A Cluster of Thin Tubular Structures Mediates Transformation of the Endoplasmic Reticulum to Autophagic Isolation Membrane

Abstract

Recent findings have suggested that the autophagic isolation membrane (IM) might originate from a domain of the endoplasmic reticulum (ER) called the omegasome. However, the morphological relationships between ER, omegasome, and IM remain unclear. In the present study, we found that hybrid structures composed of a double FYVE domain-containing protein 1 (DFCP1)-positive omegasome and the IM accumulated in Atg3-deficient mouse embryonic fibroblasts (MEFs). Moreover, correlative light and electron microscopy and immunoelectron microscopy revealed that green fluorescent protein (GFP)-tagged DFCP1 was localized on tubular or vesicular elements adjacent to the IM rims. Through detailed morphological analyses, including optimization of a fixation method and electron tomography, we observed a cluster of thin tubular structures between the IM edges and ER, part of which were continuous with IM and/or ER. The formation of these thin tubular clusters was observed in several cell lines and MEFs deficient for Atg5, Atg7, or Atg16L1 but not in FIP200-deficient cells, suggesting that they were relevant to the earlier events in autophagosome formation. Taken together, our findings indicate that these tubular profiles represent a part of the omegasome that links the ER with the IM.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.01327-13.

ACKNOWLEDGMENTS

We thank Noboru Mizushima for providing Atg5^−/− MEF and anti-Atg16L antibody and Shizuo Akira and Jun-Lin Guan for providing Atg16L1^−/− and FIP200^−/− MEFs, respectively. We also thank K. Kanno (Fukushima Medical University School of Medicine) for his help in electron microscopy and all the members of our department for helpful discussion.

This work was supported by JSPS KAKENHI grants 21659045 and 24390048 (to S. Waguri) and by grants from Fukushima Medical University (to A. Kametaka and T. Uemura).