MIGHTY MOUSE

By Christine Gorman

From the outside, the new strain of mice looked a little, well, lumpy. But when scientists peeled back their fur and skin, what had seemed like extra baggage in the shoulders and hips turned out to be pure muscle--two to three times the muscle mass of the average pip-squeak rodent. These were not your ordinary genetically engineered laboratory mice; these were Mighty Mice.

Dr. Se-Jin Lee and his colleagues at the Johns Hopkins School of Medicine didn't set out to create muscle-bound lab specimens. As reported in last week's Nature, they wanted to find out how a particular protein, a growth factor called myostatin, regulates the development of tissue. So they produced a strain of mice in which the gene that codes for myostatin had been deleted, or "knocked out." The resulting mutant animals grew up normal in every way--except for their extraordinarily well-developed musculature.

Why hasn't evolution produced more mice with rippling chests? "We're just starting to look into this," Lee explains. The burly mice seem to be a little slower and less timid than their normal counterparts. "That probably wouldn't be much of an advantage in the wild," says Lee.

It could prove to be an advantage to farmers, however, since chickens and cows make their own myostatin. In the future, artificially brawny beef cattle could be a profitable source of fat-free meat.

Humans make myostatin as well, and researchers speculate that a myostatin-blocking drug could one day add muscles to the frames of people wasting away from cancer or AIDS. A drug that could triple muscle mass might also find a market among body builders, but that's a long way off. Scientists today know only what myostatin does in mice, and they still haven't determined at what cost to the animals' health or longevity.

--By Christine Gorman