Delayed Recovery of Skeletal Muscle Mass following Hindlimb Immobilization in mTOR Heterozygous Mice

University of Rome La Sapienza, Italy
PLoS ONE (Impact Factor: 3.23). 06/2012; 7(6):e38910. DOI: 10.1371/journal.pone.0038910
Source: PubMed


The present study addressed the hypothesis that reducing mTOR, as seen in mTOR heterozygous (+/-) mice, would exaggerate the changes in protein synthesis and degradation observed during hindlimb immobilization as well as impair normal muscle regrowth during the recovery period. Atrophy was produced by unilateral hindlimb immobilization and data compared to the contralateral gastrocnemius. In wild-type (WT) mice, the gradual loss of muscle mass plateaued by day 7. This response was associated with a reduction in basal protein synthesis and development of leucine resistance. Proteasome activity was consistently elevated, but atrogin-1 and MuRF1 mRNAs were only transiently increased returning to basal values by day 7. When assessed 7 days after immobilization, the decreased muscle mass and protein synthesis and increased proteasome activity did not differ between WT and mTOR(+/-) mice. Moreover, the muscle inflammatory cytokine response did not differ between groups. After 10 days of recovery, WT mice showed no decrement in muscle mass, and this accretion resulted from a sustained increase in protein synthesis and a normalization of proteasome activity. In contrast, mTOR(+/-) mice failed to fully replete muscle mass at this time, a defect caused by the lack of a compensatory increase in protein synthesis. The delayed muscle regrowth of the previously immobilized muscle in the mTOR(+/-) mice was associated with a decreased raptor•4EBP1 and increased raptor•Deptor binding. Slowed regrowth was also associated with a sustained inflammatory response (e.g., increased TNFα and CD45 mRNA) during the recovery period and a failure of IGF-I to increase as in WT mice. These data suggest mTOR is relatively more important in regulating the accretion of muscle mass during recovery than the loss of muscle during the atrophy phase, and that protein synthesis is more sensitive than degradation to the reduction in mTOR during muscle regrowth.

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Available from: Abid A Kazi, Jan 02, 2014
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    • "Genes involved in protein degradation that are transcriptionally regulated by FoXO [38] [39] [128] include effectors of UPS-and ALPmediated proteolysis, i.e. atrogin-1 and MuRF1 [31,32,113], and BNIP3 [37,39,40], respectively, which are known to increase under muscle atrophying conditions [122]. As expected [122] [127] both atrogin-1 and MuRF-1 mRNA expressions decreased during RL. Conversely, BNIP3 expression transiently increased, in line with literature [127], and subsequently decreased throughout further RL. "
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    • "Total RNA was extracted using Tri-reagent (Molecular Research Center, Inc., Cincinnati, OH) and RNeasy mini kit (Qiagen, Valencia, CA) following manufacturers’ protocols and previous reports [14], [28], [32]. Briefly, tissue was homogenized in Tri-reagent followed by phenol/chloroform extraction according to the manufacturer’s instruction. "
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    • "Similarly, we have shown that 14 days of glucocorticoid treatment did not result in an increase in activity for any of the three catalytic subunits of the proteasome despite significant upregulation of MuRF1 and MAFbx expression (Baehr et al., 2011). Conversely, when mice were allowed to recover following 7 days of hindlimb unloading, MuRF1 and MAFbx expression was not increased at any of the time points analyzed, but 20S β5 proteasome activity was significantly increased on the first day of recovery (Lang et al., 2012). Lastly, under denervation conditions, the lack of MuRF1 resulted in greater activation of the proteasome, not less (Gomes et al., 2012). "
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