The mammalian intestine is covered by a single layer of epithelial cells that is renewed every 4-5 days. This high cell turnover makes it a very attractive and comprehensive adult organ system for ...the study of cell proliferation and differentiation. The intestine is composed of proliferative crypts, which contain intestinal stem cells, and villi, which contain differentiated specialized cell types. Through the recent identification of Lgr5, an intestinal stem cell marker, it is now possible to visualize stem cells and study their behavior and differentiation in a much broader context. In this review we describe the identification of intestinal stem cells. We also discuss genetic studies that have helped to elucidate those signals important for progenitor cells to differentiate into one of the specialized intestinal epithelial cell types. These studies describe a genetic hierarchy responsible for cell fate commitment in normal gut physiology. Where relevant we also mention aberrant deregulation of these molecular pathways that results in colon cancer.
The Wnt signaling pathway controls stem cell identity in the intestinal epithelium and in many other adult organs. The transcription factor Ascl2 (a Wnt target gene) is a master regulator of ...intestinal stem cell identity. It is unclear how the continuous Wnt gradient along the crypt axis is translated into discrete expression of Ascl2 and discrete specification of stem cells at crypt bottoms. We show that (1) Ascl2 is regulated in a direct autoactivatory loop, leading to a distinct on/off expression pattern, and (2) Wnt/R-spondin can activate this regulatory loop. This mechanism interprets the Wnt levels in the intestinal crypt and translates the continuous Wnt signal into a discrete Ascl2 “on” or “off” decision. In turn, Ascl2, together with β-catenin/Tcf, activates the genes fundamental to the stem cell state. In this manner, Ascl2 forms a transcriptional switch that is both Wnt responsive and Wnt dependent to define stem cell identity.
Display omitted
•Wnt and Ascl2 activate a gene signature fundamental to the intestinal stem cell state•β-catenin/Tcf4 and Ascl2 co-occupy DNA and synergistically activate transcription•Ascl2 forms an autoactivating loop that leads to an on/off expression pattern•This loop translates the Wnt gradient into a discrete transcriptional decision
The Wnt signaling pathway controls stem cell identity in the intestinal epithelium, but it has remained unclear how the continuous Wnt gradient is translated into discrete cell type specification. Schuijers et al. show that the transcription factor Ascl2 forms a bimodal switch that interprets Wnt levels and specifies stem cells.
Intestinal stem cells, characterized by high Lgr5 expression, reside between Paneth cells at the small intestinal crypt base and divide every day. We have carried out fate mapping of individual stem ...cells by generating a multicolor Cre-reporter. As a population, Lgr5hi stem cells persist life-long, yet crypts drift toward clonality within a period of 1–6 months. We have collected short- and long-term clonal tracing data of individual Lgr5hi cells. These reveal that most Lgr5hi cell divisions occur symmetrically and do not support a model in which two daughter cells resulting from an Lgr5hi cell division adopt divergent fates (i.e., one Lgr5hi cell and one transit-amplifying TA cell per division). The cellular dynamics are consistent with a model in which the resident stem cells double their numbers each day and stochastically adopt stem or TA fates. Quantitative analysis shows that stem cell turnover follows a pattern of neutral drift dynamics.
Display omitted
► An Intestinal crypt contains 14 equal Lgr5hi stem cells that all divide each day ► A multicolor Cre-reporter mouse was developed for lineage tracing of stem cells ► Most Lgr5hi stem cell divisions yield a symmetric cell fate outcome ► Homeostasis results from competition between symmetrically dividing Lgr5hi stem cells
The small intestinal epithelium is the most rapidly self-renewing tissue of mammals. Proliferative cells are confined to crypts, while differentiated cell types predominantly occupy the villi. We ...recently demonstrated the existence of a long-lived pool of cycling stem cells defined by
Lgr5 expression and intermingled with post-mitotic Paneth cells at crypt bottoms. We have now determined a gene signature for these Lgr5 stem cells. One of the genes within this stem cell signature is the Wnt target
Achaete scute-like 2 (
Ascl2). Transgenic expression of the Ascl2 transcription factor throughout the intestinal epithelium induces crypt hyperplasia and ectopic crypts on villi. Induced deletion of the
Ascl2 gene in adult small intestine leads to disappearance of the Lgr5 stem cells within days. The combined results from these gain- and loss-of-function experiments imply that Ascl2 controls intestinal stem cell fate.
The epithelium of the small intestine is the most rapidly self-renewing tissue in mammals. We previously demonstrated the existence of a long-lived pool of cycling stem cells defined by Lgr5 ...expression at the bottom of intestinal crypts. An Lgr5-eGFP-IRES-CreERT2 knockin allele has been instrumental in characterizing and profiling these cells, yet its low level expression and its silencing in patches of adjacent crypts have not allowed quantitative gene deletion. Olfactomedin-4 (Olfm4) has emerged from a gene signature of Lgr5 stem cells as a robust marker for murine small intestinal stem cells. We observe that Olfm4null animals show no phenotype and report the generation of an Olfm4-IRES-eGFPCreERT2 knockin mouse model that allows visualization and genetic manipulation of Lgr5+ stem cells in the epithelium of the small intestine. The eGFPCreERT2 fusion protein faithfully marks all stem cells in the small intestine and induces the activation of a conditional LacZ reporter with robust efficiency.
•Deletion of Olfm4 does not lead to a phenotype•Olfm4-IRES-eGFPCreERT2 robustly activates a LacZ reporter in vivo•Olfm4-driven eGFPCreERT2 expression marks the stem cells of the small intestine
Clevers and colleagues report that Olfm4null animals show no phenotype, and report the generation of an Olfm4-IRES-eGFPCreERT2 knockin mouse model that allows visualization and genetic manipulation of Lgr5+ stem cells in the epithelium of the small intestine. The eGFPCreERT2 fusion protein faithfully marks all stem cells in the small intestine and induces the activation of a conditional LacZ reporter with robust efficiency.
In immune responses, activated T cells migrate to B-cell follicles and develop into follicular T-helper (TFH) cells, a recently identified subset of CD4(+) T cells specialized in providing help to B ...lymphocytes in the induction of germinal centres. Although Bcl6 has been shown to be essential in TFH-cell function, it may not regulate the initial migration of T cells or the induction of the TFH program, as exemplified by C-X-C chemokine receptor type 5 (CXCR5) upregulation. Here we show that expression of achaete-scute homologue 2 (Ascl2)--a basic helix-loop-helix (bHLH) transcription factor--is selectively upregulated in TFH cells. Ectopic expression of Ascl2 upregulates CXCR5 but not Bcl6, and downregulates C-C chemokine receptor 7 (CCR7) expression in T cells in vitro, as well as accelerating T-cell migration to the follicles and TFH-cell development in vivo in mice. Genome-wide analysis indicates that Ascl2 directly regulates TFH-related genes whereas it inhibits expression of T-helper cell 1 (TH1) and TH17 signature genes. Acute deletion of Ascl2, as well as blockade of its function with the Id3 protein in CD4(+) T cells, results in impaired TFH-cell development and germinal centre response. Conversely, mutation of Id3, known to cause antibody-mediated autoimmunity, greatly enhances TFH-cell generation. Thus, Ascl2 directly initiates TFH-cell development.
Colorectal cancers are believed to arise predominantly from adenomas. Although these precancerous lesions have been subjected to extensive clinical, pathologic, and molecular analyses, little is ...currently known about the global gene expression changes accompanying their formation. To characterize the molecular processes underlying the transformation of normal colonic epithelium, we compared the transcriptomes of 32 prospectively collected adenomas with those of normal mucosa from the same individuals. Important differences emerged not only between the expression profiles of normal and adenomatous tissues but also between those of small and large adenomas. A key feature of the transformation process was the remodeling of the Wnt pathway reflected in patent overexpression and underexpression of 78 known components of this signaling cascade. The expression of 19 Wnt targets was closely correlated with clear up-regulation of KIAA1199, whose function is currently unknown. In normal mucosa, KIAA1199 expression was confined to cells in the lower portion of intestinal crypts, where Wnt signaling is physiologically active, but it was markedly increased in all adenomas, where it was expressed in most of the epithelial cells, and in colon cancer cell lines, it was markedly reduced by inactivation of the beta-catenin/T-cell factor(s) transcription complex, the pivotal mediator of Wnt signaling. Our transcriptomic profiles of normal colonic mucosa and colorectal adenomas shed new light on the early stages of colorectal tumorigenesis and identified KIAA1199 as a novel target of the Wnt signaling pathway and a putative marker of colorectal adenomatous transformation.
The Intestinal Wnt/TCF Signature Van der Flier, Laurens G; Sabates–Bellver, Jacob; Oving, Irma ...
Gastroenterology,
02/2007, Letnik:
132, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Background & Aims: In colorectal cancer, activating mutations in the Wnt pathway transform epithelial cells through the inappropriate expression of a TCF4 target gene program, which is ...physiologically expressed in intestinal crypts. Methods: We have now performed an exhaustive array-based analysis of this target gene program in colorectal cancer cell lines carrying an inducible block of the Wnt cascade. Independently, differential gene-expression profiles of human adenomas and adenocarcinomas vs normal colonic epithelium were obtained. Results Expression analyses of approximately 80 genes common between these data sets were performed in a murine adenoma model. The combined data sets describe a core target gene program, the intestinal Wnt/TCF signature gene set, which is responsible for the transformation of human intestinal epithelial cells. Conclusions The genes were invariably expressed in adenomas, yet could be subdivided into 3 modules, based on expression in distinct crypt compartments. A module of 17 genes was specifically expressed at the position of the crypt stem cell.
PDF
