Scholarly article on topic 'Home Sweet Home: a Foxl1+ Mesenchymal Niche for Intestinal Stem Cells'

Home Sweet Home: a Foxl1+ Mesenchymal Niche for Intestinal Stem Cells Academic research paper on "Biological sciences"

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Academic research paper on topic "Home Sweet Home: a Foxl1+ Mesenchymal Niche for Intestinal Stem Cells"

CELLULAR AND MOLECULAR GASTROENTEROLOGY AND HEPATOLOGY

EDITORIAL_

Home Sweet Home: a FoxM + Mesenchymal Niche (ft

CrossMark

for Intestinal Stem Cells

The intestinal epithelium undergoes rapid and constant renewal accomplished by intestinal stem cells (ISCs) every 3-5 days. ISCs divide and give rise to highly proliferative transit-amplifying progenitor cells that differentiate into the multiple differentiated cell types present in the crypt and villus.1 ISCs reside in a microenvironment surrounded by epithelial and underlying mesenchymal cells within the lamina propria populated by multiple cell types. Over the past decade, considerable advances have shown that the ISC niche is composed of molecular mediators that act in a paracrine or autocrine manner to maintain proper ISC behavior marked by self-renewal and multipotency.2 Within this niche, it is well established that Wnt and Notch signaling are active, bone morphogenic protein (BMP) signaling is inhibited, and that this collective regulation is essential for proper intestinal epithelial renewal.3,4 Blocking Wnt signaling by genetic deletion of Wnt pathway components or ectopic expression of Wnt antagonist Dickkopf-1 disrupts intestinal epithelial homeostasis, marked by crypt loss, decreased proliferation, and altered differentiation.5-7

Paneth cells (PCs) secrete Wnt and Notch ligands whereas mesenchymal cells secrete Wnts and Gremlin proteins (BMP antagonists).2 R-spondins, proteins that enhance basal Wnt signaling, may be important for ISC maintenance because R-spondins promote intestinal orga-noid growth in vitro, regulate Lgr5+ ISC in vivo,8-11 and are expressed in the intestinal stroma.12 However, the definitive identification of niche cell types responsible for ISC ho-meostasis has been elusive.

It has been suggested that PCs are essential for the ISC niche based on Wnt expression and support of in vitro ISC self-renewal.13 However, subsequent studies in animal models lacking PCs showed no ISC dysfunction, suggesting that PCs are dispensable for ISC homeostasis.14,15 Conditional ablation of Porcn, required for Wnt secretion, in myosin heavy chain 11 (Myh11)-expressing myofibroblasts16 or villin-expressing epithelial cells,12 resulted in normal intestinal homeostasis, suggesting that Wnts from these cell types are not responsible for maintenance of ISCs and the intestinal epithelium. In this issue of Cellular and Molecular Gastroenterology and Hepatology, Aoki et al17 showed that a subset of mesenchymal cells marked by the winged-helix transcription factor Foxl1 are critical in maintaining ISC by contributing to the ISC niche.

By using anti-Foxl1 antibodies and Foxl1-Cre; Rosa-mT/ mG mice, the investigators confirmed that Foxl1 marks a novel mesenchymal population that is closely apposed to crypts and is distinct from a-smooth muscle actin-positive and Myh11+ smooth muscle cells and myofibroblasts. Foxl1+ cells also express high levels of growth factors capable of inducing Wnt signaling such as Wnt2b, Wnt5a, Rspo3, Grem1,

and Grem2, and are de-enriched for the ISC marker Lgr5. Based on their location and ability to regulate Wnt signaling, the investigators hypothesized that Foxl1+ mesenchymal cells could regulate the ISC niche. To explore this, Aoki et al used 2 independent models mediated by diphtheria toxin administration to ablate Foxl1+ cells: (1) transgenic animals expressing a bacterial artificial chromosome in which the Foxl1 coding region was replaced with the human diphtheria toxin receptor (hDTR), and (2) Foxl1-Cre-mediated activation of simian DTR. Loss of Foxl1 resulted in shorter intestines associated with decreased villus height, crypt depth, intestinal proliferation, and loss of Olfm4+ ISCs, although Lgr5 expression was notassessed directly. Despite loss of ISCs, Foxl1-hDTR animals continued to harbor Paneth and goblet cells. However, Foxl1-hDTR animals showed decreased nuclear b-catenin and Sox9+ crypt cells and ablation of Wnt2b, Wnt4, and Wnt5a messenger RNA. Interestingly, the loss of ISCs despite the presence of Lyz+ PCs indicated that Foxl1+ cells and PCs are not functionally redundant and that Foxl1+ mesenchymal cells are critical for ISC maintenance, potentially via production of Wnt2b, Wnt4, and Wnt5a or other factors.

Findings from this seminal study on Foxl1+ mesenchymal cells provide a long-sought definitive identification of a niche cell type required for ISC maintenance. This work also provides a mechanistic basis for prior observations of normal ISC homeostasis upon Porcn deletion in Myh11+ smooth muscle cells or in villin-expressing intestinal epithelial cells or PC ablation.12,14-16 Future characterization of the identity of Foxl1+ mesenchymal cells and their Wnt- and non-Wnt-secreted factors should greatly illuminate our knowledge of specific mediators responsible for maintaining ISCs. The current identification of the crucial Foxl1+ population also should enable the systematic Foxl1+-specific deletion of ISC regulatory factors and subsequent phenotypic analysis. In addition, as the investigators speculated, the Foxl1+ mesenchymal population now represents an attractive cellular target for ISC-based therapeutic strategies aimed at restoration of the intestinal epithelium through modulation of the ISC niche.

AMANDA T. MAH, PhD, MPH

CALVIN J. KUO, MD, PhD

Department of Medicine, Hematology Division

Stanford University

Stanford, California

References

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17. Aoki R, Shoshkes-Carmel M, Gao N, et al. Foxl1-expressing mesenchymal cells constitute the intestinal stem cell niche. Cell Mol Gastroenterol Hepatol 2016;2: 175-188.

Correspondence

Address correspondence to: Calvin J. Kuo, MD, PhD, Department of Medicine, Hematology Division, Stanford University, Lokey Stem Cell G2034A, 265 Campus Drive, Stanford, California 94305. e-mail: cjkuo@stanford.edu.

Conflicts of interest

The authors disclose no conflicts.

Funding

Supported by National Institute of Diabetes and Digestive and Kidney Diseases Intestinal Stem Cell Consortium (2U01DK085527-06 to C.J.K.).

(j) Most current article

© 2016 The Authors. Published by Elsevier Inc. on behalf of the AGA Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2352-345X

http://dx.doi.org/10.1016/j.jcmgh.2016.01.004