Changes in ganglioside content affect the binding of Clostridium perfringens epsilon-toxin to detergent-resistant membranes of Madin-Darby canine kidney cells

Central residues of the amphipathic β-hairpin loop control the properties of Clostridium perfringens epsilon-toxin channel

Clostridium perfringens epsilon toxin (ETX) is a heptameric pore-forming toxin of the aerolysin toxin family. ETX is the most potent toxin of this toxin family and the third most potent bacterial toxin with high cytotoxic and lethal activities in animals. In addition, ETX shows a demyelinating activity in nervous tissue leading to devastating multifocal central nervous system white matter disease in ruminant animals. Pore formation in target cell membrane is most likely the initial critical step in ETX biological activity. Eight single to quadruple ETX mutants were generated by replacement of polar residues (serine, threonine, glutamine) in middle positions of the β-strands forming the β-barrel and facing the channel lumen with charged glutamic residues. Channel activity and ion selectivity were monitored in artificial lipid monolayer membranes and cytotoxicity was investigated in MDCK cells by the viability MTT test and propidium iodide entry. All the mutants formed channels with similar conductance in artificial lipid membranes and increasing cation selectivity for increasing number of mutations. Here, we show that residues in the central position of each β-strand of the amphipathic β-hairpin loop that forms the transmembrane pore, control the size and ion selectivity of the channel. While the highest cationic ETX mutants were not cytotoxic, no strict correlation was observed between ion selectivity and cytotoxicity.

Endogenous pore-forming protein complex targets acidic glycosphingolipids in lipid rafts to initiate endolysosome regulation

Bacterial pore-forming toxin aerolysin-like proteins (ALPs) are widely distributed in animals and plants. However, functional studies on these ALPs remain in their infancy. βγ-CAT is the first example of a secreted pore-forming protein that functions to modulate the endolysosome pathway via endocytosis and pore formation on endolysosomes. However, the specific cell surface molecules mediating the action of βγ-CAT remain elusive. Here, the actions of βγ-CAT were largely attenuated by either addition or elimination of acidic glycosphingolipids (AGSLs). Further study revealed that the ALP and trefoil factor (TFF) subunits of βγ-CAT bind to gangliosides and sulfatides, respectively. Additionally, disruption of lipid rafts largely impaired the actions of βγ-CAT. Finally, the ability of βγ-CAT to clear pathogens was attenuated in AGSL-eliminated frogs. These findings revealed a previously unknown double binding pattern of an animal-secreted ALP in complex with TFF that initiates ALP-induced endolysosomal pathway regulation, ultimately leading to effective antimicrobial responses.

Clostridium perfringens Sialidases: Potential Contributors to Intestinal Pathogenesis and Therapeutic Targets

Clostridium perfringens is a major cause of histotoxic and intestinal infections of humans and other animals. This Gram-positive anaerobic bacterium can produce up to three sialidases named NanH, NanI, and NanJ. The role of sialidases in histotoxic infections, such as gas gangrene (clostridial myonecrosis), remains equivocal. However, recent in vitro studies suggest that NanI may contribute to intestinal virulence by upregulating production of some toxins associated with intestinal infection, increasing the binding and activity of some of those toxins, and enhancing adherence of C. perfringens to intestinal cells. Possible contributions of NanI to intestinal colonization are further supported by observations that the C. perfringens strains causing acute food poisoning in humans often lack the nanI gene, while other C. perfringens strains causing chronic intestinal infections in humans usually carry a nanI gene. Certain sialidase inhibitors have been shown to block NanI activity and reduce C. perfringens adherence to cultured enterocyte-like cells, opening the possibility that sialidase inhibitors could be useful therapeutics against C. perfringens intestinal infections. These initial in vitro observations should be tested for their in vivo significance using animal models of intestinal infections.

The Sialidase NanS Enhances Non-TcsL Mediated Cytotoxicity of Clostridium sordellii

The clostridia produce an arsenal of toxins to facilitate their survival within the host environment. TcsL is one of two major toxins produced by Clostridium sordellii, a human and animal pathogen, and is essential for disease pathogenesis of this bacterium. C. sordellii produces many other toxins, but the role that they play in disease is not known, although previous work has suggested that the sialidase enzyme NanS may be involved in the characteristic leukemoid reaction that occurs during severe disease. In this study we investigated the role of NanS in C. sordellii disease pathogenesis. We constructed a nanS mutant and showed that NanS is the only sialidase produced from C. sordellii strain ATCC9714 since sialidase activity could not be detected from the nanS mutant. Complementation with the wild-type gene restored sialidase production to the nanS mutant strain. Cytotoxicity assays using sialidase-enriched culture supernatants applied to gut (Caco2), vaginal (VK2), and cervical cell lines (End1/E6E7 and Ect1/E6E7) showed that NanS was not cytotoxic to these cells. However, the cytotoxic capacity of a toxin-enriched supernatant to the vaginal and cervical cell lines was substantially enhanced in the presence of NanS. TcsL was not the mediator of the observed cytotoxicity since supernatants harvested from a TcsL-deficient strain displayed similar cytotoxicity levels to TcsL-containing supernatants. This study suggests that NanS works synergistically with an unknown toxin or toxins to exacerbate C. sordellii-mediated tissue damage in the host.

Effects of Mycotoxins on mucosal microbial infection and related pathogenesis

Mycotoxins are fungal secondary metabolites detected in many agricultural commodities and water-damaged indoor environments. Susceptibility to mucosal infectious diseases is closely associated with immune dysfunction caused by mycotoxin exposure in humans and other animals. Many mycotoxins suppress immune function by decreasing the proliferation of activated lymphocytes, impairing phagocytic function of macrophages, and suppressing cytokine production, but some induce hypersensitive responses in different dose regimes. The present review describes various mycotoxin responses to infectious pathogens that trigger mucosa-associated diseases in the gastrointestinal and respiratory tracts of humans and other animals. In particular, it focuses on the effects of mycotoxin exposure on invasion, pathogen clearance, the production of cytokines and immunoglobulins, and the prognostic implications of interactions between infectious pathogens and mycotoxin exposure.

Correlation between in vitro cytotoxicity and in vivo lethal activity in mice of epsilon toxin mutants from Clostridium perfringens

Epsilon toxin (Etx) from Clostridium perfringens is a pore-forming protein with a lethal effect on livestock, producing severe enterotoxemia characterized by general edema and neurological alterations. Site-specific mutations of the toxin are valuable tools to study the cellular and molecular mechanism of the toxin activity. In particular, mutants with paired cysteine substitutions that affect the membrane insertion domain behaved as dominant-negative inhibitors of toxin activity in MDCK cells. We produced similar mutants, together with a well-known non-toxic mutant (Etx-H106P), as green fluorescent protein (GFP) fusion proteins to perform in vivo studies in an acutely intoxicated mouse model. The mutant (GFP-Etx-I51C/A114C) had a lethal effect with generalized edema, and accumulated in the brain parenchyma due to its ability to cross the blood-brain barrier (BBB). In the renal system, this mutant had a cytotoxic effect on distal tubule epithelial cells. The other mutants studied (GFP-Etx-V56C/F118C and GFP-Etx-H106P) did not have a lethal effect or cross the BBB, and failed to induce a cytotoxic effect on renal epithelial cells. These data suggest a direct correlation between the lethal effect of the toxin, with its cytotoxic effect on the kidney distal tubule cells, and the ability to cross the BBB.

Attack of the nervous system by Clostridium perfringens Epsilon toxin: from disease to mode of action on neural cells

Epsilon toxin (ET), produced by Clostridium perfringens types B and D, ranks among the four most potent poisonous substances known so far. ET-intoxication is responsible for enterotoxaemia in animals, mainly sheep and goats. This disease comprises several manifestations indicating the attack of the nervous system. This review aims to summarize the effects of ET on central nervous system. ET binds to endothelial cells of brain capillary vessels before passing through the blood-brain barrier. Therefore, it induces perivascular oedema and accumulates into brain. ET binding to different brain structures and to different component in the brain indicates regional susceptibility to the toxin. Histological examination has revealed nerve tissue and cellular lesions, which may be directly or indirectly caused by ET. The naturally occurring disease caused by ET-intoxication can be reproduced experimentally in rodents. In mice and rats, ET recognizes receptor at the surface of different neural cell types, including certain neurons (e.g. the granule cells in cerebellum) as well as oligodendrocytes, which are the glial cells responsible for the axons myelination. Moreover, ET induces release of glutamate and other transmitters, leading to firing of neural network. The precise mode of action of ET on neural cells remains to be determined.

Identification of amino acids important for binding of Clostridium perfringens epsilon toxin to host cells and to HAVCR1

Clostridium perfringens epsilon toxin belongs to the aerolysin-like family of pore-forming toxins and is one of the most potent bacterial toxins known. The epsilon toxin causes fatal enterotoxemia in sheep, goats, and possibly humans. Evidence indicates that the toxin binds to protein receptors including hepatitis A virus cellular receptor 1 (HAVCR1), but the region of the toxin responsible for cell binding has not been identified. In the present study, we identify amino acids within the epsilon toxin important for this cell interaction. Site-specific mutagenesis was used to investigate the role of a surface-accessible cluster of aromatic amino acids, and purified mutant proteins were tested in a series of cell-culture assays to assess cytotoxic activity and cell binding. When added to cells, four mutant proteins (Etx-Y29E, Etx-Y30E, Etx-Y36E and Etx-Y196E) were severely impaired in their ability to not only kill host cells, but also in their ability to permeabilize the plasma membrane. Circular dichroism spectroscopy and thermal stability studies revealed that the wild-type and mutant proteins were similarly folded. Additional experiments revealed that these mutant proteins were defective in binding to host cells and to HAVCR1. These data indicate that an amino acid motif including Y29, Y30, Y36, and Y196 is important for the ability of epsilon toxin to interact with cells and HAVCR1.

A single injection of the anabolic bone agent, parathyroid hormone-collagen binding domain (PTH-CBD), results in sustained increases in bone mineral density for up to 12 months in normal female mice

Parathyroid hormone (PTH) is the most effective osteoporosis treatment, but it is only effective if administered by daily injections. We fused PTH(1-33) to a collagen binding domain (PTH-CBD) to extend its activity, and have shown an anabolic bone effect with monthly dosing. We tested the duration of action of this compound with different routes of administration. Normal young C57BL/6J mice received a single intraperitoneal injection of PTH-CBD (320 μg/kg). PTH-CBD treated mice showed a 22.2 % increase in bone mineral density (BMD) at 6 months and 12.8 % increase at 12 months. When administered by subcutaneous injection, PTH-CBD again caused increases in BMD, 15.2 % at 6 months and 14.3 % at 12 months. Radiolabeled PTH-CBD was concentrated in bone and skin after either route of administration. We further investigated skin effects of PTH-CBD, and histological analysis revealed an apparent increase in anagen VI hair follicles. A single dose of PTH-CBD caused sustained increases in BMD by >10 % for 1 year in normal mice, regardless of the route of administration, thus showing promise as a potential osteoporosis therapy.

Treatment for chemotherapy-induced alopecia in mice using parathyroid hormone agonists and antagonists linked to a collagen binding domain

Parathyroid hormone (PTH) agonists and antagonists have been shown to improve hair growth after chemotherapy; however, rapid clearance and systemic side-effects complicate their usage. To facilitate delivery and retention to skin, we fused PTH agonists and antagonists to the collagen binding domain (CBD) of Clostridium histolyticum collagenase. in-vitro studies showed that the agonist fusion protein, PTH-CBD, bound collagen and activated the PTH/parathyroid hormone-related peptide receptor in SaOS-2 cells. The antagonist fusion proteins, PTH(7-33)-CBD and PTH([-1]-33)-CBD, also bound collagen and antagonized PTH(1-34) effect in SaOS-2 cells; however, PTH(7-33)-CBD had lower intrinsic activity. Distribution studies confirmed uptake of PTH-CBD to the skin at 1 and 12 hr after subcutaneous injection. We assessed in vivo efficacy of PTH-CBD and PTH(7-33)-CBD in C57BL/6J mice. Animals were depilated to synchronize the hair follicles; treated on Day 7 with agonist, antagonist, or vehicle; treated on Day 9 with cyclophosphamide (150 mg/kg i.p.) or vehicle; and sacrificed on Day 39. Normal mice (no chemo and no treatment) showed rapid regrowth of hair and normal histology. Chemo+Vehicle mice showed reduced hair regrowth and decreased pigmentation; histology revealed reduced number and dystrophic anagen/catagen follicles. Chemo+Antagonist mice were grossly and histologically indistinguishable from Chemo+Vehicle mice. Chemo+Agonist mice showed more rapid regrowth and repigmentation of hair; histologically, there was a normal number of hair follicles, most of which were in the anagen phase. Overall, the agonist PTH-CBD had prominent effects in reducing chemotherapy-induced damage of hair follicles and may show promise as a therapy for chemotherapy-induced alopecia.

Characterization of the high affinity binding of epsilon toxin from Clostridium perfringens to the renal system

Epsilon toxin (ε-toxin), produced by Clostridium perfringens types B and D, causes fatal enterotoxaemia in livestock. In the renal system, the toxin binds to target cells before oligomerization, pore formation and cell death. Still, there is little information about the cellular and molecular mechanism involved in the initial steps of the cytotoxic action of ε-toxin, including the specific binding to the target sensitive cells. In the present report, the binding step of ε-toxin to the MDCK cell line is characterized by means of an ELISA-based binding assay with recombinant ε-toxin-green fluorescence protein (ε-toxin-GFP) and ε-prototoxin-GFP. In addition, different treatments with Pronase E, detergents, N-glycosidase F and beta-elimination on MDCK cells and renal cryosections have been performed to further characterize the ε-toxin binding. The ELISA assays revealed a single binding site with a similar dissociation constant (K(d)) for ε-toxin-GFP and ε-prototoxin-GFP, but a three-fold increase in B(max) levels in the case of ε-toxin-GFP. Double staining on kidney cryoslices with lectins and ε-prototoxin-GFP revealed specific binding to distal and collecting tubule cells. In addition, experiments on kidney and bladder cryoslices demonstrated the specific binding to distal tubule of a range of mammalian renal systems. Pronase E and beta-elimination treatments on kidney cryoslices and MDCK cells revealed that the binding of ε-toxin in renal system is mediated by a O-glycoprotein. Detergent treatments revealed that the integrity of the plasma membrane is required for the binding of ε-toxin to its receptor.

Sialidases affect the host cell adherence and epsilon toxin-induced cytotoxicity of Clostridium perfringens type D strain CN3718

Clostridium perfringens type B or D isolates, which cause enterotoxemias or enteritis in livestock, produce epsilon toxin (ETX). ETX is exceptionally potent, earning it a listing as a CDC class B select toxin. Most C. perfringens strains also express up to three different sialidases, although the possible contributions of those enzymes to type B or D pathogenesis remain unclear. Type D isolate CN3718 was found to carry two genes (nanI and nanJ) encoding secreted sialidases and one gene (nanH) encoding a cytoplasmic sialidase. Construction in CN3718 of single nanI, nanJ and nanH null mutants, as well as a nanI/nanJ double null mutant and a triple sialidase null mutant, identified NanI as the major secreted sialidase of this strain. Pretreating MDCK cells with NanI sialidase, or with culture supernatants of BMC206 (an isogenic CN3718 etx null mutant that still produces sialidases) enhanced the subsequent binding and cytotoxic effects of purified ETX. Complementation of BMC207 (an etx/nanH/nanI/nanJ null mutant) showed this effect is mainly attributable to NanI production. Contact between BMC206 and certain mammalian cells (e.g., enterocyte-like Caco-2 cells) resulted in more rapid sialidase production and this effect involved increased transcription of BMC206 nanI gene. BMC206 was shown to adhere to some (e.g. Caco-2 cells), but not all mammalian cells, and this effect was dependent upon sialidase, particularly NanI, expression. Finally, the sialidase activity of NanI (but not NanJ or NanH) could be enhanced by trypsin. Collectively these in vitro findings suggest that, during type D disease originating in the intestines, trypsin may activate NanI, which (in turn) could contribute to intestinal colonization by C. perfringens type D isolates and also increase ETX action.

Clostridium perfringens type D strains cause enteritis and enterotoxemias in livestock after colonizing the intestines and then producing toxins, notably epsilon toxin (ETX). Initially produced and secreted in an inactive form, ETX can be rapidly proteolytically-activated by trypsin and other intestinal proteases. While most C. perfringens strains produce three sialidases, no pathogenic role has yet been identified for these enzymes that remove terminal sialic acid residues from glycoproteins and glycolipids. Our current study found that trypsin increases the activity of the NanI sialidase made by type D strain CN3718. This effect enhanced the ability of NanI to modify the surface of MDCK cells, leading to increased ETX binding and cytotoxicity. We also found that modification of the host cell surface by NanI sialidase allows efficient attachment of CN3718 cells to Caco-2 cells. These results identify interactions between intestinal proteases, ETX, sialidases, and ETX-producing bacteria, whereby trypsin activates not only ETX but also NanI sialidase. If similar effects occur in the intestines, the activated NanI sialidase may modify the host cell surface to facilitate bacterial attachment and thereby worsen disease by facilitating intestinal colonization by type D strains to prolong toxin delivery and, in some species, increase ETX binding.

Monthly administration of a novel PTH-collagen binding domain fusion protein is anabolic in mice

We synthesized fusion proteins of parathyroid hormone (PTH) (1-33) and the collagen binding domain of ColH (CBD) and tested them for anabolic bone activity in mice. Two fusion proteins were synthesized, linking the carboxy terminus of PTH(1-33) either directly to the amino terminal of the CBD or to the CBD through an adjacent ColH domain (PTH-PKD-CBD). Both PTH-CBD and PTH-PKD-CBD increased cAMP accumulation in cells stably transfected with the PTH/PTHrP receptor, and both peptides bound to type 1 collagen in flow-through assays. Distribution studies indicated that the PTH-CBD was concentrated in the bone and skin, tissues with abundant collagen and blood flow. Administration of 320 μg/kg PTH-CBD either weekly (for 8 weeks) or monthly (for 6 months) to 7-week-old C57BL/6J mice resulted in a sustained increase in bone mineral density (BMD) (15% for weekly studies, 13% for monthly studies; P < 0.05). PTH-PKD-CBD showed only 5% increases in BMD after weekly administration, and, as expected, neither weekly nor monthly PTH(1-34) affected BMD. PTH-CBD increased serum alkaline phosphatase levels. Importantly, there were no significant increases in serum calcium observed. Collectively, the data suggest that PTH-CBD has a sustained anabolic effect in bone with either weekly or monthly administration. This approach of targeted delivery of PTH to bone may show promise for the treatment of disorders of low bone mass, such as postmenopausal osteoporosis.

Molecular basis of toxicity of Clostridium perfringens epsilon toxin

Clostridium perfringens ε-toxin is produced by toxinotypes B and D strains. The toxin is the aetiological agent of dysentery in newborn lambs but is also associated with enteritis and enterotoxaemia in goats, calves and foals. It is considered to be a potential biowarfare or bioterrorism agent by the US Government Centers for Disease Control and Prevention. The relatively inactive 32.9 kDa prototoxin is converted to active mature toxin by proteolytic cleavage, either by digestive proteases of the host, such as trypsin and chymotrypsin, or by C. perfringens λ-protease. In vivo, the toxin appears to target the brain and kidneys, but relatively few cell lines are susceptible to the toxin, and most work has been carried out using Madin-Darby canine kidney (MDCK) cells. The binding of ε-toxin to MDCK cells and rat synaptosomal membranes is associated with the formation of a stable, high molecular weight complex. The crystal structure of ε-toxin reveals similarity to aerolysin from Aeromonas hydrophila, parasporin-2 from Bacillus thuringiensis and a lectin from Laetiporus sulphureus. Like these toxins, ε-toxin appears to form heptameric pores in target cell membranes. The exquisite specificity of the toxin for specific cell types suggests that it binds to a receptor found only on these cells.

Gene-trap mutagenesis identifies mammalian genes contributing to intoxication by Clostridium perfringens ε-toxin

The Clostridium perfringens ε-toxin is an extremely potent toxin associated with lethal toxemias in domesticated ruminants and may be toxic to humans. Intoxication results in fluid accumulation in various tissues, most notably in the brain and kidneys. Previous studies suggest that the toxin is a pore-forming toxin, leading to dysregulated ion homeostasis and ultimately cell death. However, mammalian host factors that likely contribute to ε-toxin-induced cytotoxicity are poorly understood. A library of insertional mutant Madin Darby canine kidney (MDCK) cells, which are highly susceptible to the lethal affects of ε-toxin, was used to select clones of cells resistant to ε-toxin-induced cytotoxicity. The genes mutated in 9 surviving resistant cell clones were identified. We focused additional experiments on one of the identified genes as a means of validating the experimental approach. Gene expression microarray analysis revealed that one of the identified genes, hepatitis A virus cellular receptor 1 (HAVCR1, KIM-1, TIM1), is more abundantly expressed in human kidney cell lines than it is expressed in human cells known to be resistant to ε-toxin. One human kidney cell line, ACHN, was found to be sensitive to the toxin and expresses a larger isoform of the HAVCR1 protein than the HAVCR1 protein expressed by other, toxin-resistant human kidney cell lines. RNA interference studies in MDCK and in ACHN cells confirmed that HAVCR1 contributes to ε-toxin-induced cytotoxicity. Additionally, ε-toxin was shown to bind to HAVCR1 in vitro. The results of this study indicate that HAVCR1 and the other genes identified through the use of gene-trap mutagenesis and RNA interference strategies represent important targets for investigation of the process by which ε-toxin induces cell death and new targets for potential therapeutic intervention.

Mega assemblages of oligomeric aerolysin-like toxins stabilized by toxin-associating membrane proteins

Most β pore-forming toxins need to be oligomerized via receptors in order to form membrane pores. Though oligomerizing toxins frequently form SDS-resistant oligomers, it was questionable whether SDS-resistant oligomers reflected native functional toxin complexes. In order to elucidate the essence of the cytocidal assemblages, oligomers of aerolysin-like toxins, aerolysin, parasporin-2 and epsilon toxin, were examined with or without SDS. On Blue Native PAGE, each toxin, which had been solubilized from target cells with mild detergent, was a much larger complex (nearly 1 MDa) than the typical SDS-resistant oligomers (∼200 kDa). Size exclusion chromatography confirmed the huge toxin complexes. While a portion of the huge complexes were sensitive to proteases, SDS-resistant oligomers resist the proteolysis. Presumably the core toxin complexes remained intact while the cellular proteins were degraded. Moreover, intermediate complexes, which included no SDS-resistant oligomers, could be detected at lower temperatures. This study provides evidence for huge functional complexes of β pore-forming toxins and emphasizes their potential variance in composition.

Clostridium perfringens epsilon toxin targets granule cells in the mouse cerebellum and stimulates glutamate release

Epsilon toxin (ET) produced by C. perfringens types B and D is a highly potent pore-forming toxin. ET-intoxicated animals express severe neurological disorders that are thought to result from the formation of vasogenic brain edemas and indirect neuronal excitotoxicity. The cerebellum is a predilection site for ET damage. ET has been proposed to bind to glial cells such as astrocytes and oligodendrocytes. However, the possibility that ET binds and attacks the neurons remains an open question. Using specific anti-ET mouse polyclonal antibodies and mouse brain slices preincubated with ET, we found that several brain structures were labeled, the cerebellum being a prominent one. In cerebellar slices, we analyzed the co-staining of ET with specific cell markers, and found that ET binds to the cell body of granule cells, oligodendrocytes, but not astrocytes or nerve endings. Identification of granule cells as neuronal ET targets was confirmed by the observation that ET induced intracellular Ca(2+) rises and glutamate release in primary cultures of granule cells. In cultured cerebellar slices, whole cell patch-clamp recordings of synaptic currents in Purkinje cells revealed that ET greatly stimulates both spontaneous excitatory and inhibitory activities. However, pharmacological dissection of these effects indicated that they were only a result of an increased granule cell firing activity and did not involve a direct action of the toxin on glutamatergic nerve terminals or inhibitory interneurons. Patch-clamp recordings of granule cell somata showed that ET causes a decrease in neuronal membrane resistance associated with pore-opening and depolarization of the neuronal membrane, which subsequently lead to the firing of the neuronal network and stimulation of glutamate release. This work demonstrates that a subset of neurons can be directly targeted by ET, suggesting that part of ET-induced neuronal damage observed in neuronal tissue is due to a direct effect of ET on neurons.

Clostridium perfringens epsilon toxin is cytotoxic for human renal tubular epithelial cells

Clostridium perfringens epsilon toxin (ETX) is responsible for a fatal enterotoxemia in different animal species, producing extensive renal damage, neurological disturbance and edema of lungs, heart and kidneys. However, there is no information about the susceptibility of humans to ETX. Here, we report that primary cultures of human renal tubular epithelial cells (HRTEC) exposed to ETX showed a marked swelling with subsequent large blebs surrounding most cells. The incubation of HRTEC with ETX produced a reduction of cell viability in a dose- and time-dependent manner. The CD(50) after 1-hour and 24-hour incubation were 3 µg/mL and 0.5 µg/mL, respectively. The pulse with ETX for 3 min was enough to produce a significant cytotoxic effect on HRTEC after 1-hour incubation. ETX binds to HRTEC forming a large complex of about 160 kDa similar to what was found in the Madin-Darby canine kidney (MDCK) cell line. The HRTEC could be a useful cell model to improve the understanding of the mechanisms involved on the cell damage mediated by ETX.

Clostridial toxins

Clostridia produce the highest number of toxins of any type of bacteria and are involved in severe diseases in humans and other animals. Most of the clostridial toxins are pore-forming toxins responsible for gangrenes and gastrointestinal diseases. Among them, perfringolysin has been extensively studied and it is the paradigm of the cholesterol-dependent cytolysins, whereas Clostridium perfringens epsilon-toxin and Clostridium septicum alpha-toxin, which are related to aerolysin, are the prototypes of clostridial toxins that form small pores. Other toxins active on the cell surface possess an enzymatic activity, such as phospholipase C and collagenase, and are involved in the degradation of specific cell-membrane or extracellular-matrix components. Three groups of clostridial toxins have the ability to enter cells: large clostridial glucosylating toxins, binary toxins and neurotoxins. The binary and large clostridial glucosylating toxins alter the actin cytoskeleton by enzymatically modifying the actin monomers and the regulatory proteins from the Rho family, respectively. Clostridial neurotoxins proteolyse key components of neuroexocytosis. Botulinum neurotoxins inhibit neurotransmission at neuromuscular junctions, whereas tetanus toxin targets the inhibitory interneurons of the CNS. The high potency of clostridial toxins results from their specific targets, which have an essential cellular function, and from the type of modification that they induce. In addition, clostridial toxins are useful pharmacological and biological tools.

Identification of the channel-forming domain of Clostridium perfringens Epsilon-toxin (ETX)

Epsilon-toxin (ETX) is a potent toxin produced by Clostridium perfringens strains B and D. The bacteria are important pathogens in domestic animals and cause edema mediated by ETX. This toxin acts most likely by heptamer formation and rapid permeabilization of target cell membranes for monovalent anions and cations followed by a later entry of calcium. In this study, we compared the primary structure of ETX with that of the channel-forming stretches of a variety of binding components of A-B-types of toxins such as Anthrax protective antigen (PA), C2II of C2-toxin and Ib of Iota-toxin and found a remarkable homology to amino acids 151-180 of ETX. Site-directed mutagenesis of amino acids within the putative channel-forming domain resulted in changes of cytotoxicity and effects on channel characteristics in lipid bilayer experiments including changes of selectivity and partial channel block by methanethiosulfonate (MTS) reagents and antibodies against His(6)-tags from the trans-side of the lipid bilayer membranes.

The NanI and NanJ sialidases of Clostridium perfringens are not essential for virulence

The essential toxin in Clostridium perfringens-mediated gas gangrene or clostridial myonecrosis is alpha-toxin, although other toxins and extracellular enzymes may also be involved. In many bacterial pathogens extracellular sialidases are important virulence factors, and it has been suggested that sialidases may play a role in gas gangrene. C. perfringens strains have combinations of three different sialidase genes, two of which, nanI and nanJ, encode secreted sialidases. The nanI and nanJ genes were insertionally inactivated by homologous recombination in derivatives of sequenced strain 13 and were shown to encode two functional secreted sialidases, NanI and NanJ. Analysis of these derivatives showed that NanI was the major sialidase in this organism. Mutation of nanI resulted in loss of most of the secreted sialidase activity, and the residual activity was eliminated by subsequent mutation of the nanJ gene. Only a slight reduction in the total sialidase activity was observed in a nanJ mutant. Cytotoxicity assays using the B16 melanoma cell line showed that supernatants containing NanI or overexpressing NanJ enhanced alpha-toxin-mediated cytotoxicity. Finally, the ability of nanI, nanJ, and nanIJ mutants to cause disease was assessed in a mouse myonecrosis model. No attenuation of virulence was observed for any of these strains, providing evidence that neither the NanI sialidase nor the NanJ sialidase is essential for virulence.

Functional analysis of neutralizing antibodies against Clostridium perfringens epsilon-toxin

The Clostridium perfringens epsilon-toxin causes a severe, often fatal illness (enterotoxemia) characterized by cardiac, pulmonary, kidney, and brain edema. In this study, we examined the activities of two neutralizing monoclonal antibodies against the C. perfringens epsilon-toxin. Both antibodies inhibited epsilon-toxin cytotoxicity towards cultured MDCK cells and inhibited the ability of the toxin to form pores in the plasma membranes of cells, as shown by staining cells with the membrane-impermeant dye 7-aminoactinomycin D. Using an antibody competition enzyme-linked immunosorbent assay (ELISA), a peptide array, and analysis of mutant toxins, we mapped the epitope recognized by one of the neutralizing monoclonal antibodies to amino acids 134 to 145. The antibody competition ELISA and analysis of mutant toxins suggest that the second neutralizing monoclonal antibody also recognizes an epitope in close proximity to this region. The region comprised of amino acids 134 to 145 overlaps an amphipathic loop corresponding to the putative membrane insertion domain of the toxin. Identifying the epitopes recognized by these neutralizing antibodies constitutes an important first step in the development of therapeutic agents that could be used to counter the effects of the epsilon-toxin.

Preventing the binding of pathogens to the host by controlling sphingolipid metabolism

The binding of many pathogens and toxins to human cells can be inhibited by (1) depleting host cells of their surface glycosphingolipids; (2) coating the binding sites on pathogens (adhesins) with glycosphingolipid-like substances (decoys); (3) coating the host's glycosphingolipids with substances that compete with the pathogen for binding. Details of using these methods are described.

Use of RNA interference in Drosophila S2 cells to identify host pathways controlling compartmentalization of an intracellular pathogen

Three genome-wide RNA interference screens were performed in Drosophila S2 cells to dissect the contribution of host processes to Listeria monocytogenes entry, vacuolar escape, and intracellular growth. Among the 116 genes identified, several host pathways previously unrecognized as playing a role in listerial pathogenesis were identified: knockdowns affecting vacuolar trafficking to and from the multivesicular body bypassed the requirement for the essential pore-forming toxin listeriolysin O in mediating escape from phagocytic vacuoles and knockdowns affecting either subunit of serine palmitoyltransferase, a key enzyme in ceramide and sphingolipid biosynthesis, enhanced the toxicity of listeriolysin O expressed in the host cell cytosol, leading to lack of appropriate toxin activity compartmentalization and host cell death. Genome-wide RNA interference screens using Drosophila S2 cells proved to be a powerful approach to dissect host-pathogen interactions.