Synaptojanin1 regulates lysosomal functions in ventral midbrain neurons

A major pathological hallmark of Parkinsons Disease (PD) is the manifestation of Lewy bodies comprised of alpha-synuclein (-syn). The accumulation of -syn enriched protein aggregates is thought to arise from dysfunction in degradation systems within the brain. Recently, missense mutations of SYNJ1 encoding the SAC1 and 5-phosphatase domains have been found in families with hereditary early-onset Parkinsonism. Previous studies showed that Synj1 haploinsufficiency (Synj1+/-) leads to PD-like behavioral and pathological changes in mice, including the accumulation of the autophagy substrate p62 and pathological -syn proteins in the midbrain (MB) and striatum. In this study, we aim to investigate the neuronal degradation pathway using the Synj1+/- MB culture as a model. Our data suggests that autophagy flux and cumulative autophagosome formation is unaltered at baseline in Synj1+/- MB neurons. However, lysosome number is reduced with a similar decrease in lysosomal proteins, including LAMP1, LAMP2, and LAMP2A. Lysosomes are hyperacidified with enhanced enzymatic activity in Synj1+/- MB neurons. Using a combination of light and electron microscopy, we show that lysosomal changes are primarily associated with a lack of SAC1 activity. Consistently, expressing the SYNJ1 R258Q mutant in N2a cells reduces the lysosome number. Interestingly, the lysosomal defects in Synj1+/- neurons does not impact the clearance of exogenously expressed wild-type -syn; however, the clearance of -syn A53T was impaired in the axons of Synj1+/- MB neurons. Taken together, our results suggest axonal vulnerability to lysosomal defects in Synj1 deficient MB neurons. Significance StatementIn the study, Zhu et al. discovered a previously uncharacterized role of Synj1 in regulating lysosomal number, protein, and acidity in ventral midbrain neurons. These alterations are associated with a specific impairment in the clearance of -syn A53T, but not WT -syn in axons, suggesting an essential role of Synj1 in axonal degradative capacity under pathological stress. This work in cultured mammalian neurons complements recent research efforts in Drosophila, C. elegans and zebra fish, and provides a novel insight for the role Synj1 in neuronal autolysosomal function.