Listeria monocytogenes Invades the Epithelial Junctions at Sites of Cell Extrusion

Abstract

Listeria monocytogenes causes invasive disease by crossing the intestinal epithelial barrier. This process depends on the interaction between the bacterial surface protein Internalin A and the host protein E-cadherin, located below the epithelial tight junctions at the lateral cell-to-cell contacts. We used polarized MDCK cells as a model epithelium to determine how L. monocytogenes breaches the tight junctions to gain access to this basolateral receptor protein. We determined that L. monocytogenes does not actively disrupt the tight junctions, but finds E-cadherin at a morphologically distinct subset of intercellular junctions. We identified these sites as naturally occurring regions where single senescent cells are expelled and detached from the epithelium by extrusion. The surrounding cells reorganize to form a multicellular junction that maintains epithelial continuity. We found that E-cadherin is transiently exposed to the lumenal surface at multicellular junctions during and after cell extrusion, and that L. monocytogenes takes advantage of junctional remodeling to adhere to and subsequently invade the epithelium. In intact epithelial monolayers, an anti-E-cadherin antibody specifically decorates multicellular junctions and blocks L. monocytogenes adhesion. Furthermore, an L. monocytogenes mutant in the Internalin A gene is completely deficient in attachment to the epithelial apical surface and is unable to invade. We hypothesized that L. monocytogenes utilizes analogous extrusion sites for epithelial invasion in vivo. By infecting rabbit ileal loops, we found that the junctions at the cell extrusion zone of villus tips are the specific target for L. monocytogenes adhesion and invasion. Thus, L. monocytogenes exploits the dynamic nature of epithelial renewal and junctional remodeling to breach the intestinal barrier. Studies in microbial pathogenesis are just beginning to address how epithelial cell polarity and host tissue architecture influence host-pathogen interactions. It has long been known that Listeria monocytogenes invasion of epithelial cells requires binding of host cell E-cadherin, a cell-to-cell junction protein. However, an ongoing question has been how L. monocytogenes reaches this receptor, since E-cadherin is located in the basolateral membrane of enterocytes and is inaccessible to bacteria in the intestinal lumen. By examining polarized epithelial monolayers in tissue culture and rabbit intestine in vivo, the authors investigate how and where L. monocytogenes breaches the tight junctions to interact with E-cadherin and invade. They show that L. monocytogenes takes advantage of a temporal window of junctional reorganization that exposes E-cadherin at sites where senescent cells are extruded and removed from the epithelium. In the small intestine, cell extrusion is confined to the tips of the intestinal villi, and the authors show that junctions at the villus tips are the sites of L. monocytogenes invasion in vivo. If basolateral proteins are exposed at cell extrusion sites, do other microbes that utilize basolateral cellular receptors also find them?