P.11.4 Altered mechano-transduction in dystrophic muscle is accompanied by changes in function and expression of ClC-1 chloride channel

Abstract

The understanding of the molecular mechanisms underlying exercise susceptibility of mdx mice may help to identify new drug targets for muscular dystrophy. An enhancement of in vivo weakness and fatigability occurs in mdx mice undergoing 6–12weeks of treadmill running; in parallel ex vivo studies show that exercise increases the resistance to eccentric contraction in wildtype (wt) extensor digitorum longus (EDL) muscles, while the adaptation is not observed in mdx ones, which remain weaker than controls. For clarifying the mechanism involved in the different exercise adaptations of the two genotypes, we focused on chloride channel conductance (gCl), crucial for excitation–contraction coupling and myofiber-type profile. A decrease in gCl is a typical hallmark of mdx diaphragm (DIA) myofibers, while it occurs in EDL ones only as a consequence of exercise. RT-PCR experiments show a 30–35% reduction of CLC-1 mRNA in both DIA and EDL of mdx mice, irrespective to the exercise regimen. In parallel, genes involved in slow-oxidative phenotype such as type I myosin heavy chain, peroxisome proliferator-activated receptor <i>γ</i> coactivator 1<i>α</i> and sirtuin-1 are more expressed in both EDL and gastrocnemius muscles of non-exercised mdx mice vs. wt ones. The exercise-selective alteration of gCl in mdx EDL muscle suggests post-transcriptional mechanisms of channel modulation. We investigated the role of mechano-sensitive mediators of inflammation and oxidative stress, such as Angiotensin II (Ang-II). The application of Ang-II to wt EDL fibers reduces gCl in a concentration-dependent manner (IC50=60nM). The effect is mediated by activation of the AT1-receptor and inhibited by chelerythrine, a protein kinase C-inhibitor, N-acetylcysteine and apocynin, an inhibitor of NADPH-oxidase. Then, transcriptional and post-transcriptional changes may occur in mechano-sensitive targets in mdx muscles in relation to metabolism, inflammation and oxidative stress (supported by DPP/NL).