Molecular biology of distal nephron sodium transport mechanisms

The biogenesis of potassium transporters: implications of disease-associated mutations

The concentration of intracellular and extracellular potassium is tightly regulated due to the action of various ion transporters, channels, and pumps, which reside primarily in the kidney. Yet, potassium transporters and cotransporters play vital roles in all organs and cell types. Perhaps not surprisingly, defects in the biogenesis, function, and/or regulation of these proteins are linked to range of catastrophic human diseases, but to date, few drugs have been approved to treat these maladies. In this review, we discuss the structure, function, and activity of a group of potassium-chloride cotransporters, the KCCs, as well as the related sodium-potassium-chloride cotransporters, the NKCCs. Diseases associated with each of the four KCCs and two NKCCs are also discussed. Particular emphasis is placed on how these complex membrane proteins fold and mature in the endoplasmic reticulum, how non-native forms of the cotransporters are destroyed in the cell, and which cellular factors oversee their maturation and transport to the cell surface. When known, we also outline how the levels and activities of each cotransporter are regulated. Open questions in the field and avenues for future investigations are further outlined.

Protein Quality Control of NKCC2 in Bartter Syndrome and Blood Pressure Regulation

Mutations in NKCC2 generate antenatal Bartter syndrome type 1 (type 1 BS), a life-threatening salt-losing nephropathy characterized by arterial hypotension, as well as electrolyte abnormalities. In contrast to the genetic inactivation of NKCC2, inappropriate increased NKCC2 activity has been associated with salt-sensitive hypertension. Given the importance of NKCC2 in salt-sensitive hypertension and the pathophysiology of prenatal BS, studying the molecular regulation of this Na-K-2Cl cotransporter has attracted great interest. Therefore, several studies have addressed various aspects of NKCC2 regulation, such as phosphorylation and post-Golgi trafficking. However, the regulation of this cotransporter at the pre-Golgi level remained unknown for years. Similar to several transmembrane proteins, export from the ER appears to be the rate-limiting step in the cotransporter's maturation and trafficking to the plasma membrane. The most compelling evidence comes from patients with type 5 BS, the most severe form of prenatal BS, in whom NKCC2 is not detectable in the apical membrane of thick ascending limb (TAL) cells due to ER retention and ER-associated degradation (ERAD) mechanisms. In addition, type 1 BS is one of the diseases linked to ERAD pathways. In recent years, several molecular determinants of NKCC2 export from the ER and protein quality control have been identified. The aim of this review is therefore to summarize recent data regarding the protein quality control of NKCC2 and to discuss their potential implications in BS and blood pressure regulation.

Effect of low sodium and high potassium diet on lowering blood pressure and cardiovascular events

Incorporating aggressive lifestyle modifications along with antihypertensive medication therapy is a crucial treatment strategy to enhance the control rate of hypertension. Dietary modification is one of the important lifestyle interventions for hypertension, and it has been proven to have a clear effect. Among food ingredients, sodium and potassium have been found to have the strongest association with blood pressure. The blood pressure-lowering effect of a low sodium diet and a high potassium diet has been well established, especially in hypertensive population. A high intake of potassium, a key component of the Dietary Approaches to Stop Hypertension (DASH) diet, has also shown a favorable impact on the risk of cardiovascular events. Additionally, research conducted with robust measurement methods has shown cardiovascular benefits of low-sodium intake. In this review, we aim to discuss the evidence regarding the relationship between the low sodium and high potassium diet and blood pressure and cardiovascular events.

Greater natriuretic response to ENaC inhibition in male versus female Sprague-Dawley rats

Genes for the epithelial sodium channel (ENaC) subunits are expressed in a circadian manner, but whether this results in time-of-day differences in activity is not known. Recent data show that protein expression of ENaC subunits is higher in kidneys from female rats, yet females are more efficient in excreting an acute salt load. Thus, our in vivo study determined whether there is a time-of-day difference as well as a sex difference in the response to ENaC inhibition by benzamil. Our results showed that the natriuretic and diuretic responses to a single dose of benzamil were significantly greater in male compared with female rats whether given at the beginning of the inactive period [Zeitgeber time 0 (ZT0), 7 AM] or active period (ZT12, 7 PM). However, the response to benzamil was not significantly different between ZT0 and ZT12 dosing in either male or female rats. There was no difference in renal cortical α-ENaC protein abundance between ZT0 and ZT12 or males and females. Given previous reports of flow-induced stimulation of endothelin-1 (ET-1) production and sex differences in the renal endothelin system, we measured urinary ET-1 excretion to assess the effects of increased urine flow on intrarenal ET-1. ET-1 excretion was significantly increased following benzamil administration in both sexes, but this increase was significantly greater in females. These results support the hypothesis that ENaC activity is less prominent in maintaining Na⁺ balance in females independent of renal ET-1. Because ENaC subunit genes and protein expression vary by time of day and are greater in female rat kidneys, this suggests a clear disconnect between ENaC expression and channel activity.

Arg913Gln variation of SLC12A3 gene is associated with diabetic nephropathy in type 2 diabetes and Gitelman syndrome: a systematic review

Abstract

Background

Diabetic nephropathy is a global common cause of chronic kidney disease and end-stage renal disease. A lot of research has been conducted in biomedical sciences, which has enhanced understanding of the pathophysiology of diabetic nephropathy and has expanded the potential available therapies. An increasing number of evidence suggests that genetic alterations play a major role in development and progression of diabetic nephropathy. This systematic review was focused on searching an association between Arg913Gln variation in SLC12A3 gene with diabetic nephropathy in individuals with Type 2 Diabetes and Gitelman Syndrome.

Methods

An extensive systematic review of the literature was completed using PubMed, EBSCO and Cochrane Library, from their inception to January 2018. The PRISMA guidelines were followed and the search strategy ensured that all possible studies were identified to compile the review. Inclusion criteria for this review were: 1) Studies that analyzed the SLC12A3 gene in individuals with Type 2 Diabetes and Gitelman Syndrome. 2) Use of at least one analysis investigating the association between the Arg913Gln variation of SLC12A3 gene with diabetic nephropathy. 3) Use of a case–control or follow-up design. 4) Investigation of type 2 diabetes mellitus in individuals with Gitelman’s syndrome, with a history of diabetic nephropathy.

Results

The included studies comprised 2106 individuals with diabetic nephropathy. This review shows a significant genetic association in most studies in the Arg913Gln variation of SLC12A3 gene with the diabetic nephropathy, pointing out that the mutations of this gene could be a key predictor of end-stage renal disease.

Conclusions

The results showed in this systematic review contribute to better understanding of the association between the Arg913Gln variation of SLC12A3 gene with the pathogenesis of diabetic nephropathy in individuals with T2DM and GS.

Studying Na+ and K+ channels in aldosterone-sensitive distal nephrons

Aldosterone-sensitive distal nephron (ASDN) including the distal convoluted tubule (DCT), connecting tubule (CNT) and collecting duct (CD) plays an important role in the regulation of hormone-dependent Na⁺ reabsorption and dietary K⁺-intake dependent K⁺ excretion. The major Na⁺ transporters in the ASDN are thiazide-sensitive Na-Cl cotransporter (NCC), epithelial Na⁺ channel (ENaC), pendrin/Na⁺-dependent Cl^--bicarbonate exchanger (NDCBE). Whereas major K⁺ channels in the ASDN are Kir4.1 and Kir5.1 in the basolateral membrane; and Kir1.1 (ROMK) and Ca²⁺ activated big conductance K⁺ channel (BK) in the apical membrane. Although a variety of in vitro cell lines of the ASDN is available and these cell models have been employed for studying Na⁺ and K⁺ channels, the biophysical properties and the regulation of Na⁺ and K⁺ channels in vitro cell models may not be able to recapitulate those in vivo conditions. Thus, the studies performed in the native ASDN are essential for providing highly physiological relevant information and for understanding the Na⁺ and K⁺ transport in the ASDN. Here we provide a detailed methodology describing how to perform the electrophysiological measurement in the native DCT, CNT and cortical collecting duct (CCD).

Physiological role of SLC12 family members in the kidney

The solute carrier family 12, as numbered according to Human Genome Organisation (HUGO) nomenclature, encodes the electroneutral cation-coupled chloride cotransporters that are expressed in many cells and tissues; they play key roles in important physiological events, such as cell volume regulation, modulation of the intracellular chloride concentration, and transepithelial ion transport. Most of these family members are expressed in specific regions of the nephron. The Na-K-2Cl cotransporter NKCC2, which is located in the thick ascending limb, and the Na-Cl cotransporter, which is located in the distal convoluted tubule, play important roles in salt reabsorption and serve as the receptors for loop and thiazide diuretics, respectively (Thiazide diuretics are among the most commonly prescribed drugs in the world.). The activity of these transporters correlates with blood pressure levels; thus, their regulation has been a subject of intense research for more than a decade. The K-Cl cotransporters KCC1, KCC3, and KCC4 are expressed in several nephron segments, and their role in renal physiology is less understood but nevertheless important. Evidence suggests that they are involved in modulating proximal tubule glucose reabsorption, thick ascending limb salt reabsorption and collecting duct proton secretion. In this work, we present an overview of the physiological roles of these transporters in the kidney, with particular emphasis on the knowledge gained in the past few years.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

Analysis of Kif5b expression during mouse kidney development

Recent studies showed that kidney-specific inactivation of Kif3a produces kidney cysts and renal failure, suggesting that kinesin-mediated intracellular transportation is important for the establishement and maintenance of renal epithelial cell polarity and normal nephron functions. Kif5b, one of the most conserved kinesin heavy chain, is the mouse homologue of the human ubiquitous Kinesin Heavy Chain (uKHC). In order to elucidate the role of Kif5b in kidney development and function, it is essential to establish its expression profile within the organ. Therefore, in this study, we examined the expression pattern of Kif5b in mouse kidney. Kidneys from embryonic (E) 12.5-, 16.5-dpc (days post coitus) mouse fetuses, from postnatal (P) day 0, 10, 20 pups and from adult mice were collected. The distribution of Kif5b was analyzed by immunostaining. The possible involvement of Kif5b in kidney development was investigated in conditional mutant mice by using a Cre-LoxP strategy. This study showed that the distribution of Kif5b displayed spatiotemporal changes during postnatal kidney development. In kidneys of new born mice, Kif5b was strongly expressed in all developing tubules and in the ureteric bud, but not in the glomerulus or in other early-developing structures, such as the cap mesenchyme, the comma-shaped body, and the S-shaped body. In kidneys of postnatal day 20 or of older mice, however, Kif5b was localized selectively in the basolateral domain of epithelial cells of the thick ascending loop of Henle, as well as of the distal convoluted tubule, with little expression being observed in the proximal tubule or in the collecting duct. Conditional knock-down of Kif5b in mouse kidney did not result in detectable morphological defects, but it did lead to a decrease in cell proliferation rate and also to a mislocalization of Na⁺/K⁺/-ATPase, indicating that although Kif5b is non-essential for kidney morphogenesis, it is important for nephron maturation.

Src family protein tyrosine kinase regulates the basolateral K channel in the distal convoluted tubule (DCT) by phosphorylation of KCNJ10 protein

The loss of function of the basolateral K channels in the distal nephron causes electrolyte imbalance. The aim of this study is to examine the role of Src family protein tyrosine kinase (SFK) in regulating K channels in the basolateral membrane of the mouse initial distal convoluted tubule (DCT1). Single-channel recordings confirmed that the 40-picosiemen (pS) K channel was the only type of K channel in the basolateral membrane of DCT1. The suppression of SFK reversibly inhibited the basolateral 40-pS K channel activity in cell-attached patches and decreased the Ba(2+)-sensitive whole-cell K currents in DCT1. Inhibition of SFK also shifted the K reversal potential from -65 to -43 mV, suggesting a role of SFK in determining the membrane potential in DCT1. Western blot analysis showed that KCNJ10 (Kir4.1), a key component of the basolateral 40-pS K channel in DCT1, was a tyrosine-phosphorylated protein. LC/MS analysis further confirmed that SFK phosphorylated KCNJ10 at Tyr(8) and Tyr(9). The single-channel recording detected the activity of a 19-pS K channel in KCNJ10-transfected HEK293T cells and a 40-pS K channel in the cells transfected with KCNJ10+KCNJ16 (Kir.5.1) that form a heterotetramer in the basolateral membrane of the DCT. Mutation of Tyr(9) did not alter the channel conductance of the homotetramer and heterotetramer. However, it decreased the whole-cell K currents, the probability of finding K channels, and surface expression of KCNJ10 in comparison to WT KCNJ10. We conclude that SFK stimulates the basolateral K channel activity in DCT1, at least partially, by phosphorylating Tyr(9) on KCNJ10. We speculate that the modulation of tyrosine phosphorylation of KCNJ10 should play a role in regulating membrane transport function in DCT1.

Identification of moesin as NKCC2-interacting protein and analysis of its functional role in the NKCC2 apical trafficking

BACKGROUND INFORMATION

The renal Na(+) -K(+) -2Cl(-) co-transporter (NKCC2) is expressed in kidney thick ascending limb cells, where it mediates NaCl re-absorption regulating body salt levels and blood pressure.

RESULTS

In this study, we used a well-characterised NKCC2 construct (c-NKCC2) to identify NKCC2-interacting proteins by an antibody shift assay coupled with blue native/SDS-PAGE and mass spectrometry. Among the interacting proteins, we identified moesin, a protein belonging to ezrin, eadixin and moesin family. Co-immunoprecipitation experiments confirmed that c-NKCC2 interacts with the N-terminal domain of moesin in LLC-PK1 cells. Moreover, c-NKCC2 accumulates in intracellular and sub-apical vesicles in cells transfected with a moesin dominant negative green fluorescent protien (GFP)-tagged construct. In addition, moesin knock-down by short interfering RNA decreases by about 50% c-NKCC2 surface expression. Specifically, endocytosis and exocytosis assays showed that moesin knock-down does not affect c-NKCC2 internalisation but strongly reduces exocytosis of the co-transporter.

CONCLUSIONS

Our data clearly demonstrate that moesin plays a critical role in apical membrane insertion of NKCC2, suggesting a possible involvement of moesin in regulation of Na(+) and Cl(-) absorption in the kidney.

[Renal physiology]

The kidneys are responsible for the urinary excretion of uremic toxins and the regulation of several body systems such as intra and extracellular volume status, acid-base status, calcium and phosphate metabolism or erythropoiesis. They adapt quantitative and qualitative composition of the urine to keep these systems in balance. The flow of plasma is filtered in the range of 120 mL/min, and depends on the systemic and renal hemodynamics which is subject to self-regulation. The original urine will then be modified in successive segments of the nephron. The proximal nephron is to lead the massive reabsorption of water and essential elements such as sodium, bicarbonates, amino-acids and glucose. The distal nephron includes the distal convoluted tubule, the connector tube and the collecting duct. Its role is to adapt the quality composition of urine to the needs of the body.

Cellular distribution of NKCC2 in the gastric mucosa and its response to short-term osmotic shock

The Na(+)-K(+)-2Cl(-) cotransporter-2 (NKCC2) has long been recognized as a "kidney-specific" transporter and is important in salt reabsorption. NKCC2 has been found in the gastric mucosa; however, its cellular distribution and function remain obscure. The present study characterized the distribution pattern of NKCC2 in mammalian gastric mucosa and investigated its response to osmotic challenge. Reverse transcription with the polymerase chain reaction, Western blot and immunofluorescence were used to determine NKCC2 expression and localization. The effect of osmotic shock on NKCC2 expression was studied in isolated gastric mucosa with variable osmolarity treatment. Results from all of the above studies were compared with those of NKCC1. Our data indicated that NKCC1 and NKCC2 were expressed in the gastric mucosa of rat, mouse and human. The mRNA transcripts and proteins for NKCC1 and NKCC2 were broadly expressed in the rat gastric mucosa. In rat and mouse, NKCC1 was largely confined to the lower part of the oxyntic and pyloric gland areas, whereas NKCC2 extended throughout the gastric glands. NKCC1 immunoreactivity was strongly expressed in the parietal and chief cells but was weaker in the mucous cells. NKCC2 was abundantly located in the parietal and mucous cells but faintly distributed in the chief cells. Hypertonic treatment increased the protein level of NKCC1 and caused evident membrane translocation. In contrast, NKCC2 was significantly downregulated and no obvious membrane translocation was observed. Thus, NKCC2 displayed a more ubiquitous distribution in the gastric mucosa and might work coordinately with NKCC1 to maintain cell volume homeostasis under hypertonic conditions.

Secretory carrier membrane protein 2 regulates exocytic insertion of NKCC2 into the cell membrane

The renal-specific Na-K-2Cl co-transporter, NKCC2, plays a pivotal role in regulating body salt levels and blood pressure. NKCC2 mutations lead to type I Bartter syndrome, a life-threatening kidney disease. Regulation of NKCC2 trafficking behavior serves as a major mechanism in controlling NKCC2 activity across the plasma membrane. However, the identities of the protein partners involved in cell surface targeting of NKCC2 are largely unknown. To gain insight into these processes, we used a yeast two-hybrid system to screen a kidney cDNA library for proteins that interact with the NKCC2 C terminus. One binding partner we identified was SCAMP2 (secretory carrier membrane protein 2). Microscopic confocal imaging and co-immunoprecipitation assays confirmed NKCC2-SCAMP2 interaction in renal cells. SCAMP2 associated also with the structurally related co-transporter NCC, suggesting that the interaction with SCAMP2 is a common feature of sodium-dependent chloride co-transporters. Heterologous expression of SCAMP2 specifically decreased cell surface abundance as well as transport activity of NKCC2 across the plasma membrane. Co-immunolocalization experiments revealed that intracellularly retained NKCC2 co-localizes with SCAMP2 in recycling endosomes. The rate of NKCC2 endocytic retrieval, assessed by the sodium 2-mercaptoethane sulfonate cleavage assay, was not affected by SCAMP2. The surface-biotinylatable fraction of newly inserted NKCC2 in the plasma membrane was reduced by SCAMP2, demonstrating that SCAMP2-induced decrease in surface NKCC2 is due to decreased exocytotic trafficking. Finally, a single amino acid mutation, cysteine 201 to alanine, within the conserved cytoplasmic E peptide of SCAMP2, which is believed to regulate exocytosis, abolished SCAMP2-mediated down-regulation of the co-transporter. Taken together, these data are consistent with a model whereby SCAMP2 regulates NKCC2 transit through recycling endosomes and limits the cell surface targeting of the co-transporter by interfering with its exocytotic trafficking.

A highly conserved motif at the COOH terminus dictates endoplasmic reticulum exit and cell surface expression of NKCC2

Mutations in the apically located Na(+)-K(+)-2Cl(-) co-transporter, NKCC2, lead to type I Bartter syndrome, a life-threatening kidney disorder, yet the mechanisms underlying the regulation of mutated NKCC2 proteins in renal cells have not been investigated. Here, we identified a trihydrophobic motif in the distal COOH terminus of NKCC2 that was required for endoplasmic reticulum (ER) exit and surface expression of the co-transporter. Indeed, microscopic confocal imaging showed that a naturally occurring mutation depriving NKCC2 of its distal COOH-terminal region results in the absence of cell surface expression. Biotinylation assays revealed that lack of cell surface expression was associated with abolition of mature complex-glycosylated NKCC2. Pulse-chase analysis demonstrated that the absence of mature protein was not caused by reduced synthesis or increased rates of degradation of mutant co-transporters. Co-immunolocalization experiments revealed that these mutants co-localized with the ER marker protein-disulfide isomerase, demonstrating that they are retained in the ER. Cell treatment with proteasome or lysosome inhibitors failed to restore the loss of complex-glycosylated NKCC2, further eliminating the possibility that mutant co-transporters were processed by the Golgi apparatus. Serial truncation of the NKCC2 COOH terminus, followed by site-directed mutagenesis, identified hydrophobic residues (1081)LLV(1083) as an ER exit signal necessary for maturation of NKCC2. Mutation of (1081)LLV(1083) to AAA within the context of the full-length protein prevented NKCC2 ER exit independently of the expression system. This trihydrophobic motif is highly conserved in the COOH-terminal tails of all members of the cation-chloride co-transporter family, and thus may function as a common motif mediating their transport from the ER to the cell surface. Taken together, these data are consistent with a model whereby naturally occurring premature terminations that interfere with the LLV motif compromise co-transporter surface delivery through defective trafficking.

Regulation of NKCC2 by a chloride-sensing mechanism involving the WNK3 and SPAK kinases

The Na(+):K(+):2Cl(-) cotransporter (NKCC2) is the target of loop diuretics and is mutated in Bartter's syndrome, a heterogeneous autosomal recessive disease that impairs salt reabsorption in the kidney's thick ascending limb (TAL). Despite the importance of this cation/chloride cotransporter (CCC), the mechanisms that underlie its regulation are largely unknown. Here, we show that intracellular chloride depletion in Xenopus laevis oocytes, achieved by either coexpression of the K-Cl cotransporter KCC2 or low-chloride hypotonic stress, activates NKCC2 by promoting the phosphorylation of three highly conserved threonines (96, 101, and 111) in the amino terminus. Elimination of these residues renders NKCC2 unresponsive to reductions of [Cl(-)](i). The chloride-sensitive activation of NKCC2 requires the interaction of two serine-threonine kinases, WNK3 (related to WNK1 and WNK4, genes mutated in a Mendelian form of hypertension) and SPAK (a Ste20-type kinase known to interact with and phosphorylate other CCCs). WNK3 is positioned upstream of SPAK and appears to be the chloride-sensitive kinase. Elimination of WNK3's unique SPAK-binding motif prevents its activation of NKCC2, as does the mutation of threonines 96, 101, and 111. A catalytically inactive WNK3 mutant also completely prevents NKCC2 activation by intracellular chloride depletion. Together these data reveal a chloride-sensing mechanism that regulates NKCC2 and provide insight into how increases in the level of intracellular chloride in TAL cells, as seen in certain pathological states, could drastically impair renal salt reabsorption.

Cellular distribution of the renal bumetanide-sensitive Na-K-2Cl cotransporter BSC-1 in the inner stripe of the outer medulla during the development of hypertension in the spontaneously hypertensive rat

1. The renal bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1) is expressed only in the thick ascending limb and selectively traffics from intracellular vesicles (IVs) to apical plasma membranes (PMs), where BSC-1 regulates sodium reabsorption. We showed previously that in kidneys from adult spontaneously hypertensive rats (SHR; model of essential hypertension) total protein expression of BSC-1 was higher compared with kidneys from normotensive Wistar-Kyoto (WKY) rats. However, whether this change is associated with an increased trafficking of BSC-1 from IVs to PMs is unknown. The goal of the present study was to test the hypothesis that the increase in total renal BSC-1 protein expression in SHR is accompanied by an augmented distribution of BSC-1 from IVs to PMs. 2. To test the hypothesis, we obtained renal tissue from the inner stripe of the outer medulla (ISOM; enriched in thick ascending limbs) and isolated IVs and PMs from this tissue by differential centrifugation. Total BSC-1 protein expression in ISOM and BSC-1 protein expression in ISOM IVs and PMs were measured by semiquantitative western blotting in SHR and aged-matched WKY rats at different ages and stages of hypertension. 3. At 5 weeks of age, SHR were prehypertensive (mean arterial blood pressure (MABP) 97 mmHg). At this age, both the total abundance and cellular distribution of BSC-1 were similar in ISOM from SHR and WKY rats. 4. As SHR aged, their hypertension progressed (MABP 137 and 195 mmHg at 8 and 14 weeks of age, respectively). Associated with the increase in MABP was an increase in both steady state protein levels of ISOM BSC-1 and the distribution of ISOM BSC-1 to PMs (four- and sixfold increases at 8 and 14 weeks of age, respectively, compared with age-matched WKY rats; P < 0.001). 5. Using semiquantitative reverse transcription-polymerase chain reaction, BSC-1 mRNA was measured and was found not to differ between SHR and WKY rat ISOM at any age or level of MABP. 6. We conclude that as SHR transition from prehypertensive to established hypertension, there is a marked increase in the total expression of BSC-1 in ISOM that is not related to increases in steady state levels of BSC-1 mRNA and therefore unlikely to be due to changes in either the rate of BSC-1 gene transcription or the stability of BSC-1 mRNA. This suggests changes in either translational efficiency or BSC-1 protein stability in SHR. 7. We also conclude that the age/hypertension-related increase in BSC-1 protein levels in ISOM is accompanied by an equally marked increased trafficking of BSC-1 to PMs in SHR ISOM.

Tubular mitochondrial alterations in neonatal rats subjected to RAS inhibition

Pharmacological interruption of the angiotensin II (ANG II) type 1 receptor signaling during nephrogenesis in rats perturbs renal tubular development. This study aimed to further investigate tubular developmental defects in neonatal rats subjected to ANG II inhibition with enalapril. We evaluated tubular ultrastructural changes using electron microscopy and estimated spectrophotometrically activity or concentrations of succinate dehydrogenase (SDH), cytochromes a and c, which are components of mitochondrial respiratory chain, on postnatal days 2 and 9 (PD2 and PD9). Renal expression of sodium-potassium adenosinetriphosphatase (Na+-K+-ATPase) and two reflectors of mitochondrial biogenesis [mitochondrial transcription factor A (TFAM) and translocase of outer mitochondrial membrane 20 (TOM20)] also were studied using Western immunoblotting and immunohistochemistry. Enalapril disrupted inner mitochondrial membranes of developing cortical and medullary tubular cells on PD2 and PD9. These findings were paralleled by impaired mitochondrial respiratory function, as revealed from the changes in components of the mitochondrial respiratory chain, such as decreased cytochrome c level in the cortex and medulla on PD2 and PD9, decreased cytochrome a level in the cortex and medulla on PD2, and diminished cortical SDH activity on PD2 and PD9. Moreover, tubular expression of the most active energy-consuming pump Na+-K+-ATPase was decreased by enalapril treatment. Renal expression of TFAM and TOM20 was not altered by neonatal enalapril treatment. Because nephrogenesis is a highly energy-demanding biological process, with the energy being utilized for renal growth and transport activities, the structural-functional alterations of the mitochondria induced by neonatal enalapril treatment may provide the propensity for the tubular developmental defect.

Channels, carriers, and pumps in the pathogenesis of sodium-sensitive hypertension

Sodium-sensitive hypertension is thought to be dependent on primary alterations in renal tubular sodium reabsorption. The major apical plasma membrane Na(+) transporters include the proximal tubular Na(+)-H(+) exchanger, the thick ascending limb Na(+)-K(+)-2Cl(-) cotransport system, the distal tubular Na(+)-Cl(-) cotransporter, and the collecting duct epithelial sodium channel (ENaC). This article explores the role of each transporter in the pathogenesis of hypertension. Although the contribution of the proximal tubule Na(+)-H(+) exchanger is not yet defined completely, more convincing data have been generated about the importance of the Na(+)-K(+)-2Cl(-). Indeed at least 2 forms of hypertension appear to be related to the up-regulation of the transporter: the so-called programmed hypertension induced by low-protein diet during pregnancy and the early phase of hypertension in the Milan strain of rats. With respect to the Na(+)-Cl(-) cotransporter this may be overactive caused by inactivating mutation of WNK4 as in the Gordon syndrome, although it is the main actor for the maintenance phase of the hypertension found in the Milan strain of rats. Finally, the contribution of the ENaC has been established clearly; indeed, in the Liddle syndrome the mutation of the ENaC gene leads to a longer retention of the channel on the cell surface of collecting duct principal cells, thus inducing stronger sodium reabsorption along this segment. All these examples clearly indicate that renal sodium transporters may be responsible for various types of sodium-sensitive hypertension.

Activity of the renal Na+-K+-2Cl- cotransporter is reduced by mutagenesis of N-glycosylation sites: role for protein surface charge in Cl- transport

The renal-specific Na(+)-K(+)-2Cl(-) cotransporter NKCC2 belongs to the SLC12 gene family; it is the target for loop diuretics and the cause of type I Bartter's syndrome. Because the NKCC2 sequence contains two putative N-linked glycosylation sites, one of which is conserved with the renal Na(+)-Cl(-) cotransporter in which glycosylation affects thiazide affinity, we assessed the role of glycosylation on NKCC2 functional properties. One (N442Q or N452Q) or both (N442,452Q) N-glycosylation sites were eliminated by site-directed mutagenesis. Wild-type NKCC2 and mutant clones were expressed in Xenopus laevis oocytes and analyzed by (86)Rb(+) influx, Western blotting, and confocal microscopy. Inhibition of glycosylation with tunicamycin in wild-type NKCC2-injected oocytes resulted in an 80% reduction of NKCC2 activity. Immunoblot of injected oocytes revealed that glycosylation of NKCC2 was completely prevented in N442,452Q-injected oocytes. Functional activity was reduced by 50% in N442Q- and N452Q-injected oocytes and by 80% in oocytes injected with N442,452Q, whereas confocal microscopy of oocytes injected with wild-type or mutant enhanced green fluorescent protein-tagged NKCC2 clones revealed that surface fluorescence intensity was reduced approximately 20% in single mutants and 50% in the double mutant. Ion transport kinetic analyses revealed no changes in cation affinity and a small increase in Cl(-) affinity by N442Q and N442,452Q. However, a slight decrease in bumetanide affinity was observed. Our data demonstrate that NKCC2 is glycosylated and suggest that prevention of glycosylation reduces its functional expression by affecting insertion into the plasma membrane and the intrinsic activity of the cotransporter.

Altered expression of renal apical plasma membrane Na+ transporters in the early phase of genetic hypertension

The present study explores whether the development of hypertension in the Milan strain of rats (MHS) rats is preceded or paralleled by alterations of mRNA and/or protein levels of the major luminal Na+ transporters. MHS rats were studied at 23-25 days after birth; age-matched Milan normotensive (MNS) rats were used as controls. The glomerular filtration rate (GFR), measured by inulin clearance, was higher in MHS than in MNS rats, while the mean blood pressure was not different in the two strains of animals indicating that the MHS rats were still in the prehypertensive state. Type 3 sodium/hydrogen exchanger (NHE3), bumetanide-sensitive sodium-potassium-2 chloride cotransporter (NKCC2), sodium-chloride cotransporter (NCC) and alpha-ENaC mRNA abundances were quantified by competitive PCR. In MHS compared with MNS, mRNA abundance was unchanged for NHE3 in proximal tubules, higher for NKCC2 in medullary thick ascending limbs of Henle's loops (TAL) and lower for NCC in distal convoluted tubules (DCT) and for alpha-ENaC along collecting ducts (CD). Western blot experiments revealed 1) unchanged NHE3; 2) a significant increase in NKCC2 in the outer medulla; 3) a significant decrease in NCC in the renal cortex and of alpha-ENaC in both the renal cortex and outer medulla, whereas beta- and gamma-ENaC remained unchanged. These data indicate that, in MHS rats, there is a strong upregulation of NKCC2 along the TAL associated with increased GFR, robust inhibition of NCC cotransporter along the DCT and modest downregulation of alpha-ENaC along the CD. The interplay of the various Na+ transporters may well explain why, at this age, the rats are still in the prehypertensive state.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.

Functional properties of the apical Na+-K+-2Cl- cotransporter isoforms

The bumetanide-sensitive Na(+):K(+):2Cl(-) cotransporter (BSC1) is the major pathway for salt reabsorption in the apical membrane of the mammalian thick ascending limb of Henle. Three isoforms of the cotransporter, known as A, B, and F, exhibit axial expression along the thick ascending limb. We report here a functional comparison of the three isoforms from mouse kidney. When expressed in Xenopus oocytes the mBSC1-A isoform showed higher capacity of transport, with no difference in the amount of surface expression. Kinetic characterization revealed divergent affinities for the three cotransported ions. The observed EC(50) values for Na(+), K(+), and Cl(-) were 5.0 +/- 3.9, 0.96 +/- 0.16, and 22.2 +/- 4.8 mm for mBSC1-A; 3.0 +/- 0.6, 0.76 +/- 0.07, and 11.6 +/- 0.7 mm for mBSC1-B; and 20.6 +/- 7.2, 1.54 +/- 0.16, and 29.2 +/- 2.1 mm for mBSC1-F, respectively. Bumetanide sensitivity was higher in mBSC1-B compared with the mBSC1-A and mBSC1-F isoforms. All three transporters were partially inhibited by hypotonicity but to different extents. The cell swelling-induced inhibition profile was mBSC1-F > mBSC1-B > mBSC1-A. The function of the Na(+):K(+):2Cl(-) cotransporter was not affected by extracellular pH or by the addition of metolazone, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), or R(+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1-H-indenyl-5-yl)-oxy]acetic acid (DIOA) to the extracellular medium. In contrast, exposure of oocytes to HgCl(2) before the uptake period reduced the activity of the cotransporter. The effect of HgCl(2) was dose-dependent, and mBSC1-A and mBSC1-B exhibited higher affinity than mBSC1-F. Overall, the functional comparison of the murine apical renal-specific Na(+):K(+):2Cl(-) cotransporter isoforms A, B, and F reveals important functional, pharmacological, and kinetic differences, with both physiological and structural implications.

Functional comparison of the K+-Cl- cotransporters KCC1 and KCC4

The K(+)-Cl(-) cotransporters (KCCs) are members of the cation-chloride cotransporter gene family and fall into two phylogenetic subgroups: KCC2 paired with KCC4 and KCC1 paired with KCC3. We report a functional comparison in Xenopus oocytes of KCC1 and KCC4, widely expressed representatives of these two subgroups. KCC1 and KCC4 exhibit differential sensitivity to transport inhibitors, such that KCC4 is much less sensitive to bumetanide and furosemide. The efficacy of these anion inhibitors is critically dependent on the concentration of extracellular K(+), with much higher inhibition in 50 mm K(+) versus 2 mm K(+). KCC4 is also uniquely sensitive to 10 mm barium and to 2 mm trichlormethiazide. Kinetic characterization reveals divergent affinities for K(+) (K(m) values of approximately 25.5 and 17.5 mm for KCC1 and KCC4, respectively), probably due to variation within the second transmembrane segment. Although the two isoforms have equivalent affinities for Cl(-), they differ in the anion selectivity of K(+) transport (Cl(-) > SCN(-) = Br(-) > PO(4)(-3) > I(-) for KCC1 and Cl(-) > Br(-) > PO(4)(-3) = I(-) > SCN(-) for KCC4). Both KCCs express minimal K(+)-Cl(-) cotransport under isotonic conditions, with significant activation by cell swelling under hypotonic conditions. The cysteine-alkylating agent N-ethylmaleimide activates K(+)-Cl(-) cotransport in isotonic conditions but abrogates hypotonic activation, an unexpected dissociation of N-ethylmaleimide sensitivity and volume sensitivity. Although KCC4 is consistently more volume-sensitive, the hypotonic activation of both isoforms is critically dependent on protein phosphatase 1. Overall, the functional comparison of these cloned K(+)-Cl(-) cotransporters reveals important functional, pharmacological, and kinetic differences with both physiological and mechanistic implications.