Renal K+ channels: structure and function

Kir5.1 regulates Kir4.2 expression and is a key component of the 50-pS inwardly rectifying potassium channel in basolateral membrane of mouse proximal tubules

Kir5.1 encoded by Kcnj16 is an inwardly rectifying K+ channel subunit, and it possibly interacts with Kir4.2 subunit encoded by Kcnj15 for assembling a Kir4.2/Kir5.1 heterotetramer in the basolateral membrane of mouse proximal tubule. We now used patch clamp technique to examine basolateral K+ channels of mouse proximal tubule (PT) and an immunoblotting/immunofluorescence (IF) staining microscope to examine Kir4.2 expression in wild-type and Kir5.1-knockout mice. IF staining shows that Kir4.2 was exclusively expressed in the proximal tubule, whereas Kir5.1 was expressed in the proximal tubule and distal nephrons including distal convoluted tubule. Immunoblotting showed that the expression of Kir4.2 monomer was lower in Kir5.1-knockout mice than that in the wild-type mice. In contrast, Kir4.1 monomer expression was increased in Kir5.1 knockout mice. IF images further demonstrated that the basolateral membrane staining of Kir4.2 was significantly decreased in Kir5.1 knockout mice. This is in sharp contrast to Kir4.1, which also interacts with Kir5.1 in the distal nephron, and IF images show that Kir4.1 membrane expression was still visible and unchanged in Kir5.1 knockout mice. The single channel recording detected a 50-pS inwardly rectifying K+ channel, presumably a Kir4.2/Kir5.1 heterotetramer, in the basolateral membrane of the proximal tubule of Kir5.1 wild-type mice. However, this 50-pS K+ channel was completely absent in the basolateral membrane of the proximal tubule of Kir5.1 knockout mice. Moreover, the membrane potential of the proximal tubule was less negative in Kir5.1 knockout mice than wild-type mice. We conclude that Kir5.1 is essential for assembling basolateral 50-pS K+ channel in proximal tubule and that deletion of Kir5.1 decreased Kir4.2 expression in the proximal tubule thereby decreasing the basolateral K+ conductance and the membrane potentials.NEW & NOTEWORTHY Our study provides direct evidence for the notion that Kir5.1 is a key component of a 50-60 pS inwardly-rectifying-K+ channel, a main type K+ channel in the basolateral-membrane of PT. Also, we demonstrate that deletion of Kir5.1 decreased Kir4.2 protein expression including the basolateral-membrane in PT. Finally, depolarization of PT-membrane- potential in Kir5.1-knockout mice suggests that Kir4.2 alone is not able to sustain basolateral K+ conductance of the PT in the absence of Kir5.1.

Role of calcineurin in regulating renal potassium (K+) excretion: Mechanisms of calcineurin inhibitor-induced hyperkalemia

Calcineurin, protein phosphatase 2B (PP2B) or protein phosphatase 3 (PP3), is a calcium-dependent serine/threonine protein phosphatase. Calcineurin is widely expressed in the kidney and regulates renal Na+ and K+ transport. In the thick ascending limb, calcineurin plays a role in inhibiting NKCC2 function by promoting the dephosphorylation of the cotransporter and an intracellular sorting receptor, called sorting-related-receptor-with-A-type repeats (SORLA), is involved in modulating the effect of calcineurin on NKCC2. Calcineurin also participates in regulating thiazide-sensitive NaCl-cotransporter (NCC) in the distal convoluted tubule. The mechanisms by which calcineurin regulates NCC include directly dephosphorylation of NCC, regulating Kelch-like-3/CUL3 E3 ubiquitin-ligase complex, which is responsible for WNK (with-no-lysin-kinases) ubiquitination, and inhibiting Kir4.1/Kir5.1, which determines NCC expression/activity. Finally, calcineurin is also involved in regulating ROMK (Kir1.1) channels in the cortical collecting duct and Cyp11 2 expression in adrenal zona glomerulosa. In summary, calcineurin is involved in the regulation of NKCC2, NCC, and inwardly rectifying K+ channels in the kidney, and it also plays a role in modulating aldosterone synthesis in adrenal gland, which regulates epithelial-Na+-channel expression/activity. Thus, application of calcineurin inhibitors (CNIs) is expected to abrupt calcineurin-mediated regulation of transepithelial Na+ and K+ transport in the kidney. Consequently, CNIs cause hypertension, compromise renal K+ excretion, and induce hyperkalemia.

Adenosine stimulates the basolateral 50 pS K+ channel in renal proximal tubule via adenosine-A1 receptor

Background: The basolateral potassium channels play an important role in maintaining the membrane transport in the renal proximal tubules (PT) and adenosine receptors have been shown to regulate the trans-epithelial Na+ absorption in the PT. The aim of the present study is to explore whether adenosine also regulates the basolateral K+ channel of the PT and to determine the adenosine receptor type and the signaling pathway which mediates the effect of adenosine on the K+ channel. Methods: We have used the single channel recording to examine the basolateral K+ channel activity in the proximal tubules of the mouse kidney. All experiments were performed in cell-attached patches. Results: Single channel recording has detected a 50 pS inwardly-rectifying K+ channel with high channel open probability and this 50 pS K+ channel is a predominant type K+ channel in the basolateral membrane of the mouse PT. Adding adenosine increased 50 pS K+ channel activity in cell-attached patches, defined by NPo (a product of channel Numbers and Open Probability). The adenosine-induced stimulation of the 50 pS K+ channel was absent in the PT pretreated with DPCPX, a selective inhibitor of adenosine A1 receptor. In contrast, adenosine was still able to stimulate the 50 pS K+ channel in the PT pretreated with CP-66713, a selective adenosine A2 receptor antagonist. This suggests that the stimulatory effect of adenosine on the 50 pS K+ channel of the PT was mediated by adenosine-A1 receptor. Moreover, the effect of adenosine on the 50 pS K+ channel was blocked in the PT pretreated with U-73122 or Calphostin C, suggesting that adenosine-induced stimulation of the 50 pS K+ channels of the PT was due to the activation of phospholipase C (PLC) and protein kinase C (PKC) pathway. In contrast, the inhibition of phospholipase A2 (PLA2) with AACOCF3 or inhibition of protein kinase A (PKA) with H8 failed to block the adenosine-induced stimulation of the 50 pS K+ channel of the PT. Conclusion: We conclude that adenosine activates the 50 pS K+ channels in the basolateral membrane of PT via adenosine-A1 receptor. Furthermore, the effect of adenosine on the 50 pS K+ channel is mediated by PLC-PKC signaling pathway.

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.

ROMK channels are inhibited in the aldosterone-sensitive distal nephron of renal tubule Nedd4-2-deficient mice

We used whole cell recording to examine the renal outer medullary K+ channel (ROMK or Kir1.1) and epithelial Na+ channel (ENaC) in the late distal convoluted tubule (DCT2)/initial connecting tubule (iCNT) and in the cortical collecting duct (CCD) of kidney tubule-specific neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2) knockout mice (Ks-Nedd4-2 KO) and floxed neural precursor cell-expressed developmentally downregulated 4-like (Nedd4l) mice (control). Tertiapin Q (TPNQ)-sensitive K+ currents (ROMK) were smaller in both the DCT2/iCNT and CCD of Ks-Nedd4-2 KO mice on a normal diet than in control mice. Neither high dietary salt intake nor low dietary salt intake had a significant effect on ROMK activity in the DCT2/iCNT and CCD of control and Ks-Nedd4-2 KO mice. In contrast, high dietary K+ intake (HK) increased, whereas low dietary K+ intake (LK) decreased TPNQ-sensitive K+ currents in floxed Nedd4l mice. However, the effects of dietary K+ intake on ROMK channel activity were absent in Ks-Nedd4-2 KO mice since neither HK nor LK significantly affected TPNQ-sensitive K+ currents in the DCT2/iCNT and CCD. Moreover, TPNQ-sensitive K+ currents in the DCT2/iCNT and CCD of Ks-Nedd4-2 KO mice on HK were similar to those of control mice on LK. Amiloride-sensitive Na+ currents in the DCT2/iCNT and CCD were significantly higher in Ks-Nedd4-2 KO mice than in floxed Nedd4l mice on a normal K+ diet. HK increased ENaC activity of the DCT2/iCNT only in control mice, but HK stimulated ENaC of the CCD in both control and Ks-Nedd4-2 KO mice. Moreover, the HK-induced increase in amiloride-sensitive Na+ currents was larger in Ks-Nedd4-2 KO mice than in control mice. Deletion of Nedd4-2 increased with no lysine kinase 1 expression and abolished HK-induced inhibition of with no lysine kinase 1. We conclude that deletion of Nedd4-2 increases ENaC activity but decreases ROMK activity in the aldosterone-sensitive distal nephron and that HK fails to stimulate ROMK, but robustly increases ENaC activity in the CCD of Nedd4-2-deficient mice.NEW & NOTEWORTHY We demonstrate that renal outer medullary K+ (ROMK) channel activity is inhibited in the late distal convoluted tubule/initial connecting tubule and cortical collecting duct of neural precursor cell-expressed developmentally downregulated protein 4-2 (Nedd4-2)-deficient mice. Also, deletion of Nedd4-2 abolishes the stimulatory effect of dietary K+ intake on ROMK. The lack of high K+-induced stimulation of ROMK is associated with the absence of high K+-induced inhibition of with no lysine kinase 1.

Inhibitory effect of tumor necrosis factor-α on the basolateral Kir4.1/Kir5.1 channels in the thick ascending limb during diabetes

Diabetic nephropathy is a major contributor to the morbidity and mortality of patients with diabetes. TNF-α expression is elevated during diabetes and is implicated in the pathogenesis of diabetic nephropathy; however, its underlying molecular mechanisms remain unclear. The present study aimed to investigate the effect and molecular mechanism of TNF-α on the basolateral inwardly rectifying potassium (Kir)4.1/Kir5.1 channels in the thick ascending limb (TAL) of rat kidneys using western blotting and the patch clamp technique to provide a theoretical basis for the cause of the decrease in kidney concentrating capacity during diabetes. The results demonstrated that urinary TNF-α excretion and protein TNF-α expression in the TAL increased and basolateral Kir4.1/Kir5.1 channel activity decreased during diabetes; however, diabetic rats exhibited amelioration of Kir4.1/Kir5.1 activity with a soluble TNF-α antagonist, TNF receptor fusion protein (TNFR:Fc). These results suggested that TNF-α inhibited the activity of the basolateral Kir4.1/Kir5.1 channel in the TAL of rat kidneys during diabetes. In addition, the protein expression levels of phospholipase A₂ (PLA₂) and cyclooxygenase-₂ (COX₂) increased in diabetic rats, the effects of which deceased following treatment with TNFR:Fc compared with the diabetic group. Furthermore, an agonist of PLA₂ (melittin) and COX₂ production [prostaglandin E₂ (PGE₂)] inhibited the basolateral Kir4.1/Kir5.1 channels. Taken together, the results of the present study suggested that the inhibitory effect of TNF-α on the basolateral Kir4.1/Kir5.1 channels in the TAL during diabetes is mediated by the PLA₂/COX₂/PGE₂ signaling pathway.

Adult Patients Affected by Cystic Fibrosis in Therapy with Cystic Fibrosis Transmembrane Regulator Modulators and Lung Transplant: Renal Function, Metabolic and Nutritional Status

Background. Cystic fibrosis (CF) is one of the most frequent genetic diseases. The discovery and implementation of new therapies prolonged the survival of CF patients in the last years. Evaluation of long-term complications could be useful to improve the outcome of these patients. Aim of the Study. To evaluate renal function, metabolic, nutritional, and inflammatory status in CF patients on cystic fibrosis transmembrane regulator (CFTR) modulators therapy as well as lung transplant recipients (LRs) and patients on conservative therapy (control group). Materials and Methods. We performed a prospective, longitudinal study on 69 CF patients. Clinical and laboratory parameters (metabolic and nutritional indices and inflammatory markers) were evaluated in all patients before starting CFTR therapy or transplant (T0) and after 3 years (T1). Results. We enrolled 69 CF patients (42 males). Patients were distributed into three groups. The average age was 35.01 ± 10.57 years for the control group (group 0), 32.47 ± 9.40 years for patients on CFTR modulators therapy (group 1), and 38.93 ± 7.14 years for LRs (group 2). At T1, we showed a significant difference among the three groups in terms of renal function indices: creatinine, eGFR, serum nitrogen as well as serum uric acid, sodium, and potassium ( $p<0.001$ , $p<0.001$ , $p<0.001$ , $p<0.001$ , $p<0.001$ , and $p<0.001$ , respectively), particularly in LRs patients. Significant differences were found in nutritional status parameters among the three groups: total protein, serum albumin, serum fibrinogen, serum transferrin, and white blood cell counts ( $p<0.001$ , $p=0.037$ , $p=0.04$ , $p=0.003$ , and $p=0.007$ , respectively), particularly in LRs compared with other groups. Moreover, we found significant differences in metabolic profile (HbA1c, $p=0.026$ ) and inflammatory status, with a significant difference in C-reactive protein values, neutrophil counts, and neutrophil-lymphocyte ratio (NLR) among the three groups ( $p<0.001$ , $p=0.005$ , and $p=0.026$ , respectively). Conclusions. Our study showed a reduced renal function and poor nutritional status in LRs, along with worse metabolic control. Moreover, we showed a lower inflammatory status in patients on CFTR modulators therapy. Therefore, we suggest early and careful monitoring of renal function, metabolic, and nutritional parameters in CF patients, whether they are on conservative therapy, CFTR modulators therapy, and LRs patients.

Expression, localization, and functional properties of inwardly rectifying K+ channels in the kidney

Inwardly rectifying K+ (Kir) channels are expressed in multiple organs and cell types and play critical roles in cellular function. Most notably, Kir channels are major determinants of the resting membrane potential and K+ homeostasis. The renal outer medullary K+ channel (Kir1.1) was the first renal Kir channel identified and cloned in the kidney over two decades ago. Since then, several additional members, including classical and ATP-regulated Kir family classes, have been identified to be expressed in the kidney and to contribute to renal ion transport. Although the ATP-regulated Kir channel class remains the most well known due to severe pathological phenotypes associated with their mutations, progress is being made in defining the properties, localization, and physiological functions of other renal Kir channels, including those localized to the basolateral epithelium. This review is primarily focused on the current knowledge of the expression and localization of renal Kir channels but will also briefly describe their proposed functions in the kidney.

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

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

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.

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.

Role and mechanisms of regulation of the basolateral Kir 4.1/Kir 5.1K+ channels in the distal tubules

Epithelial K+ channels are essential for maintaining electrolyte and fluid homeostasis in the kidney. It is recognized that basolateral inward-rectifying K+ (Kir ) channels play an important role in the control of resting membrane potential and transepithelial voltage, thereby modulating water and electrolyte transport in the distal part of nephron and collecting duct. Monomeric Kir 4.1 (encoded by Kcnj10 gene) and heteromeric Kir 4.1/Kir 5.1 (Kir 4.1 together with Kir 5.1 (Kcnj16)) channels are abundantly expressed at the basolateral membranes of the distal convoluted tubule and the cortical collecting duct cells. Loss-of-function mutations in KCNJ10 cause EAST/SeSAME tubulopathy in humans associated with salt wasting, hypomagnesaemia, metabolic alkalosis and hypokalaemia. In contrast, mice lacking Kir 5.1 have severe renal phenotype that, apart from hypokalaemia, is the opposite of the phenotype seen in EAST/SeSAME syndrome. Experimental advances using genetic animal models provided critical insights into the physiological role of these channels in electrolyte homeostasis and the control of kidney function. Here, we discuss current knowledge about K+ channels at the basolateral membrane of the distal tubules with specific focus on the homomeric Kir 4.1 and heteromeric Kir 4.1/Kir 5.1 channels. Recently identified molecular mechanisms regulating expression and activity of these channels, such as cell acidification, dopamine, insulin and insulin-like growth factor-1, Src family protein tyrosine kinases, as well as the role of these channels in NCC-mediated transport in the distal convoluted tubules, are also described.

Dominant-Negative Effect of a Missense Variant in the TASK-2 (KCNK5) K+ Channel Associated with Balkan Endemic Nephropathy

TASK-2, a member of the Two-Pore Domain (K2P) subfamily of K⁺ channels, is encoded by the KCNK5 gene. The channel is expressed primarily in renal epithelial tissues and a potentially deleterious missense variant in KCNK5 has recently been shown to be prevalent amongst patients predisposed to the development of Balkan Endemic Nephropathy (BEN), a chronic tubulointerstitial renal disease of unknown etiology. In this study we show that this variant (T108P) results in a complete loss of channel function and is associated with a major reduction in TASK-2 channel subunits at the cell surface. Furthermore, these mutant subunits have a suppressive or ‘dominant-negative’ effect on channel function when coexpressed with wild-type subunits. This missense variant is located at the extracellular surface of the M2 transmembrane helix and by using a combination of structural modelling and further functional analysis we also show that this highly-conserved threonine residue is critical for the correct function of other K2P channels. These results therefore provide further structural and functional insights into the possible pathophysiological effects of this missense variant in TASK-2.

Inhibitors of the renal outer medullary potassium channel: a patent review

INTRODUCTION

Hypertension represents a substantial cardiovascular risk factor. Among anti-hypertensive drugs, diuretics play an important role. Nevertheless, they present adverse effects such as hypokalemia or hyperkalemia. In this panorama, inhibitors of the renal outer medullary potassium (ROMK) channels are emerging because they are predicted to give a diuretic/natriuretic activity higher than that provided by loop diuretics, without hypokaliemic and hyperkaliemic side effects.

AREAS COVERED

This article reviews the current literature, including all the patents published in the field of inhibitors of the ROMK channels for the treatment of hypertension, heart failure and correlated diseases. The patent examination has been carried out using electronic databases Espacenet.

EXPERT OPINION

Although anti-hypertensive drugs armamentarium enumerates a plethora of therapeutic classes, including diuretics, the novel class of ROMK inhibitors may find a place in this crowded market, because of the diuretic/natriuretic effects, devoid of worrying influence on potassium balance. The patent examination highlights, as a strength, the individuation of a successful template: almost all the compounds show noteworthy potency. However, only few selected compounds underwent an in vivo investigation of diuretic and anti-hypertensive activities, and no data on the hERG channel are given in these patents.

Caveolin-1 Deficiency Inhibits the Basolateral K+ Channels in the Distal Convoluted Tubule and Impairs Renal K+ and Mg2+ Transport

Kcnj10 encodes the inwardly rectifying K(+) channel Kir4.1 in the basolateral membrane of the distal convoluted tubule (DCT) and is activated by c-Src. However, the regulation and function of this K(+) channel are incompletely characterized. Here, patch-clamp experiments in Kcnj10-transfected HEK293 cells demonstrated that c-Src-induced stimulation of Kcnj10 requires coexpression of caveolin-1 (cav-1), and immunostaining showed expression of cav-1 in the basolateral membrane of parvalbumin-positive DCT. Patch-clamp experiments detected a 40-pS inwardly rectifying K(+) channel, a heterotetramer of Kir4.1/Kir5.1, in the basolateral membrane of the early DCT (DCT1) in both wild-type (WT) and cav-1-knockout (KO) mice. However, the activity of this basolateral 40-pS K(+) channel was lower in KO mice than in WT mice. Moreover, the K(+) reversal potential (an indication of membrane potential) was less negative in the DCT1 of KO mice than in the DCT1 of WT mice. Western blot analysis demonstrated that cav-1 deficiency decreased the expression of the Na(+)/Cl(-) cotransporter and Ste20-proline-alanine-rich kinase (SPAK) but increased the expression of epithelial Na(+) channel-α. Furthermore, the urinary excretion of Mg(2+) and K(+) was significantly higher in KO mice than in WT mice, and KO mice developed hypomagnesemia, hypocalcemia, and hypokalemia. We conclude that disruption of cav-1 decreases basolateral K(+) channel activity and depolarizes the cell membrane potential in the DCT1 at least in part by suppressing the stimulatory effect of c-Src on Kcnj10. Furthermore, the decrease in Kcnj10 and Na(+)/Cl(-) cotransporter expression induced by cav-1 deficiency may underlie the compromised renal transport of Mg(2+), Ca(2+), and K(+).

A High-Throughput Electrophysiology Assay Identifies Inhibitors of the Inwardly Rectifying Potassium Channel Kir7.1

Kir7.1 is an inwardly rectifying potassium channel that has been implicated in controlling the resting membrane potential of the myometrium. Abnormal uterine activity in pregnancy plays an important role in postpartum hemorrhage, and novel therapies for this condition may lie in manipulation of membrane potential. This work presents an assay development and screening strategy for identifying novel inhibitors of Kir7.1. A cell-based automated patch-clamp electrophysiology assay was developed using the IonWorks Quattro (Molecular Devices, Sunnyvale, CA) system, and the iterative optimization is described. In total, 7087 compounds were tested, with a hit rate (>40% inhibition) of 3.09%. During screening, average Z' values of 0.63 ± 0.09 were observed. After chemistry triage, lead compounds were resynthesized and activity confirmed by IC50 determinations. The most potent compound identified (MRT00200769) gave rise to an IC50 of 1.3 µM at Kir7.1. Compounds were assessed for selectivity using the inwardly rectifying potassium channel Kir1.1 (ROMK) and hERG (human Ether-à-go-go Related Gene). Pharmacological characterization of known Kir7.1 inhibitors was also carried out and analogues of VU590 tested to assess selectivity at Kir7.1.

KCNJ10 determines the expression of the apical Na-Cl cotransporter (NCC) in the early distal convoluted tubule (DCT1)

The renal phenotype induced by loss-of-function mutations of inwardly rectifying potassium channel (Kir), Kcnj10 (Kir4.1), includes salt wasting, hypomagnesemia, metabolic alkalosis and hypokalemia. However, the mechanism by which Kir.4.1 mutations cause the tubulopathy is not completely understood. Here we demonstrate that Kcnj10 is a main contributor to the basolateral K conductance in the early distal convoluted tubule (DCT1) and determines the expression of the apical Na-Cl cotransporter (NCC) in the DCT. Immunostaining demonstrated Kcnj10 and Kcnj16 were expressed in the basolateral membrane of DCT, and patch-clamp studies detected a 40-pS K channel in the basolateral membrane of the DCT1 of p8/p10 wild-type Kcnj10(+/+) mice (WT). This 40-pS K channel is absent in homozygous Kcnj10(-/-) (knockout) mice. The disruption of Kcnj10 almost completely eliminated the basolateral K conductance and decreased the negativity of the cell membrane potential in DCT1. Moreover, the lack of Kcnj10 decreased the basolateral Cl conductance, inhibited the expression of Ste20-related proline-alanine-rich kinase and diminished the apical NCC expression in DCT. We conclude that Kcnj10 plays a dominant role in determining the basolateral K conductance and membrane potential of DCT1 and that the basolateral K channel activity in the DCT determines the apical NCC expression possibly through a Ste20-related proline-alanine-rich kinase-dependent mechanism.

CFTR and TNR-CFTR expression and function in the kidney

The cystic fibrosis transmembrane conductance regulator (CFTR) is abundantly expressed in the kidney. CFTR mRNA is detected in all nephron segments of rats and humans and its expression is higher in the renal cortex and outer medulla than in the inner medulla. CFTR protein is detected at the apical surface of both proximal and distal tubules of rat kidney but not in the outer medullary collecting ducts. The localization of CFTR in the proximal tubules is compatible with that of endosomes, suggesting that CFTR might regulate pH in endocytic vesicles by equilibrating H+ accumulation due to H+-ATPase activity. Many studies have also demonstrated that CFTR also regulates channel pore opening and the transport of sodium, chloride and potassium. The kidneys also express a CFTR splicing variant, called TNR-CFTR, in a tissue-specific manner, primarily in the renal medulla. This splicing variant conserves the functional characteristics of wild-type CFTR. The functional significance of TNR-CFTR remains to be elucidated, but our group proposes that TNR-CFTR may have a basic function in intracellular organelles, rather than in the plasma membrane. Also, this splicing variant is able to partially substitute CFTR functions in the renal medulla of Cftr-/- mice and CF patients. In this review we discuss the major functions that have been proposed for CFTR and TNR-CFTR in the kidney.

Angiotensin II stimulates basolateral 50-pS K channels in the thick ascending limb

We used the patch-clamp technique to examine the effect of angiotensin II (ANG II) on the basolateral K channels in the thick ascending limb (TAL) of the rat kidney. Application of ANG II increased the channel activity and the current amplitude of the basolateral 50-pS K channel. The stimulatory effect of ANG II on the K channels was completely abolished by losartan, an inhibitor of type 1 angiotensin receptor (AT1R), but not by PD123319, an AT2R antagonist. Moreover, inhibition of phospholipase C (PLC) and protein kinase C (PKC) also abrogated the stimulatory effect of ANG II on the basolateral K channels in the TAL. This suggests that the stimulatory effect of ANG II on the K channels was induced by activating PLC and PKC pathways. Western blotting demonstrated that ANG II increased the phosphorylation of c-Src at tyrosine residue 416, an indication of c-Src activation. This effect was mimicked by PKC stimulator but abolished by calphostin C. Moreover, inhibition of NADPH oxidase (NOX) also blocked the effect of ANG II on c-Src tyrosine phosphorylation. The role of Src-family protein tyrosine kinase (SFK) in mediating the effect of ANG II on the basolateral K channel was further suggested by the experiments in which inhibition of SFK abrogated the stimulatory effect of ANG II on the basolateral 50-pS K channel. We conclude that ANG II increases basolateral 50-pS K channel activity via AT1R and that activation of AT1R stimulates SFK by a PLC-PKC-NOX-dependent mechanism.

Opposite actions of alcohol on tonic GABA(A) receptor currents mediated by nNOS and PKC activity

The molecular mechanisms that mediate genetic variability in response to alcohol are unclear. We found that alcohol had opposite actions (enhancement or suppression) on GABA(A) receptor (GABA(A)R) inhibition in granule cells from the cerebellum of behaviorally sensitive, low alcohol-consuming Sprague-Dawley rats and DBA/2 mice and behaviorally insensitive, high alcohol-consuming C57BL/6 mice, respectively. The effect of alcohol on granule cell GABA(A)R inhibition was determined by a balance between two opposing effects: enhanced presynaptic vesicular release of GABA via alcohol inhibition of nitric oxide synthase (NOS) and a direct suppression of the activity of postsynaptic GABA(A)Rs. The balance of these two processes was determined by differential expression of neuronal NOS (nNOS) and postsynaptic PKC activity, both of which varied across the rodent genotypes. These findings identify opposing molecular processes that differentially control the magnitude and polarity of GABA(A)R responses to alcohol across rodent genotypes.

In vivo and ex vivo analysis of tubule function

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

Novel diuretic targets

As the molecular revolution continues to inform a deeper understanding of disease mechanisms and pathways, there exist unprecedented opportunities for translating discoveries at the bench into novel therapies for improving human health. Despite the availability of several different classes of antihypertensive medications, only about half of the 67 million Americans with hypertension manage their blood pressure appropriately. A broader selection of structurally diverse antihypertensive drugs acting through different mechanisms would provide clinicians with greater flexibility in developing effective treatment regimens for an increasingly diverse and aging patient population. An emerging body of physiological, genetic, and pharmacological evidence has implicated several renal ion-transport proteins, or regulators thereof, as novel, yet clinically unexploited, diuretic targets. These include the renal outer medullary potassium channel, ROMK (Kir1.1), Kir4.1/5.1 potassium channels, ClC-Ka/b chloride channels, UTA/B urea transporters, the chloride/bicarbonate exchanger pendrin, and the STE20/SPS1-related proline/alanine-rich kinase (SPAK). The molecular pharmacology of these putative targets is poorly developed or lacking altogether; however, recent efforts by a few academic and pharmaceutical laboratories have begun to lessen this critical barrier. Here, we review the evidence in support of the aforementioned proteins as novel diuretic targets and highlight examples where progress toward developing small-molecule pharmacology has been made.

Regulatory volume decrease of rat kidney principal cells after successive hypo-osmotic shocks

Outer Medullary Collecting Duct (OMCD) principal cells are exposed to significant changes of the extracellular osmolarity and thus the analysis of their regulatory volume decrease (RVD) function is of great importance in order to avoid cell membrane rupture and subsequent death. In this paper we provide a sub-second temporal analysis of RVD events occurring after two successive hypo-osmotic challenges in rat kidney OMCD principal cells. We performed experimental cell volume measurements and created a mathematical model based on our experimental results. As a consequence of RVD the cell expels part of intracellular osmolytes and reduces the permeability of the plasma membrane to water. The next osmotic challenge does not cause significant RVD if it occurs within a minute after the primary shock. In such a case the cell reacts as an ideal osmometer. Through our model we provide the basis for further detailed studies on RVD dynamical modeling.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Stimulation of Ca2+-sensing receptor inhibits the basolateral 50-pS K channels in the thick ascending limb of rat kidney

We used the patch-clamp technique to study the effect of changing the external Ca2+ on the basolateral 50-pS K channel in the thick ascending limb (TAL) of rat kidney. Increasing the external Ca2+ concentration from 1 mM to 2 or 3 mM inhibited the basolateral 50-pS K channels while decreasing external Ca2+ to 10 μM increased the 50-pS K channel activity. The effect of the external Ca2+ on the 50-pS K channels was observed only in cell-attached patches but not in excised patches. Moreover, the inhibitory effect of increasing external Ca2+ on the 50-pS K channels was absent in the presence of NPS2390, an antagonist of Ca2+-sensing receptor (CaSR), suggesting that the inhibitory effect of the external Ca2+ was the result of stimulation of the CaSR. Application of the membrane-permeable cAMP analog increased the 50-pS K channel activity but did not block the effect of raising the external Ca2+ on the K channels. Neither inhibition of phospholipase A2 (PLA2) nor suppression of cytochrome P450-ω-hydroxylation-dependent metabolism of arachidonic acid was able to abolish the effect of raising the external Ca2+ on the 50-pS K channels. In contrast, inhibition of phospholipase C (PLC) or blocking protein kinase C (PKC) completely abolished the inhibition of the basolateral 50-pS K channels induced by raising the external Ca2+. We conclude that the external Ca2+ concentration plays an important role in the regulation of the basolateral K channel activity in the TAL and that the effect of the external Ca2+ is mediated by the CaSR which stimulates PLC-PKC pathways. The regulation of the basolateral K channels by the CaSR may be the mechanism by which extracellular Ca2+ level modulates the reabsorption of divalent cations.

Renal phenotype in mice lacking the Kir5.1 (Kcnj16) K+ channel subunit contrasts with that observed in SeSAME/EAST syndrome

The heteromeric inwardly rectifying Kir4.1/Kir5.1 K(+) channel underlies the basolateral K(+) conductance in the distal nephron and is extremely sensitive to inhibition by intracellular pH. The functional importance of Kir4.1/Kir5.1 in renal ion transport has recently been highlighted by mutations in the human Kir4.1 gene (KCNJ10) that result in seizures, sensorineural deafness, ataxia, mental retardation, and electrolyte imbalance (SeSAME)/epilepsy, ataxia, sensorineural deafness, and renal tubulopathy (EAST) syndrome, a complex disorder that includes salt wasting and hypokalemic alkalosis. Here, we investigated the role of the Kir5.1 subunit in mice with a targeted disruption of the Kir5.1 gene (Kcnj16). The Kir5.1(-/-) mice displayed hypokalemic, hyperchloremic metabolic acidosis with hypercalciuria. The short-term responses to hydrochlorothiazide, an inhibitor of ion transport in the distal convoluted tubule (DCT), were also exaggerated, indicating excessive renal Na(+) absorption in this segment. Furthermore, chronic treatment with hydrochlorothiazide normalized urinary excretion of Na(+) and Ca(2+), and abolished acidosis in Kir5.1(-/-) mice. Finally, in contrast to WT mice, electrophysiological recording of K(+) channels in the DCT basolateral membrane of Kir5.1(-/-) mice revealed that, even though Kir5.1 is absent, there is an increased K(+) conductance caused by the decreased pH sensitivity of the remaining homomeric Kir4.1 channels. In conclusion, disruption of Kcnj16 induces a severe renal phenotype that, apart from hypokalemia, is the opposite of the phenotype seen in SeSAME/EAST syndrome. These results highlight the important role that Kir5.1 plays as a pH-sensitive regulator of salt transport in the DCT, and the implication of these results for the correct genetic diagnosis of renal tubulopathies is discussed.

Interaction between Calcineurin and Ca/Calmodulin Kinase-II in Modulating Cellular Functions

Roles of calcineurin (CaN), a Ca²⁺/calmodulin- (CaM-) dependent protein phosphatase, and Ca²⁺/CaM-dependent protein kinase-II (CaMKII) in modulating K⁺ channel activity and the intracellular Ca²⁺ concentration ([Ca²⁺]_i) have been investigated in renal tubule epithelial cells. The channel current through the cell membrane was recorded with the patch-clamp technique, and [Ca²⁺]_i was monitored using fura-2 imaging. We found that a CaN-inhibitor, cyclosporin A (CyA), lowered the K⁺ channel activity and elevated [Ca²⁺]_i, suggesting that CyA closes K⁺ channels and opens Ca²⁺-release channels of the cytosolic Ca²⁺-store. Moreover, both of these responses were blocked by KN-62, an inhibitor of CaMKII. It is suggested that the CyA-mediated response results from the activation of CaMKII. Indeed, Western blot analysis revealed that CyA increased phospho-CaMKII, an active form of CaMKII. These findings suggest that CaN-dependent dephosphorylation inhibits CaMKII-mediated phosphorylation, and the inhibition of CaN increases phospho-CaMKII, which results in the stimulation of CaMKII-dependent cellular actions.

A nicardipine-sensitive Ca2+ entry contributes to the hypotonicity-induced increase in [Ca2+]i of principal cells in rat cortical collecting duct

We examined the mechanisms involved in the [Ca(2+)](i) response to the extracellular hypotonicity in the principal cells of freshly isolated rat cortical collecting duct (CCD), using Fura-2/AM fluorescence imaging. Reduction of extracellular osmolality from 305 (control) to 195 mosmol/kgH(2)O (hypotonic) evoked transient increase in [Ca(2+)](i) of principal cells of rat CCDs. The [Ca(2+)](i) increase was markedly attenuated by the removal of extracellular Ca(2+)(.) The application of a P(2) purinoceptor antagonist, suramin failed to inhibit the hypotonicity-induced [Ca(2+)](i) increase. The [Ca(2+)](i) increase in response to extracellular hypotonicity was not influenced by application of Gd(3+) and ruthenium red. On the other hand, a voltage-gated Ca(2+) channel inhibitor, nicardipine, significantly reduced the peak amplitude of [Ca(2+)](i) increase in the principal cells. In order to assess Ca(2+) entry during the hypotonic stimulation, we measured the quenching of Fura-2 fluorescence intensity by Mn(2+). The hypotonic stimulation enhanced quenching of Fura-2 fluorescence by Mn(2+), indicating that a Ca(2+)-permeable pathway was activated by the hypotonicity. The hypotonicity-mediated enhancement of Mn(2+) quenching was significantly inhibited by nicardipine. These results strongly suggested that a nicardipine-sensitive Ca(2+) entry pathway would contribute to the mechanisms underlying the hypotonicity-induced [Ca(2+)](i) elevation of principal cells in rat CCD.

The hypertonic environment differentially regulates wild-type CFTR and TNR-CFTR chloride channels

This study tested the hypotheses that the hypertonic environment of the renal medulla regulates the expression of cystic fibrosis transmembrane conductance regulator protein (CFTR) and its natural splice variant, TNR-CFTR. To accomplish this, Madin-Darby canine kidney (MDCK) stable cell lines expressing TNR-CFTR or CFTR were used. The cells were treated with hypertonic medium made with either NaCl or urea or sucrose (480 mOsm/kg or 560 mOsm/kg) to mimic the tonicity of the renal medulla environment. Western blot data showed that CFTR and TNR-CFTR total cell protein is increased by hypertonic medium, but using the surface biotinylation technique, only CFTR was found to be increased in cell plasma membrane. Confocal microscopy showed TNR-CFTR localization primarily at the endoplasmic reticulum and plasma membrane. In conclusion, CFTR and TNR-CFTR have different patterns of distribution in MDCK cells and they are modulated by a hypertonic environment, suggesting their physiological importance in renal medulla.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

A mathematical model of rat ascending Henle limb. II. Epithelial function

A mathematical model of ascending Henle limb (AHL) epithelium has been fashioned using kinetic representations of Na+-K+-2Cl- cotransporter (NKCC2), KCC4, and type 3 Na+/H+ exchanger (NHE3), with transporter densities selected to yield the reabsorptive Na+ flux expected for rat tubules in vivo. Of necessity, this model predicts fluxes that are higher than those measured in vitro. The kinetics of the NKCC and KCC are such that Na+ reabsorption by the model tubule is responsive to variation in luminal NaCl concentration over the range of 30 to 130 mM, with only minor changes in cell volume. Peritubular KCC accounts for about half the reabsorptive Cl- flux, with the remainder via peritubular Cl- channels. Transcellular Na+ flux is turned off by increasing peritubular KCl, which produces increased cytosolic Cl- and thus inhibits NKCC2 transport. In the presence of physiological concentrations of ammonia, there is a large acid challenge to the cell, due primarily to NH4+ entry via NKCC2, with diffusive NH3 exit to both lumen and peritubular solutions. When NHE3 density is adjusted to compensate this acid challenge, the model predicts luminal membrane proton secretion that is greater than the HCO3(-)-reabsorptive fluxes measured in vitro. The model also predicts luminal membrane ammonia cycling, with uptake via NKCC2 or K+ channel, and secretion either as NH4+ by NHE3 or as diffusive NH3 flux in parallel with a secreted proton. If such luminal ammonia cycling occurs in vivo, it could act in concert with luminal K+ cycling to facilitate AHL Na+ reabsorption via NKCC2. With physiological ammonia, peritubular KCl also blunts NHE3 activity by inhibiting NH4+ uptake on the Na-K-ATPase, and alkalinizing the cell.

Role of calcineurin-mediated dephosphorylation in modulation of an inwardly rectifying K+ channel in human proximal tubule cells

Activity of an inwardly rectifying K(+) channel with inward conductance of about 40 pS in cultured human renal proximal tubule epithelial cells (RPTECs) is regulated at least in part by protein phosphorylation and dephosphorylation. In this study, we examined involvement of calcineurin (CaN), a Ca(2+)/calmodulin (CaM)-dependent phosphatase, in modulating K(+) channel activity. In cell-attached mode of the patch-clamp technique, application of a CaN inhibitor, cyclosporin A (CsA, 5 microM) or FK520 (5 microM), significantly suppressed channel activity. Intracellular Ca(2+) concentration ([Ca(2+)]( i )) estimated by fura-2 imaging was elevated by these inhibitors. Since inhibition of CaN attenuates some dephosphorylation with increase in [Ca(2+)]( i ), we speculated that inhibiting CaN enhances Ca(2+)-dependent phosphorylation, which might result in channel suppression. To verify this hypothesis, we examined effects of inhibitors of PKC and Ca(2+)/CaM-dependent protein kinase-II (CaMKII) on CsA-induced channel suppression. Although the PKC inhibitor GF109203X (500 nM) did not influence the CsA-induced channel suppression, the CaMKII inhibitor KN62 (20 microM) prevented channel suppression, suggesting that the channel suppression resulted from CaMKII-dependent processes. Indeed, Western blot analysis showed that CsA increased phospho-CaMKII (Thr286), an activated CaMKII in inside-out patches, application of CaM (0.6 microM) and CaMKII (0.15 U/ml) to the bath at 10(-6) M Ca(2+) significantly suppressed channel activity, which was reactivated by subsequent application of CaN (800 U/ml). These results suggest that CaN plays an important role in supporting K(+) channel activity in RPTECs by preventing CaMKII-dependent phosphorylation.

POSH stimulates the ubiquitination and the clathrin-independent endocytosis of ROMK1 channels

POSH (plenty of SH3) is a scaffold protein that has been shown to act as an E3 ubiquitin ligase. Here we report that POSH stimulates the ubiquitination of Kir1.1 (ROMK) and enhances the internalization of this potassium channel. Immunostaining reveals the expression of POSH in the renal cortical collecting duct. Immunoprecipitation of renal tissue lysate with ROMK antibody and glutathione S-transferase pulldown experiments demonstrated the association between ROMK and POSH. Moreover, immunoprecipitation of lysates of HEK293T cells transfected with ROMK1 or with constructs encoding the ROMK-N terminus or ROMK1-C-Terminus demonstrated that POSH binds to ROMK1 on its N terminus. To study the effect of POSH on ROMK1 channels, we measured potassium currents with electrophysiological methods in HEK293T cells and in oocytes transfected or injected with ROMK1 and POSH. POSH decreased potassium currents, and the inhibitory effect of POSH on ROMK channels was dose-dependent. Biotinylation assay further showed that POSH decreased surface expression of ROMK channels in HEK293T cells transfected with ROMK1 and POSH. The effect of POSH on ROMK1 channels was specific because POSH did not inhibit sodium current in oocytes injected with ENaC-alpha, beta, and gamma subunits. Moreover, POSH still decreased the potassium current in oocytes injected with a ROMK1 mutant (R1Delta373-378), in which a clathrin-dependent tyrosine-based internalization signal residing between amino acid residues 373 and 378 is deleted. However, the inhibitory effect of POSH on ROMK channels was absent in cells expressing with dominant negative dynamin and POSHDeltaRING, in which the RING domain was deleted. Expression of POSH also increased the ubiquitination of ROMK1, whereas expression of POSHDeltaRING diminished its ubiquitination in HEK293T cells. The notion that POSH may serve as an E3 ubiquitin ligase is also supported by in vitro ubiquitination assays in which adding POSH increased the ROMK ubiquitination. We conclude that POSH inhibits ROMK channels by enhancing dynamin-dependent and clathrin-independent endocytosis and by stimulating ubiquitination of ROMK channels.

A comprehensive guide to the ROMK potassium channel: form and function in health and disease

The discovery of the renal outer medullary K+ channel (ROMK, K(ir)1.1), the founding member of the inward-rectifying K+ channel (K(ir)) family, by Ho and Hebert in 1993 revolutionized our understanding of potassium channel biology and renal potassium handling. Because of the central role that ROMK plays in the regulation of salt and potassium homeostasis, considerable efforts have been invested in understanding the underlying molecular mechanisms. Here we provide a comprehensive guide to ROMK, spanning from the physiology in the kidney to the organization and regulation by intracellular factors to the structural basis of its function at the atomic level.

CFTR structure and function: is there a role in the kidney?

Cystic fibrosis (CF) is a lethal autosomal recessive genetic disease caused by mutations in the CF transmembrane conductance regulator (CFTR). Mutations in the CFTR gene may result in a defective protein processing that leads to changes in function and regulation of this chloride channel. Despite of the expression of CFTR in the kidney, patients with CF do not present major renal dysfunction, but it is known that both the urinary excretion of proteins and renal capacity to concentrate and dilute urine are altered in these patients. CFTR mRNA is expressed in all nephron segments of rat and human, and this abundance is more prominent in renal cortex and outer medulla renal areas. CFTR protein was detected in apical surface of both proximal and distal tubules of rat kidney but not in the outer medullary collecting ducts. Studies have demonstrated that CFTR does not only transport Cl- but also ATP. ATP transport by CFTR could be involved in the control of other ion transporters such as Na+ (ENaC) and K+ (renal outer medullary potassium) channels, especially in TAL and CCD. In the kidney, CFTR also might be involved in the endocytosis of low-molecular-weight proteins by proximal tubules. This review is focused on the CFTR function and structure, its role in the renal physiology, and its modulation by hormones involved in the control of extracellular fluid volume.

Female ROMK null mice manifest more severe Bartter II phenotype on renal function and higher PGE2 production

ROMK null mice with a high survival rate and varying severity of hydronephrosis provide a good model to study type II Bartter syndrome pathophysiology (26). During the development of such a colony, we found that more male than female null mice survived, 58.7% vs. 33.3%. To investigate the possible mechanism of this difference, we compared the survival rates, renal functions, degree of hydronephrosis, as well as PGE(2) and TXB(2) production between male and female ROMK wild-type and null mice. We observed that female ROMK Bartter's mice exhibited lower GFR (0.37 vs. 0.54 ml.min(-1).100 g BW(-1), P < 0.05) and higher fractional Na(+) excretion (0.66% vs. 0.48%, P < 0.05) than male Bartter's. No significant differences in acid-base parameters, urinary K(+) excretion, and plasma electrolyte concentrations were observed between sexes. In addition, we assessed the liquid retention rate in the kidney to evaluate the extent of hydronephrosis and observed that 67% of male and 90% of female ROMK null mice were hydronephrotic mice. Urinary PGE(2) excretion was higher in both sexes of ROMK null mice: 1.35 vs. 1.10 ng/24 h in males and 2.90 vs. 0.87 ng/24 h in females. TXB(2) excretion was higher in female mice in both wild-type and ROMK null mice. The increments of urinary PGE(2) and TXB(2) were significantly higher in female null mice than males, 233.33% vs. 22.74% of PGE(2) and 85.67% vs. 20.36% of TXB(2). These data demonstrate a more severe Bartter phenotype in female ROMK null mice, and higher PGE(2) and TXB(2) production may be one of the mechanisms of this manifestation.

Molecular physiology of the WNK kinases

Mutations in the serine-threonine kinases WNK1 and WNK4 cause a Mendelian disease featuring hypertension and hyperkalemia. In vitro and in vivo studies have revealed that these proteins are molecular switches that have discrete functional states that impart different effects on downstream ion channels, transporters, and the paracellular pathway. These effects enable the distal nephron to allow either maximal NaCl reabsorption or maximal K+ secretion in response to hypovolemia or hyperkalemia, respectively. The related kinase WNK3 has reciprocal actions on the primary mediators of cellular Cl(-) influx and efflux, effects that can serve to regulate cell volume during growth and in response to osmotic stress as well as to modulate neuronal responses to GABA. These findings define a versatile new family of kinases that coordinate the activities of diverse ion transport pathways to achieve and maintain fluid and electrolyte homeostasis.

Arachidonic acid inhibits basolateral K channels in the cortical collecting duct via cytochrome P-450 epoxygenase-dependent metabolic pathways

We used the patch-clamp technique to study the effect of arachidonic acid (AA) on basolateral 18-pS K channels in the principal cell of the cortical collecting duct (CCD) of the rat kidney. Application of AA inhibited the 18-pS K channels in a dose-dependent manner and 10 microM AA caused a maximal inhibition. The effect of AA on the 18-pS K channel was specific because application of 11,14,17-eicosatrienoic acid had no effect on channel activity. Also, the inhibitory effect of AA on the 18-pS K channels was abolished by blocking cytochrome P-450 (CYP) epoxygenase with N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide (MS-PPOH) but was not affected by inhibiting CYP omega-hydroxylase or cyclooxygenase. The notion that the inhibitory effect of AA was mediated by CYP epoxygenase-dependent metabolites was further supported by the observation that application of 100 nM 11,12-epoxyeicosatrienoic acid (EET) mimicked the effect of AA and inhibited the basolateral 18-pS K channels. In contrast, addition of either 5,6-, 8,9-, or 14,15-EET failed to inhibit the 18-pS K channels. Moreover, application of 11,12-EET was still able to inhibit the 18-pS K channels in the presence of MS-PPOH. This suggests that 11,12-EET is a mediator for the AA-induced inhibition of the 18-pS K channels. We conclude that AA inhibits basolateral 18-pS K channels by a CYP epoxygenase-dependent pathway and that 11,12-EET is a mediator for the effect of AA on basolateral K channels in the CCD.

A novel protein kinase signaling pathway essential for blood pressure regulation in humans

The discovery that mutations in WNK4 [encoding a member of the WNK family - so named because of the unique substitution of cysteine for lysine at a nearly invariant residue within subdomain II of its catalytic core: with no K (lysine)] cause pseudohypoaldosteronism type II, an autosomal dominant form of human hypertension, provided the initial clue that this serine/threonine kinase is a crucial part of a complex renal salt regulatory system. Recent findings from physiological studies of WNK4 in Xenopus laevis oocytes, mammalian cell systems and in vivo in mouse models have provided novel insights into the mechanisms by which the kidney regulates salt homeostasis, and therefore blood pressure, downstream of aldosterone signaling in mammals. The current evidence supports a model in which WNK4 coordinates the activities of diverse aldosterone-sensitive mediators of ion transport in the distal nephron to promote normal homeostasis in response to physiological perturbation.

Characterization of a long-term rat mTAL cell line

A medullary thick ascending limb (mTAL) cell line, termed raTAL, has been established from freshly isolated rat mTAL tubules and cultured continuously for up to 75 passages; it retains characteristics of mTAL cells even after retrieval from storage in liquid nitrogen for several months. The cells express Tamm-Horsfall glycoprotein (THP), a TAL-specific marker, grow to confluence, exhibit a polygonal morphology characteristic of epithelial cells, and form “domes.” Detection of THP, Na+-K+-2Cl−cotransporter (NKCC2), Na+-K+-ATPase, and renal outer medullary K+channel (ROMK) was achieved using indirect immunofluorescence and confocal microscopy. Western blot analysis of NKCC2 expression using two different antibodies revealed a band of ∼160 kDa, and RT-PCR analysis demonstrated the presence of NKCC2 isoforms A and F, which was confirmed by DNA sequencing; transport of Cl−into raTAL cells was inhibited by furosemide. Ouabain- and bumetanide-sensitive oxygen consumption, an index of ion transport activity in the mTAL, was observed in raTAL cells, and the number of domes present was reduced significantly when cells were incubated in the presence of ouabain or bumetanide. The specific activity of Na+-K+-ATPase activity was determined in raTAL cells (0.67 ± 0.18 nmol Pi·μg protein−1·min−1), primary cultures of mTAL cells (0.39 ± 0.08 nmol Pi·μg protein−1·min−1), and freshly isolated mTAL tubules (1.10 ± 0.29 nmol Pi·μg protein−1·min−1), and ∼30–50% of total cellular ATPase activity was inhibited by ouabain, in accord with other mTAL preparations. This cell line will be used in studies that address biochemical, molecular, and physiological mechanisms in the mTAL.

Inhibition of MAPK stimulates the Ca2+ -dependent big-conductance K channels in cortical collecting duct

The kidney plays a key role in maintaining potassium (K) homeostasis. K excretion is determined by the balance between K secretion and absorption in distal tubule segments such as the connecting tubule and cortical collecting duct. K secretion takes place by K entering principal cells (PC) from blood side through Na+, K+ -ATPase and being secreted into the lumen via both ROMK-like small-conductance K (SK) channels and Ca2+ -activated big-conductance K (BK) channels. K reabsorption occurs by stimulation of apical K/H-ATPase and inhibition of K recycling across the apical membrane in intercalated cells (IC). The role of ROMK channels in K secretion is well documented. However, the importance of BK channels in mediating K secretion is incompletely understood. It has been shown that their activity increases with high tubule flow rate and augmented K intake. However, BK channels have a low open probability and are mainly located in IC, which lack appropriate transporters for effective K secretion. Here we demonstrate that inhibition of ERK and P38 MAPKs stimulates BK channels in both PC and IC in the cortical collecting duct and that changes in K intake modulate their activity. Under control conditions, BK channel activity in PC was low but increased significantly by inhibition of both ERK and P38. Blocking MAPKs also increased channel open probability of BK in IC and thereby it may affect K backflux and net K absorption Thus, modulation of ERK and P38 MAPK activity is involved in controlling net K secretion in the distal nephron.

Renal nucleoside transporters: physiological and clinical implicationsThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Renal handling of physiological and pharmacological nucleosides is a major determinant of their plasma levels and tissue availabilities. Additionally, the pharmacokinetics and normal tissue toxicities of nucleoside drugs are influenced by their handling in the kidney. Renal reabsorption or secretion of nucleosides is selective and dependent on integral membrane proteins, termed nucleoside transporters (NTs) present in renal epithelia. The 7 known human NTs (hNTs) exhibit varying permeant selectivities and are divided into 2 protein families: the solute carrier (SLC) 29 (SLC29A1, SLC29A2, SLC29A3, SLC29A4) and SLC28 (SLC28A1, SLC28A2, SLC28A3) proteins, otherwise known, respectively, as the human equilibrative NTs (hENTs, hENT1, hENT2, hENT3, hENT4) and human concentrative NTs (hCNTs, hCNT1, hCNT2, hCNT3). The well characterized hENTs (hENT1 and hENT2) are bidirectional facilitative diffusion transporters in plasma membranes; hENT3 and hENT4 are much less well known, although hENT3, found in lysosomal membranes, transports nucleosides and is pH dependent, whereas hENT4–PMAT is a H+-adenosine cotransporter as well as a monoamine–organic cation transporter. The 3 hCNTs are unidirectional secondary active Na+-nucleoside cotransporters. In renal epithelial cells, hCNT1, hCNT2, and hCNT3 at apical membranes, and hENT1 and hENT2 at basolateral membranes, apparently work in concert to mediate reabsorption of nucleosides from lumen to blood, driven by Na+ gradients. Secretion of some physiological nucleosides, therapeutic nucleoside analog drugs, and nucleotide metabolites of therapeutic nucleoside and nucleobase drugs likely occurs through various xenobiotic transporters in renal epithelia, including organic cation transporters, organic anion transporters, multidrug resistance related proteins, and multidrug resistance proteins. Mounting evidence suggests that hENT1 may have a presence at both apical and basolateral membranes of renal epithelia, and thus may participate in both selective secretory and reabsorptive fluxes of nucleosides. In this review, the renal handling of nucleosides is examined with respect to physiological and clinical implications for the regulation of human kidney NTs and adenosine signaling, intracellular nucleoside transport, and nephrotoxicities associated with some nucleoside drugs.

High-conductance K channels in intercalated cells of the rat distal nephron

High-conductance (BK or maxi) K(+) channels were observed in cell-attached patches of the apical membrane of the isolated split-open rat connecting tubule (CNT). These channels were quite rare in cells identified visually as principal cells (PCs; 5/162 patches) but common in intercalated cells (ICs; 24/26 patches). The BK-expressing intercalated cells in the CNT and cortical collecting duct (CCD) were characterized by a low membrane potential (-36 mV) under short-circuit conditions, measured from the reversal potential of the channel currents with similar K(+) concentrations on both sides of the membrane. Under whole-cell clamp conditions with low intracellular Ca(2+), ICs had a very low K(+) conductance. When cell Ca(2+) was increased to 200 nM, a voltage-dependent, tetraethylammonium (TEA)-sensitive outward conductance was activated with a limiting value of 90 and 140 nS/cell in the CNT and CCD, respectively. Feeding animals a high-K diet for 1 wk did not increase these currents. TEA-sensitive currents were much smaller in PCs and usually below detection limits. To examine the possibility that the ICs participate in transepithelial K(+) secretion, we measured Na/K pump activity as a ouabain-sensitive current. Although these currents were easily observed in PCs, averaging 79 +/- 14 and 250 +/- 50 pA/cell in the CCD and CNT, respectively, they were below the level of detection in the ICs. We conclude that ICs have BK channel densities that are sufficient to support renal secretion of K(+) if cell Ca(2+) is elevated. However. a pathway for K(+) entry into these cells has not been identified.

Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions

Glia in the central nervous system (CNS) express diverse inward rectifying potassium channels (Kir). The major function of Kir is in establishing the high potassium (K+) selectivity of the glial cell membrane and strongly negative resting membrane potential (RMP), which are characteristic physiological properties of glia. The classical property of Kir is that K+ flows inwards when the RMP is negative to the equilibrium potential for K+ (E(K)), but at more positive potentials outward currents are inhibited. This provides the driving force for glial uptake of K+ released during neuronal activity, by the processes of "K+ spatial buffering" and "K+ siphoning", considered a key function of astrocytes, the main glial cell type in the CNS. Glia express multiple Kir channel subtypes, which are likely to have distinct functional roles related to their differences in conductance, and sensitivity to intracellular and extracellular factors, including pH, ATP, G-proteins, neurotransmitters and hormones. A feature of CNS glia is their specific expression of the Kir4.1 subtype, which is a major K+ conductance in glial cell membranes and has a key role in setting the glial RMP. It is proposed that Kir4.1 have a primary function in K+ regulation, both as homomeric channels and as heteromeric channels by co-assembly with Kir5.1 and probably Kir2.0 subtypes. Significantly, Kir4.1 are also expressed by oligodendrocytes, the myelin-forming cells of the CNS, and the genetic ablation of Kir4.1 results in severe hypomyelination. Hence, Kir, and in particular Kir4.1, are key regulators of glial functions, which in turn determine neuronal excitability and axonal conduction.