Engineering anthrax toxin variants that exclusively form octamers and their application to targeting tumors

Anthrax lethal toxin exerts potent metabolic inhibition of the cardiovascular system

Bacillus anthracis causes anthrax through a combination of bacterial infection and toxemia. As a major virulence factor of B. anthracis, anthrax lethal toxin (LT) is a zinc-dependent metalloproteinase, exerting its cytotoxicity through proteolytic cleavage of the mitogen-activated protein kinase kinases, thereby shutting down the MAPK pathways. Anthrax lethal toxin induces host lethality mostly by targeting the cardiovascular system. Although the enzymatic activity and the molecular targets of LT have long been known, the detailed mechanisms underlying cellular/tissue/organ toxicity are still poorly understood. In this work, we sought to investigate the mechanism of LT-induced cellular damage in the cardiovascular system. We demonstrate for the first time that anthrax lethal toxin has potent inhibitory effects on the central metabolism of cardiomyocytes and endothelial cells. This is likely due to the observed downregulating of c-Myc expression through the toxin-induced inhibition of the ERK pathway. Since c-Myc is a master transcription factor controlling the expression of many rate-limiting metabolic enzymes in glycolysis and the tricarboxylic acid cycle, LT's downregulation of c-Myc may lead to the observed bioenergetic collapse, particularly, in cardiomyocytes. Since cardiac cell contraction requires continuous production of large amounts of ATP, potent inhibition of the bioenergetics of cardiomyocytes would be incompatible with life. Thus, LT-induced lethality through targeting cardiomyocytes and endothelial cells appears to be a consequence of a bioenergetic collapse, likely due to the toxin's potent inhibitory activity on the MEK-ERK-c-Myc-metabolic/bioenergetic axis within these target cells of cardiovascular system.IMPORTANCEAnthrax lethal toxin (LT) is a major virulence factor of Bacillus anthracis, the causative pathogen of anthrax disease. Anthrax lethal toxin is a metalloproteinase that cleaves and inactivates MEKs, thereby shutting down MAPK pathways, leading to host mortality primarily through targeting of the cardiovascular system. However, the detailed mechanisms underlying the toxin's cellular and tissue toxicity are still poorly understood. Here, we found that anthrax lethal toxin has potent inhibitory activity on glycolysis and oxidative phosphorylation of cardiomyocytes and endothelial cells. These effects appear to be the consequence of downregulation of c-Myc, a master transcription factor that controls many rate-limiting enzymes of glycolysis and the tricarboxylic acid cycle. With the high demand on energy for cardiac contraction, the potent inhibition of cardiomyocyte metabolism by LT would be incompatible with life. This work provides critical insights into why the cardiovascular system is the major in vivo target of LT-induced lethality.

Anthrax Toxin: Model System for Studying Protein Translocation

Dedicated translocase channels are nanomachines that often, but not always, unfold and translocate proteins through narrow pores across the membrane. Generally, these molecular machines utilize external sources of free energy to drive these reactions, since folded proteins are thermodynamically stable, and once unfolded they contain immense diffusive configurational entropy. To catalyze unfolding and translocate the unfolded state at appreciable timescales, translocase channels often utilize analogous peptide-clamp active sites. Here we describe how anthrax toxin has been used as a biophysical model system to study protein translocation. The tripartite bacterial toxin is composed of an oligomeric translocase channel, protective antigen (PA), and two enzymes, edema factor (EF) and lethal factor (LF), which are translocated by PA into mammalian host cells. Unfolding and translocation are powered by the endosomal proton gradient and are catalyzed by three peptide-clamp sites in the PA channel: the α clamp, the ϕ clamp, and the charge clamp. These clamp sites interact nonspecifically with the chemically complex translocating chain, serve to minimize unfolded state configurational entropy, and work cooperatively to promote translocation. Two models of proton gradient driven translocation have been proposed: (i) an extended-chain Brownian ratchet mechanism and (ii) a proton-driven helix-compression mechanism. These models are not mutually exclusive; instead the extended-chain Brownian ratchet likely operates on β-sheet sequences and the helix-compression mechanism likely operates on α-helical sequences. Finally, we compare and contrast anthrax toxin with other related and unrelated translocase channels.

AB Toxins as High-Affinity Ligands for Cell Targeting in Cancer Therapy

Conventional targeted therapies for the treatment of cancer have limitations, including the development of acquired resistance. However, novel alternatives have emerged in the form of targeted therapies based on AB toxins. These biotoxins are a diverse group of highly poisonous molecules that show a nanomolar affinity for their target cell receptors, making them an invaluable source of ligands for biomedical applications. Bacterial AB toxins, in particular, are modular proteins that can be genetically engineered to develop high-affinity therapeutic compounds. These toxins consist of two distinct domains: a catalytically active domain and an innocuous domain that acts as a ligand, directing the catalytic domain to the target cells. Interestingly, many tumor cells show receptors on the surface that are recognized by AB toxins, making these high-affinity proteins promising tools for developing new methods for targeting anticancer therapies. Here we describe the structure and mechanisms of action of Diphtheria (Dtx), Anthrax (Atx), Shiga (Stx), and Cholera (Ctx) toxins, and review the potential uses of AB toxins in cancer therapy. We also discuss the main advances in this field, some successful results, and, finally, the possible development of innovative and precise applications in oncology based on engineered recombinant AB toxins.

ERK and c-Myc signaling in host-derived tumor endothelial cells is essential for solid tumor growth

The limited efficacy of the current antitumor microenvironment strategies is due in part to the poor understanding of the roles and relative contributions of the various tumor stromal cells to tumor development. Here, we describe a versatile in vivo anthrax toxin protein delivery system allowing for the unambiguous genetic evaluation of individual tumor stromal elements in cancer. Our reengineered tumor-selective anthrax toxin exhibits potent antiproliferative activity by disrupting ERK signaling in sensitive cells. Since this activity requires the surface expression of the capillary morphogenesis protein-2 (CMG2) toxin receptor, genetic manipulation of CMG2 expression using our cell-type-specific CMG2 transgenic mice allows us to specifically define the role of individual tumor stromal cell types in tumor development. Here, we established mice with CMG2 only expressed in tumor endothelial cells (ECs) and determined the specific contribution of tumor stromal ECs to the toxin's antitumor activity. Our results demonstrate that disruption of ERK signaling only within tumor ECs is sufficient to halt tumor growth. We discovered that c-Myc is a downstream effector of ERK signaling and that the MEK-ERK-c-Myc central metabolic axis in tumor ECs is essential for tumor progression. As such, disruption of ERK-c-Myc signaling in host-derived tumor ECs by our tumor-selective anthrax toxins explains their high efficacy in solid tumor therapy.

Imaging of anthrax intoxication in mice reveals shared and individual functions of surface receptors CMG-2 and TEM-8 in cellular toxin entry

Bacillus anthracis lethal toxin and edema toxin are binary toxins that consist of a common cell-binding moiety, protective antigen (PA), and the enzymatic moieties, lethal factor (LF) and edema factor (EF). PA binds to either of two receptors, capillary morphogenesis protein-2 (CMG-2) or tumor endothelial marker-8 (TEM-8), which triggers the binding and cytoplasmic translocation of LF and EF. However, the distribution of functional TEM-8 and CMG-2 receptors during anthrax toxin intoxication in animals has not been fully elucidated. Herein, we describe an assay to image anthrax toxin intoxication in animals, and we use it to visualize TEM-8- and CMG-2-dependent intoxication in mice. Specifically, we generated a chimeric protein consisting of the N-terminal domain of LF fused to a nuclear localization signal-tagged Cre recombinase (LFn-NLS-Cre). When PA and LFn-NLS-Cre were coadministered to transgenic mice expressing a red fluorescent protein in the absence of Cre and a green fluorescent protein in the presence of Cre, intoxication could be visualized at single-cell resolution by confocal microscopy or flow cytometry. Using this assay, we found that: (a) CMG-2 is critical for intoxication in the liver and heart, (b) TEM-8 is required for intoxication in the kidney and spleen, (c) CMG-2 and TEM-8 are redundant for intoxication of some organs, (d) combined loss of CMG-2 and TEM-8 completely abolishes intoxication, and (e) CMG-2 is the dominant receptor on leukocytes. The novel assay will be useful for basic and clinical/translational studies of Bacillus anthracis infection and for clinical development of reengineered toxin variants for cancer treatment.

Anthrax toxin channel: What we know based on over 30 years of research

Protective antigen channel is the central component of the deadly anthrax exotoxin responsible for binding and delivery of the toxin's enzymatic lethal and edema factor components into the cytosol. The channel, which is more than three times longer than the lipid bilayer membrane thickness and has a 6-Å limiting diameter, is believed to provide a sophisticated unfoldase and translocase machinery for the foreign protein transport into the host cell cytosol. The tripartite toxin can be reengineered, one component at a time or collectively, to adapt it for the targeted cancer therapeutic treatments. In this review, we focus on the biophysical studies of the protective antigen channel-forming activity, small ion transport properties, enzymatic factor translocation, and blockage comparing it with the related clostridial binary toxin channels. We address issues linked to the anthrax toxin channel structural dynamics and lipid dependence, which are yet to become generally recognized as parts of the toxin translocation machinery.

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.

Natural and Designed Toxins for Precise Therapy: Modern Approaches in Experimental Oncology

Cancer cells frequently overexpress specific surface receptors providing tumor growth and survival which can be used for precise therapy. Targeting cancer cell receptors with protein toxins is an attractive approach widely used in contemporary experimental oncology and preclinical studies. Methods of targeted delivery of toxins to cancer cells, different drug carriers based on nanosized materials (liposomes, nanoparticles, polymers), the most promising designed light-activated toxins, as well as mechanisms of the cytotoxic action of the main natural toxins used in modern experimental oncology, are discussed in this review. The prospects of the combined therapy of tumors based on multimodal nanostructures are also discussed.

Harnessing the Membrane Translocation Properties of AB Toxins for Therapeutic Applications

Over the last few decades, proteins and peptides have become increasingly more common as FDA-approved drugs, despite their inefficient delivery due to their inability to cross the plasma membrane. In this context, bacterial two-component systems, termed AB toxins, use various protein-based membrane translocation mechanisms to deliver toxins into cells, and these mechanisms could provide new insights into the development of bio-based drug delivery systems. These toxins have great potential as therapies both because of their intrinsic properties as well as the modular characteristics of both subunits, which make them highly amenable to conjugation with various drug classes. This review focuses on the therapeutical approaches involving the internalization mechanisms of three representative AB toxins: botulinum toxin type A, anthrax toxin, and cholera toxin. We showcase several specific examples of the use of these toxins to develop new therapeutic strategies for numerous diseases and explain what makes these toxins promising tools in the development of drugs and drug delivery systems.

Cryo-EM Uncovers Atomic Details for Loading Cell-Killing Enzymes to the Anthrax Toxin Pretranslocation Complex

In this issue of Structure, Zhou et al. report the structures of full-length lethal and edema factors, the cytotoxic components of the deadly anthrax toxin, in complex with the toxin's cell binding and delivery module, the protective antigen prechannel, providing an atomic description for the toxin recruitment prior to translocation.

Atomic Structures of Anthrax Prechannel Bound with Full-Length Lethal and Edema Factors

Pathogenesis of anthrax disease involves two cytotoxic enzymes-edema factor (EF) and lethal factor (LF)-which are individually recruited by the protective antigen heptamer (PA₇) or octamer (PA₈) prechannel and subsequently translocated across channels formed on the endosomal membrane upon exposure to low pH. Here, we report the atomic structures of PA₈ prechannel-bound full-length EF and LF. In this pretranslocation state, the N-terminal segment of both factors refolds into an α helix engaged in the α clamp of the prechannel. Recruitment to the PA prechannel exposes an originally buried β strand of both toxins and enables domain organization of EF. Many interactions occur on domain interfaces in both PA prechannel-bound EF and LF, leading to toxin compaction prior to translocation. Our results provide key insights into the molecular mechanisms of translocation-coupled protein unfolding and translocation.

Exploring the Nature of Cationic Blocker Recognition by the Anthrax Toxin Channel

Obstructing conductive pathways of the channel-forming toxins with targeted blockers is a promising drug design approach. Nearly all tested positively charged ligands have been shown to reversibly block the cation-selective channel-forming protective antigen (PA₆₃) component of the binary anthrax toxin. The cationic ligands with more hydrophobic surfaces, particularly those carrying aromatic moieties, inhibited PA₆₃ more effectively. To understand the physical basis of PA₆₃ selectivity for a particular ligand, detailed information is required on how the blocker structural elements (e.g., positively charged and aromatic groups) influence the molecular kinetics of the blocker/channel binding reactions. In this study, we address this problem using the high-resolution single-channel planar lipid bilayer technique. Several structurally distinct cationic blockers, namely per-6-S-(3-amino) propyl-β-cyclodextrin, per-6-S-(3-aminomethyl) benzyl-α-cyclodextrin, per-6-S-(3-aminomethyl) benzyl-β-cyclodextrin, per-6-S-(3-aminomethyl) benzyl-γ-cyclodextrin, methyltriphenylphosphonium ion, and G0 polyamidoamine dendrimer are tested for their ability to inhibit the heptameric and octameric PA₆₃ variants and PA₆₃F427A mutant. The F427 residues form a hydrophobic constriction region inside the channel, known as the "ϕ-clamp." We show that the cationic blockers interact with PA₆₃ through a combination of forces. Analysis of the binding reaction kinetics suggests the involvement of cation-π, Coulomb, and salt-concentration-independent π-π or hydrophobic interactions in the cationic cyclodextrin binding. It is possible that these blockers bind to the ϕ-clamp and are also stabilized by the Coulomb interactions of their terminal amino groups with the water-exposed negatively charged channel residues. In PA₆₃F427A, only the suggested Coulomb component of the cyclodextrin interaction remains. Methyltriphenylphosphonium ion and G0 polyamidoamine dendrimer, despite being positively charged, interact primarily with the ϕ-clamp. We also show that seven- and eightfold symmetric cyclodextrins effectively block the heptameric and octameric forms of PA₆₃ interchangeably, adding flexibility to the earlier formulated blocker/target symmetry match requirement.

Bismaleimide cross-linked anthrax toxin forms functional octamers with high specificity in tumor targeting

In recent years, anthrax toxin has been reengineered to act as a highly specific antiangiogenic cancer therapeutic, shown to kill tumors in animal models. This has been achieved by modifying protective antigen (PA) so that its activation and toxicity require the presence of two proteases, matrix metalloproteinase (MMP) and urokinase plasminogen activator (uPA), which are upregulated in tumor microenvironments. These therapeutics consist of intercomplementing PA variants, which are individually nontoxic, but form functional toxins upon complementary oligomerization. Here, we have created a dual-protease requiring PA targeting system which utilizes bismaleimide cross-linked PA (CLPA) rather than the intercomplementing PA variants. Three different CLPA agents were tested and, as expected, found to exclusively form octamers. Two of the CLPA agents have in vitro toxicities equal to those of previous intercomplementing agents, while the third CLPA agent had compromised in vitro cleavage and was significantly less cytotoxic. We hypothesize this difference was due to steric hindrance caused by cross-linking two PA monomers in close proximity to the PA cleavage site. Overall, this work advances the development and use of the PA and LF tumor-targeting system as a practical cancer therapeutic, as it provides a way to reduce the drug components of the anthrax toxin drug delivery system from three to two, which may lower the cost and simplify testing in clinical trials. HIGHLIGHT: Previously, anthrax toxin has been reengineered to act as a highly specific antiangiogenic cancer therapeutic. Here, we present a version, which utilizes bismaleimide cross-linked protective antigen (PA) rather than intercomplementing PA variants. This advances the development of anthrax toxin as a practical cancer therapeutic as it reduces the components of the drug delivery system to two, which may lower the cost and simplify testing in clinical trials.

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.

Functional classification of protein toxins as a basis for bioinformatic screening

Proteins are fundamental to life and exhibit a wide diversity of activities, some of which are toxic. Therefore, assessing whether a specific protein is safe for consumption in foods and feeds is critical. Simple BLAST searches may reveal homology to a known toxin, when in fact the protein may pose no real danger. Another challenge to answer this question is the lack of curated databases with a representative set of experimentally validated toxins. Here we have systematically analyzed over 10,000 manually curated toxin sequences using sequence clustering, network analysis, and protein domain classification. We also developed a functional sequence signature method to distinguish toxic from non-toxic proteins. The current database, combined with motif analysis, can be used by researchers and regulators in a hazard screening capacity to assess the potential of a protein to be toxic at early stages of development. Identifying key signatures of toxicity can also aid in redesigning proteins, so as to maintain their desirable functions while reducing the risk of potential health hazards.

Effect of endosomal acidification on small ion transport through the anthrax toxin PA63 channel

Tight regulation of pH is critical for the structure and function of cells and organelles. The pH environment changes dramatically along the endocytic pathway, an internalization transport process that is 'hijacked' by many intracellularly active bacterial exotoxins, including the anthrax toxin. Here, we investigate the role of pH on single-channel properties of the anthrax toxin protective antigen (PA₆₃ ). Using conductance and current noise analysis, blocker binding, ion selectivity, and poly(ethylene glycol) partitioning measurements, we show that the channel exists in two different open states ('maximum' and 'main') at pH ≥ 5.5, while only a maximum conductance state is detected at pH < 5.5. We describe two substantially distinct patterns of PA₆₃ conductance dependence on KCl concentration uncovered at pH 6.5 and 4.5.

Anthrax lethal toxin rapidly reduces c-Jun levels by inhibiting c-Jun gene transcription and promoting c-Jun protein degradation

Anthrax is a life-threatening disease caused by infection with Bacillus anthracis, which expresses lethal factor and the receptor-binding protective antigen. These two proteins combine to form anthrax lethal toxin (LT), whose proximal targets are mitogen-activated kinase kinases (MKKs). However, the downstream mediators of LT toxicity remain elusive. Here we report that LT exposure rapidly reduces the levels of c-Jun, a key regulator of cell proliferation and survival. Blockade of proteasome-dependent protein degradation with the 26S proteasome inhibitor MG132 largely restored c-Jun protein levels, suggesting that LT promotes degradation of c-Jun protein. Using the MKK1/2 inhibitor U0126, we further show that MKK1/2-Erk1/2 pathway inactivation similarly reduces c-Jun protein, which was also restored by MG132 pre-exposure. Interestingly, c-Jun protein rebounded to normal levels 4 h following U0126 exposure but not after LT exposure. The restoration of c-Jun in U0126-exposed cells was associated with increased c-Jun mRNA levels and was blocked by inactivation of the JNK1/2 signaling pathway. These results indicate that LT reduces c-Jun both by promoting c-Jun protein degradation via inactivation of MKK1/2-Erk1/2 signaling and by blocking c-Jun gene transcription via inactivation of MKK4-JNK1/2 signaling. In line with the known functions of c-Jun, LT also inhibited cell proliferation. Ectopic expression of LT-resistant MKK2 and MKK4 variants partially restored Erk1/2 and JNK1/2 signaling in LT-exposed cells, enabling the cells to maintain relatively normal c-Jun protein levels and cell proliferation. Taken together, these findings indicate that LT reduces c-Jun protein levels via two distinct mechanisms, thereby inhibiting critical cell functions, including cellular proliferation.

Anti-tumor activity of anthrax toxin variants that form a functional translocation pore by intermolecular complementation

Anthrax lethal toxin is a typical A-B type protein toxin secreted by Bacillus anthracis. Lethal factor (LF) is the catalytic A-subunit, a metalloprotease having MEKs as targets. LF relies on the cell-binding B-subunit, protective antigen (PA), to gain entry into the cytosol of target cells. PA binds to cell surface toxin receptors and is activated by furin protease to form an LF-binding-competent oligomer-PA pre-pore, which converts to a functional protein-conductive pore in the acidic endocytic vesicles, allowing translocation of LF into the cytosol. During PA pre-pore-to-pore conversion, the intermolecular salt bridge interactions between Lys397 and Asp426 on adjacent PA protomers play a critical role in positioning neighboring luminal Phe427 residues to form the Phe-clamp, an essential element of the PA functional pore. This essential intermolecular interaction affords the opportunity to create pairs of PA variants that depend on intermolecular complementation to form a functional pore. We have previously generated PA variants with furin-cleavage site replaced by substrate sequences of tumor-associated proteases, such as urokinase or MMPs. Here we show that PA-U2-K397Q, a urokinase-activated PA variant with Lys397 residue replaced by glutamine, and PA-L1-D426K, a MMP-activated PA variant with Asp426 changed to lysine, do not form functional pores both in vitro or in vivo unless they are used together. Further, the mixture of PA-U2-K397Q and PA-L1-D426K displayed potent anti-tumor activity in the presence of LF. Thus, PA-U2-K397Q and PA-L1-D426K form a novel intermolecular complementation system with toxin activation relying on the presence of two distinct tumor-associated proteases, i.e., urokinase and MMPs.

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

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

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

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

Tumor Targeting and Drug Delivery by Anthrax Toxin

Anthrax toxin is a potent tripartite protein toxin from Bacillus anthracis. It is one of the two virulence factors and causes the disease anthrax. The receptor-binding component of the toxin, protective antigen, needs to be cleaved by furin-like proteases to be activated and to deliver the enzymatic moieties lethal factor and edema factor to the cytosol of cells. Alteration of the protease cleavage site allows the activation of the toxin selectively in response to the presence of tumor-associated proteases. This initial idea of re-targeting anthrax toxin to tumor cells was further elaborated in recent years and resulted in the design of many modifications of anthrax toxin, which resulted in successful tumor therapy in animal models. These modifications include the combination of different toxin variants that require activation by two different tumor-associated proteases for increased specificity of toxin activation. The anthrax toxin system has proved to be a versatile system for drug delivery of several enzymatic moieties into cells. This highly efficient delivery system has recently been further modified by introducing ubiquitin as a cytosolic cleavage site into lethal factor fusion proteins. This review article describes the latest developments in this field of tumor targeting and drug delivery.

Solid tumor therapy by selectively targeting stromal endothelial cells

Engineered tumor-targeted anthrax lethal toxin proteins have been shown to strongly suppress growth of solid tumors in mice. These toxins work through the native toxin receptors tumor endothelium marker-8 and capillary morphogenesis protein-2 (CMG2), which, in other contexts, have been described as markers of tumor endothelium. We found that neither receptor is required for tumor growth. We further demonstrate that tumor cells, which are resistant to the toxin when grown in vitro, become highly sensitive when implanted in mice. Using a range of tissue-specific loss-of-function and gain-of-function genetic models, we determined that this in vivo toxin sensitivity requires CMG2 expression on host-derived tumor endothelial cells. Notably, engineered toxins were shown to suppress the proliferation of isolated tumor endothelial cells. Finally, we demonstrate that administering an immunosuppressive regimen allows animals to receive multiple toxin dosages and thereby produces a strong and durable antitumor effect. The ability to give repeated doses of toxins, coupled with the specific targeting of tumor endothelial cells, suggests that our strategy should be efficacious for a wide range of solid tumors.

Proteases in cancer drug delivery

Whereas protease inhibitors have been developed successfully against hypertension and viral infections, they have failed thus far as cancer drugs. With advances in cancer profiling we now better understand that the tumor "degradome" (i.e. the repertoire of proteases and their natural inhibitors and interaction partners) forms a complex network in which specific nodes determine the global outcome of manipulation of the protease web. However, knowing which proteases are active in the tumor micro-environment, we may tackle cancers with the use of Protease-Activated Prodrugs (PAPs). Here we exemplify this concept for metallo-, cysteine and serine proteases. PAPs not only exist as small molecular adducts, containing a cleavable substrate sequence and a latent prodrug, they are presently also manufactured as various types of nanoparticles. Although the emphasis of this review is on PAPs for treatment, it is clear that protease activatable probes and nanoparticles are also powerful tools for imaging purposes, including tumor diagnosis and staging, as well as visualization of tumor imaging during microsurgical resections.

An anthrax toxin variant with an improved activity in tumor targeting

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

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.

Prodrug applications for targeted cancer therapy

Prodrugs are widely used in the targeted delivery of cytotoxic compounds to cancer cells. To date, targeted prodrugs for cancer therapy have achieved great diversity in terms of target selection, activation chemistry, as well as size and physicochemical nature of the prodrug. Macromolecular prodrugs such as antibody-drug conjugates, targeted polymer-drug conjugates and other conjugates that self-assemble to form liposomal and micellar nanoparticles currently represent a major trend in prodrug development for cancer therapy. In this review, we explore a unified view of cancer-targeted prodrugs and highlight several examples from recombinant technology that exemplify the prodrug concept but are not identified as such. Recombinant "prodrugs" such as engineered anthrax toxin show promise in biological specificity through the conditionally targeting of multiple cellular markers. Conditional targeting is achieved by structural complementation, the spontaneous assembly of engineered inactive subunits or fragments to reconstitute functional activity. These complementing systems can be readily adapted to achieve conditionally bispecific targeting of enzymes that are used to activate low-molecular weight prodrugs. By leveraging strengths from medicinal chemistry, polymer science, and recombinant technology, prodrugs are poised to remain a core component of highly focused and tailored strategies aimed at conditionally attacking complex molecular phenotypes in clinically relevant cancer.

Anthrax lethal and edema toxins in anthrax pathogenesis

The pathophysiological effects resulting from many bacterial diseases are caused by exotoxins released by the bacteria. Bacillus anthracis, a spore-forming bacterium, is such a pathogen, causing anthrax through a combination of bacterial infection and toxemia. B. anthracis causes natural infection in humans and animals and has been a top bioterrorism concern since the 2001 anthrax attacks in the USA. The exotoxins secreted by B. anthracis use capillary morphogenesis protein 2 (CMG2) as the major toxin receptor and play essential roles in pathogenesis during the entire course of the disease. This review focuses on the activities of anthrax toxins and their roles in initial and late stages of anthrax infection.

Structural complementation of the catalytic domain of pseudomonas exotoxin A

The catalytic moiety of Pseudomonas exotoxin A (domain III or PE3) inhibits protein synthesis by ADP-ribosylation of eukaryotic elongation factor 2. PE3 is widely used as a cytocidal payload in receptor-targeted protein toxin conjugates. We have designed and characterized catalytically inactive fragments of PE3 that are capable of structural complementation. We dissected PE3 at an extended loop and fused each fragment to one subunit of a heterospecific coiled coil. In vitro ADP-ribosylation and protein translation assays demonstrate that the resulting fusions-supplied exogenously as genetic elements or purified protein fragments-had no significant catalytic activity or effect on protein synthesis individually but, in combination, catalyzed the ADP-ribosylation of eukaryotic elongation factor 2 and inhibited protein synthesis. Although complementing PE3 fragments are catalytically less efficient than intact PE3 in cell-free systems, co-expression in live cells transfected with transgenes encoding the toxin fusions inhibits protein synthesis and causes cell death comparably as intact PE3. Complementation of split PE3 offers a direct extension of the immunotoxin approach to generate bispecific agents that may be useful to target complex phenotypes.

Cytolethal distending toxin B as a cell-killing component of tumor-targeted anthrax toxin fusion proteins

Cytolethal distending toxin (Cdt) is produced by Gram-negative bacteria of several species. It is composed of three subunits, CdtA, CdtB, and CdtC, with CdtB being the catalytic subunit. We fused CdtB from Haemophilus ducreyi to the N-terminal 255 amino acids of Bacillus anthracis toxin lethal factor (LFn) to design a novel, potentially potent antitumor drug. As a result of this fusion, CdtB was transported into the cytosol of targeted cells via the efficient delivery mechanism of anthrax toxin. The fusion protein efficiently killed various human tumor cell lines by first inducing a complete cell cycle arrest in the G2/M phase, followed by induction of apoptosis. The fusion protein showed very low toxicity in mouse experiments and impressive antitumor effects in a Lewis Lung carcinoma model, with a 90% cure rate. This study demonstrates that efficient drug delivery by a modified anthrax toxin system combined with the enzymatic activity of CdtB has great potential as anticancer treatment and should be considered for the development of novel anticancer drugs.

The E. coli CNF1 as a pioneering therapy for the central nervous system diseases

The Cytotoxic Necrotizing Factor 1 (CNF1), a protein toxin from pathogenic E. coli, modulates the Rho GTPases, thus, directing the organization of the actin cytoskeleton. In the nervous system, the Rho GTPases play a key role in several processes, controlling the morphogenesis of dendritic spines and synaptic plasticity in brain tissues. This review is focused on the peculiar property of CNF1 to enhance brain plasticity in in vivo animal models of central nervous system (CNS) diseases, and on its possible application in therapy.