ClC-6 and ClC-7 are two novel broadly expressed members of the CLC chloride channel family

pH and external Ca(2+) regulation of a small conductance Cl(-) channel in kidney distal tubule

A single channel characterization of the Cl(-) channels in distal nephron was undertaken using vesicles prepared from plasma membranes of isolated rabbit distal tubules. The presence in this vesicle preparation of ClC-K type Cl(-) channels was first established by immunodetection using an antibody raised against ClC-K isoforms. A ClC-K1 based functional characterization was next performed by investigating the pH and external Ca(2+) regulation of a small conductance Cl(-) channel which we identified previously by channel incorporation experiments. Acidification of the cis (external) solution from pH 7.4 to 6.5 led to a dose-dependent inhibition of the channel open probability P(O). Similarly, changing the trans pH from 7.4 to 6.8 resulted in a 4-fold decrease of the channel P(O) with no effect on the channel conductance. Channel activity also appeared to be regulated by cis (external) Ca(2+) concentration, with a dose-dependent increase in channel activity as a function of the cis Ca(2+) concentration. It is concluded on the basis of these results that the small conductance Cl(-) channel present in rabbit distal tubules is functionally equivalent to the ClC-K1 channel in the rat. In addition, the present work constitutes the first single channel evidence for a chloride channel regulated by external Ca(2+).

Into ion channel and transporter function. Caenorhabditis elegans ClC-type chloride channels: novel variants and functional expression

Six ClC-type chloride channel genes have been identified in Caenorhabditis elegans, termed clh-1 through clh-6. cDNA sequences from these genes suggest that clh-2, clh-3, and clh-4 may code for multiple channel variants, bringing the total to at least nine channel types in this nematode. Promoter-driven green fluorescent protein (GFP) expression in transgenic animals indicates that the protein CLH-5 is expressed ubiquitously, CLH-6 is expressed mainly in nonneuronal cells, and the remaining isoforms vary from those restricted to a single cell to those expressed in over a dozen cells of the nematode. In an Sf9 cell expression system, recombinant CLH-2b, CLH-4b, and CLH-5 did not form functional plasma membrane channels. In contrast, both CLH-1 and CLH-3b produced strong, inward-rectifying chloride currents similar to those arising from mammalian ClC2, but which operate over different voltage ranges. Our demonstration of multiple CLH protein variants and comparison of expression patterns among the clh gene family provides a framework, in combination with the electrical properties of the recombinant channels, to further examine the physiology and cell-specific role each isoform plays in this simple model system.

Isolation and characterization of a Ca(2+)-activated chloride channel from human corneal epithelium

PURPOSE

Transparency of the cornea is maintained through the activity of secretory mechanisms in the epithelium and endothelium, which offset the tendency of the stroma to imbibe fluid and swell. These secretory mechanisms establish osmotic gradients thereby providing the osmotic driving forces for coupled fluid transport from the stroma into both the tears and the anterior chamber. To further characterize the mechanism of epithelial Cl secretion, we cloned a cDNA encoding a Ca(2+)-dependent chloride channel, an abundant mRNA in human corneal epithelium. We investigated the abundance of all known human chloride channels in corneal epithelium to identify those responsible for regulating chloride conductance in this tissue.

METHODS

For the isolation of a full-length cDNA clone, a probe was selected from a set of expressed sequenced tag (EST) clones classified as unique to corneal epithelium (http://bodymap. ims.u-tokyo.ac.jp). The expression patterns of the corresponding gene encoding novel chloride channel gene in human cornea and other tissues were examined by reverse transcription-polymerase chain reaction (RT-PCR). Quantitative PCR was performed to clarify the expression level of the novel chloride channel gene in cornea relative to that in other human tissues.

RESULTS

We cloned a new Ca(2+)-activated chloride channel, CLCA2, from corneal epithelium. The full length cDNA clone encoded 943 amino acids with 62% identity to bovine Ca(2+)-activated chloride channel. The CLCA2 gene mapped to human chromosome 1p32. Quantitative expression analysis by RT-PCR showed that it is the most abundant chloride channel in corneal epithelium.

CONCLUSION

High and tissue specific expression of the CLCA2 gene in human corneal epithelium implies an important role in corneal transparency maintenance.

In vivo role of CLC chloride channels in the kidney

Chloride channels in the kidney are involved in important physiological functions such as cell volume regulation, acidification of intracellular vesicles, and transepithelial chloride transport. Among eight mammalian CLC chloride channels expressed in the kidney, three (CLC-K1, CLC-K2, and CLC-5) were identified to be related to kidney diseases in humans or mice. CLC-K1 mediates a transepithelial chloride transport in the thin ascending limb of Henle's loop and is essential for urinary concentrating mechanisms. CLC-K2 is a basolateral chloride channel in distal nephron segments and is necessary for chloride reabsorption. CLC-5 is a chloride channel in intracellular vesicles of proximal tubules and is involved in endocytosis. This review will cover the recent advances in research on the CLC chloride channels of the kidney with a special focus on the issues most necessary to understand their physiological roles in vivo, i.e., their intrarenal and cellular localization and their phenotypes of humans and mice that have their loss-of-function mutations.

The human and mouse methylenetetrahydrofolate reductase (MTHFR) genes: genomic organization, mRNA structure and linkage to the CLCN6 gene

Methylenetetrahydrofolate reductase (MTHFR), a pivotal enzyme in folate metabolism, regulates the proportional distribution of one-carbon moieties between cellular methylation reactions and nucleic acid synthesis. The organization of the MTHFR gene and the structure of its mRNA were characterized in human and mouse. There are three mRNA transcripts of 2.8, 7.2 and 9.8 kb in human and two of 3.2 and 7.5 kb in mouse. Northern blot analysis revealed that human MTHFR MRNA is only present at low abundance in most tissues tested. Five kilobases of sequence flanking the 3' end of the human gene were isolated, and polyadenylation sites were defined by 3' RACE. The shorter 2.8 kb transcript and the two larger 7.2 and 9.8 kb transcripts utilize different polyadenylation signal sequences, 629 and 4937 bp downstream of the stop codon, respectively. The two mRNA species in mouse also result from differential polyadenylation. Approximately 7 and 3.5 kb upstream of the human and mouse genes, respectively, were isolated and sequenced. Transcription start sites in human MTHFR were mapped using 5' RACE. The 2.8 and 7.2 kb mRNAs originate from one of two transcription start sites that are 206 and 243 bp upstream of the ATG initiation codon, whereas transcription of the 9.8 kb mRNA is initiated at a start site located 2.8 kb upstream of the translation start codon. The putative MTHFR promoter does not have a TATA box but contains CpG islands and multiple potential Sp1 binding sites. The MTHFR gene was finely mapped to interval 16 of chromosome 1p36.3, a region deleted in many tumors, by establishing a close linkage to CLCN6, a putative chloride channel gene. A novel CA-repeat polymorphism identified within intron 2 of the CLCN6 gene may be useful in assessing loss of heterozygosity in such tumors. The multiple MTHFR mRNA species identified in this report may reflect an underlying complex set of gene regulatory mechanisms acting through an alternative transcription start site and/or polyadenylation signal sequence utilization.

Distinct ion channel classes are expressed on the outer nuclear envelope of T- and B-lymphocyte cell lines

The outer nuclear membrane, endoplasmic reticulum, and mitochondrial membrane ion channels are poorly understood, although they are important in the control of compartmental calcium levels, cell division, and apoptosis. Few direct recordings of these ion channels have been made because of the difficulty of accessing these intracellular membranes. Using patch-clamp techniques on isolated nuclei, we measured distinct ion channel classes on the outer nuclear envelope of T-cell (human Jurkat) and BFL5 cell (murine promyelocyte) lines. We first imaged the nuclear envelopes of both Jurkat and FL5 cells with atomic force microscopy to determine the density of pore proteins. The nuclear pore complex was intact at roughly similar densities in both cell types. In patch-clamp recordings of Jurkat nuclear membranes, Cl channels (105 +/- 5 pS) predominated and inactivated with negative pipette potentials. Nucleotides transiently inhibited the anion channel. In contrast, FL5 nuclear channels were cation selective (52 +/- 2 pS), were inactivated with positive membrane potentials, and were insensitive to GTPgammaS applied to the bath. We hypothesize that T- and B-cell nuclear membrane channels are distinct, and that this is perhaps related to their unique roles in the immune system.

From tonus to tonicity: physiology of CLC chloride channels

Chloride channels are involved in a multitude of physiologic processes ranging from basal cellular functions such as cell volume regulation and acidification of intracellular vesicles to more specialized mechanisms such as vectorial transepithelial transport and regulation of cellular excitability. This plethora of functions is accomplished by numerous functionally highly diverse chloride channels that are only partially identified at the molecular level. The CLC family of chloride channels comprises at present nine members in mammals that differ with respect to biophysical properties, cellular compartmentalization, and tissue distribution. Their common structural features include a predicted topology model with 10 to 12 transmembrane regions together with two C-terminal CBS domains. Loss of function mutations affecting three different members of the CLC channel family lead to three human inherited diseases : myotonia congenita, Dent's disease, and Bartter's syndrome. These diseases, together with the diabetes insipidus symptoms of a knockout mouse model, emphasize the physiologic relevance of this ion channel family.

GOLAC: an endogenous anion channel of the Golgi complex

The Golgi complex is present in every eukaryotic cell and functions in posttranslational modifications and sorting of proteins and lipids to post-Golgi destinations. Both functions require an acidic lumenal pH and transport of substrates into and by-products out of the Golgi lumen. Endogenous ion channels are expected to be important for these features, but none has been described. Ion channels from an enriched Golgi fraction cleared of transiting proteins were incorporated into planar lipid bilayers. Eighty percent of the single-channel recordings revealed the same anion channel. This channel has novel properties and has been named GOLAC (Golgi anion channel). The channel has six subconductance states with a maximum conductance of 130 pS, is open over 95% of the time, and is not voltage-gated. Significant for Golgi function, the channel conductance is increased by reduction of pH on the lumenal surface. This channel may serve two nonexclusive functions: providing counterions for the acidification of the Golgi lumen by the H(+)-ATPase and removal of inorganic phosphate generated by glycosylation and sulfation of proteins and lipids in the Golgi.

Mapping of a locus for a familial autosomal recessive idiopathic myoclonic epilepsy of infancy to chromosome 16p13

Myoclonic epilepsies with onset in infancy and childhood are clinically and etiologically heterogeneous. Although genetic factors are thought to play an important role, to date very little is known about the etiology of these disorders. We ascertained a large Italian pedigree segregating a recessive idiopathic myoclonic epilepsy that starts in early infancy as myoclonic seizures, febrile convulsions, and tonic-clonic seizures. We typed 304 microsatellite markers spanning the 22 autosomes and mapped the locus on chromosome 16p13 by linkage analysis. A maximum LOD score of 4.48 was obtained for marker D16S3027 at recombination fraction 0. Haplotype analysis placed the critical region within a 3.4-cM interval between D16S3024 and D16S423. The present report constitutes the first example of an idiopathic epilepsy that is inherited as an autosomal recessive trait.

Anion channels in higher plants: functional characterization, molecular structure and physiological role

Anion channels are well documented in various tissues, cell types and membranes of algae and higher plants, and current evidence supports their central role in cell signaling, osmoregulation, plant nutrition and metabolism. It is the aim of this review to illustrate through a few selected examples the variety of anion channels operating in plant cells and some of their regulation properties and unique physiological functions. In contrast, information on the molecular structure of plant anion channels has only recently started to emerge. Only a few genes coding for putative plant anion channels from the large chloride channel (CLC) family have been isolated, and current molecular data on these plant CLCs are presented and discussed. A major challenge remains to identify the genes encoding the various anion channels described so far in plant cells. Future prospects along this line are briefly outlined, as well as recent advances based on the use of knockout mutants in the model plant Arabidopsis thaliana to explore the physiological functions of anion channels in planta.

Pathogenesis of Dent's disease and related syndromes of X-linked nephrolithiasis

Renal stone disease, which affects 12% of males and 5% of females by the seventh decade, occurs as an inherited disorder in 45% of patients and is most commonly associated with hypercalciuria. The biochemical basis for hereditary nephrolithiasis and hypercalciuria is unknown, and this has therefore been investigated by a "positional cloning" approach. As a first step in this approach, the chromosomal locations of two disorders referred to as Dent's disease and X-linked recessive nephrolithiasis (XRN) were determined. These two disorders, which represent unusual forms of the renal Fanconi syndrome, are characterized by a low molecular weight proteinuria, hypercalciuria, nephrocalcinosis, nephrolithiasis and renal failure. An X-linked inheritance for XRN was established by studies of a North American kindred, and a similar inheritance for Dent's disease was indicated by the observation of a greater disease severity in males and an absence of male-to-male transmission in five British families. X-linked polymorphic genetic markers were used in linkage studies of these families, and the genes causing Dent's disease and XRN were mapped to Xp11. In addition, in one family with Dent's disease, a microdeletion involving the DNA probe M27 beta was identified. This microdeletion was further characterized by using yeast artificial chromosomes (YACs) and its size was estimated to be 515 Kb. A search for renal-expressed genes from this region identified a novel gene encoding a chloride channel (CLCN5) with similarities to a family of voltage-gated chloride channels. Molecular genetic studies of CLCN5 demonstrated that mutations, which resulted in a functional loss, were associated with Dent's disease and XRN. In addition, such CLCN5 mutations that would result in a functional loss have also been demonstrated in Japanese children with idiopathic low molecular weight proteinuria, hypercalciuria and nephrocalcinosis, and an Italian kindred with X-linked recessive hypophosphatemic rickets (XLRH) and hypercalciuria. Thus, four hereditary disorders of nephrolithiasis are due to mutations of the novel chloride channel, CLCN5.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Identification of an acid-activated Cl(-) channel from human skeletal muscles

ClC-4 gene was isolated as a putative Cl(-) channel. Due to a lack of functional expression of ClC-4, its physiological role remains unknown. We isolated a human ClC-4 clone (hClC-4sk) from human skeletal muscles and stably transfected it to Chinese hamster ovary cells. Whole cell patch-clamp studies showed that the hClC-4sk channel was activated by external acidic pH and inhibited by DIDS. It passed a strong outward Cl(-) current with a permeability sequence of I(-) > Cl(-) > F(-). The hClC-4sk has consensus sites for phosphorylation by protein kinase A (PKA); however, stimulation of PKA had no effect on the currents. hClC-4sk mRNA was expressed in excitable tissues, such as heart, brain, and skeletal muscle. These functional characteristics of hClC-4sk provide a clue to its physiological role in excitable cells.

Complete genomic structure of the CLCN6 and CLCN7 putative chloride channel genes(1)

The CLC family of voltage-gated chloride channels comprises nine members in mammals. CLCN6 and CLCN7 belong to a novel, poorly characterized subbranch of this family. We investigated the genomic organization of the human CLCN6 gene, as well as the murine CLCN6 and CLCN7 genes. The human and murine CLCN6 genes both consist of 23 exons and share a nearly identical genomic structure. The coding region of mouse CLCN7 is composed of 25 exons. Comparison of the genomic organization of CLCN6 and CLCN7 genes shows that just eight introns are located at corresponding cDNA positions. Moreover, no significant gene structure homology to other members of the CLC family could be detected indicating a great structural diversity of mammalian CLC genes.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Regulation of the human skeletal muscle chloride channel hClC-1 by protein kinase C

1. The regulation of a recombinant human muscle chloride channel, hClC-1, by protein kinase C (PKC) was investigated in human embryonic kidney (HEK 293) cells. 2. External application of 4beta-phorbol esters (4beta-PMA) reduced the instantaneous whole-cell current amplitude over the entire voltage range tested. This effect was abolished when the cells were intracellularly perfused with a specific protein kinase C inhibitor, chelerythine. Inactive 4alpha-phorbolesters did not affect the chloride currents. We conclude that the effect of 4beta-phorbol esters is mediated by protein kinase C (PKC). 3. Activation of PKC resulted in changes in macroscopic current kinetics. The time course of current deactivation determined in the presence and absence of 4beta-phorbol esters could be fitted with the sum of two exponentials and a constant value. In the presence of phorbol esters, the fast time constants and the minimum value of the fraction of non-deactivating current were increased, whereas the voltage dependence of all fractional current amplitudes remained unchanged. PKC-induced phosphorylation had only small effects on the voltage dependence of the relative open probability and the maximum absolute open probability was unaffected by treatment with 4beta-PMA, as shown by non-stationary noise analysis. 4. The kinetic changes indicate that phosphorylation alters functional properties of active channels. Since the absolute open probability is not reduced, the observed macroscopic current reduction implies alterations of the ion permeation process. 5. Phosphorylation by PKC appears to affect ion transfer and gating processes. It is postulated that the phosphorylation site may be located at the cytoplasmic vestibule face of the pore.

Characterization of the putative chloride channel xClC-5 expressed in Xenopus laevis oocytes and comparison with endogenous chloride currents

1. We recently cloned a putative chloride channel (xClC-5) from the renal cell line A6, which induced the appearance of a Cl- conductance not found in control oocytes after homologous expression in Xenopus oocytes. With the aim of increasing the Xenopus oocyte xClC-5 expression, we constructed a new plasmid in which the native 5' and 3' non-coding regions of xClC-5 were replaced by the non-coding regions of the Xenopus beta-globin sequence and in which a Kozak consensus site was introduced before the initiator ATG. 2. We then compared the induced currents Inative (induced by injection of cRNA presenting the native non-coding regions of xClC-5) and Ibeta-globin (induced by injection of cRNA presenting the non-coding regions of the Xenopus beta-globin sequence) investigating anion selectivity and anion blocker sensitivity. Several differences were found: (1) expression yield and oocyte surviving rate were largely increased by injecting (beta) xClC-5 cRNA, (2) the Ibeta-globin outward rectification score was 2.6 times that of Inative, (3) the anion conductivity sequence was nitrate > bromide > chloride > iodide >> gluconate for Ibeta-globin and iodide > bromide > nitrate > chloride >> gluconate for Inative, (4) 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), anthracene-9-carboxylic acid (9-AC), DIDS, lanthanum ions, cAMP and ionomycin-induced [Ca2+]i increase inhibited Inative but had no effect on Ibeta-globin, and (5) Inative showed considerable similarity to the previously reported endogenous current appearing after ClC-6 or pICln cRNA injection. 3. Comparison of Inative with the endogenous chloride current ICl,swell which develops under hyposmotic conditions demonstrated several similarities in their electrophysiological and pharmacological characteristics but were nevertheless distinguishable. 4. In vitro translation assays demonstrated that protein synthesis was much greater using the (beta) xClC-5 construct than that of xClC-5. Furthermore, immunoreactivity of membrane preparations of Xenopus oocytes was only observed with the (beta) xClC-5 construct, its intensity being positively correlated with Ibeta-globin levels. 5. In addition, the current induced in (beta) xClC-5 cRNA-injected oocytes presented a very marked pH dependence (inhibition by acid external media) with a pKa value (negative log of the acid dissociation constant) of 5.67. 6. In conclusion, Ibeta-globin may be due to the presence of xClC-5 in the oocyte plasma membrane playing a role as an anion channel whereas Inative may represent an endogenous current induced by xClC-5 cRNA injection. The use of antibodies will facilitate the tissue and subcellular localization of xClC-5 and the identification of its physiological role.

Molecular physiology of renal chloride channels

Chloride channels are present in all cells of the kidney. Physiological studies have revealed a bewildering variety of kidney chloride channels, but only in the past few years has molecular information on some of these channels emerged. This review will focus on cloned chloride channels expressed in renal cells.

Ontogeny of cation-Cl- cotransporter expression in rat neocortex

Neuronal precursors and immature cortical neurons actively accumulate Cl- and as a consequence depolarize in response to GABAA receptor activation. With maturity, intracellular Cl- decreases resulting in a shift towards GABAA inhibition. These observations suggest that changes in expression of cation-Cl- cotransporters may have a significant role in the ontogeny of neuronal Cl- homeostasis. Using ribonuclease protection analysis and in situ hybridization we examined the developmental expression of all presently known members of the cation-Cl- cotransporter gene family in rat brain. Of the inwardly directed cotransporters, NKCC-1, NKCC-2, and NCC-1, only NKCC-1 was detected at significant levels in brain. NKCC-1 was expressed in neurons, appearing first in cortical plate but not in ventricular or subventricular zone. Expression levels peaked by the third postnatal week and were maintained into adulthood. The outwardly directed cotransporters, KCC-1 and KCC-2, demonstrated significantly different levels and time courses of expression. KCC-1 was expressed prenatally at very low levels which increased little over the course of development. In contrast, KCC-2 expression appeared perinatally and increased dramatically after the first week of postnatal life. Differential changes in expression of this gene family occurred during periods of critical shifts in chloride homeostasis and GABA response suggestive of a role in these processes. Furthermore the absence of expression of known inwardly directed cotransporters in Cl- accumulating neuroepithelia and lack of evidence for glial expression suggests that as yet unidentified members of this gene family may be involved in chloride homeostasis in immature neuronal precursors and neuroglia.

The role of renal chloride channel mutations in kidney stone disease and nephrocalcinosis

Recent advances in molecular biology have characterised a new class of chloride channels (CLCs) that are referred to as voltage-gated CLCs. To date nine such voltage-gated CLCs (CLC-1 to CLC-7, CLC-Ka and CLC-Kb, which are encoded by the genes CLCN1 to CLCN7, CLC-Ka and CLC-Kb, respectively) have been identified in mammals. Mutations in two of these, CLC-5 and CLC-Kb, have been defined in the hypercalciuric nephrolithiasis disorders of Dent's disease and a form of Bartter's syndrome, respectively. In addition, other forms of Bartter's syndrome have been defined with mutations involving the bumetanide-sensitive sodium-potassium-chloride cotransporter (NKCC2) and the potassium channel ROMK. Finally, mutations of the thiazide-sensitive sodium-chloride cotransporter (NCCT) are associated with Gitelman's syndrome, in which hypocalciuria and hypomagnesaemia are notable features. These molecular genetic studies have increased our understanding of the renal tubular mechanisms that regulate mineral homeostasis.

Developmental expression of C1C-2 in the rat nervous system

Regulation of expression of the voltage-gated chloride channel, C1C-2, was investigated during development and adult life in rat brain. RNase protection assays demonstrated a marked increase in levels of expression of C1C-2 in brain during early postnatal development which was also detected in adult brain. In situ hybridization of E15 and E18 rat brains demonstrated C1C-2 expression in deep brain nuclei and scattered cells within the neuroepithelial layers, but not in the regions of subventricular zone that primarily give rise to glial populations. By E18 all neurons within the emerging cortical plate and its equivalent in other areas of the CNS were heavily labeled. During the first postnatal week, C1C-2 was highly expressed in most neurons. By P7 a pattern of differential expression emerged with evidence of decreased expression of C1C-2 mRNA in many neuronal populations. In adult rat brain, C1C-2 was expressed at highest levels in large neurons as found within layer V of cortex, Ammon's Horn of hippocampus, or mitral cells of the olfactory bulb and Purkinje cells within the cerebellum. Many smaller neurons within the diencephalon maintained significant levels of expression. A functional conductance was readily detected in hippocampal neurons during the first postnatal week, which had the same characteristic properties as the conductance observed in adult neurons. The observed expression and functional presence of C1C-2 suggest a widespread role in neuronal chloride homeostasis in early postnatal life, and demonstrated that cell specific shut-down resulted in the adult pattern of expression.

Golgi localization and functionally important domains in the NH2 and COOH terminus of the yeast CLC putative chloride channel Gef1p

GEF1 encodes the single CLC putative chloride channel in yeast. Its disruption leads to a defect in iron metabolism (Greene, J. R., Brown, N. H., DiDomenico, B. J., Kaplan, J., and Eide, D. (1993) Mol. Gen. Genet. 241, 542-553). Since disruption of GEF2, a subunit of the vacuolar H+-ATPase, leads to a similar phenotype, it was previously suggested that the chloride conductance provided by Gef1p is necessary for vacuolar acidification. We now show that gef1 cells indeed grow less well at less acidic pH. However, no defect in vacuolar acidification is apparent from quinacrine staining, and Gef1p co-localizes with Mnt1p in the medial Golgi. Thus, Gef1p may be important in determining Golgi pH. Systematic alanine scanning of the amino and the carboxyl terminus revealed several regions essential for Gef1p localization and function. One sequence (FVTID) in the amino terminus conforms to a class of sorting signals containing aromatic amino acids. This was further supported by point mutations. Alanine scanning of the carboxyl terminus identified a stretch of roughly 25 amino acids which coincides with the second CBS domain, a conserved protein motif recently identified. Mutations in the first CBS domain also destroyed proper function and localization. The second CBS domain can be transplanted to the amino terminus without loss of function, but could not be replaced by the corresponding domain of the homologous mammalian channel ClC-2.

The exon-intron architecture of human chloride channel genes is not conserved

The human CLCN6 gene contains a 167 bp exon that is optionally included or excluded in ClC-6 mRNAs. The corresponding region (3.4 kbp) of the human CLCN7 gene has now been cloned and sequenced. A comparison of the human CLCN1, CLCN5, CLCN6 and CLCN7 genes indicates that there is no homologue of the optional CLCN6 exon in the CLCN1, CLCN5 or CLCN7 genes. Thus, the CLCN6 type of alternative splicing and the ensuing structural diversity is not conserved within the CLC gene family.

ClC and CFTR chloride channel gating

Chloride channels are widely expressed and play important roles in cell volume regulation, transepithelial transport, intracellular pH regulation, and membrane excitability. Most chloride channels have yet to be identified at a molecular level. The ClC gene family and the cystic fibrosis transmembrane conductance regulator (CFTR) are distinct chloride channels expressed in many cell types, and mutations in their genes are the cause of several diseases including myotonias, cystic fibrosis, and kidney stones. Because of their molecular definition and roles in disease, these channels have been studied intensively over the past several years. The focus of this review is on recent studies that have provided new insights into the mechanisms governing the opening and closing, i.e. gating, of the ClC and CFTR chloride channels.

Evidence for the intracellular location of chloride channel (ClC)-type proteins: co-localization of ClC-6a and ClC-6c with the sarco/endoplasmic-reticulum Ca2+ pump SERCA2b

Chloride channel protein (ClC)-6a and ClC-6c, a kidney-specific splice variant with a truncated C-terminus, are proteins that belong structurally to the family of voltage-dependent chloride channels. Attempts to characterize functionally ClC-6a or ClC-6c in Xenopus oocytes have so far been negative. Similarly, expression of both ClC-6 isoforms in mammalian cells failed to provide functional information. One possible explanation of these negative results is that ClC-6 is an intracellular chloride channel rather than being located in the plasma membrane. We therefore studied the subcellular location of ClC-6 isoforms by transiently transfecting COS and CHO cells with epitope-tagged versions of ClC-6a and ClC-6c. Confocal imaging of transfected cells revealed for both ClC-6 isoforms an intracellular distribution pattern that clearly differed from the peripheral location of CD2, a plasma-membrane glycoprotein. Furthermore, dual-labelling experiments of COS cells co-transfected with ClC-6a or -6c and the sarco/endoplasmic-reticulum Ca2+ pump (SERCA2b) indicated that the ClC-6 isoforms co-localized with the SERCA2b Ca2+ pump. Thus ClC-6a and ClC-6c are intracellular membrane proteins, most likely residing in the endoplasmic reticulum. In view of their structural similarity to proven chloride channels, ClC-6 isoforms are molecular candidates for intracellular chloride channels.

Isolation and characterization of kidney-specific ClC-K1 chloride channel gene promoter

The rat ClC-K1 chloride channel is a kidney-specific member of the ClC chloride channel family found exclusively in the thin ascending limb of Henle's loop in the kidney. To gain insight into the mechanism(s) of kidney-specific expression of ClC-K1, a genomic clone that contains the 5'-flanking region of the rat ClC-K1 gene was isolated. A single transcription start site was located 84 bp upstream of the start codon. The sequence of the proximal 5'-flanking region contained an activator protein (AP)-3 site, a glucocorticoid-responsive element, several AP-2 sites, and several E-boxes, but it lacked a TATA box. To functionally express the promoter, the approximately 2.5-kb pair 5'-flanking region was ligated to a luciferase reporter gene and transfected into inner medullary (IM) cells, a stable ClC-K1-expressing cell line derived from the inner medulla of simian virus 40 transgenic mouse, and ClC-K1-nonexpressing cell lines. Luciferase activity was 7-to 24-fold greater in IM cells than those in nonexpressing cell lines, suggesting that the approximately 2.5-kb fragment contained cis-acting regulatory elements for cell-specific expression of the ClC-K1 gene. Deletion analysis revealed that this cell-specific promoter activity in IM cells was still present in the construct containing 51 bp of the 5'-flanking region but was lost in the -29 construct, clearly demonstrating that the 22 bp from -51 to -30 have a major role in the cell-specific activity of the ClC-K1 promoter. These 22 bp consist of purine-rich sequence (GGGGAGGGG-GAGGGGAG), and gel-retardation analysis demonstrated the existence of a specific protein(s) binding to this element in IM cells. These results suggest that the novel purine-rich element may play a key role in the activity of the ClC-K1 gene promoter.

Linkage of infantile Bartter syndrome with sensorineural deafness to chromosome 1p

Bartter syndrome (BS) is a family of disorders manifested by hypokalemic hypochloremic metabolic alkalosis with normotensive hyperreninemic hyperaldosteronism. We evaluated a unique, inbred Bedouin kindred in which sensorineural deafness (SND) cosegregates with an infantile variant of the BS phenotype. Using a DNA-pooling strategy, we screened the human genome and successfully demonstrated linkage of this unique syndrome to chromosome 1p31. The genes for two kidney-specific chloride channels and a sodium/hydrogen antiporter, located near this region, were excluded as candidate genes. Although the search for the disease-causing gene in this family continues, this linkage further demonstrates the genetic heterogeneity of BS. In addition, the cosegregation of these phenotypes allows us to postulate that a single genetic alteration may be responsible for the SND and the BS phenotype. The identification and characterization of this gene would lead to a better understanding of the normal physiology of the kidney and the inner ear.

The swelling-activated chloride channel ClC-2, the chloride channel ClC-3, and ClC-5, a chloride channel mutated in kidney stone disease, are expressed in distinct subpopulations of renal epithelial cells

The mammalian genome encodes at least nine different members of the ClC family of chloride channels. So far only two of them could be localized on a cellular level in the kidney. We now report on the precise intrarenal localization of the mRNAs coding for the chloride channels ClC-2, ClC-3 and ClC-5. Expression of ClC-2 mRNA, encoding a swelling-activated chloride channel, could be demonstrated in the S3 segment of the proximal tubule. The chloride channel ClC-3 mRNA and ClC-5 mRNA, coding for a chloride channel mutated in kidney stone disease, were both expressed in intercalated cells of the connecting tubule and collecting duct. Whereas ClC-3 mRNA expression was most prominent in the cortex of rat kidneys, ClC-5 mRNA was expressed from the cortex through the upper portion of the inner medulla. A detailed analysis revealed that ClC-3 was expressed by type B intercalated cells, whereas ClC-5 was expressed by type A intercalated cells. These findings have important implications for the pathogenesis of hereditary kidney stone disease caused by mutations in the CLCN5 gene.

Alternative mRNA splice variants of the rat ClC-2 chloride channel gene are expressed in lung: genomic sequence and organization of ClC-2

The ClC-2 epithelial cell chloride channel is a voltage-, tonicity- and pH-regulated member of the ClC super family. We have previously shown that rat lung ClC-2 (rClC-2) is down-regulated at birth, and molecular diversity is generated by alternative splicing [Murray et al. (1995) Am. J. Respir. Cell Mol. Biol. 12, 597-604; Murray et al. (1996) Am. J. Physiol. 271, L829-L837; Chu et al . (1996) Nucleic Acids Res. 24, 3453-3457]. To investigate other possible mRNA splice variations, we sequenced the entire rClC-2 gene and found that ClC-2Sa (formerly ClC-2S) results from the deletion of exon 20. The preceding intron 19 has an unusually high CT content and a rare AAG acceptor site. Because both features were also found in intron 13, we next tested the hypothesis that intron 13 would be involved in alternative splicing. As predicted, a second splice product, ClC-2Sb, was found by RT-PCR, but only in lung. When we compared the genomic maps of rClC-2 and human ClC-1 (hClC-1), striking similarities were found in each exon except for rClC-2 exon 20, which is absent in hClC-1. These observations suggest that ClC-1 and ClC-2 may have evolved by gene duplication, mutation and DNA rearrangement.

Mutations of CLCN5 in Japanese children with idiopathic low molecular weight proteinuria, hypercalciuria and nephrocalcinosis

The annual urinary screening of Japanese children above three years of age has identified a progressive renal tubular disorder characterized by low molecular weight proteinuria, hypercalciuria and nephrocalcinosis. The disorder has been observed in over 60 patients and has a familial predisposition. Mutations of a renal chloride channel gene, CLCN5, have been reported in four such families, and we have undertaken studies in additional patients from 10 unrelated, non-consanguineous Japanese families to further characterize such CLCN5 mutations and to ascertain their prevalence. CLCN5 abnormalities we identified in 7 of the 10 unrelated patients and consisted of 5 mutations (2 nonsense, 1 frameshift and 2 missense), 1 deletion and 1 silent polymorphism. A clustering of these mutations in CLCN5 exons 8 and 10 was observed. Over 80% of the CLCN5 mutations could be readily detected by single stranded conformational polymorphism (SSCP) analysis, thereby providing a useful mutation screening method. Our results, which indicate that over 70% of Japanese patients with this renal tubulopathy have CLCN5 mutations, will help in the genetic and clinical evaluation of children at risk from this disorder.

Cloning and functional expression of a ClC Cl- channel from the renal cell line A6

Cl- channels are important for ion transport and cell volume regulation in A6 renal cells. In the present study, we used reverse transcriptase (RT)-polymerase chain reaction (PCR) and rapid amplification of cDNA ends (RACE) to identify proteins homologous to ClC Cl- channel proteins in A6 cells. Using degenerate primers designed on consensus sequences for members of the ClC family, we amplified an RT-PCR product that had significant homology to the ClC sequences. RACE-PCR was then used to isolate several full-length clones that had total lengths from 2,764 to 3,016 base pairs. Although the coding regions were identical, sequence differences occurred in the 5' noncoding regions. The amino acid sequences of the clones had high homologies to rat and human ClC-5 (85 and 84%, respectively, if the 5th methionine of the open reading frame represents the start codon). Three parts of the protein (53, 80, and 63 amino acids in length) were 97-100% homologous to the mammalian sequences. Ribonuclease protection assay analysis revealed mRNA for this protein in oocytes, kidney, intestine, liver, brain, and blood, with lower amounts in stomach, muscle, and skin. Expression of the clones in Xenopus laevis oocytes resulted in an outwardly rectifying Cl- current that was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and possessed an anion selectivity of I- > Cl- >> gluconate.

Inward rectification in ClC-0 chloride channels caused by mutations in several protein regions

Several cloned ClC-type Cl- channels open and close in a voltage-dependent manner. The Torpedo electric organ Cl- channel, ClC-0, is the best studied member of this gene family. ClC-0 is gated by a fast and a slow gating mechanism of opposite voltage direction. Fast gating is dependent on voltage and on the external and internal Cl- concentration, and it has been proposed that the permeant anion serves as the gating charge in ClC-0 (Pusch, M., U. Ludewig, A. Rehfeldt, and T.J. Jentsch. 1995. Nature (Lond.). 373:527-531). The deactivation at negative voltages of the muscular ClC-1 channel is similar but not identical to ClC-0. Different from the extrinsic voltage dependence suggested for ClC-0, an intrinsic voltage sensor had been proposed to underlie the voltage dependence in ClC-1 (Fahlke, C., R. Rüdel, N. Mitrovic, M. Zhou, and A.L. George. 1995. Neuron. 15:463-472; Fahlke, C., A. Rosenbohm, N. Mitrovic, A.L. George, and R. Rüdel. 1996. Biophys. J. 71:695-706). The gating model for ClC-1 was partially based on the properties of a point-mutation found in recessice myotonia (D136G). Here we investigate the functional effects of mutating the corresponding residue in ClC-0 (D70). Both the corresponding charge neutralization (D70G) and a charge conserving mutation (D70E) led to an inwardly rectifying phenotype resembling that of ClC-1 (D136G). Several other mutations at very different positions in ClC-0 (K165R, H472K, S475T, E482D, T484S, T484Q), however, also led to a similar phenotype. In one of these mutants (T484S) the typical wild-type gating, characterized by a deactivation at negative voltages, can be partially restored by using external perchlorate (ClO4-) solutions. We conclude that gating in ClC-0 and ClC-1 is due to similar mechanisms. The negative charge at position 70 in ClC-0 does not specifically confer the voltage sensitivity in ClC-channels, and there is no need to postulate an intrinsic voltage sensor in ClC-channels.

Alternative splicing of ClC-6 (a member of the CIC chloride-channel family) transcripts generates three truncated isoforms one of which, ClC-6c, is kidney-specific

ClC-6 is a protein that structurally belongs to the family of ClC-type chloride channels. We now report the identification of three additional ClC-6 isoforms that are truncated because of alternative splicing. We have isolated, from human K562 cells, four types of ClC-6 cDNAs that encode four distinct ClC-6 protein isoforms. ClC-6a (869 amino acids) corresponds to the previously published ClC-6 protein [Brandt and Jentsch (1995) FEBS Lett. 377, 15-20] and it has a canonical ClC structure. However, ClC-6b (320 amino acids), ClC-6c (353 amino acids) and ClC-6d (308 amino acids) are truncated at their C-termini. Hydropathy-plot analysis indicates that the shortened isoforms contain maximally four (ClC-6b and -6d) or seven (ClC-6c) transmembrane domains. Sequence analysis of a human genomic ClC-6 fragment indicates that the cDNA variability arises from alternative splicing at two different positions: the first alternative site consists of an intron flanked by two alternative donor sites and two alternative acceptor sites, the second being due to an exon that is optionally included or excluded. Reverse-transcription-PCR analysis of ClC-6 expression in human cell lines and tissues shows that the majority (83%) of ClC-6 mRNAs consists of ClC-6a or ClC-6c messengers. Furthermore, in a mouse tissue panel, the ClC-6a mRNA has a relatively broad tissue expression pattern, since it could be detected in brain, kidney, testis, skeletal muscle, thymus and pancreas. In contrast, expression of ClC-6c is more restricted, since it was only detected in kidney.

Molecular dissection of gating in the ClC-2 chloride channel

The ClC-2 chloride channel is probably involved in the regulation of cell volume and of neuronal excitability. Site-directed mutagenesis was used to understand ClC-2 activation in response to cell swelling, hyperpolarization and acidic extracellular pH. Similar to equivalent mutations in ClC-0, neutralizing Lys566 at the end of the transmembrane domains results in outward rectification and a shift in voltage dependence, but leaves the basic gating mechanism, including swelling activation, intact. In contrast, mutations in the cytoplasmic loop between transmembrane domains D7 and D8 abolish all three modes of activation by constitutively opening the channel without changing its pore properties. These effects resemble those observed with deletions of an amino-terminal inactivation domain, and suggest that it may act as its receptor. Such a 'ball-and-chain' type mechanism may act as a final pathway in the activation of ClC-2 elicited by several stimuli.

Idiopathic low molecular weight proteinuria associated with hypercalciuric nephrocalcinosis in Japanese children is due to mutations of the renal chloride channel (CLCN5)

The annual urinary screening of Japanese children above 3 yr of age has identified a progressive proximal renal tubular disorder characterized by low molecular weight proteinuria, hypercalciuria, and nephrocalcinosis. The disorder, which has a familial predisposition and occurs predominantly in males, has similarities to three X-linked proximal renal tubular disorders that are due to mutations in the renal chloride channel gene, CLCN5. We have investigated four unrelated Japanese kindreds with this tubulopathy and have identified four different CLCN5 mutations (two nonsense, one missense, and one frameshift). These are predicted to lead to a loss of chloride channel function, and heterologous expression of the missense CLCN5 mutation in Xenopus oocytes demonstrated a 70% reduction in channel activity when compared with the wild-type. In addition, single-stranded conformation polymorphism (SSCP) analysis was found to be a sensitive and specific mutational screening method that detected > 75% of CLCN5 mutations. Thus, the results of our study expand the spectrum of clinical phenotypes associated with CLCN5 mutations to include this proximal renal tubular disorder of Japanese children. In addition, the mutational screening of CLCN5 by SSCP will help to supplement the clinical evaluation of the annual urinary screening program for this disorder.

Expression of human pICln and ClC-6 in Xenopus oocytes induces an identical endogenous chloride conductance

pICln is a protein that induces an outwardly rectifying, nucleotide-sensitive chloride current (ICln) when expressed in Xenopus oocytes, but its precise function (plasma-membrane anion channel versus cytosolic regulator of a channel) remains controversial. We now report that a chloride current identical to ICln is induced when Xenopus oocytes are injected with human ClC-6 RNA. Indeed, both the pICln and the ClC-6 induced current are outwardly rectifying, they inactivate slowly at positive potentials and have an anion permeability sequence NO3- > I- > Br- > Cl- > gluconate. Cyclamate, NPPB, and extracellular cAMP block the induced currents. The success rate of current expression is significantly increased when the injected Xenopus oocytes are incubated at a higher temperature (24 or 37 degrees C) prior to the analysis. In addition, the ICln current was detected in 6.2% of noninjected control Xenopus oocytes. We therefore conclude that the ICln current in Xenopus oocytes corresponds to an endogenous conductance that can be activated by expression of structurally unrelated proteins. Furthermore, functional, biochemical, and morphological observations did not support the notion that pICln resides in the plasma membrane either permanently or transiently after cell swelling. Thus, it is unlikely that pICln forms the channel that is responsible for the ICln current in Xenopus oocytes.

Chloride channels: an emerging molecular picture

Chloride channels are probably found in every cell, from bacteria to mammals. Their physiological tasks range from cell volume regulation to stabilization of the membrane potential, signal transduction, transepithelial transport and acidification of intracellular organelles. These different functions require the presence of many distinct chloride channels, which are differentially expressed and regulated by various stimuli. These include various intracellular messengers (like calcium and cyclic AMP), pH, extracellular ligands and transmembrane voltage. Three major structural classes of chloride channels are known to date, but there may be others not yet identified. After an overview of the general functions of chloride channels, this review will focus on these cloned chloride channels: the CLC chloride channel family, which includes voltage-gated chloride channels, and the cystic fibrosis transmembrane regulator (CFTR), which performs other functions in addition to being a chloride channel. Finally, a short section deals with GABA and glycine receptors. Diseases resulting from chloride channel defects will be specially emphasized, together with the somewhat limited information about how these proteins work at the molecular level.

A family of putative chloride channels from Arabidopsis and functional complementation of a yeast strain with a CLC gene disruption

We have cloned four novel members of the CLC family of chloride channels from Arabidopsis thaliana. The four plant genes are homologous to a recently isolated chloride channel gene from tobacco (CLC-Nt1; Lurin, C., Geelen, D., Barbier-Brygoo, H., Guern, J., and Maurel, C. (1996) Plant Cell 8, 701-711) and are about 30% identical in sequence to the most closely related CLC-6 and CLC-7 putative chloride channels from mammalia. AtCLC transcripts are broadly expressed in the plant. Similarly, antibodies against the AtCLC-d protein detected the protein in all tissues, but predominantly in the silique. AtCLC-a and AtCLC-b are highly homologous to each other ( approximately 87% identity), while being approximately 50% identical to either AtCLC-c or AtCLC-d. None of the four cDNAs elicited chloride currents when expressed in Xenopus oocytes, either singly or in combination. Among these genes, only AtCLC-d could functionally substitute for the single yeast CLC protein, restoring iron-limited growth of a strain disrupted for this gene. Introduction of disease causing mutations, identified in human CLC genes, abolished this capacity. Consistent with a similar function of both proteins, the green fluorescent protein-tagged AtCLC-d protein showed the identical localization pattern as the yeast ScCLC protein. This suggests that in Arabidopsis AtCLC-d functions as an intracellular chloride channel.

Heteromultimeric CLC chloride channels with novel properties

The skeletal muscle chloride channel CLC-1 and the ubiquitous volume-activated chloride channel CLC-2 belong to a large gene family whose members often show overlapping expression patterns. CLC-1 and CLC-2 are coexpressed in skeletal and smooth muscle and in the heart. By coexpressing CLC-1 and CLC-2 in Xenopus oocytes, we now show the formation of novel CLC-1/CLC-2 heterooligomers that yield time-independent linear chloride currents with a chloride-->bromide-->iodide selectivity sequence. Formation of heterooligomeric CLC channels increases the number and possible functions of chloride channels.

Molecular basis for decreased muscle chloride conductance in the myotonic goat

Certain forms of myotonia, a condition characterized by delayed relaxation of muscle secondary to sarcolemmal hyperexcitability, are caused by diminished chloride conductance in the muscle cell membrane. We have investigated the molecular basis for decreased muscle chloride conductance in the myotonic goat, an historically important animal model for the elucidation of the role of chloride in muscle excitation. A single nucleotide change causing the substitution of proline for a conserved alanine residue in the carboxyl terminus of the goat muscle chloride channel (gCIC-1) was discovered. Heterologous expression of the mutation demonstrated a substantial (+47 mV) shift in the midpoint of steady-state activation of the channel, resulting in a diminished channel open probability at voltages near the resting membrane potential of skeletal muscle. These results provide a molecular basis for the decreased chloride conductance in myotonic muscle.

Two physically distinct pores in the dimeric ClC-0 chloride channel

The Torpedo chloride channel ClC-0 is the prototype of a large family of chloride channels that have roles in transepithelial transport and in regulating electrical excitability and cell volume. ClC-0 opens in bursts with two identical conductance levels of approximately 8pS. Hyperpolarization slowly increases the probability of bursts ('slow gating'), and depolarization increases channel opening within bursts ('fast gating'). Replacing serine 123 by threonine changes rectification, ion selectivity and gating, but retains the typical bursting behaviour with two identical independent albeit reduced, conductance states (approximately 1.5 pS). Coexpression with wild-type ClC-0, either as covalently linked concatamers or as independent proteins, leads to bursting channels with two different pores. Our experiments strongly suggest that conductance, ion selectivity and 'fast' gating are determined only by the single subunit forming a single pore, independent from the attached pore; in contrast, 'slow' gating is a function of both subunits. Thus ClC-0 is a homodimer with two largely independent pores.

Chloride channels: a molecular perspective

Plasma membrane Cl- channels perform a variety of functions, including control of excitability in neurons and muscle, cell volume regulation and transepithelial transport. Structurally, three classes of Cl- channels have been identified: ligand-gated, postsynaptic Cl- channels (e.g. GABA and glycine receptors); the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels (which belong to the traffic ATPase superfamily); and the CLC family of Cl- channels. Recent developments of note include further characterization of the expanding CLC Cl- channel family, advances in understanding the regulation of the CFTR Cl- channel and its emergent role as a regulator of other channels, clarification of issues related to swelling-activated Cl- channels, and the discovery that several co-transporter molecules are now known to induce Cl- currents in Xenopus oocytes.

A common molecular basis for three inherited kidney stone diseases

Kidney stones (nephrolithiasis), which affect 12% of males and 5% of females in the western world, are familial in 45% of patients and are most commonly associated with hypercalciuria. Three disorders of hypercalciuric nephrolithiasis (Dent's disease, X-linked recessive nephrolithiasis (XRN), and X-linked recessive hypophosphataemic rickets (XLRH)) have been mapped to Xp11.22 (refs 5-7). A microdeletion in one Dent's disease kindred allowed the identification of a candidate gene, CLCN5 (refs 8,9) which encodes a putative renal chloride channel. Here we report the investigation of 11 kindreds with these renal tubular disorders for CLCN5 abnormalities; this identified three nonsense, four missense and two donor splice site mutations, together with one intragenic deletion and one microdeletion encompassing the entire gene. Heterologous expression of wild-type CLCN5 in Xenopus oocytes yielded outwardly rectifying chloride currents, which were either abolished or markedly reduced by the mutations. The common aetiology for Dent's disease, XRN and XLRH indicates that CLCN5 may be involved in other renal tubular disorders associated with kidney stones.