Signaling and epigenetic mechanisms of intestinal stem cells and progenitors: insight into crypt homeostasis, plasticity, and niches.

The rapid turnover of intestinal epithelial cells is maintained by a small number of stem cells located in pocket-like gland structures called crypts. While our understanding of the identity and function of intestinal stem cells (ISCs) has rapidly progressed, epigenetic and transcriptional regulation in crypt stem cell and progenitor pools remains an active field of investigation. Surrounded by various types of cells in the stroma, crypt progenitors display high levels of plasticity, harboring the ability to interconvert in the face of epithelial damage. Recent studies analyzing epigenetic patterns of intestinal epithelial cells have provided evidence that plasticity is maintained by a broadly permissive epigenomic state, wherein cell-lineage specification is directed through activation of signaling pathways and transcription factor (TF) expression. New studies also have shown that the ISC niche, which is comprised of surrounding epithelial and mesenchymal tissues, plays a crucial role in supporting the maintenance and differentiation of stem cells by providing contextual information in the form of signaling cascades, such as Wnt, Notch, and Hippo. These cascades ultimately govern TF expression to promote early cell-lineage decisions in both crypt stem cells and progenitors. Highlighting recent studies investigating signaling, transcriptional, and epigenetic mechanisms of intestinal epithelial cells, we will discuss the mechanisms underlying crypt homeostasis, plasticity, and niches. (C) 2017 Wiley Periodicals, Inc.