ATG5 selectively engages virus-tethered BST2/tetherin in an LC3C-associated pathway

Bone marrow stromal antigen 2 (BST2)/tetherin is a restriction factor that reduces HIV-1 dissemination by tethering virus at the cell surface. BST2 also acts as a sensor of HIV-1 budding, establishing a cellular antiviral state. The HIV-1 Vpu protein antagonizes BST2 antiviral functions via multiple mechanisms, including the subversion of an LC3C-associated pathway, a key cell intrinsic antimicrobial mechanism. Here, we describe the first step of this viral-induced LC3C-associated process. This process is initiated at the plasma membrane through the recognition and internalization of virus-tethered BST2 by ATG5, an autophagy protein. ATG5 and BST2 assemble as a complex, independently of the viral protein Vpu and ahead of the recruitment of the ATG protein LC3C. The conjugation of ATG5 with ATG12 is dispensable for this interaction. ATG5 recognizes cysteine-linked homodimerized BST2 and specifically engages phosphorylated BST2 tethering viruses at the plasma membrane, in an LC3C-associated pathway. We also found that this LC3C-associated pathway is used by Vpu to attenuate the inflammatory responses mediated by virion retention. Overall, we highlight that by targeting BST2 tethering viruses, ATG5 acts as a signaling scaffold to trigger an LC3C-associated pathway induced by HIV-1 infection.

Measles virus-imposed remodeling of the autophagy machinery determines the outcome of bacterial coinfection

Although it is admitted that secondary infection can complicate viral diseases, the consequences of viral infection on cell susceptibility to other infections remain underexplored at the cellular level. We though to examine whether the sustained macroautophagy/autophagy associated with measles virus (MeV) infection could help cells oppose invasion by Salmonella Typhimurium, a bacterium sensitive to autophagic restriction. We report here the unexpected finding that Salmonella markedly replicated in MeV-infected cultures due to selective growth within multinucleated cells. Hyper-replicating Salmonella localized outside of LAMP1-positive compartments to an extent that equaled that of the predominantly cytosolic sifA mutant Salmonella. Bacteria were subjected to effective ubiquitination but failed to be targeted by LC3 despite an ongoing productive autophagy. Such a phenotype could not be further aggravated upon silencing of the selective autophagy regulator TBK1 or core autophagy factors ATG5 or ATG7. MeV infection also conditioned primary human epithelial cells for augmented Salmonella replication. The analysis of selective autophagy receptors able to target Salmonella revealed that a lowered expression level of SQSTM1/p62 and TAX1BP1/T6BP autophagy receptors prevented effective anti-Salmonella autophagy in MeV-induced syncytia. Conversely, as SQSTM1/p62 is promoting the cytosolic growth of Shigella flexneri, MeV infection led to reduced Shigella replication. The results indicate that the rarefaction of dedicated autophagy receptors associated with MeV infection differentially affects the outcome of bacterial coinfection depending on the nature of the functional relationship between bacteria and such receptors. Thus, virus-imposed reconfiguration of the autophagy machinery can be instrumental in determining the fate of bacterial coinfection.Abbreviations: ACTB/β-ACTIN: actin beta; ATG: autophagy related; BAFA1: bafilomycin A1; CFU: colony-forming units; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; FIP: fusion inhibitory peptide; GFP: green fluorescent protein; LAMP1: lysosomal associated membrane protein 1; LIR: MAP1LC3/LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MeV: measles virus; MOI: multiplicity of infection; OPTN: optineurin; PHH: primary human hepatocyte; SCV: Salmonella-containing vacuoles; SQSTM1/p62: sequestosome 1; S. flexneri: Shigella flexneri; S. Typhimurium: Salmonella enterica serovar Typhimurium; TAX1BP1/T6BP: Tax1 binding protein 1; TBK1: TANK binding kinase 1.

TBK1 phosphorylation activates LIR-dependent degradation of the inflammation repressor TNIP1

Limitation of excessive inflammation due to selective degradation of pro-inflammatory proteins is one of the cytoprotective functions attributed to autophagy. In the current study, we highlight that selective autophagy also plays a vital role in promoting the establishment of a robust inflammatory response. Under inflammatory conditions, here TLR3-activation by poly(I:C) treatment, the inflammation repressor TNIP1 (TNFAIP3 interacting protein 1) is phosphorylated by Tank-binding kinase 1 (TBK1) activating an LIR motif that leads to the selective autophagy-dependent degradation of TNIP1, supporting the expression of pro-inflammatory genes and proteins. This selective autophagy efficiently reduces TNIP1 protein levels early (0-4 h) upon poly(I:C) treatment to allow efficient initiation of the inflammatory response. At 6 h, TNIP1 levels are restored due to increased transcription avoiding sustained inflammation. Thus, similarly as in cancer, autophagy may play a dual role in controlling inflammation depending on the exact state and timing of the inflammatory response.

--Atg9 interactions via its transmembrane domains are required for phagophore expansion during autophagy

During macroautophagy/autophagy, precursor cisterna known as phagophores expand and sequester portions of the cytoplasm and/or organelles, and subsequently close resulting in double-membrane transport vesicles called autophagosomes. Autophagosomes fuse with lysosomes/vacuoles to allow the degradation and recycling of their cargoes. We previously showed that sequential binding of yeast Atg2 and Atg18 to Atg9, the only conserved transmembrane protein in autophagy, at the extremities of the phagophore mediates the establishment of membrane contact sites between the phagophore and the endoplasmic reticulum. As the Atg2-Atg18 complex transfers lipids between adjacent membranes in vitro, it has been postulated that this activity and the scramblase activity of the trimers formed by Atg9 are required for the phagophore expansion. Here, we present evidence that Atg9 indeed promotes Atg2-Atg18 complex-mediated lipid transfer in vitro, although this is not the only requirement for its function in vivo. In particular, we show that Atg9 function is dramatically compromised by a F627A mutation within the conserved interface between the transmembrane domains of the Atg9 monomers. Although Atg9^F627A self-interacts and binds to the Atg2-Atg18 complex, the F627A mutation blocks the phagophore expansion and thus autophagy progression. This phenotype is conserved because the corresponding human ATG9A mutant severely impairs autophagy as well. Importantly, Atg9^F627A has identical scramblase activity in vitro like Atg9, and as with the wild-type protein enhances Atg2-Atg18-mediated lipid transfer. Collectively, our data reveal that interactions of Atg9 trimers via their transmembrane segments play a key role in phagophore expansion beyond Atg9's role as a lipid scramblase.Abbreviations: BafA1: bafilomycin A₁; Cvt: cytoplasm-to-vacuole targeting; Cryo-EM: cryo-electron microscopy; ER: endoplasmic reticulum; GFP: green fluorescent protein; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MCS: membrane contact site; NBD-PE: N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine; PAS: phagophore assembly site; PE: phosphatidylethanolamine; prApe1: precursor Ape1; PtdIns3P: phosphatidylinositol-3-phosphate; SLB: supported lipid bilayer; SUV: small unilamellar vesicle; TMD: transmembrane domain; WT: wild type.

HSBP1 Is a Novel Interactor of FIP200 and ATG13 That Promotes Autophagy Initiation and Picornavirus Replication

ATG13 and FIP200 are two subunits of the ULK kinase complex, a key regulatory component of the autophagy machinery. We have previously found that the FIP200-ATG13 subcomplex controls picornavirus replication outside its role in the ULK kinase complex and autophagy. Here, we characterized HSBP1, a very small cytoplasmic coiled-coil protein, as a novel interactor of FIP200 and ATG13 that binds these two proteins via FIP200. HSBP1 is a novel pro-picornaviral host factor since its knockdown or knockout, inhibits the replication of various picornaviruses. The anti-picornaviral function of the FIP200-ATG13 subcomplex was abolished when HSBP1 was depleted, inferring that this subcomplex negatively regulates HSBP1’s pro-picornaviral function during infections. HSBP1depletion also reduces the stability of ULK kinase complex subunits, resulting in an impairment in autophagy induction. Altogether, our data show that HSBP1 interaction with FIP200-ATG13-containing complexes is involved in the regulation of different cellular pathways.

ATG9A protects the plasma membrane from programmed and incidental permeabilization

The integral membrane protein ATG9A plays a key role in autophagy. It displays a broad intracellular distribution and is present in numerous compartments, including the plasma membrane (PM). The reasons for the distribution of ATG9A to the PM and its role at the PM are not understood. Here, we show that ATG9A organizes, in concert with IQGAP1, components of the ESCRT system and uncover cooperation between ATG9A, IQGAP1 and ESCRTs in protection from PM damage. ESCRTs and ATG9A phenocopied each other in protection against PM injury. ATG9A knockouts sensitized the PM to permeabilization by a broad spectrum of microbial and endogenous agents, including gasdermin, MLKL and the MLKL-like action of coronavirus ORF3a. Thus, ATG9A engages IQGAP1 and the ESCRT system to maintain PM integrity.

Crimean-Congo hemorrhagic fever virus replication imposes hyper-lipidation of MAP1LC3 in epithelial cells

Crimean-Congo hemorrhagic fever virus (CCHFV) is a virus that causes severe liver dysfunctions and hemorrhagic fever, with high mortality rate. Here, we show that CCHFV infection caused a massive lipidation of LC3 in hepatocytes. This lipidation was not dependent on ATG5, ATG7 or BECN1, and no signs for recruitment of the alternative ATG12-ATG3 pathway for lipidation was found. Both virus replication and protein synthesis were required for the lipidation of LC3. Despite an augmented transcription of SQSTM1, the amount of proteins did not show a massive and sustained increase in infected cells, indicating that degradation of SQSTM1 by macroautophagy/autophagy was still occurring. The genetic alteration of autophagy did not influence the production of CCHFV particles demonstrating that autophagy was not required for CCHFV replication. Thus, the results indicate that CCHFV multiplication imposes an overtly elevated level of LC3 mobilization that involves a possibly novel type of non-canonical lipidation. Abbreviations: BECN1: Beclin 1; CCHF: Crimean-Congo hemorrhagic fever; CCHFV: Crimean-Congo hemorrhagic fever virus; CHX: cycloheximide; ER: endoplasmic reticulum; GFP: green fluorescent protein; GP: glycoproteins; MAP1LC3: microtubule associated protein 1 light chain 3; MOI: multiplicity of infection; n.i.: non-infected; NP: nucleoprotein; p.i.: post-infection; SQSTM1: sequestosome 1.

Manipulation of selective macroautophagy by pathogens at a glance

Macroautophagy (hereafter autophagy) is a highly conserved catabolic pathway, which mediates the delivery of unwanted cytoplasmic structures and organelles to lysosomes for degradation. In numerous situations, autophagy is highly selective and exclusively targets specific intracellular components. Selective types of autophagy are a central element of our cell-autonomous innate immunity as they can mediate the turnover of viruses or bacteria, that gain access to the cytoplasm of the cell. Selective autophagy also modulates other aspects of our immunity by turning over specific immunoregulators. Throughout evolution, however, the continuous interaction between this fundamental cellular pathway and pathogens has led several pathogens to develop exquisite mechanisms to inhibit or subvert selective types of autophagy, to promote their intracellular multiplication. This Cell Science at a Glance article and the accompanying poster provides an overview of the selective autophagy of both pathogens, known as xenophagy, and of immunoregulators, and highlights a few archetypal examples that illustrate molecular strategies developed by viruses and bacteria to manipulate selective autophagy for their own benefit.

SQSTM-1/p62 potentiates HTLV-1 Tax-mediated NF-κB activation through its ubiquitin binding function

The NF-κB pathway is constitutively activated in adult T cell leukemia, an aggressive malignancy caused by Human T Leukemia Virus type 1 (HTLV-1). The viral oncoprotein Tax triggers this constitutive activation by interacting with the ubiquitin-rich IKK complex. We previously demonstrated that Optineurin and TAX1BP1, two members of the ubiquitin-binding, Sequestosome-1 (SQSTM-1/p62)-like selective autophagy receptor family, are involved in Tax-mediated NF-κB signaling. Here, using a proximity-dependent biotinylation approach (BioID), we identify p62 as a new candidate partner of Tax and confirm the interaction in infected T cells. We then demonstrate that p62 knock-out in MEF cells as well as p62 knock-down in HEK293T cells significantly reduces Tax-mediated NF-κB activity. We further show that although p62 knock-down does not alter NF-κB activation in Jurkat T cells nor in infected T cells, p62 does potentiate Tax-mediated NF-κB activity upon over-expression in Jurkat T cells. We next show that p62 associates with the Tax/IKK signalosome in cells, and identify the 170–206 domain of p62 as sufficient for the direct, ubiquitin-independent interaction with Tax. However, we observe that this domain is dispensable for modulating Tax activity in cells, and functional analysis of p62 mutants indicates that p62 could potentiate Tax activity in cells by facilitating the association of ubiquitin chains with the Tax/IKK signalosome. Altogether, our results identify p62 as a new ubiquitin-dependent modulator of Tax activity on NF-κB, further highlighting the importance of ubiquitin in the signaling activity of the viral Tax oncoprotein.

Probing aggrephagy using chemically-induced protein aggregates

Selective types of autophagy mediate the clearance of specific cellular components and are essential to maintain cellular homeostasis. However, tools to directly induce and monitor such pathways are limited. Here we introduce the PIM (particles induced by multimerization) assay as a tool for the study of aggrephagy, the autophagic clearance of aggregates. The assay uses an inducible multimerization module to assemble protein clusters, which upon induction recruit ubiquitin, p62, and LC3 before being delivered to lysosomes. Moreover, use of a dual fluorescent tag allows for the direct observation of cluster delivery to the lysosome. Using flow cytometry and fluorescence microscopy, we show that delivery to the lysosome is partially dependent on p62 and ATG7. This assay will help in elucidating the spatiotemporal dynamics and control mechanisms underlying aggregate clearance by the autophagy-lysosomal system.

Sorting the trash: Micronucleophagy gets selective

During micronucleophagy, the nucleolus is targeted by autophagic degradation, but although nucleolar proteins are recycled, ribosomal DNA is spared. Mostofa et al. (2018. J. Cell Biol. https://doi.org/10.1083/jcb.201706164) reveal that the separation of these two nucleolar components is mediated by the CLIP and cohibin complexes and is vital for cell survival during starvation.

The Interaction between Nidovirales and Autophagy Components

Autophagy is a conserved intracellular catabolic pathway that allows cells to maintain homeostasis through the degradation of deleterious components via specialized double-membrane vesicles called autophagosomes. During the past decades, it has been revealed that numerous pathogens, including viruses, usurp autophagy in order to promote their propagation. Nidovirales are an order of enveloped viruses with large single-stranded positive RNA genomes. Four virus families (Arterividae, Coronaviridae, Mesoniviridae, and Roniviridae) are part of this order, which comprises several human and animal pathogens of medical and veterinary importance. In host cells, Nidovirales induce membrane rearrangements including autophagosome formation. The relevance and putative mechanism of autophagy usurpation, however, remain largely elusive. Here, we review the current knowledge about the possible interplay between Nidovirales and autophagy.

2BC Non-Structural Protein of Enterovirus A71 Interacts with SNARE Proteins to Trigger Autolysosome Formation

Viruses have evolved unique strategies to evade or subvert autophagy machinery. Enterovirus A71 (EV-A71) induces autophagy during infection in vitro and in vivo. In this study, we report that EV-A71 triggers autolysosome formation during infection in human rhabdomyosarcoma (RD) cells to facilitate its replication. Blocking autophagosome-lysosome fusion with chloroquine inhibited virus RNA replication, resulting in lower viral titres, viral RNA copies and viral proteins. Overexpression of the non-structural protein 2BC of EV-A71 induced autolysosome formation. Yeast 2-hybrid and co-affinity purification assays showed that 2BC physically and specifically interacted with a N-ethylmaleimide-sensitive factor attachment receptor (SNARE) protein, syntaxin-17 (STX17). Co-immunoprecipitation assay further showed that 2BC binds to SNARE proteins, STX17 and synaptosome associated protein 29 (SNAP29). Transient knockdown of STX17, SNAP29, and microtubule-associated protein 1 light chain 3B (LC3B), crucial proteins in the fusion between autophagosomes and lysosomes) as well as the lysosomal-associated membrane protein 1 (LAMP1) impaired production of infectious EV-A71 in RD cells. Collectively, these results demonstrate that the generation of autolysosomes triggered by the 2BC non-structural protein is important for EV-A71 replication, revealing a potential molecular pathway targeted by the virus to exploit autophagy. This study opens the possibility for the development of novel antivirals that specifically target 2BC to inhibit formation of autolysosomes during EV-A71 infection.

Distinct Contributions of Autophagy Receptors in Measles Virus Replication

Autophagy is a potent cell autonomous defense mechanism that engages the lysosomal pathway to fight intracellular pathogens. Several autophagy receptors can recognize invading pathogens in order to target them towards autophagy for their degradation after the fusion of pathogen-containing autophagosomes with lysosomes. However, numerous intracellular pathogens can avoid or exploit autophagy, among which is measles virus (MeV). This virus induces a complete autophagy flux, which is required to improve viral replication. We therefore asked how measles virus interferes with autophagy receptors during the course of infection. We report that in addition to NDP52/CALCOCO₂ and OPTINEURIN/OPTN, another autophagy receptor, namely T6BP/TAXIBP1, also regulates the maturation of autophagosomes by promoting their fusion with lysosomes, independently of any infection. Surprisingly, only two of these receptors, NDP52 and T6BP, impacted measles virus replication, although independently, and possibly through physical interaction with MeV proteins. Thus, our results suggest that a restricted set of autophagosomes is selectively exploited by measles virus to replicate in the course of infection.

Dual function of CALCOCO2/NDP52 during xenophagy

During xenophagy, pathogens are selectively targeted by autophagy receptors to the autophagy machinery for their subsequent degradation. In infected cells, the autophagy receptor CALCOCO2/NDP52 targets Salmonella Typhimurium to the phagophore membrane by concomitantly interacting with LC3C and binding to ubiquitinated cytosolic bacteria or to LGALS8/GALECTIN 8 adsorbed on damaged vacuoles that contain bacteria. We recently reported that in addition, CALCOCO2 is also necessary for the maturation step of Salmonella Typhimurium-containing autophagosomes. Interestingly, the role of CALCOCO2 in maturation is independent of its role in targeting, as these functions rely on distinct binding domains and protein partners. Indeed, to mediate autophagosome maturation CALCOCO2 binds on the one hand to LC3A, LC3B, or GABARAPL2, and on the other hand to MYO6/MYOSIN VI, whereas the interaction with LC3C is dispensable. Therefore, the autophagy receptor CALCOCO2 plays a dual function during xenophagy first by targeting bacteria to nascent autophagosomes and then by promoting autophagosome maturation in order to destroy bacteria.

Autophagy receptor NDP52 regulates pathogen-containing autophagosome maturation

Xenophagy, an essential anti-microbial cell-autonomous mechanism, relies on the ability of the autophagic process to selectively entrap intracellular pathogens within autophagosomes to degrade them in autolysosomes. This selective targeting is carried out by specialized autophagy receptors, such as NDP52, but it is unknown whether the fusion of pathogen-containing autophagosomes with lysosomes is also regulated by pathogen-specific cellular factors. Here, we show that NDP52 also promotes the maturation of autophagosomes via its interaction with LC3A, LC3B, and/or GABARAPL2 through a distinct LC3-interacting region, and with MYOSIN VI. During Salmonella Typhimurium infection, the regulatory function of NDP52 in autophagosome maturation is complementary but independent of its function in pathogen targeting to autophagosomes, which relies on the interaction with LC3C. Thus, complete xenophagy is selectively regulated by a single autophagy receptor, which initially orchestrates bacteria targeting to autophagosomes and subsequently ensures pathogen degradation by regulating pathogen-containing autophagosome maturation.

Sustained autophagy contributes to measles virus infectivity

The interplay between autophagy and intracellular pathogens is intricate as autophagy is an essential cellular response to fight against infections, whereas numerous microbes have developed strategies to escape this process or even exploit it to their own benefit. The fine tuned timing and/or selective molecular pathways involved in the induction of autophagy upon infections could be the cornerstone allowing cells to either control intracellular pathogens, or be invaded by them. We report here that measles virus infection induces successive autophagy signallings in permissive cells, via distinct and uncoupled molecular pathways. Immediately upon infection, attenuated measles virus induces a first transient wave of autophagy, via a pathway involving its cellular receptor CD46 and the scaffold protein GOPC. Soon after infection, a new autophagy signalling is initiated which requires viral replication and the expression of the non-structural measles virus protein C. Strikingly, this second autophagy signalling can be sustained overtime within infected cells, independently of the expression of C, but via a third autophagy input resulting from cell-cell fusion and the formation of syncytia. Whereas this sustained autophagy signalling leads to the autophagy degradation of cellular contents, viral proteins escape from degradation. Furthermore, this autophagy flux is ultimately exploited by measles virus to limit the death of infected cells and to improve viral particle formation. Whereas CD150 dependent virulent strains of measles virus are unable to induce the early CD46/GOPC dependent autophagy wave, they induce and exploit the late and sustained autophagy. Overall, our work describes distinct molecular pathways for an induction of self-beneficial sustained autophagy by measles virus.

Autophagy is an evolutionarily conserved lysosomal dependent degradative pathway for recycling of long-lived proteins and damaged organelles. Autophagy is also an essential cellular response to fight infection by destroying infectious pathogens trapped within autophagosomes and plays a key role in the induction of both innate and adaptive immune responses. Numerous viruses have evolved strategies to counteract autophagy in order to escape from degradation or/and to inhibit immune signals. The kinetic and molecular pathways involved in the induction of autophagy upon infections might determine if cells would be able to control pathogens or would be invaded by them. We showed that measles virus (MeV) infection induces successive autophagy signallings in cells via distinct molecular pathways. A first autophagy wave is induced by the engagement of the MeV cellular receptor CD46 and the scaffold protein GOPC. A second wave is initiated after viral replication by the expression of the non-structural MeV protein C and is sustained overtime within infected cells thanks to the formation of syncytia. This sustained autophagy is exploited by MeV to limit the death of infected cells and to improve viral particle formation. We describe new molecular pathways by which MeV hijacks autophagy to promote its infectivity.

[Human retrovirus XMRV: The end of an exciting story?]

Viruses represent an important cause of cancer in humans: infections are estimated to account for close to one cancer case out of five.With the ongoing discovery of new infectious agents, this number should be raising in the near future. In 2006, the discovery of a new _-retrovirus in prostate cancer biopsies launched an intense research activity: could this new xenotropic MLV-related virus (XMRV) be the cause of prostate cancer? Five years later, the initial enthusiasm of retrovirologists has dramatically diminished. One by one, arguments favouring the hypothesis of human infection with XMRV are being refuted. The aim of this review article is to present the discovery of XMRV and to analyze recent data arguing against its existence in humans. A synthetic interpretation of XMRV literature will then be suggested.