Purified Bacillus anthracis lethal toxin complex formed in vitro and during infection exhibits functional and biological activity

Early Circulating Edema Factor in Inhalational Anthrax Infection: Does It Matter?

Anthrax toxins are critical virulence factors of Bacillus anthracis and Bacillus cereus strains that cause anthrax-like disease, composed of a common binding factor, the protective antigen (PA), and two enzymatic proteins, lethal factor (LF) and edema factor (EF). While PA is required for endocytosis and activity of EF and LF, several studies showed that these enzymatic factors disseminate within the body in the absence of PA after intranasal infection. In an effort to understand the impact of EF in the absence of PA, we used a fluorescent EF chimera to facilitate the study of endocytosis in different cell lines. Unexpectedly, EF was found inside cells in the absence of PA and showed a pole-dependent endocytosis. However, looking at enzymatic activity, PA was still required for EF to induce an increase in intracellular cAMP levels. Interestingly, the sequential delivery of EF and then PA rescued the rise in cAMP levels, indicating that PA and EF may functionally associate during intracellular trafficking, as well as it did at the cell surface. Our data shed new light on EF trafficking and the potential location of PA and EF association for optimal cytosolic delivery.

Comprehensive characterization of toxins during progression of inhalation anthrax in a non-human primate model

Inhalation anthrax has three clinical stages: early-prodromal, intermediate-progressive, and late-fulminant. We report the comprehensive characterization of anthrax toxins, including total protective antigen (PA), total lethal factor (LF), total edema factor (EF), and their toxin complexes, lethal toxin and edema toxin in plasma, during the course of inhalation anthrax in 23 cynomolgus macaques. The toxin kinetics were predominantly triphasic with an early rise (phase-1), a plateau/decline (phase-2), and a final rapid rise (phase-3). Eleven animals had shorter survival times, mean±standard deviation of 58.7±7.6 hours (fast progression), 11 animals had longer survival times, 113±34.4 hours (slow progression), and one animal survived. Median (lower-upper quartile) LF levels at the end-of-phase-1 were significantly higher in animals with fast progression [138 (54.9-326) ng/mL], than in those with slow progression [23.8 (15.6-26.3) ng/mL] (p = 0.0002), and the survivor (11.1 ng/mL). The differences were also observed for other toxins and bacteremia. Animals with slow progression had an extended phase-2 plateau, with low variability of LF levels across all time points and animals. Characterization of phase-2 toxin levels defined upper thresholds; critical levels for exiting phase-2 and entering the critical phase-3, 342 ng/mL (PA), 35.8 ng/mL (LF), and 1.10 ng/mL (EF). The thresholds were exceeded earlier in animals with fast progression (38.5±7.4 hours) and later in animals with slow progression (78.7±15.2 hours). Once the threshold was passed, toxin levels rose rapidly in both groups to the terminal stage. The time from threshold to terminal was rapid and similar; 20.8±7.4 hours for fast and 19.9±7.5 hours for slow progression. The three toxemic phases were aligned with the three clinical stages of anthrax for fast and slow progression which showed that anthrax progression is toxin- rather than time-dependent. This first comprehensive evaluation of anthrax toxins provides new insights into disease progression.

Solution Structures of Bacillus anthracis Protective Antigen Proteins Using Small Angle Neutron Scattering and Protective Antigen 63 Ion Channel Formation Kinetics

We are studying the structures of bacterial toxins that form ion channels and enable macromolecule transport across membranes. For example, the crystal structure of the Staphylococcus aureus α-hemolysin (α-HL) channel in its functional state was confirmed using neutron reflectometry (NR) with the protein reconstituted in membranes tethered to a solid support. This method, which provides sub-nanometer structural information, could also test putative structures of the Bacillus anthracis protective antigen 63 (PA63) channel, locate where B. anthracis lethal factor and edema factor toxins (LF and EF, respectively) bind to it, and determine how certain small molecules can inhibit the interaction of LF and EF with the channel. We report here the solution structures of channel-forming PA63 and its precursor PA83 (which does not form channels) obtained with small angle neutron scattering. At near neutral pH, PA83 is a monomer and PA63 a heptamer. The latter is compared to two cryo-electron microscopy structures. We also show that although the α-HL and PA63 channels have similar structural features, unlike α-HL, PA63 channel formation in lipid bilayer membranes ceases within minutes of protein addition, which currently precludes the use of NR for elucidating the interactions between PA63, LF, EF, and potential therapeutic agents.

Hydrophobic Gating and 1/f Noise of the Anthrax Toxin Channel

"Pink" or 1/f noise is a natural phenomenon omnipresent in physics, economics, astrophysics, biology, and even music and languages. In electrophysiology, the stochastic activity of a number of biological ion channels and artificial nanopores could be characterized by current noise with a 1/f power spectral density. In the anthrax toxin channel (PA₆₃), it appears as fast voltage-independent current interruptions between conducting and nonconducting states. This behavior hampers potential development of PA₆₃ as an ion-channel biosensor. On the bright side, the PA₆₃ flickering represents a mesmerizing phenomenon to investigate. Notably, similar 1/f fluctuations are observed in the channel-forming components of clostridial binary C2 and iota toxins, which share functional and structural similarities with the anthrax toxin channel. Similar to PA₆₃, they are evolved to translocate the enzymatic components of the toxins into the cytosol. Here, using high-resolution single-channel lipid bilayer experiments and all-atom molecular dynamic simulations, we suggest that the 1/f noise in PA₆₃ occurs as a result of "hydrophobic gating" at the ϕ-clamp region, the phenomenon earlier observed in several water-filled channels "fastened" inside by the hydrophobic belts. The ϕ-clamp is a narrow "hydrophobic ring" in the PA₆₃ lumen formed by seven or eight phenylalanine residues at position 427, conserved in the C2 and iota toxin channels, which catalyzes protein translocation. Notably, the 1/f noise remains undetected in the F427A PA₆₃ mutant. This finding can elucidate the functional purpose of 1/f noise and its possible role in the transport of the enzymatic components of binary toxins.

Serum-Mediated Cleavage of Bacillus anthracis Protective Antigen Is a Two-Step Process That Involves a Serum Carboxypeptidase

Our findings identify a serum-mediated modification of PA₂₀ that has not been previously described. These observations further imply that the processing of PA is more complex than currently thought. Additional study is needed to define the contribution of serum processing of PA to the host response and individual susceptibility to anthrax.

Much of our understanding of the activity of anthrax toxin is based on in vitro systems, which delineate the interaction between Bacillus anthracis toxins and the cell surface. However, these systems fail to account for the intimate association of B. anthracis with the circulatory system, including the contribution of serum proteins to the host response and processing of anthrax toxins. Using a variety of immunological techniques to inhibit serum processing of B. anthracis protective antigen (PA) along with mass spectrometry analysis, we demonstrate that serum digests PA via 2 distinct reactions. In the first reaction, serum cleaves PA₈₃ into 2 fragments to produce PA₆₃ and PA₂₀ fragments, similarly to that observed following furin digestion. This is followed by carboxypeptidase-mediated removal of the carboxy-terminal arginine and lysines from PA₂₀.

IMPORTANCE Our findings identify a serum-mediated modification of PA₂₀ that has not been previously described. These observations further imply that the processing of PA is more complex than currently thought. Additional study is needed to define the contribution of serum processing of PA to the host response and individual susceptibility to anthrax.

Bioinspired detoxification of blood: The efficient removal of anthrax toxin protective antigen using an extracorporeal macroporous adsorbent device

Whilst various remedial human monoclonal antibodies have been developed to treat the potentially life-threatening systemic complications associated with anthrax infection, an optimal and universally effective administration route has yet to be established. In the later stages of infection when antibody administration by injection is more likely to fail one possible route to improve outcome is via the use of an antibody-bound, adsorbent haemoperfusion device. We report here the development of an adsorbent macroporous polymer column containing immobilised B. anthracis exotoxin-specific antibodies, PANG (a non-glycosylated, version of a plant-produced human monoclonal antibody) and Valortim (a fully human monoclonal N-linked glycosylated antibody), for removal of anthrax protective antigen (PA) from freshly frozen human plasma and human whole blood. In addition, we have demonstrated that continuous extracorporeal blood recirculation through a Valortim-bound haemoperfusion column significantly reduced the blood plasma concentration of anthrax PA over 2 hours using an in vivo PA rat infusion model. This work provides proof-of-concept evidence to support the development of such alternative detoxification platforms.

Proteomic Methods of Detection and Quantification of Protein Toxins

Biological toxins are a heterogeneous group of compounds that share commonalities with biological and chemical agents. Among them, protein toxins represent a considerable, diverse set. They cover a broad range of molecular weights from less than 1000 Da to more than 150 kDa. This review aims to compare conventional detection methods of protein toxins such as in vitro bioassays with proteomic methods, including immunoassays and mass spectrometry-based techniques and their combination. Special emphasis is given to toxins falling into a group of selected agents, according to the Centers for Disease Control and Prevention, such as Staphylococcal enterotoxins, Bacillus anthracis toxins, Clostridium botulinum toxins, Clostridium perfringens epsilon toxin, ricin from Ricinus communis, Abrin from Abrus precatorius or control of trade in dual-use items in the European Union, including lesser known protein toxins such as Viscumin from Viscum album. The analysis of protein toxins and monitoring for biological threats, i.e., the deliberate spread of infectious microorganisms or toxins through water, food, or the air, requires rapid and reliable methods for the early identification of these agents.

Structure of anthrax lethal toxin prepore complex suggests a pathway for efficient cell entry

Anthrax toxin comprises three soluble proteins: protective antigen (PA), lethal factor (LF), and edema factor (EF). PA must be cleaved by host proteases before it oligomerizes and forms a prepore, to which LF and EF bind. After endocytosis of this tripartite complex, the prepore transforms into a narrow transmembrane pore that delivers unfolded LF and EF into the host cytosol. Here, we find that translocation of multiple 90-kD LF molecules is rapid and efficient. To probe the molecular basis of this translocation, we calculated a three-dimensional map of the fully loaded (PA63)7-(LF)3 prepore complex by cryo-electron microscopy (cryo-EM). The map shows three LFs bound in a similar way to one another, via their N-terminal domains, to the surface of the PA heptamer. The model also reveals contacts between the N- and C-terminal domains of adjacent LF molecules. We propose that this molecular arrangement plays an important role in the maintenance of translocation efficiency through the narrow PA pore.

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

Biochip for the Detection of Bacillus anthracis Lethal Factor and Therapeutic Agents against Anthrax Toxins

Tethered lipid bilayer membranes (tBLMs) have been used in many applications, including biosensing and membrane protein structure studies. This report describes a biosensor for anthrax toxins that was fabricated through the self-assembly of a tBLM with B. anthracis protective antigen ion channels that are both the recognition element and electrochemical transducer. We characterize the sensor and its properties with electrochemical impedance spectroscopy and surface plasmon resonance. The sensor shows a sensitivity similar to ELISA and can also be used to rapidly screen for molecules that bind to the toxins and potentially inhibit their lethal effects.

Molecular assembly of lethal factor enzyme and pre-pore heptameric protective antigen in early stage of translocation

During intoxication, the anthrax toxin lethal (LF) and edema (EF) factors initially assemble with the protective antigen (PA) on the plasma membrane of cells expressing the membrane-bound surface-exposed anthrax toxin receptor (ATR). This takes place at the physiological pH prior to entering the acidic environment of the endosome. We elucidated the molecular dynamics (MD) behaviors of the three-dimensional structure of the (PA63)7LF3 complex in various conformations and analyzed the dynamical properties of the fully loaded pre-pore complex on the plasma membrane at the physiological pH. The analysis points to the interaction networks of amino acids conserved between PA63 octamer and heptamer, which are not affected during the initial stage of the LFs binding. The simulations show an asymmetrical movement of the complex domains that directly affect LFs conformations. The conformational and structural alterations of the 2β2-2β3 loops of PA subunits are associated with pore formation. The early conformational changes of the loops appear as they peel off from the domain 2 toward domain 4 of each PA subunit. The LFs unfold in 1α1 segments of their N-terminal initiating the early stage of the pre-pore formation. The results indicate instable regions within the complex and provide important clues concerning the detail of fluctuating residues of the LF-PA interface regions at the early steps of toxins translocation.

Ultrasensitive detection of protease activity of anthrax and botulinum toxins by a new PCR-based assay

Anthrax and botulism are dangerous infectious diseases that can be fatal unless detected and treated quickly. Fatalities from these diseases are primarily due to endopeptidase toxins secreted by the pathogens. Rapid and sensitive detection of the presence of active toxins is the key element for protection from natural outbreaks of anthrax and botulism, as well as from the threat of bioterrorism. We describe an ultrasensitive polymerase chain reaction (PCR)-based assay for detecting proteolytic activity of anthrax and botulinum toxins using composite probes consisting of covalent peptide-DNA conjugate for the detection of anthrax, and noncovalent protein-aptamer assembly to assay botulinum toxin activity. Probes immobilized on the solid-phase support are cleaved by toxins to release DNA, which is detected by real-time PCR. Both assays can detect subpicogram quantities of active toxins isolated from composite matrices. Special procedures were developed to isolate intact toxins from the matrices under mild conditions. The assay is rapid, uses proven technologies, and can be modified to detect other proteolytic and biopolymer-degrading enzymes.

High-sensitivity MALDI-TOF MS quantification of anthrax lethal toxin for diagnostics and evaluation of medical countermeasures

Inhalation anthrax has a rapid progression and high fatality rate. Pathology and death from inhalation of Bacillus anthracis spores are attributed to the actions of secreted protein toxins. Protective antigen (PA) binds and imports the catalytic component lethal factor (LF), a zinc endoprotease, and edema factor (EF), an adenylyl cyclase, into susceptible cells. PA-LF is termed lethal toxin (LTx) and PA-EF, edema toxin. As the universal transporter for both toxins, PA is an important target for vaccination and immunotherapeutic intervention. However, its quantification has been limited to methods of relatively low analytic sensitivity. Quantification of LTx may be more clinically relevant than LF or PA alone because LTx is the toxic form that acts on cells. A method was developed for LTx-specific quantification in plasma using anti-PA IgG magnetic immunoprecipitation of PA and quantification of LF activity that co-purified with PA. The method was fast (<4 h total time to detection), sensitive at 0.033 ng/mL LTx in plasma for the fast analysis (0.0075 ng/mL LTx in plasma for an 18 h reaction), precise (6.3–9.9 % coefficient of variation), and accurate (0.1–12.7 %error; n ≥ 25). Diagnostic sensitivity was 100 % (n = 27 animal/clinical cases). Diagnostic specificity was 100 % (n = 141). LTx was detected post-antibiotic treatment in 6/6 treated rhesus macaques and 3/3 clinical cases of inhalation anthrax and as long as 8 days post-treatment. Over the course of infection in two rhesus macaques, LTx was first detected at 0.101 and 0.237 ng/mL at 36 h post-exposure and increased to 1147 and 12,107 ng/mL in late-stage anthrax. This demonstrated the importance of LTx as a diagnostic and therapeutic target. This method provides a sensitive, accurate tool for anthrax toxin detection and evaluation of PA-directed therapeutics.

Graphical Abstract

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.

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.

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.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Detecting and characterizing individual molecules with single nanopores

Single-nanometer-scale pores have demonstrated the capability for the detection, identification, and characterization of individual molecules. This measurement method could soon extend the existing commercial instrumentation or provide solutions to niche applications in many fields, including health care and the basic sciences. However, that paradigm shift requires a significantly better understanding of the physics and chemistry that govern the interactions between nanopores and analytes. We describe herein some of our methods and approaches to address this issue.

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.

Anthrax toxin protective antigen--insights into molecular switching from prepore to pore

The protective antigen is a key component of the anthrax toxin, as it allows entry of the enzymatic components edema factor and lethal factor into the host cell, through the formation of a membrane spanning pore. This event is absolutely critical for the pathogenesis of anthrax, and although we have yet to understand the mechanism of pore formation, recent developments have provided key insights into how this process may occur. Based on the available data, a model is proposed for the kinetic steps for protective antigen conversion from prepore to pore. In this model, the driving force for pore formation is the formation of the phi (ϕ)-clamp, a region that forms a leak-free seal around the translocating polypeptide. Formation of the ϕ-clamp elicits movements within the prepore that provide steric freedom for the subsequent conformational changes required to form the membrane spanning pore.

Delayed toxicity associated with soluble anthrax toxin receptor decoy-Ig fusion protein treatment

Soluble receptor decoy inhibitors, including receptor-immunogloubulin (Ig) fusion proteins, have shown promise as candidate anthrax toxin therapeutics. These agents act by binding to the receptor-interaction site on the protective antigen (PA) toxin subunit, thereby blocking toxin binding to cell surface receptors. Here we have made the surprising observation that co-administration of receptor decoy-Ig fusion proteins significantly delayed, but did not protect, rats challenged with anthrax lethal toxin. The delayed toxicity was associated with the in vivo assembly of a long-lived complex comprised of anthrax lethal toxin and the receptor decoy-Ig inhibitor. Intoxication in this system presumably results from the slow dissociation of the toxin complex from the inhibitor following their prolonged circulation. We conclude that while receptor decoy-Ig proteins represent promising candidates for the early treatment of B. anthracis infection, they may not be suitable for therapeutic use at later stages when fatal levels of toxin have already accumulated in the bloodstream.

Binding and cell intoxication studies of anthrax lethal toxin

Anthrax lethal toxin (LT) is a major virulence factor of Bacillus anthracis. The vast majority of the anthrax toxin-related literature describes the assembly of LT as a cell-dependent process. However, some reports have provided evidence for the existence of a fully assembled LT, either in vitro or in the bloodstream of anthrax-infected animals. To follow up on this work, we present studies on fully-assembled LT. We first demonstrate facile and cell-free assembly and purification of LT. We then show that fully assembled LT binds an anthrax toxin receptor with almost 100-fold higher affinity than the protective antigen (PA) alone. Quantitative cell intoxication assays were used to determine the LD(50) (lethal dose 50) for LT. The cell-binding studies revealed that LT binds mammalian cells using a different mode from PA. Even when PA-specific receptors were blocked, fully assembled LT was able to bind the cell surface. Our studies support the existing evidence that LT fully assembles in the blood stream and can bind and intoxicate mammalian cells with very high affinity and efficacy. More importantly, the data presented here invoke the possibility that LT may bind cells in a receptor-independent fashion, or recognize receptors that do not interact with PA. Hence, blood borne LT may emerge as a novel therapeutic target for combating anthrax.

Sensitive fluorescence assay of anthrax protective antigen with two new DNA aptamers and their binding properties

A homogeneous assay of the protective antigen in anthrax toxin is reported using two new PA-specific aptamers for selective and sensitive detection, based on reduction in the fluorescence emission according to the formation of the aptamer-PA ternary complex. PA at 1 nM was readily detected using OliGreen as a fluorophore in HEPES buffer. We also demonstrated that the PA detection could be performed in blood serum. The binding interaction between the aptamer and PA was strong enough to dehybridize double-stranded DNA paired completely with 12 bases at room temperature. Moreover, this fluorescence study revealed that the binding sites of the two aptamers were located differently on the PA protein. We believe our approach may lay the groundwork for the real-time detection of PA.

Quantitative mass spectrometry for bacterial protein toxins--a sensitive, specific, high-throughput tool for detection and diagnosis

Matrix-assisted laser-desorption time-of-flight (MALDI-TOF) mass spectrometry (MS) is a valuable high-throughput tool for peptide analysis. Liquid chromatography electrospray ionization (LC-ESI) tandem-MS provides sensitive and specific quantification of small molecules and peptides. The high analytic power of MS coupled with high-specificity substrates is ideally suited for detection and quantification of bacterial enzymatic activities. As specific examples of the MS applications in disease diagnosis and select agent detection, we describe recent advances in the analyses of two high profile protein toxin groups, the Bacillus anthracis toxins and the Clostridium botulinum neurotoxins. The two binary toxins produced by B. anthracis consist of protective antigen (PA) which combines with lethal factor (LF) and edema factor (EF), forming lethal toxin and edema toxin respectively. LF is a zinc-dependent endoprotease which hydrolyzes specific proteins involved in inflammation and immunity. EF is an adenylyl cyclase which converts ATP to cyclic-AMP. Toxin-specific enzyme activity for a strategically designed substrate, amplifies reaction products which are detected by MALDI-TOF-MS and LC-ESI-MS/MS. Pre-concentration/purification with toxin specific monoclonal antibodies provides additional specificity. These combined technologies have achieved high specificity, ultrasensitive detection and quantification of the anthrax toxins. We also describe potential applications to diseases of high public health impact, including Clostridium difficile glucosylating toxins and the Bordetella pertussis adenylyl cyclase.

Identification of molecular-mimicry-based ligands for cholera diagnostics using magnetic relaxation

When covalently bound to an appropriate ligand, iron oxide nanoparticles can bind to a specific target of interest. This interaction can be detected through changes in the solution's spin-spin relaxation times (T2) via magnetic relaxation measurements. In this report, a strategy of molecular mimicry was used in order to identify targeting ligands that bind to the cholera toxin B subunit (CTB). The cellular CTB-receptor, ganglioside GM1, contains a pentasaccharide moiety consisting in part of galactose and glucose units. We therefore predicted that CTB would recognize carbohydrate-conjugated iron oxide nanoparticles as GM1 mimics, thus producing a detectable change in the T2 relaxation times. Magnetic relaxation experiments demonstrated that CTB interacted with the galactose-conjugated nanoparticles. This interaction was confirmed via surface plasmon resonance studies using either the free or nanoparticle-conjugated galactose molecule. The galactose-conjugated nanoparticles were then used as CTB sensors achieving a detection limit of 40 pM. Via magnetic relaxation studies, we found that CTB also interacted with dextran-coated nanoparticles, and surface plasmon resonance studies also confirmed this interaction. Additional experiments demonstrated that the dextran-coated nanoparticle can also be used as CTB sensors and that dextran can prevent the internalization of CTB into GM1-expressing cells. Our work indicates that magnetic nanoparticle conjugates and magnetic relaxation detection can be used as a simple and fast method to identify targeting ligands via molecular mimicry. Furthermore, our results show that the dextran-coated nanoparticles represent a low-cost approach for CTB detection.

Nano aptasensor for protective antigen toxin of anthrax

We demonstrate a highly sensitive nano aptasensor for anthrax toxin through the detection of its polypeptide entity, protective antigen (PA toxin) using a PA toxin ssDNA aptamer functionalized single-walled carbon nanotubes (SWNTs) device. The aptamer was developed in-house by capillary electrophoresis systematic evolution of ligands by exponential enrichment (CE-SELEX) and had a dissociation constant (K(d)) of 112 nM. The aptasensor displayed a wide dynamic range spanning up to 800 nM with a detection limit of 1 nM. The sensitivity was 0.11 per nM, and it was reusable six times. The aptasensor was also highly selective for PA toxin with no interference from human and bovine serum albumin, demonstrating it as a potential tool for rapid and point-of-care diagnosis for anthrax.

Emerging nanotechnology-based strategies for the identification of microbial pathogenesis

Infectious diseases are still a major healthcare problem. From food intoxication and contaminated water, to hospital-acquired diseases and pandemics, infectious agents cause disease throughout the world. Despite advancements in pathogens' identification, some of the gold-standard diagnostic methods have limitations, including laborious sample preparation, bulky instrumentation and slow data readout. In addition, new field-deployable diagnostic modalities are urgently needed in first responder and point-of-care applications. Apart from compact, these sensors must be sensitive, specific, robust and fast, in order to facilitate detection of the pathogen even in remote rural areas. Considering these characteristics, researchers have utilized innovative approaches by employing the unique properties of nanomaterials in order to achieve detection of infectious agents, even in complex media like blood. From gold nanoparticles and their plasmonic shifts to iron oxide nanoparticles and changes in magnetic properties, detection of pathogens, toxins, antigens and nucleic acids has been achieved with impressive detection thresholds. Additionally, as bacteria become resistant to antibiotics, nanotechnology has achieved the rapid determination of bacterial drug susceptibility and resistance using novel methods, such as amperometry and magnetic relaxation. Overall, these promising results hint to the adoption of nanotechnology-based diagnostics for the diagnosis of infectious diseases in diverse settings throughout the globe, preventing epidemics and safeguarding human and economic wellness.

Receptors of anthrax toxin and cell entry

Anthrax toxin-receptor interactions are critical for toxin delivery to the host cell cytoplasm. This review summarizes what is known about the molecular details of the protective antigen (PA) toxin subunit interaction with either the ANTXR1 and ANTXR2 cellular receptors, and how receptor-type can dictate the low pH threshold of PA pore formation. The roles played by cellular factors in regulating the endocytosis of toxin-receptor complexes is also discussed.

Efficient neutralization of antibody-resistant forms of anthrax toxin by a soluble receptor decoy inhibitor

A soluble receptor decoy inhibitor (RDI), comprised of the extracellular I domain of ANTXR2, is a candidate anthrax therapeutic. Here we show that RDI can effectively neutralize altered forms of the protective antigen toxin subunit that are resistant to 14B7 monoclonal antibody neutralization. These data highlight the potential of RDI to act as an adjunct to existing antibody-based therapies and indicate that inhibitors based on RDI might be useful as a stand-alone treatment against specifically engineered strains of Bacillus anthracis.

Association of Bacillus anthracis capsule with lethal toxin during experimental infection

Bacillus anthracis lethal toxin (LT) was characterized in plasma from infected African Green monkeys, rabbits, and guinea pigs. In all cases, during the terminal phase of infection only the protease-activated 63-kDa form of protective antigen (PA(63)) and the residual 20-kDa fragment (PA(20)) were detected in the plasma. No uncut PA with a molecular mass of 83 kDa was detected in plasma from toxemic animals during the terminal stage of infection. PA(63) was largely associated with lethal factor (LF), forming LT. Characterization of LT by Western blotting, capture enzyme-linked immunosorbent assay, and size exclusion chromatography revealed that the antiphagocytic poly-gamma-d-glutamic acid (gamma-DPGA) capsule released from B. anthracis bacilli was associated with LT in animal blood in variable amounts. While the nature of this in vivo association is not understood, we were able to determine that a portion of these LT/gamma-DPGA complexes retained LF protease activity. Our findings suggest that the in vivo LT complexes differ from in vitro-produced LT and that including gamma-DPGA when examining the effects of LT on specific immune cells in vitro may reveal novel and important roles for gamma-DPGA in anthrax pathogenesis.

Sizing the Bacillus anthracis PA63 channel with nonelectrolyte poly(ethylene glycols)

Nonelectrolyte polymers of poly(ethylene glycol) (PEG) were used to estimate the diameter of the ion channel formed by the Bacillus anthracis protective antigen 63 (PA(63)). Based on the ability of different molecular weight PEGs to partition into the pore and reduce channel conductance, the pore appears to be narrower than the one formed by Staphylococcus aureus alpha-hemolysin. Numerical integration of the PEG sample mass spectra and the channel conductance data were used to refine the estimate of the pore's PEG molecular mass cutoff (approximately 1400 g/mol). The results suggest that the limiting diameter of the PA(63) pore is <2 nm, which is consistent with an all-atom model of the PA(63) channel and previous experiments using large ions.

Detection and quantification of anthrax lethal factor in serum by mass spectrometry

The lethal toxin produced during Bacillus anthracis infection is a complex of protective antigen, which localizes the toxin to the cell receptor, and lethal factor (LF), a zinc-dependent endoproteinase whose known targets include five members of the mitogen-activated protein kinase kinase (MAPKK) family of response regulators. We have developed a method for detecting functional LF in serum. Anti-LF murine monoclonal antibodies immobilized on magnetic protein G beads were used to capture and concentrate the LF from serum. The captured LF was exposed to an optimized MAPKK-based peptide substrate, which it hydrolyzed into two smaller peptides. The LF cleavage products were then analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MS) and quantified by isotope dilution-MS. The entire analytical method can be performed in less than 4 h with detection of LF levels as low as 0.05 ng/mL. The method was used to quantify LF levels in serum from rhesus macaques infected with B. anthracis. Serum samples obtained at day 2 postinfection contained 30-250 ng/mL LF and illustrated the clear potential to detect LF earlier in the infection cycle. This method represents a highly specific and rapid diagnostic tool for early anthrax and has a potential additional role as a research tool for understanding toxemia and effects of medical countermeasures for anthrax.

Anthrax protective antigen cleavage and clearance from the blood of mice and rats

Bacillus anthracis protective antigen (PA) is an 83-kDa (PA83) protein that is cleaved to the 63-kDa protein (PA63) as an essential step in binding and internalizing lethal factor (LF). To assess in vivo receptor saturating PA concentrations, we injected mice with PA variants and measured the PA remaining in the blood at various times using PA83- and PA63-specific enzyme-linked immunosorbent assays. We found that both wild-type PA (WT-PA) and a receptor-binding-defective mutant (Ub-PA) were cleaved to PA63 independent of their ability to bind cells. This suggested a PA-acting protease activity in the blood. The protease cleaved PA at the furin cleavage sequence because furin site-modified PA mutants were not cleaved. Cleavage measured in vitro was leupeptin sensitive and dependent on calcium. Cell surface cleavage was important for toxin clearance, however, as Ub-PA and uncleavable PA mutants were cleared at slower rates than WT-PA. The cell binding-independent cleavage of PA was also verified by using Ub-PA (which is still cleaved) to rescue mice from toxin challenge by competitively binding circulating LF. This mutant was able to rescue mice even when given 12 h before toxin challenge. Its therapeutic ability was comparable to that of dominant-negative PA, which binds cells but does not allow LF translocation, and to the protection afforded through receptor clearance by WT-PA and uncleavable receptor binding-competent mutants. The PA cleavage and clearance observed in mice did not appear to have a role in the differential mouse susceptibility as it occurred similarly in lethal toxin (LT)-resistant DBA/2J and LT-sensitive BALB/cJ mice. Interestingly, PA63 was not found in LT-resistant or -sensitive rats and PA83 clearance was slower in rats than in mice. Finally, to determine the minimum amount of PA required in circulation for LT toxicity in mice, we administered time-separated injections of PA and LF and showed that lethality of LF for mice after PA was no longer measurable in circulation, suggesting active PA sequestration at tissue surfaces.

Multiple asparagine deamidation of Bacillus anthracis protective antigen causes charge isoforms whose complexity correlates with reduced biological activity

Protective antigen is essential for the pathology of Bacillus anthracis and is the proposed immunogen for an improved human anthrax vaccine. Known since discovery to comprise differentially charged isoforms, the cause of heterogeneity has eluded specific structural definition until now. Recombinant protective antigen (rPA) contains similar isoforms that appear early in fermentation and are mostly removed through purification. By liquid chromatography-tandem mass spectrometry sequencing of the entire protein and inspection of spectral data for amino acid modifications, pharmaceutical rPA contained measurable deamidation at seven of its 68 asparagine residues. A direct association between isoform complexity and percent deamidation was observed such that each decreased with purity and increased with protein aging. Position N537 consistently showed the highest level of modification, although its predicted rate of deamidation ranked 10th by theoretical calculation, and other asparagines of higher predicted rates were observed to be unmodified. rPA with more isoforms and greater deamidation displayed lower activities for furin cleavage, heptamerization, and holotoxin formation. Lethal factor-mediated macrophage toxicity correlated inversely with deamidation at residues N466 and N408. The described method measures deamidation without employing theoretical isotopic distributions, comparison between differentially treated samples or computational predictions of reactivity rates, and is broadly applicable to the characterization of other deamidated proteins.

Anthrax toxin receptor 2 determinants that dictate the pH threshold of toxin pore formation

The anthrax toxin receptors, ANTXR1 and ANTXR2, act as molecular clamps to prevent the protective antigen (PA) toxin subunit from forming pores until exposure to low pH. PA forms pores at pH ∼6.0 or below when it is bound to ANTXR1, but only at pH ∼5.0 or below when it is bound to ANTXR2. Here, structure-based mutagenesis was used to identify non-conserved ANTXR2 residues responsible for this striking 1.0 pH unit difference in pH threshold. Residues conserved between ANTXR2 and ANTXR1 that influence the ANTXR2-associated pH threshold of pore formation were also identified. All of these residues contact either PA domain 2 or the neighboring edge of PA domain 4. These results provide genetic evidence for receptor release of these regions of PA as being necessary for the protein rearrangements that accompany anthrax toxin pore formation.

Anthrax lethal toxin impairs innate immune functions of alveolar macrophages and facilitates Bacillus anthracis survival

Alveolar macrophages (AM) are very important for pulmonary innate immune responses against invading inhaled pathogens because they directly kill the organisms and initiate a cascade of innate and adaptive immune responses. Although several factors contribute to inhalational anthrax, we hypothesized that unimpeded infection of Bacillus anthracis is directly linked to disabling the innate immune functions contributed by AM. Here, we investigated the effects of lethal toxin (LT), one of the binary complex virulence factors produced by B. anthracis, on freshly isolated nonhuman primate AM. Exposure of AM to doses of LT that killed susceptible macrophages had no effect on the viability of AM, despite complete MEK1 cleavage. Intoxicated AM remained fully capable of B. anthracis spore phagocytosis. However, pretreatment of AM with LT resulted in a significant decrease in the clearance of both the Sterne strain and the fully virulent Ames strain of B. anthracis, which may have been a result of impaired AM secretion of proinflammatory cytokines. Our data imply that cytolysis does not correlate with MEK1 cleavage, and this is the first report of LT-mediated impairment of nonhuman primate AM bactericidal activity against B. anthracis.

Past, imminent and future human medical countermeasures for anthrax

AIM

Anthrax is caused by the bacterium Bacillus anthracis. Although primarily a disease of animals, it can also infect man, sometimes with fatal consequences. As a result of concerns over the illicit use of this organism, considerable effort is focussed on the development of therapies capable of conferring protection against anthrax. This brief review will describe the efforts being made to address these issues.

METHODS AND RESULTS

A review of the literature and the proceedings of the sixth international conference on anthrax, held in Santa Fe, USA in 2005 shows intense activity, but there has been as yet no real progress. While effective antibiotics, antitoxins and vaccines are available, concerns over their toxicity and the emergence of resistant strains have driven the development of second-generation products. The principal target for vaccine development is Protective Antigen (PA), the nontoxic cell-binding component of anthrax lethal toxin. While the recombinant products currently undergoing human clinical trials will offer considerable advantages in terms of reduced side effects and ease of production, they would still require multiple, needle-based dosing, and the inclusion of the adjuvant alum makes them expensive to administer and stockpile. To address these issues, researchers are developing vaccine formulations, which stimulate rapid protection following needle-free injection (nasal, oral or transcutaneous), and are stable at room temperature to facilitate stockpiling and mass vaccination programs.

CONCLUSIONS

An array of medical countermeasures targeting B. anthracis will become available over the next 5-10 years.

SIGNIFICANCE AND IMPACT OF THE STUDY

The huge investment of research dollars is expected to dramatically expand the knowledge base. A better understanding of basic issues, such as survival in nature and pathogenesis in humans, will facilitate the development of new modalities to eliminate the threat posed by this organism.

Detection of anthrax toxin in the serum of animals infected with Bacillus anthracis by using engineered immunoassays

Several strategies that target anthrax toxin are being developed as therapies for infection by Bacillus anthracis. Although the action of the tripartite anthrax toxin has been extensively studied in vitro, relatively little is known about the presence of toxins during an infection in vivo. We developed a series of sensitive sandwich enzyme-linked immunosorbent assays (ELISAs) for detection of both the protective antigen (PA) and lethal factor (LF) components of the anthrax exotoxin in serum. The assays utilize as capture agents an engineered high-affinity antibody to PA, a soluble form of the extracellular domain of the anthrax toxin receptor (ANTXR2/CMG2), or PA itself. Sandwich immunoassays were used to detect and quantify PA and LF in animals infected with the Ames or Vollum strains of anthrax spores. PA and LF were detected before and after signs of toxemia were observed, with increasing levels reported in the late stages of the infection. These results represent the detection of free PA and LF by ELISA in the systemic circulation of two animal models exposed to either of the two fully virulent strains of anthrax. Simple anthrax toxin detection ELISAs could prove useful in the evaluation of potential therapies and possibly as a clinical diagnostic to complement other strategies for the rapid identification of B. anthracis infection.

Anthrax Biosensor, Protective Antigen Ion Channel Asymmetric Blockade

The significant threat posed by biological agents (e.g. anthrax, tetanus, botulinum, and diphtheria toxins) (Inglesby, T. V., O'Toole, T., Henderson, D. A., Bartlett, J. G., Ascher, M. S., Eitzen, E., Friedlander, A. M., Gerberding, J., Hauer, J., Hughes, J., McDade, J., Osterholm, M. T., Parker, G., Perl, T. M., Russell, P. K., and Tonat, K. (2002) J. Am. Med. Assoc. 287, 2236-2252) requires innovative technologies and approaches to understand the mechanisms of toxin action and to develop better therapies. Anthrax toxins are formed from three proteins secreted by fully virulent Bacillus anthracis, protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa), and edema factor (EF, 89 kDa). Here we present electrophysiological measurements demonstrating that full-length LF and EF convert the current-voltage relationship of the heptameric PA63 ion channel from slightly nonlinear to highly rectifying and diode-like at pH 6.6. This effect provides a novel method for characterizing functional toxin interactions. The method confirms that a previously well characterized PA63 monoclonal antibody, which neutralizes anthrax lethal toxin in animals in vivo and in vitro, prevents the binding of LF to the PA63 pore. The technique can also detect the presence of anthrax lethal toxin complex from plasma of infected animals. The latter two results suggest the potential application of PA63 nanopore-based biosensors in anthrax therapeutics and diagnostics.