Age-Dependent Effect of Myostatin Blockade on Disease Severity in a Murine Model of Limb-Girdle Muscular Dystrophy

Abstract

Myostatin (MSTN) is a muscle-specific secreted peptide that functions to limit muscle growth through an autocrine regulatory feedback loop. Loss of MSTN activity in cattle, mice, and humans leads to a profound phenotype of muscle overgrowth, associated with more and larger fibers and enhanced regenerative capacity. Deletion of MSTN in the mdx mouse model of Duchenne muscular dystrophy enhances muscle mass and reduces disease severity. In contrast, loss of MSTN activity in the dy W /dyW mouse model of laminin-deficient congenital muscular dystrophy, a much more severe and lethal disease model, does not improve all aspects of muscle pathology. Here we examined disease severity associated with myostatin (mstn Myostatin (MSTN), also known as growth and differentiation factor-8 (GDF-8), is a member of the transforming growth factor-␤ superfamily of growth factors and functions as a negative regulator of muscle mass.1 Spontaneous mutations in the MSTN gene in mice and cattle have been demonstrated to lead to significantly greater muscle mass due to both muscle hypertrophy and hyperplasia.2-5 Because of this role, inhibition of MSTN is a potentially important mechanism for treating human diseases that lead to muscle wasting and degeneration, such as muscular dystrophy.The mdx (X-chromosome-linked muscular dystrophy) mouse is a well-characterized and widely used model for the study of Duchenne muscular dystrophy (DMD). Disease in the mdx model shows a continuum of severity in different muscles based partially on use, with the diaphragm being the most affected, although these mice do not ultimately die prematurely like human DMD patients. Studies in mdx mice have suggested that inhibition of MSTN partially rescues muscular dystrophy. For example, genetic deletion of mstn in the mdx background attenuated the severity of muscular dystrophy and histopathology, as well as enhanced regeneration. 6,7 In support of this conclusion, treatment of mdx mice with a dominant-negative MSTN propeptide fusion protein or a blocking monoclonal antibody each ameliorated/attenuated dystrophic pathology. 8,9 In contrast, a recent study by Li and colleagues 10 examined loss of MSTN in a much more severe dystrophic mouse model, the laminin ␣2-deficient dy W /dy W mice. These mice have severe skeletal muscle degeneration and die at 3 to 6 weeks of age. Interestingly, loss of MSTN in dy W /dy W mice does enhance muscle formation and regeneration, but it does not ultimately rescue disease or obviate muscle pathology. 10Here we investigated another mouse model of muscular dystrophy associated with loss of ␦-sarcoglycan (scgd). This protein is a member of the sarcoglycan comSupported by the National Institutes of Health (grants to J.D.M. and E.M.M. and training grant no. 5T32 HL07382 to S.A.P.).

Inhibiting myostatin reverses muscle fibrosis through apoptosis

“…In the current study, ActRIIB.mFc increased muscle growth and regeneration in senescent mdx mice treated for 6 weeks. This was an anticipated finding since myostatin inhibitors in young mdx animals as well as gene deletion of myostatin in senescent animals have previously been shown to enhance muscle growth and regeneration (Bogdanovich et al, 2002;McCroskery et al, 2005;Parsons et al, 2006;Wagner et al, 2005). Unanticipated, were the consistent findings by radiographical, biochemical and histological analyses that ActRIIB.mFc treatment not only reduced fibrosis compared to untreated controls but reversed pre-existing fibrosis in individual senescent animals.…”

“…Reduction of myostatin signaling by a variety of genetic and pharmacological mechanisms results in amelioration of disease features in several mouse models of muscular dystrophy (Bartoli et al, 2007;Bogdanovich et al, 2002;Ohsawa et al, 2006;Parsons et al, 2006;Qiao et al, 2008;Wagner et al, 2002). Mdx mice have a nonsense mutation in the gene for dystrophin and are a genetic model of a common and fatal muscular dystrophy, Duchenne muscular dystrophy (DMD), and a phenotypic model of the less severe, allelic Becker Muscular Dystrophy (Hoffman et al, 1987).…”

Functional and morphological recovery of dystrophic muscles in mice treated with deacetylase inhibitors

“…Myostatin blockade at early stages of the disease provides a beneficial effect in both MDX and a-SG-deficient mice 4,5,12 . These functional analogies led us to think that the beneficial effect of TSA that we observed in MDX mice might be extended to a-SG-deficient mice.…”

Myostatin Directly Regulates Skeletal Muscle Fibrosis

“…Decreased fibrosis of limb muscle in myostatin null animals after acute injury with notexin has been reported (9). Recently, Parsons et al (13) studied the effect of myostatin loss or postnatal inhibition in ␦-sarcoglycan (scgd Ϫ/Ϫ ) mice, a mouse model of limb-girdle muscle dystrophy (LGMD2F). Fibrosis, as defined by morphometric analysis and hydroxyproline content, was significantly decreased in the scgd Ϫ/Ϫ myostatin null mouse although not in the scgd Ϫ/Ϫ treated with a neutralizing antibody to myostatin (13).…”

“…Recently, Parsons et al (13) studied the effect of myostatin loss or postnatal inhibition in ␦-sarcoglycan (scgd Ϫ/Ϫ ) mice, a mouse model of limb-girdle muscle dystrophy (LGMD2F). Fibrosis, as defined by morphometric analysis and hydroxyproline content, was significantly decreased in the scgd Ϫ/Ϫ myostatin null mouse although not in the scgd Ϫ/Ϫ treated with a neutralizing antibody to myostatin (13). These studies in models of acute and chronic injury all support the emerging concept that myostatin is a negative regulator of muscle precursors and that inhibition of myostatin improves muscle growth and regeneration.…”