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Satellite Cell Senescence Underlies Myopathy in a Mouse Model of Limb-Girdle Muscular Dystrophy 2H Elena Kudryashova, Irina Kramerova, and Melissa J

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Satellite Cell Senescence Underlies Myopathy in a Mouse Model of Limb-Girdle Muscular Dystrophy 2H Elena Kudryashova, Irina Kramerova, and Melissa J Research article Satellite cell senescence underlies myopathy in a mouse model of limb-girdle muscular dystrophy 2H Elena Kudryashova, Irina Kramerova, and Melissa J. Spencer Department of Neurology, Center for Duchenne Muscular Dystrophy at UCLA, David Geffen School of Medicine at UCLA, Los Angeles, California, USA. Mutations in the E3 ubiquitin ligase tripartite motif-containing 32 (TRIM32) are responsible for the disease limb-girdle muscular dystrophy 2H (LGMD2H). Previously, we generated Trim32 knockout mice (Trim32–/– mice) and showed that they display a myopathic phenotype accompanied by neurogenic features. Here, we used these mice to investigate the muscle-specific defects arising from the absence of TRIM32, which underlie the myopathic phenotype. Using 2 models of induced atrophy, we showed that TRIM32 is dispensable for muscle atrophy. Conversely, TRIM32 was necessary for muscle regrowth after atrophy. Furthermore, TRIM32-defi- cient primary myoblasts underwent premature senescence and impaired myogenesis due to accumulation of PIAS4, an E3 SUMO ligase and TRIM32 substrate that was previously shown to be associated with senescence. Premature senescence of myoblasts was also observed in vivo in an atrophy/regrowth model. Trim32–/– muscles had substantially fewer activated satellite cells, increased PIAS4 levels, and growth failure compared with wild- type muscles. Moreover, Trim32–/– muscles exhibited features of premature sarcopenia, such as selective type II fast fiber atrophy. These results imply that premature senescence of muscle satellite cells is an underlying pathogenic feature of LGMD2H and reveal what we believe to be a new mechanism of muscular dystrophy associated with reductions in available satellite cells and premature sarcopenia. Introduction and are primarily characterized by proximal muscle weakness and Tripartite motif-containing 32 (TRIM32) is a member of the tri- wasting. LGMD2H biopsies show myopathic features of central partite motif (TRIM) family of proteins (1), which share the com- nucleation, fiber splitting, Z-line streaming, and a dilated sarcotu- mon features of a RING finger, a B-box, and a coiled-coil region. bular system with vacuoles (4, 5, 8, 13–16). In the Trim32–/– mouse TRIM32 is an E3 ubiquitin ligase, the activity of which is con- model, the absence of TRIM32 resulted in a similar myopathic tained in the RING finger domain (2). TRIM32 differs from most phenotype to that described in LGMD2H/STM biopsies, demon- other TRIM family members because its C terminus contains a strating similar muscle morphology and muscle weakness on grip 6-bladed β-propeller NHL domain, which is known to mediate strength and wire hang testing. In addition, our studies revealed protein-protein interactions (3). Although the TRIM32 protein that the Trim32–/– myopathy included a neurogenic component. is ubiquitously expressed, different mutations in the Trim32 gene The observation of reduced neurofilament protein concentra- result in several different inherited diseases, with disparate phe- tion led to the discovery that Trim32–/– mice had reduced motor notypes that affect both muscle and nonmuscle tissues. For exam- axon diameters, resulting in a shift to a slow myosin fiber-type ple, all mutations that result in muscle phenotypes (limb-girdle composition (11). In agreement with our findings in the mouse, muscular dystrophy 2H [LGMD2H] and sarcotubular myopathy neuronal involvement was also confirmed in some patients with [STM]) occur in the C-terminal NHL domain of TRIM32 (4–8). LGMD2H/STM. These LGMD2H-associated neurogenic features On the other hand, a missense mutation in the B-box of TRIM32 included mixed neuropathic and myopathic elements revealed by results in another genetically inherited disorder called Bardet- electromyography, muscle weakness, paresis, paresthesia, hypoac- Biedl syndrome type 11 (BBS11). BBS11 is a multisystemic dis- tive or absent tendon reflexes, a slight dominance of type I slow order that has no skeletal muscle involvement (9). These diseases muscle fibers, and decreased motor and sensory nerve conduction are rare, and their incidence is still not known. Several muscle-spe- velocities (5, 7, 8, 14). Therefore, the Trim32–/– mouse replicates the cific substrates and/or interacting partners of TRIM32 have been LGMD2H phenotype and provides an excellent model system for identified, including myosin, actin (2), and dysbindin (10), yet the elucidating functions of TRIM32 in vivo. biological role of these interactions is not understood. In this study, we sought to investigate the myogenic compo- Previously, we created Trim32 knockout mice (Trim32–/– mice), nent of the Trim32–/– myopathy to lend insight into LGMD2H which proved to be a good model for studying pathogenic mecha- pathogenesis. We found that TRIM32 has a unique role in skeletal nisms of LGMD2H (11). LGMD2H/STM (MIM 254110) is a neu- muscle, which is different from known roles of other E3 ubiqui- romuscular disorder with a highly variable and slowly progressive tin ligases involved in skeletal muscle remodeling. In contrast to clinical course (12). Phenotypic outcomes of LGMD2H and its the muscle-specific E3 ubiquitin ligases TRIM63 (also known as more severe form STM are similar to other muscular dystrophies MURF1) and F-box protein 32 (FBXO32; also known as MAFbx/ atrogin-1), which are key regulators of atrophy (17, 18), our data Conflict of interest: The authors have declared that no conflict of interest exists. demonstrated that TRIM32 is not necessary for muscle atrophy Citation for this article: J Clin Invest. 2012;122(5):1764–1776. doi:10.1172/JCI59581. but instead participates in muscle regrowth after atrophy. In this 1764 The Journal of Clinical Investigation http://www.jci.org Volume 122 Number 5 May 2012 research article Figure 1 Fasting-induced atrophy occurs normally in Trim32–/– mice. (A) A statistically significant (P < 0.05) body weight reduction due to 24 or 48 hours of fasting was observed in mice of both genotypes. The rate of body weight reduction in Trim32–/– fasting mice is indistinguishable from the Trim32+/+ rate (n = 4). Error bars represent SEm. (B) After 48 hours of food deprivation, gastrocnemius muscle weight was reduced by 20%, similarly in Trim32–/– and Trim32+/+ mice. (C) Accumulation of high-molecular weight ubiquitin-protein conjugates occurs similarly in Trim32–/– and Trim32+/+ myofibrils, as demonstrated by anti-ubiquitin (Ub) Western blot staining. A representative blot is shown. Ponceau S staining is shown as a loading control. (D) Ratios of major myofibrillar proteins, myosin heavy chain (myHC) to actin and actin to desmin, were assessed in myofibrils isolated from Trim32+/+ and Trim32–/– fed mice and 48 hour–fasted mice by densitometry of Coomassie-stained SDS-PAGE. No difference was observed between Trim32–/– and Trim32+/+ samples. investigation, we present evidence that, in the absence of TRIM32, cell population, which in turn results in reduced growth poten- primary myogenic cells demonstrated impaired myogenesis and tial of skeletal muscle and premature sarcopenia in vivo. Sarco- underwent premature senescence mediated by accumulation of penia is defined as loss of muscle mass and function associated an E3 small ubiquitin-related modifier (SUMO) ligase called pro- with aging; its pathogenesis involves multiple factors, including tein inhibitor of activated STAT (PIAS4, also known as PIASy). a reduction of satellite cell number and selective type II fast fiber PIAS4 was previously identified as a substrate of TRIM32 in atrophy (26–28). In this study, we identified premature senescence keratinocytes (19). SUMO ligases are enzymes involved in the of myogenic cells as a mechanism that can cause adaptive growth process of sumoylation, which is a cascade of SUMO activa- deficiency and subsequent muscular dystrophy in adult muscles. tion, and transfer reactions that conjugate a SUMO moiety to In an animal model of induced muscle growth, we demonstrated a protein substrate. The sumoylation pathway regulates a wide that Trim32–/– muscles do indeed have substantially fewer satellite range of cellular events, similar to ubiquitination (20). Recently, cells and reduced muscle growth in vivo. We also present evidence it was demonstrated that overexpression of PIAS4 or SUMO-2/3 of premature sarcopenia in Trim32–/– mice. Collectively, these data led to senescence in several different cell models, thus implicat- identified premature myoblast senescence and sarcopenia as an ing PIAS4 and sumoylation in regulation of cellular senescence underlying pathogenic feature of TRIM32 deficiency and, thus, (21–23). Senescence was first described as irreversible prolifera- revealed what we believe to be a novel mechanism of muscular dys- tion arrest that limits the replicative ability of cultured cells, a trophy due to reduced replicative potential of satellite cells result- process termed “replicative senescence” (24). However, senescence ing in impaired growth capability of skeletal muscle. can also be triggered in different cell types in response to certain kinds of stress, such as DNA damage or oncogenic or oxidative Results stress, and is referred to as “premature senescence” (25). Skeletal muscles are able to atrophy in the absence of TRIM32. Previous Since adult skeletal muscle contains a population of resident, studies demonstrated an involvement of the muscle-specific E3 self-renewable muscle stem cells called “satellite cells,” the prema- ubiquitin ligases TRIM63 and FBXO32 in muscle
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