Editors' ChoiceCell Biology

Restoring the Barrier While Promoting Communication

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Sci. Signal.  18 Aug 2009:
Vol. 2, Issue 84, pp. ec273
DOI: 10.1126/scisignal.284ec273

Disruption of the intestinal epithelial cell barrier leads to inflammatory bowel disease. Connexin43 (Cx43) is a member of a family of proteins that form protein pores called gap junctions that connect cells and coordinate cellular behavior through the passage of small molecules in a process called gap junctional intercellular communication (GJIC). Alterations in Cx43 can disrupt the intestinal epithelial cell barrier and contribute to inflammatory bowel disease. Toll-like receptors (TLRs), which recognize products of pathogenic and commensal microbes, also regulate intestinal inflammation. Pam3CysSK4 (PCSK), an agonist of Toll-like receptor 2 (TLR2), is known to stabilize the intestinal epithelial cell barrier and reduce inflammation associated with colitis in a mouse model. In addition to the previously reported mechanisms involving stabilization of tight junctions, Ey et al. show that TLR2 activity also enhances GJIC in wounded epithelial cell lines, reduces inflammation in mouse colon through a Cx43-mediated mechanism, and contributes to three-dimensional organization of explanted human colonic tissue. Treatment of the human intestinal epithelial cell line Caco-2 with PCSK increased the transcription, abundance, and membrane association of Cx43. In IEC-6 cells, another intestinal epithelial cell line, PCSK in the presence of cell wounding, but not in the absence, increased the extent and duration of GJIC. Transfection of a TLR2 mutant (TLR2-R753Q) that is associated with human colitis into the Caco-2 cells increased the mRNA for Cx43 but decreased the abundance of the protein, and these cells failed to show GJIC in response to wounding. Treatment of these cells with a proteasome inhibitor restored Cx43 and GJIC and the enhancement of GJIC by PCSK. In TLR2-deficient mice, instead of forming typical gap junctional plaques, Cx43 formed large, sheetlike patches, and Cx43 abundance was diminished in the colons of TLR2-deficient mice exposed to the intestinal irritant dextran sodium sulfate (DSS). In contrast to control DSS-exposed mice, when Cx43 was locally knocked down in the colons of DSS-exposed mice, PCSK failed to reduce inflammation. In a mouse model of chronic colitis, phosphorylation of Cx43 on Ser368, which is associated with inhibition of GJIC, was increased and the localization of Cx43 was aberrant. Treatment with PCSK decreased the Cx43 Ser368 phosphorylation and delayed disease onset. When human colonic punch biopsies were cultured in three-dimensional Matrigel supplemented with PCSK, the cells formed a cuboid three-dimensional geometry. Together, these data suggest that TLR2 signaling plays an important role in promoting gap junction communication and thereby in cell polarization and organization.

B. Ey, A. Eyking, G. Gerken, D. K. Podolsky, E. Cario, TLR2 mediates gap junctional intercellular communication through Connexin-43 in intestinal epithelial barrier injury. J. Biol. Chem. 284, 22332–22343 (2009). [Abstract] [Full Text]

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