Shigella Navigates Tight Corners
Authors: Manuel Amieva
Journal: Cell Host & Microbe (2012)

In this issue, Fukumatsu et al., 2012Fukumatsu M. Ogawa M. Arakawa S. Suzuki M. Furuse M. Nakayama K. Shimizu S. Kim M. Mimuro H. Sasakawa C. Cell Host Microbe. 2012; 11 (this issue): 325-336Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar find that Shigella preferentially spread from cell-to-cell at unique intercellular junctions. Shigella protrusions invade adjacent cells at junctions where three cells meet, the tricellular junction. The tight junction protein tricellulin marks these sites and is important for Shigella spread. In this issue, Fukumatsu et al., 2012Fukumatsu M. Ogawa M. Arakawa S. Suzuki M. Furuse M. Nakayama K. Shimizu S. Kim M. Mimuro H. Sasakawa C. Cell Host Microbe. 2012; 11 (this issue): 325-336Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar find that Shigella preferentially spread from cell-to-cell at unique intercellular junctions. Shigella protrusions invade adjacent cells at junctions where three cells meet, the tricellular junction. The tight junction protein tricellulin marks these sites and is important for Shigella spread. Shigella and members of diverse bacterial genera like Listeria, Rickettsia, Burkholderia, and Mycobacteria have evolved the capacity to use the host cell actin cytoskeleton to move inside the cytosol and spread from cell to cell (Stevens et al., 2006Stevens J.M. Galyov E.E. Stevens M.P. Nat. Rev. Microbiol. 2006; 4: 91-101Crossref PubMed Scopus (164) Google Scholar). After attachment to the cell surface, Shigella inject effector proteins into the epithelial cell to induce bacterial uptake, lyse the endocytic vacuole to reach the cell cytosol, and then induce polymerization of host actin filaments at one pole of the bacteria. The force of actin polymerization pushes the invading Shigella within the cytosol, where they replicate as they move. When the bacteria are pushed against the inner surface of the host cell membrane, they protrude the membrane outward, forming finger-like pseudopodia with bacteria in their tips. When protrusions extend into adjacent cells, the recipient cell engulfs the Shigella into a double-membraned compartment, where the cycle begins again. The mechanisms by which Shigella form protrusions into adjacent epithelial cells and how they are engulfed were not well understood. In this issue of Cell Host & Microbe, Fukumatsu et al., 2012Fukumatsu M. Ogawa M. Arakawa S. Suzuki M. Furuse M. Nakayama K. Shimizu S. Kim M. Mimuro H. Sasakawa C. Cell Host Microbe. 2012; 11 (this issue): 325-336Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar begin to dissect the molecular pathways by which Shigella spread into adjacent cells. The authors used time-lapse microscopy to monitor bacterial spread within epithelial sheets (Fukumatsu et al., 2012Fukumatsu M. Ogawa M. Arakawa S. Suzuki M. Furuse M. Nakayama K. Shimizu S. Kim M. Mimuro H. Sasakawa C. Cell Host Microbe. 2012; 11 (this issue): 325-336Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar). They observed that cell-to-cell spread does not occur at random sites of cell contact. Rather, the protrusions preferentially penetrate through cell "corners" where three cells meet. These cell corners contain specialized tight junctions marked by the protein tricellulin, a transmembrane protein related to occludin. Transmembrane tight junction proteins are organized into rows that form branching strands in the plasma membrane. Their extracellular domains bind tightly to those of the adjacent cell, thus creating a tight seal. At junctions between two cells (bicellular junctions, bTj) the tight junction strands form a ring around the apical border between epithelial cells just above the adherens junctions. But where three cells meet, at tricellular contact sites, the tight junction strands extend down along the lateral membranes (Figure 1). These specialized tricellular tight junctions (tTj) are thought to form small channels that control paracellular flux (Krug et al., 2009Krug S.M. Amasheh S. Richter J.F. Milatz S. Günzel D. Westphal J.K. Huber O. Schulzke J.D. Fromm M. Mol. Biol. Cell. 2009; 20: 3713-3724Crossref PubMed Scopus (250) Google Scholar). Fukumatsu et al. show that Shigella preferentially spread at tricellulin-containing tricellular junctions (Fukumatsu et al., 2012Fukumatsu M. Ogawa M. Arakawa S. Suzuki M. Furuse M. Nakayama K. Shimizu S. Kim M. Mimuro H. Sasakawa C. Cell Host Microbe. 2012; 11 (this issue): 325-336Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar). They also determined that tricellulin is not just a marker for these sites, but that it is co-opted for the process of cell-to-cell spread. They used shRNA to knock down the expression of tricellulin in infected cells without affecting the rate of protrusion formation and found a reduction in the spread of Shigella to adjacent cells. Tricellulin depletion reduced Shigella spread more efficiently than knockdown of occludin or E-cadherin, which are enriched at bicellular tight junctions and adherens junctions respectively. After establishing that tricellular junctions are the preferred site of Shigella spread and that tricellulin participates in the process, the authors explore the downstream events in the uptake of Shigella protrusions and show that uptake depends on phosphatidylinositol 3-kinase (PI3-kinase) signaling and clathrin-mediated endocytosis (Fukumatsu et al., 2012Fukumatsu M. Ogawa M. Arakawa S. Suzuki M. Furuse M. Nakayama K. Shimizu S. Kim M. Mimuro H. Sasakawa C. Cell Host Microbe. 2012; 11 (this issue): 325-336Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar). Interestingly, clathrin-mediated endocytosis of the Shigella protrusions is distinct from classical endocytosis at the cell surface, as seen with the transferrin, EGF, or LDL receptors. In classical endocytosis, clathrin cooperates with AP-2 adaptors to form curved membranes that eventually pinch off into the cell (Reider and Wendland, 2011Reider A. Wendland B. J. Cell Sci. 2011; 124: 1613-1622Crossref PubMed Scopus (84) Google Scholar). Endocytosis of Shigella protrusions appears to be independent of the AP-2 adaptor but rather depends on another adaptor, Epsin-1. Epsin-1 is targeted to areas of endocytosis by binding the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2). It also binds ubiquitylated cargo, and is involved in modifying membrane curvature during endocytosis. The authors propose that uptake of the Shigella protrusion reflects a specialized form of endocytosis that depends on noncanonical clathrin-associated adaptors. The spread of Shigella at tricellulin-containing junctions suggests that these unique intercellular contacts have properties that allow transfer of membranes from adjacent cells by specialized endocytic processes. Is this something that Shigella actively induces in the host cell, or is it making use of a previously unrecognized mode of junctional remodeling? Does this form of endocytosis occur naturally during paraphagocytosis, the transfer of membrane bound fragments between epithelial cells (Robbins et al., 1999Robbins J.R. Barth A.I. Marquis H. de Hostos E.L. Nelson W.J. Theriot J.A. J. Cell Biol. 1999; 146: 1333-1350Crossref PubMed Scopus (129) Google Scholar)? We still do not know whether Shigella uses specialized effectors to interact with tricellulin or other components of the tricellular tight junctions. Alternatively, Shigella may not be modifying the tricellular junctions, but rather taking advantage of their inherent plasticity. In contrast to Shigella, Listeria show less of a preference for tricellular junctions when invading adjacent cells. The authors propose that Listeria uses additional mechanisms to invade near the bicellular junctions or through the lateral membranes instead. For example, the secreted Listeria protein internalin C (InlC) enhances spread of Listeria to adjacent cells by loosening the bicellular junctions and the cortical cytoskeleton through its inhibition of the actin-modifying proteins Tuba and N-WASP (Rajabian et al., 2009Rajabian T. Gavicherla B. Heisig M. Müller-Altrock S. Goebel W. Gray-Owen S.D. Ireton K. Nat. Cell Biol. 2009; 11: 1212-1218Crossref PubMed Scopus (136) Google Scholar). In the future, it may be interesting to explore whether a Listeria mutant in InlC preferentially penetrates tricellular junctions or whether ectopic expression of InlC in Shigella-infected cells (or coinfection with Listeria) abrogates the preference for spread at tricellular junctions and enables the use of bicellular junctions instead. It will also be important to document whether intercellular spread of Shigella through tricellular junctions is even more specific in fully polarized cells or in well-differentiated colonic epithelial tissue. The study by Fukumatsu and coworkers is an elegant reminder that bacterial pathogens teach us both about mechanisms of disease and about host cell biology. Shigella is a formidable pathogen, and more than 100 years after its discovery it is still responsible for over 100 million cases of dysentery each year and over half a million deaths in children (www.who.org) (Kotloff et al., 1999Kotloff K.L. Winickoff J.P. Ivanoff B. Clemens J.D. Swerdlow D.L. Sansonetti P.J. Adak G.K. Levine M.M. Bull. World Health Organ. 1999; 77: 651-666PubMed Google Scholar, Niyogi, 2005Niyogi S.K. J. Microbiol. 2005; 43: 133-143PubMed Google Scholar). Shigella's only natural environment is the human intestine, and understanding Shigella evolution in this context has led to several discoveries about how our cells function. Based on the Fukumatsu et al. study, Shigella's actions literally point us toward previously unknown properties of the junctions between epithelial cells.