Identification of surface residues mediating tissue factor binding and catalytic function of the serine protease factor VIIa

Membrane-bound model of the ternary complex between factor VIIa/tissue factor and factor X

ABSTRACT

Formation of the extrinsic complex (EC) on cell surfaces is the event that triggers the coagulation cascade. Tissue factor (TF) and factor VIIa (FVIIa) form the EC together with FX on phosphatidylserine-containing membranes, leading to FX activation by TF:FVIIa. This lipid dependence has made experimental characterization of the EC structure challenging. Using a novel computational methodology combining rigid-body protein-protein docking and extensive nonequilibrium molecular dynamics simulations in the explicit presence of a membrane, we developed, to our knowledge, the first atomic-level model of the EC, taking full account of the role of the membrane. Rigid-body docking generated 1 000 000 protein-only structures that predict the binding of key EC domains. Residue-residue contact information was then used in nonequilibrium simulations to drive the formation of the EC on a phosphatidylserine/phosphatidylcholine membrane surface, providing, to our knowledge, the first membrane-bound model for the EC. Strikingly, in our model, FX makes contact with TF:FVIIa chiefly via its γ-carboxyglutamate-rich (GLA) domain and protease domain, with the majority of the FX light chain (ie, its 2 epidermal growth factor-like domains) out in the solvent, making no direct contact with TF:FVIIa. The TF exosite makes substantial contacts with both the FX- and FVIIa-GLA domains, in which TF residue K165 engages directly with the FVIIa-GLA domain, whereas K166 plays a central role in binding to the FX-GLA domain. These findings underscore the substrate-binding exosite of TF as being pivotal in the formation of the EC, serving as a critical interface linking the GLA domains of both FVIIa and FX.

The intricate allostery in factor VIIa: triggering the trigger

In the last couple of decades, numerous investigations have shed considerable light on how precisely factor (F)VIIa mediates the initiation of blood coagulation upon association with its cofactor, tissue factor (TF). The role of the cofactor in this process is indispensable under physiological conditions, serving as a membrane-tethering allosteric activator of FVIIa also interacting with substrates (eg, FX). Available evidence reveals the induction and manifestation of complex allostery within FVIIa when stimulated by TF, involving at least 2 connected pathways spanning the interactive interface of the FVIIa-TF complex and the functional segments of FVIIa. Carefully designed FVIIa variants demonstrate corresponding modulations of their properties and response to TF-triggered allostery and activation. In addition, antibodies can stimulate FVIIa activity in both similar and distinctly different ways compared to that employed by TF. The mechanistic insights obtained through basic biochemical investigations have been validated through select engineered FVIIa constructs which, even in vivo, demonstrate beneficial, proof-of-concept effects. Altogether, we have recently gained unprecedented knowledge about and control over FVIIa allostery, enabling us to influence FVIIa activity in advanced manners and in a desired direction. Here, we summarize our current understanding of the allosteric activation of FVIIa ending up with some prospects of future investigations.

An RNA aptamer exploits exosite-dependent allostery to achieve specific inhibition of coagulation factor IXa

Hemostasis relies on a reaction network of serine proteases and their cofactors to form a blood clot. Coagulation factor IXa (protease) plays an essential role in hemostasis as evident from the bleeding disease associated with its absence. RNA aptamers specifically targeting individual coagulation factors have potential as anticoagulants and as probes of the relationship between structure and function. Here, we report X-ray structures of human factor IXa without a ligand bound to the active site either in the apo-form or in complex with an inhibitory aptamer specific for factor IXa. The aptamer binds to an exosite in the catalytic domain and allosterically distorts the active site. Our studies reveal a conformational ensemble of IXa states, wherein large movements of Trp215 near the active site drive functional transitions between the closed (aptamer-bound), latent (apo), and open (substrate-bound) states. The latent state of the apo-enzyme may bear on the uniquely poor catalytic activity of IXa compared to other coagulation proteases. The exosite, to which the aptamer binds, has been implicated in binding VIIIa and heparin, both of which regulate IXa function. Our findings reveal the importance of exosite-driven allosteric modulation of IXa function and new strategies to rebalance hemostasis for therapeutic gain.

Post-Translational Oxidative Modifications of Hemostasis Proteins: Structure, Function, and Regulation

Reactive oxygen species (ROS) are constantly generated in a living organism. An imbalance between the amount of generated reactive species in the body and their destruction leads to the development of oxidative stress. Proteins are extremely vulnerable targets for ROS molecules, which can cause oxidative modifications of amino acid residues, thus altering structure and function of intra- and extracellular proteins. The current review considers the effect of oxidation on the structural rearrangements and functional activity of hemostasis proteins: coagulation system proteins such as fibrinogen, prothrombin/thrombin, factor VII/VIIa; anticoagulant proteins - thrombomodulin and protein C; proteins of the fibrinolytic system such as plasminogen, tissue plasminogen activator and plasminogen activator inhibitor-1. Structure and function of the proteins, oxidative modifications, and their detrimental consequences resulting from the induced oxidation or oxidative stress in vivo are described. Possible effects of oxidative modifications of proteins in vitro and in vivo leading to disruption of the coagulation and fibrinolysis processes are summarized and systematized, and the possibility of a compensatory mechanism in maintaining hemostasis under oxidative stress is analyzed.

A systematic approach for evaluating the role of surface-exposed loops in trypsin-like serine proteases applied to the 170 loop in coagulation factor VIIa

Proteases play a major role in many vital physiological processes. Trypsin-like serine proteases (TLPs), in particular, are paramount in proteolytic cascade systems such as blood coagulation and complement activation. The structural topology of TLPs is highly conserved, with the trypsin fold comprising two β-barrels connected by a number of variable surface-exposed loops that provide a surprising capacity for functional diversity and substrate specificity. To expand our understanding of the roles these loops play in substrate and co-factor interactions, we employ a systematic methodology akin to the natural truncations and insertions observed through evolution of TLPs. The approach explores a larger deletion space than classical random or directed mutagenesis. Using FVIIa as a model system, deletions of 1–7 amino acids through the surface exposed 170 loop, a vital allosteric regulator, was introduced. All variants were extensively evaluated by established functional assays and computational loop modelling with Rosetta. The approach revealed detailed structural and functional insights recapitulation and expanding on the main findings in relation to 170 loop functions elucidated over several decades using more cumbersome crystallization and single deletion/mutation methodologies. The larger deletion space was key in capturing the most active variant, which unexpectedly had a six-amino acid truncation. This variant would have remained undiscovered if only 2–3 deletions were considered, supporting the usefulness of the methodology in general protease engineering approaches. Our findings shed further light on the complex role that surface-exposed loops play in TLP function and supports the important role of loop length in the regulation and fine-tunning of enzymatic function throughout evolution.

Current Targets and Bioconjugation Strategies in Photodynamic Diagnosis and Therapy of Cancer

Photodynamic diagnosis (PDD) and therapy (PDT) are emerging, non/minimally invasive techniques for cancer diagnosis and treatment. Both techniques require a photosensitizer and light to visualize or destroy cancer cells. However, a limitation of conventional, non-targeted PDT is poor selectivity, causing side effects. The bioconjugation of a photosensitizer to a tumor-targeting molecule, such as an antibody or a ligand peptide, is a way to improve selectivity. The bioconjugation strategy can generate a tumor-targeting photosensitizer conjugate specific for cancer cells, or ideally, for multiple tumor compartments to improve selectivity and efficacy, such as cancer stem cells and tumor neovasculature within the tumor microenvironment. If successful, such targeted photosensitizer conjugates can also be used for specific visualization and detection of cancer cells and/or tumor angiogenesis (an early event in tumorigenesis) with the hope of an early diagnosis of cancer. The purpose of this review is to summarize some current promising target molecules, e.g., tissue factor (also known as CD142), and the currently used bioconjugation strategies in PDT and PDD, with a focus on newly developed protein photosensitizers. These are genetically engineered photosensitizers, with the possibility of generating a fusion protein photosensitizer by recombinant DNA technology for both PDT and PDD without the need of chemical conjugation. We believe that providing an overview of promising targets and bioconjugation strategies will aid in driving research in this field forward towards more effective, less toxic, and non- or minimally invasive treatment and diagnosis options for cancer patients.

Development of Optimized Tissue-Factor-Targeted Peptide Amphiphile Nanofibers to Slow Noncompressible Torso Hemorrhage

Noncompressible torso hemorrhage accounts for a significant portion of preventable trauma deaths. We report here on the development of injectable, targeted supramolecular nanotherapeutics based on peptide amphiphile (PA) molecules that are designed to target tissue factor (TF) and, therefore, selectively localize to sites of injury to slow hemorrhage. Eight TF-targeting sequences were identified, synthesized into PA molecules, coassembled with nontargeted backbone PA at various weight percentages, and characterized via circular dichroism spectroscopy, transmission electron microscopy, and X-ray scattering. Following intravenous injection in a rat liver hemorrhage model, two of these PA nanofiber coassemblies exhibited the most specific localization to the site of injury compared to controls (p < 0.05), as quantified using immunofluorescence imaging of injured liver and uninjured organs. To determine if the nanofibers were targeting TF in vivo, a mouse saphenous vein laser injury model was performed and showed that TF-targeted nanofibers colocalized with fibrin, demonstrating increased levels of nanofiber at TF-rich sites. Thromboelastograms obtained using samples of heparinized rat whole blood containing TF demonstrated that no clots were formed in the absence of TF-targeted nanofibers. Lastly, both PA nanofiber coassemblies decreased blood loss in comparison to sham and backbone nanofiber controls by 35-59% (p < 0.05). These data demonstrate an optimal TF-targeted nanofiber that localizes selectively to sites of injury and TF exposure, and, interestingly, reduces blood loss. This research represents a promising initial phase in the development of a TF-targeted injectable therapeutic to reduce preventable deaths from hemorrhage.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Beating tissue factor at its own game: Design and properties of a soluble tissue factor-independent coagulation factor VIIa

Two decades of research have uncovered the mechanism by which the complex of tissue factor (TF) and the plasma serine protease factor VIIa (FVIIa) mediates the initiation of blood coagulation. Membrane-anchored TF directly interacts with substrates and induces allosteric effects in the protease domain of FVIIa. These properties are also recapitulated by the soluble ectodomain of TF (sTF). At least two interdependent allosteric activation pathways originate at the FVIIa:sTF interface are proposed to enhance FVIIa activity upon sTF binding. Here, we sought to engineer an sTF-independent FVIIa variant by stabilizing both proposed pathways, with one pathway terminating at segment 215-217 in the activation domain and the other pathway terminating at the N terminus insertion site. To stabilize segment 215-217, we replaced the flexible 170 loop of FVIIa with the more rigid 170 loop from trypsin and combined it with an L163V substitution (FVIIa-VY_T). The FVIIa-VYT variant exhibited 60-fold higher amidolytic activity than FVIIa, and displayed similar FX activation and antithrombin inhibition kinetics to the FVIIa.sTF complex. The sTF-independent activity of FVIIa-VY_T was partly mediated by an increase in the N terminus insertion and, as shown by X-ray crystallography, partly by Tyr-172 inserting into a cavity in the activation domain stabilizing the S1 substrate-binding pocket. The combination with L163V likely drove additional changes in a delicate hydrogen-bonding network that further stabilized S1-S3 sites. In summary, we report the first FVIIa variant that is catalytically independent of sTF and provide evidence supporting the existence of two TF-mediated allosteric activation pathways.

Orphan Three-Finger Toxins Bind at Tissue Factor-Factor VIIa Interface to Inhibit Factor X Activation: Identification of Functional Site by Docking

Three-finger toxins (3FTxs) contribute to toxicity of venomous snakes belonging to the family Elapidae. Currently, functions of a considerable proportion of 3FTxs are still unknown. Here, we describe the function of orphan group I 3FTxs consisting of four members. We also identified a new member of this group by sequencing a transcript isolated from Naja naja venom. This transcript, named najalexin, is identical to that previously described 3FTx from Naja atra venom gland, and shared high sequence identity with ringhalexin from Hemachatus haemachatus and a hypothetical protein from Ophiophagus hannah (here named as ophiolexin). The three-dimensional structure, as predicted by molecular modeling, showed that najalexin and ophiolexin share the same conserved structural organization as ringhalexin and other 3FTxs. Since ringhalexin inhibits the activation of factor X by the tissue factor-factor VIIa complex (TF-FVIIa), we evaluated the interaction of this group of 3FTxs with all components using in silico protein-protein docking studies. The binding of orphan group I 3FTxs to TF-FVIIa complex appears to be driven by their interaction with TF. They bind to fibronectin domain closer to the 170-loop of the FVIIa heavy chain to inhibit factor X activation. The docking studies reveal that functional site residues Tyr7, Lys9, Glu12, Lys26, Arg34, Leu35, Arg40, Val55, Asp56, Cys57, Cys58, and Arg65 on these 3FTxs are crucial for interaction. In silico replacement of these residues by Ala resulted in significant effects in the binding energies. Furthermore, these functional residues are not found in other groups of 3FTxs, which exhibit distinct pharmacological properties.

Targeting Tissue Factor for Immunotherapy of Triple-Negative Breast Cancer Using a Second-Generation ICON

Triple-negative breast cancer (TNBC) is a leading cause of breast cancer death and is often associated with BRCA1 and BRCA2 mutation. Due to the lack of validated target molecules, no targeted therapy for TNBC is approved. Tissue factor (TF) is a common yet specific surface target receptor for cancer cells, tumor vascular endothelial cells, and cancer stem cells in several types of solid cancers, including breast cancer. Here, we report evidence supporting the idea that TF is a surface target in TNBC. We used in vitro cancer lines and in vivo tumor xenografts in mice, all with BRCA1 or BRCA2 mutations, derived from patients' tumors. We showed that TF is overexpressed on TNBC cells and tumor neovasculature in 50% to 85% of TNBC patients (n = 161) and in TNBC cell line-derived xenografts (CDX) and patient-derived xenografts (PDX) from mice, but was not detected in adjacent normal breast tissue. We then describe the development of a second-generation TF-targeting immunoconjugate (called L-ICON1, for lighter or light chain ICON) with improved efficacy and safety profiles compared with the original ICON. We showed that L-ICON1 kills TNBC cells in vitro via antibody-dependent cell-mediated cytotoxicity and can be used to treat human and murine TNBC CDX as well as PDX in vivo in orthotopic mouse models. Thus, TF could be a useful target for the development of immunotherapeutics for TNBC patients, with or without BRCA1 and BRCA2 mutations. Cancer Immunol Res; 6(6); 671-84. ©2018 AACR.

Evolutionary conservation of the allosteric activation of factor VIIa by tissue factor in lamprey

Essentials Tissue factor (TF) enhances factor VIIa (FVIIa) activity through structural and dynamic changes. We analyzed conservation of TF-activated FVIIa allosteric networks in extant vertebrate lamprey. Lamprey Tf/FVIIa molecular dynamics show conserved Tf-induced structural/dynamic FVIIa changes. Lamprey Tf activation of FVIIa allosteric networks follows molecular pathways similar to human.

SUMMARY

Background Previous studies have provided insight into the molecular basis of human tissue factor (TF) activation of activated factor VII (FVIIa). TF-induced allosteric networks of FVIIa activation have been rationalized through analysis of the dynamic changes and residue connectivities in the human soluble TF (sTF)/FVIIa complex structure during molecular dynamics (MD) simulation. Evolutionary conservation of the molecular mechanisms for TF-induced allosteric FVIIa activation between humans and extant vertebrate jawless fish (lampreys), where blood coagulation emerged more than 500 million years ago, is unknown and of considerable interest. Objective To model the sTf/FVIIa complex from cloned Petromyzon marinus lamprey sequences, and with comparisons to human sTF/FVlla investigate conservation of allosteric mechanisms of FVIIa activity enhancement by soluble TF using MD simulations. Methods Full-length cDNAs of lamprey tf and f7 were cloned and characterized. Comparative models of lamprey sTf/FVIIa complex and free FVIIa were determined based on constructed human sTF/FVIIa complex and free FVIIa models, used in full-atomic MD simulations, and characterized using dynamic network analysis approaches. Results Allosteric paths of correlated motion from Tf contact points in lamprey sTf/FVIIa to the FVIIa active site were determined and quantified, and were found to encompass residue-residue interactions along significantly similar paths compared with human. Conclusions Despite low conservation of residues between lamprey and human proteins, 30% TF and 39% FVII, the structural and protein dynamic effects of TF activation of FVIIa appear conserved and, moreover, present in extant vertebrate proteins from 500 million years ago when TF/FVIIa-initiated extrinsic pathway blood coagulation emerged.

Engineering of a membrane-triggered activity switch in coagulation factor VIIa

Coagulation factor VIIa (FVIIa) is an intrinsically poor serine protease that requires assistance from its cofactor tissue factor (TF) to trigger the extrinsic pathway of blood coagulation. TF stimulates FVIIa through allosteric maturation of its active site and by facilitating substrate recognition. The surface dependence of the latter property allowed us to design a potent membrane-triggered activity switch in FVIIa by engineering a disulfide cross-link between an allosterically silent FVIIa variant and soluble TF. These results show that optimization of substrate recognition remote from the active site represents a promising new route to simultaneously enhance and localize the procoagulant activity of FVIIa for therapeutic purposes.

Recombinant factor VIIa (FVIIa) variants with increased activity offer the promise to improve the treatment of bleeding episodes in patients with inhibitor-complicated hemophilia. Here, an approach was adopted to enhance the activity of FVIIa by selectively optimizing substrate turnover at the membrane surface. Under physiological conditions, endogenous FVIIa engages its cell-localized cofactor tissue factor (TF), which stimulates activity through membrane-dependent substrate recognition and allosteric effects. To exploit these properties of TF, a covalent complex between FVIIa and the soluble ectodomain of TF (sTF) was engineered by introduction of a nonperturbing cystine bridge (FVIIa Q64C-sTF G109C) in the interface. Upon coexpression, FVIIa Q64C and sTF G109C spontaneously assembled into a covalent complex with functional properties similar to the noncovalent wild-type complex. Additional introduction of a FVIIa-M306D mutation to uncouple the sTF-mediated allosteric stimulation of FVIIa provided a final complex with FVIIa-like activity in solution, while exhibiting a two to three orders-of-magnitude increase in activity relative to FVIIa upon exposure to a procoagulant membrane. In a mouse model of hemophilia A, the complex normalized hemostasis upon vascular injury at a dose of 0.3 nmol/kg compared with 300 nmol/kg for FVIIa.

The role of the tissue factor pathway in haemostasis and beyond

PURPOSE OF REVIEW

The role of tissue factor (TF) in the initiation of the blood coagulation network leading to generation of a fibrin clot has been well defined over the past 50 years. Although much is known about this sequence of events and its regulation, many important questions remain unresolved. More recently, a complex role for TF in cellular processes independent of fibrin generation has emerged. This review summarizes some of the advances in this field.

RECENT FINDINGS

TF is the cellular receptor and cofactor for factor VII/VIIa; however, controversy still surrounds expression of TF within the vasculature, the role of circulating microvesicle pools of TF and mechanisms of 'encryption' of TF activity. However, there have been significant advances in the role of TF-initiated cell signalling. Lastly, an alternatively spliced TF transcript has been identified and some insights into its role in cancer cell metastasis/proliferation have been elucidated.

SUMMARY

Understanding of TF structure function has increased substantially; however, multiple controversies still surround some aspects of its regulation. TF has emerged as a pivotal player in orchestrating not only fibrin generation but wound repair. Derangement of these repair processes contributes significantly to the pathophysiology of a number of disease processes.

Molecular determinants involved in differential behaviour between soluble tissue factor and full-length tissue factor towards factor VIIa

During blood-coagulation, the transmembrane protein tissue factor (TF) binds to its ligand, factor (F)VII, activating and allosterically modifying it to form a mature active binary complex (TF–FVIIa).

Tissue-Factor Targeted Peptide Amphiphile Nanofibers as an Injectable Therapy To Control Hemorrhage

Noncompressible torso hemorrhage is a leading cause of mortality in civilian and battlefield trauma. We sought to develop an i.v.-injectable, tissue factor (TF)-targeted nanotherapy to stop hemorrhage. Tissue factor was chosen as a target because it is only exposed to the intravascular space upon vessel disruption. Peptide amphiphile (PA) monomers that self-assemble into nanofibers were chosen as the delivery vehicle. Three TF-binding sequences were identified (EGR, RLM, and RTL), covalently incorporated into the PA backbone, and shown to self-assemble into nanofibers by cryo-transmission electron microscopy. Both the RLM and RTL peptides bound recombinant TF in vitro. All three TF-targeted nanofibers bound to the site of punch biopsy-induced liver hemorrhage in vivo, but only RTL nanofibers reduced blood loss versus sham (53% reduction, p < 0.05). Increasing the targeting ligand density of RTL nanofibers yielded qualitatively better binding to the site of injury and greater reductions in blood loss in vivo (p < 0.05). In fact, 100% RTL nanofiber reduced overall blood loss by 60% versus sham (p < 0.05). Evaluation of the biocompatibility of the RTL nanofiber revealed that it did not induce RBC hemolysis, did not induce neutrophil or macrophage inflammation at the site of liver injury, and 70% remained intact in plasma after 30 min. In summary, these studies demonstrate successful binding of peptides to TF in vitro and successful homing of a TF-targeted PA nanofiber to the site of hemorrhage with an associated decrease in blood loss in vivo. Thus, this therapeutic may potentially treat noncompressible hemorrhage.

Structure-Function Relationship of the Interaction between Tissue Factor and Factor VIIa

Interactions between tissue factor and factor VIIa are the primary initiators of coagulation in hemostasis and certain thrombotic diseases. Tissue factor, an integral membrane protein expressed extensively outside of the vasculature, is the regulatory protein cofactor for coagulation factor VIIa. Factor VIIa, a trypsin-like serine protease homologous with other blood coagulation proteases, is weakly active when free in solution and must bind its membrane-bound cofactor for physiologically relevant activity. Tissue factor allosterically activates factor VIIa by several mechanisms such as active site positioning, spatial stabilization, and direct interactions with the substrate. Protein-membrane interactions between tissue factor, factor VIIa, and substrates all play critical roles in modulating the activity of this enzyme complex. Additionally, divalent cations such as Ca(2+) and Mg(2+) are critical for correct protein folding, as well as protein-membrane and protein-protein interactions. The contributions of these factors toward tissue factor-factor VIIa activity are discussed in this review.

Why Ser and not Thr brokers catalysis in the trypsin fold

Although Thr is equally represented as Ser in the human genome and as a nucleophile is as good as Ser, it is never found in the active site of the large family of trypsin-like proteases that utilize the Asp/His/Ser triad. The molecular basis of the preference of Ser over Thr in the trypsin fold was investigated with X-ray structures of the thrombin mutant S195T free and bound to an irreversible active site inhibitor. In the free form, the methyl group of T195 is oriented toward the incoming substrate in a conformation seemingly incompatible with productive binding. In the bound form, the side chain of T195 is reoriented for efficient substrate acylation without causing steric clash within the active site. Rapid kinetics prove that this change is due to selection of an active conformation from a preexisting ensemble of reactive and unreactive rotamers whose relative distribution determines the level of activity of the protease. Consistent with these observations, the S195T substitution is associated with a weak yet finite activity that allows identification of an unanticipated important role for S195 as the end point of allosteric transduction in the trypsin fold. The S195T mutation abrogates the Na(+)-dependent enhancement of catalytic activity in thrombin, activated protein C, and factor Xa and significantly weakens the physiologically important allosteric effects of thrombomodulin on thrombin and of cofactor Va on factor Xa. The evolutionary selection of Ser over Thr in trypsin-like proteases was therefore driven by the need for high catalytic activity and efficient allosteric regulation.

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.

Conformational Changes of Congenital FVII Variants with Defective Binding to Tissue Factor ARG304GLN (FVII Padua), ARG 304TRP (FVII Nagoya) and ARG79GLN (FVII Shinjo or Tondabayashi)

BACKGROUND

The relation between Factor VII (FVII) and tissue thromboplastin is not completely clarified, yet. Three FVII abnormalities, FVII Padua (Arg304Gln), FVII Nagoya (Arg304Trp) and FVII Shinjo or Tondabayshi (Arg79Gln) show different FVII activity according to the tissue Tissue Factor (TF) used in the assay system (rabbit brain, human placenta or human recombinant and ox brain).

OBJECTIVES

To investigate the possible existence of common conformational changes with regard to different tissue factors in these three FVII variants.

MATERIAL AND METHODS

Crystal structure analysis and "visual inspection" of FVII were deeply performed to select a crystallographic template for the in silico mutagenesis procedure of FVII Arg79Gln, Arg304Gln and Arg304Trp.100ns 300K NVT large-scale molecular dynamics simulation on GPU were applied to the models of FVII. The aims of this run was to describe at molecular level the influence of the mutation on the protein structure and function.

RESULTS

The molecular modelling of those three variants has shown common features in spite of the different location of the mutation involved (the first epidermal growth factor for the Arg79Gln and the catalytic region for the Arg304Gln or Arg304Trp). Molecular dynamics studies have shown in fact that the mutant FVII, shows a decreased flexibility or freezing of the protein conformation of FVIIa with regard to TF. This results in the formation of a defective FVIIa-TF complex that justifies the different clotting results observed in these variants according to the TF used.

CONCLUSIONS

The conformational studies may supply useful information on the structure- function relation of clotting factors.

Structural biology of factor VIIa/tissue factor initiated coagulation

Factor VII (FVII) consists of an N-terminal gamma-carboxyglutamic acid domain followed by two epidermal growth factor-like (EGF1 and EGF2) domains and the C-terminal protease domain. Activation of FVII results in a two-chain FVIIa molecule consisting of a light chain (Gla-EGF1-EGF2 domains) and a heavy chain (protease domain) held together by a single disulfide bond. During coagulation, the complex of tissue factor (TF, a transmembrane glycoprotein) and FVIIa activates factor IX (FIX) and factor X (FX). FVIIa is structurally "zymogen-like" and when bound to TF, it is more "active enzyme-like." FIX and FX share structural homology with FVII. Three structural biology aspects of FVIIa/TF are presented in this review. One, regions in soluble TF (sTF) that interact with FVIIa as well as mapping of Ca2+, Mg2+, Na+ and Zn2+ sites in FVIIa and their functions; two, modeled interactive regions of Gla and EGF1 domains of FXa and FIXa with FVIIa/sTF; and three, incompletely formed oxyanion hole in FVIIa/sTF and its induction by substrate/inhibitor. Finally, an overview of the recognition elements in TF pathway inhibitor is provided.

Identification of exosite residues of factor Xa involved in recognition of PAR-2 on endothelial cells

Recent results have indicated that factor Xa (FXa) cleaves protease-activated receptor 2 (PAR-2) to elicit protective intracellular signaling responses in endothelial cells. In this study, we investigated the molecular determinants of the specificity of FXa interaction with PAR-2 by monitoring the cleavage of PAR-2 by FXa in endothelial cells transiently transfected with a PAR-2 cleavage reporter construct in which the extracellular domain of the receptor was fused to cDNA encoding for alkaline phosphatase. Comparison of the cleavage efficiency of PAR-2 by a series of FXa mutants containing mutations in different surface loops indicated that the acidic residues of 39-loop (Glu-36, Glu-37, and Glu-39) and the basic residues of 60-loop (Lys-62 and Arg-63), 148-loop (Arg-143, Arg-150, and Arg-154), and 162-helix (Arg-165 and Lys-169) contribute to the specificity of receptor recognition by FXa on endothelial cells. This was evidenced by significantly reduced activity of mutants toward PAR-2 expressed on transfected cells. The extent of loss in the PAR-2 cleavage activity of FXa mutants correlated with the extent of loss in their PAR-2-dependent intracellular signaling activity. Further characterization of FXa mutants indicated that, with the exception of basic residues of 162-helix, which play a role in the recognition specificity of the prothrombinase complex, none of the surface loop residues under study makes a significant contribution to the activity of FXa in the prothrombinase complex. These results provide new insight into mechanisms through which FXa specifically interacts with its macromolecular substrates in the clotting and signaling pathways.

Considerations on a tentative classification of FVII deficiency suited for practical clinical purposes

Adequate classifications of disorders are of paramount importance in the management of congenital bleeding disorders. Classification of congenital FVII deficiency should be simple, based on few tests using thromboplastins of different origin. The first thromboplastin to be used is a rabbit brain preparation since it has been proven that this is the one that, overall, yields the lowest activity level. This is particularly so since molecular biology techniques have supplied important information with regard to the structure-function relation but have failed to supply a satisfactory classification of the defect. Mutations in the same domain have yielded different forms of FVII deficiency. Furthermore, molecular biology techniques are time consuming and are not feasible in every laboratory. A classification of FVII deficiency based on clinical, clotting, and immunological assays is proposed. This classification is suited for practical clinical purposes and may represent a useful preparatory basis for molecular biology studies.

Protease signalling: the cutting edge

Protease research has undergone a major expansion in the last decade, largely due to the extremely rapid development of new technologies, such as quantitative proteomics and in-vivo imaging, as well as an extensive use of in-vivo models. These have led to identification of physiological substrates and resulted in a paradigm shift from the concept of proteases as protein-degrading enzymes to proteases as key signalling molecules. However, we are still at the beginning of an understanding of protease signalling pathways. We have only identified a minor subset of true physiological substrates for a limited number of proteases, and their physiological regulation is still not well understood. Similarly, links with other signalling systems are not well established. Herein, we will highlight current challenges in protease research.

Is EPCR a multi-ligand receptor? Pros and cons

In the last decade, the endothelial cell protein C/activated protein C receptor (EPCR) has received considerable attention. The role initially attributed to EPCR, i.e. the enhancement of protein C (PC) activation by the thrombin-thrombomodulin complex on the surface of the large vessels, although important, did not go beyond the haemostasis scenario. However, the discovery of the cytoprotective, anti-inflammatory and anti-apoptotic features of the activated PC (APC) and the required involvement of EPCR for APC to exert such actions did place the receptor in a privileged position in the crosstalk between coagulation and inflammation. The last five years have shown that PC/APC are not the only molecules able to interact with EPCR. Factor VII/VIIa (FVII/VIIa) and factor Xa (FXa), two other serine proteases that play a central role in haemostasis and are also involved in signalling processes influencing wound healing, tissue remodelling, inflammation or metastasis, have been reported to bind to EPCR. These observations have paved the way for an exploration of unsuspected new roles for the receptor. This review aims to offer a new image of EPCR in the light of its extended panel of ligands. A brief update of what is known about the APC-evoked EPCR-dependent cell signalling mechanisms is provided, but special care has been taken to assemble all the information available about the interaction of EPCR with FVII/VIIa and FXa.

Antibody-induced enhancement of factor VIIa activity through distinct allosteric pathways

In the absence of its cofactor tissue factor (TF), coagulation factor VIIa (FVIIa) predominantly exists in a zymogen-like, catalytically incompetent state. Here we demonstrate that conformation-specific monoclonal antibodies (mAbs) can be used to characterize structural features determining the activity of FVIIa. We isolated two classes of mAbs, which both increased the catalytic efficiency of FVIIa more than 150-fold. The effects of the antibodies were retained with a FVIIa variant, which has been shown to be inert to allosteric activation by the natural activator TF, suggesting that the antibodies and TF employ distinct mechanisms of activation. The antibodies could be classified into two groups based on their patterns of affinities for different conformations of FVIIa. Whereas one class of antibodies affected both the K(m) and k(cat), the other class mainly affected the K(m). The antibody-induced activity enhancement could be traced to maturation of the S1 substrate binding pocket and the oxyanion hole, evident by an increased affinity for p-aminobenzamidine, an increased rate of antithrombin inhibition, an increased rate of incorporation of diisopropylfluorophosphate, and an enhanced fraction of molecules with a buried N terminus of the catalytic domain in the presence of antibodies. As demonstrated by site-directed mutagenesis, the two groups of antibodies appear to have overlapping, although clearly different, epitopes in the 170-loop. Our findings suggest that binding of ligands to specific residues in the 170-loop or its spatial vicinity may stabilize the S1 pocket and the oxyanion hole, and they may have general implications for the molecular understanding of FVIIa regulatory mechanisms.

Natural and engineered carboxy-terminal variants: decreased secretion and gain-of-function result in asymptomatic coagulation factor VII deficiency

We report 2 asymptomatic homozygotes for the nonsense p.R462X mutation affecting the carboxy-terminus of coagulation factor VII (FVII, 466 aminoacids). FVII levels of 3-5% and 2.7 ± 0.4% were found in prothrombin time-based and activated factor X (FXa) generation assays with human thromboplastins. Noticeably, FVII antigen levels were barely detectable (0.7 ± 0.2%) which suggested a gain-of-function effect. This effect was more pronounced with bovine thromboplastin (4.8 ± 0.9%) and disappeared with rabbit thromboplastin (0.7 ± 0.2%). This suggests that the mutation influences tissue factor/FVII interactions. Whereas the recombinant rFVII-462X variant confirmed an increase in specific activity (~400%), a panel of nonsense (p.P466X, p.F465X, p.P464X, p.A463X) and missense (p.R462A, p.R462Q, p.R462W) mutations of the FVII carboxy-terminus resulted in reduced secretion but normal specific activity. These data provide evidence for counteracting pleiotropic effects of the p.R462X mutation, which explains the asymptomatic FVII deficiency, and contributes to our understanding of the role of the highly variable carboxy-terminus of coagulation serine proteases.

Allostery in trypsin-like proteases suggests new therapeutic strategies

Trypsin-like proteases (TLPs) are a large family of enzymes responsible for digestion, blood coagulation, fibrinolysis, development, fertilization, apoptosis and immunity. A current paradigm posits that the irreversible transition from an inactive zymogen to the active protease form enables productive interaction with substrate and catalysis. Analysis of the entire structural database reveals two distinct conformations of the active site: one fully accessible to substrate (E) and the other occluded by the collapse of a specific segment (E*). The allosteric E*-E equilibrium provides a reversible mechanism for activity and regulation in addition to the irreversible zymogen to protease conversion and points to new therapeutic strategies aimed at inhibiting or activating the enzyme. In this review, we discuss relevant examples, with emphasis on the rational engineering of anticoagulant thrombin mutants.

Tissue factor expression in ovarian cancer: implications for immunotherapy with hI-con1, a factor VII-IgGF(c) chimeric protein targeting tissue factor

We evaluated the expression of tissue factor (TF) in ovarian cancer (EOC) and the potential of hI-con1, an antibody-like molecule targeting TF, as a novel form of therapy against chemotherapy-resistant ovarian disease. We studied the expression of TF in 88 EOC by immunohistochemistry (IHC) and real-time-PCR (qRT-PCR) and the levels of membrane-bound-complement-regulatory-proteins CD46, CD55 and CD59 in primary EOC cell lines by flow-cytometry. Sensitivity to hI-con1-dependent-cell-mediated-cytotoxicity (IDCC), complement-dependent-cell-cytotoxicity and inhibition of IDCC by γ-immunoglobulin were evaluated in 5-h (51)chromium-release-assays. Cytoplasmic and/or membrane TF expression was observed in 24 out of 25 (96%) of the EOC samples tested by IHC, but not in normal ovarian-tissue. EOC with clear cell histology significantly overexpress TF when compared to serous, endometrioid, or undifferentiated tumors by qRT-PCR. With a single exception, all primary EOC that overexpressed TF demonstrated high levels of CD46, CD55 and CD59 and regardless of their histology or resistance to chemotherapy, were highly sensitive to IDCC. The effect of complement and physiologic doses of γ-immunoglobulin on IDCC in ovarian cancer cell lines overexpressing TF was tumor specific and related to the overexpression of CD59 on tumor cells. Small-interfering-RNA-mediated knockdown of CD59 expression in ovarian tumors significantly increased hI-con1-mediated cytotoxic activity in vitro. Finally, low doses of interleukin-2 further increased the cytotoxic effect induced by hI-con1 (P < 0.01). hI-con1 molecule induces strong cytotoxicity against primary chemotherapy-resistant ovarian cancer cell lines overexpressing TF and may represent a novel therapeutic agent for the treatment of ovarian tumors refractory to standard treatment modalities.

Crystallographic and kinetic evidence of allostery in a trypsin-like protease

Protein allostery is based on the existence of multiple conformations in equilibrium linked to distinct functional properties. Although evidence of allosteric transitions is relatively easy to identify by functional studies, structural detection of a pre-existing equilibrium between alternative conformations remains challenging even for textbook examples of allosteric proteins. Kinetic studies show that the trypsin-like protease thrombin exists in equilibrium between two conformations where the active site is either collapsed (E*) or accessible to substrate (E). However, structural demonstration that the two conformations exist in the same enzyme construct free of ligands has remained elusive. Here we report the crystal structure of the thrombin mutant N143P in the E form, which complements the recently reported structure in the E* form, and both the E and E* forms of the thrombin mutant Y225P. The side chain of W215 moves 10.9 Å between the two forms, causing a displacement of 6.6 Å of the entire 215-217 segment into the active site that in turn opens or closes access to the primary specificity pocket. Rapid kinetic measurements of p-aminobenzamidine binding to the active site confirm the existence of the E*-E equilibrium in solution for wild-type and the mutants N143P and Y225P. These findings provide unequivocal proof of the allosteric nature of thrombin and lend strong support to the recent proposal that the E*-E equilibrium is a key property of the trypsin fold.

Expression of tissue factor in adenocarcinoma and squamous cell carcinoma of the uterine cervix: implications for immunotherapy with hI-con1, a factor VII-IgGFc chimeric protein targeting tissue factor

Background

Cervical cancer continues to be an important worldwide health problem for women. Up to 35% of patients who are diagnosed with and appropriately treated for cervical cancer will recur and treatment results are poor for recurrent disease. Given these sobering statistics, development of novel therapies for cervical cancer remains a high priority. We evaluated the expression of Tissue Factor (TF) in cervical cancer and the potential of hI-con1, an antibody-like-molecule targeted against TF, as a novel form of immunotherapy against multiple primary cervical carcinoma cell lines with squamous- and adenocarcinoma histology.

Methods

Because TF is a transmembrane receptor for coagulation factor VII/VIIa (fVII), in this study we evaluated the in vitro expression of TF in cervical carcinoma cell lines by immunohistochemistry (IHC), real time-PCR (qRT-PCR) and flow cytometry. Sensitivity to hI-con1-dependent cell-mediated-cytotoxicity (IDCC) was evaluated in 5-hrs-⁵¹chromium-release-assays against cervical cancer cell lines in vitro.

Results

Cytoplasmic and/or membrane TF expression was observed in 8 out of 8 (100%) of the tumor tissues tested by IHC and in 100% (11 out of 11) of the cervical carcinoma cell lines tested by real-time-PCR and flow cytometry but not in normal cervical keratinocytes (p = 0.0023 qRT-PCR; p = 0.0042 flow cytometry). All primary cervical cancer cell lines tested overexpressing TF, regardless of their histology, were highly sensitive to IDCC (mean killing ± SD, 56.2% ± 15.9%, range, 32.4%-76.9%, p < 0.001), while negligible cytotoxicity was seen in the absence of hI-con1 or in the presence of rituximab-control-antibody. Low doses of interleukin-2 further increased the cytotoxic effect induced by hI-con1 (p = 0.025) while human serum did not significantly decrease IDCC against cervical cancer cell lines (p = 0.597).

Conclusions

TF is highly expressed in squamous and adenocarcinoma of the uterine cervix. hI-con1 induces strong cytotoxicity against primary cervical cancer cell lines overexpressing TF and may represent a novel therapeutic agent for the treatment of cervical cancer refractory to standard treatment modalities.

3D-interologs: an evolution database of physical protein- protein interactions across multiple genomes

Background

Comprehensive exploration of protein-protein interactions is a challenging route to understand biological processes. For efficiently enlarging protein interactions annotated with residue-based binding models, we proposed a new concept "3D-domain interolog mapping" with a scoring system to explore all possible protein pairs between the two homolog families, derived from a known 3D-structure dimmer (template), across multiple species. Each family consists of homologous proteins which have interacting domains of the template for studying domain interface evolution of two interacting homolog families.

Results

The 3D-interologs database records the evolution of protein-protein interactions database across multiple species. Based on "3D-domain interolog mapping" and a new scoring function, we infer 173,294 protein-protein interactions by using 1,895 three-dimensional (3D) structure heterodimers to search the UniProt database (4,826,134 protein sequences). The 3D- interologs database comprises 15,124 species and 283,980 protein-protein interactions, including 173,294 interactions (61%) and 110,686 interactions (39%) summarized from the IntAct database. For a protein-protein interaction, the 3D-interologs database shows functional annotations (e.g. Gene Ontology), interacting domains and binding models (e.g. hydrogen-bond interactions and conserved residues). Additionally, this database provides couple-conserved residues and the interacting evolution by exploring the interologs across multiple species. Experimental results reveal that the proposed scoring function obtains good agreement for the binding affinity of 275 mutated residues from the ASEdb. The precision and recall of our method are 0.52 and 0.34, respectively, by using 563 non-redundant heterodimers to search on the Integr8 database (549 complete genomes).

Conclusions

Experimental results demonstrate that the proposed method can infer reliable physical protein-protein interactions and be useful for studying the protein-protein interaction evolution across multiple species. In addition, the top-ranked strategy and template interface score are able to significantly improve the accuracies of identifying protein-protein interactions in a complete genome. The 3D-interologs database is available at http://3D- interologs.life.nctu.edu.tw.

hI-con1, a factor VII-IgGFc chimeric protein targeting tissue factor for immunotherapy of uterine serous papillary carcinoma

Background:

Uterine serous papillary adenocarcinoma (USPC) is a highly aggressive variant of endometrial cancer. Human immuno-conjugate molecule (hI-con1) is an antibody-like molecule targeted against tissue factor (TF), composed of two human Factor VII (fVII) as the targeting domain, fused to human immunoglobulin (Ig) G1 Fc as an effector domain. We evaluated hI-con1 potential activity against primary chemotherapy-resistant USPC cell lines expressing different levels of TF.

Methods:

A total of 16 formalin-fixed, paraffin-embedded USPC samples were evaluated by immunohistochemistry (IHC) for TF expression. Six primary USPC cell lines, half of which overexpress the epidermal growth factor type II (HER2/neu) receptor at 3+ levels, were assessed by flow cytometry and real-time PCR for TF expression. Sensitivity to hI-con1-dependent cell-mediated cytotoxicity (IDCC) was evaluated in 5-hour-chromium release assays. Finally, to investigate the effect of interleukin-2 (IL-2) on IDCC, 5-h ⁵¹Cr assays were also conducted in the presence of low doses of IL-2 (i.e., 50–100 IU ml⁻¹).

Results:

Cytoplasmic and/or membrane TF expression was observed in all 16 (100%) USPC samples tested by IHC, but not in normal endometrium. High expression of TF was found in 50% (three out of six) of the USPC cell lines tested by real-time PCR and flow cytometry when compared with normal endometrial cells (NECs; P<0.001). Uterine serous papillary adenocarcinoma cell lines overexpressing TF, regardless of their high or low HER2/neu expression, were highly sensitive to IDCC (mean killing±s.d., 65.6±3.7%, range 57.5–77.0%, P<0.001), although negligible cytotoxicity against USPC was seen in the absence of hI-con1 or in the presence of Rituximab control antibody. The addition of low doses of IL-2 further increased the cytotoxic effect induced by hI-con1 against chemotherapy-resistant USPC.

Conclusion:

hI-con1 induces strong cytotoxicity against primary chemotherapy-resistant USPC cell lines overexpressing TF. The hI-con1 may represent a novel therapeutic agent for the treatment of patients harbouring advanced, recurrent and/or metastatic USPC refractory to standard treatment modalities.

Targeting tissue factor on tumour cells and angiogenic vascular endothelial cells by factor VII-targeted verteporfin photodynamic therapy for breast cancer in vitro and in vivo in mice

Background

The objective of this study was to develop a ligand-targeted photodynamic therapy (tPDT) by conjugating factor VII (fVII) protein with photosensitiser verteporfin in order to overcome the poor selectivity and enhance the effect of non-targeted PDT (ntPDT) for cancer. fVII is a natural ligand for receptor tissue factor (TF) with high affinity and specificity. The reason for targeting receptor TF for the development of tPDT is that TF is a common but specific target on angiogenic tumour vascular endothelial cells (VEC) and many types of tumour cells, including solid tumours and leukaemia.

Methods

Murine factor VII protein (mfVII) containing a mutation (Lys341Ala) was covalently conjugated via a cross linker EDC with Veterporfin (VP) that was extracted from liposomal Visudyne, and then free VP was separated by Sephadex G50 spin columns. fVII-tPDT using mfVII-VP conjugate, compared to ntPDT, was tested in vitro for the killing of breast cancer cells and VEGF-stimulated VEC and in vivo for inhibiting the tumour growth of breast tumours in a mouse xenograft model.

Results

We showed that: (i) fVII protein could be conjugated with VP without affecting its binding activity; (ii) fVII-tPDT could selectively kill TF-expressing breast cancer cells and VEGF-stimulated angiogenic HUVECs but had no side effects on non-TF expressing unstimulated HUVEC, CHO-K1 and 293 cells; (iii) fVII targeting enhanced the effect of VP PDT by three to four fold; (iii) fVII-tPDT induced significantly stronger levels of apoptosis and necrosis than ntPDT; and (iv) fVII-tPDT had a significantly stronger effect on inhibiting breast tumour growth in mice than ntPDT.

Conclusions

We conclude that the fVII-targeted VP PDT that we report here is a novel and effective therapeutic with improved selectivity for the treatment of breast cancer. Since TF is expressed on many types of cancer cells including leukaemic cells and selectively on angiogenic tumour VECs, fVII-tPDT could have broad therapeutic applications for other solid cancers and leukaemia.

Engineering of substrate selectivity for tissue factor.factor VIIa complex signaling through protease-activated receptor 2

The complex of factor VIIa (FVIIa) with tissue factor (TF) triggers coagulation by recognizing its macromolecular substrate factors IX (FIX) and X (FX) predominantly through extended exosite interactions. In addition, TF mediates unique cell-signaling properties in cancer, angiogenesis, and inflammation that involve proteolytic cleavage of protease-activated receptor 2 (PAR2). PAR2 is cleaved by FVIIa in the binary TF.FVIIa complex and by FXa in the ternary TF.FVIIa.FXa complex, but physiological roles of these signaling complexes are incompletely understood. In a screen of FVIIa protease domain mutants, three variants (Q40A, Q143N, and T151S) activated macromolecular coagulation substrates and supported signaling of the ternary TF.FVIIa-Xa complex normally but were severely impaired in binary TF.FVIIa.PAR2 signaling. The residues identified were located in the model-predicted S2' pocket of FVIIa, and complementary PAR2 P2' Leu-38 replacements demonstrated that the P2' side chain was indeed crucial for PAR2 cleavage by TF.FVIIa. In addition, PAR2 was activated more efficiently by FVIIa T99Y, consistent with further contributions from the S2 subsite. The P2 residue preference of FVIIa and FXa predicted additional PAR2 mutants that were efficiently activated by TF.FVIIa but resistant to cleavage by the alternative PAR2 activator FXa. Thus, contrary to the paradigm of exosite-assisted cleavage of PAR1 by thrombin, the cofactor-associated protease FVIIa recognizes PAR2 predominantly by catalytic cleft interactions. Furthermore, the delineated molecular details of this substrate interaction enabled protein engineering of protease-selective PAR2 receptors that will aid further studies to dissect the roles of TF signaling complexes in vivo.

The immunoconjugate "icon" targets aberrantly expressed endothelial tissue factor causing regression of endometriosis

Endometriosis is a major cause of chronic pain, infertility, medical and surgical interventions, and health care expenditures. Tissue factor (TF), the primary initiator of coagulation and a modulator of angiogenesis, is not normally expressed by the endothelium; however, prior studies have demonstrated that both blood vessels in solid tumors and choroidal tissue in macular degeneration express endothelial TF. The present study describes the anomalous expression of TF by endothelial cells in endometriotic lesions. The immunoconjugate molecule (Icon), which binds with high affinity and specificity to this aberrant endothelial TF, has been shown to induce a cytolytic immune response that eradicates tumor and choroidal blood vessels. Using an athymic mouse model of endometriosis, we now report that Icon largely destroys endometriotic implants by vascular disruption without apparent toxicity, reduced fertility, or subsequent teratogenic effects. Unlike antiangiogenic treatments that can only target developing angiogenesis, Icon eliminates pre-existing pathological vessels. Thus, Icon could serve as a novel, nontoxic, fertility-preserving, and effective treatment for endometriosis.

Functional role of O-linked and N-linked glycosylation sites present on the activation peptide of factor X

BACKGROUND

There are two O-linked and two N-linked glycosylation sites on the activation peptide of factor X (FX) involving residues Thr-17, Thr-29, Asn-39 and Asn-49.

OBJECTIVES

The purpose of this study was to explore the contribution of carbohydrates of the FX activation peptide to zymogen recognition by physiological activators.

METHODS

The putative glycosylation sites were substituted individually or in combinations with Ala and mutants were expressed in mammalian cells. The entire activation peptide up to the P3 residue was deleted in another construct.

RESULTS

It was discovered that activation of zymogen mutants by both FVIIa and FIXa on negatively charged phospholipid vesicles has been improved 2-40-fold independent of a cofactor. These mutants were activated with slightly lower catalytic efficiency (k(cat)/K(m)) by FVIIa in the extrinsic Xase complex, though both K(m) and k(cat) constants for mutants were elevated. With the exception of approximately 3-fold improvement in the activation of N49A, the catalytic efficiency of FIXa toward mutants was decreased 2-5-fold in the intrinsic Xase complex.

CONCLUSIONS

The carbohydrate chains of the FX activation peptide play an important role in restricting the specificity of zymogen recognition by both FVIIa and FIXa, thereby preventing the cofactor-independent activation of FX by these proteases. On the other hand, the carbohydrates contribute to the cofactor-dependent recognition of the zymogen by both extrinsic and intrinsic Xase complexes.

Activation loop 3 and the 170 loop interact in the active conformation of coagulation factor VIIa

The initiation of blood coagulation involves tissue factor (TF)-induced allosteric activation of factor VIIa (FVIIa), which circulates in a zymogen-like state. In addition, the (most) active conformation of FVIIa presumably relies on a number of intramolecular interactions. We have characterized the role of Gly372(223) in FVIIa, which is the sole residue in activation loop 3 that is capable of forming backbone hydrogen bonds with the unusually long 170 loop and with activation loop 2, by studying the effects of replacement with Ala [G372(223)A]. G372A-FVIIa, both in the free and TF-bound form, exhibited reduced cleavage of factor X (FX) and of peptidyl substrates, and had increased K(m) values compared with wild-type FVIIa. Inhibition of G372A-FVIIa.sTF by p-aminobenzamidine was characterized by a seven-fold higher K(i) than obtained with FVIIa.sTF. Crystallographic and modelling data suggest that the most active conformation of FVIIa depends on the backbone hydrogen bond between Gly372(223) and Arg315(170C) in the 170 loop. Despite the reduced activity and inhibitor susceptibility, native and active site-inhibited G372A-FVIIa bound sTF with the same affinity as the corresponding forms of FVIIa, and burial of the N-terminus of the protease domain increased similarly upon sTF binding to G372A-FVIIa and FVIIa. Thus Gly372(223) in FVIIa appears to play a critical role in maturation of the S1 pocket and adjacent subsites, but does not appear to be of importance for TF binding and the ensuing allostery.

Mechanism of the Ca2+-induced enhancement of the intrinsic factor VIIa activity

The intrinsic activity of coagulation factor VIIa (FVIIa) is dependent on Ca(2+) binding to a loop (residues 210-220) in the protease domain. Structural analysis revealed that Ca(2+) may enhance the activity by attenuating electrostatic repulsion of Glu(296) and/or by facilitating interactions between the loop and Lys(161) in the N-terminal tail. In support of the first mechanism, the mutations E296V and D212N resulted in similar, about 2-fold, enhancements of the amidolytic activity. Moreover, mutation of the Lys(161)-interactive residue Asp(217) or Asp(219) to Ala reduced the amidolytic activity by 40-50%, whereas the K161A mutation resulted in 80% reduction. Hence one of these Asp residues in the Ca(2+)-binding loop appears to suffice for some residual interaction with Lys(161), whereas the more severe effect upon replacement of Lys(161) is due to abrogation of the interaction with the N-terminal tail. However, Ca(2+) attenuation of the repulsion between Asp(212) and Glu(296) keeps the activity above that of apoFVIIa. Altogether, our data suggest that repulsion involving Asp(212) in the Ca(2+)-binding loop suppresses FVIIa activity and that optimal activity requires a favorable interaction between the Ca(2+)-binding loop and the N-terminal tail. Crystal structures of tissue factor-bound FVIIa(D212N) and FVIIa(V158D/E296V/M298Q) revealed altered hydrogen bond networks, resembling those in factor Xa and thrombin, after introduction of the D212N and E296V mutations plausibly responsible for tethering the N-terminal tail to the activation domain. The charge repulsion between the Ca(2+)-binding loop and the activation domain appeared to be either relieved by charge removal and new hydrogen bonds (D212N) or abolished (E296V). We propose that Ca(2+) stimulates the intrinsic FVIIa activity by a combination of charge neutralization and loop stabilization.

Targeting tissue factor-expressing tumor angiogenesis and tumors with EF24 conjugated to factor VIIa

Tissue factor (TF) is aberrantly expressed on tumor vascular endothelial cells (VECs) and on cancer cells in many malignant tumors, but not on normal VECs, making it a promising target for cancer therapy. As a transmembrane receptor for coagulation factor VIIa (fVIIa), TF forms a high-affinity complex with its cognate ligand, which is subsequently internalized through receptor-mediated endocytosis. Accordingly, we developed a method for selectively delivering EF24, a potent synthetic curcumin analog, to TF-expressing tumor vasculature and tumors using fVIIa as a drug carrier. EF24 was chemically conjugated to fVIIa through a tripeptide-chloromethyl ketone. After binding to TF-expressing targets by fVIIa, EF24 will be endocytosed along with the drug carrier and will exert its cytotoxicity. Our results showed that the conjugate inhibits vascular endothelial growth factor-induced angiogenesis in a rabbit cornea model and in a Matrigel model in athymic nude mice. The conjugate-induced apoptosis in tumor cells and significantly reduced tumor size in human breast cancer xenografts in athymic nude mice as compared with the unconjugated EF24. By conjugating potent drugs to fVIIa, this targeted drug delivery system has the potential to enhance therapeutic efficacy, while reducing toxic side effects. It may also prove to be useful for treating drug-resistant tumors and micro-metastases in addition to primary tumors.

The origins of enhanced activity in factor VIIa analogs and the interplay between key allosteric sites revealed by hydrogen exchange mass spectrometry

Factor VIIa (FVIIa) circulates in the blood in a zymogen-like state. Only upon association with membrane-bound tissue factor (TF) at the site of vascular injury does FVIIa become active and able to initiate blood coagulation. Here we used hydrogen exchange monitored by mass spectrometry to investigate the conformational effects of site-directed mutagenesis at key positions in FVIIa and the origins of enhanced intrinsic activity of FVIIa analogs. The differences in hydrogen exchange of two highly active variants, FVIIa(DVQ) and FVIIa(VEAY), imply that enhanced catalytic efficiency was attained by two different mechanisms. Regions protected from exchange in FVIIa(DVQ) include the N-terminal tail and the activation pocket, which is a subset of the regions of FVIIa protected from exchange upon TF binding. FVIIa(DVQ) appeared to adopt an intermediate conformation between the free (zymogen-like) and TF-bound (active) form of FVIIa and to attain enhanced activity by partial mimicry of TF-induced activation. In contrast, exchange-protected regions in FVIIa(VEAY) were confined to the vicinity of the active site of FVIIa. Thus, the changes in FVIIa(VEAY) appeared to optimize the active site region rather than imitate the TF-induced effect. Hydrogen exchange analysis of the FVIIa(M306D) variant, which was unresponsive to stimulation by TF, correlated widespread reductions in exchange to the single mutation in the TF-binding region. These results reveal the delicate interplay between key allosteric sites necessary to achieve the transition of FVIIa into the active form.

The haemostatic role of tissue factor pathway inhibitor

Under normal conditions the blood circulates freely within the confines of the vascular system, carrying oxygen, nutrients, and hormonal information around the body and removing metabolic waste. If blood gains access to extravascular sites, or the vasculature becomes pathologically challenged, hemostasis may be activated. This process is finely regulated by positive and negative feedback loops that modulate fibrin clot formation. Blood coagulation revolves around the activation and assembly of the components of the prothrombinase complex, which converts the inactive zymogen, prothrombin, into its active form, thrombin. This serine protease catalyzes the conversion of fibrinogen to fibrin, the structural scaffold that stabilizes platelet aggregates at sites of vascular injury. The extent of the hemostatic response is controlled by the action of inhibitory pathways, which ensure that thrombin activity and the spread of the hemostatic plug is limited to the site of vessel damage. This review article focuses on the major physiological regulator of tissue factor-induced coagulation, tissue factor pathway inhibitor, its expression, anticoagulant function, and its role in normal hemostasis.