Cdk5 is a post-mitotic kinase with complex roles in maintaining neuronal health. The various mechanisms by which Cdk5 inhibits and promotes neurodegeneration are still poorly understood. Here, we show that in Drosophila melanogaster Cdk5 regulates basal autophagy, a key mechanism suppressing neurodegeneration. In a targeted screen, Cdk5 genetically interacted with Acinus (Acn), a primarily nuclear protein, which promotes starvation-independent, basal autophagy. Loss of Cdk5, or its required cofactor p35, reduces S437-Acn phosphorylation, whereas Cdk5 gain-of-function increases pS437-Acn levels. The phospho-mimetic S437D mutation stabilizes Acn and promotes basal autophagy. In p35 mutants, basal autophagy and lifespan are reduced, but restored to near wild-type levels in the presence of stabilized AcnS437D. Expression of aggregation-prone polyQ-containing proteins or the Amyloid-β42 peptide, but not alpha-Synuclein, enhances Cdk5-dependent phosphorylation of S437-Acn. Our data indicate that Cdk5 is required to maintain the protective role of basal autophagy in the initial responses to a subset of neurodegenerative challenges.

In Drosophila, AKT1 and the Dcp-1 caspase calibrate basal autophagy levels and cellular metabolism by regulating Acn stability.

The expression of multiple growth-promoting genes is coordinated by the transcriptional co-activator Yorkie with its major regulatory input provided by the Hippo-Warts kinase cascade. Here, we identify Atg1/ULK1-mediated phosphorylation of Yorkie as an additional inhibitory input independent of the Hippo-Warts pathway. Two serine residues in Yorkie, S74 and S97, are Atg1/ULK1 consensus target sites and are phosphorylated by ULK1 in vitro, thereby preventing its binding to Scalloped. In vivo, gain of function of Atg1, or its activator Acinus, caused elevated Yorkie phosphorylation and inhibited Yorkie's growth-promoting activity. Loss of function of Atg1 or Acinus raised expression of Yorkie target genes and increased tissue size. Unlike Atg1's role in autophagy, Atg1-mediated phosphorylation of Yorkie does not require Atg13. Atg1 is activated by starvation and other cellular stressors and therefore can impose temporary stress-induced constraints on the growth-promoting gene networks under the control of Hippo-Yorkie signaling.

Acinus (Acn) is a nuclear protein that participates in the regulation of autophagy. Loss of Acn function prevents autophagy in starving cells. Conversely, Acn activation induces basal autophagy. This enhances the quality control functions of autophagy such as the removal of misfolded proteins, thereby reducing neurodegeneration and prolonging lifespan. Acn activity is enhanced by Akt1-mediated phosphorylation, which counteracts the cleavage of Acn by a caspase-3 homolog.

Cdk5 is a post-mitotic kinase with complex roles in maintaining neuronal health. The various mechanisms by which Cdk5 inhibits and promotes neurodegeneration are still poorly understood. Here, we show that in Drosophila melanogaster Cdk5 regulates basal autophagy, a key mechanism suppressing neurodegeneration. In a targeted screen, Cdk5 genetically interacted with Acinus (Acn), a primarily nuclear protein, which promotes starvation-independent, basal autophagy. Loss of Cdk5, or its required cofactor p35, reduces S437-Acn phosphorylation, whereas Cdk5 gain-of-function increases pS437-Acn levels. The phospho-mimetic S437D mutation stabilizes Acn and promotes basal autophagy. In p35 mutants, basal autophagy and lifespan are reduced, but restored to near wild-type levels in the presence of stabilized Acn S437D . Expression of aggregationprone polyQ-containing proteins or the Amyloid-b42 peptide, but not alpha-Synuclein, enhances Cdk5-dependent phosphorylation of S437-Acn. Our data indicate that Cdk5 is required to maintain the protective role of basal autophagy in the initial responses to a subset of neurodegenerative challenges.