Pathogenic Role of Mechanical Stress in Fibrosis and Tissue Remodeling in Crohn's Disease

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Stricture formation due to tissue fibrosis and smooth muscle hyperplasia is a hallmark of severe Crohn’s disease (CD). Although stricture formation is associated with chronic inflammation, no anti-inflammatory treatment is effective for it, except surgical approaches. However, post-surgery recurrences in the pre-stenotic region are almost 100%. Studies into the possible role of inflammation-independent mechanisms in fibrosis and hyperplasia are needed. Mechanical stress (MS) associated with tissue deformation, edema, fibrosis, and distention are commonly encountered in CD. We hypothesize that MS plays a critical role in fibrosis and hyperplasia in CD. We found in a well-defined rodent model of CD that intracolonic injection of TNBS induced a localized transmural inflammation with lumen narrowing in the distal colon and marked distention in the segment proximal to inflammation. We found that expression of connective tissue growth factor (CTGF) and brain-derived neurotrophic factor (BDNF) in colon smooth muscle cells (SMC) was markedly induced not only in the inflammation site but in the distended segment proximal to inflammation. We also detected significant fibrosis and hyperplasia in the inflammation site and the segment proximal to inflammation by 21 days. The non-distended segment distal to inflammation did not show any increased CTGF and BDNF, or fibrosis and hyperplasia, indicating a MS dependent mechanism. Furthermore, if mechanical distention was prevented by feeding rats with only liquid diet, which mimics exclusive enteral nutrition (EEN) in CD management, expression of CTGF and BDNF was dramatically attenuated and fibrosis was significantly improved. Mechanical stretch in vitro induced expression of CTGF and BDNF in colon SMC, and activated transcription activator yes-associated protein-1 (YAP). Moreover, YAP activity is found markedly increased in fibrostenotic CD tissues in humans. We propose that transmural inflammation in CD causes MS in the inflammation site and the distended segment proximal to inflammation, and the MS induces YAP-dependent mechanosensitive expression of CTGF and BDNF, which contribute to fibrosis and hyperplasia. The specific aims of the study are: 1. To determine if MS plays a role in intestinal fibrosis and SMC hyperplasia in CD. We will differentiate the effect of MS from inflammation by assessing site-specific changes of fibrosis, SMC growth, and expression of CTGF and BDNF in the site of inflammation (with both inflammation and MS), the segments proximal (with MS) and distal (with neither inflammation nor MS) to the inflammation site in the CD model. The role of MS in ECM production and SMC hyperplasia will be further assessed in CD without MS (rats fed with liquid diet) and in a model with MS only (mechanical obstruction). 2. To investigate the signaling mechanisms of MS-induced YAP activation and YAP-dependent expression of CTGF and BDNF. 3. To examine the pathogenic roles of YAP mediated mechanosensitive expression of CTGF and BDNF in fibrosis and hyperplasia. The possible cooperation between inflammation and MS in fibrosis and hyperplasia will be investigated as well.