Synaptojanin1 deficiency upregulates basal level autophagosome formation in astrocytes

Abstract

Macroautophagy (hereafter, autophagy) dysregulation is implicated in multiple neurological disorders. While the autophagy pathways are heavily investigated in heterologous cells and neurons, how autophagy is regulated in the astrocyte, the most abundant cell type in the mammalian brain, is less understood. Here we report that Synaptojanin1 (Synj1), a neuron enriched lipid phosphatase, is expressed in low levels in astrocytes and represses autophagy at the basal level. Synj1 is encoded by the Synj1 gene, whose missense mutations are linked to Parkinsonism with seizure. While the best-known role of Synj1 is to facilitate synaptic vesicle recycling, recent studies suggest that Synj1 also regulates autophagy. Our previous study using the Synj1 haploinsufficient (Synj1+/-) mouse demonstrated that Synj1 deficiency was associated with an age-dependent autophagy impairment in multiple brain regions. We now use cultured astrocytes from Synj1 deficient mice to investigate its role in astrocyte autophagy. We demonstrate that Synj1 deficient astrocytes exhibit increased LC3 puncta, which is more pronounced when lysosomal acidification is blocked. The increased autophagosome formation is accompanied by reduced autophagy substrate, p62, but an insensitivity to starvation induced autophagy clearance. Moreover, we show, for the first time, that the Parkinsonism associated R839C mutation impacts astrocyte autophagy. The profound impact of this mutation on Synj1s phosphatase functions results in elevated basal level autophagosome formation and clearance that mimics Synj1 deletion. We find that energy sensing molecules, including mTOR and AMPK, are altered in Synj1 deficient astrocytes, which may contribute to the enhanced basal level autophagy.