AQP5: Renal Aquaporins Family Got a New Member

In an attempt to investigate the regenerative potential of adult multipotent renal progenitor/stem cells (ARPCs) isolated from human kidneys (Sallustio et al., 2009) we characterized them for the expression of aquaporins. ARPCs expressed measurable levels of the proximal tubule-specific AQP1, both at mRNA and protein levels. When ARPCs were differentiated in vitro into epithelial cells, the expression of the collecting duct-specific AQP2 was also induced. Surprisingly, ARPCs also expressed measurable levels of AQP5, an aquaporin known to be selectively expressed in lung, salivary and lachrymal glands in mammals. This evidence prompted us to investigate the presence and the localization of AQP5 in the mammalian kidney. Total RNA was isolated from adult human, rat and mouse kidneys and subjected to RT-PCR. Interestingly, AQP5 transcripts were found in all the species tested. Western blotting analysis, revealed an AQP5 band of 27 kDa as well as a glycosylated form. Consistent with that, neither the transcript nor the protein was found in AQP5 null mice. AQP5 abundance was higher in the renal cortex than in the medulla. Immunolocalization indicated that AQP5 was expressed at the apical membrane of the cortical collecting ducts (CCDs) epithelial cells with negligible staining in the inner medulla. Triple immunostaining indicated that, in rat CCDs, AQP5 did not colocalize either with AQP2 or with the intercalated cells marker V-ATPase, suggesting a cell specific expression of AQP5 in cells not expressing AQP2 but likely involved in water reabsorption. The ratio between AQP2- and AQP5-expressing cells was approximately 3:1. In conclusion, the expression of AQP5 in the ARPCs, might suggest a role in the differentiation/regeneration processes of the collecting duct epithelial cells. Moreover, its constitutive expression at the apical membrane in the CCD, renders AQP5 a possible target for improving water reabsorption in the collecting duct when AQP2 apical expression is unpaired as in nephrogenic diabetes insipidus.