Aging is a multifactorial process with many mechanisms contributing to the decline. Mutations decreasing insulin/IGF-1 (insulin-like growth factor-1) or TOR (target of rapamycin) kinase-mediated signaling, mitochondrial activity and food intake each extend life span in divergent animal phyla. Understanding how these genetically distinct mechanisms interact to control longevity is a fundamental and fascinating problem in biology. Here we show that mutational inactivation of autophagy genes, which are involved in the degradation of aberrant, damaged cytoplasmic constituents accumulating in all aging cells, accelerates the rate at which the tissues age in the nematode Caenorhabditis elegans. According to our results Drosophila flies deficient in autophagy are also short-lived. We further demonstrate that reduced activity of autophagy genes suppresses life span extension in mutant nematodes with inherent dietary restriction, aberrant insulin/IGF-1 or TOR signaling, and lowered mitochondrial respiration. These findings suggest that the autophagy gene cascade functions downstream of and is inhibited by different longevity pathways in C. elegans, therefore, their effects converge on autophagy genes to slow down aging and lengthen life span. Thus, autophagy may act as a central regulatory mechanism of animal aging.
The Drosophila homolog of yeast Aut1, CG6877/ Draut1, is a ubiquitously expressed cytosolic protein. Draut1 loss of function was achieved by expression of an inverted repeat transgene inducing RNA interference. The e¡ect is temperaturedependent and resembles an allelic series as described by Fortier, E. and Belote, J.M. (Genesis 26 (2000) 240^244). Draut1 loss of function larvae are unable to induce autophagy and heterophagy in fat body cells before pupariation and die during metamorphosis. To our knowledge, this is the ¢rst report of a multicellular animal lacking the function of a gene participating in the protein conjugation systems of autophagy. ß
In holometabolous insects including Drosophila melanogaster a wave of autophagy triggered by 20-hydroxyecdysone is observed in the larval tissues during the third larval stage of metamorphosis. We used this model system to study the genetic regulation of autophagy. We performed a genetic screen to select P-element insertions that affect autophagy in the larval fat body. Light and electron microscopy of one of the isolated mutants (l(3)S005042) revealed the absence of autophagic vesicles in their fat body cells during the third larval stage. We show that formation of autophagic vesicles cannot be induced by 20-hydroxyecdysone in the tissues of mutant flies and represent evidence demonstrating that the failure to form autophagic vesicles is due to the insertion of a P-element into the gene coding SNF4Aγ, the Drosophila homologue of the AMPK (AMP-activated protein kinase) γ subunit. The ability to form autophagic vesicles (wild-type phenotype) can be restored by remobilization of the P-element in the mutant. Silencing of SNF4Aγ by RNAi suppresses autophagic vesicle formation in wildtype flies. We raised an antibody against SNF4Aγ and showed that this gene product is constitutively present in the wild-type larval tissues during postembryonal development. SNF4Aγ is nearly absent from the cells of homozygous mutants. SNF4Aγ translocates into the nuclei of fat body cells at the onset of the wandering stage concurrently with the beginning of the autophagic process. Our results demonstrate that SNF4Aγ has an essential role in the regulation of autophagy in Drosophila larval fat body cells.
Details of the functional mechanisms of intrinsically disordered proteins (IDPs) in living cells is an area not frequently investigated. Here, we dissect the molecular mechanism of action of an IDP in cells by detailed structural analyses based on an in-cell nuclear magnetic resonance experiment. We show that the ID stress protein (IDSP) A. thaliana Early Response to Dehydration (ERD14) is capable of protecting E. coli cells under heat stress. The overexpression of ERD14 increases the viability of E. coli cells from 38.9% to 73.9% following heat stress (50 °C × 15 min). We also provide evidence that the protection is mainly achieved by protecting the proteome of the cells. In-cell NMR experiments performed in E. coli cells show that the protective activity is associated with a largely disordered structural state with conserved, short sequence motifs (K- and H-segments), which transiently sample helical conformations in vitro and engage in partner binding in vivo. Other regions of the protein, such as its S segment and its regions linking and flanking the binding motifs, remain unbound and disordered in the cell. Our data suggest that the cellular function of ERD14 is compatible with its residual structural disorder in vivo.
Screening P-element-induced mutant collections, 52 lines were selected as potentially defected ones in endocytosis or autophagy. After excluding those which were rescued by 20-hydroxyecdysone treatment, the exact position of the inserted P-element was determined in the remaining lines. In the case of l(3)S011027 stock, the liquid facets (lqf) gene was affected which codes an epsin-homolog protein in Drosophila. We reveal that Lqf is essential to the receptor-mediated endocytosis of larval serum proteins (LSPs) in the larval fat body cells of Drosophila. In l(3)S011027 line, lack of Lqf fails the formation of autophagosomes thus leading to the arrest of destroying of trophocytes. Transgenic larvae carrying Lqf-RNAi construct were unable to generate endocytic and autophagic vacuoles and led to a prolonged larval stage. On the other hand, Lqf protein showed an exclusive colocalization with the LysoTracker Red- or GFP-Atg8a labeled autophagosomes. By using the antiserum generated against the fifth exon of lqf, we demonstrated that prior to the onset of developmental autophagy the Lqf protein was present in the nucleus of fat body cell, but thereafter the protein was localized in the territory of endocytic and autophagic vacuoles. The fact that the inhibition of the target of rapamycin (TOR) did not restore the autophagic process and the normal development in the case of lqf mutant larvae points to that the Lqf is downstream to the TOR, the central kinase of the autophagy pathway.
Changes in the specific and total activity of the lysosomal marker enzyme acid phosphatase (Acph) and in the amount of enzyme protein were examined in the fat body and the hemolymph from the last larval molt to the larval-pupal apolysis. The specific activity showed minor changes during the last larval period. In contrast, the total activity of the enzyme was low during the feeding period and higher during the wandering stage and strikingly increased at the time of puparium formation. We purified a protein having para-nitrophenyl phosphate phosphatase (Acph) activity and raised antisera against it. The amount of Acph protein in the fat body and hemolymph was examined using an ELISA. The specific Acph content showed little variation, but the total amount of the enzyme protein showed a stepwise increase in both organs during last larval stage and was markedly elevated in the pupal stage in the fat body. In contrast, a considerable decrease in the amount of Acph protein was observed in the hemolymph during this period. These data were in agreement with immunohistochemical observations showing an accumulation of the enzyme protein in fat body cells during the prepupal stage with a concomitant disappearance of the enzyme from the hemolymph. Inhibition of ecdysteroid secretion by water stress prevented the changes both in total enzyme activity and in the amount of Acph protein. However, Acph protein content and enzyme activity could be restored when the water stress was followed by a 20-hydroxyecdysone (20-HE) treatment. Taken together, our data show that Acph is secreted by fat body cells into the hemolymph during the larval stage, where it is stored in an inactive form. Increase in the 20-HE titer at the end of last larval stage reverses this process, and the enzyme is taken up by the fat body cells, where it becomes activated Acknowledgments: We are indebted to Professor S.E. Reynolds for his valuable comments and for improving the English grammar of the manuscript. We are grateful to Professor J. Kovács for critical reading of the first version and to Dr. P. Löw, Dr. L. László, and K. Molnár for many helpful discussions. We wish to thank E. Válóczi for excellent technical assistance. 370Csikós and Sass and appears in auto-and heterophagic vacuoles.
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