Bacterial Entry Into Host Cells

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[Mostowy et al., Nature Reviews Molecular Cell Biology 2012]

Cytoskeleton rearrangements during bacterial infection. a,b) Schematic of different types of infection processes. Invasive bacteria may enter non-phagocytic host cells through zippering (a) or triggering (b) mechanisms. In both cases, a cascade of events leads to actin polymerization and cytoskeletal rearrangements, which allow bacterial engulfment. Assembly of septin rings to the site of entry has been observed and is thought to promote interaction with the plasma membrane. c) Bacteria are subsequently internalized in a phagocytic vacuole. Some bacteria escape from the phagocytic vacuole to the cytoplasm. d) Once released into the cytoplasm, bacteria may form actin tails to move within the cell. Septins may form ring-like structures around bacterial actin tails (not shown). e) In the case of Shigella flexneri, bacteria may be trapped in septin cages, which are thought to restrict motility and therefore dissemination. f) Recruitment of actin and septin (SEPT9) at the site of entry for Listeria monocytogenes, a zippering bacterium. g) Recruitment of actin and septin (SEPT9) at the site of entry for S. flexneri, a triggering bacterium. h) Recruitment of septin (SEPT2) to the L. monocytogenes actin tail. i) Recruitment of septin (SEPT2) to cytosolic S. flexneri devoid of actin tails; that is, the S. flexneri septin cage.

 

 

 

 

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[Cossart, PNAS 2011]

The entry of L. monocytogenes into cells. Schematic representation of the signaling pathways triggered by InlA and InlB (Top). The three bottom images show recruitment of actin, clathrin, and septin (green fluorescence) at the site of bacterial entry (bacteria shown in red).

 

 

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[Pizarro-Cerda et al., BioEssays 2010]

Recruitment of clathrin to the entry sites of L. monocyto- genes and L. innocuaInlAþ. HeLa cells were infected for 5 min with a L. monocytogenes recombinant strain expressing InlB covalently attached to the bacterial cell wall. Immunofluorescence in non-permeabilized cells firstly allows labeling of extracellular bacteria (shown in blue); labeling of clathrin (red) and total bacteria (green) was performed subsequently after permeabilization. The area indi- cated by the arrowhead is magnified to better show how clathrin is recruited to the bacterial entry site. Bar: 6 mm.

 

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[Pizarro-Cerda-Cerda et al., Cell 2006]

Invasive Molecular Strategies of Salmonella, Shigella, Yersinia, and Listeria (A)The Yersinia outer membrane invasin interacts with b1 integrin receptors and favors activation of the small RhoGTPase Rac1, which will indirectly modulate the phosphatidylinositol metabolism to induce actin rearrangements at the site of bacterial entry, promoting invasion. Host kinases such as FAK or Src also participate in the process. (B) Salmonella translocates several effectors into target cells, some of them allowing the initial uptake of the bacterium: SipC is part of the TTSS and drives actin polymerization and actin-filament bundling; SopE activates Rho GTPases, fostering actin polimerization and membrane ruffle formation; SopB modulates inositol-polyphosphate metabolism, activating indirectly the same Rho GTPases as SopE; and SipA blocks the actin depolymerization factor cofilin, favoring also membrane ruffle formation. SptP plays a role once the internalization has taken place, inactivating the Rho GTPases, inhibiting actin polymerization, and helping the closure of the plasma membrane over internalized bacteria. (C) Shigella also translocates several TTSS effectors into target cells to induce invasion: the translocon component IpaC nucleates the formation of actin filaments; VirA indirectly stimulates the RhoGTPase Rac1 favoring actin polymerization (the host tyrosine kinases Abl/Arg also activate indirectly Cdc42 and Rac1) and inhibits microtubule polymerization; IpgD affects phosphoinositide metabolism and promotes the extension of membrane ruffles by decreasing the interactions between the plasma membrane and the actin cytoskeleton; IpaA activates the host protein vinculin, inducing actin depolymerization and recovery of the plasma membrane architecture once the bacteria are internalized. (D) Listeria invades target cells combining two molecular pathways. In the InlA-dependent pathway, the sortase-anchored bacterial protein InlA interacts with the cell adhesion molecule E-cadherin and promotes the subversion of cell adherens junction machinery (including b- and a-catenins) to induce entry. The myosin VIIA probably generates the contractile force required for bacterial engulfment. Actin polymerization relies, among other molecules, on the RhoGTPase Rac1.

(E) In the InlB-dependent pathway of Listeria, the loosely cell-wall-attached bacterial protein InlB interacts with the molecule gC1qR, and with the

signaling receptor Met, which recruits several molecular adaptors, which will perform several functions including the recruitment of a PI3K (involved in the activation of the RhoGTPase Rac1 and the polymerization of actin), and also the ubiquitination of Met and the endocytosis of the receptor via a clathrin-dependent mechanisms. A balance between actin polymerization and actin depolymerization required for efficient bacterial entry is controlled by regulation of the activities of the Lim kinase and the actin depolymerizing factor cofilin.

 

 

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Scanning electron microscopy (SEM). Adhesion of L. monocytogenes on a cell wall. Experiment performed at Pasteur Institute.

 

 

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Visualization of actin comet tails stained with two different antibodies against actin (Green and Red) in PtK2 cell infected with Listeria monocytogenes (Blue).
The image is an optical section taken from a Z-stack acquired with a Leica LSM 512 confocal microscope and a 63x oil immersion objective. Experiment was performed by Edith Gouin, Imaging was performed by Matteo Bonazzi

Updated on 13/05/2014

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