Arguing for Mitochondrial ROS to Cause Stochastic Nuclear DNA Damage that then Causes Cellular Senescence

The open access paper I'll point out today ties together a number of common themes in aging research. The authors propose that mitochondrial production of reactive oxygen species (ROS) is a significant cause of stochastic nuclear DNA damage, which in turn is a significant cause of cellular senescence. Those issues can then also disrupt mitochondrial function to increase ROS production, forming a feedback loop. In this view of the driving processes of aging, mitochondria are largely at fault for anything that can be pinned to rising levels of random mutations in nuclear DNA: cancer risk, cellular senescence, generally increased levels of cellular malfunction, and so forth.

An important caution regarding this paper is that the researchers used mice with a DNA repair deficiency in order to assemble their data. Such animals exhibit the appearance of accelerated aging, but it isn't in fact accelerated aging. It is usually an excess of cellular damage that isn't all that relevant in normal aging - any sort of global dysfunction in cells will tend to share high level similarities with aging, even if the damage is different. When it is significantly different, however, you usually can't learn much from it. So whether or not work in such mice is in fact useful in understanding normal aging depends very strongly on the low-level biochemical details in question. That can be hard to judge for those of us who are not life scientists.

The approach to the problem taken here sounds basically sensible as it is described below, but it nonetheless calls out for some form of confirming study in normal mice in order to rule out any peculiarity of DNA repair deficiency. One possibility would be to take one of the existing mitochondrially targeted antioxidant compounds and design a study that specifically evaluates reduced nuclear DNA mutation and reduced cellular senescence burden as a result of administration. Researchers have already run numerous studies in mice with these compounds, and some of that existing data might be helpful from this point of view. I note, however, that those studies didn't produce very large gains in life span where those gains were measured, which should perhaps temper our enthusiasm for this whole line of thought.

Spontaneous DNA damage to the nuclear genome promotes senescence, redox imbalance, and aging

Cellular senescence was recently established to play a causal role in aging and many age-related diseases. Senescence is a programmed cell fate characterized by growth arrest, a metabolic shift, resistance to apoptosis and often a secretory phenotype. The senescent cell burden increases with age in virtually all vertebrates. In replicating human cells, shortened telomeres drive senescence. It has become increasingly clear that non-replicating cells also undergo senescence. However, in non-dividing cells, which are the majority of cells in mammalian organisms, the cause of senescence is not clear.

A variety of cellular stressors including genotoxic, proteotoxic, inflammatory, and oxidative have been implicated in driving senescence. However, senescence itself is associated with many of these cellular stressors, making it very difficult to decipher cause and effect. For example, DNA damaging agents definitively cause increased senescence (e.g. in cancer patients). Yet senescent cells are defined by persistent activation of the DNA damage response, increased expression of surrogate markers of DNA damage and are able to trigger genotoxic stress in neighboring cells. Therefore, in vivo, the importance of DNA damage as a driver of senescence and aging is debated.

Even less is known about endogenous DNA damage as a potential driver of senescence and aging. The vast majority of evidence implicating DNA damage in senescence comes from experiments implementing very high doses of environmental genotoxins such as ionizing radiation or doxorubicin. Also of note, all genotoxins damage not only DNA, but also all cellular nucleophiles including phospholipids, proteins, and RNA. Thus, it remains unknown whether physiological levels of spontaneous DNA damage is sufficient to drive cellular senescence.

A major source of endogenous DNA damage is reactive oxygen species (ROS) produced during mitochondrial-based aerobic metabolism. Some mitochondrial-derived ROS, such as H2O2, can diffuse throughout the cell, resulting in oxidative damage to lipids, proteins, RNA and DNA. Thus, mitochondrial dysfunction, which leads to an increase in ROS production, was proposed to be central to the aging process. However, this too remains controversial.

To address these gaps in knowledge, we utilized a genetic approach to increase endogenous nuclear DNA damage in mice. ERCC1-XPF is an endonuclease complex required for nucleotide excision repair, interstrand crosslink repair and the repair of a subset of DNA double-strand breaks. Mutations that mediate reduced expression of this enzyme cause accelerated aging in humans and mice. Genetic depletion of DNA repair mechanisms does not increase the amount of damage incurred, it simply accelerates the pace at which damage triggers a demonstrable physiological impact, affording an opportunity to investigate the role of endogenous nuclear DNA damage in driving senescence.

Here, we demonstrate that Ercc1-/Δ mice accumulate oxidative DNA damage and senescent cells more rapidly than age-matched wild-type (WT) controls, yet comparable to WT mice over two years of age. Surprisingly, we found that Ercc1-/Δ mice are also under increased oxidative stress. Increased ROS production and decreased antioxidant buffering capacity contributed to the oxidative stress, which was also observed in aged WT mice. Treatment of Ercc1-/Δ mice with a mitochondrial-targeted radical scavenger (XJB-5-131) was sufficient to suppress oxidative DNA damage, senescence, and age-related pathologies. These data demonstrate that damage of the nuclear genome arising spontaneously in vivo is sufficient to drive cellular senescence. Our data also demonstrate that endogenous DNA damage, as a primary insult, is able to trigger increased reactive oxygen species (ROS) and further oxidative damage in vivo.

By definition, the primary insult in untreated Ercc1-/Δ mice is unrepaired endogenous DNA damage to the nuclear genome. Not surprisingly, the Ercc1-/Δ mice accumulate senescent cells more rapidly than WT mice. This formally demonstrates that physiologically-relevant types and levels of endogenous DNA damage are able to trigger the time-dependent accumulation of senescent cells. Chronic administration of XJB-5-131 significantly reduced both oxidative DNA damage and senescence. The reduced level of senescent cells corresponded to a reduction in age-related morbidity. The observation that suppressing oxidant production is sufficient to decreases senescence indicates that reactive species are required to ultimately cause or maintain senescence in response to genotoxic stress.

Comments

Hi, just a 2 cent.

The key word is redox imbalance, there is ambiguity about antioxidants because they could become toxic, prooxidant, or abrogate ROS emission; which means dangerous loss of signal (ROS signalling loss, such loss of nuclear errythroid response factor 2(NRF2)/Phase II Detox enzymes (GSHTransferase/Antioxidant Response Element (ARE)/(EpRE) Electrophile Response Element)). ROS are a double-edged sword because they oxidizing but signalling too, plus they are in balance
Red'ox (Red'uced/Ox'idized). In this case of oxidative stress, there is a redox Imbalance towards oxidized milieu (mV+, oxidized thiol pool, accumulation of oxidized thiols as disulphides, the cell is 'stressed' oxidatively and the mitochondrial ROS end up damaging mtDNA and nDNA. In this study, there is aging acceleration upon nuclear DNA damage, thus telomeric DNA damage (nuclear chromosome end termini DNA (telomeres)), just like in HGPS progeria where HGPS people lose 500 bp/year in their nuclear telomeric DNA vs regular people losing 50 bp/year (10 times faster aging in HGPS, these people live 10-15 years and die as teens, while regular people can reach 122 years old MLSP; demonstrating a linear correlation to speed of aging. Nuclear DNA has an impact, especially telomeric nuclear DNA since it links to epigenetics which control DNA epigenetic clock (through methyl/acetyl count in DNA CpG island decorum for gene silencing/activating. Genes that control the redox).

One study had demonstrated that mtDNA damage was far more pertinent to intrinsic aging and MLSP. The animals displayed mtDNA lesions with aging, and, especially base pair Deletions (there was direct correlation qnd causation of mtDNA lesions to mammal MLSP in several important organs, while the link was much less clear in nuclear DNA (because nDNA is much better repaired and protected, than mtDNA where it it the 'ROS weakest link site')). This in turn caused cell energy crisis over time (gradual ATP levels going down), the mitochondrias would fail to do correct OXPHOS and TCA cycle, bbecause of deleted mtDNA. In other words creation of mitochondrial errors and mutations during mitochondrial replicating. This would be manifest as increased mitochondrial ROS emission in state3/4 respiration and electron leakage. The mitochondrial Complexes I to V would diassemble or produce folds more ROS, faulty Complex with mutations would create ATP energy crisis and mtDNA Deletions (mtDNA wipe out)..
Rendering cell starved and to apoptosis. All this creates visible damage in the cell and junk accumulation to proteasome. And, on tissue level that means stem cells working double time to try rebuilding injury and damaged tissue. This means, acceleration of damage and Aging.

mtDNA damage is far more pertinent and important in regular aging, because the main limiting Element is gradual redox imbalance, mitochondrial deleterious events (accelerated aging mutator mice and HGPS human all show mtDNA deletions and, ROS elevation, Complexes failure). There is a link between what is happening in mitochondrias and nucleus. Both affect and communicate to each other. One controls genetic command and cell cycle, while the other cell energy and ROS signalling. Both are critical, but mtDNA is the main culprit (ROS caused through everysecond respiration oxidizing DNA molecules. mtDNA first) when health is maintained up to maximum lifespan.

Just a 2 cent.

Posted by: CANanonymity at May 14th, 2018 7:14 PM

There is this research. Then there is the research that is showing that the lysosomal aggregate problem is also based on bio-energetics (e.g. mitochondrial dysfunction). It is also likely that the cross-link issue is also the same.

It really does look like all roads lead to mitochondrial dysfunction as the leading cause of aging. The mitochondrial dysfunction in turn is caused by mtDNA damage of some sort.

Posted by: Abelard Lindsey at May 15th, 2018 9:51 AM

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