Proteolytic activation of human factors IX and X by recombinant human factor VIIa: effects of calcium, phospholipids, and tissue factor

Platelet C3G: a key player in vesicle exocytosis, spreading and clot retraction

C3G is a Rap1 GEF that plays a pivotal role in platelet-mediated processes such as angiogenesis, tumor growth, and metastasis by modulating the platelet secretome. Here, we explore the mechanisms through which C3G governs platelet secretion. For this, we utilized animal models featuring either overexpression or deletion of C3G in platelets, as well as PC12 cell clones expressing C3G mutants. We found that C3G specifically regulates α-granule secretion via PKCδ, but it does not affect δ-granules or lysosomes. C3G activated RalA through a GEF-dependent mechanism, facilitating vesicle docking, while interfering with the formation of the trans-SNARE complex, thereby restricting vesicle fusion. Furthermore, C3G promotes the formation of lamellipodia during platelet spreading on specific substrates by enhancing actin polymerization via Src and Rac1-Arp2/3 pathways, but not Rap1. Consequently, C3G deletion in platelets favored kiss-and-run exocytosis. C3G also controlled granule secretion in PC12 cells, including pore formation. Additionally, C3G-deficient platelets exhibited reduced phosphatidylserine exposure, resulting in decreased thrombin generation, which along with defective actin polymerization and spreading, led to impaired clot retraction. In summary, platelet C3G plays a dual role by facilitating platelet spreading and clot retraction through the promotion of outside-in signaling while concurrently downregulating α-granule secretion by restricting granule fusion.

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Lyophilized apoptotic vesicle-encapsulated adhesive hydrogel sponge as a rapid hemostat for traumatic hemorrhage in coagulopathy

Rapid hemostasis of uncontrolled bleeding following traumatic injuries, especially accompanied by coagulopathies, remains a significant clinical challenge. Extracellular vesicles (EVs) show therapeutic effects for fast clotting. However, low yield, specific storage conditions, and lack of proper carriers have hindered EVs' clinical application. Herein, we establish an optimized procedure method to generate lyophilized mesenchymal stem cell-derived apoptotic vesicles (apoVs) with adhesive hydrogel sponge to show superior procoagulant activity for traumatic hemorrhage. Mechanistically, apoVs' procoagulant ability stems from their high tissue factor (TF) and phosphatidylserine (PS) expression independent of hemocytes and circulating procoagulant microparticles (cMPs). Their stable hemostatic capability was maintained after 2-month room temperature storage. Subsequently, we mixed apoVs with both phenylboronic acid grafted oxidized hyaluronic acid (PBA-HA) and poly(vinyl alcohol) (PVA) simultaneously, followed by lyophilization to construct a novel apoV-encapsulated hydrogel sponge (apoV-HS). Compared to commercial hemostats, apoV-HS exhibits rapid procoagulant ability in liver-laceration and femoral artery hemorrhage in rat and rabbit models of coagulopathies. The combination of high productivity, physiological stability, injectability, plasticity, excellent adhesivity, biocompatibility, and rapid coagulant property indicates that apoV-HS is a promising therapeutic approach for heavy hemorrhage in civilian and military populations.

Critical evaluation of kinetic schemes for coagulation

Two well-established numerical representations of the coagulation cascade either initiated by the intrinsic system (Chatterjee et al., PLOS Computational Biology 2010) or the extrinsic system (Butenas et al., Journal of Biological Chemistry, 2004) were compared with thrombin generation assays under realistic pathological conditions. Biochemical modifications such as the omission of reactions not relevant to the case studied, the modification of reactions related to factor XI activation and auto-activation, the adaptation of initial conditions to the thrombin assay system, and the adjustment of some of the model parameters were necessary to align in vitro and in silico data. The modified models are able to reproduce thrombin generation for a range of factor XII, XI, and VIII deficiencies, with the coagulation cascade initiated either extrinsically or intrinsically. The results emphasize that when existing models are extrapolated to experimental parameters for which they have not been calibrated, careful adjustments are required.

Longitudinal observations of TFPI levels in paediatric Haemophilia A patients

INTRODUCTION

Inhibition of tissue factor pathway inhibitor (TFPI) is a potential new mode of action to achieve haemostasis in haemophilia A and B patients.

AIM

Knowledge about potential developmental changes of TFPI levels during childhood are a prerequisite to translate adult doses of TFPI inhibitors to doses in paediatric patients.

METHODS

In this study we present longitudinal data for total TFPI concentrations (TFPI-T) and TFPI activity (TFPI-A) from 48 paediatric Haemophilia A patients in the age range from 3 to 18 years (2-12 observations per patient).

RESULTS

TFPI-T and TFPI-A tend to decrease over age during childhood. Lowest values were observed between 12 and <18 years. On average, TFPI-T and TFPI-A were lower in adolescent haemophilia patients than in adult haemophilia patients.

CONCLUSION

In summary, the presented information on TFPI levels in children adds to the current knowledge of developmental haemostasis and it can be helpful in evaluating how children respond to haemophilia treatment including the new class of anti-TFPI compounds.

The Tissue Factor Pathway in Cancer: Overview and Role of Heparan Sulfate Proteoglycans

Historically, the only focus on tissue factor (TF) in clinical pathophysiology has been on its function as the initiation of the extrinsic coagulation cascade. This obsolete vessel-wall TF dogma is now being challenged by the findings that TF circulates throughout the body as a soluble form, a cell-associated protein, and a binding microparticle. Furthermore, it has been observed that TF is expressed by various cell types, including T-lymphocytes and platelets, and that certain pathological situations, such as chronic and acute inflammatory states, and cancer, may increase its expression and activity. Transmembrane G protein-coupled protease-activated receptors can be proteolytically cleaved by the TF:FVIIa complex that develops when TF binds to Factor VII (PARs). The TF:FVIIa complex can activate integrins, receptor tyrosine kinases (RTKs), and PARs in addition to PARs. Cancer cells use these signaling pathways to promote cell division, angiogenesis, metastasis, and the maintenance of cancer stem-like cells. Proteoglycans play a crucial role in the biochemical and mechanical properties of the cellular extracellular matrix, where they control cellular behavior via interacting with transmembrane receptors. For TFPI.fXa complexes, heparan sulfate proteoglycans (HSPGs) may serve as the primary receptor for uptake and degradation. The regulation of TF expression, TF signaling mechanisms, their pathogenic effects, and their therapeutic targeting in cancer are all covered in detail here.

Dysregulation of Protein S in COVID-19

Coronavirus Disease 2019 (COVID-19) has been widely associated with increased thrombotic risk, with many different proposed mechanisms. One such mechanism is acquired deficiency of protein S (PS), a plasma protein that regulates coagulation and inflammatory processes, including complement activation and efferocytosis. Acquired PS deficiency is common in patients with severe viral infections and has been reported in multiple studies of COVID-19. This deficiency may be caused by consumption, degradation, or clearance of the protein, by decreased synthesis, or by binding of PS to other plasma proteins, which block its anticoagulant activity. Here, we review the functions of PS, the evidence of acquired PS deficiency in COVID-19 patients, the potential mechanisms of PS deficiency, and the evidence that those mechanisms may be occurring in COVID-19.

Total Plasma Protein S Is a Prothrombotic Marker in People Living With HIV

BACKGROUND

HIV-1 infection is associated with multiple procoagulant changes and increased thrombotic risk. Possible mechanisms for this risk include heigthened expression of procoagulant tissue factor (TF) on circulating monocytes, extracellular vesicles, and viral particles and/or acquired deficiency of protein S (PS), a critical cofactor for the anticoagulant protein C (PC). PS deficiency occurs in up to 76% of people living with HIV-1 (PLWH). As increased ex vivo plasma thrombin generation is a strong predictor of mortality, we investigated whether PS and plasma TF are associated with plasma thrombin generation.

METHODS

We analyzed plasma samples from 9 healthy controls, 17 PLWH on first diagnosis (naive), and 13 PLWH on antiretroviral therapy (ART). Plasma thrombin generation, total and free PS, PC, C4b-binding protein, and TF activity were measured.

RESULTS

We determined that the plasma thrombin generation assay is insensitive to PS, because of a lack of PC activation, and developed a modified PS-sensitive assay. Total plasma PS was reduced in 58% of the naive and 38% of the ART-treated PLWH samples and correlated with increased thrombin generation in the modified assay. Conversely, plasma TF was not increased in our patient population, suggesting that it does not significantly contribute to ex vivo plasma thrombin generation.

CONCLUSION

These data suggest that reduced total plasma PS contributes to the thrombotic risk associated with HIV-1 infection and can serve as a prothrombotic biomarker. In addition, our refined thrombin generation assay offers a more sensitive tool to assess the functional consequences of acquired PS deficiency in PLWH.

Anticoagulation targeting membrane-bound anionic phospholipids improves outcomes of traumatic brain injury in mice

Severe traumatic brain injury (TBI) often causes an acute systemic hypercoagulable state that rapidly develops into consumptive coagulopathy. We have recently demonstrated that TBI-induced coagulopathy (TBI-IC) is initiated and disseminated by brain-derived extracellular vesicles (BDEVs) and propagated by extracellular vesicles (EVs) from endothelial cells and platelets. Here, we present results from a study designed to test the hypothesis that anticoagulation targeting anionic phospholipid-expressing EVs prevents TBI-IC and improves the outcomes of mice subjected to severe TBI. We evaluated the effects of a fusion protein (ANV-6L15) for improving the outcomes of TBI in mouse models combined with in vitro experiments. ANV-6L15 combines the phosphatidylserine (PS)-binding annexin V (ANV) with a peptide anticoagulant modified to preferentially target extrinsic coagulation. We found that ANV-6L15 reduced intracranial hematoma by 70.2%, improved neurological function, and reduced death by 56.8% in mice subjected to fluid percussion injury at 1.9 atm. It protected the TBI mice by preventing vascular leakage, tissue edema, and the TBI-induced hypercoagulable state. We further showed that the extrinsic tenase complex was formed on the surfaces of circulating EVs, with the highest level found on BDEVs. The phospholipidomic analysis detected the highest levels of PS on BDEVs, as compared with EVs from endothelial cells and platelets (79.1, 15.2, and 3.5 nM/mg of protein, respectively). These findings demonstrate that TBI-IC results from a trauma-induced hypercoagulable state and may be treated by anticoagulation targeting on the anionic phospholipid-expressing membrane of EVs from the brain and other cells.

Acute promyelocytic leukemia presenting as recurrent venous and arterial thrombotic events: a case report and review of the literature

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia characterized by a translocation of chromosomes 15 and 17, creating an alternation in the retinoic acid receptor-alpha (RAR-alpha) gene. This leads to excessive medullary production of promyelocytic blasts, which are frequently associated with the hemorrhagic complications seen in APL. In contrast, APL-associated thrombosis occurs much less frequently and is an underappreciated life-threatening manifestation of the disease. Most thrombotic events occur during induction chemotherapy with all-transretinoic acid and are rarely seen as the initial presentation on APL. Here we report an exceedingly rare case of a patient with recurrent venous and arterial thrombotic events, including deep vein thrombosis, bilateral segmental pulmonary embolism, an ischemic stroke, splenic infarcts, and renal infarcts, later found to have APL. We aim to discuss the most recent understanding of the pathogenesis of APL-associated thrombosis and to summarize the literature of this rare presentation of APL.

Very low-dose recombinant Factor VIIa administration for cardiac surgical bleeding reduces red blood cell transfusions and renal risk: a matched cohort study

Outcomes following administration of very-low-dose recombinant activated factor VIIa (vld-rFVIIa) for cardiac surgical bleeding remain debatable. We sought to determine the association of vld-rFVIIa and adverse surgical outcomes. Retrospective, cohort matching of patients undergoing cardiac surgery who received vld-rFVIIa (median 13.02 μg/kg) for perioperative bleeding were matched to cardiac surgical patients who had bleeding and received standard of care for bleeding without Factor VIIa administration. Of the 362 matched patients (182 in each group), patients who received rFVIIa required significantly less red blood cell transfusions [median 3 units (range 0--60, IQR = 4 units) versus 4 units (range 2-34, IQR = 4 units); P = 0.0004], decreased length of hospital stay (median 8 versus 9 days; P = 0.0158) and decreased renal risk (P < 0.0001). Incidence of renal failure, postoperative infection, postoperative thrombosis, prolonged ventilation, total ICU hours and 30-day mortality were not different between the two groups. Vld-rFVIIa for cardiac surgical bleeding was associated with decreased red blood cell transfusion, renal risk and length of hospital stay without increased thromboembolism or mortality when compared to patients who had cardiac surgical bleeding and received standard of care without Factor VIIa.

An integrated fluid-chemical model toward modeling the thrombus formation in an idealized model of aortic dissection

Type B aortic dissection is a major aortic catastrophe that can be acutely complicated by rapid expansion, rupture, and malperfusion syndromes. The separation of the intima from aortic walls will form a second blood-filled lumen defined as "false lumen (FL)", where the thrombus is more likely to form due to the local stasis hemodynamic conditions. Complete thrombosis of FL is associated with a beneficial outcome while patency and partial thrombosis will lead to later complications. However, the thrombosis mechanism is still unclear and little is known about the impact of chemical species transported by blood flow on this process. The proteins involved in the coagulation cascade (CC) may play an important role in the process of thrombosis, especially in the activation and stabilization of platelets. Based on this hypothesis, a reduced-order fluid-chemical model was established to simulate CC in an aortic dissection phantom with two tears. A high level of fibrin is continuously observed at the top of the FL and some time-varying areas between two tears, indicating a high likelihood of thrombus formation there. This finding is consistent with the clinical observation. The time evolution of coagulation factors is greatly affected by local hemodynamics, especially in the high disturbance zone where the evolution has characteristics of periodic changes consistent with the flow field. The ability of the proposed model to reproduce the CC response provides a potential application to integrate with a model that can simulate platelet activities, forming a biochemical-based model which would help unveil the mechanisms of thrombosis in FL and the clinical decision of appropriate treatment.

Inflammation, Infection and Venous Thromboembolism

The association between inflammation, infection, and venous thrombosis has long been recognized; yet, only in the last decades have we begun to understand the mechanisms through which the immune and coagulation systems interact and reciprocally regulate one another. These interconnected networks mount an effective response to injury and pathogen invasion, but if unregulated can result in pathological thrombosis and organ damage. Neutrophils, monocytes, and platelets interact with each other and the endothelium in host defense and also play critical roles in the formation of venous thromboembolism. This knowledge has advanced our understanding of both human physiology and pathophysiology, as well as identified mechanisms of anticoagulant resistance and novel therapeutic targets for the prevention and treatment of thrombosis. In this review, we discuss the contributions of inflammation and infection to venous thromboembolism.

Endothelial cell apoptosis and the role of endothelial cell-derived extracellular vesicles in the progression of atherosclerosis

To maintain physiological homeostasis, cell turnover occurs every day in the body via a form of programmed cell death called apoptosis. During apoptosis, cells undergo distinct morphological changes culminating in the disassembly of the dying cell into smaller fragments known as apoptotic bodies (ApoBDs). Dysregulation of apoptosis is associated with diseases including infection, cancer and atherosclerosis. Although the development of atherosclerosis is largely attributed to the accumulation of lipids and inflammatory debris in vessel walls, it is also associated with apoptosis of macrophages, smooth muscle cells (SMCs) and endothelial cells. During cellular activation and apoptosis, endothelial cells can release several types of membrane-bound extracellular vesicles (EVs) including exosomes, microvesicles (MVs)/microparticles and ApoBDs. Emerging evidence in the field suggests that these endothelial cell-derived EVs (EndoEVs) can contribute to intercellular communication during the development of atherosclerosis via the transfer of cellular contents such as protein and microRNA, which may prevent or promote disease progression depending on the context. This review provides an up-to-date overview of the known causes and consequences of endothelial cell death during atherosclerosis along with highlighting current methodological approaches to studying EndoEVs and the potential roles of EndoEVs in atherosclerosis development.

Potential role of activated factor VIII (FVIIIa) in FVIIa/tissue factor-dependent FXa generation in initiation phase of blood coagulation

Factor VIIa/tissue factor (FVIIa/TF) initiates blood coagulation by promoting FXa generation (extrinsic-Xa). Subsequent generation of intrinsic FXa (intrinsic-Xa) amplifies thrombin formation. Previous studies suggested that FVIIa/TF activates FVIII rapidly in immediate coagulation reactions, and FVIIa/TF/FXa activates FVIII prior to thrombin-dependent feedback. We investigated FVIII/FVIIa/TF/FXa relationships in early coagulation mechanisms. Total FXa generated by FVIIa/TF and FVIIa/TF-activated FVIII (FVIIIa_VIIa/TF) was 22.6 ± 1.7 nM (1 min); total FXa with FVIIa-inhibitor was 3.4 ± 0.7 nM, whereas FXa generated by FVIIa/TF or FVIII/TF was 10.4 ± 1.1 or 0.74 ± 0.14 nM, respectively. Little Xa was generated by FVIII alone, suggesting that intrinsic-Xa mechanisms were mediated by FVIIIa_VIIa/TF and FVIII/TF in the initiation phase. Intrinsic-Xa was delayed somewhat by von Willebrand factor (VWF). FVIII activation by FXa with FVIIa/TF was comparable to activation with Glu-Gly-Arg-inactivated-FVIIa/TF. TF counteracted the inhibitory effects of VWF on FXa-induced FVIII activation mediated by Arg³⁷² cleavage. The FVIII-C2 domain bound to cytoplasmic domain-deleted TF (TF^1-243), and VWF blocked this binding by > 80%, indicating an overlap between VWF- and TF^1-243-binding site(s) on C2. Overall, these data suggest that FVIII-associated intrinsic-Xa, governed by both FVIIa/TF-induced and FXa-induced FVIII activation mediated by FVIII-TF interactions, together with FVIIa-dependent extrinsic-Xa mechanisms, may be central to the initiation phase of coagulation.

Synthesis of Phosphatidylserine and Its Stereoisomers: Their Role in Activation of Blood Coagulation

Natural phosphatidylserine (PS), which contains two chiral centers, enhances blood coagulation. However, the process by which PS enhanced blood coagulation is not completely understood. An efficient and flexible synthetic route has been developed to synthesize all of the possible stereoisomers of PS. In this study, we examined the role of PS chiral centers in modulating the activity of the tissue factor (TF)-factor VIIa coagulation initiation complex. Full length TF was relipidated with phosphatidylcholine, and the synthesized PS isomers were individually used to estimate the procoagulant activity of the TF-FVIIa complex via a FXa generation assay. The results revealed that the initiation complex activity was stereoselective and had increased sensitivity to the configuration of the PS glycerol backbone due to optimal protein-lipid interactions.

Factor VIII activation by factor VIIa analog (V158D/E296V/M298Q) in tissue factor-independent mechanisms

Factor (F)VIIa with tissue factor (TF) is a primary trigger of blood coagulation. The recombinant (r)FVIIa analog, NN1731 (V158D/E296V/ M298Q) containing a thrombin/FIXa-mimicking catalytic domain, is ~30-fold more effective on activated platelets without TF, but ~1.2-fold with TF, than rFVIIa for FX activation. We have recently demonstrated the FVIIa/TF-dependent FVIII activation in the early coagulation phase. We assessed the action of NN1731 on FVIII activation. NN1731/TF increased FVIII activity ~2.9-fold within 30 seconds, followed by rapid inactivation, and was slightly more active than rFVIIa/TF. NN1731-catalysed activation, however, was enhanced ~6-fold at 5 minutes (min), and its peak level persisted for ~30 min. NN1731/TF proteolysed FVIII at Arg740, Arg372, and Arg336, similar to rFVIIa/TF, but cleavage by NN1731 alone was much slower at Arg336 than at Arg740 and Arg372. The Km and Vmax for NN1731/TF-catalysed activation were ~1.8-fold lower and ~2.3-fold greater than rFVIIa/TF. The Km for NN1731 alone was ~1.3-fold lower than rFVIIa, whilst the Vmax was ~7.9-fold greater, indicating that the efficiency of FVIII activation by NN1731 and NN1731/TF was ~11- and ~4-fold greater, respectively, than equivalent reactions with rFVIIa. In SPR-based assays, NN1731 bound to FVIII and the heavy chain (Kd; 0.62 and 1.9 nM) with ~1.4- and ~3.1-fold higher affinity than rFVIIa, and the A2 domain contributed to this increase. Von Willebrand factor moderated NN1731-catalysed activation more significantly than NN1731/TF. In conclusion, NN1731 was a greater potential than rFVIIa in up-regulating FVIII activity, and the TF-independent FVIII activation might represent a potential extra mode of its enhanced haemostatic effect.

Effects of anti-factor VIII inhibitor antibodies on factor VIIa/tissue factor-catalysed activation and inactivation of factor VIII

Factor (F)VIIa/tissue factor (TF) rapidly activates FVIII activity by proteolysis at Arg372 and Arg740, and subsequently inactivates FVIIIa activity by proteolysis at Arg336, although this activation is weaker than that by thrombin. The effects of anti-FVIII inhibitor antibodies on these reactions remain unknown, however. In this study, 13 of anti-FVIII inhibitor antibodies recognising the A2 or C2 domain were prepared. None of them, irrespective of epitope specificity, significantly affected FVIIa/TFcatalysed FVIII activation in one-stage clotting assays. Anti-A2 and anti-C2 type 2 antibodies had little effect on the inactivation phase. Anti-C2 type 1 antibodies, however, modulated inactivation by 40–60% of that seen with control IgG, suggesting that the activity of FVIIIa generated by FVIIa/TF persisted in the presence of this specific type of inhibitor. SDS-PAGE analysis demonstrated that all antibodies had little effect on FVIIa/TF-catalyzed proteolysis at Arg372 and Arg740. Anti-C2 type 1, however, significantly delayed cleavage at Arg336 in dose-dependent manners. Neither anti-A2 nor anti-C2 type 2 affected this reaction, and the findings were consistent with the results of the functional assays. In addition, anti-C2 monoclonal antibodies with type 1 and 2 demonstrated similar patterns of reaction as the anti-C2 polyclonal antibodies in FVIIa/TF-mediated FVIII mechanisms. We demonstrated that FVIIa/TF activated FVIII even in the presence of anti-FVIII antibodies, but inactivation patterns appeared to depend on inhibitor type. It could be important to determine the characteristic of these inhibitor antibodies for prediction of their effects on FVIIa-related FVIII reactions, and the results could have significant therapeutic implications.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, 2009, New Orleans, LA, USA. This work was supported by grants for MEXT KAKENHI 21591370 in Japan and Bayer Hemophilia Award program.

Recombinant human factor VIIa (rFVIIa) in hemophilia: mode of action and evidence to date

Recombinant activated factor VII (rFVIIa) is a bypassing agent widely used both in the treatment and prevention of hemorrhagic complications due to hemophilia with inhibitor. In such cases, antihemophilic factors cannot be used. The normal physiology of factor VII/ factor VIIa (FVII/FVIIa) in the hemostatic process requires the presence of tissue factor (TF) that links to FVII leading to a FVIIa-TF complex which activates both factor X and factor IX. The therapeutic use of rFVIIa requires high amount of FVIIa. Some studies demonstrate that FVIIa at high doses still requires tissue factor for function, whereas others suggest that FVIIa activates FX directly on the platelet surface, in a TF-independent manner. In the present article, we discuss the arguments supporting both TF-dependent and TF-independent modes of action. Finally, the coexistence of both TF-dependent and TF-independent mechanisms cannot be excluded.

Can the diagnostic reliability of the thrombin generation test as a global haemostasis assay be improved? The impact of calcium chloride concentration

BACKGROUND

Thrombin generation test (TGT) is a global haemostasis assay with a potential to predict bleeding tendencies and treatment effects in patients with haemophilia. Despite 15 years of clinical research, the diagnostic value of TGT remains controversial, possibly due to suboptimal sensitivity to coagulation deficiencies, robustness and reproducibility.

OBJECTIVE

The goal of this study was to explore the effect of calcium chloride (CaCl₂ ) concentration on the TGT's response to intrinsic coagulation factors (F) VIII, IX and XIa.

METHODS

Normal and factor-deficient plasmas supplemented with lacking coagulation factor and different CaCl₂ levels were tested by calibrated thrombinography assay.

RESULTS

Thrombin peak height (TPH) was strongly CaCl₂ dependent, increasing sharply from no TG at 5 mm to a peak at 13.8 mm of CaCl₂ (95% confidence interval [CI]: 13.0, 14.5) in normal and normalized deficient plasmas and at 11.9 mm (CI: 9.7, 14.2) in deficient plasmas, and then decreasing slowly to a complete inhibition at 30-40 mm. In contrast, TG lag time, time to peak and endogenous thrombin potential were nearly insensitive to CaCl₂ concentrations between 10 and 20 mm. The maximal difference between the TPH in deficient and supplemented plasmas was observed at 15.5 mm (CI: 12.8, 18.1).

CONCLUSION

Variations in CaCl₂ concentration in the assay mixture and sodium citrate concentrations in patient plasma samples may affect TGT responses, sensitivity and result in increased inter- and intra-laboratory variance. Implementation of TGT by clinical and quality control laboratories may require optimization of CaCl₂ concentration.

Reduced Prothrombinase Inhibition by Tissue Factor Pathway Inhibitor Contributes to the Factor V Leiden Hypercoagulable State

Activated factor V (FVa) and factor X (FXa) form prothrombinase, which converts prothrombin to thrombin. The α isoform of tissue factor (TF) pathway inhibitor (TFPI) dampens early procoagulant events, partly by interacting with FV. FV Leiden (FVL) is the most common genetic thrombophilia in Caucasians. Thrombosis risk is particularly elevated in women with FVL taking oral contraceptives, which produce acquired TFPIα deficiency. In mice, FVL combined with 50% reduction in TFPI causes severe thrombosis and perinatal lethality. However, a possible interaction between FVL and TFPIα has not been defined in humans. Here, we examined this interaction using samples from patients with FVL in thrombin generation and fibrin formation assays. In dilute TF- or FXa-initiated reactions, these studies exposed a TFPI-dependent activation threshold for coagulation initiation that was greatly reduced by FVL. The reduced threshold was progressively overcome with higher concentrations of TF or FXa. Plasma assays using anti-TFPI antibodies or a TFPI peptide that binds and inhibits FVa demonstrated that the decreased activation threshold resulted from reduced TFPIα inhibition of prothrombinase. In assays using purified proteins, TFPIα was a 1.7-fold weaker inhibitor of prothrombinase assembled with FVL than with FV. Thus, FVL reduces the threshold for initiating coagulation, and this threshold is further reduced in situations of low TFPIα concentration. Individuals with FVL are likely prone to thrombosis in response to weak procoagulant stimuli that would not initiate blood clot formation in individuals with FV.

The modulation of coagulation by aptamers: an up-to-date review

Coagulation and anticoagulation system is kept in balance by the orchestrated action of a variety of biological factors, and the disruption of this balance leads to the risk of hemorrhage or thrombosis. Oligonucleotide aptamers are single-stranded DNA (ssDNA) or RNA ligands that are synthesized in vitro and bind to target molecules through dimensional structure with high specificity and affinity, and thus represent attractive candidates for the development of agents to maintain the balance of coagulation and anticoagulation. In this review, we summarize recent progress in aptamer-based application in the modulation of coagulation. The aptamers with specific chemical and biological characteristics have great potential to be explored as agents for the treatment of blood coagulation abnormalities.

How it all starts: Initiation of the clotting cascade

The plasma coagulation system in mammalian blood consists of a cascade of enzyme activation events in which serine proteases activate the proteins (proenzymes and procofactors) in the next step of the cascade via limited proteolysis. The ultimate outcome is the polymerization of fibrin and the activation of platelets, leading to a blood clot. This process is protective, as it prevents excessive blood loss following injury (normal hemostasis). Unfortunately, the blood clotting system can also lead to unwanted blood clots inside blood vessels (pathologic thrombosis), which is a leading cause of disability and death in the developed world. There are two main mechanisms for triggering the blood clotting, termed the tissue factor pathway and the contact pathway. Only one of these pathways (the tissue factor pathway) functions in normal hemostasis. Both pathways, however, are thought to contribute to thrombosis. An emerging concept is that the contact pathway functions in host pathogen defenses. This review focuses on how the initiation phase of the blood clotting cascade is regulated in both pathways, with a discussion of the contributions of these pathways to hemostasis versus thrombosis.

Biology of tissue factor pathway inhibitor

Recent studies of the anticoagulant activities of the tissue factor (TF) pathway inhibitor (TFPI) isoforms, TFPIα and TFPIβ, have provided new insight into the biochemical and physiological mechanisms that underlie bleeding and clotting disorders. TFPIα and TFPIβ have tissue-specific expression patterns and anticoagulant activities. An alternative splicing event in the 5' untranslated region allows for translational regulation of TFPIβ expression. TFPIα has 3 Kunitz-type inhibitor domains (K1, K2, K3) and a basic C terminus, whereas TFPIβ has the K1 and K2 domains attached to a glycosylphosphatidyl inositol-anchored C terminus. TFPIα is the only isoform present in platelets, whereas endothelial cells produce both isoforms, secreting TFPIα and expressing TFPIβ on the cell surface. TFPIα and TFPIβ inhibit both TF-factor VIIa-dependent factor Xa (FXa) generation and free FXa. Protein S enhances FXa inhibition by TFPIα. TFPIα produces isoform-specific inhibition of prothrombinase during the initiation of coagulation, an anticoagulant activity that requires an exosite interaction between its basic C terminus and an acidic region in the factor Va B domain. Platelet TFPIα may be optimally localized to dampen initial thrombin generation. Similarly, endothelial TFPIβ may be optimally localized to inhibit processes that occur when endothelial TF is present, such as during the inflammatory response.

Comparison of natural and recombinant tissue factor proteins: new insights

Tissue factor (TF), an initiator of blood coagulation in vivo, is expressed in a variety of cells. Sufficient natural TF has been isolated to clone and express recombinant proteins ranging from full-length TF to its extracellular domain. Because of the limited availability of natural TF, recombinant proteins have been used as surrogates. Despite the differences in their post-translational modifications, it has been accepted that membrane-anchored recombinant TFs are quite similar to the natural TF. Recent studies, however, have shown that post-translational modifications play an important role in TF-triggered thrombin generation.

Overexpression of factor VII ameliorates bleeding diathesis of factor VIII-deficient mice with inhibitors

INTRODUCTION

Factor VIII (FVIII) treatment for hemophilia A has difficulties in correcting bleeding diathesis in the presence of inhibitors.

MATERIALS AND METHODS

An adeno-associated virus type 8 (AAV8) vector containing the factor VII (FVII) gene or the activated factor VII (FVIIa) gene was used to investigate the therapeutic effect of FVII or FVIIa overexpression in FVIII-deficient mice with inhibitors.

RESULTS

Following repeated human FVIII injection, FVIII-deficient mice developed anti-human FVIII antibodies that cross-reacted with mouse FVIII. High transgene expression of murine FVII or murine FVIIa was achieved using the AAV8 vector and resulted in increased blood FVII activity greater than 800% of normal murine FVII levels in vector-injected FVIII-deficient mice. Thromboelastography analysis showed significant improvements in clotting time, clot formation time, α angle, and mean clot firmness in AAV8 vector-injected FVIII-deficient mice with inhibitors. Overexpression of FVIIa ameliorated the bleeding phenotype of FVIII-deficient mice with inhibitors and significantly increased the survival rate after tail clipping. In addition, overexpression of FVII increased the survival rate of FVIII-deficient mice with inhibitors after tail clipping though it was not as efficient as FVIIa overexpression.

CONCLUSIONS

These data suggest that FVII overexpression is an alternative strategy for the treatment of hemophilia A with inhibitors.

Tissue factor structure and function

Tissue factor (TF) is an integral membrane protein that is essential to life. It is a component of the factor VIIa-TF complex enzyme and plays a primary role in both normal hemostasis and thrombosis. With a vascular injury, TF becomes exposed to blood and binds plasma factor VIIa, and the resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. Many cells, both healthy, and tumor cells, produce detectable amounts of TF, especially when they are stimulated by various agents. Despite the relative simplicity and small size of TF, there are numerous contradictory reports about the synthesis and presentation of TF on blood cells and circulation in normal blood either on microparticles or as a soluble protein. Another subject of controversy is related to the structure/function of TF. It has been almost commonly accepted that cell-surface-associated TF has low (if any) activity, that is, is "encrypted" and requires specific conditions/reagents to become active, that is, "decrypted." However there is a lack of agreement related to the mechanism and processes leading to alterations in TF function. In this paper TF structure, presentation, and function, and controversies concerning these features are discussed.

Thrombin activity propagates in space during blood coagulation as an excitation wave

Injury-induced bleeding is stopped by a hemostatic plug formation that is controlled by a complex nonlinear and spatially heterogeneous biochemical network of proteolytic enzymes called blood coagulation. We studied spatial dynamics of thrombin, the central enzyme of this network, by developing a fluorogenic substrate-based method for time- and space-resolved imaging of thrombin enzymatic activity. Clotting stimulation by immobilized tissue factor induced localized thrombin activity impulse that propagated in space and possessed all characteristic traits of a traveling excitation wave: constant spatial velocity, constant amplitude, and insensitivity to the initial stimulation once it exceeded activation threshold. The parameters of this traveling wave were controlled by the availability of phospholipids or platelets, and the wave did not form in plasmas from hemophilia A or C patients who lack factors VIII and XI, which are mediators of the two principal positive feedbacks of coagulation. Stimulation of the negative feedback of the protein C pathway with thrombomodulin produced nonstationary patterns of wave formation followed by deceleration and annihilation. This indicates that blood can function as an excitable medium that conducts traveling waves of coagulation.

Tissue factor pathway inhibitor and the revised hypothesis of blood coagulation

The recent rediscovery, isolation, and characterization of an endogenous coagulation inhibitor termed tissue factor pathway inhibitor (TFPI) has provided new insight into the regulation of in vivo coagulation. TFPI is a multivalent, Kunitz-type, protease inhibitor that directly binds and inactivates factor Xa and, in a factor-Xa-dependent fashion, produces feedback inhibition of the factor VIIa-tissue factor catalytic complex. The demonstrated in vitro properties of TFPI have led to the formulation of a revised theory of blood coagulation. In the revised model, coagulation proceeds through a single pathway rather than the alternative and redundant "extrinsic" and "intrinsic" pathways that had previously been postulated.

Decryption of tissue factor

Tissue factor (TF) is a transmembrane protein which, in complex with factor (F)VIIa, initiates blood coagulation. Numerous studies have determined TF epitopes and individual amino acids which play an important role in the TF/FVIIa complex formation and its activity towards natural substrates. However the subject of cell-surface TF activity remains controversial. It has been almost commonly accepted that TF on the cell surface has low (if any) activity, i.e. is encrypted and requires specific conditions/reagents to become active, i.e. decrypted. One of the leading theories suggests that cell membrane lipid composition plays a crucial role in TF decryption, whereas another assigns the key role to the formation of the Cys(186)-Cys(209) disulfide bond. Despite a number of studies published from several laboratories, the role of this bond in the activity of the TF/FVIIa complex remains elusive and controversial. One of the causes of this controversy could be related to the lack of specificity of the reagents used for the cell treatment leading to possible alterations in other cell surface proteins and cell membrane environment. In conclusion, the influence of the Cys(186)-Cys(209) this bond on cell surface TF function remains unclear.

Measuring the mechanical properties of blood clots formed via the tissue factor pathway of coagulation

Thrombelastography (TEG) is a method that is used to conduct global assays that monitor fibrin formation and fibrinolysis and platelet aggregation in whole blood. The purpose of this study was to use a well-characterized tissue factor (Tf) reagent and contact pathway inhibitor (corn trypsin inhibitor, CTI) to develop a reproducible thrombelastography assay. In this study, blood was collected from 5 male subjects (three times). Clot formation was initiated in whole blood with 5 pM Tf in the presence of CTI, and fibrinolysis was induced by adding tissue plasminogen activator (tPA). Changes in viscoelasticity were then monitored by TEG. In quality control assays, our Tf reagent, when used at 5 pM, induced coagulation in whole blood in 3.93 ± 0.23 min and in plasma in 5.12 ± 0.23 min (n=3). In TEG assays, tPA significantly decreased clot strength (maximum amplitude, MA) in all individuals but had no effect on clot time (R time). The intraassay variability (CVa<10%) for R time, angle, and MA suggests that these parameters reliably describe the dynamics of fibrin formation and degradation in whole blood. Our Tf reagent reproducibly induces coagulation, making it an ideal tool to quantify the processes that contribute to mechanical clot strength in whole blood.

Role of tissue factor in venous thrombosis

Venous thromboembolism (VTE) is a leading cause of morbidity and mortality worldwide. However, the mechanisms by which clots are formed in the deep veins have not been determined. Tissue factor (TF) is the primary initiator of the coagulation cascade and is essential for hemostasis. Under pathological conditions, TF is released into the circulation on small-membrane vesicles termed microparticles (MPs). Recent studies suggest that elevated levels of MP TF may trigger thrombosis. This review provides an overview of the role of TF in VTE.

Mechanisms of factor VIIa-catalyzed activation of factor VIII

BACKGROUND

Factor (F)VIIa, complexed with tissue factor (TF), is a primary trigger of blood coagulation, and has extremely restricted substrate specificity. The complex catalyzes limited proteolysis of FVIII, but these mechanisms are poorly understood.

OBJECTIVES

In the present study, we investigated the precise mechanisms of FVIIa/TF-catalyzed FVIII activation.

RESULTS

FVIII activity increased ~4-fold within 30 s in the presence of FVIIa/TF, and then decreased to initial levels within 20 min. FVIIa (0.1 nM), at concentrations present physiologically in plasma, activated FVIII in the presence of TF, and this activation was more rapid than that induced by thrombin. The heavy chain (HCh) of FVIII was proteolyzed at Arg(740) and Arg(372) more rapidly by FVIIa/TF than by thrombin, consistent with the enhanced activation of FVIII. Cleavage at Arg(336) was evident at ~1 min, whilst little cleavage of the light chain (LCh) was observed. Cleavage of the HCh by FVIIa/TF was governed by the presence of the LCh. FVIII bound to Glu-Gly-Arg-active-site-modified FVIIa (K(d), ~0.8 nM) with a higher affinity for the HCh than for the LCh (K(d), 5.9 and 18.9 nm). Binding to the A2 domain was particularly evident. Von Willebrand factor (VWF) modestly inhibited FVIIa/TF-catalyzed FVIII activation, in keeping with the concept that VWF could moderate FVIIa/TF-mediated reactions.

CONCLUSIONS

The results demonstrated that this activation mechanism was distinct from those mediated by thrombin, and indicated that FVIIa/TF functions through a 'priming' mechanism for the activation of FVIII in the initiation phase of coagulation.

Systems biology of coagulation initiation: kinetics of thrombin generation in resting and activated human blood

Blood function defines bleeding and clotting risks and dictates approaches for clinical intervention. Independent of adding exogenous tissue factor (TF), human blood treated in vitro with corn trypsin inhibitor (CTI, to block Factor XIIa) will generate thrombin after an initiation time (T_i) of 1 to 2 hours (depending on donor), while activation of platelets with the GPVI-activator convulxin reduces T_i to ∼20 minutes. Since current kinetic models fail to generate thrombin in the absence of added TF, we implemented a Platelet-Plasma ODE model accounting for: the Hockin-Mann protease reaction network, thrombin-dependent display of platelet phosphatidylserine, VIIa function on activated platelets, XIIa and XIa generation and function, competitive thrombin substrates (fluorogenic detector and fibrinogen), and thrombin consumption during fibrin polymerization. The kinetic model consisting of 76 ordinary differential equations (76 species, 57 reactions, 105 kinetic parameters) predicted the clotting of resting and convulxin-activated human blood as well as predicted T_i of human blood under 50 different initial conditions that titrated increasing levels of TF, Xa, Va, XIa, IXa, and VIIa. Experiments with combined anti-XI and anti-XII antibodies prevented thrombin production, demonstrating that a leak of XIIa past saturating amounts of CTI (and not “blood-borne TF” alone) was responsible for in vitro initiation without added TF. Clotting was not blocked by antibodies used individually against TF, VII/VIIa, P-selectin, GPIb, protein disulfide isomerase, cathepsin G, nor blocked by the ribosome inhibitor puromycin, the Clk1 kinase inhibitor Tg003, or inhibited VIIa (VIIai). This is the first model to predict the observed behavior of CTI-treated human blood, either resting or stimulated with platelet activators. CTI-treated human blood will clot in vitro due to the combined activity of XIIa and XIa, a process enhanced by platelet activators and which proceeds in the absence of any evidence for kinetically significant blood borne tissue factor.

Clotting of blood involves a series of reactions wherein at each step an inactive zymogen is converted to an active enzyme by the product of the previous step, sometimes in plasma and usually on efficient catalytic surfaces provided by the activating platelet. The protein Tissue Factor (TF) initiates this cascade when blood vessels are disrupted, but how this cascade is triggered in the absence of exogenous TF remains the subject of much debate. First, we validated a high throughput experimental system that allowed the noninvasive quantification of thrombin generation dynamics. Next, we showed that “contact activation,” despite use of the best available inhibitor (CTI) to prevent it, builds up enough autocatalytic strength to trigger coagulation without exogenous TF, particularly upon activated platelets. Further, we build an ODE based model to predict the stability of blood resulting from multiple perturbations with active enzymes at various physiologically realizable concentrations. Unlike existing models, we consider the dynamics of platelet activation on reaction rates due to phosphatiylserine exposure. The “Platelet-Plasma” model lays the groundwork for integration of coagulation reaction kinetics and donor specific descriptions of platelet function.

Molecular intercommunication between the complement and coagulation systems

The complement system as well as the coagulation system has fundamental clinical implications in the context of life-threatening tissue injury and inflammation. Associations between both cascades have been proposed, but the precise molecular mechanisms remain unknown. The current study reports multiple links for various factors of the coagulation and fibrinolysis cascades with the central complement components C3 and C5 in vitro and ex vivo. Thrombin, human coagulation factors (F) XIa, Xa, and IXa, and plasmin were all found to effectively cleave C3 and C5. Mass spectrometric analyses identified the cleavage products as C3a and C5a, displaying identical molecular weights as the native anaphylatoxins C3a and C5a. Cleavage products also exhibited robust chemoattraction of human mast cells and neutrophils, respectively. Enzymatic activity for C3 cleavage by the investigated clotting and fibrinolysis factors is defined in the following order: FXa > plasmin > thrombin > FIXa > FXIa > control. Furthermore, FXa-induced cleavage of C3 was significantly suppressed in the presence of the selective FXa inhibitors fondaparinux and enoxaparin in a concentration-dependent manner. Addition of FXa to human serum or plasma activated complement ex vivo, represented by the generation of C3a, C5a, and the terminal complement complex, and decreased complement hemolytic serum activity that defines exact serum concentration that results in complement-mediated lysis of 50% of sensitized sheep erythrocytes. Furthermore, in plasma from patients with multiple injuries (n = 12), a very early appearance and correlation of coagulation (thrombin-antithrombin complexes) and the complement activation product C5a was found. The present data suggest that coagulation/fibrinolysis proteases may act as natural C3 and C5 convertases, generating biologically active anaphylatoxins, linking both cascades via multiple direct interactions in terms of a complex serine protease system.

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.

Role of tissue factor in thrombosis in antiphospholipid antibody syndrome

Antiphospholipid syndrome (APS) is an acquired autoimmune disorder defined by the presence of an antiphospholipid antibody (aPL) and the occurrence of at least one associated clinical condition that includes venous thrombosis, arterial thrombosis or pregnancy morbidity. The aPL detected in APS have long been thought to have a direct prothrombotic effect in vivo. However, the pathophysiology underlying their coagulopathic effect has not been defined. Emerging data suggest a role for the procoagulant protein tissue factor (TF). In this review we provide an overview of TF, describe mouse models used in the evaluation of the role of TF in thrombosis, as well as summarize recent work on TF and APS.