Safety, feasibility, and efficacy of strengthening exercise in Duchenne muscular dystrophy.

BACKGROUND: This two-part study explored the safety, feasibility, and efficacy of a mild-moderate resistance isometric leg exercise program in ambulatory boys with Duchenne muscular dystrophy (DMD). METHODS: First, we used a dose escalation paradigm with varying intensity and frequency of leg isometric exercise to determine the dose response and safety in 10 boys. Second, we examined safety and feasibility of a 12-wk in-home, remotely supervised, mild-moderate intensity strengthening program in eight boys. Safety measures included T2 MRI, creatine kinase levels, and pain. Peak strength and function (time to ascend/descend four stairs) were also measured. RESULTS: Dose-escalation revealed no signs of muscle damage. Seven of the eight boys completed the 12-wk in-home program with a compliance of 84.9%, no signs of muscle damage, and improvements in strength (knee extensors P < .01; knee flexors P < .05) and function (descending steps P < .05). CONCLUSIONS: An in-home, mild-moderate intensity leg exercise program is safe with potential to positively impact both strength and function in ambulatory boys with DMD.

Walking activity in a large cohort of boys with Duchenne muscular dystrophy.

INTRODUCTION: In this study we explored walking activity in a large cohort of boys with Duchenne muscular dystrophy (DMD). METHODS: Step activity (monitored for 7 days), functional ability, and strength were quantified in ambulatory boys (5-12.9 years of age) with DMD and unaffected boys. Ambulatory status was determined 2 years later. RESULTS: Two to 5 days of activity monitoring predicted weekly step activity (adjusted R2 = 0.80-0.95). Age comparisons revealed significant declines for step activity with increasing age, and relationships were found between step activity with both function and strength (P < .01). Our regression model predicted 36.5% of the variance in step activity. Those who were still ambulatory after 2 years demonstrated baseline step activity nearly double that of those who were no longer walking 2 years later (P < .01). DISCUSSION: Step activity for DMD is related to and predictive of functional declines, which may be useful for clinical trials.

Fidelity of muscle fibre reinnervation modulates ageing muscle impact in elderly women.

KEY POINTS: Susceptibility to age-related muscle atrophy relates to the degree of muscle denervation and the capacity of successful reinnervation. However, the specific role of denervation as a determinant of the severity of muscle aging between populations with low versus high physical function has not been addressed. We show that prefrail/frail elderly women exhibited marked features of muscle denervation, whereas world class octogenarian female master athletes showed attenuated indices of denervation and greater reinnervation capacity. These findings suggest that the difference in age-related muscle impact between low- and high-functioning elderly women is the robustness of the response to denervation of myofibers. ABSTRACT: Ageing muscle degeneration is a key contributor to physical frailty; however, the factors responsible for exacerbated vs. muted ageing muscle impact are largely unknown. Based upon evidence that susceptibility to neurogenic impact is an important determinant of the severity of ageing muscle degeneration, we aimed to determine the presence and extent of denervation in pre-frail/frail elderly (FE, 77.9 ± 6.2 years) women compared to young physically inactive (YI, 24.0 ± 3.5 years) females, and contrast these findings to high-functioning world class octogenarian female masters athletes (MA, 80.9 ± 6.6 years). Muscle biopsies from vastus lateralis muscle were obtained from all three groups to assess denervation-related morphological and transcriptional markers. The FE group displayed marked grouping of slow fibres, accumulation of very small myofibres, a severe reduction in type IIa/I size ratio, highly variable inter-subject accumulation of neural cell adhesion molecule (NCAM)-positive myofibres, and an accumulation of pyknotic nuclei, indicative of recurring cycles of denervation/reinnervation and persistent denervation. The MA group exhibited a smaller decline in type IIa/I size ratio and fewer pyknotic nuclei, accompanied by a higher degree of type I fibre grouping and larger fibre group size, suggesting a greater reinnervation of denervated fibres. Consistent with this interpretation, MA had higher mRNA levels of the reinnervation-promoting cytokine fibroblast growth factor binding protein 1 (FGFBP1) than FE. Our results indicate that the muscle of FE women has significant neurogenic atrophy, whereas MA muscle exhibit superior reinnervation capacity, suggesting that the difference in age-related muscle impact between low- and high-functioning elderly women is the robustness of the response to denervation of myofibres.

Smoke-induced neuromuscular junction degeneration precedes the fibre type shift and atrophy in chronic obstructive pulmonary disease.

KEY POINTS: Chronic obstructive pulmonary disease (COPD) is largely caused by smoking, and patient limb muscle exhibits a fast fibre shift and atrophy. We show that this fast fibre shift is associated with type grouping, suggesting recurring cycles of denervation-reinnervation underlie the type shift. Compared to patients with normal fat-free mass index (FFMI), patients with low FFMI exhibited an exacerbated fibre type shift, marked accumulation of very small persistently denervated muscle fibres, and a blunted denervation-responsive transcript profile, suggesting failed denervation precipitates muscle atrophy in patients with low FFMI. Sixteen weeks of passive tobacco smoke exposure in mice caused neuromuscular junction degeneration, consistent with a key role for smoke exposure in initiating denervation in COPD. ABSTRACT: A neurological basis for the fast fibre shift and atrophy seen in limb muscle of patients with chronic obstructive pulmonary disease (COPD) has not been considered previously. The objective of our study was: (1) to determine if denervation contributes to fast fibre shift and muscle atrophy in COPD; and (2) to assess using a preclinical smoking mouse model whether chronic tobacco smoke (TS) exposure could initiate denervation by causing neuromuscular junction (NMJ) degeneration. Vastus lateralis muscle biopsies were obtained from severe COPD patients [n = 10 with low fat-free mass index (FFMI), 65 years; n = 15 normal FFMI, 65 years) and healthy age- and activity-matched non-smoker control subjects (CON; n = 11, 67 years), to evaluate morphological and transcriptional markers of denervation. To evaluate the potential for chronic TS exposure to initiate these changes, we examined NMJ morphology in male adult mice following 16 weeks of passive TS exposure. We observed a high proportion of grouped fast fibres and a denervation transcript profile in COPD patients, suggesting that motor unit remodelling drives the fast fibre type shift in COPD patient limb muscle. A further exacerbation of fast fibre grouping in patients with low FFMI, coupled with blunted reinnervation signals, accumulation of very small non-specific esterase hyperactive fibres and neural cell adhesion molecule-positive type I and type II fibres, suggests denervation-induced exhaustion of reinnervation contributes to muscle atrophy in COPD. Evidence from a smoking mouse model showed significant NMJ degeneration, suggesting that recurring denervation in COPD is probably caused by decades of chronic TS exposure.

Common data elements for clinical research in mitochondrial disease: a National Institute for Neurological Disorders and Stroke project.

OBJECTIVES: The common data elements (CDE) project was developed by the National Institute of Neurological Disorders and Stroke (NINDS) to provide clinical researchers with tools to improve data quality and allow for harmonization of data collected in different research studies. CDEs have been created for several neurological diseases; the aim of this project was to develop CDEs specifically curated for mitochondrial disease (Mito) to enhance clinical research. METHODS: Nine working groups (WGs), composed of international mitochondrial disease experts, provided recommendations for Mito clinical research. They initially reviewed existing NINDS CDEs and instruments, and developed new data elements or instruments when needed. Recommendations were organized, internally reviewed by the Mito WGs, and posted online for external public comment for a period of eight weeks. The final version was again reviewed by all WGs and the NINDS CDE team prior to posting for public use. RESULTS: The NINDS Mito CDEs and supporting documents are publicly available on the NINDS CDE website ( https://commondataelements.ninds.nih.gov/ ), organized into domain categories such as Participant/Subject Characteristics, Assessments, and Examinations. CONCLUSION: We developed a comprehensive set of CDE recommendations, data definitions, case report forms (CRFs), and guidelines for use in Mito clinical research. The widespread use of CDEs is intended to enhance Mito clinical research endeavors, including natural history studies, clinical trial design, and data sharing. Ongoing international collaboration will facilitate regular review, updates and online publication of Mito CDEs, and support improved consistency of data collection and reporting.

Reduction in single muscle fiber rate of force development with aging is not attenuated in world class older masters athletes.

Normal adult aging is associated with impaired muscle contractile function; however, to what extent cross-bridge kinetics are altered in aging muscle is not clear. We used a slacken restretch maneuver on single muscle fiber segments biopsied from the vastus lateralis of young adults (∼23 yr), older nonathlete (NA) adults (∼80 yr), and age-matched world class masters athletes (MA; ∼80 yr) to assess the rate of force redevelopment (ktr) and cross-bridge kinetics. A post hoc analysis was performed, and only the mechanical properties of "slow type" fibers based on unloaded shortening velocity (Vo) measurements are reported. The MA and NA were ∼54 and 43% weaker, respectively, for specific force compared with young. Similarly, when force was normalized to cross-sectional area determined via the fiber shape angularity data, both old groups did not differ, and the MA and NA were ∼43 and 48% weaker, respectively, compared with young (P < 0.05). Vo for both MA and NA old groups was 62 and 46% slower, respectively, compared with young. Both MA and NA adults had approximately two times slower values for ktr compared with young. The slower Vo in both old groups relative to young, coupled with a similarly reduced ktr, suggests impaired cross-bridge kinetics are responsible for impaired single fiber contractile properties with aging. These results challenge the widely accepted resilience of slow type fibers to cellular aging.

Denervation drives mitochondrial dysfunction in skeletal muscle of octogenarians.

KEY POINTS: Mitochondria are frequently implicated in the ageing of skeletal muscle, although the role of denervation in modulating mitochondrial function in ageing muscle is unknown. We show that increased sensitivity to apoptosis initiation occurs prior to evidence of persistent denervation and is thus a primary mitochondrial defect in ageing muscle worthy of therapeutic targeting. However, at more advanced age, mitochondrial function changes are markedly impacted by persistent sporadic myofibre denervation, suggesting the mitochondrion may be a less viable therapeutic target. ABSTRACT: Experimental denervation modulates mitochondrial function, where changes in both reactive oxygen species (ROS) and sensitivity to permeability transition are implicated in the resultant muscle atrophy. Notably, although denervation occurs sporadically in ageing muscle, its impact on ageing muscle mitochondria is unknown. Because this information has important therapeutic implications concerning targeting the mitochondrion in ageing muscle, we examined mitochondrial function in skeletal muscle from four groups of humans, comprising two active (mean ± SD age: 23.7 ± 2.7 years and 71.2 ± 4.9 years) and two inactive groups (64.8 ± 3.1 years and 82.5 ± 4.8 years), and compared this with a murine model of sporadic denervation. We tested the hypothesis that, although some alterations of mitochondrial function in aged muscle are attributable to a primary organelle defect, mitochondrial dysfunction would be impacted by persistent denervation in advanced age. Both ageing in humans and sporadic denervation in mice increased mitochondrial sensitivity to permeability transition (humans, P = 0.004; mice, P = 0.01). To determine the contribution of sporadic denervation to mitochondrial function, we pharmacologically inhibited the denervation-induced ROS response. This reduced ROS emission by 60% (P = 0.02) in sporadically denervated mouse muscle, which is similar to that seen in humans older than 75 years (-66%, P = 0.02) but not those younger than 75 years. We conclude that an increased sensitivity to permeability transition is a primary mitochondrial defect in ageing muscle. However, at more advanced age, when muscle atrophy becomes more clinically severe, mitochondrial function changes are markedly impacted by persistent sporadic denervation, making the mitochondrion a less viable therapeutic target.

Addressing Assumptions for the Use of Non-invasive Cardiac Output Measurement Techniques During Exercise in COPD.

The multifactorial functional limitation of COPD increasingly demonstrates the need for an integrated circulatory assessment. In this study cardiac output (Qc) derived from non-inert (CO2-RB), inert (N2O-RB) gas rebreathing approaches and bioimpedance were compared to examine the limitations of currently available non-invasive techniques for exercise Qc determination in patients with chronic lung disease. Thirteen COPD patients (GOLD II-III) completed three constant cycling bouts at 20, 35, and 50% of peak work on two occasions to assess Qc with bioimpedance as well as using CO2-RB and N2O-RB for all exercise tests. Results showed significantly lower Qc using the N2O-RB or end-tidal CO2-derived Qc compared to the PaCO2-derived CO2-RB or the bioimpedance at rest and for all exercise intensities. End-tidal CO2-derived values are however not statistically different from those obtained using inert-gas rebreathing. This study show that in COPD patients, CO2-rebreathing Qc values obtained using PaCO2 contents which account for any gas exchange impairment or inadequate gas mixing are similar to those obtained using thoracic bioimpedance. Alternately, the lower values for N2O rebreathing derived Qc indicates the inability of this technique to account for gas exchange impairment in the computation of Qc. These findings indicate that the choice of a gas rebreathing technique to measure Qc in patients must be dictated by the ability to include in the derived computations a correction for either gas exchange inadequacies and/or a vascular shunt.

Increased sensitivity to mitochondrial permeability transition and myonuclear translocation of endonuclease G in atrophied muscle of physically active older humans.

Mitochondrial dysfunction is implicated in skeletal muscle atrophy and dysfunction with aging, with strong support for an increased mitochondrial-mediated apoptosis in sedentary rodent models. Whether this applies to aged human muscle is unknown, nor is it clear whether these changes are caused by sedentary behavior. Thus, we examined mitochondrial function [respiration, reactive oxygen species (ROS) emission, and calcium retention capacity (CRC)] in permeabilized myofibers obtained from vastus lateralis muscle biopsies of healthy physically active young (23.7±2.7 yr; mean±SD) and older (71.2±4.9 yr) men. Although mitochondrial ROS and maximal respiratory capacity were unaffected, the acceptor control ratio was reduced by 18% with aging, suggesting mild uncoupling of oxidative phosphorylation. CRC was reduced by 50% with aging, indicating sensitization of the mitochondrial permeability transition pore (mPTP) to apoptosis. Consistent with the mPTP sensitization, older muscles showed a 3-fold greater fraction of endonuclease G (a mitochondrial proapoptotic factor)-positive myonuclei. Aged muscles also had lower mitophagic potential, based on a 43% reduction in Parkin to the voltage-dependent anion channel (VDAC) protein ratio. Collectively, these results show that mitochondrial-mediated apoptotic signaling is increased in older human muscle and suggest that accumulation of dysfunctional mitochondria with exaggerated apoptotic sensitivity is due to impaired mitophagy.

An official American Thoracic Society/European Respiratory Society statement: update on limb muscle dysfunction in chronic obstructive pulmonary disease.

BACKGROUND: Limb muscle dysfunction is prevalent in chronic obstructive pulmonary disease (COPD) and it has important clinical implications, such as reduced exercise tolerance, quality of life, and even survival. Since the previous American Thoracic Society/European Respiratory Society (ATS/ERS) statement on limb muscle dysfunction, important progress has been made on the characterization of this problem and on our understanding of its pathophysiology and clinical implications. PURPOSE: The purpose of this document is to update the 1999 ATS/ERS statement on limb muscle dysfunction in COPD. METHODS: An interdisciplinary committee of experts from the ATS and ERS Pulmonary Rehabilitation and Clinical Problems assemblies determined that the scope of this document should be limited to limb muscles. Committee members conducted focused reviews of the literature on several topics. A librarian also performed a literature search. An ATS methodologist provided advice to the committee, ensuring that the methodological approach was consistent with ATS standards. RESULTS: We identified important advances in our understanding of the extent and nature of the structural alterations in limb muscles in patients with COPD. Since the last update, landmark studies were published on the mechanisms of development of limb muscle dysfunction in COPD and on the treatment of this condition. We now have a better understanding of the clinical implications of limb muscle dysfunction. Although exercise training is the most potent intervention to address this condition, other therapies, such as neuromuscular electrical stimulation, are emerging. Assessment of limb muscle function can identify patients who are at increased risk of poor clinical outcomes, such as exercise intolerance and premature mortality. CONCLUSIONS: Limb muscle dysfunction is a key systemic consequence of COPD. However, there are still important gaps in our knowledge about the mechanisms of development of this problem. Strategies for early detection and specific treatments for this condition are also needed.

Physiological mechanisms of sex differences in exertional dyspnoea: role of neural respiratory motor drive.

NEW FINDINGS: What is the central question of this study? Does the combination of a higher neural respiratory drive and greater dynamic mechanical ventilatory constraints during exercise in healthy women versus men form the mechanistic basis of sex differences in activity-related dyspnoea? What is the main finding and its importance? Sex differences in activity-related dyspnoea in health primarily reflected the awareness of a higher neural respiratory drive needed to achieve any given ventilation during exercise in the setting of relatively greater dynamic mechanical ventilatory constraints in women. These findings may have implications for our understanding of the mechanisms of sex differences in exertional dyspnoea in variants of health (e.g. the elderly) and in patients with cardiorespiratory disease. The purpose of this study was to elucidate the physiological mechanisms of sex differences in exertional dyspnoea. We compared detailed measures of neural respiratory motor drive [diaphragmatic EMG (EMGdi) expressed as a percentage of maximal EMGdi (EMGdi%max)], breathing pattern, operating lung volumes, dynamic respiratory mechanics [tidal oesophageal (P(oes,tida)l%peak) and transdiaphragmatic pressure swings (P(di,tidal)%peak) expressed as a percentage of their respective peak values] and sensory intensity and unpleasantness ratings of dyspnoea during symptom-limited incremental cycle exercise in healthy young women (n = 25) and men (n = 25). The tidal volume to forced vital capacity ratio (V(T)%FVC), breathing frequency, EMGdi%max, P(oes,tidal)%peak, P(di,tidal)%peak and sensory intensity and unpleasantness ratings of dyspnoea were higher, while dynamic inspiratory capacity and inspiratory reserve volume were lower at a standardized absolute ventilation of 55 l min(-1) during submaximal exercise in women versus men (all P < 0.05). In contrast, sex had no demonstrable effect on the inter-relationships between exercise-induced increases in V(T)%FVC, EMGdi%max and sensory intensity and unpleasantness ratings of dyspnoea. The results of this study suggest that sex differences in the intensity and unpleasantness of exertional dyspnoea in health are likely to reflect the awareness of a relatively higher neural respiratory motor drive (or EMGdi%max) needed to achieve any given ventilation during exercise in the setting of relatively greater dynamic mechanical constraints on V(T) expansion in women.

Autophagy in locomotor muscles of patients with chronic obstructive pulmonary disease.

RATIONALE: Locomotor muscle atrophy develops in patients with chronic obstructive pulmonary disease (COPD) partly because of increased protein degradation by the ubiquitin-proteasome system. It is not known if autophagy also contributes to protein degradation. OBJECTIVES: To investigate whether autophagy is enhanced in locomotor muscles of stable patients with COPD, to quantify autophagy-related gene expression in these muscles, and to identify mechanisms of autophagy induction. METHODS: Muscle biopsies were obtained from two cohorts of control subjects and patients with COPD and the numbers of autophagosomes in the vastus lateralis and tibialis anterior muscles, the levels of LC3B protein lipidation, and the expression of autophagy-related genes were measured in the vastus lateralis muscle. To investigate potential pathways that might induce the activation of autophagy, measures were taken of protein kinase B (AKT), mTORC1, and AMPK pathway activation, transcription factor regulation, proteasome activation, and oxidative stress. MEASUREMENTS AND MAIN RESULTS: Autophagy is enhanced in the locomotor muscles of patients with COPD as shown by significantly higher numbers of autophagosomes in affected muscles as compared with control subjects. Autophagosome number inversely correlates with FEV1. In the vastus lateralis, LC3B protein lipidation is increased by COPD and the expression of autophagy-related gene expressions is up-regulated. LC3B lipidation inversely correlates with thigh cross-sectional area, FEV1, and FEV1/FVC ratio. Enhanced autophagy is associated with activation of the AMPK pathway and FOXO transcription factors, inhibition of the mTORC1 and AKT pathways, and the development of oxidative stress. CONCLUSIONS: Autophagy is significantly enhanced in locomotor muscles of stable patients with COPD. The degree of autophagy correlates with severity of muscle atrophy and lung function impairment.

Resting-state Functional Connectivity of the Motor and Cognitive Areas is Preserved in Masters Athletes.

Aging is characterized by a decline in physical and cognitive functions, often resulting in decreased quality of life. Physical activity has been suggested to potentially slow down various aspects of the aging process, a theory that has been supported by studies of Masters Athletes (MA). For example, MA usually have better cognitive and physical functions than age-matched sedentary and healthy older adults (OA), making them a valuable model to gain insights into mechanisms that promote physical and cognitive function with aging. The purpose of this study was to identify differences in resting-state functional connectivity (rs-FC) of motor and cognitive regions between MA and OA and determine if these differences in the resting brain are associated with differences in cognitive and physical performance between groups. Fifteen MA (9 males) and 12 age-matched OA (six males) were included. rs-FC images were compared to identify significant between-groups differences in brain connectivity. There was higher connectivity between the cognitive and motor networks for the OA group, whereas the MA group had stronger connectivity between different regions within the same network, both for the cognitive and the motor networks. These results are in line with the literature suggesting that aging reduces the segregation between functional networks and causes regions within the same network to be less strongly connected. High-level physical activity practiced by the MA most likely contributes to attenuating aging-related changes in brain functional connectivity, preserving clearer boundaries between different functional networks, which may ultimately favor maintenance of efficient cognitive and sensorimotor processing.

Increased capillaries in mitochondrial myopathy: implications for the regulation of oxygen delivery.

Human skeletal muscle respiratory chain defects restrict the ability of working muscle to extract oxygen from blood, and result in a hyperkinetic circulation during exercise in which oxygen delivery is excessive relative to oxygen uptake and oxygen levels within contracting muscle are abnormally high. To investigate the role of the muscle microcirculation in this anomalous circulatory response and possible implications for the regulation of muscle angiogenesis, we assessed muscle oxidative capacity during cycle exercise and determined capillary levels and distribution and vascular endothelial growth factor expression in quadriceps muscle biopsies in patients with mitochondrial myopathy attributable to heteroplasmic mitochondrial DNA mutations. We found that in patients with mitochondrial myopathy, muscle capillary levels were twice that of sedentary healthy subjects (3.0 ± 0.9% compared with 1.4 ± 0.3%, P < 0.001) despite the fact that oxygen utilization during peak cycle exercise was half that of control subjects (11.1 ± 4.0 ml/kg/min compared with 20.7 ± 7.9 ml/kg/min, P < 0.01); that capillary area was greatest in patients with the most severe muscle oxidative defects and was more than two times higher around muscle fibre segments with defective (i.e. cytochrome oxidase negative/succinic dehydrogenase-positive or 'ragged-red' fibres) compared with more preserved respiratory chain function; and that vascular endothelial growth factor expression paralleled capillary distribution. The increased muscle capillary levels in patients correlated directly (r(2) = 0.68, P < 0.05) with the severity of the mismatch between systemic oxygen delivery (cardiac output) and oxygen utilization during cycle exercise. Our results suggest that capillary growth is increased as a result of impaired muscle oxidative phosphorylation in mitochondrial myopathy, thus promoting increased blood flow to respiration-incompetent muscle fibres and a mismatch between oxygen delivery and utilization during exercise. Furthermore, the finding of high capillary levels despite elevated tissue oxygen levels during exercise in respiration-deficient muscle fibres implies that mitochondrial metabolism activates angiogenesis in skeletal muscle by a mechanism that is independent of hypoxia.

Mitochondrial dysfunction and lipid accumulation in the human diaphragm during mechanical ventilation.

RATIONALE: Mechanical ventilation (MV) is associated with adverse effects on the diaphragm, but the cellular basis for this phenomenon, referred to as ventilator-induced diaphragmatic dysfunction (VIDD), is poorly understood. OBJECTIVES: To determine whether mitochondrial function and cellular energy status are disrupted in human diaphragms after MV, and the role of mitochondria-derived oxidative stress in the development of VIDD. METHODS: Diaphragm and biceps specimens obtained from brain-dead organ donors who underwent MV (15-176 h) and age-matched control subjects were compared regarding mitochondrial enzymatic function, mitochondrial DNA integrity, lipid content, and metabolic gene and protein expression. In addition, diaphragmatic force and oxidative stress after exposure to MV for 6 hours were evaluated in mice under different conditions. MEASUREMENTS AND MAIN RESULTS: In human MV diaphragms, mitochondrial biogenesis and content were down-regulated, with a more specific defect of respiratory chain cytochrome-c oxidase. Laser capture microdissection of cytochrome-c oxidase-deficient fibers revealed mitochondrial DNA deletions, consistent with damage from oxidative stress. Diaphragmatic lipid accumulation and responses of master cellular metabolic sensors (AMP-activated protein kinase and sirtuins) were consistent with energy substrate excess as a possible stimulus for these changes. In mice, induction of hyperlipidemia worsened diaphragmatic oxidative stress during MV, whereas transgenic overexpression of a mitochondria-localized antioxidant (peroxiredoxin-3) was protective against VIDD. CONCLUSIONS: Our data suggest that mitochondrial dysfunction lies at the nexus between oxidative stress and the impaired diaphragmatic contractility that develops during MV. Energy substrate oversupply relative to demand, resulting from diaphragmatic inactivity during MV, could play an important role in this process.

OxPhos defects cause hypermetabolism and reduce lifespan in cells and in patients with mitochondrial diseases.

Patients with primary mitochondrial oxidative phosphorylation (OxPhos) defects present with fatigue and multi-system disorders, are often lean, and die prematurely, but the mechanistic basis for this clinical picture remains unclear. By integrating data from 17 cohorts of patients with mitochondrial diseases (n = 690) we find evidence that these disorders increase resting energy expenditure, a state termed hypermetabolism. We examine this phenomenon longitudinally in patient-derived fibroblasts from multiple donors. Genetically or pharmacologically disrupting OxPhos approximately doubles cellular energy expenditure. This cell-autonomous state of hypermetabolism occurs despite near-normal OxPhos coupling efficiency, excluding uncoupling as a general mechanism. Instead, hypermetabolism is associated with mitochondrial DNA instability, activation of the integrated stress response (ISR), and increased extracellular secretion of age-related cytokines and metabokines including GDF15. In parallel, OxPhos defects accelerate telomere erosion and epigenetic aging per cell division, consistent with evidence that excess energy expenditure accelerates biological aging. To explore potential mechanisms for these effects, we generate a longitudinal RNASeq and DNA methylation resource dataset, which reveals conserved, energetically demanding, genome-wide recalibrations. Taken together, these findings highlight the need to understand how OxPhos defects influence the energetic cost of living, and the link between hypermetabolism and aging in cells and patients with mitochondrial diseases.

Integrating Mechanisms of Exacerbated Atrophy and Other Adverse Skeletal Muscle Impact in COPD.

The normal decline in skeletal muscle mass that occurs with aging is exacerbated in patients with chronic obstructive pulmonary disease (COPD) and contributes to poor health outcomes, including a greater risk of death. There has been controversy about the causes of this exacerbated muscle atrophy, with considerable debate about the degree to which it reflects the very sedentary nature of COPD patients vs. being precipitated by various aspects of the COPD pathophysiology and its most frequent proximate cause, long-term smoking. Consistent with the latter view, recent evidence suggests that exacerbated aging muscle loss with COPD is likely initiated by decades of smoking-induced stress on the neuromuscular junction that predisposes patients to premature failure of muscle reinnervation capacity, accompanied by various alterations in mitochondrial function. Superimposed upon this are various aspects of COPD pathophysiology, such as hypercapnia, hypoxia, and inflammation, that can also contribute to muscle atrophy. This review will summarize the available knowledge concerning the mechanisms contributing to exacerbated aging muscle affect in COPD, consider the potential role of comorbidities using the specific example of chronic kidney disease, and identify emerging molecular mechanisms of muscle impairment, including mitochondrial permeability transition as a mechanism of muscle atrophy, and chronic activation of the aryl hydrocarbon receptor in driving COPD muscle pathophysiology.

Unbiased proteomics, histochemistry, and mitochondrial DNA copy number reveal better mitochondrial health in muscle of high-functioning octogenarians.

Background: Master athletes (MAs) prove that preserving a high level of physical function up to very late in life is possible, but the mechanisms responsible for their high function remain unclear. Methods: We performed muscle biopsies in 15 octogenarian world-class track and field MAs and 14 non-athlete age/sex-matched controls (NA) to provide insights into mechanisms for preserving function in advanced age. Muscle samples were assessed for respiratory compromised fibers, mitochondrial DNA (mtDNA) copy number, and proteomics by liquid-chromatography mass spectrometry. Results: MA exhibited markedly better performance on clinical function tests and greater cross-sectional area of the vastus lateralis muscle. Proteomics analysis revealed marked differences, where most of the ~800 differentially represented proteins in MA versus NA pertained to mitochondria structure/function such as electron transport capacity (ETC), cristae formation, mitochondrial biogenesis, and mtDNA-encoded proteins. In contrast, proteins from the spliceosome complex and nuclear pore were downregulated in MA. Consistent with proteomics data, MA had fewer respiratory compromised fibers, higher mtDNA copy number, and an increased protein ratio of the cristae-bound ETC subunits relative to the outer mitochondrial membrane protein voltage-dependent anion channel. There was a substantial overlap of proteins overrepresented in MA versus NA with proteins that decline with aging and that are higher in physically active than sedentary individuals. However, we also found 176 proteins related to mitochondria that are uniquely differentially expressed in MA. Conclusions: We conclude that high function in advanced age is associated with preserving mitochondrial structure/function proteins, with underrepresentation of proteins involved in the spliceosome and nuclear pore complex. Whereas many of these differences in MA appear related to their physical activity habits, others may reflect unique biological (e.g., gene, environment) mechanisms that preserve muscle integrity and function with aging. Funding: Funding for this study was provided by operating grants from the Canadian Institutes of Health Research (MOP 84408 to TT and MOP 125986 to RTH). This work was supported in part by the Intramural Research Program of the National Institute on Aging, NIH, Baltimore, MD, USA.

Chronic aryl hydrocarbon receptor activity phenocopies smoking-induced skeletal muscle impairment.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) patients exhibit skeletal muscle atrophy, denervation, and reduced mitochondrial oxidative capacity. Whilst chronic tobacco smoke exposure is implicated in COPD muscle impairment, the mechanisms involved are ambiguous. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that activates detoxifying pathways with numerous exogenous ligands, including tobacco smoke. Whereas transient AHR activation is adaptive, chronic activation can be toxic. On this basis, we tested the hypothesis that chronic smoke-induced AHR activation causes adverse muscle impact. METHODS: We used clinical patient muscle samples, and in vitro (C2C12 myotubes) and in vivo models (mouse), to perform gene expression, mitochondrial function, muscle and neuromuscular junction morphology, and genetic manipulations (adeno-associated virus-mediated gene transfer). RESULTS: Sixteen weeks of tobacco smoke exposure in mice caused muscle atrophy, neuromuscular junction degeneration, and reduced oxidative capacity. Similarly, smoke exposure reprogrammed the muscle transcriptome, with down-regulation of mitochondrial and neuromuscular junction genes. In mouse and human patient specimens, smoke exposure increased muscle AHR signalling. Mechanistically, experiments in cultured myotubes demonstrated that smoke condensate activated the AHR, caused mitochondrial impairments, and induced an AHR-dependent myotube atrophy. Finally, to isolate the role of AHR activity, expression of a constitutively active AHR mutant without smoke exposure caused atrophy and mitochondrial impairments in cultured myotubes, and muscle atrophy and neuromuscular junction degeneration in mice. CONCLUSIONS: These results establish that chronic AHR activity, as occurs in smokers, phenocopies the atrophy, mitochondrial impairment, and neuromuscular junction degeneration caused by chronic tobacco smoke exposure.