The rat intrinsic factor gene: its 5'-upstream region and chief cell-specific transcription

Isolation of CHO-K1 clones defective in cAMP-dependent proteolysis, as determined by the stability of exogenously expressed GATA-6

Degradation of the GATA-6(Delta50) protein expressed in a CHO-K1 clone (tc1-17a) is stimulated in the presence of dbcAMP through proteasome without new protein synthesis [FEBS Lett. 408 (1997) 301], whereas the intrinsic GC-box-binding protein was stable. To examine the cellular mechanism responsible for this specific degradation of GATA-6(Delta50), we initially introduced the blasticidin-S deaminase gene carrying a promoter with GATA motifs that are recognized by GATA-6. The resulting cell line (tc2G2) grew in the presence of blasticidin S. However, the presence of both blasticidin S and dbcAMP was lethal due to degradation of GATA-6. Cells resistant to such lethality were isolated by chemical mutagenesis. The GATA-6(Delta50) in these resistant cells was stable in the presence of dbcAMP in contrast to that in the parent tc2G2 cells, as determined by gel-mobility shift analysis and Western blotting. These clones could be beneficial for identification and characterization of the components participating in the signaling pathway for both protein degradation and cAMP-dependent biological processes.

Impaired gastric acid secretion in mice with a targeted disruption of the NHE4 Na+/H+ exchanger

The NHE4 Na+/H+ exchanger is abundantly expressed on the basolateral membrane of gastric parietal cells. To test the hypothesis that it is required for normal acid secretion, NHE4-null mutant (NHE4-/-) mice were prepared by targeted disruption of the NHE4 (Slc9a4) gene. NHE4-/- mice survived and appeared outwardly normal. Analysis of stomach contents revealed that NHE4-/- mice were hypochlorhydric. The reduction in acid secretion was similar in 18-day-old, 9-week-old, and 6-month-old mice, indicating that the hypochlorhydria phenotype did not progress over time, as was observed in mice lacking the NHE2 Na+/H+ exchanger. Histological abnormalities were observed in the gastric mucosa of 9-week-old NHE4-/- mice, including sharply reduced numbers of parietal cells, a loss of mature chief cells, increased numbers of mucous and undifferentiated cells, and an increase in the number of necrotic and apoptotic cells. NHE4-/- parietal cells exhibited limited development of canalicular membranes and a virtual absence of tubulovesicles, and some of the microvilli had centrally bundled actin. We conclude that NHE4, which may normally be coupled with the AE2 Cl-/HCO3- exchanger, is important for normal levels of gastric acid secretion, gastric epithelial cell differentiation, and development of secretory canalicular and tubulovesicular membranes.

Mice with a targeted disruption of the AE2 Cl-/HCO3- exchanger are achlorhydric

The AE2 Cl-/HCO3- exchanger is expressed in numerous cell types, including epithelial cells of the kidney, respiratory tract, and alimentary tract. In gastric epithelia, AE2 is particularly abundant in parietal cells, where it may be the predominant mechanism for HCO3- efflux and Cl- influx across the basolateral membrane that is needed for acid secretion. To investigate the hypothesis that AE2 is critical for parietal cell function and to assess its importance in other tissues, homozygous null mutant (AE2(-/-)) mice were prepared by targeted disruption of the AE2 (Slc4a2) gene. AE2(-/-) mice were emaciated, edentulous (toothless), and exhibited severe growth retardation, and most of them died around the time of weaning. AE2(-/-) mice exhibited achlorhydria, and histological studies revealed abnormalities of the gastric epithelium, including moderate dilation of the gastric gland lumens and a reduction in the number of parietal cells. There was little evidence, however, that parietal cell viability was impaired. Ultrastructural analysis of AE2(-/-) gastric mucosa revealed abnormal parietal cell structure, with severely impaired development of secretory canaliculi and few tubulovesicles but normal apical microvilli. These results demonstrate that AE2 is essential for gastric acid secretion and for normal development of secretory canalicular and tubulovesicular membranes in mouse parietal cells.

Characterization of the mouse TFF1 (pS2) gene promoter region

Trefoil peptides (TFFs) with a unique trefoil domain(s) are presumed to function in protection and repair of the gastrointestinal epithelial layer. Three peptide family members are differently distributed in the mouse gastrointestinal tract: TFF1/pS2 specifically in stomach, TFF2/SP mainly in stomach, pancreas and duodenum, and TFF3/ITF in intestine. We cloned and sequenced the mouse TFF1 gene 5'-upstream region by means of the genomic walking procedure. The cloned region was ligated to the luciferase reporter gene and then introduced into mouse gastric surface mucous GSM10 cells which express TFF1 and TFF2. The minimum promoter was located in the region containing the TATA-box between -39 and the transcriptional start site. Further upstream regions stimulated (-2192-- -1630bp, -641-- -243bp, -137-- -39bp) and inhibited (-1630-- -641bp, -243-- -137 bp) luciferase gene expression. These regions as well as short segments conserved in the mouse and human 5'-upstream sequences may be important for modulation of the mRNA level of the TFF1 gene.

Stomachs of mice lacking the gastric H,K-ATPase alpha -subunit have achlorhydria, abnormal parietal cells, and ciliated metaplasia

The H,K-ATPase of the gastric parietal cell is the most critical component of the ion transport system mediating acid secretion in the stomach. To study the requirement of this enzyme in the development, maintenance, and function of the gastric mucosa, we used gene targeting to prepare mice lacking the alpha-subunit. Homozygous mutant (Atp4a(-/-)) mice appeared healthy and exhibited normal systemic electrolyte and acid-base status but were achlorhydric and hypergastrinemic. Immunocytochemical, histological, and ultrastructural analyses of Atp4a(-/-) stomachs revealed the presence of chief cells, demonstrating that the lack of acid secretion does not interfere with their differentiation. Parietal cells were also present in normal numbers, and despite the absence of alpha-subunit mRNA and protein, the beta-subunit was expressed. However, Atp4a(-/-) parietal cells had dilated canaliculi and lacked typical canalicular microvilli and tubulovesicles, and subsets of these cells contained abnormal mitochondria and/or massive glycogen stores. Stomachs of adult Atp4a(-/-) mice exhibited metaplasia, which included the presence of ciliated cells. We conclude that ablation of the H,K-ATPase alpha-subunit causes achlorhydria and hypergastrinemia, severe perturbations in the secretory membranes of the parietal cell, and metaplasia of the gastric mucosa; however, the absence of the pump appears not to perturb parietal cell viability or chief cell differentiation.

An ABC transporter homologous to TAP proteins

Polymerase chain reaction amplification of cDNA from rat intestine revealed the expression of a novel ABC transporter, TAPL (TAP-like). Subsequently, the protein sequence was deduced from the nucleotide sequence of cDNA carrying the entire coding region. TAPL is transcribed ubiquitously in various rat tissues. The protein, with 762 amino acid residues, has potential transmembrane domains, and an ATP-binding domain in its amino and carboxyl terminal regions, respectively, and is highly homologous to TAP1 and TAP2 (transporters associated with antigen presentation/processing): pairwise comparisons with TAPL demonstrated 39 and 41% of the residues are identical, respectively. These numerical values are essentially the same as that for TAP1 and TAP2 (39%), and the hydropathy profiles of TAPL, TAP1 and TAP2 are quite similar. The similarity among these three proteins suggests that they could be derived from a common ancestral gene. Furthermore, we found that there is a potential splicing isoform, sharing the amino terminal 720 amino acid residues of TAPL.

Expression of intrinsic factor and pepsinogen in the rat stomach identifies a subset of parietal cells

Morphological and functional heterogeneity of parietal cells has been thought to be due to different maturation positions within the gastric gland. Morphodynamic studies have shown that 2% of parietal cells in mice derive from a pre-neck (chief) cell precursor. Intrinsic factor (IF) and pepsinogen, markers of rat chief cells, were used to determine if these proteins identified a subset of parietal cells that might reflect origin from the pre-neck cell lineage. The zymogenic region of the rat stomach and gradient-isolated fractions enriched in parietal and chief cells were fixed in 10% buffered Formalin or in Bouin's solution. Immunostaining was performed using indirect immunoperoxidase histochemistry and double-labeled immunofluorescence with antibodies raised against human IF, pepsinogen II, and H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase). In intact tissue, parietal (H(+)-K(+)-ATPase-positive) cells were found starting at the upper edge of the isthmus, but parietal cells positive for IF and pepsinogen were only found from just below the isthmus and neck region to the base of the gastric gland. Three to four percent of isolated parietal cells were positive for these ectopic markers. This subset of cells was also positive for H(+)-K(+)-ATPase. Thus products of rat chief cells are expressed in a subset of parietal cells. The percentage of positive cells is similar to that predicted to be derived from the pre-neck (chief) precursor lineage in the mouse. The distribution of these cells to the lower neck and base of the gland suggests that the expression of chief cell products is consistent with either predetermination by lineage or parietal cell maturation or with both processes.