Miklós Sass

Longevity pathways converge on autophagy genes to regulate life span in Caenorhabditis elegans

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 class III PI(3)K Vps34 promotes autophagy and endocytosis but not TOR signaling in Drosophila

Degradation of cytoplasmic components by autophagy requires the class III phosphatidylinositol 3 (PI(3))–kinase Vps34, but the mechanisms by which this kinase and its lipid product PI(3) phosphate (PI(3)P) promote autophagy are unclear. In mammalian cells, Vps34, with the proautophagic tumor suppressors Beclin1/Atg6, Bif-1, and UVRAG, forms a multiprotein complex that initiates autophagosome formation. Distinct Vps34 complexes also regulate endocytic processes that are critical for late-stage autophagosome-lysosome fusion. In contrast, Vps34 may also transduce activating nutrient signals to mammalian target of rapamycin (TOR), a negative regulator of autophagy. To determine potential in vivo functions of Vps34, we generated mutations in the single Drosophila melanogaster Vps34 orthologue, causing cell-autonomous disruption of autophagosome/autolysosome formation in larval fat body cells. Endocytosis is also disrupted in Vps34−/− animals, but we demonstrate that this does not account for their autophagy defect. Unexpectedly, TOR signaling is unaffected in Vps34 mutants, indicating that Vps34 does not act upstream of TOR in this system. Instead, we show that TOR/Atg1 signaling regulates the starvation-induced recruitment of PI(3)P to nascent autophagosomes. Our results suggest that Vps34 is regulated by TOR-dependent nutrient signals directly at sites of autophagosome formation.

The regulation of aging: does autophagy underlie longevity?

The accumulation of cellular damage is a feature common to all aging cells and leads to decreased ability of the organism to survive. The overall rate at which damage accumulates is influenced by conserved metabolic factors (longevity pathways and regulatory proteins) that control lifespan through adjusting mechanisms for maintenance and repair. Autophagy, the major catabolic process of eukaryotic cells that degrades and recycles damaged macromolecules and organelles, is implicated in aging and in the incidence of diverse age-related pathologies. Recent evidence has revealed that autophagic activity is required for lifespan extension in various long-lived mutant organisms, and that numerous autophagy-related genes or proteins are directly regulated by longevity pathways. These findings support the emerging view that autophagy is a central regulatory mechanism for aging in diverse eukaryotic species.

Gene Expression Profiling Identifies FKBP39 as an Inhibitor of Autophagy in Larval Drosophila Fat Body

In Drosophila, the fat body undergoes a massive burst of autophagy at the end of larval development in preparation for the pupal transition. To identify genes involved in this process, we carried out a microarray analysis. We found that mRNA levels of the homologs of Atg8, the coat protein of early autophagic structures, and lysosomal hydrolases were upregulated, consistent with previous results. Genes encoding mitochondrial proteins and many chaperones were downregulated, including the inhibitor of eIF2alpha kinases and the peptidyl-prolyl cis–trans isomerase FK506-binding protein of 39 kDa (FKBP39). Genetic manipulation of FKBP39 expression had a significant effect on autophagy, potentially through modulation of the transcription factor Foxo. Accordingly, we found that Foxo mutants cannot properly undergo autophagy in response to starvation, and that overexpression of Foxo induces autophagy.

Interaction of the HOPS complex with Syntaxin 17 mediates autophagosome clearance in Drosophila

Interaction of the autophagosomal SNARE Syntaxin 17 (Syx17) with the homotypic fusion and vacuole protein–sorting (HOPS) tethering complex is necessary for the fusion of autophagosomes with lysosomes. HOPS, but not Syx17, is also required for endocytic degradation and biosynthetic transport to lysosomes and eye pigment granules.

The Drosophila homolog of Aut1 is essential for autophagy and development.

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 effect is temperature‐dependent 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 first report of a multicellular animal lacking the function of a gene participating in the protein conjugation systems of autophagy.

The emerging role of acetylation in the regulation of autophagy

Autophagy is an evolutionarily conserved catabolic process through which different components of the cells are sequestered into double-membrane cytosolic vesicles called autophagosomes, and fated to degradation through fusion with lysosomes. Autophagy plays a major function in many physiological processes including response to different stress factors, energy homeostasis, elimination of cellular organelles and tissue remodeling during development. Consequently, autophagy is strictly controlled and post-translational modifications such as phosphorylation and ubiquitination have long been associated with autophagy regulation. In contrast, the importance of acetylation in autophagy control has only emerged in the last few years. In this review, we summarize how previously identified histone acetylases and deacetylases modify key autophagic effector proteins, and discuss how this has an impact on physiological and pathological cellular processes.

The autophagic roles of Rab small GTPases and their upstream regulators: A review

Macroautophagy is an evolutionarily conserved degradative process of eukaryotic cells. Double-membrane vesicles called autophagosomes sequester portions of cytoplasm and undergo fusion with the endolysosomal pathway in order to degrade their content. There is growing evidence that members of the small GTPase RAB protein family—the well-known regulators of membrane trafficking and fusion events—play key roles in the regulation of the autophagic process. Despite numerous studies focusing on the functions of RAB proteins in autophagy, the importance of their upstream regulators in this process emerged only in the past few years. In this review, we summarize recent advances on the effects of RABs and their upstream modulators in the regulation of autophagy. Moreover, we discuss how impairment of these proteins alters the autophagic process leading to several generally known human diseases.

Rab11 facilitates cross-talk between autophagy and endosomal pathway through regulation of Hook localization

During autophagy, double-membrane autophagosomes deliver sequestered cytoplasmic content to late endosomes and lysosomes for degradation. The molecular mechanism of autophagosome maturation is still poorly characterized. The small GTPase Rab11 regulates endosomal traffic and is thought to function at the level of recycling endosomes. We show that loss of Rab11 leads to accumulation of autophagosomes and late endosomes in Drosophila melanogaster. Rab11 translocates from recycling endosomes to autophagosomes in response to autophagy induction and physically interacts with Hook, a negative regulator of endosome maturation. Hook anchors endosomes to microtubules, and we show that Rab11 facilitates the fusion of endosomes and autophagosomes by removing Hook from mature late endosomes and inhibiting its homodimerization. Thus induction of autophagy appears to promote autophagic flux by increased convergence with the endosomal pathway.

Autophagy and its physiological relevance in arthropods: Current knowledge and perspectives

Autophagic process is one of the best examples of a conserved mechanism of survival in eukaryotes. At the molecular level there are impressive similarities between unicellular and multicellular organisms, but there is increasing evidence that the same process may be used for different ends, i.e., survival or death, at least at cellular levels. Arthropods encompass a wide variety of invertebrates such as insects, crustaceans and spiders, and thus represent the taxon in which most of the investigations on autophagy in non-mammalian models are performed. The present review is focused on the genetic basis and the physiological meaning of the autophagic process on key models of arthropods. The involvement of autophagy in programmed cell death, especially during oogenesis and development, is also discussed.