Coordinate Regulation of Mature Dopaminergic Axon Morphology by Macroautophagy and the PTEN Signaling Pathway

Abstract

Macroautophagy is a conserved mechanism for the bulk degradation of proteins and organelles. Pathological studies have implicated defective macroautophagy in neurodegeneration, but physiological functions of macroautophagy in adult neurons remain unclear. Here we show that Atg7, an essential macroautophagy component, regulates dopaminergic axon terminal morphology. Mature Atg7-deficient midbrain dopamine (DA) neurons harbored selectively enlarged axonal terminals. This contrasted with the phenotype of DA neurons deficient in Pten – a key negative regulator of the mTOR kinase signaling pathway and neuron size – that displayed enlarged soma but unaltered axon terminals. Surprisingly, concomitant deficiency of both Atg7 and Pten led to a dramatic enhancement of axon terminal enlargement relative to Atg7 deletion alone. Similar genetic interactions between Atg7 and Pten were observed in the context of DA turnover and DA-dependent locomotor behaviors. These data suggest a model for morphological regulation of mature dopaminergic axon terminals whereby the impact of mTOR pathway is suppressed by macroautophagy. Macroautophagy is a major recycling pathway in cells, and its dysfunction is associated with neurological disorders including Alzheimer's disease, Parkinson's disease, and frontotemporal dementia. Here we show that Atg7, an essential component of macroautophagy, regulates mature dopaminergic axon terminal morphology in coordination with the well-described role of the PI3K pathway. Deficiency of Pten, a negative regulator of the PI3K/mTOR pathway, leads primarily to enlarged dopaminergic cell soma but normal-appearing axonal terminals, whereas Atg7 deficiency primarily induces enlarged axonal terminals. Atg7 and Pten double deficiency leads to further axon terminal enlargement, suggesting that Atg7 deficiency unmasks the impact of PI3K/mTOR pathway on mature dopaminergic axon terminals. In addition, we show that Atg7 and Pten coordinately regulate striatal dopamine turnover and dopamine-dependent motor behaviors. Taken together, these data support a novel role for Atg7-dependent macroautophagy in the regulation of dopaminergic axon terminal morphology, in coordination with the PI3K/mTOR pathway.