Shigella entry unveils a calcium/calpain-dependent mechanism for inhibiting sumoylation.

Disruption of the sumoylation/desumoylation equilibrium is associated with several disease states such as cancer and infections, however the mechanisms regulating the global SUMO balance remain poorly defined. Here, we show that infection by Shigella flexneri, the causative agent of human bacillary dysentery, switches off host sumoylation during epithelial cell infection in vitro and in vivo and that this effect is mainly mediated by a calcium/calpain-induced cleavage of the SUMO E1 enzyme SAE2, thus leading to sumoylation inhibition. Furthermore, we describe a mechanism by which Shigella promotes its own invasion by altering the sumoylation state of RhoGDIα, a master negative regulator of RhoGTPase activity and actin polymerization. Together, our data suggest that SUMO modification is essential to restrain pathogenic bacterial entry by limiting cytoskeletal rearrangement induced by bacterial effectors. Moreover, these findings identify calcium-activated calpains as powerful modulators of cellular sumoylation levels with potentially broad implications in several physiological and pathological situations.

Sumoylation controls host anti-bacterial response to the gut invasive pathogen Shigella flexneri.

Shigella flexneri, the etiological agent of bacillary dysentery, invades the human colonic epithelium and causes its massive inflammatory destruction. Little is known about the post-translational modifications implicated in regulating the host defense pathway against Shigella. Here, we show that SUMO-2 impairs Shigella invasion of epithelial cells in vitro. Using mice haploinsufficient for the SUMO E2 enzyme, we found that sumoylation regulates intestinal permeability and is required to restrict epithelial invasion and control mucosal inflammation. Quantitative proteomics reveals that Shigella infection alters the sumoylation status of a restricted set of transcriptional regulators involved in intestinal functions and inflammation. Consistent with this, sumoylation restricts the pro-inflammatory transcriptional response of Shigella-infected guts. Altogether, our results show that the SUMO pathway is an essential component of host innate protection, as it reduces the efficiency of two key steps of shigellosis: invasion and inflammatory destruction of the intestinal epithelium.

Shigella effector IpaB-induced cholesterol relocation disrupts the Golgi complex and recycling network to inhibit host cell secretion.

Shigella infection causes destruction of the human colonic epithelial barrier. The Golgi network and recycling endosomes are essential for maintaining epithelial barrier function. Here we show that Shigella epithelial invasion induces fragmentation of the Golgi complex with consequent inhibition of both secretion and retrograde transport in the infected host cell. Shigella induces tubulation of the Rab11-positive compartment, thereby affecting cell surface receptor recycling. The molecular process underlying the observed damage to the Golgi complex and receptor recycling is a massive redistribution of plasma membrane cholesterol to the sites of Shigella entry. IpaB, a virulence factor of Shigella that is known to bind cholesterol, is necessary and sufficient to induce Golgi fragmentation and reorganization of the recycling compartment. Shigella infection-induced Golgi disorganization was also observed in vivo, suggesting that this mechanism affecting the sorting of cell surface molecules likely contributes to host epithelial barrier disruption associated with Shigella pathogenesis.

Shigella flexneri infection generates the lipid PI5P to alter endocytosis and prevent termination of EGFR signaling.

The phosphoinositide metabolic pathway, which regulates cellular processes implicated in survival, motility, and trafficking, is often subverted by bacterial pathogens. Shigella flexneri, a bacterium that causes dysentery, injects IpgD, a phosphoinositide phosphatase that generates the lipid phosphatidylinositol 5-phosphate (PI5P), into host cells, thereby activating the phosphoinositide 3-kinase-Akt survival pathway. We show that epidermal growth factor receptor (EGFR) is required for PI5P-dependent activation of Akt in infected HeLa cells or cells ectopically expressing IpgD. Cells treated with PI5P had increased numbers of early endosomes with activated EGFR, no detectable EGFR in the late endosomal or lysosomal compartments, and prolonged EGFR signaling. Endosomal recycling and retrograde pathways were spared, indicating that the effect of PI5P on the degradative route to the late endocytic compartments was specific. Thus, we identified PI5P, which was enriched in endosomes, as a regulator of vesicular trafficking that alters growth factor receptor signaling by impairing lysosomal degradation, a property used by S. flexneri to favor survival of host cells.

ATP-mediated Erk1/2 activation stimulates bacterial capture by filopodia, which precedes Shigella invasion of epithelial cells.

Shigella, the causative agent of bacillary dysentery in humans, invades epithelial cells, using a type III secretory system (T3SS) to inject bacterial effectors into host cells and remodel the actin cytoskeleton. ATP released through connexin hemichanels on the epithelial membrane stimulates Shigella invasion and dissemination in epithelial cells. Here, we show that prior to contact with the cell body, Shigella is captured by nanometer-thin micropodial extensions (NMEs) at a distance from the cell surface, in a process involving the T3SS tip complex proteins and stimulated by ATP- and connexin-mediated signaling. Upon bacterial contact, NMEs retract, bringing bacteria in contact with the cell body, where invasion occurs. ATP stimulates Erk1/2 activation, which controls actin retrograde flow in NMEs and their retraction. These findings reveal previously unappreciated facets of interaction of an invasive bacterium with host cells and a prominent role for Erk1/2 in the control of filopodial dynamics.

Modulation of Shigella virulence in response to available oxygen in vivo.

Bacteria coordinate expression of virulence determinants in response to localized microenvironments in their hosts. Here we show that Shigella flexneri, which causes dysentery, encounters varying oxygen concentrations in the gastrointestinal tract, which govern activity of its type three secretion system (T3SS). The T3SS is essential for cell invasion and virulence. In anaerobic environments (for example, the gastrointestinal tract lumen), Shigella is primed for invasion and expresses extended T3SS needles while reducing Ipa (invasion plasmid antigen) effector secretion. This is mediated by FNR (fumarate and nitrate reduction), a regulator of anaerobic metabolism that represses transcription of spa32 and spa33, virulence genes that regulate secretion through the T3SS. We demonstrate there is a zone of relative oxygenation adjacent to the gastrointestinal tract mucosa, caused by diffusion from the capillary network at the tips of villi. This would reverse the anaerobic block of Ipa secretion, allowing T3SS activation at its precise site of action, enhancing invasion and virulence.

Galectin-3, a marker for vacuole lysis by invasive pathogens.

Shigella bacteria invade macrophages and epithelial cells and following internalization lyse the phagosome and escape to the cytoplasm. Galectin-3, an abundant protein in macrophages and epithelial cells, belongs to a family of beta-galactoside-binding proteins, the galectins, with many proposed functions in immune response, development, differentiation, cancer and infection. Galectins are synthesized as cytosolic proteins and following non-classical secretion bind extracellular beta-galactosides. Here we analysed the localization of galectin-3 following entry of Shigella into the cytosol and detected a striking phenomenon. Very shortly after bacterial invasion, intracellular galectin-3 accumulated in structures in vicinity to internalized bacteria. By using immuno-electron microscopy analysis we identified galectin-3 in membranes localized in the phagosome and in tubules and vesicles that derive from the endocytic pathway. We also demonstrated that the binding of galectin-3 to host N-acetyllactosamine-containing glycans, was required for forming the structures. Accumulation of the structures was a type three secretion system-dependent process. More specifically, existence of structures was strictly dependent upon lysis of the phagocytic vacuole and could be shown also by Gram-positive Listeria and Salmonella sifA mutant. We suggest that galectin-3-containing structures may serve as a potential novel tool to spot vacuole lysis.

The IpaC carboxyterminal effector domain mediates Src-dependent actin polymerization during Shigella invasion of epithelial cells.

Shigella, the causative agent of bacillary dysentery, invades epithelial cells by locally reorganizing the actin cytoskeleton. Shigella invasion requires actin polymerization dependent on the Src tyrosine kinase and a functional bacterial type III secretion (T3S) apparatus. Using dynamic as well as immunofluorescence microscopy, we show that the T3S translocon component IpaC allows the recruitment of the Src kinase required for actin polymerization at bacterial entry sites during the initial stages of Shigella entry. Src recruitment occurred at bacterial-cell contact sites independent of actin polymerization at the onset of the invasive process and was still observed in Shigella strains mutated for translocated T3S effectors of invasion. A Shigella strain with a polar mutation that expressed low levels of the translocator components IpaB and IpaC was fully proficient for Src recruitment and bacterial invasion. In contrast, a Shigella strain mutated in the IpaC carboxyterminal effector domain that was proficient for T3S effector translocation did not induce Src recruitment. Consistent with a direct role for IpaC in Src activation, cell incubation with the IpaC last 72 carboxyterminal residues fused to the Iota toxin Ia (IaC) component that translocates into the cell cytosol upon binding to the Ib component led to Src-dependent ruffle formation. Strikingly, IaC also induced actin structures resembling bacterial entry foci that were enriched in activated Src and were inhibited by the Src inhibitor PP2. These results indicate that the IpaC effector domain determines Src-dependent actin polymerization and ruffle formation during bacterial invasion.

PtdIns5P activates the host cell PI3-kinase/Akt pathway during Shigella flexneri infection.

The virulence factor IpgD, delivered into nonphagocytic cells by the type III secretion system of the pathogen Shigella flexneri, is a phosphoinositide 4-phosphatase generating phosphatidylinositol 5 monophosphate (PtdIns5P). We show that PtdIns5P is rapidly produced and concentrated at the entry foci of the bacteria, where it colocalises with phosphorylated Akt during the first steps of infection. Moreover, S. flexneri-induced phosphorylation of host cell Akt and its targets specifically requires IpgD. Ectopic expression of IpgD in various cell types, but not of its inactive mutant, or addition of short-chain penetrating PtdIns5P is sufficient to induce Akt phosphorylation. Conversely, sequestration of PtdIns5P or reduction of its level strongly decreases Akt phosphorylation in infected cells or in IpgD-expressing cells. Accordingly, IpgD and PtdIns5P production specifically activates a class IA PI 3-kinase via a mechanism involving tyrosine phosphorylations. Thus, S. flexneri parasitism is shedding light onto a new mechanism of PI 3-kinase/Akt activation via PtdIns5P production that plays an important role in host cell responses such as survival.

Secretion of type III effectors into host cells in real time.

Type III secretion (T3S) systems are key features of many gram-negative bacteria that translocate T3S effector proteins directly into eukaryotic cells. There, T3S effectors exert many effects, such as cellular invasion or modulation of host immune responses. Studying spatiotemporal orchestrated secretion of various effectors has been difficult without disrupting their functions. Here we developed a new approach using Shigella flexneri T3S as a model to investigate bacterial translocation of individual effectors via multidimensional time-lapse microscopy. We demonstrate that direct fluorescent labeling of tetracysteine motif-tagged effectors IpaB and IpaC is possible in situ without loss of function. Studying the T3S kinetics of IpaB and IpaC ejection from individual bacteria, we found that the entire pools of IpaB and IpaC were released concurrently upon host cell contact, and that 50% of each effector was secreted in 240 s. This method allows an unprecedented analysis of the spatiotemporal events during T3S.

A lysine- and glutamic acid-rich protein, KERP1, from Entamoeba histolytica binds to human enterocytes.

Contact-dependent cytolysis of host cells by Entamoeba histolytica is an important hallmark of amoebiasis that points out the importance of molecules involved in the interaction between the parasite and the human cells. To decipher the molecular and cellular mechanisms supporting the invasion of the intestinal epithelium by E. histolytica, we analysed proteins involved in the interaction of the parasite with enterocytes. Affinity chromatography revealed several amoebic proteins interacting with purified brush border of differentiated Caco2 cells. Among them were found the intermediate subunit of the Gal/GalNAc lectin, an alpha-actinin-like protein and two new proteins KERP1 and KERP2 rich in lysine and glutamic acid. In silico analysis revealed the presence of KERP2 in the closely related non-pathogenic amoeba species Entamoeba dispar but not of KERP1. In additon, polymerase chain reaction analysis allowed to suggest the absence of kerp1 homologous gene in E. dispar. Therefore, we concentrated on the cellular analysis of KERP1. Cloning of the KERP1-encoding gene, production of a recombinant protein in Escherichia coli and production of a specific antibody allowed us to show the following properties: (i) purified KERP1 binds to epithelial cell surface, (ii) KERP1 is located on the plasma membrane and in vesicles of trophozoites and (iii) KERP1 is delivered in the interstitial area between the trophozoites and the intestinal cells.

Optimization of virulence functions through glucosylation of Shigella LPS.

Shigella, the leading cause of bacillary dysentery, uses a type III secretion system (TTSS) to inject proteins into human cells, leading to bacterial invasion and a vigorous inflammatory response. The bacterium is protected against the response by the O antigen of lipopolysaccharide (LPS) on its surface. We show that bacteriophage-encoded glucosylation of Shigella O antigen, the basis of different serotypes, shortens the LPS molecule by around half. This enhances TTSS function without compromising the protective properties of the LPS. Thus, LPS glucosylation promotes bacterial invasion and evasion of innate immunity, which may have contributed to the emergence of serotype diversity in Shigella.