Inactivation of muscle chloride channel by transposon insertion in myotonic mice

The skeletal muscle chloride channel in dominant and recessive human myotonia

Autosomal recessive generalized myotonia (Becker's disease) (GM) and autosomal dominant myotonia congenita (Thomsen's disease) (MC) are characterized by skeletal muscle stiffness that is a result of muscle membrane hyperexcitability. For both diseases, alterations in muscle chloride or sodium currents or both have been observed. A complementary DNA for a human skeletal muscle chloride channel (CLC-1) was cloned, physically localized on chromosome 7, and linked to the T cell receptor beta (TCRB) locus. Tight linkage of these two loci to GM and MC was found in German families. An unusual restriction site in the CLC-1 locus in two GM families identified a mutation associated with that disease, a phenylalanine-to-cysteine substitution in putative transmembrane domain D8. This suggests that different mutations in CLC-1 may cause dominant or recessive myotonia.

Right-angle light scattering to assay basal and regulated plasma membrane Cl- conductances

We describe a simple and rapid technique for assaying both constitutive and regulated plasma membrane Cl- conductances. The method uses right-angle light scattering to measure the rate of swelling of cells in suspension, in which the anion conductance is rate limiting for swelling, due to introduction of high plasma membrane cation conductance using gramicidin. The technique was verified using Chinese hamster ovary cells and mouse L cells, both stably transfected with the cystic fibrosis transmembrane conductance regulator (CFTR), to confer a specific cAMP-activated Cl- conductance not normally present in these cell types. In agreement with results obtained using other methods for assaying Cl- permeability in these cells, forskolin stimulated a significant increase in plasma membrane Cl- conductance in CFTR-expressing cells, as indicated by an increase in light scattering. That the enhanced light scattering by the cells was the result of cell swelling due to NaCl influx was shown by ion substitution experiments, in which no forskolin-induced increase in light scatter occurred in N-methyl-D-glucamine Cl- or Na+ gluconate medium. Enhanced light scattering was also observed in both CFTR-expressing and control cells stimulated with the Ca2+ ionophore, ionomycin. Extracellular anion substitution, to exploit the inwardly directed halide gradient utilized in this protocol, enabled determination of the anion selectivities of both the cAMP- and Ca(2+)-activated Cl- channels. Thus this technique provides a simple optical method for rapidly assaying not only constitutive and regulated Cl- conductance pathways but also their anion selectivities.

Chemical probes for anion transporters of mammalian cell membranes

Mammalian cell membranes harbor several types of chloride channels, chloride-cation symporters/cotransporters, and several classes of anion exchangers/antiporters. These transport systems subserve different cellular or organismic functions, depending on the nature of the cell, the spatial organization of transporters, and their functional interplay. Chemical probing has played a central role in the structural and functional delineation of the various anion transport systems. The design of specific probes or their selection from existing sources coupled with their judicious application to the most appropriate biological system had led to the identification of specific anion transporters and to the elucidation of the underlying molecular transport mechanism. In many instances, chemical probing has remained the major or exclusive analytical tool for the functional definition or identification of a given transport system, particularly for discerning among the various anion transporters which operate in highly heterogeneous cell membrane systems. This work critically reviews the present state of the chemical armamentarium available for the most common anion transporters found in mammalian cell membranes. It encompasses the description of the most useful or commonly used probes in terms of their chemical, biochemical, physiological, and pharmacological properties. The review deals primarily with what chemical probes tell about anion transporters and, most importantly, with the limitations inherent in the use of probes in transport studies.

A chloride channel widely expressed in epithelial and non-epithelial cells

Chloride channels have several functions, including the regulation of cell volume, stabilizing membrane potential, signal transduction and transepithelial transport. The plasma membrane Cl- channels already cloned belong to different structural classes: ligand-gated channels, voltage-gated channels, and possibly transporters of the ATP-binding-cassette type (if the cystic fibrosis transmembrane regulator is a Cl- channel). The importance of chloride channels is illustrated by the phenotypes that can result from their malfunction: cystic fibrosis, in which transepithelial transport is impaired, and myotonia, in which ClC-1, the principal skeletal muscle Cl- channel, is defective. Here we report the properties of ClC-2, a new member of the voltage-gated Cl- channel family. Its sequence is approximately 50% identical to either the Torpedo electroplax Cl- channel, ClC-0 (ref. 8), or the rat muscle Cl- channel, ClC-1 (ref. 9). Isolated initially from rat heart and brain, it is also expressed in pancreas, lung and liver, for example, and in pure cell lines of fibroblastic, neuronal, and epithelial origin, including tissues and cells affected by cystic fibrosis. Expression in Xenopus oocytes induces Cl- currents that activate slowly upon hyperpolarization and display a linear instantaneous current-voltage relationship. The conductivity sequence is Cl- greater than or equal to Br- greater than I-. The presence of ClC-2 in such different cell types contrasts with the highly specialized expression of ClC-1 (ref. 9) and also with the cloned cation channels, and suggests that its function is important for most cells.

The structure of the mouse parvalbumin gene

Parvalbumin (PV) is a calcium-binding protein of the EF-hand family, expressed mainly in fast contracting/relaxing muscles of vertebrates. We have isolated five overlapping genomic PV clones which overall span 28 kilobase pairs (kb) around the Pva locus on mouse Chromosome (Chr) 15. The positions of four introns were determined by DNA sequencing. They interrupt the coding sequences at positions corresponding to those in rat and human PV genes. The transcription start site, 25 bp downstream from the TATA-box, was mapped by oligonucleotide primer extension on poly(A)(+)-RNA. The analysis of 0.4 kb promoter sequence of the mouse PV gene revealed CCAAT- and TATA-box sequences and a 59 bp GC-rich stretch between positions -59 and -118. Similar motifs have been found in the parvalbumin genes of rat and human. A perfect 11-bp repeat upstream to positions -149 and -163 respectively is homologous only to the rat promoter. These results will be related to tissue and species differences in PV expression.

Primary structure and functional expression of a developmentally regulated skeletal muscle chloride channel

Skeletal muscle is unusual in that 70-85% of resting membrane conductance is carried by chloride ions. This conductance is essential for membrane-potential stability, as its block by 9-anthracene-carboxylic acid and other drugs causes myotonia. Fish electric organs are developmentally derived from skeletal muscle, suggesting that mammalian muscle may express a homologue of the Torpedo mamorata electroplax chloride channel. We have now cloned the complementary DNA encoding a rat skeletal muscle chloride channel by homology screening to the Cl- channel from Torpedo. It encodes a 994-amino-acid protein which is about 54% identical to the Torpedo channel and is predominantly expressed in skeletal muscle. Messenger RNA amounts in that tissue increase steeply in the first 3-4 weeks after birth, in parallel with the increase in muscle Cl- conductance. Expression from cRNA in Xenopus oocytes leads to 9-anthracene-carboxylic acid-sensitive currents with time and voltage dependence typical for macroscopic muscle Cl- conductance. This and the functional destruction of this channel in mouse myotonia suggests that we have cloned the major skeletal muscle chloride channel.