Mitochondrial and nuclear DNA-repair capacity of various brain regions in mouse is altered in an age-dependent manner
Introduction
Aging is an inevitable biological process, which is characterized by a general decline in physiological function that leads to morbidity and mortality [20]. The detrimental effects of aging are best observed in post-mitotic tissues, where cells that are irreversibly damaged or lost cannot be replaced by mitosis of intact ones. Among such tissues the brain is most important, as it has the main role in homeostasis of the organism. The generation of various oxidants, both reactive oxygen (ROS) and nitrogen-species (RNS), leads to macromolecular damage, a characteristic of aging [28]. Aging enhances the generation of ROS and RNS in mouse models of dopaminergic damage [9], [29]. The hippocampus and cortex of aged rats have been reported to have increased neuronal nitric oxide synthase mRNA, resulting in an increased free radical production in these brain regions [15]. Antioxidant enzymes, such as manganese superoxide dismutase, were also found to be decreased in the brain of these rats [4]. Steady state levels of 8-oxoguanine (8-oxoG) have been reported to increase significantly in the striatum of aged rats [5]. Oxidative damage to DNA may play a role in both normal aging and neurodegenerative diseases [21], [22]. Recent studies suggest a role of free radicals in neuronal degeneration [19]. Oxidative damage is a well-recognized mechanism of injury in several neurodegenerative dementias including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), and is thought to contribute to the neuronal death associated with these disorders [26].
Free radical attack on nuclear and mitochondrial DNA is believed to be a major contributing factor to aging [23]. The levels of damaged DNA and the ability of the neurons to repair such damage may be an important predictor of the maximum life span of each species [24]. Oxidative damage to DNA may be specifically important because it has the potential to affect expression of wide variety of genes. It can also stimulate cell-cycle-related intrinsic DNA repair mechanisms that may be either reparative or deleterious [14]. The free radical theory of aging postulates that increased DNA damage and/or decreased DNA repair could lead to the accumulation of DNA lesions, ultimately resulting in cellular senescence and death [2].
The base-excision repair (BER) pathway repairs small base modifications, including lesions generated by reactive oxygen species. A DNA glycosylase, which catalyzes the hydrolysis of the N-glycosyl bond, initiates BER by releasing the base and generating an abasic site. The abasic site is cleaved by an AP lyase or AP endonuclease and the one base gap is filled in by a DNA polymerase and ligated by a DNA ligase. The specificity of BER is provided by the DNA glycosylases, which have precise substrate specificities. Mammalian cells have several substrate-specific DNA glycosylases: oxoguanine DNA glycosylase (OGG1), which primarily recognizes 8-oxo-dG but is active on other oxidized purines; uracil DNA glycosylase (UDG), which removes deoxyuracil from DNA; endonuclease III homologue (NTH1), which recognizes and cleaves oxidized pyrimidines such as thymine glycol and 5-hydroxycytosine (5-OH-dC); and 3-methyl adenine DNA glycosylase, which removes alkylated bases [27]. Although various reports have shown an accumulation of different oxidative DNA lesions in brain and possibly a decrease in DNA repair in various cell-types of brain, no study has been reported so far that examines the processing of DNA lesions in different brain regions during the normal aging process. In this study, we sought to evaluate the activity of three major DNA glycosylases, OGG1, UDG and NTH1, in different brain regions of young and old wild-type mice in order to understand the age-dependent status of their capacity to incise oxidative DNA lesions. We also evaluated regional changes in nuclear and mitochondrial protein levels of two DNA-repair proteins involved in different steps of BER, as well as uracil-initiated mitochondrial repair synthesis incorporation as a function of aging.
Section snippets
Materials
HEPES, benzamidine–HCl, dithiothreitol (DTT), bovine serum albumin (BSA), and acrylamide/bis-acrylamide (19:1) were from Sigma Chemicals. Leupeptin was from Roche. Isotopes were from NEN Life Science Products; G25 spin columns were from Amersham. T4 polynucleotide kinase was from Stratagene. All other reagents were of ACS grade from Sigma.
Animals
We used male C57Bl/6 mice (6 month and 18 month) obtained from the National Institute on Aging animal colony. The animals were fed regular Purina animal chow
Results
DNA glycosylases initiate BER by recognizing and removing the damaged base. In the present study, we examined DNA glycosylase activity as a measure of DNA repair in various mouse brain regions that are implicated in different varieties of neurodegeneration with respect to age. We compared the DNA incision activity of nuclear and mitochondrial extracts of five brain regions that include caudate nucleus (CN), frontal cortex (FC), hippocampus (Hip), cerebellum (CE) and brain stem (BS) of young (6
Discussion
Different tissue/organs have different metabolic profiles and are exposed to different levels of DNA damaging agents. It is therefore not surprising that DNA repair activities vary considerably from tissue to tissue. In a previous study we demonstrated that BER activities differ significantly in both mitochondria and nuclei from 6 mouse tissues, including brain [13]. However, while the other organs (liver, heart, kidney, skeletal muscle and testis) are relatively homogeneous as to cellular
References (34)
- et al.
Mitochondrial decay in aging
Biochim Biophys Acta
(1995) - et al.
An investigation of antioxidant status, DNA repair capacity and mutation as a function of age in humans
Mutat Res
(1995) - et al.
Oxidative DNA damage processing in nuclear and mitochondrial DNA
Biochimie
(1999) - et al.
DNA damage, repair, and antioxidant systems in brain regions: a correlative study
Free Radic Biol Med
(2000) - et al.
Effects of diethylmaleate on DNA damage and repair in the mouse brain
Free Radic Biol Med
(2002) - et al.
Molecular biology of aging
Cell
(1999) - et al.
Effect of l-carnitine on nucleic acid status of aged rat brain
Exp Neurol
(2005) - et al.
Alteration of expression levels of neuronal nitric oxide synthase and haem oxygenase-2 messenger RNA in the hippocampi and cortices of young adult and aged cognitively unimpaired and impaired long-Evans rats
Neuroscience
(2000) - et al.
The molecular bases of Alzheimer's disease and other neurodegenerative disorders
Arch Med Res
(2001) - et al.
Possible relationship between conditions associated with chronic hypoxia and brain mitochondrial DNA deletions
Arch Biochem Biophys
(1996)
Senescent microstructural changes in rat cerebellum
Brain Res
Cellular determinants of reduced adaptability of the aging brain: neurotransmitter utilization and cell signaling responses after MDMA lesions
Brain Res
Oxidative damage, mitochondrial oxidant generation and antioxidant defenses during aging and in response to food restriction in the mouse
Mech Ageing Dev
Oxidative DNA damage in the aging mouse brain
Mov Disord
Age-associated decrease of oxidative repair enzymes, human 8-oxoguanine DNA glycosylases (hOgg1), in human aging
J Radiat Res (Tokyo)
Mitochondrial DNA deletions in human brain: regional variability and increase with advanced age
Nat Genet
Aging increases the susceptiblity to methamphetamine-induced dopaminergic neurotoxicity in rats: correlation with peroxynitrite production and hyperthermia
J Neurochem
Cited by (162)
Selective neuronal vulnerability to deficits in RNA processing
2023, Progress in NeurobiologyAnalysis of representative mutants for key DNA repair pathways on healthspan in Caenorhabditis elegans
2021, Mechanisms of Ageing and DevelopmentDecoding and rejuvenating human ageing genomes: Lessons from mosaic chromosomal alterations
2021, Ageing Research ReviewsThe multifaceted roles of DNA repair and replication proteins in aging and obesity
2021, DNA RepairCitation Excerpt :As a critical enzyme involved in the repair of oxidative lesions, it can be hypothesized that OGG1 plays a role in regulating the onset of aging and age-related diseases by facilitating the removal of the DNA damage caused by high levels of oxidative stress, thereby preventing the onset of cellular senescence. Indeed, several studies using various animal models have demonstrated increased levels of 8-oxoG and decreased levels of OGG1 with age [45–47]. Notably, many publications have reported that mitochondrial DNA, and not nuclear DNA, is where most oxidative damage is detected [28,45,47,48].
Mitochondrial dysfunction and autophagy in neurodegeneration
2021, Mitochondrial Dysfunction and Nanotherapeutics: Aging, Diseases, and Nanotechnology-Related Strategies in Mitochondrial MedicineGenomic instability and aging: Causes and consequences
2021, Genome Stability: From Virus to Human Application