The bHLH Transcription Factor ASCL1 Promotes Differentiation of Endocrine Cells in the Stomach and is Regulated by Notch Signaling.

Notch signaling regulates gastrointestinal stem cell proliferation and differentiation yet Notch-regulated transcriptional effectors of gastric epithelial cell differentiation are poorly understood. Here we tested the role of the bHLH transcription factor achaete-scute homolog 1 (ASCL1) in gastric epithelial cell differentiation, and its regulation by Notch. Newborn Ascl1 null mice showed a loss of expression of markers of neurogenin-3 dependent enteroendocrine cells, with normal expression of enterochromaffin-like cells, mucous cells, chief cells, and parietal cells. In adult mice, Ascl1 gene expression was observed in stomach, but not intestine, with higher expression in antral than corpus epithelium. Lineage tracing in Ascl1-CreERT2 ; Rosa26-LSL-tdTomato mice revealed single, scattered ASCL1+ cells in the gastric epithelium, with expression in antral gastrin- and serotonin-producing endocrine cells. ASCL1-expressing endocrine cells persisted for several weeks post-tamoxifen labeling with a half-life of approximately 2 months. Lineage tracing in Gastrin-CreERT2 mice demonstrated a similar lifespan for gastrin-producing cells, confirming that gastric endocrine cells are long-lived. Finally, treatment of Ascl1-CreERT2 ; Rosa26-LSL-tdTomato mice with the pan-Notch inhibitor dibenzazepine increased the number of lineage-labeledcells in the gastric antrum, suggesting that Notch signaling normally inhibits Ascl1 expression. Notch regulation of Ascl1 was also demonstrated in a genetic mouse model of Notch activation, as well as Notch-manipulated antral organoid cultures, thus suggesting that ASCL1 is a key downstream Notch pathway effector promoting endocrine cell differentiation in the gastric epithelium.