Cullin 3/with No Lysine [K] Kinase/Ste20/SPS-Related Proline Alanine Rich Kinase Signaling: Impact on NaCl Cotransporter Activity in BP Regulation.

The sodium chloride cotransporter (NCC) fine-tunes Na + balance and indirectly affects the homeostasis of other ions including K + , Mg 2+ , and Ca 2+ . Owing to its effects on Na + balance, BP is significantly affected by alterations in NCC activity. Several factors have been reported to influence the expression and activity of NCC. One critical factor is NCC phosphorylation/dephosphorylation that occurs at key serine-threonine amino acid residues of the protein. Phosphorylation, which results in increased NCC activity, is mediated by the with no lysine [K] (WNK)-SPS-related proline alanine rich kinase (SPAK)/OSR1 kinases. NCC activation stimulates reabsorption of Na + , increasing extracellular fluid volume and hence BP. On the other hand, proteasomal degradation of WNK kinases after ubiquitination by the Cullin 3-Kelch-like 3 E3 ubiquitin ligase complex and dephosphorylation pathways oppose WNK-SPAK/OSR1-mediated NCC activation. Components of the Cullin 3/Kelch-like 3-WNK-SPAK/OSR1 regulatory pathway may be targets for novel antihypertensive drugs. In this review, we outline the impact of these regulators on the activity of NCC and the consequent effect on BP.

Electroneutral Na+/Cl- cotransport activity of zebrafish Slc12a10.1 expressed in Xenopus oocytes.

Na+/Cl- cotransporter 2 (Ncc2 or Slc12a10) is a membrane transport protein that belongs to the electroneutral cation-chloride cotransporter family. The Slc12a10 gene (slc12a10) is widely present in bony vertebrates but is deleted or pseudogenized in birds, some bony fishes, and most mammals. Slc12a10 is highly homologous to Ncc (Slc12a3 or Ncc1); however, there are only a few reports measuring the activity of Slc12a10. In this study, we focused on zebrafish Slc12a10.1 (zSlc12a10.1) and analyzed its activity using Xenopus oocyte electrophysiology. Analysis using Na+-selective microelectrodes showed that intracellular sodium activity (aNai) in zSlc12a10.1 oocytes was significantly decreased in Na+- or Cl--free medium and recovered when Na+ or Cl- was readded to the medium. Similar analysis using a Cl--selective microelectrode showed that intracellular chloride activity (aCli) in zSlc12a10.1 oocytes significantly decreased in Na+- or Cl--free medium and recovered when Na+ or Cl- was readded to the medium. When a similar experiment was performed with a voltage clamp, the membrane current did not change when aNai of zSlc12a10.1 oocytes was decreased in Na+-free medium. Molecular phylogenetic and synteny analyses suggest that gene duplication between slc12a10.2 and slc12a10.3 in zebrafish is a relatively recent event, whereas gene duplication between slc12a10.1 and the ancestral gene of slc12a10.2/slc12a10.3 occurred at least about 2 million years ago. slc12a10 deficiency was observed in species belonging to Ictaluridae, Salmoniformes, Osmeriformes, Batrachoididae, Syngnathiformes, Gobiesociformes, Labriformes, and Tetraodontiformes. These results indicate that zebrafish Slc12a10.1 is an electroneutral Na+/Cl-cotransporter and establish its evolutionary position among various teleost slc12a10 paralogs.NEW & NOTEWORTHY Na+/Cl- cotransporter 2 (Slc12a10; Ncc2) is a protein highly homologous to Ncc (Slc12a3; Ncc1); however, there are only a few reports measuring the activity of Slc12a10. Electrophysiological analysis of Xenopus oocytes expressing zebrafish Slc12a10.1 showed that Slc12a10.1 acts as an electroneutral Na+/Cl-cotransporter. This is the third report on the activity of Slc12a10, following previous reports on Slc12a10 in eels.

Renal CD81 interacts with sodium potassium 2 chloride cotransporter and sodium chloride cotransporter in rats with lipopolysaccharide-induced preeclampsia.

The kidney regulates blood pressure through salt/water reabsorption affected by tubular sodium transporters. Expanding our prior research on placental cluster of differentiation 81 (CD81), this study explores the interaction of renal CD81 with sodium transporters in preeclampsia (PE). Effects of renal CD81 with sodium transporters were determined in lipopolysaccharide (LPS)-induced PE rats and immortalized mouse renal distal convoluted tubule cells. Urinary exosomal CD81, sodium potassium 2 chloride cotransporter (NKCC2), and sodium chloride cotransporter (NCC) were measured in PE patients. LPS-PE rats had hypertension from gestational days (GD) 6 to 18 and proteinuria from GD9 to GD18. Urinary CD81 in both groups tented to rise during pregnancy. Renal CD81, not sodium transporters, was higher in LPS-PE than controls on GD14. On GD18, LPS-PE rats exhibited higher CD81 in kidneys and urine exosomes, higher renal total and phosphorylated renal NKCC2 and NCC with elevated mRNAs, and lower ubiquitinated NCC than controls. CD81 was co-immunoprecipitated with NKCC2 or NCC in kidney homogenates and co-immunostained with NKCC2 or NCC in apical membranes of renal tubules. In plasma membrane fractions, LPS-PE rats had greater amounts of CD81, NKCC2, and NCC than controls with enhanced co-immunoprecipitations of CD81 with NKCC2 or NCC. In renal distal convoluted tubule cells, silencing CD81 with siRNA inhibited NCC and prevented LPS-induced NCC elevation. Further, PE patients had higher CD81 in original urines, urine exosomes and higher NKCC2 and NCC in urine exosomes than controls. Thus, the upregulation of renal CD81 on NKCC2 and NCC may contribute to the sustained hypertension observed in LPS-PE model. Urine CD81 with NKCC2 and NCC may be used as biomarkers for PE.

Urinary extracellular vesicle mRNA analysis of sodium chloride cotransporter in hypertensive patients under different conditions.

Urinary extracellular vesicles (UEV) mainly derive from cells of the urogenital tract and their cargo (proteins, nucleic acids, lipids, etc.) reflects their cells of origin. Na chloride cotransporter (NCC) is expressed at the kidney level in the distal convoluted tubule, is involved in salt reabsorption, and is the target of the diuretic thiazides. NCC protein has been recognized and quantified in UEV in previous studies; however, UEV NCC mRNA has never been studied. This study aimed to identify and analyze NCC mRNA levels in primary aldosteronism (PA). The rationale for this investigation stems from previous observations regarding NCC (protein) as a possible biomarker for the diagnosis of PA. To evaluate modulations in the expression of NCC, we analyzed NCC mRNA levels in UEV in PA and essential hypertensive (EH) patients under different conditions, that is, before and after saline infusion, anti-aldosterone pharmacological treatment, and adrenal surgery. NCC mRNA was measured by RT-qPCR in all the samples and was regulated by volume expansion. Its response to mineralocorticoid receptor antagonist was correlated with renin, and it was increased in PA patients after adrenalectomy. NCC mRNA is evaluable in UEV and it can provide insights into the pathophysiology of distal convolute tubule in different clinical conditions including PA.

The European and Japanese eel NaCl cotransporters ß exhibit chloride currents and are resistant to thiazide type diuretics.

The thiazide-sensitive Na+-Cl- cotransporter (NCC) is the major pathway for salt reabsorption in the mammalian distal convoluted tubule, and the inhibition of its function with thiazides is widely used for the treatment of arterial hypertension. In mammals and teleosts, NCC is present as one ortholog that is mainly expressed in the kidney. One exception, however, is the eel, which has two genes encoding NCC. The eNCCα is located in the kidney and eNCCß, which is present in the apical membrane of the rectum. Interestingly, the European eNCCß functions as a Na+-Cl- cotransporter that is nevertheless resistant to thiazides and is not activated by low-chloride hypotonic stress. However, in the Japanese eel rectal sac, a thiazide-sensitive NaCl transport mechanism has been described. The protein sequences between eNCCß and jNCCß are 98% identical. Here, by site-directed mutagenesis, we transformed eNCCß into jNCCß. Our data showed that jNCCß, similar to eNCCß, is resistant to thiazides. In addition, both NCCß proteins have high transport capacity with respect to their renal NCC orthologs and, in contrast to known NCCs, exhibit electrogenic properties that are reduced when residue I172 is substituted by A, G, or M. This is considered a key residue for the chloride ion-binding sites of NKCC and KCC. We conclude that NCCß proteins are not sensitive to thiazides and have electrogenic properties dependent on Cl-, and site I172 is important for the function of NCCß.

Deletion of Kir5.1 abolishes the effect of high Na+ intake on Kir4.1 and Na+-Cl- cotransporter.

High sodium (HS) intake inhibited epithelial Na+ channel (ENaC) in the aldosterone-sensitive distal nephron and Na+-Cl- cotransporter (NCC) by suppressing basolateral Kir4.1/Kir5.1 in the distal convoluted tubule (DCT), thereby increasing renal Na+ excretion but not affecting K+ excretion. The aim of the present study was to explore whether deletion of Kir5.1 compromises the inhibitory effect of HS on NCC expression/activity and renal K+ excretion. Patch-clamp experiments demonstrated that HS failed to inhibit DCT basolateral K+ channels and did not depolarize K+ current reversal potential of the DCT in Kir5.1 knockout (KO) mice. Moreover, deletion of Kir5.1 not only increased the expression of Kir4.1, phospho-NCC, and total NCC but also abolished the inhibitory effect of HS on the expression of Kir4.1, phospho-NCC, and total NCC and thiazide-induced natriuresis. Also, low sodium-induced stimulation of NCC expression/activity and basolateral K+ channels in the DCT were absent in Kir5.1 KO mice. Deletion of Kir5.1 decreased ENaC currents in the late DCT, and HS further inhibited ENaC activity in Kir5.1 KO mice. Finally, measurement of the basal renal K+ excretion rate with the modified renal clearance method demonstrated that long-term HS inhibited the renal K+ excretion rate and steadily increased plasma K+ levels in Kir5.1 KO mice but not in wild-type mice. We conclude that Kir5.1 plays an important role in mediating the effect of HS intake on basolateral K+ channels in the DCT and NCC activity/expression. Kir5.1 is involved in maintaining renal ability of K+ excretion during HS intake. NEW & NOTEWORTHY Kir5.1 plays an important role in mediating the effect of high sodium intake on basolateral K+ channels in the distal convoluted tubule and Na+-Cl- cotransporter activity/expression.

Sympathetic Regulation of the NCC (Sodium Chloride Cotransporter) in Dahl Salt-Sensitive Hypertension.

Increased sympathoexcitation and renal sodium retention during high salt intake are hallmarks of the salt sensitivity of blood pressure. The mechanism(s) by which excessive sympathetic nervous system release of norepinephrine influences renal sodium reabsorption is unclear. However, studies demonstrate that norepinephrine can stimulate the activity of the NCC (sodium chloride cotransporter) and promote the development of SSH (salt-sensitive hypertension). The adrenergic signaling pathways governing NCC activity remain a significant source of controversy with opposing studies suggesting a central role of upstream α1- and ß-adrenoceptors in the canonical regulatory pathway involving WNKs (with-no-lysine kinases), SPAK (STE20/SPS1-related proline alanine-rich kinase), and OxSR1 (oxidative stress response 1). In our previous study, α1-adrenoceptor antagonism in norepinephrine-infused male Sprague-Dawley rats prevented the development of norepinephrine-evoked SSH in part by suppressing NCC activity and expression. In these studies, we used selective adrenoceptor antagonism in male Dahl salt-sensitive rats to test the hypothesis that norepinephrine-mediated activation of the NCC in Dahl SSH occurs via an α1-adrenoceptor dependent pathway. A high-salt diet evoked significant increases in NCC activity, expression, and phosphorylation in Dahl salt-sensitive rats that developed SSH. Increases were associated with a dysfunctional WNK1/4 dynamic and a failure to suppress SPAK/OxSR1 activity. α1-adrenoceptor antagonism initiated before high-salt intake or following the establishment of SSH attenuated blood pressure in part by suppressing NCC activity, expression, and phosphorylation. Collectively, our findings support the existence of a norepinephrine-activated α1-adrenoceptor gated pathway that relies on WNK/SPAK/OxSR1 signaling to regulate NCC activity in SSH.

Distal convoluted tubule Cl- concentration is modulated via K+ channels and transporters.

Cl--sensitive with-no-lysine kinase (WNK) plays a key role in regulating the thiazide-sensitive Na+-Cl- cotransporter (NCC) in the distal convoluted tubule (DCT). Cl- enters DCT cells through NCC and leaves the cell across the basolateral membrane via the Cl- channel ClC-K2 or K+-Cl- cotransporter (KCC). While KCC is electroneutral, Cl- exit via ClC-K2 is electrogenic. Therefore, an alteration in DCT basolateral K+ channel activity is expected to influence Cl- movement across the basolateral membrane. Although a role for intracellular Cl- in the regulation of WNK and NCC has been established, intracellular Cl- concentrations ([Cl-]i) have not been directly measured in the mammalian DCT. Therefore, to measure [Cl-]i in DCT cells, we generated a transgenic mouse model expressing an optogenetic kidney-specific Cl-Sensor and measured Cl- fluorescent imaging in the isolated DCT. Basal measurements indicated that the mean [Cl-]i was ~7 mM. Stimulation of Cl- exit with low-Cl- hypotonic solutions decreased [Cl-]i, whereas inhibition of KCC by DIOA or inhibition of ClC-K2 by NPPB increased [Cl-]i, suggesting roles for both KCC and ClC-K2 in the modulation of [Cl-]i . Blockade of basolateral K+ channels (Kir4.1/5.1) with barium significantly increased [Cl-]i. Finally, a decrease in extracellular K+ concentration transiently decreased [Cl-]i, whereas raising extracellular K+ transiently increased [Cl-]i, further suggesting a role for Kir4.1/5.1 in the regulation of [Cl-]i. We conclude that the alteration in ClC-K2, KCC, and Kir4.1/5.1 activity influences [Cl-]i in the DCT.

PGF2α stimulates the 10-pS Cl- channel and thiazide-sensitive Na+-Cl- cotransporter in the distal convoluted tubule.

We used patch-clamp and Western blot analysis to test whether PGF2α stimulates the basolateral 10-pS Cl- channel and thiazide-sensitive Na+-Cl- cotransporter (NCC) in the distal convoluted tubule (DCT) via a prostaglandin F receptor (FP-R). Single channel and whole cell recordings demonstrated that PGF2α stimulated the 10-pS Cl- channel in the DCT. The stimulatory effect of PGF2α on the Cl- channel was mimicked by a FP-R agonist, latanoprost, but was abrogated by blocking FP-R with AL8810. Also, the effect of PGF2α on the Cl- channel in the DCT was recapitulated by stimulating PKC but was blocked by inhibiting PKC. Furthermore, inhibition of p38 MAPK but not ERK blocked the effect of PGF2α on the 10-pS Cl- channel. Inhibition of NADPH oxidase also abrogated the stimulatory effect of PGF2α on the 10-pS Cl- channel, while the addition of 10 µM H2O2 mimicked the stimulatory effect of PGF2α on the 10-pS Cl- channel. Moreover, superoxide-related species may mediate the stimulatory effect of PGF2α on the 10-pS Cl- channel because the stimulatory effect of PGF2α and H2O2 was not additive. Western blot analysis showed that infusion of PGF2α in vivo not only increased the expression of FP-R but also increased the expression of total NCC and phosphorylated NCC. We conclude that PGF2α stimulates the basolateral 10-pS Cl- channel in the DCT by activating FP-R through PKC/p38 MAPK and NADPH oxidase-dependent pathways. The stimulatory effects of PGF2α on the Cl- channel and NCC may contribute to PGF2α-induced increases in NaCl reabsorption in the DCT.

A novel distal convoluted tubule-specific Cre-recombinase driven by the NaCl cotransporter gene.

Cre-lox technology has revolutionized research in renal physiology by allowing site-specific genetic recombination in individual nephron segments. The distal convoluted tubule (DCT), consisting of distinct early (DCT1) and late (DCT2) segments, plays a central role in Na+ and K+ homeostasis. The only established Cre line targeting the DCT is Pvalb-Cre, which is limited by noninducibility, activity along DCT1 only, and activity in neurons. Here, we report the characterization of the first Cre line specific to the entire DCT. CRISPR/Cas9 targeting was used to introduce a tamoxifen-inducible IRES-Cre-ERT2 cassette downstream of the coding region of the Slc12a3 gene encoding the NaCl cotransporter (NCC). The resulting Slc12a3-Cre-ERT2 mice were crossed with R26R-YFP reporter mice, which revealed minimal leakiness with 6.3% of NCC-positive cells expressing yellow fluorescent protein (YFP) in the absence of tamoxifen. After tamoxifen injection, YFP expression was observed in 91.2% of NCC-positive cells and only in NCC-positive cells, revealing high recombination efficiency and DCT specificity. Crossing to R26R-TdTomato mice revealed higher leakiness (64.5%), suggesting differential sensitivity of the floxed site. Western blot analysis revealed no differences in abundances of total NCC or the active phosphorylated form of NCC in Slc12a3-Cre-ERT2 mice of either sex compared with controls. Plasma K+ and Mg2+ concentrations and thiazide-sensitive Na+ and K+ excretion did not differ in Slc12a3-Cre-ERT2 mice compared with controls when sex matched. These data suggest genetic modification had no obvious effect on NCC function. Slc12a3-Cre-ERT2 mice are the first line generated demonstrating inducible Cre recombinase activity along the entire DCT and will be a useful tool to study DCT function.

Elevated FGF23 Levels in Mice Lacking the Thiazide-Sensitive NaCl cotransporter (NCC).

Fibroblast growth factor 23 (FGF23) participates in the orchestration of mineral metabolism by inducing phosphaturia and decreasing the production of 1,25(OH)2D3. It is known that FGF23 release is stimulated by aldosterone and extracellular volume depletion. To characterize this effect further in a model of mild hypovolemia, we studied mice lacking the thiazide sensitive NaCl cotransporter (NCC). Our data indicate that NCC knockout mice (KO) have significantly higher FGF23, PTH and aldosterone concentrations than corresponding wild type (WT) mice. However, 1,25(OH)2D3, fractional phosphate excretion and renal brush border expression of the sodium/phosphate co-transporter 2a were not different between the two genotypes. In addition, renal expression of FGF23 receptor FGFR1 and the co-receptor Klotho were unaltered in NCC KO mice. FGF23 transcript was increased in the bone of NCC KO mice compared to WT mice, but treatment of primary murine osteoblasts with the NCC inhibitor hydrochlorothiazide did not elicit an increase of FGF23 transcription. In contrast, the mineralocorticoid receptor blocker eplerenone reversed excess FGF23 levels in KO mice but not in WT mice, indicating that FGF23 upregulation in NCC KO mice is primarily aldosterone-mediated. Together, our data reveal that lack of renal NCC causes an aldosterone-mediated upregulation of circulating FGF23.

Comparative transcriptomic analysis identifies evolutionarily conserved gene products in the vertebrate renal distal convoluted tubule.

Understanding the molecular basis of the complex regulatory networks controlling renal ion transports is of major physiological and clinical importance. In this study, we aimed to identify evolutionarily conserved critical players in the function of the renal distal convoluted tubule (DCT) by a comparative transcriptomic approach. We generated a transgenic zebrafish line with expression of the red fluorescent mCherry protein under the control of the zebrafish DCT-specific promoter of the thiazide-sensitive NaCl cotransporter (NCC). The mCherry expression was then used to isolate from the zebrafish mesonephric kidneys the distal late (DL) segments, the equivalent of the mammalian DCT, for subsequent RNA-seq analysis. We next compared this zebrafish DL transcriptome to the previously established mouse DCT transcriptome and identified a subset of gene products significantly enriched in both the teleost DL and the mammalian DCT, including SLCs and nuclear transcription factors. Surprisingly, several of the previously described regulators of NCC (e.g., SPAK, KLHL3, ppp1r1a) in the mouse were not found enriched in the zebrafish DL. Nevertheless, the zebrafish DL expressed enriched levels of related homologues. Functional knockdown of one of these genes, ppp1r1b, reduced the phosphorylation of NCC in the zebrafish pronephros, similar to what was seen previously in knockout mice for its homologue, Ppp1r1a. The present work is the first report on global gene expression profiling in a specific nephron portion of the zebrafish kidney, an increasingly used model system for kidney research. Our study suggests that comparative analysis of gene expression between phylogenetically distant species may be an effective approach to identify novel regulators of renal function.

Hypokalemia and Pendrin Induction by Aldosterone.

Aldosterone plays an important role in regulating Na-Cl reabsorption and blood pressure. Epithelial Na+ channel, Na+-Cl- cotransporter, and Cl-/HCO3- exchanger pendrin are the major mediators of Na-Cl transport in the aldosterone-sensitive distal nephron. Existing evidence also suggests that plasma K+ concentration affects renal Na-Cl handling. In this study, we posited that hypokalemia modulates the effects of aldosterone on pendrin in hyperaldosteronism. Chronic aldosterone infusion in mice increased pendrin levels at the plasma membrane, and correcting hypokalemia in this model almost completely blocked pendrin upregulation. However, hypokalemia induced by a low-K+ diet resulted in pendrin downregulation along with reduced plasma aldosterone levels, indicating that both hypokalemia and aldosterone excess are necessary for pendrin induction. In contrast, decreased plasma K+ levels were sufficient to increase Na+-Cl- cotransporter levels. We found that phosphorylation of mineralocorticoid receptor that prevents aldosterone binding in intercalated cells was suppressed by hypokalemia, which resulted in enhanced pendrin response to aldosterone, explaining the coordinated action of aldosterone and hypokalemia in pendrin regulation. Finally, to address the physiological significance of our observations, we administered aldosterone to mice lacking pendrin. Notably, plasma K+ levels were significantly lower in pendrin knockout mice (2.7±0.1 mmol/L) than in wild-type mice (3.0±0.1 mmol/L) after aldosterone infusion, demonstrating that pendrin alleviates hypokalemia in a state of aldosterone excess. These data indicate that the decreased plasma K+ levels promote pendrin induction by aldosterone, which, in concert with Na+-Cl- cotransporter, counteracts the progression of hypokalemia but promotes hypertension in primary aldosterone excess.

WNK signalling pathways in blood pressure regulation.

Hypertension (high blood pressure) is a major public health problem affecting more than a billion people worldwide with complications, including stroke, heart failure and kidney failure. The regulation of blood pressure is multifactorial reflecting genetic susceptibility, in utero environment and external factors such as obesity and salt intake. In keeping with Arthur Guyton's hypothesis, the kidney plays a key role in blood pressure control and data from clinical studies; physiology and genetics have shown that hypertension is driven a failure of the kidney to excrete excess salt at normal levels of blood pressure. There is a number of rare Mendelian blood pressure syndromes, which have shed light on the molecular mechanisms involved in dysregulated ion transport in the distal kidney. One in particular is Familial hyperkalemic hypertension (FHHt), an autosomal dominant monogenic form of hypertension characterised by high blood pressure, hyperkalemia, hyperchloremic metabolic acidosis, and hypercalciuria. The clinical signs of FHHt are treated by low doses of thiazide diuretic, and it mirrors Gitelman syndrome which features the inverse phenotype of hypotension, hypokalemic metabolic alkalosis, and hypocalciuria. Gitelman syndrome is caused by loss of function mutations in the thiazide-sensitive Na/Cl cotransporter (NCC); however, FHHt patients do not have mutations in the SCL12A3 locus encoding NCC. Instead, mutations have been identified in genes that have revealed a key signalling pathway that regulates NCC and several other key transporters and ion channels in the kidney that are critical for BP regulation. This is the WNK kinase signalling pathway that is the subject of this review.

Channel-transporter complexes: an emerging theme in cell signaling.

In a recent edition of Biochemical Journal, Mistry et al. described the discovery of a novel protein complex, formed from the epithelial sodium channel (ENaC) and the sodium chloride cotransporter (NCC) [Mistry et al. (2016) Biochem. J. 473, 3237­3252]. The importance of these two proteins in the regulation of salt balance and blood pressure has long been known, as has their overlapping expression in the distal convoluted tubule of the kidney. The new study by Mistry et al. now demonstrates their physical interaction in the kidney and when heterologously co-expressed. Furthermore, the authors demonstrate some degree of functional co-dependence between ENaC and NCC, with pharmacological inhibition of the latter diminishing activity of the former when the two are co-assembled. This novel and potentially important interaction adds to a growing number of recently identified channel-transporter ('chansporter') complexes, which together constitute an emerging theme in cell signaling.

The European Eel NCCß Gene Encodes a Thiazide-resistant Na-Cl Cotransporter.

The thiazide-sensitive Na-Cl cotransporter (NCC) is the major pathway for salt reabsorption in the mammalian distal convoluted tubule. NCC plays a key role in the regulation of blood pressure. Its inhibition with thiazides constitutes the primary baseline therapy for arterial hypertension. However, the thiazide-binding site in NCC is unknown. Mammals have only one gene encoding for NCC. The eel, however, contains a duplicate gene. NCCα is an ortholog of mammalian NCC and is expressed in the kidney. NCCß is present in the apical membrane of the rectum. Here we cloned and functionally characterized NCCß from the European eel. The cRNA encodes a 1043-amino acid membrane protein that, when expressed in Xenopus oocytes, functions as an Na-Cl cotransporter with two major characteristics, making it different from other known NCCs. First, eel NCCß is resistant to thiazides. Single-point mutagenesis supports that the absence of thiazide inhibition is, at least in part, due to the substitution of a conserved serine for a cysteine at position 379. Second, NCCß is not activated by low-chloride hypotonic stress, although the unique Ste20-related proline alanine-rich kinase (SPAK) binding site in the amino-terminal domain is conserved. Thus, NCCß exhibits significant functional differences from NCCs that could be helpful in defining several aspects of the structure-function relationship of this important cotransporter.

Adult nephron-specific MR-deficient mice develop a severe renal PHA-1 phenotype.

Aldosterone is the main mineralocorticoid hormone controlling sodium balance, fluid homeostasis, and blood pressure by regulating sodium reabsorption in the aldosterone-sensitive distal nephron (ASDN). Germline loss-of-function mutations of the mineralocorticoid receptor (MR) in humans and in mice lead to the "renal" form of type 1 pseudohypoaldosteronism (PHA-1), a case of aldosterone resistance characterized by salt wasting, dehydration, failure to thrive, hyperkalemia, and metabolic acidosis. To investigate the importance of MR in adult epithelial cells, we generated nephron-specific MR knockout mice (MR(Pax8/LC1)) using a doxycycline-inducible system. Under standard diet, MR(Pax8/LC1) mice exhibit inability to gain weight and significant weight loss compared to control mice. Interestingly, despite failure to thrive, MR(Pax8/LC1) mice survive but develop a severe PHA-1 phenotype with higher urinary Na(+) levels, decreased plasma Na(+), hyperkalemia, and higher levels of plasma aldosterone. This phenotype further worsens and becomes lethal under a sodium-deficient diet. Na(+)/Cl(-) co-transporter (NCC) protein expression and its phosphorylated form are downregulated in the MR(Pax8/LC1) knockouts, as well as the αENaC protein expression level, whereas the expression of glucocorticoid receptor (GR) is increased. A diet rich in Na(+) and low in K(+) does not restore plasma aldosterone to control levels but is sufficient to restore body weight, plasma, and urinary electrolytes. In conclusion, MR deletion along the nephron fully recapitulates the features of severe human PHA-1. ENaC protein expression is dependent on MR activity. Suppression of NCC under hyperkalemia predominates in a hypovolemic state.

Polymorphisms in the SLC12A3 Gene Encoding Sodium-Chloride Cotransporter are Associated with Hypertension: A Family-Based Study in the Mongolian Population.

BACKGROUND/AIMS: Hypertension or persistent high blood pressure (BP) is a leading cause of death worldwide. Extensive evidence indicates that the thiazide-sensitive Na+-Cl- cotransporter (NCC) affects BP via regulation of renal sodium reabsorption. However, the relationship between genetic variants of the NCC-encoding SLC12A3 gene and hypertension in the Mongolian population is still ambiguous. In this study, we aimed to genotype an extended cohort of hypertensive Mongolian families for polymorphisms in the SLC12A3 locus. METHODS: Eighty-eight families with a history of hypertension, including parents, offspring, and relatives underwent clinical testing. Family-based association tests and haplotype analysis were used to evaluate the association between hypertension and polymorphisms in the SLC12A3 locus. RESULTS: We identified three single nucleotide polymorphisms (SNPs), one in the SLC12A3 coding region (p = 0.05) and two in the intron (p = 0.02 and p = 0.07), which were significantly associated with the hypertension phenotype. Haplotype-specific association tests confirmed the correlation of these SNPs with hypertension (p < 0.05). CONCLUSION: These results suggest that SNPs in the SLC12A3 gene confer susceptibility to hypertension in the Mongolian population. Further research is needed to validate the functional role of SLC12A3 polymorphisms in hypertension.

Chronic Metabolic Acidosis Activates Renal Tubular Sodium Chloride Cotransporter through Angiotension II-dependent WNK4-SPAK Phosphorylation Pathway.

The mechanism by which chronic metabolic acidosis (CMA) regulates sodium (Na(+))-chloride (Cl(-)) cotransporter (NCC) in the renal distal convoluted tubules remains unexplored. We examined the role of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and with-no-lysine kinase 4 (WNK4) on expression of NCC in mouse models of CMA. CMA was induced by NH4Cl in wild type mice (WTA mice), SPAK, and WNK4 knockout mice. The quantities of Ncc mRNA, expression of total NCC, phosphorylated (p)-NCC, SPAK and WNK4 in the kidneys as well as NCC inhibition with hydrochlorothiazide and Na(+) balance were evaluated. Relative to WT mice, WTA mice had similar levels of Ncc mRNA, but increased expression of total and p-NCC, SPAK, and WNK4 and an exaggerated response to hydrochlorothiazide which could not be observed in SPAK or WNK4 knockout mice with CMA. In WTA mice, increased plasma renin activity, aldosterone and angiotensin II concentrations accompanied by a significantly negative Na(+) balance. High Na(+) diet abolished the enhanced NCC expression in WTA mice. Furthermore, an angiotensin II type 1 receptor blocker rather than a mineralocorticoid receptor antagonist exerted a marked inhibition on Na(+) reabsorption and NCC phosphorylation in WTA mice. CMA increases WNK4-SPAK-dependent NCC phosphorylation and appears to be secondary to previous natriuresis with volume-dependent angiotensin II activation.

Cortisol regulates sodium homeostasis by stimulating the transcription of sodium-chloride transporter (NCC) in zebrafish (Danio rerio).

In mammals, sodium/hydrogen exchanger (NHE) and sodium-chloride cotransporter (NCC) are expressed in renal tubules, and exhibit functional redundancy and mutual compensation in Na(+) uptake. In teleosts, the gills of the adult and skin of the embryonic stage function as external kidneys, and ionocytes are responsible for ionoregulation in these tissues. NHE- and NCC-expressing ionocytes mutually cooperate to adjust Na(+) uptake, which is analogous to the activity of the mammalian kidney. Cortisol is a hormone that controls Na(+) uptake through regulating NCC expression and activity in mammals; however, cortisol-mediated control of NCC expression is little understood in non-mammalian vertebrates, such as teleosts. It is essential for our understanding of the evolution of such regulation to determine whether cortisol has a conserved effect on NCC in vertebrates. In the present study, we treated zebrafish embryos with low Na(+) medium (LNa, 0.04 mM Na(+)) for 3 d to stimulate the mRNA expression of nhe3b, ncc, and cyp11b1 (a cortisol-synthesis enzyme) and whole body cortisol level. Exogenous cortisol treatment (20 mg/l, 3 d) resulted in an elevation of whole-body Na(+) content, ncc expression, and the density of ncc-expressing cells in zebrafish larvae. In loss-of-function experiments, microinjection of glucocorticoid receptor (gr) morpholino (MO) suppressed sodium content, ncc expression, and the density of ncc-expressing cells, but injection of mr MO had no such effects. In addition, exogenous cortisol treatment and gr MO injection also altered ncc expression and the density of ncc-expressing cells in gcm2 morphant larvae. Taken together, cortisol and GR appear to regulate Na(+) absorption through stimulating ncc expression and the differentiation of ncc-expressing ionocytes, providing new insights into the actions of cortisol on Na(+) uptake.