![]() ![]() Since IpaC and IpaD mutants are totally deficient for invasion despite the fact that they secrete IpaB ( Parsot et al., 1995), it is likely that this CD44–IpaB interaction is implicated in a discrete step of the entry process. Identical mechanisms can be proposed for the reported interaction between IpaB and the β1 integrin ( Watarai et al., 1996). ![]() Alternatively, CD44 could be involved in the adhesion of the secreton to the host cell surface. CD44 may therefore help to regulate the actin assembly/disassembly that occurrs during Shigella entry, as the CD44–ERM complex activates RhoGDI, a negative regulator of Rho proteins that were found to be implicated in Shigella entry ( Watarai et al., 1997 Mounier et al., 1999). Ezrin was shown to be recruited at the Shigella entry structure and to be required for the entry process ( Skoudy et al., 1999). CD44 has been reported to bind via its cytoplasmic tail the ezrin–radixin–moesin (ERM) family of proteins that associate with the actin cytoskeleton ( Hirao et al., 1996). CD44 is a member of the immunoglobulin superfamily that binds hyaluronan. Using several biochemical assays, IpaB was shown to bind the transmembrane receptor CD44 ( Skoudy et al., 2000). In monocyte/macrophage and dendritic populations, IpaB additionally binds to caspase I, leading to apoptosis with release of the pro-inflammatory molecules IL-1β and IL-18 ( Sansonetti, 2001). In addition, IpaA and IpaC are involved in actin polymerization and bundling, whereas an IpaB–IpaC complex is needed for the bacterium to break the phagocytic vacuole once the bacterium is inside the host cell ( Sansonetti, 2001). Among these effectors, the Ipa proteins play essential roles: IpaB and IpaD form a complex that controls the flux of proteins through the secreton, while IpaB and IpaC insert into the plasma membrane of the host cell to form a pore. The secreton is evolutionarily related to the flagellar origin but is used as a needle that injects effector proteins into the host cell cytoplasm ( Blocker et al., 2001). It contains a ‘pathogenicity island’ (30 kb) encompassing the ipa/mxi–spa operons, which encode the proteins composing the type III secretion system, or secreton, and effector proteins. The pWR100 (214 kb) plasmid of the M90T strain of Shigella flexneri 5a has been fully sequenced. To be virulent, Shigella isolates carry a plasmid encoding the ‘invasive phenotype’ of the particular species. This ‘snowball effect’ of inflammation and invasion induces severe inflammatory destruction, characteristic of the disease ( Sansonetti, 2001). As a result, polymorphonuclear cells are activated and transmigrate through the epithelium via a process called ‘fatal attraction’, thus disrupting the monolayer and allowing rapid basolateral Shigella invasion of epithelial cells. ![]() Once translocated, Shigella infect macrophages in which they trigger cell death. After oral contamination, the pathogen reaches the colon where it is translocated across the epithelial barrier by the M cells in the Peyer patches. Shigellosis is endemic throughout the world, although most of the million yearly deaths, 70% of which affect children between the ages of 1 and 5 years, occur in developing countries. Shigella is a Gram-negative enteroinvasive bacterium known as the causative agent of bacillary dysentery ( Sansonetti, 2001 Suzuki and Sasakawa, 2001). Our results show that rafts are implicated in Shigella binding and entry, suggesting that raft-associated molecular machineries are engaged in mediating the cell signalling response required for the invasion process. Finally, we find that Shigella is less invasive in sphingosid-based lipid-deficient cell lines, demonstrating the involvement of sphingolipids. Moreover, we report that Shigella entry is impaired after cholesterol depletion using methyl-β-cyclodextrin. We also document accumulation of cholesterol and raft-associated proteins at Shigella entry foci. We identified a molecular complex involving proteins of both the host, CD44 the hyaluronan receptor, and Shigella, the invasin IpaB, which partitions during infection within specialized membrane microdomains enriched in cholesterol and sphingolipids, called rafts. Deciphering the initial molecular events is crucial to understanding the infectious process. Upon host cell– Shigella interaction, major host cell signalling responses are activated. Shigellosis is an acute inflammatory bowel disease caused by the enteroinvasive bacterium Shigella. ![]()
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