Intestinal homeostasis and stem cells are not affected when Paneth cells stop producing Wnt, but Wnt is required to maintain the stem cell niche in intestinal epithelial cultures, according to a mouse study in the December issue of Gastroenterology. These findings indicate that underlying mesenchymal cells provide a secondary physiological source of Wnt to intestinal stem cells.
Wnt signaling regulates many aspects of intestinal physiology, and also maintains stem cells. Paneth cells (which reside at the base of the crypt, interspersed with Lgr5+ stem cells) support the stem cells, producing important signaling factors. These include the Wnt ligands Wnt3, Wnt6, and Wnt9b; the Notch ligands Dll-1 and Dll-4; and EGF. However, it is not clear if Paneth cell production of all these factors is required to maintain the stem cell niche.
To examine the roles of Wnt production in the intestinal epithelia, Henner Farin et al. created mice with conditional deletion of Wnt3 (Vil-CreERT2;Wnt3fl/fl mice). They examined intestinal tissues and cultured epithelial cells from adult mice.
The authors found, unexpectedly, that Wnt3 was dispensable for maintenance of intestinal stem cells in mice. In the Vil-CreERT2;Wnt3fl/fl mice, long-lived Paneth cells were unaffected, as were the other secretory lineages (goblet, enteroendocrine, and tuft cells) and Olfm4+ stem cells.
Intestinal stem cells can be cultured ex vivo over long periods to form organoid structures that undergo continuous self-renewal and differentiation. Farin et al. found that these cultured crypt organoids required Paneth cell-derived Wnt3 to grow and bud. Addition of exogenous Wnt, or co-culture with mesenchymal cells, restored growth of Vil-CreERT2;Wnt3fl/fl crypt organoids. Addition of Wnt3 also allowed growth intestinal organoids formed from mice that do not form Paneth cells.
In an editorial that accompanies the article, Nicholas Smith et al. state that these findings reveal that compensatory signaling support is provided by the underlying mesenchymal cells of the intestine.
Farin et al. also found that Wnt signaling has synergistic effects with the Lgr4 and 5 ligand R-spondin that induce formation of Paneth cells.
The authors conclude that Wnt is part of a signaling loop that affects homeostasis of intestinal stem and Paneth cells in mice (see below figure).
Smith et al. propose that further studies with this combination in vitro/in vivo system could reveal additional mesenchymal–epithelial signaling mechanisms that mediate intestinal homeostasis and disease. The model could also be used to delineate the hierarchy of intestinal stem cells, specify the niche requirements of the +4 intestinal stem cell population, study the effects of the microbiome on stem cell behavior, and help develop therapeutics and tissue regeneration models.
Read the article online.
Farin HF, Van Es JH, Clevers H. Redundant sources of wnt regulate intestinal stem cells and promote formation of paneth cells. Gastroenterology 2012;143:1518-1529.e7.
Read the accompanying editorial.
Smith NR, Davies PS, Silk AD, Wong MH. Epithelial and mesenchymal contribution to the niche: a safeguard for intestinal stem cell homeostasis. Gastroenterology 2012;143:1426-1430.