Myostatin Directly Regulates Skeletal Muscle Fibrosis

Abstract

Skeletal muscle fibrosis is a major pathological hallmark of chronic myopathies in which myofibers are replaced by progressive deposition of collagen and other extracellular matrix proteins produced by muscle fibroblasts. Recent studies have shown that in the absence of the endogenous muscle growth regulator myostatin, regeneration of muscle is enhanced, and muscle fibrosis is correspondingly reduced. We now demonstrate that myostatin not only regulates the growth of myocytes but also directly regulates muscle fibroblasts. Our results show that myostatin stimulates the proliferation of muscle fibroblasts and the production of extracellular matrix proteins both in vitro and in vivo. Further, muscle fibroblasts express myostatin and its putative receptor activin receptor IIB. Proliferation of muscle fibroblasts, induced by myostatin, involves the activation of Smad, p38 MAPK and Akt pathways. These results expand our understanding of the function of myostatin in muscle tissue and provide a potential target for anti-fibrotic therapies.Fibroblasts play an important role in the repair response of tissues to injury by secreting extracellular matrix proteins including collagen and growth factors. However, in a variety of disease states, continued fibrosis contributes to the pathological process. In chronic myopathies and muscular dystrophies, fibrosis is considered to be associated with decreased strength and elasticity of muscle and may inhibit the diffusion of nutrients to myofibers (1-3). Identification of factors that regulate fibrosis is an important goal not only in understanding the pathogenesis of muscular dystrophies but also in developing novel therapies to treat these disorders. Several potential future therapies for the muscular dystrophies, including those involving gene and myoblast transfer, may be hampered by significant fibrosis.Myostatin is a highly conserved transforming growth factor-␤ (TGF-␤) 2 family member that is expressed in skeletal muscle, which is also the primary target tissue (4). Deletion of the myostatin gene (MSTN) in mice leads to muscle hypertrophy and hyperplasia with an approximate doubling of muscle mass (4). This function of myostatin, as an endogenous inhibitor of muscle growth, is also conserved in humans, as demonstrated by the identification of a hypermuscular child with a loss-of-function mutation in the myostatin gene (5). One mechanism by which myostatin regulates muscle growth in adult animals appears to be direct inhibition of the proliferation and differentiation of resident muscle precursor cells (6 -8).The potential effect of myostatin inhibition on muscle degenerative diseases has been explored in various animal models. In the absence of myostatin, muscle regenerates more quickly and completely following acute and chronic injury (8, 9). In the mdx mouse, a model of Duchenne and Becker muscular dystrophy, myostatin deletion, or postnatal inhibition increases muscle mass and strength (10 -12). An early observation of mdx/mstn null mice was that the diaphragm muscle of t...

Smad3 signaling is required for satellite cell function and myogenic differentiation of myoblasts

“…However, other signaling pathways including Smad2 are also shown to be involved in myostatin signaling [9,13,[39][40][41][42]. In this study, higher myostatin level was shown to be, in part, responsible for the muscle atrophy in Smad3-null mice.…”

“…Mechanistically, the Smad2/3 pathway plays a significant role in myostatin and TGF-β signaling and inhibition of myogenesis [10,11]. Specifically, Smad3 is required for the inhibition of myoblast differentiation by myostatin [12] and for the activation of fibroblast proliferation by myostatin [13]. The Smad3-dependent pathway begins when activated myostatin receptors (activin type IIB receptor/ALK5) form heterotetrameric complexes, which then recruit, phosphorylate and activate the receptor-regulated SMADS (R-Smads).…”

Targeting the Activin Type IIB Receptor to Improve Muscle Mass and Function in the mdx Mouse Model of Duchenne Muscular Dystrophy

“…21,22 In vitro studies localize MSTN and ActRIIB directly on fibroblasts, supporting the idea that MSTN is a profibrotic factor that interacts directly with fibroblasts to mediate its effects. 58 In vivo, MSTN-coated beads implanted into skeletal muscle increased fibrotic tissue, and this effect was inhibited when the endogenous MSTN inhibitor follistatin was also present on the beads. 58 Diaphragmatic fibrosis was reduced by 44% in Mstn Ϫ/Ϫ mdx mice at 6 months of age, compared with Mstn ϩ/ϩ mdx mice, but was still elevated compared with the wild type.…”

Muscle Gene Expression Patterns in Human Rotator Cuff Pathology

“…In the myogenesis program, myogenic differentiation 1 (MYOD1) and myogenic factor 5 (MYF5) are important for myogenic differentiation of satellite cells, myoblast commitment, and induction of myogenin (MYOG, which helps control the formation of myotubes and normal muscle development) 20 . On the other hand, myostatin (MSTN), which is secreted by muscle, inhibits myogenesis 21 and muscle hypertrophy [22][23][24] (via regulation of mammalian target of rapamycin [MTOR]) but promotes fibrosis 25,26 and adipogenesis 21 . An animal model of rotator cuff injury has also shown that MTOR modulates fatty infiltration 27 , and it regulates fibrosis [28][29][30] .…”