Snake-venom-derived Factor IX-binding protein specifically blocks the gamma-carboxyglutamic acid-rich-domain-mediated membrane binding of human Factors IX and X

A potent anticoagulant protein, IX-bp (Factor IX binding protein), has been isolated from the venom of Trimeresurus flavoviridis (habu snake) and is known to bind specifically to the Gla (gamma-carboxyglutamic acid-rich) domain of Factor IX. To evaluate the molecular basis for its anticoagulation activity, we assessed its interactions with various clotting factors. We found that the anticoagulation activity is primarily due to binding to the Gla domains of Factors IX and X, thus preventing these factors from recognizing phosphatidylserine on the plasma membrane. The present study suggests that ligands that bind to the Gla domains of Factors IX and X may have the potential to become novel anticoagulants.

Tissue factor coagulant function is enhanced by protein-disulfide isomerase independent of oxidoreductase activity

Protein-disulfide isomerase (PDI) switches tissue factor (TF) from coagulation to signaling by targeting the allosteric Cys186-Cys209 disulfide. Here, we further characterize the interaction of purified PDI with TF. We find that PDI enhances factor VIIa-dependent substrate factor X activation 5-10-fold in the presence of wild-type, oxidized soluble TF but not TF mutants that contain an unpaired Cys186 or Cys209. PDI-accelerated factor Xa generation was blocked by bacitracin but not influenced by inhibition of vicinal thiols, reduction of PDI, changes in redox gradients, or covalent thiol modification of reduced PDI by N-ethylmaleimide or methyl-methanethiosulfonate, which abolished PDI oxidoreductase but not chaperone activity. PDI had no effect on fully active TF on either negatively charged phospholipids or in activating detergent, indicating that PDI selectively acts upon cryptic TF to facilitate ternary complex formation and macromolecular substrate turnover. PDI activation was reduced upon mutation of TF residues in proximity to the macromolecular substrate binding site, consistent with a primary interaction of PDI with TF. PDI enhanced TF coagulant activity on microvesicles shed from cells, suggesting that PDI plays a role as an activating chaperone for circulating cryptic TF.

New Pharmacologic Approaches to Prevent Thromboembolism in Patients with Atrial Fibrillation

Atrial fibrillation (AF) is associated with a 6 fold increased risk for ischemic stroke. Observational studies suggest that one in four to five strokes is due to AF. Depending on the risk profile of an individual patient, the yearly risk for ischemic stroke is between 2% and 14%. AF is accompanied by an increased propensity for atrial clot formation due to a combination of decreased atrial blood flow, increased activity of the platelet/plasmatic coagulation system and prothrombotic changes at the atrial endocardium. This review summarizes the current guidelines for thromboembolic prevention in patients with AF. In many cases, continuous oral anticoagulation therapy (OAT) with vitamin K antagonists (VitKAs) is indicated if AF is accompanied by more than one additional risk factor for thromboembolic complications. However, therapeutic range of VitKAs (Phenprocoumon, Warfarin, and others), the most commonly used oral anticoagulants, is narrow and their use requires regular anticoagulation monitoring. Possibly due to these limitations, about one third of eligible patients are not treated with VitKAs. Furthermore, in many treated patients OAT is not well controlled. Thus, in clinical practice anticoagulation therapy in AF is suboptimal. Therefore, new and more convenient pharmacologic approaches to prevent thromboembolism with i.e. direct thrombin inhibitors (DTI), synthetic polysaccharides (factor Xa Inhibitors), and others are discussed, and their possible future role in the treatment of AF is evaluated.

The quest for Factor VIIa exosite inhibitors

Coagulation proteases are involved in a highly orchestrated proteolytic cascade which is essential for haemostasis and blood clotting. In particular, the initiator of the coagulation cascade, Factor VIIa, binds to its cofactor, tissue factor, and its substrate, Factor X, via exosite interactions to form a ternary catalytic complex named extrinsic Xase. These exosite interactions have also been shown to allosterically induce the active conformation of the catalytic site of Factor VIIa. We have developed a direct continuous fluorescence polarization-based extrinsic Xase assay, which has been used to screen in excess of 1 million structurally diverse low-molecular-mass compounds as a potential starting point for the development of anticoagulants. The primary screen hits were categorized with deconvolution assays into either active-site or exosite inhibitors. The latter category of hits displayed both competitive and uncompetitive modalities of inhibition with respect to Factor X activation. An uncompetitive mechanism of action is of particular interest as it offers a hypothetical inhibitory advantage in the context of inhibiting a proteolytic cascade such as the blood coagulation pathway.

Variability in expression of Bothrops insularis snake venom proteases: an ontogenetic approach

Bothrops insularis is a threatened snake endemic to Queimada Grande Island, southern coast of São Paulo, Brazil, and the occurrence of sexual abnormalities in males, females and intersexes (females with functional ovaries and rudimentary hemipenis) has been reported in this population. The aim of this study was to identify ontogenetic shifts in protease expression of offspring of captive-bred B. insularis. Three neonates from a single litter were maintained at the facilities of Laboratory of Herpetology, Institute Butantan, for 41 months. The snakes were individually milked and venoms were analyzed both by SDS-PAGE, under reducing conditions, and for biochemical activities. The venoms from the mother and from a pool of adult specimens were used as references. In regard to the electrophoretic patterns, common bands were identified mainly between 14 and 50 kDa among snakes. The occurrence of proteolytic activity was noticed predominantly between 27 and 45 kDa in zymograms. Inhibitory assays with 1,10-phenantroline (10 mM) and PMSF (5 mM) showed that venoms possessed both metalloproteases and serine proteases. Venoms of young specimens showed a higher coagulant activity than those of adults, especially upon factors X and II. All venoms presented fibrino(geno)lytic activity, degrading Aalpha and Bbeta chains of fibrinogen, and lysing fibrin plate. These findings can reflect important individual, ontogenetic and sexual differences on venom composition and are likely correlated with diet habits of this species.

Analysis of the novel factor X gene mutation Glu51Lys in two families with factor X-Riyadh anomaly

Two families with 'factor X(FX)-Riyadh' have been identified (one of them related to the originally reported family). Affected members of both families exhibit prolongation in prothrombin time (PT) with normal partial thromboplastin time (PTT) and low assay levels of FX, when measured by PT-based assay. They do not have clinical bleeding diathesis, regardless of the PT prolongation. FX genes of the affected family members were analyzed by sequence analysis. A novel missense mutation in exon 4 of the FX gene, which causes the Glu51Lys substitution in the first epidermal growth factor-like domain of FX was found. The Glu51Lys mutation represents a type II mutation with low FX coagulant activity in the extrinsic pathway and normal FX antigen levels. This mutation may result in disruption of the predicted H-bonding between residue Glu51 of FX and the Asn199 residue of the tissue factor (TF) in the FX/TF/factor VIIa ternary complex, producing the phenotype 'FX deficiency Riyadh', with prolonged PT and normal PTT.

Substitution of the Gla domain in factor X with that of protein C impairs its interaction with factor VIIa/tissue factor: lack of comparable effect by similar substitution in factor IX

We previously reported that the first epidermal growth factor-like (EGF1) domain in factor X (FX) or factor IX (FIX) plays an important role in the factor VIIa/tissue factor (FVIIa/TF)-induced coagulation. To assess the role of gamma-carboxyglutamic acid (Gla) domains of FX and FIX in FVIIa/TF induced coagulation, we studied four new and two previously described replacement mutants: FX(PCGla) and FIX(PCGla) (Gla domain replaced with that of protein C), FX(PCEGF1) and FIX(PCEGF1) (EGF1 domain replaced with that of protein C), as well as FX(PCGla/EGF1) and FIX(PCGla/EGF1) (both Gla and EGF1 domains replaced with those of protein C). FVIIa/TF activation of each FX mutant and the corresponding reciprocal activation of FVII/TF by each FXa mutant were impaired. In contrast, FVIIa/TF activation of FIX(PCGla) was minimally affected, and the reciprocal activation of FVII/TF by FIXa(PCGla) was normal; however, both reactions were impaired for the FIX(PCEGF1) and FIX(PCGla/EGF1) mutants. Predictably, FXIa activation of FIX(PCEGF1) was normal, whereas it was impaired for the FIX(PCGla) and FIX(PCGla/EGF1) mutants. Molecular models reveal that alternate interactions exist for the Gla domain of protein C such that it is comparable with FIX but not FX in its binding to FVIIa/TF. Further, additional interactions exist for the EGF1 domain of FX, which are not possible for FIX. Importantly, a seven-residue insertion in the EGF1 domain of protein C prevents its interaction with FVIIa/TF. Cumulatively, our data provide a molecular framework demonstrating that the Gla and EGF1 domains of FX interact more strongly with FVIIa/TF than the corresponding domains in FIX.

Platelet microparticle membranes have 50- to 100-fold higher specific procoagulant activity than activated platelets

Platelet microparticles (PMPs) are small vesicles released from blood platelets upon activation. The procoagulant activity of PMPs has been previously mainly characterized by their ability to bind coagulation factors VIII and Va in reconstructed systems. It can be supposed that PMPs can contribute to the development of thrombotic complications in the pathologic states associated with the increase of their blood concentration. In this study, we compared procoagulant properties of calcium ionophore A23187-activated platelets and PMPs using several in-vitro models of hemostasis. Surface densities of phosphatidylserine, CD61, CD62P and factor X bound per surface area unit were determined by flow cytometry. They were 2.7-, 8.4-, 4.3-, and 13-fold higher for PMPs than for activated platelets, respectively. Spatial clot growth rate (V(clot)) in the reaction-diffusion experimental model and endogenous thrombin potential (ETP) were determined in plasma, which was depleted of phospholipid cell surfaces by ultra-centrifugation and supplemented with activated platelets or PMPs at different concentrations. Both V(clot) and ETP rapidly increased with the increase of PMP or platelet concentration until saturation was reached. The plateau values of V(clot) and ETP for activated platelets and PMPs were similar. In both assays, the procoagulant activity of one PMP was almost equal to that of one activated platelet despite at least two-orders-of-magnitude difference in their surface areas. This suggests that the PMP surface is approximately 50- to 100-fold more procoagulant than the surface of activated platelets.

Identification of a basic region on tissue factor that interacts with the first epidermal growth factor-like domain of factor X

Tissue factor (TF) facilitates the recognition and rapid activation of factor X (fX) by factor VIIa (fVIIa) in the extrinsic Xase pathway. TF makes extensive interactions with both light and heavy chains of fVIIa; however, with the exception of a basic recognition site for the Gla domain of fX, no other interactive site on TF for the substrate has been identified. Structural and modeling data have predicted that a basic region of TF comprised of residues Asn-199, Arg-200, and Lys-201 is located at a proper height on the membrane surface to interact with either the C-terminus of the Gla domain or the EGF-1 domain of fX. To investigate this possibility, we prepared the Ala substitution mutants of these residues and evaluated their ability to function as cofactors for fVIIa in the activation of wild-type fX and its two mutants which lack either the Gla domain (GD-fX) or both the Gla and EGF-1 domains (E2-fX). All three TF mutants exhibited normal cofactor activity in the amidolytic activity assays, but the cofactor activity of Arg-200 and Lys-201 mutants in fVIIa activation of both fX and GD-fX, but not E2-fX, was impaired approximately 3-fold. Further kinetic analysis revealed that kcat values with both TF mutants are impaired with no change in Km. These results suggest that both Arg-200 and Lys-201 of TF interact with EGF-1 of fX to facilitate the optimal docking of the substrate into the catalytic groove of the protease in the activation complex.

Functional Analysis of the Factor IX Epidermal Growth Factor-Like Domain Mutation Ile66Thr Associated with Mild Hemophilia B

The present study focused on the functional role of the mutation Ile66Thr located in the N-terminal epidermal growth factor-like domain of coagulation factor IX (FIX). This mutation causes mild hemophilia B with approximately 25% FIX coagulant activity and FIX antigen levels of around 90% of normal. In the 3-dimensional structure of porcine FIXa and in the subsequent 3-dimensional model of human FIXa that we have previously developed, residue 66 is exposed to the solvent and can be replaced by many amino acids, including Thr, without affecting the major folding/stability of the molecule. This is consistent with the basically normal antigen levels observed. We found that the FIX Ile66Thr mutant was activated to a normal extent by FVIIa/TF and FXIa. However, the ability of FIX Ile66Thr to activate FX was impaired in both the presence and absence of FVIIIa, indicating that Ile66 is not directly involved in the binding of FIX to FVIIIa.

Adenovirus 5 fibers mutated at the putative HSPG-binding site show restricted retargeting with targeting peptides in the HI loop

Adenoviral vectors are commonly used for liver-directed gene therapy following systemic administration owing to their strong propensity for hepatocyte transduction. However, many disease applications would benefit from the delivery of adenoviruses to alternate tissues via this route. Research has thus focused on stripping the virus of native hepatic tropism in conjunction with modifying virus capsid proteins to incorporate novel tropism. Recently, the KO1S* adenovirus serotype 5 fiber mutant, devoid of both coxsackie and adenovirus receptor binding in the fiber knob domain and mutated at the putative heparan sulphate proteoglycan binding site in the fiber shaft, was shown to possess strikingly poor hepatic tropism in mice, rats, and non-human primates. Thus, it is an ideal candidate for retargeting strategies. We therefore assessed the ability of peptide-modified KO1S* fibers to retarget adenovirus. Peptide insertions were well tolerated and virions produced to high titers. However, expected retargeting at the level of transduction was not observed, despite cell-binding studies showing enhanced vector targeting at the cell surface. Cy3 labeling studies showed retarded trafficking of S*-containing fibers. Taken together, our data demonstrates that KO1S* mutant fibers are ineffective for cell retargeting strategies.

A variant of recombinant factor VIIa with enhanced procoagulant and antifibrinolytic activities in an in vitro model of hemophilia

OBJECTIVE

Recombinant factor VIIa (rFVIIa, NovoSeven) has proven efficacy in treating bleeding in hemophilia patients with inhibitors. A rFVIIa analog with mutations V158D/E296V/M298Q (NN1731) exhibits increased procoagulant activity in in vitro and in vivo models. The aim of this work was to define the effects of NN1731 toward factor X activation, platelet activation, thrombin generation, and fibrin clot formation and stability.

METHODS AND RESULTS

In a cell-based in vitro model of hemophilia, rFVIIa and NN1731 similarly increased factor X activation on tissue factor-bearing cells; however, NN1731 exhibited 30-fold higher factor Xa generation on platelets than similar rFVIIa concentrations. NN1731-mediated thrombin generation depended on platelet activation, but NN1731 did not directly activate platelets. NN1731 produced 4- to 10-fold higher maximal thrombin generation rates than equal rFVIIa concentrations. Both rFVIIa and NN1731 shortened clotting times in the absence of factors IX and VIII; however, NN1731 did so at 50-fold lower concentrations than were required of rFVIIa. In fibrinolytic conditions, both rFVIIa and NN1731 increased fibrin formation and stability; however, NN1731 was effective at 50-fold lower concentrations than were required of rFVIIa.

CONCLUSIONS

By increasing factor Xa generation, NN1731 promotes the formation of thrombin and a stable clot to a greater degree than rFVIIa.

A novel anti-tissue factor monoclonal antibody with anticoagulant potency derived from synthesized multiple antigenic peptide through blocking FX combination with TF

Tissue factor (TF) has been implicated in the pathogenesis of various thrombotic disorders. Monoclonal antibodies (mAb) that specifically target TF may have potential as antithrombotic therapy. We designed a unique TF peptide (TFP) that was specific for the binding site to factor X (FX). This peptide was used to develop TF mAb that block the coagulation cascade by interfering with the combination of FX with the TF/FVIIa complex. Chemically synthesized TFP coupled to polylysine matrix was used as multiple antigenic peptide (TF-MAP) and this was used to immunize Balb/c mice for the preparation of hybridomas. One hybridoma cell line released an antibody, named TF4A12, which had high anticoagulant potency (by dilute prothrombin time assay). Western blotting showed that TF4A12 could bind TF-MAP and the soluble TF extracellular domain (sTF(1-219)). Results of FX activation assay and amidolytic activity assay showed that the anticoagulant ability of TF4A12 is due to blocking FX, but not FVII, binding to TF. Our study identified an efficient method of developing TF mAb that could block the coagulation cascade.

Structure of two genes encoding parallel prothrombin activators in Tropidechis carinatus snake: gene duplication and recruitment of factor X gene to the venom gland

BACKGROUND

Proteins with new function originate through gene duplication followed by divergence. In nature, occurrence of structurally and functionally similar proteins performing highly diverse physiological roles within an organism is rare. Several Australian elapid snakes have two parallel prothrombin activating systems with distinct physiological roles. For example, in Tropidechis carinatus, trocarin D exists in the venom and acts as toxin, whereas coagulation factor (F) X exists in plasma and plays a role in hemostasis.

RESULTS

Here, we show that FX and the trocarin D genes are expressed in a highly tissue-specific manner in T. carinatus. To understand their origin, recruitment and evolution, we determined the complete structure of their genes. Both genes have eight exons with identical exon-intron boundaries. All the introns are 92-99% identical with the exception of intron 1, indicating a recent gene duplication event. The first intron of the trocarin D gene is also nearly identical to that of the FX gene, except for two deletions (255 and 1406 bp) and three insertions (214, 1975, and 2174 bp). The third insertion has a potential scaffold/matrix attached region. The putative promoter of the snake FX gene shares similar cis-elements compared with those of human and murine FX genes. Interestingly, the trocarin D promoter has a 264-bp insertion with core promoter sequences and cis-elements that are known to induce high-level expression. This insertion might be responsible for switching from constitutive expression of the FX gene to inducible expression of trocarin D. Thus, we named this segment as VERSE (Venom Recruitment/Switch Element).

CONCLUSION

To our knowledge, this is the first molecular evidence for the recruitment of a duplicated gene for expression in venom glands by a simple insertion.

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.

The influence of sodium ion binding on factor IXa activity

Sodium plays an important role in modulating both the amidolytic and proteolytic activities of thrombin. By contrast, while the optimal amidolytic activity of factor Xa requires Na(+), the proteolytic activity of factor Xa in the prothrombinase complex is minimally affected by the monovalent cation. In this study, we analyzed the effect of Na(+) on the amidolytic and proteolytic activity of factor IXa in the absence and presence of factor VIIIa. Factor IXa exhibited normal activity towards a fIXa-specific chromogenic substrate and antithrombin in the presence of physiological concentrations of Ca(2+) with no obvious requirement for Na(+) in either reaction. Further studies revealed that factor IXa binds to its cofactor factor VIIIa with a normal affinity in the absence of Na(+) and that the catalytic function in the intrinsic Xase complex is also independent of Na(+) in the presence of physiological concentrations of Ca(2+). These results suggest that unlike the important role that Na(+) plays in modulating the macromolecular substrate specificity of thrombin, the monovalent cation is not required for the physiological function of factor Ixa in the intrinsic Xase complex.

Lopap, a prothrombin activator from Lonomia obliqua belonging to the lipocalin family: recombinant production, biochemical characterization and structure-function insights

Using a cDNA library made from Lonomia obliqua caterpillar bristles, we identified a transcript with a 603 bp open reading frame. The deduced protein corresponds to Lopap, a prothrombin activator previously isolated by our group from the bristles of this species. The mature protein is composed by 185 amino acids and shares similarity with members of the lipocalin family. The cDNA encoding the mature form was amplified by PCR, subcloned into pAE vector and used to transform Escherichia coli BL21(DE3) cells. As for the native Lopap, the recombinant fusion protein shows enzymatic activity, promotes prothrombin hydrolysis, generates fragments similar to prethrombin-2 and fragment 1.2 as intermediates, and generates thrombin as the final product. In addition, structural bioinformatics studies indicated several interesting molecular features, including the residues that could be responsible for Lopap's serine protease-like activity and the role of calcium binding in this context. Such catalytic activity has never been found in other members of the lipocalin family. This is the first report describing the recombinant production and biochemical characterization of a Lonomia obliqua lipocalin, as well as the structural features that could be responsible for its serine protease-like catalytic activity.

Raising the Active Site of Factor VIIa above the Membrane Surface Reduces Its Procoagulant Activity but Not Factor VII Autoactivation

Tissue factor, the physiologic trigger of blood clotting, is the membrane-anchored protein cofactor for the plasma serine protease, factor VIIa. Tissue factor is hypothesized to position and align the active site of factor VIIa relative to the membrane surface for optimum proteolytic attack on the scissile bonds of membrane-bound protein substrates such as factor X. We tested this hypothesis by raising the factor VIIa binding site above the membrane surface by creating chimeras containing the tissue factor ectodomain linked to varying portions of the membrane-anchored protein, P-selectin. The tissue factor/P-selectin chimeras bound factor VIIa with high affinity and supported full allosteric activation of factor VIIa toward tripeptidyl-amide substrates. That the active site of factor VIIa was raised above the membrane surface when bound to tissue factor/P-selectin chimeras was confirmed using resonance energy transfer techniques in which appropriate fluorescent dyes were placed in the active site of factor VIIa and at the membrane surface. The chimeras were deficient in supporting factor X activation by factor VIIa due to decreased k(cat). The chimeras were also markedly deficient in clotting plasma, although incubating factor VII or VIIa with the chimeras prior to the addition of plasma restored much of their procoagulant activity. Interestingly, all chimeras fully supported tissue factor-dependent factor VII autoactivation. These studies indicate that proper positioning of the factor VII/VIIa binding site on tissue factor above the membrane surface is important for efficient rates of activation of factor X by this membrane-bound enzyme/cofactor complex.

Pediatric reference intervals for uncommon bleeding and thrombotic disorders

This study provides pediatric reference intervals and median values for factors II, V, VII, X, fibrinogen, alpha-2-antiplasmin (AP), antithrombin (AT), plasminogen, protein C (PC), and protein S (PS) for children 7 to 17 years of age. All analytes exhibited at least some age dependence in late childhood and adolescence either when compared against adult values or when medians for children were regressed against age.

Traces of factor VIIa modulate thromboplastin sensitivity to factors V, VII, X, and prothrombin

BACKGROUND

Thromboplastin reagents are used to conduct prothrombin time (PT) clotting tests to monitor oral anticoagulant therapy and screen for clotting factor deficiencies. Thromboplastins made from purified, recombinant tissue factor are generally more sensitive to changes in plasma factor (F) VII levels than are thromboplastins prepared from tissue extracts. This may be problematic as FVII's short plasma half-life can result in day-to-day fluctuation during oral anticoagulant therapy. We hypothesized that trace contamination of tissue-derived thromboplastins with FVII(a) blunts sensitivity to plasma FVII levels.

METHODS

Traces of purified FVIIa were added to thromboplastin reagents prepared using recombinant human tissue factor and the effect on sensitivity to individual clotting factors was quantified in PT clotting assays.

RESULTS AND CONCLUSIONS

Adding 5-100 pm FVIIa not only decreased thromboplastin sensitivity to plasma FVII, it surprisingly increased sensitivity to plasma levels of FV, FX and prothrombin. In addition, traces of FVIIa interacted with changes in the salt content and phospholipid composition of recombinant thromboplastins to further modulate their sensitivities to individual clotting factors. These results help explain how thromboplastin reagents of differing composition exhibit differing sensitivities to individual clotting factor levels. Implications of our results for monitoring oral anticoagulant therapy and other uses of the PT assay are discussed.

Multiple vitamin K-dependent coagulation zymogens promote adenovirus-mediated gene delivery to hepatocytes

Upon local delivery, adenovirus (Ad) serotype 5 viruses use the coxsackie and Ad receptor (CAR) for cell binding and alpha(v) integrins for internalization. When administered systemically, however, their role in liver tropism is limited because CAR-permissive and mutated viruses show similar biodistribution, a finding recently attributed to blood coagulation factor (F) IX or complement protein C4BP binding to the adenovirus fiber and "bridging" to either low-density lipoprotein receptor-related protein or heparan sulfate proteoglycans. Here, we show that hepatocyte transduction in vitro can be enhanced by the vitamin K-dependent factors FX, protein C, and FVII in addition to FIX but not by prothrombin (FII), FXI, and FXII. This phenomenon was not dependent on proteolytic activation or cell signaling activity and for FX was mediated by direct virus-factor binding. Human FX substantially enhanced hepatocyte transduction by CAR-permissive and mutated viruses in an ex vivo liver perfusion model. In vivo, global down-regulation of vitamin K-dependent zymogens by warfarin significantly diminished liver uptake of CAR-deleted Ads; however, this phenomenon was fully rescued by acute infusion of human FX. Our results indicate a common and pivotal role for distinct vitamin K-dependent coagulation factors in mediating hepatocyte transduction by adenoviruses in vitro and in vivo.

Molecular evolution caught in action: gene duplication and evolution of molecular isoforms of prothrombin activators in Pseudonaja textilis (brown snake)

BACKGROUND

The evolution of structurally and functionally similar proteins with highly diverse physiological roles within a single organism is of great interest. Australian elapid snakes offer an excellent opportunity to study the molecular evolution of prothrombin activators. Venom from Pseudonaja textilis contains pseutarin C, a group C prothrombin activator. Its enzymatic subunit is structurally and functionally similar to mammalian factor (F) Xa, whereas its non-enzymatic subunit is similar to FVa. As vertebrates, the snakes also contain a system to activate prothrombin in their own blood during injury. These hemostatic factors are produced in the liver.

RESULTS

Here we describe the presence of two molecular forms of FX expressed in the liver of P. textilis. Both isoforms have molecular signatures and domain architecture of FX. However, one isoform shows approximately 94% sequence identity with the snake FX from Tropidechis carinatus, whereas the other is much closer (90% identity) to the catalytic subunit of pseutarin C (PCCS). Real-time polymerase chain reaction reveals that the latter isoform is expressed approximately 56 000 times lower in the liver of P. textilis. However, the isoforms are not expressed in the venom gland.

CONCLUSION

A detailed analysis of deletions and insertions along with the sequence indicates that the second isoform is an intermediate caught in the evolution of venom prothrombin activator from the blood coagulation FX. Thus, this isoform represents a 'molecular fossil' and reveals the likely evolutionary path of recruitment of FX in the venom gland.