Calcineurin-NFATc signaling pathway regulates AQP2 expression in response to calcium signals and osmotic stress.

The aquaporin (AQP)2 channel mediates the reabsorption of water in renal collecting ducts in response to arginine vasopressin (AVP) and hypertonicity. Here we show that AQP2 expression is induced not only by the tonicity-responsive enhancer binding protein (TonEBP)/nuclear factor of activated T cells (NFAT)5-mediated hypertonic stress response but also by the calcium-dependent calcineurin-NFATc pathway. The induction of AQP2 expression by the calcineurin-NFATc pathway can occur in the absence of TonEBP/NFAT5. Mutational and chromatin immunoprecipitation analyses revealed the existence of functional NFAT binding sites within the proximal AQP2 promoter responsible for regulation of AQP2 by NFATc proteins and TonEBP/NFAT5. Contrary to the notion that TonEBP/NFAT5 is the only Rel/NFAT family member regulated by tonicity, we found that hypertonicity promotes the nuclear translocation of NFATc proteins for the subsequent induction of AQP2 expression. Calcineurin activity was also found to be involved in the induction of TonEBP/NFAT5 expression by hypertonicity, thus further defining the signaling mechanisms that underlie the TonEBP/NFAT5 osmotic stress response pathway. The coordinate regulation of AQP2 expression by both osmotic stress and calcium signaling appears to provide a means to integrate diverse extracellular signals into optimal cellular responses.

Congenital progressive hydronephrosis (cph) is caused by an S256L mutation in aquaporin-2 that affects its phosphorylation and apical membrane accumulation.

Congenital progressive hydronephrosis (cph) is a spontaneous recessive mutation that causes severe hydronephrosis and obstructive nephropathy in affected mice. The mutation has been mapped to the distal end of mouse chromosome 15, but the mutated gene has not been found. Here, we describe the identification of a single base pair change in aquaporin-2 (Aqp2) in cph mutants through genetic linkage mapping. The C-T change led to the substitution of a Ser (S256) by a Leu in the cytoplasmic tail of the Aqp2 protein, preventing its phosphorylation at S256 and the subsequent accumulation of Aqp2 on the apical membrane of the collecting duct principal cells. The interference with normal trafficking of Aqp2 by this mutation resulted in a severe urine concentration defect. cph homozygotes demonstrated polydipsia and produced a copious amount of hypotonic urine. The urine concentration defect could not be corrected by [deamino-Cys1,D-Arg8]-vasopressin (DDAVP, a vasopressin analog), characteristic of nephrogenic diabetes insipidus. The nephrogenic diabetes insipidus symptoms and the absence of developmental defects in the pyeloureteral peristaltic machinery in the mutants before the onset of hydronephrosis suggest that the congenital obstructive nephropathy is most likely a result of the polyuria. This study has revealed the genetic basis for the classical cph mutation and has provided direct genetic evidence that S256 in Aqp2 is indispensable for the apical accumulation, but not the general glycosylation or membrane association, of Aqp2.