オートファジーによるバルク分解とそのモニター方法

Monitoring methods for autophagy

　オートファジー（自食作用）は真核生物に普遍的に備わっている主要な細胞内分解システムである。ユビキチン・プロテアソーム系が選択的なタンパク質分解を担っているのに対し，オートファジーはリソソームにおける非特異的な大規模な分解系である。オートファジーは細胞質成分の日常的な代謝回転のみならず，飢餓応答，発生分化，細胞死，変性疾患などにも深く関与していると考えられているものの，未だ具体的な意義はほとんど明らかにされていない。その理由のひとつとして，これまでオートファジーをモニターする良い方法がなかったことがあげられる。そこで本稿では，オートファジーのモニター方法を中心に，バルク分解の研究の現状を考えてみたい。

　Autophagy is an intracellular bulk degradation system that is ubiquitously found in eukaryotes. Autophagy accounts for the degradation of most long-lived proteins and some organelles；cytoplasmic constituents, including organelles, are sequestered into double-membraned autophagosomes, which subsequently fuse with lysosomes. This system is implicated in various physiological processes such as protein and organelle turnover, the starvation response, cellular differentiation, cell death and pathogenesis. However, its precise role is still poorly understood. In addition, systematic analysis describing where and when autophagy occurs has not been performed. This is largely due to a lack of good diagnostic methods. To date, electron microscopy has been the only method to monitor autophagy. Unfortunately, this is a method requiring many skills and much time, and sometimes it is difficult to distinguish autophagic vacuoles from other structures just by morphology. Based on the findings obtained from yeast genetics, we have dissected the autophagic process in mammalian cells at the molecular level. These studies also provided us several useful marker proteins for autophagic vacuoles, which enable us to detect the autophagosomes easily and accurately by fluorescent microscopy. We also applied this method toin vivostudies. We generated a transgenic mouse systemically expressing GFP-LC3（mammalian Aut7/Apg8）that labels autophagosomes. Using this transgenic mouse, we observed very active autophagy in many tissues particularly after food withdrawal. We also found that autophagy occurs almost constitutively in some type of cells such as thymic epithelial cells. The role and regulation of autophagy are discussed.

（Received：August 29, 2003）