bims-unfpre Biomed News
on Unfolded protein response
Issue of 2020‒03‒22
seven papers selected by
Susan Logue
University of Manitoba

  1. iScience. 2020 Feb 28. pii: S2589-0042(20)30136-X. [Epub ahead of print]23(3): 100952
    Ireland S, Ramnarayanan S, Fu M, Zhang X, Zhang J, Li J, Emebo D, Wang Y.
      It has been well documented that the ER responds to cellular stresses through the unfolded protein response (UPR), but it is unknown how the Golgi responds to similar stresses. In this study, we treated HeLa cells with ER stress inducers, thapsigargin (TG), tunicamycin (Tm), and dithiothreitol (DTT), and found that only TG treatment resulted in Golgi fragmentation. TG induced Golgi fragmentation at a low dose and short time when UPR was undetectable, indicating that Golgi fragmentation occurs independently of ER stress. Further experiments demonstrated that TG induces Golgi fragmentation through elevating intracellular Ca2+ and protein kinase Cα (PKCα) activity, which phosphorylates the Golgi stacking protein GRASP55. Significantly, activation of PKCα with other activating or inflammatory agents, including phorbol 12-myristate 13-acetate and histamine, modulates Golgi structure in a similar fashion. Hence, our study revealed a novel mechanism through which increased cytosolic Ca2+ modulates Golgi structure and function.
    Keywords:  Biological Sciences; Cell Biology; Functional Aspects of Cell Biology
  2. Cells. 2020 Mar 12. pii: E695. [Epub ahead of print]9(3):
    Di Conza G, Ho PC.
      The endoplasmic reticulum (ER) is a critical organelle, storing the majority of calcium and governing protein translation. Thus, it is crucial to keep the homeostasis in all ER components and machineries. The ER stress sensor pathways, including IRE1/sXBP1, PERK/EIf2 and ATF6, orchestrate the major regulatory circuits to ensure ER homeostasis. The embryonic or postnatal lethality that occurs upon genetic depletion of these sensors reveals the essential role of the ER stress pathway in cell biology. In contrast, the impairment or excessive activation of ER stress has been reported to cause or aggravate several diseases such as atherosclerosis, diabetes, NAFDL/NASH, obesity and cancer. Being part of innate immunity, myeloid cells are the first immune cells entering the inflammation site. Upon entry into a metabolically stressed disease environment, activation of ER stress occurs within the myeloid compartment, leading to the modulation of their phenotype and functions. In this review, we discuss causes and consequences of ER stress activation in the myeloid compartment with a special focus on the crosstalk between ER, innate signaling and metabolic environments.
    Keywords:  ER stress; chronic diseases; infection; innate immunity
  3. J Biol Chem. 2020 Mar 16. pii: jbc.RA120.012721. [Epub ahead of print]
    Zhang IX, Ren J, Vadrevu S, Raghavan M, Satin LS.
      Type 2 diabetes mellitus (T2DM) is characterized by impaired glucose-stimulated insulin secretion and increased peripheral insulin resistance. Unremitting ER stress can lead to b-cell apoptosis and has been linked to type 2 diabetes. Although many studies have attempted to link ER stress and T2DM, the specific effects of ER stress on b-cell function remain incompletely understood. To determine the interrelationship between ER stress and b-cell function, here we treated insulin-secreting INS-1(832/13) cells or isolated mouse islets with the ER stress inducer tunicamycin (TM). TM induced ER stress as expected, as evidenced by activation of the unfolded protein response (UPR). b Cells treated with TM also exhibited concomitant alterations in their electrical activity and cytosolic free Ca2+ oscillations. As ER stress is known to reduce ER Ca2+ levels, we tested the hypothesis that the observed increase in Ca2+ oscillations occurred because of reduced ER Ca2+ levels and, in turn, increased store-operated Ca2+ entry (or SOCE). TM-induced cytosolic Ca2+ and membrane electrical oscillations were acutely inhibited by YM58483, which blocks store-operated Ca2+ channels. Significantly, TM-treated cells secreted increased insulin under conditions normally associated with only minimal release, e.g. 5 mM glucose, YM58483 blocked this secretion. Taken together, these results support a critical role for ER Ca2+ depletion-activated Ca2+ current in mediating Ca2+-induced insulin secretion in response to ER stress.
    Keywords:  SOCE; beta cell (B-cell); calcium signaling; cellular calcium homeostasis; diabetes; endoplasmic reticulum stress (ER stress); insulin resistance; insulin secretion; pancreatic islet; store-operated calcium channel; unfolded protein response (UPR)
  4. PLoS Genet. 2020 Mar 19. 16(3): e1008638
    Haeussler S, Köhler F, Witting M, Premm MF, Rolland SG, Fischer C, Chauve L, Casanueva O, Conradt B.
      Compromising mitochondrial fusion or fission disrupts cellular homeostasis; however, the underlying mechanism(s) are not fully understood. The loss of C. elegans fzo-1MFN results in mitochondrial fragmentation, decreased mitochondrial membrane potential and the induction of the mitochondrial unfolded protein response (UPRmt). We performed a genome-wide RNAi screen for genes that when knocked-down suppress fzo-1MFN(lf)-induced UPRmt. Of the 299 genes identified, 143 encode negative regulators of autophagy, many of which have previously not been implicated in this cellular quality control mechanism. We present evidence that increased autophagic flux suppresses fzo-1MFN(lf)-induced UPRmt by increasing mitochondrial membrane potential rather than restoring mitochondrial morphology. Furthermore, we demonstrate that increased autophagic flux also suppresses UPRmt induction in response to a block in mitochondrial fission, but not in response to the loss of spg-7, which encodes a mitochondrial metalloprotease. Finally, we found that blocking mitochondrial fusion or fission leads to increased levels of certain types of triacylglycerols and that this is at least partially reverted by the induction of autophagy. We propose that the breakdown of these triacylglycerols through autophagy leads to elevated metabolic activity, thereby increasing mitochondrial membrane potential and restoring mitochondrial and cellular homeostasis.
  5. J Clin Invest. 2020 Mar 17. pii: 134874. [Epub ahead of print]
    Shrestha N, Liu T, Ji Y, Reinert R, Torres M, Li X, Zhang M, Tang CA, Hu CA, Liu C, Naji A, Liu M, Lin JD, Kersten S, Arvan P, Qi L.
      β-cell apoptosis and dedifferentiation are two hotly-debated mechanisms underlying β-cell loss in type 2 diabetes; however, the molecular drivers underlying such events remain largely unclear. Here, we performed a side-by-side comparison of mice carrying β-cell-specific deletion of endoplasmic reticulum (ER)-associated degradation (ERAD) and autophagy. We reported that while autophagy was necessary for β-cell survival, the highly conserved Sel1L-Hrd1 ERAD protein complex was required for the maintenance of β-cell maturation and identity. Using single cell RNA-sequencing, we demonstrated that Sel1L deficiency was not associated with β-cell loss, but rather loss of β-cell identity. Sel1L-Hrd1 ERAD controlled β-cell identity via TGFβ signaling, in part by mediating the degradation of TGFβ receptor 1 (TGFβRI). Inhibition of TGFβ signaling in Sel1L-deficient β-cells augmented the expression of β-cell maturation markers and increased the total insulin content. Our data revealed distinct pathogenic effects of two major proteolytic pathways in β-cells, providing a new framework for therapies targeting distinct mechanisms of protein quality control.
    Keywords:  Beta cells; Cell Biology; Diabetes; Metabolism; Protein misfolding
  6. J Virol. 2020 Mar 18. pii: JVI.00178-20. [Epub ahead of print]
    Volk A, Hackbart M, Deng X, Cruz-Pulido Y, O'Brien A, Baker SC.
      Coronaviruses encode multiple interferon antagonists that modulate the host response to virus replication. Here, we evaluated the host transcriptional response to infection with murine coronaviruses encoding independent mutations in one of two different viral antagonists: the deubiquitinase (DUB) within nonstructural protein 3 or the endoribonuclease (EndoU) within nonstructural protein 15. We used transcriptomics approaches to compare the scope and kinetics of the host response to the wild-type, DUBmut, and EndoUmut viruses in infected macrophages. We found that the EndoUmut virus activates a focused response predominantly involving type I interferons and interferon-related genes, whereas the WT and DUBmut viruses more broadly stimulate upregulation of over 2,800 genes, including networks associated with activating the unfolded protein response (UPR), and the proinflammatory response associated with viral pathogenesis. This study highlights the role of viral interferon antagonists in shaping the kinetics and magnitude of the host response during virus infection and demonstrates that inactivating a dominant viral antagonist, the coronavirus endoribonuclease, dramatically alters the host response in macrophages.ImportanceMacrophages are an important cell type during coronavirus infections because they "notice" the infection and respond by inducing type I interferons, which limits virus replication. In turn, coronaviruses encode proteins that mitigate the cell's ability to signal an interferon response. Here, we evaluated the host macrophage response to two independent mutant coronaviruses: one with reduced deubiquitinating activity (DUBmut) and the other containing an inactivated endoribonuclease (EndoUmut). We observed a rapid, robust, and focused response to the EndoUmut virus, which was characterized by enhanced expression of interferon and interferon-related genes. In contrast, wild-type virus and the DUBmut virus elicited a more limited interferon response and ultimately activated over 2,800 genes, including players in the unfolded protein response and pro-inflammatory pathways associated with progression of significant disease. This study reveals that EndoU activity substantially contributes to the ability of coronaviruses to evade the host innate response and to replicate in macrophages.
  7. Metabolism. 2020 Mar 14. pii: S0026-0495(20)30069-X. [Epub ahead of print] 154205
    Sozen E, Yazgan B, Tok OE, Demirel T, Ercan F, Proto JD, Ozer NK.
      BACKGROUND: Cardiovascular diseases (CVDs), with highest mortality and morbidity rates, are the major cause of death in the world. Due to the limited information on heart tissue changes, mediated by hypercholesterolemia, we planned to investigate molecular mechanisms of endoplasmic reticulum (ER) stress and related cell death in high cholesterol fed rabbit model and possible beneficial effects of α-tocopherol.METHODS: Molecular changes in rabbit heart tissue and cultured cardiomyocytes (H9c2 cells) were measured by western blotting, qRT-PCR, immunflouresence and flow cytometry experiments. Histological modifications were assessed by light and electron microscopes, while degradation of mitochondria was quantified through confocal microscope.
    RESULTS: Feeding rabbits 2% cholesterol diet for 8 weeks and treatment of cultured cardiomyocytes with 10 μg/ml cholesterol for 3 h induced excessive autophagic activity via IRE1/JNK pathway. While no change in ER-associated degradation (ERAD) and apoptotic cell death were determined, electron and confocal microscopy analyses in cholesterol supplemented rabbits revealed significant parameters of autophagic cell death, including cytoplasmic autophagosomes, autolysosomes and organelle loss in juxtanuclear area as well as mitochondria engulfment by autophagosome. Either inhibition of ER stress or JNK in cultured cardiomyocytes or α-tocopherol supplementation in rabbits could counteract the effects of cholesterol.
    CONCLUSION: Our findings underline the essential role of hypercholesterolemia in stimulating IRE1/JNK branch of ER stress response which then leads to autophagic cell death in heart tissue. Results also showed α-tocopherol as a promising regulator of autophagic cell death in cardiomyocytes.
    Keywords:  Autophagic cell death; Cardiomyocyte; Endoplasmic reticulum stress; Hypercholesterolemia; α-Tocopherol