Mitochondrial DNA Mutations and Aging

Kynurenine pathway, NAD+ synthesis, and mitochondrial function: Targeting tryptophan metabolism to promote longevity and healthspan

Aging is characterized by a progressive decline in the normal physiological functions of an organism, ultimately leading to mortality. Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor that plays a critical role in mitochondrial energy production as well as many enzymatic redox reactions. Age-associated decline in NAD⁺ is implicated as a driving factor in several categories of age-associated disease, including metabolic and neurodegenerative disease, as well as deficiency in the mechanisms of cellular defense against oxidative stress. The kynurenine metabolic pathway is the sole de novo NAD⁺ biosynthetic pathway, generating NAD⁺ from ingested tryptophan. Altered kynurenine pathway activity is associated with both aging and a variety of age-associated diseases. Kynurenine pathway interventions can extend lifespan in both fruit flies and nematodes, and altered NAD⁺ metabolism represents one potential mediating mechanism. Recent studies demonstrate that supplementation with NAD⁺ or NAD⁺-precursors increase longevity and promote healthy aging in fruit flies, nematodes, and mice. NAD⁺ levels and the intrinsic relationship to mitochondrial function have been widely studied in the context of aging. Mitochondrial function and dynamics have both been implicated in longevity determination in a range of organisms from yeast to humans, at least in part due to their intimate link to regulating an organism's cellular energy economy and capacity to resist oxidative stress. Recent findings support the idea that complex communication between the mitochondria and the nucleus orchestrates a series of events and stress responses involving mitophagy, mitochondrial number, mitochondrial unfolded protein response (UPR^mt), and mitochondria fission and fusion events. In this review, we discuss how mitochondrial morphological changes and dynamics operate during aging, and how altered metabolism of tryptophan to NAD⁺ through the kynurenine pathway interacts with these processes.

Mitochondria in cancer: in the aspects of tumorigenesis and targeted therapy

Mitochondria play pivotal roles in most eukaryotic cells, ranging from energy production to regulation of apoptosis. As sites of cellular respiration, mitochondria experience accumulation of reactive oxygen species (ROS) due to damage in electron transport chain carriers. Mutations in mitochondrial DNA (mtDNA) as well as nuclear DNA are reported in various cancers. Mitochondria have a dual role in cancer: the development of tumors due to mutations in mitochondrial genome and the generation of ROS. Impairment in the mitochondria-regulated apoptosis pathway accelerates tumorigenesis. Numerous strategies targeting mitochondria have been developed to induce the mitochondrial (i.e. intrinsic) apoptosis pathway in cancer cells. This review elaborates the roles of mitochondria in cancer with respect to mutations and apoptosis and discusses mitochondria-targeting strategies as cancer therapies to enhance the killing of cancer cells.

Metabolic features and regulation in cell senescence

Organismal aging is accompanied by a host of progressive metabolic alterations and an accumulation of senescent cells, along with functional decline and the appearance of multiple diseases. This implies that the metabolic features of cell senescence may contribute to the organism’s metabolic changes and be closely linked to age-associated diseases, especially metabolic syndromes. However, there is no clear understanding of senescent metabolic characteristics. Here, we review key metabolic features and regulators of cellular senescence, focusing on mitochondrial dysfunction and anabolic deregulation, and their link to other senescence phenotypes and aging. We further discuss the mechanistic involvement of the metabolic regulators mTOR, AMPK, and GSK3, proposing them as key metabolic switches for modulating senescence.

Increased mitochondrial common deletion in platelets from patients with type 2 diabetes is not associated with abnormal platelet activity or mitochondrial function

The present study examined the presence and frequency of the 4,977‑base pair mitochondrial (mt)DNA (mtDNA4977) deletion in blood platelets, and whether increased mtDNA4977 deletion was associated with abnormal mitochondrial and platelet function in type 2 diabetes mellitus. A total of 66 patients with type 2 diabetes mellitus and 23 healthy subjects were included in the present study. Patients were divided into three subgroups according to glycemic control, and the presence or absence of chronic diabetic complications: i) Good glycemic control [glycated hemoglobin (HbA1c) <7] without complications; ii) poor glycemic control (HbA1c ≥7) without complications; and iii) poor glycemic control (HbA1c ≥7) with complications. mtDNA4977 deletion, mtDNA copy number, adenine nucleotides, mitochondrial membrane potential and P‑selectin expression levels were analyzed in platelets. Although the frequency of mtDNA4977 deletion in platelets of the patient (96.9%) and control groups (95.6%) was extremely similar, the deletion level significantly increased in all the diabetic groups, compared with the healthy control group. However, the data from the present study revealed that an increased deletion frequency in platelets was not associated with disease severity, although there was clear interindividual variability. Furthermore, all other parameters were not significantly different among the groups, and there were no correlations between mtDNA4977 deletion frequency and all other studied parameters for any of the case groups. The results indicated that the mtDNA4977 deletion occurred in platelets, and increased deletion in patients with type 2 diabetes did not have a marked influence on mitochondrial and/or platelet dysfunction, when compared with the non‑diabetic subjects. Further research is required to elucidate the sources of inter‑individual variability observed in certain parameters.

Recent Progress in Vascular Aging: Mechanisms and Its Role in Age-related Diseases

As with many age-related diseases including vascular dysfunction, age is considered an independent and crucial risk factor. Complicated alterations of structure and function in the vasculature are linked with aging hence, understanding the underlying mechanisms of age-induced vascular pathophysiological changes holds possibilities for developing clinical diagnostic methods and new therapeutic strategies. Here, we discuss the underlying molecular mediators that could be involved in vascular aging, e.g., the renin-angiotensin system and pro-inflammatory factors, metalloproteinases, calpain-1, monocyte chemoattractant protein-1 (MCP-1) and TGFβ-1 as well as the potential roles of testosterone and estrogen. We then relate all of these to clinical manifestations such as vascular dementia and stroke in addition to reviewing the existing clinical measurements and potential interventions for age-related vascular dysfunction.

Mitochondrial DNA: Radically free of free-radical driven mutations

Mitochondrial DNA has long been posited as a likely target of oxidative damage induced mutation during the ageing process. Research over the past decades has uncovered the accumulation of mitochondrial DNA mutations in association with a mosaic pattern of cells displaying mitochondrial dysfunction in ageing individuals. Unfortunately, the underlying mechanisms are far less straightforward than originally anticipated. Recent research on mitochondria reveals that these genomes are far less helpless than originally envisioned. Additionally, new technologies have allowed us to analyze the mutational signatures of many more somatic mitochondrial DNA mutations, revealing surprising patterns that are inconsistent with a DNA-oxidative damage based hypothesis. In this review, we will discuss these recent observations and new insights into the eccentricities of mitochondrial genetics, and their impact on our understanding of mitochondrial mutations and their role in the ageing process. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.

Mitochondrial DNA Depletion and Deletions in Paediatric Patients with Neuromuscular Diseases: Novel Phenotypes

OBJECTIVE

To study the clinical manifestations and occurrence of mtDNA depletion and deletions in paediatric patients with neuromuscular diseases and to identify novel clinical phenotypes associated with mtDNA depletion or deletions.

METHODS

Muscle DNA samples from patients presenting with undefined encephalomyopathies or myopathies were analysed for mtDNA content by quantitative real-time PCR and for deletions by long-range PCR. Direct sequencing of mtDNA maintenance genes and whole-exome sequencing were used to study the genetic aetiologies of the diseases. Clinical and laboratory findings were collected.

RESULTS

Muscle samples were obtained from 104 paediatric patients with neuromuscular diseases. mtDNA depletion was found in three patients with severe early-onset encephalomyopathy or myopathy. Two of these patients presented with novel types of mitochondrial DNA depletion syndromes associated with increased serum creatine kinase (CK) and multiorgan disease without mutations in any of the known mtDNA maintenance genes; one patient had pathologic endoplasmic reticulum (ER) membranes in muscle. The third patient with mtDNA depletion was diagnosed with merosine-deficient muscular dystrophy caused by a homozygous mutation in the LAMA2 gene. Two patients with an early-onset Kearns-Sayre/Pearson-like phenotype harboured a large-scale mtDNA deletion, minor multiple deletions and high mtDNA content.

CONCLUSIONS

Novel encephalomyopathic mtDNA depletion syndrome with structural alterations in muscle ER was identified. mtDNA depletion may also refer to secondary mitochondrial changes related to muscular dystrophy. We suggest that a large-scale mtDNA deletion, minor multiple deletions and high mtDNA content associated with Kearns-Sayre/Pearson syndromes may be secondary changes caused by mutations in an unknown nuclear gene.

Mitochondrial function and lifespan of mice with controlled ubiquinone biosynthesis

Ubiquinone (UQ) is implicated in mitochondrial electron transport, superoxide generation and as a membrane antioxidant. Here we present a mouse model in which UQ biosynthesis can be interrupted and partially restored at will. Global loss of UQ leads to gradual loss of mitochondrial function, gradual development of disease phenotypes and shortened lifespan. However, we find that UQ does not act as antioxidant in vivo and that its requirement for electron transport is much lower than anticipated, even in vital mitochondria-rich organs. In fact, severely depressed mitochondrial function due to UQ depletion in the heart does not acutely impair organ function. In addition, we demonstrate that severe disease phenotypes and shortened lifespan are reversible upon partial restoration of UQ levels and mitochondrial function. This observation strongly suggests that the irreversible degenerative phenotypes that characterize ageing are not secondarily caused by the gradual mitochondrial dysfunction that is observed in aged animals.

Control of mitochondrial integrity in ageing and disease

Various molecular and cellular pathways are active in eukaryotes to control the quality and integrity of mitochondria. These pathways are involved in keeping a 'healthy' population of this essential organelle during the lifetime of the organism. Quality control (QC) systems counteract processes that lead to organellar dysfunction manifesting as degenerative diseases and ageing. We discuss disease- and ageing-related pathways involved in mitochondrial QC: mtDNA repair and reorganization, regeneration of oxidized amino acids, refolding and degradation of severely damaged proteins, degradation of whole mitochondria by mitophagy and finally programmed cell death. The control of the integrity of mtDNA and regulation of its expression is essential to remodel single proteins as well as mitochondrial complexes that determine mitochondrial functions. The redundancy of components, such as proteases, and the hierarchies of the QC raise questions about crosstalk between systems and their precise regulation. The understanding of the underlying mechanisms on the genomic, proteomic, organellar and cellular levels holds the key for the development of interventions for mitochondrial dysfunctions, degenerative processes, ageing and age-related diseases resulting from impairments of mitochondria.

Unique features of DNA replication in mitochondria: a functional and evolutionary perspective

Last year, we reported a new mechanism of DNA replication in mammals. It occurs inside mitochondria and entails the use of processed transcripts, termed bootlaces, which hybridize with the displaced parental strand as the replication fork advances. Here we discuss possible reasons why such an unusual mechanism of DNA replication might have evolved. The bootlace mechanism can minimize the occurrence and impact of single-strand breaks that would otherwise threaten genome stability. Furthermore, by providing an implicit mismatch recognition system, it should limit the occurrence of replication-dependent deletions and insertions, and defend against invading elements. Such a mechanism may also limit attempts to manipulate the mammalian mitochondrial genome.

Regular bottlenecks and restrictions to somatic fusion prevent the accumulation of mitochondrial defects in Neurospora

The replication and segregation of multi-copy mitochondrial DNA (mtDNA) are not under strict control of the nuclear DNA. Within-cell selection may thus favour variants with an intracellular selective advantage but a detrimental effect on cell fitness. High relatedness among the mtDNA variants of an individual is predicted to disfavour such deleterious selfish genetic elements, but experimental evidence for this hypothesis is scarce. We studied the effect of mtDNA relatedness on the opportunities for suppressive mtDNA variants in the fungus Neurospora carrying the mitochondrial mutator plasmid pKALILO. During growth, this plasmid integrates into the mitochondrial genome, generating suppressive mtDNA variants. These mtDNA variants gradually replace the wild-type mtDNA, ultimately culminating in growth arrest and death. We show that regular sequestration of mtDNA variation is required for effective selection against suppressive mtDNA variants. First, bottlenecks in the number of mtDNA copies from which a 'Kalilo' culture started significantly increased the maximum lifespan and variation in lifespan among cultures. Second, restrictions to somatic fusion among fungal individuals, either by using anastomosis-deficient mutants or by generating allotype diversity, prevented the accumulation of suppressive mtDNA variants. We discuss the implications of these results for the somatic accumulation of mitochondrial defects during ageing.

Functional and morphological impact of ER stress on mitochondria

Over the past years, knowledge and evidence about the existence of crosstalks between cellular organelles and their potential effects on survival or cell death have been constantly growing. More recently, evidence accumulated showing an intimate relationship between endoplasmic reticulum (ER) and mitochondria. These close contacts not only establish extensive physical links allowing exchange of lipids and calcium but they can also coordinate pathways involved in cell life and death. It is now obvious that ER dysfunction/stress and unfolded protein response (UPR) as well as mitochondria play major roles in apoptosis. However, while the effects of major ER stress on cell death have been largely studied and reviewed, it becomes more and more evident that cells might regularly deal with sublethal ER stress, a condition that does not necessarily lead to cell death but might affect the function/activity of other organelles such as mitochondria. In this review, we will particularly focus on these new, interesting and intriguing metabolic and morphological events that occur during the early adaptative phase of the ER stress, before the onset of cell death, and that remain largely unknown. Relevance and implication of these mitochondrial changes in response to ER stress conditions for human diseases such as type II diabetes and Alzheimer's disease will also be considered.

Thermodynamic and structural analysis of DNA damage architectures related to replication

Damaged DNA, generated by the abstraction of one of five hydrogen atoms from the 2′-deoxyribose ring of the nucleic acid, can contain a variety of lesions, some of which compromise physiological processes. Recently, DNA damage, resulting from the formation of a C3′-thymidinyl radical in DNA oligomers, was found to be dependent on nucleic acid structure. Architectures relevant to DNA replication were observed to generate larger amounts of strand-break and 1-(2′-deoxy-β-D-threo-pentofuranosyl)thymidine formation than that observed for duplex DNA. To understand how this damage can affect the integrity of DNA, the impact of C3′-thymidinyl radical derived lesions on DNA stability and structure was characterized using biophysical methods. DNA architectures evaluated include duplex DNA (dsDNA), single 3′ or 5′-overhangs (OvHgs), and forks. Thermal melting analysis and differential scanning calorimetry measurements indicate that an individual 3′-OvHg is more destabilizing than a 5′-OvHg. The presence of a terminal 3′ or 5′ phosphate decreases the ΔG₂₅ to the same extent, while the effect of the phosphate at the ss-dsDNA junction of OvHgs is dependent on sequence. Additionally, the effect of 1-(2′-deoxy-β-D-threo-pentofuranosyl)thymidine is found to depend on DNA architecture and proximity to the 3′ end of the damaged strand.

Mitochondrial DNA polymorphisms specifically modify cerebral β-amyloid proteostasis

Several lines of evidence link mutations and deletions in mitochondrial DNA (mtDNA) and its maternal inheritance to neurodegenerative diseases in the elderly. Age-related mutations of mtDNA modulate the tricarboxylic cycle enzyme activity, mitochondrial oxidative phosphorylation capacity and oxidative stress response. To investigate the functional relevance of specific mtDNA polymorphisms of inbred mouse strains in the proteostasis regulation of the brain, we established novel mitochondrial congenic mouse lines of Alzheimer's disease (AD). We crossed females from inbred strains (FVB/N, AKR/J, NOD/LtJ) with C57BL/6 males for at least ten generations to gain specific mitochondrial conplastic strains with pure C57BL/6 nuclear backgrounds. We show that specific mtDNA polymorphisms originating from the inbred strains differentially influence mitochondrial energy metabolism, ATP production and ATP-driven microglial activity, resulting in alterations of cerebral β-amyloid (Aβ) accumulation. Our findings demonstrate that mtDNA-related increases in ATP levels and subsequently in microglial activity are directly linked to decreased Aβ accumulation in vivo, implicating reduced mitochondrial function in microglia as a causative factor in the development of age-related cerebral proteopathies such as AD.

Mitochondrial myopathy caused by arsenic trioxide therapy

Arsenic trioxide (ATO) has been successfully used as a treatment for acute promyelocytic leukemia (APL) for more than a decade. Here we report a patient with APL who developed a mitochondrial myopathy after treatment with ATO. Three months after ATO therapy withdrawal, the patient was unable to walk without assistance and skeletal muscle studies showed a myopathy with abundant cytoplasmic lipid droplets, decreased activities of the mitochondrial respiratory chain complexes, multiple mitochondrial DNA (mtDNA) deletions, and increased muscle arsenic content. Six months after ATO treatment was interrupted, the patient recovered normal strength, lipid droplets had decreased in size and number, respiratory chain complex activities were partially restored, but multiple mtDNA deletions and increased muscle arsenic content persisted. ATO therapy may provoke a delayed, severe, and partially reversible mitochondrial myopathy, and a long-term careful surveillance for muscle disease should be instituted when ATO is used in patients with APL.

α-Lipoic acid attenuates vascular calcification via reversal of mitochondrial function and restoration of Gas6/Axl/Akt survival pathway

Vascular calcification is prevalent in patients with chronic kidney disease and leads to increased cardiovascular morbidity and mortality. Although several reports have implicated mitochondrial dysfunction in cardiovascular disease and chronic kidney disease, little is known about the potential role of mitochondrial dysfunction in the process of vascular calcification. This study investigated the effect of α-lipoic acid (ALA), a naturally occurring antioxidant that improves mitochondrial function, on vascular calcification in vitro and in vivo. Calcifying vascular smooth muscle cells (VSMCs) treated with inorganic phosphate (Pi) exhibited mitochondrial dysfunction, as demonstrated by decreased mitochondrial membrane potential and ATP production, the disruption of mitochondrial structural integrity and concurrently increased production of reactive oxygen species. These Pi-induced functional and structural mitochondrial defects were accompanied by mitochondria-dependent apoptotic events, including release of cytochrome c from the mitochondria into the cytosol, subsequent activation of caspase-9 and -3, and chromosomal DNA fragmentation. Intriguingly, ALA blocked the Pi-induced VSMC apoptosis and calcification by recovery of mitochondrial function and intracellular redox status. Moreover, ALA inhibited Pi-induced down-regulation of cell survival signals through the binding of growth arrest-specific gene 6 (Gas6) to its cognate receptor Axl and subsequent Akt activation, resulting in increased survival and decreased apoptosis. Finally, ALA significantly ameliorated vitamin D(3) -induced aortic calcification and mitochondrial damage in mice. Collectively, the findings suggest ALA attenuates vascular calcification by inhibiting VSMC apoptosis through two distinct mechanisms; preservation of mitochondrial function via its antioxidant potential and restoration of the Gas6/Axl/Akt survival pathway.

Hitting the brakes: termination of mitochondrial transcription

Deficiencies in mitochondrial protein production are associated with human disease and aging. Given the central role of transcription in gene expression, recent years have seen a renewed interest in understanding the molecular mechanisms controlling this process. In this review, we have focused on the mostly uncharacterized process of transcriptional termination. We review how several recent breakthroughs have provided insight into our understanding of the termination mechanism, the protein factors that mediate termination, and the functional relevance of different termination events. Furthermore, the identification of termination defects resulting from a number of mtDNA mutations has led to the suggestion that this could be a common mechanism influencing pathogenesis in a number of mitochondrial diseases, highlighting the importance of understanding the processes that regulate transcription in human mitochondria. We discuss how these recent findings set the stage for future studies on this important regulatory mechanism. This article is part of a Special Issue entitled: Mitochondrial Gene Expression.

Mitochondrial-nuclear epistasis: implications for human aging and longevity

There is substantial evidence that mitochondria are involved in the aging process. Mitochondrial function requires the coordinated expression of hundreds of nuclear genes and a few dozen mitochondrial genes, many of which have been associated with either extended or shortened life span. Impaired mitochondrial function resulting from mtDNA and nuclear DNA variation is likely to contribute to an imbalance in cellular energy homeostasis, increased vulnerability to oxidative stress, and an increased rate of cellular senescence and aging. The complex genetic architecture of mitochondria suggests that there may be an equally complex set of gene interactions (epistases) involving genetic variation in the nuclear and mitochondrial genomes. Results from Drosophila suggest that the effects of mtDNA haplotypes on longevity vary among different nuclear allelic backgrounds, which could account for the inconsistent associations that have been observed between mitochondrial DNA (mtDNA) haplogroups and survival in humans. A diversity of pathways may influence the way mitochondria and nuclear-mitochondrial interactions modulate longevity, including: oxidative phosphorylation; mitochondrial uncoupling; antioxidant defenses; mitochondrial fission and fusion; and sirtuin regulation of mitochondrial genes. We hypothesize that aging and longevity, as complex traits having a significant genetic component, are likely to be controlled by nuclear gene variants interacting with both inherited and somatic mtDNA variability.

Oxidant stress evoked by pacemaking in dopaminergic neurons is attenuated by DJ-1

Parkinson's disease is a pervasive, ageing-related neurodegenerative disease the cardinal motor symptoms of which reflect the loss of a small group of neurons, the dopaminergic neurons in the substantia nigra pars compacta (SNc). Mitochondrial oxidant stress is widely viewed as being responsible for this loss, but why these particular neurons should be stressed is a mystery. Here we show, using transgenic mice that expressed a redox-sensitive variant of green fluorescent protein targeted to the mitochondrial matrix, that the engagement of plasma membrane L-type calcium channels during normal autonomous pacemaking created an oxidant stress that was specific to vulnerable SNc dopaminergic neurons. The oxidant stress engaged defences that induced transient, mild mitochondrial depolarization or uncoupling. The mild uncoupling was not affected by deletion of cyclophilin D, which is a component of the permeability transition pore, but was attenuated by genipin and purine nucleotides, which are antagonists of cloned uncoupling proteins. Knocking out DJ-1 (also known as PARK7 in humans and Park7 in mice), which is a gene associated with an early-onset form of Parkinson's disease, downregulated the expression of two uncoupling proteins (UCP4 (SLC25A27) and UCP5 (SLC25A14)), compromised calcium-induced uncoupling and increased oxidation of matrix proteins specifically in SNc dopaminergic neurons. Because drugs approved for human use can antagonize calcium entry through L-type channels, these results point to a novel neuroprotective strategy for both idiopathic and familial forms of Parkinson's disease.

What role does mitochondrial stress play in neurodegenerative diseases?

The essential need for mitochondrial function has been extensively shown to relate to neuronal health. Neurodegeneration and neurodegeneration-related diseases have been associated with multiple mitochondrial dysfunctions. This review highlights key findings related to commonly studied mitochondrial dysfunctions: imbalance of mitochondrial dynamics, mutations in the mitochondrial genome, excessive reactive oxygen species, and misfolded protein associations/interactions with the mitochondria. Future research in mitochondrial function will help elucidate complex neurodegenerative events while impacting both individual and societal health.

Drosophila melanogaster in the study of human neurodegeneration

Human neurodegenerative diseases are devastating illnesses that predominantly affect elderly people. The majority of the diseases are associated with pathogenic oligomers from misfolded proteins, eventually causing the formation of aggregates and the progressive loss of neurons in the brain and nervous system. Several of these proteinopathies are sporadic and the cause of pathogenesis remains elusive. Heritable forms are associated with genetic defects, suggesting that the affected protein is causally related to disease formation and/or progression. The limitations of human genetics, however, make it necessary to use model systems to analyse affected genes and pathways in more detail. During the last two decades, research using the genetically amenable fruitfly has established Drosophila melanogaster as a valuable model system in the study of human neurodegeneration. These studies offer reliable models for Alzheimer's, Parkinson's, and motor neuron diseases, as well as models for trinucleotide repeat expansion diseases, including ataxias and Huntington's disease. As a result of these studies, several signalling pathways including phosphatidylinositol 3-kinase (PI3K)/Akt and target of rapamycin (TOR), c-Jun N-terminal kinase (JNK) and bone morphogenetic protein (BMP) signalling, have been shown to be deregulated in models of proteinopathies, suggesting that two or more initiating events may trigger disease formation in an age-related manner. Moreover, these studies also demonstrate that the fruitfly can be used to screen chemical compounds for their potential to prevent or ameliorate the disease, which in turn can directly guide clinical research and the development of novel therapeutic strategies for the treatment of human neurodegenerative diseases.

Caloric restriction delays cardiac ageing in rats: role of mitochondria

AIMS

We tested whether long-term caloric restriction (CR) corrects pre-existing manifestations of cardiac ageing in rats.

METHODS AND RESULTS

The age-specific effects of CR (-40%, 6 months) on mortality, left ventricular (LV) function, mitochondrial function, oxidative damage, and apoptosis were analysed in young (6 + 6 months) and senescent rats (24 + 6 months). CR in senescent rats significantly reduced mortality. LV and cardiomyocyte hypertrophy were reduced together with the mRNA expression and plasma concentrations of overload indicators BNP/ANP. Mitochondrial function was improved, resulting in lower oxidative damage and apoptotic activation. In particular, the pro-apoptotic Bcl-xS/Bcl-xL isoform pattern, mitochondrial translocation of Bax, release of cytochrome C into cytosol, and caspase-9 activation were reduced in comparison to age-matched rats on the control diet. However, CR resulted only in minor changes in young rats. Serum obtained from old control or CR rats was used for in vitro experiments. H9C2 cardiomyoblasts and adult rat ventricular cardiomyocytes preconditioned with CR serum demonstrated a low Bcl-xS/Bcl-xL ratio. H9C2 cells were resistant against H(2)O(2)-mediated loss of mitochondrial membrane potential, apoptosis activation, and reduced cell viability. Thus, beneficial effects of CR are mediated through circulating factors and can be mimicked with CR serum. However, this protection critically depended on a high Bcl-xL protein expression as seen after siRNA-mediated Bcl-xL knockdown.

CONCLUSION

CR is cardioprotective in senescent myocardium by correcting pre-existing mitochondrial dysfunction and apoptotic activation and by preventing deterioration in LV function. Therefore, interventions that mimic these effects of CR may represent an additional therapeutic option for the aged or failing heart.

Mitochondrial fusion is required for mtDNA stability in skeletal muscle and tolerance of mtDNA mutations

Mitochondria are highly mobile and dynamic organelles that continually fuse and divide. These processes allow mitochondria to exchange contents, including mitochondrial DNA (mtDNA). Here we examine the functions of mitochondrial fusion in differentiated skeletal muscle through conditional deletion of the mitofusins Mfn1 and Mfn2, mitochondrial GTPases essential for fusion. Loss of the mitofusins causes severe mitochondrial dysfunction, compensatory mitochondrial proliferation, and muscle atrophy. Mutant mice have severe mtDNA depletion in muscle that precedes physiological abnormalities. Moreover, the mitochondrial genomes of the mutant muscle rapidly accumulate point mutations and deletions. In a related experiment, we find that disruption of mitochondrial fusion strongly increases mitochondrial dysfunction and lethality in a mouse model with high levels of mtDNA mutations. With its dual function in safeguarding mtDNA integrity and preserving mtDNA function in the face of mutations, mitochondrial fusion is likely to be a protective factor in human disorders associated with mtDNA mutations.

Mitochondrial fusion is required for mtDNA stability in skeletal muscle and tolerance of mtDNA mutations

Mitochondria are highly mobile and dynamic organelles that continually fuse and divide. These processes allow mitochondria to exchange contents, including mitochondrial DNA (mtDNA). Here we examine the functions of mitochondrial fusion in differentiated skeletal muscle through conditional deletion of the mitofusins Mfn1 and Mfn2, mitochondrial GTPases essential for fusion. Loss of the mitofusins causes severe mitochondrial dysfunction, compensatory mitochondrial proliferation, and muscle atrophy. Mutant mice have severe mtDNA depletion in muscle that precedes physiological abnormalities. Moreover, the mitochondrial genomes of the mutant muscle rapidly accumulate point mutations and deletions. In a related experiment, we find that disruption of mitochondrial fusion strongly increases mitochondrial dysfunction and lethality in a mouse model with high levels of mtDNA mutations. With its dual function in safeguarding mtDNA integrity and preserving mtDNA function in the face of mutations, mitochondrial fusion is likely to be a protective factor in human disorders associated with mtDNA mutations.

Role of Drosophila alkaline ceramidase (Dacer) in Drosophila development and longevity

Ceramidases catalyze the hydrolysis of ceramides to generate sphingosine (SPH) and fatty acids, and ceramide metabolism is implicated in various biological responses in Drosophila melanogaster. Here we report the cloning, biochemical characterization, and functional analysis of a Drosophila alkaline ceramidase (Dacer). Dacer, a membrane-bound protein of 284 amino acids, shares homology with yeast and mammalian alkaline ceramidases. Overexpression of Dacer in High Five insect cells increases ceramidase activity in the alkaline pH range, indicating that Dacer is a bona fide alkaline ceramidase. Dacer mRNA is highly expressed in the midgut and at the pupal stage. An inactivation of Dacer by insertional mutagenesis increases the levels of ceramides in both Drosophila pupae and adult flies. Dacer inactivation increases Drosophila pre-adult development time, lifespan, and anti-oxidative stress capacity. Collectively, these results suggest that Dacer plays an important role in the Drosophila development and longevity by controlling the metabolism of ceramides.

Zen and the art of mitochondrial DNA maintenance

Because mitochondrial genes encode proteins essential for aerobic ATP production, mitochondrial DNA defects can cause an energy crisis. These defects fall into two broad categories: primary mutations in mitochondrial DNA and mutations in nuclear genes, whose protein products are involved in mitochondrial DNA maintenance. Evidence is accumulating that both types of defects can cause mitochondrial DNA loss. Hence, regulatory factors, which determine whether mitochondrial DNA molecules are maintained or lost, potentially play a more important role in these disorders than hitherto recognised. Candidates include reactive oxygen species (ROS) and the tumour suppressor p53. The cell might not always be the best judge of when to dispense with the services of mitochondrial DNA, and so interventions that favour its retention could potentially limit the adverse effects of pathological mitochondrial DNAs.

Role of reactive oxygen species-elicited apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases associated with mitochondrial DNA mutations

A wide spectrum of pathogenic mutations of mitochondrial DNA (mtDNA) has been demonstrated to cause mitochondrial dysfunction and overproduction of reactive oxygen species (ROS), in relation to mitochondrial and neurodegenerative diseases. Our previous studies have shown that large-scale deletions of mtDNA not only serve as an indicator of oxidative damage, but also result in greater susceptibility of human cells to apoptosis triggered by UV irradiation and other apoptotic stimuli. In this review, we focus on the involvement of mtDNA-mutation-associated oxidative stress and susceptibility to apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases. Different lines of research have provided concordant data to suggest that the mtDNA-mutation-elicited energy insufficiency and enhanced oxidative stress and damage lead to cell dysfunction, and increase the susceptibility of affected cells to apoptosis in patients with these diseases. Moreover, accumulating experimental evidence has shown that antioxidant therapy is a good strategy for decreasing intracellular ROS and alleviating oxidative-stress-induced apoptosis in cells of patients that harbor pathogenic mtDNA mutations.

Telomerase, mitochondria and oxidative stress

Telomerase plays an important role in cellular proliferation capacity and survival under conditions of stress. A large part of this protective function is due to telomere capping and maintenance. Thus it contributes to cellular immortality in stem cells and cancer. Recently, evidence has accumulated that telomerase can contribute to cell survival and stress resistance in a largely telomere-independent manner. Telomerase has been shown to shuttle dynamically between different cellular locations. Under increased oxidative stress telomerase is excluded from the nucleus and can be found within the mitochondria. This phenotype correlates with decreased oxidative stress within telomerase expressing cells and improved mitochondrial function by currently largely unknown mechanisms. Our data suggest that mitochondrial protection could be an important non-canonical function for telomerase in cell survival and ageing. This review summarises briefly our knowledge about extra-telomeric functions of telomerase and discusses the potential significance of its mitochondrial localisation.

Mitochondrial DNA mutations in disease, aging, and neurodegeneration

Patients with disorders from mutations in the mitochondrial genome have variable phenotypes, but common to many of these disorders are underlying changes in postmitotic cells, particularly neurons and muscle fibers. The mitochondrial dysfunction caused by these mutations has been shown to be associated with signs of apoptosis and to cause cell loss. Mutations of the mitochondrial genome have also been shown to accumulate with age and in common neurodegenerative diseases, such as Parkinson's disease. This review presents recent data to show that the information gained from studying patients with mitochondrial disorders can help our understanding of the role of mitochondrial DNA mutations in brain aging and neurodegeneration.

The conserved translocase Tim17 prevents mitochondrial DNA loss

Maintenance of an intact mitochondrial genome is essential for oxidative phosphorylation in all eukaryotes. Depletion of mitochondrial genome copy number can have severe pathological consequences due to loss of respiratory capacity. In Saccharomyces cerevisiae, several bifunctional metabolic enzymes have been shown to be required for mitochondrial DNA (mtDNA) maintenance. For example, Ilv5 is required for branched chain amino acid biosynthesis and mtDNA stability. We have identified OXA1 and TIM17 as novel multicopy suppressors of mtDNA instability in ilv5 cells. In addition, overexpression of TIM17, but not OXA1, prevents the complete loss of mtDNA in cells lacking the TFAM homologue Abf2. Introduction of the disease-associated A3243G mutant mtDNA into human NT2 teratocarcinoma cells frequently causes mtDNA loss. Yet when human TIM17A is overexpressed in NT2 cybrids carrying A3243G mtDNA, the proportion of cybrid clones maintaining mtDNA increases significantly. TIM17A overexpression results in long-term mtDNA stabilization, since NT2 cybrids overexpressing TIM17A maintain mtDNA at levels similar to controls for several months. Tim17 is a conserved suppressor of mtDNA instability and is the first factor to be identified that can prevent mtDNA loss in a human cellular model of mitochondrial disease.

Renin-angiotensin system inhibitors protect against age-related changes in rat liver mitochondrial DNA content and gene expression

Chronic renin-angiotensin system inhibition protects against liver fibrosis, ameliorates age-associated mitochondrial dysfunction and increases rodent lifespan. We hypothesized that life-long angiotensin-II-mediated stimulation of oxidant generation might participate in mitochondrial DNA "common deletion" formation, and the resulting impairment of bioenergetic capacity. Enalapril (10 mg/kg/d) or losartan (30 mg/kg/d) administered during 16.5 months were unable to prevent the age-dependent accumulation of rat liver mitochondrial DNA "common deletion", but attenuated the decrease of mitochondrial DNA content. This evidence - together with the enhancement of NRF-1 and PGC-1 mRNA contents - seems to explain why enalapril and losartan improved mitochondrial functioning and lowered oxidant production, since both the absolute number of mtDNA molecules and increased NRF-1 and PGC-1 transcription are positively related to mitochondrial respiratory capacity, and PGC-1 protects against increases in ROS production and damage. Oxidative stress evoked by abnormal respiratory function contributes to the pathophysiology of mitochondrial disease and human aging. If the present mitochondrial actions of renin-angiotensin system inhibitors are confirmed in humans they may modify the therapeutic significance of that strategy.

Mitochondrial DNA injury and mortality in hemodialysis patients

The role of mitochondrial injury in the pathogenesis of complications of uremia is incompletely defined, although diminished bioenergetic capacity and the accumulation of mitochondrial DNA (mtDNA) mutations have been reported. This study was undertaken to evaluate the prevalence of mtDNA injury in 180 patients who had ESRD and were enrolled into the baseline phase of the HEMO study and to relate these markers to all-cause mortality. The mitochondrial injury markers studied in peripheral blood mononuclear cells were the mtDNA copy number per cell, measured by quantitative PCR, and the presence of the mtDNA(4977) mutation. After frequency-matching healthy control subjects for age, mtDNA copy number was lower among older dialysis patients compared with older healthy subjects (P = 0.01). A one-log increase in mtDNA copy number was independently associated with a decreased hazard for mortality (adjusted hazard ratio 0.64; 95% confidence interval 0.46 to 0.89). The mtDNA(4977) deletion was present in 48 (31%) patients and was independently associated with a decreased hazard for mortality (adjusted hazard ratio 0.33; 95% confidence interval 0.19 to 0.56). In summary, the mtDNA(4977) seems to predict survival in ESRD, but a reduced mitochondrial copy number seems to predict a poor outcome. Although further exploration of these associations is needed, evaluation of mitochondrial DNA copy number and somatic mtDNA mutations may provide simple genomic biomarkers to predict clinical outcomes among patients with ESRD.

Mitochondrial dysfunction increases oxidative stress and decreases chronological life span in fission yeast

Background

Oxidative stress is a probable cause of aging and associated diseases. Reactive oxygen species (ROS) originate mainly from endogenous sources, namely the mitochondria.

Methodology/Principal Findings

We analyzed the effect of aerobic metabolism on oxidative damage in Schizosaccharomyces pombe by global mapping of those genes that are required for growth on both respiratory-proficient media and hydrogen-peroxide-containing fermentable media. Out of a collection of approximately 2700 haploid yeast deletion mutants, 51 were sensitive to both conditions and 19 of these were related to mitochondrial function. Twelve deletion mutants lacked components of the electron transport chain. The growth defects of these mutants can be alleviated by the addition of antioxidants, which points to intrinsic oxidative stress as the origin of the phenotypes observed. These respiration-deficient mutants display elevated steady-state levels of ROS, probably due to enhanced electron leakage from their defective transport chains, which compromises the viability of chronologically-aged cells.

Conclusion/Significance

Individual mitochondrial dysfunctions have often been described as the cause of diseases or aging, and our global characterization emphasizes the primacy of oxidative stress in the etiology of such processes.

Mitochondrial fragmentation in neurodegeneration

Mitochondria are remarkably dynamic organelles that migrate, divide and fuse. Cycles of mitochondrial fission and fusion ensure metabolite and mitochondrial DNA mixing and dictate organelle shape, number and bioenergetic functionality. There is mounting evidence that mitochondrial dysfunction is an early and causal event in neurodegeneration. Mutations in the mitochondrial fusion GTPases mitofusin 2 and optic atrophy 1, neurotoxins and oxidative stress all disrupt the cable-like morphology of functional mitochondria. This results in impaired bioenergetics and mitochondrial migration, and can trigger neurodegeneration. These findings suggest potential new treatment avenues for neurodegenerative diseases.

mt-Nd2a suppresses reactive oxygen species production by mitochondrial complexes I and III

Reactive oxygen species (ROS) play a critical role in the pathogenesis of human diseases. A cytosine to adenine transversion in the mitochondrially encoded NADH dehydrogenase subunit 2 (mt-ND2, human; mt-Nd2, mouse) gene results in resistance against type 1 diabetes and several additional ROS-associated conditions. Our previous studies have demonstrated that the adenine-containing allele (mt-Nd2(a)) is also strongly associated with resistance against type 1 diabetes in mice. In this report we have confirmed that the cytosine-containing allele (mt-Nd2(c)) results in elevated mitochondrial ROS production. Using inhibitors of the electron transport chain, we show that when in combination with nuclear genes from the alloxan-resistant (ALR) strain, mt-Nd2(c) increases ROS from complex III. Furthermore, by using alamethicin-permeabilized mitochondria, we measured a significant increase in electron transport chain-dependent ROS production from all mt-Nd2(c)-encoding strains including ALR.mt(NOD), non-obese diabetic (NOD), and C57BL/6 (B6). Studies employing alamethicin and inhibitors were able to again localize the heightened ROS production in ALR.mt(NOD) to complex III and identified complex I as the site of elevated ROS production from NOD and B6 mitochondria. Using submitochondrial particles, we confirmed that in the context of the NOD or B6 nuclear genomes, mt-Nd2(c) elevates complex I-specific ROS production. In all assays mitochondria from mt-Nd2(a)-encoding strains exhibited low ROS production. Our data suggest that lowering overall mitochondrial ROS production is a key mechanism of disease protection provided by mt-Nd2(a).

Intra-mitochondrial degradation of Tim23 curtails the survival of cells rescued from apoptosis by caspase inhibitors

Caspase inhibition can extend the survival of cells undergoing apoptosis beyond the point of mitochondrial outer membrane permeabilisation (MOMP), but this does not confer long-term protection because caspase-independent death pathways emerge. Here, we describe a novel mechanism of mitochondrial self-destruction in caspase-inhibited cells, whose hallmark is the degradation of Tim23, the essential pore-forming component of the TIM23 inner membrane translocase. We show that Tim23 degradation occurs in cycling and post-mitotic cells, it is caspase-independent but Bax/Bak dependent, and it follows cytochrome c release. The proteolytic degradation of Tim23 is induced by MOMP and is mitochondrion-autonomous, as it also occurs in isolated mitochondria undergoing permeability transition. Degradation of Tim23 is selective, as expression of several other inner membrane proteins that regulate respiratory chain function is unaffected, and is not autophagic, as it occurs similarly in autophagy-proficient and -deficient (Atg-5 knockout) cells. Depleting Tim23 with siRNA is sufficient to inhibit cell proliferation and prevent long-term survival, while expression of degradation-resistant Tim23-GFP in mitochondria delays caspase-independent cell death. Thus, mitochondrial autodigestion of Tim23 joins the array of processes contributing to caspase-independent cell death. Because mitochondrial biogenesis requires a functional protein-import machinery, preventing Tim23 degradation might, therefore, be essential for repairing damaged mitochondria in chronic degenerative diseases.