Renewal and patterning of the intestinal epithelium is coordinated by intestinal stem cells (ISCs); dietary and metabolic factors provide signals to the niche that control ISC activity. Bile acids (BAs), metabolites in the gut, signal nutrient availability by activating the G protein-coupled bile acid receptor 1 (GPBAR1, also called TGR5). TGR5 is expressed in the intestinal epithelium, but it is not clear how its activation affects ISCs and regeneration of the intestinal epithelium. We studied the role of BAs and TGR5 in intestinal renewal, and regulation of ISC function in mice and intestinal organoids.
We derived intestinal organoids from wild-type mice and Tgr5 mice, incubated them with BAs or the TGR5 agonist INT-777, and monitored ISC function by morphologic analyses and colony forming assays. We disrupted Tgr5 specifically in Lgr5-positive ISCs in mice (Tgr5 mice) and analyzed ISC number, proliferation, and differentiation by flow cytometry, immunofluorescence, and organoid assays. Tgr5 mice were given cholecystokinin; we measured the effects of BA release into the intestinal lumen and on cell renewal. We induced colitis in Tgr5 mice by administration of dextran sulfate sodium; disease severity was determined based on body weight, colon length, and histopathology analysis of colon biopsies.
BAs and TGR5 agonists promoted growth of intestinal organoids. Administration of cholecystokinin to mice resulted in acute release of BAs into the intestinal lumen and increased proliferation of the intestinal epithelium. BAs and Tgr5 expression in ISCs were required for homeostatic intestinal epithelial renewal and fate specification, and for regeneration after colitis induction. Tgr5 mice developed more severe colitis than mice without Tgr5 disruption in ISCs. ISCs incubated with INT-777 increased activation of yes-associated protein 1 (YAP1) and of its upstream regulator SRC. Inhibitors of YAP1 and SRC prevented organoid growth induced by TGR5 activation.
BAs promote regeneration of the intestinal epithelium via activation of TGR5 in ISCs, resulting in activation of SRC and YAP and activation of their target genes. Release of endogenous BAs in the intestinal lumen is sufficient to promote ISC renewal and proliferation in response to injury.
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