Overexpression of CLC-3 in HEK293T cells yields novel currents that are pH dependent

The PDE4 Inhibitors Roflumilast and Rolipram Rescue ADO2 Osteoclast Resorption Dysfunction

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption caused by heterozygous missense mutations in the chloride channel 7 (CLCN7). Adenylate cyclase, which catalyzes the formation of cAMP, is critical for lysosomal acidification in osteoclasts. We found reduced cAMP levels in ADO2 osteoclasts compared to wild-type (WT) osteoclasts, leading us to examine whether regulating cAMP would improve ADO2 osteoclast activity. Although forskolin, a known activator of adenylate cyclase and cAMP levels, negatively affected osteoclast number, it led to an overall increase in ADO2 and WT osteoclast resorption activity in vitro. Next, we examined cAMP hydrolysis by the phosphodiesterase 4 (PDE4) proteins in ADO2 versus WT osteoclasts. QPCR analysis revealed higher expression of the three major PDE4 subtypes (4a, 4b, 4d) in ADO2 osteoclasts compared in WT, consistent with reduced cAMP levels in ADO2 osteoclasts. In addition, we found that the PDE4 antagonists, rolipram and roflumilast, stimulated ADO2 and WT osteoclast formation in a dose-dependent manner. Importantly, roflumilast and rolipram displayed a concentration-dependent increase in osteoclast resorption activity which was greater in ADO2 than WT osteoclasts. Moreover, treatment with roflumilast rescued cAMP levels in ADO2 OCLs. The key findings from our studies demonstrate that osteoclasts from ADO2 mice exhibit reduced cAMP levels and PDE4 inhibition rescues cAMP levels and ADO2 osteoclast activity dysfunction in vitro. The mechanism of action of PDE4 inhibitors and their ability to reduce the high bone mass of ADO2 mice in vivo are currently under investigation. Importantly, these studies advance the understanding of the mechanisms underlying the ADO2 osteoclast dysfunction which is critical for the development of therapeutic approaches to treat clinically affected ADO2 patients.

Structural basis of pH-dependent activation in a CLC transporter

CLCs are dimeric chloride channels and anion/proton exchangers that regulate processes such as muscle contraction and endo-lysosome acidification. Common gating controls their activity; its closure simultaneously silences both protomers, and its opening allows them to independently transport ions. Mutations affecting common gating in human CLCs cause dominant genetic disorders. The structural rearrangements underlying common gating are unknown. Here, using single-particle cryo-electron microscopy, we show that the prototypical Escherichia coli CLC-ec1 undergoes large-scale rearrangements in activating conditions. The slow, pH-dependent remodeling of the dimer interface leads to the concerted opening of the intracellular H+ pathways and is required for transport. The more frequent formation of short water wires in the open H+ pathway enables Cl- pore openings. Mutations at disease-causing sites favor CLC-ec1 activation and accelerate common gate opening in the human CLC-7 exchanger. We suggest that the pH activation mechanism of CLC-ec1 is related to the common gating of CLC-7.

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.

Chloride channels with ClC-1-like properties differentially regulate the excitability of dopamine receptor D1- and D2-expressing striatal medium spiny neurons

Dynamic chloride (Cl^-) regulation is critical for synaptic inhibition. In mature neurons, Cl^- influx and extrusion are primarily controlled by ligand-gated anion channels (GABA_A and glycine receptors) and the potassium chloride cotransporter K⁺-Cl^- cotransporter 2 (KCC2), respectively. Here, we report for the first time, to our knowledge, a presence of a new source of Cl^- influx in striatal neurons with properties similar to chloride voltage-gated channel 1 (ClC-1). Using whole cell patch-clamp recordings, we detected an outwardly rectifying voltage-dependent current that was impermeable to the large anion methanesulfonate (MsO^-). The anionic current was sensitive to the ClC-1 inhibitor 9-anthracenecarboxylic acid (9-AC) and the nonspecific blocker phloretin. The mean fractions of anionic current inhibition by MsO^-, 9-AC, and phloretin were not significantly different, indicating that anionic current was caused by active ClC-1-like channels. In addition, we found that Cl^- current was not sensitive to the transmembrane protein 16A (TMEM16A; Ano1) inhibitor Ani9 and that the outward Cl^- rectification was preserved even at a very high intracellular Ca²⁺ concentration (2 mM), indicating that TMEM16B (Ano2) did not contribute to the total current. Western blotting and immunohistochemical analyses confirmed the presence of ClC-1 channels in the striatum mainly localized to the somata of striatal neurons. Finally, we found that 9-AC decreased action potential firing frequencies and increased excitability in medium spiny neurons (MSNs) expressing dopamine type 1 (D1) and type 2 (D2) receptors in the brain slices, respectively. We conclude that ClC-1-like channels are preferentially located at the somata of MSNs, are functional, and can modulate neuronal excitability.

Clues and new evidences in arterial hypertension: unmasking the role of the chloride anion

The present review will focus on the role of chloride anion in cardiovascular disease, with special emphasis in the development of hypertensive disease and vascular inflammation. It is known that acute and chronic overload of sodium chloride increase blood pressure and have pro-inflammatory and pro-fibrotic effects on different target organs, but it is unknown how chloride may influence these processes. Chloride anion is the predominant anion in the extracellular fluid and its intracellular concentration is dynamically regulated. As the queen of the electrolytes, it is of crucial importance to understand the physiological mechanisms that regulate the cellular handling of this anion including the different transporters and cellular chloride channels, which exert a variety of functions, such as regulation of cellular proliferation, differentiation, migration, apoptosis, intracellular pH and cellular redox state. In this article, we will also review the relationship between dietary, serum and intracellular chloride and how these different sources of chloride in the organism are affected in hypertension and their impact on cardiovascular disease. Additionally, we will discuss the approach of potential strategies that affect chloride handling and its potential effect on cardiovascular system, including pharmacological blockade of chloride channels and non-pharmacological interventions by replacing chloride by another anion.

Neurodegeneration Upon Dysfunction of Endosomal/Lysosomal CLC Chloride Transporters

The regulation of luminal ion concentrations is critical for the function of, and transport between intracellular organelles. The importance of the acidic pH in the compartments of the endosomal-lysosomal pathway has been well-known for decades. Besides the V-ATPase, which pumps protons into their lumen, a variety of ion transporters and channels is involved in the regulation of the organelles' complex ion homeostasis. Amongst these are the intracellular members of the CLC family, ClC-3 through ClC-7. They localize to distinct but overlapping compartments of the endosomal-lysosomal pathway, partially with tissue-specific expression. Functioning as 2Cl⁻/H⁺ exchangers, they can support the vesicular acidification and accumulate luminal Cl⁻. Mutations in the encoding genes in patients and mouse models underlie severe phenotypes including kidney stones with CLCN5 and osteopetrosis or hypopigmentation with CLCN7. Dysfunction of those intracellular CLCs that are expressed in neurons lead to neuronal defects. Loss of endosomal ClC-3, which heteromerizes with ClC-4, results in neurodegeneration. Mutations in ClC-4 are associated with epileptic encephalopathy and intellectual disability. Mice lacking the late endosomal ClC-6 develop a lysosomal storage disease with reduced pain sensitivity. Human gene variants have been associated with epilepsy, and a gain-of-function mutation causes early-onset neurodegeneration. Dysfunction of the lysosomal ClC-7 leads to a lysosomal storage disease and neurodegeneration in mice and humans. Reduced luminal chloride, as well as altered calcium regulation, has been associated with lysosomal storage diseases in general. This review discusses the properties of endosomal and lysosomal Cl⁻/H⁺ exchange by CLCs and how various alterations of ion transport by CLCs impact organellar ion homeostasis and function in neurodegenerative disorders.

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.

A chloride channel in rat pancreatic acinar AR42J cells is sensitive to extracellular acidification and dependent on ROS

Extracellular acidification, playing a promoting role in the process of acute pancreatitis, has been reported to activate Cl^- channels in several types of cells. However, whether extracellular acidification aggravates acute pancreatitis via activating Cl^- channels remains unclear. Here, we investigated the effects of extracellular acidification on Cl^- channels in rat pancreatic acinar AR42J cells using whole-cell patch-clamp recordings. We found that extracellular acidification induced a moderately outward-rectified Cl^- current, with a selectivity sequence of I^- > Br^- ≥ Cl^- > gluconate^-, while intracellular acidification failed to induce the currents. The acid-sensitive currents were inhibited by Cl^- channel blockers, 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt hydrate and 5-Nitro-2-(3-phenylpropylamino) benzoic acid. After ClC-3 was silenced by ClC-3 shRNA, the acid-sensitive Cl^- currents were attenuated significantly, indicating that ClC-3 plays a vital role in the induction of acid-sensitive Cl^- currents. Extracellular acid elevated the intracellular level of reactive oxygen species (ROS) significantly, prior to inducing Cl^- currents. When ROS production was scavenged, the acid-sensitive Cl^- currents were abolished. Whereas, the level of acid-induced ROS was unaffected with silence of ClC-3. Our findings above demonstrate that extracellular acidification induces a Cl^- current in pancreatic acinar cells via promoting ROS generation, implying an underlying mechanism that extracellular acidification might aggravate acute pancreatitis through Cl^- channels.

Uncoupling endosomal CLC chloride/proton exchange causes severe neurodegeneration

CLC chloride/proton exchangers may support acidification of endolysosomes and raise their luminal Cl⁻ concentration. Disruption of endosomal ClC‐3 causes severe neurodegeneration. To assess the importance of ClC‐3 Cl⁻/H⁺ exchange, we now generate Clcn3^unc/unc mice in which ClC‐3 is converted into a Cl⁻ channel. Unlike Clcn3^−/− mice, Clcn3^unc/unc mice appear normal owing to compensation by ClC‐4 with which ClC‐3 forms heteromers. ClC‐4 protein levels are strongly reduced in Clcn3^−/−, but not in Clcn3^unc/unc mice because ClC‐3^unc binds and stabilizes ClC‐4 like wild‐type ClC‐3. Although mice lacking ClC‐4 appear healthy, its absence in Clcn3^unc/unc/Clcn4^−/− mice entails even stronger neurodegeneration than observed in Clcn3^−/− mice. A fraction of ClC‐3 is found on synaptic vesicles, but miniature postsynaptic currents and synaptic vesicle acidification are not affected in Clcn3^unc/unc or Clcn3^−/− mice before neurodegeneration sets in. Both, Cl⁻/H⁺‐exchange activity and the stabilizing effect on ClC‐4, are central to the biological function of ClC‐3.

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.

Starvation-induced autophagy is up-regulated via ROS-mediated ClC-3 chloride channel activation in the nasopharyngeal carcinoma cell line CNE-2Z

Nutrient deficiency develops frequently in nasopharyngeal carcinoma cell (CNE-2Z) due to the characteristics of aggregation and uncontrolled proliferation. Therefore, starvation can induce autophagy in these cells. Chloride channel 3 (ClC-3), a member of the chloride channel family, is involved in various biological processes. However, whether ClC-3 plays an important role in starvation-induced autophagy is unclear. In this study, Earle's balanced salt solution (EBSS) was used to induce autophagy in CNE-2Z cells. We found that autophagy and the chloride current induced by EBSS were inhibited by chloride channel blockers. ClC-3 knockdown inhibited the degradation of LC3-II and P62. Furthermore, when reactive oxygen species (ROS) generation was suppressed by antioxidant N-acetyl-l-cysteine (L-NAC) pretreatment, EBSS-induced autophagy was inhibited, and the chloride current was unable to be activated. Nevertheless, ClC-3 knockdown had little effect on ROS levels, indicating that ROS acted upstream of ClC-3 and that both ROS and ClC-3 participated in EBSS-induced autophagy regulation in CNE-2Z.

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.

Bactridine 2 effect in DRG neurons. Identification of NHE as a second target

Bactridine 2 (Bact-2) is an antibacterial toxin from Tityus discrepans venom which modifies isoforms 1.2, 1.4 and 1.6 voltage-dependent sodium (Na_v) channels. Bactridine-induced Na⁺ outflow in Yersinia enterocolitica was blocked by amiloride, suggesting that Bact-2 effect was mediated by an amiloride sensitive sodium channel. In this study we show that Bact-2 increases also an outward rectifying current in rat dorsal root ganglia (DRG) sensory neurons; therefore, the nature of the outward rectifying currents was characterized and then the effect of Bact-2 on these currents was studied. These currents are enhanced by amiloride, are decreased by Na⁺ when an outward pH gradient is present and its reversal potential coincides with that of a Cl^-/H⁺ exchanger, suggesting that rectifying currents are produced by the electrogenic Cl^-/H⁺ exchanger modulated by the Na⁺/H⁺ antiporter. Bact-2 also leads to an increase of the outward currents in a similar way to the produced by the inhibition of the Na⁺/H⁺ exchanger. Additionally, the subsequent application of Bact-2 after blocking the Na⁺/H⁺ exchanger does not produce any further effect, suggesting that Bact-2 modifies the outward current by modulating the activity of the Na⁺/H⁺ exchanger. The effect of Bact-2 on pH_i regulation was determined using the pH indicator BCECF. The results show that the Na⁺/H⁺ exchanger is blocked by amiloride and Na⁺ free solutions and is modulated by Bact-2 in a similar way as cariporide. This study validates that besides Na_v channels, Bact-2 modulates the activity of the Na⁺/H⁺ exchanger.

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, ISS ), 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/ISS . An M531A mutation predicted to increase water-wire stability and cytoplasmic proton supply improves efficiency. Protonation (pH 5.0) of the outer glutamate gate (Gluext ; 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 (ISS ), gating charge (Q) movement, and cytoplasmic alkalization rate. ClC-3 exhibited lower transport efficiency than ClC-5, as reflected by a larger Q/ISS 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 ("Gluext ") (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/ISS ratio by 50% and enhanced H+ transport. External protons (pH 5.0) inhibited ISS 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 Gluext adopting an outward/open position. Internal "anion gate" removal (Y572S) dramatically increased ISS 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/ISS 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.

Molecular Identities and ATP Release Activities of Two Types of Volume-Regulatory Anion Channels, VSOR and Maxi-Cl

An elaborate volume regulation system based on interplay of ion channels and transporters was evolved to cope with constant osmotic challenges caused by intensive metabolism, transport and other physiological/pathophysiological events. In animal cells, two types of anion channels are directly activated by cell swelling and involved in the regulatory volume decrease (RVD): volume-sensitive outwardly rectifying anion channel (VSOR), also called volume-regulated anion channel (VRAC), and Maxi-Cl which is the most major type of maxi-anion channel (MAC). These two channels have very different biophysical profiles and exhibit opposite dependence on intracellular ATP. After several decades of verifying many false-positive candidates for VSOR and Maxi-Cl, LRRC8 family proteins emerged as major VSOR components, and SLCO2A1 protein as a core of Maxi-Cl. Still, neither of these proteins alone can fully reproduce the native channel phenotypes suggesting existence of missing components. Although both VSOR and Maxi-Cl have pores wide enough to accommodate bulky ATP^4- and MgATP^2- anions, evidence accumulated hitherto, based on pharmacological and gene silencing experiments, suggests that Maxi-Cl, but not VSOR, serves as one of the major pathways for the release of ATP from swollen and ischemic/hypoxic cells. Relations of VSOR and Maxi-Cl with diseases and their selective pharmacology are the topics promoted by recent advance in molecular identification of the two volume-activated, volume-regulatory anion channels.

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.

ClC Channels and Transporters: Structure, Physiological Functions, and Implications in Human Chloride Channelopathies

The discovery of ClC proteins at the beginning of the 1990s was important for the development of the Cl^- transport research field. ClCs form a large family of proteins that mediate voltage-dependent transport of Cl^- ions across cell membranes. They are expressed in both plasma and intracellular membranes of cells from almost all living organisms. ClC proteins form transmembrane dimers, in which each monomer displays independent ion conductance. Eukaryotic members also possess a large cytoplasmic domain containing two CBS domains, which are involved in transport modulation. ClC proteins function as either Cl^- channels or Cl^-/H⁺ exchangers, although all ClC proteins share the same basic architecture. ClC channels have two gating mechanisms: a relatively well-studied fast gating mechanism, and a slow gating mechanism, which is poorly defined. ClCs are involved in a wide range of physiological processes, including regulation of resting membrane potential in skeletal muscle, facilitation of transepithelial Cl^- reabsorption in kidneys, and control of pH and Cl^- concentration in intracellular compartments through coupled Cl^-/H⁺ exchange mechanisms. Several inherited diseases result from C1C gene mutations, including myotonia congenita, Bartter's syndrome (types 3 and 4), Dent's disease, osteopetrosis, retinal degeneration, and lysosomal storage diseases. This review summarizes general features, known or suspected, of ClC structure, gating and physiological functions. We also discuss biophysical properties of mammalian ClCs that are directly involved in the pathophysiology of several human inherited disorders, or that induce interesting phenotypes in animal models.

Proximate causes for diet-induced obesity in laboratory mice: a case study

BACKGROUND/OBJECTIVES

Detailed protocols and recommendations for the assessment of energy balance have been provided to address the problems associated with different body mass and body composition as apparent for mouse models in obesity research. Here, we applied these guidelines to investigate energy balance in two inbred mouse strains with contrasting susceptibilities for diet-induced obesity (DIO). Mice of the AKR/J strain are highly susceptible, whereas the SWR/J mice are almost completely resistant. The proximate mechanisms responsible for this striking phenotypic difference are only partially understood.

SUBJECTS/METHODS

Body mass and body composition, metabolizable energy, energy expenditure (EE), body temperature and spontaneous physical activity behavior were first assessed in a cohort of male AKR/J (N=29) and SWR/J (N=30) mice fed on a low-fat control diet (CD) to identify metabolic adaptations determining resistance to DIO. Thereafter, the immediate metabolic responses to high-fat diet (HFD) feeding for 3 days were investigated. Groups of weight-matched AKR/J (N=8) and SWR/J (N=8) mice were selected from the initial cohort for this intervention.

RESULTS

Strain differences in body mass, fat mass and lean mass were adjusted by body mass as this was the only covariate significantly correlated with metabolizable energy and EE. On the CD, EE and fat oxidation was higher in SWR/J than in AKR/J mice, whereas no difference was found for metabolizable energy. In response to HFD feeding, both strains increased metabolizable energy intake, but also increased EE, body temperature, and fat oxidation. The catabolic adaptations to HFD feeding opposed the development of positive energy balance. Increased EE was not due to increased spontaneous physical activity. A significant strain difference was found when balancing metabolizable energy and daily energy expenditure (DEE).

CONCLUSIONS

The guidelines were applicable with some limitations related to the adjustment of differences in body composition. Metabolic phenotyping revealed that metabolizable energy, DEE and metabolic fuel selection all contribute to the development of DIO. Therefore, assessing both sides of the energy balance equation is essential to identify the proximate mechanisms.

CaMKII-mediated phosphorylation of GluN2B regulates recombinant NMDA receptor currents in a chloride-dependent manner

Some forms of long-term synaptic plasticity require docking of Ca²⁺/calmodulin-dependent protein kinase II α (CaMKIIα) to residues 1290-1309 within the intracellular C-terminal tail of the N-methyl-d-aspartate (NMDA) receptor GluN2B subunit. The phosphorylation of Ser1303 within this region destabilizes CaMKII binding. Interestingly, Ser1303 is a substrate for CaMKII itself, as well as PKC and DAPK1, but these kinases have been reported to have contradictory effects on the activity of GluN2B-containing NMDA receptors. Here, we re-assessed the effect of CaMKII on NMDA receptor desensitization in heterologous cells, as measured by the ratio of steady-state to peak currents induced during 3s agonist applications. CaMKIIα co-expression or infusion of constitutively active CaMKII limits the extent of desensitization and preserves current amplitude with repeated stimulation of recombinant GluN1A/GluN2B when examined using low intracellular chloride (Cl^-) levels, characteristic of neurons beyond the first postnatal week. In contrast, CaMKIIα enhances the acute rate and extent of desensitization when intracellular Cl^- concentrations are high. The apparent dependence of CaMKIIα effects on NMDA receptor desensitization on Cl^- concentrations is consistent with the presence of a Ca²⁺-activated Cl^- conductance endogenous to HEK 293 cells, which was confirmed by photolysis of caged-Ca²⁺. However, Ca²⁺-activated Cl^- conductances are unaffected by CaMKIIα expression, indicating that CaMKII affects agonist-induced whole cell currents via modulation of the NMDA receptor. In support of this idea, CaMKIIα modulation of GluN2B-NMDA receptors is abrogated by the phospho-null mutation of Ser1303 in GluN2B to alanine and occluded by phospho-mimetic mutation of Ser1303 to aspartate regardless of intracellular Cl^- concentration. Thus, CaMKII-mediated phosphorylation of GluN2B-containing NMDA receptors reduces desensitization at physiological (low) intracellular Cl^-, perhaps serving as a feed-forward mechanism to sustain NMDA-mediated Ca²⁺ entry and continued CaMKII activation during learning and memory.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Distribution and dynamics of branchial ionocytes in houndshark reared in full-strength and diluted seawater environments

In teleost fishes, it is well-established that the gill serves as an important ionoregulatory organ in addition to its primary function of respiratory gas exchange. In elasmobranchs, however, the ionoregulatory function of the gills is still incompletely understood. Although two types of ionocytes, Na(+)/K(+)-ATPase (NKA)-rich (type-A) cell and vacuolar-type H(+)-ATPase (V-ATPase)-rich (type-B) cell, have been found in elasmobranch fishes, these cells were considered to function primarily in acid-base regulation. In the present study, we examined ion-transporting proteins expressed in ionocytes of Japanese-banded houndshark, Triakis scyllium, reared in full-strength seawater (SW) and transferred to diluted (30%) SW. In addition to the upregulation of NKA and Na(+)/H(+) exchanger type 3 (NHE3) mRNAs in the type-A ionocytes, we found that Na(+), Cl(-) cotransporter (NCC, Slc12a3) is expressed in a subpopulation of the type-B ionocytes, and that the expression level of NCC mRNA was enhanced in houndsharks transferred to a low-salinity environment. These results suggest that elasmobranch gill ionocytes contribute to NaCl uptake in addition to the already described function of acid-base regulation, and that NCC is most probably one of the key molecules for hyper-osmoregulatory function of elasmobranch gills. The existence of two types of ionocytes (NHE3- and NCC-expressing cells) that are responsible for NaCl absorption seems to be a common feature in both teleosts and elasmobranchs for adaptation to a low salinity environment. A possible driving mechanism for NCC in type-B ionocytes is discussed.

Chloride channel-3 promotes tumor metastasis by regulating membrane ruffling and is associated with poor survival

The chloride channel-3 (ClC-3) protein is known to be a component of Cl⁻ channels involved in cell volume regulation or acidification of intracellular vesicles. Here, we report that ClC-3 was highly expressed in the cytoplasm of metastatic carcinomatous cells and accelerated cell migration in vitro and tumor metastasis in vivo. High-grade expression of cytoplasmic ClC-3 predicted poor survival in cancer patients. We found that independent of its volume-activated Cl⁻ channel properties, ClC-3 was able to promote cell membrane ruffling, required for tumor metastasis. ClC-3 mediated membrane ruffling by regulating keratin 18 phosphorylation to control β1 Integrin recycling. Therefore, cytoplasmic ClC-3 plays an active and key role in tumor metastasis and may be a valuable prognostic biomarker and a therapeutic target to prevent tumor spread.

Chloride Channel 3 Channels in the Activation and Migration of Human Blood Eosinophils in Allergic Asthma

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is responsible for respiratory burst in immune cells. Chloride channel 3 (CLC3) has been linked to the respiratory burst in eosinophils and neutrophils. The effect of cytokines and the involvement of CLC3 in the regulation of NADPH-dependent oxidative stress and on cytokine-mediated migration of eosinophils are not known. Human peripheral blood eosinophils were isolated from healthy individuals and from individuals with asthma by negative selection. Real-time PCR was used to detect the expression of NADPH oxidases in eosinophils. Intracellular reactive oxygen species (ROS) measurement was done with flow cytometry. Superoxide generation was measured with transforming growth factor (TGF)-β, eotaxin, and CLC3 blockers. CLC3 dependence of eosinophils in TGF-β- and eotaxin-induced migration was also examined. The messenger RNA (mRNA) transcripts of NADPH oxidase (NOX) 2, dual oxidase (DUOX) 1, and DUOX2 were detected in blood eosinophils, with very low expression of NOX1, NOX3, and NOX5 and no NOX4 mRNA. The level of NOX2 mRNA transcripts increased with disease severity in the eosinophils of subjects with asthma compared with healthy nonatopic volunteers. Change in granularity and size in eosinophils, but no change in intracellular ROS, was observed with phorbol myristate acetate (PMA). PMA, TGF-β, and eotaxin used the CLC3-dependent pathway to increase superoxide radicals. TGF-β and eotaxin induced CLC3-dependent chemotaxis of eosinophils. These findings support the requirement of CLC3 in the activation and migration of human blood eosinophils and may provide a potential novel therapeutic target to regulate eosinophil hyperactivity in allergic airway inflammation in asthma.

Discovery of CLC transport proteins: cloning, structure, function and pathophysiology

After providing a personal description of the convoluted path leading 25 years ago to the molecular identification of the Torpedo Cl(-) channel ClC-0 and the discovery of the CLC gene family, I succinctly describe the general structural and functional features of these ion transporters before giving a short overview of mammalian CLCs. These can be categorized into plasma membrane Cl(-) channels and vesicular Cl(-) /H(+) -exchangers. They are involved in the regulation of membrane excitability, transepithelial transport, extracellular ion homeostasis, endocytosis and lysosomal function. Diseases caused by CLC dysfunction include myotonia, neurodegeneration, deafness, blindness, leukodystrophy, male infertility, renal salt loss, kidney stones and osteopetrosis, revealing a surprisingly broad spectrum of biological roles for chloride transport that was unsuspected when I set out to clone the first voltage-gated chloride channel.

Molecular Physiology of an Extra-renal Cl(-) Uptake Mechanism for Body Fluid Cl(-) Homeostasis

The development of an ion regulatory mechanism for body fluid homeostasis was an important trait for vertebrates during the evolution from aquatic to terrestrial life. The homeostatic mechanism of Cl^- in aquatic fish appears to be similar to that of terrestrial vertebrates; however, the mechanism in non-mammalian vertebrates is poorly understood. Unlike in mammals, in which the kidney plays a central role, in most fish species, the gill is responsible for the maintenance of Cl^- homeostasis via Cl^- transport uptake mechanisms. Previous studies in zebrafish identified Na⁺-Cl^- cotransporter (NCC) 2b-expressing cells in the gills and skin as the major ionocytes responsible for Cl^- uptake, similar to distal convoluted tubular cells in mammalian kidney. However, the mechanism by which basolateral ions exit from NCC cells is still unclear.

Of the in situ hybridization signals of twelve members of the clc Cl^- channel family, only that of clc-2c exhibited an ionocyte pattern in the gill and embryonic skin. Double in situ hybridization/immunocytochemistry confirmed colocalization of apical NCC2b with basolateral CLC-2c. Acclimation to a low Cl^- environment increased mRNA expression of both clc-2c and ncc2b, and also the protein expression of CLC-2c in embryos and adult gills. Loss-of-function of clc-2c resulted in a significant decrease in whole body Cl^- content in zebrafish embryos, a phenotype similar to that of ncc2b mutants; this finding suggests a role for CLC-2c in Cl^- uptake. Translational knockdown of clc-2c stimulated ncc2b mRNA expression and vice versa, revealing cooperation between these two transporters in the context of zebrafish Cl^- homeostasis. Further comparative genomic and phylogenetic analyses revealed that zebrafish CLC-2c is a fish-specific isoform that diverged from a kidney-predominant homologue, in the same manner as NCC2b and its counterparts (NCCs).

Several lines of molecular and cellular physiological evidences demonstrated the cofunctional role of apical NCC2b and basolateral CLC-2c in the gill/skin Cl^- uptake pathway. Taking the phylogenetic evidence into consideration, fish-specific NCC2b and CLC-2c may have coevolved to perform extra-renal Cl^- uptake during the evolution of vertebrates in an aquatic environment.

Novel CLC3 transcript variants in blood eosinophils and increased CLC3 expression in nasal lavage and blood eosinophils of asthmatics

Eosinophilia is a characteristic feature of allergic airway inflammation and remodeling. Chloride channel-3 (CLC3) in eosinophils has been associated with superoxide generation and respiratory burst. The CLC3 gene may produce multiple transcript variants through alternative splicing. However, the presence of CLC3 variants in human eosinophils is unknown. We examined the expression of CLC3 transcript variants in peripheral blood eosinophils of allergic asthmatics and healthy individuals. Potential of these obligatory dimers to form homo- or hetero-dimers was examined in HEK293 cells co-transfected with CLC3b-GFP and CLC3e-RFP. Eosinophils were isolated from peripheral blood by negative selection. Expression of CLC3 and CLC3 transcript variants was examined by qPCR, Western blot, and immunofluorescence. Confocal micrographs were analyzed with Image J software. Higher levels of novel transcript variants of CLC3 (CLC3b and CLC3e) were found in peripheral blood eosinophils of asthmatics compared to healthy non-atopic subjects. We also found higher CLC3 protein expression in the blood and nasal lavage eosinophils of asthmatics than healthy subjects. Both membranous and intracellular CLC3 expression were observed. Also, we found the presence of both homodimers and heterodimers of CLC3b-GFP and CLC3e-RFP in HEK293 cells. Higher and differential expression of novel CLC3 transcript variants in mild-to-moderate and moderate-to-severe asthmatic eosinophils suggest their critical role in allergic asthma. Membranous and intracellular (granular) expression of CLC3 in nasal lavage and peripheral blood eosinophils suggest their involvement in the activation and migration of eosinophils in allergic asthma. Moreover, homo- and hetero-dimerization of these transcript variants may change the channel properties to exhibit these states. Presence of CLC3 variants may serve as a biomarker in allergic asthma and additional knowledge of interaction between CLC3 transcript variants and their specific role in the activation and migration of eosinophils will allow to explore novel therapeutic approach in allergic asthma.

Structural requirements of steroidal agonists of transient receptor potential melastatin 3 (TRPM3) cation channels

BACKGROUND AND PURPOSE

Transient receptor potential melastatin 3 (TRPM3) proteins form non-selective but calcium-permeable membrane channels, rapidly activated by extracellular application of the steroid pregnenolone sulphate and the dihydropyridine nifedipine. Our aim was to characterize the steroid binding site by analysing the structural chemical requirements for TRPM3 activation.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp recordings and measurements of intracellular calcium concentrations were performed on HEK293 cells transfected with TRPM3 (or untransfected controls) during superfusion with pharmacological substances.

KEY RESULTS

Pregnenolone sulphate and nifedipine activated TRPM3 channels supra-additively over a wide concentration range. Other dihydropyridines inhibited TRPM3 channels. The natural enantiomer of pregnenolone sulphate was more efficient in activating TRPM3 channels than its synthetic mirror image. However, both enantiomers exerted very similar inhibitory effects on proton-activated outwardly rectifying anion channels. Epiallopregnanolone sulphate activated TRPM3 almost equally as well as pregnenolone sulphate. Exchanging the sulphate for other chemical moieties showed that a negative charge at this position is required for activating TRPM3 channels.

CONCLUSIONS AND IMPLICATIONS

Our data demonstrate that nifedipine and pregnenolone sulphate act at different binding sites when activating TRPM3. The latter activates TRPM3 by binding to a chiral and thus proteinaceous binding site, as inferred from the differential effects of the enantiomers. The double bond between position C5 and C6 of pregnenolone sulphate is not strictly necessary for the activation of TRPM3 channels, but a negative charge at position C3 of the steroid is highly important. These results provide a solid basis for understanding mechanistically the rapid chemical activation of TRPM3 channels.

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.

Genome-wide identification and functional analyses of microRNA signatures associated with cancer pain

Cancer pain remains a major challenge and there is an urgent demand for the development of specific mechanism-based therapies. Various diseases are associated with unique signatures of expression of microRNAs (miRNAs), which reveal deep insights into disease pathology. Using a comprehensive approach combining genome-wide miRNA screening, molecular and in silico analyses with behavioural approaches in a clinically relevant model of metastatic bone-cancer pain in mice, we now show that tumour-induced conditions are associated with a marked dysregulation of 57 miRNAs in sensory neurons corresponding to tumour-affected areas. By establishing protocols for interference with disease-induced miRNA dysregulation in peripheral sensory neurons in vivo, we functionally validate six dysregulated miRNAs as significant modulators of tumour-associated hypersensitivity. In silico analyses revealed that their predicted targets include key pain-related genes and we identified Clcn3, a gene encoding a chloride channel, as a key miRNA target in sensory neurons, which is functionally important in tumour-induced nociceptive hypersensitivity in vivo. Our results provide new insights into endogenous gene regulatory mechanisms in cancer pain and open up attractive and viable therapeutic options.

Involvement of tumor acidification in brain cancer pathophysiology

Gliomas, primary brain cancers, are characterized by remarkable invasiveness and fast growth. While they share many qualities with other solid tumors, gliomas have developed special mechanisms to convert the cramped brain space and other limitations afforded by the privileged central nervous system into pathophysiological advantages. In this review we discuss gliomas and other primary brain cancers in the context of acid-base regulation and interstitial acidification; namely, how the altered proton (H⁺) content surrounding these brain tumors influences tumor development in both autocrine and paracrine manners. As proton movement is directly coupled to movement of other ions, pH serves as both a regulator of cell activity as well as an indirect readout of other cellular functions. In the case of brain tumors, these processes result in pathophysiology unique to the central nervous system. We will highlight what is known about pH-sensitive processes in brain tumors in addition to gleaning insight from other solid tumors.

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.

Tamoxifen inhibits migration of estrogen receptor-negative hepatocellular carcinoma cells by blocking the swelling-activated chloride current

Tamoxifen is a triphenylethylene non-steroidal antiestrogen anticancer agent. It also shows inhibitory effects on metastasis of estrogen receptor (EsR)-independent tumors, but the underlying mechanism is unclear. It was demonstrated in this study that, in EsR-negative and highly metastatic human hepatocellular carcinoma MHCC97H cells, tamoxifen-inhibited cell migration, volume-activated Cl(-) currents (I(Cl,vol)) and regulatory volume decrease (RVD) in a concentration-dependent manner with a similar IC(50). Analysis of the relationships between migration, I(Cl,vol) and RVD showed that cell migration was positively correlated with I(Cl,vol) and RVD. Knockdown of the expression of ClC-3 Cl(-) channel proteins by ClC-3 shRNA or siRNA inhibited I(Cl,vol), and cell migration, and these inhibitory effects could not be increased further by addition of tamoxifen in the medium. The results suggest that knockdown of ClC-3 expression may deplete the effects of tamoxifen; tamoxifen may inhibit cell migration by modulating I(Cl,vol) and cell volume. Moreover, tamoxifen decreased the activity of protein kinase C (PKC) and the effects were reversed by the PKC activator PMA. Activation of PKC by PMA could competitively downregulate the inhibitory effects of tamoxifen on I(Cl,vol). PMA promoted cell migration, and knockdown of ClC-3 expression by ClC-3 siRNA abolished the PMA effect on cell migration. The results suggest that tamoxifen may inhibit I(Cl,vol) by suppressing PKC activation; I(Cl,vol) may be an EsR-independent target for tamoxifen in the anti-metastatic action on cancers, especially on EsR-negative cancers. The finding may have an implication in the clinical use of tamoxifen in the treatments of both EsR-positive and EsR-negative cancers.

The GmCLC1 protein from soybean functions as a chloride ion transporter

Soil salinization is a global issue that hampers agricultural production. Chloride is one of the prominent anions on saline land that cause toxicity to the plant. We previously identified the GmCLC1 gene from soybean (Glycine max) that encodes a putative tonoplast-localized chloride transporter. In this study, using electrophysiological analysis, we demonstrated the chloride transport function of GmCLC1. Interestingly, this chloride transport activity is pH dependent, suggesting that GmCLC1 is probably a chloride/proton antiporter. When the cDNA of GmCLC1 was expressed in tobacco BY-2 cells under the control of a constitutive promoter, the protective effect against salinity stress in transgenic tobacco BY-2 cells was also found to be pH sensitive. In the native host soybean, the expression of GmCLC1 gene is regulated by pH. All these findings support the notion that the function of GmCLC1 is regulated by pH.

Chloride channels in stroke

Vascular remodeling of cerebral arterioles, including proliferation, migration, and apoptosis of vascular smooth muscle cells (VSMCs), is the major cause of changes in the cross-sectional area and diameter of the arteries and sudden interruption of blood flow or hemorrhage in the brain, ie, stroke. Accumulating evidence strongly supports an important role for chloride (Cl(-)) channels in vascular remodeling and stroke. At least three Cl(-) channel genes are expressed in VSMCs: 1) the TMEM16A (or Ano1), which may encode the calcium-activated Cl(-) channels (CACCs); 2) the CLC-3 Cl(-) channel and Cl(-)/H(+) antiporter, which is closely related to the volume-regulated Cl(-) channels (VRCCs); and 3) the cystic fibrosis transmembrane conductance regulator (CFTR), which encodes the PKA- and PKC-activated Cl(-) channels. Activation of the CACCs by agonist-induced increase in intracellular Ca(2+) causes membrane depolarization, vasoconstriction, and inhibition of VSMC proliferation. Activation of VRCCs by cell volume increase or membrane stretch promotes the production of reactive oxygen species, induces proliferation and inhibits apoptosis of VSMCs. Activation of CFTR inhibits oxidative stress and may prevent the development of hypertension. In addition, Cl(-) current mediated by gamma-aminobutyric acid (GABA) receptor has also been implicated a role in ischemic neuron death. This review focuses on the functional roles of Cl(-) channels in the development of stroke and provides a perspective on the future directions for research and the potential to develop Cl(-) channels as new targets for the prevention and treatment of stroke.

CLC-3 chloride channels moderate long-term potentiation at Schaffer collateral-CA1 synapses

The chloride channel CLC-3 is expressed in the brain on synaptic vesicles and postsynaptic membranes. Although CLC-3 is broadly expressed throughout the brain, the CLC-3 knockout mouse shows complete, selective postnatal neurodegeneration of the hippocampus, suggesting a crucial role for the channel in maintaining normal brain function. CLC-3 channels are functionally linked to NMDA receptors in the hippocampus; NMDA receptor-dependent Ca(2+) entry, activation of Ca(2+)/calmodulin kinase II and subsequent gating of CLC-3 link the channels via a Ca(2+)-mediated feedback loop. We demonstrate that loss of CLC-3 at mature synapses increases long-term potentiation from 135 ± 4% in the wild-type slice preparation to 154 ± 7% above baseline (P < 0.001) in the knockout; therefore, the contribution of CLC-3 is to reduce synaptic potentiation by ∼40%. Using a decoy peptide representing the Ca(2+)/calmodulin kinase II phosphorylation site on CLC-3, we show that phosphorylation of CLC-3 is required for its regulatory function in long-term potentiation. CLC-3 is also expressed on synaptic vesicles; however, our data suggest functionally separable pre- and postsynaptic roles. Thus, CLC-3 confers Cl(-) sensitivity to excitatory synapses, controls the magnitude of long-term potentiation and may provide a protective limit on Ca(2+) influx.

Molecular mechanism of proton transport in CLC Cl-/H+ exchange transporters

CLC proteins underlie muscle, kidney, bone, and other organ system function by catalyzing the transport of Cl(-) ions across cell and organellar membranes. Some CLC proteins are ion channels while others are pumps that exchange Cl(-) for H(+). The pathway through which Cl(-) ions cross the membrane has been characterized, but the transport of H(+) and the principle by which their movement is coupled to Cl(-) movement is not well understood. Here we show that H(+) transport depends not only on the presence of a specific glutamate residue but also the presence of Cl(-) ions. H(+) transport, however, can be isolated and analyzed in the absence of Cl(-) by mutating the glutamate to alanine and adding carboxylate-containing molecules to solution, consistent with the notion that H(+) transfer is mediated through the entry of a carboxylate group into the anion pathway. Cl(-) ions and carboxylate interact with each other strongly. These data support a mechanism in which the glutamate carboxylate functions as a surrogate Cl(-) ion, but it can accept a H(+) and transfer it between the external solution and the central Cl(-) binding site, coupled to the movement of 2 Cl(-) ions.

Uncoupling of K+ and Cl- transport across the cell membrane in the process of regulatory volume decrease

It is accepted that K(+) and Cl(-) flows are coupled tightly in regulatory volume decrease (RVD). However, using self referencing microelectrodes, we proved that K(+) and Cl(-) transport mainly by channels in RVD was uncoupled in nasopharyngeal carcinoma CNE-2Z cells, with the transient K(+) efflux activated earlier and sustained Cl(-) efflux activated later. Hypotonic challenges decreased intracellular pH (pH(i)), and activated a proton pump-dependent H(+) efflux, resulting in a decline of extracellular pH (pH(o)). Modest decreases of pH(o) inhibited the volume-activated K(+) outflow and RVD, but not the Cl(-) outflow, while inhibition of H(+) efflux or increase of pH(o) buffer ability promoted K(+) efflux and RVD. The results suggest that the temporal dynamics of K(+) channel activities is different from that of Cl(-) channels in RVD, due to differential sensitivity of K(+) and Cl(-) channels to pH(o). H(+) efflux may play important roles in cell volume regulation, and may be a therapeutic target for human nasopharyngeal carcinoma.

Synergistic substrate binding determines the stoichiometry of transport of a prokaryotic H(+)/Cl(-) exchanger

Active exchangers dissipate the gradient of one substrate to accumulate nutrients, export xenobiotics and maintain cellular homeostasis. Mechanistic studies suggested that all exchangers share two fundamental properties: substrate binding is antagonistic and coupling is maintained by preventing shuttling of the empty transporter. The CLC Cl⁻: H⁺ exchangers control the homeostasis of cellular compartments in most living organisms but their transport mechanism remains unclear. We show that substrate binding to CLC-ec1 is synergistic rather than antagonistic: chloride binding induces protonation of a critical glutamate. The simultaneous binding of H⁺ and Cl⁻ gives rise to a fully-loaded state incompatible with conventional mechanisms. Mutations in the Cl⁻ transport pathway identically alter the stoichiometries of Cl⁻: H⁺ exchange and binding. We propose that the thermodynamics of synergistic substrate binding determine the stoichiometry of transport rather than the kinetics of conformational changes and ion binding.

Cell cycle-dependent subcellular distribution of ClC-3 in HeLa cells

Chloride channel-3 (ClC-3) is suggested to be a component and/or a regulator of the volume-activated Cl(-) channel in the plasma membrane. However, ClC-3 is predominantly located inside cells and the role of intracellular ClC-3 in tumor growth is unknown. In this study, we found that the subcellular distribution of endogenous ClC-3 varied in a cell cycle-dependent manner in HeLa cells. During interphase, ClC-3 was distributed throughout the cell and it accumulated at various positions in different stages. In early G1, ClC-3 was mainly located in the nucleus. In middle G1, ClC-3 gathered around the nuclear periphery as a ring. In late G1, ClC-3 moved back into the nucleus, where it remained throughout S phase. In G2, ClC-3 was concentrated in the cytoplasm. When cells progressed from G2 to the prophase of mitosis, ClC-3 from the cytoplasm translocated into the nucleus. During metaphase and anaphase, ClC-3 was distributed throughout the cell except for around the chromosomes and was aggregated at the spindle poles and in between two chromosomes, respectively. ClC-3 was then again concentrated in the nucleus upon the progression from telophase to cytokinesis. These results reveal a cell cycle-dependent change of the subcellular distribution of ClC-3 and strongly suggest that ClC-3 has nucleocytoplasmic shuttling dynamics that may play key regulatory roles during different stages of the cell cycle in tumor cells.

Determinants of coupled transport and uncoupled current by the electrogenic SLC26 transporters

Members of the SLC26 family of anion transporters mediate the transport of diverse molecules ranging from halides to carboxylic acids and can function as coupled transporters or as channels. A unique feature of the two members of the family, Slc26a3 and Slc26a6, is that they can function as both obligate coupled and mediate an uncoupled current, in a channel-like mode, depending on the transported anion. To identify potential features that control the two modes of transport, we performed in silico modeling of Slc26a6, which suggested that the closest potential fold similarity of the Slc26a6 transmembrane domains is to the CLC transporters, despite their minimal sequence identity. Examining the predicted Slc26a6 fold identified a highly conserved glutamate (Glu⁻; Slc26a6(E357)) with the predicted spatial orientation similar to that of the CLC-ec1 E148, which determines coupled or uncoupled transport by CLC-ec1. This raised the question of whether the conserved Glu⁻ in Slc26a6(E357) and Slc26a3(E367) have a role in the unique transport modes by these transporters. Reversing the Glu⁻ charge in Slc26a3 and Slc26a6 resulted in the inhibition of all modes of transport. However, most notably, neutralizing the charge in Slc26a6(E357A) eliminated all forms of coupled transport without affecting the uncoupled current. The Slc26a3(E367A) mutation markedly reduced the coupled transport and converted the stoichiometry of the residual exchange from 2Cl⁻/1HCO₃⁻ to 1Cl⁻/1HCO₃⁻, while completely sparing the current. These findings suggest the possibility that similar structural motif may determine multiple functional modes of these transporters.

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.

Expression and localization of CLC chloride transport proteins in the avian retina

Members of the ubiquitously expressed CLC protein family of chloride channels and transporters play important roles in regulating cellular chloride and pH. The CLCs that function as Cl⁻/H⁺ antiporters, ClCs 3–7, are essential in particular for the acidification of endosomal compartments and protein degradation. These proteins are broadly expressed in the nervous system, and mutations that disrupt their expression are responsible for several human genetic diseases. Furthermore, knock-out of ClC3 and ClC7 in the mouse result in the degeneration of the hippocampus and the retina. Despite this evidence of their importance in retinal function, the expression patterns of different CLC transporters in different retinal cell types are as yet undescribed. Previous work in our lab has shown that in chicken amacrine cells, internal Cl⁻ can be dynamic. To determine whether CLCs have the potential to participate, we used PCR and immunohistochemical techniques to examine CLC transporter expression in the chicken retina. We observed a high level of variation in the retinal expression levels and patterns among the different CLC proteins examined. These findings, which represent the first systematic investigation of CLC transporter expression in the retina, support diverse functions for the different CLCs in this tissue.