Current View: Intestinal Stem Cells and Signaling David H. Scoville, Toshiro Sato, Xi C. He, Linheng Li  Gastroenterology  Volume 134, Issue 3, Pages 849-864 (March 2008) DOI: 10.1053/j.gastro.2008.01.079 Copyright © 2008 AGA Institute Terms and Conditions

Figure 1 Model for 2 types of intestinal stem cells. (A) Pictorial representation of crypt-villus structure in the small intestine. A gradient of BMP signaling, known to inhibit proliferation, is established along the crypt-villus axis, with relatively high activity throughout the villus and correspondingly less activity within the crypt. An opposite gradient of Wnt signaling, providing an important proliferative stimulus, is highest at the crypt base and decreases toward the crypt-villus junction. In addition a very restricted gradient of BMP antagonists originates from stromal cells near the crypt base and assists Wnt signaling in ISCs. (B) An enlarged view of a small intestinal crypt depicting 2 different stem cell regions; a quiescent stem cell zone and an active stem cell zone. +4 LRCs are normally maintained in a quiescent state through direct interaction with and signals generated from the niche, such as pericryptal myofibroblasts and adjacent enteroendocrine cells within the +4 annulus. CBCs, continually activated by signals generated from stromal cells at the crypt base, such as pericryptal myofibroblasts and smooth muscle cells, are responsible for most of the regenerative capacity of the intestine under homeostatic conditions. (C) Under various conditions of stress or injury, +4 LRCs may undergo transient activation to generate progenitors as well as CBCs. In addition, CBCs may be capable of regenerating lost +4 LRCs. Both intrinsic and extrinsic molecules known to associate with +4 LRCs, in both quiescent and transiently active states, as well as CBCs are listed. Gastroenterology 2008 134, 849-864DOI: (10.1053/j.gastro.2008.01.079) Copyright © 2008 AGA Institute Terms and Conditions

Figure 2 Signaling pathways within the crypt – ISC activation. Outline of Notch, Wnt, PI3K, and BMP pathways and their potential points of interaction. As mentioned within the review, most of the data regarding pathways involved in regulating ISC activation are based on +4 LRCs. Normally +4 LRCs are maintained in a quiescent state through canonical BMP signaling via the transcriptional effects of SMADs and/or possible regulation of PTEN and subsequent inhibition of PI3K signaling. In addition, Wnt inhibitors, such as secreted frizzled-related proteins (sFRPs), act to hinder Wnt mediated effects. Transient activation of +4 LRCs is coordinately regulated by Wnt pathway activation, expression of BMP antagonists, such as Noggin that abrogate BMP inhibition, as well as PI3K activity. Induction of the PI3K pathway results in Akt activation and subsequent C-terminal S552 phosphorylation of β-catenin which likely affects the nuclear activity of this molecule. Thus, Akt assists Wnt-induced β-catenin activation that promotes cell cycle entry and progression within these putative ISCs. Evidence also suggests that Notch pathway activation may be permissive for Wnt induced crypt cell proliferation. Gastroenterology 2008 134, 849-864DOI: (10.1053/j.gastro.2008.01.079) Copyright © 2008 AGA Institute Terms and Conditions

Figure 3 Signaling pathways within the crypt – migration. Interaction between Ephrin B ligands and EphB receptors direct cellular localization and migratory behavior within the crypt. Cell location dictates cell state by defining exposure levels to secreted molecules and niche interactions. A gradient of Ephrin B1/B2 ligands exists within the crypt with cells at the crypt-villus junction expressing high levels of these molecules. An opposing gradient of EphB2 expression exists beginning at the crypt base. Thus the level of Ephrin B ligand and Wnt-induced EphB expression determines cell location. Interestingly, Paneth cells express EphB3 and no Ephrin B ligands, thereby restricting these cells to the crypt base. CBCs express both EphB3 and EphB2 while +4 LRCs express high levels of EphB2, thereby restricting upward migration of these cell types and ensuring that they are localized near their respective niches. Gastroenterology 2008 134, 849-864DOI: (10.1053/j.gastro.2008.01.079) Copyright © 2008 AGA Institute Terms and Conditions

Figure 4 Signaling pathways within the crypt – differentiation. Intestinal stem cells are thought to give rise to both secretory and absorptive progenitors, which are responsible for generating the mature cell types within their respective lineages. As putative ISCs, both +4 LRCs and CBCs may be able to produce both progenitor types as well as regenerate one another; however, thus far only CBCs have been shown to directly contribute to the major intestinal epithelial lineages. Secretory progenitors are directed and maintained through expression of Math-1 and Wnt-induced Sox9. Further expression of lineage specific transcription factors, such as Klf-4 for goblet cells, Ngn-3 for enteroendocrine cells, and β-catenin for Paneth cells promote maturation of these terminally differentiated secretory cells. Although Wnt signals are important for secretory progenitor maintenance and terminal Paneth cell maturation, BMP signals have recently been implicated in directing maturation of all 3 secretory cell types. Notch signaling activates Hes1, a bHLH transcription factor, which inhibits Math-1 and directs development of an absorptive progenitor and ultimately mature enterocytes. Gastroenterology 2008 134, 849-864DOI: (10.1053/j.gastro.2008.01.079) Copyright © 2008 AGA Institute Terms and Conditions