Exercise intolerance in mitochondrial myopathy is not related to lactic acidosis

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.

Mitochondrial DNA replication and repair defects: Clinical phenotypes and therapeutic interventions

Mitochondria is a unique cellular organelle involved in multiple cellular processes and is critical for maintaining cellular homeostasis. This semi-autonomous organelle contains its circular genome - mtDNA (mitochondrial DNA), that undergoes continuous cycles of replication and repair to maintain the mitochondrial genome integrity. The majority of the mitochondrial genes, including mitochondrial replisome and repair genes, are nuclear-encoded. Although the repair machinery of mitochondria is quite efficient, the mitochondrial genome is highly susceptible to oxidative damage and other types of exogenous and endogenous agent-induced DNA damage, due to the absence of protective histones and their proximity to the main ROS production sites. Mutations in replication and repair genes of mitochondria can result in mtDNA depletion and deletions subsequently leading to mitochondrial genome instability. The combined action of mutations and deletions can result in compromised mitochondrial genome maintenance and lead to various mitochondrial disorders. Here, we review the mechanism of mitochondrial DNA replication and repair process, key proteins involved, and their altered function in mitochondrial disorders. The focus of this review will be on the key genes of mitochondrial DNA replication and repair machinery and the clinical phenotypes associated with mutations in these genes.

Impaired aerobic capacity and premature fatigue preceding muscle weakness in the skeletal muscle Tfam-knockout mouse model

Mitochondrial diseases are genetic disorders that lead to impaired mitochondrial function, resulting in exercise intolerance and muscle weakness. In patients, muscle fatigue due to defects in mitochondrial oxidative capacities commonly precedes muscle weakness. In mice, deletion of the fast-twitch skeletal muscle-specific Tfam gene (Tfam KO) leads to a deficit in respiratory chain activity, severe muscle weakness and early death. Here, we performed a time-course study of mitochondrial and muscular dysfunctions in 11- and 14-week-old Tfam KO mice, i.e. before and when mice are about to enter the terminal stage, respectively. Although force in the unfatigued state was reduced in Tfam KO mice compared to control littermates (wild type) only at 14 weeks, during repeated submaximal contractions fatigue was faster at both ages. During fatiguing stimulation, total phosphocreatine breakdown was larger in Tfam KO muscle than in wild-type muscle at both ages, whereas phosphocreatine consumption was faster only at 14 weeks. In conclusion, the Tfam KO mouse model represents a reliable model of lethal mitochondrial myopathy in which impaired mitochondrial energy production and premature fatigue occur before muscle weakness and early death.

Summary: A time-course study of mitochondrial and muscular dysfunctions in a mouse model of mitochondrial myopathy reveals that decreased resistance to fatigue together with decreased oxidative capacities arise ahead of muscle weakness.

Exercise Testing, Physical Training and Fatigue in Patients with Mitochondrial Myopathy Related to mtDNA Mutations

Mutations in mitochondrial DNA (mtDNA) cause disruption of the oxidative phosphorylation chain and impair energy production in cells throughout the human body. Primary mitochondrial disorders due to mtDNA mutations can present with symptoms from adult-onset mono-organ affection to death in infancy due to multi-organ involvement. The heterogeneous phenotypes that patients with a mutation of mtDNA can present with are thought, at least to some extent, to be a result of differences in mtDNA mutation load among patients and even among tissues in the individual. The most common symptom in patients with mitochondrial myopathy (MM) is exercise intolerance. Since mitochondrial function can be assessed directly in skeletal muscle, exercise studies can be used to elucidate the physiological consequences of defective mitochondria due to mtDNA mutations. Moreover, exercise tests have been developed for diagnostic purposes for mitochondrial myopathy. In this review, we present the rationale for exercise testing of patients with MM due to mutations in mtDNA, evaluate the diagnostic yield of exercise tests for MM and touch upon how exercise tests can be used as tools for follow-up to assess disease course or effects of treatment interventions.

Mitochondrial functions and rare diseases

Mitochondria are dynamic cellular organelles responsible for a large variety of biochemical processes as energy transduction, REDOX signaling, the biosynthesis of hormones and vitamins, inflammation or cell death execution. Cell biology studies established that 1158 human genes encode proteins localized to mitochondria, as registered in MITOCARTA. Clinical studies showed that a large number of these mitochondrial proteins can be altered in expression and function through genetic, epigenetic or biochemical mechanisms including the interaction with environmental toxics or iatrogenic medicine. As a result, pathogenic mitochondrial genetic and functional defects participate to the onset and the progression of a growing number of rare diseases. In this review we provide an exhaustive survey of the biochemical, genetic and clinical studies that demonstrated the implication of mitochondrial dysfunction in human rare diseases. We discuss the striking diversity of the symptoms caused by mitochondrial dysfunction and the strategies proposed for mitochondrial therapy, including a survey of ongoing clinical trials.

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.

Therapeutic strategies for mitochondrial disorders

OBJECTIVES

There is currently no curative therapy for mitochondrial disorders, although symptomatic measures can be highly effective and greatly improve the quality of life and outcome of these patients. This review highlights potential strategies for the therapeutic management of mitochondrial disorders.

METHODS

Data for this review were identified by searches of MEDLINE, Current Contents, using various relevant search terms.

RESULTS

Strategies to establish a therapeutic regimen aim to enhance respiratory chain function, eliminate noxious compounds, shift the heteroplasmy rate, alter mitochondrial dynamics, transfer cytoplasm, and promote gene therapy. Symptomatic measures rely on drugs (e.g., antiepileptics), avoidance of mitochondrion-toxic agents, substitution of blood cells, hemodialysis, invasive measures (such as a pacemaker), surgery (e.g., ptosis correction), physiotherapy, speech therapy, occupational therapy, dietary measures (e.g., ketogenic diet, anaplerotic diet), and the avoidance of mitochondrion-toxic agents (e.g., ozone). With the increasing awareness of mitochondrial disorders, the number of treatment studies is growing and its quality is improving. If high quality studies (high Jadad score) yield statistical significance for end points, a treatment is more reliable than with lower quality studies.

CONCLUSIONS

Despite the lack of a proven treatment for mitochondrial disorders, a nihilistic attitude toward treatment is not justified. A number of studies are seeking targeted therapies, and highly effective symptomatic measures are available.

Development of pharmacological strategies for mitochondrial disorders

Mitochondrial diseases are an unusually genetically and phenotypically heterogeneous group of disorders, which are extremely challenging to treat. Currently, apart from supportive therapy, there are no effective treatments for the vast majority of mitochondrial diseases. Huge scientific effort, however, is being put into understanding the mechanisms underlying mitochondrial disease pathology and developing potential treatments. To date, a variety of treatments have been evaluated by randomized clinical trials, but unfortunately, none of these has delivered breakthrough results. Increased understanding of mitochondrial pathways and the development of many animal models, some of which are accurate phenocopies of human diseases, are facilitating the discovery and evaluation of novel prospective treatments. Targeting reactive oxygen species has been a treatment of interest for many years; however, only in recent years has it been possible to direct antioxidant delivery specifically into the mitochondria. Increasing mitochondrial biogenesis, whether by pharmacological approaches, dietary manipulation or exercise therapy, is also currently an active area of research. Modulating mitochondrial dynamics and mitophagy and the mitochondrial membrane lipid milieu have also emerged as possible treatment strategies. Recent technological advances in gene therapy, including allotopic and transkingdom gene expression and mitochondrially targeted transcription activator-like nucleases, have led to promising results in cell and animal models of mitochondrial diseases, but most of these techniques are still far from clinical application.

Strategies for treating mitochondrial disorders: an update

Mitochondrial diseases are a heterogeneous group of disorders resulting from primary dysfunction of the respiratory chain due to both nuclear and mitochondrial DNA mutations. The wide heterogeneity of biochemical dysfunctions and pathogenic mechanisms typical of this group of diseases has hindered therapy trials; therefore, available treatment options remain limited. Therapeutic strategies aimed at increasing mitochondrial functions (by enhancing biogenesis and electron transport chain function), improving the removal of reactive oxygen species and noxious metabolites, modulating aberrant calcium homeostasis and repopulating mitochondrial DNA could potentially restore the respiratory chain dysfunction. The challenge that lies ahead is the translation of some promising laboratory results into safe and effective therapies for patients. In this review we briefly update and discuss the most feasible therapeutic approaches for mitochondrial diseases.

Lactate metabolism during exercise in patients with mitochondrial myopathy

Patients with mitochondrial DNA mutations often have elevated plasma lactate at rest and during exercise, but it is unknown whether the high lactate levels are caused by a high production, an impaired oxidation or a combination. We studied lactate kinetics in 10 patients with mtDNA mutations and 10 matched healthy control subjects at rest and during cycle exercise with a combination of femoral arterio-venous differences of lactate, and lactate tracer dilution methodology. During exercise, lactate concentration and production rates were several-fold higher in patients, but despite mitochondrial dysfunction, lactate was oxidized in muscle to the same extent as in healthy control subjects. This surprisingly high ability to burn lactate in working muscle with defective mitochondria, probably relates to the variability of oxidative capacity among muscle fibers. The data suggests that lactate is not solely an indicator of impaired oxidative capacity, but an important fuel for oxidative metabolism, even in muscle with severely impaired mitochondrial function.

Defining cardiac adaptations and safety of endurance training in patients with m.3243A>G-related mitochondrial disease

Background

Cardiac hypertrophic remodelling and systolic dysfunction are common in patients with mitochondrial disease and independent predictors of morbidity and early mortality. Endurance exercise training improves symptoms and skeletal muscle function, yet cardiac adaptations are unknown.

Methods and results

Before and after 16-weeks of training, exercise capacity, cardiac magnetic resonance imaging and phosphorus-31 spectroscopy, disease burden, fatigue, quality of life, heart rate variability (HRV) and blood pressure variability (BPV) were assessed in 10 adult patients with m.3243A>G-related mitochondrial disease, and compared to age- and gender-matched sedentary control subjects. At baseline, patients had increased left ventricular mass index (LVMI, p < 0.05) and LV mass to end-diastolic volume ratio, and decreased longitudinal shortening and myocardial phosphocreatine/adenosine triphosphate ratio (all p < 0.01). Peak arterial–venous oxygen difference (p < 0.05), oxygen uptake (VO₂) and power were decreased in patients (both p < 0.01) with no significant difference in cardiac power output. All patients remained stable and completed ≥ 80% sessions. With training, there were similar proportional increases in peak VO₂, anaerobic threshold and work capacity in patients and controls. LVMI increased in both groups (p < 0.01), with no significant effect on myocardial function or bioenergetics. Pre- and post-exercise training, HRV and BPV demonstrated increased low frequency and decreased high frequency components in patients compared to controls (all p < 0.05).

Conclusion

Patients with mitochondrial disease and controls achieved similar proportional benefits of exercise training, without evidence of disease progression, or deleterious effects on cardiac function. Reduced exercise capacity is largely mediated through skeletal muscle dysfunction at baseline and sympathetic over-activation may be important in pathogenesis.

The elusive magic pill: finding effective therapies for mitochondrial disorders

The incidence of mitochondrial diseases has been estimated at 11.5/100,000 (1:8500) worldwide. In the USA up to 4000 newborns annually are expected to develop a mitochondrial disease. More than 50 million adults in the USA also suffer from diseases in which primary or secondary mitochondrial dysfunction is involved. Mitochondrial dysfunction has been identified in cancer, infertility, diabetes, heart diseases, blindness, deafness, kidney disease, liver disease, stroke, migraine, dwarfism, and resulting from numerous medication toxicities. Mitochondrial dysfunction is also involved in normal aging and age-related neurodegenerative diseases, such as Parkinson and Alzheimer diseases. Yet most treatments available are based on empiric data and clinician experience because of the lack of randomized controlled clinical trials to provide evidence-based treatments for these disorders. Here we explore the current state of research for the treatment of mitochondrial disorders.

Fatigue and exercise intolerance in mitochondrial diseases. Literature revision and experience of the Italian Network of mitochondrial diseases

Fatigue and exercise intolerance are common symptoms of mitochondrial diseases, but difficult to be clinically assessed. New methods to quantify these rather common complaints are strongly needed in the clinical practice. Coenzyme Q10 administration and aerobic exercise may improve exercise intolerance, but more definite studies are still pending. Herein, we have revised "how to measure" and "how to treat" these symptoms of mitochondrial patients. Subsequently, we reviewed the clinical data of the 1164 confirmed mitochondrial patients present in the Italian nation-wide database of mitochondrial disease, with special regard to exercise intolerance. We observed that more of 20% of mitochondrial patients complain of exercise intolerance. This symptom seems to be frequently associated with specific patient groups and/or genotypes. Ragged red fibers and COX-negative fibers are more often present in subjects with exercise intolerance, whereas lactate levels could not predict this symptom. Multicenter efforts are strongly needed for rare disorders such as mitochondrial diseases, and may represent the basis for more rigorous longitudinal studies.

Diagnosis and treatment of mitochondrial myopathies

Mitochondrial disorders are a heterogeneous group of disorders resulting from primary dysfunction of the respiratory chain. Muscle tissue is highly metabolically active, and therefore myopathy is a common element of the clinical presentation of these disorders, although this may be overshadowed by central neurological features. This review is aimed at a general medical and neurologist readership and provides a clinical approach to the recognition, investigation, and treatment of mitochondrial myopathies. Emphasis is placed on practical management considerations while including some recent updates in the field.

Treatment for mitochondrial disorders

BACKGROUND

Mitochondrial respiratory chain disorders are the most prevalent group of inherited neurometabolic diseases. They present with central and peripheral neurological features usually in association with other organ involvement including the eye, the heart, the liver, and kidneys, diabetes mellitus and sensorineural deafness. Current treatment is largely supportive and the disorders progress relentlessly causing significant morbidity and premature death. Vitamin supplements, pharmacological agents and exercise therapy have been used in isolated cases and small clinical trials, but the efficacy of these interventions is unclear. The first review was carried out in 2003, and identified six clinical trials. This major update was carried out to identify new studies and grade the original studies for potential bias in accordance with revised Cochrane Collaboration guidelines.

OBJECTIVES

To determine whether there is objective evidence to support the use of current treatments for mitochondrial disease.

SEARCH METHODS

We searched the Cochrane Neuromuscular Disease Group Specialized Register (4 July 2011), CENTRAL (2011, Issue 2, MEDLINE (1966 to July 2011), and EMBASE (January 1980 to July 2011), and contacted experts in the field.

SELECTION CRITERIA

We included randomised controlled trials (including cross-over studies). Two of the authors independently selected abstracts for further detailed review. Further review was performed independently by all five authors to decide which trials fit the inclusion criteria and graded risk of bias. Participants included males and females of any age with a confirmed diagnosis of mitochondrial disease based upon muscle histochemistry, respiratory chain complex analysis of tissues or cell lines or DNA studies. Interventions included any pharmacological agent, dietary modification, nutritional supplement, exercise therapy or other treatment. The review authors excluded studies at high risk of bias in any category. The primary outcome measures included an change in muscle strength and/or endurance, or neurological clinical features. Secondary outcome measures included quality of life assessments, biochemical markers of disease and negative outcomes.

DATA COLLECTION AND ANALYSIS

Two of the authors (GP and PFC) independently identified studies for further evaluation from all abstracts within the search period. For those studies identified for further review, all five authors then independently assessed which studies met the entry criteria. For the included studies, we extracted details of the number of randomised participants, treatment, study design, study category, allocation concealment and other risk of bias criteria, and participant characteristics. Analysis was based on intention-to-treat data. We planned to use meta-analysis, but this did not prove necessary.

MAIN RESULTS

The authors reviewed 1335 abstracts, and from these identified 21 potentially eligible abstracts. Upon detailed review, 12 studies fulfilled the entry criteria. Of these, eight were new studies that had been published since the previous version of this review. Two studies which were included in the previous version of this review were excluded because of potential for bias. The comparability of the included studies is extremely low because of differences in the specific diseases studied, differences in the therapeutic agents used, dosage, study design, and outcomes. The methodological quality of included studies was generally high, although risk of bias was unclear in random sequence generation and allocation concealment for most studies. Otherwise, the risk of bias was low for most studies in the other categories. Serious adverse events were uncommon, except for peripheral nerve toxicity in a long-term trial of dichloroacetate (DCA) in adults.One trial studied high-dose coenzyme Q10 without clinically meaningful improvement (although there were multiple biochemical, physiologic, and neuroimaging outcomes, in 30 participants). Three trials used creatine monohydrate alone, with one reporting evidence of improved measures of muscle strength and post-exercise lactate, but the other two reported no benefit (total of 38 participants). One trial studied the effects of a combination of coenzyme Q10, creatine monohydrate, and lipoic acid and reported a statistically significant improvement in biochemical markers and peak ankle dorsiflexion strength, but overall no clinical improvement in 16 participants. Five trials studied the effects of DCA: three trials in children showed a statistically significant improvement in secondary outcome measures of mitochondrial metabolism (venous lactate in three trials, and magnetic resonance spectroscopy (MRS) in one trial; total of 63 participants). One trial of short-term DCA in adults demonstrated no clinically relevant improvement (improved venous lactate but no change in physiologic, imaging, or questionnaire findings, in eight participants). One longer-term DCA trial in adults was terminated prematurely due to peripheral nerve toxicity without clinical benefit (assessments included the GATE score, venous lactate and MRS, in 30 participants). One trial using dimethylglycine showed no significant effect (measurements of venous lactate and oxygen consumption (VO(2)) in five participants). One trial using a whey-based supplement showed statistically significant improvement in markers of free radical reducing capacity but no clinical benefit (assessments included the Short Form 36 Health Survey (SF-36) questionnaire and UK Medical Research Council (MRC) muscle strength, in 13 participants).

AUTHORS' CONCLUSIONS

Despite identifying eight new trials there is currently no clear evidence supporting the use of any intervention in mitochondrial disorders. Further research is needed to establish the role of a wide range of therapeutic approaches. We suggest further research should identify novel agents to be tested in homogeneous study populations with clinically relevant primary endpoints.

Disruption of BCAA metabolism in mice impairs exercise metabolism and endurance

Exercise enhances branched-chain amino acid (BCAA) catabolism, and BCAA supplementation influences exercise metabolism. However, it remains controversial whether BCAA supplementation improves exercise endurance, and unknown whether the exercise endurance effect of BCAA supplementation requires catabolism of these amino acids. Therefore, we examined exercise capacity and intermediary metabolism in skeletal muscle of knockout (KO) mice of mitochondrial branched-chain aminotransferase (BCATm), which catalyzes the first step of BCAA catabolism. We found that BCATm KO mice were exercise intolerant with markedly decreased endurance to exhaustion. Their plasma lactate and lactate-to-pyruvate ratio in skeletal muscle during exercise and lactate release from hindlimb perfused with high concentrations of insulin and glucose were significantly higher in KO than wild-type (WT) mice. Plasma and muscle ammonia concentrations were also markedly higher in KO than WT mice during a brief bout of exercise. BCATm KO mice exhibited 43-79% declines in the muscle concentration of alanine, glutamine, aspartate, and glutamate at rest and during exercise. In response to exercise, the increments in muscle malate and alpha-ketoglutarate were greater in KO than WT mice. While muscle ATP concentration tended to be lower, muscle IMP concentration was sevenfold higher in KO compared with WT mice after a brief bout of exercise, suggesting elevated ammonia in KO is derived from the purine nucleotide cycle. These data suggest that disruption of BCAA transamination causes impaired malate/aspartate shuttle, thereby resulting in decreased alanine and glutamine formation, as well as increases in lactate-to-pyruvate ratio and ammonia in skeletal muscle. Thus BCAA metabolism may regulate exercise capacity in mice.

Thermal instability of compound variants of carnitine palmitoyltransferase II and impaired mitochondrial fuel utilization in influenza-associated encephalopathy

Influenza-associated encephalopathy (IAE) is characterized by persistent high fever, febrile convulsions, severe brain edema, and high mortality in otherwise apparently healthy individuals. We have reported that a large proportion of patients suffering from disabling or fatal IAE, with transiently elevated serum acylcarnitine during high fever, exhibit a thermolabile phenotype of compound homo-/heterozygous variants of carnitine palmitoyltransferase II (CPT II, gene symbol CPT2). We characterized the enzymatic properties of five single and three compound CPT II variants in patients with IAE. The kinetic characteristics of WT and variant CPT IIs, expressed in COS-7 cells, indicated that the variants exert a dominant-negative effect on the homotetrameric protein of the enzyme. Among the variants, three compound variations found in patients with severe encephalopathy; [c.1055T>G (p.Phe352Cys); c.1102G>A (p.Val368Ile)], [c.1511C>T (p.Pro504Leu); c.1813G>C (p.Val605Leu)], and [c.1055T>G (p.Phe352Cys); c.1102G>A (p.Val368Ile); c.1813G>C (p.Val605Leu)], showed reduced activities, thermal instability, and short half-lives compared with the WT. Like other disease-causing mutant proteins, these variant proteins were poly-ubiquitinated and rapidly degraded by a lactacystin-sensitive proteasome pathway. COS-7 cells transfected with the compound variants had their fatty acid beta-oxidation decreased to 30-59% and intracellular ATP levels to 48-79%, and a marked reduction of mitochondrial membrane potential at 41 degrees C, compared with control cells transfected with WT at 37 degrees C. The unstable CPT II variants with decreased enzymatic activities may bring mitochondrial fuel utilization below the phenotypic threshold during high fever, and thus may play an important etiopathological role in the development of brain edema of IAE.

A novel mitochondrial ND5 (MTND5) gene mutation giving isolated exercise intolerance

We describe a patient with isolated exercise intolerance caused by a new, maternally inherited mutation in mitochondrial DNA. The heteroplasmic T>C transition at position 13271 in MTND5 affects a highly conserved base and segregates with the disease, being present at highest levels in skeletal muscle fibres showing abnormal mitochondrial accumulation. This is the 15th mutation affecting the MTND5 subunit of respiratory chain complex I and confirms this protein as an important site for disease with phenotypes ranging from MELAS and infantile encephalopathies to isolated syndromes affecting a single tissue such as Leber hereditary optic neuropathy and now skeletal muscle.

The evidence basis for coenzyme Q therapy in oxidative phosphorylation disease

The evidence supporting a treatment benefit for coenzyme Q10 (CoQ10) in primary mitochondrial disease (mitochondrial disease) whilst positive is limited. Mitochondrial disease in this context is defined as genetic disease causing an impairment in mitochondrial oxidative phosphorylation (OXPHOS). There are no treatment trials achieving the highest Level I evidence designation. Reasons for this include the relative rarity of mitochondrial disease, the heterogeneity of mitochondrial disease, the natural cofactor status and easy 'over the counter availability' of CoQ10 all of which make funding for the necessary large blinded clinical trials unlikely. At this time the best evidence for efficacy comes from controlled trials in common cardiovascular and neurodegenerative diseases with mitochondrial and OXPHOS dysfunction the etiology of which is most likely multifactorial with environmental factors playing on a background of genetic predisposition. There remain questions about dosing, bioavailability, tissue penetration and intracellular distribution of orally administered CoQ10, a compound which is endogenously produced within the mitochondria of all cells. In some mitochondrial diseases and other commoner disorders such as cardiac disease and Parkinson's disease low mitochondrial or tissue levels of CoQ10 have been demonstrated providing an obvious rationale for supplementation. This paper discusses the current state of the evidence supporting the use of CoQ10 in mitochondrial disease.

Skeletal muscle energetics with PNMR: personal views and historic perspectives

This article reviews historical and current NMR approaches to describing in vivo bioenergetics of skeletal muscles in normal and diseased populations. It draws upon the first author's more than 70 years of personal experience in enzyme kinetics and the last author's physiological approaches. The development of in vivo PNMR jointly with researchers around the world is described. It is explained how non-invasive PNMR has advanced human exercise biochemistry, physiology and pathology. Further, after a brief explanation of bioenergetics with PNMR on creatine kinase, anerobic glycolysis and mitochondrial oxidative phosphorylation, some basic and controversial subjects are focused upon, and the authors' view of the subjects are offered, with questions and answers. Some of the research has been introduced in exercise physiology. Future directions of NMR on bioenergetics, as a part of system biological approaches, are indicated.

Attenuation of free radical production and paracrystalline inclusions by creatine supplementation in a patient with a novel cytochrome b mutation

Mitochondrial cytopathies are associated with increased free radical generation and paracrystalline inclusions. Paracrystalline inclusions were serendipitously found in a young male athlete with a very high respiratory exchange ratio during steady-state exercise; he also had an unusually low aerobic capacity. Direct sequencing of the mitochondrial DNA (mtDNA) coding regions revealed a novel missense mutation (G15497A) resulting in a glycine-->serine conversion at a highly conserved site in the cytochrome b gene in the subject, his mother, and sister. Cybrids, prepared by fusion of the subject's platelets with either U87MG rho degrees or SH-SY5Y rho degrees cells, generated higher basal levels of reactive oxygen species (ROS), had a lower adenosine triphosphate (ATP) content, and were more sensitive to oxygen and glucose deprivation and peroxynitrite generation compared to control cybrids with wild-type mtDNA. Cell survival was significantly enhanced with 50 mmol/L creatine monohydrate (CM) administration. The subject was also treated with CM (10 g/d) for a period of 5 weeks and a repeat muscle biopsy showed no paracrystalline inclusions. The results suggest that the development of exercise-induced paracrystalline inclusions may be influenced by the G15497A mtDNA mutation, and that CM mitigates against the pathological consequences of this mutation.

Dichloroacetate therapy attenuates the blood lactate response to submaximal exercise in patients with defects in mitochondrial energy metabolism

We determined acute and chronic effects of dichloroacetate (DCA) on maximal (MAX) and submaximal (SUB) exercise responses in patients with abnormal mitochondrial energetics. Subjects (n = 9) completed a MAX treadmill bout 1 h after ingesting 25 mg/kg DCA or placebo (PL). A 15-min SUB bout was completed the next day while receiving the same treatment. After a 1-d washout, MAX and SUB were repeated while receiving the alternate treatment (acute). Gas exchange and heart rate were measured throughout all tests. Blood lactate (Bla) was measured 0, 3, and 10 min after MAX, and 5, 10, and 15 min during SUB. MAX and SUB were repeated after 3 months of daily DCA or PL. After a 2-wk washout, a final MAX and SUB were completed after 3 months of alternate treatment (chronic). Average Bla during SUB was lower (P < 0.05) during both acute (1.99 +/- 1.10 vs. 2.49 +/- 1.52 mmol/liter) and chronic (1.71 +/- 1.37 vs. 2.39 +/- 1.32 mmol/liter) DCA vs. PL despite similar exercise intensities between conditions ( approximately 75 and 70% maximal exercise capacity during acute and chronic treatment). Thus, although DCA does not alter MAX responses, acute and chronic DCA attenuate the Bla response to moderate exercise in patients with abnormal mitochondrial energetics.

A diagnostic cycle test for McArdle's disease

We investigated whether the second wind phenomenon (ie, a decrease in heart rate and perceived exertion during exercise) is pathognomonic for McArdle's disease. Twenty-four patients with McArdle's disease, 17 healthy subjects, and 25 patients with other inborn errors of muscle metabolism cycled a constant workload for 15 minutes. In McArdle's disease patients, heart rate consistently decreased by 35 +/- 3 beats per minute from the 7(th) to the 15(th) minute of exercise, whereas heart rate increased progressively with exercise in all 42 control subjects. The findings indicate that cycling at a moderate, constant workload provides a specific, sensitive, and simple diagnostic test for McArdle's disease.

Role of fatigue in limiting physical activities in humans with neuromuscular diseases

New methods of examining both central and peripheral fatigue are now available. A broader understanding of the mechanisms of fatigue in healthy human subjects has begun to emerge. The mechanisms of fatigue in patients with various neuromuscular diseases are even more complex than in healthy persons. Examples of both central and peripheral fatigue in various neuromuscular diseases and other disorders are presented, including metabolic myopathy, chronic fatigue syndrome, postpolio syndrome, and amyotrophic lateral sclerosis.