GLI1-expressing mesenchymal cells form the essential Wnt-secreting niche for colon stem cells

Wnt-β-catenin signalling plays a pivotal role in the homeostasis of the intestinal epithelium by promoting stem cell renewal^1,2. In the small intestine, epithelial Paneth cells secrete Wnt ligands and thus adopt the function of the stem cell niche to maintain epithelial homeostasis^3,4. It is unclear which cells comprise the stem cell niche in the colon. Here we show that subepithelial mesenchymal GLI1-expressing cells form this essential niche. Blocking Wnt secretion from GLI1-expressing cells prevents colonic stem cell renewal in mice: the stem cells are lost and, as a consequence, the integrity of the colonic epithelium is corrupted, leading to death. GLI1-expressing cells also play an important role in the maintenance of the small intestine, where they serve as a reserve Wnt source that becomes critical when Wnt secretion from epithelial cells is prevented. Our data suggest a mechanism by which the stem cell niche is adjusted to meet the needs of the intestine via adaptive changes in the number of mesenchymal GLI1-expressing cells.