Plasticity of TOM complex assembly in skeletal muscle mitochondria in response to chronic contractile activity

Endurance training increases mitochondrial myoglobin and enhances its interaction with complex IV in rat plantaris muscle

AIM

Endurance exercise training is known to increase mitochondrial respiration in skeletal muscle. However, the molecular mechanisms behind this are not fully understood. Myoglobin (Mb) is a member of the globin family, which is highly expressed in skeletal and cardiac muscles. We recently found that Mb localizes inside mitochondria in skeletal muscle and interacts with cytochrome c oxidase subunit IV (COXIV), a subunit of mitochondrial complex IV, which regulates respiration by augmenting complex IV activity. In the present study, we investigated the effect of endurance training on Mb-COXIV interaction within mitochondria in rat skeletal muscle.

METHODS

Eight-week-old male Wistar rats were subjected to 6-week treadmill running training. Forty-eight hours after the last training session, the plantaris muscle was removed under anesthesia and used for biochemical analysis.

RESULTS

The endurance training increased mitochondrial content in the skeletal muscle. It also augmented complex IV-dependent oxygen consumption and complex IV activity in isolated mitochondria from skeletal muscle. Furthermore, endurance training increased Mb expression at the whole muscle level. Importantly, mitochondrial Mb content and Mb-COXIV binding were increased by endurance training.

CONCLUSION

These findings suggest that an increase in mitochondrial Mb and the concomitant enhancement of Mb interaction with COXIV may contribute to the endurance training-induced upregulation of mitochondrial respiration by augmenting complex IV activity.

Mitochondrial protein import and UPRmt in skeletal muscle remodeling and adaptation

The biogenesis of mitochondria requires the coordinated expression of the nuclear and the mitochondrial genomes. However, the vast majority of gene products within the organelle are encoded in the nucleus, synthesized in the cytosol, and imported into mitochondria via the protein import machinery, which permit the entry of proteins to expand the mitochondrial network. Once inside, proteins undergo a maturation and folding process brought about by enzymes comprising the unfolded protein response (UPR^mt). Protein import and UPR^mt activity must be synchronized and matched with mtDNA-encoded subunit synthesis for proper assembly of electron transport chain complexes to avoid proteotoxicity. This review discusses the functions of the import and UPR^mt systems in mammalian skeletal muscle, as well as how exercise alters the equilibrium of these pathways in a time-dependent manner, leading to a new steady state of mitochondrial content resulting in enhanced oxidative capacity and improved muscle health.

Mitochondrial Protein Import Dysfunction in Pathogenesis of Neurodegenerative Diseases

Mitochondria play an essential role in maintaining energy homeostasis and cellular survival. In the brain, higher ATP production is required by mature neurons for communication. Most of the mitochondrial proteins transcribe in the nucleus and import in mitochondria through different pathways of the mitochondrial protein import machinery. This machinery plays a crucial role in determining mitochondrial morphology and functions through mitochondrial biogenesis. Failure of this machinery and any alterations during mitochondrial biogenesis underlies neurodegeneration resulting in Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), and Parkinson's disease (PD) etc. Current knowledge has revealed the different pathways of mitochondrial protein import machinery such as translocase of the outer mitochondrial membrane complex, the presequence pathway, carrier pathway, β-barrel pathway, and mitochondrial import and assembly machinery etc. In this review, we have discussed the recent studies regarding protein import machinery, beyond the well-known effects of increased oxidative stress and bioenergetics dysfunctions. We have elucidated in detail how these types of machinery help to import and locate the precursor proteins to their specific location inside the mitochondria and play a major role in mitochondrial biogenesis. We further discuss their involvement in mitochondrial dysfunctioning and the induction of toxic aggregates in neurodegenerative diseases like AD and PD. The review supports the importance of import machinery in neuronal functions and its association with toxic aggregated proteins in mitochondrial impairment, suggesting a critical role in fostering and maintaining neurodegeneration and therapeutic response.

The intersection of exercise and aging on mitochondrial protein quality control

Skeletal muscle quality and quantity are negatively impacted with age. Part of this decline in function can be attributed to alterations in mitochondrial turnover, and in the mechanisms that regulate mitochondrial homeostasis. Protein quality control within the mitochondria relies on a number of interconnected processes, namely the mitochondrial unfolded protein response (UPR^mt), protein import and mitophagy. In particular, the post-transcriptional regulation of protein import into the organelle has generated considerable recent interest in view of its dynamic versatility. The capacity for import can be increased by chronic exercise, and diminished by muscle disuse, and defects in the import pathway can be rescued by exercise. Within mitochondria, the unfolded protein response (UPR) is activated if protein import is altered, or if protein misfolding takes place. This UPR generates retrograde signaling to the nucleus to activate compensatory gene expression and protein synthesis. Mitophagy is also elevated with age, contributing to the lower mitochondrial content in aging muscle. However, mitophagy is amenable to exercise adaptations, as it is activated with each exercise bout, presumably to mediate mitochondrial quality control. However, this response is attenuated in older subjects. Although not yet completely elucidated, numerous molecular processes involved in mitochondrial biogenesis and turnover are affected with age. The contrasting and often opposite consequences of exercise and age suggest that exercise can serve as non-pharmacological "mitochondrial medicine" for aging muscle to ameliorate mitochondrial content and function, via pathways that implicate organelle protein quality control mechanisms.

Resistance Exercise Improves Mitochondrial Quality Control in a Rat Model of Sporadic Inclusion Body Myositis

Background: Mitochondrial dysfunction is implicated in the pathogenesis of multiple muscular diseases, including sporadic inclusion body myositis (s-IBM), the most common aging-related muscle disease. However, the factors causing mitochondrial dysfunction in s-IBM are unknown. Objective: We hypothesized that resistance exercise (RE) may alleviate muscle impairment by improving mitochondrial function via reducing amyloid-beta (Aβ) accumulation. Methods: Twenty-four male Wistar rats were randomized to a saline-injection control group (sham, n = 8), a chloroquine (CQ) control group (CQ-CON, n = 8), and a CQ plus RE group (CQ-RE, n = 8) in which rats climbed a ladder with weight attached to their tails 9 weeks after starting CQ treatment. Results: RE markedly inhibited soleus muscle atrophy and muscle damage. RE reduced CQ-induced Aβ accumulation, which resulted in decreased formation of rimmed vacuoles and mitochondrial-mediated apoptosis. Most importantly, the decreased Aβ accumulation improved both mitochondrial quality control (MQC) through increased mitochondrial biogenesis and mitophagy, and mitochondrial dynamics. Furthermore, RE-mediated reduction of Aβ accumulation elevated mitochondrial oxidative capacity by upregulating superoxide dismutase-2, catalase, and citrate synthase via activating sirtuin 3 signaling. Conclusion: RE enhances mitochondrial function by improving MQC and mitochondrial oxidative capacity via reducing Aβ accumulation, thereby inhibiting CQ-induced muscle impairment, in a rat model of s-IBM.

Maintenance of Skeletal Muscle Mitochondria in Health, Exercise, and Aging

Mitochondria are critical organelles responsible for regulating the metabolic status of skeletal muscle. These organelles exhibit remarkable plasticity by adapting their volume, structure, and function in response to chronic exercise, disuse, aging, and disease. A single bout of exercise initiates signaling to provoke increases in mitochondrial biogenesis, balanced by the onset of organelle turnover carried out by the mitophagy pathway. This accelerated turnover ensures the presence of a high functioning network of mitochondria designed for optimal ATP supply, with the consequence of favoring lipid metabolism, maintaining muscle mass, and reducing apoptotic susceptibility over the longer term. Conversely, aging and disuse are associated with reductions in muscle mass that are in part attributable to dysregulation of the mitochondrial network and impaired mitochondrial function. Therefore, exercise represents a viable, nonpharmaceutical therapy with the potential to reverse and enhance the impaired mitochondrial function observed with aging and chronic muscle disuse.

Treadmill exercise alleviates motor deficits and improves mitochondrial import machinery in an MPTP-induced mouse model of Parkinson's disease

Alpha-synuclein (α-Syn) accumulation is significantly correlated with motor deficits and mitochondrial dysfunction in Parkinson's disease (PD), but the molecular mechanism underlying its pathogenesis is unclear. In this study, we investigated the effects of treadmill exercise on motor deficits and mitochondrial dysfunction in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD. Treadmill exercise inhibited dopaminergic neuron loss by promoting the expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT) and seemed to improve cell survival by reducing α-Syn expression. Most importantly, treadmill exercise increased expression of the mitochondrial import machinery proteins TOM-40, TOM-20, and TIM-23. This was associated with decreased α-Syn expression and subsequent upregulation of the mitochondrial proteins COX-I, COX-IV, and mtHSP70. Taken together, these results indicate that treadmill exercise may ameliorate motor deficits and improve mitochondrial dysfunction by reducing α-Syn expression in the MPTP-induced mouse model of PD.

Chronology of UPR activation in skeletal muscle adaptations to chronic contractile activity

The mitochondrial and endoplasmic reticulum unfolded protein responses (UPR(mt) and UPR(ER)) are important for cellular homeostasis during stimulus-induced increases in protein synthesis. Exercise triggers the synthesis of mitochondrial proteins, regulated in part by peroxisome proliferator activator receptor-γ coactivator 1α (PGC-1α). To investigate the role of the UPR in exercise-induced adaptations, we subjected rats to 3 h of chronic contractile activity (CCA) for 1, 2, 3, 5, or 7 days followed by 3 h of recovery. Mitochondrial biogenesis signaling, through PGC-1α mRNA, increased 14-fold after 1 day of CCA. This resulted in 10-32% increases in cytochrome c oxidase activity, indicative of mitochondrial content, between days 3 and 7, as well as increases in the autophagic degradation of p62 and microtubule-associated proteins 1A/1B light chain 3A (LC3)-II protein. Before these adaptations, the UPR(ER) transcripts activating transcription factor-4, spliced X-box-binding protein 1, and binding immunoglobulin protein were elevated (1.3- to 3.8-fold) at days 1-3, while CCAAT/enhancer-binding protein homologous protein (CHOP) and chaperones binding immunoglobulin protein and heat shock protein (HSP) 70 were elevated at mRNA and protein levels (1.5- to 3.9-fold) at days 1-7 of CCA. The mitochondrial chaperones 10-kDa chaperonin, HSP60, and 75-kDa mitochondrial HSP, the protease ATP-dependent Clp protease proteolytic subunit, and the regulatory protein sirtuin-3 of the UPR(mt) were concurrently induced 10-80% between days 1 and 7 To test the role of the UPR in CCA-induced remodeling, we treated animals with the endoplasmic reticulum stress suppressor tauroursodeoxycholic acid and subjected them to 2 or 7 days of CCA. Tauroursodeoxycholic acid attenuated CHOP and HSP70 protein induction; however, this failed to impact mitochondrial remodeling. Our data indicate that signaling to the UPR is rapidly activated following acute contractile activity, that this is attenuated with repeated bouts, and that the UPR is involved in chronic adaptations to CCA; however, this appears to be independent of CHOP signaling.

Exercise and the Regulation of Mitochondrial Turnover

Exercise is a well-known stimulus for the expansion of the mitochondrial pool within skeletal muscle. Mitochondria have a remarkable ability to remodel their networks and can respond to an array of signaling stimuli following contractile activity to adapt to the metabolic demands of the tissue, synthesizing proteins to expand the mitochondrial reticulum. In addition, when they become dysfunctional, these organelles can be recycled by a specialized intracellular system. The signals regulating this mitochondrial life cycle of synthesis and degradation during exercise are still an area of great research interest. As mitochondrial turnover has valuable consequences in physical performance, in addition to metabolic health, disease, and aging, consideration of the signals which control this cycle is vital. This review focuses on the regulation of mitochondrial turnover in skeletal muscle and summarizes our current understanding of the impact that exercise has in modulating this process.

Recent advances in mitochondrial turnover during chronic muscle disuse

Chronic muscle disuse, such as that resulting from immobilization, denervation, or prolonged physical inactivity, produces atrophy and a loss of mitochondria, yet the molecular relationship between these events is not fully understood. In this review we attempt to identify the key regulatory steps mediating the loss of muscle mass and the decline in mitochondrial content and function. An understanding of common intracellular signaling pathways may provide much-needed insight into the possible therapeutic targets for treatments that will maintain aerobic energy metabolism and preserve muscle mass during disuse conditions.

Altered mitochondrial morphology and defective protein import reveal novel roles for Bax and/or Bak in skeletal muscle

The function Bax and/or Bak in constituting a gateway for mitochondrial apoptosis in response to apoptotic stimuli has been unequivocally demonstrated. However, recent work has suggested that Bax/Bak may have unrecognized nonapoptotic functions related to mitochondrial function in nonstressful environments. Wild-type (WT) and Bax/Bak double knockout (DKO) mice were used to determine alternative roles for Bax and Bak in mitochondrial morphology and protein import in skeletal muscle. The absence of Bax and/or Bak altered mitochondrial dynamics by regulating protein components of the organelle fission and fusion machinery. Moreover, DKO mice exhibited defective mitochondrial protein import, both into the matrix and outer membrane compartments, which was consistent with our observations of impaired membrane potential and attenuated expression of protein import machinery (PIM) components in intermyofibrillar mitochondria. Furthermore, the cytosolic chaperones heat-shock protein 90 (Hsp90) and binding immunoglobulin protein (BiP) were markedly increased with the deletion of Bax/Bak, indicating that the cytosolic environment related to protein folding may be changed in DKO mice. Interestingly, endurance training fully restored the deficiency of protein import in DKO mice, likely via the upregulation of PIM components and through improved cytosolic chaperone protein expression. Thus our results emphasize novel roles for Bax and/or Bak in mitochondrial function and provide evidence, for the first time, of a curative function of exercise training in ameliorating a condition of defective mitochondrial protein import.

Skeletal muscle aging and the mitochondrion

Decline in human muscle mass and strength (sarcopenia) is a hallmark of the aging process. A growing body of research in the areas of bioenergetics and protein turnover has placed the mitochondria at the center of this process. It is now clear that, unless an active lifestyle is rigorously followed, skeletal muscle mitochondrial decline occurs as humans age. Increasing research on mitochondrial biology has elucidated the regulatory pathways involved in mitochondrial biogenesis, many of which are potential therapeutic targets, and highlight the beneficial effects of vigorous physical activity on skeletal muscle health for an aging population.