Genetic ablation of aquaporin-2 in the mouse connecting tubules results in defective renal water handling

Key points center dot The water channel aquaporin-2 (AQP2) is regulated by the hormone vasopressin, and is essential for renal water handling and overall body water balance. center dot AQP2 is expressed in the renal connecting tubule (CNT) and collecting duct (CD). The role of AQP2 in the CD is well established. center dot Here we generate a novel mouse model with gene deletion of AQP2 in the mouse CNT and use this model to examine the role of AQP2 in this segment. center dot Knockout (KO) mice have defective renal water handling under basal conditions, with higher urine volume and reduced urine osmolality, but are able to decrease urine volume under conditions of high circulating vasopressin. center dot KO mice have no obvious compensatory mechanisms in other transporters. center dot KO mice develop a urinary-concentrating defect similar to control mice following lithium chloride treatment. However, the defect in KO mice continued to be more severe than in the control mice, suggesting that the CNT does not play a significant role in the pathology of lithium-induced nephrogenic diabetes insipidus. center dot Our studies indicate that the CNT plays a role in regulating body water balance under basal conditions, but not for maximal concentration of the urine during antidiuresis. Abstract Body water balance is regulated via the water channel aquaporin-2 (AQP2), which is expressed in the renal connecting tubule (CNT) and collecting duct (CD). The relative roles of AQP2 in the CNT and CD are not fully understood. To study the role of AQP2 in the CNT we generated a mouse model with CNT-specific AQP2 deletion (AQP2-CNT-knockout (KO)). Confocal laser scanning microscopy and immunogold electron microscopy demonstrated an absence of AQP2 in the CNT of AQP2-CNT-KO mice. Twenty-four hour urine output was significantly increased (KO: 3.0 +/- 0.3 ml (20 g body weight (BW))1; wild-type (WT): 1.9 +/- 0.3 ml (20 g BW)1) and urine osmolality decreased (KO: 1179 +/- 107 mosmol kg1; WT: 1790 +/- 146 mosmol kg1) in AQP2-CNT-KO mice compared with controls. After 24 h water restriction, urine osmolality was still significantly lower in AQP2-CNT-KO mice (KO: 2087 +/- 169 mosmol kg1; WT: 2678 +/- 144 mosmol kg1). A significant difference in urine osmolality between groups before desmopressin (dDAVP) (KO: 873 +/- 129 mosmol kg1; WT: 1387 +/- 163 mosmol kg1) was not apparent 2 h after injection, with urine osmolality increased significantly in both groups (KO: 2944 +/- 41 mosmol kg1; WT: 3133 +/- 66 mosmol kg1). Cortical kidney fractions from AQP2-CNT-KO mice had significantly reduced AQP2, with no compensatory changes in sodium potassium chloride cotransporter (NKCC2), AQP3 or AQP4. Lithium chloride treatment increased urine volume and decreased osmolality in both WT and AQP2-CNT-KO mice. After 8 days of treatment, the AQP2-CNT-KO mice still had a significantly higher urine volume and lower urine osmolality, suggesting that the CNT does not play a significant role in the pathology of lithium-induced nephrogenic diabetes insipidus. Our studies indicate that the CNT plays a role in regulating body water balance under basal conditions, but not for maximal concentration of the urine during antidiuresis.