β-Hydroxy-β-methylbutyrate enhances fast-twitch muscle and mitochondrial function, histopathology and mTORC1 signalling in the <i>mdx</i> dystrophic mouse

Duchenne muscular dystrophy (DMD) is one of the most severe forms of inheritable muscular dystrophies, caused by a genetic mutation resulting in the loss of dystrophin. Dystrophin loss initiates a cascade of negative mechanistic changes in skeletal muscle, such as disrupted protein homeostasis and mitochondrial dysfunction. Recent evidence suggests the leucine metabolite, β-hydroxy-β-methylbutyrate (HMB), may improve physical function in DMD boys and improve aspects of the dystrophic phenotype in preclinical <i>mdx</i> mice. HMB has been shown to modulate protein turnover and mitochondrial function, both of which are dysregulated in DMD. Therefore, this study examined the effect of 3-wk of HMB supplementation (0.75 mg/g/day via drinking water), starting at 3-wk of age in <i>mdx</i> mice. HMB-treated <i>mdx</i> mice exhibited increased full-body grip strength and holding impulse, compared with <i>mdx</i> controls. HMB treatment also increased normalized muscle mass of the fast-twitch extensor digitorum longus (EDL) muscle, which coincided with increased average fiber size and improved absolute/specific in vitro force production. Moreover, HMB-treated EDL muscles displayed increased mitochondrial complex II succinate dehydrogenase activity, alongside upregulated markers of mammalian target of rapamycin complex 1 (mTORC1) signalling (p70S6K1 and 4EBP1 phosphorylation), suggestive of increased protein synthesis. Finally, muscle fibers isolated from HMB-treated <i>mdx</i> mice showed improved mitochondrial efficiency that was associated with increased maximal respiration, spare respiratory capacity, and ATP synthesis. This study is the first to show HMB-induced improvements on in vitro and in vivo measures of <i>mdx</i> skeletal muscle force production that are coupled with improved mitochondrial function, suggesting that HMB may be a viable treatment option for DMD.<b>NEW & NOTEWORTHY</b> This is the first study to examine the effect of HMB in 3-wk-old <i>mdx</i> mice undergoing extensive muscle damage and regeneration both in vivo and in vitro. HMB treatment increased voluntary grip strength and holding impulse, while elevating force production of isolated <i>mdx</i> EDL muscles, which were associated with improved muscle mass, muscle fiber size, and succinate dehydrogenase activity. Finally, these improvements coincided with increased markers of mTORC1 signalling, mitochondrial respiration, and ATP production.