Publications by authors named "Aeid Igbaria"

14 Publications

  • Page 1 of 1

Reflux of Endoplasmic Reticulum proteins to the cytosol inactivates tumor suppressors.

EMBO Rep 2021 05 12;22(5):e51412. Epub 2021 Mar 12.

Inserm U1242, University of Rennes, Rennes, France.

In the past decades, many studies reported the presence of endoplasmic reticulum (ER)-resident proteins in the cytosol. However, the mechanisms by which these proteins relocate and whether they exert cytosolic functions remain unknown. We find that a subset of ER luminal proteins accumulates in the cytosol of glioblastoma cells isolated from mouse and human tumors. In cultured cells, ER protein reflux to the cytosol occurs upon ER proteostasis perturbation. Using the ER luminal protein anterior gradient 2 (AGR2) as a proof of concept, we tested whether the refluxed proteins gain new functions in the cytosol. We find that refluxed, cytosolic AGR2 binds and inhibits the tumor suppressor p53. These data suggest that ER reflux constitutes an ER surveillance mechanism to relieve the ER from its contents upon stress, providing a selective advantage to tumor cells through gain-of-cytosolic functions-a phenomenon we name ER to Cytosol Signaling (ERCYS).
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http://dx.doi.org/10.15252/embr.202051412DOI Listing
May 2021

Control of Protein Homeostasis in the Early Secretory Pathway: Current Status and Challenges.

Cells 2019 10 29;8(11). Epub 2019 Oct 29.

Proteostasis & Cancer Team INSERM U1242 « Chemistry, Oncogenesis Stress Signaling », Université de Rennes, CEDEX, 35042 Rennes, France.

Discrimination between properly folded proteins and those that do not reach this state is necessary for cells to achieve functionality. Eukaryotic cells have evolved several mechanisms to ensure secretory protein quality control, which allows efficiency and fidelity in protein production. Among the actors involved in such process, both endoplasmic reticulum (ER) and the Golgi complex play prominent roles in protein synthesis, biogenesis and secretion. ER and Golgi functions ensure that only properly folded proteins are allowed to flow through the secretory pathway while improperly folded proteins have to be eliminated to not impinge on cellular functions. Thus, complex quality control and degradation machineries are crucial to prevent the toxic accumulation of improperly folded proteins. However, in some instances, improperly folded proteins can escape the quality control systems thereby contributing to several human diseases. Herein, we summarize how the early secretory pathways copes with the accumulation of improperly folded proteins, and how insufficient handling can cause the development of several human diseases. Finally, we detail the genetic and pharmacologic approaches that could be used as potential therapeutic tools to treat these diseases.
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http://dx.doi.org/10.3390/cells8111347DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6912474PMC
October 2019

A guide to assessing endoplasmic reticulum homeostasis and stress in mammalian systems.

FEBS J 2020 01 11;287(1):27-42. Epub 2019 Nov 11.

Inserm U1242, University of Rennes, France.

The endoplasmic reticulum (ER) is a multifunctional organelle that constitutes the entry into the secretory pathway. The ER contributes to the maintenance of cellular calcium homeostasis, lipid synthesis and productive secretory, and transmembrane protein folding. Physiological, chemical, and pathological factors that compromise ER homeostasis lead to endoplasmic reticulum stress (ER stress). To cope with this situation, cells activate an adaptive signaling pathway termed the unfolded protein response (UPR) that aims at restoring ER homeostasis. The UPR is transduced through post-translational, translational, post-transcriptional, and transcriptional mechanisms initiated by three ER-resident sensors, inositol-requiring protein 1α, activating transcription factor 6α, and PRKR-like endoplasmic reticulum kinase. Determining the in and out of ER homeostasis control and UPR activation still represents a challenge for the community. Hence, standardized criteria and methodologies need to be proposed for monitoring ER homeostasis and ER stress in different model systems. Here, we summarize the pathways that are activated during ER stress and provide approaches aimed at assess ER homeostasis and stress in vitro and in vivo mammalian systems that can be used by researchers to plan and interpret experiments. We recommend the use of multiple assays to verify ER stress because no individual assay is guaranteed to be the most appropriate one.
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http://dx.doi.org/10.1111/febs.15107DOI Listing
January 2020

Chaperone-mediated reflux of secretory proteins to the cytosol during endoplasmic reticulum stress.

Proc Natl Acad Sci U S A 2019 06 17;116(23):11291-11298. Epub 2019 May 17.

Department of Medicine, University of California, San Francisco, CA 94143;

Diverse perturbations to endoplasmic reticulum (ER) functions compromise the proper folding and structural maturation of secretory proteins. To study secretory pathway physiology during such "ER stress," we employed an ER-targeted, redox-responsive, green fluorescent protein-eroGFP-that reports on ambient changes in oxidizing potential. Here we find that diverse ER stress regimes cause properly folded, ER-resident eroGFP (and other ER luminal proteins) to "reflux" back to the reducing environment of the cytosol as intact, folded proteins. By utilizing eroGFP in a comprehensive genetic screen in , we show that ER protein reflux during ER stress requires specific chaperones and cochaperones residing in both the ER and the cytosol. Chaperone-mediated ER protein reflux does not require E3 ligase activity, and proceeds even more vigorously when these ER-associated degradation (ERAD) factors are crippled, suggesting that reflux may work in parallel with ERAD. In summary, chaperone-mediated ER protein reflux may be a conserved protein quality control process that evolved to maintain secretory pathway homeostasis during ER protein-folding stress.
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http://dx.doi.org/10.1073/pnas.1904516116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6561268PMC
June 2019

Regulation of tumor-stroma interactions by the unfolded protein response.

FEBS J 2019 01 29;286(2):279-296. Epub 2017 Dec 29.

Inserm U1242 'Chemistry, Oncogenesis, Stress & Signaling', Université de Rennes, Rennes, France.

The unfolded protein response (UPR) is a conserved adaptive pathway that helps cells cope with the protein misfolding burden within the endoplasmic reticulum (ER). Imbalance between protein folding demand and capacity in the ER leads to a situation called ER stress that is often observed in highly proliferative and secretory tumor cells. As such, activation of the UPR signaling has emerged as a key adaptive mechanism promoting cancer progression. It is becoming widely acknowledged that, in addition to its intrinsic effect on tumor biology, the UPR can also regulate tumor microenvironment. In this review, we discuss how the UPR coordinates the crosstalk between tumor and stromal cells, such as endothelial cells, normal parenchymal cells, and immune cells. In addition, we further describe the involvement of ER stress signaling in the response to current treatments as well as its impact on antitumor immunity mainly driven by immunogenic cell death. Finally, in this context, we discuss the relevance of targeting ER stress/UPR signaling as a potential anticancer approach.
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http://dx.doi.org/10.1111/febs.14359DOI Listing
January 2019

Endoplasmic Reticulum Transport of Glutathione by Sec61 Is Regulated by Ero1 and Bip.

Mol Cell 2017 Sep 14;67(6):962-973.e5. Epub 2017 Sep 14.

Institute for Integrative Biology of the Cell (I2BC), CEA-Saclay, CNRS, Université Paris-Saclay, ISVJC/SBIGEM, Laboratoire Stress Oxydant et Cancer, 91191 Gif-sur-Yvette, France. Electronic address:

In the endoplasmic reticulum (ER), Ero1 catalyzes disulfide bond formation and promotes glutathione (GSH) oxidation to GSSG. Since GSSG cannot be reduced in the ER, maintenance of the ER glutathione redox state and levels likely depends on ER glutathione import and GSSG export. We used quantitative GSH and GSSG biosensors to monitor glutathione import into the ER of yeast cells. We found that glutathione enters the ER by facilitated diffusion through the Sec61 protein-conducting channel, while oxidized Bip (Kar2) inhibits transport. Increased ER glutathione import triggers HO-dependent Bip oxidation through Ero1 reductive activation, which inhibits glutathione import in a negative regulatory loop. During ER stress, transport is activated by UPR-dependent Ero1 induction, and cytosolic glutathione levels increase. Thus, the ER redox poise is tuned by reciprocal control of glutathione import and Ero1 activation. The ER protein-conducting channel is permeable to small molecules, provided the driving force of a concentration gradient.
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http://dx.doi.org/10.1016/j.molcel.2017.08.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5772773PMC
September 2017

Light-sensing via hydrogen peroxide and a peroxiredoxin.

Nat Commun 2017 03 24;8:14791. Epub 2017 Mar 24.

Department of Chemistry and Molecular Biology, University of Gothenburg, Box 462, S-413 90 Göteborg, Sweden.

Yeast lacks dedicated photoreceptors; however, blue light still causes pronounced oscillations of the transcription factor Msn2 into and out of the nucleus. Here we show that this poorly understood phenomenon is initiated by a peroxisomal oxidase, which converts light into a hydrogen peroxide (HO) signal that is sensed by the peroxiredoxin Tsa1 and transduced to thioredoxin, to counteract PKA-dependent Msn2 phosphorylation. Upon HO, the nuclear retention of PKA catalytic subunits, which contributes to delayed Msn2 nuclear concentration, is antagonized in a Tsa1-dependent manner. Conversely, peroxiredoxin hyperoxidation interrupts the HO signal and drives Msn2 oscillations by superimposing on PKA feedback regulation. Our data identify a mechanism by which light could be sensed in all cells lacking dedicated photoreceptors. In particular, the use of HO as a second messenger in signalling is common to Msn2 oscillations and to light-induced entrainment of circadian rhythms and suggests conserved roles for peroxiredoxins in endogenous rhythms.
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http://dx.doi.org/10.1038/ncomms14791DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5376668PMC
March 2017

Allosteric inhibition of the IRE1α RNase preserves cell viability and function during endoplasmic reticulum stress.

Cell 2014 Jul 10;158(3):534-48. Epub 2014 Jul 10.

Department of Medicine, University of California, San Francisco, San Francisco, CA 94143, USA; Diabetes Center, University of California, San Francisco, San Francisco, CA 94143, USA; Lung Biology Center, University of California, San Francisco, San Francisco, CA 94143, USA; California Institute for Quantitative Biosciences, University of California, San Francisco, San Francisco, CA 94143, USA. Electronic address:

Depending on endoplasmic reticulum (ER) stress levels, the ER transmembrane multidomain protein IRE1α promotes either adaptation or apoptosis. Unfolded ER proteins cause IRE1α lumenal domain homo-oligomerization, inducing trans autophosphorylation that further drives homo-oligomerization of its cytosolic kinase/endoribonuclease (RNase) domains to activate mRNA splicing of adaptive XBP1 transcription factor. However, under high/chronic ER stress, IRE1α surpasses an oligomerization threshold that expands RNase substrate repertoire to many ER-localized mRNAs, leading to apoptosis. To modulate these effects, we developed ATP-competitive IRE1α Kinase-Inhibiting RNase Attenuators-KIRAs-that allosterically inhibit IRE1α's RNase by breaking oligomers. One optimized KIRA, KIRA6, inhibits IRE1α in vivo and promotes cell survival under ER stress. Intravitreally, KIRA6 preserves photoreceptor functional viability in rat models of ER stress-induced retinal degeneration. Systemically, KIRA6 preserves pancreatic β cells, increases insulin, and reduces hyperglycemia in Akita diabetic mice. Thus, IRE1α powerfully controls cell fate but can itself be controlled with small molecules to reduce cell degeneration.
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http://dx.doi.org/10.1016/j.cell.2014.07.002DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4244221PMC
July 2014

Functions and cellular compartmentation of the thioredoxin and glutathione pathways in yeast.

Antioxid Redox Signal 2013 May 5;18(13):1699-711. Epub 2013 Feb 5.

Laboratoire Stress Oxydants et Cancer, IBITECS, CEA-Saclay, Gif-sur-Yvette, France.

Significance: The thioredoxin (TRX) and glutathione (GSH) pathways are universally conserved thiol-reductase systems that drive an array of cellular functions involving reversible disulfide formation. Here we consider these pathways in Saccharomyces cerevisiae, focusing on their cell compartment-specific functions, as well as the mechanisms that explain extreme differences of redox states between compartments.

Recent Advances: Recent work leads to a model in which the yeast TRX and GSH pathways are not redundant, in contrast to Escherichia coli. The cytosol possesses full sets of both pathways, of which the TRX pathway is dominant, while the GSH pathway acts as back up of the former. The mitochondrial matrix also possesses entire sets of both pathways, in which the GSH pathway has major role in redox control. In both compartments, GSH has also nonredox functions in iron metabolism, essential for viability. The endoplasmic reticulum (ER) and mitochondrial intermembrane space (IMS) are sites of intense thiol oxidation, but except GSH lack thiol-reductase pathways.

Critical Issues: What are the thiol-redox links between compartments? Mitochondria are totally independent, and insulated from the other compartments. The cytosol is also totally independent, but also provides reducing power to the ER and IMS, possibly by ways of reduced and oxidized GSH entering and exiting these compartments.

Future Directions: Identifying the mechanisms regulating fluxes of GSH and oxidized glutathione between cytosol and ER, IMS, and possibly also peroxisomes, vacuole is needed to establish the proposed model of eukaryotic thiol-redox homeostasis, which should facilitate exploration of this system in mammals and plants.
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http://dx.doi.org/10.1089/ars.2012.5033DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3771550PMC
May 2013

IRE1α induces thioredoxin-interacting protein to activate the NLRP3 inflammasome and promote programmed cell death under irremediable ER stress.

Cell Metab 2012 Aug;16(2):250-64

Department of Medicine, University of California, San Francisco, San Francisco, CA 94143, USA.

When unfolded proteins accumulate to irremediably high levels within the endoplasmic reticulum (ER), intracellular signaling pathways called the unfolded protein response (UPR) become hyperactivated to cause programmed cell death. We discovered that thioredoxin-interacting protein (TXNIP) is a critical node in this "terminal UPR." TXNIP becomes rapidly induced by IRE1α, an ER bifunctional kinase/endoribonuclease (RNase). Hyperactivated IRE1α increases TXNIP mRNA stability by reducing levels of a TXNIP destabilizing microRNA, miR-17. In turn, elevated TXNIP protein activates the NLRP3 inflammasome, causing procaspase-1 cleavage and interleukin 1β (IL-1β) secretion. Txnip gene deletion reduces pancreatic β cell death during ER stress and suppresses diabetes caused by proinsulin misfolding in the Akita mouse. Finally, small molecule IRE1α RNase inhibitors suppress TXNIP production to block IL-1β secretion. In summary, the IRE1α-TXNIP pathway is used in the terminal UPR to promote sterile inflammation and programmed cell death and may be targeted to develop effective treatments for cell degenerative diseases.
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http://dx.doi.org/10.1016/j.cmet.2012.07.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4014071PMC
August 2012

Glutathione revisited: a vital function in iron metabolism and ancillary role in thiol-redox control.

EMBO J 2011 May 8;30(10):2044-56. Epub 2011 Apr 8.

LSOC, DSV, IBITECS, CEA-Saclay, France.

Glutathione contributes to thiol-redox control and to extra-mitochondrial iron-sulphur cluster (ISC) maturation. To determine the physiological importance of these functions and sort out those that account for the GSH requirement for viability, we performed a comprehensive analysis of yeast cells depleted of or containing toxic levels of GSH. Both conditions triggered an intense iron starvation-like response and impaired the activity of extra-mitochondrial ISC enzymes but did not impact thiol-redox maintenance, except for high glutathione levels that altered oxidative protein folding in the endoplasmic reticulum. While iron partially rescued the ISC maturation and growth defects of GSH-depleted cells, genetic experiments indicated that unlike thioredoxin, glutathione could not support by itself the thiol-redox duties of the cell. We propose that glutathione is essential by its requirement in ISC assembly, but only serves as a thioredoxin backup in cytosolic thiol-redox maintenance. Glutathione-high physiological levels are thus meant to insulate its cytosolic function in iron metabolism from variations of its concentration during redox stresses, a model challenging the traditional view of it as prime actor in thiol-redox control.
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http://dx.doi.org/10.1038/emboj.2011.105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3098478PMC
May 2011

Proteome screens for Cys residues oxidation: the redoxome.

Methods Enzymol 2010 ;473:199-216

Laboratoire Stress Oxydants et Cancer, DSV, IBITECS, CEA-Saclay, Gif-sur-Yvette, France.

The oxidation of the cysteine (Cys) residue to sulfenic (-S-OH), disulfide (-S-S-), or S-nitroso (S-NO) forms are thought to be a posttranslational modifications that regulate protein function. However, despite a few solid examples of its occurrence, thiol-redox regulation of protein function is still debated and often seen as an exotic phenomenon. A systematic and exhaustive characterization of all oxidized Cys residues, an experimental approach called redox proteomics or redoxome analysis, should help establish the physiological scope of Cys residue oxidation and give clues to its mechanisms. Redox proteomics still remains a technical challenge, mainly because of the labile nature of thiol-redox reactions and the lack of tools to directly detect the modified residues. Here we consider recent technical advances in redox proteomics, focusing on a gel-based fluorescent method and on the shotgun OxICAT technique.
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http://dx.doi.org/10.1016/S0076-6879(10)73010-XDOI Listing
September 2010

Distinct and combined roles of the MAP kinases of Cochliobolus heterostrophus in virulence and stress responses.

Mol Plant Microbe Interact 2008 Jun;21(6):769-80

Department of Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel.

Pathogenicity mitogen-activated protein kinases (MAPKs), related to yeast FUS3/KSS1, are essential for virulence in fungi, including Cochliobolus heterostrophus, a necrotrophic pathogen causing Southern corn leaf blight. We compared the phenotypes of mutants in three MAPK genes: HOG1, MPS1, and CHK1. The chk1 and mps1 mutants show autolytic appearance, light pigmentation, and dramatic reduction in virulence and conidiation. Similarity of mps1 and chk1 mutants is reflected by coregulation by these two MAPKs of several genes. Unlike chk1, mps1 mutants are female-fertile and form normal-looking appressoria. HOG1 mediates resistance to hyperosmotic and, to a lesser extent, oxidative stress, and is required for stress upregulation of glycerol-3-phosphate phosphatase, transaldolase, and a monosaccharide transporter. Hog1, but not Mps1 or Chk1, was rapidly phosphorylated in response to increased osmolarity. The hog1 mutants have smaller appressoria and cause decreased disease symptoms on maize leaves. Surprisingly, loss of MPS1 in a wild-type or hog1 background improved resistance to some stresses. All three MAPKs contribute to the regulation of central developmental functions under normal and stress conditions, and full virulence cannot be achieved without appropriate input from all three pathways.
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http://dx.doi.org/10.1094/MPMI-21-6-0769DOI Listing
June 2008

Melanin biosynthesis in the maize pathogen Cochliobolus heterostrophus depends on two mitogen-activated protein kinases, Chk1 and Mps1, and the transcription factor Cmr1.

Eukaryot Cell 2007 Mar 19;6(3):421-9. Epub 2007 Jan 19.

Department of Biology, Technion-Israel Institute of Technology, Haifa, Israel.

The maize pathogen Cochliobolus heterostrophus requires two mitogen-activated protein kinases (MAPKs), Chk1 and Mps1, to produce normal pigmentation. Young colonies of mps1 and chk1 deletion mutants have a white and autolytic appearance, which was partially rescued by a hyperosmotic environment. We isolated the transcription factor Cmr1, an ortholog of Colletotrichum lagenarium Cmr1 and Magnaporthe grisea Pig1, which regulates melanin biosynthesis in C. heterostrophus. Deletion of CMR1 in C. heterostrophus resulted in mutants that lacked dark pigmentation and acquired an orange-pink color. In cmr1 deletion strains the expression of putative scytalone dehydratase (SCD1) and hydroxynaphthalene reductase (BRN1 and BRN2) genes involved in melanin biosynthesis was undetectable, whereas expression of PKS18, encoding a polyketide synthase, was only moderately reduced. In chk1 and mps1 mutants expression of PKS18, SCD1, BRN1, BRN2, and the transcription factor CMR1 itself was very low in young colonies, slightly up-regulated in aging colonies, and significantly induced in hyperosmotic conditions, compared to invariably high expression in the wild type. These findings indicate that two MAPKs, Chk1 and Mps1, affect Cmr1 at the transcriptional level and this influence is partially overridden in stress conditions including aging culture and hyperosmotic environment. Surprisingly, we found that the CMR1 gene was transcribed in both sense and antisense directions, apparently producing mRNA as well as a long noncoding RNA transcript. Expression of the antisense CMR1 was also Chk1 and Mps1 dependent. Analysis of chromosomal location of the melanin biosynthesis genes in C. heterostrophus resulted in identification of a small gene cluster comprising BRN1, CMR1, and PKS18. Since expression of all three genes depends on Chk1 and Mps1 MAPKs, we suggest their possible epigenetic regulation.
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http://dx.doi.org/10.1128/EC.00264-06DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1828933PMC
March 2007