The HOPS complex mediates autophagosome-lysosome fusion through interaction with syntaxin 17

Impairment of autophagosome-lysosome fusion in the buff mutant mice with the VPS33A(D251E) mutation

The HOPS (homotypic fusion and protein sorting) complex functions in endocytic and autophagic pathways in both lower eukaryotes and mammalian cells through its involvement in fusion events between endosomes and lysosomes or autophagosomes and lysosomes. However, the differential molecular mechanisms underlying these fusion processes are largely unknown. Buff (bf) is a mouse mutant that carries an Asp251-to-Glu point mutation (D251E) in the VPS33A protein, a tethering protein and a core subunit of the HOPS complex. Bf mice showed impaired spontaneous locomotor activity, motor learning, and autophagic activity. Although the gross anatomy of the brain was apparently normal, the number of Purkinje cells was significantly reduced. Furthermore, we found that fusion between autophagosomes and lysosomes was defective in bf cells without compromising the endocytic pathway. The direct association of mutant VPS33A(D251E) with the autophagic SNARE complex, STX17 (syntaxin 17)-VAMP8-SNAP29, was enhanced. In addition, the VPS33A(D251E) mutation enhanced interactions with other HOPS subunits, namely VPS41, VPS39, VPS18, and VPS11, except for VPS16. Reduction of the interactions between VPS33A(Y440D) and several other HOPS subunits led to decreased association with STX17. These results suggest that the VPS33A(D251E) mutation plays dual roles by increasing the HOPS complex assembly and its association with the autophagic SNARE complex, which selectively affects the autophagosome-lysosome fusion that impairs basal autophagic activity and induces Purkinje cell loss.

Cholesterol and ORP1L-mediated ER contact sites control autophagosome transport and fusion with the endocytic pathway

Autophagy is the main homeostatic pathway guiding cytosolic materials for degradation by the lysosome. Maturation of autophagosomes requires their transport towards the perinuclear region of the cell, with key factors underlying both processes still poorly understood. Here we show that transport and positioning of late autophagosomes depends on cholesterol by way of the cholesterol-sensing Rab7 effector ORP1L. ORP1L localizes to late autophagosomes and—under low-cholesterol conditions—contacts the ER protein VAP-A, forming ER-autophagosome contact sites, which prevent minus-end transport by the Rab7–RILP–dynein complex. ORP1L-mediated contact sites also inhibit localization of PLEKHM1 to Rab7. PLEKHM1, together with RILP, then recruits the homotypic fusion and vacuole protein-sorting (HOPS) complex for fusion of autophagosomes with late endosomes and lysosomes. Thus, ORP1L, via its liganding by lipids and the formation of contacts between autophagic vacuoles and the ER, governs the last steps in autophagy that lead to the lysosomal degradation of cytosolic material.

Autophagy requires transport of autophagosomes to the perinuclear region. Here, the authors show that ORP1L localizes to autophagosomes and mediates formation of ER contact sites that prevent autophagosome transport and fusion with endocytic vesicles when cholesterol levels are low.

Interactions between Autophagy and Inhibitory Cytokines

Autophagy is a degradative pathway that plays an essential role in maintaining cellular homeostasis. Most early studies of autophagy focused on its involvement in age-associated degeneration and nutrient deprivation. However, the immunological functions of autophagy have become more widely studied in recent years. Autophagy has been shown to be an intrinsic cellular defense mechanism in the innate and adaptive immune responses. Cytokines belong to a broad and loose category of proteins and are crucial for innate and adaptive immunity. Inhibitory cytokines have evolved to permit tolerance to self while also contributing to the eradication of invading pathogens. Interactions between inhibitory cytokines and autophagy have recently been reported, revealing a novel mechanism by which autophagy controls the immune response. In this review, we discuss interactions between autophagy and the regulatory cytokines IL-10, transforming growth factor-β, and IL-27. We also mention possible interactions between two newly discovered cytokines, IL-35 and IL-37, and autophagy.

MiniCORVET is a Vps8-containing early endosomal tether in Drosophila

Yeast studies identified two heterohexameric tethering complexes, which consist of 4 shared (Vps11, Vps16, Vps18 and Vps33) and 2 specific subunits: Vps3 and Vps8 (CORVET) versus Vps39 and Vps41 (HOPS). CORVET is an early and HOPS is a late endosomal tether. The function of HOPS is well known in animal cells, while CORVET is poorly characterized. Here we show that Drosophila Vps8 is highly expressed in hemocytes and nephrocytes, and localizes to early endosomes despite the lack of a clear Vps3 homolog. We find that Vps8 forms a complex and acts together with Vps16A, Dor/Vps18 and Car/Vps33A, and loss of any of these proteins leads to fragmentation of endosomes. Surprisingly, Vps11 deletion causes enlargement of endosomes, similar to loss of the HOPS-specific subunits Vps39 and Lt/Vps41. We thus identify a 4 subunit-containing miniCORVET complex as an unconventional early endosomal tether in Drosophila.

DOI:http://dx.doi.org/10.7554/eLife.14226.001

A Genetic Screen Identifies a Critical Role for the WDR81-WDR91 Complex in the Trafficking and Degradation of Tetherin

Tetherin (BST2/CD317) is a viral restriction factor that anchors enveloped viruses to host cells and limits viral spread. The HIV‐1 Vpu accessory protein counteracts tetherin by decreasing its cell surface expression and targeting it for ubiquitin‐dependent endolysosomal degradation. Although the Vpu‐mediated downregulation of tetherin has been extensively studied, the molecular details are not completely elucidated. We therefore used a forward genetic screen in human haploid KBM7 cells to identify novel genes required for tetherin trafficking. Our screen identified WDR81 as a novel gene required for tetherin trafficking and degradation in both the presence and absence of Vpu. WDR81 is a BEACH‐domain containing protein that is also required for the degradation of EGF‐stimulated epidermal growth factor receptor (EGFR) and functions in a complex with the WDR91 protein. In the absence of WDR81 the endolysosomal compartment appears swollen, with enlarged early and late endosomes and reduced delivery of endocytosed dextran to cathepsin‐active lysosomes. Our data suggest a role for the WDR81‐WDR91 complex in the fusion of endolysosomal compartments and the absence of WDR81 leads to impaired receptor trafficking and degradation.

Membrane Tethering Complexes in the Endosomal System

Vesicles that are generated by endocytic events at the plasma membrane are destined to early endosomes. A prerequisite for proper fusion is the tethering of two membrane entities. Tethering of vesicles to early endosomes is mediated by the class C core vacuole/endosome tethering (CORVET) complex, while fusion of late endosomes with lysosomes depends on the homotypic fusion and vacuole protein sorting (HOPS) complex. Recycling through the trans-Golgi network (TGN) and to the plasma membrane is facilitated by the Golgi associated retrograde protein (GARP) and endosome-associated recycling protein (EARP) complexes, respectively. However, there are other tethering functions in the endosomal system as there are multiple pathways through which proteins can be delivered from endosomes to either the TGN or the plasma membrane. Furthermore, proteins that may be part of novel tethering complexes have been recently identified. Thus, it is likely that more tethering factors exist. In this review, I will provide an overview of different tethering complexes of the endosomal system and discuss how they may provide specificity in membrane traffic.

A Founder Mutation in VPS11 Causes an Autosomal Recessive Leukoencephalopathy Linked to Autophagic Defects

Genetic leukoencephalopathies (gLEs) are a group of heterogeneous disorders with white matter abnormalities affecting the central nervous system (CNS). The causative mutation in ~50% of gLEs is unknown. Using whole exome sequencing (WES), we identified homozygosity for a missense variant, VPS11: c.2536T>G (p.C846G), as the genetic cause of a leukoencephalopathy syndrome in five individuals from three unrelated Ashkenazi Jewish (AJ) families. All five patients exhibited highly concordant disease progression characterized by infantile onset leukoencephalopathy with brain white matter abnormalities, severe motor impairment, cortical blindness, intellectual disability, and seizures. The carrier frequency of the VPS11: c.2536T>G variant is 1:250 in the AJ population (n = 2,026). VPS11 protein is a core component of HOPS (homotypic fusion and protein sorting) and CORVET (class C core vacuole/endosome tethering) protein complexes involved in membrane trafficking and fusion of the lysosomes and endosomes. The cysteine 846 resides in an evolutionarily conserved cysteine-rich RING-H2 domain in carboxyl terminal regions of VPS11 proteins. Our data shows that the C846G mutation causes aberrant ubiquitination and accelerated turnover of VPS11 protein as well as compromised VPS11-VPS18 complex assembly, suggesting a loss of function in the mutant protein. Reduced VPS11 expression leads to an impaired autophagic activity in human cells. Importantly, zebrafish harboring a vps11 mutation with truncated RING-H2 domain demonstrated a significant reduction in CNS myelination following extensive neuronal death in the hindbrain and midbrain. Thus, our study reveals a defect in VPS11 as the underlying etiology for an autosomal recessive leukoencephalopathy disorder associated with a dysfunctional autophagy-lysosome trafficking pathway.

Genetic leukoencephalopathies (gLEs) are a group of heterogeneous disorders with white matter abnormalities in the central nervous system (CNS). Patients affected with gLEs have brain white matter defects that can be seen on MRI and exhibit variable neurologic phenotypes including motor impairment, hypotonia, pyramidal dysfunction, dystonia and/or dyskinesias, ataxia, seizures, cortical blindness, optic atrophy, and impaired cognitive development. The exact etiology of half of gLEs is unknown. We studied three unrelated families affected with an undiagnosed gLE and discovered a homozygous germline mutation c.2536T>G in VPS11, a gene involved in membrane trafficking and fusion of lysosomes and endosomes, as a novel cause of a new gLE syndrome. The mutation in VPS11 results in protein instability and impaired protein complex assembly. In addition, we show that VPS11 is required for proper autophagic activities in human cells. Importantly, we characterized a zebrafish line carrying a vps11 mutation and confirmed its essential role in brain white matter development and neuron survival.

ESCRT-0 dysfunction compromises autophagic degradation of protein aggregates and facilitates ER stress-mediated neurodegeneration via apoptotic and necroptotic pathways

Endosomal sorting required for transport (ESCRT) complexes orchestrate endo-lysosomal sorting of ubiquitinated proteins, multivesicular body formation and autophagic degradation. Defects in the ESCRT pathway have been implicated in many neurodegenerative diseases, but the underlying molecular mechanisms that link them to neurodegeneration remain unknown. In this study, we showed that forebrain-specific ablation of ESCRT-0/Hrs induced marked hippocampal neuronal cell loss accompanied by the accumulation of ubiquitinated proteins, including α-synuclein, TDP-43 and huntingtin as well as the autophagic substrate SQSTM1/p62. Consistent with this, silencing of Hrs in cultured cells not only led to α-synuclein and TDP-43 accumulation in addition to impaired autophagic flux but also suppressed cell viability through the induction of ER stress followed by the activation of JNK and RIPK1, a key regulator of necroptosis. Moreover, necrostatin-1, a specific inhibitor of RIPK1, and pan-caspase inhibitors partially reduced the neurotoxicity in the Hrs-silenced cells. Altogether, these findings suggest that the disruption of ESCRT-0/Hrs in the nervous system compromises autophagic/lysosomal degradation of neurodegenerative disease-related proteins, which thereby triggers ER stress-mediated apoptotic and necroptotic cell death.

Autophagy and Mammalian Viruses: Roles in Immune Response, Viral Replication, and Beyond

Autophagy is an important cellular catabolic process conserved from yeast to man. Double-membrane vesicles deliver their cargo to the lysosome for degradation. Hence, autophagy is one of the key mechanisms mammalian cells deploy to rid themselves of intracellular pathogens including viruses. However, autophagy serves many more functions during viral infection. First, it regulates the immune response through selective degradation of immune components, thus preventing possibly harmful overactivation and inflammation. Additionally, it delivers virus-derived antigens to antigen-loading compartments for presentation to T lymphocytes. Second, it might take an active part in the viral life cycle by, eg, facilitating its release from cells. Lastly, in the constant arms race between host and virus, autophagy is often hijacked by viruses and manipulated to their own advantage. In this review, we will highlight key steps during viral infection in which autophagy plays a role. We have selected some exemplary viruses and will describe the molecular mechanisms behind their intricate relationship with the autophagic machinery, a result of host-pathogen coevolution.

Global Analysis of Cellular Protein Flux Quantifies the Selectivity of Basal Autophagy

In eukaryotic cells, macroautophagy is a catabolic pathway implicated in the degradation of long-lived proteins and damaged organelles. Although it has been demonstrated that macroautophagy can selectively degrade specific targets, its contribution to the basal turnover of cellular proteins has not been quantified on proteome-wide scales. In this study, we created autophagy-deficient primary human fibroblasts and quantified the resulting changes in basal degradative flux by dynamic proteomics. Our results provide a global comparison of protein half-lives between wild-type and autophagy-deficient cells. The data indicate that in quiescent fibroblasts, macroautophagy contributes to the basal turnover of a substantial fraction of the proteome at varying levels. As contrasting examples, we demonstrate that the proteasome and CCT/TRiC chaperonin are robust substrates of basal autophagy, whereas the ribosome is largely protected under basal conditions. This selectivity may establish a proteostatic feedback mechanism that stabilizes the proteasome and CCT/TRiC when autophagy is inhibited.

Mechanistically Dissecting Autophagy: Insights from In Vitro Reconstitution

Autophagy is a fundamental cellular mechanism responsible for bulk turnover of cytoplasmic components. It is broadly related to many cellular activities, physiological processes, and pathological conditions. Autophagy entails a spatiotemporal interaction between cytosolic factors and membranes that are remodeled to encapsulate autophagic cargo within an autophagosome. Although majority of the factors [autophagy-related gene (Atg) proteins] involved in autophagy have been identified by genetic studies, the mechanism accounting for how these factors act upon the membrane to remodel it and efficiently recruit cargo for degradation is unclear. In vitro reconstitution of several different aspects of autophagy has provided important insights into the understanding of the mechanistic details underlying autophagic membrane remodeling and cargo recruitment. Here, we highlight these efforts toward studying autophagy through in vitro approaches.

Membrane dynamics in autophagosome biogenesis

Bilayered phospholipid membranes are vital to the organization of the living cell. Based on fundamental principles of polarity, membranes create borders allowing defined spaces to be encapsulated. This compartmentalization is a prerequisite for the complex functional design of the eukaryotic cell, yielding localities that can differ in composition and operation. During macroautophagy, cytoplasmic components become enclosed by a growing double bilayered membrane, which upon closure creates a separate compartment, the autophagosome. The autophagosome is then primed for fusion with endosomal and lysosomal compartments, leading to degradation of the captured material. A large number of proteins have been found to be essential for autophagy, but little is known about the specific lipids that constitute the autophagic membranes and the membrane modeling events that are responsible for regulation of autophagosome shape and size. In this Commentary, we review the recent progress in our understanding of the membrane shaping and remodeling events that are required at different steps of the autophagy pathway. This article is part of a Focus on Autophagosome biogenesis. For further reading, please see related articles: 'ERES: sites for autophagosome biogenesis and maturation?' by Jana Sanchez-Wandelmer et al. (J. Cell Sci. 128, 185-192) and 'WIPI proteins: essential PtdIns3P effectors at the nascent autophagosome' by Tassula Proikas-Cezanne et al. (J. Cell Sci. 128, 207-217).

Stem-cell-specific endocytic degradation defects lead to intestinal dysplasia in Drosophila

UV radiation resistance-associated gene (UVRAG) is a tumor suppressor involved in autophagy, endocytosis and DNA damage repair, but how its loss contributes to colorectal cancer is poorly understood. Here, we show that UVRAG deficiency in Drosophila intestinal stem cells leads to uncontrolled proliferation and impaired differentiation without preventing autophagy. As a result, affected animals suffer from gut dysfunction and short lifespan. Dysplasia upon loss of UVRAG is characterized by the accumulation of endocytosed ligands and sustained activation of STAT and JNK signaling, and attenuation of these pathways suppresses stem cell hyperproliferation. Importantly, the inhibition of early (dynamin-dependent) or late (Rab7-dependent) steps of endocytosis in intestinal stem cells also induces hyperproliferation and dysplasia. Our data raise the possibility that endocytic, but not autophagic, defects contribute to UVRAG-deficient colorectal cancer development in humans.

Drosophila Collection: Intestinal-stem-cell-specific loss of the Drosophila ortholog of the tumor suppressor UVRAG, which is implicated in colorectal cancer, leads to endocytic defects and dysplasia.

Uvrag targeting by Mir125a and Mir351 modulates autophagy associated with Ewsr1 deficiency

The EWSR1 (EWS RNA-binding protein 1/Ewing Sarcoma Break Point Region 1) gene encodes a RNA/DNA binding protein that is ubiquitously expressed and involved in various cellular processes. EWSR1 deficiency leads to impairment of development and accelerated senescence but the mechanism is not known. Herein, we found that EWSR1 modulates the Uvrag (UV radiation resistance associated) gene at the post-transcription level. Interestingly, EWSR1 deficiency led to the activation of the DROSHA-mediated microprocessor complex and increased the level of Mir125a and Mir351, which directly target Uvrag. Moreover, the Mir125a- and Mir351-mediated reduction of Uvrag was associated with the inhibition of autophagy that was confirmed in ewsr1 knockout (KO) MEFs and ewsr1 KO mice. Taken together, our data indicate that EWSR1 is involved in the post-transcriptional regulation of Uvrag via a miRNA-dependent pathway, resulting in the deregulation of autophagy inhibition. The mechanism of Uvrag and autophagy regulation by EWSR1 provides new insights into the role of EWSR1 deficiency-related cellular dysfunction.

Autophagy in the test tube: In vitro reconstitution of aspects of autophagosome biogenesis

Autophagy is a versatile recycling pathway that delivers cytoplasmic contents to lysosomal compartments for degradation. It involves the formation of a cup-shaped membrane that expands to capture cargo. After the cargo has been entirely enclosed, the membrane is sealed to generate a double-membrane-enclosed compartment, termed the autophagosome. Depending on the physiological state of the cell, the cargo is selected either specifically or non-specifically. The process involves a highly conserved set of autophagy-related proteins. Reconstitution of their action on model membranes in vitro has contributed tremendously to our understanding of autophagosome biogenesis. This review will focus on various in vitro techniques that have been employed to decipher the function of the autophagic core machinery.

Mitophagy programs: mechanisms and physiological implications of mitochondrial targeting by autophagy

Mitochondria are an essential source of ATP for cellular function, but when damaged, mitochondria generate a plethora of stress signals, which lead to cellular dysfunction and eventually programmed cell death. Thus, a major component of maintaining cellular homeostasis is the recognition and removal of dysfunctional mitochondria through autophagy-mediated degradation, i.e., mitophagy. Mitophagy further constitutes a developmental program, and undergoes a high degree of crosstalk with apoptosis. Reduced mitochondrial quality control is linked to disease pathogenesis, suggesting the importance of process elucidation as a clinical target. Recent work has revealed multiple mitophagy programs that operate independently or undergo crosstalk, and require modulated autophagy receptor activities at outer membranes of mitochondria. Here, we review these mitophagy programs, focusing on pathway mechanisms which recognize and target mitochondria for sequestration by autophagosomes, as well as mechanisms controlling pathway activities. Furthermore, we provide an introduction to the currently available methods for detecting mitophagy.

The Autophagic Machinery in Enterovirus Infection

The Enterovirus genus of the Picornaviridae family comprises many important human pathogens, including polioviruses, rhinovirus, enterovirus A71, and enterovirus D68. They cause a wide variety of diseases, ranging from mild to severe life-threatening diseases. Currently, no effective vaccine is available against enteroviruses except for poliovirus. Enteroviruses subvert the autophagic machinery to benefit their assembly, maturation, and exit from host. Some enteroviruses spread between cells via a process described as autophagosome-mediated exit without lysis (AWOL). The early and late phases of autophagy are regulated through various lipids and their metabolizing enzymes. Some of these lipids and enzymes are specifically regulated by enteroviruses. In the present review, we summarize the current understanding of the regulation of autophagic machinery by enteroviruses, and provide updates on recent developments in this field.

TECPR2 Cooperates with LC3C to Regulate COPII-Dependent ER Export

Hereditary spastic paraplegias (HSPs) are a diverse group of neurodegenerative diseases that are characterized by axonopathy of the corticospinal motor neurons. A mutation in the gene encoding for Tectonin β-propeller containing protein 2 (TECPR2) causes HSP that is complicated by neurological symptoms. While TECPR2 is a human ATG8 binding protein and positive regulator of autophagy, the exact function of TECPR2 is unknown. Here, we show that TECPR2 associates with several trafficking components, among them the COPII coat protein SEC24D. TECPR2 is required for stabilization of SEC24D protein levels, maintenance of functional ER exit sites (ERES), and efficient ER export in a manner dependent on binding to lipidated LC3C. TECPR2-deficient HSP patient cells display alterations in SEC24D abundance and ER export efficiency. Additionally, TECPR2 and LC3C are required for autophagosome formation, possibly through maintaining functional ERES. Collectively, these results reveal that TECPR2 functions as molecular scaffold linking early secretion pathway and autophagy.

TDP-43 loss of function increases TFEB activity and blocks autophagosome-lysosome fusion

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that is characterized by selective loss of motor neurons in brain and spinal cord. TAR DNA-binding protein 43 (TDP-43) was identified as a major component of disease pathogenesis in ALS, frontotemporal lobar degeneration (FTLD), and other neurodegenerative disease. Despite the fact that TDP-43 is a multi-functional protein involved in RNA processing and a large number of TDP-43 RNA targets have been discovered, the initial toxic effect and the pathogenic mechanism underlying TDP-43-linked neurodegeneration remain elusive. In this study, we found that loss of TDP-43 strongly induced a nuclear translocation of TFEB, the master regulator of lysosomal biogenesis and autophagy, through targeting the mTORC1 key component raptor. This regulation in turn enhanced global gene expressions in the autophagy-lysosome pathway (ALP) and increased autophagosomal and lysosomal biogenesis. However, loss of TDP-43 also impaired the fusion of autophagosomes with lysosomes through dynactin 1 downregulation, leading to accumulation of immature autophagic vesicles and overwhelmed ALP function. Importantly, inhibition of mTORC1 signaling by rapamycin treatment aggravated the neurodegenerative phenotype in a TDP-43-depleted Drosophila model, whereas activation of mTORC1 signaling by PA treatment ameliorated the neurodegenerative phenotype. Taken together, our data indicate that impaired mTORC1 signaling and influenced ALP may contribute to TDP-43-mediated neurodegeneration.

Autophagy in the light of sphingolipid metabolism

Maintenance of cellular homeostasis requires tight and coordinated control of numerous metabolic pathways, which are governed by interconnected networks of signaling pathways and energy-sensing regulators. Autophagy, a lysosomal degradation pathway by which the cell self-digests its own components, has over the past decade been recognized as an essential part of metabolism. Autophagy not only rids the cell of excessive or damaged organelles, misfolded proteins, and invading microorganisms, it also provides nutrients to maintain crucial cellular functions. Besides serving as essential structural moieties of biomembranes, lipids including sphingolipids are increasingly being recognized as central regulators of a number of important cellular processes, including autophagy. In the present review we describe how sphingolipids, with special emphasis on ceramides and sphingosine-1-phosphate, can act as physiological regulators of autophagy in relation to cellular and organismal growth, survival, and aging.

The Function of Autophagy in Neurodegenerative Diseases

Macroautophagy, hereafter referred to as autophagy, is a bulk degradation process performed by lysosomes in which aggregated and altered proteins as well as dysfunctional organelles are decomposed. Autophagy is a basic cellular process that maintains homeostasis and is crucial for postmitotic neurons. Thus, impaired autophagic processes in neurons lead to improper homeostasis and neurodegeneration. Recent studies have suggested that impairments of the autophagic process are associated with several neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and static encephalopathy of childhood with neurodegeneration in adulthood. In this review, we focus on the recent findings regarding the autophagic process and the involvement of autophagy in neurodegenerative diseases.

Characterization of the Mammalian CORVET and HOPS Complexes and Their Modular Restructuring for Endosome Specificity

Trafficking of cargo through the endosomal system depends on endosomal fusion events mediated by SNARE proteins, Rab-GTPases, and multisubunit tethering complexes. The CORVET and HOPS tethering complexes, respectively, regulate early and late endosomal tethering and have been characterized in detail in yeast where their sequential membrane targeting and assembly is well understood. Mammalian CORVET and HOPS subunits significantly differ from their yeast homologues, and novel proteins with high homology to CORVET/HOPS subunits have evolved. However, an analysis of the molecular interactions between these subunits in mammals is lacking. Here, we provide a detailed analysis of interactions within the mammalian CORVET and HOPS as well as an additional endosomal-targeting complex (VIPAS39-VPS33B) that does not exist in yeast. We show that core interactions within CORVET and HOPS are largely conserved but that the membrane-targeting module in HOPS has significantly changed to accommodate binding to mammalian-specific RAB7 interacting lysosomal protein (RILP). Arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome-associated mutations in VPS33B selectively disrupt recruitment to late endosomes by RILP or binding to its partner VIPAS39. Within the shared core of CORVET/HOPS, we find that VPS11 acts as a molecular switch that binds either CORVET-specific TGFBRAP1 or HOPS-specific VPS39/RILP thereby allowing selective targeting of these tethering complexes to early or late endosomes to time fusion events in the endo/lysosomal pathway.

Arf-like GTPase Arl8: Moving from the periphery to the center of lysosomal biology

Lysosomes are dynamic organelles that not only mediate degradation of cellular substrates but also play critical roles in processes such as cholesterol homeostasis, plasma membrane repair, antigen presentation, and cell migration. The small GTPase Arl8, a member of Arf-like (Arl) family of proteins, has recently emerged as a crucial regulator of lysosome positioning and membrane trafficking toward lysosomes. Through interaction with its effector SKIP, the human Arl8 paralog (Arl8b) mediates kinesin-1 dependent motility of lysosomes on microtubule tracks toward the cell periphery. Arl8b-mediated kinesin-driven motility is also implicated in regulating lytic granule polarization in NK cells, lysosome tubulation in macrophages, cell spreading, and migration. Moreover, Arl8b regulates membrane traffic toward lysosomes by recruiting subunits of the HOPS complex, a multi-subunit tethering complex that mediates endo-lysosome fusion. Here we provide a brief review on this recently characterized lysosomal GTPase and summarize the studies focusing on its known functions in regulating lysosomal motility and delivery of endocytic cargo to the lysosomes. We also explore the role of human Arl8b and its orthologs upon infection by intracellular pathogens.

mTOR activates the VPS34-UVRAG complex to regulate autolysosomal tubulation and cell survival

Lysosomes are essential organelles that function to degrade and recycle unwanted, damaged and toxic biological components. Lysosomes also act as signalling platforms in activating the nutrient-sensing kinase mTOR. mTOR regulates cellular growth, but it also helps to maintain lysosome identity by initiating lysosomal tubulation through a process termed autophagosome-lysosome reformation (ALR). Here we identify a lysosomal pool of phosphatidylinositol 3-phosphate that, when depleted by specific inhibition of the class III phosphoinositide 3-kinase VPS34, results in prolonged lysosomal tubulation. This tubulation requires mTOR activity, and we identified two direct mTOR phosphorylation sites on UVRAG (S550 and S571) that activate VPS34. Loss of these phosphorylation sites reduced VPS34 lipid kinase activity and resulted in an increase in number and length of lysosomal tubules. In cells in which phosphorylation at these UVRAG sites is disrupted, the result of impaired lysosomal tubulation alongside ALR activation is massive cell death. Our data imply that ALR is critical for cell survival under nutrient stress and that VPS34 is an essential regulatory element in this process.

Role of Autophagy in the Maintenance of Intestinal Homeostasis

Genome-wide association studies of inflammatory bowel disease have identified several risk loci in genes that regulate autophagy, and studies have provided insight into the functional effects of these polymorphisms. We review the mechanisms by which autophagy contributes to intestinal homeostasis, focusing on its cell type-specific roles in regulating gut ecology, restricting pathogenic bacteria, and controlling inflammation. Based on this information, we are beginning to understand how alterations in autophagy can contribute to intestinal inflammation.

Contribution of autophagy to antiviral immunity

Although identified in the 1960's, interest in autophagy has significantly increased in the past decade with notable research efforts oriented at understanding as to how this multi-protein complex operates and is regulated. Autophagy is commonly defined as a "self-eating" process evolved by eukaryotic cells to recycle senescent organelles and expired proteins, which is significantly increased during cellular stress responses. In addition, autophagy can also play important roles during human diseases, such as cancer, neurodegenerative and autoimmune disorders. Furthermore, novel findings suggest that autophagy contributes to the host defense against microbial infections. In this article, we review the role of macroautophagy in antiviral immune responses and discuss molecular mechanisms evolved by viral pathogens to evade this process. A role for autophagy as an effector mechanism used both, by innate and adaptive immunity is also discussed.

Recruitment of VPS33A to HOPS by VPS16 Is Required for Lysosome Fusion with Endosomes and Autophagosomes

The mammalian homotypic fusion and vacuole protein sorting (HOPS) complex is comprised of six subunits: VPS11, VPS16, VPS18, VPS39, VPS41 and the Sec1/Munc18 (SM) family member VPS33A. Human HOPS has been predicted to be a tethering complex required for fusion of intracellular compartments with lysosomes, but it remains unclear whether all HOPS subunits are required. We showed that the whole HOPS complex is required for fusion of endosomes with lysosomes by monitoring the delivery of endocytosed fluorescent dextran to lysosomes in cells depleted of individual HOPS proteins. We used the crystal structure of the VPS16/VPS33A complex to design VPS16 and VPS33A mutants that no longer bind each other and showed that, unlike the wild-type proteins, these mutants no longer rescue lysosome fusion with endosomes or autophagosomes in cells depleted of the endogenous proteins. There was no effect of depleting either VIPAR or VPS33B, paralogs of VPS16 and VPS33A, on fusion of lysosomes with either endosomes or autophagosomes and immunoprecipitation showed that they form a complex distinct from HOPS. Our data demonstrate the necessity of recruiting the SM protein VPS33A to HOPS via its interaction with VPS16 and that HOPS proteins, but not VIPAR or VPS33B, are essential for fusion of endosomes or autophagosomes with lysosomes.

iFly: The eye of the fruit fly as a model to study autophagy and related trafficking pathways

Autophagy is a process by which eukaryotic cells degrade and recycle their intracellular components within lysosomes. Autophagy is induced by starvation to ensure survival of individual cells, and it has evolved to fulfill numerous additional roles in animals. Autophagy not only provides nutrient supply through breakdown products during starvation, but it is also required for the elimination of damaged or surplus organelles, toxic proteins, aggregates, and pathogens, and is essential for normal organelle turnover. Because of these roles, defects in autophagy have pathological consequences. Here we summarize the current knowledge of autophagy and related trafficking pathways in a convenient model: the compound eye of the fruit fly Drosophila melanogaster. In our review, we present a general introduction of the development and structure of the compound eye. This is followed by a discussion of various neurodegeneration models including retinopathies, with special emphasis on the protective role of autophagy against these diseases.

The small GTPase Rab7 as a central regulator of hepatocellular lipophagy

Autophagy is a central mechanism by which hepatocytes catabolize lipid droplets (LDs). Currently, the regulatory mechanisms that control this important process are poorly defined. The small guanosine triphosphatase (GTPase) Rab7 has been implicated in the late endocytic pathway and is known to associate with LDs, although its role in LD breakdown has not been tested. In this study, we demonstrate that Rab7 is indispensable for LD breakdown ("lipophagy") in hepatocytes subjected to nutrient deprivation. Importantly, Rab7 is dramatically activated in cells placed under nutrient stress; this activation is required for the trafficking of both multivesicular bodies and lysosomes to the LD surface during lipophagy, resulting in the formation of a lipophagic "synapse." Depletion of Rab7 leads to gross morphological changes of multivesicular bodies, lysosomes, and autophagosomes, consequently leading to attenuation of hepatocellular lipophagy.

CONCLUSION

These findings provide additional support for the role of autophagy in hepatocellular LD catabolism while implicating the small GTPase Rab7 as a key regulatory component of this essential process.

The small GTPase Arl8b regulates assembly of the mammalian HOPS complex on lysosomes

The homotypic fusion and protein sorting (HOPS) complex is a multi-subunit complex conserved from yeast to mammals that regulates late endosome and lysosome fusion. However, little is known about how the HOPS complex is recruited to lysosomes in mammalian cells. Here, we report that the small GTPase Arl8b, but not Rab7 (also known as RAB7A), is essential for membrane localization of the human (h)Vps41 subunit of the HOPS complex. Assembly of the core HOPS subunits to Arl8b- and hVps41-positive lysosomes is guided by their subunit-subunit interactions. RNA interference (RNAi)-mediated depletion of hVps41 resulted in the impaired degradation of EGFR that was rescued upon expression of wild-type but not an Arl8b-binding-defective mutant of hVps41, suggesting that Arl8b-dependent lysosomal localization of hVps41 is required for its endocytic function. Furthermore, we have also identified that the Arl8b effector SKIP (also known as PLEKHM2) interacts with and recruits HOPS subunits to Arl8b and kinesin-positive peripheral lysosomes. Accordingly, RNAi-mediated depletion of SKIP impaired lysosomal trafficking and degradation of EGFR. These findings reveal that Arl8b regulates the association of the human HOPS complex with lysosomal membranes, which is crucial for the function of this tethering complex in endocytic degradation.

Eaten alive: novel insights into autophagy from multicellular model systems

Autophagy delivers cytoplasmic material to lysosomes for degradation. First identified in yeast, the core genes that control this process are conserved in higher organisms. Studies of mammalian cell cultures have expanded our understanding of the core autophagy pathway, but cannot reveal the unique animal-specific mechanisms for the regulation and function of autophagy. Multicellular organisms have different types of cells that possess distinct composition, morphology, and organization of intracellular organelles. In addition, the autophagic machinery integrates signals from other cells and environmental conditions to maintain cell, tissue and organism homeostasis. Here, we highlight how studies of autophagy in flies and worms have identified novel core autophagy genes and mechanisms, and provided insight into the context-specific regulation and function of autophagy.

The endosomal pathway in Parkinson's disease

Parkinson's disease is primarily a movement disorder with predilection for the nigral dopaminergic neurons and is often associated with widespread neurodegeneration and diffuse Lewy body deposition. Recent advances in molecular genetics and studies in model organisms have transformed our understanding of Parkinson's pathogenesis and suggested unifying biochemical pathways despite the clinical heterogeneity of the disease. In this review, we summarized the evidence that a number of Parkinson's associated genetic mutations or polymorphisms (LRRK2, VPS35, GBA, ATP13A2, ATP6AP2, DNAJC13/RME-8, RAB7L1, GAK) disrupt protein trafficking and degradation via the endosomal pathway and discussed how such defects could arise from or contribute to the accumulation and misfolding of α-synuclein in Lewy bodies. We propose that an age-related pathological depletion of functional endolysosomes due to neuromelanin deposition in dopaminergic neurons may increase their susceptibility to stochastic molecular defects in this pathway and we discuss how enzymes that regulate ubiquitin signaling, as exemplified by the ubiquitin ligase Nedd4, could provide the missing link between genetic and acquired defects in endosomal trafficking. This article is part of a Special Issue entitled 'Neuronal Protein'.

Regulators of autophagosome formation in Drosophila muscles

Given the diversity of autophagy targets and regulation, it is important to characterize autophagy in various cell types and conditions. We used a primary myocyte cell culture system to assay the role of putative autophagy regulators in the specific context of skeletal muscle. By treating the cultures with rapamycin (Rap) and chloroquine (CQ) we induced an autophagic response, fully suppressible by knockdown of core ATG genes. We screened D. melanogaster orthologs of a previously reported mammalian autophagy protein-protein interaction network, identifying several proteins required for autophagosome formation in muscle cells, including orthologs of the Rab regulators RabGap1 and Rab3Gap1. The screen also highlighted the critical roles of the proteasome and glycogen metabolism in regulating autophagy. Specifically, sustained proteasome inhibition inhibited autophagosome formation both in primary culture and larval skeletal muscle, even though autophagy normally acts to suppress ubiquitin aggregate formation in these tissues. In addition, analyses of glycogen metabolic genes in both primary cultured and larval muscles indicated that glycogen storage enhances the autophagic response to starvation, an important insight given the link between glycogen storage disorders, autophagy, and muscle function.

Since the identification of the core autophagy genes in yeast, tissue culture cell lines have been the primary tool to evaluate the role and regulation of autophagy in higher organisms. However, since autophagy is a tissue-specific, context dependent process, stable cell lines can only give a limited view of the autophagic process. Here, we focus on the role of putative autophagy regulators in the specific context of the skeletal muscle, which has one of the most robust autophagy responses in mammals. We describe a fruitfly model of autophagy for skeletal muscles that integrates rapid genetic screening in primary cultured cells with robust in vivo validation in the larval muscle. We screened a set of genes previously linked to the autophagy pathway in humans, and identified both positive and negative regulators of autophagy. Our observation that genes involved in sugar metabolism impact muscle autophagy has important implications for both skeletal and cardiac myopathies associated with aberrant sugar storage. Surprisingly, we found that the proteasome is required to maintain autophagy in the muscle, suggesting that therapeutic treatments aiming to induce autophagy by proteasome inhibition must be carefully calibrated to ensure that the opposite phenotype does not occur.

Starvation-induced MTMR13 and RAB21 activity regulates VAMP8 to promote autophagosome-lysosome fusion

Autophagy, the process for recycling cytoplasm in the lysosome, depends on membrane trafficking. We previously identified Drosophila Sbf as a Rab21 guanine nucleotide exchange factor (GEF) that acts with Rab21 in endosomal trafficking. Here, we show that Sbf/MTMR13 and Rab21 have conserved functions required for starvation-induced autophagy. Depletion of Sbf/MTMR13 or Rab21 blocked endolysosomal trafficking of VAMP8, a SNARE required for autophagosome-lysosome fusion. We show that starvation induces Sbf/MTMR13 GEF and RAB21 activity, as well as their induced binding to VAMP8 (or closest Drosophila homolog, Vamp7). MTMR13 is required for RAB21 activation, VAMP8 interaction and VAMP8 endolysosomal trafficking, defining a novel GEF-Rab-effector pathway. These results identify starvation-responsive endosomal regulators and trafficking that tunes membrane demands with changing autophagy status.

Autophagy machinery in the context of mammalian mitophagy

Autophagy is an intracellular catabolic system that degrades cytoplasmic proteins and organelles. Damaged mitochondria can be degraded by a selective type of autophagy, which is termed mitophagy. PINK1-Parkin-dependent mitophagy has been extensively studied in the mammalian system. PINK1 accumulates on damaged mitochondria to recruit Parkin, which subsequently ubiquitinates a broad range of outer mitochondrial membrane proteins. Ubiquitinated mitochondria associate with the autophagosome formation site, and are selectively incorporated into autophagosomes. During this process, damaged mitochondria first associate with the autophagosome formation site together with upstream autophagy factors, then are efficiently incorporated into autophagosomes through binding with autophagosome adaptors. This "two-step model" may be applied to other selective types of autophagy.

Dual roles of an Arabidopsis ESCRT component FREE1 in regulating vacuolar protein transport and autophagic degradation

Protein turnover can be achieved via the lysosome/vacuole and the autophagic degradation pathways. Evidence has accumulated revealing that efficient autophagic degradation requires functional endosomal sorting complex required for transport (ESCRT) machinery. However, the interplay between the ESCRT machinery and the autophagy regulator remains unclear. Here, we show that FYVE domain protein required for endosomal sorting 1 (FREE1), a recently identified plant-specific ESCRT component essential for multivesicular body (MVB) biogenesis and plant growth, plays roles both in vacuolar protein transport and autophagic degradation. FREE1 also regulates vacuole biogenesis in both seeds and vegetative cells of Arabidopsis. Additionally, FREE1 interacts directly with a unique plant autophagy regulator SH3 domain-containing protein2 and associates with the PI3K complex, to regulate the autophagic degradation in plants. Thus, FREE1 plays multiple functional roles in vacuolar protein trafficking and organelle biogenesis as well as in autophagic degradation via a previously unidentified regulatory mechanism of cross-talk between the ESCRT machinery and autophagy process.

Teaching the basics of autophagy and mitophagy to redox biologists--mechanisms and experimental approaches

Autophagy is a lysosomal mediated degradation activity providing an essential mechanism for recycling cellular constituents, and clearance of excess or damaged lipids, proteins and organelles. Autophagy involves more than 30 proteins and is regulated by nutrient availability, and various stress sensing signaling pathways. This article provides an overview of the mechanisms and regulation of autophagy, its role in health and diseases, and methods for its measurement. Hopefully this teaching review together with the graphic illustrations will be helpful for instructors teaching graduate students who are interested in grasping the concepts and major research areas and introducing recent developments in the field.

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mTOR, Beclin–VPS34, LC3 homologs, and adaptor proteins in autophagy.

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Autophagosomal membranes may derive from multiple sources.

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Autophagosomal–lysosomal fusion contributes to the control of autophagic flux.

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Assess autophagy by autophagosomal and protein turnover, and morphological alterations.

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Autophagy adysfunction in cancer, aging, neurodegeneration and infection.

Phosphorylation of LC3 by the Hippo kinases STK3/STK4 is essential for autophagy

The protein LC3 is indispensible for the cellular recycling process of autophagy and plays critical roles during cargo recruitment, autophagosome biogenesis, and completion. Here, we report that LC3 is phosphorylated at threonine 50 (Thr(50)) by the mammalian Sterile-20 kinases STK3 and STK4. Loss of phosphorylation at this site blocks autophagy by impairing fusion of autophagosomes with lysosomes, and compromises the ability of cells to clear intracellular bacteria, an established cargo for autophagy. Strikingly, mutation of LC3 mimicking constitutive phosphorylation at Thr(50) reverses the autophagy block in STK3/STK4-deficient cells and restores their capacity to clear bacteria. Loss of STK3/STK4 impairs autophagy in diverse species, indicating that these kinases are conserved autophagy regulators. We conclude that phosphorylation of LC3 by STK3/STK4 is an essential step in the autophagy process. Since several pathological conditions, including bacterial infections, display aberrant autophagy, we propose that pharmacological agents targeting this regulatory circuit hold therapeutic potential.

mTORC1 phosphorylates UVRAG to negatively regulate autophagosome and endosome maturation

mTORC1 plays a key role in autophagy as a negative regulator. The currently known targets of mTORC1 in the autophagy pathway mainly function at early stages of autophagosome formation. Here, we identify that mTORC1 inhibits later stages of autophagy by phosphorylating UVRAG. Under nutrient-enriched conditions, mTORC1 binds and phosphorylates UVRAG. The phosphorylation positively regulates the association of UVRAG with RUBICON, thereby enhancing the antagonizing effect of RUBICON on UVRAG-mediated autophagosome maturation. Upon dephosphorylation, UVRAG is released from RUBICON to interact with the HOPS complex, a component for the late endosome and lysosome fusion machinery, and enhances autophagosome and endosome maturation. Consequently, the dephosphorylation of UVRAG facilitates the lysosomal degradation of epidermal growth factor receptor (EGFR), reduces EGFR signaling, and suppresses cancer cell proliferation and tumor growth. These results demonstrate that mTORC1 engages in late stages of autophagy and endosome maturation, defining a broader range of mTORC1 functions in the membrane-associated processes.

PLEKHM1 regulates autophagosome-lysosome fusion through HOPS complex and LC3/GABARAP proteins

The lysosome is the final destination for degradation of endocytic cargo, plasma membrane constituents, and intracellular components sequestered by macroautophagy. Fusion of endosomes and autophagosomes with the lysosome depends on the GTPase Rab7 and the homotypic fusion and protein sorting (HOPS) complex, but adaptor proteins that link endocytic and autophagy pathways with lysosomes are poorly characterized. Herein, we show that Pleckstrin homology domain containing protein family member 1 (PLEKHM1) directly interacts with HOPS complex and contains a LC3-interacting region (LIR) that mediates its binding to autophagosomal membranes. Depletion of PLEKHM1 blocks lysosomal degradation of endocytic (EGFR) cargo and enhances presentation of MHC class I molecules. Moreover, genetic loss of PLEKHM1 impedes autophagy flux upon mTOR inhibition and PLEKHM1 regulates clearance of protein aggregates in an autophagy- and LIR-dependent manner. PLEKHM1 is thus a multivalent endocytic adaptor involved in the lysosome fusion events controlling selective and nonselective autophagy pathways.

Autophagy and regulation of cilia function and assembly

Motile and primary cilia (PC) are microtubule-based structures located at the cell surface of many cell types. Cilia govern cellular functions ranging from motility to integration of mechanical and chemical signaling from the environment. Recent studies highlight the interplay between cilia and autophagy, a conserved cellular process responsible for intracellular degradation. Signaling from the PC recruits the autophagic machinery to trigger autophagosome formation. Conversely, autophagy regulates ciliogenesis by controlling the levels of ciliary proteins. The cross talk between autophagy and ciliated structures is a novel aspect of cell biology with major implications in development, physiology and human pathologies related to defects in cilium function.

Human GABARAP can restore autophagosome biogenesis in a C. elegans lgg-1 mutant

We recently described in C. elegans embryos, the acquisition of specialized functions for orthologs of yeast Atg8 (e.g., mammalian MAP1LC3/LC3) in allophagy, a selective and developmentally regulated autophagic process. During the formation of double-membrane autophagosomes, the ubiquitin-like Atg8/LC3 proteins are recruited to the membrane through a lipidation process. While at least 6 orthologs and paralogs are present in mammals, C. elegans only possesses 2 orthologs, LGG-1 and LGG-2, corresponding to the GABARAP-GABARAPL2/GATE-16 and the MAP1LC3 families, respectively. During allophagy, LGG-1 acts upstream of LGG-2 and is essential for autophagosome biogenesis, whereas LGG-2 facilitates their maturation. We demonstrated that LGG-2 directly interacts with the HOPS complex subunit VPS-39, and mediates the tethering between autophagosomes and lysosomes, which also requires RAB-7. In the present addendum, we compared the localization of autophagosomes, endosomes, amphisomes, and lysosomes in vps-39, rab-7, and lgg-2 depleted embryos. Our results suggest that lysosomes interact with autophagosomes or endosomes through a similar mechanism. We also performed a functional complementation of an lgg-1 null mutant with human GABARAP, its closer homolog, and showed that it localizes to autophagosomes and can rescue LGG-1 functions in the early embryo.

Getting ready for building: signaling and autophagosome biogenesis

Autophagy is the main cellular catabolic process responsible for degrading organelles and large protein aggregates. It is initiated by the formation of a unique membrane structure, the phagophore, which engulfs part of the cytoplasm and forms a double-membrane vesicle termed the autophagosome. Fusion of the outer autophagosomal membrane with the lysosome and degradation of the inner membrane contents complete the process. The extent of autophagy must be tightly regulated to avoid destruction of proteins and organelles essential for cell survival. Autophagic activity is thus regulated by external and internal cues, which initiate the formation of well-defined autophagy-related protein complexes that mediate autophagosome formation and selective cargo recruitment into these organelles. Autophagosome formation and the signaling pathways that regulate it have recently attracted substantial attention. In this review, we analyze the different signaling pathways that regulate autophagy and discuss recent progress in our understanding of autophagosome biogenesis.

The role of the selective adaptor p62 and ubiquitin-like proteins in autophagy

The ubiquitin-proteasome system and autophagy were long viewed as independent, parallel degradation systems with no point of intersection. By now we know that these degradation pathways share certain substrates and regulatory molecules and show coordinated and compensatory function. Two ubiquitin-like protein conjugation pathways were discovered that are required for autophagosome biogenesis: the Atg12-Atg5-Atg16 and Atg8 systems. Autophagy has been considered to be essentially a nonselective process, but it turned out to be at least partially selective. Selective substrates of autophagy include damaged mitochondria, intracellular pathogens, and even a subset of cytosolic proteins with the help of ubiquitin-binding autophagic adaptors, such as p62/SQSTM1, NBR1, NDP52, and Optineurin. These proteins selectively recognize autophagic cargo and mediate its engulfment into autophagosomes by binding to the small ubiquitin-like modifiers that belong to the Atg8/LC3 family.

Atg6/UVRAG/Vps34-containing lipid kinase complex is required for receptor downregulation through endolysosomal degradation and epithelial polarity during Drosophila wing development

Atg6 (Beclin 1 in mammals) is a core component of the Vps34 PI3K (III) complex, which promotes multiple vesicle trafficking pathways. Atg6 and Vps34 form two distinct PI3K (III) complexes in yeast and mammalian cells, either with Atg14 or with UVRAG. The functions of these two complexes are not entirely clear, as both Atg14 and UVRAG have been suggested to regulate both endocytosis and autophagy. In this study, we performed a microscopic analysis of UVRAG, Atg14, or Atg6 loss-of-function cells in the developing Drosophila wing. Both autophagy and endocytosis are seriously impaired and defective endolysosomes accumulate upon loss of Atg6. We show that Atg6 is required for the downregulation of Notch and Wingless signaling pathways; thus it is essential for normal wing development. Moreover, the loss of Atg6 impairs cell polarity. Atg14 depletion results in autophagy defects with no effect on endocytosis or cell polarity, while the silencing of UVRAG phenocopies all but the autophagy defect of Atg6 depleted cells. Thus, our results indicate that the UVRAG-containing PI3K (III) complex is required for receptor downregulation through endolysosomal degradation and for the establishment of proper cell polarity in the developing wing, while the Atg14-containing complex is involved in autophagosome formation.

Autophagy in Drosophila: from historical studies to current knowledge

The discovery of evolutionarily conserved Atg genes required for autophagy in yeast truly revolutionized this research field and made it possible to carry out functional studies on model organisms. Insects including Drosophila are classical and still popular models to study autophagy, starting from the 1960s. This review aims to summarize past achievements and our current knowledge about the role and regulation of autophagy in Drosophila, with an outlook to yeast and mammals. The basic mechanisms of autophagy in fruit fly cells appear to be quite similar to other eukaryotes, and the role that this lysosomal self-degradation process plays in Drosophila models of various diseases already made it possible to recognize certain aspects of human pathologies. Future studies in this complete animal hold great promise for the better understanding of such processes and may also help finding new research avenues for the treatment of disorders with misregulated autophagy.