Lethal factor active-site mutations affect catalytic activity in vitro

Anthrax lethal factor as an immune target in humans and transgenic mice and the impact of HLA polymorphism on CD4+ T cell immunity

Bacillus anthracis produces a binary toxin composed of protective antigen (PA) and one of two subunits, lethal factor (LF) or edema factor (EF). Most studies have concentrated on induction of toxin-specific antibodies as the correlate of protective immunity, in contrast to which understanding of cellular immunity to these toxins and its impact on infection is limited. We characterized CD4⁺ T cell immunity to LF in a panel of humanized HLA-DR and DQ transgenic mice and in naturally exposed patients. As the variation in antigen presentation governed by HLA polymorphism has a major impact on protective immunity to specific epitopes, we examined relative binding affinities of LF peptides to purified HLA class II molecules, identifying those regions likely to be of broad applicability to human immune studies through their ability to bind multiple alleles. Transgenics differing only in their expression of human HLA class II alleles showed a marked hierarchy of immunity to LF. Immunogenicity in HLA transgenics was primarily restricted to epitopes from domains II and IV of LF and promiscuous, dominant epitopes, common to all HLA types, were identified in domain II. The relevance of this model was further demonstrated by the fact that a number of the immunodominant epitopes identified in mice were recognized by T cells from humans previously infected with cutaneous anthrax and from vaccinated individuals. The ability of the identified epitopes to confer protective immunity was demonstrated by lethal anthrax challenge of HLA transgenic mice immunized with a peptide subunit vaccine comprising the immunodominant epitopes that we identified.

Anthrax is of concern with respect to human exposure in endemic regions, concerns about bioterrorism and the considerable global burden of livestock infections. The immunology of this disease remains poorly understood. Vaccination has been based on B. anthracis filtrates or attenuated spore-based vaccines, with more recent trials of next-generation recombinant vaccines. Approaches generally require extensive vaccination regimens and there have been concerns about immunogenicity and adverse reactions. An ongoing need remains for rationally designed, effective and safe anthrax vaccines. The importance of T cell stimulating vaccines is inceasingly recognized. An essential step is an understanding of immunodominant epitopes and their relevance across the diverse HLA immune response genes of human populations. We characterized CD4 T cell immunity to anthrax Lethal Factor (LF), using HLA transgenic mice, as well as testing candidate peptide epitopes for binding to a wide range of HLA alleles. We identified anthrax epitopes, noteworthy in that they elicit exceptionally strong immunity with promiscuous binding across multiple HLA alleles and isotypes. T cell responses in humans exposed to LF through either natural anthrax infection or vaccination were also examined. Epitopes identified as candidates were used to protect HLA transgenic mice from anthrax challenge.

Small molecule inhibitors of anthrax lethal factor toxin

This manuscript describes the preparation of new small molecule inhibitors of Bacillus anthracis lethal factor. Our starting point was the symmetrical, bis-quinolinyl compound 1 (NSC 12155). Optimization of one half of this molecule led to new LF inhibitors that were desymmetrized to afford more drug-like compounds.

Anthrax lethal toxin and the induction of CD4 T cell immunity

Bacillus anthracis secretes exotoxins which act through several mechanisms including those that can subvert adaptive immunity with respect both to antigen presenting cell and T cell function. The combination of Protective Antigen (PA) and Lethal Factor (LF) forming Lethal Toxin (LT), acts within host cells to down-regulate the mitogen activated protein kinase (MAPK) signaling cascade. Until recently the MAPK kinases were the only known substrate for LT; over the past few years it has become evident that LT also cleaves Nlrp1, leading to inflammasome activation and macrophage death. The predicted downstream consequences of subverting these important cellular pathways are impaired antigen presentation and adaptive immunity. In contrast to this, recent work has indicated that robust memory T cell responses to B. anthracis antigens can be identified following natural anthrax infection. We discuss how LT affects the adaptive immune response and specifically the identification of B. anthracis epitopes that are both immunogenic and protective with the potential for inclusion in protein sub-unit based vaccines.

Alkaline earth metals are not required for the restoration of the apoform of anthrax lethal factor to its holoenzyme state

Anthrax lethal factor (LF) is a zinc-dependent metalloendopeptidase previously shown to require calcium and magnesium for the restoration of its catalytic function upon exposure of the apoprotein (apoLF) to Zn(2+). Since concrete Ca(2+)/Mg(2+) binding sites have not been identified in LF, the effects of alkaline earth metals on the enzymatic function of holoLF (ZnLF) and on the reconstitution of apoLF were reinvestigated. The current study reveals alkaline earth metals to be inhibitory at concentrations higher than 1mM. A combination of activity/inhibition assays and Tb(3+) luminescence spectroscopy was employed to unequivocally establish the presence of at least one inhibitory low-affinity Ca(2+)-site in LF. A comparative analysis of apoLF preparations obtained by dialysis and centrifugal filtration (following treatment of ZnLF with chelators) revealed the exposure of apoLF to low equimolar amounts of Zn(2+) to be sufficient for the full restoration of the protein's catalytic competence, a finding constistent with the picomolar dissociation constant of ZnLF determined in this study. The previously documented requirement of Ca(2+) and Mg(2+) in apoLF reconstitution may be explicable on the basis of contamination of dialyzed apoprotein preparations with residual chelator, a phenomenon not encountered with apoLF obtained by centrifugal filtration.

Yeast-hybrid based high-throughput assay for identification of anthrax lethal factor inhibitors

Inhibitors of anthrax lethal factor (LF) are currently being sought as effective therapeutics for the treatment of anthrax. Here we report a novel screening approach for inhibitors of LF, a yeast-hybrid-based assay system in which the expression of reporter genes from a Gal4 promoter is repressed by LF proteolytic activity. Yeast cells were co-transformed with LF and a chimeric transcription factor that contains an LF substrate sequence inserted between the DNA-binding and activation domains of Gal4. In the resulting yeast cells, LF cleaves the substrate, thus inactivating the chimeric Gal4 and resulting in lack of expression of reporter genes. Compounds that inhibit LF cleavage of its substrate are identified by changes in reporter gene activity. Relative to in vitro screens for inhibitors of LF proteolytic activity, this screen has the advantage of excluding compounds that are toxic or non-permeable to eukaryotic cells. Additionally, the screen has the advantage of being fast, easy and cheap because exogenous LF and substrate are not needed. An initial chemical library screen with this system has identified four candidate inhibitors which were confirmed to inhibit LF protease activity in an in vitro assay. Furthermore, FBS-00831, one of the compounds identified, protects Raw 264.7 macrophages from anthrax lethal toxin and the possible binding site on LF was also evaluated by molecular docking.

Residue histidine 669 is essential for the catalytic activity of Bacillus anthracis lethal factor

The lethal factor (LF) of Bacillus anthracis is a Zn(2+)-dependent metalloprotease which plays an important role in anthrax virulence. This study was aimed at identifying the histidine residues that are essential to the catalytic activities of LF. The site-directed mutagenesis was employed to replace the 10 histidine residues in domains II, III, and IV of LF with alanine residues, respectively. The cytotoxicity of these mutants was tested, and the results revealed that the alanine substitution for His-669 completely abolished toxicity to the lethal toxin (LT)-sensitive RAW264.7 cells. The reason for the toxicity loss was further explored. The zinc content of this LF mutant was the same as that of the wild type. Also this LF mutant retained its protective antigan (PA)-binding activity. Finally, the catalytic cleavage activity of this mutant was demonstrated to be drastically reduced. Thus, we conclude that residue His-669 is crucial to the proteolytic activity of LF.

Anthrax lethal toxin induced lysosomal membrane permeabilization and cytosolic cathepsin release is Nlrp1b/Nalp1b-dependent

NOD-like receptors (NLRs) are a group of cytoplasmic molecules that recognize microbial invasion or 'danger signals'. Activation of NLRs can induce rapid caspase-1 dependent cell death termed pyroptosis, or a caspase-1 independent cell death termed pyronecrosis. Bacillus anthracis lethal toxin (LT), is recognized by a subset of alleles of the NLR protein Nlrp1b, resulting in pyroptotic cell death of macrophages and dendritic cells. Here we show that LT induces lysosomal membrane permeabilization (LMP). The presentation of LMP requires expression of an LT-responsive allele of Nlrp1b, and is blocked by proteasome inhibitors and heat shock, both of which prevent LT-mediated pyroptosis. Further the lysosomal protease cathepsin B is released into the cell cytosol and cathepsin inhibitors block LT-mediated cell death. These data reveal a role for lysosomal membrane permeabilization in the cellular response to bacterial pathogens and demonstrate a shared requirement for cytosolic relocalization of cathepsins in pyroptosis and pyronecrosis.

Bacillus anthracis capsule activates caspase-1 and induces interleukin-1beta release from differentiated THP-1 and human monocyte-derived dendritic cells

The poly-gamma-d-glutamic acid (PGA) capsule is one of the major virulence factors of Bacillus anthracis, which causes a highly lethal infection. The antiphagocytic PGA capsule disguises the bacilli from immune surveillance and allows unimpeded growth of bacilli in the host. Recently, efforts have been made to include PGA as a component of anthrax vaccine; however, the innate immune response of PGA itself has been poorly investigated. In this study, we characterized the innate immune response elicited by PGA in the human monocytic cell line THP-1, which was differentiated into macrophages with phorbol 12-myristate 13-acetate (PMA) and human monocyte-derived dendritic cells (hMoDCs). PGA capsules were isolated from the culture supernatant of either the pXO1-cured strain of B. anthracis H9401 or B. licheniformis ATCC 9945a. PGA treatment of differentiated THP-1 cells and hMoDCs led to the specific extracellular release of interleukin-1beta (IL-1beta) in a dose-dependent manner. Evaluation of IL-1beta processing by Western blotting revealed that cleaved IL-1beta increased in THP-1 cells and hMoDCs after PGA treatment. Enhanced processing of IL-1beta directly correlated with increased activation of its upstream regulator, caspase-1, also known as IL-1beta-converting enzyme (ICE). The extracellular release of IL-1beta in response to PGA was ICE dependent, since the administration of an ICE inhibitor prior to PGA treatment blocked induction of IL-1beta. These results demonstrate that B. anthracis PGA elicits IL-1beta production through activation of ICE in PMA-differentiated THP-1 cells and hMoDCs, suggesting the potential for PGA as a therapeutic target for anthrax.

Substrate recognition of anthrax lethal factor examined by combinatorial and pre-steady-state kinetic approaches

Lethal factor (LF), a zinc-dependent protease of high specificity produced by Bacillus anthracis, is the effector component of the binary toxin that causes death in anthrax. New therapeutics targeting the toxin are required to reduce systemic anthrax-related fatalities. In particular, new insights into the LF catalytic mechanism will be useful for the development of LF inhibitors. We evaluated the minimal length required for formation of bona fide LF substrates using substrate phage display. Phage-based selection yielded a substrate that is cleaved seven times more efficiently by LF than the peptide targeted in the protein kinase MKK6. Site-directed mutagenesis within the metal-binding site in the LF active center and within phage-selected substrates revealed a complex pattern of LF-substrate interactions. The elementary steps of LF-mediated proteolysis were resolved by the stopped-flow technique. Pre-steady-state kinetics of LF proteolysis followed a four-step mechanism as follows: initial substrate binding, rearrangement of the enzyme-substrate complex, a rate-limiting cleavage step, and product release. Examination of LF interactions with metal ions revealed an unexpected activation of the protease by Ca(2+) and Mn(2+). Based on the available structural and kinetic data, we propose a model for LF-substrate interaction. Resolution of the kinetic and structural parameters governing LF activity may be exploited to design new LF inhibitors.

Anti-toxin antibodies in prophylaxis and treatment of inhalation anthrax

The CDC recommend 60 days of oral antibiotics combined with a three-dose series of the anthrax vaccine for prophylaxis after potential exposure to aerosolized Bacillus anthracis spores. The anthrax vaccine is currently not licensed for anthrax postexposure prophylaxis and has to be made available under an Investigational New Drug protocol. Postexposure prophylaxis based on antibiotics can be problematic in cases where the use of antibiotics is contraindicated. Furthermore, there is a concern that an exposure could involve antibiotic-resistant strains of B. anthracis. Availability of alternate treatment modalities that are effective in prophylaxis of inhalation anthrax is therefore highly desirable. A major research focus toward this end has been on passive immunization using polyclonal and monoclonal antibodies against B. anthracis toxin components. Since 2001, significant progress has been made in isolation and commercial development of monoclonal and polyclonal antibodies that function as potent neutralizers of anthrax lethal toxin in both a prophylactic and therapeutic setting. Several new products have completed Phase I clinical trials and are slated for addition to the National Strategic Stockpile. These rapid advances were possible because of major funding made available by the US government through programs such as Bioshield and the Biomedical Advanced Research and Development Authority. Continued government funding is critical to support the development of a robust biodefense industry.

Inactivation of rho GTPases by statins attenuates anthrax lethal toxin activity

Anthrax lethal factor (LF), secreted by Bacillus anthracis, interacts with protective antigen to form a bipartite toxin (lethal toxin [LT]) that exerts pleiotropic biological effects resulting in subversion of the innate immune response. Although the mitogen-activated protein kinase kinases (MKKs) are the major intracellular protein targets of LF, the pathology induced by LT is not well understood. The statin family of HMG-coenzyme A reductase inhibitors have potent anti-inflammatory effects independent of their cholesterol-lowering properties, which have been attributed to modulation of Rho family GTPase activity. The Rho GTPases regulate vesicular trafficking, cytoskeletal dynamics, and cell survival and proliferation. We hypothesized that disruption of Rho GTPase function by statins might alter LT action. We show here that statins delay LT-induced death and MKK cleavage in RAW macrophages and that statin-mediated effects on LT action are attributable to disruption of Rho GTPases. The Rho GTPase-inactivating toxin, toxin B, did not significantly affect LT binding or internalization, suggesting that the Rho GTPases regulate trafficking and/or localization of LT once internalized. The use of drugs capable of inhibiting Rho GTPase activity, such as statins, may provide a means to attenuate intoxication during B. anthracis infection.

Catalytically inactive anthrax toxin(s) are potential prophylactic agents

The anthrax exotoxin, which is a key mediator of anthrax related pathogenesis, is composed of two separate toxins formed by pairwise combinations of three proteins that are encoded on the pXO1 plasmid of Bacillus anthracis. Lethal toxin is composed of protective antigen (PA) combined with lethal factor (LF) while edema toxin is composed of PA and edema factor (EF). The present study found that the catalytic mutants of LF (LFE687A) and EF (EFH351A) competitively inhibited lethal toxin and edema toxin-mediated activity in vitro and lethality in vivo and were non-toxic to sensitive cell lines when combined with PA. While PA combined with EFH351A was non-lethal in mice, PA combined with LFE687A was of reduced virulence. Full protection of mice against a lethal toxin challenge required injection of mice with PA combined with both LFE687A and EFH351A. The potential use of these full-length, biologically inactive mutant proteins combined with PA as prophylactics or therapeutics is discussed.

Development of a cell-based fluorescence resonance energy transfer reporter for Bacillus anthracis lethal factor protease

We report the construction of a cell-based fluorescent reporter for anthrax lethal factor (LF) protease activity using the principle of fluorescence resonance energy transfer (FRET). This was accomplished by engineering an Escherichia coli cell line to express a genetically encoded FRET reporter and LF protease. Both proteins were encoded in two different expression plasmids under the control of different tightly controlled inducible promoters. The FRET-based reporter was designed to contain a LF recognition sequence flanked by the FRET pair formed by CyPet and YPet fluorescent proteins. The length of the linker between both fluorescent proteins was optimized using a flexible peptide linker containing several Gly-Gly-Ser repeats. Our results indicate that this FRET-based LF reporter was readily expressed in E. coli cells showing high levels of FRET in vivo in the absence of LF. The FRET signal, however, decreased five times after inducing LF expression in the same cell. These results suggest that this cell-based LF FRET reporter may be used to screen genetically encoded libraries in vivo against LF.

Development of high-throughput assay of lethal factor using native substrate

The design of inhibitors for anthrax lethal factor (LF) is currently of interest as an approach for the treatment of anthrax because LF plays a major role in the cytotoxicity of target cells. LF is a zinc-dependent metalloprotease that specifically cleaves the mitogen-activated protein kinase kinase (MKK) family. Current assay systems for the screening of LF inhibitor use the optimized synthetic peptide coupled with various kinds of fluorophores, enabling fast, sensitive, and robust assays suited to high-throughput screening. However, evidence suggests that the regions beside the cleavage site are also involved in specificity and proteolytic activity of LF. In the current study, we tried to develop a high-throughput assay for LF activity based on native substrate, mitogen-activated ERK kinase 1 (MEK1). The assay system relies on the enhanced chemiluminescence signal resulting from a specific antibody against the C-terminal region of native substrate. A glutathione-coated multiwell plate was used as a solid support to immobilize the native substrate by its N-terminal glutathione-S-transferase moiety. Immobilized substrate increases the specificity and sensitivity of LF-catalyzed substrate hydrolysis compared with the solution phase assay. This assay system might be used to discover a wide spectrum of anthrax inhibitors.

The evolving field of biodefence: therapeutic developments and diagnostics

Bioweapons are a clear threat to both military and civilian populations. Here, the latest advances in the pursuit of inhibitors against biothreat threat toxins, current therapeutic strategies for treating biodefence related pathogens, and strategies for improving detection and exposure survivability are covered.

There are numerous lead therapeutics that have emerged from drug discovery efforts. However, many of these are toxic and/or fail to possess conventional drug-like properties. One clear advantage of small (non-peptidic) molecules is that they possess scaffolds that are inherently more likely to evolve into real therapeutics.

One of the major obstacles impeding the translation of these lead therapeutics into viable drugs is the lack of involvement of the pharmaceutical industry, which has been discovering leads and translating them into drugs for decades. The expertise of the pharmaceutical industry therefore needs to be more effectively engaged in developing drugs against biothreat agents.

New methods for rapidly detecting and diagnosing biothreat agents are also in development. The detection and diagnosis of biothreats is inherently linked with treatment. The means for detecting the release of bioweapons are being deployed, and new technologies are shortening the timeframe between initial sample collection and conclusive agent determination. However, the organization of this process is imperfect.

At present, a unifying entity that orchestrates the biodefence response is clearly needed to reduce the time-to-drug process and redundancies in drug development efforts. Such a central entity could formulate and implement plans to coordinate all participants, including academic institutions, government agencies and the private sector. This could accelerate the development of countermeasures against high probability biothreat agents.

The threat of bioterrorism and the potential use of biological weapons against both military and civilian populations has become a major concern for governments around the world. For example, in 2001 anthrax-tainted letters resulted in several deaths, caused widespread public panic and exerted a heavy economic toll. If such a small-scale act of bioterrorism could have such a huge impact, then the effects of a large-scale attack would be catastrophic. This review covers recent progress in developing therapeutic countermeasures against, and diagnostics for, such agents.

Novel repression of the glucocorticoid receptor by anthrax lethal toxin

Death from anthrax has been reported to occur from systemic shock. The lethal toxin (LeTx) is the major effector of anthrax mortality. Although the mechanism of entry of this toxin into cells is well understood, its actions once inside the cell are not as well understood. LeTx is known to cleave and inactivate MAPKKs. We have recently shown that LeTx represses the glucocorticoid receptor (GR) both in vitro and in vivo. This repression is partial and specific, repressing the glucocorticoid, progesterone, and estrogen receptor alpha, but not the mineralocorticoid or estrogen receptor beta. This toxin does not affect GR ligand or DNA binding, and we have suggested that it may function by removing/inactivating one or more of the many cofactors involved in nuclear hormone receptor signaling. Although the precise involvement of this nuclear hormone receptor repression in LeTx toxicity is unknown, examples of blunted HPA axis and glucocorticoid signaling in numerous autoimmune/inflammatory diseases suggest that such repression of critically important receptors could have deleterious effects on health.

Enhancement of anthrax lethal toxin cytotoxicity: a subset of monoclonal antibodies against protective antigen increases lethal toxin-mediated killing of murine macrophages

We investigated the ability of using monoclonal antibodies (MAbs) against anthrax protective antigen (PA), an anthrax exotoxin component, to modulate exotoxin cytotoxic activity on target macrophage cell lines. Anthrax PA plays a critical role in the pathogenesis of Bacillus anthracis infection. PA is the cell-binding component of the two anthrax exotoxins: lethal toxin (LeTx) and edema toxin. Several MAbs that bind the PA component of LeTx are known to neutralize LeTx-mediated killing of target macrophages. Here we describe for the first time an overlooked population of anti-PA MAbs that, in contrast, function to increase the potency of LeTx against murine macrophage cell lines. The results support a possible mechanism of enhancement: binding of MAb to PA on the macrophage cell surface stabilizes the PA by interaction of MAb with macrophage Fcgamma receptors. This results in an increase in the amount of PA bound to the cell surface, which in turn leads to an enhancement in cell killing, most likely due to increased internalization of LF. Blocking of PA-receptor binding eliminates enhancement by MAb, demonstrating the importance of this step for the observed enhancement. The additional significance of these results is that, at least in mice, immunization with PA appears to elicit a poly-clonal response that has a significant prevalence of MAbs that enhance LeTx-mediated killing in macrophages.

Implication of pH in the catalytic properties of anthrax lethal factor

The anthrax lethal factor (LF) is a Zn(2+)-endopeptidase specific for mitogen-activated protein kinase kinases (MAPKKs), which are cleaved within their N-terminal region. Much line of effort was carried out to elucidate the catalytic activity of LF for designing the inhibitor and to understand the cellular mechanism of its cytotoxicity. Current assay methods to analyze the LF activity have been based on a synthetic peptide, consisting of 15-20 residues around being cleaved. However, there are accumulating reports that the region distal to cleavage site is required for the LF-mediated proteolysis of substrate. In this study, we demonstrate the catalytic properties of LF, using the full-length native substrate, MEK. We described the catalytic properties of LF focused on the effects of the pH alteration, which was encountered during the endocytosis of lethal toxin, and of the requirement for metal ions. We present the first evidence that additional metal ions are required for the LF catalyzed hydrolysis of native substrate, and that the pH alteration causes a significant change of catalytic properties of LF.

Tyrosine-728 and glutamic acid-735 are essential for the metalloproteolytic activity of the lethal factor of Bacillus anthracis

The lethal factor (LF) of Bacillus anthracis is a Zn2+-endopeptidase specific for the MAPK-kinase family of proteins. The catalytic zinc atom is coordinated by a first shell of residues including the two histidines and the glutamate of the zinc-binding motif HExxH and by Glu-735. A characteristic feature of LF is the presence, within the second shell of residues, of a tyrosine (Tyr-728) in close proximity (3.3 A) to the zinc atom. To investigate the role of Tyr-728 and Glu-735, LF mutants with one or both of these two residues replaced by Ala were cloned, expressed, and purified from Escherichia coli. A fourth mutant was obtained by replacing Tyr-728 with Phe. Spectroscopic analysis of these mutants indicates that they fold in the same way as the parental molecule and that zinc stabilizes the structure of LF. These mutants have neither proteolytic activity nor in vivo toxicity. The possible role of Tyr-728 in catalysis is discussed.

The structural basis for substrate and inhibitor selectivity of the anthrax lethal factor

Recent events have created an urgent need for new therapeutic strategies to treat anthrax. We have applied a mixture-based peptide library approach to rapidly determine the optimal peptide substrate for the anthrax lethal factor (LF), a metalloproteinase with an important role in the pathogenesis of the disease. Using this approach we have identified peptide analogs that inhibit the enzyme in vitro and that protect cultured macrophages from LF-mediated cytolysis. The crystal structures of LF bound to an optimized peptide substrate and to peptide-based inhibitors provide a rationale for the observed selectivity and may be exploited in the design of future generations of LF inhibitors.

Identification of small molecule inhibitors of anthrax lethal factor

The virulent spore-forming bacterium Bacillus anthracis secretes anthrax toxin composed of protective antigen (PA), lethal factor (LF) and edema factor (EF). LF is a Zn-dependent metalloprotease that inactivates key signaling molecules, such as mitogen-activated protein kinase kinases (MAPKK), to ultimately cause cell death. We report here the identification of small molecule (nonpeptidic) inhibitors of LF. Using a two-stage screening assay, we determined the LF inhibitory properties of 19 compounds. Here, we describe six inhibitors on the basis of a pharmacophoric relationship determined using X-ray crystallographic data, molecular docking studies and three-dimensional (3D) database mining from the US National Cancer Institute (NCI) chemical repository. Three of these compounds have K(i) values in the 0.5-5 microM range and show competitive inhibition. These molecular scaffolds may be used to develop therapeutically viable inhibitors of LF.

An enzymatic electrochemiluminescence assay for the lethal factor of anthrax

The lethal factor (LF) of anthrax toxin is the toxic component of the exotoxin (lethal toxin) secreted by toxic strains of Bacillus anthracis. The lethal factor is a zinc-dependent metalloprotease that specifically cleaves the mitogen-activated protein kinase kinase (MAPKK) family of enzymes. We took advantage of this substrate specificity to develop an electrochemiluminescence (ECL) peptide cleavage assay. The ECL assay uses the stable ruthenium (Ru) metal chelate that, in the presence of tripropylamine, generates a light reaction triggered by the application of an electric potential. The Ru label is specifically incorporated into the C-terminal CYS residue of a synthetic peptide (23mer) containing the MAPKK2 cleavage sequence of LF. Streptavidin-coated paramagnetic beads were the solid phase and facilitated separation and characterization of the enzymatic reaction products based upon N-terminal biotinylation of the peptide substrate. Intact peptide bound via the biotin moiety generated high signal due to the Ru label, whereas binding of the cleaved peptide fragment devoid of Ru label reduced the ECL signal. The proposed assay provides a novel opportunity for the screening of potential therapeutics against anthrax.

Production and proteolytic assay of lethal factor from Bacillus anthracis

Bacillus anthracis is the causative agent of anthrax. The major virulence factors are a poly-D-glutamic acid capsule and three-protein component exotoxin, protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa), and edema factor (EF, 89 kDa), respectively. These three proteins individually have no known toxic activities, but in combination with PA form two toxins (lethal toxin or edema toxin), causing different pathogenic responses in animals and cultured cells. In this study, we constructed and produced rLF as a form of GST fusion protein in Escherichia coli. rLF was rapidly purified through a single affinity purification step to near homogeneity. Furthermore, we developed an in vitro immobilized proteolytic assay of LF under the condition containing full-length native substrate, MEK1, rather than short synthetic peptide. The availability of full-length substrate and of an immobilized LF assay could facilitate not only the in-depth investigation of structure-function relationship of the enzyme toward its substrate but also wide spectrum screening of inhibitor collections based on the 96-well plate system.

Genetic and biochemical characterization of PrtA, an RTX-like metalloprotease from Photorhabdus

Proteases play a key role in the interaction between pathogens and their hosts. The bacterial entomopathogen Photorhabdus lives in symbiosis with nematodes that invade insects. Following entry into the insect, the bacteria are released from the nematode gut into the open blood system of the insect. Here they secrete factors which kill the host and also convert the host tissues into food for the replicating bacteria and nematodes. One of the secreted proteins is PrtA, which is shown here to be a repeats-in-toxin (RTX) alkaline zinc metalloprotease. PrtA has high affinity for artificial substrates such as casein and gelatin and can be inhibited by zinc metalloprotease inhibitors. The metalloprotease also shows a calcium- and temperature-dependent autolysis. The prtA gene carries the characteristic RTX repeated motifs and predicts high similarity to proteases from Erwinia chrysanthemi, Pseudomonas aeruginosa and Serratia marcescens. The prtA gene resides in a locus encoding both the protease ABC transporter (prtBCD) and an intervening ORF encoding a protease inhibitor (inh). PrtA activity is detectable 24 h after artificial bacterial infection of an insect, suggesting that the protease may play a key role in degrading insect tissues rather than in overcoming the insect immune system. Purified PrtA also shows cytotoxicity to mammalian cell cultures, supporting its proposed role in bioconversion of the insect cadaver into food for bacterial and nematode development.

Macrophage interactions

B. anthracis virulence is the sum of the contributions of factors involved in toxicity, growth and persistence in the host. Recent data has revealed that the interactions between B. anthracis and macrophage is central to the B. anthracis pathogenesis. This review presents and describes tactics by which B. anthracis not only overcomes and avoids macrophages but also perverts the host defense immune system and defense-related products to its advantage. The understanding of the complex network of such interactions is likely to allow new therapeutic and preventative strategies to be developed.

A zinc metalloprotease inhibitor, Inh, from the insect pathogen Photorhabdus luminescens

The entomopathogen Photorhabdus luminescens secretes many proteins during the late stages of insect larvae infection and during in vitro laboratory culture. The authors have previously characterized and purified a 55 kDa zinc metalloprotease, PrtA, from culture supernatants of P. luminescens. PrtA is secreted via a classical type I secretory pathway and is encoded within the operon prtA-inh-prtBCD. The 405 bp inh gene encodes a 14.8 kDa pre-protein that is translocated to the periplasm by the classical signal-peptide-dependent sec pathway, yielding the mature 11.9 kDa inhibitor Inh. Inh is a specific inhibitor of the protease PrtA. This study describes the purification of Inh and the initial characterization of its in vitro protease inhibition properties.

A peptide-based fluorescence resonance energy transfer assay for Bacillus anthracis lethal factor protease

A fluorescence resonance energy transfer assay has been developed for monitoring Bacillus anthracis lethal factor (LF) protease activity. A fluorogenic 16-mer peptide based on the known LF protease substrate MEK1 was synthesized and found to be cleaved by the enzyme at the anticipated site. Extension of this work to a fluorogenic 19-mer peptide, derived, in part, from a consensus sequence of known LF protease targets, produced a much better substrate, cleaving approximately 100 times more efficiently. This peptide sequence was modified further on resin to incorporate donor/quencher pairs to generate substrates for use in fluorescence resonance energy transfer-based appearance assays. All peptides cleaved at similar rates with signal/background ranging from 9-16 at 100% turnover. One of these substrates, denoted (Cou)Consensus(K(QSY-35)GG)-NH(2), was selected for additional assay optimization. A plate-based assay requiring only low nanomolar levels of enzyme was developed for screening and inhibitor characterization.

Kif1C, a kinesin-like motor protein, mediates mouse macrophage resistance to anthrax lethal factor

BACKGROUND

Inbred mouse strains exhibit striking differences in the susceptibility of their macrophages to the effects of anthrax lethal toxin (LeTx). Previous data has shown that this difference in susceptibility lies downstream of toxin entry into macrophages. A locus controlling this phenotype, called Ltxs1, has been mapped to chromosome 11, but the responsible gene has not been identified.

RESULTS

Here, we report the identification of the Ltxs1 gene as Kif1C, which encodes a kinesin-like motor protein of the UNC104 subfamily. Kif1C is the only gene in the Ltxs1 interval exhibiting polymorphisms between susceptible and resistant strains. Multiple alleles of Kif1C determine the susceptibility or resistance of cultured mouse macrophages to LeTx. Treatment of resistant macrophages with brefeldin-A (which alters the cellular localization of Kif1C) induces susceptibility to LeTx, while ectopic expression of a resistance allele of Kif1C in susceptible macrophages causes a 4-fold increase in the number of cells surviving LeTx treatment. We also show that cleavage of map kinase kinase 3, a target of LeTx proteolysis, occurs in resistant cells.

CONCLUSIONS

We conclude that mutations in Kif1C are responsible for the differences in the susceptibility of inbred mouse macrophages to LeTx and that proper Kif1C function is required for LeTx resistance. Since the LeTx-mediated proteolysis of map kinase kinase 3 occurs even in resistant cells, Kif1C does not affect cellular entry or processing of LeTx and likely influences events occurring later in the intoxication pathway.

Susceptibility of mitogen-activated protein kinase kinase family members to proteolysis by anthrax lethal factor

The lethal factor (LF) produced by toxigenic strains of Bacillus anthracis is a Zn(2+)-endopeptidase that cleaves the mitogen-activated protein kinase kinases (MAPKKs) MEK1, MEK2 and MKK3. Using genetic and biochemical approaches, we have extended the study of LF proteolytic specificity to all known MAPKK family members and found that LF also cleaves MKK4, MKK6 and MKK7, but not MEK5. The peptide bonds hydrolysed by LF within all MAPKKs were identified. Cleavage invariably occurs within the N-terminal proline-rich region preceding the kinase domain, thus disrupting a sequence involved in directing specific protein-protein interactions necessary for the assembly of signalling complexes. Alignment of the sequences flanking the site of cleavage reveals the occurrence of some consensus motifs: position P2 and P1' are occupied by hydrophobic residues and at least one basic residue is present between P4 and P7. The implications of these findings for the biochemical activity and functional specificity of LF are discussed.

Extracellular targeting of choline-binding proteins in Streptococcus pneumoniae by a zinc metalloprotease

A genetic-based search for surface proteins of Streptococcus pneumoniae involved in adhesion identified a putative zinc metalloprotease (ZmpB). ZmpB shared high amino acid sequence similarities with IgA1 proteases of Gram-positive bacteria, but ZmpB had neither IgA1 nor IgA2 protease activity. Analysis of a family of surface-expressed proteins, the choline-binding proteins (Cbp's), in a zmpB-deficient mutant demonstrated a global loss of surface expression of CbpA, CbpE, CbpF and CbpJ. CbpA was detected within the cytoplasm. The zmpB-deficient mutant also failed to lyse with penicillin, a sign of lack of function of the Cbp LytA. Immunodetection studies revealed that the autolysin (LytA), normally located on the cell wall, was trapped in the cytoplasm colocalized with DNA and the transformation protein CinA. Trafficking of CinA and RecA to the cell membrane during genetic competence was also not observed in the zmpB-deficient mutant. These results suggest a protease dependent regulatory mechanism governing the translocation of CinA and the Cbp's LytA and CbpA of S. pneumoniae.

Microbial metalloproteases and pathogenesis

Zinc metalloproteases produced by human pathogenic microorganisms show a wide variety of pathological actions. In local infections, the proteases cause necrotic or hemorrhagic tissue damage through digestion of structural components of the ground substance, and also form edematous lesions through generation of inflammatory mediators, while in systemic infections, the proteases act as a synergistic virulence factor through disordered proteolysis of many plasma proteins. Clostridial neurotoxins, Bacteroides fragilis enterotoxin and Bacillus anthracis lethal factor are also zinc metalloproteases.

Lethal toxin actions and their consequences

After entry of infectious anthrax spores into the body, host-specific signals induce spore germination, outgrowth of vegetative bacilli and the expression of lethal toxin and other virulence factors. Anthrax lethal toxin (LeTx) is a virulence factor responsible for the major pathologies seen during systemic anthrax infections. Injection of sterile LeTx into test animals mimics the shock and sudden death seen during active bacterial infections. Once large levels of LeTx are produced within the body, destruction of bacteria by administration of antibiotics is usually unsuccessful. The LeTx is believed to be secreted into the bloodstream where it circulates freely throughout the body and binds and enters host cells. Once in the cytoplasm, the lethal factor acts as a zinc-metalloprotease disrupting normal homoeostatic functions. Macrophages are a uniquely sensitive cell type that seem to be vital global mediators of toxin-induced pathologies. Removal of macrophages from mice renders them insensitive to LeTx challenge. Low levels of lethal toxin induce macrophage production, in vitro, of the shock-inducing cytokines TNF and Il-1beta. Higher levels of LeTx cause over-production of reactive oxygen intermediates, bursting of macrophages and release of mediators of shock. We believe that agents capable of blocking key steps of the lethal toxin cascade may prove useful in combating anthrax pathologies.

Anthrax lethal factor causes proteolytic inactivation of mitogen-activated protein kinase kinase

A search of the National Cancer Institute's Anti-Neoplastic Drug Screen for compounds with an inhibitory profile similar to that of the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD098059 yielded anthrax lethal toxin. Anthrax lethal factor was found to inhibit progesterone-induced meiotic maturation of frog oocytes by preventing the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Similarly, lethal toxin prevented the activation of MAPK in serum stimulated, ras-transformed NIH3T3 cells. In vitro analyses using recombinant proteins indicated that lethal factor proteolytically modified the NH2-terminus of both MAPKK1 and 2, rendering them inactive and hence incapable of activating MAPK. The consequences of this inactivation upon meiosis and transformed cells are also discussed.

Disruption of anthrax toxin binding with the use of human antibodies and competitive inhibitors

The protective antigen (PA83) of Bacillus anthracis is integral to the mechanism of anthrax toxicity. We have isolated a human single-chain Fv antibody fragment (scFv) that blocks binding of a fluorescently tagged protective antigen (PA) moiety to cell surface receptors. Several phage-displayed scFv were isolated from a naive library biopanned against PA83. Soluble, monomeric scFv were characterized for affinity and screened for their capacity to disrupt receptor-mediated binding of PA. Four unique scFv bound to PA83, as determined by surface plasmon resonance, the tightest binder exhibiting a Kd of 50 nM. Two scFv had similar affinities for natural PA83 and a novel, recombinant, 32-kDa carboxy-terminal PA fragment (PA32). Binding of scFv to green fluorescent protein fused to the amino-terminal 32-kDa fragment of B. anthracis edema factor, EGFP-EF32, was used to confirm specificity. Fusion of EGFP to PA32 facilitated development of a novel flow cytometric assay that showed that one of the scFv disrupted PA receptor binding. This method can now be used as a rapid assay for small molecule inhibitors of PA binding to cell receptors. The combined data presented suggest the potential utility of human scFv as prophylactics against anthrax poisoning. Moreover, recombinant PA32 may also be useful as a therapeutic agent to compete with anthrax toxins for cellular receptors during active infection.

Proteasome activity is required for anthrax lethal toxin to kill macrophages

Anthrax lethal toxin (LeTx), consisting of protective antigen (PA) and lethal factor (LF), rapidly kills primary mouse macrophages and macrophage-like cell lines such as RAW 264.7. LF is translocated by PA into the cytosol of target cells, where it acts as a metalloprotease to cleave mitogen-activated protein kinase kinase 1 (MEK1) and possibly other proteins. In this study, we show that proteasome inhibitors such as acetyl-Leu-Leu-norleucinal, MG132, and lactacystin efficiently block LeTx cytotoxicity, whereas other protease inhibitors do not. The inhibitor concentrations that block LF cytotoxicity are similar to those that inhibit the proteasome-dependent IkappaB-alpha degradation induced by lipopolysaccharide. The inhibitors did not interfere with the proteolytic cleavage of MEK1 in LeTx-treated cells, indicating that they do not directly block the proteolytic activity of LF. However, the proteasome inhibitors did prevent ATP depletion, an early effect of LeTx. No overall activation of the proteasome by LeTx was detected, as shown by the cleavage of fluorogenic substrates of the proteasome. All of these results suggest that the proteasome mediates a toxic process initiated by LF in the cell cytosol. This process probably involves degradation of unidentified molecules that are essential for macrophage homeostasis. Moreover, this proteasome-dependent process is an early step in LeTx intoxication, but it is downstream of the cleavage by LF of MEK1 or other putative substrates.

Molecular pathogenesis of Bacillus anthracis infection

This review summarizes the current knowledge pertaining to the pathogenesis of infection with Bacillus anthracis relative to the two exotoxins and the capsule. Emphasis is given to the structure and activities of the individual components of the exotoxins, their interaction with cells, and the response of macrophages to lethal toxin. Finally, results from vaccination studies are reviewed.