Stephen E. Alway

Rehabilitative activity can improve injury repair, but it risks additional damage and reduces the functional recovery of regenerating muscle. This study tested the hypothesis that moderate electrically evoked contractions would slow restoration of neuromuscular function after cardiotoxin‐induced injury; however exogenous mitochondrial transplantation (MT) would enhance recovery of contractile function after injury. Cardiotoxin was injected into the tibialis anterior of C57BL/6 mice (10–12 weeks of age) to induce muscle necrosis. Exogenous mitochondria or phosphate‐buffered saline (PBS) were injected into the mouse tail vein after cardiotoxin injury. Injured muscles were either rested or given 40 Hz submaximal electrically evoked contractions to cardiotoxin‐injured muscles during the recovery period. Relative to intra‐animal non‐damaged control muscles restoration of peak tetanic torque after both rested and evoked contractions during recovery and twitch torque was greater, and the difference between control and injured muscle twitch one‐half relaxation time was lower in injured muscles that were rested for 10 days after injury and received MT compared to PBS‐treated muscles. Neuromuscular junction efficiency in cardiotoxin‐injured muscles was ∼70% of control undamaged muscles, but MT improved the recovery of neuromuscular junction efficiency to produce torque by 14 days after cardiotoxin injury in muscles that received additional damage induced by evoked contractions during the recovery period. These data suggest that MT enhances the recovery of neuromuscular function when the muscle is rested after injury, but it provides limited improvement in muscle function when the muscle is challenged with electrically evoked contractions in the recovery period after injury. image Key points: Mitochondrial transplantation by systemically infusing healthy donor mitochondria into injured mice improved the recovery of maximal torque production of injured muscles when evoked contractions were provided to the regenerating muscle during the recovery period after injury. Mitochondrial transplantation improved the restoration of neuromuscular junction efficiency after muscle injury. The recovery of maximal torque capabilities function following cardiotoxin‐induced tibialis anterior muscle injury was attenuated by electrically evoked muscle contractions conducted every other day during the recovery period in young adult mice.

Electrically evoked muscle contractions conducted every other day after muscle injury caused additional damage and slowed recovery of muscle regeneration; however, mitochondria transplantation attenuated the negative regenerative effects of rehabilitative contractions on muscle fiber regeneration. This suggests that optimizing mitochondrially regulated repair may improve muscle regeneration. Furthermore, injured muscles treated by mitochondrial transplantation had lower collagen content/fibrosis than those treated with PBS after injury. Mitochondrial transplantation blunts fibrosis and improves muscle fiber repair after injury.

The present study demonstrates that in colon tumor-bearing mice, the administration of FOLFOX chemotherapy alters plasma inflammatory cytokines’ relationship to tumor mass, body composition, and physical function. Furthermore, sex influences these responses. These findings have implications for mechanistically understanding sex-specific muscle wasting and metabolic complications experienced by patients with colorectal cancer undergoing chemotherapy treatment.

Skeletal muscle is a primary tissue of dysfunction during both aging and obesity. Recently, the coincidence of obesity and aging has gained attention due to the intersection of the obesity epidemic with an aging demographic. Both aging and obesity are associated with marked defects in skeletal muscle metabolic health. Despite these findings, we have a poor understanding of how obesity and aging may interact to impact skeletal muscle mass and metabolic health. Therefore, we investigated the impact of high-fat diet (HFD)-induced obesity on skeletal muscle mass, mitochondrial function, transcriptomics, and whole-body metabolism in young and aged mice. We observed main effects of diet and age on several measures of whole-body metabolic function (VO ₂ , VCO ₂ , and RER). Complex I-driven mitochondrial proton leak was significantly elevated by HFD-induced obesity across both age groups; however, a main effect of aging for reduced complex I leak was detected in the soleus muscle. Interestingly, aged animals fed a HFD did not exhibit lower muscle mass than chow-fed young animals, but did present with stark increases in muscle triglyceride content and a unique transcriptional response to HFD. HFD-induced obesity impacted the muscle transcriptome differently in the muscles of young and aged mice, indicating that obesity can change altered gene expression with age. Our findings suggest that the presence of obesity can both compound and counteract age-associated changes to muscle mass, gene expression, and mitochondrial function.

Skeletal muscle health and function deteriorate with age, ultimately leading to impaired mobility and disability. Exercise is among the most effective interventions to mitigate muscle dysfunction in aging and reverse deficits. However, low attrition and an impaired capacity to exercise may limit its utility in improving muscle function in aged persons. Therefore, it is crucial to advance our mechanistic understanding of the molecular transducers of exercise to identify new and innovative drug targets to improve muscle health. Transcriptomic profiling of the human response to exercise has revealed that the nuclear receptor NR4A3 (NOR‐1) is among the most responsive genes to acute exercise. Previously, we observed that in vitro knockdown of NOR‐1 alters metabolic signaling in C2C12 myotubes. Specifically, we found that expression of PERM1, CKMT2, myoglobin, and mTORC1 signaling were perturbed during the knockdown of NOR‐1. Herein, we extend these findings and observe that a NOR‐1‐PERM1‐myoglobin axis regulates myoglobin expression in vitro. Furthermore, we found that aging is associated with reduced skeletal muscle NOR‐1 expression. Although it is well known that exercise improves aged muscle function, whether overexpression of the exercise‐responsive gene NOR‐1 can confer benefits and improve muscle function in an aged context has not been evaluated. We found that the overexpression of NOR‐1 in aged muscle results in enhanced muscle endurance, mitochondrial respiration, and elevated expression of NOR‐1 responsive genes that we previously identified in loss of function studies. However, we also observed that overexpression of NOR‐1 did not improve maximal muscle torque production and resulted in a small but significant loss of muscle wet weight that was concomitant with elevated autophagy signaling. Our data suggest that NOR‐1 expression may reduce muscle fatigability and that NOR‐1 drives myoglobin expression in a PERM1‐dependent manner.

Neuromuscular diseases and damage affect many people of all ages and are responsible for an exorbitant medical cost, more than $200 million annually. Finding an appropriate model to investigate potential curative interventions is necessary. One currently used involves the application of toxic agents on skeletal muscle followed by treatment interventions. A question regarding this model is whether such toxins impact not only muscle tissue, but also the neuromuscular junctions (NMJs) responsible for exciting the muscle tissue. This question was addressed here by forming 4 experimental groups: 1) Controls whose muscles had not been injured or treated, 2) muscles taken from mice that were injured and then treated with mitochondrial supplement, 3) muscles that had not been injured but still treated with mitochondria, and 4) muscles that were injured and received no mitochondrial treatment. Several pre- and post-synaptic features of NMJs received immunofluorescent staining before having morphological features assessed with confocal microscopy. Results revealed that only pre-synaptic acetylcholine (ACh) receptors showed any significant (p0.05) among treatment groups and structural features were not different between groups with the exception of dispersion of post-synaptic receptors. Overall, these results suggest that skeletal muscles damaged with toxin accurately mimic what occurs during toxin-induced damage and post-injury recovery.

Recent studies have indicated a role for circulating extracellular vesicles (EVs) in the pathogenesis of multiple diseases. However, most in vitro studies have used variable and arbitrary doses of EVs rather than interpreting EVs as an existing component of standard skeletal muscle cell culture media. The current study provides an initial investigation into the effects of circulating EVs on the metabolic phenotype of C2C12 myotubes by replacing EVs from fetal bovine serum with circulating EVs from control mice or mice with obesity and type 2 diabetes (OT2D). We report that EVs associated with OT2D decrease 2‐NBDG uptake (a proxy measure of glucose uptake) in the insulin‐stimulated state compared to controls. OT2D associated EV treatment also significantly decreased myosin heavy chain type 1 (MHCI) mRNA abundance in myotubes but had no effect on mRNA expression of any other myosin heavy chain isoforms. OT2D‐associated circulating EVs also significantly increased lipid accumulation within myotubes without altering the expression of a selection of genes important for lipid entry, synthesis, or catabolism. The data indicate that, in a severely diabetic state, circulating EVs may contribute to insulin resistance and alter gene expression in myotubes in a manner consistent with the skeletal muscle phenotype observed in OT2D.

The present study demonstrates that FOLFOX chemotherapy induces long-lasting deficits in systemic metabolism. Interestingly, FOLFOX modestly suppressed skeletal muscle AMPK and autophagy signaling in vivo and in vitro. The FOLFOX-induced suppression of muscle metabolic signaling recovered after treatment cessation, independent of systemic metabolic dysfunction. Future research should investigate if activating AMPK during treatment can prevent long-term toxicities to improve health and quality of life of patients with cancer and survivors.