Aquaporin-6 expression in rat gastrointestinal tract

Aquaporins (AQPs) are a family of channel proteins belonging to the broad super family of the Major Intrinsic Protein (MIP) transmembrane channels, that facilitate the osmotic movement of water and/or small solutes across biological membranes. Thirteen distinct aquaporins have been identified in mammals and they have been functionally subdivided in orthodox aquaporins (AQP1, 2, 4, and 5), aquaglyceroporins (AQP3, 7, 9 and 10) and unorthodox aquaporins (AQP6, 8, 11 and 12) on the basis of their permeability to water, uncharged or charged solutes. Differently from other AQPs, AQP6 has a preeminent intracellular localization and exhibits an extremely low water permeability; its H+ or Hg2+ activation is accompanied by an increased conductance to inorganic anions (NO3−> I−> Br−> Cl−). For these characteristics it has been suggested that AQP6 works more as an anion-selective channel than as a water channel. Some AQPs, have been identified in the gastrointestinal tract suggesting their involvement in fluid and small solutes movements. AQP6 have been previously identified in renal duct type A intercalated cells and in some extra renal tissues. Here, using RT-PCR, in situ hybridization and immunohistochemical techniques, we demonstrate AQP6 expression also in the gastrointestinal tract. In the stomach AQP6 mRNA is expressed in the isthmus, neck and basal region, while in the small and large intestine the transcript is distributed all along the crypta-villus axis. We detect AQP6 protein expression in mucous neck cells of the stomach, and at lower levels in chief cells and in some parietal cells, while in the small and large intestine we obtain a strong staining on the apical membrane of the surface epithelial cells with no labelling of crypt cells. Our data, together with the unique functional properties of AQP6, suggest for the first time a role for AQP6 in ions and water absorption in the gastrointestinal tract epithelium and its up regulation by feeding.