Italian scientists at the universities of Pavia and Turin discovered in recent experiments that mouse neurons, when taken out of their short-lived mouse bodies and sustained elsewhere, can actually live much longer than their original bodies would have allowed. Bodies die because their cells die. But brains, their new study suggests, would last much longer if they didn’t depend on those same, pesky bodies to survive.
Most cells in the body reproduce themselves by replicating, and “age” by losing their ability to replicate—a phenomenon known as “replicative senescence.” As described in this Genes and Development article from 2010, replicative senescence, scientists believe, happens because cells do not fully reproduce their telomeres when they divide, whose job is to protect a cell's chromosomes from deteriorating. With each division, that telomere chain gets shorter and shorter until it reaches a “critical minimal length.”
(The classic comparison for telomeres are the plastic tips on shoelaces; telomeres act as the protective end of genetic material that shrinks as it's copied. When it's gone, the replication process begins to degrade genomic information.)
In that sense, authors of the G&D article write, it acts as a sort of “molecular clock.” When the telomere gets too short, it can no longer protect the cell’s chromosomes. The clock runs out, cells stop reproducing, and eventually they die.
The evil Krang, from Teenage Mutant Ninja Turtles: Giving
weakling nerds hope for the future since the late 1980s.
Image via Chicago Now
Not so with neurons, which do not reproduce by dividing, but are generated thanks to stem cells—a process known as neurogenesis (which activities from fasting to regular exercise have been shown to promote). Scientists didn't know about neurogenesis for a long time, which is partly why scientists for decades believed that we were stuck with the same number of brain cells for most of our lives—a concept so taken for granted that a lot of good science suggesting otherwise was mostly ignored until the 1990s.
Because they don’t divide, they also don’t have the same problems with senescence that other cells do. And cells that don’t undergo senescence—as in some plants, like the hydra—could potentially live forever under the right conditions (aging is complex process that’s still not fully understood, so the idea is still very theoretical).
The Italian scientists behind the new study, published by the Proceedings of the National Academy of Sciences, made an early step toward testing that theory by transplanting embryonic mouse brain cells into the brains of Wistar rats, which live significantly longer.
Would the mouse neurons die in accordance with the life-span of their original mouse bodies? Or might they last longer? Turns out they lasted much longer where they were grafted inside those rat heads—as long as 36 months, which was twice as long as the average lifespan of a mouse.
Mice aren’t perfect analogs to humans because certain of their genes behave differently from ours, even when those genes share a common ancestor (known as genetic orthologs). Comparative studies of inflammatory diseases, for example, have found as much. But comparing neurons between species tends to be especially robust because neuronal properties are more similar. It’s their number and the ways they’re arranged that make the biggest difference.
Richard Nixon, crushing resistance for all time in Futurama.
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They’re also quite manipulable—a bit like LEGOs that scientists can effectively piece together or (as evidenced in the Italian study) occasionally transpose from one species to another, or alter to do cool things like emit a fluorescent green spark whenever a synapse fires (see here and here).
In an interview last year with Motherboard’s Derek Mead, professor Kevin Warwick described an experiment in which he and a team of scientists created a very simple, two-dimensional, living brain of about 100,000 neurons and connected it to robots (a human brain, by contrast, approaches 100 billion). It’s just a matter of time, he suggests, before scientists can build one in 3-D that’s much bigger. He also thinks most of us will one day be cyborgs to some extent. For these and other reasons, futurists like Ray Kurzweil believe that immortality is only about 40 years away.
It’s all speculation until you see the neurons in action. And although we still aren’t testing human neurons in such aggressive ways, studied like this suggest the Futurama-style head-in-jar fantasy may not be so fantastic after all.
Lead image by Gaetan Lee, via National Geographic