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

Abstract

December 13, 2006; doi:10.1152/ajpcell.00588.2005.-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 tonicityresponsive 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. aquaporin; nuclear factor of activated T cells; tonicity-responsive enhancer binding protein; osmotic response CELLS OF THE RENAL MEDULLA exist in a unique tissue environment characterized by extremes of hypertonicity during periods of antidiuresis, which allows for the efficient reabsorption of water from the glomerular filtrate (3). The hypertonic interstitial environment allows water to diffuse through the tubular epithelial cell membrane via aquaporin (AQP) water channels. Genetic studies in both mice and humans indicate that multiple AQPs, including AQP2, play a critical role in water transport in the kidney (21). AQP2 is expressed in cells of the renal collecting ducts that are exposed to the potentially hypertonic environment. Binding of arginine vasopressin (AVP) to AVP receptor 2 (AVPR2) induces the translocation of AQP2 from storage vesicles to the plasma membrane (21). Mutations in components of this pathway, especially in AVPR2 and AQP2, cause defects in urine concentration manifested as nephrogenic diabetes insipidus (NDI) (23). While AVP/AVPR2 signaling can regulate the long-term expression of AQP2 (35), regulation of AQP2 by AVP-independent mechanisms has also been reported (14,20,42).Although the hypertonic environment within the kidney allows for reabsorption of water, it also subjects cells to a potentially lethal osmotic stress. The cellular response that permits cells of the renal medulla to adapt and survive within a hypertonic environment is mediated by the tonicity-responsive enhancer binding protein (TonEBP)/nuclear factor of activated T cells (NFAT)5 transcription factor (13). TonEBP/ NF...

Activation of TonEBP by Calcium Controls β1,3-Glucuronosyltransferase-I Expression, a Key Regulator of Glycosaminoglycan Synthesis in Cells of the Intervertebral Disc

“…In T cells and kidney cells, there is some evidence to indicate that regulation may be mediated by a phosphatase, Cn, which is activated by calcium ions (24,25). The results of the studies described herein suggest that this mechanism may be cell type-specific.…”

Hypotonicity-induced Reduction of Aquaporin-2 Transcription in mpkCCD Cells Is Independent of the Tonicity Responsive Element, Vasopressin, and cAMP

“…Li et al (14) reported that the effect of hypertonicity on AQP2 transcription depends on NFATc sites in the AQP2 promoter. The NFATc sites tested by Li et al are localized in segments 1, 6, and 7 of the Ϫ0.4 kb promoter.…”

“…This protein is up-regulated with hyperto-nicity in cultured cells and in the kidney medulla (11,12) and TonEBP knockdown or inhibition, but also mutation of its tonicity-responsive element (TonE) in the AQP2 promoter, reduced AQP2 expression in mpkCCD cells (13,14). Moreover, in TonEBP Ϫ/Ϫ mice and mice transgenic for dominant-negative TonEBP, AQP2 expression was decreased (15,16).…”

“…In addition to TonE, other promoter elements have been suggested to be involved in hypertonicity-induced AQP2 expression, as Li et al (14) reported that NFATc binding sites downstream of TonE are also important for hypertonicity-induced AQP2 expression in mpkCCD cells, because following mutation of all five sites, hypertonicity-induced AQP2 expression was lost to a large extent.…”

Regulation of TRPC6 ion channels in podocytes — Implications for focal segmental glomerulosclerosis and acquired forms of proteinuric diseases

“…NFATc transcription factors are extensively studied calcineurin substrates and are major regulators of transcription in response to Ca 2+ / calcineurin signals (11,43). Upon activation by increased intracellular Ca…”