Synaptojanin1 regulates lysosomal functions in ventral midbrain neurons

Abstract

AbstractA major pathological hallmark of Parkinson’s Disease (PD) is the manifestation of Lewy bodies comprised of alpha-synuclein (α-syn). The accumulation of α-syn enriched protein aggregates is thought to arise from dysfunction in degradation systems within the brain. Recently, missense mutations ofSYNJ1encoding the SAC1 and 5’-phosphatase domains have been found in families with hereditary early-onset Parkinsonism. Previous studies showed thatSynj1haploinsufficiency (Synj1+/-) leads to PD-like behavioral and pathological changes in mice, including the accumulation of the autophagy substrate p62 and pathological α-syn proteins in the midbrain (MB) and striatum. In this study, we aim to investigate the neuronal degradation pathway using theSynj1+/-MB culture as a model. Our data suggests that autophagy flux and cumulative autophagosome formation is unaltered at baseline inSynj1+/-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 inSynj1+/-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 α-syn; however, the clearance of α-syn A53T was impaired in the axons ofSynj1+/-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 α-syn A53T, but not WT α-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 inDrosophila, C. elegans andzebra fish, and provides a novel insight for the role Synj1 in neuronal autolysosomal function.