von Willebrand factor is a cofactor for thrombin-catalyzed cleavage of the factor VIII light chain

Isolated Variable Domains of an Antibody Can Assemble on Blood Coagulation Factor VIII into a Functional Fv-like Complex

Single-chain variable fragments (scFv) are antigen-recognizing variable fragments of antibodies (FV) where both subunits (V_L and V_H) are connected via an artificial linker. One particular scFv, iKM33, directed against blood coagulation factor VIII (FVIII) was shown to inhibit major FVIII functions and is useful in FVIII research. We aimed to investigate the properties of iKM33 enabled with protease-dependent disintegration. Three variants of iKM33 bearing thrombin cleavage sites within the linker were expressed using a baculovirus system and purified by two-step chromatography. All proteins retained strong binding to FVIII by surface plasmon resonance, and upon thrombin cleavage, dissociated into V_L and V_H as shown by size-exclusion chromatography. However, in FVIII activity and low-density lipoprotein receptor-related protein 1 binding assays, the thrombin-cleaved iKM33 variants were still inhibitory. In a pull-down assay using an FVIII-affinity sorbent, the isolated V_H, a mixture of V_L and V_H, and intact iKM33 were carried over via FVIII analyzed by electrophoresis. We concluded that the isolated V_L and V_H assembled into scFv-like heterodimer on FVIII, and the isolated V_H alone also bound FVIII. We discuss the potential use of both protease-cleavable scFvs and isolated Fv subunits retaining high affinity to the antigens in various practical applications such as therapeutics, diagnostics, and research.

Activated protein C has a regulatory role in factor VIII function

Mechanisms thought to regulate activated factor VIII (FVIIIa) cofactor function include A2-domain dissociation and activated protein C (APC) cleavage. Unlike A2-domain dissociation, there is no known phenotype associated with altered APC cleavage of FVIII, and biochemical studies have suggested APC plays a marginal role in FVIIIa regulation. However, the in vivo contribution of FVIIIa inactivation by APC is unexplored. Here we compared wild-type B-domainless FVIII (FVIII-WT) recombinant protein with an APC-resistant FVIII variant (FVIII-R336Q/R562Q; FVIII-QQ). FVIII-QQ demonstrated expected APC resistance without other changes in procoagulant function or A2-domain dissociation. In plasma-based studies, FVIII-WT/FVIIIa-WT demonstrated dose-dependent sensitivity to APC with or without protein S, whereas FVIII-QQ/FVIIIa-QQ did not. Importantly, FVIII-QQ demonstrated approximately fivefold increased procoagulant function relative to FVIII-WT in the tail clip and ferric chloride injury models in hemophilia A (HA) mice. To minimize the contribution of FV inactivation by APC in vivo, a tail clip assay was performed in homozygous HA/FV Leiden (FVL) mice infused with FVIII-QQ or FVIII-WT in the presence or absence of monoclonal antibody 1609, an antibody that blocks murine PC/APC hemostatic function. FVIII-QQ again demonstrated enhanced hemostatic function in HA/FVL mice; however, FVIII-QQ and FVIII-WT performed analogously in the presence of the PC/APC inhibitory antibody, indicating the increased hemostatic effect of FVIII-QQ was APC specific. Our data demonstrate APC contributes to the in vivo regulation of FVIIIa, which has the potential to be exploited to develop novel HA therapeutics.

Acidic Region Residues 1680-1684 in the A3 Domain of Factor VIII Contain a Thrombin-Interactive Site Responsible for Proteolytic Cleavage at Arg1689

Factor VIII (FVIII) is activated by thrombin-catalyzed cleavage at Arg³⁷², Arg⁷⁴⁰, and Arg¹⁶⁸⁹. Our previous studies suggested that thrombin interacted with the FVIII C2 domain specific for cleavage at Arg¹⁶⁸⁹. An alternative report demonstrated, however, that a recombinant (r)FVIII mutant lacking the C2 domain retained >50% cofactor activity, indicating the presence of other thrombin-interactive site(s) associated with cleavage at Arg¹⁶⁸⁹. We have focused, therefore, on the A3 acidic region of FVIII, similar to the hirugen sequence specific for thrombin interaction (54-65 residues). Two synthetic peptides, spanning residues 1659-1669 with sulfated Tyr¹⁶⁶⁴ and residues 1675-1685 with sulfated Try¹⁶⁸⁰, inhibited thrombin-catalyzed FVIII activation and cleavage at Arg¹⁶⁸⁹. Treatment with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide to cross-link thrombin with either peptide showed possible contributions of both 1664-1666 and 1683-1684 residues for thrombin interaction. Thrombin-catalyzed activation and cleavage at Arg¹⁶⁸⁹ in the alanine-substituted rFVIII mutants within 1663-1666 residues were similar to those of wild type (WT). Similar studies of 1680-1684 residues, however, demonstrated that activation and cleavage by thrombin of the FVIII mutant with Y1680A or D1683A/E1684A, in particular, were severely or moderately reduced to 20 to 30% or 60 to 70% of WT, respectively. Surface plasmon resonance-based analysis revealed that thrombin interacted with both Y1680A and D1683A/E1684A mutants with approximately sixfold weaker affinities of WT. Cleavage at Arg¹⁶⁸⁹ in the isolated light-chain fragments from both mutants was similarly depressed, independently of the heavy-chain subunit. In conclusion, the 1680-1684 residues containing sulfated Tyr¹⁶⁸⁰ in the A3 acidic region also contribute to a thrombin-interactive site responsible for FVIII activation through cleavage at Arg¹⁶⁸⁹.

FVIII-VWF dos-à-dos

In this issue of Blood, back-to-back (dos-à-dos) papers by Chiu et al and Yee et al present complementary findings of structural investigations into the interaction between factor VIII (FVIII) and von Willebrand factor (VWF). The binding of FVIII to VWF contributes in a major way to the regulation of hemostasis.

Dynamics of pathologic clot formation: a mathematical model

Recent studies have provided evidence of a significant role of the Hageman factor in pathologic clot formation. Since auto-activation of the Hageman factor triggers the intrinsic coagulation pathway, we study the dynamics of pathologic clot formation considering the intrinsic pathway as the predominant mechanism of this process. Our methodological approach to studying the dynamics of clot formation is based on mathematical modelling. Activation of the blood coagulation cascade, particularly its intrinsic pathway, is known to involve platelets. Therefore, equations accounting for the effects of activated platelets on the intrinsic pathway activation are included in our model. This brings about a considerable increase in the values of kinetic constants involved in the model of the principal biochemical processes resulting in clot formation. The purpose of this study is to elucidate the mechanism of pathologic clot formation. Since the time window of thrombolysis is 3-6h, we hypothesize that in many cases the rate of pathologic clot formation is much lower than that of haemostatic clot. This assumption is used to simplify the mathematical model and to estimate kinetic constants of biochemical reactions that initiate pathologic clot formation. The insights we gained from our mathematical model may lead to new approaches to the prophylaxis of pathologic clot formation. We believe that one of the most efficient ways to prevent pathologic clot formation is simultaneous inhibition of activated factors ХII and ХI.

Residues flanking scissile bonds in Factor VIII modulate rates of cleavage and proteolytic activation catalyzed by Factor Xa

Factor Xa (FXa) proteolytically activates Factor VIII (FVIII) by cleaving P1 residues Arg(372), Arg(740), and Arg(1689). The Arg(372) site represents the rate-limiting step for procofactor activation, whereas cleavage at Arg(740) is a fast step. FXa also catalyzes inactivating cleavages that occur on a slower time scale than the activating ones. To assess the role of sequences flanking the Arg(372) and Arg(740) sites, recombinant FVIII variants in which P3-P3' sequences were swapped individually or in combination were prepared. Replacing the Arg(372) flanking sequence with that from the Arg(740) site increased the rate of cleavage at Arg(372), as judged by the ~5-fold increased rate in A1 subunit generation, and reduced the FVIIIa-dependent lag time for in situ FXa generation. The reciprocal swap yielded a nearly 2-fold increase in the rate of Arg(372) cleavage, while the combined double-swap variant showed a 10-fold rate increase at that site, consistent with the individual effects being additive. Although this cleavage represents the slow step for activation, the rate of this reaction appeared to be ~9-fold greater than the rate of the primary inactivating cleavage at Arg(336) in generating the A1(336) product. Interestingly, replacement of the Arg(372) flanking sequence with the Arg(740) sequence combined with an Arg(740)Gln mutation yielded both more rapid cleavage of the Arg(372) site and accelerated inactivating cleavages within the A1 subunit. These results indicate that flanking sequences in part modulate the reaction rates required for procofactor activation and influence the capacity of FXa as an initial activator of FVIII rather than an inactivator.

The diversity of the immune response to the A2 domain of human factor VIII

Approximately 30% of patients with severe hemophilia A develop inhibitory anti-factor VIII (fVIII) antibodies (Abs). We characterized 29 anti-human A2 monoclonal Abs (mAbs) produced in a murine hemophilia A model. A basis set of nonoverlapping mAbs was defined by competition enzyme-linked immunosorbent assay, producing 5 major groups. The overlapping epitopes covered nearly the entire A2 surface when mapped by homolog-scanning mutagenesis. Most group A mAbs recognized a previously described epitope bounded by Arg484-Ile508 in the N-terminal A2 subdomain, resulting in binding to activated fVIII and noncompetitive inhibition of the intrinsic fXase complex. Group B and C mAbs displayed little or no inhibitory activity. Group D and E mAbs recognized epitopes in the C-terminal A2 subdomain. A subset of group D mAbs inhibited the activation of fVIII by interfering with thrombin-catalyzed cleavage at Arg372 at the A1-A2 domain junction. Other group D mAbs displayed indeterminate or no inhibitory activity despite inhibiting cleavage at Arg740 at the A2-B domain junction. Group E mAbs inhibited fVIII light-chain cleavage at Arg1689. Inhibition of cleavages at Arg372 and Arg1689 represent novel mechanisms of inhibitor function and, along with the extensive epitope spectrum identified in this study, reveal hitherto unrecognized complexity in the immune response to fVIII.

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.

Distinct roles of Ser-764 and Lys-773 at the N terminus of von Willebrand factor in complex assembly with coagulation factor VIII

Complex formation between coagulation factor VIII (FVIII) and von Willebrand factor (VWF) is of critical importance to protect FVIII from rapid in vivo clearance and degradation. We have now employed a chemical footprinting approach to identify regions on VWF involved in FVIII binding. To this end, lysine amino acid residues of VWF were chemically modified in the presence of FVIII or activated FVIII, which does not bind VWF. Nano-LC-MS analysis showed that the lysine residues of almost all identified VWF peptides were not differentially modified upon incubation of VWF with FVIII or activated FVIII. However, Lys-773 of peptide Ser-766-Leu-774 was protected from chemical modification in the presence of FVIII. In addition, peptide Ser-764-Arg-782, which comprises the first 19 amino acid residues of mature VWF, showed a differential modification of both Lys-773 and the α-amino group of Ser-764. To verify the role of Lys-773 and the N-terminal Ser-764 in FVIII binding, we employed VWF variants in which either Lys-773 or Ser-764 was replaced with Ala. Surface plasmon resonance analysis and competition studies revealed that VWF(K773A) exhibited reduced binding to FVIII and the FVIII light chain, which harbors the VWF-binding site. In contrast, VWF(S764A) revealed more effective binding to FVIII and the FVIII light chain compared with WT VWF. The results of our study show that the N terminus of VWF is critical for the interaction with FVIII and that Ser-764 and Lys-773 have opposite roles in the binding mechanism.

Thermodynamic analysis of the interaction of factor VIII with von Willebrand factor

Factor VIII (FVIII) is a glycoprotein that plays an important role in the intrinsic pathway of coagulation. In circulation, FVIII is protected upon binding to von Willebrand factor (VWF), a chaperone molecule that regulates its half-life, distribution, and activity. Despite the biological significance of this interaction, its molecular mechanisms are not fully characterized. We determined the equilibrium and activation thermodynamics of the interaction between FVIII and VWF. The equilibrium affinity determined by surface plasmon resonance was temperature-dependent with a value of 0.8 nM at 35 °C. The FVIII-VWF interaction was characterized by very fast association (8.56 × 10(6) M(-1) s(-1)) and fast dissociation (6.89 × 10(-3) s(-1)) rates. Both the equilibrium association and association rate constants, but not the dissociation rate constant, were dependent on temperature. Binding of FVIII to VWF was characterized by favorable changes in the equilibrium and activation entropy (TΔS° = 89.4 kJ/mol, and -TΔS(++) = -8.9 kJ/mol) and unfavorable changes in the equilibrium and activation enthalpy (ΔH° = 39.1 kJ/mol, and ΔH(++) = 44.1 kJ/mol), yielding a negative change in the equilibrium Gibbs energy. Binding of FVIII to VWF in solid-phase assays demonstrated a high sensitivity to acidic pH and a sensitivity to ionic strength. Our data indicate that the interaction between FVIII and VWF is mediated mainly by electrostatic forces, and that it is not accompanied by entropic constraints, suggesting the absence of conformational adaptation but the presence of rigid "pre-optimized" binding surfaces.

P3-P3' residues flanking scissile bonds in factor VIII modulate rates of substrate cleavage and procofactor activation by thrombin

Thrombin-catalyzed activation of factor VIII (FVIII) occurs through proteolysis at three P1 Arg residues: Arg(372) and Arg(740) in the FVIII heavy chain and Arg(1689) in the FVIII light chain. Cleavage at the latter two sites is relatively fast compared with cleavage at Arg(372), which appears to be rate-limiting. Examination of the P3-P3' residues flanking each P1 site revealed that those sequences at Arg(740) and Arg(1689) are more optimal for thrombin cleavage than at Arg(372), suggesting these sequences may impact reaction rates. Recombinant FVIII variants were prepared with mutations swapping scissile bond flanking sequences in the heavy chain individually and in combination with a second swap or with a P1 point mutation. Rates of generation of A1 and A3-C1-C2 subunits were determined by Western blotting and correlated with rates of cleavage at Arg(372) and Arg(1689), respectively. Rates of thrombin cleavage at Arg(372) were increased ~10- and ~3-fold compared with that of wild-type FVIII when it was replaced with P3-P3' residues flanking Arg(740) and Arg(1689), respectively, and these values paralleled increased rates of A2 subunit generation and procofactor activation. Positioning of more optimal residues flanking Arg(372) abrogated the need for initial cleavage at Arg(740) to facilitate this step. These results show marked changes in cleavage rates correlate with the extent of cleavage-optimal residues flanking the scissile bond and modulate the mechanism for procofactor activation.

Cleavage at Arg-1689 influences heavy chain cleavages during thrombin-catalyzed activation of factor VIII

The procofactor, factor VIII, is activated by thrombin or factor Xa-catalyzed cleavage at three P1 residues: Arg-372, Arg-740, and Arg-1689. The catalytic efficiency for thrombin cleavage at Arg-740 is greater than at either Arg-1689 or Arg-372 and influences reaction rates at these sites. Because cleavage at Arg-372 appears rate-limiting and dependent upon initial cleavage at Arg-740, we investigated whether cleavage at Arg-1689 influences catalysis at this step. Recombinant B-domainless factor VIII mutants, R1689H and R1689Q were prepared and stably expressed to slow and eliminate cleavage, respectively. Specific activity values for the His and Gln mutations were approximately 50 and approximately 10%, respectively, that of wild type. Thrombin activation of the R1689H variant showed an approximately 340-fold reduction in the rate of Arg-1689 cleavage, whereas the R1689Q variant was resistant to thrombin cleavage at this site. Examination of heavy chain cleavages showed approximately 4- and 11-fold reductions in A2 subunit generation and approximately 3- and 7-fold reductions in A1 subunit generation for the R1689H and R1689Q mutants, respectively. These results suggest a linkage between light chain cleavage and cleavages in heavy chain. Results obtained evaluating proteolysis of the factor VIII mutants by factor Xa revealed modest rate reductions (<5-fold) in generating A2 and A1 subunits and in cleaving light chain at Arg-1721 from either variant, suggesting little dependence upon prior cleavage at residue 1689 as compared with thrombin. Overall, these results are consistent with a competition between heavy and light chains for thrombin exosite binding and subsequent proteolysis with binding of the former chain preferred.

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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Function of von Willebrand factor in haemostasis and thrombosis

The physiological protection against bleeding is secured by platelet adhesion to the site of injury and sealing of the defect. The first step involves the arrest of platelets that have adhered to subendothelial structures, primarily collagen, at the site of injury. Under conditions of low shear rates, platelet adhesion to the damaged vessel wall is mediated by several proteins, including von Willebrand factor (VWF). However, under conditions of high shear, aggregation occurs only in the presence of soluble VWF. In solution, VWF becomes immobilized via its A3 domain on the fibrillar collagen of the vessel wall and acts as an intermediary between collagen and the platelet receptor glycoprotein Ibalpha (GPIbalpha), which is the only platelet receptor that does not require prior activation for bond formation. After GPIbalpha binds to the A1 domain of its main ligand VWF, further activation of the platelet via intracellular signalling occurs, allowing other receptors to engage VWF and collagen and thereby reinforcing permanent adhesion. On this first layer of adherent platelets, soluble VWF binds and uncoils, thereby attracting more platelets. Platelet interaction with immobilized and soluble VWF may also generate platelet-derived microparticles that exhibit pro-coagulant activity. Full growth of a multilayered platelet aggregate comprises binding of the platelet receptor integrin alphaIIbbeta3 to VWF and fibrinogen. In addition, the surface of the activated platelets accelerates the coagulation cascade, which, by its end product fibrin, stabilizes the growing platelet thrombus. This article summarizes the characteristics and role of VWF in the coagulation cascade.

Thrombin hydrolysis of human osteopontin is dependent on thrombin anion-binding exosites

The cytokine osteopontin (OPN) can be hydrolyzed by thrombin exposing a cryptic alpha(4)beta(1)/alpha(9)beta(1) integrin-binding motif (SVVYGLR), thereby acting as a potent cytokine for cells bearing these activated integrins. We show that purified milk OPN is a substrate for thrombin with a k(cat)/K(m) value of 1.14 x 10(5) m(-1) s(-1). Thrombin cleavage of OPN was inhibited by unsulfated hirugen (IC(50) = 1.2 +/- 0.2 microm), unfractionated heparin (IC(50) = 56.6 +/- 8.4 microg/ml) and low molecular weight (5 kDa) heparin (IC(50) = 31.0 +/- 7.9 microg/ml), indicating the involvement of both anion-binding exosite I (ABE-I) and anion-binding exosite II (ABE-II). Using a thrombin mutant library, we mapped residues important for recognition and cleavage of OPN within ABE-I and ABE-II. A peptide (OPN-(162-197)) was designed spanning the OPN thrombin cleavage site and a hirudin-like C-terminal tail domain. Thrombin cleaved OPN-(162-197) with a specificity constant of k(cat)/K(m) = 1.64 x 10(4) m(-1) s(-1). Representative ABE-I mutants (K65A, H66A, R68A, Y71A, and R73A) showed greatly impaired cleavage, whereas the ABE-II mutants were unaffected, suggesting that ABE-I interacts principally with the hirudin-like OPN domain C-terminal and contiguous to the thrombin cleavage site. Debye-Hückel slopes for milk OPN (-4.1 +/- 1.0) and OPN-(162-197) (-2.4 +/- 0.2) suggest that electrostatic interactions play an important role in thrombin recognition and cleavage of OPN. Thus, OPN is a bona fide substrate for thrombin, and generation of thrombin-cleaved OPN with enhanced pro-inflammatory properties provides another molecular link between coagulation and inflammation.

Persistent factor VIII-dependent factor X activation on endothelial cells is independent of von Willebrand factor

Endothelial cells are able to support the activation of coagulation factor X by activated factor IX in the presence of its cofactor, factor VIII. We have previously reported that this reaction is persistent on endothelial cells, but transient on activated platelets and phospholipid vesicles when activated factor X (Xa) is used as activator of factor VIII. Aim of the present study was to explore the influence of von Willebrand factor and that of the factor VIII activator, either factor Xa or thrombin, on the decay of factor X activation on the endothelial cell surface. Kinetics of factor X activation on human umbilical vein endothelial cells was compared with that on phospholipid vesicles employing purified coagulation factors from plasma as well as recombinant factor VIII variants. Employing factor Xa as factor VIII activator, rate constants for decay of membrane-bound factor X activation were consistently low on endothelial cells (0.02 min) as compared with phospholipid vesicles (0.2 min). Activation of factor VIII by thrombin resulted in two-fold increased decay rates. In the presence of excess of von Willebrand factor over factor VIII, decay rates were not significantly changed. Factor VIII variants with and without a Tyr to Phe substitution, which abolishes high-affinity binding to von Willebrand factor, displayed the same factor X activation decay kinetics. Although previous studies have shown that von Willebrand factor modulates factor VIII activation and stabilisation, this apparently does not affect the progression of factor X activation at the endothelium.

Factor VIII accelerates proteolytic cleavage of von Willebrand factor by ADAMTS13

Proteolytic processing of von Willebrand factor (VWF) by ADAMTS13 metalloproteinase is crucial for normal hemostasis. In vitro, cleavage of VWF by ADAMTS13 is slow even at high shear stress and is typically studied in the presence of denaturants. We now show that, under shear stress and at physiological pH and ionic strength, coagulation factor VIII (FVIII) accelerates, by a factor of approximately 10, the rate of specific cleavage at the Tyr(1605)-Met(1606) bond in VWF. Multimer analysis reveals that FVIII preferentially accelerates the cleavage of high-molecular-weight multimers. This rate enhancement is not observed with VWF predenatured with 1.5 M guanidine. The ability of FVIII to enhance VWF cleavage by ADAMTS13 is rapidly lost after pretreatment of FVIII with thrombin. A FVIII derivative lacking most of the B domain behaves equivalently to full-length FVIII. In contrast, a derivative lacking both the B domain and the acidic region a3 that contributes to the high-affinity interaction of FVIII with VWF exhibits a greatly reduced ability to enhance VWF cleavage. Our data suggest that FVIII plays a role in regulating proteolytic processing of VWF by ADAMTS13 under shear stress, which depends on the high-affinity interaction between FVIII and its carrier protein, VWF.

Factor VIII inhibitors: role of von Willebrand factor on the uptake of factor VIII by dendritic cells

In patients with haemophilia A, factor VIII (FVIII) therapy leads to the development of anti-FVIII alloantibodies that inhibit FVIII pro-coagulant activity, in up to 25% of the cases. At a time when efficient viral screening procedures are at place, development of inhibitors poses the greatest threat to haemophilia A patients. Various risk factors, both patient and product-related, are responsible for the development of inhibitory antibodies. The role of FVIII-specific CD4+ T lymphocytes in the initiation of the humoral immune response to exogenous FVIII has been well. In view of their capacity to stimulate naïve T cells, dendritic cells (DCs) play a central role in the initiation of the primary immune response. Thus, in the context of a primary alloimmunization against FVIII, i.e. when FVIII-specific B lymphocytes are not there to take up FVIII from the circulation and to serve as antigen presenting cells (APCs), DCs are the only cell type that internalize FVIII, leading to activation of FVIII-specific CD4+ T lymphocytes. von Willebrand factor (VWF) present in plasma-derived FVIII therapeutic concentrates, is known to act as a chaperone molecule for procoagulant FVIII. In addition to its role in reducing the 'antigenicity' of FVIII, the role of VWF in the reduction of the 'immunogenicity' of therapeutic FVIII in patients with haemophilia A has also been suggested. We have recently demonstrated that VWF protects FVIII from being endocytosed by human DCs and subsequently being presented to FVIII-specific T cells. We propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen presenting cells.

Mathematical modeling of blood coagulation cascade: kinetics of intrinsic and extrinsic pathways in normal and deficient conditions

A mathematical model has been developed to simulate the generation of thrombin through intrinsic and extrinsic pathways. The time course of clotting factor activation during coagulation was calculated, and the sensitivity of the kinetics due to a decrease or deficiency in the concentrations of coagulation proteins or their activities was studied. The model gives reasonable predictions without the adjustment of any parameter values. The calculated clotting time was approximately 44 s for the intrinsic pathway and approximately 8.6 s for the extrinsic pathway using normal protein concentrations in plasma. Various prolonged clotting times were observed in different factor-deficient disorders using this model.

Antihuman factor VIII C2 domain antibodies in hemophilia A mice recognize a functionally complex continuous spectrum of epitopes dominated by inhibitors of factor VIII activation

The diversity of factor VIII (fVIII) C2 domain antibody epitopes was investigated by competition enzyme-linked immunosorbent assay (ELISA) using a panel of 56 antibodies. The overlap patterns produced 5 groups of monoclonal antibodies (MAbs), designated A, AB, B, BC, and C, and yielded a set of 18 distinct epitopes. Group-specific loss of antigenicity was associated with mutations at the Met2199/Phe2200 phospholipid binding beta-hairpin (group AB MAbs) and at Lys2227 (group BC MAbs), which allowed orientation of the epitope structure as a continuum that covers one face of the C2 beta-sandwich. MAbs from groups A, AB, and B inhibit the binding of fVIIIa to phospholipid membranes. Group BC was the most common group and displayed the highest specific fVIII inhibitor activities. MAbs in this group are type II inhibitors that inhibit the activation of fVIII by either thrombin or factor Xa and poorly inhibit the binding of fVIII to phospholipid membranes or von Willebrand factor (VWF). Group BC MAbs are epitopically and mechanistically distinct from the extensively studied group C MAb, ESH8. These results reveal the structural and functional complexity of the anti-C2 domain antibody response and indicate that interference with fVIII activation is a major attribute of the inhibitor landscape.

Proteolysis at Arg740 facilitates subsequent bond cleavages during thrombin-catalyzed activation of factor VIII

Thrombin activates factor VIII by proteolysis at three P1 residues: Arg372, Arg740, and Arg1689. Cleavage at Arg372 and Arg1689 are essential for procofactor activation; however cleavage at Arg740 has not been rigorously studied. To evaluate the role for cleavage at Arg740, we prepared and stably expressed two recombinant B-domainless factor VIII mutants, R740H and R740Q to slow and eliminate, respectively, cleavage at this site. Specific activity values for the variants were approximately 50 and 20%, respectively, that of wild-type factor VIII. Activation of factor VIII R740H by thrombin showed an approximately 40-fold reduction in the rate of A2 subunit generation, which reflected an approximately 20-fold reduction in cleavage rate at Arg372. Similarly, a approximately 40-fold rate reduction in cleavage at Arg1689 and consequent generation of the A3-C1-C2 subunit were observed. Rate values for A2 and A3-C1-C2 subunit generation were reduced by >700-fold and approximately 140-fold, respectively, in the R740Q variant. These results suggest that initial cleavage at Arg740 affects cleavage at both Arg372 and Arg1689 sites. Results obtained evaluating proteolysis of the factor VIII mutants by factor Xa revealed more modest rate reductions (<10-fold) in generating A2 and A3-C1-C2 subunits from either variant, suggesting that factor Xa-catalyzed activation of factor VIII was significantly less dependent upon prior cleavage at residue 740 than thrombin. Overall, these results support a model whereby cleavage of factor VIII by thrombin is an ordered pathway with cleavage at Arg740 facilitating cleavages at Arg372 and Arg1689, which result in procofactor activation.

Hemostasis as an optimal system

Zymogen and procofactor concentrations in physiological biochemical systems (PBS) have not yet been explained. The problem in question is to determine optimal plasma clotting factor (factors II, VII, IX, and X and cofactors V and VIII) concentrations for coagulation system (CS) as a whole. Constrained optimization technique is used to solve this problem. The constraint is determined by the ability of the CS to perform its physiological function--thrombin generation (and hence clot formation)--under vessel injury conditions. The constraint statement is based on the CS dynamics equations. In solving the problem the Lagrange multiplier is used. A hypothesis is advanced that this problem can be solved based on principle of minimum protein consumption subject to an above constraint. The results obtained indicate that the optimal clotting factor concentrations are in good agreement with those measured by biochemical techniques. A comparison between the theoretical results and experimental data lends support for our hypothesis that zymogen and procofactor concentrations in the CS (and, probably, for other biochemical systems) are determined by the principle of minimum protein consumption.

Catalytic IgG from Patients with Hemophilia A Inactivate Therapeutic Factor VIII

Factor VIII (FVIII) inhibitors are anti-FVIII IgG that arise in up to 50% of the patients with hemophilia A, upon therapeutic administration of exogenous FVIII. Factor VIII inhibitors neutralize the activity of the administered FVIII by sterically hindering its interaction with molecules of the coagulation cascade, or by forming immune complexes with FVIII and accelerating its clearance from the circulation. We have shown previously that a subset of anti-factor VIII IgG hydrolyzes FVIII. FVIII-hydrolyzing IgG are detected in over 50% of inhibitor-positive patients with severe hemophilia A, and are not found in inhibitor-negative patients. Although human proficient catalytic Abs have been described in a number of inflammatory and autoimmune disorders, their pathological relevance remains elusive. We demonstrate here that the kinetics of FVIII degradation by FVIII-hydrolyzing IgG are compatible with a pathogenic role for IgG catalysts. We also report that FVIII-hydrolyzing IgG from each patient exhibit multiple cleavage sites on FVIII and that, while the specificity of cleavage varies from one patient to another, catalytic IgG preferentially hydrolyze peptide bonds containing basic amino acids.

Factor V C2 domain contains a major thrombin-binding site responsible for thrombin-catalyzed factor V activation

Factor (F)V is converted into its active form, FVa, by limited proteolysis. Thrombin-catalyzed activation of FV is essential for its full cofactor activation. Previously, we reported that thrombin was bound to the C2 domain in the light chain of FVIII. As FV has a similar domain structure to FVIII, we focused on the FV C2 domain as a possible binding region for thrombin. Kinetic parameters, measured by surface plasmon resonance, revealed that the K(d) values of anhydro-thrombin for FV, FVa, and the FV C2 domain were 66, 240, and 670 nmol L(-1), respectively. FV activation was increased by approximately 9-fold by the addition of thrombin. In the presence of the FV C2 domain, this increase of the FV activation was inhibited. However, FV activation was not inhibited by the addition of the FVIII C2 domain. FV was cleaved into a 105-kDa heavy chain and a 71/74-kDa light chain by thrombin-catalyzed proteolysis at Arg709, Arg1018 and Arg1545. In the presence of the FV C2 domain, the cleavage was inhibited at all sites. Proteolysis was not affected by the addition of the FVIII C2 domain. These results indicated that the FV C2 domain contains a major binding site for thrombin and that this domain is necessary for the proteolysis at all cleavage sites. Furthermore, the present results also suggested that thrombin has an independent binding site for FV different from that for FVIII.

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.

Thrombin-catalyzed activation of factor VIII with His substituted for Arg372 at the P1 site

Thrombin-catalyzed proteolysis at Arg372 of factor VIII is essential for procofactor activation. However, hemophilia A patients with the missense mutation Arg372 to His possess a mild to moderate phenotype yet show no detectable cleavage at this bond. To evaluate this discrepancy, we prepared and stably expressed a recombinant, B-domainless factor VIII mutant (R372H) that possessed approximately 1% the specific activity of wild type. Cleavage at R372H by thrombin occurred with an approximately 80-fold decreased rate compared with wild type. N-terminal sequence analysis of the derived A2 subunit confirmed that cleavage occurred at the His372-Ser373 bond. Factor VIII R372H was activated slowly, attained lower activity levels, and exhibited an apparent reduced inactivation rate compared with factor VIII wild type. These observations were attributed to a reduced cleavage rate at His372. Factor Xa generation assays showed similar Michaelis-Menten constant (K(m), apparent) values for thrombin-catalyzed activation for either factor VIII form, but suggested an approximately 70-fold reduced maximum velocity (V(max)) for factor VIII R372H. However, prolonged reaction with thrombin yielded similar activity and stability values for the mutant and wild-type factor VIIIa forms. These results indicate a markedly reduced rate of cleavage following substitution at the P(1)Arg, and this property likely reflects the severity of the hemophilia A phenotype.

Activation of factor VIII and mechanisms of cofactor action

The factor VIII procofactor circulates as a metal ion-dependent heterodimer of a heavy chain and light chain. Activation of factor VIII results from limited proteolysis catalyzed by thrombin or factor Xa, which binds the factor VIII substrate over extended interactive surfaces. The proteases efficiently cleave factor VIII at three sites, two within the heavy and one within the light chain resulting in alteration of its covalent structure and conformation and yielding the active cofactor, factor VIIIa. The role of factor VIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. This effect is manifested in a dramatic increase in the catalytic rate constant, k(cat), by mechanisms that remain poorly understood.

Structural requirements for the activation of human factor VIII by thrombin

The coagulation factors V (FV) and VIII (FVIII) are important at sites of vascular injury for the amplification of the clotting cascade. Natural variants of these factors frequently lead to severe bleeding disorders. To understand the mechanisms of activation of FVIII by thrombin, we used a bank of mutant thrombins to define residues important for its activation. From the initial screening of 53 mutant thrombins for the activation of human recombinant FVIII, we mapped thrombin mutants with 50% or less activity to anion-binding exosite-I (Lys21Ala, His66Ala, Lys65Ala, Arg68Ala, Arg70Ala, and Tyr71Ala) and anion-binding exosite-II (Arg98Ala), the Na(+)-binding site (Glu229Ala, Arg233Ala, Asp234Ala, and Asp193Ala/Lys196Ala), and the 50-insertion loop (Trp50Ala), which were similar to our results for the activation of FV. The role of these residues for cleavage at Arg372 and Arg1689 was investigated using plasma FVIII. Anion-binding exosite-I appears to be important for cleavage at both sites, whereas the anion-binding exosite-II residue Arg98Ala is important for cleavage at Arg372 alone. The Glu229Ala mutant, which contributes to the Na(+)-binding site, and the 50-insertion loop mutant W50A have severely impaired cleavage at Arg372 and Arg1689. This suggests that the integrity of the active site and the Na(+)-bound form of thrombin are important for its procoagulant activity against FVIII. Detailed mutagenic analysis of thrombin can assist in understanding the pathogenesis of bleeding disorders and may lead to the rational design of selective thrombin inhibitors.

Molecular defects in coagulation Factor VIII and their impact on Factor VIII function

Molecular defects in Factor VIII (FVIII), such as haemophilia A-related mutations or denaturative conformational changes, may affect the stability of FVIII as well as its interactions with physiological activators, von Willebrand Factor, phospholipid, or conformationally sensitive antibodies. We summarize the contemporary assays which allow identification of impaired functional interactions of FVIII that cause a reduction or loss of its cofactor activity and/or increased immunogenicity. These assays can potentially be used for detection of molecular defects in FVIII and elucidation of the function impaired by these defects.

In vitro factor XI activation mechanism according to an optimized model of activated partial thromboplastin time test

Whether the in vitro activation of factor XI in plasma is mediated by thrombin or by auto-activation remains a controversial question. In this context, we have simulated theoretical activated partial thromboplastin time (aPTT) by means of a program based on a body of 22 essential elementary reactions implemented with rate constants quoted in current literature. To meet self-consistency in input data issued from varying sources, the results were optimized using the simplex treatment. The performance of the model was systematically evaluated considering the extent of the deviations observed between predicted aPTT and laboratory measurements conducted on normal and factor VIII, IX, XI and XII single-factor deficient plasma. The influence of the auto-activation or thrombin-mediated activation of factor XI on these aPTTs was tested separately after insertion of these reactions in the model. According to the best fits, a mechanism accounting for an auto-activation reaction of activated factor XI rather than a positive feedback reaction mediated by thrombin seemed more likely. Based on this conclusion, a chart of self-consistent rate constant values accounting for the intrinsic pathway of coagulation under static conditions is proposed.

Surface-mediated control of blood coagulation: the role of binding site densities and platelet deposition

A mathematical model of the extrinsic or tissue factor (TF) pathway of blood coagulation is formulated and results from a computational study of its behavior are presented. The model takes into account plasma-phase and surface-bound enzymes and zymogens, coagulation inhibitors, and activated and unactivated platelets. It includes both plasma-phase and membrane-phase reactions, and accounts for chemical and cellular transport by flow and diffusion, albeit in a simplified manner by assuming the existence of a thin, well-mixed fluid layer, near the surface, whose thickness depends on flow. There are three main conclusions from these studies. (i) The model system responds in a threshold manner to changes in the availability of particular surface binding sites; an increase in TF binding sites, as would occur with vascular injury, changes the system's production of thrombin dramatically. (ii) The model suggests that platelets adhering to and covering the subendothelium, rather than chemical inhibitors, may play the dominant role in blocking the activity of the TF:VIIa enzyme complex. This, in turn, suggests that a role of the IXa-tenase pathway for activating factor X to Xa is to continue factor Xa production after platelets have covered the TF:VIIa complexes on the subendothelium. (iii) The model gives a kinetic explanation of the reduced thrombin production in hemophilias A and B.

Analysis of the activated partial thromboplastin time test using mathematical modeling

Activated partial thromboplastin time (APTT) is a laboratory test for the diagnosis of blood coagulation disorders. The test consists of two stages: The first one is the preincubation of a plasma sample with negatively charged materials (kaolin, ellagic acid etc.) to activate factors XII and XI; the second stage begins after the addition of calcium ions that triggers a chain of calcium-dependent enzymatic reactions resulting in fibrinogen clotting. Mathematical modeling was used for the analysis of the APTT test. The process of coagulation was described by a set of coupled differential equations that were solved by the numerical method. It was found that as little as 2.3 x 10(-9) microM of factor XIIa (1/10000 of its plasma concentration) is enough to cause the complete activation of factor XII and prekallikrein (PK) during the first 20 s of the preincubation phase. By the end of this phase, kallikrein (K) is completely inhibited, residual activity of factor XIIa is 54%, and factor XI is activated by 26%. Once a clot is formed, factor II is activated by 4%, factor X by 5%, factor IX by 90%, and factor XI by 39%. Calculated clotting time using protein concentrations found in the blood of healthy people was 40.5 s. The most pronounced prolongation of APTT is caused by a decrease in factor X concentration.

Reduction of the antigenicity of factor VIII toward complex inhibitory antibody plasmas using multiply-substituted hybrid human/porcine factor VIII molecules

Factor VIII (fVIII) circulates as a heavy chain/light chain (A1-A2-B/ap-A3-C1-C2) heterodimer. The 41-residue light chain activation peptide, ap, is cleaved from fVIII during proteolytic activation by thrombin or factor Xa. We constructed 7 active recombinant hybrid B-domainless human/porcine fVIII molecules that contained combinations of porcine sequence replacements within the A2, ap-A3, and C2 domains. The cross-reactivity of 23 high-titer inhibitory antibodies between human fVIII and the hybrids was inversely related to the degree of porcine substitution. In all plasmas, the substitution of all 3 regions yielded cross-reactivities that were not significantly different from those of porcine fVIII. To differentiate between inhibitor binding to the ap region and the A3 domain, we constructed 2 additional hybrids that contained porcine A2 and C2 domain substitutions and either porcine A3 or porcineap substitutions. The porcine ap segment was less antigenic than the human ap segment in several plasmas that had activity against the ap-A3 region. This indicates that some inhibitor plasmas contain antibodies directed against the fVIIIap segment in addition to A2, A3, and C2 domain epitopes identified in previous studies. Substitution of porcine sequences within the A2, A3, C2, and ap regions of human fVIII is necessary and sufficient to achieve a maximal reduction in antigenicity relative to porcine fVIII with respect to most inhibitory antibody plasmas.

Role of activation of the coagulation factor VIII in interaction with vWf, phospholipid, and functioning within the factor Xase complex

Blood coagulation factor VIII (fVIII) in its nonactivated form circulates in plasma in a complex with von Willebrand factor (vWf). Upon activation by thrombin- or factor Xa-mediated site-specific proteolysis, activated fVIII (fVIIIa) serves as a cofactor for factor IXa. This protein complex assembled on a phospholipid surface (factor Xase) activates factor X. This complex plays the key role in the intrinsic pathway of blood coagulation. We reviewed the molecular events triggered by fVIII activation, which are required for the assembly and functioning of the Xase complex, including fVIIIa dissociation from vWf and a significant increase of fVIII affinity for binding to the phospholipid surface. Both events are mediated by activation-related cleavage within fVIII light chain (LCh), releasing the 40 amino-acid N-terminal LCh peptide, which is followed by a conformational change within the C2 domain. The conformational change within LCh is also required for the optimal fVIII cofactor functioning within the factor Xase complex, exerted via fVIIIa interactions with phospholipid, factor IXa, and factor X. Since factor IXa not only stabilizes but also proteolytically inactivates fVIIIa within the factor Xase complex, the stability of the membrane-bound fVIIIa in the presence and absence of factor IXa is discussed. In conclusion, we outline some new possible directions of the research. One of them arises from the recently demonstrated ability of plasma lipoproteins to provide a phospholipid surface for the assembly of the factor Xase complex in vitro. This finding raises a possibility that lipoproteins participate in factor Xase functioning in vivo and suggests a direct link between elevated levels of lipoproteins associated with atherosclerosis and increased thrombogenicity associated with this disease.

Catalytic activity of antibodies against factor VIII in patients with hemophilia A

Hemophilia A is an X chromosome-linked recessive disorder resulting in defective or deficient factor VIII (FVIII) molecules, which, in its severe form, is a life-threatening and crippling hemorrhagic disease. Infusion of homologous FVIII to patients with severe hemophilia A results, in 25% of patients, in the emergence of alloantibodies against FVIII (inhibitors)( ref. 1) that inhibit FVIII procoagulant activity by steric hindrance of the interaction of FVIII either with stabilizing molecules, with molecules essential for its activity or with activating molecules. Here, we report on the proteolysis of FVIII by alloantibodies of two patients with severe hemophilia A, demonstrating a previously unknown mechanism by which FVIII inhibitors may prevent the pro-coagulant function of FVIII. The kinetic parameters of FVIII hydrolysis indicate a functional role for the catalytic immune response in the inactivation of FVIII in vivo. The characterization of alloantibodies against FVIII as site-specific proteases may provide new approaches to the treatment of FVIII inhibitors.

A Novel Mutation in the D3 Domain of von Willebrand Factor Markedly Decreases Its Ability to Bind Factor VIII and Affects Its Multimerization

In type 2N von Willebrand disease (vWD), von Willebrand factor (vWF) is characterized by normal multimeric pattern, normal platelet-dependent function, but a markedly decreased affinity for factor VIII (FVIII). In this report, we describe the case of a vWD patient who has an abnormal vWF multimers distribution associated with a markedly decreased vWF ability to bind FVIII. Sequencing analysis of patient’s vWF gene showed, at heterozygous state, a G→A transition resulting in the substitution of Asn for Asp at position 116 of the mature vWF subunit and a C→T transition, changing the codon for Arg 896 into a stop codon. His sister who has a subnormal vWF level, but a normal FVIII/vWF interaction, was found to be heterozygous for the Arg896ter mutation only. Recombinant vWF (rvWF) containing the candidate (Asn116) missense mutation was expressed in COS-7 cells. The expression level of Asn116rvWF was significantly decreased compared with wild-type rvWF. The multimeric pattern of Asn116rvWF was greatly impaired as shown by the decrease in high molecular weight forms. The FVIII binding ability of Asn116rvWF was dramatically decreased. These data show that the Asp116Asn substitution is the cause of both the defective FVIII/vWF interaction and the impaired multimeric pattern observed in the patient’s vWF. The monoclonal antibody 31H3 against D’ domain of vWF (epitope aa 66-76) that partially inhibits the FVIII binding and recognizes only nonreduced vWF, showed a decreased ability to bind Asn116rvWF when used as capture-antibody in enzyme-linked immunosorbent assay (ELISA). This result suggests that a potential conformation change in the D’ domain is induced by the Asp116Asn substitution, which is localized in the D3 domain.

A Novel Mutation in the D3 Domain of von Willebrand Factor Markedly Decreases Its Ability to Bind Factor VIII and Affects Its Multimerization

In type 2N von Willebrand disease (vWD), von Willebrand factor (vWF) is characterized by normal multimeric pattern, normal platelet-dependent function, but a markedly decreased affinity for factor VIII (FVIII). In this report, we describe the case of a vWD patient who has an abnormal vWF multimers distribution associated with a markedly decreased vWF ability to bind FVIII. Sequencing analysis of patient’s vWF gene showed, at heterozygous state, a G→A transition resulting in the substitution of Asn for Asp at position 116 of the mature vWF subunit and a C→T transition, changing the codon for Arg 896 into a stop codon. His sister who has a subnormal vWF level, but a normal FVIII/vWF interaction, was found to be heterozygous for the Arg896ter mutation only. Recombinant vWF (rvWF) containing the candidate (Asn116) missense mutation was expressed in COS-7 cells. The expression level of Asn116rvWF was significantly decreased compared with wild-type rvWF. The multimeric pattern of Asn116rvWF was greatly impaired as shown by the decrease in high molecular weight forms. The FVIII binding ability of Asn116rvWF was dramatically decreased. These data show that the Asp116Asn substitution is the cause of both the defective FVIII/vWF interaction and the impaired multimeric pattern observed in the patient’s vWF. The monoclonal antibody 31H3 against D’ domain of vWF (epitope aa 66-76) that partially inhibits the FVIII binding and recognizes only nonreduced vWF, showed a decreased ability to bind Asn116rvWF when used as capture-antibody in enzyme-linked immunosorbent assay (ELISA). This result suggests that a potential conformation change in the D’ domain is induced by the Asp116Asn substitution, which is localized in the D3 domain.

The role of platelet von Willebrand factor in the binding of factor VIII to activated platelets

Factor VIII binds to activated platelets and contributes to the tenase complex assembled on the platelet membrane surface. We have examined the role of platelet von Willebrand factor in the binding of factor VIII to platelets using a platelet captured enzyme-linked immunosorbent assay. Purified factor VIII bound to activated normal platelets in a dose dependent manner. Factor VIII also bound to platelets obtained from a patient with Type 2N von Willebrand disease, although in this case the binding was reduced to approximately 50% of that seen with control platelets. Furthermore, factor VIII bound to Type 3 von Willebrand disease platelets in the absence of detectable von Willebrand factor. In this instance the binding reaction appeared to be approximately 30% of that seen with the same number of normal platelets. An anti-A3 domain monoclonal antibody, NMC-VIII/10, which recognizes the amino-terminal acidic region of the factor VIII light chain, and an anti-C2 domain monoclonal antibody, NMC-VIII/5, which also moderates the binding of factor VIII to phosphatidylserine, inhibited the association between factor VIII and platelets. Inhibition was more remarkable with NMC-VIII/5 than with NMC-VIII/ 10 but not complete. The findings suggest that the binding of factor VIII to activated platelets is not based on a single ligand-receptor relationship, although a predominant role exists for the platelet von Willebrand factor. Furthermore, both the amino-terminal acidic region of the A3 domain and the C2 domain participate in the binding of factor VIII to activated platelets.

Mathematical model for the blood coagulation prothrombin time test

A mathematical model for the prothrombin time test is proposed. The time course of clotting factor activation during coagulation was calculated, and the sensitivity of the test to a decrease in the concentrations of coagulation proteins or their activities was studied. The model predicts that only severe coagulation disorders connected with a more than five-fold decrease in the concentrations or activities of the blood coagulation factors can be revealed by the test.

Slowed release of thrombin-cleaved factor VIII from von Willebrand factor by a monoclonal and a human antibody is a novel mechanism for factor VIII inhibition

The anti-factor VIII (fVIII) C2 domain monoclonal antibody ESH8 inhibits fVIII activity only when fVIII is bound to von Willebrand factor (vWf). However, ESH8 binds with similar affinity to fVIII and fVIII.vWf complex, and it does not affect the kinetics of thrombin cleavage at positions 372 and 740 within the fVIII heavy chain and at 1689 within the light chain. The latter is required for fVIII release from vWf. We showed that ESH8 reduced the initial rate of thrombin-activated fVIII (fVIIIa) release from vWf by 4.3-fold compared to that in the absence of antibody. The complex of vWf. fVIII.ESH8 was activated, and the rate constant determined for fVIIIa dissociation from vWf was 4 x 10(-3) s-1. We constructed a mathematical model incorporating the measured rates for fVIIIa release from vWf and for inactivation of heterotrimeric fVIIIa due to the spontaneous loss of the A2 subunit and found that the decreased release rate is sufficient to explain our experimentally observed inhibition of fVIII activity by ESH8. We hypothesize that the slowed rate of fVIIIa release from vWf in the presence of ESH8 allows time for inactivation of unstable fVIIIa prior its participation in the formation of the factor Xase complex. The relevance of these findings is illustrated by our observation that reduction of fVIIIa release from vWf represents an additional mechanism of fVIII inhibition by an anti-C2 domain antibody (epitope 2218-2307) from a hemophilia A patient. This rare antibody binds to a more amino-terminal epitope than other human anti-C2 inhibitors, resulting in its lack of inhibition of fVIII binding to vWf but not to phospholipid. These two fVIII ligands therefore bind to C2 sites which do not overlap completely.

Activated factor VIII in therapeutic preparations: analysis by ultracentrifugation

Activation of factor VIII is signaled by an increase in the 1-stage factor VIII activity and release from von Willebrand factor. Ultracentrifugation in 10-40% sucrose gradients was used to identify such activation in therapeutic concentrates. Plasma-derived factor VIII lots were examined and factor VIII sedimenting independently of von Willebrand factor was identified in some of the preparations. In addition there was slower sedimentation of factor VIII by the 1-stage assay than by the chromogenic assay. These results are consistent with a factor VIII cleaved at residue 1689, a site important for von Willebrand binding. This activated form leads to some of the assay discrepancies between the 1-stage assay and the chromogenic or the 2-stage assay. There is more rapid sedimentation of the factor VIII measured by the chromogenic assay than the von Willebrand factor in some manufacturers' samples indicating that the method of fractionation may select a low molecular weight von Willebrand factor which does not bind factor VIII. Routine comparison between the 1-stage and chromogenic assays during fractionation may be able to identify such activated preparations. Other assay discrepancies may be due to structural differences between the standards and the tested product.

Kinetics of factor VIII light-chain cleavage by thrombin and factor Xa. A regulatory role of the factor VIII heavy-chain region Lys713-Arg740

Activation and limited proteolysis of factor VIII have been investigated with respect to the role of the heavy-chain region Lys713-Arg740. The kinetics of factor VIII activation have been analyzed in a system consisting of human factor VIII, factor IXa, factor X phospholipids, and thrombin or factor Xa. Plasma-derived factor VIII is activated by thrombin with a second-order rate constant of 3.3 +/- 0.3 x 10(6) M-1 s-1, which proved to be slightly higher than for activation by factor Xa. The second-order rate constant of activation by thrombin of plasma-derived factor VIII in the presence of a monoclonal antibody against the sequence Lys713-Arg740 is markedly reduced. The same result was obtained for activation by thrombin and factor Xa of factor VIII with a deletion including the sequence Lys713-Arg740, des-(713-1637)-factor VIII. This suggests that the region Lys713-Arg740 promotes factor VIII activation by both thrombin and factor Xa. Since factor VIII activation is associated with proteolysis, cleavage of factor VIII heavy and light chains was analyzed quantitatively. These studies indicated that heavy-chain cleavage of des-(713-1637)-factor VIII is similar to that of plasma-derived factor VIII. In contrast, cleavage of the light chain of des-(713-1637)-factor VIII is clearly reduced. Furthermore, the secondorder rate constant (0.2 +/- 0.1 x 10(6) M-1 s-1) of des-(713-1637)-factor VIII light-chain cleavage by thrombin was reduced tenfold compared with that of plasma-derived factor VIII. Proteolysis by factor Xa yielded similar results. The rate of des-(713-1637)-factor VIII light-chain cleavage by thrombin is similar to that of isolated light-chain, but isolated light-chain is cleaved by factor Xa 20-fold more efficiently than the light chain in des-(713-1637)-factor VIII. We conclude that activation of factor VIII by both thrombin and factor Xa is closely associated with light-chain cleavage. Furthermore, within the factor VIII heterodimer, the heavy-chain sequence Lys713-Arg740 promotes both activation and light-chain proteolysis.

Involvement of thrombin anion-binding exosites 1 and 2 in the activation of factor V and factor VIII

The role of anion-binding exosites of thrombin in the activation of factor V and factor VIII was studied using thrombin Arg93 --> Ala, Arg97 --> Ala, and Arg101 --> Ala (thrombin RA), a recombinant exosite 2 defective mutant, and a synthetic N-acetylated dodecapeptide, Ac-Asn-Gly-Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-O-SO4Leu (hirugen), which competitively inhibits binding of macromolecules to exosite 1. The catalytic efficiency of the activation of factor VIII or of the first step of factor V activation by thrombin RA was approximately 10% that of wild-type thrombin. The overall rate of conversion to factor Va was not influenced by the mutation. In contrast to factor V, the slow activation of factor VIII by thrombin RA was associated with a decreased rate of cleavage at all three proteolytic sites (Arg372, Arg740, and Arg1689). Hirugen inhibited factor V and factor VIII activation. These results indicate that both anion-binding exosites of thrombin are involved in the recognition of factor V and factor VIII.

Mathematical model of serine protease inhibition in the tissue factor pathway to thrombin

A mathematical model has been developed to simulate the generation of thrombin by the tissue factor pathway. The model gives reasonable predictions of published experimental results without the adjustment of any parameter values. The model also accounts explicitly for the effects of serine protease inhibitors on thrombin generation. Simulations to define the optimum affinity profile of an inhibitor in this system indicate that for an inhibitor simultaneously potent against VIIa, IXa, and Xa, inhibition of thrombin generation decreases dramatically as the affinity for thrombin increases. Additional simulations show that the reason for this behavior is the sequestration of the inhibitor by small amounts of thrombin generated early in the reaction. This model is also useful for predicting the potency of compounds that inhibit thrombosis in rats. We believe that this is the first mathematical model of blood coagulation that considers the effects of exogenous inhibitors. Such a model, or extensions thereof, should be useful for evaluating targets for therapeutic intervention in the processes of blood coagulation.

Autoantibody to factor VIII that has less reactivity to factor VIII/von Willebrand factor complex

To determine the difference in reactivity of factor VIII (FVIII) inhibitor to FVIII/von Willebrand Factor (vWF) complex and FVIII free of vWF, an autoantibody to FVIII light chain was tested. A patient (1-3) suffered from autoimmune hemolytic anemia with autoantibody to FVIII. Epitope specificity of the patient's IgG (I-3 IgG) was shown to be the C2 domain of FVIII light chain (2170-2332) by Western blotting using recombinant FVIII deletions expressed in Escherichia coli. The inhibitory effect on FVIII procoagulant activity (VIII:C) of I-3 IgG was tested against a conventional FVIII concentrate; Haemate P, a monoclonal antibody-purified FVIII concentrate; Hemofil M, and a recombinant FVIII (rFVIII); Kogenate. I-3 IgG showed only 1.3 BU/mgIgG for Haemate P, in contrast to 20 BU/mgIgG for both Hemofil M and Kogenate. The ratio of VIII:C/vWF:Ag in Haemate P and Hemofil M was 1/3.43 and 1/0.01, respectively, while Kogenate did not contain vWF. The inhibitory effect of the I-3 IgG was then compared with Kogenate and its complex with vWF. The inhibitory effect was decreased against the rFVIII by forming a complex with vWF from 22 BU/mgIgG to 0.5 BU/mgIgG. Fab from the I-3 IgG had the same effect. In addition, vWF showed a protective effect on FVIII inactivation by the I-3 IgG in a dose dependent manner. Fifty-nine percent of residual VIII:C was retained in the presence of 8 U/ml of vWF after 1 hr incubation with I-3 IgG. These results suggested that vWF could compete with the I-3 IgG for binding to FVIII.

Cleavage of factor VIII light chain is required for maximal generation of factor VIIIa activity

Thrombin-catalyzed activation of heterodimeric factor VIII occurs by limited proteolysis, yielding subunits A1 and A2 derived from the heavy chain (HC) and A3-C1-C2 derived from the light chain (LC). The roles of these cleavages in the function of procoagulant activity are poorly understood. To determine whether LC cleavage contributes to the potentiation of factor VIII activity, factor VIII heterodimers were reconstituted from native HC and either thrombin-cleaved LC (A3-C1-C2) or intact LC and purified by Mono S chromatography. The reconstituted factor VIII form containing the A3-C1-C2 subunit had a specific activity (2 units/micrograms) that was approximately 3-fold greater than that of the reconstituted factor VIII form containing native LC (0.6 units/microgram). Factor Xa generation assays using the hybrid heterodimer showed an initial rate that was unaffected by the presence of von Willebrand factor and a reduced lag time when compared with the native heterodimer. The A1/A3-C1-C2 dimer was dissociated by chelation, and the purified A1 subunit was reacted with either the A3-C1-C2 subunit or the LC in the presence of Mn2+ to reconstitute the dimer. Factor VIIIa heterotrimers were reconstituted from either A1/A3-C1-C2 or A1/LC plus the A2 subunit. The authentic factor VIIIa heterotrimer (A1/A3-C1-C2/A2) had 3-fold greater activity than the form containing the LC. However, upon reaction with thrombin, the activity of the latter form was increased to that of the factor VIIIa form containing native subunits. The incremental increase in fluorescence anisotropy of fluorescein-Phe-Phe-Arg chloromethyl ketone-modified factor IXa was markedly greater in the presence of HC/A3-C1-C2 (delta r = 0.037) compared with HC/LC (delta r = 0.011) and approached the value obtained with factor VIIIa (delta r = 0.051). These results suggest that cleavage of factor VIII LC directly contributes to the potentiation of coagulant activity by modulating the conformation of the factor IXa active site.