Ultra-high-resolution scanning electron microscopy of mitochondria and sarcoplasmic reticulum arrangement in human red, white, and intermediate muscle fibers

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

Mitochondrial Epigenetics and Environmental Exposure

The rising toll of chronic and debilitating diseases brought about by the exposure to an ever expanding number of environmental pollutants and socio-economic factors is calling for action. The understanding of the molecular mechanisms behind the effects of environmental exposures can lead to the development of biomarkers that can support the public health fields of both early diagnosis and intervention to limit the burden of environmental diseases. The study of mitochondrial epigenetics carries high hopes to provide important biomarkers of exposure and disease. Mitochondria are in fact on the frontline of the cellular response to the environment. Modifications of the epigenetic factors regulating the mitochondrial activity are emerging as informative tools that can effectively report on the effects of the environment on the phenotype. Here, we will discuss the emerging field of mitochondrial epigenetics. This review describes the main epigenetic phenomena that modify the activity of the mitochondrial DNA including DNA methylation, long and short non-coding RNAs. We will discuss the unique pattern of mitochondrial DNA methylation, describe the challenges of correctly measuring it, and report on the existing studies that have analysed the correlation between environmental exposures and mitochondrial DNA methylation. Finally, we provide a brief account of the therapeutic approaches targeting mitochondria currently under consideration.

Physical Exercise for Muscle Atrophy

The most direct characteristic of muscle atrophy is reduction in muscle mass, which is due to increased protein degradation or reduced protein synthesis in skeletal muscle. The loss of muscle mass can directly affect the quality of daily life, prolong the recovery period, and become the main risk factor for chronic diseases. However, there is currently no effective way to prevent and treat this disease, and therefore it is imperative to explore effective therapeutic approaches for muscle atrophy. It is well known that physical exercise is important for maintaining good health and long-term adherence to exercise can reduce the risk of cardiovascular diseases, obesity, and diabetes. It is also well established that exercise training can promote the synthesis of muscle protein and activate signaling pathways that regulate the metabolism and function of muscle fibers. Therefore, exercise can be used as a method to treat muscle atrophy in many of these conditions. Mitochondria play an important role in skeletal muscle homeostasis and bioenergy metabolism. Mitochondria are sensitive to contractile signals, and hence exercise can improve mitochondrial function and promote biosynthesis, which ultimately maintains the healthy state of cells and the whole body. On the other hand, frequent unaccustomed exercise will change the structure and function of skeletal muscle fibers, which is called exercise-induced muscle damage. When the exercise-induced muscle damage happens, it can cause temporary muscle damage and soreness, giving a negative effect on the muscle function.

Antioxidant MMCC ameliorates catch-up growth related metabolic dysfunction

Postnatal catch-up growth may be related to reduce mitochondrial content and oxidation capacity in skeletal muscle. The aim of this study is to explore the effect and mechanism of antioxidant MitoQuinone mesylate beta cyclodextrin complex (MMCC) ameliorates catch-up growth related metabolic disorders. Catch-up growth mice were created by restricting maternal food intake during the last week of gestation and providing high fat diet after weaning. Low birthweight mice and normal birthweight controls were randomly subjected to normal fat diet, high fat diet and high fat diet with MMCC drinking from the 4th week. MMCC treatment for 21 weeks slowed down the catch up growth and ameliorated catch-up growth related obesity, glucose intolerance and insulin resistance. MMCC administration significantly inhibited the peroxidation of the membrane lipid and up-regulated the antioxidant enzymes Catalase and MnSOD. In addition, MMCC treatment effectively enhanced mitochondrial functions in skeletal muscle through the up-regulation of the ATP generation, and the promotion of mitochondrial replication and remodeling. To conclude, this study demonstrates that antioxidant MMCC effectively ameliorates catch-up growth related metabolic dysfunctions by increasing mitochondrial functions in skeletal muscle.

Mapping the human skeletal muscle proteome: progress and potential

INTRODUCTION

Human skeletal muscle represents 40% of our body mass and deciphering its proteome composition to further understand mechanisms regulating muscle function under physiological and pathological conditions has proved a challenge. The inter-individual variability, the presence of structurally and functionally different muscle types and the high protein dynamic range require carefully selected methodologies for the assessment of the muscle proteome. Furthermore, physiological studies are understandingly hampered by ethical issues related to biopsies on healthy subjects, making it difficult to recruit matched controls essential for comparative studies. Areas covered: This review critically analyses studies performed on muscle to date and identifies what still remains unknown or poorly investigated in physiological and pathological states, such as training, aging, metabolic disorders and muscular dystrophies. Expert commentary: Efforts should be made on biological fluid analyses targeting low abundant/low molecular weight fragments generated from muscle cell disruption to improve diagnosis and clinical monitoring. From a methodological point of view, particular attention should be paid to improve the characterization of intact proteins and unknown post translational modifications to better understand the molecular mechanisms of muscle disorders.

Photothermal imaging of skeletal muscle mitochondria

The morphology and topology of mitochondria provide useful information about the physiological function of skeletal muscle. Previous studies of skeletal muscle mitochondria are based on observation with transmission, scanning electron microscopy or fluorescence microscopy. In contrast, photothermal (PT) microscopy has advantages over the above commonly used microscopic techniques because of no requirement for complex sample preparation by fixation or fluorescent-dye staining. Here, we employed the PT technique using a simple diode laser to visualize skeletal muscle mitochondria in unstained and stained tissues. The fine mitochondrial network structures in muscle fibers could be imaged with the PT imaging system, even in unstained tissues. PT imaging of tissues stained with toluidine blue revealed the structures of subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria and the swelling behavior of mitochondria in damaged muscle fibers with sufficient image quality. PT image analyses based on fast Fourier transform (FFT) and Grey-level co-occurrence matrix (GLCM) were performed to derive the characteristic size of mitochondria and to discriminate the image patterns of normal and damaged fibers.

Latent mitochondrial DNA deletion mutations drive muscle fiber loss at old age

With age, somatically derived mitochondrial DNA (mtDNA) deletion mutations arise in many tissues and species. In skeletal muscle, deletion mutations clonally accumulate along the length of individual fibers. At high intrafiber abundances, these mutations disrupt individual cell respiration and are linked to the activation of apoptosis, intrafiber atrophy, breakage, and necrosis, contributing to fiber loss. This sequence of molecular and cellular events suggests a putative mechanism for the permanent loss of muscle fibers with age. To test whether mtDNA deletion mutation accumulation is a significant contributor to the fiber loss observed in aging muscle, we pharmacologically induced deletion mutation accumulation. We observed a 1200% increase in mtDNA deletion mutation-containing electron transport chain-deficient muscle fibers, an 18% decrease in muscle fiber number and 22% worsening of muscle mass loss. These data affirm the hypothesized role for mtDNA deletion mutation in the etiology of muscle fiber loss at old age.

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.

Effects of aging on mitochondrial function in skeletal muscle of American American Quarter Horses

Skeletal muscle function, aerobic capacity, and mitochondrial (Mt) function have been found to decline with age in humans and rodents. However, not much is known about age-related changes in Mt function in equine skeletal muscle. Here, we compared fiber-type composition and Mt function in gluteus medius and triceps brachii muscle between young (age 1.8 ± 0.1 yr, n = 24) and aged (age 17-25 yr, n = 10) American Quarter Horses. The percentage of myosin heavy chain (MHC) IIX was lower in aged compared with young muscles (gluteus, P = 0.092; triceps, P = 0.012), while the percentages of MHC I (gluteus; P < 0.001) and MHC IIA (triceps; P = 0.023) were increased. Mass-specific Mt density, indicated by citrate synthase activity, was unaffected by age in gluteus, but decreased in aged triceps (P = 0.023). Cytochrome-c oxidase (COX) activity per milligram tissue and per Mt unit decreased with age in gluteus (P < 0.001 for both) and triceps (P < 0.001 and P = 0.003, respectively). Activity of 3-hydroxyacyl-CoA dehydrogenase per milligram tissue was unaffected by age, but increased per Mt unit in aged gluteus and triceps (P = 0.023 and P < 0.001, respectively). Mt respiration of permeabilized muscle fibers per milligram tissue was unaffected by age in both muscles. Main effects of age appeared when respiration was normalized to Mt content, with increases in LEAK, oxidative phosphorylation capacity, and electron transport system capacity (P = 0.038, P = 0.045, and P = 0.007, respectively), independent of muscle. In conclusion, equine skeletal muscle aging was accompanied by a shift in fiber-type composition, decrease in Mt density and COX activity, but preserved Mt respiratory function.

Exercise Promotes Healthy Aging of Skeletal Muscle

Primary aging is the progressive and inevitable process of bodily deterioration during adulthood. In skeletal muscle, primary aging causes defective mitochondrial energetics and reduced muscle mass. Secondary aging refers to additional deleterious structural and functional age-related changes caused by diseases and lifestyle factors. Secondary aging can exacerbate deficits in mitochondrial function and muscle mass, concomitant with the development of skeletal muscle insulin resistance. Exercise opposes deleterious effects of secondary aging by preventing the decline in mitochondrial respiration, mitigating aging-related loss of muscle mass and enhancing insulin sensitivity. This review focuses on mechanisms by which exercise promotes "healthy aging" by inducing modifications in skeletal muscle.

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.

Modelling in vivo creatine/phosphocreatine in vitro reveals divergent adaptations in human muscle mitochondrial respiratory control by ADP after acute and chronic exercise

KEY POINTS

Mitochondrial respiratory sensitivity to ADP is thought to influence muscle fitness and is partly regulated by cytosolic-mitochondrial diffusion of ADP or phosphate shuttling via creatine/phosphocreatine (Cr/PCr) through mitochondrial creatine kinase (mtCK). Previous measurements of respiration in vitro with Cr (saturate mtCK) or without (ADP/ATP diffusion) show mixed responses of ADP sensitivity following acute exercise vs. less sensitivity after chronic exercise. In human muscle, modelling in vivo 'exercising' [Cr:PCr] during in vitro assessments revealed novel responses to exercise that differ from detections with or without Cr (±Cr). Acute exercise increased ADP sensitivity when measured without Cr but had no effect ±Cr or with +Cr:PCr, whereas chronic exercise increased sensitivity ±Cr but lowered sensitivity with +Cr:PCr despite increased markers of mitochondrial oxidative capacity. Controlling in vivo conditions during in vitro respiratory assessments reveals responses to exercise that differ from typical ±Cr comparisons and challenges our understanding of how exercise improves metabolic control in human muscle.

ABSTRACT

Mitochondrial respiratory control by ADP (Kmapp ) is viewed as a critical regulator of muscle energy homeostasis. However, acute exercise increases, decreases or has no effect on Kmapp in human muscle, whereas chronic exercise surprisingly decreases sensitivity despite greater mitochondrial content. We hypothesized that modelling in vivo mitochondrial creatine kinase (mtCK)-dependent phosphate-shuttling conditions in vitro would reveal increased sensitivity (lower Kmapp ) after acute and chronic exercise. The Kmapp was determined in vitro with 20 mm Cr (+Cr), 0 mm Cr (-Cr) or 'in vivo exercising' 20 mm Cr/2.4 mm PCr (Cr:PCr) on vastus lateralis biopsies sampled from 11 men before, immediately after and 3 h after exercise on the first, fifth and ninth sessions over 3 weeks. Dynamic responses to acute exercise occurred throughout training, whereby the first session did not change Kmapp with in vivo Cr:PCr despite increases in -Cr. The fifth session decreased sensitivity with Cr:PCr or +Cr despite no change in -Cr. Chronic exercise increased sensitivity ±Cr in association with increased electron transport chain content (+33-62% complexes I-V), supporting classic proposals that link increased sensitivity to oxidative capacity. However, in vivo Cr:PCr reveals a perplexing decreased sensitivity, contrasting the increases seen ±Cr. Functional responses occurred without changes in fibre type or proteins regulating mitochondrial-cytosolic energy exchange (mtCK, VDAC and ANT). Despite the dynamic responses seen with ±Cr, modelling in vivo phosphate-shuttling conditions in vitro reveals that ADP sensitivity is unchanged after high-intensity exercise and is decreased after training. These findings challenge our understanding of how exercise regulates skeletal muscle energy homeostasis.

Endurance exercise ameliorates low birthweight developed catch-up growth related metabolic dysfunctions in a mouse model

Low birthweight is known to predict high risk of metabolic diseases in adulthood, while regular endurance exercises are believed sufficient to improve metabolic dysfunction. In this study, we established a mouse model to determine whether long-term exercise training could ameliorate catch-up growth, and we explored the possible underlying mechanisms. By restricting maternal food intake during the last week of gestation, we successfully produced low birthweight pups. Further, normal birthweight mice and low birthweight mice were randomly distributed into one of three groups receiving either a normal fat diet, high fat diet, or high fat diet with exercise training. The growth/metabolism, mitochondrial content and functions were assessed at 6 months of age. Through group comparisons and correlation analyses, the 4th week was demonstrated to be the period of crucial growth and chosen to be the precise point of intervention, as the growth rate at this point is significantly correlated with body weight, intraperitoneal glucose tolerance test (IPGTT), Lee's index and fat mass in adulthood. In addition, regular endurance exercises when started from 4 weeks remarkably ameliorated low birthweight outcomes and induced catch-up growth and glucose intolerance in the 25th week. Furthermore, real-time PCR and western blot results indicated that the effect of long-term exercise on mitochondrial functions alleviated catch-up related metabolic dysfunction. To conclude, long-term exercise training from the 4th week is sufficient to ameliorate catch-up growth and related metabolic disturbances in adulthood by promoting mitochondrial functions in skeletal muscle.

Structure and function of ER membrane contact sites with other organelles

The endoplasmic reticulum (ER) is the largest organelle in the cell, and its functions have been studied for decades. The past several years have provided novel insights into the existence of distinct domains between the ER and other organelles, known as membrane contact sites (MCSs). At these contact sites, organelle membranes are closely apposed and tethered, but do not fuse. Here, various protein complexes can work in concert to perform specialized functions such as binding, sensing and transferring molecules, as well as engaging in organelle biogenesis and dynamics. This Review describes the structure and functions of MCSs, primarily focusing on contacts of the ER with mitochondria and endosomes.

Mitochondrial Dynamics is a Distinguishing Feature of Skeletal Muscle Fiber Types and Regulates Organellar Compartmentalization

Skeletal muscle fibers differentiate into specific fiber types with distinct metabolic properties determined by their reliance on oxidative phosphorylation (OXPHOS). Using in vivo approaches, we find that OXPHOS-dependent fibers, compared to glycolytic fibers, contain elongated mitochondrial networks with higher fusion rates that are dependent on the mitofusins Mfn1 and Mfn2. Switching of a glycolytic fiber to an oxidative IIA type is associated with elongation of mitochondria, suggesting that mitochondrial fusion is linked to metabolic state. Furthermore, we reveal that mitochondrial proteins are compartmentalized to discrete domains centered around their nuclei of origin. The domain dimensions are dependent on fiber type and are regulated by the mitochondrial dynamics proteins Mfn1, Mfn2, and Mff. Our results indicate that mitochondrial dynamics is tailored to fiber type physiology and provides a rationale for the segmental defects characteristic of aged and diseased muscle fibers.

In vivo microscopy reveals extensive embedding of capillaries within the sarcolemma of skeletal muscle fibers

OBJECTIVE

To provide insight into mitochondrial function in vivo, we evaluated the 3D spatial relationship between capillaries, mitochondria, and muscle fibers in live mice.

METHODS

3D volumes of in vivo murine TA muscles were imaged by MPM. Muscle fiber type, mitochondrial distribution, number of capillaries, and capillary-to-fiber contact were assessed. The role of Mb-facilitated diffusion was examined in Mb KO mice. Distribution of GLUT4 was also evaluated in the context of the capillary and mitochondrial network.

RESULTS

MPM revealed that 43.6 ± 3.3% of oxidative fiber capillaries had ≥50% of their circumference embedded in a groove in the sarcolemma, in vivo. Embedded capillaries were tightly associated with dense mitochondrial populations lateral to capillary grooves and nearly absent below the groove. Mitochondrial distribution, number of embedded capillaries, and capillary-to-fiber contact were proportional to fiber oxidative capacity and unaffected by Mb KO. GLUT4 did not preferentially localize to embedded capillaries.

CONCLUSIONS

Embedding capillaries in the sarcolemma may provide a regulatory mechanism to optimize delivery of oxygen to heterogeneous groups of muscle fibers. We hypothesize that mitochondria locate to PV regions due to myofibril voids created by embedded capillaries, not to enhance the delivery of oxygen to the mitochondria.

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.

Label-free microscopy and stress responses reveal the functional organization of Pseudodiaptomus marinus copepod myofibrils

Pseudodiaptomus marinus copepods are small crustaceans living in estuarine areas endowed with exceptional swimming and adaptative performances. Since the external cuticle acts as an impermeable barrier for most dyes and molecular tools for labeling copepod proteins with fluorescent tags are not available, imaging cellular organelles in these organisms requires label free microscopy. Complementary nonlinear microscopy techniques have been used to investigate the structure and the response of their myofibrils to abrupt changes of temperature or/and salinity. In contrast with previous observations in vertebrates and invertebrates, the flavin autofluorescence which is a signature of mitochondria activity and the Coherent Anti-Stokes Raman Scattering (CARS) pattern assigned to T-tubules overlapped along myofibrils with the second harmonic generation (SHG) striated pattern generated by myosin tails in sarcomeric A bands. Temperature jumps from 18 to 4 °C or salinity jumps from 30 to 15 psu mostly affected flavin autofluorescence. Severe salinity jumps from 30 to 0 psu dismantled myofibril organization with major changes both in the SHG and CARS patterns. After a double stress (from 18 °C/30 psu to 4° C/0 psu) condensed and distended regions appeared within single myofibrils, with flavin autofluorescence bands located between sarcomeric A bands. These results shed light on the interactions between the different functional compartments which provide fast acting excitation-contraction coupling and adequate power supply in copepods muscles.

The pleiotropic effect of physical exercise on mitochondrial dynamics in aging skeletal muscle

Decline in human muscle mass and strength (sarcopenia) is one of the principal hallmarks of the aging process. Regular physical exercise and training programs are certain powerful stimuli to attenuate the physiological skeletal muscle alterations occurring during aging and contribute to promote health and well-being. Although the series of events that led to these muscle adaptations are poorly understood, the mechanisms that regulate these processes involve the “quality” of skeletal muscle mitochondria. Aerobic/endurance exercise helps to maintain and improve cardiovascular fitness and respiratory function, whereas strength/resistance-exercise programs increase muscle strength, power development, and function. Due to the different effect of both exercises in improving mitochondrial content and quality, in terms of biogenesis, dynamics, turnover, and genotype, combined physical activity programs should be individually prescribed to maximize the antiaging effects of exercise.

Muscle-specific Drp1 overexpression impairs skeletal muscle growth via translational attenuation

Mitochondrial fission and fusion are essential processes in the maintenance of the skeletal muscle function. The contribution of these processes to muscle development has not been properly investigated in vivo because of the early lethality of the models generated so far. To define the role of mitochondrial fission in muscle development and repair, we have generated a transgenic mouse line that overexpresses the fission-inducing protein Drp1 specifically in skeletal muscle. These mice displayed a drastic impairment in postnatal muscle growth, with reorganisation of the mitochondrial network and reduction of mtDNA quantity, without the deficiency of mitochondrial bioenergetics. Importantly we found that Drp1 overexpression activates the stress-induced PKR/eIF2α/Fgf21 pathway thus leading to an attenuated protein synthesis and downregulation of the growth hormone pathway. These results reveal for the first time how mitochondrial network dynamics influence muscle growth and shed light on aspects of muscle physiology relevant in human muscle pathologies.

Preparation of "functional" mitochondria: a challenging business

As the powerhouse of the cell, mitochondria play a crucial role in many aspects of life, whereby mitochondrial dysfunctions are associated with pathogenesis of many diseases, like neurodegenerative diseases, obesity, cancer, and metabolic as well as cardiovascular disorders. Mitochondria analysis frequently starts with isolation and enrichment procedures potentially affecting mitochondrial morphology having impact on their function. Due to the complex mitochondrial morphology, the major task is to preserve their structural integrity. Here we critically review a commonly used isolation procedure for mitochondria utilizing differential (gradient) centrifugation and depict major challenges to achieve "functional" mitochondria as basis for comprehensive physiological studies.

Usage of a localised microflow device to show that mitochondrial networks are not extensive in skeletal muscle fibres

In cells, such as neurones and immune cells, mitochondria can form dynamic and extensive networks that change over the minute timescale. In contrast, mitochondria in adult mammalian skeletal muscle fibres show little motility over several hours. Here, we use a novel three channelled microflow device, the multifunctional pipette, to test whether mitochondria in mouse skeletal muscle connect to each other. The central channel in the pipette delivers compounds to a restricted region of the sarcolemma, typically 30 µm in diameter. Two channels on either side of the central channel use suction to create a hydrodynamically confined flow zone and remove compounds completely from the bulk solution to internal waste compartments. Compounds were delivered locally to the end or side of single adult mouse skeletal muscle fibres to test whether changes in mitochondrial membrane potential were transmitted to more distant located mitochondria. Mitochondrial membrane potential was monitored with tetramethylrhodamine ethyl ester (TMRE). Cytosolic free [Ca²⁺] was monitored with fluo-3. A pulse of carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, 100 µM) applied to a small area of the muscle fibre (30 µm in diameter) produced a rapid decrease in the mitochondrial TMRE signal (indicative of depolarization) to 38% of its initial value. After washout of FCCP, the TMRE signal partially recovered. At distances greater than 50 µm away from the site of FCCP application, the mitochondrial TMRE signal was unchanged. Similar results were observed when two sites along the fibre were pulsed sequentially with FCCP. After a pulse of FCCP, cytosolic [Ca²⁺] was unchanged and fibres contracted in response to electrical stimulation. In conclusion, our results indicate that extensive networks of interconnected mitochondria do not exist in skeletal muscle. Furthermore, the limited and reversible effects of targeted FCCP application with the multifunctional pipette highlight its advantages over bulk application of compounds to isolated cells.

The role of tubulin in the mitochondrial metabolism and arrangement in muscle cells

Tubulin, a well-known component of the microtubule in the cytoskeleton, has an important role in the transport and positioning of mitochondria in a cell type dependent manner. This review describes different functional interactions of tubulin with cellular protein complexes and its functional interaction with the mitochondrial outer membrane. Tubulin is present in oxidative as well as glycolytic type muscle cells, but the kinetics of the in vivo regulation of mitochondrial respiration in these muscle types is drastically different. The interaction between VDAC and tubulin is probably influenced by such factors as isoformic patterns of VDAC and tubulin, post-translational modifications of tubulin and phosphorylation of VDAC. Important factor of the selective permeability of VDAC is the mitochondrial creatine kinase pathway which is present in oxidative cells, but is inactive or missing in glycolytic muscle and cancer cells. As the tubulin-VDAC interaction reduces the permeability of the channel by adenine nucleotides, energy transfer can then take place effectively only through the mitochondrial creatine kinase/phosphocreatine pathway. Therefore, closure of VDAC by tubulin may be one of the reasons of apoptosis in cells without the creatine kinase pathway. An important question in tubulin regulated interactions is whether other proteins are interacting with tubulin. The functional interaction may be direct, through other proteins like plectins, or influenced by simultaneous interaction of other complexes with VDAC.

Exercise training decreases activation of the mitochondrial fission protein dynamin-related protein-1 in insulin-resistant human skeletal muscle

Defects in mitochondrial dynamics, the processes of fission, fusion, and mitochondrial autophagy, may contribute to metabolic disease including type 2 diabetes. Dynamin-related protein-1 (Drp1) is a GTPase protein that plays a central role in mitochondrial fission. We hypothesized that aerobic exercise training would decrease Drp1 Ser(616) phosphorylation and increase fat oxidation and insulin sensitivity in obese (body mass index: 34.6 ± 0.8 kg/m(2)) insulin-resistant adults. Seventeen subjects performed supervised exercise for 60 min/day, 5 days/wk at 80-85% of maximal heart rate for 12 wk. Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp, and fat oxidation was determined by indirect calorimetry. Skeletal muscle biopsies were obtained from the vastus lateralis muscle before and after the 12-wk program. The exercise intervention increased insulin sensitivity 2.1 ± 0.2-fold (P < 0.01) and fat oxidation 1.3 ± 0.3-fold (P < 0.01). Phosphorylation of Drp1 at Ser(616) was decreased (pre vs. post: 0.81 ± 0.15 vs. 0.58 ± 0.14 arbitrary units; P < 0.05) following the intervention. Furthermore, reductions in Drp1 Ser(616) phosphorylation were negatively correlated with increases in fat oxidation (r = -0.58; P < 0.05) and insulin sensitivity (rho = -0.52; P < 0.05). We also examined expression of genes related to mitochondrial dynamics. Dynamin1-like protein (DNM1L; P < 0.01), the gene that codes for Drp1, and Optic atrophy 1 (OPA1; P = 0.05) were significantly upregulated following the intervention, while there was a trend towards an increase in expression of both mitofusin protein MFN1 (P = 0.08) and MFN2 (P = 0.07). These are the first data to suggest that lifestyle-mediated improvements in substrate metabolism and insulin sensitivity in obese insulin-resistant adults may be regulated through decreased activation of the mitochondrial fission protein Drp1.

Mitochondrial fusion is frequent in skeletal muscle and supports excitation-contraction coupling

Mitochondrial fusion is frequent in skeletal muscle, and its disruption jeopardizes excitation–contraction coupling and may contribute to the pathology of myopathies.

Genetic targeting experiments indicate a fundamental role for mitochondrial fusion proteins in mammalian physiology. However, owing to the multiple functions of fusion proteins, their related phenotypes are not necessarily caused by altered mitochondrial fusion. Perhaps the biggest mystery is presented by skeletal muscle, where mostly globular-shaped mitochondria are densely packed into the narrow intermyofilamental space, limiting the interorganellar interactions. We show here that mitochondria form local networks and regularly undergo fusion events to share matrix content in skeletal muscle fibers. However, fusion events are less frequent and more stable in the fibers than in nondifferentiated myoblasts. Complementation among muscle mitochondria was suppressed by both in vivo genetic perturbations and chronic alcohol consumption that cause myopathy. An Mfn1-dependent pathway is revealed whereby fusion inhibition weakens the metabolic reserve of mitochondria to cause dysregulation of calcium oscillations during prolonged stimulation. Thus, fusion dynamically connects skeletal muscle mitochondria and its prolonged loss jeopardizes bioenergetics and excitation–contraction coupling, providing a potential pathomechanism contributing to myopathies.

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.

Defective mitochondrial dynamics is an early event in skeletal muscle of an amyotrophic lateral sclerosis mouse model

Mitochondria are dynamic organelles that constantly undergo fusion and fission to maintain their normal functionality. Impairment of mitochondrial dynamics is implicated in various neurodegenerative disorders. Amyotrophic lateral sclerosis (ALS) is an adult-onset neuromuscular degenerative disorder characterized by motor neuron death and muscle atrophy. ALS onset and progression clearly involve motor neuron degeneration but accumulating evidence suggests primary muscle pathology may also be involved. Here, we examined mitochondrial dynamics in live skeletal muscle of an ALS mouse model (G93A) harboring a superoxide dismutase mutation (SOD1^G93A). Using confocal microscopy combined with overexpression of mitochondria-targeted photoactivatable fluorescent proteins, we discovered abnormal mitochondrial dynamics in skeletal muscle of young G93A mice before disease onset. We further demonstrated that similar abnormalities in mitochondrial dynamics were induced by overexpression of mutant SOD1^G93A in skeletal muscle of normal mice, indicating the SOD1 mutation drives ALS-like muscle pathology in the absence of motor neuron degeneration. Mutant SOD1^G93A forms aggregates inside muscle mitochondria and leads to fragmentation of the mitochondrial network as well as mitochondrial depolarization. Partial depolarization of mitochondrial membrane potential in normal muscle by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) caused abnormalities in mitochondrial dynamics similar to that in the SOD1^G93A model muscle. A specific mitochondrial fission inhibitor (Mdivi-1) reversed the SOD1^G93A action on mitochondrial dynamics, indicating SOD1^G93A likely promotes mitochondrial fission process. Our results suggest that accumulation of mutant SOD1^G93A inside mitochondria, depolarization of mitochondrial membrane potential and abnormal mitochondrial dynamics are causally linked and cause intrinsic muscle pathology, which occurs early in the course of ALS and may actively promote ALS progression.

Skeletal muscle mitochondria: a major player in exercise, health and disease

BACKGROUND

Maintaining skeletal muscle mitochondrial content and function is important for sustained health throughout the lifespan. Exercise stimulates important key stress signals that control skeletal mitochondrial biogenesis and function. Perturbations in mitochondrial content and function can directly or indirectly impact skeletal muscle function and consequently whole-body health and wellbeing.

SCOPE OF REVIEW

This review will describe the exercise-stimulated stress signals and molecular mechanisms positively regulating mitochondrial biogenesis and function. It will then discuss the major myopathies, neuromuscular diseases and conditions such as diabetes and ageing that have dysregulated mitochondrial function. Finally, the impact of exercise and potential pharmacological approaches to improve mitochondrial function in diseased populations will be discussed.

MAJOR CONCLUSIONS

Exercise activates key stress signals that positively impact major transcriptional pathways that transcribe genes involved in skeletal muscle mitochondrial biogenesis, fusion and metabolism. The positive impact of exercise is not limited to younger healthy adults but also benefits skeletal muscle from diseased populations and the elderly. Impaired mitochondrial function can directly influence skeletal muscle atrophy and contribute to the risk or severity of disease conditions. Pharmacological manipulation of exercise-induced pathways that increase skeletal muscle mitochondrial biogenesis and function in critically ill patients, where exercise may not be possible, may assist in the treatment of chronic disease.

GENERAL SIGNIFICANCE

This review highlights our understanding of how exercise positively impacts skeletal muscle mitochondrial biogenesis and function. Exercise not only improves skeletal muscle mitochondrial health but also enables us to identify molecular mechanisms that may be attractive targets for therapeutic manipulation. This article is part of a Special Issue entitled Frontiers of mitochondrial research.

Role of mitochondria-cytoskeleton interactions in respiration regulation and mitochondrial organization in striated muscles

The aim of this work was to study the regulation of respiration and energy fluxes in permeabilized oxidative and glycolytic skeletal muscle fibers, focusing also on the role of cytoskeletal protein tubulin βII isotype in mitochondrial metabolism and organization. By analyzing accessibility of mitochondrial ADP, using respirometry and pyruvate kinase-phosphoenolpyruvate trapping system for ADP, we show that the apparent affinity of respiration for ADP can be directly linked to the permeability of the mitochondrial outer membrane (MOM). Previous studies have shown that MOM permeability in cardiomyocytes can be regulated by VDAC interaction with cytoskeletal protein, βII tubulin. We found that in oxidative soleus skeletal muscle the high apparent Km for ADP is associated with low MOM permeability and high expression of non-polymerized βII tubulin. Very low expression of non-polymerized form of βII tubulin in glycolytic muscles is associated with high MOM permeability for adenine nucleotides (low apparent Km for ADP).

Ontogeny of muscle bioenergetics in Adélie penguin chicks (Pygoscelis adeliae)

The ontogeny of pectoralis muscle bioenergetics was studied in growing Adélie penguin chicks during the first month after hatching and compared with adults using permeabilized fibers and isolated mitochondria. With pyruvate-malate-succinate or palmitoyl-carnitine as substrates, permeabilized fiber respiration markedly increased during chick growth (3-fold) and further rose in adults (1.4-fold). Several markers of muscle fiber oxidative activity (cytochrome oxidase, citrate synthase, hydroxyl-acyl-CoA dehydrogenase) increased 6- to 19-fold with age together with large rises in intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondrial content (3- to 5-fold) and oxidative activities (1.5- to 2.4-fold). The proportion of IMF relative to SS mitochondria increased with chick age but markedly dropped in adults. Differences in oxidative activity between mitochondrial fractions were reduced in adults compared with hatched chicks. Extrapolation of mitochondrial to muscle respirations revealed similar figures with isolated mitochondria and permeabilized fibers with carbohydrate-derived but not with lipid-derived substrates, suggesting diffusion limitations of lipid substrates with permeabilized fibers. Two immunoreactive fusion proteins, mitofusin 2 (Mfn2) and optic atrophy 1 (OPA1), were detected by Western blots on mitochondrial extracts and their relative abundance increased with age. Muscle fiber respiration was positively related with Mfn2 and OPA1 relative abundance. Present data showed by two complementary techniques large ontogenic increases in muscle oxidative activity that may enable birds to face thermal emancipation and growth in childhood and marine life in adulthood. The concomitant rise in mitochondrial fusion protein abundance suggests a role of mitochondrial networks in the skeletal muscle processes of bioenergetics that enable penguins to overcome harsh environmental constraints.

Can we optimise the exercise training prescription to maximise improvements in mitochondria function and content?

BACKGROUND

While there is agreement that exercise is a powerful stimulus to increase both mitochondrial function and content, we do not know the optimal training stimulus to maximise improvements in mitochondrial biogenesis.

SCOPE OF REVIEW

This review will focus predominantly on the effects of exercise on mitochondrial function and content, as there is a greater volume of published research on these adaptations and stronger conclusions can be made.

MAJOR CONCLUSIONS

The results of cross-sectional studies, as well as training studies involving rats and humans, suggest that training intensity may be an important determinant of improvements in mitochondrial function (as determined by mitochondrial respiration), but not mitochondrial content (as assessed by citrate synthase activity). In contrast, it appears that training volume, rather than training intensity, may be an important determinant of exercise-induced improvements in mitochondrial content. Exercise-induced mitochondrial adaptations are quickly reversed following a reduction or cessation of physical activity, highlighting that skeletal muscle is a remarkably plastic tissue. Due to the small number of studies, more research is required to verify the trends highlighted in this review, and further studies are required to investigate the effects of different types of training on the mitochondrial sub-populations and also mitochondrial adaptations in different fibre types. Further research is also required to better understand how genetic variants influence the large individual variability for exercise-induced changes in mitochondrial biogenesis.

GENERAL SIGNIFICANCE

The importance of mitochondria for both athletic performance and health underlines the importance of better understanding the factors that regulate exercise-induced changes in mitochondrial biogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Redefining the major contributors to superoxide production in contracting skeletal muscle. The role of NAD(P)H oxidases

The production of reactive oxygen and nitrogen species (RONS) by skeletal muscle is important as it (i) underlies oxidative damage in many degenerative muscle pathologies and (ii) plays multiple regulatory roles by fulfilling important cellular functions. Superoxide and nitric oxide (NO) are the primary radical species produced by skeletal muscle and studies in the early 1980s demonstrated that their generation is augmented during contractile activity. Over the past 30 years considerable research has been undertaken to identify the major sites that contribute to the increased rate of RONS generation in response to contractions. It is widely accepted that NO is regulated by the nitric oxide synthases, however the sites that modulate changes in superoxide during exercise remain unclear. Despite the initial indications that the mitochondrial electron transport chain was the predominant source of superoxide during activity, with the development of analytical methods a number of alternative potential sites have been identified including the NAD(P)H oxidases, xanthine oxidase, cyclooxygenases, and lipoxygenases linked to the activity of the phospholipase A2 enzymes. In the present review we outline the subcellular sites that modulate intracellular changes in superoxide in skeletal muscle and based on the available experimental evidence in the literature we conclude that the NAD(P)H oxidases are likely to be the major superoxide generating sources in contracting skeletal muscle.

Functional and morphological impact of ER stress on mitochondria

Over the past years, knowledge and evidence about the existence of crosstalks between cellular organelles and their potential effects on survival or cell death have been constantly growing. More recently, evidence accumulated showing an intimate relationship between endoplasmic reticulum (ER) and mitochondria. These close contacts not only establish extensive physical links allowing exchange of lipids and calcium but they can also coordinate pathways involved in cell life and death. It is now obvious that ER dysfunction/stress and unfolded protein response (UPR) as well as mitochondria play major roles in apoptosis. However, while the effects of major ER stress on cell death have been largely studied and reviewed, it becomes more and more evident that cells might regularly deal with sublethal ER stress, a condition that does not necessarily lead to cell death but might affect the function/activity of other organelles such as mitochondria. In this review, we will particularly focus on these new, interesting and intriguing metabolic and morphological events that occur during the early adaptative phase of the ER stress, before the onset of cell death, and that remain largely unknown. Relevance and implication of these mitochondrial changes in response to ER stress conditions for human diseases such as type II diabetes and Alzheimer's disease will also be considered.

Skeletal muscle mitochondrial and lipid droplet content assessed with standardized grid sizes for stereology

Skeletal muscle mitochondrial (Mito) and lipid droplet (Lipid) content are often measured in human translational studies. Stereological point counting allows computing Mito and Lipid volume density (Vd) from micrographs taken with transmission electron microscopes. Former studies are not specific as to the size of individual squares that make up the grids, making reproducibility difficult, particularly when different magnifications are used. Our objective was to determine which size grid would be best at predicting fractional volume efficiently without sacrificing reliability and to test a novel method to reduce sampling bias. Methods: ten subjects underwent vastus lateralis biopsies. Samples were fixed, embedded, and cut longitudinally in ultrathin sections of 60 nm. Twenty micrographs from the intramyofibrillar region were taken per subject at ×33,000 magnification. Different grid sizes were superimposed on each micrograph: 1,000 × 1,000 nm, 500 × 500 nm, and 250 × 250 nm. Results: mean Mito and Lipid Vd were not statistically different across grids. Variability was greater when going from 1,000 × 1,000 to 500 × 500 nm grid than from 500 × 500 to 250 × 250 nm grid. Discussion: this study is the first to attempt to standardize grid size while keeping with the conventional stereology principles. This is all in hopes of producing replicable assessments that can be obtained universally across different studies looking at human skeletal muscle mitochondrial and lipid droplet content.

Redox regulation of insulin sensitivity due to enhanced fatty acid utilization in the mitochondria

Obesity enhances the risk for the development of type 2 diabetes and cardiovascular disease. Loss in insulin sensitivity and diminished ability of muscle to take up and use glucose are characteristics of type 2 diabetes. Paradoxically, regulatory mechanisms that promote utilization of fatty acids appear to initiate diet-induced insulin insensitivity. In this review, we discuss recent findings implicating increased mitochondrial production of the prooxidant H2O2 due to enhanced utilization of fatty acids, as a signal to diminish reliance on glucose and its metabolites for energy. In the short term, the ability to preferentially use fatty acids may be beneficial, promoting a metabolic shift that ensures use of available fat by skeletal muscle and heart while preventing intracellular glucose accumulation and toxicity. However, with prolonged consumption of high dietary fat and ensuing obesity, the near exclusive dependence on fatty acid oxidation for production of energy by the mitochondria drives insulin resistance, diabetes, and cardiovascular disease.

Acute exercise remodels mitochondrial membrane interactions in mouse skeletal muscle

A unique property of mitochondria in mammalian cells is their ability to physically interact and undergo dynamic events of fusion/fission that remodel their morphology and possibly their function. In cultured cells, metabolic perturbations similar to those incurred during exercise influence mitochondrial fusion and fission processes, but it is unknown whether exercise acutely alters mitochondrial morphology and/or membrane interactions in vivo. To study this question, we subjected mice to a 3-h voluntarily exercise intervention following their normal physical activity patterns, and quantified mitochondrial morphology and membrane interactions in the soleus using a quantitative electron microscopy approach. A single exercise bout effectively decreased blood glucose (P < 0.05) and intramyocellular lipid content (P < 0.01), indicating increased muscle metabolic demand. The number of mitochondria spanning Z-lines and proportion of electron-dense contact sites (EDCS) between adjacent mitochondrial membranes were increased immediately after exercise among both subsarcolemmal (+116%, P < 0.05) and intermyofibrillar mitochondria (+191%, P < 0.001), indicating increased physical interactions. Mitochondrial morphology, and abundance of the mitochondrial pro-fusion proteins Mfn2 and OPA1 were unchanged. Collectively, these results support the notion that mitochondrial membrane dynamics are actively remodelled in skeletal muscle, which may be regulated by contractile activity and the metabolic state. Future studies are required to understand the implications of mitochondrial dynamics in skeletal muscle physiology during exercise and inactivity.

Methods for assessing mitochondrial function in diabetes

A growing body of research is investigating the potential contribution of mitochondrial function to the etiology of type 2 diabetes. Numerous in vitro, in situ, and in vivo methodologies are available to examine various aspects of mitochondrial function, each requiring an understanding of their principles, advantages, and limitations. This review provides investigators with a critical overview of the strengths, limitations and critical experimental parameters to consider when selecting and conducting studies on mitochondrial function. In vitro (isolated mitochondria) and in situ (permeabilized cells/tissue) approaches provide direct access to the mitochondria, allowing for study of mitochondrial bioenergetics and redox function under defined substrate conditions. Several experimental parameters must be tightly controlled, including assay media, temperature, oxygen concentration, and in the case of permeabilized skeletal muscle, the contractile state of the fibers. Recently developed technology now offers the opportunity to measure oxygen consumption in intact cultured cells. Magnetic resonance spectroscopy provides the most direct way of assessing mitochondrial function in vivo with interpretations based on specific modeling approaches. The continuing rapid evolution of these technologies offers new and exciting opportunities for deciphering the potential role of mitochondrial function in the etiology and treatment of diabetes.

Expression profile and mitochondrial colocalization of Tdp1 in peripheral human tissues

Tyrosyl-DNA phosphodiesterase 1 (Tdp1) is a DNA repair enzyme that processes blocked 3' ends of DNA breaks. Functional loss of Tdp1 causes spinocerebellar ataxia with axonal neuropathy type 1 (SCAN1). Based on the prominent cytoplasmic expression of Tdp1 in the neurons presumably affected in SCAN1, we hypothesized that Tdp1 participates in the repair of mitochondrial DNA. As a step toward testing this hypothesis, we profiled Tdp1 expression in different human tissues by immunohistochemistry and immunofluorescence respectively and determined whether Tdp1 was expressed in the cytoplasm of tissues other than the neurons. We found that Tdp1 was ubiquitously expressed and present in the cytoplasm of many cell types. Within human skeletal muscle and multiple mouse tissues, Tdp1 partially colocalized with the mitochondria. In cultured human dermal fibroblasts, Tdp1 redistributed to the cytoplasm and partially colocalized with mitochondria following oxidative stress. These studies suggest that one role of cytoplasmic Tdp1 is the repair of mitochondrial DNA lesions arising from oxidative stress.

Over-expressing mitofusin-2 in healthy mature mammalian skeletal muscle does not alter mitochondrial bioenergetics

The role of mitofusin-2 (MFN-2) in regulating mitochondrial dynamics has been well-characterized in lower order eukaryotic cell lines through the complete ablation of MFN-2 protein. However, to support the contractile function of mature skeletal muscle, the subcellular architecture and constituent proteins of this tissue differ substantially from simpler cellular organisms. Such differences may also impact the role of MFN-2 in mature mammalian muscle, and it is unclear if minor fluctuations in MFN-2, as observed in response to physiological perturbations, has a functional consequence. Therefore, we have transiently transfected MFN-2 cDNA into rat tibialis anterior muscle to determine the effect of physiolgically relevant increases in MFN-2 protein on mitochondrial bioenergetics. Permeabilized muscle fibres generated from muscle following MFN-2-transfection were used for functional assessments of mitochondrial bioenergetics. In addition, we have further established a novel method for selecting fibre bundles that are positively transfected, and using this approach transient transfection increased MFN-2 protein ∼2.3 fold in selected muscle fibres. However, this did not alter maximal rates of oxygen consumption or the sensitivity for ADP-stimulated respiration. In addition, MFN-2 over-expression did not alter rates of H₂O₂ emission. Altogether, and contrary to evidence from lower order cell lines, our results indicate that over-expressing MFN-2 in healthy muscle does not influence mitochondrial bioenergetics in mature mammalian skeletal muscle.

The C57Bl/6 mouse serves as a suitable model of human skeletal muscle mitochondrial function

It is debatable whether differences in mitochondrial function exist across skeletal muscle types and whether mouse skeletal muscle mitochondrial function can serve as a valid model for human skeletal muscle mitochondrial function. The aims of this study were to compare and contrast three different mouse skeletal muscles and to identify the mouse muscle that most closely resembles human skeletal muscle respiratory capacity and control. Mouse quadriceps (QUAD(M)), soleus (SOL(M)) and gastrocnemius (GAST(M)) skeletal muscles were obtained from 8- to 10-week-old healthy mice (n = 8), representing mixed, oxidative and glycolytic muscle, respectively. Skeletal muscle samples were also collected from young, active, healthy human subjects (n = 8) from the vastis lateralis (QUAD(H)). High-resolution respirometry was used to examine mitochondrial function in all skeletal muscle samples, and mitochondrial content was quantified with citrate synthase activity. Mass-specific respiration was higher across all respiratory states in SOL(M) versus both GAST(M) and QUAD(H) (P < 0.01). When controlling for mitochondrial content, however, SOL(M) respiration was lower than GAST(M) and QUAD(H) (P < 0.05 and P < 0.01, respectively). When comparing respiratory capacity between mouse and human muscle, QUAD(M) exhibited only one different respiratory state when compared with QUAD(H). These results demonstrate that qualitative differences in mitochondrial function exist between different mouse skeletal muscles types when respiratory capacity is normalized to mitochondrial content, and that skeletal muscle respiratory capacity in young, healthy QUAD(M) does correspond well with that of young, healthy QUAD(H).

Mitochondrial morphology, topology, and membrane interactions in skeletal muscle: a quantitative three-dimensional electron microscopy study

Dynamic remodeling of mitochondrial morphology through membrane dynamics are linked to changes in mitochondrial and cellular function. Although mitochondrial membrane fusion/fission events are frequent in cell culture models, whether mitochondrial membranes dynamically interact in postmitotic muscle fibers in vivo remains unclear. Furthermore, a quantitative assessment of mitochondrial morphology in intact muscle is lacking. Here, using electron microscopy (EM), we provide evidence of interacting membranes from adjacent mitochondria in intact mouse skeletal muscle. Electron-dense mitochondrial contact sites consistent with events of outer mitochondrial membrane tethering are also described. These data suggest that mitochondrial membranes interact in vivo among mitochondria, possibly to induce morphology transitions, for kiss-and-run behavior, or other processes involving contact between mitochondrial membranes. Furthermore, a combination of freeze-fracture scanning EM and transmission EM in orthogonal planes was used to characterize and quantify mitochondrial morphology. Two subpopulations of mitochondria were studied: subsarcolemmal (SS) and intermyofibrillar (IMF), which exhibited significant differences in morphological descriptors, including form factor (means ± SD for SS: 1.41 ± 0.45 vs. IMF: 2.89 ± 1.76, P < 0.01) and aspect ratio (1.97 ± 0.83 vs. 3.63 ± 2.13, P < 0.01) and circularity (0.75 ± 0.16 vs. 0.45 ± 0.22, P < 0.01) but not size (0.28 ± 0.31 vs. 0.27 ± 0.20 μm(2)). Frequency distributions for mitochondrial size and morphological parameters were highly skewed, suggesting the presence of mechanisms to influence mitochondrial size and shape. In addition, physical continuities between SS and IMF mitochondria indicated mixing of both subpopulations. These data provide evidence that mitochondrial membranes interact in vivo in mouse skeletal muscle and that factors may be involved in regulating skeletal muscle mitochondrial morphology.

Defects in mitochondrial localization and ATP synthesis in the mdx mouse model of Duchenne muscular dystrophy are not alleviated by PDE5 inhibition

Given the crucial roles for mitochondria in ATP energy supply, Ca(2+) handling and cell death, mitochondrial dysfunction has long been suspected to be an important pathogenic feature in Duchenne muscular dystrophy (DMD). Despite this foresight, mitochondrial function in dystrophin-deficient muscles has remained poorly defined and unknown in vivo. Here, we used the mdx mouse model of DMD and non-invasive spectroscopy to determine the impact of dystrophin-deficiency on skeletal muscle mitochondrial localization and oxidative phosphorylation function in vivo. Mdx mitochondria exhibited significant uncoupling of oxidative phosphorylation (reduced P/O) and a reduction in maximal ATP synthesis capacity that together decreased intramuscular ATP levels. Uncoupling was not driven by increased UCP3 or ANT1 expression. Dystrophin was required to maintain subsarcolemmal mitochondria (SSM) pool density, implicating it in the spatial control of mitochondrial localization. Given that nitric oxide-cGMP pathways regulate mitochondria and that sildenafil-mediated phosphodiesterase 5 inhibition ameliorates dystrophic pathology, we tested whether sildenafil's benefits result from decreased mitochondrial dysfunction in mdx mice. Unexpectedly, sildenafil treatment did not affect mitochondrial content or oxidative phosphorylation defects in mdx mice. Rather, PDE5 inhibition decreased resting levels of ATP, phosphocreatine and myoglobin, suggesting that sildenafil improves dystrophic pathology through other mechanisms. Overall, these data indicate that dystrophin-deficiency disrupts SSM localization, promotes mitochondrial inefficiency and restricts maximal mitochondrial ATP-generating capacity. Together these defects decrease intramuscular ATP and the ability of mdx muscle mitochondria to meet ATP demand. These findings further understanding of how mitochondrial bioenergetic dysfunction contributes to disease pathogenesis in dystrophin-deficient skeletal muscle in vivo.

Intrinsic aerobic capacity correlates with greater inherent mitochondrial oxidative and H2O2 emission capacities without major shifts in myosin heavy chain isoform

Exercise capacity and performance strongly associate with metabolic and biophysical characteristics of skeletal muscle, factors that also relate to overall disease risk. Despite its importance, the exact mechanistic features that connect aerobic metabolism with health status are unknown. To explore this, we applied artificial selection of rats for intrinsic (i.e., untrained) aerobic treadmill running to generate strains of low- and high-capacity runners (LCR and HCR, respectively), subsequently shown to diverge for disease risk. Concurrent breeding of LCR and HCR per generation allows the lines to serve as reciprocal controls for unknown environmental changes. Here we provide the first direct evidence in mitochondria isolated from skeletal muscle that intrinsic mitochondrial capacity is higher in HCR rats. Maximal phosphorylating respiration was ~40% greater in HCR mitochondria, independent of substrate and without altered proton leak or major changes in protein levels or muscle fiber type, consistent with altered control of phosphorylating respiration. Unexpectedly, H(2)O(2) emission was ~20% higher in HCR mitochondria, due to greater reduction of more harmful reactive oxygen species to H(2)O(2); indeed, oxidative modification of mitochondrial proteins was lower. When the higher mitochondrial yield was considered, phosphorylating respiration and H(2)O(2) emission were 70-80% greater in HCR muscle. Greater capacity of HCR muscle for work and H(2)O(2) signaling may result in enhanced and more immediate cellular repair, possibly explaining lowered disease risks.

Mouse lines with photo-activatable mitochondria to study mitochondrial dynamics

Many pathological states involve dysregulation of mitochondrial fusion, fission, or transport. These dynamic events are usually studied in cells lines because of the challenges in tracking mitochondria in tissues. To investigate mitochondrial dynamics in tissues and disease models, we generated two mouse lines with photo-activatable mitochondria (PhAM). In the PhAM(floxed) line, a mitochondrially localized version of the photo-convertible fluorescent protein Dendra2 (mito-Dendra2) is targeted to the ubiquitously expressed Rosa26 locus, along with an upstream loxP-flanked termination signal. Expression of Cre in PhAM( floxed) cells results in bright mito-Dendra2 fluorescence without adverse effects on mitochondrial morphology. When crossed with Cre drivers, the PhAM(floxed) line expresses mito-Dendra2 in specific cell types, allowing mitochondria to be tracked even in tissues that have high cell density. In a second line (PhAM(excised) ), the expression of mito-Dendra2 is ubiquitous, allowing mitochondria to be analyzed in a wide range of live and fixed tissues. By using photo-conversion techniques, we directly measured mitochondrial fusion events in cultured cells as well as tissues such as skeletal muscle. These mouse lines facilitate analysis of mitochondrial dynamics in a wide spectrum of primary cells and tissues, and can be used to examine mitochondria in developmental transitions and disease states.