The unique way in which some squirrels burn almost no energy during hibernation without losing muscle mass, could have major consequences for space travel.

In the Netherlands, most squirrels stay awake all winter. But that’s the thirteen-striped ground squirrel (Ictidomys tridecemlineatus) too much of a good thing. At least six months a year, this ground squirrel goes into a deep hibernation, in which it burns almost no energy. You would therefore think that he loses a lot of muscle mass during this period. Yet the animal wakes up surprisingly fit after six months. How does he manage that? Researchers now think they know. And that just might be a trick for future space travelers.

hibernation

When the squirrels enter hibernation, they stop feeding and continue to live on their fat reserves until spring. Usually, this prolonged fasting and inactivity would significantly affect muscle mass and function. Yet the thirteen-lined ground squirrel, which is common in North America, does not suffer this disastrous fate.

Theory

An explanation for this remarkable phenomenon was first suggested in the 1980s. It was stated that hibernating squirrels perform a metabolic trick. In doing so, they recycle the nitrogen in urea – a waste product that is excreted in urine – and use it to keep their body tissues intact during hibernation. “During the process, the nitrogen atom present in urea is released by gut microbes,” explains Matthew Regan in an interview with Scientias.nl from. “The animal then uses this to build new proteins. Instead of the nitrogen being lost in urine, it is stored and put to good use. This is important during hibernation, because the animal does not eat and thus has no nutritional source of nitrogen, which is a limiting factor in its ability to build new tissue protein.”

Confirmed

In a new study researchers put it to the test. After all, does this metabolic trick really underlie the ground squirrel’s ability to wake up fit and vital from hibernation? The team designed a series of techniques and experiments to explore key steps in the proposed theory. And guess what? “The storage of nitrogen from urea does indeed ensure that the animals come out of hibernation in good shape,” says Regan. And in doing so, the researchers were the first to definitively confirm the long-held theory.

Astronauts

How could this discovery be useful in space? Theoretically, Regan argues, by helping astronauts deal with muscle loss problems. It is known that space travel seriously affects the muscles of astronauts. This has to do with gravity – or rather, a lack of it. Here on Earth, our muscles have to work hard to compete against gravity. But in space – where gravity is very limited – this is not necessary. The result: the muscles relax. Weight-bearing muscles have to endure the most, such as the plaice muscle in the lower leg. “So the microgravity of space reduces the load on the muscles and then also suppresses the rate at which new proteins synthesize,” explains Regan. “As a result, astronauts’ muscles shrink. This is why when they are in space they exercise for a large part of the day, minimizing this ‘muscle atrophy’.”

Far space travel

During the fairly short stays on the ISS, where there is room for fitness equipment, this is not much of a problem. But what if in the future we undertake manned space missions to Mars, or to exotic planets even more distant? It remains to be seen whether an elliptical cross trainer will fit on board at all. And of course the last thing you want as a Mars colonist is to sink through your legs and break a leg during that first walk on Mars.

Solution

But Regan has come up with something for that. “In theory, it should be possible to apply the same process that squirrels use during their winter nap to minimize the rate of muscle loss to astronauts,” he says. “Research from the 1990s has provided some evidence that humans are able to recycle small amounts of urea nitrogen using the same trick. This suggests that the necessary mechanisms are in place. It just needs to be optimized.” However, according to Regan, some restraint is in order. “It’s important to note that this potential application is only theoretical at this point,” he underlines. “There is still a lot of work to be done before this naturally evolved mechanism during squirrel hibernation can be safely and effectively translated to humans, especially humans aboard spacecraft.”

Either way, the study sheds light on the process some hibernating mammals use to cope with the challenge of nitrogen deficiency and tissue or organ mass decline. Important knowledge, which is not only applicable to future space travelers, but which could also have important consequences for medical treatments on Earth. Hundreds of millions of people worldwide experience muscle loss due to old age, but also due to various conditions, such as malnutrition and cancer. “These people could benefit greatly from enhanced tissue protein synthesis capabilities,” Regan said. “I therefore suspect that only after the method has been safely and reliably applied on Earth, will it be looked at how a comparable technique can be implemented in the challenging environment of the universe.”