Adam Lightfoot

Muscle in Defense

The physical inactivity of the critically ill patient is an abnormal state that compromises muscle function to an extent that the immobile skeletal muscle may not simply be a bystander in the disease process. Skeletal muscle seems to maintain a system of defense against inflammation through heat shock proteins and the production of myokines. It contributes in a bidirectional role in systemic inflammatory signaling and the modulation of the inflammatory response. Skeletal muscle is a major contributor to whole-body glucose and protein metabolism, exemplified by its role in nutrient provision for the immune system and other rapidly dividing tissues through glutamine production.

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.

Mechanisms of skeletal muscle ageing; avenues for therapeutic intervention

Age-related loss of muscle mass and function, termed sarcopenia, is a catastrophic process, which impacts severely on quality of life of older people. The mechanisms underlying sarcopenia are unclear and the development of optimal therapeutic interventions remains elusive. Impaired regenerative capacity, attenuated ability to respond to stress, elevated reactive oxygen species production and low-grade systemic inflammation are all key contributors to sarcopenia. Pharmacological intervention using compounds such as 17AAG, SS-31 and Bimagrumab or naturally occurring polyphenols to target specific pathways show potential benefit to combat sarcopenia although further research is required, particularly to identify the mechanisms by which muscle fibres are completely lost with increasing age.

The role of myokines in muscle health and disease.

Purpose of reviewThis article updates on the concept that muscle-derived cytokines (myokines) play important roles in muscle health and disease. Recent findingsInterleukin-6 (IL-6) is released from normal skeletal muscle in response to exercise, mediating both anti-inflammatory responses and metabolic adaptations, actions contradictory to the prevailing view that IL-6 is a proinflammatory cytokine that is inducing and propagating disease. The anti-inflammatory effects of IL-6 result from its trans-membrane signalling capability, via membrane-bound receptors, whereas its proinflammatory effects result instead from signalling via the soluble IL-6 receptor and gp130. IL-15 is elevated following exercise, promoting muscle fibre hypertrophy in some circumstances, while inducing fibre apoptosis in others. This functional divergence appears because of variations in expression of IL-15 receptor isoforms. Decorin, a recently described myokine, is also elevated following exercise in normal muscle, and promotes muscle fibre hypertrophy by competitively binding to, and thus inhibiting, myostatin, a negative regulator of muscle protein synthesis. Exercise-induced myostatin downregulation thus promotes muscle fibre growth, prompting recent trials of a biological myostatin inhibitor in inclusion body myositis. SummaryMyokines appear to exert diverse beneficial effects, though their mechanistic roles in myositis and other myopathologies remain poorly understood.

Understanding the origin of non-immune cell-mediated weakness in the idiopathic inflammatory myopathies - potential role of ER stress pathways.

Purpose of reviewDiscussion of endoplasmic reticulum (ER) stress pathway activation in idiopathic inflammatory myopathies (IIM), and downstream mechanisms causative of muscle weakness. Recent findingsIn IIM, ER stress is an important pathogenic process, but how it causes muscle dysfunction is unknown. We discuss relevant pathways modified in response to ER stress in IIM: reactive oxygen species (ROS) generation and mitochondrial dysfunction, and muscle cytokine (myokine) generation. First, ER stress pathway activation can induce changes in mitochondrial bioenergetics and ROS production. ROS can oxidize cellular components, causing muscle contractile dysfunction and energy deficits. Novel compounds targeting ROS generation and/or mitochondrial dysfunction can improve muscle function in several myopathologies. Second, recent research has demonstrated that skeletal muscle produces multiple myokines. It is suggested that these play a role in causing muscle weakness. Myokines are capable of immune cell recruitment, thus contributing to perturbed muscle function. A characterization of myokines in IIM would clarify their pathogenic role, and so identify new therapeutic targets. SummaryER stress pathway activation is clearly of etiological relevance in IIM. Research to better understand mechanisms of weakness downstream of ER stress is now required, and which may discover new therapeutic targets for nonimmune cell-mediated weakness.

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

In the idiopathic inflammatory myopathies (IIM), do reactive oxygen species (ROS) contribute to muscle weakness?

The idiopathic inflammatory myopathies (IIMs) are a group of rare autoimmune disorders, collectively known as myositis. Affected patients present with proximal muscle weakness, which usually improves following treatment with immunosuppressants, but often incompletely so, thus many patients remain weak. IIMs are characterised histologically by inflammatory cell infiltrates into skeletal muscle and overexpression of major histocompatibility complex I on muscle cell surfaces. Although inflammatory cell infiltrates represent a major feature of myositis there is growing evidence that muscle weakness correlates only poorly with the degree of cellular infiltration, while weakness may in fact precede such infiltrations. The mechanisms underpinning such non-immune cell mediated weakness in IIM are poorly understood. Activation of the endoplasmic reticulum stress pathways appears to be a potential contributor. Data from non-muscle cells indicate that endoplasmic reticulum stress results in altered redox homeostasis capable of causing oxidative damage. In myopathological situations other than IIM, as seen in ageing and sepsis, evidence supports an important role for reactive oxygen species (ROS). Modified ROS generation is associated with mitochondrial dysfunction, depressed force generation and activation of muscle catabolic and autophagy pathways. Despite the growing evidence demonstrating a key role for ROS in skeletal muscle dysfunction in myopathologies other than IIM, no research has yet investigated the role of modified generation of ROS in inducing the weakness characteristic of IIM. This article reviews current knowledge regarding muscle weakness in the absence of immune cells in IIM, and provides a background to the potential role of modified ROS generation as a mechanism of muscle dysfunction. The authors suggest that ROS-mediated mechanisms are potentially involved in non-immune cell mediated weakness seen in IIM and outline how these mechanisms might be investigated in this context. This appears a timely strategy, given recent developments in targeted therapies which specifically modify ROS generation.

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.

Redox homeostasis and age‐related deficits in neuromuscular integrity and function

Skeletal muscle is a major site of metabolic activity and is the most abundant tissue in the human body. Age‐related muscle atrophy (sarcopenia) and weakness, characterized by progressive loss of lean muscle mass and function, is a major contributor to morbidity and has a profound effect on the quality of life of older people. With a continuously growing older population (estimated 2 billion of people aged >60 by 2050), demand for medical and social care due to functional deficits, associated with neuromuscular ageing, will inevitably increase. Despite the importance of this ‘epidemic’ problem, the primary biochemical and molecular mechanisms underlying age‐related deficits in neuromuscular integrity and function have not been fully determined. Skeletal muscle generates reactive oxygen and nitrogen species (RONS) from a variety of subcellular sources, and age‐associated oxidative damage has been suggested to be a major factor contributing to the initiation and progression of muscle atrophy inherent with ageing. RONS can modulate a variety of intracellular signal transduction processes, and disruption of these events over time due to altered redox control has been proposed as an underlying mechanism of ageing. The role of oxidants in ageing has been extensively examined in different model organisms that have undergone genetic manipulations with inconsistent findings. Transgenic and knockout rodent studies have provided insight into the function of RONS regulatory systems in neuromuscular ageing. This review summarizes almost 30 years of research in the field of redox homeostasis and muscle ageing, providing a detailed discussion of the experimental approaches that have been undertaken in murine models to examine the role of redox regulation in age‐related muscle atrophy and weakness.

Editorial: Endurance Exercise: An Important Therapeutic Adjuvant in the Overall Treatment of Myositis?

Idiopathic inflammatory myopathy (myositis) is an autoimmune disorder characterized by myositis-specific and -associated autoantibody production as well as the histologic detection of inflammatory cell infiltrates between and within skeletal muscle fibers in diagnostic muscle biopsy samples from affected patients. The infiltrates are associated with fiber atrophy and myonecrosis, causing weakness and disability. Given the inflammatory nature of myositis, it is no surprise that treatment currently consists of glucocorticoid use in combination with various “diseasemodifying” immunosuppressive agents. Interestingly, no one immunosuppressive regimen has yet proved superior to any other in myositis. Even with aggressive immunosuppression, muscle damage may still progress, with cumulative muscle atrophy and fatty replacement, which are usually irreversible. Even when immunosuppression has clearly reduced inflammatory cell loads, muscle weakness often persists; therefore, most treated patients have detectable if stable weakness. The magnitude of inflammation detected in muscle biopsy samples often correlates only poorly with the degree of detected muscle weakness (1), while in an established murine model of myositis, muscle weakness appears to actually precede the infiltration of inflammatory cells (2). These observations highlight the paucity of understanding regarding the mechanisms of weakness induction in myositis. It is, however, increasingly recognized that myositis-associated muscle weakness may arise from non–immune cell–mediated mechanisms (i.e., over and above that caused by immune cell infiltration). For instance, endoplasmic reticulum (ER) stress pathway activation has received much recent attention regarding its potential role in noninflammatory weakness induction (3). Given that immunosuppression is incompletely effective in myositis, and that non–immune cell– mediated pathways are likely also involved in weakness induction, it follows that for future myositis treatments to become more effective, they must address noninflammatory as well as inflammatory pathologies. A key question arising in myositis is what is/are the precise cause/s of “residual” weakness (i.e., over and above that due to inflammation). Unsuppressed disease activity or irreversible damage may play a role, although both scenarios would be detectable by repeat neurophysiologic examinations (electromyography), muscle biopsy, and magnetic resonance imaging, and subsequent treatment would be tailored accordingly. If retesting excludes disease activity and irreversible damage, the cause of ongoing weakness could instead be a myositis-induced intrinsic disturbance of intracellular bioenergetic and/or proteomic pathways (e.g., as postulated to result from chronic ER stress induction) (3,4). Moreover, the ongoing weakness may be attributed to a lower grade of muscle atrophy that has not, for whatever reason, improved with the expected return toward normal activity levels following suppression of inflammation. The latter possibility could suggest that disuse atrophy also represents a specific problem. Ongoing noninflammatory weakness could obviously also have iatrogenic components (e.g., due to long-term glucocorticoid use). Interrogating the issue of residual weakness by the use of an exercise program may enable us to distinguish between these possibilities.