Novel molecular variants of the Na-K-2Cl cotransporter gene are responsible for antenatal Bartter syndrome

Recurrent transient severe hypocalcaemia in two siblings with type 1 Bartter syndrome

Type 1 Bartter syndrome causes hypokalaemia and metabolic alkalosis owing to mutation in the SLC12A1 gene. Meanwhile, hypocalcaemia is rare in Bartter syndrome, except in type 5 Bartter syndrome. Herein, we describe two siblings with type 1 Bartter syndrome with recurrent transient severe hypocalcaemia. They each visited our hospital several times with chief complaints of numbness in the limbs, shortness of breath and tetany after stresses such as exercise or fever. Severe hypocalcaemia was also observed with a serum calcium level of approximately 6.0 mg/dL at each visit. The clinical symptoms and abnormalities in laboratory findings quickly improved with rest and intravenous treatment. In a steady state, no severe hypocalcaemia was evident, but serum intact parathyroid hormone (PTH) levels were high. In recent years, a large-scale study has revealed that type 1 and type 2 Bartter syndrome have high PTH values. In addition, there are reports that these patients develop hypocalcaemia due to PTH resistance. Therefore, our patient was also in a PTH-resistant state, and hypocalcaemia was thought to be exacerbated by physical stress. It is not well known that Bartter syndrome patients other than those with type 5 suffer from hypocalcaemia. And hypocalcaemia was not detected in normal examinations under steady-state conditions. Therefore, in patients with type 1 and type 2 Bartter syndrome, severe hypocalcaemia may occur, but may go unnoticed. When following up these patients, the attending physician must keep in mind that such patients are in a PTH-resistant state and that physical stress can cause severe hypocalcaemia.

AUP1 Regulates the Endoplasmic Reticulum-Associated Degradation and Polyubiquitination of NKCC2

Inactivating mutations of kidney Na-K-2Cl cotransporter NKCC2 lead to antenatal Bartter syndrome (BS) type 1, a life-threatening salt-losing tubulopathy. We previously reported that this serious inherited renal disease is linked to the endoplasmic reticulum-associated degradation (ERAD) pathway. The purpose of this work is to characterize further the ERAD machinery of NKCC2. Here, we report the identification of ancient ubiquitous protein 1 (AUP1) as a novel interactor of NKCC2 ER-resident form in renal cells. AUP1 is also an interactor of the ER lectin OS9, a key player in the ERAD of NKCC2. Similar to OS9, AUP1 co-expression decreased the amount of total NKCC2 protein by enhancing the ER retention and associated protein degradation of the cotransporter. Blocking the ERAD pathway with the proteasome inhibitor MG132 or the α-mannosidase inhibitor kifunensine fully abolished the AUP1 effect on NKCC2. Importantly, AUP1 knock-down or inhibition by overexpressing its dominant negative form strikingly decreased NKCC2 polyubiquitination and increased the protein level of the cotransporter. Interestingly, AUP1 co-expression produced a more profound impact on NKCC2 folding mutants. Moreover, AUP1 also interacted with the related kidney cotransporter NCC and downregulated its expression, strongly indicating that AUP1 is a common regulator of sodium-dependent chloride cotransporters. In conclusion, our data reveal the presence of an AUP1-mediated pathway enhancing the polyubiquitination and ERAD of NKCC2. The characterization and selective regulation of specific ERAD constituents of NKCC2 and its pathogenic mutants could open new avenues in the therapeutic strategies for type 1 BS treatment.

Clinical Findings and Genetic Analysis of Nine Mexican Families with Bartter Syndrome

BACKGROUND

Bartter's syndrome (BS) is a group of salt-wasting tubulopathies characterized by hypokalemia, metabolic alkalosis, hypercalciuria, secondary hyperaldosteronism, and low or normal blood pressure. Loss-of-function variants in genes encoding for five proteins expressed in the thick ascending limb of Henle in the nephron, produced different genetic types of BS.

AIM

Clinical and genetic analysis of families with Antenatal Bartter syndrome (ABS) and with Classic Bartter syndrome (CBS).

METHODS

Nine patients from unrelated non-consanguineous Mexican families were studied. Massive parallel sequencing of a gene panel or whole-exome sequencing was used to identify the causative gene.

RESULTS

Proband 1 was homozygous for the pathogenic variant p.Arg302Gln in the SLC12A1 gene encoding for the sodium-potassium-chloride NKCC2 cotransporter. Proband 3 was homozygous for the nonsense variant p.Cys308* in the KCNJ1 gene encoding for the ROMK potassium channel. Probands 7, 8, and 9 showed variants in the CLCKNB gene encoding the chloride channel ClC-Kb: proband 7 was compound heterozygous for the deletion of the entire gene and the missense change p.Arg438Cys; proband 8 presented a homozygous deletion of the whole gene and proband 9 was homozygous for the nonsense mutation p.Arg595*. A heterozygous variant of unknown significance was detected in the SLC12A1 gene in proband 2, and no variants were found in SLC12A1, KCNJ1, BSND, CLCNKA, CLCNKB, and MAGED2 genes in probands 4, 5, and 6.

CONCLUSIONS

Genetic analysis identified loss-of-function variants in the SLC12A1, KCNJ1, and CLCNKB genes in four patients with ABS and in the CLCNKB gene in two patients with CBS.

Twelve exonic variants in the SLC12A1 and CLCNKB genes alter RNA splicing in a minigene assay

Background: Bartter syndrome (BS) is a rare renal tubular disease caused by gene variants in SLC12A1, KCNJ1, CLCNKA, CLCNKB, BSND or MAGED2 genes. There is growing evidence that many exonic mutations can affect the pre-mRNA normal splicing and induce exon skipping by altering various splicing regulatory signals. Therefore, the aim of this study was to gain new insights into the consequences of exonic mutations associated with BS on pre-mRNA splicing.

Methods: We analyzed all the missense, nonsense and synonymous variants described in six pathogenic genes by bioinformatics programs and identified candidate mutations that may promote exon skipping through a minigene system.

Results: Results of the study showed that 12 of 14 candidate variants distributed in SLC12A1 (c.728G>A, C.735C>G, c.904C>T, c.905G>A, c.1304C>T, c.1493C>T, c.2221A>T) and CLCNKB (c.226C>T, c.228A>C, c.229G>A, c.229G>C, c.1979C>A) were identified to induce splicing alterations. These variants may not only disrupt exonic splicing enhancers (ESEs) but also generate new exonic splicing silencers (ESSs), or disturb the classic splicing sites.

Conclusion: To our knowledge, this is a comprehensive study regarding alterations in pre-mRNA of exonic variants in BS pathogenic genes. Our results reinforce the necessity of assessing the consequences of exonic variants at the mRNA level.

Pathophysiological aspects of the thick ascending limb and novel genetic defects: HELIX syndrome and transient antenatal Bartter syndrome

The thick ascending limb plays a central role in human kidney physiology, participating in sodium reabsorption, urine concentrating mechanisms, calcium and magnesium homeostasis, bicarbonate and ammonium homeostasis, and uromodulin synthesis. This review aims to illustrate the importance of these roles from a pathophysiological point of view by describing the interactions of the key proteins of this segment and by discussing how recently identified and long-known hereditary diseases affect this segment. The descriptions of two recently described salt-losing tubulopathies, transient antenatal Bartter syndrome and HELIX syndrome, which are caused by mutations in MAGED2 and CLDN10 genes, respectively, highlight the role of new players in the modulation of sodium reabsorption the thick ascending limb.

Golgi Alpha1,2-Mannosidase IA Promotes Efficient Endoplasmic Reticulum-Associated Degradation of NKCC2

Mutations in the apically located kidney Na-K-2Cl cotransporter NKCC2 cause type I Bartter syndrome, a life-threatening kidney disorder. We previously showed that transport from the ER represents the limiting phase in NKCC2 journey to the cell surface. Yet very little is known about the ER quality control components specific to NKCC2 and its disease-causing mutants. Here, we report the identification of Golgi alpha1, 2-mannosidase IA (ManIA) as a novel binding partner of the immature form of NKCC2. ManIA interaction with NKCC2 takes place mainly at the cis-Golgi network. ManIA coexpression decreased total NKCC2 protein abundance whereas ManIA knock-down produced the opposite effect. Importantly, ManIA coexpression had a more profound effect on NKCC2 folding mutants. Cycloheximide chase assay showed that in cells overexpressing ManIA, NKCC2 stability and maturation are heavily hampered. Deleting the cytoplasmic region of ManIA attenuated its interaction with NKCC2 and inhibited its effect on the maturation of the cotransporter. ManIA-induced reductions in NKCC2 expression were offset by the proteasome inhibitor MG132. Likewise, kifunensine treatment greatly reduced ManIA effect, strongly suggesting that mannose trimming is involved in the enhanced ERAD of the cotransporter. Moreover, depriving ManIA of its catalytic domain fully abolished its effect on NKCC2. In summary, our data demonstrate the presence of a ManIA-mediated ERAD pathway in renal cells promoting retention and degradation of misfolded NKCC2 proteins. They suggest a model whereby Golgi ManIA contributes to ERAD of NKCC2, by promoting the retention, recycling, and ERAD of misfolded proteins that initially escape protein quality control surveillance within the ER.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

New insights into the role of endoplasmic reticulum-associated degradation in Bartter Syndrome Type 1

Mutations in Na-K-2Cl co-transporter, NKCC2, lead to type I Bartter syndrome (BS1), a life-threatening kidney disease. Yet, our knowledge of the molecular regulation of NKCC2 mutants remains poor. Here, we aimed to identify the molecular pathogenic mechanisms of one novel and three previously reported missense NKCC2 mutations. Co-immunolocalization studies revealed that all NKCC2 variants are not functional because they are not expressed at the cell surface due to retention in the endoplasmic reticulum (ER). Cycloheximide chase assays together with treatment by protein degradation and mannose trimming inhibitors demonstrated that the defect in NKCC2 maturation arises from ER retention and associated degradation (ERAD). Small interfering RNA (siRNA) knock-down experiments revealed that the ER lectin OS9 is involved in the ERAD of NKCC2 mutants. 4-phenyl butyric acid (4-PBA) treatment mimicked OS9 knock-down effect on NKCC2 mutants by stabilizing their immature forms. Importantly, out of the four studied mutants, only one showed an increased protein maturation upon treatment with glycerol. In summary, our study reveals that BS1 is among diseases linked to the ERAD pathway. Moreover, our data open the possibility that maturation of some ER retained NKCC2 variants is correctable by chemical chaperones offering, therefore, promising avenues in elucidating the molecular pathways governing the ERAD of NKCC2 folding mutants.

Differential Effects of STCH and Stress-Inducible Hsp70 on the Stability and Maturation of NKCC2

Mutations in the Na-K-2Cl co-transporter NKCC2 lead to type I Bartter syndrome, a life-threatening kidney disease. We previously showed that export from the ER constitutes the limiting step in NKCC2 maturation and cell surface expression. Yet, the molecular mechanisms involved in this process remain obscure. Here, we report the identification of chaperone stress 70 protein (STCH) and the stress-inducible heat shock protein 70 (Hsp70), as two novel binding partners of the ER-resident form of NKCC2. STCH knock-down increased total NKCC2 expression whereas Hsp70 knock-down or its inhibition by YM-01 had the opposite effect. Accordingly, overexpressing of STCH and Hsp70 exerted opposite actions on total protein abundance of NKCC2 and its folding mutants. Cycloheximide chase assay showed that in cells over-expressing STCH, NKCC2 stability and maturation are heavily impaired. In contrast to STCH, Hsp70 co-expression increased NKCC2 maturation. Interestingly, treatment by protein degradation inhibitors revealed that in addition to the proteasome, the ER associated degradation (ERAD) of NKCC2 mediated by STCH, involves also the ER-to-lysosome-associated degradation pathway. In summary, our data are consistent with STCH and Hsp70 having differential and antagonistic effects with regard to NKCC2 biogenesis. These findings may have an impact on our understanding and potential treatment of diseases related to aberrant NKCC2 trafficking and expression.

Differential Effects of STCH and Stress-Inducible Hsp70 on the Stability and Maturation of NKCC2

Mutations in the Na-K-2Cl co-transporter NKCC2 lead to type I Bartter syndrome, a life-threatening kidney disease. We previously showed that export from the ER constitutes the limiting step in NKCC2 maturation and cell surface expression. Yet, the molecular mechanisms involved in this process remain obscure. Here, we report the identification of chaperone stress 70 protein (STCH) and the stress-inducible heat shock protein 70 (Hsp70), as two novel binding partners of the ER-resident form of NKCC2. STCH knock-down increased total NKCC2 expression whereas Hsp70 knock-down or its inhibition by YM-01 had the opposite effect. Accordingly, overexpressing of STCH and Hsp70 exerted opposite actions on total protein abundance of NKCC2 and its folding mutants. Cycloheximide chase assay showed that in cells over-expressing STCH, NKCC2 stability and maturation are heavily impaired. In contrast to STCH, Hsp70 co-expression increased NKCC2 maturation. Interestingly, treatment by protein degradation inhibitors revealed that in addition to the proteasome, the ER associated degradation (ERAD) of NKCC2 mediated by STCH, involves also the ER-to-lysosome-associated degradation pathway. In summary, our data are consistent with STCH and Hsp70 having differential and antagonistic effects with regard to NKCC2 biogenesis. These findings may have an impact on our understanding and potential treatment of diseases related to aberrant NKCC2 trafficking and expression.

The structural basis of function and regulation of neuronal cotransporters NKCC1 and KCC2

NKCC and KCC transporters mediate coupled transport of Na++K++Cl- and K++Cl- across the plasma membrane, thus regulating cell Cl- concentration and cell volume and playing critical roles in transepithelial salt and water transport and in neuronal excitability. The function of these transporters has been intensively studied, but a mechanistic understanding has awaited structural studies of the transporters. Here, we present the cryo-electron microscopy (cryo-EM) structures of the two neuronal cation-chloride cotransporters human NKCC1 (SLC12A2) and mouse KCC2 (SLC12A5), along with computational analysis and functional characterization. These structures highlight essential residues in ion transport and allow us to propose mechanisms by which phosphorylation regulates transport activity.

A long-term study of the effects of SLC12A1 homozygous mutation (g.62382825G>A, p.Pro372Leu) in Japanese Black cattle

Recessive missense mutation in the solute carrier family 12, member 1 (SLC12A1) gene (g.62382825G>A) is associated with hydrallantois, which is the accumulation of fluid in the allantoic cavity of a pregnant animal, and usually causes fetal death in Japanese Black cattle. However, the symptoms of a homozygote with this mutation that do not result in fetal death have not previously been tracked and evaluated. In the present study, we observed a homozygote with the SLC12A1 risk allele over a long-term period. The calf did not show any obvious clinical symptoms, although it did exhibit a slight growth retardation that accompanied mild calciuria. At 28 months of age, the homozygote showed renal dysfunction, which in turn resulted in hydronephrosis. The time course of the symptoms was consistent with the phenotype of Bartter syndrome in humans. Additionally, the risk heterozygous genotype did not any effects on carcass traits, which indicates that eliminating the risk allele would not have any unfavorable effects. Therefore, we emphasize that both the fetal- and late-stage symptoms associated with the SLC12A1 risk allele compromise animal welfare, and consequently may result in severe economic losses for individual farmers if the SLC12A1 risk allele is not eliminated from the population.

Role of the Cation-Chloride-Cotransporters in Cardiovascular Disease

The SLC12 family of cation-chloride-cotransporters (CCCs) is comprised of potassium chloride cotransporters (KCCs), which mediate Cl- extrusion and sodium-potassium chloride cotransporters (N[K]CCs), which mediate Cl- loading. The CCCs play vital roles in cell volume regulation and ion homeostasis. The functions of CCCs influence a variety of physiological processes, many of which overlap with the pathophysiology of cardiovascular disease. Although not all of the cotransporters have been linked to Mendelian genetic disorders, recent studies have provided new insights into their functional role in vascular and renal cells in addition to their contribution to cardiovascular diseases. Particularly, an imbalance in potassium levels promotes the pathogenesis of atherosclerosis and disturbances in sodium homeostasis are one of the causes of hypertension. Recent findings suggest hypothalamic signaling as a key signaling pathway in the pathophysiology of hypertension. In this review, we summarize and discuss the role of CCCs in cardiovascular disease with particular emphasis on knowledge gained in recent years on NKCCs and KCCs.

Performance of ion chromatography to measure picomole amounts of magnesium in nanolitre samples

KEY POINTS

An UHPLC method to measure picomole amounts of magnesium has been developed. The method is sensitive, specific, accurate and reproducible. The method is suitable for quantifying magnesium transport across intact epithelia.

ABSTRACT

Magnesium is involved in many biological processes. Extracellular magnesium homeostasis mainly depends on the renal handling of magnesium, the study of which requires measurement of low concentrations of magnesium in renal tubular fluid. We developed an ultra-high-performance liquid chromatography method to measure millimolar concentrations of magnesium in nanolitre samples. Within-assay and between-assay coefficients of variation were lower than 5% and 6.6%, respectively. Measurement of magnesium concentration was linear (r²  = 0.9998) over the range 0-4 mmol/l. Absolute bias ranged from -0.03 to 0.05 mmol/l. The lower limit of quantification was 0.2 mmol/l. Recovery was 97.5-100.3%. No significant interference with calcium, another divalent cation present in the same samples, was detected. The method was successfully applied to quantify transepithelial magnesium transport by medullary and cortical thick ascending limbs during ex vivo microperfusion experiments. In conclusion, ultra-high-performance liquid chromatography is suitable for measurement of picomole amounts of magnesium in renal tubular fluid. The method allows detailed studies of transepithelial magnesium transport across native epithelium.

Molecular characteristics and physiological roles of Na+ -K+ -Cl- cotransporter 2

Na⁺–K⁺–Cl⁻ cotransporter 2 (NKCC2; SLC12A1) is an integral membrane protein that comes as three splice variants and mediates the cotranslocation of Na⁺, K⁺, and Cl⁻ ions through the apical membrane of the thick ascending loop of Henle (TALH). In doing so, and through the involvement of other ion transport systems, it allows this nephron segment to reclaim a large fraction of the ultrafiltered Na⁺, Cl⁻, Ca²⁺, Mg²⁺, and HCO₃⁻ loads. The functional relevance of NKCC2 in human is illustrated by the many abnormalities that result from the inactivation of this transport system through the use of loop diuretics or in the setting of inherited disorders. The following presentation aims at discussing the physiological roles and molecular characteristics of Na⁺–K⁺–Cl⁻ cotransport in the TALH and those of the individual NKCC2 splice variants more specifically. Many of the historical and recent data that have emerged from the experiments conducted will be outlined and their larger meaning will also be placed into perspective with the aid of various hypotheses.

Molecular regulation of NKCC2 in blood pressure control and hypertension

PURPOSE OF REVIEW

The apical Na/K/2Cl cotransporter (NKCC2) mediates NaCl reabsorption by the thick ascending limb, contributing to maintenance of blood pressure (BP). Despite effective NKCC2 inhibition by loop diuretics, these agents are not viable for long-term management of BP due to side effects. Novel molecular mechanisms that control NKCC2 activity reveal an increasingly complex picture with interacting layers of NKCC2 regulation. Here, we review the latest developments that shine new light on NKCC2-mediated control of BP and potential new long-term therapies to treat hypertension.

RECENT FINDINGS

Emerging molecular NKCC2 regulators, often binding partners, reveal a complex overlay of interacting mechanisms aimed at fine tuning NKCC2 activity. Different factors achieve this by shifting the balance between trafficking steps like exocytosis, endocytosis, recycling and protein turnover, or by balancing phosphorylation vs. dephosphorylation. Further molecular details are also emerging on previously known pathways of NKCC2 regulation, and recent in-vivo data continues to place NKCC2 regulation at the center of BP control.

SUMMARY

Several layers of emerging molecular mechanisms that control NKCC2 activity may operate simultaneously, but they can also be controlled independently. This provides an opportunity to identify new pharmacological targets to fine-tune NKCC2 activity for BP management.

Regulation of NKCC2B by TNF-α in response to salt restriction

We have previously shown that TNF-α produced by renal epithelial cells inhibits Na⁺-K⁺-2Cl^- cotransporter (NKCC2) activity as part of a mechanism that attenuates increases in blood pressure in response to high NaCl intake. As the role of TNF-α in the kidney is still being defined, the effects of low salt intake on TNF-α and NKCC2B expression were determined. Mice given a low-salt (0.02% NaCl) diet (LSD) for 7 days exhibited a 62 ± 7.4% decrease in TNF-α mRNA accumulation in the renal cortex. Mice that ingested the LSD also exhibited an ~63% increase in phosphorylated NKCC2 expression in the cortical thick ascending limb of Henle's loop and a concomitant threefold increase in NKCC2B mRNA abundance without a concurrent change in NKCC2A mRNA accumulation. NKCC2B mRNA levels increased fivefold in mice that ingested the LSD and also received an intrarenal injection of a lentivirus construct that specifically silenced TNF-α in the kidney (U6-TNF-ex4) compared with mice injected with control lentivirus. Administration of a single intrarenal injection of murine recombinant TNF-α (5 ng/g body wt) attenuated the increases of NKCC2B mRNA by ~50% and inhibited the increase in phosphorylated NKCC2 by ~54% in the renal cortex of mice given the LSD for 7 days. Renal silencing of TNF-α decreased urine volume and NaCl excretion in mice given the LSD, effects that were reversed when NKCC2B was silenced in the kidney. Collectively, these findings demonstrate that downregulation of renal TNF-α production in response to low-salt conditions contributes to the regulation of NaCl reabsorption via an NKCC2B-dependent mechanism.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

A novel compound heterozygous KCNJ1 gene mutation presenting as late-onset Bartter syndrome: Case report

RATIONALE

Bartter syndrome is an autosomal-recessive inherited disease in which patients present with hypokalemia and metabolic alkalosis. We present 1 case with Bartter syndrome, due to a novel compound heterozygous mutation in the KCNJ1 gene encoding the ATP-sensitive inward rectifier potassium channel in the thick ascending limb of the loop of Henle.

PATIENT CONCERNS

A patient was admitted to our hospital because of weakness, polyuria, and polydipsia. At presentation to our hospital, the female Chinese patient was 34 years old and her physical examination was normal. Laboratory studies revealed hypokalemia, metabolic alkalosis, hypercalciuria, hyperparathyroidemia, and hyper-reninemia. In addition, urinary potassium was obviously higher. Computer tomography scan confirmed the patient had the bilateral medullary nephrocalcinosis.

DIAGNOSIS

Blood samples were received from the patient and her parents, and deoxyribonucleic acid was extracted. The genetic analysis of SLC12A1, SLC12A3, KCNJ1, CLCNKB, BSND, and CASR was performed. The compound heterozygous KCNJ1 gene mutation was validated using conventional Sanger sequencing methods.

INTERVENTIONS

The patient was treated with potassium supplementation. Her blood and urine chemistries improved over the next week. Serum potassium normalized with improvement in polyuria and polydipsia over the next month.

OUTCOMES

Our patient was compound heterozygous for Thr234Ile and Thr71Met in the KCNJ1 gene. The c.701C>T variant predicted a change from a threonine codon to an isoleucine codon (p.Thr234Ile). The c.212C>T variant predicted a change from a threonine codon to a methionine codon (p.Thr71Met). The unaffected mother was heterozygous for the Thr234Ile mutation, whereas unaffected father was heterozygous for the Thr71Met mutation.

LESSONS

The phenotypes of the patient were similar to other patients with Bartter syndrome. The phenotypes of the patient could eventually be explained by the presence of the novel compound heterozygous p.Thr234Ile/p.Thr71Met variants in the KCNJ1 gene.

Eleven novel SLC12A1 variants and an exonic mutation cause exon skipping in Bartter syndrome type I

INTRODUCTION

Bartter syndrome type I (BS1) has been rarely reported in large groups. On the other hand, the phenomenon of exon skipping, in which exonic mutations result in abnormal splicing, has been reported to be associated with various diseases. Specifically, mutations that result in the disruption of exonic splicing enhancers (ESEs) and/or the creation of exonic splicing silencers (ESSs) can promote exon skipping. However, the aberrant exon skipping caused by an exonic variant in such splicing regulatory elements (SREs) sequences has never been reported in the causal gene of SLC12A1 in BS1.

METHODS

We analyze the variants in nine Chinese families with BS1, including eight with antenatal BS (aBS) and one presenting as classical BS (cBS), by next-generation sequencing. Then we used bioinformatics programs to analyze all these variants found in this study and identify candidate mutations that may induce exon skipping. Furthermore, the effects of identified variants were classified according to the 2015 American College of Medical Genetics and Genomics (ACMG) standards and guidelines.

RESULTS

Fifteen different variants of SLC12A1 gene were identified, including 11 novel ones. Two of the nine probands were homozygotes, the rest seven ones were compound heterozygotes. One candidate variant (c.1435C>G), not only significantly reduced ESEs scores but also markedly increased ESSs scores, were further investigated by mini-gene splicing assay, and found this single-nucleotide substitution causes abnormal splicing in vitro (exclusion of exon 11). Finally, among 15 variants, 9, 3, and 3 were classified as "pathogenic variants", "likely pathogenic variants", "variants with uncertain significance", respectively.

CONCLUSION

These data would enrich the human gene mutation database (HGMD) and would provide valuable references to the genetic counseling and diagnosis of BS1 for Chinese population. Additionally, our results suggest that aberrant exon skipping is one previously unrecognized mechanism by which an exonic variant in SLC12A1 can lead to BS1.

A novel SLC12A1 mutation in Bedouin kindred with antenatal Bartter syndrome type I

Four affected individuals of consanguineous kindred presented at infancy with an apparently autosomal recessive syndrome of polyuria and hypokalemic metabolic alkalosis, following maternal polyhydramnios and premature delivery, culminating in severe failure to thrive. Hypercalciuria, nephrocalcinosis, and hyperaldosteronism were further apparent as well as an unusual finding of intermittent hypernatremia. Additionally, all patients demonstrated variable micrognathia with upper respiratory airway abnormalities. As neither postnatal hyperkalemia nor permanent hearing deficits were shown, clinical assessment was consistent with antenatal Bartter syndrome (ABS) type I, which was never described before in the Israeli Bedouin population. Through genome-wide linkage analysis, we identified a single ∼3.3 Mbp disease-associated locus on chromosome 15q21.1, segregating within the pedigree. Whole-exome sequencing revealed a single novel homozygous missense mutation within this locus, in SLC12A1, encoding the Na-K-Cl cotransporter, NKCC2, in accordance with the clinical diagnosis. In this concise study, we report a novel missense mutation within the SLC12A1 gene, causing a severe form of ABS type I, the first to be described in Israeli Bedouins, with unusual clinical features of hypernatremia caused by nephrogenic diabetes insipidus and putatively related micrognathia with upper airway abnormalities .

Regulation of Na+-K+-Cl- cotransporter type 2 by the with no lysine kinase-dependent signaling pathway

Na⁺-K⁺-Cl^- cotransporter type 2 (NKCC2) is confined to the apical membrane of the thick ascending limb of Henle, where it reabsorbs a substantial fraction of the ultrafiltered NaCl load. It is expressed along this nephron segment as three main splice variants (called NKCC2A, NKCC2B, and NKCC2F) that differ in residue composition along their second transmembrane domain and first intracellular cytosolic connecting segment (CS2). NKCC2 is known to be activated by cell shrinkage and intracellular [Cl^-] reduction. Although the with no lysine (WNK) kinases could play a role in this response, the mechanisms involved are ill defined, and the possibility of variant-specific responses has not been tested thus far. In this study, we have used the Xenopus laevis oocyte expression system to gain further insight in these regards. We have found for the first time that cell shrinkage could stimulate NKCC2A- and NKCC2B-mediated ion transport by increasing carrier abundance at the cell surface and that this response was achieved (at least in part) by the enzymatic function of a WNK kinase. Interestingly, we have also found that the activity and cell surface abundance of NKCC2F were less affected by cell shrinkage compared with the other variants and that ion transport by certain variants could be stimulated through WNK kinase expression in the absence of carrier redistribution. Taken together, these results suggest that the WNK kinase-dependent pathway can affect both the trafficking as well as intrinsic activity of NKCC2 and that CS2 plays an important role in carrier regulation.

Salt stress in the renal tubules is linked to TAL-specific expression of uromodulin and an upregulation of heat shock genes

Previously, our comprehensive cardiovascular characterization study validated Uromodulin as a blood pressure gene. Uromodulin is a glycoprotein exclusively synthesized at the thick ascending limb of the loop of Henle and is encoded by the Umod gene. Umod-/- mice have significantly lower blood pressure than Umod+/+ mice, are resistant to salt-induced changes in blood pressure, and show a leftward shift in pressure-natriuresis curves reflecting changes of sodium reabsorption. Salt stress triggers transcription factors and genes that alter renal sodium reabsorption. To date there are no studies on renal transcriptome responses to salt stress. Here we aimed use RNA-Seq to delineate salt stress pathways in tubules isolated from Umod+/+ mice (a model of sodium retention) and Umod-/- mice (a model of sodium depletion) ± 300 mosmol sodium chloride ( n = 3 per group). In response to salt stress, the tubules of Umod+/+ mice displayed an upregulation of heat shock transcripts. The greatest changes occurred in the expression of: Hspa1a (Log2 fold change 4.35, P = 2.48 e-12) and Hspa1b (Log2 fold change 4.05, P = 2.48 e-12). This response was absent in tubules of Umod-/- mice. Interestingly, seven of the genes discordantly expressed in the Umod-/- tubules were electrolyte transporters. Our results are the first to show that salt stress in renal tubules alters the transcriptome, increasing the expression of heat shock genes. This direction of effect in Umod+/+ tubules suggest the difference is due to the presence of Umod facilitating greater sodium entry into the tubule cell reflecting a specific response to salt stress.

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.

Mutation spectrum of Chinese patients with Bartter syndrome

OBJECTIVE

Bartter syndrome (BS) has been rarely reported in Chinese population except for a few case reports. This investigation was aimed to analyze the mutations of the causal genes in sixteen Chinese patients with BS, and review their followup and treatment.

METHODS

Identify mutations by the next generation sequencing and the multiplex ligation-dependent probe amplification (MLPA). Clinical characteristics and biochemical findings at the first presentation as well as follow-up were reviewed.

RESULTS

15 different CLCNKB gene mutations were identified in fourteen patients with BS, including 11 novel ones. A novel missense mutation and a novel small deletion were found from SLC12A1 gene. A novel gross deletion was found in CLCNKA gene. A recurrent missense mutation was identified from BSND gene. We found that the whole gene deletion mutation of CLCNKB gene was the most frequent mutation (32%), and the rate of gross deletion was up to 50 percent in this group of Chinese patients.

CONCLUSION

The present study has found 19 mutations, including 14 novel ones, which would enrich the human gene mutation database (HGMD) and provide valuable references to the genetic counseling and diagnosis of the Chinese population.

Multiplex epithelium dysfunction due to CLDN10 mutation: the HELIX syndrome

PurposeWe aimed to identify the genetic cause to a clinical syndrome encompassing hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX syndrome), and to comprehensively delineate the phenotype.MethodsWe performed homozygosity mapping, whole-genome sequencing, gene sequencing, expression studies, functional tests, protein bioinformatics, and histological characterization in two unrelated families with HELIX syndrome.ResultsWe identified biallelic missense mutations (c.386C>T, p.S131L and c.2T>C, p.M1T) in CLDN10B in six patients from two unrelated families. CLDN10B encodes Claudin-10b, an integral tight junction (TJ) membrane-spanning protein expressed in the kidney, skin, and salivary glands. All patients had hypohidrosis, renal loss of NaCl with secondary hyperaldosteronism and hypokalemia, as well as hypolacrymia, ichthyosis, xerostomia, and severe enamel wear. Functional testing revealed that patients had a decreased NaCl absorption in the thick ascending limb of the loop of Henle and a severely decreased secretion of saliva. Both mutations resulted in reduced or absent Claudin-10 at the plasma membrane of epithelial cells.ConclusionCLDN10 mutations cause a dysfunction in TJs in several tissues and, subsequently, abnormalities in renal ion transport, ectodermal gland homeostasis, and epidermal integrity.

Role of renal transporters and novel regulatory interactions in the TAL that control blood pressure

Hypertension (HTN), a major public health issue is currently the leading factor in the global burden of disease, where associated complications account for 9.4 million deaths worldwide every year. Excessive dietary salt intake is among the environmental factors that contribute to HTN, known as salt sensitivity. The heterogeneity of salt sensitivity and the multiple mechanisms that link high salt intake to increases in blood pressure are of upmost importance for therapeutic application. A continual increase in the kidney's reabsorption of sodium (Na⁺) relies on sequential actions at various segments along the nephron. When the distal segments of the nephron fail to regulate Na⁺, the effects on Na⁺ homeostasis are unfavorable. We propose that the specific nephron region where increased active uptake occurs as a result of variations in Na⁺ reabsorption is at the thick ascending limb of the loop of Henle (TAL). The purpose of this review is to urge the consideration of the TAL as contributing to the pathophysiology of salt-sensitive HTN. Further research in this area will enable development of a therapeutic application for targeted treatment.

A novel SLC12A1 gene mutation associated with hyperparathyroidism, hypercalcemia, nephrogenic diabetes insipidus, and nephrocalcinosis in four patients

Solute Carrier Family 12 member 1 (SLC12A1) gene encodes the sodium-potassium-chloride co-transporter (NKCC2) at the apical membrane of the thick ascending loop of Henle (TAL). Bartter's syndrome (BS) type I is a rare, autosomal recessive, renal tubular disorder associated with mutation of the SLC12A1 gene. Presenting features include: hypokalemic metabolic alkalosis, hypercalciuria and nephrocalcinosis. The many allelic variants reported present with a spectrum of phenotypes, biochemical abnormalities and clinical severities. However, to date, only two reports have described hyperparathyroidism and hypercalcemia in patients with SLC12A1 gene mutations. We describe 4 patients with 4 novel mutation variants in the SLC12A1 gene (c.735C>G, c.1137del, c.2498-2499del, and c.1833delT) presenting with variable degrees of hyperparathyroidism, hypercalcemia, hypokalemic metabolic alkalosis, nephrocalcinosis, and nephrogenic diabetes insipidus. The link between calcium and parathyroid hormone abnormalities in patients with SLC12A1 mutations is unclear; the cases described suggest an association between primary hyperparathyroidism and loss of function mutation of SLC12A1, which may result in an aberrant threshold of the calcium sensing receptor at the level of the kidney.

A novel variant in the SLC12A1 gene in two families with antenatal Bartter syndrome

AIM

Bartter syndrome is an autosomal-recessive inherited disease in which patients present with hypokalaemia and metabolic alkalosis. We present two apparently nonrelated cases with antenatal Bartter syndrome type I, due to a novel variant in the SLC12A1 gene encoding the bumetanide-sensitive sodium-(potassium)-chloride cotransporter 2 in the thick ascending limb of the loop of Henle.

METHODS

Blood samples were received from the two cases and 19 of their relatives, and deoxyribonucleic acid was extracted. The coding regions of the SLC12A1 gene were amplified using polymerase chain reaction, followed by bidirectional direct deoxyribonucleic acid sequencing.

RESULTS

Each affected child in the two families was homozygous for a novel inherited variant in the SLC12A1gene, c.1614T>A. The variant predicts a change from a tyrosine codon to a stop codon (p.Tyr538Ter). The two cases presented antenatally and at six months of age, respectively.

CONCLUSION

The two cases were homozygous for the same variant in the SLC12A1 gene, but presented clinically at different ages. This could eventually be explained by the presence of other gene variants or environmental factors modifying the phenotypes. The phenotypes of the patients were similar to other patients with antenatal Bartter syndrome.

Genetic heterogeneity in patients with Bartter syndrome type 1

Bartter syndrome (BS) type 1 is an autosomal recessive kidney disorder caused by loss-of-function mutations in the solute carrier family 12 member 1 (SLC12A1) gene. To date, 72 BS type 1 patients harboring SLC12A1 mutations have been documented. Of these 144 alleles studied, 68 different disease-causing mutations have been detected in 129 alleles, and no mutation was detected in the remaining 15 alleles. The mutation types included missense/nonsense mutations, splicing mutations and small insertions and deletions ranging from 1 to 4 nucleotides. A large deletion encompassing a whole exon in the SLC12A1 gene has not yet been reported. The current study initially identified an undocumented homozygous frameshift mutation (c.1833delT) by Sanger sequencing analysis of a single infant with BS type 1. However, in a subsequent analysis, the mutation was detected only in the father's DNA. Upon further investigation using a next-generation sequencing approach, a deletion in exons 14 and 15 in both the patient and patient's mother was detected. The deletion was subsequently confirmed by use of a long-range polymerase chain reaction and was determined to be 3.16 kb in size based on sequencing of the junction fragment. The results of the present study demonstrated that pathogenic variants of SLC12A1 are heterogeneous. Large deletions appear to serve an etiological role in BS type 1, and may be more prevalent than previously thought.

A missense mutation in solute carrier family 12, member 1 (SLC12A1) causes hydrallantois in Japanese Black cattle

Background

Hydrallantois is the excessive accumulation of fluid within the allantoic cavity in pregnant animals and is associated with fetal mortality. Although the incidence of hydrallantois is very low in artificial insemination breeding programs in cattle, recently 38 cows with the phenotypic appearance of hydrallantois were reported in a local subpopulation of Japanese Black cattle. Of these, 33 were traced back to the same sire; however, both their parents were reported healthy, suggesting that hydrallantois is a recessive inherited disorder. To identify autozygous chromosome segments shared by individuals with hydrallantois and the causative mutation in Japanese Black cattle, we performed autozygosity mapping using single-nucleotide polymorphism (SNP) array and exome sequencing.

Results

Shared haplotypes of the affected fetuses spanned 3.52 Mb on bovine chromosome 10. Exome sequencing identified a SNP (g.62382825G > A, p.Pro372Leu) in exon 10 of solute carrier family 12, member 1 (SLC12A1), the genotype of which was compatible with recessive inheritance. SLC12A1 serves as a reabsorption molecule of Na⁺-K⁺-2Cl⁻ in the apical membrane of the thick ascending limb of the loop of Henle in the kidney. We observed that the concentration of Na⁺-Cl⁻ increased in allantoic fluid of homozygous SLC12A1 (g.62382825G > A) in a hydrallantois individual. In addition, SLC12A1-positive signals were localized at the apical membrane in the kidneys of unaffected fetuses, whereas they were absent from the apical membrane in the kidneys of affected fetuses. These results suggested that p.Pro372Leu affects the membrane localization of SLC12A1, and in turn, may impair its transporter activity. Surveillance of the risk-allele frequency revealed that the carriers were restricted to the local subpopulation of Japanese Black cattle. Moreover, we identified a founder individual that carried the mutation (g.62382825G > A).

Conclusions

In this study, we mapped the shared haplotypes of affected fetuses using autozygosity mapping and identified a de novo mutation in the SLC12A1 gene that was associated with hydrallantois in Japanese Black cattle. In kidneys of hydrallantois-affected fetuses, the mutation in SLC12A1 impaired the apical membrane localization of SLC12A1 and reabsorption of Na⁺-K⁺-2Cl⁻ in the thick ascending limb of the loop of Henle, leading to a defect in the concentration of urine via the countercurrent mechanism. Consequently, the affected fetuses exhibited polyuria that accumulated in the allantoic cavity. Surveillance of the risk-allele frequency indicated that carriers were not widespread throughout the Japanese Black cattle population. Moreover, we identified the founder individual, and thus could effectively manage the disorder in the population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3035-1) contains supplementary material, which is available to authorized users.

Vesicle-associated Membrane Protein 3 (VAMP3) Mediates Constitutive Trafficking of the Renal Co-transporter NKCC2 in Thick Ascending Limbs: ROLE IN RENAL FUNCTION AND BLOOD PRESSURE

Renal cells of the thick ascending limb (TAL) reabsorb NaCl via the apical Na⁺/K⁺/2Cl^- co-transporter NKCC2. Trafficking of NKCC2 to the apical surface regulates NKCC2-mediated NaCl absorption and blood pressure. The molecular mechanisms by which NKCC2 reaches the apical surface and their role in renal function and maintenance of blood pressure are poorly characterized. Here we report that NKCC2 interacts with the vesicle fusion protein VAMP3, and they co-localize at the TAL apical surface. We observed that silencing VAMP3 in vivo blocks constitutive NKCC2 exocytic delivery, decreasing the amount of NKCC2 at the TAL apical surface. VAMP3 is not required for cAMP-stimulated NKCC2 exocytic delivery. Additionally, genetic deletion of VAMP3 in mice decreased total expression of NKCC2 in the TAL and lowered blood pressure. Consistent with these results, urinary excretion of water and electrolytes was higher in VAMP3 knock-out mice, which produced more diluted urine. We conclude that VAMP3 interacts with NKCC2 and mediates its constitutive exocytic delivery to the apical surface. Additionally, VAMP3 is required for normal NKCC2 expression, renal function, and blood pressure.

Prenatal diagnosis of Bartter syndrome: amniotic fluid aldosterone

OBJECTIVE

Bartter syndrome is a severe inherited tubulopathy characterized by postnatal salt wasting, severe polyuria, dehydration, failure to thrive and secondary hyperaldosteronism. Prenatally, the disease is usually discovered following the onset of severe polyhydramnios in the second trimester. We studied amniotic fluid aldosterone concentration in Bartter syndrome and in controls.

METHODS

Amniotic fluid aldosterone was assayed by radioimmunoassay. We undertook a retrospective case-control study based on 36 cases of prenatally suspected and postnatally confirmed Bartter syndrome (22 with identified mutations): and 72 gestational age matched controls presenting with polyhydramnios and 72 without polyhydramnios. Amniotic fluid aldosterone was compared between the three groups.

RESULTS

The median amniotic fluid aldosterone concentration in the Bartter syndrome group (90 pg/mL) was not different from that in the controls with polyhydramnios (90 pg/mL, P = 0.33) or without polyhydramnios (87 pg/mL, P = 0.41).

CONCLUSION

Amniotic fluid aldosterone assay cannot be used for prenatal diagnosis of Bartter syndrome. © 2015 John Wiley & Sons, Ltd.

NKCC2 activity is inhibited by the Bartter's syndrome type 5 gain-of-function CaR-A843E mutant in renal cells

BACKGROUND INFORMATION

The gain-of-function A843E mutation of the calcium sensing receptor (CaR) causes Bartter syndrome type 5. Patients carrying this CaR variant show a remarkably reduced renal NaCl reabsorption in the thick ascending limb (TAL) of Henle's loop resulting in renal loss of NaCl in the absence of mutations in renal Na(+) and Cl(-) ion transporters. The molecular mechanisms underlying this clinical phenotype are incompletely understood. We investigated, in human embryonic kidney 293 (HEK 293) cells and porcine kidney epithelial (LLC-PK1) cells, the functional cross-talk of CaR-A843E with the Na(+):K(+):2Cl(-) co-transporter, NKCC2, which provides NaCl reabsorption in the TAL.

RESULTS

The expression of the CaR mutant did not alter the apical localisation of NKCC2 in LLC-PK1 cells. However, the steady-state NKCC2 phosphorylation and activity were decreased in cells transfected with CaR-A843E compared with the control wild-type CaR (CaR WT)-transfected cells. Of note, low-Cl(-)-dependent NKCC2 activation was also strongly inhibited upon the expression of CaR-A843E mutant. The use of either P450 ω-hydroxylase (CYP4)- or phospholipase A2 (PLA2)-blockers suggests that this effect is likely mediated by arachidonic acid (AA) metabolites.

CONCLUSIONS

The data suggested that the activated CaR affects intracellular pathways modulating NKCC2 activity rather than NKCC2 intracellular trafficking in renal cells, and throw further light on the pathological role played by active CaR mutants in Bartter syndrome type 5.

Molecular and evolutionary insights into the structural organization of cation chloride cotransporters

Cation chloride cotransporters (CCC) play an essential role for neuronal chloride homeostasis. K(+)-Cl(-) cotransporter (KCC2), is the principal Cl(-)-extruder, whereas Na(+)-K(+)-Cl(-) cotransporter (NKCC1), is the major Cl(-)-uptake mechanism in many neurons. As a consequence, the action of the inhibitory neurotransmitters gamma-aminobutyric acid (GABA) and glycine strongly depend on the activity of these two transporters. Knowledge of the mechanisms involved in ion transport and regulation is thus of great importance to better understand normal and disturbed brain function. Although no overall 3-dimensional crystal structures are yet available, recent molecular and phylogenetic studies and modeling have provided new and exciting insights into structure-function relationships of CCC. Here, we will summarize our current knowledge of the gross structural organization of the proteins, their functional domains, ion binding and translocation sites, and the established role of individual amino acids (aa). A major focus will be laid on the delineation of shared and distinct organizational principles between KCC2 and NKCC1. Exploiting the richness of recently generated genome data across the tree of life, we will also explore the molecular evolution of these features.

Physiology and pathophysiology of the renal Na-K-2Cl cotransporter (NKCC2)

The Na-K-2Cl cotransporter (NKCC2; BSC1) is located in the apical membrane of the epithelial cells of the thick ascending limb of the loop of Henle (TAL). NKCC2 facilitates ∼20–25% of the reuptake of the total filtered NaCl load. NKCC2 is therefore one of the transport proteins with the highest overall reabsorptive capacity in the kidney. Consequently, even subtle changes in NKCC2 transport activity considerably alter the renal reabsorptive capacity for NaCl and eventually lead to perturbations of the salt and water homoeostasis. In addition to facilitating the bulk reabsorption of NaCl in the TAL, NKCC2 transport activity in the macula densa cells of the TAL constitutes the initial step of the tubular-vascular communication within the juxtaglomerular apparatus (JGA); this communications allows the TAL to modulate the preglomerular resistance of the afferent arteriole and the renin secretion from the granular cells of the JGA. This review provides an overview of our current knowledge with respect to the general functions of NKCC2, the modulation of its transport activity by different regulatory mechanisms, and new developments in the pathophysiology of NKCC2-dependent renal NaCl transport.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Dietary salt intake modulates differential splicing of the Na-K-2Cl cotransporter NKCC2

Both sodium reabsorption in the thick ascending limb of the loop of Henle (TAL) and macula densa salt sensing crucially depend on the function of the Na/K/2Cl cotransporter NKCC2. The NKCC2 gene gives rise to at least three different full-length NKCC2 isoforms derived from differential splicing. In the present study, we addressed the influence of dietary salt intake on the differential splicing of NKCC2. Mice were subjected to diets with low-salt, standard salt, and high-salt content for 7 days, and NKCC2 isoform mRNA abundance was determined. With decreasing salt intake, we found a reduced abundance of the low-affinity isoform NKCC2A and an increase in the high-affinity isoform NKCC2B in the renal cortex and the outer stripe of the outer medulla. This shift from NKCC2A to NKCC2B during a low-salt diet could be mimicked by furosemide in vivo and in cultured kidney slices. Furthermore, the changes in NKCC2 isoform abundance during a salt-restricted diet were partly mediated by the actions of angiotensin II on AT1 receptors, as determined using chronic angiotensin II infusion. In contrast to changes in oral salt intake, water restriction (48 h) and water loading (8% sucrose solution) increased and suppressed the expression of all NKCC2 isoforms, without changing the distribution pattern of the single isoforms. In summary, the differential splicing of NKCC2 pre-mRNA is modulated by dietary salt intake, which may be mediated by changes in intracellular ion composition. Differential splicing of NKCC2 appears to contribute to the adaptive capacity of the kidney to cope with changes in reabsorptive needs.

Physiology of SLC12 transporters: lessons from inherited human genetic mutations and genetically engineered mouse knockouts

Among the over 300 members of the solute carrier (SLC) group of integral plasma membrane transport proteins are the nine electroneutral cation-chloride cotransporters belonging to the SLC12 gene family. Seven of these transporters have been functionally described as coupling the electrically silent movement of chloride with sodium and/or potassium. Although in silico analysis has identified two additional SLC12 family members, no physiological role has been ascribed to the proteins encoded by either the SLC12A8 or the SLC12A9 genes. Evolutionary conservation of this gene family from protists to humans confirms their importance. A wealth of physiological, immunohistochemical, and biochemical studies have revealed a great deal of information regarding the importance of this gene family to human health and disease. The sequencing of the human genome has provided investigators with the capability to link several human diseases with mutations in the genes encoding these plasma membrane proteins. The availability of bacterial artificial chromosomes, recombination engineering techniques, and the mouse genome sequence has simplified the creation of targeting constructs to manipulate the expression/function of these cation-chloride cotransporters in the mouse in an attempt to recapitulate some of these human pathologies. This review will summarize the three human disorders that have been linked to the mutation/dysfunction of the Na-Cl, Na-K-2Cl, and K-Cl cotransporters (i.e., Bartter's, Gitleman's, and Andermann's syndromes), examine some additional pathologies arising from genetically modified mouse models of these cotransporters including deafness, blood pressure, hyperexcitability, and epithelial transport deficit phenotypes.

Multiple evolutionarily conserved Di-leucine like motifs in the carboxyl terminus control the anterograde trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the (1081)LLV(1083) motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, (1038)LL(1039) and (1048)LI(1049), that are required for ER exit and surface expression of the co-transporter. Double mutations of (1038)LL(1039) or (1048)LI(1049) to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that (1038)AA(1039) and (1048)AA(1049) were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.

Multiple evolutionarily conserved Di-leucine like motifs in the carboxyl terminus control the anterograde trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the (1081)LLV(1083) motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, (1038)LL(1039) and (1048)LI(1049), that are required for ER exit and surface expression of the co-transporter. Double mutations of (1038)LL(1039) or (1048)LI(1049) to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that (1038)AA(1039) and (1048)AA(1049) were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.

Status epilepticus as the only presentation of the neonatal Bartter syndrome

Bartter syndrome is a rare hereditary (autosomal recessive) salt-losing tubulopathy characterized by hypokalemia, hypochloremia, metabolic alkalosis, and normal blood pressure with hyperreninemia, The underlying renal abnormality results in excessive urinary losses of sodium, chloride, and potassium. We report a case of a four-month-old infant with neonatal Bartter syndrome, who presented only with status epilepticus. To the best of our present knowledge, there is no reported case of Bartter syndrome who presented with status epilepticus.

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Differential regulation of NFAT5 by NKCC2 isoforms in medullary thick ascending limb (mTAL) cells

The effects of Na(+)-K(+)-2Cl(-) cotransporter type 2 (NKCC2) isoforms on the regulation of nuclear factor of activated T cells isoform 5 (NFAT5) were determined in mouse medullary thick ascending limb (mTAL) cells exposed to high NaCl concentration. Primary cultures of mTAL cells and freshly isolated mTAL tubules, both derived from the outer medulla (outer stripe>inner stripe), express NKCC2 isoforms A and F. The relative expression of NKCC2A mRNA was approximately twofold greater than NKCC2F in these preparations. The abundance of NKCC2A mRNA, but not NKCC2F mRNA, increased approximately twofold when mTAL cells were exposed for 2 h to a change in osmolality from 300 to 500 mosmol/kgH₂O, produced with NaCl. Total NKCC2 protein expression also increased. Moreover, a 2.5-fold increase in NFAT5 mRNA accumulation was observed after cells were exposed to 500 mosmol/kgH₂O for 4 h. Laser-scanning cytometry detected a twofold increase in endogenous NFAT5 protein expression in response to high NaCl concentration. Pretreatment with the loop diuretic bumetanide dramatically reduced transcriptional activity of the NFAT5-specific reporter construct TonE-Luc in mTAL cells exposed to high NaCl. Transient transfection of mTAL cells with shRNA vectors targeting NKCC2A prevented increases in NFAT5 mRNA abundance and protein expression and inhibited NFAT5 transcriptional activity in response to hypertonic stress. Silencing of NKCC2F mRNA did not affect NFAT5 mRNA accumulation but partially inhibited NFAT5 transcriptional activity. These findings suggest that NKCC2A and NKCC2F exhibit differential effects on NFAT5 expression and transcriptional activity in response to hypertonicity produced by high NaCl concentration.

Rare mutations in the human Na-K-Cl cotransporter (NKCC2) associated with lower blood pressure exhibit impaired processing and transport function

The Na-K-Cl cotransporter (NKCC2) is the major salt transport pathway in the thick ascending limb of Henle's loop and is part of the molecular mechanism for blood pressure regulation. Recent screening of ∼3,000 members of the Framingham Heart Study identified nine rare independent mutations in the gene encoding NKCC2 (SLC12A1) associated with clinically reduced blood pressure and protection from hypertension (Ji WZ, Foo JN, O'Roak BJ, Zhao H, Larson MG, Simon DB, Newton-Cheh C, State M, Levy D, Lifton RP. Nat Genet 40: 592-599, 2008). To investigate their functional consequences, we introduced the nine mutations in human NKCC2A and examined protein function, expression, localization, regulation, and ion transport kinetics using heterologous expression in Xenopus laevis oocytes and HEK-293 cells. When expressed in oocytes, four of the mutants (T235M, R302W, L505V, and P569H) exhibited reduced transport function compared with wild-type. In HEK-293 cells, the same four mutants exhibited reduced function, and in addition N399S and P1083A had significantly lower activity than wild-type. The two most functionally impaired mutants (R302W and L505V) exhibited dramatically diminished production of complex-glycosylated protein and a decrease in or absence of plasma membrane localization, indicative of a processing defect. All of the functional human (h) NKCC2A variants were regulated by changes in oocyte volume and intracellular chloride in HEK cells, but P254A and N399S exhibited a lower constitutive activity in HEK cells. The P569H mutant exhibited a 50% reduction in sodium affinity compared with wild-type, predicting lower transport activity at lower intratubular salt concentrations, while the P254A mutant exhibited a 35% increase in rubidium affinity. We conclude that defects in NKCC2 processing, transport turnover rate, regulation, and ion affinity contribute to impaired transport function in six of the nine identified mutants, providing support for the predictive approach of Ji et al. to identify functionally important residues by sequence conservation. Such mutations in hNKCC2A are likely to reduce renal salt reabsorption, providing a mechanism for lower blood pressure.

A case of Bartter syndrome type I with atypical presentations

Bartter syndrome (BS) is an autosomal recessively inherited rare renal tubular disorder characterized by hypokalemic metabolic alkalosis and hyperreninemic hyperaldosteronism with normal to low blood pressure due to a renal loss of sodium. Genetically, BS is classified into 5 subtypes according to the underlying genetic defects, and BS is clinically categorized into antenatal BS and classical BS according to onset age. BS type I is caused by loss-of-function mutations in the SLC12A1 gene and usually manifests as antenatal BS. This report concerns a male patient with compound heterozygous missense mutations on SLC12A1 (p.C436Y and p.L560P) and atypical clinical and laboratory features. The patient had low urinary sodium and chloride levels without definite metabolic alkalosis until the age of 32 months, which led to confusion between BS and nephrogenic diabetes insipidus (NDI). In addition, the clinical onset of the patient was far beyond the neonatal period. Genetic study eventually led to the diagnosis of BS type I. The low urinary sodium and chloride concentrations may be caused by secondary NDI, and the later onset may suggest the existence of a genotype-phenotype correlation.

In summary, BS type I may have phenotype variability including low urine sodium and chloride levels and later onset. A definitive diagnosis can be confirmed by genetic testing.

Localization and functional characterization of the human NKCC2 isoforms

AIM

Salt reabsorption across the apical membrane of cells in the thick ascending limb (TAL) of Henle is primarily mediated by the bumetanide-sensitive Na(+)/K(+)/2Cl(-) cotransporter NKCC2. Three full-length splice variants of NKCC2 (NKCC2B, NKCC2A and NKCC2F) have been described. The NKCC2 isoforms have specific localizations and transport characteristics, as assessed for rabbit, rat and mouse. In the present study, we aimed to address the localization and transport characteristics of the human NKCC2 isoforms.

METHODS

RT-PCR, in situ hybridization and uptake studies in Xenopus oocytes were performed to characterize human NKCC2 isoforms.

RESULTS

All three classical NKCC2 isoforms were detected in the human kidney; in addition, we found splice variants with tandem duplicates of the variable exon 4. Contrary to rodents, in which NKCC2F is the most abundant NKCC2 isoform, NKCC2A was the dominant isoform in humans; similarly, isoform-specific in situ hybridization showed high expression levels of human NKCC2A along the TAL. Compared to NKCC2B and NKCC2F, human NKCC2A had the lowest Cl(-) affinity as determined by (86)Rb(+) uptake studies in oocytes. All NKCC2 isoforms were more efficiently inhibited by bumetanide than by furosemide. A sequence analysis of the amino acids encoded by exon 4 variants revealed high similarities between human and rodent NKCC2 isoforms, suggesting that differences in ion transport characteristics between species may be related to sequence variations outside the highly conserved sequence encoded by exon 4.

CONCLUSION

The human NKCC2 is an example of how differential splicing forms the basis for a diversification of transporter protein function.