Abmormal expression of mitochondrial gene in disuse-induced muscle atrophy

为了阐明微重力诱导肌肉萎缩的机制，我们研究了暴露于太空飞行16天的新生大鼠萎缩腓肠肌中肌球蛋白重链（myosin heavy chain， MHC）的降解和基因表达。太空飞行刺激了蛋白质的泛素化，包括MHC分子，以及MHC降解碎片在肌肉中的积累。与地面对照大鼠相比，航天飞行显著增加了肌肉组织蛋白酶L、蛋白酶体组分（RC2和RC9）、多泛素、泛素结合酶、NADH脱氢酶和16S核糖体RNA的mRNA水平。μ-calpain、m-calpain、cathepsin B和cathepsin H mrna的表达水平未见明显变化。大鼠尾巴悬吊10天或更长时间也会引起类似的变化，在航天大鼠中观察到，这表明MHC降解与蛋白酶体依赖性蛋白水解的激活和线粒体基因的异常表达密切相关。然后，我们尝试通过补充氨基酸来改善悬液诱导的肌肉萎缩。饲喂半胱氨酸140 mg，可抑制肌肉蛋白泛素化，显著预防后肢肌肉重量的减少。谷氨酰胺、丙氨酸或半胱氨酸蛋白酶抑制剂对萎缩作用不大。半胱氨酸可以清除线粒体产生的活性氧自由基，因此补充半胱氨酸可以防止悬浮诱导的肌肉蛋白泛素化。

To elucidate the mechanisms of microgravity-induced muscle atrophy, we focused on myosin heavy chain (MHC) degradation and gene expression in atrophied gastrocnemius muscles of neonatal rats exposed to 16-day spaceflight. The spaceflight stimulated ubiquitination of proteins, including a MHC molecule, and accumulation of MHC degradation fragments in the muscles. The spaceflight significantly increased mRNA levels of cathepsin L, proteasome components (RC2 and RC9), polyubiquitin, ubiquitin-conjugating enzyme, NADH dehydrogenase and 16S ribosome RNA in the muscles, compared with those of ground control rats. The levels of μ-calpain, m-calpain, cathepsin B and cathepsin H mRNAs were not changed by the spaceflight. Tail-suspension of rats for 10 days or longer also caused similar changes, as was observed in the spaceflight rats, suggesting that MHC degradation were closely associated with activation of proteasome-dependent proteolysis and abnormal expression of mitochondrial gene. We then tried to improve the suspension-induced muscle atrophy by amino acid supplementation. Intragastric supplementation of 140 mg of cysteine to suspended rats significantly prevented loss of the hindlimb muscle weight by suppressing ubiquitination of muscle protein. Adminstraton of glutamine, alanine or cysteine protease inhibitor had little effect on the atrophy. Cysteine may scavenge reactive oxygen radicals produced from mitochondria so that cysteine supplementation prevented suspension-induced ubiquitination of muscle protein.