Møller Andreas Buch, Vendelbo Mikkel Holm, Schjerling Peter, Couppé Christian, Møller Niels, Kjær Michael, Hansen Mette, Jessen Niels
Research Laboratory for Biochemical Pathology, Institute for Clinical Medicine, Aarhus University, Aarhus, Denmark.
Steno Diabetes Center Aarhus, Aarhus University Hospital, Aarhus, Denmark.
Front Physiol. 2019 Jun 14;10:736. doi: 10.3389/fphys.2019.00736. eCollection 2019.
Immobilization of the lower limbs promotes a catabolic state that reduces muscle mass, whereas physical training promotes an anabolic state that increases muscle mass. Understanding the molecular mechanisms underlying this is of clinical interest, as loss of muscle mass is a major complication to critical illness in humans. To determine the molecular regulation of protein synthesis and degradation during muscle loss and hypertrophy, we examined skeletal muscle biopsies from healthy human subjects after 2 weeks unilateral immobilization of a lower limb and during 6 weeks of physical rehabilitation. We have previously shown that cross-sectional area of the knee muscle-extensors decreased by ∼10% during immobilization and was completely restored during rehabilitation. Here we provide novel data to suggest that autophagy is an important underlying mechanism involved in regulation of muscle mass. Protein expression of MuRF1 and ATROGIN-1 did not change during the study, indicating that the recruitment of substrates to the proteasomes was unaltered. Phosphorylation of mTORat Ser did not change during the study, and neither did phosphorylation of the mTORC1 substrates 4EBP1 Thr and p70S6K Thr, suggesting that this pathway does not suppress protein synthesis during muscle wasting. Protein levels of p62 and ULK1 increased during immobilization and returned to baseline levels during rehabilitation. Same pattern was observed for FOXO3a phosphorylation at Ser, suggesting transcriptional activation during immobilization and inactivation during rehabilitation. To investigate this further, we analyzed mRNA expression of seven autophagy-related genes controlled by FOXO3a. Five of these (p62, LC3B, BECLIN-1, ATG12, and BNIP3) increased during immobilization and returned to baseline during rehabilitation. In conclusion, immobilization of a lower limb increases autophagy-related gene and protein expression in human skeletal muscle in a pattern that mirrors FOXO3a phosphorylation. These findings could imply that FOXO3a dependent transcriptional regulation of autophagy is involved in the regulation of muscle mass in humans.
The study was approved by the Ethics Committee of Copenhagen (j.no. H-1-2010-016).
下肢固定会促进分解代谢状态，导致肌肉量减少，而体育锻炼则促进合成代谢状态，增加肌肉量。了解其潜在的分子机制具有临床意义，因为肌肉量减少是人类危重病的主要并发症。为了确定肌肉减少和肥大期间蛋白质合成和降解的分子调控，我们检查了健康人类受试者在下肢单侧固定2周后以及6周身体康复期间的骨骼肌活检样本。我们之前已经表明，在固定期间，膝部伸肌的横截面积减少了约10%，而在康复期间完全恢复。在这里，我们提供了新的数据表明自噬是参与肌肉量调节的重要潜在机制。在研究期间，MuRF1和ATROGIN-1的蛋白质表达没有变化，表明蛋白酶体底物的募集没有改变。在研究期间，mTOR丝氨酸位点的磷酸化没有变化，mTORC1底物4EBP1苏氨酸位点和p70S6K苏氨酸位点的磷酸化也没有变化，这表明该途径在肌肉消耗期间不会抑制蛋白质合成。在固定期间，p62和ULK1的蛋白质水平升高，在康复期间恢复到基线水平。FOXO3a丝氨酸位点的磷酸化也观察到相同的模式，表明在固定期间转录激活，在康复期间失活。为了进一步研究这一点，我们分析了由FOXO3a控制的七个自噬相关基因的mRNA表达。其中五个基因（p62、LC3B、BECLIN-1、ATG12和BNIP3）在固定期间增加，在康复期间恢复到基线水平。总之，下肢固定会增加人类骨骼肌中自噬相关基因和蛋白质的表达，其模式与FOXO3a磷酸化情况一致。这些发现可能意味着FOXO3a依赖的自噬转录调控参与了人类肌肉量的调节。
该研究已获得哥本哈根伦理委员会批准（编号：H-1-2010-016）。
