The evidence for other players came from experiments with mighty mice themselves. Because these mice don't have any myostatin, any effects of injecting the new agent would come from its effects on other proteins, explains Lee. After five injections over four weeks, mighty mice injected with the new agent had muscles 24 percent larger than their counterparts that didn't get the new agent.
"In some ways this was supposed to be a control experiment," says Lee. "We weren't really expecting to see an effect, let alone an effect that sizeable."
In other experiments with normal female mice, weekly injections of the new agent provided the biggest effect on muscle growth after just two weeks at the highest dose given (50 milligrams per kilogram mouse weight). Depending on the muscle group analyzed, the treated mice's muscles were bigger than untreated mice by 39 percent (the gastrocnemius [calf] muscle) to 61 percent (the triceps).
After just one week, mice given a fifth of that highest dose had muscles 16 percent to 25 percent bigger than untreated mice, depending on the muscle group analyzed, and mice treated with one injection a week for two, three or four weeks continued to gain muscle mass.
But although the new agent seems quite promising, its advantage in potency also requires extra caution. "We don't know what else the new agent is affecting or whether those effects will turn out to be entirely beneficial," says Lee.
Lee says they also are conducting experiments with the mice now to see whether the effect lasts after injections cease and whether it helps a mouse model of muscular dystrophy retain enough muscle strength to prolong life.