NF-kappaB modulates aquaporin-2 transcription in renal collecting duct principal cells

'Aquaporin-omics': mechanisms of aquaporin-2 loss in polyuric disorders

Animal models of a variety of acquired nephrogenic diabetes insipidus (NDI) disorders have identified a common feature: all such models are associated with the loss of aquaporin-2 (AQP2) from collecting duct principal cells, explaining the associated polyuria. To discover mechanisms of AQP2 loss, previous investigators have carried out either transcriptomics (lithium-induced NDI, unilateral ureteral obstruction, endotoxin-induced NDI) or proteomics (hypokalaemia-associated NDI, hypercalcaemia-associated NDI, bilateral ureteral obstruction), yielding contrasting views. Here, to address whether there may be common mechanisms underlying loss of AQP2 in acquired NDI disorders, we have used bioinformatic data integration techniques to combine information from all transcriptomic and proteomic data sets. The analysis reveals roles for autophagy/apoptosis, oxidative stress and inflammatory signalling as key elements of the mechanism that results in loss of AQP2. These processes can cause AQP2 loss through the combined effects of repression of Aqp2 gene transcription, generalized translational repression, and increased autophagic degradation of proteins including AQP2. Two possible types of stress-sensor proteins, namely death receptors and stress-sensitive protein kinases of the EIF2AK family, are discussed as potential triggers for signalling processes that result in loss of AQP2. KEY POINTS: Prior studies have shown in a variety of animal models of acquired nephrogenic diabetes insipidus (NDI) that loss of the aquaporin-2 (AQP2) protein is a common feature. Investigations of acquired NDI using transcriptomics (RNA-seq) and proteomics (protein mass spectrometry) have led to differing conclusions regarding mechanisms of AQP2 loss. Bioinformatic integration of transcriptomic and proteomic data from these prior studies now reveals that acquired NDI models map to three core processes: oxidative stress, apoptosis/autophagy and inflammatory signalling. These processes cause loss of AQP2 through translational repression, accelerated degradation of proteins, and transcriptional repression.

TNF inhibits AQP2 expression via a miR137-dependent pathway

As miR-137 is a regulator of aquaporin (AQP)2 expression and tumor necrosis factor (TNF) inhibits the expression of several extrarenal AQPs, we tested the hypothesis that TNF inhibits AQP2 in the kidney via a miR-137-dependent mechanism. AQP2 mRNA and protein expression decreased ∼70% and 53%, respectively, in primary renal inner medullary collecting duct (IMCD) cells transfected with a miRNA mimic of mmu-miR-137, suggesting that miR-137 directly targets AQP2 mRNA in these cells. Exposure of IMCD cells for 2 h to 400 mosmol/kgH2O medium increased mmu-miR-137 mRNA expression about twofold, conditions that also increased TNF production approximately fourfold. To determine if the increase in mmu-miR-137 mRNA expression was related to the concomitant increase in TNF, IMCD cells were transfected with a lentivirus construct to silence TNF. This construct decreased mmu-miR-137 mRNA expression by ∼63%, suggesting that TNF upregulates the expression of miR-137. Levels of miR-137 also increased approximately twofold in IMCD tubules isolated from male mice given 1% NaCl in the drinking water for 3 days. Intrarenal lentivirus silencing of TNF increased AQP2 mRNA levels and protein expression concomitant with a decrease in miR-137 levels in tubules isolated from mice given NaCl. The changes in AQP2 expression levels affected the diluting ability of the kidney, which was assessed by measuring urine osmolality and urine volume, as the decrease in these parameters after renal silencing of TNF was prevented on intrarenal administration of miR-137. The study reveals a novel TNF function via a miR-137-dependent mechanism that regulates AQP2 expression and function.NEW & NOTEWORTHY An emerging intratubular tumor necrosis factor system, functioning during normotensive noninflammatory conditions, acts as a breaking mechanism that attenuates both the increases in Na+-K+-2Cl- cotransporter and aquaporin-2 induced by arginine vasopressin, thereby contributing to the regulation of electrolyte balance and blood pressure. A greater appreciation for the role of cytokines as mediators of immunophysiological responses may help reveal the relationship between the immune system and other physiological systems.

Neutral amino acid transporter SLC38A2 protects renal medulla from hyperosmolarity-induced ferroptosis

Hyperosmolarity of the renal medulla is essential for urine concentration and water homeostasis. However, how renal medullary collecting duct (MCD) cells survive and function under harsh hyperosmotic stress remains unclear. Using RNA-Seq, we identified SLC38A2 as a novel osmoresponsive neutral amino acid transporter in MCD cells. Hyperosmotic stress-induced cell death in MCD cells occurred mainly via ferroptosis, and it was significantly attenuated by SLC38A2 overexpression but worsened by Slc38a2-gene deletion or silencing. Mechanistic studies revealed that the osmoprotective effect of SLC38A2 is dependent on the activation of mTORC1. Moreover, an in vivo study demonstrated that Slc38a2-knockout mice exhibited significantly increased medullary ferroptosis following water restriction. Collectively, these findings reveal that Slc38a2 is an important osmoresponsive gene in the renal medulla and provide novel insights into the critical role of SLC38A2 in protecting MCD cells from hyperosmolarity-induced ferroptosis via the mTORC1 signalling pathway.

Expression Regulation and Trafficking of Aquaporins

Aquaporins (AQPs) mediate the bidirectional water flow driven by an osmotic gradient. Either gating or trafficking allows for rapid and specific AQP regulation in a tissue-dependent manner. The regulatory mechanisms of AQP2 are discussed mainly in this chapter, as the mechanisms controlling the regulation and trafficking of AQP2 have been very well studied. The targeting of AQP2 to the apical plasma membrane of collecting duct principal cells is mainly regulated by the action of arginine vasopressin (AVP) on the type 2 AVP receptor (V2R), which cause increased intracellular cAMP or elevated intracellular calcium levels. Activation of these intracellular signaling pathways results in vesicles bearing AQP2 transport, docking and fusion with the apical membrane, which increase density of AQP2 on the membrane. The removal of AQP2 from the membrane requires dynamic cytoskeletal remodeling. AQP2 is degraded through the ubiquitin proteasome pathway and lysosomal proteolysis pathway. Finally, we review updated findings in transcriptional and epigenetic regulation of AQP2.

Genetic deletion of the nuclear factor of activated T cells 5 in collecting duct principal cells causes nephrogenic diabetes insipidus

Water homeostasis is tightly regulated by the kidneys via the process of urine concentration. During reduced water intake, the antidiuretic hormone arginine vasopressin (AVP) binds to the vasopressin receptor type II (V2R) in the kidney to enhance countercurrent multiplication and medullary osmolality, and increase water reabsorption via aquaporin-2 (AQP2) water channels. The importance of this AVP, V2R, and AQP2 axis is highlighted by low urine osmolality and polyuria in people with various water balance disorders, including nephrogenic diabetes insipidus (NDI). ELF5 and nuclear factor of activated T cells 5 (NFAT5) are two transcription factors proposed to regulate Aqp2 expression, but their role is poorly defined. Here we generated two novel mouse lines with principal cell (PC)-specific deletion of ELF5 or NFAT5 and phenotyped them in respect to renal water handling. ELF5-deficient mice (ELF5^PC-KO ) had a very mild phenotype, with no clear differences in AQP2 abundance, and mild differences in renal water handling and maximal urinary concentrating capacity. In contrast, NFAT5 (NFAT5^PC-KO ) mice had significantly higher water intake and their 24 h urine volume was almost 10-fold greater than controls. After challenging with dDAVP or 8 h water restriction, NFAT5^PC-KO mice were unable to concentrate their urine, demonstrating that they suffer from NDI. The abundance of AQP2, other AQPs, and the urea transporter UT-A1 were greatly decreased in NFAT5^PC-KO mice. In conclusion, NFAT5 is a major regulator of not only Aqp2 gene transcription, but also other genes important for water homeostasis and its absence leads to the development of NDI.

Early Molecular Events Mediating Loss of Aquaporin-2 during Ureteral Obstruction in Rats

BACKGROUND

Ureteral obstruction is marked by disappearance of the vasopressin-dependent water channel aquaporin-2 (AQP2) in the renal collecting duct and polyuria upon reversal. Most studies of unilateral ureteral obstruction (UUO) models have examined late time points, obscuring the early signals that trigger loss of AQP2.

METHODS

We performed RNA-Seq on microdissected rat cortical collecting ducts (CCDs) to identify early signaling pathways after establishment of UUO.

RESULTS

Vasopressin V2 receptor (AVPR2) mRNA was decreased 3 hours after UUO, identifying one cause of AQP2 loss. Collecting duct principal cell differentiation markers were lost, including many not regulated by vasopressin. Immediate early genes in CCDs were widely induced 3 hours after UUO, including Myc, Atf3, and Fos (confirmed at the protein level). Simultaneously, expression of NF-κB signaling response genes known to repress Aqp2 increased. RNA-Seq for CCDs at an even earlier time point (30 minutes) showed widespread mRNA loss, indicating a "stunned" profile. Immunocytochemical labeling of markers of mRNA-degrading P-bodies DDX6 and 4E-T indicated an increase in P-body formation within 30 minutes.

CONCLUSIONS

Immediately after establishment of UUO, collecting ducts manifest a stunned state with broad disappearance of mRNAs. Within 3 hours, there is upregulation of immediate early and inflammatory genes and disappearance of the V2 vasopressin receptor, resulting in loss of AQP2 (confirmed by lipopolysaccharide administration). The inflammatory response seen rapidly after UUO establishment may be relevant to both UUO-induced polyuria and long-term development of fibrosis in UUO kidneys.

Comparative quantitation of aquaporin-2 and arginine vasopressin receptor-2 localizations among chronic kidney disease and healthy kidney in dogs

BACKGROUND AND AIM

Aquaporin-2 (AQP2) and arginine vasopressin receptor-2 (AVPR2) are proteins that control water homeostasis in principal cells. Chronic kidney disease (CKD) is defined as the impairment and irreversible loss of kidney function and/or structure, which causes water imbalances and polyuria. The study aimed to know the expression of AQPs and AVPR2 in the kidneys of a canine with CKD.

MATERIALS AND METHODS

The kidneys were collected from two dog carcasses from Small Animal Teaching Hospital, Faculty of Veterinary Medicine, Chiang Mai University. The kidney tissue was prepared for immunohistochemistry and investigated the expression and localization of tissue's AQP2 and AVPR2. For statistical analysis, the Mann-Whitney U-test was applied to the data.

RESULTS

By immunohistochemistry, AQP2 was expressed strongly in the basolateral and apical membranes of the principal cells, whereas AVPR2 was localized in the principal cell's basolateral membrane in both renal cortex and renal medulla. In the normal kidney, the semi-quantitative immunohistochemistry for the percentage of protein expression of AQP2 and AVPR2 was 5.062±0.4587 and 4.306±0.7695, respectively. In contrast, protein expression of AQP2 and AVPR2 in CKD was found to be 1.218±0.1719 and 0.8536±0.1396, respectively. The data shows that the percentage of AQP2 and AVPR2 expression was decreased, corresponding to a 4-fold and 5-fold in CKD (p<0.001).

CONCLUSION

Our findings revealed that CKD was a marked decrease in AQP2 and AVPR2 expression. The central role of specific AQP2 and AVPR2 in regulating water homeostasis will provide correlations in case of CKD with polyuria.

Bayesian identification of candidate transcription factors for the regulation of Aqp2 gene expression

Aquaporin-2 (Aqp2) gene transcription is strongly regulated by vasopressin in the renal collecting duct. However, the transcription factors (TFs) responsible for the regulation of expression of Aqp2 remain largely unknown. We used Bayes' theorem to integrate several -omics data sets to stratify the 1,344 TFs present in the mouse genome with regard to probabilities of regulating Aqp2 gene transcription. Also, we carried out new RNA sequencing experiments mapping the time course of vasopressin-induced changes in the transcriptome of mpkCCD cells to identify TFs that change in tandem with Aqp2. The analysis identified 17 of 1,344 TFs that are most likely to be involved in the regulation of Aqp2 gene transcription. These TFs included eight that have been proposed in prior studies to play a role in Aqp2 regulation, viz., Cebpb, Elf1, Elf3, Ets1, Jun, Junb, Nfkb1, and Sp1. The remaining nine represent new candidates for future studies (Atf1, Irf3, Klf5, Klf6, Mef2d, Nfyb, Nr2f6, Stat3, and Nr4a1). Conspicuously absent is CREB (Creb1), which has been widely proposed to mediate vasopressin-induced regulation of Aqp2 gene transcription (Nielsen S, Frokiaer J, Marples D, Kwon TH, Agre P, Knepper MA. Physiol Rev 82: 205-244, 2002; Kortenoeven ML, Fenton RA. Biochim Biophys Acta 1840: 1533-1549, 2014; Bockenhauer D, Bichet DG. Nat Rev Nephrol 11: 576-588, 2015; Pearce D, Soundararajan R, Trimpert C, Kashlan OB, Deen PM, Kohan DE. Clin J Am Soc Nephrol 10: 135-146, 2015). Instead, another CREB-like TF, Atf1, ranked fourth among all TFs. RNA sequencing time-course experiments showed a rapid increase in Aqp2 mRNA, within 3 h of vasopressin exposure. This response was matched by an equally rapid increase in the abundance of the mRNA coding for Cebpb, which we have shown by chromatin immunoprecipitation-sequencing studies to bind downstream from the Aqp2 gene. The identified TFs provide a roadmap for future studies to understand regulation of Aqp2 gene expression.NEW & NOTEWORTHY Abetted by the advent of systems biology-based ("-omics") techniques in the 21st century, there has been a massive expansion of published data relevant to virtually every physiological question. The authors have developed a large-scale data integration approach based on the application of Bayes'' theorem. In the current work, they integrated 12 different -omics data sets to identify the transcription factors most likely to mediate vasopressin-dependent regulation of transcription of the aquaporin-2 gene.

Role of Aquaporins in Inflammation-a Scientific Curation

Inflammation is a universal response mechanism existing as inter-communicator of biological systems. Uncontrolled or dysregulated inflammation addresses chronic low-grade effects eventually resulting in multimorbidity. Active solute transport across the membrane establishes varying osmotic gradients. Aquaporins (AQPs) are a class of critical ubiquitously expressed transmembrane proteins that aid in fluid and small solute transport via facilitated diffusion over established osmotic gradients. Numerous significant data features the biological functions of AQPs rendering them as an appropriate biomarker of health and diseases. Besides their physiological role in well-balanced inflammatory responses, it is worth noting the dysregulation of AQPs during any undesirable inflammatory event. Most literature to date clearly sets out AQPs as potential drug targets instigating AQP-based therapies. In light of this conception, the current review provides a compendious overview on the propitious and portentous out-turns of AQPs under inflammation.

Pdcd10-Stk24/25 complex controls kidney water reabsorption by regulating Aqp2 membrane targeting

PDCD10, also known as CCM3, is a gene found to be associated with the human disease cerebral cavernous malformations (CCMs). PDCD10 forms a complex with GCKIII kinases including STK24, STK25, and MST4. Studies in C. elegans and Drosophila have shown a pivotal role of the PDCD10-GCKIII complex in maintaining epithelial integrity. Here, we found that mice deficient of Pdcd10 or Stk24/25 in the kidney tubules developed polyuria and displayed increased water consumption. Although the expression levels of aquaporin genes were not decreased, the levels of total and phosphorylated aquaporin 2 (Aqp2) protein in the apical membrane of tubular epithelial cells were decreased in Pdcd10- and Stk24/25-deficient mice. This loss of Aqp2 was associated with increased expression and membrane targeting of Ezrin and phosphorylated Ezrin, Radixin, Moesin (p-ERM) proteins and impaired intracellular vesicle trafficking. Treatment with Erlotinib, a tyrosine kinase inhibitor promoting exocytosis and inhibiting endocytosis, normalized the expression level and membrane abundance of Aqp2 protein, and partially rescued the water reabsorption defect observed in the Pdcd10-deficient mice. Our current study identified the PDCD10-STK-ERM signaling pathway as a potentially novel pathway required for water balance control by regulating vesicle trafficking and protein abundance of AQP2 in the kidneys.

Rapid development of vasopressin resistance in dietary K+ deficiency

The association between diabetes insipidus (DI) and chronic dietary K+ deprivation is well known, but it remains uncertain how the disorder develops and whether it is influenced by the sexual dimorphism in K+ handling. Here, we determined the plasma K+ (PK) threshold for DI in male and female mice and ascertained if DI is initiated by polydipsia or by a central or nephrogenic defect. C57BL6J mice were randomized to a control diet or to graded reductions in dietary K+ for 8 days, and kidney function and transporters involved in water balance were characterized. We found that male and female mice develop polyuria and secondary polydipsia. Altered water balance coincided with a decrease in aquaporin-2 (AQP2) phosphorylation and apical localization despite increased levels of the vasopressin surrogate marker copeptin. No change in the protein abundance of urea transporter-A1 was observed. The Na+-K+-2Cl- cotransporter decreased only in males. Desmopressin treatment failed to reverse water diuresis in K+-restricted mice. These findings indicate that even a small fall in PK is associated with nephrogenic DI (NDI), coincident with the development of altered AQP2 regulation, implicating low PK as a causal trigger of NDI. We found that PK decreased more in females, and, consequently, females were more prone to develop NDI. Together, these data indicate that AQP2 regulation is disrupted by a small decrease in PK and that the response is influenced by sexual dimorphism in K+ handling. These findings provide new insights into the mechanisms linking water and K+ balances and support defining the disorder as "potassium-dependent NDI."NEW & NOTEWORTHY This study shows that aquaporin-2 regulation is disrupted by a small fall in plasma potassium levels and the response is influenced by sexual dimorphism in renal potassium handling. The findings provided new insights into the mechanisms by which water balance is altered in dietary potassium deficiency and support defining the disorder as "potassium-dependent nephrogenic diabetes insipidus."

Maren Pills Improve Constipation via Regulating AQP3 and NF-κB Signaling Pathway in Slow Transit Constipation In Vitro and In Vivo

Background

Maren pills have been used to treat constipation. Aquaporin 3 (AQP3) plays a vital role in regulating water transfer in the colon. It has been reported that the downregulation of AQP3 can regulate liquid water metabolism and intestinal permeability in irritable bowel syndrome (IBS) rats' colon via NF-κB pathway. In this study, we investigated whether the laxative effect of Maren pills is associated with the regulation of AQP3 and NF-κB signaling pathway in the colon.

Methods

The compound diphenoxylate suspension-induced STC rats received Maren pills intragastrically for 1 consecutive week to evaluate the laxative effect of Maren pills involving the regulation of AQP3 and NF-κB signaling pathway. Moreover, human intestinal epithelial cells (HT-29) were treated with drug serum to obtain in vitro data.

Results

Our results revealed that treatment with Maren pills increased the stool number, moisture content of feces, and intestinal transit rate in a dose-dependent manner. Maren pills significantly increased the AQP3, fibrosis transmembrane conductance regulator (CFTR), and protein kinase A (PKA) proteins in the colon of rats and in HT-29 cells. Mechanistically, Maren pills obviously inhibited the activation of NF-κB pathway in the colon of rats and in HT-29 cells.

Conclusion

These results suggest that the laxative effect of Maren pills is associated with the increased expression of AQP3 by downregulating NF-κB signal pathway.

Stevia rebaudiana extract attenuate metabolic disorders in diabetic rats via modulation of glucose transport and antioxidant signaling pathways and aquaporin-2 expression in two extrahepatic tissues

Today, plant-based therapies have been attracted attention to overcome diabetes complications. This study was an attempt to evaluate whether antidiabetic and nephroprotective effects of Stevia Rebaudiana Bertoni (SRB) can be exerted via upregulation of GLUT-4, SNAP23, and Stx4 in skeletal muscles or modulation of AQP2 mRNA expression and antioxidant signaling pathway activity (Nrf2/Keap1) in kidneys. To achieve this aim, diabetes was induced via STZ-nicotinamide (STZ-NA). Diabetes increased the level of Blood Urea Nitrogen (BUN), serum creatinine, Fasting Blood Sugar (FBS), and Keap1 mRNA expression, which was coincide with reduction in mRNA levels of Nrf2, GLUT4, SNAP23, and Stx4. SRB and metformin compensate mentioned variables. However, SRB extract was more effective than metformin to increase the levels of GLUT4 and Nrf2 mRNA. It seems that SRB might attenuate the diabetic complications via manipulating the glucose uptake components in peripheral tissues and might exert the nephroprotective effects by modulation of AQP2, and Nrf2/Keap1 mRNA expression. PRACTICAL APPLICATIONS: Synthetic antidiabetic drugs have been only partially successful in controlling the diabetic complications. Moreover, use of these drugs is associated with a number of adverse effects. Over the past few years, a renewed attention has been paid to the prevention and treatment of diabetes using medicinal plants and functional foods. SRB that have been known as natural sweetener for centuries, is a such natural agent that has high source of various phytochemicals with antidiabetic, renal protective, antitumor, and antioxidant properties. In the current study, possible molecular mechanisms of insulin-mimetic and nephroprotective effects of SRB extract was evaluated in diabetic rats. Due to powerful antihyperglycemic and nephroprotective effects of SRB extract that were showed in this study and previous studies, hence the fact that SRB is to be highlighted for future research as a new therapeutic agent for diabetes.

Pax2 and Pax8 Proteins Regulate Urea Transporters and Aquaporins to Control Urine Concentration in the Adult Kidney

BACKGROUND

As the glomerular filtrate passes through the nephron and into the renal medulla, electrolytes, water, and urea are reabsorbed through the concerted actions of solute carrier channels and aquaporins at various positions along the nephron and in the outer and inner medulla. Proliferating stem cells expressing the nuclear transcription factor Pax2 give rise to renal epithelial cells. Pax2 expression ends once the epithelial cells differentiate into mature proximal and distal tubules, whereas expression of the related Pax8 protein continues. The collecting tubules and renal medulla are derived from Pax2-positive ureteric bud epithelia that continue to express Pax2 and Pax8 in adult kidneys. Despite the crucial role of Pax2 in renal development, functions for Pax2 or Pax8 in adult renal epithelia have not been established.

METHODS

To examine the roles of Pax2 and Pax8 in the adult mouse kidney, we deleted either Pax2, Pax8, or both genes in adult mice and examined the resulting phenotypes and changes in gene expression patterns. We also explored the mechanism of Pax8-mediated activation of potential target genes in inner medullary collecting duct cells.

RESULTS

Mice with induced deletions of both Pax2 and Pax8 exhibit severe polyuria that can be attributed to significant changes in the expression of solute carriers, such as the urea transporters encoded by Slc14a2, as well as aquaporins within the inner and outer medulla. Furthermore, Pax8 expression is induced by high-salt levels in collecting duct cells and activates the Slc14a2 gene by recruiting a histone methyltransferase complex to the promoter.

CONCLUSIONS

These data reveal novel functions for Pax proteins in adult renal epithelia that are essential for retaining water and concentrating urine.

Phosphoproteomic Identification of Vasopressin/cAMP/Protein Kinase A-Dependent Signaling in Kidney

Water excretion by the kidney is regulated by the neurohypophyseal peptide hormone vasopressin through actions in renal collecting duct cells to regulate the water channel protein aquaporin-2. Vasopressin signaling is initiated by binding to a G-protein-coupled receptor called V2R, which signals through heterotrimeric G-protein subunit G_s α, adenylyl cyclase 6, and activation of the cAMP-regulated protein kinase (PKA). Signaling events coupling PKA activation and aquaporin-2 regulation were largely unknown until the advent of modern protein mass spectrometry techniques that allow proteome-wide quantification of protein phosphorylation changes (phosphoproteomics). This short review documents phosphoproteomic findings in collecting duct cells describing the response to V2R-selective vasopressin agonists and antagonists, the response to CRISPR-mediated deletion of PKA, results from in vitro phosphorylation studies using recombinant PKA, the response to the broad-spectrum kinase inhibitor H89 (N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulphonamide), and the responses underlying lithium-induced nephrogenic diabetes insipidus. These phosphoproteomic data sets have been made available online for modeling vasopressin signaling and signaling downstream from other G-protein-coupled receptors. SIGNIFICANCE STATEMENT: New developments in protein mass spectrometry are facilitating progress in identification of signaling networks. Using mass spectrometry, it is now possible to identify and quantify thousands of phosphorylation sites in a given cell type (phosphoproteomics). The authors describe the use of phosphoproteomics technology to identify signaling mechanisms downstream from a G-protein-coupled receptor, the vasopressin V2 subtype receptor, and its role of the regulation and dysregulation of water excretion in the kidney. Data from multiple phosphoproteomic data sets are provided as web-based resources.

Time-course of sodium transport along the nephron in nephrotic syndrome: The role of potassium

The mechanism of sodium retention and its location in kidney tubules may vary with time in nephrotic syndrome (NS). We studied the mechanisms of sodium retention in transgenic POD-ATTAC mice, which display an inducible podocyte-specific apoptosis. At day 2 after the induction of NS, the increased abundance of NHE3 and phosphorylated NCC in nephrotic mice compared with controls suggest that early sodium retention occurs mainly in the proximal and distal tubules. At day 3, the abundance of NHE3 normalized, phosphorylated NCC levels decreased, and cleavage and apical localization of γ-ENaC increased in nephrotic mice. These findings indicate that sodium retention shifted from the proximal and distal tubules to the collecting system. Increased cleavage and apical localization of γ-ENaC persisted at day 5 in nephrotic mice when hypovolemia resolved and steady-state was reached. Sodium retention and γ-ENaC cleavage were independent of the increased plasma levels of aldosterone. Nephrotic mice displayed decreased glomerular filtration rate and urinary potassium excretion associated with hyperkaliemia at day 3. Feeding nephrotic mice with a low potassium diet prevented hyperkaliemia, γ-ENaC cleavage, and led to persistent increased phosphorylation of NCC. These results suggest that potassium homeostasis is a major determinant of the tubular site of sodium retention in nephrotic mice.

Inverse Regulation of Lipocalin-2/24p3 Receptor/SLC22A17 and Lipocalin-2 Expression by Tonicity, NFAT5/TonEBP and Arginine Vasopressin in Mouse Cortical Collecting Duct Cells mCCD(cl.1): Implications for Osmotolerance

The rodent collecting duct (CD) expresses a 24p3/NGAL/lipocalin-2 (LCN2) receptor (SLC22A17) apically, possibly to mediate high-affinity reabsorption of filtered proteins by endocytosis, although its functions remain uncertain. Recently, we showed that hyperosmolarity/-tonicity upregulates SLC22A17 in cultured mouse inner-medullary CD cells, whereas activation of toll-like receptor 4 (TLR4), via bacterial lipopolysaccharides (LPS), downregulates SLC22A17. This is similar to the upregulation of Aqp2 by hyperosmolarity/-tonicity and arginine vasopressin (AVP), and downregulation by TLR4 signaling, which occur via the transcription factors NFAT5 (TonEBP or OREBP), cAMP-responsive element binding protein (CREB), and nuclear factor-kappa B, respectively. The aim of the study was to determine the effects of osmolarity/tonicity and AVP, and their associated signaling pathways, on the expression of SLC22A17 and its ligand, LCN2, in the mouse (m) cortical collecting duct cell line mCCD(cl.1). Normosmolarity/-tonicity corresponded to 300 mosmol/L, whereas the addition of 50–100 mmol/L NaCl for up to 72 h induced hyperosmolarity/-tonicity (400–500 mosmol/L). RT-PCR, qPCR, immunoblotting and immunofluorescence microscopy detected Slc22a17/SLC22A17 and Lcn2/LCN2 expression. RNAi silenced Nfat5, and the pharmacological agent 666-15 blocked CREB. Activation of TLR4 was induced with LPS. Similar to Aqp2, hyperosmotic/-tonic media and AVP upregulated Slc22a17/SLC22A17, via activation of NFAT5 and CREB, respectively, and LPS/TLR4 signaling downregulated Slc22a17/SLC22A17. Conversely, though NFAT5 mediated the hyperosmolarity/-tonicity induced downregulation of Lcn2/LCN2 expression, AVP reduced Lcn2/LCN2 expression and predominantly apical LCN2 secretion, evoked by LPS, through a posttranslational mode of action that was independent of CREB signaling. In conclusion, the hyperosmotic/-tonic upregulation of SLC22A17 in mCCD(cl.1) cells, via NFAT5, and by AVP, via CREB, suggests that SLC22A17 contributes to adaptive osmotolerance, whereas LCN2 downregulation could counteract increased proliferation and permanent damage of osmotically stressed cells.

Inhibition of IL-1β by Aliskiren Improved Renal AQP2 Expression and Urinary Concentration Defect in Ureteral Obstruction and Release

We previously demonstrated that ureteral obstruction is associated with a urinary concentrating defect and reduced expression of renal aquaporins (AQPs), in which the renin–angiotensin system (RAS) may play an important role. The aims of the present study were to examine whether the renin inhibitor aliskiren could prevent the reduction in AQP expression and improve the urinary concentrating capacity in mice with bilateral ureteral obstruction (BUO) and BUO release. BUO was performed for 24 h, and BUO release was performed for 1 (B-R1D) or 3 days (B-R3D) with or without aliskiren treatment. Aliskiren prevented polyuria and decreased urine osmolality induced by B-R3D. In mice with BUO and BUO release, aliskiren attenuated the reduction in AQP2 protein and mRNA expression in the obstructed kidneys. B-R3D increased the protein expression of NLRP3 inflammasome components ASC, caspase-1, and interleukin-1β in the obstructed kidneys, which was markedly prevented by aliskiren. Moreover, the NF-κB inhibitor Bay 11-7082 blocked NLRP3 inflammasome activation and attenuated the decrease in AQP2 protein expression in primary cultured rat inner medullary collecting duct cells treated with angiotensin II. These results indicate that the renin inhibitor aliskiren increases water channel AQP2 expression at least partially by suppressing NLRP3 inflammasome activation in the obstructed kidneys of mice with BUO and BUO release.

RNA-Seq and protein mass spectrometry in microdissected kidney tubules reveal signaling processes initiating lithium-induced nephrogenic diabetes insipidus

Lithium salts, used for treating bipolar disorder, frequently induce nephrogenic diabetes insipidus (NDI) thereby limiting therapeutic success. NDI is associated with loss of expression of the gene coding for the molecular water channel, aquaporin-2, in the renal collecting duct (CD). Here, we use systems biology methods in a well-established rat model of lithium-induced NDI to identify signaling pathways activated at the onset of polyuria. Using single-tubule RNA-Seq, full transcriptomes were determined in microdissected cortical collecting ducts (CCDs) of rats after 72 hours without or with initiation of lithium chloride administration. Transcriptome-wide changes in mRNA abundances were mapped to gene sets associated with curated canonical signaling pathways, showing evidence for activation of NF-κB signaling with induction of genes coding for multiple chemokines and most components of the Major Histocompatibility Complex Class I antigen-presenting complex. Administration of anti-inflammatory doses of dexamethasone to lithium chloride-treated rats countered the loss of aquaporin-2. RNA-Seq also confirmed prior evidence of a shift from quiescence into the cell cycle with arrest. Time course studies demonstrated an early (12 hour) increase in multiple immediate early response genes including several transcription factors. Protein mass spectrometry in microdissected CCDs provided corroborative evidence and identified decreased abundance of several anti-oxidant proteins. Thus, in the context of prior observations, our study can be best explained by a model in which lithium increases ERK activation leading to induction of NF-κB signaling and an inflammatory-like response that represses Aqp2 transcription.

Tonicity-responsive enhancer binding protein (TonEBP) regulates TNF-α-induced hypothalamic inflammation

Tonicity-responsive enhancer binding protein (TonEBP) is a widely expressed transcription factor and is important in the regulation of inflammatory cytokines. Here, we have identified TonEBP expression in the hypothalamus, which is particularly high in proopiomelanocortin (POMC) neurons. TonEBP overexpression stimulates POMC transcription, and TonEBP haploinsufficiency in TonEBP (+/-) mice results in a decrease in hypothalamic POMC expression. TonEBP (+/-) mice show reduced sickness responses, which include anorexia and hyperthermia, that are initially induced by tumor necrosis factor (TNF)-α. TonEBP (+/-) mice also show lower levels of TNF-α-induced hypothalamic expression of POMC and pro-inflammatory cytokines. These results suggest that TonEBP is an important molecular regulator in the development of inflammatory sickness responses through the control of POMC and pro-inflammatory cytokine expression in the hypothalamus.

New insights into the transcriptional regulation of aquaporin-2 and the treatment of X-linked hereditary nephrogenic diabetes insipidus

The kidney collecting duct (CD) is a tubular segment of the kidney where the osmolality and final flow rate of urine are established, enabling urine concentration and body water homeostasis. Water reabsorption in the CD depends on the action of arginine vasopressin (AVP) and a transepithelial osmotic gradient between the luminal fluid and surrounding interstitium. AVP induces transcellular water reabsorption across CD principal cells through associated signaling pathways after binding to arginine vasopressin receptor 2 (AVPR2). This signaling cascade regulates the water channel protein aquaporin-2 (AQP2). AQP2 is exclusively localized in kidney connecting tubules and CDs. Specifically, AVP stimulates the intracellular translocation of AQP2-containing vesicles to the apical plasma membrane, increasing the osmotic water permeability of CD cells. Moreover, AVP induces transcription of the Aqp2 gene, increasing AQP2 protein abundance. This review provides new insights into the transcriptional regulation of the Aqp2 gene in the kidney CD with an overview of AVP and AQP2. It summarizes current therapeutic approaches for X-linked nephrogenic diabetes insipidus caused by AVPR2 gene mutations.

Suppression of fatty acid β-oxidation and energy deficiency as a cause of inhibitory effect of E. coli lipopolysaccharide on osmotic water transport in the frog urinary bladder

Previously we showed that arginine-vasotocin (AVT)-stimulated osmotic water permeability (OWP) of the frog urinary bladder was decreased if the mucosal side of the bladder has been naturally colonized by Gram-negative bacteria, or if bacterial lipopolysaccharide (LPS) was introduced into the lumen of the isolated bladder (J. Exp. Zool., 2013, 319, 487-494). Taking into account that in different tissues and cell types, challenge with LPS causes significant metabolic shift and energy deficiency, we hypothesized that an LPS-induced decrease of AVT-stimulated OWP could depend on the reduction of fatty acid oxidation (FAO), which is important for generation of ATP in epithelia. Using an isolated frog Rana temporaria urinary bladder we showed that the AVT-induced increase of OWP did not depend on the external glucose, but was inhibited by oligomycin, an ATP-synthase inhibitor, and by etomoxir, an inhibitor of carnitine palmitoyltransferase-1. In primary cultured epithelial cells isolated from the bladder mucosa, LPS E. coli (25 μg/ml, 21 h), as well as etomoxir (100 μM), decreased FAO accompanied by triacylglycerol accumulation. Both drugs impaired mitochondrial functions demonstrated by decreased ATP production and a reduced maximal oxygen consumption rate (OCR) and OCR directed at ATP synthesis. Additionally, we found that LPS decreased the expression of peroxisome proliferator-activated receptor alpha, a key player in the regulation of FAO. These data indicate that the impairment of AVT-induced water transport in osmoregulatory epithelium caused by LPS depends at least partly on defects in FAO and FAO-dependent energy production.

The Deubiquitylase USP4 Interacts with the Water Channel AQP2 to Modulate Its Apical Membrane Accumulation and Cellular Abundance

Aquaporin 2 (AQP2) mediates the osmotic water permeability of the kidney collecting duct in response to arginine vasopressin (VP) and is essential for body water homeostasis. VP effects on AQP2 occur via long-term alterations in AQP2 abundance and short-term changes in AQP2 localization. Several of the effects of VP on AQP2 are dependent on AQP2 phosphorylation and ubiquitylation; post-translational modifications (PTM) that modulate AQP2 subcellular distribution and function. Although several protein kinases, phosphatases, and ubiquitin E3 ligases have been implicated in AQP2 PTM, how AQP2 is deubiquitylated or the role of deubiquitylases (DUBS) in AQP2 function is unknown. Here, we report a novel role of the ubiquitin-specific protease USP4 in modulating AQP2 function. USP4 co-localized with AQP2 in the mouse kidney, and in mpkCCD14 cells USP4 and AQP2 abundance are increased by VP. AQP2 and USP4 co-immunoprecipitated from mpkCCD14 cells and mouse kidney, and in vitro, USP4 can deubiquitylate AQP2. In mpkCCD14 cells, shRNA mediated knockdown of USP4 decreased AQP2 protein abundance, whereas no changes in AQP2 mRNA levels or VP-induced cAMP production were detected. VP-induced AQP2 membrane accumulation in knockdown cells was significantly reduced, which was associated with higher levels of ubiquitylated AQP2. AQP2 protein half-life was also significantly reduced in USP4 knockdown cells. Taken together, the data suggest that USP4 is a key regulator of AQP2 deubiquitylation and that loss of USP4 leads to increased AQP2 ubiquitylation, decreased AQP2 levels, and decreased cell surface AQP2 accumulation upon VP treatment. These studies have implications for understanding body water homeostasis.

Sequential and synchronized hypertonicity-induced activation of Rel-family transcription factors is required for osmoprotection in renal cells

NF-κB and TonEBP belong to the Rel-superfamily of transcription factors. Several specific stimuli, including hypertonicity which is a key factor for renal physiology, are able to activate them. It has been reported that, after hypertonic challenge, NF-κB activity can be modulated by TonEBP, considered as the master regulator of transcriptional activity in the presence of changes in environmental tonicity. In the present work we evaluated whether hypertonicity-induced gene transcription mediated by p65/RelA and TonEBP occurs by an independent action of each transcription factor or by acting together. To do this, we evaluated the expression of their specific target genes and cyclooxygenase-2 (COX-2), a common target of both transcription factors, in the renal epithelial cell line Madin-Darby canine kidney (MDCK) subjected to hypertonic environment. The results herein indicate that hypertonicity activates the Rel-family transcription factors p65/RelA and TonEBP in MDCK cells, and that both are required for hypertonic induction of COX-2 and of their specific target genes. In addition, present data show that p65/RelA modulates TonEBP expression and both colocalize in nuclei of hypertonic cultures of MDCK cells. Thus, a sequential and synchronized action p65/RelA → TonEBP would be necessary for the expression of hypertonicity-induced protective genes.

NFAT5 up-regulates expression of the kidney-specific ubiquitin ligase gene Rnf183 under hypertonic conditions in inner-medullary collecting duct cells

We previously reported that among the 37 RING finger protein (RNF) family members, RNF183 mRNA is specifically expressed in the kidney under normal conditions. However, the mechanism supporting its kidney-specific expression pattern remains unclear. In this study, we elucidated the mechanism of the transcriptional activation of murine Rnf183 in inner-medullary collecting duct cells. Experiments with anti-RNF183 antibody revealed that RNF183 is predominantly expressed in the renal medulla. Among the 37 RNF family members, Rnf183 mRNA expression was specifically increased in hypertonic conditions, a hallmark of the renal medulla. RNF183 up-regulation was consistent with the activation of nuclear factor of activated T cells 5 (NFAT5), a transcription factor essential for adaptation to hypertonic conditions. Accordingly, siRNA-mediated knockdown of NFAT5 down-regulated RNF183 expression. Furthermore, the -3,466 to -3,136-bp region upstream of the mouse Rnf183 promoter containing the NFAT5-binding motif is conserved among mammals. A luciferase-based reporter vector containing the NFAT5-binding site was activated in response to hypertonic stress, but was inhibited by a mutation at the NFAT5-binding site. ChIP assays revealed that the binding of NFAT5 to this DNA site is enhanced by hypertonic stress. Of note, siRNA-mediated RNF183 knockdown increased hypertonicity-induced caspase-3 activation and decreased viability of mIMCD-3 cells. These results indicate that (i) RNF183 is predominantly expressed in the normal renal medulla, (ii) NFAT5 stimulates transcriptional activation of Rnf183 by binding to its cognate binding motif in the Rnf183 promoter, and (iii) RNF183 protects renal medullary cells from hypertonicity-induced apoptosis.

Tonicity inversely modulates lipocalin-2 (Lcn2/24p3/NGAL) receptor (SLC22A17) and Lcn2 expression via Wnt/β-catenin signaling in renal inner medullary collecting duct cells: implications for cell fate and bacterial infection

Background

We have previously evidenced apical expression of the 24p3/NGAL/lipocalin-2 receptor (Lcn2-R; SLC22A17) in inner medullary collecting duct (IMCD) cells, which are present in vivo in a hyperosmotic/-tonic environment that activates canonical Wnt/β-catenin signaling. The localization of Lcn2-R in the inner medulla is intriguing considering local bacterial infections trigger toll-like receptor-4 (TLR-4)-mediated secretion of the bacteriostatic Fe³⁺-free (apo-)Lcn2.

Aim

To determine the effects of osmolarity/tonicity changes, Wnt/β-catenin and TLR-4 activation on Lcn2-R and Lcn2 expression and cell viability in rat primary IMCD and mouse (m)IMCD3 cells.

Methods

Normosmolarity/-tonicity was 300 mosmol/l whereas hyperosmolarity/-tonicity was induced by adding 100 mmol/l NaCl + 100 mmol/l urea (600 mosmol/l, 1-7 days). Lcn2-R and Lcn2 expression were determined by qPCR, immunoblotting, flow cytometry and immunofluorescence microscopy. β-catenin was silenced by RNAi. Cell viability/death was determined with MTT and LDH release assays. TLR-4 was activated by bacterial lipopolysaccharides (LPS).

Results

Hyperosmotic/-tonic media upregulated Lcn2-R by ~4-fold and decreased Lcn2 expression/secretion, along with Wnt/β-catenin activation, in IMCD cells. These effects of hyperosmotic/-tonic media on Lcn2-R/Lcn2 expression were reverted by normosmolarity/-tonicity, β-catenin silencing and/or LPS. Exposure of cells with endogenous or stably overexpressing Lcn2-R to apo-Lcn2 or LPS decreased cell viability.

Conclusions

Lcn2-R upregulation and Lcn2 downregulation via Wnt/β-catenin may promote adaptive osmotolerant survival of IMCD cells in response to hyperosmolarity/-tonicity whereas Lcn2 upregulation and Lcn2-R downregulation via TLR-4 and/or normosmolarity/-tonicity may protect IMCD cells against bacterial infections and prevent autocrine death induction by Lcn2.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0285-3) contains supplementary material, which is available to authorized users.

Inhibition of inflammation using diacerein markedly improved renal function in endotoxemic acute kidney injured mice

BACKGROUND

Inflammation is an important pathogenic component of endotoxemia-induced acute kidney injury (AKI), finally resulting in renal failure. Diacerein is an interleukin-1β (IL-1β) inhibitor used for osteoarthritis treatment by exerting anti-inflammatory effects. This study aims to investigate the effects of diacerein on endotoxemia-induced AKI.

METHODS

Male C57BL/6 mice were intraperitoneally injected with lipopolysaccharide (LPS, 10 mg/kg) for 24 h prior to diacerein treatment (15 mg/kg/day) for another 48 h. Mice were examined by histological, molecular and biochemical approaches.

RESULTS

LPS administration showed a time-dependent increase of IL-1β expression and secretion in kidney tissues. Diacerein treatment normalized urine volume and osmolarity, reduced blood urea nitrogen (BUN), fractional excretion of sodium (FENa), serum creatinine and osmolarity, and protected renal function in an endotoxemic AKI mice model. In the histopathologic study, diacerein also improved renal tubular damage such as necrosis of the tubular segment. Moreover, diacerein inhibited LPS-induced increase of inflammatory cytokines, such as IL-1β, tumor necrosis factor-α, monocyte chemoattractant protein-1 and nitric oxide synthase 2. In addition, LPS administration markedly decreased aquaporin 1 (AQP1), AQP2, AQP3, Na,K-ATPase α1, apical type 3 Na/H exchanger and Na-K-2Cl cotransporter expression in the kidney, which was reversed by diacerein treatment. We also found that diacerein or IL-1β inhibition prevented the secretion of inflammatory cytokines and the decrease of AQP and sodium transporter expression induced by LPS in HK-2 cells.

CONCLUSION

Our study demonstrates for the first time that diacerein improves renal function efficiently in endotoxemic AKI mice by suppressing inflammation and altering tubular water and sodium handing. These results suggest that diacerein may be a novel therapeutic agent for the treatment of endotoxemic AKI.

Soluble (pro)renin receptor as a potential therapy for diabetes insipidus

The antidiuretic hormone vasopressin (VP) is produced by the hypothalamus and is stored and secreted from the posterior pituitary. VP acts via VP type 2 receptors (V2Rs) on the basolateral membrane of principal cells of the collecting duct (CD) to regulate fluid permeability. The VP-evoked endocrine pathway is essential in determining urine concentrating capability. For example, a defect in any component of the VP signaling pathway can result in polyuria, polydipsia, and hypotonic urine, collectively termed diabetes insipidus (DI). A lack of VP production precipitates central diabetes insipidus (CDI), which can be managed effectively by VP supplementation. A majority of cases of nephrogenic diabetes insipidus (NDI) result from V2R mutations that impair receptor sensitivity. No specific therapy is currently available for management of NDI. Evidence is evolving that (pro)renin receptor (PRR), a newly identified member of the renin-angiotensin system, is capable of regulating VP production and action. As such, PRR should be considered strongly as a therapeutic target for treating CDI and NDI. The current review will summarize recent advances in understanding the physiology of renal and central PRR as it relates to the two types of DI.

Altered expression of Aquaporin-2 in one-kidney, one-clip hypertension

AIMS

The aim of the present study was to evaluate the regulation of Aquaporin-2 (AQP2) water channel in the kidney of one-kidney, one-clip rats (Goldblatt-1 model). In addition, some mechanisms that underlie the role of AQP2 in the Goldblatt-1 model were evaluated.

MAIN METHODS

Sprague-Dawley rats were divided in three groups: control two-kidney, no clip (C, 2 K-NC); nephrectomized one-kidney, no clip (N, 1 K-NC) and Goldblatt one-kidney, one-clip (G, 1 K-1C). AQP2 expression (by westernblot, real time PCR, immunohistochemistry and immunofluorescence), vasopressin V2 receptor expression (by real time PCR), cAMP concentration, NFkB and TonEBP (cytosol to nucleus ratio) were evaluated in the renal medulla.

KEY FINDINGS

AQP2 expression, V2 receptor expression and cAMP concentration were decreased in the renal medulla of 1 K-1C rats, NFkB translocation was favoured towards the nucleus suggesting its activation while TonEBP translocation was not altered in this model of hypertension.

SIGNIFICANCE

In this model of hypertension the decrease of AQP2 expression could be a mechanism that counteracts the high blood pressure promoting water excretion and this may be consequence of decreased vasopressin sensitivity and/or the increased activity of NFkB at renomedullary collecting duct level. Given that renovascular hypertension is among the most common causes of secondary hypertension, it is important to elucidate all the relevant mechanisms involved in the generation or in the compensation of the hypertensive state in order to improve the diagnoses and treatment of the patients.

Aquaporins 1, 3 and 8 expression in irritable bowel syndrome rats' colon via NF-κB pathway

Objective

Our research was to detect the expression of aquaporins. NF-κB in Irritable bowel syndrome (IBS) rat models’ colon so as to find novel pathogenesisof IBS.

Results

The expression of AQP1, AQP3, and AQP8 of IBS model group was down-regulated while NF-κB p65 was up-regulated comparing with control group (p < 0.05), and the expression of AQP1, AQP3, and AQP8 of inhibitor group was up-regulated while NF-κB p65 was down-regulated comparing with IBS model group (p < 0.05).

Materials and Methods

18 adult female SD big rats were divided into three groups:the rats in control group were normal rats, the rats in IBS model group and the rats of inhibitor group were injected with the inhibitor of NF-κB (PDTC). Immunohistochemical technique and western blot were performed to detect the expression of AQP1, AQP3, AQP8 and NF-κB p65. RT-PCR was performed to detect the expression of AQP1, AQP3, and AQP8.

Conclusions

Liquid water metabolic abnormalities and intestine permeability alteration might be the mechanism of IBS by down-regulating AQP1, AQP3 and AQP8 via NF-κB pathway.

Aquaporin Membrane Channels in Oxidative Stress, Cell Signaling, and Aging: Recent Advances and Research Trends

Reactive oxygen species (ROS) are produced as a result of aerobic metabolism and as by-products through numerous physiological and biochemical processes. While ROS-dependent modifications are fundamental in transducing intracellular signals controlling pleiotropic functions, imbalanced ROS can cause oxidative damage, eventually leading to many chronic diseases. Moreover, increased ROS and reduced nitric oxide (NO) bioavailability are main key factors in dysfunctions underlying aging, frailty, hypertension, and atherosclerosis. Extensive investigation aims to elucidate the beneficial effects of ROS and NO, providing novel insights into the current medical treatment of oxidative stress-related diseases of high epidemiological impact. This review focuses on emerging topics encompassing the functional involvement of aquaporin channel proteins (AQPs) and membrane transport systems, also allowing permeation of NO and hydrogen peroxide, a major ROS, in oxidative stress physiology and pathophysiology. The most recent advances regarding the modulation exerted by food phytocompounds with antioxidant action on AQPs are also reviewed.

Genome-Wide Mapping of DNA Accessibility and Binding Sites for CREB and C/EBPβ in Vasopressin-Sensitive Collecting Duct Cells

Background Renal water excretion is controlled by vasopressin, in part through regulation of the transcription of the aquaporin-2 gene (Aqp2).Methods To identify enhancer regions likely to be involved in the regulation of Aqp2 and other principal cell-specific genes, we used several next generation DNA-sequencing techniques in a well characterized cultured cell model of collecting duct principal cells (mpkCCD). To locate enhancers, we performed the assay for transposase-accessible chromatin using sequencing (ATAC-Seq) to identify accessible regions of DNA and integrated the data with data generated by chromatin immunoprecipitation followed by next generation DNA-sequencing (ChIP-Seq) for CCCTC binding factor (CTCF) binding, histone H3 lysine-27 acetylation, and RNA polymerase II.Results We identified two high-probability enhancers centered 81 kb upstream and 5.8 kb downstream from the Aqp2 transcriptional start site. Motif analysis of these regions and the Aqp2 promoter identified several potential transcription factor binding sites, including sites for two b-ZIP transcription factors: CCAAT/enhancer binding protein-β (C/EBPβ) and cAMP-responsive element binding protein (CREB). To identify genomic binding sites for both, we conducted ChIP-Seq using well characterized antibodies. In the presence of vasopressin, C/EBPβ, a pioneer transcription factor critical to cell-specific gene expression, bound strongly at the identified enhancer downstream from Aqp2 However, over multiple replicates, we found no detectable CREB binding sites within 390 kb of Aqp2 Thus, any role for CREB in the regulation of Aqp2 gene transcription is likely to be indirect.Conclusions The analysis identified two enhancer regions pertinent to transcriptional regulation of the Aqp2 gene and showed C/EBPβ (but not CREB) binding.

New Perspectives on the Potential Role of Aquaporins (AQPs) in the Physiology of Inflammation

Aquaporins (AQPs) are emerging, in the last few decades, as critical proteins regulating water fluid homeostasis in cells involved in inflammation. AQPs represent a family of ubiquitous membrane channels that regulate osmotically water flux in various tissues and sometimes the transport of small solutes, including glycerol. Extensive data indicate that AQPs, working as water channel proteins, regulate not only cell migration, but also common events essential for inflammatory response. The involvement of AQPs in several inflammatory processes, as demonstrated by their dysregulation both in human and animal diseases, identifies their new role in protection and response to different noxious stimuli, including bacterial infection. This contribution could represent a new key to clarify the dilemma of host-pathogen communications, and opens up new scenarios regarding the investigation of the modulation of specific AQPs, as target for new pharmacological therapies. This review provides updated information on the underlying mechanisms of AQPs in the regulation of inflammatory responses in mammals and discusses the broad spectrum of options that can be tailored for different diseases and their pharmacological treatment.

Integrin linked kinase regulates the transcription of AQP2 by NFATC3

Two processes are associated with progressive loss of renal function: 1) decreased aquaporin-2 (AQP2) expression and urinary concentrating capacity (Nephrogenic Diabetes Insipidus, NDI); and 2) changes in extracellular matrix (ECM) composition, e.g. increased collagen I (Col I) deposition, characteristic of tubule-interstitial fibrosis. AQP2 expression is regulated by both the ECM-to-intracellular scaffold protein integrin-linked kinase (ILK) by NFATc/AP1 and other transcription factors. In the present work, we used in vivo and in vitro approaches to examine ILK participation in NFATc3/AP-1-mediated increases in AQP2 gene expression. Both NFATc3 knock-out mice and ILK conditional-knockdown mice (cKD-ILK) display symptoms of NDI (polyuria and reduced AQP2 expression). NFATc3 is upregulated in the renal medulla tubular cells of cKD-ILK mice but with reduced nuclear localization. Inner medullary collecting duct mIMCD3 cells were subjected to ILK depletion and transfected with reporter plasmids. Pharmacological activators or inhibitors determined the effect of ILK activity on NFATc/AP-1-dependent increases in transcription of AQP2. Finally, mIMCD3 cultured on Col I showed reduced activity of the ILK/GSK3β/NFATc/AQP2 axis, suggesting this pathway is a potential target for therapeutic treatment of NDI.

Identification of β-catenin-interacting proteins in nuclear fractions of native rat collecting duct cells

The gene encoding the aquaporin-2 water channel is regulated transcriptionally in response to vasopressin. In the renal collecting duct, vasopressin stimulates the nuclear translocation and phosphorylation (at Ser⁵⁵²) of β-catenin, a multifunctional protein that acts as a transcriptional coregulator in the nucleus. The purpose of this study was to identify β-catenin-interacting proteins that might be involved in transcriptional regulation in rat inner medullary collecting duct (IMCD) cells, using experimental and computational approaches. We used a standard chromatin immunoprecipitation procedure coupled to mass spectrometry (ChIP-MS) in a nuclear fraction isolated from rat IMCD suspensions. Over four biological replicates, we reproducibly identified 43 β-catenin-binding proteins, including several known β-catenin-binding partners as well as novel interacting proteins. Multiple proteins involved in transcriptional regulation were identified (Taf1, Jup, Tdrd3, Cdh1, Cenpj, and several histones). Many of the identified β-catenin-binding partners were found in prior studies to translocate to the nucleus in response to vasopressin. There was only one DNA-binding transcription factor (TF), specifically Taf1, part of the RNA-polymerase II preinitiation complex. To identify sequence-specific TFs that might interact with β-catenin, Bayes' theorem was used to integrate data from several information sources. The analysis identified several TFs with potential binding sites in the Aqp2 gene promoter that could interact with β-catenin in the regulation of Aqp2 gene transcription, specifically Jun, Junb, Jund, Atf1, Atf2, Mef2d, Usf1, Max, Pou2f1, and Rxra. The findings provide information necessary for modeling the transcriptional response to vasopressin.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

Sirtuin1 (SIRT1) Regulates Tumor Necrosis Factor-alpha (TNF-α-Induced) Aquaporin-2 (AQP2) Expression in Renal Medullary Collecting Duct Cells Through Inhibiting the NF-κB Pathway

Background

Aquaporin-2 (AQP2) plays a major role in water reabsorption in the renal collecting duct, and is involved in a variety of renal disease. Recent studies have indicate that sirtuin1 (SIRT1) exerts renoprotective properties against kidney diseases. This study aimed to determine the potential role of SIRT1 in AQP2 expression induced by tumor necrosis factor-alpha (TNF-α) and to disclose the underlying mechanism in renal inner medullary collecting duct (IMCD) cells.

Material/Methods

Quantitative real-time PCR and Western blotting were respectively identified mRNA and protein expression. Immunofluorescence staining was used to detect the localization of AQP2. Small-interfering RNA (siRNA) was carried out for mechanism study.

Results

Results showed that AQP2 was clearly increased in the plasma membrane and decreased in the cytoplasm of IMCD cells treated with AVP. TNF-α treatment in IMCD cells significantly reduced SIRT1 and AQP2 expression, and increased acetylated NF-κBp65 protein level in time- and concentration-dependent manners. Moreover, SIRT1 overexpression or the activator SRT1720 augmented AQP2 expression and reduced the acetylation of NF-κBp65, which was reversed by SIRT1 siRNA or the inhibitors Ex527 and sirtinol in TNF-α-induced IMCD cells. Knockdown of NF-κBp65 or NF-κBp65 inhibition by pyrrolidine dithiocarbamate (PDTC) enhanced AQP2 expression in IMCD cells exposed to TNF-α. Importantly, knockdown of NF-κBp65 augmented the up-regulation of SIRT1 on AQP2 expression in IMCD cells induced by TNF-α.

Conclusions

These findings indicate that SIRT1 increases AQP2 expression in TNF-α-induced IMCD cells via the NF-κB-dependent signalling pathway, which might provide novel insight to understanding the renoprotective effects of SIRT1 in kidney diseases.

Resveratrol attenuates 4-hydroxy-2-hexenal-induced oxidative stress in mouse cortical collecting duct cells

Resveratrol (RSV) may provide numerous protective eff ects against chronic inflammatory diseases. Due to local hypoxia and hypertonicity, the renal medulla is subject to extreme oxidative stress, and aldehyde products formed during lipid peroxidation, such as 4-hydroxy-2-hexenal (HHE), might be responsible for tubular injury. This study aimed at investigating the eff ects of RSV on renal and its signaling mechanisms. While HHE treatment resulted in decreased expression of Sirt1, AQP2, and nuclear factor erythroid 2-related factor 2 (Nrf2), mouse cortical collecting duct cells (M1) cells treated with HHE exhibited increased activation of p38 MAPK, extracellular signal regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and increased expression of NOX4, p47^phox, Kelch ECH associating protein 1 (Keap1) and COX2. HHE treatment also induced NF-κB activation by promoting IκB-α degradation. Meanwhile, the observed increases in nuclear NF-κB, NOX4, p47^phox, and COX2 expression were attenuated by treatment with Bay 117082, N-acetyl-l-cysteine (NAC), or RSV. Our findings indicate that RSV inhibits the expression of inflammatory proteins and the production of reactive oxygen species in M1 cells by inhibiting NF-κB activation.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

Deep proteomic profiling of vasopressin-sensitive collecting duct cells. I. Virtual Western blots and molecular weight distributions

The mouse mpkCCD cell line is a continuous cultured epithelial cell line with characteristics of renal collecting duct principal cells. This line is widely used to study epithelial transport and its regulation. To provide a data resource useful for experimental design and interpretation in studies using mpkCCD cells, we have carried out "deep" proteomic profiling of these cells using three levels of fractionation (differential centrifugation, SDS-PAGE, and HPLC) followed by tandem mass spectrometry to identify and quantify proteins. The analysis of all resulting samples generated 34.6 gigabytes of spectral data. As a result, we identified 6,766 proteins in mpkCCD cells at a high level of stringency. These proteins are expressed over eight orders of magnitude of protein abundance. The data are provided to users as a public data base (https://helixweb.nih.gov/ESBL/Database/mpkFractions/). The mass spectrometry data were mapped back to their gel slices to generate "virtual Western blots" for each protein. For most of the 6,766 proteins, the apparent molecular weight from SDS-PAGE agreed closely with the calculated molecular weight. However, a substantial fraction (>15%) of proteins was found to run aberrantly, with much higher or much lower mobilities than predicted. These proteins were analyzed to identify mechanisms responsible for altered mobility on SDS-PAGE, including high or low isoelectric point, high or low hydrophobicity, physiological cleavage, residence in the lysosome, posttranslational modifications, and expression of alternative isoforms due to alternative exon usage. Additionally, this analysis identified a previously unrecognized isoform of aquaporin-2 with apparent molecular mass <20 kDa.

Aquaporins, vasopressin, and aging: current perspectives

Functioning of the hypothalamic-neurohypophyseal-vasopressin axis is altered in aging, and the pathway may represent a plausible target to slow the process of aging. Arginine vasopressin, a nine-amino acid peptide that is secreted from the posterior pituitary in response to high plasma osmolality and hypotension, is central in this pathway. Vasopressin has important roles in circulatory and water homoeostasis mediated by vasopressin receptor subtypes V1a (vascular), V1b (pituitary), and V2 (vascular, renal). A dysfunction in this pathway as a result of aging can result in multiple abnormalities in several physiological systems. In addition, vasopressin plasma concentration is significantly higher in males than in females and vasopressin-mediated effects on renal and vascular targets are more pronounced in males than in females. These findings may be caused by sex differences in vasopressin secretion and action, making men more susceptible than females to diseases like hypertension, cardiovascular and chronic kidney diseases, and urolithiasis. Recently the availability of new, potent, orally active vasopressin receptor antagonists, the vaptans, has strongly increased the interest on vasopressin and its receptors as a new target for prevention of age-related diseases associated with its receptor-altered signaling. This review summarizes the recent literature in the field of vasopressin signaling in age-dependent abnormalities in kidney, cardiovascular function, and bone function.

A novel therapeutic effect of statins on nephrogenic diabetes insipidus

Statins competitively inhibit hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase, resulting in reduced plasma total and low-density lipoprotein cholesterol levels. Recently, it has been shown that statins exert additional ‘pleiotropic’ effects by increasing expression levels of the membrane water channels aquaporin 2 (AQP2). AQP2 is localized mainly in the kidney and plays a critical role in determining cellular water content. This additional effect is independent of cholesterol homoeostasis, and depends on depletion of mevalonate-derived intermediates of sterol synthetic pathways, i.e. farnesylpyrophosphate and geranylgeranylpyrophosphate. By up-regulating the expression levels of AQP2, statins increase water reabsorption by the kidney, thus opening up a new avenue in treating patients with nephrogenic diabetes insipidus (NDI), a hereditary disease that yet lacks high-powered and limited side effects therapy. Aspects related to water balance determined by AQP2 in the kidney, as well as standard and novel therapeutic strategies of NDI are discussed.

Angiotensin II increases the expression of (pro)renin receptor during low-salt conditions

BACKGROUND

Evidence indicates that chronic angiotensin II (AngII) infusion increases (pro)renin receptor ((P)RR) expression in renal inner medullary collecting duct (IMCD) cells. Recently, it has been shown that renal (P)RR expression is augmented during a low-salt (LS) diet. However, the role of AngII in mediating the stimulation of (P)RR during LS conditions is unknown. We hypothesized that AngII mediates the increased expression of (P)RR during low-salt conditions in IMCDs.

METHODS

(P)RR expression and AngII levels were evaluated in Sprague-Dawley rats fed a LS diet (0.03% NaCl) and normal salt (NS; 0.4% NaCl) for 7 days. We examined the effects of sodium reduction (130 mM NaCl) and AngII on (P)RR expression in IMCDs isolated in hypertonic conditions (640 mOsmol/L with 280 mM NaCl).

RESULTS

Plasma renin activity in LS rats was significantly higher than rats fed with NS (28.1 ± 2.2 versus 6.7 ± 1.1 ng AngI·mL⁻¹·hr⁻¹; P < 0.05), as well as renin content in renal cortex and medulla. The (P)RR mRNA and protein levels were higher in medullary tissues from LS rats but did not change in the cortex. Intrarenal AngII was augmented in LS compared with NS rats (cortex: 710 ± 113 versus 277 ± 86 fmol/g, P < 0.05; medulla: 2093 ± 125 versus 1426 ± 126 fmol/g, P < 0.05). In cultured IMCDs, (P)RR expression was increased in response to LS or AngII treatment and potentiated by both treatments (both at 640 mOsmol/L).

CONCLUSIONS

These data indicate that (P)RR is augmented in medullary collecting ducts in response to LS and that this effect is further enhanced by the increased intrarenal AngII content.

Epithelial sodium channel abundance is decreased by an unfolded protein response induced by hyperosmolality

Large shifts of osmolality occur in the kidney medulla as part of the urine concentrating mechanism. Hyperosmotic stress profoundly challenges cellular homeostasis and induces endoplasmic reticulum (ER) stress. Here, we examined the unfolded protein response (UPR) in hyperosmotically-challenged principal cells of the kidney collecting duct (CD) and show its relevance in controlling epithelial sodium channel (ENaC) abundance, responsible for the final adjustment of Na(+) excretion. Dehydration increases medullary but not cortical osmolality. Q-PCR analysis of microdissected CD of water-deprived mice revealed increased aquaporin-2 (AQP2) expression in outer medullary and cortical CD while ENaC abundance decreased in outer medullary but not cortical CD. Immunoblotting, Q-PCR and immunofluorescence revealed that hyperosmolality induced a transient ER stress-like response both ex vivo and in cultured CD principal cells and increased activity of the canonical UPR mediators PERK and ATF6. Both hyperosmolality and chemical induction of ER stress decreased ENaC expression in vitro. ENaC depletion by either stimulus was abolished by transcriptional inhibition and by the chemical chaperone 4-phenylbutyric acid and was partly abrogated by either PERK or ATF6 silencing. Our data suggest that induction of the UPR by hyperosmolality may help preserve body fluid homeostasis under conditions of dehydration by uncoupling AQP2 and ENaC abundance in outer medullary CD.

Integrin-linked kinase regulates tubular aquaporin-2 content and intracellular location: a link between the extracellular matrix and water reabsorption

One of the clinical alterations observed in chronic renal disease (CRD) is the impaired urine concentration, known as diabetes insipidus (DI). Tubulointerstitial fibrosis of the kidney is also a pathological finding observed in CRD and involves composition of extracellular matrix (ECM). However, an association between these two events has not been elucidated. In this study, we showed that the extracellular-to-intracellular scaffold protein integrin-linked kinase (ILK) regulates expression of tubular water channel aquaporin-2 (AQP2) and its apical membrane presence in the renal tubule. Basally, polyuria and decreased urine osmolality were present in ILK conditional-knockdown (cKD-ILK) adult mice compared with nondepleted ILK littermates. No changes were observed in arginine-vasopressin (AVP) blood levels, renal receptor (V2R), or AQP3 expression. However, tubular AQP2 was decreased in expression and apical membrane presence in cKD-ILK mice, where the canonical V2R/cAMP axis activation is still functional, but independent of the absence of ILK. Thus, cKD-ILK constitutes a nephrogenic diabetes insipidus (NDI) model. AQP2 and ILK colocalize in cultured inner medullary collecting duct (mIMCD3) cells. Specific ILK siRNAs and collagen I (Col) decrease ILK and AQP2 levels and AQP2 presence on the membrane of tubular mIMCD3 cells, which impairs the capacity of the cells to transport water under hypotonic stress. The present work points to ILK as a therapeutic target in NDI.

Role of angiotensin II and oxidative stress on renal aquaporins expression in hypernatremic rats

The aim of this study was to assess whether endogenous Ang II and oxidative stress produced by acute hypertonic sodium overload may regulate the expression of aquaporin-1 (AQP-1) and aquaporin-2 (AQP-2) in the kidney. Groups of anesthetized male Sprague-Dawley rats were infused with isotonic saline solution (control) or with hypertonic saline solution (Na group, 1 M NaCl), either alone or with losartan (10 mg kg(-1)) or tempol (0.5 mg min(-1) kg(-1)) during 2 h. Renal function parameters were measured. Groups of unanesthetized animals were injected intraperitoneally with hypertonic saline solution, with or without free access to water intake, Na+W, and Na-W, respectively. The expression of AQP-1, AQP-2, Ang II, eNOS, and NF-kB were evaluated in the kidney by Western blot and immunohistochemistry. AQP-2 distribution was assessed by immunofluorescence. Na group showed increased natriuresis and diuresis, and Ang II and NF-kB expression, but decreased eNOS expression. Losartan or tempol enhanced further the diuresis, and AQP-2 and eNOS expression, as well as decreased Ang II and NF-kB expression. Confocal immunofluorescence imaging revealed labeling of AQP-2 in the apical plasma membrane with less labeling in the intracellular vesicles than the apical membrane in kidney medullary collecting duct principal cells both in C and Na groups. Importantly, our data also show that losartan and tempol induces a predominantly accumulation of AQP-2 in intracellular vesicles. In unanesthetized rats, Na+W group presented increased diuresis, natriuresis, and AQP-2 expression (112 ± 25 vs 64 ± 16; *p < 0.05). Water deprivation increased plasma sodium and diuresis but decreased AQP-2 (46 ± 22 vs 112 ± 25; §p < 0.05) and eNOS expression in the kidney. This study is a novel demonstration that renal endogenous Ang II-oxidative stress, induced in vivo in hypernatremic rats by an acute sodium overload, regulates AQP-2 expression.

Farnesoid X receptor (FXR) gene deficiency impairs urine concentration in mice

The farnesoid X receptor (FXR) is a ligand-activated transcription factor belonging to the nuclear receptor superfamily. FXR is mainly expressed in liver and small intestine, where it plays an important role in bile acid, lipid, and glucose metabolism. The kidney also has a high FXR expression level, with its physiological function unknown. Here we demonstrate that FXR is ubiquitously distributed in renal tubules. FXR agonist treatment significantly lowered urine volume and increased urine osmolality, whereas FXR knockout mice exhibited an impaired urine concentrating ability, which led to a polyuria phenotype. We further found that treatment of C57BL/6 mice with chenodeoxycholic acid, an FXR endogenous ligand, significantly up-regulated renal aquaporin 2 (AQP2) expression, whereas FXR gene deficiency markedly reduced AQP2 expression levels in the kidney. In vitro studies showed that the AQP2 gene promoter contained a putative FXR response element site, which can be bound and activated by FXR, resulting in a significant increase of AQP2 transcription in cultured primary inner medullary collecting duct cells. In conclusion, the present study demonstrates that FXR plays a critical role in the regulation of urine volume, and its activation increases urinary concentrating capacity mainly via up-regulating its target gene AQP2 expression in the collecting ducts.

NADPH oxidase 4 deficiency reduces aquaporin-2 mRNA expression in cultured renal collecting duct principal cells via increased PDE3 and PDE4 activity

The final control of renal water reabsorption occurs in the collecting duct (CD) and relies on regulated expression of aquaporin-2 (AQP2) in principal CD cells. AQP2 transcription is primarily induced by type 2 vasopressin receptor (V₂R)-cAMP-protein kinase A (PKA) signaling but also by other factors, including TonEBP and NF-κB. NAPDH oxidase 4 (NOX4) represents a major source of reactive oxygen species (ROS) in the kidney. Because NOX-derived ROS may alter PKA, TonEBP and NF-κB activity, we examined the effects of NOX4 depletion on AQP2 expression. Depleted NOX4 expression by siRNA (siNOX4) in mpkCCD_cl4 cells attenuated increased AQP2 mRNA expression by arginine vasopressin (AVP) but not by hypertonicity, which induces both TonEBP and NF-κB activity. AVP-induced AQP2 expression was similarly decreased by the flavoprotein inhibitor diphenyleneiodonium. siNOX4 altered neither TonEBP nor NF-κB activity but attenuated AVP-inducible cellular cAMP concentration, PKA activity and CREB phosphorylation as well as AQP2 mRNA expression induced by forskolin, a potent activator of adenylate cyclase. The repressive effect of siNOX4 on AVP-induced AQP2 mRNA expression was abolished by the non-selective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) and was significantly decreased by selective PDE antagonists cilostamide and rolipram, but not vinpocetine, which respectively target PDE3, PDE4 and PDE1. Thus, by inhibiting PDE3 and PDE4 activity NOX4-derived ROS may contribute to V₂R-cAMP-PKA signaling and enhance AQP2 transcription.