Space Mice Return to Earth, Mighty as Ever

In December 2019, a group of 40 mice boarded a SpaceX rocket to join the crew at the International Space Station. But these weren’t regular mice—among the group were mutant “mighty mice” with double the muscle mass of the average mouse. They traveled through space for 33 days as part of an experiment to better understand loss of muscle and bone mass that occurs in zero gravity conditions.

The findings from this study, published Monday in the journal Proceedings of the National Academy of Sciences, show that the “mighty mice” retained their muscle and bone mass throughout their time in space, reports Ashley Strickland for CNN.

The results show promise for developing a treatment for the millions of Earth-bound people living with debilitating medical conditions that can weaken their muscles and bones, such as cancer, brittle bone disease and muscular dystrophy. This will also help alleviate muscle and bone loss for astronauts in space, who can lose up to 20 percent of their muscle mass in less than two weeks, according to research from NASA.

The experiment was led by geneticist Se-Jin Lee and rare bone disease expert Emily L. Germain-Lee, both researchers at the University of Connecticut School of Medicine. They examined the role of two key proteins—myostatin and activin A—in regulating muscle and bone growth. In the 1990s, Lee genetically engineered mice to lack the myostatin gene. Without it to keep muscle growth in check, the mutant mice grew to be twice the size of normal mice and with a particularly buff physique, earning the name “mighty mice.”

During their time in space, the “mighty mice” remained healthy while regular lab mice in the control group lost up to 18 percent of their muscle and bone mass, reports Marcia Dunn for the Associated Press. Even after spending more than a month in space, the “mighty mice had similar body metrics to those that stayed behind at NASA’s Kennedy Space Center.

"Mice that were hypermuscular as a result of having a mutation in the myostatin gene were able to retain most, if not all, of that extra muscle during spaceflight," Lee and Germain-Lee tell CNN in an email.

Lee also used this spacefaring opportunity to test a drug with the potential to simultaneously strengthen both muscle and bone. A few years ago, Lee and Germain-Lee tested this drug on mice diagnosed with a version of brittle bone disease. The experiments were successful, but they wondered how the mice might in fare in space, they told Jon Hamilton’s NPR in January.

At the International Space Station, a few of the normal mice were treated with a drug designed to block myostatin and activin A, which work together to limit muscle growth. By blocking these pathways, the researchers hypothesized that they could induce muscle and bone growth. According to a press release, the mice treated with the drug, lean body weight, muscle mass and bone mineral density all increased, demonstrating that the drug could be used to successfully treat muscle and bone loss—on Earth, at least.

While Lee and Germain-Lee say these strategies show promising results, they are years away from testing on humans. “But that’s how everything is when you go from mouse to human studies,” Germain-Lee tells the AP.

Moving forward, the researchers will be fine-tuning the drug and trying to understand any possible side effects. But this study also revealed new questions and “an embarrassment of riches,” says Lee. He has plans to further this investigation, hopefully by sending more mice to space for a longer stay.

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Rasha Aridi is a science journalist based in Richmond, Virginia. She has written for Science magazine and Science News for Students. You can find her portfolio at rashaaridi.com.