Clostridium perfringens type A enterotoxin forms mepacrine-sensitive pores in pure phospholipid bilayers in the absence of putative receptor proteins

Antibody-mediated targeting of Claudins in cancer

Tight junctions (TJs) are large intercellular adhesion complexes that maintain cell polarity in normal epithelia and endothelia. Claudins are critical components of TJs, forming homo- and heteromeric interaction between adjacent cells, which have emerged as key functional modulators of carcinogenesis and metastasis. Numerous epithelial-derived cancers display altered claudin expression patterns, and these aberrantly expressed claudins have been shown to regulate cancer cell proliferation/growth, metabolism, metastasis and cell stemness. Certain claudins can now be used as biomarkers to predict patient prognosis in a variety of solid cancers. Our understanding of the distinct roles played by claudins during the cancer progression has progressed significantly over the last decade and claudins are now being investigated as possible diagnostic markers and therapeutic targets. In this review, we will summarize recent progress in the use of antibody-based or related strategies for targeting claudins in cancer treatment. We first describe pre-clinical studies that have facilitated the development of neutralizing antibodies and antibody-drug-conjugates targeting Claudins (Claudins-1, -3, -4, -6 and 18.2). Next, we summarize clinical trials assessing the efficacy of antibodies targeting Claudin-6 or Claudin-18.2. Finally, emerging strategies for targeting Claudins, including Chimeric Antigen Receptor (CAR)-T cell therapy and Bi-specific T cell engagers (BiTEs), are also discussed.

Pore-forming toxins of foodborne pathogens

Pore-forming toxins (PFTs) are water-soluble molecules that have been identified as the most crucial virulence factors during bacterial pathogenesis. PFTs disrupt the host cell membrane to internalize or to deliver other bacterial or virulence factors for establishing infections. Disruption of the host cell membrane by PFTs can lead to uncontrollable exchanges between the extracellular and the intracellular matrix, thereby disturbing the cellular homeostasis. Recent studies have provided insights into the molecular mechanism of PFTs during pathogenesis. Evidence also suggests the activation of several signal transduction pathways in the host cell on recognition of PFTs. Additionally, numerous distinctive host defense mechanisms as well as membrane repair mechanisms have been reported; however, studies reveal that PFTs aid in host immune evasion of the bacteria through numerous pathways. PFTs have been primarily associated with foodborne pathogens. Infection and death from diseases by consuming contaminated food are a constant threat to public health worldwide, affecting socioeconomic development. Moreover, the emergence of new foodborne pathogens has led to the rise of bacterial antimicrobial resistance affecting the population. Hence, this review focuses on the role of PFTs secreted by foodborne pathogens. The review highlights the molecular mechanism of foodborne bacterial PFTs, assisting bacterial survival from the host immune responses and understanding the downstream mechanism in the activation of various signaling pathways in the host upon PFT recognition. PFT research is a remarkable and an important field for exploring novel and broad applications of antimicrobial compounds as therapeutics.

Potential Therapeutic Effects of Mepacrine against Clostridium perfringens Enterotoxin in a Mouse Model of Enterotoxemia

Clostridium perfringens enterotoxin (CPE) is a pore-forming toxin that causes the symptoms of common bacterial food poisoning and several non-foodborne human gastrointestinal diseases, including antibiotic-associated diarrhea and sporadic diarrhea. In some cases, CPE-mediated disease can be very severe or fatal due to the involvement of enterotoxemia. Therefore, the development of potential therapeutics against CPE action during enterotoxemia is warranted. Mepacrine, an acridine derivative drug with broad-spectrum effects on pores and channels in mammalian membranes, has been used to treat protozoal intestinal infections in human patients. A previous study showed that the presence of mepacrine inhibits CPE-induced pore formation and activity in enterocyte-like Caco-2 cells, reducing the cytotoxicity caused by this toxin in vitro Whether mepacrine is similarly protective against CPE action in vivo has not been tested. When the current study evaluated whether mepacrine protects against CPE-induced death and intestinal damage using a murine ligated intestinal loop model, mepacrine protected mice from the enterotoxemic lethality caused by CPE. This protection was accompanied by a reduction in the severity of intestinal lesions induced by the toxin. Mepacrine did not reduce CPE pore formation in the intestine but inhibited absorption of the toxin into the blood of some mice. Protection from enterotoxemic death correlated with the ability of this drug to reduce CPE-induced hyperpotassemia. These in vivo findings, coupled with previous in vitro studies, support mepacrine as a potential therapeutic against CPE-mediated enterotoxemic disease.

Clostridium perfringens Enterotoxin: The Toxin Forms Highly Cation-Selective Channels in Lipid Bilayers

One of the numerous toxins produced by Clostridium perfringens is Clostridium perfringens enterotoxin (CPE), a polypeptide with a molecular mass of 35.5 kDa exhibiting three different domains. Domain one is responsible for receptor binding, domain two is involved in hexamer formation and domain three has to do with channel formation in membranes. CPE is the major virulence factor of this bacterium and acts on the claudin-receptor containing tight junctions between epithelial cells resulting in various gastrointestinal diseases. The activity of CPE on Vero cells was demonstrated by the entry of propidium iodide (PI) in the cells. The entry of propidium iodide caused by CPE was well correlated with the loss of cell viability monitored by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test. CPE formed ion-permeable channels in artificial lipid bilayer membranes with a single-channel conductance of 620 pS in 1 M KCl. The single-channel conductance was not a linear function of the bulk aqueous salt concentration indicating that point-negative charges at the CPE channel controlled ion transport. This resulted in the high cation selectivity of the CPE channels, which suggested that anions are presumably not permeable through the CPE channels. The possible role of cation transport by CPE channels in disease caused by C. perfringens is discussed.

Planar lipid bilayers in recombinant ion channel research

There are a number of methods of investigating the function of recombinant proteins such as ion channels. However, after channel purification there are few methods to guarantee that the protein still functions. For ion channels, reconstituting back into planar lipid bilayers and demonstrating preserved function is a convenient and trusted method. It is cell free and even inaccessible, intracellular ion channels can be studied. We have used this method to study the function of recombinant channels of known subunit composition and have found it convenient for investigating the mode of action of ion channel modulators.

The Potential Therapeutic Agent Mepacrine Protects Caco-2 Cells against Clostridium perfringens Enterotoxin Action

Clostridium perfringens enterotoxin (CPE) causes the gastrointestinal (GI) symptoms of a common bacterial food poisoning and several nonfoodborne human GI diseases. A previous study showed that, via an undetermined mechanism, the presence of mepacrine blocks CPE-induced electrophysiologic activity in artificial membranes. The current study now demonstrates that mepacrine also inhibits CPE-induced cytotoxicity in human enterocyte-like Caco-2 cells and that mepacrine does not directly inactivate CPE. Instead, this drug reduces both CPE pore formation and CPE pore activity in Caco-2 cells. These results suggest mepacrine as a therapeutic candidate for treating CPE-mediated GI diseases.

Clostridium perfringens enterotoxin (CPE) causes the diarrhea associated with a common bacterial food poisoning and many antibiotic-associated diarrhea cases. The severity of some CPE-mediated disease cases warrants the development of potential therapeutics. A previous study showed that the presence of mepacrine inhibited CPE-induced electrophysiology effects in artificial lipid bilayers lacking CPE receptors. However, that study did not assess whether mepacrine inactivates CPE or, instead, inhibits a step in CPE action. Furthermore, CPE action in host cells is complex, involving the toxin binding to receptors, receptor-bound CPE oligomerizing into a prepore on the membrane surface, and β-hairpins in the CPE prepore inserting into the membrane to form a pore that induces cell death. Therefore, the current study evaluated the ability of mepacrine to protect cells from CPE. This drug was found to reduce CPE-induced cytotoxicity in Caco-2 cells. This protection did not involve mepacrine inactivation of CPE, indicating that mepacrine affects one or more steps in CPE action. Western blotting then demonstrated that mepacrine decreases CPE pore levels in Caco-2 cells. This mepacrine-induced reduction in CPE pore levels did not involve CPE binding inhibition but rather an increase in CPE monomer dissociation due to mepacrine interactions with Caco-2 membranes. In addition, mepacrine was also shown to inhibit CPE pores when already present in Caco-2 cells. These in vitro studies, which identified two mepacrine-sensitive steps in CPE-induced cytotoxicity, add support to further testing of the therapeutic potential of mepacrine against CPE-mediated disease.

IMPORTANCEClostridium perfringens enterotoxin (CPE) causes the gastrointestinal (GI) symptoms of a common bacterial food poisoning and several nonfoodborne human GI diseases. A previous study showed that, via an undetermined mechanism, the presence of mepacrine blocks CPE-induced electrophysiologic activity in artificial membranes. The current study now demonstrates that mepacrine also inhibits CPE-induced cytotoxicity in human enterocyte-like Caco-2 cells and that mepacrine does not directly inactivate CPE. Instead, this drug reduces both CPE pore formation and CPE pore activity in Caco-2 cells. These results suggest mepacrine as a therapeutic candidate for treating CPE-mediated GI diseases.

Structure of a C. perfringens enterotoxin mutant in complex with a modified Claudin-2 extracellular loop 2

CPE (Clostridium perfringens enterotoxin) is the major virulence determinant for C. perfringens type-A food poisoning, the second most common bacterial food-borne illness in the UK and USA. After binding to its receptors, which include particular human claudins, the toxin forms pores in the cell membrane. The mature pore apparently contains a hexamer of CPE, claudin and, possibly, occludin. The combination of high binding specificity with cytotoxicity has resulted in CPE being investigated, with some success, as a targeted cytotoxic agent for oncotherapy. In this paper, we present the X-ray crystallographic structure of CPE in complex with a peptide derived from extracellular loop 2 of a modified, CPE-binding Claudin-2, together with high-resolution native and pore-formation mutant structures. Our structure provides the first atomic-resolution data on any part of a claudin molecule and reveals that claudin's CPE-binding fingerprint (NPLVP) is in a tight turn conformation and binds, as expected, in CPE's C-terminal claudin-binding groove. The leucine and valine residues insert into the binding groove while the first residue, asparagine, tethers the peptide via an interaction with CPE's aspartate 225 and the two prolines are required to maintain the tight turn conformation. Understanding the structural basis of the contribution these residues make to binding will aid in engineering CPE to target tumor cells.

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.

Structure of the food-poisoning Clostridium perfringens enterotoxin reveals similarity to the aerolysin-like pore-forming toxins

Clostridium perfringens enterotoxin (CPE) is a major cause of food poisoning and antibiotic-associated diarrhea. Upon its release from C. perfringens spores, CPE binds to its receptor, claudin, at the tight junctions between the epithelial cells of the gut wall and subsequently forms pores in the cell membranes. A number of different complexes between CPE and claudin have been observed, and the process of pore formation has not been fully elucidated. We have determined the three-dimensional structure of the soluble form of CPE in two crystal forms by X-ray crystallography, to a resolution of 2.7 and 4.0 Å, respectively, and found that the N-terminal domain shows structural homology with the aerolysin-like β-pore-forming family of proteins. We show that CPE forms a trimer in both crystal forms and that this trimer is likely to be biologically relevant but is not the active pore form. We use these data to discuss models of pore formation.

Demonstration of a cholesterol-dependent cytolysin in a noninsecticidal Bacillus sphaericus strain and evidence for widespread distribution of the toxin within the species

During the course of screening Bacillus species from food and water in Norway, we isolated a strain of Bacillus sphaericus of DNA homology group V, not previously recognized to contain entomopathogenic strains, that was cytotoxic to Vero cell epithelia. Peptide mass fingerprinting of a protein purified from the culture supernatant of B. sphaericus B354 identified a cholesterol-dependent cytolysin (CDC) with high amino acid sequence identity with sphaericolysin, a CDC identified recently in B. sphaericus DNA homology group IIA. The toxin was haemolytic against erythrocytes from a range of species. Haemolysis was potentiated by dithiothreitol and inhibited by preincubation with cholesterol. The toxin induced lactate dehydrogenase release from Vero cells and formed pores in planar lipid bilayers. The distribution of CDC genes in B. sphaericus was examined, with CDC gene products obtained in 13 out of 17 strains representing four of the six DNA homology groups. Thus, we demonstrate the presence of a CDC in a nonentomopathogenic DNA homology group of B. sphaericus (group V) with typical CDC characteristics. CDCs appear to be present in a high proportion of B. sphaericus strains and are not restricted to group IIA insecticidal strains.

Bacillus cereus Nhe is a pore-forming toxin with structural and functional properties similar to the ClyA (HlyE, SheA) family of haemolysins, able to induce osmotic lysis in epithelia

The mechanism by which Bacillus cereus causes diarrhoea is unknown. Three putative enterotoxins have been proposed, haemolysin BL (Hbl), cytotoxin K and non-haemolytic enterotoxin (Nhe). Both Hbl and Nhe are three-component cytotoxins and maximal cytotoxicity of Nhe against epithelia is dependent on all three components. However, little is known of the mechanism of cytotoxicity. Markers of plasma membrane disruption, namely propidium iodide uptake, loss of cellular ATP and release of lactate dehydrogenase (LDH), were observed in epithelia exposed to Nhe from culture supernatants of B. cereus, but not in those exposed to supernatants from a mutant strain lacking NheB and NheC. Consistent with an exogenous cause of membrane damage, purified Nhe components combined to form large conductance pores in planar lipid bilayers. The inhibition of LDH release by osmotic protectants and the increase in cell size caused by Nhe indicate that epithelia lyse following osmotic swelling. Nhe and Hbl show sequence homology, and Hbl component B has remarkable structural similarities to cytolysin A (ClyA), with both structures possessing an alpha-helix bundle and a unique subdomain containing a hydrophobic beta-hairpin. Correspondingly, we show that Nhe has haemolytic activity against erythrocytes from a variety of species. We propose that the common structural and functional properties indicate that the Hbl/Nhe and ClyA families of toxins constitute a superfamily of pore-forming cytotoxins.

Compositional and stoichiometric analysis of Clostridium perfringens enterotoxin complexes in Caco-2 cells and claudin 4 fibroblast transfectants

Clostridium perfringens enterotoxin (CPE) binds to host cell receptors, forming a small complex precursor for two large complexes reportedly having molecular masses of approximately 155 or approximately 200 kDa. Formation of the approximately 155 kDa complex causes a Ca(2+) influx that leads to apoptosis or oncosis. CPE complex composition is currently poorly understood, although occludin was identified in the approximately 200 kDa complex. The current study used heteromer gel shift analysis to show both CPE large complexes contain six CPE molecules. Ferguson plots and size exclusion chromatography re-sized the approximately 155 and approximately 200 kDa complexes as approximately 425-500 kDa and approximately 550-660 kDa respectively. Co-immunoprecipitation and electroelution studies demonstrated both CPE-binding and non-CPE-binding claudins are associated with all three CPE complexes in Caco-2 cells and with small complex and approximately 425-500 kDa complex of claudin 4 transfectants. Fibroblast transfectants expressing claudin 4 or C-terminal truncated claudin 4 were CPE-sensitive and formed the approximately 425 kDa complex, indicating claudin-induced cell signalling is not required for CPE action and that expression of a single receptor claudin suffices for approximately 425-500 kDa CPE complex formation. These results identify CPE as a unique toxin that combines with tight junction proteins to form high-molecular-mass hexameric pores and alter membrane permeability.

Identification of a prepore large-complex stage in the mechanism of action of Clostridium perfringens enterotoxin

Clostridium perfringens enterotoxin (CPE) is the etiological agent of the third most common food-borne illness in the United States. The enteropathogenic effects of CPE result from formation of large CPE-containing complexes in eukaryotic cell membranes. Formation of these approximately 155- and approximately 200-kDa complexes coincides with plasma membrane permeability changes in eukaryotic cells, causing a Ca2+ influx that drives cell death pathways. CPE contains a stretch of amino acids (residues 81 to 106) that alternates markedly in side chain polarity (a pattern shared by the transmembrane domains of the beta-barrel pore-forming toxin family). The goal of this study, therefore, was to investigate whether this CPE region is involved in pore formation. Complete deletion of the CPE region from 81 to 106 produced a CPE variant that was noncytotoxic for Caco-2 cells and was unable to form CPE pores. However, this variant maintained the ability to form the approximately 155-kDa large complex. This large complex appears to be a prepore present on the plasma membrane surface since it showed greater susceptibility to proteases, increased complex instability, and a higher degree of dissociation from membranes compared to the large complex formed by recombinant CPE. When a D48A mutation was engineered into this prepore-forming CPE variant, the resultant variant was unable to form any prepore approximately 155-kDa large complex. Collectively these findings reveal a new step in CPE action, whereby receptor binding is followed by formation of a prepore large complex, which then inserts into membranes to form a pore.

Cytotoxicity in Bacillus mojavensis is abolished following loss of surfactin synthesis: implications for assessment of toxicity and food poisoning potential

Bacillus subtilis and the closely related species Bacillus pumilus and Bacillus licheniformis have periodically been suggested to play a role in the aetiology of food poisoning despite the fact that the organisms do not possess the genes associated with enteropathogenicity in Bacillus cereus. We show here that Bacillus mojavensis, an organism closely related to B. subtilis, is able to produce toxic components which identify as a complex of three different surfactin analogues. These cyclic lipopeptides were soluble in methanol, heat stable after treatment in a boiling water bath for 10 min, resistant to enzymatic degradation by pepsin, trypsin, endoprotease V8 and proteinase K and formed pores in planar lipid bilayers. They were cytotoxic when tested in a series of commonly used laboratory cytotoxicity assays, namely, lactate dehydrogenase release, haemolysis, inhibition of both protein synthesis in Vero cells and motility in boar sperm. We show that such in vitro markers of enterotoxicity are due entirely to production of cyclic lipopeptides since deletion of sfp, a gene essential for surfactin synthesis which abolished the cytotoxicity to Vero cells, boar sperm motility and haemolytic activity. Thus, the relevance of cyclic lipopeptides as food poisoning toxins needs to be evaluated in assays other than the cell cytotoxicity assays in common use.

Modulation of the BK channel by estrogens: examination at single channel level

BK channels regulate vascular tone by hyperpolarizing smooth muscle in response to fluctuating calcium concentrations. Oestrogen has been reported to lower blood pressure by increasing BK channel open probability through direct binding to the regulatory beta1-subunit(s) associated with the channel. The present investigation demonstrates that 17beta-oestradiol activates the BK channel complex by increasing the burst duration of channel openings. A subconductance state was observed in 25% of recordings following the addition of 17beta-oestradiol and could reflect uncoupling between the pore forming alpha1-subunit and the regulatory beta1-subunit. We also present evidence that more than one beta1-subunit is required to facilitate binding of 17beta-oestradiol to the channel complex.

Clostridium perfringens: insight into virulence evolution and population structure

Clostridium perfringens is an important pathogen in veterinary and medical fields. Diseases caused by this organism are in many cases life threatening or fatal. At the same time, it is part of the ecological community of the intestinal tract of man and animals. Virulence in this species is not fully understood and it does seem that there is erratic distribution of the toxin/enzyme genes within C. perfringens population. We used the recently developed multiple-locus variable-number tandem repeat analysis (MLVA) scheme to investigate the evolution of virulence and population structure of this species. Analysis of the phylogenetic signal indicates that acquisition of the major toxin genes as well as other plasmid-borne toxin genes is a recent evolutionary event and their maintenance is essentially a function of the selective advantage they confer in certain niches under different conditions. In addition, it indicates the ability of virulent strains to cause disease in different host species. More interestingly, there is evidence that certain normal flora strains are virulent when they gain access to a different host species. Analysis of the population structure indicates that recombination events are the major tool that shapes the population and this panmixia is interrupted by frequent clonal expansion that mostly corresponds to disease processes. The signature of positive selection was detected in alpha toxin gene, suggesting the possibility of adaptive alleles on the other chromosomally encoded determinants. Finally, C. perfringens proved to have a dynamic population and availability of more genome sequences and use of comparative proteomics and animal modeling would provide more insight into the virulence of this organism.

Fine mapping of the N-terminal cytotoxicity region of Clostridium perfringens enterotoxin by site-directed mutagenesis

Clostridium perfringens enterotoxin (CPE) has a unique mechanism of action that results in the formation of large, sodium dodecyl sulfate-resistant complexes involving tight junction proteins; those complexes then induce plasma membrane permeability alterations in host intestinal epithelial cells, leading to cell death and epithelial desquamation. Previous deletion and point mutational studies mapped CPE receptor binding activity to the toxin's extreme C terminus. Those earlier analyses also determined that an N-terminal CPE region between residues D45 and G53 is required for large complex formation and cytotoxicity. To more finely map this N-terminal cytotoxicity region, site-directed mutagenesis was performed with recombinant CPE (rCPE). Alanine-scanning mutagenesis produced one rCPE variant, D48A, that failed to form large complexes or induce cytotoxicity, despite having normal ability to bind and form the small complex. Two saturation variants, D48E and D48N, also had a phenotype resembling that of the D48A variant, indicating that both size and charge are important at CPE residue 48. Another alanine substitution rCPE variant, I51A, was highly attenuated for large complex formation and cytotoxicity, but rCPE saturation variants I51L and I51V displayed a normal large complex formation and cytotoxicity phenotype. Collectively, these mutagenesis results identify a core CPE sequence extending from residues G47 to I51 that directly participates in large complex formation and cytotoxicity.

Single channel evidence for innate pore-formation byVibrio parahaemolyticusthermostable direct haemolysin (TDH) in phospholipid bilayers

Vibrio parahaemolyticus thermostable direct haemolysin (TDH) is widely considered to be a pore-forming toxin. The protein has no significant homology to other known pore-forming toxins and its mechanism of action in vivo remains undefined. We demonstrate single channel pore-forming activity of V. parahaemolyticus TDH in planar lipid bilayers. Channel conductance ranged between 30-450 pS in 0.5 M KCl with a calculated cation selectivity (P(K)/P(Cl)) of 2.7. Channels were formed in NaCl and choline-Cl with and without cholesterol present and in the presence of neutral or negatively charged phospholipids. Zinc ions did not block pore formation. Whilst various techniques have previously suggested that TDH is a pore-forming toxin, the data in this study provide direct single channel evidence and indicate several features of pore formation in synthetic phospholipid membranes.

The enteric toxins of Clostridium perfringens

The Gram-positive pathogen Clostridium perfringens is a major cause of human and veterinary enteric disease largely because this bacterium can produce several toxins when present inside the gastrointestinal tract. The enteric toxins of C. perfringens share two common features: (1) they are all single polypeptides of modest (approximately 25-35 kDa) size, although lacking in sequence homology, and (2) they generally act by forming pores or channels in plasma membranes of host cells. These enteric toxins include C. perfringens enterotoxin (CPE), which is responsible for the symptoms of a common human food poisoning and acts by forming pores after interacting with intestinal tight junction proteins. Two other C. perfringens enteric toxins, epsilon-toxin (a bioterrorism select agent) and beta-toxin, cause veterinary enterotoxemias when absorbed from the intestines; beta- and epsilon-toxins then apparently act by forming oligomeric pores in intestinal or extra-intestinal target tissues. The action of a newly discovered C. perfringens enteric toxin, beta2 toxin, has not yet been defined but precedent suggests it might also be a pore-former. Experience with other clostridial toxins certainly warrants continued research on these C. perfringens enteric toxins to develop their potential as therapeutic agents and tools for cellular biology.

The role of bacterial and non-bacterial toxins in the induction of changes in membrane transport: implications for diarrhea

Bacterial toxins induce changes in membrane transport which underlie the loss of electrolyte homeostasis associated with diarrhea. Bacterial- and their secreted toxin-types which have been linked with diarrhea include: (a) Vibrio cholerae (cholera toxin, E1 Tor hemolysin and accessory cholera enterotoxin); (b) Escherichia coli (heat stable enterotoxin, heat-labile enterotoxin and colicins); (c) Shigella dysenteriae (shiga-toxin); (d) Clostridium perfringens (C. perfringens enterotoxin, alpha-toxin, beta-toxin and theta-toxin); (e) Clostridium difficile (toxins A and B); (f) Staphylococcus aureus (alpha-haemolysin); (g) Bacillus cereus (cytotoxin K and haemolysin BL); and (h) Aeromonas hydrophila (aerolysin, heat labile cytotoxins and heat stable cytotoxins). The mechanisms of toxin-induced diarrhea include: (a) direct effects on ion transport in intestinal epithelial cells, i.e. direct toxin interaction with intrinsic ion channels in the membrane and (b) indirect interaction with ion transport in intestinal epithelial cells mediated by toxin binding to a membrane receptor. These effects consequently cause the release of second messengers, e.g. the release of adenosine 3',5'-cyclic monophosphate/guanosine 3',5'-monophosphate, IP(3), Ca2+ and/or changes in second messengers that are the result of toxin-formed Ca2+ and K+ permeable channels, which increase Ca2+ flux and augment changes in Ca2+ homeostasis and cause depolarisation of the membrane potential. Consequently, many voltage-dependent ion transport systems, e.g. voltage-dependent Ca2+ influx, are affected. The toxin-formed ion channels may act as a pathway for loss of fluid and electrolytes. Although most of the diarrhea-causing toxins have been reported to act via cation and anion channel formation, the properties of these channels have not been well studied, and the available biophysical properties that are needed for the characterization of these channels are inadequate.