In the current issue of the Proceedings of the National Academy of Sciences, scientists from the Johns Hopkins University School of Medicine in Baltimore; Baylor College of Medicine in Houston; Harvard Medical School; two units of Madison, N.J.-based Wyeth; and Beltsville, Md.-based MetaMorphix Inc. reported on a new compound that shows potential in treating muscle-wasting diseases.

The compound is a soluble receptor for myostatin, but the results led first author Se-Jin Lee, professor at the Johns Hopkins University and a consultant to MetaMorphix, to conclude that "clearly, myostatin is not the whole story."

Myostatin is a regulatory protein that inhibits the formation of muscle mass. In 1997, Lee and his colleagues first reported that knocking out the myostatin gene led to mice that were twice as muscular as their normal siblings, giving them the moniker "mighty mice." Highly muscular cattle breeds also have a mutated myostatin gene. (See BioWorld Today, May 2, 1997.)

No word is available on Arnold Schwarzenegger's myostatin status but apparently superhero-like strength would also ensue in humans when myostatin is not working. In 2004, a report in the New England Journal of Medicine described a boy who had no functional copies of the myostatin gene and twice the normal muscle mass for his age. While it was not possible to genotype other members of the boy's family, the change was likely inherited: His mother was a professional athlete, and, according to the paper, an uncle was "a construction worker who was able to unload curbstones by hand."

As a result of Lee's earlier work on growth factors, Johns Hopkins, together with Genetics Institute, spun out MetaMorphix in 1994. A myostatin antibody - called JA16 antibody - was licensed to MetaMorphix by Johns Hopkins and sublicensed to Wyeth. It now is in clinical trials, but in preclinical experiments that antibody led to a two- or threefold increase in muscle mass in mice.

For the new work, Lee and his colleagues decided to see whether a broader-spectrum inhibition of myostatin would be even more effective in stimulating muscle growth.

The scientists constructed a soluble form of the activin type 2B receptor, which normally binds myostatin in cell membranes, that they then coupled to the FC region of antibodies. The FC receptor domain usually activates the immune system, but Lee said the domain used in those experiments had point mutations that would allow it to stabilize the receptor without activating the immune system.

Lee told BioWorld Today that the receptor "can produce dramatic effects in a short period of time." In fact, two injections, given one week apart, of the receptor increased muscle mass by up to 60 percent in wild-type mice.

Myostatin appears to work by capping the size of muscle fibers, and the activin 2b described in the PNAS paper increased the size of those fibers. When the researchers measured the diameters of muscle fibers, they were increased by about 20 percent. While they did not directly measure volume, Lee said that the larger diameter, together with increases in protein and DNA content that they did measure, would be consistent with the idea of larger muscle fibers and sufficient to account for the increases they saw.

The results also suggested that myostatin is not the only agent that inhibits muscle growth. The scientists administered the activin antibodies to mice lacking the myostatin gene - if myostatin were the only protein inhibiting muscle growth, the antibody should be without effect in such mice, because they have no myostatin for it to bind to. Instead, those mice showed less of an effect when the activin receptors were administered, but still formed more muscle than controls. That led the researchers to conclude that one or more additional protein or proteins must also be involved in regulating muscle growth.

There are two activin receptors that bind myostatin, and teasing out the relative effects of the type 2 and type 2b receptors proved somewhat challenging, as double mutants mice showed the most frequent knockout phenotype known to man: embryonic death. Nevertheless, the scientists were able to make a mouse that lacked three of the four genes for the receptors: both copies of the type 2 receptor plus one copy of the type 2b receptor. Those mice had further increases in muscle mass compared to mice lacking only the type 2 receptor, suggesting that the receptors are somewhat but not completely redundant.