Quantitative change in mitochondrial DNA content in various mouse tissues during aging

Mitochondrial DNA content in human omental adipose tissue

BACKGROUND

Impairment of mitochondrial function plays an important role in obesity and the development of insulin resistance. The aim of this project was to investigate the mitochondrial DNA copy number in human omental adipose tissue with respect to obesity.

METHODS

The mitochondrial DNA (mtDNA) content per single adipocyte derived from abdominal omental adipose tissue was determined by quantitative RT-PCR in a group of 75 patients, consisting of obese and morbidly obese subjects, as well as non-obese controls. Additionally, basal metabolic rate and fat oxidation rate were recorded and expressed as total values or per kilogram fat mass.

RESULTS

MtDNA content is associated with obesity. Higher body mass index (BMI) resulted in a significantly elevated mtDNA count (ratio = 1.56; p = 0.0331) comparing non-obese (BMI < 30) to obese volunteers (BMI >or= 30). The mtDNA count per cell was not correlated with age or gender. Diabetic patients showed a trend toward reduced mtDNA content. A seasonal change in mtDNA copy number could not be identified. In addition, a substudy investigating the basal metabolic rate and the fasting fat oxidation did not reveal any associations to the mtDNA count.

CONCLUSIONS

The mtDNA content per cell of omental adipose tissue did not correlate with various clinical parameters but tended to be reduced in patients with diabetes, which may partly explain the impairment of mitochondrial function observed in insulin resistance. Furthermore, the mtDNA content was significantly increased in patients suffering from obesity (BMI above 30). This might reflect a compensatory response to the development of obesity, which is associated with impairment of mitochondrial function.

Autophagy is essential for mitochondrial clearance in mature T lymphocytes

Macroautophagy plays an important role in the regulation of cell survival, metabolism, and the lysosomal degradation of cytoplasmic material. In the immune system, autophagy contributes to the clearance of intracellular pathogens, MHCII cross-presentation of endogenous Ags, as well as cell survival. We and others have demonstrated that autophagy occurs in T lymphocytes and contributes to the regulation of their cellular function, including survival and proliferation. Here we show that the essential autophagy gene Atg7 is required in a cell-intrinsic manner for the survival of mature primary T lymphocytes. We also find that mitochondrial content is developmentally regulated in T but not in B cells, with exit from the thymus marking a transition from high mitochondrial content in thymocytes to lower mitochondrial content in mature T cells. Macroautophagy has been proposed to play an important role in the clearance of intracellular organelles, and autophagy-deficient mature T cells fail to reduce their mitochondrial content in vivo. Consistent with alterations in mitochondrial content, autophagy-deficient T cells have increased reactive oxygen species production as well as an imbalance in pro- and antiapoptotic protein expression. With much recent interest in the possibility of autophagy-dependent developmentally programmed clearance of organelles in lens epithelial cells and erythrocytes, our data demonstrate that autophagy may have a physiologically significant role in the clearance of superfluous mitochondria in T lymphocytes as part of normal T cell homeostasis.

Region-specific changes in mitochondrial D-loop in aged rat CNS

Impaired mitochondrial oxidative phosphorylation (OXPHOS) is considered a cause of aging. A reduction in mitochondrial DNA (mtDNA) replication and/or transcription may contribute to this OXPHOS diminution. Impairments in the displacement (D) loop, or non-coding, region of the mitochondrial genome, or accumulation of mtDNA mutations, may affect mtDNA replication and transcription. We determined the effects of age on the D-loop and on mtDNA deletion mutations in the spinal cord, medulla, midbrain, cerebellum, striatum, and cerebral cortex of Fischer 344 rats. D-loop, 7S DNA levels were reduced by 3-fold in striatum, 2.5-fold in cortex, and 2-fold in the spinal cord of older animals. We did not detect a population of mtDNA affected by the most prevalent known (ND4-containing) deletions, indicating they do not comprise a significant portion of total mtDNA. However, we detected an age-related and region-specific increase in the common deletion, which comprised 0.0003-0.0007% of total mtDNA. Mitochondrial genome copy number varied between regions, in addition to an overall 18% decrease with age across the whole brain. These results suggest the age-related decline in OXPHOS may be related to a reduction in D-loop function.

Mitochondrial DNA content in paired normal and cancerous breast tissue samples from patients with breast cancer

INTRODUCTION

We develop a multiplex quantitative real-time PCR for synchronized analysis of mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) to investigate relative mtDNA abundance in paired normal and cancerous breast tissues.

MATERIALS AND METHODS

The amounts of nDNA and mtDNA in 102 tissue samples were quantified for both glyceraldehype-3-phosphodehydrogenase (GAPDH) gene and mtDNA encoded ATPase (MTATP) 8 gene. The average threshold cycle (Ct) number values of the nDNA and mtDNA were used to calculate relative mtDNA content in breast tissues.

RESULTS

The median delta Ct (DeltaCt) and the median mtDNA content for normal and cancerous breast tissues were 6.73 and 2.54, as well as 106.50 and 5.80 (P = 0.000, respectively). The mtDNA content was decreased in 82% of cancerous breast tissues compared with the normal ones. The changes were associated with hormone receptor status.

CONCLUSION

Our finding suggests that decreased mtDNA content in breast cancer may have diagnostic and prognostic value for the disease.

Early-onset liver mtDNA depletion and late-onset proteinuric nephropathy in Mpv17 knockout mice

In humans, MPV17 mutations are responsible for severe mitochondrial depletion syndrome, mainly affecting the liver and the nervous system. To gain insight into physiopathology of MPV17-related disease, we investigated an available Mpv17 knockout animal model. We found severe mtDNA depletion in liver and, albeit to a lesser extent, in skeletal muscle, whereas hardly any depletion was detected in brain and kidney, up to 1 year after birth. Mouse embryonic fibroblasts did show mtDNA depletion, but only after several culturing passages, or in a serumless culturing medium. In spite of severe mtDNA depletion, only moderate decrease in respiratory chain enzymatic activities, and mild cytoarchitectural alterations, were observed in the Mpv17^−/− livers, but neither cirrhosis nor failure ever occurred in this organ at any age. The mtDNA transcription rate was markedly increased in liver, which could contribute to compensate the severe mtDNA depletion. This phenomenon was associated with specific downregulation of Mterf1, a negative modulator of mtDNA transcription. The most relevant clinical features involved skin, inner ear and kidney. The coat of the Mpv17^−/− mice turned gray early in adulthood, and 18-month or older mice developed focal segmental glomerulosclerosis (FSGS) with massive proteinuria. Concomitant degeneration of cochlear sensory epithelia was reported as well. These symptoms were associated with significantly shorter lifespan. Coincidental with the onset of FSGS, there was hardly any mtDNA left in the glomerular tufts. These results demonstrate that Mpv17 controls mtDNA copy number by a highly tissue- and possibly cytotype-specific mechanism.

Caloric restriction retards the age-related decline in mitochondrial function of brown adipose tissue

Caloric restriction (CR) has been shown to prevent the age-associated loss of mitochondrial function and biogenesis in several tissues such as liver, heart, and skeletal muscle. However, little is known about the effects of CR on a tissue in which the mitochondria have no adenosine triphosphate (ATP)-producing purpose but show a high degree of uncoupling, namely brown adipose tissue (BAT). Hence, the aim of the present study was to analyze the effect of long-term CR on BAT mitochondrial function and biogenesis. BAT mitochondria obtained from 24-month-old male and female rats previously subjected to 40% CR for 12 months were compared with mitochondria from old (24 months) and young (6 months) ad libitum fed rats. Old restricted rats compared to old ad libitum fed ones showed a reduction in BAT size with respect to fat content and adipocyte number. Mitochondrial DNA content in BAT increased with age and even more so in restricted rats, indicating a summative effect of age and CR on mitochondrial proliferation. CR induced resistance to lose total and mitochondrial protein, COX activity, and uncoupling capacity with advancing age, in relation with a lower decrease of mitochondrial transcription factor A (TFAM). In summary, our results demonstrate CR prevents the age-associated decline in mitochondrial function in BAT, probably in relation with a lower impairment of mitochondrial biogenesis.

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 structure and function in the untreated Jackson toxic milk (tx-j) mouse, a model for Wilson disease

Structural changes in hepatocellular mitochondria are characteristic of Wilson disease (WD). Features include variability in size and shape, increased density of matrix, discreet inclusions, and cystic dilatation of the cristae. We examined the functional basis for these mitochondrial changes in the toxic milk (tx-j) mouse model for WD. Its normal syngeic strain, C3H, served as control. Hepatic histology was near-normal in tx-j mice at 3-4-months-old and showed mild inflammation and steatosis at 6-months-old. Transmission electron microscopy showed typical mitochondrial abnormalities, specifically cystic dilatation of tips of cristae, in 3, 4, and 6-month-old tx-j mice and none in normal 3-month-old C3H mice. Citrate synthase (CS) activity was initially lower in tx-j mice than age-matched controls but increased over the first 6 months such that it was significantly greater at 5 and 6-months-old (p<0.003). No evidence for hepatic mtDNA depletion was found by long-PCR analysis. NB-PAGE showed preservation of all complexes in the oxidative-phosphorylation chain except complex IV which declined markedly from 5-months-old onwards. Hepatic complex IV activity was significantly decreased in 5-month-old tx-j mice (p<0.04). Expression of mitochondrial transfer factor A (TFAM) mRNA declined progressively in 6-8-month-old tx-j mice; immunodetectable protein levels declined in parallel. Expression of mtSSB mRNA was uniformly low in tx-j mice from 1-8-months-old. Levels of two mitochondrial antioxidant proteins capable of binding copper, thioredoxin-2 and peroxiredoxin-3, rose over the first 6 months of life. Mitochondrial changes occur early in WD and reflect complex, probably oxidative, injury.

Identification of age-dependently expressed genes in mouse liver by differential display-PCR analysis

The aim of this study was to identify genes expressed in an age-dependent manner in mouse (Mus musculus) liver. To search for age-dependently expressed genes, we used a fluorescence differential display-PCR (FDD-PCR) technique on total RNA extracted from mouse livers collected at seven different developmental stages. All differentially expressed cDNAs detected by FDD-PCR were reamplified, subcloned and sequenced, and six genes were confirmed to show age-dependent expression by quantitative real-time PCR analysis. Nucleotide sequence analyses showed that four of them had high homology with known genes (mitochondrial DNA, cytosolic aldehyde dehydrogenase, cell division cycle 2-like 5 and complement component 8 alpha polypeptide), and two with expressed sequence tags of unknown genes. The FDD-PCR technique was effective for detecting novel age-dependently expressed genes, and also for newly characterizing individual expression patterns of known genes. The age-dependent expression patterns of known genes revealed in this study may provide an opportunity to investigate the unknown physiological roles of the proteins they encode.

Mitochondrial-nuclear cross-talk in the aging and failing heart

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.

Mitochondrial toxicity in hearts of CD-1 mice following perinatal exposure to AZT, 3TC, or AZT/3TC in combination

Antiretroviral therapies based on nucleoside reverse transcriptase inhibitors (NRTIs), like zidovudine (3'-azido-3'-deoxythymidine; AZT) and lamivudine ((-)2',3'-dideoxy-3'-thiacytidine; 3TC), markedly reduce mother-to-child transmission of the human immunodeficiency virus (HIV). However, AZT induces damage in nuclear DNA of mice exposed in utero and postnatally, and mitochondrial DNA (mtDNA) damage has been observed in both human and mouse neonates following perinatal exposure to AZT and AZT/3TC in combination. To provide animal data modeling the NRTI-induced heart damage reported in human infants, we treated pregnant CD-1 mice throughout gestation and treated their pups by direct gavage from postnatal day (PND) 4 through PND 28 with daily doses of 150 mg/kg body weight (bw)/day AZT, 75 mg/kg bw/day 3TC, 125/62.5 mg/kg bw/day AZT/3TC, or the vehicle control. Half the pups were euthanized on PND 28; the remainder received no further dosing, and were euthanized at week 10. Heart tissue was collected, total DNA was extracted, and mtDNA copy number relative to nuclear DNA copy number, mtDNA damage, and mtDNA mutation assays were performed using PCR-based methods. Analyses revealed increases in mtDNA lesions in 4-week-old males and females treated with AZT or 3TC, but not in 10-week-old mice, suggesting that the damage resolved after treatment ceased. Interestingly, 10-week-old females treated with AZT/3TC had significant increases in mtDNA damage. Point mutations were elevated in 10-week-old females treated with AZT or AZT/3TC, but not 3TC; no increases in mutations were seen in either gender at 4 weeks of age. Our data suggest that AZT/3TC combination treatment produces greater mtDNA damage than either agent individually, and that female mice are more sensitive than males to AZT/3TC-induced mtDNA damage.

Mitochondrial DNA alterations in blood of the humans exposed to N,N-dimethylformamide

N,N-Dimethylformamide (DMF) has been widely used in industries because of its extensive miscibility with water and solvents. Its health effects include hepatotoxicity and male reproductoxicity, possibly linked with mitochondrial DNA (mtDNA) alterations including mtDNA common deletion (DeltamtDNA(4977)) and mtDNA copy number. The relationship between DMF exposure and mtDNA alterations, however, has not been postulated yet. The purposes of this study were to investigate whether the DMF exposure is associated with DeltamtDNA(4977) and mtDNA copy number and to evaluate the DMF-derived mtDNA alterations are more associated with exposure to the airborne DMF concentrations or to the levels of two urinary DMF biomarkers of N-methylformamide (NMF) and N-acetyl-S-(N-methylcarbamoryl) cysteine(AMCC). Thirteen DMF-exposed workers and 13 age and seniority-matched control workers in a synthetic leather factory were monitored on their airborne DMF, NMF and AMCC in the urine as well as DeltamtDNA(4977) and mtDNA copy number in blood cells. We found that the frequencies of relative DeltamtDNA(4977) in DMF-exposed group were significantly higher than those in the control group. Moreover, elevation in the proportion of DeltamtDNA(4977) of individuals with high urine AMCC (U-AMCC) and airborne DMF levels were significantly higher than those without. We conclude that long-term exposure to DMF is highly associated with the alterations of mtDNA in urine and blood cells. The DeltamtDNA(4977) was more significantly related to repeated exposure to DMF and mtDNA copy number was more closely related to short-term DMF exposure. We also confirmed that U-AMCC is more appropriate to serve as a toxicity biomarker for DMF exposure than U-NMF. Further study with a larger number of subjects is warranted.

Mitochondrial DNA involvement in human longevity

The main message of this review can be summarized as follows: aging and longevity, as complex traits having a significant genetic component, likely depend on a number of nuclear gene variants interacting with mtDNA variability both inherited and somatic. We reviewed the data available in the literature with particular attention to human longevity, and argued that what we hypothesize for aging and longevity could have a more general relevance and be extended to other age-related complex traits such as Alzheimer's and Parkinson's diseases. The genetics which emerges for complex traits, including aging and longevity, is thus even more complicated than previously thought, as epistatic interactions between nuclear gene polymorphisms and mtDNA variability (both somatic and inherited) as well as between mtDNA somatic mutations (tissue specific) and mtDNA inherited variants (haplogroups and sub-haplogroups) must be considered as additional players capable of explaining a part of the aging and longevity phenotype. To test this hypothesis is one of the main challenge in the genetics of aging and longevity in the next future.

Mitochondrial DNA copy number is regulated by cellular proliferation: a role for Ras and p66(Shc)

The abundance of mitochondria is regulated by biogenesis and division. These processes are controlled by cellular factors, given that, for example, mitochondria have to replicate their DNA prior to cell division. However, the mechanisms that allow a synchronization of cell proliferation with mitochondrial genome replication are still obscure. We report here our investigations on the role of proliferation and the contribution of Ras and p66Shc in the regulation of mitochondrial DNA copy number. Ras proteins mediate a variety of receptor-transduced mitogenic signals and appear to play an essential role in the cellular response to growth factors. P66Shc is a genetic determinant of life span in mammals and has been implicated in the regulation of receptor signaling and various mitochondrial functions. First, we confirmed previous reports showing that mitochondrial DNA is replicated during a specific phase of the cell cycle (the pre-S phase) and provided novel evidences that this process is regulated by mitogenic growth factors. Second, we showed that mitochondrial DNA replication is activated following Ras-induced cellular hyper-proliferation. Finally, we showed that p66Shc expression induces mitochondrial DNA replication, both in vitro and in vivo. We suggest that mitochondria are target of intracellular signaling pathways leading to proliferation, involving Ras and p66Shc, which might function to integrate cellular bio-energetic requirements and the inheritance of mitochondrial DNA in a cell cycle-dependent manner.