cAMP motifs regulating transcription in the aquaporin 2 gene

V2 vasopressin receptor mutations: future personalized therapy based on individual molecular biology

The diluting and concentrating function of the kidney plays a crucial role in regulating the water homeostasis of the body. This function is regulated by the antidiuretic hormone, arginine vasopressin through the type 2 vasopressin receptor (V2R), allowing the body to adapt to periods of water load or water restriction. Loss-of-function mutations of the V2R cause X-linked nephrogenic diabetes insipidus (XNDI), which is characterized by polyuria, polydipsia, and hyposthenuria. Gain-of-function mutations of the V2R lead to nephrogenic syndrome of inappropriate antidiuresis disease (NSIAD), which results in hyponatremia. Various mechanisms may be responsible for the impaired receptor functions, and this review provides an overview of recent findings about the potential therapeutic interventions in the light of the current experimental data.

Kidney tubular transcription co-activator, Yes-associated protein 1 (YAP), controls the expression of collecting duct aquaporins and water homeostasis

Final urine volume and concentration are defined by water reabsorption through the water channel proteins aquaporin (AQP)-2, -3 and -4 in the collecting duct. However, the transcriptional regulation of these AQPs is not well understood. The Hippo/Yes-associated protein 1 (YAP) pathway plays an important role in organ size control and tissue homeostasis. When the Hippo pathway including the Mst1/Mst2 kinases is inhibited, YAP is activated and functions as a transcription co-activator. Our previous work revealed a pathological role of tubular YAP activation in chronic kidney disease, but the physiological role of YAP in the kidney remains to be established. Here, we found that tubule-specific Yap knockout mice showed increased urine output and decreased urinary osmolality. Decreases in Aqp2, -3 and -4 mRNA and protein abundance in the kidney were evident in Yap knockout mice. Analysis of Mst1/Mst2 double knockout and Mst1/Mst2/Yap triple knockout mice showed that expression of Aqp2 and Aqp4 but not Aqp3 was dependent on YAP. Furthermore, YAP was recruited to the promoters of the Aqp2 and Aqp4 genes and stimulated their transcription. Interestingly, YAP was found to interact with transcription factors GATA2, GATA3 and NFATc1. These three factors promoted Aqp2 transcription in a YAP dependent manner in collecting duct cells. These three factors also promoted Aqp4 transcription whereas only GATA2 and GATA3 enhanced Aqp3 transcription. Thus, our results suggest that YAP promotes Aqp2 and Aqp4 transcription, interacts with GATA2, GATA3 and NFATc1 to control Aqp2 expression, while Aqp-2, -3 and -4 exploit overlapping mechanisms for their baseline transcriptional regulation.

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.

Glucocorticoid Receptor Maintains Vasopressin Responses in Kidney Collecting Duct Cells

Water permeability of the kidney collecting ducts is regulated in part by the amount of the molecular water channel protein aquaporin-2 (AQP2), whose expression, in turn, is regulated by the pituitary peptide hormone vasopressin. We previously showed that stable glucocorticoid receptor knockdown diminished the vasopressin-induced Aqp2 gene expression in the collecting duct cell model mpkCCD. Here, we investigated the pathways regulated by the glucocorticoid receptor by comparing transcriptomes of the mpkCCD cells with or without stable glucocorticoid receptor knockdown. Glucocorticoid receptor knockdown downregulated 5,394 transcripts associated with 55 KEGG pathways including "vasopressin-regulated water reabsorption," indicative of positive regulatory roles of these pathways in the vasopressin-induced Aqp2 gene expression. Quantitative RT-PCR confirmed the downregulation of the vasopressin V2 receptor transcript upon glucocorticoid receptor knockdown. Glucocorticoid receptor knockdown upregulated 3,785 transcripts associated with 42 KEGG pathways including the "TNF signaling pathway" and "TGFβ signaling pathway," suggesting the negative regulatory roles of these pathways in the vasopressin-induced Aqp2 gene expression. Quantitative RT-PCR confirmed the upregulation of TNF and TGFβ receptor transcripts upon glucocorticoid receptor knockdown. TNF or TGFβ inhibitor alone, in the absence of vasopressin, did not induce Aqp2 gene transcription. However, TNF or TGFβ blunted the vasopressin-induced Aqp2 gene expression. In particular, TGFβ reduced vasopressin-induced increases in Akt phosphorylation without inducing epithelial-to-mesenchymal transition or interfering with vasopressin-induced apical AQP2 trafficking. In summary, our RNA-seq transcriptomic comparison revealed positive and negative regulatory pathways maintained by the glucocorticoid receptor for the vasopressin-induced Aqp2 gene expression.

HDAC3 inhibition improves urinary-concentrating defect in hypokalaemia by promoting AQP2 transcription

AIM

This study investigated whether enhanced histone acetylation, achieved by inhibiting histone deacetylases (HDACs), could prevent decreased aquaporin-2 (AQP2) expression during hypokalaemia.

METHODS

Male Wistar rats were fed a potassium-free diet with or without 4-phenylbutyric acid (4-PBA) or the selective HDAC3 inhibitor RGFP966 for 4 days. Primary renal inner medullary collecting duct (IMCD) cells and immortalized mouse cortical collecting duct (mpkCCD) cells were cultured in potassium-deprivation medium with or without HDAC inhibitors.

RESULTS

4-PBA increased the levels of AQP2 mRNA and protein in the kidney inner medullae in hypokalaemic (HK) rats, which was associated with decreased urine output and increased urinary osmolality. The level of acetylated H3K27 (H3K27ac) protein was decreased in the inner medullae of HK rat kidneys; this decrease was mitigated by 4-PBA. The H3K27ac levels were decreased in IMCD and mpkCCD cells cultured in potassium-deprivation medium. Decreased H3K27ac in the Aqp2 promoter region was associated with reduced Aqp2 mRNA levels. HDAC3 protein expression was upregulated in mpkCCD and IMCD cells in response to potassium deprivation, and the binding of HDAC3 to the Aqp2 promoter was also increased. RGFP966 increased the levels of H3K27ac and AQP2 proteins and enhanced binding between H3K27ac and AQP2 in mpkCCD cells. Furthermore, RGFP966 reversed the hypokalaemia-induced downregulation of AQP2 and H3K27ac and alleviated polyuria in rats. RGFP966 increased interstitial osmolality in the kidney inner medullae of HK rats but did not affect urinary cAMP levels.

CONCLUSION

HDAC inhibitors prevented the downregulation of AQP2 induced by potassium deprivation, probably by enhancing H3K27 acetylation.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

The aquaporins (AQPs) are a family of small integral membrane proteins that facilitate the bidirectional transport of water across biological membranes in response to osmotic pressure gradients as well as enable the transmembrane diffusion of small neutral solutes (such as urea, glycerol, and hydrogen peroxide) and ions. AQPs are expressed throughout the human body. Here, we review their key roles in fluid homeostasis, glandular secretions, signal transduction and sensation, barrier function, immunity and inflammation, cell migration, and angiogenesis. Evidence from a wide variety of studies now supports a view of the functions of AQPs being much more complex than simply mediating the passive flow of water across biological membranes. The discovery and development of small-molecule AQP inhibitors for research use and therapeutic development will lead to new insights into the basic biology of and novel treatments for the wide range of AQP-associated disorders.

α-Actinin 4 Links Vasopressin Short-Term and Long-Term Regulation of Aquaporin-2 in Kidney Collecting Duct Cells

Water permeability of the kidney collecting ducts is regulated by the peptide hormone vasopressin. Between minutes and hours (short-term), vasopressin induces trafficking of the water channel protein aquaporin-2 to the apical plasma membrane of the collecting duct principal cells to increase water permeability. Between hours and days (long-term), vasopressin induces aquaporin-2 gene expression. Here, we investigated the mechanisms that bridge the short-term and long-term vasopressin-mediated aquaporin-2 regulation by α-actinin 4, an F-actin crosslinking protein and a transcription co-activator of the glucocorticoid receptor. Vasopressin induced F-actin depolymerization and α-actinin 4 nuclear translocation in the mpkCCD collecting duct cell model. Co-immunoprecipitation followed by immunoblotting showed increased interaction between α-actinin 4 and glucocorticoid receptor in response to vasopressin. ChIP-PCR showed results consistent with α-actinin 4 and glucocorticoid receptor binding to the aquaporin-2 promoter. α-actinin 4 knockdown reduced vasopressin-induced increases in aquaporin-2 mRNA and protein expression. α-actinin 4 knockdown did not affect vasopressin-induced glucocorticoid receptor nuclear translocation, suggesting independent mechanisms of vasopressin-induced nuclear translocation of α-actinin 4 and glucocorticoid receptor. Glucocorticoid receptor knockdown profoundly reduced vasopressin-induced increases in aquaporin-2 mRNA and protein expression. In the absence of glucocorticoid analog dexamethasone, vasopressin-induced increases in glucocorticoid receptor nuclear translocation and aquaporin-2 mRNA were greatly reduced. α-actinin 4 knockdown further reduced vasopressin-induced increase in aquaporin-2 mRNA in the absence of dexamethasone. We conclude that glucocorticoid receptor plays a major role in vasopressin-induced aquaporin-2 gene expression that can be enhanced by α-actinin 4. In the absence of vasopressin, α-actinin 4 crosslinks F-actin underneath the apical plasma membrane, impeding aquaporin-2 membrane insertion. Vasopressin-induced F-actin depolymerization in one hand facilitates aquaporin-2 apical membrane insertion and in the other hand frees α-actinin 4 to enter the nucleus where it binds glucocorticoid receptor to enhance aquaporin-2 gene expression.

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.

The physiological and molecular mechanisms to maintain water and salt homeostasis in response to high salt intake in Mongolian gerbils (Meriones unguiculatus)

Desert rodents are faced with many challenges such as high dietary salt in their natural habitats and they have evolved abilities to conserve water and tolerate salt. However, the physiological and molecular mechanisms involved in water and salt balances in desert rodents are unknown. We hypothesized that desert rodents regulated water and salt balances by altering the expression of AQP2 and α-ENaC in the kidney. Mongolian gerbils (Meriones unguiculatus), a desert species, were acclimated to drinking water with different salt contents: (0, control; 4% NaCl, moderate salt, MS; 8% NaCl, high salt, HS) for 4 weeks. The gerbils drinking salty water had lower body mass, food intake, water intake, metabolic water production and urine volume. The HS gerbils increased the expression of arginine vasopressin (AVP) in the hypothalamus, and also enhanced the expression of AQP2 and cAMP/PKA/CREB signaling pathway in the kidney. In addition, these gerbils reduced serum aldosterone levels and α-ENaC expression in the kidney. Creatinine clearance was lower in the HS group than that in the control group, but serum and urine creatinine levels did not change. These data indicate that desert rodents rely on AVP-dependent upregulation of AQP2 and aldosterone-dependent downregulation of α-ENaC in the kidney to promote water reabsorption and sodium excretion under high salt intake.

Targeting the Trafficking of Kidney Water Channels for Therapeutic Benefit

The ability to regulate water movement is vital for the survival of cells and organisms. In addition to passively crossing lipid bilayers by diffusion, water transport is also driven across cell membranes by osmotic gradients through aquaporin water channels. There are 13 aquaporins in human tissues, and of these, aquaporin-2 (AQP2) is the most highly regulated water channel in the kidney: The expression and trafficking of AQP2 respond to body volume status and plasma osmolality via the antidiuretic hormone, vasopressin (VP). Dysfunctional VP signaling in renal epithelial cells contributes to disorders of water balance, and research initially focused on regulating the major cAMP/PKA pathway to normalize urine concentrating ability. With the discovery of novel and more complex signaling networks that regulate AQP2 trafficking, promising therapeutic targets have since been identified. Several strategies based on data from preclinical studies may ultimately translate to the care of patients with defective water homeostasis.

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.

Transcription Factor Elf3 Modulates Vasopressin-Induced Aquaporin-2 Gene Expression in Kidney Collecting Duct Cells

Aquaporin-2 (AQP2) is a molecular water channel protein responsible for water reabsorption by the kidney collecting ducts. Many water balance disorders are associated with defects in AQP2 gene expression regulated by the peptide hormone vasopressin. Here, we studied roles of Elf3 (E26 transformation-specific (Ets)-related transcription factor 3) in AQP2 gene expression in the collecting duct cells (mpkCCD). Vasopressin increased AQP2 mRNA and protein levels without affecting AQP2 mRNA degradation, indicative of transcriptional regulation. Elf3 knockdown and overexpression, respectively, reduced and increased AQP2 gene expression under basal and vasopressin-stimulated conditions. However, the vasopressin-to-basal ratios of AQP2 gene expression levels remained constant, indicating that Elf3 does not directly mediate vasopressin response but modulates the level of AQP2 gene expression inducible by vasopressin. The Elf3-modulated AQP2 gene expression was associated with AQP2 promoter activity, in line with Elf3’s ability to bind an Ets element in the AQP2 promoter. Mutation in the Ets element reduced both basal and vasopressin-stimulated AQP2 promoter activity, again without affecting vasopressin-to-basal ratios of the AQP2 promoter activity. Lithium chloride reduced both Elf3 and AQP2 mRNA in the mpkCCD cells as well as in mouse kidney inner medulla. We conclude that Elf3 modulates AQP2 promoter activity thereby gauging vasopressin-inducible AQP2 gene expression levels. Our data provide a potential explanation to lithium-induced nephrogenic diabetes insipidus where lithium reduces Elf3 and hence AQP2 abundance.

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.

The contribution of collecting duct NOS1 to the concentrating mechanisms in male and female mice

The collecting duct (CD) concentrates the urine, thereby maintaining body water volume and plasma osmolality within a normal range. The endocrine hormone arginine vasopressin acts in the CD to increase water permeability via the vasopressin 2 receptor (V2R)-aquaporin (AQP) axis. Recent studies have suggested that autocrine factors may also contribute to the regulation of CD water permeability. Nitric oxide is produced predominantly by nitric oxide synthase 1 (NOS1) in the CD and acts as a diuretic during salt loading. The present study sought to determine whether CD NOS1 regulates diuresis during changes in hydration status. Male and female control and CD NOS1 knockout (CDNOS1KO) mice were hydrated (5% sucrose water), water deprived, or acutely challenged with the V2R agonist desmopressin. In male mice, water deprivation resulted in decreased urine flow and increased plasma osmolality, copeptin concentration, and kidney AQP2 abundance independent of CD NOS1. In female control mice, water deprivation reduced urine flow, increased plasma osmolality and copeptin, but did not significantly change total AQP2; however, there was increased basolateral AQP3 localization. Surprisingly, female CDNOS1KO mice while on the sucrose water presented with symptoms of dehydration. Fibroblast growth factor 21, an endocrine regulator of sweetness preference, was significantly higher in female CDNOS1KO mice, suggesting that this was reducing their drive to drink the sucrose water. With acute desmopressin challenge, female CDNOS1KO mice failed to appropriately concentrate their urine, resulting in higher plasma osmolality than controls. In conclusion, CD NOS1 plays only a minor role in urine-concentrating mechanisms.

Adenine acts in the kidney as a signaling factor and causes salt- and water-losing nephropathy: early mechanism of adenine-induced renal injury

Chronic adenine feeding is extensively used to develop animal models of chronic renal failure with metabolic features resembling those observed in humans. However, the mechanism by which adenine induces renal failure is poorly understood. In this study, we examined the early effects of adenine on water metabolism and salt balance in rats placed in metabolic cages and fed control or adenine-containing diets for 7 days. Molecular and functional studies demonstrated that adenine-fed rats exhibited a significant reduction in food intake, polyuria, polydipsia, decreased urine osmolality, and increased salt wasting. These effects are independent of changes in food intake and result from a coordinated downregulation of water channel aquaporin-2 (AQP2) and salt transporter (Na⁺-K⁺-Cl^- cotransporter 2; NKCC2) in the collecting duct and medullary thick ascending limb, respectively. As a result, adenine-fed rats exhibited massive volume depletion, as indicated by a significant body weight loss, increased blood urea nitrogen, and increased hematocrit and hemoglobin levels, all of which were significantly corrected with NaCl replacement. Adenine-induced urinary concentrating defect was not corrected by exogenous arginine vasopressin (AVP), and it correlated with reduced cAMP production in vivo and in vitro. In conclusion, adenine acts on renal tubules as a signaling molecule and causes nephrogenic diabetes insipidus with salt wasting, at least, by directly interfering with AVP V2 receptor signaling with subsequent downregulation of NKCC2 and AQP2 in the kidney. The combination of renal fluid loss and decreased food intake with subsequent massive volume depletion likely plays an important role in the development of early prerenal failure that progresses to chronic kidney disease in long-term adenine feeding.

Progranulin Deficient Mice Develop Nephrogenic Diabetes Insipidus

Loss-of-function mutations of progranulin are associated with frontotemporal dementia in humans, and its deficiency in mice is a model for this disease but with normal life expectancy and mild cognitive decline on aging. The present study shows that aging progranulin deficient mice develop progressive polydipsia and polyuria under standard housing conditions starting at middle age (6-9 months). They showed high water licking behavior and doubling of the normal daily drinking volume, associated with increased daily urine output and a decrease of urine osmolality, all maintained during water restriction. Creatinine clearance, urine urea, urine albumin and glucose were normal. Hence, there were no signs of osmotic diuresis or overt renal disease, other than a concentrating defect. In line, the kidney morphology and histology revealed a 50% increase of the kidney weight, kidney enlargement, mild infiltrations of the medulla with pro-inflammatory cells, widening of tubules but no overt signs of a glomerular or tubular pathology. Plasma vasopressin levels were on average about 3-fold higher than normal levels, suggesting that the water loss resulted from unresponsiveness of the collecting tubules towards vasopressin, and indeed aquaporin-2 immunofluorescence in collecting tubules was diminished, whereas renal and hypothalamic vasopressin were increased, the latter in spite of substantial astrogliosis in the hypothalamus. The data suggest that progranulin deficiency causes nephrogenic diabetes insipidus in mice during aging. Possibly, polydipsia in affected patients - eventually interpreted as psychogenic polydipsia - may point to a similar concentrating defect.

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.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

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.

Wiryeongtang regulates hypertonicity-induced expression of aquaporin-2 water channels in mIMCD-3 cells

The kidneys have a key role in the homeostasis of water excretion and reabsorption. Water channels, particularly aquaporin-2 (AQP2), are important proteins in water homeostasis in the body through the short‑term and long-term regulation of water permeability. Wiryeongtang (WRT) is a well-known traditional oriental medicine, which is used for the treatment of chronic edema and dysuresia. The aim of the present study was to evaluate the inhibitory effect of WRT on the hypertonicity-induced expression of AQP2 in the inner medullary collecting duct cell line (IMCD‑3). Western blotting, reverse transcription‑polymerase chain reaction and immunofluorescence analysis were performed to determine the effect of WRT under hypertonic stress. WRT attenuated the 175 mM NaCl hypertonic stress‑induced increases in protein and mRNA levels of AQP2 and apical membrane insertion in a concentration‑dependent manner. However, no differences were observed in the levels of AQP1, AQP3 or AQP4 between the hypertonic stress and WRT groups. WRT attenuated the hypertonicity-induced phosphorylation of glucocorticoid-inducible protein kinase 1. In addition, the mRNA expression of tonicity‑responsive enhancer binding protein was attenuated by WRT under hypertonic stress. Pretreatment with WRT also decreased the hypertonic stress‑induced expression of AQP2, as with KT5720, a protein kinase A inhibitor. These results provided evidence of the beneficial effect of the traditional formula WRT in regulating water balance in hypertonic stress of the renal collecting ducts.

Postobstructive diuresis in cats with naturally occurring lower urinary tract obstruction: incidence, severity and association with laboratory parameters on admission

OBJECTIVES

The objectives of this retrospective study were to investigate the actual incidence of postobstructive diuresis after relief of urethral obstruction in cats, as well as to identify changes in blood and urine parameters that might be associated with postobstructive diuresis (POD), and to assess the impact of fluid therapy.

METHODS

The medical records of 57 male cats with urethral obstruction that were treated with an indwelling urinary catheter were retrospectively analysed. Absolute urine output in ml/kg/h every 4 h and the incidence of cats with polyuria (urine volume >2 ml/kg/h) at any time point over a 48 h period after the re-establishment of urine flow were investigated. In addition, postobstructive diuresis in relation to fluid therapy (PODFR) was defined as urine output greater than the administered amount of intravenous fluids on at least two subsequent time points. Polyuria and PODFR were investigated for their association with blood and urine laboratory parameters.

RESULTS

After 4 h, 74.1% (40/54) of the cats had polyuria, with a urine output of >2 ml/kg/h. Metabolic acidosis was present in 46.2% of the cats. Venous blood pH and bicarbonate were inversely correlated with urine output in ml/kg/h after 4 h. The overall incidence of POD within 48 h of catheterisation was 87.7%. There was a significant correlation between intravenous fluid rate at time point x and urine output at time point x + 1 at all the time points except for the fluid rate at time point 0 and the urine output after 4 h. PODFR was seen in 21/57 cats (36.8%).

CONCLUSIONS AND RELEVANCE

POD is a frequent finding in cats treated for urethral obstruction, and can be very pronounced. Further studies are required to determine whether or not a change in venous blood pH actually interferes with renal concentrating ability. The discrepancy between the frequency of cats with polyuria and PODFR (87.7% vs 36.8%) in the present study indicates that administered intravenous fluid therapy might be the driving force for the high incidence of polyuria in some cats with naturally occurring obstructive feline lower urinary tract disease.

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.

Endocannabinoid Regulation of Neuroendocrine Systems

The hypothalamus is a part of the brain that is critical for sustaining life through its homeostatic control and integrative regulation of the autonomic nervous system and neuroendocrine systems. Neuroendocrine function in mammals is mediated mainly through the control of pituitary hormone secretion by diverse neuroendocrine cell groups in the hypothalamus. Cannabinoid receptors are expressed throughout the hypothalamus, and endocannabinoids have been found to exert pronounced regulatory effects on neuroendocrine function via modulation of the outputs of several neuroendocrine systems. Here, we review the physiological regulation of neuroendocrine function by endocannabinoids, focusing on the role of endocannabinoids in the neuroendocrine regulation of the stress response, food intake, fluid homeostasis, and reproductive function. Cannabis sativa (marijuana) has a long history of recreational and/or medicinal use dating back to ancient times. It was used as an analgesic, anesthetic, and antianxiety herb as early as 2600 B.C. The hedonic, anxiolytic, and mood-elevating properties of cannabis have also been cited in ancient records from different cultures. However, it was not until 1964 that the psychoactive constituent of cannabis, Δ(9)-tetrahydrocannabinol, was isolated and its chemical structure determined (Gaoni & Mechoulam, 1964).

Proteomic profiling of nuclear fractions from native renal inner medullary collecting duct cells

The control of renal water excretion occurs in part by regulation of transcription in response to vasopressin in cells of the collecting duct. A systems biology-based approach to understanding transcriptional control in renal collecting duct cells depends on knowledge of what transcription factors and other regulatory proteins are present in the cells' nuclei. The goal of this article is to report comprehensive proteomic profiling of cellular fractions enriched in nuclear proteins from native inner medullary collecting duct (IMCD) cells of the rat. Multidimensional separation procedures and state-of-the art protein mass spectrometry produced 18 GB of spectral data that allowed the high-stringency identification of 5,048 proteins in nuclear pellet (NP) and nuclear extract (NE) fractions of biochemically isolated rat IMCD cells (URL: https://helixweb.nih.gov/ESBL/Database/IMCD_Nucleus/). The analysis identified 369 transcription factor proteins out of the 1,371 transcription factors coded by the rat genome. The analysis added 1,511 proteins to the recognized proteome of rat IMCD cells, now amounting to 8,290 unique proteins. Analysis of samples treated with the vasopressin analog dDAVP (1 nM for 30 min) or its vehicle revealed 99 proteins in the NP fraction and 88 proteins in the NE fraction with significant changes in spectral counts (Fisher exact test, P < 0.005). Among those altered by vasopressin were seven distinct histone proteins, all of which showed decreased abundance in the NP fraction, consistent with a possible effect of vasopressin to induce chromatin remodeling. The results provide a data resource for future studies of vasopressin-mediated transcriptional regulation in the renal collecting duct.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

Pathophysiology, diagnosis and management of nephrogenic diabetes insipidus

Healthy kidneys maintain fluid and electrolyte homoeostasis by adjusting urine volume and composition according to physiological needs. The final urine composition is determined in the last tubular segment: the collecting duct. Water permeability in the collecting duct is regulated by arginine vasopressin (AVP). Secretion of AVP from the neurohypophysis is regulated by a complex signalling network that involves osmosensors, barosensors and volume sensors. AVP facilitates aquaporin (AQP)-mediated water reabsorption via activation of the vasopressin V2 receptor (AVPR2) in the collecting duct, thus enabling concentration of urine. In nephrogenic diabetes insipidus (NDI), inability of the kidneys to respond to AVP results in functional AQP deficiency. Consequently, affected patients have constant diuresis, resulting in large volumes of dilute urine. Primary forms of NDI result from mutations in the genes that encode the key proteins AVPR2 and AQP2, whereas secondary forms are associated with biochemical abnormalities, obstructive uropathy or the use of certain medications, particularly lithium. Treatment of the disease is informed by identification of the underlying cause. Here we review the clinical aspects and diagnosis of NDI, the various aetiologies, current treatment options and potential future developments.

Regulation of aquaporins by vasopressin in the kidney

Vasopressin is the main hormone that regulates water conservation in mammals and one of its major targets is the principal cells in the renal collecting duct. Vasopressin increases the apical water permeability of principal cells, mediated by apical accumulation of aquaporin-2 (AQP2), a water channel protein, thus facilitating water reabsorption by the kidney. The mechanisms underlying the accumulation of AQP2 in response to vasopressin include vesicular trafficking from intracellular storage vesicles expressing AQP2 within several tens of minutes (short-term regulation) and protein expression of AQP2 over a period of hours to days (long-term regulation). This chapter reviews vasopressin signaling in the kidney, focusing on the molecular mechanisms of short- and long-term regulations of AQP2 expression.

Glycogen synthase kinase 3α regulates urine concentrating mechanism in mice

In mammals, glycogen synthase kinase (GSK)3 comprises GSK3α and GSK3β isoforms. GSK3β has been shown to play a role in the ability of kidneys to concentrate urine by regulating vasopressin-mediated water permeability of collecting ducts, whereas the role of GSK3α has yet to be discerned. To investigate the role of GSK3α in urine concentration, we compared GSK3α knockout (GSK3αKO) mice with wild-type (WT) littermates. Under normal conditions, GSK3αKO mice had higher water intake and urine output. GSK3αKO mice also showed reduced urine osmolality and aquaporin-2 levels but higher urinary vasopressin. When water deprived, they failed to concentrate their urine to the same level as WT littermates. The addition of 1-desamino-8-d-arginine vasopressin to isolated inner medullary collecting ducts increased the cAMP response in WT mice, but this response was reduced in GSK3αKO mice, suggesting reduced responsiveness to vasopressin. Gene silencing of GSK3α in mpkCCD cells also reduced forskolin-induced aquaporin-2 expression. When treated with LiCl, an isoform nonselective inhibitor of GSK3 and known inducer of polyuria, WT mice developed significant polyuria within 6 days. However, in GSK3αKO mice, the polyuric response was markedly reduced. This study demonstrates, for the first time, that GSK3α could play a crucial role in renal urine concentration and suggest that GSK3α might be one of the initial targets of Li(+) in LiCl-induced nephrogenic diabetes insipidus.

Expression of a dominant negative PKA mutation in the kidney elicits a diabetes insipidus phenotype

PKA plays a critical role in water excretion through regulation of the production and action of the antidiuretic hormone arginine vasopressin (AVP). The AVP prohormone is produced in the hypothalamus, where its transcription is regulated by cAMP. Once released into the circulation, AVP stimulates antidiuresis through activation of vasopressin 2 receptors in renal principal cells. Vasopressin 2 receptor activation increases cAMP and activates PKA, which, in turn, phosphorylates aquaporin (AQP)2, triggering apical membrane accumulation, increased collecting duct permeability, and water reabsorption. We used single-minded homolog 1 (Sim1)-Cre recombinase-mediated expression of a dominant negative PKA regulatory subunit (RIαB) to disrupt kinase activity in vivo and assess the role of PKA in fluid homeostasis. RIαB expression gave rise to marked polydipsia and polyuria; however, neither hypothalamic Avp mRNA expression nor urinary AVP levels were attenuated, indicating a primary physiological effect on the kidney. RIαB mice displayed a marked deficit in urinary concentrating ability and greatly reduced levels of AQP2 and phospho-AQP2. Dehydration induced Aqp2 mRNA in the kidney of both control and RIαB-expressing mice, but AQP2 protein levels were still reduced in RIαB-expressing mutants, and mice were unable to fully concentrate their urine and conserve water. We conclude that partial PKA inhibition in the kidney leads to posttranslational effects that reduce AQP2 protein levels and interfere with apical membrane localization. These findings demonstrate a distinct physiological role for PKA signaling in both short- and long-term regulation of AQP2 and characterize a novel mouse model of diabetes insipidus.

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.

Renal aquaporins and water balance disorders

BACKGROUND

Aquaporins (AQPs) are a family of proteins that can act as water channels. Regulation of AQPs is critical to osmoregulation and the maintenance of body water homeostasis. Eight AQPs are expressed in the kidney of which five have been shown to play a role in body water balance; AQP1, AQP2, AQP3, AQP4 and AQP7. AQP2 in particular is regulated by vasopressin.

SCOPE OF REVIEW

This review summarizes our current knowledge of the underlying mechanisms of various water balance disorders and their treatment strategies.

MAJOR CONCLUSIONS

Dysfunctions of AQPs are involved in disorders associated with disturbed water homeostasis. Hyponatremia with increased AQP levels can be caused by diseases with low effective circulating blood volume, such as congestive heart failure, or osmoregulation disorders such as the syndrome of inappropriate secretion of antidiuretic hormone. Treatment consists of fluid restriction, demeclocycline and vasopressin type-2 receptor antagonists. Decreased AQP levels can lead to diabetes insipidus (DI), characterized by polyuria and polydipsia. In central DI, vasopressin production is impaired, while in gestational DI, levels of the vasopressin-degrading enzyme vasopressinase are abnormally increased. Treatment consists of the vasopressin analogue dDAVP. Nephrogenic DI is caused by the inability of the kidney to respond to vasopressin and can be congenital, but is most commonly acquired, usually due to lithium therapy. Treatment consists of sufficient fluid supply, low-solute diet and diuretics.

GENERAL SIGNIFICANCE

In recent years, our understanding of the underlying mechanisms of water balance disorders has increased enormously, which has opened up several possible new treatment strategies. This article is part of a Special Issue entitled Aquaporins.

Vasopressin and the regulation of aquaporin-2

Water excretion is regulated in large part through the regulation of osmotic water permeability of the renal collecting duct epithelium. Water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: (1) regulation of AQP2 trafficking to the apical plasma membrane; and (2) regulation of the total amount of the AQP2 protein in the cells. Regulation of AQP2 abundance is defective in several water-balance disorders, including many polyuric disorders and the syndrome of inappropriate antidiuresis. Here we review vasopressin signaling in the renal collecting duct that is relevant to the two modes of water permeability regulation.

Demeclocycline attenuates hyponatremia by reducing aquaporin-2 expression in the renal inner medulla

Binding of vasopressin to its type 2 receptor in renal collecting ducts induces cAMP signaling, transcription and translocation of aquaporin (AQP)2 water channels to the plasma membrane, and water reabsorption from the prourine. Demeclocycline is currently used to treat hyponatremia in patients with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Demeclocycline's mechanism of action, which is poorly understood, is studied here. In mouse cortical collecting duct (mpkCCD) cells, which exhibit deamino-8-D-arginine vasopressin (dDAVP)-dependent expression of endogenous AQP2, demeclocycline decreased AQP2 abundance and gene transcription but not its protein stability. Demeclocycline did not affect vasopressin type 2 receptor localization but decreased dDAVP-induced cAMP generation and the abundance of adenylate cyclase 3 and 5/6. The addition of exogenous cAMP partially corrected the demeclocycline effect. As in patients, demeclocycline increased urine volume, decreased urine osmolality, and reverted hyponatremia in an SIADH rat model. AQP2 and adenylate cyclase 5/6 abundances were reduced in the inner medulla but increased in the cortex and outer medulla, in the absence of any sign of toxicity. In conclusion, our in vitro and in vivo data indicate that demeclocycline mainly attenuates hyponatremia in SIADH by reducing adenylate cyclase 5/6 expression and, consequently, cAMP generation, AQP2 gene transcription, and AQP2 abundance in the renal inner medulla, coinciding with a reduced vasopressin escape response in other collecting duct segments.

Proteome-wide measurement of protein half-lives and translation rates in vasopressin-sensitive collecting duct cells

Vasopressin regulates water excretion, in part, by controlling the abundances of the water channel aquaporin-2 (AQP2) protein and regulatory proteins in the renal collecting duct. To determine whether vasopressin-induced alterations in protein abundance result from modulation of protein production, protein degradation, or both, we used protein mass spectrometry with dynamic stable isotope labeling in cell culture to achieve a proteome-wide determination of protein half-lives and relative translation rates in mpkCCD cells. Measurements were made at steady state in the absence or presence of the vasopressin analog, desmopressin (dDAVP). Desmopressin altered the translation rate rather than the stability of most responding proteins, but it significantly increased both the translation rate and the half-life of AQP2. In addition, proteins associated with vasopressin action, including Mal2, Akap12, gelsolin, myosin light chain kinase, annexin-2, and Hsp70, manifested altered translation rates. Interestingly, desmopressin increased the translation of seven glutathione S-transferase proteins and enhanced protein S-glutathionylation, uncovering a previously unexplored vasopressin-induced post-translational modification. Additional bioinformatic analysis of the mpkCCD proteome indicated a correlation between protein function and protein half-life. In particular, processes that are rapidly regulated, such as transcription, endocytosis, cell cycle regulation, and ubiquitylation are associated with proteins with especially short half-lives. These data extend our understanding of the mechanisms underlying vasopressin signaling and provide a broad resource for additional investigation of collecting duct function (http://helixweb.nih.gov/ESBL/Database/ProteinHalfLives/index.html).

Glycogen synthase kinase-3 regulation of urinary concentrating ability

PURPOSE OF REVIEW

Glycogen synthase kinase-3 (GSK3) is an enzyme that is gaining prominence as a critical signaling molecule in the epithelial cells of renal tubules. This review will focus on recent findings exploring the role of GSK3 in renal collecting ducts, especially its role in urine concentration involving vasopressin signaling.

RECENT FINDINGS

Recent studies using inhibition or tissue-specific gene deletion of GSK3 revealed the mechanism by which GSK3 regulates aquaporin 2 water channels via adenylate cyclase or the prostaglandin-E2 pathway. In other studies, postnatal treatment with lithium, an inhibitor of GSK3, increased cell proliferation and led to microcyst formation in rat kidneys. These studies suggest that loss of GSK3 activity could interfere with renal water transport at two levels. In the short term, it could disrupt vasopressin signaling in collecting duct cells and in the long term it could alter the structure of the collecting ducts, making them less responsive to the hydro-osmotic effects of vasopressin.

SUMMARY

Ongoing studies reveal the crucial role played by GSK3 in the regulation of vasopressin action in the renal collecting ducts and suggest a possible use of GSK3 inhibitors in disease conditions associated with disrupted vasopressin signaling.

Aquaporin-2 regulation in health and disease

Aquaporin-2 (AQP2), the vasopressin-regulated water channel of the renal collecting duct, is dysregulated in numerous disorders of water balance in people and animals, including those associated with polyuria (urinary tract obstruction, hypokalemia, inflammation, and lithium toxicity) and with dilutional hyponatremia (syndrome of inappropriate antidiuresis, congestive heart failure, cirrhosis). Normal regulation of AQP2 by vasopressin involves 2 independent regulatory mechanisms: (1) short-term regulation of AQP2 trafficking to and from the apical plasma membrane, and (2) long-term regulation of the total abundance of the AQP2 protein in the cells. Most disorders of water balance are the result of dysregulation of processes that regulate the total abundance of AQP2 in collecting duct cells. In general, the level of AQP2 in a collecting duct cell is determined by a balance between production via translation of AQP2 mRNA and removal via degradation or secretion into the urine in exosomes. AQP2 abundance increases in response to vasopressin chiefly due to increased translation subsequent to increases in AQP2 mRNA. Vasopressin-mediated regulation of AQP2 gene transcription is poorly understood, although several transcription factor-binding elements in the 5' flanking region of the AQP2 gene have been identified, and candidate transcription factors corresponding to these elements have been discovered in proteomics studies. Here, we review progress in this area and discuss elements of vasopressin signaling in the collecting duct that may impinge on regulation of AQP2 in health and in the context of examples of polyuric diseases.

Effect of Atractylodes macrocephala on Hypertonic Stress-Induced Water Channel Protein Expression in Renal Collecting Duct Cells

Edema is a symptom that results from the abnormal accumulation of fluid in the body. The cause of edema is related to the level of aquaporin (AQP)2 protein expression, which regulates the reabsorption of water in the kidney. Edema is caused by overexpression of the AQP2 protein when the concentration of Na⁺ in the blood increases. The rhizome of Atractylodes macrocephala has been used in traditional oriental medicine as a diuretic drug; however, the mechanism responsible for the diuretic effect of the aqueous extract from A. macrocephala rhizomes (AAMs) has not yet been identified. We examined the effect of the AAM on the regulation of water channels in the mouse inner medullary collecting duct (mIMCD)-3 cells under hypertonic stress. Pretreatment of AAM attenuates a hypertonicity-induced increase in AQP2 expression as well as the trafficking of AQP2 to the apical plasma membrane. Tonicity-responsive enhancer binding protein (TonEBP) is a transcription factor known to play a central role in cellular homeostasis by regulating the expression of some proteins, including AQP2. Western immunoblot analysis demonstrated that the protein and mRNA expression levels of TonEBP also decrease after AAM treatment. These results suggest that the AAM has a diuretic effect by suppressing water reabsorption via the downregulation of the TonEBP-AQP2 signaling pathway.

Aquaporin-4 distribution in control and stressed astrocytes in culture and in the cerebrospinal fluid of patients with traumatic brain injuries

Distribution of aquaporin-4 (AQP4) was studied by western analysis and immunofluorescence in rat astrocytes exposed to either hypothermic (30 °C) or hyperosmolar (0.45 M sucrose) stress, and in the cerebrospinal fluid (CSF) of patients who suffered traumatic brain injury (TBI). CSF was obtained from 5 healthy subjects and from 20 patients suffering from severe TBI. CSF samples were taken at admission and on days 3 and 5-7. Here we report that, in response to both hypothermia and hyperosmolar stress, AQP4 was markedly reduced in cultured astrocytes. We also found that AQP4 significantly increased in patients with severe brain injury in respect to healthy subjects (P < 0.002). AQP4 in CSF remained unchanged in patients with elevated intracranial pressure (ICP), whereas there was a clear tendency to further increase in those patients whose ICP could be controlled within the normal range. We conclude that AQP4 levels in CSF are elevated after TBI and it might serve as a useful biochemical marker to assess brain water metabolism in clinical settings.

Quantitative phosphoproteomics in nuclei of vasopressin-sensitive renal collecting duct cells

Vasopressin regulates transport across the collecting duct epithelium in part via effects on gene transcription. Transcriptional regulation occurs partially via changes in phosphorylation of transcription factors, transcriptional coactivators, and protein kinases in the nucleus. To test whether vasopressin alters the nuclear phosphoproteome of vasopressin-sensitive cultured mouse mpkCCD cells, we used stable isotope labeling and mass spectrometry to quantify thousands of phosphorylation sites in nuclear extracts and nuclear pellet fractions. Measurements were made in the presence and absence of the vasopressin analog dDAVP. Of the 1,251 sites quantified, 39 changed significantly in response to dDAVP. Network analysis of the regulated proteins revealed two major clusters ("cell-cell adhesion" and "transcriptional regulation") that were connected to known elements of the vasopressin signaling pathway. The hub proteins for these two clusters were the transcriptional coactivator β-catenin and the transcription factor c-Jun. Phosphorylation of β-catenin at Ser552 was increased by dDAVP [log(2)(dDAVP/vehicle) = 1.79], and phosphorylation of c-Jun at Ser73 was decreased [log(2)(dDAVP/vehicle) = -0.53]. The β-catenin site is known to be targeted by either protein kinase A or Akt, both of which are activated in response to vasopressin. The c-Jun site is a canonical target for the MAP kinase Jnk2, which is downregulated in response to vasopressin in the collecting duct. The data support the idea that vasopressin-mediated control of transcription in collecting duct cells involves selective changes in the nuclear phosphoproteome. All data are available to users at http://helixweb.nih.gov/ESBL/Database/mNPPD/.

Chronic use of chloroquine disrupts the urine concentration mechanism by lowering cAMP levels in the inner medulla

Chloroquine, a widely used anti-malaria drug, has gained popularity for the treatment of rheumatoid arthritis, systemic lupus erythematosus (SLE), and human immunodeficiency virus (HIV). Unfortunately, chloroquine may also negatively impact renal function for patients whose fluid and electrolyte homeostasis is already compromised by diseases. Chronic administration of chloroquine also results in polyuria, which may be explained by suppression of the antidiuretic response of vasopressin. Several of the transporters responsible for concentrating urine are vasopressin-sensitive including the urea transporters UT-A1 and UT-A3, the water channel aquaporin-2 (AQP2), and the Na(+)-K(+)-2Cl(-) cotransporter (NKCC2). To examine the effect of chloroquine on these transporters, Sprague-Dawley rats received daily subcutaneous injections of 80 mg·kg(-1)·day(-1) of chloroquine for 4 days. Twenty-four hour urine output was twofold higher, and urine osmolality was decreased by twofold in chloroquine-treated rats compared with controls. Urine analysis of treated rats detected the presence chloroquine as well as decreased urine urea and cAMP levels compared with control rats. Western blot analysis showed a downregulation of AQP2 and NKCC2 transporters; however, UT-A1 and UT-A3 abundances were unaffected by chloroquine treatment. Immunohistochemistry showed a marked reduction of UT-A1 and AQP2 in the apical membrane in inner medullary collecting ducts of chloroquine-treated rats. In conclusion, chloroquine-induced polyuria likely occurs as a result of lowered cAMP production. These findings suggest that chronic chloroquine treatment would exacerbate the already compromised fluid homeostasis observed in diseases like chronic kidney disease.

Lack of protein kinase C-α leads to impaired urine concentrating ability and decreased aquaporin-2 in angiotensin II-induced hypertension

Regulation of water and urea transport in the inner medullary collecting duct is essential for urine concentration. Aquaporin (AQP)2 water channels and urea transporter (UT)-A1 are inserted into the apical membrane upon phosphorylation of the channels to allow the transcellular movement of water and urea. Since ANG II activates PKC in many cell types, we tested the hypothesis that ANG II-induced regulation of water and urea transport is mediated by PKC. Osmotic minipumps delivered ANG II to wild-type (WT) or PKC-α(-/-) mice for 7 days. Inner medullas were harvested, and protein abundance was determined by immunoblot. ANG II increased systolic blood pressure to a similar degree in WT and PKC-α(-/-) mice. ANG II had no effect on the urine output of WT mice but increased that of PKC-α(-/-) mice. In accordance with observed differences in urine output, AQP2 abundance was unchanged in ANG II-treated WT animals but was decreased in PKC-α(-/-) mice. No change in membrane accumulation was seen. Phosphorylation of the cAMP-induced transcription factor CREB was decreased in PKC-α(-/-) mice in response to ANG II with no change in overall CREB abundance. ANG II did not alter the abundance of UT-A1 protein in WT or PKC-α(-/-) mice. Phosphorylation and overall abundance of tonicity-responsive enhancer-binding protein, a transcription factor that regulates UT-A1, were also unaltered by ANG II in either group. We conclude that PKC-α protects against ANG II-induced decreases in urine concentrating ability by maintaining AQP2 levels through CREB phosphorylation.

Treatment with the vascular disrupting agent combretastatin is associated with impaired AQP2 trafficking and increased urine output

Combretastatin A-4 disodium phosphate (CA4P) is a vascular disrupting agent known to mediate its effects primarily on tumor blood vessels. CA4P has previously been shown to induce a significant increase in mean arterial blood pressure and in hemoglobin concentration in mice. In the present study, we examined whether this is associated with a general leakage of water into certain tissues or with changes in renal water handling. Munich-Wistar rats received either CA4P (30 mg/kg body wt) or saline intraperitoneally as a bolus injection. One hour later, hemoglobin concentration and mean blood pressure increased significantly. MRI showed no significant changes in tissue water content following CA4P administration. However, urine output and salt excretion increased 1 h after CA4P treatment, without changes in urinary and medullary osmolality. Aquaporin 2 (AQP2) mRNA levels in kidney inner medulla did not change 1 h after CA4P treatment, but semiquantitative confocal laser-scanning microscopy analysis demonstrated a decrease in phosphorylated AQP2 (pS256-AQP2) apical distribution within the collecting ducts of CA4P-treated rats compared with the characteristic apical localization in control rats. Furthermore, we demonstrated that CA4P cause disruption of microtubules and a weaker apical labeling of pS256-AQP2 in collecting duct principal cells within 1 h. In conclusion, our data indicate that water escapes from the vascular system after CA4P treatment, and it may take place primarily through a renal mechanism. The CA4P-mediated increase in urine output seems to be a local effect in the collecting ducts due to reduced AQP2 trafficking to the apical plasma membrane.

Quantitative proteomics identifies vasopressin-responsive nuclear proteins in collecting duct cells

Vasopressin controls transport in the renal collecting duct, in part, by regulating transcription. This complex process, which can involve translocation and/or modification of transcriptional regulators, is not completely understood. Here, we applied a method for large-scale profiling of nuclear proteins to quantify vasopressin-induced changes in the nuclear proteome of cortical collecting duct (mpkCCD) cells. Using stable isotope labeling and tandem mass spectrometry, we quantified 3987 nuclear proteins and identified significant changes in the abundance of 65, including previously established targets of vasopressin signaling in the collecting duct. Vasopressin-induced changes in the abundance of the transcription factors JunB, Elf3, Gatad2b, and Hmbox1; transcriptional co-regulators Ctnnb1 (β-catenin) and Crebbp; subunits of the Mediator complex; E3 ubiquitin ligase Nedd4; nuclear transport regulator RanGap1; and several proteins associated with tight junctions and adherens junctions. Bioinformatic analysis showed that many of the quantified transcription factors have putative binding sites in the 5'-flanking regions of genes coding for the channel proteins Aqp2, Aqp3, Scnn1b (ENaCβ), and Scnn1g (ENaCγ), which are known targets of vasopressin. Immunoblotting demonstrated that the increase in β-catenin in nuclear fractions was accompanied by an even larger increase in its phosphorylated form (pSer552). The findings provide a new online database resource for nuclear proteomics (http://helixweb.nih.gov/ESBL/Database/mNPD/) and generate new hypotheses regarding vasopressin-mediated transcriptional regulation in the collecting duct.

In mpkCCD cells, long-term regulation of aquaporin-2 by vasopressin occurs independent of protein kinase A and CREB but may involve Epac

Urine concentration involves the hormone vasopressin (AVP), which stimulates cAMP production in renal principal cells, resulting in translocation and transcription of aquaporin-2 (AQP2) water channels, greatly increasing the water permeability, leading to a concentrated urine. As cAMP levels decrease shortly after AVP addition, whereas AQP2 levels still increase and are maintained for days, we investigated in the present study the mechanism responsible for the AQP2 increase after long-term 1-desamino-8-d-arginine vasopressin (dDAVP) application using mouse collecting duct (mpkCCD) cells. While 30 min of dDAVP incubation strongly increased cAMP, cAMP was lower with 1 day and was even further reduced with 4 days of dDAVP, although still significantly higher than in control cells. One day of dDAVP incubation increased AQP2 promoter-dependent transcription, which was blocked by the protein kinase A (PKA) inhibitor H89. Moreover, phosphorylation of the cAMP-responsive element binding protein (CREB) and CRE-dependent transcription was observed after short-term dDAVP stimulation. With 4 days of dDAVP, AQP2 transcription remained elevated, but this was not blocked by H89, and CRE-dependent transcription and CREB phosphorylation were not increased. Exchange factor directly activated by cAMP (Epac) 1 and 2 were found to be endogenously expressed in mpkCCD cells. Application of dDAVP increased the expression of Epac1, while Epac2 was reduced. Incubation with a specific Epac activator after dDAVP pretreatment increased both AQP2 abundance and transcription compared with cells left unstimulated the last day. In conclusion, the PKA-CRE pathway is involved in the initial rise in AQP2 levels after dDAVP stimulation but not in the long-term effect of dDAVP. Instead, long-term regulation of AQP2 may involve the activation of Epac.

PKCα regulates vasopressin-induced aquaporin-2 trafficking in mouse kidney collecting duct cells in vitro via altering microtubule assembly

AIM

Aquaporin-2 (AQP2) is a vasopressin-regulated water channel located in the collecting tubule and collecting duct cells of mammalian kidney. The aim of this study is to investigate whether PKCα plays a role in vasopressin-induced AQP2 trafficking in mouse inner medullary collecting duct 3 (mIMCD3) cells.

METHODS

AQP2-mIMCD3 stable cell line was constructed by transfection of mouse inner medullary collecting duct 3 (mIMCD3) cells with AQP2-GFP construct. Then the cells were transfected with PKCα shRNA, PKCα A/25E, or PKCα scrambled shRNA. The expression levels of PKCα, AQP2, and phospho-S256-AQP2 were analyzed using Western blot. The interaction between AQP2 and PKCα was examined using immunoprecipitation. The distribution of AQP2 and microtubules was studied using immunocytochemistry. The AQP2 trafficking was examined using the biotinylation of surface membranes.

RESULTS

Treatment of AQP2-mIMCD3 cells with 100 μmol/L of 1-desamino-8-D-arginine vasopressin (DdAVP) for 30 min stimulated the translocation of AQP2 from the cytoplasm to plasma membrane through influencing the microtubule assembly. Upregulation of active PKCα by transfection with PKCα A/25E plasmids resulted in de-polymerization of α-tubulin and redistributed AQP2 in the cytoplasm. Down-regulation of PKCα by PKCα shRNA partially inhibited DdAVP-stimulated AQP2 trafficking without altering α-tubulin distribution. Although 100 μmol/L of DdAVP increased AQP2 phosphorylation at serine 256, down-regulation of PKCα by PKCα shRNA did not influence DdAVP-induced AQP2 phosphorylation, suggesting that AQP2 phosphorylation at serine 256 was independent of PKCα. Moreover, PKCα did not physically interact with AQP2 in the presence or absence of DdAVP.

CONCLUSION

Our results suggested that PKCα regulates AQP2 trafficking induced by DdAVP via microtubule assembly.

Anthrax edema toxin impairs clearance in mice

The anthrax edema toxin (ET) of Bacillus anthracis is composed of the receptor-binding component protective antigen (PA) and of the adenylyl cyclase catalytic moiety, edema factor (EF). Uptake of ET into cells raises intracellular concentrations of the secondary messenger cyclic AMP, thereby impairing or activating host cell functions. We report here on a new consequence of ET action in vivo. We show that in mouse models of toxemia and infection, serum PA concentrations were significantly higher in the presence of enzymatically active EF. These higher concentrations were not caused by ET-induced inhibition of PA endocytosis; on the contrary, ET induced increased PA binding and uptake of the PA oligomer in vitro and in vivo through upregulation of the PA receptors TEM8 and CMG2 in both myeloid and nonmyeloid cells. ET effects on protein clearance from circulation appeared to be global and were not limited to PA. ET also impaired the clearance of ovalbumin, green fluorescent protein, and EF itself, as well as the small molecule biotin when these molecules were coinjected with the toxin. Effects on injected protein levels were not a result of general increase in protein concentrations due to fluid loss. Functional markers for liver and kidney were altered in response to ET. Concomitantly, ET caused phosphorylation and activation of the aquaporin-2 water channel present in the principal cells of the collecting ducts of the kidneys that are responsible for fluid homeostasis. Our data suggest that in vivo, ET alters circulatory protein and small molecule pharmacokinetics by an as-yet-undefined mechanism, thereby potentially allowing a prolonged circulation of anthrax virulence factors such as EF during infection.

AQP2 is necessary for vasopressin- and forskolin-mediated filamentous actin depolymerization in renal epithelial cells

Remodeling of the actin cytoskeleton is required for vasopressin (VP)-induced aquaporin 2 (AQP2) trafficking. Here, we asked whether VP and forskolin (FK)-mediated F-actin depolymerization depends on AQP2 expression. Using various MDCK and LLC-PK1 cell lines with different AQP2 expression levels, we performed F-actin quantification and immunofluorescence staining after VP/FK treatment. In MDCK cells, in which AQP2 is delivered apically, VP/FK mediated F-actin depolymerization was significantly correlated with AQP2 expression levels. A decrease of apical membrane associated F-actin was observed upon VP/FK treatment in AQP2 transfected, but not in untransfected cells. There was no change in basolateral actin staining under these conditions. In LLC-PK₁ cells, which deliver AQP2 basolaterally, a significant VP/FK mediated decrease in F-actin was also detected only in AQP2 transfected cells. This depolymerization response to VP/FK was significantly reduced by siRNA knockdown of AQP2. By immunofluorescence, an inverse relationship between plasma membrane AQP2 and membrane-associated F-actin was observed after VP/FK treatment again only in AQP2 transfected cells. This is the first report showing that VP/FK mediated F-actin depolymerization is dependent on AQP2 protein expression in renal epithelial cells, and that this is not dependent on the polarity of AQP2 membrane insertion.

Hypotonicity-induced reduction of aquaporin-2 transcription in mpkCCD cells is independent of the tonicity responsive element, vasopressin, and cAMP

The syndrome of inappropriate antidiuretic hormone secretion is characterized by excessive water uptake and hyponatremia. The extent of hyponatremia, however, is less than anticipated, which is ascribed to a defense mechanism, the vasopressin-escape, and is suggested to involve a tonicity-determined down-regulation of the water channel aquaporin-2 (AQP2). The underlying mechanism, however, is poorly understood. To study this, we used the mouse cortical collecting duct (mpkCCD) cell line. MpkCCD cells, transfected with an AQP2-promoter luciferase construct showed a reduced and increased AQP2 abundance and transcription following culture in hypotonic and hypertonic medium, respectively. This depended on tonicity rather than osmolality and occurred independently of the vasopressin analog dDAVP, cAMP levels, or protein kinase A activity. Although prostaglandins and nitric oxide reduced AQP2 abundance, inhibition of their synthesis did not influence tonicity-induced AQP2 transcription. Also, cells in which the cAMP or tonicity-responsive element (CRE/TonE) in the AQP2-promoter were mutated showed a similar response to hypotonicity. Instead, the tonicity-responsive elements were pin-pointed to nucleotides -283 to -252 and -157 to -126 bp. In conclusion, our data indicate that hypotonicity reduces AQP2 abundance and transcription, which occurs independently of vasopressin, cAMP, and the known TonE and CRE in the AQP2-promoter. Increased prostaglandin and nitric oxide, as found in vivo, may contribute to reduced AQP2 in vasopressin-escape, but do not mediate the effect of hypotonicity on AQP2 transcription. Our data suggest that two novel segments (-283 to -252 and -157 to -126 bp) in the AQP2-promoter mediate the hypotonicity-induced AQP2 down-regulation during vasopressin-escape.

Calcitonin has a vasopressin-like effect on aquaporin-2 trafficking and urinary concentration

The most common cause of hereditary nephrogenic diabetes insipidus is a nonfunctional vasopressin (VP) receptor type 2 (V2R). Calcitonin, another ligand of G-protein-coupled receptors, has a VP-like effect on electrolytes and water reabsorption, suggesting that it may affect AQP2 trafficking. Here, calcitonin increased intracellular cAMP and stimulated the membrane accumulation of AQP2 in LLC-PK1 cells. Pharmacologic inhibition of protein kinase A (PKA) and deficiency of a critical PKA phosphorylation site on AQP2 both prevented calcitonin-induced membrane accumulation of AQP2. Fluorescence assays showed that calcitonin led to a 70% increase in exocytosis and a 20% decrease in endocytosis of AQP2. Immunostaining of rat kidney slices demonstrated that calcitonin induced a significant redistribution of AQP2 to the apical membrane of principal cells in cortical collecting ducts and connecting segments but not in the inner stripe or inner medulla. Calcitonin-treated VP-deficient Brattleboro rats had a reduced urine flow and two-fold higher urine osmolality during the first 12 hours of treatment compared with control groups. Although this VP-like effect of calcitonin diminished over the following 72 hours, the tachyphylaxis was reversible. Taken together, these data show that calcitonin induces cAMP-dependent AQP2 trafficking in cortical collecting and connecting tubules in parallel with an increase in urine concentration. This suggests that calcitonin has a potential therapeutic use in nephrogenic diabetes insipidus.