Interactions of mitochondrial and skeletal muscle biology in mitochondrial myopathy.

Mitochondrial dysfunction in skeletal muscle fibres occurs with both healthy aging and a range of neuromuscular diseases. The impact of mitochondrial dysfunction in skeletal muscle and the way muscle fibres adapt to this dysfunction is important to understand disease mechanisms and to develop therapeutic interventions. Furthermore, interactions between mitochondrial dysfunction and skeletal muscle biology, in mitochondrial myopathy, likely have important implications for normal muscle function and physiology. In this review, we will try to give an overview of what is known to date about these interactions including metabolic remodelling, mitochondrial morphology, mitochondrial turnover, cellular processes and muscle cell structure and function. Each of these topics is at a different stage of understanding, with some being well researched and understood, and others in their infancy. Furthermore, some of what we know comes from disease models. Whilst some findings are confirmed in humans, where this is not yet the case, we must be cautious in interpreting findings in the context of human muscle and disease. Here, our goal is to discuss what is known, highlight what is unknown and give a perspective on the future direction of research in this area.

A coordinated multiorgan metabolic response contributes to human mitochondrial myopathy.

Mitochondrial diseases are a heterogeneous group of monogenic disorders that result from impaired oxidative phosphorylation (OXPHOS). As neuromuscular tissues are highly energy-dependent, mitochondrial diseases often affect skeletal muscle. Although genetic and bioenergetic causes of OXPHOS impairment in human mitochondrial myopathies are well established, there is a limited understanding of metabolic drivers of muscle degeneration. This knowledge gap contributes to the lack of effective treatments for these disorders. Here, we discovered fundamental muscle metabolic remodeling mechanisms shared by mitochondrial disease patients and a mouse model of mitochondrial myopathy. This metabolic remodeling is triggered by a starvation-like response that evokes accelerated oxidation of amino acids through a truncated Krebs cycle. While initially adaptive, this response evolves in an integrated multiorgan catabolic signaling, lipid store mobilization, and intramuscular lipid accumulation. We show that this multiorgan feed-forward metabolic response involves leptin and glucocorticoid signaling. This study elucidates systemic metabolic dyshomeostasis mechanisms that underlie human mitochondrial myopathies and identifies potential new targets for metabolic intervention.

OMA1 mediates local and global stress responses against protein misfolding in CHCHD10 mitochondrial myopathy.

Mitochondrial stress triggers a response in the cell's mitochondria and nucleus, but how these stress responses are coordinated in vivo is poorly understood. Here, we characterize a family with myopathy caused by a dominant p.G58R mutation in the mitochondrial protein CHCHD10. To understand the disease etiology, we developed a knockin (KI) mouse model and found that mutant CHCHD10 aggregated in affected tissues, applying a toxic protein stress to the inner mitochondrial membrane. Unexpectedly, the survival of CHCHD10-KI mice depended on a protective stress response mediated by the mitochondrial metalloendopeptidase OMA1. The OMA1 stress response acted both locally within mitochondria, causing mitochondrial fragmentation, and signaled outside the mitochondria, activating the integrated stress response through cleavage of DAP3-binding cell death enhancer 1 (DELE1). We additionally identified an isoform switch in the terminal complex of the electron transport chain as a component of this response. Our results demonstrate that OMA1 was critical for neonatal survival conditionally in the setting of inner mitochondrial membrane stress, coordinating local and global stress responses to reshape the mitochondrial network and proteome.

Effect of training on skeletal muscle bioenergetic system in patients with mitochondrial myopathies: A computational study.

A computer model of the skeletal muscle bioenergetic system, involving the "Pi double-threshold" mechanism of muscle fatigue, was used to investigate the effect of muscle training on system kinetic properties in mitochondrial myopathies (MM) patients with inborn OXPHOS deficiencies. An increase in OXPHOS activity and decrease in peak Pi can account for the training-induced increase in V̇O2max, acceleration of the primary phase II of the V̇O2 on-kinetics, delay of muscle fatigue and prolongation of exercise at a given work intensity encountered in experimental studies. Depending on the mutation load and work intensity, training can bring the muscle from severe- to very-heavy- to moderate-exercise-like behavior, thus lessening the exertional fatigue and lengthening the physical activity of a given intensity. Training significantly increases critical power (CP) and slightly decreases the curvature constant (W') of the power-duration relationship. Generally, a mechanism underlying the training-induced changes in the skeletal muscle bioenergetic system in MM patients is proposed.

NAD+ Repletion Reverses Heart Failure With Preserved Ejection Fraction.

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Diminished muscle oxygen uptake and fatigue in spinal muscular atrophy.

OBJECTIVE: To estimate muscle oxygen uptake and quantify fatigue during exercise in ambulatory individuals with spinal muscular atrophy (SMA) and healthy controls. METHODS: Peak aerobic capacity (VO2peak ) and workload (Wpeak ) were measured by cardiopulmonary exercise test (CPET) in 19 ambulatory SMA patients and 16 healthy controls. Submaximal exercise (SME) at 40% Wpeak was performed for 10 minutes. Change in vastus lateralis deoxygenated hemoglobin, measured by near-infrared spectroscopy, determined muscle oxygen uptake (ΔHHb) at rest and during CPET and SME. Dual energy X-ray absorptiometry assessed fat-free mass (FFM%). Fatigue was determined by percent change in workload or distance in the first compared to the last minute of SME (FatigueSME ) and six-minute walk test (Fatigue6MWT ), respectively. RESULTS: ΔHHb-PEAK, ΔHHb-SME, VO2peak , Wpeak , FFM%, and 6MWT distance were lower (P < 0.001), and Fatigue6MWT and FatigueSME were higher (P < 0.001) in SMA compared to controls. ΔHHb-PEAK correlated with FFM% (r = 0.50) and VO2peak (r = 0.41) only in controls. Only in SMA, Fatigue6MWT was inversely correlated with Wpeak (r = -0.69), and FatigueSME was inversely correlated with FFM% (r = -0.55) and VO2peak (r = -0.69). INTERPRETATION: This study provides further support for muscle mitochondrial dysfunction in SMA patients. During exercise, we observed diminished muscle oxygen uptake but no correlation with aerobic capacity or body composition. We also observed increased fatigue which correlated with decreased aerobic capacity, workload, and body composition. Understanding the mechanisms underlying diminished muscle oxygen uptake and increased fatigue during exercise in SMA may identify additional therapeutic targets that rescue symptomatic patients and mitigate their residual disease burden.

Genomic integrity and mitochondrial metabolism defects in Warsaw syndrome cells: a comparison with Fanconi anemia.

Warsaw breakage syndrome (WABS), is caused by biallelic mutations of DDX11, a gene coding a DNA helicase. We have recently reported two affected sisters, compound heterozygous for a missense (p.Leu836Pro) and a frameshift (p.Lys303Glufs*22) variant. By investigating the pathogenic mechanism, we demonstrate the inability of the DDX11 p.Leu836Pro mutant to unwind forked DNA substrates, while retaining DNA binding activity. We observed the accumulation of patient-derived cells at the G2/M phase and increased chromosomal fragmentation after mitomycin C treatment. The phenotype partially overlaps with features of the Fanconi anemia cells, which shows not only genomic instability but also defective mitochondria. This prompted us to examine mitochondrial functionality in WABS cells and revealed an altered aerobic metabolism. This opens the door to the further elucidation of the molecular and cellular basis of an impaired mitochondrial phenotype and sheds light on this fundamental process in cell physiology and the pathogenesis of these diseases.

A mitochondrial myopathy-associated tRNASer(UCN) 7453G>A mutation alters tRNA metabolism and mitochondrial function.

BACKGROUND: Mitochondrial disorders are a group of heterogeneous diseases characterized by biochemical disturbances in oxidative phosphorylation (OXPHOS). Mutations in mitochondrial transfer RNA (mt-tRNA) genes are the most frequently in mitochondrial disease. However, few studies have detailed the molecular mechanisms behind these mutations. METHODS: We performed clinical evaluation, genetic analysis, muscle histochemistry, and molecular and biochemical investigations in muscle tissue and proband-derived cybrid cell lines. RESULTS: We found a mitochondrial tRNASer(UCN) mutation (m.7453G>A) in a 15-year-old patient with severe mitochondrial myopathy. We demonstrated that this mutation caused impairment of mitochondrial translation, respiratory deficiency, overproduction of reactive oxygen species (ROS), and decreased mitochondrial membrane potential (MMP), which ultimately led to severe mitochondrial myopathy. CONCLUSION: Our findings offer valuable new insights into the tRNASer(UCN) m.7453G>A mutation for both the pathogenic mechanism and functional consequences.

GDF15 as a central mediator for integrated stress response and a promising therapeutic molecule for metabolic disorders and NASH.

BACKGROUND: Mitochondria is a key organelle for energy production and cellular adaptive response to intracellular and extracellular stresses. Mitochondrial stress can be evoked by various stimuli such as metabolic stressors or pathogen infection, which may lead to expression of 'mitokines' such as growth differentiation factor 15 (GDF15). SCOPE OF REVIEW: This review summarizes the mechanism of GDF15 expression in response to organelle stress such as mitochondrial stress, and covers pathophysiological conditions or diseases that are associated with elevated GDF15 level. This review also illustrates the in vivo role of GDF15 expression in those stress conditions or diseases, and a potential of GDF15 as a therapeutic agent against metabolic disorders such as NASH. MAJOR CONCLUSIONS: Mitochondrial unfolded protein response (UPRmt) is a critical process to recover from mitochondrial stress. UPRmt can induce expression of secretory proteins that can exert systemic effects (mitokines) as well as mitochondrial chaperons. GDF15 can have either protective or detrimental systemic effects in response to mitochondrial stresses, suggesting its role as a mitokine. Mounting evidence shows that GDF15 is also induced by stresses of organelles other than mitochondria such as endoplasmic reticulum (ER). GDF15 level is increased in serum or tissue of mice and human subjects with metabolic diseases such as obesity or NASH. GDF15 can modulate metabolic features of those diseases. GENERAL SIGNIFICANCE: GDF15 play a role as an integrated stress response (ISR) beyond mitochondrial stress response. GDF15 is involved in the pathogenesis of metabolic diseases such as NASH, and also could be a candidate for therapeutic agent against those diseases.

Molecular imaging for mitochondrial metabolism and oxidative stress in mitochondrial diseases and neurodegenerative disorders.

BACKGROUND: Increasing evidence from pathological and biochemical investigations suggests that mitochondrial metabolic impairment and oxidative stress play a crucial role in the pathogenesis of mitochondrial diseases, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, and various neurodegenerative disorders. Recent advances in molecular imaging technology with positron emission tomography (PET) and functional magnetic resonance imaging (MRI) have accomplished a direct and non-invasive evaluation of the pathophysiological changes in living patients. SCOPE OF REVIEW: In this review, we focus on the latest achievements of molecular imaging for mitochondrial metabolism and oxidative stress in mitochondrial diseases and neurodegenerative disorders. MAJOR CONCLUSIONS: Molecular imaging with PET and MRI exhibited mitochondrial metabolic changes, such as enhanced glucose utilization with lactic acid fermentation, suppressed fatty acid metabolism, decreased TCA-cycle metabolism, impaired respiratory chain activity, and increased oxidative stress, in patients with MELAS syndrome. In addition, PET imaging clearly demonstrated enhanced cerebral oxidative stress in patients with Parkinson's disease or amyotrophic lateral sclerosis. The magnitude of oxidative stress correlated well with clinical severity in patients, indicating that oxidative stress based on mitochondrial dysfunction is associated with the neurodegenerative changes in these diseases. GENERAL SIGNIFICANCE: Molecular imaging is a promising tool to improve our knowledge regarding the pathogenesis of diseases associated with mitochondrial dysfunction and oxidative stress, and this would facilitate the development of potential antioxidants and mitochondrial therapies.

Mitochondrial neuropathy and neurogenic features in mitochondrial myopathy.

Mitochondrial diseases (MIDs) involve multiple organs including peripheral nerves and skeletal muscle. Mitochondrial neuropathy (MN) and mitochondrial myopathy (MM) are commonly associated and linked at the neuromuscular junction (NMJ). Herein we review MN in connection with neurogenic features of MM, and pathological evidence for the involvement of the peripheral nerve and NMJ in MID patients traditionally assumed to have predominantly MM. MN is not uncommon, but still likely under-reported, and muscle biopsies of MM commonly exhibit neurogenic features. Pathological examination remains the gold standard to assess the nerve and muscle changes in patients with MIDs. Ultrastructural studies by electron microscopy are often necessary to fully characterize the pathology of mitochondrial cytopathy in MN and MM.

Metabolic shift underlies recovery in reversible infantile respiratory chain deficiency.

Reversible infantile respiratory chain deficiency (RIRCD) is a rare mitochondrial myopathy leading to severe metabolic disturbances in infants, which recover spontaneously after 6-months of age. RIRCD is associated with the homoplasmic m.14674T>C mitochondrial DNA mutation; however, only ~ 1/100 carriers develop the disease. We studied 27 affected and 15 unaffected individuals from 19 families and found additional heterozygous mutations in nuclear genes interacting with mt-tRNAGlu including EARS2 and TRMU in the majority of affected individuals, but not in healthy carriers of m.14674T>C, supporting a digenic inheritance. Our transcriptomic and proteomic analysis of patient muscle suggests a stepwise mechanism where first, the integrated stress response associated with increased FGF21 and GDF15 expression enhances the metabolism modulated by serine biosynthesis, one carbon metabolism, TCA lipid oxidation and amino acid availability, while in the second step mTOR activation leads to increased mitochondrial biogenesis. Our data suggest that the spontaneous recovery in infants with digenic mutations may be modulated by the above described changes. Similar mechanisms may explain the variable penetrance and tissue specificity of other mtDNA mutations and highlight the potential role of amino acids in improving mitochondrial disease.

Unique Transcriptome Signature Distinguishes Patients With Heart Failure With Myopathy.

Background People with chronic heart failure (CHF) experience severe skeletal muscle dysfunction, characterized by mitochondrial abnormalities, which exacerbates the primary symptom of exercise intolerance. However, the molecular triggers and characteristics underlying mitochondrial abnormalities caused by CHF remain poorly understood. Methods and Results We recruited 28 patients with CHF caused by reduced ejection fraction and 9 controls. We simultaneously biopsied skeletal muscle from the pectoralis major in the upper limb and from the vastus lateralis in the lower limb. We phenotyped mitochondrial function in permeabilized myofibers from both sites and followed this by complete RNA sequencing to identify novel molecular abnormalities in CHF skeletal muscle. Patients with CHF presented with upper and lower limb skeletal muscle impairments to mitochondrial function that were of a similar deficit and indicative of a myopathy. Mitochondrial abnormalities were strongly correlated to symptoms. Further RNA sequencing revealed a unique transcriptome signature in CHF skeletal muscle characterized by a novel triad of differentially expressed genes related to deficits in energy metabolism including adenosine monophosphate deaminase 3, pyridine nucleotide-disulphide oxidoreductase domain 2, and lactate dehydrogenase C. Conclusions Our data suggest an upper and lower limb metabolic myopathy that is characterized by a unique transcriptome signature in skeletal muscle of humans with CHF.

Successful treatment of a patient with mitochondrial myopathy with alirocumab.

A 48-year-old man presented to our lipid clinic with statin intolerance and elevated serum creatine kinase levels, being affected by mitochondrial myopathy because of heteroplasmic mitochondrial DNA missense mutation in MTCO1 gene (m.7671T>A). He had just been treated with a coronary artery bypass 4 years before because of acute coronary syndrome, and he had consistently high levels of both low-density lipoprotein cholesterol and triglycerides. Dyslipidemia was successfully treated using 75 mg of alirocumab subcutaneously every 2 weeks, 10 mg of ezetimibe daily, 2 g of marine omega-3 fatty acids daily, and 145 mg of micronized fenofibrate every 2 days. Although muscle weakness persisted, myalgia did not reoccur and serum creatine kinase levels remained almost stable over the time.

Other Myopathies.

Healthy muscle relies on a complex and interdependent network that includes, but is not limited to, proteins, ion channels, and the production and utilization of ATP. Disruptions to the system can occur for a number of reasons (genetic mutations, toxins, systemic disease, inflammation), yet they clinically present with symptoms that are nonspecific and common to myopathies: weakness, muscle pain, cramping, hypotonia. This article uses a case-based format to review the clinical reasoning and diagnostic tools that guide the accurate diagnosis of myopathies. We specifically focus on toxic, metabolic, mitochondrial, and late-onset congenital myopathies.

Niacin Cures Systemic NAD+ Deficiency and Improves Muscle Performance in Adult-Onset Mitochondrial Myopathy.

NAD+ is a redox-active metabolite, the depletion of which has been proposed to promote aging and degenerative diseases in rodents. However, whether NAD+ depletion occurs in patients with degenerative disorders and whether NAD+ repletion improves their symptoms has remained open. Here, we report systemic NAD+ deficiency in adult-onset mitochondrial myopathy patients. We administered an increasing dose of NAD+-booster niacin, a vitamin B3 form (to 750-1,000 mg/day; clinicaltrials.govNCT03973203) for patients and their matched controls for 10 or 4 months, respectively. Blood NAD+ increased in all subjects, up to 8-fold, and muscle NAD+ of patients reached the level of their controls. Some patients showed anemia tendency, while muscle strength and mitochondrial biogenesis increased in all subjects. In patients, muscle metabolome shifted toward controls and liver fat decreased even 50%. Our evidence indicates that blood analysis is useful in identifying NAD+ deficiency and points niacin to be an efficient NAD+ booster for treating mitochondrial myopathy.

Proteome profile of peripheral myelin in healthy mice and in a neuropathy model.

Proteome and transcriptome analyses aim at comprehending the molecular profiles of the brain, its cell-types and subcellular compartments including myelin. Despite the relevance of the peripheral nervous system for normal sensory and motor capabilities, analogous approaches to peripheral nerves and peripheral myelin have fallen behind evolving technical standards. Here we assess the peripheral myelin proteome by gel-free, label-free mass-spectrometry for deep quantitative coverage. Integration with RNA-Sequencing-based developmental mRNA-abundance profiles and neuropathy disease genes illustrates the utility of this resource. Notably, the periaxin-deficient mouse model of the neuropathy Charcot-Marie-Tooth 4F displays a highly pathological myelin proteome profile, exemplified by the discovery of reduced levels of the monocarboxylate transporter MCT1/SLC16A1 as a novel facet of the neuropathology. This work provides the most comprehensive proteome resource thus far to approach development, function and pathology of peripheral myelin, and a straightforward, accurate and sensitive workflow to address myelin diversity in health and disease.

Metabolic effects of bezafibrate in mitochondrial disease.

Mitochondrial disorders affect 1/5,000 and have no cure. Inducing mitochondrial biogenesis with bezafibrate improves mitochondrial function in animal models, but there are no comparable human studies. We performed an open-label observational experimental medicine study of six patients with mitochondrial myopathy caused by the m.3243A>G MTTL1 mutation. Our primary aim was to determine the effects of bezafibrate on mitochondrial metabolism, whilst providing preliminary evidence of safety and efficacy using biomarkers. The participants received 600-1,200 mg bezafibrate daily for 12 weeks. There were no clinically significant adverse events, and liver function was not affected. We detected a reduction in the number of complex IV-immunodeficient muscle fibres and improved cardiac function. However, this was accompanied by an increase in serum biomarkers of mitochondrial disease, including fibroblast growth factor 21 (FGF-21), growth and differentiation factor 15 (GDF-15), plus dysregulation of fatty acid and amino acid metabolism. Thus, although potentially beneficial in short term, inducing mitochondrial biogenesis with bezafibrate altered the metabolomic signature of mitochondrial disease, raising concerns about long-term sequelae.

Myopathy reversion in mice after restauration of mitochondrial complex I.

Myopathies are common manifestations of mitochondrial diseases. To investigate whether gene replacement can be used as an effective strategy to treat or cure mitochondrial myopathies, we have generated a complex I conditional knockout mouse model lacking NDUFS3 subunit in skeletal muscle. NDUFS3 protein levels were undetectable in muscle of 15-day-old smKO mice, and myopathy symptoms could be detected by 2 months of age, worsening over time. rAAV9-Ndufs3 delivered systemically into 15- to 18-day-old mice effectively restored NDUFS3 levels in skeletal muscle, precluding the development of the myopathy. To test the ability of rAAV9-mediated gene replacement to revert muscle function after disease onset, we also treated post-symptomatic, 2-month-old mice. The injected mice showed a remarkable improvement of the mitochondrial myopathy and biochemical parameters, which remained for the duration of the study. Our results showed that muscle pathology could be reversed after restoring complex I, which was absent for more than 2 months. These findings have far-reaching implications for the ability of muscle to tolerate a mitochondrial defect and for the treatment of mitochondrial myopathies.