The binding of human factor IX to endothelial cells is mediated by residues 3-11

Unresolved hemostasis issues in haemophilia

Despite rapid technological advancement in factor and nonfactor products in the prevention and treatment of bleeding in haemophilia patients, it is imperative that we acknowledge gaps in our understanding of how hemostasis is achieved. The authors will briefly review three unresolved issues in persons with haemophilia (PwH) focusing on the forgotten function that red blood cells play in hemostasis, the critical role of extravascular (outside circulation) FIX in hemostasis in the context of unmodified and extended half-life FIX products and finally on the role that skeletal muscle myosin plays in prothrombinase assembly and subsequent thrombin generation that could mitigate breakthrough muscle hematomas.

Extravascular factor IX pool fed by prophylaxis is a true hemostatic barrier against bleeding

BACKGROUND

Factor (F)IX can bind to type IV collagen in the endothelial basement membrane and diffuse into extravascular spaces. Previous studies in rodents have reported a large biodistribution of FIX.

OBJECTIVES

The aim of the study was to evaluate the potential hemostatic activity of extravascular FIX and its role in protecting against joint bleeds.

METHODS

The capacity of 4 different FIX molecules (plasma-derived and recombinant) to bind type I and type IV collagen was studied here. FIX molecules were also administered intravenously at doses of 50 to 3000 IU/kg in FIX knockout mice.

RESULTS

A specific FIX signal was detected in immunohistochemistry in the liver as well as in muscles and knee joints with recombinant FIX molecules injected at 1000 and 3000 IU/kg but not at the usual clinical doses of 50 to 100 IU/kg, while plasma-derived FIX generated a FIX signal at all doses, including 50 IU/kg. Such a signal was also detected after five 100 IU/kg daily infusions of recombinant FIX, suggesting that FIX can accumulate in the extravascular space during prophylaxis. The extravascular procoagulant activity of FIX, assessed in saphenous vein bleeding assays, was significantly higher in hemophilia B mice after these 5 days of prophylaxis compared to a single infusion of 100 IU/kg of FIX and assessment of FIX activity 7 days later.

CONCLUSION

Taken together, these results show that in individuals with severe hemophilia B receiving regular prophylaxis with FIX, extravascular accumulation of FIX over time may have a significant impact on the coagulation capacity and protection toward bleeding.

Modulation of Extravascular Binding of Recombinant Factor IX Impacts the Duration of Efficacy in Mouse Models

BACKGROUND

 There is an emerging concept that in addition to circulating coagulation factor IX (FIX), extravascular FIX contributes to hemostasis.

OBJECTIVE

 Our objective was to evaluate the efficacy of extravascular FIX using animal models of tail clip bleeding and ferric chloride-induced thrombosis.

METHODS

 Mutant rFIX proteins with described enhanced (rFIXK5R) or reduced (rFIXK5A) binding to extracellular matrix were generated and characterized using in vitro aPTT, one-stage clotting, and modified FX assays. Using hemophilia B mice, pharmacokinetic (PK) parameters and in vivo efficacy of these proteins were compared against rFIX wild-type protein (rFIXWT) in a tail clip bleeding and FeCl3-induced thrombosis model. Respective tissue disposition of FIX was evaluated using immunofluorescence.

RESULTS

 In vitro characterization demonstrated comparable clotting activity of rFIX proteins. The PK profile showed that rFIXK5A displayed the highest plasma exposure compared to rFIXWT and rFIXK5R. Immunofluorescence evaluation of liver tissue showed that rFIXK5R was detectable up to 24 hours, whereas rFIXWT and rFIXK5A were detectable only up to 15 minutes. In the tail clip bleeding model, rFIXK5R displayed significant hemostatic protection against bleeding incidence for up to 72 hours postintravenous administration of 50 IU/kg, whereas the efficacy of rFIXK5A was already reduced at 24 hours. Similarly, in the mesenteric artery thrombus model, rFIXK5R and rFIXWT demonstrated prolonged efficacy compared to rFIXK5A.

CONCLUSION

 Using two different in vivo models of hemostasis and thrombosis, we demonstrate that mutated rFIX protein with enhanced binding (rFIXK5R) to extravascular space confers prolonged hemostatic efficacy in vivo despite lower plasma exposure, whereas rFIXK5A rapidly lost its efficacy despite higher plasma exposure.

Recent advances in the production of recombinant factor IX: bioprocessing and cell engineering

Appropriate treatment of Hemophilia B is vital for patients' quality of life. Historically, the treatment used was the administration of coagulation Factor IX derived from human plasma. Advancements in recombinant technologies allowed Factor IX to be produced recombinantly. Successful recombinant production has triggered a gradual shift from the plasma derived origins of Factor IX, as it provides extended half-life and expanded production capacity. However, the complex post-translational modifications of Factor IX have made recombinant production at scale difficult. Considerable research has therefore been invested into understanding and optimizing the recombinant production of Factor IX. Here, we review the evolution of recombinant Factor IX production, focusing on recent developments in bioprocessing and cell engineering to control its post-translational modifications in its expression from Chinese Hamster Ovary (CHO) cells.

The Function of extravascular coagulation factor IX in haemostasis

INTRODUCTION

The majority of clotting factor IX (FIX) resides extravascularly, in the subendothelial basement membrane, where it is important for haemostasis.

AIM

We summarize preclinical studies demonstrating extravascular FIX and its role in haemostasis and discuss clinical observations supporting this. We compare the in vivo binding of BeneFIX^® and the extended half-life FIX, Alprolix^® , to extravascular type IV collagen (Col4).

METHODS

Three mouse models of haemophilia were used: the FIX knockout as the CRM^- model and two knock-in mice, representing a CRM⁺ model of a commonly occurring patient mutation (FIX_R333Q ) or a mutation that binds poorly to Col4 (FIX_K5A ). The murine saphenous vein bleeding model was used to assess haemostatic competency. Clinical publications were reviewed for relevance to extravascular FIX.

RESULTS

CRM status affects recovery and prophylactic efficacy. Prophylactic protection decreases ~5X faster in CRM⁺ animals. Extravascular haemostasis can explain unexpected breakthrough bleeding in patients treated with some EHL-FIX therapeutics. In mice, both Alprolix^® and BeneFIX^® bind Col4 with similar affinities (Kd~20-40 nM) and show dose-dependent recoveries. As expected, the concentration of binding sites in the mouse calculated for Alprolix^® (574 nM) was greater than for BeneFIX^® (405 nM), due to Alprolix^® binding to both Col4 and the endothelial cell neonatal Fc receptor.

CONCLUSION

Preclinical and clinical results support the interpretation that FIX plays a role in haemostasis from its extravascular location. We believe that knowing the CRM status of haemophilia B patients is important for optimizing prophylactic dosing with less trial and error, thereby decreasing clinical morbidity.

Dysfunctional endogenous FIX impairs prophylaxis in a mouse hemophilia B model

Factor IX (FIX) binds to collagen IV (Col4) in the subendothelial basement membrane. In hemophilia B, this FIX-Col4 interaction reduces the plasma recovery of infused FIX and plays a role in hemostasis. Studies examining the recovery of infused BeneFix (FIX_WT) in null (cross-reactive material negative, CRM^-) hemophilia B mice suggest the concentration of Col4 readily available for binding FIX is ∼405 nM with a 95% confidence interval of 374 to 436 nM. Thus, the vascular cache of FIX bound to Col4 is several-fold the FIX level measured in plasma. In a mouse model of prophylactic therapy (testing hemostasis by saphenous vein bleeding 7 days after infusion of 150 IU/kg FIX), FIX_WT and the increased half-life FIXs Alprolix (FIX_FC) and Idelvion (FIX_Alb) produce comparable hemostatic results in CRM^- mice. In bleeding CRM^- hemophilia B mice, the times to first clot at a saphenous vein injury site after the infusions of the FIX agents are significantly different, at FIX_WT < FIX_FC < FIX_Alb Dysfunctional forms of FIX, however, circulate in the majority of patients with hemophilia B (CRM⁺). In the mouse prophylactic therapy model, none of the FIX products improves hemostasis in CRM⁺ mice expressing a dysfunctional FIX, FIX_R333Q, that nevertheless competes with infused FIX for Col4 binding and potentially other processes involving FIX. The results in this mouse model of CRM⁺ hemophilia B demonstrate that the endogenous expression of a dysfunctional FIX can deleteriously affect the hemostatic response to prophylactic therapy.

Protein-Engineered Coagulation Factors for Hemophilia Gene Therapy

Hemophilia A (HA) and hemophilia B (HB) are X-linked bleeding disorders due to inheritable deficiencies in either coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Recently, gene therapy clinical trials with adeno-associated virus (AAV) vectors and protein-engineered transgenes, B-domain deleted (BDD) FVIII and FIX-Padua, have reported near-phenotypic cures in subjects with HA and HB, respectively. Here, we review the biology and the clinical development of FVIII-BDD and FIX-Padua as transgenes. We also examine alternative bioengineering strategies for FVIII and FIX, as well as the immunological challenges of these approaches. Other engineered proteins and their potential use in gene therapy for hemophilia with inhibitors are also discussed. Continued advancement of gene therapy for HA and HB using protein-engineered transgenes has the potential to alleviate the substantial medical and psychosocial burdens of the disease.

Extravascular FIX and coagulation

This review summarizes the evidence that collagen IV binding is physiologically important, and that the extravascular compartment of FIX is composed of type IV collagen. As we have previously demonstrated, 7 days post-infusion, FIX_WT (BeneFIX) is able to control bleeding as well as the same dosage of Alprolix in hemophilia B mice, tested using the saphenous vein bleeding model (Alprolix is a chimeric FIX molecule joined at its C terminus to a Fc domain). Furthermore, we have shown that in hemophilia B mice, doses of BeneFIX or Alprolix (up to a dose of 150 IU/kg) have increased bleeding-control effectiveness in proportion to the dose up to a certain limit: higher doses are no more effective than the 150 IU/kg dose. These studies suggest that in hemophilia B mice, tested using the saphenous vein bleeding model, three things are true: first, extravascular FIX is at least as important for coagulation as is circulating FIX; second, measuring circulating levels of FIX may not be the best criterion for designing new “longer lasting” FIX molecules; and third, trough levels are less diagnostic for FIX therapy than they are for FVIII therapy.

Prophylactic efficacy of BeneFIX vs Alprolix in hemophilia B mice

FIX binds tightly to collagen IV. Furthermore, a FIX mutant, FIXK5R, which binds better than wild-type FIX to collagen IV, provides better hemostasis than wild-type FIX, long after both are undetectable in the plasma. There is also credible evidence of extravascular FIX. Here, we use the saphenous vein bleeding model to compare the efficacy of recombinant FIXFc (Alprolix) and wild-type FIX (BeneFIX) in hemophilia B mice 7 days postinfusion. Although the terminal half-life of Alprolix is significantly longer than that of BeneFIX, at equal doses Alprolix is not better at controlling bleeding 7 days postinfusion, presumably because of the extravascular FIX. Both BeneFIX and Alprolix exhibit a linear response in clotting efficacy up to 150 IU/kg, where they appear to saturate an extravascular compartment, because there is no additional prophylactic benefit from higher doses. A robust pool of extravascular FIX is clearly observed surrounding blood vessels, localized to the same region as collagen IV, in 2 representative human tissues: liver and skeletal muscle. We see no increased risk for thrombosis at 250 IU/kg FIX at 6 hours postinfusion. In summary, 7 days postinfusion into hemophilia B mice, BeneFIX and Alprolix are hemostatically indistinguishable despite the latter's increased half-life. We predict that doses of FIX ∼3 times higher than the currently recommended 40 to 50 IU/kg will, because of FIX's large extravascular compartment, efficiently prolong prophylactic hemostasis without thrombotic risk.

Factor IXa as a target for anticoagulation in thrombotic disorders and conditions

From acute coronary syndrome (ACS) to the prevention of cardioembolic events in patients with atrial fibrillation and thrombosis of mechanical heart valves, there is a quest to develop a new generation of anticoagulants. Perhaps the 'holy grail' of antithrombotic therapy is not only a drug that will prevent coagulation without promoting bleeding but also an anticoagulant that is easily reversible should the clinical need arise. Further, an optimally designed anticoagulant would have broad applications to include arterial, venous, hybrid conditions (atrial flutter and fibrillation) and nonbiological materials. Factor (F)IXa plays a pivotal part in tissue factor (TF)-mediated thrombin generation, and therefore represents a potentially promising target for drug development. FIXa activity has been targeted by multiple modalities, including oral inhibitors, RNA aptamers, monoclonal antibodies and synthetic active-site-blocking competitive inhibitors. Herein, we summarize the biochemistry of FIXa as it applies to thrombotic disorders and conditions, as well as the evolution of targeted therapies.

FVIIa as used pharmacologically is not TF dependent in hemophilia B mice

Activated factor VII is approved for treating hemophilia patients with autoantibodies to their factor IX or FVIII; however, its mechanism of action remains controversial. Some studies suggest that FVIIa requires tissue factor (TF) for function and that the reason for the high dose requirement is that it must compete with endogenous FVII for tissue factor. Others suggest that FVIIa binds platelets where it activates FX directly; the high concentration required would result from FVIIa's weak affinity for phospholipids. We address this question by infusing a chimera of mouse FIX (Gla and EGF1) with FVIIa (EGF2 and catalytic domain) into hemophilia B mice. This mutant has no TF-dependent activity because it cannot functionally bind TF at physiologically relevant concentrations. In vivo, this mutant is as effective as mouse FVIIa in controlling bleeding in hemophilia B mice. Our results suggest that the hemostatic effect of pharmacologic doses of FVIIa is TF independent.

Glycoengineered factor IX variants with improved pharmacokinetics and subcutaneous efficacy

BACKGROUND

The rapid clearance of factor IX (FIX) necessitates frequent intravenous administration to achieve effective prophylaxis for patients with hemophilia B. Subcutaneous administration would be a preferred route of administration but is limited by bioavailability.

OBJECTIVES

To improve the pharmacokinetics (PK) and bioavailability of FIX, a screen was performed to identify positions for the introduction of novel glycosylation sites with maximal effect on PK and maintenance of coagulation activity.

METHODS

Two hundred fifty-one variants, each containing one additional N-linked glycosylation site, were screened in vitro, and the PK profiles of selected variants mapping to spatially distinct regions of FIX were evaluated in mice. Optimal variants were combined, and their PK and efficacy were determined in mice with hemophilia B.

RESULTS

Variants that mapped to spatially distinct regions of the FIX structure exhibited different degrees of improved PK and enabled selection of optimized sites while minimizing the loss of FIX activity. Combining the most effective N-glycan sites in the same FIX molecule resulted in further improvements in PK. An optimized variant containing three novel N-glycan sites (at amino acids 103, 151, and 228), and the activity enhancing 338A variant had double the specific activity of wild-type FIX, exhibited 4.5-fold reduced clearance and 2.4-fold increased subcutaneous bioavailability, and was efficacious at a fivefold lower mass dose than wild-type FIX after subcutaneous injection in a bleeding model in mice with hemophilia B.

CONCLUSIONS

Glycoengineering was used to significantly improve the subcutaneous PK and efficacy of FIX and may have advantages for subcutaneous dosing.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

Prolonged half-life and preserved enzymatic properties of factor IX selectively PEGylated on native N-glycans in the activation peptide

Current management of hemophilia B entails multiple weekly infusions of factor IX (FIX) to prevent bleeding episodes. In an attempt to make a longer acting recombinant FIX (rFIX), we have explored a new releasable protraction concept using the native N-glycans in the activation peptide as sites for attachment of polyethylene glycol (PEG). Release of the activation peptide by physiologic activators converted glycoPEGylated rFIX (N9-GP) to native rFIXa and proceeded with normal kinetics for FXIa, while the K(m) for activation by FVIIa-tissue factor (TF) was increased by 2-fold. Consistent with minimal perturbation of rFIX by the attached PEG, N9-GP retained 73%-100% specific activity in plasma and whole-blood-based assays and showed efficacy comparable with rFIX in stopping acute bleeds in hemophilia B mice. In animal models N9-GP exhibited up to 2-fold increased in vivo recovery and a markedly prolonged half-life in mini-pig (76 hours) and hemophilia B dog (113 hours) compared with rFIX (16 hours). The extended circulation time of N9-GP was reflected in prolonged correction of coagulation parameters in hemophilia B dog and duration of effect in hemophilia B mice. Collectively, these results suggest that N9-GP has the potential to offer efficacious prophylactic and acute treatment of hemophilia B patients at a reduced dosing frequency.

Abnormal hemostasis in a knock-in mouse carrying a variant of factor IX with impaired binding to collagen type IV

BACKGROUND

Factor IX binds to collagen type IV, but this binding has no known consequence.

OBJECTIVES

To determine the effect of reduced binding of FIX to collagen IV.

METHODS

We constructed and characterized 'knock-in' mice containing the mutation lysine 5 to alanine (K5A) in the Gla domain of their FIX. The K5A mutation dramatically reduced the affinity of FIX for collagen type IV, but had no measurable effect on platelet binding, phospholipid binding, or in vitro clotting activity. However, K5AFIX mice had a mild bleeding tendency, despite their in vitro clotting activity being normal. Hemostatic protection from delayed rebleeding was intermediate between wild-type and hemophilia B mice (which had no detectable clotting activity); moreover, survival of K5A FIX mice after nascent clot removal was dramatically improved as compared with hemophilia B mice. Importantly, there was no detectable difference between K5AFIX and wild-type mice in either a laser-induced thrombosis model or the chromogenic FIX activity assay. In contrast, after ferric chloride injury, which exposes collagen IV as well as other basement membrane proteins, intravital microscopy revealed that vessel occlusion was significantly slower in K5AFIX mice than in wild-type mice.

CONCLUSIONS

Our results indicate that the FIX molecule with decreased affinity for collagen IV has altered hemostatic properties in vivo and that the binding of FIX to collagen IV probably plays a significant functional role in hemostasis.

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 IX variants improve gene therapy efficacy for hemophilia B

Intramuscular injection of adeno-associated viral (AAV) vector to skeletal muscle of humans with hemophilia B is safe, but higher doses are required to achieve therapeutic factor IX (F.IX) levels. The efficacy of this approach is hampered by the retention of F.IX in muscle extracellular spaces and by the limiting capacity of muscle to synthesize fully active F.IX at high expression rates. To overcome these limitations, we constructed AAV vectors encoding F.IX variants for muscle- or liver-directed expression in hemophilia B mice. Circulating F.IX levels following intramuscular injection of AAV-F.IX-K5A/V10K, a variant with low-affinity to extracellular matrix, were 2-5 fold higher compared with wild-type (WT) F.IX, while the protein-specific activities remained similar. Expression of F.IX-R338A generated a protein with 2- or 6-fold higher specific activity than F.IX-WT following vector delivery to skeletal muscle or liver, respectively. F.IX-WT and variant forms provide effective hemostasis in vivo upon challenge by tail-clipping assay. Importantly, intramuscular injection of AAV-F.IX variants did not trigger antibody formation to F.IX in mice tolerant to F.IX-WT. These studies demonstrate that F.IX variants provide a promising strategy to improve the efficacy for a variety of gene-based therapies for hemophilia B.

Discontinuous residues of factor IX constitute a surface for binding the anti-factor IX monoclonal antibody A-5

Anti-human factor IX monoclonal antibody, A-5 (Mab A-5), has been widely used in structure-function studies for factor IX. Mab A-5 recognizes the catalytic domain of human factor IX (FIX). Regions important for Mab A-5 binding have previously been localized to the amino terminus of the heavy chain of factor IX, encompassing amino acid residues 181-310 [Blood (74) 971]. We have selected 20 positions in this region for alanine-scanning mutagenesis. We found that Mab A-5 failed to react with the recombinant factor IX mutants with substitutions at positions 228 and 252. Mab A-5 also reacted to a lesser extent to FIXD276A (factor IX with alanine substitution for aspartic acid at residue 276) and FIXK201A/D203A (double alanine substitutions at residues 201 and 203). The apparent dissociation rate constants (K(D)) in binding Mab A-5 were 6.0 x 10(-9), 1.4 x 10(-8) and 2.0 x 10(-8) M, for wild-type FIX, FIXK201A/D203A and FIXD276A, respectively. The increased K(D) values of the two FIX mutants are mainly owing to the increased dissociation rates. These affected residues constitute a surface opposite from the factor VIIIa binding surface. We conclude that the epitope for Mab A-5 is on a surface composed of residues 228, 252, 276, and 201 or 203. This surface, which may not be important for factor VIII binding, contains a strong antigenic region on factor IX.

Protein S Gla-domain mutations causing impaired Ca(2+)-induced phospholipid binding and severe functional protein S deficiency

We have identified 2 PROS1 missense mutations in the exon that encodes the vitamin K-dependent Gla domain of protein S (Gly11Asp and Thr37Met) in kindred with phenotypic protein S deficiency and thrombosis. In studies using recombinant proteins, substitution of Gly11Asp did not affect production of protein S but resulted in 15.2-fold reduced protein S activity in a factor Va inactivation assay. Substitution of Thr37Met reduced expression by 33.2% (P <.001) and activity by 3.6-fold. The Gly11Asp variant had 5.4-fold reduced affinity for anionic phospholipid vesicles (P <.0001) and decreased affinity for an antibody specific for the Ca(2+)-dependent conformation of the protein S Gla domain (HPS21). Examination of a molecular model suggested that this could be due to repositioning of Gla29. In contrast, the Thr37Met variant had only a modest 1.5-fold (P <.001), reduced affinities for phospholipid and HPS21. This mutation seems to disrupt the aromatic stack region. The proposita was a compound heterozygote with free protein S antigen levels just below the lower limit of the normal range, and this is now attributed to the partial expression defect of the Thr37Met mutation. The activity levels were strongly reduced to 15% of normal, probably reflecting the functional deficit of both protein S variants. Her son (who was heterozygous only for Thr37Met) had borderline levels of protein S antigen and activity, reflecting the partial secretion and functional defect associated with this mutation. This first characterization of natural protein S Gla-domain variants highlights the importance of the high affinity protein S-phospholipid interaction for its anticoagulant role.

Circulating and binding characteristics of wild-type factor IX and certain Gla domain mutants in vivo

Residue K5 in factor IX gamma-carboxyglutamic acid (Gla) domain participates in binding endothelial cells/collagen IV. We injected recombinant factor IX containing mutations at residue 5 (K5A, K5R) into factor IX-deficient mice and compared their behavior with that of wild-type factor IX. The plasma concentration of factor IX that binds to endothelial cells/collagen IV (recombinant wild type and K5R) was consistently lower than that of the one that does not bind (K5A). Mice treated with wild type or K5R had 79% of the injected factor IX in the liver after 2 minutes, whereas 17% remained in circulation. In mice injected with K5A, 59% of the injected factor IX was found in liver and 31% was found in plasma. When we blocked the liver circulation before factor IX injection, 74% of K5A and 64% of K5R remained in the blood. When we treated the mouse with EDTA after injecting exogenous factor IX, the blood levels of factor IX that bind to endothelial cells/collagen IV increased, presumably because of release from endothelial cell/collagen IV binding sites. In contrast, the levels of the mutants that do not bind were unaffected by EDTA. In immunohistochemical studies, factor IX appears on the endothelial surfaces of mouse arteries after factor IX injection and of human arteries from surgical specimens. Thus, we have demonstrated that factor IX binds in vivo to endothelial cell-collagen IV surfaces. Our results suggest that factor IX Gla-domain mediated binding to endothelial cells/collagen IV plays a role in controlling factor IX concentration in the blood.

Phospholipid-binding domain of factor VIII is involved in endothelial cell-mediated activation of factor X by factor IXa

Apparently quiescent, nonapoptotic endothelial cells mediate the activation of factor X by activated factor IX in the presence of its cofactor, activated factor VIII. In a previous study, we reported that during the activation of factor X, the interaction of the cofactor with the endothelial cell membrane clearly differs from the interaction of the cofactor with artificial lipid membranes. In the present study, we identified the peptide domain of factor VIII involved in the assembly of the enzyme-cofactor complex on the endothelial cell surface. With the use of monoclonal antibodies against different peptide sequences on the factor VIII light chain, it was observed that the lipid-binding region of the C2 domain on the factor VIII light chain mediates the assembly of the factor X-activating complex on the endothelial cell surface. In addition, a synthetic peptide that constitutes region Ala2318-Tyr2332 of the C2 domain and that is known for its ability to inhibit the binding of factor VIII to artificial lipid membranes also showed inhibition of the cofactor activity of factor VIII on endothelial cells. Thus, the carboxy-terminal part of the factor VIII light chain not only contains sites involved in lipid binding but also contains sites involved in complex assembly on the endothelial cell membrane.

A human antibody that inhibits factor IX/IXa function potently inhibits arterial thrombosis without increasing bleeding

10C12, a human antibody F(ab')2, which specifically binds to the gamma-carboxyglutamic acid domain of factor IX/factor IXa (F.IX/IXa), interferes with all known coagulation processes in which F.IX/IXa is involved. In a rabbit model of carotid artery injury, intravenous administration of 10C12 or heparin decreased thrombosis dose dependently. The dose that resulted in a 90% reduction of thrombus mass (ED90) was a 30-microg/kg bolus of 10C12 or a 100-U/kg bolus plus 1.0 U x kg(-1) x min(-1) infusion of heparin. Heparin, at and below the ED90, significantly prolonged coagulation times and cuticle bleeding times. In contrast, 10C12 had no effect on coagulation or bleeding times at doses up to 4 times the ED90. To further evaluate the effect of 10C12 on bleeding, it was compared with heparin in a novel model of blood loss. At the ED90 of heparin, blood loss induced by a standardized injury to the vasculature of the rabbit tibia increased to more than 2 times that of saline controls. In contrast, the dose of 10C12 required to produce a similar increase in blood loss was more than 30 times the ED90. The antithrombotic potency and relative safety of this fully human antibody suggests that it may have therapeutic value for treatment of thrombotic disorders.

The factor IXa second epidermal growth factor (EGF2) domain mediates platelet binding and assembly of the factor X activating complex

Previously we have determined that residues 88-109 (but not Arg(94)) in the second epidermal growth factor (EGF2)-like domain of factor IXa (FIXa) are important for assembly of the factor X (FX) activating complex on phospholipid vesicles (Wilkinson, F. H., London, F. S., and Walsh, P. N. (2002) J. Biol. Chem. 277, 5725-5733). Here we report that these residues are important for platelet binding affinity, stoichiometry, and assembly of the FX activating complex. We prepared several chimeric FIXa proteins using homologous sequences from factor VII (FVII): FIXa(FVIIEGF2) (FIX Delta 88-124,inverted Delta FVII91-127), FIXa(loop1) (FIX Delta 88-99,inverted Delta FVII91-102), FIXa(loop2) (FIX Delta 95-109,inverted Delta FVII98-112), and FIXa(loop3) (FIX Delta 111-124,inverted Delta FVII114-127) and tested their ability to bind to thrombin-activated platelets. Binding affinities (K(d) values in 10(-9) m) for the proteins were as follows in the presence and absence of FVIIIa, respectively: FIXa(N) (0.55 +/- 0.06, 2.9 +/- 0.45), FIXa(WT) (0.80 +/- 0.08, 3.5 +/- 0.5), FIXa(loop1) (19 +/- 4.0, 27 +/- 5.0), FIXa(loop2) (35 +/- 9.0, 65 +/- 12.0), and FIXa(loop3) (1.1 +/- 0.09, 5.0 +/- 0.90). These K(d) values are in good agreement with K((d)(app)) values (in 10(-9) m) determined from the activation of FX (in the presence and absence of FVIIIa, respectively): FIXa(N) (0.46 +/- 0.05, 1.40 +/- 0.14), FIXa(WT) (0.72 +/- 0.08, 3.8 +/- 0.08), FIXa(loop1) (3.2 +/- 0.72, 14.0 +/- 1.60), FIXa(loop2) (18.4 +/- 1.60, 26.3 +/- 3.40), and FIXa(loop3) (0.7 +/- 0.05, 3.0 +/- 0.15). Moreover, the stoichiometry of binding (sites/platelet) showed an agreement with V(max) of FX activation and was reduced in those proteins that also showed a decreased platelet binding affinity. A peptide corresponding to the FIX EGF2 domain (Leu(84)-Val(128)) was an effective inhibitor of FIXa binding to platelets in both the presence (K(i) = 0.7 x 10(-6) m) and the absence (K(i) = 1.5 x 10(-6) m) of FVIIIa and FX. We conclude that residues 88-109 of the FIXa EGF2 domain mediate binding to platelets and assembly of the FX activating complex.ut not Ar

Location of the platelet binding site in zymogen coagulation factor IX

The assembly of the tenase complex on the surface of the platelet is an essential step in maintaining normal hemostasis as evidenced by the serious hemorrhagic diathesis associated with either factor IX (FIX) or factor VIII deficiencies. Understanding the regions and or residues of FIX crucial for proper binding to platelets has important clinical implications. The ability of FIX to bind activated platelets in the presence of 4 mmol/l CaCl2 was examined using electrophoretic light-scattering experiments. Wild-type FIX binds to activated platelets with dissociation constant Kd = 7.9 nmol/l. Activated FIX binds to activated platelets with Kd = 2 nmol/l. Activated factor VII does not bind activated platelets at physiological concentrations. The Gla domain of FIX is important for the binding of FIX to activated platelets since a chimera with a factor VII (FVII) template and FIX Gla [FVII(FIXGla)] has Kd = 9.6 nmol/l, and a chimera with a FVII template and FIX Gla, A and the first epidermal growth factor domain (EGF1) [FVII(FIXGla,A,EGF1)] has Kd = 9.7 nmol/l, but a chimera with a FIX template and a FVII Gla [FIX(FVIIGla)] does not bind activated platelets. Altering the fifth residue of FIX from a lysine to an alanine (Lys5<--Ala) abolishes the mutant from binding to collagen but does not affect FIX binding to the activated platelet (Kd = 9.8 nmol/l). Point mutations involved with residues 4 and 5 (Gly4<--Phe and Lys5<--no residue), residue 9 (Phe9<--Ala), residue 10 (Val10<--Lys) and residues 9-11 (Phe9<--Met, Val10<--Lys, Glu11<--Lys) do not bind to activated platelets.

Identification of twenty-one new mutations in the factor IX gene by SSCP analysis

In this study we have analyzed the factor IX gene from 84 hemophilia B patients of Spanish origin. It included single-strand conformation polymorphism (SSCP) analysis of all functional regions of the gene and further sequencing of all fragments showing abnormal migration. In 76 patients (90.4%), it was possible to identify molecular alterations leading to the appearance of the disease. Twenty-one new mutations were identified, including 13 missense mutations, two nonsense mutations, three splice-site mutations, one frameshift deletion, one frameshift insertion, and one non-frameshift deletion. The approach appears to be very suitable for molecular diagnosis of hemophilia B.

Identification of residues in the Gla-domain of human factor IX involved in the binding to conformation specific antibodies

The binding of Ca2+ induces a conformational change in factor IX which can be monitored with conformation specific antibodies. Anti-FIX:Mg(II) antibodies recognize a conformational epitope (FIX') that can be induced by several metal ions such as Ca2+, Mg2+, Mn2+ and Ba2+, while anti-FIX:Ca(II) antibodies recognize a conformational epitope (FIX*) that can be only induced by Ca2+ and Sr2+ ions (Liebman et al., J. Biol. Chem., vol. 262 (1987) pp. 7605-7612). The latter conformation is essential for the function of factor IX. In this study we tried to identify residues in the Gla-domain of factor IX which are involved in binding to anti-factor IX:Mg(II) and anti-factor IX:Ca(II) antibodies. For this we substituted residues in recombinant human factor IX for those of factor X or factor VII. The substitution of residues 1-40 of factor IX by those of factor VII eliminated binding to both types of antibodies. Re-introduction of factor IX specific residues increased the binding to conformation specific anti-factor IX antibodies, but reduced the binding to conformation specific anti-factor VII antibodies, indicating that the structural integrity of the Gla-domain was not seriously affected by the mutations. We provide evidence that residues 33, 39 and 40 of human factor IX are important for binding to anti-factor IX:Mg(II) antibodies, while residues 1-11 are important for binding to anti-factor IX:Ca(II) antibodies.

A Coagulation Factor IX-Deficient Mouse Model for Human Hemophilia B

Coagulation factor IX deficiency causes hemophilia B in humans. We have used gene targeting to develop a coagulation factor IX-deficient (factor IX-knockout) mouse strain. Mouse embryonic stem (ES) cells were targeted by a socket-containing vector that replaces the promoter through exon 3 of the factor IX gene by neoΔHPRT, which is a functional neo gene plus a partially deleted hypoxanthine phosphoribosyl transferase minigene. Chimeric mice generated using these socket-containing ES cells transmitted the targeted factor IX gene to their female offspring. Male offspring from these females were characterized and shown to exhibit a phenotype similar to hemophilia B. This factor IX-deficient mouse strain will be useful for studying gene therapy methods and structure-function relationships of recombinant factor IX proteins in vivo.

The interaction between the endothelial cell protein C receptor and protein C is dictated by the gamma-carboxyglutamic acid domain of protein C

The endothelial cell protein C receptor (EPCR) binds to both protein C and activated protein C (APC) with similar affinity. Removal of the Gla domain of protein C results in the loss of most of the binding affinity. This observation is compatible with at least two models: 1) the Gla domain of protein C interacts with phospholipid on cell surfaces to stabilize interaction with EPCR or 2) the Gla domain of protein C makes specific protein-protein interactions with EPCR. The latter model predicts that chimeric proteins containing the protein C Gla domain should interact with EPCR. To test this, we constructed a prothrombin chimera in which the Gla domain and aromatic stack of prothrombin were replaced with the corresponding region of protein C. The 125I-labeled chimera (Kd = 176 nM) and 125I-APC (Kd = 65 nM) both bound specifically to 293 cells stably transfected with EPCR, but both bound poorly to sham-transfected cells. The chimera also blocked APC binding to EPCR-transfected cells in a dose-dependent fashion (Ki approximately 139 nM) similarly to protein C (Ki approximately 75 nM). Chimera binding to EPCR-transfected cells was blocked by soluble EPCR, demonstrating direct protein-protein interaction between the chimera and EPCR. Consistent with this conclusion, the isolated Gla domain of protein C blocked APC binding to EPCR-transfected cells (IC50 = 2 microM). No inhibition was observed with the isolated prothrombin Gla domain. A protein C chimera with the prothrombin Gla domain and aromatic stack failed to bind to EPCR detectably. These data suggest that the Gla domain of protein C is responsible for much of the binding energy and specificity of the protein C-EPCR interaction.

Human factor IX binds to specific sites on the collagenous domain of collagen IV

The primary region of factor IX that mediates binding to bovine aortic endothelial cells resides in residues 3-11 of the N-terminal region known as the Gla domain. Recently, it was proposed that the observed binding to endothelial cells is actually a measure of the interaction between factor IX and collagen IV (Cheung, W. F., van den Born, J., Kuhn, K., Kjellen, L., Hudson, B. G., and Stafford, D. W. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 11068-11073). To confirm that factor IX binds to collagen IV and to examine the specificity of this interaction, we used scanning force microscopy to examine factor IX binding to collagen IV. We imaged collagen IV in the presence and the absence of factor IX and observed specific interactions between factor IX and collagen IV. Our results demonstrate that factor IX binds to collagen IV at specific sites in the collagenous domain approximately 98 and approximately 50 nm from the C-terminal pepsin-cleaved end.

Stable gene transfer and expression of human blood coagulation factor IX after intramuscular injection of recombinant adeno-associated virus

We sought to determine whether intramuscular injection of a recombinant adeno-associated virus (rAAV) vector expressing human factor IX (hF.IX) could direct expression of therapeutic levels of the transgene in experimental animals. High titer (10(12)-10(13) vector genomes/ml) rAAV expressing hF.IX was prepared, purified, and injected into hindlimb muscles of C57BL/6 mice and Rag 1 mice. In the immunocompetent C57BL/6 mice, immunofluorescence staining of muscle harvested 3 months after injection demonstrated the presence of hF.IX protein, and PCR analysis of muscle DNA was positive for AAV DNA, but no hF.IX was detected in mouse plasma. Further studies showed that these mice had developed circulating antibodies to hF.IX. In follow-up experiments in Rag 1 mice, which carry a mutation in the recombinase activating gene-1 and thus lack functional B and T cells, similar results were seen on DNA analysis of muscle, but these mice also demonstrated therapeutic levels (200-350 ng/ml) of F. IX in the plasma. The time course of F.IX expression demonstrates that levels gradually increase over a period of several weeks before reaching a plateau that is stable 6 months after injection. In other experiments we demonstrate colocalization of hF.IX and collagen IV in intersitial spaces between muscle fibers. Collagen IV has recently been identified as a F.IX-binding protein; this finding explains the unusual pattern of immunofluorescent staining for F.IX shown in these experiments. Thus rAAV can be used to direct stable expression of therapeutic levels of F.IX after intramuscular injection and is a feasible strategy for treatment of patients with hemophilia B.

Refinement of the NMR solution structure of the gamma-carboxyglutamic acid domain of coagulation factor IX using molecular dynamics simulation with initial Ca2+ positions determined by a genetic algorithm

A genetic algorithm (GA) successfully identified the calcium positions in the crystal structure of bovine prothrombin fragment 1 bound with calcium ions (bf1/Ca). The same protocol was then used to determine the calcium positions in a closely related fragment, the Gla domain of coagulation factor IX, the structure of which had previously been determined by NMR spectroscopy in the presence of calcium ions. The most frequently occurring low-energy structure found by GA was used as the starting structure for a molecular dynamics refinement. The molecular dynamics simulation was performed using explicit water and the Particle-Mesh Ewald method to accommodate the long-range electrostatic forces. While the overall conformation of the NMR structure was preserved, significant refinement is apparent when comparing the simulation average structure with its NMR precursor in terms of the N-terminal (Tyr1-N) network, the total number of hydrogen bonds, the calcium ion coordinations, and the compactness of the structure. It is likely that the placement of calcium ions in the protein is critical for refinement. The calcium ions apparently induce structural changes during the course of the simulation that result in a more compact structure.

gamma-Carboxyglutamic acids 36 and 40 do not contribute to human factor IX function

The gamma-carboxyglutamic acid (Gla) domains of the vitamin K-dependent blood coagulation proteins contain 10 highly conserved Gla residues within the first 33 residues, but factor IX is unique in possessing 2 additional Gla residues at positions 36 and 40. To determine their importance, factor IX species lacking these Gla residues were isolated from heterologously expressed human factor IX. Using ion-exchange chromatography, peptide mapping, mass spectrometry, and N-terminal sequencing, we have purified and identified two partially carboxylated recombinant factor IX species; factor IX/gamma 40E is uncarboxylated at residue 40 and factor IX/gamma 36,40E is uncarboxylated at both residues 36 and 40. These species were compared with the fully gamma-carboxylated recombinant factor IX, unfractionated recombinant factor IX, and plasma-derived factor IX. As monitored by anti-factor IX:Ca (II)-specific antibodies and by the quenching of intrinsic fluorescence, all these factor IX species underwent the Ca(II)-induced conformational transition required for phospholipid membrane binding and bound equivalently to phospholipid vesicles composed of phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine. Endothelial cell binding was also similar in all species, with half-maximal inhibition of the binding of 125I-labeled plasma-derived factor IX at concentrations of 2-6 nM. Functionally, factor IX/gamma 36,40E and factor IX/gamma 40E were similar to fully gamma-carboxylated recombinant factor IX and plasma-derived factor IX in their coagulant activity and in their ability to participate in the activation of factor X in the tenase complex both with synthetic phospholipid vesicles and activated platelets. However, Gla 36 and Gla 40 represent part of the epitope targeted by anti-factor IX:Mg(II)-specific antibodies because these antibodies bound factor IX preferentially to factor IX/gamma 36,40E and factor IX/gamma 40E. These results demonstrate that the gamma-carboxylation of glutamic acid residues 36 and 40 in human factor IX is not required for any function of factor IX examined.

The entire gamma-carboxyglutamic acid- and helical stack-domains of human coagulation factor IX are required for optimal binding to its endothelial cell receptor

The minimal region of the gamma-carboxyglutamic acid (Gla) domain of human factor (f) IX that interacted with its putative bovine aortic endothelial cell (BAEC) receptor was examined by chemical synthesis of peptides with sequence counterparts in this region of the protein, and assessment of their relative abilities to compete with fIX for receptor binding. We found that IC50 values (total peptide concentrations needed to achieve 50% inhibition of binding of [125I]-fIX to BAEC) were ca. 18 nM for unlabeled fIX and 23 nM for the peptide consisting of the entire Gla domain/helical stack (HS) region (residues 1-47) of fIX. The peptide containing only the Gla domain of fIX (residues 1-38) displayed an IC50 value of > 500 nM for this same competitive binding, whereas peptides containing sequences present in positions 1-14 and 1-24 of the Gla domain of human fIX did not significantly compete with [125I]-fIX for BAEC binding. We conclude that whereas a specific receptor recognition element is present within residues 1-14 of fIX, as has previously been concluded by others and by us, full expression of this epitope requires its presence within the entire Gla domain and HS for proper folding. All determinants for proper folding of fIX that lead to BAEC receptor binding appear to be present within these two domains.

The molecular basis of haemophilia A and B

Most families with haemophilia A or B carry gene defects of independent origin. Haemophilia B is mostly due to small changes in the factor IX gene affecting either its transcription, mRNA maturation, mRNA translation or the fine structure of factor IX. Only 2-3% of patients show gross deletions or rearrangements. The great variety of missense mutations reported to cause haemophilia B indicates that this multidomain protein is highly constrained. Less is known about the factor VIII gene as fully efficient mutation detection procedures only became available in 1991. This, however, led to the discovery that almost half the severe cases of haemophilia A or a fifth of all cases are due to frequently occurring inversions caused by homologous intra-chromosome (-chromatid) recombination between repeated sequences 9.5 kb long. Of the three repeats one is in intron 22 of the factor VIII gene and two are 400-500 kb more telomeric. They are 99.8% similar to each other. The spectrum of the other haemophilia A mutations is similar to that of haemophilia B. Since 1983 mounting evidence has shown that in both haemophilias the nature of the mutation is important in predisposing to the inhibitor complication.

Structural integrity of the gamma-carboxyglutamic acid domain of human blood coagulation factor IXa Is required for its binding to cofactor VIIIa

This report describes the analysis of a novel mutant human factor IX protein from a patient with hemophilia B (factor IX activity <1%; factor IX antigen 45%). Enzymatic amplification of all eight exons of the factor IX gene followed by direct sequence analysis reveals a single nucleotide change (a guanine --> adenine transition) in exon 2 at nucleotide 6409 which results in a glycine --> arginine substitution at amino acid 12 in the gamma-carboxyglutamic acid rich (Gla) domain of the mature protein. Factor IX was isolated by immunoaffinity chromatography from plasma obtained from the proband. The purified protein is indistinguishable from normal factor IX by polyacrylamide gel electrophoresis. Characterization of the variant in purified component assays reveals that it is activated normally by its physiologic activator factor XIa, but its phospholipid-dependent activation by the factor VIIa-tissue factor complex is diminished. In the presence of phospholipid and 5 mM Ca2+, the activities of variant and normal plasma-derived factor IX are similar; however, in the presence of activated factor VIIIa (intrinsic tenase complex), the normal augmentation of the cleavage of the specific substrate of factor IX, factor X, is not observed. The determination of the association constants for normal and variant factor IXa with factor VIIIa shows that the affinity of the activated variant factor IX for the cofactor factor VIIIa is 172-fold lower than normal. Competition studies using active site-inactivated factor IXas in the intrinsic tenase complex confirm that the defect in the variant protein is in its binding to factor VIIIa. We conclude that the structural integrity of the Gla domain of human factor IX is critical for the normal binding of factor IXa to factor VIIIa in the intrinsic tenase complex. In addition, a glycine at amino acid 12 is necessary for normal activation of factor IX by the factor VIIa-tissue factor complex.

Localization of a calcium-dependent epitope to the amino terminal region of the Gla domain of human factor IX

We have used site-directed mutagenesis to define the epitope of calcium-dependent monoclonal antibodies to human factor IX. We demonstrate that the calcium-specific epitope includes residues 1-11 of factor IX, with apparent contributions from other regions of the protein. Antibodies JK.IX-1, -3, and -4 had critical portions of their epitopes in the first eleven amino acids of the Gla domain. These three antibodies could bind to a chimera containing the factor VII Gla domain, in which surface residues 3, 4, 5, 9, 10, and 11 were modified to those of factor IX. In contrast, the epitope of JK.IX-2 was unaffected by mutations in residues 3-11 of factor IX, but was dependent on the amino terminal tyrosine residue. Furthermore, the calcium-dependent monoclonal antibodies, JK.IX-1, -3, and -4, whose epitope include residues 3 through 11. inhibit factor IX's binding to endothelial cells, for which the binding site on factor IX has been localized to this region. Our results, together with previous studies, confirm the existence of discrete calcium and metal-dependent epitopes within the Gla domain of factor IX and show that the calcium-specific epitope lies near the amino terminus region of this domain.

Localization of a metal-dependent epitope to the amino terminal residues 33-40 of human factor IX

Metal binding sites within the Gla domain of vitamin K-dependent coagulation factors have been divided into nonspecific metal sites and calcium-specific sites. We demonstrate here that five residues within the Gla domain of factor IX are responsible for the reactivity with the metal-dependent factor IX monoclonal antibody, A-7. First we demonstrate that modifying any one of three residues within this site in factor IX abolishes the binding of A-7. To confirm the specificity of the antibody, the Gla domain of factor VII was changed at residues 32, 33, 34, 38 and 39 to the homologous residues of human factor IX. These changes were sufficient to generate a factor VII Gla domain with an A-7 binding site of the same affinity as that in factor IX. The site identified is one of the two major surfaces of the Gla domain and may represent the metal-dependent binding site.

Homology modeling and molecular dynamics simulation of human prothrombin fragment 1

The crystallographic structure of bovine prothrombin fragment 1 bound with calcium ions was used to construct the corresponding human prothrombin structure (hf1/Ca). The model structure was refined by molecular dynamics to estimate the average solution structure. Accommodation of long-range ionic forces was essential to reach a stable solution structure. The gamma-carboxyglutamic acid (Gla) domain and the kringle domain of hf1/Ca independently equilibrated. Likewise, the hydrogen bond network and the calcium ion coordinations were well preserved. A discussion of the phospholipid binding of the vitamin K-dependent coagulation proteins in the context of the structure and mutational data of the Gla domain is presented.

Localization of an epitope of a calcium-dependent monoclonal antibody to the N-terminal region of the Gla domain of human factor VII

The epitope of a calcium-dependent murine monoclonal antibody specific for human factor VII (Thim et.al., Biochemistry (1988) 27, 7785-7793) has been determined. Site-directed mutagenesis of residues 3 through 10 of factor VII eliminates the binding of this antibody but does not disturb the binding of a second antibody which binds nearby.

Transfer of specific endothelial cell-binding properties from the procoagulant protein human factor IX into the anticoagulant protein human protein C

A series of recombinant (r) chimeric mutants of human coagulation protein C (PC) and activated protein C (APC) containing replacements of homologous PC domains by those of human coagulation factor IX (fIX) were generated, with the intention of determining whether the specific bovine aortic endothelial cell (BAEC) receptor-binding characteristics of fIX could be incorporated into the chimeric r-PC while maintaining the essential properties of PC and APC. Using a competitive BAEC displacement assay with [125I]fIX, we found that a chimeric r-PC (r[delta PC1-46/delta fIX1-47]PC), consisting of the entire gamma-carboxyglutamic domain ([GDIX], residues 1-38) and helical stack ([HSIX], residues 38-47) of fIX as replacements for these same domains of PC, provided an IC50 for fIX-related BAEC binding of 13 nM, as compared to 10 nM for that of unlabeled fIX. This showed that all of the BAEC tight binding determinants for fIX existed within the [GDIX/HSIX]. Additionally, this chimera reacted to the same extent as fIX with the Ca(2+)-dependent, [GDIX]-specific monoclonal antibody H5B7 and lost its reactivity to a similar antibody specific for the [GDPC], JTC1. A synthetic peptide containing residues 1-47 of fIX also competed effectively (IC50 = 16 nM) with intact fIX for BAEC binding. Displacement of [125I]fIX from BAEC did not occur with a chimera containing the [HSIX] alone or with another mutant protein possessing a replacement of the two epidermal growth factor (EGF) homology regions of r-PC (residues 47-137) with those same domains of fIX.(ABSTRACT TRUNCATED AT 250 WORDS)

Cleavage at arginine 145 in human blood coagulation factor IX converts the zymogen into a factor VIII binding enzyme

The transition of the factor IX zymogen into the enzyme factor IXa beta was investigated. For this purpose, the activation intermediate factors IX alpha and IXa alpha were purified after cleavage of the Arg145-Ala146 and Arg180-Val181 bonds, respectively. These intermediates were compared for a number of functional properties with factor IXa beta, which is cleaved at both positions. Factor IXa alpha was equal to factor IXa beta in hydrolyzing the synthetic substrate CH3SO2-Leu-Gly-Arg-p-nitroanilide (kcat/Km approximately 120 s-1 M-1) but was less efficient in factor X activation. Factor IX alpha was incapable of generating factor Xa but displayed reactivity toward p-nitrophenol p-guanidinobenzoate and the peptide substrate. The catalytic efficiency, however, was 4-fold lower compared with factor IXa alpha and factor IXa beta. Factor IX alpha and factor IXa beta had similar affinity for the inhibitor benzamidine (Ki approximately 2.5 mM), and amidolytic activity of both species was inhibited by Glu-Gly-Arg-chloromethyl ketone and antithrombin III. Unlike factor IXa beta, factor IX alpha was unable to form SDS stable complexes with antithrombin III. Moreover, inhibition of factor IXa beta and factor IX alpha by Glu-Gly-Arg-chloromethyl ketone followed distinct pathways, because factor IX alpha was inhibited in a nonirreversible manner and displayed only minor incorporation of the dansylated inhibitor into its catalytic site. These data demonstrate that the catalytic site of factor IX alpha differs from that of the fully activated factor IXa beta. Factor IX and its derivatives were also compared with regard to complex assembly with factor VIII in direct binding studies employing the immobilized factor VIII light chain. Factor IX alpha and factor IXa beta displayed a 30-fold higher affinity for the factor VIII light chain (Kd approximately 12 nM) than the factor IX zymogen. Factor IXa alpha showed lower affinity (Kd approximately 50 nM) than factor IX alpha and factor IXa beta, which may explain the lower efficiency of factor X activation by factor IXa alpha. Collectively, our data indicate that cleavage of the Arg180-Val181 bond develops full amidolytic activity but results in suboptimal binding to the factor VIII light chain. With regard to cleavage of the Arg145-Ala146 bond, we have demonstrated that this results in the transition of the factor IX zymogen into an enzyme that lacks proteolytic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

High-affinity, specific factor IXa binding to platelets is mediated in part by residues 3-11

To identify the amino acids in the Gla domain that mediate factor IXa binding to human platelets, we have used chimeric molecules and point mutations in the Gla domain of recombinant factor IX, based on molecular modeling using the coordinates of the Gla domain of bovine prothrombin, which reveals two surface structures whose sequences differ among factor IX, factor X, and factor VII. Binding to thrombin-activated platelets of factor IXa in the presence of factor VIIIa (2 units/mL) and factor X (1.5 microM) revealed a stoichiometry of approximately 550 sites per platelet with a Kd of approximately 0.65 nM compared with a Kd of approximately 2.5 nM in the absence of factor VIIIa and factor X. In contrast, mutations of factor IX to factor X residues at positions 4 and 5 or at positions 9, 10, and 11 results in decreases in the number of sites and affinity of factor IXa binding in the presence or absence of factor VIIIa and factor X. A chimera consisting of the Gla domain of factor VII with factor IX residues at positions 33, 34, 35, 39, and 40 displayed abnormal factor IXa binding and a decreased Vmax and a normal Km for factor X activation, and the replacement of amino acid residues 3-10 with those of factor IX restored normal binding and factor X activation kinetics to this chimeric protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of recombinant chimeric human protein C and activated protein C containing the gamma-carboxyglutamic acid and trailing helical stack domains of protein C replaced by those of human coagulation factor IX

The properties of a recombinant (r) chimeric human protein C (PC) containing replacement of its gamma-carboxyglutamic acid (Gla) and helical stack (HS) domains by those of human coagulation factor IX (fIX) have been examined. Titration with Ca2+ of the divalent cation-induced intrinsic fluorescence quenching of this chimera (r-GDIX/PC) allowed determination of the [Ca2+], of 1.8 mM, required to produce this alteration in 50% of the protein molecules. These values were 0.41 and 0.61 mM for wtr-PC and fIX, respectively. The chimera did not react with a Ca(2+)-dependent, Gla domain-directed conformational monoclonal antibody (MAb) to r-PC but did interact with a similar MAb (H5B7) to fIX. The [Ca2+] required to induced H5B7 binding to 50% of the r-GDIX/PC molecules was 6.6 mM, while this same value for fIX was a nearly identical 7.2 mM. The [Ca2+] needed for binding of 50% of r-GDIX/PC to acidic phospholipid (PL) vesicles was 0.58 mM, while that for wtr-PC and fIX were 1.2 and 0.55 mM, respectively. The [protein] required for 50% binding of r-GDIX/PC to PL at 20 mM Ca2+ was 0.29 microM. These same values for r-PC and fIX were 0.38 and 1.8 microM, respectively. The Ca(2+)-mediated inhibition of the thrombin-catalyzed activation of r-GDIX/PC was characterized by a Ki of 118 microM, a value similar to that of 125 microM obtained for this same inhibition of wtr-PC activation. The thrombin-catalyzed activation of both r-GDIX/PC and wtr-PC was stimulated by soluble r-thrombomodulin. Similar to the case of wtr-PC, Ca2+ initially enhanced and, at higher concentrations, inhibited the activation of r-GDIX/PC. The Km and kcat values for this latter activation at optimal [Ca2+] (100 microM) were 4.1 microM and 2.5 s-1, respectively. These same kinetic constants for activation of wtr-PC were 4.3 microM and 2.9 s-1, respectively. These results show that many of the features needed for functional integrity of the Ca2+-bound Gla/HS domains of PC are also present in those same modules of fIX, a finding that points to a generalized functional role for the Ca2+-induced conformation of the structural unit consisting of the Gla and HS domains. The data also suggest that the Ca2+-bound form of the Gla/HS region is an independently folded unit in PC and perhaps in fIX.(ABSTRACT TRUNCATED AT 400 WORDS)

The binding of natural variants of human factor IX to endothelial cells

The Gla-domain of human factor IX contains a specific element required for the binding of factor IX to an endothelial cell surface protein. We have investigated the dependence of this interaction on the structural integrity of the adjacent hydrophobic stack and epidermal growth factor-like domains. The ability of purified natural variants of human factor IX to compete with wild-type factor IX binding to the endothelial cell surface was used to obtain apparent Ki values of the variants. Our data suggest that the functional integrity of the Gla domain, enabling factor IX to specifically interact with an endothelial cell surface protein, depends on the structural and functional integrity of both the hydrophobic stack domain and the first epidermal growth factor-like domain.