Regulation of gene expression in gastric epithelial cell populations of fetal, neonatal, and adult transgenic mice.

Little is known about lineage relationships and differentiation programs of various epithelial cells present in mouse gastric units. We have previously used rat liver fatty acid binding protein/human growth hormone (L-FABP/hGH) transgenes to define epithelial cell lineages relationships in the small intestine of fetal and adult mice and to examine regulation of their terminal differentiation programs along the crypt-to-villus and duodenal-to-ileal axes. We have now used these transgenes to explore similar issues in the stomach. Immunocytochemical studies of fetal and adult transgenic L-FABP/hGH animals and their normal littermates revealed that the intact endogenous mouse L-FABP gene (Fabpl) is not expressed in gastric epithelium. Nucleotides-596 to +21 of the rat L-FABP gene direct "inappropriate" expression of hGH in the gastric epithelium as early as fetal day 15. From 1 to 13 mo, L-FABP-596 to +21/hGH expression occurs only in surface mucous cells of zymogenic and mucous gastric units; the reporter is not detectable in the enteroendocrine, parietal and chief cell populations of zymogenic glands. Electron microscopic immunocytochemistry revealed that hGH is directed to apical secretory granules in surface and pit mucous cells expressing the transgene. hGH levels vary widely among surface mucous cells both within single pits and between gastric units in a given animal. The heterogeneity noted in reporter expression suggests that there are marked differences in the regulatory environments of individual cells of a single type within a given gastric unit. This raises the possibility that cell differentiation programs in the stomach may not be as tightly coupled to cellular translocation as in the small intestine. Finally, the lack of expression of L-FABP-596 to +21/hGH in gastrin- and serotonin-immunoreactive cells of the stomach contrasts with its efficient expression in comparable cell types located in the duodenum; providing a model system for examining differential regulation of gene expression in terminally differentiated cell types represented in both gastric and intestinal epithelium.