Sensitivity of a renal K+ channel (ROMK2) to the inhibitory sulfonylurea compound glibenclamide is enhanced by coexpression with the ATP-binding cassette transporter cystic fibrosis transmembrane regulator

Interaction of ROMK2 channel with lipid kinases DGKE and AGK: Potential channel activation by localized anionic lipid synthesis

In this study, we utilized enzyme-catalyzed proximity labeling with the engineered promiscuous biotin ligase Turbo-ID to identify the proxisome of the ROMK2 channel. This channel resides in various cellular membrane compartments of the cell including the plasma membrane, endoplasmic reticulum and mitochondria. Within mitochondria, ROMK2 has been suggested as a pore-forming subunit of mitochondrial ATP-regulated potassium channel (mitoKATP). We found that ROMK2 proxisome in addition to previously known protein partners included two lipid kinases: acylglycerol kinase (AGK) and diacylglycerol kinase ε (DGKE), which are localized in mitochondria and the endoplasmic reticulum, respectively. Through co-immunoprecipitation, we confirmed that these two kinases are present in complexes with ROMK2 channels. Additionally, we found that the products of AGK and DGKE, lysophosphatidic acid (LPA) and phosphatidic acid (PA), stimulated the activity of ROMK2 channels in artificial lipid bilayers. Our molecular docking studies revealed the presence of acidic lipid binding sites in the ROMK2 channel, similar to those previously identified in Kir2 channels. Based on these findings, we propose a model wherein localized lipid synthesis, mediated by channel-bound lipid kinases, contributes to the regulation of ROMK2 activity within distinct intracellular compartments, such as mitochondria and the endoplasmic reticulum.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Calumenin contributes to ER-Ca2+ homeostasis in bronchial epithelial cells expressing WT and F508del mutated CFTR and to F508del-CFTR retention

Cystic Fibrosis (CF) is the most frequent fatal genetic disease in Caucasian populations. Mutations in the chloride channel CF Transmembrane Conductance Regulator (CFTR) gene are responsible for functional defects of the protein and multiple associated dysregulations. The most common mutation in patients with CF, F508del-CFTR, causes defective CFTR protein folding. Thus minimal levels of the receptor are expressed at the cell surface as the mutated CFTR is retained in the endoplasmic reticulum (ER) where it correlates with defective calcium (Ca²⁺) homeostasis. In this study, we discovered that the Ca²⁺ binding protein Calumenin (CALU) is a key regulator in the maintenance of ER-Ca²⁺ calcium homeostasis in both wild type and F508del-CFTR expressing cells. Calumenin modulates SERCA pump activity without drastically affecting ER-Ca²⁺ concentration. In addition, reducing Calumenin expression in CF cells results in a partial restoration of CFTR activity, highlighting a potential function of Calumenin in CFTR maturation. These findings demonstrate a pivotal role for Calumenin in CF cells, providing insights into how modulation of Calumenin expression or activity may be used as a potential therapeutic tool to correct defects in F508del-CFTR.

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Uses its C-Terminus to Regulate the A2B Adenosine Receptor

CFTR is an apical membrane anion channel that regulates fluid homeostasis in many organs including the airways, colon, pancreas and sweat glands. In cystic fibrosis, CFTR dysfunction causes significant morbidity/mortality. Whilst CFTR's function as an ion channel has been well described, its ability to regulate other proteins is less understood. We have previously shown that plasma membrane CFTR increases the surface density of the adenosine 2B receptor (A2BR), but not of the β2 adrenergic receptor (β2AR), leading to an enhanced, adenosine-induced cAMP response in the presence of CFTR. In this study, we have found that the C-terminal PDZ-domain of both A2BR and CFTR were crucial for this interaction, and that replacing the C-terminus of A2BR with that of β2AR removed this CFTR-dependency. This observation extended to intact epithelia and disruption of the actin cytoskeleton prevented A2BR-induced but not β2AR-induced airway surface liquid (ASL) secretion. We also found that CFTR expression altered the organization of the actin cytoskeleton and PDZ-binding proteins in both HEK293T cells and in well-differentiated human bronchial epithelia. Furthermore, removal of CFTR's PDZ binding motif (ΔTRL) prevented actin rearrangement, suggesting that CFTR insertion in the plasma membrane results in local reorganization of actin, PDZ binding proteins and certain GPCRs.

Investigating CFTR and KCa3.1 Protein/Protein Interactions

In epithelia, Cl^- channels play a prominent role in fluid and electrolyte transport. Of particular importance is the cAMP-dependent cystic fibrosis transmembrane conductance regulator Cl^- channel (CFTR) with mutations of the CFTR encoding gene causing cystic fibrosis. The bulk transepithelial transport of Cl^- ions and electrolytes needs however to be coupled to an increase in K⁺ conductance in order to recycle K⁺ and maintain an electrical driving force for anion exit across the apical membrane. In several epithelia, this K⁺ efflux is ensured by K⁺ channels, including KCa3.1, which is expressed at both the apical and basolateral membranes. We show here for the first time that CFTR and KCa3.1 can physically interact. We first performed a two-hybrid screen to identify which KCa3.1 cytosolic domains might mediate an interaction with CFTR. Our results showed that both the N-terminal fragment M1-M40 of KCa3.1 and part of the KCa3.1 calmodulin binding domain (residues L345-A400) interact with the NBD2 segment (G1237-Y1420) and C- region of CFTR (residues T1387-L1480), respectively. An association of CFTR and F508del-CFTR with KCa3.1 was further confirmed in co-immunoprecipitation experiments demonstrating the formation of immunoprecipitable CFTR/KCa3.1 complexes in CFBE cells. Co-expression of KCa3.1 and CFTR in HEK cells did not impact CFTR expression at the cell surface, and KCa3.1 trafficking appeared independent of CFTR stimulation. Finally, evidence is presented through cross-correlation spectroscopy measurements that KCa3.1 and CFTR colocalize at the plasma membrane and that KCa3.1 channels tend to aggregate consequent to an enhanced interaction with CFTR channels at the plasma membrane following an increase in intracellular Ca²⁺ concentration. Altogether, these results suggest 1) that the physical interaction KCa3.1/CFTR can occur early during the biogenesis of both proteins and 2) that KCa3.1 and CFTR form a dynamic complex, the formation of which depends on internal Ca²⁺.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

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.

Cystic fibrosis transmembrane conductance regulator in the endolymphatic sac of the rat

OBJECTIVE

Na(+) and Cl(-) are dominant ions in the endolymphatic fluid in the endolymphatic sac and are important for volume regulation in the endolymphatic sac. An epithelial sodium channel (ENaC) and other Na(+) transporters have been identified in the endolymphatic sac epithelia, and they are involved in the regulation of endolymph. Although the presence of Cl(-) channels in the endolymphatic sac epithelia has been speculated, no Cl(-) channels have been identified. In this study, we confirmed the expression of cystic fibrosis transmembrane conductance regulator (CFTR) in the endolymphatic sac by reverse transcriptase polymerase chain reaction (RT-PCR) and by immunohistochemical staining.

METHODS

Pure mRNA from endolymphatic sac epithelia was prepared using laser capture microdissection (LCM) and examined using RT-PCR. Localization of CFTR and ENaC in the endolymphatic sac was examined using immunohistochemistry.

RESULTS

mRNA of the CFTR was expressed in the endolymphatic sac. Immunohistochemical analysis showed the expression of the CFTR on apical side of the endolymphatic sac epithelia and co-localization with the ENaC.

CONCLUSION

RT-PCR and immunohistochemistry were used to identify the expression of CFTR in the endolymphatic sac epithelia, which gives us a clue for understanding Cl(-) transport in the endolymphatic sac. These results suggest a pathway for Cl(-), possibly through interaction with the ENaC, which may regulate the endolymph in the endolymphatic sac.

In vitro analysis of PDZ-dependent CFTR macromolecular signaling complexes

Cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel located primarily at the apical membranes of epithelial cells, plays a crucial role in transepithelial fluid homeostasis(1-3). CFTR has been implicated in two major diseases: cystic fibrosis (CF)(4) and secretory diarrhea(5). In CF, the synthesis or functional activity of the CFTR Cl- channel is reduced. This disorder affects approximately 1 in 2,500 Caucasians in the United States(6). Excessive CFTR activity has also been implicated in cases of toxin-induced secretory diarrhea (e.g., by cholera toxin and heat stable E. coli enterotoxin) that stimulates cAMP or cGMP production in the gut(7). Accumulating evidence suggest the existence of physical and functional interactions between CFTR and a growing number of other proteins, including transporters, ion channels, receptors, kinases, phosphatases, signaling molecules, and cytoskeletal elements, and these interactions between CFTR and its binding proteins have been shown to be critically involved in regulating CFTR-mediated transepithelial ion transport in vitro and also in vivo(8-19). In this protocol, we focus only on the methods that aid in the study of the interactions between CFTR carboxyl terminal tail, which possesses a protein-binding motif [referred to as PSD95/Dlg1/ZO-1 (PDZ) motif], and a group of scaffold proteins, which contain a specific binding module referred to as PDZ domains. So far, several different PDZ scaffold proteins have been reported to bind to the carboxyl terminal tail of CFTR with various affinities, such as NHERF1, NHERF2, PDZK1, PDZK2, CAL (CFTR-associated ligand), Shank2, and GRASP(20-27). The PDZ motif within CFTR that is recognized by PDZ scaffold proteins is the last four amino acids at the C terminus (i.e., 1477-DTRL-1480 in human CFTR)(20). Interestingly, CFTR can bind more than one PDZ domain of both NHERFs and PDZK1, albeit with varying affinities(22). This multivalency with respect to CFTR binding has been shown to be of functional significance, suggesting that PDZ scaffold proteins may facilitate formation of CFTR macromolecular signaling complexes for specific/selective and efficient signaling in cells(16-18). Multiple biochemical assays have been developed to study CFTR-involving protein interactions, such as co-immunoprecipitation, pull-down assay, pair-wise binding assay, colorimetric pair-wise binding assay, and macromolecular complex assembly assay(16-19,28,29). Here we focus on the detailed procedures of assembling a PDZ motif-dependent CFTR-containing macromolecular complex in vitro, which is used extensively by our laboratory to study protein-protein or domain-domain interactions involving CFTR(16-19,28,29).

Cystic fibrosis: insight into CFTR pathophysiology and pharmacotherapy

Cystic fibrosis is the most common life-threatening recessively inherited disease in Caucasians. Due to early provision of care in specialized reference centers and more comprehensive care, survival has improved over time. Despite great advances in supportive care and in our understanding of its pathophysiology, there is still no cure for the disease. Therapeutic strategies aimed at rescuing the abnormal protein are either being sought after or under investigation. This review highlights salient insights into pathophysiology and candidate molecules suitable for CFTR pharmacotherapy. Clinical trials using Ataluren, VX-809 and ivacaftor have provided encouraging data. Preclinical data with inhibitors of phosphodiesterase type 5, such as sildenafil and analogs, have highlighted their potential for CFTR pharmacotherapy. Because sildenafil and analogs are in clinical use for other clinical applications, research on this class of drugs might speed up the development of new therapies for CF.

Current status of inflammasome blockers as anti-inflammatory drugs

INTRODUCTION

The inflammasomes have emerged as key mediators of inflammation and immunity, yet clinical application of this knowledge has been limited by a lack of specific and drug-like antagonists. Recent studies using inflammasome knockout mice have shown that different inflammasomes control immunity in different pathologies. Drug-like antagonists acting up- or down-stream of the inflammasome pathway have been successfully used in clinics as important therapeutics to treat different inflammatory diseases.

AREAS COVERED

The current literature has been reviewed on the role of inflammasomes in inflammatory disease, focusing on potential therapeutic applications of selective inflammasome antagonists as anti-inflammatory agents. Particular emphasis has been placed on the potential role of the different inflammasomes in common inflammatory diseases. The latest clinical developments for drugs targeting inflammasome pathways are covered.

EXPERT OPINION

Recent studies using inflammasome knockout mice suggest its importance as a potential therapeutic target for the treatment of inflammatory disease. However, efficacious antagonists for the inflammasome for use in clinical studies are still at an early stage of development. Developing selective inflammasome antagonists is a challenge that if met, offers promise for the treatment of chronic inflammatory diseases. Major developments in this area will include the identification of reliable high-throughput screening methods for compounds directly targeting inflammasome assembly.

Functional interaction between CFTR and the sodium-phosphate co-transport type 2a in Xenopus laevis oocytes

Background

A growing number of proteins, including ion transporters, have been shown to interact with Cystic Fibrosis Transmembrane conductance Regulator (CFTR). CFTR is an epithelial chloride channel that is involved in Cystic Fibrosis (CF) when mutated; thus a better knowledge of its functional interactome may help to understand the pathophysiology of this complex disease. In the present study, we investigated if CFTR and the sodium-phosphate co-transporter type 2a (NPT2a) functionally interact after heterologous expression of both proteins in Xenopus laevis oocytes.

Methodology/Findings

NPT2a was expressed alone or in combination with CFTR in X. laevis oocytes. Using the two-electrode voltage-clamp technique, the inorganic phosphate-induced current (IPi) was measured and taken as an index of NPT2a activity. The maximal IPi for NPT2a substrates was reduced when CFTR was co-expressed with NPT2a, suggesting a decrease in its expression at the oolemna. This was consistent with Western blot analysis showing reduced NPT2a plasma membrane expression in oocytes co-expressing both proteins, whereas NPT2a protein level in total cell lysate was the same in NPT2a- and NPT2a+CFTR-oocytes. In NPT2a+CFTR- but not in NPT2a-oocytes, IPi and NPT2a surface expression were increased upon PKA stimulation, whereas stimulation of Exchange Protein directly Activated by cAMP (EPAC) had no effect. When NPT2a-oocytes were injected with NEG2, a short amino-acid sequence from the CFTR regulatory domain that regulates PKA-dependent CFTR trafficking to the plasma membrane, IPi values and NPT2a membrane expression were diminished, and could be enhanced by PKA stimulation, thereby mimicking the effects of CFTR co-expression.

Conclusion/Perspectives

We conclude that when both CFTR and NPT2a are expressed in X. laevis oocytes, CFTR confers to NPT2a a cAMPi-dependent trafficking to the membrane. This functional interaction raises the hypothesis that CFTR may play a role in phosphate homeostasis.

The glycolytic enzymes glyceraldehyde 3-phosphate dehydrogenase and enolase interact with the renal epithelial K+ channel ROMK2 and regulate its function

BACKGROUND/AIMS

ROMK channels mediate potassium secretion and regulate NaCl reabsorption in the kidney. The aim was to study the functional implications of the interaction between ROMK2 (Kir1.1b) and two glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and enolase-α, which were identified as potential regulatory subunits of the channel complex.

METHODS

We performed a membrane yeast-two-hybrid screen of a human kidney cDNA library with ROMK2 as a bait. Interaction of ROMK2 with GAPDH and enolase was verified using GST pull-down, co-immunoprecipitation, immunohistochemistry and co-expression in Xenopus oocytes.

RESULTS

Confocal imaging showed co-localisation of enolase and GAPDH with ROMK2 in the apical membrane of the renal epithelial cells of the thick ascending limb. Over-expression of GAPDH or enolase-α in Xenopus oocytes markedly reduced the amplitude of ROMK2 currents but did not affect the surface expression of the channels. Co-expression of the glycolytically inactive GAPDH mutant C149G did not have any effect on ROMK2 current amplitude.

CONCLUSION

Our results suggest that the glycolytic enzymes GAPDH and enolase are part of the ROMK2 channel supramolecular complex and may serve to couple salt reabsorption in the thick ascending limb of the loop of Henle to the metabolic status of the renal epithelial cells.

Differential contribution of TM6 and TM12 to the pore of CFTR identified by three sulfonylurea-based blockers

Previous studies suggested that four transmembrane domains 5, 6, 11, 12 make the greatest contribution to forming the pore of the CFTR chloride channel. We used excised, inside-out patches from oocytes expressing CFTR with alanine-scanning mutagenesis in amino acids in TM6 and TM12 to probe CFTR pore structure with four blockers: glibenclamide (Glyb), glipizide (Glip), tolbutamide (Tolb), and Meglitinide. Glyb and Glip blocked wildtype (WT)-CFTR in a voltage-, time-, and concentration-dependent manner. At V (M) = -120 mV with symmetrical 150 mM Cl(-) solution, fractional block of WT-CFTR by 50 μM Glyb and 200 μM Glip was 0.64 ± 0.03 (n = 7) and 0.48 ± 0.02 (n = 7), respectively. The major effects on block by Glyb and Glip were found with mutations at F337, S341, I344, M348, and V350 of TM6. Under similar conditions, fractional block of WT-CFTR by 300 μM Tolb was 0.40 ± 0.04. Unlike Glyb, Glip, and Meglitinide, block by Tolb lacked time-dependence (n = 7). We then tested the effects of alanine mutations in TM12 on block by Glyb and Glip; the major effects were found at N1138, T1142, V1147, N1148, S1149, S1150, I1151, and D1152. From these experiments, we infer that amino acids F337, S341, I344, M348, and V350 of TM6 face the pore when the channel is in the open state, while the amino acids of TM12 make less important contributions to pore function. These data also suggest that the region between F337 and S341 forms the narrow part of the CFTR pore.

Glyburide is anti-inflammatory and associated with reduced mortality in melioidosis

Patients with diabetes have better survival from septic melioidosis than patients who without diabetes. This difference was seen only in patients taking glyburide prior to presentation and was associated with an anti-inflammatory effect of glyburide.

Background. Patients with diabetes mellitus are more prone to bacterial sepsis, but there are conflicting data on whether outcomes are worse in diabetics after presentation with sepsis. Glyburide is an oral hypoglycemic agent used to treat diabetes mellitus. This K_ATP-channel blocker and broad-spectrum ATP-binding cassette (ABC) transporter inhibitor has broad-ranging effects on the immune system, including inhibition of inflammasome assembly and would be predicted to influence the host response to infection.

Methods. We studied a cohort of 1160 patients with gram-negative sepsis caused by a single pathogen (Burkholderia pseudomallei), 410 (35%) of whom were known to have diabetes. We subsequently studied prospectively diabetics with B. pseudomallei infection (n = 20) to compare the gene expression profile of peripheral whole blood leukocytes in patients who were taking glyburide against those not taking any sulfonylurea.

Results. Survival was greater in diabetics than in nondiabetics (38% vs 45%, respectively, P = .04), but the survival benefit was confined to the patient group taking glyburide (adjusted odds ratio .47, 95% confidence interval .28–.74, P = .005). We identified differential expression of 63 immune-related genes (P = .001) in patients taking glyburide, the sum effect of which we predict to be antiinflammatory in the glyburide group.

Conclusions. We present observational evidence for a glyburide-associated benefit during human melioidosis and correlate this with an anti-inflammatory effect of glyburide on the immune system.

Analysis of CFTR interactome in the macromolecular complexes

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel localized primarily at the apical surface of epithelial cells lining the airway, gut, exocrine glands, etc., where it is responsible for transepithelial salt and water transport. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might regulate the activities of other ion channels, receptors, and transporters, in addition to its role as a chloride conductor. Most interactions occur primarily between the opposing terminal tails (N or C) of CFTR and its binding partners, either directly or mediated through various PDZ domain-containing proteins. This chapter describes methods we developed to cross-link CFTR into a macromolecular complex to identify and analyze the assembly and regulation of CFTR-containing complexes in the plasma membrane.

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.

Mouse cystic fibrosis transmembrane conductance regulator forms cAMP-PKA-regulated apical chloride channels in cortical collecting duct

The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in many segments of the mammalian nephron, where it may interact with and modulate the activity of a variety of apical membrane proteins, including the renal outer medullary potassium (ROMK) K(+) channel. However, the expression of CFTR in apical cell membranes or its function as a Cl(-) channel in native renal epithelia has not been demonstrated. Here, we establish that CFTR forms protein kinase A (PKA)-activated Cl(-) channels in the apical membrane of principal cells from the cortical collecting duct obtained from mice. These Cl(-) channels were observed in cell-attached apical patches of principal cells after stimulation by forskolin/3-isobutyl-1-methylxanthine. Quiescent Cl(-) channels were present in patches excised from untreated tubules because they could be activated after exposure to Mg-ATP and the catalytic subunit of PKA. The single-channel conductance, kinetics, and anion selectivity of these Cl(-) channels were the same as those of recombinant mouse CFTR channels expressed in Xenopus laevis oocytes. The CFTR-specific closed-channel blocker CFTR(inh)-172 abolished apical Cl(-) channel activity in excised patches. Moreover, apical Cl(-) channel activity was completely absent in principal cells from transgenic mice expressing the DeltaF508 CFTR mutation but was present and unaltered in ROMK-null mice. We discuss the physiologic implications of open CFTR Cl(-) channels on salt handling by the collecting duct and on the functional CFTR-ROMK interactions in modulating the metabolic ATP-sensing of ROMK.

CFTR chloride channel in the apical compartments: spatiotemporal coupling to its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel located primarily at the apical or luminal surfaces of epithelial cells in the airway, intestine, pancreas, kidney, sweat gland, as well as male reproductive tract, where it plays a crucial role in transepithelial fluid homeostasis. CFTR dysfunction can be detrimental and may result in life-threatening disorders. CFTR hypofunctioning because of genetic defects leads to cystic fibrosis, the most common lethal genetic disease in Caucasians, whereas CFTR hyperfunctioning resulting from various infections evokes secretory diarrhea, the leading cause of mortality in early childhood. Therefore, maintaining a dynamic balance between CFTR up-regulating processes and CFTR down-regulating processes is essential for maintaining fluid and body homeostasis. Accumulating evidence suggests that protein-protein interactions play a critical role in the fine-tuned regulation of CFTR function. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might be coupled spatially and temporally to a wide variety of interacting partners including ion channels, receptors, transporters, scaffolding proteins, enzyme molecules, signaling molecules, and effectors. Most interactions occur primarily between the opposing terminal tails (amino or carboxyl) of CFTR protein and its binding partners, either directly or mediated through various PDZ scaffolding proteins. These dynamic interactions impact the channel function, as well as localization and processing of CFTR protein within cells. This article reviews the most recent progress and findings about the interactions between CFTR and its binding partners through PDZ scaffolding proteins, as well as the spatiotemporal regulation of CFTR-containing macromolecular signaling complexes in the apical compartments of polarized cells lining the secretory epithelia.

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.

Cl transport in complemented CF bronchial epithelial cells correlates with CFTR mRNA expression levels

Little is known about the relationship between CF transmembrane conductance regulator (CFTR) gene expression and the corresponding transport of Cl. The phenotypic characteristics of polarized DeltaF508 homozygote CF bronchial epithelial (CFBE41o-) cells were evaluated following transfection with episomal expression vector containing either full-length (6.2kb) wild type (wt) and (4.7kb) DeltaF508CFTR cDNA. Forskolin-stimulated Cl secretion in two clones expressing the full-length wild type CFTR was assessed; clone c7-6.2wt gave 13.4+/-2.5 microA/cm(2) and clone c10-6.2wt showed 41.3+/-25.3 microA/cm(2). Another clone (c4-4.7DeltaF) complemented with the DeltaF508 CFTR cDNA showed high and stable expression of vector-derived DeltaF508 CFTR mRNA and a small cAMP-stimulated Cl current (4.7+/-0.7 microA/cm(2)) indicating DeltaF508CFTR trafficking to the plasma membrane at physiological temperatures. Vector-driven CFTR mRNA levels were 5-fold (c7-6.2wt), 14-fold (c10-6.2wt), and 27-fold (c7-4.7DeltaF) higher than observed in normal bronchial epithelial cells (16HBE14o-) endogenously expressing wtCFTR. Assessment of CFTR mRNA levels and CFTR function showed that cAMP-stimulated CFTR Cl currents were 33%, 167% and 24%, respectively, of those in 16HBE14o- cells. The data suggest that transgene expression needs to be significantly higher than endogenously expressed CFTR to restore functional wtCFTR Cl transport to levels sufficient to reverse CF pathology.

Regulatory mechanism of "K+recycling" for Na +reabsorption in renal tubules

Hypertension is one of the predominant risk factors for the progression of renal impairment, and the most common disorder in industrialized societies. Because reduction of the systemic blood pressure in hypertension can halt the progression of renal impairment, it is imperative to appropriately control the systemic blood pressure. Recent genetic analysis has reconfirmed that renal maladjustment of Na(+)-homeostasis, which determines the extracellular fluid volume, is a key element in the pathogenesis of hypertension. The distal tubules adjust the net Na(+)-excretion according to Na(+)-ingestion and maintain the Na(+)-homeostasis. The distal convoluted tubules and the connecting tubules are the predominant sites for the adjustment in individuals with a modern lifestyle. In these tubules, Na(+)-reabsorption depends on "K(+)-recycling", which is conducted through K(+) channels. Because the functional expression of K(+) channels in these tubules is regulated by signal motifs for intracellular localization, the adjustment of "K(+)-recycling" through the modification of signal motifs could be a new target for the treatment of hypertension.

ΔF508 Mutation Results in Impaired Gastric Acid Secretion

The cystic fibrosis transmembrane conductance regulator (CFTR) is recognized as a multifunctional protein that is involved in Cl(-) secretion, as well as acting as a regulatory protein. In order for acid secretion to take place a complex interaction of transport proteins and channels must occur at the apical pole of the parietal cell. Included in this process is at least one K(+) and Cl(-) channel, allowing for both recycling of K(+) for the H,K-ATPase, and Cl(-) secretion, necessary for the generation of concentrated HCl in the gastric gland lumen. We have previously shown that an ATP-sensitive potassium channel (K(ATP)) is expressed in parietal cells. In the present study we measured secretagogue-induced acid secretion from wild-type and DeltaF508-deficient mice in isolated gastric glands and whole stomach preparations. Secretagogue-induced acid secretion in wild-type mouse gastric glands could be significantly reduced with either glibenclamide or the specific inhibitor CFTR-inh172. In DeltaF508-deficient mice, however, histamine-induced acid secretion was significantly less than in wild-type mice. Furthermore, immunofluorescent localization of sulfonylurea 1 and 2 failed to show expression of a sulfonylurea receptor in the parietal cell, thus further implicating CFTR as the ATP-binding cassette transporter associated with the K(ATP) channels. These results demonstrate a regulatory role for the CFTR protein in normal gastric acid secretion.

Sulphonylurea sensitivity and enriched expression implicate inward rectifier K+ channels in Drosophila melanogaster renal function

Insect Malpighian (renal) tubules are capable of transporting fluid at remarkable rates. Secondary active transport of potassium at the apical surface of the principal cell must be matched by a high-capacity basolateral potassium entry route. A recent microarray analysis of Drosophila tubule identified three extremely abundant and enriched K(+) channel genes encoding the three inward rectifier channels of Drosophila: ir, irk2 and irk3. Enriched expression of inward rectifier channels in tubule was verified by quantitative RT-PCR, and all three IRKs localised to principal cells of the main segment (and ir and irk3 to the lower tubule) by in situ hybridisation, suggesting roles both in primary secretion and reabsorption. A new splice form of irk2 was also identified. The role of inward rectifiers in fluid secretion was assessed with a panel of selective inhibitors of inward rectifier channels, the antidiabetic sulphonylureas. All completely inhibited fluid secretion, with IC(50)s of 0.78 mmol l(-1) for glibenclamide and approximately 5 mmol l(-1) for tolbutamide, 0.01 mmol l(-1) for minoxidil and 0.1 mmol l(-1) for diazoxide. This pharmacology is consistent with a lower-affinity class of inward rectifier channel that does not form an obligate multimer with the sulphonylurea receptor (SUR), although effects on non-IRK targets cannot be excluded. Glibenclamide inhibited fluid secretion similarly to basolateral K(+)-free saline. Radiolabelled glibenclamide is both potently transported and metabolised by tubule. Furthermore, glibenclamide is capable of blocking transport of the organic dye amaranth (azorubin S), at concentrations of glibenclamide much lower than required to impact on fluid secretion. Glibenclamide thus interacts with tubule in three separate ways; as a potent inhibitor of fluid secretion, as an inhibitor (possibly competitive) of an organic solute transporter and as a substrate for excretion and metabolism.

CFTR is required for PKA-regulated ATP sensitivity of Kir1.1 potassium channels in mouse kidney

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel plays vital roles in fluid transport in many epithelia. While CFTR is expressed along the entire nephron, its function in renal tubule epithelial cells remains unclear, as no specific renal phenotype has been identified in cystic fibrosis. CFTR has been proposed as a regulator of the 30 pS, ATP-sensitive renal K channel (Kir1.1, also known as renal outer medullar K [ROMK]) that is critical for K secretion by cells of the thick ascending limb (TAL) and distal nephron segments responsive to aldosterone. We report here that both ATP and glibenclamide sensitivities of the 30 pS K channel in TAL cells were absent in mice lacking CFTR and in mice homozygous for the deltaF508 mutation. Curcumin treatment in deltaF508-CFTR mice partially reversed the defect in ATP sensitivity. We demonstrate that the effect of CFTR on ATP sensitivity was abrogated by increasing PKA activity. We propose that CFTR regulates the renal K secretory channel by providing a PKA-regulated functional switch that determines the distribution of open and ATP-inhibited K channels in apical membranes. We discuss the potential physiological role of this functional switch in renal K handling during water diuresis and the relevance to renal K homeostasis in cystic fibrosis.

Regulation of ROMK (Kir1.1) channels: new mechanisms and aspects

This brief review attempts to provide an overview regarding recent developments in the regulation of ROMK channels. Studies performed in ROMK null mice suggest that ROMK cannot only form hometetramers such as the small-conductance (30-pS) K channels but also construct heterotetramers such as the 70-pS K channel in the thick ascending limb (TAL). The expression of ROMK channels in the plasma membrane is regulated by protein tyrosine kinase (PTK), serum and glucorticoid-induced kinase (SGK), and with-no-lysine-kinase 4. PTK is involved in mediating the effect of low K intake on ROMK channel activity. Increases in superoxide anions induced by low dietary K intake are responsible for the stimulation of PTK expression and tyrosine phosphorylation of ROMK channels. Finally, a recent study indicated that ROMK channels can be monoubiquitinated and monoubiquitination regulates the surface expression of ROMK channels.

Emerging role of AMP-activated protein kinase in coupling membrane transport to cellular metabolism

PURPOSE OF REVIEW

It has long been recognized that the coupling of membrane transport to underlying cellular metabolic status is critical because transport processes consume a large portion of total cellular energy. Recently, the finely tuned metabolic sensor AMP-activated protein kinase (AMPK) has emerged as a membrane transport regulator, which may permit sensitive transport-metabolism crosstalk. This review will discuss how AMPK may play an important role in the regulation of ion and solute transport across the plasma membrane under both physiological and pathological conditions in epithelia and other tissues.

RECENT FINDINGS

Recent studies have found that AMPK, which becomes activated during cellular metabolic stress, promotes the cellular uptake of fuel sources such as glucose and fatty acids to promote ATP generation and inhibits ion-transport proteins such as the cystic fibrosis transmembrane conductance regulator Cl channel and the epithelial Na channel, thereby limiting the dissipation of transmembrane ion gradients. An understanding of the underlying cellular and molecular mechanisms for AMPK-dependent regulation of transport proteins is beginning to emerge.

SUMMARY

As earlier studies have focused on the role of nucleotides such as ATP in regulating transport-protein activities, the regulation of membrane transport by AMPK represents a novel and more-sensitive mechanism for the coupling of membrane transport to cellular metabolic status. Identifying new membrane-transport targets of AMPK and elucidating the mechanisms involved in their AMPK-dependent regulation are fruitful areas for new investigation that should yield valuable insights into the pathophysiology of hypoxic and ischemic tissue injury.

Characterization of novel airway submucosal gland cell models for cystic fibrosis studies

Cultured airway epithelial cells are widely used in cystic fibrosis (CF) research as in vitro models that mimic the in vivo manifestations of the disease and help to define a specific cellular phenotype. Recently, a number of in vitro studies have used an airway adenocarcinoma cell line, Calu-3 that expresses submucosal gland cell features and significant levels of the wild-type CFTR mRNA and protein. We further characterized previously described CF tracheobronchial gland cell lines, CFSMEo- and 6CFSMEo- and determined that these cell lines are compound heterozygotes for the F508del and Q2X mutations, produce vestigial amounts of CFTR mRNA, and do not express detectable CFTR protein. Electrophysiologically, both cell lines are characteristically CF in that they lack cAMP-induced Cl- currents. In this study the cell lines are evaluated in the context of their role as the CF correlate to the Calu-3 cells. Together these cell systems provide defined culture systems to study the biology and pathology of CF. These airway epithelial cell lines may also be a useful negative protein control for numerous studies involving gene therapy by cDNA complementation or gene targeting.

Chloride channel diseases resulting from impaired transepithelial transport or vesicular function

The transport of anions across cellular membranes is crucial for various functions, including the control of electrical excitability of muscle and nerve, transport of salt and water across epithelia, and the regulation of cell volume or the acidification and ionic homeostasis of intracellular organelles. Given this broad range of functions, it is perhaps not surprising that mutations in Cl- channels lead to a large spectrum of diseases. These diverse pathologies include the muscle disorder myotonia, cystic fibrosis, renal salt loss in Bartter syndrome, kidney stones, deafness, and the bone disease osteopetrosis. This review will focus on diseases related to transepithelial transport and on disorders involving vesicular Cl- channels.

Macromolecular complexes of cystic fibrosis transmembrane conductance regulator and its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is the product of the gene mutated in patients with cystic fibrosis (CF). CFTR is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells lining the airway, gut, exocrine glands, etc., where it is responsible for transepithelial salt and water transport. CFTR chloride channel belongs to the superfamily of the ATP-binding cassette (ABC) transporters, which bind ATP and use the energy to drive the transport of a wide variety of substrates across extra- and intracellular membranes. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might regulate the activities of other ion channels, receptors, or transporters, in addition to its role as a chloride conductor. The molecular assembly of CFTR with these interacting proteins is of great interest and importance because several human diseases are attributed to altered regulation of CFTR, among which cystic fibrosis is the most serious one. Most interactions primarily occur between the opposing terminal tails (N- or C-) of CFTR and its binding partners, either directly or mediated through various PDZ domain-containing proteins. These dynamic interactions impact the channel function as well as the localization and processing of CFTR protein within cells. This review focuses on the recent developments in defining the assembly of CFTR-containing complexes in the plasma membrane and its interacting proteins.

Steady-state interactions of glibenclamide with CFTR: evidence for multiple sites in the pore

The objective of the present study was to clarify the mechanism by which the sulfonylurea drug, glibenclamide, inhibits single CFTR channels in excised patches from Xenopus oocytes. Glibenclamide blocks the open pore of the channel via binding at multiple sites with varying kinetics. In the absence of glibenclamide, open-channel bursts exhibited a flickery intraburst closed state (C1); this is due to block of the pore by the pH buffer, TES. Application of 25 microM glibenclamide to the cytoplasmic solution resulted in the appearance of two drug-induced intraburst closed states (C2, C3) of widely different duration, which differed in pH-dependence. The kinetics of interaction with the C3 state, but not the C2 state, were strongly voltage-dependent. The durations of both the C2 and C3 states were concentration-dependent, indicating a non-linear reaction scheme. Application of drug also increased the burst duration, which is consistent with an open-channel blocking mechanism. A kinetic model is proposed. These results indicate that glibenclamide interacts with open CFTR channels in a complex manner, involving interactions with multiple binding sites in the channel pore.

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

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

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Ion channels: function unravelled by dysfunction

Ion channels allow the passage of specific ions and electrical charge. Plasma membrane channels are, for example, important for electrical excitability and transepithelial transport, whereas intracellular channels have roles in acidifying endosomes or in releasing Ca(2+) from stores. The function of several channels emerged from mutations in humans or mice. The resulting phenotypes include kidney stones resulting from impaired endocytosis, hypertension, defective insulin secretion, cardiac arrhythmias, neurological diseases like epilepsy or deafness and even 'developmental' defects such as osteopetrosis.

The cystic fibrosis transmembrane conductance regulator: an intriguing protein with pleiotropic functions

Cystic fibrosis is a frequent autosomal recessive disorder that is caused by the malfunctioning of a small chloride channel, the cystic fibrosis transmembrane conductance regulator. The protein is found in the apical membrane of epithelial cells lining exocrine glands. Absence of this channel results in imbalance of ion concentrations across the cell membrane. As a result, fluids secreted through these glands become more viscous and, in the end, ducts become plugged and atrophic. Little is known about the pathways that link the malfunctioning of the CFTR protein with the observed clinical phenotype. Moreover, there is no strict correlation between specific CFTR mutations and the CF phenotype. This might be explained by the fact that environmental and additional genetic factors may influence the phenotype. The CFTR protein itself is regulated at the maturational level by chaperones and SNARE proteins and at the functional level by several protein kinases. Moreover, CFTR functions also as a regulator of other ion channels and of intracellular membrane transport processes. In order to be able to function as a protein with pleiotropic actions, CFTR seems to be linked with other proteins and with the cytoskeleton through interaction with PDZ-domain-containing proteins at the apical pole of the cell. Progress in cystic fibrosis research is substantial, but still leaves many questions unanswered.

Regulation of renal K transport by dietary K intake

Extracellular K must be kept within a narrow concentration range for the normal function of neurons, skeletal muscle, and cardiac myocytes. Maintenance of normal plasma K is achieved by a dual mechanism that includes extrarenal factors such as insulin and beta-adrenergic agonists, which stimulate the movement of K from extracellular to intracellular fluid and modulate renal K excretion. Dietary K intake is an important factor for the regulation of K secretion: An increase in K intake stimulates secretion, whereas a decrease inhibits K secretion and enhances absorption. This effect of changes in dietary K intake on tubule K transport is mediated by aldosterone-dependent and -independent mechanisms. Recently, it has been demonstrated that the protein tyrosine kinase (PTK)-dependent signal transduction pathway is an important aldosterone-independent regulatory mechanism that mediates the effect of altered K intake on K secretion. A low-K intake stimulates PTK activity, which leads to increase in phosphorylation of cloned inwardly rectifying renal K (ROMK) channels, whereas a high-K intake has the opposite effect. Stimulation of tyrosine phosphorylation also suppresses K secretion in principal cell by facilitating the internalization of apical K channels in the collecting duct.

Pharmacotherapy of the ion transport defect in cystic fibrosis: role of purinergic receptor agonists and other potential therapeutics

Cystic fibrosis (CF), is an autosomal recessive disease frequently seen in the Caucasian population. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CF is characterized by enhanced airway Na(+) absorption, mediated by epithelial Na(+) channels (ENaC), and deficient Cl(-) transport. In addition, other mechanisms may contribute to the pathophysiological changes in the CF lung, such as defective regulation of HCO(3)(-) secretion. In other epithelial tissues, epithelial Na(+) conductance is either increased (intestine) or decreased (sweat duct) in CF. CFTR is a cyclic AMP-regulated epithelial Cl(-) channel, and appears to control the activity of several other transport proteins. Accordingly, defective epithelial ion transport in CF is likely to be a combination of defective Cl(-) channel function and impaired regulator function of CFTR, which in turn is linked to impaired mucociliary clearance and development of chronic lung disease. As the clinical course of CF is determined primarily by progressive lung disease, novel pharmacological strategies for the treatment of CF focus on correction of the ion transport defect in the airways. In recent years, it has been demonstrated that activation of purinergic receptors in airway epithelia by extracellular nucleotides (adenosine triphosphate/uridine triphosphate) has beneficial effects on mucus clearance in CF. Activation of the dominant class of metabotropic purinergic receptors, P2Y(2) receptors, appears to have a 2-fold benefit on ion transport in CF airways; excessive Na(+) absorption is attenuated, most likely by inhibition of the ENaC and, simultaneously, an alternative Ca(2+)-dependent Cl(-) channel is activated that may compensate for the CFTR Cl(-) channel defect. Thus activation of P2Y(2) receptors is expected to lead to improved hydration of the airway surface liquid in CF. Furthermore, purinergic activation has been shown to promote other components of mucociliary clearance such as ciliary beat frequency and mucus secretion. Clinical trials are under way to test the effect of synthetic purinergic compounds, such as the P2Y(2) receptor agonist INS37217, on the progression of lung disease in patients with CF. Administration of these compounds alone, or in combination with other drugs that inhibit accelerated Na(+) transport and help recover or increase residual activity of mutant CFTR, is most promising as successful therapy to counteract the ion transport defect in the airways of CF patients.

Evidence for up-regulation of the endogenous Na-K-2Cl co-transporter by molecular interactions with the anion exchanger tAE1 expressed in Xenopus oocyte

Expression of trout anion exchanger 1 (tAE1) in Xenopus oocyte led to the stimulation of a Na(+)- and Cl(-)-dependent Rb influx. Functional features and pharmacological data strongly suggest that this Rb influx is mediated by the endogenous Na-K-2Cl (NKCC) co-transporter. The functional relationship between expression of tAE1 and activation of the NKCC co-transporter was investigated. Indeed, it was shown previously that tAE1 expressed in Xenopus oocyte induces a strong anion conductance which is correlated with an increased taurine permeability. Measurements of intracellular ion contents ruled out the involvement of any modification of known electrochemical parameters in NKCC co-transporter activation by tAE1. Furthermore, using chimera of tAE1 made with AE1 from other species unable to exhibit anion conductance led to the conclusion that there was no correlation between tAE1 anion conductance and NKCC co-transporter stimulation. Therefore, a possible molecular interaction between tAE1 and the NKCC co-transporter was investigated. Our results clearly show that NKCC activation is dependent upon the C-terminal part of tAE1. Chimeric constructions where tAE1 C-terminal part was substituted by the corresponding part of mouse AE1 abolished co-transporter activation. Moreover, steric encumbrance on the C-terminal end of tAE1 with a specific antibody or with a protein fusion also prevented the co-transporter activation. These data suggest a new role for some anion exchangers in controlling other transporter activity by molecular interactions.

Assembly and trafficking of a multiprotein ROMK (Kir 1.1) channel complex by PDZ interactions

The ROMK subtypes of inward rectifier K+ channels (Kir 1.1, KCNJ1) mediate potassium secretion and regulate NaCl reabsorption in the kidney. In the present study, the role of the PDZ binding motif in ROMK function is explored. Here we identify the Na/H exchange regulatory factors, NHERF-1 and NHERF-2, as PDZ domain interaction partners of the ROMK channel. Characterization of the basis and consequences of NHERF association with ROMK reveals a PDZ interaction-dependent trafficking process and a coupling mechanism for linking ROMK to a channel modifier protein, the cystic fibrosis transmembrane regulator (CFTR). As measured by antibody binding of external epitope-tagged forms of Kir 1.1 in intact cells, NHERF-1 or NHERF-2 coexpression increased cell surface expression of ROMK. Channel interaction with NHERF proteins and effects of NHERF on ROMK localization were dependent on the presence of the PDZ domain binding motif in ROMK. Both NHERF proteins contain two PDZ domains; recombinant protein-protein binding assays and yeast-two-hybrid studies revealed that ROMK preferentially associates with the second PDZ domain of NHERF-1 and with the first PDZ domain of NHERF-2, precisely opposite of what has been reported for CFTR. Consistent with the scaffolding capacity of the NHERF proteins, coexpression of NHERF-2 with ROMK and CFTR dramatically increases the amount of ROMK protein that coimmunopurifies and functionally interacts with CFTR. Thus NHERF facilitates assembly of a ternary complex containing ROMK and CFTR. These observations raise the possibility that PDZ-based interactions may underscore physiological regulation and membrane targeting of ROMK in the kidney.

CFTR-dependent and -independent swelling-activated K+ currents in primary cultures of mouse nephron

The role of CFTR in the control of K(+) currents was studied in mouse kidney. Whole cell clamp was used to identify K(+) currents on the basis of pharmacological sensitivities in primary cultures of proximal (PCT) and distal convoluted tubule (DCT) and cortical collecting tubule (CCT) from wild-type (WT) and CFTR knockout (KO) mice. In DCT and CCT cells, forskolin activated a 293B-sensitive K(+) current in WT, but not in KO, mice. In these cells, a hypotonic shock induced K(+) currents blocked by charybdotoxin in WT, but not in KO, mice. In PCT cells from WT and KO mice, the hypotonicity-induced K(+) currents were insensitive to these toxins and were activated at extracellular pH 8.0 and inhibited at pH 6.0, suggesting that the corresponding channel was TASK2. In conclusion, CFTR is implicated in the control of KCNQ1 and Ca(2+)-sensitive swelling-activated K(+) conductances in DCT and CCT, but not in proximal convoluted tubule, cells. In KO mice, impairment of the regulatory volume decrease process in DCT and CCT could be due to the loss of an autocrine mechanism, implicating ATP and adenosine, which controls swelling-activated Cl(-) and K(+) channels.

Chloride channels in the kidney: lessons learned from knockout animals

Cl- channels are involved in a range of functions, including regulation of cell volume and/or intracellular pH, acidification of intracellular vesicles, and vectorial transport of NaCl across many epithelia. Numerous Cl- channels have been identified in the kidney, based on single-channel properties such as conductance, anion selectivity, gating, and response to inhibitors. The molecular counterpart of many of these Cl- channels is still not known. This review will focus on gene-targeted mouse models disrupting two structural classes of Cl- channels that are relevant for the kidney: the CLC family of voltage-gated Cl- channels and the CFTR. Disruption of several members of the CLC family in the mouse provided useful models for various inherited diseases of the kidney, including Dent's disease and diabetes insipidus. Mice with disrupted CFTR are valuable models for cystic fibrosis (CF), the most common autosomal recessive, lethal disease in Caucasians. Although CFTR is expressed in various nephron segments, there is no overt renal phenotype in CF. Analysis of CF mice has been useful to identify the role and potential interactions of CFTR in the kidney. Furthermore, observations made in CF mice are potentially relevant to all other models of Cl- channel knockouts because they emphasize the importance of alternative Cl- pathways in such models.

Protein kinase C (PKC)-induced phosphorylation of ROMK1 is essential for the surface expression of ROMK1 channels

We carried out in vitro phosphorylation assays to determine whether ROMK1 is a substrate of protein kinase C (PKC) and used the two-electrode voltage clamp method to investigate the role of serine residues 4, 183, and 201, the three putative PKC phosphorylation sites, in the regulation of ROMK1 channel activity. Incubation of the purified His-tagged ROMK1 protein with PKC and radiolabeled ATP resulted in (32)P incorporation into ROMK1 detected by autoradiography. Moreover, the in vitro phosphorylation study of three synthesized peptides corresponding to amino acids 1-16, 174-189, and 196-211 of ROMK1 revealed that serine residues 4 and 201 of ROMK1 were the two main PKC phosphorylation sites. In contrast, (32)P incorporation of peptide 174-189 was absent. In vitro phosphorylation studies with ROMK1 mutants, R1S4/201A, R1S4/183A, and R1S183/201A, demonstrated that the phosphorylation levels of R1S4/201A were significantly lower than those of the other two mutants. Also, the Ba(2+)-sensitive K(+) current in oocytes injected with green fluorescent protein (GFP)-R1S4/201A was only 5% of that in oocytes injected with wild type GFP-ROMK1. In contrast, the K(+) current in oocytes injected with GFP-ROMK1 mutants containing either serine residue 4 or 201 was similar to those injected with wild type ROMK1. Confocal microscope imaging shows that the surface expression of the K(+) channels was significantly diminished in oocytes injected with R1S4/201A and completely absent in oocytes injected with R1S4/183/201A. Furthermore, the biotin labeling technique confirmed that the membrane fraction of ROMK channels was almost absent in HEK293 cells transfected with either R1S4/201A or R1S4/183/201A. However, when serine residues 4 and 201 were mutated to aspartate, the K(+) currents and the surface expression were completely restored. Finally, addition of calphostin C in the incubation medium significantly decreased the K(+) current in comparison with that under control conditions. Biotin labeling technique further indicated that inhibition of PKC decreases the surface ROMK1 expression in human embryonic kidney (HEK) cells transfected with ROMK1. We conclude that ROMK1 is a substrate of PKC and that serine residues 4 and 201 are the two main PKC phosphorylation sites that are essential for the expression of ROMK1 in the cell surface.

A trail of research on potassium

A complex pump-leak system involving both active and passive transport mechanisms is responsible for the appropriate distribution of potassium (K) between the intra- and extracellular fluid compartments. In addition, the kidneys, and to a lesser extent the colon, safeguard maintenance of the narrow range of low K concentrations in the extracellular fluid. Early renal clearance studies showed that K is normally both reabsorbed and secreted by renal tubules, and that regulated secretion is the major source of K excretion. Studies at the tubule and cell level have localized secretion and reabsorption of K to principal and intercalated cells in the collecting ducts. Measurements of the electrochemical driving forces across individual cell membranes have permitted the characterization of specific ATPases, K channels and K cotransporters and also provided insights into the molecular structure of individual transporters that regulate K excretion.

Sulfonylurea-sensitive K(+) transport is involved in Cl(-) secretion and cyst trowth by cultured ADPKD cells

Transepithelial chloride and fluid secretion by many types of epithelia involves activation of a conductive K(+) pathway that serves to support the electrochemical driving force for Cl(-) secretion. This study sought to determine if such a pathway is involved in Cl(-) and fluid secretion by the cystic epithelia in autosomal dominant polycystic kidney disease (ADPKD). Primary cultures of cells derived from the cysts of patients with ADPKD were used. Confluent monolayers of these cells, mounted in Ussing chambers, were stimulated to secrete Cl(-) by application of the adenylyl cyclase agonist, forskolin. The effects of various K(+) channel blockers on the increase in short-circuit current (I(sc)) generated by active Cl(-) secretion were determined. Charybdotoxin, an inhibitor of Ca(2+)-sensitive K(+) channels exerted no effect. Similarly, the chromanole 293B, an inhibitor of cAMP-induced K(+) conductance, exerted no effect on cAMP-dependent anion secretion. Glibenclamide, an inhibitor of ATP-sensitive K(+) channels and the cystic fibrosis transmembrane conductance regulator (CFTR), modestly inhibited the forskolin-stimulated current when applied to the apical surface of the monolayers, suggesting a relatively weak effect on CFTR. Basolateral application of glibenclamide inhibited I(sc) to a greater extent. This latter effect may be due to inhibition of a K(+)-conductive transport step. Glibenclamide exerted little effect on the I(sc) of nonstimulated monolayers. Cyst growth in ADPKD is driven by cell proliferation and Cl(-) and fluid secretion. The effect of glibenclamide on the growth of cysts formed within a collagen gel by cultured ADPKD cells was tested. Addition of glibenclamide to the media bathing the cysts inhibited their growth. Glibenclamide also blocked the formation of cysts when it was added to the media at the time the cells were seeded within the collagen gel. Glibenclamide was also found to inhibit the proliferation of ADPKD cells. RT-PCR analysis demonstrated that the ATP-sensitive K(+) channel, K(ir) 6.2, is expressed in cultured ADPKD cells and in normal human kidney. These results suggest that ATP-sensitive K(+) channel blockers should be investigated as possible therapeutic agents to inhibit cyst growth in ADPKD.