Synaptojanin1 regulates synaptic dopamine release and axonal integrity via retromer-dependent endosomal sorting

Synaptic dysfunction is increasingly recognized as an early feature of Parkinson's disease (PD); however, synaptic mechanisms contributing to early dopamine release defects and neurodegeneration remains poorly understood. Here we identify a presynaptic endosomal-dependent mechanism supporting dopamine release and axonal integrity. Loss of the PD-associated lipid enzyme Synaptojanin1 impairs dopamine release due to endosomal retention of the dopamine D2 autoreceptor and dopamine transporter (DAT). Conditional deletion of Synaptojanin1 in mouse dopamine neurons results in endosomal swelling within striatal DAT clusters and PD-like locomotor deficits. Mechanistically, Synaptojanin1 remodels endosomal phosphatidylinositol 4-phosphate to facilitate the recruitment of the PD-associated retromer component VPS35. Notably, overexpressing VPS35 rescues presynaptic sorting defects in Synaptojanin1-deficient dopamine neurons despite lipid impairments. Furthermore, Synaptojanin1 and VPS35 exhibit correlated expression and dopamine-induced co-clustering in axons, supporting their broader roles in regulating synaptic surface proteins. Our work demonstrates a lipid-dependent endosomal mechanism that may contribute to motor deficits in early PD.