Commentary | Open | Published:
MG53′s new identity
Skeletal Musclevolume 3, Article number: 25 (2013)
Mitsugumin 53 (MG53) is a relatively newly identified tripartite motif-containing (TRIM) family muscle-specific E3 ubiquitin ligase that is expressed in skeletal muscle and the heart. It has been postulated to facilitate repair by targeting the site of an injury, and acting as a scaffold for assembly of a repair complex made up of dysferlin, annexin V, caveolin-3, and polymerase I and transcript release factor (PTRF). A recent letter published in Nature by Song et al. proposes an alternate function for MG53: as an E3 ligase that targets the insulin receptor and insulin receptor substrate 1 (IRS1) for degradation, therefore regulating muscle insulin signaling. This work is exciting, as it not only presents a novel role for MG53, but also suggests that muscle insulin signaling has a systemic influence on insulin resistance and the metabolic syndrome.
Metabolic syndrome is a disorder of increasing prevalence, especially in developed countries; it is associated with an increase in the incidence of obesity and the often-associated type 2 diabetes. It is defined as a cluster of conditions comprising hypertension, high blood glucose levels, abnormal cholesterol levels and central obesity that, when they occur in combination, increase the risk for cardiovascular disease and type 2 diabetes . The majority of insulin-stimulated glucose disposal occurs in skeletal muscle [2, 3], and therefore it is likely that insulin resistance in skeletal muscle is a precursor to systemic metabolic dysfunction.
Mitsugumin 53 (MG53) is a cardiac and skeletal muscle-specific protein, which was identified in a screen using an immuno-proteomic monoclonal antibody library against proteins present in striated muscle . MG53, also known as tripartite motif 72 (TRIM72), has a molecular weight of 53 kD, and contains the prototypical tripartite motif, which includes a RING, B-box and coiled-coil moiety. MG53 also has a SPRY domain at the carboxy terminus . Although MG53 contains a canonical E3 ligase RING finger domain, its E3 ligase activity was not confirmed until recently .
Cai et al. generated an MG53-null mouse, which developed slowly progressive muscle pathology: mice aged 10 to 11 months had increased numbers of central nuclei and decreased muscle fiber diameter. The skeletal muscle from MG53-null mice had increased Evans blue dye uptake after downhill running, suggesting that these mice have increased susceptibility to membrane injury. Further experiments by this group of researchers implicated MG53 as a central player in the repair of membrane damage; it acts by binding to phosphatidylserine at the damage site and acts as a scaffold to recruit a complex of repair proteins that includes dysferlin, annexin V, caveolin-3 and polymerase I and transcript release factor (PTRF) [5, 7]. Although MG53 has been found to be upregulated in muscular dystrophy patients , no pathogenic MG53 mutations have been identified.
Recently, Song et al. published a series of experiments that identify a novel role for MG53 as a mediator of insulin signaling . The authors observed that animal models for metabolic syndrome (high-fat diet (HFD)-induced obese mice, db/db diabetic mice, spontaneously hypertensive rats and non-human primate models of metabolic syndrome), as well as obese humans, have increased expression of MG53 compared to control animals and non-obese humans.
To determine if MG53 is required for pathogenesis of metabolic syndrome, the authors fed MG53-null mice and wild-type littermates a HFD. MG53-null mice were protected from increases in body weight, blood pressure, and serum cholesterol and triglyceride levels. Further, the loss of MG53 blocked HFD-induced systemic insulin resistance. Conversely, a transgenic mouse model that overexpresses MG53 had the hallmarks of metabolic syndrome (increased body weight, blood pressure and serum cholesterol levels) and insulin resistance in the absence of a HFD .
The authors delved into the mechanism for the regulation of insulin signaling by MG53. Insulin stimulation leads to autophosphorylation of the insulin receptor. This leads to phosphorylation of insulin receptor substrate 1 (IRS1) and insulin receptor substrate 2 (IRS2), which in turn activates the phosphatidylinositol-3-OH kinase (PI(3)K)-Akt-GSK3-beta signaling pathway. This pathway mediates glucose homeostasis in skeletal muscle . Previously, Ko and colleagues  showed that MG53 overexpression prevented IRS1 phosphorylation and myogenesis in C2C12 myoblasts. The current study found that MG53-null tissues had increased insulin signaling, whereas MG53 overexpression suppresses insulin signaling. Changes in insulin signaling accompanied marked changes in insulin receptor and IRS1 protein levels, but not mRNA levels. Surprisingly, skeletal muscle insulin resistance in MG53 transgenic mice preceded whole-body insulin disorders (obesity and multi-organ insulin resistance). Specific inhibition of the E3 ubiquitin ligase activity of MG53 by deletion of the RING finger domain or by mutation of a cysteine to an alanine at position 14 suppresses the effects of MG53 on insulin receptor activity and ubiquitination. This supports a model for MG53 targeting insulin receptor degradation via its E3 ubiquitination ligase activity; the downstream ubiquitination of IRS1 seems dependent on the initial insulin receptor degradation, since IRS1 was not degraded upon IGF1 stimulation. Therefore it seems that IRS1 may be a 'bystander’ – ubiquitinated due to its proximity to the insulin receptor/MG53 complex. The negative regulation of insulin signaling in skeletal muscle by MG53 triggers systemic changes in insulin function and metabolism.
The compelling results presented in Song et al. suggest a novel role for MG53. It is interesting to speculate on how to reconcile its new identity as an E3 ubiquitin ligase that targets insulin signaling pathways with its canonical role as a membrane repair protein. It is possible that MG53 performs two disparate functions in skeletal muscle – it could exist in two functional pools, perhaps mediated by localization or post-translational modification. Alternatively, its roles may be synergistic. For instance, the direct interaction of MG53 with the insulin receptor and IRS1  could direct MG53 to the cell surface, where it will be readily available when sarcolemmal disruptions require its activity as a repair protein. In support of a correlation between muscle membrane repair defects and insulin resistance, myocytes from db/db diabetic mice show increased dye uptake after laser damage, indicating reduced efficiency of membrane repair. Downhill running of db/db mice showed increased permeability of myocytes from these mice to Evans blue dye compared to wild-type mice . Interestingly, deletion of the MG53-interacting proteins PTRF or caveolin-3 cause glucose intolerance and insulin resistance, as opposed to the protective effect seen in MG53-null mice, suggesting that members of the membrane repair complex have opposing roles in insulin signaling [12, 13].
Alternatively, the E3 ubiquitin ligase activity of MG53 may act on targets in addition to insulin receptors and IRS1. The ubiquitin-mediated degradation of these potential targets may facilitate membrane repair using a mechanism that is not yet known. Interestingly, MG53 has recently been shown to interact with and attenuate the activity of sarcoplasmic reticulum calcium-ATPase 1a (SERCA1a)  – it is possible that SERCA is an additional target for the E3 ubiquitin-ligase activity of MG53.
One of the more surprising findings by Song et al. is that the insulin resistance observed in MG53 transgenic mice preceded changes in body weight and composition, suggesting that MG53 action in muscle alters the whole-body energy balance. The effects of insulin signaling specifically in muscle have been studied previously in mice with muscle-specific inactivation of the insulin receptor gene (muscle-specific insulin receptor knockout (MIRKO) mice) . These mice show relatively minor changes in glucose homeostasis , possibly because the redistribution of substrates to adipose tissue causes increased adiposity in these mice . It is possible that Song et al. observed a similar phenomenon in the MG53-transgenic mice: overexpression of MG53 targets insulin receptors for degradation, which could lead to redistribution of substrates to adipose tissue and, subsequently, metabolic syndrome.
Song et al. found elevated levels of the MG53 protein in obese humans and in several animal models for insulin resistance and metabolic disorders, including HFD-induced obese mice. It is unclear what mechanism is responsible for increased MG53 expression in situations of nutritional overload. Interestingly, a recent study by Ko and colleagues  confirms that MG53 is a ubiquitin E3 ligase that induces IRS-1 ubiquitination, and that MG53-null mice fed a high-fat and high-sucrose diet are protected from insulin resistance. However, this group found that muscle from HFD-fed mice and type 2 diabetes patients had normal levels of the MG53 protein. Ko and colleagues also challenged the finding by Song et al. that insulin receptor is a substrate of MG53 because insulin receptor protein levels and phosphorylation are unchanged when MG53 is overexpressed in myoblasts or genetically disrupted in knockout mice. These inconsistencies highlight a need for further study of the role of MG53 in insulin signaling and metabolism.
Recently, Weisleder et al. explored recombinant MG53 protein therapy in mouse models of muscular dystrophy . They found that exogenous recombinant human MG53 reduced muscle membrane damage and decreased muscle pathology in mdx mice. While these results suggest MG53 protein administration could be therapeutic for Duchenne and potentially other muscular dystrophies, the results from Song et al. should serve to caution researchers to examine insulin resistance and other hallmarks of metabolic disorders in treatment groups. It is even possible that changes in insulin signaling are somehow driving the improved pathology observed in MG53-treated mdx mice.
In addition to discovering a new cellular role for MG53, Song et al. identified MG53-mediated suppression of insulin signaling as having the capacity to underlie systemic insulin resistance and metabolic syndrome. Muscle researchers should be excited to consider MG53 and skeletal muscle insulin signaling as central players in the development of metabolic syndrome.
Insulin receptor substrate
Muscle-specific insulin receptor knockout
Polymerase I and transcript release factor
Sarcoplasmic reticulum calcium-ATPase
Eckel RH, Grundy SM, Zimmet PZ: The metabolic syndrome Lancet 2005,365(9468):1415-1428. 10.1016/S0140-6736(05)66378-7
DeFronzo RA, Jacot E, Jequier E, Maeder E, Wahren J, Felber JP: The effect of insulin on the disposal of intravenous glucose. Results from indirect calorimetry and hepatic and femoral venous catheterization Diabetes 1981,30(12):1000-1007. 10.2337/diab.30.12.1000
Shulman GI, Rothman DL, Jue T, Stein P, DeFronzo RA, Shulman RG: Quantitation of muscle glycogen synthesis in normal subjects and subjects with non-insulin-dependent diabetes by 13C nuclear magnetic resonance spectroscopy New Eng J Med 1990,322(4):223-228. 10.1056/NEJM199001253220403
Weisleder N, Takeshima H, Ma J: Immuno-proteomic approach to excitation–contraction coupling in skeletal and cardiac muscle: molecular insights revealed by the mitsugumins Cell calcium 2008,43(1):1-8. 10.1016/j.ceca.2007.10.006
Cai C, Masumiya H, Weisleder N, Matsuda N, Nishi M, Hwang M, Ko JK, Lin P, Thornton A, Zhao X, Pan Z, Komazaki S, Brotto M, Takeshima H, Ma J: MG53 nucleates assembly of cell membrane repair machinery Nat Cell Biol 2009,11(1):56-64. 10.1038/ncb1812
Song R, Peng W, Zhang Y, Lv F, Wu HK, Guo J, Cao Y, Pi Y, Zhang X, Jin L, Zhang M, Jiang P, Liu F, Meng S, Zhang X, Jiang P, Cao CM, Xiao RP: Central role of E3 ubiquitin ligase MG53 in insulin resistance and metabolic disorders Nature 2013,494(7437):375-379. 10.1038/nature11834
Cai C, Masumiya H, Weisleder N, Pan Z, Nishi M, Komazaki S, Takeshima H, Ma J: MG53 regulates membrane budding and exocytosis in muscle cells J Biol Chem 2009,284(5):3314-3322.
Waddell LB, Lemckert FA, Zheng XF, Tran J, Evesson FJ, Hawkes JM, Lek A, Street NE, Lin P, Clarke NF, Landstrom AP, Ackerman MJ, Weisleder N, Ma J, North KN, Cooper ST: Dysferlin, annexin A1, and mitsugumin 53 are upregulated in muscular dystrophy and localize to longitudinal tubules of the T-system with stretch J Neuropathol Exper Neurol 2011,70(4):302-313. 10.1097/NEN.0b013e31821350b0
Glass DJ: PI3 kinase regulation of skeletal muscle hypertrophy and atrophy Curr Top Microbiol Immunol 2010, 346 267-278. 10.1007/82_2010_78
Lee CS, Yi JS, Jung SY, Kim BW, Lee NR, Choo HJ, Jang SY, Han J, Chi SG, Park M, Lee JH, Ko KG: TRIM72 negatively regulates myogenesis via targeting insulin receptor substrate-1 Cell Death Differ 2010,178):1254-1265. 10.1038/cdd.2010.1
Howard AC, McNeil AK, Xiong F, Xiong WC, McNeil PL: A novel cellular defect in diabetes: membrane repair failure Diabetes 2011,6011):3034-3043. 10.2337/db11-0851
Liu L, Brown D, McKee M, Lebrasseur NK, Yang D, Albrecht KH, Ravid K, Pilch PF: Deletion of Cavin/PTRF causes global loss of caveolae, dyslipidemia, and glucose intolerance Cell metabolism 2008,84):310-317. 10.1016/j.cmet.2008.07.008
Capozza F, Combs TP, Cohen AW, Cho YR, Park SY, Schubert W, Williams TM, Brasaemle DL, Jelicks LA, Scherer PE, Kim JK, Lisanti MP: Caveolin-3 knockout mice show increased adiposity and whole body insulin resistance, with ligand-induced insulin receptor instability in skeletal muscle Am J Physiol Cell Physiol 2005,2886):C1317-C1331. 10.1152/ajpcell.00489.2004
Lee KJ, Park CS, Woo JS, Kim do H, Ma J, Lee EH: Mitsugumin 53 attenuates the activity of sarcoplasmic reticulum Ca2-ATPase 1a (SERCA1a) in skeletal muscle Biochem Biophys Res Comm 2012,4283):383-388. 10.1016/j.bbrc.2012.10.063
Bruning JC, Michael MD, Winnay JN, Hayashi T, Horsch D, Accili D, Goodyear LJ, Kahn CR: A muscle-specific insulin receptor knockout exhibits features of the metabolic syndrome of NIDDM without altering glucose tolerance Molecular cell 1998,25):559-569. 10.1016/S1097-2765(00)80155-0
Kim JK, Michael MD, Previs SF, Peroni OD, Mauvais-Jarvis F, Neschen S, Kahn BB, Kahn CR, Shulman GI: Redistribution of substrates to adipose tissue promotes obesity in mice with selective insulin resistance in muscle J Clin Invest 2000,10512):1791-1797. 10.1172/JCI8305
Yi JS, Park JS, Ham YM, Nguyen N, Lee NR, Hong J, Kim BW, Lee H, Lee CS, Jeong BC, Song HK, Cho H, Kim YK, Lee JS, Park KS, Shin H, Choi I, Lee SH, Park WJ, Park SY, Choi CS, Lin P, Karunasiri M, Tan T, Duann P, Zhu H, Ma J, Ko YG: MG53-induced IRS-1 ubiquitination negatively regulates skeletal myogenesis and insulin signalling Nat Comm 2013, 4 2354.
Weisleder N, Takizawa N, Lin P, Wang X, Cao C, Zhang Y, Tan T, Ferrante C, Zhu H, Chen PJ, Yan R, Sterling M, Zhao X, Hwang M, Takeshima M, Cai C, Cheng H, Takeshima H, Xiao RP, Ma J: Recombinant MG53 protein modulates therapeutic cell membrane repair in treatment of muscular dystrophy Sci Trans Med 2012,4139):139-185.
JRL is supported by a Muscular Dystrophy Association Development grant (MDA200826). This work was supported in part by a Paul D Wellstone Muscular Dystrophy Cooperative Research Center grant (1U54NS053672 to KPC). KPC is an investigator of the Howard Hughes Medical Institute. Work in DJG’s laboratory is supported by Novartis.
DJG is an employee of Novartis.
JRL, KPC and DJG wrote the commentary. All authors read and approved the final manuscript.