New Atg9 phosphorylation sites regulate autophagic trafficking in glia

We previously identified a role for dAuxilin (dAux), the fly homolog of Cyclin G-associated kinase (GAK), in glial autophagy contributing to Parkinsons disease (PD). To further dissect the mechanism, we present evidence here that lack of glial dAux enhanced the phosphorylation of the autophagy-related protein Atg9 at two newly identified threonine residues, T62 and T69. Enhanced Atg9 phosphorylation in the absence of glial dAux is potentially regulated through the master autophagy regulator Atg1, as the presence of which is required for Atg9 to interact with dAux in an otherwise separate condition. The enhanced Atg9 phosphorylation promotes autophagosome formation and Atg9 trafficking to the autophagosomes in glia. Whereas the expression of the non-phosphorylatable Atg9 variants suppresses the lack of dAux-induced increase in both autophagosome formation and Atg9 trafficking to autophagosome, the expression of the phophomimetic Atg9 variants restores the lack of Atg1-induced decrease in both events. Notably, Atg9 phosphorylation at T62 and T69 contributes to DA neurodegeneration and locomotor dysfunction implicated in PD. Thus, we have identified a dAux-Atg1-Atg9 axis relaying signals through the Atg9 phosphorylation at T62 and T69; these findings further elaborate the mechanism of dAux regulating glial autophagy and highlight the significance of protein degradation pathway in glia contributing to PD.