Glial cell-line derived neurotrophic factor attenuates inflammation-induced breakdown of intestinal epithelial barrier function by stabilization of Dsg2-dependent intercellular adhesion

The glial‐cell line derived neurotrophic factor (GDNF) has been reported to be critically involved in intestinal epithelial barrier (IEB) maturation by affecting phosphorylation of p38 mitogen‐associated protein kinase (MAPK). Furthermore it has been demonstrated that inflammation‐induced loss of the intestinal barrier is mediated by increased phosphorylation of p38MAPK leading to a reduction of the desmosomal junctional protein desmoglein2 (Dsg2) at the cell borders. We hypothesized that GDNF acts in a p38MAPK/Dsg2‐dependent manner and thereby prevents inflammation‐induced breakdown of intestinal epithelial barrier function. Caco2 and HT29B6 monolayers mimicking in vitro models of the IEB showed after application of recombinant GDNF an increase of transepithelial electrical resistance (TEER) and increased mechanical stability of intercellular adhesion as revealed by dispase‐based enterocyte dissociation assays. Under steady state conditions GDNF induced strengthening of the cortical intermediate filament system as shown in immunostaining of cytokeratin18 (K18). This effect was paralleled by reduction of p38MAPK and K18 phosphorylation in Western blot analyses. Simultaneously, both immunostaining and triton‐based membrane protein extraction assays provided a significant augmentation of Dsg2 at the cell borders. Vice versa, TNFα‐induced breakdown of intestinal epithelial barrier confirmed by reduced TEER and loss of intercellular adhesion was accompanied by reduced Dsg2 at the cell borders and retraction of the cortical intermediate filament ring. Furthermore TNFα increased phosphorylation of both p38MAPK and K18. Interestingly, GDNF was able to block all TNFα‐induced changes. Comparable with GDNF also the application of the p38MAPK inhibitor SB202190 abrogated all TNFα‐induced changes in enterocytes underlining that the barrier protective effects of GDNF are based on a reduction of phosphorylated p38MAPK. To investigate the clinical relevance of these findings we analyzed human specimen of patients suffering from Crohn's disease that underwent a surgical resection of the terminal ileum and compared them with specimen of healthy patients. Western Blot analyses and ELISAs revealed a reduction of GDNF in highly inflamed parts of the terminal ileum compared to uninflamed areas of the same patient and control specimen of healthy patients. Taken together GNDF‐induced protective effects on IEB under resting conditions are mediated by a reduction of phosphorylated p38MAPK and K18 leading to augmented Dsg2 at the cell borders and increased intercellular adhesion. This mechanism appears to be of relevance in inflammation since all of the observed effects on IEB by TNFα were blocked by GDNF application. Our findings provide novel insights in the mechanism of GDNF in the regulation of epithelial barrier function under steady state and inflammatory conditions. Thus, GDNF might become important as a therapeutic agent to achieve barrier stabilization and in inflammatory bowel diseases. Support or Funding Information DFG Schl1962/5‐1IZKF Z‐2/63