Mitochondrial and nuclear DNA base excision repair are affected differently by caloric restriction

Aging is strongly correlated with the accumulation of oxidative damage in DNA, particularly in mitochondria. Oxidative damage to both mitochondrial and nuclear DNA is repaired by the base excision repair (BER) pathway. The "mitochondrial theory of aging" suggests that aging results from declining mitochondrial function, due to high loads of damage and mutation in mitochondrial DNA (mtDNA). Restriction of caloric intake is the only intervention so far proven to slow the aging rate. However, the molecular mechanisms underlying such effects are still unclear. We used caloric-restricted (CR) mice to investigate whether lifespan extension is associated with changes in mitochondrial BER activities. Mice were divided into two groups, receiving 100% (PF) or 60% (CR) of normal caloric intake, a regime that extends mean lifespan by approximately 40% in CR mice. Mitochondria isolated from CR mice had slightly higher uracil (UDG) and oxoguanine DNA glycosylase (OGG1) activities but marginally lower abasic endonuclease and polymerase gamma gap-filling activities, although these differences were tissue-specific. Uracil-initiated BER synthesis incorporation activities were significantly lower in brain and kidney from CR mice but marginally enhanced in liver. However, nuclear repair synthesis activities were increased by CR, indicating differential regulation of BER in the two compartments. The results indicate that a general up-regulation of mitochondrial BER does not occur in CR.

Mitochondrial toxin 3-nitropropionic acid induces cardiac and neurotoxicity differentially in mice

We investigated the effects of 3-nitropropionic acid (3NPA), a previously characterized neurotoxin, in four strains of mice to better understand the molecular basis of variable host responses to this agent. Unexpectedly, we found significant cardiac toxicity that always accompanied the neurotoxicity in all strains of mice in acute and subacute/chronic toxicity testing. Caudate putamen infarction never occurred without cardiac toxicity. All mouse strains tested are sensitive to 3NPA although the C57BL/6 and BALB/c mice require more exposure than 129SVEMS and FVB/n mice. Cardiac toxicity alone was found in 50% of symptomatic mice tested and morphologically, the cardiac toxicity is characterized by diffuse swelling of cardiomyocytes and multifocal coagulative contraction band necrosis. In subacute to chronic exposure, atrial thrombosis, cardiac mineralization, cell loss, and fibrosis are combined with cardiomyocyte swelling and necrosis. Ultrastructurally, mitochondrial swelling occurs initially, followed by disruption of myofilaments. Biochemically, isolated heart mitochondria from the highly sensitive 129SVEMS mice have a significant reduction of succinate dehydrogenase activity, succinate oxygen consumption rates, and heart adenosine triphosphate after 3NPA treatment. The severity of morphological changes parallels the biochemical alterations caused by 3NPA, consistent with cardiac toxicity being a consequence of the effects of 3NPA on succinate dehydrogenase. These experiments show, for the first time, that 3NPA has important cardiotoxic effects as well as neurotoxic effects, and that cardiac toxicity possibly resulting from inhibition of the succinate dehydrogenase in heart mitochondria, contributes to the cause of death in 3NPA poisoning in acute and subacute/chronic studies in mice.

Repair of 8-oxodeoxyguanosine lesions in mitochondrial dna depends on the oxoguanine dna glycosylase (OGG1) gene and 8-oxoguanine accumulates in the mitochondrial dna of OGG1-defective mice

Mitochondria are not only the major site for generation of reactive oxygen species, but also one of the main targets of oxidative damage. One of the major products of DNA oxidation, 8-oxodeoxyguanosine (8-oxodG), accumulates in mitochondrial DNA (mtDNA) at levels three times higher than in nuclear DNA. The main pathway for the repair of 8-oxodG is the base excision repair pathway initiated by oxoguanine DNA glycosylase (OGG1). We previously demonstrated that mammalian mitochondria from mice efficiently remove 8-oxodG from their genomes and isolated a protein from rat liver mitochondria with 8-oxoguanine (8-oxodG) DNA glycosylase/apurinic DNA lyase activity. In the present study, we demonstrated that the mitochondrial 8-oxodG DNA glycosylase/apurinic DNA lyase activity is the mitochondrial isoform of OGG1. Using mouse liver mitochondria isolated from ogg1(-/-) mice, we showed that the OGG1 gene encodes for the mitochondrial 8-oxodG glycosylase because these extracts have no incision activity toward an oligonucleotide containing a single 8-oxodG DNA base lesion. Consistent with an important role for the OGG1 protein in the removal of 8-oxodG from the mitochondrial genome, we found that mtDNA isolated from liver from OGG1-null mutant animals contained 20-fold more 8-oxodG than mtDNA from wild-type animals.

DNA repair and aging in mouse liver: 8-oxodG glycosylase activity increase in mitochondrial but not in nuclear extracts

8-oxo-deoxyguanosine (8-oxodG) is one of the major DNA lesions formed upon oxidative attack of DNA. It is a mutagenic adduct that has been associated with pathological states such as cancer and aging. Base excision repair (BER) is the main pathway for the repair of 8-oxodG. There is a great deal of interest in the question about age-associated accumulation of this DNA lesion and its intracellular distribution, particularly with respect to mitochondrial or nuclear localization. We have previously shown that 8-oxodG-incision activity increases with age in rat mitochondria obtained from both liver and heart. In this study, we have investigated the age-associated changes in DNA repair activities in both mitochondrial and nuclear extracts obtained from mouse liver. We observed that 8-oxodG incision activity of mitochondrial extracts increases significantly with age, from 13.4 + or - 2.2 fmoles of oligomer/100 microg of protein/16 h at 6 to 18.6 + or - 4.9 at 14 and 23.7 + or - 3.8 at 23 months of age. In contrast, the nuclear 8-oxodG incision activity showed no significant change with age, and in fact slightly decreased from 11.8 + or - 3 fmoles/50 microg of protein/2 h at 6 months to 9.7 + or - 0.8 at 14 months. Uracil DNA glycosylase and endonuclease G activities did not change with age in nucleus or mitochondria. Our results show that the repair of 8-oxodG is regulated differently in nucleus and mitochondria during the aging process. The specific increase in 8-oxodG-incision activity in mitochondria, rather than a general up-regulation of DNA metabolizing enzymes in those organelles, suggests that this pathway may be up regulated during aging in mice.

Age-associated change in mitochondrial DNA damage

There is an age-associated decline in the mitochondrial function of the Wistar rat heart. Previous reports from this lab have shown a decrease in mitochondrial cytochrome c oxidase (COX) activity associated with a reduction in COX gene and protein expression and a similar decrease in the rate of mitochondrial protein synthesis. Damage to mitochondrial DNA may contribute to this decline. Using the HPLC-Coularray system (ESA, USA), we measured levels of nuclear and mitochondrial 8-oxo-2'-deoxyguanosine (8-oxodG) from 6-month (young) and 23-month-old (senescent) rat liver DNA. We measured the sensitivity of the technique by damaging calf thymus DNA with photoactivated methylene blue for 30s up to 2h. The levels of damage were linear over the entire time course including the shorter times which showed levels comparable to those expected in liver. For the liver data, 8-oxodG was reported as a fraction of 2-deoxyguanosine (2-dG). There was no change in the levels of 8-oxodG levels in the nuclear DNA from 6 to 23-months of age. However, the levels of 8-oxodG increased 2.5-fold in the mitochondrial DNA with age. At 6 months, the level of 8-oxodG in mtDNA was 5-fold higher than nuclear and increased to approximately 12-fold higher by 23 months of age. These findings agree with other reports showing an age-associated increase in levels of mtDNA damage; however, the degree to which it increases is smaller. Such damage to the mitochondrial DNA may contribute to the age-associated decline in mitochondrial function.

Dependence of H2O2 formation by rat heart mitochondria on substrate availability and donor age

We have examined the substrate specificity and inhibitor sensitivity of H2O2 formation by rat heart mitochondria. Active H2O2 production requires both a high fractional reduction of Complex I (indexed by NADH/NAD+ + NADH ratio) and a high membrane potential, delta psi. These conditions are achieved with supraphysiological concentrations of succinate. With physiological concentrations of NAD-linked substrates, rates of H2O2 formation are much lower (less than 0.1% of respiratory chain electron flux) but may be stimulated by the Complex III inhibitor antimycin A, but not by myxothiazol. Addition of Mn2+ to give 10 nmol/mg of mitochondrial protein enhances H2O2 production with all substrate combinations, possibly by repleting mitochondrial superoxide dismutase with this cation. Contrary to previously published work, no increased activity of H2O2 production was found with heart mitochondria from senescent (24 month) rats, relative to young adults (6 month).

Effects of acidosis on resting cytosolic and mitochondrial Ca2+ in mammalian myocardium

Acidosis increases resting cytosolic [Ca2+], (Cai) of myocardial preparations; however, neither the Ca2+ sources for the increase in Cai nor the effect of acidosis on mitochondrial free [Ca2+], (Cam) have been characterized. In this study cytosolic pH (pHi) was monitored in adult rat left ventricular myocytes loaded with the acetoxymethyl ester (AM form) of SNARF-1. A stable decrease in the pHi of 0.52 +/- 0.05 U (n = 16) was obtained by switching from a bicarbonate buffer equilibrated with 5% CO2 to a buffer equilibrated with 20% CO2. Electrical stimulation at either 0.5 or 1.5 Hz had no effect on pHi in 5% CO2, nor did it affect the magnitude of pHi decrease in response to hypercarbic acidosis. Cai was measured in myocytes loaded with indo-1/free acid and Cam was monitored in cells loaded with indo-1/AM after quenching cytosolic indo-1 fluorescence with MnCl2. In quiescent intact myocytes bathed in 1.5 mM [Ca2+], hypercarbia increased Cai from 130 +/- 5 to 221 +/- 13 nM. However, when acidosis was effected in electrically stimulated myocytes, diastolic Cai increased more than resting Cai in quiescent myocytes, and during pacing at 1.5 Hz diastolic Cai was higher (285 +/- 17 nM) than at 0.5 Hz (245 +/- 18 nM; P < 0.05). The magnitude of Cai increase in quiescent myocytes was not affected either by sarcoplasmic reticulum (SR) Ca2+ depletion with ryanodine or by SR Ca2+ depletion and concomitant superfusion with a Ca(2+)-free buffer. In unstimulated intact myocytes hypercarbia increased Cam from 95 +/- 12 to 147 +/- 19 nM and this response was not modified either by ryanodine and a Ca(2+)-free buffer or by 50 microM ruthenium red in order to block the mitochondrial uniporter. In mitochondrial suspensions loaded either with BCECF/AM or indo-1/AM, acidosis produced by lactic acid addition decreased both intra- and extramitochondrial pH and increased Cam. Studies of mitochondrial suspensions bathed in indo-1/free acid-containing solution showed an increase in extramitochondrial Ca2+ after the addition of lactic acid. Thus, in quiescent myocytes, cytoplasmic and intramitochondrial buffers, rather than transsarcolemmal Ca2+ influx or SR Ca2+ release, are the likely Ca2+ sources for the increase in Cai and Cam, respectively; additionally, Ca2+ efflux from the mitochondria may contribute to the raise in Cai. In contrast, in response to acidosis, diastolic Cai in electrically stimulated myocytes increases more than resting Cai in quiescent cells; this suggests that during pacing, net cell Ca2+ gain contributes to enhance diastolic Cai.

Altered pyruvate dehydrogenase control and mitochondrial free Ca2+ in hearts of cardiomyopathic hamsters

The fraction of total pyruvate dehydrogenase in the active, dephosphorylated form is much lower in the glucose-perfused isolated hearts of two myopathic strains of Syrian hamster (BIO 14.6 and TO-2) than in the hearts of healthy control animals (F1B). The myopathic hearts also develop significantly less pressure under these conditions. Experiments with isolated myocytes from the BIO 14.6 heart reveal that intramitochondrial free Ca2+ ([Ca2+]m), a positive effector of pyruvate dehydrogenase interconversion, rises much less in response to a protocol of increased frequency of electrical stimulation and adrenergic stimulation than does [Ca2+]m in cells from the healthy control animals (viz from 248 +/- 15 to 348 +/- 44 nM in BIO 14.6 vs. from 241 +/- 35 to 830 +/- 124 nM in F1B, at 4 Hz). As the concentration of Ca2+ that produces half-maximal activation of pyruvate dehydrogenase within mitochondria is 650 nM, this difference between strains is likely the mechanism of the altered enzyme interconversion. The lesser response of [Ca2+]m to electrical stimulation in the BIO 14.6 cells probably results mainly from smaller systolic transients in cytosolic free Ca2+ in response to excitation of single myocytes from the BIO 14.6 animal. Lowered values of [Ca2+]m within the range described would compromise not only pyruvate dehydrogenase activity, but also flux through the tricarboxylate cycle in the myopathic heart, owing to the sensitivity of 2-oxoglutarate dehydrogenase to Ca2+. This may explain the decreased activity of oxidative phosphorylation and performance of work in the myopathic heart.

Cytosolic pH measurements in single cardiac myocytes using carboxy-seminaphthorhodafluor-1

This study examines the use of carboxy-seminaphthorhodafluor-1 (C-SNARF-1) as an indicator of cytosolic pH in isolated rat cardiac myocytes. The emission spectrum of C-SNARF-1 when excited at 530 nm contains two well-separated peaks at approximately 590 and 640 nm, corresponding to the acidic and basic forms of the indicator. This spectral feature allows the indicator to be used in the single excitation, dual emission ratio mode. When C-SNARF-1 is loaded into rat cardiac myocytes as the membrane permeant ester derivative, C-SNARF-1/AM, the indicator localizes within the cytosol with virtually no partitioning into the mitochondria. C-SNARF-1 does not load into isolated mitochondria in suspension. There was no evidence for the presence of non-deesterified C-SNARF-1 within the cells. C-SNARF-1 can be calibrated in situ using a technique that abolishes all transsarcolemmal pH gradients. A 0.7-unit shift in the apparent pK (pKapp = pK-log10) between the in vitro calibration and the in situ calibration is consistent with a change in beta (I640 to pH 9/I640 at pH 5) in the cytosolic environment (beta in situ/beta in vitro = 0.21) and not a change in the true pK of the indicator. The contribution of cellular autofluorescence to the total signal can be made negligible. There is no effect of C-SNARF-1 on the contractile properties of rat cardiac myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of substrate oxidation in mitochondria by the peripheral-type benzodiazepine receptor ligand AHN 086

The effects, of the benzodiazepines RO5-4864, AHN 086, PK 11195 and clonazepam on respiration of mitochondria from heart, kidney, and liver were studied. ADP-stimulated respiration of heart mitochondria was the most sensitive to inhibition by AHN 086; clonazepam was not inhibitory. Several respiratory chain segment activities of submitochondrial particles were insensitive to AHN 086, except for NADH oxidase which was partially inhibited. However, in contrast to submitochondrial particles, the succinate-cytochrome c oxidoreductase activity in intact mitochondria was inhibited by AHN 086, suggesting an effect at the substrate transport level. Phosphate-induced, succinate-dependent swelling was also inhibited by AHN 086 it was not affected by clonazepam. Uncoupled ATP hydrolysis was partially inhibited by RO5-4864, AHN 086, and clonazepam. It is suggested that there is an unspecific inhibition of NADH oxidase and ATP hydrolysis by these benzodiazepines and a specific inhibition on oxidizable substrate transport by the peripheral-type benzodiazepine AHN 086.

Influence of NAD-linked dehydrogenase activity on flux through oxidative phosphorylation

1. We have examined systematically the relationship between the percentage reduction of cardiac mitochondrial NAD and the flux through oxidative phosphorylation, as measured by O2 uptake. Reduction of NAD was varied by varying the concentration of palmitoyl-L-carnitine, pyruvate, 2-oxoglutarate or glutamate in the presence of malate as the oxidizable substrate. 2. In the presence of ADP (State 3 respiration) there was a substantially linear positive relationship between O2 uptake and the percentage reduction of NAD. Coupled respiration in the absence of ADP also showed an increase with increasing NADH, with the exact shape of the relationship being variable. 3. When pyruvate and 2-oxoglutarate dehydrogenase activity were increased by increasing medium Ca2+ concentration within the range 5 nM to 1.23 microM, at non-saturating substrate concentrations, there was again a positive relationship between O2 uptake and the reduction of NAD; however, rates of O2 uptake tended to be higher at given values of NAD reduction when the incubation medium contained Ca2+. This is taken to indicate an activation by Ca2+ of the enzymes of phosphorylation or of the respiratory chain, in addition to the dehydrogenase activation. 4. When carboxyatractyloside plus ADP were used to generate 50% State 3 rates of O2 uptake with pyruvate or 2-oxoglutarate, sensitivity to Ca2+ was retained. However, when oligomycin plus 1 mM-ADP and 1 mM-ATP were used to generate 50% State 3, no such dependence was seen. 5. The results are interpreted to indicate a substantial role for substrate dehydrogenation in the overall regulation of oxidative phosphorylation when substrates are available at near-physiological concentrations.