The CDC25 protein phosphatases (CDC25A, B, and C) drive cell cycle transitions by activating key components of the cell cycle engine. CDC25A and CDC25B are frequently overproduced in human cancers. Disruption of Cdc25B or Cdc25C individually or in combination has no effect on mouse viability. Here we report that CDC25A is the only family member to provide an essential function during early embryonic development, and that other family members compensate for its loss in adult mice. In contrast, conditional disruption of the entire family is lethal in adults due to a loss of small intestinal epithelial cell proliferation in crypts of Lieberkü hn. Cdc25 loss induced Wnt signaling, and overall crypt structures were preserved. In the face of continuous Wnt signaling, nearly all crypt epithelial progenitors differentiated into multiple cell lineages, including crypt base columnar cells, a proposed stem cell. A small population of Musashi/Dcamkl-1/nuclear ␤-catenin-positive epithelial cells was retained in these crypts. These findings have implications for the development of novel, less cytotoxic cancer chemotherapeutic drugs that specifically target the cell cycle.CDC25 phosphatases ͉ cell cycle ͉ stem cells T he CDC25 phosphatases promote cell cycle progression by activating cyclin-dependent protein kinases (1-6). In mice, Cdc25A, B, and C exhibit overlapping but distinct patterns of expression during development and in adult tissues (7-10). This suggests that they have distinct biological functions in embryonic and adult mice. Mice lacking CDC25B and CDC25C, individually or in combination, are viable and develop normally, and embryonic fibroblasts derived from these mice exhibit normal cell cycle parameters in culture (11-13). These findings demonstrate that mice can survive throughout embryogenesis and adulthood with a single member of the family, CDC25A. Here we report the consequences of deleting Cdc25A alone and in combination with Cdc25B and Cdc25C in mice. Our data demonstrate that CDC25A provides an essential function during early embryogenesis, and that CDC25B and/or CDC25C compensate for CDC25A loss in adult mice. In contrast, mice lacking all 3 CDC25s die within 1 week due to complete loss of epithelial cell proliferation in the small intestinal crypts.We used this model to explore how small intestine stem and progenitor cells respond to the acute disruption of cell division. The self-renewing epithelium of the adult small intestine contains tetrapotent stem cells that give rise to rapidly proliferating committed daughter cells, which in turn produce terminally differentiated cells (14). One of these lineages, Paneth cells, are located in crypts of Lieberkühn along with stem and progenitor cells. Analysis of gene expression and cellular morphology, as well as lineage tracing experiments, suggest that stem cells are intermingled with or lie just above Paneth cells at the crypt base (15-18). These stem cells are considered to provide the source of recovery after damage to the epithelial lining from such facto...

The CDC25 protein phosphatases drive cell cycle advancement by activating cyclin-dependent protein kinases (CDKs). Humans and mice encode three family members denoted CDC25A, -B and -C and genes encoding these family members can be disrupted individually with minimal phenotypic consequences in adult mice. However, adult mice globally deleted for all three phosphatases die within one week after Cdc25 disruption. A severe loss of absorptive villi due to a failure of crypt epithelial cells to proliferate was observed in the small intestines of these mice. Because the Cdc25s were globally deleted, the small intestinal phenotype and loss of animal viability could not be solely attributed to an intrinsic defect in the inability of small intestinal stem and progenitor cells to divide. Here, we report the consequences of deleting different combinations of Cdc25s specifically in intestinal epithelial cells. The phenotypes arising in these mice were then compared with those arising in mice globally deleted for the Cdc25s and in mice treated with irinotecan, a chemotherapeutic agent commonly used to treat colorectal cancer. We report that the phenotypes arising in mice globally deleted for the Cdc25s are due to the failure of small intestinal stem and progenitor cells to proliferate and that blocking cell division by inhibiting the cell cycle engine (through Cdc25 loss) versus by inducing DNA damage (via irinotecan) provokes a markedly different response of small intestinal epithelial cells. Finally, we demonstrate that CDC25A and CDC25B but not CDC25C compensate for each other to maintain the proliferative capacity of intestinal epithelial stem and progenitor cells.