Genetic ablation of aquaporin-2 in the mouse connecting tubules results in defective renal water handling

Cannabinoid receptor type 1 activation causes a water diuresis by inducing an acute central diabetes insipidus in mice

Cannabis and synthetic cannabinoid consumption are increasing worldwide. Cannabis contains numerous phytocannabinoids that act on the G protein-coupled cannabinoid receptor type 1 (CB1R) and cannabinoid receptor type 2 expressed throughout the body, including the kidney. Essentially every organ, including the kidney, produces endocannabinoids, which are endogenous ligands to these receptors. Cannabinoids acutely increase urine output in rodents and humans, thus potentially influencing total body water and electrolyte homeostasis. As the kidney collecting duct (CD) regulates total body water, acid/base, and electrolyte balance through specific functions of principal cells (PCs) and intercalated cells (ICs), we examined the cell-specific immunolocalization of CB1R in the mouse CD. Antibodies against either the C-terminus or N-terminus of CB1R consistently labeled aquaporin 2 (AQP2)-negative cells in the cortical and medullary CD and thus presumably ICs. Given the well-established role of ICs in urinary acidification, we used a clearance approach in mice that were acid loaded with 280 mM NH4Cl for 7 days and nonacid-loaded mice treated with the cannabinoid receptor agonist WIN55,212-2 (WIN) or a vehicle control. Although WIN had no effect on urinary acidification, these WIN-treated mice had less apical + subapical AQP2 expression in PCs compared with controls and developed acute diabetes insipidus associated with the excretion of large volumes of dilute urine. Mice maximally concentrated their urine when WIN and 1-desamino-8-d-arginine vasopressin [desmopressin (DDAVP)] were coadministered, consistent with central rather than nephrogenic diabetes insipidus. Although ICs express CB1R, the physiological role of CB1R in this cell type remains to be determined.NEW & NOTEWORTHY The CB1R agonist WIN55,212-2 induces central diabetes insipidus in mice. This research integrates existing knowledge regarding the diuretic effects of cannabinoids and the influence of CB1R on vasopressin secretion while adding new mechanistic insights about total body water homeostasis. Our findings provide a deeper understanding about the potential clinical impact of cannabinoids on human physiology and may help identify targets for novel therapeutics to treat water and electrolyte disorders such as hyponatremia and volume overload.

Aquaporins alteration revealed kidney damages in cerebral ischemia/reperfusion rats

Background

Restoration of blood supply is a desired goal for the treatment of acute ischemic stroke. However, the restoration often leads to cerebral ischemia-reperfusion injury (CIR/I), which greatly increases the risk of non-neural organ damage. In particular, the acute kidney injury might be one of the most common complications.

Aims

The study aimed to understand the damage occurred and the potential molecular mechanisms.

Methods

The study was explored on the CIR/I rats generated by performing middle cerebral artery occlusion/reperfusion (MCAO/Reperfusion). The rats were evaluated with injury on the brains, followed by the non-neural organs including kidneys, livers, colons and stomachs. They were examined further with histopathological changes, and gene expression alterations by using RT-qPCR of ten aquaporins (Aqps) subtypes including Aqp1～Aqp9 and Aqp11. Furthermore, the Aqps expression profiles were constructed for each organ and analyzed by performing Principle Component Analysis. In addition, immunohistochemistry was explored to look at the protein expression of Aqp1, Aqp2, Aqp3 and Aqp4 in the rat kidneys.

Results

There was a prominent down-regulation profile in the MCAO/Reperfusion rat kidneys. The protein expression of Aqp1, Aqp2, Aqp3 and Aqp4 was decreased in the kidneys of the MCAO/Reperfusion rats. We suggested that the kidney was in the highest risk to be damaged following the CIR/I. Down-regulation of Aqp2, Aqp3 and Aqp4 was involved in the acute kidney injury induced by the CIR/I.

Renal water transport in health and disease

Saving body water by optimal reabsorption of water filtered by the kidney leading to excretion of urine with concentrations of solutes largely above that of plasma allowed vertebrate species to leave the aquatic environment to live on solid ground. Filtered water is reabsorbed for 70% and 20% by proximal tubules and thin descending limbs of Henle, respectively. These two nephron segments express the water channel aquaporin-1 located along both apical and basolateral membranes. In the proximal tubule, the paracellular pathway accounts for at least 30% of water reabsorption, and the tight-junction core protein claudin-2 plays a key role in this permeability. The ascending limb of Henle and the distal convoluted tubule are impermeant to water and are responsible for urine dilution. The water balance is adjusted along the collecting system, i.e. connecting tubule and the collecting duct, under the control of arginine-vasopressin (AVP). AVP is synthesized by the hypothalamus and released in response to an increase in extracellular osmolality or stimulation of baroreceptors by decreased blood pressure. In response to AVP, aquaporin-2 water channels stored in subapical intracellular vesicles are translocated to the apical plasma membrane and raise the water permeability of the collecting system. The basolateral step of water reabsorption is mediated by aquaporin-3 and -4, which are constitutively expressed. Drugs targeting water transport include classical diuretics, which primarily inhibit sodium transport; the new class of SGLT2 inhibitors, which promotes osmotic diuresis and the non-peptidic antagonists of the V2 receptor, which are pure aquaretic drugs. Disturbed water balance includes diabetes insipidus and hyponatremias. Diabetes insipidus is characterized by polyuria and polydipsia. It is either related to a deficit in AVP secretion called central diabetes insipidus that can be treated by AVP analogs or to a peripheral defect in AVP response called nephrogenic diabetes insipidus. Diabetes insipidus can be either of genetic origin or acquired. Hyponatremia is a common disorder most often related to free water excess relying on overstimulated or inappropriate AVP secretion. The assessment of blood volume is key for the diagnosis and treatment of hyponatremia, which can be classified as hypo-, eu-, or hypervolemic.

Organ Specific Differences in Alteration of Aquaporin Expression in Rats Treated with Sennoside A, Senna Anthraquinones and Rhubarb Anthraquinones

Senna and rhubarb are often used as routine laxatives, but there are differences in mechanism of action and potential side effects. Here, we studied metabolites of senna anthraquinones (SAQ), rhubarb anthraquinones (RAQ) and their chemical marker, sennoside A (SA), in a rat diarrhea model. In in vitro biotransformation experiments, SAQ, RAQ and SA were incubated with rat fecal flora solution and the metabolites produced were analyzed using HPLC. In in vivo studies, the same compounds were investigated for purgation induction, with measurement of histopathology and Aqps gene expression in six organs. The results indicated that SAQ and RAQ had similar principal constituents but could be degraded into different metabolites. A similar profile of Aqps down-regulation for all compounds was seen in the colon, suggesting a similar mechanism of action for purgation. However, in the kidneys and livers of the diarrhea-rats, down-regulation of Aqps was found in the RAQ-rats whereas up-regulation of Aqps was seen in the SAQ-rats. Furthermore, the RAQ-rats showed lower Aqp2 protein expression in the kidneys, whilst the SA-rats and SAQ-rats had higher Aqp2 protein expression in the kidneys. This may have implications for side effects of SAQ or RAQ in patients with chronic kidney or liver diseases.

Aquaporin regulation in metabolic organs

Aquaporins (AQPs) are a family of 13 small trans-membrane proteins, which facilitate shuttling of glycerol, water and urea. The peculiar role of AQPs in glycerol transport makes them attractive targets in metabolic organs since glycerol represents the backbone of triglyceride synthesis. Importantly, AQPs are known to be regulated by various nuclear receptors which in turn govern lipid and glucose metabolism as well as inflammatory cascades. Here, we review the role of AQPs regulation in metabolic organs exploring their physiological impact in health and disease.

Two Mineralocorticoid Receptor-Mediated Mechanisms of Pendrin Activation in Distal Nephrons

BACKGROUND

Regulation of sodium chloride transport in the aldosterone-sensitive distal nephron is essential for fluid homeostasis and BP control. The chloride-bicarbonate exchanger pendrin in β-intercalated cells, along with sodium chloride cotransporter (NCC) in distal convoluted tubules, complementarily regulate sodium chloride handling, which is controlled by the renin-angiotensin-aldosterone system.

METHODS

Using mice with mineralocorticoid receptor deletion in intercalated cells, we examined the mechanism and roles of pendrin upregulation via mineralocorticoid receptor in two different models of renin-angiotensin-aldosterone system activation. We also used aldosterone-treated NCC knockout mice to examine the role of pendrin regulation in salt-sensitive hypertension.

RESULTS

Deletion of mineralocorticoid receptor in intercalated cells suppressed the increase in renal pendrin expression induced by either exogenous angiotensin II infusion or endogenous angiotensin II upregulation via salt restriction. When fed a low-salt diet, intercalated cell-specific mineralocorticoid receptor knockout mice with suppression of pendrin upregulation showed BP reduction that was attenuated by compensatory activation of NCC. In contrast, upregulation of pendrin induced by aldosterone excess combined with a high-salt diet was scarcely affected by deletion of mineralocorticoid receptor in intercalated cells, but depended instead on hypokalemic alkalosis through the activated mineralocorticoid receptor-epithelial sodium channel cascade in principal cells. In aldosterone-treated NCC knockout mice showing upregulation of pendrin, potassium supplementation corrected alkalosis and inhibited the pendrin upregulation, thereby lowering BP.

CONCLUSIONS

In conjunction with NCC, the two pathways of pendrin upregulation, induced by angiotensin II through mineralocorticoid receptor activation in intercalated cells and by alkalosis through mineralocorticoid receptor activation in principal cells, play important roles in fluid homeostasis during salt depletion and salt-sensitive hypertension mediated by aldosterone excess.

Dietary sodium modulates nephropathy in Nedd4-2-deficient mice

Salt homeostasis is maintained by tight control of Na⁺ filtration and reabsorption. In the distal part of the nephron the ubiquitin protein ligase Nedd4-2 regulates membrane abundance and thus activity of the epithelial Na⁺ channel (ENaC), which is rate-limiting for Na⁺ reabsorption. Nedd4-2 deficiency in mouse results in elevated ENaC and nephropathy, however the contribution of dietary salt to this has not been characterized. In this study we show that high dietary Na⁺ exacerbated kidney injury in Nedd4-2-deficient mice, significantly perturbing normal postnatal nephrogenesis and resulting in multifocal areas of renal dysplasia, increased markers of kidney injury and a decline in renal function. In control mice, high dietary Na⁺ resulted in reduced levels of ENaC. However, Nedd4-2-deficient kidneys maintained elevated ENaC even after high dietary Na⁺, suggesting that the inability to efficiently downregulate ENaC is responsible for the salt-sensitivity of disease. Importantly, low dietary Na⁺ significantly ameliorated nephropathy in Nedd4-2-deficient mice. Our results demonstrate that due to dysregulation of ENaC, kidney injury in Nedd4-2-deficient mice is sensitive to dietary Na⁺, which may have implications in the management of disease in patients with kidney disease.

Zika Virus Infection Induces Acute Kidney Injury Through Activating NLRP3 Inflammasome Via Suppressing Bcl-2

Zika virus (ZIKV) is a newly emerging flavivirus that broadly exhibits in various bodily tissues and fluids, especially in the brain, and ZIKV infection often causes microcephaly. Previous studies have been reported that ZIKV can infect renal cells and can be detected in the urine samples of infected individuals. However, whether ZIKV infection causes renal diseases and its pathogenic mechanisms remains unknown. Here, we identified that ZIKV infection resulted in acute kidney injury (AKI) in both newborn and adult mouse models by increasing the levels of AKI-related biomarkers [e.g., serum creatinine (Scr), kidney injury molecular-1 (Kim-1), and neutrophil gelatinase-associated lipocalin (NGAL)]. ZIKV infection triggered the inflammatory response and renal cell injury by activating Nod-like receptor 3 (NLRP3) inflammasome and secreting interleukin-1β (IL-1β). IL-1β inhibited aquaporins expression and led to water re-absorption disorder. Furthermore, ZIKV infection induced a decreased expression of B-cell lymphoma-2 (Bcl-2) in the kidney. Overexpression of Bcl-2 attenuated ZIKV-induced NLRP3 inflammasome activation in renal cells and down-regulated PARP/caspase-3-mediated renal apoptosis. Overall, our findings demonstrated that ZIKV infection induced AKI by activating NLRP3 inflammasome and apoptosis through suppressing Bcl-2 expression, which provided potential therapeutic targets for ZIKV-associated renal diseases.

Generation of Human PSC-Derived Kidney Organoids with Patterned Nephron Segments and a De Novo Vascular Network

Human pluripotent stem cell-derived kidney organoids recapitulate developmental processes and tissue architecture, but intrinsic limitations, such as lack of vasculature and functionality, have greatly hampered their application. Here we establish a versatile protocol for generating vascularized three-dimensional (3D) kidney organoids. We employ dynamic modulation of WNT signaling to control the relative proportion of proximal versus distal nephron segments, producing a correlative level of vascular endothelial growth factor A (VEGFA) to define a resident vascular network. Single-cell RNA sequencing identifies a subset of nephron progenitor cells as a potential source of renal vasculature. These kidney organoids undergo further structural and functional maturation upon implantation. Using this kidney organoid platform, we establish an in vitro model of autosomal recessive polycystic kidney disease (ARPKD), the cystic phenotype of which can be effectively prevented by gene correction or drug treatment. Our studies provide new avenues for studying human kidney development, modeling disease pathogenesis, and performing patient-specific drug validation.

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

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

Lack of urea transporters, UT-A1 and UT-A3, increases nitric oxide accumulation to dampen medullary sodium reabsorption through ENaC

Although the role of urea in urine concentration is known, the effect of urea handling by the urea transporters (UTs), UT-A1 and UT-A3, on sodium balance remains elusive. Serum and urinary sodium concentration is similar between wild-type mice (WT) and UT-A3 null (UT-A3 KO) mice; however, mice lacking both UT-A1 and UT-A3 (UT-A1/A3 KO) have significantly lower serum sodium and higher urinary sodium. Protein expression of renal sodium transporters is unchanged among all three genotypes. WT, UT-A3 KO, and UT-A1/A3 KO acutely respond to hydrochlorothiazide and furosemide; however, UT-A1/A3 KO fail to show a diuretic or natriuretic response following amiloride administration, indicating that baseline epithelial Na+ channel (ENaC) activity is impaired. UT-A1/A3 KO have more ENaC at the apical membrane than WT mice, and single-channel analysis of ENaC in split-open inner medullary collecting duct (IMCD) isolated in saline shows that ENaC channel density and open probability is higher in UT-A1/A3 KO than WT. UT-A1/A3 KO excrete more urinary nitric oxide (NO), a paracrine inhibitor of ENaC, and inner medullary nitric oxide synthase 1 mRNA expression is ~40-fold higher than WT. Because endogenous NO is unstable, ENaC activity was reassessed in split-open IMCD with the NO donor PAPA NONOate [1-propanamine-3-(2-hydroxy-2-nitroso-1-propylhydrazine)], and ENaC activity was almost abolished in UT-A1/A3 KO. In summary, loss of both UT-A1 and UT-A3 (but not UT-A3 alone) causes elevated medullary NO production and salt wasting. NO inhibition of ENaC, despite elevated apical accumulation of ENaC in UT-A1/A3 KO IMCD, appears to be the main contributor to natriuresis in UT-A1/A3 KO mice.

Progranulin Deficient Mice Develop Nephrogenic Diabetes Insipidus

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

H+-ATPase B1 subunit localizes to thick ascending limb and distal convoluted tubule of rodent and human kidney

The vacuolar-type H⁺-ATPase B1 subunit is heavily expressed in the intercalated cells of the collecting system, where it contributes to H⁺ transport, but has also been described in other segments of the renal tubule. This study aimed to determine the localization of the B1 subunit of the vacuolar-type H⁺-ATPase in the early distal nephron, encompassing thick ascending limbs (TAL) and distal convoluted tubules (DCT), in human kidney and determine whether the localization differs between rodents and humans. Antibodies directed against the H⁺-ATPase B1 subunit were used to determine its localization in paraffin-embedded formalin-fixed mouse, rat, and human kidneys by light microscopy and in sections of Lowicryl-embedded rat kidneys by electron microscopy. Abundant H⁺-ATPase B1 subunit immunoreactivity was observed in the human kidney. As expected, intercalated cells showed the strongest signal, but significant signal was also observed in apical membrane domains of the distal nephron, including TAL, macula densa, and DCT. In mouse and rat, H⁺-ATPase B1 subunit expression could also be detected in apical membrane domains of these segments. In rat, electron microscopy revealed that the H⁺-ATPase B1 subunit was located in the apical membrane. Furthermore, the H⁺-ATPase B1 subunit colocalized with other H⁺-ATPase subunits in the TAL and DCT. In conclusion, the B1 subunit is expressed in the early distal nephron. The physiological importance of H⁺-ATPase expression in these segments remains to be delineated in detail. The phenotype of disease-causing mutations in the B1 subunit may also relate to its presence in the TAL and DCT.

Deficiency of Carbonic Anhydrase II Results in a Urinary Concentrating Defect

Carbonic anhydrase II (CAII) is expressed along the nephron where it interacts with a number of transport proteins augmenting their activity. Aquaporin-1 (AQP1) interacts with CAII to increase water flux through the water channel. Both CAII and aquaporin-1 are expressed in the thin descending limb (TDL); however, the physiological role of a CAII-AQP1 interaction in this nephron segment is not known. To determine if CAII was required for urinary concentration, we studied water handling in CAII-deficient mice. CAII-deficient mice demonstrate polyuria and polydipsia as well as an alkaline urine and bicarbonaturia, consistent with a type III renal tubular acidosis. Natriuresis and hypercalciuria cause polyuria, however, CAII-deficient mice did not have increased urinary sodium nor calcium excretion. Further examination revealed dilute urine in the CAII-deficient mice. Urinary concentration remained reduced in CAII-deficient mice relative to wild-type animals even after water deprivation. The renal expression and localization by light microscopy of NKCC2 and aquaporin-2 was not altered. However, CAII-deficient mice had increased renal AQP1 expression. CAII associates with and increases water flux through aquaporin-1. Water flux through aquaporin-1 in the TDL of the loop of Henle is essential to the concentration of urine, as this is required to generate a concentrated medullary interstitium. We therefore measured cortical and medullary interstitial concentration in wild-type and CAII-deficient mice. Mice lacking CAII had equivalent cortical interstitial osmolarity to wild-type mice: however, they had reduced medullary interstitial osmolarity. We propose therefore that reduced water flux through aquaporin-1 in the TDL in the absence of CAII prevents the generation of a maximally concentrated medullary interstitium. This, in turn, limits urinary concentration in CAII deficient mice.

Hypercalcemia induces targeted autophagic degradation of aquaporin-2 at the onset of nephrogenic diabetes insipidus

Hypercalcemia can cause renal dysfunction such as nephrogenic diabetes insipidus (NDI), but the mechanisms underlying hypercalcemia-induced NDI are not well understood. To elucidate the early molecular changes responsible for this disorder, we employed mass spectrometry-based proteomic analysis of inner medullary collecting ducts (IMCD) isolated from parathyroid hormone-treated rats at onset of hypercalcemia-induced NDI. Forty-one proteins, including the water channel aquaporin-2, exhibited significant changes in abundance, most of which were decreased. Bioinformatic analysis revealed that many of the downregulated proteins were associated with cytoskeletal protein binding, regulation of actin filament polymerization, and cell-cell junctions. Targeted LC-MS/MS and immunoblot studies confirmed the downregulation of 16 proteins identified in the initial proteomic analysis and in additional experiments using a vitamin D treatment model of hypercalcemia-induced NDI. Evaluation of transcript levels and estimated half-life of the downregulated proteins suggested enhanced protein degradation as the possible regulatory mechanism. Electron microscopy showed defective intercellular junctions and autophagy in the IMCD cells from both vitamin D- and parathyroid hormone-treated rats. A significant increase in the number of autophagosomes was confirmed by immunofluorescence labeling of LC3. Colocalization of LC3 and Lamp1 with aquaporin-2 and other downregulated proteins was found in both models. Immunogold electron microscopy revealed aquaporin-2 in autophagosomes in IMCD cells from both hypercalcemia models. Finally, parathyroid hormone withdrawal reversed the NDI phenotype, accompanied by termination of aquaporin-2 autophagic degradation and normalization of both nonphoshorylated and S256-phosphorylated aquaporin-2 levels. Thus, enhanced autophagic degradation of proteins plays an important role in the initial mechanism of hypercalcemic-induced NDI.

Role of adenylyl cyclase 6 in the development of lithium-induced nephrogenic diabetes insipidus

Psychiatric patients treated with lithium (Li⁺) may develop nephrogenic diabetes insipidus (NDI). Although the etiology of Li⁺-induced NDI (Li-NDI) is poorly understood, it occurs partially due to reduced aquaporin-2 (AQP2) expression in the kidney collecting ducts. A mechanism postulated for this is that Li⁺ inhibits adenylyl cyclase (AC) activity, leading to decreased cAMP, reduced AQP2 abundance, and less membrane targeting. We hypothesized that Li-NDI would not develop in mice lacking AC6. Whole-body AC6 knockout (AC6^-/-) mice and potentially novel connecting tubule/principal cell-specific AC6 knockout (AC6^loxloxCre) mice had approximately 50% lower urine osmolality and doubled water intake under baseline conditions compared with controls. Dietary Li⁺ administration increased water intake and reduced urine osmolality in control, AC6^-/-, and AC6^loxloxCre mice. Consistent with AC6^-/- mice, medullary AQP2 and pS256-AQP2 abundances were lower in AC6^loxloxCre mice compared with controls under standard conditions, and levels were further reduced after Li⁺ administration. AC6^loxloxCre and control mice had a similar increase in the numbers of proliferating cell nuclear antigen-positive cells in response to Li⁺. However, AC6^loxloxCre mice had a higher number of H⁺-ATPase B1 subunit-positive cells under standard conditions and after Li⁺ administration. Collectively, AC6 has a minor role in Li-NDI development but may be important for determining the intercalated cell-to-principal cell ratio.

Renal tubular NHE3 is required in the maintenance of water and sodium chloride homeostasis

The sodium/proton exchanger isoform 3 (NHE3) is expressed in the intestine and the kidney, where it facilitates sodium (re)absorption and proton secretion. The importance of NHE3 in the kidney for sodium chloride homeostasis, relative to the intestine, is unknown. Constitutive tubule-specific NHE3 knockout mice (NHE3^loxloxCre) did not show significant differences compared to control mice in body weight, blood pH or bicarbonate and plasma sodium, potassium, or aldosterone levels. Fluid intake, urinary flow rate, urinary sodium/creatinine, and pH were significantly elevated in NHE3^loxloxCre mice, while urine osmolality and GFR were significantly lower. Water deprivation revealed a small urinary concentrating defect in NHE3^loxloxCre mice on a control diet, exaggerated on low sodium chloride. Ten days of low or high sodium chloride diet did not affect plasma sodium in control mice; however, NHE3^loxloxCre mice were susceptible to low sodium chloride (about -4 mM) or high sodium chloride intake (about +2 mM) versus baseline, effects without differences in plasma aldosterone between groups. Blood pressure was significantly lower in NHE3^loxloxCre mice and was sodium chloride sensitive. In control mice, the expression of the sodium/phosphate co-transporter Npt2c was sodium chloride sensitive. However, lack of tubular NHE3 blunted Npt2c expression. Alterations in the abundances of sodium/chloride cotransporter and its phosphorylation at threonine 58 as well as the abundances of the α-subunit of the epithelial sodium channel, and its cleaved form, were also apparent in NHE3^loxloxCre mice. Thus, renal NHE3 is required to maintain blood pressure and steady-state plasma sodium levels when dietary sodium chloride intake is modified.

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.

Nuclear Receptor Regulation of Aquaporin-2 in the Kidney

Aquaporin-2 (AQP2) is a vasopressin-regulated water channel responsible for regulating water reabsorption through the apical plasma membrane of the principal cells of renal collecting ducts. It has been found that dysregulation and dysfunction of AQP2 cause many disorders related to water balance in people and animals, including polyuria and dilutional hyponatremia. Classically, AQP2 mRNA and protein expression and its membrane translocation are regulated by systemic vasopressin involving short-term regulation of AQP2 trafficking to and from the apical plasma membrane and long-term regulation of the total amount of the AQP2 protein in the cell. Recently, increasing evidence has demonstrated that collecting duct AQP2 expression and membrane translocation are also under the control of many other local factors, especially nuclear receptors. Here, we briefly review the progress of studies in this area and discuss the role of nuclear receptors in the regulation of water reabsorption via affecting AQP2 expression and function.

Reducing αENaC expression in the kidney connecting tubule induces pseudohypoaldosteronism type 1 symptoms during K+ loading

Genetic inactivation of the epithelial Na+ channel α-subunit (αENaC) in the renal collecting duct (CD) does not interfere with Na+ and K+ homeostasis in mice. However, inactivation in the CD and a part of the connecting tubule (CNT) induces autosomal recessive pseudohypoaldosteronism type 1 (PHA-1) symptoms in subjects already on a standard diet. In the present study, we further examined the importance of αENaC in the CNT. Knockout mice with αENaC deleted primarily in a part of the CNT (CNT-KO) were generated using Scnn1alox/lox mice and Atp6v1b1:: Cre mice. With a standard diet, plasma Na+ concentration ([Na+]) and [K+], and urine Na+ and K+ output were unaffected. Seven days of Na+ restriction (0.01% Na+) led to a higher urine Na+ output only on days 3–5, and after 7 days plasma [Na+] and [K+] were unaffected. In contrast, the CNT-KO mice were highly susceptible to a 2-day 5% K+ diet and showed lower food intake and relative body weight, lower plasma [Na+], higher fractional excretion (FE) of Na+, higher plasma [K+], and lower FE of K+. The higher FE of Na+ coincided with lower abundance and phosphorylation of the Na+-Cl− cotransporter. In conclusion, reducing ENaC expression in the CNT induces clear PHA-1 symptoms during high dietary K+ loading.

Autophagic degradation of aquaporin-2 is an early event in hypokalemia-induced nephrogenic diabetes insipidus

Hypokalemia (low serum potassium level) is a common electrolyte imbalance that can cause a defect in urinary concentrating ability, i.e., nephrogenic diabetes insipidus (NDI), but the molecular mechanism is unknown. We employed proteomic analysis of inner medullary collecting ducts (IMCD) from rats fed with a potassium-free diet for 1 day. IMCD protein quantification was performed by mass spectrometry using a label-free methodology. A total of 131 proteins, including the water channel AQP2, exhibited significant changes in abundance, most of which were decreased. Bioinformatic analysis revealed that many of the down-regulated proteins were associated with the biological processes of generation of precursor metabolites and energy, actin cytoskeleton organization, and cell-cell adhesion. Targeted LC-MS/MS and immunoblotting studies further confirmed the down regulation of 18 selected proteins. Electron microscopy showed autophagosomes/autophagolysosomes in the IMCD cells of rats deprived of potassium for only 1 day. An increased number of autophagosomes was also confirmed by immunofluorescence, demonstrating co-localization of LC3 and Lamp1 with AQP2 and several other down-regulated proteins in IMCD cells. AQP2 was also detected in autophagosomes in IMCD cells of potassium-deprived rats by immunogold electron microscopy. Thus, enhanced autophagic degradation of proteins, most notably including AQP2, is an early event in hypokalemia-induced NDI.

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.

Rab-GAP TBC1D4 (AS160) is dispensable for the renal control of sodium and water homeostasis but regulates GLUT4 in mouse kidney

The Rab GTPase-activating protein TBC1D4 (AS160) controls trafficking of the glucose transporter GLUT4 in adipocytes and skeletal muscle cells. TBC1D4 is also highly abundant in the renal distal tubule, although its role in this tubule is so far unknown. In vitro studies suggest that it is involved in the regulation of renal transporters and channels such as the epithelial sodium channel (ENaC), aquaporin-2 (AQP2), and the Na+-K+-ATPase. To assess the physiological role of TBC1D4 in the kidney, wild-type (TBC1D4+/+) and TBC1D4-deficient (TBC1D4-/-) mice were studied. Unexpectedly, neither under standard nor under challenging conditions (low Na+/high K+, water restriction) did TBC1D4-/- mice show any difference in urinary Na+ and K+ excretion, urine osmolarity, plasma ion and aldosterone levels, and blood pressure compared with TBC1D4+/+ mice. Also, immunoblotting did not reveal any change in the abundance of major renal sodium- and water-transporting proteins [Na-K-2Cl cotransporter (NKCC2) NKCC2, NaCl cotransporter (NCC), ENaC, AQP2, and the Na+-K+-ATPase]. However, the abundance of GLUT4, which colocalizes with TBC1D4 along the distal nephron of TBC1D4+/+ mice, was lower in whole kidney lysates of TBC1D4-/- mice than in TBC1D4+/+ mice. Likewise, primary thick ascending limb (TAL) cells isolated from TBC1D4-/- mice showed an increased basal glucose uptake and an abrogated insulin response compared with TAL cells from TBC1D4+/+ mice. Thus, TBC1D4 is dispensable for the regulation of renal Na+ and water transport, but may play a role for GLUT4-mediated basolateral glucose uptake in distal tubules. The latter may contribute to the known anaerobic glycolytic capacity of distal tubules during renal ischemia.

Urinary concentration: different ways to open and close the tap

Nephrogenic diabetes insipidus (NDI) provides an excellent model for the benefits and insights that can be gained from studying rare diseases. The discovery of underlying genes identified key molecules involved in urinary concentration, including the type 2 vasopressin receptor AVPR2 and the water channel AQP2, which constitute obvious pharmacologic targets. Subsequently developed drugs targeting AVPR2 not only provide potential benefit to some patients with NDI, but are now used for much more common clinical applications as diverse as nocturnal enuresis and heart failure. Yet, the story is still evolving: clinical observations and animal experiments continue to discover new ways to affect urinary concentration. These novel pathways can potentially be exploited for therapeutic gain. Here we review the (patho)physiology of water homoeostasis, the current status of clinical management, and potential new treatments.

Renal aquaporins and water balance disorders

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Aquaporins in avian kidneys: function and perspectives

For terrestrial vertebrates, water economy is a prerequisite for survival, and the kidney is their major osmoregulatory organ. Birds are the only vertebrates other than mammals that can concentrate urine in adaptation to terrestrial environments. Aquaporin (AQP) and glyceroporin (GLP) are phylogenetically old molecules and have been found in plants, microbial organisms, invertebrates, and vertebrates. Currently, 13 AQPs/aquaGLPs and isoforms are known to be present in mammals. AQPs 1, 2, 3, 4, 6, 7, 8, and 11 are expressed in the kidney; of these, AQPs 1, 2, 3, 4, and 7 are shown to be involved in fluid homeostasis. In avian kidneys, AQPs 1, 2, 3, and 4 have been identified and characterized. Also, gene and/or amino acid sequences of AQP5, AQP7, AQP8, AQP9, AQP11, and AQP12 have been reported in birds. AQPs 2 and 3 are expressed along cortical and medullary collecting ducts (CDs) and are responsible, respectively, for the water inflow and outflow of CD epithelial cells. While AQP4 plays an important role in water exit in the CD of mammalian kidneys, it is unlikely to participate in water outflow in avian CDs. This review summarizes current knowledge on structure and function of avian AQPs and compares them to those in mammalian and nonmammalian vertebrates. Also, we aim to provide input into, and perspectives on, the role of renal AQPs in body water homeostasis during ontogenic and phylogenetic advancement.