Mice and cattle with genetic deficiencies in myostatin exhibit dramatic increases in skeletal muscle mass, suggesting that myostatin normally suppresses muscle growth. Whether this increased muscling results from prenatal or postnatal lack of myostatin activity is unknown. Here we show that myostatin circulates in the blood of adult mice in a latent form that can be activated by acid treatment. Systemic overexpression of myostatin in adult mice was found to induce profound muscle and fat loss analogous to that seen in human cachexia syndromes. These data indicate that myostatin acts systemically in adult animals and may be a useful pharmacologic target in clinical settings such as cachexia, where muscle growth is desired.

Myostatin is a secreted protein that normally functions as a negative regulator of muscle growth. Agents capable of blocking the myostatin signaling pathway could have important applications for treating human muscle degenerative diseases as well as for enhancing livestock production. Here we describe a potent myostatin inhibitor, a soluble form of the activin type IIB receptor (ACVR2B), which can cause dramatic increases in muscle mass (up to 60% in 2 weeks) when injected into wild-type mice. Furthermore, we show that the effect of the soluble receptor is attenuated but not eliminated in Mstn ؊/؊ mice, suggesting that at least one other ligand in addition to myostatin normally functions to limit muscle growth. Finally, we provide genetic evidence that these ligands signal through both activin type II receptors, ACVR2 and ACVR2B, to regulate muscle growth in vivo. Mice carrying a targeted mutation in the myostatin gene have muscles that are about twice the normal size as a result of a combination of muscle fiber hyperplasia and hypertrophy (2). Myostatin appears to play a similar role in other species as well; naturally occurring mutations in the myostatin gene have been shown to be responsible for the double-muscling phenotype in cattle (3-6), and recent studies have demonstrated that a human baby with approximately twice the normal muscle mass is also homozygous for a loss-of-function mutation in the MSTN gene (7). These findings have raised the possibility that agents capable of targeting the myostatin signaling pathway may be useful for increasing muscle mass for both agricultural and human therapeutic applications. In this regard, loss of myostatin signaling has been shown to have beneficial effects in mouse models of muscle degenerative (8, 9) and metabolic (10) diseases.Various myostatin-binding proteins have been identified that are capable of inhibiting myostatin activity in vitro (8,(11)(12)(13)(14)(15)(16). Two of these proteins, the JA16 neutralizing monoclonal antibody (Ab) directed against myostatin (8, 15) and a mutant form of the myostatin propeptide resistant to members of the BMP-1͞tolloid family of metalloproteases (16), have been shown to be capable of increasing muscle mass by Ϸ25% when administered to wild-type (WT) mice. To determine whether these increases in muscle growth are the maximal achievable by targeting this signaling pathway, we sought additional myostatin inhibitors that might have a broader specificity in their ability to target additional members of the TGF-␤ superfamily. Previous studies have demonstrated that myostatin is capable of binding the two activin type II receptors, ACVR2B and, to a lesser extent, ACVR2, in transfected COS cells (11,17). Moreover, transgenic mice in which a myosin light chain promoter͞ enhancer was used to express a truncated form of ACVR2B in skeletal muscle were found to have dramatic increases in muscle mass (11). Because the activin type II receptors have been shown to be capable of binding a number of other TGF-␤ family members in addition to ...

Myostatin is a transforming growth factor ␤ family member that acts as a negative regulator of skeletal muscle growth. Myostatin circulates in the blood of adult mice in a noncovalently held complex with other proteins, including its propeptide, which maintain the C-terminal dimer in a latent, inactive state. This latent form of myostatin can be activated in vitro by treatment with acid; however, the mechanisms by which latent myostatin is activated in vivo are unknown. Here, we show that members of the bone morphogenetic protein-1͞tolloid (BMP-1͞TLD) family of metalloproteinases can cleave the myostatin propeptide in this complex and can thereby activate latent myostatin. Furthermore, we show that a mutant form of the propeptide resistant to cleavage by BMP-1͞TLD proteinases can cause significant increases in muscle mass when injected into adult mice. These findings raise the possibility that members of the BMP-1͞TLD family may be involved in activating latent myostatin in vivo and that molecules capable of inhibiting these proteinases may be effective agents for increasing muscle mass for both human therapeutic and agricultural applications.

Myostatin, also known as growth and differentiation factor 8, is a member of the transforming growth factor ␤ superfamily that negatively regulates skeletal muscle mass (1). Recent experiments have shown that myostatin activity is detected in serum by a reporter gene assay only after activation by acid, suggesting that native myostatin circulates as a latent complex (2). We have used a monoclonal myostatin antibody, JA16, to isolate the native myostatin complex from normal mouse and human serum. Analysis by mass spectrometry and Western blot shows that circulating myostatin is bound to at least two major proteins, the myostatin propeptide and the follistatin-related gene (FLRG). The myostatin propeptide is known to bind and inhibit myostatin in vitro (3). Here we show that this interaction is relevant in vivo, with a majority (>70%) of myostatin in serum bound to its propeptide. Studies with recombinant V5-His-tagged FLRG protein confirm a direct interaction between mature myostatin and FLRG. Functional studies show that FLRG inhibits myostatin activity in a reporter gene assay. These experiments suggest that the myostatin propeptide and FLRG are major negative regulators of myostatin in vivo.