Emerging roles of ATG7 in human health and disease
2021; Springer Nature; Volume: 13; Issue: 12 Linguagem: Inglês
10.15252/emmm.202114824
ISSN1757-4684
AutoresJack J. Collier, Fumi Suomi, Monika Oláhová, Thomas G. McWilliams, Robert W. Taylor,
Tópico(s)Endoplasmic Reticulum Stress and Disease
ResumoReview2 November 2021Open Access Emerging roles of ATG7 in human health and disease Jack J Collier Corresponding Author Jack J Collier [email protected] orcid.org/0000-0001-6282-0301 Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK Search for more papers by this author Fumi Suomi Fumi Suomi orcid.org/0000-0001-8883-3338 Translational Stem Cell Biology & Metabolism Program, Research Programs Unit, University of Helsinki, Helsinki, Finland Search for more papers by this author Monika Oláhová Monika Oláhová orcid.org/0000-0002-4082-3875 Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK Search for more papers by this author Thomas G McWilliams Thomas G McWilliams orcid.org/0000-0002-9570-4152 Translational Stem Cell Biology & Metabolism Program, Research Programs Unit, University of Helsinki, Helsinki, Finland Department of Anatomy, Faculty of Medicine, University of Helsinki, Helsinki, Finland Search for more papers by this author Robert W Taylor Corresponding Author Robert W Taylor [email protected] orcid.org/0000-0002-7768-8873 Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children, Newcastle University, Newcastle upon Tyne, UK Search for more papers by this author Jack J Collier Corresponding Author Jack J Collier [email protected] orcid.org/0000-0001-6282-0301 Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK Search for more papers by this author Fumi Suomi Fumi Suomi orcid.org/0000-0001-8883-3338 Translational Stem Cell Biology & Metabolism Program, Research Programs Unit, University of Helsinki, Helsinki, Finland Search for more papers by this author Monika Oláhová Monika Oláhová orcid.org/0000-0002-4082-3875 Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK Search for more papers by this author Thomas G McWilliams Thomas G McWilliams orcid.org/0000-0002-9570-4152 Translational Stem Cell Biology & Metabolism Program, Research Programs Unit, University of Helsinki, Helsinki, Finland Department of Anatomy, Faculty of Medicine, University of Helsinki, Helsinki, Finland Search for more papers by this author Robert W Taylor Corresponding Author Robert W Taylor [email protected] orcid.org/0000-0002-7768-8873 Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children, Newcastle University, Newcastle upon Tyne, UK Search for more papers by this author Author Information Jack J Collier *,1,†, Fumi Suomi2, Monika Oláhová1, Thomas G McWilliams2,3 and Robert W Taylor *,1,4 1Wellcome Centre for Mitochondrial Research, Translational and Clinical Research Institute, Newcastle University, Newcastle upon Tyne, UK 2Translational Stem Cell Biology & Metabolism Program, Research Programs Unit, University of Helsinki, Helsinki, Finland 3Department of Anatomy, Faculty of Medicine, University of Helsinki, Helsinki, Finland 4NHS Highly Specialised Service for Rare Mitochondrial Disorders of Adults and Children, Newcastle University, Newcastle upon Tyne, UK †Present address: Department of Neurology and Neurosurgery, Montreal Neurological Institute, McGill University, Montreal, QC, Canada *Corresponding author. Tel: +1 5143982795; E-mail: [email protected] *Corresponding author. Tel: +44 191 2083685; E-mail: [email protected] EMBO Mol Med (2021)13:e14824https://doi.org/10.15252/emmm.202114824 See the Glossary for abbreviations used in this article. PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Abstract The cardinal stages of macroautophagy are driven by core autophagy-related (ATG) proteins, whose ablation largely abolishes intracellular turnover. Disrupting ATG genes is paradigmatic of studying autophagy deficiency, yet emerging data suggest that ATG proteins have extensive biological importance beyond autophagic elimination. An important example is ATG7, an essential autophagy effector enzyme that in concert with other ATG proteins, also regulates immunity, cell death and protein secretion, and independently regulates the cell cycle and apoptosis. Recently, a direct association between ATG7 dysfunction and disease was established in patients with biallelic ATG7 variants and childhood-onset neuropathology. Moreover, a prodigious body of evidence supports a role for ATG7 in protecting against complex disease states in model organisms, although how dysfunctional ATG7 contributes to manifestation of these diseases, including cancer, neurodegeneration and infection, in humans remains unclear. Here, we systematically review the biological functions of ATG7, discussing the impact of its impairment on signalling pathways and human pathology. Future studies illuminating the molecular relationship between ATG7 dysfunction and disease will expedite therapies for disorders involving ATG7 deficiency and/or impaired autophagy. Glossary Autophagic flux The amount of autophagic degradation or activity that occurs over a specific time, typically referring to non-selective (bulk) degradation or “macroautophagy”. Flux is typically measured by treating cells or tissues with various compounds that inhibit or activate autophagy. Autophagic turnover occurs at steady state within mammals. Autophagosome A transient double-membrane-bound organelle that sequesters cytoplasmic cargo and fuses with the endolysosomal system whose hydrolytic enzymes degrade its constituents. Autophagy conjugation system The group of proteins (including ATG3, ATG4, ATG5, ATG7, ATG10 and ATG12) that drive the lipidation of ATG8 homologues. Autophagy A homeostatic and developmental process that drives the delivery of damaged or unwanted cytoplasmic material to the endolysosomal system for degradation. Biallelic When both copies of an individual gene are affected by DNA variants. Complex disorder A disorder that cannot be explained by variants affecting a single gene. These are thought to be caused by the interactions between variants on a number of genes and environment. Conditional knockout When a gene and its protein are selectively eliminated or depleted from a specific tissue. LC3-associated phagocytosis A process whereby extracellular material (e.g. a pathogen) is engulfed into an LC3-positive single-membrane structure that is delivered to the endolysosomal system for degradation. LIR motif The LC3-interacting region (LIR) motif [(W/F/Y)XX(L/I/V)] is an amino acid sequence within proteins that enable them to interact directly with ATG8 homologues. Recessive Heritable characteristics that have an effect when a variant that controls the characteristic is present on both copies of a single gene. The same variant could be present on both alleles, or each allele could harbour a different variant that together has an effect. Selective autophagy Refers to a growing number of pathways that target specific cargo (e.g. mitochondria) for autophagic elimination. These pathways rely on specific adaptors that interact with both the cargo and autophagosome-bound ATG8 homologues. Introduction The degradation of encapsulated cytoplasmic material via the endolysosomal system provides a first-principle definition of autophagy. Numerous specialised autophagy pathways have been discovered and categorised, including macroautophagy, which can be selective or non-selective, and other variants such as chaperone-mediated autophagy (CMA) and microautophagy. CMA recognises protein substrates with KFERQ motifs and translocates these to lysosomes (Kaushik & Cuervo, 2018). Microautophagy involves the direct engulfment and destruction of cytoplasmic substrates by lysosomal membrane invagination (Schuck, 2020). Macroautophagy (hereafter, “autophagy”) remains the most widely studied pathway. During autophagy, a transient double-membrane-bound autophagosome engulfs cytoplasmic constituents, eventually fusing with acidic endolysosomal compartments where hydrolysis degrades cargo (Yorimitsu & Klionsky, 2005). The fundamental morphological and molecular signatures of autophagy have remained largely unchanged over the past 10 years (Levine & Kroemer, 2008, 2019). Autophagy functions constitutively under basal conditions (Mizushima et al, 2004), but can be induced further by a number of stimuli, including starvation, hypoxia and DNA damage (Kroemer et al, 2010). This triggers the de novo nucleation of a phagophore, a double-membrane cup-shaped structure that matures via the incorporation of supplementary lipids (Nakamura & Yoshimori, 2017). A variety of cytoplasmic cargoes, including organelles, microbes and cytotoxic protein aggregates can be sequestered within this transient structure (Johansen & Lamark, 2020). Autophagy can be a non-selective or selective process (Mizushima & Komatsu, 2011). “Bulk” autophagy involves the non-selective sequestration of cytoplasmic material, ensuring the degradation of long-lived proteins and replenishment of essential building blocks. During selective autophagy, specific cellular components are decorated with specialised signals that recruit the autophagic machinery to the target entity for elimination. Selectivity is conferred upon the pathway by LC3-interacting region (LIR) motifs [(W/F/Y)XX(L/I/V)]) that are present within cargo or specialised adaptor proteins, enabling them to interact with ATG8 proteins that are embedded within the inner and outer membrane of the phagophore (Martens & Fracchiolla, 2020). Following cargo sequestration, the leading edges of the double-membrane structure fuse to generate an autophagosome which merges with acidic compartments of the endolysosomal system. After the inner autophagosomal membrane is degraded, the resident lysosomal acid hydrolases degrade the autophagic cargo which is then recycled (Koyama-Honda et al, 2017) (Fig 1A). Figure 1. ATG7 drives the fundamental stages of degradative autophagy (A) An overview of classical degradative autophagy showing the early, middle and late stages of the process. (B) Phagophore expansion is stimulated by ATG7, which facilitates ATG8 lipidation through its E1-like enzymatic activity. (C) ATG8 lipidation is also critical for selective autophagy and (D) contributes to the late stages of autophagy. Download figure Download PowerPoint In the 1990s, pioneering studies using yeast genetic screens helped to define the molecular basis of autophagy (Tsukada & Ohsumi, 1993; Harding et al, 1995). This approach led to the identification of autophagy-related (ATG) genes, and whilst the exact number is debated, approximately 20 of these are “core” ATG genes, conserved across eukaryotes and encoding proteins essential for both non-selective and selective autophagy (Tsukada & Ohsumi, 1993). Auxiliary ATG proteins enhance the autophagic process and/or participate in selective autophagy, though degradative autophagy in any case requires functional lysosomal proteins (Tanaka et al, 2000). One of the key molecular signatures of autophagy is ATG8 lipidation, a process whereby ATG8 is conjugated to phosphatidylethanolamine (PE) embedded in the emerging phagophore, thus enabling ATG8 to become an integral part of the autophagic membrane (Martens & Fracchiolla, 2020). ATG8 lipidation is a particularly important event because ATG8 facilitates several stages of autophagy including phagophore expansion, cargo recruitment, autophagosome transport and lysosomal fusion. In mammals, there are six ATG8 homologues, classified in the LC3 or GABARAP protein subfamilies. A large body of evidence has demonstrated that ATG proteins contribute to a diverse range of biological processes that extend far beyond autophagy (Levine & Kroemer, 2019). ATG7 is one such multifaceted core ATG protein that drives the cardinal stages of classical degradative autophagy through ATG8 lipidation. ATG7 also makes pivotal contributions to innate immunity via LC3-associated phagocytosis, unconventional protein secretion, receptor recycling, exocytosis of secretory granules and modulation of p53-dependent cell cycle arrest and apoptosis (Mizushima & Levine, 2020; Mizushima, 2020) (Figs 1 and 2). This review will describe the role of ATG7 in these pathways, and the breakthrough genetic models that have led to our understanding of how ATG7 deficiency affects mammalian physiology. We will explore the association of impaired ATG7 activity with human pathologies including neurodegeneration, cancer and infection and pay particular attention to the recently identified recessive congenital disorder of autophagy caused by inherited ATG7 dysfunction leading to neurological manifestations (Collier et al, 2021). Recent breakthroughs in delineating the role of ATG7 in cell biology and human disease have important implications for the development of therapeutics that regulate autophagy. Whereas activation of autophagy provides an attractive therapeutic approach to treat human neuropathology where impaired autophagy is implicated, evidence has also emerged that autophagy inhibition can improve cancer treatment outcomes (Mizushima & Levine, 2020). Figure 2. Overview of the autophagy-related and autophagy-independent biological functions of ATG7 Through its E1-like enzymatic activity, ATG7 drives the conjugation of phosphatidylethanolamine (PE) to LC3-I in a process termed “lipidation”, generating LC3-II which is important for a number of physiological pathways beyond degradative autophagy. Independently of its E1-like enzymatic activity, ATG7 modulates P53 activity, thus affecting cell cycle arrest and apoptosis via control of gene expression. ROS = reactive oxygen species. Adapted from Levine and Kroemer (2019). Download figure Download PowerPoint Biological functions of ATG7 Classical degradative autophagy ATG7 impairment classically renders cells and tissues as “autophagy deficient”, and the study of autophagy has underpinned the majority of ATG7-focused research (Komatsu et al, 2005; Komatsu et al, 2007a; Matsumoto et al, 2008; Collier et al, 2021). During autophagy, the phagophore membrane is enriched with phosphatidylethanolamine (PE), an abundant phospholipid that has been reported to positively regulate autophagic activity (Rockenfeller et al, 2015). PE is important because it acts as the anchor for recruitment of cytosolic ATG8 to the emerging phagophore membrane (Fig 1B). There are two mammalian ATG8 subfamilies encoding six homologues to yeast atg8 protein. The first, LC3, has three members, LC3A, LC3B and LC3C, and the second, GABARAP, represents the remaining three homologues, GABARAP, GABARAPL1 and GABARAPL2 (Lee & Lee, 2016). By convention, ATG8 refers to both LC3 and GABARAP subfamilies, and upon lipidation becomes “ATG8-PE”. Whereas conjugated GABARAP is similarly referred to as “GABARAP-PE”, the lipidated form of LC3 is termed “LC3-II”. Lipidation of ATG8 remains an important marker for evaluating levels of autophagy in tissue and cells (Mizushima et al, 1998, 2010; Ichimura et al, 2000). In mammalian cells, levels of LC3 lipidation in particular are used to estimate autophagic flux via immunoblotting, yet this should not detract from the biological importance of GABARAP proteins. ATG8 lipidation is a multistep process, driven by the E1-like enzymatic activity of homodimeric ATG7 (Tanida et al, 1999, 2001; Komatsu et al, 2001) (Fig 1B). First, the protease ATG4 exposes the C-terminus glycine residue of ATG8, generating form I (e.g. LC3-I) (Kirisako et al, 1999). This form is then activated by ATG7 via adenylation, before it is transferred to ATG3 where it is conjugated to PE to generate form II (e.g. LC3-II) (Tanida et al, 1999; Ichimura et al, 2000; Taherbhoy et al, 2011) which localises to both the inner (IAM) and outer autophagosomal membranes (OAM), and is subsequently degraded upon autolysosome formation (Kabeya et al, 2004). ATG7 is also involved in a second autophagy conjugation reaction that supports ATG8 lipidation. During this reaction, ATG12 is adenylated by ATG7 then transferred to ATG5 via E2-like enzyme ATG10, generating ATG5-ATG12 conjugates (Mizushima et al, 1998; Shintani et al, 1999; Tanida et al, 1999; Yamaguchi et al, 2012) which are restricted to the OAM and removed prior to sealing of the autophagosome (Koyama-Honda et al, 2013). Although LC3-II can be generated in vitro in the presence of ATG3, ATG7, LC3, ATP and liposomes containing PE, ATG5-ATG12 forms a complex with ATG16L that promotes LC3 lipidation in vivo (Mizushima et al, 1999, 2003; Kuma et al, 2002; Hanada et al, 2007; Lystad et al, 2019). Evidence currently suggests that WIPI2 localises to PI3P-rich regions of the phagophore membrane, recruiting the ATG5-ATG12-ATG16L1 complex that binds ATG3, which transfers activated ATG8 to membrane-bound PE. Oxidation of ATG3 and ATG7 facilitates autophagy inhibition (Frudd et al, 2018). Endogenous ATG7 deletion prevents ATG8 lipidation, so ATG7 knockout (KO) models are commonly used to study the biological significance of the autophagy conjugation systems. ATG8 has diverse roles in the autophagy pathway (Lee & Lee, 2016; Johansen & Lamark, 2020). First, mammalian LC3-II is important for autophagosome maturation, with levels of lipidated LC3 correlating with the extent of autophagosome formation (Kabeya et al, 2004) and autophagic structures generated in the absence of ATG8 homologues are smaller (Nguyen et al, 2016). It was recently demonstrated that attachment of ATG8 to the phagophore membrane stimulates membrane deformation, leading to expansion of this structure and underpinning efficient autophagosome formation (Maruyama et al, 2021). Blockade of mammalian ATG8 lipidation through ATG3 deletion caused delayed autophagosome maturation and a significant reduction in the success rate of autophagosome formation (Tsuboyama et al, 2016). In fact, a number of proteins involved in the early and late stages of autophagy have LIRs, emphasising the ability of ATG8 family proteins to coordinate multiple stages of the autophagic process (Martens & Fracchiolla, 2020) (Fig 1D). Beyond autophagic membrane expansion, ATG8 homologues facilitate transport of autophagosomes along microtubules via interactions with motor proteins via adaptor proteins and contribute to the binding of autophagosomes to lysosomes (Lorincz & Juhasz, 2020; Martens & Fracchiolla, 2020). LC3B can also be phosphorylated to regulate autophagosome transport (Nieto-Torres et al, 2021). Moreover, loss of ATG7 impairs inner autophagosomal membrane (IAM) degradation after autophagosome–lysosome fusion (Tsuboyama et al, 2016). Consequently, autophagy is severely impaired by ATG7 deletion as evidenced in yeast, mouse and humans (Tanida et al, 1999; Komatsu et al, 2005; Luhr et al, 2018). One of the most widely studied aspects of ATG8 lipidation is its requirement for selective autophagy which is currently defined by the recognition, sequestration and elimination of specific cytoplasmic cargo. This selectivity is achieved through the interaction of cargo with ATG8 via LIR motifs within specific receptors that act as “eat-me” signals for damaged or excess cellular components (Johansen & Lamark, 2020) (Fig 1C). Cargo types include organelles such as mitochondria (termed mitophagy), endoplasmic reticulum (ER-phagy or reticulophagy) and peroxisomes (pexophagy), proteins and protein aggregates (aggrephagy), and intracellular pathogens (xenophagy). Consequently, selective autophagy is an important homeostatic mechanism, preventing the accumulation of dysfunctional organelles, cytotoxic aggregates and providing innate immune support, as well as a developmental tool facilitating the removal of mitochondria from maturing reticulocytes, cardiomyocytes, kidney cells and ocular tissues, for example (Sandoval et al, 2008; Mortensen et al, 2010; McWilliams et al, 2016, 2019; Esteban-Martinez et al, 2017). LIR motifs can be regulated through masking and activating/inhibitory phosphorylation to prevent promiscuous cargo sequestration under conditions where the autophagic degradation of that substrate or organelle has not been stimulated (Chen et al, 2014, 2016; Lv et al, 2017; Wei et al, 2017). Autophagic sequestration of mitochondrial proteins can also be regulated by acetylation (Webster et al, 2013). Although the fundamental mechanisms driving cargo selection are shared between ATG8 homologues, there is evidence of homologue-specific autophagy adaptor proteins (Wirth et al, 2019). Autophagy-related functions Autophagy-related functions of ATG7 involve membrane trafficking events that are dependent on LC3 lipidation. Consequently, these processes require the activity of other core ATG proteins that drive lipidation of ATG8 homologues, including ATG3, ATG5 and ATG12—members of the autophagy conjugation system (Mizushima, 2020) (Fig 1). This includes the innate immune process LC3-associated phagocytosis (LAP) (Heckmann & Green, 2019; Inomata et al, 2020) (Fig 2). During LAP, extracellular pathogens, dead/dying cells and other extracellular substrates are recognised by cell surface receptors then endocytosed, generating an intracellular single-membrane structure called the phagosome. Then, LC3-II, generated in an ATG7-dependent manner, is recruited to the phagosome in a process dependent on NOX2-derived ROS (Sanjuan et al, 2007; Martinez et al, 2015). This structure, the LAPosome, is now decorated with LC3-II and able to fuse with lysosomes for elimination (Sanjuan et al, 2007). A related process, termed entosis, is also dependent on LC3 lipidation. During entosis, viable cells are engulfed by epithelial cells. This process is regulated by the cell being engulfed, after which this cell undergoes non-apoptotic cell death driven by autophagosomes and lysosomes of the host cell (a process termed “non-cell autonomous autophagy”). When autophagy in the host cell is inhibited, the engulfed cell largely undergoes apoptosis (Florey et al, 2011), whereas others have been observed to divide inside the host cell, or escape into culture (Overholtzer et al, 2007). Tumour cells can also undergo entosis (Overholtzer et al, 2007; Fais & Overholtzer, 2018). The activities of ATG7 can also be non-degradative. For example, LAP also facilitates Toll-like receptor 9 (TLR9) trafficking and converges with the classical autophagy pathway to regulate IFN-alpha production (Henault et al, 2012). ATG7-mediated LC3 lipidation is also required for the exocytic release of cathepsin K by osteoblasts (DeSelm et al, 2011), and LC3-positive lysozyme-containing granules are released by Paneth cells upon infection (Bel et al, 2017). Related to this, autophagosomes facilitate the unconventional secretion of proteins including IL1B and ferritin in response to lysosomal damage (Kimura et al, 2017), and loss of ATG7 causes accumulation of mucin granules in Goblet cells (Patel et al, 2013). Autophagosomes are also able to sequester TBC1D5, thus freeing the retromer complex to mediate the return shuttling of GLUT1 transporters to the plasma membrane from endosomes (Roy et al, 2017). As part of the autophagic machinery, ATG7 is also involved in regulating the switch between apoptosis and necrosis (Goodall et al, 2016). This important study demonstrated that the necrosome (a protein complex that leads to rapid plasma membrane rupture and inflammation) can assemble on autophagosomes at selective autophagy receptor p62 sites and that loss of p62 can switch cell death mechanisms towards apoptosis. Autophagy-independent functions ATG7 also participates in cellular functions that are independent of its E1-like enzymatic activity. Consequently, these functions do not require the other ATG machinery required for autophagy-associated signalling. A number of autophagy-independent functions of ATG7 have been described, with two involving modulation of p53 activity. Upon starvation, Atg7 has been reported to interact with p53 to inhibit the expression of pro-apoptotic genes Noxa, Puma and Bax. Accordingly, Atg7-null mouse embryonic fibroblasts demonstrated augmented DNA damage under basal conditions. The proliferation of Atg7-null cells proceeded at a far greater rate than control cells due to diminished p53-mediated p21 expression, which usually promotes cell cycle arrest (Lee et al, 2012). In another study, Atg7 was also shown to repress the pro-apoptotic properties of caspase 9 (Han et al, 2014). An isoform of ATG7 that lacks E1-like enzymatic activity has also been discovered, and this variant cannot lipidate ATG8 homologues (Ogmundsdottir et al, 2018). The biological function of this intriguing isoform is unknown, but it may negatively regulate autophagy by potentially disrupting the formation of functional ATG7 homodimers. Mouse models of Atg7 deficiency A prodigious body of evidence, largely attained through studying mouse genetic models, has demonstrated that faithful ATG7 function is essential for the development, maintenance and adaptation of mammalian tissues (Xiong, 2015). Embryonic Atg7 deletion in mice results in perinatal lethality, and the subsequent characterisation of conditional Atg7 deficiency in mice has illuminated the contribution of ATG7 to mammalian physiology (Table 1). It is notable that manipulation of other core Atg genes causes similar phenotypes to those observed in Atg7 KO models, supporting the role of autophagy in these discoveries. Here, we discuss these key genetic mouse models, exploring the phenotypic and cellular consequences of endogenous inhibition of mammalian ATG7. Table 1. Overview of Atg7-deficient mouse models. Knockout Phenotypes References Whole body (embryonic) Perinatal lethal Komatsu et al (2005) Whole body (adult) Neurodegeneration Susceptibility to infection Karsli-Uzunbas et al (2014) Central nervous system Neurodegeneration Ataxia Behavioural abnormalities Kim et al (2017), Komatsu et al (2006), Komatsu et al (2007b) Liver Liver enlargement Multiple adenomas Komatsu et al (2007a), Komatsu et al (2005) Skeletal muscle Loss of muscle mass and strength Impaired exercise adaptation Lo Verso et al (2014), Masiero et al (2009) Circulatory system Diabetic cardiomyopathy Susceptible to ischaemic injury Saito et al (2019), Tong et al (2019) Pancreas Premature death Hyperglycaemia Insulin deficiency Endotoxin-induced chronic pancreatitis Xia et al (2020), Zhou et al (2017) Adipose Loss of white adipose tissue mass Insulin sensitivity Singh et al (2009b), Zhang et al (2009b) Haematopoiesis Severe anaemia Inability to reconstitute irradiated mice Mortensen et al (2010), Mortensen et al (2011) Bone Reduced bone mass Short tibia and femur Li et al (2018) Intestine Susceptible to infection Inoue et al (2012b) Ear Early-onset hearing loss Zhou et al (2020) Eye Retinal degeneration Zhang et al (2017) Systemic or whole-body Atg7 deletion Similarly to the majority of other core Atg genes, systemic knockout of Atg7 in mice causes death within 24 h after birth (Komatsu et al, 2005). The neonatal lethal phenotype is recapitulated across other core Atg genes, including Atg5 (Kuma et al, 2004). It was then demonstrated that neural reconstitution of Atg5 activity in Atg5-null mice prevents neonatal death (mice die between 8 weeks and 8 months after birth), revealing that neural dysfunction is the primary cause of perinatal death in whole-body knockout animals. This is possibly due to a suckling defect (Yoshii et al, 2016), although Atg7- and Atg5-null mice died before wild-type mice, even under non-suckling conditions (Kuma et al, 2004; Komatsu et al, 2005). Conditional models such as tamoxifen-inducible whole-body Atg7 deletion in adult mice impaired glucose metabolism, causing death 2–3 months post-knockout due to neurodegeneration, and fasting these mice caused fatal hypoglycaemia (low blood glucose levels) and cachexia (muscle wasting) (Karsli-Uzunbas et al, 2014). Amino acid levels were also diminished in Atg7 KO mice (Komatsu et al, 2005). A combination of defective LAP and autophagy may underlie the susceptibility of inducible adult Atg7 KO mice to Streptococcus infection (Karsli-Uzunbas et al, 2014). Adult mice with concurrent Atg7 and p53 deletion have similar lifespan to p53 KO mice, and the double KO mice died from neurodegeneration without the tumour development that was observed in p53 KO mice (Yang et al, 2020). The double KO mice were more resistant to fasting, and liver and brain injury was decreased due to protection against apoptosis and DNA damage (Yang et al, 2020). Central nervous system Perhaps the most striking physiological effects of Atg7 deficiency manifest in the central nervous system, where conditional Atg7 KO caused neurodegeneration resulting in premature death (Komatsu et al, 2006). Mice also displayed an ataxic phenotype caused by selective vulnerability of cerebellar Purkinje neurons to Atg7 deficiency (Komatsu et al, 2006; Komatsu et al, 2007b) and behavioural abnormalities that are recapitulated in mice with myeloid-specific Atg7 deletion via impaired microglial synaptic refinement (Kim et al, 2017). Other regions in the brain affected by Atg7 deletion include the hypothalamus through impaired lipolysis and glucose homeostasis (Cou
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