Daily heat stress treatment rescues denervation-activated mitochondrial clearance and atrophy in skeletal muscle

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Abstract

Traumatic nerve injury or motor neuron disease leads to denervation and severe muscle atrophy. Recent evidence indicates that loss of mitochondria and the related reduction in oxidative capacity could be key mediators of skeletal muscle atrophy. As our previous study showed that heat stress increased the numbers of mitochondria in skeletal muscle, we evaluated whether heat stress treatment could have a beneficial impact on denervation-induced loss of mitochondria and subsequent muscle atrophy. Here, we report that daily heat stress treatment (mice placed in a chamber with a hot environment; 40°C, 30 min day−1, for 7 days) rescues the following parameters: (i) muscle atrophy (decreased gastrocnemius muscle mass); (ii) loss of mitochondrial content (decreased levels of ubiquinol–cytochrome c reductase core protein II, cytochrome c oxidase subunits I and IV and voltage-dependent anion channel protein); and (iii) reduction in oxidative capacity (reduced maximal activities of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase) in denervated muscle (produced by unilateral sciatic nerve transection). In order to gain a better understanding of the above mitochondrial adaptations, we also examined the effects of heat stress on autophagy-dependent mitochondrial clearance (mitophagy). Daily heat stress normalized denervation-activated induction of mitophagy (increased mitochondrial microtubule-associated protein 1A/1B-light chain3-II (LC3-II) with and without blocker of autophagosome clearance). The molecular basis of this observation was explained by the results that heat stress attenuated the denervation-induced increase in key proteins that regulate the following steps: (i) the tagging step of mitochondrial clearance (increased mitochondrial Parkin, ubiquitin-conjugated, P62/sequestosome 1 (P62/SQSTM1)); and (ii) the elongation step of autophagosome formation (increased Atg5–Atg12 conjugate and Atg16L). Overall, our results contribute to the better understanding of mitochondrial quality control and the mechanisms behind the attenuation of muscle wasting by heat stress in denervated skeletal muscle.

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