An experimental drug has dramatically increased muscle and bone mass in mice that spent a month on the International Space Station. The hope is that a human version could help people who spend long periods in bed due to illnesses, as well as astronauts.
鈥淚t could help any condition, any prolonged illness, in which someone is bedridden,鈥� says Emily Germain-Lee at the University of Connecticut.
Maintaining muscle and bone is costly, so they lose mass when we don鈥檛 use them, whether due to spending time in bed or in microgravity. This effect is partly due to a protein called myostatin, which binds to receptors on the surface of muscle cells and blocks their growth.
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Many groups have been trying for decades to develop drugs that block myostatin鈥檚 effects, with little success. That may be because myostatin isn鈥檛 the only growth factor involved, says team member Se-Jin Lee at the Jackson Laboratory in Connecticut. Another protein, called activin A, binds to the same receptor and has similar effects.
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Disabling the myostatin gene in mice doubles muscle mass, says Lee, while disabling both the myostatin and activin A genes quadruples it.
The experimental drug is a modified form of the receptor that both proteins bind to. When it is injected into the bloodstream, myostatin and activin A bind to it instead of the real receptors, reducing the inhibitory effect.
鈥淚t acts as a decoy,鈥� says Lee. High doses produce a 50 per cent increase in muscle mass throughout the bodies of mice in just two weeks.
When mice were given a dose of the decoy receptor two days before being launched to the International Space Station, and then weekly while on the station, their muscle and bone mass increased by around 20 per cent, similar to ground-based mice given the same doses. In untreated mice, muscle mass fell around 10 per cent during the month in space.
Mice that weren鈥檛 treated while in space recovered their muscle and bone mass faster if given the drug on their return.
The fact that the drug boosts bone mass as well as muscle is crucial, says Germain-Lee, as both are needed to help bedridden people get back on their feet. 鈥淭his is why this is so exciting,鈥� she says. 鈥淢uscle and bone are so intertwined.鈥�
A company called Acceleron has already tested a human version of the decoy protein in people, but had to stop trials because of side effects including nose bleeds. Lee hopes it will be possible to modify the decoy to avoid these problems.
PNAS
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