Anion- and proton-dependent gating of ClC-4 anion/proton transporter under uncoupling conditions

Molecular basis of ClC-6 function and its impairment in human disease

ClC-6 is a late endosomal voltage-gated chloride-proton exchanger that is predominantly expressed in the nervous system. Mutated forms of ClC-6 are associated with severe neurological disease. However, the mechanistic role of ClC-6 in normal and pathological states remains largely unknown. Here, we present cryo-EM structures of ClC-6 that guided subsequent functional studies. Previously unrecognized ATP binding to cytosolic ClC-6 domains enhanced ion transport activity. Guided by a disease-causing mutation (p.Y553C), we identified an interaction network formed by Y553/F317/T520 as potential hotspot for disease-causing mutations. This was validated by the identification of a patient with a de novo pathogenic variant p.T520A. Extending these findings, we found contacts between intramembrane helices and connecting loops that modulate the voltage dependence of ClC-6 gating and constitute additional candidate regions for disease-associated gain-of-function mutations. Besides providing insights into the structure, function, and regulation of ClC-6, our work correctly predicts hotspots for CLCN6 mutations in neurodegenerative disorders.

Backbone amides are determinants of Cl- selectivity in CLC ion channels

Chloride homeostasis is regulated in all cellular compartments. CLC-type channels selectively transport Cl- across biological membranes. It is proposed that side-chains of pore-lining residues determine Cl- selectivity in CLC-type channels, but their spatial orientation and contributions to selectivity are not conserved. This suggests a possible role for mainchain amides in selectivity. We use nonsense suppression to insert α-hydroxy acids at pore-lining positions in two CLC-type channels, CLC-0 and bCLC-k, thus exchanging peptide-bond amides with ester-bond oxygens which are incapable of hydrogen-bonding. Backbone substitutions functionally degrade inter-anion discrimination in a site-specific manner. The presence of a pore-occupying glutamate side chain modulates these effects. Molecular dynamics simulations show backbone amides determine ion energetics within the bCLC-k pore and how insertion of an α-hydroxy acid alters selectivity. We propose that backbone-ion interactions are determinants of Cl- specificity in CLC channels in a mechanism reminiscent of that described for K+ channels.

Loss of Chloride Channel 6 (CLC-6) Affects Vascular Smooth Muscle Contractility and Arterial Stiffness via Alterations to Golgi Calcium Stores

Genome-wide association studies have found a number of potential genes involved in blood pressure regulation; however, the functional role of many of these candidates has yet to be established. One such candidate gene is CLCN6, which encodes the transmembrane protein, chloride channel 6 (ClC-6). Although the CLCN6 locus has been widely associated with human blood pressure regulation, the mechanistic role of ClC-6 in blood pressure homeostasis at the molecular, cellular, and physiological levels is completely unknown. In this study, we demonstrate that rats with a functional knockout of ClC-6 on the Dahl Salt-Sensitive rat background (SS-Clcn6) have lower diastolic but not systolic blood pressures. The effect of diastolic blood pressure attenuation was independent of dietary salt exposure in knockout animals. Moreover, SS-Clcn6 rats are protected from hypertension-induced cardiac hypertrophy and arterial stiffening; however, they have impaired vasodilation and dysregulated intracellular calcium handling. ClC-6 is highly expressed in vascular smooth muscle cells where it is targeted to the Golgi apparatus. Using bilayer electrophysiology, we provide evidence that recombinant human ClC-6 protein can function as a channel. Last, we demonstrate that loss of ClC-6 function reduces Golgi calcium stores, which may play a previously unidentified role in vascular contraction and relaxation signaling in vascular smooth muscle cells. Collectively, these data indicate that ClC-6 may modulate blood pressure by regulating Golgi calcium reserves, which in turn contribute to vascular smooth muscle function.

Metabolic energy sensing by mammalian CLC anion/proton exchangers

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.

Extracellular pH and intracellular phosphatidylinositol 4,5-bisphosphate control Cl- currents in guinea pig detrusor smooth muscle cells

Cl^- channels serve as key regulators of excitability and contractility in vascular, intestinal, and airway smooth muscle cells. We recently reported a Cl^- conductance in detrusor smooth muscle (DSM) cells. Here, we used the whole cell patch-clamp technique to further characterize biophysical properties and physiological regulators of the Cl^- current in freshly isolated guinea pig DSM cells. The Cl^- current demonstrated outward rectification arising from voltage-dependent gating of Cl^- channels rather than the Cl^- transmembrane gradient. An exposure of DSM cells to hypotonic extracellular solution (Δ 165 mOsm challenge) did not increase the Cl^- current providing strong evidence that volume-regulated anion channels do not contribute to the Cl^- current in DSM cells. The Cl^- current was monotonically dependent on extracellular pH, larger and lower in magnitude at acidic (5.0) and basic pH (8.5) values, respectively. Additionally, intracellularly applied phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] analog [PI(4,5)P₂-diC8] increased the average Cl^- current density by approximately threefold in a voltage-independent manner. The magnitude of the DSM whole cell Cl^- current did not depend on the cell surface area (cell capacitance) regardless of the presence or absence of PI(4,5)P₂-diC8, an intriguing finding that underscores the complex nature of Cl^- channel expression and function in DSM cells. Removal of both extracellular Ca²⁺ and Mg²⁺ did not affect the DSM whole cell Cl^- current, whereas Gd³⁺ (1 mM) potentiated the current. Collectively, our recent and present findings strongly suggest that Cl^- channels are critical regulators of DSM excitability and are regulated by extracellular pH, Gd³⁺, and PI(4,5)P₂.

Properties of single-channel and whole cell Cl- currents in guinea pig detrusor smooth muscle cells

Multiple types of Cl^- channels regulate smooth muscle excitability and contractility in vascular, gastrointestinal, and airway smooth muscle cells. However, little is known about Cl^- channels in detrusor smooth muscle (DSM) cells. Here, we used inside-out single channel and whole cell patch-clamp recordings for detailed biophysical and pharmacological characterizations of Cl^- channels in freshly isolated guinea pig DSM cells. The recorded single Cl^- channels displayed unique gating with multiple subconductive states, a fully opened single-channel conductance of 164 pS, and a reversal potential of -41.5 mV, which is close to the E_Cl of -65 mV, confirming preferential permeability to Cl^-. The Cl^- channel demonstrated strong voltage dependence of activation (half-maximum of mean open probability, V_0.5, ~-20 mV) and robust prolonged openings at depolarizing voltages. The channel displayed similar gating when exposed intracellularly to solutions containing Ca²⁺-free or 1 mM Ca²⁺. In whole cell patch-clamp recordings, macroscopic current demonstrated outward rectification, inhibitions by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) and niflumic acid, and insensitivity to chlorotoxin. The outward current was reversibly reduced by 94% replacement of extracellular Cl^- with I^-, Br^-, or methanesulfonate (MsO^-), resulting in anionic permeability sequence: Cl^->Br^->I^->MsO^-. While intracellular Ca²⁺ levels (0, 300 nM, and 1 mM) did not affect the amplitude of Cl^- current and outward rectification, high Ca²⁺ slowed voltage-step current activation at depolarizing voltages. In conclusion, our data reveal for the first time the presence of a Ca²⁺-independent DIDS and niflumic acid-sensitive, voltage-dependent Cl^- channel in the plasma membrane of DSM cells. This channel may be a key regulator of DSM excitability.

Modulating the Chemical Transport Properties of a Transmembrane Antiporter via Alternative Anion Flux

ClC-ec1 is a prototype of the ClC antiporters, proteins that stoichiometrically exchange Cl^– and H⁺ ions in opposite directions across a membrane. It has been shown that other polyatomic anions, such as NO₃^– and SCN^–, can also be transported by ClC-ec1, but with partially or completely uncoupled proton flux. Herein, with the help of multiscale computer simulations in which the Grotthuss mechanism of proton transport (PT) is treated explicitly, we demonstrate how the chemical nature of these anions alters the coupling mechanism and qualitatively explain the shifts in the ion stoichiometry. Multidimensional free energy profiles for PT and the coupled changes in hydration are presented for NO₃^– and SCN^–. The calculated proton conductances agree with experiment, showing reduced or abolished proton flux. Surprisingly, the proton affinity of the anion is less influential on the PT, while its size and interactions with the protein significantly alter hydration and shift its influence on PT from facilitating to inhibiting. We find that the hydration of the cavity below the anion is relatively fast, but connecting the water network past the steric hindrance of these polyatomic anions is quite slow. Hence, the most relevant coordinate to the PT free energy barrier is the water connectivity along the PT pathway, but importantly only in the presence of the excess proton, and this coordinate is significantly affected by the nature of the bound anion. This work again demonstrates how PT is intrinsically coupled with protein cavity hydration changes as well as influenced by the protein environment. It additionally suggests ways in which ion exchange can be modulated and exchange stoichiometries altered.

Modulation of ClC-3 gating and proton/anion exchange by internal and external protons and the anion selectivity filter

KEY POINTS

The ClC-3 2Cl^- /1H⁺ exchanger modulates endosome pH and Cl^- concentration. We investigated the relationships between ClC-3-mediated ion transport (steady-state transport current, I_SS ), gating charge (Q) and cytoplasmic alkalization. ClC-3 transport is functionally unidirectional. ClC-5 and ClC-3 display indistinguishable exchange ratios, but ClC-3 cycling is less "efficient", as reflected by a large Q/I_SS . An M531A mutation predicted to increase water-wire stability and cytoplasmic proton supply improves efficiency. Protonation (pH 5.0) of the outer glutamate gate (Glu_ext ; E224) reduces Q, inhibits transport, and weakens coupling. Removal of the central tyrosine anion gate (Y572S) greatly increases uncoupled anion current. Tyrosine -OH removal (Y572F) alters anion selectivity and impairs coupling. E224 and Y572 act as anion barriers, and contribute to gating. The Y572 side chain and -OH regulate Q movement kinetics and voltage dependence. E224 and Y572 interact to create a "closed" inner gate conformation that maintains coupling during cycling.

ABSTRACT

We utilized plasma membrane-localized ClC-3 to investigate relationships between steady-state transport current (I_SS ), gating charge (Q) movement, and cytoplasmic alkalization rate. ClC-3 exhibited lower transport efficiency than ClC-5, as reflected by a larger Q/I_SS ratio, but an indistinguishable Cl^- /H⁺ coupling ratio. External SCN^- reduced H⁺ transport rate and uncoupled anion/H⁺ exchange by 80-90%. Removal of the external gating glutamate ("Glu_ext ") (E224A mutation) reduced Q and abolished H⁺ transport. We hypothesized that Methionine 531 (M531) impedes "water wire" H⁺ transfer from the cytoplasm to E224. Accordingly, an M531A mutation decreased the Q/I_SS ratio by 50% and enhanced H⁺ transport. External protons (pH 5.0) inhibited I_SS and markedly reduced Q while shifting the Q-voltage (V) relationship positively. The Cl^- /H⁺ coupling ratio at pH 5.0 was significantly increased, consistent with externally protonated Glu_ext adopting an outward/open position. Internal "anion gate" removal (Y572S) dramatically increased I_SS and impaired coupling, without slowing H⁺ transport rate. Loss of both gates (Y572S/E224A) resulted in a large "open pore" conductance. Y572F (removing only the phenolic hydroxide) and Y572S shortened Q duration similarly, resulting in faster Q kinetics at all voltages. These data reveal a complex relationship between Q and ion transport. Q/I_SS must be assessed together with coupling ratio to properly interpret efficiency. Coupling and transport rate are influenced by the anion, internal proton supply and external protons. Y572 regulates H⁺ coupling as well as anion selectivity, and interacts directly with E224. Disruption of this "closed gate" conformation by internal protons may represent a critical step in the ClC-3 transport cycle.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Neurotoxic Effects of Linalool and β-Pinene on Tribolium castaneum Herbst

Effective, ethical pest control requires the use of chemicals that are highly specific, safe, and ecofriendly. Linalool and β-pinene occur naturally as major constituents of the essential oils of many plant species distributed throughout the world, and thus meet these requirements. These monoterpenes were tested as repellents against Tribolium castaneum, using the area preference method, after four hours of exposure and the effect transcriptional of genes associated with neurotransmission. Changes in gene expression of acetylcholinesterase (Ace1), GABA-gated anion channel splice variant 3a6a (Rdl), GABA-gated ion channel (Grd), glutamate-gated chloride channel (Glucl), and histamine-gated chloride channel 2 (Hiscl2) were assessed and the interaction with proteins important for the insect using in silico methods was also studied. For linalool and β-pinene, the repellent concentration 50 (RC₅₀) values were 0.11 µL/cm² and 0.03 µL/cm², respectively. Both compounds induced overexpression of Hiscl2 gen in adult insects, and β-pinene also promoted the overexpression of Grd and the Ace1 gene. However, β-pinene and linalool had little potential to dock on computer-generated models for GABA-gated ion channel LCCH3, nicotinic acetylcholine receptor subunits alpha1 and alpha2, and putative octopamine/tyramine receptor proteins from T. castaneum as their respective binding affinities were marginal, and therefore the repellent action probably involved mechanisms other than direct interaction with these targets. Results indicated that β-pinene was more potent than linalool in inducing insect repellency, and also had a greater capacity to generate changes in the expression of genes involved in neuronal transmission.

Molecular Basis for Differential Anion Binding and Proton Coupling in the Cl(-)/H(+) Exchanger ClC-ec1

Cl–/H+ transporters of the CLC superfamily form a ubiquitous class of membrane proteins that catalyze stoichiometrically coupled exchange of Cl– and H+ across biological membranes. CLC transporters exchange H+ for halides and certain polyatomic anions, but exclude cations, F–, and larger physiological anions, such as PO43– and SO42–. Despite comparable transport rates of different anions, the H+ coupling in CLC transporters varies significantly depending on the chemical nature of the transported anion. Although the molecular mechanism of exchange remains unknown, studies on bacterial ClC-ec1 transporter revealed that Cl– binding to the central anion-binding site (Scen) is crucial for the anion-coupled H+ transport. Here, we show that Cl–, F–, NO3–, and SCN– display distinct binding coordinations at the Scen site and are hydrated in different manners. Consistent with the observation of differential bindings, ClC-ec1 exhibits markedly variable ability to support the formation of the transient water wires, which are necessary to support the connection of the two H+ transfer sites (Gluin and Gluex), in the presence of different anions. While continuous water wires are frequently observed in the presence of physiologically transported Cl–, binding of F– or NO3– leads to the formation of pseudo-water-wires that are substantially different from the wires formed with Cl–. Binding of SCN–, however, eliminates the water wires altogether. These findings provide structural details of anion binding in ClC-ec1 and reveal a putative atomic-level mechanism for the decoupling of H+ transport to the transport of anions other than Cl–.

Neuronal ClC-3 Splice Variants Differ in Subcellular Localizations, but Mediate Identical Transport Functions

ClC-3 is a member of the CLC family of anion channels and transporters, for which multiple functional properties and subcellular localizations have been reported. Since alternative splicing often results in proteins with diverse properties, we investigated to what extent alternative splicing might influence subcellular targeting and function of ClC-3. We identified three alternatively spliced ClC-3 isoforms, ClC-3a, ClC-3b, and ClC-3c, in mouse brain, with ClC-3c being the predominant splice variant. Whereas ClC-3a and ClC-3b are present in late endosomes/lysosomes, ClC-3c is targeted to recycling endosomes via a novel N-terminal isoleucine-proline (IP) motif. Surface membrane insertion of a fraction of ClC-3c transporters permitted electrophysiological characterization of this splice variant through whole-cell patch clamping on transfected mammalian cells. In contrast, neutralization of the N-terminal dileucine-like motifs was required for functional analysis of ClC-3a and ClC-3b. Heterologous expression of ClC-3a or ClC-3b carrying mutations in N-terminal dileucine motifs as well as WTClC-3c in HEK293T cells resulted in outwardly rectifying Cl(-) currents with significant capacitive current components. We conclude that alternative splicing of Clcn3 results in proteins with different subcellular localizations, but leaves the transport function of the proteins unaffected.

Mutations associated with Dent's disease affect gating and voltage dependence of the human anion/proton exchanger ClC-5

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 toward more depolarized potentials. The shift can be explained by impeded movements of the neighboring gating glutamate Glu_ext, 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 Glu_in 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.

Conformational changes required for H(+)/Cl(-) exchange mediated by a CLC transporter

CLC-type exchangers mediate transmembrane Cl(-) transport. Mutations altering their gating properties cause numerous genetic disorders. However, their transport mechanism remains poorly understood. In conventional models, two gates alternatively expose substrates to the intra- or extracellular solutions. A glutamate was identified as the only gate in the CLCs, suggesting that CLCs function by a nonconventional mechanism. Here we show that transport in CLC-ec1, a prokaryotic homolog, is inhibited by cross-links constraining movement of helix O far from the transport pathway. Cross-linked CLC-ec1 adopts a wild-type-like structure, indicating stabilization of a native conformation. Movements of helix O are transduced to the ion pathway via a direct contact between its C terminus and a tyrosine that is a constitutive element of the second gate of CLC transporters. Therefore, the CLC exchangers have two gates that are coupled through conformational rearrangements outside the ion pathway.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

The Concise Guide to PHARMACOLOGY 2013/14: ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

A single point mutation reveals gating of the human ClC-5 Cl-/H+ antiporter

ClC-5 is a 2Cl⁻/1H⁺ antiporter highly expressed in endosomes of proximal tubule cells. It is essential for endocytosis and mutations in ClC-5 cause Dent's disease, potentially leading to renal failure. However, the physiological role of ClC-5 is still unclear. One of the main issues is whether the strong rectification of ClC-5 currents observed in heterologous systems, with currents elicited only at positive voltages, is preserved in vivo and what is the origin of this rectification. In this work we identified a ClC-5 mutation, D76H, which, besides the typical outward currents of the wild-type (WT), shows inward tail currents at negative potentials that allow the estimation of the reversal of ClC-5 currents for the first time. A detailed analysis of the dependence of these inward tail currents on internal and external pH and [Cl⁻] shows that they are generated by a coupled transport of Cl⁻ and H⁺ with a 2 : 1 stoichiometry. From this result we conclude that the inward tail currents are caused by a gating mechanism that regulates ClC-5 transport activity and not by a major alteration of the transport mechanism itself. This implies that the strong rectification of the currents of WT ClC-5 is at least in part caused by a gating mechanism that activates the transporter at positive potentials. These results elucidate the biophysical properties of ClC-5 and contribute to the understanding of its physiological role.

ClC-3 is an intracellular chloride/proton exchanger with large voltage-dependent nonlinear capacitance

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.

Glutamate 268 regulates transport probability of the anion/proton exchanger ClC-5

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.

ClC-7 is a slowly voltage-gated 2Cl(-)/1H(+)-exchanger and requires Ostm1 for transport activity

Mutations in the ClC-7/Ostm1 ion transporter lead to osteopetrosis and lysosomal storage disease. Its lysosomal localization hitherto precluded detailed functional characterization. Using a mutated ClC-7 that reaches the plasma membrane, we now show that both the aminoterminus and transmembrane span of the Ostm1 β-subunit are required for ClC-7 Cl(-)/H(+)-exchange, whereas the Ostm1 transmembrane domain suffices for its ClC-7-dependent trafficking to lysosomes. ClC-7/Ostm1 currents were strongly outwardly rectifying owing to slow gating of ion exchange, which itself displays an intrinsically almost linear voltage dependence. Reversal potentials of tail currents revealed a 2Cl(-)/1H(+)-exchange stoichiometry. Several disease-causing CLCN7 mutations accelerated gating. Such mutations cluster to the second cytosolic cystathionine-β-synthase domain and potential contact sites at the transmembrane segment. Our work suggests that gating underlies the rectification of all endosomal/lysosomal CLCs and extends the concept of voltage gating beyond channels to ion exchangers.