Abstract
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.
Highlights
The plasticity of skeletal muscle is evidenced by the hypertrophy seen with strength training and, the atrophy seen with disuse [1,2]
The most widely used of these is likely hindlimb suspension, but an atrophic response is seen in response to extended bed rest, denervation, or hindlimb immobilization
As we are aware of no investigation which reports the temporal progression of changes in both muscle protein synthesis and degradation during both a period of immobilization and recovery in the same animal model, our initial studies focused on the metabolic characterization of this new murine model
Summary
The plasticity of skeletal muscle is evidenced by the hypertrophy seen with strength training and, the atrophy seen with disuse [1,2] This atrophic response can be generalized when produced by extended periods of bed rest or localized when a single limb is immobilized [3,4,5,6,7,8,9]. This muscle wasting when sustained and severe can be a determinant of morbidity and mortality in chronic catabolic states, and results from an imbalance between rates of protein synthesis and breakdown. As restoration of muscle mass is necessary to re-establish normal function, a greater understanding of mechanisms by which muscle is lost and regained is of clinical importance
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