Interactions of Campylobacter jejuni cytolethal distending toxin subunits CdtA and CdtC with HeLa cells

Campylobacter jejuni response to human mucin MUC2: modulation of colonization and pathogenicity determinants

Campylobacter jejuni is the main cause of bacterial acute gastroenteritis worldwide. In its colonization of the host intestinal tract, it encounters secreted mucins in the mucus layer and surface mucins in the epithelial cells. Mucins are complex glycoproteins that comprise the major component of mucus and give mucus its viscous consistency. MUC2 is the most abundant secreted mucin in the human intestine; it is a major chemoattractant for C. jejuni, and the bacterium binds to it. There are no studies on the transcriptional response of the bacterium to this mucin. Here, cell-culture techniques and quantitative RT-PCR were used to characterize in vitro the effects of MUC2 on C. jejuni growth and the changes in expression of 20 C. jejuni genes related to various functions. The genes encoding cytolethal distending toxin protein (cdtABC), vacuolating cytotoxin (vacB), C. jejuni lipoprotein (jlpA), Campylobacter invasion antigen (ciaB), the multidrug efflux system (cmeAB), putative mucin-degrading enzymes (cj1344c, cj0843c, cj0256 and cj1055c), flagellin A (flaA) and putative rod-shape-determining proteins (mreB and mreC) were upregulated, whereas those encoding Campylobacter adhesion fibronectin-binding protein (cadF) and sialic acid synthase (neuB1) were downregulated. These results showed that C. jejuni utilizes MUC2 as an environmental cue for the modulation of expression of genes with various functions including colonization and pathogenicity.

Role of aromatic amino acids in receptor binding activity and subunit assembly of the cytolethal distending toxin of Aggregatibacter actinomycetemcomitans

The periodontal pathogen Aggregatibacter actinomycetemcomitans produces a cytolethal distending toxin (Cdt) that inhibits the proliferation of oral epithelial cells. Structural models suggest that the CdtA and CdtC subunits of the Cdt heterotrimer form two putative lectin domains with a central groove. A region of CdtA rich in heterocyclic amino acids (aromatic patch) appears to play an important role in receptor recognition. In this study site-specific mutagenesis was used to assess the contributions of aromatic amino acids (tyrosine and phenylalanine) to receptor binding and CdtA-CdtC assembly. Predominant surface-exposed aromatic residues that are adjacent to the aromatic patch region in CdtA or are near the groove located at the junction of CdtA and CdtC were studied. Separately replacing residues Y105, Y140, Y188, and Y189 with alanine in CdtA resulted in differential effects on binding related to residue position within the aromatic region. The data indicate that an extensive receptor binding domain extends from the groove across the entire face of CdtA that is oriented 180 degrees from the CdtB subunit. Replacement of residue Y105 in CdtA and residues Y61 and F141 in CdtC, which are located in or at the periphery of the groove, inhibited toxin assembly. Taken together, these results, along with the lack of an aromatic amino acid-rich region in CdtC similar to that in CdtA, suggest that binding of the heterotoxin to its cell surface receptor is mediated predominantly by the CdtA subunit. These findings are important for developing strategies designed to block the activity of this prominent virulence factor.

Campylobacter jejuni-mediated disease pathogenesis: an update

Infection by Campylobacter jejuni is considered to be the most prevalent cause of bacterial-mediated diarrhoeal disease worldwide. Both in the developing and the developed world, young children remain most susceptible. Although disease is generally mild and self-limiting, severe post-infectious complications such as Gullain-Barré syndrome may occur. Despite the significant health burden caused by the organism, our current understanding of disease pathogenesis remains in its infancy. Elucidation of the genome sequences of many different C. jejuni strains in recent years has started to accelerate research in Campylobacter genetics, pathogenesis and host immunity in response to infection. Campylobacter jejuni is the first prokaryote shown to code for both O- and N-linked glycosylation systems, a feature that is likely to not only modulate bacterial virulence and survival, but also influence host-pathogen interactions and disease outcome. Here recent developments in C. jejuni research, with a particular focus on disease pathogenesis including early host immune responses, are highlighted.

Campylobacter jejuni: molecular biology and pathogenesis

Campylobacter jejuni is a foodborne bacterial pathogen that is common in the developed world. However, we know less about its biology and pathogenicity than we do about other less prevalent pathogens. Interest in C. jejuni has increased in recent years as a result of the growing appreciation of its importance as a pathogen and the availability of new model systems and genetic and genomic technologies. C. jejuni establishes persistent, benign infections in chickens and is rapidly cleared by many strains of laboratory mouse, but causes significant inflammation and enteritis in humans. Comparing the different host responses to C. jejuni colonization should increase our understanding of this organism.

The contribution of cytolethal distending toxin to bacterial pathogenesis

Cytolethal distending toxin (CDT) is a bacterial toxin that initiates a eukaryotic cell cycle block at the G2 stage prior to mitosis. CDT is produced by a number of bacterial pathogens including: Campylobacter species, Escherichia coli, Salmonella enterica serovar Typhi, Shigella dystenteriae, enterohepatic Helicobacter species, Actinobacillus actinomycetemcomitans (the cause of aggressive periodontitis), and Haemophilus ducreyi (the cause of chancroid). The functional toxin is composed of three proteins; CdtB potentiates a cascade leading to cell cycle block, and CdtA and CdtC function as dimeric subunits, which bind CdtB and delivers it to the mammalian cell interior. Once inside the cell, CdtB enters the nucleus and exhibits a DNase I-like activity that results in DNA double-strand breaks. The eukaryotic cell responds to the DNA double-strand breaks by initiating a regulatory cascade that results in cell cycle arrest, cellular distension, and cell death. Mutations in CdtABC that cause any of the three subunits to lose function prevent the bacterial cell from inducing cytotoxicity. The result of CDT activity can differ somewhat depending on the eukaryotic cell types affected. Epithelial cells, endothelial cells, and keratinocytes undergo G2 cell cycle arrest, cellular distension, and death; fibroblasts undergo G1 and G2 arrest, cellular distension, and death; and immune cells undergo G2 arrest followed by apoptosis. CDT contributes to pathogenesis by inhibiting both cellular and humoral immunity via apoptosis of immune response cells, and by generating necrosis of epithelial-type cells and fibroblasts involved in the repair of lesions produced by pathogens resulting in slow healing and production of disease symptoms. Thus, CDT may function as a virulence factor in pathogens that produce the toxin.

Dynamics and assembly of the cytolethal distending toxin

The cytolethal distending toxin (CDT) is a widespread bacterial toxin that consists of an active subunit CdtB with nuclease activity and two ricin-like lectin domains, CdtA and CdtC, that are involved in the delivery of CdtB into the host cell. The three subunits form a tripartite complex that is required to achieve the fully active holotoxin. In the present study we investigate the assembly and dynamic properties of the CDT holotoxin using molecular dynamics simulations and binding free energy calculations. The results have revealed that CdtB likely adopts a different conformation in the unbound state with a closed DNA binding site. The two characterized structural elements of the aromatic patch and groove on the CdtA and CdtC protein surfaces exhibit high mobility, and free energy calculations show that the heterodimeric complex CdtA-CdtC, as well as the CdtA-CdtB and CdtB-CdtC sub-complexes are less energetically stable as compared to the binding in the tripartite complex. Analysis of the dynamical cross-correlation map reveals information on the correlated motions and long-range interplay among the CDT subunits associated with complex formation. Finally, the estimated binding free energies of subunit interactions are presented, together with the free energy decomposition to determine the contributions of residues for both binding partners, providing insight into the protein-protein interactions in the CDT holotoxin.

Comparative analysis of cytolethal distending toxin (cdt) genes among Campylobacter jejuni, C. coli and C. fetus strains

The cytolethal distending toxin (cdt) gene clusters of Campylobacter coli strain Co1-243 and C. fetus strain Col-187 were cloned and sequenced to understand the importance of Cdt as a virulence factor. The cdt genes of C. coli and C. fetus consist of three closely linked genes termed cdtA, cdtB, cdtC whose sizes are 774, 801, and 570 bp, and 702, 798, and 546 bp, respectively. The homologies of each subunit of cdt genes between C. jejuni and C. coli, C. jejuni and C. fetus, or C. coli and C. fetus are 59.6%, 40.3%, or 46.5% for cdtA, 70.2%, 62.4%, or 61.3% for cdtB, 61.3%, 52.3%, or 50.1% for cdtC, respectively. Colony hybridization assay revealed that the genes homologous to the cdtABC gene were distributed in all 27, 19, 20 strains of C. jejuni, C. coli, and C. fetus, respectively, isolated from patients and animals in species-specific manner. Furthermore, nucleotide sequence of the cdt operon, including flanking region, of 10 strains of each species indicated that though the size of the cdtB gene was conserved in each species, those of cdtA and cdtC genes varied particularly among C. coli strains. Amino acid residues demonstrated to be important for toxin activity in CdtB, corresponding to H152, D185, D222, D258, H259 in Cj-CdtB, were also conserved in Cc-CdtB and Cf-CdtB. The cdt gene cluster was located in different sites among different species but in the same site of genomes of the same species. Cdt activity produced by C. jejuni and C. fetus varied among strains, however, any C. coli strains exhibited Cdt activity on HeLa cells. These data indicate that the cdt gene may have a potential for virulence factor at least in C. jejuni and C. fetus.

Escherichia coli cyclomodulin Cif induces G2arrest of the host cell cycle without activation of the DNA-damage checkpoint-signalling pathway

The cycle inhibiting factor (Cif) belongs to a family of bacterial toxins and effector proteins, the cyclomodulins, that deregulate the host cell cycle. Upon injection into HeLa cells by the enteropathogenic Escherichia coli (EPEC) type III secretion system, Cif induces a cytopathic effect characterized by the recruitment of focal adhesion plates and the formation of stress fibres, an irreversible cell cycle arrest at the G(2)/M transition, and sustained inhibitory phosphorylation of mitosis inducer, CDK1. Here, we report that the reference typical EPEC strain B171 produces a functional Cif and that lipid-mediated delivery of purified Cif into HeLa cells induces cell cycle arrest and actin stress fibres, implying that Cif is necessary and sufficient for these effects. EPEC infection of intestinal epithelial cells (Caco-2, IEC-6) also induces cell cycle arrest and CDK1 inhibition. The effect of Cif is strikingly similar to that of cytolethal distending toxin (CDT), which inhibits the G(2)/M transition by activating the DNA-damage checkpoint pathway. However, in contrast to CDT, Cif does not cause phosphorylation of histone H2AX, which is associated with DNA double-stranded breaks. Following EPEC infection, the checkpoint effectors ATM/ATR, Chk1 and Chk2 are not activated, the levels of the CDK-activating phosphatases Cdc25B and Cdc25C are not affected, and Cdc25C is not sequestered in host cell cytoplasm. Hence, Cif activates a DNA damage-independent signalling pathway that leads to inhibition of the G(2)/M transition.

Role of intrachain disulfides in the activities of the CdtA and CdtC subunits of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans

The cytolethal distending toxin (Cdt) of Actinobacillus actinomycetemcomitans is an atypical A-B-type toxin consisting of a heterotrimer composed of the cdtA, cdtB, and cdtC gene products. The CdtA and CdtC subunits form two heterogeneous ricin-like lectin domains which bind the holotoxin to the target cell. Point mutations were used to study CdtC structure and function. One (mutC216(F97C)) of eight single-amino-acid replacement mutants identified yielded a gene product that failed to form biologically active holotoxin. Based on the possibility that the F97C mutation destabilized a predicted disulfide, targeted mutagenesis was used to examine the contribution of each of four cysteine residues, in two predicted disulfides (C96/C107 and C135/C149), to CdtC activities. Cysteine replacement mutations in two predicted disulfides (C136/C149 and C178/C197) in CdtA were also characterized. Flow cytometry and CHO cell proliferation assays showed that changing either C96 or C149 in CdtC to alanine abolished the biological activity of holotoxin complexes. However, replacing C107 or C135 in CdtC and any of the four cysteines in CdtA with alanine or serine resulted in only partial or no loss of holotoxin activity. Changes in the biological activities of the mutant holotoxins correlated with altered subunit binding. In contrast to elimination of the B chain of ricin, the elimination of intrachain disulfides in CdtC and CdtA by genetic replacement of cysteines destabilizes these subunit proteins but not to the extent that cytotoxicity is lost. Reduction of the wild-type holotoxin did not affect cytotoxicity, and the reduced form of wild-type CdtA exhibited a statistically significant increase in binding to ligand. A diminished role for intrachain disulfides in stabilizing CdtA and CdtC may have clinical relevance for the A. actinomycetemcomitans Cdt. The cdt gene products secreted by this pathogen assemble and bind to target cells in periodontally involved sites, which are decidedly reduced environments in the human oral cavity.

Single nucleotide polymorphism in the cytolethal distending toxin B gene confers heterogeneity in the cytotoxicity of Actinobacillus actinomycetemcomitans

Clinical Actinobacillus actinomycetemcomitans produces cytolethal distending toxin (CDT) with titers ranging from 10(2) to 10(8) U/mg. Single nucleotide polymorphism analysis of the cdt gene in clinical isolates identified a variation of a single amino acid at residue 281 of CdtB, which significantly affected CDT toxicity by modulating the chromatin-degrading activity of CdtB.

Comparison of polymerase chain reaction systems for detection of different cdt genes in Escherichia coli strains

AIMS

Cytolethal distending toxins (CDT) are tripartite toxins encoded by three adjacent or overlapping genes (cdtA, cdtB, cdtC) and found in multiple pathogens. The present knowledge regarding heterogeneity of cdt genes and our previous study revealed that the available polymerase chain reaction (PCR) systems lack adequate specificity. The detection of various cdt genes present in Escherichia coli strains, from different geographical regions demands further assays for wide-range coverage. On the basis of these observations, we were prompted to undertake the present study; hence the specificity of existing PCR systems was addressed using E. coli prototype strains with known cdt gene sequences.

METHODS AND RESULTS

A multiplex PCR designed for the detection of E. coli cdt genes was found to be sensitive and specific enough for initial screening. However, for subtyping, the PCR systems yielded nonspecific products upon amplification. These primers are usually designed for sequences of the cdtB locus (the most conserved region of the gene), and since CDT-producing E. coli strains carry different cdt genes, none of the systems are really type specific. Furthermore, PCR systems with type-specific primers for other regions of the gene, i.e. ORF A or ORF C are found to be strain specific and their applications in different geographical regions have limitations.

CONCLUSIONS

In conclusion, based on our observations, using these available primers, it seems that the existing PCR systems are not sufficiently accurate to differentiate between different types of cdt genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained from this study revealed that so-far reported PCR systems are short in specificity. These PCR protocols were not found to be specific enough to detect various cdt genes and have a limited range of application. Moreover, due to similarities in cdt genes the cross-reaction between different sets of primers exists. Hence for epidemiological studies, some additional PCR protocols are required for screening clinical isolates for cdt genes.

Comparative structure-function analysis of cytolethal distending toxins

Cytolethal distending toxins (CDTs) constitute a family of bacterial proteins that enter eukaryotic cells with genotoxic activity leading to cell cycle arrest and apoptosis. CDTs are widespread, having been found in a variety of Gram-negative pathogens with a broad tissue tropism. The recently determined crystal structure of the Haemophilus ducreyi CDT provides a powerful starting point for analysis of the structure and function in this toxin family. In this study, we apply comparative modeling and structural analysis to extend the experimental structural information to multiple CDT toxins from a diverse species. Analysis of structurally and functionally important residues in the active subunit, CdtB, and putative cell delivery elements, CdtA and CdtC, begins to establish the fundamental, mechanistic elements of this unique holotoxin. The results reveal that key structural features with important functional consequences are highly conserved across different CDTs, providing a blueprint for directed examination of functional hypotheses in a variety of pathogenic contexts.

Variation of loop sequence alters stability of cytolethal distending toxin (CDT): crystal structure of CDT from Actinobacillus actinomycetemcomitans

Cytolethal distending toxin (CDT) secreted by Actinobacillus actinomycetemcomitans induces cell cycle arrest of cultured cells in the G2 phase. The crystal structure of the natural form of A. actinomycetemcomitans DCT (aCDT) has been determined at 2.4 A resolution. aCDT is a heterotrimer consisting of the three subunits, aCdtA, aCdtB, and aCdtC. Two crystallographically independent aCDTs form a dimer through interactions of the aCdtB subunits. The primary structure of aCDT has 94.3% identity with that of Haemophilus ducreyi CDT (hCDT), and the structure of aCDT is quite similar to that of hCDT reconstituted from the three subunits determined recently. However, the molecular packings in the crystal structures of aCDT and hCDT are quite different. A careful analysis of molecular packing suggests that variation of the amino acid residues in a nonconserved loop (181TSSPSSPERRGY192 of aCdtB and 181NSSSSPPERRVY192 of hCdtB) is responsible for the different oligomerization of very similar CDTs. The loop of aCdtB has a conformation to form a dimer, while the loop conformation of hCdtB prevents hCDT from forming a dimer. Although dimerization of aCDT does not affect toxic activity, it changes the stability of protein. aCDT rapidly aggregates and loses toxic activity in the absence of sucrose in a buffered solution, while hCDT is stable and retains toxic activity. Another analysis of crystal structures of aCDT and hCDT suggests that the receptor contact area is in the deep groove between CdtA and CdtC, and the characteristic "aromatic patch" on CdtA.

Crystallization of Escherichia coli CdtB, the biologically active subunit of cytolethal distending toxin

Cytolethal distending toxin (CDT) is a secreted protein toxin produced by several bacterial pathogens. The biologically active CDT subunit CdtB is an active homolog of mammalian type I DNase. Internalization of CdtB and subsequent translocation into the nucleus of target cells results in DNA-strand breaks, leading to cell-cycle arrest and apoptosis. CdtB crystals were grown using microbatch methods with polyethylene glycol 8000 as the precipitant. The CdtB crystals contain one molecule of MW 30.5 kDa per asymmetric unit, belong to space group P2(1)2(1)2(1) and diffract to 1.72 A.

Characterization of point mutations in the cdtA gene of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans

The Cdt is a family of gram-negative bacterial toxins that typically arrest eukaryotic cells in the G0/G1 or G2/M phase of the cell cycle. The toxin is a heterotrimer composed of the cdtA, cdtB and cdtC gene products. Although it has been shown that the CdtA protein subunit binds to cells in culture and in an enzyme-linked immunosorbent assay (CELISA) the precise mechanisms by which CdtA interacts with CdtB and CdtC has not yet been clarified. In this study we employed a random mutagenesis strategy to construct a library of point mutations in cdtA to assess the contribution of individual amino acids to binding activity and to the ability of the subunit to form biologically active holotoxin. Single unique amino acid substitutions in seven CdtA mutants resulted in reduced binding of the purified recombinant protein to Chinese hamster ovary cells and loss of binding to the fucose-containing glycoprotein, thyroglobulin. These mutations clustered at the 5'- and 3'-ends of the cdtA gene resulting in amino acid substitutions that resided outside of the aromatic patch region and a conserved region in CdtA homologues. Three of the amino acid substitutions, at positions S165N (mutA81), T41A (mutA121) and C178W (mutA221) resulted in gene products that formed holotoxin complexes that exhibited a 60% reduction (mutA81) or loss (mutA121, mutA221) of proliferation inhibition. A similar pattern was observed when these mutant holotoxins were tested for their ability to induce cell cycle arrest and to convert supercoiled DNA to relaxed and linear forms in vitro. The mutations in mutA81 and mutA221 disrupted holotoxin formation. The positions of the amino acid substitutions were mapped in the Haemophilus ducreyi Cdt crystal structure providing some insight into structure and function.

The cytolethal distending toxin among Helicobacter pullorum strains from human and poultry origin

Helicobacter pullorum has been associated with diarrhoea, gastroenteritis and liver disease in humans and with hepatitis and enteritis in poultry. The purpose of the present study was to examine whether cytolethal distending toxin was present among 10 poultry and three human H. pullorum isolates and whether a different level of cytolethal distending toxin activity was noted. A PCR assay was performed to detect the cdtB gene. In addition, epithelial Hep-2 cells inoculated with sonicate from all strains were observed microscopically and DNA analysis of these cells was done by flow cytometry. All H. pullorum isolates harboured the cdtB gene, but functional cytolethal distending toxin activity was only demonstrated in the human H. pullorum strain CCUG 33839. A significant number of cells treated with sonicate from this strain were enlarged. The nuclei were distended proportionally. Giant cells and multinucleated cells were observed as well. In addition, stress fibers accumulated. DNA analysis by flow cytometry revealed 31.0% of these cells at the S/G2 stage of the cell cycle. The tested poultry and human H. pullorum isolates all possess the cdtB gene, but under the circumstances adopted in this study only the human strain CCUG 33839 seems to show biological activity typically for CDT in vitro.

Mechanisms of assembly and cellular interactions for the bacterial genotoxin CDT

Many bacterial pathogens that cause different illnesses employ the cytolethal distending toxin (CDT) to induce host cell DNA damage, leading to cell cycle arrest or apoptosis. CDT is a tripartite holotoxin that consists of a DNase I family nuclease (CdtB) bound to two ricin-like lectin domains (CdtA and CdtC). Through the use of structure-based mutagenesis, biochemical and cellular toxicity assays, we have examined several key structural elements of the CdtA and CdtC subunits for their importance to toxin assembly, cell surface binding, and activity. CdtA and CdtC possess N- and C-terminal nonglobular polypeptides that extensively interact with each other and CdtB, and we have determined the contribution of each to toxin stability and activity. We have also functionally characterized two key binding elements of the holotoxin revealed from its crystal structure. One is an aromatic cluster in CdtA, and the other is a long and deep groove that is formed at the interface of CdtA and CdtC. We demonstrate that mutations of the aromatic patch or groove residues impair toxin binding to HeLa cells and that cell surface binding is tightly correlated with intoxication of cultured cells. These results establish several structure-based hypotheses for the assembly and function of this toxin family.

The cytolethal distending toxin is used by many bacteria to damage the DNA of infected organisms. This DNA damage prevents cells from dividing and eventually leads to cell death, which raises the possibility that this genomic damage may be a contributing factor to carcinogenesis. The cytolethal distending toxin is composed of three proteins that form a tightly associated complex. After secretion by the bacterium, two proteins in this complex adhere to the cell surface and achieve the delivery of the third protein into the cell, where it causes DNA lesions. This report examines how this toxin is assembled and how it adheres to host cell surfaces. A set of molecular features on the toxin is shown to be critical for this cell adherence and for the ability of the cytolethal distending toxin to inhibit cell division. These results tie together for the first time aspects of the molecular structure of the cytolethal distending toxin and its ability to adhere to host cell surfaces, contributing to mechanistic understanding of the activity of this genotoxin.

Involvement of ganglioside GM3 in G(2)/M cell cycle arrest of human monocytic cells induced by Actinobacillus actinomycetemcomitans cytolethal distending toxin

Actinobacillus actinomycetemcomitans produces a toxin called cytolethal distending toxin (CDT), which causes host cell DNA damage leading to the induction of DNA damage checkpoint pathways. CDT consists of three subunits, CdtA, CdtB, and CdtC. CdtB is the active subunit of CDT and exerts its effect as a nuclease that damages nuclear DNA, triggering cell cycle arrest. In the present study, we confirmed that the only combination of toxin proteins causing cell cycle arrest was that of all three recombinant CDT (rCDT) protein subunits. Furthermore, in order for rCDT to demonstrate toxicity, it was necessary for CdtA and CdtC to access the cell before CdtB. The coexistence of CdtA and CdtC was necessary for these subunits to bind to the cell. Cells treated with the glucosylceramide synthesis inhibitor 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol showed resistance to the cytotoxicity induced by rCDT. Furthermore, LY-B cells, which are deficient in the biosynthesis of sphingolipid, also showed resistance to the cytotoxicity induced by rCDT. To evaluate the binding of each subunit for glucosylceramides, we performed thin-layer chromatography immunostaining. The results indicated that each subunit reacted with the glycosphingolipids GM1, GM2, GM3, Gb3, and Gb4. The rCDT mixture incubated with liposomes containing GM3 displayed partially reduced toxicity. These results indicate that GM3 can act as a CDT receptor.

Cytolethal distending toxin generates cell death by inducing a bottleneck in the cell cycle

Cytolethal distending toxin (CDT) is a bacterial protein that is widely distributed among gram-negative bacteria including Escherichia coli, Campylobacter spp., enterohepatic Helicobacter spp., Actinobacillus actinomycetemcomitans and Haemophilus ducreyi. In vitro studies demonstrated that it is able to stop proliferation of various cell lines. The toxin is composed of three subunits designated CDTs A, B and C. The B subunit targets the eukaryotic DNA and triggers a signalling pathway involving different protein kinases which results in a cell block before entering into mitosis. To date, the individual role of the A and C subunits has not been totally elucidated. There are indications that the CDT is also produced in vivo. Its exact role in pathogenesis is not yet clear, but possible actions include inhibition of epithelial cell proliferation, apoptosis of immune cells and inhibition of a fibrotic response.

Mechanism of internalization of the cytolethal distending toxin of Actinobacillus actinomycetemcomitans

Cytolethal distending toxin (CDT), which is encoded by three genes, cdtA, cdtB and cdtC, is now recognized to have a growing list of biological actions, including inhibition of cell cycle progression, promotion of apoptosis and stimulation of cytokine secretion. It appears that internalization of CDT is essential, at least for cell cycle blockade. Using purified recombinant CDT proteins from the periodontopathic bacterium Actinobacillus actinomycetemcomitans, the authors investigated which combination of toxin proteins produce cell cycle inhibition and which bound and/or entered into host cells. No evidence was found that CdtB bound to HEp-2 human epithelial cells. In contrast, both CdtA and CdtC bound to these cells. Induction of cell cycle arrest required that cells be exposed to both CdtB and CdtC. Pre-exposure of cells to CdtC for as little as 10 min, followed by removal of the free CdtC and addition of exogenous CdtB, resulted in the inhibition of cell cycle progression, suggesting that CdtB could bind to cell-surface-located CdtC. Using various methods to follow internalization of the CDT proteins it was concluded that CdtC acts to bind CdtB at the cell surface and transports it into the cell as a complex via an endosomal pathway blockable by monensin and brefeldin A.

Cyclomodulins: bacterial effectors that modulate the eukaryotic cell cycle

Microbial pathogens have developed a variety of strategies to manipulate host-cell functions, presumably for their own benefit. We propose the term "cyclomodulins" to describe the growing family of bacterial toxins and effectors that interfere with the eukaryotic cell cycle. Inhibitory cyclomodulins, such as cytolethal distending toxins (CDTs) and the cycle inhibiting factor (Cif), block mitosis and might constitute powerful weapons for immune evasion by inhibiting clonal expansion of lymphocytes. Cell-cycle inhibitors might also impair epithelial-barrier integrity, allowing the entry of pathogenic bacteria into the body or prolonging their local existence by blocking the shedding of epithelia. Conversely, cyclomodulins that promote cellular proliferation, such as the cytotoxic necrotizing factor (CNF), exemplify another subversion mechanism by interfering with pathways of cell differentiation and development. The role of these cyclomodulins in bacterial virulence and carcinogenesis awaits further study and will delineate new perspectives in basic research and therapeutic applications.

Bacterial toxins that modulate host cell-cycle progression

The mammalian cell cycle is involved in many processes--such as immune responses, maintenance of epithelial barrier functions, and cellular differentiation--that affect the growth and colonization of pathogenic bacteria. Therefore it is not surprising that many bacterial pathogens manipulate the host cell cycle with respect to these functions. Cyclomodulins are a growing family of bacterial toxins and effectors that interfere with the eukaryotic cell cycle. Here, we review some of these cyclomodulins such as cytolethal distending toxins, vacuolating cytotoxin, the polyketide-derived macrolide mycolactone, cycle-inhibiting factor, cytotoxic necrotizing factors, dermonecrotic toxin, Pasteurella multocida toxin and cytotoxin-associated antigen A. We describe and compare their effects on the mammalian cell cycle and their putative role in disease, commensalism and symbiosis. We also discuss a possible link between these cyclomodulins and cancer.

Cellular internalization of cytolethal distending toxin: a new end to a known pathway

The cytolethal distending toxins (CDTs) are unique in their ability to induce DNA damage, activate checkpoint responses and cause cell cycle arrest or apoptosis in intoxicated cells. However, little is known about their cellular internalization pathway. We demonstrate that binding of the Haemophilus ducreyi CDT (HdCDT) on the plasma membrane of sensitive cells was abolished by cholesterol extraction with methyl-beta-cyclodextrin. The toxin was internalized via the Golgi complex, and retrogradely transported to the endoplasmic reticulum (ER), as assessed by N-linked glycosylation. Further translocation from the ER did not require the ER-associated degradation (ERAD) pathway, and was Derlin-1 independent. The genotoxic activity of HdCDT was dependent on its internalization and its DNase activity, as induction of DNA double-stranded breaks was prevented in Brefeldin A-treated cells and in cells exposed to a catalytically inactive toxin. Our data contribute to a better understanding of the CDT mode of action and highlight two important aspects of the biology of this bacterial toxin family: (i) HdCDT translocation from the ER to the nucleus does not involve the classical pathways followed by other retrogradely transported toxins and (ii) toxin internalization is crucial for execution of its genotoxic activity.

Carbohydrate-binding specificity of the Escherichia coli cytolethal distending toxin CdtA-II and CdtC-II subunits

Intoxication by cytolethal distending toxin depends on assembly of CdtB, the active A component of this AB toxin, with the cell surface-binding (B) component, composed of the CdtA-CdtC heterodimer, to form the active holotoxin. Here we examine the cell surface binding properties of Escherichia coli-derived CdtA-II (CdtA-II(Ec)) and CdtC-II(Ec) and their capacity to provide a binding platform for CdtB-II(Ec). Using a flow cytometry-based binding assay, we demonstrate that CdtB-II(Ec) binds to the HeLa cell surface in a CdtA-II(Ec)- and CdtC-II(Ec)-dependent manner and that CdtA-II(Ec) and CdtC-II(Ec) compete for the same structure on the HeLa cell surface. Preincubation of cells with glycoproteins (thyroglobulin and fetuin), but not simple sugars, blocks surface binding of CdtA-II(Ec) and CdtC-II(Ec). Moreover, CdtA-II(Ec) and CdtC-II(Ec) bind immobilized fetuin and thyroglobulin as well as fucose and to a lesser degree N-acetylgalactoseamine and N-acetylglucoseamine. Removal of N- but not O-linked carbohydrates from fetuin and thyroglobulin prevents binding of CdtA-II(Ec) and CdtC-II(Ec) to these glycoproteins. In addition, removal of N- but not O-linked surface sugar attachments prevents CDT-II(Ec) intoxication. To characterize the cell surface ligand recognized by CdtA-II(Ec) and CdtC-II(Ec), lectins having various carbohydrate specificities were used to block CDT activity and the cell surface binding of CdtA-II(Ec) and CdtC-II(Ec). Pretreatment of cells with AAA, SNA-I, STA, UEA-I, GNA, and NPA partially or completely blocked CDT activity. AAA, EEA, and UEA-I lectins, all having specificity for fucose, blocked surface binding of CdtA-II(Ec) and CdtC-II(Ec). Together, our data indicate that CdtA-II(Ec) and CdtC-II(Ec) bind an N-linked fucose-containing structure on HeLa cells.

Cytolethal distending toxin: creating a gap in the cell cycle

Cytolethal distending toxin (CDT) is a novel bacterial toxin that is produced by a variety of pathogenic bacteria. The mechanism of cytotoxicity of CDT is unique in that it enters into eukaryotic cells and breaks double-stranded DNA. This initiates the cell's own DNA damage-response mechanisms, resulting in the arrest of the cell cycle at the G2/M boundary. Affected cells enlarge until they finally undergo programmed cell death. This review encompasses recent work on CDT and focuses on the molecular mechanisms used by this toxin to block cell-cycle progression, the benefit to the bacterium of possession of this toxin and the clinical relevance of intoxication.

Evolutionary genetics of a new pathogenic Escherichia species: Escherichia albertii and related Shigella boydii strains

A bacterium originally described as Hafnia alvei induces diarrhea in rabbits and causes epithelial damage similar to the attachment and effacement associated with enteropathogenic Escherichia coli. Subsequent studies identified similar H. alvei-like strains that are positive for an intimin gene (eae) probe and, based on DNA relatedness, are classified as a distinct Escherichia species, Escherichia albertii. We determined sequences for multiple housekeeping genes in five E. albertii strains and compared these sequences to those of strains representing the major groups of pathogenic E. coli and Shigella. A comparison of 2,484 codon positions in 14 genes revealed that E. albertii strains differ, on average, at approximately 7.4% of the nucleotide sites from pathogenic E. coli strains and at 15.7% from Salmonella enterica serotype Typhimurium. Interestingly, E. albertii strains were found to be closely related to strains of Shigella boydii serotype 13 (Shigella B13), a distant relative of E. coli representing a divergent lineage in the genus Escherichia. Analysis of homologues of intimin (eae) revealed that the central conserved domains are similar in E. albertii and Shigella B13 and distinct from those of eae variants found in pathogenic E. coli. Sequence analysis of the cytolethal distending toxin gene cluster (cdt) also disclosed three allelic groups corresponding to E. albertii, Shigella B13, and a nontypeable isolate serologically related to S. boydii serotype 7. Based on the synonymous substitution rate, the E. albertii-Shigella B13 lineage is estimated to have split from an E. coli-like ancestor approximately 28 million years ago and formed a distinct evolutionary branch of enteric pathogens that has radiated into groups with distinct virulence properties.

Introns in the cytolethal distending toxin gene of Actinobacillus actinomycetemcomitans

In eukaryotic cells, genes are interrupted by intervening sequences called introns. Introns are transcribed as part of a precursor RNA that is subsequently removed by splicing, giving rise to mature mRNA. However, introns are rarely found in bacteria. Actinobacillus actinomycetemcomitans is a periodontal pathogen implicated in aggressive forms of periodontal disease. This organism has been shown to produce cytolethal distending toxin (CDT), which causes sensitive eukaryotic cells to become irreversibly blocked at the G2/M phase of the cell cycle. In this study, we report the presence of introns within the cdt gene of A. actinomycetemcomitans. By use of reverse transcription-PCR, cdt transcripts of 2.123, 1.572, and 0.882 kb (RTA1, RTA2, and RTA3, respectively) were detected. In contrast, a single 2.123-kb amplicon was obtained by PCR with the genomic DNA. Similar results were obtained when a plasmid carrying cdt was cloned into Escherichia coli. Sequence analysis of RTA1, RTA2, and RTA3 revealed that RTA1 had undergone splicing, giving rise to RTA2 and RTA3. Two exon-intron boundaries, or splice sites, were identified at positions 863 to 868 and 1553 to 1558 of RTA1. Site-directed and deletion mutation studies of the splice site sequence indicated that sequence conservation was important in order for accurate splicing to occur. The catalytic region of the cdt RNA was located within the cdtC gene. This 0.56-kb RNA behaved independently as a catalytically active RNA molecule (a ribozyme) in vitro, capable of splicing heterologous RNA in both cis and trans configurations.

Cytolethal distending toxin from Shiga toxin-producing Escherichia coli O157 causes irreversible G2/M arrest, inhibition of proliferation, and death of human endothelial cells

Recently, cytolethal distending toxin V (CDT-V), a new member of the CDT family, was identified in Shiga toxin-producing Escherichia coli (STEC) O157 and particular non-O157 serotypes. Here we investigated the biological effects of CDT-V from STEC O157:H(-) (strain 493/89) on human endothelial cells, which are believed to be major pathogenetic targets in severe STEC-mediated diseases. CDT-V caused dose-dependent G(2)/M cell cycle arrest leading to distension, inhibition of proliferation, and death in primary human umbilical vein endothelial cells (HUVEC) and two endothelial cell lines, EA.hy 926 cells (HUVEC derived) and human brain microvascular endothelial cells (HBMEC). The cell cycle effects of CDT-V were cell type specific. In HUVEC and EA.hy 926 cells, CDT-V caused a slowly developing but persistent G(2)/M block which resulted in delayed nonapoptotic cell death. In contrast, in HBMEC, CDT-V induced a rapidly evolving but transient G(2)/M block which was followed by progressive, mostly apoptotic cell death. In both HBMEC and EA.hy 926 cells, G(2)/M arrest was preceded by the early accumulation of a phosphorylated inactive form of cdc2 kinase. Significant G(2)/M arrest and inhibition of proliferation in both HUVEC and each of the endothelial cell lines were induced by 2 to 15 min of exposure to CDT-V, indicating that the effects of the toxin are irreversible. CDT-V-treated HBMEC and EA.hy 926 cells displayed fragmented nuclei and expressed phosphorylated histone protein H2AX, indicative of DNA damage followed by a DNA repair response. Our data demonstrate that CDT-V causes irreversible damage to human endothelial cells and thus may contribute to the pathogenesis of STEC-mediated diseases.

Delivery of cytolethal distending toxin B induces cell cycle arrest and apoptosis in gingival squamous cell carcinoma in vitro

The cytolethal distending toxin (Cdt) from Actinobacillus actinomycetemcomitans consists of three proteins, CdtA, CdtB, and CdtC, which are responsible for cell cycle arrest and apoptosis. In the present study, local delivery systems of recombinant CdtB and CdtB-expressing plasmid were established using Ca9-22, human gingival squamous cell carcinoma cell line. When CdtB was delivered to Ca9-22 cells using a BioPORTER, a 32-kDa protein was detected by Western blotting, and G2 cell cycle arrest and apoptosis occurred. In addition, the CdtB delivered upregulated the expression of phosphorylated p53 and the cyclin-dependent kinase inhibitor p21(CIP1/WAF1) in Ca9-22 cells, suggesting that these intracellular molecules might contribute to the induction of G2 cell cycle arrest and apoptosis. When the CdtB-expressing plasmid was transfected into Ca9-22 cells by lipofection or electroporation, CdtB (32 kDa) was clearly detected. Further, TdT-mediated dUTP nick end labeling positive cells were observed after transfection of the CdtB-expressing plasmid. These findings indicated that delivery of the CdtB protein and transfection of the cdtB gene induced cell cycle arrest and apoptosis in Ca9-22 cells in vitro, and we conclude that it may be possible to induce apoptosis in human gingival squamous cell carcinoma by electroporation of the cdtB gene.

Cytolethal distending toxins

The cytolethal distending toxins (CDTs) constitute the most recently discovered family of bacterial protein toxins. CDTs are unique among bacterial toxins as they have the ability to induce DNA double strand breaks (DSBs) in both proliferating and nonproliferating cells, thereby causing irreversible cell cycle arrest or death of the target cells. CDTs are encoded by three linked genes ( cdtA, cdtB and cdtC) which have been identified among a variety of Gram-negative pathogenic bacteria. All three of these gene products are required to constitute the fully active holotoxin, and this is in agreement with the recently determined crystal structure of CDT. The CdtB component has functional homology with mammalian deoxyribonuclease I (DNase I). Mutation of the conserved sites necessary for this catalytic activity prevents the induction of DSBs as well as all subsequent intoxication responses of target cells. CDT is endocytosed via clathrin-coated pits and requires an intact Golgi complex to exert the cytotoxic activity. Several issues remain to be elucidated regarding CDT biology, such as the detailed function(s) of the CdtA and CdtC subunits, the identity of the cell surface receptor(s) for CDT, the final steps in the cellular internalization pathway, and a molecular understanding of how CDT interacts with DNA. Moreover, the role of CDTs in the pathogenesis of diseases still remains unclear.

Assembly and function of a bacterial genotoxin

The tripartite cytolethal distending toxin (CDT) induces cell cycle arrest and apoptosis in eukaryotic cells. The subunits CdtA and CdtC associate with the nuclease CdtB to form a holotoxin that translocates CdtB into the host cell, where it acts as a genotoxin by creating DNA lesions. Here we show that the crystal structure of the holotoxin from Haemophilus ducreyi reveals that CDT consists of an enzyme of the DNase-I family, bound to two ricin-like lectin domains. CdtA, CdtB and CdtC form a ternary complex with three interdependent molecular interfaces, characterized by globular, as well as extensive non-globular, interactions. The lectin subunits form a deeply grooved, highly aromatic surface that we show to be critical for toxicity. The holotoxin possesses a steric block of the CdtB active site by means of a non-globular extension of the CdtC subunit, and we identify putative DNA binding residues in CdtB that are essential for toxin activity.

Nuclear localization of the Escherichia coli cytolethal distending toxin CdtB subunit

Cytolethal distending toxin (CDT) is a heterotrimeric protein toxin produced by several bacterial pathogens. Cells exposed to CDT die from either activation of the mitotic checkpoint cascade or apoptosis. Introduction of the purified CdtB subunit, a homologue of mammalian type I DNase, into cells mimics the action of the CDT holotoxin. Mutant CdtBs lacking DNase activity are devoid of biological activity. Chromosomal DNA appears to be the CDT target; thus, nuclear translocation of CdtB must precede cytolethal activity. Examination of the CdtB sequence indicates the presence of putative candidate bipartite nuclear localization signals (NLS). Here, we examine the functionality of the two potential NLS sequences found in the Escherichia coli CdtB-II. Nuclear translocation of EcCdtB-II was examined by monitoring the localization of an EcCdtB-II-EGFP fusion in Cos-7 cells. Our results indicated that EGFP-EcCdtB-II localized to the nucleus. The candidate EcCdtB-II-II NLS sequences were modified by site-directed mutagenesis such that tandem arginine residues were changed to threonine and serine respectively. Mutation of both putative NLS sequences had no effect on EcCdtB-II-associated DNase activity; however, cell cycle arrest and nuclear localization were significantly impaired in cells that received CDT reconstituted from the EcCdtB-II-DeltaNLS mutants. When HeLa cells were electroporated with the EcCdtB-II-DeltaNLS1 and the EcCdtB-II-NLS double mutants, toxicity was not observed, whereas the activity of EcCdtB-II-DeltaNLS2 was similar to that of wild-type EcCdtB-II. These data indicate that the putative NLS sequences are important for CDT-mediated action arrest and that they are likely to function in the nuclear translocation of EcCdtB-II.

Characterization of cytolethal distending toxin genes and expression in shiga toxin-producing Escherichia coli strains of non-O157 serogroups

We identified cytolethal distending toxin and its gene (cdt) in 17 of 340 non-O157 Shiga toxin-producing Escherichia coli (STEC) strains (serotypes O73:H18, O91:H21, O113:H21, and O153:H18), all of which were eae negative. cdt is either chromosomal and homologous to cdt-V (serotypes O73:H18, O91:H21, and O113:H21) or plasmidborne and identical to cdt-III (serotype O153:H18). Among eae-negative STEC, cdt was associated with disease (P = 0.003).

Patterns of variations in Escherichia coli strains that produce cytolethal distending toxin

A collection of 20 Escherichia coli strains that produce cytolethal distending toxin (CDT) were analyzed for their virulence-associated genes. All of these strains were serotyped, and multiplex PCR analysis was used to ascertain the presence of genes encoding other virulence factors, including Shiga toxin, intimin, enterohemolysin, cytotoxic necrotizing factor type 1 (CNF1) and CNF2, heat-stable toxin, and heat-labile toxin. These CDT-producing strains possessed various combinations of known virulence genes, some of which have not been noted before. Partial cdtB sequences were obtained from 10 of these strains, and their predicted CdtB sequences were compared to known E. coli CdtB sequences; some of the sequences were identical to known CdtB sequences, but two were not. PCR primers based on sequence differences between the known cdt sequences were tested for their ability to detect CDT producers and to determine CDT type. Correlations between the type of CDT produced, the presence of other virulence properties, and overall strain relatedness revealed that the CDT producers studied here can be divided into three general groups, with distinct differences in CDT type and in their complement of virulence-associated genes.