Models of blood coagulation

Activated protein C inhibitor for correction of thrombin generation in hemophilia A blood and plasma1

BACKGROUND

Replacement therapy for hemophilic patient treatment is costly, because of the high price of pharmacologic products, and is not affordable for the majority of patients in developing countries.

OBJECTIVE

To generate and evaluate low molecular weight agents that could be useful for hemophilia treatment.

METHODS

Potential agents were generated by synthesizing specific inhibitors [6-(Lys-Lys-Thr-[homo]Arg)amino-2-(Lys[carbobenzoxy]-Lys[carbobenzoxy]-O-benzyl)naphthalenesulfonamide] (PNASN-1)] for activated protein C (APC) and tested in plasma and fresh blood from hemophilia A patients.

RESULTS

In the activated partial thromboplastin time-based APC resistance assay, PNASN-1 partially neutralized the effect of APC. In calibrated automated thrombography, PNASN-1 neutralized the effect of APC on thrombin generation in normal and congenital factor VIII-deficient plasma (FVIII:C < 1%). The addition of PNASN-1 to tissue factor-triggered (5 pm) contact pathway-inhibited fresh blood from 15 hemophilia A patients with various degrees of FVIII deficiency (FVIII:C < 1-51%) increased the maximum level of thrombin generated from 78 to 162 nm, which approached that observed in blood from a healthy individual (201 nm). PNASN-1 also caused a 47% increase in clot weight in hemophilia A blood.

CONCLUSIONS

Specific APC inhibitors compensate to a significant extent for FVIII deficiency, and could be used for hemophilia treatment.

Anticoagulant effect of polyphenols-rich extracts from black chokeberry and grape seeds

Blood coagulation consists of a series of zymogens that can be converted by limited proteolysis to active enzymes leading to the generation of thrombin. Fresh plasma and human thrombin was incubated with extracts from berries of Aronia melanocarpa or seeds of Vitis vinifera (0.5; 5; 50 μg/ml). The in vitro experiments showed that both extracts prolonged clotting time and decreased the maximal velocity of fibrin polymerization in human plasma. Moreover thrombin incubation with both extracts results in the inhibition of amidolytic activity of this enzyme. It gives hopes for development of diet supplements, which may be preventing thrombosis in pathological states.

Tissue Factor Pathway Inhibitor

Objective—

To investigate in women older than 60 whether aortic stiffness or pulse pressure (PP) is associated with selected procoagulant or anticoagulant factors and to examine whether pulsatile stretch influences these factors in human vascular smooth muscle cells (VSMCs) in vitro.

Methods and Results—

Aortic pulse wave velocity (PWV) and carotid PP were studied in 123 apparently healthy postmenopausal women. PWV, PP, von Willebrand factor, and free tissue factor pathway inhibitor (TFPI), but not mean arterial pressure, increased with age. Free TFPI and PWV were positively correlated, even after adjustment for age and PP and other confounding parameters. In vitro, 5% or 10% pulsatile stretch (at 1 Hz) enhanced TFPI synthesis and secretion by VSMCs in a time-independent manner (1 to 48 hours) without changes in protein level of smooth muscle myosin heavy chain. Application of 5% static stretch had no effect.

Conclusion—

In postmenopausal women, free TFPI increases as vascular wall function deteriorates and PP increases. These findings are supported by the increase in TFPI synthesized by VSMCs in response to cyclic stress in vitro. They suggest that VSMCs require pulsatility to interfere with the coagulation process and highlight the relevance of plasma free TFPI levels to cardiovascular diseases.

Biophysical investigation of GpIbalpha binding to thrombin anion binding exosite II

Substrates and cofactors of the serine protease thrombin (IIa) employ two anion binding exosites (ABE-I and -II) to aid in binding. On the surface of platelets resides the GpIbalpha/beta-GpIX-GpV membrane-bound receptor complex. IIa's ABE-II is proposed to interact with an anionic portion of GpIbalpha which enhances IIa cleavage of PAR-1 and subsequent activation of platelets. In this work, one-dimensional (1D) and two-dimensional (2D) NMR, analytical ultracentrifugation (AUC), and hydrogen-deuterium exchange (HDX) coupled with MALDI-TOF MS were performed to further characterize the features of binding to IIa's ABEs. The described work builds upon investigations performed in a prior study with fibrin(ogen)'s gamma' peptide and IIa [Sabo, T. M., Farrell, D. H., and Maurer, M. C. (2006) Biochemistry 45, 7434-7445]. 1D line broadening NMR (1H and 31P) and 2D trNOESY NMR studies indicate that GpIbalpha residues D274-E285 interact extensively with the IIa surface in an extended conformation. AUC demonstrates that both GpIbalpha (269-286) and gamma' (410-427) peptides interact with IIa with a 1:1 stoichiometry. When the HDX results are compared to those for the ABE-I targeting peptide hirudin (54-65), the data imply that GpIbalpha (269-286), GpIbalpha (1-290), and gamma' (410-427) are indeed directed to ABE-II. The ABE-II binding fragments reduce HDX for sites distant from the interface, suggesting long-range conformational effects. These studies illustrate that GpIbalpha and gamma' target ABE-II with similar consequences on IIa dynamics, albeit with differing structural features.

Review article: Coagulation cascade and therapeutics update: relevance to nephrology. Part 1: Overview of coagulation, thrombophilias and history of anticoagulants

Coagulation involves the regulated sequence of proteolytic activation of a series of zymogens to achieve appropriate and timely haemostasis in an injured vessel, in an environment that overwhelmingly favours an anticoagulant state. In the non-pathological state, the inciting event involves exposure of circulating factor VII/VIIa to extravascularly expressed tissue factor, which brings into motion the series of steps which results in amplification of the initial stimulus, culminating in the conversion of fibrinogen to fibrin and clot formation. The precisely synchronized cascade of events is counter-balanced by a system of anticoagulant mechanisms, which serve to ensure that the haemostatic effect is regulated and does not extend inappropriately. Conversely, in pathological states, these events can escape normal control mechanisms, due to either inherited or acquired defects, which lead to thrombosis. Current anticoagulant therapy, although based on medications that have been in existence for upwards of 80 years, is moving towards targeted therapy for specific coagulation factors and events in the coagulation cascade, based on the current knowledge of the main triggers and key events within the series of reactions that culminates in haemostasis. It remains to be seen whether these newer medications will become first-line therapies for thrombosis in the coming decade. This review aims to elucidate the main events within the coagulation cascade as it is currently understood to operate in vivo, with a brief discussion focusing on hypercoagulable states, and also a short review of the history of anticoagulants as they relate to this model.

The influence of von Willebrand factor on factor VIII activity measurements

BACKGROUND

It has been reported by multiple laboratories that the quantitation of factor (F)VIII by activity-based assays is influenced by the method, procedure and the quality of reagents used in the assays.

OBJECTIVE

To evaluate the influence of von Willebrand factor (VWF) on FVIII activity in vitro.

METHODS

The activated partial thromboplastin time (APTT) and synthetic coagulation proteome assays were used. Citrated FVIII/VWF-depleted substrate plasma (SP) (both antigens < 0.5%) was used in all APTT assays.

RESULTS

The concentration of FVIII antigen in pooled plasma from healthy donors [normal plasma (NP)] was 1.5 nm. The SP reconstituted with 1.5 nm recombinant (r)FVIII clotted in 23.8 +/- 0.2 s (standard deviation). The addition of 10 microg mL(-1) VWF to the SP increased the clotting time to 28.7 +/- 0.1 s; that is, the activity of rFVIII (FVIIIc) decreased to 50%. This inhibitory effect of VWF decreased with decreasing rFVIII concentration in SP, and became negligible at rFVIII

CONCLUSIONS

VWF has an inhibitory effect on the measurement of FVIII clotting activity. This effect depends upon the structure and formulation of the FVIII product.

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Key Words

• factor viii
• von willebrand factor
• clotting activity
• factor viii products
• tissue factor
• synthetic coagulation proteome

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MESH Headings

• Blood Coagulation/drug effects
• Blood Coagulation Tests
• Dose-Response Relationship, Drug
• Factor VIII/antagonists & inhibitors
• Factor VIII/pharmacology
• Humans
• Partial Thromboplastin Time
• von Willebrand Factor/pharmacology

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Grants

• P01 HL046703 NHLBI NIH HHS
• P01 HL046703-16A1 NHLBI NIH HHS
• P01 HL046703-17 NHLBI NIH HHS
• P01 HL46703 NHLBI NIH HHS

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Affiliation(s)

S Butenas Department of Biochemistry, College of Medicine, University of Vermont, Burlington, VT 05446, USA.
B Parhami-Seren
K G Mann

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Effects of plasma dilution on tissue-factor-induced thrombin generation and thromboelastography: partly compensating role of platelets

BACKGROUND

Bleeding upon major surgery or severe trauma is treated by transfusion with crystalloids, colloids, or plasma. This treatment, however, can lead to dilutional coagulopathy and impaired hemostasis. We investigated the suitability of two integrative coagulation tests to measure the hemostatic activity of diluted plasma.

STUDY DESIGN AND METHODS

Plasma from healthy donors was diluted in vitro with saline or colloid (venofundin or gelofusin). Coagulant activity in response to tissue factor was monitored by calibrated automated thrombin (CAT) generation and rotational thromboelastography (TEG), detecting formation of elastic fibrin clots. Plasma from patients receiving fluid infusion during coronary artery bypass grafting (CABG) was analyzed with the same assays.

RESULTS

Optimal activity of CAT and TEG assays required the presence of 10 pmol per L tissue factor and 4 micromol per L phospholipid vesicles or 100 x 10(9) platelets (PLTs) per L. Strikingly, thrombin generation and clot formation became impaired at a higher extent of dilution with PLTs present (< or =40% plasma) than with phospholipid vesicles present (< or =60% plasma). Colloids aggravated the dilution effect on clot formation, but FFP antagonized the dilution effect on thrombin and clot formation. In contrast, fibrinogen and Factor (F)XIII only restored the impaired clot formation. In plasma samples from patients undergoing CABG, CAT and TEG assay variables were altered to an extent corresponding with the volume of fluid infusion.

CONCLUSION

Thrombin generation and clot formation are reduced at a plasma dilution of more than 40 percent. In either process, PLTs can partly compensate for the dilution effect. In vitro dilution with colloids impaired fibrin clot elasticity compared to saline.

Advances in understanding pathogenic mechanisms of thrombophilic disorders

Venous thromboembolism is a major medical problem, annually affecting 1 in 1000 individuals. It is a typical multifactorial disease, involving both genetic and circumstantial risk factors that affect a delicate balance between procoagulant and anticoagulant forces. In the last 50 years, the molecular basis of blood coagulation and the anticoagulant systems that control it have been elucidated. This has laid the foundation for discoveries of both common and rare genetic traits that tip the natural balance in favor of coagulation, with a resulting lifelong increased risk of venous thrombosis. Multiple mutations in the genes for anticoagulant proteins such as antithrombin, protein C, and protein S have been identified and constitute important risk factors. Two single mutations in the genes for coagulation factor V (FV Leiden) and prothrombin (20210G>A), resulting from approximately 20,000-year-old mutations with subsequent founder effects, are common in the general population and constitute major genetic risk factors for thrombosis. In celebration of the 50-year anniversary of the American Society of Hematology, this invited review highlights discoveries that have contributed to our present understanding of the systems that control blood coagulation and the genetic factors that are involved in the pathogenesis of venous thrombosis.

A model for the formation, growth, and lysis of clots in quiescent plasma. A comparison between the effects of antithrombin III deficiency and protein C deficiency

A mathematical model comprised of 23 reaction-diffusion equations is used to simulate the biochemical changes and transport of various reactants involved in coagulation and fibrinolysis in quiescent plasma. The growth and lysis of a thrombus, as portrayed by the model equations, is governed by boundary conditions that include the surface concentration of TF-VIIa, the generation of XIa by contact activation (in vitro), and the secretion of tPA due to endothelial activation. We apply the model to two clinically relevant hypercoagulable states, caused by deficiency of either antithrombin III or protein C. These predictions are compared with published experimental data which validate the utility of the developed model under the special case of static conditions. The incorporation of varying hemodynamic conditions in to the current fluid static model remains to be performed.

Rate of mixing controls rate and outcome of autocatalytic processes: theory and microfluidic experiments with chemical reactions and blood coagulation

This article demonstrates that the rate of mixing can regulate the rate and outcome of both biological and nonbiological autocatalytic reaction systems that display a threshold response to the concentration of an activator. Plug-based microfluidics was used to control the timing of reactions, the rate of mixing, and surface chemistry in blood clotting and its chemical model. Initiation of clotting of human blood plasma required addition of a critical concentration of thrombin. Clotting could be prevented by rapid mixing when thrombin was added near the critical concentration, and mixing also affected the rate of clotting when thrombin was added at concentrations far above the critical concentration in two clinical clotting assays for human plasma. This phenomenon was modeled by a simple mechanism--local and global competition between the clotting reaction, which autocatalytically produces an activator, and mixing, which removes the activator. Numerical simulations showed that the Damköhler number, which describes this competition, predicts the effects of mixing. Many biological systems are controlled by thresholds, and these results shed light on the dynamics of these systems in the presence of spatial heterogeneities and provide simple guidelines for designing and interpreting experiments with such systems.

'Deep phenotyping': characterizing populations in the era of genomics and systems biology

PURPOSE OF REVIEW

Large-scale genomic studies establish genotype-phenotype associations, but they use phenotypes that represent current views of disease. There is an opportunity to enhance our understanding of genotype-phenotype associations by extending phenotypes into much greater detail ('deep phenotyping').

RECENT FINDINGS

We should engage in deep phenotyping for the following reasons. First, the current emphasis on clinical outcomes, although necessary for the advancement of clinical medicine, is not sufficient. Second, analytical and biological variance embedded in traditional phenotypes dilutes statistical power and strength of association. Finally, even relatively precise phenotypes may vary in terms of underlying pathophysiology across an individual's life history. Deep phenotyping focuses on the biological relevance of pathways and metabolic flux, increasing the 'granularity' of phenotypes.

SUMMARY

Focus on medical phenotypes is critical, but long-term interests require additional studies that illuminate underlying biology. Deep phenotyping is less likely to yield dramatic changes in current medical practice but it offers an opportunity to gain scientific insight in an incremental manner and to make progress in redefining clinical outcomes with greater precision. It is expensive, and debate is needed to determine when and how it should be applied.

Fibrinogen gamma' chain carboxy terminal peptide selectively inhibits the intrinsic coagulation pathway

The minor gammaA/gamma' isoform of fibrinogen contains a high affinity binding site for thrombin exosite II that is lacking in the major fibrinogen isoform, gammaA/gammaA fibrinogen. The biological consequences of gamma' chain binding to thrombin were therefore investigated. Coagulation assays, thrombin activity assays, and a primate thrombosis model were used to characterize the biological effects of the gamma' 410-427 peptide. The gamma' peptide had little effect on thrombin cleavage of the small peptidyl substrate tosyl-glycyl-prolyl-arginine-4-nitranilide acetate. However, in vitro assays demonstrated that the gamma' peptide inhibited thrombin cleavage of larger proteinaceous substrates, including fibrinogen and factor VIII. The gamma' peptide inhibited the activated partial thromboplastin time in plasma and showed greater inhibition of activated partial thromboplastin time assays than prothrombin time assays, consistent with the inhibition of factor VIII cleavage. Studies in a baboon thrombosis model showed that the gamma' 410-427 peptide inhibited fibrin-rich thrombus formation (typical of venous thrombi) and, to a lesser extent, platelet-rich thrombus formation (typical of arterial thrombi). These results indicate that binding of thrombin exosite II by the gamma' peptide has selective effects on the intrinsic pathway.