Paneth cell mitochondria and intestinal homeostasis

About This Grant

Project Summary Mitochondrial dysfunction is rapidly emerging as a mediator in the pathogenesis of inflammatory bowel disease (IBD), including ulcerative colitis and Crohn's disease (CD). Recent mitochondrial studies from my lab have implicated chronic dysfunction in Paneth cells as a mediator of ileitis in mice (9). Paneth cells are secretory intestinal epithelial cells (IECs) most abundant in the ileum that contribute to gut homeostasis and innate immunity through secretion of antimicrobial peptides and intestinal stem cell factors (10). Paneth cell subsets have been identified with distinguishing transcriptomic signatures and secretomes based on intestinal location or disease state. 20- 50% of CD patients exhibit PCs with altered secretory granule phenotype that correlate with worse clinical outcomes, gut microbiota dysbiosis, and decreased expression of many mitochondrial-related genes including subunits of electron transport chain complex I. Our preliminary data suggest that these altered PCs also associate with increased CD74 expression, a marker of an inflammatory PC subset. As a novel model to mimic this mitochondrial dampening at complex I, we generated mice with deletion of Prohibitin 1 (PHB1) specifically in PCs driven by Defa6Cre (Phb1∆PC). PHB1 serves as a mitochondrial chaperone protein of the electron transport chain complex I subunits. Loss of PHB1 drives increased mitochondrial ROS (mtROS) due to malfunction of the electron transport chain. ROS are well-established signaling molecules often originating in the mitochondria, driving cell responses that are initiated by this organelle. Phb1∆PC mice develop spontaneous ileitis at 16-20 weeks of age that is preceded by Paneth cell abnormalities similar to those in CD ileitis patients and that our new preliminary data suggest is the manifestation of an inflammatory Cd74+ PC subtype dependent on mtROS. Counterintuitively, at 8 weeks of age prior to the onset of spontaneous ileitis, Phb1∆PC mice are protected when challenged with acute Salmonella typhimurium infection. Recently, host IL-18 has emerged as crucial for PC-induced defense, with specificity driven by predominant IL-18 receptor expression in PCs versus other IECs (23). Our preliminary data demonstrate that IL-18 induces TRAF6-dependent mtROS production and expression of Cd74 in PCs. PHB1 is K63-linked ubiquitinated during IL-18 treatment dependent on TRAF6, suggesting that PHB1 is a novel target of TRAF6-mediated ubiquitination, a post-translational modification linked to altered protein-protein interaction and altered protein trafficking (85). We will test the hypothesis is that IL-18 induces an inflammatory PC subtype driven by mitochondrial-derived reactive oxygen species (mtROS) signaling that is beneficial during acute infection, but when persistent, drives ileitis. We propose 2 specific aims: Aim 1) Define mitochondrial-induced expansion of CD74+ inflammatory PCs to impact intestinal homeostasis; Aim 2) Identify IL-18 mechanism driving mitochondrial signaling to induce ileal inflammatory PCs.