Expression of the thiazide-sensitive Na-Cl cotransporter in rat and human kidney

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Enriched Single-Nucleus RNA-Sequencing Reveals Unique Attributes of Distal Convoluted Tubule Cells

SIGNIFICANCE STATEMENT

High-resolution single-nucleus RNA-sequencing data indicate a clear separation between primary sites of calcium and magnesium handling within distal convoluted tubule (DCT). Both DCT1 and DCT2 express Slc12a3, but these subsegments serve distinctive functions, with more abundant magnesium-handling genes along DCT1 and more calcium-handling genes along DCT2. The data also provide insight into the plasticity of the distal nephron-collecting duct junction, formed from cells of separate embryonic origins. By focusing/changing gradients of gene expression, the DCT can morph into different physiological cell states on demand.

BACKGROUND

The distal convoluted tubule (DCT) comprises two subsegments, DCT1 and DCT2, with different functional and molecular characteristics. The functional and molecular distinction between these segments, however, has been controversial.

METHODS

To understand the heterogeneity within the DCT population with better clarity, we enriched for DCT nuclei by using a mouse line combining "Isolation of Nuclei Tagged in specific Cell Types" and sodium chloride cotransporter-driven inducible Cre recombinase. We sorted the fluorescently labeled DCT nuclei using Fluorescence-Activated Nucleus Sorting and performed single-nucleus transcriptomics.

RESULTS

Among 25,183 DCT cells, 75% were from DCT1 and 25% were from DCT2. In addition, there was a small population (<1%) enriched in proliferation-related genes, such as Top2a , Cenpp , and Mki67 . Although both DCT1 and DCT2 expressed sodium chloride cotransporter, magnesium transport genes were predominantly expressed along DCT1, whereas calcium, electrogenic sodium, and potassium transport genes were more abundant along DCT2. The transition between these two segments was gradual, with a transitional zone in which DCT1 and DCT2 cells were interspersed. The expression of the homeobox genes by DCT cells suggests that they develop along different trajectories.

CONCLUSIONS

Transcriptomic analysis of an enriched rare cell population using a genetically targeted approach clarifies the function and classification of distal cells. The DCT segment is short, can be separated into two subsegments that serve distinct functions, and is speculated to derive from different origins during development.

Combination Diuretic Therapy to Counter Renal Sodium Avidity in Acute Heart Failure: Trials and Tribulations

In contrast to significant advances in the management of patients with chronic heart failure over the past few years, there has been little change in how patients with acute heart failure are treated. Symptoms and signs of fluid overload are the primary reason for hospitalization of patients who experience acute decompensation of heart failure. Intravenous loop diuretics remain the mainstay of therapy in this patient population, with a significant subset of them showing suboptimal response to these agents leading to incomplete decongestion at the time of discharge. Combination diuretic therapy, that is, using loop diuretics along with an add-on agent, is a widely applied strategy to counter renal sodium avidity through sequential blockade of sodium absorption within renal tubules. The choice of the second diuretic is affected by several factors, including the site of action, the anticipated secondary effects, and the available evidence on their efficacy and safety. While the current guidelines recommend combination diuretic therapy as a viable option to overcome suboptimal response to loop diuretics, it is also acknowledged that this strategy is not supported by strong evidence and remains an area of uncertainty. The recent publication of landmark studies has regenerated the interest in sequential nephron blockade. In this article, we provide an overview of the results of the key studies on combination diuretic therapy in the setting of acute heart failure and discuss their findings primarily with regard to the effect on renal sodium avidity and cardiorenal outcomes.

The effect of high-dietary K+ (HK) on Kir4.1/Kir5.1 and ROMK in the distal convoluted tubule (DCT) is not affected by gender and Cl- content of the diet

Basolateral potassium channels in the distal convoluted tubule (DCT) are composed of inwardly-rectifying potassium channel 4.1 (Kir4.1) and Kir5.1. Kir4.1 interacts with Kir5.1 to form a 40 pS K⁺ channel which is the only type K⁺ channel expressed in the basolateral membrane of the DCT. Moreover, Kir4.1/Kir5.1 heterotetramer plays a key role in determining the expression and activity of thiazide-sensitive Na-Cl cotransport (NCC). In addition to Kir4.1/Kir5.1, Kir1.1 (ROMK) is expressed in the apical membrane of the late DCT (DCT2) and plays a key role in mediating epithelial Na⁺ channel (ENaC)-dependent K⁺ excretion. High dietary-K⁺-intake (HK) stimulates ROMK and inhibits Kir4.1/Kir5.1 in the DCT. Inhibition of Kir4.1/Kir5.1 is essential for HK-induced suppression of NCC whereas the stimulation of ROMK is important for increasing ENaC-dependent K⁺ excretion during HK. We have now used the patch-clamp-technique to examine whether gender and Cl^- content of K⁺-diet affect HK-induced inhibition of basolateral Kir4.1/Kir5.1 and HK-induced stimulation of ROMK. Single-channel-recording shows that basolateral 40 pS K⁺ channel (Kir4.1/Kir5.1) activity of the DCT defined by NP_o was 1.34 (1% KCl, normal K, NK), 0.95 (5% KCl) and 1.03 (5% K⁺-citrate) in male mice while it was 1.47, 1.02 and 1.05 in female mice. The whole-cell recording shows that Kir4.1/Kir5.1-mediated-K⁺ current of the early-DCT (DCT1) was 1,170 pA (NK), 725 pA (5% KCl) and 700 pA (5% K⁺-citrate) in male mice whereas it was 1,125 pA, 674 pA and 700 pA in female mice. Moreover, K⁺-currents (I_K) reversal potential of DCT (an index of membrane potential) was -63 mV (NK), -49 mV (5% KCl) and -49 mV (5% K-citrate) in the male mice whereas it was -63 mV, -50 mV and -50 mV in female mice. Finally, TPNQ-sensitive whole-cell ROMK-currents in the DCT2 /initial-connecting tubule (CNT) were 910 pA (NK), 1,520 pA (5% KCl) and 1,540 pA (5% K⁺-citrate) in male mice whereas the ROMK-mediated K⁺ currents were 1,005 pA, 1,590 pA and 1,570 pA in female mice. We conclude that the effect of HK intake on Kir4.1/Kir5.1 of the DCT and ROMK of DCT2/CNT is similar between male and female mice. Also, Cl^- content in HK diets has no effect on HK-induced inhibition of Kir4.1/Kir5.1 of the DCT and HK-induced stimulation of ROMK in DCT2/CNT.

Inwardly rectifying K+ channels 4.1 and 5.1 (Kir4.1/Kir5.1) in the renal distal nephron

The inwardly rectifying potassium channel (Kir) 4.1 (encoded by KCNJ10) interacts with Kir5.1 (encoded by KCNJ16) to form a major basolateral K⁺ channel in the renal distal convoluted tubule (DCT), connecting tubule (CNT), and the cortical collecting duct (CCD). Kir4.1/Kir5.1 heterotetramer plays an important role in regulating Na⁺ and K⁺ transport in the DCT, CNT, and CCD. A recent development in the field has firmly established the role of Kir4.1/Kir5.1 heterotetramer of the DCT in the regulation of thiazide-sensitive Na-Cl cotransporter (NCC). Changes in Kir4.1/Kir5.1 activity of the DCT are an essential step for the regulation of NCC expression/activity induced by dietary K⁺ and Na⁺ intakes and play a role in modulating NCC by type 2 angiotensin II receptor (AT2R), bradykinin type II receptor (BK2R), and β-adrenergic receptor. Since NCC activity determines the Na⁺ delivery rate to the aldosterone-sensitive distal nephron (ASDN), a distal nephron segment from late DCT to CCD, Kir4.1/Kir5.1 activity plays a critical role not only in the regulation of renal Na⁺ absorption but also in modulating renal K⁺ excretion and maintaining K⁺ homeostasis. Thus, Kir4.1/Kir5.1 activity serves as an important component of renal K⁺ sensing mechanism. The main focus of this review is to provide an overview regarding the role of Kir4.1 and Kir5.1 of the DCT and CCD in the regulation of renal K⁺ excretion and Na⁺ absorption.

Elevated blood pressure, antihypertensive medications and bone health in the population: revisiting old hypotheses and exploring future research directions

Blood pressure and bone metabolism appear to share commonalities in their physiologic regulation. Specific antihypertensive drug classes may also influence bone mineral density. However, current evidence from existing observational studies and randomised trials is insufficient to establish causal associations for blood pressure and use of blood pressure-lowering drugs with bone health outcomes, particularly with the risks of osteoporosis and fractures. The availability and access to relevant large-scale biomedical data sources as well as developments in study designs and analytical approaches provide opportunities to examine the nature of the association between blood pressure and bone health more reliably and in greater detail than has ever been possible. It is unlikely that a single source of data or study design can provide a definitive answer. However, with appropriate considerations of the strengths and limitations of the different data sources and analytical techniques, we should be able to advance our understanding of the role of raised blood pressure and its drug treatment on the risks of low bone mineral density and fractures. As elevated blood pressure is highly prevalent and blood pressure-lowering drugs are widely prescribed, even small effects of these exposures on bone health outcomes could be important at a population level.

Plasma metabolomic profiles associated with hypertension and blood pressure in response to thiazide diuretics

This study aimed to identify the metabolomic alterations associated with hypertension (HTN) and the response of blood pressure (BP) to thiazide diuretics. A total of 50 participants previously untreated for HTN were prospectively recruited. After a 2-week lifestyle adjustment, 30 participants with systolic BP ≥ 140 mmHg and/or diastolic BP ≥ 90 mmHg were classified into the HTN group and prescribed hydrochlorothiazide (HCTZ) at 50 mg per day for 2 weeks. The remaining 20 participants, who had relatively normal BP, were assigned to the normotension group. Metabolomic profiles related to the response of BP to thiazide diuretics were analyzed. A total of 73 differential metabolites were found to be associated with HTN, and 27 metabolites were significantly changed upon HCTZ treatment (HCTZ-sensitive metabolites). Among the identified metabolites, 7 (aspartate, histidine, C5-DC, C5-M-DC, C14:1, phosphatidylcholine ae C34:1, and phosphatidylcholine ae C34:3) were positively associated with HTN and decreased in abundance upon HCTZ treatment (HCTZ-reduced/HTN-associated metabolites). Moreover, multivariate analysis of 20 metabolites whose baseline levels were associated with the response of BP revealed that aspartate, glutamate, lysophosphatidylcholine C16:0, lysophosphatidylcholine C20:3, and sphingomyelin C24:1 were independently related to systolic BP reduction, and lysophosphatidylcholine C20:3 was independently associated with diastolic BP reduction. In conclusion, we identified 5 metabolites independently related to BP changes with HCTZ treatment. An advanced biomarker profile of thiazide-induced metabolomic changes may provide a clue with which to further explore the complex and mixed effects of thiazide treatment in a clinical setting.

Renal cell markers: lighthouses for managing renal diseases

Kidneys, one of the vital organs in our body, are responsible for maintaining whole body homeostasis. The complexity of renal function (e.g., filtration, reabsorption, fluid and electrolyte regulation, and urine production) demands diversity not only at the level of cell types but also in their overall distribution and structural framework within the kidney. To gain an in depth molecular-level understanding of the renal system, it is imperative to discern the components of kidney and the types of cells residing in each of the subregions. Recent developments in labeling, tracing, and imaging techniques have enabled us to mark, monitor, and identify these cells in vivo with high efficiency in a minimally invasive manner. In this review, we summarize different cell types, specific markers that are uniquely associated with those cell types, and their distribution in the kidney, which altogether make kidneys so special and different. Cellular sorting based on the presence of certain proteins on the cell surface allowed for the assignment of multiple markers for each cell type. However, different studies using different techniques have found contradictions in cell type-specific markers. Thus, the term "cell marker" might be imprecise and suboptimal, leading to uncertainty when interpreting the data. Therefore, we strongly believe that there is an unmet need to define the best cell markers for a cell type. Although the compendium of renal-selective marker proteins presented in this review is a resource that may be useful to researchers, we acknowledge that the list may not be necessarily exhaustive.

Mechanisms Underlying Calcium Nephrolithiasis

Nephrolithiasis is a worldwide problem with increasing prevalence, enormous costs, and significant morbidity. Calcium-containing kidney stones are by far the most common kidney stones encountered in clinical practice. Consequently, hypercalciuria is the greatest risk factor for kidney stone formation. Hypercalciuria can result from enhanced intestinal absorption, increased bone resorption, or altered renal tubular transport. Kidney stone formation is complex and driven by high concentrations of calcium-oxalate or calcium-phosphate in the urine. After discussing the mechanism mediating renal calcium salt precipitation, we review recent discoveries in renal tubular calcium transport from the proximal tubule, thick ascending limb, and distal convolution. Furthermore, we address how calcium is absorbed from the intestine and mobilized from bone. The effect of acidosis on bone calcium resorption and urinary calcium excretion is also considered. Although recent discoveries provide insight into these processes, much remains to be understood in order to provide improved therapies for hypercalciuria and prevent kidney stone formation. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Targeted Single-Cell RNA-seq Identifies Minority Cell Types of Kidney Distal Nephron

BACKGROUND

Proximal tubule cells dominate the kidney parenchyma numerically, although less abundant cell types of the distal nephron have disproportionate roles in water and electrolyte balance.

METHODS

Coupling of a FACS-based enrichment protocol with single-cell RNA-seq profiled the transcriptomes of 9099 cells from the thick ascending limb (CTAL)/distal convoluted tubule (DCT) region of the mouse nephron.

RESULTS

Unsupervised clustering revealed Slc12a3 ⁺/Pvalb ⁺ and Slc12a3 ⁺/Pvalb ^- cells, identified as DCT1 and DCT2 cells, respectively. DCT1 cells appear to be heterogeneous, with orthogonally variable expression of Slc8a1, Calb1, and Ckb. An additional DCT1 subcluster showed marked enrichment of cell cycle-/cell proliferation-associated mRNAs (e.g., Mki67, Stmn1, and Top2a), which fit with the known plasticity of DCT cells. No DCT2-specific transcripts were found. DCT2 cells contrast with DCT1 cells by expression of epithelial sodium channel β- and γ-subunits and much stronger expression of transcripts associated with calcium transport (Trpv5, Calb1, S100g, and Slc8a1). Additionally, scRNA-seq identified three distinct CTAL (Slc12a1 ⁺) cell subtypes. One of these expressed Nos1 and Avpr1a, consistent with macula densa cells. The other two CTAL clusters were distinguished by Cldn10 and Ptger3 in one and Cldn16 and Foxq1 in the other. These two CTAL cell types were also distinguished by expression of alternative Iroquois homeobox transcription factors, with Irx1 and Irx2 in the Cldn10 ⁺ CTAL cells and Irx3 in the Cldn16 ⁺ CTAL cells.

CONCLUSIONS

Single-cell transcriptomics revealed unexpected diversity among the cells of the distal nephron in mouse. Web-based data resources are provided for the single-cell data.

Generation of a human induced pluripotent stem cell line (CMCi002-A) from a patient with Gitelman's syndrome

We established a human induced pluripotent stem cells (hiPSC) line (CMCi002-A) from peripheral blood mononuclear cells (PBMCs) of 29-year-old male with Gitelman's syndrome (GIT) caused by the mutation of solute carrier family 12 member 3 (SLC12A3) gene using Sendai virus. The GIT-hiPSCs showed a typical human embryonic stem cell like morphology and expressed all pluripotency-associated markers, exhibited normal karyotype and were capable of differentiating into cells representative of three germ layers.

Epoxyeicosatrienoic acid metabolites inhibit Kir4.1/Kir5.1 in the distal convoluted tubule

Cytochrome P-450 (Cyp) epoxygenase-dependent metabolites of arachidonic acid (AA) have been shown to inhibit renal Na+ transport, and inhibition of Cyp-epoxygenase is associated with salt-sensitive hypertension. We used the patch-clamp technique to examine whether Cyp-epoxygenase-dependent AA metabolites inhibited the basolateral 40-pS K+ channel (Kir4.1/Kir5.1) in the distal convoluted tubule (DCT). Application of AA inhibited the basolateral 40-pS K+ channel in the DCT. The inhibitory effect of AA on the 40-pS K+ channel was specific because neither linoleic nor oleic acid was able to mimic the effect of AA on the K+ channel. Inhibition of Cyp-monooxygenase with N-methylsulfonyl-12,12-dibromododec-11-enamide or inhibition of cyclooxygenase with indomethacin failed to abolish the inhibitory effect of AA on the 40-pS K+ channel. However, the inhibition of Cyp-epoxygenase with N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide abolished the effect of AA on the 40-pS K+ channel in the DCT. Moreover, addition of either 11,12-epoxyeicosatrienoic acid (EET) or 14,15-EET also inhibited the 40-pS K+ channel in the DCT. Whole cell recording demonstrated that application of AA decreased, whereas N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide treatment increased, Ba2+-sensitive K+ currents in the DCT. Finally, application of 14,15-EET but not AA was able to inhibit the basolateral 40-pS K+ channel in the DCT of Cyp2c44-/- mice. We conclude that Cyp-epoxygenase-dependent AA metabolites inhibit the basolateral Kir4.1/Kir5.1 in the DCT and that Cyp2c44-epoxygenase plays a role in the regulation of the basolateral K+ channel in the mouse DCT.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

A review of the prescribing trend of thiazide-type and thiazide-like diuretics in hypertension: A UK perspective

Thiazide diuretics have been the cornerstone of hypertension treatment for >5 decades. Most recent European and American guidelines recommend both thiazide-type and thiazide-like diuretics as first-line drugs for all patients with hypertension. In contrast, diuretics are not regarded as first-line treatment in the UK and in patients who are to be initiated on a diuretic treatment, thiazide-like molecules, such as chlortalidone and indapamide are the preferred option. This review examines the prescribing trend of the 4 most commonly prescribed thiazide diuretics for the treatment of hypertension in the UK. Prescription cost analysis data were obtained for both 2010 and 2016/2017 for each region of the UK to analyse the impact of the 2011 National Institute for Health and Care Excellence hypertension guidelines on the trend in thiazide diuretic prescribing. Overall, the prescriptions of thiazide diuretics declined over the years. Bendroflumethiazide is the most commonly prescribed diuretic in the UK and despite some geographical differences, thiazide-type diuretics are more widely used than thiazide-like. The use of indapamide increased significantly between 2010 and 2016/2017 while chlortalidone was rarely employed. Of the many factors affecting trends in prescriptions, clinical inertia, treatment adherence, availability of the products and the lack of fixed dose combinations may play a role.

MST3 is involved in ENaC-mediated hypertension

Liddle syndrome is an inherited form of human hypertension caused by increasing epithelial Na⁺ channel (ENaC) expression. Increased Na⁺ retention through ENaC with subsequent volume expansion causes hypertension. In addition to ENaC, the Na⁺-K⁺-Cl^- cotransporter (NKCC) and Na⁺-Cl^- symporter (NCC) are responsible for Na⁺ reabsorption in the kidneys. Several Na⁺ transporters are evolutionarily regulated by the Ste20 kinase family. Ste20-related proline/alanine-rich kinase and oxidative stress-responsive kinase-1 phosphorylate downstream NKCC2 and NCC to maintain Na⁺ and blood pressure (BP) homeostasis. Mammalian Ste20 kinase 3 (MST3) is another member of the Ste20 family. We previously reported that reduced MST3 levels were found in the kidneys in spontaneously hypertensive rats and that MST3 was involved in Na⁺ regulation. To determine whether MST3 is involved in BP stability through Na⁺ regulation, we generated a MST3 hypomorphic mutation and designated MST3^+/- and MST3^-/- mice to examine BP and serum Na⁺ and K⁺ concentrations. MST3^-/- mice exhibited hypernatremia, hypokalemia, and hypertension. The increased ENaC in the kidney played roles in hypernatremia. The reabsorption of more Na⁺ promoted more K⁺ secretion in the kidney and caused hypokalemia. The hypernatremia and hypokalemia in MST3^-/- mice were significantly reversed by the ENaC inhibitor amiloride, indicating that MST3^-/- mice reabsorbed more Na⁺ through ENaC. Furthermore, Madin-Darby canine kidney cells stably expressing kinase-dead MST3 displayed elevated ENaC currents. Both the in vivo and in vitro results indicated that MST3 maintained Na⁺ homeostasis through ENaC regulation. We are the first to report that MST3 maintains BP stability through ENaC regulation.

Kir4.1/Kir5.1 Activity Is Essential for Dietary Sodium Intake-Induced Modulation of Na-Cl Cotransporter

BACKGROUND

Dietary sodium intake regulates the thiazide-sensitive Na-Cl cotransporter (NCC) in the distal convoluted tubule (DCT). Whether the basolateral, inwardly rectifying potassium channel Kir4.1/Kir5.1 (a heterotetramer of Kir4.1/Kir5.1) in the DCT is essential for mediating the effect of dietary sodium intake on NCC activity is unknown.

METHODS

We used electrophysiology, renal clearance techniques, and immunoblotting to examine effects of Kir4.1/Kir5.1 in the DCT and NCC in wild-type and kidney-specific Kir4.1 knockout mice.

RESULTS

Low sodium intake stimulated basolateral Kir4.1/Kir5.1 activity, increased basolateral K⁺ conductance, and hyperpolarized the membrane. Conversely, high sodium intake inhibited the potassium channel, decreased basolateral K⁺ currents, and depolarized the membrane. Low sodium intake increased total and phosphorylated NCC expression and augmented hydrochlorothiazide-induced natriuresis; high sodium intake had opposite effects. Thus, elevated NCC activity induced by low sodium intake was associated with upregulation of Kir4.1/Kir5.1 activity in the DCT, whereas inhibition of NCC activity by high sodium intake was associated with diminished Kir4.1/Kir5.1 activity. In contrast, dietary sodium intake did not affect NCC activity in knockout mice. Further, Kir4.1 deletion not only abolished basolateral K⁺ conductance and depolarized the DCT membrane, but also abrogated the stimulating effects induced by low sodium intake on basolateral K⁺ conductance and hyperpolarization. Finally, dietary sodium intake did not alter urinary potassium excretion rate in hypokalemic knockout and wild-type mice.

CONCLUSIONS

Stimulation of Kir4.1/Kir5.1 by low intake of dietary sodium is essential for NCC upregulation, and inhibition of Kir4.1/Kir5.1 induced by high sodium intake is a key step for downregulation of NCC.

Kir5.1 regulates Nedd4-2-mediated ubiquitination of Kir4.1 in distal nephron

Kir4.1/5.1 heterotetramer participates in generating the negative cell membrane potential in distal convoluted tubule (DCT) and plays a critical role in determining the activity of Na-Cl cotransporter (NCC). Kir5.1 contains a phosphothreonine motif at its COOH terminus (AA249-252). Coimmunoprecipitation showed that Nedd4-2 was associated with Kir5.1 in HEK293 cells cotransfected with Kir5.1 or Kir4.1/Kir5.1. GST pull-down further confirmed the association between Nedd4-2 and Kir5.1. Ubiquitination assay showed that Nedd4-2 increased the ubiquitination of Kir4.1/Kir5.1 heterotetramer in the cells cotransfected with Kir4.1/Kir5.1, but it has no effect on Kir4.1 or Kir5.1 alone. Patch-clamp and Western blot also demonstrated that coexpression of Nedd4-2 but not Nedd4-1 decreased K currents and Kir4.1 expression in the cells cotransfected with Kir4.1 and Kir5.1. In contrast, Nedd4-2 fails to inhibit Kir4.1 in the absence of Kir5.1 or in the cells transfected with the inactivated form of Nedd4-2 (Nedd4-2C821A). Moreover, the mutation of TPVT motif in the COOH terminus of Kir5.1 largely abolished the association of Nedd4-2 with Kir5.1 and abolished the inhibitory effect of Nedd4-2 on K currents in HEK293 cells transfected with Kir4.1 and Kir5.1 mutant (Kir5.1T249A). Finally, the basolateral K conductance in the DCT and Kir4.1 expression is significantly increased in the kidney-specific Nedd4-2 knockout or in Kir5.1 knockout mice in comparison to their corresponding wild-type littermates. We conclude that Nedd4-2 binds to Kir5.1 at the phosphothreonine motif of the COOH terminus, and the association of Nedd4-2 with Kir5.1 facilitates the ubiquitination of Kir4.1, thereby regulating its plasma expression in the DCT.

AT2R (Angiotensin II Type 2 Receptor)-Mediated Regulation of NCC (Na-Cl Cotransporter) and Renal K Excretion Depends on the K Channel, Kir4.1

AT2R (AngII [angiotensin II] type 2 receptor) is expressed in the distal nephrons. The aim of the present study is to examine whether AT2R regulates NCC (Na-Cl cotransporter) and Kir4.1 of the distal convoluted tubule. AngII inhibited the basolateral 40 pS K channel (a Kir4.1/5.1 heterotetramer) in the distal convoluted tubule treated with losartan but not with PD123319. AT2R agonist also inhibits the K channel, indicating that AT2R was involved in tonic regulation of Kir4.1. The infusion of PD123319 stimulated the expression of tNCC (total NCC) and pNCC (phosphorylated NCC; Thr53) by a time-dependent way with the peak at 4 days. PD123319 treatment (4 days) stimulated the basolateral 40 pS K channel activity, augmented the basolateral K conductance, and increased the negativity of distal convoluted tubule membrane. The stimulation of Kir4.1 was essential for PD123319-induced increase in NCC because inhibiting AT2R increased the expression of tNCC and pNCC only in wild-type but not in the kidney-specific Kir4.1 knockout mice. Renal clearance study showed that thiazide-induced natriuretic effect was larger in PD123319-treated mice for 4 days than untreated mice. However, this effect was absent in kidney-specific Kir4.1 knockout mice which were also Na wasting under basal conditions. Finally, application of AT2R antagonist decreased the renal ability of K excretion and caused hyperkalemia in wild-type but not in kidney-specific Kir4.1 knockout mice. We conclude that AT2R-dependent regulation of NCC requires Kir4.1 in the distal convoluted tubule and that AT2R plays a role in stimulating K excretion by inhibiting Kir4.1 and NCC.

Aldosterone Modulates the Association between NCC and ENaC

Distal sodium transport is a final step in the regulation of blood pressure. As such, understanding how the two main sodium transport proteins, the thiazide-sensitive sodium chloride cotransporter (NCC) and the epithelial sodium channel (ENaC), are regulated is paramount. Both are expressed in the late distal nephron; however, no evidence has suggested that these two sodium transport proteins interact. Recently, we established that these two sodium transport proteins functionally interact in the second part of the distal nephron (DCT2). Given their co-localization within the DCT2, we hypothesized that NCC and ENaC interactions might be modulated by aldosterone (Aldo). Aldo treatment increased NCC and αENaC colocalization (electron microscopy) and interaction (coimmunoprecipitation). Finally, with co-expression of the Aldo-induced protein serum- and glucocorticoid-inducible kinase 1 (SGK1), NCC and αENaC interactions were increased. These data demonstrate that Aldo promotes increased interaction of NCC and ENaC, within the DCT2 revealing a novel method of regulation for distal sodium reabsorption.

PGF2α regulates the basolateral K channels in the distal convoluted tubule

Our aim is to examine the role of PGF_2α receptor (FP), a highly expressed prostaglandin receptor in the distal convoluted tubule (DCT) in regulating the basolateral 40-pS K channel. The single-channel studies demonstrated that PGF_2α had a biphasic effect on the 40-pS K channel in the DCT-PGF_2α stimulated at low concentrations (less than 500 nM), while at high concentrations (above 1 µM), it inhibited the 40-pS K channels. Moreover, neither 13,14-dihydro-15-keto-PGF_2α (a metabolite of PGF_2α) nor PGE₂ was able to mimic the effect of PGF_2α on the 40-pS K channel in the DCT. The inhibition of PKC had no significant effect on the 40-pS K channel; however, it abrogated the inhibitory effect of 5 µM PGF_2α on the K channel. Moreover, stimulation of PKC inhibited the 40-pS K channel in the DCT, suggesting that PKC mediates the inhibitory effect of PGF_2α on the 40-pS K channel. Conversely, the stimulatory effect of PGF_2α on the 40-pS K channel was absent in the DCT treated with DPI, a NADPH oxidase (NOX) inhibitor. Also, adding 100 µM H₂O₂ mimicked the stimulatory effect of PGF_2α and increased the 40-pS K channel activity in DCT. Moreover, the stimulatory effect of 500 nM PGF_2α and H₂O₂ was not additive, suggesting the role of superoxide-related species in mediating the stimulatory effect of PGF_2α on the 40-pS K channel. The inhibition of Src family tyrosine protein kinase (SFK) not only inhibited the 40-pS K channel in the DCT but also completely abolished the stimulatory effects of PGF_2α and H₂O₂ on the 40-pS K channel. We conclude that PGF_2α at low doses stimulates the basolateral 40-pS K channel by a NOX- and SFK-dependent mechanism, while at high concentrations, it inhibits the K channel by a PKC-dependent pathway.

Basolateral Kir4.1 activity in the distal convoluted tubule regulates K secretion by determining NaCl cotransporter activity

PURPOSE OF REVIEW

Renal potassium (K) secretion plays a key role in maintaining K homeostasis. The classic mechanism of renal K secretion is focused on the connecting tubule and cortical collecting duct, in which K is uptaken by basolateral Na-K-ATPase and is secreted into the lumen by apical ROMK (Kir1.1) and Ca-activated big conductance K channel. Recently, genetic studies and animal models have indicated that inwardly rectifying K channel 4.1 (Kir4.1 or Kcnj10) in the distal convoluted tubule (DCT) may play a role in the regulation of K secretion in the aldosterone-sensitive distal nephron by targeting the NaCl cotransporter (NCC). This review summarizes recent progresses regarding the role of Kir4.1 in the regulation of NCC and K secretion.

RECENT FINDINGS

Kir4.1 is expressed in the basolateral membrane of the DCT, and plays a predominant role in contributing to the basolateral K conductance and in participating in the generation of negative membrane potential. Kir4.1 is also the substrate of src-family tyrosine kinase and the stimulation of src-family tyrosine kinase activates Kir4.1 activity in the DCT. The genetic deletion or functional inhibition of Kir4.1 depolarizes the membrane of the DCT, inhibits ste20-proline-alanine rich kinase, and suppresses NCC activity. Moreover, the downregulation of Kir4.1 increases epithelial Na channel expression in the collecting duct and urinary K excretion. Finally, mice with low Kir4.1 activity in the DCT are hypomagnesemia and hypokalemia.

SUMMARY

Recent progress in exploring the regulation and the function of Kir4.1 in the DCT strongly indicates that Kir4.1plays an important role in initiating the regulation of renal K secretion by targeting NCC and it may serves as a K sensor in the kidney.

The sodium chloride cotransporter (NCC) and epithelial sodium channel (ENaC) associate

The thiazide-sensitive sodium chloride cotransporter (NCC) and the epithelial sodium channel (ENaC) are two of the most important determinants of salt balance and thus systemic blood pressure. Abnormalities in either result in profound changes in blood pressure. There is one segment of the nephron where these two sodium transporters are coexpressed, the second part of the distal convoluted tubule. This is a key part of the aldosterone-sensitive distal nephron, the final regulator of salt handling in the kidney. Aldosterone is the key hormonal regulator for both of these proteins. Despite these shared regulators and coexpression in a key nephron segment, associations between these proteins have not been investigated. After confirming apical localization of these proteins, we demonstrated the presence of functional transport proteins and native association by blue native PAGE. Extensive coimmunoprecipitation experiments demonstrated a consistent interaction of NCC with α- and γ-ENaC. Mammalian two-hybrid studies demonstrated direct binding of NCC to ENaC subunits. Fluorescence resonance energy transfer and immunogold EM studies confirmed that these transport proteins are within appropriate proximity for direct binding. Additionally, we demonstrate that there are functional consequences of this interaction, with inhibition of NCC affecting the function of ENaC. This novel finding of an association between ENaC and NCC could alter our understanding of salt transport in the distal tubule.

Potassium and Its Discontents: New Insight, New Treatments

Hyperkalemia is common in patients with impaired kidney function or who take drugs that inhibit the renin-angiotensin-aldosterone axis. During the past decade, substantial advances in understanding how the body controls potassium excretion have been made, which may lead to improved standard of care for these patients. Renal potassium disposition is primarily handled by a short segment of the nephron, comprising part of the distal convoluted tubule and the connecting tubule, and regulation results from the interplay between aldosterone and plasma potassium. When dietary potassium intake and plasma potassium are low, the electroneutral sodium chloride cotransporter is activated, leading to salt retention. This effect limits sodium delivery to potassium secretory segments, limiting potassium losses. In contrast, when dietary potassium intake is high, aldosterone is stimulated. Simultaneously, potassium inhibits the sodium chloride cotransporter. Because more sodium is then delivered to potassium secretory segments, primed by aldosterone, kaliuresis results. When these processes are disrupted, hyperkalemia results. Recently, new agents capable of removing potassium from the body and treating hyperkalemia have been tested in clinical trials. This development suggests that more effective and safer approaches to the prevention and treatment of hyperkalemia may be on the horizon.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

An adolescent with tingling and numbness of hand: gitelman syndrome

CONTEXT

Gitelman syndrome is an inherited autosomal recessive disorder. It is usually diagnosed incidentally during adolescence or early adulthood based on clinical and biochemical findings.

CASE REPORT

We present a case of 16 years old adolescent female presenting with recurrent chest pain, tingling, and numbness of bilateral hands. Diagnosis was established by the typical biochemical abnormalities with hypokalemia, hypomagnesemia, hypocalciuria, metabolic alkalosis, and hyperreninemic hyperaldosteronism. Genetic diagnosis was confirmed by sequence analysis of the SLC12A3 gene showing the compound heterozygous mutation encoding the thiazide-sensitive sodium chloride co-transporter. The patient was treated with oral potassium, magnesium, and amiloride with complete improvement of symptoms and biochemical profile.

CONCLUSION

Gitelman syndrome should be considered as a differential diagnosis in work up of hypokalemia, especially in adolescent age group. The presence of hypokalemia, metabolic alkalosis, hypomagnesaemia, hypocalciuria, and mutation analysis provides the final diagnosis.

Antihypertensive medications, bone mineral density, and fractures: a review of old cardiac drugs that provides new insights into osteoporosis

Osteoporosis is increasing in prevalence and importance as society's age, with the clinical consequence of fractures of the hip, spine, and upper extremity, leading to impaired quality of life, loss of function and independence, and increased morbidity and mortality. A major risk factor for osteoporosis is older age, and cardiovascular diseases also share this risk factor; therefore, osteoporosis and cardiovascular disease often coexist and share risk factors. Medications used for the treatment of cardiovascular diseases, in particular antihypertensive drugs, have been shown in a variety of studies of varying designs to modulate bone health in both a positive or negative manner. In this article, we reviewed the pharmacology, potential mechanisms, and possible effects on bone mineral density and fracture risk of commonly prescribed antihypertensive medications, including thiazide and non-thiazide diuretics, beta-blockers, calcium channel blockers, renin-angiotensin-aldosterone system agents, and nitrates.

Prolactin and teleost ionocytes: new insights into cellular and molecular targets of prolactin in vertebrate epithelia

The peptide hormone prolactin is a functionally versatile hormone produced by the vertebrate pituitary. Comparative studies over the last six decades have revealed that a conserved function for prolactin across vertebrates is the regulation of ion and water transport in a variety of tissues including those responsible for whole-organism ion homeostasis. In teleost fishes, prolactin was identified as the "freshwater-adapting hormone", promoting ion-conserving and water-secreting processes by acting on the gill, kidney, gut and urinary bladder. In mammals, prolactin is known to regulate renal, intestinal, mammary and amniotic epithelia, with dysfunction linked to hypogonadism, infertility, and metabolic disorders. Until recently, our understanding of the cellular mechanisms of prolactin action in fishes has been hampered by a paucity of molecular tools to define and study ionocytes, specialized cells that control active ion transport across branchial and epidermal epithelia. Here we review work in teleost models indicating that prolactin regulates ion balance through action on ion transporters, tight-junction proteins, and water channels in ionocytes, and discuss recent advances in our understanding of ionocyte function in the genetically and embryonically accessible zebrafish (Danio rerio). Given the high degree of evolutionary conservation in endocrine and osmoregulatory systems, these studies in teleost models are contributing novel mechanistic insight into how prolactin participates in the development, function, and dysfunction of osmoregulatory systems across the vertebrate lineage.

A molecular update on pseudohypoaldosteronism type II

The DCT (distal convoluted tubule) is the site of microregulation of water reabsorption and ion handling in the kidneys, which is mainly under the control of aldosterone. Aldosterone binds to and activates mineralocorticoid receptors, which ultimately lead to increased sodium reabsorption in the distal part of the nephron. Impairment of mineralocorticoid signal transduction results in resistance to aldosterone and mineralocorticoids, and, therefore, causes disturbances in electrolyte balance. Pseudohypoaldosteronism type II (PHAII) or familial hyperkalemic hypertension (FHHt) is a rare, autosomal dominant syndrome characterized by hypertension, hyperkalemia, metabolic acidosis, elevated or low aldosterone levels, and decreased plasma renin activity. PHAII is caused by mutations in the WNK isoforms (with no lysine kinase), which regulate the Na-Cl and Na-K-Cl cotransporters (NCC and NKCC2, respectively) and the renal outer medullary potassium (ROMK) channel in the DCT. This review focuses on new candidate genes such as KLHL3 and Cullin3, which are instrumental to unraveling novel signal transductions pathways involving NCC, to better understand the cause of PHAII along with the molecular mechanisms governing the pathophysiology of PHAII and its clinical manifestations.

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.

Direct activation of ENaC by angiotensin II: recent advances and new insights

Angiotensin II (Ang II) is the principal effector of the renin-angiotensin-aldosterone system (RAAS). It initiates myriad processes in multiple organs integrated to increase circulating volume and elevate systemic blood pressure. In the kidney, Ang II stimulates renal tubular water and salt reabsorption causing antinatriuresis and antidiuresis. Activation of the RAAS is known to enhance activity of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. In addition to its well described stimulatory actions on aldosterone secretion, Ang II is also capable of directly increasing ENaC activity. In this brief review, we discuss recent findings about non-classical Ang II actions on ENaC and speculate about its relevance for renal sodium handling.

Molecular physiology of SPAK and OSR1: two Ste20-related protein kinases regulating ion transport

SPAK (Ste20-related proline alanine rich kinase) and OSR1 (oxidative stress responsive kinase) are members of the germinal center kinase VI subfamily of the mammalian Ste20 (Sterile20)-related protein kinase family. Although there are 30 enzymes in this protein kinase family, their conservation across the fungi, plant, and animal kingdom confirms their evolutionary importance. Already, a large volume of work has accumulated on the tissue distribution, binding partners, signaling cascades, and physiological roles of mammalian SPAK and OSR1 in multiple organ systems. After reviewing this basic information, we will examine newer studies that demonstrate the pathophysiological consequences to SPAK and/or OSR1 disruption, discuss the development and analysis of genetically engineered mouse models, and address the possible role these serine/threonine kinases might have in cancer proliferation and migration.

Effects of genetic variation in H3K79 methylation regulatory genes on clinical blood pressure and blood pressure response to hydrochlorothiazide

BACKGROUND

Nearly one-third of the United States adult population suffers from hypertension. Hydrochlorothiazide (HCTZ), one of the most commonly used medications to treat hypertension, has variable efficacy. The renal epithelial sodium channel (ENaC) provides a mechanism for fine-tuning sodium excretion, and is a major regulator of blood pressure homeostasis. DOT1L, MLLT3, SIRT1, and SGK1 encode genes in a pathway that controls methylation of the histone H3 globular domain at lysine 79 (H3K79), thereby modulating expression of the ENaCα subunit. This study aimed to determine the role of variation in these regulatory genes on blood pressure response to HCTZ, and secondarily, untreated blood pressure.

METHODS

We investigated associations between genetic variations in this candidate pathway and HCTZ blood pressure response in two separate hypertensive cohorts (clinicaltrials.gov NCT00246519 and NCT00005520). In a secondary, exploratory analysis, we measured associations between these same genetic variations and untreated blood pressure. Associations were measured by linear regression, with only associations with P ≤ 0.01 in one cohort and replication by P ≤ 0.05 in the other cohort considered significant.

RESULTS

In one cohort, a polymorphism in DOT1L (rs2269879) was strongly associated with greater systolic (P = 0.0002) and diastolic (P = 0.0016) blood pressure response to hydrochlorothiazide in Caucasians. However, this association was not replicated in the other cohort. When untreated blood pressure levels were analyzed, we found directionally similar associations between a polymorphism in MLLT3 (rs12350051) and greater untreated systolic (P < 0.01 in both cohorts) and diastolic (P < 0.05 in both cohorts) blood pressure levels in both cohorts. However, when further replication was attempted in a third hypertensive cohort and in smaller, normotensive samples, significant associations were not observed.

CONCLUSIONS

Our data suggest polymorphisms in DOT1L, MLLT3, SIRT1, and SGK1 are not likely associated with blood pressure response to HCTZ. However, a possibility exists that rs2269879 in DOT1L could be associated with HCTZ response in Caucasians. Additionally, exploratory analyses suggest rs12350051 in MLLT3 may be associated with untreated blood pressure in African-Americans. Replication efforts are needed to verify roles for these polymorphisms in human blood pressure regulation.

A new look at electrolyte transport in the distal tubule

The distal nephron plays a critical role in the renal control of homeostasis. Until very recently most studies focused on the control of Na(+), K(+), and water balance by principal cells of the collecting duct and the regulation of solute and water by hormones from the renin-angiotensin-aldosterone system and by antidiuretic hormone. However, recent studies have revealed the unexpected importance of renal intercalated cells, a subtype of cells present in the connecting tubule and collecting ducts. Such cells were thought initially to be involved exclusively in acid-base regulation. However, it is clear now that intercalated cells absorb NaCl and K(+) and hence may participate in the regulation of blood pressure and potassium balance. The second paradigm-challenging concept we highlight is the emerging importance of local paracrine factors that play a critical role in the renal control of water and electrolyte balance.

Aldosterone paradox: differential regulation of ion transport in distal nephron

The mechanisms through which aldosterone promotes apparently opposite effects like salt reabsorption and K(+) secretion remain poorly understood. The identification, localization, and physiological analysis of ion transport systems in distal nephron have revealed an intricate network of interactions between several players, revealing the complex mechanism behind the aldosterone paradox. We review the mechanisms involved in differential regulation of ion transport that allow the fine tuning of salt and K(+) balance.

The WNKs: atypical protein kinases with pleiotropic actions

WNKs are serine/threonine kinases that comprise a unique branch of the kinome. They are so-named owing to the unusual placement of an essential catalytic lysine. WNKs have now been identified in diverse organisms. In humans and other mammals, four genes encode WNKs. WNKs are widely expressed at the message level, although data on protein expression is more limited. Soon after the WNKs were identified, mutations in genes encoding WNK1 and -4 were determined to cause the human disease familial hyperkalemic hypertension (also known as pseudohypoaldosteronism II, or Gordon's Syndrome). For this reason, a major focus of investigation has been to dissect the role of WNK kinases in renal regulation of ion transport. More recently, a different mutation in WNK1 was identified as the cause of hereditary sensory and autonomic neuropathy type II, an early-onset autosomal disease of peripheral sensory nerves. Thus the WNKs represent an important family of potential targets for the treatment of human disease, and further elucidation of their physiological actions outside of the kidney and brain is necessary. In this review, we describe the gene structure and mechanisms regulating expression and activity of the WNKs. Subsequently, we outline substrates and targets of WNKs as well as effects of WNKs on cellular physiology, both in the kidney and elsewhere. Next, consequences of these effects on integrated physiological function are outlined. Finally, we discuss the known and putative pathophysiological relevance of the WNKs.

Mechanisms for blood pressure lowering and metabolic effects of thiazide and thiazide-like diuretics

Thiazide and thiazide-like diuretics are among the most commonly used antihypertensives and have been available for over 50 years. However, the mechanism by which these drugs chronically lower blood pressure is poorly understood. Possible mechanisms include direct endothelial- or vascular smooth muscle-mediated vasodilation and indirect compensation to acute decreases in cardiac output. In addition, thiazides are associated with adverse metabolic effects, particularly hyperglycemia, and the mechanistic underpinnings of these effects are also poorly understood. Thiazide-induced hypokalemia, as well as other theories to explain these metabolic disturbances, including increased visceral adiposity, hyperuricemia, decreased glucose metabolism and pancreatic beta-cell hyperpolarization, may play a role. Understanding genetic variants with differential responses to thiazides could reveal new mechanistic candidates for future research to provide a more complete understanding of the blood pressure and metabolic response to thiazide diuretics.

The Na+-dependent chloride-bicarbonate exchanger SLC4A8 mediates an electroneutral Na+ reabsorption process in the renal cortical collecting ducts of mice

Regulation of sodium balance is a critical factor in the maintenance of euvolemia, and dysregulation of renal sodium excretion results in disorders of altered intravascular volume, such as hypertension. The amiloride-sensitive epithelial sodium channel (ENaC) is thought to be the only mechanism for sodium transport in the cortical collecting duct (CCD) of the kidney. However, it has been found that much of the sodium absorption in the CCD is actually amiloride insensitive and sensitive to thiazide diuretics, which also block the Na-Cl cotransporter (NCC) located in the distal convoluted tubule. In this study, we have demonstrated the presence of electroneutral, amiloride-resistant, thiazide-sensitive, transepithelial NaCl absorption in mouse CCDs, which persists even with genetic disruption of ENaC. Furthermore, hydrochlorothiazide (HCTZ) increased excretion of Na+ and Cl- in mice devoid of the thiazide target NCC, suggesting that an additional mechanism might account for this effect. Studies on isolated CCDs suggested that the parallel action of the Na+-driven Cl-/HCO3- exchanger (NDCBE/SLC4A8) and the Na+-independent Cl-/HCO3- exchanger (pendrin/SLC26A4) accounted for the electroneutral thiazide-sensitive sodium transport. Furthermore, genetic ablation of SLC4A8 abolished thiazide-sensitive NaCl transport in the CCD. These studies establish what we believe to be a novel role for NDCBE in mediating substantial Na+ reabsorption in the CCD and suggest a role for this transporter in the regulation of fluid homeostasis in mice.

The thiazide-sensitive Na+-Cl- cotransporter: molecular biology, functional properties, and regulation by WNKs

The thiazide-sensitive Na+-Cl(-) cotransporter is the major salt reabsorption pathway in the distal convoluted tubule, which is located just after the macula densa at the beginning of the aldosterone-sensitive nephron. This cotransporter was identified at the molecular level in the early 1990s by the pioneering work of Steven C. Hebert and coworkers, opening the molecular area, not only for the Na+-Cl(-) cotransporter but also for the family of electroneutral cation-coupled chloride cotransporters that includes the loop diuretic-sensitive Na+-K+-2Cl(-) cotransporter of the thick ascending limb of Henle's loop. This work honoring the memory of Steve Hebert presents a brief review of our current knowledge about salt and water homeostasis generated as a consequence of cloning the cotransporter, with particular emphasis on the molecular biology, physiological properties, human disease due to decreased or increased activity of the cotransporter, and regulation of the cotransporter by a family of serine/threonine kinases known as WNK. Thus one of the legacies of Steve Hebert is a better understanding of salt and water homeostasis.

Expression of the RNA-stabilizing protein HuR in ischemia-reperfusion injury of rat kidney

The RNA-binding protein human antigen R (HuR) participates in the posttranscriptional regulation of mRNAs bearing 3' AU-rich and U-rich elements, which HuR can stabilize under conditions of cellular stress. Using the LLC-PK(1) proximal tubule cell line model, we recently suggested a role for HuR in protecting kidney epithelia from injury during ischemic stress (Jeyaraj S, Dakhlallah D, Hill SR, Lee BS. J Biol Chem 280: 37957-37964, 2005; Jeyaraj SC, Dakhlallah D, Hill SR, Lee BS. Am J Physiol Renal Physiol 291: F1255-F1263, 2006). Here, we have extended this work to show that small interfering RNA-mediated suppression of HuR in LLC-PK(1) cells increased apoptosis during energy depletion, while overexpression of HuR diminished apoptosis. Suppression of HuR also resulted in diminished levels of key cell survival proteins such as Bcl-2 and Hsp70. Furthermore, rat kidneys were subjected in vivo to transient ischemia followed by varying periods of reperfusion. Ischemia and reperfusion (I/R) affected intensity and distribution of HuR in a nephron segment-specific manner. Cells of the proximal tubule, which are most sensitive to I/R injury, demonstrated a transient shift of HuR to the cytoplasm immediately following ischemia. Over a 14-day period following the onset of reperfusion, nuclear and total HuR protein gradually increased in cortical and medullary proximal tubules, but not in non-proximal tubule cells. HuR mRNA was expressed in two forms with alternate transcriptional start sites that increased over a 14-day I/R period, and in vitro studies suggest selective translatability of these two mRNAs. Baseline and I/R-stimulated levels of HuR mRNA did not parallel those of HuR protein, suggesting translational control of HuR expression, particularly in medullary proximal tubules. These findings suggest that alterations in distribution and expression of the antiaptotic protein HuR specifically in cells of the proximal tubule effect a protective mechanism during and following I/R injury in kidney.

Case-control study of the role of the Gitelman's syndrome gene in essential hypertension

BACKGROUND

Gitelman's syndrome is an inherited tubular disorder characterized by sodium wasting, low blood pressure, secondary hyperaldosteronism, metabolic alkalosis, hypokalemia, hypomagnesemia of renal origin, and hypocalciuria. The majority of patients with this syndrome carry inactivating mutations in the SLC12A3 gene encoding the thiazide-sensitive Na (+)-Cl (-) cotransporter (NCC) located in the distal convoluted tubule, which is involved in renal sodium reabsorption. This suggests that the SLC12A3 gene is involved in mediation of blood pressure levels. The aim of the present study was to investigate relationships between single nucleotide polymorphisms (SNPs) in the human SLC12A3 gene and essential hypertension (EH) in Japanese.

METHOD

We selected 3 SNPs in the human SLC12A3 gene (T180K, A569V, L849H), and performed a case-control study of 315 EH patients and 305 normotensive (NT) individuals.

RESULTS

There was no significant difference in overall distribution of genotypes or alleles of any of the SNPs between the EH and NT groups.

CONCLUSION

We conclude that the causal gene of Gitelman's syndrome is not involved in determining blood pressure levels.

ANG II provokes acute trafficking of distal tubule Na+-Cl(-) cotransporter to apical membrane

The distal convoluted tubule (DCT) Na+-Cl(-) cotransporter (NCC), the target of thiazide diuretics, is responsible for the reabsorption of 5-10% of filtered NaCl. The aim of this study was to test the hypothesis that acute infusion of the angiotensin-converting enzyme (ACE) inhibitor captopril (at 12 microg/min) for 20 min provokes trafficking of NCC from apical plasma membranes (APM) to subapical cytoplasmic vesicles (SCV), which is reversed by acute ANG II infusion (ANG II at 20 ng.kg(-1).min(-1) along with 12 microg/min captopril) for 20 min in male Sprague-Dawley rats (250-350 g). By immuno-electron microscopy using an anti-NCC (D. Ellison) 71.5 +/- SD 4.9% of the NCC gold labeling was associated with the APM in control, sham operated, and infused rats, while captopril infusion reduced NCC in APM to 54.9 +/- 6.9% (P < 0.001) and markedly increased immunogold labeling of SCV. Subsequent infusion of ANG II with captopril restored NCC immunogold labeling of APM to 72.4 +/- 4.2%, that is, 20% of the total NCC trafficked between APM and SCV. Likewise, on density gradients of cortex, captopril provoked redistribution of 27.3% of total NCC from low-density APM-enriched membranes to higher-density membranes and ANG II+captopril restored 20.3% of the NCC to APM-enriched fractions. Redistribution occurred independent of a change in NCC total abundance. In conclusion, this study demonstrates that ACE inhibition provokes acute trafficking of NCC out of the plasma membrane, which likely decreases DCT Na+ reabsorption, while ANG II provokes rapid trafficking of NCC from stores in subapical vesicles to the plasma membrane, which likely increases DCT Na+ reabsorption.

WNK4 enhances TRPV5-mediated calcium transport: potential role in hypercalciuria of familial hyperkalemic hypertension caused by gene mutation of WNK4

The epithelial Ca(2+) channel TRPV5 serves as a gatekeeper for active Ca(2+) reabsorption in the distal convoluted tubule and connecting tubule of the kidney. WNK4, a protein serine/threonine kinase with gene mutations that cause familial hyperkalemic hypertension (FHH), including a subtype with hypercalciuria, is also localized in the distal tubule of the nephron. To understand the role of WNK4 in modulation of Ca(2+) reabsorption, we evaluated the effect of WNK4 on TRPV5-mediated Ca(2+) transport in Xenopus laevis oocytes. Coexpression of TRPV5 with WNK4 resulted in a twofold increase in TRPV5-mediated Ca(2+) uptake. The increase in Ca(2+) uptake was due to the increase in surface expression of TRPV5. When the thiazide-sensitive Na(+)-Cl(-) cotransporter NCC was coexpressed, the effect of WNK4 on TRPV5 was weakened by NCC in a dose-dependent manner. Although the WNK4 disease-causing mutants E562K, D564A, Q565E, and R1185C retained their ability to upregulate TRPV5, the blocking effect of NCC was further strengthened when wild-type WNK4 was replaced by the Q565E mutant, which causes FHH with hypercalciuria. We conclude that WNK4 positively regulates TRPV5-mediated Ca(2+) transport and that the inhibitory effect of NCC on this process may be involved in the pathogenesis of hypercalciuria of FHH caused by gene mutation in WNK4.

Novel mouse strain with Cre recombinase in 11beta-hydroxysteroid dehydrogenase-2-expressing cells

Here we describe the generation and characterization of a mouse strain that expresses an improved Cre (iCre) recombinase (48) under the control of the endogenous 11beta-hydroxysteroid dehydrogenase type 2 (11HSD2) promoter. Progeny of 11HSD2/iCre and ROSA26 reporter mice were used to determine the pattern of iCre expression by measuring the activity of the LacZ gene product beta-galactosidase in a panel of tissues. On Cre recombinase activity, intense beta-galactosidase activity (X-gal staining) was observed in the classic mineralocorticoid target segments of the kidney, as well as in the colon, and both female and male reproductive organs. Weaker iCre expression was detected in the lung and heart. In the brain, strong iCre activity was present in cardiovascular centers that are known to express 11HSD2 and mineralocorticoid receptors [nucleus tractus solitarius (NTS) and amygdala] as well as in the granular layer of the cerebellum. iCre expression was weaker in neonatal kidney and colon than in the adult but was present in the hair follicles and cartilage. These results indicate that in the 11HSD2/iCre strain iCre expression faithfully represents the expression pattern of endogenous 11HSD2. Thus this mouse model represents the first Cre deleter strain that can be used to eliminate desired genes in every mineralocorticoid target tissue. This mouse model should serve as a useful resource for investigators who want to study the function of genes involved in aldosterone action and genes in other pathways that are selectively expressed in these cells.

Urinary aquaporin-2 excretion during early human development

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

Regulation of the renal thiazide-sensitive Na-Cl cotransporter, blood pressure, and natriuresis in obese Zucker rats treated with rosiglitazone

Previously, we showed an increase in protein abundance of the renal thiazide-sensitive Na-Cl cotransporter (NCC) in young, prediabetic, obese Zucker rats relative to lean age mates (Bickel CA, Verbalis JF, Knepper MA, and Ecelbarger CA. Am J Physiol Renal Physiol 281: F639–F648, 2001). To test whether this increase correlated with increased thiazide sensitivity (NCC activity) and blood pressure, and could be modified by insulin-sensitizing agents, we treated lean and obese Zucker rats (9 wk old) with either a control diet or this diet supplemented with 3 mg/kg body wt rosiglitazone (RGZ), a peroxisomal proliferator-activated receptor subtype γ agonist and potent insulin-sensitizing agent, for 12 wk ( n = 9/group). The rise in blood pressure, measured continuously by radiotelemetry, was significantly blunted in the RGZ-treated obese rats. Similarly, blood glucose and urinary albumin were markedly decreased in these rats. RGZ-treated rats whether lean or obese excreted a NaCl load faster but excreted less sodium in response to hydrochlorothiazide, applied as a novel in vivo measure of NCC activity. Obese rats had increased renal protein abundance and urinary excretion of NCC; however, this was not significantly reduced by RGZ (densitometry in cortex homogenate − %lean control): 100 ± 9, 93 ± 4, 124 ± 9, and 141 ± 14 for lean control, lean RGZ, obese control, and obese RGZ, respectively. Subcellular localization, as evaluated by confocal microscopy and immunoblotting following differential centrifugation, of NCC was not different between rat groups. Overall, RGZ reduced blood pressure and thiazide sensitivity; however, the mechanism(s) did not seem to involve a decrease in NCC protein abundance or cellular location. Decreased NCC activity may have contributed to the maintenance of normotension in RGZ-treated obese rats.

Salt handling in the distal nephron: lessons learned from inherited human disorders

The molecular basis of inherited salt-losing tubular disorders with secondary hypokalemia has become much clearer in the past two decades. Two distinct segments along the nephron turned out to be affected, the thick ascending limb of Henle's loop and the distal convoluted tubule, accounting for two major clinical phenotypes, hyperprostaglandin E syndrome and Bartter-Gitelman syndrome. To date, inactivating mutations have been detected in six different genes encoding for proteins involved in renal transepithelial salt transport. Careful examination of genetically defined patients (“human knockouts”) allowed us to determine the individual role of a specific protein and its contribution to the overall process of renal salt reabsorption. The recent generation of several genetically engineered mouse models that are deficient in orthologous genes further enabled us to compare the human phenotype with the animal models, revealing some unexpected interspecies differences. As the first line treatment in hyperprostaglandin E syndrome includes cyclooxygenase inhibitors, we propose some hypotheses about the mysterious role of PGE2in the etiology of renal salt-losing disorders.