Changes in mitochondrial DNA during aging

The aging paradox: free radical theory of aging

There are more than 300 theories to explain the aging phenomenon. Many of them originate from the study of changes that accumulate with time. Among all the theories, the free radical theory of aging, postulated first by Harman, is the most popular and widely tested, and is based on the chemical nature and ubiquitous presence of free radicals. This review aims to recapitulate various studies on the role of free radicals in DNA damage-both nuclear as well as mitochondrial-the oxidative stress they impose on cells, the role of antioxidants, the presence of autoantibodies, and their overall impact on the aging process.

Evidence for and against the causal involvement of mitochondrial DNA mutation in mammalian ageing

Current experimental evidence on the role of mitochondrial DNA mutation in ageing is assessed alongside reports implicating other genetic and non-genetic causes, including inter-relationships between the mitochondrial and nuclear genomes and their potential effect on mitochondrial structure and function. The role of a 5-kb mtDNA deletion, identified as age-dependent in a variety of human and other mammalian species, is specifically evaluated in the context of its functional effect in mitotic and non-mitotic adult tissue. Downstream effects of mitochondrial decline are considered in terms of the maintenance of ATP production. Associated sequelae then are discussed specifically with reference to restrictions in the supply of ribose moieties for DNA and RNA synthesis, and to disruption of NADPH production and hence cellular anti-oxidant defences.

An update on the mitochondrial-DNA mutation hypothesis of cell aging

Our electron microscopic study of aging insects and mammals suggests that metazoan senescence is linked to a gradual process of mitochondrial breakdown (and lipofuscin accumulation) in fixed postmitotic cells. This led us to propose in the early 1980s an oxyradical-mitochondrial DNA damage hypothesis, according to which metazoan aging may be caused by mutation, inactivation or loss of the mitochondrial genome (mtDNA) in irreversibly differentiated cells. This extranuclear somatic gene mutation concept of aging is in agreement with the fact that mtDNA synthesis takes place at the inner mitochondrial membrane near the sites of formation of highly reactive oxygen species and their products. Mitochondrial DNA may be unable to counteract the damage inflicted by those by-products of respiration because, in contrast to the nuclear genome, it lacks excision and recombination repair. Since mtDNA contains the structural genes for 13 hydrophobic proteins of the respiratory chain and ATP synthase as well as mitochondrial rRNAs and tRNAs, damage to this organellar genome will decrease or prevent the 'rejuvenation' of the mitochondria through the process of macromolecular turnover and organelle fission. Thus deprived of the ability to regenerate their mitochondria, the fixed postmitotic cells will sustain a decrease in the number of functional organelles, with resulting decline in ATP production. At higher levels of biological organization, this will lead to a loss in the bioenergetic capacity of cells, with concomitant decreases in ATP dependent protein synthesis and specialized physiological function, thus paving the way for age related degenerative diseases. The above concept is supported by a wealth of recent observations confirming the genomic instability of mitochondria and suggesting that animal and human aging is accompanied by mtDNA deletions and other types of injury to the mitochondrial genome. Our hypothesis of mtDNA damage is integrated with the classic concepts of Weissman and Minot in order to provide a preliminary explanation of the evolutionary roots of aging and reconcile the programed and stochastic views of metazoan senescence.

Oxygen-induced mitochondrial damage and aging

Although the views of Harman and Gerschman provide a reasonable explanation for many of the effects of aging, they fail to explain why many cell types, from amoebae to mammalian spermatogonia, do not show a time-related involution, while other cells (especially the neurons) change with age. We feel that a better understanding of senescence (from the molecular to the organ and organismic levels) can be gained by integrating the free radical theory of aging with the classic concepts of Minot and Pearl on the role of cell differentiation and metabolic rate in, respectively, triggering and pacing senescence. In agreement with the above, we maintain that aging is the non-programmed but unavoidable "side effect" of oxy-radical damage to the membrane and genome of the mitochondria of irreversibly differentiated cells. If oxy-radical damage to mtDNA occurs, it will block the rejuvenation of the mitochondrial population by the process of organelle division, thus leading to bioenergetic decline and cellular death.

Changes in the rat liver mitochondrial DNA upon aging

During experiments on the molecular basis of morphological and functional changes observed in rat liver mitochondria upon aging, we found that the buoyant density profile of mitochondrial DNA (mtDNA) shows a wide distribution pattern especially in the lighter region than that of young rat liver mtDNA. The heterogeneous pattern may be partly recovered to become similar to that of young rat liver mtDNA by treatment with proteinase K. Therefore, it is quite likely that mtDNA of old rat liver contains firmly bound protein(s) or peptides. During the morphological observation of mtDNA by electron microscopy, we found that mtDNA of old rat had a novel property, that is, the ability to attach to negatively charged mica in the absence of magnesium ions, although their morphological features showing circular 5 microns contour length form did not change. Further, mtDNA gained resistance against EcoRI digestion during aging. This property was not shared by the DNA from young animal, and might be due to the binding protein(s).

Morphological and functional studies during aging at mitochondrial level. Action of drugs

1. Mitochondria show morphological changes on aging: the volume density decreases and the total surface area or volume increase. 2. Biochemical studies indicated a decrease in the rate of oxygen consumption and ATP levels for increasing age. Measurements of calcium transport across the mitochondrial membrane revealed a decrease in accumulated calcium and an alteration in the uptake kinetics of the ion in older animals. 3. Theophylline and calcitonin action were also studied. In both age groups (young and old) theophylline shows an inhibitory effect on all the parameters studied (both morphological and biochemical). 4. Calcitonin showed no effect on morphological and respiratory parameters, although it increased calcium uptake into the mitochondrion to a lesser extent in the 24 month old animals.

Restriction enzyme analysis of mitochondrial DNA in aging human cells

Human diploid fibroblasts show a limited lifespan in vitro. To investigate the integrity of mitochondrial DNA (mtDNA) in aging fibroblasts, whole cell DNA samples from the human cell line IMR-90 have been prepared at 36, 22, and 3 population doublings (PD) from the end of the lifespan (63 PD). These DNA samples were then digested separately with 19 different restriction endonucleases, and the resulting fragments were separated by agarose gel electrophoresis and transferred to nitrocellulose filters. Fragment sizes were revealed by hybridization to 32P-labelled mouse mtDNA and autoradiography, and were compared with computer maps of fragments generated from the known sequence of human mtDNA. These 19 enzymes recognize a total of 297 recognition sites comprising 1315 nucleotide base pairs (bp), approximately 8% of the human mtDNA (16 569 bp). Control experiments reveal that a minor component representing as little as 5% of the total mtDNA can be detected. No changes were seen in the restriction fragment pattern with fibroblast cell age. It is concluded that there are no large deletions, insertions, or rearrangements in human mtDNA, and no single base changes in the detectable regions. This suggests efficient maintenance of mtDNA molecules and/or elimination of damaged mtDNA during fibroblast cell lifespan.

Age changes of chromatin. A review

The age-related studies of chromatin and DNA has attracted significant interest in recent years. However, individual works describe only some and a few of the many changes of chromatin. It is often difficult to decide whether these changes have secondary or primary nature. The overview of these studies makes it possible to realize how many very complex and interdependent changes occur in chromatin during ageing. Chromatin is the most complex among self-reproducible parts of the cell. A very sophisticated structure of chromatin makes possible the differential transcription of a genetic programme which supports the accurate specialized functions of each cell in interphase and also provides a mechanism for perfect reproduction of this complex machinery of genetic information during cell division. It is known that chromatin proteins, more than chromatin DNA show tissue specificity and developmental changes. There are many theories of cellular ageing which select some special types of DNA, RNA or protein changes and to promote them as the main or primary causes of cellular senescence. However, if these changes are considered within the more comprehensive picture of functional structure of chromatin the results show the interdependence of individual alterations and their proper place in the complex, multichannel, species and tissue-specific character of actual ageing. An attempt to summarize the basic facts and theories about age changes of the two main parts of chromatin structure, proteins and DNA is being made in this review. At the same time the author tried to develop a concept of non-random distribution of the age changes in chromatin and a possible higher rate of accumulation of different alteration and lesions in the transcribed and functionally active parts of chromatin.