AP-1B-Mediated Protein Sorting Regulates Polarity and Proliferation of Intestinal Epithelial Cells in Mice

BACKGROUND & AIMS: In epithelial cells, protein sorting mechanisms regulate localization of plasma membrane proteins that generate and maintain cell polarity. The clathrin-adaptor protein (AP) complex AP-1B is expressed specifically in polarized epithelial cells, where it regulates basolateral sorting of membrane proteins. However, little is known about its physiological significance. METHODS: We analyzed the intestinal epithelia of mice deficient in Ap1m2 (Ap1m2(-/-) mice), which encodes the AP-1B mu 1B subunit, and compared it with 129/B6/CD1 littermates (controls). Notch signaling was inhibited by intraperitoneal injection of dibenzazepine, and beta-catenin signaling was inhibited by injection of IWR1. Intestinal tissue samples were collected and analyzed by immunofluorescence analysis. RESULTS: Ap1m2(-/-) mice developed intestinal epithelial cell hyperplasia. The polarity of intestinal epithelial cells was disrupted, as indicated by the appearance of ectopic microvilli-like structures on the lateral plasma membrane and mislocalization of basolateral membrane proteins, including the low-density lipoprotein receptor and E-cadherin. The E-cadherin-beta-catenin complex therefore was disrupted at the adherens junction, resulting in nuclear translocation of beta-catenin. This resulted in up-regulation of genes regulated by beta-catenin/transcription factor 4 (Tcf4) complex, and increased the proliferation of intestinal epithelial cells. CONCLUSIONS: AP-1B is required for protein sorting and polarization of intestinal cells in mice. Loss of AP-1B in the intestinal epithelia results in mislocalization of E-cadherin, activation of beta-catenin/Tcf4 complex, proliferation, and hyperplasia.