Mitochondrial myopathies: diagnosis, exercise intolerance, and treatment options

Optimized Nutrition in Mitochondrial Disease Correlates to Improved Muscle Fatigue, Strength, and Quality of Life

We sought to prospectively characterize the nutritional status of adults ≥ 19 years (n = 22, 27% males) and children (n = 38, 61% male) with genetically-confirmed primary mitochondrial disease (PMD) to guide development of precision nutritional support strategies to be tested in future clinical trials. We excluded subjects who were exclusively tube-fed. Daily caloric requirements were estimated using World Health Organization (WHO) equations to predict resting energy expenditure (REE) multiplied by an activity factor (AF) based on individual activity levels. We developed a Mitochondrial Disease Activity Factors (MOTIVATOR) score to encompass the impact of muscle fatigue typical of PMD on physical activity levels. PMD cohort daily diet intake was estimated to be 1,143 ± 104.1 kcal in adults (mean ± SEM, 76.2% of WHO-MOTIVATOR predicted requirement), and 1,114 ± 62.3 kcal in children (86.4% predicted). A total of 11/22 (50%) adults and 18/38 (47.4%) children with PMD consumed ≤ 75% predicted daily Kcal needs. Malnutrition was identified in 16/60 (26.7%) PMD subjects. Increased protein and fat intake correlated with improved muscle strength in those with insufficient daily Kcal intake (≤ 75% predicted); higher protein and fat intake correlated with decreased muscle fatigue; and higher protein, fat, and carbohydrate intake correlated with improved quality of life (QoL). These data demonstrate the frequent occurrence of malnutrition in PMD and emphasize the critical need to devise nutritional interventions to optimize clinical outcomes.

Mitochondrial impairment but not peripheral inflammation predicts greater Gulf War illness severity

Gulf War illness (GWI) is an important exemplar of environmentally-triggered chronic multisymptom illness, and a potential model for accelerated aging. Inflammation is the main hypothesized mechanism for GWI, with mitochondrial impairment also proposed. No study has directly assessed mitochondrial respiratory chain function (MRCF) on muscle biopsy in veterans with GWI (VGWI). We recruited 42 participants, half VGWI, with biopsy material successfully secured in 36. Impaired MRCF indexed by complex I and II oxidative phosphorylation with glucose as a fuel source (CI&CIIOXPHOS) related significantly or borderline significantly in the predicted direction to 17 of 20 symptoms in the combined sample. Lower CI&CIIOXPHOS significantly predicted GWI severity in the combined sample and in VGWI separately, with or without adjustment for hsCRP. Higher-hsCRP (peripheral inflammation) related strongly to lower-MRCF (particularly fatty acid oxidation (FAO) indices) in VGWI, but not in controls. Despite this, whereas greater MRCF-impairment predicted greater GWI symptoms and severity, greater inflammation did not. Surprisingly, adjusted for MRCF, higher hsCRP significantly predicted lesser symptom severity in VGWI selectively. Findings comport with a hypothesis in which the increased inflammation observed in GWI is driven by FAO-defect-induced mitochondrial apoptosis. In conclusion, impaired mitochondrial function-but not peripheral inflammation-predicts greater GWI symptoms and severity.

Repeatability assessment for simultaneous measurement of arterial blood flow, venous oxygen saturation, and muscle perfusion following dynamic exercise

The purpose of the present study was to demonstrate a new sequence and determine the repeatability of simultaneous dynamic measurements of blood flow, venous oxygen saturation (SvO₂ ), and relative perfusion (change from resting perfusion) in calf muscle during recovery from plantar flexion exercise. The feasibility of near simultaneous measurement of bio-energetic parameters was also demonstrated. A sequence was developed to simultaneously measure arterial blood flow using flow-encoded projection, SvO₂ using susceptibility-based oximetry, and relative perfusion using arterial spin labeling in combination with dynamic plantar flexion exercise. The parameters were determined at rest and during recovery from single leg plantar flexion exercise. Test-retest repeatability was analyzed using Bland-Altman analysis and intraclass correlation coefficients (ICC). The mitochondrial capacity of skeletal muscle was also measured immediately afterwards with dynamic phosphorus magnetic resonance spectroscopy. Eight healthy subjects participated in the study for test-retest repeatability. Popliteal artery blood flow at rest was 1.79 ± 0.58 ml/s and increased to 11.18 ± 3.02 ml/s immediately after exercise. Popliteal vein SvO₂ decreased to 45.93% ± 6.5% from a resting value of 70.46% ± 4.76% following exercise. Relative perfusion (change from rest value) was 51.83 ± 15.00 ml/100 g/min at the cessation of exercise. The recovery of blood flow and SvO₂ was modeled as a single exponential with time constants of 38.03 ± 6.91 and 71.19 ± 14.53 s, respectively. All the measured parameters exhibited good repeatability with ICC ranging from 0.8 to 0.95. Bioenergetics measurements were within normal range, demonstrating the feasibility of near simultaneous measurement of hemodynamic and energetic parameters. Clinical feasibility was assessed with Barth syndrome patients, demonstrating reduced oxygen extraction from the blood and reduced mitochondrial oxidative capacity compared with healthy controls. The proposed protocol allows rapid imaging of multiple parameters in skeletal muscle that might be affected in disease.

Mitochondrial respiratory quiescence: A new model for examining the role of mitochondrial metabolism in development

Mitochondria are vital organelles with a central role in all aspects of cellular metabolism. As a means to support the ever-changing demands of the cell, mitochondria produce energy, drive biosynthetic processes, maintain redox homeostasis, and function as a hub for cell signaling. While mitochondria have been widely studied for their role in disease and metabolic dysfunction, this organelle has a continually evolving role in the regulation of development, wound repair, and regeneration. Mitochondrial metabolism dynamically changes as tissues transition through distinct phases of development. These organelles support the energetic and biosynthetic demands of developing cells and function as key structures that coordinate the nutrient status of the organism with developmental progression. This review will examine the mechanisms that link mitochondria to developmental processes. We will also examine the process of mitochondrial respiratory quiescence (MRQ), a novel mechanism for regulating cellular metabolism through the biochemical and physiological remodeling of mitochondria. Lastly, we will examine MRQ as a system to discover the mechanisms that drive mitochondrial remodeling during development.

Currently available therapies in mitochondrial disease

Mitochondrial diseases are a heterogeneous group of multisystem disorders caused by impaired mitochondrial function. These disorders occur at any age and involve any tissue, typically affecting organs highly dependent on aerobic metabolism. Diagnosis and management are extremely difficult due to various underlying genetic defects and a wide range of clinical symptoms. Preventive care and active surveillance are strategies to try to reduce morbidity and mortality by timely treatment of organ-specific complications. More specific interventional therapies are in early phases of development and no effective treatment or cure currently exists. A variety of dietary supplements have been utilized based on biological logic. For several reasons, few randomized controlled trials have been completed to assess the efficacy of these supplements. The majority of the literature on supplement efficacy represents case reports, retrospective analyses and open-label studies. We briefly review selected supplements that have some degree of clinical research support. In mitochondrial diseases, potential triggers of metabolic decompensation or medications that are potentially toxic to mitochondrial function should be avoided. We shortly summarize current recommendations on safe medication in mitochondrial diseases. Finally, we focus on the frequent and debilitating symptoms of exercise intolerance and fatigue and their management including physical training strategies.

Unexplained dyspnea linked to mitochondrial myopathy following military deployment to Southwest Asia and Afghanistan

We identified a case of probable mitochondrial myopathy (MM) in a soldier with dyspnea and reduced exercise tolerance through cardiopulmonary exercise testing (CPET) following Southwest Asia (SWA) deployment. Muscle biopsy showed myopathic features. We compared demographic, occupational exposure, and clinical characteristics in symptomatic military deployers with and without probable MM diagnosed by CPET criteria. We evaluated 235 symptomatic military personnel who deployed to SWA and/or Afghanistan between 2010 and 2021. Of these, 168 underwent cycle ergometer maximal CPET with an indwelling arterial line. We defined probable MM based on five CPET criteria: arterial peak exercise lactate >12 mEq/L, anaerobic threshold (AT) ≤50%, maximum oxygen consumption (VO2max ) <95% predicted, oxygen (O2) pulse percent predicted (pp) at least 10% lower than heart rate pp, and elevated ventilatory equivalent for O2 at end exercise (VE/VO2 ≥ 40). We characterized demographics, smoking status/pack-years, spirometry, and deployment exposures, and used descriptive statistics to compare findings in those with and without probable MM. We found 9/168 (5.4%) deployers with probable MM. Compared to symptomatic deployers without probable MM, they were younger (p = 0.0025) and had lower mean BMI (p = 0.02). They had a higher mean forced expiratory volume (FEV1)pp (p = 0.02) and mean arterial oxygen partial pressure (PaO2) at maximum exercise (p = 0.0003). We found no significant differences in smoking status, deployment frequency/duration, or inhalational exposures. Our findings suggest that mitochondrial myopathy may be a cause of dyspnea and reduced exercise tolerance in a subset of previously deployed military personnel. CPET with arterial line may assist with MM diagnosis and management.

Mitochondrial Dysfunction as an Underlying Cause of Skeletal Muscle Disorders

Mitochondria are an important energy source in skeletal muscle. A main function of mitochondria is the generation of ATP for energy through oxidative phosphorylation (OXPHOS). Mitochondrial defects or abnormalities can lead to muscle disease or multisystem disease. Mitochondrial dysfunction can be caused by defective mitochondrial OXPHOS, mtDNA mutations, Ca2+ imbalances, mitochondrial-related proteins, mitochondrial chaperone proteins, and ultrastructural defects. In addition, an imbalance between mitochondrial fusion and fission, lysosomal dysfunction due to insufficient biosynthesis, and/or defects in mitophagy can result in mitochondrial damage. In this review, we explore the association between impaired mitochondrial function and skeletal muscle disorders. Furthermore, we emphasize the need for more research to determine the specific clinical benefits of mitochondrial therapy in the treatment of skeletal muscle disorders.

Impact of nutraceuticals and dietary supplements on mitochondria modifications in healthy aging: a systematic review of randomized controlled trials

BACKGROUND

To date, the mitochondrial function has been related to several pathways involved in the cellular aging process. Dietary supplements might have reciprocal and multilevel interactions with mitochondria network; however, no systematic review assessed the role of different nutraceuticals in mitochondria modification of healthy older adults.

AIM

To assess the effects of different dietary supplements on mitochondria modifications in older adults.

METHODS

On February 22, 2022, PubMed, Scopus, Web of Science, and Cochrane were systematically searched from inception for randomized controlled trials (RCTs). According to PICO model, we considered healthy older adults as participants, nutraceutical treatment as intervention, any treatment as comparator, mitochondrial modifications as outcome. Jadad scale was used for the quality assessment.

RESULTS

Altogether, 8489 records were identified and screened until 6 studies were included. A total of 201 healthy older adults were included in the systematic review (mean age ranged from 67.0 ± 1.0 years to 76.0 ± 5.6 years). The dietary supplements assessed were sodium nitrite, N-3 polyunsaturated fatty acids, hydrogen-rich water, nicotinamide riboside, urolithin A, and whey protein powder. Positive effects were reported in terms of mitochondrial oxidative and antioxidant capacity, volume, bioenergetic capacity, and mitochondrial transcriptome based on the nutritional supplements. The quality assessment underlined that all the studies included were of good quality.

DISCUSSION

Although dietary supplements might provide positive effects on mitochondria modifications, few studies are currently available in this field.

CONCLUSION

Further studies are needed to better elucidate the reciprocal and multilevel interactions between nutraceuticals, mitochondria, and environmental stressors in healthy older adults.

Mitochondrial Stress Responses in Duchenne muscular dystrophy: Metabolic Dysfunction or Adaptive Reprogramming?

Mitochondrial stress may be a secondary contributor to muscle weakness in inherited muscular dystrophies. Duchenne muscular dystrophy has received the majority of attention whereby most discoveries suggest mitochondrial ATP synthesis may be reduced. However, not all studies support this finding. Furthermore, some studies have reported increased mitochondrial reactive oxygen species and propensity for permeability transition pore formation as an inducer of apoptosis, although divergent findings have also been described. A closer examination of the literature suggests the degree and direction of mitochondrial stress responses may depend on the progression of the disease, the muscle type examined, the mouse model employed with regards to pre-clinical research, the precise metabolic pathways in consideration, and in some cases the in vitro technique used to assess a given mitochondrial bioenergetic function. One intent of this review is to provide careful considerations for future experimental designs to resolve the heterogeneous nature of mitochondrial stress during the progression of Duchenne muscular dystrophy. Such considerations have implications for other muscular dystrophies as well which are addressed briefly herein. A renewed perspective of the term 'mitochondrial dysfunction' is presented whereby stress responses might be re-explored in future investigations as direct contributors to myopathy vs an adaptive 'reprogramming' intended to maintain homeostasis in the face of disease stressors themselves. In so doing, the prospective development of mitochondrial enhancement therapies can be driven by advances in perspectives as much as experimental approaches when resolving the precise relationships between mitochondrial remodelling and muscle weakness in Duchenne and, indeed, other muscular dystrophies.

In vivo assessment of OXPHOS capacity using 3 T CrCEST MRI in Friedreich's ataxia

BACKGROUND

Friedreich's ataxia (FRDA) is a neurodegenerative disease caused by decreased expression of frataxin, a protein involved in many cellular metabolic processes, including mitochondrial oxidative phosphorylation (OXPHOS). Our objective was to assess skeletal muscle oxidative metabolism in vivo in adults with FRDA as compared to adults without FRDA using chemical exchange saturation transfer (CrCEST) MRI, which measures free creatine (Cr) over time following an in-magnet plantar flexion exercise.

METHODS

Participants included adults with FRDA (n = 11) and healthy adults (n = 25). All underwent 3-Tesla CrCEST MRI of the calf before and after in-scanner plantar flexion exercise. Participants also underwent whole-body dual-energy X-ray absorptiometry (DXA) scans to measure body composition and completed questionnaires to assess physical activity.

RESULTS

We found prolonged post-exercise exponential decline in CrCEST (τCr) in the lateral gastrocnemius (LG, 274 s vs. 138 s, p = 0.01) in adults with FRDA (vs. healthy adults), likely reflecting decreased OXPHOS capacity. Adults with FRDA (vs. healthy adults) also engaged different muscle groups during exercise, as indicated by muscle group-specific changes in creatine with exercise (∆CrCEST), possibly reflecting decreased coordination. Across all participants, increased adiposity and decreased usual physical activity were associated with smaller ∆CrCEST.

CONCLUSION

In FRDA, CrCEST MRI may be a useful biomarker of muscle-group-specific decline in OXPHOS capacity that can be leveraged to track within-participant changes over time. Appropriate participant selection and further optimization of the exercise stimulus will enhance the utility of this technique.

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 "P_i 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 P_i can account for the training-induced increase in V̇O_2max, 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.

Mechanisms of the effect of oxidative phosphorylation deficiencies on the skeletal muscle bioenergetic system in patients with mitochondrial myopathies

Simulations carried out using a previously developed model of the skeletal muscle bioenergetic system, involving the "inorganic phosphate (P_i) double-threshold" mechanism of muscle fatigue, lead to the conclusion that a decrease in the oxidative phosphorylation (OXPHOS) activity, caused by mutations in mitochondrial or nuclear DNA, is the main mechanism underlying the changes in the kinetic properties of the system in mitochondrial myopathies (MM). These changes generally involve the very-heavy-exercise-like behavior and exercise termination because of fatigue at low work intensities. In particular, a sufficiently large (at a given work intensity) decrease in OXPHOS activity leads to slowing of the primary phase II of the oxygen uptake (V̇o₂) on-kinetics, decrease in maximal V̇o₂ (V̇o_2max), appearance of the slow component of the V̇o₂ on-kinetics, exercise intolerance, and lactic acidosis at relatively low power outputs encountered in experimental studies in patients with MM. Thus, the "P_i double-threshold" mechanism of muscle fatigue is able to account, at least semiquantitatively, for various kinetic effects of inborn OXPHOS deficiencies of the skeletal muscle bioenergetic system. Exercise can be potentially lengthened and V̇o_2max elevated in patients with MM through an increase in peak P_i (Pi_peak), at which exercise is terminated because of fatigue. Generally, a mechanism underlying the kinetic effects of OXPHOS deficiencies on the skeletal muscle bioenergetic system in MM is proposed that was absent in the literature.NEW & NOTEWORTHY A mechanism of the OXPHOS deficiencies-induced changes of the skeletal muscle bioenergetic system in patients with mitochondrial myopathies (MM), namely, appearance of the slow component of the V̇o₂ on-kinetics at relatively low work intensities, slowed primary phase II of the V̇o₂ on-kinetics, lowered V̇o_2max, and lactic acidosis is proposed. It involves a decrease in OXPHOS activity acting through the "P_i double-threshold" mechanism of muscle fatigue comprising initiation of the additional ATP usage and termination of exercise.

A novel exercise testing algorithm to diagnose mitochondrial myopathy

INTRODUCTION

Oxygen uptake efficiency slope (OUES) is a noninvasive cardiopulmonary exercise testing (CPET) measurement based on oxygen uptake (V˙O₂ ) and minute ventilation (V˙E) and is a marker of the efficiency of oxygen utilization by the body. However, it has not been studied in mitochondrial disorders. We explored noninvasive CPET parameters, including OUES, as a way to reliably diagnose mitochondrial myopathy.

METHODS

We performed cycle ergometer maximal exercise testing on definite and suspected mitochondrial myopathy subjects (MM-D and MM-S) and their age- and sex-matched controls. OUES was corrected for body surface area (OUES/BSA) to eliminate the effect of body size.

RESULTS

A total of 40 participants, including 20 MM-D (n = 13; 6 males; aged 14-64 years) and 7 MM-S (5 males, aged 11-30 years) subjects and 20 controls, completed the study. MM-D subjects showed lower aerobic fitness than controls. OUES/BSA was lower in MM-D subjects, suggesting inefficient oxygen utilization. Area under the curve (AUC) and 95% confidence interval (CI) for OUES/BSA (AUC, 0.91; 95% CI, 0.80-1.00), peak V˙O₂ percent predicted (AUC, 0.95; 95% CI, 0.86-1.00), and V˙O₂ /work slope (AUC, 0.94; 95% CI, 0.85-1.00) showed excellent ability to diagnose mitochondrial myopathy in MM-D subjects. We applied a diagnostic approach based on the parameters just noted to MM-S subjects and their controls and were able to support or disprove the diagnosis of mitochondrial myopathy.

DISCUSSION

We proposed and applied an approach based on the aformentioned three CPET parameters to diagnose mitochondrial myopathy reliably and found it to be clinically useful.

Skeletal muscle mitochondria in health and disease

Mitochondrial activity warrants energy supply to oxidative myofibres to sustain endurance workload. The maintenance of mitochondrial homeostasis is ensured by the control of fission and fusion processes and by the mitophagic removal of aberrant organelles. Many diseases are due to or characterized by dysfunctional mitochondria, and altered mitochondrial dynamics or turnover trigger myopathy per se. In this review, we will tackle the role of mitochondrial dynamics, turnover and metabolism in skeletal muscle, both in health and disease.

Targeted Biomedical Treatment for Autism Spectrum Disorders

BACKGROUND

A diagnosis of autism spectrum disorders (ASD) represents presentations with impairment in communication and behaviour that vary considerably in their clinical manifestations and etiology as well as in their likely pathophysiology. A growing body of data indicates that the deleterious effect of oxidative stress, mitochondrial dysfunction, immune dysregulation and neuroinflammation, as well as their interconnections are important aspects of the pathophysiology of ASD. Glutathione deficiency decreases the mitochondrial protection against oxidants and tumor necrosis factor (TNF)-α; immune dysregulation and inflammation inhibit mitochondrial function through TNF-α; autoantibodies against the folate receptors underpin cerebral folate deficiency, resulting in disturbed methylation, and mitochondrial dysfunction. Such pathophysiological processes can arise from environmental and epigenetic factors as well as their combined interactions, such as environmental toxicant exposures in individuals with (epi)genetically impaired detoxification. The emerging evidence on biochemical alterations in ASD is forming the basis for treatments aimed to target its biological underpinnings, which is of some importance, given the uncertain and slow effects of the various educational interventions most commonly used.

METHODS

Literature-based review of the biomedical treatment options for ASD that are derived from established pathophysiological processes.

RESULTS

Most proposed biomedical treatments show significant clinical utility only in ASD subgroups, with specified pre-treatment biomarkers that are ameliorated by the specified treatment. For example, folinic acid supplementation has positive effects in ASD patients with identified folate receptor autoantibodies, whilst the clinical utility of methylcobalamine is apparent in ASD patients with impaired methylation capacity. Mitochondrial modulating cofactors should be considered when mitochondrial dysfunction is evident, although further research is required to identify the most appropriate single or combined treatment. Multivitamins/multiminerals formulas, as well as biotin, seem appropriate following the identification of metabolic abnormalities, with doses tapered to individual requirements. A promising area, requiring further investigations, is the utilization of antipurinergic therapies, such as low dose suramin.

CONCLUSION

The assessment and identification of relevant physiological alterations and targeted intervention are more likely to produce positive treatment outcomes. As such, current evidence indicates the utility of an approach based on personalized and evidence-based medicine, rather than treatment targeted to all that may not always be beneficial (primum non nocere).

Carnitine Palmitoyl Transferase Deficiency in a University Immunology Practice

PURPOSE

This report describes the clinical manifestations of 35 patients sent to a University Immunology clinic with a diagnosis of fatigue and exercise intolerance who were identified to have low carnitine palmitoyl transferase activity on muscle biopsies.

RECENT FINDINGS

All of the patients presented with fatigue and exercise intolerance and many had been diagnosed with fibromyalgia. Their symptoms responded to treatment of the metabolic disease. Associated symptoms included bloating, diarrhea, constipation, gastrointestinal reflux symptoms, recurrent infections, arthritis, dyspnea, dry eye, visual loss, and hearing loss. Associated medical conditions included Hashimoto thyroiditis, Sjogren's syndrome, seronegative arthritis, food hypersensitivities, asthma, sleep apnea, and vasculitis. This study identifies clinical features that should alert physicians to the possibility of an underlying metabolic disease. Treatment of the metabolic disease leads to symptomatic improvement.

Expressing a Z-disk nebulin fragment in nebulin-deficient mouse muscle: effects on muscle structure and function

BACKGROUND

Nebulin is a critical thin filament-binding protein that spans from the Z-disk of the skeletal muscle sarcomere to near the pointed end of the thin filament. Its massive size and actin-binding property allows it to provide the thin filaments with structural and regulatory support. When this protein is lost, nemaline myopathy occurs. Nemaline myopathy causes severe muscle weakness as well as structural defects on a sarcomeric level. There is no known cure for this disease.

METHODS

We studied whether sarcomeric structure and function can be improved by introducing nebulin's Z-disk region into a nebulin-deficient mouse model (Neb cKO) through adeno-associated viral (AAV) vector therapy. Following this treatment, the structural and functional characteristics of both vehicle-treated and AAV-treated Neb cKO and control muscles were studied.

RESULTS

Intramuscular injection of this AAV construct resulted in a successful expression of the Z-disk fragment within the target muscles. This expression was significantly higher in Neb cKO mice than control mice. Analysis of protein expression revealed that the nebulin fragment was localized exclusively to the Z-disks and that Neb cKO expressed the nebulin fragment at levels comparable to the level of full-length nebulin in control mice. Additionally, the Z-disk fragment displaced full-length nebulin in control mice, resulting in nemaline rod body formation and a worsening of muscle function. Neb cKO mice experienced a slight functional benefit from the AAV treatment, with a small increase in force and fatigue resistance. Disease progression was also slowed as indicated by improved muscle structure and myosin isoform expression.

CONCLUSIONS

This study reveals that nebulin fragments are well-received by nebulin-deficient mouse muscles and that limited functional benefits are achievable.

Unexplained exertional intolerance associated with impaired systemic oxygen extraction

PURPOSE

The clinical investigation of exertional intolerance generally focuses on cardiopulmonary diseases, while peripheral factors are often overlooked. We hypothesize that a subset of patients exists whose predominant exercise limitation is due to abnormal systemic oxygen extraction (SOE).

METHODS

We reviewed invasive cardiopulmonary exercise test (iCPET) results of 313 consecutive patients presenting with unexplained exertional intolerance. An exercise limit due to poor SOE was defined as peak exercise (Ca-vO₂)/[Hb] ≤ 0.8 and VO_2max < 80% predicted in the absence of a cardiac or pulmonary mechanical limit. Those with peak (Ca-vO₂)/[Hb] > 0.8, VO_2max ≥ 80%, and no cardiac or pulmonary limit were considered otherwise normal. The otherwise normal group was divided into hyperventilators (HV) and normals (NL). Hyperventilation was defined as peak PaCO₂ < [1.5 × HCO₃ + 6].

RESULTS

Prevalence of impaired SOE as the sole cause of exertional intolerance was 12.5% (32/257). At peak exercise, poor SOE and HV had less acidemic arterial blood compared to NL (pHa = 7.39 ± 0.05 vs. 7.38 ± 0.05 vs. 7.32 ± 0.02, p < 0.001), which was explained by relative hypocapnia (PaCO₂ = 29.9 ± 5.4 mmHg vs. 31.6 ± 5.4 vs. 37.5 ± 3.4, p < 0.001). For a subset of poor SOE, this relative alkalemia, also seen in mixed venous blood, was associated with a normal PvO₂ nadir (28 ± 2 mmHg vs. 26 ± 4, p = 0.627) but increased SvO₂ at peak exercise (44.1 ± 5.2% vs. 31.4 ± 7.0, p < 0.001).

CONCLUSIONS

We identified a cohort of patients whose exercise limitation is due only to systemic oxygen extraction, due to either an intrinsic abnormality of skeletal muscle mitochondrion, limb muscle microcirculatory dysregulation, or hyperventilation and left shift the oxyhemoglobin dissociation curve.

Neuromuscular and Muscle Metabolic Functions in MELAS Before and After Resistance Training: A Case Study

Mitochondrial encephalomyopathy, lactic acidosis, and recurrent stroke-like episodes syndrome (MELAS) is a rare degenerative disease. Recent studies have shown that resistant training (RT) can ameliorate muscular force in mitochondrial diseases. However, the effects of RT in MELAS are unknown. The aim of this case report was to investigate the effects of RT on skeletal muscle and mitochondrial function in a 21-years old patient with MELAS. RT included 12 weeks of RT at 85% of 1 repetition maximum. Body composition (DXA), in vivo mitochondrial respiration capacity (mVO₂) utilizing Near-infrared spectroscopy on the right plantar-flexor muscles, maximal voluntary torque (MVC), electrically evoked resting twitch (EET) and maximal voluntary activation (VMA) of the right leg extensors (LE) muscles were measured with the interpolated twitch technique. The participant with MELAS exhibited a marked increase in body mass (1.4 kg) and thigh muscle mass (0.3 kg). After the training period MVC (+5.5 Nm), EET (+2.1 N⋅m) and VMA (+13.1%) were ameliorated. Data of mVO₂ revealed negligible changes in the end-exercise mVO₂ (0.02 mM min^-1), Δ mVO₂ (0.09 mM min^-1), while there was a marked amelioration in the kinetics of mVO₂ (τ mVO₂; Δ70.2 s). This is the first report of RT-induced ameliorations on skeletal muscle and mitochondrial function in MELAS. This case study suggests a preserved plasticity in the skeletal muscle of a patient with MELAS. RT appears to be an effective method to increase skeletal muscle function, and this effect is mediated by both neuromuscular and mitochondrial adaptations.

Perspectives on Exertional Rhabdomyolysis

Exertional (exercise-induced) rhabdomyolysis is a potentially life threatening condition that has been the subject of research, intense discussion, and media attention. The causes of rhabdomyolysis are numerous and can include direct muscle injury, unaccustomed exercise, ischemia, extreme temperatures, electrolyte abnormalities, endocrinologic conditions, genetic disorders, autoimmune disorders, infections, drugs, toxins, and venoms. The objective of this article is to review the literature on exertional rhabdomyolysis, identify precipitating factors, and examine the role of the dietary supplement creatine monohydrate. PubMed and SPORTDiscus databases were searched using the terms rhabdomyolysis, muscle damage, creatine, creatine supplementation, creatine monohydrate, and phosphocreatine. Additionally, the references of papers identified through this search were examined for relevant studies. A meta-analysis was not performed. Although the prevalence of rhabdomyolysis is low, instances still occur where exercise is improperly prescribed or used as punishment, or incomplete medical history is taken, and exertional rhabdomyolysis occurs. Creatine monohydrate does not appear to be a precipitating factor for exertional rhabdomyolysis. Healthcare professionals should be able to recognize the basic signs of exertional rhabdomyolysis so prompt treatment can be administered. For the risk of rhabdomyolysis to remain low, exercise testing and prescription must be properly conducted based on professional standards.

Cardiopulmonary Exercise Testing and Metabolic Myopathies

Skeletal muscle requires a large increase in its ATP production to meet the energy needs of exercise. Normally, most of this increase in ATP is supplied by the aerobic process of oxidative phosphorylation. The main defects in muscle metabolism that interfere with production of ATP are (1) disorders of glycogenolysis and glycolysis, which prevent both carbohydrate entering the tricarboxylic acid cycle and the production of lactic acid; (2) mitochondrial myopathies where the defect is usually within the electron transport chain, reducing the rate of oxidative phosphorylation; and (3) disorders of lipid metabolism. Gas exchange measurements derived from exhaled gas analysis during cardiopulmonary exercise testing can identify defects in muscle metabolism because [Formula: see text]o₂ and [Formula: see text]co₂ are abnormal at the level of the muscle. Cardiopulmonary exercise testing may thus suggest a likely diagnosis and guide additional investigation. Defects in glycogenolysis and glycolysis are identified by a low peak [Formula: see text]o₂ and absence of excess [Formula: see text]co₂ from buffering of lactic acid by bicarbonate. Defects in the electron transport chain also result in low peak [Formula: see text]o₂, but because there is an overreliance on anaerobic processes, lactic acid accumulation and excess carbon dioxide from buffering occur early during exercise. Defects in lipid metabolism result in only minor abnormalities during cardiopulmonary exercise testing. In defects of glycogenolysis and glycolysis and in mitochondrial myopathies, other features may include an exaggerated cardiovascular response to exercise, a low oxygen-pulse, and excessive ammonia release.

A pivotal role of AMPK signaling in medicarpin-mediated formation of brown and beige

Obesity poses a substantial threat of a worldwide epidemic and requires better understanding of adipose-tissue biology as well as necessitates research into the etiology and therapeutic interventions. In this study, Medicarpin (Med), a natural pterocarpan, was selected (by screening) as a small-molecule inducer of adipocyte differentiation among 854 candidates by using C3H10T1/2 mesenchymal stem cell; a cellular model of adipogenesis. Med induced the expression of brown-adipocyte commitment marker Bmp7 as well as the early regulators of brown fat fate Pparγ, Prdm16, and Pgc-1α during differentiation of C3H10T1/2 mesenchymal stem cells. Med also induced the expression of a key thermogenic marker-uncoupling protein 1 (UCP1)-along with expression of other brown-fat-specific markers and beige-fat-specific markers. Of note, Med significantly reduced the expression of white fat markers too. Furthermore, Med treatment promoted formation of multilocular lipid droplets (LDs), expression of mitochondrial-biogenesis-related genes, and increased oxygen consumption. Gene silencing study revealed that Med promotes the development of brown- and beige-adipocyte characteristics in C3H10T1/2 mesenchymal stem cells through activation of the AMPK pathway, and our data allow us to propose Med as a candidate for therapeutics against obesity or related metabolic disorders. © 2017 BioFactors, 44(2):168-179, 2018.

Side alternating vibration training in patients with mitochondrial disease: a pilot study

Background

Side alternating vibration training (SAVT) is a mechanical oscillation using a vibrating platform that simulates exercise. We hypothesized that patients with mitochondrial myopathies, who experience muscle weakness, may see an improvement in muscle power with SAVT.

Methods

Patients with mitochondrial disease started either a treatment (SAVT) or control phase (standing without vibration) for 12 weeks, then 12 weeks of washout, and then a 12-week cross-over. The main outcome measure was peak jump power (PJP). We compared this to a natural history cohort from clinic.

Results

Seven out of 13 patients completed at least 80% of their SAVT sessions and were analyzed. The ΔPJP after the control phase was -2.7 ± 1.7 W/kg (mean ± SEM), SAVT was +2.8 ± 0.6 W/kg (p < 0.05) and from the natural history cohort was -2.4 ± 0.8 W/kg/year.

Conclusions

SAVT is well tolerated and may improve muscle power in mitochondrial disease patients.

Cryptotanshinone promotes commitment to the brown adipocyte lineage and mitochondrial biogenesis in C3H10T1/2 mesenchymal stem cells via AMPK and p38-MAPK signaling

Although white adipose tissue (WAT) stores triglycerides and contributes to obesity, brown adipose tissue (BAT) dissipates energy as heat. Therefore, browning of WAT is regarded as an attractive way to counteract obesity. Our previous studies have revealed that treatment with cryptotanshinone (CT) during adipogenesis of 3T3-L1 cells inhibits their differentiation. Here, we found that pretreatment of C3H10T1/2 mesenchymal stem cells with CT before exposure to adipogenic hormonal stimuli promotes the commitment of these mesenchymal stem cells to the adipocyte lineage as confirmed by increased triglyceride accumulation. Furthermore, CT treatment induced the expression of early B-cell factor 2 (Ebf2) and bone morphogenetic protein 7 (Bmp7), which are known to drive differentiation of C3H10T1/2 mesenchymal stem cells toward preadipocytes and to the commitment to brown adipocytes. Consequently, CT treatment yielded brown-adipocyte-like features as evidenced by elevated expression of brown-fat signature genes including Ucp1, Prdm16, Pgc-1α, Cidea, Zic1, and beige-cell-specific genes such as CD137, Hspb7, Cox2, and Tmem26. Additionally, CT treatment induced mitochondrial biogenesis through upregulation of Sirt1, Tfam, Nrf1, and Cox7a and increased mitochondrial mass and DNA content. Our data also showed that cotreatment with CT and BMP4 was more effective at activating brown-adipocyte-specific genes. Mechanistic experiments revealed that treatment with CT activated AMPKα and p38-MAPK via their phosphorylation: the two major signaling pathways regulating energy metabolism. Thus, these findings suggest that CT is a candidate therapeutic agent against obesity working via activation of browning and mitochondrial biogenesis in C3H10T1/2 mesenchymal stem cells.

The use of high-throughput screening techniques to evaluate mitochondrial toxicity

Toxicologists and chemical regulators depend on accurate and effective methods to evaluate and predict the toxicity of thousands of current and future compounds. Robust high-throughput screening (HTS) experiments have the potential to efficiently test large numbers of chemical compounds for effects on biological pathways. HTS assays can be utilized to examine chemical toxicity across multiple mechanisms of action, experimental models, concentrations, and lengths of exposure. Many agricultural, industrial, and pharmaceutical chemicals classified as harmful to human and environmental health exert their effects through the mechanism of mitochondrial toxicity. Mitochondrial toxicants are compounds that cause a decrease in the number of mitochondria within a cell, and/or decrease the ability of mitochondria to perform normal functions including producing adenosine triphosphate (ATP) and maintaining cellular homeostasis. Mitochondrial dysfunction can lead to apoptosis, necrosis, altered metabolism, muscle weakness, neurodegeneration, decreased organ function, and eventually disease or death of the whole organism. The development of HTS techniques to identify mitochondrial toxicants will provide extensive databases with essential connections between mechanistic mitochondrial toxicity and chemical structure. Computational and bioinformatics approaches can be used to evaluate compound databases for specific chemical structures associated with toxicity, with the goal of developing quantitative structure-activity relationship (QSAR) models and mitochondrial toxicophores. Ultimately these predictive models will facilitate the identification of mitochondrial liabilities in consumer products, industrial compounds, pharmaceuticals and environmental hazards.

Altered skeletal muscle (mitochondrial) properties in patients with mitochondrial DNA single deletion myopathy

Background

Mitochondrial myopathy severely affects skeletal muscle structure and function resulting in defective oxidative phosphorylation. However, the major pathomechanisms and therewith effective treatment approaches remain elusive. Therefore, the aim of the present study was to investigate disease-related impairments in skeletal muscle properties in patients with mitochondrial myopathy. Accordingly, skeletal muscle biopsies were obtained from six patients with moleculargenetically diagnosed mitochondrial myopathy (one male and five females, 53 ± 9 years) and eight age- and gender-matched healthy controls (two males and six females, 58 ± 14 years) to determine mitochondrial respiratory capacity of complex I-V, mitochondrial volume density and fiber type distribution.

Results

Mitochondrial volume density (4.0 ± 0.5 vs. 5.1 ± 0.8 %) as well as respiratory capacity of complex I-V were lower (P < 0.05) in mitochondrial myopathy and associated with a higher (P < 0.001) proportion of type II fibers (65.2 ± 3.6 vs. 44.3 ± 5.9 %). Additionally, mitochondrial volume density and maximal oxidative phosphorylation capacity correlated positively (P < 0.05) to peak oxygen uptake.

Conclusion

Mitochondrial myopathy leads to impaired mitochondrial quantity and quality and a shift towards a more glycolytic skeletal muscle phenotype.

Primary Mitochondrial Disease and Secondary Mitochondrial Dysfunction: Importance of Distinction for Diagnosis and Treatment

Mitochondrial disease refers to a heterogeneous group of disorders resulting in defective cellular energy production due to abnormal oxidative phosphorylation (oxphos). Primary mitochondrial disease (PMD) is diagnosed clinically and ideally, but not always, confirmed by a known or indisputably pathogenic mitochondrial DNA (mtDNA) or nuclear DNA (nDNA) mutation. The PMD genes either encode oxphos proteins directly or they affect oxphos function by impacting production of the complex machinery needed to run the oxphos process. However, many disorders have the 'mitochondrial' phenotype without an identifiable mtDNA or nDNA mutation or they have a variant of unknown clinical significance. Secondary mitochondrial dysfunction (SMD) can be caused by genes encoding neither function nor production of the oxphos proteins and accompanies many hereditary non-mitochondrial diseases. SMD may also be due to nongenetic causes such as environmental factors. In our practice, we see many patients with clinical signs of mitochondrial dysfunction based on phenotype, biomarkers, imaging, muscle biopsy, or negative/equivocal mtDNA or nDNA test results. In these cases, it is often tempting to assign a patient's phenotype to 'mitochondrial disease', but SMD is often challenging to distinguish from PMD. Fortunately, rapid advances in molecular testing, made possible by next generation sequencing, have been effective at least in some cases in establishing accurate diagnoses to distinguish between PMD and SMD. This is important, since their treatments and prognoses can be quite different. However, even in the absence of the ability to distinguish between PMD and SMD, treating SMD with standard treatments for PMD can be effective. We review the latest findings regarding mitochondrial disease/dysfunction and give representative examples in which differentiation between PMD and SMD has been crucial for diagnosis and treatment.

Mitochondrial disorders: challenges in diagnosis & treatment

Mitochondrial dysfunctions are known to be responsible for a number of heterogenous clinical presentations with multi-systemic involvement. Impaired oxidative phosphorylation leading to a decrease in cellular energy (ATP) production is the most important cause underlying these disorders. Despite significant progress made in the field of mitochondrial medicine during the last two decades, the molecular mechanisms underlying these disorders are not fully understood. Since the identification of first mitochondrial DNA (mtDNA) mutation in 1988, there has been an exponential rise in the identification of mtDNA and nuclear DNA mutations that are responsible for mitochondrial dysfunction and disease. Genetic complexity together with ever widening clinical spectrum associated with mitochondrial dysfunction poses a major challenge in diagnosis and treatment. Effective therapy has remained elusive till date and is mostly efficient in relieving symptoms. In this review, we discuss the important clinical and genetic features of mitochondrials disorders with special emphasis on diagnosis and treatment.

Mitochondrial dysfunction in inflammatory bowel disease

Inflammatory Bowel Disease (IBD) represents a group of idiopathic disorders characterized by chronic or recurring inflammation of the gastrointestinal tract. While the exact etiology of disease is unknown, IBD is recognized to be a complex, multifactorial disease that results from an intricate interplay of genetic predisposition, an altered immune response, changes in the intestinal microbiota, and environmental factors. Together, these contribute to a destruction of the intestinal epithelial barrier, increased gut permeability, and an influx of immune cells. Given that most cellular functions as well as maintenance of the epithelial barrier is energy-dependent, it is logical to assume that mitochondrial dysfunction may play a key role in both the onset and recurrence of disease. Indeed several studies have demonstrated evidence of mitochondrial stress and alterations in mitochondrial function within the intestinal epithelium of patients with IBD and mice undergoing experimental colitis. Although the hallmarks of mitochondrial dysfunction, including oxidative stress and impaired ATP production are known to be evident in the intestines of patients with IBD, it is as yet unclear whether these processes occur as a cause of consequence of disease. We provide a current review of mitochondrial function in the setting of intestinal inflammation during IBD.

Assessment of pharmacokinetic interaction between piracetam and l-carnitine in healthy subjects

A rapid, sensitive and specific method for quantifying piracetam in human plasma using Piracetam d-8 as the internal standard (IS) is described. The analyte and the IS were extracted from plasma by one-step precipitation of protein using an acetonitrile (100%). The extracts were analyzed by high-performance liquid chromatography coupled with electrospray tandem mass spectrometry (HPLC-MS/MS). The method had a chromatographic run time of 3.8 min and a linear calibration curve over the range 0.5-50 µg/mL (r > 0.99). This LC-MS-MS procedure was used to assess the bioavailability of two piracetam formulations: piracetam + l-carnitine (Piracar®; 270/330 mg tablet) and piracetam (Nootropil®; 800 mg tablet) in healthy volunteers of both sexes. The geometric means with corresponding 90% confidence interval (CI) for test/reference percentage ratios were 88.49% (90% CI = 81.19 - 96.46) for peak concentration/dose and 102.55% (90% CI = 100.62 - 104.51) for AUCinf /dose. The limit of quantitation of 0.5 µg/mL is well suited for pharmacokinetic studies in healthy volunteers. It was concluded that piracetam (Piracar®; 270/330 mg tablet) has a bioavailability equivalent to the piracetam (Nootropil®; 800 mg tablet) formulation with regard to both the rate and the extent of absorption.

Exercise and the Regulation of Mitochondrial Turnover

Exercise is a well-known stimulus for the expansion of the mitochondrial pool within skeletal muscle. Mitochondria have a remarkable ability to remodel their networks and can respond to an array of signaling stimuli following contractile activity to adapt to the metabolic demands of the tissue, synthesizing proteins to expand the mitochondrial reticulum. In addition, when they become dysfunctional, these organelles can be recycled by a specialized intracellular system. The signals regulating this mitochondrial life cycle of synthesis and degradation during exercise are still an area of great research interest. As mitochondrial turnover has valuable consequences in physical performance, in addition to metabolic health, disease, and aging, consideration of the signals which control this cycle is vital. This review focuses on the regulation of mitochondrial turnover in skeletal muscle and summarizes our current understanding of the impact that exercise has in modulating this process.

Treatment options for lactic acidosis and metabolic crisis in children with mitochondrial disease

The mitochondrial pyruvate oxidation route is a tightly regulated process, which is essential for aerobic cellular energy production. Disruption of this pathway may lead to severe neurometabolic disorders with onset in early childhood. A frequent finding in these patients is acute and chronic lactic acidemia, which is caused by increased conversion of pyruvate via the enzyme lactate dehydrogenase. Under stable clinical conditions, this process may remain well compensated and does not require specific therapy. However, especially in situations with altered energy demands, such as febrile infections or longer periods of fasting, children with mitochondrial disorders have a high risk of metabolic decompensation with exacerbation of hyperlactatemia and severe metabolic acidosis. Unfortunately, no controlled studies regarding therapy of this critical condition are available and clinical outcome is often unfavorable. Therefore, the aim of this review was to formulate expert-based suggestions for treatment of these patients, including dietary recommendations, buffering strategies and specific drug therapy. However, it is important to keep in mind that a specific therapy for the underlying metabolic cause in children with mitochondrial diseases is usually not available and symptomatic therapy especially of severe lactic acidosis has its ethical limitations.

Scoliosis in mitochondrial myopathy: case report and review of the literature

The mitochondrial myopathies include a diverse group of disorders characterized by morphological abnormalities of muscle mitochondria. Little is reported about spinal deformity associated with this syndrome.This study presents a case of scoliosis occurring in the setting of mitochondrial myopathies and explores the possible mechanisms between the 2 diseases.A previously unreported scoliosis in mitochondrial myopathies is described. The patient was a 16-year-old Chinese adolescent boy undergoing a posterior correction at thoracic 2-lumbar 3 (T2-L3) levels using the Moss-SI spinal system. At 48-month follow-up, the patient was clinically pain free and well balanced. Plain radiographs showed solid spine fusion with no loss of deformity correction. After evaluating 60 mitochondrial myopathies, patients referred to Peking Union Medical College Hospital from February 2009 to May 2013, the prevalence of scoliosis among patients with mitochondrial myopathies was 5% (3/60), much higher than that among general population (2%).The scoliosis in mitochondrial myopathies is usually extensive and progressively aggravated and the correction of the associated spinal deformities is generally difficult. Therefore, the exact role of mitochondrial myopathy in the development of scoliosis requires further study for a better understanding of the disease, as well as adequate and effective patient care.

Diagnosis and management of mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society

PURPOSE

The purpose of this statement is to review the literature regarding mitochondrial disease and to provide recommendations for optimal diagnosis and treatment. This statement is intended for physicians who are engaged in diagnosing and treating these patients.

METHODS

The Writing Group members were appointed by the Mitochondrial Medicine Society. The panel included members with expertise in several different areas. The panel members utilized a comprehensive review of the literature, surveys, and the Delphi method to reach consensus. We anticipate that this statement will need to be updated as the field continues to evolve.

RESULTS

Consensus-based recommendations are provided for the diagnosis and treatment of mitochondrial disease.

CONCLUSION

The Delphi process enabled the formation of consensus-based recommendations. We hope that these recommendations will help standardize the evaluation, diagnosis, and care of patients with suspected or demonstrated mitochondrial disease.

Treatment of mitochondrial disorders: antioxidants and beyond

Although mitochondrial disorders are among the most common inherited conditions that cause neurologic impairment, there are currently no U.S. Food and Drug Administration (FDA)-approved medications designed to treat primary mitochondrial disease. This is in part related to the lack of biomarkers to monitor disease status or response to treatment and the paucity of randomized, controlled clinical trials focused on mitochondrial disease therapies. Despite this discouraging historical precedent, a number of new approaches to mitochondrial disease therapy are on the horizon. By studying metabolites central to redox chemistry, investigators are gaining new insights into potential noninvasive biomarkers. Controlled clinical trials designed to study the effects of novel redox-modulating therapies, such as EPI-743, in patients with inherited mitochondrial disease are also underway. Furthermore, several new compounds with potential effects on inner mitochondrial membrane function and mitochondrial biogenesis are in development. Such advances are providing the foundation for a new era in mitochondrial disease therapeutics.

Skeletal Muscle Mitochondrial Function and Fatigability in Older Adults

BACKGROUND

Fatigability increases while the capacity for mitochondrial energy production tends to decrease significantly with age. Thus, diminished mitochondrial function may contribute to higher levels of fatigability in older adults.

METHODS

The relationship between fatigability and skeletal muscle mitochondrial function was examined in 30 participants aged 78.5 ± 5.0 years (47% female, 93% white), with a body mass index of 25.9 ± 2.7 kg/m(2) and usual gait-speed of 1.2 ± 0.2 m/s. Fatigability was defined using rating of perceived exertion (6-20 point Borg scale) after a 5-minute treadmill walk at 0.72 m/s. Phosphocreatine recovery in the quadriceps was measured using (31)P magnetic resonance spectroscopy and images of the quadriceps were captured to calculate quadriceps volume. ATPmax (mM ATP/s) and oxidative capacity of the quadriceps (ATPmax·Quadriceps volume) were calculated. Peak aerobic capacity (VO2peak) was measured using a modified Balke protocol.

RESULTS

ATPmax·Quadriceps volume was associated with VO2peak and was 162.61mM ATP·mL/s lower (p = .03) in those with high (rating of perceived exertion ≥10) versus low (rating of perceived exertion ≤9) fatigability. Participants with high fatigability required a significantly higher proportion of VO2peak to walk at 0.72 m/s compared with those with low fatigability (58.7 ± 19.4% vs 44.9 ± 13.2%, p < .05). After adjustment for age and sex, higher ATPmax was associated with lower odds of having high fatigability (odds ratio: 0.34, 95% CI: 0.11-1.01, p = .05).

CONCLUSIONS

Lower capacity for oxidative phosphorylation in the quadriceps, perhaps by contributing to lower VO2peak, is associated with higher fatigability in older adults.

Targeting mitochondria-derived reactive oxygen species to reduce epithelial barrier dysfunction and colitis

Epithelial permeability is often increased in inflammatory bowel diseases. We hypothesized that perturbed mitochondrial function would cause barrier dysfunction and hence epithelial mitochondria could be targeted to treat intestinal inflammation. Mitochondrial dysfunction was induced in human colon-derived epithelial cell lines or colonic biopsy specimens using dinitrophenol, and barrier function was assessed by transepithelial flux of Escherichia coli with or without mitochondria-targeted antioxidant (MTA) cotreatment. The impact of mitochondria-targeted antioxidants on gut permeability and dextran sodium sulfate (DSS)-induced colitis in mice was tested. Mitochondrial superoxide evoked by dinitrophenol elicited significant internalization and translocation of E. coli across epithelia and control colonic biopsy specimens, which was more striking in Crohn's disease biopsy specimens; the mitochondria-targeted antioxidant, MitoTEMPO, inhibited these barrier defects. Increased gut permeability and reduced epithelial mitochondrial voltage-dependent anion channel expression were observed 3 days after DSS. These changes and the severity of DSS-colitis were reduced by MitoTEMPO treatment. In vitro DSS-stimulated IL-8 production by epithelia was reduced by MitoTEMPO. Metabolic stress evokes significant penetration of commensal bacteria across the epithelium, which is mediated by mitochondria-derived superoxide acting as a signaling, not a cytotoxic, molecule. MitoTEMPO inhibited this barrier dysfunction and suppressed colitis in DSS-colitis, likely via enhancing barrier function and inhibiting proinflammatory cytokine production. These novel findings support consideration of MTAs in the maintenance of epithelial barrier function and the management of inflammatory bowel diseases.

The "second wind" in McArdle's disease patients during a second bout of constant work rate submaximal exercise

Patients with McArdle's disease (McA) typically show the "second-wind" phenomenon, a sudden decrease in heart rate (HR) and an improved exercise tolerance occurring after a few minutes of exercise. In the present study, we investigated whether in McA a first bout of exercise determines a second wind during a second bout, separated by the first by a few minutes of recovery. Eight McA (44 ± 4 yr) and a control group of six mitochondrial myopathy patients (51 ± 6 yr) performed two repetitions (CWR1 and CWR2) of 6-min constant work rate exercise (∼50% of peak work rate) separated by 6-min (SHORT) or 18-min (LONG) recovery. Pulmonary O2 uptake (Vo2), HR, cardiac output, rates of perceived exertion, vastus lateralis oxygenation {changes in deoxygenated Hb and myoglobin Mb concentrations, Δ[deoxy(Hb+Mb)], by near-infrared spectroscopy} were determined. In McA, Vo2 (0.86 ± 0.2 vs. 0.95 ± 0.1 l/min), HR (113 ± 10 vs. 150 ± 13 beats/min), cardiac output (11.6 ± 0.6 vs. 15.0 ± 0.8 l/min), and rates of perceived exertion (11 ± 2 vs. 14 ± 3) were lower, whereas Δ[deoxy(Hb+Mb)] was higher (14.7 ± 2.3 vs. -0.1 ± 4.6%) in CWR2-SHORT vs. CWR1; the "overshoot" of Δ[deoxy(Hb+Mb)] and the "slow component" of Vo2 kinetics disappeared in CWR2-SHORT. No differences (vs. CWR1) were observed in McA during CWR2-LONG, or in mitochondrial myopathy patients during both CWR2-SHORT and -LONG. A second-wind phenomenon was observed in McA during the second of two consecutive 6-min constant-work rate submaximal exercises. The second wind was associated with changes of physiological variables, suggesting an enhanced skeletal muscle oxidative metabolism. The second wind was not described after a longer (18-min) recovery period.

Effects of gas exchange on acid-base balance

This paper describes the interactions between ventilation and acid-base balance under a variety of conditions including rest, exercise, altitude, pregnancy, and various muscle, respiratory, cardiac, and renal pathologies. We introduce the physicochemical approach to assessing acid-base status and demonstrate how this approach can be used to quantify the origins of acid-base disorders using examples from the literature. The relationships between chemoreceptor and metaboreceptor control of ventilation and acid-base balance summarized here for adults, youth, and in various pathological conditions. There is a dynamic interplay between disturbances in acid-base balance, that is, exercise, that affect ventilation as well as imposed or pathological disturbances of ventilation that affect acid-base balance. Interactions between ventilation and acid-base balance are highlighted for moderate- to high-intensity exercise, altitude, induced acidosis and alkalosis, pregnancy, obesity, and some pathological conditions. In many situations, complete acid-base data are lacking, indicating a need for further research aimed at elucidating mechanistic bases for relationships between alterations in acid-base state and the ventilatory responses.

Nuclear recruitment of neuronal nitric-oxide synthase by α-syntrophin is crucial for the induction of mitochondrial biogenesis

Neuronal nitric-oxide synthase (nNOS) has various splicing variants and different subcellular localizations. nNOS can be found also in the nucleus; however, its exact role in this compartment is still not completely defined. In this report, we demonstrate that the PDZ domain allows the recruitment of nNOS to nuclei, thus favoring local NO production, nuclear protein S-nitrosylation, and induction of mitochondrial biogenesis. In particular, overexpression of PDZ-containing nNOS (nNOSα) increases S-nitrosylated CREB with consequent augmented binding on cAMP response element consensus sequence on peroxisome proliferator-activated receptor γ co-activator (PGC)-1α promoter. The resulting PGC-1α induction is accompanied by the expression of mitochondrial genes (e.g., TFAM, MtCO1) and increased mitochondrial mass. Importantly, full active nNOS lacking PDZ domain (nNOSβ) does not localize in nuclei and fails in inducing the expression of PGC-1α. Moreover, we substantiate that the mitochondrial biogenesis normally accompanying myogenesis is associated with nuclear translocation of nNOS. We demonstrate that α-Syntrophin, which resides in nuclei of myocytes, functions as the upstream mediator of nuclear nNOS translocation and nNOS-dependent mitochondrial biogenesis. Overall, our results indicate that altered nNOS splicing and nuclear localization could be contributing factors in human muscular diseases associated with mitochondrial impairment.

Dysregulation of mitochondrial quality control processes contribute to sarcopenia in a mouse model of premature aging

Mitochondrial DNA (mtDNA) mutations lead to decrements in mitochondrial function and accelerated rates of these mutations has been linked to skeletal muscle loss (sarcopenia). The purpose of this study was to investigate the effect of mtDNA mutations on mitochondrial quality control processes in skeletal muscle from animals (young; 3–6 months and older; 8–15 months) expressing a proofreading-deficient version of mtDNA polymerase gamma (PolG). This progeroid aging model exhibits elevated mtDNA mutation rates, mitochondrial dysfunction, and a premature aging phenotype that includes sarcopenia. We found increased expression of the mitochondrial biogenesis regulator peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) and its target proteins, nuclear respiratory factor 1 (NRF-1) and mitochondrial transcription factor A (Tfam) in PolG animals compared to wild-type (WT) (P<0.05). Muscle from older PolG animals displayed higher mitochondrial fission protein 1 (Fis1) concurrent with greater induction of autophagy, as indicated by changes in Atg5 and p62 protein content (P<0.05). Additionally, levels of the Tom22 import protein were higher in PolG animals when compared to WT (P<0.05). In contrast, muscle from normally-aged animals exhibited a distinctly different expression profile compared to PolG animals. Older WT animals appeared to have higher fusion (greater Mfn1/Mfn2, and lower Fis1) and lower autophagy (Beclin-1 and p62) compared to young WT suggesting that autophagy is impaired in aging muscle. In conclusion, muscle from mtDNA mutator mice display higher mitochondrial fission and autophagy levels that likely contribute to the sarcopenic phenotype observed in premature aging and this differs from the response observed in normally-aged muscle.

Impaired mitochondrial respiration and decreased fatigue resistance followed by severe muscle weakness in skeletal muscle of mitochondrial DNA mutator mice

Mitochondrial dysfunction can drastically impair muscle function, with weakness and exercise intolerance as key symptoms. Here we examine the time course of development of muscle dysfunction in a mouse model of premature ageing induced by defective proofreading function of mitochondrial DNA (mtDNA) polymerase (mtDNA mutator mouse). Isolated fast-twitch muscles and single muscle fibres from young (3-5 months) and end-stage (11 months) mtDNA mutator mice were compared to age-matched control mice. Force and free myoplasmic [Ca(2+)] ([Ca(2+)](i)) were measured under resting conditions and during fatigue induced by repeated tetani. Muscles of young mtDNA mutator mice displayed no weakness in the rested state, but had lower force and [Ca(2+)](i) than control mice during induction of fatigue. Muscles of young mtDNA mutator mice showed decreased activities of citrate synthase and β-hydroxyacyl-coenzyme A dehydrogenase, reduced expression of cytochrome c oxidase, and decreased expression of triggers of mitochondrial biogenesis (PGC-1α, PPARα, AMPK). Muscles from end-stage mtDNA mutator mice showed weakness under resting conditions with markedly decreased tetanic [Ca(2+)](i), force per cross-sectional area and protein expression of the sarcoplasmic reticulum Ca(2+) pump (SERCA1). In conclusion, fast-twitch muscles of prematurely ageing mtDNA mutator mice display a sequence of deleterious mitochondrial-to-nucleus signalling with an initial decrease in oxidative capacity, which was not counteracted by activation of signalling to increase mitochondrial biogenesis. This was followed by severe muscle weakness in the end stage. These results have implication for normal ageing and suggest that decreased mitochondrial oxidative capacity due to a sedentary lifestyle may predispose towards muscle weakness developing later in life.

Use of quantitative membrane proteomics identifies a novel role of mitochondria in healing injured muscles

Skeletal muscles are proficient at healing from a variety of injuries. Healing occurs in two phases, early and late phase. Early phase involves healing the injured sarcolemma and restricting the spread of damage to the injured myofiber. Late phase of healing occurs a few days postinjury and involves interaction of injured myofibers with regenerative and inflammatory cells. Of the two phases, cellular and molecular processes involved in the early phase of healing are poorly understood. We have implemented an improved sarcolemmal proteomics approach together with in vivo labeling of proteins with modified amino acids in mice to study acute changes in the sarcolemmal proteome in early phase of myofiber injury. We find that a notable early phase response to muscle injury is an increased association of mitochondria with the injured sarcolemma. Real-time imaging of live myofibers during injury demonstrated that the increased association of mitochondria with the injured sarcolemma involves translocation of mitochondria to the site of injury, a response that is lacking in cultured myoblasts. Inhibiting mitochondrial function at the time of injury inhibited healing of the injured myofibers. This identifies a novel role of mitochondria in the early phase of healing injured myofibers.

Defective mitochondrial morphology and bioenergetic function in mice lacking the transcription factor Yin Yang 1 in skeletal muscle

The formation, distribution, and maintenance of functional mitochondria are achieved through dynamic processes that depend strictly on the transcription of nuclear genes encoding mitochondrial proteins. A large number of these mitochondrial genes contain binding sites for the transcription factor Yin Yang 1 (YY1) in their proximal promoters, but the physiological relevance is unknown. We report here that skeletal-muscle-specific YY1 knockout (YY1mKO) mice have severely defective mitochondrial morphology and oxidative function associated with exercise intolerance, signs of mitochondrial myopathy, and short stature. Gene set enrichment analysis (GSEA) revealed that the top pathways downregulated in YY1mKO mice were assigned to key metabolic and regulatory mitochondrial genes. This analysis was consistent with a profound decrease in the level of mitochondrial proteins and oxidative phosphorylation (OXPHOS) bioenergetic function in these mice. In contrast to the finding for wild-type mice, inactivation of the mammalian target of rapamycin (mTOR) did not suppress mitochondrial genes in YY1mKO mice. Mechanistically, mTOR-dependent phosphorylation of YY1 resulted in a strong interaction between YY1 and the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1α (PGC1α), a major regulator of mitochondrial function. These results underscore the important role of YY1 in the maintenance of mitochondrial function and explain how its inactivation might contribute to exercise intolerance and mitochondrial myopathies.

Mitochondrial diseases and the heart: an overview of molecular basis, diagnosis, treatment and clinical course

The mitochondrion is the main site of production of ATP that represents the source of energy for a large number of cellular processes. Mitochondrial diseases that result in a deficit in ATP production can affect almost every organ system with a large spectrum of clinical phenotypes. Cardiomyocytes are particularly vulnerable to limited ATP supply because of their large energy requirement. Abnormalities in the mitochondrial function are increasingly recognized in association with dilated and hypertrophic cardiomyopathy, cardiac conduction defects, endothelial dysfunction and coronary artery disease. Cardiologists should, therefore, be alerted to symptoms and signs suggestive of mitochondrial diseases and become familiar with the general issues related to multisystem disease management, genetic counseling and testing.