The vacuolating toxin of Helicobacter pylori: a few answers, many questions

Abstract

kJanrrr&We: Or V. Ricci, lstituto di Fisiologia Umana, Universitti di Pavia. via Forlanini 6, 27’100 Pavia, italy. Fax: +39-CB92-507SS4. E-meil: wicci@unipv. it Helicobacterpylori (H. pylori) has developed properties that allow it to survive and grow in a hostile environment like the human stomach where it causes an inflammatory reaction and epithelial damage with cellular swelling, cytoplasmic vacuolation, and expansion of endosomal compartments ‘. Bacterial extracts from H. pylori induce cytoplasmic vacuolar degeneration in cultured cells 2-4. A pivotal role in cell damage induced by H. pylori seems to be played by the vacuolating toxin (VacA) 5m7. VacA is a protein toxin (thus far the only protein toxin known to be produced by H. pylori), encoded by the vacA gene, able to induce cytoplasmic vacuoles in eukaryotic cells in culture although sensitivity to VacA differs greatly in the different cell types tested *. When given to mice, VacA causes gastric epithelial damage closely resembling that found in H. pyZori-colonized patients s. VacA toxin is composed of monomers of about 90 kDa synthesized as a 140 kDa precursor with a N-terminal signal sequence of 33 amino acids 5 6 The structure of the vacA gene varies, es. pecially in the region encoding the signal sequence (which may be type s la, s lb or s2) and in the mid-region (which may be type ml or m2) (‘. VacA is released by the bacterium both as soluble secreted protein and as VacA-containing vesicles derived from outer membrane blebs * O. Both soluble secreted VacA and VacA-containing vesicles bind to, and are internalized by, gastric epithelial cells in culture and are detectable in the gastric mucosa from H. pylori-infected patients * 9. Therefore, the release of outer membrane vesicles by H. pylori may represent a mechanism, in addition to secretory pathway, for the delivery of bacterial virulence factors and antigens into the gastric mucosa”. However, the role of VacAcontaining vesicles in VacA-induced cell damage is still unknown. Purified toxin forms high molecular mass (about 1,000 kDa) oligomers which need to be disassembled in monomers by acid or alkaline treatment to become active ‘” ‘I. Nevertheless, the findings that both bacterium-associated toxin and bacterial broth culture filtrates are constitutively very active without requiring acid/alkaline pretreatment ‘O suggest that VacA may be present in the juxtacellular microenvironment of the human gastric mucosa in its active monomeric form. Moreover, it has been suggested that monomers are further processed to produce a 34-37 kDa Nterminal fragment and a 58 kDa C-terminal fragment and that these fragments remain associated after cleavage and may represent two distinct subunits of the toxin (j. This type of cleavage in subunits is characteristic of the A-B family of bacterial toxins in which subunit A exhibits enzymatic activity accounting for the overall toxin action, while subunit B is devoted to cell binding and membrane translocation I*. VacA binds plasma membrane of epithelial cells and is internalized by the cells via an active cellular process ‘j. Some authors have presented data suggesting the existence of a specific, high affinity cell surface receptor for VacA, although they failed to agree on its molecular characteristics. For instance, one group I’ reported that the 250 kDa receptor protein tyrosine phosphatase beta may act as a VacA receptor even if VacA also binds to an unidentified 140 kDa protein on the surface of cultured cells. Another group I4 reported that the 170 kDa epidermal growth factor receptor may