A structural overview of the Helicobacter cytotoxin

Helicobacter pylori: phenotypes, genotypes and virulence genes

Helicobacter pylori is a Gram-negative, microaerophilic bacterium that colonizes the gastric mucus overlying the epithelium of the stomach in more than 50% of the world's population. This gastric colonization induces chronic gastric inflammation in all infected individuals, but only induces clinical diseases in 10-20% of infected individuals. These include peptic ulcers, acute and atrophic gastritis, intestinal metaplasia, gastric adenocarcinoma and gastric B-cell lymphoma. Various bacterial virulence factors are associated with the development of such gastric diseases, and the characterization of these markers could aid medical prognosis, which could be extremely important in predicting clinical outcomes. The purpose of this review is to summarize the role of the phenotypes, virulence-related genes and genotypes of H. pylori in the establishment of gastric colonization and the development of associated diseases.

Host-bacterial interactions in Helicobacter pylori infection

Helicobacter pylori are spiral-shaped gram-negative bacteria with polar flagella that live near the surface of the human gastric mucosa. They have evolved intricate mechanisms to avoid the bactericidal acid in the gastric lumen and to survive near, to attach to, and to communicate with the human gastric epithelium and host immune system. This interaction sometimes results in severe gastric pathology. H pylori infection is the strongest known risk factor for the development of gastroduodenal ulcers, with infection being present in 60%-80% of gastric and 95% of duodenal ulcers.(1)H pylori is also the first bacterium to be classified as a definite carcinogen by the World Health Organization's International Agency for Research on Cancer because of its epidemiologic relationship to gastric adenocarcinoma and gastric mucosa-associated lymphoid tissue lymphoma.(2) In the last 25 years, since H pylori was first described and cultured, a complete paradigm shift has occurred in our clinical approach to these gastric diseases, and more than 20,000 scientific publications have appeared on the subject. From the medical point of view, H pylori is a formidable pathogen responsible for much morbidity and mortality worldwide. However, H pylori infection occurs in approximately half of the world population, with disease being an exception rather than the rule. Understanding how this organism interacts with its host is essential for formulating an intelligent strategy for dealing with its most important clinical consequences. This review offers an insight into H pylori host-bacterial interactions.

Of microbe and man: determinants ofHelicobacter pylori-related diseases

The human gastric pathogen Helicobacterpylori infects the human gastric mucus layer of approximately half of the world's population. Colonization with this bacterium results in superficial gastritis without clinical symptoms, but can progress into gastric or duodenal ulcers, gastric malignancies and mucosa-associated lymphoid tissue-lymphomas. Disease outcome is affected by a complex interplay between host, environmental and bacterial factors. Irrespective of disease outcome, the majority of H. pylori infected individuals remain colonized for life. Changing conditions in the human gastric mucosa may alter gene expression and/or result in the outgrowth of more fit H. pylori variants. As such, H. pylori is a highly flexible organism that is optimally adapted to its host. the heterogeneity in H. pylori populations make predictions on H. pylori-related pathogenesis difficult. In this review, we discuss host, environmental and bacterial factors that are important in disease progression. Moreover, H. pylori adaptive mechanisms, which allow its life-long survival and growth in the gastric mucosa are considered.

Molecular biology of gastric cancer: Helicobacter infection and gastric adenocarcinoma: bacterial and host factors responsible for altered growth signaling

Gastric cancer remains the second most common cause of cancer-related mortality worldwide. The single most common cause of gastric cancer is chronic infection with the gram-negative microaerophilic spiral bacterium: Helicobacter pylori. Recent advances in this field have identified host factors which predispose to gastric cancer formation via modulation of the host immune response. In addition, recent work has explored bacterial virulence factors which may directly cause tissue damage, and lead to gastric carcinogenesis, as well as factors responsible for enhanced immune response. Environmental factors, long associated with a predilection for gastric cancer, are recognized as modifiers of key growth signalling pathways within the gastric mucosa and as such lead to growth alterations. This review focuses on exploring new advances in our understanding of bacterial factors, host genetic polymorphisms and the interaction between the bacterium and host at the level of the immune response and the regulation of proliferative and apoptotic signal transduction cascades. Modulation of the pivotal balance between cell growth and cell death leads to the formation of gastric adenocarcinoma.

Helicobacter pylori-induced homotypic phagosome fusion in human monocytes is independent of the bacterial vacA and cag status

Following reports that a VacA+cag+ toxigenic but not a VacA-cag- non-toxigenic Helicobacter pylori strain induced homotypic phagosome fusion in murine macrophages, we addressed that phenomenon in human cells. Mononuclear phagocytes and epitheloid cells were challenged with H. pylori strains of different vacA and cag genotypes and with VacA- and Cag- isogenic mutants, and chased in the absence or presence of signal transduction modulators. Electron microscopy revealed that, in monocytes: (i) homotypic phagosome fusion was frequently induced by all live H. pylori strains investigated but not by exogenous VacA; (ii) phagosomes containing bacteria fused, but not those containing latex beads; (iii) fusion resulted in communal compartments resembling giant multivesicular bodies; and (iv) formation of these compartments was blocked by inhibiting the host cell regulators PI 3-kinase, phospholipase C and p42 MAP kinase. Whereas some internalized bacteria remained viable 1 h after uptake, none survived a 24 h period. In contrast to monocytes, infected epitheloid cells rarely developed communal compartments. In combination, these results demonstrate that, in human monocytes, the H. pylori-induced homotypic phagosome fusion depends on neither the vacuolating cytotoxin VacA nor the cag pathogenicity island of H. pylori and does not result in prolonged intracellular survival.

The Helicobacter pylori vacuolating toxin inhibits T cell activation by two independent mechanisms

Helicobacter pylori toxin, VacA, damages the gastric epithelium by erosion and loosening of tight junctions. Here we report that VacA also interferes with T cell activation by two different mechanisms. Formation of anion-specific channels by VacA prevents calcium influx from the extracellular milieu. The transcription factor NF-AT thus fails to translocate to the nucleus and activate key cytokine genes. A second, channel-independent mechanism involves activation of intracellular signaling through the mitogen-activated protein kinases MKK3/6 and p38 and the Rac-specific nucleotide exchange factor, Vav. As a consequence of aberrant Rac activation, disordered actin polymerization is stimulated. The resulting defects in T cell activation may help H. pylori to prevent an effective immune response leading to chronic colonization of its gastric niche.

Helicobacter pylori virulence factors and the host immune response: implications for therapeutic vaccination

Helicobacter pylori colonizes the human gastric mucosa and is associated with specific gastric disease. Virulence factors, such as urease, the vacuolating toxin (VacA), the cytotoxin-associated antigen CagA or blood-group-antigen-binding adhesin (BabA), an adherence factor, might account for the development of different diseases. Vaccination trials exploiting the antigenic properties of some of these proteins have not been successful in preventing infection in humans. A more in-depth understanding of the immune response to H. pylori infection as well as additional information on suitable epitopes and adjuvants will be required before a successful vaccine can be developed.

Molecular typing of Helicobacter pylori by virulence-gene based multiplex PCR and RT-PCR analysis

BACKGROUND

In recent years, numerous PCR amplification typing methods have been developed for the identification of H. pylori specific virulence genes. To reduce the number of PCR amplifications needed we have previously developed virulence gene based multiplex PCR and RT-PCR assays.

AIM

The aim of the present study was to characterise Helicobacter pylori strains by means of virulence-gene based multiplex PCR and reverse-transcription PCR.

SUBJECTS

Helicobacter pylori clinical reference strains HP-HJM 1-25 originally obtained from routine cultures of clinical gastric biopsy specimens from dyspeptic patients were used in the present study.

METHODS

Helicobacter pylori multiplex PCR and RT-PCR assays were carried out using primer pairs targeting the cagA, vacA, ureA, ureI, hspA (hsp60), and sodB genes and their cDNA.

RESULTS

Unexpected multiplex PCR and RT-PCR patterns were observed by ethidium bromide staining of agarose gels and Southern blot hybridisation analysis. DNA sequence analysis revealed a complex pattern of point mutations, deletions and insertions in the sodB, cagA and vacA genes, respectively. Mutations occurring in the PCR and hybridisation primer binding sites might explain some of the discrepancies previously observed in the expression of these genes. Furthermore, the present data show that coexpression of cagA and vacA transcripts of different sizes may take place in the same strain.

CONCLUSIONS

The multiplex PCR and RT-PCR assay described allows rapid characterisation of H. pylori virulence genes at the DNA and RNA (cDNA) levels. However, extensive DNA sequence analysis seems necessary if one wants to reveal details of mutations occurring in the cagA and vacA genes.

Review article: Helicobacter pylori infection from pathogenesis to treatment--a critical reappraisal

The main areas of this review are Helicobacter pylori and disease pathogenesis; the relationship of H. pylori to lower gastrointestinal diseases, liver disease and extra-gastrointestinal conditions; the relationship of H. pylori to gastro-oesophageal reflux disease; infection in the very young and very old; diagnostic techniques; and management of H. pylori infections with particular emphasis on eradication regimens and antibiotic resistance.

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

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

Proteome analysis of secreted proteins of the gastric pathogen Helicobacter pylori

Secreted proteins (the secretome) of the human pathogen Helicobacter pylori may mediate important pathogen-host interactions, but such proteins are technically difficult to analyze. Here, we report on a comprehensive secretome analysis that uses protein-free culture conditions to minimize autolysis, an efficient recovery method for extracellular proteins, and two-dimensional gel electrophoresis followed by peptide mass fingerprinting for protein resolution and identification. Twenty-six of the 33 separated secreted proteins were identified. Among them were six putative oxidoreductases that may be involved in the modification of protein-disulfide bonds, three flagellar proteins, three defined fragments of the vacuolating toxin VacA, the serine protease HtrA, and eight proteins of unknown function. A cleavage site for the amino-terminal passenger domain of VacA between amino acids 991 and 992 was determined by collision-induced dissociation mass spectrometry. Several of the secreted proteins are interesting targets for antimicrobial chemotherapy and vaccine development.