The hairpin-type tail-anchored SNARE syntaxin 17 targets to autophagosomes for fusion with endosomes/lysosomes.

The lysosome is a degradative organelle, and its fusion with other organelles is strictly regulated. In contrast to fusion with the late endosome, the mechanisms underlying autophagosome-lysosome fusion remain unknown. Here, we identify syntaxin 17 (Stx17) as the autophagosomal SNARE required for fusion with the endosome/lysosome. Stx17 localizes to the outer membrane of completed autophagosomes but not to the isolation membrane (unclosed intermediate structures); for this reason, the lysosome does not fuse with the isolation membrane. Stx17 interacts with SNAP-29 and the endosomal/lysosomal SNARE VAMP8. Depletion of Stx17 causes accumulation of autophagosomes without degradation. Stx17 has a unique C-terminal hairpin structure mediated by two tandem transmembrane domains containing glycine zipper-like motifs, which is essential for its association with the autophagosomal membrane. These findings reveal a mechanism by which the SNARE protein is available to the completed autophagosome.

Ultrastructural insights into pathogen clearance by autophagy.

Autophagy defends cells against proliferation of bacteria such as Salmonella in the cytosol. After escape from a damaged Salmonella-containing vacuole (SCV) exposing luminal glycans that bind to Galectin-8, the host cell ubiquitination machinery deposits a dense layer of ubiquitin around the cytosolic bacteria. The nature and spatial distribution of this ubiquitin coat in relation to other autophagy-related membranes are unknown. Using transmission electron microscopy, we determined the exact localisation of ubiquitin, the ruptured SCV membrane and phagophores around cytosolic Salmonella. Ubiquitin was not predominantly present on the Salmonella surface, but enriched on the fragmented SCV. Cytosolic bacteria without SCVs were less efficiently targeted by phagophores. Single bacteria were contained in single phagophores but multiple bacteria could be within large autophagic vacuoles reaching 30 µm in circumference. These large phagophores followed the contour of the engulfed bacteria, they were frequently in close association with endoplasmic reticulum membranes and, within them, remnants of the SCV were seen associated with each engulfed particle. Our data suggest that the Salmonella SCV has a major role in the formation of autophagic phagophores and highlight evolutionary conserved parallel mechanisms between xenophagy and mitophagy with the fragmented SCV and the damaged outer mitochondrial membrane serving similar functions.

Targeting of early endosomes by autophagy facilitates EGFR recycling and signalling.

Despite recently uncovered connections between autophagy and the endocytic pathway, the role of autophagy in regulating endosomal function remains incompletely understood. Here, we find that the ablation of autophagy-essential players disrupts EGF-induced endocytic trafficking of EGFR. Cells lacking ATG7 or ATG16L1 exhibit increased levels of phosphatidylinositol-3-phosphate (PI(3)P), a key determinant of early endosome maturation. Increased PI(3)P levels are associated with an accumulation of EEA1-positive endosomes where EGFR trafficking is stalled. Aberrant early endosomes are recognised by the autophagy machinery in a TBK1- and Gal8-dependent manner and are delivered to LAMP2-positive lysosomes. Preventing this homeostatic regulation of early endosomes by autophagy reduces EGFR recycling to the plasma membrane and compromises downstream signalling and cell survival. Our findings uncover a novel role for the autophagy machinery in maintaining early endosome function and growth factor sensing.

Ultrastructural analysis of autophagosome organization using mammalian autophagy-deficient cells.

Autophagy is mediated by a unique organelle, the autophagosome. Autophagosome formation involves a number of autophagy-related (ATG) proteins and complicated membrane dynamics. Although the hierarchical relationships of ATG proteins have been investigated, how individual ATG proteins or their complexes contribute to the organization of the autophagic membrane remains largely unknown. Here, systematic ultrastructural analysis of mouse embryonic fibroblasts (MEFs) and HeLa cells deficient in various ATG proteins reveals that the emergence of the isolation membrane (phagophore) requires FIP200 (also known as RB1CC1), ATG9A and phosphatidylinositol (PtdIns) 3-kinase activity. By contrast, small premature isolation-membrane-like and autophagosome-like structures were generated in cells lacking VMP1 and both ATG2A and ATG2B, respectively. The isolation membranes could elongate in cells lacking ATG5, but did not mature into autophagosomes. We also found that ferritin clusters accumulated at the autophagosome formation site together with p62 (also known as SQSTM1) in autophagy-deficient cells. These results reveal the specific functions of these representative ATG proteins in autophagic membrane organization and ATG-independent recruitment of ferritin to the site of autophagosome formation.

Structures containing Atg9A and the ULK1 complex independently target depolarized mitochondria at initial stages of Parkin-mediated mitophagy.

Mitochondria can be degraded by autophagy in a process termed mitophagy. The Parkinson-disease-associated ubiquitin ligase Parkin can trigger mitophagy of depolarized mitochondria. However, it remains to be determined how the autophagy machinery is involved in this specific type of autophagy. It has been speculated that adaptor proteins such as p62 might mediate the interaction between the autophagosomal LC3 family of proteins and ubiquitylated proteins on mitochondria. Here, we describe our systematic analysis of the recruitment of Atg proteins in Parkin-dependent mitophagy. Structures containing upstream Atg proteins, including ULK1, Atg14, DFCP1, WIPI-1 and Atg16L1, can associate with depolarized mitochondria even in the absence of membrane-bound LC3. Atg9A structures are also recruited to these damaged mitochondria as well as to the autophagosome formation site during starvation-induced canonical autophagy. In the initial steps of Parkin-mediated mitophagy, the structures containing the ULK1 complex and Atg9A are independently recruited to depolarized mitochondria and both are required for further recruitment of downstream Atg proteins except LC3. Autophagosomal LC3 is important for efficient incorporation of damaged mitochondria into the autophagosome at a later stage. These findings suggest a process whereby the isolation membrane is generated de novo on damaged mitochondria as opposed to one where a preformed isolation membrane recognizes mitochondria.

GABARAP sequesters the FLCN-FNIP tumor suppressor complex to couple autophagy with lysosomal biogenesis.

Adaptive changes in lysosomal capacity are driven by the transcription factors TFEB and TFE3 in response to increased autophagic flux and endolysosomal stress, yet the molecular details of their activation are unclear. LC3 and GABARAP members of the ATG8 protein family are required for selective autophagy and sensing perturbation within the endolysosomal system. Here, we show that during the conjugation of ATG8 to single membranes (CASM), Parkin-dependent mitophagy, and Salmonella-induced xenophagy, the membrane conjugation of GABARAP, but not LC3, is required for activation of TFEB/TFE3 to control lysosomal capacity. GABARAP directly binds to a previously unidentified LC3-interacting motif (LIR) in the FLCN/FNIP tumor suppressor complex and mediates sequestration to GABARAP-conjugated membrane compartments. This disrupts FLCN/FNIP GAP function toward RagC/D, resulting in impaired substrate-specific mTOR-dependent phosphorylation of TFEB. Thus, the GABARAP-FLCN/FNIP-TFEB axis serves as a molecular sensor that coordinates lysosomal homeostasis with perturbations and cargo flux within the autophagy-lysosomal network.

Selective Autophagy of Mitochondria on a Ubiquitin-Endoplasmic-Reticulum Platform.

The dynamics and coordination between autophagy machinery and selective receptors during mitophagy are unknown. Also unknown is whether mitophagy depends on pre-existing membranes or is triggered on the surface of damaged mitochondria. Using a ubiquitin-dependent mitophagy inducer, the lactone ivermectin, we have combined genetic and imaging experiments to address these questions. Ubiquitination of mitochondrial fragments is required the earliest, followed by auto-phosphorylation of TBK1. Next, early essential autophagy proteins FIP200 and ATG13 act at different steps, whereas ULK1 and ULK2 are dispensable. Receptors act temporally and mechanistically upstream of ATG13 but downstream of FIP200. The VPS34 complex functions at the omegasome step. ATG13 and optineurin target mitochondria in a discontinuous oscillatory way, suggesting multiple initiation events. Targeted ubiquitinated mitochondria are cradled by endoplasmic reticulum (ER) strands even without functional autophagy machinery and mitophagy adaptors. We propose that damaged mitochondria are ubiquitinated and dynamically encased in ER strands, providing platforms for formation of the mitophagosomes.

The Use of Correlative Light-Electron Microscopy (CLEM) to Study PINK1/Parkin-Mediated Mitophagy.

In this chapter we describe the use of correlative light-electron microscopy (CLEM) to study, in cultured cells, the turnover of damaged mitochondria by PINK1/Parkin-dependent mitophagy. CLEM combines the advantages of light microscopy, which allows to image and rapidly screen a large number of the cells, while electron microscopy provides high-resolution imaging of these selected cells and a detailed structural analysis of their cellular organelles. We describe in detail how to prepare the cell cultures for optimum preservation of their cellular ultrastructure for CLEM using the most suitable buffers, fixatives, and embedding resins. These protocols are applicable for detailed ultrastructural analysis in a wide variety of organisms and cells, ranging from prokaryotic bacteria to mammalian cells.

Myosin VI-Dependent Actin Cages Encapsulate Parkin-Positive Damaged Mitochondria.

Mitochondrial quality control is essential to maintain cellular homeostasis and is achieved by removing damaged, ubiquitinated mitochondria via Parkin-mediated mitophagy. Here, we demonstrate that MYO6 (myosin VI), a unique myosin that moves toward the minus end of actin filaments, forms a complex with Parkin and is selectively recruited to damaged mitochondria via its ubiquitin-binding domain. This myosin motor initiates the assembly of F-actin cages to encapsulate damaged mitochondria by forming a physical barrier that prevents refusion with neighboring populations. Loss of MYO6 results in an accumulation of mitophagosomes and an increase in mitochondrial mass. In addition, we observe downstream mitochondrial dysfunction manifesting as reduced respiratory capacity and decreased ability to rely on oxidative phosphorylation for energy production. Our work uncovers a crucial step in mitochondrial quality control: the formation of MYO6-dependent actin cages that ensure isolation of damaged mitochondria from the network.

Loss of functional MYO1C/myosin 1c, a motor protein involved in lipid raft trafficking, disrupts autophagosome-lysosome fusion.

MYO1C, a single-headed class I myosin, associates with cholesterol-enriched lipid rafts and facilitates their recycling from intracellular compartments to the cell surface. Absence of functional MYO1C disturbs the cellular distribution of lipid rafts, causes the accumulation of cholesterol-enriched membranes in the perinuclear recycling compartment, and leads to enlargement of endolysosomal membranes. Several feeder pathways, including classical endocytosis but also the autophagy pathway, maintain the health of the cell by selective degradation of cargo through fusion with the lysosome. Here we show that loss of functional MYO1C leads to an increase in total cellular cholesterol and its disrupted subcellular distribution. We observe an accumulation of autophagic structures caused by a block in fusion with the lysosome and a defect in autophagic cargo degradation. Interestingly, the loss of MYO1C has no effect on degradation of endocytic cargo such as EGFR, illustrating that although the endolysosomal compartment is enlarged in size, it is functional, contains active hydrolases, and the correct pH. Our results highlight the importance of correct lipid composition in autophagosomes and lysosomes to enable them to fuse. Ablating MYO1C function causes abnormal cholesterol distribution, which has a major selective impact on the autophagy pathway.

Autophagy-related Atg8 localizes to the apicoplast of the human malaria parasite Plasmodium falciparum.

Autophagy is a membrane-mediated degradation process, which is governed by sequential functions of Atg proteins. Although Atg proteins are highly conserved in eukaryotes, protozoa possess only a partial set of Atg proteins. Nonetheless, almost all protozoa have the complete factors belonging to the Atg8 conjugation system, namely, Atg3, Atg4, Atg7, and Atg8. Here, we report the biochemical properties and subcellular localization of the Atg8 protein of the human malaria parasite Plasmodium falciparum (PfAtg8). PfAtg8 is expressed during intra-erythrocytic development and associates with membranes likely as a lipid-conjugated form. Fluorescence microscopy and immunoelectron microscopy show that PfAtg8 localizes to the apicoplast, a four membrane-bound non-photosynthetic plastid. Autophagosome-like structures are not observed in the erythrocytic stages. These data suggest that, although Plasmodium parasites have lost most Atg proteins during evolution, they use the Atg8 conjugation system for the unique organelle, the apicoplast.