bims-unfpre 2019-06-23 papers

Issue of 2019–06–23
five papers selected by
Susan Logue, University of Manitoba

EMBO J. 2019 Jun 17. pii: e100990. [Epub ahead of print]

ERp18 regulates activation of ATF6α during unfolded protein response.

Ojore Bv Oka, Marcel van Lith, Jana Rudolf, Wanida Tungkum, Marie Anne Pringle, Neil J Bulleid.

Activation of the ATF6α signaling pathway is initiated by trafficking of ATF6α from the ER to the Golgi apparatus. Its subsequent proteolysis releases a transcription factor that translocates to the nucleus causing downstream gene activation. How ER retention, Golgi trafficking, and proteolysis of ATF6α are regulated and whether additional protein partners are required for its localization and processing remain unresolved. Here, we show that ER-resident oxidoreductase ERp18 associates with ATF6α following ER stress and plays a key role in both trafficking and activation of ATF6α. We find that ERp18 depletion attenuates the ATF6α stress response. Paradoxically, ER stress accelerates trafficking of ATF6α to the Golgi in ERp18-depleted cells. However, the translocated ATF6α becomes aberrantly processed preventing release of the soluble transcription factor. Hence, we demonstrate that ERp18 monitors ATF6α ER quality control to ensure optimal processing following trafficking to the Golgi.

Keywords: ATF6α; ER stress; ERp18; protein trafficking; unfolded protein response

DOI: https://doi.org/10.15252/embj.2018100990

Cell Death Dis. 2019 Jun 21. 10(7): 490

Expression of UPR effector proteins ATF6 and XBP1 reduce colorectal cancer cell proliferation and stemness by activating PERK signaling.

Claudia N Spaan, Wouter L Smit, Jooske F van Lidth de Jeude, Bartolomeus J Meijer, Vanesa Muncan, Gijs R van den Brink, Jarom Heijmans.

The unfolded protein response (UPR) acts through its downstream branches, PERK-eIF2α signaling, IRE1α-XBP1 signaling and ATF6 signaling. In the intestine, activation of the UPR through the kinase PERK results in differentiation of intestinal epithelial stem cells and colon cancer stem cells, whereas deletion of XBP1 results in increased stemness and adenomagenesis. How downstream activation of XBP1 and ATF6 influences intestinal stemness and proliferation remains largely unknown. We generated colorectal cancer cells (LS174T) that harbor doxycycline inducible expression of the active forms of either XBP1(s) or ATF61-373. Activation of either XBP1 or ATF6 resulted in reduced cellular proliferation and reduced expression of markers of intestinal epithelial stemness. Moreover, XBP1 and ATF6 activation reduced global protein synthesis and lowered the threshold for UPR activation. XBP1-mediated loss of stemness and proliferation resulted from crossactivation of PERK-eIF2α signaling and could be rescued by constitutive expression of eIF2α phosphatase GADD34. We thus find that enforced activation of XBP1 and ATF6 results in reduction of stemness and proliferation. We expose a novel interaction between XBP1 and PERK-eIF2α signaling.

DOI: https://doi.org/10.1038/s41419-019-1729-4

Autophagy. 2019 Jun 17.

WDR45 contributes to neurodegeneration through regulation of ER homeostasis and neuronal death.

Huida Wan, Qi Wang, Xiuting Chen, Qiufang Zeng, Yanjiao Shao, Houqin Fang, Xun Liao, Hu-Song Li, Ming-Gang Liu, Tian-Le Xu, Miaomiao Diao, Dali Li, Bo Meng, Bin Tang, Zhuohua Zhang, Lujian Liao.

Mutations in the macroautophagy/autophagy gene WDR45 cause β-propeller protein-associated neurodegeneration (BPAN); however the molecular and cellular mechanism of the disease process is largely unknown. Here we generated constitutive wdr45 knockout (KO) mice that displayed cognitive impairments, abnormal synaptic transmission and lesions in several brain regions. Immunohistochemistry analysis showed loss of neurons in prefrontal cortex and basal ganglion in aged mice, and increased apoptosis in prefrontal cortex, recapitulating a hallmark of neurodegeneration. Quantitative proteomic analysis showed accumulation of endoplasmic reticulum (ER) proteins in KO mouse. At the cellular level, accumulation of ER proteins due to WDR45 deficiency resulted in increased ER stress and impaired ER quality control. The unfolded protein response (UPR) was elevated through ERN1/IRE1 or EIF2AK3/PERK pathway, and eventually led to neuronal apoptosis. Suppression of ER stress or activation of autophagy through MTOR inhibition alleviated cell death. Thus, the loss of WDR45 cripples macroautophagy machinery in neurons and leads to impairment in organelle autophagy, which provides a mechanistic understanding of cause of BPAN and a potential therapeutic strategy to treat this genetic disorder.

Keywords: ER stress; UPR; WDR45; neuronal apoptosis; quantitative proteomics

DOI: https://doi.org/10.1080/15548627.2019.1630224

Proc Natl Acad Sci U S A. 2019 Jun 18. pii: 201906275. [Epub ahead of print]

Genome-wide CRISPR screen identifies suppressors of endoplasmic reticulum stress-induced apoptosis.

Ronald A Panganiban, Hae-Ryung Park, Maoyun Sun, Maya Shumyatcher, Blanca E Himes, Quan Lu.

Sensing misfolded proteins in the endoplasmic reticulum (ER), cells initiate the ER stress response and, when overwhelmed, undergo apoptosis. However, little is known about how cells prevent excessive ER stress response and cell death to restore homeostasis. Here, we report the identification and characterization of cellular suppressors of ER stress-induced apoptosis. Using a genome-wide CRISPR library, we screen for genes whose inactivation further increases ER stress-induced up-regulation of C/EBP homologous protein 10 (CHOP)-the transcription factor central to ER stress-associated apoptosis. Among the top validated hits are two interacting components of the polycomb repressive complex (L3MBTL2 [L(3)Mbt-Like 2] and MGA [MAX gene associated]), and microRNA-124-3 (miR-124-3). CRISPR knockout of these genes increases CHOP expression and sensitizes cells to apoptosis induced by multiple ER stressors, while overexpression confers the opposite effects. L3MBTL2 associates with the CHOP promoter in unstressed cells to repress CHOP induction but dissociates from the promoter in the presence of ER stress, whereas miR-124-3 directly targets the IRE1 branch of the ER stress pathway. Our study reveals distinct mechanisms that suppress ER stress-induced apoptosis and may lead to a better understanding of diseases whose pathogenesis is linked to overactive ER stress response.

Keywords: UPR (unfolded protein response); environmental toxicant; genetic screen; proteotoxicity

DOI: https://doi.org/10.1073/pnas.1906275116

Sci Rep. 2019 Jun 20. 9(1): 8836

A sensitive and simple targeted proteomics approach to quantify transcription factor and membrane proteins of the unfolded protein response pathway in glioblastoma cells.

Chi D L Nguyen, Sebastian Malchow, Stefan Reich, Sascha Steltgens, Konstantin V Shuvaev, Stefan Loroch, Christin Lorenz, Albert Sickmann, Christiane B Knobbe-Thomsen, Björn Tews, Jan Medenbach, Robert Ahrends.

Many cellular events are driven by changes in protein expression, measurable by mass spectrometry or antibody-based assays. However, using conventional technology, the analysis of transcription factor or membrane receptor expression is often limited by an insufficient sensitivity and specificity. To overcome this limitation, we have developed a high-resolution targeted proteomics strategy, which allows quantification down to the lower attomol range in a straightforward way without any prior enrichment or fractionation approaches. The method applies isotope-labeled peptide standards for quantification of the protein of interest. As proof of principle, we applied the improved workflow to proteins of the unfolded protein response (UPR), a signaling pathway of great clinical importance, and could for the first time detect and quantify all major UPR receptors, transducers and effectors that are not readily detectable via antibody-based-, SRM- or conventional PRM assays. As transcription and translation is central to the regulation of UPR, quantification and determination of protein copy numbers in the cell is important for our understanding of the signaling process as well as how pharmacologic modulation of these pathways impacts on the signaling. These questions can be answered using our newly established workflow as exemplified in an experiment using UPR perturbation in a glioblastoma cell lines.

DOI: https://doi.org/10.1038/s41598-019-45237-5