Lgr5 + crypt base columnar cells, the operational intestinal stem cells (ISCs), are thought to be dispensable for small intestinal (SI) homeostasis. Using a Lgr5-2A-DTR (diphtheria toxin receptor) model, which ablates Lgr5 + cells with near-complete efficiency and retains endogenous levels of Lgr5 expression, we show that persistent depletion of Lgr5 + ISCs in fact compromises SI epithelial integrity and reduces epithelial turnover in vivo. In vitro, Lgr5-2A-DTR SI organoids are unable to establish or survive when Lgr5 + ISCs are continuously eliminated by adding DT to the media. However, transient exposure to DT at the start of culture allows organoids to form, and the rate of outgrowth reduces with the increasing length of DT presence. Our results indicate that intestinal homeostasis requires a constant pool of Lgr5 + ISCs, which is supplied by rapidly reprogrammed non-Lgr5 + crypt populations when preexisting Lgr5 + ISCs are ablated.

Mitchell-riley syndrome (MRS) is caused by recessive mutations in the Regulatory Factor X, 6 (RFX6) gene and is characterised by pancreatic hypoplasia and neonatal diabetes. To determine why MRS patients specifically lack pancreatic endocrine cells, we micro-CT imaged a 12-week old foetus homozygous for the nonsense mutation RFX6 c.1129C>T, which revealed loss of the pancreas body and tail. From this foetus, we derived iPSC and show that differentiation of these cells in vitro proceeds normally until generation of pancreatic endoderm, which is significantly reduced. We additionally generated an RFX6HA reporter allele by gene targeting in wild-type H9 cells to precisely define RFX6 expression and in parallel performed in situ hybridization for RFX6 in the dorsal pancreatic bud of a Carnegie Stage 14 human embryo. Both in vitro and in vivo, we find that RFX6 specifically labels a subset of PDX1-expressing pancreatic endoderm. In summary, RFX6 is essential for efficient differentiation of pancreatic endoderm, and its absence in MRS patients specifically impairs formation of endocrine cells of the pancreas head and tail.

Mutated in Colorectal Cancer (MCC) encodes a coiled-coil protein implicated, as its name suggests, in the pathogenesis of hereditary human colon cancer. To date, however, the contributions of MCC to intestinal homeostasis and disease remain unclear. Here, we examine the subcellular localization of MCC, both at the mRNA and protein levels, in the adult intestinal epithelium. Our findings reveal that Mcc transcripts are restricted to proliferating crypt cells, including Lgr5+ stem cells, where the Mcc protein is distinctly associated with the centrosome. Upon intestinal cellular differentiation, Mcc is redeployed to the apical domain of polarized villus cells where non-centrosomal microtubule organizing centers (ncMTOCs) are positioned. Using intestinal organoids, we show that the shuttling of the Mcc protein depends on phosphorylation by Casein Kinases 1δ/ɛ, which are critical modulators of WNT signaling. Together, our findings support a role for MCC in establishing and maintaining the cellular architecture of the intestinal epithelium as a component of both the centrosome and ncMTOC.

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