Kuczmarski J Matthew, Hord Jeff M, Lee Yang, Guzzoni Vinicius, Rodriguez Dinah, Lawler Matthew S, Garcia-Villatoro Erika L, Holly Dylan, Ryan Patrick, Falcon Kristian, Garcia Marcela, Janini Gomes Mariana, Fluckey James D, Lawler John M
Redox Biology & Cell Signaling Laboratory, Department of Health and Kinesiology, Texas A&M University, College Station, TX, USA.
Heart and Vascular Institute, Penn State College of Medicine, Hershey, PA, USA.
Exp Physiol. 2018 Apr 1;103(4):545-558. doi: 10.1113/EP086649. Epub 2018 Feb 28.
What is the central question of this study? Translocation of nNOSμ initiates catabolic signalling via FoxO3a and skeletal muscle atrophy during mechanical unloading. Recent evidence suggests that unloading-induced muscle atrophy and FoxO3a activation are redox sensitive. Will a mimetic of superoxide dismutase and catalase (i.e. Eukarion-134) also mitigate suppression of the Akt-mTOR pathway? What is the main finding and its importance? Eukarion-134 rescued Akt-mTOR signalling and sarcolemmal nNOSμ, which were linked to protection against the unloading phenotype, muscle fibre atrophy and partial fibre-type shift from slow to fast twitch. The loss of nNOSμ from the sarcolemma appears crucial to Akt phosphorylation and is redox sensitive, although the mechanisms remain unresolved.
Mechanical unloading stimulates rapid changes in skeletal muscle morphology, characterized by atrophy of muscle fibre cross-sectional area and a partial fibre-type shift from slow to fast twitch. Recent studies revealed that oxidative stress contributes to activation of forkhead box O3a (FoxO3a), proteolytic signalling and unloading-induced muscle atrophy via translocation of the μ-splice variant of neuronal nitric oxide synthase (nNOSμ) and activation of FoxO3a. There is limited understanding of the role of reactive oxygen species in the Akt-mammalian target of rapamycin (mTOR) pathway signalling during unloading. We hypothesized that Eukarion-134 (EUK-134), a mimetic of the antioxidant enzymes superoxide dismutase and catalase, would protect Akt-mTOR signalling in the unloaded rat soleus. Male Fischer 344 rats were separated into the following three study groups: ambulatory control (n = 11); 7 days of hindlimb unloading + saline injections (HU, n = 11); or 7 days of HU + EUK-134; (HU + EUK-134, n = 9). EUK-134 mitigated unloading-induced dephosphorylation of Akt, as well as FoxO3a, in the soleus. Phosphorylation of mTOR in the EUK-treated HU rats was not different from that in control animals. However, EUK-134 did not significantly rescue p70S6K phosphorylation. EUK-134 attenuated translocation of nNOSμ from the membrane to the cytosol, reduced nitration of tyrosine residues and suppressed upregulation of caveolin-3 and dysferlin. EUK-134 ameliorated HU-induced remodelling, atrophy of muscle fibres and the 12% increase in type II myosin heavy chain-positive fibres. Attenuation of the unloaded muscle phenotype was associated with decreased reactive oxygen species, as assessed by ethidium-positive nuclei. We conclude that oxidative stress affects Akt-mTOR signalling in unloaded skeletal muscle. Direct linkage of abrogation of nNOSμ translocation with Akt-mTOR signalling during unloading is the subject of future investigation.
本研究的核心问题是什么？在机械性卸载过程中，神经元型一氧化氮合酶μ亚型（nNOSμ）的易位通过叉头框蛋白O3a（FoxO3a）启动分解代谢信号并导致骨骼肌萎缩。最近的证据表明，卸载诱导的肌肉萎缩和FoxO3a激活对氧化还原敏感。超氧化物歧化酶和过氧化氢酶模拟物（即Eukarion-134）是否也能减轻对Akt-雷帕霉素哺乳动物靶蛋白（mTOR）通路的抑制作用？主要发现及其重要性是什么？Eukarion-134挽救了Akt-mTOR信号传导和肌膜nNOSμ，这与预防卸载表型、肌纤维萎缩以及肌纤维类型从慢肌向快肌的部分转变有关。肌膜nNOSμ的丧失似乎对Akt磷酸化至关重要，并且对氧化还原敏感，尽管其机制仍未明确。
机械性卸载会刺激骨骼肌形态迅速发生变化，其特征为肌纤维横截面积萎缩以及肌纤维类型从慢肌向快肌的部分转变。最近的研究表明，氧化应激通过神经元型一氧化氮合酶μ剪接变体（nNOSμ）的易位和FoxO3a的激活，促进叉头框O3a（FoxO3a）的激活、蛋白水解信号传导以及卸载诱导的肌肉萎缩。对于卸载过程中活性氧在Akt-雷帕霉素哺乳动物靶蛋白（mTOR）通路信号传导中的作用，人们了解有限。我们假设，抗氧化酶超氧化物歧化酶和过氧化氢酶的模拟物Eukarion-134（EUK-134）能够保护卸载大鼠比目鱼肌中的Akt-mTOR信号传导。将雄性Fischer 344大鼠分为以下三个研究组：活动对照组（n = 11）；后肢卸载7天+注射生理盐水组（HU，n = 11）；或后肢卸载7天+EUK-134组（HU + EUK-134，n = 9）。EUK-134减轻了比目鱼肌中卸载诱导的Akt以及FoxO3a的去磷酸化。EUK处理的HU大鼠中mTOR的磷酸化与对照动物无异。然而，EUK-134并未显著挽救p70S6K的磷酸化。EUK-134减弱了nNOSμ从膜向细胞质的易位，减少了酪氨酸残基的硝化，并抑制了小窝蛋白3和dysferlin的上调。EUK-134改善了HU诱导的重塑、肌纤维萎缩以及II型肌球蛋白重链阳性纤维增加12%的情况。通过乙锭阳性核评估，卸载肌肉表型的减轻与活性氧的减少有关。我们得出结论，氧化应激影响卸载骨骼肌中的Akt-mTOR信号传导。卸载过程中nNOSμ易位的消除与Akt-mTOR信号传导的直接联系是未来研究的主题。
