The SAC1 domain in synaptojanin is required for autophagosome maturation at presynaptic terminals

Abstract

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.

Role of the endolysosomal system in Parkinson’s disease

“…This discrepancy and the precise relationship between SV endocytosis and synaptic autophagy remains to be clarified. However, the Drosophila model revealed loss of dopaminergic neurons, providing evidence for the contribution of synaptic autophagy defects in pathogenesis of PD (Vanhauwaert et al ). Considering its dual enzymatic activity, SynJ1 is also believed to play a role in fast endophilin‐mediated, clathrin‐independent endocytosis which is an alternative to CME (Watanabe et al ) and several other processes.…”

Parkinson's disease: convergence on synaptic homeostasis

“…In particular, we will review how these proteins regulate protein or organellar homeostasis, often uniquely at synapses. This idea mostly stems from studies of genes causative to familial forms of PD (Burré et al , ; Nemani et al , ; Zimprich et al , ; Edvardson et al , ; Matta et al , ; Krebs et al , ; Quadri et al , ; Soukup et al , ; Vanhauwaert et al , ). However, given that α‐Synuclein aggregates, a proxy for protein homeostasis defects, are observed both in familial and sporadic cases of PD, this idea is most likely also relevant to the sporadic cases of the disease.…”

“…This autophagosome fuses with the lysosome to degrade and recycle the contents. Autophagosomes were recently shown to form at presynaptic endings and to be transported back to the cell body (Hernandez et al , ; Maday & Holzbaur, ; Binotti et al , ; Soukup et al , ; Oerlundk et al , ; Vanhauwaert et al , ). Macroautophagy can be non‐selective or selective.…”

“…These mutations block SAC1 function while leaving the 5‐phosphatase activity intact (Krebs et al , ). Flies expressing only Synj R228Q (the corresponding pathogenic mutation to R258Q in flies) show largely normal synaptic endocytosis at excitatory glutamatergic neurons and histaminergic photoreceptors (Vanhauwaert et al , ). In contrast, inhibitory neurons of Synj1 R258Q knock‐in mice show an accumulation of coated vesicles, suggesting slowed vesicle recycling.…”

“…In this context, it would also be interesting to analyze autophagic function at the synapses of Synj1 R258Q knock‐in mice. Nonetheless, the data in flies and induced patient neurons indicate the role of Synj1 in autophagy is evolutionary conserved (Vanhauwaert et al , ).…”