Angiotensin II and hypertonicity modulate proximal tubular aquaporin 1 expression

Therapeutic effects of intermittent fasting on high-fat, high-fructose diet; involvement of jejunal aquaporin 1, 3, and 7

Background

Aquaporins (AQPs) are transmembrane channel proteins. Aquaporin 1 (AQP1), Aquaporin 3 (AQP3), and Aquaporin 7 (AQP7) are expressed in the jejunum. The purpose of this study was to ascertain how a high-fat high-fructose diet (HFFD) and intermittent fasting (IF) affect AQP1, AQP3, and AQP7 expression in the rat jejunum.

Methods

Sixteen adult male rats were divided into control rats (n = 4) fed on a basal diet and water ad libitum for 12 weeks; IF control rats (n = 4) followed the IF protocol, HFFD-fed rats (n = 8) fed HFFD for eight weeks, and rats were randomized into two groups: HFFD only or HFFD and IF protocol from the beginning of the 9th week until the end of the experiment. The lipid profile values were assessed after 12 weeks. Jejunal oxidative markers (malondialdehyde and reduced glutathione) and AQP1, AQP3, and AQP7 mRNA expression were measured. Jejunal sections were used for morphometric analysis of villus length and crypt depth. Immunohistochemical evaluation of AQP1, AQP3, and AQP7 expression was also performed.

Results

IF ameliorates HFFD-induced lipid profile, oxidative stress, and jejunal morphometric changes. The results of both mRNA expression using PCR and immunohistochemistry showed a significant increase in AQP1, AQP3, and AQP7 expression in HFFD, whereas IF caused a decline in this expression.

Conclusion

These findings suggest that IF can reduce inflammation, and oxidative stress and restore jejunal morphology caused by HFFD.

Potential role for vitamin D vs. intermittent fasting in controlling aquaporin-1 and aquaporin-3 expression in HFD-induced urinary bladder alterations in rats

Background: High-fat diet-induced obesity is linked to suppression of aquaporins (AQPs) expression in different tissues. Both vitamin D and intermittent fasting were identified to enhance AQPs expression. In the urinary bladder, AQP-1 and AQP-3 mRNA transcripts were identified. Vitamin D has an impact on a variety of genes that encode proteins that control cell proliferation, differentiation, and death. Aim: To assess potential benefits of vitamin D and intermittent fasting (IF) and to explore alterations to the urinary bladder triggered by high-fat diet (HFD) in a rat model of obesity. Methods: Each of the 4 groups contained six adult male albino rats; control: a standard rodent chew for 12 weeks, HFD: HFD and fructose were administered orally via gastric gavage for 12 weeks, and vitamin D: HFD and fructose were administered orally for 8 weeks, then 4 weeks of intraperitoneal injection of vitamin D (5 microns/Kg/2 days) and IF group: Received intraperitoneal injections of vitamin D (5 microns/Kg/2 days) for 4 weeks after consumption of HFD and fructose orally for 8 weeks. The serum lipid profile was conducted at end of the experiment. In the bladder homogenates, the levels of oxidative stress indicators were assessed. Quantitative real-time PCR was performed on recently collected bladder samples. AQP-1 and AQP-3 immunohistochemistry was done. Results: When compared to the HFD group, the vitamin D and IF groups both demonstrated a substantial improvement in histopathological, immunohistochemical, biochemical, and molecular markers. Conclusion: In all examined parameters, IF exceeded vitamin D as a preventive factor for the urinary bladder deterioration.

Effect of SGLT2 Inhibitor on Cardiomyopathy in a Rat Model of T2DM: Possible involvement of Cardiac Aquaporins

Diabetic cardiomyopathy (DCM) causes arrhythmia, heart failure, and sudden death. Empagliflozin, an SGLT-2 (Sodium glucose co-transporter) inhibitor, is an anti-diabetic medication that decreases blood glucose levels by stimulating urinary glucose excretion. Several aquaporins (AQPs) including AQP-1-3 and - 4 and their involvement in the pathogenesis in different cardiac diseases were detected. In the current study the effect of Empagliflozin on diabetic cardiomyopathy and the possible involvement of cardiac AQPs were investigated.

METHODS

56 adult male Sprague-Dawley rats were divided into 4 groups: Control, DCM: type 2 diabetic rats, low EMPA+DCM received empagliflozin (10 mg/kg/day) and high EMPA+DCM received empagliflozin (30 mg/kg/day) for 6 weeks.

RESULTS

Administration of both EMPA doses, especially in high dose group, led to significant improvement in ECG parameters. Also, a significant improvement in biochemical and cardiac oxidative stress markers (significant decrease in serum CK-MB, and malondialdehyde while increasing catalase) with decreased fibrosis and edema in histopathological examination and a significant attenuation in apoptosis (caspase-3) and edema (AQP-1& -4).

CONCLUSION

Both doses of Empagliflozin have a cardioprotective effect and reduced myocardial tissue edema with high dose having a greater effect. This might be due to attenuation of oxidative stress, fibrosis and edema mediated through AQP-1, - 3& - 4 expression.

Alteration of Aquaporins 1 and 4 immunohistochemical and gene expression in the cerebellum of diabetic albino rat

Aquaporins (AQPs) are a family of transmembrane channel proteins. AQP1 and AQP4 are expressed in cerebellum amongst others. This study was designed to assess the effect of diabetes on AQP1 and AQP4 expression in cerebellum of rats. Diabetes was induced by a single intraperitoneal injection of Streptozotocin 45 mg/kg in 24 adult male Sprague Dawley rats. Six rats from control and diabetic groups were sacrificed at one, four, and eight weeks post diabetic confirmation. After eight weeks, measurement of malondialdehyde (MDA), reduced glutathione (GSH) concentrations, and cerebellar mRNA expression for AQP1 and AQP4 genes were performed. Immunohistochemical evaluation of AQP1, AQP4, and glial fibrillary acidic protein (GFAP) for cerebellar sections was performed for all groups. Diabetes caused degenerative changes in Purkinje cells with a significant increase in the cerebellar level of MDA and AQP1 immunoreactivity and a significant decrease in GSH level and AQP4 expression levels. However, the alteration in the AQP1 mRNA level was not statistically significant. GFAP immunoreactivity was increased in 8 W diabetic rats following its decrease in 1 W diabetic rats. Diabetes caused some alteration in the AQPs 1 and 4 expression in the cerebellum of diabetic rats which may contribute to diabetes-induced cerebellar complications.

Proteomic analysis of urinary extracellular vesicles highlights specific signatures for patients with primary aldosteronism

Background

Urinary extracellular vesicles (uEVs) can be released by different cell types facing the urogenital tract and are involved in cellular trafficking, differentiation and survival. UEVs can be easily detected in urine and provide pathophysiological information "in vivo" without the need of a biopsy. Based on these premises, we hypothesized that uEVs proteomic profile may serve as a valuable tool in the differential characterization between Essential Hypertension (EH) and primary aldosteronism (PA).

Methods

Patients with essential hypertension (EH) and PA were enrolled in the study (EH= 12, PA=24: 11 Bilateral Primary Aldosteronism subtype (BPA) and 13 Aldosterone Producing Adenoma (APA)). Clinical and biochemical parameters were available for all the subjects. UEVs were isolated from urine by ultracentrifugation and analysed by Transmission Electron Microscopy (TEM) and nanotrack particle analysis (NTA). UEVs protein content was investigated through an untargeted MS-based approach. Statistical and network analysis was performed to identify potential candidates for the identification and classification of PA.

Results

MS analysis provided more than 300 protein identifications. Exosomal markers CD9 and CD63 were detected in all samples. Several molecules characterizing EH vs PA patients as well as BPA and APA subtypes were identified after statistical elaboration and filtering of the results. In particular, some key proteins involved in water reabsorption mechanisms, such as AQP1 and AQP2, were among the best candidates for discriminating EH vs PA, as well as A1AG1 (AGP1).

Conclusion

Through this proteomic approach, we identified uEVs molecular indicators that can improve PA characterization and help in the gain of insights of the pathophysiological features of this disease. In particular, PA was characterized by a reduction of AQP1 and AQP2 expression as compared with EH.

XinLi formula, a traditional Chinese decoction, alleviates chronic heart failure via regulating the interaction of AGTR1 and AQP1

BACKGROUND

XinLi formula (XLF) is a traditional Chinese medicine used in clinical practice to treat chronic heart failure (CHF) in humans, with remarkable curative effect. However, the mechanism remains unknown.

PURPOSE

The goal of the current investigation was to determine how XLF affected CHF in a rat model of the condition brought on by ligation of the left anterior descending coronary artery, and to investigate the underlying mechanism.

STUDY DESIGN AND METHODS

Cardiac function was detected by echocardiography. The contents of myocardial enzymes, Ang II, ALD, TGF-β1, and inflammatory factors were measured by ELISA. Myocardial injury and myocardial fibrosis were evaluated by HE and Masson staining. Myocardial edema was assessed by cardiac mass index and transmission electron microscopy. Using Western blot and immunohistochemistry to examining the protein expression of inflammasome, TGF-β1, AGTR1, and AQP1 in the left ventricle. Furthermore, the interaction of AGTR1 and AQP1 was evaluated by co-immunoprecipitation.

RESULTS

XLF attenuated myocardial enzymes and myocardial injury, and improved cardiac function in rats with CHF after myocardial infarction. It also reduced Ang II and ALD levels in CHF rats, and suppressed the expression of AGTR1 and TGF-β1, finally alleviated myocardial fibrosis. By mechanism, XLF inhibited the expression of NLRP3 inflammasome proteins, reduced the plasma contents of IL-1β, IL-18, IL-6 and TNF-α. Additionally, XLF inhibited the expression of AQP1 and the interaction of AGTR1 and AQP1, alleviating myocardial edema. The common structure of the main chemical constituents of XLF were glycoside compounds with glycosyl.

CONCLUSION

XLF ameliorated CHF, which was evidenced by the alleviation of myocardial fibrosis by inhibiting AGTR1/NLRP3 signal, as well as the attenuation of myocardial edema by suppressing the interaction of AGTR1 and AQP1.

Role of Cannabinoid Type 2 Receptor Activation in Renal Fibrosis Induced by Unilateral Ureteric Obstruction in Rats

Background

Chronic kidney disease (CKD) ends mostly with renal fibrosis. The effect of CB2 receptor on renal fibrosis has been unclear. The aim of this study was to investigate the effect of CB2 receptor on renal fibrosis and the mechanisms behind it.

Methods

50 adult male Sprague-Dawley rats were divided into 5 groups; normal, sham; rats had their ureters only manipulated, UUO; rats had their left ureters ligated, and JWH post; rats had their left ureters ligated and they received JWH 133 for 14 days, JWH pre+post; rats received JWH 133 for 14 days before and after UUO procedure. Serum creatinine and BUN were assessed together with tissue MDA, GSH, and catalase. Histopathological evaluation of the renal tissue by H&E and Masson's trichrome was done. Immunohistochemical staining for TGF-β1, AQP1, Caspase-3, LC3B and p62 was performed. AQP1 and CB2 receptors genes expression was detected by quantitative RT-PCR.

Results

UUO had caused severe damage in the renal tissue with reduction of the renal function parameter accompanied by increase in the collagen deposition with increase TGF-β1 and decrease AQP1 expression.

Conclusions

The improvement of these parameters with JWH-133 suggests an anti-fibrotic role of CB2 receptor activation through reduction of oxidative stress, apoptosis, and autophagy.

The pathogenesis of idiopathic normal pressure hydrocephalus based on the understanding of AQP1 and AQP4

Idiopathic normal pressure hydrocephalus (iNPH) is a neurological disorder without a recognized cause. Aquaporins (AQPs) are transmembrane channels that carry water through cell membranes and are critical for cerebrospinal fluid circulation and cerebral water balance. The function of AQPs in developing and maintaining hydrocephalus should be studied in greater detail as a possible diagnostic and therapeutic tool. Recent research indicates that patients with iNPH exhibited high levels of aquaporin 1 and low levels of aquaporin 4 expression, suggesting that these AQPs are essential in iNPH pathogenesis. To determine the source of iNPH and diagnose and treat it, it is necessary to examine and appreciate their function in the genesis and maintenance of hydrocephalus. The expression, function, and regulation of AQPs in iNPH are reviewed in this article, in order to provide fresh targets and suggestions for future research.

PPAR-α knockout leads to elevated blood pressure response to angiotensin II infusion associated with an increase in renal α-1 Na+/K+ ATPase protein expression and activity

Peroxisome proliferator activated receptor alpha (PPAR-α) deletion has been shown to increase blood pressure (BP). We hypothesized that the BP increase in PPAR-α KO mice was mediated by increased expression and activity of basolateral Na+/K+ ATPase (NKA) pump. To address this hypothesis, we treated wild-type (WT) and PPAR-α knockout (KO) mice with a slow-pressor dose of angiotensin II (400 ng/kg·min) for 12 days by osmotic minipump. Radiotelemetry showed no significant differences in baseline mean arterial pressure (MAP) between WT and PPAR-α KO mice; however, by day 12 of infusion, MAP was significantly higher in PPAR-α KO mice (156 ± 16) compared to WT mice (138 ± 11 mmHg). NKA activity and protein expression (α1 subunit) were significantly higher in PPAR-α KO mice compared to WT mice. There was no significant difference in NKA mRNA levels. Angiotensin II further increased the expression and activity of the NKA in both genotypes along with the water channel, aquaporin 1 (Aqp1). In contrast, angiotensin II decreased the expression (64-97% reduction in band density) of sodium‑hydrogen exchanger-3 (NHE3), NHE regulatory factor-1 (NHERF1, Slc9a3r1), sodium‑potassium-2-chloride cotransporter (NKCC2), and epithelial sodium channel (ENaC) β- and γ- subunits in the renal cortex of both WT and PPAR-α KO mice, with no difference between genotypes. The sodium-chloride cotransporter (NCC) was also decreased by angiotensin II, but significantly more in PPAR-α KO (59% WT versus 77% KO reduction from their respective vehicle-treated mice). Our results suggest that PPAR-α attenuates angiotensin II-mediated increased blood pressure potentially via reducing expression and activity of the NKA.

Aestivation motifs explain hypertension and muscle mass loss in mice with psoriatic skin barrier defect

AIM

Recent evidence suggests that arterial hypertension could be alternatively explained as a physiological adaptation response to water shortage, termed aestivation, which relies on complex multi-organ metabolic adjustments to prevent dehydration. Here, we tested the hypothesis that chronic water loss across diseased skin leads to similar adaptive water conservation responses as observed in experimental renal failure or high salt diet.

METHODS

We studied mice with keratinocyte-specific overexpression of IL-17A which develop severe psoriasis-like skin disease. We measured transepidermal water loss and solute and water excretion in the urine. We quantified glomerular filtration rate (GFR) by intravital microscopy, and energy and nitrogen pathways by metabolomics. We measured skin blood flow and transepidermal water loss (TEWL) in conjunction with renal resistive indices and arterial blood pressure.

RESULTS

Psoriatic animals lost large amounts of water across their defective cutaneous epithelial barrier. Metabolic adaptive water conservation included mobilization of nitrogen and energy from muscle to increase organic osmolyte production, solute-driven maximal anti-diuresis at normal GFR, increased metanephrine and angiotensin 2 levels, and cutaneous vasoconstriction to limit TEWL. Heat exposure led to cutaneous vasodilation and blood pressure normalization without parallel changes in renal resistive index, albeit at the expense of further increased TEWL.

CONCLUSION

Severe cutaneous water loss predisposes psoriatic mice to lethal dehydration. In response to this dehydration stress, the mice activate aestivation-like water conservation motifs to maintain their body hydration status. The circulatory water conservation response explains their arterial hypertension. The nitrogen-dependency of the metabolic water conservation response explains their catabolic muscle wasting.

Higher Expression Levels of Aquaporin Family of Proteins in the Kidneys of Arid-Desert Living Lepus yarkandensis

Lepus yarkandensis specifically lives in arid climate with rare precipitation of Tarim Basin in western China. Aquaporins (AQPs) are a family of channel proteins that facilitate water transportation across cell membranes. Kidney AQPs play vital roles in renal tubule water permeability and maintenance of body water homeostasis. This study aimed to investigate whether kidney AQPs exhibit higher expression in arid-desert living animals. Immunohistochemistry results revealed localization of AQP1 to the capillary endothelial cells in glomerulus and epithelial cells in proximal tubule and descending thin limbs, AQP2 to the apical plasma membrane of principal cells in the cortical collecting duct (CCD), outer medullary collecting duct (OMCD), and IMCD cells in the initial inner medullary collecting duct (IMCD1) and middle IMCD (IMCD2), and AQP3 and AQP4 to the basolateral plasma membrane of principal cells and IMCD cells in CCD, OMCD, IMCD1, and IMCD2 in L. yarkandensis kidneys. Quantitative real-time PCR analysis showed higher mRNA levels of AQP1, AQP2, AQP3, and AQP4 in L. yarkandensis kidneys compared with Oryctolagus cuniculus. Similar results were obtained by western blotting. Our results suggested that higher expression levels of AQP1, AQP2, AQP3, and AQP4 in L. yarkandensis kidneys favored for drawing more water from the tubular fluid.

Physical Processes in Polymeric Filters Used for Dialysis

The key physical processes in polymeric filters used for the blood purification include transport across the capillary wall and the interaction of blood cells with the polymer membrane surface. Theoretical modeling of membrane transport is an important tool which provides researchers with a quantification of the complex phenomena involved in dialysis. In the paper, we present a dense review of the most successful theoretical approaches to the description of transport across the polymeric membrane wall as well as the cell⁻polymer surface interaction, and refer to the corresponding experimental methods while studying these phenomena in dialyzing filters.

Angiotensin II Signal Transduction: An Update on Mechanisms of Physiology and Pathophysiology

The renin-angiotensin-aldosterone system plays crucial roles in cardiovascular physiology and pathophysiology. However, many of the signaling mechanisms have been unclear. The angiotensin II (ANG II) type 1 receptor (AT₁R) is believed to mediate most functions of ANG II in the system. AT₁R utilizes various signal transduction cascades causing hypertension, cardiovascular remodeling, and end organ damage. Moreover, functional cross-talk between AT₁R signaling pathways and other signaling pathways have been recognized. Accumulating evidence reveals the complexity of ANG II signal transduction in pathophysiology of the vasculature, heart, kidney, and brain, as well as several pathophysiological features, including inflammation, metabolic dysfunction, and aging. In this review, we provide a comprehensive update of the ANG II receptor signaling events and their functional significances for potential translation into therapeutic strategies. AT₁R remains central to the system in mediating physiological and pathophysiological functions of ANG II, and participation of specific signaling pathways becomes much clearer. There are still certain limitations and many controversies, and several noteworthy new concepts require further support. However, it is expected that rigorous translational research of the ANG II signaling pathways including those in large animals and humans will contribute to establishing effective new therapies against various diseases.

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.

Overexpression of angiotensinogen downregulates aquaporin 1 expression via modulation of Nrf2-HO-1 pathway in renal proximal tubular cells of transgenic mice

Introduction:

We aimed to examine the regulation of aquaporin 1 expression in an angiotensinogen transgenic mouse model, focusing on underlying mechanisms.

Methods:

Male transgenic mice overexpressing rat angiotensinogen in their renal proximal tubular cells (RPTCs) and rat immortalised RPTCs stably transfected with rat angiotensinogen cDNA were used.

Results:

Angiotensinogen-transgenic mice developed hypertension and nephropathy, changes that were either partially or completely attenuated by treatment with losartan or dual renin–angiotensin system blockade (losartan and perindopril), respectively, while hydralazine prevented hypertension but not nephropathy. Decreased expression of aquaporin 1 and heme oxygenase-1 and increased expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and sodium–hydrogen exchanger 3 were observed in RPTCs of angiotensinogen-transgenic mice and in angiotensinogen-transfected immortalised RPTCs. These parameters were normalised by dual renin–angiotensin system blockade. Both in vivo and in vitro studies identified a novel mechanism in which angiotensinogen overexpression in RPTCs enhances the cytosolic accumulation of Nrf2 via the phosphorylation of pGSK3β Y216. Consequently, lower intranuclear Nrf2 levels are less efficient to trigger heme oxygenase-1 expression as a defence mechanism, which subsequently diminishes aquaporin 1 expression in RPTCs.

Conclusions:

Angiotensinogen-mediated downregulation of aquaporin 1 and Nrf2 signalling may play an important role in intrarenal renin–angiotensin system-induced hypertension and kidney injury.

Na(+), K(+), Cl(-), acid-base or H2O homeostasis in children with urinary tract infections: a narrative review

Guidelines on the diagnosis and management of urinary tract infections in childhood do not address the issue of abnormalities in Na(+), K(+), Cl(-) and acid-base balance. We have conducted a narrative review of the literature with the aim to describe the underlying mechanisms of these abnormalities and to suggest therapeutic maneuvers. Abnormalities in Na(+), K(+), Cl(-) and acid-base balance are common in newborns and infants and uncommon in children of more than 3 years of age. Such abnormalities may result from factitious laboratory results, from signs and symptoms (such as excessive sweating, poor fluid intake, vomiting and passage of loose stools) of the infection itself, from a renal dysfunction, from improper parenteral fluid management or from the prescribed antimicrobials. In addition, two transient renal tubular dysfunctions may occur in infants with infectious renal parenchymal involvement: a reduced capacity to concentrate urine and pseudohypoaldosteronism secondary to renal tubular unresponsiveness to aldosterone that presents with hyponatremia, hyperkalemia and acidosis. In addition to antimicrobials, volume resuscitation with an isotonic solution is required in these children. In secondary pseudohypoaldosteronism, isotonic solutions (such as 0.9 % saline or lactated Ringer) correct not only the volume depletion but also the hyperkalemia and acidosis. In conclusion, our review suggests that in infants with infectious renal parenchymal involvement, non-renal and renal causes concur to cause fluid volume depletion and abnormalities in electrolyte and acid-base balance, most frequently hyponatremia.

Long-term administration of tolvaptan increases myocardial remodeling and mortality via exacerbation of congestion in mice heart failure model after myocardial infarction

BACKGROUND

In contrast to loop diuretics, tolvaptan does not cause neurohormonal activation in several animal heart failure models. However, it remains unknown whether chronic vasopressin type 2 receptor blockade exerts beneficial effects on mortality in murine heart failure after myocardial infarction (MI). In an experimental heart failure model, we tested the hypothesis that tolvaptan reduces myocardial remodeling and mortality.

METHODS AND RESULTS

MI was induced in 9-week-old male C57Bl6/J by the left coronary artery ligation. In study 1, animals were randomly assigned to treatment with placebo or tolvaptan starting 14days post-MI. In study 2, animals were randomized to tolvaptan or furosemide+tolvaptan starting 14days post-MI. Interestingly, results showed lower survival rate in tolvaptan group compared to placebo. Tolvaptan group had higher serum osmolality, heavier body weight, more severe myocardial remodeling, and lung congestion at day 28 of drug administration compared to placebo. In study 2, addition of furosemide significantly reduced mortality rate seen with tolvaptan, and presented with decreased osmolality, myocardial remodeling, and lung congestion compared to tolvaptan-treated mice. Increase in proximal tubular expression of aquaporin 1, Angiotensin II, and vasopressin seen with tolvaptan treatments were normalized to basal levels, similar to levels in placebo-treated mice.

CONCLUSIONS

Contrary to our hypothesis, tolvaptan was associated with increased mortality in murine heart failure after MI. This increase in lung congestion, myocardial remodeling, could be prevented by co-administration of furosemide, which resulted in normalized serum osmolality, neurohormonal activation, and renal aquaporin 1 expression, and hence decreased mortality post-MI.

Cyclophosphamide-induced vasopressin-independent activation of aquaporin-2 in the rat kidney

Because cyclophosphamide-induced hyponatremia was reported to occur without changes in plasma vasopressin in a patient with central diabetes insipidus, we hypothesized that cyclophosphamide or its active metabolite, 4-hydroperoxycyclophosphamide (4-HC), may directly dysregulate the expression of water channels or sodium transporters in the kidney. To investigate whether intrarenal mechanisms for urinary concentration are activated in vivo and in vitro by treatment with cyclophosphamide and 4-HC, respectively, we used water-loaded male Sprague-Dawley rats, primary cultured inner medullary collecting duct (IMCD) cells, and IMCD suspensions prepared from male Sprague-Dawley rats. In cyclophosphamide-treated rats, significant increases in renal expression of aquaporin-2 (AQP2) and Na-K-2Cl cotransporter type 2 (NKCC2) were shown by immunoblot analysis and immunohistochemistry. Apical translocation of AQP2 was also demonstrated by quantitative immunocytochemistry. In both rat kidney and primary cultured IMCD cells, significant increases in AQP2 and vasopressin receptor type 2 (V2R) mRNA expression were demonstrated by real-time quantitative PCR analysis. Confocal laser-scanning microscopy revealed that apical translocation of AQP2 was remarkably increased when primary cultured IMCD cells were treated with 4-HC in the absence of vasopressin stimulation. Moreover, AQP2 upregulation and cAMP accumulation in response to 4-HC were significantly reduced by tolvaptan cotreatment in primary cultured IMCD cells and IMCD suspensions, respectively. We demonstrated that, in the rat kidney, cyclophosphamide may activate V2R and induce upregulation of AQP2 in the absence of vasopressin stimulation, suggesting the possibility of drug-induced nephrogenic syndrome of inappropriate antidiuresis (NSIAD).

Sex-dependent expression of water channel AQP1 along the rat nephron

In the mammalian kidney, nonglycosylated and glycosylated forms of aquaporin protein 1 (AQP1) coexist in the luminal and basolateral plasma membranes of proximal tubule and descending thin limb. Factors that influence AQP1 expression in (patho)physiological conditions are poorly known. Thus far, only angiotensin II and hypertonicity were found to upregulate AQP1 expression in rat proximal tubule in vivo and in vitro (Bouley R, Palomino Z, Tang SS, Nunes P, Kobori H, Lu HA, Shum WW, Sabolic I, Brown D, Ingelfinger JR, Jung FF. Am J Physiol Renal Physiol 297: F1575-F1586, 2009), a phenomenon that may be relevant for higher blood pressure observed in men and male experimental animals. Here we investigated the sex-dependent AQP1 protein and mRNA expression in the rat kidney by immunochemical methods and qRT-PCR in tissue samples from prepubertal and intact gonadectomized animals and sex hormone-treated gonadectomized adult male and female animals. In adult rats, the overall renal AQP1 protein and mRNA expression was ∼80% and ∼40% higher, respectively, in males than in females, downregulated by gonadectomy in both sexes and upregulated strongly by testosterone and moderately by progesterone treatment; estradiol treatment had no effect. In prepubertal rats, the AQP1 protein expression was low compared with adults and slightly higher in females, whereas the AQP1 mRNA expression was low and similar in both sexes. The observed differences in AQP1 protein expression in various experiments mainly reflect changes in the glycosylated form. The male-dominant expression of renal AQP1 in rats, which develops after puberty largely in the glycosylated form of the protein, may contribute to enhanced fluid reabsorption following the androgen- or progesterone-stimulated activities of sodium-reabsorptive mechanisms in proximal tubules.

The water channel aquaporin-1 contributes to renin cell recruitment during chronic stimulation of renin production

Both the processing and release of secretory granules involve water movement across granule membranes. It was hypothesized that the water channel aquaporin (AQP)1 directly contributes to the recruitment of renin-positive cells in the afferent arteriole. AQP1−/− and AQP1+/+ mice were fed a low-salt (LS) diet [0.004% (wt/wt) NaCl] for 7 days and given enalapril [angiotensin-converting enzyme inhibitor (ACEI), 0.1 mg/ml] in drinking water for 3 days. There were no differences in plasma renin concentration at baseline. After LS-ACEI, plasma renin concentrations increased markedly in both genotypes but was significantly lower in AQP1−/− mice compared with AQP1+/+ mice. Tissue renin concentrations were higher in AQP1−/− mice, and renin mRNA levels were not different between genotypes. Mean arterial blood pressure was not different at baseline and during LS diet but decreased significantly in both genotypes after the addition of ACEI; the response was faster in AQP1−/− mice but then stabilized at a similar level. Renin release after 200 μl blood withdrawal was not different. Isoprenaline-stimulated renin release from isolated perfused kidneys did not differ between genotypes. Cortical tissue norepinephrine concentrations were lower after LS-ACEI compared with baseline with no difference between genotypes. Plasma nitrite/nitrate concentrations were unaffected by genotype and LS-ACEI. In AQP1−/− mice, the number of afferent arterioles with recruitment was significantly lower compared with AQP1+/+ mice after LS-ACEI. We conclude that AQP1 is not necessary for acutely stimulated renin secretion in vivo and from isolated perfused kidneys, whereas recruitment of renin-positive cells in response to chronic stimulation is attenuated or delayed in AQP1−/− mice.

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.

Decrease of renal aquaporins 1-4 is associated with renal function impairment in pediatric congenital hydronephrosis

BACKGROUND

Renal aquaporins (AQP1-4) concentration is downregulated and is in proportion to the degree of hydronephrosis graded by ultrasound in pediatric congenital hydronephrosis (CH). However, the relationship between the expression of AQP1-4 with the changes of renal function impairment (RFI) evaluated by (99m)Tc-DTPA renal dynamic imaging is still unclear. This study aimed to investigate the relationship between AQP1-4 expression and degree of RFI in children with CH.

METHODS

The expression of AQP1-4 was evaluated in 45 children with unilateral ureteropelvic junction obstruction (28 boys and 17 girls, average age: 28±10 months) and 15 children undergoing nephrectomy for nephroblastoma (8 boys and 7 girls, average age: 26±8 months) by immunoblotting and immunohistochemistry. Renal function was graded into mild and severe RFI by (99m)Tc-DTPA renal dynamic imaging.

RESULTS

One-way analysis of variance with Bonferonni's correction showed a significantly reduced protein expression of AQP1-4 in the severe RFI group compared with those in both mild RFI group and controls (AQP1: 0.52±0.09 vs. 0.91±0.06 vs. 1.23±0.033; AQP2: 0.68±0.12 vs. 1.09±0.06 vs. 1.52±0.08; AQP3: 0.59±0.16 vs. 0.94±0.08 vs. 1.31±0.07; AQP4: 0.64±0.06 vs. 1.14±0.07 vs. 1.61±0.07; P<0.001, respectively). In kidneys with severe RFI, there was a reduction in the protein concentration of all four AQP isoforms which was more pronounced compared with those seen in kidneys with mild RFI and in the controls.

CONCLUSION

AQP1-4 expression is reduced in proportion with the impairment degree of renal function graded by (99m)Tc-DTPA renal dynamic imaging in human CH.

Early alteration of urinary exosomal aquaporin 1 and transforming growth factor β1 after release of unilateral pelviureteral junction obstruction

BACKGROUND/PURPOSE

Down-regulation of aquaporin 1 (AQP1) and up-regulation of transforming growth factor β(1) (TGF-β(1)) in the renal parenchyma have been demonstrated in children who underwent pyeloplasty for pelviureteral junction obstruction. However, no information about urinary exosomal AQP1 and TGF-β(1) during postobstructive polyuria in children with congenital unilateral hydronephrosis is available. The aim of the present study is to evaluate the urine concentration of exosomal AQP1 and TGF-β(1) on the first and the second day after surgery in children who underwent pyeloplasty.

METHODS

Twenty-two patients (age, 36.2 ± 17.1 months) with unilateral pelviureteral junction obstruction were examined in the study. For the first 2 days after the operation, the urine was collected separately from pyelostomy draining only from the postobstructed kidney and from the bladder catheter draining mostly from the contralateral kidney, which was used as an internal control. Urinary output, urinary osmolality, sodium, β(2)-microglobulin (β(2)-MG), and creatinine, as well as urinary exosomal AQP1 and TGF-β(1) excretion, were tested in each sample.

RESULTS

After pyeloplasty, a significantly decreased urinary excretion of exosomal AQP1 (≈ 64%) was found in the postobstructed kidney. The patients developed polyuria (807 ± 216 mL/24 h vs 484 ± 144 mL/24 h at day 1, 1021 ± 348 mL/24 h vs 603 ± 228 mL/24 h at day 2; P < .01) and reduced urine osmolality (115 ± 44 mOsm/kg vs 282 ± 61 mOsm/kg at day 1, 139 ± 39 vs 303 ± 46 mOsm/kg at day 2; P < .01) that persisted for 48 hours. In parallel, urinary TGF-β(1) and β(2)-MG (normalized for creatinine) from the postobstructed kidney were significantly higher compared with the contralateral kidney. The urine output and urinary sodium concentration from the postobstructed kidney elevated significantly on the second day after the release of obstruction compared with those on the first day. The contralateral kidney also showed same trends.

CONCLUSIONS

The down-regulation of urinary exosomal AQP1 in the postobstructed kidney may account for the polyuria, hypotonic urine, and elevated urinary β(2)-MG. The urinary TGF-β(1) level locally increased in the postobstructed kidney may be involved in renal AQP1 down-regulation.

Salt-induced downregulation of renal aquaporins is prevented by losartan

AIMS

The purpose of this study was to investigate the expression of aquaporin-1 (AQP-1) and aquaporin-2 (AQP-2) in the renal tubule of rats fed with a high-salt diet and its modulation by the AT1 receptor blocker losartan.

MAIN METHODS

The experiments were performed in four groups of rats fed for 3 weeks with the following diets: regular rat chow (NS); high-salt (8% NaCl) chow (HS), NS plus losartan (NS-L) and HS plus losartan (HS-L). Losartan (40 mg x kg(-1)) was administered in the drinking water. Systolic blood pressure (SBP) and renal function were evaluated. The intrarenal levels of angiotensin II (Ang II), TGF-β(1), α-smooth muscle actin (α-SMA), endothelial nitric oxide synthase (eNOS), AQP-1 and AQP-2 were determined by immunohistochemistry. AQP-1 and AQP-2 protein levels were measured by western blot analysis.

KEY FINDINGS

A high-sodium diet downregulated AQP-1 and AQP-2 expression levels in the proximal tubule and collecting duct, respectively. The high-sodium diet also induced Ang II, TGF-β(1) and α-SMA overexpression and decreased eNOS expression in the renal cortex and medulla. Losartan increased the diuresis and natriuresis, favoring urinary sodium concentration. Additionally, losartan prevented the profibrogenic response, decreasing Ang II, TGF-β(1) and α-SMA levels and normalizing AQP-2 expression in the HS-L group. AQP-1 expression was upregulated by losartan in both the NS-L and HS-L groups.

SIGNIFICANCE

These results show that increased intrarenal Ang II in rats fed with a high-salt diet downregulates renal AQP-1 and AQP-2 expressions. In addition, although losartan increased diuresis and natriuresis, it prevented the downregulation of aquaporins, favoring urinary sodium concentration.

Rapid aquaporin translocation regulates cellular water flow: mechanism of hypotonicity-induced subcellular localization of aquaporin 1 water channel

The control of cellular water flow is mediated by the aquaporin (AQP) family of membrane proteins. The structural features of the family and the mechanism of selective water passage through the AQP pore are established, but there remains a gap in our knowledge of how water transport is regulated. Two broad possibilities exist. One is controlling the passage of water through the AQP pore, but this only has been observed as a phenomenon in some plant and microbial AQPs. An alternative is controlling the number of AQPs in the cell membrane. Here, we describe a novel pathway in mammalian cells whereby a hypotonic stimulus directly induces intracellular calcium elevations through transient receptor potential channels, which trigger AQP1 translocation. This translocation, which has a direct role in cell volume regulation, occurs within 30 s and is dependent on calmodulin activation and phosphorylation of AQP1 at two threonine residues by protein kinase C. This direct mechanism provides a rationale for the changes in water transport that are required in response to constantly changing local cellular water availability. Moreover, because calcium is a pluripotent and ubiquitous second messenger in biological systems, the discovery of its role in the regulation of AQP translocation has ramifications for diverse physiological and pathophysiological processes, as well as providing an explanation for the rapid regulation of water flow that is necessary for cell homeostasis.

Altered expression of renal aquaporins and α-adducin polymorphisms may contribute to the establishment of salt-sensitive hypertension

BACKGROUND

Sodium-sensitive hypertension is caused by renal tubular dysfunction, leading to increased retention of sodium and water. Previous findings have suggested that single-nucleotide polymorphisms of the cytoskeletal protein, α-adducin, are associated with increased membrane expression of the Na/K pump and abnormal renal sodium transport in Milan hypertensive strain (MHS) rats and in humans. However, the possible contribution of renal aquaporins (AQPs) to water retention remains undefined in MHS rats.

METHODS

Kidneys from MHS rats were analyzed and compared with those from age-matched Milan normotensive strain (MNS) animals by quantitative-PCR, immunoblotting, and immunoperoxidase. Endocytosis assay was performed on renal cells stably expressing AQP4 and co-transfected either with wild-type normotensive (NT) or with mutated hypertensive (HT) α-adducin.

RESULTS

Semiquantitative immunoblotting revealed that AQP1 abundance was significantly decreased only in HT MHS whereas AQP2 was reduced in both young pre-HT and adult-HT animals. On the other hand, AQP4 was dramatically upregulated in MHS regardless of the age. These results were confirmed by immunoperoxidase microscopy. Endocytosis assays clearly showed that the expression of mutated adducin strongly reduced the rate of constitutive AQP4 endocytosis, thereby increasing its abundance at the plasma membrane.

CONCLUSIONS

We provide here the first evidence that AQP1, AQP2, and AQP4 are dysregulated in the kidneys of MHS animals. In particular, we provide evidence that α-adducin mutations may be responsible for AQP4 upregulation. The downregulation of AQP1 and AQP2 and the upregulation of AQP4 may be relevant for the onset and maintenance of salt-sensitive hypertension.

Molecular imaging of the kidneys

Radionuclide imaging of the kidneys with gamma cameras involves the use of labeled molecules seeking functionally critical molecular mechanisms to detect the pathophysiology of the diseased kidneys and achieve an early, sensitive, and accurate diagnosis. The most recent imaging technology, positron emission tomography, permits quantitative imaging of the kidney at a spatial resolution appropriate for the organ. H(2)(15)O, (82)RbCl, and [(64)Cu] ETS are the most important radiopharmaceuticals for measuring renal blood flow. The renin angiotensin system is the most important regulator of renal blood flow; this role is being interrogated by detecting angiotensin receptor subtype angiotensin subtype 1 receptor by the use of in vivo positron emission tomography. Membrane organic anion transporters are important for the function of the tubular epithelium; therefore, Tc99m MAG3 as well as some novel radiopharmaceuticals, such as copper-64 labeled mono oxo-tetraazamacrocyclic ligands, have been used for molecular renal imaging. In addition, other radioligands that interact with the organic cation transporters or peptide transporters have been developed. Focusing on early detection of kidney injury at the molecular level is an evolving field of great significance. Potential imaging targets are the kidney injury molecule 1, which is highly expressed in kidney injury and renal cancer but not in normal kidneys. Although pelvic clearance, in addition to parenchymal transport, is an important measure in obstructive nephropathy, techniques that focus on up-regulated molecules in response to tissue stress resulting from obstruction will be of great implication. Monocyte chemoattractant protein-1 is a well-suited molecule here. The greatest advances in molecular imaging of the kidneys have been recently achieved in detecting renal cancer. In addition to the ubiquitous [(18)F] fluorodeoxyglucose, other radioligands, such as [(11)C] acetate and anti-1-amino-3-[18F]fluorocyclobutane-1-carboxylic acid, have emerged. Radioimmunoimaging with [(124)I] G250 could lead to radioimmunotherapy for renal cancer. Considering the increasing age of general population, the incidence of kidney diseases, such as atherosclerosis, diabetic nephropathy, and cancer, is expected to increase. Successful management of these diseases offers an opportunity and a challenge for development of novel molecular imaging technologies.

Aquaporin-1: New Developments and Perspectives for Peritoneal Dialysis

Peritoneal dialysis involves diffusive and convective transport and osmosis through the highly vascularized peritoneal membrane. Several lines of evidence have demonstrated that the water channel aquaporin-1 (AQP1) corresponds to the ultrasmall pore predicted by the model of peritoneal transport. Proof-of-principle studies have shown that upregulation of the expression of AQP1 in peritoneal capillaries results in increased water permeability and ultrafiltration, without affecting the osmotic gradient or small solute permeability. Conversely, studies in Aqp1 mice have shown that haplo-insufficiency for AQP1 results in significant attenuation of water transport. Recent studies have demonstrated that AQP1 is involved in the migration of different cell types, including endothelial cells. In parallel, chemical screening has identified lead compounds that could act as antagonists or agonists of AQPs, with description of putative binding sites and potential mechanisms of gating the water channel. By modulating water transport, these pharmacological agents could have clinically relevant effects in targeting specific tissues or disease states.