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

Structural Analysis of Membrane-associated Forms of Helicobacter pylori VacA Toxin

Helicobacter pylori colonizes the stomach in about half of the human population, leading to an increased risk of peptic ulcer disease and gastric cancer. H. pylori secretes an 88 kDa VacA toxin that contributes to pathogenesis. VacA assembles into oligomeric complexes in solution and forms anion-selective channels in cell membranes. Cryo-electron microscopy (cryo-EM) analyses of VacA oligomers in solution provided insights into VacA oligomerization but failed to reveal the structure of the hydrophobic N-terminal region predicted to be a pore-forming domain. In this study, we incubated VacA with liposomes and used single particle cryo-EM to analyze detergent-extracted VacA oligomers. A 3D structure of detergent-solubilized VacA hexamers revealed the presence of six α-helices extending from the center of the oligomers, a feature not observed in previous studies of water-soluble VacA oligomers. Cryo-electron tomography analysis and 2D averages of VacA associated with liposomes confirmed that central regions of the membrane-associated VacA oligomers can insert into the lipid bilayer. However, insertion is heterogenous, with some membrane-associated oligomers appearing only partially inserted and others sitting on top of the bilayer. These studies indicate that VacA undergoes a conformational change when contacting the membrane and reveal an α-helical region positioned to extend into the membrane. Although the reported VacA 3D structure does not represent a selective anion channel, our combined single particle 3D analysis, cryo-electron tomography, and modeling allow us to propose a model for the structural organization of the VacA N-terminus in the context of a hexamer as it inserts into the membrane.

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.

Host cell sensing and restoration of mitochondrial function and metabolism within Helicobacter pylori VacA intoxicated cells

IMPORTANCE

Persistent human gastric infection with Helicobacter pylori is the single most important risk factor for development of gastric malignancy, which is one of the leading causes of cancer-related deaths worldwide. An important virulence factor for Hp colonization and severity of gastric disease is the protein exotoxin VacA, which is secreted by the bacterium and modulates functional properties of gastric cells. VacA acts by damaging mitochondria, which impairs host cell metabolism through impairment of energy production. Here, we demonstrate that intoxicated cells have the capacity to detect VacA-mediated damage, and orchestrate the repair of mitochondrial function, thereby restoring cellular health and vitality. This study provides new insights into cellular recognition and responses to intracellular-acting toxin modulation of host cell function, which could be relevant for the growing list of pathogenic microbes and viruses identified that target mitochondria as part of their virulence strategies.

Cell lipid biology in infections: an overview

Lipids are a big family of molecules with a vast number of functions in the cell membranes, within the cytoplasm, and extracellularly. Lipid droplets (LDs) are the most common storage organelles and are present in almost every tissue type in the body. They also have structural functions serving as building blocks of cellular membranes and may be precursors of other molecules such as hormones, and lipoproteins, and as messengers in signal transduction. Fatty acids (FAs), such as sterol esters and triacylglycerols, are stored in LDs and are used in β-oxidation as fuel for tricarboxylic acid cycle (TCA) and adenosine triphosphate (ATP) generation. FA uptake and entrance in the cytoplasm are mediated by membrane receptors. After a cytoplasmic round of α- and β-oxidation, FAs are guided into the mitochondrial matrix by the L-carnitine shuttle system, where they are fully metabolized, and enter the TCA cycle. Pathogen infections may lead to impaired lipid metabolism, usage of membrane phospholipids, and LD accumulation in the cytoplasm of infected cells. Otherwise, bacterial pathogens may use lipid metabolism as a carbon source, thus altering the reactions and leading to cellular and organelles malfunctioning. This review aims to describe cellular lipid metabolism and alterations that occur upon infections.

Restoration of mitochondrial structure and function within Helicobacter pylori VacA intoxicated cells

The Helicobacter pylori vacuolating cytotoxin (VacA) is an intracellular, mitochondrial-targeting exotoxin that rapidly causes mitochondrial dysfunction and fragmentation. Although VacA targeting of mitochondria has been reported to alter overall cellular metabolism, there is little known about the consequences of extended exposure to the toxin. Here, we describe studies to address this gap in knowledge, which have revealed that mitochondrial dysfunction and fragmentation are followed by a time-dependent recovery of mitochondrial structure, mitochondrial transmembrane potential, and cellular ATP levels. Cells exposed to VacA also initially demonstrated a reduction in oxidative phosphorylation, as well as increase in compensatory aerobic glycolysis. These metabolic alterations were reversed in cells with limited toxin exposure, congruent with the recovery of mitochondrial transmembrane potential and the absence of cytochrome c release from the mitochondria. Taken together, these results are consistent with a model that mitochondrial structure and function are restored in VacA-intoxicated cells.

The CPNE Family and Their Role in Cancers

Lung cancer is the leading cause of cancer-related deaths worldwide. Despite significant advances in cancer research and treatment, the overall prognosis of lung cancer patients remains poor. Therefore, the identification for novel therapeutic targets is critical for the diagnosis and treatment of lung cancer. CPNEs (copines) are a family of membrane-bound proteins that are highly conserved, soluble, ubiquitous, calcium dependent in a variety of eukaryotes. Emerging evidences have also indicated CPNE family members are involved in cancer development and progression as well. However, the expression patterns and clinical roles in cancer have not yet been well understood. In this review, we summarize recent advances concerning CPNE family members and provide insights into new potential mechanism involved in cancer development.

Chronic in vivo exposure to Helicobacter pylori VacA: Assessing the efficacy of automated and long-term intragastric toxin infusion

Helicobacter pylori (Hp) secrete VacA, a diffusible pore-forming exotoxin that is epidemiologically linked to gastric disease in humans. In vitro studies indicate that VacA modulates gastric epithelial and immune cells, but the in vivo contributions of VacA as an important determinant of Hp colonization and chronic infection remain poorly understood. To identify perturbations in the stomachs of C57BL/6 or BALB/C mice that result specifically from extended VacA exposure, we evaluated the efficacy of administering purified toxin using automated infusion via surgically-implanted, intragastric catheters. At 3 and 30 days of interrupted infusion, VacA was detected in association with gastric glands. In contrast to previously-reported tissue damage resulting from short term exposure to Hp extracts administered by oral gavage, extended infusion of VacA did not damage stomach, esophageal, intestinal, or liver tissue. However, several alterations previously reported during Hp infection were detected in animals infused with VacA, including reduction of the gastric mucus layer, and increased vacuolation of parietal cells. VacA infusion invoked an immune response, as indicated by the detection of circulating VacA antibodies. These foundational studies support the use of VacA infusion for identifying gastric alterations that are unambiguously attributable to long-term exposure to toxin.

Functional Properties of Helicobacter pylori VacA Toxin m1 and m2 Variants

Helicobacter pylori colonizes the gastric mucosa and secretes a pore-forming toxin (VacA). Two main types of VacA, m1 and m2, can be distinguished by phylogenetic analysis. Type m1 forms of VacA have been extensively studied, but there has been relatively little study of m2 forms. In this study, we generated H. pylori strains producing chimeric proteins in which VacA m1 segments of a parental strain were replaced by corresponding m2 sequences. In comparison to the parental m1 VacA protein, a chimeric protein (designated m2/m1) containing m2 sequences in the N-terminal portion of the m region was less potent in causing vacuolation of HeLa cells, AGS gastric cells, and AZ-521 duodenal cells and had reduced capacity to cause membrane depolarization or death of AZ-521 cells. Consistent with the observed differences in activity, the chimeric m2/m1 VacA protein bound to cells at reduced levels compared to the binding levels of the parental m1 protein. The presence of two strain-specific insertions or deletions within or adjacent to the m region did not influence toxin activity. Experiments with human gastric organoids grown as monolayers indicated that m1 and m2/m1 forms of VacA had similar cell-vacuolating activities. Interestingly, both forms of VacA bound preferentially to the basolateral surface of organoid monolayers and caused increased cell vacuolation when interacting with the basolateral surface compared to the apical surface. These data provide insights into functional correlates of sequence variation in the VacA midregion (m region).

Activity and Functional Importance of Helicobacter pylori Virulence Factors

Helicobacter pylori is a very successful Gram-negative pathogen colonizing the stomach of humans worldwide. Infections with this bacterium can generate pathologies ranging from chronic gastritis and peptic ulceration to gastric cancer. The best characterized H. pylori virulence factors that cause direct cell damage include an effector protein encoded by the cytotoxin-associated gene A (CagA), a type IV secretion system (T4SS) encoded in the cag-pathogenicity island (cag PAI), vacuolating cytotoxin A (VacA), γ-glutamyl transpeptidase (GGT), high temperature requirement A (HtrA, a serine protease) and cholesterol glycosyl-transferase (CGT). Since these H. pylori factors are either surface-exposed, secreted or translocated, they can directly interact with host cell molecules and are able to hijack cellular functions. Studies on these bacterial factors have progressed substantially in recent years. Here, we review the current status in the characterization of signaling cascades by these factors in vivo and in vitro, which comprise the disruption of cell-to-cell junctions, induction of membrane rearrangements, cytoskeletal dynamics, proliferative, pro-inflammatory, as well as, pro-apoptotic and anti-apoptotic responses or immune evasion. The impact of these signal transduction modules in the pathogenesis of H. pylori infections is discussed.

A Comprehensive Review on the Manipulation of the Sphingolipid Pathway by Pathogenic Bacteria

Bacterial pathogens have developed many different strategies to hijack host cell responses to promote their own survival. The manipulation of lipid biogenesis and cell membrane stability is emerging as a key player in bacterial host cell control. Indeed, many bacterial pathogens such as Legionella, Pseudomonas, Neisseria, Staphylococci, Mycobacteria, Helicobacter, or Clostridia are able to manipulate and use host sphingolipids during multiple steps of the infectious process. Sphingolipids have long been considered only as structural components of cell membranes, however, it is now well known that they are also intracellular and intercellular signaling molecules that play important roles in many eukaryotic cell functions as well as in orchestrating immune responses. Furthermore, they are important to eliminate invading pathogens and play a crucial role in infectious diseases. In this review, we focus on the different strategies employed by pathogenic bacteria to hijack the sphingolipid balance in the host cell to promote cellular colonization.

Molecular evolution of the VacA p55 binding domain of Helicobacter pylori in mestizos from a high gastric cancer region of Colombia

The stomach bacterium Helicobacter pylori is one of the most prevalent pathogens in humans, closely linked with serious diseases such as gastric cancer. The microbe has been associated with its host for more than 100,000 years and escorted modern humans out of Africa. H. pylori is predominantly transmitted within families and dispersed globally, resulting in distinct phylogeographic patterns, which can be utilized to investigate migrations and bioturbation events in human history. Latin America was affected by several human migratory waves due to the Spanish colonisation that drastically changed the genetic load and composition of the bacteria and its host. Genetic evidence indicates that independent evolutionary lines of H. pylori have evolved in mestizos from Colombia and other countries in the region during more than 500 years since colonisation. The vacuolating cytotoxin VacA represents a major virulence factor of the pathogen comprising two domains, p33 and p55, the latter of which is essential for binding to the host epithelial cell. The evolution of the VacA toxin in Colombia has been strongly biased due to the effects of Spanish colonization. However, the variation patterns and microevolution of the p55 domain have not yet been described for this population. In the present study, we determined the genetic polymorphisms and deviations in the neutral model of molecular evolution in the p55 domain of 101 clinical H. pylori isolates collected in Bogotá, a city located in Andean mountains characterized by its high gastric cancer risk and its dominant mestizo population. The microevolutionary patterns of the p55 domain were shaped by recombination, purifying and episodic diversifying positive selection. Furthermore, amino acid positions 261 and 321 in the p55 domain of VacA show a high variability among mestizos clinical subsets, suggesting that natural selection in H. pylori may operate differentially in patients with different gastric diseases.

Carbohydrate-Dependent and Antimicrobial Peptide Defence Mechanisms Against Helicobacter pylori Infections

The human stomach is a harsh and fluctuating environment for bacteria with hazards such as gastric acid and flow through of gastric contents into the intestine. H. pylori gains admission to a stable niche with nutrient access from exudates when attached to the epithelial cells under the mucus layer, whereof adherence to glycolipids and other factors provides stable and intimate attachment. To reach this niche, H. pylori must overcome mucosal defence mechanisms including the continuously secreted mucus layer, which provides several layers of defence: (1) mucins in the mucus layer can bind H. pylori and transport it away from the gastric niche with the gastric emptying, (2) mucins can inhibit H. pylori growth, both via glycans that can have antibiotic like function and via an aggregation-dependent mechanism, (3) antimicrobial peptides (AMPs) have antimicrobial activity and are retained in a strategic position in the mucus layer and (4) underneath the mucus layer, the membrane-bound mucins provide a second barrier, and can function as releasable decoys. Many of these functions are dependent on H. pylori interactions with host glycan structures, and both the host glycosylation and concentration of antimicrobial peptides change with infection and inflammation, making these interactions dynamic. Here, we review our current understanding of mucin glycan and antimicrobial peptide-dependent host defence mechanisms against H. pylori infection.

Helicobacter pylori Infection Modulates Host Cell Metabolism through VacA-Dependent Inhibition of mTORC1

Helicobacter pylori (Hp) vacuolating cytotoxin (VacA) is a bacterial exotoxin that enters host cells and induces mitochondrial dysfunction. However, the extent to which VacA-dependent mitochondrial perturbations affect overall cellular metabolism is poorly understood. We report that VacA perturbations in mitochondria are linked to alterations in cellular amino acid homeostasis, which results in the inhibition of mammalian target of rapamycin complex 1 (mTORC1) and subsequent autophagy. mTORC1, which regulates cellular metabolism during nutrient stress, is inhibited during Hp infection by a VacA-dependent mechanism. This VacA-dependent inhibition of mTORC1 signaling is linked to the dissociation of mTORC1 from the lysosomal surface and results in activation of cellular autophagy through the Unc 51-like kinase 1 (Ulk1) complex. VacA intoxication results in reduced cellular amino acids, and bolstering amino acid pools prevents VacA-mediated mTORC1 inhibition. Overall, these studies support a model that Hp modulate host cell metabolism through the action of VacA at mitochondria.

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.

Helicobacter pylori VacA, acting through receptor protein tyrosine phosphatase α, is crucial for CagA phosphorylation in human duodenum carcinoma cell line AZ-521

Helicobacter pylori, a major cause of gastroduodenal diseases, produces vacuolating cytotoxin (VacA) and cytotoxin-associated gene A (CagA), which seem to be involved in virulence. VacA exhibits pleiotropic actions in gastroduodenal disorders via its specific receptors. Recently, we found that VacA induced the phosphorylation of cellular Src kinase (Src) at Tyr418 in AZ-521 cells. Silencing of receptor protein tyrosine phosphatase (RPTP)α, a VacA receptor, reduced VacA-induced Src phosphorylation. Src is responsible for tyrosine phosphorylation of CagA at its Glu-Pro-Ile-Tyr-Ala (EPIYA) variant C (EPIYA-C) motif in Helicobacterpylori-infected gastric epithelial cells, resulting in binding of CagA to SHP-2 phosphatase. Challenging AZ-521 cells with wild-type H. pylori induced phosphorylation of CagA, but this did not occur when challenged with a vacA gene-disrupted mutant strain. CagA phosphorylation was observed in cells infected with a vacA gene-disrupted mutant strain after addition of purified VacA, suggesting that VacA is required for H. pylori-induced CagA phosphorylation. Following siRNA-mediated RPTPα knockdown in AZ-521 cells, infection with wild-type H. pylori and treatment with VacA did not induce CagA phosphorylation. Taken together, these results support our conclusion that VacA mediates CagA phosphorylation through RPTPα in AZ-521 cells. These data indicate the possibility that Src phosphorylation induced by VacA is mediated through RPTPα, resulting in activation of Src, leading to CagA phosphorylation at Tyr972 in AZ-521 cells.

Summary: The authors show a newly identified role of VacA in Helicobacter pylori infection through induction of tyrosine phosphorylation of CagA acting through the VacA receptor RPTPα.

Helicobacter pylori targets mitochondrial import and components of mitochondrial DNA replication machinery through an alternative VacA-dependent and a VacA-independent mechanisms

Targeting mitochondria is a powerful strategy for pathogens to subvert cell physiology and establish infection. Helicobacter pylori is a bacterial pathogen associated with gastric cancer development that is known to target mitochondria directly and exclusively through its pro-apoptotic and vacuolating cytotoxin VacA. By in vitro infection of gastric epithelial cells with wild-type and VacA-deficient H. pylori strains, treatment of cells with purified VacA proteins and infection of a mouse model, we show that H. pylori deregulates mitochondria by two novel mechanisms, both rather associated with host cell survival. First, early upon infection VacA induces transient increase of mitochondrial translocases and a dramatic accumulation of the mitochondrial DNA replication and maintenance factors POLG and TFAM. These events occur when VacA is not detected intracellularly, therefore do not require the direct interaction of the cytotoxin with the organelle, and are independent of the toxin vacuolating activity. In vivo, these alterations coincide with the evolution of gastric lesions towards severity. Second, H. pylori also induces VacA-independent alteration of mitochondrial replication and import components, suggesting the involvement of additional H. pylori activities in mitochondria-mediated effects. These data unveil two novel mitochondrial effectors in H. pylori-host interaction with links on gastric pathogenesis.

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.

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.

Relationship between VacA Toxin and Host Cell Autophagy in Helicobacter pylori Infection of the Human Stomach: A Few Answers, Many Questions

Helicobacter pylori is a Gram-negative bacterium that colonizes the stomach of about half the global population and represents the greatest risk factor for gastric malignancy. The relevance of H. pylori for gastric cancer development is equivalent to that of tobacco smoking for lung cancer. VacA toxin seems to play a pivotal role in the overall strategy of H. pylori towards achieving persistent gastric colonization. This strategy appears to involve the modulation of host cell autophagy. After an overview of autophagy and its role in infection and carcinogenesis, I critically review current knowledge about the action of VacA on host cell autophagy during H. pylori infection of the human stomach. Although VacA is a key player in modulation of H. pylori-induced autophagy, a few discrepancies in the data are also evident and many questions remain to be answered. We are thus still far from a definitive understanding of the molecular mechanisms through which VacA affects autophagy and the consequences of this toxin action on the overall pathogenic activity of H. pylori.

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.

Vacuolating cytotoxin A (VacA) - A multi-talented pore-forming toxin from Helicobacter pylori

Helicobacter pylori is associated with severe and chronic diseases of the stomach and duodenum such as peptic ulcer, non-cardial adenocarcinoma and gastric lymphoma, making Helicobacter pylori the only bacterial pathogen which is known to cause cancer. The worldwide rate of incidence for these diseases is extremely high and it is estimated that about half of the world's population is infected with H. pylori. Among the bacterial virulence factors is the vacuolating cytotoxin A (VacA), which represents an important determinant of pathogenicity. Intensive characterization of VacA over the past years has provided insight into an ample variety of mechanisms contributing to host-pathogen interactions. The toxin is considered as an important target for ongoing research for several reasons: i) VacA displays unique features and structural properties and its mechanism of action is unrelated to any other known bacterial toxin; ii) the toxin is involved in disease progress and colonization by H. pylori of the stomach; iii) VacA is a potential and promising candidate for the inclusion as antigen in a vaccine directed against H. pylori and iv) the vacA gene is characterized by a high allelic diversity, and allelic variants contribute differently to the pathogenicity of H. pylori. Despite the accumulation of substantial data related to VacA over the past years, several aspects of VacA-related activity have been characterized only to a limited extent. The biologically most significant effect of VacA activity on host cells is the formation of membrane pores and the induction of vacuole formation. This review discusses recent findings and advances on structure-function relations of the H. pylori VacA toxin, in particular with a view to membrane channel formation, oligomerization, receptor binding and apoptosis.

Co-development of diagnostic vectors to support targeted therapies and theranostics: essential tools in personalized cancer therapy

Novel technologies are being developed to improve patient therapy through the identification of targets and surrogate molecular signatures that can help direct appropriate treatment regimens for efficacy and drug safety. This is particularly the case in oncology whereby patient tumor and biofluids are routinely isolated and analyzed for genetic, immunohistochemical, and/or soluble markers to determine if a predictive biomarker signature (i.e., mutated gene product, differentially expressed protein, altered cell surface antigen, etc.) exists as a means for selecting optimal treatment. These biomarkers may be drug-specific targets and/or differentially expressed nucleic acids, proteins, or cell lineage profiles that can directly affect the patient’s disease tissue or immune response to a therapeutic regimen. Improvements in diagnostics that can prescreen predictive response biomarker profiles will continue to optimize the ability to enhance patient therapy via molecularly defined disease-specific treatment. Conversely, patients lacking predictive response biomarkers will no longer needlessly be exposed to drugs that are unlikely to provide clinical benefit, thereby enabling patients to pursue other therapeutic options and lowering overall healthcare costs by avoiding futile treatment. While patient molecular profiling offers a powerful tool to direct treatment options, the difficulty in identifying disease-specific targets or predictive biomarker signatures that stratify a significant fraction within a disease indication remains challenging. A goal for drug developers is to identify and implement new strategies that can rapidly enable the development of beneficial disease-specific therapies for broad patient-specific targeting without the need of tedious predictive biomarker discovery and validation efforts, currently a bottleneck for development timelines. Successful strategies may gain an advantage by employing repurposed, less-expensive existing agents while potentially improving the therapeutic activity of novel, target-specific therapies that may otherwise have off-target toxicities or less efficacy in cells exhibiting certain pathways. Here, we discuss the use of co-developing diagnostic-targeting vectors to identify patients whose malignant tissue can specifically uptake a targeted anti-cancer drug vector prior to treatment. Using this system, a patient can be predetermined in real-time as to whether or not their tumor(s) can specifically uptake a drug-linked diagnostic vector, thus inferring the uptake of a similar vector linked to an anti-cancer agent. If tumor-specific uptake is observed, then the patient may be suitable for drug-linked vector therapy and have a higher likelihood of clinical benefit while patients with no tumor uptake should consider other therapeutic options. This approach offers complementary opportunities to rapidly develop broad tumor-specific agents for use in personalized medicine.

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.

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.

Lipid sorting by ceramide structure from plasma membrane to ER for the cholera toxin receptor ganglioside GM1

The glycosphingolipid GM1 binds cholera toxin (CT) on host cells and carries it retrograde from the plasma membrane (PM) through endosomes, the trans-Golgi (TGN), and the endoplasmic reticulum (ER) to induce toxicity. To elucidate how a membrane lipid can specify trafficking in these pathways, we synthesized GM1 isoforms with alternate ceramide domains and imaged their trafficking in live cells. Only GM1 with unsaturated acyl chains sorted efficiently from PM to TGN and ER. Toxin binding, which effectively crosslinks GM1 lipids, was dispensable, but membrane cholesterol and the lipid raft-associated proteins actin and flotillin were required. The results implicate a protein-dependent mechanism of lipid sorting by ceramide structure and provide a molecular explanation for the diversity and specificity of retrograde trafficking by CT in host cells.

Remodeling the host environment: modulation of the gastric epithelium by the Helicobacter pylori vacuolating toxin (VacA)

Virulence mechanisms underlying Helicobacter pylori persistence and disease remain poorly understood, in part, because the factors underlying disease risk are multifactorial and complex. Among the bacterial factors that contribute to the cumulative pathophysiology associated with H. pylori infections, the vacuolating cytotoxin (VacA) is one of the most important. Analogous to a number of H. pylori genes, the vacA gene exhibits allelic mosaicism, and human epidemiological studies have revealed that several families of toxin alleles are predictive of more severe disease. Animal model studies suggest that VacA may contribute to pathogenesis in several ways. VacA functions as an intracellular-acting protein exotoxin. However, VacA does not fit the current prototype of AB intracellular-acting bacterial toxins, which elaborate modulatory effects through the action of an enzymatic domain translocated inside host cells. Rather, VacA may represent an alternative prototype for AB intracellular acting toxins that modulate cellular homeostasis by forming ion-conducting intracellular membrane channels. Although VacA seems to form channels in several different membranes, one of the most important target sites is the mitochondrial inner membrane. VacA apparently take advantage of an unusual intracellular trafficking pathway to mitochondria, where the toxin is imported and depolarizes the inner membrane to disrupt mitochondrial dynamics and cellular energy homeostasis as a mechanism for engaging the apoptotic machinery within host cells. VacA remodeling of the gastric environment appears to be fine-tuned through the action of the Type IV effector protein CagA which, in part, limits the cytotoxic effects of VacA in cells colonized by H. pylori.

Intoxication strategy of Helicobacter pylori VacA toxin

VacA toxin from the cancer-inducing bacterium Helicobacter pylori is currently classified as a pore-forming toxin but is also considered a multifunctional toxin, apparently causing many pleiotropic cell effects. However, an increasing body of evidence suggests that VacA could be the prototype of a new class of monofunctional A-B toxins in which the A subunit exhibits pore-forming instead of enzymatic activity. Thus, VacA may use a peculiar mechanism of action, allowing it to intoxicate the human stomach. By combining the action of a cell-binding domain, a specific intracellular trafficking pathway and a novel mitochondrion-targeting sequence, the VacA pore-forming domain is selectively delivered to the inner mitochondrial membrane to efficiently kill target epithelial cells by apoptosis.

Recent advances in Helicobacter pylori pathogenesis

PURPOSE OF REVIEW

Helicobacter pylori is implicated in numerous gastric pathologies; however, the prevalence of infection is declining in developed countries. Therefore, it is important to understand the complex mechanism of its interaction with the host and how the changing epidemiology of infection may impact on disease. In this review, we systemically revisit the major novel discoveries of the last year relating to H. pylori disease pathogenesis.

RECENT FINDINGS

Novel pathways have been implicated in H. pylori cytotoxin-associated gene (CagA) mediated carcinogenesis, highlighting the aberrant regulation of proliferation and apoptosis. Furthermore, the human microbiome was implicated as having a key role in H. pylori-related disease development. Several studies have begun to delineate the mechanisms behind the epidemiologically inverse correlation of H. pylori infection with asthma and inflammatory bowel disease.

SUMMARY

The recent findings enable researchers to focus on novel and previously unsuspected mechanisms in the development of disease, and prompt further research into possible therapeutic approaches. The potential beneficial aspects of H. pylori colonization and the role bacterial flora play in promoting disease have yet to be elucidated, but promise to have a great impact on patient care.

Alzheimer's Disease: APP, Gamma Secretase, APOE, CLU, CR1, PICALM, ABCA7, BIN1, CD2AP, CD33, EPHA1, and MS4A2, and Their Relationships with Herpes Simplex, C. Pneumoniae, Other Suspect Pathogens, and the Immune System

Alzheimer's disease susceptibility genes, APP and gamma-secretase, are involved in the herpes simplex life cycle, and that of other suspect pathogens (C. pneumoniae, H. pylori, C. neoformans, B. burgdorferri, P. gingivalis) or immune defence. Such pathogens promote beta-amyloid deposition and tau phosphorylation and may thus be causative agents, whose effects are conditioned by genes. The antimicrobial effects of beta-amyloid, the localisation of APP/gamma-secretase in immunocompetent dendritic cells, and gamma secretase cleavage of numerous pathogen receptors suggest that this network is concerned with pathogen disposal, effects which may be abrogated by the presence of beta-amyloid autoantibodies in the elderly. These autoantibodies, as well as those to nerve growth factor and tau, also observed in Alzheimer's disease, may well be antibodies to pathogens, due to homology between human autoantigens and pathogen proteins. NGF or tau antibodies promote beta-amyloid deposition, neurofibrillary tangles, or cholinergic neuronal loss, and, with other autoantibodies, such as anti-ATPase, are potential agents of destruction, whose formation is dictated by sequence homology between pathogen and human proteins, and thus by pathogen strain and human genes. Pathogen elimination in the ageing population and removal of culpable autoantibodies might reduce the incidence and offer hope for a cure in this affliction.

Membrane interaction of Pasteurella multocida toxin involves sphingomyelin

Pasteurella multocida toxin (PMT) is an AB toxin that causes pleiotropic effects in targeted host cells. The N-terminus of PMT (PMT-N) is considered to harbor the membrane receptor binding and translocation domains responsible for mediating cellular entry and delivery of the C-terminal catalytic domain into the host cytosol. Previous studies have implicated gangliosides as the host receptors for PMT binding. To gain further insight into the binding interactions involved in PMT binding to cell membranes, we explored the role of various membrane components in PMT binding, utilizing four different approaches: (a) TLC-overlay binding experiments with (125) I-labeled PMT, PMT-N or the C-terminus of PMT; (b) pull-down experiments using reconstituted membrane liposomes with full-length PMT; (c) surface plasmon resonance analysis of PMT-N binding to reconstituted membrane liposomes; (d) and surface plasmon resonance analysis of PMT-N binding to HEK-293T cell membranes without or with sphingomyelinase, phospholipase D or trypsin treatment. The results obtained revealed that, in our experimental system, full-length PMT and PMT-N did not bind to gangliosides, including monoasialogangliosides GM(1) , GM(2) or GM(3) , but instead bound to membrane phospholipids, primarily the abundant sphingophospholipid sphingomyelin or phosphatidylcholine with other lipid components. Collectively, these studies demonstrate the importance of sphingomyelin for PMT binding to membranes and suggest the involvement of a protein co-receptor.

Helicobacter pylori vacuolating cytotoxin A (VacA) engages the mitochondrial fission machinery to induce host cell death

A number of pathogenic bacteria target mitochondria to modulate the host's apoptotic machinery. Studies here revealed that infection with the human gastric pathogen Helicobacter pylori disrupts the morphological dynamics of mitochondria as a mechanism to induce host cell death. The vacuolating cytotoxin A (VacA) is both essential and sufficient for inducing mitochondrial network fragmentation through the mitochondrial recruitment and activation of dynamin-related protein 1 (Drp1), which is a critical regulator of mitochondrial fission within cells. Inhibition of Drp1-induced mitochondrial fission within VacA-intoxicated cells inhibited the activation of the proapoptotic Bcl-2-associated X (Bax) protein, permeabilization of the mitochondrial outer membrane, and cell death. Our data reveal a heretofore unrecognized strategy by which a pathogenic microbe engages the host's apoptotic machinery.

Molecular cross-talk between Helicobacter pylori and human gastric mucosa

Helicobacter pylori (H. pylori) has co-evolved with humans to be transmitted from person to person and to colonize the stomach persistently. A well-choreographed equilibrium between the bacterial effectors and host responses permits microbial persistence and health of the host, but confers a risk for serious diseases including gastric cancer. During its long coexistence with humans, H. pylori has developed complex strategies to limit the degree and extent of gastric mucosal damage and inflammation, as well as immune effector activity. The present editorial thus aims to introduce and comment on major advances in the rapidly developing area of H. pylori/human gastric mucosa interaction (and its pathological sequelae), which is the result of millennia of co-evolution of, and thus of reciprocal knowledge between, the pathogen and its human host.

A Tale of Two Toxins: Helicobacter Pylori CagA and VacA Modulate Host Pathways that Impact Disease

Helicobacter pylori is a pathogenic bacterium that colonizes more than 50% of the world's population, which leads to a tremendous medical burden. H. pylori infection is associated with such varied diseases as gastritis, peptic ulcers, and two forms of gastric cancer: gastric adenocarcinoma and mucosa-associated lymphoid tissue lymphoma. This association represents a novel paradigm for cancer development; H. pylori is currently the only bacterium to be recognized as a carcinogen. Therefore, a significant amount of research has been conducted to identify the bacterial factors and the deregulated host cell pathways that are responsible for the progression to more severe disease states. Two of the virulence factors that have been implicated in this process are cytotoxin-associated gene A (CagA) and vacuolating cytotoxin A (VacA), which are cytotoxins that are injected and secreted by H. pylori, respectively. Both of these virulence factors are polymorphic and affect a multitude of host cellular pathways. These combined facts could easily contribute to differences in disease severity across the population as various CagA and VacA alleles differentially target some pathways. Herein we highlight the diverse types of cellular pathways and processes targeted by these important toxins.