Liu S, Sarkar C, Dinizo M, Faden A I, Koh E Y, Lipinski M M, Wu J
Department of Orthopaedics, University of Maryland School of Medicine, Baltimore, MD, USA.
Department of Anesthesiology, Shock, Trauma and Anesthesiology Research (STAR) Center, University of Maryland School of Medicine, Baltimore, MD, USA.
Cell Death Dis. 2015 Jan 8;6(1):e1582. doi: 10.1038/cddis.2014.527.
Autophagy is a catabolic mechanism facilitating degradation of cytoplasmic proteins and organelles in a lysosome-dependent manner. Autophagy flux is necessary for normal neuronal homeostasis and its dysfunction contributes to neuronal cell death in several neurodegenerative diseases. Elevated autophagy has been reported after spinal cord injury (SCI); however, its mechanism, cell type specificity and relationship to cell death are unknown. Using a rat model of contusive SCI, we observed accumulation of LC3-II-positive autophagosomes starting at posttrauma day 1. This was accompanied by a pronounced accumulation of autophagy substrate protein p62, indicating that early elevation of autophagy markers reflected disrupted autophagosome degradation. Levels of lysosomal protease cathepsin D and numbers of cathepsin-D-positive lysosomes were also decreased at this time, suggesting that lysosomal damage may contribute to the observed defect in autophagy flux. Normalization of p62 levels started by day 7 after SCI, and was associated with increased cathepsin D levels. At day 1 after SCI, accumulation of autophagosomes was pronounced in ventral horn motor neurons and dorsal column oligodendrocytes and microglia. In motor neurons, disruption of autophagy strongly correlated with evidence of endoplasmic reticulum (ER) stress. As autophagy is thought to protect against ER stress, its disruption after SCI could contribute to ER-stress-induced neuronal apoptosis. Consistently, motor neurons showing disrupted autophagy co-expressed ER-stress-associated initiator caspase 12 and cleaved executioner caspase 3. Together, these findings indicate that SCI causes lysosomal dysfunction that contributes to autophagy disruption and associated ER-stress-induced neuronal apoptosis.
自噬是一种分解代谢机制，以溶酶体依赖的方式促进细胞质蛋白和细胞器的降解。自噬通量对于正常的神经元稳态是必需的，其功能障碍在几种神经退行性疾病中导致神经元细胞死亡。脊髓损伤（SCI）后已有自噬增加的报道；然而，其机制、细胞类型特异性以及与细胞死亡的关系尚不清楚。使用挫伤性SCI大鼠模型，我们观察到自创伤后第1天开始LC3-II阳性自噬体的积累。这伴随着自噬底物蛋白p62的显著积累，表明自噬标志物的早期升高反映了自噬体降解的破坏。此时溶酶体蛋白酶组织蛋白酶D的水平和组织蛋白酶D阳性溶酶体的数量也降低，提示溶酶体损伤可能导致观察到的自噬通量缺陷。SCI后第7天p62水平开始恢复正常，并与组织蛋白酶D水平升高相关。SCI后第1天，自噬体在腹角运动神经元、背柱少突胶质细胞和小胶质细胞中显著积累。在运动神经元中，自噬的破坏与内质网（ER）应激的证据密切相关。由于自噬被认为可抵御ER应激，SCI后其破坏可能导致ER应激诱导的神经元凋亡。一致地，显示自噬破坏的运动神经元共表达ER应激相关的起始半胱天冬酶12和裂解的执行半胱天冬酶3。总之，这些发现表明SCI导致溶酶体功能障碍，进而导致自噬破坏以及相关的ER应激诱导的神经元凋亡。
