Endocytosis and toxicity of clostridial binary toxins depend on a clathrin-independent pathway regulated by Rho-GDI

Alpha-1 antitrypsin inhibits Clostridium botulinum C2 toxin, Corynebacterium diphtheriae diphtheria toxin and B. anthracis fusion toxin

The bacterium Clostridium botulinum, well-known for producing botulinum neurotoxins, which cause the severe paralytic illness known as botulism, produces C2 toxin, a binary AB-toxin with ADP-ribosyltranferase activity. C2 toxin possesses two separate protein components, an enzymatically active A-component C2I and the binding and translocation B-component C2II. After proteolytic activation of C2II to C2IIa, the heptameric structure binds C2I and is taken up via receptor-mediated endocytosis into the target cells. Due to acidification of endosomes, the C2IIa/C2I complex undergoes conformational changes and consequently C2IIa forms a pore into the endosomal membrane and C2I can translocate into the cytoplasm, where it ADP-ribosylates G-actin, a key component of the cytoskeleton. This modification disrupts the actin cytoskeleton, resulting in the collapse of cytoskeleton and ultimately cell death. Here, we show that the serine-protease inhibitor α1-antitrypsin (α1AT) which we identified previously from a hemofiltrate library screen for PT from Bordetella pertussis is a multitoxin inhibitor. α1AT inhibits intoxication of cells with C2 toxin via inhibition of binding to cells and inhibition of enzyme activity of C2I. Moreover, diphtheria toxin and an anthrax fusion toxin are inhibited by α1AT. Since α1AT is commercially available as a drug for treatment of the α1AT deficiency, it could be repurposed for treatment of toxin-mediated diseases.

Structural and Functional Insights into the Delivery Systems of Bacillus and Clostridial Binary Toxins

Pathogenic Bacillus and clostridial (i.e., Clostridium and Clostridioides) bacteria express a diverse repertoire of effector proteins to promote disease. This includes production of binary toxins, which enter host epithelial cells and seriously damage the intestinal tracts of insects, animals, and humans. In particular, binary toxins form an AB-type complex composed of a catalytic subunit that is toxic (A) and an oligomeric cell-binding and delivery subunit (B), where upon delivery of A into the cytoplasm of the host cell it catalytically ADP-ribosylates actin and rapidly induces host cell death. In this review, binary toxins expressed by Bacillus thuringiensis, Clostridioides difficile, and Clostridium perfringens will be discussed, with particular focus placed upon the structural elucidations of their respective B subunits and how these findings help to deconvolute how toxic enzyme delivery into target host cells is achieved by these deadly bacteria.

An Updated View on the Cellular Uptake and Mode-of-Action of Clostridioides difficile Toxins

Research on the human gut pathogen Clostridioides (C.) difficile and its toxins continues to attract much attention as a consequence of the threat to human health posed by hypervirulent strains. Toxin A (TcdA) and Toxin B (TcdB) are the two major virulence determinants of C. difficile. Both are single-chain proteins with a similar multidomain architecture. Certain hypervirulent C. difficile strains also produce a third toxin, namely binary toxin CDT (C. difficile transferase). C. difficile toxins are the causative agents of C. difficile-associated diseases (CDADs), such as antibiotics-associated diarrhea and pseudomembranous colitis. For that reason, considerable efforts have been expended to unravel their molecular mode-of-action and the cellular mechanisms responsible for their uptake. Many of these studies have been conducted in European laboratories. Here, we provide an update on our previous review (Papatheodorou et al. Adv Exp Med Biol, 2018) on important advances in C. difficile toxins research.

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.

Clostridioides difficile infection: traversing host-pathogen interactions in the gut

C. difficile is the primary cause for nosocomial infective diarrhoea. For a successful infection, C. difficile must navigate between resident gut bacteria and the harsh host environment. The perturbation of the intestinal microbiota by broad-spectrum antibiotics alters the composition and the geography of the gut microbiota, deterring colonization resistance, and enabling C. difficile to colonize. This review will discuss how C. difficile interacts with and exploits the microbiota and the host epithelium to infect and persist. We provide an overview of C. difficile virulence factors and their interactions with the gut to aid adhesion, cause epithelial damage and mediate persistence. Finally, we document the host responses to C. difficile, describing the immune cells and host pathways that are associated and triggered during C. difficile infection.

The Pore-Forming Subunit C2IIa of the Binary Clostridium botulinum C2 Toxin Reduces the Chemotactic Translocation of Human Polymorphonuclear Leukocytes

The binary C2 toxin of Clostridium (C.) botulinum consists of two non-linked proteins, the enzyme subunit C2I and the separate binding/transport subunit C2II. To exhibit toxic effects on mammalian cells, proteolytically activated C2II (C2IIa) forms barrel-shaped heptamers that bind to carbohydrate receptors which are present on all mammalian cell types. C2I binds to C2IIa and the toxin complexes are internalized via receptor-mediated endocytosis. In acidified endosomal vesicles, C2IIa heptamers change their conformation and insert as pores into endosomal membranes. These pores serve as translocation-channels for the subsequent transport of C2I from the endosomal lumen into the cytosol. There, C2I mono-ADP-ribosylates G-actin, which results in depolymerization of F-actin and cell rounding. Noteworthy, so far morphological changes in cells were only observed after incubation with the complete C2 toxin, i.e., C2IIa plus C2I, but not with the single subunits. Unexpectedly, we observed that the non-catalytic transport subunit C2IIa (but not C2II) alone induced morphological changes and actin alterations in primary human polymorphonuclear leukocytes (PMNs, alias neutrophils) from healthy donors ex vivo, but not macrophages, epithelial and endothelial cells, as detected by phase contrast microscopy and fluorescent microscopy of the actin cytoskeleton. This suggests a PMN selective mode of action for C2IIa. The cytotoxicity of C2IIa on PMNs was prevented by C2IIa pore blockers and treatment with C2IIa (but not C2II) rapidly induced Ca²⁺ influx in PMNs, suggesting that pore-formation by C2IIa in cell membranes of PMNs is crucial for this effect. In addition, incubation of primary human PMNs with C2IIa decreased their chemotaxis ex vivo through porous culture inserts and in co-culture with human endothelial cells which is closer to the physiological extravasation process. In conclusion, the results suggest that C2IIa is a PMN-selective inhibitor of chemotaxis. This provides new knowledge for a pathophysiological role of C2 toxin as a modulator of innate immune cells and makes C2IIa an attractive candidate for the development of novel pharmacological strategies to selectively down-modulate the excessive and detrimental PMN recruitment into organs after traumatic injuries.

1HN, 13C, and 15N resonance assignments of the Clostridioides difficile receptor binding domain 2 (CDTb, residues 757-876)

Clostridioides difficile is a bacterial pathogen responsible for the majority of nosocomial infections in the developed world. C. difficile infection (CDI) is difficult to treat in many cases because hypervirulent strains have evolved that contain a third toxin, termed the C. difficile toxin (CDT), in addition to the two enterotoxins TcdA and TcdB. CDT is a binary toxin comprised of an enzymatic, ADP-ribosyltransferase (ART) toxin component, CDTa, and a pore-forming or delivery subunit, CDTb. In the absence of CDTa, CDTb assembles into two distinct di-heptameric states, a symmetric and an asymmetric form with both states having two surface-accessible host cell receptor-binding domains, termed RBD1 and RBD2. RBD1 has a unique amino acid sequence, when aligned to other well-studied binary toxins (i.e., anthrax), and it contains a novel Ca2+-binding site important for CDTb stability. The other receptor binding domain, RBD2, is critically important for CDT toxicity, and a domain such as this is missing altogether in other binary toxins and shows further that CDT is unique when compared to other binary toxins. In this study, the 1H, 13C, and 15N backbone and sidechain resonances of the 120 amino acid RBD2 domain of CDTb (residues 757-876) were assigned sequence-specifically and provide a framework for future NMR-based drug discovery studies directed towards targeting the most virulent strains of CDI.

The Caenorhabditis elegans CUB-like-domain containing protein RBT-1 functions as a receptor for Bacillus thuringiensis Cry6Aa toxin

Plant-parasitic nematodes cause huge agricultural economic losses. Two major families of Bacillus thuringiensis crystal proteins, Cry5 and Cry6, show nematicidal activity. Previous work showed that binding to midgut receptors is a limiting step in Cry toxin mode of action. In the case of Cry5Ba, certain Caenorhabditis elegans glycolipids were identified as receptors of this toxin. However, the receptors for Cry6 toxin remain unknown. In this study, the C. elegans CUB-like-domain containing protein RBT-1, released by phosphatidylinositol-specific phospholipase C (PI-PLC), was identified as a Cry6Aa binding protein by affinity chromatography. RBT-1 contained a predicted glycosylphosphatidylinositol (GPI) anchor site and was shown to locate in lipid rafts in the surface of the midgut cells. Western ligand blot assays and ELISA binding analysis confirmed the binding interaction between Cry6Aa and RBT-1 showing high affinity and specificity. In addition, the mutation of rbt-1 gene decreased the susceptibility of C. elegans to Cry6Aa but not that of Cry5Ba. Furthermore, RBT-1 mediated the uptake of Cry6Aa into C. elegans gut cells, and was shown to be involved in triggering pore-formation activity, indicating that RBT-1 is required for the interaction of Cry6Aa with the nematode midgut cells. These results support that RBT-1 is a functional receptor for Cry6Aa.

Bacillus thuringiensis (Bt) crystal proteins belong to pore-forming toxins (PFTs), which display virulence against target hosts by forming holes in the cell membrane. Cry6A is a nematicidal PFT, which exhibits unique protein structure and different mode of action than Cry5B, another nematicidal PFT. However, little is known about the mode of action of Cry6A. Although an intracellular nematicidal necrosis pathway of Cry6A was reported, its extracellular mode of action remains unknown. We here demonstrate that the CUB-like-domain containing protein RBT-1 acts as a functional receptor of Cry6A, which mediates the intestinal cell interaction and nematicidal activity of this toxin. RBT-1 represents a new class of crystal protein receptors. RBT-1 is dispensable for Cry5B toxicity against nematodes, consistent with that Cry6A and Cry5B have different nematicidal mechanisms. We also find that Cry6A kills nematodes by complex mechanism since rbt-1 mutation did not affect Cry6A-mediated necrosis signaling pathway. This work not only enhances the understanding of Bt crystal protein-nematode mechanism, but is also in favor for the application of Cry6A in nematode control.

Structure of the cell-binding component of the Clostridium difficile binary toxin reveals a di-heptamer macromolecular assembly

Targeting Clostridium difficile infection is challenging because treatment options are limited, and high recurrence rates are common. One reason for this is that hypervirulent C. difficile strains often have a binary toxin termed the C. difficile toxin, in addition to the enterotoxins TsdA and TsdB. The C. difficile toxin has an enzymatic component, termed CDTa, and a pore-forming or delivery subunit termed CDTb. CDTb was characterized here using a combination of single-particle cryoelectron microscopy, X-ray crystallography, NMR, and other biophysical methods. In the absence of CDTa, 2 di-heptamer structures for activated CDTb (1.0 MDa) were solved at atomic resolution, including a symmetric (SymCDTb; 3.14 Å) and an asymmetric form (AsymCDTb; 2.84 Å). Roles played by 2 receptor-binding domains of activated CDTb were of particular interest since the receptor-binding domain 1 lacks sequence homology to any other known toxin, and the receptor-binding domain 2 is completely absent in other well-studied heptameric toxins (i.e., anthrax). For AsymCDTb, a Ca2+ binding site was discovered in the first receptor-binding domain that is important for its stability, and the second receptor-binding domain was found to be critical for host cell toxicity and the di-heptamer fold for both forms of activated CDTb. Together, these studies represent a starting point for developing structure-based drug-design strategies to target the most severe strains of C. difficile.

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.

Receptor-Binding and Uptake of Binary Actin-ADP-Ribosylating Toxins

Binary actin-ADP-ribosylating toxins (e.g., Clostridium botulinum C2 toxin or Clostridium perfringens iota toxin ) consist of two separate proteins: An ADP-ribosyltransferase, which modifies actin thereby inhibiting actin polymerization, and a binding component that forms heptamers after proteolytic activation. While C2 toxin interacts with carbohydrate structures on host cells, the group of iota-like toxins binds to lipolysis-stimulated lipoprotein receptor (LSR). Here, we review LSR and discuss the role and function of LSR in interaction of iota-like toxins with host cells.

Human alpha-defensin-1 protects cells from intoxication with Clostridium perfringens iota toxin

Iota toxin is produced by Clostridium perfringens type E strains and associated with diarrhea in cattle and lambs. This binary protein toxin comprises the enzyme component iota a (Ia), which ADP-ribosylates G-actin, and the separate transport component iota b (Ib), which delivers Ia into the cytosol of target cells. Ib binds to cell receptors and forms biologically active toxin complexes with Ia, which cause rounding of adherent cells due to the destruction of the actin cytoskeleton. Here, we report that the human peptide α-defensin-1 protects cultured cells including human colon cells from intoxication with iota toxin. In contrast, the related ß-defensin-1 had no effect, indicating a specific mode of action. The α-defensin-1 did not inhibit ADP-ribosylation of actin by Ia in vitro. Pretreatment of Ib with α-defensin-1 prior to addition of Ia prevented intoxication. Additionally, α-defensin-1 protected cells from cytotoxic effects mediated by Ib in the absence of Ia, implicating that α-defensin-1 interacts with Ib to prevent the formation of biologically active iota toxin on cells. In conclusion, the findings contribute to a better understanding of the functions of α-defensin-1 and suggest that this human peptide might be an attractive starting point to develop novel pharmacological options to treat/prevent diseases associated with iota toxin-producing Clostridium perfringens strains.

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.

Binary Clostridium difficile toxin (CDT) - A virulence factor disturbing the cytoskeleton

Clostridium difficile infection causes antibiotics-associated diarrhea and pseudomembranous colitis. Major virulence factors of C. difficile are the Rho-glucosylating toxins TcdA and TcdB. In addition, many, so-called hypervirulent C. difficile strains produce the binary actin-ADP-ribosylating toxin CDT. CDT causes depolymerization of F-actin and rearrangement of the actin cytoskeleton. Thereby, many cellular functions, which depend on actin, are altered. CDT disturbs the dynamic balance between actin and microtubules in target cells. The toxin increases microtubule polymerization and induces the formation of microtubule-based protrusions at the plasma membrane of target cells. Moreover, CDT causes a redistribution of vesicles from the basolateral side to the apical side, where extracellular matrix proteins are released. These processes may increase the adherence of clostridia to target cells. Here, we review the effects of the action of CDT on the actin cytoskeleton and on the microtubule system.

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.

Pore-forming activity of clostridial binary toxins

Clostridial binary toxins (Clostridium perfringens Iota toxin, Clostridium difficile transferase, Clostridium spiroforme toxin, Clostridium botulinum C2 toxin) as Bacillus binary toxins, including Bacillus anthracis toxins consist of two independent proteins, one being the binding component which mediates the internalization into cell of the intracellularly active component. Clostridial binary toxins induce actin cytoskeleton disorganization through mono-ADP-ribosylation of globular actin and are responsible for enteric diseases. Clostridial and Bacillus binary toxins share structurally and functionally related binding components which recognize specific cell receptors, oligomerize, form pores in endocytic vesicle membrane, and mediate the transport of the enzymatic component into the cytosol. Binding components retain the global structure of pore-forming toxins (PFTs) from the cholesterol-dependent cytotoxin family such as perfringolysin. However, their pore-forming activity notably that of clostridial binding components is more related to that of heptameric PFT family including aerolysin and C. perfringens epsilon toxin. This review focuses upon pore-forming activity of clostridial binary toxins compared to other related PFTs. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Uptake of clostridium botulinum C3 exoenzyme into intact HT22 and J774A.1 cells

The Clostridium botulinum C3 exoenzyme selectively ADP-ribosylates low molecular weight GTP-binding proteins RhoA, B and C. This covalent modification inhibits Rho signaling activity, resulting in distinct actin cytoskeleton changes. Although C3 exoenzyme has no binding, the translocation domain assures that C3 enters cells and acts intracellularly. C3 uptake is thought to occur due to the high concentration of the C3 enzyme. However, recent work indicates that C3 is selectively endocytosed, suggesting a specific endocytotic pathway, which is not yet understood. In this study, we show that the C3 exoenzyme binds to cell surfaces and is internalized in a time-dependent manner. We show that the intermediate filament, vimentin, is involved in C3 uptake, as indicated by the inhibition of C3 internalization by acrylamide, a known vimentin disruption agent. Inhibition of C3 internalization was not observed by chemical inhibitors, like bafilomycin A, methyl-β-cyclodextrin, nocodazole or latrunculin B. Furthermore, the internalization of C3 exoenzyme was markedly inhibited in dynasore-treated HT22 cells. Our results indicate that C3 internalization depends on vimentin and does not depend strictly on both clathrin and caveolae.

Clostridium perfringens type A-E toxin plasmids

Clostridium perfringens relies upon plasmid-encoded toxin genes to cause intestinal infections. These toxin genes are associated with insertion sequences that may facilitate their mobilization and transfer, giving rise to new toxin plasmids with common backbones. Most toxin plasmids carry a transfer of clostridial plasmids locus mediating conjugation, which likely explains the presence of similar toxin plasmids in otherwise unrelated C. perfringens strains. The association of many toxin genes with insertion sequences and conjugative plasmids provides virulence flexibility when causing intestinal infections. However, incompatibility issues apparently limit the number of toxin plasmids maintained by a single cell.

Challenging the roles of CD44 and lipolysis stimulated lipoprotein receptor in conveying Clostridium perfringens iota toxin cytotoxicity in breast cancer

Background

Translational exploration of bacterial toxins has come to the forefront of research given their potential as a chemotherapeutic tool. Studies in select tissues have demonstrated that Clostridium perfringens iota toxin binds to CD44 and lipolysis stimulated lipoprotein receptor (LSR) cell-surface proteins. We recently demonstrated that LSR expression correlates with estrogen receptor positive breast cancers and that LSR signaling directs aggressive, tumor-initiating cell behaviors. Herein, we identify the mechanisms of iota toxin cytotoxicity in a tissue-specific, breast cancer model with the ultimate goal of laying the foundation for using iota toxin as a targeted breast cancer therapy.

Methods

In vitro model systems were used to determine the cytotoxic effect of iota toxin on breast cancer intrinsic subtypes. The use of overexpression and knockdown technologies confirmed the roles of LSR and CD44 in regulating iota toxin endocytosis and induction of cell death. Lastly, cytotoxicity assays were used to demonstrate the effect of iota toxin on a validated set of tamoxifen resistant breast cancer cell lines.

Results

Treatment of 14 breast cancer cell lines revealed that LSR+/CD44- lines were highly sensitive, LSR+/CD44+ lines were slightly sensitive, and LSR-/CD44+ lines were resistant to iota cytotoxicity. Reduction in LSR expression resulted in a significant decrease in toxin sensitivity; however, overexpression of CD44 conveyed toxin resistance. CD44 overexpression was correlated with decreased toxin-stimulated lysosome formation and decreased cytosolic levels of iota toxin. These findings indicated that expression of CD44 drives iota toxin resistance through inhibition of endocytosis in breast cancer cells, a role not previously defined for CD44. Moreover, tamoxifen-resistant breast cancer cells exhibited robust expression of LSR and were highly sensitive to iota-induced cytotoxicity.

Conclusions

Collectively, these data are the first to show that iota toxin has the potential to be an effective, targeted therapy for breast cancer.

Toxin plasmids of Clostridium perfringens

In both humans and animals, Clostridium perfringens is an important cause of histotoxic infections and diseases originating in the intestines, such as enteritis and enterotoxemia. The virulence of this Gram-positive, anaerobic bacterium is heavily dependent upon its prolific toxin-producing ability. Many of the ∼16 toxins produced by C. perfringens are encoded by large plasmids that range in size from ∼45 kb to ∼140 kb. These plasmid-encoded toxins are often closely associated with mobile elements. A C. perfringens strain can carry up to three different toxin plasmids, with a single plasmid carrying up to three distinct toxin genes. Molecular Koch's postulate analyses have established the importance of several plasmid-encoded toxins when C. perfringens disease strains cause enteritis or enterotoxemias. Many toxin plasmids are closely related, suggesting a common evolutionary origin. In particular, most toxin plasmids and some antibiotic resistance plasmids of C. perfringens share an ∼35-kb region containing a Tn916-related conjugation locus named tcp (transfer of clostridial plasmids). This tcp locus can mediate highly efficient conjugative transfer of these toxin or resistance plasmids. For example, conjugative transfer of a toxin plasmid from an infecting strain to C. perfringens normal intestinal flora strains may help to amplify and prolong an infection. Therefore, the presence of toxin genes on conjugative plasmids, particularly in association with insertion sequences that may mobilize these toxin genes, likely provides C. perfringens with considerable virulence plasticity and adaptability when it causes diseases originating in the gastrointestinal tract.

Internalization of Clostridium perfringens α-toxin leads to ERK activation and is involved on its cytotoxic effect

Clostridium perfringens phospholipase C (CpPLC), also called α-toxin, plays a key role in the pathogenesis of gas gangrene. CpPLC may lead to cell lysis at concentrations that cause extensive degradation of plasma membrane phospholipids. However, at sublytic concentrations it induces cytotoxicity without inducing evident membrane damage. The results of this work demonstrate that CpPLC becomes internalized in cells by a dynamin-dependent mechanism and in a time progressive process: first, CpPLC colocalizes with caveolin both at the plasma membrane and in vesicles, and later it colocalizes with early and late endosomes and lysosomes. Lysosomal damage in the target cells is evident 9 h after CpPLC exposure. Our previous work demonstrated that CpPLCinduces ERK1/2 activation, which is involved in its cytotoxic effect. In this work we found that cholesterol sequestration, dynamin inhibition, as well as inhibition of actin polymerization, prevent CpPLC internalization and ERK1/2 activation, involving endocytosis in the signalling events required for CpPLC cytotoxic effect at sublytic concentrations. These results provide new insights about the mode of action of this bacterial phospholipase C, previously considered to act only locally on cell membrane.

[Clathrin-independent endocytosis: free the way!]

Along the years, the interest paid to the study of endocytosis has never wavered as this process plays such an essential role in many cellular functions. Cell growth, adhesion and differentiation, regulation of signaling induced by membrane receptors or infection by viral particles are all dependent on the entry of molecules into the cell. Once the clathrin-dependent endocytosis well characterized, it has become apparent that other entry pathways also existed in the cell. This review is intended to provide an update on recent advances that establish with certainty the existence of endocytic pathways independent of clathrin and highlight their specific regulators.

Interferon gamma receptor: the beginning of the journey

Our view of endocytosis and membrane trafficking of transmembrane receptors has dramatically changed over the last 20 years. Several new endocytic routes have been discovered and mechanistically characterized in mammalian cells. Long considered as a passive means to terminate signaling through down-regulation of the number of activated receptors at the plasma membrane, it is now established that receptor endocytosis and endosomal sorting can be directly linked to the regulation of intracellular signaling pathways. The functional links between membrane trafficking of interferon receptors and JAK/STAT signaling have recently begun to be unraveled. These studies raise the exciting possibility that a certain level of signal specificity can be achieved through endocytosis and selective localization of the activated complexes within cellular membranes. The ongoing development of high-resolution cell imaging techniques with better spatial and temporal resolution gives new means of deciphering the inherent complexity of membrane trafficking and signaling. This should help to better comprehend the molecular mechanisms by which endocytosis and endosomal sorting of interferon receptors can orchestrate signaling selectivity within the JAK/STAT pathway that can be activated by as many as 60 different cytokines, growth factors, and hormones.

Clostridium difficile binary toxin CDT induces clustering of the lipolysis-stimulated lipoprotein receptor into lipid rafts

Clostridium difficile is the leading cause of antibiotics-associated diarrhea and pseudomembranous colitis. Hypervirulent C. difficile strains produce the binary actin-ADP-ribosylating toxin CDT (C. difficile transferase), in addition to the Rho-glucosylating toxins A and B. We recently identified the lipolysis-stimulated lipoprotein receptor (LSR) as the host receptor that mediates uptake of CDT into target cells. Here we investigated in H1-HeLa cells, which ectopically express LSR, the influence of CDT on the plasma membrane distribution of the receptor. We found by fluorescence microscopy that the binding component of CDT (CDTb) induces clustering of LSR into subcompartments of the plasma membrane. Detergent extraction of cells treated with CDTb, followed by sucrose gradient fractionation, uncovered accumulation of LSR in detergent-resistant membranes (DRMs) that contained typical marker proteins of lipid rafts. Membrane cholesterol depletion with methyl-β-cyclodextrin inhibited the association of LSR with DRMs upon addition of CDTb. The receptor-binding domain of CDTb also triggered LSR clustering into DRMs. CDTb-triggered clustering of LSR into DRMs could be confirmed in Caco-2 cells. Our data suggest that CDT forces its receptor to cluster into lipid rafts and that oligomerization of the B component might enhance but is not essential for this process.

C. difficile binary toxin CDT is a member of the iota-like, actin ADP-ribosylating toxin family. The mechanism that mediates endocytic uptake of these toxins still remains elusive. Previous studies highlighted the importance of lipid rafts for oligomerization of the binding component of these toxins and for cell entry. Recently, the host cell receptor for this toxin family, namely, the lipolysis-stimulated lipoprotein receptor (LSR), has been identified. Our study now demonstrates that the binding component of CDT (CDTb) induces clustering of LSR into lipid rafts. Importantly, LSR clustering is efficiently induced also by the receptor-binding domain of CDTb, suggesting that oligomerization of the B component of CDT is not the main trigger of this process. The current work extends our knowledge on the cooperative play between iota-like toxins and their receptor.

Endocytosis of gene delivery vectors: from clathrin-dependent to lipid raft-mediated endocytosis

The ideal nonviral vector delivers its nucleic acid cargo to a specific intracellular target. Vectors enter cells mainly through endocytosis and are distributed to various intracellular organelles. Recent advances in microscopy, lipidomics, and proteomics confirm that the cell membrane is composed of clusters of lipids, organized in the form of lipid raft domains, together with non-raft domains that comprise a generally disordered lipid milieu. The binding of a nonviral vector to either region can determine the pathway for its endocytic uptake and subsequent intracellular itinerary. Given this model of the cell membrane structure, endocytic pathways should be reclassified in relation to lipid rafts. In this review, we attempt to assess the currently recognized endocytic pathways in mammalian cells. The endocytic pathways are classified in relation to the membrane regions that make up the primary endocytic vesicles. This review covers the well-recognized clathrin-mediated endocytosis (CME), phagocytosis, and macropinocytosis in addition to the less addressed pathways that take place in lipid rafts. These include caveolae-mediated, flotillin-dependent, GTPase regulator associated with focal adhesion kinase-1 (GRAF1)-dependent, adenosine diphosphate-ribosylation factor 6 (Arf6)-dependent, and RhoA-dependent endocytic pathways. We summarize the regulators associated with each uptake pathway and methods for interfering with these regulators are discussed. The fate of endocytic vesicles resulting from each endocytic uptake pathway is highlighted.

Host cell cytotoxicity and cytoskeleton disruption by CerADPr, an ADP-ribosyltransferase of Bacillus cereus G9241

Bacillus cereus G9241 was isolated from a welder suffering from an anthrax-like inhalation illness. B. cereus G9241 encodes two megaplasmids, pBCXO1 and pBC210, which are analogous to the toxin- and capsule-encoding virulence plasmids of Bacillus anthracis. Protein modeling predicted that the pBC210 LF homologue contained an ADP-ribosyltransferase (ADPr) domain. This putative bacterial ADP-ribosyltransferase domain was denoted CerADPr. Iterative modeling showed that CerADPr possessed several conserved ADP-ribosyltransferase features, including an α-3 helix, an ADP-ribosyltransferase turn-turn loop, and a "Gln-XXX-Glu" motif. CerADPr ADP-ribosylated an ~120 kDa protein in HeLa cell lysates and intact cells. EGFP-CerADPr rounded HeLa cells, elicited cytoskeletal changes, and yielded a cytotoxic phenotype, indicating that CerADPr disrupts cytoskeletal signaling. CerADPr(E431D) did not possess ADP-ribosyltransferase or NAD glycohydrolase activities and did not elicit a phenotype in HeLa cells, implicating Glu431 as a catalytic residue. These experiments identify CerADPr as a cytotoxic ADP-ribosyltransferase that disrupts the host cytoskeleton.

CD44 Promotes intoxication by the clostridial iota-family toxins

Various pathogenic clostridia produce binary protein toxins associated with enteric diseases of humans and animals. Separate binding/translocation (B) components bind to a protein receptor on the cell surface, assemble with enzymatic (A) component(s), and mediate endocytosis of the toxin complex. Ultimately there is translocation of A component(s) from acidified endosomes into the cytosol, leading to destruction of the actin cytoskeleton. Our results revealed that CD44, a multifunctional surface protein of mammalian cells, facilitates intoxication by the iota family of clostridial binary toxins. Specific antibody against CD44 inhibited cytotoxicity of the prototypical Clostridium perfringens iota toxin. Versus CD44⁺ melanoma cells, those lacking CD44 bound less toxin and were dose-dependently resistant to C. perfringens iota, as well as Clostridium difficile and Clostridium spiroforme iota-like, toxins. Purified CD44 specifically interacted in vitro with iota and iota-like, but not related Clostridium botulinum C2, toxins. Furthermore, CD44 knockout mice were resistant to iota toxin lethality. Collective data reveal an important role for CD44 during intoxication by a family of clostridial binary toxins.

Intracellular trafficking of Clostridium perfringens iota-toxin b

Clostridium perfringens iota-toxin is composed of an enzymatic component (Ia) and a binding component (Ib). Ib binds to a cell surface receptor, undergoes oligomerization in lipid rafts, and binds Ia. The resulting complex is then endocytosed. Here, we show the intracellular trafficking of iota-toxin. After the binding of the Ib monomer with cells at 4°C, oligomers of Ib formed at 37°C and later disappeared. Immunofluorescence staining of Ib revealed that the internalized Ib was transported to early endosomes. Some Ib was returned to the plasma membrane through recycling endosomes, whereas the rest was transported to late endosomes and lysosomes for degradation. Degraded Ib was delivered to the plasma membrane by an increase in the intracellular Ca(2+) concentration caused by Ib. Bafilomycin A1, an endosomal acidification inhibitor, caused the accumulation of Ib in endosomes, and both nocodazole and colchicine, microtubule-disrupting agents, restricted Ib's movement in the cytosol. These results indicated that an internalized Ia and Ib complex was delivered to early endosomes and that subsequent delivery of Ia to the cytoplasm occurs mainly in early endosomes. Ib was either sent back to the plasma membranes through recycling endosomes or transported to late endosomes and lysosomes for degradation. Degraded Ib was transported to plasma membranes.

Clostridial binary toxins: iota and C2 family portraits

There are many pathogenic Clostridium species with diverse virulence factors that include protein toxins. Some of these bacteria, such as C. botulinum, C. difficile, C. perfringens, and C. spiroforme, cause enteric problems in animals as well as humans. These often fatal diseases can partly be attributed to binary protein toxins that follow a classic AB paradigm. Within a targeted cell, all clostridial binary toxins destroy filamentous actin via mono-ADP-ribosylation of globular actin by the A component. However, much less is known about B component binding to cell-surface receptors. These toxins share sequence homology amongst themselves and with those produced by another Gram-positive, spore-forming bacterium also commonly associated with soil and disease: Bacillus anthracis. This review focuses upon the iota and C2 families of clostridial binary toxins and includes: (1) basics of the bacterial source; (2) toxin biochemistry; (3) sophisticated cellular uptake machinery; and (4) host–cell responses following toxin-mediated disruption of the cytoskeleton. In summary, these protein toxins aid diverse enteric species within the genus Clostridium.

Multifaceted interactions of bacterial toxins with the gastrointestinal mucosa

The digestive tract is one of the ecosystems that harbors the largest number and greatest variety of bacteria. Among them, certain bacteria have developed various strategies, including the synthesis of virulence factors such as toxins, to interact with the intestinal mucosa, and are responsible for various pathologies. A large variety of bacterial toxins of different sizes, structures and modes of action are able to interact with the gastrointestinal mucosa. Some toxins, termed enterotoxins, directly stimulate fluid secretion in enterocytes or cause their death, whereas other toxins pass through the intestinal barrier and disseminate by the general circulation to remote organs or tissues, where they are active. After recognition of a membrane receptor on target cells, toxins can act at the cell membrane by transducing a signal across the membrane in a hormone-like manner, by pore formation or by damaging membrane compounds. Other toxins can enter the cells and modify an intracellular target leading to a disregulation of certain physiological processes or disorganization of some structural architectures and cell death. Toxins are fascinating molecules, which mimic or interfere with eukaryotic physiological processes. Thereby, they have permitted the identification and characterization of new natural hormones or regulatory pathways. Besides use as protective antigens in vaccines, toxins offer multiple possibilities in pharmacology, such as immune modulation or specific delivery of a protein of interest into target cells.

Membrane trafficking and signaling: two sides of the same coin

Recent findings on clathrin-dependent and non clathrin-dependent endocytic routes are currently changing our classical view of endocytosis. Originally seen as a way for the cell to internalize membrane, receptors or various soluble molecules, this process is in fact directly linked to complex signaling pathways. Here, we review new insights in endocytosis and present latest development in imaging techniques that allow us to visualize and follow the dynamics of membrane-associated signaling events at the plasma membrane and other intracellular compartments. The immune synapse is taken as an illustration of the importance of membrane reorganization and proteins clustering to initiate and maintain signaling. Future challenges include understanding the crosslink between traffic and signaling and how all compartmentalized signals are integrated inside the cell at a higher level.

Lipolysis-stimulated lipoprotein receptor (LSR) is the host receptor for the binary toxin Clostridium difficile transferase (CDT)

Clostridium difficile infection (CDI) causes antibiotic-associated diarrhea and pseudomembranous colitis. Hypervirulent strains of the pathogen, which are responsible for increased morbidity and mortality of CDI, produce the binary actin-ADP ribosylating toxin Clostridium difficile transferase (CDT) in addition to the Rho-glucosylating toxins A and B. CDT depolymerizes the actin cytoskeleton, increases adherence and colonization of Clostridia by induction of microtubule-based cell protrusions and, eventually, causes death of target cells. Using a haploid genetic screen, we identified the lipolysis-stimulated lipoprotein receptor as the membrane receptor for CDT uptake by target cells. Moreover, we show that Clostridium perfringens iota toxin, which is a related binary actin-ADP ribosylating toxin, enters target cells via the lipolysis-stimulated lipoprotein receptor. Identification of the toxin receptors is essential for understanding of the toxin uptake and provides a most valuable basis for antitoxin strategies.

Dynamin inhibition blocks botulinum neurotoxin type A endocytosis in neurons and delays botulism

The botulinum neurotoxins (BoNTs) are di-chain bacterial proteins responsible for the paralytic disease botulism. Following binding to the plasma membrane of cholinergic motor nerve terminals, BoNTs are internalized into an endocytic compartment. Although several endocytic pathways have been characterized in neurons, the molecular mechanism underpinning the uptake of BoNTs at the presynaptic nerve terminal is still unclear. Here, a recombinant BoNT/A heavy chain binding domain (Hc) was used to unravel the internalization pathway by fluorescence and electron microscopy. BoNT/A-Hc initially enters cultured hippocampal neurons in an activity-dependent manner into synaptic vesicles and clathrin-coated vesicles before also entering endosomal structures and multivesicular bodies. We found that inhibiting dynamin with the novel potent Dynasore analog, Dyngo-4a(TM), was sufficient to abolish BoNT/A-Hc internalization and BoNT/A-induced SNAP25 cleavage in hippocampal neurons. Dyngo-4a also interfered with BoNT/A-Hc internalization into motor nerve terminals. Furthermore, Dyngo-4a afforded protection against BoNT/A-induced paralysis at the rat hemidiaphragm. A significant delay of >30% in the onset of botulism was observed in mice injected with Dyngo-4a. Dynamin inhibition therefore provides a therapeutic avenue for the treatment of botulism and other diseases caused by pathogens sharing dynamin-dependent uptake mechanisms.

Cholesterol- and sphingolipid-rich microdomains are essential for microtubule-based membrane protrusions induced by Clostridium difficile transferase (CDT)

Clostridium difficile toxin (CDT) is a binary actin-ADP-ribosylating toxin that causes depolymerization of the actin cytoskeleton and formation of microtubule-based membrane protrusions, which are suggested to be involved in enhanced bacterial adhesion and colonization of hypervirulent C. difficile strains. Here, we studied the involvement of membrane lipid components of human colon adenocarcinoma (Caco-2) cells in formation of membrane protrusions. Depletion of cholesterol by methyl-β-cyclodextrin inhibited protrusion formation in a concentration-dependent manner but had no major effect on the toxin-catalyzed modification of actin in target cells. Repletion of cholesterol reconstituted formation of protrusions and increased velocity and total amount of protrusion formation. Methyl-β-cyclodextrin had no effect on the CDT-induced changes in the dynamics of microtubules. Formation of membrane protrusions was also inhibited by the cholesterol-binding polyene antibiotic nystatin. Degradation or inhibition of synthesis of sphingolipids by sphingomyelinase and myriocin, respectively, blocked CDT-induced protrusion formation. Benzyl alcohol, which increases membrane fluidity, prevented protrusion formation. CDT-induced membrane protrusions were stained by flotillin-2 and by the fluorescent-labeled lipid raft marker cholera toxin subunit B, which selectively interacts with GM1 ganglioside mainly located in lipid microdomains. The data suggest that formation and especially the initiation of CDT-induced microtubule-based membrane protrusions depend on cholesterol- and sphingolipid-rich lipid microdomains.