Altered expression of major renal Na transporters in rats with unilateral ureteral obstruction

Obstructive nephropathy and molecular pathophysiology of renal interstitial fibrosis

The kidneys play a key role in maintaining total body homeostasis. The complexity of this task is reflected in the unique architecture of the organ. Ureteral obstruction greatly affects renal physiology by altering hemodynamics, changing glomerular filtration and renal metabolism, and inducing architectural malformations of the kidney parenchyma, most importantly renal fibrosis. Persisting pathological changes lead to chronic kidney disease, which currently affects ∼10% of the global population and is one of the major causes of death worldwide. Studies on the consequences of ureteral obstruction date back to the 1800s. Even today, experimental unilateral ureteral obstruction (UUO) remains the standard model for tubulointerstitial fibrosis. However, the model has certain limitations when it comes to studying tubular injury and repair, as well as a limited potential for human translation. Nevertheless, ureteral obstruction has provided the scientific community with a wealth of knowledge on renal (patho)physiology. With the introduction of advanced omics techniques, the classical UUO model has remained relevant to this day and has been instrumental in understanding renal fibrosis at the molecular, genomic, and cellular levels. This review details key concepts and recent advances in the understanding of obstructive nephropathy, highlighting the pathophysiological hallmarks responsible for the functional and architectural changes induced by ureteral obstruction, with a special emphasis on renal fibrosis.

Diagnostic Accuracy of Copeptin in the Differential Diagnosis of patients with Diabetes Insipidus: A Systematic Review and Meta-analysis

OBJECTIVE

Accurate diagnosis of diabetes insipidus (DI) is of significant importance for correct management. We aimed to evaluate the diagnostic accuracy of copeptin measurements in the differential diagnosis between DI and primary polydipsia (PP).

METHODS

A literature search of electronic databases from Jan. 1st, 2005 to July 13th, 2022 was performed. Primary studies that evaluated the diagnostic accuracy of copeptin in patients with DI and PP were eligible. Two reviewers independently screened relevant articles and extracted data. The Quality Assessment of Diagnostic Accuracy Studies (QUADAS)-2 tool was used to assess the quality of the included studies. The hierarchical summary receiver operating characteristic model and bivariate method were used.

RESULTS

Seven studies including 422 patients with polydipsia-polyuria syndrome (PPS), 189 (44.79%) of whom with arginine vasopressin-deficiency (AVP-D, cranial DI) and 212 (50.24%) with PP, were included. The summary estimates of the diagnostic performance of stimulated copeptin to differentiate between PP and AVP-D were 0.93 (95% confidence interval [CI] 0.89-0.97) for sensitivity, 0.96 (95% CI 0.88-1.00) for specificity, respectively. Baseline copeptin level had high performance to identify AVP- resistance (AVP-R, nephrogenic DI), with pooled sensitivity of 1.00 (95% CI 0.82-1.00) and specificity of 1.00 (95% CI 0.98-1.00); however, it showed little value in the differentiation between PP and AVP-D.

CONCLUSION

Copeptin measurement is a useful tool for the differential diagnosis of patients with DI and PP. Stimulation before copeptin measurement is necessary in the diagnosis of AVP-D.

Anti-Na+/K+-ATPase DR antibody attenuates UUO-induced renal fibrosis through inhibition of Na+/K+-ATPase α1-dependent HMGB1 release

Reduced Na⁺/K⁺-ATPase (NKA) activity and NKAα1 expression are engaged in the pathologies of renal diseases. NKA-mediated Src activation is not the only reason for NKA-related renal fibrosis. In this study, we found that genetic reduction of NKAα1 exhibited exacerbated tubulointerstitial lesions and fibrosis in the UUO mice model. Activation of NKAα1 with an antibody against the extracellular DR region of the NKAα1 subunit (DRm217) prevented UUO-induced tubulointerstitial lesions, preserved kidney function, and decrease renal fibrosis. Further studies revealed that NKAα1 deficiency mice exhibited high inflammation factors expression when they suffered UUO surgery, compared with NKAα1^+/+ (WT) mice. DRm217 alleviated inflammatory cell infiltration, suppress NF-κB phosphorylation, and decreased inflammatory factors expression in the UUO mice model. Released HMGB1 can trigger the inflammatory response and contribute to renal fibrosis. Knockdown of NKA in renal tubular cells or in NKAα1^+/- mice was associated with more susceptibility to HMGB1 release in the UUO mice model. DRm217 exerted its antifibrotic effect via inhibiting HMGB1 release. Furthermore, AMPK activation participates in the effect of DRm217 on inhibiting HMGB1 release. Our findings suggest that NKAα1 is a regulator of renal fibrosis and its DR-region is a novel target on it.

Ion channels as a therapeutic target for renal fibrosis

Renal ion channel transport and electrolyte disturbances play an important role in the process of functional impairment and fibrosis in the kidney. It is well known that there are limited effective drugs for the treatment of renal fibrosis, and since a large number of ion channels are involved in the renal fibrosis process, understanding the mechanisms of ion channel transport and the complex network of signaling cascades between them is essential to identify potential therapeutic approaches to slow down renal fibrosis. This review summarizes the current work of ion channels in renal fibrosis. We pay close attention to the effect of cystic fibrosis transmembrane conductance regulator (CFTR), transmembrane Member 16A (TMEM16A) and other Cl− channel mediated signaling pathways and ion concentrations on fibrosis, as well as the various complex mechanisms for the action of Ca2+ handling channels including Ca2+-release-activated Ca2+ channel (CRAC), purinergic receptor, and transient receptor potential (TRP) channels. Furthermore, we also focus on the contribution of Na+ transport such as epithelial sodium channel (ENaC), Na+, K+-ATPase, Na+-H+ exchangers, and K+ channels like Ca2+-activated K+ channels, voltage-dependent K+ channel, ATP-sensitive K+ channels on renal fibrosis. Proposed potential therapeutic approaches through further dissection of these mechanisms may provide new therapeutic opportunities to reduce the burden of chronic kidney disease.

Prenatal cyanuric acid exposure disrupts cognitive flexibility and mGluR1-mediated hippocampal long-term depression in male rats

Cyanuric acid is one of the most widely used classes of industrial chemicals and is now well known as food adulterant and contaminant in pet food and infant formula. Previously, it was reported that animals prenatally exposed to cyanuric acid showed neurotoxic effects that impaired memory consolidating and suppressed long-term potentiation (LTP) in the hippocampus. However, it is not clear if prenatal exposure to cyanuric acid induces deficits in reversal learning and long-term depression (LTD), which is required for the developmental reorganization of synaptic circuits and updating learned behaviors. Here, pregnant rats were i.p. injected with cyanuric acid (20 mg/kg) during the whole of gestation, and male offspring were selected to examine the levels of hippocampal mGluR1 and mGluR2/3 in young adulthood. The LTD at the Schaffer collateral-CA1 pathway was induced by low-frequency stimulation (LFS) and recorded. Reversal learning and hippocampus-dependent learning strategy were tested in Morris-water maze (MWM) and T-maze tasks, respectively. To further confirm the potential mechanism, selective agonists of mGluR1 and mGluR2/3 and antagonists of mGluR were intra-hippocampal infused before behavioral and neuronal recording. We found the levels of alkaline phosphatase were markedly increased in the maternal placenta and fetal brain following prenatal exposure. The expression of mGluR1 but not mGluR2/3 was significantly decreased and mGluR1-mediated LTD was selectively weakened. Prenatal cyanuric acid impaired reversal learning ability, without changing place learning strategy. The mGluR1 agonist could effectively enhance LFS-induced LTD and mitigate reversal learning deficits. Meanwhile, the reductions in the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR)-mediated spontaneous excitatory postsynaptic currents (sEPSCs) amplitude and frequency of cyanuric acid offspring were simultaneously alleviated by mGluR1 agonist infusions. Therefore, the results indicate the cognitive and synaptic impairments induced by prenatal cyanuric acid exposure are attributed to the disruption of the hippocampal mGluR1 signaling. Our findings provided the first evidence for the deteriorated effects of cyanuric acid on synaptic depression and advanced cognitive performance.

Copeptin in fluid disorders and stress

Copeptin, a glycosylated peptide of 39 amino acids, is the C-terminal segment of arginine vasopressin (AVP) precursor peptide, which is consisted of two other fragments, vasopressin and neurophysin Ⅱ. The main physiological functions of AVP are fluid and osmotic balance, cardiovascular homeostasis and regulation of the endocrine stress response. Numerous studies have demonstrated that the endogenous AVP in plasma is a meaningful biomarker to guide diagnosis and therapy of diseases associated with fluids disorders and stress. However, due to its instability, short half-time life in circulation and lack of readily available AVP assays, clinical measurement of AVP is restricted. In contrast to AVP, copeptin which is released in an equimolar mode with AVP from the pituitary, has emerged as a stable and simple-to-measure surrogate marker of AVP and displays excellent potential in diagnosis, differentiation and prognosis of various diseases. This review will discuss the studies on the clinical value of copeptin in different diseases, especially in AVP-dependent fluids disorders, as well as issues and prospects of the application of this potential biomarker.

Prenatal cyanuric acid exposure depresses hippocampal synaptic plasticity and induces spatial learning and memory deficits

The infant and fetus may be exposed to cyanuric acid (CA) via several different routes into the diet or milk product as well as deliberate contamination. Previous findings indicated chronic CA treatment caused neurotransmission and synaptic impairment in the early developing hippocampus. This study was designed to characterize the effects of different doses (10 mg/kg, 20 mg/kg and 40 mg/kg) of CA exposure on the developing fetus. Pregnant rats were intraperitoneally exposed to CA during the entire period of gestation and male offspring were selected for water maze task, neural recording and N-methyl-d-aspartate (NMDA) receptor detection around the eighth postnatal week. We found that CA exposure impaired the learning and memory function in a dose-dependent manner. The paired-pulse ratio (PPR) and GluN2A-dependent long-term potentiation (LTP) at the Schaffer collateral-CA1 pathway were affected in CA-exposed rats. Remarkably, hippocampal levels of NMDA-GluN2A, but not NMDA-GluN2B, were significantly decreased. Meanwhile, the spine density of hippocampal CA1 neurons was not altered by the CA exposure. Our findings are consistent with the hypothesis that CA treatment during the prenatal period produces deficits in spatial cognition by disrupting hippocampal synaptic function.

The use of urinary osmolality to evaluate postoperative renal function in children with ureteropelvic junction obstruction

INTRODUCTION

Split renal function measured in a diuretic renogram is the most popular tool in initial assessment and follow-up of patients with ureteropelvic junction obstruction (UPJO). This study aims to evaluate the use of maximum urinary osmolality after desmopressin administration (DDAVP) to detect renal dysfunction.

PATIENTS AND METHODS

56 children (33 males, 23 females) diagnosed with UPJO underwent quantification of the maximum urinary osmolality (UOsm) at diagnosis. 41 of these children (28 males, 13 females) underwent surgery for UPJO and quantification of the UOsm before and after the surgical intervention (six to 18 months postoperatively) and were included in this longitudinal study.

RESULTS AND DISCUSSION

At diagnosis, UOsm measured after desmopressin administration was abnormal in 64% of patients. After surgical intervention, this rate decreased to 53%. At initial assessment, high creatinine levels were found in 32% of infants younger than one year of age. Albumin/Cr and NAG/Cr ratios were elevated in 12% and 7% of cases, respectively. After surgical intervention, an improvement in the NAG/creatinine ratio and creatinine levels was observed. Preoperative split renal function of the affected kidney was less than 45% in 39% of cases, normal in 44%, and greater than 55% in 17%; in these three subgroups, no differences in renal function markers were found.

CONCLUSIONS

The most sensitive parameter to detect alterations in renal function in children with UPJO is the UOsm and, therefore, the most useful in the follow-up after surgery. No correlation was found between other functional and morphological parameters obtained on renal ultrasound and renogram.

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

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

Chronic Cyanuric Acid Exposure Depresses Hippocampal LTP but Does Not Disrupt Spatial Learning or Memory in the Morris Water Maze

Exposure to cyanuric acid (CA) causes multiple organ failure accompanied by the involvement in kinds of target proteins, which are detectable and play central roles in the CNS. The hippocampus has been identified as a brain area which was especially vulnerable in developmental condition associated with cognitive dysfunction. No studies have examined the effects of CA on hippocampal function after in vitro or in vivo treatment. Here, we aimed to examine hippocampal synaptic function and adverse behavioral effects using a rat model administered CA intraperitoneally or intrahippocampally. We found that infusion of CA induced a depression in the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), miniature excitatory postsynaptic currents (mEPSCs), or N-methyl-D-aspartate (NMDA)-mediated excitatory postsynaptic currents (EPSCs) of the CA1 neurons in dose-dependent pattern. Both intraperitoneal and intrahippocampal injections of CA suppressed hippocampal LTP from Schaffer collaterals to CA1 regions. Paired-pulse facilitation (PPF), a presynaptic phenomenon, was enhanced while the total and phosphorylated expression of NMDA-GluN1, NMDA-GluN2A, and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-GluA1 subunits were comparable between CA-treated and control groups. In Morris water maze test, both groups could effectively learn and retain spatial memory. Our studies provide the first evidence for the neurotoxic effect of CA and the insight into its potential mechanisms.

Ameliorating effect of troxerutin in unilateral ureteral obstruction induced renal oxidative stress, inflammation, and apoptosis in male rats

Unilateral ureteral obstruction (UUO) induces renal injury and troxerutin attenuates the inflammatory parameters and decreases oxidative stress. Accordingly, this study explored the renoprotective effect of troxerutin in UUO-induced renal oxidative stress, inflammation, and apoptosis in male Wistar rats. Animals were randomly separated into five groups (n = 8): control, UUO, and three UUO groups treated with troxerutin (1, 10, and 100 mg/kg). UUO-induced and vehicle/troxerutin administration was continued for 3 days. Then serum creatinine, mean arterial pressure (MAP), renal perfusion pressure (RPP), renal vascular resistance (RVR), and renal blood flow (RBF) were measured. Superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase activities, total antioxidant capacity (TAC), and malondialdehyde (MDA) levels as some oxidative stress parameters were measured in the left kidney. The immunoblotting method was applied to evaluate the cleaved caspase-3 Bax, Bcl-2, and TNF-α proteins level. The hematoxylin and eosin method was used to assess the kidney tissue damage score (KTDS). In 3 days, UUO significantly increased serum creatinine level, KTDS, RVR, MDA, Bax, cleaved caspase-3, and TNF-α protein levels (p < 0.05); and decreased RBF, TAC, SOD, catalase, GPx activity levels and Bcl-2 protein expression level in the left kidney (p < 0.05). Troxerutin (100 mg/kg) significantly attenuates the indicators alteration induced by UUO. Our findings represented that the renoprotective effect of troxerutin may be related to its anti-oxidative stress, anti-inflammation, anti-apoptosis, and RBF improver properties.

Bilateral ureteral obstruction is rapidly accompanied by ER stress and activation of autophagic degradation of IMCD proteins, including AQP2

After the release of bilateral ureteral obstruction (BUO), postobstructive diuresis from an impaired urine concentration mechanism is associated with reduced aquaporin 2 (AQP2) abundance in the inner medullary collecting duct (IMCD). However, the underlying molecular mechanism of this AQP2 reduction is incompletely understood. To elucidate the mechanisms responsible for this phenomenon, we studied molecular changes in IMCDs isolated from rats with 4-h BUO or sham operation at the early onset of AQP2 downregulation using mass spectrometry-based proteomic analysis. Two-hundred fifteen proteins had significant changes in abundances, with 65% of them downregulated in the IMCD of 4-h BUO rats compared with sham rats. Bioinformatic analysis revealed that significantly changed proteins were associated with functional Gene Ontology terms, including “cell-cell adhesion,” “cell-cell adherens junction,” “mitochondrial inner membrane,” “endoplasmic reticulum chaperone complex,” and the KEGG pathway of glycolysis/gluconeogenesis. Targeted liquid chromatography-tandem mass spectrometry or immunoblot analysis confirmed the changes in 19 proteins representative of each predominant cluster, including AQP2. Electron microscopy demonstrated disrupted tight junctions, disorganized adherens junctions, swollen mitochondria, enlargement of the endoplasmic reticulum lumen, and numerous autophagosomes/lysosomes in the IMCD of rats with 4-h BUO. AQP2 and seven proteins chosen as representative of the significantly altered clusters had a significant increase in immunofluorescence-based colocalization with autophagosomes/lysosomes. Immunogold electron microscopy confirmed colocalization of AQP2 with the autophagosome marker microtubule-associated protein 1A/1B-light chain 3 and the lysosomal marker cathepsin D in IMCD cells of rats with 4-h BUO. We conclude that enhanced autophagic degradation of AQP2 and other critical proteins, as well as endoplasmic reticulum stress in the IMCD, are initiated shortly after BUO.

The effect of oleuropein on unilateral ureteral obstruction induced-kidney injury in rats: the role of oxidative stress, inflammation and apoptosis

Unilateral ureteral obstruction (UUO) induces kidney injury. Oleuropein as a major compound of olive leaves modulates the inflammatory parameters and decreases oxidative stress. Accordingly, we evaluate the renoprotective effect of oleuropein against 3-day UUO rats. Forty rats were randomly divided into five groups (n = 8) including control, UUO and UUO + oleuropein groups (50, 100 and 200 mg/kg). UUO model was induced by left ureter ligation and continued for 3-day. Rats were treated synchronic daily for 3-day, then mean arterial pressure (MAP), renal perfusion pressure (RPP), renal blood flow (RBF), serum creatinine level, and also superoxide dismutase (SOD), glutathione peroxidase (GPx) activity levels and malondialdehyde (MDA) concentration (in the obstructed kidney) were measured. The western blotting method was applied to evaluate the Bax, Bcl-2, cleaved caspase-3 and TNF-α proteins expression level. The hematoxylin and eosin method was applied to evaluate the kidney tissue damage score (KTDS). UUO significantly increased RVR, KTDS, and MDA, cleaved caspase-3, Bax, serum creatinine and TNF-α protein levels (P < 0.05), and also significantly decreased RBF, SOD, and GPx and Bcl-2 protein expression levels (P < 0.001) in the obstructed kidney and oleuropein (200 mg/kg) significantly ameliorated the changes induced by UUO. Our findings showed that oleuropein has a renoprotective effect against 3-day UUO. The mechanisms underlying the observed effects may be related to its antioxidative stress, anti-apoptotic, and anti-inflammatory effects.

Combination exposure of melamine and cyanuric acid is associated with polyuria and activation of NLRP3 inflammasome in rats

The molecular mechanisms of melamine-induced renal toxicity have not been fully understood. The purpose of the study aimed to investigate whether melamine and cyanuric acid induced NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome activation in the kidney, which may contribute to abnormal water and sodium handling in a rat model. Wistar rats received melamine (Mel; 200 mg·kg body wt−1·day−1), cyanuric acid (CA; 200 mg·kg body wt−1·day−1), or Mel plus CA (Mel + CA; 100 mg·kg body wt−1·day−1, each) for 2 wk. Mel + CA caused damaged tubular epithelial structure and organelles, dilated tubular lumen, and inflammatory responses. Crystals were observed in urine and serum specimen, also in the lumen of dilated distal renal tubules. The combined ingestion of Mel and CA in rats caused a markedly impaired urinary concentration, which was associated with reduced protein expression of aquaporin (AQP)1, 2, and 3 in inner medulla and α-Na-K-ATPase and Na-K-2Cl transporters in cortex and outer medulla. Mel + CA treatment was associated with increased protein expression of CD3 and mRNA levels of CD68 and F4/80 as well as phosphorylation of NF-κB in the kidney. Mel + CA treatment increased protein and mRNA expression of NLRP3 inflammasome components apoptosis-associated speck-like protein containing a caspase recruitment domain, caspase-1, and IL-1β in the inner medulla of rats. NF-κB inhibitor Bay 11-7082 reduced IL-1β expression induced by Mel + CA and prevented downregulation of AQP2 in inner medullary collecting duct cell suspensions. In conclusion, Mel + CA treatment caused urinary-concentrating defects and reduced expression of renal AQPs and key sodium transporters, which is likely due to the inflammatory responses and activation of NLRP3 inflammasome induced by crystals formed in the kidney.

Bartter Syndrome Type 1 Presenting as Nephrogenic Diabetes Insipidus

Bartter syndrome (BS) type 1 (OMIM #601678) is a hereditary salt-losing renal tubular disorder characterized by hypokalemic metabolic alkalosis, hypercalciuria, nephrocalcinosis, polyuria, recurrent vomiting, and growth retardation. It is caused by loss-of-function mutations of the SLC12A1 gene, encoding the furosemide-sensitive Na-K-Cl cotransporter. Recently, a phenotypic variability has been observed in patients with genetically determined BS, including absence of nephrocalcinosis, hypokalemia, and/or metabolic alkalosis in the first year of life as well as persistent metabolic acidosis mimicking distal renal tubular acidosis. We report the case of a child with a genetically determined diagnosis of Bartter syndrome type 1 who presented with a phenotype of nephrogenic diabetes insipidus, with severe hypernatremia and urinary concentrating defect. In these atypical cases, molecular analysis is mandatory to define the diagnosis, in order to establish the correct clinical and therapeutic management.

The molecular biology of pelvi-ureteric junction obstruction

Over recent years routine ultrasound scanning has identified increasing numbers of neonates as having hydronephrosis and pelvi-ureteric junction obstruction (PUJO). This patient group presents a diagnostic and management challenge for paediatric nephrologists and urologists. In this review we consider the known molecular mechanisms underpinning PUJO and review the potential of utilising this information to develop novel therapeutics and diagnostic biomarkers to improve the care of children with this disorder.

MnTBAP therapy attenuates the downregulation of sodium transporters in obstructive kidney disease

Ureteral obstruction is associated with reduced expressions of renal sodium transporters, which contributes to impaired urinary concentrating capacity. In this study, we employed a synthetic mitochondrial superoxide dismutase 2 (SOD2) mimic MnTBAP to investigate the role of mitochondrial oxidative stress in modulating the sodium transporters in obstructive kidney disease. Following unilateral ureteral obstruction (UUO) for 7 days, a global reduction of sodium transporters including NHE3, NCC, NKCC2, and ENaCα was observed as determined by qRT-PCR, Western Blotting or immunohistochemistry. Among these sodium transporters, the downregulation of NHE3, NCC, and NKCC2 was partially reversed by MnTBAP treatment. In contrast, the reduction of ENaCα was not affected by MnTBAP. The β and γ subunits of ENaC were not significantly altered by ureteral obstruction or MnTBAP therapy. To further confirm the anti-oxidant effect of MnTBAP, we examined the levels of TBARs in the urine collected from the obstructed ureters of UUO mice and bladder of sham mice. As expected, the increment of urinary TBARs in UUO mice was entirely abolished by MnTBAP therapy, indicating an amelioration of oxidative stress. Meantime, we found that three types of SOD were all reduced in obstructed kidneys determined by qRT-PCR, which was unaffected by MnTBAP. Collectively, these results demonstrated an important role of mitochondrial oxidative stress in mediating the downregulation of sodium transporters in obstructive kidney disease.

Renal Expression and Urinary Excretion of Na-K-2Cl Cotransporter in Obstructive Nephropathy

Renal damage due to urinary tract obstruction accounts for up to 30% of acute kidney injury in paediatrics and adults. Bilateral ureteral obstruction (BUO) is associated with polyuria and reduced urinary concentrating capacity. We investigated the renal handling of water and electrolytes together with the renal expression and the urinary excretion of the Na-K-Cl cotransporter (NKCC2) after 1 (BUO-1), 2 (BUO-2), and 7 (BUO-7) days of release of BUO. Immunoblotting and immunohistochemical studies showed that NKCC2 expression was upregulated in apical membranes both from BUO-2 and from BUO-7 rats. The apical membrane expression, where NKCC2 is functional, may be sufficient to normalize water, potassium, sodium, and osmolytes tubular handling. NKCC2 abundance in homogenates and mRNA levels of NKCC2 was significantly decreased in almost all groups suggesting a decrease in the synthesis of the transporter. Urinary excretion of NKCC2 was increased in BUO-7 groups. These data suggest that the upregulation in the expression of NKCC2 in apical membranes during the postobstructive phase of BUO could contribute to improving the excretion of sodium and consequently also the excretion of potassium, osmolytes, and water. Moreover, the increase in urinary excretion of NKCC2 in BUO-7 group could be a potential additional biomarker of renal function recovery.

Deficiency of mPGES-1 exacerbates renal fibrosis and inflammation in mice with unilateral ureteral obstruction

Microsomal prostaglandin E₂ synthase-1 (mPGES-1), an inducible enzyme that converts prostaglandin H₂ to prostaglandin E₂ (PGE₂), plays an important role in a variety of inflammatory diseases. We investigated the contribution of mPGES-1 to renal fibrosis and inflammation in unilateral ureteral obstruction (UUO) for 7 days using wild-type (WT) and mPGES-1 knockout (KO) mice. UUO induced increased mRNA and protein expression of mPGES-1 and cyclooxygenase-2 in WT mice. UUO was associated with increased renal PGE₂ content and upregulated PGE₂ receptor (EP) 4 expression in obstructed kidneys of both WT and mPGES-1 KO mice; EP4 expression levels were higher in KO mice with UUO than those in WT mice. Protein expression of NLRP3 inflammasome components ASC and interleukin-1β was significantly increased in obstructed kidneys of KO mice compared with that in WT mice. mRNA expression levels of fibronectin, collagen III, and transforming growth factor-β1 (TGF-β1) were significantly higher in obstructed kidneys of KO mice than that in WT mice. In KO mice, protein expression of fibronectin and collagen III was markedly increased in obstructed kidneys compared with WT mice, which was associated with increased phosphorylation of protein kinase B (AKT). EP4 agonist CAY10598 attenuated increased expression of collagen I and fibronectin induced by TGF-β1 in inner medullary collecting duct 3 cells. Moreover, CAY10598 prevented the activation of NLRP3 inflammasomes induced by angiotensin II in human proximal tubule cells (HK2). In conclusion, these findings suggested that mPGES-1 exerts a potentially protective effect against renal fibrosis and inflammation induced by UUO in mice.

Aliskiren restores renal AQP2 expression during unilateral ureteral obstruction by inhibiting the inflammasome

Ureteral obstruction is associated with reduced expression of renal aquaporins (AQPs), urinary concentrating defects, and an enhanced inflammatory response, in which the renin-angiotensin system (RAS) may play an important role. We evaluated whether RAS blockade by a direct renin inhibitor, aliskiren, would prevent the decreased renal protein expression of AQPs in a unilateral ureteral obstruction (UUO) model and what potential mechanisms may be involved. UUO was performed for 3 days (3UUO) and 7 days (7UUO) in C57BL/6 mice with or without aliskiren injection. In 3UUO and 7UUO mice, aliskiren abolished the reduction of AQP2 protein expression but not AQP1, AQP3, and AQP4. mRNA levels of renal AQP2 and vasopressin type 2 receptor were decreased in obstructed kidneys of 7UUO mice, which were prevented by aliskiren treatment. Aliskiren treatment was also associated with a reduced inflammatory response in obstructed kidneys of UUO mice. Aliskiren significantly decreased mRNA levels of several proinflammatory factors, such as transforming growth factor-β and tumor necrosis factor-α, seen in obstructed kidneys of UUO mice. Interestingly, mRNA and protein levels of the NOD-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome components apoptosis-associated speck-like protein containing a caspase recruitment domain, caspase-1, and IL-1β were dramatically increased in obstructed kidneys of 7UUO mice, which were significantly suppressed by aliskiren. In primary cultured inner medullary collecting duct cells, IL-1β significantly decreased AQP2 expression. In conclusions, RAS blockade with the direct renin inhibitor aliskiren increased water channel AQP2 expression in obstructed kidneys of UUO mice, at least partially by preventing NLRP3 inflammasome activation in association with ureteral obstruction.

Regulation of proximal tubular cell differentiation and proliferation in primary culture by matrix stiffness and ECM components

To explore whether matrix stiffness affects cell differentiation, proliferation, and transforming growth factor (TGF)-β1-induced epithelial-mesenchymal transition (EMT) in primary cultures of mouse proximal tubular epithelial cells (mPTECs), we used a soft matrix made from monomeric collagen type I-coated polyacrylamide gel or matrigel (MG). Both kinds of soft matrix benefited primary mPTECs to retain tubular-like morphology with differentiation and growth arrest and to evade TGF-β1-induced EMT. However, the potent effect of MG on mPTEC differentiation was suppressed by glutaraldehyde-induced cross-linking and subsequently stiffening MG or by an increasing ratio of collagen in the soft mixed gel. Culture media supplemented with MG also helped mPTECs to retain tubular-like morphology and a differentiated phenotype on stiff culture dishes as soft MG did. We further found that the protein level and activity of ERK were scaled with the matrix stiffness. U-0126, a MEK inhibitor, abolished the stiff matrix-induced dedifferentiation and proliferation. These data suggest that the ERK signaling pathway plays a vital role in matrix stiffness-regulated cell growth and differentiation. Taken together, both compliant property and specific MG signals from the matrix are required for the regulation of epithelial differentiation and proliferation. This study provides a basic understanding of how physical and chemical cues derived from the extracellular matrix regulate the physiological function of proximal tubules and the pathological development of renal fibrosis.

Downregulation of renal type IIa sodium-dependent phosphate cotransporter during lipopolysaccharide-induced acute inflammation

The type IIa sodium-dependent phosphate cotransporter (Npt2a) plays a critical role in reabsorption of inorganic phosphate (Pi) by renal proximal tubular cells. Pi abnormalities during early stages of sepsis have been reported, but the mechanisms regulating Pi homeostasis during acute inflammation are poorly understood. We examined the regulation of Pi metabolism and renal Npt2a expression during lipopolysaccharide (LPS)-induced inflammation in mice. Dose-response and time-course studies with LPS showed significant increases of plasma Pi and intact parathyroid hormone (iPTH) levels and renal Pi excretion, while renal calcium excretion was significantly decreased. There was no difference in plasma 1,25-dihydroxyvitamin D levels, but the induction of plasma intact fibroblast growth factor 23 levels peaked 3 h after LPS treatment. Western blotting, immunostaining, and quantitative real-time PCR showed that LPS administration significantly decreased Npt2a protein expression in the brush border membrane (BBM) 3 h after injection, but there was no change in renal Npt2a mRNA levels. Moreover, tumor necrosis factor-α injection also increased plasma iPTH and decreased renal BBM Npt2a expression. Importantly, we revealed that parathyroidectomized rats had impaired renal Pi excretion and BBM Npt2a expression in response to LPS. These results suggest that the downregulation of Npt2a expression in renal BBM through induction of plasma iPTH levels alter Pi homeostasis during LPS-induced acute inflammation.

Pre- or post-treatment with aminoguanidine attenuates a renal distal acidification defect induced by acute ureteral obstruction in rats

Acute unilateral ureteral obstruction (UUO) impairs distal nephron acid secretion and stimulates expression of inducible nitric oxide synthase (iNOS) in post-obstructed kidney (POK). This study investigated the influence of pre- or post-treatment with aminoguanidine as a selective iNOS inhibitor on UUO-induced renal functional disturbances. To induce acute UUO, the left ureter in rats was ligated and released after 24 h. Then, a 3 h clearance period followed by bicarbonate loading and thereafter a 30 min clearance period were allocated. Aminoguanidine was administered either prior to the UUO induction or after release of the obstruction in the different rat groups, while untreated and sham groups received normal saline. During the first clearance period, fractional bicarbonate excretion and urinary pH increased markedly in the POK of the untreated group compared with the left kidney of sham group, and a large drop in the difference between urine and blood pCO2 (U–B pCO2) was observed after bicarbonate loading; all of these parameters were ameliorated in the pre-treated and post-treated groups. However, the UUO-induced decreases in creatinine clearance, sodium reabsorption, urine osmolality, and free-water reabsorption in the POK were attenuated only in the post-treated group. Therefore, the in vivo application of a selective iNOS inhibitor partially improved the acute UUO-induced distal nephron acidification defect, while post-treatment but not pre-treatment with aminoguanidine ameliorated decrements of glomerular filtration, sodium reabsorption, and urine-concentrating ability.

Alteration of renal cyclooxygenase expression due to partial unilateral ureteral obstruction in neonatal

INTRODUCTION

Impaired renal water handling in response to neonatally-induced partial unilateral ureteral obstruction (PUUO) may be associated with altered expression of cyclooxygenase (COX). The purpose of the present study was to examine whether long-term PUUO induced at birth was associated with changes of COX-2.

METHODS

Rats were subjected to PUUO (n = 14) or a sham operation (n = 12) within the first 48 hours of life. The rats were divided into 4 groups: (1) PUUO at 9 weeks (n = 7); (2) the sham operation at 9 weeks (n = 6); (3) PUUO at 15 weeks (n = 7); and (4) the sham operation at 15 weeks (n = 6). Urine and blood samples were collected before sacrificing the animals. Plasma potassium, creatinine and urea, as well as the osmolality and sodium of plasma and urine were tested in each sample. The expression of renal COX-1 and COX-2 was examined at 9 and 15 weeks in rats with neonatally-induced PUUO within the first 48 hours of life by immunoblotting and immunocytochemistry.

RESULTS

PUUO caused a marked decrease in urine osmolality and a significant increase in urinary sodium of the obstructed kidney compared with the sham-operated kidney at 9 and 15 weeks. Immunoblotting analysis showed that an abundance of COX-2 in the obstructed kidney significantly increased compared with the non-obstructed kidney and sham-operated kidney at 9 weeks (p < 0.05) and 15 weeks (p < 0.05) in rats with PUUO. In contrast, COX-1 abundance in the obstructed kidney was similar to that in the non-obstructed kidney. Immunocytochemistry confirmed these findings.

CONCLUSION

Renal COX-2 expression in the obstructed kidney is significantly altered in response to neonatally-induced PUUO. A marked increase in COX-2 indicates that it may be an important factor in reducing renal handling of sodium and water in response to PUUO.

Toward the optimal clinical use of the fraction excretion of solutes in oliguric azotemia

While the fractional excretion of solutes have long been considered excellent research tools to investigate tubular physiology, their clinical use has become common over the last 40 years in the diagnoses of many disorders; however, none have reached the clinical utility of the fractional excretion of sodium in the ability to distinguish pre-renal azotemia from acute tubular necrosis. Nevertheless, there are many drugs and medical conditions that interfere with that utility and recently other solutes, including urea, uric acid and lithium, have been recently investigated to improve the diagnostic ability in clinical situations where the fractional excretion of sodium is known to be unreliable. We review the tubular physiology of these solutes and show how the differences in tubular physiology might be exploited to develop a strategy for their optimal clinical use.

Oxygen tension correlates with regional blood flow in obstructed rat kidney

As renal tissue oxygen tension (P(O(2))) is determined by the balance between oxygen supply and consumption, direct tissue P(O(2)) measurements are essential when evaluating the presence of hypoxia. The present study aimed at evaluating invasively and continuously the renal medullary and cortical tissue P(O(2)) by novel fibre-optic probes in rats subjected to acute unilateral ureteral obstruction (AUUO). In parallel, regional blood flow measurements were obtained by MRI to investigate the relationship between regional blood flow and tissue oxygen tension. The abundance of transport proteins was determined by immunoblotting. In the obstructed kidney, AUUO caused a prompt decrease in medullary tissue P(O(2)) to 60% of baseline level whereas cortical tissue P(O(2)) was unchanged. By contrast, tissue P(O(2)) slightly increased in the non-obstructed kidney. These changes developed during the first 30 min after AUUO and persisted for the 3 h observation period. Medullary blood flow declined 1.5-2 h after induction of AUUO to 61% of baseline level in the obstructed kidney. By contrast, cortical blood flow increased to 108% of baseline level in the non-obstructed kidney. Finally, the abundance of phosphorylated aquaporin 2 decreased significantly in the obstructed kidney medulla, but increased in the obstructed kidney cortex. The Na(+)/K(+)-ATPase abundance increased in the obstructed kidney medulla whereas the Na(+)/K(+)/2Cl(-) co-transporter abundance remained unchanged in the obstructed kidney. In conclusion, measurements of regional blood flow reflect tissue P(O(2)) changes during AUUO suggesting that reduced regional blood flow is a predictor of local hypoxia. Furthermore, the abundance of major transport protein is independent of tissue P(O(2)).

Unilateral ureteral obstruction alters expression of acid-base transporters in rat kidney

PURPOSE

Unilateral ureteral obstruction is a common clinical problem that is often associated with a urinary acidification defect caused by decreased net H(+) secretion and/or HCO(3)(-) reabsorption. To clarify the molecular mechanisms of these defects we examined expression levels of key acid-base transporters along the renal nephron segments and collecting duct.

MATERIALS AND METHODS

Wistar rats (Møllegard Breeding Centre, Eiby, Denmark) underwent 24-hour unilateral ureteral obstruction, unilateral ureteral obstruction release followed for 4 days or unilateral ureteral obstruction release followed for 4 days plus experimental acidosis induced by NH(4)Cl oral administration. After sacrifice kidneys were processed for immunoblotting and immunohistochemistry.

RESULTS

Semiquantitative immunoblotting revealed that unilateral ureteral obstruction caused significant mean +/- SE down-regulation of type 3 Na(+)/H(+) exchanger to 53% +/- 9%, electrogenic Na(+)/HCO(3)(-) cotransporter to 60% +/- 9%, type 1 bumetanide sensitive Na(+)-K(+)(NH(4)(+)) -2Cl(-) cotransporter to 64% +/- 7%, electroneutral Na(+)/HCO(3)(-) cotransporter to 43% +/- 4% and anion exchanger (pendrin) to 53% +/- 10% in the obstructed kidney, which was confirmed by immunohistochemistry. After release of unilateral ureteral obstruction down-regulation of these transporters persisted together with marked down-regulation of H(+)-adenosine triphosphatase in the obstructed kidney. In rats with unilateral ureteral obstruction release followed for 4 days with experimental acidosis induced by NH(4)Cl oral administration plasma pH and HCO(3)(-) were dramatically decreased in response to NH(4)Cl for 2 days compared with those in sham operated rats with acid loading, indicating a defect in H(+) excretion and HCO(3)(-) reabsorption after obstruction release. Expression of these transporters did not change in the contralateral nonobstructed kidney of rats with unilateral ureteral obstruction and unilateral ureteral obstruction release followed for 4 days.

CONCLUSIONS

The expression of renal acid-base transporters is markedly decreased in the obstructed kidney, which may be responsible for the contribution of impaired renal H(+) excretion and HCO(3)(-) reabsorption to the urinary acidification defect in response to unilateral ureteral obstruction.

Age-dependent renal expression of acid-base transporters in neonatal ureter obstruction

Congenital obstructive nephropathy accounts for a major proportion of renal insufficiency in infancy and childhood. In an earlier investigation we demonstrated that bilateral complete ureteral obstruction (BUO) in rats is associated with inadequate urinary acidification [Am J Physiol Renal Physiol. 295(2):F497-506, 2008]. The aim of the study reported here was to determine whether this defect is also associated with unilateral ureteral obstruction (UUO), which is clinically more common than BUO. The time-course of the changes in protein expression levels of major renal acid-base transporters was examined at 7 and 14 weeks in rats with neonatally induced partial unilateral ureteral obstruction (PUUO), which was performed within the first 48 h of life. We observed that protein expression of the renal acid-base transporters NHE3, NBC1, NBCn1, pendrin and Na(+)-K(+)-ATPase was increased in both obstructed and non-obstructed kidneys 7 weeks after the induction of neonatal PUUO. This was confirmed by immunocytochemistry. In contrast, 14 weeks after the induction of PUUO, there was a significant downregulation of the renal acid-base transporters NBC1, NBCn1 and Na(+)-K(+)-ATPase in the obstructed kidneys. These time/age-dependent changes in protein expression were associated with parallel changes in renal function resulting in urine acidification in response to exogenous acid loading. In conclusion, these results show that downregulation of protein expression is a time/age-dependent response to PUUO, which could contribute to the decreased net acid excretion and development of metabolic acidosis in neonatal rats with PUUO.

Epithelial cell surface polarity: the early steps

Establishment and maintenance of epithelial cell surface polarity is of vital importance for the correct function of transporting epithelia. To maintain normal cell function, the distribution of apical and basal-lateral proteins is highly regulated and defects in expression levels or plasma membrane targeting can have severe consequences. It has been shown recently that initiation of cell-surface polarity occurs immediately upon cell-cell contact, and requires components of the lateral targeting patch, the Exocyst and the lateral SNARE complex to specify delivery of basolateral proteins to the site of cell-cell adhesion. The Exocyst and SNARE complex are present in the cytoplasm in single epithelial cells before adhesion. Upon initial cell-cell adhesion, E-cadherin accumulates at the forming contact between cells. Shortly hereafter, components of the lateral targeting patch, the Exocyst and the lateral SNARE complex, co-localize with E-cadherin at the forming contact, where they function in specifying the delivery of basal-lateral.

Implications of oxidative stress in the pathophysiology of obstructive uropathy

Although the functional and clinical alterations occurring in patients with obstructive uropathy are not well understood, it has been suggested that oxidative stress could contribute in the mechanism responsible for the impairment of sodium and water balance. This study aimed to test the hypothesis that red wine administration causes an amelioration of both the renal damage and impairment of renal Na(+), K(+)-ATPase activity occurring after ureteral obstruction in the rat. Twenty-four male Wistar adult rats weighting 200-250 g were used. Half of them received a 10-week treatment with wine as the sole fluid source, while the other group received water. Both groups were subjected to 24-h unilateral ureteral obstruction (UUO). Kidney tissue was collected following the relief of the ligature to perform the biochemical assessments. Urine and blood samples were taken at baseline and after the relief. Results show that the treatment with red wine significantly enhances the activity of antioxidant enzymes, and thus reduces renal lipid peroxidation secondary to UUO, which correlated negatively with Na(+), K(+)-ATPase activity. Based on this and other previous data, it could be suggested that red wine administration may prevent renal damage secondary to UUO by inducing enhanced antioxidant potential.

Aquaporin 2 mutations in nephrogenic diabetes insipidus

Water reabsorption in the renal collecting duct is regulated by the antidiuretic hormone vasopressin (AVP). When the vasopressin V2 receptor, present on the basolateral site of the renal principal cell, becomes activated by AVP, aquaporin-2 (AQP2) water channels will be inserted in the apical membrane, and in this fashion, water can be reabsorbed from the pro-urine into the interstitium. The essential role of the vasopressin V2 receptor and AQP2 in the maintenance of body water homeostasis became clear when it was shown that mutations in their genes cause nephrogenic diabetes insipidus, a disorder in which the kidney is unable to concentrate urine in response to AVP. This review describes the current knowledge on AQP2 mutations in nephrogenic diabetes insipidus.

Altered expression of epithelial sodium channel in rats with bilateral or unilateral ureteral obstruction

The roles of epithelial sodium channel (ENaC) subunits (α, β, and γ) in the impaired renal reabsorption of sodium and water were examined in rat models with bilateral (BUO) or unilateral ureteral obstruction (UUO) for 24 h or with BUO followed by release of obstruction and 3 days of observation (BUO-3dR). In BUO rats, plasma osmolality was increased dramatically, whereas plasma sodium concentration was decreased. Immunoblotting revealed a significantly decreased expression of α-ENaC (57 ± 7%), β-ENaC (19 ± 5%), and γ-ENaC (51 ± 10%) as well as 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) in the cortex and outer medulla (C+OM) compared with sham-operated controls. This was confirmed by immunohistochemistry. BUO-3dR was associated with polyuria and impaired renal sodium handling. The protein abundance and the apical labeling of α-ENaC were significantly increased, whereas β- and γ-ENaC as well as 11β-HSD2 expression remained decreased. In UUO rats, expression of α- and β-ENaC and 11β-HSD2 decreased in the C+OM in the obstructed kidney. In contrast, the abundance and the apical labeling of α-ENaC in the nonobstructed kidneys were markedly increased, suggesting compensatory upregulation in this kidney. In conclusion, α-, β-, and γ-ENaC expression levels are downregulated in the obstructed kidney. The expression and apical labeling of α-ENaC were increased in BUO-3dR rats and in the nonobstructed kidneys from UUO rats. These results suggest that altered expression of α-, β-, and γ-ENaC may contribute to impaired renal sodium and water handling in response to ureteral obstruction.

Candesartan prevents long-term impairment of renal function in response to neonatal partial unilateral ureteral obstruction

Angiotensin II (ANG II) plays an important role in the development of obstructive nephropathy. Here, we examined the effects of the ANG II receptor type 1 (AT1R) blockade using candesartan on long-term renal molecular and functional changes in response to partial unilateral ureteral obstruction (PUUO). Newborn rats were subjected to severe PUUO or sham operation (Sham) within the first 48 h of life. Candesartan was provided in the drinking water (10 mg·kg−1·day−1) from day 21 of life until 10 wk of age. Renal blood flow (RBF) was evaluated by MRI, glomerular filtration rate (GFR) was measured using the renal clearance of51Cr-EDTA, and the renal expression of Na-K-ATPase and the collecting duct water channel aquaporin-2 (AQP2) was examined by immunoblotting and immunocytochemistry. At 10 wk of age, PUUO significantly reduced RBF (0.8 ± 0.1 vs. 1.6 ± 0.1 ml·min−1·100 g body wt−1; P < 0.05) and GFR (37 ± 16 vs. 448 ± 111 μl·min−1·100 g body wt−1; P < 0.05) compared with Sham. Candesartan prevented the RBF reduction (PUUO+CAN: 1.6 ± 0.2 vs. PUUO: 0.8 ± 0.1 ml·min−1·100 g body wt−1; P < 0.05) and attenuated the GFR reduction (PUUO+CAN: 265 ± 68 vs. PUUO: 37 ± 16 μl·min−1·100 g body wt−1; P < 0.05). PUUO was also associated with a significant downregulation in the expression of Na-K-ATPase (75 ± 12 vs. 100 ± 5%, P < 0.05) and AQP2 (52 ± 15 vs. 100 ± 4%, P < 0.05), which were also prevented by candesartan (Na-K-ATPase: 103 ± 8 vs. 100 ± 5% and AQP2: 74 ± 13 vs. 100 ± 4%). These findings were confirmed by immunocytochemistry. Consistent with this, candesartan treatment partly prevented the reduction in solute free water reabsorption and attenuated fractional sodium excretion in rats with PUUO. In conclusion, candesartan prevents or attenuates the reduction in RBF, GFR and dysregulation of AQP2 and Na-K-ATPase in response to congenital PUUO in rats, suggesting that AT1R blockade may protect the neonatally obstructed kidney against development of obstructive nephropathy.

Natural History of Postobstructive Nephropathy: A Single-Center Retrospective Study

BACKGROUND

There is extensive published material on the effects of experimental ureteric obstruction. In contrast, there is very little literature on the natural history of postobstructive renal disease in patients.

METHODS

Patients presenting with obstructive nephropathy and renal impairment in a tertiary renal unit, and whose obstruction was successfully relieved, were identified, and the Modification of Diet in Renal Disease equation was used to calculate the glomerular filtration rate (GFR) at presentation and after relief of obstruction.

RESULTS

104 patients were identified (79 male). There were 9 deaths at 12 months and a further 4 deaths by 3 years; losses to follow-up were 7 and 20 at the same time points. The mean GFR was 9.2 +/- (SEM) 0.9 ml/min at presentation (n = 104), 27.8 +/- 1.7 ml/min at 3 months (n = 98), 29.0 +/- 1.8 ml/min at 12 months (n = 79), and 30.0 +/- 2.2 ml/min at 3 years (n = 61). Of 28 patients initially requiring dialysis, 6 were dialysis dependent at 3 months; 3 further patients, all with GFR <10 ml/min after relief of obstruction, required dialysis by 12 months and 1 further patient by 3 years.

CONCLUSIONS

Progressive deterioration in renal function is unusual after relief of obstruction in the majority of patients over a 3-year period. This relative stability in human postobstructive nephropathy contrasts with the course of remnant nephropathy in animal models.

Angiotensin II mediates downregulation of aquaporin water channels and key renal sodium transporters in response to urinary tract obstruction

The renin-angiotensin system is well known to be involved in the pathophysiological changes in renal function after obstruction of the ureter. Previously, we demonstrated that bilateral ureteral obstruction (BUO) is associated with dramatic changes in the expression of both renal sodium transporters and aquaporin water channels (AQPs). We now examined the effects of the AT1-receptor antagonist candesartan on the dysregulation of AQPs and key renal sodium transporters in rats subjected to 24-h BUO and followed 2 days after release of BUO (BUO-2R). Consistent with previous observations, BUO-2R resulted in a significantly decreased expression of AQP1, -2, and -3 compared with control rats. Concomitantly, the rats developed polyuria and reduced urine osmolality. Moreover, expression of the type 2 Na-phosphate cotransporter (NaPi-2) and type 1 bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) was markedly reduced, consistent with postobstructive natriuresis. Candesartan treatment from the onset of obstruction attenuated the reduction in GFR (3.1 ± 0.4 vs. 1.7 ± 0.3 ml·min−1·kg−1) and partially prevented the reduction in the expression of AQP2 (66 ± 21 vs. 13 ± 2%, n = 7; P < 0.05), NaPi-2 (84 ± 6 vs. 57 ± 10%, n = 7; P < 0.05), and NKCC2 (89 ± 12 vs. 46% ± 11, n = 7; P < 0.05). Consistent with this, candesartan treatment attenuated the increase in urine output (58 ± 4 vs. 97 ± 5 μl·min−1·kg−1, n = 7; P < 0.01) and the reduction in sodium reabsorption (433 ± 62 vs. 233 ± 45 μmol·min−1·kg−1, n = 7; P < 0.05) normally found in rats subjected to BUO. Moreover, candesartan treatment attenuated induction of cyclooxygenase 2 (COX-2) expression in the inner medulla, suggesting that COX-2 induction in response to obstruction is regulated by ANG II. In conclusion, candesartan prevents dysregulation of AQP2, sodium transporters, and development of polyuria seen in BUO. This strongly supports the view that candesartan protects kidney function in response to urinary tract obstruction.

Cell biological aspects of the vasopressin type-2 receptor and aquaporin 2 water channel in nephrogenic diabetes insipidus

In the renal collecting duct, water reabsorption is regulated by the antidiuretic hormone vasopressin (AVP). Binding of this hormone to the vasopressin V2 receptor (V2R) leads to insertion of aquaporin-2 (AQP2) water channels in the apical membrane, thereby allowing water reabsorption from the pro-urine to the interstitium. The disorder nephrogenic diabetes insipidus (NDI) is characterized by the kidney's inability to concentrate pro-urine in response to AVP, which is mostly acquired due to electrolyte disturbances or lithium therapy. Alternatively, NDI is inherited in an X-linked or autosomal fashion due to mutations in the genes encoding V2R or AQP2, respectively. This review describes the current knowledge of the cell biological causes of NDI and how these defects may explain the patients' phenotypes. Also, the increased understanding of these cellular defects in NDI has opened exciting initiatives in the development of novel therapies for NDI, which are extensively discussed in this review.

Na+/H+ exchangers and the regulation of volume

The regulation of volume is fundamental to life. There exist numerous conditions that can produce perturbations of cell volume. The cell has developed mechanisms to directly counteract these perturbations so as to maintain its physiological volume. Directed influx of the major extracellular cation, sodium, serves to counteract a decreased cell volume through the subsequent osmotically coupled movement of water to the intracellular space. This process, termed regulatory volume increase is often mediated by the ubiquitous sodium/hydrogen ion exchanger, NHE1. Similarly, the maintenance of intravascular volume is essential for the maintenance of blood pressure and consequently the proper perfusion of vital organs. Numerous mechanisms exist to counterbalance alterations in intravascular volume, not the least of which is the renal absorption of sodium filtered at the glomerulus. Two-thirds of filtered sodium and water are absorbed in the renal proximal tubule, a mechanism that intimately involves the apical sodium/hydrogen ion exchanger, NHE3. This isoform is fundamental to the maintenance and regulation of intravascular volume and blood pressure. In this article, the effects of cell volume on the activity of these different isoforms, NHE1 and NHE3, will be described and the consequences of their activity on intracellular and intravascular volume will be explored.

alpha-MSH prevents impairment in renal function and dysregulation of AQPs and Na-K-ATPase in rats with bilateral ureteral obstruction

The purpose of this study was to evaluate the effects of the anti-inflammatory hormone alpha-melanocyte-stimulating hormone (alpha-MSH) treatment on renal function and expression of aquaporins (AQPs) and Na-K-ATPase in the kidney in response to 24 h of bilateral ureteral obstruction (BUO) or release of BUO (BUO-R). In rats with 24-h BUO, immunoblotting revealed that downregulation of AQP2 and AQP3 was attenuated (AQP2: 38 +/- 5 vs. 13 +/- 4%; AQP3: 44 +/- 3 vs. 19 +/- 4% of sham levels; P < 0.05), whereas downregulation of Na-K-ATPase was prevented by alpha-MSH treatment (Na-K-ATPase: 94 +/- 7 vs. 35 +/- 5% of sham levels; P < 0.05). Immunocytochemistry confirmed the changes in AQP1 and Na-K-ATPase expression. Renal tubular cell apoptosis was confirmed in BUO kidneys, and alpha-MSH treatment virtually completely abolished apoptosis. Furthermore, we measured glomerular filtration rate (GFR) and effective renal plasma flow (ERPF), respectively. Forty-eight hours after BUO-R demonstrated that alpha-MSH treatment almost completely prevented the decrease in GFR (nontreated: 271 +/- 50; alpha-MSH: 706 +/- 85; sham: 841 +/- 105 microl x min(-1).100 g body wt(-1), P < 0.05) and ERPF (nontreated: 1,139 +/- 217; alpha-MSH: 2,598 +/- 129; sham: 2,633 +/- 457 microl x min(-1).100 g body wt(-1), P < 0.05). alpha-MSH treatment also partly prevented the downregulation of AQP1 and Na-K-ATPase expression in rats after BUO-R for 48 h. In conclusion, alpha-MSH treatment significantly prevents impairment in renal function and also prevents downregulation of AQP2, AQP3, and Na-K-ATPase during BUO or AQP1 and Na-K-ATPase after BUO-R, demonstrating a marked renoprotective effect of alpha-MSH treatment in conditions with urinary tract obstruction.

Validation of quantitative BOLD MRI measurements in kidney: application to unilateral ureteral obstruction

BACKGROUND

Blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI) provides a measure of deoxyhemoglobin content and therefore an indirect measure of the partial oxygen pressure (pO(2)). The main purpose of this study was to examine the relationship between the apparent relaxation rate (R2*) in the pig kidney by BOLD imaging and renal tissue pO(2) levels measured directly by oxygen-sensitive microelectrodes. Second, BOLD imaging was applied to kidneys in pigs subjected to acute unilateral ureteral obstruction (UUO) to examine whether this condition is associated with changes in intrarenal oxygenation.

METHODS

Oxygen-sensitive microelectrodes were inserted in the cortex and medulla of pig kidneys (N= 6). Different arterial and intrarenal levels of pO(2) were obtained by stepwise changing the oxygen-to-nitrogen ratio supplied by a respirator. Simultaneous BOLD MRI measurements using an R2*-sensitive Echo Planar Imaging (EPI) sequence were performed on the contralateral kidney. In another group of pigs (N= 3) BOLD imaging was performed following 24 hours of UUO.

RESULTS

When the inhaled oxygen fraction was 5% to 70%, R2* was linearly related to pO(2) levels (cortex DeltaR2*/DeltapO(2)=-1.2 ms(-1)kPa(-1), and medulla DeltaR2*/DeltapO2 =-1.7 ms(-1)kPa(-1)). Twenty-four hours of UUO was associated with an increased R2* in the cortex and a decreased R2* in medulla as compared with baseline, which remained augmented after the release of UUO, indicating that pO(2) levels were reduced in the cortex and increased in the medulla during and after release of obstruction.

CONCLUSION

BOLD MRI provides noninvasive estimates of regional renal oxygen content and our study demonstrates that this technique may provide a useful tool in UUO which is associated with altered renal oxygen consumption.

Neonatal ureteral obstruction alters expression of renal sodium transporters and aquaporin water channels

BACKGROUND

Congenital urinary tract obstruction is a common cause of renal insufficiency in the neonate and during infancy. Recently, we demonstrated that ureteral obstruction in adult rats is associated with reduction in the abundance of renal aquaporins (AQPs) and renal sodium transporters, which paralleled an impaired urinary concentrating capacity.

METHODS

In the present study, renal handling of sodium and water, together with the expression of renal aquaporins and major renal sodium transporters, was examined in rats with neonatally induced partial unilateral ureteral obstruction (PUUO) within the first 48 hours of life to clarify the molecular mechanisms involved in the tubular functional defects in response to congenital obstruction. Rats were then followed for 12 or 24 weeks.

RESULTS

Neonatal PUUO caused a progressive reduction in single kidney glomerular filtration rate (SKGFR) on the obstructed side to 43% of controls at 12 weeks (115 +/- 28 vs. 267 +/- 36 microL/min/100g bw, P < 0.05), and 31% of controls at 24 weeks (106 +/- 24 vs. 343 +/- 41 microL/min/100g bw, P < 0.05). Na-K-ATPase abundance was decreased in the obstructed kidney compared with the nonobstructed kidney at 24 weeks (79 +/- 6%, P < 0.05), and the abundance of bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1) located to the medullary thick ascending limb (mTAL) of the obstructed kidney was significantly reduced both at 12 weeks (42 +/- 10%, P < 0.05) and 24 weeks (50 +/- 10%, P < 0.05). Immunohistochemistry confirmed down-regulation of BSC-1 both at 12 and 24 weeks after onset of obstruction. Consistent with this, sodium excretion from the obstructed kidney was increased at 12 weeks (0.13 +/- 0.03 vs. 0.04 +/- 0.01 micromol/min/100g bw, P < 0.05), and persisted 24 weeks after onset of PUUO (0.15 +/- 0.02 vs. 0.06 +/- 0.01 micromol/min/100g bw, P < 0.05). AQP2 abundance in the collecting duct was also reduced both at 12 weeks (68 +/- 5%, P < 0.05) and at 24 weeks (69 +/- 13%, P < 0.05). Consistent with this, solute-free water reabsorption was decreased in the obstructed kidney at 12 weeks (0.61 +/- 0.42 vs. 1.97 +/- 0.63 microL/min/100g bw, P < 0.05) and remained decreased after 24 weeks of PUUO (0.42 +/- 0.04 vs. 1.56 +/- 0.39 microL/min/100g bw, P < 0.05).

CONCLUSION

Major sodium transporters and aquaporins in the obstructed kidney are down-regulated in response to neonatally induced PUUO, which indicates that these transporters may play a crucial role for the persistent reduction in renal handling of sodium and water in response to PUUO.

Characterization of an animal model of spontaneous congenital unilateral obstructive uropathy by cDNA microarray analysis

INTRODUCTION

The pathophysiology of congenital obstructive uropathy has been studied intensively in several animal models. A reliable parameter for early detection of relevant obstruction is not yet identified. For further investigation of the complex regulatory events of obstructive uropathy, we used a cDNA microarray technology for a parallel differential expression analysis of about 15000 different genes of obstructed, contralateral and healthy kidneys of rats with spontaneous congenital obstructive uropathy.

METHODS

Total cellular RNA of obstructed, contralateral and healthy control kidneys of 32-35 days old rats was extracted and pooled. RNA quality and quantity was assessed using a Bioanalyzer 2100. mRNA expression for selected marker genes was assessed by real time PCR. High throughput gene expression profiling was performed with cDNA microarrays with differentially labeled cDNA targets.

RESULTS

Beside expected typical alterations of several growth factors such as TGF-beta, EGF, IGF-1, PDGF, we observed overexpression of extracellular matrix proteins and a decreased activity of antioxidant enzymes. Additionally, NFkappaB, a nuclear transcription factor involved in the development of interstitial fibrosis, was slightly up-regulated in the obstructed kidneys. Down-regulation of genes involved in tubular sodium transport indicated impaired concentration ability of the obstructed kidneys. Furthermore, we found up-regulation of pro-apoptotic genes including transcripts for the proapoptotic protein Siva. Interestingly, TNFalpha, another factor involved in the pathophysiology of congenital obstructive uropathy, was not differentially regulated.

CONCLUSIONS

The alterations of the genes for growth factors, extracellular matrix proteins, antioxidant enzymes, sodium transport and genes involved in apoptosis support the representative character of this animal model for congenital obstructive uropathy. In the rather advanced stage of congenital renal obstruction a possible explanation of the lacking differential regulation of TNFalpha highlights it as a possible marker of earlier stages of obstruction. Furthermore, the possible involvement of the Siva/CD27 pathway in the apoptotic cascade also offers a new possibility to find a sensitive marker to detect renal obstruction already in an earlier stage.

Altered expression of urea transporters in response to ureteral obstruction

Urea plays an important role in the urinary concentrating capacity. Renal inner medullary (IM) urea transporter expression was examined in rats with bilateral (BUO) or unilateral ureteral obstruction (UUO). BUO (24 h) was associated with markedly increased plasma urea (42.4 +/- 1.0 vs. 5.2 +/- 0.2 mmol/l) and a significant decrease in expression of UT-A1 (28 +/- 8% of sham levels), UT-A3 (45 +/- 11%), and UT-B (70 +/- 8%). Immunocytochemistry confirmed downregulation of UT-A1 and UT-A3 in IM collecting duct and UT-B in the descending vasa recta. Three days after release of BUO, UT-A1, UT-A3, and UT-B remained significantly downregulated (UT-A1: 37 +/- 6%; UT-A3: 25 +/- 6%; and UT-B: 10 +/- 5% of sham levels; P < 0.05) concurrent with a persistent polyuria and a marked reduction in solute-free water reabsorption (115 +/- 11 vs. 196 +/- 8 microl.min(-1).kg(-1), P < 0.05). Moreover, 14 days after release of BUO, total UT-A1, UT-A3, and UT-B remained significantly decreased compared with sham-operated controls and urine urea remained reduced (588 +/- 43 vs. 1,150 +/- 94 mmol/l). Consistent with increased levels of plasma urea 24 h after onset of UUO (7.4 +/- 0.3 vs. 4.8 +/- 0.3 mmol/l), the protein abundance of UT-A1, UT-A3, and UT-B in IM was markedly reduced in the obstructed kidney, which was confirmed by immunocytochemistry. In the nonobstructed kidney, the expression of urea transporters did not change. In conclusion, reduced expression of UT-A1, UT-A3, and UT-B levels in both BUO and UUO rats suggests that urea transporters play important roles in the impaired urinary concentrating capacity in response to urinary tract obstruction.

Molecular physiology of cation-coupled Cl- cotransport: the SLC12 family

The electroneutral cation-chloride-coupled cotransporter gene family ( SLC12) was identified initially at the molecular level in fish and then in mammals. This nine-member gene family encompasses two major branches, one including two bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporters and the thiazide-sensitive Na(+):Cl(-) cotransporter. Two of the genes in this branch ( SLC12A1 and SLC12A3), exhibit kidney-specific expression and function in renal salt reabsorption, whereas the third gene ( SLC12A2) is expressed ubiquitously and plays a key role in epithelial salt secretion and cell volume regulation. The functional characterization of both alternatively-spliced mammalian Na(+)-K(+)-2Cl(-) cotransporter isoforms and orthologs from distantly related species has generated important structure-function data. The second branch includes four genes ( SLC12A4- 7) encoding electroneutral K(+)-Cl(-) cotransporters. The relative expression level of the neuron-specific SLC12A5 and the Na(+)-K(+)-2Cl(-) cotransporter SLC12A2 appears to determine whether neurons respond to GABA with a depolarizing, excitatory response or with a hyperpolarizing, inhibitory response. The four K(+)-Cl(-) cotransporter genes are co-expressed to varying degrees in most tissues, with further roles in cell volume regulation, transepithelial salt transport, hearing, and function of the peripheral nervous system. The transported substrates of the remaining two SLC12 family members, SLC12A8 and SLC12A9, are as yet unknown. Inactivating mutations in three members of the SLC12 gene family result in Mendelian disease; Bartter syndrome type I in the case of SLC12A1, Gitelman syndrome for SLC12A3, and peripheral neuropathy in the case of SLC12A6. In addition, knockout mice for many members of this family have generated important new information regarding their respective physiological roles.