Chloride conductance in mouse muscle is subject to post‐transcriptional compensation of the functional Cl⁻ channel 1 gene dosage

Abstract

In mature mammalian muscle, the muscular chloride channel ClC‐1 contributes about 75% of the sarcolemmal resting conductance (Gm). In mice carrying two defective alleles of the corresponding Clc1 gene, chloride conductance (GCl) is reduced to less than 10% of that of wild‐type, and this causes hyperexcitability, the salient feature of the disease myotonia. Potassium conductance (GK) values in myotonic mouse muscle fibres are lowered by about 60% compared with wild‐type. The defective Clcadr allele causes loss of the 4.5 kb ClC‐1 mRNA. Mice heterozygous for the defective Clc1adr allele contain about 50% functional mRNA in their muscles compared with homozygous wild‐type mice. Despite a halved functional gene dosage, heterozygous muscles display an average GCl which is not significantly different from that of homozygous wild‐type animals. The GK values in heterozygotes are also indistinguishable from homozygous wild‐type animals. These results indicate that a regulatory mechanism acting at the post‐transcriptional level limits the density of ClC‐1 channels. GK is probably indirectly regulated by muscle activity.

Huntington disease skeletal muscle is hyperexcitable owing to chloride and potassium channel dysfunction

“…Consistent with this, we found reduced levels of normal mature Clcn1 and Kcnj2 mRNA. A disruption of mRNA splicing has been shown to cause a decrease in ClC-1 expression in myotonic dystrophy type 1, another trinucleotide repeat disorder (27)(28)(29)(30)(31)(32)(33)(34)(35)(36). We measured elevated levels of aberrant Clcn1 mRNA containing exon 7a in HD muscle, which indicates a similar disruption in Clcn1 pre-mRNA splicing.…”

“…The loss of chloride channels in myotonic dystrophy is thought to be due to an accumulation of RNA with CUG or CCUG repeats in the nucleus that disrupt the function of RNA binding proteins, such as muscleblind-like 1 and 2; consequently, aberrantly spliced Clcn1 mRNA that contains exon 7a is degraded via nonsense-mediated decay (27)(28)(29)(30)(31)(32)(33)(34)(35)(36). We tested for this mechanism in HD interosseous muscle (Fig.…”

Myotonia caused by mutations in the muscle chloride channel geneCLCN1

“…This observation also shows that a gene dosage of 50% is not sufficient to cause myotonia. Actually, at least in mice, it seems that a 50% gene dosage is almost fully compensated at the functional level [Chen et al, 1997]. Even a pharmacological reduction of the muscle Cl conductance by 50% is not sufficient to induce myotonia [Furman and Barchi, 1978], indicating a relatively large safetymargin for the prevention of myotonic runs.…”

Maturation and myotonia influence the abundance of cation channels KDR, KIR and CIR differently: a patch-clamp study on mouse interosseus muscle fibres

“…First, during postnatal development in rats, G K decreases whereas G Cl increases [9]. Second, in the myotonic mouse mutant ADR ("arrested development of righting response"), lowered G Cl values, responsible for muscle hyperexcitability, are accompanied by a reduction of G K to less than half [7]. Third, whole-cell, inwards-rectifying K + currents (K IR ), thought to contribute to resting G K , are up-regulated postnatally in mouse muscle.…”

“…The contribution of K DR to the macroscopic sarcolemmal G K can be calculated according to G=N·P o ·γ·A -1 (where A is the area of the membrane patch). Since, in contrast to our protocol, membrane conductances are usually measured at 37 °C [7,19], so that K DR unitary conductance must be corrected by a Q 10 value of 1.45 [20], yielding approximately 19 pS. Assuming a patch membrane area of about 5 µm 2 for 3-to 5-MΩ pipettes [27], K DR would contribute 8 µS cm -2 or less to total G K , for which a value of 369 µS cm -2 for extensor digitorum longus (EDL) muscle fibres [19] has been reported.…”