Differential expression of KvLQT1 and its regulator IsK in mouse epithelia

SGK1 dependence of insulin induced hypokalemia

Insulin stimulates cellular K+ uptake leading to hypokalemia. Cellular K+ uptake is accomplished by parallel stimulation of Na+/H+ exchange, Na+,K+,2Cl- co-transport, and Na+/K+ ATPase and leads to cell swelling, a prerequisite for several metabolic effects of the hormone. Little is known about underlying signaling. Insulin is known to activate the serum and glucocorticoid-inducible kinase SGK1, which in turn enhances the activity of all three transport proteins. The present study thus explored the contribution of SGK1 to insulin-induced hypokalemia. To this end, gene-targeted mice lacking SGK1 (sgk1-/-) and their wild-type littermates (sgk1+/+) have been infused with insulin (2 mU kg(-1) min(-1)) and glucose at rates leaving the plasma glucose concentration constant. Moreover, isolated liver perfusion experiments have been performed to determine stimulation of cellular K+ uptake by insulin (100 nM). As a result, combined glucose and insulin infusion significantly decreased plasma K+ concentration despite a significant decrease of urinary K+ excretion in sgk1+/+ but not in sgk1-/- mice. Accordingly, the plasma K+ concentration was within 60 min significantly lower in sgk1+/+ than in sgk1-/- mice. In isolated liver perfusion experiments, cellular K+ uptake was stimulated by insulin (100 nM), an effect blunted by 72% in sgk1-/- mice as compared to sgk1+/+ mice. Accordingly, insulin-induced cell hydration was 63% lower in sgk1-/- mice than in sgk1+/+ mice. Moreover, volume regulatory K+ release was 31% smaller in sgk1-/- mice than in sgk1+/+ mice. In conclusion, the serum and glucocorticoid-inducible kinase SGK1 participates in the signaling mediating the hypokalemic effect of insulin.

KCNE4 can co-associate with the I(Ks) (KCNQ1-KCNE1) channel complex

Voltage-gated potassium (K(V)) channels can form heteromultimeric complexes with a variety of accessory subunits, including KCNE proteins. Heterologous expression studies have demonstrated diverse functional effects of KCNE subunits on several K(V) channels, including KCNQ1 (K(V)7.1) that, together with KCNE1, generates the slow-delayed rectifier current (I(Ks)) important for cardiac repolarization. In particular, KCNE4 exerts a strong inhibitory effect on KCNQ1 and other K(V) channels, raising the possibility that this accessory subunit is an important potassium current modulator. A polyclonal KCNE4 antibody was developed to determine the human tissue expression pattern and to investigate the biochemical associations of this protein with KCNQ1. We found that KCNE4 is widely and variably expressed in several human tissues, with greatest abundance in brain, liver and testis. In heterologous expression experiments, immunoprecipitation followed by immunoblotting was used to establish that KCNE4 directly associates with KCNQ1, and can co-associate together with KCNE1 in the same KCNQ1 complex to form a 'triple subunit' complex (KCNE1-KCNQ1-KCNE4). We also used cell surface biotinylation to demonstrate that KCNE4 does not impair plasma membrane expression of either KCNQ1 or the triple subunit complex, indicating that biophysical mechanisms probably underlie the inhibitory effects of KCNE4. The observation that multiple KCNE proteins can co-associate with and modulate KCNQ1 channels to produce biochemically diverse channel complexes has important implications for understanding K(V) channel regulation in human physiology.

Evidence for KCNQ1 K+ channel expression in rat zymogen granule membranes and involvement in cholecystokinin-induced pancreatic acinar secretion

Secretion of enzymes and fluid induced by Ca(2+) in pancreatic acini is not completely understood and may involve activation of ion conductive pathways in zymogen granule (ZG) membranes. We hypothesized that a chromanol 293B-sensitive K(+) conductance carried by a KCNQ1 protein is expressed in ZG membranes (ZGM). In suspensions of rat pancreatic ZG, ion flux was determined by ionophore-induced osmotic lysis of ZG suspended in isotonic salts. The KCNQ1 blocker 293B selectively blocked K(+) permeability (IC(50) of approximately 10 microM). After incorporation of ZGM into planar bilayer membranes, cation channels were detected in 645/150 mM potassium gluconate cis/trans solutions. Channels had linear current-voltage relationships, a reversal potential (E(rev)) of -20.9 +/- 0.9 mV, and a single-channel K(+) conductance (g(K)) of 265.8 +/- 44.0 pS (n = 39). Replacement of cis 500 mM K(+) by 500 mM Na(+) shifted E(rev) to -2.4 +/- 3.6 mV (n = 3), indicating K(+) selectivity. Single-channel analysis identified several K(+) channel groups with distinct channel behaviors. K(+) channels with a g(K) of 651.8 +/- 88.0 pS, E(rev) of -22.9 +/- 2.2 mV, and open probability (P(open)) of 0.43 +/- 0.06 at 0 mV (n = 6) and channels with a g(K) of 155.0 +/- 11.4 pS, E(rev) of -18.3 +/- 1.8 mV, and P(open) of 0.80 +/- 0.03 at 0 mV (n = 3) were inhibited by 100 microM 293B or by the more selective inhibitor HMR-1556 but not by the maxi-Ca(2+)-activated K(+) channel (BK channel) inhibitor charybdotoxin (5 nM). KCNQ1 protein was demonstrated by immunoperoxidase labeling of pancreatic tissue, immunogold labeling of ZG, and immunoblotting of ZGM. 293B also inhibited cholecystokinin-induced amylase secretion of permeabilized acini (IC(50) of approximately 10 microM). Thus KCNQ1 may account for ZG K(+) conductance and contribute to pancreatic hormone-stimulated enzyme and fluid secretion.

Modification of K+ channel-drug interactions by ancillary subunits

Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.

Absence of KCNQ1-dependent K+ fluxes in proximal tubular cells of frog kidney

The present study was designed to investigate the functional significance of KCNQ1-mediated K+ secretory fluxes in proximal tubular cells of the frog kidney. To this end, we investigated the effects on rapid depolarization and slow repolarization of the peritubular membrane potential after luminal addition of L-phenylalanine or L-alanine plus/minus KCNQ1 channel blockers. Perfusing the lumen with 10 mmol/L L-phenylalanine plus/minus luminal 293B, a specific blocker of KCNQ1, did not modify the rapid depolarization and the rate of slow repolarization. Perfusing the lumen with 10 mmol/L L-alanine plus/minus luminal HMR-1556, a more potent KCNQ1 channel blocker, did not also alter the rapid depolarization and the rate of slow repolarization. Pretreatment (1 h) of the lumen with HMR-1556 also failed to modify rapid depolarization and rate of slow repolarization upon luminal 10 mmol/L L-alanine. Perfusing the lumen with 1 mmol/L L-alanine plus/minus luminal HMR-1556 did not change the rapid depolarization and the rate of slow repolarization. The pretreatment (1 h) with luminal HMR-1556 did not modify the rapid depolarization and the rate of slow repolarization upon luminal 1 mmol/L L-alanine. The pretreatment (1 h) of the lumen with HMR-1556 did not change transference number for K+ of peritubular cell membrane. Finally, luminal barium blunted the rapid depolarization upon application of luminal 1 mmol/L L-alanine. RT-PCR showed that KCNQ1 mRNA was not expressed in frog kidney. In conclusion, the KCNQ1-dependent K+ secretory fluxes are absent in proximal tubule of frog kidney.

Phenotypic Analysis of Vertigo 2 Jackson Mice with a Kcnq1 Potassium Channel Mutation

The KCNQ1 gene encodes a voltage-dependent potassium ion channel, and mutations in this gene are the most common cause of congenital long QT syndrome (LQTS). In the present study, we investigated the various phenotypic characteristics of vertigo 2 Jackson (C3H/HeJCrl-Kcnq1(vtg-2J)/J) mice with a Kcnq1 mutation. Both heterozygotes (vtg-2J/+) and homozygotes (vtg-2J/vtg-2J) showed prolonged QT intervals in electrocardiograms (ECGs) compared to C3H/HeJ control (+/+) mice. Furthermore, vtg-2J/vtg-2J mice showed gastric achlorhydria associated with elevation of their serum gastrin levels. The serum corticosterone levels were also significantly increased in vtg-2J/vtg-2J mice. In addition, vtg-2J/vtg-2J mice exhibited significantly higher blood pressure. These findings indicate that the Kcnq1 mutation in vtg-2J mice alters various physiological functions in the cardiac, gastric and adrenocortical systems, and suggest that vtg-2J mice may represent a useful model for studying Kcnq1 functions.

Regulation of ENaC and CFTR expression with K+channel modulators and effect on fluid absorption across alveolar epithelial cells

In a recent study (Leroy C, Dagenais A, Berthiaume Y, and Brochiero E. Am J Physiol Lung Cell Mol Physiol 286: L1027–L1037, 2004), we identified an ATP-sensitive K+(KATP) channel in alveolar epithelial cells, formed by inwardly rectifying K+channel Kir6.1/sulfonylurea receptor (SUR)2B subunits. We found that short applications of KATP, voltage-dependent K+channel KvLQT1, and calcium-activated K+(KCa) channel modulators modified Na+and Cl−currents in alveolar monolayers. In addition, it was shown previously that a KATPopener increased alveolar liquid clearance in human lungs by a mechanism possibly related to epithelial sodium channels (ENaC). We therefore hypothesized that prolonged treatment with K+channel modulators could induce a sustained regulation of ENaC activity and/or expression. Alveolar monolayers were treated for 24 h with inhibitors of KATP, KvLQT1, and KCachannels identified by PCR. Glibenclamide and clofilium (KATPand KvLQT1 inhibitors) strongly reduced basal transepithelial current, amiloride-sensitive Na+current, and forskolin-activated Cl−currents, whereas pinacidil, a KATPactivator, increased them. Interestingly, K+inhibitors or membrane depolarization (induced by valinomycin in high-K+medium) decreased α-, β-, and γ-ENaC and CFTR mRNA. α-ENaC and CFTR proteins also declined after glibenclamide or clofilium treatment. Conversely, pinacidil augmented ENaC and CFTR mRNAs and proteins. Since alveolar fluid transport was found to be driven, at least in part, by Na+transport through ENaC, we tested the impact of K+channel modulators on fluid absorption across alveolar monolayers. We found that glibenclamide and clofilium reduced fluid absorption to a level similar to that seen in the presence of amiloride, whereas pinacidil slightly enhanced it. Long-term regulation of ENaC and CFTR expression by K+channel activity could benefit patients with pulmonary diseases affecting ion transport and fluid clearance.

Cellular distribution of the potassium channel KCNQ1 in normal mouse kidney

Mechanisms of K(+) secretion and absorption along the collecting duct are not understood fully. Because KCNQ1 participates in K(+) secretion within the inner ear and stomach, distribution of KCNQ1 in mouse kidney was studied using Northern and Western analyses, RT-PCR of isolated tubules, and immunohistochemistry. Northern blots demonstrated KCNQ1 transcripts in whole kidney. RT-PCR showed KCNQ1 mRNA in isolated distal convoluted tubule (DCT), connecting segment (CNT), collecting ducts (CD), and glomeruli. Immunoblots of kidney and stomach revealed a approximately 75-kDa protein, the expected mobility for KCNQ1. KCNQ1 was detected by immunohistochemistry throughout the distal nephron and CD. Thick ascending limbs exhibited weak basolateral immunolabel. In DCT and CNT cells, immunolabel was intense and basolateral, although KCNQ1 label was stronger in late than in early DCT. Initial collecting tubule and cortical CD KCNQ1 immunolabel was predominantly diffuse, but many cells exhibited discrete apical label. Double-labeling experiments demonstrated that principal cells, type B intercalated cells, and a few type A intercalated cells exhibited distinct apical KCNQ1 immunolabel. In inner medullary CD, principal cells exhibited distinct basolateral KCNQ1 immunolabel, whereas intercalated cells showed diffuse cytoplasmic staining. Thus KCNQ1 protein is widely distributed in mouse distal nephron and CD, with significant axial and cellular heterogeneity in location and intensity. These findings suggest that KCNQ1 has cell-specific roles in renal ion transport and may participate in K(+) secretion and/or absorption along the thick ascending limb, DCT, connecting tubule, and CD.

Ancillary subunits and stimulation frequency determine the potency of chromanol 293B block of the KCNQ1 potassium channel

KCNQ1 (Kv7.1 or KvLQT1) encodes the alpha-subunit of a voltage-gated potassium channel found in tissues including heart, brain, epithelia and smooth muscle. Tissue-specific characteristics of KCNQ1 current are diverse, due to modification by ancillary subunits. In heart, KCNQ1 associates with KCNE1 (MinK), producing a slowly activating voltage-dependent channel. In epithelia, KCNQ1 co-assembles with KCNE3 (Mirp2) producing a constitutively open channel. Chromanol 293B is a selective KCNQ1 blocker. We studied drug binding and frequency dependence of 293B on KCNQ1 and ancillary subunits expressed in Xenopus oocytes. Ancillary subunits altered 293B potency up to 100-fold (IC(50) for KCNQ1 = 65.4 +/- 1.7 microm; KCNQ1/KCNE1 = 15.1 +/- 3.3 microm; KCNQ1/KCNE3 = 0.54 +/- 0.18 microm). Block of KCNQ1 and KCNQ1/KCNE3 was time independent, but 293B altered KCNQ1/KCNE1 activation. We therefore studied frequency-dependent block of KCNQ1/KCNE1. Repetitive rapid stimulation increased KCNQ1/KCNE1 current biphasically, and 293B abolished the slow component. KCNQ1/KCNE3[V72T] activates slowly with a KCNQ1/KCNE1-like phenotype, but retains the high affinity binding of KCNQ1/KCNE3, demonstrating that subunit-mediated changes in gating can be dissociated from subunit-mediated changes in affinity. This study demonstrates the KCNQ1 pharmacology is significantly altered by ancillary subunits. The response of KCNQ1 to specific blockers will therefore be critically dependent on the electrical stimulation pattern of the target organ. Furthermore, the dissociation between gating and overall affinity suggests that mutations in ancillary subunits can potentially strongly alter drug sensitivity without obvious functional changes in gating behaviour, giving rise to unexpected side-effects such as a predisposition to acquired long QT syndrome.

WTC deafness Kyoto (dfk): a rat model for extensive investigations ofKcnq1functions

KCNQ1 forms K+channels by assembly with regulatory subunit KCNE proteins and plays a key role in the K+homeostasis in a variety of tissues. In the heart, KCNQ1 is coassembled with KCNE1 to produce a cardiac delayed rectifier K+current. In the inner ear, the KCNQ1/KCNE1 complex maintains the high concentration of K+in the endolymph. In the stomach, KCNQ1 is coassembled with KCNE2 to form the K+exflux channel that is essential for gastric acid secretion. In the colon and small intestine, KCNQ1 is coassembled with KCNE3 to play an important role in transepithelial cAMP-stimulated Cl−secretion. For further understanding of Kcnq1 function in vivo, an animal model has been required. Here we reported the identification of a coisogenic Kcnq1 mutant rat, named deafness Kyoto ( dfk), and the characterization of its phenotypes. WTC- dfk rats carried intragenic deletion at the Kcnq1 gene and showed impaired gain of weight, deafness, and imbalance resulting from the marked reduction of endolymph, prolonged QT interval in the electrocardiogram (ECG), and gastric achlorhydria associated with hypertrophic gastric mucosa. Surprisingly, WTC- dfk rats showed hypertension, which suggested that Kcnq1 might be involved in the regulation of blood pressure. These findings suggest that WTC- dfk rats could represent a powerful tool for studying the physiological functions of KCNQ1 and for the establishment of new therapeutic procedures for Kcnq1-related diseases.

The KCNQ1 potassium channel: from gene to physiological function

The voltage-gated KCNQ1 (KvLQT1, Kv7.1) potassium channel plays a crucial role in shaping the cardiac action potential as well as in controlling the water and salt homeostasis in several epithelial tissues. KCNQ1 channels in these tissues are tightly regulated by auxiliary proteins and accessory factors, capable of modulating the properties of the channel complexes. This paper reviews the current knowledge about the KCNQ1 channel with a major focus on interacting proteins and physiological functions.

Protein distribution of Kcnq1, Kcnh2, and Kcne3 potassium channel subunits during mouse embryonic development

Voltage-dependent potassium channels consist of a pore-forming alpha-subunit, which is modulated by additional beta-ancillary or regulatory subunits. Kcnq1 and Kcnh2 alpha-channel subunits play pivotal roles in the developing and adult heart. However, Kcnq1 and Kcnh2 have a much wider expression profile than strictly confined to the myocardium, similar to their putative regulatory Kcne1-5 beta-subunits. At present, the distribution of distinct potassium channel subunits has been partially mapped in adult tissues, whereas almost no information is available during embryonic development. In this study, we report a detailed analysis of Kcnq1, Kcnh2, and Kcne3 protein expression during mouse embryogenesis. Our results demonstrate that Kcnq1 and Kcnh2 are widely distributed. Coexpression of both alpha-subunits is observed in a wide variety of organs, such as heart and the skeletal muscle, whereas others display unique Kcnq1 or Knch2 expression. Interestingly, Kcne3 expression is also widely observed in distinct tissue layers during embryogenesis, supporting the notion that an exquisite balance of alpha- and beta-subunit expression is required for modulating potassium conductance in distinct organs and tissue layers.

Attenuation of apoptosis in enterocytes by blockade of potassium channels

Apoptosis plays an important role in maintaining the balance between proliferation and cell loss in the intestinal epithelium. Apoptosis rates may increase in intestinal pathologies such as inflammatory bowel disease and necrotizing enterocolitis, suggesting pharmacological prevention of apoptosis as a therapy for these conditions. Here, we explore the feasibility of this approach using the rat epithelial cell line IEC-6 as a model. On the basis of the known role of K+ efflux in apoptosis in various cell types, we hypothesized that K+ efflux is essential for apoptosis in enterocytes and that pharmacological blockade of this efflux would inhibit apoptosis. By probing intracellular [K+] with the K+-sensitive fluorescent dye and measuring the efflux of 86Rb+, we found that apoptosis-inducing treatment with the proteasome inhibitor MG-132 leads to a twofold increase in K+ efflux from IEC-6 cells. Blockade of K+ efflux with tetraethylammonium, 4-aminopyridine, stromatoxin, chromanol 293B, and the recently described K+ channel inhibitor 48F10 prevents DNA fragmentation, caspase activation, release of cytochrome c from mitochondria, and loss of mitochondrial membrane potential. Thus K+ efflux occurs early in the apoptotic program and is required for the execution of later events. Apoptotic K+ efflux critically depends on activation of p38 MAPK. These results demonstrate for the first time the requirement of K+ channel-mediated K+ efflux for progression of apoptosis in enterocytes and suggest the use of K+ channel blockers to prevent apoptotic cell loss occurring in intestinal pathologies.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

KCNQ1/KCNE1 channels during germ-cell differentiation in the rat: expression associated with testis pathologies

KCNQ1/KCNE1 channels are responsible for the Jervell-Lange-Nielsen cardiac syndrome, which is also characterized by congenital deafness. KCNQ1/KCNE1 is crucial for K+ transport in the inner ear. We show that KCNQ1 and KCNE1 are associated in testis and that their expression is closely regulated during development. Both genes were expressed in undifferentiated germ cells in 21-day-old rats and mostly confined to basal immature germ cells in adulthood. Leydig and Sertoli cells were negative. KCNQ1 and KCNE1 were also studied in various germ-cell pathologies. First, in spontaneous unilateral rat testis atrophy, hematoxylin-eosin analysis revealed massive germ-cell aplasia with only Sertoli cells and groups of interstitial Leydig cells. In these samples, KCNQ1 and KCNE1 were not expressed. In human seminoma samples characterized by a proliferation of undifferentiated germ cells, KCNQ1/KCNE1 protein levels were higher than in healthy samples. Our results demonstrate that the expression of KCNQ1 and KCNE1 is associated with early stages of spermatogenesis and with the presence of undifferentiated healthy or neoplastic germ cells. The presence of a K+ rich-fluid in the seminiferous tubule suggests that KCNQ1/KCNE1 is involved in K+ transport, probably during germ-cell development.

The MinK-related peptides

Voltage-gated potassium (Kv) channels mediate rapid, selective diffusion of K+ ions through the plasma membrane, controlling cell excitability, secretion and signal transduction. KCNE genes encode a family of single transmembrane domain proteins called MinK-related peptides (MiRPs) that function as ancillary or beta subunits of Kv channels. When co-expressed in heterologous systems, MiRPs confer changes in Kv channel conductance, gating kinetics and pharmacology, and are fundamental to recapitulation of the properties of some native currents. Inherited mutations in KCNE genes are associated with diseases of cardiac and skeletal muscle, and the inner ear. This article reviews our current understanding of MiRPs--their functional roles, the mechanisms underlying their association with Kv alpha subunits, their patterns of native expression and emerging evidence of the potential roles of MiRPs in the brain. The ubiquity of MiRP expression and their promiscuous association with Kv alpha subunits suggest a prominent role for MiRPs in channel dependent systems.

Basolateral localisation of KCNQ1 potassium channels in MDCK cells: molecular identification of an N-terminal targeting motif

KCNQ1 potassium channels are expressed in many epithelial tissues as well as in the heart. In epithelia KCNQ1 channels play an important role in salt and water transport and the channel has been reported to be located apically in some cell types and basolaterally in others. Here we show that KCNQ1 channels are located basolaterally when expressed in polarised MDCK cells. The basolateral localisation of KCNQ1 is not affected by co-expression of any of the five KCNE β-subunits. We characterise two independent basolateral sorting signals present in the N-terminal tail of KCNQ1. Mutation of the tyrosine residue at position 51 resulted in a non-polarized steady-state distribution of the channel. The importance of tyrosine 51 in basolateral localisation was emphasized by the fact that a short peptide comprising this tyrosine was able to redirect the p75 neurotrophin receptor, an otherwise apically located protein, to the basolateral plasma membrane. Furthermore, a di-leucine-like motif at residues 38-40 (LEL) was found to affect the basolateral localisation of KCNQ1. Mutation of these two leucines resulted in a primarily intracellular localisation of the channel.

Volume regulation is defective in renal proximal tubule cells isolated from KCNE1 knockout mice

The membrane protein KCNE1 has been implicated in cell volume regulation. Using a knockout mouse model, this study examined the role of KCNE1 in regulatory volume decrease (RVD) in freshly isolated renal proximal tubule cells. Cell diameter was measured using an optical technique in response to hypotonic shock and stimulation of Na(+)-alanine cotransport in cells isolated from wild-type and KCNE1 knockout mice. In HEPES buffered solutions 64% of wild-type and 56% of knockout cells demonstrated RVD. In HCO3- buffered solutions 100% of the wild-type cells showed RVD, while in the knockout cells the proportion of cells displaying RVD remained unchanged. RVD in the knockout cells was rescued by valinomycin, a K+ ionophore. In wild-type HCO3- dependent cells the K+ channel inhibitors barium and clofilium inhibited RVD. These data suggest that mouse renal proximal tubule is comprised of two cell populations. One cell population is capable of RVD in the absence of HCO3-, whereas RVD in the other cell population has an absolute requirement for HCO3-. The HCO3- dependent RVD requires the normal expression of KCNE1.

Molecular identity and function in transepithelial transport of K(ATP) channels in alveolar epithelial cells

K(+) channels play a crucial role in epithelia by repolarizing cells and maintaining electrochemical gradient for Na(+) absorption and Cl(-) secretion. In the airway epithelium, the most frequently studied K(+) channels are KvLQT1 and K(Ca). A functional role for K(ATP) channels has been also suggested in the lung, where K(ATP) channel openers activate alveolar clearance and attenuate ischemia-reperfusion injury. However, the molecular identity of this channel is unknown in airway and alveolar epithelial cells (AEC). We adopted an RT-PCR strategy to identify, in AEC, cDNA transcripts for Kir channels (Kir6.1 or 6.2) and sulfonylurea receptors (SUR1, 2A, or 2B) forming K(ATP) channels. Only Kir6.1 and SUR2B were detected in freshly isolated and cultured alveolar cells. To determine the physiological role of K(+) channels in the transepithelial transport of alveolar monolayers, we studied the effect, on total short-circuit currents (I(sc)), of basolateral application of glibenclamide, an inhibitor of K(ATP) channels, as well as clofilium, charybdotoxin, clotrimazole, and iberiotoxin, inhibitors of KvLQT1 and K(Ca) channels, respectively. Interestingly, activity of the three types of K(+) channels was detected, since all tested inhibitors decreased I(sc). Furthermore, these K(+) channel inhibitors reduced amiloride-sensitive Na(+) currents (mediated by ENaC) and completely abolished stimulation of Cl(-) currents by forskolin. Conversely, pinacidil, an activator of K(ATP) channels, increased Na(+) and Cl(-) transepithelial transport by 33-35%. These results suggest the presence, in AEC, of a K(ATP) channel, formed from Kir6.1 and SUR2B subunits, which plays a physiological role, with KvLQT1 and K(Ca) channels, in Na(+) and Cl(-) transepithelial transport.

Swelling-activated chloride and potassium conductance in primary cultures of mouse proximal tubules. Implication of KCNE1 protein

Volume-sensitive chloride and potassium currents were studied, using the whole-cell clamp technique, in cultured wild-type mouse proximal convoluted tubule (PCT) epithelial cells and compared with those measured in PCT cells from null mutant kcne1 -/- mice. In wild-type PCT cells in primary culture, a Cl- conductance activated by cell swelling was identified. The initial current exhibited an outwardly rectifying current-voltage (I-V) relationship, whereas steady-state current showed decay at depolarized membrane potentials. The ion selectivity was I- > Br- > Cl- > > gluconate. This conductance was sensitive to 1 mM 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 0.1 mM 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and 1 mM diphenylamine-2-carboxylate (DPC). Osmotic stress also activated K+ currents. These currents are time-independent, activated at depolarized potentials, and inhibited by 0.5 mM quinidine, 5 mM barium, and 10 microM clofilium but are insensitive to 1 mM tetraethylammonium (TEA), 10 nM charybdotoxin (CTX), and 10 microM 293B. In contrast, the null mutation of kcne1 completely impaired volume-sensitive chloride and potassium currents in PCT. The transitory transfection of kcne1 restores both Cl- and K+ swelling-activated currents, confirming the implication of KCNE1 protein in the cell-volume regulation in PCT cells in primary cultures.

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.

Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins

Regulated secretion in exocrine and neuroendocrine cells occurs through exocytosis of secretory granules and the subsequent release of stored small molecules and proteins. The introduction of biophysical techniques with high temporal and spatial resolution, and the identification of Ca(2+)-dependent and -independent "docking" and "fusion" proteins, has greatly enhanced our understanding of exocytosis. The cloning of families of ion channel proteins, including intracellular ion channels, has also revived interest in the role of secretory granule ion channels in exocytotic secretion. Thus secretory granules of pancreatic acinar cell express a ClC-2 Cl(-) channel, a HCO-permeable member of the CLCA Ca(2+)-dependent anion channel family, and a KCNQ1 K(+) channel. Evidence suggests that these channels may facilitate the release of digestive enzymes and/or prevent exocytosed granules from collapsing during "kiss and run" recycling. In pancreatic beta-cells, a granular ClC-3 Cl(-) channel provides a shunt pathway for a vacuolar-type H(+)-ATPase. Acidification "primes" the granules for Ca(2+)-dependent exocytosis and release of insulin. In summary, secretory granules are equipped with specific sets of ion channels, which modulate regulated exocytosis and the release of macromolecules. These channels could represent excellent targets for therapeutic interventions to control exocytotic secretion in relevant diseases, such as pancreatitis, cystic fibrosis, or diabetes mellitus.

KCNE4 is an inhibitory subunit to the KCNQ1 channel

KCNE4 is a membrane protein belonging to a family of single transmembrane domain proteins known to have dramatic effect on the gating of certain potassium channels. However, no functional role of KCNE4 has been suggested so far. In the present paper we demonstrate that KCNE4 is an inhibitory subunit to KCNQ1 channels. Co-expression of KCNQ1 and KCNE4 in Xenopus oocytes completely inhibited the KCNQ1 current. This was reproduced in mammalian CHO-K1 cells. Experiments with delayed expression of mRNA coding for KCNE4 in KCNQ1-expressing oocytes suggested that KCNE4 exerts its effect on KCNQ1 channels already expressed in the plasma membrane. This notion was supported by immunocytochemical studies and Western blotting, showing no significant difference in plasma membrane expression of KCNQ1 channels in the presence or absence of KCNE4. The impact of KCNE4 on KCNQ1 was specific since no effect of KCNE4 could be detected if co-expressed with KCNQ2-5 channels or hERG1 channels. RT-PCR studies revealed high KCNE4 expression in embryos and adult uterus, where significant expression of KCNQ1 channels has also been demonstrated.

Expression and coassociation of ERG1, KCNQ1, and KCNE1 potassium channel proteins in horse heart

In dogs and in humans, potassium channels formed by ether-a-go-go-related gene 1 protein ERG1 (KCNH2) and KCNQ1 alpha-subunits, in association with KCNE beta-subunits, play a role in normal repolarization and may contribute to abnormal repolarization associated with long QT syndrome (LQTS). The molecular basis of repolarization in horse heart is unknown, although horses exhibit common cardiac arrhythmias and may receive drugs that induce LQTS. In horse heart, we have used immunoblotting and immunostaining to demonstrate the expression of ERG1, KCNQ1, KCNE1, and KCNE3 proteins and RT-PCR to detect KCNE2 message. Peptide N-glycosidase F-sensitive forms of horse ERG1 (145 kDa) and KCNQ1 (75 kDa) were detected. Both ERG1 and KCNQ1 coimmunoprecipitated with KCNE1. Cardiac action potential duration was prolonged by antagonists of either ERG1 (MK-499, cisapride) or KCNQ1/KCNE1 (chromanol 293B). Patch-clamp analysis confirmed the presence of a slow delayed rectifier current. These data suggest that repolarizing currents in horses are similar to those of other species, and that horses are therefore at risk for acquired LQTS. The data also provide unique evidence for coassociation between ERG1 and KCNE1 in cardiac tissue.

The multifaceted phenotype of the knockout mouse for the KCNE1 potassium channel gene

Mutations of the KCNE1 gene (IsK, minK) are related to hereditary forms of cardiac arrhythmias, so-called long QT syndromes (LQT). Here we review the phenotype of a mouse model for the recessive form of LQT known as Jervell and Lange-Nielsen syndrome. KCNE1 knockout mice exhibit an enhanced QT-RR adaptability, which is probably part of the pathophysiological mechanism leading to life-threatening tachyarrhythmia in patients. Like patients, knockout mice are deaf and show vestibular symptoms due to an impaired endolymph production. Knockout mice show urinary and fecal salt wasting and volume depletion. The renal phenotype is due to diminished reabsorption of Na(+) and glucose. The mice are hypokalemic and have increased aldosterone levels. Besides volume depletion, aldosterone is elevated via a set-point shift for sensing of extracellular K(+) in aldosterone-secreting glomerulosa cells, which physiologically express KCNE1. In conclusion, KCNE1 knockout leads to a complex phenotype resulting from direct loss of KCNE1 and compensatory mechanisms. Murine KCNE1 physiology could be helpful for the pathophysiological understanding and perhaps gene-specific treatment of long QT patients.

Genetic manipulation of cardiac K(+) channel function in mice: what have we learned, and where do we go from here?

In the mammalian myocardium, potassium (K(+)) channels control resting potentials, action potential waveforms, automaticity, and refractory periods and, in most cardiac cells, multiple types of K(+) channels that subserve these functions are expressed. Molecular cloning has revealed the presence of a large number of K(+) channel pore forming (alpha) and accessory (beta) subunits in the heart, and considerable progress has been made recently in defining the relationships between expressed K(+) channel subunits and functional cardiac K(+) channels. To date, more than 20 mouse models with altered K(+) channel expression/functioning have been generated using dominant-negative transgenic and targeted gene deletion approaches. In several instances, the genetic manipulation of K(+) channel subunit expression has revealed the role of specific K(+) channel subunit subfamilies or individual K(+) channel subunit genes in the generation of myocardial K(+) channels. In other cases, however, the phenotypic consequences have been unexpected. This review summarizes what has been learned from the in situ genetic manipulation of cardiac K(+) channel functioning in the mouse, discusses the limitations of the models developed to date, and explores the likely directions of future research.

Altered potassium balance and aldosterone secretion in a mouse model of human congenital long QT syndrome

The voltage-dependent K(+) channel responsible for the slowly activating delayed K(+) current I(Ks) is composed of pore-forming KCNQ1 and regulatory KCNE1 subunits, which are mutated in familial forms of cardiac long QT syndrome. Because KCNQ1 and KCNE1 genes also are expressed in epithelial tissues, such as the kidneys and the intestine, we have investigated the adaptation of KCNE1-deficient mice to different K(+) and Na(+) intakes. On a normal K(+) diet, homozygous kcne1(-/-) mice exhibit signs of chronic volume depletion associated with fecal Na(+) and K(+) wasting and have lower plasma K(+) concentration and higher levels of aldosterone than wild-type mice. Although plasma aldosterone can be suppressed by low K(+) diets or stimulated by low Na(+) diets, a high K(+) diet provokes a tremendous increase of plasma aldosterone levels in kcne1(-/-) mice as compared with wild-type mice (7.1-fold vs. 1.8-fold) despite lower plasma K(+) in kcne1(-/-) mice. This exacerbated aldosterone production in kcne1(-/-) mice is accompanied by an abnormally high plasma renin concentration, which could partly explain the hyperaldosteronism. In addition, we found that KCNE1 and KCNQ1 mRNAs are expressed in the zona glomerulosa of adrenal glands where I(Ks) may directly participate in the control of aldosterone production by plasma K(+). These results, which show that KCNE1 and I(Ks) are involved in K(+) homeostasis, might have important implications for patients with I(Ks)-related long QT syndrome, because hypokalemia is a well known risk factor for the occurrence of torsades de pointes ventricular arrhythmia.

AKAP proteins anchor cAMP-dependent protein kinase to KvLQT1/IsK channel complex

In cardiac myocytes, the slow component of the delayed rectifier K(+) current (I(Ks)) is regulated by cAMP. Elevated cAMP increases I(Ks) amplitude, slows its deactivation kinetics, and shifts its activation curve. At the molecular level, I(Ks) channels are composed of KvLQT1/IsK complexes. In a variety of mammalian heterologous expression systems maintained at physiological temperature, we explored cAMP regulation of recombinant KvLQT1/IsK complexes. In these systems, KvLQT1/IsK complexes were totally insensitive to cAMP regulation. cAMP regulation was not restored by coexpression with the dominant negative isoform of KvLQT1 or with the cystic fibrosis transmembrane regulator. In contrast, coexpression of the neuronal A kinase anchoring protein (AKAP)79, a fragment of a cardiac AKAP (mAKAP), or cardiac AKAP15/18 restored cAMP regulation of KvLQT1/IsK complexes inasmuch as cAMP stimulation increased the I(Ks) amplitude, increased its deactivation time constant, and negatively shifted its activation curve. However, in cells expressing an AKAP, the effects of cAMP stimulation on the I(Ks) amplitude remained modest compared with those previously reported in cardiac myocytes. The effects of cAMP stimulation were fully prevented by including the Ht31 peptide (a global disruptor of protein kinase A anchoring) in the intracellular medium. We concluded that cAMP regulation of I(Ks) requires protein kinase A anchoring by AKAPs, which therefore participate with the channel protein complex underlying I(Ks).

Mice disrupted for the KvLQT1 potassium channel regulator IsK gene accumulate mature T cells

The IsK protein associates with KvLQT1 potassium channels to generate the slow component of the outward rectifying K(+) current involved in human cardiac repolarization. Mutations in either KCNE1 (encoding IsK) or KCNQ1 (encoding KvLQT1) genes have been associated with the long QT syndrome, a genetic disorder leading to prolonged cardiac repolarization and sudden death. We now report that the IsK protein is also involved in mature T cell homeostasis. In KCNE1 gene knockout mice, we observed a significant increase in the T cell compartment. Thymus and peripheral lymphoid organs of KCNE1-/- mice displayed a significant increase in mature T cells. The immunological phenotype of KCNE1-/- is age-dependent and only expressed in adult mice. Both IsK and KvLQT1 mRNA are expressed in murine thymus. Our data suggest that, in addition to its role in myocardial repolarization, the IsK-KvLQT1 tandem also plays a crucial role in T cell homeostasis.