Aquaporin-6 is expressed along the rat gastrointestinal tract and upregulated by feeding in the small intestine

BACKGROUND: Several aquaporins (a family of integral membrane proteins) have been recently identified in the mammalian gastrointestinal tract, and their involvement in the movement of fluid and small solutes has been suggested. In this direction we investigated, in some regions of the rat gastrointestinal tract, the presence and localization of aquaporin-6, given its peculiar function as an ion selective channel. RESULTS: RT-PCR and immunoblotting experiments showed that aquaporin-6 was expressed in all the investigated portions of the rat gastrointestinal tract. The RT-PCR experiments showed that aquaporin-6 transcript was highly expressed in small intestine and rectum, and less in stomach, caecum and colon. In addition, jejunal mRNA expression was specifically stimulated by feeding. Immunoblotting analysis showed a major band with a molecular weight of about 55 kDa corresponding to the aquaporin-6 protein dimer; this band was stronger in the stomach and large intestine than in the small intestine. Immunoblotting analysis of brush border membrane vesicle preparations showed an intense signal for aquaporin-6 protein. The results of in situ hybridization experiments demonstrate that aquaporin-6 transcript is present in the isthmus, neck and basal regions of the stomach lining, and throughout the crypt-villus axis in both small and large intestine. In the latter regions, immunohistochemistry revealed strong aquaporin-6 labelling in the apical membrane of the surface epithelial cells, while weak or no labelling was observed in the crypt cells. In the stomach, an intense staining was observed in mucous neck cells and lower signal in principal cells and some parietal cells. CONCLUSION: The results indicate that aquaporin-6 is distributed throughout the gastrointestinal tract. Aquaporin-6 localization at the apical pole of the superficial epithelial cells and its upregulation by feeding suggest that it may be involved in movements of water and anions through the epithelium of the villi.