Characterization of Atg6 function in autophagy and growth control during Drosophila melanogaster development

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Hill, Jahda Hope
Wu, Louisa P
Baehrecke, Eric H
The tumor suppressor Beclin 1 mitigates cell stress by regulating the lysosomal degradation pathway known as autophagy. This process involves formation of intracellular double-membraned vesicles, known as autophagosomes, which engulf proteins and damaged organelles and fuse with lysosomes, where the contents are degraded. It is unclear whether the function of Beclin 1 in autophagy is related to cell transformation in <italic>beclin 1<super>+/-</super> </italic> animals. Using the fruit fly, <italic>Drosophila melanogaster</italic>, I investigated the function of the Beclin 1 ortholog Atg6 in autophagy and growth control. Through transgenic experiments, I found that Atg6, like Beclin 1, induces autophagy by functioning in a complex consisting of the lipid kinase Vps34 and the serine&ndash;threonine kinase Vps15. I also generated a strong loss of function mutant, <italic>Atg6<super>1</super></italic>, and found that Atg6 is required for development. <italic>Atg6</italic> mutant animals contained an excess of blood cells, which surrounded melanotic tumors prior to death. At the cellular level, Atg6 is required for autophagy and endocytosis, and cells lacking <italic>Atg6</italic> accumulate high levels of the endoplasmic reticulum stress protein Hsc3 and the adaptor protein p62. I also showed that <italic>Atg6</italic> mutant cells displayed mis-regulated nuclear localization of NF &kappa;B proteins, transcription factors whose downstream targets include regulators of innate immunity. Significantly, my results suggest that Atg6 may regulate growth independent of its function in autophagy, as mosaic loss of <italic> Atg6</italic> in the eye resulted in over-representation of <italic>Atg6</italic> mutant cells, a phenotype not shared by other autophagy gene mutant mosaics. Finally, through a collaborative effort, our lab identified a novel function for Atg6 in regulation of epithelial cell polarity. This finding is significant, as epithelial tumor cells are known to lose polarity during metastasis. Our studies have provided a significant contribution to the Beclin 1 field, by providing the first characterization of a <italic>Drosophila Atg6</italic> mutant, and demonstrating its function in novel cellular processes.