Mechanisms of Interactions of Factor X and Factor Xa with the Acidic Region in the Factor VIII A1 Domain

Factor VIII A3 domain residues 1793-1795 represent a factor IXa-interactive site in the tenase complex

BACKGROUND

Factor (F)VIII functions as a cofactor in the tenase complex responsible for conversion of FX to FXa by FIXa. Earlier studies indicated that one of the FIXa-binding sites is located in residues 1811-1818 (crucially F1816) of the FVIII A3 domain. A putative, three-dimensional structure model of the FVIIIa molecule suggested that residues 1790-1798 form a V-shaped loop, and juxtapose residues 1811-1818 on the extended surface of FVIIIa.

AIM

To examine FIXa molecular interactions in the clustered acidic sites of FVIII including residues 1790-1798.

METHODS AND RESULTS

Specific ELISA's demonstrated that the synthetic peptides, encompassing residues 1790-1798 and 1811-1818, competitively inhibited the binding of FVIII light chain to active-site-blocked Glu-Gly-Arg-FIXa (EGR-FIXa) (IC50; 19.2 and 42.9 μM, respectively), in keeping with a possible role for the 1790-1798 in FIXa interactions. Surface plasmon resonance-based analyses demonstrated that variants of FVIII, in which the clustered acidic residues (E1793/E1794/D1793) or F1816 contained substituted alanine, bound to immobilized biotin labeled-Phe-Pro-Arg-FIXa (bFPR-FIXa) with a 1.5-2.2-fold greater KD compared to wild-type FVIII (WT). Similarly, FXa generation assays indicated that E1793A/E1794A/D1795A and F1816A mutants increased the Km by 1.6-2.8-fold relative to WT. Furthermore, E1793A/E1794A/D1795A/F1816A mutant showed that the Km was increased by 3.4-fold and the Vmax was decreased by 0.75-fold, compared to WT. Molecular dynamics simulation analyses revealed the subtle changes between WT and E1793A/E1794A/D1795A mutant, supportive of the contribution of these residues for FIXa interaction.

CONCLUSION

The 1790-1798 region in the A3 domain, especially clustered acidic residues E1793/E1794/D1795, contains a FIXa-interactive site.

Factor VIII: A Dynamic Modulator of Hemostasis and Thrombosis in Trauma

A trace amount of thrombin cleaves factor VIII (FVIII) into an active form (FVIIIa), which catalyzes FIXa-mediated activation of FX on the activated platelet surface. FVIII rapidly binds to von Willebrand factor (VWF) after secretion and becomes highly concentrated via VWF-platelet interaction at a site of endothelial inflammation or injury. Circulating levels of FVIII and VWF are influenced by age, blood type (nontype O > type O), and metabolic syndromes. In the latter, hypercoagulability is associated with chronic inflammation (known as thrombo-inflammation). In acute stress including trauma, releasable pools of FVIII/VWF are secreted from the Weibel-Palade bodies in the endothelium and then augment local platelet accumulation, thrombin generation, and leukocyte recruitment. Early systemic increases of FVIII/VWF (>200% of normal) levels in trauma result in a lower sensitivity of contact-activated clotting time (activated partial thromboplastin time [aPTT] or viscoelastic coagulation test [VCT]). However, in severely injured patients, multiple serine proteases (FXa plasmin and activated protein C [APC]) are locally activated and may be systemically released. Severity of traumatic injury correlates with prolonged aPTT and elevated activation markers of FXa, plasmin, and APC, culminating in a poor prognosis. In a subset of acute trauma patients, cryoprecipitate that contains fibrinogen, FVIII/VWF, and FXIII is theoretically advantageous over purified fibrinogen concentrate to promote stable clot formation, but comparative efficacy data are lacking. In chronic inflammation or subacute phase of trauma, elevated FVIII/VWF contributes to the pathogenesis of venous thrombosis by enhancing not only thrombin generation but also augmenting inflammatory functions. Future developments in coagulation monitoring specific to trauma patients, and targeted to enhancement or inhibition of FVIII/VWF, are likely to help clinicians gain better control of hemostasis and thromboprophylaxis. The main goal of this narrative is to review the physiological functions and regulations of FVIII and implications of FVIII in coagulation monitoring and thromboembolic complications in major trauma patients.

The C-terminal acidic region in the A1 domain of factor VIII facilitates thrombin-catalyzed activation and cleavage at Arg372

BACKGROUND

Factor VIII (FVIII) is activated by thrombin-catalyzed cleavage at three sites. Previous reports indicated that the A2 domain contained thrombin-interactive sites responsible for cleavage at Arg372 . We have also found that the A1 domain of FVIII bound to the anion-binding exosite I of thrombin. The present study focused, therefore, on thrombin interaction with A1 residues 337-372 containing clustered acidic and hirugen-like sequences.

AIM

To identify specific thrombin-interactive site(s) within the A1 acidic region of FVIII.

METHODS AND RESULTS

The synthetic peptide of residues 337-353 with sulfated Tyr346 (337-353S) significantly blocked thrombin-catalyzed FVIII activation and cleavage at Arg372 , while a corresponding peptide of residues 354-372 had no significant effect. Treatment with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide to cross-link thrombin and 340-350S suggested that the 344-349 clustered acidic region was involved in thrombin interaction. Alanine-substituted FVIII mutants, Y346A and D347A/D348A/D349A, depressed thrombin-catalyzed activation and cleavage at Arg372 , with peak activation at ~ 50% and cleavage rates of ~ 10% to 20% compared to wild type (WT). The peak level of thrombin-catalyzed activation and the cleavage rate at Arg372 using FVIII mutants with 337-346 residues substituted with hirugen-sequences (MKNNEEAEDY337-346GDFEEIPEEY) were ~ 1.5- and ~ 2.5-fold of WT, respectively. Surface plasmon resonance-based analysis demonstrated that the Kd for active-site modified thrombin interactions using Y346A and D347A/D348A/D349A mutants was ~ 3- to 6-fold higher than that of WT, and that the hirugen-hybrid mutant facilitated association kinetics ~ 1.8-fold of WT.

CONCLUSION

Residues 346-349 with sulfated Tyr provided a thrombin-interactive site responsible for activation and cleavage at Arg372 . A hirugen-hybrid A1 mutant showed more efficient thrombin-catalyzed cleavage at Arg372 .

Spectrum of F8 Genotype and Genetic Impact on Inhibitor Development in Patients with Hemophilia A from Multicenter Cohort Studies (J-HIS) in Japan

Some genetic and treatment-related factors are risk factors for inhibitor development in patients with hemophilia A (PwHA). However, the genotype distribution of the factor VIII gene (F8) and genetic impact on inhibitor development in Japanese PwHA remain unknown. In 2007, the Japan Hemophilia Inhibitor Study 2 (J-HIS2) was organized to establish a nationwide registry system for hemophiliacs and to elucidate risk factors for inhibitor development, designed for prospective investigation following a retrospective study (J-HIS1) which had already finished. Patients, newly diagnosed after January 2007, were enrolled in J-HIS2 and followed up for inhibitor development and clinical environments since 2008 onward. In the present study, F8 genotypes of PwHA were investigated in the patients recruited from the J-HIS2 cohort as well as those with inhibitor from the J-HIS1 cohort. F8 variants identified in 59 PwHA with inhibitor in J-HIS1 were: 20 intron-22 inversions, 5 intron-1 inversions, 9 large deletions, 4 nonsense, 8 missense, 11 small in/del, and 2 splice-site variants. F8 variants identified in 267 (67 with inhibitor) PwHA in J-HIS2 were: 76(28) intron-22 inversions, 3(2) intron-1 inversion, 1(0) duplication, 8(5) large deletions, 21(7) nonsense, 109(7) missense, 40(11) small in/del, and 9(7) splice-site variants. Forty variants were novel. The cumulative inhibitor incidence rate in the severe group with null changes was 42.4% (95% confidence interval [CI]: 33.7-50.8), higher than that with nonnull changes (15.6% [95%CI: 6.8-27.8]), in J-HIS2. Relative risk for inhibitor development of null changes was 2.89. The spectrum of F8 genotype and genetic impact on inhibitor development in Japanese PwHA were consistent with the previous reports.

Interactions between residues 2228–2240 within factor VIIIa C2 domain and factor IXa Gla domain contribute to propagation of clot formation

Factor (F)VIII functions as a cofactor in the tenase complex responsible for phospholipid (PL)-dependent FXa generation by FIXa. We have recently reported that the FVIIIa C2 domain (residues 2228–2240) interacts with the FIXa Gla domain in this complex. We examined the role of this interaction in the generation of tenase activity during the process of clot formation, using a synthetic peptide corresponding to residues 2228–2240. The peptide 2228–2240 inhibited FVIIIa/FIXa-mediated FX activation dose-dependently in the presence of PL by >95% (IC50; ~10 μM). This effect was significantly greater than that obtained by peptide 1804–1818 (IC50; ~180 μM) which corresponds to another FIXa-interactive site in the light chain that provides the majority of binding energy for FIXa interaction. Peptide 2228–2240 had little effect on the prothrombin time and did not inhibit FIX activation in the coagulation process mediated by FVIIa/tissue factor or FXIa, suggesting specific inhibition of the intrinsic tenase complex. Clot waveform analysis, a plasma based-assay used to evaluate the process of intrinsic coagulation, demonstrated that peptide 2228–2240 significantly depressed both maximum coagulation velocity (|min1|) and acceleration (|min2|), reflecting the propagation of clot formation, although the clotting time was only marginally prolonged. Thromboelastography, an alternative whole blood based-assay, demonstrated that the peptide inhibited clot formation time, α-angle and maximal clot firmness, but had little effect on the clotting time. Interactions of the FVIIIa C2 domain (residues 2228–2240) with the FIXa Gla domain in the tenase complex appeared to contribute essentially to the propagation of clot formation.

Mechanisms of human neutrophil elastase-catalysed inactivation of factor VIII(a)

Mechanisms of inflammation and coagulation are linked through various pathways. Human neutrophil elastase (HNE), can bind to activated platelets, might be localised on platelet membranes that provide negatively-charged phospholipid essential for the optimum function of tenase complex. In this study, we examined the effect of HNE on factor (F)VIII. FVIII activity was rapidly diminished in the presence of HNE and was undetectable within 10 minutes. The inactivation rate waŝ8-fold greater than that of activated protein C (APC). This time-dependent inactivation was moderately affected by von Willebrand factor. HNE proteolysed the heavy chain (HCh) of FVIII into two terminal products, A11–358 and A2375–708, by limited proteolysis at Val358, Val374, and Val708. Cleavage at Val708 was much slower than that at Val358 in the >90-kDa A1-A2-B compared to the 90-kDa A1-A2. The 80-kDa light chain (LCh) was proteolysed to 75-kDa product by cleavage at Val1670. HNE-cata- lysed FVIIIa inactivation was markedly slower than that of native FVIII (by ~25-fold), due to delayed cleavage at Val708 in FVIIIa. The inactivation rate mediated by HNE was ~8-fold lower than that by APC. Cleavages at Val358 and Val708 were regulated by the presence of LCh and HCh, respectively. In conclusion, HNE-catalysed FVIII inactivation was associated with the limited-proteolysis that led to A11–358, A2375–708, and A3-C1-C21671–2332, and subsequently to critical cleavage at Val708. HNE-related FVIII(a) reaction might play a role in inactivation of HNE-induced coagulation process, and appeared to depend on the amounts of inactivated FVIII and active FVIIIa which is predominantly resistant to HNE inactivation.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, December 10, 2009, New Orleans, LA, USA.

The mild phenotype in severe hemophilia A with Arg1781His mutation is associated with enhanced binding affinity of factor VIII for factor X

The clinical severity in some patients with haemophilia A appears to be unrelated to the levels of factor (F)VIII activity (FVIII:C), but mechanisms are poorly understood. We have investigated a patient with a FVIII gene mutation at Arg1781 to His (R1781H) presenting with a mild phenotype despite FVIII:C of 0.9 IU/dl. Rotational thromboelastometry using the patient’s whole blood demonstrated that the clot time and clot firmness were comparable to those usually observed at FVIII:C 5–10 IU/dl. Thrombin and FXa assays using plasma samples also showed that the peak levels of thrombin formation and the initial rate of FXa generation were comparable to those observed at FVIII:C 5–10 IU/dl. The results suggested a significantly greater haemostatic potential in this individual than in those with severe phenotype. The addition of incremental amounts of FX to control plasma with FVIII:C 0.9 IU/dl in clot waveform analyses suggested that the enhanced functional tenase assembly might have been related to changes in association between FVIII and FX. To further investigate this mechanism, we prepared a stably expressed, recombinant, B-domainless FVIII R1781H mutant. Thrombin generation assays using mixtures of control plasma and FVIII revealed that the coagulation function observed with the R1781H mutant (0.9 IU/dl) was comparable to that seen with wild-type FVIII:C at ∼5 IU/dl. In addition, the R1781H mutant demonstrated an ∼1.9-fold decrease in K m for FX compared to wild type. These results indicated that relatively enhanced binding affinity of FVIII R1781H for FX appeared to moderate the severity of the haemophilia A phenotype.

Note: An account of this work was presented, in part, at the 23rd Congress of the International Society of Thrombosis and Haemostasis, July 27, 2011, Kyoto, Japan.

Functional characterization of a novel missense mutation, His147Arg, in A1 domain of FV protein causing type II deficiency

INTRODUCTION

Congenital factor V (FV) deficiency is a rare inherited disorder. Three compound heterozygous missense mutations, Asp68His, His147Arg, and Arg2074Cys, were observed in a Taiwanese patient with moderately severe FV deficiency.

METHOD

The novel His147Arg mutation in the A1 domain was investigated by protein modeling, followed by in vitro expression studies in COS-1 cells, to elucidate the molecular pathology associated with FV deficiency.

RESULTS

The His147Arg mutation was associated with normal antigen levels, both in cell lysates and conditioned media, whereas FV activity was significantly reduced to 63.5 ± 17.0%. These observations correspond to a type II FV deficiency mutation. Protein modeling by short-duration molecular dynamics (MD) simulation showed that the His147Arg mutation was associated with a conformational change, which could disrupt the stability of FVa by interfering with His1817 coordination of the copper ion. In functional activation assays, the His147Arg mutation did not affect FV protein activation by thrombin; however, reduced cofactor activity of the FVa protein, due to an increased rate of dissociation of heavy and light chains, was observed.

CONCLUSION

Our results show that the His147Arg mutation in the A1 domain of FV does not impair synthesis or procoagulant activity. Instead, the His147Arg mutation appears to disrupt the stability of FVa, providing a potential explanation for the functional deficiency.

Stable complex formation between serine protease inhibitor and zymogen: coagulation factor X cleaves the Arg393-Ser394 bond in a reactive centre loop of antithrombin in the presence of heparin

Antithrombin (AT) inhibits several blood coagulation proteases, including activated factor X (FXa), by forming stable complexes with these proteases. Herein, we demonstrate that AT forms a stable complex with zymogen factor X (FX). Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and size-exclusion chromatography analyses showed that AT and FX formed an SDS-stable complex, which is distinct in apparent molecular mass from an FXa-AT complex, in the presence of heparin. Amino-terminal sequence analysis of the complex following SDS-PAGE under reducing conditions provided clear evidence that AT forms this complex with the heavy chain of FX, because two sequences, HGSPVDI (residues 1-7 of AT) and SVAQATS (residues 1-7 of the heavy chain of FX), were identified. Furthermore, sequence SLNPNRV, which corresponds to residues 394-400 of AT, was identified in the non-reduced FX-AT complex, indicating that FX cleaved the Arg393-Ser394 bond in a reactive centre loop of AT. Unfractionated heparin induced FX-AT complex formation more effectively than low-molecular weight heparin or AT-binding pentasaccharide, and appeared to promote complex formation mainly via a template effect. These data suggest that AT is capable of forming a stable complex with zymogen FX by acting as an inhibitor in the presence of heparin.

Sequences flanking Arg336 in factor VIIIa modulate factor Xa-catalyzed cleavage rates at this site and cofactor function

Factor (F)VIII can be activated to FVIIIa by FXa following cleavages at Arg(372), Arg(740), and Arg(1689). FXa also cleaves FVIII/FVIIIa at Arg(336) and Arg(562) resulting in inactivation of the cofactor. These inactivating cleavages occur on a slower time scale than the activating ones. We assessed the contributions to cleavage rate and cofactor function of residues flanking Arg(336), the primary site yielding FVIII(a) inactivation, following replacement of these residues with those flanking the faster-reacting Arg(740) and Arg(372) sites and the slower-reacting Arg(562) site. Replacing P4-P3' residues flanking Arg(336) with those from Arg(372) or Arg(740) resulted in ∼4-6-fold increases in rates of FXa-catalyzed inactivation of FVIIIa, which paralleled the rates of proteolysis at Arg(336). Examination of partial sequence replacements showed a predominant contribution of prime residues flanking the scissile bonds to the enhanced rates. Conversely, replacement of this sequence with residues flanking the slow-reacting Arg(562) site yielded inactivation and cleavage rates that were ∼40% that of the WT values. The capacity for FXa to activate FVIII variants where cleavage at Arg(336) was accelerated due to flanking sequence replacement showed marked reductions in peak activity, whereas reducing the cleavage rate at this site enhanced peak activity. Furthermore, plasma-based thrombin generation assays employing the variants revealed significant reductions in multiple parameter values with acceleration of Arg(336) cleavage suggesting increased down-regulation of FXase. Overall, these results are consistent with a model of competition for activating and inactivating cleavages catalyzed by FXa that is modulated in large part by sequences flanking the scissile bonds.

Interaction between mouse adenovirus type 1 and cell surface heparan sulfate proteoglycans

Application of human adenovirus type 5 (Ad5) derived vectors for cancer gene therapy has been limited by the poor cell surface expression, on some tumor cell types, of the primary Ad5 receptor, the coxsackie-adenovirus-receptor (CAR), as well as the accumulation of Ad5 in the liver following interaction with blood coagulation factor X (FX) and subsequent tethering of the FX-Ad5 complex to heparan sulfate proteoglycan (HSPG) on liver cells. As an alternative vector, mouse adenovirus type 1 (MAV-1) is particularly attractive, since this non-human adenovirus displays pronounced endothelial cell tropism and does not use CAR as a cellular attachment receptor. We here demonstrate that MAV-1 uses cell surface heparan sulfate proteoglycans (HSPGs) as primary cellular attachment receptor. Direct binding of MAV-1 to heparan sulfate-coated plates proved to be markedly more efficient compared to that of Ad5. Experiments with modified heparins revealed that the interaction of MAV-1 to HSPGs depends on their N-sulfation and, to a lesser extent, 6-O-sulfation rate. Whereas the interaction between Ad5 and HSPGs was enhanced by FX, this was not the case for MAV-1. A slot blot assay demonstrated the ability of MAV-1 to directly interact with FX, although the amount of FX complexed to MAV-1 was much lower than observed for Ad5. Analysis of the binding of MAV-1 and Ad5 to the NCI-60 panel of different human tumor cell lines revealed the preference of MAV-1 for ovarian carcinoma cells. Together, the data presented here enlarge our insight into the HSPG receptor usage of MAV-1 and support the development of an MAV-1-derived gene vector for human cancer therapy.

Does plasmin have anticoagulant activity?

The coagulation and fibrinolytic pathways regulate hemostasis and thrombosis, and an imbalance in these pathways may result in pathologic hemophilia or thrombosis. The plasminogen system is the primary proteolytic pathway for fibrinolysis, but also has important proteolytic functions in cell migration, extracellular matrix degradation, metalloproteinase activation, and hormone processing. Several studies have demonstrated plasmin cleavage and inactivation of several coagulation factors, suggesting plasmin may be not only be the primary fibrinolytic enzyme, but may have anticoagulant properties as well. The objective of this review is to examine both in vitro and in vivo evidence for plasmin inactivation of coagulation, and to consider whether plasmin may act as a physiological regulator of coagulation. While several studies have demonstrated strong evidence for plasmin cleavage and inactivation of coagulation factors FV, FVIII, FIX, and FX in vitro, in vivo evidence is lacking for a physiologic role for plasmin as an anticoagulant. However, inactivation of coagulation factors by plasmin may be useful as a localized anticoagulant therapy or as a combined thrombolytic and anticoagulant therapy.

Activated factor X cleaves factor VIII at arginine 562, limiting its cofactor efficiency

BACKGROUND

Factor VIII (FVIII) and its activated form (FVIIIa) are subject to proteolysis that dampens their cofactor function. Among the proteases that attack FVIII (activated factor X (FXa), activated protein C (APC) and plasmin), only APC cleaves within the FVIII A2 domain at R562 to fully abolish FVIII activity.

OBJECTIVES

We investigated the possible involvement of the FXa cleavage at R562 within the A2 domain in the process of FVIII inactivation.

METHODS

An antibody (GMA012/R8B12) that recognizes the carboxy-terminus extremity of the A2 domain (A2C) was used to evaluate FXa action. A molecule mutated at R562 was also generated to assess the functional role of this particular residue.

RESULTS AND CONCLUSIONS

The appearance of the A2C domain as a function of time evidenced the identical cleavage within the A2 domain of FVIII and FVIIIa by FXa. This cleavage required phospholipids and occurred within minutes. In contrast, the isolated A2 domain was not cleaved by FXa. Von Willebrand factor and activated FIX inhibited the cleavage in a dose-dependent manner. Mutation R562K increased both the FVIII specific activity and the generation of FXa due to an increase in FVIII catalytic efficiency. Moreover, A2C fragment could not be identified from FVIII-R562K cleavage. In summary, this study defines a new mechanism for A2 domain-mediated FVIII degradation by FXa and implicates the bisecting of the A2 domain at R562.

Identification of 31 novel mutations in the F8 gene in Spanish hemophilia A patients: structural analysis of 20 missense mutations suggests new intermolecular binding sites

Hemophilia A (HA) is an X-linked bleeding disorder caused by a wide variety of mutations in the factor 8 (F8) gene, leading to absent or deficient factor VIII (FVIII). We analyzed the F8 gene of 267 unrelated Spanish patients with HA. After excluding patients with the common intron-1 and intron-22 inversions and large deletions, we detected 137 individuals with small mutations, 31 of which had not been reported previously. Eleven of these were nonsense, frameshift, and splicing mutations, whereas 20 were missense changes. We assessed the impact of the 20 substitutions based on currently available information about FV and FVIII structure and function relationship, including previously reported results of replacements at these and topologically equivalent positions. Although most changes are likely to cause gross structural perturbations and concomitant cofactor instability, p.Ala375Ser is predicted to affect cofactor activation. Finally, 3 further mutations (p.Pro64Arg, p.Gly494Val, and p.Asp2267Gly) appear to affect cofactor interactions with its carrier protein, von Willebrand factor, with the scavenger receptor low-density lipoprotein receptor-related protein (LRP), and/or with the substrate of the FVIIIapi*FIXa (Xase) complex, factor X. Characterization of these novel mutations is important for adequate genetic counseling in HA families, but also contributes to a better understanding of FVIII structure-function relationship.

Ixolaris binding to factor X reveals a precursor state of factor Xa heparin-binding exosite

Ixolaris is a two-Kunitz tick salivary gland tissue factor pathway inhibitor (TFPI). In contrast to human TFPI, Ixolaris specifically binds to factor Xa (FXa) heparin-binding exosite (HBE). In addition, Ixolaris interacts with zymogen FX. In the present work we characterized the interaction of Ixolaris with human FX quantitatively, and identified a precursor state of the heparin-binding exosite (proexosite, HBPE) as the Ixolaris-binding site on the zymogen. Gel-filtration chromatography demonstrated 1:1 complex formation between fluorescein-labeled Ixolaris and FX. Isothermal titration calorimetry confirmed that the binding of Ixolaris to FX occurs at stoichiometric concentrations in a reaction which is characteristically exothermic, with a favorable enthalpy (DeltaH) of -10.78 kcal/mol. ELISA and plasmon resonance experiments also indicate that Ixolaris binds to plasma FX and FXa, or to recombinant Gla domain-containing FX/FXa with comparable affinities ( approximately 1 nM). Using a series of mutants on the HBPE, we identified the most important amino acids involved in zymogen/Ixolaris interaction-Arg-93 >>> Arg-165 > or = Lys-169 > Lys-236 > Arg-125-which was identical to that observed for FXa/Ixolaris interaction. Remarkably, Ixolaris strongly inhibited FX activation by factor IXa in the presence but not in the absence of factor VIIIa, suggesting a specific interference in the cofactor activity. Further, solid phase assays demonstrated that Ixolaris inhibits FX interaction with immobilized FVIIIa. Altogether, Ixolaris is the first inhibitor characterized to date that specifically binds to FX HBPE. Ixolaris may be a useful tool to study the physiological role of the FX HBPE and to evaluate this domain as a target for anticoagulant drugs.

Study on the efficacy and safety of Xueyou Mixture in treating hemophilia

OBJECTIVE

To observe the effect of Xueyou Mixture (, XYM) on blood coagulation factors and its safety in treating hemophilia.

METHODS

To the randomly selected 65 inpatients of hemophilia, XYM was administered accompanied with intravenous dripping of liver cell growth factor 60-100 mg once a day to protect the liver, with no blood products like concentrated VIII and FIX factors or blood plasma given. The treatment lasted for 3 weeks. The short-term efficacy and adverse reactions were observed. The long-term efficacy in patients was observed in a follow-up study of 6-12 months after they were discharged from the hospital but continuously took XYM orally.

RESULTS

The short-term markedly effective rate in the patients was 95.38% (62/65). After they were treated for 3 weeks, the level of FVIII factor activity increased in 56 patients of type A from (3.32+/-2.21) % to (4.18+/-2.23) %, and in 9 of type B from (4.92+/-1.81) % to (5.64+/-1.96) %. Compared with that before treatment, the difference was significant in both of them (P<0.01). No obvious adverse reaction was found in the treatment period. The follow-up study showed that in 22 patients of type A, the FVIII factor activity ratio increased from (3.25+/-2.11) % to (6.31+/-2.16) %, (8.36+/-1.05) %, and (16.38+/-2.71) % in the 2nd, 3rd and 6th month after discharge respectively, all showing significant difference to that before treatment (P<0.01); and in 4 patients of type B, it increased from (4.15+/-2.26) % to 7.8% and 11.6% (mean value) in the 2nd and 6th month respectively.

CONCLUSION

XYM could raise the activity of factors VIII and IX in patients with hemophilia, and the degree of the rise is related with the duration of the therapy, with no obvious adverse reaction, which strikes out a new path and new train of thinking for the treatment of the disease by nonblood preparation.

Mechanisms of Plasmin-catalyzed Inactivation of Factor VIII

Plasmin not only functions as a key enzyme in the fibrinolytic system but also directly inactivates factor VIII and other clotting factors such as factor V. However, the mechanisms of plasmin-catalyzed factor VIII inactivation are poorly understood. In this study, levels of factor VIII activity increased approximately 2-fold within 3 min in the presence of plasmin, and subsequently decreased to undetectable levels within 45 min. This time-dependent reaction was not affected by von Willebrand factor and phospholipid. The rate constant of plasmin-catalyzed factor VIIIa inactivation was approximately 12- and approximately 3.7-fold greater than those mediated by factor Xa and activated protein C, respectively. SDS-PAGE analysis showed that plasmin cleaved the heavy chain of factor VIII into two terminal products, A1(37-336) and A2 subunits, by limited proteolysis at Lys(36), Arg(336), Arg(372), and Arg(740). The 80-kDa light chain was converted into a 67-kDa subunit by cleavage at Arg(1689) and Arg(1721), identical to the pattern induced by factor Xa. Plasmin-catalyzed cleavage at Arg(336) proceeded faster than that at Arg(372), in contrast to proteolysis by factor Xa. Furthermore, breakdown was faster than that in the presence of activated protein C, consistent with rapid inactivation of factor VIII. The cleavages at Arg(336) and Lys(36) occurred rapidly in the presence of A2 and A3-C1-C2 subunits, respectively. These results strongly indicated that cleavage at Arg(336) was a central mechanism of plasmin-catalyzed factor VIII inactivation. Furthermore, the cleavages at Arg(336) and Lys(36) appeared to be selectively regulated by the A2 and A3-C1-C2 domains, respectively, interacting with plasmin.

In silico prediction of FVIII epitopes recognised by natural autoantibodies in polyvalent immunoglobulin concentrates

Inhibitory antibodies directed against blood coagulation factor VIII (FVIII) impair FVIII replacement therapy, constituting a serious complication in haemophilic and autoimmune patients. Identifying B-cell FVIII epitopes and mapping them on the molecule remain important challenges. Using a combination of different algorithms, more than 30 hypothetical linear epitopes were predicted on the FVIII molecule surface. We selected several major predicted sequences, spanning all FVIII domains, for specific antibody induction in rabbits. All peptides tested successfully induced production of specific anti-FVIII rabbit antibodies, supporting the relevance of our approach. To investigate the presence of FVIII-reactive antibodies in the healthy donor population, a pooled fraction rich in all IgG subclasses was purified on peptide-Sepharose columns. Substantial amounts of Ig, specific for each FVIII peptide, were purified with yields ranging from 8 to 223 ng/mg immunoglobulins. Our results confirm the diversity of FVIII epitopes recognised by natural human anti-FVIII autoantibodies. All IgG subclasses were found in the affinity-isolated anti-peptide material, with overrepresentation of IgG2 and IgG4. Evidence was also found for new FVIII epitopes. Five human anti-peptide preparations displayed FVIII-neutralising activity, ranging from 1.3 to 5.3 BU/mg. Although the presence of naturally occurring anti-FVIII antibodies in healthy donors has been previously described, our methodology has allowed, for the first time, a fine mapping of several inhibitory and non-inhibitory epitopes. Our observations support the hypothesis that FVIII inhibitors in haemophilia A and autoimmune disease may originate from the proliferation of natural FVIII-specific B-cell clones.

Role of P1 residues Arg336 and Arg562 in the activated-Protein-C-catalysed inactivation of Factor VIIIa

APC (activated Protein C) inactivates human Factor VIIIa following cleavage at residues Arg336 and Arg562 within the A1 and A2 subunits respectively. The role of the P1 arginine in APC-catalysed inactivation of Factor VIIIa was examined by employing recombinant Factor VIIIa molecules where residues 336 and 562 were replaced with alanine and/or glutamine. Stably expressed Factor VIII proteins were activated by thrombin and resultant Factor VIIIa was reacted at high concentration with APC to minimize cofactor inactivation due to A2 subunit dissociation. APC cleaved wild-type Factor VIIIa at the A1 site with a rate approximately 25-fold greater than that for the A2 site. A1 mutants R336A and R336Q were inactivated approximately 9-fold slower than wild-type Factor VIIIa, whereas the A2 mutant R562A was inactivated approximately 2-fold slower. No cleavage at the mutated sites was observed. Taken together, these results suggested that cleavage at the A1 site was the dominant mechanism for Factor VIIIa inactivation catalysed by the proteinase. On the basis of cleavage at Arg336, a K(m) value for wild-type Factor VIIIa of 102 nM was determined, and this value was significantly greater than K(i) values (approximately 9-18 nM) obtained for an R336Q/R562Q Factor VIIIa. Furthermore, evaluation of a series of cluster mutants in the C-terminal region of the A1 subunit revealed a role for acidic residues in segment 341-345 in the APC-catalysed proteolysis of Arg336. Thus, while P1 residues contribute to catalytic efficiency, residues removed from these sites make a primary contribution to the overall binding of APC to Factor VIIIa.

Factor VIIIa regulates substrate delivery to the intrinsic factor X-activating complex

Activation of coagulation factor X (fX) by activated factors IX (fIXa) and VIII (fVIIIa) requires the assembly of the enzyme-cofactor-substrate fIXa-fVIIIa-fX complex on negatively charged phospholipid membranes. Using flow cytometry, we explored formation of the intermediate membrane-bound binary complexes of fIXa, fVIIIa, and fX. Studies of the coordinate binding of coagulation factors to 0.8-microm phospholipid vesicles (25/75 phosphatidylserine/phosphatidylcholine) showed that fVIII (fVIIIa), fIXa, and fX bind to 32 700 +/- 5000 (33 200 +/- 14 100), 20 000 +/- 4500, and 30 500 +/- 1300 binding sites per vesicle with apparent K(d) values of 76 +/- 23 (71 +/- 5), 1510 +/- 430, and 223 +/- 79 nm, respectively. FVIII at 10 nm induced the appearance of additional high-affinity sites for fIXa (1810 +/- 370, 20 +/- 5 nm) and fX (12 630 +/- 690, 14 +/- 4 nm), whereas fX at 100 nm induced high-affinity sites for fIXa (541 +/- 67, 23 +/- 5 nm). The effects of fVIII and fVIIIa on the binding of fIXa or fX were similar. The apparent Michaelis constant of the fX activation by fIXa was a linear function of the fVIIIa concentration with a slope of 1.00 +/- 0.12 and an intrinsic K(m) value of 8.0 +/- 1.5 nm, in agreement with the hypothesis that the reaction rate is limited by the fVIIIa-fX complex formation. In addition, direct correlation was observed between the fX activation rate and formation of the fVIIIa-fX complex. Titration of fX, fVIIIa, phospholipid concentration and phosphatidylserine content suggested that at high fVIIIa concentration the reaction rate is regulated by the concentration of free fX rather than of membrane-bound fX. The obtained results reveal formation of high-affinity fVIIIa-fX complexes on phospholipid membranes and suggest their role in regulating fX activation by anchoring and delivering fX to the enzymatic complex.

Factor VIII Structure and Function

Factor VIII, a non-covalent heterodimer comprised of a heavy chain (A1-A2-B domains) and light chain (A3-C1-C2 domains), circulates as an inactive procofactor in complex with von Willebrand factor. Metal ions are critical to the integrity of factor VIII, with Cu and Ca ions stabilizing the heterodimer and generating the active conformation, respectively. Activation of factor VIII catalyzed by thrombin appears dependent upon interactions with both anion-binding exosites I and II, and converts the heterodimer to the active cofactor, factor VIIIa. This protein, comprised of A1, A2, and A3-C1-C2 subunits, is labile due to weak affinity of the A2 subunit. Association of factor VIIIa with factor IXa to form the intrinsic factor Xase complex is membrane-dependent and involves multiple inter-protein contacts that remain poorly characterized. This complex catalyzes the conversion of factor X to factor Xa, a reaction that is essential for the propagation phase of coagulation. The role of factor VIIIa in this complex is to increase the catalytic efficiency for factor Xa generation by several orders of magnitude. Mechanisms for the down-regulation of factor Xase focus upon inactivation of the cofactor and include dissociation of the A2 subunit as well as activated protein C-catalyzed proteolysis.

Ecotin modulates thrombin activity through exosite-2 interactions

Ecotin is a Escherichia coli-derived protein that has been characterized as a potent inhibitor of serine-proteases. This protein is highly effective against several mammalian enzymes, which includes pancreatic and neutrophil-derived elastases, chymotrypsin, trypsin, factor Xa, and kallikrein. In this work we showed that ecotin binds to human alpha-thrombin via its secondary binding site, and modulates thrombin catalytic activity. Formation of wild type ecotin-alpha-thrombin complex was observed by native PAGE and remarkably, gel filtration chromatography showed an unusual 2:1 ecotin:enzyme stoichiometry. Analysis of the protease inhibitor effects on thrombin biological activities showed that (i) it decreases the inhibition of thrombin by heparin/antithrombin complex (IC50=3.2 microM); (ii) it produces a two-fold increase in the thrombin-induced fibrinogen clotting; and (iii) it inhibits thrombin-induced platelet aggregation (IC50=4.5 microM). Allosteric changes on thrombin structure were then evaluated. Complex formation with ecotin caused a three-fold increase in the rate of thrombin inhibition by BPTI, suggesting a displacement of the enzyme's 60-loop. In addition, ecotin modulated the enzyme's catalytic site, as demonstrated by changes in the fluorescence emission of fluorescein-FPRCK-alpha-thrombin (EC50=3.5 microM). Finally, solid phase competition assays demonstrated that heparin and prothrombin fragment 2 prevents thrombin interaction with ecotin. Altogether, these observations strongly support an ecotin interaction with thrombin anion-binding exosite-2, resulting in modulation of its biological activities. At this point, ecotin might be useful as a new tool for studying thrombin allosteric modulation.

Targeting exosites on blood coagulation proteases

The high specificity of blood coagulation proteases has been attributed not only to residues surrounding the active site but also to other surface domains that are involved in recognizing and interacting with macromolecular substrates and inhibitors. Specific blood coagulation inhibitors obtained from exogenous sources such as blood sucking salivary glands and snake venoms have been identified. Some of these inhibitors interact with exosites on coagulation enzymes. Two examples are discussed in this short revision. Bothrojaracin is a snake venom-derived protein that binds to thrombin exosites 1 and 2. Complex formation impairs several exosite-dependent activities of thrombin including fibrinogen cleavage and platelet activation. Bothrojaracin also interacts with proexosite 1 on prothrombin thus decreasing the zymogen activation by the prothrombinase complex (FXa/FVa). Ixolaris is a two Kunitz tick salivary gland inhibitor, that is homologous to tissue factor pathway inhibitor. Recently it was demonstrated that ixolaris binds to heparin-binding exosite of FXa, thus impairing the recognition of prothrombin by the enzyme. In addition, ixolaris interacts with FX possibly through the heparin-binding proexosite. Differently from FX, the ixolaris-FX complex is not recognized as substrate by the intrinsic tenase complex (FIXa/FVIIIa). We conclude that these inhibitors may serve as tools for the study of coagulation exosites as well as prototypes for new anticoagulant drugs.

Exosite-interactive regions in the A1 and A2 domains of factor VIII facilitate thrombin-catalyzed cleavage of heavy chain

Thrombin catalyzes the proteolytic activation of factor VIII, cleaving two sites in the heavy chain and one site in the light chain of the procofactor. Evaluation of thrombin binding the reaction products from heavy chain cleavage by steady state fluorescence energy transfer using a fluorophore-labeled, active site-modified thrombin as well as by solid phase binding assays using a thrombin Ser(205) --> Ala mutant indicated a high affinity site in the A1 subunit (K(d) approximately 5 nm) that was dependent upon the Na(+)-bound form of thrombin, whereas a moderate affinity site in the A2 subunit (K(d) approximately 100 nm) was observed for both Na(+)-bound and -free forms. The solid phase assay also indicated that hirudin blocked thrombin interaction with the A1 subunit and had little, if any, effect on its interaction with the A2 subunit. Conversely, heparin blocked thrombin interaction with the A2 subunit and showed a marginal effect on A1 binding. Evaluation of the A2 sequence revealed two regions rich in acidic residues that are localized close to the N and C termini of this domain. Peptides encompassing these clustered acidic regions, residues 373-395 and 719-740, blocked thrombin cleavage of the isolated heavy chain at Arg(372) and Arg(740) and inhibited A2 binding to thrombin Ser(205) --> Ala, suggesting that both A2 domain regions potentially support interaction with thrombin. A B-domainless, factor VIII double mutant Asp(392) --> Ala/Asp(394) --> Ala was constructed, expressed, and purified and possessed specific activity equivalent to a severe hemophilia phenotype. This mutant was resistant to cleavage at Arg(740), whereas cleavage at Arg(372) was not affected. These data suggest the acidic region comprising residues 389-394 in factor VIII A2 domain interacts with thrombin via its heparin-binding exosite and facilitates cleavage at Arg(740) during procofactor activation.