Identification of the minimal intracellular vacuolating domain of the Helicobacter pylori vacuolating toxin

Essential domain of receptor tyrosine phosphatase beta (RPTPbeta) for interaction with Helicobacter pylori vacuolating cytotoxin

Helicobacter pylori produces a potent exotoxin, VacA, which causes progressive vacuolation as well as gastric injury. Although VacA was able to interact with two receptor-like protein tyrosine phosphatases, RPTPbeta and RPTPalpha, RPTPbeta was found to be responsible for gastric damage caused by VacA. To define the region of RPTPbeta involved in VacA binding, we made mutants of human cDNA RPTPbeta-B, a short receptor form of RPTPbeta. Immunoprecipitation experiments to assess VacA binding to RPTPbeta-B mutants indicated that five residues (QTTQP) at positions 747-751 of the extracellular domain of RPTPbeta-B (which is commonly retained in RPTPbeta-A, a long form of RPTPbeta) play a crucial role in its interaction with VacA, resulting in vacuolation as well as Git-1 phosphorylation. Transfected cells expressing deletion mutant Delta752, which lacks QTTQP, or the double point mutant Delta747 (T748A,T749A) had diminished vacuolation in response to VacA. Treatment of RPTPbeta-B and Delta747 (which have QTTQP at 747-751) with neuraminidase and O-glycosidase diminished their VacA binding, whereas chondroitinase ABC did not have an effect. No inhibitory effect of pleiotrophin, a natural RPTPbeta ligand, on VacA binding to RPTPbeta-B or Delta747 was observed, supporting the conclusion that the extracellular region of RPTPbeta-B responsible for VacA binding is different from that involved in binding pleiotrophin. These data define the region in the RPTPbeta extracellular domain critical for VacA binding, in particular the sequence QTTQP at positions 747-751 with crucial threonines at positions 748 and 749 and are consistent with a role for terminal sialic acids possibly because of threonine glycosylation.

Novel activities of the Helicobacter pylori vacuolating cytotoxin: from epithelial cells towards the immune system

H. pylori has developed a unique set of virulence factors, which allow its survival in a unique ecological niche, the human stomach. The vacuolating cytotoxin (VacA) and the cytotoxin-associated antigen (CagA) are major bacterial factors involved in modulating the host. VacA, so far mainly regarded as a cytotoxin for the gastric epithelial cell layer, apparently has profound effects in modulating the immune response. In this review we discuss some of the classical effects of VacA, such as cell vacuolation, and compare them with more recently identified mechanisms of VacA on immune cells.

The multiple cellular activities of the VacA cytotoxin of Helicobacter pylori

Helicobacter pylori has elaborated a unique set of virulence factors that allow it to colonize the stomach wall. These factors include urease, helicoidal shape, flagella, adhesion and pro-inflammatory molecules. Here we discuss the molecular and cellular mechanisms of action of the vacuolating cytotoxin VacA. Its activities are discussed in terms of tissue alterations which promote the release of nutrients necessary to the growth and survival of the bacterium in its nutrient-poor ecological niche. This toxin also shows some pro-inflammatory and immunosuppressive activities which may be functional to the establishment of a chronic type of inflammation.

The Helicobacter pylori vacuolating cytotoxin: from cellular vacuolation to immunosuppressive activities

Helicobacter pylori is a highly successful bacterial pathogen of humans, infecting the stomach of more than half of the world's population. The H. pylori infection results in chronic gastritis, eventually followed by peptic ulceration and, more rarely, gastric cancer. H. pylori has developed a unique set of virulence factors, actively supporting its survival in the special ecological niche of the human stomach. Vacuolating cytotoxin (VacA) and cytotoxin-associated antigen A (CagA) are two major bacterial virulence factors involved in host cell modulation. VacA, so far mainly regarded as a cytotoxin of the gastric epithelial cell layer, now turns out to be a potent immunomodulatory toxin, targeting the adapted immune system. Thus, in addition to the well-known vacuolating activity, VacA has been reported to induce apoptosis in epithelial cells, to affect B lymphocyte antigen presentation, to inhibit the activation and proliferation of T lymphocytes, and to modulate the T cell-mediated cytokine response.

Glycosylphosphatidylinositol-anchored Proteins and Actin Cytoskeleton Modulate Chloride Transport by Channels Formed by the Helicobacter pylori Vacuolating Cytotoxin VacA in HeLa Cells

The vacuolating cytotoxin VacA is an important virulence factor of Helicobacter pylori. Removing glycosylphosphatidylinositol-anchored proteins (GPI-Ps) from the cell surface by phosphatidylinositol-phospholipase C or disrupting the cell actin cytoskeleton by cytochalasin D reduced VacA-induced vacuolation of cells. Using the fluorescent dye 6-methoxy-N-ethylquinolinium chloride, an indicator for cytosolic chloride, we have investigated the role of either GPI-Ps or actin cytoskeleton in the activity of the selective anionic channel formed by VacA at the plasma membrane level. Removal of GPI-Ps from HeLa cell surfaces did not impair VacA localization into lipid rafts but strongly reduced VacA channel-mediated cell influx and efflux of chloride. Disruption of the actin cytoskeleton of HeLa cells by cytochalasin D did not affect VacA localization in lipid rafts but blocked VacA cell internalization and inhibited cell vacuolation while increasing the overall chloride transport by the toxin channel at the cell surface. Specific enlargement of Rab7-positive compartments induced by VacA could be mimicked by the weak base chloroquine alone, and the vacuolating activities of either chloroquine alone or VacA were blocked with the same potency by the anion channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid shown to inhibit VacA channel activity. We suggest that formation of functional VacA channels at the cell surface required GPI-Ps and that endocytosis of these channels by an actin-dependent process increases the chloride content of late endosomes that accumulate weak bases, provoking their enlargement by osmotic swelling.

Clustering and redistribution of late endocytic compartments in response to Helicobacter pylori vacuolating toxin

Helicobacter pylori VacA is a secreted protein toxin that may contribute to the pathogenesis of peptic ulcer disease and gastric adenocarcinoma. When added to cultured mammalian cells in the presence of weak bases (e.g., ammonium chloride), VacA induces the formation of large cytoplasmic vacuoles. Here, we report a previously unrecognized capacity of VacA to induce clustering and perinuclear redistribution of late endocytic compartments. In contrast to VacA-induced cell vacuolation, VacA-induced clustering and redistribution of late endocytic compartments are not dependent on the presence of weak bases and are not inhibited by bafilomycin A1. VacA mutant toxins defective in the capacity to form anion-selective membrane channels fail to cause clustering and redistribution. VacA-induced clusters of late endocytic compartments undergo transformation into vacuoles after the addition of ammonium chloride. VacA-induced clustering and redistribution of late endocytic compartments occur in cells expressing wild-type or constitutively active Rab7, but not in cells expressing dominant-negative mutant Rab7. In VacA-treated cells containing clustered late endocytic compartments, overexpression of dominant-negative Rab7 causes reversion to a nonclustered distribution. Redistribution of late endocytic compartments to the perinuclear region requires a functional microtubule cytoskeleton, whereas clustering of these compartments and vacuole formation do not. These data provide evidence that clustering of late endocytic compartments is a critical mechanistic step in the process of VacA-induced cell vacuolation. We speculate that VacA-induced alterations in late endocytic membrane traffic contribute to the capacity of H. pylori to persistently colonize the human gastric mucosa.

Cellular vacuolation and mitochondrial cytochrome c release are independent outcomes of Helicobacter pylori vacuolating cytotoxin activity that are each dependent on membrane channel formation

Helicobacter pylori vacuolating toxin (VacA) is a secreted toxin that is reported to produce multiple effects on mammalian cells. In this study, we explored the relationship between VacA-induced cellular vacuolation and VacA-induced cytochrome c release from mitochondria. Within intoxicated cells, vacuolation precedes cytochrome c release and occurs at lower VacA concentrations, indicating that cellular vacuolation is not a downstream consequence of cytochrome c release. Conversely, bafilomycin A1 blocks VacA-induced vacuolation but not VacA-induced cytochrome c release, which indicates that cytochrome c release is not a downstream consequence of cellular vacuolation. Acid activation of purified VacA is required for entry of VacA into cells, and correspondingly, acid activation of the toxin is required for both vacuolation and cytochrome c release, which suggests that VacA must enter cells to produce these two effects. Single amino acid substitutions (P9A and G14A) that ablate vacuolating activity and membrane channel-forming activity render VacA unable to induce cytochrome c release. Channel blockers known to inhibit cellular vacuolation and VacA membrane channel activity also inhibit cytochrome c release. These data indicate that cellular vacuolation and mitochondrial cytochrome c release are two independent outcomes of VacA intoxication and that both effects are dependent on the formation of anion-selective membrane channels.

Helicobacter pylori VacA activates the p38/activating transcription factor 2-mediated signal pathway in AZ-521 cells

Persistent Helicobacter pylori colonization in the stomach induces gastritis and peptic ulcer and interferes with ulcer healing. Most strains of H. pylori produce a cytotoxin, VacA, that induces cytoplasmic vacuolation in epithelial cells with structural and functional changes, leading to gastric injury. VacA is known to cause cell death by mitochondrial damage. We hypothesized that VacA might disrupt other signaling pathways; to that end, we examined the effects of VacA on MAPKs to elucidate their role in the abnormalities seen in VacA-treated cells. VacA stimulated phosphorylation of p38 and Erk1/2, but not JNK, in AZ-521 cells. Both phosphorylation and kinase activation of p38 were maximal 10-30 min after addition of VacA and declined thereafter. Treatment with anti-VacA antibody or the p38 inhibitor SB203580 blocked p38 phosphorylation caused by VacA and inhibited VacA-induced phosphorylation of activating transcription factor 2 (ATF-2), which is implicated in transcriptional control of stress-responsive genes. These data indicate that VacA stimulates a p38/ATF-2-mediated signal pathway. However, 10 microM SB203580, which is sufficient to decrease p38 phosphorylation, did not inhibit VacA-induced cellular vacuolation, decrease in mitochondrial membrane potential, or cytochrome c release from mitochondria. These results suggest that VacA-induced activation of p38/ATF-2-mediated signal pathway is independent of cellular vacuolation, decrease in mitochondrial membrane potential, or cytochrome c release from mitochondria caused by VacA. The cytotoxin may thus act independently on several cellular targets, leading to disruption of signaling, regulatory, and metabolic pathways.

Interactions between p-33 and p-55 domains of the Helicobacter pylori vacuolating cytotoxin (VacA)

The VacA toxin secreted by Helicobacter pylori is considered to be an important virulence factor in the pathogenesis of peptic ulcer disease and gastric cancer. VacA monomers self-assemble into water-soluble oligomeric structures and can form anion-selective membrane channels. The goal of this study was to characterize VacA-VacA interactions that may mediate assembly of VacA monomers into higher order structures. We investigated potential interactions between two domains of VacA (termed p-33 and p-55) by using a yeast two-hybrid system. p-33/p-55 interactions were detected in this system, whereas p-33/p-33 and p-55/p-55 interactions were not detected. Several p-33 proteins containing internal deletion mutations were unable to interact with wild-type p-55 in the yeast two-hybrid system. Introduction of these same deletion mutations into the H. pylori vacA gene resulted in secretion of mutant VacA proteins that failed to assemble into large oligomeric structures and that lacked vacuolating toxic activity for HeLa cells. Additional mapping studies in the yeast two-hybrid system indicated that only the N-terminal portion of the p-55 domain is required for p-33/p-55 interactions. To characterize further p-33/p-55 interactions, we engineered an H. pylori strain that produced a VacA toxin containing an enterokinase cleavage site located between the p-33 and p-55 domains. Enterokinase treatment resulted in complete proteolysis of VacA into p-33 and p-55 domains, which remained physically associated within oligomeric structures and retained vacuolating cytotoxin activity. These results provide evidence that interactions between p-33 and p-55 domains play an important role in VacA assembly into oligomeric structures.

Structure-function analysis of the enteroaggregative Escherichia coli plasmid-encoded toxin autotransporter using scanning linker mutagenesis

The plasmid-encoded toxin (Pet) from enteroaggregative Escherichia coli is a cytopathic serine protease, which is prototypical of a large family of bacterial autotransporter toxins. To further elucidate the structure-function relationships of this toxin, we employed transposon-based scanning linker mutagenesis. A subset of insertions throughout the Pet mature toxin (passenger) domain reduced secretion to the extracellular space. Many of these mutants were undetectable, but secretion of a subset of mutants with insertions in the N-terminal half of the toxin could be restored to wild type secretion levels if cultured in the presence of 0.1% Triton X-100. Secretion of two mutants with insertions at the extreme C terminus was partially restored when co-expressed with a minimal clone of EspP, a related autotransporter protein. Several well secreted mutants with insertions in the N-terminal third of the molecule reduced protease activity over 20-fold, suggesting that the protease domain is located within this N-terminal region of Pet. We have also identified two insertional mutants in the middle of the passenger domain that were proteolytic but no longer cytopathic; these mutants displayed decreased binding and internalization upon incubation with HEp-2 cells. Our data suggest the existence of separate functional domains mediating Pet proteolysis, secretion, and cell interaction.

Determinants of non-toxicity in the gastric pathogen Helicobacter pylori

The Helicobacter pylori vacuolating cytotoxin gene, vacA, is naturally polymorphic, the two most diverse regions being the signal region (which can be type s1 or s2) and the mid region (m1 or m2). Previous work has shown which features of vacA make peptic ulcer and gastric cancer-associated type s1/m1 and s1/m2 strains toxic. vacA s2/m2 strains are associated with lower peptic ulcer and gastric cancer risk and are non-toxic. We now define the features of vacA that determine the non-toxicity of these strains. To do this, we deleted parts of vacA and constructed isogenic hybrid strains in which regions of vacA were exchanged between toxigenic and non-toxigenic strains. We showed that a naturally occurring 12-amino acid hydrophilic N-terminal extension found on s2 VacA blocks vacuolating activity as its removal (to make the strain s1-like) confers activity. The mid region of s2/m2 vacA does not cause the non-vacuolating phenotype, but if VacA is unblocked, it confers cell line specificity of vacuolation as in natural s1/m2 strains. Chromosomal replacement of vacA in a non-toxigenic strain with vacA from a toxigenic strain confers full vacuolating activity proving that this activity is entirely controlled by elements within vacA. This work defines why H. pylori strains with different vacA allelic structures have differing toxicity and provides a rational basis for vacA typing schemes.

Molecular and cellular mechanisms of action of the vacuolating cytotoxin (VacA) and neutrophil-activating protein (HP-NAP) virulence factors of Helicobacter pylori

Helicobacter pylori has elaborated a unique set of virulence factors that allow it to colonise the stomach wall. These factors include urease, helicoidal shape, flagella and adhesion molecules. Here we discuss the molecular characteristics and mechanisms of action of the vacuolating cytotoxin, VacA, and the neutrophil-activating protein, HP-NAP. Their activities are discussed in terms of tissue alterations, which promote the release of nutrients necessary for the growth and survival of the bacterium in its nutrient-poor ecological niche.

Essential role of a GXXXG motif for membrane channel formation by Helicobacter pylori vacuolating toxin

Helicobacter pylori secretes a toxin, VacA, that can form anion-selective membrane channels. Within a unique amino-terminal hydrophobic region of VacA, there are three tandem GXXXG motifs (defined by glycines at positions 14, 18, 22, and 26), which are characteristic of transmembrane dimerization sequences. The goals of the current study were to investigate whether these GXXXG motifs are required for membrane channel formation and cytotoxicity and to clarify the role of membrane channel formation in the biological activity of VacA. Six different alanine substitution mutations (P9A, G13A, G14A, G18A, G22A, and G26A) were introduced into the unique hydrophobic region located near the amino terminus of VacA. The effects of these mutations were first analyzed using the TOXCAT system, which permits the study of transmembrane oligomerization of proteins in a natural membrane environment. None of the mutations altered the capacity of ToxR-VacA-maltose-binding protein fusion proteins to insert into a membrane, but G14A and G18A mutations markedly diminished the capacity of the fusion proteins to oligomerize. We then introduced the six alanine substitution mutations into the vacA chromosomal gene of H. pylori and analyzed the properties of purified mutant VacA proteins. VacA-G13A, VacA-G22A, and VacA-G26A induced vacuolation of HeLa cells, whereas VacA-P9A, VacA-G14A, and VacA-G18A did not. Subsequent experiments examined the capacity of each mutant toxin to form membrane channels. In a planar lipid bilayer assay, VacA proteins containing G13A, G22A, and G26A mutations formed anion-selective membrane channels, whereas VacA proteins containing P9A, G14A, and G18A mutations did not. Similarly, VacA-G13A, VacA-G22A, and VacA-G26A induced depolarization of HeLa cells, whereas VacA-P9A, VacA-G14A, and VacA-G18A did not. These data indicate that an intact proline residue and an intact G(14)XXXG(18) motif within the amino-terminal hydrophobic region of VacA are essential for membrane channel formation, and they also provide strong evidence that membrane channel formation is essential for VacA cytotoxicity.

Expression of Helicobacter pylori vacuolating toxin in Escherichia coli

VacA is a secreted toxin that plays a role in Helicobacter pylori colonization of the stomach and that contributes to the pathogenesis of peptic ulcer disease. Studies of VacA structure and function have been hindered by the lack of an efficient system for expression and genetic manipulation of this toxin. In this study, we developed methodology for expression of a functionally active VacA toxin in Escherichia coli. We then used a high-throughput screen to analyze a library of mutant toxins with pentapeptide insertions and identified six mutants that lacked the capacity to induce vacuolation of HeLa cells. The capacity to analyze VacA in this heterologous-expression system should greatly facilitate efforts to elucidate the structure and function of this toxin.

Association of Helicobacter pylori vacuolating toxin (VacA) with lipid rafts

A variety of extracellular ligands and pathogens interact with raft domains in the plasma membrane of eukaryotic cells. In this study, we examined the role of lipid rafts and raft-associated glycosylphosphatidylinositol (GPI)-anchored proteins in the process by which Helicobacter pylori vacuolating toxin (VacA) intoxicates cells. We first investigated whether GPI-anchored proteins are required for VacA toxicity by analyzing wild-type Chinese hamster ovary (CHO) cells and CHO-LA1 mutant cells that are defective in production of GPI-anchored proteins. Whereas wild-type and mutant cells differed markedly in susceptibility to aerolysin (a bacterial toxin that binds to GPI-anchored proteins), they were equally susceptible to VacA. We next determined whether VacA physically associates with lipid rafts. CHO or HeLa cells were incubated with VacA, and Triton-insoluble membranes then were separated by sucrose density gradient centrifugation. Immunoblot analysis revealed that a substantial proportion of cell-associated toxin was associated with detergent-resistant membranes (DRMs). DRM association required acid activation of the purified toxin prior to contact with cells, and acid activation also was required for VacA cytotoxicity. Treatment of cells with methyl-beta-cyclodextrin (a cholesterol-depleting agent) did not inhibit VacA-induced depolarization of the plasma membrane, but interfered with the internalization or intracellular localization of VacA and inhibited the capacity of the toxin to induce cell vacuolation. Treatment of cells with nystatin also inhibited VacA-induced cell vacuolation. These data indicate that VacA associates with lipid raft microdomains in the absence of GPI-anchored proteins and suggest that association of the toxin with lipid rafts is important for VacA cytotoxicity.

Plasma membrane cholesterol modulates cellular vacuolation induced by the Helicobacter pylori vacuolating cytotoxin

The Helicobacter pylori vacuolating cytotoxin (VacA) induces the degenerative vacuolation of mammalian cells both in vitro and in vivo. Here, we demonstrate that plasma membrane cholesterol is essential for vacuolation of mammalian cells by VacA. Vacuole biogenesis in multiple cell lines was completely blocked when cholesterol was extracted selectively from the plasma membrane by using beta-cyclodextrins. Moreover, increasing plasma membrane cholesterol levels strongly potentiated VacA-induced vacuolation. In contrast, inhibiting de novo biosynthesis of cholesterol with lovastatin or compactin had no detectable effect on vacuolation. While depletion of plasma membrane cholesterol has been shown to interfere with both clathrin-mediated endocytosis and caveola-dependent endocytosis, neither of these two internalization pathways was found to be essential for vacuolation of cells by VacA. Depleting plasma membrane cholesterol attenuated the entry of VacA into HeLa cells. In addition, beta-cyclodextrin reagents blocked vacuolation of cells that were either preloaded with VacA or had VacA directly expressed within the cytosol. Collectively, our results suggest that plasma membrane cholesterol is important for both the intoxication mechanism of VacA and subsequent vacuole biogenesis.

Fluorescence resonance energy transfer microscopy of the Helicobacter pylori vacuolating cytotoxin within mammalian cells

The Helicobacter pylori vacuolating cytotoxin (VacA) binds and enters mammalian cells to induce cellular vacuolation. To investigate the quaternary structure of VacA within the intracellular environment where toxin cytotoxicity is elaborated, we employed fluorescence resonance energy transfer (FRET) microscopy. HeLa cells coexpressing full-length and truncated forms of VacA fused to cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP) were analyzed for FRET to indicate direct associations. These studies revealed that VacA-CFP and VacA-YFP interact within vacuolated cells, supporting the belief that monomer associations at an intracellular site are important for the toxin's vacuolating activity. In addition, the two fragments of proteolytically nicked VacA, p37 and p58, interact when coexpressed within mammalian cells. Because p37 and p58 function in trans when expressed separately within mammalian cells, these data suggest that the mechanism by which these two fragments induce vacuolation requires direct association. FRET microscopy also demonstrated interactions between mutant forms of VacA, as well as wild-type VacA with mutant forms of the toxin within vacuolated cells. Finally, a dominant-negative form of the toxin directly associates with wild-type VacA in cells where vacuolation was not detectable, suggesting that the formation of complexes comprising wild-type and dominant-negative forms of toxin acts to block intracellular toxin function.

Functional complementation reveals the importance of intermolecular monomer interactions for Helicobacter pylori VacA vacuolating activity

The Helicobacter pylori vacuolating cytotoxin (VacA) induces degenerative vacuolation of sensitive mammalian cell lines. Although evidence is accumulating that VacA enters cells and functions from an intracellular site of action, the biochemical mechanism by which VacA mediates cellular vacuolation has not been established. In this study, we used functional complementation and biochemical approaches to probe the structure of VacA. VacA consists of two discrete fragments, p37 and p58, that are both required for vacuolating activity. Using a transient transfection system, we expressed genetically modified forms of VacA and identified mutations in either p37 or p58 that inactivated the toxin. VacA with an inactivating single-residue substitution in the p37 domain [VacA (P9A)] functionally complemented a second mutant form of VacA with an inactivating two-residue deletion in the p58 domain [VacA Delta(346-347)]. VacA (P9A) and VacA Delta(346-347) also co-immunoprecipitated from vacuolated monolayers, supporting the hypothesis that these two inactive mutants associate directly to function in trans. p37 and p58 interact directly when expressed as separate fragments within HeLa cells, suggesting that p37-p58 inter-actions facilitate VacA monomer associations. Collectively, these results support a model in which the active form of VacA requires assembly into a complex of two or more monomers to elaborate toxin function.

Cell vacuolization induced by Helicobacter pylori VacA cytotoxin does not depend on late endosomal SNAREs

Cellular vacuoles induced by the Helicobacter pylori vacuolating cytotoxin VacA originate from late endosomal compartments. Their biogenesis requires the activity of both rab7 GTPase and the ATPase proton pump. The toxin has been suggested to cause an increased luminal osmotic pressure via its anion-specific channel activity localized on late endosomal compartments after endocytosis. Here, we show that the extensive membrane fusion that takes place in the transition from the small late endosomal compartments to the large vacuoles does not depend on soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) proteins. The process of vacuolization leads to disappearance of the large array of internal membranes of late endosomes. We suggest that most of the vacuole-limiting membrane derives from internal membranes.

RACK1 protein interacts with Helicobacter pylori VacA cytotoxin: the yeast two-hybrid approach

The VacA toxin is the major virulence factor of Helicobacter pylori. The studies on VacA intracellular expression suggest that it interacts with cytosolic proteins and that this interaction contributes significantly to vacuolization. The aim of this study was to identify the host protein(s) that interacts with the VacA protein. We used the fragments of VacA protein fused with GAL4-BD as the baits in the yeast two-hybrid approach. The yeast transformed with plasmids encoding bait proteins were screened with human gastric mucosa cDNA library, encoded C-terminal fusion proteins with GAL4-AD. Three independent His-beta-Gal-positive clones were identified in VacA-b1 screen; they matched two different lengths of cDNA encoding RACK1 protein. The specific activity of beta-galactosidase found in the yeast expressing both VacA-b1 and RACK1 fusion proteins was 12-19 times higher compared to all negative controls used. VacA is capable of binding the RACK1 in vitro as was confirmed by the pull-down assay with GST fusion VacA protein and [(35)S]Met-labeled RACK1 protein fragments.

A 12-amino-acid segment, present in type s2 but not type s1 Helicobacter pylori VacA proteins, abolishes cytotoxin activity and alters membrane channel formation

Helicobacter pylori, a gram-negative bacterium associated with gastritis, peptic ulceration, and gastric adenocarcinoma in humans, secretes a protein toxin, VacA, that causes vacuolar degeneration of epithelial cells. Several different families of H. pylori vacA alleles can be distinguished based on sequence diversity in the "middle" region (i.e., m1 and m2) and in the 5' end of the gene (i.e., s1 and s2). Type s2 VacA toxins contain a 12-amino-acid amino-terminal hydrophilic segment, which is absent from type s1 toxins. To examine the functional properties of VacA toxins containing this 12-amino-acid segment, we analyzed a wild-type s1/m1 VacA and a chimeric s2/m1 VacA protein. Purified s1/m1 VacA from H. pylori strain 60190 induced vacuolation in HeLa and Vero cells, whereas the chimeric s2/m1 toxin (in which the s1 sequence of VacA from strain 60190 was replaced with the s2 sequence from strain Tx30a) lacked detectable cytotoxic activity. Type s1/m1 VacA from strain 60190 formed membrane channels in a planar lipid bilayer assay at a significantly higher rate than did s2/m1 VacA. However, membrane channels formed by type s1 VacA and type s2 VacA proteins exhibited similar anion selectivities (permeability ratio, P(Cl)/P(Na) = 5). When an equimolar mixture of the chimeric s2/m1 toxin and the wild-type s1/m1 toxin was added to HeLa cells, the chimeric toxin completely inhibited the activity of the s1/m1 toxin. Thus, the s2/m1 toxin exhibited a dominant-negative phenotype similar to that of a previously described mutant toxin, VacA-(Delta6-27). Immunoprecipitation experiments indicated that both s2/m1 VacA and VacA-(Delta6-27) could physically interact with a c-myc epitope-tagged s1/m1 VacA, which suggests that the dominant-negative phenotype results from the formation of heterooligomeric VacA complexes with defective functional activity. Despite detectable differences in the channel-forming activities and cytotoxic properties of type s1 and type s2 VacA proteins, the conservation of type s2 sequences in many H. pylori isolates suggests that type s2 VacA proteins retain an important biological activity.

In search of the Helicobacter pylori VacA mechanism of action

Helicobacter pylori secretes an approximately 88 kDa VacA toxin that is considered to be an important virulence factor in the pathogenesis of peptic ulcer disease. Over the past decade, research on the molecular mechanisms and biological functions of VacA has generated a complex and often puzzling scenario. VacA is secreted into the extracellular space and also is partially retained on the bacterial cell surface, exists in monomeric and oligomeric forms, and binds to multiple eukaryotic cell-surface receptors. The cellular effects induced by VacA include vacuolation, alteration of endo-lysosomal function, pore formation in the plasma membrane, apoptosis, and epithelial monolayer permeabilisation. VacA has been reported to target several different cell components, including endocytic vesicles, mitochondria, the cytoskeleton, and epithelial cell-cell junctions. It remains unclear whether VacA should be classified as an A/B type toxin, a channel-forming toxin, or both. This review is intended to summarise our current knowledge about VacA, and to orient the reader to this fascinating and challenging research area.

The design of vaccines against Helicobacter pylori and their development

Helicobacter pylori is a gram negative, spiral, microaerophylic bacterium that infects the stomach of more than 50% of the human population worldwide. It is mostly acquired during childhood and, if not treated, persists chronically, causing chronic gastritis, peptic ulcer disease, and in some individuals, gastric adenocarcinoma and gastric B cell lymphoma. The current therapy, based on the use of a proton-pump inhibitor and antibiotics, is efficacious but faces problems such as patient compliance, antibiotic resistance, and possible recurrence of infection. The development of an efficacious vaccine against H. pylori would thus offer several advantages. Various approaches have been followed in the development of vaccines against H. pylori, most of which have been based on the use of selected antigens known to be involved in the pathogenesis of the infection, such as urease, the vacuolating cytotoxin (VacA), the cytotoxin-associated antigen (CagA), the neutrophil-activating protein (NAP), and others, and intended to confer protection prophylactically and/or therapeutically in animal models of infection. However, very little is known of the natural history of H. pylori infection and of the kinetics of the induced immune responses. Several lines of evidence suggest that H. pylori infection is accompanied by a pronounced Th1-type CD4(+) T cell response. It appears, however, that after immunization, the antigen-specific response is predominantly polarized toward a Th2-type response, with production of cytokines that can inhibit the activation of Th1 cells and of macrophages, and the production of proinflammatory cytokines. The exact effector mechanisms of protection induced after immunization are still poorly understood. The next couple of years will be crucial for the development of vaccines against H. pylori. Several trials are foreseen in humans, and expectations are that most of the questions being asked now on the host-microbe interactions will be answered.

Antigenic diversity among Helicobacter pylori vacuolating toxins

Helicobacter pylori vacuolating cytotoxin (VacA) is a secreted protein that induces vacuolation of epithelial cells. To study VacA structure and function, we immunized mice with purified type s1-m1 VacA from H. pylori strain 60190 and generated a panel of 10 immunoglobulin G1kappa anti-VacA monoclonal antibodies. All of the antibodies reacted with purified native VacA but not with denatured VacA, suggesting that these antibodies react with conformational epitopes. Seven of the antibodies reacted with both native and acid-treated VacA, which suggests that epitopes present on both oligomeric and monomeric forms of the toxin were recognized. Two monoclonal antibodies, both reactive with epitopes formed by amino acids in the carboxy-terminal portion of VacA (amino acids 685 to 821), neutralized the cytotoxic activity of type s1-m1 VacA when toxin and antibody were mixed prior to cell contact but failed to neutralize the cytotoxic activity of type s1-m2 VacA. Only 3 of the 10 antibodies consistently recognized type s1-m1 VacA toxins from multiple H. pylori strains, and none of the antibodies recognized type s2-m2 VacA toxins. These results indicate that there is considerable antigenic diversity among VacA toxins produced by different H. pylori strains.

Multiple domains are required for the toxic activity of Pseudomonas aeruginosa ExoU

Expression of ExoU by Pseudomonas aeruginosa is correlated with acute cytotoxicity in a number of epithelial and macrophage cell lines. In vivo, ExoU is responsible for epithelial injury. The absence of a known motif or significant homology with other proteins suggests that ExoU may possess a new mechanism of toxicity. To study the intracellular effects of ExoU, we developed a transient-transfection system in Chinese hamster ovary cells. Transfection with full-length but not truncated forms of ExoU inhibited reporter gene expression. Inhibition of reporter activity after cotransfection with ExoU-encoding constructs was correlated with cellular permeability and death. The toxicity of truncated versions of ExoU could be restored by coexpression of the remainder of the molecule from separate plasmids in trans. This strategy was used to map N- and C-terminal regions of ExoU that are necessary but not sufficient for toxicity. Disruption of a middle region of the protein reduces toxicity. This portion of the molecule is postulated to allow the N- and C-terminal regions to functionally complement one another. In contrast to ExoS and ExoT, native and recombinant ExoU molecules do not oligomerize or form aggregates. The complex domain structure of ExoU suggests that, like other P. aeruginosa-encoded type III effectors (ExoS and ExoT), ExoU toxicity may result from a molecule that possesses more than one activity.

Living dangerously: how Helicobacter pylori survives in the human stomach

Helicobacter pylori was already present in the stomach of primitive humans as they left Africa and spread through the world. Today, it still chronically infects more than 50% of the human population, causing, in some cases, severe diseases such as peptic ulcers and stomach cancer. To succeed in these long-term associations, H. pylori has developed a unique set of virulence factors, which allow survival in a unique and hostile ecological niche--the human stomach.

Helicobacter pylori vacuolating cytotoxin binding to a putative cell surface receptor, heparan sulfate, studied by surface plasmon resonance

The Helicobacter pylori vacuolating cytotoxin or VacA toxin is a major virulence factor in H. pylori infection and type B gastritis. We predicted heparin/heparan sulfate (H/HS) binding properties of the 58-kDa subunit of VacA cytotoxin using bioinformatics tools and showed this by surface plasmon resonance (SPR)-based biosensor studies. Putative H/HS binding peptides were synthesized and binding to HS was shown by SPR in the absence or presence of trifluoroethanol. We found that a recombinant cytotoxin VacA polypeptide binds to surface-immobilized HS and propose that HS might be a receptor/co-receptor for H. pylori VacA cytotoxin.

Amino-terminal hydrophobic region of Helicobacter pylori vacuolating cytotoxin (VacA) mediates transmembrane protein dimerization

Helicobacter pylori VacA is a secreted protein toxin that forms channels in lipid bilayers and induces multiple structural and functional alterations in eukaryotic cells. A unique hydrophobic segment at the amino terminus of VacA contains three tandem repeats of a GxxxG motif that is characteristic of transmembrane dimerization sequences. To examine functional properties of this region, we expressed and analyzed ToxR-VacA-maltose binding protein fusions using the TOXCAT system, which was recently developed by W. P. Russ and D. M. Engelman (Proc. Natl. Acad. Sci. USA 96:863-868, 1999) to study transmembrane helix-helix associations in a natural membrane environment. A wild-type VacA hydrophobic region mediated insertion of the fusion protein into the inner membrane of Escherichia coli and mediated protein dimerization. A fusion protein containing a mutant VacA hydrophobic region (in which glycine 14 of VacA was replaced by alanine) also inserted into the inner membrane but dimerized significantly less efficiently than the fusion protein containing the wild-type VacA sequence. Based on these results, we speculate that the wild-type VacA amino-terminal hydrophobic region contributes to oligomerization of the toxin within membranes of eukaryotic cells.

The N-terminal 34 kDa fragment of Helicobacter pylori vacuolating cytotoxin targets mitochondria and induces cytochrome c release

The pathogenic bacterium Helicobacter pylori produces the cytotoxin VacA, which is implicated in the genesis of gastric epithelial lesions. By transfect ing HEp-2 cells with DNAs encoding either the N-terminal (p34) or the C-terminal (p58) fragment of VacA, p34 was found localized specifically to mitochondria, whereas p58 was cytosolic. Incubated in vitro with purified mitochondria, VacA and p34 but not p58 translocated into the mitochondria. Microinjection of DNAs encoding VacA-GFP and p34-GFP, but not GFP-VacA or GFP-p34, induced cell death by apoptosis. Transient transfection of HeLa cells with p34-GFP or VacA-GFP induced the release of cytochrome c from mitochondria and activated the executioner caspase 3, as determined by the cleavage of poly(ADP-ribose) polymerase (PARP). PARP cleavage was antagonized specifically by co-transfection of DNA encoding Bcl-2, known to block mitochondria-dependent apoptotic signals. The relevance of these observations to the in vivo mechanism of VacA action was supported by the fact that purified activated VacA applied externally to cells induced cytochrome c release into the cytosol.

Expression and binding analysis of GST-VacA fusions reveals that the C-terminal approximately 100-residue segment of exotoxin is crucial for binding in HeLa cells

The Helicobacter pylori toxin VacA induces intracellular vacuolation and plays an essential role in H. pylori-related diseases. The mature exotoxin is divided into two domains, P37 and P58. A soluble form of VacA fused with GST was expressed in Escherichia coli. Although the soluble fusion lacked vacuolating activity after cleavage by thrombin, it had a binding affinity similar to that of the native VacA. Moreover, it blocked the vacuolating activity induced by the native toxin. Different C-terminal truncated fusions were generated (GST-P72, GST-P53, and GST-P37) and were also produced in a soluble form. A significantly reduced binding activity was seen for GST-P72 and nearly no specific association was detected for GST-P37. Our results suggested that the whole P58 fragment contributed to the cell binding activity in HeLa cells, particularly in the C-terminal approximately 100-residue region.

A structural overview of the Helicobacter cytotoxin

VacA, the major exotoxin produced by Helicobacter pylori, is composed of identical 87-kDa monomers that assemble into flower-shaped oligomers. The monomers can be proteolytically cleaved into two moieties of 37 and 58 kDa, or P37 and P58. The most studied property of VacA is the alteration of intracellular vesicular trafficking in eukaryotic cells leading to the formation of large vacuoles containing markers of late endosomes and lysosomes. However, VacA also causes a reduction in trans-epithelial electrical resistance in polarized monolayers and forms ion channels in lipid bilayers. The ability to induce vacuoles is localized mostly but not entirely in P37, while P58 is involved in cell targeting. Here, we review the structural aspects of VacA biology.

Membrane topology of VacA cytotoxin from H. pylori

The interaction of VacA with membranes involves: (i) a low pH activation that induces VacA monomerization in solution, (ii) binding of the monomers to the membrane, (iii) oligomerization and (iv) channel formation. To better understand the structure-activity relationship of VacA, we determined its topology in a lipid membrane by a combination of proteolytic, structural and fluorescence techniques. Residues 40-66, 111-169, 205-266, 548-574 and 723-767 were protected from proteolysis because of their interaction with the membrane. This last peptide was shown to most probably adopt a surface orientation. Both alpha-helices and beta-sheets were found in the structure of the protected peptides.

Acid activation of Helicobacter pylori vacuolating cytotoxin (VacA) results in toxin internalization by eukaryotic cells

Helicobacter pylori VacA is a secreted toxin that induces multiple structural and functional alterations in eukaryotic cells. Exposure of VacA to either acidic or alkaline pH ('activation') results in structural changes in the protein and a marked enhancement of its cell-vacuolating activity. However, the mechanism by which activation leads to increased cytotoxicity is not well understood. In this study, we analysed the binding and internalization of [125I]-VacA by HeLa cells. We detected no difference in the binding of untreated and activated [125I]-VacA to cells. Binding of acid-activated [125I]-VacA to cells at 4 degrees C was not saturable, and was only partially inhibited by excess unlabelled toxin. These results suggest that VacA binds either non-specifically or to an abundant, low-affinity receptor on HeLa cells. To study internalization of VacA, we used a protease protection assay. Analysis by SDS-PAGE and autoradiography indicated that the intact 87 kDa toxin was internalized in a time-dependent process at 37 degrees C but not at 4 degrees C. Furthermore, internalization of the intact toxin was detected only if VacA was acid or alkaline activated before being added to cells. The internalization of activated [125I]-VacA was not substantially inhibited by the presence of excess unlabelled toxin, but was blocked if cells were depleted of cellular ATP by the addition of sodium azide and 2-deoxy-D-glucose. These results indicate that acid or alkaline pH-induced structural changes in VacA are required for VacA entry into cells, and that internalization of the intact 87 kDa toxin is required for VacA cytotoxicity.

Mutational analysis of the Helicobacter pylori vacuolating toxin amino terminus: identification of amino acids essential for cellular vacuolation

The functional importance of the amino terminus of the Helicobacter pylori vacuolating cytotoxin (VacA) was investigated by analyzing the relative levels of vacuolation of HeLa cells transfected with plasmids encoding wild-type and mutant forms of the toxin. Notably, VacA's intracellular activity was found to be sensitive to small truncations and internal deletions at the toxin's amino terminus. Moreover, alanine-scanning mutagenesis revealed the first VacA point mutations (at proline 9 or glycine 14) that completely abolish the toxin's intracellular activity.

Natural diversity in the N terminus of the mature vacuolating cytotoxin of Helicobacter pylori determines cytotoxin activity

Naturally occurring noncytotoxic vacA type s2 strains of Helicobacter pylori have a 12-residue extension to the vacuolating cytotoxin (VacA) compared with cytotoxic type s1 strains. We show that adding the region encoding this extension to type s1 vacA completely abolishes vacuolating cytotoxin activity but has no effect on VacA production.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori, a gram-negative, microaerophilic, motile, spiral-shaped bacterium, has been established as the etiologic agent of gastritis and peptic ulcers and is a major risk factor for gastric adenocarcinoma and mucosa-associated lymphoid tissue lymphoma (MALT). The ability of H. pylori to cause this spectrum of diseases depends on host, bacterial, and environmental factors. Bacterial factors critical for H. pylori colonization of the gastric mucosa include urease, flagella, adhesins, and delta-glutamyltranspeptidase. Lipopolysaccharide, urease, and vacuolating cytotoxin are among the factors that allow H. pylori to persist for decades and invoke an intense inflammatory response, leading to damaged host cells. Genes in the cag pathogenicity island also contribute to the inflammatory response by initiating a signal transduction cascade, resulting in interleukin-8 production. Proinflammatory cytokines and a Th-1 cytokine response further exacerbates the inflammation. Products of the enzymes nitric oxide synthase (iNOS) and cyclooxygenase may perturb the balance between gastric epithelial cell apoptosis (ulcer formation) and proliferation (cancer). The host Th-1 response and antibodies directed against H. pylori do not eliminate the organism, which presents challenges to vaccine development. Vaccines that include urease have shown some promise, but improved adjuvants and animal models should hasten progress in vaccine research. H. pylori is the most genetically diverse organism known, and the panmictic population structure may contribute to the varying ranges of disease severity produced by different strains. The complete genome sequence of two strains of H. pylori has propelled this field forward, and numerous groups are now using genomic, proteomic, and mutagenetic approaches to identify new virulence genes. Discovered only in 1982, H. pylori is now among the most intensely investigated organisms. This review summarizes recent progress in this rapidly moving field.

A dominant negative mutant of Helicobacter pylori vacuolating toxin (VacA) inhibits VacA-induced cell vacuolation

Most Helicobacter pylori strains secrete a toxin (VacA) that causes structural and functional alterations in epithelial cells and is thought to play an important role in the pathogenesis of H. pylori-associated gastroduodenal diseases. The amino acid sequence, ultrastructural morphology, and cellular effects of VacA are unrelated to those of any other known bacterial protein toxin, and the VacA mechanism of action remains poorly understood. To analyze the functional role of a unique strongly hydrophobic region near the VacA amino terminus, we constructed an H. pylori strain that produced a mutant VacA protein (VacA-(Delta6-27)) in which this hydrophobic segment was deleted. VacA-(Delta6-27) was secreted by H. pylori, oligomerized properly, and formed two-dimensional lipid-bound crystals with structural features that were indistinguishable from those of wild-type VacA. However, VacA-(Delta6-27) formed ion-conductive channels in planar lipid bilayers significantly more slowly than did wild-type VacA, and the mutant channels were less anion-selective. Mixtures of wild-type VacA and VacA-(Delta6-27) formed membrane channels with properties intermediate between those formed by either isolated species. VacA-(Delta6-27) did not exhibit any detectable defects in binding or uptake by HeLa cells, but this mutant toxin failed to induce cell vacuolation. Moreover, when an equimolar mixture of purified VacA-(Delta6-27) and purified wild-type VacA were added simultaneously to HeLa cells, the mutant toxin exhibited a dominant negative effect, completely inhibiting the vacuolating activity of wild-type VacA. A dominant negative effect also was observed when HeLa cells were co-transfected with plasmids encoding wild-type and mutant toxins. We propose a model in which the dominant negative effects of VacA-(Delta6-27) result from protein-protein interactions between the mutant and wild-type VacA proteins, thereby resulting in the formation of mixed oligomers with defective functional activity.

Inhibition of the vacuolating and anion channel activities of the VacA toxin of Helicobacter pylori

VacA, the vacuolating cytotoxin secreted by Helicobacter pylori, is believed to be a major causative factor in the development of gastroduodenal ulcers. This toxin causes vacuolation of cultured cells and it has recently been found to form anion-selective channels upon insertion into planar bilayers as well as in the plasma membrane of HeLa cells. Here, we identify a series of inhibitors of VacA channels and we compare their effectiveness as channel blockers and as inhibitors of VacA-induced vacuolation, confirming that the two phenomena are linked. This characterization opens the way to studies in other experimental systems and to the search for a specific inhibitor of VacA action.

Towards deciphering the Helicobacter pylori cytotoxin

VacA, the major exotoxin produced by Helicobacter pylori, is composed of identical 87 kDa monomers that assemble into flower-shaped oligomers. The monomers can be proteolytically cleaved into two moieties, one of 37 and the other of 58 kDa, named P37 and P58 respectively. The most studied property of VacA is the alteration of intracellular vesicular trafficking in eukaryotic cells leading to the formation of large vacuoles containing markers of late endosomes and lysosomes. However, VacA also causes a reduction in transepithelial electrical resistance in polarized monolayers and forms ion channels in lipid bilayers. The ability to induce vacuoles is localized mostly but not entirely in P37, whereas P58 is mostly involved in cell targeting. Until recently, H. pylori isolates were classified as tox+ or tox-, depending on whether they induced vacuoles in HeLa cells or not. Today, we know that almost all strains are cytotoxic. The major difference between tox+ and tox- resides in the cell binding domain, which exists in two allelic forms, only one of which is toxic for HeLa cells. The two forms, named m1 and m2, are found predominantly in Western and Chinese isolates respectively.

Molecular and cellular activities of Helicobacter pylori pathogenic factors

Stomach infection with pathogenic strains of Helicobacter pylori causes in some patients severe gastroduodenal diseases. These bacteria produce various virulence factors and, here, we review the recent acquisition on the biochemical mode of action of three major factors. We discuss the role of urease both as buffer of the stomach pH and as source of ammonia. The vacuolating toxin alters the endocytic pathway of non-polarized cells, inducing the release of acid hydrolases, the depression of extracellular ligand degradation and of antigen processing and, in the presence of ammonia, swelling of late-prelysosomal compartments. In polarized epithelial monolayers, vacuolating toxin induces an increase of the paracellular permeability, independent of vacuolation. The neutrophil activating protein induces the production of oxygen radicals in human neutrophils and could contribute to the damage of the stomach mucosa. The activities of these factors are discussed in terms of the need of the bacterium of increasing the supply of nutrients from the stomach lumen and from the mucosa.