Apoptosis of motor neurons after spinal cord injury

脊髓损伤后，脊髓运动神经元选择性地变得脆弱。最近的研究表明，他们经历凋亡后，半胱天冬酶激活后，通过一个依赖于细胞凋亡途径，氧化应激后SCI可能发挥作用。然而，SCI后涉及线粒体的其他凋亡信号通路尚未得到深入研究。凋亡诱导因子（Apoptosis-inducing factor，AIF）和核酸内切酶G（EndoG）是线粒体凋亡诱导蛋白，通过非半胱天冬酶依赖性途径从线粒体转移到细胞核，诱导神经元凋亡。在这项研究中，我们研究了这些蛋白质的易位和运动神经元的细胞凋亡。氧化应激的作用也进行了研究，使用过表达的内在抗氧化剂铜/锌-超氧化物歧化酶（SOD 1）的转基因（Tg）大鼠。Western印迹和活性测定结果表明，与野生型（Wt）大鼠相比，Tg大鼠线粒体中SOD 1的含量和活性均显著增加。免疫组化和Western印迹显示，脊髓损伤后1天，两组大鼠运动神经元的EndoG和AIF从线粒体易位到细胞核。然而，与Wt大鼠相比，Tg大鼠中的EndoG易位显著较少。SCI后3天，Tg大鼠中检测到的凋亡细胞死亡少于Wt大鼠。这些结果表明，EndoG和AIF从线粒体到细胞核的易位可能启动凋亡的半胱天冬酶非依赖性途径。线粒体中SOD 1水平的增加可以降低氧化应激，从而减弱EndoG易位，并导致半胱天冬酶非依赖性细胞凋亡的减少。

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.