Why Do Old Brains Prefer Young Blood?

A paper published today in Nature finds that when younger mice are exposed to the blood of older mice, their brain cells behave more like those found in aging brains, and vice versa. The researchers who carried out the work also uncovered chemical signals in aged blood that can dampen the growth of new brain cells, suggesting that the decline in brain function with age could be caused in part by blood-borne factors rather than an intrinsic failure of brain cells. _TechnologyReview

Many things change in the human body as we age. Our cells lose their ability to repair incidental damage, and produce less and less energy for our ever-less efficient muscles. We produce lower levels of hormones which help us, and higher levels of chemicals that cause inflammation and cellular damage.

It has been found that young blood can reverse certain signs of aging in the circulatory systems of old mice. Now there is evidence that young blood can help rejuvenate old brains.

To arrive at the discovery, the researchers studied pairs of old and young mice that were literally joined at the hip. They used a technique called parabiosis, in which two mice are surgically joined together along the flank, which causes them to develop a shared circulatory system. The technique has been used to study the development of the blood system, and more recently has been used to investigate the effects of age by joining old and young mice.

Lead author Tony Wyss-Coray, a neuroscientist at Stanford University, says that five weeks after creating these May-December pairings, "we found striking effects both on the young and old brains." The young mice had a reduction in the production of new neurons (neurogenesis), an increase in brain inflammation, and less activity in synapses connecting neurons.

The older mice, in contrast, had an increase in new neurons, less inflammation, and greater activity at synapses. "You could almost call this a rejuvenation effect," Wyss-Coray says.

...To see whether the effect could influence behavior, they injected, in separate experiments, young mice with plasma from older mice and vice versa, and found that old plasma impaired the younger animals' ability to perform learning and memory tasks, whereas young plasma improved the abilities of older mice.

Blood cells from one mouse cannot travel into the brain of the other because of the blood-brain barrier, so the team concluded that free-floating molecules in the blood, capable of passing through, must be responsible for the effects. By comparing more than 60 chemokines—chemical messengers secreted by cells that circulate in the blood—the researchers identified several associated with the detrimental effect of old blood. Administering one of these chemicals, called CCL11, to young mice dampened neurogenesis and impaired learning and memory. CCL11 has been studied for its role in allergies and asthma, but it's not clear how it influences neurons. _TechnologyReview

Does this mean that those of us who wish to stay young will have to prey on our young like vampires, sucking their life's blood for our own sustenance? No. For we are learning how to take our old cells and make them young again, in vitro -- in the test tube. The goal is to do the same thing, only better, and in vivo.

Such cellular rejuvenation treatments are likely to excellent stopgap methods of anti-aging, with significant -- but limited -- effects. The lifespans we live will be lived as younger, more vital monkey-men. And that is worth a very great deal.

But if we wish to live significantly longer lives, at significantly higher levels of awareness, intellect, and invention, we will need to go deeper than cellular replacement and humoral replacement therapies of this type.