bims-auttor Biomed News
on Autophagy and mTOR
Issue of 2019‒06‒23
twenty papers selected by
Viktor Korolchuk, Newcastle University

  1. Autophagy. 2019 Jun 17.
      ULK1 (unc-51 like autophagy activating kinase 1) is the key mediator of MTORC1 signaling to macroautophagy/autophagy. ULK1 functions as a protein complex by interacting with ATG13, RB1CC1/FIP200, and ATG101. How the ULK1 complex is regulated to trigger autophagy induction remains unclear. In this study, we have determined roles of Atg8-family proteins (ATG8s) in regulating ULK1 activity and autophagy. Using human cells depleted of each subfamily of ATG8, we found that the GABARAP subfamily positively regulates ULK1 activity and phagophore and autophagosome formation in response to starvation. In contrast, the LC3 subfamily negatively regulates ULK1 activity and phagophore formation. By reconstituting ATG8-depleted cells with individual ATG8 members, we identified GABARAP and GABARAPL1 as positive and LC3B and LC3C as negative regulators of ULK1 activity. To address the role of ATG8 binding to ULK1, we mutated the LIR of endogenous ULK1 to disrupt the ATG8-ULK1 interaction by genome editing. The mutation drastically reduced the activity of ULK1, autophagic degradation of SQSTM1, and phagophore formation in response to starvation. The mutation also suppressed the formation and turnover of autophagosomes in response to starvation. Similar to the mutation of the ULK1 LIR, disruption of the ATG13-ATG8 interaction suppressed ULK1 activity and autophagosome formation. In contrast, RB1CC1 did not show any specific binding to ATG8s, and mutation of its LIR did not affect ULK1 activity. Together, this study demonstrates differential binding and opposite regulation of the ULK1 complex by GABARAPs and LC3s, and an important role of the ULK1- and ATG13-ATG8 interactions in autophagy induction.
    Keywords:  ATG8; GABARAP; LC3; LIR; ULK1
  2. J Mol Biol. 2019 Jun 13. pii: S0022-2836(19)30352-3. [Epub ahead of print]
      Eukaryotic cells have the capacity to degrade intracellular components through a lysosomal degradation pathway called macroautophagy (henceforth referred to as autophagy) in which superfluous or damaged cytosolic entities are engulfed and separated from the rest of the cell constituents into double membraned vesicles known as autophagosomes. Autophagosomes then fuse with endosomes and lysosomes, where cargo is broken down into basic building blocks that are released to the cytoplasm for the cell to reuse. Autophagic degradation can target either cytoplasmic material in bulk (non-selective autophagy) or particular cargo in what is called selective autophagy. Proper autophagic turnover requires the orchestrated participation of several players that need to be tightly and temporally coordinated. Whereas a large number of autophagy-related (ATG) proteins have been identified and their functions and regulation are starting to be understood, there is substantially less knowledge regarding the specific lipids constituting the autophagic membranes as well as their role in initiating, enabling or regulating the autophagic process. This review focuses on lipids and their corresponding binding proteins that are crucial in the process of selective autophagy.
    Keywords:  lipid regulation; lipid-binding domain; lipid-protein interaction; membrane; mitophagy
  3. Cell Signal. 2019 Jun 12. pii: S0898-6568(19)30140-8. [Epub ahead of print]62 109344
      Target of rapamycin complex 1 (TORC1) protein kinase responds to various stresses including genotoxic stress. However, its molecular mechanism is poorly understood. Here, we show that DNA damage induces nonselective and selective autophagy in budding yeast. DNA damage caused the attenuation of TORC1 activity, dephosphorylation of Atg13, and autophagy induction. The TORC1-upstream Rag GTPase Gtr1 was not required for TORC1 inactivation and autophagy induction after DNA damage. Furthermore, DNA damage responsive protein kinases Mec1/ATM and Tel1/ATR, and stress-responsive mitogen-activated protein kinase Mpk1/Slt2 were required for the full induction of autophagy. Autophagic proteolysis was required for DNA damage tolerance in TORC1 inactive conditions. This study revealed that multiple protein kinases regulate DNA damage-induced autophagy.
    Keywords:  Autophagy; DNA damage; Mec1; Mpk1; TORC1; Tel1
  4. Proc Natl Acad Sci U S A. 2019 Jun 18. pii: 201901039. [Epub ahead of print]
      BRUCE/Apollon is a membrane-associated inhibitor of apoptosis protein that is essential for viability and has ubiquitin-conjugating activity. On initiation of apoptosis, the ubiquitin ligase Nrdp1/RNF41 promotes proteasomal degradation of BRUCE. Here we demonstrate that BRUCE together with the proteasome activator PA28γ causes proteasomal degradation of LC3-I and thus inhibits autophagy. LC3-I on the phagophore membrane is conjugated to phosphatidylethanolamine to form LC3-II, which is required for the formation of autophagosomes and selective recruitment of substrates. SIP/CacyBP is a ubiquitination-related protein that is highly expressed in neurons and various tumors. Under normal conditions, SIP inhibits the ubiquitination and degradation of BRUCE, probably by blocking the binding of Nrdp1 to BRUCE. On DNA damage by topoisomerase inhibitors, Nrdp1 causes monoubiquitination of SIP and thus promotes apoptosis. However, on starvation, SIP together with Rab8 enhances the translocation of BRUCE into the recycling endosome, formation of autophagosomes, and degradation of BRUCE by optineurin-mediated autophagy. Accordingly, deletion of SIP in cultured cells reduces the autophagic degradation of damaged mitochondria and cytosolic protein aggregates. Thus, by stimulating proteasomal degradation of LC3-I, BRUCE also inhibits autophagy. Conversely, SIP promotes autophagy by blocking BRUCE-dependent degradation of LC3-I and by enhancing autophagosome formation and autophagic destruction of BRUCE. These actions of BRUCE and SIP represent mechanisms that link the regulation of autophagy and apoptosis under different conditions.
    Keywords:  BRUCE/Apollon; CacyBP/SIP; LC3; apoptosis; autophagy
  5. J Mol Biol. 2019 Jun 17. pii: S0022-2836(19)30377-8. [Epub ahead of print]
      Cells are constantly challenged by endogenous and exogenous stress sources. To cope with them, organisms have developed a series of defensive mechanism to prevent and intercept the threats and to repair the generated damage. Autophagy, once defined as a waste-disposal or non-specific degradative pathway, has arisen as a new organizer of the different physiological stress responses. In the present review we will discuss how autophagy is capable of orchestrate these pathways by the specific degradation of individual autophagosomal LC3/GABARAP-binding proteins, rather than the bulk degradation of harmful products or organelles.
    Keywords:  Autophagy; CRY1; NCoR1; TRIM5; p62/SQSTM1
  6. Nat Chem Biol. 2019 Jun 20.
      Autophagy mediates the degradation of damaged proteins, organelles and pathogens, and plays a key role in health and disease. Thus, the identification of new mechanisms involved in the regulation of autophagy is of major interest. In particular, little is known about the role of lipids and lipid-binding proteins in the early steps of autophagosome biogenesis. Using target-agnostic, high-content, image-based identification of indicative phenotypic changes induced by small molecules, we have identified autogramins as a new class of autophagy inhibitor. Autogramins selectively target the recently discovered cholesterol transfer protein GRAM domain-containing protein 1A (GRAMD1A, which had not previously been implicated in autophagy), and directly compete with cholesterol binding to the GRAMD1A StART domain. GRAMD1A accumulates at sites of autophagosome initiation, affects cholesterol distribution in response to starvation and is required for autophagosome biogenesis. These findings identify a new biological function of GRAMD1A and a new role for cholesterol in autophagy.
  7. Autophagy. 2019 Jun 17.
      Genetic screens have identified two sets of genes that act at distinct steps of basal autophagy in higher eukaryotes: the pan-eukaryotic ATG genes and the metazoan-specific EPG genes. Very little is known about whether these core macroautophagy/autophagy genes are differentially employed during multicellular organism development. Here we analyzed the function of core autophagy genes in autophagic removal of SQST-1/SQSTM1 during C. elegans development. We found that loss of function of genes acting at distinct steps in the autophagy pathway causes different patterns of SQST-1 accumulation in different tissues and developmental stages. We also identified that the calpain protease clp-2 acts in a cell context-specific manner in SQST-1 degradation. clp-2 is required for degradation of SQST-1 in the hypodermis and neurons, but is dispensable in the body wall muscle and intestine. Our results indicate that autophagy genes are differentially employed in a tissue- and stage-specific manner during the development of multicellular organisms.
    Keywords:  ; gene; gene, ; aggrephagy
  8. J Biol Chem. 2019 Jun 17. pii: jbc.RA118.007020. [Epub ahead of print]
      Autophagy, a membrane-dependent catabolic process, ensures survival of aging cells and depends on the cellular energetic status. Acetyl-coenzyme A carboxylase 1 (Acc1) connects central energy- to lipid metabolism and is rate limiting for the de novo synthesis of lipids. However, it is unclear how de novo lipogenesis and its metabolic consequences affect autophagic activity. Here we show that in aging yeast, autophagy levels highly depend on the activity of Acc1. Constitutively active Acc1 or a deletion of the Acc1 negative regulator, Snf1 (yeast AMPK), show elevated autophagy levels, which can be reversed by the Acc1 inhibitor soraphen A. Vice versa, pharmacological inhibition of Acc1 drastically reduces cell survival and results in the accumulation of Atg8-positive structures at the vacuolar membrane, suggesting late defects in the autophagic cascade. As expected, acc1S/A cells exhibit a reduction in acetate/acetyl-CoA availability along with elevated cellular lipid content. However, concomitant administration of acetate fails to fully revert the increase in autophagy exerted by acc1S/A. Instead, administration of oleate - while mimicking constitutively active Acc1 in wild-type cells - alleviates the vacuolar fusion defects induced by Acc1 inhibition. Our results argue for a largely lipid-dependent process of autophagy regulation downstream of Acc1. We present a versatile genetic model to investigate the complex relationship between acetate metabolism, lipid homeostasis and autophagy, and propose Acc1-dependent lipogenesis as a fundamental metabolic path downstream of Snf1 to maintain autophagy and survival during cellular aging.
    Keywords:  AMPK; Acc1; Acetyl-CoA carboxylase 1; Snf1; acetyl coenzyme A (acetyl-CoA); aging; autophagy; lipid metabolism; oleate; yeast
  9. Pflugers Arch. 2019 Jun 21.
      Senescent cells, which are cells in a post-proliferative state, show an increased number of dysfunctional mitochondria and oxidatively damaged and aggregated proteins. The mitochondrial-lysosomal axis theory of aging proposes that the autophago-lysosomal system is unable to cope with the rising amount of damaged organelles and proteins. We used human umbilical vein endothelial cells (HUVEC) as in vitro model system to determine which part/s of the autophago-lysosomal pathway become deficient by aging. Senescent HUVEC contained a much larger population of autophagosomes and lysosomes compared to young cells. Transcriptome analysis comparing young and old cells demonstrated several age-related changes of autophagy gene expression. One reason for the observed increase of autophagosomes was an impairment of the autophagic flux in senescent cells due to reduced V-ATPase activity required for acidification of the lysosomes and thus functionality of lysosomal hydrolases. The hypothesis that reduced mitochondrial ATP production underlies low V-ATPase activity was supported by addition of exogenous ATP. This procedure rescued the lysosomal acidification and restored the autophagic flux. Thus, we propose impaired lysosomal acidification due to ATP shortage which may result from mitochondrial dysfunction as a mechanism underlying the accumulation of dysfunctional cellular constituents during aging.
    Keywords:  ATP; Aging; Autophagic flux; Lysosomes; Mitochondria; V-ATPase
  10. Oxid Med Cell Longev. 2019 ;2019 1283075
      Reactive oxygen species (ROS) and autophagy are two highly complex and interrelated components of cell physiopathology, but our understanding of their integration and their contribution to cell homeostasis and disease is still limited. Sestrins (SESNs) belong to a family of highly conserved stress-inducible proteins that orchestrate antioxidant and autophagy-regulating functions protecting cells from various noxious stimuli, including DNA damage, oxidative stress, hypoxia, and metabolic stress. They are also relevant modulators of metabolism as positive regulators of the key energy sensor AMP-dependent protein kinase (AMPK) and inhibitors of mammalian target of rapamycin complex 1 (mTORC1). Since perturbations in these pathways are central to multiple disorders, SESNs might constitute potential novel therapeutic targets of broad interest. In this review, we discuss the current understanding of regulatory and effector networks of SESNs, highlighting their significance as potential biomarkers and therapeutic targets for different diseases, such as aging-related diseases, metabolic disorders, neurodegenerative diseases, and cancer.
  11. EMBO J. 2019 Jun 17. pii: e100978. [Epub ahead of print]
      Viral infection triggers the formation of mitochondrial antiviral signaling protein (MAVS) aggregates, which potently promote immune signaling. Autophagy plays an important role in controlling MAVS-mediated antiviral signaling; however, the exact molecular mechanism underlying the targeted autophagic degradation of MAVS remains unclear. Here, we investigated the mechanism by which RNF34 regulates immunity and mitophagy by targeting MAVS RNF34 binds to MAVS in the mitochondrial compartment after viral infection and negatively regulates RIG-I-like receptor (RLR)-mediated antiviral immunity. Moreover, RNF34 catalyzes the K27-/K29-linked ubiquitination of MAVS at Lys 297, 311, 348, and 362 Arg, which serves as a recognition signal for NDP52-dependent autophagic degradation. Specifically, RNF34 initiates the K63- to K27-linked ubiquitination transition on MAVS primarily at Lys 311, which facilitates the autophagic degradation of MAVS upon RIG-I stimulation. Notably, RNF34 is required for the clearance of damaged mitochondria upon viral infection. Thus, we elucidated the mechanism by which RNF34-mediated autophagic degradation of MAVS regulates the innate immune response, mitochondrial homeostasis, and infection.
    Keywords:   MAVS ; RNF34; innate immune response; selective mitophagy; ubiquitination
  12. Cell Stress. 2018 Mar 07. 2(3): 55-65
      Autophagy is an evolutionarily conserved process that degrades subcellular constituents. Mammalian cells undergo two types of autophagy; Atg5-dependent conventional autophagy and Atg5-independent alternative autophagy, and the molecules required for the latter type of autophagy are largely unknown. In this study, we analyzed the molecular mechanisms of genotoxic stress-induced alternative autophagy, and identified the essential role of p53 and damage-regulated autophagy modulator (Dram1). Dram1 was sufficient to induce alternative autophagy. In the mechanism of alternative autophagy, Dram1 functions in the closure of isolation membranes downstream of p53. These findings indicate that Dram1 plays a pivotal role in genotoxic stress-induced alternative autophagy.
    Keywords:  DNA damage response; Dram1; alternative autophagy; genotoxic stress; p53
  13. Dev Cell. 2019 May 31. pii: S1534-5807(19)30420-4. [Epub ahead of print]
      Serum starvation stimulates cilia growth in cultured cells, yet serum factors associated with ciliogenesis are unknown. Previously, we showed that starvation induces rapid Rab11-dependent vesicular trafficking of Rabin8, a Rab8 guanine-nucleotide exchange factor (GEF), to the mother centriole, leading to Rab8 activation and cilium growth. Here, we demonstrate that through the LPA receptor 1 (LPAR1), serum lysophosphatidic acid (LPA) inhibits Rab11a-Rabin8 interaction and ciliogenesis. LPA/LPAR1 regulates ciliogenesis initiation via downstream PI3K/Akt activation, independent of effects on cell cycle. Akt stabilizes Rab11a binding to its effector, WDR44, and a WDR44-pAkt-phosphomimetic mutant blocks ciliogenesis. WDR44 depletion promotes Rabin8 preciliary trafficking and ciliogenesis-initiating events at the mother centriole. Our work suggests disruption of Akt signaling causes a switch from Rab11-WDR44 to the ciliogenic Rab11-FIP3-Rabin8 complex. Finally, we demonstrate that Akt regulates downstream ciliogenesis processes associated with Rab8-dependent cilia growth. Together, this study uncovers a mechanism whereby serum mitogen signaling regulates Rabin8 preciliary trafficking and ciliogenesis initiation.
    Keywords:  Akt; FIP3; LPA; MC; Rab11 effector switch; Rabin8; WDR44; ciliogenesis; lysophosphatidic acid; mother centriole; phosphorylation; preciliary trafficking
  14. Genes Cells. 2019 Jun 18.
      Maintaining protein homeostasis is central to cell survival. The ubiquitin-proteasome system and autophagy play pivotal roles in protein quality control through protein degradation. Activities of these degradative pathways are carefully orchestrated, and autophagy is upregulated during proteasome dysfunction for cellular homeostasis. However, the mechanism by which proteasome impairment induces compensatory autophagy has remained largely elusive. Here, we show that FAM48A mediates autophagy induction during proteasome inhibition. FAM48A is degraded by the proteasome and accumulates in cells by proteasome inhibition. Knockdown of FAM48A led to defective induction of autophagy during proteasome inhibition, accompanied by defective localization of Atg9 on recycling endosomes. Our results indicate that FAM48A is a kind of sensor that is required for compensatory autophagy induction upon proteasome impairment. This article is protected by copyright. All rights reserved.
  15. Cell Stress. 2019 Mar 15. 3(4): 110-114
      Mitochondria have relatively independent protein quality control systems, including their own chaperones for protein folding and AAA proteases for protein degradation. Accumulating evidence has shown that cytosolic proteins and disease-causing misfolded proteins can be translocated into mitochondria and then impinge upon their function. It is important to understand the interplay between cellular proteostasis and mitochondria, as impaired proteostasis and mitochondrial dysfunction are causally linked with aging and age-related disorders. This review highlights our recent finding showing that the outer mitochondrial membrane protein FUNDC1, a previously reported mitophagy receptor, interacts with the chaperone protein HSC70 to mediate the mitochondrial translocation of cytosolic proteasomal substrates via the TOM/TIM complex into the mitochondrial matrix where they can be degraded by LONP1 protease. Excessive accumulation of unfolded proteins within mitochondria triggers the formation of Mitochondrion-Associated Protein Aggregates (MAPAs), which are subsequently autophagically degraded in a FUNDC1-dependent manner. We suggest that mitochondria actively organize the cellular proteostatic response and that the interaction between FUNDC1 and HSC70 may represent a new link between impaired proteostasis, mitochondrial dysfunction and cellular aging.
    Keywords:  cellular senescence; mitochondrial quality control; proteostasis
  16. Cell Metab. 2019 Jun 07. pii: S1550-4131(19)30255-4. [Epub ahead of print]
      Aging is a significant risk factor for impaired tissue functions and chronic diseases. Age-associated decline in systemic NAD+ availability plays a critical role in regulating the aging process across many species. Here, we show that the circulating levels of extracellular nicotinamide phosphoribosyltransferase (eNAMPT) significantly decline with age in mice and humans. Increasing circulating eNAMPT levels in aged mice by adipose-tissue-specific overexpression of NAMPT increases NAD+ levels in multiple tissues, thereby enhancing their functions and extending healthspan in female mice. Interestingly, eNAMPT is carried in extracellular vesicles (EVs) through systemic circulation in mice and humans. EV-contained eNAMPT is internalized into cells and enhances NAD+ biosynthesis. Supplementing eNAMPT-containing EVs isolated from young mice significantly improves wheel-running activity and extends lifespan in aged mice. Our findings have revealed a novel EV-mediated delivery mechanism for eNAMPT, which promotes systemic NAD+ biosynthesis and counteracts aging, suggesting a potential avenue for anti-aging intervention in humans.
    Keywords:  EV; NAD+; adipose tissue; aging; eNAMPT; exosome; extracellular vesicle; hypothalamus; longevity; metabolism
  17. Autophagy. 2019 Jun 16. 1-11
      Chondrogenesis is accompanied by not only cellular renovation, but also metabolic stress. Therefore, macroautophagy/autophagy is postulated to be involved in this process. Previous reports have shown that suppression of autophagy during chondrogenesis causes mild growth retardation. However, the role of autophagy in chondrocyte differentiation still largely remains unclear. Here, we show the important role of autophagy on chondrogenesis. The transition of mesenchymal cells to chondrocytes was severely impaired by ablation of Atg7, a gene essential for autophagy. Mice lacking Atg7 after the transition exhibited phenotypes severer than mutant mice in which Atg7 was removed before the transition. Atg7-deficient chondrocytes accumulated large numbers of glycogen granules, hardly proliferate and died specifically in the proliferative zone without any ER-stress signal. Our results suggest that the suppression of autophagy in prechondrogenic cells drives compensatory mechanism(s) that mitigate defective chondrogenesis, and that autophagy participates in glycogenolysis to supply glucose in avascular growth plates. Abbreviations: DDIT3/CHOP: DNA damage inducible transcript 3; ER: endoplasmic reticulum; NFE2L2/NRF2: nuclear factor, erythroid derived 2, like 2; SQSTM1/p62: sequestosome 1; STBD1: starch-binding domain-containing protein 1.
    Keywords:  Atg7; autophagy; cartilage; chondrogenesis; glycogenolysis
  18. FEBS Lett. 2019 Jun 18.
      Cellular senescence and mitochondrial dysfunction have both been defined as classical hallmarks of the ageing process. Here, we review the intricate relationship between the two. In the context of ageing, it is now well regarded that cellular senescence is a key driver in both ageing and the onset of a number of age-related pathologies. Emerging evidence has pinpointed mitochondria as one of the key modulators in the development of the senescence phenotype, particularly the pro-inflammatory senescence associated secretory phenotype (SASP). This review focuses on the contribution of homeostatic mechanisms, as well as of reactive oxygen species (ROS) and mitochondrial metabolites in the senescence programme. Furthermore, we discuss emerging pathways and mitochondrial-mediated mechanisms that may be influencing the SASP and, subsequently, explore how these may be exploited to open up new therapeutic avenues. This article is protected by copyright. All rights reserved.
    Keywords:  Ageing; Mitochondria; Senescence; Senolytics; Senostatics
  19. Cell Stress. 2018 Oct 25. 2(11): 282-291
      In this review we analyze the ability of antipsychotic medications to modulate macroautophagy, a process of controlled lysosomal digestion of cellular macromolecules and organelles. We focus on its molecular mechanisms, consequences for the function/survival of neuronal and other cells, and the contribution to the beneficial and side-effects of antipsychotics in the treatment of schizophrenia, neurodegeneration, and cancer. A wide range of antipsychotics was able to induce neuronal autophagy as a part of the adaptive stress response apparently independent of mammalian target of rapamycin and dopamine receptor blockade. Autophagy induction by antipsychotics could contribute to reducing neuronal dysfunction in schizophrenia, but also to the adverse effects associated with their long-term use, such as brain volume loss and weight gain. In neurodegenerative diseases, antipsychotic-stimulated autophagy might help to increase the clearance and reduce neurotoxicity of aggregated proteotoxins. However, the possibility that some antipsychotics might block autophagic flux and potentially contribute to proteotoxin-mediated neurodegeneration must be considered. Finally, the anticancer effects of autophagy induction by antipsychotics make plausible their repurposing as adjuncts to standard cancer therapy.
  20. Free Radic Res. 2019 Jun 21. 1-261
      Acute kidney injury (AKI) is a major kidney disease associated with poor clinical outcomes. Oxidative stress is predominantly involved in the pathogenesis of AKI. Autophagy and the Keap1-Nrf2 signaling pathway are both involved in the oxidative-stress response. However, the cross talk between these two pathways in AKI remains unknown. Here, we found that autophagy is upregulated during cisplatin-induced AKI. In contrast with previous studies, we observed a marked increase in p62. We also found that p62 knockdown reduces autophagosome formation and the expression of LC3II. To explore the cross talk between p62 and the Keap1-Nrf2 signaling pathway, HK-2 cells were transfected with siRNA targeting Nrf2, and we found that Nrf2 knockdown significantly reduced cisplatin-induced p62 expression. Moreover, p62 knockdown significantly decreased the protein expression of Nrf2, as well as Heme Oxygenase-1 (HO-1) and NAD(P)H:quinone oxidoreductase l (NQO1), whereas the expression of kelch-like ECH-associated protein 1 (Keap1) was upregulated. These results indicate that p62 creates a positive feedback loop in the Keap1-Nrf2 signaling pathway. Finally, we examined the role of p62 in cell protection during cisplatin-induced oxidative stress, and we found that p62 silencing in HK-2 cells increases apoptosis and ROS levels, which further indicates the protective role of p62 under oxidative stress and suggests that the cytoprotection 62 mediated is in part by regulating autophagic activity or the Keap1-Nrf2 signaling pathway. Taken together, our results have demonstrated a reciprocal regulation of p62, autophagy and the Keap1-Nrf2 signaling pathway under oxidative stress, which may be a potential therapeutic target against AKI.
    Keywords:  Acute kidney injury; Keap1-Nrf2 signaling pathway; autophagy; oxidative stress; p62/SQSTM1