Autophagy is an essential cellular process that maintains homeostasis through the degradation and recycling of cytoplasmic organelles and macromolecules. Substrates targeted for autophagy are engulfed in a double-membrane structure called the autophagosome which is then targeted to the lysosome for degradation. The initiation of autophagy requires two separate ubiquitin-like protein (UBL) systems that regulate autophagosome assembly. In these systems Atg7 acts as an E1-like enzyme for the UBLs Atg12 and Atg8. Atg7 binds to and activates these UBLs to allow their transfer to an E2 enzyme and eventually their targets. Upon initiation of autophagosome assembly Atg7 binds to the UBL Atg12 and transfers it to its final binding partner Atg5 via the E2 enzyme Atg10. The Atg12-Atg5 conjugate assembles into a large multimeric complex along with Atg16. This Atg12-Atg5/Atg16 complex localizes to the expanding phagopore where it acts as an E3 ligase during the lipidation of the UBL Atg8. During Atg8 lipidation, Atg7 binds to and activates Atg8 to allow its transfer to the E2 enzyme Atg3. The Atg12-Atg5/Atg16 complex then functions as an E3 ligase to facilitate the final transfer step of Atg8 to its lipid target phosphotidylethanolamine (PE). This Atg8-PE conjugate is inserted into phagopore double-membrane where it recruits autophagy machinery including cargo binding proteins. Atg7's role in both of these autophagy-specific UBL systems makes it an essential regulator of autophagosome assembly. Defects in this pathway can lead to neurodegenerative diseases or tumor formation.
Yang et al. discovered elevated levels of autophagy in pancreatic cancer cells (1). Through qPCR and immunohistochemistry using the Atg7 antibody they showed elevated levels of autophagy related proteins. They also confirmed that pharmacological inhibition of autophagy can effectively suppress cancer cell growth. The Gao group of UCSF published research on the potential contribution of autophagosome accumulation to neuronal cell death and its implications in neurodegenerative disorders (2). By targeting Atg7 with siRNA and reducing autophagy induction they were able to delay cell loss. Knockdowns were confirmed with the Atg7 antibody. The Tooze group demonstrated that inhibition of early endosome function causes the accumulation of immature autophagosomes and blocks autophagy (3). By depleting Atg7 and using the Atg7 antibody monitor protein levels they were able to show the accumulation of immature autophagosomes caused by early endosome inhibition was dependent on Atg7. Park et al. identified a method to induce autophagic cell death in cancer cells using an antibody fragment targeting TRAIL receptor 2 (4). To confirm the mechanism of cell death induced by the antibody fragment they blocked autophagy by depleting Atg7 and using the Atg7 antibody to confirm a reduction in protein levels. The Maeda group examined the role of autophagy in light induced retinal degeneration (5). The Atg7 antibody was used to monitor Atg7 protein levels in response to intense lighting conditions. Their experiments demonstrated the importance of autophagy in protecting rod photoreceptor cells from light-induced cell stress.
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