Resistance exercise following short term muscle disuse leads to enhanced mitochondrial activity and dynamics

Introduction: Short-term muscle unloading (~7-10 days) can be usually experienced following injury or hospitalisation, leading to muscle disuse atrophy, muscle weakness and changes in motor control. In such contexts, exercise countermeasures can be employed to overcome these neuromuscular impairments. Surprisingly, the effects of exercise countermeasures following the exposure to muscle unloading has been scarcely investigated. Hence, this study aimed to investigate the functional and molecular adaptations to a 21-day resistance training (RT) program following short term muscle disuse. We hypothesised that a period of resistance exercise following short-term unloading would speed up the recovery on muscle mass and function mostly by an effcient counteraction of the molecular regulators of muscle disuse atrophy consistent with the exercise modality employed. Methods: A 10-day unilateral lower limb suspension (ULLS) model was applied to eleven healthy young males (22.09 ± 2.91 y), followed by a 21-day RT program for the knee extensors performed at 70% of the 1RM (3 times/wk, 9 sessions in total). Data acquisition was performed before the ULLS period, after 10 days of unloading and after 21 days of resistance exercise. Quadriceps femoris isometric maximum voluntary contraction (MVC) was evaluated by dynamometry and quadriceps CSA was assessed by ultrasonography. Succinate dehydrogenase (SDH, a key component of the citric acid cycle and of the respiratory electron transfer chain) staining was performed on frozen cryosections obtained from VL biopsies, whereas western blot analyses were carried out for several mitochondrial-related proteins and transcriptomics data were obtained by RNA-seq. Results: Following ULLS, QF MVC and CSA decreased (-29.3%, p<0.001, and -4.5%, p<0.05, respectively). After the 21-d of RT, MVC was fully brought back (+42% compared to post-ULLS) whereas QF CSA showed a very marked hypertrophy (+18.6%, p<0.001, compared to post-ULLS). SDH immunoreactivity was not affected by unloading but revealed a significant increase after exercise compared to post ULLS values (~+20%, p<0.001). In addition, mitochondrial dynamics were highly influenced by resistance exercise, as mitochondrial pro-fusion (MFN1, MFN2, OPA1) and fission (DRP1, pDRP1616) proteins were markedly increased in response to RT compared to post-ULLS values (p<0.05). Further, PGC-1a levels showed a trend to decrease after ULLS but even a stronger trend to increase after resistance exercise compared to post-ULLS (p=0.056). Transcriptomics analyses showed that the most differentially expressed genes were identified in the pathways related to oxidative phosphorylation, in which the top regulated genes after resistance training were mitochondrial related genes (CKMT2, UQCRH, ACSS1, MDH1, COX7A2, CYCS, SLC16A1), presenting a marked reduction after ULLS and being significantly enhanced after RT. When compared to baseline, the genes which showed a positive “rebound effect” after resistance exercise were mostly related to mitochondrial function (MT-ATP6, MT-CO2, MT-ND3). Noteworthy, among these latter genes, the top upregulated one was APLNR, coding for apelin receptor protein, a positive regulator of inter-myofibrillar mitochondrial content in sarcopenia and ageing. Conclusions: Resistance exercise was effective in recovering muscle mass and function after 10 days of disuse, but it enhanced mitochondrial activity, biogenesis, and dynamics far beyond the expected effects for such exercise modality. In fact, mitochondrial-related genes expression was the most affected by exercise. This suggests that mitochondrial dynamics could play an essential role in the recovery of neuromuscular alterations induced by short term disuse. We put forward the hypothesis that the prior level of physical activity and a prior exposure to short-term muscle disuse could be driving the nature of adaptations to subsequent resistance training.