The endoplasmic reticulum and the unfolded protein response

The endoplasmic reticulum (ER) is the site where proteins enter the secretory pathway. Proteins are translocated into the ER lumen in an unfolded state and require protein chaperones and catalysts of protein folding to attain their final appropriate conformation. A sensitive surveillance mechanism exists to prevent misfolded proteins from transiting the secretory pathway and ensures that persistently misfolded proteins are directed towards a degradative pathway. In addition, those processes that prevent accumulation of unfolded proteins in the ER lumen are highly regulated by an intracellular signaling pathway known as the unfolded protein response (UPR). The UPR provides a mechanism by which cells can rapidly adapt to alterations in client protein-folding load in the ER lumen by expanding the capacity for protein folding. In addition, a variety of insults that disrupt protein folding in the ER lumen also activate the UPR. These include changes in intralumenal calcium, altered glycosylation, nutrient deprivation, pathogen infection, expression of folding-defective proteins, and changes in redox status. Persistent protein misfolding initiates apoptotic cascades that are now known to play fundamental roles in the pathogenesis of multiple human diseases including diabetes, atherosclerosis and neurodegenerative diseases.     

Connecting endoplasmic reticulum stress to autophagy by unfolded protein response and calcium

Eukaryotic cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER) either by unfolded protein response that leads to an increase in the capacity of the ER to fold its client proteins or by apoptosis when the function of ER cannot be restored. Emerging data now indicate that ER stress is also a potent inducer of macroautophagy, a process whereby eukaryotic cells recycle their macromolecules and organelles. Depending on the context, autophagy counterbalances ER stress-induced ER expansion, enhances cell survival or commits the cell to non-apoptotic death. Here, we discuss the signaling pathways linking ER stress to autophagy and possibilities for their clinical exploitation.     

Signal integration in the endoplasmic reticulum unfolded protein response

The endoplasmic reticulum (ER) responds to the accumulation of unfolded proteins in its lumen (ER stress) by activating intracellular signal transduction pathways - cumulatively called the unfolded protein response (UPR). Together, at least three mechanistically distinct arms of the UPR regulate the expression of numerous genes that function within the secretory pathway but also affect broad aspects of cell fate and the metabolism of proteins, amino acids and lipids. The arms of the UPR are integrated to provide a response that remodels the secretory apparatus and aligns cellular physiology to the demands imposed by ER stress.     

The unfolded protein response coordinates the production of endoplasmic reticulum protein and endoplasmic reticulum membrane.

The endoplasmic reticulum (ER) is a multifunctional organelle responsible for production of both lumenal and membrane components of secretory pathway compartments. Secretory proteins are folded, processed, and sorted in the ER lumen and lipid synthesis occurs on the ER membrane itself. In the yeast Saccharomyces cerevisiae, synthesis of ER components is highly regulated: the ER-resident proteins by the unfolded protein response and membrane lipid synthesis by the inositol response. We demonstrate that these two responses are intimately linked, forming different branches of the same pathway. Furthermore, we present evidence indicating that this coordinate regulation plays a role in ER biogenesis.     

Reshaping endoplasmic reticulum quality control through the unfolded protein response

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Evaluating endoplasmic reticulum stress and unfolded protein response through the lens of ecology and evolution

Considerable progress has been made in understanding the physiological basis for variation in the life-history patterns of animals, particularly with regard to the roles of oxidative stress and hormonal regulation. However, an underappreciated and understudied area that could play a role in mediating inter- and intraspecific variation of life history is endoplasmic reticulum (ER) stress, and the resulting unfolded protein response (UPR^ER). ER stress response and the UPR^ER maintain proteostasis in cells by reducing the intracellular load of secretory proteins and enhancing protein folding capacity or initiating apoptosis in cells that cannot recover. Proper modulation of the ER stress response and execution of the UPR^ER allow animals to respond to intracellular and extracellular stressors and adapt to constantly changing environments. ER stress responses are heritable and there is considerable individual variation in UPR^ER phenotype in animals, suggesting that ER stress and UPR^ER phenotype can be subjected to natural selection. The variation in UPR^ER phenotype presumably reflects the way animals respond to ER stress and environmental challenges. Most of what we know about ER stress and the UPR^ER in animals has either come from biomedical studies using cell culture or from experiments involving conventional laboratory or agriculturally important models that exhibit limited genetic diversity. Furthermore, these studies involve the assessment of experimentally induced qualitative changes in gene expression as opposed to the quantitative variations that occur in naturally existing populations. Almost all of these studies were conducted in controlled settings that are often quite different from the conditions animals experience in nature. Herein, we review studies that investigated ER stress and the UPR^ER in relation to key life-history traits including growth and development, reproduction, bioenergetics and physical performance, and ageing and senescence. We then ask if these studies can inform us about the role of ER stress and the UPR^ER in mediating the aforementioned life-history traits in free-living animals. We propose that there is a need to conduct experiments pertaining to ER stress and the UPR^ER in ecologically relevant settings, to characterize variation in ER stress and the UPR^ER in free-living animals, and to relate the observed variation to key life-history traits. We urge others to integrate multiple physiological systems and investigate how interactions between ER stress and oxidative stress shape life-history trade-offs in free-living animals.     

HCV causes chronic endoplasmic reticulum stress leading to adaptation and interference with the unfolded protein response

Background

The endoplasmic reticulum (ER) is the cellular site for protein folding. ER stress occurs when protein folding capacity is exceeded. This stress induces a cyto-protective signaling cascades termed the unfolded protein response (UPR) aimed at restoring homeostasis. While acute ER stress is lethal, chronic sub-lethal ER stress causes cells to adapt by attenuation of UPR activation. Hepatitis C virus (HCV), a major human pathogen, was shown to cause ER stress, however it is unclear whether HCV induces chronic ER stress, and if so whether adaptation mechanisms are initiated. We wanted to characterize the kinetics of HCV-induced ER stress during infection and assess adaptation mechanisms and their significance.

Methods and Findings

The HuH7.5.1 cellular system and HCV-transgenic (HCV-Tg) mice were used to characterize HCV-induced ER stress/UPR pathway activation and adaptation. HCV induced a wave of acute ER stress peaking 2–5 days post-infection, which rapidly subsided thereafter. UPR pathways were activated including IRE1 and EIF2α phosphorylation, ATF6 cleavage and XBP-1 splicing. Downstream target genes including GADD34, ERdj4, p58ipk, ATF3 and ATF4 were upregulated. CHOP, a UPR regulated protein was activated and translocated to the nucleus. Remarkably, UPR activity did not return to baseline but remained elevated for up to 14 days post infection suggesting that chronic ER stress is induced. At this time, cells adapted to ER stress and were less responsive to further drug-induced ER stress. Similar results were obtained in HCV-Tg mice. Suppression of HCV by Interferon-α 2a treatment, restored UPR responsiveness to ER stress tolerant cells.

Conclusions

Our study shows, for the first time, that HCV induces adaptation to chronic ER stress which was reversed upon viral suppression. These finding represent a novel viral mechanism to manipulate cellular response pathways.     

Mitochondrial TRAP1 regulates the unfolded protein response in the endoplasmic reticulum

Stress in mitochondria or the endoplasmic reticulum (ER) independently causes cell death. Recently, it was reported that ER stress causes mitochondrial dysfunction via p53-upregulated modulator of apoptosis (PUMA). However, little is known regarding the mitochondria molecules that mediate ER dysfunction. The present study revealed that tumor necrosis factor receptor-associated protein 1 (TRAP1), which localizes in the mitochondria, is associated with the unfolded protein response (UPR) in the ER. TRAP1 knockdown activated the ER-resident caspase-4, which is activated by ER stress, to induce cell death in humans. However, TRAP1 knockdown cells did not show a significant increase in the level of cell death at least within 24 h after early phase of ER stress in comparison with that of the control cells. This finding could be attributed to a number of reasons. TRAP1 knockdown failed to activate caspase-9, which is activated by activated caspase-4. In addition, TRAP1 knockdown increased the basal level of GRP78/BiP expression, which protects cells, and decreased the basal level of C/EBP homologous protein (CHOP) expression, which induces cell death, even under ER stress. Thus, the present study revealed that mitochondria could be a potential regulator of the UPR in the ER through mitochondrial TRAP1.     

The unfolded protein response regulates glutamate receptor export from the endoplasmic reticulum

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors mediate the majority of excitatory signaling in the CNS, and the functional properties and subcellular fate of these receptors depend on receptor subunit composition. Subunit assembly is thought to occur in the endoplasmic reticulum (ER), although we are just beginning to understand the underlying mechanism. Here we examine the trafficking of Caenorhabditis elegans glutamate receptors through the ER. Our data indicate that neurons require signaling by the unfolded protein response (UPR) to move GLR-1, GLR-2, and GLR-5 subunits out of the ER and through the secretory pathway. In contrast, other neuronal transmembrane proteins do not require UPR signaling for ER exit. The requirement for the UPR pathway is cell type and age dependent: impairment for receptor trafficking increases as animals age and does not occur in all neurons. Expression of XBP-1, a component of the UPR pathway, is elevated in neurons during development. Our results suggest that UPR signaling is a critical step in neural function that is needed for glutamate receptor assembly and secretion.