A friend sent me a link to this paper in which researchers found a way to reduce the apparent age of human cells. This is encouraging, because I like to see advances in anti-aging research, but I'm skeptical. I'm not a biologist, so can someone who is (or has experience in some related field) tell me how much of a big deal this is?

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It might be a big deal. They used Yamanaka factors (chemical signals used to allow children to be young, rather than a thirty year old mother giving birth to a thirty year old child) to deage cells. That’s not new; deaging cells has been around for a few years. What is new is that they managed to deage some cells in a tissue, and then have them relearn to do their job from the untreated cells around them.
The thing about Yamanaka deaging is that if it’s done far enough, the cells in question become stem cells, and lose their specialization-they don’t know whether to be a brain or a nerve or what. Thus, there’s been a tradeoff between aggressive deaging (which can kill if you push it too far; an animal can’t usually survive as a blob of stem cells; also if the stem cells get confused they can cause cancers called teratomas) and insufficient deaging (a much more gentle Yamanaka regime has been shown to safely extend mouse lives, but it’s not powerful enough to stop age from eventually killing them). The breakthrough here is that a highly aggressive Yamanaka regime, powerful enough to heal thirty years of age, and powerful enough to cause stem cell reversion, can be used without leaving the stem cells confused and unintegrated: they become cells like those around them once more, and rejuvenated.

The optimistic view is that we could use this sort of thing to gradually refresh the entire body, only converting some cells at a time to ensure the others can guide them back to proper specialization. That could be true immortality.

The pessimistic view is that we won’t be able to deliver it reliably throughout the body, won’t be able to target which cells to revert so some won’t receive the treatment or too many are converted at once, that it might cause cancer eventually even though it seemed safe in this trial, that sometimes the respecialization process will fail, that you can’t turn parts of your brain to stem cells without losing important information and potentially dying if you do this often enough to defeat aging, or that other forms of aging damage will still kill you even if this works as advertised.

Realistically, in my opinion at least, this is probably either a way to reset the epigenetic clock in most tissues or very close to it. That’s fantastic news, but if we want true immortality from it, we probably still need to treat transposon propagation (genes that copy themselves in your genome, basically the genetic equivalent of cancer), possibly need to address system-level aging dynamics (are there aging markers between cells that wouldn’t be cured solely by healing the individual cells?), and need to see if this can be used on the brain safely.

transposon propagation

How is this prevented from being passed from a parent to a child? (Analogy to "Yamanaka factors (chemical signals used to allow children to be young, rather than a thirty year old mother giving birth to a thirty year old child)".)

Transposons are aggressively suppressed in the gonads, but not in the rest of the body. It is possible that this is even the reason for the epigenetic clock, that it evolved to kill the elderly so that they wouldn't take resources away from their less transposon-burdened offspring, as opposed to trying to live with transposons or spend the energy needed to protect the entire body from them.

Thanks for the solid response. I didn't realize that they were using surrounding cells to guide the new stem cells back into their original roles and I could see how that would limit the use of a technique like this.

What about in neurons/post-mitotic brain cells?

That's unclear yet. It's not obvious how to revert brain cells without losing important information, which is a potential way this treatment could fail. Hopefully there's a way to do it, or else the brain can survive getting turned into stem cells by parts, but that's a clear obstacle.
2Alex K. Chen4mo
My guess is that rejuvenation of the rest of the body [esp kidney/liver] will probably give the brain an extra 2 decades [but this is an imprecise estimate].

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