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A number of observations have shown that mitochondria are at the center of the pathophysiology of the failing heart and mitochondrial-based oxidative stress (OS), myocardial apoptosis, and cardiac bioenergetic dysfunction are implicated in the progression of heart failure (HF), as shown by both clinical studies and animal models. In this manuscript, we review the body of evidence that multiple defects in mitochondria are central and primary to HF progression. In addition, novel approaches to therapeutic targeting of mitochondrial bioenergetic, biogenic, and signaling abnormalities that can impact HF are discussed. [ABSTRACT FROM AUTHOR]

Autism spectrum disorder (ASD) has been associated with mitochondrial dysfunction but few studies have examined the relationship between mitochondrial function and ASD symptoms. We measured Complex I and IV and citrate synthase activities in 76 children with ASD who were not receiving vitamin supplementation or medication. We also measured language using the Preschool Language Scales or Clinical Evaluation of Language Fundamentals, adaptive behavior using the Vineland Adaptive Behavioral Scale, social function using the Social Responsiveness Scale and behavior using Aberrant Behavior Checklist, Childhood Behavior Checklist and the Ohio Autism Clinical Impression Scale. Children with ASD demonstrated significantly greater variation in mitochondrial activity compared to controls with more than expected ASD children having enzyme activity outside of the normal range for Citrate Synthase (24%), Complex I (39%) and Complex IV (11%). Poorer adaptive skills were associated with Complex IV activity lower or higher than average and lower Complex I activity. Poorer social function and behavior was associated with relatively higher Citrate Synthase activity. Similar to previous studies we find both mitochondrial underactivity and overactivity in ASD. This study confirms an expanded variation in mitochondrial activity in ASD and demonstrates, for the first time, that such variations are related to ASD symptoms.

Treatment for mitochondrial dysfunction is typically guided by expert opinion with a paucity of empirical evidence of the effect of treatment on mitochondrial activity. We examined citrate synthase and Complex I and IV activities using a validated buccal swab method in 127 children with autism spectrum disorder with and without mitochondrial disease, a portion of which were on common mitochondrial supplements. Mixed-model linear regression determined whether specific supplements altered the absolute mitochondrial activity as well as the relationship between the activities of mitochondrial components. Complex I activity was increased by fatty acid and folate supplementation, but folate only effected those with mitochondrial disease. Citrate synthase activity was increased by antioxidant supplementation but only for the mitochondrial disease subgroup. The relationship between Complex I and IV was modulated by folate while the relationship between Complex I and Citrate Synthase was modulated by both folate and B12. This study provides empirical support for common mitochondrial treatments and demonstrates that the relationship between activities of mitochondrial components might be a marker to follow in addition to absolute activities. Measurements of mitochondrial activity that can be practically repeated over time may be very useful to monitor the biochemical effects of treatments.

As the heart is an energy-demanding organ, impaired cardiac energy metabolism and mitochondrial function have been inexorably linked to cardiac dysfunction. There is a growing recognition that mitochondrial dysfunction contributes to impaired myocardial energetics and increased oxidative stress in cardiomyopathies, cardiac ischemic damage and heart failure (HF), and mitochondrial permeability transition pore opening has been reported a critical trigger of myocyte death and myocardial remodeling. It is well established that mitochondria play pivotal roles in intracellular signaling in both cell death as well as in cardioprotective pathways. Moreover, recent studies have shown that defects in mitochondrial dynamics affecting biogenesis and turnover are linked to cardiac senescence and HF. Accordingly, there has been an increasing interest in targeting mitochondria for HF therapy. This article reviews the background and recent evidence of mitochondrial involvement in several types of cell death (apoptosis, necrosis and autophagy) occurring in HF. In addition, potential strategies for targeting mitochondria are examined, and their utility in HF therapy considered.

Phelan-McDermid Syndrome (PMS), which is defined by a deletion within 22q13, demonstrates significant phenotypic variation. Given that six mitochondrial genes are located within 22q13, including complex I and IV genes, we hypothesize that mitochondrial complex activity abnormalities may explain phenotypic variation in PMS symptoms. Complex I, II, II + III and IV activity was measured in 51 PMS participants. Caretakers completed questionnaires and provided genetic information through the PMS foundation registry. Complex activity was abnormal in 59% of PMS participants. Abnormalities were found in complex I and IV but not complex II + III and II activity, consistent with disruption of genes within the 22q13 region. However, complex activity abnormalities were not related to specific gene deletions suggesting a "neighboring effect" of regional deletions on adjacent gene expression. A specific combination of symptoms (autism spectrum disorder, developmental regression, failure-to-thrive, exercise intolerance/fatigue) was associated with complex activity abnormalities. 64% of 106 individuals in the PMS foundation registry who did not have complex activity measured also endorsed this pattern of symptoms. These data suggest that mitochondrial abnormalities, specifically abnormalities in complex I and IV activity, may explain some phenotypic variation in PMS individuals. These results point to novel pathophysiology mechanisms and treatment targets for PMS patients.

Objectives: The objectives of our study were to compare the mitochondrial enzyme activity between obese and non-obese children and to assess the association between mitochondrial DNA content and function and markers of metabolic syndrome., Methods: Clinical and anthropometric data of obese and normal-weight children ages 2-18 years were collected. We collected buccal swabs for mitochondrial respiratory enzymes (complex I, IV, and Citrate Synthase). In obese children only, serum levels of metabolic parameters and mitochondrial DNA from mononuclear cells were quantitated., Results: We recruited 75 obese and 65 normal-weight children. There was no difference in respiratory complex enzyme activity levels between obese and normal-weight subjects. In obese subjects, mitochondrial to nuclear DNA (mt/nDNA) ratio was significantly correlated with BMI Z-score and BMI percentile (p < 0.05, and p < 0.01, respectively), and the strength of this correlation was proportionate to the degree of obesity. We did not find any association between mt/nDNA ratio and metabolic parameters. We observed a significant positive association between complex IV activity and fasting insulin level (p < 0.05). Finally, fasting insulin explained 45% of the variation in the complex IV activity level (p < 0.05)., Conclusion: Our findings indicate that mitochondrial DNA content is directly related to obesity, but not to the markers of metabolic syndrome/insulin resistance in children. Longitudinal studies involving larger samples are needed to confirm our findings and help elucidate the relationship between mitochondrial function, adiposity, and insulin resistance.

Aim: Mitochondrial function studies in autism spectrum disorders (ASD) have detected skeletal muscle mitochondrial enzyme deficiencies in respiratory complex (RC) activities. As a muscle biopsy is expensive and invasive, we assessed RC-I and RC-IV activities in buccal swabs., Methods: 92 children with ASD and 68 controls were studied with immunocapture for RC-I and microspectrophotometry for RC-IV., Results: Significant RC activity deficiencies were found in 39 (42%) ASD patients (p < 0.01) and more prevalent in more severe cases. Aberrant RC overactivity was seen in 9 children. RC-I/RC-IV activity ratio was significantly increased in 64% of the entire ASD cohort including 76% of those more severely affected (p < 0.05)., Conclusion: Buccal swab analysis revealed extensive RC abnormalities in ASD providing a noninvasive biomarker to assess mitochondrial function in ASD patients.

Epilepsy is the most common neurologic disorder worldwide and is characterized by recurrent unprovoked seizures. The mitochondrial (mt) respiratory chain is the final common pathway for cellular energy production through the process of oxidative phosphorylation. As neurons are terminally differentiated cells that lack significant regenerative capacity and have a high energy demand, they are more vulnerable to mt dysfunction. Therefore, epileptic seizures have been well described in several diseases such as mt encephalomyopathy, lactic acidosis, and stroke-like episodes and myoclonic epilepsy and ragged red fibers, which are caused by gene mutations in mtDNA, among others. Mutations in nuclear DNA regulating mt function are also being described (eg, POLG gene mutation). The role of mitochondria (mt) in acquired epilepsies, which account for about 60% of all epilepsies, is equally important but less well understood. Oxidative stress is one of the possible mechanisms in the pathogenesis of epilepsy resulting from mt dysfunction gradually disrupting the intracellular Ca(2+) homeostasis, which modulates neuronal excitability and synaptic transmission, making neurons more vulnerable to additional stress, and leading to energy failure and neuronal loss in epilepsy. Antiepileptic drugs (AEDs) also affect mt function in several ways. There must be caution when treating epilepsy in patients with known mt disorders as some AEDs are toxic to the mt. This review summarizes our current knowledge of the effect of mt disorders on epilepsy, of epileptic seizures on mt, and of AEDs on mt function and the implications of all these interactions for the management of epilepsy in patients with or without mt disease., (© 2013 Published by Elsevier Inc.)

Using data of the current prevalence of autism as 200:10,000 and a 1:2000 incidence of definite mitochondrial (mt) disease, if there was no linkage of autism spectrum disorder (ASD) and mt disease, it would be expected that 1 in 110 subjects with mt disease would have ASD and 1 in 2000 individuals with ASD would have mt disease. The co-occurrence of autism and mt disease is much higher than these figures, suggesting a possible pathogenetic relationship. Such hypothesis was initially suggested by the presence of biochemical markers of abnormal mt metabolic function in patients with ASD, including elevation of lactate, pyruvate, or alanine levels in blood, cerebrospinal fluid, or brain; carnitine level in plasma; and level of organic acids in urine, and by demonstrating impaired mt fatty acid β-oxidation. More recently, mtDNA genetic mutations or deletions or mutations of nuclear genes regulating mt function have been associated with ASD in patients or in neuropathologic studies on the brains of patients with autism. In addition, the presence of dysfunction of the complexes of the mt respiratory chain or electron transport chain, indicating abnormal oxidative phosphorylation, has been reported in patients with ASD and in the autopsy samples of brains. Possible pathogenetic mechanisms linking mt dysfunction and ASD include mt activation of the immune system, abnormal mt Ca(2+) handling, and mt-induced oxidative stress. Genetic and epigenetic regulation of brain development may also be disrupted by mt dysfunction, including mt-induced oxidative stress. The role of the purinergic system linking mt dysfunction and ASD is currently under investigation. In summary, there is genetic and biochemical evidence for a mitochondria (mt) role in the pathogenesis of ASD in a subset of children. To determine the prevalence and type of genetic and biochemical mt defects in ASD, there is a need for further research using the latest genetic technology such as next-generation sequencing, microarrays, bioinformatics, and biochemical assays. Because of the availability of potential therapeutic options for mt disease, successful research results could translate into better treatment and outcome for patients with mt-associated ASD. This requires a high index of suspicion of mt disease in children with autism who are diagnosed early., (© 2013 Published by Elsevier Inc.)

Making a diagnosis of mitochondrial disease (MD) is extremely challenging and often employs the analysis of respiratory complex (RC) activities in biopsied skeletal muscle. Given both the invasive nature and expense of biopsied-muscle based testing for mitochondrial defects, buccal swab enzyme analysis has been explored as an alternative approach to the more invasive muscle biopsy. Case studies have recently suggested that buccal swabs from patients can be used to accurately assess mitochondrial enzyme activities including RC I and RC IV using a dipstick methodology combined with spectrophotometric analysis. In this study, forty patients with suspected MD who have previously been found to have significant defects in either RC I or RC IV in skeletal muscle were assessed by buccal swab analysis and compared to enzyme values obtained with unaffected controls (n=106) in the same age range. Buccal citrate synthase was used as an indicator of overall mitochondrial content, correlating well with overall buccal mitochondrial frataxin levels and was found to be elevated above control levels in 28% of the patients in this cohort. Of 26 cases with significant muscle RC I deficiency, 20 displayed significantly reduced levels of buccal RC I activity. All 7 of the patients with muscle RC IV deficiency showed significant buccal RC IV defect and 6 of the 7 patients with combined defects in muscle RC I and IV activity levels also exhibited analogous deficiencies in both buccal RC I and RC IV activities. In conclusion, the relatively high correlation (over 82%) of buccal and muscle RC deficiencies further supports the validity of this non-invasive approach as a potentially useful tool in the diagnosis of MD., (Copyright © 2011 Elsevier Inc. All rights reserved.)

The authors describe mitochondrial studies in a 6-year-old patient with a seizure disorder that can be seen in myoclonic epilepsy and ragged red fibers. Using a recently developed noninvasive approach, analysis of buccal mitochondrial enzyme function revealed severe respiratory complex I and IV deficiencies in the patient. In addition, analysis of buccal mitochondrial DNA showed significant amounts of the common 5 kb and 7.4 kb mitochondrial DNA deletions, also detectable in blood. This study suggests that a buccal swab approach can be used to informatively examine mitochondrial dysfunction in children with seizures and may be applicable to screening mitochondrial disease with other clinical presentations.

Background: Specific myocardial mitochondrial enzymatic dysfunction and apoptotic remodeling occur in pacing-induced heart failure. We sought to define their regional distribution and molecular basis in the failing heart., Methods and Results: Enzyme dysfunction was assessed in mitochondrial subpopulations and immunoblot analysis was performed using homogenate proteins from the left atria (LA) and left ventricle (LV) of paced and control mongrel dogs. A greater range of enzymatic defects (complex I, III, and V) was found in mitochondria subpopulations from the LV as compared with the LA (where only complex V was defective). Analysis of paced LV proteins demonstrated a downregulated expression of both mitochondrial genes (eg, cytochrome b) and nuclear genes (eg, ATP synthase beta subunit, mitochondrial creatine kinase). Protease-activated products of both mitochondrial (eg, apoptosis inducing factor) and cytosolic (eg, caspase-3) apoptogenic proteins were increased in both the LA and LV. Nuclear-localized apoptotic markers (eg, p53, p21) were also significantly increased in the LV of paced dogs., Conclusion: Abnormal activity of several mitochondrial enzymes and increased apoptogenic pathway appear to be mediated, at least in part, by an orchestrated shift in expression (both nuclear and mitochondrial DNA) of respiratory chain subunits (eg, cyt b, ATP-beta), mitochondrial bioenergetic enzymes (eg, mitochondrial creatine kinase), global transcription factor (eg, PGC-1), and apoptotic proteins (eg, p53, p21) with distinct differences in their regional distribution and in the subpopulations of mitochondria affected.

p53 is an important regulator of cell growth and apoptosis and its activity is regulated by phosphorylation. Accordingly, in neonatal rat cardiomyocytes we examined the involvement of p53 in H(2)O(2)-induced apoptosis. Treatment with 50-100 microM H(2)O(2) markedly induced apoptosis in cardiomyocytes, as assessed by gel electrophoresis of genomic DNA. To examine whether H(2)O(2) increases p53 phosphorylation in cardiomyocytes, we utilized an antibody that specifically recognizes phosphorylated p53 at serine-15. The level of phosphorylated p53 was markedly increased by 100 microM H(2)O(2) at 30 and 60 min. Using specific protein kinase inhibitors we examined the involvement of protein kinases in p53 phosphorylation in response to H(2)O(2) treatment. However, staurosporine, a broad spectrum inhibitor of protein kinases, SB202190, a specific p38 kinase inhibitor, PD98059, a MAP kinase inhibitor, wortmannin, an inhibitor of DNA-PK and PI3 kinase, SP600125, a JNK inhibitor and caffeine,an inhibitor of ATM and ATR, failed to prevent the H(2)O(2)-induced phosphorylation of p53. cDNA microarray revealed that H(2)O(2) markedly increased expression of several p53 upstream modifiers such as the p300 coactivator protein and several downstream effectors such as gadd45, but decreased the expression of MDM2, a negative regulator of p53. Our results suggest that phosphorylation of p53 at serine-15 may be an important signaling event in the H(2)O(2)-mediated apoptotic process.

Defects in ion channels (channelopathies) are increasingly found in a large spectrum of human pathologies including aging. Mutations in genes encoding ion channel proteins, which disrupt channel function, are the most commonly identified cause of channelopathies. Mutations in associated proteins, alterations in the expression of ion channels, or changes in the activity of non-mutated channel genes or associated proteins can also produce acquired channelopathies. Mitochondria, the powerhouse of the cells, are considered to be the most important cellular organelles to contribute to aging mainly because of their role in the production of reactive oxygen species in the initiation of apoptotic cell remodeling and in efficient ATP synthesis. During the past 50 years, multiple ion channels or transporters have been found in mitochondria, and the relationship between the activity of these channels and cellular aging, as well as the overall cellular biological function, has been intensively studied in a number of cell types and animal models. In this review, we discuss the better characterized mitochondrial ion channels whose dysfunction (mitochondrial channelopathies) may affect or accelerate the aging processes. These channels include the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)), Ca(2+) transporters, voltage-dependent anion channel, and the mitochondrial permeability transition pore (mitoPTP).

Studies in animal models of myocardial ischemia-reperfusion revealed that the administration of insulin-like growth factor (IGF-1) can provide substantial cardioprotective effect. However, the mechanisms by which IGF-1 prevents myocardial ischemia-reperfusion injury are not fully understood. This study addresses whether mitochondrial bioenergetic pathways are involved in the cardioprotective effects of IGF-1. Single cardiomyocytes from adult rats were incubated in the absence or presence of IGF-1 for 60 min and subjected to 60 min hypoxia followed by 30 min reoxygenation at 37 degrees C. Mitochondrial function was evaluated by assessment of enzyme activities of oxidative phosphorylation and Krebs cycle pathways. Hypoxia/reoxygenation (HR) caused significant inhibition of mitochondrial respiratory complex IV and V activities and of the Krebs cycle enzyme citrate synthase, whereas pretreatment with IGF-1 maintained enzyme activities in myocytes at or near control levels. Mitochondrial membrane potential, evaluated with JC-1 staining, was significantly higher in IGF-1 + HR- treated myocytes than in HR alone, with levels similar to those found in normal control cardiomyocytes. In addition, IGF-1 reduced both HR-induced lactate dehydrogenase (LDH) release and malondialdehyde production (an indicator of lipid peroxidation) in cardiomyocytes. These results suggest that IGF-1 protects cardiomyocytes from HR injury via stabilizing mitochondria and reducing reactive oxidative species (ROS) damage.

Hypothesis: Damage to heart mitochondrial structure and function occur with aging, and in heart failure (HF). However, the extent of mitochondrial dysfunction, the expression of mitochondrial and nuclear genes, and their cross-talk is not known., Observations: Several observations have suggested that somatic mutations in mitochondrial DNA (mtDNA), induced by reactive oxygen species (ROS), appear to be the primary cause of energy decline, and that the generation of ROS is mainly the product of the mitochondrial respiratory chain. The free radical theory of aging, that could also be applied to HF, and in particular the targeting of mtDNA is supported by a plurality of observations from both animal and clinical studies showing decreased mitochondrial function, increased ROS levels and mtDNA mutations in the aging heart., Discussion: Aging and HF with their increased ROS-induced defects in mtDNA, including base modifications and frequency of mtDNA deletions, might be expected to cause increased errors or mutations in mtDNA-encoded enzyme subunits, resulting in impaired oxidative phosphorylation and defective electron transport chain (ETC) activity which in turn creates more ROS. These events in both the aging and failing heart involve substantial nuclear-mitochondrial interaction, which is further illustrated in the progression of myocardial apoptosis. In this review the cross-talk between the nucleus and the mitochondrial organelle will be examined based on a number of animal and clinical studies, including our own.

Heart disease including myocardial infarction and ischemia is associated with the irreversible loss of cardiomyocytes and vasculature, both via apoptosis or necrosis. However, the native capacity for the renewal and repair of myocardial tissue is inadequate as have been current therapeutic measures to prevent left ventricular remodeling. Cell transplantation has emerged as a potentially viable therapeutic approach to directly repopulate and repair the damaged myocardium. A detailed analysis and a vision for future progress in stem cell applications, both in research and clinical cardiology are presented in this review, highlighting the use of a wide spectrum of stem/progenitor cell types including embryonic or fetal stem cells, myoblasts, and adult bone marrow stem cells. An up-to-date comparison of donor cell-types used, and evaluation of the myocardial disorders that might be most amenable to stem cell therapy are discussed. The roles that myocardial cell fusion and transdifferentiation play in stem cell transplantation, the specific shortcomings of available technologies, and recommendations for practical ways that these concerns might be overcome, are also presented.

Thyroid hormone (TH) induces marked changes in the biochemical and physiological functioning of cardiac muscle affecting its bioenergetics, contractility and structure. Using a time-course analysis of in vitro treatment of neonatal rat cardiomyocytes with triiodothyronine (T3), mitochondrial biogenesis, functional bioenergetics and cardiomyocyte hypertrophic phenotype were assessed. Activity of respiratory complexes II, IV, V and citrate synthase (CS), levels of mitochondrial enzyme subunits (e.g. COXI, COXIV) and nuclear-encoded transcription factors, involved in mitochondrial biogenesis (e.g. PGC-1, mtTFA and PPAR-alpha), were significantly elevated with 72 h T3 treatment. A time-course analysis showed an early increase (between 3 and 12 h) in activity and levels of subunits of complex IV and V, mitochondrial Ca2+ accumulation and a late increase (at 72 h) in complex II and CS activities, mitochondrial protein content and mitochondrial respiration. Based on overall protein content and specific peptide levels (e.g. actin or myosin) only mild cardiomyocyte hypertrophy was detected. T3 mediates an early stimulation of enzymes containing mtDNA encoded subunits (e.g. complex IV and V) in contrast to a different regulatory pattern for the entirely nuclear-encoded enzymes (e.g. CS and complex II). T3-regulation was similar in both neonatal and young adult cardiomyocytes (ARCM) but absent in the senescent cardiomyocytes. This model offer an opportunity to study the rapid timing of events involved in myocardial cell signaling, bioenergetics and growth dynamics in a timeframe not available with whole animal studies.

Marked changes in energy substrate utilization occur during the progression of congestive heart failure (CHF) where fatty acid utilization, as the primary source of cardiac energy, is severely diminished, oxidative phosphorylation is down-regulated, and glucose uptake and utilization increase. Neither the signaling events or the molecular basis for the shift in substrate utilization have yet been elucidated. This study was designed to examine in the canine model of paced-induced CHF, the potential role of the Akt pathway in signaling the metabolic transitions central to progression to heart failure. Myocardial Akt levels were elevated in early heart failure (after 1-2 weeks of pacing) accompanied by increased levels of oxidative stress, cytokine tumor necrosis factor-alpha (TNF-alpha) and free fatty acid accumulation, reduced activity levels of mitochondrial respiratory complexes III and V and apoptosis initiation. At severe heart failure (3-4 weeks of pacing), there was significant further increase in myocardial apoptosis, with pronounced decline in myocardial Akt kinase activity. At this later stage, there were no further changes in free fatty acid accumulation, complex V activity or in oxidative stress levels indicating that these changes primarily occurred in the earlier stage of evolving heart failure. In contrast, during severe heart failure, both the reduction in complex III activity and increase in TNF-alpha level became more pronounced. Our data provide critical support for the hypothesis that the Akt signaling pathway is a contributory element in the early signaling events leading to the progression of pacing-induced heart failure, accompanying the shift in substrate utilization.

Changes in thyroid status are associated with profound alterations in biochemical and physiological functioning of cardiac muscle impacting metabolic rate, contractility and structural hypertrophy. Using an in vivo model of chronic treatment with thyroid hormone (T4, 0.3 mg/kg/day), we evaluated how mitochondria are regulated in response to T4, and assessed the relationship of T4-induced mitochondrial biogenesis and bioenergetics to overall cardiac hypertrophy. The role of thyroid hormone in cardiac bioenergetic remodeling was addressed in rats treated with T4 for 5, 10 and 15 days. Over that time, myocardial oxygen consumption substantially increased as did cardiac hypertrophy. Myocardial levels of mitochondrial enzyme activities, mitochondrial DNA (mtDNA), specific proteins and transcript were assessed. Activity levels of respiratory complexes I-V and citrate synthase significantly increased with 15 but not with 5 or 10-day T4 treatment. Myocardial levels of mtDNA, mitochondrial proteins (e.g. cytochrome c, cytochrome b, ATPase subunits, MnSOD) and the global transcription factor PPARalpha were significantly elevated with 15-day T4. Transcript analysis revealed increased expression of transcription factors and cofactors involved in mitochondrial biogenesis including PPARalpha, mtTFA, ErbAalpha and PGC-1alpha. Our findings indicate parallel increases in myocardial mitochondrial bioenergetic capacity, oxygen consumption and markers of mitochondrial biogenesis with 15-day T4; these changes were not present with 10-day T4 even with significant cardiac hypertrophy. The marked, parallel increases in PPARalpha levels suggest its potential involvement in mediating myocardial-specific remodeling of mitochondria in response to T4.

The contribution that mitochondria make to cardiac function extends well beyond their critical bioenergetic role as a supplier of ATP. The organelle plays an integral part in the regulatory and signaling events that occur in response to physiological stresses, including but not limited to myocardial ischemia and reperfusion, hypoxia, oxidative stress, and hormonal and cytokine stimuli. Research on both intact cardiac muscle tissue and cultured cardiomyocytes has just begun to probe the nature and the extent of mitochondrial involvement in interorganelle communication, hypertropic growth, and cell death. This review covers particular aspects of the newly emerging field of mitochondrial medicine offering a critical guide in the assessment of mitochondrial participation at the molecular and biochemical levels in the multiple and interrelated signaling pathways, gauging the effect that mitochondria have as a receiver, integrator, and transmitter of signals on cardiac phenotype. We also discuss future directions that may impact on the treatment of cardiac diseases.

Oxidative stress is involved in mitochondrial apoptosis, and plays a critical role in ischemic heart disease and cardiac failure. Exposure of cardiomyocytes to H(2)O(2) leads to oxidative stress and mitochondrial dysfunction. In this study, we investigated the temporal order of mitochondrial-related events in the neonatal rat cardiomyocyte response to H(2)O(2) treatment. At times ranging from 10 to 90 min after H(2)O(2) treatment, levels were determined for respiratory complexes I, II, IV and V, and citrate synthase activities, mitochondrial Ca(2+) flux, intracellular oxidation, mitochondrial membrane potential and apoptotic progression. Complexes II and IV activity levels were significantly reduced within 20 min of H(2)O(2) exposure while complexes I and V, and citrate synthase were unaffected. Mitochondrial membrane potential declined after 20 and 60 min of H(2)O(2) exposure while intracellular oxidation, declining complex I activity and apoptotic progression were detectable only after 60 min. Measurement of mitochondrial Ca(2+) ([Ca(2+)](m)) using rhodamine 2 detected an early accumulation of [Ca(2+)](m) occurring between 5 and 10 min. Pretreatment of cardiomyocytes with either ruthenium red or cyclosporin A abrogated the H(2)O(2)-induced decline in complexes II and IV activities, indicating that [Ca(2+)](m) flux and onset of mitochondrial permeability transition pore opening likely precede the observed early enzymatic decline. Our findings suggest that [Ca(2+)](m) flux represents an early pivotal event in H(2)O(2)-induced cardiomyocyte damage, preceding and presumably leading to reduced mitochondrial respiratory activity levels followed by accumulation of intracellular oxidation, mitochondrial membrane depolarization and apoptotic progression concomitant with declining complex I activity.

Research over the last decade has extended the prevailing view of cell mitochondrial function well beyond its critical bioenergetic role in supplying ATP. Recently, it has been recognized that the mitochondria play a critical role in cell regulatory and signaling events, in the responses of cells to a multiplicity of physiological and genetic stresses, inter-organelle communication, cell proliferation and cell death. Nevertheless, a broad-based review on mitochondrial signaling is not presently available. To bridge that gap, this review examines the perspective of mitochondria as the receiver integrator and transmitter of signals, dissecting the multiple and interrelated signaling pathways at both the molecular and biochemical levels with particular focus on nuclear and cytoplasmic factors, fundamentally involved in the shaping of the organelles' responses. We examine evidence that the mitochondria act as a dynamic receiver and integrator of numerous translocated signaling proteins (including protein kinases and transcription factors), regulatory Ca2+ fluxes and membrane phospholipids as well the transmission of mitochondrial-generated oxidative stress and energy-related signaling. Novel experimental approaches studying mitochondrial signaling including cell studies using metabolic inhibitors and genetic stresses (e.g. mtDNA depletion) are discussed. While there is abundant interest and information concerning its integral role to apoptosis, mitochondrial signaling also plays a fundamental role in proliferative pathways, nutrient sensing, inter-organellar cross-talk and in the responses of cells to metabolic transition and physiological stresses which remain relatively unexplored.

Background: There is increasing evidence documenting the capacity of myocardial cells exposed to a variety of insults to mount a cardioprotective response. Although this cardioprotection has been most well characterized with respect to ischemic preconditioning, other chemical and metabolic stressors have been shown to share features of the ischemic preconditioning model, including the involvement of mitochondria in the triggering, signaling, and mediation of the cardioprotective response., Methods: In this article, we review the evidence showing that mitochondria play a critical role in cardioprotection from multiple (often interrelated) standpoints: its primary function in producing the cellular bioenergetic supply, its control over events in apoptosis, its contribution to myocardial signal transducing processes, and its role in producing reactive oxidative species and in providing an appropriate antioxidant response to a variety of cellular insults., Conclusions: Although our understanding of cytoprotection has increased substantially within the last few years, the mechanisms mediating mitochondrial resistance to insults leading to cardiac protection remain to be fully delineated, and represents a significant approach in the clinical treatment of heart disease.

Multisystemic disorders with predominantly neurologic manifestations often present with mitochondrial abnormalities in striated muscle biopsies. Decreased respiratory complex activities and abnormalities in mitochondrial structure and DNA constitute the spectrum of mitochondrial changes used as diagnostic and prognostic indicators in patients with neuromuscular disorders. This study assessed mitochondrial defects present in a cohort of 154 young patients to determine diagnostic efficiency and probe the relationship of mitochondrial to clinical phenotype. Striated muscle biopsies were analyzed for mitochondrial structure and number, levels of enzyme activities of complex I-V and citrate synthase, mitochondrial DNA and specific mitochondrial DNA deletions, and presence of 15 pathogenic mitochondrial DNA point mutations. Reduced complex I, III, IV, and V activities were the most ubiquitous finding, with complex III most commonly affected. Mitochondrial structural defects (39%) included changes in mitochondria sizes/shapes and number and aberrant cristae formation. Mitochondrial DNA deletions were evident in 15 patients, three displayed mitochondrial DNA depletion, and only two harbored pathogenic point mutations. Reductions in specific enzyme activities may be the most sensitive diagnostic indicator, whereas defects in ultrastructure and mitochondrial DNA integrity were frequently accompanied by the full spectrum of mitochondrial abnormalities. Some phenotypes displayed specific mitochondrial abnormalities; however, most clinical phenotypes displayed little specificity with regard to mitochondrial phenotype.

The use of stem cells has proved to be an important tool in investigating the events of early cardiac development, differentiation, and morphogenesis. In addition, stem cell transplantation in the treatment of certain cardiac disorders has shown early promise. We have attempted to present a balanced review of both basic studies and clinical-therapeutic potential of stem cells transplantation in the damaged heart.

The heart is highly dependent for its function on oxidative energy generated in mitochondria, primarily by fatty acid beta-oxidation, respiratory electron chain and oxidative phosphorylation. Defects in mitochondrial structure and function have been found in association with cardiovascular diseases such as dilated and hypertrophy cardiomyopathy, cardiac conduction defects and sudden death, ischemic and alcoholic cardiomyopathy, as well as myocarditis. While a subset of these mitochondrial abnormalities have a defined genetic basis (e.g. mitochondrial DNA changes leading to oxidative phosphorylation dysfunction,fatty acid beta-oxidation defects due to specific nuclear DNA mutations), other abnormalities appear to be due to a more sporadic or environmental cardiotoxic insult or have not yet been characterized.This review focuses on abnormalities in mitochondrial bioenergetic function and mitochondrial DNA defects associated with cardiovascular diseases, their significance in cardiac pathogenesis as well as on the available diagnostic and therapeutic options. A concise background concerning mitochondrial biogenesis and bioenergetic pathways during cardiac growth,development and aging will also be provided.

Objective: Defects in mitochondrial structure and function have been found in association with cardiovascular diseases such as dilated and hypertrophic cardiomyopathy, cardiac conduction defects and sudden death, ischemic and alcoholic cardiomyopathy, and myocarditis. A genetic basis has been established for some mitochondrial abnormalities (eg, mitochondrial DNA changes leading to oxidative phosphorylation dysfunction, fatty acid beta-oxidation (FAO) defects resulting from specific nuclear mutations) whereas other abnormalities appear to be due to a more sporadic or environmental cardiotoxic insult or have not yet been characterized., Methods: This article reviews mitochondrial abnormalities in structure or function reported in cardiac diseases highlighting information about their potential etiology, significance in cardiac pathogenesis, and diagnostic and therapeutic options available to the clinician. We also provide a brief background concerning mitochondrial biogenesis and bioenergetic pathways in cardiac growth, development, and aging., Conclusions: Although aberrations in bioenergetic functioning of mitochondria appear to be most often related to cardiac dysfunction, the primary defect(s) causing bioenergetic dysfunction may reside in a nonbioenergetic pathway (eg, signaling between mitochondria and nucleus) or in overall mitochondrial biogenesis or degradation pathways.

Cardiomyopathy and neuromuscular abnormalities may simultaneously coexist and present with defects in mitochondrial DNA and bioenergetic function. We sought to evaluate the relationship between clinical and mitochondrial phenotypes in 28 young patients with both cardiomyopathy and neurologic disorders including seizures, dystonia, ophthalmoplegia, Kearns-Sayre syndrome, Leigh disease, and Friedreich's ataxia. All tissues examined displayed marked defects in respiratory complex activities. Five patients had abundant large-scale mitochondrial DNA deletions and one patient displayed a pathogenic point mutation previously reported with mitochondrial cytopathy. In this cohort, patients with hypertrophic cardiomyopathy displayed a higher incidence of complex I defects, fewer DNA deletions and mitochondrial structural abnormalities and were less often associated with developmental delay phenotype compared with patients with dilated cardiomyopathy. Although structural abnormalities are present in a subset of patients, evaluation of respiratory enzyme activity appears to be most informative whether tissues examined were derived from heart or skeletal muscle. Defects in mitochondrial DNA and bioenergetics are frequently present in children with cardiomyopathy presenting with a variety of neurologic abnormalities and are amenable to biochemical and molecular analysis.

Objective: Mitochondrial enzymatic activity reductions in both myocardial and skeletal muscle tissues have been reported in a canine model of pacing-induced congestive heart failure (CHF). Endothelin-1 (ET-1), a vasoconstrictor peptide with diverse biological properties, has been implicated in CHF pathogenesis, and ET-1 receptor blockade has been shown to attenuate CHF progression. We hypothesized that the beneficial effect of ET-1 receptor blockade may be mediated in part by improved mitochondrial function., Methods: Myocardium and skeletal muscle tissues were evaluated for respiratory complex I-V and citrate synthase activity levels in paced animals treated with and without LU 135252, a specific type A ET-1 receptor (ET(A)) antagonist., Results: Specific activity levels of complex V and III, which were 65% to 85% lower in both cardiac and skeletal muscle in paced compared to unpaced animals, were significantly increased in ET(A) antagonist-treated animals (50%-300% compared to untreated paced animals). Levels of other mitochondrial respiratory complex activities including complex I, II, and IV as well as citrate synthase were not significantly changed., Conclusions: These findings suggest that endothelin activation may be involved in the myocardial dysfunction and mitochondrial enzyme deficiencies observed in pacing-induced CHF. Improvement of mitochondrial function may be a novel mechanism mediating the beneficial effect of ET(A) receptor blockade in CHF.

We describe a 17-year-old male with neurologic and cardiovascular disorders characterized by complete atrioventricular block and a mitochondrial cytopathy with clinical, structural, biochemical, and molecular features shared by chronic progressive external ophthalmoplegia and Kearns-Sayre syndrome. The patient's manifestations included progressive external ophthalmoplegia, bilateral ptosis, muscle weakness, delayed development, and progressive hearing loss with multiple mitochondrial DNA deletions, including an abundant 11-kb novel deletion and reduced specific activities of respiratory complexes I, III, and IV present in skeletal muscle. Ultrastructural analysis of biopsied muscle revealed a heterogenous mixture of normal and abnormal mitochondria with unusual cristae. This unique mitochondrial DNA deletion, which eliminates the origin of mitochondrial DNA replication for the light strand, may be responsible for generating an intermediate clinical phenotype.

A group of 12 children clinically presenting with hypotonia, intractable epilepsy, autism, and developmental delay, who did not fall into previously described categories of mitochondrial encephalomyopathy, were evaluated for mitochondrial respiratory enzyme activity levels, mitochondrial DNA, and mitochondrial structural abnormalities. Reduced levels in specific respiratory activities were found solely in enzymes with subunits encoded by mitochondrial DNA in seven of eight biopsied skeletal muscle specimens evaluated. Five cases exhibited increased levels of large-scale mitochondrial DNA deletions, whereas pathogenic point mutations previously described in association with mitochondrial encephalomyopathies were not found. Mitochondrial structural abnormalities were present in three of four patients examined. Our findings suggest that mitochondrial dysfunction, including extensive abnormalities in specific enzyme activities, mitochondrial structure, and mitochondrial DNA integrity, may be present in children with a clinical constellation including hypotonia, epileptic seizures, autism, and developmental delay. The acronym HEADD is presented here to facilitate pursuit of mitochondrial defects in patients with this clinical constellation after other causes have been excluded.

Background: An age-dependent accumulation of point mutations in the noncoding control region of human mitochondrial D-loop has been found in cultured fibroblasts and muscle cells. Damage in mitochondrial DNA (mtDNA) coding genes and decreased bioenergetic generation have also been found in human cardiac tissues with aging and in cardiac disease., Methods and Results: We analyzed cardiac mtDNA for the incidence and distribution of point mutations in the D-loop control region involved in mtDNA replication (from nucleotides 110 to 570) in 47 patients with cardiomyopathy and 40 subjects with no history of cardiac disease. The nucleotide changes in the control region were compared with changes in a cytb fragment of roughly the same size in controls and patients. Frequency and distribution of mutations in relation to age and cardiac disease were assessed. No significant accumulation of point mutations in the D-loop control region or in cytb was found as a function of age. However, mutations in important sites within the D-loop control region were present in 8 patients (17%)., Conclusions: We found specific mutations at critical sites in the D-loop in cardiomyopathy that may play a role in cardiac pathogenesis. Age does not appear to be a factor in mutation accumulation.

Leigh syndrome is a heterogenous neurologic disease characterized by seizures, developmental delay, muscle weakness, respiratory abnormalities, optic abnormalities, including atrophy and ophthalmoplegia, and progressive cranial nerve degeneration with early onset in infants and children. Diagnosis can be confirmed by characteristic pathologic findings of necrosis in the basal ganglia, thalamus, and brainstem. Severe dysfunction of mitochondrial energy metabolism is generally present and involved in the etiology of this degenerative central nervous system disease. At the molecular level, a number of point mutations have been located in mitochondrial DNA genes, including ATPase6 and tRNA(Lys) genes, and in nuclear genes encoding subunits of oxidative enzymes, such as pyruvate dehydrogenase. Biochemically these mutations are responsible for enzymatic defects in either respiratory complexes (I, IV, or V) or pyruvate dehydrogenase. We describe here the first case of Leigh syndrome with marked depletion of mitochondrial DNA levels in skeletal muscle and abnormal activities in skeletal muscle of mitochondrial respiratory complexes I, III, IV, and V.

Background: Previous studies have shown that marked changes in myocardial mitochondrial structure and function occur in human cardiac failure. To further understand the cellular events and to clarify their role in the pathology of cardiac failure, we have examined mitochondrial enzymatic function and peptide content, and mitochondrial DNA (mtDNA) integrity in a canine model of pacing-induced cardiac failure., Methods: Myocardium and skeletal muscle tissues were evaluated for levels of respiratory complex I-V and citrate synthase activities, large-scale mtDNA deletions as well as peptide content of specific mitochondrial enzyme subunits. Levels of circulating and cardiac tumor necrosis factor-alpha (TNF-alpha), and of total aldehyde content in left ventricle were also assessed., Results: Specific activity levels of complex III and V were significantly lower in both myocardial and skeletal muscle tissues of paced animals compared to controls. In contrast, activity levels of complex I, II, IV and citrate synthase were unchanged, as was the peptide content of specific mitochondrial enzyme subunits. Large-scale mtDNA deletions were found to be more likely present in myocardial tissue of paced as compared to control animals, albeit at a relatively low proportion of mtDNA molecules (<0.01% of wild-type). In addition, the reduction in complex III and V activities was correlated with elevated plasma and cardiac TNF-alpha levels. Significant increases in left ventricle aldehyde levels were also found., Conclusions: Our data show reductions in specific mitochondrial respiratory enzyme activities in pacing-induced heart failure which is not likely due to overall decreases in mitochondrial number, or necrosis. Our findings suggest a role for mitochondrial dysfunction in the pathogenesis of cardiac failure and may indicate a commonality in the signaling for pacing-induced mitochondrial dysfunction in myocardial and skeletal muscle. Increased levels of TNF-alpha and oxidative stress appear to play a contributory role.

Background: Previous studies have shown that mitochondrial DNA (mtDNA) mutations are often present in patients with myocardial dysfunction. We sought to assess the prevalence and significance of heart mtDNA sequence changes in patients with idiopathic dilated cardiomyopathy (DCM)., Methods and Results: DNA sequence of all the transfer ribonucleic acid (tRNA), ribosomal RNA (rRNA), and structural genes in cardiac mtDNA of 28 patients with DCM was determined and compared with a control group that had no evidence of heart disease. An increased number of point mutations were found in DCM cardiac mtDNA when compared with controls. Both novel and previously reported mutations were found in mitochondrial tRNA and structural genes. One of these mutations was heteroplasmic and resulted in changing a highly conserved nucleotide in tRNAArg. Novel, heteroplasmic mtDNA mutations (n = 4) specifying changes in moderate to highly conserved amino acid residues were found in COII, COIII, ND5, and cytb. These novel mtDNA mutations were found only in patients with severe reduction in mitochondrial enzyme activities., Conclusions: Our results indicate that a high incidence of mtDNA nucleotide sequence changes in both tRNA and structural genes are present in DCM. Five heteroplasmic mutations were detected that both changed evolutionarily conserved residues (which may impair the function of proteins or tRNAs) and were associated with specific enzymatic defects. These mutations could play an important role in the pathogenesis of cardiomyopathy.

Defects in myocardial bioenergetics have been reported in patients with cardiomyopathy but their molecular basis and role in pathophysiology remain unclear. We sought to establish a molecular basis for cardiac mitochondrial respiratory enzyme abnormalities frequently present (75%) in a group of 16 children (including 2 neonates) with end-stage cardiomyopathy. Decreased specific activity levels were found in complexes I, III, IV and V but not in II, the only complex that is entirely nuclear encoded. Sequence analysis of cardiac mtDNA revealed 4 patients harbouring heteroplasmic mtDNA mutations in cytb, tRNAArg, and ND5 at highly conserved positions. These mutations were present neither in controls nor in patients without enzymatic defect. In addition, 4 patients exhibited marked reduction in cardiac mtDNA levels. The basis for respiratory enzyme abnormalities can be explained in a subset of our patients as a result of either pathogenic mtDNA mutation or depletion. Patients harbouring both DNA and enzymatic defects fulfil rigorous criteria defining mitochondrial cardiomyopathy.

We describe a 17-year-old boy with a clinical neurologic picture consistent with Kearns-Sayre syndrome. His manifestations included progressive external ophthalmoplegia, bilateral ptosis, retinitis pigmentosa, and muscle weakness. He was found to harbor an abundant novel deletion in skeletal muscle mitochondrial DNA. Biochemical analysis of the patient's biopsied skeletal muscle showed that the specific activities of all four respiratory complexes with mitochondrial DNA-encoded subunits were markedly reduced in contrast to normal activity levels of entirely nuclear DNA-encoded enzyme activities (eg, complex II and citrate synthase). Ultrastructural analysis also indicated the presence of strikingly abnormal mitochondria with both unusual cristae and frequent paracrystalline inclusions. The great amount of the deleted mitochondrial DNA in this patient's muscle, as well as the concomitant reduction in specific respiratory complex activity, suggests that the mitochondrial DNA deletion plays a role in the pathogenesis of this neurologic disease.

Previous studies in our laboratory demonstrated significant changes in bovine heart mitochondrial bioenergetics during fetal growth and development. To further understand mitochondrial biogenesis in early human development, the activity and subunit content levels of specific mitochondrial enzymes in fetal and neonatal heart were determined. Comparing early gestation (EG, 45-65 day) later gestation (LG, 85-110 day) and neonate (birth-1 month), specific activity of citrate synthase (CS), a Krebs cycle enzyme showed a 2 fold increase from EG to LG and a 2 fold increase from LG to neonate. Specific activities of complex IV and complex V increased similarly 1.8-2 fold from EG to LG. However during the later fetal period from LG to neonate, complex IV activity increased only 1.3 fold and complex V showed no significant increase. Peptide content of COX-II subunit increased 2 fold from EG to LG and by 3.5 fold from LG to neonate. Levels of COX-IV and ATP synthase alpha subunits were undetectable in EG hearts, clearly detectable in LG heart and 3 fold increased from LG to neonate. Unexpectedly, mitochondrial transcription factor A (mt-TFA) levels were not significantly different during these developmental stages. Mitochondrial DNA (mtDNA) levels increased 1.8 fold from EG to LG, and 3.8 fold increase from EG to neonate and correlated with CS activity levels. In conclusion, these data indicate coordinated regulation of some nuclear-encoded (COX-IV and CS activity) and mitochondrial components (COX-II and mtDNA), and strongly suggest that mitochondrial content increases particularly during the early fetal cardiac development and reveal a distinct pattern of regulation for mt-TFA.

A variety of mitochondrial DNA (mtDNA) defects, ranging from point mutations and large-scale deletions to severe reduction in the overall quantity of mtDNA (mtDNA depletion), may be associated with neuromuscular disorders. The nuclear genome, which encodes most of the proteins involved in mitochondrial biogenesis (regulation of maintenance, replication, and transcription of mtDNA), appears to be implicated in many of the mtDNA defects. In this review, we describe some of the mtDNA defects discovered by our laboratory and others in patients with neurologic disorders and analyze their potential relationship with the pathways and mechanisms involved in mitochondrial biogenesis.

A group of 25 children (5 months to 20 years of age) presenting with intractable seizures, developmental delay, and severe hypotonia, who did not fall into the known categories of mitochondrial encephalomyopathies, underwent muscle biopsy for evaluation of mitochondrial function and were compared with age-matched control subjects. Biopsied skeletal muscle was analyzed for six mitochondrial enzyme-specific activities, mitochondrial DNA point mutations and deletions, and mitochondrial DNA levels. The data reveal a high incidence of specific mitochondrial enzyme activity defects. Reduced activity levels were evident in complex I (11 patients), III (24 patients), IV (nine patients), and V (10 patients). Two patients also exhibited pronounced reduction in mitochondrial DNA levels (80% reduction compared with control subjects). Two patients manifested increased levels of 5-kb and 7.4-kb mitochondrial DNA deletions. Pathogenic mutations previously described in association with mitochondrial encephalomyopathies were not evident. The data suggest that mitochondrial dysfunction, including extensive defects in specific enzyme activities, may be frequently present in children with seizures, developmental delay, and hypotonia that do not fall within the known mitochondrial encephalomyopathies. These mitochondrial deficiencies can be primarily ascertained by biochemical analysis and are rarely accompanied by mitochondrial ultrastructural changes. The molecular basis of these defects, their role in these disorders, and potential treatment warrant further study.

While mitochondrial abnormalities are increasingly recognized in cardiac diseases including hypertrophic cardiomyopathy, their presence in idiopathic dilated cardiomyopathy and the role that age plays in their incidence and severity have yet not been assessed. Levels of cardiac respiratory enzyme activities and mitochondrial DNA (mtDNA) were examined in 55 subjects with idiopathic dilated cardiomyopathy divided into 3 age groups. Respiratory enzyme activity levels were significantly lower in 37 patients (67%) compared to age-matched controls and increased activity levels were noted in 9 (16%). Decreased activities were found in complex I (n = 11), III (n = 16), IV (n = 12) and V (n = 13), but not in II, the only respiratory complex entirely nuclear-encoded. No age-specific differences were found in the overall frequency of enzymatic abnormalities. However, older patients had significantly increased multiple enzyme activity defects as well as increases in abundance and frequency of the 7.4 kb deletion. In addition, 3 patients were noted with marked reduction in mtDNA levels. None of the pathogenic mtDNA mutations previously associated with hypertrophic cardiomyopathy were found, nor was there any relationship that could be established between levels of specific mtDNA deletions and enzyme activities. In summary, specific mitochondrial abnormalities are heterogenous and frequent in both adults and children with idiopathic dilated cardiomyopathy. Older patients are more likely to have mtDNA deletions and multiple enzyme activity defects. The molecular basis for these abnormalities remains undefined.

Objectives: This study sought to examine skeletal muscle of children with cardiomyopathy (CM) for changes in mitochondrial enzyme activities and in mitochondrial DNA (mtDNA)., Background: Heart mitochondrial enzymatic activity defects have been often found in dilated and hypertrophic CM. The defects primarily involve the activities of the electron transport system and oxidative phosphorylation pathway including respiratory complexes I, III, IV, and V., Methods: Skeletal muscle biopsies of 8 children with CM were examined for specific mitochondrial enzyme activities, mtDNA copy number and the presence of pathogenic mutations and deletions in mtDNA., Results: A marked deficiency in specific mitochondrial enzyme activities was found in 6 of 8 patients in skeletal muscle as well as in 2 of 3 hearts of those in whom cardiac tissue was available. Specific activity defects were found in complex I (2 cases), complex III (5 cases), complex IV (3 cases), and complex V (4 cases). Complex II and citrate synthase activities were unaffected. None of the previously reported pathogenic mutations associated with CM were detected, nor was there any evidence of mtDNA depletion. The incidence of defective respiratory complex activities in skeletal muscle was similar to the incidence of defective complex activities previously reported in cardiac tissue., Conclusions: Mitochondrial analysis of skeletal muscle is warranted in the overall clinical evaluation of children with CM, and particularly before consideration for cardiac transplantation.

The nucleotide sequence encoding the citrate synthase (CS) gene was determined from the sequencing of the CS cDNA isolated from a human heart cDNA library. The primary sequence of CS deduced from its nucleotide sequence reveals a highly conserved, albeit slightly larger, protein of 466 amino acids, with 95% homology to its pig homologue. The data also indicate that the human genomic CS gene contains no introns, and confirms the location of the human CS gene on chromosome 12.

Abnormalities in mitochondrial DNA (mtDNA) including specific deletions and point mutations have been found in an increasing number of cases of both dilated and hypertrophic cardiomyopathy. The role that these mutations may play in contributing to the cardiomyopathic phenotype is discussed in this survey of the recent literature.

We report the case of a child with severe hypertrophic cardiomyopathy, with decreased activity levels of cardiac mitochondrial respiratory complex I and III, and with a pronounced reduction in cardiac mitochondrial DNA copy number level. Mitochondrial DNA depletion has not been previously reported in hypertrophic cardiomyopathy and it may play a role in its pathogenesis.

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.

Abnormalities in specific mitochondrial respiratory enzymes and DNA (mtDNA) have been reported in cardiomyopathy. In this study, we report 4 cases of severe hypertrophic cardiomyopathy (HCM) in which specific cardiac mitochondrial enzyme activity defects were found, including complex I (n = 2), complex III (n = 2), complex IV (n = 2) and complex V (n = 1). Other abnormalities were also noted including a marked depletion of mtDNA (n = 1) and decreased content of subunit 2 of cytochrome c oxidase (n = 1). None of the mtDNA point mutations and common deletions previously found in association with cardiomyopathy were detected in these patients. These data indicate that specific respiratory enzyme activity defects are frequently present in HCM. Also, our finding of a marked depletion of mtDNA in 1 patient suggests that cardiac mtDNA depletion, previously unreported in HCM, needs further examination in order to establish whether it plays a primary role in its pathogenesis.