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内容記述 |
Autophagy is a non-selective degradation process in which long-lived proteins and organelles are sequestered within double-membrane vesicles, termed autophagosomes that deliver the contents to the lysosome/vacuole for degradation. The lysosome/vacuole is an acidic compartment that contains various hydrolytic enzymes. Autophagy is fundamental and evolutionarily conserved function in eukaryotes. More than 20 genes, which are essential for autophagy (ATG genes; <U>A</U>u<U>T</U>opha<U>G</U>y), have been identified by genetic screens in yeast, and most of the ATG genes are conserved in higher eukaryotes. <br /> The major function of autophagy is response to starvation. For example, autophagy is suppressed to undetectable levels under nutrient rich conditions in yeast, but it is rapidly induced under nutrient starvation conditions. In autophagy deficient mutants of yeast, the survival rate decreases under starvation conditions. Similarly, autophagy also plays a response to starvation in mammals. Autophagy is induced in many tissues in response to food withdrawal in adult mice. Autophagy-defective mutant mice, Atg5<SUP>-/-</SUP> mice, appear almost normal at birth, but most of the Atg5<SUP>-/-</SUP> neonates died within 1 day of delivery. I thought that phenotype of Atg5<SUP>-/-</SUP> mice is related to nutrient starvation. However, the kinetics of autophagy is unclear in wild-type embryos and neonates. In this study, to study the significance of mammalian autophagy in vivo, I observed occurrence of autophagy in embryos and neonates using transgenic mouse model in which autophagosomes are labelled with GFP-LC3. <br /> I observed that autophagy remained at low levels throughout the embryonic period. Formation of autophagosomes was extensively induced in various tissues after a natural birth and is maintained at high levels for 3-12 h after birth. The number of autophagosomes gradually decreased to basal levels by day one or two. Soon after birth, mammals face with the first, and probably the most severe, starvation during their lifespan, because trans-placental nutrients supply is suddenly terminated. These results suggest that induction of autophagy is important for survival during neonatal starvation. <br /> In mammal, embryos are under the nutrient rich conditions. However, I observed that low levels of autophagy occur constantly at embryonic period. It is unclear that the basal autophagy plays an important role during mouse embryogenesis. In this study, I examined the role of basal autophagy using the autophagy indicator mice and autophagy deficient mice. In histological examination of Atg5<SUP>-/-</SUP> neonates, I detected ubiquitin-positive aggregates (inclusion bodies; IBs) only in a few tissues, including neural cells, hepatocytes, and anterior pituitary gland cells. These results suggest that function of autophagy is not only maintenance of viability during starvation, but also intracellular quality control. Recently, a similar observation was reported in the hepatocytes of liver-specific conditional Atg7<SUP>-/-</SUP> mice. I demonstrated that time from the liver genesis to the birth is sufficient to generate large IBs in hepatocytes. In addition, I revealed that protein quality control is highly dependent on basal levels of autophagy in hepatocytes, neural cells and anterior pituitary cells. <br /> Autophagy can degrade not only proteins but also intracellular organelles such as mitochondria, peroxisomes and endoplasmic reticulum. Dramatic degradation of organelles is observed in the processes of lens organelle free zone (OFZ) formation. The mechanism by which these organelles are destroyed during this process is not fully understood. It is suggested that autophagy is involved in lens organelle degradation. However, organelle degradation during the differentiation of lens occurred normally in Atg5<SUP>-/-</SUP> mice. Therefore, autophagy is dispensable for OFZ formation in lens epithelial cells. <br /> In this study, I showed significance of basal- and induced-autophagy during embryogenesis and neonatal starvation. These findings of in vivo analyses will help to understand physiological significance of autophagy in mammals. |
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