bims-auttor Biomed News
on Autophagy and mTOR
Issue of 2020–04–12
27 papers selected by
Viktor Korolchuk, Newcastle University
  1. Identification of a phosphorylation site on Ulk1 required for genotoxic stress-induced alternative autophagy.
  2. Acetylation of STX17 (syntaxin 17) controls autophagosome maturation.
  3. The Dynein Adaptor RILP Controls Neuronal Autophagosome Biogenesis, Transport, and Clearance.
  4. Lysosomal Exocytosis, Exosome Release and Secretory Autophagy: The Autophagic- and Endo-Lysosomal Systems Go Extracellular.
  5. The AMPK-MFN2 Axis Regulates MAM Dynamics and Autophagy Induced by Energy Stresses.
  6. A Conserved LIR Motif in Connexins Mediates Ubiquitin-Independent Binding to LC3/GABARAP Proteins.
  7. Pathophysiological Role of Transient Receptor Potential Mucolipin Channel 1 in Calcium-Mediated Stress-Induced Neurodegenerative Diseases.
  8. Recent advances in the understanding of autophagosome biogenesis.
  9. The cholesterol 24-hydroxylase activates autophagy and decreases mutant huntingtin build-up in a neuroblastoma culture model of Huntington's disease.
  10. Mitophagy: An Emerging Role in Aging and Age-Associated Diseases.
  11. Selective degradation of IKKα by autophagy is essential for arsenite-induced cancer cell apoptosis.
  12. Autophagy Suppresses Breast Cancer Metastasis By Degrading NBR1.
  13. Mammalian Mitophagosome Formation: A Focus on the Early Signals and Steps.
  14. The Warburg Micro Syndrome associated Rab3GAP-Rab18 module promotes autolysosome maturation through the Vps34 Complex I.
  15. Autophagy: an essential but limited cellular process for timely skeletal muscle recovery from injury.
  16. Pharmacological inhibition of mTOR attenuates replicative cell senescence and improves cellular function via regulating the STAT3-PIM1 axis in human cardiac progenitor cells.
  17. Autophagy in the mammalian nervous system: a primer for neuroscientists.
  18. Advanced glycation end products reduce macrophage-mediated killing of Staphylococcus aureus by ARL8 upregulation and inhibition of autolysosome formation.
  19. Differential expression of MAGEA6 toggles autophagy to promote pancreatic cancer progression.
  20. The clearance of dead cells by efferocytosis.
  21. Mitochondria-Derived H2O2 Promotes Symmetry Breaking of the C. elegans Zygote.
  22. Altered Proteostasis in Neurodegenerative Tauopathies.
  23. Autophagy-related protein PIK3C3/VPS34 controls T cell metabolism and function.
  24. p53-mediated control of aspartate-asparagine homeostasis dictates LKB1 activity and modulates cell survival.
  25. The functions of Atg8-family proteins in autophagy and cancer: linked or unrelated?
  26. Inhibition of HDAC6 activity protects dopaminergic neurons from alpha-synuclein toxicity.
  27. Lysosome-targeting ratiometric fluorescent pH probes based on long-wavelength BODIPY.
  1. Nat Commun. 2020 Apr 09. 11(1): 1754
    Identification of a phosphorylation site on Ulk1 required for genotoxic stress-induced alternative autophagy.
    Satoru Torii, Hirofumi Yamaguchi, Akira Nakanishi, Satoko Arakawa, Shinya Honda, Kenta Moriwaki, Hiroyasu Nakano, Shigeomi Shimizu.
      Alternative autophagy is an autophagy-related protein 5 (Atg5)-independent type of macroautophagy. Unc51-like kinase 1 (Ulk1) is an essential initiator not only for Atg5-dependent canonical autophagy but also for alternative autophagy. However, the mechanism as to how Ulk1 differentially regulates both types of autophagy has remained unclear. In this study, we identify a phosphorylation site of Ulk1 at Ser746, which is phosphorylated during genotoxic stress-induced alternative autophagy. Phospho-Ulk1746 localizes exclusively on the Golgi and is required for alternative autophagy, but not canonical autophagy. We also identify receptor-interacting protein kinase 3 (RIPK3) as the kinase responsible for genotoxic stress-induced Ulk1746 phosphorylation, because RIPK3 interacts with and phosphorylates Ulk1 at Ser746, and loss of RIPK3 abolishes Ulk1746 phosphorylation. These findings indicate that RIPK3-dependent Ulk1746 phosphorylation on the Golgi plays a pivotal role in genotoxic stress-induced alternative autophagy.
    DOI:  https://doi.org/10.1038/s41467-020-15577-2
  2. Autophagy. 2020 Apr 07.
    Acetylation of STX17 (syntaxin 17) controls autophagosome maturation.
    Qiuhong Shen, Yin Shi, Jiaqi Liu, Hua Su, Jingtao Huang, Yi Zhang, Chao Peng, Tianhua Zhou, Qiming Sun, Wei Wan, Wei Liu.
      The fusion of autophagosomes and endosomes/lysosomes, also called autophagosome maturation, ensures the degradation of autophagic cargoes. It is an important regulatory step of the macroautophagy/autophagy process. STX17 is the key autophagosomal SNARE protein that mediates autophagosome maturation. Here, we report that the acetylation of STX17 regulates its SNARE activity and autophagic degradation. The histone acetyltransferase CREBBP/CBP and the deacetylase HDAC2 specifically regulate the acetylation of STX17. In response to cell starvation and MTORC1 inhibition, the inactivation of CREBBP leads to the deacetylation of STX17 at its SNARE domain. This deacetylation promotes the interaction between STX17 and SNAP29 and the formation of the STX17-SNAP29-VAMP8 SNARE complex with no effect on the recruitment of STX17 to autophagosomal membranes. Deacetylation of STX17 also enhances the interaction between STX17 and the tethering complex HOPS, thereby further promoting autophagosome-lysosome fusion. Our study suggests a mechanism by which acetylation regulates the late-stage of autophagy, and possibly other STX17-related intracellular membrane fusion events.
    Keywords:  CREBBP; HDAC2; HOPS; SNARE complex; STX17; acetylation; autophagosome maturation; autophagy
    DOI:  https://doi.org/10.1080/15548627.2020.1752471
  3. Dev Cell. 2020 Apr 02. pii: S1534-5807(20)30218-5. [Epub ahead of print]
    The Dynein Adaptor RILP Controls Neuronal Autophagosome Biogenesis, Transport, and Clearance.
    Noopur V Khobrekar, Sebastian Quintremil, Tiago J Dantas, Richard B Vallee.
      Autophagy plays critical roles in neurodegeneration and development, but how this pathway is organized and regulated in neurons remains poorly understood. Here, we find that the dynein adaptor RILP is essential for retrograde transport of neuronal autophagosomes, and surprisingly, their biogenesis as well. We find that induction of autophagy by mTOR inhibition specifically upregulates RILP expression and its localization to autophagosomes. RILP depletion or mutations in its LC3-binding LIR motifs strongly decrease autophagosome numbers suggesting an unexpected RILP role in autophagosome biogenesis. We find that RILP also interacts with ATG5 on isolation membranes, precluding premature dynein recruitment and autophagosome transport. RILP inhibition impedes autophagic turnover and causes p62/sequestosome-1 aggregation. Together, our results identify an mTOR-responsive neuronal autophagy pathway, wherein RILP integrates the processes of autophagosome biogenesis and retrograde transport to control autophagic turnover. This pathway has important implications for understanding how autophagy contributes to neuronal function, development, and disease.
    Keywords:  RILP; autophagosome biogenesis; dynein; isolation membranes; mTOR regulation; neuronal autophagy; retrograde transport; sequestosome-1/p62
    DOI:  https://doi.org/10.1016/j.devcel.2020.03.011
  4. Int J Mol Sci. 2020 Apr 08. pii: E2576. [Epub ahead of print]21(7):
    Lysosomal Exocytosis, Exosome Release and Secretory Autophagy: The Autophagic- and Endo-Lysosomal Systems Go Extracellular.
    Sandra Buratta, Brunella Tancini, Krizia Sagini, Federica Delo, Elisabetta Chiaradia, Lorena Urbanelli, Carla Emiliani.
      Beyond the consolidated role in degrading and recycling cellular waste, the autophagic- and endo-lysosomal systems play a crucial role in extracellular release pathways. Lysosomal exocytosis is a process leading to the secretion of lysosomal content upon lysosome fusion with plasma membrane and is an important mechanism of cellular clearance, necessary to maintain cell fitness. Exosomes are a class of extracellular vesicles originating from the inward budding of the membrane of late endosomes, which may not fuse with lysosomes but be released extracellularly upon exocytosis. In addition to garbage disposal tools, they are now considered a cell-to-cell communication mechanism. Autophagy is a cellular process leading to sequestration of cytosolic cargoes for their degradation within lysosomes. However, the autophagic machinery is also involved in unconventional protein secretion and autophagy-dependent secretion, which are fundamental mechanisms for toxic protein disposal, immune signalling and pathogen surveillance. These cellular processes underline the crosstalk between the autophagic and the endosomal system and indicate an intersection between degradative and secretory functions. Further, they suggest that the molecular mechanisms underlying fusion, either with lysosomes or plasma membrane, are key determinants to maintain cell homeostasis upon stressing stimuli. When they fail, the accumulation of undigested substrates leads to pathological consequences, as indicated by the involvement of autophagic and lysosomal alteration in human diseases, namely lysosomal storage disorders, age-related neurodegenerative diseases and cancer. In this paper, we reviewed the current knowledge on the functional role of extracellular release pathways involving lysosomes and the autophagic- and endo-lysosomal systems, evaluating their implication in health and disease.
    Keywords:  amphisomes; autophagosomes; exosomes; extracellular vesicles; lysosomal exocytosis; secretory autophagy; unconventional protein secretion
    DOI:  https://doi.org/10.3390/ijms21072576
  5. Autophagy. 2020 Apr 05.
    The AMPK-MFN2 Axis Regulates MAM Dynamics and Autophagy Induced by Energy Stresses.
    Yongquan Hu, Hao Chen, Luying Zhang, Xiaoying Lin, Xia Li, Haixia Zhuang, Hualin Fan, Tian Meng, Zhengjie He, Haofeng Huang, Qing Gong, Dongxing Zhu, Yiming Xu, Pengcheng He, Longxuan Li, Du Feng.
      Energy deprivation activates the cellular energy sensor AMP-activated protein kinase (AMPK), which in turn induces macroautophagy/autophagy. The mitochondrial-associated ER membrane (MAM) plays a key role in mitochondrial division and autophagy, and the mitochondrial fusion protein MFN2 (mitofusin 2) tethers the MAM, but the mechanism by which AMPK and MFN2 regulate autophagy in response to energy stress remains unclear. Here, we found that energy stress not only triggers mitochondrial fission and autophagy, but more importantly increases the number of MAMs, a process that requires AMPK. Interestingly, under energy stress, considerable amounts of AMPK translocate from cytosol to the MAM and the mitochondrion as mitochondrial fission occurs. Unexpectedly, AMPK interacts directly with MFN2. The autophagic ability of mouse embryonic fibroblasts (MEFs) lacking MFN2 (mfn2-/-) is significantly attenuated in response to energy stress as compared to wild-type MEFs (WT MEFs), while re-expression of MFN2 in mfn2-/- cells rescues the autophagy defects of these cells. The abundance of MAMs is also greatly reduced in MFN2-deficient cells. Functional experiments show that the oxygen consumption rate and the glycolytic function of cells lacking MFN2 but not MFN1 are obviously attenuated, and MFN2 is important for cell survival under energy stress. In conclusion, our study establishes the molecular link between the energy sensor AMPK and the MAM tether MFN2, and reveals the important role of AMPK and MFN2 in energy stress-induced autophagy and MAM dynamics.
    Keywords:  AMPK; MAM; MFN2; autophagy; fusion; metformin; mitochondrial energy stress; mitochondrial fission; rotenone
    DOI:  https://doi.org/10.1080/15548627.2020.1749490
  6. Cells. 2020 Apr 07. pii: E902. [Epub ahead of print]9(4):
    A Conserved LIR Motif in Connexins Mediates Ubiquitin-Independent Binding to LC3/GABARAP Proteins.
    Steve Catarino, Teresa M Ribeiro-Rodrigues, Rita Sá Ferreira, José Ramalho, Christine Abert, Sascha Martens, Henrique Girão.
      Gap junctions (GJ) are specialized cell-cell contacts formed by connexins (Cxs), which provide direct communication between adjacent cells. Cx43 ubiquitination has been suggested to induce the internalization of GJs, as well as the recruitment of the autophagy receptor p62 to mediate binding to LC3B and degradation by macroautophagy. In this report, we describe a functional LC3 interacting region (LIR), present in the amino terminal of most Cx protein family members, which can mediate the autophagy degradation of Cx43 without the need of ubiquitin. Mutation of the LIR motif on Cx37, Cx43, Cx46 and Cx50 impairs interaction with LC3B and GABARAP without compromising protein ubiquitination. Through in vitro protein-protein interaction assays, we demonstrate that this LIR motif is required for the binding of Cx43 to LC3B and GABARAP. Overall, our findings describe an alternative mechanism whereby Cxs interact with LC3/GABARAP proteins, envisioning a new model for the autophagy degradation of connexins.
    Keywords:  Cx43; GABARAP; MAPLC3; autophagy; gap junction
    DOI:  https://doi.org/10.3390/cells9040902
  7. Front Physiol. 2020 ;11 251
    Pathophysiological Role of Transient Receptor Potential Mucolipin Channel 1 in Calcium-Mediated Stress-Induced Neurodegenerative Diseases.
    Giorgio Santoni, Federica Maggi, Consuelo Amantini, Oliviero Marinelli, Massimo Nabissi, Maria Beatrice Morelli.
      Mucolipins (TRPML) are endosome/lysosome Ca2+ permeable channels belonging to the family of transient receptor potential channels. In mammals, there are three TRPML proteins, TRPML1, 2, and 3, encoded by MCOLN1-3 genes. Among these channels, TRPML1 is a reactive oxygen species sensor localized on the lysosomal membrane that is able to control intracellular oxidative stress due to the activation of the autophagic process. Moreover, genetic or pharmacological inhibition of the TRPML1 channel stimulates oxidative stress signaling pathways. Experimental data suggest that elevated levels of reactive species play a role in several neurological disorders. There is a need to gain better understanding of the molecular mechanisms behind these neurodegenerative diseases, considering that the main sources of free radicals are mitochondria, that mitochondria/endoplasmic reticulum and lysosomes are coupled, and that growing evidence links neurodegenerative diseases to the gain or loss of function of proteins related to lysosome homeostasis. This review examines the significant roles played by the TRPML1 channel in the alterations of calcium signaling responsible for stress-mediated neurodegenerative disorders and its potential as a new therapeutic target for ameliorating neurodegeneration in our ever-aging population.
    Keywords:  Ca2+ signaling; TRPML1; autophagy; lysosomal storage disease; mitochondria; neurodegenerative disease; oxidative stress
    DOI:  https://doi.org/10.3389/fphys.2020.00251
  8. F1000Res. 2020 ;pii: F1000 Faculty Rev-212. [Epub ahead of print]9
    Recent advances in the understanding of autophagosome biogenesis.
    Martin Graef.
      Autophagy is a conserved catabolic process critical for cell homeostasis with broad implications for aging and age-associated diseases. A defining feature of autophagy is the de novo formation of a specialized transient organelle, the double-membrane autophagosome. Autophagosomes originate from small vesicular precursors after rapid membrane expansion resulting in the engulfment of a broad spectrum of cytoplasmic cargoes within a few minutes for vacuolar or lysosomal degradation. Recent advances have provided exciting new insights into the molecular mechanisms underlying the assembly of autophagic membranes during autophagosome biogenesis. Specifically, the phospholipid biosynthesis activity of the endoplasmic reticulum and a dedicated membrane-tethering complex between nascent autophagosomes and the endoplasmic reticulum have emerged as key factors in autophagosome formation.
    Keywords:  Autophagy; autophagosome biogenesis; membrane contact sites; phospholipids
    DOI:  https://doi.org/10.12688/f1000research.22111.1
  9. BMC Res Notes. 2020 Apr 10. 13(1): 210
    The cholesterol 24-hydroxylase activates autophagy and decreases mutant huntingtin build-up in a neuroblastoma culture model of Huntington's disease.
    Clévio Nóbrega, André Conceição, Rafael G Costa, Rebekah Koppenol, Raquel L Sequeira, Ricardo Nunes, Sara Carmo-Silva, Adriana Marcelo, Carlos A Matos, Sandrine Betuing, Jocelyne Caboche, Nathalie Cartier, Sandro Alves.
       OBJECTIVE: Compromised brain cholesterol turnover and altered regulation of brain cholesterol metabolism have been allied with some neurodegenerative diseases, including Huntington's disease (HD). Following our previous studies in HD, in this study we aim to investigate in vitro in a neuroblastoma cellular model of HD, the effect of CYP46A1 overexpression, an essential enzyme in cholesterol metabolism, on huntingtin aggregation and levels.
    RESULTS: We found that CYP46A1 reduces the quantity and size of mutant huntingtin aggregates in cells, as well as the levels of mutant huntingtin protein. Additionally, our results suggest that the observed beneficial effects of CYP46A1 in HD cells are linked to the activation of autophagy. Taken together, our results further demonstrate that CYP46A1 is a pertinent target to counteract HD progression.
    Keywords:  Autophagy; CYP46A1; Cholesterol; Huntingtin; Neuroblastoma cells
    DOI:  https://doi.org/10.1186/s13104-020-05053-x
  10. Front Cell Dev Biol. 2020 ;8 200
    Mitophagy: An Emerging Role in Aging and Age-Associated Diseases.
    Guo Chen, Guido Kroemer, Oliver Kepp.
      Mitochondrial dysfunction constitutes one of the hallmarks of aging and is characterized by irregular mitochondrial morphology, insufficient ATP production, accumulation of mitochondrial DNA (mtDNA) mutations, increased production of mitochondrial reactive oxygen species (ROS) and the consequent oxidative damage to nucleic acids, proteins and lipids. Mitophagy, a mitochondrial quality control mechanism enabling the degradation of damaged and superfluous mitochondria, prevents such detrimental effects and reinstates cellular homeostasis in response to stress. To date, there is increasing evidence that mitophagy is significantly impaired in several human pathologies including aging and age-related diseases such as neurodegenerative disorders, cardiovascular pathologies and cancer. Therapeutic interventions aiming at the induction of mitophagy may have the potency to ameliorate these dysfunctions. In this review, we summarize recent findings on mechanisms controlling mitophagy and its role in aging and the development of human pathologies.
    Keywords:  ROS; aging; caloric restriction; mitochondria; mitophagy
    DOI:  https://doi.org/10.3389/fcell.2020.00200
  11. Cell Death Dis. 2020 Apr 07. 11(4): 222
    Selective degradation of IKKα by autophagy is essential for arsenite-induced cancer cell apoptosis.
    Qixing Tan, Shuxian Zou, Rui Jin, Yongliang Hu, Huan Xu, Hongli Wang, Mengnan Ding, Meiru Hu, Changyuan Wei, Lun Song.
      Two catalytic subunits of the IKK complex, IKKα and IKKβ, trigger NF-κB activation as well as NF-κB-independent signaling events under both physiological and pathological conditions. Here we identified the NF-κB-unrelated cytoprotective function of IKKα in promoting autophagy by triggering p53 transactivation and upregulation of its downstream autophagic mediator, DRAM1, in the arsenite-treated hepatoma cells, which responses depended on IKKα kinase activity. Furthermore, IKKα triggered p53/DRAM1-dependent autophagy by inducing CHK1 activation and CHK1/p53 interaction. Interestingly, after provoking autophagy, IKKα could be specifically recognized by the autophagic machinery via directly binding with LC3B, resulting in selective degradation of IKKα by autophagy. Unexpectedly, the selectivity of autophagic sequestration towards IKKα was mediated by novel mechanism independent of the classical LC3-interacting regions (LIRs) within IKKα, while C-terminal arm of LIR was involved in mediating IKKα/LC3B interaction. Taken together, we conclude that IKKα attenuates arsenite-induced apoptosis by inducing p53-dependent autophagy, and then selective feedback degradation of IKKα by autophagy contributes to the cytotoxic response induced by arsenite.
    DOI:  https://doi.org/10.1038/s41419-020-2420-5
  12. Autophagy. 2020 Apr 08.
    Autophagy Suppresses Breast Cancer Metastasis By Degrading NBR1.
    Timothy Marsh, Jayanta Debnath.
      Macroautophagy/autophagy plays complex, context-dependent roles in cancer. How autophagy governs the emergence of metastatic disease has been incompletely understood. We recently uncovered that genetic autophagy inhibition strongly attenuates primary tumor growth in mammary cancer models, yet paradoxically promotes spontaneous metastasis to the lung and enables the outgrowth of disseminated tumor cells (DTCs) into overt macro-metastases. Furthermore, at both primary and metastatic sites, genetic autophagy inhibition leads to the marked expansion of tumor cells exhibiting aggressive and pro-metastatic basal epithelial differentiation. These pro-metastatic effects of autophagy inhibition are due to the cytosolic accumulation of the autophagy cargo receptor NBR1 in autophagy-deficient tumor cells.
    Keywords:  Autophagy; Keratin14; NBR1; Rubicon; TP63; chloroquine; metastasis
    DOI:  https://doi.org/10.1080/15548627.2020.1753001
  13. Front Cell Dev Biol. 2020 ;8 171
    Mammalian Mitophagosome Formation: A Focus on the Early Signals and Steps.
    Maria Zachari, Nicholas T Ktistakis.
      Mitophagy, a conserved intracellular process by which mitochondria are eliminated via the autophagic machinery, is a quality control mechanism which facilitates maintenance of a functional mitochondrial network and cell homeostasis, making it a key process in development and longevity. Mitophagy has been linked to multiple human disorders, especially neurodegenerative diseases where the long-lived neurons are relying on clearance of old/damaged mitochondria to survive. During the past decade, the availability of novel tools to study mitophagy both in vitro and in vivo has significantly advanced our understanding of the molecular mechanisms governing this fundamental process in normal physiology and in various disease models. We here give an overview of the known mitophagy pathways and how they are induced, with a particular emphasis on the early events governing mitophagosome formation.
    Keywords:  autophagy; endoplasmic reticulum; mTOR; membrane potential; mitophagy
    DOI:  https://doi.org/10.3389/fcell.2020.00171
  14. FEBS J. 2020 Apr 04.
    The Warburg Micro Syndrome associated Rab3GAP-Rab18 module promotes autolysosome maturation through the Vps34 Complex I.
    Szabolcs Takáts, Luca Lévay, Attila Boda, Sarolta Tóth, Zsófia Simon-Vecsei, András Rubics, Ágnes Varga, Mónika Lippai, Péter Lőrincz, Gábor Glatz, Gábor Juhász.
      Warburg Micro Syndrome (WMS) is a hereditary autosomal neuromuscular disorder in humans caused by mutations in Rab18, Rab3GAP1 or Rab3GAP2 genes. Rab3GAP1/2 form a heterodimeric complex, which acts as a guanosine nucleotide exchange factor and activates Rab18. Although the genetic causes of WMS are known, it is still unclear whether loss of the Rab3GAP-Rab18 module affects neuronal or muscle cell physiology or both, and how. In this work, we characterize a Rab3GAP2 mutant Drosophila line to establish a novel animal model for WMS. Similarly to symptoms of WMS, loss of Rab3GAP2 leads to highly decreased motility in Drosophila that becomes more serious with age. We demonstrate that these mutant flies are defective for autophagic degradation in multiple tissues including fat cells and muscles. Loss of Rab3GAP-Rab18 module members leads to perturbed autolysosome morphology due to destabilization of Rab7-positive autophagosomal and late endosomal compartments and perturbation of lysosomal biosynthetic transport. Importantly, overexpression of UVRAG or loss of Atg14, two alternative subunits of the Vps34/PI3K (Vacuole protein sorting 34/Phosphatidylinositol-3-Kinase) complexes in fat cells, mimics the autophagic phenotype of Rab3GAP-Rab18 module loss. We find that GTP-bound Rab18 binds to Atg6/Beclin1, a permanent subunit of Vps34 complexes. Finally, we show that Rab3GAP2 and Rab18 are present on autophagosomal and autolysosomal membranes and colocalize with Vps34 Complex I subunits. Our data suggest that the Rab3GAP-Rab18 module regulates autolysosomal maturation through its interaction with the Vps34 Complex I, and perturbed autophagy due to loss of the Rab3GAP-Rab18 module may contribute to the development of WMS.
    Keywords:  Atg14; Rab18; Rab3GAP1; Rab3GAP2; Vps34; autophagy
    DOI:  https://doi.org/10.1111/febs.15313
  15. Autophagy. 2020 Apr 08.
    Autophagy: an essential but limited cellular process for timely skeletal muscle recovery from injury.
    Jarrod A Call, Anna S Nichenko.
      Macroautophagy/autophagy induction, i.e., the formation of autophagosomes, is robust following many forms of muscle injury. Autophagy inhibition studies strongly indicate that autophagy is necessary for successful muscle fiber recovery. Now, there are accumulating pieces of evidence indicating that autophagosome clearance, i.e., autophagy flux, does not increase to match the burden of accumulating damaged proteins and organelles after muscle fiber damage, creating a bottleneck effect. Some potential consequences of the bottleneck effect are reduced regenerative capacity marked by the inadequate activation of muscle stem cells (i.e., satellite cells) and a lesser commitment towards differentiation due to a deficiency in energetic substrates and/or molecular signaling pathways. These findings highlight an emerging area of investigation for both autophagy and muscle regeneration fields. The identification of the molecular mechanisms governing autophagy and autophagy flux may serve as targets for future therapies to enhance the recovery of its function in healthy and diseased muscle.
    Keywords:  ULK1; mitochondria; mitophagy; muscle regeneration; muscle strength; satellite cell; two-photon microscopy
    DOI:  https://doi.org/10.1080/15548627.2020.1753000
  16. Exp Mol Med. 2020 Apr 09.
    Pharmacological inhibition of mTOR attenuates replicative cell senescence and improves cellular function via regulating the STAT3-PIM1 axis in human cardiac progenitor cells.
    Ji Hye Park, Na Kyoung Lee, Hye Ji Lim, Seung Taek Ji, Yeon-Ju Kim, Woong Bi Jang, Da Yeon Kim, Songhwa Kang, Jisoo Yun, Jong Seong Ha, Hyungtae Kim, Dongjun Lee, Sang Hong Baek, Sang-Mo Kwon.
      The mammalian target of rapamycin (mTOR) signaling pathway efficiently regulates the energy state of cells and maintains tissue homeostasis. Dysregulation of the mTOR pathway has been implicated in several human diseases. Rapamycin is a specific inhibitor of mTOR and pharmacological inhibition of mTOR with rapamycin promote cardiac cell generation from the differentiation of mouse and human embryonic stem cells. These studies strongly implicate a role of sustained mTOR activity in the differentiating functions of embryonic stem cells; however, they do not directly address the required effect for sustained mTOR activity in human cardiac progenitor cells. In the present study, we evaluated the effect of mTOR inhibition by rapamycin on the cellular function of human cardiac progenitor cells and discovered that treatment with rapamycin markedly attenuated replicative cell senescence in human cardiac progenitor cells (hCPCs) and promoted their cellular functions. Furthermore, rapamycin not only inhibited mTOR signaling but also influenced signaling pathways, including STAT3 and PIM1, in hCPCs. Therefore, these data reveal a crucial function for rapamycin in senescent hCPCs and provide clinical strategies based on chronic mTOR activity.
    DOI:  https://doi.org/10.1038/s12276-020-0374-4
  17. Health Psychol Behav Med. 2019 Sep;3(3): NS20180134
    Autophagy in the mammalian nervous system: a primer for neuroscientists.
    Fumi Suomi, Thomas G McWilliams.
      Autophagy refers to the lysosomal degradation of damaged or superfluous components and is essential for metabolic plasticity and tissue integrity. This evolutionarily conserved process is particularly vital to mammalian post-mitotic cells such as neurons, which face unique logistical challenges and must sustain homoeostasis over decades. Defective autophagy has pathophysiological importance, especially for human neurodegeneration. The present-day definition of autophagy broadly encompasses two distinct yet related phenomena: non-selective and selective autophagy. In this minireview, we focus on established and emerging concepts in the field, paying particular attention to the physiological significance of macroautophagy and the burgeoning world of selective autophagy pathways in the context of the vertebrate nervous system. By highlighting established basics and recent breakthroughs, we aim to provide a useful conceptual framework for neuroscientists interested in autophagy, in addition to autophagy enthusiasts with an eye on the nervous system.
    Keywords:  autophagy; metabolism; mitochondria; neurodegeneration; neurons; organelles
    DOI:  https://doi.org/10.1042/NS20180134
  18. Eur J Immunol. 2020 Apr 06.
    Advanced glycation end products reduce macrophage-mediated killing of Staphylococcus aureus by ARL8 upregulation and inhibition of autolysosome formation.
    Xiaoying Xie, Chuan Yang, Chaohui Duan, Hongxing Chen, Tingting Zeng, Songyin Huang, Hongyu Li, Meng Ren, Wei-Jye Lin, Li Yan.
      Staphylococcus aureus (S. aureus), a pathogen most frequently found in diabetic foot ulcer infection (DFI), was recently suggested as an intracellular pathogen. Autophagy in professional phagocytes like macrophages allows selective destruction of intracellular pathogens, and its dysfunction can increase the survival of internalized pathogens, causing infections to worsen and spread. Previous works have shown that S. aureus infections in diabetes appeared more severe and invasive, coincided with the suppressed autophagy in dermal tissues of diabetic rat, but the exact mechanisms are unclear. Here, we demonstrated that accumulation of advanced glycation end products (AGEs) contributed to the diminished autophagy-mediated clearance of S. aureus in the macrophages differentiated from PMA-treated human monocytic cell line THP-1. Importantly, infected macrophages showed increased S. aureus-containing autophagosome, but the subsequent fusion of S. aureus-containing autophagosome and lysosome was suppressed in AGEs-pretreated cells, suggesting AGEs blocked the autophagic flux and enabled S. aureus survival and escape. At the molecular level, elevated lysosomal ARL8 expression in AGEs-treated macrophages was required for AGEs-mediated inhibition of autophagosome-lysosome fusion. Silencing ARL8 in AGEs-treated macrophages restored autophagic flux and increased S. aureus clearance. Our results therefore demonstrate a new mechanism in which AGEs accelerate S. aureus immune evasion in macrophages by ARL8-dependent suppression of autophagosome-lysosome fusion and bactericidal capability. This article is protected by copyright. All rights reserved.
    Keywords:  ARL8; Advanced glycation end products (AGEs); Autophagy; Macrophage; Staphylococcus aureus
    DOI:  https://doi.org/10.1002/eji.201948477
  19. Elife. 2020 Apr 09. pii: e48963. [Epub ahead of print]9
    Differential expression of MAGEA6 toggles autophagy to promote pancreatic cancer progression.
    Yiu Huen Tsang, Yumeng Wang, Kathleen Kong, Caitlin Grzeskowiak, Oksana Zagorodna, Turgut Dogruluk, Hengyu Lu, Nicole Villafane, Venkata Hemanjani Bhavana, Daniela Moreno, Sarah H Elsea, Han Liang, Gordon B Mills, Kenneth L Scott.
      The melanoma-associated antigen family A (MAGEA) antigens are expressed in a wide variety of malignant tumors but not in adult somatic cells, rendering them attractive targets for cancer immunotherapy. Here we show that a number of cancer-associated MAGEA mutants that undergo proteasome-dependent degradation in vitro could negatively impact their utility as immunotherapeutic targets. Importantly, in pancreatic ductal adenocarcinoma cell models, MAGEA6 suppresses macroautophagy (autophagy). The inhibition of autophagy is released upon MAGEA6 degradation, which can be induced by nutrient deficiency or by acquisition of cancer-associated mutations. Using xenograft mouse models, we demonstrated that inhibition of autophagy is critical for tumor initiation whereas reinstitution of autophagy as a consequence of MAGEA6 degradation contributes to tumor progression. These findings could inform cancer immunotherapeutic strategies for targeting MAGEA antigens and provide mechanistic insight into the divergent roles of MAGEA6 during pancreatic cancer initiation and progression.
    Keywords:  cancer biology; genetics; genomics; human
    DOI:  https://doi.org/10.7554/eLife.48963
  20. Nat Rev Mol Cell Biol. 2020 Apr 06.
    The clearance of dead cells by efferocytosis.
    Emilio Boada-Romero, Jennifer Martinez, Bradlee L Heckmann, Douglas R Green.
      Multiple modes of cell death have been identified, each with a unique function and each induced in a setting-dependent manner. As billions of cells die during mammalian embryogenesis and daily in adult organisms, clearing dead cells and associated cellular debris is important in physiology. In this Review, we present an overview of the phagocytosis of dead and dying cells, a process known as efferocytosis. Efferocytosis is performed by macrophages and to a lesser extent by other 'professional' phagocytes (such as monocytes and dendritic cells) and 'non-professional' phagocytes, such as epithelial cells. Recent discoveries have shed light on this process and how it functions to maintain tissue homeostasis, tissue repair and organismal health. Here, we outline the mechanisms of efferocytosis, from the recognition of dying cells through to phagocytic engulfment and homeostatic resolution, and highlight the pathophysiological consequences that can arise when this process is abrogated.
    DOI:  https://doi.org/10.1038/s41580-020-0232-1
  21. Dev Cell. 2020 Apr 07. pii: S1534-5807(20)30192-1. [Epub ahead of print]
    Mitochondria-Derived H2O2 Promotes Symmetry Breaking of the C. elegans Zygote.
    Sasha De Henau, Marc Pagès-Gallego, Willem-Jan Pannekoek, Tobias B Dansen.
      Symmetry breaking is an essential step in cell differentiation and early embryonic development. However, the molecular cues that trigger symmetry breaking remain largely unknown. Here, we show that mitochondrial H2O2 acts as a symmetry-breaking cue in the C. elegans zygote. We find that symmetry breaking is marked by a local H2O2 increase and coincides with a relocation of mitochondria to the cell cortex. Lowering endogenous H2O2 levels delays the onset of symmetry breaking, while artificially targeting mitochondria to the cellular cortex using a light-induced heterodimerization technique is sufficient to initiate symmetry breaking in a H2O2-dependent manner. In wild-type development, both sperm and maternal mitochondria contribute to symmetry breaking. Our findings reveal that mitochondrial H2O2-signaling promotes the onset of polarization, a fundamental process in development and cell differentiation, and this is achieved by both mitochondrial redistribution and differential H2O2-production.
    Keywords:  C. elegans; H(2)O(2); cell polarity; embryonic development; mitochondria; redox signaling; symmetry breaking
    DOI:  https://doi.org/10.1016/j.devcel.2020.03.008
  22. Adv Exp Med Biol. 2020 ;1233 177-194
    Altered Proteostasis in Neurodegenerative Tauopathies.
    Katerina Papanikolopoulou, Efthimios M C Skoulakis.
      Tauopathies are a heterogeneous group of neurodegenerative dementias involving perturbations in the levels, phosphorylation or mutations of the neuronal microtubule-binding protein Tau. Tauopathies are characterized by accumulation of hyperphosphorylated Tau leading to formation of a range of aggregates including macromolecular ensembles such as Paired Helical filaments and Neurofibrilary Tangles whose morphology characterizes and differentiates these disease states. Why nonphysiological Tau proteins elude the surveillance normal proteostatic mechanisms and eventually form these macromolecular assemblies is a central mostly unresolved question of cardinal importance for diagnoses and potential therapeutic interventions. We discuss the response of the Ubiquitin-Proteasome system, autophagy and the Endoplasmic Reticulum-Unfolded Protein response in Tauopathy models and patients, revealing interactions of components of these systems with Tau, but also of the effects of pathological Tau on these systems which eventually lead to Tau aggregation and accumulation. These interactions point to potential disease biomarkers and future potential therapeutic targets.
    Keywords:  Alzheimer’s disease; Neurodegeneration; Proteostasis; Tau toxicity; Tauopathies
    DOI:  https://doi.org/10.1007/978-3-030-38266-7_7
  23. Autophagy. 2020 Apr 09.
    Autophagy-related protein PIK3C3/VPS34 controls T cell metabolism and function.
    Guan Yang, Wenqiang Song, J Luke Postoak, Jin Chen, Jennifer Martinez, Jianhua Zhang, Lan Wu, Luc Van Kaer.
      The PIK3C3/VPS34 subunit of the class III phosphatidylinositol 3-kinase (PtdIns3K) complex is a key early player in macroautophagy/autophagy. In this study, we assessed the contribution of PIK3C3 to T cell metabolism and function. We found that Pik3c3-deficient T cells exhibited impaired cellular metabolism, and Pik3c3-deficient CD4+ T cells failed to differentiate into T helper 1 cells. These alterations were associated with reduced levels of active mitochondria upon T cell activation. In addition, conditional Pik3c3-deficient animals failed to mount autoreactive T cell responses and were resistant to experimental autoimmune encephalomyelitis (EAE). Interestingly, the deletion of Pik3c3 had little effect on the capacity of animals to clear tumor metastases. Collectively, our studies have revealed a critical role of PIK3C3 in T cell metabolism and the pathogenicity of these cells during EAE. Our findings also have important implications for the development of immunotherapies to treat multiple sclerosis and other inflammatory diseases by targeting PIK3C3.
    Keywords:  PIK3C3/VPS34; T cell differentiation; T cell metabolism; autophagy; experimental autoimmune encephalomyelitis
    DOI:  https://doi.org/10.1080/15548627.2020.1752979
  24. Nat Commun. 2020 Apr 09. 11(1): 1755
    p53-mediated control of aspartate-asparagine homeostasis dictates LKB1 activity and modulates cell survival.
    Longfei Deng, Pengbo Yao, Le Li, Fansen Ji, Shuang Zhao, Chang Xu, Xun Lan, Peng Jiang.
      Asparagine synthetase (ASNS) catalyses the ATP-dependent conversion of aspartate to asparagine. However, both the regulation and biological functions of asparagine in tumour cells remain largely unknown. Here, we report that p53 suppresses asparagine synthesis through the transcriptional downregulation of ASNS expression and disrupts asparagine-aspartate homeostasis, leading to lymphoma and colon tumour growth inhibition in vivo and in vitro. Moreover, the removal of asparagine from culture medium or the inhibition of ASNS impairs cell proliferation and induces p53/p21-dependent senescence and cell cycle arrest. Mechanistically, asparagine and aspartate regulate AMPK-mediated p53 activation by physically binding to LKB1 and oppositely modulating LKB1 activity. Thus, we found that p53 regulates asparagine metabolism and dictates cell survival by generating an auto-amplification loop via asparagine-aspartate-mediated LKB1-AMPK signalling. Our findings highlight a role for LKB1 in sensing asparagine and aspartate and connect asparagine metabolism to the cellular signalling transduction network that modulates cell survival.
    DOI:  https://doi.org/10.1038/s41467-020-15573-6
  25. Autophagy. 2020 Apr 07.
    The functions of Atg8-family proteins in autophagy and cancer: linked or unrelated?
    Marine Jacquet, Michaël Guittaut, Annick Fraichard, Gilles Despouy.
      The Atg8-family proteins are subdivided into two subfamilies: the GABARAP and LC3 subfamilies. These proteins, which are major players of the autophagy pathway, present a conserved glycine in their C-terminus necessary for their association to the autophagosome membrane. This family of proteins present multiple roles from autophagy induction to autophagosome-lysosome fusion and have been described to play a role during cancer progression. Indeed, GABARAPs are described to be downregulated in cancers, and high expression has been linked to a good prognosis. Regarding LC3s, their expression does not correlate to a particular tumor type or stage. The involvement of Atg8-family proteins during cancer, therefore, remains unclear, and it appears that their anti-tumor role may be associated with their implication in selective protein degradation by autophagy but might also be independent, in some cases, of their conjugation to autophagosomes. In this review, we will then focus on the involvement of GABARAP and LC3 subfamilies during autophagy and cancer and highlight the similarities but also the differences of action of each subfamily member.
    Keywords:  Atg8; GABARAP; GABARAPL1; GABARAPL2; LC3; autophagy; cancer
    DOI:  https://doi.org/10.1080/15548627.2020.1749367
  26. Sci Rep. 2020 Apr 08. 10(1): 6064
    Inhibition of HDAC6 activity protects dopaminergic neurons from alpha-synuclein toxicity.
    Laetitia Francelle, Tiago F Outeiro, Gudrun A Rappold.
      The neuropathological hallmarks of Parkinson's disease include preferential vulnerability of dopaminergic neurons of the substantia nigra pars compacta, and accumulation of intraneuronal protein inclusions known as Lewy bodies. These inclusions contain, among other proteins, aggregated alpha-synuclein and histone deacetylase 6 (HDAC6). In our study we found that selective inhibition of HDAC6 activity by Tubastatin A has protective effects in a rat model of Parkinson's disease. We provide evidence that this protection may be due to the activation of chaperone-mediated autophagy through the up-regulation of key members of this pathway. Moreover, Tubastatin A significantly inhibited the expression of a toxic form of alpha-synuclein that is phosphorylated at serine position 129. Tubastatin A treatment also permitted to partially modulate neuroinflammation. Taken together, our study highlights the neuroprotective effects of Tubastatin A in a rat model of Parkinson's disease and provides mechanistic insight in Tubastatin A-mediated protection against alpha-synuclein toxicity and substantia nigra degeneration. These findings are of potential therapeutic value in Parkinson's disease and other synucleinopathies.
    DOI:  https://doi.org/10.1038/s41598-020-62678-5
  27. J Mater Chem B. 2018 Jul 21. 6(27): 4422-4426
    Lysosome-targeting ratiometric fluorescent pH probes based on long-wavelength BODIPY.
    Mengliang Zhu, Peipei Xing, Yabin Zhou, Lei Gong, Jinghui Zhang, Dongdong Qi, Yongzhong Bian, Hongwu Du, Jianzhuang Jiang.
      BODIPY-based probes were developed for the ratiometric fluorescence detection of pH values. They showed high sensitivity under acidic conditions via a protonation-modulated ICT mechanism and could selectively stain lysosomes, and thus could be applied for imaging and detecting abnormal pH decreases in live cells.
    DOI:  https://doi.org/10.1039/c8tb00883c
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