Functional characterization and localization of AQP3 in the human colon

Water channels or aquaporins (AQPs) have been identified in a large variety of tissues. Nevertheless, their role in the human gastrointestinal tract, where their action is essential for the reabsorption and secretion of water and electrolytes, is still unclear. The purpose of the present study was t o investigate the structure and function of water channels expressed in the human colon. A cDNA fragment of about 420 bp with a 98% identity to human AQP3 was amplified from human stomach, small intestine and colon by reverse transcription polymerase chain reaction (RT-PCR) and a transcript of 2.2 k b was expressed more abundantly in colon than in jejunum, ileum and stomach as indicated by Northern blots. Expression of mRNA from the colon of adult s and children but not from other gastrointestinal regions in Xenopus oocyt es enhanced the osmotic water permeability, and the urea and glycerol trans port in a manner sensitive to an antisense AQP3 oligonucleotide, indicating the presence of functional AQP3. Immunocytochemistry and immunofluorescenc e studies in human colon revealed that the AQP3 protein is restricted to th e villus epithelial cells. The immunostaining within these cells was more i ntense in the apical than in the basolateral membranes. The presence of AQP 3 in villus epithelial cells suggests that AQP3 is implicated in water abso rption across human colonic surface cells.