Manuscript 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...

Presynaptic terminals are metabolically active and accrue damage through continuous vesicle cycling. How synapses locally regulate protein homeostasis is poorly understood. We show that the presynaptic lipid phosphatase synaptojanin is required for macroautophagy, and this role is inhibited by the Parkinson's disease mutation R258Q. Synaptojanin drives synaptic endocytosis by dephosphorylating PI(4,5)P 2 , but this function appears normal in Synaptojanin RQ knock-in flies. Instead, R258Q affects the synaptojanin SAC1 domain that dephosphorylates PI(3)P and PI(3,5)P 2 , two lipids found in autophagosomal membranes. Using advanced imaging, we show that Synaptojanin RQ mutants accumulate the PI(3)P/PI(3,5)P 2 -binding protein Atg18a on nascent synaptic autophagosomes, blocking autophagosome maturation at fly synapses and in neurites of human patient induced pluripotent stem cell-derived neurons. Additionally, we observe neurodegeneration, including dopaminergic neuron loss, in Synaptojanin RQ flies.Thus, synaptojanin is essential for macroautophagy within presynaptic terminals, coupling protein turnover with synaptic vesicle cycling and linking presynaptic-specific autophagy defects to Parkinson's disease.

No abstract

Parkinson's disease, the second most common neurodegenerative disorder, affects millions of people globally. There is no cure, and its prevalence will double by 2030. In recent years, numerous causative genes and risk factors for Parkinson's disease have been identified and more than half appear to function at the synapse. Subtle synaptic defects are thought to precede blunt neuronal death, but the mechanisms that are dysfunctional at synapses are only now being unraveled. Here, we review recent work and propose a model where different Parkinson proteins interact in a cell compartment-specific manner at the synapse where these proteins regulate endocytosis and autophagy. While this field is only recently emerging, the work suggests that the loss of synaptic homeostasis may contribute to neurodegeneration and is a key player in Parkinson's disease.