Endosomal Microautophagy in Drosophila

Endosomal Microautophagy in Drosophila Proper turnover of proteins and organelles is essential for normal cell function. Damaged or altered cytosolic proteins are cleared by the proteasome and autophagy. Importantly, autophagy has the additional role of providing nutrients to cells under stress conditions such as starvation, and is thus essential for energy balance. The liver is one of the main regulators of lipids in the body and has major roles in metabolism such as gluconeogenesis, a process that is particularly dependent on amino acids generated by autophagic degradation of cellular proteins under starvation or stress. Furthermore, removal of damaged organelles and aggregated proteins is essential to protect liver and kidneys against age related disorders. Macroautophagy (MA), Chaperone mediated Autophagy (CMA) and endosomal Microautophagy (eMI) are the three major forms of autophagy. MA engulfs bulk-regions of cytoplasm including organelles in a double membrane vesicle (autophagosome). Autophagosome fusion with lysosomes leads to the degradation of the engulfed material. Less is known about CMA and eMI, which mostly degrade proteins containing a targeting motif (KFERQ related sequences) that is recognized by the cytoplasmic Hsc70. During eMI, which to date has only been characterized biochemically and by EM, KFERQ containing substrates bound to Hsc70 are taken up into multivesicular bodies/late endosomes in an ESCRT machinery dependent process and degraded. Previously, the existence of eMI beyond mammals was unknown and there is currently no in vivo system to study mammalian eMI. Hence, the genetic power of model organisms such as Drosophila have not been exploited for the study of KFERQ- dependent forms of autophagy. Using a fluorescently tagged model substrate expressed in transgenic flies, we developed a model system to study starvation inducible eMI in vivo. Using this system, we will assess the physiological function and regulation of eMI by starvation and other forms of cellular stress. Furthermore, we will characterize regulators of eMI that we have identified in a genetic screen.