Myostatin and muscle fiber size. Focus on "Smad2 and 3 transcription factors control muscle mass in adulthood" and "Myostatin reduces Akt/TORC1/p70S6K signaling, inhibiting myoblast differentiation and myotube size".

THE PLASTICITY OF SKELETAL muscle continues to fascinate physiologists and cell biologists. Many of the key molecules in the pathways that signal growth or atrophy of muscle have been identified, along with a number of hormones and growth factors that regulate these pathways. This knowledge may enable us to more effectively prevent or reverse muscle atrophy associated with a number of conditions: cancer, muscular dystrophies, old age, inability to maintain normal activity, glucocorticoid therapy, arthritis, or settings associated with high levels of catabolic hormones and cytokines. Of course, there is much more to learn about muscle growth and atrophy pathways, and two articles (9, 12) contribute to our understanding of one of the important regulators of muscle growth: myostatin [also known as growth differentiation factor-8 (GDF-8)]. The discovery of myostatin, a member of the transforming growth factor(TGF) superfamily, was reported 12 years ago (7). When myostatin was knocked out in mice, their skeletal muscles were two or more times the normal size. Remarkably, no other major effects of myostatin deficiency were noted, although subsequent research has revealed a few. Expression of the myostatin gene was high in muscle, but absent or very low in other tissues. It was soon realized that naturally occurring mutations in this gene, which is highly conserved across species, are responsible for hypermuscularity in certain breeds of cattle, sheep, and dogs. The active domain of the peptide is identical in mice and humans, and a homozygous loss of function mutation in a boy was associated with marked hypermuscularity in infancy (10). Anti-myostatin antibodies and other proteins that bind myostatin were reported to increase muscle mass and strength in normal and dystrophic mice (2, 18). All of this information suggested that myostatin could be an ideal target for drugs or biological compounds to combat muscle atrophy. The first human trial of an antimyostatin antibody did not demonstrate functional improvement or significant muscle hypertrophy (at low doses) in adults with muscular dystrophies (14). This negative result should not discourage more research on myostatin; one of the main problems with that first trial was that no molecular markers were assessed to confirm that the administered doses of antibody had an impact on the relevant signaling pathways in muscle. A deeper understanding of the molecular and phenotypic effects of myostatin is needed before advocating or rejecting myostatin inhibition as an anti-atrophy strategy. From the standpoint of translating basic knowledge into biomedical advances, it is especially important to study postdevelopmental effects of myostatin because anti-myostatin agents generally would be used in adults. It has been assumed that signaling by myostatin is the same as that of TGF, i.e., binding of the ligand to a type II receptor (activin receptor 2B appears to be the critical one for myostatin) induces phosphorylation of a type I receptor, which phosphorylates Smad2 and Smad3, thereby facilitating formation of a complex of these phospho-Smads with Smad4. This complex enters the nucleus and alters gene transcription. It had not been proven that activation or inhibition of this pathway is sufficient to alter myofiber size. In their article, Sartori et al. (9) show that activation of the pathway in vivo in adult mice, which the authors accomplish by electroporating genes encoding a constitutively active form of either of the TGF/activin type I receptors [activin receptor-like kinase 4 (ALK4) or ALK5], induces myofiber atrophy. This effect was blocked by small hairpin RNAs (shRNAs) targeting Smad2 and Smad3. Administration of Smad2 and Smad3 shRNAs together induced fiber hypertrophy to an extent similar to previously reported effects of anti-myostatin treatments in adult mice (16, 18). In their article, Trendelenburg et al. (12) also demonstrated that Smad2 and Smad3 small interfering RNAs (siRNAs) block myostatin-induced and TGF1-induced atrophy of human myotubes. There is strong evidence that other ligands besides myostatin contribute significantly to the basal level of myostatin-like activity in muscle, although it remains to be determined which ones are involved (4). The study by Trendelenburg et al. shows that both TGF1 and GDF-11 are more potent activators of this pathway (as reflected by inhibition of myoblast differentiation) in human myotubes than is myostatin, although it is unknown whether endogenous levels of these specific ligands in muscle are sufficient to influence muscle mass. Many conditions that induce myofiber atrophy do so by increasing the expression of “atrogenes,” which promote degradation of muscle proteins primarily through the ubiquitinproteasome proteolytic pathway. Two key atrogenes are those encoding the E3 ubiquitin ligases muscle atrophy F-box (MAFbx) (atrogin-1) and muscle RING-finger 1 (MuRF1). Trendelenburg et al. report that myostatin causes atrophy of human myotubes while inhibiting the expression of these genes. This effect occurred at myostatin concentrations of 1–100 ng/ml and differs from the response to a high concentration of myostatin (3,000 ng/ml) in C2C12 myotubes, which increases atrogin-1 (but not MuRF1) expression and the ubiquitination of muscle proteins (6). A modest increase in myostatin levels in rat muscle in vivo also caused atrophy without Address for reprint requests and other correspondence: S. L. Welle, School of Medicine and Dentistry, Univ. of Rochester, 601 Elmwood Ave., Box 693, Rochester, NY 14642 ([email protected]). Am J Physiol Cell Physiol 296: C1245–C1247, 2009; doi:10.1152/ajpcell.00154.2009.