Aquaporin-2 regulation in health and disease

Aquaporin 2 in Cerebral Edema: Potential Prognostic Marker in Craniocerebral Injuries

Despite continuous medical advancements, traumatic brain injury (TBI) remains a leading cause of death and disability worldwide. Consequently, there is a pursuit for biomarkers that allow non-invasive monitoring of patients after cranial trauma, potentially improving clinical management and reducing complications and mortality. Aquaporins (AQPs), which are crucial for transmembrane water transport, may be significant in this context. This study included 48 patients, with 27 having acute (aSDH) and 21 having chronic subdural hematoma (cSDH). Blood plasma samples were collected from the participants at three intervals: the first sample before surgery, the second at 15 h, and the third at 30 h post-surgery. Plasma concentrations of AQP1, AQP2, AQP4, and AQP9 were determined using the sandwich ELISA technique. CT scans were performed on all patients pre- and post-surgery. Correlations between variables were examined using Spearman's nonparametric rank correlation coefficient. A strong correlation was found between aquaporin 2 levels and the volume of chronic subdural hematoma and midline shift. However, no significant link was found between aquaporin levels (AQP1, AQP2, AQP4, and AQP9) before and after surgery for acute subdural hematoma, nor for AQP1, AQP4, and AQP9 after surgery for chronic subdural hematoma. In the chronic SDH group, AQP2 plasma concentration negatively correlated with the midline shift measured before surgery (Spearman's ρ -0.54; p = 0.017) and positively with hematoma volume change between baseline and 30 h post-surgery (Spearman's ρ 0.627; p = 0.007). No statistically significant correlation was found between aquaporin plasma levels and hematoma volume for AQP1, AQP2, AQP4, and AQP9 in patients with acute SDH. There is a correlation between chronic subdural hematoma volume, measured radiologically, and serum AQP2 concentration, highlighting aquaporins' potential as clinical biomarkers.

Effect of intermittent fasting on adriamycin-induced nephropathy: Possible underlying mechanisms

PURPOSE

Intermittent fasting (IF) has been shown to induce a well-organized adaptive defense against stress inside the cells, which increases the production of anti-oxidant defenses, repair of DNA, biogenesis of mitochondria, and genes that combat inflammation. So, the goal of the current investigation was to identify the effects of IF on rats with adriamycin (ADR)-induced nephropathy and any potential underlying mechanisms.

METHODS

Four groups of 40 mature Sprague-Dawley male rats were allocated as follow; control, fasting, ADR, and ADR plus fasting. After 8 weeks of ADR administration urine, blood samples and kidneys were taken for assessment of serum creatinine (Cr), BUN, urinary proteins, indicators of oxidative damage (malondialdehyde (MDA), reduced glutathione (GSH) and Catalase (CAT) levels), histopathological examinations, immunohistochemical examinations for caspase-3, Sirt1, aquaporin2 (AQP2) and real time PCR for antioxidant genes; Nrf2, HO-1 in kidney tissues.

RESULTS

IF significantly improved serum creatinine, BUN and urinary protein excretion, oxidative stress (low MDA with high CAT and GSH), in addition to morphological damage to the renal tubules and glomeruli as well as caspase-3 production during apoptosis. Moreover, IF stimulates significantly the expression of Sirt1 and Nrf2/HO-1 and AQP2.

CONCLUSION

AQP2, Sirt1, Nrf2/HO-1 signaling may be upregulated and activated by IF, which alleviates ADR nephropathy. Enhancing endogenous antioxidants, reducing apoptosis and tubulointerstitial damage, and maintaining the glomerular membrane's integrity are other goals.

Using CRISPR-Cas9/phosphoproteomics to identify substrates of calcium/calmodulin-dependent kinase 2δ

Ca2+/Calmodulin-dependent protein kinase 2 (CAMK2) family proteins are involved in the regulation of cellular processes in a variety of tissues including brain, heart, liver, and kidney. One member, CAMK2δ (CAMK2D), has been proposed to be involved in vasopressin signaling in the renal collecting duct, which controls water excretion through regulation of the water channel aquaporin-2 (AQP2). To identify CAMK2D target proteins in renal collecting duct cells (mpkCCD), we deleted Camk2d and carried out LC-MS/MS-based quantitative phosphoproteomics. Specifically, we used CRISPR/Cas9 with two different guide RNAs targeting the CAMK2D catalytic domain to create multiple CAMK2D KO cell lines. AQP2 protein abundance was lower in the CAMK2D KO cells than in CAMK2D-intact controls. AQP2 phosphorylation at Ser256 and Ser269 (normalized for total AQP2) was decreased. However, trafficking of AQP2 to and from the apical plasma membrane was sustained. Large-scale quantitative phosphoproteomic analysis (TMT-labeling) in the presence of the vasopressin analog dDAVP (0.1 nM, 30 min) allowed quantification of 11,570 phosphosites of which 169 were significantly decreased, while 206 were increased in abundance in CAMK2D KO clones. These data are available for browsing or download at https://esbl.nhlbi.nih.gov/Databases/CAMK2D-proteome/. Motif analysis of the decreased phosphorylation sites revealed a target preference of -(R/K)-X-X-p(S/T)-X-(D/E), matching the motif identified in previous in vitro phosphorylation studies using recombinant CAMK2D. Thirty five of the significantly downregulated phosphorylation sites in CAMK2D KO cells had exactly this motif and are judged to be likely direct CAMK2D targets. This adds to the list of known CAMK2D target proteins found in prior reductionist studies.

Malignancy diseases and kidneys: A nephrologist prospect and updated review

Acute kidney injury (AKI), chronic renal failure, and tubular abnormalities represent the kidney disease spectrum of malignancy. Prompt diagnosis and treatment may prevent or reverse these complications. The pathogenesis of AKI in cancer is multifactorial. AKI affects outcomes in cancer, oncological therapy withdrawal, increased hospitalization rate, and hospital stay. Renal function derangement can be recovered with early detection and targeted therapy of cancers. Identifying patients at higher risk of renal damage and implementing preventive measures without sacrificing the benefits of oncological therapy improve survival. Multidisciplinary approaches, such as relieving obstruction, hydration, etc., are required to minimize the kidney injury rate. Different keywords, texts, and phrases were used to search Google, EMBASE, PubMed, Scopus, and Google Scholar for related original and review articles that serve the article's aim well. In this nonsystematic article, we aimed to review the published data on cancer-associated kidney complications, their pathogenesis, management, prevention, and the latest updates. Kidney involvement in cancer occurs due to tumor therapy, direct kidney invasion by tumor, or tumor complications. Early diagnosis and therapy improve the survival rate. Pathogenesis of cancer-related kidney involvement is different and complicated. Clinicians' awareness of all the potential causes of cancer-related complications is essential, and a kidney biopsy should be conducted to confirm the kidney pathologies. Chronic kidney disease is a known complication in malignancy and therapies. Hence, avoiding nephrotoxic drugs, dose standardization, and early cancer detection are mandatory measures to prevent renal involvement.

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.

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.

The novel role of the aquaporin water channel in lycopene preventing DEHP-induced renal ionic homeostasis disturbance in mice

Di-(2-ethylhexyl) phthalate (DEHP), an extensively used plasticizer, can cause environmental pollution and organ injury. Lycopene (LYC) is a natural carotene that has the potential to prevent chronic diseases. To reveal the effect of DEHP and/or LYC on the kidney, male mice were treated with LYC (5 mg/kg) and/or DEHP (500 mg/kg or 1000 mg/kg) by gavage for 28 days. The study indicated that DEHP caused glomerular atrophy, tubular expansion, disappearance of the mitochondrial membrane, and cristae rupture. DEHP exposure can increase the expression of aquaporin (AQP) subunits and the activity of Ca²⁺-Mg²⁺-ATPase and decrease the activity of Na⁺-K⁺-ATPase, which results in ion disorder. However, LYC can relieve kidney injury by regulating the activity of ATPase, the expression of ATPase subunits, and AQP subunit expression. The results indicated that AQP was a target for LYC in antagonizing the disturbance of DEHP-induced renal damage.

Immunohistochemical Expression of AQP2 and HSP70 in Broiler Kidney Tissue Treated with Salix tetrasperma Roxb. Extract under Heat Exposure

The administration of plant extracts to broilers may be a way to mitigate the effects of heat stress. The importance of AQP2 and HSP70 compounds in maintaining the homeostasis of the chicken body when it is subjected to heat stress is well established. This study aims to determine the effect of giving the ethanolic extract of the leaves of Salix tetrasperma Roxb. on the immunohistochemical expression of AQP2 and HSP70 in exposed and unexposed broiler kidney tissue. This study used 36 samples of 28-day-old chicken kidneys. Chickens were kept in individual cages, provided with feed and drinking water ad libitum. The design used was a completely randomized design with 6 treatments and 6 replications: (a) chickens were reared in conditions exposed to heat (HS + 0); (b) chickens were reared in conditions exposed to heat and given Salix extract at a dose of 50 mg/L drinking water (HS + 50); (c) chickens were reared under heat-exposed conditions and given Salix extract at a dose of 100 mg/L drinking water (HS + 100); (d) chickens were reared in conditions without exposure to heat (n-HS + 0); (e) chickens were reared in conditions without exposure to heat and given Salix extract at a dose of 50 mg/L drinking water (nHS + 50); and (f) chickens were reared in conditions exposed without exposure to heat and given 100 mg/L drinking water (nHS + 100) of Salix extract. Salix extract was given for 24 hours and was renewed every 6 hours. The results showed that giving Salix extract 100 mg/L in drinking water to chickens exposed to heat (HS + 100) reduced the value of the H/L ratio. Giving Salix extract 50–100 mg/L in drinking water caused an upregulated AQP2 expression; on the other hand, it downregulated HSP-70 expression, in chicken kidney tubules both exposed to heat stress and nonexposed to heat stress. In conclusion, exposure to heat stress in broiler chickens and giving Salix extract can increase the formation of aquaporin 2 compounds and suppress the formation of HSP70.

Aquaporins in insulin resistance and diabetes: More than channels!

Aquaporins (AQPs) are part of the family of the integral membrane proteins. Their function is dedicated to the transport of water, glycerol, ammonia, urea, H2O2, and other small molecules across the biological membranes. Although for many years they were scarcely considered, AQPs have a relevant role in the development of many diseases. Recent discoveries suggest, that AQPs may play an important role in the process of fat accumulation and regulation of oxidative stress, two crucial aspects of insulin resistance and type-2 diabetes (T2D). Insulin resistance (IR) and T2D are multi-faceted systemic diseases with multiple connections to obesity and other comorbidities such as hypertension, dyslipidemia and metabolic syndrome. Both IR and T2D transcends different tissues and organs, creating the maze of mutual relationships between adipose fat depots, skeletal muscle, liver and other insulin-sensitive organs. AQPs with their heterogenous properties, distinctive tissue distribution and documented involvement in both the lipid metabolism and regulation of the oxidative stress appear to be feasible candidates in the search for the explanation to this third-millennium plague. A lot of research has been assigned to adipose tissue AQP7 and liver tissue AQP9, clarifying their relationship and coordinated work in the induction of hepatic insulin resistance. Novel research points also to other aquaporins, such as AQP11 which may be associated with the induction of insulin resistance and T2D through its involvement in hydrogen peroxide transport. In this review we collected recent discoveries in the field of AQP's involvement in the insulin resistance and T2D. Novel paths which connect AQPs with metabolic disorders can give new fuel to the research on obesity, insulin resistance and T2D - one of the most worrying problems of the modern society.

The combined effect of hypomagnesemia and hypokalemia inducing nephrogenic diabetes insipidus in a patient with type 1 diabetes mellitus

Nephrogenic diabetes insipidus (NDI) is rarely considered against more common differentials such as diabetes mellitus in patients presenting with polydipsia and polyuria. Hypokalemia and hypercalcemia are known to induce NDI, but not much is known about hypomagnesemia. Hypokalemia refractory to therapy should prompt consideration of hypomagnesemia.

Renal expression and urinary excretion of aquaporin-2 in postobstructive uropathy in rats

This work assessed the time course of water renal management together with aquaporin-2 (AQP2) kidney expression and urinary AQP2 levels (AQP2u) in obstructive nephropathy. Adult male Wistar rats were monitored after 1, 2, and 7 days of bilateral ureteral release (bilateral ureteral obstruction (BUO); BUO-1, BUO-2 and BUO-7). Renal water handling was evaluated using conventional clearance techniques. AQP2 levels were assessed by immunoblotting and immunohistochemical techniques. AQP2 expression in apical membranes was downregulated in BUO-1 rats and upregulated both in BUO-2 and BUO-7 animals. AQP2 protein expression in whole cell lysate fraction from kidney cortex and medulla were significantly decreased in all the experimental groups. Concomitantly, mRNA levels of AQP2 decreased in renal medulla of all groups and in renal cortex from BUO-1; however, in renal cortex from BUO-2 and BUO-7 a recovery and an increase in the level of AQP2 mRNA were, respectively, observed. BUO-7 group showed a significant increase in AQP2u. The alterations observed in apical membranes AQP2 expression could explain, at least in part, the evolution time of water kidney management in the postobstructive phase of BUO. Additionally, the AQP2u increase after 7 days of ureteral release may be postulated as a biomarker of improvement in the kidney function.

Human Aquaporins: Functional Diversity and Potential Roles in Infectious and Non-infectious Diseases

Aquaporins (AQPs) are integral membrane proteins and found in all living organisms from bacteria to human. AQPs mainly involved in the transmembrane diffusion of water as well as various small solutes in a bidirectional manner are widely distributed in various human tissues. Human contains 13 AQPs (AQP0-AQP12) which are divided into three sub-classes namely orthodox aquaporin (AQP0, 1, 2, 4, 5, 6, and 8), aquaglyceroporin (AQP3, 7, 9, and 10) and super or unorthodox aquaporin (AQP11 and 12) based on their pore selectivity. Human AQPs are functionally diverse, which are involved in wide variety of non-infectious diseases including cancer, renal dysfunction, neurological disorder, epilepsy, skin disease, metabolic syndrome, and even cardiac diseases. However, the association of AQPs with infectious diseases has not been fully evaluated. Several studies have unveiled that AQPs can be regulated by microbial and parasitic infections that suggest their involvement in microbial pathogenesis, inflammation-associated responses and AQP-mediated cell water homeostasis. This review mainly aims to shed light on the involvement of AQPs in infectious and non-infectious diseases and potential AQPs-target modulators. Furthermore, AQP structures, tissue-specific distributions and their physiological relevance, functional diversity and regulations have been discussed. Altogether, this review would be useful for further investigation of AQPs as a potential therapeutic target for treatment of infectious as well as non-infectious diseases.

Analysis of urinary exosomes applications for rare kidney disorders

Introduction: Exosomes are nanovesicles that play important functions in a variety of physiological and pathological conditions. They are powerful cell-to-cell communication tool thanks to the protein, mRNA, miRNA, and lipid cargoes they carry. They are also emerging as valuable diagnostic and prognostic biomarker sources. Urinary exosomes carry information from all the cells of the urinary tract, downstream of the podocyte. Rare kidney diseases are a subset of an inherited diseases whose genetic diagnosis can be unclear, and presentation can vary due to genetic, epigenetic, and environmental factors. Areas covered: In this review, we focus on a group of rare and often neglected kidney diseases, for which we have sufficient available literature data on urinary exosomes. The analysis of their content can help to comprehend pathological mechanisms and to identify biomarkers for diagnosis, prognosis, and therapeutic targets. Expert opinion: The foreseeable large-scale application of system biology approach to the profiling of exosomal proteins as a source of renal disease biomarkers will be also useful to stratify patients with rare kidney diseases whose penetrance, phenotypic presentation, and age of onset vary sensibly. This can ameliorate the clinical management.

Coordinate adaptations of skeletal muscle and kidney to maintain extracellular [K+] during K+-deficient diet

Extracellular fluid (ECF) potassium concentration ([K+]) is maintained by adaptations of kidney and skeletal muscle, responses heretofore studied separately. We aimed to determine how these organ systems work in concert to preserve ECF [K+] in male C57BL/6J mice fed a K+-deficient diet (0K) versus 1% K+ diet (1K) for 10 days (n = 5-6/group). During 0K feeding, plasma [K+] fell from 4.5 to 2 mM; hindlimb muscle (gastrocnemius and soleus) lost 28 mM K+ (from 115 ± 2 to 87 ± 2 mM) and gained 27 mM Na+ (from 27 ± 0.4 to 54 ± 2 mM). Doubling of muscle tissue [Na+] was not associated with inflammation, cytokine production or hypertension as reported by others. Muscle transporter adaptations in 0K- versus 1K-fed mice, assessed by immunoblot, included decreased sodium pump α2-β2 subunits, decreased K+-Cl- cotransporter isoform 3, and increased phosphorylated (p) Na+,K+,2Cl- cotransporter isoform 1 (NKCC1p), Ste20/SPS-1-related proline-alanine rich kinase (SPAKp), and oxidative stress-responsive kinase 1 (OSR1p) consistent with intracellular fluid (ICF) K+ loss and Na+ gain. Renal transporters' adaptations, effecting a 98% reduction in K+ excretion, included two- to threefold increased phosphorylated Na+-Cl- cotransporter (NCCp), SPAKp, and OSR1p abundance, limiting Na+ delivery to epithelial Na+ channels where Na+ reabsorption drives K+ secretion; and renal K sensor Kir 4.1 abundance fell 25%. Mass balance estimations indicate that over 10 days of 0K feeding, mice lose ~48 μmol K+ into the urine and muscle shifts ~47 μmol K+ from ICF to ECF, illustrating the importance of the concerted responses during K+ deficiency.

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.

Roles for Exosome in Various Kidney Diseases and Disorders

Exosome is a nanoscale vesicle with a size range of 30–100 nm. It is secreted from cell to extracellular space by exocytosis after fusion of multivesicular body (MVB) (formed by endocytic vesicles) with plasma membrane. Exosome plays several important roles in cellular homeostasis and intercellular communications. During the last two decades, exosome has acquired a wide attention to explore its additional roles in various aspects of cell biology and function in several organ systems. For the kidney, several lines of evidence have demonstrated 1that exosome is involved in the renal physiology and pathogenic mechanisms of various kidney diseases/disorders. This article summarizes roles of the exosome as the potential source of biomarkers, pathogenic molecules, and therapeutic biologics that have been extensively investigated in many kidney diseases/disorders, including lupus nephritis (LN), other glomerular diseases, acute kidney injury (AKI), diabetic nephropathy (DN), as well as in the process of renal fibrosis and chronic kidney disease (CKD) progression, in addition to polycystic kidney disease (PKD), kidney transplantation, and renal cell carcinoma (RCC). Moreover, the most recent evidence has shown its emerging role in kidney stone disease (or nephrolithiasis), involving inflammasome activation and inflammatory cascade frequently found in kidney stone pathogenesis.

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.

Bile acid signaling in renal water regulation

Emerging evidence has shown that bile acids play important roles in renal physiology and diseases by activating two major receptors, the nuclear farnesoid X receptor (FXR) and the membrane G protein-coupled bile acid receptor-1 (Gpbar1; also known as TGR5). Both FXR and TGR5 have been identified in human and rodent kidneys, where they are deeply involved in renal water handling. In mice, FXR- or TGR5-related gene deficiency has been associated with reduced aquaporin-2 expression accompanied with impaired urinary concentration ability. In this mini-review, we briefly discuss the current understanding of FXR/TGR5 signaling in the kidneys, with a special focus on the regulation of aquaporin-2 expression by bile acids in the collecting ducts and its potential significance in disease conditions.

Inhibitors of Mammalian Aquaporin Water Channels

Aquaporins (AQPs) are water channel proteins that are essential to life, being expressed in all kingdoms. In humans, there are 13 AQPs, at least one of which is found in every organ system. The structural biology of the AQP family is well-established and many functions for AQPs have been reported in health and disease. AQP expression is linked to numerous pathologies including tumor metastasis, fluid dysregulation, and traumatic injury. The targeted modulation of AQPs therefore presents an opportunity to develop novel treatments for diverse conditions. Various techniques such as video microscopy, light scattering and fluorescence quenching have been used to test putative AQP inhibitors in both AQP-expressing mammalian cells and heterologous expression systems. The inherent variability within these methods has caused discrepancy and many molecules that are inhibitory in one experimental system (such as tetraethylammonium, acetazolamide, and anti-epileptic drugs) have no activity in others. Some heavy metal ions (that would not be suitable for therapeutic use) and the compound, TGN-020, have been shown to inhibit some AQPs. Clinical trials for neuromyelitis optica treatments using anti-AQP4 IgG are in progress. However, these antibodies have no effect on water transport. More research to standardize high-throughput assays is required to identify AQP modulators for which there is an urgent and unmet clinical need.

Atg7-dependent canonical autophagy regulates the degradation of aquaporin 2 in prolonged hypokalemia

Prolonged hypokalemia induces a decrease of urinary concentrating ability via down-regulation of aquaporin 2 (AQP2); however, the precise mechanisms remain unknown. To investigate the role of autophagy in the degradation of AQP2, we generated the principal cell-specific Atg7 deletion (Atg7^Δpc) mice. In hypokalemic Atg7-floxed (Atg7^f/f) mice, huge irregular shaped LC3-positive autophagic vacuoles accumulated mainly in inner medullary collecting duct (IMCD) cells. Total- and pS261-AQP2 were redistributed from apical and subapical domains into these vacuoles, which were not co-localized with RAB9. However, in the IMCD cells of hypokalemic Atg7^Δpc mice, these canonical autophagic vacuoles were markedly reduced, whereas numerous small regular shaped LC3-negative/RAB9-positive non-canonical autophagic vacuoles were observed along with diffusely distributed total- and pS261-AQP2 in the cytoplasm. The immunoreactivity of pS256-AQP2 in the apical membrane of IMCD cells was markedly decreased, and no redistribution was observed in both hypokalemic Atg7^f/f and Atg7^Δpc mice. These findings suggest that AQP2 down regulation in hypokalemia was induced by reduced phosphorylation of AQP2, resulting in a reduction of apical plasma labeling of pS256-AQP2 and degradation of total- and pS261-AQP2 via an LC3/ATG7-dependent canonical autophagy pathway.

Zhen-wu-tang attenuates Adriamycin-induced nephropathy via regulating AQP2 and miR-92b

Nephrotic syndrome (NS) is characterized by proteinuria, hypoalbuminemia and edema. The disorder of sodium and water metabolism is a critical mechanism regulating the origination and progression of NS. Zhen-wu-tang (ZWT) has been traditionally used to treat edema disease in China and Japan. The present study was carried out to assess the protective effect of ZWT in Adriamycin-induced (ADR) NS rats and investigate the potential anti-NS mechanisms of ZWT. We found that ZWT treatment ameliorate impaired kidney function and regulate water balance of kidney. Importantly, ZWT increased the expression of Aquaporin-2 (AQP2) which play key roles in maintaining body water homeostasis. Additionally, we determined miRNAs expression patterns in NS rats. Using bioinformatics prediction and miR-92b mimic or inhibitor in vitro, we identified miR-92b as a possible modulator of AQP2. Also we found that ZWT can decrease the expression of miR-92b and reverse the effect of miR-92b on AQP2 in vitro. We further demonstrated that miR-92b directly regulated AQP2 expression by targeting 3'-UTR of AQP2. These finding suggest that ZWT may reduce renal edema in Adriamycin-induced nephropathy via regulating AQP2 and miR-92b.

miR-137 Affects Vaginal Lubrication in Female Sexual Dysfunction by Targeting Aquaporin-2

Introduction

Female sexual dysfunction (FSD) is a common disease with serious potential hazards, but it has not received much attention. The pathogenesis of FSD is urgently needed for the diagnosis and treatment of FSD.

Aim

To investigate the role of microribonucleic acid (mRNA, miR)-137 in FSD.

Methods

Vaginal epithelium tissues from 15 women with lubrication disorder and 15 women with normal function were collected for this study. The expression level of miR-137 in lubrication disorder and normal function women were measured by microarray analysis and Real-time Quantitative Polymerase Chain Reaction (PCR, qPCR). miR-137 was overexpressed in vaginal epithelial cells VK2/E6E7 by lentivirus infection. The cell water permeability was measured using the calcein-quenching method. Cell apoptosis was analyzed by flow cytometry. The potential target of miR-137 was predicted by bioinformatic analysis, then verified by luciferase reporter assays.

Main Outcome Measure

The expression level of miR-137 and aquaporin-2 (AQP2), cell water permeability, cell apoptosis, and luciferase reporter assays were examined.

Results

miR-137 was found to be highly expressed in vaginal epithelial tissues of women with lubrication disorder. Additionally, functional in vitro studies suggested that overexpression of miR-137 leads to a decrease in cell permeability. By combining target prediction and examination, we identified AQP2 as the direct mechanistic target of miR-137 that affected the water permeability of vaginal epithelial cells.

Conclusion

Our results point to a novel role for miR-137 and its downstream effector AQP2 in vaginal lubrication, which can be manipulated as therapeutic targets against lubrication disorder and its related disorders.

Zhang H, Liu T, Zhou Z. miR-137 affects vaginal lubrication in female sexual dysfunction by targeting Aquaporin-2. Sex Med 2018;6:339–347.

The relationship between Aquaporin-2 protein levels in amniotic fluid and the fetal kidney in humans

OBJECTIVES

Aquaporin-2 (AQP2) is an important water channel protein that is expressed in the renal collecting duct and plays a key role in urine concentration and body water homeostasis. It has been demonstrated that the urinary excretion of AQP2 correlates strongly with its expression in the kidney in adult humans and rats. However, there have been no studies on the urinary excretion of AQP2 in human fetuses during development. Fetal urine is the main source of the amniotic fluid; we speculate that the level of AQP2 in the amniotic fluid could reflect the expression level of the AQP2 protein in the fetal kidney. The purpose of the present study was to explore the relationship between AQP2 in the amniotic fluid and that in the fetal kidney.

METHODS

In the present study, the concentration of the AQP2 protein in human amniotic fluid was measured by Enzyme-linked immunosorbent assay (ELISA) and its expression level in human fetal kidneys were examined by wastern blot and immunohistochemistry.

RESULTS

Both the expression level of AQP2 in the fetal kidney (F = 195.9, P < 0.001) and the concentration of AQP2 in the amniotic fluid increased with gestational age (F = 1098, P < 0.001). Moreover, the concentration of AQP2 in the amniotic fluid was positively correlated with its expression level in the fetal kidney (r = 0.872, P < 0.0001).

CONCLUSIONS

Our research indicates that AQP2 levels in the amniotic fluid may be used as a marker for AQP2 expression in the fetal kidney.

Steviol slows renal cyst growth by reducing AQP2 expression and promoting AQP2 degradation

Overexpression of aquaporin 2 (AQP2) was observed and suggested to be involved in fluid secretion leading to cyst enlargement in polycystic kidney disease (PKD). The cyst expansion deteriorates the renal function and, therefore, therapies targeting cyst enlargement are of clinical interest. Of note, inhibition of vasopressin function using vasopressin 2 receptor (V2R) antagonist which decreased cAMP production along with AQP2 production and function can slow cyst growth in ADPKD. This finding supports the role of AQP2 in cyst enlargement. Steviol, a major metabolite of the sweetening compound stevioside, was reported to retard MDCK cyst growth and enlargement by inhibiting CFTR activity. Interestingly, its efficacy was found to be higher than that of CFTR_inh-172. Since steviol was also found to produce diuresis in rodent, it is likely that steviol might have an additional effect in retarding cyst progression, such as inhibition of AQP2 expression and function. Here, we investigated the effect of steviol on AQP2 function and on cyst growth using an in vitro cyst model (MDCK and Pkd1^-/- cells). We found that steviol could markedly inhibit cyst growth by reducing AQP2 expression in both Pkd1^-/- and MDCK cells. Real-time PCR also revealed that steviol decreased AQP2 mRNA expression level as well. Moreover, a proteasome inhibitor, MG-132, and the lysosomotropic agent, hydroxychloroquine (HCQ) were found to abolish the inhibitory effect of steviol in Pkd1^-/- cells. Increased lysosomal enzyme marker (LAMP2) expression following steviol treatment clearly confirmed the involvement of lysosomes in steviol action. In conclusion, our finding showed for the first time that steviol slowed cyst growth, in part, by reducing AQP2 transcription, promoted proteasome, and lysosome-mediated AQP2 degradation. Due to its multiple actions, steviol is a promising compound for further development in the treatment of PKD.

Different Effects of Tolvaptan in Patients with Idiopathic Membranous Nephropathy with Nephrotic Syndrome

This case report discusses the clinical indication for immunosuppressants in patients with idiopathic membranous nephropathy (IMN). Because this disease occasionally shows spontaneous remission, it is necessary to determine the predictive values for a therapeutic effect in order to provide appropriate treatment. Two distinct cases described herein illustrate the different effects of tolvaptan in responders and non-responders, according to the pre-treatment levels of AQP-2 immunostaining in the samples from renal biopsy and urinary levels of AQP-2 and osmolality, suggesting that these values may be useful predictors of response to tolvaptan in patients with nephrotic IMN.

Disruption of Membranes of Extracellular Vesicles Is Necessary for ELISA Determination of Urine AQP2: Proof of Disruption and Epitopes of AQP2 Antibodies

Aquaporin-2 (AQP2) is present in urine extracellular vesicles (EVs) and is a useful biomarker for water balance disorders. We previously found that pre-treatment of urine with alkali/detergent or storage at −25 °C is required for enzyme-linked immunosorbent assay (ELISA) measurement. We speculated that disruptions of EVs membranes are necessary to allow for the direct contact of antibodies with their epitopes. Human urine EVs were prepared using an ultracentrifugation method. Urine EV samples were stored at different temperatures for a week. Electron microscopy showed abundant EVs with diameters of 20–100 nm, consistent with those of exosomes, in normal urine, whereas samples from alkali/detergent pre-treated urine showed fewer EVs with large swollen shapes and frequent membrane disruptions. The abundance and structures of EVs were maintained during storage at −80 °C, but were severely damaged at −25 °C. Binding and competitive inhibition assays showed that epitopes of monoclonal antibody and polyclonal antibody were the hydrophilic Loop D and C-terminus of AQP2, respectively, both of which are present on the inner surface of EVs. Thus, urine storage at −25 °C or pre-treatment with alkali/detergent disrupt EVs membranes and allow AQP2 antibodies to bind to their epitopes located inside EVs.

Urinary excretion of the water channel aquaporin 2 correlated with the pharmacological effect of tolvaptan in cirrhotic patients with ascites

BACKGROUND

The water channel aquaporin 2 (AQP2) at the apical membrane of renal collecting duct cells mediates water reabsorption. The expression of AQP2 at the apical membrane is tightly regulated by vasopressin and was quantitated by measurement of the urinary form by a recently developed ELISA. Tolvaptan, an antagonist of vasopressin type 2 receptor, inhibits water reabsorption in cirrhosis. The aim of this study was to determine the correlation between the pharmacological effect of tolvaptan and the dynamics of urinary AQP2 levels.

METHODS

Tolvaptan was administered to 41 cirrhotic patients with ascites unresponsive to standard diuretic therapy. Urinary excretion of AQP2 and urinary osmolarity were measured at the baseline and at 4, 8, and 24 h after administration of tolvaptan.

RESULTS

At the baseline, urinary AQP2/creatinine ratios were significantly higher in cirrhotic patients with ascites than in healthy controls (P < 0.0001). After administration of tolvaptan, urinary AQP2/creatinine ratios decreased by 45.0 % at 4 h and 77.0 % at 8 h. Similarly, urinary osmolarity decreased by 42.0 % at 4 h and 41.5 % at 8 h. Urinary AQP2 levels and urinary osmolarity significantly correlated at the baseline and at all time points after tolvaptan administration. The degree of the decrease in urinary AQP2 levels and degree of the decrease in urinary osmolarity correlated significantly at 4 h (r = 0.452, P = 0.009) and 8 h (r = 0.384, P = 0.030) after tolvaptan administration.

CONCLUSIONS

These results indicate that the vasopressin-AQP2 system plays a major role in fluid retention in cirrhosis and that the pharmacological effect of tolvaptan to inhibit water reabsorption can be monitored by measurement of the dynamics of urinary AQP2 levels.

Tolvaptan regulates aquaporin-2 and fecal water in cirrhotic rats with ascites

AIM: To investigate the role of tolvaptan in regulating aquaporin (AQP)-2 expression and fecal water content in cirrhotic rats with ascites.

METHODS: Cirrhosis with ascites was induced in rats by repetitive dorsal injection of CCl₄ for 14 wk. In total, 84 cirrhotic rats with ascites divided into three groups (vehicle, 3 mg/kg and 5 mg/kg tolvaptan), and then further divided into five subgroups (days 1, 2, 3, 4, and 5). Blood samples were obtained to measure vasopressin and sodium concentrations. Rats were killed and colonic mucosa was scraped for analysis of protein expression and AQP-2 transcriptional level. The whole layer was fixed for hematoxylin&eosin (HE) staining and feces were collected for determination of fecal water content.

CONCLUSION: Compared with vehicle, vasopressin decreased significantly in the tolvaptan groups from day 2 to a similar level in each treatment group. AQP-2 showed significant upregulation in cirrhotic rats with ascites compared with an untreated control group (100% ± 22.9% vs 22.2% ± 10.23%, P < 0.01). After administration of tolvaptan, AQP-2 expression began to decrease significantly from day 2 in each treatment group, but no significant difference was finally found between the treatment groups. Fecal water content in the distal colon was increased by 5 mg/kg tolvaptan on day 1 (66.8% ± 9.3% vs 41.4% ± 6.3%, in the vehicle group, P < 0.05). Fecal water content returned to baseline at day 4 at the latest in both treatment groups, and did not correspond to the change in AQP-2 expression. HE staining of the colonic mucosa showed no mucosal damage related to tolvaptan.

CONCLUSION: Upregulation of AQP-2 in the distal colon is found in cirrhotic rats with ascites. Tolvaptan inhibits its expression and may decrease water reabsorption and induce diarrhea.

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.

Analysis of mutations of two Gitelman syndrome family SLC12A3 genes and proposed treatments using Chinese medicine

OBJECTIVE

To determine the gene location of two Gitelman syndrome (GS) family SLC12A3 genes and explore treatments using Chinese medicine (CM) prescriptions.

METHODS

In order to locate the two GS mutations, samples were collected from 11 people from two different pedigrees for direct genetic sequencing and comparison of the 26 exons of SLC12A3. Furthermore, the change of serum potassium was monitored throughout the therapy and those two probands undertook a sequential superposition of Western medicine (including potassium, Panangin and potassium-sparing diuretics) with CM prescription based on Buyang Huanwu Decoction () and Sijunzi Decoction (). The treatment included three stages, oral potassium chloride for the first 2 weeks (stage 1), potassium-sparing diuretic and Panangin with potassium chloride for the next 2 weeks (stage 2), CM along with the medicine in stage 2 for the final 2 weeks (stage 3).

RESULTS

The three mutations occurring in proband 1 from pedigree I were Thr60Met, 965-1_976del13ins12 (small indels mutation) and Ala122Ala (homozygous silent mutation). Likewise, three mutations, Asn359Lys, Thr382Met and Arg913Gln, appeared in the proband 2 from pedigree II. The serum potassium levels increasing from baseline to sequential stages were 1.63 mmol/L (baseline), 2.5 mmol/L (stage 1), 3.1 mmol/L (stage 2) and 3.9 mmol/L (stage 3) in the proband 1, and 2.8 mmol/L (baseline), 3.1 mmol/L (stage 1), 3.5 mmol/L (stage 2) and 4.3 mmol/L (stage 3) in the proband 2, respectively. The symptoms (numbness of limbs, weakness, palpitations, etc.) of both probands were all alleviated.

CONCLUSIONS

The mutations of both GS pedigrees can be defined as compound heterozygous mutations, most of which are known as missense mutations. Applying CM could be an appropriate choice for future intervention of GS.

Dexamethasone increases aquaporin-2 protein expression in ex vivo inner medullary collecting duct suspensions

Aquaporin-2 (AQP2) is the vasopressin-regulated water channel that controls renal water reabsorption and plays an important role in the maintenance of body water homeostasis. Excessive glucocorticoid as often seen in Cushing's syndrome causes water retention. However, whether and how glucocorticoid regulates AQP2 remains unclear. In this study, we examined the direct effect of dexamethasone on AQP2 protein expression and activity. Dexamethasone increased AQP2 protein abundance in rat inner medullary collecting duct (IMCD) suspensions. This was confirmed in HEK293 cells transfected with AQP2 cDNA. Cell surface protein biotinylation showed an increase of dexamethasone-induced cell membrane AQP2 expression and this effect was blocked by glucocorticoid receptor antagonist RU486. Functionally, dexamethasone treatment of oocytes injected with an AQP2 cRNA increased water transport activity as judged by cell rupture time in a hypo-osmotic solution (66 ± 13 s in dexamethasone vs. 101 ± 11 s in control, n = 15). We further found that dexamethasone treatment reduced AQP2 protein degradation, which could result in an increase of AQP2 protein. Interestingly, dexamethasone promoted cell membrane AQP2 moving to less buoyant lipid raft submicrodomains. Taken together, our data demonstrate that dexamethasone promotes AQP2 protein expression and increases water permeability mainly via inhibition of AQP2 protein degradation. The increase in AQP2 activity promotes water reabsorption, which may contribute to glucocorticoid-induced water retention and hypertension.

Alkali treatment stabilizes fluctuations of urine AQP2 values measured by ELISA

Background

Aquaporin-2 (AQP2) in urine is now measured in many water-balance disorders and regarded as a useful biomarker for diagnosis and prognosis. An enzyme-linked immunosorbent assay (ELISA) method has been developed for measurement of large numbers of clinical samples. However, fluctuations in the measured values were sometimes observed depending on storage conditions. Urine AQP2 is present in exosome membranes and we speculated that this structural organization causes the fluctuations.

Methods

Human urine samples from healthy subjects were measured by ELISA. Effects of maneuvers to disrupt the exosome membrane mechanically (freezing and thawing at different temperatures) and chemically (treating with alkali and detergents) prior to ELISA were examined.

Results

Urine samples stored at 4 or −80 °C did not show significant AQP2 values, whereas those stored at −25 °C for more that 2 weeks provided the values. Urine samples treated with 0.4 N NaOH and 0.5 % Triton X-305 showed the consistent and comparable values to those stored at −25 °C.

Conclusion

Pretreatment with alkali (0.4 N NaOH) to disrupt exosome membranes allowed consistent ELISA measurements of urinary AQP2. This simple method is applicable to ELISA of other membrane proteins included in exosomes.

Targeting renal purinergic signalling for the treatment of lithium-induced nephrogenic diabetes insipidus

Lithium still retains its critical position in the treatment of bipolar disorder by virtue of its ability to prevent suicidal tendencies. However, chronic use of lithium is often limited by the development of nephrogenic diabetes insipidus (NDI), a debilitating condition. Lithium-induced NDI is due to resistance of the kidney to arginine vasopressin (AVP), leading to polyuria, natriuresis and kaliuresis. Purinergic signalling mediated by extracellular nucleotides (ATP/UTP), acting via P2Y receptors, opposes the action of AVP on renal collecting duct (CD) by decreasing the cellular cAMP and thus AQP2 protein levels. Taking a cue from this phenomenon, we discovered the potential involvement of ATP/UTP-activated P2Y2 receptor in lithium-induced NDI in rats and showed that P2Y2 receptor knockout mice are significantly resistant to Li-induced polyuria, natriuresis and kaliuresis. Extension of these studies revealed that ADP-activated P2Y12 receptor is expressed in the kidney, and its irreversible blockade by the administration of clopidogrel bisulphate (Plavix(®)) ameliorates Li-induced NDI in rodents. Parallel in vitro studies showed that P2Y12 receptor blockade by the reversible antagonist PSB-0739 sensitizes CD to the action of AVP. Thus, our studies unravelled the potential beneficial effects of targeting P2Y2 or P2Y12 receptors to counter AVP resistance in lithium-induced NDI. If established in further studies, our findings may pave the way for the development of better and safer methods for the treatment of NDI by bringing a paradigm shift in the approach from the current therapies that predominantly counter the anti-AVP effects to those that enhance the sensitivity of the kidney to AVP action.

P2Y12 Receptor Localizes in the Renal Collecting Duct and Its Blockade Augments Arginine Vasopressin Action and Alleviates Nephrogenic Diabetes Insipidus

P2Y12 receptor (P2Y12-R) signaling is mediated through Gi, ultimately reducing cellular cAMP levels. Because cAMP is a central modulator of arginine vasopressin (AVP)-induced water transport in the renal collecting duct (CD), we hypothesized that if expressed in the CD, P2Y12-R may play a role in renal handling of water in health and in nephrogenic diabetes insipidus. We found P2Y12-R mRNA expression in rat kidney, and immunolocalized its protein and aquaporin-2 (AQP2) in CD principal cells. Administration of clopidogrel bisulfate, an irreversible inhibitor of P2Y12-R, significantly increased urine concentration and AQP2 protein in the kidneys of Sprague-Dawley rats. Notably, clopidogrel did not alter urine concentration in Brattleboro rats that lack AVP. Clopidogrel administration also significantly ameliorated lithium-induced polyuria, improved urine concentrating ability and AQP2 protein abundance, and reversed the lithium-induced increase in free-water excretion, without decreasing blood or kidney tissue lithium levels. Clopidogrel administration also augmented the lithium-induced increase in urinary AVP excretion and suppressed the lithium-induced increase in urinary nitrates/nitrites (nitric oxide production) and 8-isoprostane (oxidative stress). Furthermore, selective blockade of P2Y12-R by the reversible antagonist PSB-0739 in primary cultures of rat inner medullary CD cells potentiated the expression of AQP2 and AQP3 mRNA, and cAMP production induced by dDAVP (desmopressin). In conclusion, pharmacologic blockade of renal P2Y12-R increases urinary concentrating ability by augmenting the effect of AVP on the kidney and ameliorates lithium-induced NDI by potentiating the action of AVP on the CD. This strategy may offer a novel and effective therapy for lithium-induced NDI.

Regulation of neurovascular coupling in autoimmunity to water and ion channels

Much progress has been made in understanding autoimmune channelopathies, but the underlying pathogenic mechanisms are not always clear due to broad expression of some channel proteins. Recent studies show that autoimmune conditions that interfere with neurovascular coupling in the central nervous system (CNS) can lead to neurodegeneration. Cerebral blood flow that meets neuronal activity and metabolic demand is tightly regulated by local neural activity. This process of reciprocal regulation involves coordinated actions of a number of cell types, including neurons, glia, and vascular cells. In particular, astrocytic endfeet cover more than 90% of brain capillaries to assist blood-brain barrier (BBB) function, and wrap around synapses and nodes of Ranvier to communicate with neuronal activity. In this review, we highlight four types of channel proteins that are expressed in astrocytes, regarding their structures, biophysical properties, expression and distribution patterns, and related diseases including autoimmune disorders. Water channel aquaporin 4 (AQP4) and inwardly rectifying potassium (Kir4.1) channels are concentrated in astrocytic endfeet, whereas some voltage-gated Ca(2+) and two-pore domain K(+) channels are expressed throughout the cell body of reactive astrocytes. More channel proteins are found in astrocytes under normal and abnormal conditions. This research field will contribute to a better understanding of pathogenic mechanisms underlying autoimmune disorders.

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.

Collecting duct principal cell transport processes and their regulation

The principal cell of the kidney collecting duct is one of the most highly regulated epithelial cell types in vertebrates. The effects of hormonal, autocrine, and paracrine factors to regulate principal cell transport processes are central to the maintenance of fluid and electrolyte balance in the face of wide variations in food and water intake. In marked contrast with the epithelial cells lining the proximal tubule, the collecting duct is electrically tight, and ion and osmotic gradients can be very high. The central role of principal cells in salt and water transport is reflected by their defining transporters-the epithelial Na(+) channel (ENaC), the renal outer medullary K(+) channel, and the aquaporin 2 (AQP2) water channel. The coordinated regulation of ENaC by aldosterone, and AQP2 by arginine vasopressin (AVP) in principal cells is essential for the control of plasma Na(+) and K(+) concentrations, extracellular fluid volume, and BP. In addition to these essential hormones, additional neuronal, physical, and chemical factors influence Na(+), K(+), and water homeostasis. Notably, a variety of secreted paracrine and autocrine agents such as bradykinin, ATP, endothelin, nitric oxide, and prostaglandin E2 counterbalance and limit the natriferic effects of aldosterone and the water-retaining effects of AVP. Considerable recent progress has improved our understanding of the transporters, receptors, second messengers, and signaling events that mediate principal cell responses to changing environments in health and disease. This review primarily addresses the structure and function of the key transporters and the complex interplay of regulatory factors that modulate principal cell ion and water transport.

Sclederma of Poria cocos exerts its diuretic effect via suppression of renal aquaporin-2 expression in rats with chronic heart failure

ETHNOPHARMACOLOGICAL RELEVANCE

Sclederma of Poria cocos (Hoelen) has been used as a diuretic in traditional Asian medicine. However, the underlying mechanism by which Sclederma of Poria cocos (hoelen) exerts its diuretic effect has not been well identified. The aim of the present study was to evaluate the effects of Sclederma of Poria cocos (hoelen) in rats with chronic heart failure (CHF) induced by acute myocardial infarction and to investigate the underlying mechanisms.

MATERIALS AND METHODS

An aqueous extract of Sclederma of Poria cocos (hoelen) (2.4 g/kg/d, 1.2 g/kg/d or 0.6 g/kg/d) or furosemide (20 mg/kg/d) was administered orally to male Sprague-Dawley rats starting on the day of coronary ligation. The urine output of all rats was quantified and collected every day for 1 or 4 weeks. The expression of aquaporin-2 (AQP2) was examined after treatment for 1 or 4 weeks.

RESULTS

Urinary output increased significantly and urinary osmolality decreased after oral administration of Sclederma of Poria cocos (hoelen) for both 1 and 4 weeks. Sclederma of Poria cocos (hoelen) caused less electrolyte disorder than furosemide. Furthermore, Sclederma of Poria cocos (hoelen) reduced the levels of plasma BNP in CHF rats, whereas furosemide had no effect. Importantly, both mRNA and protein expression of AQP2 were down-regulated and urinary excretion of AQP2 was decreased after administration of Sclederma of Poria cocos (hoelen) to CHF rats. Similarly, Sclederma of Poria cocos (hoelen) reduced plasma arginine vasopressin (AVP) level and down-regulated vasopressin type 2 receptor (V2R) mRNA expression.

CONCLUSIONS

Sclederma of Poria cocos (hoelen) exerts its diuretic effect and improves cardiac function in CHF rats via the AVP-V2R-AQP2 axis.

Increased urine aquaporin-2 relative to plasma arginine vasopressin is a novel marker of response to tolvaptan in patients with decompensated heart failure

BACKGROUND

Preserved function of the renal collecting duct may be essential for response to the vasopressin V2receptor antagonist, tolvaptan (TLV), but the predictors of response to TLV are unknown. METHODS AND RESULTS: Sixty consecutive patients with stage D decompensated heart failure (HF) who had received TLV on a de novo basis were retrospectively enrolled (TLV(+) group). Among them, 41 patients were responders defined according to urine volume (UV) increase after TLV initiation. In the UV-defined responders, plasma arginine vasopressin (P-AVP) had a close correlation with urine aquaporin-2 (U-AQP2; 5.42±3.54 ng/ml; r=0.843, P<0.001). In contrast, 19 were UV-defined non-responders, and they had extremely low U-AQP2 (0.76±0.59 ng/ml, P<0.001 vs. responders) regardless of P-AVP level. On receiver operating characteristic analysis, U-AQP2/P-AVP ≥0.5×10(3)clearly separated the UV-defined responders from the non-responders. We then identified AQP-defined responders as having U-AQP2/P-AVP ≥0.5×10(3). Sixty propensity score-matched HF patients without TLV treatment were examined, and exactly the same number of patients as that of the AQP-defined responders (n=41) was selected. These patients had a poorer survival without TLV than the TLV-treated responders during a 2-year observation period (73.8% vs. 94.8%, P=0.034).

CONCLUSIONS

U-AQP2/P-AVP is a novel predictor of response to TLV in patients with decompensated HF. AQP-defined responders may have a better prognosis on TLV treatment.

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.

Lithium: a versatile tool for understanding renal physiology

By virtue of its unique interactions with kidney cells, lithium became an important research tool in renal physiology and pathophysiology. Investigators have uncovered the intricate relationships of lithium with the vasopressin and aldosterone systems, and the membrane channels or transporters regulated by them. While doing so, their work has also led to 1) questioning the role of adenylyl cyclase activity and prostaglandins in lithium-induced suppression of aquaporin-2 gene transcription; 2) unraveling the role of purinergic signaling in lithium-induced polyuria; and 3) highlighting the importance of the epithelial sodium channel (ENaC) in lithium-induced nephrogenic diabetes insipidus (NDI). Lithium-induced remodeling of the collecting duct has the potential to shed new light on collecting duct remodeling in disease conditions, such as diabetes insipidus. The finding that lithium inhibits glycogen synthase kinase-3β (GSK3β) has opened an avenue for studies on the role of GSK3β in urinary concentration, and GSK isoforms in renal development. Finally, proteomic and metabolomic profiling of the kidney and urine in rats treated with lithium is providing insights into how the kidney adapts its metabolism in conditions such as acquired NDI and the multifactorial nature of lithium-induced NDI. This review provides state-of-the-art knowledge of lithium as a versatile tool for understanding the molecular physiology of the kidney, and a comprehensive view of how this tool is challenging some of our long-standing concepts in renal physiology, often with paradigm shifts, and presenting paradoxical situations in renal pathophysiology. In addition, this review points to future directions in research where lithium can lead the renal community.