bims-auttor Biomed News
on Autophagy and mTOR
Issue of 2019‒04‒07
nine papers selected by
Viktor Korolchuk
Newcastle University

  1. Cell Metab. 2019 Apr 02. pii: S1550-4131(19)30131-7. [Epub ahead of print]29(4): 803-826
      Autophagy is an evolutionarily conserved lysosome- or vacuole-dependent catabolic pathway in eukaryotes. Autophagy functions basally for cellular quality control and is induced to act as an alternative source of basic metabolites during nutrient deprivation. These functions of autophagy are intimately connected to the regulation of metabolism, and the metabolic status of the cell in turn controls the nature and extent of autophagic induction. Here, we highlight the co-regulation of autophagy and metabolism with a special focus on selective autophagy that, along with bulk autophagy, plays a central role in regulating and rewiring metabolic circuits. We outline the metabolic signals that activate these pathways, the mechanisms involved, and the downstream effects and implications while recognizing yet unanswered questions. We also discuss the role of autophagy in the development and maintenance of adipose tissue, an emerging player in systemic metabolic homeostasis, and describe what is currently known about the complex relationship between autophagy and cancer.
    Keywords:  AMPK; autophagy; ferritinophagy; homeostasis; lysosome; mTOR; macroautophagy; mitophagy; physiology; protein turnover; proteostasis; stress
  2. Autophagy. 2019 Apr 02. 1-19
      Mutations in the TBK1 (TANK binding kinase 1) gene are causally linked to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). TBK1 phosphorylates the cargo receptors OPTN and SQSTM1 regulating a critical step in macroautophagy/autophagy. Disruption of the autophagic flux leads to accumulation of cytosolic protein aggregates, which are a hallmark of ALS. hiPSC-derived TBK1-mutant motoneurons (MNs) showed reduced TBK1 levels and accumulation of cytosolic SQSTM1-positive aggresomes. By screening a library of nuclear-receptor-agonists for modifiers of the SQSTM1 aggregates, we identified 4-hydroxy(phenyl)retinamide (4HPR) as a potent modifier exerting detrimental effects on mutant-TBK1 motoneurons fitness exacerbating the autophagy overload. We have shown by TEM that TBK1-mutant motoneurons accumulate immature phagophores due a failure in the elongation phase, and 4HPR further worsens the burden of dysfunctional phagophores. 4HPR-increased toxicity was associated with the upregulation of SQSTM1 in a context of strongly reduced ATG10, while rescue of ATG10 levels abolished 4HPR toxicity. Finally, we showed that 4HPR leads to a downregulation of ATG10 and to an accumulation of SQSTM1+ aggresomes also in hiPSC-derived C9orf72-mutant motoneurons. Our data show that cultured human motoneurons harboring mutations in TBK1 gene display typical ALS features, like decreased viability and accumulation of cytosolic SQSTM1-positive aggresomes. The retinoid 4HPR appears a strong negative modifier of the fitness of TBK1 and C9orf72-mutant MNs, through a pathway converging on the mismatch of initiated autophagy and ATG10 levels. Thus, autophagy induction appears not to be a therapeutic strategy for ALS unless the specific underlying pathway alterations are properly addressed. Abbreviations: 4HPR: 4-hydroxy(phenyl)retinamide; AKT: AKT1 serine/threonine kinase 1; ALS: amyotrophic lateral sclerosis; ATG: autophagy related; AVs: autophagic vesicle; C9orf72: chromosome 9 open reading frame 72; CASP3: caspase 3; CHAT: choline O-acetyltransferase; CYCS: cytochrome c, somatic; DIV: day in vitro; FTD: frontotemporal dementia; FUS: FUS RNA binding protein; GFP: green fluorescent protein; hiPSCs: human induced pluripotent stem cells; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MNs: motoneurons; mRFP: monomeric red fluorescent protein; MTOR: mechanistic target of rapamycin kinase; NFE2L2/NRF2: nuclear factor, erythroid 2 like 2; RARA: retinoic acid receptor alpha; SLC18A3/VACHT: solute carrier family 18 (vesicular acetylcholine transporter), member 3; SQSTM1/p62: sequestosome 1; TBK1: TANK binding kinase 1; TEM: transmission electron microscopy.
    Keywords:  ALS; TBK1; autophagy; hiPSC; motoneurons
  3. Nat Commun. 2019 Apr 05. 10(1): 1567
      Selective autophagy ensures the removal of specific soluble proteins, protein aggregates, damaged mitochondria, and invasive bacteria from cells. Defective autophagy has been directly linked to metabolic disorders. However how selective autophagy regulates metabolism remains largely uncharacterized. Here we show that a deficiency in selective autophagy is associated with suppression of lipid oxidation. Hepatic loss of Atg7 or Atg5 significantly impairs the production of ketone bodies upon fasting, due to decreased expression of enzymes involved in β-oxidation following suppression of transactivation by PPARα. Mechanistically, nuclear receptor co-repressor 1 (NCoR1), which interacts with PPARα to suppress its transactivation, binds to the autophagosomal GABARAP family proteins and is degraded by autophagy. Consequently, loss of autophagy causes accumulation of NCoR1, suppressing PPARα activity and resulting in impaired lipid oxidation. These results suggest that autophagy contributes to PPARα activation upon fasting by promoting degradation of NCoR1 and thus regulates β-oxidation and ketone bodies production.
  4. Autophagy. 2019 Apr 04.
      Parkinson disease (PD) is a disabling, incurable disorder with increasing prevalence in the western world. In rare cases PD is caused by mutations in the genes for PINK1 (PTEN induced kinase 1) or PRKN (parkin RBR E3 ubiquitin protein ligase), which impair the selective autophagic elimination of damaged mitochondria (mitophagy). Mutations in the gene encoding LRRK2 (leucine rich repeat kinase 2) are the most common monogenic cause of PD. Here, we report that the LRRK2 kinase substrate RAB10 accumulates on depolarized mitochondria in a PINK1- and PRKN-dependent manner. RAB10 binds the autophagy receptor OPTN (optineurin), promotes OPTN accumulation on depolarized mitochondria and facilitates mitophagy. In PD patients with the two most common LRRK2 mutations (G2019S and R1441C), RAB10 phosphorylation at threonine 73 is enhanced, while RAB10 interaction with OPTN, accumulation of RAB10 and OPTN on depolarized mitochondria, depolarization-induced mitophagy and mitochondrial function are all impaired. These defects in LRRK2 mutant patient cells are rescued by LRRK2 knockdown and LRRK2 kinase inhibition. A phosphomimetic RAB10 mutant showed less OPTN interaction and less translocation to depolarized mitochondria than wild-type RAB10, and failed to rescue mitophagy in LRRK2 mutant cells. These data connect LRRK2 with PINK1- and PRKN-mediated mitophagy via its substrate RAB10, and indicate that the pathogenic effects of mutations in LRRK2, PINK1 and PRKN may converge on a common pathway.
    Keywords:  OPTN; PARK2; PINK1; PRKN; Parkinson disease; autophagy receptor; mitochondria; optineurin; parkin; selective autophagy
  5. Front Cell Dev Biol. 2019 ;7 38
      Autophagy is an intracellular degradation pathway for malfunctioning aggregation-prone proteins, damaged organelles, unwanted macromolecules and invading pathogens. This process is essential for maintaining cellular and tissue homeostasis that contribute to organismal survival. Autophagy dysfunction has been implicated in the pathogenesis of diverse human diseases, and therefore, therapeutic exploitation of autophagy is of potential biomedical relevance. A number of chemical screening approaches have been established for the drug discovery of autophagy modulators based on the perturbations of autophagy reporters or the clearance of autophagy substrates. These readouts can be detected by fluorescence and high-content microscopy, flow cytometry, microplate reader and immunoblotting, and the assays have evolved to enable high-throughput screening and measurement of autophagic flux. Several pharmacological modulators of autophagy have been identified that act either via the classical mechanistic target of rapamycin (mTOR) pathway or independently of mTOR. Many of these autophagy modulators have been demonstrated to exert beneficial effects in transgenic models of neurodegenerative disorders, cancer, infectious diseases, liver diseases, myopathies as well as in lifespan extension. This review describes the commonly used chemical screening approaches in mammalian cells and the key autophagy modulators identified through these methods, and highlights the therapeutic benefits of these compounds in specific disease contexts.
    Keywords:  autophagy; autophagy modulator; autophagy reporter; autophagy substrate; cancer; lifespan extension; neurodegenerative diseases; screening method
  6. J Cell Biol. 2019 Apr 05. pii: jcb.201811139. [Epub ahead of print]
      During macroautophagic stress, autophagosomes can be produced continuously and in high numbers. Many different organelles have been reported as potential donor membranes for this sustained autophagosome growth, but specific machinery to support the delivery of lipid to the growing autophagosome membrane has remained unknown. Here we show that the autophagy protein, ATG2, without a clear function since its discovery over 20 yr ago, is in fact a lipid-transfer protein likely operating at the ER-autophagosome interface. ATG2A can bind tens of glycerophospholipids at once and transfers lipids robustly in vitro. An N-terminal fragment of ATG2A that supports lipid transfer in vitro is both necessary and fully sufficient to rescue blocked autophagosome biogenesis in ATG2A/ATG2B KO cells, implying that regulation of lipid homeostasis is the major autophagy-dependent activity of this protein and, by extension, that protein-mediated lipid transfer across contact sites is a principal contributor to autophagosome formation.
  7. Acta Neuropathol. 2019 Apr 01.
      Amyotrophic lateral sclerosis (ALS) is a lethal disease characterized by motor neuron degeneration and associated with aggregation of nuclear RNA-binding proteins (RBPs), including FUS. How FUS aggregation and neurodegeneration are prevented in healthy motor neurons remain critically unanswered questions. Here, we use a combination of ALS patient autopsy tissue and induced pluripotent stem cell-derived neurons to study the effects of FUS mutations on RBP homeostasis. We show that FUS' tendency to aggregate is normally buffered by interacting RBPs, but this buffering is lost when FUS mislocalizes to the cytoplasm due to ALS mutations. The presence of aggregation-prone FUS in the cytoplasm causes imbalances in RBP homeostasis that exacerbate neurodegeneration. However, enhancing autophagy using small molecules reduces cytoplasmic FUS, restores RBP homeostasis and rescues motor function in vivo. We conclude that disruption of RBP homeostasis plays a critical role in FUS-ALS and can be treated by stimulating autophagy.
    Keywords:  Amyotrophic lateral sclerosis; FUS; Induced pluripotent stem cells; Phase transition; Protein homeostasis; RNA-binding proteins
  8. Cell. 2019 Apr 04. pii: S0092-8674(19)30162-X. [Epub ahead of print]177(2): 299-314.e16
      Autophagy is required in diverse paradigms of lifespan extension, leading to the prevailing notion that autophagy is beneficial for longevity. However, why autophagy is harmful in certain contexts remains unexplained. Here, we show that mitochondrial permeability defines the impact of autophagy on aging. Elevated autophagy unexpectedly shortens lifespan in C. elegans lacking serum/glucocorticoid regulated kinase-1 (sgk-1) because of increased mitochondrial permeability. In sgk-1 mutants, reducing levels of autophagy or mitochondrial permeability transition pore (mPTP) opening restores normal lifespan. Remarkably, low mitochondrial permeability is required across all paradigms examined of autophagy-dependent lifespan extension. Genetically induced mPTP opening blocks autophagy-dependent lifespan extension resulting from caloric restriction or loss of germline stem cells. Mitochondrial permeability similarly transforms autophagy into a destructive force in mammals, as liver-specific Sgk knockout mice demonstrate marked enhancement of hepatocyte autophagy, mPTP opening, and death with ischemia/reperfusion injury. Targeting mitochondrial permeability may maximize benefits of autophagy in aging.
    Keywords:  SGK; aging; autophagy; ischemia/reperfusion injury; longevity; mPTP; mTORC2; mitochondrial permeability
  9. EMBO J. 2019 Apr 01. pii: e100554. [Epub ahead of print]
      Membrane targeting of autophagy-related complexes is an important step that regulates their activities and prevents their aberrant engagement on non-autophagic membranes. ATG16L1 is a core autophagy protein implicated at distinct phases of autophagosome biogenesis. In this study, we dissected the recruitment of ATG16L1 to the pre-autophagosomal structure (PAS) and showed that it requires sequences within its coiled-coil domain (CCD) dispensable for homodimerisation. Structural and mutational analyses identified conserved residues within the CCD of ATG16L1 that mediate direct binding to phosphoinositides, including phosphatidylinositol 3-phosphate (PI3P). Mutating putative lipid binding residues abrogated the localisation of ATG16L1 to the PAS and inhibited LC3 lipidation. On the other hand, enhancing lipid binding of ATG16L1 by mutating negatively charged residues adjacent to the lipid binding motif also resulted in autophagy inhibition, suggesting that regulated recruitment of ATG16L1 to the PAS is required for its autophagic activity. Overall, our findings indicate that ATG16L1 harbours an intrinsic ability to bind lipids that plays an essential role during LC3 lipidation and autophagosome maturation.
    Keywords:  ATG16L1; PI3P; autophagy; coiled‐coil domain; phagophore