Novel receptors for bacterial protein toxins

Unveiling the pathogenic mechanisms of Clostridium perfringens toxins and virulence factors

Clostridium perfringens causes multiple diseases in humans and animals. Its pathogenic effect is supported by a broad and heterogeneous arsenal of toxins and other virulence factors associated with a specific host tropism. Molecular approaches have indicated that most C. perfringens toxins produce membrane pores, leading to osmotic cell disruption and apoptosis. However, identifying mechanisms involved in cell tropism and selective toxicity effects should be studied more. The differential presence and polymorphisms of toxin-encoding genes and genes encoding other virulence factors suggest that molecular mechanisms might exist associated with host preference, receptor binding, and impact on the host; however, this information has not been reviewed in detail. Therefore, this review aims to clarify the current state of knowledge on the structural features and mechanisms of action of the major toxins and virulence factors of C. perfringens and discuss the impact of genetic diversity of toxinotypes in tropism for several hosts.

Cellular Uptake and Cytotoxicity of Clostridium perfringens Iota-Toxin

Clostridium perfringens iota-toxin is composed of two separate proteins: a binding protein (Ib) that recognizes a host cell receptor and promotes the cellular uptake of a catalytic protein and (Ia) possessing ADP-ribosyltransferase activity that induces actin cytoskeleton disorganization. Ib exhibits the overall structure of bacterial pore-forming toxins (PFTs). Lipolysis-stimulated lipoprotein receptor (LSR) is defined as a host cell receptor for Ib. The binding of Ib to LSR causes an oligomer formation of Ib in lipid rafts of plasma membranes, mediating the entry of Ia into the cytoplasm. Ia induces actin cytoskeleton disruption via the ADP-ribosylation of G-actin and causes cell rounding and death. The binding protein alone disrupts the cell membrane and induces cytotoxicity in sensitive cells. Host cells permeabilized by the pore formation of Ib are repaired by a Ca2+-dependent plasma repair pathway. This review shows that the cellular uptake of iota-toxin utilizes a pathway of plasma membrane repair and that Ib alone induces cytotoxicity.

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.

Navarro M, Li J, Shrestha A, Uzal F, A. McClane B. Pathogenicity and virulence of Clostridium perfringens

Clostridium perfringens is an extremely versatile pathogen of humans and livestock, causing wound infections like gas gangrene (clostridial myonecrosis), enteritis/enterocolitis (including one of the most common human food-borne illnesses), and enterotoxemia (where toxins produced in the intestine are absorbed and damage distant organs such as the brain). The virulence of this Gram-positive, spore-forming, anaerobe is largely attributable to its copious toxin production; the diverse actions and roles in infection of these toxins are now becoming established. Most C. perfringens toxin genes are encoded on conjugative plasmids, including the pCW3-like and the recently discovered pCP13-like plasmid families. Production of C. perfringens toxins is highly regulated via processes involving two-component regulatory systems, quorum sensing and/or sporulation-related alternative sigma factors. Non-toxin factors, such as degradative enzymes like sialidases, are also now being implicated in the pathogenicity of this bacterium. These factors can promote toxin action in vitro and, perhaps in vivo, and also enhance C. perfringens intestinal colonization, e.g. NanI sialidase increases C. perfringens adherence to intestinal tissue and generates nutrients for its growth, at least in vitro. The possible virulence contributions of many other factors, such as adhesins, the capsule and biofilms, largely await future study.

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.

Characterization of Virulent netB+/tpeL+ Clostridium perfringens Strains from Necrotic Enteritis-Affected Broiler Chicken Farms

Clostridium perfringens (CP) type A and newly created type G strains are the key etiological factors in the induction of necrotic enteritis (NE), an important enteric disease that is responsible for the annual loss of $6 billion in the worldwide poultry industry. Several CP toxin genes were found to be critical in NE pathogenesis in chickens, but limited information is available on the CP lethal toxin tpeL gene. In this study, 19 CP strains isolated from NE-affected chicken farms were characterized microbiologically and molecularly and evaluated for their pathogenicity in commercial broiler chickens. Toxin typing by PCR revealed that all strains tested were positive for the netB toxin gene, but only five strains were positive for the tpeL toxin gene (LLY-TpeL 13, -TpeL 15, -TpeL 17, -TpeL 18, and -TpeL 19, simplified as TpeL 13, TpeL 15, TpeL 17, TpeL 18, and TpeL 19). High levels of TpeL proteins were detected in the concentrated culture supernatant from strains TpeL 13, 15, 17, and 19 by western blotting. Quantitative PCR showed that strains TpeL 13, 15, 17, 18, and 19 harbored a high number of copies of tpeL genes, while TpeL 18 had the highest number of copies of the tpeL gene among all CP strains tested when normalized with copy numbers of 16S rRNA gene as a housekeeping gene marker. The in vivo NE challenge test using multiple oral CP inoculations combined with a high-protein diet showed that TpeL 17 was the most virulent in inducing typical NE lesions, followed by TpeL 19 as the next most virulent, when tested in commercial broiler chickens. Infection with TpeL 17 reduced the growth rate significantly, as shown by reduced relative body weight gain percentage at day 5 postinfection. Availability of the virulent netB⁺tpeL⁺ CP strains is essential for the development of a CP-alone NE challenge model that could provide significant tools for understanding CP pathogenesis and for development of alternative to antibiotics.

Clostridial Binary Toxins: Basic Understandings that Include Cell Surface Binding and an Internal "Coup de Grâce"

Clostridium species can make a remarkable number of different protein toxins, causing many diverse diseases in humans and animals. The binary toxins of Clostridium botulinum, C. difficile, C. perfringens, and C. spiroforme are one group of enteric-acting toxins that attack the actin cytoskeleton of various cell types. These enterotoxins consist of A (enzymatic) and B (cell binding/membrane translocation) components that assemble on the targeted cell surface or in solution, forming a multimeric complex. Once translocated into the cytosol via endosomal trafficking and acidification, the A component dismantles the filamentous actin-based cytoskeleton via mono-ADP-ribosylation of globular actin. Knowledge of cell surface receptors and how these usurped, host-derived molecules facilitate intoxication can lead to novel ways of defending against these clostridial binary toxins. A molecular-based understanding of the various steps involved in toxin internalization can also unveil therapeutic intervention points that stop the intoxication process. Furthermore, using these bacterial proteins as medicinal shuttle systems into cells provides intriguing possibilities in the future. The pertinent past and state-of-the-art present, regarding clostridial binary toxins, will be evident in this chapter.

Methods to Study Rho GTPases Using Bacterial Toxins

Bacterial pathogens developed several strategies to overcome defense systems of eukaryotic hosts. Within the infection process they need to attach to and cross through epithelial layers, escape from the innate and adaptive immune response, and find a physiological niche to survive. For this purpose bacteria developed toxins that specifically target central eukaryotic proteins, for example actin or Rho GTPases as regulators of the actin cytoskeleton. Some bacterial toxins catalyze a covalent modification of Rho GTPases to keep these molecular switches in a constitutive active or inactive state. This leads to rearrangement of the actin cytoskeleton. Toxin-treated cells show typical morphological changes depending on substrate specificity and action of the toxins. In this chapter I describe methods to illustrate how bacterial toxins may help to study the involvement of Rho GTPases in physiological and pathophysiological processes.

Mechanisms of Action and Cell Death Associated with Clostridium perfringens Toxins

Clostridium perfringens uses its large arsenal of protein toxins to produce histotoxic, neurologic and intestinal infections in humans and animals. The major toxins involved in diseases are alpha (CPA), beta (CPB), epsilon (ETX), iota (ITX), enterotoxin (CPE), and necrotic B-like (NetB) toxins. CPA is the main virulence factor involved in gas gangrene in humans, whereas its role in animal diseases is limited and controversial. CPB is responsible for necrotizing enteritis and enterotoxemia, mostly in neonatal individuals of many animal species, including humans. ETX is the main toxin involved in enterotoxemia of sheep and goats. ITX has been implicated in cases of enteritis in rabbits and other animal species; however, its specific role in causing disease has not been proved. CPE is responsible for human food-poisoning and non-foodborne C. perfringens-mediated diarrhea. NetB is the cause of necrotic enteritis in chickens. In most cases, host–toxin interaction starts on the plasma membrane of target cells via specific receptors, resulting in the activation of intracellular pathways with a variety of effects, commonly including cell death. In general, the molecular mechanisms of cell death associated with C. perfringens toxins involve features of apoptosis, necrosis and/or necroptosis.

Mechanism of Apoptosis Induction by Mycoplasmal Nuclease MGA_0676 in Chicken Embryo Fibroblasts

MGA_0676 has been characterized as a Mycoplasma gallisepticum nuclease that can induce apoptosis of chicken cells. However, the mechanism by which MGA_0676 induces apoptosis has remained unclear. In this study, we evaluated MGA_0676-induced apoptosis and internalization in immortalized chicken embryo fibroblasts (DF-1) and cancer cell lines. The internalization of MGA_0676 was proven through caveolin-mediated endocytosis by blocking the endocytosis with specific inhibitors or with siRNA. We identified the Thif domain of NEDD8-activating enzyme E1 regulatory subunit (NAE) in DF-1 as the target region interacting with the SNC domain of MGA_0676. The interaction between the Thif and SNC domains was observed co-located in the perinuclear and nuclear of DF-1. We found that the interaction between NAE and MGA_0676 increased the ability of apoptosis and accelerated the process of cullin neddylation in DF-1 cells, in turn activating NF-κB. This resulted in the observed aggregation of NF-κB in the nuclei of DF-1 cells. Moreover, the apoptosis induced by MGA_0676 decreased significantly when NF-κB was inhibited by siRNA or BAY 11-7082 or when NAE was silenced by siRNA. Overall, our results demonstrate that MGA_0676 is internalized through caveolin-mediated endocytosis, interacts with SNC-dependent Thif to accelerate the process of cullin neddylation and activates NF-κB in DF-1 cells, ultimately playing a key role in apoptosis in chicken cells. Our results indicate MGA_0676 constitutes a critical etiological virulence factor of the respiratory disease caused by M. gallisepticum. This study also opens a venue to investigate MGA_0676 as a potential candidate as pro-apoptotic drug in cancer studies.

Cellular Entry of Clostridium perfringens Iota-Toxin and Clostridium botulinum C2 Toxin

Clostridium perfringens iota-toxin and Clostridium botulinum C2 toxin are composed of two non-linked proteins, one being the enzymatic component and the other being the binding/translocation component. These latter components recognize specific receptors and oligomerize in plasma membrane lipid-rafts, mediating the uptake of the enzymatic component into the cytosol. Enzymatic components induce actin cytoskeleton disorganization through the ADP-ribosylation of actin and are responsible for cell rounding and death. This review focuses upon the recent advances in cellular internalization of clostridial binary toxins.

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

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

Formation of Nanotube-Like Protrusions, Regulation of Septin Organization and Re-guidance of Vesicle Traffic by Depolymerization of the Actin Cytoskeleton Induced by Binary Bacterial Protein Toxins

A large group of bacterial protein toxins, including binary ADP-ribosylating toxins, modify actin at arginine-177, thereby actin polymerization is blocked and the actin cytoskeleton is redistributed. Modulation of actin functions largely affects other components of the cytoskeleton, especially microtubules and septins. Here, recent findings about the functional interconnections of the actin cytoskeleton with microtubules and septins, affected by bacterial toxins, are reviewed.

Interaction of the Clostridium difficile Binary Toxin CDT and Its Host Cell Receptor, Lipolysis-stimulated Lipoprotein Receptor (LSR)

CDT (Clostridium difficile transferase) is a binary, actin ADP-ribosylating toxin frequently associated with hypervirulent strains of the human enteric pathogen C. difficile, the most serious cause of antibiotic-associated diarrhea and pseudomembranous colitis. CDT leads to the collapse of the actin cytoskeleton and, eventually, to cell death. Low doses of CDT result in the formation of microtubule-based protrusions on the cell surface that increase the adherence and colonization of C. difficile. The lipolysis-stimulated lipoprotein receptor (LSR) is the host cell receptor for CDT, and our aim was to gain a deeper insight into the interplay between both proteins. We show that CDT interacts with the extracellular, Ig-like domain of LSR with an affinity in the nanomolar range. We identified LSR splice variants in the colon carcinoma cell line HCT116 and disrupted the LSR gene in these cells by applying the CRISPR-Cas9 technology. LSR truncations ectopically expressed in LSR knock-out cells indicated that intracellular parts of LSR are not essential for plasma membrane targeting of the receptor and cellular uptake of CDT. By generating a series of N- and C-terminal truncations of the binding component of CDT (CDTb), we found that amino acids 757-866 of CDTb are sufficient for binding to LSR. With a transposon-based, random mutagenesis approach, we identified potential LSR-interacting epitopes in CDTb. This study increases our understanding about the interaction between CDT and its receptor LSR, which is key to the development of anti-toxin strategies for preventing cell entry of the toxin.