Impenetrable barriers or entry portals? The role of cell-cell adhesion during infection

Abstract

Cell-cell adhesion plays a fundamental role in cell polarity and organogenesis. It also contributes to the formation and establishment of physical barriers against microbial infections. However, a large number of pathogens, from viruses to bacteria and parasites, have developed countless strategies to specifically target cell adhesion molecules in order to adhere to and invade epithelial cells, disrupt epithelial integrity, and access deeper tissues for dissemination. The study of all these processes has contributed to the characterization of molecular machineries at the junctions of eukaryotic cells that have been better understood by using pathogens as probes.

Figure 1.

Tight junctions and infection. (A) Tight junctions (TJs) form at the border between the apical and the basolateral side of neighboring cells. Their main components are occludins, claudins, the JAM family of receptors and the CAR receptors. All of these receptors interact with the actin cytoskeleton via the members of the ZO family of proteins (ZO-1, -2, and -3). (B) The Rotavirus (RotaV) toxin NSP4 is released in the host cytoplasm where it increases the concentration of intracellular calcium and disrupts the actin cytoskeleton. VP4 is cleaved into VP5 and VP8, the latter interfering with the recruitment of ZO-1, occludin, and claudin at the plasma membrane. Upon interacting with the LDL receptor at the surface of the host cell, the hepatitis C virus (HCV) uses claudins as receptors for internalization. Reovirus (ReoV) uses the viral surface protein σ1 to interact with the JAM-1 receptors and invade host cells. Adenovirus (AV) binds the coxsackievirus and adenovirus receptor (CAR) for internalization. Coxsackievirus (CXV) interaction with CD55 at the surface of host cells triggers a signaling cascade that activates the Src kinases Abl and Fyn. Abl activates Rac, inducing the rearrangements of the actin cytoskeleton that reposition coxsackie viral particles to TJs. Here the virus binds to CAR proteins to invade host cells. (C) Enteropathogenic (shown) and enterohemorragic E. coli (EPEC and EHEC) first adhere to host cells by bundling-forming pili (BFP, not depicted). Then T3SS effectors are secreted, including the bacterial protein Tir, which inserts into the host cell plasma membrane and serves as the bacterial receptor. The T3SS effectors EspF, EspG, and Map are able to activate ezrin, dephosphorylate occludin, and activate MLCK. Activated MLCK can in turn activate myosin II that might have a role in TJs destabilization by pulling on actin filaments. Vibrio cholerae secretes the metalloprotease HA/P that degrades the extracellular domain of occludin, accompanied by the dissociation of ZO-1 from TJs due to the conformational change of the cytoplasmic domain of occludin that remains associated with the plasma membrane. Clostridium perfringens secretes the enterotoxin CPE that binds occludin, claudin-3, and claudin-4 and induce their degradation. Clostridium difficile secretes toxin A and toxin B that monoglucosylate the small GTPases Rac, Cdc42, and Rho, thereby affecting cytoskeleton dynamics. Helicobacter pylori injects the effector protein CagA, which is phosphorylated by Src within the host cell. This triggers the inhibition of Src activity, the dephosphorylation of cortactin, and the interaction of CagA with SHP-2. In addition, CagA activates the Met receptor, triggering a motile response in infected cells, and independently recruits TJ proteins at the bacteria/cell adhesion sites. The Salmonella SPI1 effectors SopB, SopE, SopE2, and SipA alter TJs by decreasing ZO-1 expression levels (crooked arrow) as well as delocalizing occludin, whereas the Salmonella Typhimurium T3SS effector AvrA stabilizes TJ. Shigella infections affect the expression levels of ZO-1, claudin-1, and the phosphorylation state of occludin, leading to a severe disruption of TJs. The Listeria monocytogenes virulence factor InlC relieves cortical actin tension at cell–cell contacts by binding the mammalian adaptor protein Tuba, preventing its interaction with N-WASP.

Figure 2.

Adherens junctions, gap junctions, and infection. (A) Adherens junctions (AJs) and gap junctions (GJs) form at the lower region of cell–cell contacts. The main components of AJs are the members of the cadherin family and nectins. Both interact with the actin cytoskeleton by means of catenins (α and β) and afadin, accessory proteins of cadherins and nectins, respectively. Gap junctions are established upon interactions of connexin-based pores at the membrane of neighboring cells. (B) Herpes simplex virus (HSV) uses the glycoprotein D (gD) to interact with nectins during infection. Listeria monocytogenes adheres to host cells by means of the surface protein InlA that interacts with E-cadherin and triggers the recruitment of α- and β-catenin, myosin VIIa, vezatin, and ARHGAP10 at bacterial entry sites. E-cadherin clustering activates Src that is upstream of E-cadherin ubiquitination mediated by the ubiquitin ligase Hakai. This triggers the recruitment of clathrin at the bacterial entry site required to recruit actin by means of Hip1R and bring about bacterial internalization by recruiting myosin VI. Bacteroides fragilis produces the toxin fragilysin, which cleaves the extracellular domain of E-cadherin disrupting adherens junctions and affecting TJ integrity. Candida albicans interacts with N-cadherin on endothelial cells and E-cadherin on oral epithelial cells by means of the surface invasins Als1 and Als3. Candida hyphae internalization requires the rearrangements of the actin cytoskeleton and induces the recruitment of clathrin, dynamin (Dyn), and cortactin (Cort) at the internalization site. (C) Shigella cell-to-cell spread is facilitated by the bacterial-mediated opening of connexin 26 hemichannels that allows the diffusion of calcium from infected to noninfected neighboring cells and the release of ATP in the medium. The Shigella effectors OspG and OspF attenuate IL-8 production in infected cells; however, the opening of Gap junctions favors the cell-to-cell diffusion of the bacterial peptidoglycan (LPS) that would then activate IL-8 production in bystander, noninfected cells. Listeria monocytogenes infections induce the synthesis of reactive oxygen intermediates (ROI) that spread to neighboring, noninfected cells by a GJ-independent mechanism and induce the activation of the MAP kinase Erk. This in turn regulates the synthesis of the proinflammatory chemokine Cxcl-2.