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2Synapses are often far from the soma and independently cope with proteopathic stress induced by intense neuronal activity. However, how presynaptic compartments turnover proteins is poorly understood. We show that the synapse-enriched protein EndophilinA, thus far studied for its role in endocytosis, induces macroautophagy at presynaptic terminals. We find that EndophilinA executes this unexpected function at least partly independent of its role in synaptic vesicle endocytosis. EndophilinA-induced macroautophagy is activated when the kinase LRRK2 phosphorylates the EndophilinA-BAR domain and is blocked in animals where EndophilinA cannot be phosphorylated.
EndophilinA
INTRODUCTIONNeurons can be metabolically very active, firing at rates of more than 100 Hz.Synaptic proteins and organelles are used and re-used multiple times and accumulate damage as a result of this stress. Furthermore, synapses are often located far away from the cell body and must therefore in part operate independently. This raises the question how synapses maintain protein quality. Given that neurodegeneration is thought to start with subtle synaptic defects before evolving into blunt neuronal death (Burke and O'Malley, 2013;, the mechanisms of synaptic protein homeostasis are likely relevant for the understanding of neurodegenerative disease.Macroautophagy is well-placed to mediate protein turnover, but how the needs of synapses are served by this process has not been well-studied. Cellular signals like stress and amino acid deprivation induce macroautophagy, where cytoplasm is engulfed by double membrane structures before fusion with degradative lysosomes (Mizushima et al., 2011).Autophagosomes have been visualized using fluorescent markers in yeast, Drosophila and mammalian cells and are often observed as Atg8/LC3 positive puncta (Kabeya et al., 2000;Scott et al., 2004). At the stage of initiation, Atg9-positive vesicles fuse into elongated preautophagosomal structures with growing edges (He et al., 2006). These edges are highly curved and harbor lipid packing defects. These edges serve as protein docking sites, attracting specific autophagic factors such as Atg3, Atg14/Barkor and Atg1 that insert into such zones, recognizing specific lipids (phosphatidylinositol-3-phosphate (PI(3)P)) and lipid packing defects (Fan et al., 2011;Nath et al., 2014;Ragusa et al., 2012). The recruitment of these factors then promotes the further steps of autophagosome formation; in particular the E2-like protein Atg3 itself recruits the autophagic marker LC3/Atg8 (Nath et al., 2014). However, how these highly curved edges are formed and maintained is very poorly understood.While autophagy has been mostly analyzed in the soma of cultured cells and yeast, autophagic markers have also been observed away from the soma at neuronal synapses 4 (Hernandez et al., 2012;Maday and Holzbaur, 2014;Williamson et al., 2010) and these markers were shown to be transported along axons (Maday and Holzbaur, 2014). However, how autophagosomes are formed at synapses an...