Bidirectional regulation of tonicity-responsive enhancer binding protein in response to changes in tonicity

Extracellular sodium regulates fibroblast growth factor 23 (FGF23) formation

The bone-derived hormone fibroblast growth factor-23 (FGF23) has recently received much attention due to its association with chronic kidney disease and cardiovascular disease progression. Extracellular sodium concentration ([Na+]) plays a significant role in bone metabolism. Hyponatremia (lower serum [Na+]) has recently been shown to be independently associated with FGF23 levels in patients with chronic systolic heart failure. However, nothing is known about the direct impact of [Na+] on FGF23 production. Here, we show that an elevated [Na+] (+20 mM) suppressed FGF23 formation, whereas low [Na+] (-20 mM) increased FGF23 synthesis in the osteoblast-like cell lines UMR-106 and MC3T3-E1. Similar bidirectional changes in FGF23 abundance were observed when osmolality was altered by mannitol but not by urea, suggesting a role of tonicity in FGF23 formation. Moreover, these changes in FGF23 were inversely proportional to the expression of NFAT5 (nuclear factor of activated T cells-5), a transcription factor responsible for tonicity-mediated cellular adaptations. Furthermore, arginine vasopressin, which is often responsible for hyponatremia, did not affect FGF23 production. Next, we performed a comprehensive and unbiased RNA-seq analysis of UMR-106 cells exposed to low versus high [Na+], which revealed several novel genes involved in cellular adaptation to altered tonicity. Additional analysis of cells with Crisp-Cas9-mediated NFAT5 deletion indicated that NFAT5 controls numerous genes associated with FGF23 synthesis, thereby confirming its role in [Na+]-mediated FGF23 regulation. In line with these in vitro observations, we found that hyponatremia patients have higher FGF23 levels. Our results suggest that [Na+] is a critical regulator of FGF23 synthesis.

Hyperosmolar expansion medium improves nucleus pulposus cell phenotype

Background

Repopulating the degenerated intervertebral disc (IVD) with tissue‐specific nucleus pulposus cells (NPCs) has already been shown to promote regeneration in various species. Yet the applicability of NPCs as cell‐based therapy has been hampered by the low cell numbers that can be extracted from donor IVDs and their potentially limited regenerative capacity due to their degenerated phenotype. To optimize the expansion conditions, we investigated the effects of increasing culture medium osmolarity during expansion on the phenotype of dog NPCs and their ability to produce a healthy extracellular matrix (ECM) in a 3D culture model.

Methods

Dog NPCs were expanded in expansion medium with a standard osmolarity of 300 mOsm/L or adjusted to 400 or 500 mOsm/L in both normoxic and hypoxic conditions. Following expansion, NPCs were cultured in a 3D culture model in chondrogenic culture medium with a standard osmolarity. Read‐out parameters included cell proliferaton rate, morphology, phenotype and healthy ECM production.

Results

Increasing the expansion medium osmolarity from 300 to 500 mOsm/L resulted in NPCs with a more rounded morphology and a lower cell proliferation rate accompanied by the expression of several healthy NPC and progenitor markers at gene (KRT18, ACAN, COL2, CD73, CD90) and protein (ACAN, PAX1, CD24, TEK, CD73) level. The NPCs expanded at 500 mOsm/L were able to retain most of their phenotypic markers and produce healthy ECM during 3D culture independent of the oxygen level used during expansion.

Conclusions

Altogether, our findings show that increasing medium osmolarity during expansion results in an NPC population with improved phenotype, which could enhance the potential of cell‐based therapies for IVD regeneration.

miR-324-5p and miR-30c-2-3p Alter Renal Mineralocorticoid Receptor Signaling under Hypertonicity

The Mineralocorticoid Receptor (MR) mediates the sodium-retaining action of aldosterone in the distal nephron, but mechanisms regulating MR expression are still poorly understood. We previously showed that RNA Binding Proteins (RBPs) regulate MR expression at the post-transcriptional level in response to variations of extracellular tonicity. Herein, we highlight a novel regulatory mechanism involving the recruitment of microRNAs (miRNAs) under hypertonicity. RT-qPCR validated miRNAs candidates identified by high throughput screening approaches and transfection of a luciferase reporter construct together with miRNAs Mimics or Inhibitors demonstrated their functional interaction with target transcripts. Overexpression strategies using Mimics or lentivirus revealed the impact on MR expression and signaling in renal KC3AC1 cells. miR-324-5p and miR-30c-2-3p expression are increased under hypertonicity in KC3AC1 cells. These miRNAs directly affect Nr3c2 (MR) transcript stability, act with Tis11b to destabilize MR transcript but also repress Elavl1 (HuR) transcript, which enhances MR expression and signaling. Overexpression of miR-324-5p and miR-30c-2-3p alter MR expression and signaling in KC3AC1 cells with blunted responses in terms of aldosterone-regulated genes expression. We also confirm that their expression is increased by hypertonicity in vivo in the kidneys of mice treated with furosemide. These findings may have major implications for the pathogenesis of renal dysfunctions, sodium retention, and mineralocorticoid resistance.

Sperm preparedness and adaptation to osmotic and pH stressors relate to functional competence of sperm in Bos taurus

The adaptive ability of sperm in the female reproductive tract micromilieu signifies the successful fertilization process. The study aimed to analyze the preparedness of sperm to the prevailing osmotic and pH stressors in the female reproductive tract. Fresh bovine sperm were incubated in 290 (isosmotic-control), 355 (hyperosmotic-uterus and oviduct), and 420 (hyperosmotic-control) mOsm/kg and each with pH of 6.8 (uterus) and 7.4 (oviduct). During incubation, the changes in sperm functional attributes were studied. Sperm kinematics and head area decreased significantly (p < 0.05) immediately upon exposure to hyperosmotic stress at both pH. Proportion of sperm capacitated (%) in 355 mOsm/kg at 1 and 2 h of incubation were significantly (p < 0.05) higher than those in 290 mOsm media. The magnitude and duration of recovery of sperm progressive motility in 355 mOsm with pH 7.4 was correlated with the ejaculate rejection rate (R² = 0.7). Using this information, the bulls were divided into good (n = 5) and poor (n = 5) osmo-adapters. The osmo-responsive genes such as NFAT5, HSP90AB1, SLC9C1, ADAM1B and GAPDH were upregulated (p < 0.05) in the sperm of good osmo-adapters. The study suggests that sperm are prepared for the osmotic and pH challenges in the female reproductive tract and the osmoadaptive ability is associated with ejaculate quality in bulls.

Na+ i/K+ i imbalance contributes to gene expression in endothelial cells exposed to elevated NaCl

High-salt consumption contributes to the development of hypertension and is considered an independent risk factor for vascular remodelling, cardiac hypertrophy and stroke incidence. Alterations in NO production, inflammation and endothelial cell stiffening are considered now as plausible mediators of cardiovascular dysfunction. We studied early responses of endothelial cells (HUVEC) caused by a moderate increase in extracellular sodium concentration. Exposure of HUVEC to elevated sodium within the physiological range up to 24 h is accompanied by changes in monovalent cations fluxes and Na,K-ATPase activation, and, in turn, results in a significant decrease in the content of PTGS2, IL6 and IL1LR1 mRNAs. The expression of NOS3 and FOS genes, as well as the abundance of cytosolic and nuclear NFAT5 protein, remained unchanged. We assessed the mechanical properties of endothelial cells by estimating Young's modulus and equivalent elastic constant using atomic force and interference microscopy, respectively. These parameters were unaffected by elevated-salt exposure for 24 h. The data obtained suggest that even small and short-term elevations of extracellular sodium concentration affect the expression of genes involved in the control of endothelial function through the Na⁺_i/K⁺_i-dependent mechanism(s).

The TLR-2/TonEBP signaling pathway regulates 29-kDa fibronectin fragment-dependent expression of matrix metalloproteinases

Tonicity-responsive enhancer-binding protein (TonEBP; nuclear factor of activated T cells 5) is a transcription factor that responds to changes in osmolality. However, recent studies have shown that it also modulates immune responses under inflammatory conditions independently of hyperosmolality. Fibronectin fragments (FN-fs), which are abundant in the synovial fluid of patients with osteoarthritis (OA), induce expression of matrix metalloproteinases (MMPs) via the toll-like receptor-2 (TLR-2) signaling pathway. In this study we examined whether TonEBP is involved in 29-kDa FN-f-induced expression of MMPs. The expression of TonEBP was significantly higher in human osteoarthritis compared with normal cartilage samples. 29-kDa FN-f affected the expression of MMPs 1, 3, and 13 via TonEBP, and expression and nuclear accumulation of TonEBP were induced by activation of the phospholipase C/NF-κB/MAPK signaling pathway and, in particular, modulated by TLR-2. In addition, 29-kDa FN-f induced the expression of osmoregulatory genes, including Tau-T, SMIT, and AR, as well as voltage-dependent calcium channels via the TonEBP/TLR-2 signaling pathway. These results show that 29-kDa FN-f upregulates MMPs in chondrocytes via the TLR-2/TonEBP signaling pathway.

NFAT5 Is Involved in GRP-Enhanced Secretion of GLP-1 by Sodium

Gastrin, secreted by G-cells, and glucagon-like peptide-1 (GLP-1), secreted by L-cells, may participate in the regulation of sodium balance. We studied the effect of sodium in mice in vivo and mouse ileum and human L-cells, on GLP-1 secretion, and the role of NFAT5 and gastrin-releasing peptide receptor (GRPR) in this process. A high-sodium diet increases serum GLP-1 levels in mice. Increasing sodium concentration stimulates GLP-1 secretion from mouse ileum and L-cells. GRP enhances the high sodium-induced increase in GLP-1 secretion. High sodium increases cellular GLP-1 expression, while low and high sodium concentrations increase NFAT5 and GRPR expression. Silencing NFAT5 in L-cells abrogates the stimulatory effect of GRP on the high sodium-induced GLP-1 secretion and protein expression, and the sodium-induced increase in GRPR expression. GLP-1 and gastrin decrease the expression of Na⁺-K⁺/ATPase and increase the phosphorylation of sodium/hydrogen exchanger type 3 (NHE3) in human renal proximal tubule cells (hRPTCs). This study gives a new perspective on the mechanisms of GLP-1 secretion, especially that engendered by ingested sodium, and the ability of GLP-1, with gastrin, to decrease Na⁺-K⁺/ATPase expression and NHE3 function in hRPTCs. These results may contribute to the better utilization of current and future GLP-1-based drugs in the treatment of hypertension.

Description of a Novel Mechanism Possibly Explaining the Antiproliferative Properties of Glucocorticoids in Duchenne Muscular Dystrophy Fibroblasts Based on Glucocorticoid Receptor GR and NFAT5

Glucocorticoids are drugs of choice in Duchenne muscular dystrophy (DMD), prolonging patients’ ambulation. Their mode of action at the protein level is not completely understood. In DMD, muscle tissue is replaced by fibrotic tissue produced by fibroblasts, reducing mobility. Nuclear factor of activated T-cells 5 (NFAT5) is involved in fibroblast proliferation. By treating one DMD fibroblast cell culture and one of unaffected skeletal muscle fibroblasts with methylprednisolone (MP) or hydrocortisone (HC) for 24 h or 12 d, the antiproliferative properties of glucocorticoids could be unraveled. NFAT5 localization and expression was explored by immunocytochemistry (ICC), Western blotting (WB) and RT-qPCR. NFAT5 and glucocorticoid receptor (GR) colocalization was measured by ImageJ. GR siRNA was used, evaluating GR’s influence on NFAT5 expression during MP and HC treatment. Cell proliferation was monitored by IncuCyte ZOOM. In DMD fibroblasts, treatment with MP for 24 h induced dots (ICC) positive for NFAT5 and colocalizing with GR. After 12 d of MP or HC in DMD fibroblasts, NFAT5 expression was decreased (RT-qPCR and WB) and growth arrest was observed (Incucyte ZOOM), whereas NFAT5 expression and cell growth remained unchanged in unaffected skeletal muscle fibroblasts. This study may help understand the antiproliferative properties of glucocorticoids in DMD fibroblasts.

An osmolality/salinity-responsive enhancer 1 (OSRE1) in intron 1 promotes salinity induction of tilapia glutamine synthetase

Euryhaline tilapia (Oreochromis mossambicus) are fish that tolerate a wide salinity range from fresh water to > 3× seawater. Even though the physiological effector mechanisms of osmoregulation that maintain plasma homeostasis in fresh water and seawater fish are well known, the corresponding molecular mechanisms that control switching between hyper- (fresh water) and hypo-osmoregulation (seawater) remain mostly elusive. In this study we show that hyperosmotic induction of glutamine synthetase represents a prominent part of this switch. Proteomics analysis of the O. mossambicus OmB cell line revealed that glutamine synthetase is transcriptionally regulated by hyperosmolality. Therefore, the 5' regulatory sequence of O. mossambicus glutamine synthetase was investigated. Using an enhancer trapping assay, we discovered a novel osmosensitive mechanism by which intron 1 positively mediates glutamine synthetase transcription. Intron 1 includes a single, functional copy of an osmoresponsive element, osmolality/salinity-responsive enhancer 1 (OSRE1). Unlike for conventional enhancers, the hyperosmotic induction of glutamine synthetase by intron 1 is position dependent. But irrespective of intron 1 position, OSRE1 deletion from intron 1 abolishes hyperosmotic enhancer activity. These findings indicate that proper intron 1 positioning and the presence of an OSRE1 in intron 1 are required for precise enhancement of hyperosmotic glutamine synthetase expression.

Recurrent Amplification of the Osmotic Stress Transcription Factor NFAT5 in Adrenocortical Carcinoma

Tumorigenesis requires mitigation of osmotic stress and the transcription factor nuclear factor of activated T cells 5 (NFAT5) coordinates this response by inducing transcellular transport of ions and osmolytes. NFAT5 modulates in vitro behavior in several cancer types, but a potential role of NFAT5 in adrenocortical carcinoma (ACC) has not been studied. A discovery cohort of 28 ACCs was selected for analysis. Coverage depth analysis of whole-exome sequencing reads assessed NFAT5 copy number alterations in 19 ACCs. Quantitative real-time PCR measured NFAT5 mRNA expression levels in 11 ACCs and 23 adrenocortical adenomas. Immunohistochemistry investigated protein expression in representative adrenal samples. The Cancer Genome Atlas database was analyzed to corroborate NFAT5 findings from the discovery cohort and to test whether NFAT5 expression correlated with ion/osmolyte channel and regulatory protein expression patterns in ACC. NFAT5 was amplified in 10 ACCs (52.6%) and clustered in the top 6% of all amplified genes. mRNA expression levels were 5-fold higher compared with adrenocortical adenomas (P < 0.0001) and NFAT5 overexpression had a sensitivity and specificity of 81.8% and 82.7%, respectively, for malignancy. Increased protein expression and nuclear localization occurred in representative ACCs. The Cancer Genome Atlas analysis demonstrated concomitant NFAT5 amplification and overexpression (P < 0.0001) that correlated with increased expression of sodium/myo-inositol transporter SLC5A3 (r² = 0.237, P < 0.0001) and 14 other regulatory proteins (P < 0.05) previously shown to interact with NFAT5. Amplification and overexpression of NFAT5 and associated osmotic stress response related genes may play an important role adrenocortical tumorigenesis.

The evolving role of TonEBP as an immunometabolic stress protein

Tonicity-responsive enhancer-binding protein (TonEBP), which is also known as nuclear factor of activated T cells 5 (NFAT5), was discovered 20 years ago as a transcriptional regulator of the cellular response to hypertonic (hyperosmotic salinity) stress in the renal medulla. Numerous studies since then have revealed that TonEBP is a pleiotropic stress protein that is involved in a range of immunometabolic diseases. Some of the single-nucleotide polymorphisms (SNPs) in TONEBP introns are cis-expression quantitative trait loci that affect TONEBP transcription. These SNPs are associated with increased risk of type 2 diabetes mellitus, diabetic nephropathy, inflammation, high blood pressure and abnormal plasma osmolality, indicating that variation in TONEBP expression might contribute to these phenotypes. In addition, functional studies have shown that TonEBP is involved in the pathogenesis of rheumatoid arthritis, atherosclerosis, diabetic nephropathy, acute kidney injury, hyperlipidaemia and insulin resistance, autoimmune diseases (including type 1 diabetes mellitus and multiple sclerosis), salt-sensitive hypertension and hepatocellular carcinoma. These pathological activities of TonEBP are in contrast to the protective actions of TonEBP in response to hypertonicity, bacterial infection and DNA damage induced by genotoxins. An emerging theme is that TonEBP is a stress protein that mediates the cellular response to a range of pathological insults, including excess caloric intake, inflammation and oxidative stress.

Arp2/3 inactivation causes intervertebral disc and cartilage degeneration with dysregulated TonEBP-mediated osmoadaptation

Extracellular matrix and osmolarity influence the development and homeostasis of skeletal tissues through Rho GTPase-mediated alteration of the actin cytoskeleton. This study investigated whether the actin-branching Arp2/3 complex, a downstream effector of the Rho GTPases Cdc42 and Rac1, plays a critical role in maintaining the health of matrix-rich and osmotically loaded intervertebral discs and cartilage. Mice with constitutive intervertebral disc- and cartilage-specific deletion of the critical Arp2/3 subunit Arpc2 (Col2-Cre; Arpc2fl/fl) developed chondrodysplasia and spinal defects. Since these mice did not survive to adulthood, we generated mice with inducible Arpc2 deletion in disc and cartilage (Acan-CreERT2; Arpc2fl/fl). Inactivation of Arp2/3 at skeletal maturity resulted in growth plate closure, loss of proteoglycan content in articular cartilage, and degenerative changes in the intervertebral disc at 1 year of age. Chondrocytes with Arpc2 deletion showed compromised cell spreading on both collagen and fibronectin. Pharmacological inhibition of Cdc42 and Arp2/3 prevented the osmoadaptive transcription factor TonEBP/NFAT5 from recruiting cofactors in response to a hyperosmolarity challenge. Together, these findings suggest that Arp2/3 plays a critical role in cartilaginous tissues through the regulation of cell-extracellular matrix interactions and modulation of TonEBP-mediated osmoadaptation.

Emerging new role of NFAT5 in inducible nitric oxide synthase in response to hypoxia in mouse embryonic fibroblast cells

We previously described the protective role of the nuclear factor of activated T cells 5 (NFAT5) during hypoxia. Alternatively, inducible nitric oxide synthase (iNOS) is also induced by hypoxia. Some evidence indicates that NFAT5 is essential for the expression of iNOS in Toll-like receptor-stimulated macrophages and that iNOS inhibition increases NFAT5 expression in renal ischemia-reperfusion. Here we studied potential NFAT5 target genes stimulated by hypoxia in mouse embryonic fibroblast (MEF) cells. We used three types of MEF cells associated with NFAT5 gene: NFAT5 wild type (MEF-NFAT5+/+), NFAT5 knockout (MEF-NFAT5-/-), and NFAT5 dominant-negative (MEF-NFAT5Δ/Δ) cells. MEF cells were exposed to 21% or 1% O2 in a time course curve of 48 h. We found that, in MEF-NFAT5+/+ cells exposed to 1% O2, NFAT5 was upregulated and translocated into the nuclei, and its transactivation domain activity was induced, concomitant with iNOS, aquaporin 1 (AQP-1), and urea transporter 1 (UTA-1) upregulation. Interestingly, in MEF-NFAT5-/- or MEF-NFAT5Δ/Δ cells, the basal levels of iNOS and AQP-1 expression were strongly downregulated, but not for UTA-1. The upregulation of AQP-1, UTA-1, and iNOS by hypoxia was blocked in both NFAT5-mutated cells. The iNOS induction by hypoxia was recovered in MEF-NFAT5-/- MEF cells, when recombinant NFAT5 protein expression was reconstituted, but not in MEF-NFAT5Δ/Δ cells, confirming the dominant-negative effect of MEF-NFAT5Δ/Δ cells. We did not see the rescue effect on AQP-1 expression. This work provides novel and relevant information about the signaling pathway of NFAT5 during responses to oxygen depletion in mammalian cells and suggests that the expression of iNOS induced by hypoxia is dependent on NFAT5.

Nfat5 is involved in the hyperosmotic regulation of Tmem184b: a putative modulator of ibuprofen transport in renal MDCK I cells

Nuclear factor of activated T cells 5 (NFAT5) is a transcription factor involved in the regulation of several genes involved in the response to extracellular hyperosmolality. Recently, the uptake of ibuprofen by an as yet unknown carrier was suggested in Madin‐Darby canine kidney (MDCK) I cells exposed to hyperosmolality. We therefore speculated that Nfat5 could be involved in the regulation of this ibuprofen carrier. Reverse transfection with siRNA against Nfat5 was used to knock down Nfat5 in MDCK I cells. The uptake of both radiolabelled taurine and ibuprofen was measured in MDCK I cells, first treated with siRNA against Nfat5 and afterwards cultivated with raffinose‐supplemented normal growth medium (500 mOsm) for 24 h. The siRNA transfection resulted in knockdown of Nfat5, and uptake of both taurine and ibuprofen was significantly decreased in transfected MDCK I cells. The decrease in ibuprofen uptake indicates that Nfat5 is involved in upregulation of the ibuprofen carrier. A transcriptome analysis of MDCK I cells treated with siRNA against Nfat5 revealed 989 genes upregulated by Nfat5 during hyperosmotic exposure. From these genes, the gene product transmembrane protein 184b was found to be regulated by Nfat5, and Tmem184b was the only potential gene product involved in the uptake of ibuprofen in MDCK I cells.

Dataset

The RNA sequencing dataset is available from the NCBI Gene Expression 452 Omnibus (https://www.ncbi.nlm.nih.gov/geo/) with the accession number GSE122074.

NFAT5 Has a Job in the Brain

Nuclear factor of activated T cells 5 (NFAT5) has recently been classified as a new member of the Rel family. In addition, there are 5 more well-defined members (NF-κB and NFAT1-4) in the Rel family, which participate in regulating the expression of immune and inflammatory response-related genes. NFAT5 was initially identified in renal medullary cells where it regulated the expression of osmoprotective-related genes during the osmotic response. Many studies have demonstrated that NFAT5 is highly expressed in the nuclei of neurons in fetal and adult brains. Additionally, its expression is approximately 10-fold higher in fetal brains. With the development of detection technologies (laser scanning confocal microscopy, transgene technology, etc.), recent studies suggest that NFAT5 is also expressed in glial cells and plays a more diverse functional role. This article aims to summarize the current knowledge regarding the expression of NFAT5, its regulation of activation, and varied biological functions in the brain.

VEGF-C attenuates renal damage in salt-sensitive hypertension

Salt-sensitive hypertension is a major risk factor for renal impairment leading to chronic kidney disease. High-salt diet leads to hypertonic skin interstitial volume retention enhancing the activation of the tonicity-responsive enhancer-binding protein (TonEBP) within macrophages leading to vascular endothelial growth factor C (VEGF-C) secretion and NOS3 modulation. This promotes skin lymphangiogenesis and blood pressure regulation. Whether VEGF-C administration enhances renal and skin lymphangiogenesis and attenuates renal damage in salt-sensitive hypertension remains to be elucidated. Hypertension was induced in BALB/c mice by a high-salt diet. VEGF-C was administered subcutaneously to high-salt-treated mice as well as control animals. Analyses of kidney injury, inflammation, fibrosis, and biochemical markers were performed in vivo. VEGF-C reduced plasma inflammatory markers in salt-treated mice. In addition, VEGF-C exhibited a renal anti-inflammatory effect with the induction of macrophage M2 phenotype, followed by reductions in interstitial fibrosis. Antioxidant enzymes within the kidney as well as urinary RNA/DNA damage markers were all revelatory of abolished oxidative stress under VEGF-C. Furthermore, VEGF-C decreased the urinary albumin/creatinine ratio and blood pressure as well as glomerular and tubular damages. These improvements were associated with enhanced TonEBP, NOS3, and lymphangiogenesis within the kidney and skin. Our data show that VEGF-C administration plays a major role in preserving renal histology and reducing blood pressure. VEGF-C might constitute an interesting potential therapeutic target for improving renal remodeling in salt-sensitive hypertension.

Search for Upstream Cell Volume Sensors: The Role of Plasma Membrane and Cytoplasmic Hydrogel

The plasma membrane plays a prominent role in the regulation of cell volume by mediating selective transport of extra- and intracellular osmolytes. Recent studies show that upstream sensors of cell volume changes are mainly located within the cytoplasm that displays properties of a hydrogel and not in the plasma membrane. Cell volume changes occurring in anisosmotic medium as well as in isosmotic environment affect properties of cytoplasmic hydrogel that, in turn, trigger rapid regulatory volume increase and decrease (RVI and RVD). The downstream signaling pathways include reorganization of 2D cytoskeleton and altered composition of polyphosphoinositides located on the inner surface of the plasma membrane. In addition to its action on physico-chemical properties of cytoplasmic hydrogel, cell volume changes in anisosmotic conditions affect the ionic strength of the cytoplasm and the [Na⁺]_i/[K⁺]_i ratio. Elevated intracellular ionic strength evoked by long term exposure of cells to hypertonic environment resulted in the activation of TonEBP and augmented expression of genes controlling intracellular organic osmolyte levels. The role of Na⁺_i/K⁺_i -sensitive, Ca²⁺_i -mediated and Ca²⁺_i-independent mechanisms of excitation-transcription coupling in cell volume-adjustment remains unknown.

TNF-α promotes nuclear enrichment of the transcription factor TonEBP/NFAT5 to selectively control inflammatory but not osmoregulatory responses in nucleus pulposus cells

Intervertebral disc degeneration (IDD) causes chronic back pain and is linked to production of proinflammatory molecules by nucleus pulposus (NP) and other disc cells. Activation of tonicity-responsive enhancer-binding protein (TonEBP)/NFAT5 by non-osmotic stimuli, including proinflammatory molecules, occurs in cells involved in immune response. However, whether inflammatory stimuli activate TonEBP in NP cells and whether TonEBP controls inflammation during IDD is unknown. We show that TNF-α, but not IL-1β or LPS, promoted nuclear enrichment of TonEBP protein. However, TNF-α-mediated activation of TonEBP did not cause induction of osmoregulatory genes. RNA sequencing showed that 8.5% of TNF-α transcriptional responses were TonEBP-dependent and identified genes regulated by both TNF-α and TonEBP. These genes were over-enriched in pathways and diseases related to inflammatory response and inhibition of matrix metalloproteases. Based on RNA-sequencing results, we further investigated regulation of novel TonEBP targets CXCL1, CXCL2, and CXCL3 TonEBP acted synergistically with TNF-α and LPS to induce CXCL1-proximal promoter activity. Interestingly, this regulation required a highly conserved NF-κB-binding site but not a predicted TonE, suggesting cross-talk between these two members of the Rel family. Finally, analysis of human NP tissue showed that TonEBP expression correlated with canonical osmoregulatory targets TauT/SLC6A6, SMIT/SLC5A3, and AR/AKR1B1, supporting in vitro findings that the inflammatory milieu during IDD does not interfere with TonEBP osmoregulation. In summary, whereas TonEBP participates in the proinflammatory response to TNF-α, therapeutic strategies targeting this transcription factor for treatment of disc disease must spare osmoprotective, prosurvival, and matrix homeostatic activities.

NFAT5 and SLC4A10 Loci Associate with Plasma Osmolality

Disorders of water balance, an excess or deficit of total body water relative to body electrolyte content, are common and ascertained by plasma hypo- or hypernatremia, respectively. We performed a two-stage genome-wide association study meta-analysis on plasma sodium concentration in 45,889 individuals of European descent (stage 1 discovery) and 17,637 additional individuals of European descent (stage 2 replication), and a transethnic meta-analysis of replicated single-nucleotide polymorphisms in 79,506 individuals (63,526 individuals of European descent, 8765 individuals of Asian Indian descent, and 7215 individuals of African descent). In stage 1, we identified eight loci associated with plasma sodium concentration at P<5.0 × 10^-6 Of these, rs9980 at NFAT5 replicated in stage 2 meta-analysis (P=3.1 × 10^-5), with combined stages 1 and 2 genome-wide significance of P=5.6 × 10^-10 Transethnic meta-analysis further supported the association at rs9980 (P=5.9 × 10^-12). Additionally, rs16846053 at SLC4A10 showed nominally, but not genome-wide, significant association in combined stages 1 and 2 meta-analysis (P=6.7 × 10^-8). NFAT5 encodes a ubiquitously expressed transcription factor that coordinates the intracellular response to hypertonic stress but was not previously implicated in the regulation of systemic water balance. SLC4A10 encodes a sodium bicarbonate transporter with a brain-restricted expression pattern, and variant rs16846053 affects a putative intronic NFAT5 DNA binding motif. The lead variants for NFAT5 and SLC4A10 are cis expression quantitative trait loci in tissues of the central nervous system and relevant to transcriptional regulation. Thus, genetic variation in NFAT5 and SLC4A10 expression and function in the central nervous system may affect the regulation of systemic water balance.

Osmolality/salinity-responsive enhancers (OSREs) control induction of osmoprotective genes in euryhaline fish

Fish respond to salinity stress by transcriptional induction of many genes, but the mechanism of their osmotic regulation is unknown. We developed a reporter assay using cells derived from the brain of the tilapia Oreochromis mossambicus (OmB cells) to identify osmolality/salinity-responsive enhancers (OSREs) in the genes of Omossambicus Genomic DNA comprising the regulatory regions of two strongly salinity-induced genes, inositol monophosphatase 1 (IMPA1.1) and myo-inositol phosphate synthase (MIPS), was isolated and analyzed with dual luciferase enhancer trap reporter assays. We identified five sequences (two in IMPA1.1 and three in MIPS) that share a common consensus element (DDKGGAAWWDWWYDNRB), which we named "OSRE1." Additional OSREs that were less effective in conferring salinity-induced trans-activation and do not match the OSRE1 consensus also were identified in both MIPS and IMPA1.1 Although OSRE1 shares homology with the mammalian osmotic-response element/tonicity-responsive enhancer (ORE/TonE) enhancer, the latter is insufficient to confer osmotic induction in fish. Like other enhancers, OSRE1 trans-activates genes independent of orientation. We conclude that OSRE1 is a cis-regulatory element (CRE) that enhances the hyperosmotic induction of osmoregulated genes in fish. Our study also shows that tailored reporter assays developed for OmB cells facilitate the identification of CREs in fish genomes. Knowledge of the OSRE1 motif allows affinity-purification of the corresponding transcription factor and computational approaches for enhancer screening of fish genomes. Moreover, our study enables targeted inactivation of OSRE1 enhancers, a method superior to gene knockout for functional characterization because it confines impairment of gene function to a specific context (salinity stress) and eliminates pitfalls of constitutive gene knockouts (embryonic lethality, developmental compensation).

A Transporter of Ibuprofen is Upregulated in MDCK I Cells under Hyperosmotic Culture Conditions

Ibuprofen is a widely used drug. It has been identified as an inhibitor of several transporters, but it is not clear if ibuprofen is a substrate of any transporter itself. In the present work, we have characterized a transporter of ibuprofen, which is upregulated by hyperosmotic culture conditions in Madin-Darby canine kidney I (MDCK I) renal cells. [(3)H]-Ibuprofen uptake rate was measured in MDCK I cell cultured under normal (300 mOsm) and hyperosmotic (500 mOsm) conditions. Hyperosmotic conditions were obtained by supplementing urea, NaCl, mannitol, or raffinose to culture medium. The effect of increased osmolarity was investigated for different incubation times. [(3)H]-Ibuprofen uptake in MDCK I cells was upregulated by hyperosmotic culture condition, and was saturable with a Km value of 0.37 ± 0.08 μM and a Vmax of 233.1 ± 17.2 pmol· cm(-2)· min(-1). Racemic [(3)H]-ibuprofen uptake could be inhibited by (R)-(-)- and (S)-(+)-ibuprofen with IC50 values of 19 μM (Log IC50 1.39 ± 0.34) and 0.47 μM (Log IC50 -0.36 ± 0.41), respectively. Furthermore, the [(3)H]-ibuprofen uptake rate was increased by decreased extracellular pH but not dependent on Na(+) or Cl(-) ions. The mRNA of Mct1, -2, -4, and -6 as well as Oat1 and -3 were not upregulated by hyperosmolarity. Our findings present strong evidence for the presence of a yet unknown ibuprofen transporter in MDCK I cells. The transporter was upregulated under hyperosmotic culture conditions, and the present study is therefore a starting point for identification of the molecular correlate and potential impact on ibuprofen disposition.

NFAT5 genes are part of the osmotic regulatory system in Atlantic salmon (Salmo salar)

The anadromous Atlantic salmon utilizes both fresh and salt water (FW and SW) habitats during its life cycle. The parr-smolt transformation (PST) is an important developmental transition from a FW adapted juvenile parr to a SW adapted smolt. Physiological changes in osmoregulatory tissues, particularly the gill, are key in maintaining effective ion regulation during PST. Changes are initiated prior to SW exposure (preparative phase), and are completed when smolts enter the sea (activational phase) where osmotic stress may directly stimulate changes in gene expression. In this paper we identify 4 nuclear factor of activated T cells (NFAT5, an osmotic stress transcription factor) paralogues in Atlantic salmon, which showed strong homology in characterized functional domains with those identified in other vertebrates. Two of the identified paralogues (NFAT5b1 and NFAT5b2) showed increased expression following transfer from FW to SW. This effect was largest in parr that were maintained under short day photoperiod, and showed the highest increases in chloride ion levels in response to SW exposure. The results of this study suggest that NFAT5 is involved in the osmotic stress response of Atlantic salmon.

Mitogen-activated protein kinases as key players in osmotic stress signaling

BACKGROUND

Osmotic stress arises from the difference between intracellular and extracellular osmolality. It induces cell swelling or shrinkage as a consequence of water influx or efflux, which threatens cellular activities. Mitogen-activated protein kinases (MAPKs) play central roles in signaling pathways in osmotic stress responses, including the regulation of intracellular levels of inorganic ions and organic osmolytes.

SCOPE OF REVIEW

The present review summarizes the cellular osmotic stress response and the function and regulation of the vertebrate MAPK signaling pathways involved. We also describe recent findings regarding apoptosis signal-regulating kinase 3 (ASK3), a MAP3K member, to demonstrate its regulatory effects on signaling molecules beyond MAPKs.

MAJOR CONCLUSIONS

MAPKs are rapidly activated by osmotic stress and have diverse roles, such as cell volume regulation, gene expression, and cell survival/death. There is significant cell type specificity in the function and regulation of MAPKs. Based on its activity change during osmotic stress and its regulation of the WNK1-SPAK/OSR1 pathway, ASK3 is expected to play important roles in osmosensing mechanisms and cellular functions related to osmoregulation.

GENERAL SIGNIFICANCE

MAPKs are essential for various cellular responses to osmotic stress; thus, the identification of the upstream regulators of MAPK pathways will provide valuable clues regarding the cellular osmosensing mechanism, which remains elusive in mammals. The elucidation of in vivo MAPK functions is also important because osmotic stress in physiological and pathophysiological conditions often results from changes in the intracellular osmolality. These studies potentially contribute to the establishment of therapeutic strategies against diseases that accompany osmotic perturbation.

SIRT1 contributes to aldose reductase expression through modulating NFAT5 under osmotic stress: In vitro and in silico insights

So far, a myriad of molecules were characterized to modulate NFAT5 and its downstream targets. Among these NFAT5 modifiers, SIRT1 was proposed to have a promising role in NFAT5 dependent events, yet the exact underlying mechanism still remains obscure. Hence, the link between SIRT1 and NFAT5-aldose reductase (AR) axis under osmotic stress, was aimed to be delineated in this study. A unique osmotic stress model was generated and its mechanistic components were deciphered in U937 monocytes. In this model, AR expression and nuclear NFAT5 stabilization were revealed to be positively regulated by SIRT1 through utilization of pharmacological modulators. Overexpression and co-transfection studies of NFAT5 and SIRT1 further validated the contribution of SIRT1 to AR and NFAT5. The involvement of SIRT1 activity in these events was mediated via modification of DNA binding of NFAT5 to AR ORE region. Besides, NFAT5 and SIRT1 were also shown to co-immunoprecipitate under isosmotic conditions and this interaction was disrupted by osmotic stress. Further in silico experiments were conducted to investigate if SIRT1 directly targets NFAT5. In this regard, certain lysine residues of NFAT5, when kept deacetylated, were found to contribute to its DNA binding and SIRT1 was shown to directly bind K282 of NFAT5. Based on these in vitro and in silico findings, SIRT1 was identified, for the first time, as a novel positive regulator of NFAT5 dependent AR expression under osmotic stress in U937 monocytes.

NFAT5 promotes proliferation and migration of lung adenocarcinoma cells in part through regulating AQP5 expression

The osmoregulated transcription factor nuclear factor of activated T-cells 5(NFAT5), has been found to play important roles in the development of many kinds of human cancers, including breast cancer, colon carcinoma, renal cell carcinoma and melanoma. The aim of the present study was to determine whether NFAT5 is involved in the proliferation and migration of lung adenocarcinoma cells. We found that NFAT5 was upregulated in lung adenocarcinoma cells and knockdown of NFAT5 decreased proliferation and migration of the cells, accompanied by a significant reduction in the expression of AQP5. AQP5 was upregulated in lung adenocarcinoma cells and knockdown of AQP5 also inhibited proliferation and migration of the cells as knockdown of NFAT5 did. Moreover, overexpression of NFAT5 promoted proliferation and migration of lung adenocarcinoma cells, accompanied by a significant increase in the expression of AQP5. These results indicate that NFAT5 plays important roles in proliferation and migration of human lung adenocarcinoma cells through regulating AQP5 expression, providing a new therapeutic option for lung adenocarcinoma therapy.

Expression, fermentation and purification of a predicted intrinsically disordered region of the transcription factor, NFAT5

Hypertonicity stimulates Nuclear Factor of Activated T-cells 5 (NFAT5) nuclear localization and transactivating activity. Many transcription factors are known to contain intrinsically disordered regions (IDRs) which become more structured with local environmental changes such as osmolality, temperature and tonicity. The transactivating domain of NFAT5 is predicted to be intrinsically disordered under normal tonicity, and under high NaCl, the activity of this domain is increased. To study the binding of co-regulatory proteins at IDRs a cDNA construct expressing the NFAT5 TAD was created and transformed into Escherichia coli cells. Transformed E. coli cells were mass produced by fermentation and extracted by cell lysis to release the NFAT5 TAD. The NFAT5 TAD was subsequently purified using a His-tag column, cation exchange chromatography as well as hydrophobic interaction chromatography and then characterized by mass spectrometry (MS).

Hypotonic activation of the myo-inositol transporter SLC5A3 in HEK293 cells probed by cell volumetry, confocal and super-resolution microscopy

Swelling-activated pathways for myo-inositol, one of the most abundant organic osmolytes in mammalian cells, have not yet been identified. The present study explores the SLC5A3 protein as a possible transporter of myo-inositol in hyponically swollen HEK293 cells. To address this issue, we examined the relationship between the hypotonicity-induced changes in plasma membrane permeability to myo-inositol P_ino [m/s] and expression/localization of SLC5A3. P_ino values were determined by cell volumetry over a wide tonicity range (100–275 mOsm) in myo-inositol-substituted solutions. While being negligible under mild hypotonicity (200–275 mOsm), P_ino grew rapidly at osmolalities below 200 mOsm to reach a maximum of ∼3 nm/s at 100–125 mOsm, as indicated by fast cell swelling due to myo-inositol influx. The increase in P_ino resulted most likely from the hypotonicity-mediated incorporation of cytosolic SLC5A3 into the plasma membrane, as revealed by confocal fluorescence microscopy of cells expressing EGFP-tagged SLC5A3 and super-resolution imaging of immunostained SLC5A3 by direct stochastic optical reconstruction microscopy (dSTORM). dSTORM in hypotonic cells revealed a surface density of membrane-associated SLC5A3 proteins of 200–2000 localizations/μm². Assuming SLC5A3 to be the major path for myo-inositol, a turnover rate of 80–800 myo-inositol molecules per second for a single transporter protein was estimated from combined volumetric and dSTORM data. Hypotonic stress also caused a significant upregulation of SLC5A3 gene expression as detected by semiquantitative RT-PCR and Western blot analysis. In summary, our data provide first evidence for swelling-mediated activation of SLC5A3 thus suggesting a functional role of this transporter in hypotonic volume regulation of mammalian cells.

Camelid genomes reveal evolution and adaptation to desert environments

Bactrian camel (Camelus bactrianus), dromedary (Camelus dromedarius) and alpaca (Vicugna pacos) are economically important livestock. Although the Bactrian camel and dromedary are large, typically arid-desert-adapted mammals, alpacas are adapted to plateaus. Here we present high-quality genome sequences of these three species. Our analysis reveals the demographic history of these species since the Tortonian Stage of the Miocene and uncovers a striking correlation between large fluctuations in population size and geological time boundaries. Comparative genomic analysis reveals complex features related to desert adaptations, including fat and water metabolism, stress responses to heat, aridity, intense ultraviolet radiation and choking dust. Transcriptomic analysis of Bactrian camels further reveals unique osmoregulation, osmoprotection and compensatory mechanisms for water reservation underpinned by high blood glucose levels. We hypothesize that these physiological mechanisms represent kidney evolutionary adaptations to the desert environment. This study advances our understanding of camelid evolution and the adaptation of camels to arid-desert environments.

Extracellular osmolarity regulates matrix homeostasis in the intervertebral disc and articular cartilage: evolving role of TonEBP

Degeneration of the intervertebral disc is characterized by changes in proteoglycan status, loss of bound water molecules, decreased tissue osmotic pressure and a resulting mechanical failure of the disc. A similar spectrum of changes is evident in osteoarthritic articular cartilage. When healthy, resident cells in these skeletal tissues respond to applied mechanical loads by regulating their own osmotic state and the hydration of the extracellular matrix. The transcription factor Tonicity-Responsive Enhancer Binding Protein (TonEBP or NFAT5) is known to mediate the osmoadaptive response in these and other tissues. While the molecular basis of how osmotic loading controls matrix homeostasis is not completely understood, TonEBP regulates the expression of aggrecan and β1,3-glucoronosyltransferase in nucleus pulposus cells, in addition to targets that allow for survival under hypertonic stress. Moreover, in chondrocytes, TonEBP controls expression of several collagen subtypes and Sox9, a master regulator of aggrecan and collagen II expression. Thus, TonEBP-mediated regulation of the matrix composition allows disc cells and chondrocytes to modify the extracellular osmotic state itself. On the other hand, TonEBP in immune cells induces expression of TNF-α, IL-6 and MCP-1, pro-inflammatory molecules closely linked to matrix catabolism and pathogenesis of both disc degeneration and osteoarthritis, warranting investigations of this aspect of TonEBP function in skeletal cells. In summary, the TonEBP system, through its effects on extracellular matrix and osmoregulatory genes can be viewed primarily as a protective or homeostatic response to physiological loading.

The betaine/GABA transporter and betaine: roles in brain, kidney, and liver

The physiological roles of the betaine/GABA transporter (BGT1; slc6a12) are still being debated. BGT1 is a member of the solute carrier family 6 (the neurotransmitter, sodium symporter transporter family) and mediates cellular uptake of betaine and GABA in a sodium- and chloride-dependent process. Most of the studies of BGT1 concern its function and regulation in the kidney medulla where its role is best understood. The conditions here are hostile due to hyperosmolarity and significant concentrations of NH₄Cl and urea. To withstand the hyperosmolarity, cells trigger osmotic adaptation, involving concentration of a transcriptional factor TonEBP/NFAT5 in the nucleus, and accumulate betaine and other osmolytes. Data from renal cells in culture, primarily MDCK, revealed that transcriptional regulation of BGT1 by TonEBP/NFAT5 is relatively slow. To allow more acute control of the abundance of BGT1 protein in the plasma membrane, there is also post-translation regulation of BGT1 protein trafficking which is dependent on intracellular calcium and ATP. Further, betaine may be important in liver metabolism as a methyl donor. In fact, in the mouse the liver is the organ with the highest content of BGT1. Hepatocytes express high levels of both BGT1 and the only enzyme that can metabolize betaine, namely betaine:homocysteine –S-methyltransferase (BHMT1). The BHMT1 enzyme removes a methyl group from betaine and transfers it to homocysteine, a potential risk factor for cardiovascular disease. Finally, BGT1 has been proposed to play a role in controlling brain excitability and thereby represents a target for anticonvulsive drug development. The latter hypothesis is controversial due to very low expression levels of BGT1 relative to other GABA transporters in brain, and also the primary location of BGT1 at the surface of the brain in the leptomeninges. These issues are discussed in detail.

14-3-3-β and -{varepsilon} contribute to activation of the osmoprotective transcription factor NFAT5 by increasing its protein abundance and its transactivating activity

Having previously found that high NaCl causes rapid exit of 14‐3‐3 isoforms from the nucleus, we used siRNA‐mediated knockdown to test whether 14‐3‐3s contribute to the high NaCl‐induced increase in the activity of the osmoprotective transcription factor NFAT5. We find that, when NaCl is elevated, knockdown of 14‐3‐3‐β and/or 14‐3‐3‐ε decreases NFAT5 transcriptional activity, as assayed both by luciferase reporter and by the mRNA abundance of the NFAT5 target genes aldose reductase and the sodium‐ and chloride‐dependent betaine transporter, BGT1. Knockdown of other 14‐3‐3 isoforms does not significantly affect NFAT5 activity. 14‐3‐3‐β and/or 14‐3‐3‐ε do not act by affecting the nuclear localization of NFAT5, but by at least two other mechanisms: (1) 14‐3‐3‐β and 14‐3‐3‐ε increase protein abundance of NFAT5 and (2) they increase NFAT5 transactivating activity. When NaCl is elevated, knockdown of 14‐3‐3‐β and/or 14‐3‐3‐ε reduces the protein abundance of NFAT5, as measured by Western blot, without affecting the level of NFAT5 mRNA, and the knockdown also decreases NFAT5 transactivating activity, as measured by luciferase reporter. The 14‐3‐3s increase NFAT5 protein, not by increasing its translation, but by decreasing the rate at which it is degraded, as measured by cycloheximide chase. It is not clear at this point whether the 14‐3‐3s affect NFAT5 directly or indirectly through their effects on other proteins that signal activation of NFAT5.

e12000

When NaCl is elevated, knockdown of 14‐3‐3‐β and/or 14‐3‐3‐ε reduces the protein abundance of NFAT5, as measured by Western blot, without affecting the level of NFAT5 mRNA, and the knockdown also decreases NFAT5 transactivating activity, as measured by luciferase reporter. The 14‐3‐3s increase NFAT5 protein, not by increasing its translation, but by decreasing the rate at which it is degraded, as measured by cycloheximide chase.

NFAT5 in cellular adaptation to hypertonic stress - regulations and functional significance

The Nuclear Factor of Activated T Cells-5 (NFAT5), also known as OREBP or TonEBP, is a member of the nuclear factors of the activated T cells family of transcription factors. It is also the only known tonicity-regulated transcription factor in mammals. NFAT5 was initially known for its role in the hypertonic kidney inner medulla for orchestrating a genetic program to restore the cellular homeostasis. Emerging evidence, however, suggests that NFAT5 might play a more diverse functional role, including a pivotal role in blood pressure regulation and the development of autoimmune diseases. Despite the growing significance of NFAT5 in physiology and diseases, our understanding of how its activity is regulated remains very limited. Furthermore, how changes in tonicities are converted into functional outputs via NFAT5 remains elusive. Therefore, this review aims to summarize our current knowledge on the functional roles of NFAT5 in osmotic stress adaptation and the signaling pathways that regulate its activity.

Mutations that reduce its specific DNA binding inhibit high NaCl-induced nuclear localization of the osmoprotective transcription factor NFAT5

The transcription factor nuclear factor of activated T cell 5 (NFAT5) is activated by the stress of hypertonicity (e.g., high NaCl). Increased expression of NFAT5 target genes causes accumulation of protective organic osmolytes and heat shock proteins. Under normotonic conditions (∼300 mosmol/kgH(2)O), NFAT5 is distributed between the nucleus and cytoplasm, hypertonicity causes it to translocate into the nucleus, and hypotonicity causes it to translocate into the cytoplasm. The mechanism of translocation is complex and not completely understood. NFAT5-T298 is a known contact site of NFAT5 with its specific DNA element [osmotic response element (ORE)]. In the present study, we find that mutation of NFAT5-T298 to alanine or aspartic acid not only reduces binding of NFAT5 to OREs (EMSA) but also proportionately reduces high NaCl-induced nuclear translocation of NFAT5. Combined mutation of other NFAT5 DNA contact sites (R293A/E299A/R302A) also greatly reduces both specific DNA binding and nuclear localization of NFAT5. NFAT5-T298 is a potential phosphorylation site, but, using protein mass spectrometry, we do not find phosphorylation at NFAT5-T298. Further, decreased high NaCl-induced nuclear localization of NFAT5 mutated at T298 does not involve previously known regulatory mechanisms, including hypotonicity-induced export of NFAT5, regulated by phosphorylation of NFAT5-S155, XPO1 (CRM1/exportin1)-mediated export of NFAT5 from the nucleus, or hypertonicity-induced elevation of NUP88, which enhances nuclear localization of NFAT5. We conclude that specific DNA binding of NFAT5 contributes to its nuclear localization, by mechanisms, as yet undetermined, but independent of ones previously described to regulate NFAT5 distribution.

The mineralocorticoid signaling pathway throughout development: expression, regulation and pathophysiological implications

The mineralocorticoid signaling pathway has gained interest over the past few years, considering not only its implication in numerous pathologies but also its emerging role in physiological processes during kidney, brain, heart and lung development. This review aims at describing the setting and regulation of aldosterone biosynthesis and the expression of the mineralocorticoid receptor (MR), a nuclear receptor mediating aldosterone action in target tissues, during the perinatal period. Specificities concerning MR expression and regulation during the development of several major organs are highlighted. We provide evidence that MR expression is tightly controlled in a tissue-specific manner during development, which could have major pathophysiological implications in the neonatal period.

NFAT5 is activated by hypoxia: role in ischemia and reperfusion in the rat kidney

The current hypothesis postulates that NFAT5 activation in the kidney's inner medulla is due to hypertonicity, resulting in cell protection. Additionally, the renal medulla is hypoxic (10–18 mmHg); however there is no information about the effect of hypoxia on NFAT5. Using in vivo and in vitro models, we evaluated the effect of reducing the partial pressure of oxygen (PO₂) on NFAT5 activity. We found that 1) Anoxia increased NFAT5 expression and nuclear translocation in primary cultures of IMCD cells from rat kidney. 2) Anoxia increased transcriptional activity and nuclear translocation of NFAT5 in HEK293 cells. 3) The dose-response curve demonstrated that HIF-1α peaked at 2.5% and NFAT5 at 1% of O₂. 4) At 2.5% of O₂, the time-course curve of hypoxia demonstrated earlier induction of HIF-1α gene expression than NFAT5. 5) siRNA knockdown of NFAT5 increased the hypoxia-induced cell death. 6) siRNA knockdown of HIF-1α did not affect the NFAT5 induction by hypoxia. Additionally, HIF-1α was still induced by hypoxia even when NFAT5 was knocked down. 7) NFAT5 and HIF-1α expression were increased in kidney (cortex and medulla) from rats subjected to an experimental model of ischemia and reperfusion (I/R). 7) Experimental I/R increased the NFAT5-target gene aldose reductase (AR). 8) NFAT5 activators (ATM and PI3K) were induced in vitro (HEK293 cells) and in vivo (I/R kidneys) with the same timing of NFAT5. 8) Wortmannin, which inhibits ATM and PI3K, reduces hypoxia-induced NFAT5 transcriptional activation in HEK293 cells. These results demonstrate for the first time that NFAT5 is induced by hypoxia and could be a protective factor against ischemic damage.

Placental TonEBP/NFAT5 osmolyte regulation in an ovine model of intrauterine growth restriction

TonEBP/NFAT5 (the tonicity-responsive enhancer binding protein/nuclear factor of activated T cells) modulates cellular response to osmotic changes by accumulating inositol and sorbitol inside the cells. Our objective was to assess placental osmolytes, TonEBP/NFAT5 RNA and protein expression, and signaling molecules across gestation between control and intrauterine growth restriction (IUGR) ovine pregnancies. Pregnant sheep were placed in hyperthermic conditions to induce IUGR. Placental tissues were collected at 55, 95, and 130 days gestational age (dGA) to measure inositol, sorbitol, TonEBP/NFAT5 (NFAT5), sodium-dependent myo-inositol transporter (SMIT; official symbol SLC5A3), aldose reductase (AR), and NADPH (official symbol DE-CR1). Placental weight was reduced in IUGR compared to controls at 95 and 130 dGA. Osmolyte concentrations were similar between control and IUGR placentas, but both groups demonstrated a significant decrease in inositol concentration and an increase in sorbitol concentration with advancing gestation. Cytosolic NFAT5 protein decreased significantly from 55 to 95 dGA in both groups, and nuclear NFAT5 protein increased only at 130 dGA in the IUGR group, but no differences were seen between groups for either cytosolic or nuclear NFAT5 protein concentrations. DE-CR1 concentrations were similar between groups and increased significantly with advancing gestational age. AR was lowest at 55dGA, and SLC5A3 increased with advancing gestational age. We conclude that both placental osmolytes inositol and sorbitol (and their corresponding proteins SLC5A3 and AR) change with gestational age and are regulated, at least in part, by NFAT5 and DE-CR1 (NADPH). The inverse relationship between each osmolyte across gestation (e.g., inositol higher in early gestation and sorbitol higher in late gestation) may reflect nutritional needs that change across gestation.

Downregulation of the taurine transporter TauT during hypo-osmotic stress in NIH3T3 mouse fibroblasts

The present work was initiated to investigate regulation of the taurine transporter TauT by reactive oxygen species (ROS) and the tonicity-responsive enhancer binding protein (TonEBP) in NIH3T3 mouse fibroblasts during acute and long-term (4 h) exposure to low-sodium/hypo-osmotic stress. Taurine influx is reduced following reduction in osmolarity, keeping the extracellular Na(+) concentration constant. TonEBP activity is unaltered, whereas TauT transcription as well as TauT activity are significantly reduced under hypo-osmotic conditions. In contrast, TonEBP activity and TauT transcription are significantly increased following hyperosmotic exposure. Swelling-induced ROS production in NIH3T3 fibroblasts is generated by NOX4 and by increasing total ROS, by either exogenous application of H(2)O(2) or overexpressing NOX4, we demonstrate that TonEBP activity and taurine influx are regulated negatively by ROS under hypo-osmotic, low-sodium conditions, whereas the TauT mRNA level is unaffected. Acute exposure to ROS reduces taurine uptake as a result of modulated TauT transport kinetics. Thus, swelling-induced ROS production could account for the reduced taurine uptake under low-sodium/hypo-osmotic conditions by direct modulation of TauT.

NFAT5 contributes to osmolality-induced MCP-1 expression in mesothelial cells

Increased expression of the C-C chemokine monocyte chemoattractant protein-1 (MCP-1) in mesothelial cells in response to high glucose concentrations and/or high osmolality plays a crucial role in the development of peritoneal fibrosis during continuous ambulatory peritoneal dialysis (CAPD). Recent studies suggest that in kidney cells osmolality-induced MCP-1 upregulation is mediated by the osmosensitive transcription factor, nuclear factor of activated T cells 5 (NFAT5). The present study addressed the question of whether activation of NFAT5 by hyperosmolality, as present in PD fluids, contributes to MCP-1 expression in the mesothelial cell line Met5A. Hyperosmolality, induced by addition of glucose, NaCl, or mannitol to the growth medium, increased NFAT5 activity and stimulated MCP-1 expression in Met5A cells. siRNA-mediated knockdown of NFAT5 attenuated osmolality-induced MCP-1 upregulation substantially. Hyperosmolality also induced activation of nuclear factor-κB (NF-κB). Accordingly, pharmacological inhibition of NF-κB significantly decreased osmolality-induced MCP-1 expression. Taken together, these results indicate that high osmolalities activate the transcription factor NFAT5 in mesothelial cells. NFAT5 in turn upregulates MCP-1, likely in combination with NF-κB, and thus may participate in the development of peritoneal fibrosis during CAPD.

Dual role of the ddx5/ddx17 RNA helicases in the control of the pro-migratory NFAT5 transcription factor

Ddx5 and ddx17 are two highly related RNA helicases involved in both transcription and splicing. These proteins coactivate transcription factors involved in cancer such as the estrogen receptor alpha, p53 and beta-catenin. Ddx5 and ddx17 are part of the splicing machinery and can modulate alternative splicing, the main mechanism increasing the proteome diversity. Alternative splicing also has a role in gene expression level regulation when it is coupled to the nonsense-mediated mRNA decay (NMD) pathway. In this work, we report that ddx5 and ddx17 have a dual role in the control of the pro-migratory NFAT5 transcription factor. First, ddx5 and ddx17 act as transcriptional coactivators of NFAT5 and are required for activating NFAT5 target genes involved in tumor cell migration. Second, at the splicing level, ddx5 and ddx17 increase the inclusion of NFAT5 exon 5. As exon 5 contains a pre-mature translation termination codon, its inclusion leads to the regulation of NFAT5 mRNAs by the NMD pathway and to a decrease in NFAT5 protein level. Therefore, we demonstrated for the first time that a transcriptional coregulator can simultaneously regulate the transcriptional activity and alternative splicing of a transcription factor. This dual regulation, where ddx5 and ddx17 enhance the transcriptional activity of NFAT5 although reducing its protein expression level, suggests a critical role for ddx5 and ddx17 in tumor cell migration through the fine regulation of NFAT5 pathway.

Nuclear factor of activated T cells 5 deficiency increases the severity of neuronal cell death in ischemic injury

Nuclear factor of activated T cells 5 (NFAT5) has been implicated in regulating several genes that are thought to be neuroprotective in ischemic injury. Because of the embryonic lethality of NFAT5 knockout (NFAT5(-/-)) mice, the heterozygous (NFAT5(+/-)) mice were used to study the in vivo role of NFAT5 in hypoxia/ischemia (H/I) condition. The NFAT5(+/-) mice exhibited more severe neurological deficits, larger infarct area and edema formation associated with increased aquaporin 4 expressions in the brain. Under in vitro H/I condition, increased apoptotic cell death was found in NFAT5(-/-) neurons. Moreover, SMIT, a downstream to NFAT5, was upregulated in NFAT5(+/+) neurons, while the SMIT level could not be upregulated in NFAT5(-/-) neurons under H/I condition. The elevation of reactive oxygen species generation in NFAT5(-/-) neurons under H/I condition further confirmed that NFAT5(-/-) neurons were more susceptible to oxidative stress. The present study demonstrated that activation of NFAT5 and its downstream SMIT induction is important in protecting neurons from ischemia-induced oxidative stress.

Roles of cell volume in molecular and cellular biology

Extracellular tonicity and volume regulation control a great number of molecular and cellular functions including: cell proliferation, apoptosis, migration, hormone and neuromediator release, gene expression, ion channel and transporter activity and metabolism. The aim of this review is to describe these effects and to determine if they are direct or are secondarily the result of the activity of second messengers.

Tonicity-independent regulation of the osmosensitive transcription factor TonEBP (NFAT5)

Tonicity-responsive enhancer binding protein (TonEBP/nuclear factor of activated T-cells 5 [NFAT5]) is a Rel homology transcription factor classically known for its osmosensitive role in regulating cellular homeostasis during states of hypo- and hypertonic stress. A recently growing body of research indicates that TonEBP is not solely regulated by tonicity, but that it can be stimulated by various tonicity-independent mechanisms in both hypertonic and isotonic tissues. Physiological and pathophysiological stimuli such as cytokines, growth factors, receptor and integrin activation, contractile agonists, ions, and reactive oxygen species have been implicated in the positive regulation of TonEBP expression and activity in diverse cell types. These new data demonstrate that tonicity-independent stimulation of TonEBP is critical for tissue-specific functions like enhanced cell survival, migration, proliferation, vascular remodeling, carcinoma invasion, and angiogenesis. Continuing research will provide a better understanding as to how these and other alternative TonEBP stimuli regulate gene expression in both health and disease.

Role of NFAT5 in inflammatory disorders associated with osmotic stress

Nuclear factor of activated T cells 5 (NFAT5) is the most recently described member of the Rel family of transcription factors, including NF-κB and NFAT1-4, which play central roles in inducible gene expression during the immune response. NFAT5 was initially described to drive osmoprotective gene expression in renal medullary cells, which are routinely faced by high extracellular osmolalities. Recent data however indicate profound biological importance of the mammalian osmotic stress response in view of NFAT5 dependent gene regulation in non-renal tissues. In mononuclear cells and epithelial cells, NFAT5 stimulates the expression of various pro-inflammatory cytokines during elevated ambient tonicity. Accordingly, compared to plasma, the interstitial tonicity of lymphoid organs like spleen and thymus and that of liver is substantially hypertonic under physiological conditions. In addition, anisotonic disorders (hypernatremia, diabetes mellitus, dehydration) entail systemic hyperosmolality, and, in inflammatory disorders, the skin, intestine, and cornea are sites of local hyperosmolality. This article summarizes the current knowledge regarding systemic and local osmotic stress in anisotonic and inflammatory disorders in view of NFAT5 activation and regulation, and NFAT5 dependent cytokine production.

Hippocampal betaine/GABA transporter mRNA expression is not regulated by inflammation or dehydration post-status epilepticus

Seizure activity can alter GABA transporter and osmoprotective gene expression, which may be involved in the pathogenesis of epilepsy. However, the response of the betaine/GABA transporter (BGT1) is unknown. The goal of the present study was to compare the expression of BGT1 mRNA to that of other osmoprotective genes and GABA transporters following status epilepticus (SE). The possible contributory role of dehydration and inflammation was also investigated because both have been shown to be involved in the regulation of GABA transporter and/or osmoprotective gene expression. BGT1 mRNA was increased 24 h post-SE, as were osmoprotective genes. BGT1 was decreased 72 h and 4 weeks post-SE, as were the GABA transporter mRNAs. The mRNA values for osmoprotective genes following 24-h water withdrawal were significantly lower than the values obtained 24 h post-SE despite similarities in their plasma osmolality values. BGT1 mRNA was not altered by lipopolysaccharide-induced inflammation while the transcription factor tonicity-responsive enhancer binding protein and the GABA transporters 1 and 3 were. These results suggest that neither plasma osmolality nor inflammation fully account for the changes seen in BGT1 mRNA expression post-SE. However, it is evident that BGT1 mRNA expression is altered by SE and displays a temporal pattern with similarities to both GABA and osmolyte transporters. Further investigation of BGT1 regulation in the brain is warranted.

The expression of aquaporin-1 in the medulla of the kidney is dependent on the transcription factor associated with hypertonicity, TonEBP

Expression of aquaporin-1 (AQP1) and -2 (AQP2) channels in the kidney are critical for the maintenance of water homeostasis and the operation of the urinary concentrating mechanism. Hypertonic stress induced in inner medullary (IMCD3) cells by addition of NaCl to the medium substantially up-regulated the mRNA and protein expression of AQP1, suggesting that its activation occurs at a transcriptional and a translational levels. In contrast, no up-regulation of AQP1 was observed when these cells were exposed to the same tonicity by addition of urea. To explore the transcriptional activation of aqp1 under hypertonic stress, we examined the role of the transcription factor associated with hypertonicity, TonEBP. Treatment of IMCD3 cells with the TonEBP inhibitor rottlerin or silencing its expression with specific shRNA technology led to a substantial reduction in AQP1 expression under hypertonic conditions. Moreover, we defined a conserved TonEBP binding site located 811 bp upstream of the aqp1 exon that is essential for its expression. Single site-directed mutation of this TonE site led to a 54 ± 5% (p < 0.01) decrease in AQP1 luciferase-driven activity under hypertonic stress. TonEBP mutant mice display marked decrement in the expression of AQP1 in the inner medulla. In conclusion, these data demonstrate that TonEBP is necessary for the regulation of AQP1 expression in the inner medulla of the kidney under hypertonic conditions.

Physiological tonicity improves human chondrogenic marker expression through nuclear factor of activated T-cells 5 in vitro

Introduction

Chondrocytes experience a hypertonic environment compared with plasma (280 mOsm) due to the high fixed negative charge density of cartilage. Standard isolation of chondrocytes removes their hypertonic matrix, exposing them to nonphysiological conditions. During in vitro expansion, chondrocytes quickly lose their specialized phenotype, making them inappropriate for cell-based regenerative strategies. We aimed to elucidate the effects of tonicity during isolation and in vitro expansion on chondrocyte phenotype.

Methods

Human articular chondrocytes were isolated and subsequently expanded at control tonicity (280 mOsm) or at moderately elevated, physiological tonicity (380 mOsm). The effects of physiological tonicity on chondrocyte proliferation and chondrogenic marker expression were evaluated. The role of Tonicity-responsive Enhancer Binding Protein in response to physiological tonicity was investigated using nuclear factor of activated T-cells 5 (NFAT5) RNA interference.

Results

Moderately elevated, physiological tonicity (380 mOsm) did not affect chondrocyte proliferation, while higher tonicities inhibited proliferation and diminished cell viability. Physiological tonicity improved expression of chondrogenic markers and NFAT5 and its target genes, while suppressing dedifferentiation marker collagen type I and improving type II/type I expression ratios >100-fold. Effects of physiological tonicity were similar in osteoarthritic and normal (nonosteoarthritic) chondrocytes, indicating a disease-independent mechanism. NFAT5 RNA interference abolished tonicity-mediated effects and revealed that NFAT5 positively regulates collagen type II expression, while suppressing type I.

Conclusions

Physiological tonicity provides a simple, yet effective, means to improve phenotypical characteristics during cytokine-free isolation and in vitro expansion of human articular chondrocytes. Our findings will lead to the development of improved cell-based repair strategies for chondral lesions and provides important insights into mechanisms underlying osteoarthritic progression.

Determination of the NFAT5/TonEBP transcription factor in the human and ovine placenta

Osmotic stress results in the accumulation of osmolytes in tissues. Synthesis of these osmolytes is mediated by the transcription factor NFAT5/TonEBP in the human kidney. We tested for the presence of NFAT5 mRNA and protein in the human and ovine placenta and confirmed sorbitol and inositol osmolyte concentrations in these tissues. To determine NFAT5 protein, human and ovine placenta were tested for inositol, sorbitol and glucose using high performance liquid chromatography (HPLC). Additionally, RNA was extracted and cDNA was made from these tissues. PCR was performed and products were sequenced. Western blotting was used to assess the expression of the NFAT5 protein. Human and ovine placenta demonstrated: 1) high concentrations of sorbitol and inositol, 2) presence of NFAT5 mRNA, 3) confirmation by NFAT5 sequence identity, and 4) presence of NFAT5 protein. NFAT5 is present in the ovine and human placenta at the RNA and protein levels that suggest a role for this protein in the induction of these osmolytes. Further trophoblast studies of osmotic stress effects on osmolytes are planned.

TonEBP regulates hyperosmolality-induced arginine vasotocin gene expression in the chick (Gallus domesticus)

Arginine vasotocin (AVT) is expressed mainly in the paraventircular and supraoptic nuclei of the hypothalamus in chicken. This peptide is known to act as an antidiuretic hormone and its gene expression is stimulated by hyperosmolality. However, the transcription factors that regulate the AVT gene expression induced by hyperosmolality are still unknown. In this study, we examined the role of hyper-tonicity enhancer binding protein (TonEBP) in the transcriptional regulation of AVT gene in chicken. TonEBP mRNA expression levels increased at 1h after salt-loading treatment in the hypothalamus. This increase preceded that in AVT and c-fos mRNA expression. Intracerebroventricular injections of TonEBP antisense oligonucleotides, before the salt-loading treatment, prevented the increase in AVT gene expression. These results, all together, suggest that the transcription factor TonEBP may be involved in the regulation of AVT genes expression in response to a hyperosmotic environment in chicken.