MITOCHONDRIAL TURNOVER IS ELEVATED IN RESPONSE TO INTESTINAL INFLAMMATION AND IS AMENABLE TO THERAPUTIC TARGETING VIA UROLITHIN A

Abstract Inflammatory bowel disease (IBD) is a complex, multifactorial set of diseases defined by chronic and relapsing inflammation of the gut. Changes in oxidative stress, inflammation, and metabolism are central to the disease process, linking mitochondrial health to IBD pathogenesis. Mitophagy is a quality-control mechanism that selectively removes dysfunctional mitochondria. Multiple mitophagy pathways co-exist in the intestine, including the canonical PINK1/PARKIN pathway and NIX. We have previously shown that NIX is increased in IBD, removing damaged mitochondria in the epithelium. Whether this translates to a physiologic increase in mitochondrial turnover remains unanswered. We hypothesized that mitophagy within the intestinal epithelium represents a key homeostatic response to inflammation. To study this hypothesis, we employed the use of a novel transgenic mouse expressing the pH-dependent fluorescent protein, mt-Keima, tethered to the mitochondria. Adult mt-Keima mice were subjected to chemically induced colitis (3% DSS) for 7 days to induce intestinal inflammation. In vivo mitophagic flux, measured via live-cell confocal microscopy, was increased nearly 4-fold (n=3, p<0.05) in DSS-treated animals compared to controls. Despite this consistent elevation, NIX expression varied in both human and mouse tissue, suggesting that other mitophagy pathways, like PARKIN, may be involved. Interestingly, activated PARKIN (pSer65-PARKIN) was higher in inflamed tissues of humans and mice as compared to controls, though variation was present. To determine the functional importance of PARKIN, Parkin-/- mice were subjected to a 7-day treatment of 2% DSS. These mice developed a more severe colitis, suggesting that Parkin-mediated mitophagy contributes to the mitochondrial damage response in IBD. These data collectively indicate that mitophagy is elevated in IBD, and both NIX and PARKIN likely work in parallel to respond to the elevated mitophagic demand. Interestingly, both NIX and pSer65-PARKIN expression are co-expressed in human control tissues, while both were generally expressed in inflamed biopsies, and at higher levels. We speculate that the mechanism of mitophagy may not necessarily be important, but that it is elevated in response to intestinal inflammation. To address this aim, we administered a known mitophagy inducer, urolithin A, to mice on a 7-day course of DSS. Urolithin A + DSS-treated mice had elevated NIX and pSer65-PARKIN expression compared to vehicle + DSS controls. This elevation correlated with decreased disease activity and improved O2 mitochondria respiration. We conclude that mitophagy is an imperative homeostatic response to remove superfluously damaged mitochondria in IBD and is amenable to therapeutic intervention. Elevating mitophagic flux might be a potential therapeutic option to help improve disease outcomes in IBD.