Renal aquaporins and water balance disorders

Lagopsis supina exerts its diuretic effect via inhibition of aquaporin-1, 2 and 3 expression in a rat model of traumatic blood stasis

ETHNOPHARMACOLOGICAL RELEVANCE

Lagopsis supina has been used as a traditional medicinal herb for centuries in China. In folk medicine, it is used for promoting blood circulation and removing blood stasis (PBCRBS), anti-inflammatory and diuretic activities. Modern pharmacological investigation have shown that L. supina have an improvement in blood and lymphatic microcirculation, myocardioprotective, and antioxidative activities. Although the pharmacological research of L. supina was more, there was no report on the diuretic activity.

AIM OF THE STUDY

This study was to evaluate the diuretic activity and the underlying mechanism of an ethanol extract of L. supina (LS) in a rat model of traumatic blood stasis (TBS).

MATERIALS AND METHODS

There were 30 male Sprague-Dawley rats that were randomly assigned to the control group, TBS group, and LS group (10 animals in each group). LS was administered orally (460 mg/kg) once daily for 7 successive days. The control group and TBS group were given an equal amount of 0.3% sodium carboxymethyl cellulose (CMC-Na). For the efficacy evaluation, the urine output volume, the urinary electrolyte concentrations (Na⁺, K⁺, Cl^- and Ca²⁺) and pH value, the levels of angiotensin II (Ang II), atriopeptin (ANP), anti-diuretic hormone (ADH) and aldosterone (ALD), as well as aquaporin (AQP)-1, 2 and 3 protein expressions were detected in a rat model of TBS. The protein expressions of AQP-1, 2 and 3 were detected by quantitative immunohistochemistry (IHC) and Western blot analysis.

RESULTS

In the efficacy evaluation, rat models treated with LS showed a significant increase in the total urine output (p < 0.01). The urinary electrolyte and the acid-base disturbances, including the decrease of Na⁺ and Ca²⁺ levels and the Na⁺/K⁺ value together with the increase in the Cl^- level and the pH value, in the urine of the LS group were compared with the TBS group. Moreover, the levels of Ang II, ADH and ALD of rat model were decreased after being treated with LS (p < 0.05 or p < 0.01), while the ANP level was increased (p < 0.05). In addition, the results of the quantitative IHC and the Western blot analysis showed that the expression levels of AQP-1, 2 and 3 proteins decreased significantly compared with those of the TBS group.

CONCLUSIONS

This is the first reported notable diuretic effect by LS, which probably was through the suppression of the renin-angiotensin-aldosterone system (RAAS) and the regulation of the signaling pathways of AQP-1, 2 and 3 protein expressions. Based on our results, we conclude that L. supina carries out its diuretic effect mainly by down-regulating the levels of AQP-1, 2 and 3 expressions in TBS rat model. These data also embody the traditional Chinese medicine (TCM) application principle of Huo xue li shui. These findings suggest that LS may warrant further evaluation as a possible agent for the diuretic drug in clinical applications. Further research is underway to elucidate the active compounds responsible for the diuretic activity of LS.

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.

Genetic deletion of ADP-activated P2Y12 receptor ameliorates lithium-induced nephrogenic diabetes insipidus in mice

AIM

Therapeutic use of lithium in bipolar disorder is limited by the development of nephrogenic diabetes insipidus (NDI). We reported that pharmacological blockade of P2Y₁₂ receptor (R) with clopidogrel or prasugrel significantly ameliorated lithium-induced NDI in rodents. Using mice genetically lacking P2Y₁₂ -R we evaluated whether the observed amelioration is mediated through P2Y₁₂ -R METHODS: P2ry12^-/- mouse line (C57/BL6) was rederived from cryopreserved embryos of the knockout (KO) mice generated by Deltagen Inc. Syngeneic wild type (WT) mice obtained by heterozygous crossing were inbred. Groups of adult WT and KO mice were fed lithium-added (40 mmol LiCl/kg food) or regular diet, and euthanized after 2 or 4 weeks. Twenty-four hour urine samples and terminal blood and kidney samples were analyzed.

RESULTS

At both time points, lithium-induced polyuria and decrease in aquaporin-2 (AQP2) protein abundance in the kidney medulla were less marked in KO vs WT mice. Immunofluorescence microscopy revealed that lithium-induced alterations in the cellular disposition of AQP2 protein in the medullary collecting ducts of WT mice were blunted in KO mice. Serum lithium, sodium and osmolality were similar in both genotypes after lithium treatment. After 2 weeks, lithium induced marked increases in urinary excretion of Na, K, and arginine vasopressin in WT mice but not in KO mice.

CONCLUSION

Taken together, our data show that similar to pharmacological blockade, deletion of P2Y₁₂ -R significantly ameliorates lithium-induced NDI, without reducing serum lithium levels. Hence, targeting P2Y₁₂ -R with currently available drugs in the market offers a novel and safer method for treating NDI.

Aquaporins in Renal Diseases

Aquaporins (AQPs) are a family of highly selective transmembrane channels that mainly transport water across the cell and some facilitate low-molecular-weight solutes. Eight AQPs, including AQP1, AQP2, AQP3, AQP4, AQP5, AQP6, AQP7, and AQP11, are expressed in different segments and various cells in the kidney to maintain normal urine concentration function. AQP2 is critical in regulating urine concentrating ability. The expression and function of AQP2 are regulated by a series of transcriptional factors and post-transcriptional phosphorylation, ubiquitination, and glycosylation. Mutation or functional deficiency of AQP2 leads to severe nephrogenic diabetes insipidus. Studies with animal models show AQPs are related to acute kidney injury and various chronic kidney diseases, such as diabetic nephropathy, polycystic kidney disease, and renal cell carcinoma. Experimental data suggest ideal prospects for AQPs as biomarkers and therapeutic targets in clinic. This review article mainly focuses on recent advances in studying AQPs in renal diseases.

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.

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.

CHIP Regulates Aquaporin-2 Quality Control and Body Water Homeostasis

The importance of the kidney distal convoluted tubule (DCT) and cortical collecting duct (CCD) is highlighted by various water and electrolyte disorders that arise when the unique transport properties of these segments are disturbed. Despite this critical role, little is known about which proteins have a regulatory role in these cells and how these cells can be regulated by individual physiologic stimuli. By combining proteomics, bioinformatics, and cell biology approaches, we found that the E3 ubiquitin ligase CHIP is highly expressed throughout the collecting duct; is modulated in abundance by vasopressin; interacts with aquaporin-2 (AQP2), Hsp70, and Hsc70; and can directly ubiquitylate the water channel AQP2 in vitro shRNA knockdown of CHIP in CCD cells increased AQP2 protein t_1/2 and reduced AQP2 ubiquitylation, resulting in greater levels of AQP2 and phosphorylated AQP2. CHIP knockdown increased the plasma membrane abundance of AQP2 in these cells. Compared with wild-type controls, CHIP knockout mice or novel CRISPR/Cas9 mice without CHIP E3 ligase activity had greater AQP2 abundance and altered renal water handling, with decreased water intake and urine volume, alongside higher urine osmolality. We did not observe significant changes in other water- or sodium-transporting proteins in the gene-modified mice. In summary, these results suggest that CHIP regulates AQP2 and subsequently, renal water handling.

Prostaglandin E2 in the Regulation of Water Transport in Renal Collecting Ducts

The kidney plays a central role in the regulation of the body water balance. The process of targeting the water channel aquaporin-2 (AQP2) on the apical plasma membrane of the collecting duct (CD) principal cells is mainly regulated by the antidiuretic peptide hormone arginine vasopressin (AVP), which is responsible for the maintenance of water homeostasis. Recently, much attention has been focused on the local factors modulating renal water reabsorption by AQP2 in the collecting ducts, especially prostaglandin E2 (PGE₂). PGE₂ is a lipid mediator involved in a variety of physiological and pathophysiological processes in the kidney. The biological function of PGE₂ is mainly mediated by four G-protein-coupled receptors, namely EP1-4, which couple to drive separate intracellular signaling pathways. Increasing evidence demonstrates that PGE₂ is essential for renal water transport regulation via multiple mechanisms. Each EP receptor plays a unique role in regulating water reabsorption in renal collecting ducts. This brief review highlights the role of PGE₂ in the regulation of water reabsorption and discusses the involvement of each EP receptor subtype in renal collecting duct. A better understanding of the role of PGE₂ in renal water transport process may improve disease management strategies for water balance disorders, including nephrogenic diabetes insipidus.

Hereditary Nephrogenic Diabetes Insipidus: Pathophysiology and Possible Treatment. An Update

Under physiological conditions, excessive loss of water through the urine is prevented by the release of the antidiuretic hormone arginine-vasopressin (AVP) from the posterior pituitary. In the kidney, AVP elicits a number of cellular responses, which converge on increasing the osmotic reabsorption of water in the collecting duct. One of the key events triggered by the binding of AVP to its type-2 receptor (AVPR2) is the exocytosis of the water channel aquaporin 2 (AQP2) at the apical membrane the principal cells of the collecting duct. Mutations of either AVPR2 or AQP2 result in a genetic disease known as nephrogenic diabetes insipidus, which is characterized by the lack of responsiveness of the collecting duct to the antidiuretic action of AVP. The affected subject, being incapable of concentrating the urine, presents marked polyuria and compensatory polydipsia and is constantly at risk of severe dehydration. The molecular bases of the disease are fully uncovered, as well as the genetic or clinical tests for a prompt diagnosis of the disease in newborns. A real cure for nephrogenic diabetes insipidus (NDI) is still missing, and the main symptoms of the disease are handled with s continuous supply of water, a restrictive diet, and nonspecific drugs. Unfortunately, the current therapeutic options are limited and only partially beneficial. Further investigation in vitro or using the available animal models of the disease, combined with clinical trials, will eventually lead to the identification of one or more targeted strategies that will improve or replace the current conventional therapy and grant NDI patients a better quality of life. Here we provide an updated overview of the genetic defects causing NDI, the most recent strategies under investigation for rescuing the activity of mutated AVPR2 or AQP2, or for bypassing defective AVPR2 signaling and restoring AQP2 plasma membrane expression.

Aquaporin Protein-Protein Interactions

Aquaporins are tetrameric membrane-bound channels that facilitate transport of water and other small solutes across cell membranes. In eukaryotes, they are frequently regulated by gating or trafficking, allowing for the cell to control membrane permeability in a specific manner. Protein–protein interactions play crucial roles in both regulatory processes and also mediate alternative functions such as cell adhesion. In this review, we summarize recent knowledge about aquaporin protein–protein interactions; dividing the interactions into three types: (1) interactions between aquaporin tetramers; (2) interactions between aquaporin monomers within a tetramer (hetero-tetramerization); and (3) transient interactions with regulatory proteins. We particularly focus on the structural aspects of the interactions, discussing the small differences within a conserved overall fold that allow for aquaporins to be differentially regulated in an organism-, tissue- and trigger-specific manner. A deep knowledge about these differences is needed to fully understand aquaporin function and regulation in many physiological processes, and may enable design of compounds targeting specific aquaporins for treatment of human disease.

Investigation of aquaporins and apparent diffusion coefficient from ultra-high b-values in a rat model of diabetic nephropathy

Background

To assess kidney damage in a rat model of type-2 diabetic nephropathy based on apparent diffusion coefficient (ADC) data obtained from ultra-high b-values and discuss its relationship to the expression of aquaporins (AQPs).

Methods

This study was approved by the institutional Animal Care and Use Committee. Thirty male Sprague-Dawley rats were randomised into two groups: (1) untreated controls and (2) diabetes mellitus (DM). All rats underwent diffusion-weighted imaging (DWI) with 18 b-values (0–4500 s/mm²). Maps of low ADC (ADC_low), standard ADC (ADC_st) and ultra-high ADC (ADC_uh) were calculated from low b-values (0–200 s/mm²), standard b-values (300–1500 s/mm²) and ultra-high b-values (1700–4500 s/mm²), respectively. The expression of AQPs in the kidneys was studied using immunohistochemistry. Laboratory parameters of diabetic and kidney functions, ADC_low, ADC_st, ADC_uh, and the optical density (OD) of AQP expression in the two groups were compared using an independent t test. Correlations between ADCs and the OD of AQP expression were evaluated by Pearson’s correlation analysis.

Results

ADC_uh were significantly higher in the cortex (CO), outer stripe of the outer medulla (OS) and inner stripe of the outer medulla (IS), and the OD values of AQ-2 were significantly higher in the OS, IS and inner medulla (IM) in DM animals compared with control animals. ADC_uh and OD values of AQP-2 expression were positively correlated in the OS, IS and IM of the kidney.

Conclusions

ADC_uh may work as useful metrics for early detection of kidney damage in diabetic nephropathy and may be associated with AQP-2 expression.

How impaired efficacy happened between Gancao and Yuanhua: Compounds, targets and pathways

As recorded in Traditional Chinese Medicine (TCM) theory, Gancao (Glycyrrhizae Radix et Rhizoma) could weaken the pharmacological effect or increase the toxicity of Yuanhua (Genkwa Flos). However, the theory has been suspected due to lack of evidence. Here, we investigate whether Gancao could weaken Yuanhua’s diuretic effect, if so, which chemicals and which targets may be involved. Results showed that Yuanhua exerted diuretic effect through down-regulating renal AQP 2, without electrolyte disturbances such as K⁺ loss which has been observed as side-effect of most diuretics. Gancao had no diuretic effect, but could impair Yuanhua’s diuretic effect through up-regulating renal AQP 2. Glycyrrhetinic acid (GRA) in Gancao could up-regulate AQP 2 and counteract the AQP 2 regulation effect of Yuanhuacine (YHC) and Ginkwanin (GKW) in Yuanhua. Network pharmacology method suggested that YHC, GKW and GRA could bind to MEK1/FGFR1 protein and influence ERK-MAPK pathway, which was verified by Western blotting. This study supports TCM theory and reminds that more attention should be paid to the safety and efficacy problems induced by improper combination between herbs. Moreover, we suggested that promising diuretics with less side effects can be developed from Chinese Medicines such as Yuanhua.

Na+/HCO3- Cotransporter NBCn2 Mediates HCO3- Reclamation in the Apical Membrane of Renal Proximal Tubules

The kidney maintains systemic acid-base balance by reclaiming from the renal tubule lumen virtually all HCO₃^- filtered in glomeruli and by secreting additional H⁺ to titrate luminal buffers. For proximal tubules, which are responsible for about 80% of this activity, it is believed that HCO₃^- reclamation depends solely on H⁺ secretion, mediated by the apical Na⁺/H⁺ exchanger NHE₃ and the vacuolar proton pump. However, _NHE3 and the proton pump cannot account for all HCO₃^- reclamation. Here, we investigated the potential contribution of two variants of the electroneutral Na⁺/HCO₃^- cotransporter NBCn2, the amino termini of which start with the amino acids MCDL (MCDL-NBCn2) and MEIK (MEIK-NBCn2). Western blot analysis and immunocytochemistry revealed that MEIK-NBCn2 predominantly localizes at the basolateral membrane of medullary thick ascending limbs in the rat kidney, whereas MCDL-NBCn2 localizes at the apical membrane of proximal tubules. Notably, NH₄Cl-induced systemic metabolic acidosis or hypokalemic alkalosis downregulated the abundance of MCDL-NBCn2 and reciprocally upregulated NHE₃ Conversely, NaHCO₃-induced metabolic alkalosis upregulated MCDL-NBCn2 and reciprocally downregulated NHE₃ We propose that the apical membrane of the proximal tubules has two distinct strategies for HCO₃^- reclamation: the conventional indirect pathway, in which NHE₃ and the proton pump secrete H⁺ to titrate luminal HCO₃^-, and the novel direct pathway, in which NBCn2 removes HCO₃^- from the lumen. The reciprocal regulation of NBCn2 and NHE₃ under different physiologic conditions is consistent with our mathematical simulations, which suggest that HCO₃^- uptake and H⁺ secretion have reciprocal efficiencies for HCO₃^- reclamation versus titration of luminal buffers.

Comparison of aquaporin-1 expression between yak (Bos grunniens) and indigenous cattle (Bos taurus) in the Qinghai–Tibetan Plateau

Aquaporins (AQPs) are a large family of integral membrane proteins that facilitate the transport of water through the biomembranes. AQP1, one of the 13 AQPs identified in mammals, is distributed in various tissues and organs, and plays an important role in body water homeostasis. The objectives of the present study were to identify the expression of aquaporin-1 (AQP1) in the kidney, rumen and parotid gland of yaks, and to quantify whether the protein abundance of AQP1 is species specific between yak (Bos grunniens) and indigenous cattle (Bos taurus). Three 3-year-old castrated males (156 ± 6.8 kg of bodyweight) of each of three genotypes, namely, yak, indigenous cattle and the crossbred between the two (Bos taurus♂ × Bos grunniens♀), were used; all animals were grazed in the same autumn pasture of the Qinghai–Tibetan Plateau, China. Western blot results detected (1) 28-kDa unglycosylated AQP1 in the kidney, rumen and parotid gland of all three genotypes, 40-kDa glycosylated AQP1 in renal cortex and medulla. (2) Yaks expressed less 28-kDa AQP1 protein in the outer cortex (P &lt; 0.05), significantly more in the outer medulla (P &lt; 0.01), and slightly more in the ventral rumen (P = 0.088) than did cattle. No difference was observed in the dorsal rumen and parotid gland (P &gt; 0.10). In conclusion, the present study is the first to confirm the presence of AQP1 in bovine rumen and parotid gland and identified its expression in yaks. Abundance of AQP1 protein in yak kidney showed some difference from indigenous cattle. This could provide a new perspective to explain some adaptive mechanisms of yaks to the harsh environment in the Qinghai–Tibetan Plateau.

Ion transport in the zebrafish kidney from a human disease angle: possibilities, considerations, and future perspectives

Unique experimental advantages, such as its embryonic/larval transparency, high-throughput nature, and ease of genetic modification, underpin the rapid emergence of the zebrafish ( Danio rerio) as a preeminent model in biomedical research. Particularly in the field of nephrology, the zebrafish provides a promising model for studying the physiological implications of human solute transport processes along consecutive nephron segments. However, although the zebrafish might be considered a valuable model for numerous renal ion transport diseases and functional studies of many channels and transporters, not all human renal electrolyte transport mechanisms and human diseases can be modeled in the zebrafish. With this review, we explore the ontogeny of zebrafish renal ion transport, its nephron structure and function, and thereby demonstrate the clinical translational value of this model. By critical assessment of genomic and amino acid conservation of human proteins involved in renal ion handling (channels, transporters, and claudins), kidney and nephron segment conservation, and renal electrolyte transport physiology in the zebrafish, we provide researchers and nephrologists with an indication of the possibilities and considerations of the zebrafish as a model for human renal ion transport. Combined with advanced techniques envisioned for the future, implementation of the zebrafish might expand beyond unraveling pathophysiological mechanisms that underlie distinct genetic or environmentally, i.e., pharmacological and lifestyle, induced renal transport deficits. Specifically, the ease of drug administration and the exploitation of improved genetic approaches might argue for the adoption of the zebrafish as a model for preclinical personalized medicine for distinct renal diseases and renal electrolyte transport proteins.

Aquaporins in Urinary System

Several aquaporin (AQP )-type water channels are expressed in kidney: AQP1 in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2 -6 in the collecting duct; AQP7 in the proximal tubule; AQP8 in the proximal tubule and collecting duct; and AQP11 in the endoplasmic reticulum of proximal tubule cells. AQP2 is the vasopressin-regulated water channel that is important in hereditary and acquired diseases affecting urine-concentrating ability. The roles of AQPs in renal physiology and transepithelial water transport have been determined using AQP knockout mouse models. This chapter describes renal physiologic insights revealed by phenotypic analysis of AQP knockout mice and the prospects for further basic and clinical studies.

Urinary Excretion of Kidney Aquaporins as Possible Diagnostic Biomarker of Diabetic Nephropathy

Diabetic nephropathy (DN) is a microangiopathic complication of diabetes mellitus (DM) affecting one-third of diabetic patients. The large variability in the clinical presentation of renal involvement in patients with DM makes kidney biopsy a prerequisite for a correct diagnosis. However, renal biopsy is an invasive procedure associated with risk of major complications. Numerous studies aimed to identify a noninvasive biomarker of DN but, so far, none of these is considered to be sufficiently specific and sensitive. Water channel aquaporins (AQPs), expressed at the plasma membrane of epithelial tubular cells, are often dysregulated during DN. In this work, we analyzed the urine excretion of AQP5 and AQP2 (uAQP5 and uAQP2), via exosomes, in 35 diabetic patients: 12 normoalbuminuric with normal renal function (DM), 11 with proteinuric nondiabetic nephropathy (NDN), and 12 with histological diagnosis and classification of DN. ELISA and WB analysis independently showed that uAQP5 was significantly increased in DN patients. Interestingly, linear regression analysis showed a positive correlation between uAQP5 and the histological class of DN. The same analysis, focusing on uAQP2, showed comparable results. Taken together, these data suggest a possible use of AQP5 and AQP2 as novel noninvasive biomarkers to help in classifying the clinical stage of DN.

The gene-expression profile of renal medulla in ISIAH rats with inherited stress-induced arterial hypertension

Background

The changes in the renal function leading to a reduction of medullary blood flow can have a great impact on sodium and water homeostasis and on the long-term control of arterial blood pressure. The RNA-Seq approach was used for transcriptome profiling of the renal medulla from hypertensive ISIAH and normotensive WAG rats to uncover the genetic basis of the changes underlying the renal medulla function in the ISIAH rats being a model of the stress-sensitive arterial hypertension and to reveal the genes which possibly may contribute to the alterations in medullary blood flow.

Results

Multiple DEGs specifying the function of renal medulla in ISIAH rats were revealed. The group of DEGs described by Gene Ontology term ‘oxidation reduction’ was the most significantly enriched one. The other groups of DEGs related to response to external stimulus, response to hormone (endogenous) stimulus, response to stress, and homeostatic process provide the molecular basis for integrated responses to homeostasis disturbances in the renal medulla of the ISIAH rats. Several DEGs, which may modulate the renal medulla blood flow, were detected. The reduced transcription of Nos3 pointed to the possible reduction of the blood flow in the renal medulla of ISIAH rats.

Conclusions

The generated data may be useful for comparison with those from different models of hypertension and for identifying the common molecular determinants contributing to disease manifestation, which may be potentially used as new pharmacological targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0462-6) contains supplementary material, which is available to authorized users.

ILK and cytoskeletal architecture: an important determinant of AQP2 recycling and subsequent entry into the exocytotic pathway

Within the past decade tremendous efforts have been made to understand the mechanism behind aquaporin-2 (AQP2) water channel trafficking and recycling, to open a path toward effective diabetes insipidus therapeutics. A recent study has shown that integrin-linked kinase (ILK) conditional-knockdown mice developed polyuria along with decreased AQP2 expression. To understand whether ILK also regulates AQP2 trafficking in kidney tubular cells, we performed in vitro analysis using LLCPK1 cells stably expressing rat AQP2 (LLC-AQP2 cells). Upon treatment of LLC-AQP2 cells with ILK inhibitor cpd22 and ILK-siRNA, we observed increased accumulation of AQP2 in the perinuclear region, without any significant increase in the rate of endocytosis. This perinuclear accumulation did not occur in cells expressing a serine-256-aspartic acid mutation that retains AQP2 in the plasma membrane. We then examined clathrin-mediated endocytosis after ILK inhibition using rhodamine-conjugated transferrin. Despite no differences in overall transferrin endocytosis, the endocytosed transferrin also accumulated in the perinuclear region where it colocalized with AQP2. These accumulated vesicles also contained the recycling endosome marker Rab11. In parallel, the usual vasopressin-induced AQP2 membrane accumulation was prevented after ILK inhibition; however, ILK inhibition did not measurably affect AQP2 phosphorylation at serine-256 or its dephosphorylation at serine-261. Instead, we found that inhibition of ILK increased F-actin polymerization. When F-actin was depolymerized with latrunculin, the perinuclear located AQP2 dispersed. We conclude that ILK is important in orchestrating dynamic cytoskeletal architecture during recycling of AQP2, which is necessary for its subsequent entry into the exocytotic pathway.

Depletion of vacuolar protein sorting-associated protein 35 is associated with increased lysosomal degradation of aquaporin-2

The carboxyl terminus of aquaporin-2 (AQP2c) undergoes posttranslational modifications, including phosphorylation and ubiquitination, in the process of regulating aquaporin-2 (AQP2) translocation and protein abundance. We aimed to identify novel proteins interacting with AQP2c. Recombinant AQP2c protein was made in Escherichia coli BL21 (DE3) cells by exploiting the pET32 TrxA fusion system. Lysates of rat kidney inner medullary collecting duct (IMCD) tubule suspensions interacted with rat AQP2c bound to Ni²⁺-resin were subjected to LC-MS/MS proteomic analysis. Potential interacting proteins were identified, including vacuolar protein sorting-associated protein 35 (Vps35). Coimmunoprecipitation assay demonstrated that Vps35 interacted with AQP2c. Immunohistochemistry of rat kidney revealed that AQP2 and Vps35 were partly colocalized at the intracellular vesicles in collecting duct cells. The role of Vps35 in AQP2 regulation induced by 1-deamino-8D-arginine vasopressin (dDAVP) was examined in mpkCCDc14 cells. Cell surface biotinylation assay demonstrated that dDAVP-induced apical translocation of AQP2 was significantly decreased under siRNA-mediated Vps35 knockdown. dDAVP-induced AQP2 upregulation was less prominent in the cells with Vps35 knockdown. Moreover, AQP2 protein abundance was decreased to a greater extent during the withdrawal period after dDAVP stimulation under Vps35 knockdown, which was significantly inhibited by chloroquine (a blocker of the lysosomal pathway) but not by MG132 (a proteasome inhibitor). Immunocytochemistry demonstrated that internalized AQP2 was more associated with lysosomal-associated membrane protein 1 (LAMP-1) in primary cultured IMCD cells under a Vps35 knockdown situation. Taken together, our results show that Vps35 interacts with AQP2c, and depletion of Vps35 is likely to be associated with decreased AQP2 trafficking and increased lysosomal degradation of AQP2 protein.

Systems-level analysis reveals selective regulation of Aqp2 gene expression by vasopressin

Vasopressin-mediated regulation of renal water excretion is defective in a variety of water balance disorders in humans. It occurs in part through long-term mechanisms that regulate the abundance of the aquaporin-2 water channel in renal collecting duct cells. Here, we use deep DNA sequencing in mouse collecting duct cells to ask whether vasopressin signaling selectively increases Aqp2 gene transcription or whether it triggers a broadly targeted transcriptional network. ChIP-Seq quantification of binding sites for RNA polymerase II was combined with RNA-Seq quantification of transcript abundances to identify genes whose transcription is regulated by vasopressin. (View curated dataset at https://helixweb.nih.gov/ESBL/Database/Vasopressin/). The analysis revealed only 35 vasopressin-regulated genes (of 3659) including Aqp2. Increases in RNA polymerase II binding and mRNA abundances for Aqp2 far outstripped corresponding measurements for all other genes, consistent with the conclusion that vasopressin-mediated transcriptional regulation is highly selective for Aqp2. Despite the overall selectivity of the net transcriptional response, vasopressin treatment was associated with increased RNA polymerase II binding to the promoter proximal region of a majority of expressed genes, suggesting a nearly global positive regulation of transcriptional initiation with transcriptional pausing. Thus, the overall net selectivity appears to be a result of selective control of transcriptional elongation.

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.

Remote ischemic perconditioning attenuates ischemia/reperfusion-induced downregulation of AQP2 in rat kidney

Renal ischemia/reperfusion (I/R) can lead to impaired urine concentration ability and increased fractional excretion of sodium (FeNa). Local ischemic preconditioning improves renal water and sodium handling after I/R injury. Here, we investigate whether remote ischemic perconditioning (rIPeC) prevents dysregulation of renal water and salt handling in response to I/R injury and mechanisms that may be involved. Rats were subjected to right nephrectomy and randomized into a sham group or an I/R group. In the I/R group, rats were subjected to 37 min of renal ischemia and 3 days of reperfusion. rIPeC was applied to the abdominal aorta. Blood and urine were collected on day 3 postoperatively for clearance studies. The expression of aquaporins (AQPs) and the sodium transporter Na-K-ATPase were analyzed using immunoblotting and immunohistochemistry. I/R injury resulted in polyuria, increased FeNa, and decreased urine osmolality compared to sham rats. rIPeC attenuated the increase in FeNa and the decrease in urine osmolality. Expression of AQP1, AQP2, phosphorylated AQP2 (pAQP2), and Na-K-ATPase was downregulated in I/R rats. rIPeC attenuated the reductions in AQP2 and pAQP2 expression. Immunohistochemistry revealed decreased labeling of Na-K-ATPase in the outer medulla in I/R kidneys compared to kidneys from sham and I/R + rIPeC rats. After renal ischemia, the expression of Na-K-ATPase was substantially reduced in the outer medullary thick ascending limb. In conclusion, our data suggest that rIPeC might prevent dysregulation of renal water and salt handling via regulation of AQP2 expression and phosphorylation as well as via regulation of Na-K-ATPase expression in I/R rat kidneys.

Influence of sex on aquaporin1-4 and vasopressin V2 receptor expression in the pig kidney during development

BACKGROUND

The ability of the immature kidney to concentrate urine is lower than in adults. The aquaporin (AQP) family and the vasopressin V2 receptor (V2R) play a critical role in the urinary concentrating capacity. Here we investigated a possible sex difference in AQP1, AQP2, AQP3, and AQP4 as well as V2R expression in the fetal pig kidney at different gestation stages.

METHODS

Pig fetuses were divided into three groups according to gestation age of 60, 80, and 100 d. Quantitative PCR and immunohistochemistry were used to determine the regulation of AQP1, AQP2, AQP3, and AQP4 as well as V2R in the fetal pig kidneys.

RESULTS

Renal AQP1, AQP2 and AQP3, and V2R expression was increased with gestation age in both sexes, whereas AQP4 expression was unchanged over time. We observed neither sex differences in the AQPs nor V2R expression in the fetal pig kidneys.

CONCLUSION

AQP1, AQP2, and AQP3, and V2R expression increased with gestation age in the fetal kidney, suggesting that this induction might contribute to the maturation of urinary concentrating capacity. However, no sex differences were observed indicating that sex might not play a role for the maturation of the urinary concentrating activity during kidney development in fetal pig.

Activation of the metabolic sensor AMP-activated protein kinase inhibits aquaporin-2 function in kidney principal cells

Aquaporin-2 (AQP2) is essential to maintain body water homeostasis. AQP2 traffics from intracellular vesicles to the apical membrane of kidney collecting duct principal cells in response to vasopressin [arginine vasopressin (AVP)], a hormone released with low intravascular volume, which causes decreased kidney perfusion. Decreased kidney perfusion activates AMP-activated kinase (AMPK), a metabolic sensor that inhibits the activity of several transport proteins. We hypothesized that AMPK activation also inhibits AQP2 function. These putative AMPK effects could protect interstitial ionic gradients required for urinary concentration during metabolic stress when low intravascular volume induces AVP release. Here we found that short-term AMPK activation by treatment with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; 75 min) in kidney tissue prevented baseline AQP2 apical accumulation in principal cells, but did not prevent AQP2 apical accumulation in response to the AVP analog desmopressin (dDAVP). Prolonged AMPK activation prevented AQP2 cell membrane accumulation in response to forskolin in mouse collecting duct mpkCCD_c14 cells. Moreover, AMPK inhibition accelerated hypotonic lysis of Xenopus oocytes expressing AQP2. We performed phosphorylation assays to elucidate the mechanism by which AMPK regulates AQP2. Although AMPK weakly phosphorylated immunoprecipitated AQP2 in vitro, no direct AMPK phosphorylation of the AQP2 COOH-terminus was detected by mass spectrometry. AMPK promoted Ser-261 phosphorylation and antagonized dDAVP-dependent phosphorylation of other AQP2 COOH-terminal sites in cells. Our findings suggest an increasing, time-dependent antagonism of AMPK on AQP2 regulation with AICAR-dependent inhibition of cAMP-dependent apical accumulation and AVP-dependent phosphorylation of AQP2. This inhibition likely occurs via a mechanism that does not involve direct AQP2 phosphorylation by AMPK.

4-PBA improves lithium-induced nephrogenic diabetes insipidus by attenuating ER stress

Endoplasmic reticulum (ER) stress has been implicated in some types of glomerular and tubular disorders. The objectives of this study were to elucidate the role of ER stress in lithium-induced nephrogenic diabetes insipidus (NDI) and to investigate whether attenuation of ER stress by 4-phenylbutyric acid (4-PBA) improves urinary concentrating defect in lithium-treated rats. Wistar rats received lithium (40 mmol/kg food), 4-PBA (320 mg/kg body wt by gavage every day), or no treatment (control) for 2 wk, and they were dehydrated for 24 h before euthanasia. Lithium treatment resulted in increased urine output and decreased urinary osmolality, which was significantly improved by 4-PBA. 4-PBA also prevented reduced protein expression of aquaporin-2 (AQP2), pS256-AQP2, and pS261-AQP2 in the inner medulla of kidneys from lithium-treated rats after 24-h dehydration. Lithium treatment resulted in increased expression of ER stress markers in the inner medulla, which was associated with dilated cisternae and expansion of ER in the inner medullary collecting duct (IMCD) principal cells. Confocal immunofluorescence studies showed colocalization of a molecular chaperone, binding IgG protein (BiP), with AQP2 in principal cells. Immunohistochemistry demonstrated increased intracellular expression of BiP and decreased AQP2 expression in IMCD principal cells of kidneys from lithium-treated rats. 4-PBA attenuated expression of ER stress markers and recovered ER morphology. In IMCD suspensions isolated from lithium-treated rats, 4-PBA incubation was also associated with increased AQP2 expression and ameliorated ER stress. In conclusion, in experimental lithium-induced NDI, 4-PBA improved the urinary concentrating defect and increased AQP2 expression, likely via attenuating ER stress in IMCD principal cells.

Autoantibodies Targeting a Collecting Duct-Specific Water Channel in Tubulointerstitial Nephritis

Tubulointerstitial nephritis is a common cause of kidney failure and may have diverse etiologies. This form of nephritis is sometimes associated with autoimmune disease, but the role of autoimmune mechanisms in disease development is not well understood. Here, we present the cases of three patients with autoimmune polyendocrine syndrome type 1 who developed tubulointerstitial nephritis and ESRD in association with autoantibodies against kidney collecting duct cells. One of the patients developed autoantibodies targeting the collecting duct-specific water channel aquaporin 2, whereas autoantibodies of the two other patients reacted against the HOXB7 or NFAT5 transcription factors, which regulate the aquaporin 2 promoter. Our findings suggest that tubulointerstitial nephritis developed in these patients as a result of an autoimmune insult on the kidney collecting duct cells.

Renal aquaporins and water balance disorders

Aquaporins (AQPs) are a 13 member family (AQP0-12) of proteins that act as channels, through which water and, for some family members, glycerol, urea and other small solutes can be transported. Aquaporins are highly abundant in kidney epithelial cells where they play a critical role with respect to water balance. In this review we summarize the current knowledge with respect to the localization and function of AQPs within the kidney tubule, and their role in mammalian water homeostasis and the water balance disorders. Overviews of practical aspects with regard to differential diagnosis for some of these disorders, alongside treatment strategies are also discussed.

Genetic forms of nephrogenic diabetes insipidus (NDI): Vasopressin receptor defect (X-linked) and aquaporin defect (autosomal recessive and dominant)

Nephrogenic diabetes insipidus (NDI), which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone, arginine vasopressin (AVP). Polyuria with hyposthenuria and polydipsia are the cardinal clinical manifestations of the disease. About 90% of patients with congenital NDI are males with X-linked NDI who have mutations in the vasopressin V2 receptor (AVPR2) gene encoding the vasopressin V2 receptor. In less than 10% of the families studied, congenital NDI has an autosomal recessive or autosomal dominant mode of inheritance with mutations in the aquaporin-2 (AQP2) gene. When studied in vitro, most AVPR2 and AQP2 mutations lead to proteins trapped in the endoplasmic reticulum and are unable to reach the plasma membrane. Prior knowledge of AVPR2 or AQP2 mutations in NDI families and perinatal mutation testing is of direct clinical value and can avert the physical and mental retardation associated with repeated episodes of dehydration.

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.

Detecting Aquaporin Function and Regulation

Water is the major component of cells and tissues throughout all forms of life. Fluxes of water and solutes through cell membranes and epithelia are essential for osmoregulation and energy homeostasis. Aquaporins are membrane channels expressed in almost every organism and involved in the bidirectional transfer of water and small solutes across cell membranes. Aquaporins have important biological roles and have been implicated in several pathophysiological conditions suggesting a great translational potential in aquaporin-based diagnostics and therapeutics. Detecting aquaporin function is critical for assessing regulation and screening for new activity modulators that can prompt the development of efficient medicines. Appropriate methods for functional analysis comprising suitable cell models and techniques to accurately evaluate water and solute membrane permeability are essential to validate aquaporin function and assess short-term regulation. The present review describes established assays commonly used to assess aquaporin function in cells and tissues, as well as the experimental biophysical strategies required to reveal functional regulation and identify modulators, the first step for aquaporin drug discovery.

Water deprivation up-regulates urine osmolality and renal aquaporin 2 in Mongolian gerbils (Meriones unguiculatus)

To better understand how desert rodents adapt to water scarcity, we examined urine osmolality, renal distribution and expression of aquaporins (AQPs) in Mongolian gerbils (Meriones unguiculatus) during 7 days of water deprivation (WD). Urine osmolality of the gerbils during WD averaged 7503 mOsm kg(-1). Renal distributions of AQP1, AQP2, and AQP3 were similar to that described in other rodents. After the 7 day WD, renal AQP2 was up-regulated, while resting metabolic rate and total evaporative water loss decreased by 43% and 36%, respectively. Our data demonstrated that Mongolian gerbils showed high urine concentration, renal AQPs expression and body water conservation to cope with limited water availability, which may be critical for their survival during dry seasons in cold deserts.

Inflammation and Edema in the Lung and Kidney of Hemorrhagic Shock Rats Are Alleviated by Biliary Tract External Drainage via the Heme Oxygenase-1 Pathway

The lung and kidney are two organs that are easily affected by hemorrhagic shock (HS). We investigated roles of biliary tract external drainage (BTED) in inflammation and edema of the lung and kidney in HS and its relationship with the heme oxygenase-1 (HO-1) pathway. Rat models of HS were induced by drawing blood from the femoral artery until a mean arterial pressure (MAP) of 40 ± 5 mmHg was achieved. A MAP of 40 ± 5 mmHg was maintained for 60 min. Thirty-six Sprague-Dawley rats were randomized to the following groups: sham group; HS group; HS + zinc protoporphyrin IX (ZnPP), a specific HO-1 inhibitor, group; HS + BTED group; HS + BTED + ZnPP group; and HS + BTED + bile infusion (BI) group. HO-1 levels, aquaporin-1 levels, and ratios of dry/wet in the lung and kidney increased markedly after BTED, but tumor necrosis factor-α and myeloperoxidase levels in the lung and kidney decreased significantly after BTED under HS conditions. Under the condition that HO-1 was inhibited by ZnPP, all these effects induced by BTED disappeared in the lung and kidney. These results demonstrated that inflammation and edema of the lung and kidney of HS rats are alleviated by BTED via the HO-1 pathway.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Differences in physiological traits associated with water balance among rodents, and their relationship to tolerance of habitat fragmentation

Physiological concepts and tools can help us to understand why organisms and populations respond to habitat fragmentation in the way they do, and allow us to determine the mechanisms or individual characteristics underlying this differential sensitivity. Here, we examine food intake, relative medullary thickness and distribution/expression of water channel aquaporin-1 in three species of South American rodents that have been reported to have different levels of tolerance to habitat fragmentation (Akodon montensis, Oligoryzomys nigripes, and Euryoryzomys russatus), using a classic water deprivation experiment to assess their abilities to cope with water shortage. We believe the mechanisms underlying this differential sensitivity are related to the organisms' capacities to maintain water balance, and therefore the species more tolerant to habitat fragmentation (A. montensis and O. nigripes) should have a higher capacity to maintain water balance. We found that A. montensis and O. nigripes were more tolerant to water deprivation than E. russatus, and this difference appears to be unrelated to differences in food ingestion rate. O. nigripes showed the highest values for RMT, followed by A. montensis and E. russatus. However all species showed RMT values that were 2.2% to 14.1% below the lower prediction limit when compared to other rodents through allometric relationships. Water deprivation seems to trigger changes in the distribution of aquaporin-1, mostly for O. nigripes and E. russatus, which may contribute to water balance maintenance. Our data suggest that these intrinsic physiological differences among these species could provide a mechanism for their differential tolerance of habitat fragmentation. J. Exp. Zool. 323A: 731-744, 2015. © 2015 Wiley Periodicals, Inc.

Vasopressin receptors V1a and V2 are not osmosensors

Herein, we investigated whether G protein-coupled signaling via the vasopressin receptors of the V1_a and V2 subtypes (V1_aR and V2R) could be obtained as a direct response to hyperosmolar challenges and/or whether hyperosmolar challenges could augment classical vasopressin-dependent V1_aR signaling. The V1_aR-dependent response was monitored indirectly via its effects on aquaporin 4 (AQP4) when heterologously expressed in Xenopus oocytes and V1_aR and V2R function was directly monitored following heterologous expression in COS-7 cells. A tendency toward an osmotically induced, V1_aR-mediated reduction in AQP4-dependent water permeability was observed, although osmotic challenges failed to mimic vasopressin-dependent V1_aR-mediated internalization of AQP4. Direct monitoring of inositol phosphate (IP) production of V1_aR-expressing COS-7 cells demonstrated an efficient vasopressin-dependent response that was, however, independent of hyperosmotic challenges. Similarly, the cAMP production by the V2R was unaffected by hyperosmotic challenges although, in contrast to the V1_aR, the V2R displayed an ability to support alternative signaling (IP production) at higher concentration of vasopressin. V1_aR and V2R respond directly to vasopressin exposure, but they do not have an ability to act as osmo- or volume sensors when exposed to an osmotic gradient in the absence or presence of vasopressin.

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.

Polyphenols as Modulators of Aquaporin Family in Health and Disease

Polyphenols are bioactive molecules widely distributed in fruits, vegetables, cereals, and beverages. Polyphenols in food sources are extensively studied for their role in the maintenance of human health and in the protection against development of chronic/degenerative diseases. Polyphenols act mainly as antioxidant molecules, protecting cell constituents against oxidative damage. The enormous number of polyphenolic compounds leads to huge different mechanisms of action not fully understood. Recently, some evidence is emerging about the role of polyphenols, such as curcumin, pinocembrin, resveratrol, and quercetin, in modulating the activity of some aquaporin (AQP) isoforms. AQPs are integral, small hydrophobic water channel proteins, extensively expressed in many organs and tissues, whose major function is to facilitate the transport of water or glycerol over cell plasma membranes. Here we summarize AQP physiological functions and report emerging evidence on the implication of these proteins in a number of pathophysiological processes. In particular, this review offers an overview about the role of AQPs in brain, eye, skin diseases, and metabolic syndrome, focusing on the ability of polyphenols to modulate AQP expression. This original analysis can contribute to elucidating some peculiar effects exerted by polyphenols and can lead to the development of an innovative potential preventive/therapeutic strategy.

Molecular machinery for vasotocin-dependent transepithelial water movement in amphibians: aquaporins and evolution

Amphibians represent the first vertebrates to adapt to terrestrial environments, and are successfully distributed around the world. The ventral skin, kidney, and urinary bladder are important osmoregulatory organs for adult anuran amphibians. Water channel proteins, called aquaporins (AQPs), play key roles in transepithelial water absorption/reabsorption in these organs. At least 43 types of AQPs were identified in anurans; a recent phylogenetic analysis categorized anuran AQPs among 16 classes (AQP0-14, 16). Anuran-specific AQPa2 was assigned to AQP6, then was further subdivided into the ventral skin-type (AQP6vs; AQPa2S), whose expression is confined to the ventral skin, and the urinary bladder-type (AQP6ub; AQPa2U), which is basically expressed in the urinary bladder. For the osmoregulatory organs, AQP3 is constitutively located in the basolateral plasma membrane of tight-junctioned epithelial cells. AQP6vs, AQP2 and/or AQP6ub are also expressed in these epithelial cells and are translocated to the apical membrane in response to arginine vasotocin, thereby regulating water absorption/reabsorption. It was suggested recently that two subtypes of AQP6vs contribute to cutaneous water absorption in Ranid species. In addition, AQP5 (AQP5a) and AQP5L (AQP5b) were identified from Xenopus tropicalis Gray, 1864, and AQP5 was localized to the apical membrane of luminal epithelial cells of the urinary bladder in dehydrated Xenopus. This finding suggested that AQP5 may be involved in water reabsorption from this organ under dehydration. Based on the hitherto reported information, we propose models for the evolution of water-absorbing/reabsorbing mechanisms in anuran osmoregulatory organs in association with AQPs.

Water transport and functional dynamics of aquaporins in osmoregulatory organs of fishes

Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.

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.

Thiazolidinediones and Edema: Recent Advances in the Pathogenesis of Thiazolidinediones-Induced Renal Sodium Retention

Thiazolidinediones (TZDs) are one of the major classes of antidiabetic drugs that are used widely. TZDs improve insulin resistance by activating peroxisome proliferator-activated receptor gamma (PPARγ) and ameliorate diabetic and other nephropathies, at least, in experimental animals. However, TZDs have side effects, such as edema, congestive heart failure, and bone fracture, and may increase bladder cancer risk. Edema and heart failure, which both probably originate from renal sodium retention, are of great importance because these side effects make it difficult to continue the use of TZDs. However, the pathogenesis of edema remains a matter of controversy. Initially, upregulation of the epithelial sodium channel (ENaC) in the collecting ducts by TZDs was thought to be the primary cause of edema. However, the results of other studies do not support this view. Recent data suggest the involvement of transporters in the proximal tubule, such as sodium-bicarbonate cotransporter and sodium-proton exchanger. Other studies have suggested that sodium-potassium-chloride cotransporter 2 in the thick ascending limb of Henle and aquaporins are also possible targets for TZDs. This paper will discuss the recent advances in the pathogenesis of TZD-induced sodium reabsorption in the renal tubules and edema.

Extracellular pH affects phosphorylation and intracellular trafficking of AQP2 in inner medullary collecting duct cells

Kidney collecting duct cells are continuously exposed to the changes of extracellular pH (pHe). We aimed to study the effects of altered pHe on desmopressin (dDAVP)-induced phosphorylation (Ser256, Ser261, Ser264, and Ser269) and apical targeting of aquaporin-2 (AQP2) in rat kidney inner medullary collecting duct (IMCD) cells. When freshly prepared IMCD tubule suspensions exposed to HEPES buffer with pH 5.4, 6.4, 7.4, or 8.4 for 1 h were stimulated with dDAVP (10−10 M, 3 min), AQP2 phosphorylation at Ser256, Ser264, and Ser269 was significantly attenuated under acidic conditions. Next, IMCD cells primary cultured in transwell chambers were exposed to a transepithelial pH gradient for 1 h (apical pH 6.4, 7.4, or 8.4 vs. basolateral pH 7.4 and vice versa). Immunocytochemistry and cell surface biotinylation assay revealed that exposure to either apical pH 6.4 or basolateral pH 6.4 for 1 h was associated with decreased dDAVP (10−9 M, 15 min, basolateral)-induced apical targeting of AQP2 and surface expression of AQP2. Fluorescence resonance energy transfer analysis revealed that the dDAVP (10−9 M)-induced increase of PKA activity was significantly attenuated when LLC-PK1 cells were exposed to pHe 6.4 compared with pHe 7.4 and 8.4. In contrast, forskolin (10−7 M)-induced PKA activation and dDAVP (10−9 M)-induced increases of intracellular Ca2+ were not affected. Taken together, dDAVP-induced phosphorylation and apical targeting of AQP2 are attenuated in IMCD cells under acidic pHe, likely via an inhibition of vasopressin V2 receptor-G protein-cAMP-PKA actions.

Aquaporins as diagnostic and therapeutic targets in cancer: how far we are?

Aquaporins (AQPs) are a family of water channel proteins distributed in various human tissues, responsible for the transport of small solutes such as glycerol, even gas and ions. The expression of AQPs has been found in more than 20 human cancer types and is significantly correlated with the severity of histological tumors and prognosis of patients with cancer. More recent evidence showed that AQPs could also play a role in tumor-associated edema, tumor cell proliferation and migration, and tumor angiogenesis in solid and hematological tumors. Inhibitors of AQPs in tumor cells and microvessels have been suggested as new therapeutic strategies. The present review overviews AQPs structures, expression variation among normal tissues and tumors, AQPs functions and roles in the development of cancer with special focuses on lung, colorectal, liver, brain and breast cancers, and potential AQPs-target inhibitors. We call the special attention to consider AQPs important as diagnostic and therapeutic biomarkers. It may be a novel anticancer therapy by the AQPs inhibition.

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.