Plasma and recombinant thrombin-activable fibrinolysis inhibitor (TAFI) and activated TAFI compared with respect to glycosylation, thrombin/thrombomodulin-dependent activation, thermal stability, and enzymatic properties

Thrombin-activatable fibrinolysis inhibitor is degraded by Salmonella enterica and Yersinia pestis

BACKGROUND

 Pathogenic bacteria modulate the host coagulation system to evade immune responses or to facilitate dissemination through extravascular tissues. In particular, the important bacterial pathogens Salmonella enterica and Yersinia pestis intervene with the plasminogen/fibrinolytic system. Thrombin-activatable fibrinolysis inhibitor (TAFI) has anti-fibrinolytic properties as the active enzyme (TAFIa) removes C-terminal lysine residues from fibrin, thereby attenuating accelerated plasmin formation.

RESULTS

 Here, we demonstrate inactivation and cleavage of TAFI by homologous surface proteases, the omptins Pla of Y. pestis and PgtE of S. enterica. We show that omptin-expressing bacteria decrease TAFI activatability by thrombin-thrombomodulin and that the anti-fibrinolytic potential of TAFIa was reduced by recombinant Escherichia coli expressing Pla or PgtE. The functional impairment resulted from C-terminal cleavage of TAFI by the omptins.

CONCLUSIONS

 Our results indicate that TAFI is degraded directly by the omptins PgtE of S. enterica and Pla of Y. pestis. This may contribute to the ability of PgtE and Pla to damage tissue barriers, such as fibrin, and thereby to enhance spread of S. enterica and Y. pestis during infection.

Flexibility of the thrombin-activatable fibrinolysis inhibitor pro-domain enables productive binding of protein substrates

We have previously reported that thrombin-activatable fibrinolysis inhibitor (TAFI) exhibits intrinsic proteolytic activity toward large peptides. The structural basis for this observation was clarified by the crystal structures of human and bovine TAFI. These structures evinced a significant rotation of the pro-domain away from the catalytic moiety when compared with other pro-carboxypeptidases, thus enabling access of large peptide substrates to the active site cleft. Here, we further investigated the flexible nature of the pro-domain and demonstrated that TAFI forms productive complexes with protein carboxypeptidase inhibitors from potato, leech, and tick (PCI, LCI, and TCI, respectively). We determined the crystal structure of the bovine TAFI-TCI complex, revealing that the pro-domain was completely displaced from the position observed in the TAFI structure. It protruded into the bulk solvent and was disordered, whereas TCI occupied the position previously held by the pro-domain. The authentic nature of the presently studied TAFI-inhibitor complexes was supported by the trimming of the C-terminal residues from the three inhibitors upon complex formation. This finding suggests that the inhibitors interact with the active site of TAFI in a substrate-like manner. Taken together, these data show for the first time that TAFI is able to form a bona fide complex with protein carboxypeptidase inhibitors. This underlines the unusually flexible nature of the pro-domain and implies a possible mechanism for regulation of TAFI intrinsic proteolytic activity in vivo.

Ensembles of uncertain mathematical models can identify network response to therapeutic interventions

The role of mechanistic modeling and systems biology in molecular medicine remains unclear. In this study, we explored whether uncertain models could be used to understand how a network responds to a therapeutic intervention. As a proof of concept, we modeled and analyzed the response of the human coagulation cascade to recombinant factor VIIa (rFVIIa) and prothrombin (fII) addition in normal and hemophilic plasma. An ensemble of parametrically uncertain human coagulation models was developed (N = 437). Each model described the time evolution of 193 proteins and protein complexes interconnected by 301 interactions under quiescent flow. The 467 unknown model parameters were estimated, using multiobjective optimization, from published in vitro coagulation studies. The model ensemble was validated using published in vitro thrombin measurements and thrombin measurements taken from coronary artery disease patients. Sensitivity analysis was then used to rank-order the importance of model parameters as a function of experimental or physiological conditions. A novel strategy for the systematic comparison of ranks identified a family of fX/FXa and fII/FIIa interactions that became more sensitive with decreasing fVIII/fIX. The fragility of these interactions was preserved following the addition of exogenous rFVIIa and fII. This suggested that exogenous rFVIIa did not alter the qualitative operation of the cascade. Rather, exogenous rFVIIa and fII took advantage of existing fluid and interfacial fX/FXa and fII/FIIa sensitivity to restore normal coagulation in low fVIII/fIX conditions. The proposed rFVIIa mechanism of action was consistent with experimental literature not used in model training. Thus, we demonstrated that an ensemble of uncertain models could unravel key facets of the mechanism of action of a focused intervention. Whereas the current study was limited to coagulation, perhaps the general strategy used could be extended to other molecular networks relevant to human health.

Thrombin activatable fibrinolysis inhibitor (TAFI): relationship to hemostatic alteration in patients with beta-thalassemia

Profound hemostatic changes have been observed among thalassemic patients. Thrombin activatable fibrinolysis inhibitor (TAFI) is a newly discovered protein that potentially attenuates fibrinolysis. The authors aimed to investigate plasma level of TAFI in beta-thalassemia patients in relation to clinical severity and hemostatic alteration. Fifty-one thalassemic patients (mean age 10.79 +/- 5.59 years) (21 splenectomized thalassemia major patients, 18 nonsplenectomized thalassemia major patients, 12 nonsplenectomized thalassemia intermedia) were recruited from Pediatric Hematology Clinic, Ain Shams University; in addition, 32 healthy age- and sex-matched controls (10.31 +/- 5.58 years) were also included. In addition to clinical assessment, laboratory investigations included complete blood count (CBC), hemoglobin electrophoresis, prothrombin time (PT), activated partial thromboplastin time (PTT), liver function tests, viral hepatitis markers, serum ferritin, and plasma TAFI levels. Nine out of 51 patients (17.5%) suffered from bleeding manifestations mainly in the form of epistaxis; none of the studied patients had thromboembolism. Significant reduction in TAFI levels was shown in thalassemic patients compared to controls (P < .0001), in splenectomized compared to nonsplenectomized thalassemia group (P < .0001), and in thalassemia major compared to thalassemia intermedia group (P < .0001). Negative correlation was present between TAFI levels and both liver enzymes and serum ferritin levels (P < .05). Thalassemic patients suffering from bleeding showed lower mean TAFI levels compared to those not suffering from bleeding (P < .001). Marked reduction in TAFI levels was observed in thalassemic patients with splenectomy, altered liver functions, and poor chelation who therefore might be at a higher risk for altered hemostasis.

Progress in metallocarboxypeptidases and their small molecular weight inhibitors

In what corresponds to a life span, metallocarboxypeptidases (MCPs) have jumped from being mere contaminants in animal pancreas powders (in depression year 1929) to be key players in cellular and molecular processes (in yet-another-depression years 2009-2010). MCPs are unique zinc-dependent enzymes that catalyze the breakdown of the amide bond at the C-terminus of peptide and protein substrates and participate in the recovery of dietary amino acids, tissue organogenesis, neurohormone and cytokine maturation and other important physiological processes. More than 26 genes code for MCPs in the human genome, many of them still waiting to be fully understood in terms of physiological function. A variety of MCPs have been linked to diseases in man: acute pancreatitis and pancreas cancer, type 2 diabetes, Alzheimer's Disease, various types of cancer, and fibrinolysis and inflammation. Many of these discoveries have been made possible thanks to recent advances, as exemplified by plasma carboxypeptidases N and B, known for fifty and twenty years, respectively, which have had their structures released only very recently. Plasma carboxypeptidase B is a biological target for therapy because of its involvement in the coagulation/fibrinolysis processes. Besides, the widespread use of carboxypeptidase A as a benchmark metalloprotease since the early days of Biochemistry has allowed the identification and design of an increasingly vast repertory of small molecular weight inhibitors. With these two examples we wish to emphasize that MCPs have become part of the drug discovery portfolio of pharmaceutical companies and academic research laboratories. This paper will review key developments in the discovery and design of MCP small molecular weight inhibitors, with an emphasis on the discovery of chemically diverse entities. Although encouraging advances have been achieved in the last few years, the specificity and oral bioavailability of the new chemotherapeutic agents seem to pose a challenge to medicinal chemists.

Insights into the molecular inactivation mechanism of human activated thrombin-activatable fibrinolysis inhibitor

SUMMARY BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a validated target for thrombotic diseases. TAFI is converted in vivo to activated TAFI (TAFIa) by removal of its pro-domain. Whereas TAFI is stable and persists in the circulation, possibly in complex with plasminogen, TAFIa is unstable and poorly soluble, with a half-life of minutes.

OBJECTIVES

In order to study the molecular determinants of this instability, we studied the influence of protein inhibitors on human TAFIa.

RESULTS

We found that protein inhibitors significantly reduced the instability and insolubility of TAFIa. In addition, we solved the 2.5-A resolution crystal structure of human TAFIa in complex with a potent protein inhibitor, tick-derived carboxypeptidase inhibitor, which gives rise to a stable and soluble TAFIa species. The structure revealed a significant reduction in the flexibility of dynamic segments when compared with the structures of bovine and human TAFI. We also identified two latent hotspots, loop Lbeta2beta3 and segment alpha5-Lalpha5beta7-beta7, where conformational destabilization may begin. These hotspots are also present in TAFI, but the pro-domain may provide sufficient stabilization and solubility to guarantee protein persistence in vivo. When the pro-domain is removed, the free TAFIa moiety becomes unstable, its activity is suppressed, and the molecule becomes insoluble.

CONCLUSIONS

The present study corroborates the function of protein inhibitors in stabilizing human TAFIa and it provides a rigid and high-resolution mold for the design of small molecule inhibitors of this enzyme, thus paving the way for novel therapy for thrombotic disorders.

Carboxypeptidase U (TAFIa): a new drug target for fibrinolytic therapy?

Procarboxypeptidase U (TAFI) is a recently discovered plasma procarboxypeptidase that upon activation by thrombin or thrombin-thrombomodulin turns into a potent antifibrinolytic enzyme. Its prominent bridging function between coagulation and fibrinolysis raised the interest of many research groups and of the pharmaceutical industry. The development of carboxypeptidase U (CPU) inhibitors as profibrinolytic agents is an attractive concept and possibilities for rational drug design will become more readily available in the near future as a result of the recently published crystal structure. Numerous studies have been performed and many of them show beneficial effects of CPU inhibitors for the improvement of endogenous fibrinolysis in different animal sepsis and thrombosis models. CPU inhibitors combined with tissue-type plasminogen activator (t-PA) seem to increase the efficiency of pharmacological thrombolysis allowing lower dosing of t-PA and subsequently fewer bleeding complications. This review will focus on recently obtained in vivo data and the benefits/risks of targeting CPU for the treatment of thrombotic disorders.

Functional analysis of mutant variants of thrombin-activatable fibrinolysis inhibitor resistant to activation by thrombin or plasmin

BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) defines a pathway that functionally links the coagulation and fibrinolytic cascades. TAFI is activated by proteolytic cleavage, a reaction that can be performed by thrombin and plasmin, but most efficiently by thrombin in complex with the endothelial cofactor thrombomodulin (TM). The respective roles of these activators in regulating the TAFI pathway are largely unknown.

OBJECTIVE AND METHODS

In the present study, we constructed and expressed mutant variants of TAFI that have key substitutions in the amino acids surrounding the scissile Arg92-Ala93 bond.

RESULTS AND CONCLUSIONS

We identified variants that showed patterns of resistance to specific activators. For example, the P91S, R92K and S90P variants exhibited specific impairment of activation by thrombin or thrombin-TM, thrombin alone, and thrombin alone or plasmin, respectively. The variants that we tested also showed antifibrinolytic potentials that can be rationalized in terms of which enzymes are capable of activating them. On the other hand, certain predictions from peptide studies of mutations that would be expected to interfere with plasmin cleavage were not satisfied by our data, indicating that protein context, as well as the identity of amino acids at protease cleavage sites, dictates protease specificity.

The activation peptide of thrombin-activatable fibrinolysis inhibitor: a role in activity and stability of the enzyme?

BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a 56-kDa procarboxypeptidase. Proteolytic enzymes activate TAFI into TAFIa, an inhibitor of fibrinolysis, by cleaving off the N-terminal activation peptide (amino acids 1-92), from the enzyme moiety. Activated TAFI is unstable, with a half-life of approximately 10 min at 37 degrees C. So far, it is unknown whether the activation peptide is released or remains attached to the catalytic domain, and whether it influences TAFIa's properties. The current study was performed to clarify these issues.

METHODS

TAFI was activated, and the activity and half-life of the enzyme were determined in the presence and absence of the activation peptide.

RESULTS

TAFIa was active both before and after removal of the activation peptide, and the half-life of TAFIa was identical in the two preparations. Furthermore, we observed that intrinsically inactivated TAFIa (TAFIai) aggregated into large, insoluble complexes that could be removed by centrifugation.

CONCLUSIONS

The data presented in this article show that the activation peptide of TAFI is not required for TAFIa activity and that the activation peptide has no effect on the stability of the enzyme. These results are in favour of a model in which the activation peptide solely stabilizes the structure of the proenzyme. After activation of TAFI and subsequent breakage of interactions between the activation peptide and the catalytic domain, the activation peptide is no longer capable of performing this stabilizing task, and the integrity of the catalytic domain is lost rapidly. The resulting TAFIai is more prone to proteolysis and aggregation.

Activated thrombin activatable fibrinolysis inhibitor levels are associated with the risk of cardiovascular death in patients with coronary artery disease: the AtheroGene study

BACKGROUND

Thrombin activatable fibrinolysis inhibitor (TAFI) attenuates fibrinolysis. Results on the association between TAFI levels and the risk of coronary artery disease (CAD) are inconsistent.

OBJECTIVES

We investigated the association between TAFI levels and the risk of cardiovascular events in CAD.

PATIENTS/METHODS

1668 individuals with angiographically proven CAD at baseline were followed for a median of 2.3 years, as part of the prospective AtheroGene cohort. Fifty-six deaths from cardiovascular (CV) causes and 35 non-fatal CV events were observed.

RESULTS

At baseline, three TAFI measurements were available: one evaluating the total amount of TAFI (t-TAFI), one measuring the TAFIa/TAFIai amount, and the last the released activated peptide (TAFI-AP). TAFIa/TAFIai levels were associated with increased risk of CV death [hazard ratio (HR) for one tertile increase, 2.38 (1.56-3.63); P < 10(-4)]. This association remained significant after adjustment for conventional risk factors, CRP levels, white blood count and markers of thrombin generation and fibrinolysis [HR = 1.69 (1.07-2.67); P = 0.01]. In addition, CPB2 gene polymorphisms explained 12%, 6%, and 3% of t-TAFI, TAFIa/TAFIai and TAFI-AP levels, respectively, but none was associated with CV events.

CONCLUSIONS

The amount of activated TAFI, measured by TAFIa/TAFIai ELISA, but not of the t-TAFI is independently associated with the risk of CV death.

The roles of selected arginine and lysine residues of TAFI (Pro-CPU) in its activation to TAFIa by the thrombin-thrombomodulin complex

Thrombomodulin (TM) increases the catalytic efficiency of thrombin (IIa)-mediated activation of thrombin-activable fibrinolysis inhibitor (TAFI) 1250-fold. Negatively charged residues of the C-loop of TM-EGF-like domain 3 are required for TAFI activation. Molecular models suggested several positively charged residues of TAFI with which the C-loop residues could interact. Seven TAFI mutants were constructed to determine if these residues are required for efficient TAFI activation. TAFI wild-type or mutants were activated in the presence or absence of TM and the kinetic parameters of TAFI activation were determined. When the three consecutive lysine residues in the activation peptide of TAFI were substituted with alanine (K42/43/44A), the catalytic efficiencies for TAFI activation with TM decreased 8-fold. When other positively charged surface residues of TAFI (Lys-133, Lys-211, Lys-212, Arg-220, Lys-240, or Arg-275) were mutated to alanine, the catalytic efficiencies for TAFI activation with TM decreased by 1.7-2.7-fold. All decreases were highly statistically significant. In the absence of TM, catalytic efficiencies ranged from 2.8-fold lower to 1.24-fold higher than wild-type. None of these, except the 2.8-fold lower value, was statistically significant. The average half-life of the TAFIa mutants was 8.1+/-0.6 min, and that of wild type was 8.4+/-0.3 min at 37 degrees C. Our data show that these residues are important in the activation of TAFI by IIa, especially in the presence of TM. Whether the mutated residues promote a TAFI-TM or TAFI-IIa interaction remains to be determined. In addition, these residues do not influence spontaneous inactivation of TAFIa.

Discovery of novel mechanisms and molecular targets for the inhibition of activated thrombin activatable fibrinolysis inhibitor

BACKGROUND

Thrombin activatable fibrinolysis inhibitor (TAFI) is an important regulator of fibrinolysis and an attractive target to develop profibrinolytic drugs.

OBJECTIVE

To analyze the (inhibitory) properties of five monoclonal antibodies (mAbs) directed towards rat TAFI (i.e. MA-RT13B2, MA-RT30D8, MA-RT36A3F5, MA-RT36B2 and MA-RT82F12).

METHODS AND RESULTS

Direct interference of the mAb with rat activated TAFI (TAFIa) activity was assayed using a chromogenic activity assay. This revealed reductions of 79% +/- 1%, 54% +/- 4%, and 19% +/- 2% in activity in the presence of a 16-fold molar excess of MA-RT13B2, MA-RT36A3F5, and MA-RT82F12, respectively whereas MA-RT30D8 and MA-RT36B2 had no direct inhibitory effect. Additionally, MA-RT13B2 and MA-RT36A3F5 reduced rat TAFIa half-life by 56% +/- 2% and 61% +/- 3%. Tissue-type plasminogen activator mediated in vitro clot lysis was determined using rat plasma. Compared to potato tuber carboxypeptidase inhibitor, MA-RT13B2, MA-RT30D8, MA-RT36A3F5, and MA-RT82F12 reduced clot lysis times by 86% +/- 14%, 100% +/- 5%, 100% +/- 10%, and 100% +/- 11%, respectively. During epitope mapping, Arg(227) and Ser(251) were identified as major residues interacting with MA-RT13B2. Arg(188) and His(192) contribute to the interaction with MA-RT36A3F5. Arg(227), Ser(249), Ser(251), and Tyr(260) are involved in the binding of MA-RT30D8 and MA-RT82F12 with rat TAFI(a). The following mechanisms of inhibition have been deduced: MA-RT13B2 and MA-RT36A3F5 have a destabilizing effect on rat TAFIa whereas MA-RT30D8 and MA-RT82F12 partially block the access to the active site of TAFIa or interact with the binding of TAFIa to the blood clot.

CONCLUSIONS

The described inhibitory mAb towards rat TAFIa will facilitate TAFI research in murine models. Additionally, we reveal novel molecular targets for the direct inhibition of TAFIa through different mechanisms.

The crystal structure of thrombin-activable fibrinolysis inhibitor (TAFI) provides the structural basis for its intrinsic activity and the short half-life of TAFIa

Mature thrombin-activable fibrinolysis inhibitor (TAFIa) is a highly unstable metallocarboxypeptidase that stabilizes blood clots by clipping C-terminal lysine residues from partially degraded fibrin. In accordance with its in vitro antifibrinolytic activity, animal studies have reported that inhibition of mature TAFI aids in the prevention of thrombosis. The level of TAFI activity is stringently regulated through (i) controlled proteolytic truncation of the zymogen (TAFI), generating the mature enzyme, TAFIa, and (ii) the short half-life of TAFIa. TAFI itself exhibits an intrinsic enzymatic activity, which is likely required to provide a baseline level of antifibrinolytic activity. The novel crystal structure presented here reveals that the active site of TAFI is accessible, providing the structural explanation for the its intrinsic activity. It also supports the notion that an "instability region" exists, in agreement with site-directed mutagenesis studies. Sulfate ions, bound to this region, point toward a potential heparin-binding site and could explain how heparin stabilizes TAFIa.

Structure of activated thrombin-activatable fibrinolysis inhibitor, a molecular link between coagulation and fibrinolysis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a metallocarboxypeptidase (MCP) that links blood coagulation and fibrinolysis. TAFI hampers fibrin-clot lysis and is a pharmacological target for the treatment of thrombotic conditions. TAFI is transformed through removal of its prodomain by thrombin-thrombomodulin into TAFIa, which is intrinsically unstable and has a short half-life in vivo. Here we show that purified bovine TAFI activated in the presence of a proteinaceous inhibitor renders a stable enzyme-inhibitor complex. Its crystal structure reveals that TAFIa conforms to the alpha/beta-hydrolase fold of MCPs and displays two unique flexible loops on the molecular surface, accounting for structural instability and susceptibility to proteolysis. In addition, point mutations reported to enhance protein stability in vivo are mainly located in the first loop and in another surface region, which is a potential heparin-binding site. The protein inhibitor contacts both the TAFIa active site and an exosite, thus contributing to high inhibitory efficiency.

Thrombin-activable fibrinolysis inhibitor zymogen does not play a significant role in the attenuation of fibrinolysis

Activated thrombin-activable fibrinolysis inhibitor (TAFIa) plays a significant role in the prolongation of fibrinolysis. During fibrinolysis, plasminogen is activated to plasmin, which lyses a clot by cleaving fibrin after selected arginine and lysine residues. TAFIa attenuates fibrinolysis by removing the exposed C-terminal lysine residues. It was recently reported that TAFI zymogen possesses sufficient carboxypeptidase activity to attenuate fibrinolysis through a mechanism similar to TAFIa. Here, we show with a recently developed TAFIa assay that when thrombin is used to clot TAFI-deficient plasma supplemented with TAFI, there is some TAFI activation. The extent of activation was dependent upon the concentration of zymogen present in the plasma, and lysis times were prolonged by TAFIa in a concentration-dependent manner. Potato tuber carboxypeptidase inhibitor, an inhibitor of TAFIa but not TAFI, abolished the prolongation of lysis in TAFI-deficient plasma supplemented with TAFI zymogen. In addition, TAFIa but not TAFI catalyzed release of plasminogen bound to soluble fibrin degradation products. The data presented confirm that TAFI zymogen is effective in cleaving a small substrate but does not play a role in the attenuation of fibrinolysis because of its inability to cleave plasmin-modified fibrin degradation products.

Biochemical importance of glycosylation in thrombin activatable fibrinolysis inhibitor

Activated Thrombin Activatable Fibrinolysis Inhibitor (TAFIa) exerts an antifibrinolytic effect by removing C-terminal lysines from partially degraded fibrin. These lysines are essential for a rapid conversion of plasminogen to plasmin by tissue type plasminogen activator. TAFI is heavily glycosylated at Asn22, Asn51, Asn63, and Asn86. Although the glycans occurring at the glycosylation sites have previously been identified, the biochemical role of these glycans is not known yet. Therefore, we have determined the biochemical importance of the glycosylation in TAFI. Four single, 6 double, 4 triple, and 1 quadruple mutant, in which asparagine was replaced by glutamine, were constructed and transfected into HEK293T cells. Based on the determination of antigen and activity levels on conditioned medium, 4 single and 1 triple mutant were purified and their biochemical properties were determined. The glycosylation knockout mutants did neither reveal an altered fragmentation pattern nor differences in TAFIa stability, but TAFI-N51Q, TAFI-N63Q, and TAFI-N22Q-N51Q-N63Q revealed a decreased TAFIa activity, an increased intrinsic catalytic activity of the zymogen, and a decreased antifibrinolytic potential compared with TAFI-wild-type, whereas TAFI-N22Q and TAFI-N86Q revealed an increased antifibrinolytic potential probably because of an increased catalytic efficiency toward the physiological substrate. From these data it can be concluded that mainly the glycosylation at Asn86 contributes to the biochemical characteristics of TAFI. Furthermore we provide evidence that the activation peptide stays in close proximity to the TAFIa moiety after activation.

An assay for measuring functional activated thrombin-activatable fibrinolysis inhibitor in plasma

Thrombin-activatable fibrinolysis inhibitor (TAFI), also called procarboxypeptidase U (proCPU), is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa, CPU) removes C-terminal lysine residues of plasmin-modified fibrin (FN') that mediates a positive feedback mechanism in plasminogen (Pg) activation, thereby attenuating fibrinolysis. The plasma concentration of TAFI is approximately 75 nM. Because the half-maximal effect of TAFIa occurs at 1 nM, only approximately 1.3% of TAFI needs to be activated to exert an effect on clot lysis. The assay is performed by mixing soluble FN' covalently attached to a quencher and fluorescein-labeled Pg. The sample containing TAFIa is then added, and the rate of fluorescence increase due to removal of C-terminal lysine from FN' and loss of Pg binding is measured with a fluorescence plate reader. The assay was shown to be sensitive for TAFIa at a concentration as low as 12 pM. The intraassay variability and interassay variability of the assay were 6.3 and 8.3%, respectively. This assay was not confounded by the naturally occurring TAFI Thr325Leu polymorphism that affects the thermal stability of TAFIa or endogenous plasminogen in plasma.

Thrombin activatable fibrinolysis inhibitor (TAFI): A role in pre-eclampsia?

Pre-eclampsia (P-Ec) is a complex multisystem disorder of unknown aetiology reported to occur in about 6% to 8% of all pregnancies throughout the world. This disease is associated with fibrin deposition and occlusive lesions in placental vessels. Pro-thrombin activatable fibrinolysis inhibitor (pro-TAFI) is a relatively recently described glycoprotein that can be converted into its active form (TAFIa) by thrombin, thrombin-thrombomodulin and plasmin. TAFIa potentially inhibits fibrinolysis by removing C-terminal lysine and arginine residues from fibrin. These residues are required for adsorption of tissue-type plasminogen activator (t-PA) and plasminogen to fibrin. Therefore, TAFIa decreases plasmin formation and protects the fibrin clot against lysis. An increased of pro-TAFI/TAFIa levels has been reported in some clinical conditions associated with thrombotic tendency, as type II diabetes mellitus, deep vein thrombosis and symptomatic artery disease. Few studies have investigated pro-TAFI/TAFIa in normal or complicated pregnancy but contrasting results were reported. Understanding the role of pro-TAFI/TAFIa in the pathogenesis of P-Ec can hold great promise for improving P-Ec management. In this context, a large-scale study evaluating plasma TAFI antigen and activity, its synthesis and metabolism in pre-eclamptic women is required. Recently new selective TAFIa inhibitors have been developed. The design of a new therapy to treat and/or prevent P-Ec, based on successful use of TAFIa inhibitors, may have significant clinical ramifications.

The role of the S1 binding site of carboxypeptidase M in substrate specificity and turn-over

The influence of the P1 amino acid on the substrate selectivity, the catalytic parameters K(m) and k(cat), of carboxypeptidase M (CPM) (E.C. 3.4.17.12) was systematically studied using a series of benzoyl-Xaa-Arg substrates. CPM had the highest catalytic efficiency (k(cat)/K(m)) for substrates with Met, Ala and aromatic amino acids in the penultimate position and the lowest with amino acids with branched side-chains. Substrates with Pro in P1 were not cleaved in similar conditions. The P1 substrate preference of CPM differed from that of two other members of the carboxypeptidase family, CPN (CPN/CPE subfamily) and CPB (CPA/CPB subfamily). Aromatic P1 residues discriminated most between CPM and CPN. The type of P2 residue also influenced the k(cat) and K(m) of CPM. Extending the substrate up to P7 had little effect on the catalytic parameters. The substrates were modelled in the active site of CPM. The results indicate that P1-S1 interactions play a role in substrate binding and turn-over.

Curiouser and curiouser: recent advances in measurement of thrombin-activatable fibrinolysis inhibitor (TAFI) and in understanding its molecular genetics, gene regulation, and biological roles

The thrombin-activatable fibrinolysis inhibitor (TAFI) pathway defines a novel molecular connection between blood coagulation and both fibrinolysis and inflammation. TAFI is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa) attenuates fibrinolysis by removing the carboxyl-terminal lysine residues from partially degraded fibrin that mediate positive feedback in the fibrinolytic cascade. A role for TAFIa in modulating inflammation is suggested by the ability of this enzyme to down-regulate pericellular plasminogen activation and to inactivate the inflammatory peptides bradykinin and the anaphylatoxins C3a and C5a. The focus of this review is on recent advances in the clinical measurement of the TAFI pathway in human subjects and what this has revealed in terms of the molecular genetics of TAFI, the biological variation in plasma TAFI antigen levels, potential regulators of expression of the gene encoding TAFI, and the TAFI pathway as a risk factor for the development of vascular diseases. Although this field is in its infancy, much recent progress has been made and the available data suggest that the TAFI pathway is an intriguing new player in a variety of physiological and pathophysiological contexts.

Regulation of fibrinolysis by thrombin activatable fibrinolysis inhibitor, an unstable carboxypeptidase B that unites the pathways of coagulation and fibrinolysis

The coagulation and fibrinolytic systems safeguard the patency of the vasculature and surrounding tissue. Cross regulation of coagulation and fibrinolysis plays an important role in preserving a balanced hemostatic process. Identification of Thrombin Activatable Fibrinolysis Inhibitor (TAFI) as an inhibitor of fibrinolysis and one of the main intermediates between coagulation and fibrinolysis, greatly improved our understanding of cross regulation of coagulation and fibrinolysis. As TAFI is an enzyme that is activated by thrombin generated by the coagulation system, its activation is sensitive to the dynamics of the coagulation system. Defects in coagulation, such as in thrombosis or hemophilia, resonate in TAFI-mediated regulation of fibrinolysis and imply that clinical symptoms of coagulation defects are amplified by unbalanced fibrinolysis. Thrombomodulin promotes the generation of both antithrombotic activated protein C (APC) and prothrombotic (antifibrinolytic) activated TAFI, illustrating the paradoxical effects of thrombomodulin on the regulation of coagulation and fibrinolysis. This review will discuss the role of TAFI in the regulation of fibrinolysis and detail its regulation of activation and its potential therapeutic applications in thrombotic disease and bleeding disorders.

A rapid and sensitive assay for the quantitation of carboxypeptidase N, an important regulator of inflammation

BACKGROUND

Carboxypeptidase N is a plasma zinc metallocarboxypeptidase which is constitutively expressed in the liver and was identified as the enzyme responsible for inactivating bradykinin and kallidin by removing the C-terminal arginine. Because CPN can cleave the C-terminal arginine of C3a, C4a and C5a it is often referred to as anaphylatoxin inactivator. Markedly reduced levels of circulating CPN are associated with recurrent angioedema and abnormal cutaneous polymorphonuclear cell infiltration.

METHODS

In this paper we describe a fast kinetic coupled enzymatic assay for the sensitive measurement of carboxypeptidase N activities in serum samples. The assay makes use of the excellent CPN substrate Benzoyl-L-Alanyl-L-Arginine.

RESULTS

This novel assay is very fast, easy to perform and combines good reliability and reproducibility with excellent correlation with the HPLC-assisted assay (r=0.927; n=140).

CONCLUSION

The presented assay can be used for high throughput screening of this important regulator of inflammation in clinical plasma or serum samples.

Fine mapping of quantitative trait nucleotides underlying thrombin-activatable fibrinolysis inhibitor antigen levels by a transethnic study

Recent studies revisiting the association between plasma thrombin-activatable fibrinolysis inhibitor (TAFI) Ag levels and polymorphisms of the CPB2 gene (coding for TAFI) suggested that TAFI Ag levels were influenced by 2 major quantitative trait nucleotides (QTNs) in European whites. However, the strong linkage disequilibrium (LD) between CPB2 polymorphisms in European whites did not allow one to distinguish which polymorphisms could be the putative QTNs. To get a better insight into the identification of QTNs, a transethnic haplotype analysis contrasting 2 populations of African and European subjects was performed using 13 CPB2 polymorphisms. Results of the haplotype analyses suggested that 3 QTNs had independent effects and explained about 15% of the TAFI variability, consistently in the 2 populations. The lower LD observed in the African population enabled us to identify the 1583T>A SNP located in 3'UTR as one of these QTNs, whereas the -2599C>G and -2345--2344insG SNPs located in the 5' region might be the 2 other QTNs. A phylogenetic study suggested that these 3 polymorphisms occurred before the period of migration "out of Africa." Although this transethnic comparison contributed to better map the putative CPB2 QTNs, further studies are required to clarify the role of the promoter region.

Post-translational modifications of human thrombin-activatable fibrinolysis inhibitor (TAFI): evidence for a large shift in the isoelectric point and reduced solubility upon activation

Thrombin-activable fibrinolysis inhibitor (TAFI) is distinct from pancreatic procarboxypeptidase B in several ways. The enzymatic activity of TAFIa is unstable and decays with a half-life of a few minutes. During this study, we observed that (i) the isoelectric point (pI) of TAFI shifts dramatically from pH 5 toward pH 8 upon activation and (ii) TAFIa is significantly less soluble than TAFI. The structural bases for these observations were investigated by characterizing all post-translational modifications, including attached glycans and disulfide connectivity. The analyses revealed that all five potential N-glycosylation sites were utilized including Asn22, Asn51, Asn63, Asn86 (located in the activation peptide), and Asn219 (located in the catalytic domain). Asn219 was also found in an unglycosylated variant. Four of the glycans, Asn51, Asn63, Asn86, and Asn219 displayed microheterogeneity, while the glycan attached to Asn22 appeared to be homogeneous. In addition, bisecting GlcNAc attached to the trimannose core was detected, suggesting an origin other than the liver. Monosaccharide composition and LC-MS/MS analyses did not produce evidence for O glycosylation. TAFI contains eight cysteine residues, of which two, Cys69 and Cys383, are not involved in disulfides and contain free sulfhydryl groups. The remaining six cystines form disulfides, including Cys156-Cys169, Cys228-Cys252, and Cys243-Cys257. This pattern is homologous to pancreatic procarboxypeptidase B, and it is therefore unlikely that permutations in the cysteine connectivity are responsible for the enzymatic instability. LC-MS/MS analyses covering more than 90% of the TAFI amino acid sequence revealed no additional modifications. When these results are taken together, they suggest that the inherent instability of TAFIa is not caused by post-translational modifications. However, after activation, TAFIa loses 80% of the attached glycans, generating a large shift in pI and a propensity to precipitate. These changes are likely to significantly affect the properties of TAFIa as compared to TAFI.

Generation of a stable activated thrombin activable fibrinolysis inhibitor variant

Activated thrombin activable fibrinolysis inhibitor (TAFIa), generated upon activation of TAFI, exerts an antifibrinolytic effect. TAFIa is a thermolabile enzyme, inactivated through a conformational change. The objective of the current study was to generate a stable variant of human TAFIa. Using a site-directed as well as a random mutagenesis approach to generate a library of TAFI mutants, we identified two mutations that increase TAFIa stability, i.e. a Ser305 to Cys and a Thr329 to Ile mutation, respectively. Combining these mutations in TAFI-Ala147-Ile325, the most stable isoform of TAFIa (half-life of 9.4 +/- 0.4 min), revealed a TAFIa half-life of 70 +/- 3.1 min (i.e. an 11-fold increase versus 6.3 +/- 0.3 min for TAFIa-Ala147-Thr325, the most frequently occurring isoform of TAFI in humans) at 37 degrees C. Moreover, clot lysis (induced by tissue plasminogen activator) experiments in which TAFI-Ala147-Cys305-Ile325-Ile329 was added to TAFI-depleted plasma revealed a 50% clot lysis time of 313 +/- 77 min (i.e. a 3.0-fold increase versus 117 +/- 10 min for TAFI-Ala147-Thr325). The availability of a more stable TAFIa variant will facilitate the search for inhibitors and allow further structural analysis to elucidate the mechanisms of the instability of TAFIa.

Demonstration of enhanced endogenous fibrinolysis in thrombin activatable fibrinolysis inhibitor-deficient mice

To investigate the importance of thrombin activatable fibrinolysis inhibitor (TAFI) in the stabilization of plasma clots, we have compared fibrinolysis in TAFI-deficient (KO) and wild-type (WT) littermate mice. TAFI-deficient mice were previously generated by targeted gene disruption. The level of TAFI activity generated in plasma from WT mice in the presence of added thrombin and thrombomodulin (activatable TAFI) is twice that of plasma from TAFI heterozygous mice (HET); no activatable TAFI is detected in TAFI KO plasma. In vitro, TAFI KO plasma clots lysed faster than WT plasma clots, and HET plasma clots lysed at an intermediate rate. The rate of clot lysis for KO mice is not changed in the presence of potato carboxypeptidase inhibitor, a specific inhibitor of TAFIa, whereas the WT and HET clot lysis rates are increased in the presence of potato carboxypeptidase inhibitor. C-terminal lysine residues are preserved on partially degraded clots from KO mice, but are absent from partially degraded WT clots. In vivo, in a batroxobin-induced pulmonary embolism model, KO mice displayed a lower retention of fibrin in the lungs than did WT mice. These results are the first demonstration of enhanced endogenous fibrinolysis in an in vivo model without the addition of exogenous thrombolytic.

High dose factor VIIa improves clot structure and stability in a model of haemophilia B

Factor IX (FIX) deficiency results in haemophilia B and high dose recombinant activated factor VII (rFVIIa) can decrease bleeding. Previously, we showed that FIX deficiency results in a reduced rate and peak of thrombin generation. We have now used plasma and an in vitro coagulation model to examine the effect of these changes in thrombin generation on fibrin clot structure and stability. Low FIX delayed the clot formation onset and reduced the fibrin polymerisation rate. Clots formed without FIX were composed of thicker fibrin fibres than normal. rFVIIa shortened the clot formation onset time and improved the fibre structure of haemophilic clots. We also examined clot formation in the presence of a fibrinolytic challenge by including tissue plasminogen activator or plasmin in the reaction milieu. In these assays, normal FIX levels supported clot formation; however, clots did not form in the absence of FIX. rFVIIa partially restored haemophilic clot formation. These results were independent of the effects of the thrombin-activatable fibrinolysis inhibitor. Our data suggest that rFVIIa enhances haemostasis in haemophiliacs by increasing the thrombin generation rate to both promote formation of a structurally normal clot and improve clot formation and stability at sites with high endogenous fibrinolytic activities.

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.

Development of ELISAs measuring the extent of TAFI activation

OBJECTIVE

To date, quantitation of TAFI antigen levels has been mainly focused on "total" antigen levels and has been shown to yield ambiguous results because of the existence of different isoforms and various degrees of activation. Our objective was to develop assays that allow measuring the extent of TAFI activation.

METHODS AND RESULTS

A variety of enzyme-linked immunosorbent assays (ELISAs) were evaluated for their preferential reactivity toward TAFI before and after activation, and toward the recombinantly expressed activation peptide. Three ELISAs with distinct reactivities were selected: recognizing either exclusively nonactivated TAFI, the released activation peptide, or exclusively TAFIa (activated TAFI). Evaluation of TAFI activation during clot lysis revealed that decreases of TAFI levels are associated with increases of the released activation peptide and TAFIa levels. In addition, antigenic measurement of TAFIa parallels activity measured by chromogenic assay. Analyzing plasma samples revealed that subjects with hyperlipidemia had significantly higher plasma levels of both the activation peptide (109.2 versus 95.5; P<0.001) and TAFIa (112.1 versus 103.3; P=0.03), and not of TAFI antigen (92.5 versus 87.9; P=0.07) (results in % of plasma pooled from normolipidemic subjects).

CONCLUSIONS

ELISAs that allow to measure the extent of TAFI activation were developed. These ELISAs constitute more sensitive markers in studies on the relationship between TAFI and cardiovascular diseases.

Modulation of TAFI function through different pathways--implications for the development of TAFI inhibitors

OBJECTIVE

To elucidate the mechanism and the binding regions of monoclonal antibodies (MA) that interfere with thrombin-activatable fibrinolysis inhibitor (TAFI)/activated thrombin-activatable fibrinolysis inhibitor (TAFIa) activity.

RESULTS

Of 42 MA, 19 interfere with the TAFI activation/TAFIa activity resulting in an inhibition of up to 92%. Characterization of the mechanism of inhibition revealed that 14 MA blocked the activation of TAFI by thrombin/thrombomodulin completely whereas five MA interfered directly with the enzymatic activity of TAFIa. Surprisingly, the former, except one, induced a significant reduction of clot lysis time whereas the latter did not. Affinity studies using a human/murine TAFI chimer revealed that the binding region of the 14 activation blocking MA is located between AA1 and AA67. MA that inhibit exclusively the activation of TAFI by thrombin/thrombomodulin bind to Gly66. A MA that inhibits the activation of TAFI by both thrombin/thrombomodulin and plasmin binds to Val41. The MA that interfere with the enzymatic activity bind to the TAFIa moiety.

CONCLUSIONS

The current study reveals at least three different putative molecular targets in the search for pharmacologically active compounds to modulate TAFIa activity.

Measurement of procarboxypeptidase U (TAFI) in human plasma: a laboratory challenge

BACKGROUND

The importance of carboxypeptidase U (CPU) as a novel regulator of the fibrinolytic rate has attracted much interest during recent years. CPU circulates in plasma as a zymogen, proCPU, that can be activated by thrombin, thrombin-thrombomodulin (T-Tm), or plasmin. Given that the proCPU concentration in plasma is far below its K(m) for activation by the T-Tm complex, the formation of CPU will be directly proportional to the proCPU concentration. A low or high proCPU plasma concentration might therefore tip the balance between profibrinolytic and antifibrinolytic pathways and thereby cause a predisposition to bleeding or thrombosis.

CONTENT

To measure plasma proCPU concentrations, different methods have been developed based on 2 different principles: antigen determination and measurement of CPU activity after quantitative conversion of the proenzyme to its active form by addition of T-Tm. The major drawbacks that should be kept in mind when analyzing clinical samples by both principles are reviewed.

CONCLUSIONS

proCPU is a potential prothrombotic risk factor. Evaluation of its relationship with thrombosis requires accurate assays. Many assays used in different clinical settings are inadequately validated, forcing reconsideration of conclusions made in these reports.

Does an enzyme other than thrombin contribute to unexpected changes in the levels of the different forms of thrombin activatable fibrinolysis inhibitor in patients with hemophilia A, hemophilia B and von Willebrand disease?

Pro-thrombin activatable fibrinolysis inhibitor (pro-TAFI), also called plasma procarboxypeptidase B or U, is one of the modulators of fibrinolysis in blood. Pro-TAFI is activated by thrombin/thrombomodulin complex or by plasmin to a carboxypeptidase B-like enzyme (TAFI) of 35.8 kD molecular weight. TAFI spontaneously becomes inactive as a result of a temperature-dependent conformational change in the protein (TAFIi). In this study, pro-TAFI, total TAFI antigen and TAFI-TAFIi antigen levels were measured in 32 patients with hemophilia A, 4 patients with hemophilia B, 21 patients with von Willebrand disease (VWD) and 13 healthy controls. A statistically significant decrease in pro-TAFI was found in all groups (10.72+/-4.57 mg/L (p<0.001); 8.00+/-2.35 mg/L (p<0.01) and 8.98+/-2.33 mg/L (p <0.001) for hemophilia A, hemophilia B and VWD, respectively) compared to controls (17.85+4.61 mg/L). A statistically significant increase in TAFI-TAFIi antigen was found in hemophilia A (1.05+/-1.01 mg/L) (p<0.05) and in VWD patients (0.96+/-1.01 mg/L) (p<0.05) compared to controls (0.55+/-0.36 mg/L). There was no difference in total TAFI antigen levels between any group of patients and the controls. Neither did pro-TAFI nor TAFI-TAFIi levels differ within the group of hemophilia A patients in relation to severity (mild, moderate and severe) or among the VWD patients in relation to subtype (type 1, type 2A and type 3). These findings indicate an increased conversion of pro-TAFI to TAFI and/or TAFIi in patients with bleeding disorders. As thrombin generation is seriously impaired in these patients and almost absent in hemophilia A and B and in type 3 VWD, it is possible that plasmin mediates pro-TAFI activation in these patients. Enhanced fibrinolysis via generation of plasmin has previously been reported in hemophilia and VWD. Activation of pro-TAFI by plasmin may be a feedback mechanism that counterbalances increased fibrinolysis in patients with bleeding disorders. The relationship between the TAFI activation pathway and bleeding complications associated with hemophilia A, hemophilia B and VWD requires further investigation.

Crystal Structures of Potent Thiol-Based Inhibitors Bound to Carboxypeptidase B,

This paper presents the crystal structure of porcine pancreatic carboxypeptidase B (pp-CpB) in complex with a variety of thiol-based inhibitors that were developed as antagonists of activated thrombin-activatable fibrinolysis inhibitor (TAFIa). Recent studies have indicated that a selective inhibitor of TAFIa could enhance the efficacy of existing thrombolytic agents for the treatment of acute myocardial infarction, one of the most prevalent forms of heart attacks. Unfortunately, activated TAFIa rapidly degrades in solution and cannot be used for crystallographic studies. In contrast, porcine pancreatic CpB is stable at room temperature and is available from commercial sources. Both pancreatic CpB and TAFIa are zinc-based exopeptidases, and the proteins share a 47% sequence identity. The homology improves considerably in the active site where nearly all of the residues are conserved. The inhibitors used in this study were designed to mimic a C-terminal arginine residue, one of the natural substrates of TAFIa. The X-ray structures show that the thiol group chelates the active site zinc, the carboxylic acid forms a salt bridge to Arg145, and the guanidine group forms two hydrogen bonds to Asp255. A meta-substituted phenyl was introduced into our inhibitors to reduce conformational freedom. This modification vastly improved the selectivity of compounds against other exopeptidases that cleave basic residues. Comparisons between structures indicate that selectivity derives from the interaction between the guanidine group in the inhibitors and an acidic active site residue. The location of this acidic residue is not conserved in the various carboxypeptidases.

Role of isoleucine residues 182 and 183 in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a procarboxypeptidase that, once activated, can attenuate fibrinolysis. The active form, TAFIa, is a labile enzyme, with a half-life of a few minutes at 37 degrees C. Understanding the molecular mechanisms of TAFIa inactivation will allow the development of compounds that modulate TAFIa activity. Based on their three-dimensional model of TAFI, Barbosa Pereira et al. [J Mol Biol (2002), vol. 321, pp. 537-547] suggested that Ile182 and Ile183 were involved in the instability of TAFIa. However, these carboxypeptidases are, unlike TAFIa, stable proteases. Therefore, we constructed, expressed and characterized a TAFI mutant in which Ile182 and Ile183 were changed into the residues found in pancreas carboxypeptidase B at corresponding positions, Arg and Glu. The active form of the mutant, TAFIa-I182R-I183E, had a similar half-life as wild-type TAFIa, showing that Ile182 and Ile183 were not involved in the regulation of TAFIa stability. Remarkably, however, TAFI-I182R-I183E was activated at a lower rate by thrombin-thrombomodulin (mutant: 45 +/- 2 U L(-1) s(-1) and wild type: 103 +/- 3 U L(-1) s(-1)), thrombin (mutant: 1 +/-0.1 U L(-1) s(-1) and wild type 3 +/- 0.2 U L(-1) s(-1)) and plasmin (mutant: 0.8 +/- 0.04 U L(-1) s(-1) and wild type: 5.0 +/-0.2 U L(-1) s(-1)) compared with wild-type TAFI. Accordingly, it had a sixfold reduced antifibrinolytic potential. In conclusion, analysis of TAFI-I182R-I183E showed that I182 and I183 are not involved in TAFIa inactivation by conformational instability but that these residues may be involved in the activation of TAFI and stabilization of the fibrin clot.

A functional assay for measuring activated thrombin-activatable fibrinolysis inhibitor in plasma

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a procarboxypeptidase found in plasma that is activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The active carboxypeptidase, TAFIa, attenuates fibrinolysis by removing newly exposed carboxy-terminal lysine residues on fibrin. The half-maximal effect of TAFIa on clot lysis occurs at 1 nM and the maximal effect occurs at 20 nM. Since the circulating concentration of the procarboxypeptidase is approximately 75 nM, only a small portion needs to be activated to have a significant effect on clot lysis. Several assays to measure total plasma TAFI levels and plasma TAFIa levels after it is fully activated exist. However, no currently available assay is sufficiently sensitive and specific to measure endogenous TAFIa in plasma. We have devised a new sensitive and specific assay for TAFIa in plasma that is based on physiologic function. This assay is based on the fact that TAFIa decreases the cofactor activity of high-molecular-weight fibrin degradation products in the stimulation of plasminogen cleavage in a concentration-dependent fashion. With this assay, we can measure TAFIa concentrations as low as 10 pM in plasma and it is not affected by variability in other hemostatic factors. This assay is reliable and repeatable with intra- and interassay variabilities of 6.5 and 6.1%, respectively.

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.

Antithrombotic agents in the treatment of severe sepsis

Sepsis, a systemic inflammatory syndrome, is a response to infection and when associated with multiple organ dysfunction is termed severe sepsis. It remains a leading cause of mortality in the critically ill. The response to the invading microorganisms may be considered as a balance between a pro-inflammatory and an anti-inflammatory reaction. While an inadequate pro-inflammatory reaction and a strong anti-inflammatory response could lead to overwhelming infection and the death of the patient, a strong and uncontrolled pro-inflammatory response, manifested by the release of pro-inflammatory mediators may lead to microvascular thrombosis and multiple organ failure. Endotoxin triggers sepsis via the release of various mediators such as tumour necrosis factor-alpha and interleukin-1 (IL-1). These cytokines activate the complement and coagulation systems, release adhesion molecules, prostaglandins, leukotrienes, reactive oxygen species and nitric oxide. Other mediators involved in the sepsis syndrome include IL-1, -6 and -8; arachidonic acid metabolites; platelet activating factor; histamine; bradykinin; angiotensin; complement components and vasoactive intestinal peptide. These pro-inflammatory responses are counteracted by IL-10. Most of the trials targeting the different mediators of the pro-inflammatory response have failed due to a lack of correct definition of sepsis. Understanding the exact pathophysiology of the disease will enable more advanced treatment options. Targeting the coagulation system with various anticoagulant agents including, activated protein C, and tissue factor pathway inhibitor (TFPI) is a rational approach. Many clinical trials have been conducted to evaluate these agents in severe sepsis. While trials on antithrombin and TFPI were not so successful, the double-blind, placebo-controlled, Phase III trial of recombinant human activated Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) was successful, creating a significant decrease in mortality when compared to the placebo group. A better understanding of the pathophysiologic mechanism of severe sepsis will provide better treatment options, and combination antithrombotic treatment may provide a multipronged approach for the treatment of severe sepsis.

Properties of recombinant human plasma procarboxypeptidase U produced in mammalian and insect cells

BACKGROUND

Carboxypeptidase U (EC 3.4.17.20, TAFIa) is a new member of the metallocarboxypeptidase family circulating in human plasma as a zymogen. It is activated during coagulation and is considered as an important player in the regulation of fibrinolysis.

METHODS

Heterologous expression of human plasma procarboxypeptidase U (proCPU, TAFI) was obtained in mammalian cells (C127 and DON) and in insect cells (Sf21 and H5 cells). Conditioned media were purified by cation-exchange chromatography and plasminogen affinity chromatography to yield an essentially pure protein.

RESULTS

All systems gave high expression levels (6-20 mg/l). Due to differences in glycosylation of the activation peptide, the recombinant variants of proCPU migrated differently on SDS-PAGE (52-65 kDa). However, after activation, all active recombinant enzymes migrated at 35 kDa, similar to native CPU and no evidence for post-translational modification of the catalytic domains could be detected. For the mammalian cell produced variants, activation was more efficient after desialylation. After activation, CPU showed low solubility (0.2 mg/ml) but was inhibited similarly as native CPU.

CONCLUSIONS

Mammalian cell systems were the most efficient for the production of human plasma recombinant proCPU. The obtained zymogen differs with respect to the extent and the heterogeneity of glycosylation but, after activation, the experiments did not reveal any alteration between the recombinant and native protein.

Role of mRNA transcript stability in modulation of expression of the gene encoding thrombin activable fibrinolysis inhibitor

Regulation of mRNA stability has emerged as a major control point in eukaryotic gene expression. The abundance of a particular mRNA can be rapidly regulated in response to a stimulus by altering the stability of existing translatable transcripts rather than by altering the rate of transcription initiation. Alternative polyadenylation of transcripts during mRNA processing can be important in determining transcript abundance if the different forms of mRNA possess different stabilities or translatability. The mRNA transcript encoding thrombin activable fibrinolysis inhibitor (TAFI) is an attractive candidate for regulation of mRNA stability because of the relatively long length of its 3'-untranslated region and because the transcript can be polyadenylated at three different sites. As well, we have previously reported that treatment of HepG2 cells with interleukins (IL) - 1beta and - 6 destabilizes the endogenous TAFI mRNA expressed in this cell line. In the current study, we report that the TAFI 3'-untranslated region contains cis-acting instability element(s) and that these elements in fact determine the intrinsic stability of the TAFI transcript. Moreover, we found that the three different polyadenylated mRNA forms have different intrinsic stabilities, with the mRNA half-life increasing from the longest to the shortest transcript. Interestingly, treatment with IL-1beta plus IL-6 not only resulted in a 2-fold decrease in stability of the transcript produced using the 3'-most polyadenylation site but also resulted in profound shifts in the relative abundances of the respective polyadenylated forms through changes in the frequency of utilization of the three polyadenylation sites. As such, in the presence of IL-1beta and IL-6, the longest transcript is over a thousand times more abundant than the two shorter transcripts whereas in the absence of the stimulus it comprises only 1% of the total TAFI transcripts.

The intrinsic threshold of the fibrinolytic system is modulated by basic carboxypeptidases, but the magnitude of the antifibrinolytic effect of activated thrombin-activable fibrinolysis inhibitor is masked by its instability

Activated thrombin-activable fibrinolysis inhibitor (TAFIa) is intrinsically unstable, a property that complicates the study of its role in regulating fibrinolysis. To investigate the effect of basic carboxypeptidases on fibrinolysis under conditions of constant carboxypeptidase activity, we employed pancreatic carboxypeptidase B (CPB), a homologous, stable basic carboxypeptidase, as a surrogate for TAFIa. Clots formed from TAFI-depleted plasma or from purified components were supplemented with tissue-type plasminogen activator and either CPB or TAFIa. The clot lysis data indicate that the down-regulation of fibrinolysis mediated by basic carboxypeptidases involves a threshold mechanism. At carboxypeptidase concentrations above the threshold, plasminogen activation is maintained in a fully down-regulated state; experiments in plasma showed that fibrinolysis is essentially halted by saturating concentrations of TAFIa and that fibrinolysis can be prolonged more than 45-fold by a stable carboxypeptidase. The threshold carboxypeptidase concentration was dependent on tissue-type plasminogen activator and antiplasmin concentrations, indicating that the threshold is determined by the steady-state plasmin concentration. Although obvious with CPB, the threshold was masked by the intrinsic instability of TAFIa and became apparent only when the effect of TAFIa was investigated over the picomolar concentration range. Because of the threshold effect and the instability of TAFIa, exponential increases in TAFIa concentration generate linear increases in lysis time. A model relating lysis time to TAFIa concentration, TAFIa half-life, and the threshold concentration of TAFIa is provided. The threshold effect has potentially important implications regarding the role of TAFIa and the regulation of clot lysis in vivo.

Migration of the activation peptide of thrombin-activatable fibrinolysis inhibitor (TAFI) during SDS-polyacrylamide gel electrophoresis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma zymogen, which upon activation is capable of delaying fibrinolysis. We investigated the migration and detection of the activation peptide of TAFI during SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Purified TAFI before and after activation by thrombin/thrombomodulin was electrophoresed on 4-20% polyacrylamide gels and stained with Coomassie blue as well as Western blotting. Before activation, Coomassie blue staining resulted in one main band of TAFI. After activation, a sharp band corresponding to TAFIa was observed. No distinct activation peptide was detected, in agreement with the literature. Western blotting using a polyclonal anti-TAFI antibody, on the other hand, showed one additional broad band with an Mr of about 33 000 after TAFI activation. N-terminal sequence analysis confirmed that this band represented the activation peptide of TAFI. In addition, we tested the reactivity of two anti-TAFI monoclonal antibodies (MA-T3D8 and MA-T18A8) towards TAFI before and after activation by Western blotting. Both monoclonal antibodies recognized TAFI. After activation of TAFI, MA-T3D8 reacted with TAFIa, while MA-T18A8 reacted with the activation peptide. We identify the 33 000 band as the activation peptide of TAFI and exemplify the use of this information for the characterization of monoclonal antibodies against TAFI.

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.