Interaction of Helicobacter pylori with host cells: function of secreted and translocated molecules

Prediction of extracellular proteases of the human pathogen Helicobacter pylori reveals proteolytic activity of the Hp1018/19 protein HtrA

Exported proteases of Helicobacter pylori (H. pylori) are potentially involved in pathogen-associated disorders leading to gastric inflammation and neoplasia. By comprehensive sequence screening of the H. pylori proteome for predicted secreted proteases, we retrieved several candidate genes. We detected caseinolytic activities of several such proteases, which are released independently from the H. pylori type IV secretion system encoded by the cag pathogenicity island (cagPAI). Among these, we found the predicted serine protease HtrA (Hp1019), which was previously identified in the bacterial secretome of H. pylori. Importantly, we further found that the H. pylori genes hp1018 and hp1019 represent a single gene likely coding for an exported protein. Here, we directly verified proteolytic activity of HtrA in vitro and identified the HtrA protease in zymograms by mass spectrometry. Overexpressed and purified HtrA exhibited pronounced proteolytic activity, which is inactivated after mutation of Ser205 to alanine in the predicted active center of HtrA. These data demonstrate that H. pylori secretes HtrA as an active protease, which might represent a novel candidate target for therapeutic intervention strategies.

Regulation of cell growth during serum starvation and bacterial survival in macrophages by the bifunctional enzyme SpoT in Helicobacter pylori

In Helicobacter pylori the stringent response is mediated solely by spoT. The spoT gene is known to encode (p)ppGpp synthetase activity and is required for H. pylori survival in the stationary phase. However, neither the hydrolase activity of the H. pylori SpoT protein nor the role of SpoT in the regulation of growth during serum starvation and intracellular survival of H. pylori in macrophages has been determined. In this study, we examined the effects of SpoT on these factors. Our results showed that the H. pylori spoT gene encodes a bifunctional enzyme with both a hydrolase activity and the previously described (p)ppGpp synthetase activity, as determined by introducing the gene into Escherichia coli relA and spoT defective strains. Also, we found that SpoT mediates a serum starvation response, which not only restricts the growth but also maintains the helical morphology of H. pylori. Strikingly, a spoT null mutant was able to grow to a higher density in serum-free medium than the wild-type strain, mimicking the "relaxed" growth phenotype of an E. coli relA mutant during amino acid starvation. Finally, SpoT was found to be important for intracellular survival in macrophages during phagocytosis. The unique role of (p)ppGpp in cell growth during serum starvation, in the stress response, and in the persistence of H. pylori is discussed.

Targeting focal adhesions:Helicobacter pylori-host communication in cell migration

Highly dynamic integrin-based focal adhesions provide an important structural basis for anchoring the cellular actin cytoskeleton to the surrounding extracellular matrix. The human pathogen Helicobacter pylori (H. pylori) directly targets integrins with drastic consequences on the epithelial cell morphology and migration, which might contribute to the disruption of the gastric epithelium in vivo. In this review, we summarize the recent findings concerning the complex mechanism through which H. pylori interferes with host integrin signaling thereby deregulating focal adhesions and the actin cytoskeleton of motile epithelial cells.

Molecular mechanisms of epithelial-barrier disruption by Helicobacter pylori

Intact intercellular junctions and cell-matrix contacts are important structures in the formation and maintenance of epithelial-barrier functions against microbes. The human gastric pathogen Helicobacter pylori developed a remarkable network of strategies to alter these epithelial cell-cell and cell-matrix adhesions, which are implicated in inflammation, proliferation, cell migration and invasive growth. This review focuses on recent findings on H. pylori-induced host-cell signaling. We propose a stepwise model for how H. pylori interacts with components of focal adhesions and intercellular tight and adherens junctions to disrupt the epithelial layer, providing novel insights into the pathogenesis of H. pylori.

Inactivation of the type IV secretion system reduces the Th1 polarization of the immune response to Brucella abortus infection

The Brucella abortus type IV secretion system (T4SS), encoded by the virB operon, is essential for establishing persistent infection in the murine reticuloendothelial system. To gain insight into the in vivo interactions mediated by the T4SS, we compared host responses elicited by B. abortus with those of an isogenic mutant in the virB operon. Mice infected with the B. abortus virB mutant elicited smaller increases in serum levels of immunoglobulin G2a, gamma interferon (IFN-gamma), and interleukin-12p40 than did mice infected with wild-type B. abortus. Despite equal bacterial loads in the spleen, at 3 to 4 days postinfection, levels of IFN-gamma were higher in mice infected with wild-type B. abortus than in mice infected with the virB mutant, as shown by real-time PCR, intracellular cytokine staining, and cytokine levels. IFN-gamma-producing CD4(+) T cells were more abundant in spleens of mice infected with wild-type B. abortus than in virB mutant-infected mice. Similar numbers of IFN-gamma-secreting CD8(+) T cells were observed in the spleens of mice infected with B. abortus 2308 or a virB mutant. These results suggest that early differences in cytokine responses contribute to a stronger Th1 polarization of the immune response in mice infected with wild-type B. abortus than in mice infected with the virB mutant.

The Helicobacter pylori's protein VacA has direct effects on the regulation of cell cycle and apoptosis in gastric epithelial cells

In this study, we have evaluated the effects on cell cycle regulation of VacA alone and in combination with other two Helicobacter pylori proteins, cytotoxin-associated protein (CagA) and HspB, using the human gastric epithelial cells (AGS). Our results indicate that VacA alone was able to inhibit the G1 to S progression of the cell cycle. The VacA capacity of inhibiting cell progression from G1 to S phase was also observed when cells were co-transfected with CagA or HspB. Moreover, VacA over-expression caused apoptosis in AGS cells through activation of caspase 8 and even more of caspase 9, thus indicating an involvement of both the receptor-mediated and the mitochondrial pathways of apoptosis. Indeed, the two pathways probably can co-operate to execute cell death with a prevalence of the mitochondrial pathways. Our data taken together provide additional information to further enhance our understanding of the molecular mechanism by which H. pylori proteins alter the growth status of human gastric epithelial cells.

The Helicobacter pylori CagF protein is a type IV secretion chaperone-like molecule that binds close to the C-terminal secretion signal of the CagA effector protein

Type IV secretion systems are common bacterial macromolecule transporters that have been adapted to various functions, such as effector protein translocation to eukaryotic cells, nucleoprotein transfer to bacterial or eukaryotic cells, and DNA transport into and out of bacterial cells. Helicobacter pylori, the causative agent of bacterial gastritis, peptic ulcers, gastric adenocarcinoma and mucosa-associated lymphoid tissue (MALT) lymphoma, uses the Cag type IV secretion system to inject the CagA protein into host cells, thereby altering gene expression profiles and the host cell cytoskeleton. The molecular mechanism of CagA recognition as a type IV substrate is only poorly understood, but seems to be more complex than that of other type IV secretion systems. Apart from 14 essential components of the secretion apparatus, CagA translocation specifically requires the presence of four additional Cag proteins. Here we show that the CagA-binding protein CagF is a secretion chaperone-like protein that interacts with a 100 aa region that is adjacent to the C-terminal secretion signal of CagA. The interaction between CagA and CagF takes place at the bacterial cytoplasmic membrane, and is independent of a functional type IV secretion apparatus and other cag-encoded factors. Our data indicate that CagF binding precedes recognition of the C-terminal CagA translocation signal, and that both steps are required to recruit CagA to the type IV translocation channel.

Helicobacter pylori vaccine development: optimisation of strategies and importance of challenging strain and animal model

Gastric infection with the gram-negative bacterial pathogen Helicobacter pylori is widespread (approximately 50% of the human population is affected) and is associated with the induction of specific gastroduodenal disease. Although extensive studies in the H. pylori mouse model have demonstrated the feasibility of both therapeutic and prophylactic immunisations, the mechanism of vaccine-induced protection is still poorly understood. We report here on novel strategies to optimise the generation of H. pylori ghosts as vaccine candidates and highlight the need to concentrate on alternative animal models and the use of fully virulent H. pylori type I strains for vaccination. An effective vaccine strategy against H. pylori has the potential to significantly improve population health worldwide.

Specific serum immunoglobulin G to H pylori and CagA in healthy children and adults (south-east of Iran)

AIM: To evaluate the serologic IgG response to H pylori and CagA across age groups and in healthy children and adults.

METHODS: Totally, 386 children aged 1-15 years and 200 adults aged 20-60 years, were enrolled to study. The serum samples of participant were tested for presence of anti-H pylori and anti-CagA IgG by using ELISA method.

RESULTS: The seroprevalence of H pylori in adults was significantly higher than that observed in children (67.5% vs 46.6%; P < 0.000003). In children, the seropositivity rate in males (51.9%) was significantly (P < 0.05) higher than that observed in females (41.7%). The prevalence of serum anti-CagA antibody was 72.8% and 67.4% in infected children and adults, respectively. The mean titer of serum anti-CagA antibodies was significantly higher among children in comparison to adults (64.1 Uarb/mL vs 30.7; P < 0.03). In infected children and adults the prevalence of serum anti-CagA antibody was higher in males compared to females (78.4% vs 66.3%; P = 0.07 and 75.6% vs 54.71%; P < 0.04, respectively). The age-specific prevalence of anti-H pylori and anti-CagA antibody (in infected subjects) was 37.6% and 59.57% at age 1-5 years, 46.9% and 75% at age 6-10 years, 54.9% and 79.45% at age 11-15, 59.01% and 83.33% at age 20-30 years, 66.6% and 60.52% at age 31-40 years, 73.46% and 63.88% at age 41-50 years and 75.75% and 60% at age 51-60 years with mean titer of anti-CagA antibody of 75.94, 63.32, 57.11, 52.06, 23.62, 21.52 and 21.80 Uarb/mL, respectively. There was significant difference between mean serum anti-CagA antibody in age subgroups (P < 0.001).

CONCLUSION: These results showed that anti-H pylori and anti-CagA antibodies were common in the children and adults. The H pylori-specific antibodies influenced by age and sex of subjects. Moreover, it seems that males are more susceptible to infection with CagA⁺ strains compared to females. The seroprevalence of anti-CagA antibody was increased with age, up to 30 years and then decreased. It was also found that the magnitude of the IgG response to CagA decreased with advanced age.

Ureases display biological effects independent of enzymatic activity: is there a connection to diseases caused by urease-producing bacteria?

Ureases are enzymes from plants, fungi and bacteria that catalyze the hydrolysis of urea to form ammonia and carbon dioxide. While fungal and plant ureases are homo-oligomers of 90-kDa subunits, bacterial ureases are multimers of two or three subunit complexes. We showed that some isoforms of jack bean urease, canatoxin and the classical urease, bind to glycoconjugates and induce platelet aggregation. Canatoxin also promotes release of histamine from mast cells, insulin from pancreatic cells and neurotransmitters from brain synaptosomes. In vivo it induces rat paw edema and neutrophil chemotaxis. These effects are independent of ureolytic activity and require activation of eicosanoid metabolism and calcium channels. Helicobacter pylori, a Gram-negative bacterium that colonizes the human stomach mucosa, causes gastric ulcers and cancer by a mechanism that is not understood. H. pylori produces factors that damage gastric epithelial cells, such as the vacuolating cytotoxin VacA, the cytotoxin-associated protein CagA, and a urease (up to 10% of bacterial protein) that neutralizes the acidic medium permitting its survival in the stomach. H. pylori whole cells or extracts of its water-soluble proteins promote inflammation, activate neutrophils and induce the release of cytokines. In this paper we review data from the literature suggesting that H. pylori urease displays many of the biological activities observed for jack bean ureases and show that bacterial ureases have a secretagogue effect modulated by eicosanoid metabolites through lipoxygenase pathways. These findings could be relevant to the elucidation of the role of urease in the pathogenesis of the gastrointestinal disease caused by H. pylori.

The transmission of Helicobacter pylori: the effects of analysis method and study population on inference

Although much is known about the virulence of Helicobacter pylori, the transmission pathways for this bacterium are still unresolved. Transmission has been addressed through: (1) prevalence within families; (2) detection in fecal/oral environments; (3) detection in the abiotic/biotic environment; and (4) direct inference from strain similarity. Here, we review the molecular and biochemical methods used and discuss the relative merits of each. Furthermore, as there are differences between developing and developed nations, we discuss the results obtained from transmission studies in light of the study population. We conclude that H. pylori is probably transmitted person-to-person, facilitated by fecal-oral transmission during episodes of diarrhea or gastro-oral contact during periods of vomiting. The persistence of H. pylori in abiotic and biotic environments remains unproven but possible reactivation from viable, non-culturable coccoid forms should be further investigated. Finally, we speculate on the effect of host-pathogen interactions in confounding the inference of transmission.

The role of bacterial pathogens in cancer

The association of Helicobacter pylori with gastric cancer is the best-studied relationship between a bacterial infection and cancer. Other bacterial pathogens in humans and rodents are now being recognized as potentially having a direct role in carcinogenesis. Thus, it might be possible to understand the pathogenesis and prevention of certain cancers by studying the bacterial infections associated with them, and their effects on the host. However, the mechanisms by which bacteria contribute to cancer formation are complex, and recent investigations show that they involve the interplay between chronic inflammation, direct microbial effects on host cell physiology and, ultimately, changes in tissue stem cell homeostasis.

The inflammatory and immune response to Helicobacter pylori infection

Lifelong Helicobacter pylori infection and its associated gastric inflammation underlie peptic ulceration and gastric carcinogenesis. The immune and inflammatory responses to H. pylori are doubly responsible: gastric inflammation is the main mediator of pathology, and the immune and inflammatory response is ineffective, allowing lifelong bacterial persistence. However, despite inducing gastric inflammation, most infections do not cause disease, and bacterial, host and environmental factors determine individual disease risk. Although H. pylori avoids many innate immune receptors, specific virulence factors (including those encoded on the cag pathogenicity island) stimulate innate immunity to increase gastric inflammation and increase disease risk. An acquired T helper 1 response upregulates local immune effectors. The extent to which environmental factors (including parasite infection), host factors and H. pylori itself influence T-helper differentiation and regulatory T-cell responses remains controversial. Finally, effective vaccines have still not been developed: a better understanding of the immune response to H. pylori may help.

Characterization of TccP-mediated N-WASP activation during enterohaemorrhagic Escherichia coli infection

Subversion of the host cell cytoskeleton is the hallmark of enterohaemorrhagic Escherichia coli (EHEC) infection. EHEC translocates the trans-membrane receptor protein Tir (translocated intimin receptor), which links the extracellular bacterium to the eukaryotic cell actin cytoskeleton, triggering formation of actin-rich pedestals beneath adherent bacteria. Tir-mediated actin accretion by EHEC requires TccP (Tir cytoskeleton coupling protein), a recently discovered type III secretion system effector protein which, following translocation, binds and activates Wiskott-Aldrich syndrome protein (N-WASP), which in turn activates the actin-related protein 2/3 complex leading to localized polymerization of actin. In this study, truncated N-WASP and TccP derivatives were generated and tested in in vitro actin polymerization and epithelial cell infection assays. The C-terminal amino acids 253-276 of the GTPase binding domain (GBD) of N-WASP were identified as essential, although not sufficient, for TccP:N-WASP protein:protein interaction, TccP-mediated N-WASP activation and induction of actin polymerization. TccP from EHEC O157:H7 strain EDL933 consists of a unique N-terminal domain and six proline-rich repeats. Progressive deletions within the N-terminus of TccP revealed that residues 1-21 are necessary and sufficient for its translocation, while amino acids 1-181, encompassing the N-terminal translocation signal and two proline-rich repeats, are sufficient for triggering actin polymerization in EHEC-infected epithelial cells and in in vitro actin polymerization assays. This study defines the modular domain structure of TccP and the molecular basis of TccP-mediated N-WASP activation and EHEC-induced remodelling of the host actin cytoskeleton.

Helicobacter pylori VacA toxin promotes bacterial intracellular survival in gastric epithelial cells

Helicobacter pylori colonizes the gastric epithelium of at least 50% of the world's human population, playing a causative role in the development of chronic gastritis, peptic ulcers, and gastric adenocarcinoma. Current evidence indicates that H. pylori can invade epithelial cells in the gastric mucosa. However, relatively little is known about the biology of H. pylori invasion and survival in host cells. Here, we analyze both the nature of and the mechanisms responsible for the formation of H. pylori's intracellular niche. We show that in AGS cells infected with H. pylori, bacterium-containing vacuoles originate through the fusion of late endocytic organelles. This process is mediated by the VacA-dependent retention of the small GTPase Rab7. In addition, functional interactions between Rab7 and its downstream effector, Rab-interacting lysosomal protein (RILP), are necessary for the formation of the bacterial compartment since expression of mutant forms of RILP or Rab7 that fail to bind each other impaired the formation of this unique bacterial niche. Moreover, the VacA-mediated sequestration of active Rab7 disrupts the full maturation of vacuoles as assessed by the lack of both colocalization with cathepsin D and degradation of internalized cargo in the H. pylori-containing vacuole. Based on these findings, we propose that the VacA-dependent isolation of the H. pylori-containing vacuole from bactericidal components of the lysosomal pathway promotes bacterial survival and contributes to the persistence of infection.

Bacterial genomics and pathogen evolution

The availability of hundreds of bacterial genome sequences has altered the study of bacterial pathogenesis, affecting both design of experiments and analysis of results. Comparative genomics and genomic tools have been used to identify virulence factors and genes involved in environmental persistence of pathogens. However, a major stumbling block in the genomics revolution has been the large number of genes with unknown function that have been identified in every organism sequenced to date.

Interaction with CagF is required for translocation of CagA into the host via the Helicobacter pylori type IV secretion system

Development of severe gastric diseases is strongly associated with those strains of Helicobacter pylori that contain the cag pathogenicity island (PAI) inserted into the chromosome. The cag PAI encodes a type IV secretion system that translocates the major disease-associated virulence protein, CagA, into the host epithelial cell. CagA then affects host signaling pathways, leading to cell elongations and inflammation. Since the precise mechanism by which the CagA toxin is translocated by the type IV secretion system remained elusive, we used fusion proteins and immunoprecipitation studies to identify CagA-interacting secretion components. Here we demonstrate that CagA, in addition to other yet-unidentified proteins, interacts with CagF, presumably at the inner bacterial membrane. This interaction is required for CagA translocation, since an isogenic nonpolar cagF mutant was translocation deficient. Our results suggest that CagF may be a protein with unique chaperone-like function that is involved in the early steps of CagA recognition and delivery into the type IV secretion channel.

The Helicobacter felis model of adoptive transfer gastritis

The bacterium Helicobacter pylori is a major human pathogen and the principal cause of acute and chronic gastritis, gastric and duodenal ulcer disease, and gastric adenocarcinoma. Infection with gastric Helicobacter results in an early infiltration of neutrophils, monocytes, and natural killer cells, followed by an influx of T cells and plasma cells. Although the critical components of this gastric infiltrate that lead to disease are unclear, the Helicobacter felis-infected mouse and other mouse models of Helicobacter-associated gastritis have demonstrated the critical nature of adaptive immunity in the development of gastric epithelial pathology. To further investigate the role of adaptive immunity in this disease, adoptive transfer models of disease have also been utilized. These models clearly demonstrate that it is the host CD4+ T lymphocyte response that is crucial for the development of Helicobacter-associated gastric epithelial changes.

Interaction of CagA with Crk plays an important role in Helicobacter pylori-induced loss of gastric epithelial cell adhesion

CagA protein is a major virulence factor of Helicobacter pylori, which is delivered into gastric epithelial cells and elicits growth factor-like responses. Once within the cells, CagA is tyrosine phosphorylated by Src family kinases and targets host proteins required to induce the cell responses. We show that the phosphorylated CagA binds Crk adaptor proteins (Crk-II, Crk-I, and Crk-L) and that the interaction is important for the CagA-mediated host responses during H. pylori infection. H. pylori-induced scattering of gastric epithelial cells in culture was blocked by overexpression of dominant-negative Crk and by RNA interference-mediated knockdown of endogenous Crk. H. pylori infection of the gastric epithelium induced disruption of E-cadherin/catenin-containing adherens junctions, which was also dependent on CagA/Crk signaling. Furthermore, inhibition of the SoS1/H-Ras/Raf1, C3G/Rap1/B-Raf, or Dock180/Rac1/Wiskott-Aldrich syndrome protein family verprolin homologous protein pathway, all of which are involved downstream of Crk adaptors, greatly diminished the CagA-associated host responses. Thus, CagA targeting of Crk plays a central role in inducing the pleiotropic cell responses to H. pylori infection that cause several gastric diseases, including gastric cancer.

Microbial strategies to target, cross or disrupt epithelia

Epithelia are highly organized structures adapted to protect the underlying tissues from external aggressions, including microbial infections. Consequently, pathogens have evolved various strategies to target directly or indirectly intercellular junctions and/or components that maintain the structure of epithelia. Interestingly, some extracellular pathogens secrete enzymes that modify the extracellular part of junction components. Others produce toxins that are endocytosed and act from the inside of the cell to disrupt epithelial junctions. Other pathogens may directly inject into cells factors that are targeted to and destabilize the junctions, or that interact with signaling cascades that affect junction stability. Finally invasive bacteria or viruses may, by entering into cells, destabilize the junctions by targeting junction components directly or by inducing a series of events that lead to chemokine secretion, polymorphonuclear recruitment and inflammation.

Bacteria-host-cell interactions at the plasma membrane: stories on actin cytoskeleton subversion

Exploitation of the host-cell actin cytoskeleton is pivotal for many microbial pathogens to enter cells, to disseminate within and between infected tissues, to prevent their uptake by phagocytic cells, or to promote intimate attachment to the cell surface. To accomplish this, these pathogens have evolved common as well as unique strategies to modulate actin dynamics at the plasma membrane, which will be discussed here, exemplified by a number of well-studied bacterial pathogens.