Wnt Ligand Expression and Regulation of Stem Cells During Postnatal Growth of the Small Intestine in Rats

Abstract

Background: The sequential proliferation, lineage-specific differentiation, crypt-villus migration, and cell death of the epithelial cells of the intestinal mucosa is tightly regulated by regulatory peptides, differentiation signals, and luminal stimuli, including nutrients and pathogenic/commensal organisms. Despite its importance for understanding normal homeostasis and pathogenesis of disease states, the intracellular signal transduction mechanisms involved remain incompletely understood. Here, we tested the hypothesis that PKD1 plays a key role in intestinal crypt cell proliferation In Vivo. Results: To clarify the role of PKD1 in intestinal epithelial cell proliferation In Vivo, we generated transgenic mice that express elevated PKD1 protein in the distal small intestinal and proximal colonic epithelium. The catalytic activity and multi-site phosphorylation (Ser744, Ser748 and Ser916) of PKD1 was strikingly higher in extracts from ileal mucosa of transgenic mice as compared with non-transgenic littermates. These results indicate that transgenic PKD1 is functional in the intestinal epithelium. PKD1 signaling stimulated intestinal cell proliferation, as shown by detection of 5-bromo-2-deoxyuridine (BrdU) incorporated into the cell nuclei of crypt cells of the ileum and proximal colon, where PKD1 protein is maximally expressed. Our results demonstrated a highly statistically significant increase (p<0.005) in DNA synthesizing cells in the crypts of the PKD1 transgenic mice as compared with non-transgenic littermates. In the intestine, normal cell numbers are maintained by balancing rates of cell proliferation, differentiation, migration and apoptosis. Consequently, we determined whether transgenic PKD1 leads to a change in tissue architecture, manifested by an increase in the size and total number of epithelial cells in the crypts. We measured crypt height (in micrometer and cell number) and crypt circumference (in micrometer and cell number) in histological sections of control and PKD1 transgenic mice. The data was used to calculate the size of individual cells and the total number of cells per crypt. Our results show a significant increase in the depth (either in μm or in number of cells) and in the total number of cells per crypt in the transgenic PKD1 mice as compared with the nontransgenic littermates (276 total cells per ileal crypt in transgenic mice versus 192 in nontransgenic mice; p< 0.005). These results indicate that the expression of the PKD1 transgene led to a marked increase (44%) in the total number of intestinal epithelial cells per crypt. Conclusion: Transgenic PKD1 expression increases the number of cells per crypt by stimulating the rate of crypt cell proliferation. These results support the hypothesis that PKD1 signaling plays a role in a pathway leading to proliferation in intestinal epithelial cells.