New study argues against young blood reversing aging in old mice

A new study indicates that tissue health and repair dramatically decline in young mice when half of their blood is replaced with blood from old mice, and old mice show either slight or no significant improvements after receiving half of their blood from young ones.

The study by Irina Conboy, associate professor in the Department of Bioengineering at the University of California, Berkeley, and her colleagues argues against the rejuvenating properties of young blood and points to old blood, or molecules within, as driving the aging process.

"Our study suggests that young blood by itself will not work as effective medicine," said Conboy, whose latest work was described in a paper published in the journal Nature Communications on Tuesday. "It's more accurate to say that there are inhibitors in old blood that we need to target to reverse aging."

Conboy and colleagues reported in the journal Nature in 2005 about evidence for tissue rejuvenation in older mice when they are surgically joined to younger mice so that blood is exchanged between the two.

Despite remaining questions about the mechanism underlying the rejuvenation, media coverage of the study fixated on the potential of young blood to reverse the aging process, Conboy said.

"What we showed in 2005 was evidence that aging is reversible and is not set in stone," she was quoted as saying in a news release from UC Berkeley. "Under no circumstances were we saying that infusions of young blood into elderly is medicine."

While the experimental model used in the 2005 study found evidence that some aspects of aging may be reversed, the techniques used then do not allow researchers to precisely control the exchange of blood, which is necessary to dig deeper into blood's effect on aging.

When two mice are sutured together, a technique called parabiosis, blood is not the only thing that is exchanged; organs are also shared, so old mice get access to younger lungs, thymus-immune system, heart, liver and kidneys.

In the new study, the researchers developed an experimental technique to exchange blood between mice without joining them so that they can control blood circulation and conduct precise measurements on how old mice respond to young blood, and vice versa.

In the new system, mice are connected and disconnected at will, removing the influence of shared organs or of any adaptation to being joined, and the onset of the effects of blood on the health and repair of multiple tissues, including muscle, liver and brain, were seen around 24 hours after exchange.

In repeating the experiments from 2005, blood was exchanged between an old mouse and a young mouse until each mouse had half its blood from the other.

The researchers then tested various indicators of aging in each mouse, such as liver cell growth as well as liver fibrosis and adiposity (fat), brain cell development in the region that is needed for learning and memory, muscle strength and muscle tissue repair.

In many of these experiments, older mice that received younger blood saw either slight or no significant improvements compared to old mice with old blood, and young mice that received older blood, however, saw large declines in most of these tissues or organs.

In addition, about blood's impact on new neuron production in the area of the brain where memory and learning are formed, older mice showed no significant improvement in brain neuron stem cells after receiving younger blood, but younger mice that received older blood saw a more than twofold drop in brain cell development compared to normal young mice.

The researchers think that many benefits seen in old mice after receiving young blood might be due to the young blood diluting the concentration of inhibitors in the old blood.

"Under no circumstances did young blood improve brain neurogenesis in our experiments," Conboy noted. "Old blood appears to have inhibitors of brain cell health and growth, which we need to identify and remove if we want to improve memory."

The team has begun to investigate specific molecules in old blood that might cause inhibition of cell development. Endit