Probing thrombin's ability to accommodate a V34F substitution within the factor XIII activation peptide segment (28-41).

In blood coagulation, thrombin helps to activate factor XIII (FXIII) by cleaving the activation peptide (AP) at the R37-G38 peptide bond. The common polymorphism V34L yields a FXIII that is more easily activated than the wild type enzyme. Peptides based on the FXIII (28-41) (28TVELQGVVPRGVNL41) sequence serve as an important model system to evaluate the substrate specificity of thrombin and thus how to regulate FXIII activation. Our previous kinetic and nuclear magnetic resonance (NMR) studies have suggested that the P4-P1 amino acids on this FXIII segment provide key anchors to the thrombin active site surface. Furthermore, the most effective amino acid to have at the P4 position is a leucine. In the current work, a peptide containing V34F was examined to probe the ability to accommodate an aromatic residue at this position. Kinetic parameters for thrombin-catalyzed hydrolysis of FXIII AP (28-41) V34F are comparable with that of the wild type V34. One-dimensional proton line-broadening studies reveal that the 34FVPR37 segment encompassing the P4-P1 positions makes the most contact with the thrombin surface. Two-dimensional transferred-nuclear overhauser effect spectroscopy (NOESY) studies indicate that when the peptide is bound to thrombin, the F34 aromatic ring is oriented to promote P4-P2 interactions with P36. This characteristic has been viewed as a hallmark for V34L. An ability to generate this interaction may promote the ability of FXIII AP (28-41) V34F to remain a viable substrate for thrombin.     

Cloning and expression in COS-1 cells of a full-length cDNA encoding human coagulation factor X

A 1.5-kb cDNA (FX) encoding full-length human coagulation factor X was isolated from a human fetal liver cDNA library. The identity of the insert in a selected phage lambda clone was confirmed to be FX by nucleotide (nt) sequence analysis and restriction mapping. This FX cDNA clone contained 1467 bp of coding sequence, no 5'-untranslated sequence, a short 3'-untranslated sequence of 10 nt and a poly(A) tail at the 3'-end. The FX cDNA was inserted into a mammalian expression vector and transfected into COS-1 monkey kidney cells. Media from transfected cells showed evidence of factor X antigen and, following addition of Russel's viper venom factor X activator, enhanced amidolytic activity toward a synthetic peptide rho-nitroanilide substrate. Immunoprecipitation with an anti-factor X monoclonal antibody of [35S]methionine-labeled cell-conditioned media showed evidence of polypeptides of 74, 55, and 17 kDa, as determined by SDS-PAGE followed by autoradiography. Together, these results indicate that an active factor X can be successfully expressed in a recombinant DNA expression system. This approach will allow the systematic structure/function investigation of this important blood-clotting enzyme.     

Role of the Gla and first epidermal growth factor-like domains of factor X in the prothrombinase and tissue factor-factor VIIa complexes

Factor X (FX) has high structure homology with other proteins of blood coagulation such as factor IX (FIX) and factor VII (FVII). These proteins present at their amino-terminal extremity a gamma-carboxyglutamic acid containing domain (Gla domain), followed by two epidermal growth factor-like (EGF1 and EGF2) domains, an activation peptide, and a serine protease domain. After vascular damage, the tissue factor-FVIIa (TF-FVIIa) complex activates both FX and FIX. FXa interacts stoichiometrically with tissue pathway inhibitor (TFPI), regulating TF-FVIIa activity by forming the TF-FVIIa-TFPI-FXa quaternary complex. Conversely, FXa boosts coagulation by its association with its cofactor, factor Va (FVa). To investigate the contribution of the Gla and EGF1 domains of FX in these complexes, FX chimeras were produced in which FIX Gla and EGF1 domains substituted the corresponding domains of FX. The affinity of the two chimeras, FX/FIX(Gla) and FX/FIX(EGF1), for the TF-FVIIa complex was markedly reduced compared with that of wild-type-FX (wt-FX) independently of the presence of phospholipids. Furthermore, the association rate constants of preformed FX/FIX(Gla)-TFPI and FX/FIX(EGF1)-TFPI complexes with TF-FVIIa were, respectively, 10- and 5-fold slower than that of wt-FXa-TFPI complex. Finally, the apparent affinity of FVa was 2-fold higher for the chimeras than for wt-FX in the presence of phospholipids and equal in their absence. These data demonstrate that FX Gla and EGF1 domains contain residues, which interact with TF-FVIIa exosites contributing to the formation of the TF-FVIIa-FX and TF-FVIIa-TFPI-FXa complexes. On the opposite, FXa Gla and EGF1 domains are not directly involved in FVa binding.     

The endothelial protein C receptor supports tissue factor ternary coagulation initiation complex signaling through protease-activated receptors

Protease-activated receptor (PAR) signaling is closely linked to the cellular activation of the pro- and anticoagulant pathways. The endothelial protein C receptor (EPCR) is crucial for signaling by activated protein C through PAR1, but EPCR may have additional roles by interacting with the 4-carboxyglutamic acid domains of procoagulant coagulation factors VII (FVII) and X (FX). Here we show that soluble EPCR regulates the interaction of FX with human or mouse tissue factor (TF)-FVIIa complexes. Mutagenesis of the FVIIa 4-carboxyglutamic acid domain and dose titrations with FX showed that EPCR interacted primarily with FX to attenuate FX activation in lipid-free assay systems. In human cell models of TF signaling, antibody inhibition of EPCR selectively blocked PAR activation by the ternary TF-FVIIa-FXa complex but not by the non-coagulant TF-FVIIa binary complex. Heterologous expression of EPCR promoted PAR1 and PAR2 cleavage by FXa in the ternary complex but did not alter PAR2 cleavage by TF-FVIIa. In murine smooth muscle cells that constitutively express EPCR and TF, thrombin and FVIIa/FX but not FVIIa alone induced PAR1-dependent signaling. Although thrombin signaling was unchanged, cells with genetically reduced levels of EPCR no longer showed a signaling response to the ternary complex. These results demonstrate that EPCR interacts with the ternary TF coagulation initiation complex to enable PAR signaling and suggest that EPCR may play a role in regulating the biology of TF-expressing extravascular and vessel wall cells that are exposed to limited concentrations of FVIIa and FX provided by ectopic synthesis or vascular leakage.     

Coagulation Factor X Interaction with Macrophages through Its N-Glycans Protects It from a Rapid Clearance

Factor X (FX), a plasma glycoprotein playing a central role in coagulation has a long circulatory half-life compared to closely related coagulation factors. The activation peptide of FX has been shown to influence its clearance with two N-glycans as key determinants of FX’s relatively long survival. To decipher FX clearance mechanism, organ biodistribution and cellular interactions of human plasma FX (pd-FX), recombinant FX (rFX), N-deglycosylated FX (N-degly-FX) and recombinant FX mutated at both N-glycosylation sites (rFX^{N181A–N191A}) were evaluated. Biodistribution analysis of ¹²⁵I-labelled FX proteins after administration to mice revealed liver as major target organ for all FX variants. Liver tissue sections analysis showed an interaction of pd-FX and N-degly-FX to different cell types. These findings were confirmed in cell binding studies revealing that FX and FX without N-glycans interact with macrophages and hepatocytes, respectively. N-degly-FX appeared to be degraded in hepatocytes while interestingly pd-FX was not by macrophages. Furthermore, the chemical inactivation of macrophages by gadolinium chloride resulted in a significant decrease of circulating pd-FX into mice and not of N-degly-FX. Altogether our data lead to the conclusion that FX interaction with macrophages through its N-glycans protects it from a rapid clearance explaining its relatively long circulatory half-life.     

Inhibitors of the tissue factor/factor VIIa-induced coagulation: synthesis and in vitro evaluation of novel 2-aryl substituted pyrid

The synthesis of a series of 2-aryl substituted hetero annulated 1,3-oxazin-4-ones and their evaluation as specific inhibitors of the tissue factor (TF)/factor VIIa (FVIIa)-induced pathway of coagulation is reported. Inhibitory activities (IC₅₀ values) in the range 0.64 to >40 μM on the activation of factor X (FX) by the TF/FVIIa complex were found for compounds having one or two electronegative substituents such as F and NO₂ in the 2-aryl substituent. Some of the compounds showed a selectivity ratio towards FX and thrombin of >50, thus being similar in specificity to 2-aryl substituted 4H-3,1-benzoxazin-4-ones described as potential drugs for oral antithrombotic treatment without side effects, such as bleeding, which is observed especially with thrombin inhibitors.     

The role of calcium ions in factor X activation by thrombin E192Q.

Despite considerable sequence similarities, blood coagulation serine proteases exhibit remarkable specificity with respect to which zymogen they activate. The basis for this specificity presumably involves recognition of a short sequence within the extended binding pocket of the enzyme, other interactions remote from the catalytic groove, and modulation by a definite protein cofactor. In addition, Ca2+ plays a major role in most activation processes, but, because both the enzyme and its substrate interact with Ca2+, whether Ca2+ influences the substrate, the enzyme, or both remains an open question. Thrombin is not a factor X-activating enzyme, but when Glu192, 3 residues remote from the active Ser195, is replaced with glutamine, the resultant serine protease (E192Q) becomes a bovine, but not human, factor X activator. Kinetic experiments with peptides corresponding to human and bovine factor X activating sites reveal that threonine at position P2 in human (versus a valine in bovine) accounts for the species specificity. Substitution of the threonine in P2 of the human sequence with valine allows E192Q to cleave the human peptide whereas substitution of the valine in P2 of the bovine sequence with threonine hinders E192Q catalysis. Thrombin has no high affinity Ca2+ binding sites, and E192Q proteolysis of these peptides is not altered by Ca2+. The influence of Ca2+ in E192Q-mediated factor X activation provides therefore new insights into the role of the different Ca2+ binding sites in factor X. With factor X as substrate, the addition of Ca2+ enhances Kcat 4-fold but increases Km 10-fold. When the vitamin K-dependent gamma-carboxyglutamic acid domain of factor X is removed, the Km remains high with or without Ca2+ whereas Kcat still increases upon addition of the metal ion. These results suggest that factor X undergoes two metal-dependent suggest that factor X undergoes two metal-dependent transitions that influence the presentation of the activation site to activators.     

Possible involvement of PPARγ in the regulation of basal channel opening of P2X7 receptor in cultured mouse astrocytes

AimsRecently, we demonstrated that cultured mouse astrocytes exhibited basal channel opening of P2X7 receptor (P2X7R) in the absence of any exogenous ligand, but the regulatory mechanism involved was not elucidated. Since our preliminary experiments suggested possible involvement of peroxisome proliferator-activated receptor (PPAR) γ in the regulation, we examined whether PPARγ regulated P2X7R basal channel opening in mouse astrocytes.Main methodsP2X7R channel opening was assessed as to the uptake of a marker dye, YO-PRO-1^® (YP), in the presence or absence of agonists and antagonists for PPARγ under a fluorescence microscope. Expression of PPARγ was evaluated by Western blotting and immunocytochemistry.Key findingsNSAIDs such as flufenamic acid (FFA) and indomethacin, which are a cyclooxygenase inhibitor and a PPARγ agonist, showed enhancing and inhibiting effects on YP uptake at low and high concentrations, respectively, and the enhanced uptake was abolished by periodate-oxidized ATP (oxATP), a selective P2X7R antagonist. The PPARγ agonists 15-deoxy-Δ^12,14-prostaglandin J₂ and ciglitazone decreased the basal and FFA-enhanced YP uptake, while the antagonist GW9662 increased YP uptake, this effect being blocked by the agonists and also by oxATP. PPARγ was distributed in the nucleus and cytosolic/membrane fraction of cultured mouse astrocytes.SignificanceThese findings indicate that basal channel opening of P2X7R in mouse astrocytes is at least in part regulated by PPARγ.     

Screening for new peptide substrates for the development of albumin binding anticancer pro-drugs that are cleaved by prostate-specific antigen (PSA) to improve the anti tumor efficacy

Several attempts have been made over the past decade to explore the concept of prodrug strategies that exploit PSA as a molecular target for the release of anticancer drugs in prostate tumors using various prostate specific antigen (PSA)-cleavable peptide linkers, but the desired antitumor and antimetastatic efficacy has not yet been fully achieved. We set out to look for new PSA-cleavable peptide substrates that could be cleaved more rapidly and efficiently than the previously used peptides. To look for the most susceptible PSA-cleavable peptide substrates, we used the so-called spot technology. With the following general formula, we designed 25 different fluorogenic heptapeptides; Cellulose-P5-P4-P3-P2-P1-P1′-P2’ (Fluorophore). The increase of the fluorescence in the supernatant of the reaction mixture was monitored using a 96-well fluorometric plate reader with excitation of λ_ex 485 nm and λ_em 535 nm. Three sequences showed a high fluorogenic liberation after incubation with PSA, i.e., Arg-Arg-Leu-His-Tyr-Ser-Leu (7), Arg-Arg-Leu-Asn-Tyr-Ser-Leu (8) and Arg-Ser-Ser-Tyr-Arg-Ser-Leu (23). Future incorporation of these optimized substrates in the PSA-cleavable prodrug formulations could further optimize the cleavage pattern and so the release characteristics of these prodrugs to rapidly and efficiently liberate the free cytotoxic agents inside the tumor tissues.     

Thrombin activity is unaltered by N-terminal truncation of factor XIII activation peptides

In blood coagulation, thrombin helps to activate factor XIII by cleaving the activation peptide at the R37-G38 peptide bond. The residues N-terminal to the scissile bond are important in determining rates of hydrolysis. Solution studies of wild-type and mutant peptides of factor XIII AP (28-37) suggest residues P(4)-P(1) are most critical in substrate recognition. By contrast, the X-ray crystal structure of FXIII AP (28-37) displays all of the residues, P(10)-P(1), interacting with the thrombin active site in a conformation similar to that of fibrinogen Aalpha (7-16) [Sadasivan, C., and Yee, V. C. (2000) J. Biol. Chem. 275, 36942-36948]. Peptides were therefore synthesized with the N-terminal P(10)-P(6) residues removed to further characterize interactions of thrombin with factor XIII activation peptides. The truncations have no adverse effects on thrombin's ability to bind and to hydrolyze the shortened peptides. The wild-type FXIII AP (33-41) V34 sequence actually exhibits a decrease in K(m) relative to the longer (28-41) sequence whereas the cardioprotective FXIII AP (33-41) V34L exhibits a further increase in k(cat) relative to its longer parent sequence. One-dimensional proton line broadening NMR and 2D transferred-NOESY studies indicate that the shortened peptides maintain similar bound conformations as their FXIII AP (28-37) counterparts. Furthermore, the distinctive NOE between the L34 and P36 side chains is preserved. Kinetic and NMR studies thus reveal that the N-terminal portions of FXIII AP (28-37) (V34 and V34L) are not necessary for effective interaction with the thrombin active site surface. FXIII activation peptides bind to thrombin in a manner more like PAR1 than fibrinogen Aalpha.     

Thrombin-activable factor X re-establishes an intrinsic amplification in tenase-deficient plasmas

Classical hemophilia results from a defect of the intrinsic tenase complex, the main factor X (FX) activator. Binding of factor VIIa to tissue factor triggers coagulation, but little amplification of thrombin production occurs. Handling of hemophilia by injection of the deficient or missing (thus foreign) factor often causes immunological complications. Several strategies have been designed to bypass intrinsic tenase complex, but none induce true auto-amplification of thrombin production. In an attempt to re-establish a cyclic amplification of prothrombin activation in the absence of tenase, we prepared a chimera of FX having fibrinopeptide A for the activation domain (FX(FpA)). We reasoned that cascade initiation would produce traces of thrombin that would activate FX(FpA) (contrary to its normal homologue). Given that the activation domain of FX is released upon activation, thrombin cleavage would produce authentic FXa that would produce more thrombin, which in turn would activate more chimeras. FX(FpA) was indeed activable by thrombin, albeit at a relatively low rate (5 x 10(3) M(-1) s(-1)). Nevertheless, FX(FpA) allowed in vitro amplification of thrombin production, and 100 nM efficiently corrected thrombin generation in tenase-deficient plasmas. A decisive advantage of FX(FpA) could be that the artificial cascade is self-regulating: FX(FpA) had little influence on the clotting time of normal plasma, yet corrected that of tenase deficiency. Another advantage could be the half-life of FX(FpA) in blood; FX has a half-life of about 30 h (less than 3 h for FVIIa). It is also reasonable to expect little or no immunogenicity, because FX and fibrinopeptide A both circulate normally in the blood of hemophiliacs.     

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.     

P2X7 receptor-Pannexin1 complex: pharmacology and signaling

Pannexin 1 (Panx1), an ortholog to invertebrate innexin gap junctions, has recently been proposed to be the pore induced by P2X(7) receptor (P2X(7)R) activation. We explored the pharmacological action of compounds known to block gap junctions on Panx1 channels activated by the P2X(7)R and the mechanisms involved in the interaction between these two proteins. Whole cell recordings revealed distinct P2X(7)R and Panx1 currents in response to agonists. Activation of Panx1 currents following P2X(7)R stimulation or by membrane depolarization was blocked by Panx1 small-interfering RNA (siRNA) and with mefloquine > carbenoxolone > flufenamic acid. Incubation of cells with KN-62, a P2X(7)R antagonist, prevented current activation by 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP). Membrane permeabilization to dye induced by BzATP was also prevented by Panx1 siRNA and by carbenoxolone and mefloquine. Membrane permeant (TAT-P2X(7)) peptides, provided evidence that the Src homology 3 death domain of the COOH-terminus of the P2X(7)R is involved in the initial steps of the signal transduction events leading to Panx1 activation and that a Src tyrosine kinase is likely involved in this process. Competition assays indicated that 20 muM TAT-P2X(7) peptide caused 50% reduction in Src binding to the P2X(7)R complex. Src tyrosine phosphorylation following BzATP stimulation was reduced by KN-62, TAT-P2X(7) peptide, and by the Src tyrosine inhibitor PP2 and these compounds prevented both large-conductance Panx1 currents and membrane permeabilization. These results together with the lack Panx1 tyrosine phosphorylation in response to P2X(7)R stimulation indicate the involvement of an additional molecule in the tyrosine kinase signal transduction pathway mediating Panx1 activation through the P2X(7)R.     

Structure and dynamics of zymogen human blood coagulation factor X

The solution structure and dynamics of the human coagulation factor X (FX) have been investigated to understand the key structural elements in the zymogenic form that participates in the activation process. The model was constructed based on the 2.3-A-resolution x-ray crystallographic structure of active-site inhibited human FXa (PDB:1XKA). The missing gamma-carboxyglutamic acid (GLA) and part of epidermal growth factor 1 (EGF1) domains of the light chain were modeled based on the template of GLA-EGF1 domains of the tissue factor (TF)-bound FVIIa structure (PDB:1DAN). The activation peptide and other missing segments of FX were introduced using homology modeling. The full calcium-bound model of FX was subjected to 6.2 ns of molecular dynamics simulation in aqueous medium using the AMBER6.0 package. We observed significant reorientation of the serine-protease (SP) domain upon activation leading to a compact multi-domain structure. The solution structure of zymogen appears to be in a well-extended conformation with the distance between the calcium ions in the GLA domain and the catalytic residues estimated to be approximately 95 A in contrast to approximately 83 A in the activated form. The latter is in close agreement with fluorescence studies on FXa. The S1-specificity residues near the catalytic triad show significant differences between the zymogen and activated structures.     

Mass spectrometry based analysis of human plasma‐derived factor X revealed novel post‐translational modifications

Human coagulation factor X is a central component of the blood coagulation cascade that converts, under its activated form, prothrombin into thrombin. Generation of thrombin is the final step of the clotting cascade that leads to the clot by polymerization of fibrinogen molecules into a fibrin network. Today, research of new by‐passing agents of the coagulation may contribute to an increased interest for human factor X, which may, in consequence, lead to the need of a more exhaustive picture of its structural features. Several post‐translational modifications of human factor X such as γ‐carboxylation/β‐hydroxylation of the N‐terminal light chain and N‐/O‐glycosylation of the activation peptide have been described. But, so far as we know, no comprehensive studies of its post‐translational modifications have been reported. In this article we report an exhaustive structural analysis of human factor X by mass spectrometry using successive protein and peptide mapping. Surprisingly, human factor X was found to be mostly O‐glucosylated on its light chain at Ser₁₀₆ position, Ser₉ of its activation peptide is phosphorylated at about 30% and its C‐terminal heavy chain is fully O‐glycosylated at Thr₂₄₉ by a mucin‐type O‐glycan (HexNAc‐Hex‐NeuAc). The knowledge of these post‐translational modifications is mandatory for the development of recombinant molecules.     

Factor X/Xa Elicits Protective Signaling Responses in Endothelial Cells Directly via PAR-2 and Indirectly via Endothelial Protein C Receptor-dependent Recruitment of PAR-1

We recently demonstrated that the Gla domain-dependent interaction of protein C with endothelial protein C receptor (EPCR) leads to dissociation of the receptor from caveolin-1 and recruitment of PAR-1 to a protective signaling pathway. Thus, the activation of PAR-1 by either thrombin or PAR-1 agonist peptide elicited a barrier-protective response if endothelial cells were preincubated with protein C. In this study, we examined whether other vitamin K-dependent coagulation protease zymogens can modulate PAR-dependent signaling responses in endothelial cells. We discovered that the activation of both PAR-1 and PAR-2 in endothelial cells pretreated with factor FX (FX)-S195A, but not other procoagulant protease zymogens, also results in initiation of protective intracellular responses. Interestingly, similar to protein C, FX interaction with endothelial cells leads to dissociation of EPCR from caveolin-1 and recruitment of PAR-1 to a protective pathway. Further studies revealed that, FX activated by factor VIIa on tissue factor bearing endothelial cells also initiates protective signaling responses through the activation of PAR-2 independent of EPCR mobilization. All results could be recapitulated by the receptor agonist peptides to both PAR-1 and PAR-2. These results suggest that a cross-talk between EPCR and an unknown FX/FXa receptor, which does not require interaction with the Gla domain of FX, recruits PAR-1 to protective signaling pathways in endothelial cells.     

Molecular defect (Gla+14----Lys) and its functional consequences in a hereditary factor X deficiency (factor X "Vorarlberg")

Factor X (FX) "Vorarlberg" is a congenital FX deficiency characterized clinically by a mild bleeding tendency. Homozygous individuals have a FX activity of less than 10% in the extrinsic system and 25% in the intrinsic system. FX antigen is 20%. Using molecular techniques, two point mutations were detected in the coding sequence of the FX Vorarlberg gene: a G----A at base pair 160 in exon II resulting in a change of Gla14 (GAA) to Lys (AAA); a G----A at base pair 424 in exon V resulting in a change from Glu102 (GAG) to Lys (AAG). The mutations abolished a TaqI restriction site in exon II and an MnlI site in exon V. To determine whether these mutations are present on one or on both alleles, restriction analyses of amplified exon II and exon V fragments were performed. Analysis of the pedigree showed that the genotype for the mutation on exon II (homozygous versus heterozygous) correlates with the severity of the phenotypic coagulation defect. We therefore conclude that the mutation in exon II is responsible for the functional defect in FX Vorarlberg. We have also purified the mutant FX protein from patient plasma. Purified FX Vorarlberg is indistinguishable from normal FX on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Its activity is 15% of normal FX upon activation with factor VIIa/tissue factor, 75% upon activation with factor IXa/factor VIIIa, and 100% upon activation with RVV. Activation at varying Ca2+ concentrations shows that the affinity of FX Vorarlberg for Ca2+ is decreased. Factor Xa Vorarlberg is able to convert prothrombin at a normal rate but also shows decreased affinity for Ca2+ in this interaction. Upon addition of Ca2+, FX Vorarlberg does not undergo the same conformational change as normal FX. Our data show that FX Vorarlberg has a decreased affinity for Ca2+ which impedes a normal conformational change. This leads to a decreased rate of activation by factor VIIa/tissue factor and by factor IXa. The decrease is much more marked for the extrinsic than for the intrinsic pathway.     

Influence of coagulation factor zymogens on the infectivity of adenoviruses pseudotyped with fibers from subgroup D

Recent evidence supports a role for vitamin K-dependent coagulation zymogens in adenovirus serotype 5 (Ad5, subgroup C) infection of hepatocytes. Here, we assessed the effect of virus-zymogen interaction on cellular transduction using a panel of fiber (f)-pseudotyped viruses derived from subgroup D (f47, f33, f24, f45, f17, f30). Each virus directly bound factor X (FX) as determined by surface plasmon resonance, resulting in enhanced cell surface binding. Infection of HepG2 cells was promoted by FX but not by FVII or FIX, while transduction of CHO cells was blocked in heparan sulfate proteoglycan-deficient cells. This suggests a broad role for FX in adenovirus infectivity.     

Activity, folding, misfolding, and aggregation in vitro of the naturally occurring human tissue factor mutant R200W

Tissue factor (TF), a small transmembrane receptor, binds factor VIIa (FVIIa), and the formed complex initiates blood coagulation by proteolytic activation of substrate factors IX and X. A naturally occurring mutation in the human TF gene was recently reported, where a single-base substitution results in an R200W mutation in the TF extracellular domain [Zawadzki, C., Preudhomme, C., Gaveriaux, V., Amouyel, P., and Jude, B. (2002) Thromb. Haemost. 87, 540-541]. This mutation appears to be associated with low monocyte TF expression and may protect against thrombosis but has not been associated with any pathological condition, and individuals who present the heterozygous trait appear healthy. Here, we report the activity, folding, and aggregation behavior of the R200W mutant of the 219-residue soluble extracellular domain of TF (sTF(R200W)) compared to that of the wild-type protein (sTF(wt)). No differences in stability or FVIIa cofactor activity but an impaired ability to promote FX activation at physiological conditions between the sTF(R200W) mutant and sTF(wt) were evident. Increased binding of 1-anilino-8-naphthalene-sulfonic acid (ANS) to sTF(R200W) indicated a population of partially folded intermediates during denaturation. sTF(R200W) showed a dramatically increased propensity for aggregate formation compared to sTF(wt) at mildly acidic pHs, with an increased rate of aggregation during conditions, promoting the intermediate state. The lowered pH resistance could explain the loss of sTF(R200W) in vivo because of aggregation of the mutant. The intrinsic structure of the sTF aggregates appears reminiscent of amyloid fibrils, as revealed by thioflavin T fluorescence, atomic force microscopy, and transmission electron microscopy. We conclude that the lowered activity for FX activation and the propensity of the mutant protein to misfold and aggregate will both contribute to decreased coagulation activity in TF(R200W) carriers, which could protect from thrombotic disease.     

A cluster of basic amino acids in the factor X serine protease mediates surface attachment of adenovirus/FX complexes

Hepatocyte transduction following intravenous administration of adenovirus 5 (Ad5) is mediated by interaction between coagulation factor X (FX) and the hexon. The FX serine protease (SP) domain tethers the Ad5/FX complex to hepatocytes through binding heparan sulfate proteoglycans (HSPGs). Here, we identify the critical HSPG-interacting residues of FX. We generated an FX mutant by modifying seven residues in the SP domain. Surface plasmon resonance demonstrated that mutations did not affect binding to Ad5. FX-mediated, HSPG-associated cell binding and transduction were abolished. A cluster of basic amino acids in the SP domain therefore mediates surface interaction of the Ad/FX complex.