Macroautophagy is a process that leads to the bulk degradation of subcellular constituents by producing autophagosomes/autolysosomes1,2,3. It is believed that Atg5 (ref. 4) and Atg7 (ref. 5) are essential genes for mammalian macroautophagy. Here we show, however, that mouse cells lacking Atg5 or Atg7 can still form autophagosomes/autolysosomes and perform autophagy-mediated protein degradation when subjected to certain stressors. Although lipidation of the microtubule-associated protein light chain 3 (LC3, also known as Map1lc3a) to form LC3-II is generally considered to be a good indicator of macroautophagy6, it did not occur during the Atg5/Atg7-independent alternative process of macroautophagy. We also found that this alternative process of macroautophagy was regulated by several autophagic proteins, including Unc-51-like kinase 1 (Ulk1) and beclin 1. Unlike conventional macroautophagy, autophagosomes seemed to be generated in a Rab9-dependent manner by the fusion of isolation membranes with vesicles derived from the trans-Golgi and late endosomes. In vivo, Atg5-independent alternative macroautophagy was detected in several embryonic tissues. It also had a function in clearing mitochondria during erythroid maturation. These results indicate that mammalian macroautophagy can occur through at least two different pathways: an Atg5/Atg7-dependent conventional pathway and an Atg5/Atg7-independent alternative pathway.
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We thank M. Narita and A. R. J. Young for critical reading of the manuscript; N. Mizushima for providing Atg5+/- mice and the expression plasmid of GFP–LC3; T. Yoshimori for the human beclin 1 expression plasmid; and T. Kitamura for providing Plat-E cells. This study was supported in part by the Program for Promotion of Fundamental Studies in Health Sciences of the National Institute of Biomedical Innovation (NIBIO), a grant for Creative Scientific Research, a grant for the 21st Century COE Program from the Japanese Ministry of Education, Science, Sports and Culture, a grant for Comprehensive Research on Aging and Health from the Japanese Ministry of Health, Labor and Welfare, and a grant for Solution-Oriented Research for Science and Technology (SORST) from the Japan Science and Technology Corporation. This study was also supported by grants from the Uehara Memorial Foundation, the Sagawa Foundation for Promotion of Cancer Research, the YASUDA Medical Foundation, the Astellas foundation for research on metabolic disorders, and the Foundation for Promotion of Cancer Research.
Author Contributions Y.N. performed the biochemical analyses. S.A. and T.K. performed the electron microscopy analyses. K.F. and H.Y. performed the Rab9 study. T.M. developed the Lamp2 immunofluorescence assay. M.K. provided the Atg7-/- cells. K.O. contributed data analysis. Y.T. supervised data interpretation. S.S. designed the research and wrote the paper.
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Nishida, Y., Arakawa, S., Fujitani, K. et al. Discovery of Atg5/Atg7-independent alternative macroautophagy. Nature 461, 654–658 (2009). https://doi.org/10.1038/nature08455
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