The relationship between cytotoxic effect and binding to mammalian cultured cells ofClostridium perfringensenterotoxin

The biology and pathogenicity of Clostridium perfringens type F: a common human enteropathogen with a new(ish) name

SUMMARYIn the 2018-revised Clostridium perfringens typing classification system, isolates carrying the enterotoxin (cpe) and alpha toxin genes but no other typing toxin genes are now designated as type F. Type F isolates cause food poisoning and nonfoodborne human gastrointestinal (GI) diseases, which most commonly involve type F isolates carrying, respectivefooly, a chromosomal or plasmid-borne cpe gene. Compared to spores of other C. perfringens isolates, spores of type F chromosomal cpe isolates often exhibit greater resistance to food environment stresses, likely facilitating their survival in improperly prepared or stored foods. Multiple factors contribute to this spore resistance phenotype, including the production of a variant small acid-soluble protein-4. The pathogenicity of type F isolates involves sporulation-dependent C. perfringens enterotoxin (CPE) production. C. perfringens sporulation is initiated by orphan histidine kinases and sporulation-associated sigma factors that drive cpe transcription. CPE-induced cytotoxicity starts when CPE binds to claudin receptors to form a small complex (which also includes nonreceptor claudins). Approximately six small complexes oligomerize on the host cell plasma membrane surface to form a prepore. CPE molecules in that prepore apparently extend β-hairpin loops to form a β-barrel pore, allowing a Ca2+ influx that activates calpain. With low-dose CPE treatment, caspase-3-dependent apoptosis develops, while high-CPE dose treatment induces necroptosis. Those effects cause histologic damage along with fluid and electrolyte losses from the colon and small intestine. Sialidases likely contribute to type F disease by enhancing CPE action and, for NanI-producing nonfoodborne human GI disease isolates, increasing intestinal growth and colonization.

Disruption of Claudin-Made Tight Junction Barriers by Clostridium perfringens Enterotoxin: Insights from Structural Biology

Claudins are a family of integral membrane proteins that enable epithelial cell/cell interactions by localizing to and driving the formation of tight junctions. Via claudin self-assembly within the membranes of adjoining cells, their extracellular domains interact, forming barriers to the paracellular transport of small molecules and ions. The bacterium Clostridium perfringens causes prevalent gastrointestinal disorders in mammals by employing an enterotoxin (CpE) that targets claudins. CpE binds to claudins at or near tight junctions in the gut and disrupts their barrier function, potentially by disabling their assembly or via cell signaling means—the mechanism(s) remain unclear. CpE ultimately destroys claudin-expressing cells through the formation of a cytotoxic membrane-penetrating β-barrel pore. Structures obtained by X-ray crystallography of CpE, claudins, and claudins in complex with CpE fragments have provided the structural bases of claudin and CpE functions, revealing potential mechanisms for the CpE-mediated disruption of claudin-made tight junctions. This review highlights current progress in this space—what has been discovered and what remains unknown—toward efforts to elucidate the molecular mechanism of CpE disruption of tight junction barriers. It further underscores the key insights obtained through structure that are being applied to develop CpE-based therapeutics that combat claudin-overexpressing cancers or modulate tight junction barriers.

Clostridium perfringens enterotoxin interacts with claudins via electrostatic attraction

Clostridium perfringens enterotoxin (CPE), a causative agent of food poisoning, is a pore-forming toxin disrupting the selective permeability of the plasma membrane of target cells, resulting in cell death. We previously identified claudin as the cell surface receptor for CPE. Claudin, a component of tight junctions, is a tetratransmembrane protein and constitutes a large family of more than 20 members, not all of which serve as the receptor for CPE. The mechanism by which the toxin distinguishes the sensitive claudins is unknown. In this study, we localized the region of claudin responsible for interaction with CPE to the C-terminal part of the second extracellular loop and found that the isoelectric point of this region in sensitive claudins was higher than insensitive claudins. Amino acid substitutions to lower the pI resulted in reduced sensitivity to CPE among sensitive claudins, whereas substitutions to raise the pI endowed CPE-insensitive claudins with sensitivity. The steric structure of the claudin-binding domain of CPE reveals an acidic cleft surrounded by Tyr(306), Tyr(310), Tyr(312), and Leu(315), which were reported to be essential for interaction with the sensitive claudins. These results imply that an electrostatic attraction between the basic claudin region and the acidic CPE cleft is involved in their interaction.

Clostridium perfringens enterotoxin as a novel-targeted therapeutic for brain metastasis

Brain metastasis is the most commonly occurring intracranial tumor whose incidence seems to be increasing. With standard therapy, the average survival time of patients is approximately 8 months, and treatment often leads to neurologic dysfunction in long-term survivors, emphasizing the need for novel therapeutics. Clostridium perfringens enterotoxin (CPE) has recently been shown to rapidly and specifically destroy cancer cells expressing CPE receptors claudin-3 and claudin-4. Unfortunately, the utility of CPE is precluded by systemic toxicity because its receptors are expressed in numerous organs. Here, we provide the first preclinical evidence that CPE may be uniquely suited to the local treatment of brain metastasis. By immunohistochemical analysis, claudin-3 and claudin-4 were expressed frequently in metastases from breast (15 of 18), lung (15 of 20), and colon (12 of 14) carcinoma, and infrequently in metastases from renal cell carcinoma (2 of 16) and melanoma (2 of 16). In contrast, expression of claudin-3 and claudin-4 was absent in adjacent normal brain tissue. Further examination of the central nervous system (CNS) revealed low or undetectable levels of claudin-3 and claudin-4 in all regions tested by Western and immunohistochemical analysis. Treatment of breast cancer cell lines (MCF-7, MDA-MB-468, NT2.5-luc) and normal human astrocytes with CPE in vitro resulted in rapid and dose-dependent cytolysis exclusively in breast cancer cells, correlating with claudin-3 and claudin-4 expression. Moreover, intracranial CPE treatment significantly inhibited tumor growth and increased survival in two murine models of breast cancer brain metastasis, without any apparent local or systemic toxicity. These data suggest that CPE therapy may have efficacy against a wide variety of brain metastases without CNS toxicity.

The importance of calcium influx, calpain and calmodulin for the activation of CaCo-2 cell death pathways by Clostridium perfringens enterotoxin

CaCo-2 cells exhibit apoptosis when treated with low doses of Clostridium perfringens enterotoxin (CPE), but develop oncosis when treated with high CPE doses. This study reports that the presence of extracellular Ca(2+) in treatment buffers is important for normal activation of both those cell death pathways in CPE-treated CaCo-2 cells. Normal development of CPE-induced cell death pathway effects, such as morphologic damage, DNA fragmentation, caspase activation, mitochondrial membrane depolarization and cytochrome c release, was strongly inhibited when CaCo-2 cells were CPE-treated in Ca(2+)-free buffers. When treatment buffers contained Ca(2+), CPE caused a rapid increase in CaCo-2 cell Ca(2+) levels, apparently because of increased Ca(2+) influx through a CPE pore. High CPE doses caused massive changes in cellular Ca(2+) levels that appear responsible for activating oncosis, whereas low CPE doses caused less perturbations in cellular Ca(2+) levels that appear responsible for activating apoptosis. Both CPE-induced apoptosis and oncosis were found to be calmodulin- and calpain-dependent processes. As Ca(2+) levels present in the intestinal lumen resemble those of Ca(2+)-containing treatment buffers used in this study, perturbations in cellular Ca(2+) levels and calpain/calmodulin-dependent processes are also probably important for inducing enterocyte cell death during CPE-mediated gastrointestinal disease.

Clostridium perfringens enterotoxin elicits rapid and specific cytolysis of breast carcinoma cells mediated through tight junction proteins claudin 3 and 4

Clostridium perfringens enterotoxin (CPE) induces cytolysis very rapidly through binding to its receptors, the tight junction proteins CLDN 3 and 4. In this study, we investigated CLDN 3 and 4 expression in breast cancer and tested the potential of CPE-mediated therapy. CLDN 3 and 4 proteins were detected in all primary breast carcinomas tested (n = 21) and, compared to normal mammary epithelium, were overexpressed in approximately 62% and 26%, respectively. Treatment of breast cancer cell lines in culture with CPE resulted in rapid and dose-dependent cytolysis exclusively in cells that expressed CLDN 3 and 4. Intratumoral CPE treatment of xenografts of T47D breast cancer cells in immunodeficient mice resulted in a significant reduction in tumor volume (P = 0.007), with accompanying necrosis. Necrotic reactions were also seen in three freshly resected primary breast carcinoma samples treated with CPE for 12 hours, while isolated primary breast carcinoma cells underwent rapid and complete cytolysis within 1 hour. Thus, expression of CLDN 3 and 4 sensitizes primary breast carcinomas to CPE-mediated cytolysis and emphasizes the potential of CPE in breast cancer therapy.

The complex interactions between Clostridium perfringens enterotoxin and epithelial tight junctions

Clostridium perfringens enterotoxin (CPE) is responsible for the diarrheal symptoms of C. perfringens type A food poisoning and antibiotic-associated diarrhea. The CPE protein consists of a single 35 kDa polypeptide with a C-terminal receptor-binding region and an N-terminal toxicity domain. Under appropriate conditions, CPE can interact with structural components of the epithelial tight junctions, including certain claudins and occludin. Those interactions can affect tight junction structure and function, thereby altering paracellular permeability and (possibly) contributing to CPE-induced diarrhea. However, the tight junction effects of CPE require cellular damage as a prerequisite. CPE induces cellular damage via its cytotoxic activity, which results from plasma membrane permeability alterations caused by formation of a approximately 155 kDa CPE-containing complex that may correspond to a pore. Thus, CPE appears to be a bifunctional toxin that first induces plasma membrane permeability alterations; using the resultant cell damage, CPE then gains access to tight junction proteins and affects tight junction structure and function.

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.

CaCo-2 cells treated with Clostridium perfringens enterotoxin form multiple large complex species, one of which contains the tight junction protein occludin

The previous model for the action of Clostridium perfringens enterotoxin (CPE) proposed that (i) CPE binds to host cell receptor(s), forming a small ( approximately 90 kDa) complex, (ii) the small complex interacts with other eucaryotic protein(s), forming a large ( approximately 160 kDa) complex, and (iii) the large complex triggers massive permeability changes, thereby inducing enterocyte death. In the current study, Western immunoblot analysis demonstrated that CPE bound to CaCo-2 human intestinal cells at 37 degrees C forms multiple large complex species, with apparent sizes of approximately 200, approximately 155, and approximately 135 kDa. These immunoblot experiments also revealed that occludin, an approximately 65-kDa tight junction protein, is present in the approximately 200-kDa large complex but absent from the other large complex species. Immunoprecipitation studies confirmed that occludin physically associates with CPE in large complex material and also indicated that occludin is absent from small complex. These results strongly suggest that occludin becomes associated with CPE during formation of the approximately 200-kDa large complex. A postbinding association between CPE and occludin is consistent with the failure of rat fibroblast transfectants expressing occludin to bind CPE in the current study. Those occludin transfectants were also insensitive to CPE, strongly suggesting that occludin expression is not sufficient to confer CPE sensitivity. However, the occludin-containing, approximately 200-kDa large complex may contribute to CPE-induced cytotoxicity, because nontoxic CPE point mutants did not form any large complex species. By showing that large complex material is comprised of several species (one containing occludin), the current studies indicate that CPE action is more complicated than previously appreciated and also provide additional evidence for CPE interactions with tight junction proteins, which could be important for CPE-induced pathophysiology.

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 enterotoxin utilizes two structurally related membrane proteins as functional receptors in vivo

Human and mouse cDNAs showing homology to the Clostridium perfringens enterotoxin (CPE) receptor gene (CPE-R) from Vero cells (DDBJ/EMBL/GenBankTM accession no. D88492) (Katahira, J., Inoue, N., Horiguchi, Y., Matsuda, M., and Sugimoto, N. (1997) J. Cell Biol. 136, 1239-1247) were cloned. They were classified into two groups, the Vero cell CPE receptor homologues and rat androgen withdrawal apoptosis protein (RVP1; accession no. M74067) homologues, based on the similarities of primary amino acid sequences. L929 cells that were originally insensitive to CPE became sensitive to CPE on their transfection with cDNAs encoding either the CPE receptor or RVP1 homologues, indicating that these gene products are not only structurally similar but also functionally active as receptors for CPE. By binding assay, the human RVP1 homologue showed differences in affinity and capacity of binding from those of the human CPE receptor. Northern blot analysis showed that mouse homologues of the CPE receptor and RVP1 are expressed abundantly in mouse small intestine. The expression of CPE-R mRNA in the small intestine was restricted to cryptic enterocytes, indicating that the CPE receptor is expressed in intestinal epithelial cells. These results are consistent with reports that CPE binds to the small intestinal cells via two different kinds of receptors. High levels of expression of CPE-R and/or RVP1 mRNA were also detected in other organs, including the lungs, liver, and kidneys, but only low levels were expressed in heart and skeletal muscles. These results indicate that CPE uses structurally related cellular proteins as functional receptors in vivo and that organs that have not so far been recognized as CPE-sensitive have the potential to be targets of CPE.

Molecular cloning and functional characterization of the receptor for Clostridium perfringens enterotoxin

A cDNA encoding the Clostridium perfringens enterotoxin receptor gene (CPE-R) was cloned from an expression library of enterotoxin-sensitive Vero cells. The nucleotide sequence of CPE-R showed that the enterotoxin receptor consists of 209 amino acids with a calculated molecular mass of 22,029 D. This receptor is highly hydrophobic, contains four putative transmembrane segments, and has significant similarity to the rat androgen withdrawal apoptosis protein RVP1 and the mouse oligodendrocyte specific protein, the functions of which are unknown. The expression of CPE-R was detected in the enterotoxin-sensitive Vero, Hep3B, and Intestine 407 cell lines, but not in the enterotoxin-insensitive K562 and JY cell lines. The CPE-R gene product expressed in enterotoxin-resistant L929 cells bound to enterotoxin specifically and directly and with high affinity and rendered the cells sensitive to the toxin, indicating that the cloned receptor is functional. Results showed that enterotoxin could not assemble into a complex with a defined structure unless it interacted with the receptor. From these results, it is proposed that the enterotoxin receptor is required for both target cell recognition and pore formation in the cell membrane.

Evidence that a region(s) of the Clostridium perfringens enterotoxin molecule remains exposed on the external surface of the mammalian plasma membrane when the toxin is sequestered in small or large complexes

In studies performed to investigate the topology of Clostridium perfringens enterotoxin (CPE) when this toxin is associated with intestinal brush border membrane (BBMs), it was shown that radiolabeled CPE antibodies react more strongly against intact CPE-treated BBMs than against control BBMs. Immunoprecipitation studies then demonstrated that CPE antibodies are able to react with both small and large CPE-containing complexes while these complexes are still present in intact BBMs. Therefore, at least a portion of the CPE molecule appears to remain surface exposed in BBMs throughout the action of this toxin.

Clostridium perfringens enterotoxin acts by producing small molecule permeability alterations in plasma membranes

Clostridium perfringens enterotoxin (CPE) appears to utilize a unique mechanism of action to directly affect the plasma membrane permeability of mammalian cells. CPE action involves a multi-step action which culminates in cytotoxicity. Initially CPE binds to a protein receptor on mammalian plasma membranes. The membrane-bound CPE then becomes progressively more resistant to release by proteases (a phenomenon consistent with the insertion of CPE into membranes). This 'inserted' CPE then participates in the formation of a large complex in plasma membranes which contains one CPE: one 70 kDa membrane protein: one 50 kDa membrane protein. Upon formation of large complex, plasma membranes become freely permeable to small molecules such as ions and amino acids. This CPE-induced disruption of the cellular colloid-osmotic equilibrium then causes secondary cellular effects and cell death.

Vero cell assay for rapid detection of Clostridium perfringens enterotoxin

A rapid assay which measured the biological activity of Clostridium perfringens enterotoxin was developed. The method involved the rapid killing of Vero cells by enterotoxin produced by C. perfringens grown in Duncan and Strong sporulation medium. Serial dilutions of toxin were added to Vero cells either in suspension or grown as monolayers in wells of a 96-well cell tissue culture cluster plate. Vital staining of Vero cells with neutral red, followed by extraction of the dye, allowed toxin levels to be determined either visually or by optical density measurements with a micro-ELISA M580 computer program. The toxin produced was confirmed as different from the Vero toxin of Escherichia coli and the alpha and theta toxins of C. perfringens.

Clostridium perfringens enterotoxin

Current knowledge of CPE action is briefly summarized in Figure 1. After specific binding to a protein receptor(s), the entire CPE molecule rapidly inserts into membranes forming a complex of 150,000 Mr. Almost simultaneously with insertion, there is a sudden change in ion fluxes. The molecular events behind the induction of ion flux changes remain undefined, but might involve either direct formation of membrane pores by CPE or activation of pre-existing membrane pores. As intracellular ion levels change, cellular metabolism is affected and processes such as macromolecular syntheses are inhibited. One of the ion flux effects resulting from CPE treatment involves increased Ca2+ influx; as more Ca2+ enters the cell, morphologic damage and permeability alterations for larger molecules occur. It remains to be determined if both morphologic damage and larger permeability alterations are necessarily linked but, for example, it could be envisioned that CPE-induced Ca2+ influx causes a cytoskeletal collapse leading to altered membrane permeability. The cytoskeleton has been shown to be sensitive to intracellular Ca2+ levels and is important in normal membrane structure/function relationships. As the cumulative effects of CPE inhibit cellular metabolism, cell death occurs. The precise irreversible CPE lethal action still must be identified. As CPE-treated intestinal epithelial cells die in vivo, histopathologic damage appears. This damage results in loss of normal intestinal function causing secretion of fluids and electrolytes. This effect is clinically manifested as diarrhea. The strongly cytotoxic action of CPE clearly distinguished the action enterotoxin from STa or CT.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Ca2+ and other cations on the action of Clostridium perfringens enterotoxin

We investigated the role of extracellular Ca2+ in the Clostridium perfringens enterotoxin-induced alteration of the permeability of the plasma membrane. Enterotoxin released 86Rb and 51Cr from the Vero cells preloaded with the isotope. In the presence of EGTA, however, it released 86Rb but not 51Cr. The binding of enterotoxin to the cells was not influenced by Ca2+ or Mg2+. The effects of various cations on the enterotoxin-induced 51Cr release was also studied. The release depended on extracellular Ca2+ but not on Mg2+; it was inhibited by each of Zn2+, La3+ and Co2+. Zn2+ and Co2+ also inhibited 51Cr release caused by the enterotoxin previously bound to the cell membrane. In contrast, antibody against enterotoxin did not neutralize the toxin once it was bound to the Vero cells. When the cells were treated with enterotoxin, 45Ca influx occurred and reached the plateau in a few minutes, as did 86Rb release.

Purification and characterization of an Aeromonas hydrophila hemolysin

A hemolysin produced by Aeromonas hydrophila CA-11, isolated from an environmental source, was purified by sulfopropyl-Sephadex C-25 chromatography at pH 5.0. This hemolysin caused fluid accumulation in infant mouse intestines and rabbit intestinal loops and killed Vero cells, as did the hemolysin produced by strain AH-1, isolated from a diarrheal case. In polyacrylamide gel electrophoreses at pHs 4.0 and 9.4 and in thin-layer isoelectric focusing, CA-11 hemolysin migrated as a single band to a position different from that of AH-1 hemolysin. Immunodiffusion tests indicated that CA-11 hemolysin was immunologically related to AH-1 hemolysin but possessed unique antigenic determinants. Neutralization tests with antihemolysin sera also demonstrated immunological cross-reactivity between AH-1 and CA-11 hemolysins. These results apparently indicate that the hemolysins produced by the two strains of A. hydrophila are immunologically and physicochemically different from each other.