FZD5 is an essential regulator of intestinal crypt homeostasis and surrogate Wnt agonist response
The rapid regeneration of the small intestinal epithelium is sustained by crypt intestinal stem cells (ISCs).  Wnt/b-catenin signaling is essential for intestinal crypt homeostasis and maintenance of Lgr5+ ISC, and yet no single or combinatorial knockout of Frizzled (FZD) genes, representing Wnt receptors, has phenocopied the severe intestinal epithelial effects of Wnt signaling blockade.  The elusive identification of specific Frizzled (Fzd) receptor(s) underlying homeostatic proliferation and ISC function would greatly inform therapeutic mucosal repair strategies.  In prior pharmacologic studies, bioengineered antagonists that block Wnt binding to both FZD5 and FZD8 receptors induced lethal crypt and villus loss, implicating FZD5 and/or FZD8 as essential for ISCs maintenance.  Here, the potential function of Fzd5 in during intestinal homeostasis was examined by epithelial-specific Fzd5 ko, which rapidly elicited lethal pan-intestinal crypt and villus loss, and by Lgr5-specific Fzd5 cKO, which strongly reduced Lgr5+ ISC while inducing their premature differentiation. In parallel, Fzd5 cKO potently repressed Wnt target gene expression, with phenotypic rescue by constitutive activation of b-catenin in vivo and confirmation upon in vitro organoid culture.  Fzd5 cKO but not Fzd8 cKO in organoids ablated responsiveness to dual specificity bioengineered FZD5/FZD8-selective Wnt surrogate agonists, which reversed DSS-induced colitis phenotypes in both wild-type and Fzd8 cKO mice.  Overall, our results implicate the FZD5 receptor as an essential regulator of crypt homeostasis, Lgr5+ ISCs and intestinal response to bioengineered Wnt surrogate agonists. Overall design: Lgr5+ Intestinal stem cells were isolated by flow cytometry from the jejunum of Lgr5-eGFP-IRES-CreER mice (n=3 mice per condition). RNA isolation used the PicoPure RNA Isolation Kit (Applied Biosystems, Cat: KIT0204), followed by 10x Genomics Chromium 3' library preparation and NGS sequencing. Sequences from scRNA-seq were processed with Cell Ranger software (10x Genomics) with UMI (unique molecular identifier) collapsing and alignment to the GRCm38 mouse transcriptome. scRNA-seq data were loaded into Seurat objects following their standard pipeline