Apoptosis of motor neurons after spinal cord injury

脊髓损伤后，脊髓运动神经元具有选择性易损。最近的研究表明，它们通过线粒体依赖的凋亡途径激活caspase后发生凋亡，并且脊髓损伤后的氧化应激可能起作用。然而，脊髓损伤后涉及线粒体的其他凋亡信号通路尚未被深入研究。凋亡诱导因子（AIF）和内切酶G （EndoG）是线粒体凋亡蛋白，通过caspase不依赖的途径从线粒体转移到细胞核，能够诱导神经元凋亡。在这项研究中，我们检测了这些蛋白的易位和运动神经元的凋亡细胞死亡。利用转基因（Tg）大鼠过度表达内在抗氧化剂铜/锌超氧化物歧化酶（SOD1），研究了氧化应激的作用。Western blots和活性分析表明，Tg大鼠线粒体中SOD1含量和SOD活性均高于野生型大鼠。免疫组化和Western blot结果显示，两组大鼠脊髓损伤后1天运动神经元内EndoG和AIF均从线粒体向细胞核转移。然而，与Wt大鼠相比，Tg大鼠的EndoG易位明显减少。脊髓损伤后3天，Tg大鼠的凋亡细胞死亡少于Wt大鼠。这些结果表明，EndoG和AIF从线粒体转移到细胞核可能启动了一个不依赖于caspase的凋亡途径。线粒体中SOD1水平的升高可以减轻氧化应激，从而减弱EndoG易位，并导致caspase非依赖性凋亡的减少。

Spinal motor neurons are selectively vulnerable after spinal cord injury (SCI). Recent studies suggest they undergo apoptosis after caspase activation through a mitochondria-dependent apoptosis pathway, and that oxidative stress after SCI is likely to play a role. However, other signaling pathways of apoptosis that involve mitochondria have not been thoroughly studied after SCI. Apoptosis-inducing factor (AIF) and endonuclease G (EndoG) are mitochondrial apoptogenic proteins that are capable of inducing neuronal apoptosis when translocated from mitochondria to nuclei through a caspase-independent pathway. In this study, we examined translocation of these proteins and apoptotic cell death of motor neurons. The role of oxidative stress was also studied using transgenic (Tg) rats that overexpress the intrinsic antioxidant copper/zinc-superoxide dismutase (SOD1). Western blots and an activity assay demonstrated that a greater amount of SOD1 and higher activity of SOD presented in mitochondria of Tg rats compared with wild-type (Wt) rats. Immunohistochemistry and Western blots showed translocation of EndoG and AIF from mitochondria to nuclei in motor neurons 1 day after SCI in both groups of rats. However, there was significantly less translocation of EndoG in the Tg rats compared with the Wt rats. Less apoptotic cell death was detected in the Tg rats than in the Wt rats 3 days after SCI. These results suggest that translocation of EndoG and AIF from mitochondria to nuclei may initiate a caspase-independent pathway of apoptosis. An increased level of SOD1 in mitochondria conceivably reduces oxidative stress, thereby attenuating EndoG translocation, and resulting in reduction of caspase-independent apoptosis.