Analyses of mitochondrial respiratory chain function and mitochondrial DNA deletion in human skeletal muscle: effect of ageing

Age-related mitochondrial DNA mutations in the human larynx

OBJECTIVE

To determine whether age-related mitochondrial DNA mutations occur in the human larynx.

STUDY DESIGN

Genetic study of cadaveric larynx specimens.

METHODS

Vocal fold mucosa, thyroarytenoid muscle, and cricoarytenoidjoint tissue were harvested from 13 fresh postmortem larynges (age range, 2 d-82 y). DNA was extracted from each sample, and the polymerase chain reaction (PCR) was used to amplify a target DNA sequence resulting from the common age-associated, 4977-base-pair (bp) mitochondrial DNA deletion. PCR products were visualized by agarose gel electrophoresis. Automated sequencing determined the sequence of identified PCR products.

SUBJECTS

Thirteen cadaveric larynges were obtained through the University of Kentucky Medical Center (Lexington, KY). Specimens from patients with a history of head and neck cancer, previous laryngeal trauma, or surgery were excluded.

RESULTS

Strongly positive bands were identified in samples from three individuals. Weaker bands were seen in samples from four other samples. No band was noted from the two pediatric larynges. Different band patterns were seen among the three different tissue sites in the larynges with positive PCR products, but no consistent pattern was seen. Sequencing of the identified PCR products from selected samples confirmed that they were products of the age-associated, 4977-bp mitochondrial DNA deletion.

CONCLUSIONS

An age-associated mitochondrial DNA deletion was detected in several post-mortem human larynges. Its presence seemed to increase in appearance with age. In the larynges in which the deletion occurred, there were individual regional differences in the occurrence of the deletion, but no consistent pattern was noted across all individuals who carried the deletion.

Mitochondria and ageing

Mitochondria have long been proposed as a perpetrator of aging. We review here the accumulating evidence for chronological alterations in the mitochondrion and discuss how these changes may cause cellular dysfunction and death. We conclude that although the evidence for aging changes in the level of oxidative stress, DNA mutations and biochemical deficiencies in mitochondria are convincing, there is little experimental evidence to link these changes directly with the cellular pathology of aging. Promising avenues for addressing this problem include the investigation of established mitochondrial DNA disorders and the development of animal models with mitochondrial defects.

High-abundance mRNAs in human muscle: comparison between young and old

To gain a better understanding of the potential role of altered gene expression in the diminished muscle function in old age, we performed a broad search for transcripts expressed at quantitatively different levels in younger (21-24 yr) and older (66-77 yr) human vastus lateralis muscle by serial analysis of gene expression (SAGE). Because SAGE was based on RNA pooled from muscle of several different subjects, relative concentrations of selected mRNAs also were determined in individual muscle samples by quantitative RT-PCR. There were 702 SAGE tags detected at least 10 times in one or both mRNA pools, and the detection frequency was different (at P < 0.01) between young and older muscle for 89 of these. The ratio of myosin heavy chain 2a mRNA to myosin heavy chain 1 mRNA was reduced in older muscle. The mRNAs encoding several mitochondrial proteins involved in electron transport (including several subunits of cytochrome-c oxidase and NADH dehydrogenase) and subunits of ATP synthase were approximately 30% less abundant in older muscle. Several mRNAs encoding enzymes involved in glucose metabolism also were less abundant in older muscle. Analysis of individual samples revealed that the differences suggested by SAGE were not artifacts of atypical gene expression in one or a few individuals. These data suggest that some of the phenotypic changes in senescent muscle may be related to altered gene transcription.

Oxidative capacity and ageing in human muscle

This study determined the decline in oxidative capacity per volume of human vastus lateralis muscle between nine adult (mean age 38.8 years) and 40 elderly (mean age 68.8 years) human subjects (age range 25-80 years). We based our oxidative capacity estimates on the kinetics of changes in creatine phosphate content ([PCr]) during recovery from exercise as measured by (31)P magnetic resonance (MR) spectroscopy. A matched muscle biopsy sample permitted determination of mitochondrial volume density and the contribution of the loss of mitochondrial content to the decline in oxidative capacity with age. The maximal oxidative phosphorylation rate or oxidative capacity was estimated from the PCr recovery rate constant (k(PCr)) and the [PCr] in accordance with a simple electrical circuit model of mitochondrial respiratory control. Oxidative capacity was 50 % lower in the elderly vs. the adult group (0.61 +/- 0.04 vs. 1.16 +/- 0.147 mM ATP s(-1)). Mitochondrial volume density was significantly lower in elderly compared with adult muscle (2.9 +/- 0.15 vs. 3.6 +/- 0.11 %). In addition, the oxidative capacity per mitochondrial volume (0.22 +/- 0.042 vs. 0.32 +/- 0.015 mM ATP (s %)(-1)) was reduced in elderly vs. adult subjects. This study showed that elderly subjects had nearly 50 % lower oxidative capacity per volume of muscle than adult subjects. The cellular basis of this drop was a reduction in mitochondrial content, as well as a lower oxidative capacity of the mitochondria with age.

Role of mitochondrial DNA mutations in disease and aging

Since Harman in 1972 first proposed a role in the process of aging for the mitochondrial genome, a wealth of evidence has been accumulated to support this theory. We discuss the hereditary mitochondrial DNA disorders, which we believe may give insight into both normal aging and neurodegenerative conditions. We then review the evidence for the role of mitochondrial DNA mutations in both aging and age-related disorders and also discuss new approaches for investigating the mitochondrial genome at a single cell level, by observing the activity of the mitochondrial enzyme cytochrome c oxidase.

Age-related changes in activities of mitochondrial electron transport complexes in various tissues of the mouse

The purpose of the present study was to examine the role of mitochondria in the aging process by determining whether the activities of various electron transport chain oxidoreductases are deleteriously affected during aging and whether the hypothesized age-related alterations in different tissues follow a common pattern. Activities of respiratory complexes I, II, III, and IV were measured in mitochondria isolated from brain, heart, skeletal muscle, liver, and kidney of young (3.5 months), adult (12-14 months), and old (28-30 months) C57BL/6 mice. Activities of some individual complexes were decreased in old animals, but no common pattern can be discerned among various tissues. In general, activities of the complexes were more adversely affected in tissues such as brain, heart, and skeletal muscle, whose parenchyma is composed of postmitotic cells, than those in the liver and kidney, which are composed of slowly dividing cells. The main feature of age-related potentially dysfunctional alterations in tissues was the development of a shift in activity ratios among different complexes, such that it would tend to hinder the ability of mitochondria to effectively transfer electrons down the respiratory chain and thus adversely affect oxidative phosphorylation and/or autooxidizability of the respiratory components.

Changes as the result of ageing in extraocular muscles: a post-mortem study

BACKGROUND

Extraocular muscles differ from typical skeletal muscle in many respects such as smaller fibre size, high mitochondrial content, unusual contractile/innervation patterns and highly developed microvascular bed. Changes in typical skeletal muscle with ageing have been well documented yet the reports on ageing changes in extraocular muscles is limited. The aim of this study was to examine these changes.

METHODS

The right inferior and medial rectus muscles were removed at post-mortem from patients with no history of prior ocular or neuromuscular disease. These included 25 patients aged between 23 and 88 years (14 male, 11 female). The median age of the patients was 69 years and 72% of patients were aged older than 66 years. The median post-mortem interval was 28 h,

RESULTS

In young adults between 20 and 30 years of age, the muscle fibres were regular, of fairly uniform size with minimal endomysial connective tissue and no lipofuscin was detected in fibres. In adults between 40 and 50 years of age, a small amount of subsarcolemmal lipofuscin was detected. Between 50 and 60 years of age, a few fibres with reduced density of myofibrils were noted. All patients over 65 years had definite changes of ageing and those between 70 and 80 years of age showed similar features to those between 60 and 80 years of age. These changes included variation in fibre size, increased endomysial fibrous tissue and increased endomysial adipose tissue, and loss of myofibrils with some fibres devoid of fibrils and comprising 'bags' of mitochondria. There was increased lipofuscin in a number of fibres, increased 'degenerative' changes such as vacuolation of fibres, scattered 'eosinophilic intracytoplasmic bodies' and increased numbers of 'ringbinden'.

Mitochondrial respiratory chain activity in the human brain as a function of age

Age-associated changes in mitochondrial respiratory chain activity were investigated in human brain tissue collected at autopsy. Four brain regions, the frontal cortex, superior temporal cortex, cerebellum and putamen, were studied to map any regional variation. A significant decrease in cytochrome c oxidase activity was seen in all regions studied with increasing age (P<0.05). Although a small decrease in succinate dehydrogenase-cytochrome c oxidoreductase and NADH: ubiquinone oxidoreductase activities was observed, this was not statistically significant. This study has shown that the age-related fall in cytochrome c oxidase activity affects the frontal cortex, superior temporal cortex, cerebellum and putamen. The variation in the extent of age-related oxidative phosphorylation decline was striking. We hypothesize that individuals with more severe age-related decline may be predisposed to neuronal dysfunction, whereas individuals with well preserved oxidative phosphorylation may enjoy some degree of neuronal protection.

Estimation of age at death based on quantitation of the 4977-bp deletion of human mitochondrial DNA in skeletal muscle

The 4977-bp deletion in human mitochondrial DNA (mtDNA) is known to accumulate in various tissues with age. Since this deletion in mtDNA correlates closest with age in muscle tissue, iliopsoas muscle tissue was taken at autopsy from 50 persons aged 24-97 years to determine whether age at death can be estimated based on the amount of the 4977-bp deletion in skeletal muscle. Total DNA (nuclear and mtDNA) was extracted from 100 mg tissue and the 4977-bp deletion quantified using a kinetic polymerase chain reaction (PCR) followed by visualization of the products on silver stained polyacrylamide gels. The amount of the 4977-bp deletion of mtDNA ranged from 0.00049% to 0.14% depending on age, with a correlation coefficient of r = 0.83 (P = 0.0001). In forensic practice this method can aid in the estimation of age at death with a relatively wide confidence interval, thus enabling a discrimination between young and elderly persons in the identification of human remains based solely on skeletal muscle.

A missense mutation of cytochrome oxidase subunit II causes defective assembly and myopathy

We report the first missense mutation in the mtDNA gene for subunit II of cytochrome c oxidase (COX). The mutation was identified in a 14-year-old boy with a proximal myopathy and lactic acidosis. Muscle histochemistry and mitochondrial respiratory-chain enzymology demonstrated a marked reduction in COX activity. Immunohistochemistry and immunoblot analyses with COX subunit-specific monoclonal antibodies showed a pattern suggestive of a primary mtDNA defect, most likely involving CO II, for COX subunit II (COX II). mtDNA-sequence analysis demonstrated a novel heteroplasmic T-->A transversion at nucleotide position 7,671 in CO II. This mutation changes a methionine to a lysine residue in the middle of the first N-terminal membrane-spanning region of COX II. The immunoblot studies demonstrated a severe reduction in cross-reactivity, not only for COX II but also for the mtDNA-encoded subunit COX III and for nuclear-encoded subunits Vb, VIa, VIb, and VIc. Steady-state levels of the mtDNA-encoded subunit COX I showed a mild reduction, but spectrophotometric analysis revealed a dramatic decrease in COX I-associated heme a3 levels. These observations suggest that, in the COX protein, a structural association of COX II with COX I is necessary to stabilize the binding of heme a3 to COX I.

Age-dependent changes of antioxidant activities and markers of free radical damage in human skeletal muscle

This study was conducted in order to provide evidence for the role of reactive oxygen species (ROS) in human skeletal muscle aging. We used human muscle samples obtained from hospitalized patients in an open study with matched pairs of individuals of different ages. The subjects, ranging in age from 17 to 91 years, were grouped as follows: 17-25-, 26-35-, 36-45-, 46-55-, 56-65-, 66-75-, 76-85-, and 86-91-year-old groups. To investigate the relationship between muscle aging and oxidative damage we measured total and Mn-dependent superoxide dismutase (total SOD, MnSOD), glutathione peroxidase (GSHPx), and catalase (CAT) activities; total reduced and oxidized glutathione (GSHtot, GSH, and GSSG) levels; lipid peroxidation (LPO), and protein carbonyl content (PrC). Total SOD activity decreases significantly with age in the 66-75-year-old group, although MnSOD activity increases significantly in the 76-85-year-old group. The activity of the two H2O2 detoxifying enzymes (GSHPx and CAT) did not change with age, as do GSHtot and GSH levels. GSSG levels increased significantly (76-85- and 86-91-year-old groups) with age. We observed a significant increase in LPO levels (66-75- and 76-85-year-old groups), although the PrC content shows a trend of increase without gaining the statistical significance. These results support the idea that ROS play an important role in the human muscle aging process.

Aging and acute exercise enhance free radical generation in rat skeletal muscle

Reactive oxygen species (ROS) are implicated in the mechanism of biological aging and exercise-induced oxidative damage. The present study examined the effect of an acute bout of exercise on intracellular ROS production, lipid and protein peroxidation, and GSH status in the skeletal muscle of young adult (8 mo, n = 24) and old (24 mo, n = 24) female Fischer 344 rats. Young rats ran on a treadmill at 25 m/min and 5% grade until exhaustion (55.4 +/- 2.7 min), whereas old rats ran at 15 m/min and 5% grade until exhaustion (58.0 +/- 2.7 min). Rate of dichlorofluorescin (DCFH) oxidation, an indication of ROS and other intracellular oxidants production in the homogenate of deep vastus lateralis, was 77% (P < 0.01) higher in rested old vs. young rats. Exercise increased DCFH oxidation by 38% (P < 0.09) and 50% (P < 0.01) in the young and old rats, respectively. DCFH oxidation in isolated deep vastus lateralis mitochondria with site 1 substrates was elevated by 57% (P < 0.01) in old vs. young rats but was unaltered with exercise. Significantly higher DCFH oxidation rate was also found in aged-muscle mitochondria (P < 0.01), but not in homogenates, when ADP, NADPH, and Fe(3+) were included in the assay medium without substrates. Lipid peroxidation in muscle measured by malondialdehyde content showed no age effect, but was increased by 20% (P < 0.05) with exercise in both young and old rats. Muscle protein carbonyl formation was unaffected by either age or exercise. Mitochondrial GSH/ GSSG ratio was significantly higher in aged vs. young rats (P < 0.05), whereas exercise increased GSSG content and decreased GSH/GSSG in both age groups (P < 0.05). These data provided direct evidence that oxidant production in skeletal muscle is increased in old age and during prolonged exercise, with both mitochondrial respiratory chain and NADPH oxidase as potential sources. The alterations of muscle lipid peroxidation and mitochondrial GSH status were consistent with these conclusions.

Mitochondrial medicine--recent advances

Mitochondria contain the respiratory chain enzyme complexes that carry out oxidative phosphorylation and produce the main part of cellular energy in the form of ATP. Mitochondrial DNA (mtDNA) encodes essential subunits of the respiratory chain and is thus critical for maintaining cellular energy production. The first pathogenic mtDNA mutations were reported in 1988, and today more than 50 disease-causing mtDNA mutations have been identified. In addition, mtDNA mutations have been implicated in ageing and in common disorders such as diabetes mellitus, heart failure and Parkinson's disease. This review will summarize recent advances in the rapidly expanding field of mitochondrial medicine.

Mitochondrial involvement in bladder function and dysfunction

Benign bladder pathology resulting from prostatic hypertrophy or other causes is a significant problem associated with ageing in humans. This condition is characterized by increased bladder mass, decreased urinary flow rate, decreased compliance, and these and other changes in bladder function often subject patients to increased risk of urinary tract infection. While the physiologic attributes of benign bladder pathology have been extensively described in humans and in various animal model systems, the biochemical and molecular genetic bases for that pathology have only recently been investigated in detail. Studies demonstrate that mitochondrial energy production and utilization are severely impaired in bladder smooth muscle during benign bladder disease, and to a large extent this realization has provided a rational basis for understanding the characteristic alterations in urinary flow and compliance in bladder tissue. Recent investigations targeting the detailed molecular basis for impaired mitochondrial function in the disease have shown that performance of the organellar genetic system, and to a large extent that of relevant portions of the nuclear genetic system as well, is severely aberrant in bladder tissue. In this article, we discuss the physiologic aspects of benign bladder disease, summarize biochemical evidence for the altered mitochondrial energy metabolism that appears to underlie bladder pathology, review the structure and function of the mitochondrial genetic system, and discuss molecular genetic studies of that system which have begun to provide a mechanistic explanation for the biochemical and physiological abnormalities that characterize the disease. We also discuss areas for further research which will be critically important in increasing our understanding of the detailed causes of benign bladder pathology.

Skeletal muscle mitochondrial DNA injury in patients with unilateral peripheral arterial disease

BACKGROUND

Patients with peripheral arterial disease (PAD) have exercise limitation due to claudication-limited pain and metabolic alterations in skeletal muscle. PAD is also associated with oxidative stress, which is a known cause of mitochondrial DNA (mtDNA) injury. The present study was designed to test the hypothesis that PAD is associated with mtDNA injury, as reflected by an increased frequency of a specific 4977-base pair (bp) mtDNA deletion mutation.

METHODS AND RESULTS

The deletion frequency was quantified in gastrocnemius muscle of 8 patients with unilateral PAD and 10 age-matched control subjects with the use of polymerase chain reaction methodologies. Muscle from the hemodynamically unaffected (less affected) PAD limb showed an 8-fold increased deletion frequency and the hemodynamically affected (worse affected) PAD limb had a 17-fold increased deletion frequency compared with muscle from control subjects. The frequency of the 4977-bp deletion in the worse-affected limb was positively correlated with the age of the patients but not the claudication-limited exercise performance of the patients. Total mtDNA content, citrate synthase activity, and cytochrome c oxidase activity were not different in the muscle from the 3 limb populations. However, the ratio of citrate synthase to cytochrome c oxidase was higher in the worse- versus less-affected limbs of PAD patients.

CONCLUSIONS

The present study demonstrates a large increase in the frequency of the mtDNA 4977-bp deletion in patients with PAD but in a distribution not limited to the hemodynamically affected limb.

Age-dependent increases in oxidative damage to DNA, lipids, and proteins in human skeletal muscle

A role for oxidative damage in normal aging is supported by studies in experimental animals, but there is limited evidence in man. We examined markers of oxidative damage to DNA, lipids, and proteins in 66 muscle biopsy specimens from humans aged 25 to 93 years. There were age-dependent increases in 8-hydroxy-2-deoxyguanosine (OH8dG), a marker of oxidative damage to DNA, in malondialdehyde (MDA), a marker of lipid peroxidation, and to a lesser extent in protein carbonyl groups, a marker of protein oxidation. The increases in OH8dG were significantly correlated with increases in MDA. These results provide evidence for a role of oxidative damage in human aging which may contribute to age-dependent losses of muscle strength and stamina.

Oxidative damage and mutation to mitochondrial DNA and age-dependent decline of mitochondrial respiratory function

Mitochondrial respiration and oxidative phosphorylation are gradually uncoupled, and the activities of the respiratory enzymes are concomitantly decreased in various human tissues upon aging. An immediate consequence of such gradual impairment of the respiratory function is the increase in the production of the reactive oxygen species (ROS) and free radicals in the mitochondria through the increased electron leak of the electron transport chain. Moreover, the intracellular levels of antioxidants and free radical scavenging enzymes are gradually altered. These two compounding factors lead to an age-dependent increase in the fraction of the ROS and free radical that may escape the defense mechanism and cause oxidative damage to various biomolecules in tissue cells. A growing body of evidence has established that the levels of ROS and oxidative damage to lipids, proteins, and nucleic acids are significantly increased with age in animal and human tissues. The mitochondrial DNA (mtDNA), although not protected by histones or DNA-binding proteins, is susceptible to oxidative damage by the ever-increasing levels of ROS and free radicals in the mitochondrial matrix. In the past few years, oxidative modification (formation of 8-hydroxy-2'-deoxyguanosine) and large-scale deletion and point mutation of mtDNA have been found to increase exponentially with age in various human tissues. The respiratory enzymes containing the mutant mtDNA-encoded defective protein subunits inevitably exhibit impaired respiratory function and thereby increase electron leak and ROS production, which in turn elevates the oxidative stress and oxidative damage of the mitochondria. This vicious cycle operates in different tissue cells at different rates and thereby leads to the differential accumulation of mutation and oxidative damage to mtDNA in human aging. This may also play some role in the pathogenesis of degenerative diseases and the age-dependent progression of the clinical course of mitochondrial diseases.

Mitochondrial DNA mutations and age

Apopotic cell death is reported to be prominent in the stable tissues of the failing heart, in cardiomyopathies (CM), in the sinus node of complete heart block, in B cells of diabetes mellitus, and in neurodegenerative diseases. Recently, mitochondrial (mt) control of nuclear apoptosis was demonstrated in the cell-free system. The mt bioenergetic crisis induced by exogenously added factors such as respiratory inhibitors leads to the collapse of mt transmembrane potential, to the opening of the inner membrane pore, to the release of the apoptotic protease activating factors into cytosol, and subsequently to nuclear DNA fragmentation. However, the endogenous factor for the mt bioenegertic crisis in naturally occurring cell death under the physiological conditions without vascular involvement has remained unknown. Recently devised, the total detection system for deletion demonstrates the extreme fragmentation of mtDNA in the cardiac myocytes of senescence, and mt CM harboring maternally inherited point mutations in mtDNA and on the cultured cell line with or without mtDNA disclosed that mtDNA is unexpectedly fragile to hydroxyl radial damage and hence to oxygen stress. The great majority of wild-type mtDNA fragmented into over two hundreds types of deleted mtDNA related to oxidative damage, resulting in pleioplasmic defects in the mt energy transducing system. The mtDNA fragmentation to this level is demonstrated in cardiac myocytes of normal subjects over age 80, of an mtCM patient who died at age 20 and one who died at age 19, of a recipient of heart transplantation at age 7 with severe mtCM, and in mtDNA of a cultured cell line under hyperbaric oxygen stress for two days, leading a majority of cells to apoptotic death on the third day. The extreme fragility of mtDNA could be the missing link in the apoptosis cascade that is the physiological basis of aging and geriatrics of such stable tissues as nerve and muscle.

Association of age-related mitochondrial abnormalities with skeletal muscle fiber atrophy

The hypothesis that mitochondrial dysfunction contributes to the senescent loss of skeletal muscle was investigated in quadriceps from 2- to 39-year old rhesus monkeys. Histological approaches, both cross-sectional (a single cross-section of the muscle) and longitudinal (multiple cross-sections of individual fibers spanning a 350-1600 microm region), were used to identify muscle fibers with abnormal mitochondrial electron transport system (ETS) enzyme activities and mitochondrial DNA deletions. Fibers were examined for two ETS activities, succinate dehydrogenase (SDH, ETS complex II) and cytochrome c oxidase (COX, ETS complex IV). The number of individual fibers containing ETS abnormalities (predominately negative for cytochrome c oxidase activity and/or hyperreactive for succinate dehydrogenase) increased with age. Deletions of the mitochondrial genome were observed in 89% of these ETS abnormal fibers. Longitudinal analysis allowed characterization of the ETS abnormal phenotype along their length. A decrease in cross-sectional area in 14% of the ETS abnormal fibers supports the hypothesis that deleted mitochondrial genomes may contribute to age-related fiber atrophy.

Age-related biology and diseases of muscle and nerve

Age-related biological changes in neurons and skeletal muscle commonly affect neuromuscular function and strongly influence the expression of neuromuscular disease. Of primary importance is the attrition of entire motor units, with resultant neurogenic atrophy of skeletal muscle. Other age-related processes are sensory neuron loss, distal axonal degeneration, axonal atrophy, accumulation of multiple mitochondrial DNA mutations in muscle, and physical inactivity and deconditioning. The decline for most of these begins in early life and proceeds steadily; the curious lack of an abrupt falloff with age is not yet accounted for by any theory of pathogenesis.

Oxidative stress and mitochondrial function in skeletal muscle: Effects of aging and exercise training

The rate of oxidative phosphorylation was investigated in isolated mitochondria from hindlimb muscles of young (4.5 mo) and old (26.5 mo) male Fischer 344 rats with or without endurance training. Further, the susceptibility of the muscle mitochondria to exogenous reactive oxygen species was examined. State 3 and 4 respiration, as well as the respiratory control index (RCI), were significantly lower in muscle mitochondria from aged vs. young rats (P<0.05), using either the site 1 substrates malate-pyruvate (M-P) and 2-oxoglutarate (2-OG), or the site 2 substrate succinate. In both young and old rats, training increased state 4 respiration with M-P, but had no effect on state 3 respiration, resulting in a reduction of RCI. Training also increased state 4 respiration with 2-OG and decreased RCI in young rats. When muscle mitochondria were exposed to superoxide radicals (O2 (·-)) and hydrogen peroxide (H2O2) generated by xanthine oxidase and hypoxanthine, or H2O2 alone in vitro, state 3 respiration and RCI in both age groups were severely hampered, but those from the old rats were inhibited to a less extent than the young rats. In contrast, state 4 respiration was impaired by O2 (·-) and/or H2O2 to a greater extent in the old rats. Muscle mitochondria from trained young rats showed a greater resistance to the O2 (· -) and/or H2O2-induced state 3 and RCI inhibition than those from untrained young rats. Muscle from aged rats had significantly higher total activities of superoxide dismutase (SOD), catalase, glutathione peroxidase (GPX), and glutathione reductase than that from young rats, however, training increased SOD and GPX activities in young but not old rats. The results of this study suggest that mitochondrial capacity for oxidative phosphorylation is compromised in aging skeletal muscle. Further, the increased mitochondrial resistance to reactive oxygen species demonstrated in aged and young trained muscles may be attributed to enhanced antioxidant enzyme activities.

Human mitochondrial function during cardiac growth and development

Little information is presently available concerning mitochondrial respiratory and oxidative phosphorylation function in the normal human heart during growth and development. We investigated the levels of specific mitochondrial enzyme activities and content during cardiac growth and development from the early neonatal period (10-20 days) to adulthood (67 years). Biochemical analysis of enzyme specific activities and content and mitochondrial DNA (mtDNA) copy number was performed with left ventricular tissues derived from 30 control individuals. The levels of cytochrome c oxidase (COX) and complex V specific activity, mtDNA copy number and COX subunit II content remained unchanged in contrast to increased citrate synthase (CS) activity and content. The developmental increase in CS activity paralleled increasing CS polypeptide content, but was neither related to overall increases in mitochondrial number nor coordinately regulated with mitochondrial respiratory enzyme activities. Our findings of unchanged levels of cardiac mitochondrial respiratory enzyme activity during the progression from early childhood to older adult contrasts with the age-specific regulation found with CS, a Krebs cycle mitochondrial enzyme.

Role of mitochondrial DNA mutations in human aging: implications for the central nervous system and muscle

It has been proposed that one mechanism for nerve and muscle dysfunction with age involves the mitochondria. Mitochondria contain the only DNA outside the nucleus in mammalian cells. Mitochondrial DNA (mtDNA) has a high mutation rate, and low levels of pathogenic mutations have been found in tissues from elderly subjects. However, the role of these mutations in the aging process is uncertain unless a mechanism can be identified that would lead to a biochemical defect. In muscle tissue from normal elderly subjects we show that there are muscle fibers with very low activity of cytochrome c oxidase, suggestive of a mtDNA defect. In these cytochrome c oxidase-deficient fibers we have found very high levels of mutant mtDNA. In addition, different mtDNA mutations are present in different fibers, which explains why there is a low overall incidence of an individual mutation in tissues from elderly subjects. These studies show a direct age-related correlation between a biochemical and genetic defect in normal human tissues and that mtDNA abnormalities are involved in the aging process in human muscle.

Oxidative phosphorylation dysfunction does not increase the rate of accumulation of age-related mtDNA deletions in skeletal muscle

Several reports described an age-related accumulation of a particular mitochondrial DNA (mtDNA) deletion ('common deletion') in post-mitotic tissues. These findings led to the hypothesis that free radicals generated inside the mitochondria could damage mtDNA during a normal life span. The impaired electron transfer function resulting from mtDNA damage would increase the production of free radicals creating a vicious cycle. If this vicious cycle is an important player in the somatic accumulation of mtDNA deletions, patients with impaired oxidative phosphorylation (regardless of the primary defect) should have an accelerated accumulation of mtDNA deletions. We tested this hypothesis by performing three analyses: (a) comparing the amounts of the mtDNA 'common deletion' in normal controls and patients with genetically characterized mitochondrial disorders associated with pathogenic mtDNA point mutations or deletions other than the common deletion; (b) analyzing the co-segregation of the age-related mtDNA common deletion with a pathogenic mtDNA point mutation; and (c) by the detection of multiple mtDNA deletions by long PCR in controls and patients with mitochondrial disorders. We observed a positive correlation between age and common deletion levels in controls (r = 0.80) and patients (r = 0.69). The slopes of the curves were similar, suggesting that the rate of accumulation of the age-related common deletion was the same in both groups. We could not find a co-segregation of the pathogenic point mutated mtDNA molecules with the common deletion nor increased number of age-related deletions in patients. Our data do not support the hypothesis that a vicious cycle (damage to mtDNA would affect the respiratory function, leading to the generation of more free radicals, which in turn would provoke additional mtDNA damage) is an important factor in the accumulation of age-related mtDNA deletions.

Protonophoric activity of NADH coenzyme Q reductase and ATP synthase in coupled submitochondrial particles from horse platelets

A method to prepare coupled submitochondrial particles from horse platelets is described. The method allowed us to study the protonophoric activities of both complex I and complex V following the fluorescence quenching of the monoamine 9-amino-6-chloro-2 methoxyacridine (ACMA), a probe highly sensitive to the generation of a transmembrane delta pH. We carried out a kinetic analysis of each enzyme complex studying the proton translocation and the electron transfer activities of complex I as well as the proton translocation and the ATP hydrolytic activities of complex V. A micromethod to prepare coupled submitochondrial particles from platelets might be useful to investigate cell bioenergetic damage occurring in mitochondrial diseases and ageing.

S1 nuclease hybrid analysis of mitochondrial DNA amplified by long-distance PCR: rapid screening for small-scale rearrangements

We report on a method suitable for screening large regions (>3 kb) of mtDNA for structural changes of <500 bp and their localization. Heteroduplexes consisting of a wild-type and a mutant strand are cleaved by S1nuclease when single-stranded loops are present due to deletions or duplications/insertions. This strategy was successfully applied to screen the muscle mtDNA of 20 patients with mitochondrial encephalomyopathies. In three of them, an altered cleavage pattern was observed caused by a homoplasmic 9 bp deletion as shown by subsequent mapping and sequencing studies.

Fragile mitochondrial DNA: the missing link in the apoptotic neuronal cell death in Parkinson's disease

The oxidative stress theory, the mitochondrial (mt) hypothesis, and the apoptosis hypothesis are proposed as the cause of neuronal cell death in Parkinson's disease (PD). However, the direct link between them has remained unknown. Recently, the mt control of nuclear apoptosis is documented that collapse of mt transmembrane potential due to energy crisis leads to release of apoptotic protease activating-factors into cytosol and subsequently nuclear DNA fragmentation. However, an endogenous factor responsible for the energy crisis under physiological conditions is missing. Here we report the missing factor as that mtDNA in the striatum of a parkinsonian patient fragments into 134 types of deleted pieces, being detected by the total detection system for mtDNA deletion. The system has documented that the mtDNA is extremely susceptible to hydroxyl radical damage, hence to oxidative stress, enough to cause the cellular energy crisis. The extensive fragility of mtDNA in brain stem could link the oxidative stress up with the apoptotic neuronal cell-death of PD.

Lactic acidosis and other mitochondrial disorders

The ability of mitochondria to oxidize substrates and generate energy is integral to normal homeostasis and to the ability of cells to survive in the face of impending energy failure. Lactic acidosis is a common and readily apparent biochemical marker for mitochondrial dysfunction. However, lactic acidosis represents only the most obvious example in which acquired or congenital abnormalities of mitochondrial oxidative phosphorylating capacity contribute to the pathobiology and phenotypic expression of a broad spectrum of clinical disorders. Consequently, interventions that improve mitochondrial function or prevent mitochondrial energy failure may have widespread therapeutic implications.

Quantitative decrease of human cytochrome c oxidase during development: evidences for a post-transcriptional regulation

In an earlier study, we showed that cytochrome c oxidase activity, measured in mitochondria isolated from human muscular biopsies, decreased steadily and substantially between the age of four years and adulthood (P < 0.05), whereas complexes I and III activity remained constant. The present study investigates a number of possible causes for this change in activity: although there is a drop in the apparent Vmax, neither the apparent enzyme Km, nor the cellular mtDNA concentration shows any variations over the studied period. Steady-state concentrations of mitochondrial gene transcripts (CO I. CO II, CO III, but also 12S, cytochrome b, or ND4) increase within this age group, indicating an overall increase in mitochondrial genome expression. Concentrations of transcripts of nuclear genes CO IV, CO Vb, and CO VIaH likewise show an increase, albeit less marked. On the other hand, heme aa3 levels and concentrations of mitochondrial (CO II) or nuclear (CO IV, CO VIIaH) subunits, estimated using specific antibodies, correlate closely with enzymatic activity and show a parallel decrease between 4 and 20 years. The observed decrease in complex IV activity is thus quantitative, and subject to post-transcriptional and/or post-translational regulation.

Idebenone induces oxygen consumption rate modifications in aged rat brain mitochondria

Idebenone effects on oxygen consumption in brain mitochondria, obtained from young and aged rats, were evaluated. Sixty rats (3 and 20 months of age) were treated for 3 months with 30 mg/kg of idebenone and compared to a placebo group. Brain mitochondria oxygen consumption rate was measured by polarographic techniques in basal (State 4), ADP-stimulated (State 3) and uncoupled conditions. When Complex I substrates (pyruvate + malate) were used, aged non-treated rats showed a significant increase in State 4 (175%) and uncoupled (152%) O(2)-uptake rate; no difference was found in State 3 respiration and in ADP/O(2) ratio. Idebenone was able to reverse these age-related effects probably acting on lipid peroxidation and the mitochondrial respiratory chain. No differences were found in mitochondrial enzymatic activities.

Mitochondrial function in muscle from elderly athletes

The extent to which mitochondria are involved in the aging process is controversial; much of the reported decline in mitochondrial oxidations in human skeletal muscle may be due to disease and inactivity rather than age. To study true aging, mitochondrial respiratory chain function was studied in 9 young and 12 elderly athletes. No significant deterioration with age was observed. If mitochondria are involved in aging, it must be through a more subtle mechanism than a global decline in respiratory chain activity.

Age-associated alterations of the mitochondrial genome

Age-associated alterations of the mitochondrial genome occur in several different species; however, their physiological relevance remains unclear. The age-associated changes of mitochondrial DNA (mtDNA) include nucleotide point mutations and modifications, as well as deletions. In this review, we summarize the current literature on age-associated mtDNA mutations and deletions and comment on their abundance. A clear need exists for a more thorough evaluation of the total damage to the mitochondrial genome that accumulates in aged tissues.

Platelet mitochondrial respiratory chain function in Parkinson's disease

Reports on mitochondrial respiratory chain (MRC) complex I (CI) dysfunction in the substantia nigra in Parkinson's disease (PD) support the oxidative stress hypothesis in the neuropathogenesis of PD. Studies in peripheral tissue have found variable decreased CI and occasionally other complex activity suggestive of systemic impairment of MRC function in PD; however, MRC activity may be influenced by numerous variables. We conducted spectrophotometric measurements of MRC function in platelet mitochondrial preparations in 13 individuals with PD and 9 age-matched controls (CON) and have identified additional variables that may affect MRC activity. Mean CI, CIII, CIV, and citrate synthase (CS) activities were similar between PD and CON. CIII and CIV, specific and CS-corrected, activities were significantly positively correlated with CI in combined and individual group data, with the exception of CIII CS-corrected and CIV specific activities in CON and PD, respectively. CIII and CS specific activities were negatively correlated with age in CON, but varied randomly in PD. In PD, CIII specific activity was 1.4-fold higher in those with a history of environmental risk factors for PD and CIV specific activity was lower in those with a positive family history of PD [8.34 +/- 0.74 (n = 4) vs. 12.4 +/- 1.1 (SEM) min-1 mg-1; p = 0.046]. Group heterogeneity, variables affecting enzyme activity, and intrinsic properties of cells may thus contribute to conflicting data in studies of MRC function in platelets and other tissues.

Effect of age on in vivo rates of mitochondrial protein synthesis in human skeletal muscle

A progressive decline in muscle performance in the rapidly expanding aging population is causing a dramatic increase in disability and health care costs. A decrease in muscle endurance capacity due to mitochondrial decay likely contributes to this decline in muscle performance. We developed a novel stable isotope technique to measure in vivo rates of mitochondrial protein synthesis in human skeletal muscle using needle biopsy samples and applied this technique to elucidate a potential mechanism for the age-related decline in the mitochondrial content and function of skeletal muscle. The fractional rate of muscle mitochondrial protein synthesis in young humans (24 +/- 1 year) was 0.081 +/- 0.004%.h-1, and this rate declined to 0.047 +/- 0.005%.h-1 by middle age (54 +/- 1 year; P < 0.01). No further decline in the rate of mitochondrial protein synthesis (0.051 +/- 0.004%.h-1) occurred with advancing age (73 +/- 2 years). The mitochondrial synthesis rate was about 95% higher than that of mixed protein in the young, whereas it was approximately 35% higher in the middle-aged and elderly subjects. In addition, decreasing activities of mitochondrial enzymes were observed in muscle homogenates (cytochrome c oxidase and citrate synthase) and in isolated mitochondria (citrate synthase) with increasing age, indicating declines in muscle oxidative capacity and mitochondrial function, respectively. The decrease in the rates of mitochondrial protein synthesis is likely to be responsible for this decline in muscle oxidative capacity and mitochondrial function. These changes in muscle mitochondrial protein metabolism may contribute to the age-related decline in aerobic capacity and muscle performance.

Large deletion (7.2 kb) of mitochondrial DNA with novel boundaries in a case of progressive external ophthalmoplegia

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Defects of the respiratory chain in oxyphil and chief cells of the normal parathyroid and in hyperfunction

Immunohistochemical detection of complex HIII (ubiquinone- cytochrome-c-oxidoreductase) and complex IV (cytochrome-c-oxidase) of the respiratory chain was performed in parathyroids of 164 humans with normal renal function (group I) and in 55 patients with chronic renal insufficiency (group II) obtained at autopsy. In group I, 33 of the 164 cases showed defects of the respiratory chain (20%). Eighty-five percent of the defects occurred in advanced age (> 50 years). In group II, 39 of 55 cases (70%) had defects, and about 70% of the defects occurred after age 50. In both groups, more than 80% of the defects were localized in oxyphil cell nodules. However, not every oxyphil nodule was involved. In group I, selective defects of complex IV predominated and were found in 47 of 86 defects (55%). Combined defects of complexes III and IV were present in 25 of 86 defects (29%). In contrast, in group II combined defects predominated and were found in 45% (107 of 240 defects), whereas single defects of complex IV existed in 38% (93 of 240 defects). The frequency of selective defects of complex III was about 16% to 17% in both groups. In situ hybridization and PCR studies for the detection of the common deletion (4.977 base pairs) and of various point mutations of mitochondrial of (m)DNA revealed no consistent molecular genetic abnormalities. A point mutation in the tRNALeu(UUR) at nucleotide (nt) 3.260 was found in only one probe. The results show that defects of the respiratory chain occur already in normal parathyroids, most probably during cell aging, especially in oxyphil cells and at a higher rate in hyperfunction. The high predominance of respiratory chain defects in oxyphil cells and their random distribution favors mutations of mtDNA as a possible cause of oxyphilic cell transformation and of the respiratory chain defects. However, the mutations of mtDNA in the parathyroids are apparently different from those in other ageing tissues.

Mitochondrial electron transport chain activities and DNA deletions in regions of the rat brain

Deletions in human mitochondrial DNA cause various mitochondrial myopathies and increase markedly with age in highly oxidative tissues, but exhibit a differential distribution in the brain. In order to determine whether a similar pattern occurs in rat brain the levels of a 4.8 kb deletion and electron transport complex activities were measured in the striatum, hippocampus, cerebellum, and cerebral cortex of young adult and senescent male Wistar rats. Deletion-containing mtDNA was present at relatively similar levels (0.0003%) in all regions in 6 mo rats, but increased 25-, 7-, 3-, and 2-fold in the striatum, hippocampus, cerebral cortex, and cerebellum, respectively, of 22-23 mo old rats. To assess the relationship between fractional occurrence of a deletion and oxidative phosphorylation capacity, the activities of mitochondrial respiratory chain complexes I, III, IV and V, the mitochondrial ATP-ase, each of which contains subunits encoded in mtDNA, were determined in homogenates. No age-related decrements in activity were observed in any of the brain regions. Thus, while mtDNA deletions increase with age and to a large extent mirror the pattern observed in the human brain, they appear to have no effect on capacity for oxidative phosphorylation of distinct brain regions. Any reductions in capacity that may be present are likely to occur only at the level of individual cells.

Defects of the respiratory chain in various tissues of old monkeys: a cytochemical-immunocytochemical study

The aim of the present study was to evaluate if defects of the respiratory chain known to occur in humans, also exist in lower primates. Cytochemical-immunocytochemical studies of the respiratory chain enzymes in five monkeys (10-25 years of age) showed defects of ubiquinone cytochrome-c-oxidoreductase (complex III), of cytochrome-c-oxidase (complex IV) and of ATP-synthase (complex V) in the limb muscles, diaphragm, heart muscle and extraocular muscles of three old animals (about 25 years) and also in the heart muscle of two younger animals (10 and 15 years). Characteristically, the defects were randomly distributed and there was no loss of succinate-dehydrogenase (complex II) in the fibres. Ultracytochemistry-immunocytochemistry of complex IV disclosed that in an involved fibre segment all the mitochondria exhibited the defect. The highest number of defects was observed in the extraocular muscle (up to 340/cm2) while the lowest defect density was present in the limb muscles (2-5/cm2). Defects of complex IV occurred two to three times more often than defects of complex III and besides isolated defects of complex III and IV, combined defects of both complexes were also observed. Defects of complex V occurred exclusively in combination and were rarely seen. Using subunit specific antisera against complex IV, it could be demonstrated at light and electron microscopic level that loss of activity of cytochrome-c-oxidase was associated with a loss both of mitochondrially and nuclearly coded subunits of the enzyme. In summary, aging in lower primates and humans is characterised by a highly similar defect expression of the respiratory chain enzymes, with intercellular and interorgan differences of the aging process, underlining the universal nature of the involved pathogenetic mechanisms.

Steady-state kinetics of the reduction of coenzyme Q analogs by complex I (NADH:ubiquinone oxidoreductase) in bovine heart mitochondria and submitochondrial particles

The reduction kinetics of coenzyme Q (CoQ, ubiquinone) by NADH:ubiquinone oxidoreductase (complex I, EC 1.6.99.3) was investigated in bovine heart mitochondrial membranes using water-soluble homologs and analogs of the endogenous ubiquinone acceptor CoQ10 [the lower homologs from CoQ0 to CoQ3, the 6-pentyl (PB) and 6-decyl (DB) analogs, and duroquinone]. By far the best substrates in bovine heart submitochondrial particles are CoQ1 and PB. The kinetics of NADH-CoQ reductase was investigated in detail using CoQ1 and PB as acceptors. The kinetic pattern follows a ping-pong mechanism; the Km for CoQ1 is in the range of 20 microM but is reversibly increased to 60 microM by extraction of the endogenous CoQ10. The increased Km in CoQ10-depleted membranes indicates that endogenous ubiquinone not only does not exert significant product inhibition but rather is required for the appropriate structure of the acceptor site. The much lower Vmax with CoQ2 but not with DB as acceptor, associated with an almost identical Km, suggests that the sites for endogenous ubiquinone bind 6-isoprenyl- and 6-alkylubiquinones with similar affinity, but the mode of electron transfer is less efficient with CoQ2. The Kmin (kcat/Km) for CoQ1 is 4 orders of magnitude lower than the bimolecular collisional constant calculated from fluorescence quenching of membrane probes; moreover, the activation energy calculated from Arrhenius plots of kmin is much higher than that of the collisional quenching constants. These observations strongly suggest that the interaction of the exogenous quinones with the enzyme is not diffusion-controlled. Contrary to other systems, in bovine submitochondrial particles, CoQ1 usually appears to be able to support a rate approaching that of endogenous CoQ10, as shown by application of the "pool equation" [Kröger, A., & Klingenberg, M. (1973) Eur. J. Biochem. 39, 313-323] relating the rate of ubiquinone reduction to the rate of ubiquinol oxidation and the overall rate through the ubiquinone pool.

Inhibitor sensitivity of respiratory complex I in human platelets: a possible biomarker of ageing

NADH-Coenzyme Q reductase was assayed in platelet mitochondrial membranes obtained from 19 pools of two venous blood samples from female young (19-30 years) individuals and 18 pools from aged ones (66-107 years). The enzyme activities were not significantly changed in the two groups, but a decrease of sensitivity to the specific inhibitor, rotenone, occurred in a substantial number of aged individuals. The results are in agreement with the predictions of the mitochondrial theory of ageing and may be used to develop a sensitive biomarker of the ageing process.

Mitochondrial DNA sequence analysis of four Alzheimer's and Parkinson's disease patients

The mitochondrial DNA (mtDNA) sequence was determined on 3 patients with Alzheimer's disease (AD) exhibiting AD plus Parkinson's disease (PD) neuropathologic changes and one patient with PD. Patient mtDNA sequences were compared to the standard Cambridge sequence to identify base changes. In the first AD+PD patient, 2 of the 15 nucleotide substitutions may contribute to the neuropathology, a nucleotide pair (np) 4336 transition in the tRNA(Gln) gene found 7.4 times more frequently in patients than in controls, and a unique np 721 transition in the 12S rRNA gene which was not found in 70 other patients or 905 controls. In the second AD+PD patient, 27 nucleotide substitutions were detected, including an np 3397 transition in the ND1 gene which converts a conserved methionine to a valine. In the third AD+PD patient, 2 polymorphic base substitutions frequently found at increased frequency in Leber's hereditary optic neuropathy patients were observed, an np 4216 transition in ND1 and an np 13708 transition in the ND5 gene. For the PD patient, 2 novel variants were observed among 25 base substitutions, an np 1709 substitution in the 16S rRNA gene and an np 15851 missense mutation in the cytb gene. Further studies will be required to demonstrate a causal role for these base substitutions in neurodegenerative disease.

Ageing is associated in females with a decline in the content and activity on the b-c1 complex in skeletal muscle mitochondria

The activity of cytochrome-c oxidase [E.C. 1.9.3.1] and b-c1 complex [E.C. 1.10.2.2] and the content of cytochromes b, c + c1 and a + a3 in human skeletal muscle mitochondria from orthopaedic patients (108 women and 68 males), of age ranging between 10 and 50 years, have been analyzed. The activity of cytochrome c-oxidase declines with age both in females and males. The activity of b-c1 complex, which in young females is significantly higher than in young males, declines sharply in females, but not in males, with ageing. These results reveal that the content of active b-c1 complex in muscle mitochondria is specifically controlled by female sex hormones.

Age-associated mitochondrial DNA deletions in mouse skeletal muscle: comparison of different regions of the mitochondrial genome

The abundance of mitochondrial DNA (mtDNA) deletions has been shown to increase with age in a number of species and may contribute to the aging process. Estimating the total mtDNA deletion load of an individual is essential in evaluating the potential physiological impact. In this study, we compared three 5-kb regions of the mitochondrial genome: one in the major arc, one in the minor arc, and a third containing the light strand origin of replication. Through PCR analysis of mouse skeletal muscle, we have determined that not all regions produce equal numbers of age-associated deletions. There are, on average, twofold more detectable deletions in the major arc region than in the minor arc region. Deletions that result in the loss of the light strand origin of replication are rarely detected. Furthermore, the mechanism of deletion formation seems to be similar in both the major and minor arcs, with direct repeats playing an important, although not essential, role.

Late-onset mitochondrial neuromyopathy: an age-related phenomenon?

Peripheral neuropathy has been described in a number of cases of mitochondrial diseases. In these patients the onset of neuropathy varies from childhood to adulthood, whereas late onset is quite rare. We report here three males, ranging from 71 to 75 years with onset of peripheral neuropathy between 64 and 74 years of age. They complain of ataxic gait, muscle aches, weakness and mild muscle atrophy, sensory impairment with predominant glove and stocking distribution, reduced or absent deep tendon reflexes. Neurophysiological examinations and sural nerve biopsy studies showed a sensorimotor neuropathy with axonal degeneration in two cases and demyelination in one. Peroneus brevis muscle biopsy revealed, apart from frank neurogenic changes, presence of ragged-red fibers and cytochrome c oxidase negative fibers. Electron microscopy confirmed an abnormally increased presence of subsarcolemmal and intermyofibrillar mitochondria in muscle samples. These morphological features suggested a mitochondrial disease that was confirmed by biochemical investigations on muscle homogenate showing that the mitochondrial respiratory chain (MRC) enzyme activities were all reduced when compared to citrate synthase activity. In addition the presence of a partially inactive cytochrome c oxidase protein by ELISA was demonstrated in two cases. According to a recent "mitochondrial theory of aging", we think that a progressive decline of MRC function has affected either skeletal muscle or peripheral nerves in our patients. Being energy-requiring processes, muscle metabolism as well as active axonal transport may become progressively defective with age resulting in a late-onset neuropathy.