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International audience; Dent disease is an X‐linked recessive renal tubular disorder characterized by low‐molecular‐weight proteinuria, hypercalciuria, nephrolithiasis, nephrocalcinosis, and progressive renal failure. Inactivating mutations of CLCN5, the gene encoding the 2Cl−/H+ exchanger ClC‐5, have been reported in patients with Dent disease 1. In vivo studies in mice harboring an artificial mutation in the “gating glutamate” of ClC‐5 (c.632A > C, p.Glu211Ala) and mathematical modeling suggest that endosomal chloride concentration could be an important parameter in endocytosis, rather than acidification as earlier hypothesized. Here, we described a novel pathogenic mutation affecting the “gating glutamate” of ClC‐5 (c.632A>G, p.Glu211Gly) and investigated its molecular consequences. In HEK293T cells, the p.Glu211Gly ClC‐5 mutant displayed unaltered N‐glycosylation and normal plasma membrane and early endosomes localizations. In Xenopus laevis oocytes and HEK293T cells, we found that contrasting with wild‐type ClC‐5, the mutation abolished the outward rectification, the sensitivity to extracellular H+ and converted ClC‐5 into a Cl− channel. Investigation of endosomal acidification in HEK293T cells using the pH‐sensitive pHluorin2 probe showed that the luminal pH of cells expressing a wild‐type or p.Glu211Gly ClC‐5 was not significantly different. Our study further confirms that impaired acidification of endosomes is not the only parameter leading to defective endocytosis in Dent disease 1.

CLC anion/proton exchangers control the pH and [Cl−] of the endolysosomal system that is essential for cellular nutrient uptake. Here, we use heterologous expression and whole‐cell electrophysiology to investigate the regulation of the CLC isoforms ClC‐3, ClC‐4, and ClC‐5 by the adenylic system components ATP, ADP, and AMP. Our results show that cytosolic ATP and ADP but not AMP and Mg2+‐free ADP enhance CLC ion transport. Biophysical analysis reveals that adenine nucleotides alter the ratio between CLC ion transport and CLC gating charge and shift the CLC voltage‐dependent activation. The latter effect is suppressed by blocking the intracellular entrance of the proton transport pathway. We suggest, therefore, that adenine nucleotides regulate the internal proton delivery into the CLC transporter machinery and alter the probability of CLC transporters to undergo silent non‐transporting cycles. Our findings suggest that the CBS domains in mammalian CLC transporters serve as energy sensors that regulate vesicular Cl−/H+ exchange by detecting changes in the cytosolic ATP/ADP/AMP equilibrium. Such sensing mechanism links the endolysosomal activity to the cellular metabolic state., Electrophysiology investigations reveal that ATP, ADP and AMP differentially regulate the ion transport of the vesicular anion/proton exchangers ClC‐3, ClC‐4 and ClC‐5, coupling the activity of the endolysosomal system to the cellular metabolic state.

Dent disease 1 (DD1) is a renal salt-wasting tubulopathy associated with mutations in the Cl-/H+ antiporter ClC-5. The disease typically manifests with proteinuria, hypercalciuria, nephrocalcinosis, and nephrolithiasis but is characterized by large phenotypic variability of no clear origin. Several DD1 cases have been reported lately with additional atypical hypokalemic metabolic alkalosis and hyperaldosteronism, symptoms usually associated with another renal disease termed Bartter syndrome (BS). Expression of the Bartter-like DD1 mutant ClC-5 G261E in HEK293T cells showed that it is retained in the ER and lacks the complex glycosylation typical for ClC-5 WT. Accordingly, the mutant abolished CLC ionic transport. Such phenotype is not unusual and is often observed also in DD1 ClC-5 mutants not associated with Bartter like phenotype. We noticed, therefore, that one type of BS is associated with mutations in the protein barttin that serves as an accessory subunit regulating the function and subcellular localization of ClC-K channels. The overlapping symptomatology of DD1 and BS, together with the homology between the proteins of the CLC family, led us to investigate whether barttin might also regulate ClC-5 transport. In HEK293T cells, we found that barttin cotransfection impairs the complex glycosylation and arrests ClC-5 in the endoplasmic reticulum. As barttin and ClC-5 are both expressed in the thin and thick ascending limbs of the Henle’s loop and the collecting duct, interactions between the two proteins could potentially contribute to the phenotypic variability of DD1. Pathologic barttin mutants differentially regulated trafficking and processing of ClC-5, suggesting that the interaction between the two proteins might be relevant also for the pathophysiology of BS. Our findings show that barttin regulates the subcellular localization not only of kidney ClC-K channels but also of the ClC-5 transporter, and suggest that ClC-5 might potentially play a role not only in kidney proximal tubules but also in tubular kidney segments expressing barttin. In addition, they demonstrate that the spectrum of clinical, genetic and molecular pathophysiology investigation of DD1 should be extended.

Toxin A and Toxin B (TcdA/TcdB) are large glucosyltransferases produced by Clostridium difficile. TcdB but not TcdA induces reactive oxygen species-mediated early cell death (ECD) when applied at high concentrations. We found that nonglucosylated Rac1 is essential for induction of ECD since inhibition of Rac1 impedes this effect. ECD only occurs when TcdB is rapidly endocytosed. This was shown by generation of chimeras using the trunk of TcdB from a hypervirulent strain. TcdB from hypervirulent strain has been described to translocate from endosomes at higher pH values and thus, meaning faster than reference type TcdB. Accordingly, intracellular delivery of the glucosyltransferase domain of reference TcdB by the trunk of TcdB from hypervirulent strain increased ECD. Furthermore, proton transporters such as sodium/proton exchanger (NHE) or the ClC-5 anion/proton exchanger, both of which contribute to endosomal acidification, also affected cytotoxic potency of TcdB: Specific inhibition of NHE reduced cytotoxicity, whereas transfection of cells with the endosomal anion/proton exchanger ClC-5 increased cytotoxicity of TcdB. Our data suggest that both the uptake rate of TcdB into the cytosol and the status of nonglucosylated Rac1 are key determinants that are decisive for whether ECD or delayed apoptosis is triggered.

Most mammalian chloride channels and transporters in the CLC family display pronounced voltage-dependent gating. Surprisingly, despite the complex nature of the gating process and the large contribution to it by the transport substrates, experimental investigations of the fast gating process usually produce canonical Boltzmann activation curves that correspond to a simple two-state activation. By using nonlinear capacitance measurements of two mutations in the ClC-5 transporter, here we are able to discriminate and visualize discrete transitions along the voltage-dependent activation pathway. The strong and specific dependence of these transitions on internal and external [Cl−] suggest that CLC gating involves voltage-dependent conformational changes as well as coordinated movement of transported substrates.

Eukaryotic ClC channels are dimeric proteins with each subunit forming an individual protopore. Single protopores are gated by a fast gate, whereas the slow gate is assumed to control both protopores through a cooperative movement of the two carboxy-terminal domains. We here study the role of the carboxy-terminal domain in modulating fast and slow gating of human ClC-2 channels, a ubiquitously expressed ClC-type chloride channel involved in transepithelial solute transport and in neuronal chloride homeostasis. Partial truncation of the carboxy-terminus abolishes function of ClC-2 by locking the channel in a closed position. However, unlike other isoforms, its complete removal preserves function of ClC-2. ClC-2 channels without the carboxy-terminus exhibit fast and slow gates that activate and deactivate significantly faster than in WT channels. In contrast to the prevalent view, a single carboxy-terminus suffices for normal slow gating, whereas both domains regulate fast gating of individual protopores. Our findings demonstrate that the carboxy-terminus is not strictly required for slow gating and that the cooperative gating resides in other regions of the channel protein. ClC-2 is expressed in neurons and believed to open at negative potentials and increased internal chloride concentrations after intense synaptic activity. We propose that the function of the ClC-2 carboxy-terminus is to slow down the time course of channel activation in order to stabilize neuronal excitability

Dent’s disease is associated with impaired renal endocytosis and endosomal acidification. It is linked to mutations in the membrane chloride/proton exchanger ClC-5, however, a direct link between localization in the protein and functional phenotype of the mutants has not been established until now. Here, two Dent’s disease mutations, G212A and E267A, were investigated using heterologous expression in HEK293T cells, patch-clamp measurements and confocal imaging. WT and, mutant ClC-5 exhibited mixed cell membrane and vesicular distribution. Reduced ion currents were measured for both mutants and both exhibited reduced capability to support endosomal acidification. Functionally, mutation G212A was capable of mediating anion/proton antiport but dramatically shifted the activation of ClC-5 towards more depolarized potentials. The shift can be explained by impeded movements of the neighboring gating glutamate Gluext, a residue that confers major part of the voltage dependence of ClC-5 and serves as a gate at the extracellular entrance of the anion transport pathway. Cell surface abundance of E267A was reduced by ~50% but also dramatically increased gating currents were detected for this mutant and accordingly reduced probability to undergoing cycles associated with electrogenic ion transport. Structurally, the gating alternations correlate to the proximity of E267A to the proton glutamate Gluin that serves as intracellular gate in the proton transport pathway and regulates the open probability of ClC-5. Remarkably, two other mammalian isoforms, ClC-3 and ClC-4, also differ from ClC-5 in gating characteristics affected by the here investigated disease-causing mutations. This evolutionary specialization, together with the functional defects arising from mutations G212A and E267A, demonstrate that the complex gating behavior exhibited by most of the mammalian CLC transporters is an important determinant of their cellular function.

Cytoplasmic S4‐S5 loops have been shown to be involved in fast inactivation of voltagegate di on channels. We studied mutations in these loops and their potential cooperative effects in domains D3 (N1151C, A1152C, I1160C/A) and D4 (F1473C, L1482C/A) of the human skeletal muscle Na + channel α-subunit (hNav1.4) using expression in tsA201 cells and the whole cell patch-clamp technique. All cysteine mutations were accessible to intracellularly applied sulfhydryl reagents which considerably destabilized fast inactivation. For different combinations of corresponding D3/D4 double mutations, fast inactivation could be almost completely removed. Thermodynamic cycle analysis indicated an additive effect for N1151C/F1473C and a significant cooperative effect for I1160/L1482 double mutations. Application of oxidizing reagents such as Cu-phenanthroline to link two cysteines via a disulfide bridge did not reveal evidence for a direct physical interaction of cysteines in D3 and D4. In addition to the pronounced alterations of fast inactivation, mutations of I1160 shifted steady-state activation in the hyperpolarizing direction and slowed the kinetics of both activation and deactivation. Sulfhydryl reagents had charge-dependent effects on I1160C suggesting interaction with negative charges in another protein region. We conclude that fast inactivation of the Na + channel involves both S4‐S5 loops in D3 and D4 in a cooperative manner. D3/S4‐S5 also plays an important role in activation and deactivation.

Idiopathic generalized epilepsy (IGE) is an inherited neurological disorder affecting about 0.4% of the world's population. Mutations in ten genes causing distinct forms of idiopathic epilepsy have been identified so far, but the genetic basis of many IGE subtypes is still unknown. Here we report a gene associated with the four most common IGE subtypes: childhood and juvenile absence epilepsy (CAE and JAE), juvenile myoclonic epilepsy (JME), and epilepsy with grand mal seizures on awakening (EGMA; ref. 8). We identified three different heterozygous mutations in the chloride-channel gene CLCN2 in three unrelated families with IGE. These mutations result in (i) a premature stop codon (M200fsX231), (ii) an atypical splicing (del74-117) and (iii) a single amino-acid substitution (G715E). All mutations produce functional alterations that provide distinct explanations for their pathogenic phenotypes. M200fsX231 and del74-117 cause a loss of function of ClC-2 channels and are expected to lower the transmembrane chloride gradient essential for GABAergic inhibition. G715E alters voltage-dependent gating, which may cause membrane depolarization and hyperexcitability.

ClC-3 is a member of the CLC family of anion channels and transporters that localizes to early and late endosomes as well as to synaptic vesicles (SV). Its genetic disruption in mouse models results in pronounced hippocampal and retinal neurodegeneration, suggesting that ClC-3 might be important for normal excitatory and/or inhibitory neurotransmission in central neurons. To characterize the role of ClC-3 in glutamate accumulation in SV we compared glutamatergic synaptic transmission in cultured hippocampal neurons from WT and Clcn3-/- mice. In Clcn3-/- neurons the amplitude and frequency of miniature as well as the amplitudes of action-potential evoked EPSCs were significantly increased as compared to WT neurons. The low-affinity competitive AMPA receptor antagonist γ-DGG reduced the quantal size of synaptic events more effectively in WT than in Clcn3-/- neurons, whereas no difference was observed for the high-affinity competitive non-NMDA antagonist NBQX. Paired pulse ratios of evoked EPSCs were significantly reduced, whereas the size of the readily releasable pool was not affected by the genetic ablation of ClC-3. Electron microscopy revealed increased volumes of SV in hippocampi of Clcn3-/- mice. Our findings demonstrate that ClC-3 controls fast excitatory synaptic transmission by regulating the amount of neurotransmitter as well as the release probability of SV. These results provide novel insights into the role of ClC-3 in synaptic transmission and identify excessive glutamate release as a likely basis of neurodegeneration in Clcn3-/-.

Generalized epilepsy with febrile seizures-plus (GEFS+) is a benign Mendelian syndrome characterized by childhood-onset febrile and afebrile seizures. Three point mutations within two voltage-gated sodium channel genes have been identified so far: in GEFS+ type 1 a mutation in the beta1-subunit gene SCN1B, and in GEFS+ type 2 two mutations within the neuronal alpha-subunit gene SCN1A. Functional expression of the SCN1B and one of the SCN1A mutations revealed defects in fast channel inactivation which are in line with previous findings on myotonia causing mutations in SCN4A, the skeletal muscle sodium channel alpha-subunit gene, all showing an impaired fast inactivation. We now studied the second GEFS+ mutation (T875M in SCN1A), using the highly homologous SCN4A gene (mutation T685M). Unexpectedly, the experiments revealed a pronounced enhancement of both fast and slow inactivation and a defect of channel activation for T685M compared to wild-type channels. Steady-state fast and slow inactivation curves were shifted in the hyperpolarizing direction, entry into slow inactivation was threefold accelerated, recovery from slow inactivation was slowed by threefold and the time course of activation was slightly but significantly accelerated. In contrast to other disease-causing mutations in SCN1A, SCN1B and SCN4A, the only mechanism that could explain hyperexcitability of the cell membrane would be the acceleration of activation. Because the enhancement of slow inactivation was the most obvious alteration in gating found for T685M, this might be the disease-causing mechanism for that mutation. In this case, the occurrence of epileptic seizures could be explained by a decrease of excitability of inhibitory neurons.

Fast and slow inactivation (FI, SI) of the voltage-gated Na+ channel are two kinetically distinct and structurally dissociated processes. The voltage sensor IV/S4 and the intracellular IV/S4-S5 loop have been shown to play an important role in FI mediating the coupling between activation and inactivation. Two mutations in IV/S4-S5 of the human muscle Na+ channel, L1482C/A, disrupt FI by inducing a persistent Na+ current, shifting steady-state inactivation in the depolarizing direction and accelerating its recovery. These effects were more pronounced for L1482A. In contrast, SI of L1482C/A channels was enhanced showing a more complete SI and a 3-fold slowing of its recovery. Effects on SI were more pronounced for L1482C. The results indicate an important role of the IV/S4-S5 loop not only in FI but also in SI of the Na+ channel.

1. Generalized epilepsy with febrile seizures plus (GEFS + ) is a benign epileptic syndrome of humans. It is characterized by febrile and afebrile generalized seizures that occur predominantly in childhood and respond well to standard antiepileptic therapy. A mutation in the b 1-subunit of the voltage-gated sodium channel, linked to chromosome 19q13 (GEFS + type 1) has been found in one family. For four other families, linkage was found to chromosome 2q21–33 (GEFS + type 2) where three genes encoding neuronal sodium channel a-subunits are located (SCN1–3A). Recently, the first two mutations were identified in SCN1A. 2. We introduced one of these mutations, which is highly conserved to SCN1A, into the cDNA of the gene SCN4A encoding the a-subunit of the human skeletal muscle sodium channel (hSkm1). The mutation is located in the S4 voltage sensor of domain IV, predicting substitution of histidine for the fifth of eight arginines (R1460H in hSkm1). Functional studies were performed by expressing the a-subunit alone in the mammalian tsA201 cell line using the whole-cell patch clamp technique. 3. Compared to wild-type (WT), mutant R1460H channels showed small defects in fast inactivation. The time course of inactivation was slightly (1.5-fold) slowed and its voltage dependence reduced, and recovery from inactivation was accelerated 3-fold. However, there was no increase in persistent sodium current as observed for SCN4A mutations causing myotonia or periodic paralysis. The activation time course of R1460H channels was slightly accelerated. Slow inactivation was slightly but significantly stabilized, confirming the importance of this region for slow inactivation. 4. The combination of activation and fast inactivation defects can explain the occurrence of epileptic seizures, but the effects were much more subtle than the inactivation defects described previously for mutations in SCN4A causing disease in skeletal muscle. Hence, with regard to pathological excitability, our results suggest a greater vulnerability of the central nervous system compared to muscle tissue.

Low-frequencyanomalous electro-optic behavior of colloidal systems (sign reversal and deviations from Kerr low) is considered in the light of electrically induced acoustic modes. The latter were recently detected and investigated in samples of isotropic spherical particles. Their linear dependence on field intensity explains the low-field "permanent dipole" behavior of charged colloids. The coupling of anisotropy and density fluctuations results in the complicated frequency curves of the electro-optic responses of anisometric particles. Copyright 1999 Academic Press.

Benign familial neonatal convulsions (BFNC) is a rare dominantly inherited epileptic syndrome characterized by frequent brief seizures within the first days of life. The disease is caused by mutations in one of two recently identified voltage-gated potassium channel genes, KCNQ2 or KCNQ3. Here, we describe a four-generation BFNC family carrying a novel mutation within the distal, unconserved C-terminal domain of KCNQ2, a 1-bp deletion, 2513delG, in codon 838 predicting substitution of the last seven and extension by another 56 amino acids. Three family members suffering from febrile but not from neonatal convulsions do not carry the mutation, confirming that febrile convulsions and BFNC are of different pathogenesis. Functional expression of the mutant channel in Xenopus oocytes revealed a reduction of the potassium current to 5% of the wild-type current, but the voltage sensitivity and kinetics were not significantly changed. To find out whether the loss of the last seven amino acids or the C-terminal extension because of 2513delG causes the phenotype, a second, artificial mutation was constructed yielding a stop codon at position 838. This truncation increased the potassium current by twofold compared with the wild type, indicating that the pathological extension produces the phenotype, and suggesting an important role of the distal, unconserved C-terminal domain of this channel. Our results indicate that BFNC is caused by a decreased potassium current impairing repolarization of the neuronal cell membrane, which results in hyperexcitability of the central nervous system.

Low frequency anomalous electro-optic behaviour of colloids (deviations from Kerr law) concerns a series of phenomena in different systems widely studied by researchers involved in the field of colloids, (bio)polymers, (bio)polyelectrolytes, etc. Investigations on samples of isotropic spherical particles display electro-optic effects of similar characteristics, which are recognized as electrically induced acoustic modes. Therefore the analysis of the colloidal electro-optic spectra requires separation of anisotropy and density fluctuations prior to application of the method for determination of single particle constants.

The chloride/proton exchangers ClC-3, ClC-4 and ClC-5 are localized in distinct intracellular compartments and regulate their luminal acidity. We used electrophysiology combined with fluorescence pH measurements to compare the functions of these three transporters. Since the expression of WT ClC-3 in the surface membrane was negligible, we removed an N-terminal retention signal for standard electrophysiological characterization of this isoform. This construct (ClC-313-19A) mediated outwardly rectifying coupled Cl(-)/H(+) antiport resembling the properties of ClC-4 and ClC-5. In addition, ClC-3 exhibited large electric capacitance, exceeding the nonlinear capacitances of ClC-4 and ClC-5. Mutations of the proton glutamate, a conserved residue at the internal side of the protein, decreased ion transport but increased nonlinear capacitances in all three isoforms. This suggests that nonlinear capacitances in mammalian ClC transporters are regulated in a similar manner. However, the voltage dependence and the amplitudes of these capacitances differed strongly between the investigated isoforms. Our results indicate that ClC-3 is specialized in mainly performing incomplete capacitive nontransporting cycles, that ClC-4 is an effective coupled transporter, and that ClC-5 displays an intermediate phenotype. Mathematical modeling showed that such functional differences would allow differential regulation of luminal acidification and chloride concentration in intracellular compartments.

The Cl(-)/H(+) exchange mediated by ClC transporters can be uncoupled by external SCN(-) and mutations of the proton glutamate, a conserved residue at the internal side of the protein. We show here for the mammalian ClC transporter ClC-5 that acidic internal pH led to a greater increase in currents upon exchanging extracellular Cl(-) for SCN(-). However, transport uncoupling, unitary current amplitudes, and the voltage dependence of the depolarization-induced activation were not altered by low pH values. Therefore, it is likely that an additional gating process regulates ClC-5 transport. Higher internal [H(+)] and the proton glutamate mutant E268H altered the ratio between ClC-5 transport and nonlinear capacitance, indicating that the gating charge movements in ClC-5 arise from incomplete transport cycles and that internal protons increase the transport probability of ClC-5. This was substantiated by site-directed sulfhydryl modification of the proton glutamate mutant E268C. The mutation exhibited small transport currents together with prominent gating charge movements. The charge restoration using a negatively charged sulfhydryl reagent reinstated also the WT phenotype. Neutralization of the charge of the gating glutamate 211 by the E211C mutation abolished the effect of internal protons, showing that the increased transport probability of ClC-5 results from protonation of this residue. S168P (a mutation that decreases the anion affinity of the central binding site) reduced also the internal pH dependence of ClC-5. These results support the idea that protonation of the gating glutamate 211 at the central anion-binding site of ClC-5 is mediated by the proton glutamate 268.

The ClC family encompasses two classes of proteins with distinct transport functions: anion channels and transporters. ClC-type transporters usually mediate secondary active anion–proton exchange. However, under certain conditions they assume slippage mode behavior in which proton and anion transport are uncoupled, resulting in passive anion fluxes without associated proton movements. Here, we use patch clamp and intracellular pH recordings on transfected mammalian cells to characterize exchanger and slippage modes of human ClC-4, a member of the ClC transporter branch. We found that the two transport modes differ in transport mechanisms and transport rates. Nonstationary noise analysis revealed a unitary transport rate of 5 × 105 s−1 at +150 mV for the slippage mode, indicating that ClC-4 functions as channel in this mode. In the exchanger mode, unitary transport rates were 10-fold lower. Both ClC-4 transport modes exhibit voltage-dependent gating, indicating that there are active and non-active states for the exchanger as well as for the slippage mode. ClC-4 can assume both transport modes under all tested conditions, with exchanger/channel ratios determined by the external anion. We propose that binding of transported anions to non-active states causes transition from slippage into exchanger mode. Binding and unbinding of anions is very rapid, and slower transitions of liganded and non-liganded states into active conformations result in a stable distribution between the two transport modes. The proposed mechanism results in anion-dependent conversion of ClC-type exchanger into an anion channel with typical attributes of ClC anion channels.

Barttin is an accessory subunit of a subgroup of ClC-type chloride channels expressed in renal and inner ear epithelia. In this study, we examined the effects of barttin on two ClC-K channel isoforms, rat ClC-K1 and human ClC-Kb, using heterologous expression, patch clamping, confocal imaging, and flow cytometry. In the absence of barttin, only a small percentage of rClC-K1 and hClC-Kb channels are inserted into the plasma membrane. Coexpression of barttin enhances surface membrane insertion and furthermore modifies permeation and gating of ClC-K channels. hClC-Kb channels are nonfunctional without barttin and require the coexpressed accessory subunit to become anion conducting. In contrast, rClC-K1 channels are active without barttin, but at the cost of reduced unitary conductance as well as altered voltage dependence of activation. We mapped the separate functions of barttin to structural domains by a deletion analysis. Whereas the transmembrane core is necessary and sufficient to promote ClC-K channel exit from the endoplasmic reticulum, a short cytoplasmic segment following the second transmembrane helix modifies the unitary conductance. The entire cytoplasmic carboxyl terminus affects the open probability of ClC-K channels. The multiple functions of barttin might be necessary for a tight adjustment of epithelial Cl − conductances to ensure a precise regulation of body salt content and endocochlear potential.

The effects of extracellular pH (6.2, 7.4 and 8.2) and 0.1 mM mexiletine, a channel blocker of the lidocaine type, are studied on two mutations of the fourth voltage sensor of the Nav1.4 sodium channel, R1448H/C. The fast inactivated channel state to which mexiletine preferentially binds is destabilized by the mutations. By contrast to the expected low response of R1448H/C carriers, mexiletine is particularly effective in preventing exercise-induced stiffness and paralysis from which these patients suffer. Our measurements performed in the whole-cell mode on stably transfected HEK cells show for the first time that the mutations strikingly accelerate closed-state inactivation and, as steady-state fast inactivation is shifted to more negative potentials, stabilize the fast inactivated channel state in the potential range around the resting potential. At pH 7.4 and 8.2, the phasic mexiletine block is larger for R1448C (55%) and R1448H (47%) than for wild-type channels (31%) due to slowed recovery from block (tau is approximately 520 ms for R1448C versus 270 ms for wild-type at pH 7.4) although the recovery from inactivation is slightly faster for the mutants (tau is approximately 1.9 ms for R1448C versus 3.8 ms for wild-type at pH 7.4). At pH 6.2, recovery from block is relatively fast (tau is approximately 35 ms for R1448H/C and 14 ms for wild-type) and thus shows no use-dependence. We conclude that enhanced closed-state inactivation expands the concept of a mutation-induced uncoupling of channel inactivation from activation to a new potential range and that the higher mexiletine efficacy in R1448H/C carriers compared to other myotonic patients offers a pharmacogenetic strategy for mutation-specific treatment.

Human malignant mesotheliomas (HMMs) are aggressive tumors that arise from the mesothelium. They respond poorly to conventional tumor treatment and outcome is often fatal. Inactivating mutations of the neurofibromatosis type 2 (NF2) tumor suppressor gene merlin have been described in nearly 60% of primary malignant mesothelioma and in approximately 20% of the mesothelioma cell lines. Studies regarding human NF2 schwannoma cells revealed a higher proliferation and a larger noninactivating K(+) outward current compared with controls. The enhanced proliferation of merlin-deficient NF2 schwannoma cells could be reduced in the presence of quinidine, a K(+) channel blocker, whereas the proliferation of normal Schwann cells is not affected. The current study was undertaken to evaluate the effect of quinidine on the proliferation of HMM cell lines in relation to their NF2 status.Proliferation analyses using bromodeoxyuridine incorporation was performed by immunocytochemical staining and fluorescence assisted cell sorting. The patch-clamp technique was applied for electrophysiologic characterization of the HMM cell lines. The cytochrome P450 2D6 locus, known to be mutated at high frequencies in NF2 patients and to be specifically inhibited by quinidine, was screened for mutations by cycle sequencing.Quinidine selectively reduces the proliferation of merlin-deficient HMM cell lines by causing a G(0)/G(1) arrest, whereas the proliferation rates of merlin-expressing HMM cell lines remain unchanged. The effect of quinidine on the proliferation of HMM cell lines appears to be correlated with the NF2 gene status but not with the K(+) outward current. No relation to cytochrome P450 2D6 mutations was detected.Quinidine or quinidine analogs are of potential therapeutic interest for the subset of merlin-deficient mesothelioma tumors.

Coexpression of KCNQ2 and KCNQ3 channels results in a 10-fold increased current amplitude compared to that of KCNQ2 alone, suggesting the formation of heteromultimeric channels. There is no interaction of either channel with KCNQ1. We evaluated the C-terminus as a potential interaction domain by construction of chimeras with interchanged C-termini of KCNQ1, KCNQ2 and KCNQ3 and functional expression in Xenopus oocytes. The chimera of KCNQ1 with a KCNQ2 C-terminus (Q1ctQ2) showed an 8-fold increase in current amplitude, and Q1ctQ3 a 3-fold increase when coexpressed with KCNQ3 and KCNQ2, respectively, indicating that the C-terminus contains an interaction domain. To characterize this interacting region, we studied further chimeras of KCNQ1 containing different parts of the KCNQ3 C-terminus for interaction with KCNQ2. We also evaluated short sequences of the KCNQ2 C-terminus for a dominant-negative effect on Q1ctQ3. According to the results of these experiments, functional interaction of KCNQ2 and KCNQ3 requires a highly conserved region of about 80 amino acids, previously called the A-domain, plus either 40 residues downstream of the A-domain (B-domain) or the proximal C-terminus between S6 and the A-domain. Furthermore, the chimeras Q1ctQ3 and Q2ctQ3 showed > 10-fold increased current amplitudes compared to KCNQ1 or KCNQ2 alone and a strong depolarizing shift of voltage-dependent activation. The proximal part of the KCNQ3 C-terminus was necessary to produce these effects. Our results indicate that specific parts of the C-terminus enable the interaction between KCNQ2 and KCNQ3 channels and that different parts of the KCNQ3 C-terminus are important for regulating current amplitude.

Idiopathic generalized epilepsy (IGE) is an inherited neurological disorder affecting about 0.4% of the world's population. Mutations in ten genes causing distinct forms of idiopathic epilepsy have been identified so far, but the genetic basis of many IGE subtypes is still unknown. Here we report a gene associated with the four most common IGE subtypes: childhood and juvenile absence epilepsy (CAE and JAE), juvenile myoclonic epilepsy (JME), and epilepsy with grand mal seizures on awakening (EGMA; ref. 8). We identified three different heterozygous mutations in the chloride-channel gene CLCN2 in three unrelated families with IGE. These mutations result in (i) a premature stop codon (M200fsX231), (ii) an atypical splicing (del74-117) and (iii) a single amino-acid substitution (G715E). All mutations produce functional alterations that provide distinct explanations for their pathogenic phenotypes. M200fsX231 and del74-117 cause a loss of function of ClC-2 channels and are expected to lower the transmembrane chloride gradient essential for GABAergic inhibition. G715E alters voltage-dependent gating, which may cause membrane depolarization and hyperexcitability.

Mutations in the voltage-gated sodium channel hNav1.4 have been correlated to various muscle diseases, such as Paramyotonia Congenita (PC), a disease clinically characterized by attacks of muscle stiffness and long lasting weakness mainly triggered by exposure to cold. Mutations causing PC share a common gating defect: slowed inactivation from the open state that mainly accounts for the disease symptoms. Previous studies show only slight effects of PC mutations on slow inactivation and its temperature dependence and significance remain unclear.Therefore, we performed a detailed study of WT and R1448H, a typical PC mutation, gating in a broad temperature range (10 - 30°C) by performing whole-cell patch-clamp experiments on HEK-293 cells, stably expressing hNav1.4. Our focus was especially on the transitions from and into the slow inactivated state. Cooling slows entry and recovery from slow inactivation and shifts the steady-state inactivation to depolarized potentials. R1448H does not change the temperature dependency of slow inactivation, but shifts the steady-state slow inactivation curve about 5mV to hyperpolarized potentials at all investigated temperatures. This shift is mainly caused by a slowed recovery from slow inactivation together with an unchanged entry into slow inactivation.In WT, the cooling induced depolarizing shift of the steady-state slow inactivation curve prevents channels from entering the slow inactivated state to maintain excitability in the cold. This effect is diminished in the mutant, which may contribute to cold induced paralysis.

Ins(3,4,5,6)P4 inhibits plasma membrane Cl− flux in secretory epithelia [1]. However, in most other mammalian cells, receptor-dependent elevation of Ins(3,4,5,6)P4 levels is an “orphan” response that lacks biological significance [2]. We set out to identify Cl− channel(s) and/or transporter(s) that are regulated by Ins(3,4,5,6)P4 in vivo. Several candidates [3-5] were excluded through biophysical criteria, electrophysiological analysis, and confocal immunofluorescence microscopy. Then, we heterologously expressed ClC-3 in the plasma membrane of HEK293-tsA201 cells; whole-cell patch-clamp analysis showed Ins(3,4,5,6)P4 to inhibit Cl− conductance through ClC-3. Next, we heterologously expressed ClC-3 in the early endosomal compartment of BHK cells; by fluorescence ratio imaging of endocytosed FITC-transferrin, we recorded intra-endosomal pH, an in situ biosensor for Cl− flux across endosomal membranes [6]. A cell-permeant, bioactivatable Ins(3,4,5,6)P4 analog elevated endosomal pH from 6.1 to 6.6, reflecting inhibition of ClC-3. Finally, Ins(3,4,5,6)P4 inhibited endogenous ClC-3 conductance in postsynaptic membranes of neonatal hippocampal neurones. Among other ClC-3 functions that could be regulated by Ins(3,4,5,6)P4 are tumor cell migration [7], apoptosis [8], and inflammatory responses [9]. Ins(3,4,5,6)P4 is a ubiquitous cellular signal with diverse biological actions.

Voltage-gated Na(+) channels play a fundamental role in the excitability of nerve and muscle cells. Defects in fast Na(+) channel inactivation can cause hereditary muscle diseases with hyper- or hypoexcitability of the sarcolemma. To explore the kinetics and gating mechanisms of noninactivating muscle Na(+) channels on a molecular level, we analyzed single channel currents from wild-type and five mutant Na(+) channels. The mutations were localized in different protein regions which have been previously shown to be important for fast inactivation (D3-D4-linker, D3/S4-S5, D4/S4-S5, D4/S6) and exhibited distinct grades of defective fast inactivation with varying levels of persistent Na(+) currents caused by late channel reopenings. Different gating schemes were fitted to the data using hidden Markov models with a correction for time interval omission and compared statistically. For all investigated channels including the wild-type, two open states were necessary to describe our data. Whereas one inactivated state was sufficient to fit the single channel behavior of wild-type channels, modeling the mutants with impaired fast inactivation revealed evidence for several inactivated states. We propose a single gating scheme with two open and three inactivated states to describe the behavior of all five examined mutants. This scheme provides a biological interpretation of the collected data, based on previous investigations in voltage-gated Na(+) and K(+) channels.

ClC-5 transports anions and protons across intracellular membranes and is necessary for endosomal/lysosomal acification. Similar to other ClC transporters, ClC-5 can switch between two different transport modes and operate either as anion-proton exchanger or as anion channel, depending on the type of external anion. So far, only some regulatory mechanisms that affect the functional mode of these transporters have been described. We here use combined whole-cell patch clamp and fluorescent intracellular pH recordings in transfected tsA201 cells to characterize ClC-5 mediated transport under various ionic conditions. Asymmetric lowering of intracellular pH results in small increases of both total and proton current amplitudes that might be well explained by changes in the electrochemical gradient imposed by such a maneuver. With internal Cl--based solution (pH 7.4) exchange of external Cl- with SCN- results in ∼5-fold increase in total ClC-5 currents at positive voltages. At higher internal proton concentration (pH 6) the same change of external anion composition results in a 70-fold increase in total currents. The voltage dependence of channel opening is not significantly altered, excluding shifts of the activation curve as a mechanistic explanation. For mutant ClC-5 lacking the “proton glutamate”, E268H ClC-5, the simultaneous effect of internal protons and external SCN is significantly less pronounced. These findings imply that acidic internal pHs change the protonation state of the proton glutamate and promote the action of external uncoupling anions. In addition, they imply that ClC-5 might even mediate uncoupled anion transport at physiological anion compositions. ClC exchangers may utilize part of the proton electrochemical driving force between the intra/extravesicular sites of intracellular compartments to establish anion concentration gradients. The described effects of intracellular pH might be an important part in this pathway and contribute to regulation of endosomal/lysosomal physiological function.

The CLC family encompasses Cl− channels and coupled Cl−/H+ exchangers. CLC channels and transporters both exhibit voltage-dependent gating, the physiological importance of which is illustrated by multiple disease-causing mutations that specifically result in altered voltage sensing. Despite a large body of available functional and structural data, the molecular mechanisms underlying voltage gating in CLCs still remain unclear. Using electrophysiological admittance measurements, we recently decomposed voltage-dependent activation of the human ClC-5 transporter into multiple discrete electrogenic transitions (Grieschat M & Alekov AK (2014) Biophys J 107, L13–L15). A key player of the gating machinery appears to be the so-called gating glutamate (Gluext; Glu148 in EcClC), which is thought to move upon changes in voltage. To better understand the basis of voltage-dependent gating, we conducted molecular dynamics simulations of the bacterial homologue EcClC. Using a double-bilayer system, we applied small ionic concentration gradients across the membrane that resulted in charge imbalances Δqion and we calculated the resulting potential differences Vm. Analysis of the dependence of Vm on Δqion for various conformations that differed in Cl− binding site occupancy and the protonation state of Gluext was used to determine charge displacements of the transporter along the electric field. Multiple processes, including protonation and conformational changes of Gluext, binding of Cl− ions and refocusing of the electric field triggered by changes in water accessibility are associated with charge transfer across the membrane and therefore exhibit intrinsic voltage dependence. We calculated gating charges that underlie these conformational transitions of the CLC transporter. Based on these calculations, gating charge recordings and capacitance measurements on ClC-5, we propose a molecular description of CLC voltage sensing which attributes fractional gating charges to these partial reactions of the transport cycle.