STANFORD, Calif. - Any older person can attest that aging muscles don't heal like young ones. But it turns out that's not the muscle's fault. A study in the Feb. 17 issue of Nature shows that it's old blood that keeps the muscles down.
The study, led by Thomas Rando, MD, PhD, associate professor of neurology and neurological sciences at the Stanford University School of Medicine, built on previous work showing that old muscles have the capacity to repair themselves but fail to do so. Rando and his group studied specialized cells called satellite cells, the muscle stem cells, that dot muscle tissue. These normally lie dormant but come to the rescue in response to damaged muscle-at least they do in young mice and humans.
In older mice the satellite cells hold the same position, but are deaf to the muscle's cry for help. In the Nature study, Rando and his group first attached old mice to their younger lab-mates in a way that caused the two mice to share a blood supply. They then induced muscle damage only in the older mice. Bathed in the presence of younger blood, the old muscles healed normally. In contrast, when old mice were connected to other old mice they healed slowly.
In similar work, the group examined the livers of older mice connected to younger lab-mates. The cells that help liver tissue regenerate are less active in older animals, but again the cells responded more robustly when the livers in older mice were bathed in the younger blood. Clearly, something in the youthful blood revived the regenerative cells in muscle and liver.
Of course another possibility is that something in the aged blood is suppressing stem cells and repair mechanisms. Does their work rule out that possibility? I don't see that it does. But I haven't read the original paper.
It would be interesting to know how the effect of the young versus old blood scaled as they were blended in different ratios. For example, does one quarter young blood mixed with three quarters old blood have a quarter the effect of pure young blood or more or less than a quarter of the effect?
There is a potential bright side to this report: If blood could be made young again then possibly cells thoroughout the body in many tissue types would act young again.
"We need to consider the possibility that the niche in which stem cells sit is as important in terms of stem cell aging as the cells themselves," said Rando, who is also an investigator at the Veterans Affairs Palo Alto Health Care System. It could be the chemical soup surrounding the cells, not the cells themselves, that's at fault in aging.
One clue to what might be going on also comes from previous work. Rando had found that satellite cells in younger muscles begin producing a protein dubbed Delta in response to muscle damage. Older muscles maintained the same pre-injury levels of Delta even after muscle damage. However, in the current study he found that satellite cells in elderly mice joined to younger partners ramped up Delta production to youthful levels after an injury.
However, there is a less optimistic interpretation to this result: The body may have evolved to produce stem cell growth suppressor compounds as the body ages in order to suppress cell divisions that could produce cancer cells. So blood that causes old stem cells to grow and repair tissue more vigorously might increase the risk of cancer. My guess is young blood would do that to older people.
The young blood effect was confirmed using cells grown in culture.
The group confirmed their results by putting satellite cells from old and young mice in a lab dish with either old or young blood serum. Old satellite cells in old serum and young satellite cells in young serum both behaved as expected. But when old satellite cells were bathed in young serum they cranked up their production of Delta and began dividing. Likewise, young satellite cells decreased the amount of Delta they produced when in a dish with older serum and divided less frequently.
Rando said that it may be a general phenomenon that a person's inability to repair tissues with age-whether it's muscle, liver, skin or brain-is a matter of the regenerative cell's environment rather than the cells themselves.
Rando said that finding the youth-promoting factors in the blood is no small task. "It's as big a fishing expedition as you can possibly imagine," he said. With thousands of proteins, lipids, sugars and other small molecules in the blood serum, deciding where to look first would be tantamount to a roll of the dice. What's more, there's no evidence that the same blood component is responsible for reviving the different types of cells.
"Another approach is to pick factors that are good candidates and see if any of them or some combination recapitulate the effect of the younger blood," Rando said. His group is now looking for likely targets. He said that for some degenerative diseases such as Alzheimer's or muscular dystrophy, such blood-borne factors may be able to reactivate the regenerative cell's ability to repair tissue that has been damaged.
This is an important report. But I repeat my caution above: If the presence or absence of some compound(s) in the blood is reducing the repair ability of a variety of tissue types (and it seems likely other tissue types will also be found to be affected by young versus old blood) then there is a decent chance that this reduction in repair ability was selected for to achieve some benefit, most likely a reduction in cancer risk.
Having stated the caution the ability to turn up repair capabilities could still be therapeutically useful for people who have dire needs for repair of some organ or tissue type. For example, turning up repair temporarily after major surgery or an accident could be worth the increased risk of cancer in some cases.
Suppose that changes in levels (either increases in suppressor molecules or decreases in cell growth stimulating molecules) of one or more compounds in the blood as we age happens in order to reduce the risk of cancer. Well, this is problematic for hopes to derive maximal benefits from replacing aged stem cell reservoirs with youthful stem cells. The old stem cells could be replaced with younger cells. There'd be immediate gains from lowered risk of cancer and relative improvements in the vigor and health of adult stem cells. So that is still worth doing. Yet the young replacement stem cells would still be restrained by levels of compounds in the aged blood. Here's the problem: If some but not all stem cell reservoirs have their stem cells replaced with younger stem cells it might not be safe to change the blood to make it more like young blood. It might be necessary to rejuvenate all stem cell reservoirs before the blood can safely be made more like young blood.
Here is an analogy: Imagine you have a car. It is old and it has 4 bad axles that will fall off if the car is driven too fast as well as a steering column that will fall apart at high speeds. Suppose you know how to replace the 4 axles but not the steering column. Well, if you replace only the 4 axles you still can't safely drive your car at high speeds. But with humans this problem is even tougher because there are many stem cell reservoirs located near every muscle and organ that would need to be rejuvenated before they could all have their level of stimulation by the blood safely raised to youthful levels.
Once really effective anti-cancer treatments (even treatments that kill all precancerous cells) are developed then most (all?) safety worries from making blood young again would go away. Any cancers that popped up in response to having youthful and growth-stimulating blood could quickly be slain or they could be slain even before the blood was rejuvenated. So great cancer-slaying treatments would make rejuvenation treatments easier to implement.
|Share |||Randall Parker, 2005 February 16 09:34 PM Aging Reversal|