Stroma provides an intestinal stem cell niche in the absence of epithelial Wnts

Abstract

Wnt/β-catenin signaling supports intestinal homeostasis by regulating proliferation in the crypt. Multiple Wnts are expressed in Paneth cells as well as other intestinal epithelial and stromal cells. Ex vivo, Wnts secreted by Paneth cells can support intestinal stem cells when Wnt signaling is enhanced with supplemental R-Spondin 1 (RSPO1). However, in vivo, the source of Wnts in the stem cell niche is less clear. Genetic ablation of Porcn, an endoplasmic reticulum resident O-acyltransferase that is essential for the secretion and activity of all vertebrate Wnts, confirmed the role of intestinal epithelial Wnts in ex vivo culture. Unexpectedly, mice lacking epithelial Wnt activity (Porcn Del /Villin-Cre mice) had normal intestinal proliferation and differentiation, as well as successful regeneration after radiation injury, indicating that epithelial Wnts are dispensable for these processes. Consistent with a key role for stroma in the crypt niche, intestinal stromal cells endogenously expressing Wnts and Rspo3 support the growth of Porcn Del organoids ex vivo without RSPO1 supplementation. Conversely, increasing pharmacologic PORCN inhibition, affecting both stroma and epithelium, reduced Lgr5 intestinal stem cells, inhibited recovery from radiation injury, and at the highest dose fully blocked intestinal proliferation. We conclude that epithelial Wnts are dispensable and that stromal production of Wnts can fully support normal murine intestinal homeostasis.

Targeting Wnts at the Source—New Mechanisms, New Biomarkers, New Drugs

“…Consistent with this, tankyrase inhibitors demonstrated severe, mechanism-based GI toxicity when given orally (14). In contrast with tankyrase inhibitors, PORCN inhibitors and the recombinant Frizzled inhibitory antibodies have not shown significant toxic effects on the intestine or skin, at least in mice, at doses that effectively inhibit cancer proliferation (30,34,37). Doses of the PORCN inhibitor Wnt C-59 10-fold higher than the therapeutic dose cause extensive loss of intestinal proliferation (37).…”

“…In contrast with tankyrase inhibitors, PORCN inhibitors and the recombinant Frizzled inhibitory antibodies have not shown significant toxic effects on the intestine or skin, at least in mice, at doses that effectively inhibit cancer proliferation (30,34,37). Doses of the PORCN inhibitor Wnt C-59 10-fold higher than the therapeutic dose cause extensive loss of intestinal proliferation (37). The reason for this therapeutic window and the differences in toxicity of these different drugs is unclear, but may be due to redundant pathways, differences in tissue penetration, or dosing schedules.…”

Ret receptor tyrosine kinase sustains proliferation and tissue maturation in intestinal epithelia

“…These data are consistent with the previous finding that elevated Wnt signalling promotes FEnS to organoid maturation (Fordham et al , 2013) and is reminiscent of the Wnt11/Ret autoregulatory loop promoting ureteric branching during kidney development (Majumdar et al , 2003). The relevant source of Wnt driving tissue maturation is currently unknown and is most likely not epithelial (Farin et al ., 2012; Kabiri et al , 2014; San Roman et al , 2014; Aoki et al , 2016; Valenta et al , 2016). Our Drosophila finding that Wg ligand upregulation is not transcriptional underscores the importance of considering tissue crosstalk and non‐autonomous signalling in any future studies addressing Wnt contributions to epithelial maturation in mice.…”

Single cell lineage tracing reveals a role for TgfβR2 in intestinal stem cell dynamics and differentiation

“…There are two main populations of ISCs: (i) a proliferating ISC population that is important for homeostasis of the niche residing below the +4 position and expressing a set of markers [e.g., leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5) and Olfm4] and (ii) a quiescent ISC population residing near the +4 position and expressing a different set of markers (e.g., Bmi1 and Hopx) (2). Proliferating ISCs are the workhorses during normal homeostasis and are maintained within the niche by a close relationship with Paneth cells (3) and the stroma (4). The proliferating ISC population can be further divided into a smaller number (4-8) of functional ISCs (5,6), which are located at the bottom of the crypt and are biased toward survival within the niche (7).…”