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Journal of Molecular Biology Apr 2020Macroautophagy is a conserved catabolic process observed in all eukaryotic cells, during which selected cellular components are transported to and broken down within... (Review)
Review
Macroautophagy is a conserved catabolic process observed in all eukaryotic cells, during which selected cellular components are transported to and broken down within lysosomes. The process starts with the capture of unnecessary material into autophagosomes, which is followed by autophagosome-lysosome fusion to generate autolysosomes that degrade the cargo. In the past quarter-century, our knowledge about autophagosome formation almost exponentially increased, while the later steps were much less studied. This fortunately changed in the past few years, with more and more publications focusing on the fate of the completed autophagosome. In this review, we aspire to summarize the current knowledge about the molecular mechanisms of autophagosome-lysosome fusion.
Topics: Animals; Autophagosomes; Autophagy; Humans; Lysosomes; Neurodegenerative Diseases; SNARE Proteins
PubMed: 31682838
DOI: 10.1016/j.jmb.2019.10.028 -
Bulletin Du Cancer Mar 2021Autophagy refers to the formation of autophagosomes by membrane wrapping part of the cytoplasm and the organelles and proteins that need to be degraded in the cells.... (Review)
Review
Autophagy refers to the formation of autophagosomes by membrane wrapping part of the cytoplasm and the organelles and proteins that need to be degraded in the cells. Autophagosomes are fused with lysosomes to form autophagolysosome, which degrade the contents of the inclusions, to achieve cell homeostasis and organelle renewal. The regulatory mechanism of autophagy is complex, and its upstream signaling pathway mainly involves mTOR dependent pathway and mTOR independent pathway (AMPK, PI3K, Ras-MAPK, p53, PTEN, endoplasmic reticulum stress). Autophagy is a phenomenon of "self-eating" in cells. Apoptosis is a phenomenon of "self-killing". Both of them share the same stimulating factors and regulatory proteins, but the threshold of induction is different. How to transform and coordinate is not clear at present. This paper summarizes the history of autophagy discovery, the structure and function of related molecules, the biological function of autophagy, the regulatory mechanism and the research results of the relationship between autophagy and apoptosis.
Topics: Apoptosis; Autophagosomes; Autophagy; Biomedical Research; Humans
PubMed: 33423775
DOI: 10.1016/j.bulcan.2020.11.004 -
International Journal of Molecular... Aug 2017Autophagy is a cytoplasmic degradation system, which is important for starvation adaptation and cellular quality control. Recent advances in understanding autophagy... (Review)
Review
Autophagy is a cytoplasmic degradation system, which is important for starvation adaptation and cellular quality control. Recent advances in understanding autophagy highlight its importance under physiological and pathological conditions. However, methods for monitoring autophagic activity are complicated and the results are sometimes misinterpreted. Here, we review the methods used to identify autophagic structures, and to measure autophagic flux in cultured cells and animals. We will also describe the existing autophagy reporter mice that are useful for autophagy studies and drug testing. Lastly, we will consider the attempts to monitor autophagy in samples derived from humans.
Topics: Animals; Autophagosomes; Autophagy; Autophagy-Related Proteins; Humans; Microscopy, Fluorescence
PubMed: 28846632
DOI: 10.3390/ijms18091865 -
Nature Reviews. Molecular Cell Biology Aug 2020Autophagosomes are double-membrane vesicles newly formed during autophagy to engulf a wide range of intracellular material and transport this autophagic cargo to... (Review)
Review
Autophagosomes are double-membrane vesicles newly formed during autophagy to engulf a wide range of intracellular material and transport this autophagic cargo to lysosomes (or vacuoles in yeasts and plants) for subsequent degradation. Autophagosome biogenesis responds to a plethora of signals and involves unique and dynamic membrane processes. Autophagy is an important cellular mechanism allowing the cell to meet various demands, and its disruption compromises homeostasis and leads to various diseases, including metabolic disorders, neurodegeneration and cancer. Thus, not surprisingly, the elucidation of the molecular mechanisms governing autophagosome biogenesis has attracted considerable interest. Key molecules and organelles involved in autophagosome biogenesis, including autophagy-related (ATG) proteins and the endoplasmic reticulum, have been discovered, and their roles and relationships have been investigated intensely. However, several fundamental questions, such as what supplies membranes/lipids to build the autophagosome and how the membrane nucleates, expands, bends into a spherical shape and finally closes, have proven difficult to address. Nonetheless, owing to recent studies with new approaches and technologies, we have begun to unveil the mechanisms underlying these processes on a molecular level. We now know that autophagosome biogenesis is a highly complex process, in which multiple proteins and lipids from various membrane sources, supported by the formation of membrane contact sites, cooperate with biophysical phenomena, including membrane shaping and liquid-liquid phase separation, to ensure seamless segregation of the autophagic cargo. Together, these studies pave the way to obtaining a holistic view of autophagosome biogenesis.
Topics: Animals; Autophagosomes; Autophagy; Autophagy-Related Proteins; Cell Membrane; Endoplasmic Reticulum; Humans; Lysosomes; Macroautophagy; Protein Transport
PubMed: 32372019
DOI: 10.1038/s41580-020-0241-0 -
Autophagy 2018Macroautophagy/autophagy is a conserved transport pathway where targeted structures are sequestered by phagophores, which mature into autophagosomes, and then delivered...
Macroautophagy/autophagy is a conserved transport pathway where targeted structures are sequestered by phagophores, which mature into autophagosomes, and then delivered into lysosomes for degradation. Autophagy is involved in the pathophysiology of numerous diseases and its modulation is beneficial for the outcome of numerous specific diseases. Several lysosomal inhibitors such as bafilomycin A (BafA), protease inhibitors and chloroquine (CQ), have been used interchangeably to block autophagy in in vitro experiments assuming that they all primarily block lysosomal degradation. Among them, only CQ and its derivate hydroxychloroquine (HCQ) are FDA-approved drugs and are thus currently the principal compounds used in clinical trials aimed to treat tumors through autophagy inhibition. However, the precise mechanism of how CQ blocks autophagy remains to be firmly demonstrated. In this study, we focus on how CQ inhibits autophagy and directly compare its effects to those of BafA. We show that CQ mainly inhibits autophagy by impairing autophagosome fusion with lysosomes rather than by affecting the acidity and/or degradative activity of this organelle. Furthermore, CQ induces an autophagy-independent severe disorganization of the Golgi and endo-lysosomal systems, which might contribute to the fusion impairment. Strikingly, HCQ-treated mice also show a Golgi disorganization in kidney and intestinal tissues. Altogether, our data reveal that CQ and HCQ are not bona fide surrogates for other types of late stage lysosomal inhibitors for in vivo experiments. Moreover, the multiple cellular alterations caused by CQ and HCQ call for caution when interpreting results obtained by blocking autophagy with this drug.
Topics: Animals; Autophagosomes; Autophagy; Cell Line, Tumor; Chloroquine; Endocytosis; Endosomes; ErbB Receptors; Female; Golgi Apparatus; Humans; Hydroxychloroquine; Lysosomes; Macrolides; Membrane Fusion; Mice, Inbred C57BL; Proteolysis; Sequestosome-1 Protein
PubMed: 29940786
DOI: 10.1080/15548627.2018.1474314 -
Cells Feb 2023Autophagy-the lysosomal degradation of cytoplasm-plays a central role in cellular homeostasis and protects cells from potentially harmful agents that may accumulate in... (Review)
Review
Autophagy-the lysosomal degradation of cytoplasm-plays a central role in cellular homeostasis and protects cells from potentially harmful agents that may accumulate in the cytoplasm, including pathogens, protein aggregates, and dysfunctional organelles. This process is initiated by the formation of a phagophore membrane, which wraps around a portion of cytoplasm or cargo and closes to form a double-membrane autophagosome. Upon the fusion of the autophagosome with a lysosome, the sequestered material is degraded by lysosomal hydrolases in the resulting autolysosome. Several alternative membrane sources of autophagosomes have been proposed, including the plasma membrane, endosomes, mitochondria, endoplasmic reticulum, lipid droplets, hybrid organelles, and de novo synthesis. Here, we review recent progress in our understanding of how the autophagosome is formed and highlight the proposed role of vesicles that contain the lipid scramblase ATG9 as potential seeds for phagophore biogenesis. We also discuss how the phagophore is sealed by the action of the endosomal sorting complex required for transport (ESCRT) proteins.
Topics: Autophagosomes; Macroautophagy; Autophagy; Endosomes; Cell Membrane
PubMed: 36831335
DOI: 10.3390/cells12040668 -
Autophagy Jan 2020Many diseases are caused by aberrant accumulation of certain proteins that are misfolded and cytotoxic, and lowering the level of these proteins provides promising... (Review)
Review
Many diseases are caused by aberrant accumulation of certain proteins that are misfolded and cytotoxic, and lowering the level of these proteins provides promising treatment strategies for these diseases. We hypothesized that compounds that interact with both the disease-causing protein and the phagophore (autophagosome precursor) protein LC3 may tether the former to phagophores for subsequent autophagic degradation. If true, this uophagosome-thering ompound (ATTEC) concept could be applied to many disease-causing proteins to treat diseases. We tested this hypothesis in the scenario of Huntington disease (HD), a neurodegenerative disorder that is caused by the mutant HTT (mHTT) protein with an expanded polyglutamine (polyQ) stretch. In our recent study, we designed a small-molecule microarray-based screening and identified four mHTT-lowering compounds that interact with both mHTT and LC3, but not wild-type (WT) HTT. These compounds target mHTT to phagophores for autophagic degradation without influencing the WT HTT level, and rescue HD-relevant phenotypes in HD cells and in the fly and mouse HD models. Interestingly, these compounds interact with the expanded polyQ stretch directly and are able to reduce other disease-causing proteins with expanded polyQ. In summary, our study provides the initial validation of lowering mHTT by ATTEC, providing entry points to new treatment strategies of HD and similar diseases.
Topics: Animals; Autophagosomes; Autophagy; Disease Models, Animal; Humans; Huntingtin Protein; Huntington Disease; Nerve Tissue Proteins
PubMed: 31690177
DOI: 10.1080/15548627.2019.1688556 -
Protein & Cell Sep 2023Lipophagy, the selective engulfment of lipid droplets (LDs) by autophagosomes for lysosomal degradation, is critical to lipid and energy homeostasis. Here we show that...
Lipophagy, the selective engulfment of lipid droplets (LDs) by autophagosomes for lysosomal degradation, is critical to lipid and energy homeostasis. Here we show that the lipid transfer protein ORP8 is located on LDs and mediates the encapsulation of LDs by autophagosomal membranes. This function of ORP8 is independent of its lipid transporter activity and is achieved through direct interaction with phagophore-anchored LC3/GABARAPs. Upon lipophagy induction, ORP8 has increased localization on LDs and is phosphorylated by AMPK, thereby enhancing its affinity for LC3/GABARAPs. Deletion of ORP8 or interruption of ORP8-LC3/GABARAP interaction results in accumulation of LDs and increased intracellular triglyceride. Overexpression of ORP8 alleviates LD and triglyceride deposition in the liver of ob/ob mice, and Osbpl8-/- mice exhibit liver lipid clearance defects. Our results suggest that ORP8 is a lipophagy receptor that plays a key role in cellular lipid metabolism.
Topics: Animals; Mice; Lipid Droplets; Autophagy; Autophagosomes; Homeostasis; Triglycerides
PubMed: 37707322
DOI: 10.1093/procel/pwac063 -
Cell Research Dec 2022STING, an endoplasmic reticulum (ER) transmembrane protein, mediates innate immune activation upon cGAMP stimulation and is degraded through autophagy. Here, we report...
STING, an endoplasmic reticulum (ER) transmembrane protein, mediates innate immune activation upon cGAMP stimulation and is degraded through autophagy. Here, we report that activated STING could be transferred between cells to promote antitumor immunity, a process triggered by RAB22A-mediated non-canonical autophagy. Mechanistically, RAB22A engages PI4K2A to generate PI4P that recruits the Atg12-Atg5-Atg16L1 complex, inducing the formation of ER-derived RAB22A-mediated non-canonical autophagosome, in which STING activated by agonists or chemoradiotherapy is packaged. This RAB22A-induced autophagosome fuses with RAB22A-positive early endosome, generating a new organelle that we name Rafeesome (RAB22A-mediated non-canonical autophagosome fused with early endosome). Meanwhile, RAB22A inactivates RAB7 to suppress the fusion of Rafeesome with lysosome, thereby enabling the secretion of the inner vesicle of the autophagosome bearing activated STING as a new type of extracellular vesicle that we define as R-EV (RAB22A-induced extracellular vesicle). Activated STING-containing R-EVs induce IFNβ release from recipient cells to the tumor microenvironment, promoting antitumor immunity. Consistently, RAB22A enhances the antitumor effect of the STING agonist diABZI in mice, and a high RAB22A level predicts good survival in nasopharyngeal cancer patients treated with chemoradiotherapy. Our findings reveal that Rafeesome regulates the intercellular transfer of activated STING to trigger and spread antitumor immunity, and that the inner vesicle of non-canonical autophagosome originated from ER is secreted as R-EV, providing a new perspective for understanding the intercellular communication of organelle membrane proteins.
Topics: Animals; Mice; Autophagosomes; Autophagy; Immunity, Innate; Lysosomes; Membrane Proteins; Nasopharyngeal Neoplasms; Tumor Microenvironment; Humans
PubMed: 36280710
DOI: 10.1038/s41422-022-00731-w -
Journal of Molecular Biology Jan 2020Selective autophagy relies on soluble or membrane-bound cargo receptors that recognize cargo and bring about autophagosome formation at the cargo. The cargo-bound... (Review)
Review
Selective autophagy relies on soluble or membrane-bound cargo receptors that recognize cargo and bring about autophagosome formation at the cargo. The cargo-bound receptors interact with lipidated ATG8 family proteins anchored in the membrane at the concave side of the forming autophagosome. The interaction is mediated by 15- to 20-amino-acid-long sequence motifs called LC3-interacting region (LIR) motifs that bind to the LIR docking site (LDS) of ATG8 proteins. In this review, we focus on LIR-ATG8 interactions and the soluble mammalian selective autophagy receptors. We discuss the roles of ATG8 family proteins as membrane scaffolds in autophagy and the LIR-LDS interaction and how specificity for binding to GABARAP or LC3 subfamily proteins is achieved. We also discuss atypical LIR-LDS interactions and a novel LIR-independent interaction. Recently, it has become clear that several of the soluble cargo receptors are able to recruit components of the core autophagy apparatus to aid in assembling autophagosome formation at the site of cargo sequestration. A model on phagophore recruitment and expansion on a selective autophagy receptor-coated cargo incorporating the latest findings is presented.
Topics: Animals; Apoptosis Regulatory Proteins; Autophagosomes; Autophagy; Autophagy-Related Protein 8 Family; Humans; Macroautophagy; Microtubule-Associated Proteins; Protein Interaction Domains and Motifs; Protein Interaction Maps
PubMed: 31310766
DOI: 10.1016/j.jmb.2019.07.016