BioID reveals an ATG9A interaction with ATG13-ATG101 in the degradation of p62/SQSTM1-ubiquitin clusters

AMPK regulates phagophore-to-autophagosome maturation

Autophagy is an important metabolic pathway that can non-selectively recycle cellular material or lead to targeted degradation of protein aggregates or damaged organelles. Autophagosome formation starts with autophagy factors accumulating on lipid vesicles containing ATG9. These phagophores attach to donor membranes, expand via ATG2-mediated lipid transfer, capture cargo, and mature into autophagosomes, ultimately fusing with lysosomes for their degradation. Autophagy can be activated by nutrient stress, for example, by a reduction in the cellular levels of amino acids. In contrast, how autophagy is regulated by low cellular ATP levels via the AMP-activated protein kinase (AMPK), an important therapeutic target, is less clear. Using live-cell imaging and an automated image analysis pipeline, we systematically dissect how nutrient starvation regulates autophagosome biogenesis. We demonstrate that glucose starvation downregulates autophagosome maturation by AMPK-mediated inhibition of phagophore tethering to donor membrane. Our results clarify AMPKs regulatory role in autophagy and highlight its potential as a therapeutic target to reduce autophagy.

Emerging roles of ATG9/ATG9A in autophagy: implications for cell and neurobiology

Atg9, the only transmembrane protein among many autophagy-related proteins, was first identified in the year 2000 in yeast. Two homologs of Atg9, ATG9A and ATG9B, have been found in mammals. While ATG9B shows a tissue-specific expression pattern, such as in the placenta and pituitary gland, ATG9A is ubiquitously expressed. Additionally, ATG9A deficiency leads to severe defects not only at the molecular and cellular levels but also at the organismal level, suggesting key and fundamental roles for ATG9A. The subcellular localization of ATG9A on small vesicles and its functional relevance to autophagy have suggested a potential role for ATG9A in the lipid supply during autophagosome biogenesis. Nevertheless, the precise role of ATG9A in the autophagic process has remained a long-standing mystery, especially in neurons. Recent findings, however, including structural, proteomic, and biochemical analyses, have provided new insights into its function in the expansion of the phagophore membrane. In this review, we aim to understand various aspects of ATG9 (in invertebrates and plants)/ATG9A (in mammals), including its localization, trafficking, and other functions, in nonneuronal cells and neurons by comparing recent discoveries related to ATG9/ATG9A and proposing directions for future research.Abbreviation: AP-4: adaptor protein complex 4; ATG: autophagy related; cKO: conditional knockout; CLA-1: CLArinet (functional homolog of cytomatrix at the active zone proteins piccolo and fife); cryo-EM: cryogenic electron microscopy; ER: endoplasmic reticulum; KO: knockout; PAS: phagophore assembly site; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; RB1CC1/FIP200: RB1 inducible coiled-coil 1; SV: synaptic vesicle; TGN: trans-Golgi network; ULK: unc-51 like autophagy activating kinase; WIPI2: WD repeat domain, phosphoinositide interacting 2.

The Role of ATG9 Vesicles in Autophagosome Biogenesis

Autophagy mediates the degradation and recycling of cellular material in the lysosomal system. Dysfunctional autophagy is associated with a plethora of diseases including uncontrolled infections, cancer and neurodegeneration. In macroautophagy (hereafter autophagy) this material is encapsulated in double membrane vesicles, the autophagosomes, which form upon induction of autophagy. The precursors to autophagosomes, referred to as phagophores, first appear as small flattened membrane cisternae, which gradually enclose the cargo material as they grow. The assembly of phagophores during autophagy initiation has been a major subject of investigation over the past decades. A special focus has been ATG9, the only conserved transmembrane protein among the core machinery. The majority of ATG9 localizes to small Golgi-derived vesicles. Here we review the recent advances and breakthroughs regarding our understanding of how ATG9 and the vesicles it resides in serve to assemble the autophagy machinery and to establish membrane contact sites for autophagosome biogenesis. We also highlight open questions in the field that need to be addressed in the years to come.

Noncanonical roles of ATG5 and membrane atg8ylation in retromer assembly and function

ATG5 is one of the core autophagy proteins with additional functions such as noncanonical membrane atg8ylation, which among a growing number of biological outputs includes control of tuberculosis in animal models. Here we show that ATG5 associates with retromer's core components VPS26, VPS29 and VPS35 and modulates retromer function. Knockout of ATG5 blocked trafficking of a key glucose transporter sorted by the retromer, GLUT1, to the plasma membrane. Knockouts of other genes essential for membrane atg8ylation, of which ATG5 is a component, affected GLUT1 sorting, indicating that membrane atg8ylation as a process affects retromer function and endosomal sorting. The contribution of membrane atg8ylation to retromer function in GLUT1 sorting was independent of canonical autophagy. These findings expand the scope of membrane atg8ylation to specific sorting processes in the cell dependent on the retromer and its known interactors.

Treatment of Rheumatoid Arthritis Based on the Inherent Bioactivity of Black Phosphorus Nanosheets

Rheumatoid arthritis (RA) is an autoimmune disease that affects the living quality of patients, especially the elderly population. RA-related morbidity and mortality increase significantly with age, while current clinical drugs for RA are far from satisfactory and may have serious side effects. Therefore, the development of new drugs with higher biosafety and efficacy is demanding. Black phosphorus nanosheets (BPNSs) have been widely studied because of their excellent biocompatibility. Here, we focus on the inherent bioactivity of BPNSs, report the potential of BPNSs as a therapeutic drug for RA and elucidate the underlying therapeutic mechanism. We find that BPNSs inhibit autophagy at an early stage via the AMPK-mTOR pathway, switch the energy metabolic pathway to oxidative phosphorylation, increase intracellular ATP levels, suppress apoptosis, reduce inflammation and oxidative stress, and down-regulate senescence-associated secretory phenotype (SASP)-related genes in rheumatoid arthritis synovial fibroblasts (RA-SFs). Further, BPNSs induce the apoptosis of macrophages and promote their transition from the M1 to the M2 phenotype by regulating related cytokines. Significantly, the administration of BPNSs can alleviate key pathological features of RA in mice, revealing great therapeutic potential. This study provides a novel option for treating RA, with BPNSs emerging as a promising therapeutic candidate.

ATG9B regulates bacterial internalization via actin rearrangement

Invasive bacterial pathogens are internalized by host cells through endocytosis, which is regulated by a cascade of actin rearrangement signals triggered by host cell receptors or bacterial proteins delivered into host cells. However, the molecular mechanisms that mediate actin rearrangement to promote bacterial invasion are not fully understood. Here, we show that the autophagy-related (ATG) protein ATG9B regulates the internalization of various bacteria by controlling actin rearrangement. ATG knockout screening and knockdown experiments in HeLa cells identified ATG9B as a critical factor for bacterial internalization. In particular, cells with ATG9B knockdown exhibited an accumulation of actin filaments and phosphorylated LIM kinase and cofilin, suggesting that ATG9B is involved in actin depolymerization. Furthermore, the kinase activity of Unc-51-like autophagy-activating kinase 1 was found to regulate ATG9B localization and actin remodeling. These findings revealed a newly discovered function of ATG proteins in bacterial infection rather than autophagy-mediated immunity.

ATG9 resides on a unique population of small vesicles in presynaptic nerve terminals

In neurons, autophagosome biogenesis occurs mainly in distal axons, followed by maturation during retrograde transport. Autophagosomal growth depends on the supply of membrane lipids which requires small vesicles containing ATG9, a lipid scramblase essential for macroautophagy/autophagy. Here, we show that ATG9-containing vesicles are enriched in synapses and resemble synaptic vesicles in size and density. The proteome of ATG9-containing vesicles immuno-isolated from nerve terminals showed conspicuously low levels of trafficking proteins except of the AP2-complex and some enzymes involved in endosomal phosphatidylinositol metabolism. Super resolution microscopy of nerve terminals and isolated vesicles revealed that ATG9-containing vesicles represent a distinct vesicle population with limited overlap not only with synaptic vesicles but also other membranes of the secretory pathway, uncovering a surprising heterogeneity in their membrane composition. Our results are compatible with the view that ATG9-containing vesicles function as lipid shuttles that scavenge membrane lipids from various intracellular membranes to support autophagosome biogenesis.Abbreviations: AP: adaptor related protein complex: ATG2: autophagy related 2; ATG9: autophagy related 9; DNA PAINT: DNA-based point accumulation for imaging in nanoscale topography; DyMIN STED: dynamic minimum stimulated emission depletion; EL: endosome and lysosome; ER: endoplasmic reticulum; GA: Golgi apparatus; iBAQ: intensity based absolute quantification; LAMP: lysosomal-associated membrane protein; M6PR: mannose-6-phosphate receptor, cation dependent; Minflux: minimal photon fluxes; Mito: mitochondria; MS: mass spectrometry; PAS: phagophore assembly site; PM: plasma membrane; Px: peroxisome; RAB26: RAB26, member RAS oncogene family; RAB3A: RAB3A, member RAS oncogene family; RAB5A: RAB5A, member RAS oncogene family; SNARE: soluble N-ethylmaleimide-sensitive-factor attachment receptor; SVs: synaptic vesicles; SYP: synaptophysin; TGN: trans-Golgi network; TRAPP: transport protein particle; VTI1: vesicle transport through interaction with t-SNAREs.

Molecular Mechanisms of Macroautophagy, Microautophagy, and Chaperone-Mediated Autophagy

Autophagy is a self-digestive process that is conserved in eukaryotic cells and responsible for maintaining cellular homeostasis through proteolysis. By this process, cells break down their own components in lysosomes. Autophagy can be classified into three categories: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). Macroautophagy involves membrane elongation and microautophagy involves membrane internalization, and both pathways undergo selective or non-selective processes that transport cytoplasmic components into lysosomes to be degraded. CMA, however, involves selective incorporation of cytosolic materials into lysosomes without membrane deformation. All three categories of autophagy have attracted much attention due to their involvement in various biological phenomena and their relevance to human diseases, such as neurodegenerative diseases and cancer. Clarification of the molecular mechanisms behind these processes is key to understanding autophagy and recent studies have made major progress in this regard, especially for the mechanisms of initiation and membrane elongation in macroautophagy and substrate recognition in microautophagy and CMA. Furthermore, it is becoming evident that the three categories of autophagy are related to each other despite their implementation by different sets of proteins and the involvement of completely different membrane dynamics. In this review, recent progress in macroautophagy, microautophagy, and CMA are summarized.

The role of the HORMA domain proteins ATG13 and ATG101 in initiating autophagosome biogenesis

Autophagy is a process of regulated degradation. It eliminates damaged and unnecessary cellular components by engulfing them with a de novo-generated organelle: the double-membrane autophagosome. The past three decades have provided us with a detailed parts list of the autophagy initiation machinery, have developed important insights into how these processes function and have identified regulatory proteins. It is now clear that autophagosome biogenesis requires the timely assembly of a complex machinery. However, it is unclear how a putative stable machine is assembled and disassembled and how the different parts cooperate to perform its overall function. Although they have long been somewhat enigmatic in their precise role, HORMA domain proteins (first identified in Hop1p, Rev7p and MAD2 proteins) autophagy-related protein 13 (ATG13) and ATG101 of the ULK-kinase complex have emerged as important coordinators of the autophagy-initiating subcomplexes. Here, we will particularly focus on ATG13 and ATG101 and the role of their unusual metamorphosis in initiating autophagosome biogenesis. We will also explore how this metamorphosis could potentially be purposefully rate-limiting and speculate on how it could regulate the spontaneous self-assembly of the autophagy-initiating machinery.

The formation of ubiquitin rich condensates triggers recruitment of the ATG9A lipid transfer complex to initiate basal autophagy

Autophagy is an essential cellular recycling process that maintains protein and organelle homeostasis. ATG9A vesicle recruitment is a critical early step in autophagy to initiate autophagosome biogenesis. The mechanisms of ATG9A vesicle recruitment are best understood in the context of starvation-induced non-selective autophagy, whereas less is known about the signals driving ATG9A vesicle recruitment to autophagy initiation sites in the absence of nutrient stress. Here we demonstrate that loss of ATG9A or the lipid transfer protein ATG2 leads to the accumulation of phosphorylated p62 aggregates in the context of basal autophagy. Furthermore, we show that p62 degradation requires the lipid scramblase activity of ATG9A. Lastly, we present evidence that poly-ubiquitin is an essential signal that recruits ATG9A and mediates autophagy foci assembly in nutrient replete cells. Together, our data support a ubiquitin-driven model of ATG9A recruitment and autophagosome formation during basal autophagy.

AMPK Regulates Phagophore-to-Autophagosome Maturation

Autophagy is an important metabolic pathway that can non-selectively recycle cellular material or lead to targeted degradation of protein aggregates or damaged organelles. Autophagosome formation starts with autophagy factors accumulating on lipid vesicles containing ATG9. These phagophores attach to donor membranes, expand via ATG2-mediated lipid transfer, capture cargo, and mature into autophagosomes, ultimately fusing with lysosomes for their degradation. Autophagy can be activated by nutrient stress, for example by a reduction in the cellular levels of amino acids. In contrast, how autophagy is regulated by low cellular ATP levels via the AMP-activated protein kinase (AMPK), an important therapeutic target, is less clear. Using live-cell imaging and an automated image analysis pipeline, we systematically dissect how nutrient starvation regulates autophagosome biogenesis. We demonstrate that glucose starvation downregulates autophagosome maturation by AMPK mediated inhibition of phagophores tethering to donor membranes. Our results clarify AMPK's regulatory role in autophagy and highlight its potential as a therapeutic target to reduce autophagy.

The development of proximity labeling technology and its applications in mammals, plants, and microorganisms

Protein‒protein, protein‒RNA, and protein‒DNA interaction networks form the basis of cellular regulation and signal transduction, making it crucial to explore these interaction networks to understand complex biological processes. Traditional methods such as affinity purification and yeast two-hybrid assays have been shown to have limitations, as they can only isolate high-affinity molecular interactions under nonphysiological conditions or in vitro. Moreover, these methods have shortcomings for organelle isolation and protein subcellular localization. To address these issues, proximity labeling techniques have been developed. This technology not only overcomes the limitations of traditional methods but also offers unique advantages in studying protein spatial characteristics and molecular interactions within living cells. Currently, this technique not only is indispensable in research on mammalian nucleoprotein interactions but also provides a reliable approach for studying nonmammalian cells, such as plants, parasites and viruses. Given these advantages, this article provides a detailed introduction to the principles of proximity labeling techniques and the development of labeling enzymes. The focus is on summarizing the recent applications of TurboID and miniTurbo in mammals, plants, and microorganisms. Video Abstract.

Atg8ylation as a host-protective mechanism against Mycobacterium tuberculosis

Nearly two decades have passed since the first report on autophagy acting as a cell-autonomous defense against Mycobacterium tuberculosis. This helped usher a new area of research within the field of host-pathogen interactions and led to the recognition of autophagy as an immunological mechanism. Interest grew in the fundamental mechanisms of antimicrobial autophagy and in the prophylactic and therapeutic potential for tuberculosis. However, puzzling in vivo data have begun to emerge in murine models of M. tuberculosis infection. The control of infection in mice affirmed the effects of certain autophagy genes, specifically ATG5, but not of other ATGs. Recent studies with a more complete inactivation of ATG genes now show that multiple ATG genes are indeed necessary for protection against M. tuberculosis. These particular ATG genes are involved in the process of membrane atg8ylation. Atg8ylation in mammalian cells is a broad response to membrane stress, damage and remodeling of which canonical autophagy is one of the multiple downstream outputs. The current developments clarify the controversies and open new avenues for both fundamental and translational studies.

LC3-independent autophagy is vital to prevent TNF cytotoxicity

The (macro)autophagy field is facing a paradigm shift after the recent discovery that cytosolic cargoes can still be selectively targeted to phagophores (the precursors to autophagosomes) even in the absence of LC3 or other Atg8-protein family members. Several in vitro studies have indeed reported on the existence of an unconventional selective autophagic pathway that involves the in-situ formation of an autophagosome around the cargo through the direct selective autophagy receptor-mediated recruitment of RB1CC1/FIP200, thereby bypassing the requirement of LC3. In an article recently published in Science, we demonstrate the physiological importance of this unconventional autophagic pathway in the context of TNF (tumor necrosis factor) signaling. We show that it promotes the degradation of the cytotoxic TNFRSF1A/TNFR1 (TNF receptor superfamily member 1A) complex II that assembles upon TNF sensing and thereby protects mice from TNFRSF1A-driven embryonic lethality and skin inflammation.Abbreviations: ATG: autophagy related; CASP: caspase; FIR: RB1CC1/FIP200-interacting region; LIR: LC3-interacting region; M1: linear; PAS: phagophore assembly site; PtdIns3K: phosphatidylinositol 3-kinase; TNF: tumor necrosis factor; TNFRSF1A: TNF receptor superfamily member 1A.

Data-Dependent Acquisition with Precursor Coisolation Improves Proteome Coverage and Measurement Throughput for Label-Free Single-Cell Proteomics

We combined efficient sample preparation and ultra-low-flow liquid chromatography with a newly developed data acquisition and analysis scheme termed wide window acquisition (WWA) to quantify >3,000 proteins from single cells in rapid label-free analyses. WWA employs large isolation windows to intentionally co-isolate and co-fragment adjacent precursors along with the selected precursor. Optimized WWA increased the number of MS2-identified proteins by ≈40 % relative to standard data-dependent acquisition. For a 40-min LC gradient operated at ≈15 nL/min, we identified an average of 3,524 proteins per single-cell-sized aliquot of protein digest. Reducing the active gradient to 20 min resulted in a modest 10 % decrease in proteome coverage. Using this platform, we compared protein expression between single HeLa cells having an essential autophagy gene, atg9a, knocked out, with their isogenic WT parental line. Similar proteome coverage was observed, and 268 proteins were significantly up- or downregulated. Protein upregulation primarily related to innate immunity, vesicle trafficking and protein degradation.

Kinase regulation by liquid-liquid phase separation

Liquid-liquid phase separation (LLPS) is emerging as a mechanism of spatiotemporal regulation that could answer long-standing questions about how order is achieved in biochemical signaling. In this review we discuss how LLPS orchestrates kinase signaling, either by creating condensate structures that are sensed by kinases or by direct LLPS of kinases, cofactors, and substrates - thereby acting as a mechanism to compartmentalize kinase-substrate relationships, and in some cases also sequestering the kinase away from inhibitory factors. We also examine the possibility that selective pressure promotes genomic rearrangements that fuse pro-growth kinases to LLPS-prone protein sequences, which in turn drives aberrant kinase activation through LLPS.

Quantitative analysis of autophagy reveals the role of ATG9 and ATG2 in autophagosome formation

Autophagy is a catabolic pathway required for the recycling of cytoplasmic materials. To define the mechanisms underlying autophagy it is critical to quantitatively characterize the dynamic behavior of autophagy factors in living cells. Using a panel of cell lines expressing HaloTagged autophagy factors from their endogenous loci, we analyzed the abundance, single-molecule dynamics, and autophagosome association kinetics of autophagy proteins involved in autophagosome biogenesis. We demonstrate that autophagosome formation is inefficient and ATG2-mediated tethering to donor membranes is a key commitment step in autophagosome formation. Furthermore, our observations support the model that phagophores are initiated by the accumulation of autophagy factors on mobile ATG9 vesicles, and that the ULK1 complex and PI3-kinase form a positive feedback loop required for autophagosome formation. Finally, we demonstrate that the duration of autophagosome biogenesis is ∼110 s. In total, our work provides quantitative insight into autophagosome biogenesis and establishes an experimental framework to analyze autophagy in human cells.

Metamorphic proteins at the basis of human autophagy initiation and lipid transfer

Autophagy is a conserved intracellular degradation pathway that generates de novo double-membrane autophagosomes to target a wide range of material for lysosomal degradation. In multicellular organisms, autophagy initiation requires the timely assembly of a contact site between the ER and the nascent autophagosome. Here, we report the in vitro reconstitution of a full-length seven-subunit human autophagy initiation supercomplex built on a core complex of ATG13-101 and ATG9. Assembly of this core complex requires the rare ability of ATG13 and ATG101 to switch between distinct folds. The slow spontaneous metamorphic conversion is rate limiting for the self-assembly of the supercomplex. The interaction of the core complex with ATG2-WIPI4 enhances tethering of membrane vesicles and accelerates lipid transfer of ATG2 by both ATG9 and ATG13-101. Our work uncovers the molecular basis of the contact site and its assembly mechanisms imposed by the metamorphosis of ATG13-101 to regulate autophagosome biogenesis in space and time.

Role of Macroautophagy in Mammalian Male Reproductive Physiology

Physiologically, autophagy is an evolutionarily conserved and self-degradative process in cells. Autophagy carries out normal physiological roles throughout mammalian life. Accumulating evidence shows autophagy as a mechanism for cellular growth, development, differentiation, survival, and homeostasis. In male reproductive systems, normal spermatogenesis and steroidogenesis need a balance between degradation and energy supply to preserve cellular metabolic homeostasis. The main process of autophagy includes the formation and maturation of the phagophore, autophagosome, and autolysosome. Autophagy is controlled by a group of autophagy-related genes that form the core machinery of autophagy. Three types of autophagy mechanisms have been discovered in mammalian cells: macroautophagy, microautophagy, and chaperone-mediated autophagy. Autophagy is classified as non-selective or selective. Non-selective macroautophagy randomly engulfs the cytoplasmic components in autophagosomes that are degraded by lysosomal enzymes. While selective macroautophagy precisely identifies and degrades a specific element, current findings have shown the novel functional roles of autophagy in male reproduction. It has been recognized that dysfunction in the autophagy process can be associated with male infertility. Overall, this review provides an overview of the cellular and molecular basics of autophagy and summarizes the latest findings on the key role of autophagy in mammalian male reproductive physiology.

An affinity-directed phosphatase, AdPhosphatase, system for targeted protein dephosphorylation

Reversible protein phosphorylation, catalyzed by protein kinases and phosphatases, is a fundamental process that controls protein function and intracellular signaling. Failure of phospho-control accounts for many human diseases. While a kinase phosphorylates multiple substrates, a substrate is often phosphorylated by multiple kinases. This renders phospho-control at the substrate level challenging, as it requires inhibition of multiple kinases, which would thus affect other kinase substrates. Here, we describe the development and application of the affinity-directed phosphatase (AdPhosphatase) system for targeted dephosphorylation of specific phospho-substrates. By deploying the Protein Phosphatase 1 or 2A catalytic subunits conjugated to an antigen-stabilized anti-GFP nanobody, we can promote the dephosphorylation of two independent phospho-proteins, FAM83D or ULK1, knocked in with GFP-tags using CRISPR-Cas9, with exquisite specificity. By redirecting protein phosphatases to neo-substrates through nanobody-mediated proximity, AdPhosphatase can alter the phospho-status and function of target proteins and thus, offers a new modality for potential drug discovery approaches.

Structural basis for ATG9A recruitment to the ULK1 complex in mitophagy initiation

The assembly of the autophagy initiation machinery nucleates autophagosome biogenesis, including in the PINK1- and Parkin-dependent mitophagy pathway implicated in Parkinson's disease. The structural interaction between the sole transmembrane autophagy protein, autophagy-related protein 9A (ATG9A), and components of the Unc-51-like autophagy activating kinase (ULK1) complex is one of the major missing links needed to complete a structural map of autophagy initiation. We determined the 2.4-Å x-ray crystallographic structure of the ternary structure of ATG9A carboxyl-terminal tail bound to the ATG13:ATG101 Hop1/Rev7/Mad2 (HORMA) dimer, which is part of the ULK1 complex. We term the interacting portion of the extreme carboxyl-terminal part of the ATG9A tail the "HORMA dimer-interacting region" (HDIR). This structure shows that the HDIR binds to the HORMA domain of ATG101 by β sheet complementation such that the ATG9A tail resides in a deep cleft at the ATG13:ATG101 interface. Disruption of this complex in cells impairs damage-induced PINK1/Parkin mitophagy mediated by the cargo receptor NDP52.

Synaptic vesicle proteins and ATG9A self-organize in distinct vesicle phases within synapsin condensates

Ectopic expression in fibroblasts of synapsin 1 and synaptophysin is sufficient to generate condensates of vesicles highly reminiscent of synaptic vesicle (SV) clusters and with liquid-like properties. Here we show that unlike synaptophysin, other major integral SV membrane proteins fail to form condensates with synapsin, but co-assemble into the clusters formed by synaptophysin and synapsin in this ectopic expression system. Another vesicle membrane protein, ATG9A, undergoes activity-dependent exo-endocytosis at synapses, raising questions about the relation of ATG9A traffic to the traffic of SVs. We find that both in fibroblasts and in nerve terminals ATG9A does not co-assemble into synaptophysin-positive vesicle condensates but localizes on a distinct class of vesicles that also assembles with synapsin but into a distinct phase. Our findings suggest that ATG9A undergoes differential sorting relative to SV proteins and also point to a dual role of synapsin in controlling clustering at synapses of SVs and ATG9A vesicles.

Autophagy Is Required to Sustain Increased Intestinal Cell Proliferation during Phenotypic Plasticity Changes in Honey Bee (Apis mellifera)

Tissue phenotypic plasticity facilitates rapid adaptation of organisms to biotic and/or abiotic pressure. The reproductive capacity of honey bee workers (Apis mellifera) is plastic and responsive to pheromones produced by broods and the queen. Egg laying workers (ELWs), which could reactivate their ovaries and lay haploid eggs upon queen lost, have been commonly discussed from many aspects. However, it remains unclear whether midgut homeostasis in ELWs is affected during plastic changes. Here, we found that the expression of nutrition- and autophagy-related genes was up-regulated in the midguts of ELWs, compared with that in nurse workers (NWs) by RNA-sequencing. Furthermore, the area and number of autophagosomes were increased, along with significantly increased cell death in the midguts of ELWs. Moreover, cell cycle progression in the midguts of ELWs was increased compared with that in NWs. Consistent with the up-regulation of nutrition-related genes, the body and midgut sizes, and the number of intestinal proliferation cells of larvae reared with royal jelly (RJ) obviously increased more than those reared without RJ in vitro. Finally, cell proliferation was dramatically suppressed in the midguts of ELWs when autophagy was inhibited. Altogether, our data suggested that autophagy was induced and required to sustain cell proliferation in ELWs' midguts, thereby revealing the critical role of autophagy played in the intestines during phenotypic plasticity changes.

Intracellular Citrate/acetyl-CoA flux and endoplasmic reticulum acetylation: Connectivity is the answer

BACKGROUND

Key cellular metabolites reflecting the immediate activity of metabolic enzymes as well as the functional metabolic state of intracellular organelles can act as powerful signal regulators to ensure the activation of homeostatic responses. The citrate/acetyl-CoA pathway, initially recognized for its role in intermediate metabolism, has emerged as a fundamental branch of this nutrient-sensing homeostatic response. Emerging studies indicate that fluctuations in acetyl-CoA availability within different cellular organelles and compartments provides substrate-level regulation of many biological functions. A fundamental aspect of these regulatory functions involves Nε-lysine acetylation.

SCOPE OF REVIEW

Here, we will examine the emerging regulatory functions of the citrate/acetyl-CoA pathway and the specific role of the endoplasmic reticulum (ER) acetylation machinery in the maintenance of intracellular crosstalk and homeostasis. These functions will be analyzed in the context of associated human diseases and specific mouse models of dysfunctional ER acetylation and citrate/acetyl-CoA flux. A primary objective of this review is to highlight the complex yet integrated response of compartment- and organelle-specific Nε-lysine acetylation to the intracellular availability and flux of acetyl-CoA, linking this important post-translational modification to cellular metabolism.

MAJOR CONCLUSIONS

The ER acetylation machinery regulates the proteostatic functions of the organelle as well as the metabolic crosstalk between different intracellular organelles and compartments. This crosstalk enables the cell to impart adaptive responses within the ER and the secretory pathway. However, it also enables the ER to impart adaptive responses within different cellular organelles and compartments. Defects in the homeostatic balance of acetyl-CoA flux and ER acetylation reflect different but converging disease states in humans as well as converging phenotypes in relevant mouse models. In conclusion, citrate and acetyl-CoA should not only be seen as metabolic substrates of intermediate metabolism but also as signaling molecules that direct functional adaptation of the cell to both intracellular and extracellular messages. Future discoveries in CoA biology and acetylation are likely to yield novel therapeutic approaches.

Mapping the proximity interactome of ATG9A reveals unexpected dynamics of ULK1 complex proteins

ATG9A is essential for macroautophagy/autophagy and considered to be one of the earliest ATG (autophagy related) proteins recruited to sites of autophagosome biogenesis. Recent data suggest ATG9A vesicles may even form the lipid seed of the autophagosome. However, ATG9A regulation is still poorly understood, which is likely at least partly due to challenges inherent to studying an intracellular transmembrane protein with no apparent enzymatic activity. To help overcome these challenges, we used BioID and quantitative LC-MS/MS to map the proximity interactome of ATG9A, which included entire protein complexes involved in protein trafficking, and proteins implicated in autophagy but previously lacking any physical link to core autophagy machinery. We also unexpectedly found an ATG9A interaction with an ULK1-independent ATG13-ATG101 dimer that promotes autophagy in fed cells.