Redox signaling loops in the unfolded protein response

The endoplasmic reticulum (ER) is the first compartment of secretory pathway. It plays a major role in ER chaperone-assisted folding and quality control, including post-translational modification such as disulfide bond formation of newly synthesized secretory proteins. Protein folding and assembly takes place in the ER, where redox conditions are distinctively different from the other organelles and are favorable for disulfide formation. These reactions generate the production of reactive oxygen species (ROS) as a byproduct of thiol/disulfide exchange reaction among ER oxidoreductin 1 (Ero1), protein disulfide isomerase (PDI) and ER client proteins, during the formation of disulfide bonds in nascent or incorrectly folded proteins. When uncontrolled, this phenomenon perturbs ER homeostasis, thus aggravating the accumulation of improperly folded or unfolded proteins in this compartment (ER stress). This results in the activation of an adaptive mechanism named the unfolded protein response (UPR). In mammalian cells, the UPR is mediated by three ER-resident membrane proteins (PERK, IRE1 and ATF6) and regulates the expression of the UPR target genes, which themselves encode ER chaperones, folding enzymes, pro-apoptotic proteins and antioxidants, with the objective of restoring ER homeostatic balance. In this review, we will describe redox dependent activation (ER) and amplification (cytosol) loops that control the UPR and the consequences these regulatory loops have on cell fate and physiology.     

Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly—misfolded proteins—are retained in the ER and their accumulation may constitute a form of stress to the cell—ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.     

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.     

ERp29 deficiency affects sensitivity to apoptosis via impairment of the ATF6–CHOP pathway of stress response

Endoplasmic reticulum protein 29 (ERp29) belongs to the redox-inactive PDI-Dβ-subfamily of PDI-proteins. ERp29 is expressed in all mammalian tissues examined. Especially high levels of expression were observed in secretory tissues and in some tumors. However, the biological role of ERp29 remains unclear. In the present study we show, by using thyrocytes and primary dermal fibroblasts from adult ERp29^?/? mice, that ERp29 deficiency affects the activation of the ATF6–CHOP-branch of unfolded protein response (UPR) without influencing the function of other UPR branches, like the ATF4-eIF2α-XBP1 signaling pathway. As a result of impaired ATF6 activation, dermal fibroblasts and adult thyrocytes from ERp29^?/? mice display significantly lower apoptosis sensitivities when treated with tunicamycin and hydrogen peroxide. However, in contrast to previous reports, we could demonstrate that ERp29 deficiency does not alter thyroglobulin expression levels. Therefore, our study suggests that ERp29 acts as an escort factor for ATF6 and promotes its transport from ER to Golgi apparatus under ER stress conditions.     

未折叠蛋白质应答

内质网是真核细胞中蛋白质合成、折叠与分泌的重要细胞器.细胞进化出一套完整的机制来监督和帮助内质网内蛋白质的折叠与修饰.而当错误折叠的蛋白质累积时,细胞通过一系列信号转导途径,对其进行应答,包括增强蛋白质折叠能力、停滞大多数蛋白质的翻译、加速蛋白质的降解等.如果内质网功能素乱持续,细胞将最终启动凋亡程序.这些反应被统称为未折叠蛋白质应答(unfolded protein response,UPR).UPR是多个信号转导通路的总称,包括IRE1-XBP1、PERK-ATF4以及ATF6等信号途径.除了应激条件外,UPR还被用于正常生理条件下的调节,例如胆固醇合成代谢的负反馈调控.     

Unfolded protein response

In eukaryotic cells, the endoplasmic reticulum (ER) is a membrane-enclosed interconnected organelle responsible for the synthesis, folding, modification, and quality control of numerous secretory and membrane proteins. The processes of protein folding and maturation are highly assisted and scrutinized but are also sensitive to changes in ER homeostasis, such as Ca(2+) depletion, oxidative stress, hypoxia, energy deprivation, metabolic stimulation, altered glycosylation, activation of inflammation, as well as increases in protein synthesis or the expression of misfolded proteins or unassembled protein subunits. Only properly folded proteins can traffic to the Golgi apparatus, whereas those that misfold are directed to ER-associated degradation (ERAD) or to autophagy. The accumulation of unfolded/misfolded proteins in the ER activates signaling events to orchestrate adaptive cellular responses. This unfolded protein response (UPR) increases the ER protein-folding capacity, reduces global protein synthesis, and enhances ERAD of misfolded proteins.     

The cargo receptor ERGIC-53 is a target of the unfolded protein response

The accumulation of unfolded proteins in the ER triggers a signaling response known as unfolded protein response (UPR). In yeast the UPR affects several hundred genes that encode ER chaperones and proteins operating at later stages of secretion. In mammalian cells the UPR appears to be more limited to chaperones of the ER and genes assumed to be important after cell recovery from ER stress that are not important for secretion. Here, we report that the mRNA of lectin ERGIC-53, a cargo receptor for the transport of glycoproteins from ER to ERGIC, and of its related protein VIP36 is induced by the known inducers of ER stress, tunicamycin and thapsigargin. In parallel, the rate of synthesis of the ERGIC-53 protein was induced by these agents. The response was due to the UPR since it was also triggered by castanospermine, a specific inducer of UPR, and inhibited by genistein. Thapsigargin-induced upregulation of ERGIC-53 could be fully accounted for by the ATF6 pathway of UPR. The results suggest that in mammalian cells the UPR also affects traffic from and beyond the ER.     

The Florey Lecture, 1992. The secretion of proteins by cells.

In eukaryotic cells, protein secretion provides a complex organizational problem. Secretory proteins are first transported, in an unfolded state, across the membrane of the endoplasmic reticulum (ER), and are then carried in small vesicles to the Golgi apparatus and finally to the cell membrane. The ER contains soluble proteins which catalyse the folding of newly synthesized polypeptides. These proteins are sorted from secretory proteins in the Golgi complex: they carry a sorting signal (the tetrapeptide KDEL or a related sequence) that allows them to be selectively retrieved and returned to the ER. This retrieval process also appears to be used by some bacterial toxins to aid their invasion of the cell: these toxins contain KDEL-like sequences and may, in effect, follow the secretory pathway in reverse. The membrane-bound receptor responsible for sorting luminal ER proteins has been identified in yeast by genetic means, and related receptors are found in mammalian cells. Unexpectedly, this receptor has a second role: in yeast it is required to maintain the normal size and function of the Golgi apparatus. By helping to maintain the composition of both ER and Golgi compartments, the KDEL receptor has an important role in the organization of the secretory pathway.