bims-apauto Biomed News
on Apoptosis and autophagy
Issue of 2021‒07‒25
six papers selected by
Su Hyun Lee
Seoul National University

  1. Nature. 2021 Jul 21.
      The mTOR complex 1 (mTORC1) controls cell growth in response to amino acid levels1. Here we report SAR1B as a leucine sensor that regulates mTORC1 signalling in response to intracellular levels of leucine. Under conditions of leucine deficiency, SAR1B inhibits mTORC1 by physically targeting its activator GATOR2. In conditions of leucine sufficiency, SAR1B binds to leucine, undergoes a conformational change and dissociates from GATOR2, which results in mTORC1 activation. SAR1B-GATOR2-mTORC1 signalling is conserved in nematodes and has a role in the regulation of lifespan. Bioinformatic analysis reveals that SAR1B deficiency correlates with the development of lung cancer. The silencing of SAR1B and its paralogue SAR1A promotes mTORC1-dependent growth of lung tumours in mice. Our results reveal that SAR1B is a conserved leucine sensor that has a potential role in the development of lung cancer.
  2. Cell Death Differ. 2021 Jul 20.
      SidE family of Legionella effectors catalyze non-canonical phosphoribosyl-linked ubiquitination (PR-ubiquitination) of host proteins during bacterial infection. SdeA localizes predominantly to ER and partially to the Golgi apparatus, and mediates serine ubiquitination of multiple ER and Golgi proteins. Here we show that SdeA causes disruption of Golgi integrity due to its ubiquitin ligase activity. The Golgi linking proteins GRASP55 and GRASP65 are PR-ubiquitinated on multiple serine residues, thus preventing their ability to cluster and form oligomeric structures. In addition, we found that the functional consequence of Golgi disruption is not linked to the recruitment of Golgi membranes to the growing Legionella-containing vacuoles. Instead, it affects the host secretory pathway. Taken together, our study sheds light on the Golgi manipulation strategy by which Legionella hijacks the secretory pathway and promotes bacterial infection.
  3. Amino Acids. 2021 Jul 22.
      Malignant cells often demonstrate a proliferative advantage when compared to non-malignant cells. However, the rapid growth and metabolism required for survival can also highlight vulnerabilities specific to these malignant cells. One such vulnerability exhibited by cancer is an increased demand for amino acids (AAs), which often results in a dependency on exogenous sources of AAs or requires upregulation of de novo synthesis. These metabolic alterations can be exploited by therapy, which aims to improve treatment outcome and decrease relapse and reoccurrence. One clinically utilised strategy targeting AA dependency is the use of asparaginase in the treatment of acute lymphoblastic leukaemia (ALL), which results in a depletion of exogenous asparagine and subsequent cancer cell death. Examples of other successful strategies include the exploitation of arginine deiminase and methioninase, nutrient restriction of methionine and the inhibition of glutaminase. In this review, we summarise these treatment strategies into three promising avenues: AA restriction, enzymatic depletion and inhibition of metabolism. This review provides an insight into the complexity of metabolism in cancer, whilst highlighting these three current research avenues that have support in both preclinical and clinical settings.
    Keywords:  Amino acids; Asparaginase; Cancer; Metabolism; Oncology
  4. Autophagy. 2021 Jul 19. 1-4
      Huntington disease (HD) manifests a unique macroautophagy/autophagy defect: the presense of cytosolic autophagosomes without substrates or so-called "empty" autophagosomes. It was proposed that mutant HTT (huntingtin; mHTT) disrupts cargo recognition by the selective autophagy receptor SQSTM1/p62 thus leading to the failure of cargo sequestration by phagophores, the precursors to autophagosomes. Here we looked at recent discoveries that liquid-like SQSTM1 droplets can serve as platforms for autophagosome formation, and discussed possible alternative mechanisms for "empty" autophagosome formation in HD inspired by these findings.
    Keywords:  Autophagosome; Huntington disease; SQSTM1/p62 droplet; huntingtin; phase transition
  5. Autophagy. 2021 Jul 21. 1-20
      Macroautophagy (hereafter referred to as autophagy) is a finely tuned process of programmed degradation and recycling of proteins and cellular components, which is crucial in neuronal function and synaptic integrity. Mounting evidence implicates chromatin remodeling in fine-tuning autophagy pathways. However, this epigenetic regulation is poorly understood in neurons. Here, we investigate the role in autophagy of KANSL1, a member of the nonspecific lethal complex, which acetylates histone H4 on lysine 16 (H4K16ac) to facilitate transcriptional activation. Loss-of-function of KANSL1 is strongly associated with the neurodevelopmental disorder Koolen-de Vries Syndrome (KdVS). Starting from KANSL1-deficient human induced-pluripotent stem cells, both from KdVS patients and genome-edited lines, we identified SOD1 (superoxide dismutase 1), an antioxidant enzyme, to be significantly decreased, leading to a subsequent increase in oxidative stress and autophagosome accumulation. In KANSL1-deficient neurons, autophagosome accumulation at excitatory synapses resulted in reduced synaptic density, reduced GRIA/AMPA receptor-mediated transmission and impaired neuronal network activity. Furthermore, we found that increased oxidative stress-mediated autophagosome accumulation leads to increased MTOR activation and decreased lysosome function, further preventing the clearing of autophagosomes. Finally, by pharmacologically reducing oxidative stress, we could rescue the aberrant autophagosome formation as well as synaptic and neuronal network activity in KANSL1-deficient neurons. Our findings thus point toward an important relation between oxidative stress-induced autophagy and synapse function, and demonstrate the importance of H4K16ac-mediated changes in chromatin structure to balance reactive oxygen species- and MTOR-dependent autophagy.Abbreviations: APO: apocynin; ATG: autophagy related; BAF: bafilomycin A1; BSO: buthionine sulfoximine; CV: coefficient of variation; DIV: days in vitro; H4K16ac: histone 4 lysine 16 acetylation; iPSC: induced-pluripotent stem cell; KANSL1: KAT8 regulatory NSL complex subunit 1; KdVS: Koolen-de Vries Syndrome; LAMP1: lysosomal associated membrane protein 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MEA: micro-electrode array; MTOR: mechanistic target of rapamycin kinase; NSL complex: nonspecific lethal complex; 8-oxo-dG: 8-hydroxydesoxyguanosine; RAP: rapamycin; ROS: reactive oxygen species; sEPSCs: spontaneous excitatory postsynaptic currents; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; SYN: synapsin; WRT: wortmannin.
    Keywords:  Autophagy; Koolen-de Vries syndrome; MTOR; iPSCs; neuronal development; reactive oxygen species; synaptic function
  6. Autophagy. 2021 Jul 19. 1-3
      As part of innate immune defenses, macroautophagy/autophagy targets viruses and viral components for lysosomal degradation and exposes pathogen-associated molecular patterns to facilitate recognition. However, viruses evolved sophisticated strategies to antagonize autophagy and even exploit it to promote their replication. In our recent study, we systematically analyzed the impact of individual SARS-CoV-2 proteins on autophagy. We showed that E, M, ORF3a, and ORF7a cause an accumulation of autophagosomes, whereas Nsp15 prevents the efficient formation of autophagosomes. Consequently, autophagic degradation of SQSTM1/p62 is decreased in the presence of E, ORF3a, ORF7a, and Nsp15. Notably, M does not alter SQSTM1 protein levels and colocalizes with accumulations of LC3B-positive membranes not resembling vesicles. Infection with SARS-CoV-2 prevents SQSTM1 degradation and increases lipidation of LC3B, indicating overall that the infection causes a reduction of autophagic flux. Our mechanistic analyses showed that the accessory proteins ORF3a and ORF7a both block autophagic degradation but use different strategies. While ORF3a prevents the fusion between autophagosomes and lysosomes, ORF7a reduces the acidity of lysosomes. In summary, we found that Nsp15, E, M, ORF3a, and ORF7a of SARS-CoV-2 manipulate cellular autophagy, and we determined the molecular mechanisms of ORF3a and ORF7a.
    Keywords:  Autophagy; COVID-19; Envelope protein; Membrane protein; Nsp15; ORF3a; ORF7a; SARS-CoV; SARS-CoV-2; double-membrane vesicles; innate immunity