bims-ershed Biomed News
on ER Stress in Health and Diseases
Issue of 2021‒10‒17
six papers selected by
Matías Eduardo González Quiroz
Worker’s Hospital

  1. Cell Mol Life Sci. 2021 Oct 12.
      Accumulation of misfolded proteins in ER activates the unfolded protein response (UPR), a multifunctional signaling pathway that is important for cell survival. The UPR is regulated by three ER transmembrane sensors, one of which is inositol-requiring protein 1 (IRE1). IRE1 activates a transcription factor, X-box-binding protein 1 (XBP1), by removing a 26-base intron from XBP1 mRNA that generates spliced XBP1 mRNA (XBP1s). To search for XBP1 transcriptional targets, we utilized an XBP1s-inducible human cell line to limit XBP1 expression in a controlled manner. We also verified the identified XBP1-dependent genes with specific silencing of this transcription factor during pharmacological ER stress induction with both an N-linked glycosylation inhibitor (tunicamycin) and a non-competitive inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) (thapsigargin). We then compared those results to the XBP1s-induced cell line without pharmacological ER stress induction. Using next-generation sequencing followed by bioinformatic analysis of XBP1-binding motifs, we defined an XBP1 regulatory network and identified XBP1 as a repressor of PUMA (a proapoptotic gene) and IRE1 mRNA expression during the UPR. Our results indicate impairing IRE1 activity during ER stress conditions accelerates cell death in ER-stressed cells, whereas elevating XBP1 expression during ER stress using an inducible cell line correlated with a clear prosurvival effect and reduced PUMA protein expression. Although further studies will be required to test the underlying molecular mechanisms involved in the relationship between these genes with XBP1, these studies identify a novel repressive role of XBP1 during the UPR.
    Keywords:  BBC3; ER stress; ERN1; UPR; XBP1s; XBP1u
  2. Life Sci Alliance. 2021 Dec;pii: e202101247. [Epub ahead of print]4(12):
      The intracellular bacterial pathogen Legionella pneumophila (L.p.) secretes ∼330 effector proteins into the host cell to sculpt an ER-derived replicative niche. We previously reported five L.p. effectors that inhibit IRE1, a key sensor of the homeostatic unfolded protein response (UPR) pathway. In this study, we discovered a subset of L.p. toxins that selectively activate the UPR sensor ATF6, resulting in its cleavage, nuclear translocation, and target gene transcription. In a deviation from the conventional model, this L.p-dependent activation of ATF6 does not require its transport to the Golgi or its cleavage by the S1P/S2P proteases. We believe that our findings highlight the unique regulatory control that L.p exerts upon the three UPR sensors and expand the repertoire of bacterial proteins that selectively perturb host homeostatic pathways.
  3. Shock. 2021 Nov 01. 56(5): 755-761
      ABSTRACT: After cardiac arrest (CA) and resuscitation, the unfolded protein response (UPR) is activated in various organs including the brain. However, the role of the UPR in CA outcome remains largely unknown. One UPR branch involves spliced X-box-binding protein-1 (XBP1s). Notably, XBP1s, a transcriptional factor, can upregulate expression of specific enzymes related to glucose metabolism, and subsequently boost O-linked β-N-acetylglucosamine modification (O-GlcNAcylation). The current study is focused on effects of the XBP1 UPR branch and its downstream O-GlcNAcylation on CA outcome. Using both loss-of-function and gain-of-function mouse genetic tools, we provide the first evidence that activation of the XBP1 UPR branch in the post-CA brain is neuroprotective. Specifically, neuron-specific Xbp1 knockout mice had worse CA outcome, while mice with neuron-specific expression of Xbp1s in the brain had better CA outcome. Since it has been shown that the protective role of the XBP1s signaling pathway under ischemic conditions is mediated by increasing O-GlcNAcylation, we then treated young mice with glucosamine, and found that functional deficits were mitigated on day 3 post CA. Finally, after confirming that glucosamine can boost O-GlcNAcylation in the aged brain, we subjected aged mice to 8 min CA, and then treated them with glucosamine. We found that glucosamine-treated aged mice performed significantly better in behavioral tests. Together, our data indicate that the XBP1s/O-GlcNAc pathway is a promising target for CA therapy.
  4. Trends Biochem Sci. 2021 Oct 12. pii: S0968-0004(21)00222-X. [Epub ahead of print]
      A recent study by Huang et al. unexpectedly uncovered that DAXX moonlights as a booster of protein folding, including counteracting aggregation of tumor suppressor p53. Since p53 aggregation is a common hallmark of cancer, this finding provides a potential pathway to therapeutically reactivate p53 signaling and halt tumor progression.
    Keywords:  DAXX; chaperone; disaggregase; p53; protein aggregation
  5. J Virol. 2021 Oct 13. JVI0169521
      The replication of coronaviruses, including severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV) and the recently emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is closely associated with the endoplasmic reticulum (ER) of infected cells. The unfolded protein response (UPR), which is mediated by ER stress (ERS), is a typical outcome in coronavirus-infected cells and is closely associated with the characteristics of coronaviruses. However, the interaction between virus-induced ERS and coronavirus replication is poorly understood. Here, we demonstrated that infection with the betacoronavirus porcine hemagglutinating encephalomyelitis virus (PHEV) induced ERS and triggered all three branches of the UPR signaling pathway both in vitro and in vivo. In addition, ERS suppressed PHEV replication in mouse neuro-2a (N2a) cells primarily by activating the protein kinase R-like ER kinase (PERK)-eukaryotic initiation factor 2α (eIF2α) axis of the UPR. Moreover, another eIF2α phosphorylation kinase, IFN-induced double-stranded RNA-dependent protein kinase (PKR), was also activated and acted cooperatively with PERK to decrease PHEV replication. Furthermore, we demonstrated that the PERK/PKR-eIF2α pathways negatively regulated PHEV replication by attenuating global protein translation. Phosphorylated eIF2α also promoted the formation of stress granule (SG), which in turn repressed PHEV replication. In summary, our study presents a vital aspect of the host innate response to invading pathogens and reveals attractive host targets (e.g., PERK, PKR and eIF2α) for antiviral drugs. IMPORTANCE Coronavirus diseases are caused by different coronaviruses of importance in humans and animals, and specific treatments are extremely limited. ERS, which can activate the UPR to modulate viral replication and the host innate response, is a frequent occurrence in coronavirus-infected cells. PHEV, a neurotropic β-coronavirus, causes nerve cell damage, which accounts for the high mortality rates in suckling piglets. However, it remains incompletely understood whether the highly developed ER in nerve cells plays an antiviral role in ERS and how ERS regulates viral proliferation. In this study, we found that PHEV infection induced ERS and activated the UPR both in vitro and in vivo and that the activated PERK/PKR-eIF2α axis inhibited PHEV replication through attenuating global protein translation and promoting SG formation. A better understanding of coronavirus-induced ERS and UPR activation may reveal the pathogenic mechanism of coronavirus and facilitate the development of new treatment strategies for these diseases.