Mitochondrial DNA (mtDNA) mutations lead to decrements in mitochondrial function and accelerated rates of these mutations has been linked to skeletal muscle loss (sarcopenia). The purpose of this study was to investigate the effect of mtDNA mutations on mitochondrial quality control processes in skeletal muscle from animals (young; 3–6 months and older; 8–15 months) expressing a proofreading-deficient version of mtDNA polymerase gamma (PolG). This progeroid aging model exhibits elevated mtDNA mutation rates, mitochondrial dysfunction, and a premature aging phenotype that includes sarcopenia. We found increased expression of the mitochondrial biogenesis regulator peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) and its target proteins, nuclear respiratory factor 1 (NRF-1) and mitochondrial transcription factor A (Tfam) in PolG animals compared to wild-type (WT) (P<0.05). Muscle from older PolG animals displayed higher mitochondrial fission protein 1 (Fis1) concurrent with greater induction of autophagy, as indicated by changes in Atg5 and p62 protein content (P<0.05). Additionally, levels of the Tom22 import protein were higher in PolG animals when compared to WT (P<0.05). In contrast, muscle from normally-aged animals exhibited a distinctly different expression profile compared to PolG animals. Older WT animals appeared to have higher fusion (greater Mfn1/Mfn2, and lower Fis1) and lower autophagy (Beclin-1 and p62) compared to young WT suggesting that autophagy is impaired in aging muscle. In conclusion, muscle from mtDNA mutator mice display higher mitochondrial fission and autophagy levels that likely contribute to the sarcopenic phenotype observed in premature aging and this differs from the response observed in normally-aged muscle.

Mechanical ventilation (MV) is a life-saving intervention for many critically ill patients. Unfortunately, prolonged MV results in rapid diaphragmatic atrophy and contractile dysfunction, collectively termed ventilator-induced diaphragm dysfunction (VIDD). Recent evidence reveals that endurance exercise training, performed prior to MV, protects the diaphragm against VIDD. While the mechanism(s) responsible for this exercise-induced protection against VIDD remain unknown, increased diaphragm antioxidant expression may be required. To investigate the role that increased antioxidants play in this protection, we tested the hypothesis that elevated levels of the mitochondrial antioxidant enzyme superoxide dismutase 2 (SOD2) is required to achieve exercise-induced protection against VIDD. Cause and effect was investigated in two ways. First, we prevented the exercise-induced increase in diaphragmatic SOD2 via delivery of an antisense oligonucleotide targeted against SOD2 post-exercise. Second, using transgene overexpression of SOD2, we determined the effects of increased SOD2 in the diaphragm independent of exercise training. Results from these experiments revealed that prevention of the exercise-induced increases in diaphragmatic SOD2 results in a loss of exercise-mediated protection against MV-induced diaphragm atrophy and a partial loss of protection against MV-induced diaphragmatic contractile dysfunction. In contrast, transgenic overexpression of SOD2 in the diaphragm, independent of exercise, did not protect against MV-induced diaphragmatic atrophy and provided only partial protection against MV-induced diaphragmatic contractile dysfunction. Collectively, these results demonstrate that increased diaphragmatic levels of SOD2 are essential to achieve the full benefit of exercise-induced protection against VIDD.

Background Mechanical ventilation (MV) is a life‐saving measure for patients in respiratory failure. However, prolonged MV results in significant diaphragm atrophy and contractile dysfunction, a condition referred to as ventilator‐induced diaphragm dysfunction (VIDD). While there are currently no clinically approved countermeasures to prevent VIDD, increased expression of heat shock protein 72 (HSP72) has been demonstrated to attenuate inactivity‐induced muscle wasting. HSP72 elicits cytoprotection via inhibition of NF‐κB and FoxO transcriptional activity, which contribute to VIDD. In addition, exercise‐induced prevention of VIDD is characterized by an increase in the concentration of HSP72 in the diaphragm. Therefore, we tested the hypothesis that increased HSP72 expression is required for the exercise‐induced prevention of VIDD. We also determined whether increasing the abundance of HSP72 in the diaphragm, independent of exercise, is sufficient to prevent VIDD. Methods Cause and effect was determined by inhibiting the endurance exercise‐induced increase in HSP72 in the diaphragm of exercise trained animals exposed to prolonged MV via administration of an antisense oligonucleotide targeting HSP72. Additional experiments were performed to determine if increasing HSP72 in the diaphragm via genetic (rAAV‐HSP72) or pharmacological (BGP‐15) overexpression is sufficient to prevent VIDD. Results Our results demonstrate that the exercise‐induced increase in HSP72 protein abundance is required for the protective effects of exercise against VIDD. Moreover, both rAAV‐HSP72 and BGP‐15‐induced overexpression of HSP72 were sufficient to prevent VIDD. In addition, modification of HSP72 in the diaphragm is inversely related to the expression of NF‐κB and FoxO target genes. Conclusions HSP72 overexpression in the diaphragm is an effective intervention to prevent MV‐induced oxidative stress and the transcriptional activity of NF‐κB and FoxO. Therefore, overexpression of HSP72 in the diaphragm is a potential therapeutic target to protect against VIDD.

Fatigue is a symptom of many diseases, but it can also manifest as a unique medical condition, such as idiopathic chronic fatigue (ICF). While the prevalence of ICF increases with age, mitochondrial content and function decline with age, which may contribute to ICF. The purpose of this study was to determine whether skeletal muscle mitochondrial dysregulation and oxidative stress is linked to ICF in older adults. Sedentary, old adults (n = 48, age 72.4 ± 5.3 years) were categorized into ICF and non-fatigued (NF) groups based on the FACIT-Fatigue questionnaire. ICF individuals had a FACIT score one standard deviation below the mean for non-anemic adults > 65 years and were excluded according to CDC diagnostic criteria for ICF. Vastus lateralis muscle biopsies were analyzed, showing reductions in mitochondrial content and suppression of mitochondrial regulatory proteins Sirt3, PGC-1α, NRF-1, and cytochrome c in ICF compared to NF. Additionally, mitochondrial morphology proteins, antioxidant enzymes, and lipid peroxidation were unchanged in ICF individuals. Our data suggests older adults with ICF have reduced skeletal muscle mitochondrial content and biogenesis signaling that cannot be accounted for by increased oxidative damage.

Curcumin, a polyphenol found in the spice turmeric, is shown to have antioxidant and anti‐inflammatory properties in multiple tissues, but whether it alters mitochondrial biogenesis/apoptotic pathways is unknown. The aging process is associated with impaired mitochondrial function and elevated mitochondrial apoptotic susceptibility. Potential pharmacological and/or neutraceutical therapeutic interventions capable of improving mitochondrial function, like curcumin, have been postulated to delay this process. Thus, we investigated whether short‐term (21d) dietary curcumin supplementation (5% diet) altered mitochondrial biogenesis in muscle and brown adipose tissue (BAT) of aged mice (24 month; C57BL/6) compared to control diet mice (n=4‐6/group). While curcumin supplementation increased the mitochondrial content markers cytochrome c (14%) and COX Vb (26%) and enhanced the mitochondrial regulators Tfam (25%) and NRF‐1 (14%) in BAT, it suppressed and/or caused no change in these mitochondrial indices in muscle. In contrast, curcumin treatment evoked significant decreases (P<0.05) in the pro‐apoptotic BAX protein in both BAT (20%) and muscle (55%). Our data indicate short‐term curcumin treatment in aged mice causes tissue‐specific mitochondrial biogenesis adaptations in BAT and muscle while potentially suppressing mitochondrial apoptotic susceptibility in both tissues.

Mechanical ventilation (MV) is a life‐saving intervention for patients unable to sustain adequate pulmonary gas exchange on their own. Unfortunately, prolonged MV results in rapid atrophy and contractile dysfunction of the diaphragm, collectively termed ventilator‐induced diaphragm dysfunction (VIDD). Although the mechanisms responsible for VIDD remain elusive, it is established that increased mitochondrial production of reactive oxygen species is required for the development of VIDD. Notably, recent evidence reveals that endurance exercise training performed prior to MV is sufficient to protect the diaphragm against VIDD. While the mechanisms responsible for exercise‐induced prevention of VIDD remain unknown, evidence suggests that endurance exercise training may reduce diaphragm oxidative damage by elevating endogenous antioxidant enzyme expression. Therefore, these experiments tested the hypothesis that endurance exercise‐induced protection against VIDD is dependent upon increased diaphragmatic levels of the mitochondrial antioxidant enzyme, manganese superoxide dismutase (MnSOD). Cause and effect was determined by administering an antisense oligonucleotide against MnSOD to prevent the exercise‐induced increase in diaphragmatic MnSOD. Our data confirm that exercise training performed prior to prolonged MV results in protection against VIDD. Importantly, prevention of the exercise‐induced increases in diaphragmatic MnSOD resulted in a loss of exercise‐mediated protection against MV‐induced diaphragm atrophy. In contrast, prevention of exercise‐induced increases in MnSOD did not result in a loss of exercise‐induced protection against MV‐induced diaphragm contractile dysfunction. Collectively, these results reveal that while increases in diaphragmatic levels of MnSOD contribute to exercise induced protection against VIDD, increased MnSOD is not the sole mechanism responsible for exercise‐induced protection against VIDD.Support or Funding InformationSupported by NIH R01 AR064189 awarded to SKP