Amplified anticoagulant activity of tissue factor-targeted thrombomodulin: in-vivo validation of a tissue factor-neutralizing antibody fused to soluble thrombomodulin

Tissue factor (TF) exposure is a potent pro-thrombotic trigger that initiates activation of the coagulation cascade, while thrombomodulin (TM) is a potent anticoagulant protein that limits the extent of activation. Both TF neutralizing antibodies and soluble TM (sTM) are effective anticoagulants. We have developed a novel anticoagulant fusion protein, Ab(TF)-TM, by fusing a TFneutralizing single-chain antibody, Ab(TF), to an active fragment of TM. Ab(TF)-TM is a novel anticoagulant targeting to sites of TF exposure with a dual mechanism of action. The Ab(TF) portion of the molecule inhibitsTF/factorVIIa mediated activation of FIX and FX, and the TM portion of the molecule acts as a cofactor for activation of protein C. In-vitro coagulation assays show that Ab(TF)-TM more potently inhibits TF-initiated coagulation (prothrombin time) than can its individual components, Ab(TF) (20-fold) and sTM (80-fold) alone, or in combination (10-fold). In contrast, the potency of Ab(TF)-TM in the activated partial thromboplastin and thrombin clotting time assays was similar to sTM alone. In a rat model of disseminated intravascular coagulation (DIC), intravenous injection of a human TF-containing thromboplastin reagent (0.5 ml/kg) resulted in an immediate death in approximately 60% of the animals and a clinical score of approximately 2.5. Pre-injection of Ab(TF)-TM or Ab(TF) and sTM, given alone or in combination, showed dose-dependent efficacy. At a dose of 0.7 nmol/kg, Ab(TF)-TM completely prevented death and reduced clinical scores by 79%, while neitherAb(TF) nor sTM, given alone or in combination, showed significant therapeutic effects. Calculated effective doses that reduced mortality by 50% relative to that in the control group (ED(50), nmol/kg) were 0.21 for Ab(TF)-TM, 3.2 for an equimolar mixture of Ab(TF) and sTM, 4.3 for sTM and 20 for Ab(TF). Thus, Ab(TF)-TM presented 10- to 100-fold enhancement of the anticoagulant potency, relative to the ED(50) in Ab(TF) and sTM given either alone or in combination, in a rat DIC model.

Allosteric disulfide bonds in thrombosis and thrombolysis

Allosteric disulfide bonds control protein function by mediating conformational change when they undergo reduction or oxidation. The known allosteric disulfide bonds are characterized by a particular bond geometry, the -RHStaple. A number of thrombosis and thrombolysis proteins contain one or more disulfide bonds of this type. Tissue factor (TF) was the first hemostasis protein shown to be controlled by an allosteric disulfide bond, the Cys186-Cys209 bond in the membrane-proximal fibronectin type III domain. TF exists in three forms on the cell surface: a cryptic form that is inert, a coagulant form that rapidly binds factor VIIa to initiate coagulation, and a signaling form that binds FVIIa and cleaves protease-activated receptor 2, which functions in inflammation, tumor progression and angiogenesis. Reduction and oxidation of the Cys186-Cys209 disulfide bond is central to the transition between the three forms of TF. The redox state of the bond appears to be controlled by protein disulfide isomerase and NO. Plasmin(ogen), vitronectin, glycoprotein 1balpha, integrin beta(3) and thrombomodulin also contain -RHStaple disulfides, and there is circumstantial evidence that the function of these proteins may involve cleavage/formation of these disulfide bonds.

The lectin-like domain of thrombomodulin interferes with complement activation and protects against arthritis

BACKGROUND

Thrombomodulin (TM) is predominantly a vascular endothelial cell plasma membrane glycoprotein that, via distinct structural domains, interacts with multiple ligands, thereby modulating coagulation, fibrinolysis, complement activation, inflammation and cell proliferation. We previously reported that by mediating signals that interfere with mitogen-activated protein kinase and nuclear factor kappaB pathways, the amino-terminal C-type lectin-like domain of TM has direct anti-inflammatory properties.

METHODS

In the current study, we use murine models of acute inflammatory arthritis and biochemical approaches to assess the mechanism by which the lectin-like domain of TM modifies disease progression.

RESULTS

Mice lacking the lectin-like domain of TM (TM(LeD/LeD)mice) develop inflammatory arthritis that is more rapid in onset and more severe than that developed in their wildtype counterparts. In two models of arthritis, treatment of mice with recombinant soluble lectin-like domain of TM significantly suppresses clinical evidence of disease and diminishes monocyte/macrophage infiltration into the synovium, with weaker expression of the pro-inflammatory high mobility group box chromosomal protein 1. While thrombin-TM mediated activation of thrombin activatable fibrinolysis inhibitor inactivates complement factors C3a and C5a, we show that TM has a second independent mechanism to regulate complement: the lectin-like domain of TM directly interferes with complement activation via the classical and lectin pathways.

CONCLUSIONS

These data extend previous insights into the mechanisms by which TM modulates innate immunity, and highlight its potential as a therapeutic target for inflammatory diseases.

Blood coagulation and propagation of autowaves in flow

This study analyses the effect of flow and boundary reactions on spatial propagation of waves of blood coagulation. A simple model of coagulation in plasma consisting of three differential reaction-diffusion equations was used for numerical simulations. The vessel was simulated as a two-dimensional channel of constant width, and the anticoagulant influence of thrombomodulin present on the undamaged vessel wall was taken into account. The results of the simulations showed that this inhibition could stop the coagulation process in the absence of flow in narrow channels. For the used mathematical model of coagulation this was the case if the width was below 0.2 mm. In wider vessels, the process could be stopped by the rapid blood flow. The required flow rate increased with the increase of the damage region size. For example, in a 0.5-mm wide channel with 1-mm long damage region, the propagation of coagulation may be terminated at the flow rate of more than 20 mm/min.

Activation of protein C by the thrombin-thrombomodulin complex: cooperative roles of Arg-35 of thrombin and Arg-67 of protein C

The binding of Ca(2+) to the 70-80 loop of protein C inhibits protein C activation by thrombin in the absence of thrombomodulin (TM), but the metal ion is required for activation in the presence of TM. Structural data suggests that the 70-80 loop is located between two antiparallel beta strands comprised of residues 64-69 and 81-91 on the protease domain of protein C. To test the hypothesis that a salt-bridge/hydrogen bond interaction between Arg-67 of the former strand and Asp-82 of the latter strand modulates the unique Ca(2+)-binding properties of protein C, we engineered a disulfide bond between the two strands by substituting both Arg-67 and Asp-82 with Cys residues. The activation of this mutant was enhanced 40- to 50-fold independent of TM and Ca(2+). Furthermore, the Arg-67 to Ala mutant of protein C was activated in the absence of TM by the Arg-35 to Glu mutant of thrombin with the same efficiency as wild-type protein C by wild-type thrombin-TM complex. These results suggest that TM functions by alleviating the Ca(2+)-dependent inhibitory interactions of Arg-67 of protein C and Arg-35 of thrombin.

Carbohydrate and Protein Immobilization onto Solid Surfaces by Sequential Diels−Alder and Azide−Alkyne Cycloadditions

We demonstrate the applicability of sequential Diels-Alder and azide-alkyne [3 + 2] cycloaddition reactions (click chemistry) for the immobilization of carbohydrates and proteins onto a solid surface. An alpha,omega-poly(ethylene glycol) (PEG) linker carrying alkyne and cyclodiene terminal groups was synthesized and immobilized onto an N-(epsilon-maleimidocaproyl) (EMC)-functionalized glass slide via an aqueous Diels-Alder reaction. In the process, an alkyne-terminated PEGylated surface was provided for the conjugation of azide-containing biomolecules via click chemistry, which proceeded to completion at low temperature and in aqueous solvent. As anticipated, alkyne, azide, cyclodiene, and EMC are independently stable and do not react with common organic reagents nor functional groups in biomolecules. Given an appropriate PEG linker, sequential Diels-Alder and azide-alkyne [3 + 2] cycloaddition reactions provide an effective strategy for the immobilization of a wide range of functionally complex substances onto solid surfaces.

Catalytic efficiency of a thrombomodulin-functionalized membrane-mimetic film in a flow model

The protein C anticoagulant pathway generates an "on demand" physiologic anticoagulant response, which is initiated when thrombin binds to thrombomodulin (TM), a transmembrane protein constitutively expressed by endothelial cells. A stable, protein C activating membrane-mimetic film was produced on a polyelectrolyte multilayer (PEM) by in situ photopolymerization of a phospholipid assembly containing TM. The monoacrylated phospholipid monomer was initially synthesized and prepared as unilamellar vesicles with varying molar concentrations of TM. Membrane-mimetic films were constructed on planar substrates with defined surface concentrations of catalytically active TM. 125I-labeled radiolabeling demonstrated little change in TM surface concentration over periods of up to 4 weeks. We utilized a parallel plate flow system to investigate the effects of simulated arterial (500 s(-1)) and venous (50 s(-1)) shear rates and TM surface concentration (0-1400 fmol cm(-2)) on the rate and extent of activation of protein C. The rate of production of activated protein C increased with shear rate and TM surface content. However, in agreement with an analysis of reaction kinetics and mass transfer, experimental results demonstrate that reaction rates become saturated at TM surface densities greater than or equal to 800 fmol cm(-2). We believe that the design of membrane-mimetic films that have the capacity to activate the protein C pathway will provide a useful strategy for generating "actively" antithrombogenic surfaces.

Thrombomodulin Tightens the Thrombin Active Site Loops To Promote Protein C Activation

Thrombomodulin (TM) forms a 1:1 complex with thrombin. Whereas thrombin alone cleaves fibrinogen to make the fibrin clot, the thrombin-TM complex cleaves protein C to initiate the anticoagulant pathway. Crystallographic investigations of the complex between thrombin and TMEGF456 did not show any changes in the thrombin active site. Therefore, research has focused recently on how TM may provide a docking site for the protein C substrate. Previous work, however, showed that when the thrombin active site was occupied with substrate analogues labeled with fluorophores, the fluorophores responded differently to active (TMEGF1-6) versus inactive (TMEGF56) fragments of TM. To investigate this further, we have carried out amide H/(2)H exchange experiments on thrombin in the presence of active (TMEGF45) and inactive (TMEGF56) fragments of TM. Both on-exchange and off-exchange experiments show changes in the thrombin active site loops, some of which are observed only when the active TM fragment is bound. These results are consistent with the previously observed fluorescence changes and point to a mechanism by which TM changes the thrombin substrate specificity in favor of protein C cleavage.

Soluble thrombomodulin activity and soluble thrombomodulin antigen in plasma

Endothelial cell membrane-bound thrombomodulin (TM) plays a critical role as a cofactor in the protein C pathway, important in regulating coagulation as well as inflammation. Heterogeneous soluble TM fragments circulate in the plasma and are found at increased levels in various diseases such as cardiovascular disease and diabetes, and in ischemic and/or inflammatory endothelial injuries. The anticoagulant function of these soluble fragments has not been measured in healthy individuals or in patients. Using an immobilized monoclonal antibody against TM and a microtiter plate format, an assay was designed to capture the soluble TM fragments in plasma and measure their cofactor activity in the thrombin-mediated activation of protein C. In addition, soluble TM antigen levels were measured by enzyme-linked immunosorbent assay. Both assays were used to investigate a group of healthy blood donors. TM fragments released into plasma were shown to retain significant cofactor activity, and reference intervals for healthy men and women were established. Furthermore, a statistically significant correlation was observed between soluble TM antigen levels and soluble TM cofactor activity. This notwithstanding, soluble TM activity only accounted for a minor part of all variation in soluble TM antigen levels (R2 = 22% in men and R2 = 16% in women).

Dose-response study of recombinant human soluble thrombomodulin (ART-123) in the prevention of venous thromboembolism after total hip replacement

BACKGROUND

Recombinant human soluble thrombomodulin (ART-123) is composed of the active, extracellular, domain of thrombomodulin. ART-123 binds to thrombin and this complex converts protein C into the natural anticoagulant activated protein C. This study was performed to identify an effective and safe dose of ART-123 for prevention of venous thromboembolism after elective, unilateral total hip replacement.

METHODS AND RESULTS

An open-label, sequential, dose-ranging study was performed in which 312 patients received either 0.3 mg kg(-1) or 0.45 mg kg(-1) of ART-123, subcutaneously, 2-4 h after surgery (day 1). Those who received 0.3 mg kg(-1) were given a second dose of 0.3 mg kg(-1) on day 6, and the first 29 of these patients also used intermittent pneumatic compression devices. Those who received 0.45 mg kg(-1) were not given a second dose. Primary efficacy outcome was all deep vein thrombosis on mandatory bilateral venography performed on day 9 +/- 2 and symptomatic venous thromboembolism up to day 11. Primary safety outcome was major bleeding up to day 11. Among patients who did not use intermittent pneumatic compression, venous thromboembolism occurred in 3.4% of 116 evaluable patients in the 0.3 mg kg(-1) group and 0.9% of 111 patients in the 0.45 mg kg(-1) group. Major bleeding occurred in 1.4% of 139 patients in the 0.3 mg kg(-1) group and 6.3% of 144 patients in the 0.45 mg kg(-1) group.

CONCLUSION

ART-123 is a highly effective antithrombotic agent that should be directly compared with current methods of prophylaxis in patients who have major orthopedic surgery.

Phenotyping the haemostatic system by thrombography--potential for the estimation of thrombotic risk

The aim of this paper is to review the thrombogram and its use for phenotyping the haemostatic system. The thrombogram can be readily obtained through Calibrated Automated Thrombography (CAT), using a commercially available fluorometer, dedicated software (Thrombinoscope) and a calibrator. Here we explore the possibility to use platelet-rich plasma (PRP) triggered with a low amount of recombinant human tissue factor (approximately 0.5 pM) and also explore the function of the protein C system by adding activated protein C (APC) or soluble recombinant thrombomodulin (TM). Examples are shown: inherited antithrombin (AT) and protein C deficiencies, and antiphospholipid antibodies.

Structural and functional consequences of methionine oxidation in thrombomodulin

Thrombomodulin (TM) is an endothelial cell surface glycoprotein that is responsible for switching the catalytic activity of thrombin away from fibrinogen cleavage (pro-coagulant) and towards protein C cleavage (anticoagulant). Although TM is a large protein, only the fourth and fifth epidermal growth factor-like (EGF-like) domains are required for anticoagulant function. These two domains must work together, and the linker between the two domains contains a single methionine residue, Met 388. Oxidation of Met 388 is deleterious for TM activity. Structural studies, both X-ray and NMR, of wild type and variants at position 388 show that Met 388 provides a key linkage between the two domains. Oxidation of the methionine has consequences for the structure of the fifth domain, which binds to thrombin. Oxidation also appears to disrupt the interdomain contacts resulting in structural and dynamic changes. The functional consequences of oxidation of Met 388 include decreased anticoagulant activity. Oxidative stress from several causes is reflected in lower serum levels of activated protein C and a higher thrombotic tendency, and this is thought to be linked to the oxidation of Met 388 in TM. Thus, TM structure and function are altered in a subtle but functionally critical way upon oxidation of Met 388.

Evidence of human thrombomodulin domain as a novel angiogenic factor

BACKGROUND

Thrombomodulin is an anticoagulant, endothelial-cell-membrane glycoprotein. A recombinant thrombomodulin domain containing 6 epidermal growth factor-like structures exhibits mitogenic activity. This study explored the novel angiogenic effects of the recombinant domain using in vitro and in vivo models.

METHODS AND RESULTS

Human recombinant thrombomodulin containing 6 epidermal growth factor-like structures (TMD2) and TMD2 plus a serine and threonine-rich domain (TMD23) were prepared using the Pichia pastoris expression system. Combined with purified TMD2 or TMD23, thrombin effectively activated protein C. TMD23 had higher activity than TMD2 in stimulating DNA synthesis in cultured human umbilical vein endothelial cells. Additionally, TMD23 stimulated chemotactic motility and capillarylike tube formation in human umbilical vein endothelial cells, an effect mediated through phosphorylation of extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase and the phosphatidylinositol-3 kinase/Akt/endothelial nitric oxide synthase pathway. TMD23 also stimulated endothelial cell expression of matrix metalloproteinases and plasminogen activators, which mediated extracellular proteolysis, leading to endothelial cell invasion and migration during angiogenesis. Furthermore, TMD23-containing implants in rat cornea induced ingrowth of new blood vessels from the limbus. With the murine angiogenesis assay, TMD23 not only induced neovascularization coinjected with Matrigel and heparin but also enhanced angiogenesis in Matrigel containing melanoma A2058 cells in nude mice.

CONCLUSIONS

The recombinant thrombomodulin domain TMD23 enhanced the angiogenic response in vitro and in vivo, suggesting that thrombomodulin fragments may play a role in the formation of new vessels. These findings may provide a new therapeutic option for treating ischemic diseases.

Dynamics of the fragment of thrombomodulin containing the fourth and fifth epidermal growth factor-like domains correlate with function

Thrombomodulin (TM) forms a 1:1 complex with thrombin. Whereas thrombin alone cleaves fibrinogen to make the fibrin clot, the thrombin-TM complex cleaves protein C to initiate the anticoagulant pathway. The fourth and fifth EGF-like domains of TM together form the minimal fragment with anticoagulant cofactor activity. A short linker connects the fourth and fifth EGF-like domains of TM, and Met 388 in the middle of the linker interacts with both domains. Several different structures of TMEGF45 variants are now available, and these show that mutation of Met 388 alters the structure of the fifth domain, as well as the connectivity of the two domains. To probe this phenomenon more thoroughly, NMR backbone dynamics experiments have been carried out on the individual fourth and fifth domains as well as on the wild type, the Met 388 Leu mutant, and the variant in which Met 388 is oxidized. The results presented here show that changes at Met 388 cause significant changes in backbone dynamics in both the fourth and fifth EGF-like domains of TM. Backbone dynamics within the small loop of the fourth domain Tyr 358 correlate with anticoagulant cofactor activity. Backbone dynamics of the thrombin-binding residues Tyr 413 and Ile 414 are inversely correlated with thrombin binding. The preordering of the backbone of Tyr 413 and Ile 414 only occurs in the two-domain fragments, revealing a role for the fourth domain in thrombin binding as well as in anticoagulant cofactor activity.

Thrombin-activatable fibrinolysis inhibitor

The coagulation system is a potent mechanism that prevents blood loss after vascular injury. It consists of a number of linked enzymatic reactions resulting in thrombin generation. Thrombin converts soluble fibrinogen into a fibrin clot. The clot is subsequently removed by the fibrinolytic system upon wound healing. Thrombin-activatable fibrinolysis inhibitor (TAFI), which is identical to the previously identified proteins procarboxypeptidase B, R, and U, forms a link between blood coagulation and fibrinolysis. TAFI circulates as an inactive proenzyme in the bloodstream, and becomes activated during blood clotting. The active form, TAFIa, inhibits fibrinolysis by cleaving off C-terminal lysine residues from partially degraded fibrin that stimulates the tissue-type plasminogen activator-mediated conversion of plasminogen to plasmin. Consequently, removal of these lysines leads to less plasmin formation and subsequently to protection of the fibrin clot from break down. Moreover, TAFI may also play a role in other processes such as, inflammation and tissue repair. In this review, recent developments in TAFI research are discussed.

The affinity of protein C for the thrombin.thrombomodulin complex is determined in a primary way by active site-dependent interactions

The interaction of thrombin (IIa) with thrombomodulin (TM) is essential for the efficient activation of protein C (PC). Interactions between PC and extended surfaces, likely contributed by TM within the IIa.TM complex, have been proposed to play a key role in PC activation. Initial velocities of PC activation at different concentrations of PC and TM could be accounted for by a model that did not require consideration of direct binding interactions between PC and TM. Reversible inhibitors directed toward the active site of IIa within the IIa.TM complex behaved as classic competitive inhibitors of both peptidyl substrate cleavage as well as PC activation. The ability of these small molecule inhibitors to block PC binding to the enzyme points to a principal role for active site-dependent substrate recognition in determining the affinity of IIa.TM for its protein substrate. Selective abrogation of active site docking by mutation of the P1 Arg in PC to Gln yielded an uncleavable derivative (PC(R15Q)). PC(R15Q) was a poor inhibitor (K(i) >or= 30 microm) of PC activation as well as peptidyl substrate cleavage by IIa.TM. Thus, inhibition by PC(R15Q) most likely results from its ability to weakly interfere with active site function rather than by blocking extended interactions with the enzyme complex. The data suggest a primary role for active site-dependent substrate recognition in driving the affinity of the IIa.TM complex for its protein substrate. Interactions between PC and extended surfaces contributed by IIa and/or TM within the IIa.TM complex likely contribute in a secondary or minor way to protein substrate affinity.

Molecular cloning of canine thrombomodulin cDNA and expression in normal tissues

Thrombomodulin (TM) is a glycoprotein localized mainly on endothelial cell surfaces, and is a major regulator of vascular thromboresistance. The entire open reading frame of canine TM cDNA comprises 1737 bp, encoding 578 amino acid residues. Comparison of the deduced amino acid sequence from canine TM with those of human, mouse, rat, rabbit and bovine (partial) TM sequences revealed 73.1%, 69.1%, 65.8%, 74.3% and 69.5% identity, respectively. Canine TM mRNA expression was confirmed by RT-PCR analysis in lung, liver, spleen, kidney, pancreas and lymph node, and was relatively low in heart, cerebrum, urinary bladder and uterus. The present results provide valuable data for research into canine coagulation disorders.

Thrombomodulin changes the molecular surface of interaction and the rate of complex formation between thrombin and protein C

The interaction of thrombin with protein C triggers a key down-regulatory process of the coagulation cascade. Using a panel of 77 Ala mutants, we have mapped the epitope of thrombin recognizing protein C in the absence or presence of the cofactor thrombomodulin. Residues around the Na(+) site (Thr-172, Lys-224, Tyr-225, and Gly-226), the aryl binding site (Tyr-60a), the primary specificity pocket (Asp-189), and the oxyanion hole (Gly-193) hold most of the favorable contributions to protein C recognition by thrombin, whereas a patch of residues in the 30-loop (Arg-35 and Pro-37) and 60-loop (Phe-60h) regions produces unfavorable contributions to binding. The shape of the epitope changes drastically in the presence of thrombomodulin. The unfavorable contributions to binding disappear and the number of residues promoting the thrombin-protein C interaction is reduced to Tyr-60a and Asp-189. Kinetic studies of protein C activation as a function of temperature reveal that thrombomodulin increases >1,000-fold the rate of diffusion of protein C into the thrombin active site and lowers the activation barrier for this process by 4 kcal/mol. We propose that the mechanism of thrombomodulin action is to kinetically facilitate the productive encounter of thrombin and protein C and to allosterically change the conformation of the activation peptide of protein C for optimal presentation to the thrombin active site.

Covalently immobilized thrombomodulin inhibits coagulation and complement activation of artificial surfaces in vitro

Thrombomodulin (TM) serves as the endothelial cell receptor for thrombin and alters its characteristics from pro- to anticoagulant. Additionally, it promotes the formation of activated protein C. We evaluated the conservation of the overall outcome of these functions in recombinant TM linked to artificial surfaces by incubation with human whole blood in vitro. TM was covalently immobilized through poly(ethylene glycol) (PEG) spacers onto thin films of poly(octadecene alt maleic anhydride) covering planar glass substrates. TM binding to the polymer films was achieved after active ester formation at the carboxylic acid terminus of the PEG spacers and thoroughly characterized by HPLC-based amino acid analysis, immunofluorescence and ellipsometry. TM-coated samples were incubated for 3h with freshly drawn whole human blood anticoagulated with heparin (5IU/ml) using in-house developed incubation systems. The substantially reduced activation of blood coagulation (TAT) for TM-coated samples correlates well with the degree of contact activation (bradykinin and FXIIa formation) while no significant effects were observed for the platelet activation (PF4). Further, complement activation (C5a levels), was strongly diminished at the TM-containing surfaces. We conclude that the suggested method for preparation of TM immobilization may serve to prepare model substrates for studies on TM interactions but similarly provides a promising coating strategy for blood contacting medical devices.

Thrombomodulin-protein C-EPCR system: integrated to regulate coagulation and inflammation

Late in the 18th century, William Hewson recognized that the formation of a clot is characteristic of many febrile, inflammatory diseases (Owen C. A History of Blood Coagulation. Rochester, Minnesota: Mayo Foundation; 2001). Since that time, there has been steady progress in our understanding of coagulation and inflammation, but it is only in the past few decades that the molecular mechanisms linking these 2 biologic systems have started to be delineated. Most of these can be traced to the vasculature, where the systems most intimately interact. Thrombomodulin (TM), a cell surface-expressed glycoprotein, predominantly synthesized by vascular endothelial cells, is a critical cofactor for thrombin-mediated activation of protein C (PC), an event further amplified by the endothelial cell protein C receptor (EPCR). Activated PC (APC), in turn, is best known for its natural anticoagulant properties. Recent evidence has revealed that TM, APC, and EPCR have activities that impact not only on coagulation but also on inflammation, fibrinolysis, and cell proliferation. This review highlights recent insights into the diverse functions of this complex multimolecular system and how its components are integrated to maintain homeostasis under hypercoagulable and/or proinflammatory stress conditions. Overall, the described advances underscore the usefulness of elucidating the relevant molecular pathways that link both systems for the development of novel therapeutic and diagnostic targets for a wide range of inflammatory diseases.

Thrombin generation in platelet-rich plasma as a tool for the detection of hypercoagulability in young stroke patients

The time course of the concentration of active thrombin in clotting plasma (the thrombogram) was measured by subsampling from platelet-rich plasma (PRP) and continuous chromogenic measurement of platelet-poor plasma (PPP) in 41 stroke patients under the age of 50, in whom stroke could not be attributed to cardioembolic disease, arterial dissection or vasculitis. A significant increase in the area under the thrombogram (endogenous thrombin potential, ETP) was seen in 23 patients. In 9 of them, ETP was increased in PRP but normal in PPP. High ETP in PRP was significantly associated with stroke, both in the middle and in the highest tercile of the ETP (odds ratio 5.1, range 1.8-15.1, and 3.7, range 1.3-10.3, respectively). A decreased sensitivity to the inhibitory action of thrombomodulin (TM) on thrombin generation was observed in 5 of 37 cases. No further definition of the cause of increased thrombin generation or TM resistance was attempted, except for the role of von Willebrand factor (vWF). ETP in PRP, platelet-derived procoagulant activity and vWF were correlated and higher in patients than in controls (p=0.002, p=0.045 and p=0.0006, respectively). This confirms the correlation between vWF level and stroke at young age found in epidemiological studies. It suggests that the role of vWF in thrombin generation, which has been demonstrated in vitro, may be the underlying mechanism of this correlation. In summary, hypercoagulability, defined as an increased capacity of the platelet plasma system to form thrombin, is found in over half of the patients under 50 years with an otherwise unexplained stroke. Sometimes it is due to increased plasma factor activity, sometimes to an increased procoagulant activity of the platelets.

Calibrated automated thrombin generation measurement in clotting plasma

Calibrated automated thrombography displays the concentration of thrombin in clotting plasma with or without platelets (platelet-rich plasma/platelet-poor plasma, PRP/PPP) in up to 48 samples by monitoring the splitting of a fluorogenic substrate and comparing it to a constant known thrombin activity in a parallel, non-clotting sample. Thus, the non-linearity of the reaction rate with thrombin concentration is compensated for, and adding an excess of substrate can be avoided. Standard conditions were established at which acceptable experimental variation accompanies sensitivity to pathological changes. The coefficients of variation of the surface under the curve (endogenous thrombin potential) are: within experiment approximately 3%; intra-individual: <5% in PPP, <8% in PRP; interindividual 15% in PPP and 19% in PRP. In PPP, calibrated automated thrombography shows all clotting factor deficiencies (except factor XIII) and the effect of all anticoagulants [AVK, heparin(-likes), direct inhibitors]. In PRP, it is diminished in von Willebrand's disease, but it also shows the effect of platelet inhibitors (e.g. aspirin and abciximab). Addition of activated protein C (APC) or thrombomodulin inhibits thrombin generation and reflects disorders of the APC system (congenital and acquired resistance, deficiencies and lupus antibodies) independent of concomitant inhibition of the procoagulant pathway as for example by anticoagulants.

Generation and Characterization of a Highly Stable Form of Activated Thrombin-activable Fibrinolysis Inhibitor

Activated thrombin-activable fibrinolysis inhibitor (TAFIa) is a carboxypeptidase B that can down-regulate fibrinolysis. TAFIa is a labile enzyme that can be inactivated by conformational instability or proteolysis. TAFI is approximately 40% identical to pancreatic carboxypeptidase B (CPB). In contrast to TAFIa, pancreatic CPB is a stable protease. We hypothesized that regions or residues that are not conserved in TAFIa compared with pancreatic CPB play a role in the conformational instability of TAFIa and that replacement of these non-conserved residues with residues of pancreatic CPB would lead to a TAFIa molecule with an increased stability. Therefore, we have expressed, purified, and characterized two TAFI-CPB chimeras: TAFI-CPB-(293-333) and TAFI-CPB-(293-401). TAFI-CPB-(293-333) could be activated by thrombin-thrombomodulin, but not as efficiently as wild-type TAFI. After activation, this mutant was unstable and was hardly able to prolong clot lysis of TAFI-deficient plasma. Binding of TAFI-CPB-(293-333) to both plasminogen and fibrinogen was normal compared with wild-type TAFI. TAFI-CPB-(293-401) could be activated by thrombin-thrombomodulin, although at a lower rate compared with wild-type TAFI. The activated mutant displayed a markedly prolonged half-life of 1.5 h. Plasmin could both activate and inactivate this chimera. Interestingly, this chimera did not bind to plasminogen or fibrinogen. TAFI-CPB-(293-401) could prolong the clot lysis time in TAFI-deficient plasma, although not as efficiently as wild-type TAFI. In conclusion, by replacing a region in TAFI with the corresponding region in pancreatic CPB, we were able to generate a TAFIa form with a highly stable activity.