Comparative biochemical and immunocytochemical studies reveal differences in the effects of Clostridium perfringens enterotoxin on polarized CaCo-2 cells versus Vero cells

Since most in vitro studies exploring the action of Clostridium perfringens enterotoxin (CPE) utilize either Vero or CaCo-2 cells, the current study directly compared the CPE responsiveness of those two cell lines. When CPE-treated in suspension, both CaCo-2 and Vero cells formed SDS-resistant, CPE-containing complexes of approximately 135, approximately 155, and approximately 200 kDa. However, confluent Transwell cultures of either cell line CPE-treated for 20 min formed only the approximately 155-kDa complex. Since those Transwell cultures also exhibited significant (86)Rb release, approximately 155-kDa complex formation is sufficient for CPE-induced cytotoxicity. Several differences in CPE responsiveness between the two cell lines were also detected. (i) CaCo-2 cells were more sensitive when CPE-treated on their basal surface, whereas Vero cells were more sensitive when CPE-treated on their apical surface; those sensitivity differences correlated with CPE binding the apical versus basolateral surfaces of these two cell lines. (ii) CPE-treated Vero cells released (86)Rb into both Transwell chambers, whereas CaCo-2 cells released (86)Rb only into the CPE-containing Transwell chamber. (iii) Vero cells express the tight junction (TJ) protein occludin but (unlike CaCo-2 cells) cannot form TJs. The ability of TJs to affect CPE responsiveness is supported by the similar effects of CPE on Transwell cultures of CaCo-2 cells and Madin-Darby canine kidney cells, another polarized cell forming TJs. Confluent CaCo-2 Transwell cultures CPE-treated for >1 h formed the approximately 200-kDa CPE complex (which also contains occludin), exhibited morphologic damage, and had occludin removed from their TJs. Collectively, these results identify CPE as a bifunctional toxin that, in confluent polarized cells, first exerts a cytotoxic effect mediated by the approximately 155-kDa complex. Resultant damage then provides CPE access to TJs, leading to approximately 200-kDa complex formation, internalization of some TJ proteins, and TJ damage that may increase paracellular permeability and thereby contribute to the diarrhea of CPE-induced gastrointestinal disease.

Identification of a Clostridium perfringens enterotoxin region required for large complex formation and cytotoxicity by random mutagenesis

Clostridium perfringens enterotoxin (CPE), a single polypeptide of 319 amino acids, has a unique multistep mechanism of action. In the first step, CPE binds to claudin proteins and/or a 50-kDa eukaryotic membrane protein receptor, forming a small ( approximately 90-kDa) complex. This small complex apparently then associates with a 70-kDa eukaryotic membrane protein, resulting in formation of a large complex that induces the onset of membrane permeability alterations. To better define the boundaries of CPE functional regions and to identify specific amino acid residues involved in various steps of CPE action, in this study we subjected the cloned cpe gene to random mutagenesis in XL-1 Red strains of Escherichia coli. Seven CPE random mutants with reduced cytotoxicity for Vero cells were phenotypically characterized for the ability to complete each step in CPE action. Five of these seven recombinant CPE (rCPE) random mutants (G49D, S59L, R116S, R137G, and S167P) exhibited binding characteristics similar to those of rCPE or native CPE, while the Y310C and W226Stop mutants showed reduced binding and no binding, respectively, to brush border membranes. Interestingly, two completely nontoxic mutants (G49D and S59L) were able to bind and form small complex but they did not mediate any detectable large complex formation. Another strongly attenuated mutant, R116S, formed reduced amounts of an anomalously migrating large complex. Collectively, these results provide further support for large complex formation being an essential step in CPE action and also identify the CPE region ranging from residues approximately 45 to 116 as important for large complex formation. Finally, we also report that limited removal of extreme N-terminal CPE sequences, which may occur in vivo during disease, stimulates cytotoxic activity by enhancing large complex formation.

Characterization of membrane-associated Clostridium perfringens enterotoxin following pronase treatment

After binding, Clostridium perfringens enterotoxin (CPE) initially localizes in a small (approximately 90-kDa) complex in plasma membranes. This event is followed by formation of a second membrane complex, referred to as large (160-kDa) complex. Contrary to a previous hypothesis proposing that CPE inserts into intestinal brush border membranes (BBMs) when this toxin is localized in the small complex, this study shows that BBMs do not offer CPE localized in the small complex protection from pronase. However, our experiments indicate that BBMs do substantially protect CPE from pronase when this toxin is localized in large complex. Since the onset of CPE-induced permeability alterations closely coincides with large-complex formation, these new results suggest that CPE-induced alterations in permeability may result from pore formation due to the partial membrane insertion of CPE when this toxin is present in large complex.

Clostridium perfringens type E animal enteritis isolates with highly conserved, silent enterotoxin gene sequences

Several Clostridium perfringens genotype E isolates, all associated with hemorrhagic enteritis of neonatal calves, were identified by multiplex PCR. These genotype E isolates were demonstrated to express alpha and iota toxins, but, despite carrying sequences for the gene (cpe) encoding C. perfringens enterotoxin (CPE), were unable to express CPE. These silent cpe sequences were shown to be highly conserved among type E isolates. However, relative to the functional cpe gene of type A isolates, these silent type E cpe sequences were found to contain nine nonsense and two frameshift mutations and to lack the initiation codon, promoters, and ribosome binding site. The type E animal enteritis isolates carrying these silent cpe sequences do not appear to be clonally related, and their silent type E cpe sequences are always located, near the iota toxin genes, on episomal DNA. These findings suggest that the highly conserved, silent cpe sequences present in most or all type E isolates may have resulted from the recent horizontal transfer of an episome, which also carries iota toxin genes, to several different type A C. perfringens isolates.

Evidence that an approximately 50-kDa mammalian plasma membrane protein with receptor-like properties mediates the amphiphilicity of specifically bound Clostridium perfringens enterotoxin

Previous studies suggest that Clostridium perfringens enterotoxin (CPE) inserts into mammalian membranes. Using Triton X-114 phase separation analysis and charge-shift electrophoresis, this study demonstrates that CPE exhibits the amphiphilicity required for membrane insertion, but this behavior develops only after exposure of CPE to membranes. This effect does not require proteolytic or covalent CPE modifications or formation of a previously reported 160-kDa CPE-containing complex. A novel 90-kDa CPE-containing complex with amphiphilic properties was detected in intestinal brush-border membranes and in CPE-sensitive, but not CPE-insensitive, cell lines using nondenaturing Triton X-100 electrophoresis. Immunoprecipitation analysis suggested that the 90-kDa complex is composed of CPE and a 45-50-kDa membrane protein. Since the 90-kDa complex is formed only in cells that bind and respond to CPE, these results are consistent with the 45-50-kDa protein mediating CPE amphiphilicity and serving as a functional CPE receptor. A four-step model for CPE action is proposed. 1) CPE binds to the 45-50-kDa protein to form a 90-kDa complex. 2) The 90-kDa complex undergoes some physical change corresponding to insertion or a conformational change. 3) The 90-kDa complex and a 70-kDa membrane protein interact to form a 160-kDa complex. 4) Formation of the 160-kDa complex leads to permeability alterations.

Mapping of functional regions of Clostridium perfringens type A enterotoxin

Studies were conducted to allow construction of an initial map of the structure-versus-function relationship of the Clostridium perfringens type A enterotoxin (CPE). Removal of the N-terminal 25 amino acids of CPE increased the primary cytotoxic effect of CPE but did not affect binding. CPE sequences required for at least four epitopes were also identified.