The Muscle Maker
Gene therapy offers a shortcut to bulking up: At the University of Pennsylvania, H. Lee Sweeney is developing a way to turn the liver into a factory that churns out a muscle growth promoter called myostatin propeptide. He injects a virus carrying the growth promoter gene into an animal’s veins, where it courses through the bloodstream and into the liver. There, it infects liver cells and delivers its genetic package. A signal from the virus tells the liver cells to manufacture the growth promoter, which is then secreted back into the bloodstream and ferried off to muscles throughout the body. Normally, myostatin puts the brakes on excessive muscle growth. But when there is too much myostatin propeptide around—delivered by the virus and pumped out by the liver—myostatin cannot do its job and muscles keep growing.
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