Urinary excretion of aquaporin-2 in humans: a potential marker of collecting duct responsiveness to vasopressin

Intake and food sources of sodium in the population residing in urban areas of Ecuador: results from ELANS study

BACKGROUND

In 2021, WHO notes that globally, 32% of annual deaths worldwide are due to cardiovascular causes, which have been attributed to excessive sodium intake, and therefore recommends a reduction in salt intake to less than 5 g/day. Ecuador does not have data on sodium consumption in the population. Hence, this study sought to determine the association between sodium consumption and sociodemographic variables in subjects living in urban areas of Ecuador.

OBJECTIVES

Determine the main dietary sources of sodium in subjects living in urban areas of the Coast and Highlands of Ecuador, and the association between sodium intake and sociodemographic variables such as: sex, region, marital status, socio-economic and educational level of this population.

METHODS

Sodium intake was studied in 800 subjects of both sexes aged 15 to 65 years living in urban areas in Ecuador, originating from the Latin American Nutrition and Health Study (ELANS) between 2014 and 2015. Data were obtained through two 24-hour recalls, and were accessed according to sex, region, age, marital status, socio-economic and educational levels.

RESULTS

The mean sodium intake was 4900 mg/day (SD ± 1188.32 mg/day), and both sexes exceeded the recommendations. Adjusting for energy intake, sodium consumption is higher in participants aged 50-65 years, from low socio-economic status and with basic education level. A positive relationship was found between sodium and energy intake. Around 48% of the sodium sources included the spices, condiments and herbs group. Within this group, salt itself constitutes 99% of sodium sources.

CONCLUSIONS

The Ecuadorian population consumes more than double the sodium recommendations, which vary according to gender and age. The first source of sodium is salt itself, which is part of spices and condiments food group. This data is important to formulate public health policies and interventions in Ecuador, especially in the population at risk.

Effect of controlled hypotensive hemorrhage on plasma sodium levels in anesthetized pigs: An exploratory study

Perioperative hyponatremia, due to non-osmotic release of the antidiuretic hormone arginine vasopressin, is a serious electrolyte disorder observed in connection with many types of surgery. Since blood loss during surgery contributes to the pathogenesis of hyponatremia, we explored the effect of bleeding on plasma sodium using a controlled hypotensive hemorrhage pig model. After 30-min baseline period, hemorrhage was induced by aspiration of blood during 30 min at mean arterial pressure <50 mmHg. Thereafter, the animals were resuscitated with retransfused blood and a near-isotonic balanced crystalloid solution and monitored for 180 min. Electrolyte and water balances, cardiovascular response, renal hemodynamics, and markers of volume regulation and osmoregulation were investigated. All pigs (n = 10) developed hyponatremia. All animals retained hypotonic fluid, and none could excrete net-free water. Urinary excretion of aquaporin 2, a surrogate marker of collecting duct responsiveness to antidiuretic hormone, was significantly reduced at the end of the study, whereas lysine vasopressin, i.e., the pig antidiuretic hormone remained high. In this animal model, hyponatremia developed due to net positive fluid balance and generation of electrolyte-free water by the kidneys. A decreased urinary aquaporin 2 excretion may indicate an escape from antidiuresis.

β3 Adrenergic Receptor Agonist Mirabegron Increases AQP2 and NKCC2 Urinary Excretion in OAB Patients: A Pleiotropic Effect of Interest for Patients with X-Linked Nephrogenic Diabetes Insipidus

We previously reported the novel finding that β3-AR is functionally expressed in the renal tubule and shares its cellular localization with the vasopressin receptor AVPR2, whose physiological stimulation triggers antidiuresis by increasing the plasma membrane expression of the water channel AQP2 and the NKCC2 symporter in renal cells. We also showed that pharmacologic stimulation of β3-AR is capable of triggering antidiuresis and correcting polyuria, in the knockout mice for the AVPR2 receptor, the animal model of human X-linked nephrogenic diabetes insipidus (XNDI), a rare genetic disease still missing a cure. Here, to demonstrate that the same response can be evoked in humans, we evaluated the effect of treatment with the β3-AR agonist mirabegron on AQP2 and NKCC2 trafficking, by evaluating their urinary excretion in a cohort of patients with overactive bladder syndrome, for the treatment of which the drug is already approved. Compared to baseline, treatment with mirabegron significantly increased AQP2 and NKCC2 excretion for the 12 weeks of treatment. This data is a step forward in corroborating the hypothesis that in patients with XNDI, treatment with mirabegron could bypass the inactivation of AVPR2, trigger antidiuresis and correct the dramatic polyuria which is the main hallmark of this disease.

Monosodium glutamate consumption reduces the renal excretion of trimethylamine N-oxide and the abundance of Akkermansia muciniphila in the gut

We previously demonstrated that monosodium glutamate (MSG) consumption increases trimethylamine (TMA) level in the renal tissue as well as dimethylamine and methylamine levels in urine of rats, suggesting the effects of MSG on humans. To better define the findings, we investigated whether MSG consumption alters serum trimethylamine N-oxide (TMAO) level, and as a consequence, induces kidney injury in the rat model. Adult male Wistar rats (n = 40) were randomized to be fed with a standard diet (control group) or a standard diet with 0.5, 1.5 or 3.0 g% MSG corresponding to 7, 21, or 42 g/day in 60 kg man, respectively in drinking water (MSG-treated groups), or a standard diet with 3.0 g% MSG in drinking water which was withdrawn after 4 weeks (MSG-withdrawal group). Blood and urine samples were collected to analyze the TMAO levels using 1H NMR and markers of kidney injury. Fecal samples were also collected for gut microbiota analysis. We found serum TMAO levels increased and urinary TMAO excretion decreased during MSG consumption, in parallel with the increase of the neutrophil gelatinase-associated lipocalin (NGAL) excretion which subsided with the withdrawal of MSG. The fecal 16 S rRNA analysis during MSG consumption showed gut microbiota changes with a consistent suppression of Akkermansia muciniphila, a mucin producing bacteria, but not of TMA-producing bacteria. In conclusions, our findings suggested that prolonged high dose MSG consumption may cause TMAO accumulation in the blood via reduction of renal excretion associated with acute kidney injury. The mechanisms by which MSG reduced TMAO excretion require further investigation.

Predictive Value of Urinary Aquaporin 2 for Acute Kidney Injury in Patients with Acute Decompensated Heart Failure

Acute kidney injury (AKI) is frequently encountered in people with acute decompensated heart failure (ADHF) and is associated with increased morbidity and mortality. Early detection of a urinary biomarker of kidney injury might allow a prompt diagnosis and improve outcomes. Levels of urinary aquaporin 2 (UAQP2), which is also associated with several renal diseases, are increased with ADHF. We aimed to determine whether UAQP2 predicted AKI in patients with ADHF. We conducted a prospective observation study in the coronary care unit (CCU) in a tertiary care university hospital in Taiwan. Individuals with ADHF admitted to the CCU between November 2009 and November 2014 were enrolled, and serum and urinary samples were collected. AKI was diagnosed in 69 (36.5%) of 189 adult patients (mean age: 68 years). Area under the receiver operating characteristic curve (AUROC) of biomarkers was evaluated to evaluate the diagnostic power for AKI. Both brain natriuretic peptide and UAQP2 demonstrated acceptable AUROCs (0.759 and 0.795, respectively). A combination of the markers had an AUROC of 0.802. UAQP2 is a potential biomarker of AKI in CCU patients with ADHF. Additional research on this novel biomarker is required.

Fluid Homeostasis and Diuretic Therapy in the Neonate

Understanding physiologic water balance and homeostasis mechanisms in the neonate is critical for clinicians in the NICU as pathologic fluid accumulation increases the risk for morbidity and mortality. In addition, once this process occurs, treatment is limited. In this review, we will cover fluid homeostasis in the neonate, explain the implications of prematurity on this process, discuss the complexity of fluid accumulation and the development of fluid overload, identify mitigation strategies, and review treatment options.

Emerging roles for AQP in mammalian extracellular vesicles

Recent research in the aquaporin (AQP) field has identified a role for diverse AQPs in extracellular vesicles (EV). Though still in its infancy, there is a growing body of knowledge in the area; AQPs in EV have been suggested as biomarkers for disease, as drug targets and show potential as therapeutics. To advance further in this field, AQPs in EV must be better understood. Here we summarize current knowledge of the presence and function of AQPs in EV and hypothesise their roles in health and disease.

Cystin genetic variants cause autosomal recessive polycystic kidney disease associated with altered Myc expression

Mutation of the Cys1 gene underlies the renal cystic disease in the Cys1cpk/cpk (cpk) mouse that phenocopies human autosomal recessive polycystic kidney disease (ARPKD). Cystin, the protein product of Cys1, is expressed in the primary apical cilia of renal ductal epithelial cells. In previous studies, we showed that cystin regulates Myc expression via interaction with the tumor suppressor, necdin. Here, we demonstrate rescue of the cpk renal phenotype by kidney-specific expression of a cystin-GFP fusion protein encoded by a transgene integrated into the Rosa26 locus. In addition, we show that expression of the cystin-GFP fusion protein in collecting duct cells down-regulates expression of Myc in cpk kidneys. Finally, we report the first human patient with an ARPKD phenotype due to homozygosity for a deleterious splicing variant in CYS1. These findings suggest that mutations in Cys1/CYS1 cause an ARPKD phenotype in mouse and human, respectively, and that the renal cystic phenotype in the mouse is driven by overexpression of the Myc proto-oncogene.

Urinary Aquaporin 2 as a Potential Indicator Predicting Tolvaptan Response in Patients With ADPKD

Introduction

Tolvaptan is used to treat autosomal dominant polycystic kidney disease (ADPKD) because it inhibits binding of the antidiuretic hormone vasopressin to the vasopressin V2 receptor (V2R), which suppresses the insertion of preformed water channel aquaporin 2 (AQP2) molecules in the luminal membrane of the collecting duct cells.

Methods

This single-center, prospective observational cohort study investigated whether decreased AQP2 elimination in urine affects the renal prognosis of patients who received tolvaptan. We selected 92 patients with ADPKD who were administered tolvaptan in our hospital. We evaluated correlations between changes in urinary AQP2 (U-AQP2) and clinical parameters and the annual change in total kidney volume (TKV) and estimated glomerular filtration rate (eGFR) as renal prognostic factors using univariable and multivariable multiple regression analyses.

Results

The observation period was 2.4 ± 1.5 years. U-AQP2 per milligram of urinary creatinine (U-AQP2/Cr) decreased from 67.8 ± 50.6 to 20.7 ± 15.1 fmol/mg urinary creatinine after 1 month of tolvaptan treatment. This initial change in U-AQP2/Cr was correlated with high baseline U-AQP2/Cr, low baseline eGFR, and a large initial change in eGFR (baseline to 1 month). The initial change in U-AQP2/Cr (baseline to 1 month) was strongly correlated with the annual change in TKV and eGFR in multivariable analysis.

Conclusion

Initial decrease in U-AQP2/Cr in the first month of treatment reflects the pharmacologic effect of tolvaptan and could be an indicator of renal prognosis during tolvaptan treatment.

Genetic and Physiological Effects of Insulin on Human Urate Homeostasis

Insulin and hyperinsulinemia reduce renal fractional excretion of urate (FeU) and play a key role in the genesis of hyperuricemia and gout, via uncharacterized mechanisms. To explore this association further we studied the effects of genetic variation in insulin-associated pathways on serum urate (SU) levels and the physiological effects of insulin on urate transporters. We found that urate-associated variants in the human insulin (INS), insulin receptor (INSR), and insulin receptor substrate-1 (IRS1) loci associate with the expression of the insulin-like growth factor 2, IRS1, INSR, and ZNF358 genes; additionally, we found genetic interaction between SLC2A9 and the three loci, most evident in women. We also found that insulin stimulates the expression of GLUT9 and increases [14C]-urate uptake in human proximal tubular cells (PTC-05) and HEK293T cells, transport activity that was effectively abrogated by uricosurics or inhibitors of protein tyrosine kinase (PTK), PI3 kinase, MEK/ERK, or p38 MAPK. Heterologous expression of individual urate transporters in Xenopus oocytes revealed that the [14C]-urate transport activities of GLUT9a, GLUT9b, OAT10, OAT3, OAT1, NPT1 and ABCG2 are directly activated by insulin signaling, through PI3 kinase (PI3K)/Akt, MEK/ERK and/or p38 MAPK. Given that the high-capacity urate transporter GLUT9a is the exclusive basolateral exit pathway for reabsorbed urate from the renal proximal tubule into the blood, that insulin stimulates both GLUT9 expression and urate transport activity more than other urate transporters, and that SLC2A9 shows genetic interaction with urate-associated insulin-signaling loci, we postulate that the anti-uricosuric effect of insulin is primarily due to the enhanced expression and activation of GLUT9.

Urine concentrating defect as presenting sign of progressive renal failure in Bardet-Biedl syndrome patients

BACKGROUND

Urine concentrating defect is a common dysfunction in ciliopathies, even though its underlying mechanism and its prognostic meaning are largely unknown. This study assesses renal function in a cohort of 54 Bardet-Biedl syndrome (BBS) individuals and analyses whether renal hyposthenuria is the result of specific tubule dysfunction and predicts renal disease progression.

METHODS

The estimated glomerular filtration rate (eGFR), urine albumin:creatinine ratio (ACR) and maximum urine osmolality (max-Uosm) were measured in all patients. Genetic analysis was conducted in 43 patients. Annual eGFR decline (ΔeGFR) was measured in patients with a median follow-up period of 6.5 years. Urine aquaporin-2 (uAQP2) excretion was measured and the furosemide test was performed in patients and controls.

RESULTS

At baseline, 33 (61.1%), 12 (22.2%) and 9 (16.7%) patients showed an eGFR >90, 60-90 and <60 mL/min/1.73 m2, respectively; 27.3% showed an ACR >30 mg/g and 55.8% of patients showed urine concentrating defect in the absence of renal insufficiency. Baseline eGFR, but not max-Uosm, correlated negatively with age. Conversely, truncating mutations affected max-Uosm and showed a trend towards a reduction in eGFR. Max-Uosm correlated with ΔeGFR (P < 0.005), suggesting that urine concentrating defect may predict disease progression. uAQP2 excretion and Na+ and Cl- fractional excretion after furosemide did not differ between hyposthenuric patients and controls, suggesting that specific collecting duct and thick ascending limb dysfunctions are unlikely to play a central role in the pathogenesis of hyposthenuria.

CONCLUSIONS

Hyposthenuria is a warning sign predicting poor renal outcome in BBS. The pathophysiology of this defect is most likely beyond defective tubular function.

Differential effects of sodium chloride and monosodium glutamate on kidney of adult and aging mice

Monosodium Glutamate (MSG) is used as flavour enhancer, with potential beneficial effects due to its nutritional value. Given the decline in kidney functions during aging, we investigated the impact of MSG voluntary intake on the kidney of male mice, aged 6 or 18 months. For 2 months, they freely consumed water (control group), sodium chloride (0.3% NaCl) or MSG (1% MSG) in addition to standard diet. Young animals consuming sodium chloride presented signs of proteinuria, hyperfiltration, enhanced expression and excretion of Aquaporin 2 and initial degenerative reactions suggestive of fibrosis, while MSG-consuming mice were similar to controls. In old mice, aging-related effects including proteinuria and increased renal corpuscle volume were observed in all groups. At an advanced age, MSG caused no adverse effects on the kidney compared to controls, despite the presence of a sodium moiety, similar to sodium chloride. These data show that prolonged MSG intake in mice has less impact on kidney compared to sodium chloride, that already in young animals induced some effects on kidney, possibly related to hypertension.

Monocarboxylate transporter 12 as a guanidinoacetate efflux transporter in renal proximal tubular epithelial cells

Guanidinoacetate (GAA), which is a precursor of creatine, is mainly biosynthesized in the renal proximal tubular epithelial cells (RPTECs). Plasma concentration of GAA has been reported to be reduced in patients with monocarboxylate transporter 12 (MCT12) mutation (p.Q215X). However, the mechanism underlying GAA release from the RPTECs remains unclear. Therefore, to elucidate the role of MCT12 in renal GAA release, MCT12-mediated GAA transport was evaluated using the human and rat MCT12-expressing Xenopus laevis oocytes and primary-cultured rat RPTECs. [¹⁴C]GAA uptake by the human and rat MCT12-expressing oocytes was significantly higher than that by the water-injected oocytes. Rat MCT12-mediated uptake of [¹⁴C]GAA by the oocytes was found to be sodium ion (Na⁺)-independent and exhibited saturable kinetics with a Michaelis-Menten constant of 3.38 mM. Transport activities of rat MCT12 tend to increase along with increasing of extracellular pH. In addition, the efflux transport of [¹⁴C]GAA from the human and rat MCT12-expressing oocytes was significantly higher than that from the water-injected oocytes. These results suggest that both the influx and efflux transport of GAA is mediated by MCT12. In the primary-cultured rat RPTECs, [¹⁴C]GAA efflux transport was significantly reduced by the transfection of MCT12-specific siRNAs, suggesting that MCT12 participates in GAA efflux transport in rat RPTECs. Therefore, it suggests that MCT12 is involved in GAA release from RPTECs to the circulating blood, since MCT12 is known to be localized on the basal membrane of RPTECs.

Urinary extracellular vesicles: Origin, role as intercellular messengers and biomarkers; efficient sorting and potential treatment options

Urinary extracellular vesicles (uEVs) are a heterogenous group of vesicles consisting mainly of microvesicles and exosomes that originate predominantly (99.96%) from kidney, the urinary tract epithelium and the male reproductive tract. Secreted EVs contain molecular cargo from parental cells and provide an attractive source for biomarkers, a potential readout of physiological and pathophysiological mechanisms, and events associated with the urinary system. uEVs are readily enriched and isolated from urine samples and we review 6 standard methods that allow for downstream analysis of the uEV cargo. Although the use of uEVs as a surrogate readout for physiological changes in tissue protein levels is widespread, the protein abundance in uEVs is affected significantly by mechanisms that regulate protein sorting and secretion in uEVs. Data suggest that baseline kidney tissue and uEV levels of apical membrane-associated electrolyte transport proteins are not directly related in human patients. Recent evidence indicates that EVs may contribute to physiological and pathophysiological intercellular signalling and EVs confer protection against renal ischemia-reperfusion injury. The therapeutic use of EVs as information carriers has mainly been explored in vitro and a major hurdle lies in the translation of the in vitro findings into an in vivo setting. Thus, the EV research field is moving from a technical focus to a more physiological focus, allowing for a deeper understanding of human physiology, development of diagnostic tools and potential treatment strategies for precision medicine.

Sickle cell disease up-regulates vasopressin, aquaporin 2, urea transporter A1, Na-K-Cl cotransporter 2, and epithelial Na channels in the mouse kidney medulla despite compromising urinary concentration ability

Sickle cell disease (SCD)-induced urinary concentration defect has been proposed as caused by impaired ability of the occluded vasa recta due to red blood cell sickling to serve as countercurrent exchangers and renal tubules to absorb water and solutes. However, the exact molecular mechanisms remain largely unknown. The present studies were undertaken to determine the effects of SCD on vasopressin, aquaporin2 (AQP2), urea transporter A1 (UTA1), Na-K-Cl co-transporter 2 (NKCC2), epithelial Na channels (ENaC), aquaporin1 (AQP1), nuclear factor of activated T cells 5 (NFAT5) and Src homology region-2 domain-containing phosphatase-1 (SHP-1), an important regulator of NFAT5, in the Berkeley SCD mouse kidney medulla. Under water repletion, SCD only induced a minor urinary concentration defect associated with increased urinary vasopressin level alone with the well-known effects of vasopressin: protein abundance of AQP2, UTA1 and ENaC-β and apical targeting of AQP2 as compared with non-SCD. SCD did not significantly affect AQP1 protein level. Water restriction had no further significant effect on SCD urinary vasopressin. NFAT5 is also critical to urinary concentration. Instead, water restriction-activated NFAT5 associated with inhibition of SHP-1 in the SCD mice. Yet, water restriction only elevated urinary osmolality by 28% in these mice as opposed to 104% in non-SCD mice despite similar degree increases of protein abundance of AQP2, NKCC2 and AQP2-S256-P. Water-restriction had no significant effect on protein abundance of ENaC or AQP1 in either strain. In conclusion, under water repletion SCD, only induces a minor defect in urinary concentration because of compensation from the up-regulated vasopressin system. However, under water restriction, SCD mice struggle to concentrate urine despite activating NFAT5. SCD-induced urinary concentration defect appears to be resulted from the poor blood flow in vasa recta rather than the renal tubules' ability to absorb water and solutes.

Low aquaporin-2 excretion in the nephrotic syndrome: an escape from the vasopressin regulating effect

Purpose

Experimental studies suggest that the nephrotic syndrome is associated with “vasopressin escape”, characterized by low aquaporin-2 (AQP2) expression in the collecting duct despite high vasopressin secretion. We investigated this phenomenon in patients with the nephrotic syndrome.

Patients and methods

We recruited 47 patients with proteinuric kidney disease who were distributed into the following four groups: 1) nephrotic syndrome with kidney dysfunction (n=10); 2) nephrotic syndrome with normal kidney function (n=16); 3) partial remission of nephrotic syndrome (n=10); and 4) minimal proteinuria (n=11). Nine healthy volunteers comprised a control group. Serum copeptin level (as a marker of vasopressin secretion) and urinary AQP2 were measured using ELISA.

Results

Nephrotic syndrome was associated with a significant increase in serum copeptin levels compared with those in the other groups (all P<0.05). In patients with nephrotic syndrome and a partial remission of nephrotic syndrome combined, there was more than a ten-fold decrease in the median urinary AQP2 excretion (0.03 ng/mL) compared with healthy volunteers (0.41 ng/mL; P<0.001) and more than a five-fold decrease compared with patients with minimal proteinuria (0.21 ng/mL; P<0.05). Unlike copeptin levels, the median urinary AQP2 excretion in patients with minimal proteinuria also decreased but less significantly than in those with nephrotic syndrome. There was a negative correlation between the urinary AQP2 excretion and daily proteinuria (R=−0.41; P=0.005).

Conclusion

Our clinical study was the first to demonstrate low AQP2 excretion in nephrotic syndrome that may indicate an escape from the vasopressin regulating effect.

Roles of Macrophage Exosomes in Immune Response to Calcium Oxalate Monohydrate Crystals

In kidney stone disease, macrophages secrete various mediators via classical secretory pathway and cause renal interstitial inflammation. However, whether their extracellular vesicles, particularly exosomes, are involved in kidney stone pathogenesis remained unknown. This study investigated alterations in exosomal proteome of U937-derived macrophages (by phorbol-12-myristate-13-acetate activation) after exposure to calcium oxalate monohydrate (COM) crystals for 16-h using 2-DE-based proteomics approach. Six significantly altered proteins in COM-treated exosomes were successfully identified by nanoscale liquid chromatography–electrospray ionization–electron transfer dissociation tandem mass spectrometry as proteins involved mainly in immune processes, including T-cell activation and homeostasis, Fcγ receptor-mediated phagocytosis, interferon-γ (IFN-γ) regulation, and cell migration/movement. The decreased heat shock protein 90-beta (HSP90β) and increased vimentin were confirmed by Western blotting. ELISA showed that the COM-treated macrophages produced greater level of interleukin-1β (IL-1β), one of the markers for inflammasome activation. Functional studies demonstrated that COM-treated exosomes enhanced monocyte and T-cell migration, monocyte activation and macrophage phagocytic activity, but on the other hand, reduced T-cell activation. In addition, COM-treated exosomes enhanced production of proinflammatory cytokine IL-8 by monocytes that could be restored to its basal level by small-interfering RNA targeting on vimentin (si-Vimentin). Moreover, si-Vimentin could also abolish effects of COM-treated exosomes on monocyte and T-cell migration as well as macrophage phagocytic activity. These findings provided some implications to the immune response during kidney stone pathogenesis via exosomal pathway of macrophages after exposure to COM crystals.

The Kidney in Bardet-Biedl Syndrome: Possible Pathogenesis of Urine Concentrating Defect

BACKGROUND

The ciliopathies are a growing number of disorders caused by mutations in genes involved in the function of the primary cilium. Bardet-Biedl syndrome (BBS) belongs to this group of disorders. In this setting, kidney dysfunction is highly variable, and urine concentrating defect, a common feature of multiple ciliopathies, has been described as the most frequent defect. Here we review the mechanism of urine concentration and describe the possible mechanism underling this defect in ciliopathies and especially BBS, based on the current body of literature.

SUMMARY

Active Na⁺ absorption along the thick ascending limb of the loop of Henle (TAL) is critical for generating the corticomedullary osmotic gradient, and the countercurrent anatomical arrangement of the 2 branches of the loop of Henle enhances this gradient. The vasa recta, paralleling the loop of Henle, operate into the countercurrent mechanism, minimizing washout of solutes from the interstitium. Final water reabsorption is mediated by the aquaporin 2 (AQP2) water channels along the distal nephron, and it is under hormonal control. Several studies demonstrated that hyposthenuria in BBS patients relies on kidney resistance to desmopressin, suggesting a renal origin. We recently showed that the majority of hyposthenuric BBS patients have also a defect regarding maximal urine dilution. Independent studies showed that BBS10 deficiency caused AQP2 mistrafficking in vitro; accordingly, we demonstrated impaired urinary AQP2 excretion in BBS patients with combined concentrating and diluting defect. Whether receptor signaling pathways or downstream events cause AQP2 deregulation is still unclear. In addition, reduced urinary uromodulin excretion in BBS patients opens the possibility that TAL dysfunction may also play a pathogenic role.

KEY MESSAGE

Impaired water handling in BBS is associated with AQP2 mistrafficking. The potential role of additional factors, such as the dissipation of the medullary osmotic gradient due to TAL dysfunction and/or structural anomalies, remains to be elucidated.

Effect of sodium nitrite on renal function and sodium and water excretion and brachial and central blood pressure in healthy subjects: a dose-response study

Sodium nitrite (NaNO₂) is converted to nitric oxide (NO) in vivo and has vasodilatory and natriuretic effects. Our aim was to examine the effects of NaNO₂ on hemodynamics, sodium excretion, and glomerular filtration rate (GFR). In a single-blinded, placebo-controlled, crossover study, we infused placebo (0.9% NaCl) or 0.58, 1.74, or 3.48 μmol NaNO₂·kg^-1·h^-1 for 2 h in 12 healthy subjects, after 4 days of a standard diet. Subjects were supine and water loaded. We measured brachial and central blood pressure (BP), plasma concentrations of renin, angiotensin II, aldosterone, arginine vasopressin (P-AVP), and plasma nitrite (P-[Formula: see text]), GFR by Cr-EDTA clearance, fractional excretion of sodium (FE_Na) free water clearance (C_H2O), and urinary excretion rate of guanosine 3',5'-cyclic monophosphate (U-cGMP). The highest dose reduced brachial systolic BP (5.6 mmHg, P = 0.003), central systolic BP (5.6 mmHg, P = 0.035), and C_H2O (maximum change from 3.79 to 1.27 ml/min, P = 0.031) and increased P-[Formula: see text] (from 0.065 to 0.766 μmol/l, P < 0.001), while reducing U-cGMP (from 444 to 247 pmol/min, P = 0.004). GFR, FE_Na, P-AVP, and the components in the renin-angiotensin-aldosterone system did not change significantly. In conclusion, intravenous NaNO₂ induced a dose-dependent reduction of brachial and central BP. The hemodynamic effect was not mediated by the renin-angiotensin-aldosterone system. NaNO₂ infusion resulted in a vasopressin-independent decrease in C_H2O and urine output but no change in urinary sodium excretion or GFR. The lack of increase in cGMP accompanying the increase in [Formula: see text] suggests a direct effect of nitrite or nitrate on the renal tubules and vascular bed with little or no systemic conversion to NO.

Aquaporins in Urinary Extracellular Vesicles (Exosomes)

Since the successful characterization of urinary extracellular vesicles (uEVs) by Knepper’s group in 2004, these vesicles have been a focus of intense basic and translational research worldwide, with the aim of developing novel biomarkers and therapeutics for renal disease. Along with these studies, there is growing evidence that aquaporins (AQPs), water channel proteins, in uEVs have the potential to be diagnostically useful. In this review, we highlight current knowledge of AQPs in uEVs from their discovery to clinical application.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Urine Aquaporin-2: A Promising Marker of Response to the Arginine Vasopressin Type-2 Antagonist, Tolvaptan in Patients with Congestive Heart Failure

Aquaporin-2, a member of the aquaporin family, is an arginine vasopressin-regulated water channel expressed in the renal collecting duct, and a promising marker of the concentrating and diluting ability of the kidney. The arginine vasopressin type-2 antagonist, tolvaptan, is a new-generation diuretic; it is especially indicated in patients with decompensated heart failure refractory to conventional diuretics. However, the ideal responders to tolvaptan have not yet been identified, and non-responders experience worse clinical courses despite treatment with tolvaptan. Urine aquaporin-2 has recently been demonstrated as a promising predictor of response to tolvaptan. We here validated aquaporin-2-guided tolvaptan therapy in patients with decompensated heart failure. Long-term efficacy of tolvaptan treatment in the responders defined by aquaporin-2 needs to be validated in the future prospective study.

Tolvaptan Prolongs Blockage of the Vasopressin Type II Receptor Over 24 Hours in Responders With Stage D Heart Failure

The urine aquaporin-2 (U-AQP2) level relative to the plasma arginine vasopressin (P-AVP) level is a novel predictor of the responsiveness to the vasopressin type 2 receptor (V2R) antagonist tolvaptan (TLV). However, little has been reported about the concentration-time profile of U-AQP2 after TLV treatment. We evaluated 24 patients with decompensated stage D heart failure (HF) who had received 3.75 mg/day of TLV on a de novo basis for > 7 days to treat congestion refractory to conventional diuretics. Seventeen patients were TLV-responders, whose 24-hour urine volume (UV) increased after TLV initiation; the other 7 patients were TLV-non-responders. The U-AQP2 of the TLV-responders, corrected for the urine creatinine concentration, decreased significantly at 4 hours after TLV administration without returning to the day-1 morning level on the morning of day-7. The TLV-non-responder U-AQP2 levels remained low even before the TLV treatment. On the morning of day-7, the TLV-responder U-AQP2/P-AVP ratio was comparable to that of the TLV-non-responders. Among 18 patients (11 responders and 7 non-responders), the day-7 TLV trough concentration was 64 ± 62 ng/mL and was negatively correlated with the estimated glomerular filtration rate (eGFR). TLV has antagonistic effects on the V2R over 24 hours in TLV-responders with advanced heart failure and chronic kidney disease, probably due to persistently elevated blood TLV concentration. The unresponsiveness to TLV in the TLV-non-responders is not attributable to malabsorption.

Therapeutic effects of short- and intermediate-term tolvaptan administration for refractory ascites in patients with advanced liver cirrhosis

AIM

Tolvaptan, an oral arginine vasopressin V2 receptor antagonist, became available for hepatic ascites. We evaluated the therapeutic efficacy and safety of tolvaptan administration to treat refractory ascites.

METHODS

Data were collected from 15 hospitalized patients with cirrhosis (hepatitis C, 10; alcoholism, five) after adding tolvaptan (3.75-11.25 mg/day) to conventional diuretics. Bodyweights and serum sodium and creatinine concentrations were measured. Tolvaptan was continued for 4 weeks or longer for a median follow-up period of 42 days (range, 28-56).

RESULTS

In the first week (introduction phase), tolvaptan significantly reduced median weight (66.6, 65.9 and 63.1 kg on days 0, 1 and 7, respectively; P < 0.004). The numbers of good responders (≥3 kg reduction in 4 days), responders (<3 kg weight reduction) and non-responders (no weight reduction) were seven (46.7%), six (40.0%) and two of the 15 (13.3%), respectively. The two non-responders had concomitant chylous pleural effusion or spontaneous bacterial peritonitis. All patients continued tolvaptan for 2 weeks or longer and six (40%, three good responders and three responders) were treated for a median of 42 days without additional intervention. During this intermediate-term administration of tolvaptan, the median weight reduction was statistically significant (65.4, 61.9 and 56.9 kg on days 0, 7 and 42, respectively; P < 0.030) and there was no serum sodium imbalance or renal dysfunction; but two of these six developed hepatic coma.

CONCLUSION

Tolvaptan safely alleviated fluid retention caused by hepatic cirrhosis. Intermediate-term administration of tolvaptan apparently helped maintain weight reduction achieved during the introduction phase.

Alkali treatment stabilizes fluctuations of urine AQP2 values measured by ELISA

Background

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

Methods

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

Results

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

Conclusion

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

Urinary aquaporin-2 excretion before and after transjugular intrahepatic portosystemic shunt insertion for refractory ascites

OBJECTIVE

The role of renal aquaporin-2 (AQP2) water channel turnover in patients with liver cirrhosis, portal hypertension and water retention remains unclear. Transjugular intrahepatic portosystemic shunt (TIPS) insertion reduces portal hypertension, improves water excretion and lowers plasma vasopressin. The aim of this study was to establish whether TIPS insertion decreases urinary AQP2 excretion (uAQP2) in parallel with improved water excretion.

MATERIAL AND METHODS

Fourteen cirrhosis patients with refractory ascites were studied before TIPS insertion and 4 and 12 weeks after insertion. A 24-h urine collection was followed by an oral water load (20 ml/kg body weight) with a 4-h blood and urine sampling.

RESULTS

TIPS reduced the portal pressure gradient from a median 18(4) (25-75% InterQuartile-range) to 7(2) mmHg, p < 0.05 and the need for diuretics (p < 0.05). TIPS increased plasma sodium from 136(6) mmol/l to 139(4), (p < 0.05) and diuresis from 1650(1043) ml/24 h to 2230(560) (p < 0.05), although the 24-h urinary sodium excretion did not change. There was no change in the baseline uAQP2 before 274(249) ng/(mmol creatinine/24 h) and 12 weeks after TIPS 242(201). There were no systematic changes in uAQP2, plasma vasopressin or other vasoactive substances during the water loads, before or after TIPS.

CONCLUSION

The effective amelioration of portal hypertension improved the patient's water excretion and plasma sodium, but there was no change in renal AQP2 trafficking or vasopressin. These findings do not support a primary role for renal AQP2 water channels in portal hypertensive water retention.

Differential expression and regional distribution of aquaporins in amnion of normal and gestational diabetic pregnancies

The region of the amnion overlying the placenta plays an active role in fluid exchange between amniotic fluid and fetal blood perfusing the surface of the placenta, whereas little transfer occurs across the reflected amnion that contacts the membranous chorion. Because aquaporins (AQPs) facilitate rapid movement of water across cells, we hypothesized that AQP gene expression in placental amnion is higher than in reflected amnion. Furthermore, because gestational diabetes mellitus (GDM) is often associated with polyhydramnios, we hypothesized that amnion AQP gene expression is reduced when amniotic fluid volume is elevated. Human placental and reflected amnion were obtained at cesarean delivery and subjected to relative quantitation of AQP mRNA by real-time RT-qPCR and proteins by western immunoblot. Amnion mRNA levels of five AQPs differed by up to 400-fold (P < 0.001), with AQP1 and AQP3 most abundant, AQP8 least and AQP9 and AQP11 intermediately expressed. Aquaporin proteins showed a similar profile. Aquaporin mRNA abundance was higher (P < 0.001) in placental than reflected amnion, whereas protein levels were lower (P < 0.01). In GDM pregnancies, neither AQP mRNA nor protein levels were different from normal. There was no correlation between AQP mRNA or protein levels with the amniotic fluid index in normal or GDM subjects. We conclude that there is a strong differential expression profile among individual AQPs and between regions of the amnion. These findings suggest differences in contribution of individual AQPs to water transport in the two regions of the amnion. Furthermore, AQP expression in the amnion is not altered in patients with GDM.

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.

Urinary concentration does not exclusively rely on plasma vasopressin. A study between genders. Gender and diurnal urine regulation

AIM

We investigated the influence of gender on the diurnal regulation of urine production with special focus on vasopressin, oxytocin and prostaglandin E2.

METHODS

Fifteen young women in mid-follicular phase and 22 young men (20-33 years) were included. All participants underwent a 24-h circadian inpatient study under standardized conditions for measurements of plasma vasopressin, oxytocin, sodium and osmolality. Urine was fractionally collected for measurements of electrolytes, aquaporin-2 and prostaglandin E2.

RESULTS

Plasma vasopressin expressed a diurnal rhythm with a night-time increase in both genders (P < 0.001). The ratio between mean daytime and mean night-time was 1.57 [95% CI: 1.33-1.84] P < 0.001 in men and 1.35 [95% CI: 1.11-1.64] P = 0.002 in women. P-vasopressin was higher in males during the night (P < 0.05). There was no difference in diuresis (P = 0.43), urine osmolality (P = 0.12) or aquaporin-2 excretion (P = 0.80) between genders. We found a trend towards a higher reabsorption of free water in males (P = 0.07). The excretion of prostaglandin E2 was higher in males (P < 0.001). There was no diurnal rhythm in p-oxytocin (P = 0.37) and no correlation to diuresis, urine osmolality or aquaporin-2 excretions.

CONCLUSION

Similar urinary flows and osmolalities are associated with levels of plasma vasopressin and renal PGE2, which are higher in males than in females. Oxytocin does not seem to play a role in the diurnal urine formation, whereas prostaglandin E2 could represent a mediator of the gender difference, not only as a mediator of the vasopressin response, but also as an independent factor. These findings need further elucidation.

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

BACKGROUND

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

CONCLUSIONS

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

Puberty alters renal water handling

We investigated the influence of sex and puberty stage on circadian urine production and levels of antidiuretic hormone [arginine vasopressin (AVP)] in healthy children. Thirty-nine volunteers (9 prepuberty boys, 10 prepuberty girls, 10 midpuberty boys, and 10 midpuberty girls) were included. All participants underwent a 24-h circadian inpatient study under standardized conditions regarding Na(+) and fluid intake. Blood samples were drawn every 4 h for measurements of plasma AVP, serum 17-β-estradiol, and testosterone, and urine was fractionally collected for measurements of electrolytes, aquaporin (AQP)2, and PGE2. We found a marked nighttime decrease in diuresis (from 1.69 ± 0.08 to 0.86 ± 0.06 ml·kg(-1)·h(-1), P < 0.001) caused by a significant nighttime increase in solute-free water reabsorption (TcH2O; day-to-night ratio: 0.64 ± 0.07, P < 0.001) concurrent with a significant decrease in osmotic excretion (day-to-night ratio: 1.23 ± 0.06, P < 0.001). Plasma AVP expressed a circadian rhythm (P < 0.01) with a nighttime increase and peak levels at midnight (0.49 ± 0.05 pg/ml). The circadian plasma AVP rhythm was not influenced by sex (P = 0.56) or puberty stage (P = 0.73). There was significantly higher nighttime TcH2O in prepuberty children. This concurred with increased nighttime urinary AQP2 excretion in prepuberty children. Urinary PGE2 exhibited a circadian rhythm independent of sex or puberty stage. Levels of serum 17β-estradiol and testosterone were as expected for sex and puberty stage, and no effect on the AVP-AQP2-TcH2O axis was observed. This study found a circadian rhythm of plasma AVP independent of sex and puberty stage, although nighttime TcH2O was higher and AQP2 excretion was more pronounced in prepuberty children, suggesting higher prepuberty renal AVP sensitivity.

Vasopressin and the regulation of aquaporin-2

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

Excretion of urinary exosomal AQP2 in rats is regulated by vasopressin and urinary pH

Urinary exosomes are small vesicles secreted into urine from all renal epithelial cell types and known to contain proteins that are involved in renal secretion and reabsorption. Among these proteins, urinary exosomal aquaporin-2 (AQP2) has been suggested to be useful for diagnosis of renal disease. However, the mechanisms underlying the excretion of urinary exosomal AQP2 are largely unknown. In this study, we examined the mechanisms of urinary exosomal AQP2 excretion in vivo, using diuretics including furosemide (FS), an inhibitor of the sodium-potassium-chloride symporter; acetazolamide (ACTZ), an inhibitor of carbonic anhydrase; OPC-31260 (OPC), a vasopressin type 2 receptor antagonist; and NaHCO3, a urinary alkalizing agent. Samples of urine from rats were collected for 2 h just after treatment with each diuretic, and urinary exosomes were isolated by ultracentrifugation. Urinary exosomal AQP2 excretion was dramatically increased by treatment with FS accompanied by urine acidification or with ACTZ accompanied by urine alkalization. Immunohistochemistry showed that apical localization of AQP2 was clearly evident and the plasma vasopressin level was increased after each treatment. Although treatment with OPC alone had no significant effect, coadministration of OPC completely inhibited the FS-induced and partially reduced the ACTZ-induced responses, respectively. Treatment with NaHCO3 increased the excretion of urinary exosomal AQP2 accompanied by urine alkalization. This increased response was partially inhibited by coadministration of OPC. These data suggest that an increased plasma level of vasopressin promoted the excretion of urinary exosomal AQP2 and that urine alkalinization also increased it independently of vasopressin.

Effect of volume expansion with hypertonic- and isotonic saline and isotonic glucose on sodium and water transport in the principal cells in the kidney

Background

The renal distal nephron plays an important role in the maintenance of sodium balance, extra cellular volume and blood pressure. The degree of water transport, via aquaporin2 water channels (AQP2), and sodium transport, via epithelial sodium channels (ENaC) in renal collecting duct principal cells are reflected by the level of urinary excretion of AQP2 (u-AQP2) and the γ-fraction of ENaC (u-ENaCγ). The effects of an acute intravenous volume load with isotonic saline, hypertonic saline and glucose on u-AQP2, u-ENaCγ and underlying mechanisms have never been studied in a randomized, placebo-controlled trial in healthy humans.

Methods

We studied the effects of 0.9% saline (23 ml/kg), 3% saline (7 ml/kg) and 5% glucose (23 ml/kg) on u-AQP2 and u-ENaCγ, fractional sodium excretion (FE_Na), free water clearance (C_H2O), and plasma concentrations of vasopressin (AVP), renin (PRC), angiotensin II (ANG II) and aldosterone (Aldo) in a randomized, crossover study of 23 healthy subjects, who consumed a standardized diet, regarding calories, sodium and fluid for 4 days before each examination day.

Results

After isotonic saline infusion, u-AQP2 increased (27%). C_H2O and u-ENaCγ were unchanged, whereas FE_Na increased (123%). After hypertonic saline infusion, there was an increase in u-AQP2 (25%), u-ENaCγ (19%) and FE_Na (96%), whereas C_H2O decreased (-153%). After isotonic glucose infusion, there was a decrease in u-AQP2 (-16%), ENaCγ (-10%) and FE_Na (-44%) whereas C_H2O increased (164%). AVP remained unchanged after isotonic saline and glucose, but increased after hypertonic saline (139%). PRC, AngII and p-Aldo decreased after isotonic and hypertonic saline infusion, but not after glucose infusion.

Conclusions

Volume expansion with 3% and 0.9% saline increased u-AQP2, while isotonic glucose decreased u-AQP2. Infusion of hypertonic saline increased u-ENaCγ, whereas u-ENaCγ was not significantly changed after isotonic saline and tended to decrease after glucose. Thus, the transport of water and sodium is changed both via the aquaporin 2 water channels and the epithelial sodium channels during all three types of volume expansion to regulate and maintain water- and sodium homeostasis in the body.

Trial registration

Clinical Trial no: NCT01414088

Aquaporin-2 regulation in health and disease

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

Persistent elevation of urine aquaporin-2 during water loading in a child with nephrogenic syndrome of inappropriate antidiuresis (NSIAD) caused by a R137L mutation in the V2 vasopressin receptor

Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD) is a novel disease caused by a gain-of-function mutation in the V2 vasopressin receptor (V2R), which results in water overload and hyponatremia. We report the effect of water loading in a 3-year old boy with NSIAD, diagnosed in infancy, to assess urine aquaporin-2 (AQP2) excretion as a marker for V2R activation, and to evaluate the progression of the disease since diagnosis. The patient is one of the first known NSIAD patients and the only patient with a R137L mutation. Patient underwent a standard water loading test in which serum and urine sodium and osmolality, serum AVP, and urine AQP2 excretion were measured. The patient was also evaluated for ad lib fluid intake before and after the test. This patient demonstrated persistent inability to excrete free water. Only 39% of the water load (20 ml/kg) was excreted during a 4-hour period (normal ≥ 80-90%). Concurrently, the patient developed hyponatremia and serum hypoosmolality. Serum AVP levels were detectable at baseline and decreased one hour after water loading; however, urine AQP2 levels were elevated and did not suppress normally during the water load. The patient remained eunatremic but relatively hypodipsic during ad lib intake. In conclusion, this is the first demonstration in a patient with NSIAD caused by a R137L mutation in the V2R that urine AQP2 excretion is inappropriately elevated and does not suppress normally with water loading. In addition, this is the first longitudinal report of a pediatric patient with NSIAD diagnosed in infancy who demonstrates the ability to maintain eunatremia during ad lib dietary intake.

Daily variance of urinary excretion of AQP2 determined by sandwich ELISA method

BACKGROUND

Urinary excretion of aquaporin 2 (AQP2) is a useful marker of kidney collecting duct function. A specific and convenient method to measure AQP2 in human urine would help to treat water balance disorders. It is unknown whether urinary excretion of AQP2 shows any daily variance.

METHODS

A sandwich enzyme-linked immunosorbent assay (ELISA) method for AQP2 was established using two different kinds of antibodies, and its sensitivity and specificity were examined. Daily variance of urinary excretion of AQP2 and responses to acute water load were examined.

RESULTS

The established ELISA specifically detected urine AQP2 with high sensitivity (detected as low as 0.34 pmol/mL). Urinary excretion of AQP2 did not show daily variance between 9 a.m. and 9 p.m. in healthy subjects.

CONCLUSIONS

The developed ELISA method using two different antibodies is convenient and highly sensitive, and could be useful in clinical practice. Urinary excretion of AQP2 is relatively constant from morning to night, and spot urine sampling at any time during this time period does not affect the results.

Effect of high and low sodium intake on urinary aquaporin-2 excretion in healthy humans

The degree of water transport via aquaporin-2 (AQP2) water channels in renal collecting duct principal cells is reflected by the level of the urinary excretion of AQP2 (u-AQP2). In rats, the AQP2 expression varies with sodium intake. In humans, the effect of sodium intake on u-AQP2 and the underlying mechanisms have not previously been studied. We measured the effect of 4 days of high sodium (HS) intake (300 mmol sodium/day; 17.5 g salt/day) and 4 days of low sodium (LS) intake (30 mmol sodium/day; 1.8 g salt/day) on u-AQP2, fractional sodium excretion (FE(Na)), free water clearance (C(H2O)), urinary excretion of PGE(2) (u-PGE(2)) and cAMP (u-cAMP), and plasma concentrations of vasopressin (AVP), renin (PRC), ANG II, aldosterone (Aldo), atrial natriuretic peptide (ANP), and brain natriuretic peptide (BNP) in a randomized, crossover study of 21 healthy subjects, during 24-h urine collection and after hypertonic saline infusion. The 24-h urinary sodium excretion was significantly higher during HS intake (213 vs. 41 mmol/24 h). ANP and BNP were significantly lower and PRC, ANG II, and Aldo were significantly higher during LS intake. AVP, u-cAMP, and u-PGE(2) were similar during HS and LS intake, but u-AQP2 was significantly higher during HS intake. The increases in AVP and u-AQP2 in response to hypertonic saline infusion were similar during HS and LS intake. In conclusion, u-AQP2 was increased during HS intake, indicating that water transport via AQP2 was increased. The effect was mediated by an unknown AVP-independent mechanism.

Diagnostic Potential of Urine Proteome: A Broken Mirror of Renal Diseases

This brief overview of studies into the urine proteome illustrates its potential value for diagnostic, prognostic, and pathophysiologic discovery. Hypothesis-targeted investigations of individual proteins as well as proteome-wide searches for urinary biomarkers of various diseases and their progression are reviewed. The majority of urine proteins appear as cleavage products that are found not only as free solutes but also in secreted membrane vesicles called exosomes. Described are several recent examples of important diagnostic findings using urine proteomics along with the idea that signature profiles of injury to individual nephron segments can be measured by this technology. Shared are some thoughts on the most challenging step: Integration of seemingly unrelated findings of various protein fragments into a rational pathogenetic pathway(s). The future chance that the centuries-old technique of uroscopy will reveal its secrets using modern proteomic approaches makes gradual improvement.

Urine osmolality, cyclic AMP and aquaporin-2 in urine of patients under lithium treatment in response to water loading followed by vasopressin administration

Lithium is the drug that is most frequently associated with acquired nephrogenic diabetes insipidus (NDI). The exact mechanism of lithium-induced NDI in man is unknown. The aim of the present study was to investigate the kidney response to minimal and maximal stimulation of the kidney urine concentrating mechanism by measuring urine osmolality, and urine levels of cAMP and AQP-2 in urine of patients under long-term lithium treatment. Twenty patients under long-term lithium treatment were included. The kidney urinary 3',5'-cyclic adenosine monophosphate (cyclic AMP), aquaporin-2 levels and urine osmolality were determined during a situation of minimal kidney urine concentrating activity (induced by water loading) and during a situation following maximal stimulation of kidney urine concentrating activity (induced by 1-desamino-8-D-arginine-vasopressin). Patients were classified as NDI, partial NDI and non-NDI based on maximal reached urine osmolality. The partial correlation (r) between urinary cyclic AMP levels (mol/l) and urine osmolality was 0.94 (P<0.001). No significant correlation was observed between urinary aquaporin-2 levels (mol/mol creatinine) and osmolality nor between urinary cyclic AMP and aquaporin-2 levels. The rise in urinary cyclic AMP but not aquaporin-2 levels upon 1-desamino-8-D-arginine-vasopressin administration after water loading significantly differed between the three categories, decreasing with increasing NDI category. In conclusion we found that in lithium-induced kidney urine concentrating deficit in man, the cyclic AMP generation in response to 1-desamino-8-D-arginine-vasopressin administration after water loading, is impaired. It remains to be elucidated whether principal cells, G-proteins or adenylate cyclase e.g. are the major targets for the mechanism underlying lithium-induced NDI in man.

Urinary aquaporin-2 excretion during early human development

This study was undertaken to assess one of the determinants of kidney concentrating capacity, aquaporin-2 (AQP2), in order to understand the physiopathology of water balance in newborn babies. Urinary AQP2 excretion has been shown to be proportional to AQP2 level in the apical plasma membrane of the kidney collecting ducts and has been suggested as a marker of vasopressin (AVP) action. Urinary AQP2 excretion in the early postnatal period and at 3 weeks of age was measured in 123 neonates admitted during a 6-month period to the neonatal intensive care unit of the Children's Hospital of Toulouse, France. Clinical and biochemical data were collected for each child. During the first days after birth, higher urinary AQP2 was observed in boys than in girls (P=0.01) and positively correlated with urinary sodium/potassium (Na/K) ratio (r=0.33, P=0.01). When the babies had reached 3 weeks of age, urinary AQP2 was proportional to the gestational age at birth (r=0.33, P=0.0068) and daily weight gain (r=0.36, P=0.003). It did not correlate with urinary osmolality, which was overall very low in all babies. Urinary AQP2 was decreased in conditions of impaired renal function (r=-0.42, P=0.0005) and acidosis (P=0.03). Prenatal corticosteroid treatment had no significant impact on urinary AQP2 level. Our data show that urinary AQP2 correlates with the overall maturity of tubular function in human neonates. In babies at this early age, urinary AQP2 cannot serve as a direct marker of the renal action of AVP but reflects AQP2 expression level associated with different physiopathological conditions.

Urinary aquaporin-2 in children with acute pyelonephritis

Children with acute pyelonephritis develop polyuria and have reduced maximum urinary concentration capacity. We studied whether these abnormalities are associated with altered urinary excretion of the water channel aquaporin-2 (AQP2) in the renal collecting duct. AQP2 is the main target for antidiuretic action of arginine vasopressin (AVP), and the urinary excretion of this protein is believed to be an index of AVP signaling activity in the kidney. Children with acute pyelonephritis, aged 5-14 years, were examined for urinary flow rate, creatinine clearance, unchallenged urine osmolality, and urinary ion excretion. Urinary excretion of AQP2 was measured by dot immunoblotting technique. Studies were performed in the acute phase of pyelonephritis, in the same children after treatment, and in control patients. At the onset of pyelonephritis, urinary flow rate and solute excretion were increased, but the urinary osmolality was unchanged. The urinary level and urinary excretion of AQP2 was increased in acute pyelonephritis and decreased after treatment. Excretion of aquaporin-3 was unchanged, suggesting that the increase in AQP2 urinary excretion was not due to a shedding of collecting duct cells. The results suggest that a mechanism proximal to the collecting duct may be responsible for the polyuria observed in children with acute pyelonephritis. Increased urinary AQP2 levels suggest that a compensatory activation of apical plasma membrane targeting of AQP2 may occur in pyelonephritis.

Clinical update on renal aquaporins

Following the discovery of the aquaporin-1 water channel over a decade ago, molecular techniques have been developed to examine the role of renal aquaporin water channels under numerous physiological and pathological conditions. The present article reviews current knowledge regarding the function and dysfunction of renal aquaporins in disorders of water metabolism.

Urinary excretion of aquaporin-2 water channel exaggerated dependent upon vasopressin in congestive heart failure

BACKGROUND

Impaired water excretion occurs in patients with congestive heart failure. The present study was undertaken to determine whether urinary excretion of aquaporin-2 (AQP-2) water channel is exaggerated in patients with congestive heart failure dependent upon arginine vasopressin (AVP).

METHODS

Sixty-five patients with congestive heart failure and eight age- and gender-matched control subjects were examined. The patients were divided into four groups according to the criteria of New York Heart Association (NYHA). Plasma AVP levels, urinary excretion of AQP-2, and cardiac index were determined.

RESULTS

Plasma AVP levels were progressively increased following the severity of NYHA class in the patients with congestive heart failure. Cardiac index was inversely decreased, and there was a negative correlation between plasma AVP levels and cardiac index (r=-0.430, P < 0.02). Urinary excretion of AQP-2 was 187.3 +/- 50.2 fmol/mg creatinine in the control subjects. It was markedly increased in the patients. Urinary excretion of AQP-2 was elevated to 1144.4 +/- 257.5 and 990.5 +/- 176.0 fmol/mg creatinine in the patients with NYHA class III and class IV, respectively, values significantly greater than the control subjects (P < 0.05). Urinary excretion of AQP-2 had a positive correlation with plasma AVP levels (r= 0.280, P < 0.02).

CONCLUSION

The present study indicates that exaggerated urinary excretion of AQP-2 is dependent on baroreceptor-mediated release of AVP in patients with congestive heart failure.