CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Developmental Cell Previews Autophagosome Formation: Tracing the Source Ame´ lie Bernard1 and Daniel J. Klionsky1,* 1Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2013.04.004 Although it is well appreciated that autophagy begins with phagophore formation and expansion through lipid acquisition to become the autophagosome, this process remains poorly understood, with the source of autophagosome membrane controversial. Reporting recently in Nature, Hamasaki et al. (2013) suggest that the ER-mitochondria contact sites are involved in phagophore assembly. Autophagy is a degradative process Using immunoelectron microscopy and esis, and what kind of exchange could during which portions of the cytoplasm subcellular fractionation, Hamasaki and they mediate? Mitochondrial proteins are engulfed within a double-membrane colleagues (2013) now show that were previously noted on autophago- compartment, the autophagosome, ATG14-positive punctae assemble at the somes. Therefore, MAMs might mediate which ultimately fuses with the lysosome mitochondria-associated ER membrane the incorporation of proteins required for or vacuole, allowing the cargo to be (MAM) under starvation conditions. autophagosome formation from both degraded and recycled. Morphologically, ZFYVE1/DFCP1, a marker of the omega- the ER and the mitochondria. ER-mito- autophagy starts with the nucleation of some, also shifts to the MAM upon starva- chondria contact sites also constitute the phagophore, the initial sequestering tion, whereas ATG5, a component of a platforms for lipid synthesis and lipid organelle. The phagophore expands ubiquitin-like conjugation system that is exchange between the two organelles through the acquisition of lipids and critical for autophagosome biogenesis, (Rowland and Voeltz, 2012). In particular, ultimately seals, thus completing the localizes to the ER-mitochondria junction the mitochondrial synthesis of phosphati- formation process and generating an sites during the formation process and dylethanolamine (PE), a critical lipid for autophagosome. How the phagophore dissociates from it after autophagosome autophagy, uses phosphatidylserine (PS) forms remains one of the most funda- completion. During the entire process, provided by the ER. As assessed by a mental unresolved questions in the ATG5 shows a stable association with fluorescent probe, incorporation of lipids understanding of the autophagy pathway. the ER but an oscillation in its localization from the ER into the mitochondrial mem- In recent years much effort has been at the mitochondria, suggesting that dy- brane and subsequently into the auto- focused on this issue, and indepen- namic ER-mitochondria association phagosome has been detected (Hailey dent studies have pointed to several could be required for autophagy. This et al., 2010). Hence, ER-mitochondria different organelles as potential mem- idea was further tested by disrupting the contacts could mediate the synthesis brane sources. These include the plasma ER-mitochondria contact sites, which re- and/or trafficking of lipids required for membrane, the Golgi apparatus, the sulted in severe impairment of ATG14 autophagosome formation. It is note- endoplasmic reticulum (ER), and the mito- puncta formation and reduction of auto- worthy that the ER-mitochondria contact chondria. In a recent study in Nature, phagic activity, suggesting that the reloc- sites are defined as regions of close Hamasaki et al. (2013) show that auto- alization of ATG14 to the MAM is required proximity, but not fusion, between the phagosomes form at ER-mitochondria for autophagosome formation. Together, membranes of the two organelles. Previ- interfaces in mammalian cells. this study (1) proposes a model in which ous independent studies have reported Upon autophagy induction, ATG14, the ER constitutes the platform for auto- membrane continuity between either the an autophagy-specific member of the phagosome formation and (2) proposes ER or the mitochondria and the phago- phosphatidylinositol-3-kinase complex, that exchanges between the ER and the phore. Could the phagophore membrane is rapidly recruited to the phagophore, mitochondria are necessary for this pro- fuse with the two organelles at the where it plays a role in early steps of cess. The importance of the ER-mito- MAM? Otherwise, how are components autophagosome biogenesis. Under auto- chondria contact sites in autophagy was delivered to the forming autophagosomes phagic conditions, ATG14 and phosphati- already pointed out in a previous study, from the mitochondria and/or the ER? dylinositol-3-phosphate are enriched in which proposed that mitochondria are The study by Hamasaki and colleagues specific ER subdomains from which the platform for autophagosome forma- (2013) provides compelling evidence cup-shaped structures called omega- tion (Hailey et al., 2010). The work by Ha- supporting the idea that autophago- somes emerge (Axe et al., 2008). Because masaki et al. (2013) thus reconciles two somes form at the ER-mitochondria these compartments are positive for com- conflicting hypotheses: ER or mitochon- interface. Nevertheless, this does not ponents of the autophagic machinery, dria as the origin of the autophagosomal exclude the possibility of other sites it was proposed that omegasomes con- membrane. contributing membranes for autophago- stitute the platform for autophagosome Why are the ER-mitochondria junctions some formation. Three-dimensional formation. important for autophagosome biogen- tomographic studies show that 30% of 116 Developmental Cell 25, April 29, 2013 ª2013 Elsevier Inc. Developmental Cell Previews the autophagosome precursors are not (2013) regarding STX17, an ER- and valuable steps forward in solving the associated with the ER (Hayashi-Nishino mitochondria-associated Qa SNARE, puzzle. et al., 2009), suggesting that there are and that from another recent independent other sites or membrane sources for study (Itakura et al., 2012). Whereas REFERENCES autophagosome formation. Several lines Hamasaki et al. (2013) show that STX17 of evidence have led to the proposal is required for the shift of ATG14 to the Axe, E.L., Walker, S.A., Manifava, M., Chandra, P., that the plasma membrane and the Golgi MAM and further demonstrate a direct Roderick, H.L., Habermann, A., Griffiths, G., and Ktistakis, N.T. (2008). J. Cell Biol. 182, 685–701. apparatus provide lipids to the phago- interaction between STX17 and ATG14, phore (for review, see Mari et al., 2011). STX17 was observed in Itakura et al. 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