scholarly journals N-Glycan–dependent protein folding and endoplasmic reticulum retention regulate GPI-anchor processing

2017 ◽  
Vol 217 (2) ◽  
pp. 585-599 ◽  
Author(s):  
Yi-Shi Liu ◽  
Xin-Yu Guo ◽  
Tetsuya Hirata ◽  
Yao Rong ◽  
Daisuke Motooka ◽  
...  
Keyword(s):  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Cell Surface ◽  
Er Stress ◽  
Posttranslational Modification ◽  
Acyl Chain ◽  
Genetic Screens ◽  
Gpi Anchor ◽  
Er Retention ◽  
Dependent Protein

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a conserved posttranslational modification in the endoplasmic reticulum (ER). Soon after GPI is attached, an acyl chain on the GPI inositol is removed by post-GPI attachment to proteins 1 (PGAP1), a GPI-inositol deacylase. This is crucial for switching GPI-anchored proteins (GPI-APs) from protein folding to transport states. We performed haploid genetic screens to identify factors regulating GPI-inositol deacylation, identifying seven genes. In particular, calnexin cycle impairment caused inefficient GPI-inositol deacylation. Calnexin was specifically associated with GPI-APs, dependent on N-glycan and GPI moieties, and assisted efficient GPI-inositol deacylation by PGAP1. Under chronic ER stress caused by misfolded GPI-APs, inositol-acylated GPI-APs were exposed on the cell surface. These results indicated that N-glycans participate in quality control and temporal ER retention of GPI-APs, ensuring their correct folding and GPI processing before exiting from the ER. Once the system is disrupted by ER stress, unprocessed GPI-APs become exposed on the cell surface.

scholarly journals Endoplasmic Reticulum Stress Regulators: New Drug Targets for Parkinson’s Disease

2021 ◽  
pp. 1-10
Author(s):  
Vera Kovaleva ◽  
Mart Saarma
Keyword(s):  
Parkinson’S Disease ◽  
Parkinson's Disease ◽  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Er Stress ◽  
Protein Misfolding ◽  
Drug Targets ◽  
Dopamine Neurons ◽  
Drug Candidates ◽  
The Unfolded Protein Response

Parkinson’s disease (PD) pathology involves progressive degeneration and death of vulnerable dopamine neurons in the substantia nigra. Extensive axonal arborisation and distinct functions make this type of neurons particularly sensitive to homeostatic perturbations, such as protein misfolding and Ca2 + dysregulation. Endoplasmic reticulum (ER) is a cell compartment orchestrating protein synthesis and folding, as well as synthesis of lipids and maintenance of Ca2 +-homeostasis in eukaryotic cells. When misfolded proteins start to accumulate in ER lumen the unfolded protein response (UPR) is activated. UPR is an adaptive signalling machinery aimed at relieving of protein folding load in the ER. When UPR is chronic, it can either boost neurodegeneration and apoptosis or cause neuronal dysfunctions. We have recently discovered that mesencephalic astrocyte-derived neurotrophic factor (MANF) exerts its prosurvival action in dopamine neurons and in animal model of PD through the direct binding to UPR sensor inositol-requiring protein 1 alpha (IRE1α) and attenuation of UPR. In line with this, UPR targeting resulted in neuroprotection and neurorestoration in various preclinical PD animal models. Therefore, growth factors (GFs), possessing both neurorestorative activity and restoration of protein folding capacity are attractive as drug candidates for PD treatment especially their blood-brain barrier penetrating analogs and small molecule mimetics. In this review, we discuss ER stress as a therapeutic target to treat PD; we summarize the existing preclinical data on the regulation of ER stress for PD treatment. In addition, we point out the crucial aspects for successful clinical translation of UPR-regulating GFs and new prospective in GFs-based treatments of PD, focusing on ER stress regulation.

scholarly journals Limited ER quality control for GPI-anchored proteins

2016 ◽  
Vol 213 (6) ◽  
pp. 693-704 ◽  
Author(s):  
Natalia Sikorska ◽  
Leticia Lemus ◽  
Auxiliadora Aguilera-Romero ◽  
Javier Manzano-Lopez ◽  
Howard Riezman ◽  
...  
Keyword(s):  
Quality Control ◽  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Misfolded Proteins ◽  
Control Mechanisms ◽  
Gpi Anchor ◽  
Er Quality Control ◽  
Entire Class ◽  
Er Export ◽  
Export Signal

Endoplasmic reticulum (ER) quality control mechanisms target terminally misfolded proteins for ER-associated degradation (ERAD). Misfolded glycophosphatidylinositol-anchored proteins (GPI-APs) are, however, generally poor ERAD substrates and are targeted mainly to the vacuole/lysosome for degradation, leading to predictions that a GPI anchor sterically obstructs ERAD. Here we analyzed the degradation of the misfolded GPI-AP Gas1* in yeast. We could efficiently route Gas1* to Hrd1-dependent ERAD and provide evidence that it contains a GPI anchor, ruling out that a GPI anchor obstructs ERAD. Instead, we show that the normally decreased susceptibility of Gas1* to ERAD is caused by canonical remodeling of its GPI anchor, which occurs in all GPI-APs and provides a protein-independent ER export signal. Thus, GPI anchor remodeling is independent of protein folding and leads to efficient ER export of even misfolded species. Our data imply that ER quality control is limited for the entire class of GPI-APs, many of them being clinically relevant.

scholarly journals Endoplasmic Reticulum Stress and Lipid Metabolism: Mechanisms and Therapeutic Potential

2012 ◽  
Vol 2012 ◽  
pp. 1-13 ◽  
Author(s):  
Sana Basseri ◽  
Richard C. Austin
Keyword(s):  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Lipid Metabolism ◽  
Er Stress ◽  
Therapeutic Potential ◽  
Critical Role ◽  
Signal Transduction Pathway ◽  
Unfolded Protein ◽  
The Unfolded Protein Response ◽  
Stress Dependent

The endoplasmic reticulum (ER) plays a crucial role in protein folding, assembly, and secretion. Disruption of ER homeostasis may lead to accumulation of misfolded or unfolded proteins in the ER lumen, a condition referred to as ER stress. In response to ER stress, a signal transduction pathway known as the unfolded protein response (UPR) is activated. UPR activation allows the cell to cope with an increased protein-folding demand on the ER. Recent studies have shown that ER stress/UPR activation plays a critical role in lipid metabolism and homeostasis. ER-stress-dependent dysregulation of lipid metabolism may lead to dyslipidemia, insulin resistance, cardiovascular disease, type 2 diabetes, and obesity. In this paper, we examine recent findings illustrating the important role ER stress/UPR signalling pathways play in regulation of lipid metabolism, and how they may lead to dysregulation of lipid homeostasis.

scholarly journals Obesity-Linked Variants of Melanocortin-4 Receptor Are Misfolded in the Endoplasmic Reticulum and Can Be Rescued to the Cell Surface by a Chemical Chaperone

2010 ◽  
Vol 31 (4) ◽  
pp. 605-605
Author(s):  
Susana Granell ◽  
Sameer Mohammad ◽  
Ramanagouda Ramanagoudr-Bhojappa ◽  
Giulia Baldini
Keyword(s):  
Endoplasmic Reticulum ◽  
Cell Surface ◽  
Er Stress ◽  
Binding Protein ◽  
Intracellular Localization ◽  
Specific Inhibitor ◽  
Surface Expression ◽  
Cell Surface Expression ◽  
Chemical Chaperone ◽  
Melanocortin 4 Receptor

Abstract Melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in the brain where it controls food intake. Many obesity-linked MC4R variants are poorly expressed at the plasma membrane and are retained intracellularly. We have studied the intracellular localization of four obesity-linked MC4R variants, P78L, R165W, I316S, and I317T, in immortalized neurons. We find that these variants are all retained in the endoplasmic reticulum (ER), are ubiquitinated to a greater extent than the wild-type (wt) receptor, and induce ER stress with increased levels of ER chaperones as compared with wt-MC4R and appearance of CCAAT/enhancer-binding protein homologous protein. Expression of the X-box-binding-protein-1 with selective activation of a protective branch of the unfolded protein response did not have any effect on the cell surface expression of MC4R-I316S. Conversely, the pharmacological chaperone 4-phenyl butyric acid (PBA) increased the cell surface expression of wt-MC4R, MC4R-I316S, and I317T by more than 40%. PBA decreased ubiquitination of MC4R-I316S and prevented ER stress induced by expression of the mutant, suggesting that the drug functions to promote MC4R folding. MC4R-I316S rescued to the cell surface is functional, with a 52% increase in agonist-induced cAMP production, as compared with untreated cells. Also direct inhibition of wt-MC4R and MC4R-I316S ubiquitination by a specific inhibitor of the ubiquitin-activating enzyme 1 increased by approximately 40% the expression of the receptors at the cell surface, and the effects of PBA and ubiquitin-activating enzyme 1 were additive. These data offer a cell-based rationale that drugs that improve MC4R folding or decrease ER-associated degradation of the receptor may function to treat some forms of hereditary obesity.

scholarly journals Mitochondria maintain maturation and secretion of lipoprotein lipase in the endoplasmic reticulum

2006 ◽  
Vol 396 (1) ◽  
pp. 173-182 ◽  
Author(s):  
Karin Osibow ◽  
Sasa Frank ◽  
Roland Malli ◽  
Rudolf Zechner ◽  
Wolfgang F. Graier
Keyword(s):  
Endothelial Cells ◽  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Lipoprotein Lipase ◽  
Chinese Hamster ◽  
Protein Levels ◽  
Dependent Activity ◽  
Dependent Protein ◽  
The Impact ◽  
Reversible Reduction

Considering the physiological Ca2+ dynamics within the ER (endoplasmic reticulum), it remains unclear how efficient protein folding is maintained in living cells. Thus, utilizing the strictly folding-dependent activity and secretion of LPL (lipoprotein lipase), we evaluated the impact of ER Ca2+ content and mitochondrial contribution to Ca2+-dependent protein folding. Exhaustive ER Ca2+ depletion by inhibition of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases caused strong, but reversible, reduction of cell-associated and released activity of constitutive and adenovirus-encoded human LPL in CHO-K1 (Chinese-hamster ovary K1) and endothelial cells respectively, which was not due to decline of mRNA or intracellular protein levels. In contrast, stimulation with the IP3 (inositol 1,4,5-trisphosphate)-generating agonist histamine only moderately and transiently affected LPL maturation in endothelial cells that paralleled a basically preserved ER Ca2+ content. However, in the absence of extracellular Ca2+ or upon prevention of transmitochondrial Ca2+ flux, LPL maturation discontinued upon histamine stimulation. Collectively, these data indicate that Ca2+-dependent protein folding in the ER is predominantly controlled by intraluminal Ca2+ and is largely maintained during physiological cell stimulation owing to efficient ER Ca2+ refilling. Since Ca2+ entry and mitochondrial Ca2+ homoeostasis are crucial for continuous Ca2+-dependent protein maturation in the ER, their pathological alterations may result in dysfunctional protein folding.

scholarly journals Ceapins inhibit ATF6α signaling by selectively preventing transport of ATF6α to the Golgi apparatus during ER stress

eLife ◽  
2016 ◽  
Vol 5 ◽  
Author(s):  
Ciara M Gallagher ◽  
Peter Walter
Keyword(s):  
Endoplasmic Reticulum ◽  
Transcription Factor ◽  
Protein Folding ◽  
Golgi Apparatus ◽  
Er Stress ◽  
Target Genes ◽  
Spatial Separation ◽  
Oligomeric State ◽  
Er Exit Sites ◽  
Membrane Bound

The membrane-bound transcription factor ATF6α is activated by proteolysis during endoplasmic reticulum (ER) stress. ATF6α target genes encode foldases, chaperones, and lipid biosynthesis enzymes that increase protein-folding capacity in response to demand. The off-state of ATF6α is maintained by its spatial separation in the ER from Golgi-resident proteases that activate it. ER stress induces trafficking of ATF6α. We discovered Ceapins, a class of pyrazole amides, as selective inhibitors of ATF6α signaling that do not inhibit the Golgi proteases or other UPR branches. We show that Ceapins block ATF6α signaling by trapping it in ER-resident foci that are excluded from ER exit sites. Removing the requirement for trafficking by pharmacological elimination of the spatial separation of the ER and Golgi apparatus restored cleavage of ATF6α in the presence of Ceapins. Washout of Ceapins resensitized ATF6α to ER stress. These results suggest that trafficking of ATF6α is regulated by its oligomeric state.

scholarly journals Endoplasmic Reticulum Stress: Its Role in Disease and Novel Prospects for Therapy

Scientifica ◽  
2012 ◽  
Vol 2012 ◽  
pp. 1-26 ◽  
Author(s):  
Axel H. Schönthal
Keyword(s):  
Endoplasmic Reticulum ◽  
Protein Folding ◽  
Er Stress ◽  
Prescription Drugs ◽  
Pancreatic Beta Cells ◽  
Cell Function ◽  
Tumor Cell Death ◽  
Cellular Mechanisms ◽  
Pharmacological Agents ◽  
The Unfolded Protein Response

The endoplasmic reticulum (ER) is a multifunctional organelle required for lipid biosynthesis, calcium storage, and protein folding and processing. A number of physiological and pathological conditions, as well as a variety of pharmacological agents, are able to disturb proper ER function and thereby cause ER stress, which severely impairs protein folding and therefore poses the risk of proteotoxicity. Specific triggers for ER stress include, for example, particular intracellular alterations (e.g., calcium or redox imbalances), certain microenvironmental conditions (e.g., hypoglycemia, hypoxia, and acidosis), high-fat and high-sugar diet, a variety of natural compounds (e.g., thapsigargin, tunicamycin, and geldanamycin), and several prescription drugs (e.g., bortezomib/Velcade, celecoxib/Celebrex, and nelfinavir/Viracept). The cell reacts to ER stress by initiating a defensive process, called the unfolded protein response (UPR), which is comprised of cellular mechanisms aimed at adaptation and safeguarding cellular survival or, in cases of excessively severe stress, at initiation of apoptosis and elimination of the faulty cell. In recent years, this dichotomic stress response system has been linked to several human diseases, and efforts are underway to develop approaches to exploit ER stress mechanisms for therapy. For example, obesity and type 2 diabetes have been linked to ER stress-induced failure of insulin-producing pancreatic beta cells, and current research efforts are aimed at developing drugs that ameliorate cellular stress and thereby protect beta cell function. Other studies seek to pharmacologically aggravate chronic ER stress in cancer cells in order to enhance apoptosis and achieve tumor cell death. In the following, these principles will be presented and discussed.

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