A Novel Autophagy Regulatory Mechanism that Functions During Programmed Cell Death: A Dissertation

Abstract

Autophagy is a cellular process that delivers cytoplasmic materials for degradation by the lysosomes. Autophagy-related (Atg) genes were identified in yeast genetic screens for vehicle formation under stress conditions, and Atg genes are conserved from yeast to human. When cells or animals are under stress, autophagy is induced and Atg8 (LC3 in mammal) is activated by E1 activating enzyme Atg7. Atg8-containing membranes form and surround cargos, close and mature to become the autophagosomes. Autophagosomes fuse with lysosomes, and cargos are degraded by lysosomal enzymes to sustain cell viability. Therefore, autophagy is most frequently considered to function in cell survival. Whether the Atg gene regulatory pathway that was defined in yeast is utilized for all autophagy in animals, as well as if autophagy could function in a cell death scenario, are less understood. The Drosophila larval digestive tissues, such as the midgut of the intestine and the salivary gland, are no longer required for the adult animal and are degraded during the pupal stage of development. Cells stop growing at the end of larval development, and proper cell growth arrest is required for midgut degradation. Ectopic activation of the PI3K/Akt signaling induces cell growth and inhibits autophagy and midgut degradation. Down regulating PI3K/Akt pathway by Pten mis-expression activates autophagy. In addition, mis-expression of autophagy initiator Atg1 inhibits cell growth and knocking down autophagy restore PI3K/Akt activity. Together, these results indicate that autophagy and growth signaling mutually inhibit each other. Midgut destruction relies on the autophagy gene Atg18, but not caspase activation. The intestine length shortens and the cells undergo programmed cell size reduction, a phenomenon that also requires Atg18, before cell death occurs during midgut destruction. To further investigate whether cell size reduction is cell autonomous and requires other Atg genes, we reduced the function of Atg genes in cell clones using either gene mutations or RNAi knockdowns. Indeed, many Atg genes, including Atg8, are required for autophagy and cell size reduction in a cell autonomous manner. Surprisingly, Atg7 is not required for midgut cell size reduction and autophagy even though this gene is essential for stress-induced autophagy. Therefore, we screened for known E1 enzymes that may function in the midgut, and discovered that Uba1 is required for autophagy, size reduction and clearance of mitochondria. Uba1 does not enzymatically substitute for Atg7, and Ubiquitin phenocopies Uba1, suggesting Uba1 functions through ubiquitination of unidentified molecule(s) to regulate autophagy. In conclusion, this thesis describes: First, autophagy participates in midgut degradation and cell death. Second it reveals a previously un-defined role of Uba1 in autophagy regulation. Third it shows that the Atg genes are not functionally conserved and the requirement of some Atg genes can be context dependent.