Young blood appears to boost the longevity of mice and gets stem cells to start dividing again in older mice

Young blood seems to turn back the effects of aging, potentially with few known safety concerns in humans and, so far, with corroborated results from parabiotic aging studies in multiple labs. But scientists and ethicists still worry about the treatment being tried in people outside approved clinical trials before evidence on its safety and effectiveness is in. Unlicensed stem-cell transplants are already a booming industry, warns Mattson, and unlicensed transfusion of young blood would be even easier.

For now, any claims that young blood or plasma will extend lifespan are false: the data are just not there. An experiment to test such claims would take upwards of six years — first waiting for the mice to age, then for them to die naturally, then analysing the data. “If we had funding to do this, I’d do it. But we don’t,” says Michael Conboy. Still, he adds, “I hope that someone, somewhere is.”

In the past few years, however, a small number of labs have revived parabiosis, especially in the field of ageing research. By joining the circulatory system of an old mouse to that of a young mouse, scientists have produced some remarkable results. In the heart, brain, muscles and almost every other tissue examined, the blood of young mice seems to bring new life to ageing organs, making old mice stronger, smarter and healthier. It even makes their fur shinier. Now these labs have begun to identify the components of young blood that are responsible for these changes. And last September, a clinical trial in California became the first to start testing the benefits of young blood in older people with Alzheimer’s disease.

“I think it is rejuvenation,” says Tony Wyss-Coray, a neurologist at Stanford University in California who founded a company that is running the trial. “We are restarting the ageing clock.”

Many of his colleagues are more cautious about making such claims. “We’re not de-ageing animals,” says Amy Wagers, a stem-cell researcher at Harvard University in Cambridge, Massachusetts, who has identified a muscle-rejuvenating factor in young mouse blood. Wagers argues that such factors are not turning old tissues into young ones, but are instead helping them to repair damage. “We’re restoring function to tissues.”

Clive McCay, a biochemist and gerontologist at Cornell University in Ithaca, New York, was the first to apply parabiosis to the study of ageing. In 1956, his team joined 69 pairs of rats, almost all of differing ages. The linked rats included a 1.5-month-old paired with a 16-month-old — the equivalent of pairing a 5-year-old human with a 47-year-old. It was not a pretty experiment. “If two rats are not adjusted to each other, one will chew the head of the other until it is destroyed,” the authors wrote in one description of their work4. And of the 69 pairs, 11 died from a mysterious condition termed parabiotic disease, which occurs approximately one to two weeks after partners are joined, and may be a form of tissue rejection.

Today, parabiosis is performed carefully to reduce animal discomfort and mortality.

More than 15 years later, in 1972, two researchers at the University of California studied the lifespans of old–young rat pairs. Older partners lived for four to five months longer than controls, suggesting for the first time that circulation of young blood might affect longevity.

Work recently

In more recent work, within five weeks of the start of tests, young blood restored muscle and liver cells in the older mice, notably by causing aged stem cells to start dividing again. The team also found that young blood resulted in enhanced growth of brain cells in old mice, although the work was left out of their 2005 paper describing the results. All in all, the results suggested that blood contains the elusive factor or factors that coordinate ageing in different tissues.

SOURCE – Nature