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Monday, March 4, 2013

Can our neurons outlive us?

Different organisms have different lifespans. Under normal circumstances, the cells that make up an organism will not survive that creature’s death. Each cell type therefore has the same average lifespan as the rest of the body. But is this inevitable? In particular, could neurons live past their usual expiration date?

To test this, Italian researchers, led by Lorenzo Magrassi of University of Pavia, relied on the different lifespans of particular strains of mice and rats. Wistar rats can live for over three years, whereas E12 mice hang on for an average of 18 months and a maximum of 26 months. The researchers took neurons called Purkinje cells (PCs) from E12 mouse embryos and grafted them into Wistar rat embryos. PCs were chosen because, unlike other types of neurons, they are progressively lost as individuals age, though in different proportions for different species. By the time a mouse dies of old age it will have lost about 40% of its PCs. In contrast, a rat will have lost only 11%.

The introduced mouse neurons did quite well in their new homes, integrating themselves into the appropriate positions throughout the brain. They were recognizable, however, because they remained slightly smaller than their neighboring rat cells. In other words, the mouse PCs maintained their mouse characteristics.

Despite this, the mouse neurons survived for as long as their host rat brains did, up to twice as long as they would have done if they’d stayed in the mice. Perhaps more significantly, they did not decrease in number as the hybrid rats aged. The grafted PCs looked like mouse neurons, but they lived like rat neurons.

This could mean one of two things. Either the type of brain in which a PC neuron finds itself dictates its longevity, or the brain type makes no difference (within reason), other than being alive. In the former case, mouse neurons will never live longer than two years as long as they reside inside mice. However, in the latter case, if we could coax a mouse into living for three or more years, its PC cells would do just fine. If the same is true for humans, this could have implications for human longevity. After all, we won’t want to live longer and longer if our neurons have a strict shelf-life. If, on the other hand, our neurons can chug along happily for double our average lifespans, then that would make it well worth working to extend our lives.

Graphic above: a human Purkinje cell.

Magrassi, L., Leto, K., & Rossi, F. (2013). Lifespan of neurons is uncoupled from organismal lifespan Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1217505110.