ROLE OF INNERVATION, EXCITABILITY, AND MYOGENIC FACTORS IN THE EXPRESSION OF THE MUSCULAR CHLORIDE CHANNEL CLC-1 - A STUDY ON NORMAL AND MYOTONIC MUSCLE

The muscular chloride channel(ClC-1) is essential for a normal excitab ility of mature mammalian muscle fibers; inactivation of the correspon ding gene by mutations leads to hyperexcitability of muscle, the hallm ark of the disease myotonia. In the mouse, there is very little ClC-1 mRNA in myotubes, and its concentration increases steeply during postn atal development, suggesting a role of the motor nerve in ClC-1 expres sion. We investigated the response of the expression of the correspond ing gene Clc-1 to different patterns of muscle activity as controlled by sarcolemmal excitability and by innervation. In rat and mouse, the level of ClC-1 mRNA was higher in fast (extensor digitorum longus) tha n in slow (soleus) muscle. Myotonia in the ADR mouse is caused by an i nsertional mutation leading to the adr allele of the Clc-1 gene and to grossly abnormal ClC-1 mRNAs. Nevertheless, in +/adr heterozygous, ph enotypically wild type (WT) animals, the expression levels of both all eles correspond to the gene dosage. However, in the myotonic ADR mouse in which both Clc-1 genes are defective, ClC-1 mRNA levels in slow mu scle were nearly as high as in WT fast muscle. In WT muscle, denervati on within 2 days caused a drastic reduction of the ClC-1 mRNA level an d at the same time an increase of myogenin and MyoD mRNAs. Neither eff ect of denervation was observed in myotonic mice (homozygous for the a lleles adr or adr(K)), suggesting that spontaneous electrical activity of the hyperexcitable sarcolemma may substitute for nerve activity. F urthermore, potential MyoD/myogenin-binding sequence motifs were ident ified in the 5' regulatory region of the Clc-1 gene. These findings su ggest that the activity-dependent regulation of the muscular chloride channel 1 gene is mediated by myogenic factors.