The Role of Mitochondria in Statin-Induced Myopathy.

Statins represent the primary therapy for combatting hypercholesterolemia and reducing mortality from cardiovascular events. Despite their pleiotropic effects in lowering cholesterol synthesis, circulating cholesterol, as well as reducing the risk of other systemic diseases, statins have adverse events in a small, but significant, population of treated patients. The most prominent of these adverse effects is statin-induced myopathy, which lacks precise definition but is characterised by elevations in the muscle enzyme creatine kinase alongside musculoskeletal complaints, including pain, weakness and fatigue. The exact aetiology of statin-induced myopathy remains to be elucidated, although impaired mitochondrial function is thought to be an important underlying cause. This may result from or be the consequence of several factors including statin-induced inhibition of coenzyme Q10 (CoQ10) biosynthesis, impaired Ca2+ signalling and modified reactive oxygen species (ROS) generation. The purpose of this review article is to provide an update on the information available linking statin therapy with mitochondrial dysfunction and to outline any mechanistic insights, which may be beneficial in the future treatment of myopathic adverse events.

Major histocompatibility complex I-induced endoplasmic reticulum stress mediates the secretion of pro-inflammatory muscle-derived cytokines.

Major histocompatibility complex (MHC) I is an important component of intracellular antigen presentation. However, improper expression of MHC I upon the cell surface has been associated with several autoimmune diseases. Myositis is a rare acquired autoimmune disease which targets skeletal muscle, and MHC I overexpression on the surface of muscle fibres and immune cell infiltration are clinical hallmarks. MHC I overexpression may have an important pathogenic role, mediated by the activation of the endoplasmic reticulum (ER) stress response. Given the evidence that muscle is a diverse source of cytokines, we aimed to investigate whether MHC I overexpression can modify the profile of muscle-derived cytokines and what role the ER stress pathway may play. Using C2C12 myoblasts we overexpressed MHC I with a H-2kb vector in the presence or absence of salubrinal an ER stress pathway modifying compound. MHC I overexpression induced ER stress pathway activation and elevated cytokine gene expression. MHC I overexpression caused significant release of cytokines and chemokines, which was attenuated in the presence of salubrinal. Conditioned media from MHC I overexpressing cells induced in vitro T-cell chemotaxis, atrophy of healthy myotubes and modified mitochondrial function, features which were attenuated in the presence of salubrinal. Collectively, these data suggest that MHC I overexpression can induce pro-inflammatory cytokine/chemokine release from C2C12 myoblasts, a process which appears to be mediated in-part by the ER stress pathway.

Eukarion-134 Attenuates Endoplasmic Reticulum Stress-Induced Mitochondrial Dysfunction in Human Skeletal Muscle Cells.

Maladaptive endoplasmic reticulum (ER) stress is associated with modified reactive oxygen species (ROS) generation and mitochondrial abnormalities; and is postulated as a potential mechanism involved in muscle weakness in myositis, an acquired autoimmune neuromuscular disease. This study investigates the impact of ROS generation in an in vitro model of ER stress in skeletal muscle, using the ER stress inducer tunicamycin (24 h) in the presence or absence of a superoxide dismutase/catalase mimetic Eukarion (EUK)-134. Tunicamycin induced maladaptive ER stress, which was mitigated by EUK-134 at the transcriptional level. ER stress promoted mitochondrial dysfunction, described by substantial loss of mitochondrial membrane potential, as well as a reduction in respiratory control ratio, reserve capacity, phosphorylating respiration, and coupling efficiency, which was ameliorated by EUK-134. Tunicamycin induced ROS-mediated biogenesis and fusion of mitochondria, which, however, had high propensity of fragmentation, accompanied by upregulated mRNA levels of fission-related markers. Increased cellular ROS generation was observed under ER stress that was prevented by EUK-134, even though no changes in mitochondrial superoxide were noticeable. These findings suggest that targeting ROS generation using EUK-134 can amend aspects of ER stress-induced changes in mitochondrial dynamics and function, and therefore, in instances of chronic ER stress, such as in myositis, quenching ROS generation may be a promising therapy for muscle weakness and dysfunction.

MicroRNA and mRNA profiling in the idiopathic inflammatory myopathies.

BACKGROUND: The idiopathic inflammatory myopathies (IIMs) are heterogeneous autoimmune conditions of skeletal muscle inflammation and weakness. MicroRNAs (miRNAs) are short, non-coding RNA which regulate gene expression of target mRNAs. The aim of this study was to profile miRNA and mRNA in IIM and identify miRNA-mRNA relationships which may be relevant to disease. METHODS: mRNA and miRNA in whole blood samples from 7 polymyositis (PM), 7 dermatomyositis (DM), 5 inclusion body myositis and 5 non-myositis controls was profiled using next generation RNA sequencing. Gene ontology and pathway analyses were performed using GOseq and Ingenuity Pathway Analysis. Dysregulation of miRNAs and opposite dysregulation of predicted target mRNAs in IIM subgroups was validated using RTqPCR and investigated by transfecting human skeletal muscle cells with miRNA mimic. RESULTS: Analysis of differentially expressed genes showed that interferon signalling, and anti-viral response pathways were upregulated in PM and DM compared to controls. An anti-Jo1 autoantibody positive subset of PM and DM (n = 5) had more significant upregulation and predicted activation of interferon signalling and highlighted T-helper (Th1 and Th2) cell pathways. In miRNA profiling miR-96-5p was significantly upregulated in PM, DM and the anti-Jo1 positive subset. RTqPCR replicated miR-96-5p upregulation and predicted mRNA target (ADK, CD28 and SLC4A10) downregulation. Transfection of a human skeletal muscle cell line with miR-96-5p mimic resulted in significant downregulation of ADK. CONCLUSION: MiRNA and mRNA profiling identified dysregulation of interferon signalling, anti-viral response and T-helper cell pathways, and indicates a possible role for miR-96-5p regulation of ADK in pathogenesis of IIM.

Targeting reactive oxygen species (ROS) to combat the age-related loss of muscle mass and function.

The loss of muscle mass and function with age, termed sarcopenia, is an inevitable process, which has a significant impact on quality of life. During ageing we observe a progressive loss of total muscle fibres and a reduction in cross-sectional area of the remaining fibres, resulting in a significant reduction in force output. The mechanisms which underpin sarcopenia are complex and poorly understood, ranging from inflammation, dysregulation of protein metabolism and denervation. However, there is significant evidence to demonstrate that modified ROS generation, redox dis-homeostasis and mitochondrial dysfunction may have an important role to play. Based on this, significant interest and research has interrogated potential ROS-targeted therapies, ranging from nutritional-based interventions such as vitamin E/C, polyphenols (resveratrol) and targeted pharmacological compounds, using molecules such as SS-31 and MitoQ. In this review we evaluate these approaches to target aberrant age-related ROS generation and the impact on muscle mass and function.

NF-kB and Inflammatory Cytokine Signalling: Role in Skeletal Muscle Atrophy.

Atrophy is a classical hallmark of an array of disorders that affect skeletal muscle, ranging from inherited dystrophies, acquired inflammatory myopathies, ageing (sarcopenia) and critical illness (sepsis). The loss of muscle mass and function in these instances is associated with disability, poor quality of life and in some cases mortality. The mechanisms which underpin muscle atrophy are complex; however, significant research has demonstrated an important role for inflammatory cytokines such as tumour necrosis factor-alpha (TNF-α), mediated by the generation of reactive oxygen species (ROS) in muscle wasting. Moreover, activation of the transcription factor nuclear factor kappa B (NF-κB) is a key lynchpin in the overall processes that mediate muscle atrophy. The significance of NF-κB as a key regulator of muscle atrophy has been emphasised by several in vivo studies, which have demonstrated that NF-κB-targeted therapies can abrogate muscle atrophy. In this chapter, we will summarise current knowledge on the role of cytokines (TNF-α) and NF-κB in the loss of muscle mass and function and highlight perspectives towards future research and potential therapies to combat muscle atrophy.