Acute exercise stimulates AMPK and corrects some disease characteristics in the skeletal muscle of myotonic dystrophy type 1 mice

Abstract

Myotonic Dystrophy type I (DM1) is the most prevalent adult form of muscular dystrophy and is characterized by myotonia, skeletal muscle weakness and wasting. The disease pathogenesis is characterized by a CTG microsatellite repeat expansion, leading to the dysregulation of pre-messenger RNA splicing of numerous muscle genes. There is currently no cure for this disorder, but recent evidence indicate that exercise is a safe and modestly effective therapy. AMP-activated protein kinase (AMPK) is critical to the acute responses and chronic adaptations to physical activity at the cellular and molecular level in the healthy condition, as well as in various disease states. However, the AMPK-signaling response to exercise in DM1 remains largely unknown. Therefore, the purpose of this study is to examine whether a single bout of exercise 1) activates AMPK and its downstream signalling network in DM1 skeletal muscle, and 2) modulates the DM1 molecular signature. Wild-type (WT) and HSA-LR (DM1) mice ran on a motor-driven treadmill until the inability to continue exercise was objectively determined, and the molecular response to physical activity at various timepoints post-exercise was examined using Western blotting, immunofluorescence microscopy, and qPCR techniques. WT mice ran for 937.6 ± 193.8 meters, which was significantly greater than DM1 mice at 573.3 ± 181.4meters. In the skeletal muscle of WT mice, AMPK activation status was significantly augmented immediately after exercise. This coincided with an increase (p < 0.05) in peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) transcript levels, as well as significant elevations in unc-51-like kinase 1 (ULK1) and calcium/calmodulin-dependent protein kinase type 2β (CAMKIIβ) activation. In DM1 mice, acute exercise also significantly stimulated AMPK, however, PGC-1α, ULK1 and CAMKIIβ were unaffected. In resting muscle, several markers related to autophagy downstream of AMPK, for example ULK1 activation status, p62 protein level, and the lipidated form of microtubule-associated protein 1-light chain 3, were significantly elevated in DM1 mice compared to their WT counterparts. These normalized after exercise. Acute physical activity did not impact either the prevalence of toxic myonuclear foci formed by the CTG microsatellite repeat or the proportion of mis-spliced muscle-specific chloride channel in DM1 mice. Collectively, these results indicate that while exercise-induced AMPK phosphorylation was preserved in the skeletal muscle of DM1 mice, markers of upstream and downstream AMPK signalling were attenuated in response to acute physical activity. Nonetheless, our data also suggest that exercise-evoked AMPK activation normalizes perturbations in the autophagy pathway observed in DM1 muscle. Thus, skeletal muscle AMPK stimulation after a single bout of exercise is very likely necessary but insufficient to elicit beneficial structural and functional adaptations in DM1.