Skeletal muscle consists of two distinct types of fiber. Type I fibers (also called slow-twitch) cope better with repeated muscle contractions, partly because they have higher numbers of mitochondria than type II (fast-twitch) fibers. The biochemical processes behind fiber-type determination have been unclear until now. In August 15 Nature, Jiandie Lu and colleagues at the Dana Farber Cancer Institute, and Harvard Medical School, Boston, US, show that the transcriptional co-activator PGC-1α (peroxisome-proliferator-activated receptor-γ co-activator-1), is a key player in the determination of muscle fiber type (Nature 2002, 418:797-801).

PGC-1α is normally preferentially expressed in type I fibers. Lu et al. created transgenic mice expressing PGC-1α under the control of the MCK promoter (which is preferentially activated in type II fibers). They observed that PGC-1α mRNA was expressed at an elevated level in plantaris (a muscle normally composed primarily of type II fibers), up to a level...

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