The intestinal epithelium is the most rapidly self-renewing tissue in adult mammals. It is currently believed that four to six crypt stem cells reside at the +4 position immediately above the Paneth cells in the small intestine; colon stem cells remain undefined. Lgr5 (leucine-rich-repeat-containing G-protein-coupled receptor 5, also known as Gpr49) was selected from a panel of intestinal Wnt target genes for its restricted crypt expression. Here, using two knock-in alleles, we reveal exclusive expression of Lgr5 in cycling columnar cells at the crypt base. In addition, Lgr5 was expressed in rare cells in several other tissues. Using an inducible Cre knock-in allele and the Rosa26-lacZ reporter strain, lineage-tracing experiments were performed in adult mice. The Lgr5-positive crypt base columnar cell generated all epithelial lineages over a 60-day period, suggesting that it represents the stem cell of the small intestine and colon. The expression pattern of Lgr5 suggests that it marks stem cells in multiple adult tissues and cancers.

Homeostasis of self-renewing small intestinal crypts results from neutral competition between Lgr5 stem cells, small cycling cells located at crypt bottoms1, 2. Lgr5 stem cells are interspersed between terminally differentiated Paneth cells, that are known to produce bactericidal products such as lysozyme and cryptdins/defensins3. Single Lgr5-expressing stem cells can be cultured to form long-lived, self-organizing crypt-villus organoids in the absence of non-epithelial niche cells4. Here, we note a close physical association of Lgr5 stem cells with Paneth cells in vivo and in vitro. CD24+ Paneth cells express EGF, TGFα, Wnt3 and the Notch-ligand Dll4, all essential signals for stem cell maintenance in culture. Co-culturing of sorted stem cells with Paneth cells dramatically improves organoid formation. This Paneth cell requirement can be substituted by a pulse of exogenous Wnt. Genetic removal of Paneth cells in vivo results in the concomitant loss of Lgr5 stem cells. In colon crypts, CD24+ cells residing between Lgr5 stem cells may represent the Paneth cell equivalents. We conclude that Lgr5 stem cells compete for essential niche signals provided by a specialized daughter cell, the Paneth cell.

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, Lgr5(hi) 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 Lgr5(hi) cells. These reveal that most Lgr5(hi) cell divisions occur symmetrically and do not support a model in which two daughter cells resulting from an Lgr5(hi) cell division adopt divergent fates (i.e., one Lgr5(hi) 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.

Intestinal cancer is initiated by Wnt-pathway-activating mutations in genes such as adenomatous polyposis coli (APC). As in most cancers, the cell of origin has remained elusive. In a previously established Lgr5 (leucine-rich-repeat containing G-protein-coupled receptor 5) knockin mouse model, a tamoxifen-inducible Cre recombinase is expressed in long-lived intestinal stem cells. Here we show that deletion of Apc in these stem cells leads to their transformation within days. Transformed stem cells remain located at crypt bottoms, while fuelling a growing microadenoma. These microadenomas show unimpeded growth and develop into macroscopic adenomas within 3-5weeks. The distribution of Lgr5(+) cells within stem-cell-derived adenomas indicates that a stem cell/progenitor cell hierarchy is maintained in early neoplastic lesions. When Apc is deleted in short-lived transit-amplifying cells using a different cre mouse, the growth of the induced microadenomas rapidly stalls. Even after 30weeks, large adenomas are very rare in these mice. We conclude that stem-cell-specific loss of Apc results in progressively growing neoplasia.

The transactivation of TCF target genes induced by Wnt pathway mutations constitutes the primary transforming event in colorectal cancer (CRC). We show that disruption of beta-catenin/TCF-4 activity in CRC cells induces a rapid G1 arrest and blocks a genetic program that is physiologically active in the proliferative compartment of colon crypts. Coincidently, an intestinal differentiation program is induced. The TCF-4 target gene c-MYC plays a central role in this switch by direct repression of the p21(CIP1/WAF1) promoter. Following disruption of beta-catenin/TCF-4 activity, the decreased expression of c-MYC releases p21(CIP1/WAF1) transcription, which in turn mediates G1 arrest and differentiation. Thus, the beta-catenin/TCF-4 complex constitutes the master switch that controls proliferation versus differentiation in healthy and malignant intestinal epithelial cells.

The study of gastric epithelial homeostasis and cancer has been hampered by the lack of stem cell markers and in vitro culture methods. The Wnt target gene Lgr5 marks stem cells in the small intestine, colon, and hair follicle. Here, we investigated Lgr5 expression in the stomach and assessed the stem cell potential of the Lgr5(+ve) cells by using in vivo lineage tracing. In neonatal stomach, Lgr5 was expressed at the base of prospective corpus and pyloric glands, whereas expression in the adult was predominantly restricted to the base of mature pyloric glands. Lineage tracing revealed these Lgr5(+ve) cells to be self-renewing, multipotent stem cells responsible for the long-term renewal of the gastric epithelium. With an in vitro culture system, single Lgr5(+ve) cells efficiently generated long-lived organoids resembling mature pyloric epithelium. The Lgr5 stem cell marker and culture method described here will be invaluable tools for accelerating research into gastric epithelial renewal, inflammation/infection, and cancer.

The self-renewing epithelium of the small intestine is ordered into stem/progenitor crypt compartments and differentiated villus compartments. Recent evidence indicates that the Wnt cascade is the dominant force in controlling cell fate along the crypt-villus axis. Here we show a rapid, massive conversion of proliferative crypt cells into post-mitotic goblet cells after conditional removal of the common Notch pathway transcription factor CSL/RBP-J. We obtained a similar phenotype by blocking the Notch cascade with a gamma-secretase inhibitor. The inhibitor also induced goblet cell differentiation in adenomas in mice carrying a mutation of the Apc tumour suppressor gene. Thus, maintenance of undifferentiated, proliferative cells in crypts and adenomas requires the concerted activation of the Notch and Wnt cascades. Our data indicate that gamma-secretase inhibitors, developed for Alzheimer's disease, might be of therapeutic benefit in colorectal neoplastic disease.

The concept that tumors are maintained by dedicated stem cells, the so-called cancer stem cell hypothesis, has attracted great interest but remains controversial. Studying mouse models, we provide direct, functional evidence for the presence of stem cell activity within primary intestinal adenomas, a precursor to intestinal cancer. By "lineage retracing" using the multicolor Cre-reporter R26R-Confetti, we demonstrate that the crypt stem cell marker Lgr5 (leucine-rich repeat-containing heterotrimeric guanine nucleotide-binding protein-coupled receptor 5) also marks a subpopulation of adenoma cells that fuel the growth of established intestinal adenomas. These Lgr5(+) cells, which represent about 5 to 10% of the cells in the adenomas, generate additional Lgr5(+) cells as well as all other adenoma cell types. The Lgr5(+) cells are intermingled with Paneth cells near the adenoma base, a pattern reminiscent of the architecture of the normal crypt niche.