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LifeXtenShow โ€“ Misunderstood Mitochondria

Reactive oxygen species may not be as important as previously thought.

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Misunderstood MitochondriaMisunderstood Mitochondria

There’s a scientific debate over what damaged mitochondria do to cause premature aging; in this episode of X10, we explore both sides of an interesting and intricate issue.

Script

Our current understanding of biological aging is that itโ€™s the result of different factors that interact with each other in complex ways. Malfunctioning mitochondria – which you can think of as tiny batteries that power your cells – are thought to be one of these factors and this idea has been around for a very long time. But what if mitochondria themselves arenโ€™t really the culprits

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Letโ€™s start with the basics. Mitochondria are organelles โ€“ basically, just parts of your cells โ€“ and the important thing about them for this video is that they have their own DNA. So thereโ€™s nuclear DNA in the nucleus and mitochondrial DNA in the mitochondria, and they donโ€™t necessarily replicate at the same time or speed โ€“ each replicates by itself.

Mitochondrial DNA replication is at the core of this story. See, thereโ€™s an idea that accumulating mutations in mitochondrial DNA are one of the causes of aging. You can find out more about that in our video on mitochondrial dysfunction as a hallmark of aging.

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Scientists have made mutator mice in which the enzyme that copies mitochondrial DNA doesnโ€™t proofread the copy itโ€™s making. This means that their mitochondrial DNA accumulates mutations,and, in keeping with the theory, these mice age prematurely. But a team of researchers in Finland, Sweden, and the US wanted to understand how mitochondrial mutations cause premature aging.

The theory is that the increase in mutations causes problems in the mitochondria, leading to increased production of reactive oxygen species, which cause damage. But the researchers noted that mice that have other kinds of mitochondrial malfunctions or mutations donโ€™t age prematurely. On top of that, there arenโ€™t increased levels of reactive oxygen species in the mature tissues of mutator mice.

That led them to think that reactive oxygen species arenโ€™t the link between mitochondrial mutations and aging, so they set out to find out whatโ€™s really going on. When they dug into whatโ€™s happening in the cells of mutator mice, they discovered that nuclear DNA replication was stalling for some reason, and there were also signs of increased DNA damage.

Itโ€™s not exactly obvious why a lack of proofreading in mitochondrial DNA replication would cause trouble with nuclear DNA replication. The explanation they came up with is actually pretty cool.

Mitochondrial and nuclear DNA copy themselves using building blocks known as dNTPs, and both processes draw on a shared dNTP pool. The researchers found that the common dNTP pool was drawn down in mutator mouse cells. At the same time, the mitochondrial dNTP pool was enriched. In other words, it seems dNTPs are being pulled from the common pool into the mitochondria.

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So, the idea is that increased mutations in mitochondrial DNA lead to faster turnover of mitochondrial DNA, which shifts the allocation of dNTPs to favour mitochondria. This means there arenโ€™t enough dNTPs in the common pool when nuclear DNA replication happens, causing it to stall and leading to DNA damage.

In a nutshell, mitochondrial DNA mutations donโ€™t directly cause aging. Instead, they create an imbalance that leads to nuclear DNA damage, which causes aging, but the story doesnโ€™t end there!

A response to this study appeared alongside it. A team of researchers in Sweden used a different technique to measure dNTP pools in mutator mice and found that they were normal. They also point out problems with the method used in the original study and conclude that โ€œchanges in dNTP pools are therefore unlikely to explain the premature-ageing phenotype.โ€

But in a response to the response, the original team defended their case by pointing out that the new measurements were taken using whole embryos of mutator mice. They argue that this isnโ€™t a fair comparison because โ€œit is known that dNTP pools are tightly regulated in a cell-type-, developmental-stage- and cell-cycle-dependent fashion.โ€ In other words, variation between different cell types in the embryo would hide the difference. They say that a fair comparison would use specific cell populations, and they stick by their original conclusions.

Whoโ€™s right? We donโ€™t know, and we wonโ€™t know until thereโ€™s more research. That process is the part of the beauty of science, but it also means learning to reserve judgment. In the meantime, it can be rewarding to discuss these ideas and think about their implications. You can find links to the study, the response, and the response to the response in the description.

Let us know what you think in the comments, and if you’d like to keep up with X10, donโ€™t forget to subscribe. And thanks again to all the Lifespan Heroes, whose contributions make our work possible and help Lifespan support longevity research. If youโ€™d like to join, go to lifespan.io/hero and make a pledge.

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