海角大神

Compared with humans, its heart rate is low, its little lungs are sluggish, and its metabolism is downright pokey. Its skin, pink and hairless and rugous, sags like wet paper from its tubular frame.

The naked mole rat looks from birth to death as though it were always close to the latter. But this geriatric fa莽ade belies the fact that the naked mole rat can live to be upwards of 28-years-old, about eight times longer than a mouse, its rodent equivalent in size.

For that reason, the naked mole rat has been in recent years pushed out of its East African burrows and into the scientific limelight, in hopes that it could answer a question that at face value seems an unusual one to ask of an ugly rodent: What can this critter teach humans about the nature of longevity?

鈥淭he naked mole rat isn鈥檛 just something we study out of curiosity 鈥� it鈥檚 useful,鈥� says Vera Gorbunova, a researcher at the University of Rochester and a co-author on a new paper published in Proceedings of the National Academy of Sciences that plumbs the naked mole rat鈥檚 cells for the rodent鈥檚 well-kept secret to longer life.

That secret, the paper reports, might be in the rodent鈥檚 highly unusual ribosomes, the molecular, protein-producing machines found in the cells of living organisms.

But before tucking into the unusual nature of the naked mole rat鈥檚 cells, there are some more obvious reasons for its long life. The primary explanation is the naked mole rat's adaptive traits which keep it underground, out of the reach of predators that snap up mice and away from the myriad above-ground accidents that can befall other rodents.

If there is any lethal drama to a naked mole rat鈥檚 existence, it is the sporadic civil wars that roil its own social unit. The naked mole rat is highly unusual among mammals in that it is a eurosocial animal: it lives in large, stratified groups, much as ants and bees do. Usually, these groups are cooperative 鈥� until the group鈥檚 queen dies. Then, the female rodents will tussle for the role of sole breeder, a plum position that comes with a coterie of attendant males.

But once the new queen is installed, the colony settles back into its usual, slow rhythm. And naked mole rats that weren鈥檛 felled during the competition can, well, expect to live a very, very long time.

Still, researchers have suspected that the naked mole rat鈥檚 protected habitat is an inadequate explanation for its protracted life. Something more, they say, must be keeping the animal not just alive but also healthy well into its late twenties 鈥� long after even the most hardy mice have succumbed to old age.

In June, Gorbunova鈥檚 team reported in Nature that the naked mole rat produces an unusual compound that protects it from cancer (not a single instance of the disease has never been recorded in the rodent). During that research, as they peered into the rodent鈥檚 cells, the team came upon another surprise: the naked mole rat鈥檚 ribosomes are not structured like those of almost any other animal.

A ribosome is a cell鈥檚 protein synthesizer, and a ribosome is scaffolded with molecules called ribosomal RNA, or rRNA. In vertebrates, each ribosome has three rRNAs. But a mole rat鈥檚 ribosomes have four rRNA molecules, the researchers found. That鈥檚 because one of the rRNAs in a naked mole rat鈥檚 ribosome, the molecule called 28S, cleaves into two.

Just one other mammal, a distantly related, underground rodent called the tuco-tuco, is known to depart from the three rRNAs standard to have four such molecules, Gorbunova said. Little is known about this South American mammal鈥檚 lifespan, she said.

The naked mole rat鈥檚 curiously structured ribosome then raised another research question: Why? How does this unusual ribosomal architecture change how the ribosome produces proteins? And how might this relate to the naked mole rat鈥檚 longevity?

鈥淭his upsets the entire folding of the ribosome,鈥� says Gorbunova, of the extra rRNA molecule. 鈥淲e wanted to know what kind of effect that would have on protein production.鈥�

In all living organisms examined to date, ribosomes are fallible: sometimes, a ribosome will insert an incorrect amino acid into the protein chain it鈥檚 making. But the researchers found that naked mole rat鈥檚 cells manufacture proteins that are up to 40 times less likely to contain mistakes than are a mouse cell鈥檚 proteins. In other words, a naked mole rat鈥檚 ribosomes are unusually faithful copiers, as precise as a cloistered Medieval scribe.

And in ribosomes, that precision apparently has life or death stakes, since protein errors are highly correlated with aging, says Gorbunova. While a young body has little trouble recycling out junk protein, an older body does so much less efficiently, allowing the junk to accumulate. This junk then clogs the cell and manifests as various diseases, she says.

Understanding the mechanism behind the natural fidelity in a naked mole rat's ribosomes could be important for longevity researchers, says聽Gorbunova.

And that means there's more work to be done. It鈥檚 unclear whether or not the extra rRNA molecule is the reason for the mole rat鈥檚 superior protein making, says Gorbunova. To find out, the researchers plan to cleave a mouse鈥檚 28S rRNA molecule to see if the mouse鈥檚 cells also begin to produce less junky protein, she says.