ER-phagy at a glance

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Abstract

Selective autophagy represents the major quality control mechanism that ensures proper turnover of exhausted or harmful organelles, among them the endoplasmic reticulum (ER), which is fragmented and delivered to the lysosome for degradation via a specific type of autophagy called ER-phagy. The recent discovery of ER-resident proteins that bind to mammalian Atg8 proteins has revealed that the selective elimination of ER involves different receptors that are specific for different ER subdomains or ER stresses. FAM134B (also known as RETREG1) and RTN3 are reticulon-type proteins that are able to remodel the ER network and ensure the basal membrane turnover. SEC62 and CCPG1 are transmembrane ER receptors that function in response to ER stress signals. This task sharing reflects the complexity of the ER in terms of biological functions and morphology. In this Cell Science at a Glance article and the accompanying poster, we summarize the most recent findings about ER-phagy in yeast and in mammalian cells.

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Autophagosome formation from membrane compartments enriched in phosphatidylinositol 3-phosphate and dynamically connected to the endoplasmic reticulum

Elizabeth L. Axe, Simon A. Walker, Maria Manifava … (2008)

Autophagy is the engulfment of cytosol and organelles by double-membrane vesicles termed autophagosomes. Autophagosome formation is known to require phosphatidylinositol 3-phosphate (PI(3)P) and occurs near the endoplasmic reticulum (ER), but the exact mechanisms are unknown. We show that double FYVE domain–containing protein 1, a PI(3)P-binding protein with unusual localization on ER and Golgi membranes, translocates in response to amino acid starvation to a punctate compartment partially colocalized with autophagosomal proteins. Translocation is dependent on Vps34 and beclin function. Other PI(3)P-binding probes targeted to the ER show the same starvation-induced translocation that is dependent on PI(3)P formation and recognition. Live imaging experiments show that this punctate compartment forms near Vps34-containing vesicles, is in dynamic equilibrium with the ER, and provides a membrane platform for accumulation of autophagosomal proteins, expansion of autophagosomal membranes, and emergence of fully formed autophagosomes. This PI(3)P-enriched compartment may be involved in autophagosome biogenesis. Its dynamic relationship with the ER is consistent with the idea that the ER may provide important components for autophagosome formation.

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Autophagy is activated for cell survival after endoplasmic reticulum stress.

Maiko OGATA, Shin-ichiro Hino, Atsushi Saito … (2006)

Eukaryotic cells deal with accumulation of unfolded proteins in the endoplasmic reticulum (ER) by the unfolded protein response, involving the induction of molecular chaperones, translational attenuation, and ER-associated degradation, to prevent cell death. Here, we found that the autophagy system is activated as a novel signaling pathway in response to ER stress. Treatment of SK-N-SH neuroblastoma cells with ER stressors markedly induced the formation of autophagosomes, which were recognized at the ultrastructural level. The formation of green fluorescent protein (GFP)-LC3-labeled structures (GFP-LC3 "dots"), representing autophagosomes, was extensively induced in cells exposed to ER stress with conversion from LC3-I to LC3-II. In IRE1-deficient cells or cells treated with c-Jun N-terminal kinase (JNK) inhibitor, the autophagy induced by ER stress was inhibited, indicating that the IRE1-JNK pathway is required for autophagy activation after ER stress. In contrast, PERK-deficient cells and ATF6 knockdown cells showed that autophagy was induced after ER stress in a manner similar to the wild-type cells. Disturbance of autophagy rendered cells vulnerable to ER stress, suggesting that autophagy plays important roles in cell survival after ER stress.

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Mitochondria supply membranes for autophagosome biogenesis during starvation.

Dale Hailey, Angelika Rambold, Prasanna Satpute-Krishnan … (2010)

Starvation-induced autophagosomes engulf cytosol and/or organelles and deliver them to lysosomes for degradation, thereby resupplying depleted nutrients. Despite advances in understanding the molecular basis of this process, the membrane origin of autophagosomes remains unclear. Here, we demonstrate that, in starved cells, the outer membrane of mitochondria participates in autophagosome biogenesis. The early autophagosomal marker, Atg5, transiently localizes to punctae on mitochondria, followed by the late autophagosomal marker, LC3. The tail-anchor of an outer mitochondrial membrane protein also labels autophagosomes and is sufficient to deliver another outer mitochondrial membrane protein, Fis1, to autophagosomes. The fluorescent lipid NBD-PS (converted to NBD-phosphotidylethanolamine in mitochondria) transfers from mitochondria to autophagosomes. Photobleaching reveals membranes of mitochondria and autophagosomes are transiently shared. Disruption of mitochondria/ER connections by mitofusin2 depletion dramatically impairs starvation-induced autophagy. Mitochondria thus play a central role in starvation-induced autophagy, contributing membrane to autophagosomes. Copyright (c) 2010 Elsevier Inc. All rights reserved.