Biochemical parameters for the diagnosis of mitochondrial respiratory chain deficiency in humans, and their lack of age-related changes

MITO-FIND: A study in 390 patients to determine a diagnostic strategy for mitochondrial disease

Mitochondrial diseases, due to nuclear or mitochondrial genome mutations causing mitochondrial dysfunction, have a wide range of clinical features involving neurologic, muscular, cardiac, hepatic, visual, and auditory symptoms. Making a diagnosis of a mitochondrial disease is often challenging since there is no gold standard and traditional testing methods have required tissue biopsy which presents technical challenges and most patients prefer a non-invasive approach. Since a diagnosis invariably involves finding a disease-causing DNA variant, new approaches such as next generation sequencing (NGS) have the potential to make it easier to make a diagnosis. We evaluated the ability of our traditional diagnostic pathway (metabolite analysis, tissue neuropathology and respiratory chain enzyme activity) in 390 patients. The traditional diagnostic pathway provided a diagnosis of mitochondrial disease in 115 patients (29.50%). Analysis of mtDNA, tissue neuropathology, skin electron microscopy, respiratory chain enzyme analysis using inhibitor assays, blue native polyacrylamide gel electrophoresis were all statistically significant in distinguishing patients between a mitochondrial and non-mitochondrial diagnosis. From these 390 patients who underwent traditional analysis, we recruited 116 patients for the NGS part of the study (36 patients who had a mitochondrial diagnosis (MITO) and 80 patients who had no diagnosis (No-Dx)). In the group of 36 MITO patients, nuclear whole exome sequencing (nWES) provided a second diagnosis in 2 cases who already had a pathogenic variant in mtDNA, and a revised diagnosis (GLUL) in one case that had abnormal pathology but no pathogenic mtDNA variant. In the 80 NO-Dx patients, nWES found non-mitochondrial diagnosis in 26 patients and a mitochondrial diagnosis in 1 patient. A genetic diagnosis was obtained in 53/116 (45.70%) cases that were recruited for NGS, but not in 11/116 (9.48%) of cases with abnormal mitochondrial neuropathology. Our results show that a non-invasive, bigenomic sequencing (BGS) approach (using both a nWES and optimized mtDNA analysis to include large deletions) should be the first step in investigating for mitochondrial diseases. There may still be a role for tissue biopsy in unsolved cases or when the diagnosis is still not clear after NGS studies.

Quantification of mitochondrial DNA from peripheral tissues: Limitations in predicting the severity of neurometabolic disorders and proposal of a novel diagnostic test

Neurometabolic disorders stem from errors in metabolic processes yielding a neurological phenotype. A subset of those disorders encompasses mitochondrial abnormalities partially due to mitochondrial DNA (mtDNA) depletion. mtDNA depletion can be attributed to inheritance, spontaneous mutations or acquired from drug-related toxicities. In the armamentarium of diagnostic procedures, mtDNA quantification is a standard for disease classification. However, alterations in mtDNA obtained from peripheral tissues such as skin fibroblasts and blood cells do not often reflect the severity of the affected organ, in this case, the brain. The purpose of this review is to highlight the pitfalls of quantitating mtDNA from peripheral -and not limited to-tissues for diagnosing patients suffering from a variety of mtDNA depletion syndromes exhibiting neurologic abnormalities. In lieu, a qualitative test of mitochondrial substrate-level phosphorylation -even from peripheral tissues-reflecting the ability of mitochondria to rely on glutaminolysis in the presence of respiratory chain defects is proposed as a novel diagnostic assessment of mitochondrial functionality.

The effects of relative gain and age on peripheral blood mononuclear cell mitochondrial enzyme activity in preweaned Holstein and Jersey calves

Mitochondria are central to metabolism, nutrition, and health, but many factors can influence their efficiency. The objectives of this study were to determine if the mitochondrial enzyme activities of citrate synthase, complex I, complex IV, and complex V from peripheral blood mononuclear cells in Holstein and Jersey dairy calves were affected by age or relative gain as a percent of initial weight. Twenty-three Holstein and 23 Jersey heifer calves were enrolled between 3 and 6 d of age and whole blood samples were collected via jugular venipuncture at 1, 2, and 8 wk of age. Crude mitochondrial extracts were obtained from the peripheral blood mononuclear cell fraction at each time point and subsequently assayed for enzymatic activity. Age-dependent changes in activity were observed in complex V for both breeds. In Jersey calves complex IV and citrate synthase activity differed with age. Complex I activity was greater for high relative gain Jerseys and tended to be greater for high relative gain Holstein calves. Holstein calves had greater incidence of scours compared with Jersey calves, and in both breeds scouring calves exhibited greater complex V activity compared with those without scours. These data suggest that age and immune challenge in the form of scours affect mitochondrial complex V activity. Additionally, complex I activity may serve as a marker for calf growth potential because it was influenced by relative gain and not age.

Effects of Exercise and Aging on Skeletal Muscle

A substantial loss of muscle mass and strength (sarcopenia), a decreased regenerative capacity, and a compromised physical performance are hallmarks of aging skeletal muscle. These changes are typically accompanied by impaired muscle metabolism, including mitochondrial dysfunction and insulin resistance. A challenge in the field of muscle aging is to dissociate the effects of chronological aging per se on muscle characteristics from the secondary influence of lifestyle and disease processes. Remarkably, physical activity and exercise are well-established countermeasures against muscle aging, and have been shown to attenuate age-related decreases in muscle mass, strength, and regenerative capacity, and slow or prevent impairments in muscle metabolism. We posit that exercise and physical activity can influence many of the changes in muscle during aging, and thus should be emphasized as part of a lifestyle essential to healthy aging.

Evaluating clinical mitochondrial respiratory chain enzymes from biopsy specimens presenting skewed probability distribution of activity data

Due to the relative rarity of mitochondrial diseases, generating reference ranges is problematic in evaluation of respiratory chain activities particularly in pediatric cases. We determined the sample distribution of respiratory chain enzyme activities in skeletal muscle biopsies collected from pediatric patients suspected of neuromuscular disorders. Activities of NADH-ubiquinone reductase, NADH-cytochrome c reductase, succinate-cytochrome c reductase; ubiquinol-cytochrome c reductase and cytochrome c oxidase activities have log-normal distributions even when confirmed mitochondrial diseases were ruled out. Impact of the log-normal distribution of the respiratory chain enzyme activities on clinical diagnostics is discussed.

Mitochondrial structure and function are not different between nonfailing donor and end-stage failing human hearts

During human heart failure, the balance of cardiac energy use switches from predominantly fatty acids (FAs) to glucose. We hypothesized that this substrate shift was the result of mitochondrial degeneration; therefore, we examined mitochondrial oxidation and ultrastructure in the failing human heart by using respirometry, transmission electron microscopy, and gene expression studies of demographically matched donor and failing human heart left ventricular (LV) tissues. Surprisingly, respiratory capacities for failing LV isolated mitochondria (n = 9) were not significantly diminished compared with donor LV isolated mitochondria (n = 7) for glycolysis (pyruvate + malate)- or FA (palmitoylcarnitine)-derived substrates, and mitochondrial densities, assessed via citrate synthase activity, were consistent between groups. Transmission electron microscopy images also showed no ultrastructural remodeling for failing vs. donor mitochondria; however, the fraction of lipid droplets (LDs) in direct contact with a mitochondrion was reduced, and the average distance between an LD and its nearest neighboring mitochondrion was increased. Analysis of FA processing gene expression between donor and failing LVs revealed 0.64-fold reduced transcript levels for the mitochondrial-LD tether, perilipin 5, in the failing myocardium (P = 0.003). Thus, reduced FA use in heart failure may result from improper delivery, potentially via decreased perilipin 5 expression and mitochondrial-LD tethering, and not from intrinsic mitochondrial dysfunction.-Holzem, K. M., Vinnakota, K. C., Ravikumar, V. K., Madden, E. J., Ewald, G. A., Dikranian, K., Beard, D. A., Efimov, I. R. Mitochondrial structure and function are not different between nonfailing donor and end-stage failing human hearts.

A novel method for determining human ex vivo submaximal skeletal muscle mitochondrial function

The present study utilized a novel method aiming to investigate mitochondrial function in human skeletal muscle at submaximal levels and at a predefined membrane potential. The effect of age and training status was investigated using a cross-sectional design. Ageing was found to be related to decreased leak regardless of training status. Increased training status was associated with increased mitochondrial hydrogen peroxide emission. Despite numerous studies, there is no consensus about whether mitochondrial function is altered with increased age. The novelty of the present study is the determination of mitochondrial function at submaximal activity rates, which is more physiologically relevant than the ex vivo functionality protocols used previously. Muscle biopsies were taken from 64 old or young male subjects (aged 60-70 or 20-30 years). Aged subjects were recruited as trained or untrained. Muscle biopsies were used for the isolation of mitochondria and subsequent measurements of DNA repair, anti-oxidant capacity and mitochondrial protein levels (complexes I-V). Mitochondrial function was determined by simultaneous measurement of oxygen consumption, membrane potential and hydrogen peroxide emission using pyruvate + malate (PM) or succinate + rotenone (SR) as substrates. Proton leak was lower in aged subjects when determined at the same membrane potential and was unaffected by training status. State 3 respiration was lower in aged untrained subjects. This effect, however, was alleviated in aged trained subjects. H2 O2 emission with PM was higher in aged subjects, and was exacerbated by training, although it was not changed when using SR. However, with a higher manganese superoxide dismuthase content, the trained aged subjects may actually have lower or similar mitochondrial superoxide emission compared to the untrained subjects. We conclude that ageing and the physical activity level in aged subjects are both related to changes in the intrinsic functionality of the mitochondrion in skeletal muscle. Both of these changes could be important factors in determining the metabolic health of the aged skeletal muscle cell.

Two weeks of one-leg immobilization decreases skeletal muscle respiratory capacity equally in young and elderly men

Physical inactivity affects human skeletal muscle mitochondrial oxidative capacity but the influence of aging combined with physical inactivity is not known. This study investigates the effect of two weeks of immobilization followed by six weeks of supervised cycle training on muscle oxidative capacity in 17 young (23±1years) and 15 elderly (68±1years) healthy men. We applied high-resolution respirometry in permeabilized fibers from muscle biopsies at inclusion after immobilization and training. Furthermore, protein content of mitochondrial complexes I-V, mitochondrial heat shock protein 70 (mtHSP70) and voltage dependent anion channel (VDAC) were measured in skeletal muscle by Western blotting. The elderly men had lower content of complexes I-V and mtHSP70 but similar respiratory capacity and content of VDAC compared to the young. In both groups the respiratory capacity and protein content of VDAC, mtHSP70 and complexes I, II, IV and V decreased with immobilization and increased with retraining. Moreover, there was no overall difference in the response between the groups. When the intrinsic mitochondrial capacity was evaluated by normalizing respiration to citrate synthase activity, the respiratory differences with immobilization and training disappeared. In conclusion, aging is not associated with a decrease in muscle respiratory capacity in spite of lower complexes I-V and mtHSP70 protein content. Furthermore, immobilization decreased and aerobic training increased the respiratory capacity and protein contents of complexes I-V, mtHSP70 and VDAC similarly in the two groups. This suggests that inactivity and training alter mitochondrial biogenesis equally in young and elderly men.

Mitochondrial and skeletal muscle health with advancing age

With increasing age there is a temporal relationship between the decline of mitochondrial and skeletal muscle volume, quality and function (i.e., health). Reduced mitochondrial mRNA expression, protein abundance, and protein synthesis rates appear to promote the decline of mitochondrial protein quality and function. Decreased mitochondrial function is suspected to impede energy demanding processes such as skeletal muscle protein turnover, which is critical for maintaining protein quality and thus skeletal muscle health with advancing age. The focus of this review was to discuss promising human physiological systems underpinning the decline of mitochondrial and skeletal muscle health with advancing age while highlighting therapeutic strategies such as aerobic exercise and caloric restriction for combating age-related functional impairments.

Skeletal muscle aging and the mitochondrion

Decline in human muscle mass and strength (sarcopenia) is a hallmark of the aging process. A growing body of research in the areas of bioenergetics and protein turnover has placed the mitochondria at the center of this process. It is now clear that, unless an active lifestyle is rigorously followed, skeletal muscle mitochondrial decline occurs as humans age. Increasing research on mitochondrial biology has elucidated the regulatory pathways involved in mitochondrial biogenesis, many of which are potential therapeutic targets, and highlight the beneficial effects of vigorous physical activity on skeletal muscle health for an aging population.

Cultured senescent myoblasts derived from human vastus lateralis exhibit normal mitochondrial ATP synthesis capacities with correlating concomitant ROS production while whole cell ATP production is decreased

The free radical theory of aging says that increased oxidative stress and mitochondrial dysfunction are associated with old age. In the present study we have investigated the effects of cellular senescence on muscle energetic by comparing mitochondrial content and function in cultured muscle satellite cells at early and late passage numbers. We show that cultured muscle satellite cells undergoing senescence express a reduced mitochondrial mass, decreased whole cell ATP level, normal to increased mitochondrial ATP production under ATP utilization, increased mitochondrial membrane potential and increased superoxide/mitochondrial mass and hydrogen peroxide/mitochondrial mass ratios. Moreover, the increased ROS production correlates with the corresponding mitochondrial ATP production. Thus, myotubes differentiated from human myoblasts undergoing senescence have a reduced mitochondrial content, but the existent mitochondria express normal to increased functional capabilities. The present data suggest that the origin of aging lies outside the mitochondria and that a malfunction in the cell might be preceding and initiating the increase of mitochondrial ATP synthesis and concomitant ROS production in the single mitochondrion in response to decreased mitochondrial mass and reduced extra-mitochondrial energy supply. This then can lead to the increased damage of DNA, lipids and proteins of the mitochondria as postulated by the free radical theory of aging.

Age-related changes in oxidative capacity differ between locomotory muscles and are associated with physical activity behavior

There is discrepancy in the literature regarding the degree to which old age affects muscle bioenergetics. These discrepancies are likely influenced by several factors, including variations in physical activity (PA) and differences in the muscle group investigated. To test the hypothesis that age may affect muscles differently, we quantified oxidative capacity of tibialis anterior (TA) and vastus lateralis (VL) muscles in healthy, relatively sedentary younger (8 YW, 8 YM; 21-35 years) and older (8 OW, 8 OM; 65-80 years) adults. To investigate the effect of physical activity on muscle oxidative capacity in older adults, we compared older sedentary women to older women with mild-to-moderate mobility impairment and lower physical activity (OIW, n = 7), and older sedentary men with older active male runners (OAM, n = 6). Oxidative capacity was measured in vivo as the rate constant, k(PCr), of postcontraction phosphocreatine recovery, obtained by (31)P magnetic resonance spectroscopy following maximal isometric contractions. While k(PCr) was higher in TA of older than activity-matched younger adults (28%; p = 0.03), older adults had lower k(PCr) in VL (23%; p = 0.04). In OIW compared with OW, k(PCr) was lower in VL (∼45%; p = 0.01), but not different in TA. In contrast, OAM had higher k(PCr) than OM (p = 0.03) in both TA (41%) and VL (54%). In older adults, moderate-to-vigorous PA was positively associated with k(PCr) in VL (r = 0.65, p < 0.001) and TA (r = 0.41, p = 0.03). Collectively, these results indicate that age-related changes in oxidative capacity vary markedly between locomotory muscles, and that altered PA behavior may play a role in these changes.

Importance of muscle light microscopic mitochondrial subsarcolemmal aggregates in the diagnosis of respiratory chain deficiency

The purpose of this study was to evaluate relationships between subsarcolemmal mitochondrial aggregates and electron transport chain deficiencies in skeletal muscle with the objective of establishing an association between mitochondrial accumulation and electron transport chain complex deficiency. We conducted a large-scale, retrospective study to evaluate factors associated with subsarcolemmal mitochondrial aggregates (percent) in pediatric patients who received muscle biopsies for suspected respiratory chain disorders. Patients were included if they had histochemical stains for assessment of mitochondrial pathology and had biochemical testing for muscle electron transport chain complex activities. Significant positive bivariate correlations (n = 337) were found between subsarcolemmal mitochondrial aggregate percentage and electron transport chain complexes II, IV, I + III, and II + III activities. Evaluation showed that a cutoff value of > 2% subsarcolemmal mitochondrial aggregates had poor overall diagnostic accuracy (mean, 32%), compared with a < 5% cutoff (mean, 60%). To better evaluate the effects of subsarcolemmal mitochondrial aggregates percentages, patients were stratified according to lower one-third (group 1, n = 120 plus ties) and upper one-third (group 2, n = 115 plus ties) of subsarcolemmal mitochondrial aggregates values. Although only minor clinical and pathologic differences were observed, group 1 participants had significantly lower electron transport chain complex activities than group 2 for all enzymes except complex III. Logistic regression showed over 2-fold greater odds of deficiency for electron transport chain complexes I + III (P = .01) and II + III (P = .03) for group 1 participants compared with group 2. We conclude that, contrary to the previous > 2.0% subsarcolemmal mitochondrial aggregates cutoff for respiratory chain disorder, patients with a low subsarcolemmal mitochondrial aggregates percentage (≤4%) are significantly more likely to have electron transport chain complex deficiency than patients with increased subsarcolemmal mitochondrial aggregates percentage (≥10%). This morphological approach for assessment of mitochondrial proliferation may assist clinicians to select further testing to rule out an electron transport chain complex deficiency in children by other methods, including direct biochemical testing of electron transport chain complex activities, measurement of muscle coenzyme Q10 content, or evaluation for a mitochondrial DNA depletion syndrome.

Mitochondrial respiratory chain: biochemical analysis and criterion for deficiency in diagnosis

Spectrophotometric evaluation of mitochondrial respiratory chain (MRC) enzymatic complexes is the main approach to the biochemical investigation and diagnosis in oxidative phosphorylation disorders (also known as mitochondrial cytopathies). Regular dual beam spectrophotometers may be used, but we describe the protocols for double wavelength devices, allowing the analysis of complex activities from a small amount of tissue, with high sensitivity. An important concern is which tissue should be selected for analysis. Accordingly, we present the results obtained with different tissues and control values to be used. There are no standards available for the determinations and no interlaboratory quality control schemes are implemented. Additionally, different laboratories may use different protocols and comparison of results may be difficult. Currently, there is no consensus in literature for defining a criterion of an MRC deficiency to be used in biochemical diagnosis. There is statistical evidence that the most adequate criterion to define an MRC deficiency is below 40% of the mean control value normalized to citrate synthase activity.

Biomarkers for mitochondrial respiratory chain disorders

Mitochondrial respiratory chain deficiencies are a group of more than 100 disorders of adults and children, with highly variable phenotypes. Their diagnosis is a great challenge, in spite of the fact that knowledge on their molecular genetic background has increased considerably during the last 20 years. Muscle biopsy is the key diagnostic procedure, including histological and biochemical analysis of mitochondria. Less invasive, specific and sensitive diagnostic tools based on serum biomarkers are still lacking. Recent technological developments, especially in mass spectrometry, enable novel tools for identification of local and global molecular consequences of mitochondrial respiratory chain dysfunction in patient samples. Furthermore, emerging disease models, especially genetically modified mice, offer unique materials to tackle pathophysiology with modern transcriptomic, proteomic, and metabolomic approaches. Identified molecular signals or metabolic fingerprints have the potential to be highly useful biomarkers for future diagnosis of mitochondrial respiratory chain disorders.

Regulation of skeletal muscle mitochondrial function: genes to proteins

The impact of ageing on mitochondrial function and the deterministic role of mitochondria on senescence continue to be topics of vigorous debate. Many studies report that skeletal muscle mitochondrial content and function are reduced with ageing and metabolic diseases associated with insulin resistance. However, an accumulating body of literature suggests that physical inactivity typical of ageing may be a more important determinant of mitochondrial function than chronological age, per se. Reports of age-related declines in mitochondrial function have spawned a vast body of literature devoted to understanding the underlying mechanisms. These mechanisms include decreased abundance of mtDNA, reduced mRNA levels, as well as decreased synthesis and expression of mitochondrial proteins, ultimately resulting in decreased function of the whole organelle. Effective therapies to prevent, reverse or delay the onset of the aforementioned mitochondrial changes, regardless of their inevitability or precise underlying causes, require an intimate understanding of the processes that regulate mitochondrial biogenesis, which necessitates the coordinated regulation of nuclear and mitochondrial genomes. Herein we review the current thinking on regulation of mitochondrial biogenesis by transcription factors and transcriptional co-activators and the role of hormones and exercise in initiating this process. We review how exercise may help preserve mitochondrial content and functionality across the lifespan, and how physical inactivity is emerging as a major determinant of many age-associated changes at the level of the mitochondrion. We also review evidence that some mitochondrial changes with ageing are independent of exercise or physical activity and appear to be inevitable consequences of old age.

Aberrant mitochondrial homeostasis in the skeletal muscle of sedentary older adults

The role of mitochondrial dysfunction and oxidative stress has been extensively characterized in the aetiology of sarcopenia (aging-associated loss of muscle mass) and muscle wasting as a result of muscle disuse. What remains less clear is whether the decline in skeletal muscle mitochondrial oxidative capacity is purely a function of the aging process or if the sedentary lifestyle of older adult subjects has confounded previous reports. The objective of the present study was to investigate if a recreationally active lifestyle in older adults can conserve skeletal muscle strength and functionality, chronic systemic inflammation, mitochondrial biogenesis and oxidative capacity, and cellular antioxidant capacity. To that end, muscle biopsies were taken from the vastus lateralis of young and age-matched recreationally active older and sedentary older men and women (N = 10/group; female symbol = male symbol). We show that a physically active lifestyle is associated with the partial compensatory preservation of mitochondrial biogenesis, and cellular oxidative and antioxidant capacity in skeletal muscle of older adults. Conversely a sedentary lifestyle, associated with osteoarthritis-mediated physical inactivity, is associated with reduced mitochondrial function, dysregulation of cellular redox status and chronic systemic inflammation that renders the skeletal muscle intracellular environment prone to reactive oxygen species-mediated toxicity. We propose that an active lifestyle is an important determinant of quality of life and molecular progression of aging in skeletal muscle of the elderly, and is a viable therapy for attenuating and/or reversing skeletal muscle strength declines and mitochondrial abnormalities associated with aging.

Evaluation of the mitochondrial respiratory chain and oxidative phosphorylation system using polarography and spectrophotometric enzyme assays

The oxidative phosphorylation (OXPHOS) system consists of five multimeric complexes embedded in the mitochondrial inner membrane. They work in concert to drive the aerobic synthesis of ATP. Mitochondrial and nuclear DNA mutations affecting the accumulation and function of these enzymes are the most common cause of mitochondrial diseases and have also been associated with neurodegeneration and aging. For this reason, several approaches for the assessment of the OXPHOS system enzymes have been developed. Based on the methods described elsewhere, the assays describe methods that form a biochemical characterization of the OXPHOS system in cells and mitochondria isolated from cultured cells or tissues.

The coordination of nuclear and mitochondrial communication during aging and calorie restriction

Mitochondria are dynamic organelles that integrate environmental signals to regulate energy production, apoptosis and Ca(2+) homeostasis. Not surprisingly, mitochondrial dysfunction is associated with aging and the pathologies observed in age-related diseases. The vast majority of mitochondrial proteins are encoded in the nuclear genome, and so communication between the nucleus and mitochondria is essential for maintenance of appropriate mitochondrial function. Several proteins have emerged as major regulators of mitochondrial gene expression, capable of increasing transcription of mitochondrial genes in response to the physiological demands of the cell. In this review, we will focus on PGC-1alpha, SIRT1, AMPK and mTOR and discuss how these proteins regulate mitochondrial function and their potential involvement in aging, calorie restriction and age-related disease. We will also discuss the pathways through which mitochondria signal to the nucleus. Although such retrograde signaling is not well studied in mammals, there is growing evidence to suggest that it may be an important area for future aging research. Greater understanding of the mechanisms by which mitochondria and the nucleus communicate will facilitate efforts to slow or reverse the mitochondrial dysfunction that occurs during aging.

Rare presentation of familial paraganglioma without evidence of mutation in the SDH, RET and VHL genes: towards further genetic heterogeneity

OBJECTIVE

Mutations in genes encoding succinate dehydrogenase and its anchoring subunits (SDH genes) are at the origin of hereditary head and neck paraganglioma (PGL) and a subset of apparently sporadic pheochromocytoma.

METHODS

We describe a family including three patients harbouring bilateral head and neck PGL diagnosed before 25 years of age. Multiple hypervascular hepatic lesions were subsequently discovered in two of them. In both, liver biopsy confirmed the diagnosis of PGL. In addition, in one patient, MRI disclosed multiple target-like lesions of the spine, highly suggestive of metastatic PGL. Family history was compatible with autosomal dominant inheritance with possible maternal imprinting.

RESULTS

Combined single-strand conformation polymorphism and heteroduplex analysis followed by sequencing did not show any mutation of the coding parts of SDHB, SDHC, SDHD, RET or VHL genes. Screening of copy number alterations and loss of heterozygosity in the three affected family members showed no deletion or amplification of the SDH, RET and VHL genes. Furthermore, succinate dehydrogenase activity measured in a liver PGL sample was not significantly decreased in the affected patient as compared with controls, underscoring the exclusion of the SDH genes.

CONCLUSIONS

To our knowledge, this is the first reported family of hereditary head and neck PGL with metastatic dissemination in the liver and the spine. A large body of evidence supports the absence of mutations in SDH, RET and VHL genes, which suggests the existence of a yet unknown gene at the origin of this particular form of familial PGL.

Resistance training, sarcopenia, and the mitochondrial theory of aging

Skeletal muscle aging is associated with a significant loss of muscle mass, strength, function, and quality of life. In addition, the healthcare cost of aging and age-related disease is growing, and will continue to grow as a larger proportion of our population reaches retirement age and beyond. The mitochondrial theory of aging has been identified as a leading explanation of the aging process and describes a path leading to cellular senescence that includes electron transport chain deficiency, reactive oxygen species production, and the accumulation of mitochondrial DNA deletions and mutations. It is also quite clear that regular resistance exercise is a potent and effective countermeasure for skeletal muscle aging. In this review, we discuss age-related sarcopenia, the mitochondrial theory of aging, and how resistance exercise may directly affect key components of the mitochondrial theory. It is clear from the data discussed that regular resistance training can effectively disturb processes that contribute to the progression of aging as it pertains to the mitochondrial theory.

Systems analysis of energy metabolism elucidates the affected respiratory chain complex in Leigh's syndrome

Leigh's syndrome is a complex neurological disease with little known correlation between causes and symptoms. Mutations in pyruvate dehydrogenase and electron transport chain complexes have been associated with this syndrome, although the identification of affected enzymes is difficult, if not impossible, with non-invasive clinical tests. In this study, isotopomer analysis is used to characterize the metabolic phenotype of normal and Leigh's syndrome fibroblasts (GM01503), thereby identifying affected enzymes in the diseased cells. Fibroblasts are grown with DMEM media enriched with (13)C labeled glucose. Amino acids from media and proteins as well as lactate are analyzed with GC-MS to identify their label distributions. A computational model accounting for all major pathways in fibroblast metabolism (including 430 metabolites and 508 reactions) is built to determine the metabolic steady states of the normal and Leigh's cell lines based on measured substrate uptake and secretion rates and isotopomer data. Results show that (i) Leigh's syndrome affected cells have slower metabolism than control fibroblasts as evidenced by their overall slower substrate utilization and lower secretion of end products; (ii) intracellular fluxes predicted by the models, some of which are validated by biochemical studies published in the literature, show that the respiratory chain in Leigh's affected cells can produce ATP at a similar rate as the controls, but with a more restricted flux range; and (iii) mutations causing the defects observed in the Leigh's cells are likely to be in succinate cytochrome c reductase.

Human epidermal keratinocytes accumulate superoxide due to low activity of Mn-SOD, leading to mitochondrial functional impairment

The energy metabolism of the epidermis has been the subject of controversy; thus we characterized the mitochondrial phenotype of human primary keratinocytes and fibroblasts, in cell culture and in human skin sections. We found that keratinocytes respire as much as fibroblasts, however, maximal activities of the respiratory chain (RC) complexes were 2- to 5-fold lower, whereas expression levels of RC proteins were similar. Maximal activities of aconitase and isocitrate dehydrogenase, two mitochondrial enzymes especially vulnerable to superoxide, were lower than in fibroblasts. Indeed, superoxide anion levels were much higher in keratinocytes, and keratinocytes displayed higher lipid peroxidation levels and a lower reduced glutathione/oxidized glutathione ratio, indicating enhanced oxidative stress. Although superoxide dismutase activity and especially expression of the mitochondrial superoxide dismutase, Mn-SOD, were drastically lower in keratinocytes, explaining the high superoxide levels, glutathione peroxidase activity and protein were almost undetectable in fibroblasts. Catalase activity and hydrogen peroxide levels were similar. In summary, we could show that keratinocytes actively use the mitochondrial RC not only for adenosine 5' triphosphate synthesis but also for the accumulation of superoxide anions, even at the expense of mitochondrial functional capacity, indicating that superoxide-driven mitochondrial impairment might be a prerequisite for keratinocyte differentiation.

Three spectrophotometric assays for the measurement of the five respiratory chain complexes in minuscule biological samples

BACKGROUND

The measurement of the activities of the five complexes comprising the respiratory chain has proven to be a major challenge when a limiting amount of biological material is available. Here we report a set of three convenient assays that allows this measurement under such circumstances.

METHODS

One assay relies on the sequential addition of reagents to measure first complex IV activity, followed by complex II+III, and then glycerol-3-phosphate dehydrogenase+complex III activities and finally isolated complex III activity. A second assay measures the activity of complex II followed by glycerol-3-phosphate dehydrogenase and isocitrate dehydrogenase. A third assay measures rotenone-sensitive complex I activity and subsequently oligomycin-sensitive complex V activity.

RESULTS

These assays have been successfully used on extracts of small numbers of human cells displaying various defects in the respiratory chain, and on frozen tissue homogenates of retina and very early mouse embryos.

CONCLUSIONS

The strength of this set of assays lies both in its rapid and simple execution and its capacity for immediate detection of partial defects, because each activity can be compared with one or two other activities measured in the same sample.

Different pathophysiological mechanisms of intramitochondrial iron accumulation in acquired and congenital sideroblastic anemia caused by mitochondrial DNA deletion

Sideroblastic anemias (SA) are characterized by iron accumulation in the mitochondria of erythroblasts. Although we have evidence of mitochondrial gene alterations in sporadic congenital cases, the origin of acquired forms [refractory anemia with ring sideroblasts (RARS)], is still largely unknown. Here, we report the analysis of respiratory chain function in a patient with a large mitochondrial deletion and in patients with RARS. A young boy with SA showed symptoms typical of a mitochondrial disease with metabolic acidosis, muscle weakness and cerebral involvement. His bone marrow DNA was analyzed for the presence of mitochondrial deletions. We found a new mitochondrial (mt)DNA deletion spanning 3,614 bp and including all the mt genes encoding complex IV, plus ATPase 6 and 8, and several transfer (t)RNAs. All tissues analyzed (liver, skeletal muscle, brain, pancreas) showed a heteroplasmic distribution of this mutant DNA. Bone marrow homogenates were obtained from five patients with RARS and from three patients with normal bone marrow and respiratory chain function assayed by spectrophotometric analysis. Cytochrome c oxidase (CCO) activity was greatly reduced in the patient's bone marrow. In contrast, CCO activity and global respiratory chain function were conserved in patients with RARS. We conclude that deficient CCO activity secondary to mtDNA deletions is related to intramitochondrial iron accumulation, as in our patient or in those with Pearson's syndrome, whereas other mechanisms, e.g. nuclear DNA mutations, have to be proposed to be involved in the acquired forms of SA.

Loss of Aif function causes cell death in the mouse embryo, but the temporal progression of patterning is normal

Apoptosis-inducing factor (AIF) is an evolutionarily conserved, ubiquitously expressed flavoprotein with NADH oxidase activity that is normally confined to mitochondria. In mammalian cells, AIF is released from mitochondria in response to apoptotic stimuli and translocates to the nucleus where it is thought to bind DNA and contribute to chromatinolysis and cell death in a caspase-independent manner. Here we describe the consequences of inactivating Aif in the early mouse embryo. Unexpectedly, we found that both the apoptosis-dependent process of cavitation in embryoid bodies and apoptosis associated with embryonic neural tube closure occur in the absence of AIF, indicating that Aif function is not required for apoptotic cell death in early mouse embryos. By embryonic day 9 (E9), loss of Aif function causes abnormal cell death, presumably because of reduced mitochondrial respiratory chain complex I activity. Because of this cell death, Aif null embryos fail to increase significantly in size after E9. Remarkably, patterning processes continue on an essentially normal schedule, such that E10 Aif null embryos with only approximately 1/10 the normal number of cells have the same somite number as their wild-type littermates. These observations show that pattern formation in the mouse can occur independent of embryo size and cell number.

Investigation of children for mitochondriopathy confirms need for strict patient selection, improved morphological criteria, and better laboratory methods

We studied muscle biopsies of 103 pediatric patients in whom clinical suspicion for disorder of energy metabolism was highest in 13 patients, intermediate in 8 patients, and lowest in 82 patients. Electron transport complex (ETC) enzyme activity measurements were available in 96 of 103 patients. Most children with unclassified encephalopathy before biopsy had negative or equivocal morphological and biochemical evaluation for disorder of energy metabolism (72/85). The incidence of ETC abnormality and morphological abnormality in muscle from 39 patients with clinical encephalomyopathy (groups I, II, and III) was 20% and 38%, respectively. In 21 children with high or intermediate clinical suspicion of mitochondriopathy, light microscopy was confirmative in 12, ultrastructure was confirmative in 15, and major ETC abnormality was present in only 4 (29%) of 14. In 82 children with lower clinical suspicion of mitochondriopathy, morphological criteria at both the light and electron microscopic level were absent, and major abnormality of ETC activity was uncommon, in 9 (11%) of 82. Partial reductions of ETC activity occurred in 15 (18%) of 82, but are of uncertain significance. Ragged blue fibers were more prevalent in infants with mitochondriopathy than ragged red fibers. Increase of large, but not small, subsarcolemmal mitochondrial aggregates based on succinate dehydrogenase histochemistry is a useful indicator for mitochondriopathy. Thus, a distinction should be made between small aggregates (normal) and large aggregates. Using strict criteria to define pathological mitochondria, we concluded that electron microscopy is a powerful tool in the diagnosis of mitochondriopathy mainly when clinical suspicion is high. We found no consistent difference in the frequency of mitochondrial "proliferation" as currently defined or in citrate synthase activity in any group. Better patient selection in infants and children and better methods for investigation of mitochondriopathy are needed.

Antioxidant enzyme activity is up-regulated after unilateral resistance exercise training in older adults

Cellular antioxidant capacity and oxidative stress are postulated to be critical factors in the aging process. The effects of resistance exercise training on the level of skeletal muscle oxidative stress and antioxidant capacity have not previously been examined in older adults. Muscle biopsies from both legs were obtained from the vastus lateralis muscle of 12 men 71 +/- 7 years of age. Subjects then engaged in a progressive resistance exercise-training program with only one leg for 12 weeks. After 12 weeks, the nontraining leg underwent an acute bout of exercise (exercise session identical to that of the trained leg at the same relative intensity) at the same time as the last bout of exercise in the training leg. Muscle biopsies were collected from the vastus lateralis of both legs 48 h after the final exercise bout. Electron transport chain enzyme activity was unaffected by resistance training and acute resistance exercise (p < 0.05). Training resulted in a significant increase in CuZnSOD (pre--7.2 +/- 4.2, post--12.6 +/- 5.6 U.mg protein(-1); p = 0.02) and catalase (pre--8.2 +/- 2.3, post--14.9 +/- 7.6 micromol.min(-1).mg protein(-1); p = 0.02) but not MnSOD activity, whereas acute exercise had no effect on the aforementioned antioxidant enzyme activities. Furthermore, basal muscle total protein carbonyl content did not change as a result of exercise training or acute exercise. In conclusion, unilateral resistance exercise training is effective in enhancing the skeletal muscle cellular antioxidant capacity in older adults. The potential long-term benefits of these adaptations remain to be evaluated.

Age-Related Changes of Mitochondrial DNA Content and Mitochondrial Genotypic and Phenotypic Alterations in Rat Hind-Limb Skeletal Muscles

Mitochondrial DNA (mtDNA) content relative to nuclear DNA content as well as mitochondrial transcription factor A (TFAM) content was measured in four hind-limb skeletal muscles, namely soleus (S), tibialis anterior (TA), gastrocnemius (G), and extensor digitorum longus (EDL) of adult rats. Content of mtDNA in 6-month-old rats is in the rank order of S > TA > G > EDL, and TFAM content is higher in S than in the other studied muscles. After the rat is 6 months of age, the mtDNA content decreases only in S and TA, whereas the TFAM content increases only in S. Deletions in mtDNA appear quite early in life in S and later on in the other muscles. Fibers defective for mitochondrial respiratory enzymes appear in rats at 15 months of age. In the oldest animals, the highest frequencies of occurrence of mtDNA deletions as well as of mitochondrial phenotypic alterations are found in S according to its highest mtDNA content and oxidative potential.

Aging muscle

Age causes structural and functional changes in skeletal muscle in a wide range of species, including humans. Muscle changes in humans start in the fourth decade of life and cause frailty and disabilities. Associated changes in body composition form the basis of many metabolic disorders, such as insulin resistance, type 2 diabetes, hypertension, and hyperlipidemia, which result in an increased incidence of cardiovascular death. Decreases in the synthesis rates of many muscle proteins, specifically of myosin heavy chain and mitochondrial proteins, occur with age. The underlying causes of the reduction in mitochondrial biogenesis and ATP production seem to be decreases in mitochondrial DNA and messenger RNA. Reduced ATP production could be the basis of reduced muscle protein turnover, which requires energy. Both aerobic exercise and resistance exercise enhance muscle protein synthesis and mitochondrial biogenesis. Insulin and amino acids have also been shown to enhance muscle mitochondrial biogenesis and mitochondrial protein synthesis. However, the insulin-induced increase in muscle mitochondrial ATP production is defective in type 2 diabetic patients with insulin resistance. Moreover, a dissociation between increases in muscle mitochondrial biogenesis and insulin sensitivity after exercise has been noted in older persons. It remains to be determined whether muscle mitochondrial dysfunction causes or results from insulin resistance. Exercise seems to enhance the efficiency of muscle mitochondrial DNA in rodents. Reduced physical activity as a contributor of age-related mitochondrial dysfunction remains to be determined. It is proposed that a reduction in tissue mitochondrial ATP production signals the hypothalamic centers to reduce spontaneous physical activities. Voluntary physical activity is regulated by cognitive centers and could attenuate the progressive decline in mitochondrial functions that occurs with age.

Decline in skeletal muscle mitochondrial function with aging in humans

Cumulative mtDNA damage occurs in aging animals, and mtDNA mutations are reported to accelerate aging in mice. We determined whether aging results in increased DNA oxidative damage and reduced mtDNA abundance and mitochondrial function in skeletal muscle of human subjects. Studies performed in 146 healthy men and women aged 18-89 yr demonstrated that mtDNA and mRNA abundance and mitochondrial ATP production all declined with advancing age. Abundance of mtDNA was positively related to mitochondrial ATP production rate, which in turn, was closely associated with aerobic capacity and glucose tolerance. The content of several mitochondrial proteins was reduced in older muscles, whereas the level of the oxidative DNA lesion, 8-oxo-deoxyguanosine, was increased, supporting the oxidative damage theory of aging. These results demonstrate that age-related muscle mitochondrial dysfunction is related to reduced mtDNA and muscle functional changes that are common in the elderly.

Oxidative stress and the mitochondrial theory of aging in human skeletal muscle

According to the mitochondrial theory of aging, an age-related increase in oxidative stress is responsible for cellular damage and ultimately cell death. Despite compelling evidence that supports the mitochondrial theory of aging in some tissues, data regarding aging skeletal muscle are inconsistent. We collected resting muscle biopsies from the vastus lateralis, and 24 h urine samples from, young (N = 12, approximately 22 yr), and older (N = 12 approximately 72 yr) men. Urinary 8-OHdG was significantly higher in older as compared to younger men (Old: 7714 +/- 1402, Young: 5333 +/- 1191 ng g(-1) creatinine: p = 0.005), as were levels of protein carbonyls (Old: 0.72 +/- 0.42, Young: 0.26 +/- 0.14 nmol mg(-1) protein: p = 0.007). MnSOD activity (Old: 7.1 +/- 0.8, Young: 5.2 +/- 1.8 U mg(-1) protein: p = 0.04) and catalase activity (Old: 8.5 +/- 2.0, Young: 6.2 +/- 2.4 micro mol min(-1) mg(-1) protein: p = 0.03) were significantly higher in old as compared to young men, respectively, with no differences observed for total or CuZnSOD. Full-length mtDNA appeared lower in old as compared to young men, and mtDNA deletions were present in 6/8 old and 0/6 young men (p = 0.003). The maximal activities of citrate synthase, and complex II+III, and IV were not different between young and old men, however, complex I+III activity was marginally higher in older as compared to younger men (Old: 2.5 +/- 0.5, Young: 1.9 +/- 0.5 micromol min(-1) g(-1) w.w: p = 0.03) respectively. In conclusion, healthy aging is associated with oxidative damage to proteins and DNA, a compensatory up-regulation of antioxidant enzymes, and aberrations of mtDNA, with no reduction in electron transport chain maximal enzyme activity.

Understanding aging: revealing order out of chaos

Aging is often described as an extremely complex process affecting all of the vital parameters of an individual. In this article, we review how understanding of aging evolved from the first analyses of population survival to the identification of the molecular mechanisms regulating life span. Abundant evidence implicates mitochondria in aging and we focus on the three main components of the mitochondrial theory of aging: (1) increased reactive oxygen species (ROS) production, (2) mitochondrial DNA (mtDNA) damage accumulation, and (3) progressive respiratory chain dysfunction. Experimental evidence shows a relationship between respiratory chain dysfunction, ROS damage, and aging in most of the model organisms. However, involvement of the mtDNA mutations in the aging process is still debated. We recently created a mutant mouse strain with increased levels of somatic mtDNA mutations causing a progressive respiratory chain deficiency and premature aging. These mice demonstrate the fundamental importance of the accumulation of mtDNA alterations in aging. We present here an integrative model where aging is provoked by a single primary event leading to a variety of effects and secondary causes.

Is defective electron transport at the hub of aging?

The bulwark of the mitochondrial theory of aging is that a defective respiratory chain initiates the death cascade. The increased production of superoxide is suggested to result in progressive oxidant damage to cellular components and particularly to mtDNA that encodes subunits assembled in respiratory complexes. Earlier studies of respiration in muscle mitochondria obtained from large cohorts of patients supported this notion by showing that either singly or in combinations, the respiratory complexes exhibited decreased activity in the elderly. The following critique of the most cited publications over the past decade points out the systematic errors that put earlier work at odds with recent findings. These later investigations indicate that aging has no overt effect on either the electron transport system or oxidative phosphorylation.

Mitochondrial activities in human cultured skin fibroblasts contaminated by Mycoplasma hyorhinis

Background

Mycoplasma contaminations are a recurrent problem in the use of cultured cells, including human cells, especially as it has been shown to impede cell cycle, triggering cell death under various conditions. More specific consequences on cell metabolism are poorly known.

Results

Here we report the lack of significant consequence of a heavy contamination by the frequently encountered mycoplasma strain, M. hyorhinis, on the determination of respiratory chain activities, but the potential interference when assaying citrate synthase. Contamination by M. hyorhinis was detected by fluorescent imaging and further quantified by the determination of the mycoplasma-specific phosphate acetyltransferase activity. Noticeably, this latter activity was not found equally distributed in various mycoplasma types, being exceptionally high in M. hyorhinis.

Conclusion

While we observed a trend for respiration reduction in heavily contaminated cells, no significant and specific targeting of any respiratory chain components could be identified. This suggested a potential interference with cell metabolism rather than direct interaction with respiratory chain components.

Persistent mitochondrial dysfunction in HIV-1-exposed but uninfected infants: clinical screening in a large prospective cohort

BACKGROUND

Antiretroviral prevention of mother to child HIV-1 is established but tolerance remains to be assessed. AIM To determine the risk for persistent mitochondrial dysfunction in HIV-uninfected children born to seropositive mothers.

METHOD

An exhaustive study in a large prospective cohort with predetermined algorithm of the unexplained symptoms compatible with mitochondrial dysfunction. A total of 2644 of 4392 children were exposed to antiretrovirals. Complementary investigations were carried out on a case-by-case basis using classification with a diagnostic probability scale, based on experience with constitutional diseases. A spontaneous notification register for children not included in the cohort was created.

RESULTS

Good circumstantial evidence of mitochondrial dysfunction was found for twelve children. Seven were from the cohort. All presented neurological symptoms, often associated with abnormal magnetic resonance image (10 of 12) and/or a significant episode of hyperlactatemia (seven of 12). All had either a profound deficit in one of the respiratory chain complexes (11 of 12) and/or a typical histological pattern (two of 12). All were perinatally exposed to antiretrovirals. None of them had perinatal morbidity that could explain this symptomatology. The 18-month incidence was 0.26% (95% confidence interval, 0.10-0.54) in exposed children, in comparison with the general figure of 0.01% for paediatric neuro-mitochondrial diseases in the general population. Fourteen other children in the cohort, all exposed to antiretrovirals, had unexplained symptoms, mostly neurological, for which one of the possible differential diagnoses was mitochondrial dysfunction. Close similarities in clinical, neuroradiological and histological findings strongly suggest a common pathological process in all these 26 children.

CONCLUSION

Children exposed to nucleoside analogues during the perinatal period are at risk of a neurological syndrome associated with persistent mitochondrial dysfunction.

Experimental evidence against the mitochondrial theory of aging. A study of isolated human skeletal muscle mitochondria

The mitochondrial theory of aging was tested with optimised preparation techniques. Mitochondria were isolated from approximately 90 mg quadriceps muscle from healthy humans at age 70+ and 20+. The content of mitochondrial protein was approximately 10 mg g(-1) muscle and the yields were approximately 40%. The mitochondrial integrity was high as judged from the respiratory control and P/O ratios. No general membrane alterations or changes in the cytochrome contents were observed. BSA decreased the non-phosphorylating rates of respiration equally in both age groups. Thirteen different enzyme activities were assayed and normalised to protein content and citrate synthase activity. Most of the critical levels for detection of declines were <10%. In the 70+ group, the activity for fatty acid oxidation was decreased by approximately 20%. Two inherently low activities associated with oxidation of sarcoplasmic NADH were also decreased, probably related to the age change of fibre types. The remaining activities measured, e.g. those of pyruvate dehydrogenase, tricarboxylic acid cycle, respiratory chain, and ATP synthesis, were not observed to be lowered. Thus, the central bioenergetic systems appeared unaltered with age. The obvious discord with reported age declines of human skeletal muscle mitochondrial function is discussed. It is concluded that the present results are incompatible with the mitochondrial theory of aging.

Antenatal manifestations of mitochondrial respiratory chain deficiency

OBJECTIVE

To review the antenatal manifestations of disorders of oxidative phosphorylation.

STUDY DESIGN

A total of 300 cases of proven respiratory chain enzyme deficiency were retrospectively reviewed for fetal development, based on course and duration of pregnancy, antenatal ultrasonography and birth weight, length, and head circumference. Particular attention was given to fetal movements, oligo/hydramnios, fetal cardiac rhythm, fetal heart ultrasound, and ultrasonography/echo Doppler signs of brain, facial, trunk, limb, and organ anomalies.

RESULTS

Retrospective analyses detected low birth weight (<3rd percentile for gestational age) in 22.7% of cases (68/300, P<.000001). Intrauterine growth retardation was either isolated (48/300, 16%) or associated with otherwise unexplained anomalies (20/300, 6.7%, P<.0001). Antenatal anomalies were usually multiple and involved several organs sharing no common function or embryologic origin. They included polyhydramnios (6/20), oligoamnios (2/20), arthrogryposis (1/20), decreased fetal movements (1/20), ventricular septal defects (2/20), hypertrophic cardiomyopathy (4/20), cardiac rhythm anomalies (4/20), hydronephrosis (3/20), vertebral abnormalities, anal atresia, cardiac abnormalities, tracheoesophageal fistula/atresia, renal agenesis and dysplasia, and limb defects (VACTERL) association (2/20), and a complex gastrointestinal malformation (1/20).

CONCLUSIONS

Although a number of metabolic diseases undergo a symptom-free period, respiratory chain deficiency may have an early antenatal expression, presumably related to the time course of the disease gene expression in the embryofetal period. The mechanism triggering malformations is unknown and may include decreased ATP formation and/or an alteration of apoptotic events controlled by the mitochondria.

Effect of smoking cessation on mitochondrial respiratory chain function

OBJECTIVE

Chronic smoking has been associated with diverse mitochondrial respiratory chain (MRC) dysfunction in lymphocytes, although inhibition of complex IV activity is the most consistent and relevant finding. These mitochondrial abnormalities have been proposed to contribute to pathogenesis of diseases associated with tobacco consumption. We assessed MRC function in peripheral lymphocytes from heavy smokers after cessation in smoking habit.

PATIENTS AND METHODS

We studied MRC function from peripheral lymphocytes of 10 healthy chronic smoker individuals (age 43 +/- 6 years; 50% women) before cessation of tobacco consumption (t0), and 7 (t1) and 28 (t2) days after cessation. Smoking abstinence was ascertained by measuring carboxyhemoglobin levels and carbon monoxide (CO) concentration in exhaled breath. Ten healthy nonsmoker individuals matched by age and gender were used as controls. Lymphocytes were isolated by Ficoll's gradient, and protein content was determined by Bradford's technique. MRC function was studied through double means: 1) individual enzyme activities of complex II, III, and IV were analyzed by means of spectrophotometry; 2) oxygen consumption was measured polarographically using pyruvate, succinate, and glycerol-3-phosphate (complex I, II, and III substrates, respectively) after lymphocyte permeabilization. Enzyme and oxidative activities were corrected by citrate synthase activity.

RESULTS

Smokers showed a significant decrease in complex IV activity (p = 0.05) and also in respiration of intact lymphocytes (p = 0.05) compared to controls. Eight chronic smokers remained abstinent during the study. Smoking cessation was associated with a significant recovery of complex IV (p = 0.01) and complex III (p = 0.05) activities. Oxidative activities did not show any change during the study.

CONCLUSION

Chronic smoking is associated with a decrease of complex IV and III activities of MRC, which return to normal values after cessation of tobacco smoking.

Mutant NDUFV2 subunit of mitochondrial complex I causes early onset hypertrophic cardiomyopathy and encephalopathy

Respiratory chain complex I deficiencies represent a genetically heterogeneous group of diseases resulting from mutations in either mitochondrial or nuclear DNA. Combination of denaturing high performance liquid chromatography and sequence analysis allowed us to show that a 4-bp deletion in intron 2 (IVS2+5_+8delGTAA) of the NDUFV2 gene (encoding NADH dehydrogenase ubiquinone flavoprotein 2) causes complex I deficiency and early onset hypertrophic cardiomyopathy with trunk hypotonia in three affected sibs of a consanguineous family. The homozygous mutation altering the consensus splice-donor site of exon 2 resulted in 70% decreased NDUFV2 protein and complex I deficiency. While mutation in a number of genes encoding complex I subunits essentially result in neurological symptoms, this first mutation in NDUFV2 is strikingly associated with cardiomyopathy, as previously observed in the unique case of NDFUS2 mutations.

Human skeletal muscle mitochondrial metabolism in youth and senescence: no signs of functional changes in ATP formation and mitochondrial oxidative capacity

The mitochondrial theory of ageing was tested. Isolated mitochondria from the quadriceps muscle from normal, healthy, young (age 20+ years, n=12) and elderly (70+ years, n=11) humans were studied in respiratory experiments and the data expressed as activities of the muscle. In each group, the subjects exhibited a variation of physical activity but, on average, the groups were representative for their age with maximum O(2) consumption rate of 50+/-9 and 34+/-13 ml min(-1) kg(-1) (mean+/-SD), respectively. Thirteen different activities were assayed. alpha-Glycerophosphate oxidation was lower in the 70+ group (38%, P~0.001), as was the respiratory capacity for fatty acids (19%, P~0.03). The remaining eleven activities, including those of the central bioenergetic reactions, were not lower in the 70+ group. Pyruvate and alpha-ketoglutarate dehydrogenase activities (i.e. the tricarboxylic acid cycle turnover) and the respiratory chain activity could all account for ~14 mmol O(2) min(-1) kg(-1) muscle (37 degrees C). The capacity for aerobic ATP synthesis was ~35 mmol ATP min(-1) kg(-1). The mitochondrial capacities were far in excess of whole-body performance. They were related to physical activity, but not to age. The mitochondrial theory of ageing, which attributes the age-related decline of muscle performance to decreased mitochondrial function, is incompatible with these results.

Mitochondrial respiratory activity is altered in osteoarthritic human articular chondrocytes

OBJECTIVE

Osteoarthritis (OA) is a degenerative rheumatic disease that is associated with extracellular matrix degradation and chondrocyte apoptosis in the articular cartilage. The role of mitochondria in degenerative diseases is widely recognized. We undertook this study to evaluate mitochondrial function in normal and OA chondrocytes and to examine age-related changes in mitochondria.

METHODS

Mitochondrial function was evaluated by analyzing respiratory chain enzyme complexes and citrate synthase (CS) activities as well as changes in mitochondrial membrane potential (Delta Psi m). The activities of mitochondrial respiratory chain complexes (complex I: rotenone-sensitive NADH-coenzyme Q(1) reductase; complex II: succinate dehydrogenase; complex III: antimycin-sensitive ubiquinol cytochrome c reductase; and complex IV: cytochrome c oxidase) and CS were measured in human articular chondrocytes isolated from OA and normal cartilage. Delta Psi m was measured by JC-1 using flow cytometry. Statistical analysis was performed using the Mann-Whitney U test and Student's t-test as well as several models of multiple linear regression.

RESULTS

OA articular chondrocytes had reduced activities of complexes II and III compared with cells from normal cartilage. However, the mitochondrial mass was increased in OA. Cultures of OA chondrocytes contained a higher proportion of cells with de-energized mitochondria. We found no relationship between mitochondrial function and donor age either in normal or in OA chondrocytes.

CONCLUSION

These findings suggest the involvement of mitochondrial function in the pathophysiology of OA. Cartilage degradation by OA and cartilage aging may be two different processes.

Mitochondrial oxidative phosphorylation: pitfalls and tips in measuring and interpreting enzyme activities

Mitochondrial oxidative phosphorylation (OXPHOS) is fundamental in all aspects of cellular life in aerobic cells and organisms. It is therefore not surprising that a variety of diseases have been attributed to dysfunction of the OXPHOS enzymes. Assessment of OXPHOS in human samples has proved to be a difficult task over years, even when relying on well-established methods. The complexity and the flexibility of the mitochondrial organization in cells account for a large part in the difficulties encountered in assessing OXPHOS activity. Nevertheless, a careful and detailed analysis of OXPHOS enzyme activity in cells or biopsy samples from patients at risk provides diagnosis of potential OXPHOS deficiency. Problems inherent in the use of human material, mostly the small size of the samples to be analysed, are difficult to resolve. However, cautious handling of these samples permits reasonable confidence to be reached in the interpretation of the data.

Mitochondrial abnormalities in muscle and other aging cells: classification, causes, and effects

The involvement of mitochondria and of mitochondrial DNA (mtDNA) in the aging process has generated much interest and even more controversy. The mitochondrial theory of aging considers a vicious circle consisting of: (1) accumulation of somatic mtDNA mutations; (2) impairment of respiratory chain function; (3) increased production of reactive oxygen species (ROS) in mitochondria; and (4) further damage to mtDNA. We review the evidence for and against the belief that these steps occur in aging muscle and brain, considering separately morphological, biochemical, and molecular data. The relationship between mitochondrial aging and late-onset neurodegenerative diseases is briefly reviewed. We conclude that mitochondrial dysfunction does play a crucial role in the aging process of both muscle and brain, but it remains unclear whether mitochondria are the culprits or mere accomplices.

Oxidative phosphorylation enzyme complexes in caloric restriction

Free radicals, generated especially by electron leakage from mitochondrial electron transport chain (ETC), are accepted as one of the possible causes of aging. Long-term caloric restriction (CR) is known to increase the species specific average and maximum life spans. Thus it provides a means for investigating mechanisms of aging. There is evidence suggesting a decrease in the free radical production with CR. In this study, Blue-Native PAGE (BN-PAGE) technique was used to investigate the effect of CR on the oxidative phosphorylation enzyme complexes. Of the total 30 female Swiss Albino balb/c mice, 15 were used as control and the other 15 as CR group. Alternate day feeding regimen was used in the CR group for 66 weeks beginning at the end of 3rd month. In the control group, 5 (33.3%) mice died, 3 (20%) of them of breast cancer, 2 (13.3%) of unknown causes and no death cases were observed in the CR group during the study. BN-PAGE was performed on the extracts from brain mitochondrial fractions. Complexes II and V were excluded from the study due to some analytical limitations. No difference was found in the levels of complexes I and III between the groups. In the CR group, complex IV level was found increased and the ratio of complex III-IV decreased compared with the control group. Since there is a slight increase (108%) in the level of complex IV in the CR group, our results could suggest possible partial compensation of electron leakage in the upstream complexes in ETC, and the decrease of free radical production with CR.

Laboratory diagnosis of metabolic myopathies

The metabolic myopathies are a heterogeneous group of disorders inherited by a variety of modes that include gene defects in both the nuclear and mitochondrial genomes. Many factors impact on the expression of the pathogenic mutations that cause these disorders including genetic background, environmental factors, and coexisting disorders. Molecular technology has greatly improved the ability to make definitive diagnoses in many of the metabolic myopathies in the last decade and particularly has demonstrated that the wide diversity in the severity of mutations contributes to understanding genotype-phenotype correlations. In some cases, molecular testing obviates the necessity to perform an invasive muscle biopsy. However, it is also clear that the diagnostic yield from molecular testing is incomplete and particularly low among the mitochondrial myopathies as a group, ranging from approximately 6% to 19% in well-classified high-risk groups. Therefore, it is often essential to combine clinical, biochemical, histopathologic, and molecular data for each patient in order to arrive at a definitive diagnosis. The approach to the laboratory diagnosis of metabolic myopathies is described emphasizing both noninvasive and invasive testing, highlighting the molecular methodologies with the benefits and disadvantages of each technology, and documenting how to determine whether patients have coexisting disorders.