Where and how do anthrax toxins exit endosomes to intoxicate host cells?

Effect of late endosomal DOBMP lipid and traditional model lipids of electrophysiology on the anthrax toxin channel activity

Anthrax toxin action requires triggering of natural endocytic transport mechanisms whereby the binding component of the toxin forms channels (PA₆₃) within endosomal limiting and intraluminal vesicle membranes to deliver the toxin's enzymatic components into the cytosol. Membrane lipid composition varies at different stages of anthrax toxin internalization, with intraluminal vesicle membranes containing ~70% of anionic bis(monoacylglycero)phosphate lipid. Using model bilayer measurements, we show that membrane lipids can have a strong effect on the anthrax toxin channel properties, including the channel-forming activity, voltage-gating, conductance, selectivity, and enzymatic factor binding. Interestingly, the highest PA₆₃ insertion rate was observed in bis(monoacylglycero)phosphate membranes. The molecular dynamics simulation data show that the conformational properties of the channel are different in bis(monoacylglycero)phosphate compared to PC, PE, and PS lipids. The anthrax toxin protein/lipid bilayer system can be advanced as a novel robust model to directly investigate lipid influence on membrane protein properties and protein/protein interactions.

Amiodarone impairs trafficking through late endosomes inducing a Niemann-Pick C-like phenotype

Patients treated with amiodarone accumulate lysobisphosphatidic acid (LBPA), also known as bis(monoacylglycero)phosphate, in airway secretions and develop in different tissues vacuoles and inclusion bodies thought to originate from endosomes. To clarify the origin of these changes, we studied in vitro the effects of amiodarone on endosomal activities like transferrin recycling, Shiga toxin processing, ESCRT-dependent lentivirus budding, fluid phase endocytosis, proteolysis and exosome secretion. Furthermore, since the accumulation of LBPA might point to a broader disturbance in lipid homeostasis, we studied the effect of amiodarone on the distribution of LBPA, unesterified cholesterol, sphingomyelin and glycosphyngolipids. Amiodarone analogues were also studied, including the recently developed derivative dronedarone. We found that amiodarone does not affect early endosomal activities, like transferrin recycling, Shiga toxin processing and lentivirus budding. Amiodarone, instead, interferes with late compartments of the endocytic pathway, blocking the progression of fluid phase endocytosis and causing fusion of organelles, collapse of lumenal structures, accumulation of undegraded substrates and amassing of different types of lipids. Not all late endocytic compartments are affected, since exosome secretion is spared. These changes recall the Niemann-Pick type-C phenotype (NPC), but originate by a different mechanism, since, differently from NPC, they are not alleviated by cholesterol removal. Studies with analogues indicate that basic pKa and high water-solubility at acidic pH are crucial requirements for the interference with late endosomes/lysosomes and that, in this respect, dronedarone is at least as potent as amiodarone. These findings may have relevance in fields unrelated to rhythm control.

Blockage of anthrax PA63 pore by a multicharged high-affinity toxin inhibitor

Single channels of Bacillus anthracis protective antigen, PA(63), were reconstituted into planar lipid membranes and their inhibition by cationic aminopropylthio-beta-cyclodextrin, AmPrbetaCD, was studied. The design of the highly efficient inhibitor, the sevenfold symmetrical cyclodextrin molecule chemically modified to add seven positive charges, was guided by the symmetry and predominantly negative charge of the PA(63) pore. The protective action of this compound has been demonstrated earlier at both single-molecule and whole-organism levels. In this study, using noise analysis, statistics of time-resolved single-channel closure events, and multichannel measurements, we find that AmPrbetaCD action is bimodal. The inhibitor, when added to the cis side of the membrane, blocks the channel reversibly. At high salt concentrations, the AmPrbetaCD blockage of the channel is well described as a two-state Markov process, in which both the on- and off-rates are functions of the salt concentration, whereas the applied voltage affects only the off-rate. At salt concentrations smaller than 1.5 M, the second mode of AmPrbetaCD action on the channel is discovered: addition of the inhibitor enhances voltage gating, making the closed states of the channel more favorable. The effect depends on the lipid composition of the membrane.

Imaging the cell entry of the anthrax oedema and lethal toxins with fluorescent protein chimeras

To investigate the cell entry and intracellular trafficking of anthrax oedema factor (EF) and lethal factor (LF), they were C-terminally fused to the enhanced green fluorescent protein (EGFP) and monomeric Cherry (mCherry) fluorescent proteins. Both chimeras bound to the surface of BHK cells treated with protective antigen (PA) in a patchy mode. Binding was followed by rapid internalization, and the two anthrax factors were found to traffic along the same endocytic route and with identical kinetics, indicating that their intracellular path is essentially dictated by PA. Colocalization studies indicated that anthrax toxins enter caveolin-1 containing compartments and then endosomes marked by phoshatidylinositol 3-phoshate and Rab5, but not by early endosome antigen 1 and transferrin. After 40 min, both EF and LF chimeras were observed to localize within late compartments. Eventually, LF and EF appeared in the cytosol with a time-course consistent with translocation from late endosomes. Only the EGFP derivatives reached the cytosol because they are translocated by the PA channel, while the mCherry derivatives are not. This difference is attributed to a higher resistance of mCherry to unfolding. After translocation, LF disperses in the cytosol, while EF localizes on the cytosolic face of late endosomes.

Inhibition of tumor angiogenesis by the matrix metalloproteinase-activated anthrax lethal toxin in an orthotopic model of anaplastic thyroid carcinoma

Patients with anaplastic thyroid carcinoma (ATC) typically succumb to their disease months after diagnosis despite aggressive therapy. A large percentage of ATCs have been shown to harbor the V600E B-Raf point mutation, leading to the constitutive activation of the mitogen-activated protein kinase pathway. ATC invasion, metastasis, and angiogenesis are in part dependent on the gelatinase class of matrix metalloproteinases (MMP). The explicit targeting of these two tumor markers may provide a novel therapeutic strategy for the treatment of ATC. The MMP-activated anthrax lethal toxin (LeTx), a novel recombinant protein toxin combination, shows potent mitogen-activated protein kinase pathway inhibition in gelatinase-expressing V600E B-Raf tumor cells in vitro. However, preliminary in vivo studies showed that the MMP-activated LeTx also exhibited dramatic antitumor activity against xenografts that did not show significant antiproliferative responses to the LeTx in vitro. Here, we show that the MMP-activated LeTx inhibits orthotopic ATC xenograft progression in both toxin-sensitive and toxin-resistant ATC cells via reduced endothelial cell recruitment and subsequent tumor vascularization. This in turn translates to an improved long-term survival that is comparable with that produced by the multikinase inhibitor sorafenib. Our results also indicate that therapy with the MMP-activated LeTx is extremely effective against advanced tumors with well-established vascular networks. Taken together, these results suggest that the MMP-activated LeTx-mediated endothelial cell targeting is the primary in vivo antitumor mechanism of this novel toxin. Therefore, the MMP-activated LeTx could be used not only in the clinical management of V600E B-Raf ATC but potentially in any solid tumor.

Anthrax toxin triggers the activation of src-like kinases to mediate its own uptake

AB-type toxins, like other bacterial toxins, are notably opportunistic molecules. They rely on target cell receptors to reach the appropriate location within the target cell where translocation of their enzymatic subunits occurs. The anthrax toxin, however, times its own uptake, suggesting that toxin binding triggers specific signaling events. Here we show that the anthrax toxin triggers tyrosine phosphorylation of its own receptors, capillary morphogenesis gene 2 and tumor endothelial marker 8, which are not endowed with intrinsic kinase activity. This is required for efficient toxin uptake because endocytosis of the mutant receptor lacking the cytoplasmic tyrosine residues is strongly delayed. Phosphorylation of the receptors was dependent on src-like kinases, which where activated upon toxin binding. Importantly, src-dependent phosphorylation of the receptor was required for its subsequent ubiquitination, which in turn was required for clathrin-mediated endocytosis. Consistently, we found that uptake of the anthrax toxin and processing of the lethal factor substrate MEK1 are inhibited by silencing of src and fyn, as well as in src and fyn knockout cells.

Cholesterol specificity of some heptameric beta-barrel pore-forming bacterial toxins: structural and functional aspects

Apart from the thiol-specific/cholesterol-dependent cytolysin family of toxins (see Chapter 20) there are a number of other unrelated bacterial toxins that also have an affinity for plasma membrane cholesterol. Emphasis is given here on the Vibrio cholerae cytolysin (VCC) and the cytolysins from related Vibrio species. The inhibition of the cytolytic activity of these toxins by prior incubation with extracellular cholesterol or low density lipoprotein emerges as a unifying feature, as does plasma membrane cholesterol depletion. Incubation of VCC with cholesterol produces a heptameric oligomer, which is not equivalent to the pre-pore since it is unable to penetrate the plasma membrane. In structural terms, the precise sequence of VCC monomer binding to membrane, oligomer formation and pore insertion through the bilayer has yet to be fully defined. Several other bacterial toxins have a dependency for cholesterol, although the available data is limited in most cases.

Membrane translocation by anthrax toxin

Much attention has been focused on anthrax toxin recently, both because of its central role in the pathogenesis of Bacillus anthracis and because it has proven to be one of the most tractable toxins for studying how enzymic moieties of intracellularly acting toxins traverse membranes. The Protective Antigen (PA) moiety of the toxin, after being proteolytically activated at the cell surface, self-associates to form a heptameric pore precursor (prepore). The prepore binds up to three molecules of Edema Factor (EF), Lethal Factor (LF), or both, forming a series of complexes that are then endocytosed. Under the influence of acidic pH within the endosome, the prepore undergoes a conformational transition to a mushroom-shaped pore, with a globular cap and 100A-long stem that spans the membrane. Electrophysiological studies in planar bilayers indicate that EF and LF translocate through the pore in unfolded form and in the N- to C-terminal direction. The pore serves as an active transporter, which translocates its proteinaceous cargo across the endosomal membrane in response to DeltapH and perhaps, to a degree, Deltapsi. A ring of seven Phe residues (Phe427) in the lumen of the pore forms a seal around the translocating polypeptide and blocks the passage of ions, presumably preserving the pH gradient. A charge state-dependent Brownian ratchet mechanism has been proposed to explain how the pore translocates EF and LF. This transport mechanism of the pore may function in concert with molecular chaperonins to effect delivery of effector proteins in catalytically active form to the cytosolic compartment of host cells.

Matrix metalloproteinase-activated anthrax lethal toxin inhibits endothelial invasion and neovasculature formation during in vitro morphogenesis

Solid tumor growth is dependent on angiogenesis, the formation of neovasculature from existing vessels. Endothelial activation of the extracellular signal-regulated kinase 1/2, c-jun NH(2)-terminal kinase, and p38 mitogen-activated protein kinase pathways is central to this process, and thus presents an attractive target for the development of angiogenesis inhibitors. Anthrax lethal toxin (LeTx) has potent catalytic mitogen-activated protein kinase inhibition activity. Preclinical studies showed that LeTx induced potent tumor growth inhibition via the inhibition of xenograft vascularization. However, LeTx receptors and the essential furin-like activating proteases are expressed in many normal tissues, potentially limiting the specificity of LeTx as an antitumor agent. To circumvent nonspecific LeTx activation and simultaneously enhance tumor vascular targeting, a substrate preferably cleaved by the gelatinases class of matrix metalloproteinases (MMP) was substituted for the furin LeTx activation site. In vivo efficacy studies showed that this MMP-activated LeTx inhibited tumor xenografts growth via the reduced migration of endothelial cells into the tumor parenchyma. Here we have expanded on these initial findings by showing that this MMP-activated LeTx reduces endothelial proangiogenic MMP expression, thus causing a diminished proteolytic capacity for extracellular matrix remodeling and endothelial differentiation into capillary networks. Additionally, our data suggest that inhibition of the c-jun NH(2)-terminal kinase and p38, but not extracellular signal-regulated kinase-1/2, pathways is significant in the antiangiogenic activity of the MMP-activated LeTx. Collectively, these results support the clinical development of the MMP-activated LeTx for the treatment of solid tumors.

Thioamide hydroxypyrothiones supersede amide hydroxypyrothiones in potency against anthrax lethal factor

Anthrax lethal factor (LF) is a critical virulence factor in the pathogenesis of anthrax. A structure-activity relationship (SAR) of potential lethal factor inhibitors (LFi) is presented in which the zinc-binding group (ZBG), linker, and backbone moieties for a series of hydroxypyrone-based compounds were systematically varied. It was found that hydroxypyrothione ZBGs generate more potent inhibitors than hydroxypyrone ZBGs. Furthermore, coupling the hydroxypyrothione to a backbone group via a thioamide bond improves potency when compared to an amide linker. QM/MM studies show that the thioamide bond in these inhibitors allows for the formation of two additional hydrogen bonds with the protein active site. In both types of hydroxypyrothione compounds, ligand efficiencies of 0.29-0.54 kcal mol(-1) per heavy atom were achieved. The results highlight the need for a better understanding to optimize the interplay between the ZBG, linker, and backbone to get improved LFi.

The CMG2 ELISA for evaluating inhibitors of the binding of anthrax toxin protective antigen to its receptor

INTRODUCTION

Anthrax toxin comprises a protective antigen (PA) of MW 83 kDa, a lethal factor (LF) and an edema factor (EF). Upon binding to its receptor on cell surfaces, PA(83) is enzymatically cleaved to a 63 kDa product (PA(63)), followed by binding of LF or EF, receptor-mediated internalisation of these factors, and production of their toxic effects. The high-affinity binding of PA(83) to its receptor is essential for the intoxication process. To study the interaction between the PA and its receptor, and inhibition of the binding, an enzyme-linked immunosorbent assay (ELISA) was developed.

METHODS

One of the two known anthrax toxin receptors (capillary morphogenesis factor 2; CMG2) was adsorbed onto wells of a 96-well plate. Either PA(83) or PA(63) was then added to the receptor-coated wells, followed by sequential addition of anti-PA antibody, anti-species antibody-enzyme conjugate, and enzyme substrate at appropriate time intervals.

RESULTS

Best results were obtained by overnight incubation of CMG2 in PBS at 4 degrees C. CMG2 was used at 1 microg/ml because of the cost of the commercial product. The rate of change of absorbance was low, and was measured over 3 h to obtain accurate results. The assay results increased almost linearly with CMG2 concentration to 10 microg/ml. PA(83) was also used at 1 microg/ml, but the assay values reached a plateau at approx. 10 microg/ml. Binding was divalent cation-dependent, almost irreversible, and free CMG2 was a potent inhibitor of binding (I(50) in the nM range). Binding of PA(63) was similar to that of PA(83).

DISCUSSION

The high-affinity binding and divalent cation dependence confirm the validity of the assay as a model for toxin-receptor binding in vivo and as a means of evaluating toxin-receptor binding and inhibitors of the binding. Attempts to use crude lysates of J774A.1 cells or von Willebrand factor as an alternative source of anthrax toxin receptor were not successful.

Anthrax edema toxin modulates PKA- and CREB-dependent signaling in two phases

Background

Anthrax edema toxin (EdTx) is an adenylate cyclase which operates in the perinuclear region of host cells. However, the action of EdTx is poorly understood, especially at molecular level. The ability of EdTx to modulate cAMP-dependent signaling was studied in Jurkat T cells and was compared with that of other cAMP-rising agents: Bordetella pertussis adenylate cyclase toxin, cholera toxin and forskolin.

Methodology/Principal Findings

EdTx caused a prolonged increase of the intracellular cAMP concentration. This led to nuclear translocation of the cAMP-dependent protein kinase (PKA) catalytic subunit, phosphorylation of cAMP response element binding protein (CREB) and expression of a reporter gene under control of the cAMP response element. Neither p90 ribosomal S6 kinase nor mitogen- and stress-activated kinase, which mediate CREB phosphorylation during T cell activation, were involved. The duration of phospho-CREB binding to chromatin correlated with the spatio-temporal rise of cAMP levels. Strikingly, EdTx pre-treated T cells were unresponsive to other stimuli involving CREB phosphorylation such as addition of forskolin or T cell receptor cross-linking.

Conclusions/Significance

We concluded that, in a first intoxication phase, EdTx induces PKA-dependent signaling, which culminates in CREB phosphorylation and activation of gene transcription. Subsequently CREB phosphorylation is impaired and therefore T cells are not able to respond to cues involving CREB. The present data functionally link the perinuclear localization of EdTx to its intoxication mechanism, indicating that this is a specific feature of its intoxication mechanism.

cAMP imaging of cells treated with pertussis toxin, cholera toxin, and anthrax edema toxin

The enzymatic activity of the three most studied bacterial toxins that increase the cytosolic cAMP level: pertussis toxin (PT), cholera toxin (CT), and anthrax edema toxin (ET), was imaged by fluorescence videomicroscopy. Three different cell lines were transfected with a fluorescence resonance energy transfer biosensor based on the PKA regulatory and catalytic subunits fused to CFP and YFP, respectively. Real-time imaging of cells expressing this cAMP biosensor provided time and space resolved pictures of the toxins action. The time course of the PT-induced cAMP increase suggests that its active subunit enters the cytosol more rapidly than that deduced by biochemical experiments. ET generated cAMP concentration gradients decreasing from the nucleus to the cell periphery. On the contrary, CT, which acts on the plasma membrane adenylate cyclase, did not. The potential of imaging methods in studying the mode of entry and the intracellular action of bacterial toxins is discussed.

Cytotoxicity of the matrix metalloproteinase-activated anthrax lethal toxin is dependent on gelatinase expression and B-RAF status in human melanoma cells

Anthrax lethal toxin (LeTx) shows potent mitogen-activated protein kinase pathway inhibition and apoptosis in melanoma cells that harbor the activating V600E B-RAF mutation. LeTx is composed of two proteins, protective antigen and lethal factor. Uptake of the toxin into cells is dependent on proteolytic activation of protective antigen by the ubiquitously expressed furin or furin-like proteases. To circumvent nonspecific LeTx activation, a substrate preferably cleaved by gelatinases was substituted for the furin LeTx activation site. Here, we have shown that the toxicity of this matrix metalloproteinase (MMP)-activated LeTx is dependent on host cell surface MMP-2 and MMP-9 activity as well as the presence of the activating V600E B-RAF mutation, making this toxin dual specific. This additional layer of tumor cell specificity would potentially decrease systemic toxicity from the reduction of nonspecific toxin activation while retaining antitumor efficacy in patients with V600E B-RAF melanomas. Moreover, our results indicate that cell surface-associated gelatinase expression can be used to predict sensitivity among V600E B-RAF melanomas. This finding will aid in the better selection of patients that will potentially respond to MMP-activated LeTx therapy.

Co-evolution and exploitation of host cell signaling pathways by bacterial pathogens

Bacterial pathogens have evolved by combinations of gene acquisition, deletion, and modification, which increases their fitness. Additionally, bacteria are able to evolve in "quantum leaps" via the ability to promiscuously acquire new genes. Many bacterial pathogens - especially Gram-negative enteric pathogens - have evolved mechanisms by which to subvert signal transduction pathways of eukaryotic cells by expressing genes that mimic or regulate host protein factors involved in a variety of signaling cascades. This results in the ability to cause diseases ranging from tumor formation in plants to gastroenteritis and bubonic plague. Here, we present recent advances on mechanisms of bacterial pathogen evolution, including specific signaling cascades targeted by their virulence genes with an emphasis on the ubiquitin modification system, Rho GTPase regulators, cytoskeletal modulators, and host innate immunity. We also comment briefly on evolution of host defense mechanisms in place that limit disease caused by bacterial pathogens.

A novel pathway for exotoxin delivery by an intracellular pathogen

Fundamental to the biology of many bacterial pathogens are bacterial proteins with the capacity to modulate host cellular functions. These bacterial proteins are delivered to the host's molecular targets by a great diversity of mechanisms of varying complexity. The different delivery mechanisms are adapted to the specific biology of the pathogen. Here we focus our attention on a recently described delivery pathway adapted to the biology of an intracellular pathogen, in which an exotoxin is delivered from an intracellular location to its molecular target through autocrine and paracrine pathways.