Changing residue 338 in human factor IX from arginine to alanine causes an increase in catalytic activity

The heparin-binding exosite of factor IXa is a critical regulator of plasma thrombin generation and venous thrombosis

The role of the factor IXa heparin-binding exosite in coagulation was assessed with mutations that enhance (R170A) or reduce (R233A) stability of the protease-factor VIIIa A2 domain interaction. After tissue factor (TF) addition to reconstituted factor IX-deficient plasma, factor IX R170A supported a 2-fold increase in velocity index (slope) and peak thrombin concentration, whereas factor IX R233A had a 4- to 10-fold reduction relative to factor IX wild-type. In the absence of TF, 5 to 100 pM of factor IXa increased thrombin generation to approach TF-stimulated thrombin generation at 100% factor IX. Factor IXa R170A demonstrated a 2- to 3-fold increase in peak thrombin concentration and 5-fold increase in velocity index, whereas the response for factor IXa R233A was blunted and delayed relative to wild-type protease. In hemophilia B mice, factor IX replacement reduced the average time to hemostasis after saphenous vein incision, and the time to occlusion after FeCl(3)-induced saphenous vein injury. At 5% factor IX, the times to occlusion for factor IX wild-type, R170A, and R233A were 15.7 minutes, 9.1 minutes (P </= .003), and more than 45 minutes. These data support the role of the factor IXa heparin-binding exosite as a critical regulator of coagulation and novel antithrombotic target.

Expression, purification and characterization of factor IX derivatives using a novel vector system

Recent studies have indicated that the loop harboring the S1 specificity site (residues 185-189 in chymotrypsin numbering) of coagulation proteases has several charged residues with important structural and functional roles for the catalytic activity of these proteases. This loop is allosterically linked to the Na(+)-binding site in both factor Xa and thrombin. There are three candidate residues (His-185, Glu-186, and Arg-188) on this loop of factor IXa (fIXa) whose side chains can influence the Na(+) binding and the catalytic function of the protease in the intrinsic Xase complex. In this study, we developed a novel expression/purification vector system, substituted all three residues of factor IX individually with Ala, and expressed the mutant zymogens in mammalian cells. Following activation, all three fIXa mutants exhibited normal activity towards a fIXa-specific chromogenic substrate in the presence of Ca(2+) with no obvious requirement for Na(+) in the reaction. Furthermore, all three mutants interacted with factor VIIIa with near normal affinity and catalyzed the activation of factor X in the intrinsic Xase complex with a normal catalytic efficiency. These results suggest that, unlike thrombin and factor Xa, the charged residues of this loop do not play a functional role in modulating the catalytic function of fIXa in the intrinsic Xase complex.

Transgenic mice can express mutant human coagulation factor IX with higher level of clotting activity

To improve the available values of transgenic animals, we produced a mutant human coagulation factor IX minigene (including cDNA and intron I) with arginine at 338 changed to alanine (R338A-hFIX) by using a direct mutation technique. The R338A-hFIX minigene was then cloned into a plasmid carrying the goat beta-casein promoter to get a mammary gland-specific expression vector. The clotting activity in the supernatant of the transfected HC-11 cells increased to approximately three times more than that of wild-type hFIX. Nine transgenic mice (three females and six males) were produced, and the copy number of the foreign gene was very different, ranging from 1 to 43 in different lines. ELISA, Western blot, and clotting assay experiments showed that the transgenic mice could express R338A-hFIX, showing higher average levels of clotting activity than wild-type hFIX in the milk (103.76% vs. 49.95%). The highest concentration and clotting activity of hFIX reached 26 mug/mL and 1287% in one founder (F(0)-7), which was over 10 times higher than that in human plasma. Furthermore, RT-PCR, APTT assay, and histological analysis indicated that hFIX was expressed specifically in the mammary gland without affecting the intrinsic coagulation pathway and physiologic performance of the local tissue.

Depolymerized holothurian glycosaminoglycan and heparin inhibit the intrinsic tenase complex by a common antithrombin-independent mechanism

Depolymerized holothurian glycosaminoglycan (DHG) is a fucosylated chrondroitin sulfate that possesses antithrombin-independent antithrombotic properties and inhibits factor X activation by the intrinsic tenase complex (factor IXa-factor VIIIa). The mechanism and molecular target for intrinsic tenase inhibition were determined and compared with inhibition by low-molecular-weight heparin (LMWH). DHG inhibited factor X activation in a noncompetitive manner (reduced V(max(app))), with 50-fold higher apparent affinity than LMWH. DHG did not affect factor VIIIa half-life or chromogenic substrate cleavage by factor IXa-phospholipid but reduced the affinity of factor IXa for factor VIIIa. DHG competed factor IXa binding to immobilized LMWH with an EC(50) 35-fold lower than soluble LWMH. Analysis of intrinsic tenase inhibition, employing factor IXa with mutations in the heparin-binding exosite, demonstrated that relative affinity (K(i)) for DHG was as follows: wild type > K241A > H92A > R170A > > R233A, with partial rather than complete inhibition of the mutants. This rank order for DHG potency correlated with the effect of these mutations on factor IXa-LMWH affinity and the potency of LMWH for intrinsic tenase. DHG also accelerated decay of the intact intrinsic tenase complex. Thus, DHG binds to an exosite on factor IXa that overlaps with the binding sites for LMWH and factor VIIIa, disrupting critical factor IXa-factor VIIIa interactions.

The promise and challenges of bioengineered recombinant clotting factors

The past 10 years of clinical experience have demonstrated the safety and efficacy of recombinant clotting factors. With the adoption of prophylactic strategies, there has been considerable progress in avoiding the complications of hemophilia. Now, insights from our understanding of clotting factor structure and function, mechanisms of hemophilia and inhibitors, gene therapy advances and a worldwide demand for clotting factor concentrates leave us on the brink of embracing targeted bioengineering strategies to further improve hemophilia therapeutics. The ability to bioengineer recombinant clotting factors with improved function holds promise to overcome some of the limitations in current treatment, the high costs of therapy and increase availability to a broader world hemophilia population. Most research has been directed at overcoming the inherent limitations of rFVIII expression and the inhibitor response. This includes techniques to improve rFVIII biosynthesis and secretion, functional activity, half-life and antigenicity/immunogenicity. Some of these proteins have already reached commercialization and have been utilized in gene therapy strategies, while others are being evaluated in pre-clinical studies. These novel proteins partnered with advances in gene transfer vector design and delivery may ultimately achieve persistent expression of FVIII leading to an effective long-term treatment strategy for hemophilia A. In addition, these novel FVIII proteins could be partnered with new advances in alternative recombinant protein production in transgenic animals yielding an affordable, more abundant supply of rFVIII. Novel rFIX proteins are being considered for gene therapy strategies whereas novel rVIIa proteins are being evaluated to improve the potency and extend their plasma half-life. This review will summarize the status of current recombinant clotting factors and the development and challenges of recombinant clotting factors bioengineered for improved function.

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.

Effects of genetic fusion of factor IX to albumin on in vivo clearance in mice and rabbits

Individuals with haemophilia B require replacement therapy with recombinant or plasma-derived coagulation factor IX (fIX). More benefit per injected dose might be obtained if fIX clearance could be slowed. The contribution of overall size to fIX clearance was explored, using genetic fusion to albumin. Recombinant murine fIX (MIX), and three proteins with C-terminal epitope tags were expressed in HEK 293 cells: tagged MIX (MIXT), tagged mouse serum albumin (MSAT) and MFUST, in which MIX and MSAT were fused in a single polypeptide chain. Proteins MFUST and MIXT were two- to threefold less active in clotting assays than MIX. In mice, the area under the clearance curve (AUC) was reduced for MFUST compared with MSAT or plasma-derived MSA (pd-MSA); the terminal catabolic half-life (t(0.5)) did not differ amongst the three proteins. Two minutes after injection, >40% of the injected MFUST was found in the liver, compared with <10% of either MSAT or pd-MSA. In rabbits, the AUC for MFUST was reduced compared to MIXT, MSAT, or pd-MSA, while the t(0.5) of the fusion protein fell between that of MIXT and MSAT or pd-MSA. Similar results were obtained with non-radioactive fused or non-fused recombinant human fIX in fIX knockout mice. The clearance behaviour of the fusion protein thus more closely resembled that of fIX than that of albumin despite a modest increase in terminal half-life, suggesting that fIX-specific interactions that are important in determining clearance were maintained in spite of the increased size of the fusion protein.

An antibody specific for coagulation factor IX enhances the activity of the intrinsic factor X-activating complex

During hemostasis the zymogen factor X (FX) is converted into its enzymatically active form factor Xa by the intrinsic FX-activating complex. This complex consists of the protease factor IXa (FIXa) that assembles, together with its cofactor, factor VIIIa, on a phospholipid surface. We have studied the functional properties of a FIXa-specific monoclonal antibody, 224AE3, which has the potential to enhance intrinsic FX activation. Binding of the antibody to FIXa improved the catalytic properties of the intrinsic FX-activating complex in two ways: (i) factor VIIIa bound to the FIXa-antibody complex with a more than 18-fold higher affinity than to FIXa, and (ii) the turnover number (kcat) of the enzyme complex increased 2- to 3-fold whereas the Km for FX remained unaffected. The ability of 224AE3 to increase the FXa-generation potential (called the "booster effect") was confirmed in factor VIII (FVIII)-depleted plasma, which was supplemented with different amounts of recombinant FVIII. In the presence of antibody 224AE3 the coagulant activity was increased 2-fold at physiological FVIII concentration and up to 15-fold at low FVIII concentrations. The booster effect that we describe demonstrates the ability of antibodies to function as an additional cofactor in an enzymatic reaction and might open up a new principle for improving the treatment of hemophilia.

Reconstructing the binding site of factor Xa in trypsin reveals ligand-induced structural plasticity

In order to investigate issues of selectivity and specificity in protein-ligand interactions, we have undertaken the reconstruction of the binding pocket of human factor Xa in the structurally related rat trypsin by site-directed mutagenesis. Three sequential regions (the "99"-, the "175"- and the "190"- loops) were selected as representing the major structural differences between the ligand binding sites of the two enzymes. Wild-type rat trypsin and variants X99rT and X(99/175/190)rT were expressed in yeast, and analysed for their interaction with factor Xa and trypsin inhibitors. For most of the inhibitors studied, progressive loop replacement at the trypsin surface resulted in inhibitory profiles akin to factor Xa. Crystals of the variants were obtained in the presence of benzamidine (3), and could be soaked with the highly specific factor Xa inhibitor (1). Binding of the latter to X99rT results in a series of structural adaptations to the ligand, including the establishment of an "aromatic box" characteristic of factor Xa. In X(99/175/190)rT, introduction of the 175-loop results in a surprising re-orientation of the "intermediate helix", otherwise common to trypsin and factor Xa. The re-orientation is accompanied by an isomerisation of the Cys168-Cys182 disulphide bond, and burial of the critical Phe174 side-chain. In the presence of (1), a major re-organisation of the binding site takes place to yield a geometry identical to that of factor Xa. In all, binding of (1) to trypsin and its variants results in significant structural rearrangements, inducing a binding surface strongly reminiscent of factor Xa, against which the inhibitor was optimised. The structural data reveal a plasticity of the intermediate helix, which has been implicated in the functional cofactor dependency of many trypsin-like serine proteinases. This approach of grafting loops onto scaffolds of known related structures may serve to bridge the gap between structural genomics and drug design.

Hypersulfated low molecular weight heparin with reduced affinity for antithrombin acts as an anticoagulant by inhibiting intrinsic tenase and prothrombinase

In buffer systems, heparin and low molecular weight heparin (LMWH) directly inhibit the intrinsic factor X-activating complex (intrinsic tenase) but have no effect on the prothrombin-activating complex (prothrombinase). Although chemical modification of LMWH, to lower its affinity for antithrombin (LA-LMWH) has no effect on its ability to inhibit intrinsic tenase, N-desulfation of LMWH reduces its activity 12-fold. To further explore the role of sulfation, hypersulfated LA-LMWH was synthesized (sLA-LMWH). sLA-LMWH is not only a 32-fold more potent inhibitor of intrinsic tenase than LA-LMWH; it also acquires prothrombinase inhibitory activity. A direct correlation between the extent of sulfation of LA-LMWH and its inhibitory activity against intrinsic tenase and prothrombinase is observed. In plasma-based assays of tenase and prothrombinase, sLA-LMWH produces similar prolongation of clotting times in plasma depleted of antithrombin and/or heparin cofactor II as it does in control plasma. In contrast, heparin has no effect in antithrombin-depleted plasma. When the effect of sLA-LMWH on various components of tenase and prothrombinase was examined, its inhibitory activity was found to be cofactor-dependent (factors Va and VIIIa) and phospholipid-independent. These studies reveal that sLA-LMWH acts as a potent antithrombin-independent inhibitor of coagulation by attenuating intrinsic tenase and prothrombinase.

Thrombin reduces cerebral arterial contractions caused by cerebrospinal fluid from patients with subarachnoid hemorrhage

BACKGROUND AND PURPOSE

We observed that the second application of cerebrospinal fluid (CSF) from subarachnoid hemorrhage (SAH) patients onto cerebral arterial segments in vitro produces a greater contraction than does the initial application. It was hypothesized that the difference between the first and second applications of SAH CSF was due to the activity of thrombin.

METHODS

Canine vertebrobasilar artery was removed under general anesthesia, cut into rings, and suspended in tissue culture baths so as to measure isometric tension. CSF was taken from patients 1 to 3 days after SAH via ventricular drains. CSF was administered in 10(-5) to 10(-1) dilutions. The thrombin antagonist hirudin (5 U) was administered before CSF in some experiments. The arterial tension response to pure oxyhemoglobin (10(-4) to 3.2 g/dL) and thrombin (10(-4) to 3.2 U/mL), administered alone or in combination, was also examined.

RESULTS

Hirudin increased arterial tension generated on the initial application of SAH CSF but had no effect on the tension generated by the second application of the SAH CSF, suggesting that thrombin limits the tension generated by vasoconstrictive agents in the CSF. Thrombin and pure oxyhemoglobin administered together produced less tension than that generated in response to oxyhemoglobin administered alone; no additive response was observed by coadministering the 2 vasoconstrictive agents.

CONCLUSIONS

In the presence of oxyhemoglobin, thrombin acts to reduce cerebral arterial tension. This interaction between thrombin and hemoglobin may account for the observation that the second application of CSF from SAH patients onto cerebral arterial segments in vitro produces a greater contraction than does the initial application.

Surface loop 199-204 in blood coagulation factor IX is a cofactor-dependent site involved in macromolecular substrate interaction

In factor IX residues 199-204 encompass one of six surface loops bordering its substrate-binding groove. To investigate the contribution of this loop to human factor IX function, a series of chimeric factor IX variants was constructed, in which residues 199-204 were replaced by the corresponding sequence of factor VII, factor X, or prothrombin. The immunopurified and activated chimeras were indistinguishable from normal factor IXa in hydrolyzing a small synthetic substrate, indicating that this region is not involved in the interaction with substrate residues on the N-terminal side of the scissile bond. In contrast, replacement of loop 199-204 resulted in a 5-25-fold reduction in reactivity toward the macromolecular substrate factor X. This reduction was due to a combination of increased K(m) and reduced k(cat). In the presence of factor VIIIa the impaired reactivity toward factor X was largely restored for all factor IXa variants, resulting in a more pronounced stimulation by factor VIIIa compared with normal factor IXa (3 to 5 x 10(4)-fold versus 5 x 10(3)-fold). Inhibition by antithrombin was only slightly affected for the factor IXa variant with the prothrombin loop sequence, whereas factor IXa variants containing the analogous residues of factor VII or factor X were virtually insensitive to antithrombin inhibition. In the presence of heparin, however, all chimeric factor IXa variants formed complexes with antithrombin. Thus the cofactors heparin and factor VIIIa have in common that they both alleviate the deleterious effects of mutations in the factor IX loop 199-204. Collectively, our data demonstrate that loop 199-204 plays an important role in the interaction of factor IXa with macromolecular substrates.

Protease and EGF1 domains of factor IXa play distinct roles in binding to factor VIIIa. Importance of helix 330 (helix 162 in chymotrypsin) of protease domain of factor IXa in its interaction with factor VIIIa

Previous studies revealed that cleavage at Arg-318-Ser-319 in the protease domain autolysis loop of factor IXa results in its diminished binding to factor VIIIa. Now, we have investigated the importance of adjacent surface-exposed helix 330-338 (162-170 in chymotrypsin numbering) of IXa in its interaction with VIIIa. IXWT, eight point mutants mostly based on hemophilia B patients, and a replacement mutant (IXhelixVII in which helix 330-338 is replaced by that of factor VII) were expressed, purified, and characterized. Each mutant was activated normally by VIIa-tissue factor-Ca2+ or XIa-Ca2+. However, in both the presence and absence of phospholipid, interaction of each activated mutant with VIIIa was impaired. The role of IXa EGF1 domain in binding to VIIIa was also examined. Two mutants (IXQ50P and IXPCEGF1, in which EGF1 domain is replaced by that of protein C) were used. Strikingly, interactions of the activated EGF1 mutants with VIIIa were impaired only in the presence of phospholipid. We conclude that helix 330 in IXa provides a critical binding site for VIIIa and that the EGF1 domain in this context primarily serves to correctly position the protease domain above the phospholipid surface for optimal interaction with VIIIa.