Introduction

      There are two major known routes for clearance of aberrant components in eukaryotic cells: (i) the ubiquitin–proteasomal pathway and (ii) the autophagy–lysosomal pathway. The ubiquitin–proteasomal pathway is responsible for degradation of short-lived proteins and has been studied intensively during the past decades, including in brain injury (Hu et al, 2000; Hu, 2006; Ciechanover, 2006). The autophagy pathway, originally described as a stress response to nutrient deprivation, is now emerging as the chief route for bulk degradation of aberrant organelles, protein aggregates, and invading foreign materials (Nixon, 2006). There are three basic types of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy. Bulk degradation of cytoplasmic organelles is largely mediated bymacroautophagy,

which is commonly referred to as autophagy (hereafter). Autophagy is a nonstop lifesustaining

renewal process that is active under normal conditions and is further enhanced in response to tissue injury (Klionsky, 2005).

     The molecular mechanisms underlying autophagy have just begun to emerge: (i) a group of autophagyrelated genes (atgs) and their encoded proteins (ATGs) have been identified by genetic screens in yeast and fungi; and (ii) human homologues of the highly conserved ATGs have been discovered and tied to specific human genetic diseases (Kiselyov et al, 2007). Autophagy starts with the formation of double-membraned cisternae that subsequently engulf cytoplasmic materials or whole organelles to become double-membrane bubblelike vacuoles known as autophagosomes (APs). After maturation, APs merge with lysosomes for bulk egradation of the cargo contents (Yorimitsu and Klionsky, 2005).Hence, the appearance of APs under transmission electron microscopy (EM) is a morphologic hallmark unique to autophagy. Two biochemical markers, also unique to autophagy, are the covalent conjugates of (i) ATG12-ATG5 and (ii) microtubule-associated protein light chain 3 (LC3)-phosphatidylethanolamine (PE). Microtubule-associated protein light chain 3 is a mammalian homologue of yeast ATG8 and it is synthesized as a pro-LC3. After synthesis, pro-LC3 is cleaved by ATG4 protease and becomes the 16 to 18 kDa LC3-I. On activation of autophagy, LC3-I is conjugated with PE (lipidated). The lipidated form is referred to as LC3-II (Kabeya et al,2004). The conjugation to create ATG12-ATG5 or LC3-II is performed by two consecutive ubiquitinationlike enzyme systems in an ATP-dependent manner, involving ATG7 and ATG10 for ATG12-ATG5 conjugation, and ATG7 and ATG3 for LC3-II conjugation (Klionsky, 2005). After conjugation,both ATG12-ATG5 and LC3-II become structural components of the double-membraned cisterns or APs and are thus redistributed among the membrane fractions. Therefore, the protein levels and redistribution of ATG5-ATG12 and/or LC3-II conjugates in the membrane fractions are often used as a measure to determine autophagic activity (Kabeya et al, 2004).

    Traumatic brain injury (TBI) is a serious and debilitating health problem affecting millions of people each year (http://www.ninds.nih.gov). Traumatic brain injury leads to brain tissue damage and cognitive impairment (Bramlett and Dietrich,2004). Although remarkable progress has been made in pathophysiology, molecular events after TBI are still incompletely understood. This study used the rat fluid percussion injury model to investigate whether the autophagy pathway is involved in TBI tissue damage and repair. The results clearly show that autophagy is induced significantly after TBI. Induction of autophagy after TBI may be responsible for eliminating aberrant cellular components, thus maintaining cellular homeostasis after TBI.

前言

     有两条主要的已知途径以清除在真核细胞中的异常成分:泛素-蛋白酶体途径和自噬-溶酶体途径。泛素-蛋白酶体途径与短命蛋白质相联系,这条通路在过去的十年已经被很好的研究了,包括脑外伤。自噬-溶酶体途径最初被认为是一种在营养缺乏状态下的应激反应,现在被认为是大量降解异常细胞器、蛋白聚集还有外来侵入物质的主要通路。自噬有三种类型:大自噬,小自噬和分子伴侣介导的自噬。细胞内细胞器大量降解主要受大自噬所调节,就是通常所说的自噬。自噬是一个不断的持续更新的过程,也可以更进一步恢复常态下的组织伤害反应。

    自噬潜在的分子机制已经逐渐被揭示出来：1）一组自噬相关基因(atgs)和及其所编码的蛋白(ATGs)可以在酵母和真菌的遗传基因中识别；2）与人类相同的高度保守的ATGS基因被发现,并且跟人类特殊遗传疾病相关联。自噬开始先形成泡状的双层膜结构,随后吞噬细胞质的物质及所有的细胞器,称为自噬小体 (APs),成熟后,自噬小体吞噬大量降解的溶酶体里的包含物。微管相关蛋白轻链（LC3）是哺乳动物中酵母ATG8基因的同源物，合成后,前LC3自ATG4蛋白酶中分裂成为16-18KDa的LC3-1。激活自噬，LC3-1与脂质性的PE相连接，这种脂质性的形式就是所谓的 LC3-II ，形成ATG12-ATG5 或 LC3-II 的连接由两个相关的泛素样酶体系统以依赖ATP的形式参与，包括ATG7和ATG10 参与ATG12-ATG5连接，ATG7和ATG3 参与LC3-II 的连接。连接后，ATG12-ATG5 和LC3-II 均成为自噬双层膜结构或自噬小体的结构性组成部分，并在膜成分中重新分部。膜成分中ATG5-ATG12 和/或LC3-II 连接体的蛋白水平和再分布通常被用来衡量自噬活力。

   脑外伤是一个严重影响数百万人民的健康问题,脑外伤可以导致脑组织损伤和认知能力的缺损。虽然脑外伤从病理生理学的角度来说已经取得了明显进步,但是在分子水平上对于TBI的了解还不清楚。该研究利用大鼠液压冲击脑外伤模型探查脑外伤后的组织损伤和修复过程是否包含细胞的自噬过程,研究结果显示：细胞的自噬作用在脑外伤发生后起了显著作用。在TBI发生后,细胞的自噬可能起了消除细胞中的异常成分,同时保持细胞中物质动态平衡的作用。