Effects of forced expression of an NH2-terminal truncated β-catenin on mouse intestinal epithelial homeostasis

beta-Catenin functions as a downstream component of the Wnt/Wingless signal transduction pathway and as an effector of cell-cell adhesion through its association with cadherins. To explore the in vivo effects of beta-catenin on proliferation, cell fate specification, adhesion, and migration in a mammalian epithelium, a human NH2-terminal truncation mutant (Delta N89 beta-catenin) was expressed in the 129/Sv embryonic stem cell-derived component of the small intestine of adult C57Bl/6-ROSA26<->129/Sv chimeric mice. Delta N89 beta-Catenin was chosen because mutants of this type are more stable than the wild-type protein, and phenocopy activation of the Wnt/Wingless signaling pathway in Xenopus and Drosophila. Delta N89 beta-Catenin had several effects. Cell division was stimulated fourfold in undifferentiated cells located in the proliferative compartment of the intestine (crypts of Lieberkuhn). The proliferative response was not associated with any discernible changes in cell fate specification but was accompanied by a three-to fourfold increase in crypt apoptosis. There was a marked augmentation of E-cadherin at the adherens junctions and basolateral surfaces of 129/Sv (Delta N89 beta-catenin) intestinal epithelial cells and an accompanying slowing of cellular migration along crypt-villus units. 1-2% of 129/Sv (Delta N89 beta-catenin) villi exhibited an abnormal branched architecture. Forced expression of Delta N89 beta-catenin expression did not perturb the level or intracellular distribution of the tumor suppressor adenomatous polyposis coli (APC). The ability of Delta N89 beta-catenin to interact with normal cellular pools of APC and/or augmented pools of E-cadherin may have helped prevent the 129/Sv gut epithelium from undergoing neoplastic transformation during the 10-mo period that animals were studied. Together, these in vivo studies emphasize the importance of beta-catenin in regulating normal adhesive and signaling functions within this epithelium.