Anthrax toxin protective antigen is activated by a cell surface protease with the sequence specificity and catalytic properties of furin

The proprotein convertase furin in tumour progression

Proprotein convertases are proteases that have been implicated in the activation of a wide variety of proteins. These proteins are generally synthesised as precursor proteins and require limited proteolysis for conversion into their mature bioactive counterparts. Many of these proteins, including metalloproteases, growth factors and their receptors or adhesion molecules, have been shown to facilitate tumour formation and progression. Hence, this review will focus on the proprotein convertase furin and its role in cancer. The expression of furin has been confirmed in a large spectrum of cancers such as head and neck squamous cell carcinoma, breast cancer and rhabdomyosarcoma. Functional studies modulating furin activity uncovered its importance for the processing of many cancer-related substrates and strongly indicate that high furin activity promotes the malignant phenotype of cancer cells. In this review, we summarise the expression and function of furin in different cancer types, discuss its role in processing cancer-related proproteins and give examples of potential therapeutic approaches that take advantage of the proteolytic activity of furin in cancer cells.

Targeted delivery of an ADP-ribosylating bacterial toxin into cancer cells

The actin cytoskeleton is an attractive target for bacterial toxins. The ADP-ribosyltransferase TccC3 from the insect bacterial pathogen Photorhabdus luminescence modifies actin to force its aggregation. We intended to transport the catalytic part of this toxin preferentially into cancer cells using a toxin transporter (Protective antigen, PA) which was redirected to Epidermal Growth Factor Receptors (EGFR) or to human EGF receptors 2 (HER2), which are overexpressed in several cancer cells. Protective antigen of anthrax toxin forms a pore through which the two catalytic parts (lethal factor and edema factor) or other proteins can be transported into mammalian cells. Here, we used PA as a double mutant (N682A, D683A; mPA) which cannot bind to the two natural anthrax receptors. Each mutated monomer is fused either to EGF or to an affibody directed against the human EGF receptor 2 (HER2). We established a cellular model system composed of two cell lines representing HER2 overexpressing esophageal adenocarcinomas (EACs) and EGFR overexpressing esophageal squamous cell carcinomas (ESCCs). We studied the specificity and efficiency of the re-directed anthrax pore for transport of TccC3 toxin and established Photorhabdus luminescence TccC3 as a toxin suitable for the development of a targeted toxin selectively killing cancer cells.

Alternative pre-approved and novel therapies for the treatment of anthrax

BACKGROUND

Bacillus anthracis, the causative agent of anthrax, is a spore forming and toxin producing rod-shaped bacterium that is classified as a category A bioterror agent. This pathogenic microbe can be transmitted to both animals and humans. Clinical presentation depends on the route of entry (direct contact, ingestion, injection or aerosolization) with symptoms ranging from isolated skin infections to more severe manifestations such as cardiac or pulmonary shock, meningitis, and death. To date, anthrax is treatable if antibiotics are administered promptly and continued for 60 days. However, if treatment is delayed or administered improperly, the patient's chances of survival are decreased drastically. In addition, antibiotics are ineffective against the harmful anthrax toxins and spores. Therefore, alternative therapeutics are essential. In this review article, we explore and discuss advances that have been made in anthrax therapy with a primary focus on alternative pre-approved and novel antibiotics as well as anti-toxin therapies.

METHODS

A literature search was conducted using the University of Manitoba search engine. Using this search engine allowed access to a greater variety of journals/articles that would have otherwise been restricted for general use. In order to be considered for discussion for this review, all articles must have been published later than 2009.

RESULTS

The alternative pre-approved antibiotics demonstrated high efficacy against B. anthracis both in vitro and in vivo. In addition, the safety profile and clinical pharmacology of these drugs were already known. Compounds that targeted underexploited bacterial processes (DNA replication, RNA synthesis, and cell division) were also very effective in combatting B. anthracis. In addition, these novel compounds prevented bacterial resistance. Targeting B. anthracis virulence, more specifically the anthrax toxins, increased the length of which treatment could be administered.

CONCLUSIONS

Several novel and pre-existing antibiotics, as well as toxin inhibitors, have shown increasing promise. A combination treatment that targets both bacterial growth and toxin production would be ideal and probably necessary for effectively combatting this armed bacterium.

Cross-inhibition of pathogenic agents and the host proteins they exploit

The major limitations of pathogen-directed therapies are the emergence of drug-resistance and their narrow spectrum of coverage. A recently applied approach directs therapies against host proteins exploited by pathogens in order to circumvent these limitations. However, host-oriented drugs leave the pathogens unaffected and may result in continued pathogen dissemination. In this study we aimed to discover drugs that could simultaneously cross-inhibit pathogenic agents, as well as the host proteins that mediate their lethality. We observed that many pathogenic and host-assisting proteins belong to the same functional class. In doing so we targeted a protease component of anthrax toxin as well as host proteases exploited by this toxin. We identified two approved drugs, ascorbic acid 6-palmitate and salmon sperm protamine, that effectively inhibited anthrax cytotoxic protease and demonstrated that they also block proteolytic activities of host furin, cathepsin B, and caspases that mediate toxin's lethality in cells. We demonstrated that these drugs are broad-spectrum and reduce cellular sensitivity to other bacterial toxins that require the same host proteases. This approach should be generally applicable to the discovery of simultaneous pathogen and host-targeting inhibitors of many additional pathogenic agents.

Targeting the membrane-anchored serine protease testisin with a novel engineered anthrax toxin prodrug to kill tumor cells and reduce tumor burden

The membrane-anchored serine proteases are a unique group of trypsin-like serine proteases that are tethered to the cell surface via transmembrane domains or glycosyl-phosphatidylinositol-anchors. Overexpressed in tumors, with pro-tumorigenic properties, they are attractive targets for protease-activated prodrug-like anti-tumor therapies. Here, we sought to engineer anthrax toxin protective antigen (PrAg), which is proteolytically activated on the cell surface by the proprotein convertase furin to instead be activated by tumor cell-expressed membrane-anchored serine proteases to function as a tumoricidal agent. PrAg's native activation sequence was mutated to a sequence derived from protein C inhibitor (PCI) that can be cleaved by membrane-anchored serine proteases, to generate the mutant protein PrAg-PCIS. PrAg-PCIS was resistant to furin cleavage in vitro, yet cytotoxic to multiple human tumor cell lines when combined with FP59, a chimeric anthrax toxin lethal factor-Pseudomonas exotoxin fusion protein. Molecular analyses showed that PrAg-PCIS can be cleaved in vitro by several serine proteases including the membrane-anchored serine protease testisin, and mediates increased killing of testisin-expressing tumor cells. Treatment with PrAg-PCIS also potently attenuated the growth of testisin-expressing xenograft tumors in mice. The data indicates PrAg can be engineered to target tumor cell-expressed membrane-anchored serine proteases to function as a potent tumoricidal agent.

The BR domain of PsrP interacts with extracellular DNA to promote bacterial aggregation; structural insights into pneumococcal biofilm formation

The major human pathogen Streptococcus pneumoniae is a leading cause of disease and death worldwide. Pneumococcal biofilm formation within the nasopharynx leads to long-term colonization and persistence within the host. We have previously demonstrated that the capsular surface-associated pneumococcal serine rich repeat protein (PsrP), key factor for biofilm formation, binds to keratin-10 (KRT10) through its microbial surface component recognizing adhesive matrix molecule (MSCRAMM)-related globular binding region domain (BR_187–385). Here, we show that BR_187–385 also binds to DNA, as demonstrated by electrophoretic mobility shift assays and size exclusion chromatography. Further, heterologous expression of BR_187–378 or the longer BR_120–378 construct on the surface of a Gram-positive model host bacterium resulted in the formation of cellular aggregates that was significantly enhanced in the presence of DNA. Crystal structure analyses revealed the formation of BR_187–385 homo-dimers via an intermolecular β-sheet, resulting in a positively charged concave surface, shaped to accommodate the acidic helical DNA structure. Furthermore, small angle X-ray scattering and circular dichroism studies indicate that the aggregate-enhancing N-terminal region of BR_120–166 adopts an extended, non-globular structure. Altogether, our results suggest that PsrP adheres to extracellular DNA in the biofilm matrix and thus promotes pneumococcal biofilm formation.

Anthrax Toxin Protective Antigen Variants That Selectively Utilize either the CMG2 or TEM8 Receptors for Cellular Uptake and Tumor Targeting

The protective antigen (PA) moiety of anthrax toxin binds to cellular receptors and mediates the translocation of the two enzymatic moieties of the toxin to the cytosol. Two PA receptors are known, with capillary morphogenesis protein 2 (CMG2) being the more important for pathogenesis and tumor endothelial marker 8 (TEM8) playing a minor role. The C-terminal PA domain 4 (PAD4) has extensive interactions with the receptors and is required for binding. Our previous study identified PAD4 variants having enhanced TEM8 binding specificity. To obtain PA variants that selectively bind to CMG2, here we performed phage display selections using magnetic beads having bound CMG2. We found that PA residue isoleucine 656 plays a critical role in PA binding to TEM8 but has a much lesser effect on PA binding to CMG2. We further characterized the role of residue 656 in distinguishing PA binding to CMG2 versus TEM8 by substituting it with the other 19 amino acids. Of the resulting variants, PA I656Q and PA I656V had significantly reduced activity on TEM8-expressing CHO cells but maintained their activity on CMG2-expressing CHO cells. The preference of these PA mutants for CMG2 over TEM8 was further demonstrated using mouse embryonic fibroblast cells and mice deficient in the CMG2 and/or the TEM8 receptors. The structural basis of the alterations in the receptor binding activities of these mutants is also discussed.

Protection of the Furin Cleavage Site in Low-Toxicity Immunotoxins Based on Pseudomonas Exotoxin A

Recombinant immunotoxins (RITs) are fusions of an Fv-based targeting moiety and a toxin. Pseudomonas exotoxin A (PE) has been used to make several immunotoxins that have been evaluated in clinical trials. Immunogenicity of the bacterial toxin and off-target toxicity have limited the efficacy of these immunotoxins. To address these issues, we have previously made RITs in which the Fv is connected to domain III (PE24) by a furin cleavage site (FCS), thereby removing unneeded sequences of domain II. However, the PE24 containing RITs do not contain the naturally occurring disulfide bond around the furin cleavage sequence, because it was removed when domain II was deleted. This could potentially allow PE24 containing immunotoxins to be cleaved and inactivated before internalization by cell surface furin or other proteases in the blood stream or tumor microenvironment. Here, we describe five new RITs in which a disulfide bond is engineered to protect the FCS. The most active of these, SS1-Fab-DS3-PE24, shows a longer serum half-life than an RIT without the disulfide bond and has the same anti-tumor activity, despite being less cytotoxic in vitro. These results have significance for the production of de-immunized, low toxicity, PE24-based immunotoxins with a longer serum half-life.

Temperature-mediated recombinant anthrax protective antigen aggregate development: Implications for toxin formation and immunogenicity

Anthrax vaccines containing recombinant PA (rPA) as the only antigen face a stability issue: rPA forms aggregates in solution after exposure to temperatures ⩾40°C, thus losing its ability to form lethal toxin (LeTx) with Lethal Factor. To study rPA aggregation's impact on immune response, we subjected rPA to several time and temperature combinations. rPA treated at 50°C for 30min formed high mass aggregates when analyzed by gel electrophoresis and failed to form LeTx as measured by a macrophage lysis assay (MLA). Aggregated rPA-formed LeTx was about 30 times less active than LeTx containing native rPA. Mice immunized with heat-treated rPA combined with Al(OH)3 developed antibody titers about 49 times lower than mice immunized with native rPA, as measured by a Toxicity Neutralization Assay (TNA). Enzyme Linked Immunosorbent Assay (ELISA) of the same immune sera showed anti-rPA titers only 2-7 times lower than titers elicited by native rPA. Thus, rPA's ability to form LeTx correlates with its production of neutralizing antibodies, and aggregation significantly impairs the protein's antibody response. However, while these findings suggest MLA has some value as an in-process quality test for rPA in new anthrax vaccines, they also confirm the superiority of TNA for use in vaccine potency.

Delivery of Non-Native Cargo into Mammalian Cells Using Anthrax Lethal Toxin

The intracellular delivery of peptide and protein therapeutics is a major challenge due to the plasma membrane, which acts as a barrier between the extracellular environment and the intracellular milieu. Over the past two decades, a nontoxic PA/LFN delivery platform derived from anthrax lethal toxin has been developed for the transport of non-native cargo into the cytosol of cells in order to understand the translocation process through a protective antigen (PA) pore and to probe intracellular biological functions. Enzyme-mediated ligation using sortase A and native chemical ligation are two facile methods used to synthesize these non-native conjugates, inaccessible by recombinant technology. Cargo molecules that translocate efficiently include enzymes from protein toxins, antibody mimic proteins, and peptides of varying lengths and non-natural amino acid compositions. The PA pore has been found to effectively convey over 30 known cargos other than native lethal factor (LF; i.e., non-native) with diverse sequences and functionalities on the LFN transporter protein. All together these studies demonstrated that non-native cargos must adopt an unfolded or extended conformation and contain a suitable charge composition in order to efficiently pass through the PA pore. This review provides insight into design parameters for the efficient delivery of new cargos using PA and LFN.

The Ins and Outs of Anthrax Toxin

Anthrax is a severe, although rather rare, infectious disease that is caused by the Gram-positive, spore-forming bacterium Bacillus anthracis. The infectious form is the spore and the major virulence factors of the bacterium are its poly-γ-D-glutamic acid capsule and the tripartite anthrax toxin. The discovery of the anthrax toxin receptors in the early 2000s has allowed in-depth studies on the mechanisms of anthrax toxin cellular entry and translocation from the endocytic compartment to the cytoplasm. The toxin generally hijacks the endocytic pathway of CMG2 and TEM8, the two anthrax toxin receptors, in order to reach the endosomes. From there, the pore-forming subunit of the toxin inserts into endosomal membranes and enables translocation of the two catalytic subunits. Insertion of the pore-forming unit preferentially occurs in intraluminal vesicles rather than the limiting membrane of the endosome, leading to the translocation of the enzymatic subunits in the lumen of these vesicles. This has important consequences that will be discussed. Ultimately, the toxins reach the cytosol where they act on their respective targets. Target modification has severe consequences on cell behavior, in particular on cells of the immune system, allowing the spread of the bacterium, in severe cases leading to host death. Here we will review the literature on anthrax disease with a focus on the structure of the toxin, how it enters cells and its immunological effects.

Soluble Expression and Characterization of Biologically Active Bacillus anthracis Protective Antigen in Escherichia coli

Bacillus anthracis secretory protein protective antigen (PA) is primary candidate for subunit vaccine against anthrax. Attempts to obtain large quantity of PA from Escherichia coli expression system often result in the formation of insoluble inclusion bodies. Therefore, it is always better to produce recombinant proteins in a soluble form. In the present study, we have obtained biologically active recombinant PA in small scale E. coli shake culture system using three different expression constructs. The PA gene was cloned in expression vectors bearing trc, T5, and T7 promoters and transformed into their respective E. coli hosts. The growth conditions were optimized to obtain maximum expression of PA in soluble form. The expression construct PA-pET32c in DE3-pLysS E. coli host resulted in a maximum production of soluble PA (15 mg L(-1)) compared to other combinations. Purified PA was subjected to trypsin digestion and binding assay with lethal factor to confirm the protein's functionality. Biological activity was confirmed by cytotoxicity assay on J774.1 cells. Balb/c mice were immunized with PA and the immunogenicity was tested by ELISA and toxin neutralization assay. This study highlights the expression of soluble and biologically active recombinant PA in larger quantity using simpler E. coli production platform.

Roles of Anthrax Toxin Receptor 2 in Anthrax Toxin Membrane Insertion and Pore Formation

Interaction between bacterial toxins and cellular surface receptors is an important component of the host-pathogen interaction. Anthrax toxin protective antigen (PA) binds to the cell surface receptor, enters the cell through receptor-mediated endocytosis, and forms a pore on the endosomal membrane that translocates toxin enzymes into the cytosol of the host cell. As the major receptor for anthrax toxin in vivo, anthrax toxin receptor 2 (ANTXR2) plays an essential role in anthrax toxin action by providing the toxin with a high-affinity binding anchor on the cell membrane and a path of entry into the host cell. ANTXR2 also acts as a molecular clamp by shifting the pH threshold of PA pore formation to a more acidic pH range, which prevents premature pore formation at neutral pH before the toxin reaches the designated intracellular location. Most recent studies have suggested that the disulfide bond in the immunoglobulin (Ig)-like domain of ANTXR2 plays an essential role in anthrax toxin action. Here we will review the roles of ANTXR2 in anthrax toxin action, with an emphasis on newly updated knowledge.

Furin as proprotein convertase and its role in normal and pathological biological processes

Furin belongs to serine intracellular Ca2+-dependent endopeptidases of the subtilisin family, also known as proprotein convertase (PC). Human furin is synthesized as zymogen with a molecular weight of 104 kDa, which is then activated by autocatalytic in two stages. This process can occur when zymogen migrates from the endoplasmic reticulum to the Golgi apparatus, where a large part of furin is accumulated. The molecular weigh t of the active furin is 98 kDa. Furin relates to enzymes with a narrow substrate specificity: it hydrolyzes peptide bonds at the site of paired basic amino acids and furin activity exhibits in a wide pH range 5-8. Its main biological function is activation of the functionally important protein precursors. It is accompanied by the launch of a cascade of reactions, which lead to appearance of biologically active molecules involved in realization of specific biological functions both in normal and in some patologicheskih processes. Furin substrates are biologically important proteins such as enzymes, hormones, growth factors and differentiation, receptors, adhesion proteins, proteins of blood plasma. Furin plays an important role in the development of processes such as proliferation, invasion, cell migration, survival, maintenance of homeostasis, embryogenesis, as well as the development of a number of pathologies, including cardiovascular, oncologic and neurodegenerative diseases. Furin and furin-like proprotein convertases participate as key factors in the realization of the regulatory functions of proteolytic enzymes, the value of which is currently being evaluated as most important in comparison with the degradative function of proteases.

A d-Amino Acid at the N-Terminus of a Protein Abrogates Its Degradation by the N-End Rule Pathway

Eukaryotes have evolved the ubiquitin (Ub)/proteasome system to degrade polypeptides. The Ub/proteasome system is one way that cells regulate cytosolic protein and amino acids levels through the recognition and ubiquitination of a protein's N-terminus via E1, E2, and E3 enzymes. The process by which the N-terminus stimulates intracellular protein degradation is referred to as the N-end rule. Characterization of the N-end rule has been limited to only the natural l-amino acids. Using a cytosolic delivery platform derived from anthrax lethal toxin, we probed the stability of mixed chirality proteins, containing one d-amino acid on the N-terminus of otherwise all l-proteins. In all cases, we observed that one N-terminal d-amino acid stabilized the cargo protein to proteasomal degradation with respect to the N-end rule. We found that since the mixed chirality proteins were not polyubiquitinated, they evaded N-end-mediated proteasomal degradation. Evidently, a subtle change on the N-terminus of a natural protein can enhance its intracellular lifetime.

Neutralizing antibody and functional mapping of Bacillus anthracis protective antigen-The first step toward a rationally designed anthrax vaccine

Anthrax is defined by the Centers for Disease Control and Prevention as a Category A pathogen for its potential use as a bioweapon. Current prevention treatments include Anthrax Vaccine Adsorbed (AVA). AVA is an undefined formulation of Bacillus anthracis culture supernatant adsorbed to aluminum hydroxide. It has an onerous vaccination schedule, is slow and cumbersome to produce and is slightly reactogenic. Next-generation vaccines are focused on producing recombinant forms of anthrax toxin in a well-defined formulation but these vaccines have been shown to lose potency as they are stored. In addition, studies have shown that a proportion of the antibody response against these vaccines is focused on non-functional, non-neutralizing regions of the anthrax toxin while some essential functional regions are shielded from eliciting an antibody response. Rational vaccinology is a developing field that focuses on designing vaccine antigens based on structural information provided by neutralizing antibody epitope mapping, crystal structure analysis, and functional mapping through amino acid mutations. This information provides an opportunity to design antigens that target only functionally important and conserved regions of a pathogen in order to make a more optimal vaccine product. This review provides an overview of the literature related to functional and neutralizing antibody epitope mapping of the Protective Antigen (PA) component of anthrax toxin.

An anthrax toxin variant with an improved activity in tumor targeting

Anthrax lethal toxin (LT) is an A-B type toxin secreted by Bacillus anthracis, consisting of the cellular binding moiety, protective antigen (PA), and the catalytic moiety, lethal factor (LF). To target cells, PA binds to cell-surface receptors and is then proteolytically processed forming a LF-binding competent PA oligomer where each LF binding site is comprised of three subsites on two adjacent PA monomers. We previously generated PA-U2-R200A, a urokinase-activated PA variant with LF-binding subsite II residue Arg200 mutated to Ala, and PA-L1-I210A, a matrix metalloproteinase-activated PA variant with subsite III residue Ile210 mutated to Ala. PA-U2-R200A and PA-L1-I210A displayed reduced cytotoxicity when used singly. However, when combined, they formed LF-binding competent heterogeneous oligomers by intermolecular complementation, and achieved high specificity in tumor targeting. Nevertheless, each of these proteins, in particular PA-L1-I210A, retained residual LF-binding ability. In this work, we screened a library containing all possible amino acid substitutions for LF-binding site to find variants with activity strictly dependent upon intermolecular complementation. PA-I207R was identified as an excellent replacement for the original clockwise-side variant, PA-I210A. Consequently, the new combination of PA-L1-I207R and PA-U2-R200A showed potent anti-tumor activity and low toxicity, exceeding the performance of the original combination, and warranting further investigation.

Presentation of peptides from Bacillus anthracis protective antigen on Tobacco Mosaic Virus as an epitope targeted anthrax vaccine

The current anthrax vaccine requires improvements for rapidly invoking longer-lasting neutralizing antibody responses with fewer doses from a well-defined formulation. Designing antigens that target neutralizing antibody epitopes of anthrax protective antigen, a component of anthrax toxin, may offer a solution for achieving a vaccine that can induce strong and long lasting antibody responses with fewer boosters. Here we report implementation of a strategy for developing epitope focused virus nanoparticle vaccines against anthrax by using immunogenic virus particles to present peptides derived from anthrax toxin previously identified in (1) neutralizing antibody epitope mapping studies, (2) toxin crystal structure analyses to identify functional regions, and (3) toxin mutational analyses. We successfully expressed two of three peptide epitopes from anthrax toxin that, in previous reports, bound antibodies that were partially neutralizing against toxin activity, discovered cross-reactivity between vaccine constructs and toxin specific antibodies raised in goats against native toxin and showed that antibodies induced by our vaccine constructs also cross-react with native toxin. While protection against intoxication in cellular and animal studies were not as effective as in previous studies, partial toxin neutralization was observed in animals, demonstrating the feasibility of using plant-virus nanoparticles as a platform for epitope defined anthrax vaccines.

Recombinant expression of Bacillus anthracis lethal toxin components of Indian isolate in Escherichia coli and determination of its acute toxicity level in mouse model

Bacillus anthracis lethal toxin (LeTx) is the principle factor responsible for toxaemia and anthrax related death. Lethal toxin consist of two proteins viz protective antigen (PA) and lethal factor which combines in a typical fashion similar to other toxins belonging to A-B toxin super family. The amount of LeTx required to kill a particular organism generally differs among strains owing to their geographical distributions and genetic variation. In the present study, we have cloned PA and LF genes from B. anthracis clinical isolate of Indian origin and expressed them in soluble form employing Escherichia coli expression system. Both the proteins were purified to near homogeneity level using Immobilized metal ion affinity chromatography (IMAC). Further we have used equal ratio of both the proteins to form LeTx and determined its acute toxicity level in Balb/c mice by graphical method of Miller and Tainter. The LD50 value of LeTx by intravenous (i.v) route was found to be 0.97 ± 0.634 mg kg(-1) Balb/c mice. This study highlights the expression of recombinant LeTx from E. coli and assessing its acute toxicity level in experimental mouse model.

Identification of agents effective against multiple toxins and viruses by host-oriented cell targeting

A longstanding and still-increasing threat to the effective treatment of infectious diseases is resistance to antimicrobial countermeasures. Potentially, the targeting of host proteins and pathways essential for the detrimental effects of pathogens offers an approach that may discover broad-spectrum anti-pathogen countermeasures and circumvent the effects of pathogen mutations leading to resistance. Here we report implementation of a strategy for discovering broad-spectrum host-oriented therapies against multiple pathogenic agents by multiplex screening of drugs for protection against the detrimental effects of multiple pathogens, identification of host cell pathways inhibited by the drug, and screening for effects of the agent on other pathogens exploiting the same pathway. We show that a clinically used antimalarial drug, Amodiaquine, discovered by this strategy, protects host cells against infection by multiple toxins and viruses by inhibiting host cathepsin B. Our results reveal the practicality of discovering broadly acting anti-pathogen countermeasures that target host proteins exploited by pathogens.

Pore-forming activity of clostridial binary toxins

Clostridial binary toxins (Clostridium perfringens Iota toxin, Clostridium difficile transferase, Clostridium spiroforme toxin, Clostridium botulinum C2 toxin) as Bacillus binary toxins, including Bacillus anthracis toxins consist of two independent proteins, one being the binding component which mediates the internalization into cell of the intracellularly active component. Clostridial binary toxins induce actin cytoskeleton disorganization through mono-ADP-ribosylation of globular actin and are responsible for enteric diseases. Clostridial and Bacillus binary toxins share structurally and functionally related binding components which recognize specific cell receptors, oligomerize, form pores in endocytic vesicle membrane, and mediate the transport of the enzymatic component into the cytosol. Binding components retain the global structure of pore-forming toxins (PFTs) from the cholesterol-dependent cytotoxin family such as perfringolysin. However, their pore-forming activity notably that of clostridial binding components is more related to that of heptameric PFT family including aerolysin and C. perfringens epsilon toxin. This review focuses upon pore-forming activity of clostridial binary toxins compared to other related PFTs. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Translocation of Non-Canonical Polypeptides into Cells Using Protective Antigen

A variety of pathogenic bacteria infect host eukaryotic cells using protein toxins, which enter the cytosol and exert their cytotoxic effects. Anthrax lethal toxin, for example, utilizes the membrane-spanning translocase, protective antigen (PA) pore, to deliver the protein toxin lethal factor (LF) from the endosome into the cytosol of cells. Previous work has investigated the delivery of natural peptides and enzymatic domains appended to the C-terminus of the PA-binding domain of lethal factor (LFN) into the cytosol via PA pore. Here, we move beyond natural amino acids and systematically investigate the translocation of polypeptide cargo containing non-canonical amino acids and functionalities through PA pore. Our results indicate translocation is not perturbed with alterations to the peptide backbone or side-chain. Moreover, despite their structural complexity, we found that the small molecule drugs, doxorubicin and monomethyl auristatin F (MMAF) translocated efficiently through PA pore. However, we found cyclic peptides and the small molecule drug docetaxel abrogated translocation due to their large size and structural rigidity. For cargos that reached the cytosol, we demonstrated that each remained intact after translocation. These studies show PA is capable of translocating non-canonical cargo provided it is in a conformational state conducive for passage through the narrow pore.

A Novel Chimeric Anti-PA Neutralizing Antibody for Postexposure Prophylaxis and Treatment of Anthrax

Anthrax is a highly lethal infectious disease caused by the bacterium Bacillus anthracis, and the associated shock is closely related to the lethal toxin (LeTx) produced by the bacterium. The central role played by the 63 kDa protective antigen (PA63) region of LeTx in the pathophysiology of anthrax makes it an excellent therapeutic target. In the present study, a human/murine chimeric IgG mAb, hmPA6, was developed by inserting murine antibody variable regions into human constant regions using antibody engineering technology. hmPA6 expressed in 293F cells could neutralize LeTx both in vitro and in vivo. At a dose of 0.3 mg/kg, it could protect all tested rats from a lethal dose of LeTx. Even administration of 0.6 mg/kg hmPA6 48 h before LeTx challenge protected all tested rats. The results indicate that hmPA6 is a potential candidate for clinical application in anthrax treatment.

[Molecular model of anthrax toxin translocation into target-cells]

Anthrax toxin is formed from three components: protective antigen (PA), lethal (LF) and edema (EF) factors. PA83 is cleaved by cell surface protease furin to produce a 63-kDa fragment (PA63). PA63 and LF/EF molecules are assembled to anthrax toxin complexes: oligomer PA63 x 7 + LF/EF x 3. Assembly is occurred during of binding with cellular receptor or near surface of target-cell. This toxin complex forms pore and induces receptor-mediated endocytosis. Formed endosome consists extracellular liquid with LF/EF and membrane-associated ferments (H+ and K+/Na+-ATPases) and proteins (receptors and others). H+ concentration is increased into endosome as result of K/Na-ATPase-dependent- activity of H+-ATPase. Difference of potentials (between endosome and intracellular liquid) is increased and LF/EF molecules are moved to pore and bound with PA63-oligomer to PA63 x 7 + LF/EF x 7 and full block pore (ion-selective channel). Endosome is increased in volume and induces increasing of PA63-oligomer pore to.size of effector complex: LF/EF x 7 + PAl7 x 7 = 750 kDa. Effector complex is translocated from endosome to cytosol by means high difference of potentials (H+) and dissociates from PA47 x 7 complex after cleavage of FFD315-sait by intracellular chymotrypsin-like proteases in all 7 molecules PA63. PA47 x 7 complex (strongly fixed in membrane with debris of hydrophobic loops) return into endosome and pore is destroyed. Endosome pH is decreased rapidly and PA47 x 7 complex is destroyed by endosomal/lysosomal proteases. Receptor-mediated endocytosis is ended by endosome recycling in cell-membrane.

Delivery of antibody mimics into mammalian cells via anthrax toxin protective antigen

Antibody mimics have significant scientific and therapeutic utility for the disruption of protein-protein interactions inside cells; however, their delivery to the cell cytosol remains a major challenge. Here we show that protective antigen (PA), a component of anthrax toxin, efficiently transports commonly used antibody mimics to the cytosol of mammalian cells when conjugated to the N-terminal domain of LF (LFN). In contrast, a cell-penetrating peptide (CPP) was not able to deliver any of these antibody mimics into the cell cytosol. The refolding and binding of a transported tandem monobody to Bcr-Abl (its protein target) in chronic myeloid leukemia cells were confirmed by co-immunoprecipitation. We also observed inhibition of Bcr-Abl kinase activity and induction of apoptosis caused by the monobody. In a separate case, we show disruption of key interactions in the MAPK signaling pathway after PA-mediated delivery of an affibody binder that targets hRaf-1. We show for the first time that PA can deliver bioactive antibody mimics to disrupt intracellular protein-protein interactions. This technology adds a useful tool to expand the applications of these modern agents to the intracellular milieu.

Polylysine-mediated translocation of the diphtheria toxin catalytic domain through the anthrax protective antigen pore

The protective antigen (PA) moiety of anthrax toxin forms oligomeric pores in the endosomal membrane, which translocate the effector proteins of the toxin to the cytosol. Effector proteins bind to oligomeric PA via their respective N-terminal domains and undergo N- to C-terminal translocation through the pore. Earlier we reported that a tract of basic amino acids fused to the N-terminus of an unrelated effector protein (the catalytic domain diphtheria toxin, DTA) potentiated that protein to undergo weak PA-dependent translocation. In this study, we varied the location of the tract (N-terminal or C-terminal) and the length of a poly-Lys tract fused to DTA and examined the effects of these variations on PA-dependent translocation into cells and across planar bilayers in vitro. Entry into cells was most efficient with ∼12 Lys residues (K12) fused to the N-terminus but also occurred, albeit 10-100-fold less efficiently, with a C-terminal tract of the same length. Similarly, K12 tracts at either terminus occluded PA pores in planar bilayers, and occlusion was more efficient with the N-terminal tag. We used biotin-labeled K12 constructs in conjunction with streptavidin to show that a biotinyl-K12 tag at either terminus is transiently exposed to the trans compartment of planar bilayers at 20 mV; this partial translocation in vitro was more efficient with an N-terminal tag than a C-terminal tag. Significantly, our studies with polycationic tracts fused to the N- and C-termini of DTA suggest that PA-mediated translocation can occur not only in the N to C direction but also in the C to N direction.

Lethal factor, but not edema factor, is required to cause fatal anthrax in cynomolgus macaques after pulmonary spore challenge

Inhalational anthrax is caused by inhalation of Bacillus anthracis spores. The ability of B. anthracis to cause anthrax is attributed to the plasmid-encoded A/B-type toxins, edema toxin (edema factor and protective antigen) and lethal toxin (lethal factor and protective antigen), and a poly-d-glutamic acid capsule. To better understand the contribution of these toxins to the disease pathophysiology in vivo, we used B. anthracis Ames strain and isogenic toxin deletion mutants derived from the Ames strain to examine the role of lethal toxin and edema toxin after pulmonary spore challenge of cynomolgus macaques. Lethal toxin, but not edema toxin, was required to induce sustained bacteremia and death after pulmonary challenge with spores delivered via bronchoscopy. After intravenous challenge with bacilli to model the systemic phase of infection, lethal toxin contributed to bacterial proliferation and subsequent host death to a greater extent than edema toxin. Deletion of protective antigen resulted in greater loss of virulence after intravenous challenge with bacilli than deletion of lethal toxin or edema toxin alone. These findings are consistent with the ability of anti-protective antigen antibodies to prevent anthrax and suggest that lethal factor is the dominant toxin that contributes to the escape of significant numbers of bacilli from the thoracic cavity to cause anthrax after inhalation challenge with spores.

Delivery of mirror image polypeptides into cells

Mirror image peptides have unique stability and immunogenic properties in mammals, making them attractive agents to investigate. Their properties inside cells have been mostly unexplored because biopolymers are difficult to transport across cellular membranes. Here, we used protective antigen (PA) from anthrax toxin to deliver mirror image polypeptide cargo into the cytosol of mammalian cells when conjugated to the C-terminus of the PA-binding domain of lethal factor, LF_N. We found mirror image polypeptides and proteins were translocated as efficiently into cells as their L counterparts. Once in the cytosol, by the use of western blot, we found that d peptides at the C-terminus of LF_N were able to achieve higher steady state concentrations when compared to the l-peptide conjugate. With this platform, we delivered a d-peptide MDM2 antagonist to disrupt the p53/MDM2 interaction in cancer cells. For the first time, we show the PA/LF_N system is adaptable for the intracellular delivery of mirror image peptides and proteins.

Anthrax toxin-induced rupture of artificial lipid bilayer membranes

We demonstrate experimentally that anthrax toxin complexes rupture artificial lipid bilayer membranes when isolated from the blood of infected animals. When the solution pH is temporally acidified to mimic that process in endosomes, recombinant anthrax toxin forms an irreversibly bound complex, which also destabilizes membranes. The results suggest an alternative mechanism for the translocation of anthrax toxin into the cytoplasm.

Long-range stabilization of anthrax protective antigen upon binding to CMG2

Protective antigen (PA) mediates entry of edema factor (EF) and lethal factor (LF) into the cytoplasmic space of the cells through the formation of a membrane-spanning pore. To do this, PA must initially bind to a host cellular receptor. Recent mass spectrometry analysis of PA using histidine hydrogen–deuterium exchange (His-HDX) has shown that binding of the von Willebrand factor A (vWA) domain of the receptor capillary morphogenesis protein-2 (CMG2) lowers the exchange rates of the imidazole C₂ hydrogen of several histidines, suggesting that receptor binding decreases the structural flexibility of PA. Here, using His-HDX and fluorescence as a function of denaturant, and protease susceptibility, we show that binding of the vWA domain of CMG2 largely increases the stability of PA and the effect reaches up to 70 Å from the receptor binding interface. We also show that the pK_a values and HDX rates of histidines located in separate domains change upon receptor binding. These results indicate that when one end of the protein is anchored, the structure of PA is tightened, noncovalent interactions are strengthened, and the global stability of the protein increases. These findings suggest that CMG2 may be used to stabilize PA in future anthrax vaccines.

Channel-forming bacterial toxins in biosensing and macromolecule delivery

To intoxicate cells, pore-forming bacterial toxins are evolved to allow for the transmembrane traffic of different substrates, ranging from small inorganic ions to cell-specific polypeptides. Recent developments in single-channel electrical recordings, X-ray crystallography, protein engineering, and computational methods have generated a large body of knowledge about the basic principles of channel-mediated molecular transport. These discoveries provide a robust framework for expansion of the described principles and methods toward use of biological nanopores in the growing field of nanobiotechnology. This article, written for a special volume on "Intracellular Traffic and Transport of Bacterial Protein Toxins", reviews the current state of applications of pore-forming bacterial toxins in small- and macromolecule-sensing, targeted cancer therapy, and drug delivery. We discuss the electrophysiological studies that explore molecular details of channel-facilitated protein and polymer transport across cellular membranes using both natural and foreign substrates. The review focuses on the structurally and functionally different bacterial toxins: gramicidin A of Bacillus brevis, α-hemolysin of Staphylococcus aureus, and binary toxin of Bacillus anthracis, which have found their "second life" in a variety of developing medical and technological applications.

Cationic PAMAM dendrimers as pore-blocking binary toxin inhibitors

Dendrimers are unique highly branched macromolecules with numerous groundbreaking biomedical applications under development. Here we identified poly(amido amine) (PAMAM) dendrimers as novel blockers for the pore-forming B components of the binary anthrax toxin (PA63) and Clostridium botulinum C2 toxin (C2IIa). These pores are essential for delivery of the enzymatic A components of the internalized toxins from endosomes into the cytosol of target cells. We demonstrate that at low μM concentrations cationic PAMAM dendrimers block PA63 and C2IIa to inhibit channel-mediated transport of the A components, thereby protecting HeLa and Vero cells from intoxication. By channel reconstitution and high-resolution current recording, we show that the PAMAM dendrimers obstruct transmembrane PA63 and C2IIa pores in planar lipid bilayers at nM concentrations. These findings suggest a new potential role for the PAMAM dendrimers as effective polyvalent channel-blocking inhibitors, which can protect human target cells from intoxication with binary toxins from pathogenic bacteria.

Comparative toxicity and efficacy of engineered anthrax lethal toxin variants with broad anti-tumor activities

We have previously designed and characterized versions of anthrax lethal toxin that are selectively cytotoxic in the tumor microenvironment and which display broad and potent anti-tumor activities in vivo. Here, we have performed the first direct comparison of the safety and efficacy of three engineered anthrax lethal toxin variants requiring activation by either matrix-metalloproteinases (MMPs), urokinase plasminogen activator (uPA) or co-localized MMP/uPA activities. C57BL/6J mice were challenged with six doses of engineered toxins via intraperitoneal (I.P.) or intravenous (I.V.) dose routes to determine the maximum tolerated dose for six administrations (MTD6) and dose-limiting toxicities. Efficacy was evaluated using the B16-BL6 syngraft model of melanoma; mice bearing established tumors were treated with six I.P. doses of toxin and tumor measurements and immunohistochemistry, paired with terminal blood work, were used to elaborate upon the anti-tumor mechanism and relative efficacy of each variant. We found that MMP-, uPA- and dual MMP/uPA-activated anthrax lethal toxins exhibited the same dose-limiting toxicity; dose-dependent GI toxicity. In terms of efficacy, all three toxins significantly reduced primary B16-BL6 tumor burden, ranging from 32% to 87% reduction, and they also delayed disease progression as evidenced by dose-dependent normalization of blood work values. While target organ toxicity and effective doses were similar amongst the variants, the dual MMP/uPA-activated anthrax lethal toxin exhibited the highest I.P. MTD6 and was 1.5-3-fold better tolerated than the single MMP- and uPA-activated toxins. Overall, we demonstrate that this dual MMP/uPA-activated anthrax lethal toxin can be administered safely and is highly effective in a preclinical model of melanoma. This modified bacterial cytotoxin is thus a promising candidate for further clinical development and evaluation for use in treating human cancers.

Anthrax vaccines: present status and future prospects

The management of anthrax remains a top priority among the biowarfare/bioterror agents. It was the Bacillus anthracis spore attack through the US mail system after the September 11, 2001, terrorist attacks in the USA that highlighted the potential of B. anthracis as a bioterrorism agent and the threat posed by its deliberate dissemination. These attacks invigorated the efforts toward understanding the anthrax pathogenesis and development of more comprehensive medical intervention strategies for its containment in case of both natural disease and manmade, accidental or deliberate infection of a non-suspecting population. Currently, efforts are directed toward the development of safe and efficacious vaccines as well as intervention tools for controlling the disease in the advanced fulminant stage when toxemia has already developed. This work presents an overview of the current understanding of anthrax pathogenesis and recent advances made, particularly after 2001, for the successful management of anthrax and outlines future perspectives.

Proprotein convertases subtilisin/kexin type 9, an enzyme turned escort protein: hepatic and extra hepatic functions

Proprotein Convertases Subtilisin/Kexin Type 9 (PCSK9) is a serine endoproteinase. Biosynthesized as a zymogen, it cleaves itself once, and then turns into an escort protein for transmembrane proteins, leading them into lysosomes for degradation. It is primarily produced and secreted by the liver. It attaches to the low-density lipoprotein receptor (LDLR) at the surface of hepatocytes and, after co-endocytosis, directs it into lysosomes where it is degraded. By downregulating LDLR, PCSK9 reduces hepatic clearance of LDL-cholesterol. Inborn or induced increase of this function causes hypercholesterolemia; its decrease causes hypocholesterolemia. This has been experimentally demonstrated ex vivo and in vivo, and corroborated by epidemiological studies associating PCSK9 genetic variations with plasma cholesterol levels. PCSK9 is now a proven target for inactivation in the treatment of hypercholesterolemia and associated atherosclerosis. However, it is still uncertain whether its severe or complete inactivation, combined with other predispositions, will be without undesirable side-effects. Some experimental data suggest that PCSK9 could contribute positively to the physiology of non-hepatic cells such as pancreatic islets β cells, adipocytes and macrophages, protecting them from excessive lipid uptake, in an endocrine, autocrine, or paracrine manner. Genetic variations that attenuate PCSK9 anti-LDLR activity are common in human populations. Their evolutionary significance still needs to be evaluated on the background of environmental pressures, such as infectious diseases, cold weather and famine, which have threatened survival and reproduction in the course of human prehistory and history.

Modulation of the Bacillus anthracis secretome by the immune inhibitor A1 protease

The Bacillus anthracis secretome includes protective antigen, lethal factor, and edema factor, which are the components of anthrax toxin, and other proteins with known or potential roles in anthrax disease. Immune inhibitor A1 (InhA1) is a secreted metalloprotease that is unique to pathogenic members of the Bacillus genus and has been associated with cleavage of host proteins during infection. Here, we report the effect of InhA1 on the B. anthracis secretome. Differential in-gel electrophoresis of proteins present in culture supernatants from a parent strain and an isogenic inhA1-null mutant revealed multiple differences. Of the 1,340 protein spots observed, approximately one-third were less abundant and one-third were more abundant in the inhA1 secretome than in the parent strain secretome. Proteases were strongly represented among those proteins exhibiting a 9-fold or greater change. InhA1 purified from a B. anthracis culture supernatant directly cleaved each of the anthrax toxin proteins as well as an additional secreted protease, Npr599. The conserved zinc binding motif HEXXH of InhA1 (HEYGH) was critical for its proteolytic activity. Our data reveal that InhA1 directly and indirectly modulates the form and/or abundance of over half of all the secreted proteins of B. anthracis. The proteolytic activity of InhA1 on established secreted virulence factors, additional proteases, and other secreted proteins suggests that this major protease plays an important role in virulence not only by cleaving mammalian substrates but also by modulating the B. anthracis secretome itself.

In vitro and in vivo activities of recombinant anthrax protective antigen co-expressed with thioredoxin in Escherichia coli

Because of the central role it plays in the formation of lethal toxin and edema toxin, protective antigen (PA) is the principal target for the development of vaccines against anthrax. In the present study, we explored and compared the in vitro and in vivo activities of recombinant anthrax protective antigen (rPA) and receptor binding domain of protective antigen (PA4). As a result, the fully soluble rPA and PA4 proteins were successfully expressed in Escherichia coli by co-expression with thioredoxin (Trx), and the rPA was active in forming cytotoxic lethal toxins, indicating that the rPA protein retains a functionally biological activity. Furthermore, immunization with rPA protein induced stronger PA-specific immune responses in mice than PA4 protein. The protection elicited by immunization with PA4 suggests the presence of common neutralizing epitopes between rPA and PA4, but the immunization with rPA protein induced stronger neutralizing antibodies and protective levels against challenge with the B. anthracis strain A16R than the PA4 protein. The sera neutralizing antibodies titers correlated well with anti-PA group ELISA antibodies titers and the in vivo protective potency. Based on the results of cell cytotoxicity assays and the observed immune responses and protective potency, we concluded that the soluble rPA protein retains the in vitro and in vivo functionally biological activity and can be developed into a highly effective human subunit vaccine candidate against anthrax.

Targeting HER2-positive cancer cells with receptor-redirected anthrax protective antigen

Targeted therapeutics have emerged in recent years as an attractive approach to treating various types of cancer. One approach is to modify a cytocidal protein toxin to direct its action to a specific population of cancer cells. We created a targeted toxin in which the receptor-binding and pore-forming moiety of anthrax toxin, termed Protective Antigen (PA), was modified to redirect its receptor specificity to HER2, a marker expressed at the surface of a significant fraction of breast and ovarian tumors. The resulting fusion protein (mPA-ZHER2) delivered cytocidal effectors specifically into HER2-positive tumor cells, including a trastuzumab-resistant line, causing death of the cells. No off-target killing of HER2-negative cells was observed, either with homogeneous populations or with mixtures of HER2-positive and HER2-negative cells. A mixture of mPA variants targeting different receptors mediated killing of cells bearing either receptor, without affecting cells devoid of these receptors. Anthrax toxin may serve as an effective platform for developing therapeutics to ablate cells bearing HER2 or other tumor-specific cell-surface markers.

Identification of new dominant-negative mutants of anthrax protective antigen using directed evolution

The anthrax toxin is composed of three proteins: protective antigen (PA), lethal factor (LF), and edema toxin (EF). The PA moiety carries EF and LF into the cytosol of mammalian cells via a mechanism that depends on the oligomerization of PA and transmembrane pore formation by the PA oligomer. Certain mutants of PA, termed dominant-negative (DN) mutants, can co-oligomerize with wild-type PA and disrupt the translocation ability of the pore. Here, we constructed a PA mutant library by introducing random mutations into domain II of PA and screened three new DN mutants of PA: V377E, T380S, and I432C. All the mutants inhibited the anthrax toxin action against sensitive cells. V377E had the strongest inhibitory effect and was further confirmed to be able to protect mice against a challenge with anthrax lethal toxin. Furthermore, we functionally characterized these mutants. The result showed that these mutations did not impair proteolytic activation or oligomer formation of PA, but impeded the prepore-pore conversion of the oligomer. These DN mutants of PA identified in our study may provide valuable information for elucidating the structure-function relationship of PA and for designing therapeutics for anthrax treatment.

Broad expression analysis of human ANTXR1/TEM8 transcripts reveals differential expression and novel splizce variants

Tumor endothelial marker 8 (TEM8; ANTXR1) is one of two anthrax toxin receptors; the other is capillary morphogenesis gene 2 protein (CMG2; ANTXR2). TEM8 shows enhanced expression in certain tumor endothelia, and is thought to be a player in tumor vasculature formation. However, a comprehensive expression profile of individual TEM8 variants in normal or cancerous tissues is lacking. In this work we carried out an extensive analysis of all splice variants of human TEM8 in 12 digestive tissues, and 8 each fetal and adult tissues, 6 of them cognate pairs. Using variant-specific primers, we first ascertained the status of full-length transcripts by nested PCR. We then carried out quantitative analysis of each transcript by real-time PCR. Three splice variants of TEM8 were reported before, two single-pass integral membrane forms (V1 and V2) and one secreted (V3). Our analysis has revealed two new variants, one encoding a membrane-bound form of the receptor and the other secreted, which we have designated V4 and V5, respectively. All tissues had V1, V2, V3, and V4, but only prostate had V5. Real-time PCR revealed that all variants are present at different levels in various tissues. V3 appeared the most abundant of all. To ascertain its functionality for anthrax toxin, we expressed the newly identified form V4 in a receptor-negative host cell, and included V1 and V2 for comparison. Cytotoxicity, toxin binding, and internalization assays showed V4 to be as efficient a receptor as V1 and V2.

Novel approaches to the treatment of systemic anthrax

Anthrax continues to generate concern as an agent of bioterrorism and as a natural cause of sporadic disease outbreaks. Despite the use of appropriate antimicrobial agents and advanced supportive care, the mortality associated with the systemic disease remains high. This is primarily due to the pathogenic exotoxins produced by Bacillus anthracis as well as other virulence factors of the organism. For this reason, new therapeutic strategies that target events in the pathogenesis of anthrax and may potentially augment antimicrobials are being investigated. These include anti-toxin approaches, such as passive immune-based therapies; non-antimicrobial drugs with activity against anthrax toxin components; and agents that inhibit binding, processing, or assembly of toxins. Adjunct therapies that target spore germination or downstream events in anthrax intoxication are also under investigation. In combination, these modalities may enhance the management of systemic anthrax.

Bacillus anthracis factors for phagosomal escape

The mechanism of phagosome escape by intracellular pathogens is an important step in the infectious cycle. During the establishment of anthrax, Bacillus anthracis undergoes a transient intracellular phase in which spores are engulfed by local phagocytes. Spores germinate inside phagosomes and grow to vegetative bacilli, which emerge from their resident intracellular compartments, replicate and eventually exit from the plasma membrane. During germination, B. anthracis secretes multiple factors that can help its resistance to the phagocytes. Here the possible role of B. anthracis toxins, phospholipases, antioxidant enzymes and capsules in the phagosomal escape and survival, is analyzed and compared with that of factors of other microbial pathogens involved in the same type of process.

Bacillus anthracis edema factor substrate specificity: evidence for new modes of action

Since the isolation of Bacillus anthracis exotoxins in the 1960s, the detrimental activity of edema factor (EF) was considered as adenylyl cyclase activity only. Yet the catalytic site of EF was recently shown to accomplish cyclization of cytidine 5'-triphosphate, uridine 5'-triphosphate and inosine 5'-triphosphate, in addition to adenosine 5'-triphosphate. This review discusses the broad EF substrate specificity and possible implications of intracellular accumulation of cyclic cytidine 3':5'-monophosphate, cyclic uridine 3':5'-monophosphate and cyclic inosine 3':5'-monophosphate on cellular functions vital for host defense. In particular, cAMP-independent mechanisms of action of EF on host cell signaling via protein kinase A, protein kinase G, phosphodiesterases and CNG channels are discussed.

From structure to solutions: the role of basic research in developing anthrax countermeasures: Microbiology Graduate Program Seminar: Anthrax toxin

Dr. John Collier traced the discoveries that elucidated the structure and function of the anthrax toxin in his talk "Anthrax Toxin," which was part of the Microbiology Graduate Program Seminar Series at Yale School of Medicine on February 23, 2012. Dr. Collier, Professor of Microbiology and Immunobiology at Harvard University, began by noting the advantages to studying anthrax pathogenesis in a biosafety level-1 lab. This designation does not merely facilitate his research, but also reflects a larger trend of basic research being leveraged to develop translational applications. Basic research on toxin structure has led to the development of a vaccine by Dr. Collier's group. Next-generation prophylactics also may stem from recent discoveries uncovering a role for cellular cofactors that mediate toxin function. Finally, basic research into the toxin substructure has facilitated efforts to change the receptor tropism to target dysregulated cells for therapeutic purposes. The urgency around biodefense agents makes the choice of research priorities a salient issue. As such, this author submits that basic research occupies a unique and lucrative niche driving clinical applications.

Changing the receptor specificity of anthrax toxin

The actions of many bacterial toxins depend on their ability to bind to one or more cell-surface receptors. Anthrax toxin acts by a sequence of events that begins when the protective-antigen (PA) moiety of the toxin binds to either one of two cell-surface proteins, ANTXR1 and ANTXR2, and is proteolytically activated. The activated PA self-associates to form oligomeric pore precursors, which, in turn, bind the enzymatic moieties of the toxin and transport them to the cytosol. We introduced a double mutation into domain 4 of PA to ablate its native receptor-binding function and fused epidermal growth factor (EGF) to the C terminus of the mutated protein. The resulting fusion protein transported enzymatic effector proteins into a cell line that expressed the EGF receptor (A431 cells), but not into a line lacking this receptor (CHO-K1 cells). Addition of excess free EGF blocked transport of effector proteins into A431 cells via the fusion protein, but not via native PA. We also showed that fusing the diphtheria toxin receptor-binding domain to the C terminus of the mutated PA channeled effector-protein transport through the diphtheria toxin receptor. PA fusion proteins with altered receptor specificity may be useful in biological research and could have practical applications, including ablation or perturbation of selected populations of cells in vivo.

Bacterial toxins that act within mammalian cells have receptor-dependent mechanisms to transport their enzymatic components to the cytoplasmic compartment. By inactivating or otherwise modifying their respective intracellular targets, these intracellular effectors disrupt metabolic pathways and in some cases cause death of the cell. Our results show that the receptor specificity of the transport protein of anthrax toxin may be readily changed, raising the possibility that receptor-redirected forms of protective antigen (PA) and PA homologs may be useful for research and medical applications requiring modification or ablation of designated populations of cells.

New developments in vaccines, inhibitors of anthrax toxins, and antibiotic therapeutics for Bacillus anthracis

Bacillus anthracis, the causative agent responsible for anthrax infections, poses a significant biodefense threat. There is a high mortality rate associated with untreated anthrax infections; specifically, inhalation anthrax is a particularly virulent form of infection with mortality rates close to 100%, even with aggressive treatment. Currently, a vaccine is not available to the general public and few antibiotics have been approved by the FDA for the treatment of inhalation anthrax. With the threat of natural or engineered bacterial resistance to antibiotics and the limited population for whom the current drugs are approved, there is a clear need for more effective treatments against this deadly infection. A comprehensive review of current research in drug discovery is presented in this article, including efforts to improve the purity and stability of vaccines, design inhibitors targeting the anthrax toxins, and identify inhibitors of novel enzyme targets. High resolution structural information for the anthrax toxins and several essential metabolic enzymes has played a significant role in aiding the structure-based design of potent and selective antibiotics.