Elimination of the slow gating of ClC-0 chloride channel by a point mutation

Unique Structure and Function of Chloride Transporting CLC Proteins

CLC proteins are a large structurally defined family of Cl- ion channels and H+/Cl- antiporters with nine distinct genes in mammals. The membrane-embedded part of CLC proteins bears no obvious similarity to any other class of membrane proteins, while the cytoplasmic C-terminus of most eukaryotic and some prokaryotic CLCs contains two regions with homology to cystathionine beta synthase (CBS) domains that are found in other proteins as well. Different members serve a broad range of physiological roles, including stabilization of the membrane potential, transepithelial ion transport, and vesicular acidification. Their physiological importance is underscored by the causative involvement in at least four different human genetic diseases. From functional studies of the Torpedo homologue ClC-0, a homodimeric architecture with two physically separate ion conduction pathways was anticipated and fully confirmed by solving the crystal structure of prokaryotic CLC homologues. The structure revealed a complex fold of 18 alpha-helices per subunit with at least two Cl- ions bound in the center of each protopore. A critical glutamic acid residue was identified whose side-chain seems to occupy a third Cl- ion binding site in the closed state and that moves away to allow Cl- binding. While the overall architecture and pore structure is certainly conserved from bacteria to humans, the bacterial proteins that were crystallized are actually not Cl- ion channels, but coupled H+/Cl- antiporters. These recent breakthroughs will allow us to study in further detail the structure, function, and the physiological and pathophysiological role of CLC proteins.

Anion pathway and potential energy profiles along curvilinear bacterial ClC Cl- pores: electrostatic effects of charged residues

X-ray structures permit theoretical study of Cl(-) permeation along bacterial ClC Cl(-) pores. We determined the lowest energy curvilinear pathway, identified anion-coordinating amino acids, and calculated the electrostatic potential energy profiles. We find that all four bacterial ClC Cl(-) crystal structures correspond to closed states. E148 and S107 side chains form steric barriers on both sides of the crystal binding site in the StClC wild-type and EcClC wild-type crystals; both the EcClC(E148A) and EcClC(E148Q) mutants are blocked at the S107 site. We studied the effect that mutating the charge of some strongly conserved pore-lining amino acids has on the electrostatic potential energy profiles. When E148 is neutralized, it creates an electrostatic trap, binding the ion near midmembrane. This suggests a possible electrostatic mechanism for controlling anion flow: neutralize E148, displace the side chain of E148 from the pore pathway to relieve the steric barrier, then trap the anion at midmembrane, and finally either deprotonate E148 and block the pore (pore closure) or bring a second Cl(-) into the pore to promote anion flow (pore conductance). Side-chain displacement may arise by competition for the binding site between the oxygens of E148 and the anion moving down the electrostatic energy gradient. We also find that the charge state of E111 and E113 may electrostatically control anion conductance and occupancy of the binding site within the cytoplasmic pore.

ClC-2 chloride secretion mediates prostaglandin-induced recovery of barrier function in ischemia-injured porcine ileum

BACKGROUND & AIMS

Ischemia results in the breakdown of the intestinal barrier, predisposing patients to sepsis and multiple organ failure. Prostaglandins play a critical role in mediating recovery of barrier function in ischemia-injured intestine through a mechanism involving stimulation of Cl - secretion. In the present study, we investigated the contributory role of individual Cl - channels in the recovery of barrier function in ischemia-injured porcine ileum.

METHODS

Ischemia-injured porcine ileal mucosa was mounted in Ussing chambers. Short-circuit current (Isc) and transepithelial resistance (TER) were measured in response to prostaglandin E 2 (PGE 2 ) and pharmacologic inhibitors of epithelial Cl - channels. Immunoassays were used to assess the expression and localization of ion channels.

RESULTS

Application of PGE 2 to ischemia-injured ileal mucosa stimulated increases in Isc, an indicator of Cl - secretion, that was followed by marked increases in TER, an indicator of barrier function recovery. In vitro studies revealed that although PGE 2 induced Cl - secretion via at least 3 distinct secretory pathways, recovery of barrier function was initiated by Cl - secretion via ClC-2 Cl - channels co-expressed with occludin and localized to tight junctions within restituting epithelium. Intravenous administration of furosemide to pigs subjected to 1 hour of ileal ischemia impaired recovery of barrier function, as evidenced by decreased TER and increased mucosal-to-serosal 3 H-mannitol flux after a 2-hour reperfusion/recovery period, confirming an important role for Cl - secretory pathways in vivo.

CONCLUSIONS

ClC-2-mediated intestinal Cl - secretion restores TER in ischemia-injured intestine. These data may provide the basis for targeted pharmacologic therapy for diseases associated with impaired barrier function.

Electrostatic control and chloride regulation of the fast gating of ClC-0 chloride channels

The opening and closing of chloride (Cl⁻) channels in the ClC family are thought to tightly couple to ion permeation through the channel pore. In the prototype channel of the family, the ClC-0 channel from the Torpedo electric organ, the opening-closing of the pore in the millisecond time range known as “fast gating” is regulated by both external and internal Cl⁻ ions. Although the external Cl⁻ effect on the fast-gate opening has been extensively studied at a quantitative level, the internal Cl⁻ regulation remains to be characterized. In this study, we examine the internal Cl⁻ effects and the electrostatic controls of the fast-gating mechanism. While having little effect on the opening rate, raising [Cl⁻]_i reduces the closing rate (or increases the open time) of the fast gate, with an apparent affinity of >1 M, a value very different from the one observed in the external Cl⁻ regulation on the opening rate. Mutating charged residues in the pore also changes the fast-gating properties—the effects are more prominent on the closing rate than on the opening rate, a phenomenon similar to the effect of [Cl⁻]_i on the fast gating. Thus, the alteration of fast-gate closing by charge mutations may come from a combination of two effects: a direct electrostatic interaction between the manipulated charge and the negatively charged glutamate gate and a repulsive force on the gate mediated by the permeant ion. Likewise, the regulations of internal Cl⁻ on the fast gating may also be due to the competition of Cl⁻ with the glutamate gate as well as the overall more negative potential brought to the pore by the binding of Cl⁻. In contrast, the opening rate of the fast gate is only minimally affected by manipulations of [Cl⁻]_i and charges in the inner pore region. The very different nature of external and internal Cl⁻ regulations on the fast gating thus may suggest that the opening and the closing of the fast gate are not microscopically reversible processes, but form a nonequilibrium cycle in the ClC-0 fast-gating mechanism.

The voltage-dependent ClC-2 chloride channel has a dual gating mechanism

Functional and structural studies demonstrate that Cl(-) channels of the ClC family have a dimeric double-barrelled structure, with each monomer contributing an identical pore. Single protopore gating is a fast process dependent on Cl(-) interaction within the selectivity filter and in ClC-0 has a low temperature coefficient over a 10 degrees C range (Q(10)). A slow gating process closes both protopores simultaneously, has a high Q(10), is facilitated by extracellular Zn(2+) and Cd(2+) and is abolished or markedly reduced by mutation of a cysteine conserved in ClC-0, -1 and -2. In order to test the hypothesis that similar slow and fast gates exist in the widely expressed ClC-2 Cl(-) channel we have investigated the effects of these manoeuvres on ClC-2. We find that the time constants of both components of the double-exponential hyperpolarization-dependent activation (and deactivation) processes have a high temperature dependence, with Q(10) values of about 4-5, suggesting important conformational changes of the channel. Mutating C256 (equivalent to C212 in ClC-0) to A, led to a significant fraction of constitutively open channels at all potentials. Activation time constants were not affected but deactivation was slower and significantly less temperature dependent in the C256A mutant. Extracellular Cd(2+), that inhibits wild-type (WT) channels almost fully, inhibited C256A only by 50%. In the WT, the time constants for opening were not affected by Cd(2+) but deactivation at positive potentials was accelerated by Cd(2+). This effect was absent in the C256A mutant. The effect of intracellular Cl(-) on channel activation was unchanged in the C256A mutant. Collectively our results strongly support the hypothesis that ClC-2 possesses a common gate and that part of the current increase induced by hyperpolarization represents an opening of the common gate. In contrast to the gating in ClC-0, the protopore gate and the common gate of ClC-2 do not appear to be independent.

Functional and structural conservation of CBS domains from CLC chloride channels

All eukaryotic CLC Cl(-) channel subunits possess a long cytoplasmic carboxy-terminus that contains two so-called CBS (cystathionine beta-synthase) domains. These domains are found in various unrelated proteins from all phylae. The crystal structure of the CBS domains of inosine monophosphate dehydrogenase (IMPDH) is known, but it is not known whether this structure is conserved in CLC channels. Working primarily with ClC-1, we used deletion scanning mutagenesis, coimmunoprecipitation and electrophysiology to demonstrate that its CBS domains interact. The replacement of CBS domains of ClC-1 with the corresponding CBS domains from other CLC channels and even human IMPDH yielded functional channels, indicating a high degree of structural conservation. Based on a homology model of the pair of CBS domains of CLC channels, we identified some residues that, when mutated, affected the common gate which acts on both pores of the dimeric channel. Thus, we propose that the structure of CBS domains from CLC channels is highly conserved and that they play a functional role in the common gate.

Conformational changes in the pore of CLC-0

The Torpedo Cl- channel, CLC-0, is inhibited by clofibric acid derivatives from the intracellular side. We used the slow gate-deficient mutant CLC-0C212S to investigate the mechanism of block by the clofibric acid-derivative p-chlorophenoxy-acetic acid (CPA). CPA blocks open channels with low affinity (KDO= 45 mM at 0 mV) and shows fast dissociation (koff = 490 s-1 at -140 mV). In contrast, the blocker binds to closed channels with higher affinity and with much slower kinetics. This state-dependent block coupled with the voltage dependence of the gating transitions results in a highly voltage-dependent inhibition of macroscopic currents (KD approximately 1 mM at -140 mV; KD approximately 65 mM at 60 mV). The large difference in CPA affinity of the open and closed state suggests that channel opening involves more than just a local conformational rearrangement. On the other hand, in a recent work (Dutzler, R., E.B. Campbell, and R. MacKinnon. 2003. Science. 300:108-112) it was proposed that the conformational change underlying channel opening is limited to a movement of a single side chain. A prediction of this latter model is that mutations that influence CPA binding to the channel should affect the affinities for an open and closed channel in a similar manner since the general structure of the pore remains largely unchanged. To test this hypothesis we introduced point mutations in four residues (S123, T471, Y512, and K519) that lie close to the intracellular pore mouth or to the putative selectivity filter. Mutation T471S alters CPA binding exclusively to closed channels. Pronounced effects on the open channel block are observed in three other mutants, S123T, Y512A, and K519Q. Together, these results collectively suggest that the structure of the CPA binding site is different in the open and closed state. Finally, replacement of Tyr 512, a residue directly coordinating the central Cl- ion in the crystal structure, with Phe or Ala has very little effect on single channel conductance and selectivity. These observations suggest that channel opening in CLC-0 consists in more than a movement of a side chain and that other parts of the channel and of the selectivity filter are probably involved.

Gating competence of constitutively open CLC-0 mutants revealed by the interaction with a small organic Inhibitor

Opening of CLC chloride channels is coupled to the translocation of the permeant anion. From the recent structure determination of bacterial CLC proteins in the closed and open configuration, a glutamate residue was hypothesized to form part of the Cl--sensitive gate. The negatively charged side-chain of the glutamate was suggested to occlude the permeation pathway in the closed state, while opening of a single protopore of the double-pore channel would reflect mainly a movement of this side-chain toward the extracellular pore vestibule, with little rearrangement of the rest of the channel. Here we show that mutating this critical residue (Glu166) in the prototype Torpedo CLC-0 to alanine, serine, or lysine leads to constitutively open channels, whereas a mutation to aspartate strongly slowed down opening. Furthermore, we investigated the interaction of the small organic channel blocker p-chlorophenoxy-acetic acid (CPA) with the mutants E166A and E166S. Both mutants were strongly inhibited by CPA at negative voltages with a >200-fold larger affinity than for wild-type CLC-0 (apparent KD at -140 mV approximately 4 micro M). A three-state linear model with an open state, a low-affinity and a high-affinity CPA-bound state can quantitatively describe steady-state and kinetic properties of the CPA block. The parameters of the model and additional mutagenesis suggest that the high-affinity CPA-bound state is similar to the closed configuration of the protopore gate of wild-type CLC-0. In the E166A mutant the glutamate side chain that occludes the permeation pathway is absent. Thus, if gating consists only in movement of this side-chain the mutant E166A should not be able to assume a closed conformation. It may thus be that fast gating in CLC-0 is more complex than anticipated from the bacterial structures.

Probing the pore of ClC-0 by substituted cysteine accessibility method using methane thiosulfonate reagents

ClC channels are a family of protein molecules containing two ion-permeation pores. Although these transmembrane proteins are important for a variety of physiological functions, their molecular operations are only superficially understood. High-resolution X-ray crystallography techniques have recently revealed the structures of two bacterial ClC channels, but whether vertebrate ClC channel pores are similar to those of bacterial homologues is not clear. To study the pore architecture of the Torpedo ClC-0 channel, we employed the substituted-cysteine-accessibility method (SCAM) and used charged methane thiosulfonate (MTS) compounds to modify the introduced cysteine. Several conclusions were derived from this approach. First, the MTS modification pattern from Y512C to E526C in ClC-0, which corresponds to residues forming helix R in bacterial ClC channels, is indeed consistent with the suggested helical structure. Second, the ClC-0 pore is more accessible to the negatively charged than to the positively charged MTS compound, a pore property that is regulated by the intrinsic electrostatic potential in the pore. Finally, attempts to modify the introduced cysteine at positions intracellular to the selectivity filter did not result in larger MTS modification rates for the open-state channel, suggesting that the fast gate of ClC-0 cannot be located at a position intracellular to the Cl- selectivity filter. Thus, the proposal that the glutamate side chain is the fast gate of the channel is applicable to ClC-0, revealing a structural and functional conservation of ClC channels between bacterial and vertebrate species.

Side-chain charge effects and conductance determinants in the pore of ClC-0 chloride channels

The charge on the side chain of the internal pore residue lysine 519 (K519) of the Torpedo ClC-0 chloride (Cl-) channel affects channel conductance. Experiments that replace wild-type (WT) lysine with neutral or negatively charged residues or that modify the K519C mutant with various methane thiosulfonate (MTS) reagents show that the conductance of the channel decreases when the charge at position 519 is made more negative. This charge effect on the channel conductance diminishes in the presence of a high intracellular Cl- concentration ([Cl-]i). However, the application of high concentrations of nonpermeant ions, such as glutamate or sulfate (SO42-), does not change the conductance, suggesting that the electrostatic effects created by the charge at position 519 are unlikely due to a surface charge mechanism. Another pore residue, glutamate 127 (E127), plays an even more critical role in controlling channel conductance. This negatively charged residue, based on the structures of the homologous bacterial ClC channels, lies 4-5 A from K519. Altering the charge of this residue can influence the apparent Cl- affinity as well as the saturated pore conductance in the conductance-Cl- activity curve. Amino acid residues at the selectivity filter also control the pore conductance but mutating these residues mainly affects the maximal pore conductance. These results suggest at least two different conductance determinants in the pore of ClC-0, consistent with the most recent crystal structure of the bacterial ClC channel solved to 2.5 A, in which multiple Cl--binding sites were identified in the pore. Thus, we suggest that the occupancy of the internal Cl--binding site is directly controlled by the charged residues located at the inner pore mouth. On the other hand, the Cl--binding site at the selectivity filter controls the exit rate of Cl- and therefore determines the maximal channel conductance.

Single channel function of inositol 1,4,5-trisphosphate receptor type-1 and -2 isoform domain-swap chimeras

The InsP3R proteins have three recognized domains, the InsP3-binding, regulatory/coupling, and channel domains (Mignery, G.A., and T.C. Südhof. 1990. EMBO J. 9:3893-3898). The InsP3 binding domain and the channel-forming domain are at opposite ends of the protein. Ligand regulation of the channel must involve communication between these different regions of the protein. This communication likely involves the interceding sequence (i.e., the regulatory/coupling domain). The single channel functional attributes of the full-length recombinant type-1, -2, and -3 InsP3R channels have been defined. Here, two type-1/type-2 InsP3R regulatory/coupling domain chimeras were created and their single channel function defined. One chimera (1-2-1) contained the type-2 regulatory/coupling domain in a type-1 backbone. The other chimera (2-1-2) contained the type-1 regulatory/coupling domain in a type-2 backbone. These chimeric proteins were expressed in COS cells, isolated, and then reconstituted in proteoliposomes. The proteoliposomes were incorporated into artificial planar lipid bilayers and the single-channel function of the chimeras defined. The chimeras had permeation properties like that of wild-type channels. The ligand regulatory properties of the chimeras were altered. The InsP3 and Ca2+ regulation had some unique features but also had features in common with wild-type channels. These results suggest that different independent structural determinants govern InsP3R permeation and ligand regulation. It also suggests that ligand regulation is a multideterminant process that involves several different regions of the protein. This study also demonstrates that a chimera approach can be applied to define InsP3R structure-function.

Conservation of chloride channel structure revealed by an inhibitor binding site in ClC-1

Crystal structures of bacterial CLC proteins were solved recently, but it is unclear to which level of detail they can be extrapolated to mammalian chloride channels. Exploiting the difference in inhibition by 9-anthracene carboxylic acid (9-AC) between ClC-0, -1, and -2, we identified a serine between helices O and P as crucial for 9-AC binding. Mutagenesis based on the crystal structure identified further residues affecting inhibitor binding. They surround a partially hydrophobic pocket close to the chloride binding site that is accessible from the cytoplasm, consistent with the observed intracellular block by 9-AC. Mutations in presumably Cl--coordinating residues yield additional insights into the structure and function of ClC-1. Our work shows that the structure of bacterial CLCs can be extrapolated with fidelity to mammalian Cl- channels.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Involvement of helices at the dimer interface in ClC-1 common gating

ClC-1 is a dimeric, double-pored chloride channel that is present in skeletal muscle. Mutations of this channel can result in the condition myotonia, a muscle disorder involving increased muscle stiffness. It has been shown that the dominant form of myotonia often results from mutations that affect the so-called slow, or common, gating process of the ClC-1 channel. Mutations causing dominant myotonia are seen to cluster at the interface of the ClC-1 channel monomers. This study has investigated the role of the H, I, P, and Q helices, which lie on this interface, as well as the G helix, which is situated immediately behind the H and I helices, on ClC-1 gating. 11 mutant ClC-1 channels (T268M, C277S, C278S, S289A, T310M, S312A, V321S, T539A, S541A, M559T, and S572V) were produced using site-directed mutagenesis, and gating properties of these channels were investigated using electrophysiological techniques. Six of the seven mutations in G, H, and I, and two of the four mutations in P and Q, caused shifts of the ClC-1 open probability. In the majority of cases this was due to alterations in the common gating process, with only three of the mutants displaying any change in fast gating. Many of the mutant channels also showed alterations in the kinetics of the common gating process, particularly at positive potentials. The changes observed in common gating were caused by changes in the opening rate (e.g. T310M), the closing rate (e.g. C277S), or both rates. These results indicate that mutations in the helices forming the dimer interface are able to alter the ClC-1 common gating process by changing the energy of the open and/or closed channel states, and hence altering transition rates between these states.

Amazing chloride channels: an overview

AIM

This review describes molecular and functional properties of the following Cl- channels: the ClC family of voltage-dependent Cl- channels, the cAMP-activated transmembrane conductance regulator (CFTR), Ca2+ activated Cl- channels (CaCC) and volume-regulated anion channels (VRAC). If structural data are available, their relationship with the function of Cl- channels will be discussed. We also describe shortly some recently discovered channels, including high conductance Cl- channels and the family of bestrophins. We illustrate the growing physiological importance of these channels in the plasma membrane and in intracellular membranes, including their involvement in transepithelial transport, pH regulation of intracellular organelles, regulation of excitability and volume regulation. Finally, we discuss the role of Cl- channels in various diseases and describe the pathological phenotypes observed in knockout mice models.

The myotonia congenita mutation A331T confers a novel hyperpolarization-activated gate to the muscle chloride channel ClC-1

Mutations in the muscle chloride channel gene CLCN1 cause myotonia congenita, an inherited disorder of skeletal muscle excitability leading to a delayed relaxation after muscle contraction. Here, we examine the functional consequences of a novel disease-causing mutation that predicts the substitution of alanine by threonine at position 331 (A331T) by whole-cell patch-clamp recording of recombinant mutant channels. A331T hClC-1 channels exhibit a novel slow gate that activates during membrane hyperpolarization and closes at positive potentials. This novel gate acts in series with fast opening and closing transitions that are common to wild-type (WT) and mutant channels. Under conditions at which this novel gate is not activated, i.e., a holding potential of 0 mV, the typical depolarization-induced activation gating of WT hClC-1 was only slightly affected by the mutation. In contrast, A331T hClC-1 channels with an open slow gate display an altered voltage dependence of open probability. These novel gating features of mutant channels produce a decreased open probability at -85 mV, the normal muscle resting potential, leading to a reduced resting chloride conductance of affected muscle fibers. The A331T mutation causes an unprecedented alteration of ClC-1 gating and reveals novel processes defining transitions between open and closed states in ClC chloride channels.

The myotonia congenita mutation A331T confers a novel hyperpolarization-activated gate to the muscle chloride channel ClC-1

Mutations in the muscle chloride channel gene CLCN1 cause myotonia congenita, an inherited disorder of skeletal muscle excitability leading to a delayed relaxation after muscle contraction. Here, we examine the functional consequences of a novel disease-causing mutation that predicts the substitution of alanine by threonine at position 331 (A331T) by whole-cell patch-clamp recording of recombinant mutant channels. A331T hClC-1 channels exhibit a novel slow gate that activates during membrane hyperpolarization and closes at positive potentials. This novel gate acts in series with fast opening and closing transitions that are common to wild-type (WT) and mutant channels. Under conditions at which this novel gate is not activated, i.e., a holding potential of 0 mV, the typical depolarization-induced activation gating of WT hClC-1 was only slightly affected by the mutation. In contrast, A331T hClC-1 channels with an open slow gate display an altered voltage dependence of open probability. These novel gating features of mutant channels produce a decreased open probability at -85 mV, the normal muscle resting potential, leading to a reduced resting chloride conductance of affected muscle fibers. The A331T mutation causes an unprecedented alteration of ClC-1 gating and reveals novel processes defining transitions between open and closed states in ClC chloride channels.

CLC chloride channels: correlating structure with function

CLC chloride channels form a large gene family that is found in bacteria, archae and eukaryotes. Previous mutagenesis studies on CLC chloride channels, combined with electrophysiology, strongly supported the theory that these channels form a homodimeric structure with one pore per subunit (a'double-barrelled' channel), and also provided clues about gating and permeation. Recently, the crystal structures of two bacterial CLC proteins have been obtained by X-ray diffraction analysis. They confirm the double-barrelled architecture, and reveal a surprisingly complex and unprecedented channel structure. At its binding site in the pore, chloride interacts with the ends of four helices that come from both sides of the membrane. A glutamate residue that protrudes into the pore is proposed to participate in gating. The structure confirms several previous conclusions from mutagenesis studies and provides an excellent framework for their interpretation.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Bacterial ion channels and their eukaryotic homologues

Due to the relative ease of obtaining their crystal structures, bacterial ion channels provide a unique opportunity to analyse structure and function of their eukaryotic homologues. This review describes prokaryotic channels whose structures have been determined. These channels are KcsA, a bacterial homologue of eukaryotic potassium channels, MscL, a bacterial mechanosensitive ion channel and ClC0, a prokaryotic homologue of the eukaryotic ClC family of anion-selective channels. General features of their structure and function are described with a special emphasis on the advantages that these channels offer for understanding the properties of their eukaryotic homologues. We present amino-acid sequences of eukaryotic proteins related in their primary sequences to bacterial mechanosensitive channels. The usefulness of bacterial mechanosensitive channels for the studies on general principles of mechanosensation is discussed.

Proline-induced hinges in transmembrane helices: possible roles in ion channel gating

A number of ion channels contain transmembrane (TM) alpha-helices that contain proline-induced molecular hinges. These TM helices include the channel-forming peptide alamethicin (Alm), the S6 helix from voltage-gated potassium (Kv) channels, and the D5 helix from voltage-gated chloride (CLC) channels. For both Alm and KvS6, experimental data implicate hinge-bending motions of the helix in an aspect of channel gating. We have compared the hinge-bending motions of these TM helices in bilayer-like environments by multi-nanosecond MD simulations in an attempt to describe motions of these helices that may underlie possible modes of channel gating. Alm is an alpha-helical channel-forming peptide, which contains a central kink associated with a Gly-x-x-Pro motif in its sequence. Simulations of Alm in a TM orientation for 10 ns in an octane slab indicate that the Gly-x-x-Pro motif acts as a molecular hinge. The S6 helix from Shaker Kv channels contains a Pro-Val-Pro motif. Modeling studies and recent experimental data suggest that the KvS6 helix may be kinked in the vicinity of this motif. Simulations (10 ns) of an isolated KvS6 helix in an octane slab and in a POPC bilayer reveal hinge-bending motions. A pattern-matching approach was used to search for possible hinge-bending motifs in the TM helices of other ion channel proteins. This uncovered a conserved Gly-x-Pro motif in TM helix D5 of CLC channels. MD simulations of a model of hCLC1-D5 spanning an octane slab suggest that this channel also contains a TM helix that undergoes hinge-bending motion. In conclusion, our simulations suggest a model in which hinge-bending motions of TM helices may play a functional role in the gating mechanisms of several different families of ion channels.

Drastic reduction of the slow gate of human muscle chloride channel (ClC-1) by mutation C277S

1. Single channel measurements suggest that the human muscle chloride channel ClC-1 presumably has a double barrelled structure, with a fast single protopore gate and a slow common pore gate similar to that of ClC-0, the chloride channel from Torpedo. The single point mutation C212S has been shown to abolish the slow gating of ClC-0 locking the slow gate in the open state. In order to test the hypothesis that the slow gating process found in ClC-1 corresponds to the well characterised slow gate found in ClC-0 we investigated the gating effects in ClC-1 of the homologous mutation corresponding to C212S, C277S. 2. We found that the mutation C277S strongly reduced the slow component of macroscopic gating relaxations at negative and at positive voltages. 3. Time constants of the fast gating relaxations were not affected by the mutation but the minimal open probability of the fast gate at negative voltages was slightly reduced to 0.08 compared with the WT value of 0.22. 4. Additionally, we characterised the block of WT ClC-1 and mutant C277S by the S(-) enantiomer of CPB (2-(p-chlorophenoxy) butyric acid), and found that the block is practically unaffected by the mutation suggesting that CPB does not interact with the slow gate of ClC-1. 5. We conclude that the slow and fast gating processes of ClC-1, respectively, reflect the slow common pore gate and the single protopore gate of the double-barrelled ClC-1 channel.

Mechanism of block of single protopores of the Torpedo chloride channel ClC-0 by 2-(p-chlorophenoxy)butyric acid (CPB)

We investigated in detail the mechanism of inhibition by the S(-) enantiomer of 2-(p-chlorophenoxy)butyric acid (CPB) of the Torpedo Cl(-)channel, ClC-0. The substance has been previously shown to inhibit the homologous skeletal muscle channel, CLC-1. ClC-0 is a homodimer with probably two independently gated protopores that are conductive only if an additional common gate is open. As a simplification, we used a mutant of ClC-0 (C212S) that has the common gate "locked open" (Lin, Y.W., C.W. Lin, and T.Y. Chen. 1999. J. Gen. Physiol. 114:1-12). CPB inhibits C212S currents only when applied to the cytoplasmic side, and single-channel recordings at voltages (V) between -120 and -80 mV demonstrate that it acts independently on individual protopores by introducing a long-lived nonconductive state with no effect on the conductance and little effect on the lifetime of the open state. Steady-state macroscopic currents at -140 mV are half-inhibited by approximately 0.5 mM CPB, but the inhibition decreases with V and vanishes for V > or = 40 mV. Relaxations of CPB inhibition after voltage steps are seen in the current responses as an additional exponential component that is much slower than the gating of drug-free protopores. For V = 60 mV) with an IC50 of approximately 30-40 mM. Altogether, these findings support a model for the mechanism of CPB inhibition in which the drug competes with Cl(-) for binding to a site of the pore where it blocks permeation. CPB binds preferentially to closed channels, and thereby also strongly alters the gating of the single protopore. Since the affinity of CPB for open WT pores is extremely low, we cannot decide in this case if it acts also as an open pore blocker. However, the experiments with the mutant K519E strongly support this interpretation. CPB block may become a useful tool to study the pore of ClC channels. As a first application, our results provide additional evidence for a double-barreled structure of ClC-0 and ClC-1.

Different fast-gate regulation by external Cl(-) and H(+) of the muscle-type ClC chloride channels

The fast gate of the muscle-type ClC channels (ClC-0 and ClC-1) opens in response to the change of membrane potential (V). This gating process is intimately associated with the binding of external Cl(-) to the channel pore in a way that the occupancy of Cl(-) on the binding site increases the channel's open probability (P(o)). External H(+) also enhances the fast-gate opening in these channels, prompting a hypothesis that protonation of the binding site may increase the Cl(-) binding affinity, and this is possibly the underlying mechanism for the H(+) modulation. However, Cl(-) and H(+), modulate the fast-gate P(o)-V curve in different ways. Varying the external Cl(-) concentrations ([Cl(-)](o)) shifts the P(o)-V curve in parallel along the voltage axis, whereas reducing external pH mainly increases the minimal P(o) of the curve. Furthermore, H(+) modulations at saturating and nonsaturating [Cl(-)](o) are similar. Thus, the H(+) effect on the fast gating appears not to be a consequence of an increase in the Cl(-) binding affinity. We previously found that a hyperpolarization-favored opening process is important to determine the fast-gate P(o) of ClC-0 at very negative voltages. This [Cl(-)](o)-independent mechanism attracted little attention, but it appears to be the opening process that is modulated by external H(+).

A decade of CLC chloride channels: structure, mechanism, and many unsettled questions

ClC-type chloride channels are ubiquitous throughout the biological world. Expressed in nearly every cell type, these proteins have a host of biological functions. With nine distinct homologues known in eukaryotes, the ClCs represent the only molecularly defined family of chloride channels. ClC channels exhibit features of molecular architecture and gating mechanisms unprecedented in other types of ion channels. They form two-pore homodimers, and their voltage-dependence arises not from charged residues in the protein, but rather via coupling of gating to the movement of chloride ions within the pore. Because the functional characteristics of only a few ClC channels have been studied in detail, we are still learning which properties are general to the whole family. New approaches, including structural analyses, will be crucial to an understanding of ClC architecture and function.

Pores formed by single subunits in mixed dimers of different CLC chloride channels

CLC chloride channels comprise a gene family with nine mammalian members. Probably all CLC channels form homodimers, and some CLC proteins may also associate to heterodimers. ClC-0 and ClC-1, the only CLC channels investigated at the single-channel level, display two conductances of equal size which are thought to result from two separate pores, formed individually by the two monomers. We generated concatemeric channels containing one subunit of ClC-0 together with one subunit of ClC-1 or ClC-2. They should display two different conductances if one monomer were sufficient to form one pore. Indeed, we found a 8-picosiemens (pS) conductance (corresponding to ClC-0) that was associated with either a 1.8-pS (ClC-1) or a 2.8-pS (ClC-2) conductance. These conductances retained their typical gating, but the slow gating of ClC-0 that affects both pores simultaneously was lost. ClC-2 and ClC-0 current components were modified by point mutations in the corresponding subunit. The ClC-2 single pore of the mixed dimer was compared with the pores in the ClC-2 homodimer and found to be unaltered. We conclude that each monomer individually forms a gated pore. CLC dimers in general must be imagined as having two pores, as shown previously for ClC-0.

Cysteine modification of a putative pore residue in ClC-0: implication for the pore stoichiometry of ClC chloride channels

The ClC channel family consists of chloride channels important for various physiological functions. Two members in this family, ClC-0 and ClC-1, share approximately 50-60% amino acid identity and show similar gating behaviors. Although they both contain two subunits, the number of pores present in the homodimeric channel is controversial. The double-barrel model proposed for ClC-0 was recently challenged by a one-pore model partly based on experiments with ClC-1 exploiting cysteine mutagenesis followed by modification with methanethiosulfonate (MTS) reagents. To investigate the pore stoichiometry of ClC-0 more rigorously, we applied a similar strategy of MTS modification in an inactivation-suppressed mutant (C212S) of ClC-0. Mutation of lysine 165 to cysteine (K165C) rendered the channel nonfunctional, but modification of the introduced cysteine by 2-aminoethyl MTS (MTSEA) recovered functional channels with altered properties of gating-permeation coupling. The fast gate of the MTSEA-modified K165C homodimer responded to external Cl(-) less effectively, so the P(o)-V curve was shifted to a more depolarized potential by approximately 45 mV. The K165C-K165 heterodimer showed double-barrel-like channel activity after MTSEA modification, with the fast-gating behaviors mimicking a combination of those of the mutant and the wild-type pore, as expected for the two-pore model. Without MTSEA modification, the heterodimer showed only one pore, and was easier to inactivate than the two-pore channel. These results showed that K165 is important for both the fast and slow gating of ClC-0. Therefore, the effects of MTS reagents on channel gating need to be carefully considered when interpreting the apparent modification rate.

Pharmacological characterization of chloride channels belonging to the ClC family by the use of chiral clofibric acid derivatives

The enantiomers of 2-(p-chlorophenoxy)propionic acid (CPP) and of its analogs with substitutions on the asymmetric carbon atom were tested on human ClC-1 channel, the skeletal muscle chloride channel, after heterologous expression in Xenopus laevis oocytes, to gain insight in the mechanism of action of these stereoselective modulators of macroscopic chloride conductance (gCl) of rat striated fibers. By means of two microelectrode voltage clamp recordings, we found that S(-)-CPP shifted the activation curve of the ClC-1 currents toward more positive potentials and decreased the residual conductance at negative membrane potential; both effects probably account for the decrease of gCl at resting potential of native muscle fibers. Experiments on expressed Torpedo marmorata ClC-0 channels and a mutant lacking the slow gate suggest that S(-)-CPP could act on the fast gate of the single protochannels constituting the double-barreled structure of ClC-0 and ClC-1. The effect of S(-)-CPP on ClC-1 was markedly increased at low external pH (pH = 6), possibly for enhanced diffusion through the membrane (i.e., because the compound was effective only when applied to the cytoplasmic side during patch clamp recordings). The R(+)-isomer had little effect at concentrations as high as 1 mM. The CPP analogs with an ethyl, a phenyl, or an n-propyl group in place of the methyl group on the asymmetric center showed a scale of potency and a stereoselective behavior on ClC-1 similar to that observed for blocking gCl in native muscle fibers. The tested compounds were selective toward the ClC-1 channel. In fact, they were almost ineffective on an N-terminal deletion mutant of ClC-2 that is volume- and pH-independent while they blocked wild-type ClC-2 currents only at high concentrations and independently of pH and drug configuration, suggesting a different mechanism of action compared with ClC-1. No effects were observed on ClC-5 that shows less than 30% homology with ClC-1. Thus, CPP-like compounds may be useful both to gain insight into biophysical properties of ClC-1 and for searching tissue-specific therapeutic agents.

Burst properties of a supergated double-barrelled chloride ion channel

The chloride selective channel from Torpedo electroplax, ClC-0, is the prototype of a large gene family of chloride channels that behave as functional dimers, with channel currents exhibiting two non-zero conductance levels. Each pore has the same conductance and is controlled by a subgate, and these have seemingly identical fast gating kinetics. However, in addition to the two subgates there is a single slower 'supergate' which simultaneously affects both channels. In the present paper, we consider a six state Markov model that is compatible with these observations and develop approximations as well as exact results for relevant properties of groupings of openings, known as bursts. Calculations with kinetic parameter values typical of ClC-0 suggest that even simple approximations can be quite accurate. Small deviations from the assumption of independence within the model lead to marked changes in certain predicted burst properties. This suggests that analysis of these properties may be helpful in assessing independence/non-independence of gating in this type of channel. Based on simulations of models of both independent and non-independent gating, tests using binomial distributions can lead to false conclusions in each situation. This is made more problematic by the difficulty of selecting an appropriate critical time in defining a burst empirically.