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

There Are No Insurmountable Barriers: Passage of the Helicobacter pylori VacA Toxin from Bacterial Cytoplasm to Eukaryotic Cell Organelle

The Gram-negative bacterium Helicobacter pylori is a very successful pathogen, one of the most commonly identified causes of bacterial infections in humans worldwide. H. pylori produces several virulence factors that contribute to its persistence in the hostile host habitat and to its pathogenicity. The most extensively studied are cytotoxin-associated gene A (CagA) and vacuolating cytotoxin A (VacA). VacA is present in almost all H. pylori strains. As a secreted multifunctional toxin, it assists bacterial colonization, survival, and proliferation during long-lasting infections. To exert its effect on gastric epithelium and other cell types, VacA undergoes several modifications and crosses multiple membrane barriers. Once inside the gastric epithelial cell, VacA disrupts many cellular-signaling pathways and processes, leading mainly to changes in the efflux of various ions, the depolarization of membrane potential, and perturbations in endocytic trafficking and mitochondrial function. The most notable effect of VacA is the formation of vacuole-like structures, which may lead to apoptosis. This review focuses on the processes involved in VacA secretion, processing, and entry into host cells, with a particular emphasis on the interaction of the mature toxin with host membranes and the formation of transmembrane pores.

Gut Commensal Parabacteroides goldsteinii MTS01 Alters Gut Microbiota Composition and Reduces Cholesterol to Mitigate Helicobacter pylori-Induced Pathogenesis

Helicobacter pylori infection is closely associated with various gastrointestinal diseases and poses a serious threat to human health owing to its increasing antimicrobial resistance. H. pylori possesses two major virulence factors, vacuolating cytotoxin A (VacA) and cytotoxin-associated gene A (CagA), which are involved in its pathogenesis. Probiotics have recently been used to eradicate H. pylori infection and reduce the adverse effects of antibiotic-based therapies. Parabacteroides goldsteinii MTS01 is a novel next-generation probiotic (NGP) with activities that can alleviate specific diseases by altering the gut microbiota. However, the mechanism by which P. goldsteinii MTS01 exerts its probiotic effects against H. pylori infection remains unclear. Our results showed that administration of P. goldsteinii MTS01 to H. pylori-infected model mice altered the composition of the gut microbiota and significantly reduced serum cholesterol levels, which mitigated H. pylori-induced gastric inflammation. In addition, the pathogenic effects of H. pylori VacA and CagA on gastric epithelial cells were markedly abrogated by treatment with P. goldsteinii MTS01. These results indicate that P. goldsteinii MTS01 can modulate gut microbiota composition and has anti-virulence factor functions, and thus could be developed as a novel functional probiotic for reducing H. pylori-induced pathogenesis.

Lipid larceny: channelizing host lipids for establishing successful pathogenesis by bacteria

Lipids are complex organic compounds made up of carbon, oxygen, and hydrogen. These play a diverse and intricate role in cellular processes like membrane trafficking, protein sorting, signal transduction, and bacterial infections. Both Gram-positive bacteria (Staphylococcus sp., Listeria monocytogenes, etc.) and Gram-negative bacteria (Chlamydia sp., Salmonella sp., E. coli, etc.) can hijack the various host-lipids and utilize them structurally as well as functionally to mount a successful infection. The pathogens can deploy with various arsenals to exploit host membrane lipids and lipid-associated receptors as an attachment for toxins' landing or facilitate their entry into the host cellular niche. Bacterial species like Mycobacterium sp. can also modulate the host lipid metabolism to fetch its carbon source from the host. The sequential conversion of host membrane lipids into arachidonic acid and prostaglandin E2 due to increased activity of cPLA-2 and COX-2 upon bacterial infection creates immunosuppressive conditions and facilitates the intracellular growth and proliferation of bacteria. However, lipids' more debatable role is that they can also be a blessing in disguise. Certain host-lipids, especially sphingolipids, have been shown to play a crucial antibacterial role and help the host in combating the infections. This review shed light on the detailed role of host lipids in bacterial infections and the current understanding of the lipid in therapeutics. We have also discussed potential prospects and the need of the hour to help us cope in this race against deadly pathogens and their rapidly evolving stealthy virulence strategies.

Functional Properties of Oligomeric and Monomeric Forms of Helicobacter pylori VacA Toxin

Helicobacter pylori VacA is a secreted toxin that assembles into water-soluble oligomeric structures and forms anion-selective membrane channels. Acidification of purified VacA enhances its activity in cell culture assays. Sites of protomer-protomer contact within VacA oligomers have been identified by cryoelectron microscopy, and in the current study, we validated several of these interactions by chemical cross-linking and mass spectrometry. We then mutated amino acids at these contact sites and analyzed the effects of the alterations on VacA oligomerization and activity. VacA proteins with amino acid charge reversals at interprotomer contact sites retained the capacity to assemble into water-soluble oligomers and retained cell-vacuolating activity. Introduction of paired cysteine substitutions at these sites resulted in formation of disulfide bonds between adjacent protomers. Negative-stain electron microscopy and single-particle two-dimensional class analysis revealed that wild-type VacA oligomers disassemble when exposed to acidic pH, whereas the mutant proteins with paired cysteine substitutions retain an oligomeric state at acidic pH. Acid-activated wild-type VacA caused vacuolation of cultured cells, whereas acid-activated mutant proteins with paired cysteine substitutions lacked cell-vacuolating activity. Treatment of these mutant proteins with both low pH and a reducing agent resulted in VacA binding to cells, VacA internalization, and cell vacuolation. Internalization of a nonoligomerizing mutant form of VacA by host cells was detected without a requirement for acid activation. Collectively, these results enhance our understanding of the molecular interactions required for VacA oligomerization and support a model in which toxin activity depends on interactions of monomeric VacA with host cells.

Statins' Regulation of the Virulence Factors of Helicobacter pylori and the Production of ROS May Inhibit the Development of Gastric Cancer

Conventionally, statins are used to treat high cholesterol levels. They exhibit pleiotropic effects, such as the prevention of cardiovascular disease and decreased cancer mortality. Gastric cancer (GC) is one of the most common cancers, ranking as the third leading global cause of cancer-related deaths, and is mainly attributed to chronic Helicobacter pylori infection. During their co-evolution with hosts, H. pylori has developed the ability to use the cellular components of the host to evade the immune system and multiply in intracellular niches. Certain H. pylori virulence factors, including cytotoxin-associated gene A (CagA), vacuolating cytotoxin A (VacA), and cholesterol-α-glucosyltransferase (CGT), have been shown to exploit host cholesterol during pathogenesis. Therefore, using statins to antagonize cholesterol synthesis might prove to be an ideal strategy for reducing the occurrence of H. pylori-related GC. This review discusses the current understanding of the interplay of H. pylori virulence factors with cholesterol and reactive oxygen species (ROS) production, which may prove to be novel therapeutic targets for the development of effective treatment strategies against H. pylori-associated GC. We also summarize the findings of several clinical studies on the association between statin therapy and the development of GC, especially in terms of cancer risk and mortality.

Vaginal Isolates of Candida glabrata Are Uniquely Susceptible to Ionophoric Killer Toxins Produced by Saccharomyces cerevisiae

Compared to other species of Candida yeasts, the growth of Candida glabrata is inhibited by many different strains of Saccharomyces killer yeasts. The ionophoric K1 and K2 killer toxins are broadly inhibitory to all clinical isolates of C. glabrata from patients with recurrent vulvovaginal candidiasis, despite high levels of resistance to clinically relevant antifungal therapeutics.

Campylobacter jejuni Cytolethal Distending Toxin C Exploits Lipid Rafts to Mitigate Helicobacter pylori-Induced Pathogenesis

Helicobacter pylori infection is associated with several gastrointestinal diseases, including gastritis, peptic ulcer, and gastrointestinal adenocarcinoma. Two major cytotoxins, vacuolating cytotoxin A (VacA) and cytotoxin-associated gene A (CagA), interact closely with lipid rafts, contributing to H. pylori-associated disease progression. The Campylobacter jejuni cytolethal distending toxin consists of three subunits: CdtA, CdtB, and CdtC. Among them, CdtA and CdtC bind to membrane lipid rafts, which is crucial for CdtB entry into cells. In this study, we employed recombinant CdtC (rCdtC) to antagonize the functions of H. pylori cytotoxin in cells. Our results showed that rCdtC alleviates cell vacuolation induced by H. pylori VacA. Furthermore, rCdtC reduces H. pylori CagA translocation, which decreases nuclear factor kappa-B activation and interleukin-8 production, resulting in the mitigation of gastric epithelial cell inflammation. These results reveal that CdtC hijacks cholesterol to compete for H. pylori cytotoxin actions via lipid rafts, ameliorating H. pylori-induced pathogenesis.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

Helicobacter pylori lipids can form ordered membrane domains (rafts)

Ordered lipid domains (rafts) are generally considered to be features of eukaryotic cells, but ordered lipid domains formed by cholesterol lipids have been identified in bacteria from the genus Borrelia, and similar cholesterol lipids exist in the bacterium Helicobacter pylori. To determine whether H. pylori lipids could form ordered membrane domains, we investigated domain formation in aqueous dispersions of H. pylori whole lipid extracts, individual H. pylori lipids, or defined mixtures of H. pylori lipids and other membrane-forming lipids. DPH (1,6-diphenyl-1,3,5-hexatriene) anisotropy measurements were used to assay membrane order and FRET (Förster resonance energy transfer) was used to detect the presence of co-existing ordered and disordered domains. We found that H. pylori membrane lipid extracts spontaneously formed lipid domains. Domain formation was more stable when lipids were extracted from H. pylori cells grown in the presence of cholesterol. Certain isolated H. pylori lipids (by themselves or when mixed with other lipids) also had the ability to form ordered domains. To be specific, H. pylori cholesteryl-6-O-tetradecanoyl-α-D-glucopyranoside (CAG) and cholesterol-6-O-phosphatidyl-α-D-glucopyranoside (CPG) had the ability to form and/or stabilize ordered domain formation, while H. pylori phosphatidylethanolamine did not, behaving similarly to unsaturated phosphatidylethanolamines. We conclude that specific H. pylori cholesterol lipids have a marked ability to form ordered lipid domains.

Unraveling the factors and mechanism involved in persistence: Host-pathogen interactions in Helicobacter pylori

Helicobacter pylori and humans have one of the most complex relationships in nature. How a bacterium manages to live in one of the harshest and hostile environments is a topic of unraveling mysteries. H. pylori is a prevalent species and it colonizes the human gut of more than 50% of the world population. It infects the epithelial region of antrum and persists there for a long period. Over the time of evolution, H. pylori has developed complex strategies to extend the degree of inflammation in gastric mucosa. H. pylori needs specific adaptations for initial colonization into the host environment like helical shape, flagellar movement, chemotaxis, and the production of urease enzyme that neutralizes acidic environment of the stomach. There are several factors from the bacterium as well as from the host that participate in these complex interactions. On the other hand, to establish the persistent infection, H. pylori escapes the immune system by mimicking the host antigens. This pathogen has the ability to dodge the immune system and then persist there in the form of host cell, which leads to immune tolerance. H. pylori has an ability to manipulate its own pathogen-associated molecular patterns, which leads to an inhibition in the binding with specific pattern recognition receptors of the host to avoid immune cell detection. Also, it manipulates the host metabolic homeostasis in the gastric epithelium. Besides, it has several genes, which may get involved in the acquisition of nutrition from the host to survive longer in the host. Due to the persistence of H. pylori, it causes chronic inflammation and raises the chances of gastric cancer. This review highlights the important elements, which are certainly responsible for the persistence of H. pylori in the human host.

Effect of environmental salt concentration on the Helicobacter pylori exoproteome

Helicobacter pylori infection and a high salt diet are each risk factors for gastric cancer. In this study, we tested the hypothesis that environmental salt concentration influences the composition of the H. pylori exoproteome. H. pylori was cultured in media containing varying concentrations of sodium chloride, and aliquots were fractionated and analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). We identified proteins that were selectively released into the extracellular space, and we identified selectively released proteins that were differentially abundant in culture supernatants, depending on the environmental salt concentration. We also used RNA-seq analysis to identify genes that were differentially expressed in response to environmental salt concentration. The salt-responsive proteins identified by proteomic analysis and salt-responsive genes identified by RNA-seq analysis were mostly non-concordant, but the secreted toxin VacA was salt-responsive in both analyses. Western blot analysis confirmed that VacA levels in the culture supernatant were increased in response to high salt conditions, and quantitative RT-qPCR experiments confirmed that vacA transcription was upregulated in response to high salt conditions. These results indicate that environmental salt concentration influences the composition of the H. pylori exoproteome, which could contribute to the increased risk of gastric cancer associated with a high salt diet. SIGNIFICANCE: Helicobacter pylori-induced alterations in the gastric mucosa have been attributed, at least in part, to the actions of secreted H. pylori proteins. In this study, we show that H. pylori growth in high salt concentrations leads to increased levels of a secreted VacA toxin. Salt-induced alterations in the composition of the H. pylori exoproteome is relevant to the increased risk of gastric cancer associated with consumption of a high salt diet.

Intracellular Degradation of Helicobacter pylori VacA Toxin as a Determinant of Gastric Epithelial Cell Viability

Helicobacter pylori VacA is a secreted pore-forming toxin that induces cell vacuolation in vitro and contributes to the pathogenesis of gastric cancer and peptic ulcer disease. We observed that purified VacA has relatively little effect on the viability of AGS gastric epithelial cells, but the presence of exogenous weak bases such as ammonium chloride (NH₄Cl) enhances the susceptibility of these cells to VacA-induced vacuolation and cell death. Therefore, we tested the hypothesis that NH₄Cl augments VacA toxicity by altering the intracellular trafficking of VacA or inhibiting intracellular VacA degradation. We observed VacA colocalization with LAMP1- and LC3-positive vesicles in both the presence and absence of NH₄Cl, indicating that NH₄Cl does not alter VacA trafficking to lysosomes or autophagosomes. Conversely, we found that supplemental NH₄Cl significantly increases the intracellular stability of VacA. By conducting experiments using chemical inhibitors, stable ATG5 knockdown cell lines, and ATG16L1 knockout cells (generated using CRISPR/Cas9), we show that VacA degradation is independent of autophagy and proteasome activity but dependent on lysosomal acidification. We conclude that weak bases like ammonia, potentially generated during H. pylori infection by urease and other enzymes, enhance VacA toxicity by inhibiting toxin degradation.

Helicobacter pylori VacA Targets Myeloid Cells in the Gastric Lamina Propria To Promote Peripherally Induced Regulatory T-Cell Differentiation and Persistent Infection

Helicobacter pylori has coexisted with humans for at least 60.000 years and has evolved persistence strategies that allow it to evade host immunity and colonize its host for life. The VacA protein is expressed by all H. pylori strains and is required for high-level persistent infection in experimental mouse models. Here, we show that VacA targets myeloid cells in the gastric mucosa to create a tolerogenic environment that facilitates regulatory T-cell differentiation, while suppressing effector T-cell priming and functionality. Tregs that are induced in the periphery during H. pylori infection can be found not only in the stomach but also in the lungs of infected mice, where they are likely to affect immune responses to allergens.

The gastric bacterium Helicobacter pylori causes a persistent infection that is directly responsible for gastric ulcers and gastric cancer in some patients and protective against allergic and other immunological disorders in others. The two outcomes of the Helicobacter-host interaction can be modeled in mice that are infected as immunocompetent adults and as neonates, respectively. Here, we have investigated the contribution of the Helicobacter immunomodulator VacA to H. pylori-specific local and systemic immune responses in both models. We found that neonatally infected mice are colonized at higher levels than mice infected as adults and fail to generate effector T-cell responses to the bacteria; rather, T-cell responses in neonatally infected mice are skewed toward Foxp3-positive (Foxp3⁺) regulatory T cells that are neuropilin negative and express RORγt. We found these peripherally induced regulatory T cells (pTregs) to be enriched, in a VacA-dependent manner, not only in the gastric mucosa but also in the lungs of infected mice. Pulmonary pTreg accumulation was observed in mice that have been infected neonatally with wild-type H. pylori but not in mice that have been infected as adults or mice infected with a VacA null mutant. Finally, we traced VacA to gastric lamina propria myeloid cells and show that it suppressed interleukin-23 (IL-23) expression by dendritic cells and induced IL-10 and TGF-β expression in macrophages. Taken together, the results are consistent with the idea that H. pylori creates a tolerogenic environment through its immunomodulator VacA, which skews T-cell responses toward Tregs, favors H. pylori persistence, and affects immunity at distant sites.

Characterization of gastric cells infection by diverse Helicobacter pylori strains through Fourier-transform infrared spectroscopy

The infection of Helicobacter pylori, covering 50% of the world-population, leads to diverse gastric diseases as ulcers and cancer along the life-time of the human host. To promote the discovery of biomarkers of bacterial infection, in the present work, Fourier-transform infrared spectra were acquired from adenocarcinoma gastric cells, incubated with H. pylori strains presenting different genotypes concerning the virulent factors cytotoxin associated gene A and vacuolating cytotoxin A. Defined absorbance ratios were evaluated by diverse methods of statistical inference, according to the fulfillment of the tests assumptions. It was possible to define from the gastric cells, diverse absorbance ratios enabling to discriminate: i) The infection; ii) the bacteria genotype; and iii) the gastric disease of the patients from which the bacteria were isolated. These biomarkers could fasten the knowledge of the complex infection process while promoting a platform for a new diagnostic method, rapid but also specific and sensitive towards the diagnosis of both infection and bacterial virulence.

Inhibition of bacterial toxin recognition of membrane components as an anti-virulence strategy

Over recent years, the development of new antibiotics has not kept pace with the rate at which bacteria develop resistance to these drugs. For this reason, many research groups have begun to design and study alternative therapeutics, including molecules to specifically inhibit the virulence of pathogenic bacteria. Because many of these pathogenic bacteria release protein toxins, which cause or exacerbate disease, inhibition of the activity of bacterial toxins is a promising anti-virulence strategy. In this review, we describe several approaches to inhibit the initial interactions of bacterial toxins with host cell membrane components. The mechanisms by which toxins interact with the host cell membrane components have been well-studied over the years, leading to the identification of therapeutic targets, which have been exploited in the work described here. We review efforts to inhibit binding to protein receptors and essential membrane lipid components, complex assembly, and pore formation. Although none of these molecules have yet been demonstrated in clinical trials, the in vitro and in vivo results presented here demonstrate their promise as novel alternatives and/or complements to traditional antibiotics.

Helicobacter pylori-binding nonacid glycosphingolipids in the human stomach

Helicobacter pylori has a number of well-characterized carbohydrate-binding adhesins (BabA, SabA, and LabA) that promote adhesion to the gastric mucosa. In contrast, information on the glycoconjugates present in the human stomach remains unavailable. Here, we used MS and binding of carbohydrate-recognizing ligands to characterize the glycosphingolipids of three human stomachs from individuals with different blood group phenotypes (O(Rh-)P, A(Rh+)P, and A(Rh+)p), focusing on compounds recognized by H. pylori We observed a high degree of structural complexity, and the composition of glycosphingolipids differed among individuals with different blood groups. The type 2 chain was the dominating core chain of the complex glycosphingolipids in the human stomach, in contrast to the complex glycosphingolipids in the human small intestine, which have mainly a type 1 core. H. pylori did not bind to the O(Rh-)P stomach glycosphingolipids, whose major complex glycosphingolipids were neolactotetraosylceramide, the Le^x, Le^a, and H type 2 pentaosylceramides, and the Le^y hexaosylceramide. Several H. pylori-binding compounds were present among the A(Rh+)P and A(Rh+)p stomach glycosphingolipids. Ligands for BabA-mediated binding of H. pylori were the Le^b hexaosylceramide, the H type 1 pentaosylceramide, and the A type 1/ALe^b heptaosylceramide. Additional H. pylori-binding glycosphingolipids recognized by BabA-deficient strains were lactosylceramide, lactotetraosylceramide, the x₂ pentaosylceramide, and neolactohexaosylceramide. Our characterization of human gastric receptors required for H. pylori adhesion provides a basis for the development of specific compounds that inhibit the binding of this bacterium to the human gastric mucosa.

Irisin protects mitochondria function during pulmonary ischemia/reperfusion injury

Limb remote ischemic preconditioning (RIPC) is an effective means of protection against ischemia/reperfusion (IR)-induced injury to multiple organs. Many studies are focused on identifying endocrine mechanisms that underlie the cross-talk between muscle and RIPC-mediated organ protection. We report that RIPC releases irisin, a myokine derived from the extracellular portion of fibronectin domain-containing 5 protein (FNDC5) in skeletal muscle, to protect against injury to the lung. Human patients with neonatal respiratory distress syndrome show reduced concentrations of irisin in the serum and increased irisin concentrations in the bronchoalveolar lavage fluid, suggesting transfer of irisin from circulation to the lung under physiologic stress. In mice, application of brief periods of ischemia preconditioning stimulates release of irisin into circulation and transfer of irisin to the lung subjected to IR injury. Irisin, via lipid raft-mediated endocytosis, enters alveolar cells and targets mitochondria. Interaction between irisin and mitochondrial uncoupling protein 2 (UCP2) allows for prevention of IR-induced oxidative stress and preservation of mitochondrial function. Animal model studies show that intravenous administration of exogenous irisin protects against IR-induced injury to the lung via improvement of mitochondrial function, whereas in UCP2-deficient mice or in the presence of a UCP2 inhibitor, the protective effect of irisin is compromised. These results demonstrate that irisin is a myokine that facilitates RIPC-mediated lung protection. Targeting the action of irisin in mitochondria presents a potential therapeutic intervention for pulmonary IR injury.

Colocalization of receptors for Shiga toxins with lipid rafts in primary human renal glomerular endothelial cells and influence of D-PDMP on synthesis and distribution of glycosphingolipid receptors

Damage of human renal glomerular endothelial cells (HRGECs) of the kidney represents the linchpin in the pathogenesis of the hemolytic uremic syndrome caused by Shiga toxins of enterohemorrhagic Escherichia coli (EHEC). We performed a comprehensive structural analysis of the Stx-receptor glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer, Galα4Galβ4Glcβ1Cer) and globotetraosylceramide (Gb4Cer, GalNAcβ3Galα4Galβ4Glcβ1Cer) and their distribution in lipid raft analog detergent-resistant membranes (DRMs) and nonDRMs prepared from primary HRGECs. Predominant receptor lipoforms were Gb3Cer and Gb4Cer with Cer (d18:1, C16:0), Cer (d18:1, C22:0) and Cer (d18:1, C24:1/C24:0). Stx-receptor GSLs co-distribute with sphingomyelin (SM) and cholesterol as well as flotillin-2 in DRMs, representing the liquid-ordered membrane phase and indicating lipid raft association. Lyso-phosphatidylcholine (lyso-PC) was identified as a nonDRM marker phospholipid of the liquid-disordered membrane phase. Exposure of primary HRGECs to the ceramide analogon d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) reduced total Gb3Cer and Gb4Cer content, roughly calculated from two biological replicates, down to half and quarter of its primordial content, respectively, but strengthened their prevalence and cholesterol preponderance in DRMs. At the same time, the distribution of PC, SM and lyso-PC to subcellular membrane fractions remained unaffected by D-PDMP treatment. Defining the GSL composition and precise microdomain structures of primary HRGECs may help to develop novel therapeutic options to combat life-threatening EHEC infections.

Determinants of Raft Partitioning of the Helicobacter pylori Pore-Forming Toxin VacA

Helicobacter pylori, a Gram-negative bacterium, is a well-known risk factor for gastric cancer. H. pylori vacuolating cytotoxin A (VacA) is a secreted pore-forming toxin that induces a wide range of cellular responses. Like many other bacterial toxins, VacA has been hypothesized to utilize lipid rafts to gain entry into host cells. Here, we used giant plasma membrane vesicles (GPMVs) as a model system to understand the preferential partitioning of VacA into lipid rafts. We show that a wild-type (WT) toxin predominantly associates with the raft phase. Acid activation of VacA enhances binding of the toxin to GPMVs but is not required for raft partitioning. VacA mutant proteins with alterations at the amino terminus (resulting in impaired membrane channel formation) and a nonoligomerizing VacA mutant protein retain the ability to preferentially associate with lipid rafts. Consistent with these results, the isolated VacA p55 domain was capable of binding to lipid rafts. We conclude that the affinity of VacA for rafts is independent of its capacity to oligomerize or form membrane channels.

Cytoplasmic vacuolization in cell death and survival

Cytoplasmic vacuolization (also called cytoplasmic vacuolation) is a well-known morphological phenomenon observed in mammalian cells after exposure to bacterial or viral pathogens as well as to various natural and artificial low-molecular-weight compounds. Vacuolization often accompanies cell death; however, its role in cell death processes remains unclear. This can be attributed to studying vacuolization at the level of morphology for many years. At the same time, new data on the molecular mechanisms of the vacuole formation and structure have become available. In addition, numerous examples of the association between vacuolization and previously unknown cell death types have been reported. Here, we review these data to make a deeper insight into the role of cytoplasmic vacuolization in cell death and survival.

Helicobacter pylori Vacuolating Toxin and Gastric Cancer

Helicobacter pylori VacA is a channel-forming toxin unrelated to other known bacterial toxins. Most H. pylori strains contain a vacA gene, but there is marked variation among strains in VacA toxin activity. This variation is attributable to strain-specific variations in VacA amino acid sequences, as well as variations in the levels of VacA transcription and secretion. In this review, we discuss epidemiologic studies showing an association between specific vacA allelic types and gastric cancer, as well as studies that have used animal models to investigate VacA activities relevant to gastric cancer. We also discuss the mechanisms by which VacA-induced cellular alterations may contribute to the pathogenesis of gastric cancer.

A New Role for Helicobacter pylori Urease: Contributions to Angiogenesis

Helicobacter pylori is a pathogen involved in gastric diseases such as ulcers and carcinomas. H. pylori's urease is an important virulence factor produced in large amounts by this bacterium. In previous studies, we have shown that this protein is able to activate several cell types like neutrophils, monocytes, platelets, endothelial cells, and gastric epithelial cells. Angiogenesis is a physiological process implicated in growth, invasion and metastization of tumors. Here, we have analyzed the angiogenic potential of H. pylori urease (HPU) in gastric epithelial cells. No cytotoxicity was observed in AGS, Kato-III, and MKN28 gastric cell lines treated with 300 nM HPU, as evaluated by the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. As we previously reported in neutrophils, treatment with 300 nM HPU also had an anti-apoptotic effect in gastric epithelial cells leading to a 2.2-fold increase in the levels of Bcl-X_L after 6 h, and a decrease of 80% in the content of BAD, after 48 h, two mitochondrial proteins involved in regulation of apoptosis. Within 10 min of exposure, HPU is rapidly internalized by gastric epithelial cells. Treatment of the gastric cells with methyl-β-cyclodextrin abolished HPU internalization suggesting a cholesterol-dependent process. HPU induces the expression of pro-angiogenic factors and the decrease of expression of anti-angiogenic factors by AGS cells. The angiogenic activity of HPU was analyzed using in vitro and in vivo models. HPU induced formation of tube-like structures by human umbilical vascular endothelial cells in a 9 h experiment. In the chicken embryo chorioallantoic membrane model, HPU induced intense neo-vascularization after 3 days. In conclusion, our results indicate that besides allowing bacterial colonization of the gastric mucosa, H. pylori's urease triggers processes that initiate pro-angiogenic responses in different cellular models. Thus, this bacterial urease, a major virulence factor, may also play a role in gastric carcinoma development.

Pleiotropic cytotoxicity of VacA toxin in host cells and its impact on immunotherapy

Introduction: In the recent decades, a number of studies have highlighted the importance of Helicobacter pylori in the initiation and development of peptic ulcer and gastric cancer. Some potential virulence factors (e.g., urease, CagA, VacA, BabA) are exploited by this microorganism, facilitating its persistence through evading human defense mechanisms. Among these toxins and enzymes, vacuolating toxin A (VacA) is of a great importance in the pathogenesis of H. pylori. VacA toxin shows different pattern of cytotoxicity through binding to different cell surface receptors in various cells.

Methods: To highlight attempts in treatment for H. pylori infection, here, we discussed the VacA potential as a candidate for development of vaccine and targeted immunotherapy. Furthermore, we reviewed the related literature to provide key insights on association of the genetic variants of VacA with the toxicity of the toxin in cells.

Results: A number of investigations on the receptor(s) binding of VacA toxin confirmed the pleiotropic nature of VacA that uses a unique mechanism for internalization through some membrane components such as lipid rafts and glycophosphatidylinositol (GPI)-anchored proteins (GPI-AP). Considering the high potency of VacA toxin in the clinical presentations in infection and assisting persistence and colonization of H. pylori, it is considered as one of the pivotal components in production vaccines and monoclonal antibodies (mAbs).

Conclusion: It is possible to generate mAbs with a considerable potential to convert into secretory immunoglobulins that could penetrate into the niche of H. pylori and inhibit its normal functionalities. Further, conjugation of H. pylori targeting Ab fragments with the toxic agents or drug delivery systems (DDSs) offers new generation of H. pylori treatments.

The Making and Taking of Lipids: The Role of Bacterial Lipid Synthesis and the Harnessing of Host Lipids in Bacterial Pathogenesis

In order to survive environmental stressors, including those induced by growth in the human host, bacterial pathogens will adjust their membrane physiology accordingly. These physiological changes also include the use of host-derived lipids to alter their own membranes and feed central metabolic pathways. Within the host, the pathogen is exposed to many stressful stimuli. A resulting adaptation is for pathogens to scavenge the host environment for readily available lipid sources. The pathogen takes advantage of these host-derived lipids to increase or decrease the rigidity of their own membranes, to provide themselves with valuable precursors to feed central metabolic pathways, or to impact host signalling and processes. Within, we review the diverse mechanisms that both extracellular and intracellular pathogens employ to alter their own membranes as well as their use of host-derived lipids in membrane synthesis and modification, in order to increase survival and perpetuate disease within the human host. Furthermore, we discuss how pathogen employed mechanistic utilization of host-derived lipids allows for their persistence, survival and potentiation of disease. A more thorough understanding of all of these mechanisms will have direct consequences for the development of new therapeutics, and specifically, therapeutics that target pathogens, while preserving normal flora.

An Overview of Helicobacter pylori VacA Toxin Biology

The VacA toxin secreted by Helicobacter pylori enhances the ability of the bacteria to colonize the stomach and contributes to the pathogenesis of gastric adenocarcinoma and peptic ulcer disease. The amino acid sequence and structure of VacA are unrelated to corresponding features of other known bacterial toxins. VacA is classified as a pore-forming toxin, and many of its effects on host cells are attributed to formation of channels in intracellular sites. The most extensively studied VacA activity is its capacity to stimulate vacuole formation, but the toxin has many additional effects on host cells. Multiple cell types are susceptible to VacA, including gastric epithelial cells, parietal cells, T cells, and other types of immune cells. This review focuses on the wide range of VacA actions that are detectable in vitro, as well as actions of VacA in vivo that are relevant for H. pylori colonization of the stomach and development of gastric disease.

New Insights into VacA Intoxication Mediated through Its Cell Surface Receptors

Helicobacter pylori (H. pylori), a major cause of gastroduodenal diseases, produces VacA, a vacuolating cytotoxin associated with gastric inflammation and ulceration. The C-terminal domain of VacA plays a crucial role in receptor recognition on target cells. We have previously identified three proteins (i.e., RPTPα, RPTPβ, and LRP1) that serve as VacA receptors. These receptors contribute to the internalization of VacA into epithelial cells, activate signal transduction pathways, and contribute to cell death and gastric ulceration. In addition, other factors (e.g., CD18, sphingomyelin) have also been identified as cell-surface, VacA-binding proteins. Since we believe that, following interactions with its host cell receptors, VacA participates in events leading to disease, a better understanding of the cellular function of VacA receptors may provide valuable information regarding the mechanisms underlying the pleiotropic actions of VacA and the pathogenesis of H. pylori-mediated disease. In this review, we focus on VacA receptors and their role in events leading to cell damage.

Hijacking and Use of Host Lipids by Intracellular Pathogens

Intracellular bacteria use a number of strategies to survive, grow, multiply, and disseminate within the host. One of the most striking adaptations that intracellular pathogens have developed is the ability to utilize host lipids and their metabolism. Bacteria such as Anaplasma, Chlamydia, or Mycobacterium can use host lipids for different purposes, such as a means of entry through lipid rafts, building blocks for bacteria membrane formation, energy sources, camouflage to avoid the fusion of phagosomes and lysosomes, and dissemination. One of the most extreme examples of lipid exploitation is Mycobacterium, which not only utilizes the host lipid as a carbon and energy source but is also able to reprogram the host lipid metabolism. Likewise, Chlamydia spp. have also developed numerous mechanisms to reprogram lipids onto their intracellular inclusions. Finally, while the ability to exploit host lipids is important in intracellular bacteria, it is not an exclusive trait. Extracellular pathogens, including Helicobacter, Mycoplasma, and Borrelia, can recruit and metabolize host lipids that are important for their growth and survival.Throughout this chapter we will review how intracellular and extracellular bacterial pathogens utilize host lipids to enter, survive, multiply, and disseminate in the host.

Inhibition of LtxA toxicity by blocking cholesterol binding with peptides

The leukotoxin (LtxA) produced by Aggregatibacter actinomycetemcomitans kills host immune cells, allowing the bacterium to establish an ecological niche in the upper aerodigestive tract of its human host. The interaction of LtxA with human immune cells is both complex and multifaceted, involving membrane lipids as well as cell-surface proteins. In the initial encounter with the host cell, LtxA associates with lymphocyte function-associated antigen-1, a cell surface adhesion glycoprotein. However, we have also demonstrated that the toxin associates strongly with the plasma membrane lipids, specifically cholesterol. This association with cholesterol is regulated by a cholesterol recognition amino acid consensus (CRAC) motif, with a sequence of (334) LEEYSKR(340), in the N-terminal region of the toxin. Here, we have demonstrated that removal of cholesterol from the plasma membrane or mutation of the LtxA CRAC motif inhibits the activity of the toxin in THP-1 cells. To inhibit LtxA activity, we designed a short peptide corresponding to the CRAC(336) motif of LtxA (CRAC(336WT)). This peptide binds to cholesterol and thereby inhibits the toxicity of LtxA in THP-1 cells. Previously, we showed that this peptide inhibits LtxA toxicity against Jn.9 (Jurkat) cells, indicating that peptides derived from the cholesterol-binding site of LtxA may have a potential clinical applicability in controlling infections of repeats-in-toxin-producing organisms.

Euphol from Euphorbia tirucalli Negatively Modulates TGF-β Responsiveness via TGF-β Receptor Segregation inside Membrane Rafts

Transforming growth factor-β (TGF-β) responsiveness in cultured cells can be modulated by TGF-β partitioning between lipid raft/caveolae- and clathrin-mediated endocytosis pathways. Lipid rafts are plasma membrane microdomains with an important role in cell survival signaling, and cholesterol is necessary for the lipid rafts’ structure and function. Euphol is a euphane-type triterpene alcohol that is structurally similar to cholesterol and has a wide range of pharmacological properties, including anti-inflammatory and anti-cancer effects. In the present study, euphol suppressed TGF-β signaling by inducing TGF-β receptor movement into lipid-raft microdomains and degrading TGF-β receptors.

Role of connexin 43 in Helicobacter pylori VacA-induced cell death

Helicobacter pylori colonizes the human stomach and confers an increased risk for the development of peptic ulceration, noncardia gastric adenocarcinoma, and gastric lymphoma. A secreted H. pylori toxin, VacA, can cause multiple alterations in gastric epithelial cells, including cell death. In this study, we sought to identify host cell factors that are required for VacA-induced cell death. To do this, we analyzed gene trap and short hairpin RNA (shRNA) libraries in AZ-521 human gastric epithelial cells and selected for VacA-resistant clones. Among the VacA-resistant clones, we identified multiple gene trap library clones and an shRNA library clone with disrupted expression of connexin 43 (Cx43) (also known as gap junction protein alpha 1 [GJA1]). Further experiments with Cx43-specific shRNAs confirmed that a reduction in Cx43 expression results in resistance to VacA-induced cell death. Immunofluorescence microscopy experiments indicated that VacA did not colocalize with Cx43. We detected production of the Cx43 protein in AZ-521 cells but not in AGS, HeLa, or RK-13 cells, and correspondingly, AZ-521 cells were the most susceptible to VacA-induced cell death. When Cx43 was expressed in HeLa cells, the cells became more susceptible to VacA. These results indicate that Cx43 is a host cell constituent that contributes to VacA-induced cell death and that variation among cell types in susceptibility to VacA-induced cell death is attributable at least in part to cell type-specific differences in Cx43 production.

Caveolin-1 protects B6129 mice against Helicobacter pylori gastritis

Caveolin-1 (Cav1) is a scaffold protein and pathogen receptor in the mucosa of the gastrointestinal tract. Chronic infection of gastric epithelial cells by Helicobacter pylori (H. pylori) is a major risk factor for human gastric cancer (GC) where Cav1 is frequently down-regulated. However, the function of Cav1 in H. pylori infection and pathogenesis of GC remained unknown. We show here that Cav1-deficient mice, infected for 11 months with the CagA-delivery deficient H. pylori strain SS1, developed more severe gastritis and tissue damage, including loss of parietal cells and foveolar hyperplasia, and displayed lower colonisation of the gastric mucosa than wild-type B6129 littermates. Cav1-null mice showed enhanced infiltration of macrophages and B-cells and secretion of chemokines (RANTES) but had reduced levels of CD25⁺ regulatory T-cells. Cav1-deficient human GC cells (AGS), infected with the CagA-delivery proficient H. pylori strain G27, were more sensitive to CagA-related cytoskeletal stress morphologies (“humming bird”) compared to AGS cells stably transfected with Cav1 (AGS/Cav1). Infection of AGS/Cav1 cells triggered the recruitment of p120 RhoGTPase-activating protein/deleted in liver cancer-1 (p120RhoGAP/DLC1) to Cav1 and counteracted CagA-induced cytoskeletal rearrangements. In human GC cell lines (MKN45, N87) and mouse stomach tissue, H. pylori down-regulated endogenous expression of Cav1 independently of CagA. Mechanistically, H. pylori activated sterol-responsive element-binding protein-1 (SREBP1) to repress transcription of the human Cav1 gene from sterol-responsive elements (SREs) in the proximal Cav1 promoter. These data suggested a protective role of Cav1 against H. pylori-induced inflammation and tissue damage. We propose that H. pylori exploits down-regulation of Cav1 to subvert the host's immune response and to promote signalling of its virulence factors in host cells.

Infection with the bacterium Helicobacter pylori (H. pylori) mainly affects children in the developing countries who are at risk to progress to gastric cancer (GC) as adults after many years of persistent infection, especially with strains which are positive for the oncogenic virulence factor CagA. Eradication of H. pylori by antibiotics is a treatment of choice but may also alter the susceptibility to allergies and other tumor types. Thus, novel diagnostic or prognostic markers are needed which detect early molecular changes in the stomach mucosa during the transition of chronic inflammation to cancer. In our study, we found that the tumor suppressor caveolin-1 (Cav1) is reduced upon infection with H. pylori, and CagA was sufficient but not necessary for this down-regulation. Loss of Cav1 was caused by H. pylori-dependent activation of sterol-responsive element-binding protein-1 (SREBP1), and this event abolished the interaction of Cav1 with p120 RhoGTPase-activating protein/deleted in liver cancer-1 (p120RhoGAP/DLC1), a second bona fide tumor suppressor in gastric tissue. Conclusively, Cav1 and DLC1 may constitute novel molecular markers in the H. pylori-infected gastric mucosa before neoplastic transformation of the epithelium.

Polymorphism in the Helicobacter pylori CagA and VacA toxins and disease

Half of the world's population is infected with Helicobacter pylori and approximately 20% of infected individuals develop overt clinical disease such as ulcers and stomach cancer. Paradoxically, despite its classification as a class I carcinogen, H. pylori has been shown to be protective against development of asthma, allergy, and esophageal disease. Given these conflicting roles for H. pylori, researchers are attempting to define the environmental, host, and pathogen interactions that ultimately result in severe disease in some individuals. From the bacterial perspective, the toxins, CagA and VacA, have each been shown to be polymorphic and to contribute to disease in an allele-dependent manner. Based on the notable advances that have recently been made in the CagA field, herein we review recent studies that have begun to shed light on the role of CagA polymorphism in H. pylori disease. Moreover, we discuss the potential interaction of CagA and VacA as a mediator of gastric disease.

Oligomerization of Clostridium perfringens epsilon toxin is dependent upon caveolins 1 and 2

Evidence from multiple studies suggests that Clostridium perfringens ε-toxin is a pore-forming toxin, assembling into oligomeric complexes in the plasma membrane of sensitive cells. In a previous study, we used gene-trap mutagenesis to identify mammalian factors contributing to toxin activity, including caveolin-2 (CAV2). In this study, we demonstrate the importance of caveolin-2 and its interaction partner, caveolin-1 (CAV1), in ε-toxin-induced cytotoxicity. Using CAV2-specific shRNA in a toxin-sensitive human kidney cell line, ACHN, we confirmed that cells deficient in CAV2 exhibit increased resistance to ε-toxin. Similarly, using CAV1-specific shRNA, we demonstrate that cells deficient in CAV1 also exhibit increased resistance to the toxin. Immunoprecipitation of CAV1 and CAV2 from ε-toxin-treated ACHN cells demonstrated interaction of both CAV1 and -2 with the toxin. Furthermore, blue-native PAGE indicated that the toxin and caveolins were components of a 670 kDa protein complex. Although ε-toxin binding was only slightly perturbed in caveolin-deficient cells, oligomerization of the toxin was dramatically reduced in both CAV1- and CAV2-deficient cells. These results indicate that CAV1 and -2 potentiate ε-toxin induced cytotoxicity by promoting toxin oligomerization – an event which is requisite for pore formation and, by extension, cell death.

The intermediate region of Helicobacter pylori VacA is a determinant of toxin potency in a Jurkat T cell assay

Colonization of the human stomach with Helicobacter pylori is a risk factor for peptic ulceration, noncardia gastric adenocarcinoma, and gastric lymphoma. The secreted VacA toxin is an important H. pylori virulence factor that causes multiple alterations in gastric epithelial cells and T cells. Several families of vacA alleles have been described, and H. pylori strains containing certain vacA types (s1, i1, and m1) are associated with an increased risk of gastric disease, compared to strains containing other vacA types (s2, i2, and m2). Thus far, there has been relatively little study of the role of the VacA intermediate region (i-region) in toxin activity. In this study, we compared the ability of i1 and i2 forms of VacA to cause functional alterations in Jurkat cells. To do this, we manipulated the chromosomal vacA gene in two H. pylori strains to introduce alterations in the region encoding the VacA i-region. We did not detect any differences in the capacity of i1 and i2 forms of VacA to cause vacuolation of RK13 cells. In comparison to i1 forms of VacA, i2 forms of VacA had a diminished capacity to inhibit the activation of nuclear factor of activated T cells (NFAT) and suppress interleukin-2 (IL-2) production. Correspondingly, i2 forms of VacA bound to Jurkat cells less avidly than did i1 forms of VacA. These results indicate that the VacA i-region is an important determinant of VacA effects on human T cell function.

Haemophilus parasuis encodes two functional cytolethal distending toxins: CdtC contains an atypical cholesterol recognition/interaction region

Haemophilus parasuis is the causative agent of Glässer's disease of pigs, a disease associated with fibrinous polyserositis, polyarthritis and meningitis. We report here H. parasuis encodes two copies of cytolethal distending toxins (Cdts), which these two Cdts showed the uniform toxin activity in vitro. We demonstrate that three Cdt peptides can form an active tripartite holotoxin that exhibits maximum cellular toxicity, and CdtA and CdtB form a more active toxin than CdtB and CdtC. Moreover, the cellular toxicity is associated with the binding of Cdt subunits to cells. Further analysis indicates that CdtC subunit contains an atypical cholesterol recognition/interaction amino acid consensus (CRAC) region. The mutation of CRAC site resulted in decreased cell toxicity. Finally, western blot analysis show all the 15 H. parasuis reference strains and 109 clinical isolates expressed CdtB subunit, indicating that Cdt is a conservative putative virulence factor for H. parasuis. This is the first report of the molecular and cellular basis of Cdt host interactions in H. parasuis.

Ceramide and Toll-like receptor 4 are mobilized into membrane rafts in response to Helicobacter pylori infection in gastric epithelial cells

Helicobacter pylori infection is thought to be involved in the development of several gastric diseases. Two H. pylori virulence factors (vacuolating cytotoxin A and cytotoxin-associated gene A) reportedly interact with lipid rafts in gastric epithelial cells. The role of Toll-like receptor (TLR)-mediated signaling in response to H. pylori infection has been investigated extensively in host cells. However, the receptor molecules in lipid rafts that are involved in H. pylori-induced innate sensing have not been well characterized. This study investigated whether lipid rafts play a role in H. pylori-induced ceramide secretion and TLR4 expression and thereby contribute to inflammation in gastric epithelial cells. We observed that both TLR4 and MD-2 mRNA and protein levels were significantly higher in H. pylori-infected AGS cells than in mock-infected cells. Moreover, significantly more TLR4 protein was detected in detergent-resistant membranes extracted from H. pylori-infected AGS cells than in those extracted from mock-infected cells. However, this effect was attenuated by the treatment of cells with cholesterol-usurping agents, suggesting that H. pylori-induced TLR4 signaling is dependent on cholesterol-rich microdomains. Similarly, the level of cellular ceramide was elevated and ceramide was translocated into lipid rafts after H. pylori infection, leading to interleukin-8 (IL-8) production. Using the sphingomyelinase inhibitor imipramine, we observed that H. pylori-induced TLR4 expression was ceramide dependent. These results indicate the mobilization of ceramide and TLR4 into lipid rafts by H. pylori infection in response to inflammation in gastric epithelial cells.

Helicobacter pylori cholesteryl glucosides interfere with host membrane phase and affect type IV secretion system function during infection in AGS cells

Helicobacter pylori infection is an aetiological cause of gastric disorders worldwide. H. pylori has been shown to assimilate and convert host cholesterol into cholesteryl glucosides (CGs) by cholesterol-α-glucosyltransferase encoded by capJ. Here, we show that CapJ-deficient (ΔcapJ) H. pylori resulted in greatly reduced type IV secretion system (TFSS)-associated activities, including the hummingbird phenotype of AGS cells, IL-8 production, CagA translocation/phosphorylation and CagA-mediated signalling events. Complementation of the ΔcapJ mutation with wild type cagJ or by adding CGs-containing lysates or exogenous fluorophore-tagged CGs reversed the mutant phenotypes. We also show that the wild-type but not ΔcapJ H. pylori recruited raft-associated components to sites of bacterial attachment. Fluorescence recovery after photobleaching (FRAP) analysis of AGS cells treated with fluorescence-tagged cholesterol/CGs revealed that there was a higher proportion of CGs associated with immobile fractions. CGs-associated membranes were also more resistant to a cold detergent extraction. Thus, we propose that CGs synthesized by H. pylori around host-pathogen contact sites partition in detergent-resistant membranes (DRMs), alters lateral-phase segregation in membrane and reorganizes membrane architecture. These processes together promote the formation of a functional TFSS and H. pylori infection.

Helicobacter pylori vacuolating toxin A and apoptosis

VacA, the vacuolating cytotoxin A of Helicobacter pylori, induces apoptosis in epithelial cells of the gastic mucosa and in leukocytes. VacA is released by the bacteria as a protein of 88 kDa. At the outer surface of host cells, it binds to the sphingomyelin of lipid rafts. At least partially, binding to the cells is facilitated by different receptor proteins. VacA is internalized by a clathrin-independent mechanism and initially accumulates in GPI-anchored proteins-enriched early endosomal compartments. Together with early endosomes, VacA is distributed inside the cells. Most of the VacA is eventually contained in the membranes of vacuoles. VacA assembles in hexameric oligomers forming an anion channel of low conductivity with a preference for chloride ions. In parallel, a significant fraction of VacA can be transferred from endosomes to mitochondria in a process involving direct endosome-mitochondria juxtaposition. Inside the mitochondria, VacA accumulates in the mitochondrial inner membrane, probably forming similar chloride channels as observed in the vacuoles. Import into mitochondria is mediated by the hydrophobic N-terminus of VacA. Apoptosis is triggered by loss of the mitochondrial membrane potential, recruitment of Bax and Bak, and release of cytochrome c.

Helicobacter pylori exploits a unique repertoire of type IV secretion system components for pilus assembly at the bacteria-host cell interface

Colonization of the human stomach by Helicobacter pylori is an important risk factor for development of gastric cancer. The H. pylori cag pathogenicity island (cag PAI) encodes components of a type IV secretion system (T4SS) that translocates the bacterial oncoprotein CagA into gastric epithelial cells, and CagL is a specialized component of the cag T4SS that binds the host receptor α5β1 integrin. Here, we utilized a mass spectrometry-based approach to reveal co-purification of CagL, CagI (another integrin-binding protein), and CagH (a protein with weak sequence similarity to CagL). These three proteins are encoded by contiguous genes in the cag PAI, and are detectable on the bacterial surface. All three proteins are required for CagA translocation into host cells and H. pylori-induced IL-8 secretion by gastric epithelial cells; however, these proteins are not homologous to components of T4SSs in other bacterial species. Scanning electron microscopy analysis reveals that these proteins are involved in the formation of pili at the interface between H. pylori and gastric epithelial cells. ΔcagI and ΔcagL mutant strains fail to form pili, whereas a ΔcagH mutant strain exhibits a hyperpiliated phenotype and produces pili that are elongated and thickened compared to those of the wild-type strain. This suggests that pilus dimensions are regulated by CagH. A conserved C-terminal hexapeptide motif is present in CagH, CagI, and CagL. Deletion of these motifs results in abrogation of CagA translocation and IL-8 induction, and the C-terminal motifs of CagI and CagL are required for formation of pili. In summary, these results indicate that CagH, CagI, and CagL are components of a T4SS subassembly involved in pilus biogenesis, and highlight the important role played by unique constituents of the H. pylori cag T4SS.

Helicobacter pylori persistently colonizes the stomach in approximately half of the human population. People who are infected with H. pylori strains harboring the cag pathogenicity island (PAI) have an increased risk of developing gastric cancer. The cag PAI encodes a type IV secretion system (T4SS) that is utilized by the bacteria to inject the bacterial oncoprotein CagA into gastric epithelial cells. Related T4SSs found in several other bacteria have been studied in detail, but thus far there has been very little study of the H. pylori cag T4SS. Here, we utilized a mass spectrometry-based approach to reveal co-purification of three constituents of the H. pylori T4SS (CagH, CagI, and CagL) that lack homology to components of T4SSs in other bacterial species. These proteins are essential for CagA translocation into host cells, and scanning electron microscope studies reveal that the proteins are involved in the formation of pili at the bacterial-host cell interface. A conserved C-terminal motif present in CagH, CagI, and CagL is essential for functionality of the T4SS. This study highlights the important role played by unique constituents of the H. pylori cag T4SS, and illustrates the marked variation that exists among bacterial T4SSs.

Cellular vacuolation induced by Clostridium perfringens epsilon-toxin

The epsilon-toxin of Clostridium perfringens forms a heptamer in the membranes of Madin-Darby canine kidney cells, leading to cell death. Here, we report that it caused the vacuolation of Madin-Darby canine kidney cells. The toxin induced vacuolation in a dose-dependent and time-dependent manner. The monomer of the toxin formed oligomers on lipid rafts in membranes of the cells. Methyl-β-cyclodextrin and poly(ethylene glycol) 4000 inhibited the vacuolation. Epsilon-toxin was internalized into the cells. Confocal microscopy revealed that the internalized toxin was transported from early endosomes (early endosome antigen 1 staining) to late endosomes and lysosomes (lysosomal-associated membrane protein 2 staining) and then distributed to the membranes of vacuoles. Furthermore, the vacuolation was inhibited by bafilomycin A1, a V-type ATPase inhibitor, and colchicine and nocodazole, microtubule-depolymerizing agents. The early endosomal marker green fluorescent protein-Rab5 and early endosome antigen 1 did not localize to vacuolar membranes. In contrast, the vacuolar membranes were specifically stained by the late endosomal and lysosomal marker green fluorescent protein-Rab7 and lysosomal-associated membrane protein 2. The vacuoles in the toxin-treated cells were stained with LysoTracker Red DND-99, a marker for late endosomes and lysosomes. A dominant negative mutant of Rab7 prevented the vacuolization, whereas a mutant form of Rab5 was less effective. These results demonstrate, for the first time, that: (a) oligomers of epsilon-toxin formed in lipid rafts are endocytosed; and (b) the vacuoles originating from late endosomes and lysosomes are formed by an oligomer of epsilon-toxin.

Cholesterol depletion reduces entry of Campylobacter jejuni cytolethal distending toxin and attenuates intoxication of host cells

Campylobacter jejuni is a common cause of pediatric diarrhea worldwide. Cytolethal distending toxin, produced by Campylobacter jejuni, is a putative virulence factor that induces cell cycle arrest and apoptosis in eukaryotic cells. Cellular cholesterol, a major component of lipid rafts, has a pivotal role in regulating signaling transduction and protein trafficking as well as pathogen internalization. In this study, we demonstrated that cell intoxication by Campylobacter jejuni cytolethal distending toxin is through the association of cytolethal distending toxin subunits and membrane cholesterol-rich microdomains. Cytolethal distending toxin subunits cofractionated with detergent-resistant membranes, while the distribution reduced upon the depletion of cholesterol, suggesting that cytolethal distending toxin subunits are associated with lipid rafts. The disruption of cholesterol using methyl-β-cyclodextrin not only reduced the binding activity of cytolethal distending toxin subunits on the cell membrane but also impaired their delivery and attenuated toxin-induced cell cycle arrest. Accordingly, cell intoxication by cytolethal distending toxin was restored by cholesterol replenishment. These findings suggest that membrane cholesterol plays a critical role in the Campylobacter jejuni cytolethal distending toxin-induced pathogenesis of host cells.

Mitochondrial ceramide-rich macrodomains functionalize Bax upon irradiation

Background

Evidence indicates that Bax functions as a “lipidic” pore to regulate mitochondrial outer membrane permeabilization (MOMP), the apoptosis commitment step, through unknown membrane elements. Here we show mitochondrial ceramide elevation facilitates MOMP-mediated cytochrome c release in HeLa cells by generating a previously-unrecognized mitochondrial ceramide-rich macrodomain (MCRM), which we visualize and isolate, into which Bax integrates.

Methodology/Principal Findings

MCRMs, virtually non-existent in resting cells, form upon irradiation coupled to ceramide synthase-mediated ceramide elevation, optimizing Bax insertion/oligomerization and MOMP. MCRMs are detected by confocal microscopy in intact HeLa cells and isolated biophysically as a light membrane fraction from HeLa cell lysates. Inhibiting ceramide generation using a well-defined natural ceramide synthase inhibitor, Fumonisin B1, prevented radiation-induced Bax insertion, oligomerization and MOMP. MCRM deconstruction using purified mouse hepatic mitochondria revealed ceramide alone is non-apoptogenic. Rather Bax integrates into MCRMs, oligomerizing therein, conferring 1–2 log enhanced cytochrome c release. Consistent with this mechanism, MCRM Bax isolates as high molecular weight “pore-forming” oligomers, while non-MCRM membrane contains exclusively MOMP-incompatible monomeric Bax.

Conclusions/Significance

Our recent studies in the C. elegans germline indicate that mitochondrial ceramide generation is obligate for radiation-induced apoptosis, although a mechanism for ceramide action was not delineated. Here we demonstrate that ceramide, generated in the mitochondrial outer membrane of mammalian cells upon irradiation, forms a platform into which Bax inserts, oligomerizes and functionalizes as a pore. We posit conceptualization of ceramide as a membrane-based stress calibrator, driving membrane macrodomain organization, which in mitochondria regulates intensity of Bax-induced MOMP, and is pharmacologically tractable in vitro and in vivo.

Helicobacter pylori VacA reduces the cellular expression of STAT3 and pro-survival Bcl-2 family proteins, Bcl-2 and Bcl-XL, leading to apoptosis in gastric epithelial cells

BACKGROUND

Helicobacter pylori vacuolating cytotoxin, VacA, stimulates apoptosis via a mitochondria-dependent pathway. VacA induces apoptosis via activation of the pro-apoptotic B-cell lymphoma (Bcl)-2 family proteins, Bcl-2-associated X protein (Bax) and Bcl-2 homologous antagonist/killer (Bak), while the implication of such pro-survival Bcl-2 family members as Bcl-2 and Bcl-XL in the VacA-induced apoptosis remains unknown. Signal transduction and activator of transcription 3 (STAT3) is a pivotal transcription factor that upregulates Bcl-2 and Bcl-XL.

AIMS

This study was conducted to elicit the implication of STAT3 and pro-survival Bcl-2 and Bcl-XL in the intrinsic apoptosis.

METHODS

Immunoblot and reverse transcriptase real-time polymerase chain reaction (RT-PCR) were employed to assess the cellular expression of STAT3, Bcl-2, and Bcl-XL in response to purified VacA in gastric adenocarcinoma cell lines. VacA-induced apoptosis was quantitated morphologically following knockdown by each specific small interfering RNA (siRNA) or in the presence of pharmacological inhibitors.

RESULTS

VacA reduced STAT3, Bcl-2, and Bcl-XL expression in a dose-dependent manner. Knockdown of STAT3, Bcl-2, and Bcl-XL by siRNA induced apoptosis to a similar extent in the case of sufficient VacA inoculation. The VacA-mediated reduction of STAT3 expression was independent of cellular vacuolization, since a vacuolar-type ATPase inhibitor, bafilomycin A1, did not inhibit VacA-induced reduction of STAT3, Bcl-2, and Bcl-XL expression. Instead, a c-JUN NH2-terminal kinase (JNK) inhibitor, SP600125, restored the VacA-induced reduction of STAT3 expression to the basal level.

CONCLUSIONS

VacA-induced apoptosis may be, in part, implicated in the reduction of STAT3 linking to the downregulation of Bcl-2 and Bcl-XL, in association with JNK activity.

Sphingomyelin is important for the cellular entry and intracellular localization of Helicobacter pylori VacA

Plasma membrane sphingomyelin (SM) binds the Helicobacter pylori vacuolating toxin (VacA) to the surface of epithelial cells. To evaluate the importance of SM for VacA cellular entry, we characterized toxin uptake and trafficking within cells enriched with synthetic variants of SM, whose intracellular trafficking properties are strictly dependent on the acyl chain lengths of their sphingolipid backbones. While toxin binding to the surface of cells was independent of acyl chain length, cells enriched with 12- or 18-carbon acyl chain variants of SM (e.g. C12-SM or C18-SM) were more sensitive to VacA, as indicated by toxin-induced cellular vacuolation, than those enriched with shorter 2- or 6-carbon variants (e.g. C2-SM or C6-SM). In C18-SM-enriched cells, VacA was taken into cells by a previously described Cdc42-dependent pinocytic mechanism, localized initially to GPI-enriched vesicles, and ultimately trafficked to Rab7/Lamp1 compartments. In contrast, within C2-SM-enriched cells, VacA was taken up at a slower rate by a Cdc42-independent mechanism and trafficked to Rab11 compartments. VacA-associated predominantly with detergent-resistant membranes (DRMs) in cells enriched with C18-SM, but predominantly with non-DRMs in C2-SM-enriched cells. These results suggest that SM is required for targeting VacA to membrane rafts important for subsequent Cdc42-dependent pinocytic cellular entry.

Uptake of Helicobacter pylori outer membrane vesicles by gastric epithelial cells

Helicobacter pylori bacteria colonize the human stomach where they stimulate a persistent inflammatory response. H. pylori is considered noninvasive; however, lipopolysaccharide (LPS)-enriched outer membrane vesicles (OMV), continuously shed from the surface of this bacterium, are observed within gastric epithelial cells. The mechanism of vesicle uptake is poorly understood, and this study was undertaken to examine the roles of bacterial VacA cytotoxin and LPS in OMV binding and cholesterol and clathrin-mediated endocytosis in vesicle uptake by gastric epithelial cells. OMV association was examined using a fluorescent membrane dye to label OMV, and a comparison was made between the associations of vesicles from a VacA(+) strain and OMV from a VacA(-) isogenic mutant strain. Within 20 min, essentially all associated OMV were intracellular, and vesicle binding appeared to be facilitated by the presence of VacA cytotoxin. Uptake of vesicles from the VacA(+) strain was inhibited by H. pylori LPS (58% inhibition with 50 μg/ml LPS), while uptake of OMV from the VacA(-) mutant strain was less affected (25% inhibition with 50 μg/ml LPS). Vesicle uptake did not require cholesterol. However, uptake of OMV from the VacA(-) mutant strain was inhibited by a reduction in clathrin-mediated endocytosis (42% with 15 μg/ml chlorpromazine), while uptake of OMV from the VacA(+) strain was less affected (25% inhibition with 15 μg/ml chlorpromazine). We conclude that VacA toxin enhances the association of H. pylori OMV with cells and that the presence of the toxin may allow vesicles to exploit more than one pathway of internalization.

Role of virulence factors and host cell signaling in the recognition of Helicobacter pylori and the generation of immune responses

Helicobacter pylori colonizes a large proportion of the world's population, with infection invariably leading to chronic, lifelong gastritis. While the infection often persists undiagnosed and without causing severe pathology, there are a number of host, bacterial and environmental factors that can influence whether infection provokes a mild inflammatory response or results in significant morbidity. Intriguingly, the most virulent H. pylori strains appear to deliberately induce the epithelial signaling cascades responsible for activating the innate immune system. While the reason for this remains unclear, the resulting adaptive immune responses are largely ineffective in clearing the bacterium once infection has become established and, as a result, inflammation likely causes more damage to the host itself.

Helicobacter pylori exploits cholesterol-rich microdomains for induction of NF-kappaB-dependent responses and peptidoglycan delivery in epithelial cells

Infection with Helicobacter pylori cag pathogenicity island (cagPAI)-positive strains is associated with more destructive tissue damage and an increased risk of severe disease. The cagPAI encodes a type IV secretion system (TFSS) that delivers the bacterial effector molecules CagA and peptidoglycan into the host cell cytoplasm, thereby inducing responses in host cells. It was previously shown that interactions between CagL, present on the TFSS pilus, and host α(5)β(1) integrin molecules were critical for CagA translocation and the induction of cytoskeletal rearrangements in epithelial cells. As the α(5)β(1) integrin is found in cholesterol-rich microdomains (known as lipid rafts), we hypothesized that these domains may also be involved in the induction of proinflammatory responses mediated by NOD1 recognition of H. pylori peptidoglycan. Indeed, not only did methyl-β-cyclodextrin depletion of cholesterol from cultured epithelial cells have a significant effect on the levels of NF-κB and interleukin-8 (IL-8) responses induced by H. pylori bacteria with an intact TFSS (P < 0.05), but it also interfered with TFSS-mediated peptidoglycan delivery to cells. Both of these effects could be restored by cholesterol replenishment of the cells. Furthermore, we demonstrated for the first time the involvement of α(5)β(1) integrin in the induction of proinflammatory responses by H. pylori. Taking the results together, we propose that α(5)β(1) integrin, which is associated with cholesterol-rich microdomains at the host cell surface, is required for NOD1 recognition of peptidoglycan and subsequent induction of NF-κB-dependent responses to H. pylori. These data implicate cholesterol-rich microdomains as a novel platform for TFSS-dependent delivery of bacterial products to cytosolic pathogen recognition molecules.

Reconstitution of Helicobacter pylori VacA toxin from purified components

Helicobacter pylori VacA is a pore-forming toxin that causes multiple alterations in human cells and contributes to the pathogenesis of peptic ulcer disease and gastric cancer. The toxin is secreted by H. pylori as an 88 kDa monomer (p88) consisting of two domains (p33 and p55). While an X-ray crystal structure for p55 exists and p88 oligomers have been visualized by cryo-electron microscopy, a detailed analysis of p33 has been hindered by an inability to purify this domain in an active form. In this study, we expressed and purified a recombinant form of p33 under denaturing conditions and optimized conditions for the refolding of the soluble protein. We show that refolded p33 can be added to purified p55 in trans to cause vacuolation of HeLa cells and inhibition of IL-2 production by Jurkat cells, effects identical to those produced by the p88 toxin from H. pylori. The p33 protein markedly enhances the cell binding properties of p55. Size exclusion chromatography experiments suggest that p33 and p55 assemble into a complex consistent with the size of a p88 monomer. Electron microscopy of these p33/p55 complexes reveals small rod-shaped structures that can convert to oligomeric flower-shaped structures in the presence of detergent. We propose that the oligomerization observed in these experiments mimics the process by which VacA oligomerizes when in contact with membranes of host cells.

Clostridial glucosylating toxins enter cells via clathrin-mediated endocytosis

Clostridium difficile toxin A (TcdA) and toxin B (TcdB), C. sordellii lethal toxin (TcsL) and C. novyi α-toxin (TcnA) are important pathogenicity factors, which represent the family of the clostridial glucosylating toxins (CGTs). Toxin A and B are associated with antibiotic-associated diarrhea and pseudomembraneous colitis. Lethal toxin is involved in toxic shock syndrome after abortion and α-toxin in gas gangrene development. CGTs enter cells via receptor-mediated endocytosis and require an acidified endosome for translocation of the catalytic domain into the cytosol. Here we studied the endocytic processes that mediate cell internalization of the CGTs. Intoxication of cells was monitored by analyzing cell morphology, status of Rac glucosylation in cell lysates and transepithelial resistance of cell monolayers. We found that the intoxication of cultured cells by CGTs was strongly delayed when cells were preincubated with dynasore, a cell-permeable inhibitor of dynamin, or chlorpromazine, an inhibitor of the clathrin-dependent endocytic pathway. Additional evidence about the role of clathrin in the uptake of the prototypical CGT family member toxin B was achieved by expression of a dominant-negative inhibitor of the clathrin-mediated endocytosis (Eps15 DN) or by siRNA against the clathrin heavy chain. Accordingly, cells that expressed dominant-negative caveolin-1 were not protected from toxin B-induced cell rounding. In addition, lipid rafts impairment by exogenous depletion of sphingomyelin did not decelerate intoxication of HeLa cells by CGTs. Taken together, our data indicate that the endocytic uptake of the CGTs involves a dynamin-dependent process that is mainly governed by clathrin.