Giorgos K. Sakellariou

Studies of Mitochondrial and Nonmitochondrial Sources Implicate Nicotinamide Adenine Dinucleotide Phosphate Oxidase(s) in the Increased Skeletal Muscle Superoxide Generation That Occurs During Contractile Activity

AIMS The sources of cytosolic superoxide in skeletal muscle have not been defined. This study examined the subcellular sites that contribute to cytosolic superoxide in mature single muscle fibers at rest and during contractile activity. RESULTS Isolated fibers from mouse flexor digitorum brevis loaded with superoxide and nitric-oxide-sensitive fluorescent probes, specific pathway inhibitors and immunolocalization techniques were used to identify subcellular sites contributing to cytosolic superoxide. Treatment with the electron transport chain complex III inhibitor, antimycin A, but not the complex I inhibitor, rotenone, caused increased cytosolic superoxide through release from the mitochondrial intermembrane space via voltage-dependent anion or Bax channels, but inhibition of these channels did not affect contraction-induced increases in cytosolic superoxide. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors decreased cytosolic superoxide at rest and following contractions. Protein and mRNA expression of NADPH oxidase subunits was demonstrated in single fibers. NOX2, NOX4, and p22(phox) subunits localized to the sarcolemma and transverse tubules; NOX4 was additionally expressed in mitochondria. Regulatory p40(phox) and p67(phox) proteins were found in the cytoplasm of resting fibers, but following contractions, p40(phox) appeared to translocate to the sarcolemma. INNOVATION Superoxide and other reactive oxygen species generated by skeletal muscle are important regulators of muscle force production and adaptations to contractions. This study has defined the relative contribution of mitochondrial and cytosolic sources of superoxide within the cytosol of single muscle fibers at rest and during contractions. CONCLUSION Muscle mitochondria do not modulate cytosolic superoxide in skeletal muscle but NADPH oxidase is a major contributor both at rest and during contractions.

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

Abstract 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.

Role of superoxide-nitric oxide interactions in the accelerated age-related loss of muscle mass in mice lacking Cu, Zn superoxide dismutase

Mice lacking Cu,Zn superoxide dismutase (SOD1) show accelerated, age‐related loss of muscle mass. Lack of SOD1 may lead to increased superoxide, reduced nitric oxide (NO), and increased peroxynitrite, each of which could initiate muscle fiber loss. Single muscle fibers from flexor digitorum brevis of wild‐type (WT) and Sod1−/− mice were loaded with NO‐sensitive (4‐amino‐5‐methylamino‐2′,7′‐difluorofluorescein diacetate, DAF‐FM) and superoxide‐sensitive (dihydroethidium, DHE) probes. Gastrocnemius muscles were analyzed for SOD enzymes, nitric oxide synthases (NOS), and 3‐nitrotyrosine (3‐NT) content. A lack of SOD1 did not increase superoxide availability at rest because no increase in ethidium or 2‐hydroxyethidium (2‐HE) formation from DHE was seen in fibers from Sod1−/− mice compared with those from WT mice. Fibers from Sod1−/− mice had decreased NO availability (decreased DAF‐FM fluorescence), increased 3‐NT in muscle proteins indicating increased peroxynitrite formation and increased content of peroxiredoxin V (a peroxynitrite reductase), compared with WT mice. Muscle fibers from Sod1−/− mice showed substantially reduced generation of superoxide in response to contractions compared with fibers from WT mice. Inhibition of NOS did not affect DHE oxidation in fibers from WT or Sod1−/− mice at rest or during contractions, but transgenic mice overexpressing nNOS showed increased DAF‐FM fluorescence and reduced DHE oxidation in resting muscle fibers. It is concluded that formation of peroxynitrite in muscle fibers is a major effect of lack of SOD1 in Sod1−/− mice and may contribute to fiber loss in this model, and that NO regulates superoxide availability and peroxynitrite formation in muscle.

Neuron-specific expression of CuZnSOD prevents the loss of muscle mass and function that occurs in homozygous CuZnSOD-knockout mice

Deletion of copper‐zinc superoxide dismutase (CuZnSOD) in Sod1–/– mice leads to accelerated loss of muscle mass and force during aging, but the losses do not occur with muscle‐specific deletion of CuZnSOD. To determine the role of motor neurons in the muscle decline, we generated transgenic Sod1–/– mice in which CuZnSOD was expressed under control of the synapsin 1 promoter (SynTgSod1–/– mice). SynTgSod1–/– mice expressed CuZnSOD in brain, spinal cord, and peripheral nerve, but not in other tissues. Sciatic nerve CuZnSOD content in SynTgSod1–/– mice was ~20% that of control mice, but no reduction in muscle mass or isometric force was observed in SynTg‐Sod1–/– mice compared with control animals, whereas muscles of age‐matched Sod1–/– mice displayed 30–40% reductions in mass and force. In addition, increased oxidative damage and adaptations in stress responses observed in muscles of Sod1–/– mice were absent in SynTgSod1–/– mice, and degeneration of neuromuscular junction (NMJ) structure and function occurred in Sod1–/– mice but not in SynTgSod1–/– mice. Our data demonstrate that specific CuZnSOD expression in neurons is sufficient to preserve NMJ and skeletal muscle structure and function in Sod1–/– mice and suggest that redox homeostasis in motor neurons plays a key role in initiating sarcopenia during aging.—Sakellariou, G. K., Davis, C. S., Shi, Y., Ivannikov, M. V., Zhang, Y., Vasilaki, A., Macleod, G. T., Richardson, A., Van Remmen, H., Jackson, M. J., McArdle, A., Brooks, S. V. Neuron‐specific expression of CuZnSOD prevents the loss of muscle mass and function that occurs in homozygous CuZnSOD‐knockout mice. FASEB J. 28, 1666‐1681 (2014). www.fasebj.org

Differential cysteine labeling and global label-free proteomics reveals an altered metabolic state in skeletal muscle aging.

The molecular mechanisms underlying skeletal muscle aging and associated sarcopenia have been linked to an altered oxidative status of redox-sensitive proteins. Reactive oxygen and reactive nitrogen species (ROS/RNS) generated by contracting skeletal muscle are necessary for optimal protein function, signaling, and adaptation. To investigate the redox proteome of aging gastrocnemius muscles from adult and old male mice, we developed a label-free quantitative proteomic approach that includes a differential cysteine labeling step. The approach allows simultaneous identification of up- and downregulated proteins between samples in addition to the identification and relative quantification of the reversible oxidation state of susceptible redox cysteine residues. Results from muscles of adult and old mice indicate significant changes in the content of chaperone, glucose metabolism, and cytoskeletal regulatory proteins, including Protein DJ-1, cAMP-dependent protein kinase type II, 78 kDa glucose regulated protein, and a reduction in the number of redox-responsive proteins identified in muscle of old mice. Results demonstrate skeletal muscle aging causes a reduction in redox-sensitive proteins involved in the generation of precursor metabolites and energy metabolism, indicating a loss in the flexibility of the redox energy response. Data is available via ProteomeXchange with identifier PXD001054.

Application of redox proteomics to skeletal muscle aging and exercise

Skeletal muscle represents a physiologically relevant model for the application of redox proteomic techniques to dissect its response to exercise and aging. Contracting skeletal muscles generate ROS (reactive oxygen species) and RNS (reactive nitrogen species) necessary for the regulation of many proteins involved in excitation-contraction coupling. The magnitude and species of ROS/RNS generated by contracting muscles will have downstream effects on specific protein targets and cellular redox signalling. Redox modifications on specific proteins are essential for the adaptive response to exercise and skeletal muscle can develop a dysregulated redox response during aging. In the present article, we discuss how redox proteomics can be applied to identify and quantify the reversible modifications on susceptible cysteine residues within those redox-sensitive proteins, and the integration of oxidative and non-oxidative protein modifications in relation to the functional proteome.

Mitochondrial ROS regulate oxidative damage and mitophagy but not age-related muscle fiber atrophy.

Age-related loss of skeletal muscle mass and function is a major contributor to morbidity and has a profound effect on the quality of life of older people. The potential role of age-dependent mitochondrial dysfunction and cumulative oxidative stress as the underlying cause of muscle aging remains a controversial topic. Here we show that the pharmacological attenuation of age-related mitochondrial redox changes in muscle with SS31 is associated with some improvements in oxidative damage and mitophagy in muscles of old mice. However, this treatment failed to rescue the age-related muscle fiber atrophy associated with muscle atrophy and weakness. Collectively, these data imply that the muscle mitochondrial redox environment is not a key regulator of muscle fiber atrophy during sarcopenia but may play a key role in the decline of mitochondrial organelle integrity that occurs with muscle aging.

Neuron specific reduction in CuZnSOD is not sufficient to initiate a full sarcopenia phenotype

Our previous studies showed that adult (8 month) mice lacking CuZn-superoxide dismutase (CuZnSOD, Sod1KO mice) have neuromuscular changes resulting in dramatic accelerated muscle atrophy and weakness that mimics age-related sarcopenia. We have further shown that loss of CuZnSOD targeted to skeletal muscle alone results in only mild weakness and no muscle atrophy. In this study, we targeted deletion of CuZnSOD specifically to neurons (nSod1KO mice) and determined the effect on muscle mass and weakness. The nSod1KO mice show a significant loss of CuZnSOD activity and protein level in brain and spinal cord but not in muscle tissue. The masses of the gastrocnemius, tibialis anterior and extensor digitorum longus (EDL) muscles were not reduced in nSod1KO compared to wild type mice, even at 20 months of age, although the quadriceps and soleus muscles showed small but statistically significant reductions in mass in the nSod1KO mice. Maximum isometric specific force was reduced by 8–10% in the gastrocnemius and EDL muscle of nSod1KO mice, while soleus was not affected. Muscle mitochondrial ROS generation and oxidative stress measured by levels of reactive oxygen/nitrogen species (RONS) regulatory enzymes, protein nitration and F2-isoprostane levels were not increased in muscle from the nSod1KO mice. Although we did not find evidence of denervation in the nSod1KO mice, neuromuscular junction morphology was altered and the expression of genes associated with denervation acetylcholine receptor subunit alpha (AChRα), the transcription factor, Runx1 and GADD45α) was increased, supporting a role for neuronal loss of CuZnSOD initiating alterations at the neuromuscular junction. These results and our previous studies support the concept that CuZnSOD deficits in either the motor neuron or muscle alone are not sufficient to initiate a full sarcopenic phenotype and that deficits in both tissues are required to recapitulate the loss of muscle observed in Sod1KO mice.

Long-term administration of the mitochondria-targeted antioxidant mitoquinone mesylate fails to attenuate age-related oxidative damage or rescue the loss of muscle mass and function associated with aging of skeletal muscle

Age‐related skeletal muscle dysfunction is the underlying cause of morbidity that affects up to half the population aged 80 and over. Considerable evidence indicates that oxidative damage and mitochondrial dysfunction contribute to the sarcopenic phenotype that occurs with aging. To examine this, we administered the mitochondria‐targeted antioxidant mitoquinone mesylate {[10‐(4,5‐dimethoxy‐2‐methyl‐3,6‐dioxo‐1,4‐cyclohexadien‐ 1‐yl)decyl] triphenylphosphonium; 100 μM} to wild‐type C57BL/6 mice for 15 wk (from 24 to 28 mo of age) and investigated the effects on age‐related loss of muscle mass and function, changes in redox homeostasis, and mitochondrial organelle integrity and function. We found that mitoquinone mesylate treatment failed to prevent agedependent loss of skeletal muscle mass associated with myofiber atrophy or alter a variety of in situ and ex vivo muscle function analyses, including maximum isometric tetanic force, decline in force after a tetanic fatiguing protocol, and single‐fiber‐specific force. We also found evidence that long‐term mitoquinone mesylate administration did not reduce mitochondrial reactive oxygen species or induce significant changes in muscle redox homeostasis, as assessed by changes in 4‐hydroxynonenal protein adducts, protein carbonyl content, protein nitration, and DNA damage determined by the content of 8‐hydroxydeoxyguanosine. Mitochondrialmembrane potential, abundance, and respiration assessed in permeabilized myofibers were not significantly altered in response to mitoquinone mesylate treatment. Collectively, these findings demonstrate that long‐term mitochondria‐targeted mitoquinone mesylate administration failed to attenuate age‐related oxidative damage in skeletal muscle of old mice or provide any protective effect in the context of muscle aging.—Sakellariou, G. K., Pearson, T., Lightfoot, A. P., Nye, G. A., Wells, N., Giakoumaki, I. I., Griffiths, R. D., McArdle, A., Jackson, M. J. Long‐term administration of the mitochondria‐targeted antioxidant mitoquinone mesylate fails to attenuate age‐related oxidative damage or rescue the loss of muscle mass and function associated with aging of skeletal muscle. FASEB J. 30, 3771–3785 (2016) www.fasebj.org

SS-31 attenuates TNF-α induced cytokine release from C2C12 myotubes

TNF-α is a key inflammatory mediator and is proposed to induce transcriptional responses via the mitochondrial generation of Reactive Oxygen Species (ROS). The aim of this study was to determine the effect of TNF-α on the production of myokines by skeletal muscle. Significant increases were seen in the release of IL-6, MCP-1/CCL2, RANTES/CCL5 and KC/CXCL1 and this release was inhibited by treatment with Brefeldin A, suggesting a golgi-mediated release of cytokines by muscle cells. An increase was also seen in superoxide in response to treatment with TNF-α, which was localised to the mitochondria and this was also associated with activation of NF-κB. The changes in superoxide, activation of NF-kB and release of myokines were attenuated following pre-treatment with SS-31 peptide indicating that the ability of TNF-α to induce myokine release may be mediated through mitochondrial superoxide, which is, at least in part, associated with activation of the redox sensitive transcription factor NF-kB.