Notch regulation of gastrointestinal stem cells

Abstract

The gastrointestinal (GI) tract epithelium is continuously replenished by actively cycling stem and progenitor cells. These cell compartments are regulated to balance proliferation and stem cell renewal with differentiation into the various mature cell types to maintain tissue homeostasis. In this topical review we focus on the role of the Notch signalling pathway to regulate GI stem cell function in adult small intestine and stomach. We first present the current view of stem and progenitor cell populations in these tissues and then summarize the studies that have established the Notch pathway as a key regulator of gastric and intestinal stem cell function. Notch signalling has been shown to be a niche factor required for maintenance of GI stem cells in both tissues. In addition, Notch has been described to regulate epithelial cell differentiation. Recent studies have revealed key similarities and differences in how Notch regulates stem cell function in the stomach compared to intestine. We summarize the literature regarding Notch regulation of GI stem cell proliferation and differentiation, highlighting tissue-specific functions to compare and contrast Notch in the stomach and intestine.

Figure 1. Notch signalling in the intestinal epithelium

Transmission of a Notch signal involves engagement of two cells: a signal‐sending cell (top) that expresses Notch ligand (DLL1 and DLL4 in intestine), and a signal‐receiving cell (bottom) that expresses Notch receptor (NOTCH1 and NOTCH2 in intestine and stomach). After ligand/receptor engagement, the Notch receptor is proteolytically cleaved: first by ADAM10, which releases the extracellular domain, and then by the γ‐secretase complex, which cleaves the receptor at the cell membrane to release the Notch intracellular domain (NICD). NICD translocates to the nucleus where it recruits a transcriptional co‐activator complex that activates downstream transcription of Notch target genes, such as Hes1 and Olfm4. RBPJ, Recombining binding protein suppressor of hairless; Mam, Mastermind (Both proteins are part of the transcriptional activation complex that interacts with NICD).

Figure 2. Stem and differentiated cells of the small intestine and stomach

Schematic diagram of the cellular composition of small intestinal crypt/villus units and gastric corpus and antral gland units, highlighting selective markers that are proposed to define distinct stem cell populations. In both stomach and intestine, stem cells proliferate to form transit‐amplifying (TA) progenitors which differentiate into the mature cell types of each tissue, as indicated. The mature cell types are replaced every several days (intestine and antrum) to several months (corpus). A, in the intestine, active Lgr5‐expressing CBC stem cells are located at the crypt base, intercalated between Paneth cells. A quiescent Bmi1‐marked stem cell (QSC) population is located approximately 4 cell positions from the crypt base. B, in the stomach, stem and progenitor cells are located in the upper mid‐region in corpus and at the gland base in antrum. LGR5 marks active stem cells in the antrum, but not corpus. Other potential gastric stem cell markers are discussed in the text.

Figure 3. Notch regulates gastointestinal epithelial cell differentiation

A, inhibition of Notch with the γ‐secretase inhibitor dibenzazepine (DBZ) induced expansion of all intestinal secretory cell lineages. Analysis of intestinal differentiated lineages by histological analysis, including goblet cells (PAS/AB), endocrine cells (chromogranin A; CgA) and Paneth cells (lysozyme) in control or Notch‐inhibited mouse duodenum. Arrows indicate CgA⁺ endocrine and lysozyme⁺ Paneth cells. Insets are magnified from lower‐powered boxed regions. B, analysis of gastric antral epithelial cells, including surface mucous cells (Muc5AC), deep mucous cells (TFF2) and endocrine cells (gastrin) in control, Notch‐inhibited and Notch‐activated (Lgr5‐CreER^T2; ROSA^NICD) mice. Notch inhibition induced differentiation of all three epithelial cell types, while Notch activation induced a generalized reduction in differentiation. Arrowheads indicate gastrin⁺ endocrine cells. Scale bars: 50 μm (B) or 100 μm (A). Figure modified from VanDussen et al. (2012) and Demitrack et al. (2015).

Figure 4. Model of Notch regulation of intestinal and gastric epithelial cell differentiation

Notch is required for renewal of both intestinal (A) and gastric antral (B) stem cells. In the intestine, Notch signalling represses the bHLH transcriptional regulator ATOH1 to form absorptive enterocytes. ATOH1⁺ secretory progenitor cells are fated towards 1 of 4 mature secretory cell types: endocrine, tuft, goblet or Paneth. Note that SPDEF and/or GFI1 regulate goblet and Paneth cell differentiation while NEUROG3 induces endocrine cell differentiation. A master transcriptional regulator, possibly a member of the bHLH family, has not been identified for the gastric antrum, although transcription factors that promote differentiation of each mature lineage are known. In the stomach, recent studies suggest that Notch functions to regulate gastric stem cell proliferation versus differentiation, and not choice between different differentiated cell types.