AQP5 is expressed in type-B intercalated cells in the collecting duct system of the rat, mouse and human kidney.

We screened human kidney-derived multipotent CD133+/CD24+ ARPCs for the possible expression of all 13 aquaporin isoforms cloned in humans. Interestingly, we found that ARPCs expressed both AQP5 mRNA and mature protein. This novel finding prompted us to investigate the presence of AQP5 in situ in kidney. We report here the novel finding that AQP5 is expressed in human, rat and mouse kidney at the apical membrane of type-B intercalated cells. AQP5 is expressed in the renal cortex and completely absent from the medulla. Immunocytochemical analysis using segment- and cell type-specific markers unambiguously indicated that AQP5 is expressed throughout the collecting system at the apical membrane of type-B intercalated cells, where it co-localizes with pendrin. No basolateral AQPs were detected in type-B intercalated cells, suggesting that AQP5 is unlikely to be involved in the net trans-epithelial water reabsorption occurring in the distal tubule. An intriguing hypothesis is that AQP5 may serve an osmosensor for the composition of the fluid coming from the thick ascending limb. Future studies will unravel the physiological role of AQP5 in the kidney.