The decoy has another effect: It traps a second protein, called activin A, which also prevents growth of muscle as well as bone. Sure enough, space mice injected with the decoy receptor gained a double benefit: They kept their muscles buff and their bone density high. Bone and muscle even grew, so that the treated mice touched down with more of those tissues than untreated mice that were left on the ground.

The team also tested whether the treatments could reverse muscle wasting and bone loss that had already happened. To do this, they gave the decoy receptor to regular mice that had lost muscle and bone mass on the ISS, after theyd splashed down in the Pacific on January 7, 2020, and been taken to land in San Diego. These animals quickly recovered both bone and muscle; their muscles even grew bigger than theyd been before takeoff.

The results suggest that there could be a way to use a drug to protect astronauts from muscle and bone loss, says David Glass, vice president for research on aging and age-related disorders at Regeneron Pharmaceuticals in Tarrytown, New York. He cautioned, however, that space atrophy is different from Earth-based forms.

Nonetheless, Lees hope is that blockers for myostatin and activin A could preserve or restore muscle on the ground. Physiologist Christoph Handschin of the University of Basel in Switzerland agrees, but he adds a caveat: This looks super-promising if this can be translated into humans. Handschin, wholaid out the landscape of therapies for muscle wastingin theAnnual Review of Physiology, notes that several drug companies have tried to treat people with compounds that interfere with myostatin, without much success.

Lee is optimistic, though. He suspects that blocking myostatin in people is not sufficient, so he hopes that compounds like the decoy receptor that engage activin A might prove more effective than earlier compounds did. Glass says that to avoid side effects, it would be important to design a medication that is unlikely to interact with molecules other than myostatin and activin A, as the decoy can.

And while supporting human space travel isnt Lees main goal, such a molecule could, perhaps, catch NASAs interest as the agencyplans for longer space missionsthat will probably not have room for extensive gym equipment.

This article originally appeared in Knowable Magazine, an independent journalistic endeavor from Annual Reviews. Sign up for the newsletter.

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Rodents in space: Keeping bone and muscle strong on the ISS - Astronomy Magazine