Effect of age on the processing and import of matrix-destined mitochondrial proteins in skeletal muscle

Mitochondrial Quantity and Quality in Age-Related Sarcopenia

Sarcopenia, the age-associated decline in skeletal muscle mass and strength, is a condition with a complex pathophysiology. Among the factors underlying the development of sarcopenia are the progressive demise of motor neurons, the transition from fast to slow myosin isoform (type II to type I fiber switch), and the decrease in satellite cell number and function. Mitochondrial dysfunction has been indicated as a key contributor to skeletal myocyte decline and loss of physical performance with aging. Several systems have been implicated in the regulation of muscle plasticity and trophism such as the fine-tuned and complex regulation between the stimulator of protein synthesis, mechanistic target of rapamycin (mTOR), and the inhibitor of mTOR, AMP-activated protein kinase (AMPK), that promotes muscle catabolism. Here, we provide an overview of the molecular mechanisms linking mitochondrial signaling and quality with muscle homeostasis and performance and discuss the main pathways elicited by their imbalance during age-related muscle wasting. We also discuss lifestyle interventions (i.e., physical exercise and nutrition) that may be exploited to preserve mitochondrial function in the aged muscle. Finally, we illustrate the emerging possibility of rescuing muscle tissue homeostasis through mitochondrial transplantation.

The Many Roles Mitochondria Play in Mammalian Aging

Significance: Aging is a natural process that affects most living organisms, resulting in increased mortality. As the world population ages, the prevalence of age-associated diseases, and their associated health care costs, has increased sharply. A better understanding of the molecular mechanisms that lead to cellular dysfunction may provide important targets for interventions to prevent or treat these diseases. Recent Advances: Although the mitochondrial theory of aging had been proposed more than 40 years ago, recent new data have given stronger support for a central role for mitochondrial dysfunction in several pathways that are deregulated during normal aging and age-associated disease. Critical Issues: Several of the experimental evidence linking mitochondrial alterations to age-associated loss of function are correlative and mechanistic insights are still elusive. Here, we review how mitochondrial dysfunction may be involved in many of the known hallmarks of aging, and how these pathways interact in an intricate net of molecular relationships. Future Directions: As it has become clear that mitochondrial dysfunction plays causative roles in normal aging and age-associated diseases, it is necessary to better define the molecular interactions and the temporal and causal relationship between these changes and the relevant phenotypes seen during the aging process. Antioxid. Redox Signal. 36, 824-843.

Regulation of muscle and mitochondrial health by the mitochondrial fission protein Drp1 in aged mice

KEY POINTS

The maintenance of mitochondrial integrity is critical for skeletal muscle health. Mitochondrial dynamics play key roles in mitochondrial quality control; however, the exact role that mitochondrial fission plays in the muscle ageing process remains unclear. Here we report that both Drp1 knockdown and Drp1 overexpression late in life in mice is detrimental to skeletal muscle function and mitochondrial health. Drp1 knockdown in 18-month-old mice resulted in severe skeletal muscle atrophy, mitochondrial dysfunction, muscle degeneration/regeneration, oxidative stress and impaired autophagy. Overexpressing Drp1 in 18-month-old mice resulted in mild skeletal muscle atrophy and decreased mitochondrial quality. Our data indicate that silencing or overexpressing Drp1 late in life is detrimental to skeletal muscle integrity. We conclude that modulating Drp1 expression is unlikely to be a viable approach to counter the muscle ageing process.

ABSTRACT

Sarcopenia, the ageing-related loss of skeletal muscle mass and function, is a debilitating process negatively impacting the quality of life of afflicted individuals. Although the mechanisms underlying sarcopenia are still only partly understood, impairments in mitochondrial dynamics, and specifically mitochondrial fission, have been proposed as an underlying mechanism. Importantly, conflicting data exist in the field and both excessive and insufficient mitochondrial fission were proposed to contribute to sarcopenia. In Drosophila melanogaster, enhancing mitochondrial fission in midlife through overexpression of dynamin-1-like protein (Drp1) extended lifespan and attenuated several key hallmarks of muscle ageing. Whether a similar outcome of Drp1 overexpression is observed in mammalian muscles remains unknown. In this study, we investigated the impact of knocking down and overexpressing Drp1 protein for 4 months in skeletal muscles of late middle-aged (18 months) mice using intra-muscular injections of adeno-associated viruses expressing shRNA targeting Drp1 or full Drp1 cDNA. We report that knocking down Drp1 expression late in life triggers severe muscle atrophy, mitochondrial dysfunctions, degeneration/regeneration, oxidative stress and impaired autophagy. Drp1 overexpression late in life triggered mild muscle atrophy and decreased mitochondrial quality. Taken altogether, our results indicate that both overexpression and silencing of Drp1 in late middle-aged mice negatively impact skeletal muscle mass and mitochondrial health. These data suggest that Drp1 content must remain within a narrow physiological range to preserve muscle and mitochondrial integrity during ageing. Altering Drp1 expression is therefore unlikely to be a viable target to counter sarcopenia.

Mitochondrial Impairment in Sarcopenia

Sarcopenia is defined by the age-related loss of skeletal muscle quality, which relies on mitochondrial homeostasis. During aging, several mitochondrial features such as bioenergetics, dynamics, biogenesis, and selective autophagy (mitophagy) are altered and impinge on protein homeostasis, resulting in loss of muscle mass and function. Thus, mitochondrial dysfunction contributes significantly to the complex pathogenesis of sarcopenia, and mitochondria are indicated as potential targets to prevent and treat this age-related condition. After a concise presentation of the age-related modifications in skeletal muscle quality and mitochondrial homeostasis, the present review summarizes the most relevant findings related to mitochondrial alterations in sarcopenia.

The redox environment and mitochondrial dysfunction in age-related skeletal muscle atrophy

Medical advancements have extended human life expectancy, which is not always accompanied by an improved quality of life or healthspan. A decline in muscle mass and function is a consequence of ageing and can result in a loss of independence in elderly individuals while increasing their risk of falls. Multiple cellular pathways have been implicated in age-related muscle atrophy, including the contribution of reactive oxygen species (ROS) and disrupted redox signalling. Aberrant levels of ROS disrupts the redox environment in older muscle, potentially disrupting cellular signalling and in some cases blunting the adaptive response to exercise. Age-related muscle atrophy is associated with disrupted mitochondrial content and function, one of the hallmarks of age-related diseases. There is a critical link between abnormal ROS generation and dysfunctional mitochondrial dynamics including mitochondrial biogenesis, fusion and fission. In order to develop effective treatments or preventative strategies, it is important to gain a comprehensive understanding of the mechanistic pathways implicated in age associated loss of muscle.

Mitochondrial Mechanisms of Neuromuscular Junction Degeneration with Aging

Skeletal muscle deteriorates with aging, contributing to physical frailty, poor health outcomes, and increased risk of mortality. Denervation is a major driver of changes in aging muscle. This occurs through transient denervation-reinnervation events throughout the aging process that remodel the spatial domain of motor units and alter fiber type. In advanced age, reinnervation wanes, leading to persistent denervation that accelerates muscle atrophy and impaired muscle contractility. Alterations in the muscle fibers and motoneurons are both likely involved in driving denervation through destabilization of the neuromuscular junction. In this respect, mitochondria are implicated in aging and age-related neurodegenerative disorders, and are also likely key to aging muscle changes through their direct effects in muscle fibers and through secondary effects mediated by mitochondrial impairments in motoneurons. Indeed, the large abundance of mitochondria in muscle fibers and motoneurons, that are further concentrated on both sides of the neuromuscular junction, likely renders the neuromuscular junction especially vulnerable to age-related mitochondrial dysfunction. Manifestations of mitochondrial dysfunction with aging include impaired respiratory function, elevated reactive oxygen species production, and increased susceptibility to permeability transition, contributing to reduced ATP generating capacity, oxidative damage, and apoptotic signaling, respectively. Using this framework, in this review we summarize our current knowledge, and relevant gaps, concerning the potential impact of mitochondrial impairment on the aging neuromuscular junction, and the mechanisms involved.

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.

Effects of Aging and Caloric Restriction on Fiber Type Composition, Mitochondrial Morphology and Dynamics in Rat Oxidative and Glycolytic Muscles

Aging is associated with a progressive decline in muscle mass and strength, a process known as sarcopenia. Evidence indicates that mitochondrial dysfunction plays a causal role in sarcopenia and suggests that alterations in mitochondrial dynamics/morphology may represent an underlying mechanism. Caloric restriction (CR) is among the most efficient nonpharmacological interventions to attenuate sarcopenia in rodents and is thought to exert its beneficial effects by improving mitochondrial function. However, CR effects on mitochondrial morphology and dynamics, especially in aging muscle, remain unknown. To address this issue, we investigated mitochondrial morphology and dynamics in the oxidative soleus (SOL) and glycolytic white gastrocnemius (WG) muscles of adult (9-month-old) ad libitum-fed (AL; A-AL), old (22-month-old) AL-fed (O-AL), and old CR (O-CR) rats. We show that CR attenuates the aging-related decline in the muscle-to-body-weight ratio, a sarcopenic index. CR also prevented the effects of aging on muscle fiber type composition in both muscles. With aging, the SOL displayed fragmented SubSarcolemmal (SS) and InterMyoFibrillar (IMF) mitochondria, an effect attenuated by CR. Aged WG displayed enlarged SS and more complex/branched IMF mitochondria. CR had marginal anti-aging effects on WG mitochondrial morphology. In the SOL, DRP1 (pro-fission protein) content was higher in O-AL vs YA-AL, and Mfn2 (pro-fusion) content was higher in O-CR vs A-AL. In the gastrocnemius, Mfn2, Drp1, and Fis1 (pro-fission) contents were higher in O-AL vs A-AL. CR reduced this aging-related increase in Mfn2 and Fis1 content. Overall, these results reveal for the first time that aging differentially impacts mitochondrial morphology and dynamics in different muscle fiber types, by increasing fission/fragmentation in oxidative fibers while enhancing mitochondrial size and branching in glycolytic fibers. Our results also indicate that although CR partially attenuates aging-related changes in mitochondrial dynamics in glycolytic fibers, its anti-aging effect on mitochondrial morphology is restricted to oxidative fibers.

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.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

Skeletal muscle is a tissue with a low mitochondrial content under basal conditions, but it is responsive to acute increases in contractile activity patterns (i.e. exercise) which initiate the signalling of a compensatory response, leading to the biogenesis of mitochondria and improved organelle function. Exercise also promotes the degradation of poorly functioning mitochondria (i.e. mitophagy), thereby accelerating mitochondrial turnover, and preserving a pool of healthy organelles. In contrast, muscle disuse, as well as the aging process, are associated with reduced mitochondrial quality and quantity in muscle. This has strong negative implications for whole-body metabolic health and the preservation of muscle mass. A number of traditional, as well as novel regulatory pathways exist in muscle that control both biogenesis and mitophagy. Interestingly, although the ablation of single regulatory transcription factors within these pathways often leads to a reduction in the basal mitochondrial content of muscle, this can invariably be overcome with exercise, signifying that exercise activates a multitude of pathways which can respond to restore mitochondrial health. This knowledge, along with growing realization that pharmacological agents can also promote mitochondrial health independently of exercise, leads to an optimistic outlook in which the maintenance of mitochondrial and whole-body metabolic health can be achieved by taking advantage of the broad benefits of exercise, along with the potential specificity of drug action.

Mitochondrial morphology is altered in atrophied skeletal muscle of aged mice

Skeletal muscle aging is associated with a progressive decline in muscle mass and strength, a process termed sarcopenia. Evidence suggests that accumulation of mitochondrial dysfunction plays a causal role in sarcopenia, which could be triggered by impaired mitophagy. Mitochondrial function, mitophagy and mitochondrial morphology are interconnected aspects of mitochondrial biology, and may coordinately be altered with aging. However, mitochondrial morphology has remained challenging to characterize in muscle, and whether sarcopenia is associated with abnormal mitochondrial morphology remains unknown. Therefore, we assessed the morphology of SubSarcolemmal (SS) and InterMyoFibrillar (IMF) mitochondria in skeletal muscle of young (8-12wk-old) and old (88-96wk-old) mice using a quantitative 2-dimensional transmission electron microscopy approach. We show that sarcopenia is associated with larger and less circular SS mitochondria. Likewise, aged IMF mitochondria were longer and more branched, suggesting increased fusion and/or decreased fission. Accordingly, although no difference in the content of proteins regulating mitochondrial dynamics (Mfn1, Mfn2, Opa1 and Drp1) was observed, a mitochondrial fusion index (Mfn2-to-Drp1 ratio) was significantly increased in aged muscles. Our results reveal that sarcopenia is associated with complex changes in mitochondrial morphology that could interfere with mitochondrial function and mitophagy, and thus contribute to aging-related accumulation of mitochondrial dysfunction and sarcopenia.

Mitochondria, muscle health, and exercise with advancing age

Skeletal muscle health is dependent on the optimal function of its mitochondria. With advancing age, decrements in numerous mitochondrial variables are evident in muscle. Part of this decline is due to reduced physical activity, whereas the remainder appears to be attributed to age-related alterations in mitochondrial synthesis and degradation. Exercise is an important strategy to stimulate mitochondrial adaptations in older individuals to foster improvements in muscle function and quality of life.

Mitochondrial Quality Control and Muscle Mass Maintenance

Loss of muscle mass and force occurs in many diseases such as disuse/inactivity, diabetes, cancer, renal, and cardiac failure and in aging-sarcopenia. In these catabolic conditions the mitochondrial content, morphology and function are greatly affected. The changes of mitochondrial network influence the production of reactive oxygen species (ROS) that play an important role in muscle function. Moreover, dysfunctional mitochondria trigger catabolic signaling pathways which feed-forward to the nucleus to promote the activation of muscle atrophy. Exercise, on the other hand, improves mitochondrial function by activating mitochondrial biogenesis and mitophagy, possibly playing an important part in the beneficial effects of physical activity in several diseases. Optimized mitochondrial function is strictly maintained by the coordinated activation of different mitochondrial quality control pathways. In this review we outline the current knowledge linking mitochondria-dependent signaling pathways to muscle homeostasis in aging and disease and the resulting implications for the development of novel therapeutic approaches to prevent muscle loss.

Mitochondrial involvement and impact in aging skeletal muscle

Atrophy is a defining feature of aging skeletal muscle that contributes to progressive weakness and an increased risk of mobility impairment, falls, and physical frailty in very advanced age. Amongst the most frequently implicated mechanisms of aging muscle atrophy is mitochondrial dysfunction. Recent studies employing methods that are well-suited to interrogating intrinsic mitochondrial function find that mitochondrial respiration and reactive oxygen species emission changes are inconsistent between aging rat muscles undergoing atrophy and appear normal in human skeletal muscle from septuagenarian physically active subjects. On the other hand, a sensitization to permeability transition seems to be a general property of atrophying muscle with aging and this effect is even seen in atrophying muscle from physically active septuagenarian subjects. In addition to this intrinsic alteration in mitochondrial function, factors extrinsic to the mitochondria may also modulate mitochondrial function in aging muscle. In particular, recent evidence implicates oxidative stress in the aging milieu as a factor that depresses respiratory function in vivo (an effect that is not present ex vivo). Furthermore, in very advanced age, not only does muscle atrophy become more severe and clinically relevant in terms of its impact, but also there is evidence that this is driven by an accumulation of severely atrophied denervated myofibers. As denervation can itself modulate mitochondrial function and recruit mitochondrial-mediated atrophy pathways, future investigations need to address the degree to which skeletal muscle mitochondrial alterations in very advanced age are a consequence of denervation, rather than a primary organelle defect, to refine our understanding of the relevance of mitochondria as a therapeutic target at this more advanced age.

Two weeks of one-leg immobilization decreases skeletal muscle respiratory capacity equally in young and elderly men

Physical inactivity affects human skeletal muscle mitochondrial oxidative capacity but the influence of aging combined with physical inactivity is not known. This study investigates the effect of two weeks of immobilization followed by six weeks of supervised cycle training on muscle oxidative capacity in 17 young (23±1years) and 15 elderly (68±1years) healthy men. We applied high-resolution respirometry in permeabilized fibers from muscle biopsies at inclusion after immobilization and training. Furthermore, protein content of mitochondrial complexes I-V, mitochondrial heat shock protein 70 (mtHSP70) and voltage dependent anion channel (VDAC) were measured in skeletal muscle by Western blotting. The elderly men had lower content of complexes I-V and mtHSP70 but similar respiratory capacity and content of VDAC compared to the young. In both groups the respiratory capacity and protein content of VDAC, mtHSP70 and complexes I, II, IV and V decreased with immobilization and increased with retraining. Moreover, there was no overall difference in the response between the groups. When the intrinsic mitochondrial capacity was evaluated by normalizing respiration to citrate synthase activity, the respiratory differences with immobilization and training disappeared. In conclusion, aging is not associated with a decrease in muscle respiratory capacity in spite of lower complexes I-V and mtHSP70 protein content. Furthermore, immobilization decreased and aerobic training increased the respiratory capacity and protein contents of complexes I-V, mtHSP70 and VDAC similarly in the two groups. This suggests that inactivity and training alter mitochondrial biogenesis equally in young and elderly men.

Mechanisms of exercise-induced mitochondrial biogenesis in skeletal muscle: implications for health and disease

Mitochondria have paradoxical functions within cells. Essential providers of energy for cellular survival, they are also harbingers of cell death (apoptosis). Mitochondria exhibit remarkable dynamics, undergoing fission, fusion, and reticular expansion. Both nuclear and mitochondrial DNA (mtDNA) encode vital sets of proteins which, when incorporated into the inner mitochondrial membrane, provide electron transport capacity for ATP production, and when mutated lead to a broad spectrum of diseases. Acute exercise can activate a set of signaling cascades in skeletal muscle, leading to the activation of the gene expression pathway, from transcription, to post-translational modifications. Research has begun to unravel the important signals and their protein targets that trigger the onset of mitochondrial adaptations to exercise. Exercise training leads to an accumulation of nuclear- and mtDNA-encoded proteins that assemble into functional complexes devoted to mitochondrial respiration, reactive oxygen species (ROS) production, the import of proteins and metabolites, or apoptosis. This process of biogenesis has important consequences for metabolic health, the oxidative capacity of muscle, and whole body fitness. In contrast, the chronic muscle disuse that accompanies aging or muscle wasting diseases provokes a decline in mitochondrial content and function, which elicits excessive ROS formation and apoptotic signaling. Research continues to seek the molecular underpinnings of how regular exercise can be used to attenuate these decrements in organelle function, maintain skeletal muscle health, and improve quality of life.

Adaptive plasticity of autophagic proteins to denervation in aging skeletal muscle

Aging muscle exhibits a progressive decline in mass and strength, known as sarcopenia, and a decrease in the adaptive response to contractile activity. The molecular mechanisms mediating this reduced plasticity have yet to be elucidated. The purposes of this study were 1) to determine whether denervation-induced muscle disuse would increase the expression of autophagy genes and 2) to examine whether selective autophagy pathways (mitophagy) are altered in aged animals. Denervation reduced muscle mass in young and aged animals by 24 and 16%, respectively. Moreover, young animals showed a 50% decrease in mitochondrial content following denervation, an adaptation that was not matched by aged animals. Basal autophagy protein expression was higher in aged animals, whereas young animals exhibited a greater induction of autophagy proteins following denervation. Localization of LC3II, Parkin, and p62 was significantly increased in the mitochondrial fraction of young and aged animals following denervation. Moreover, the unfolded protein response marker CHOP and the mitochondrial dynamics protein Fis1 were increased by 17- and 2.5-fold, respectively, in aged animals. Lipofuscin granules within lysosomes were evident with aging and denervation. Thus reductions in the adaptive plasticity of aged muscle are associated with decreases in disuse-induced autophagy. These data indicate that the expression of autophagy proteins and their localization to mitochondria are not decreased in aged muscle; however, the induction of autophagy in response to disuse, along with downstream events such as lysosome function, is impaired. This may contribute to an accumulation of dysfunctional mitochondria in aged muscle.

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

We investigated the assembly of the TOM complex within skeletal muscle under conditions of chronic contractile activity-induced mitochondrial biogenesis. Tom40 import into mitochondria was increased by chronic contractile activity, as was its time-dependent assembly into the TOM complex. These changes coincided with contractile activity-induced augmentations in the expression of key protein import machinery components Tim17, Tim23, and Tom22, as well as the cytosolic chaperone Hsp90. These data indicate the adaptability of the TOM protein import complex and suggest a regulatory role for the assembly of this complex in exercise-induced mitochondrial biogenesis.

Histochemical and ultrastructural changes of sternomastoid muscle in aged Wistar rats

The aim of this study was to evaluate histochemically and ultrastructurally the sternomastoid muscle (SM) of adults and aged rats, employing histochemic (NADH-TR reaction) and transmission electron microscopic methods. It was used 20 rats, divided into two groups: adults (n=10), animals with 4 months of age, and aged group (n=10), animals with 24 months of age. Five animals from each group were anesthetized with an overdose of urethane (3g/kg i.p.), and the muscles dissected after the samples processing for histochemical reaction (NADH-TR). Three types of fibers were identified by their metabolic characteristics: fibers with high oxidative capacity (O), intermediate oxidative capacity (OG) and low oxidative capacity (G). For transmission electron microscopic method, the animals were anesthetized and perfused by modified Karnovsky solution and the tissues were postfixed in 1% osmium tetroxide solution, dehydrated and embedded in Spurr resin. It was performed ultra-thin sections for transmission electron microscopic analysis. The SM showed heterogeneity in their composition according to the fiber types, with significant difference (p<0.05) when comparing the fibers types between the superficial and deep regions and between the adult and aged groups. It was observe a decrease between the comparison of the total fibers density and GO fiber, and an increase of the O fiber in aged group. Ultrastructural characteristics of muscle cells in aged group showed typical morphological changes, characterizing muscular atrophy. We conclude based on physiological ageing process, changes in muscle fibers classification, and ultrastructuraly, morphological alterations on muscle cells, characterizing a muscular atrophy.

Effect of denervation-induced muscle disuse on mitochondrial protein import

This study determined whether muscle disuse affects mitochondrial protein import and whether changes in protein import are related to mitochondrial content and function. Protein import was measured using a model of unilateral peroneal nerve denervation in rats for 3 ( n = 10), 7 ( n = 12), or 14 ( n = 14) days. We compared the import of preproteins into the matrix of subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria isolated from the denervated and the contralateral control tibialis anterior muscles. Denervation led to 50% and 29% reductions in protein import after 14 days of disuse in SS and IMF mitochondria, respectively. This was accompanied by significant decreases in mitochondrial state 3 respiration, muscle mass, and whole muscle cytochrome c oxidase activity. To investigate the mechanisms involved, we assessed disuse-related changes in 1) protein import machinery components and 2) mitochondrial function, reflected by respiration and reactive oxygen species (ROS) production. Denervation significantly reduced the expression of translocases localized in the inner membrane (Tim23), outer membrane (Tom20), and mitochondrial heat shock protein 70 (mtHsp70), especially in the SS subfraction. Denervation also resulted in elevated ROS generation, and exogenous ROS was found to markedly reduce protein import. Thus our data indicate that protein import kinetics are closely related to alterations in mitochondrial respiratory capacity ( r = 0.95) and are negatively impacted by ROS. Deleterious changes in the protein import system likely facilitate the reduction in mitochondrial content and the increase in organelle dysfunction (i.e., increased ROS production and decreased respiration) during chronic disuse, which likely contribute to the activation of degradative pathways leading to muscle atrophy.

Proteomic alterations of distinct mitochondrial subpopulations in the type 1 diabetic heart: contribution of protein import dysfunction

Diabetic cardiomyopathy is associated with increased risk of heart failure in type 1 diabetic patients. Mitochondrial dysfunction is suggested as an underlying contributor to diabetic cardiomyopathy. Cardiac mitochondria are characterized by subcellular spatial locale, including mitochondria located beneath the sarcolemma, subsarcolemmal mitochondria (SSM), and mitochondria situated between the myofibrils, interfibrillar mitochondria (IFM). The goal of this study was to determine whether type 1 diabetic insult in the heart influences proteomic make-up of spatially distinct mitochondrial subpopulations and to evaluate the role of nuclear encoded mitochondrial protein import. Utilizing multiple proteomic approaches (iTRAQ and two-dimensional-differential in-gel electrophoresis), IFM proteomic make-up was impacted by type 1 diabetes mellitus to a greater extent than SSM, as evidenced by decreased abundance of fatty acid oxidation and electron transport chain proteins. Mitochondrial phosphate carrier and adenine nucleotide translocator, as well as inner membrane translocases, were decreased in the diabetic IFM (P < 0.05 for both). Mitofilin, a protein involved in cristae morphology, was diminished in the diabetic IFM (P < 0.05). Posttranslational modifications, including oxidations and deamidations, were most prevalent in the diabetic IFM. Mitochondrial heat shock protein 70 (mtHsp70) was significantly decreased in diabetic IFM (P < 0.05). Mitochondrial protein import was decreased in the diabetic IFM with no change in the diabetic SSM (P < 0.05). Taken together, these results indicate that mitochondrial proteomic alterations in the type 1 diabetic heart are more pronounced in the IFM. Further, proteomic alterations are associated with nuclear encoded mitochondrial protein import dysfunction and loss of an essential mitochondrial protein import constituent, mtHsp70, implicating this process in the pathogenesis of the diabetic heart.