Parietal cell hyperstimulation and autoimmune gastritis in cholera toxin transgenic mice

The stimulation of gastric acid secretion from parietal cells involves both intracellular calcium and cAMP signaling. To understand the effect of increased cAMP on parietal cell function, we engineered transgenic mice expressing cholera toxin (Ctox), an irreversible stimulator of adenylate cyclase. The parietal cell-specific H(+),K(+)-ATPase beta-subunit promoter was used to drive expression of the cholera toxin A1 subunit (CtoxA1). Transgenic lines were established and tested for Ctox expression, acid content, plasma gastrin, tissue morphology, and cellular composition of the gastric mucosa. Four lines were generated, with Ctox-7 expressing approximately 50-fold higher Ctox than the other lines. Enhanced cAMP signaling in parietal cells was confirmed by observation of hyperphosphorylation of the protein kinase A-regulated proteins LASP-1 and CREB. Basal acid content was elevated and circulating gastrin was reduced in Ctox transgenic lines. Analysis of gastric morphology revealed a progressive cellular transformation in Ctox-7. Expanded patches of mucous neck cells were observed as early as 3 mo of age, and by 15 mo, extensive mucous cell metaplasia was observed in parallel with almost complete loss of parietal and chief cells. Detection of anti-parietal cell antibodies, inflammatory cell infiltrates, and increased expression of the Th1 cytokine IFN-gamma in Ctox-7 mice suggested that autoimmune destruction of the tissue caused atrophic gastritis. Thus constitutively high parietal cell cAMP results in high acid secretion and a compensatory reduction in circulating gastrin. High Ctox in parietal cells can also induce progressive changes in the cellular architecture of the gastric glands, corresponding to the development of anti-parietal cell antibodies and autoimmune gastritis.     

Cdx2 determines the fate of postnatal intestinal endoderm

Knock out of intestinal Cdx2 produces different effects depending upon the developmental stage at which this occurs. Early in development it produces histologically ordered stomach mucosa in the midgut. Conditional inactivation of Cdx2 in adult intestinal epithelium, as well as specifically in the Lgr5-positive stem cells, of adult mice allows long-term survival of the animals but fails to produce this phenotype. Instead, the endodermal cells exhibit cell-autonomous expression of gastric genes in an intestinal setting that is not accompanied by mesodermal expression of Barx1, which is necessary for gastric morphogenesis. Cdx2-negative endodermal cells also fail to express Sox2, a marker of gastric morphogenesis. Maturation of the stem cell niche thus appears to be associated with loss of ability to express positional information cues that are required for normal stomach development. Cdx2-negative intestinal crypts produce subsurface cystic vesicles, whereas untargeted crypts hypertrophy to later replace the surface epithelium. These observations are supported by studies involving inactivation of Cdx2 in intestinal crypts cultured in vitro. This abolishes their ability to form long-term growing intestinal organoids that differentiate into intestinal phenotypes. We conclude that expression of Cdx2 is essential for differentiation of gut stem cells into any of the intestinal cell types, but they maintain a degree of cell-autonomous plasticity that allows them to switch on a variety of gastric genes.     

Acid, bile, and CDX: the ABCs of making Barrett's metaplasia

Barrett's esophagus, a squamous-to-columnar cell metaplasia that develops as a result of chronic gastroesophageal reflux disease (GERD), is a risk factor for esophageal adenocarcinoma. The molecular events underlying the pathogenesis of Barrett's metaplasia are poorly understood, but recent studies suggest that interactions among developmental signaling pathways, morphogenetic factors, and Caudal homeobox (Cdx) genes play key roles. Strong expression of Cdx genes normally is found in the intestine but not in the esophagus and stomach. When mice are genetically engineered so that their gastric cells express Cdx, the stomach develops a metaplastic, intestinal-type epithelium similar to that of Barrett's esophagus. Exposure to acid and bile has been shown to activate the Cdx promoter in certain esophageal cell lines, and Cdx expression has been found in inflamed esophageal squamous epithelium and in the specialized intestinal metaplasia of Barrett's esophagus. Barrett's metaplasia must be sustained by stem cells, which might be identified by putative, intestinal stem cell markers like leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5) and doublecortin and CaM kinase-like-1 (DCAMKL-1). Emerging concepts in tumor biology suggest that Barrett's cancers may develop from growth-promoting mutations in metaplastic stem cells or their progenitor cell progeny. This report reviews the roles of developmental signaling pathways and the Cdx genes in the development of normal gut epithelia and the potential mechanisms whereby GERD may induce the esophageal expression of Cdx genes and other morphogenetic factors that mediate the development of Barrett's metaplasia. The role of stem cells in the development of metaplasia and in carcinogenesis and the potential for therapies directed at those stem cells also is addressed.     

A new mouse model for studying EGFR-dependent gastric polyps

Hyperactivation of the epidermal growth factor receptor (EGFR) in gastric cells due to excess of its ligand transforming growth factor-α (TGFA) is associated with hyperplastic lesions in Ménétrier's disease patients and in transgenic mice. Other EGFR ligands, however, have never been associated with stomach diseases. Here, we report that overexpression of the EGFR ligand betacellulin (BTC) results in a severe, age-dependent hyperplasia of foveolar epithelium. The stomach weight of affected mice reached up to 3g representing more than 10% of total body weight. The preexisting corpus mucosa was severely depleted, and both parietal and chief cells were replaced by proliferating foveolar epithelium. The lesions were more severe in male as compared to female transgenic mice, and partially regressed in the former after castration-mediated androgen removal. The gastric hyperplasia fully disappeared when BTC-tg mice were crossed into the Egfr(Wa5) background expressing a dominant-negative EGFR, indicating that the phenotype is EGFR-dependent. This is, to our knowledge, the first report of hyperplastic gastric lesions due to the overexpression of an EGFR ligand other than TGFA. BTC-tg mice are therefore a new, promising model for studying EGFR-dependent gastric polyps.     

3H]thymidine autoradiographic and alkaline phosphatase histochemical studies of intestinal metaplasia of the human stomach

The relationship between cell proliferation and enzyme activity in intestinal metaplasia of the human stomach was studied using a combined method of [3H]thymidine autoradiography and alkaline phosphatase histochemistry on the same section. Three types of intestinal metaplasia were observed depending on variations in both enzymatic activity and isotope labelling. One type shows alkaline phosphatase-positive cells along the entire length of the glands with [3H]thymidine-labelled cells localized only at the bottom of the glands, resembling the duodenum. In another type of intestinal metaplasia, alkaline phosphatase-positive cells are present on the surface and/or upper half of the glands with mitotically active cells occupying the lower part of the glands. The third variety of intestinal metaplasia is characterized by the absence of alkaline-phosphatase activity and [3H]thymidine-labelled cells present in an extended zone in the lower half of the glands. Differences in labelling patterns of [3H]thymidine and the activity of marker enzyme in various types of intestinal metaplasia seem to reflect variations in cell differentiation during intestinalization of gastric mucosa.