Mitochondrial dysfunction is one of the root causes of aging as described in the Hallmarks of Aging . As they age, mitochondria lose their ability to provide cellular energy and release reactive oxygen species that harm cells.
What are mitochondria?
Mitochondria, which are often called the powerhouses of cells, act like miniature factories, converting the food we eat into usable energy in the form of a chemical called adenosine triphosphate (ATP) . ATP provides energy to fuel a myriad of cellular processes, such as muscle contraction, nerve impulse propagation, and protein synthesis. ATP is common to all forms of life and is often referred to as the “molecular unit of currency” of intracellular energy transfer.
Interestingly, mitochondria did not originate as part of multicellular life; they are stowaways in our cells and have their own unique DNA, which is separate from our own. It is widely thought that they merged with a very early ancestor of all multicellular life to form a symbiotic relationship . Mitochondria become dysfunctional as we age and are host to their own separate (though similar) forms of damage.
How do mitochondria become dysfunctional?
As we age, our mitochondria go through changes that harm their ability to provide us with chemical energy while causing the release of harmful reactive oxygen species , which can cause DNA mutations leading to cancer [5-6] and even harm proteostasis . Reactive oxygen species also drive muscle weakness , a further smoldering level of background inflammation (inflammaging) , and the associated bone frailty , senescent cell load  and immune suppression  of old age. Mitochondria from elderly people even look different ; they swell while their numbers dwindle, unable to replace themselves as quickly in their dysfunctional state [14-15].
These problems aren’t all that reactive oxygen species can cause, however; they can also cause mutations in mitochondrial DNA.  While some studies suggest that this damage is not done directly , it must be remembered that reactive oxygen species can damage the very proteins that would control the reproduction of mitochondria and introduce additional errors into the copies by extension .
While most of these issues are detected by quality-control mechanisms in the cell , causing damaged mitochondria to be destroyed through a process called mitophagy, these systems become less and less effective with age, decreasing in activity and eventually allowing errors to slip through. In some cases, this isn’t so bad; each cell contains a large number of mitochondria, so ten or even a few hundred being mutated isn’t a problem. However, some of these errors can make the dysfunctional mitochondria survive longer than healthy mitochondria. In this way, some types of dysfunctional mitochondria build up and eventually become more common than healthy ones .
Additionally, as aging progresses, NAD+ levels in human cells decrease, causing a breakdown in communication between the human nucleus and mitochondrial DNA, again leading to decreased energy production and increased reactive oxygen species production .
How could we prevent or reverse this?
A number of methods have been proposed for preventing this. First of all, the issues with NAD+ could be solved by some method of NAD+ supplementation, slowing down the accumulation of this damage. In addition, the most vital parts of the mitochondrial DNA could be moved to another part of the cell – the nucleus – giving it access to better DNA repair mechanisms and keeping it away from the source of reactive oxygen species . This approach has been demonstrated for some of this vital code . Of particular note, the study proving that this is possible was funded on our crowdfunding platform Lifespan.io!
Mitochondrial dysfunction is an important part of the aging process. These miniature chemical engines, while capable of self-replication, gradually become more dysfunctional with age through a variety of mechanisms, causing harm to our cells and encouraging more dysfunction in a vicious cycle. Quality control mechanisms hold this at bay for a time, but they eventually fail, leading to multiple diseases of aging and a long-lasting, chronic background level of inflammation called inflammaging.
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