Regulation of epithelial permeability by the actin cytoskeleton

The actin cytoskeleton is a dynamic structure necessary for cell and tissue organization, including the maintenance of epithelial barriers. The epithelial barrier regulates the movement of ions, macromolecules, immune cells and pathogens, and is thus essential for normal organ function. Disruption in the epithelial barrier has been shown to coincide with alterations of the actin cytoskeleton in several disease states. These disruptions primarily manifest as increased movement through the paracellular space, which is normally regulated by tight junctions. Despite extensive research demonstrating a direct link between the actin cytoskeleton and epithelial permeability, our understanding of the physiological mechanisms that link permeability and tight junction structure are still limited. In this review we explore the role of the actin cytoskeleton at tight junctions and present several areas for future study

Epithelial barrier assembly requires coordinated activity of multiple domains of the tight junction protein ZO-1

Tight junctions (TJs) regulate the paracellular movement of ions, macromolecules and immune cells across epithelia. Zonula occludens (ZO)-1 is a multi-domain polypeptide required for the assembly of TJs. MDCK II cells lacking ZO-1, and its homolog ZO-2, have three distinct phenotypes: reduced localization of occludin and some claudins to the TJs, increased epithelial permeability, and expansion of the apical actomyosin contractile array found at the apical junction complex (AJC). However, it is unclear exactly which ZO-1 binding domains are required to coordinate these activities. We addressed this question by examining the ability of ZO-1 domain-deletion transgenes to reverse the effects of ZO depletion. We found that the SH3 domain and the U5 motif are required to recruit ZO-1 to the AJC and that localization is a prerequisite for normal TJ and cytoskeletal organization. The PDZ2 domain is not required for localization of ZO-1 to the AJC, but is necessary to establish the characteristic continuous circumferential band of ZO-1, occludin and claudin-2. PDZ2 is also required to establish normal permeability, but is not required for normal cytoskeletal organization. Finally, our results demonstrate that PDZ1 is crucial for the normal organization of both the TJ and the AJC cytoskeleton. Our results establish that ZO-1 acts as a true scaffolding protein and that the coordinated activity of multiple domains is required for normal TJ structure and function

Predominant Interferon-γ-Mediated Expression of CXCL9, CXCL10, and CCL5 Proteins in the Brain During Chronic Infection with Toxoplasma gondii in BALB/c Mice Resistant to Development of Toxoplasmic Encephalitis

We examined the role of interferon-γ (IFN-γ) in expression of chemokine mRNA and proteins in the brain during chronic infection with Toxoplasma gondii using BALB/c and BALB/c-background IFN-γ knockout (IFN-γ−/−) mice. BALB/c mice are genetically resistant to development of toxoplasmic encephalitis and establish a latent, chronic infection in the brain through IFN-γ-mediated immune responses. Amounts of mRNA for CXCL9/MIG, CXCL10/IP-10, CXCL11/I-TAC, CCL2/MCP-1, CCL3/MIP-1α, and CCL5/RANTES significantly increased in the brains of wild-type mice after infection. CXCL9/MIG, CXCL10/IP-10, and CCL5/RANTES mRNA were most abundant among these chemokines. An increase in amounts of mRNA for CXCL10/IP-10, CCL2/MCP-1, CCL3/MIP-1α, and CCL5/RANTES was also observed in the brains of IFN-γ−/− mice after infection, although CXCL10/I-10 and CCL5/RANTES mRNA levels in infected IFN-γ−/− mice were significantly lower than those of infected wild-type animals. Amounts of mRNA for CXCL9/MIG and CXCL11/I-TAC remained at the basal levels in infected IFN-γ−/− mice. When amounts of the chemokine proteins were examined in the brain homogenates of uninfected and infected mice of both strains, large amounts of CXCL9/MIG, CXCL10/IP-10, and CCL5/RANTES were detected only in infected wild-type animals. These results indicate that CXCL9/MIG, CXCL10/IP-10, and CCL5/RANTES are the chemokines predominantly induced in the brains of genetically resistant BALB/c mice during chronic infection with T. gondii, and their expression is dependent on IFN-γ