together by 29 disulfide bonds. It is 45 nm in length, with globular domains at each end and in the middle connected by alpha-helical coiled-coil rods. Both strongly and weakly bound calcium ions are important for maintenance of fibrinogen's structure and functions. The fibrinopeptides, which are in the central region, are cleaved by thrombin to convert soluble fibrinogen to insoluble fibrin polymer, via intermolecular interactions of the "knobs" exposed by fibrinopeptide removal with "holes" always exposed at the ends of the molecules. Fibrin monomers polymerize via these specific and tightly controlled binding interactions to make half-staggered oligomers that lengthen into protofibrils. The protofibrils aggregate laterally to make fibers, which then branch to yield a three-dimensional network-the fibrin clot-essential for hemostasis. X-ray crystallographic structures of portions of fibrinogen have provided some details on how these interactions occur. Finally, the transglutaminase, Factor XIIIa, covalently binds specific glutamine residues in one fibrin molecule to lysine residues in another via isopeptide bonds, stabilizing the clot against mechanical, chemical, and proteolytic insults. The gene regulation of fibrinogen synthesis and its assembly into multichain complexes proceed via a series of well-defined steps. Alternate splicing of two of the chains yields common variant molecular isoforms. The mechanical properties of clots, which can be quite variable, are essential to fibrin's functions in hemostasis and wound healing. The fibrinolytic system, with the zymogen plasminogen binding to fibrin together with tissue-type plasminogen activator to promote activation to the active enzyme plasmin, results in digestion of fibrin at specific lysine residues. Fibrin(ogen) also specifically binds a variety of other proteins, including fibronectin, albumin, thrombospondin, von Willebrand factor, fibulin, fibroblast growth factor-2, vascular endothelial growth factor, and interleukin-1. Studies of naturally occurring dysfibrinogenemias and variant molecules have increased our understanding of fibrinogen's functions. Fibrinogen binds to activated alphaIIbbeta3 integrin on the platelet surface, forming bridges responsible for platelet aggregation in hemostasis, and also has important adhesive and inflammatory functions through specific interactions with other cells. Fibrinogen-like domains originated early in evolution, and it is likely that their specific and tightly controlled intermolecular interactions are involved in other aspects of cellular function and developmental biology.

Blood Clearance and Activity of Erythrocyte-Coupled Fibrinolytics

Conjugating tissue-type plasminogen activator (tPA) to red blood cells (RBCs) endows it with features useful for thromboprophylaxis. However, the optimal intensity and duration of thromboprophylaxis vary among clinical settings. To assess how the intrinsic properties of a plasminogen activator (PA) affect functions of the corresponding RBC/PA conjugate, we coupled equal amounts of tPA or Retavase (rPA; a variant with an extended circulation time, lower fibrin affinity, and greater susceptibility to PA inhibitors). Conjugation to RBC markedly prolonged the circulation of each PA in rats and mice, without detrimental effects on carrier RBC. The initial blood clearance of RBC/tPA was faster than RBC/rPA, yet it exerted greater fibrinolytic activity, in part due to greater resistance of tPA and RBC/tPA to plasma inhibitors versus rPA and RBC/rPA observed in vitro. Soluble and RBC-coupled tPA and rPA exerted the same amidolytic activity, yet RBC/tPA lysed fibrin clots more effectively than RBC/rPA, notwithstanding comparable fibrinolytic activity of their soluble counterparts. Conjugation to RBC suppressed rPA's ability to be activated by fibrin, whereas the fibrin activation of RBC-coupled tPA was not hindered. Therefore, the functional profile of RBC/PA is influenced by: pharmacokinetic features provided by carrier RBC (e.g., prolonged circulation), intrinsic PA features (e.g., clearance rate, resistance to inhibitors), and changes imposed by conjugation to RBC (e.g., loss of cofactor stimulation). These factors, different from those guiding the design of soluble PA for lysis of existing clots, can be exploited in the rational design of RBC/PA tailored for specific prophylactic indications.

Thrombin activatable fibrinolysis inhibitor: Not just an inhibitor of fibrinolysis

OBJECTIVE

To review the activation of thrombin activatable fibrinolysis inhibitor (TAFI) and activity of activated TAFI (TAFIa) as it relates to the regulation of both fibrinolytic and proinflammatory substances.

DATA SOURCE

Published articles and reviews (from PubMed, published between 1962 and 2003) on experimental studies of coagulation, fibrinolysis, and inflammation.

DATA SYNTHESIS AND CONCLUSIONS

The principal physiologic role of TAFI is still a matter of debate. Although TAFI activation can result from proteolysis by a number of proteases, the most likely physiologic activators are thrombin (in complex with the cofactor thrombomodulin) and plasmin (in complex with polysaccharide cofactors). The activated enzyme, TAFIa, displays carboxypeptidase B-like activity and probably regulates both fibrinolysis and inflammation in response to injury and infection. At present, there is limited understanding of the role that TAFI plays in the interrelationships between coagulation, fibrinolysis, and inflammation. Although the potential therapeutic value of TAFIa inhibition/TAFI activation awaits further investigation, the data gathered to date suggest that, like activated protein C, TAFIa may play a pivotal role in regulating the crosstalk between coagulation, fibrinolysis, and inflammation.

Characterization of plasminogen as an adhesive ligand for integrins alphaMbeta2 (Mac-1) and alpha5beta1 (VLA-5)

Plasminogen (Pg) has been implicated in many biologic processes involving extracellular proteolysis. We investigated whether Pg, by virtue of its capacity to be deposited within the extracellular matrix, can serve as a ligand for cell surface integrins. We report here that Pg supports cell adhesion by engaging integrins alphaMbeta2 and alpha5beta1. The immobilized Glu-Pg, but not its derivatives with the N-terminal peptide lacking, plasmin and Lys-Pg, supported efficient adhesion that was abolished by anti-alphaMbeta2 and anti-alpha5beta1 integrin-specific monoclonal antibodies (mAbs). In addition, lysine binding sites of Glu-Pg contributed to cell adhesion inasmuch as tranexamic acid and epsilon-aminocaproic acid inhibited cell adhesion. The involvement of alphaMbeta2 and alpha5)beta1 in adhesion to Glu-Pg was demonstrable with blood neutrophils, U937 monocytoid cells, and genetically engineered alphaMbeta2-transfected human embryonic kidney (HEK) 293 cells. In alphaMbeta2, the alphaMI-domain is the binding site for Glu-Pg because the "I-less" form of alphaMbeta2 did not support cell adhesion and the recombinant alphaMI-domain bound Glu-Pg directly. In comparison with cell adhesion, the binding of soluble Glu-Pg to cells and the concomitant generation of plasmin activity was inhibited by anti-alpha5beta1 but not by anti-alphaMbeta2. These findings identify Glu-Pg as an adhesive ligand for integrins alphaMbeta2 and alpha5beta1 and suggest that alpha5beta1 may participate in the binding of soluble Glu-Pg and assist in its activation.

New insights into factors affecting clot stability: a role for thrombin activatable fibrinolysis inhibitor (TAFI; plasma procarboxypeptidase B, plasma procarboxypeptidase U, procarboxypeptidase R)

The thrombin-catalyzed conversion of plasma fibrinogen into fibrin and the development of an insoluble fibrin clot are the final steps in the coagulation cascade during hemostasis. The delicate balance between clot formation and fibrinolysis, which determines clot stability, is controlled by a complex interplay between fibrin and other molecular and cellular components of the hemostatic system, including thrombin activatable fibrinolysis inhibitor (TAFI). TAFI is activated by thrombin and has an important role in the stability of the fibrin clot, which is reviewed here. In particular, the role of TAFI in fibrinolysis and those characteristics of the protein that affect clot stability are described. In addition, the importance of TAFI in the coagulation process and how changes in its availability may contribute to bleeding or thrombotic disorders are discussed.

Prophylactic fibrinolysis through selective dissolution of nascent clots by tPA-carrying erythrocytes

A fibrinolytic agent consisting of a tissue-type plasminogen activator (tPA) coupled to the surface of red blood cells (RBCs) can dissolve nascent clots from within the clot, in a Trojan horse-like strategy, while having minimal effects on preexisting hemostatic clots or extravascular tissue. After intravenous injection, the fibrinolytic activity of RBC-tPA persisted in the bloodstream at least tenfold longer than did that of free tPA. In a model of venous thrombosis induced by intravenously injected fibrin microemboli aggregating in pulmonary vasculature, soluble tPA lysed pulmonary clots lodged before but not after tPA injection, whereas the converse was true for RBC-tPA. Free tPA failed to lyse occlusive carotid thrombosis whether injected before or after vascular trauma, whereas RBC-tPA circulating before, but not injected after, thrombus formation restored blood flow. This RBC-based drug delivery strategy alters the fibrinolytic profile of tPA, permitting prophylactic fibrinolysis.

Engineering of a staphylokinase-based fibrinolytic agent with antithrombotic activity and targeting capability toward thrombin-rich fibrin and plasma clots

Current clinically approved thrombolytic agents have significant drawbacks including reocclusion and bleeding complications. To address these problems, a staphylokinase-based thrombolytic agent equipped with antithrombotic activity from hirudin was engineered. Because the N termini for both staphylokinase and hirudin are required for their activities, a Y-shaped molecule is generated using engineered coiled-coil sequences as the heterodimerization domain. This agent, designated HE-SAKK, was produced and assembled from Bacillus subtilis via secretion using an optimized co-cultivation approach. After a simple in vitro treatment to reshuffle the disulfide bonds of hirudin, both staphylokinase and hirudin in HE-SAKK showed biological activities comparable with their parent molecules. This agent was capable of targeting thrombin-rich fibrin clots and inhibiting clot-bound thrombin activity. The time required for lysing 50% of fibrin clot in the absence or presence of fibrinogen was shortened 21 and 30%, respectively, with HE-SAKK in comparison with staphylokinase. In plasma clot studies, the HE-SAKK concentration required to achieve a comparable 50% clot lysis time was at least 12 times less than that of staphylokinase. Therefore, HE-SAKK is a promising thrombolytic agent with the capability to target thrombin-rich fibrin clots and to minimize clot reformation during fibrinolysis.

Thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B, procarboxypeptidase R, procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described, which downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to the previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. Activated TAFI (TAFIa) downregulates fibrinolysis by the removal of carboxy-terminal lysines from fibrin. These carboxy-terminal lysines are exposed upon limited proteolysis of fibrin by plasmin and act as ligands for the lysine-binding sites of plasminogen and tissue-type plasminogen activator (t-PA). Elimination of these lysines by TAFIa abrogates the fibrin cofactor function of t-PA-mediated plasminogen activation, resulting in a decreased rate of plasmin generation and thus downregulation of fibrinolysis. In this review, the characteristics of TAFI are summarized, with an emphasis on the pathways leading to activation of TAFI and the role of TAFIa in the inhibition of fibrinolysis. However, it cannot be ruled out that TAFI has other, as yet undefined, functions in biology.

ICAM-directed vascular immunotargeting of antithrombotic agents to the endothelial luminal surface

Drug targeting to a highly expressed, noninternalizable determinant up-regulated on the perturbed endothelium may help to manage inflammation and thrombosis. We tested whether inter-cellular adhesion molecule-1 (ICAM-1) targeting is suitable to deliver antithrombotic drugs to the pulmonary vascular lumen. ICAM-1 antibodies bind to the surface of endothelial cells in culture, in perfused lungs, and in vivo. Proinflammatory cytokines enhance anti-ICAM binding to the endothelium without inducing internalization. (125)I-labeled anti-ICAM and a reporter enzyme (beta-Gal) conjugated to anti-ICAM bind to endothelium and accumulate in the lungs after intravenous administration in rats and mice. Anti-ICAM is seen to localize predominantly on the luminal surface of the pulmonary endothelium by electron microscopy. We studied the pharmacological effect of ICAM-directed targeting of tissue-type plasminogen activator (tPA). Anti-ICAM/tPA, but not control IgG/tPA, conjugate accumulates in the rat lungs, where it exerts plasminogen activator activity and dissolves fibrin microemboli. Therefore, ICAM may serve as a target for drug delivery to endothelium, for example, for pulmonary thromboprophylaxis. Enhanced drug delivery to sites of inflammation and the potential anti-inflammatory effect of blocking ICAM-1 may enhance the benefit of this targeting strategy.

Engineering design of optimal strategies for blood clot dissolution

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

Disintegration and reorganization of fibrin networks during tissue-type plasminogen activator-induced clot lysis

In this study, we investigated tissue-type plasminogen activator (tPA)-induced lysis of glutamic acid (glu)-plasminogen-containing or lysine (lys)-plasminogen-containing thrombin-induced fibrin clots. We measured clot development and plasmin-mediated clot disintegration by thromboelastography, and used scanning electron microscopy (SEM) to document the structural changes taking place during clot formation and lysis. These events occurred in three overlapping stages, which were initiated by the addition of thrombin, resulting first in fibrin polymerization and clot network organization (Stage I). Autolytic plasmin cleavage of glu-plasminogen at lys-77 generates lys-plasminogen, exposing lysine binding sites in its kringle domains. The presence of lys-plasminogen within the thrombin-induced fibrin clot enhanced network reorganization to form thicker fibers as well as globular complexes containing fibrin and lys-plasminogen having a greater level of turbidity and a higher elastic modulus (G) than occurred with thrombin alone. Lys-plasminogen or glu-plasminogen that had been incorporated into the fibrin clot was activated to plasmin by tPA admixed with the thrombin, and led directly to clot disintegration (Stage II) concomitant with fibrin network reorganization. The onset of Stage III (clot dissolution) was signaled by a sustained secondary rise in turbidity that was due to the combined effects of lys-plasminogen presence or its conversion from glu-plasminogen, plus clot network reorganization. SEM images documented dynamic structural changes in the lysing fibrin network and showed that the secondary turbidity rise was due to extensive reorganization of severed fibrils and fibers to form wide, occasionally branched fibers. These degraded structures contributed little, if anything, to the structural integrity of the residual clot, and eventually collapsed completely during the course of progressive clot dissolution. These results provide new perspectives on the major structural events that occur in the fibrin clot matrix during fibrinolysis.

Basic principles in thrombolysis: regulatory role of plasminogen

During thrombolytic therapy, patients are treated with a plasminogen activator in order to stimulate the fibrinolytic system by converting the precursor plasminogen into the active enzyme plasmin. The fibrinolytic process can be divided into two phases. In the first phase, plasminogen binds to intact fibrin and initial fibrinolysis takes place. As a result, carboxyterminal lysine residues are generated, which represent new binding sites for plasminogen. In the second phase, plasminogen binds to these sites and fibrinolysis is accelerated because the local plasminogen concentration is strongly enhanced and because this plasminogen has a higher reactivity. For instance, both single-chain urokinase-type plasminogen activator (scu-PA) and staphylokinase have a high preference for this type of plasminogen, which explains their fibrin-selective action. A recently discovered thrombin-activatable fibrinolysis inhibitor (TAFI) eliminates carboxyterminal lysine residues from partially degraded fibrin and, thus, inhibits the second phase of fibrinolysis. These mechanisms show that plasminogen plays an important regulatory role in fibrinolysis and thrombolysis.

Carboxypeptidase U at the interface between coagulation and fibrinolysis

In 1988, Hendricks et al. first reported on the presence of carboxypeptidase U (U refers to the unstable nature of the enzyme) in human serum. One decade later, the importance of carboxypeptidase U (CPU) in the regulation of fibrin clot dissolution is well documented. CPU circulates in plasma as an inactive zymogen, proCPU, that is converted to its active form during coagulation and fibrinolysis. CPU cleaves off C-terminal lysine residues exposed on fibrin partially degraded by the action of plasmin. Because these C-terminal lysine residues are important for upregulating the fibrinolytic rate, CPU thus slows down fibrinolysis.

Thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B, procarboxypeptidase R, procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described. This inhibitor downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. In this overview, the protein will be referred to as TAFI. TAFI is a procarboxypeptidase and a member of the family of metallocarboxypeptidases. These enzymes are circulating in plasma and are present in several tissues such as pancreas. In this review, we will describe the properties of basic carboxypeptidases with the emphasis on the role of TAFI in coagulation and fibrinolysis. It cannot be ruled out, however, that TAFI has other, yet undefined, functions in biology.

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.

Plasminogen-enriched pulse-spray thrombolysis with tPA: further developments

PURPOSE

To further improve methods for pulsed plasminogen-enriched thrombolysis and to compare results with the best obtainable with use of tissue plasminogen activator (tPA) alone.

MATERIALS AND METHODS

Parameters of plasminogen-enriched pulse-spray thrombolysis were manipulated in groups of rabbits with inferior vena cava thrombosis, and weights of 1-hour residual thrombus were compared. Variables evaluated were (i) tPA pulse frequency, (ii) amount of plasminogen used for enrichment, (iii) tPA concentration and amount, (iv) pulsed versus infused tPA, and (v) admixture versus separation of plasminogen and tPA.

RESULTS

With use of 3 mg of tPA and approximately 0.9 mg plasminogen enrichment, efficacy varied directly with pulse frequency over a pulse range of every 15 minutes to every 30 seconds. With use of 30-second pulses of tPA at a concentration 0.125 mg/mL, efficacy also correlated directly with increasing plasminogen enrichment up to, but not beyond, approximately 1.8 mg per 1.24 g of clot. Optimized methodology yielded 89% lysis in 1 hour, as compared to 74% lysis previously reported with use of optimized low-concentration (0.01 mg/ mL) tPA alone. Plasminogen enrichment in conjunction with low concentrations of tPA, admixture of tPA and plasminogen, and fractionation of the plasminogen enrichment all proved to be nonproductive or counterproductive.

CONCLUSION

Optimized in vivo postthrombotic plasminogen enrichment significantly accelerated thrombolysis of experimental clots compared to use of optimized tPA alone.

High accumulation of plasminogen and tissue plasminogen activator at the flow surface of mural fibrin in the human arterial system

PURPOSE

We assessed the fibrinolytic activity of the organized mural thrombus lining of aneurysms and prosthetic grafts.

METHODS

Between May 1995 and April 1998, the full-thickness mural thrombi of aneurysms and the pseudointima lining of vascular grafts were obtained from 12 patients, ranging from 55 to 78 years in age, who underwent elective surgery. These included five aortic arch aneurysms, four abdominal aortic aneurysms, and three patent synthetic vascular grafts. The specimens were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/immunoblot and immunohistochemistry for human plasmin/plasminogen, tissue plasminogen activator (tPA), and fibrin degradation product (D-dimer).

RESULTS

In the SDS-PAGE/immunoblot, 25- and 27-kd bands appeared specifically in experimental fibrin plates after limited digestion by plasmin and were also recognized in the mural thrombi. The presence of bands at 25 and 27 kd, which were most prominent in sections near the flow surface layer, was consistent with the hypothesis that the mural fibrin was digested by the endogenous plasmin. Apparent immunoreactivity was found at the flow surface of the masses at a thickness of 10 to 400 micrometer suggesting the presence of a plasminogen and tPA-rich layer, with D-dimer as a consequential product of fibrinolysis.

CONCLUSION

The hypothesis that fibrin surfaces in the arterial system acquire fibrinolytic activity because of digestion by circulating endogenous plasmin was confirmed; this may contribute to the antithrombogenicity of these flow surfaces.

New thrombolytic strategy: bolus administration of tPA and urokinase-fibrinogen conjugate

Increased efficacy of thrombolytic therapy requires a comprehensive search for new and novel therapeutic strategies. Many new modified forms of plasminogen activators have been obtained by means of chemical and biological synthesis. However, clinical findings demonstrate that the reperfusion level achieved during thrombolysis remains the same for various thrombolytic agents, irrespective of an extensive search for an "ideal" thrombolytic. Thrombolytic therapy may be complicated by treatment delays, cumbersome schemes of preparation and administration, and hemorrhagic and rethrombotic events. These limitations may be overcome, at least in part, by applying combined thrombolysis with plasminogen activators exhibiting complementary actions and different pharmacokinetic profiles. The combined action of native thrombolytics allows the use of lower doses and simplified schemes of administration, yielding encouraging results in experimental models. Long-acting forms of plasminogen activators are being developed and tested in combination with tissue-type plasminogen activator as a trigger of thrombolysis. The combination of short- and long-acting plasminogen activators appears promising and potentially eligible for bolus administration to patients. On the basis of our own experimental results and data in the literature, we suggest a new thrombolytic strategy connected with the single injection of a combination of complementary and pharmacokinetically different plasminogen activators.

The molecular weights, mass distribution, chain composition, and structure of soluble fibrin degradation products released from a fibrin clot perfused with plasmin

We used a perfused clot system to study the degradation of cross-linked fibrin. Multiangle laser light scattering showed that plasmin-mediated cleavage caused the release of noncovalently associated fibrin degradation products (FDPs) with a weight-averaged molar mass (Mw) of approximately 6 x 10(6) g/mol. The Mw of FDPs is dependent on ionic strength, and the Mw observed at 0.15 M NaCl resulted from the self-association of FDPs having Mw of approximately 3.8 x 10(6) g/mol. Complete solubilization required the cleavage of approximately 25% of fragment D/fragment E connections, with 48% alpha-, 62% beta-, and 42% gamma-chains cleaved. These results showed that D-E cleavage cannot be explained by a random mechanism, implying cooperativity. Gel filtration and multiangle laser light scattering showed that FDPs range from 2.5 x 10(5) to 1 x 10(7) g/mol. In addition to fragment E, FDPs are composed of fragments ranging from 2 x 10(5) Da (D-dimer, or DD) to at least 2.3 x 10(6) Da (DX8D). FDP mass distribution is consistent with a model whereby FDPs bind to fibrin with affinities proportional to fragment mass. Root mean square radius analysis showed that small FDPs approximate rigid rods, but this relationship breaks down as FDPs size increases, suggesting that large FDPs possess significant flexibility.

Structural Studies of Fibrinolysis by Electron Microscopy

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

Structural Studies of Fibrinolysis by Electron Microscopy

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

New strategy of thrombolysis. Conjunctive effect of plasminogen activators with different pharmacokinetic profile

Combined actions of native and prolonged thrombolytics allow the use of lower doses and simplified schemes of administration, thus yielding significant results in experimental therapy regarding the efficacy and safety of thrombolysis. Development of prolonged forms of plasminogen activators and testing their effect in combination with the thrombolysis trigger are well founded and of current interest. Thrombolytic compositions on the basis of short- and long-term-acting plasminogen activators appear to be promising and potentially eligible for bolus administration.

A study of the mechanism of inhibition of fibrinolysis by activated thrombin-activable fibrinolysis inhibitor

TAFI (thrombin-activable fibrinolysis inhibitor) is a recently described plasma zymogen that, when exposed to the thrombin-thrombomodulin complex, is converted by proteolysis at Arg92 to a basic carboxypeptidase that inhibits fibrinolysis (TAFIa). The studies described here were undertaken to elucidate the molecular basis for the inhibition of fibrinolysis. When TAFIa is included in a clot undergoing fibrinolysis induced by tissue plasminogen activator and plasminogen, the time to achieve lysis is prolonged, and free arginine and lysine are released over time. In addition, TAFIa prevents a 2.5-fold increase in the rate constant for plasminogen activation which occurs when fibrin is modified by plasmin in the early course of fibrin degradation. The effect is specific for the Glu- form of plasminogen. TAFIa prevents or at least attenuates positive feedback expressed through Lys-plasminogen formation during the process of fibrinolysis initiated by tissue plasminogen activator and plasminogen. TAFIa also inhibits plasmin activity in a clot and prolongs fibrinolysis initiated with plasmin. We conclude that TAFIa suppresses fibrinolysis by removing COOH-terminal lysine and arginine residues from fibrin, thereby reducing its cofactor functions in both plasminogen activation and the positive feedback conversion of Glu-plasminogen to Lys-plasminogen. At relatively elevated concentrations, it also directly inhibits plasmin.

Augmented pulse-spray thrombolysis with tPA by early pulsed intrathrombic plasminogen enrichment

PURPOSE

This study was designed to evaluate the efficacy of plasminogen enrichment of subacute thrombus in further accelerating pulse-spray pharmacomechanical thrombolysis (PSPMT) with urokinase (UK) or tissue plasminogen activator (tPA) in a rabbit model.

MATERIALS AND METHODS

With use of a subacute rabbit inferior vena cava (IVC) thrombosis model, 78 rabbits were divided into eight groups according to the agents used for thrombolysis: (i) controls (IVC thrombosis, no lysis performed), (ii) pulse-spray thrombolysis with saline only, (iii) PSPMT with UK, (iv) PSPMT with UK, plus interim pulse-spray plasminogen enrichment after 14 minutes, (v) pulse-spray plasminogen enrichment, followed at 10 minutes by PSPMT with UK, (vi) PSPMT with tPA, (vii) PSPMT with tPA, plus interim plasminogen enrichment, and (viii) pulse-spray plasminogen enrichment, followed at 10 minutes by PSPMT with tPA.

RESULTS

Intrathrombic pulsed injection of glu-plasminogen after 14 minutes of tPA PSPMT demonstrated significant augmentation of lysis (approximately 31% decrease in residual thrombus) compared with tPA alone (P = .006). Lysis was not augmented significantly when plasminogen was sprayed into thrombus before tPA, or before or after UK.

CONCLUSION

Plasminogen enrichment of thrombus after onset of PSPMT with tPA significantly accelerated thrombolysis in a subacute in vivo rabbit model. A clinical trial of this method may be warranted.

Characterization of a Fibrin Glue-Gdnf Slow-Release Preparation

One novel method to deliver trophic factor locally in the CNS is to mix it into fibrin glue. In the present studies, [125I]-labeled GDNF-containing fibrin glue balls were used to determine binding and spread of the trophic factor. First, the binding of different concentrations of [125I]-labelèd GDNF in fibrin glue was determined in vitro. Within the six concentrations used (from 200 nM to 0.004 nM, 0 M as control), there was a strong linear correlation between the [125I]-GDNF concentration and the recovered radioactivity (r = 0.992). The mean bound radioactivity in 16 samples with 4 nM [125I]-GDNF was 71262 + 2710 CPM, and accounted for 89.8% of the mean initial count of free [125I]-GDNF (79369 + 3499 CPM). Second, [125I]-GDNF-containing glue balls were implanted into the anterior chamber of adult rats. The implanted fibrin glue balls decreased in size with time, but could still be identified on the irises 2 wk after implantation. Radioactivity was concentrated at the implantation sites in the early stages with a distribution in the surrounding iris tissue, which became separated into focal radioactive spots at the third week. Counts of radioactivity were significantly higher in the [125I]-GDNF glue ball-implanted irises than controls until 14 days after implantation. A study of the [125I] decay over time using least-squares linear regression demonstrated first-order kinetics (r = —0.98, p < 0.02) with k = 0.0091 and T 1/2 = 76 h. Finally, [125I]-GDNF–containing glue balls were implanted in the spinal cord of adult rats. Radioactivity was concentrated at the implantation sites in the early stages and was later distributed more widely in the surrounding thoracic cord. The [125I]-GDNF–containing glue degraded over time and became a porous meshwork with decreasing radioactivity at the later time points. Radioactivity in the spinal cords subjected to implantation of [125I]-GDNF–containing glue balls was higher than in controls for 14 days. Study of the [125I] decay by time with least-squares linear regression demonstrated first-order kinetics (r = -0.97, p = 0.001) with T 1/2 = 75.6 h. We conclude that the trophic factor GDNF becomes bound in the fibrin glue matrix from which it is gradually released. Our results suggest that fibrin glue is an effective substrate for keeping a trophic factor localized in situ for a finite period, protected from the circulation, surrounding aqueous humor or CSF.

On the mechanism of the antifibrinolytic activity of plasma carboxypeptidase B

The precursor of plasma carboxypeptidase B (pCPB) also known as thrombin-activable fibrinolysis inhibitor can be converted by thrombin to an active enzyme capable of eliminating C-terminal Lys- and Arg-residues from proteins. The activation is about 1000-fold more efficient in the presence of thrombomodulin (TM). We investigated the antifibrinolytic potency of maximally activated pCPB in plasma and explored the antifibrinolytic mechanism of pCPB. During clotting of plasma in the presence of 3.3 NIH units/ml thrombin and 1 microg/ml soluble TM, more than 80% pro-pCPB was converted into the active form causing an increase of plasma carboxypeptidase activity from 100 units/liter (constitutive activity ascribed to plasma carboxypeptidase N) to 430 units/liter as measured with furoylacroleyl-alanyl-arginine substrate. Under these conditions, lysis of a plasma clot induced by a range of tissue-type plasminogen activator (t-PA) concentrations (0.2-2 microg/ml) was retarded more than 4-fold. A considerable retardation of fibrinolysis was observed upon addition of as little as 12 ng/ml soluble TM, a concentration comparable with physiological concentrations of soluble TM in human plasma. The presence of Ca2+ appeared to be a critical requirement for effective activation of pro-pCPB by thrombin-TM in plasma. Plasminogen-binding sites (C-terminal lysines) on the surface of a plasmin-treated fibrin clot were eliminated within 1-3 min by plasma with maximally activated pCPB, as studied in a recently described model involving fluorescence microscopy. Confocal fluorescence microscopy showed that in the absence of TM plasminogen strongly accumulated on fibrin fibers during t-PA-induced lysis of a plasma clot. In the presence of TM (and a concomitant pro-pCPB activation), lysis was slow and was not accompanied by accumulation of plasminogen on the fibers. In conclusion, generation of active pCPB during clotting of plasma in the presence of Ca2+ and TM leads to a retardation of plasma clot lysis in a wide range of t-PA concentrations, from low to therapeutic, and to a fast elimination of plasminogen-binding sites on partially degraded fibrin. This is a likely mechanism for the antifibrinolytic effect of active pCPB.

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

A new model has been introduced to characterize the action of a fluid phase enzyme on a solid phase substrate. This approach is applied to evaluate the kinetics of fibrin dissolution with several proteases. The model predicts the rate constants for the formation and dissociation of the protease-fibrin complex, the apparent order of the association reaction between the enzyme and the substrate, as well as a global catalytic constant (kcat) for the dissolution process. These kinetic parameters show a strong dependence on the nature of the applied protease and on the structure of the polymerized substrate. The kinetic data for trypsin, PMN-elastase, and three plasminogen-derived proteases with identical catalytic domain, but with a varied N-terminal structure, are compared. The absence of kringle5 in des-kringle1-5-plasmin (microplasmin) is related to a markedly lower kcat (0.008 s-1) compared with plasmin and des-kringle1-4plasmin (miniplasmin) (0.039 s-1). The essentially identical kinetic parameters for miniplasmin and plasmin with the exception of kdiss, which is higher for miniplasmin (81.8 s-1 versus 57.6 s-1), suggest that the first four kringle domains are needed to retain the enzyme in the enzyme-fibrin complex. Trypsin, a protease of similar primary specificity to plasmin, but with a different catalytic domain, shows basically the same kcat as plasmin, but its affinity to fibrin is markedly lower compared with plasmin and even microplasmin. The latter suggests that in addition to the kringle domains, the structure of the catalytic domain in plasmin also contributes to its specificity for fibrin. The thinner and extensively branched fibers of fibrin are more efficiently dissolved than the fibers with greater diameter and lower number of branching points. When the polymer is stabilized through covalent cross-linking, the kcat for plasmin and miniplasmin is 2-4-fold higher than on non-cross-linked fibrin, but the decrease in the association rate constant for the formation of enzyme-substrate complex explains the relative proteolytic resistance of the cross-linked fibrin. Thus, the functional evaluation of the discrete steps of the fibrinolytic process reveals new aspects of the interactions between proteases and their polymer substrate.

Interactions between staphylokinase, plasmin(ogen), and fibrin. Staphylokinase discriminates between free plasminogen and plasminogen bound to partially degraded fibrin

Staphylokinase (STA), a protein of bacterial origin, induces highly fibrin-specific thrombolysis both in human plasma in vitro and in pilot clinical trials. Using fluorescence microscopy, we investigated the spatial distribution of fluorescein isothiocyanate (FITC)-labeled STA during lysis of a plasma clot and its binding to purified fibrin clots in the presence or in the absence of plasmin(ogen). STA highly accumulated in a thin superficial layer of the lysing plasma clot following the distribution of plasminogen (Pg) during lysis. Experiments with purified fibrin clots revealed that STA binds to Pg bound to partially degraded fibrin but not to Pg bound to intact fibrin. Binding of FITC-labeled STA to various forms of plasmin(ogen) in a buffer solution was studied by measuring fluorescence anisotropy. The binding constant for Glu-Pg was estimated as 7.4 microM and for Lys-Pg as 0.28 microM; for active-site blocked plasmin the binding constant was less than 0.05 microM. The much lower affinity of STA for Glu-Pg compared with that for active site-blocked plasmin was mainly due to a lower association rate constant, as assessed by real time biospecific interaction analysis. Gel filtration of a mixture of STA with a molar excess of Glu-Pg demonstrated that STA migrated as an unbound 18-kDa protein when activation of Pg into plasmin was precluded by inhibitors of plasmin. When gel-filtered under the same conditions with plasmin, STA migrated in complex with plasmin with an apparent molecular mass of 100 kDa. Confocal fluorescence microscopy finally demonstrated that when FITC-labeled STA was added to plasma before clotting, it did not bind to fibrin fibers during the first minutes (lag phase), although Pg bound to the fibers moderately. Then, both Pg and STA started to accumulate on the fibers progressively, followed by complete lysis of the clot. In conclusion, our results imply that, when STA is added to plasma, only a small percentage associates with Pg. In contrast, STA binds strongly to plasmin and to Pg, which is bound to partially degraded fibrin. These findings add a new mechanism to the known explanations for the inefficient Pg activation by STA in plasma and specify the mechanism for fibrin-dependent activation of Pg.

Rearrangements of the fibrin network and spatial distribution of fibrinolytic components during plasma clot lysis. Study with confocal microscopy

Binding of components of the fibrinolytic system to fibrin is important for the regulation of fibrinolysis. In this study, decomposition of the fibrin network and binding of plasminogen and plasminogen activators (PAs) to fibrin during lysis of a plasma clot were investigated with confocal microscopy using fluorescein-labeled preparations of fibrinogen, plasminogen, tissue-type PA (t-PA), and two-chain urokinase-type PA (tcu-PA). Lysis induced by PAs present throughout the plasma clot was accompanied by a gradual loss of fibrin content of fibers and by accumulation of plasminogen onto the fibers. Two sequential phases could be distinguished: a phase of prelysis, during which the fibrin network remained immobile, and a phase of final lysis, during which fibers moved with a tendency to shrink and eventually disappeared. The two phases occurred simultaneously but in different locations when lysis was induced by PAs present in the plasma surrounding the clot. The zone of final lysis was located within a 5-8 microns superficial layer, where fibers were mobile, a surface-associated fibrin agglomerates appeared. Plasminogen accumulated in these agglomerates up to 30-fold as compared with its concentration in the outer plasma. t-PA was also highly concentrated in the agglomerates, and tcu-PA bound to them slightly. The zone of prelysis, where plasminogen was moderately accumulated on the immobile fibers, was located deeper in the clot. This zone was much thinner in the case of t-PA-induced lysis than in the case of tcu-PA-induced lysis, reflecting the difference in penetration of the two PAs into the clot. We conclude that under conditions of diffusional transport of fibrinolytic enzymes from outside a plasma clot, extensive lysis is spatially restricted to a zone not exceeding 5-8 microns from the clot surface. In this zone the structure of the fibrin network undergoes significant changes, and strikingly high accumulation of fibrinolytic components takes place.