Potent New Targets for Autophagy Enhancement to Delay Neuronal Ageing.

Autophagy is a lysosomal-dependent degradation process of eukaryotic cells responsible for breaking down unnecessary and damaged intracellular components. Autophagic activity gradually declines with age due to genetic control, and this change contributes to the accumulation of cellular damage at advanced ages, thereby causing cells to lose their functionality and viability. This could be particularly problematic in post-mitotic cells including neurons, the mass destruction of which leads to various neurodegenerative diseases. Here, we aim to uncover new regulatory points where autophagy could be specifically activated and test these potential drug targets in neurodegenerative disease models of Drosophila melanogaster. One possible way to activate autophagy is by enhancing autophagosome-lysosome fusion that creates the autolysosome in which the enzymatic degradation happens. The HOPS (homotypic fusion and protein sorting) and SNARE (Snap receptor) protein complexes regulate the fusion process. The HOPS complex forms a bridge between the lysosome and autophagosome with the assistance of small GTPase proteins. Thus, small GTPases are essential for autolysosome maturation, and among these proteins, Rab2 (Ras-associated binding 2), Rab7, and Arl8 (Arf-like 8) are required to degrade the autophagic cargo. For our experiments, we used Drosophila melanogaster as a model organism. Nerve-specific small GTPases were silenced and overexpressed. We examined the effects of these genetic interventions on lifespan, climbing ability, and autophagy. Finally, we also studied the activation of small GTPases in a Parkinson's disease model. Our results revealed that GTP-locked, constitutively active Rab2 (Rab2-CA) and Arl8 (Arl8-CA) expression reduces the levels of the autophagic substrate p62/Ref(2)P in neurons, extends lifespan, and improves the climbing ability of animals during ageing. However, Rab7-CA expression dramatically shortens lifespan and inhibits autophagy. Rab2-CA expression also increases lifespan in a Parkinson's disease model fly strain overexpressing human mutant (A53T) α-synuclein protein. Data provided by this study suggests that Rab2 and Arl8 serve as potential targets for autophagy enhancement in the Drosophila nervous system. In the future, it might be interesting to assess the effect of Rab2 and Arl8 coactivation on autophagy, and it would also be worthwhile to validate these findings in a mammalian model and human cell lines. Molecules that specifically inhibit Rab2 or Arl8 serve as potent drug candidates to modulate the activity of the autophagic process in treating neurodegenerative pathologies. In the future, it would be reasonable to investigate which GAP enzyme can inhibit Rab2 or Arl8 specifically, but not affect Rab7, with similar medical purposes.

The evolutionary and functional divergence of the Atg8 autophagy protein superfamily.

Autophagy is a highly conserved self-degradation process of eukaryotic cells which is required for the effective elimination of damaged and unnecessary cytosolic constituents. Defects in the process can cause the intracellular accumulation of such damages, thereby leading to the senescence and subsequent loss of the affected cell. Defective autophagy hence is implicated in the development of various degenerative processes, including cancer, neurodegenerative diseases, diabetes, tissue atrophy and fibrosis, and immune deficiency, as well as in accelerated aging. The autophagic process is mediated by numerous autophagy-related (ATG) proteins, among which the ATG8/LC3/GABARAP (Microtubule-associated protein 1A/1B-light chain 3/Gammaaminobutyric acid receptor-associated protein) superfamily has a pivotal role in the formation and maturation of autophagosome, a key (macro) autophagic structure (the autophagosome sequesters parts of the cytoplasm which are destined for breakdown). While in the unicellular yeast there is only a single ATG8 protein, metazoan systems usually contain more ATG8 paralogs. ATG8 paralogs generally display tissue-specific expression patterns and their functions are not strictly restricted to autophagy. For example, GABARAP proteins also play a role in intracellular vesicle transport, and, in addition to autophagosome formation, ATG8 also functions in selective autophagy. In this review, we summarize the functional diversity of ATG8/LC3/GABARAP proteins, using tractable genetic models applied in autophagy research.