INFLAMMATION-TRIGGERED CLAUDIN-23 DOWNREGULATION IMPACTS INTESTINAL EPITHELIAL BARRIER FUNCTION

Abstract

Abstract Intestinal epithelium maintains gut homeostasis by protecting the underlying tissue compartments from harmful contents. Although it is suggested that epithelial barrier function compromise contributes to the pathogenesis of inflammatory bowel disease (IBD), the underlying mechanisms are not well understood. Claudins (CLDN) are tight junctions (TJ) proteins that determine epithelial barrier properties during homeostasis and whose expression is altered in IBD, thereby contributing to a leaky barrier. We recently identified the expression of an atypical CLDN family member, CLDN23, in the regulation of homeostatic intestinal epithelial barrier function. Analysis of the human colonic crypt-luminal axis revealed increased CLDN23 expression in differentiated intestinal epithelial cells (IEC) facing the lumen compared to the proliferative crypt-base IECs. Complementary cell biologic approaches using IEC-specific Cldn23 knockout mice (Cldn23ERΔIEC) and human colon cell lines revealed that CLDN23 strengthens epithelial barrier function. Mechanistically, CLDN23 promoted redistribution of barrier-forming CLDN3 and CLDN4 from the lateral membrane to the TJ through cis- and trans-interactions, resulting in narrowed CLDN3 and CLDN4 pore diameters. Since inflammation compromises the epithelial barrier and contributes to disease pathogenesis in IBD, we first examined CLDN23 expression in colonic mucosal tissue from IBD individuals. Immunofluorescence and RNA scope labeling revealed that CLDN23 expression was significantly decreased in colonic epithelial cells of IBD individuals. Furthermore, reduced CLDN23 expression was also detected in primary colonic epithelial cells (colonoids) from mice treated with pro-inflammatory cytokines TNFα and IFNγ compared to untreated colonoids. Barrier function assays of control Cldn23f/f and Cldn23ERΔIEC mice-derived colonoids treated with TNFα and IFNγ identified decreased transepithelial electrical resistance in colonoids lacking CLDN23 and treated with these pro-inflammatory cytokines. These findings suggest a protective role for CLDN23 in regulating barrier function under homeostatic and inflammatory conditions.