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Reviewing the Mitochondrial Contribution to Aging and Age-Related Disease

Today I'll point out a fairly readable review paper that walks through the high points of what is known of the mitochondrial contribution to degenerative aging and the common, well-studied age-related diseases that cause the greatest amounts of suffering and death. Every cell has a few hundred mitochondria swarming inside it, evolved descendants of ancient symbiotic bacteria that are now fully integrated components of the cell. They are highly active components: they replicate and fuse, pass molecular machinery between one another, are destroyed by cellular quality control mechanisms when they become damaged, and can even transfer between cells, all conducted at a rapid pace. Most of their DNA has moved into the cell nucleus, but a small number of genes remain to form the circular mitochondrial DNA. Mitochondria are primarily responsible for generating chemical energy stores, providing the power for cellular operations, but they also participate in many other fundamental cellular processes in one way or another.

There are two ways we might think of mitochondria in the context of aging. The first is the SENS view of the mitochondrial contribution to aging. The mitochondrial DNA becomes damaged, either through replication or because building energy store molecules is a process that generates potentially damaging, reactive molecules as a side-effect. Sometimes that damage cuts out an important part of the energy generation machinery, creating a mitochondrion that both runs hot, producing many more harmful molecules, but is also more competitive than its peers when it comes to replication within the cell. Perhaps it can evade quality control, perhaps it replicates more rapidly; whatever the cause, whenever this rare form of damage occurs, the descendants of the damaged mitochondrion very quickly take over the entire population within that cell.

The result is a pathological cell that churns out harmful reactive molecules in large amounts into the surrounding tissue. This can, for example, cause atherosclerosis through oxidative damage of lipids that end up in the bloodstream. There the damaged molecules irritate blood vessel walls, resulting in the lesions that will become atherosclerotic plaques and eventually rupture. This could be avoided via any reliably means of sabotaging this chain of events. The proposed SENS Research Foundation approach is to use gene therapy to copy mitochondrial DNA into the cell nucleus to provide a backup supply of protein machinery; if carried out, then it won't matter how ragged the mitochondrial DNA becomes. The mitochondria will still function correctly, and cells will remain unharmed.

The second way to think of mitochondrial in aging is given far more attention in the scientific mainstream. It is a sort of general malaise found in all cells in aged tissue, in which mitochondrial dynamics are altered, the size of mitochondria changes, and their ability to generate energy stores falters. The processes of cellular quality control responsible for destroying problematic mitochondria start to fail as well. This is well studied by researchers who specialize in neurodegenerative diseases, as the brain requires a great deal of energy to function, and lack of that energy is a real problem. Why does this happen? That remains a question; which of the forms of damage that drive aging lead to this reaction, and what exactly is the chain of cause and effect? Researchers are making some inroads in tinkering with this mitochondrial malaise, speeding it up and slowing it down somewhat, but the roots remain obscure.

The Mitochondrial Basis of Aging and Age-Related Disorders

Mitochondrial dysfunction is linked to various aspects of aging including impaired oxidative phosphorylation (OXPHOS) activity, increased oxidative damage, decline in mitochondrial quality control, reduced activity of metabolic enzymes, as well as changes in mitochondrial morphology, dynamics, and biogenesis. Mitochondrial dysfunction is also implicated in numerous age-related pathologies including neurodegenerative and cardiovascular disorders, diabetes, obesity, and cancer.

The role of mitochondria in aging was first proposed more than 40 years ago in the free radical theory of aging, suggesting that accumulation of cellular damage with increasing age results from reactive oxygen species (ROS) and mitochondria are one of the most important sources and targets of ROS that could function as an 'aging clock'. Since then, a growing body of evidence has shown that mitochondrial dysfunction contributes to aging in multiple model organisms and that several factors cause increased mitochondrial dysfunction with chronological age including accumulation of somatic mtDNA mutations, enhanced oxidative damage, decreased abundance and quality of mitochondria, as well as dysregulation of mitochondrial dynamics.

Mitochondria are unique as they harbor their own genome (mtDNA). Point mutations and deletions are the two most frequent types of mutations that arise in mtDNA genome with age mainly due to spontaneous errors during mtDNA replication or damage repair. A wealth of supportive evidence demonstrates that mitochondrial dysfunction occurs with age due to accumulation of mtDNA mutations; however, the causative role of mtDNA mutations in aging remains controversial. Various mtDNA point mutations have been shown to significantly increase with age in the human brain, heart, skeletal muscles and liver tissues. Increased frequency of mtDNA deletions/insertions have also been reported with increasing age in both animal models and humans. The strongest evidence to date that favors a causative role of mtDNA mutations in aging comes from the study of mtDNA mutator mice that exhibit significant accumulation of mtDNA mutations as well as a premature or accelerated aging phenotype.

Mitochondria are highly dynamic structures as they continuously undergo fission and fusion processes that shape their morphology and regulate mitochondrial size, number and function. Mitochondrial dynamics is essential for mitochondrial viability and response to changes in cellular bioenergetic status. Mitochondrial fission is vital for mitotic segregation of mitochondria to daughter cells, distribution of mitochondria to subcellular locations, and mitophagy. Unopposed fission leads to mitochondrial fragmentation, loss of OXPHOS function, mtDNA depletion and ROS production, which are associated with metabolic dysfunction or disease. Mitochondrial fusion is essential for maintaining mitochondrial membrane potential, ATP production, and maximal respiratory capacity. Unopposed fusion generates a network of hyperfused mitochondria associated with increased ATP production, reduced ROS generation and which exhibit an ability to counteract metabolic insults, protect against autophagy as well as apoptosis.

In the past decade, several studies have shown that mitochondrial dynamics plays a crucial role in the regulation of mitochondrial function and metabolism. Studies suggest that dysregulation of mitochondrial dynamics could contribute to aging and age-related pathologies. However, there are several outstanding questions that yet remain to be addressed regarding the link between mitochondrial dynamics and aging. For example, which factors cause altered expression of mitochondrial fission and fusion proteins during aging, and are these factors genetic or affected by environmental stimuli? Is altered mitochondrial dynamics a major cause of mitochondrial dysfunction in aged cells or tissues? Can proteins involved in mitochondrial dynamics serve as promising candidates for promoting healthy aging and/or alleviating various age-related pathologies? Future experimental studies that are designed to address these questions would help to better understand the role of mitochondrial dynamics in aging and age-related pathologies.

Comments

There is so much we can do already to help our mitochondrial function and slow their aging. We can take PQQ to get new copies of mitochondria to replace the senescent ones. We can take NAD+ to provide the extra energy needed in the correct chemical form for operation of the mitochondria. We can reduce of body temp by caloric restriction, and thus reduce ROS and slow aging of the mitochondria. I am homozygous for UCP3 uncoupling protein, that gives mitochondria a survival advantage by getting rid of excess heat, thus reducing ROS damage to the mitochondria. We can take any number of antioxidants that have shown to prolong the lifespan of mitochondria, such as Acetyl L carnitine, alpha lipoic acid, vitamin C, astraxanthin, etc. We should get an extra 5-10 years of lifespan by implementing as many of these modern improvements as possible, as well as living a relatively stress-free life and getting enough rest.

Posted by: Biotechy at December 22nd, 2017 1:22 PM

@Biotechy: I doubt that anyone is going to get ten years out of any combination of the items you mention above.

It seems unlikely that calorie restriction will do better than a five year effect, which is a little larger than median results for regular aerobic exercise, and none of the others show any signs of even small effects in human life span.

Dietary antioxidants are shown to be harmful to health, and have no beneficial effect on life span in mammals.

Posted by: Reason at December 22nd, 2017 1:34 PM

Not everyone is aware of the longevity benefits of some of the substances I mentioned above so here are some references. PQQ: Palikaras, 2015, Interfacing mitochondrial biogenesis and elimination to enhance host pathogen defenses and longevity. NAD+: Zhang, 2016, NAD+ repletion improves mitochondrial and stem cell function and enhances lifespan in mice. In terms of antioxidant benefits to longevity, you may have enough internal antioxidant pathway defenses until you are middle-age, but your mitochondria will suffer if you don't start adding some ROS protection as you start undergoing some serious aging as the elderly certainly do. And yes, as you say, regular exercise is going to benefit your lifespan, but you are not going to do much of it WITHOUT healthy mitochondria. I might add, you should keep a healthy BMI of about 21-22 or somewhat less if you are doing serious CR, all so as not to unduly stress your mitochondria and thereby increase their turnover and aging rates.

Posted by: Biotechy at December 23rd, 2017 5:24 AM

I think there are two ways in which mitochondria contribute to aging. Very broadly speaking they are,

1. ROS production over a lifetime. This is the more important one as it is one of the main regulators of max lifespan. Higher ROS leads to faster telomere attrition, cell arrest through non telomeric DNA damage and hence more senescence through this route, and greater oxidation of proteins hence more lipofuscin etc. Number 1. is mainly about the damage to other parts of the cell.

2. Dysfunctional mitochondria. This is mainly about mitochondria damaging themselves via ROS damaging mitochondrial DNA. Despite the lack of protection provided to mitochondrial DNA they seem to be able to cope with this by fusing to cope with deletions, and using fission to eliminate individual faulty mitochondrion. This mechanism seems to decline with age however, for unknown reasons (although it can certainly be made worse through poor diet and lack of exercise). However it still takes a long time for mitochondria to get so bad the cell starts starving for energy. So we only seem to see this rarely, such as in Parkinson's when >60% of mtDNA is lost. This seems to be caused by faulty mitophagy; I've never seen any evidence of the clonal expansion talked about by SENS.

There may be other mechanisms such as the inflammatory effect of floating bits of mitochondrial DNA but I'm not too sure how important this is or if it really happens.

I think it is perfectly plausible we could eliminate number 2. Maybe even through drugs/supplememts/lifestyle and keep healthy mitochondria all our lives. But although that would keep your energy levels high, you'd still age because what really matters is mechanism number 1., which is caused by the lifelong toll of ROS causing cellular senescence. That is the main reason why CR works, i believe, because ROS is reduced on a permanent basis. Even with allotopic expression you probably wouldn't make much more of a dent in 1. because even well functioning mitochondria produce ROS.

Posted by: Mark at December 23rd, 2017 12:18 PM

A few additional supplements beneficial to mitochondria that may help extend your lifespan include COQ10 and shilajet, that act synergistically to aide the function of mitochondria, and pomegranate through the action of urolithin A which helps rejuvenate mitochondria.

Posted by: Biotechy at December 25th, 2017 2:38 PM

As I understand it, mitophagy by lysosome consumption is enabled by pink and parkin, which are attracted when mitochondrial membrane potential is lowest during a fissioned state. Perhaps pink and parkin attract lysosomes. If fission can be induced in a controlled manner, NAM does this and also cinnamon upregulates pink and parkin, apigenin does this as well. Then if other conditions, a lack of stearic acid or perhaps a fasted state, draining ATP via exercise, etc, if all these conditions are met, then perhaps a greater amount of damaged mitochondria can be exposed and consumed by lysosomes. Then subsequently fused in the following period via anything that promotes fusion. Fat consumption, sulforaphane, etc. Maybe start adding in anything that promotes biogenesis at that point such as PQQ or cold shock therapy.

Also I don't think it's fair to throw blanket generalizations out there regarding antioxidants. Yes they can cause harm, they interfere with the stress response during exercise and too much too often can certainly be a bad thing. That doesn't mean they're not useful in a well timed and conditional manner. We shouldn't throw the baby out with the bathwater here but rather focus on conditions when antioxidants are most needed or beneficial. They're highly useful in reducing lipid peroxidation, for example.

Posted by: Nathan at February 22nd, 2018 12:31 PM

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