2 Recycling of the Intestinal Anion Exchanger Down-Regulated in Adenoma (DRA, SLC26A3) Depends on Its PDZ Interaction Mediating the Transfer Into Rab11a-Positive Recycling Endosomes

NHE8 is a member of the SLC9 family.This Na+/H+ exchanger is expressed on the apical membrane of intestinal epithelial cells.Although NHE8 has been shown to be important for intestinal sodium absorption early in development, its physiological role in the gastrointestinal tract remains unknown.By using NHE8 knockout (NHE8KO) mouse model, we discovered that NHE8 plays an important role in protecting the gastric mucosa against gastric acid.Stomach tissue was isolated from NHE8KO mice and their wild-type littermates.Localization of NHE8 expression in the stomach was detected using Immunohistochemistry. Mucosal surface pH was measured using a surface pH meter.Histology of the stomach of NHE8KO mice was also analyzed.RNA was isolated and used for PCR to detect the expression of PAT1 and DRA.Total tissue lysate was prepared for Western detection of NHE8, PAT1 and DRA.Immunohistochemistry indicated that NHE8 is expressed on the apical membrane of gastric epithelial cells.Histological evaluation showed no abnormality of the stomach in NHE8KO mice.Interestingly, higher incidence of gastric bleeding was observed in NHE8KO mice.The appearance of ulcers was seen in the glandular stomach.Mucosal surface pH measurement indicated that NHE8KO mice had decreased mucosal surface pH compared to wild-type mice (3.72 ± 0.13 in NHE8KO vs. 4.26 ± 0.09 in wild-type).Lower surface pH of NHE8KO gastric mucosa suggests that HCO3-secretion may be altered.Therefore, we studied the expression of PAT1 and DRA in NHE8KO mice.Indeed, PAT1 and DRA mRNA expression in the stomach of NHE8KO mice were significantly reduced by ~30% and ~90%, respectively, in NHE8KO mice compared to their wild-type littermates.At the protein expression level, PAT1 protein abundance was reduced by ~70% and DRA protein abundance was nearly abolished in NHE8KO mice.Thus NHE8 may play an important role in mucosal protection by participating bicarbonate secretion.