Abstract Ire1 and its family protein PERK are endoplasmic reticulum (ER)-stress sensors that initiate cellular responses against ER accumulation of unfolded proteins. As reviewed in this article, many publications describe molecular mechanisms by which yeast Ire1 senses ER conditions and gets regulated. We also cover recent studies which reveal that mammalian Ire1 (IRE1α) and PERK are controlled in a similar but not exactly the same manner. ER-located molecular chaperone BiP captures these ER-stress sensors and suppresses their activity. Intriguingly, Ire1 is associated with BiP not as a chaperone substrate, but as a unique ligand. Unfolded proteins accumulated in the ER promote dissociation of the Ire1-BiP complex. Moreover, Ire1 is directly bound with unfolded proteins, leading to its cluster formation and potent activation. PERK also captures unfolded proteins and then forms self-oligomers. Meanwhile, membrane-lipid aberrancy is likely to activate these ER-stress sensors independently of ER accumulation of unfolded proteins. In addition, there exist a number of reports that touch on other factors that control activity of these ER-stress sensors. Such a multiplicity of regulatory mechanisms for these ER-stress sensors is likely to contribute to fine tuning of their activity.

Abstract Cystic fibrosis (CF) is a life-shortening, genetic disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR). The primary cause of CF is reduced CFTR-mediated chloride and bicarbonate transport, due to mutations in CFTR. However, inflammation and persistent infections influence clinical outcome. Cellular stress response pathways, such as the unfolded protein response (UPR) and the integrated stress response (ISR), referred to here as cellular stress response pathways (SRPs), contribute to the pathology of human disorders. Multiple studies have indicated activation of SRPs in CF tissues. We review our present understanding of how SRPs are activated in CF and their contribution to pathology. We conclude that reduced CFTR function in CF organs establishes a tissue environment in which internal or external insults activate SRPs. SRPs contribute to CF pathogenesis by reducing CFTR expression, enhancing inflammation with consequent tissue remodeling. Understanding the contribution of SRPs to CF pathogenesis is crucial even in the era of CFTR “modulators” that are designed to potentiate, correct or amplify CFTR function, since there is an urgent need for supportive treatments. Importantly, CF patients with established pathology could benefit from the targeted use of drugs that modulate SRPs to reduce the symptoms.

Abstract Increasing evidence suggests that endoplasmic reticulum (ER) stress and autophagy play an important role in regulating brain function. ER stress activates three major branches of the unfolded protein response (UPR) pathways, namely inositol-requiring enzyme-1 (IRE1), double stranded RNA-activated protein kinase (PKR)-like ER kinase (PERK) and activating transcription factor 6 (ATF6)-mediated pathways. Recent studies have suggested that these UPR signals may be linked to autophagy. In this review article, we summarize recent evidence and discuss a possible link between ER stress and autophagy with regard to neurodegenerative diseases. Furthermore, possible pharmacological strategies targeting UPR and autophagy are discussed.

Abstract Background: Transmissible spongiform encephalopathies are a collection of rare neurodegenerative disorders characterized by loss of neuronal cells, astrocytosis, and plaque formation. The causative agent of these diseases is thought to be abnormally folded prions and is characterized by a conformational change from normal, cellular prion protein (PrPc) to the abnormal form (PrPTSE). Although, there is evidence that normal prion protein can contribute to these disorders. The unfolded protein response, a conserved series of pathways involved in resolving stress associated with unfolded protein accumulation in the Endoplasmic Reticulum (ER), has been shown to play a role in regulating the development of prion diseases. Methods: This review chose papers based on their relevance to current studies involved in prion protein synthesis and transformation, identifies various links between prion diseases and ER stress, and reports on current and potential treatments as they relate to ER stress and prion diseases. Conclusion: For the advancement of prion disease treatment, it is important to understand the mechanisms involved in prion formation, and ER stress is implicated in prion disease progression. Therefore, targeting the ER or pathways involved in response to stress in the ER may help us treat prion diseases.

Abstract The conception that protein aggregates composed by misfolded proteins underlies the occurrence of several neurodegenerative diseases suggests that this phenomenon may have a common origin, ultimately driven by disruption of proteostasis control. The unfolded protein response (UPR) embodies a major element of the proteostasis network, which is engaged by endoplasmic reticulum (ER) stress. Chronic ER stress may operate as a possible mechanism of neurodegeneration, contributing to synaptic alterations, neuroinflammation and neuronal loss. In this review we discuss most recent findings relating ER stress and the development of distinct neurodegenerative diseases, and the possible strategies for disease intervention.

The unfolded protein response (UPR) is activated in response to impairments of the folding environment in the endoplasmic reticulum (ER). The most conserved arm of the UPR, inositol-requiring ER-to-nucleus signaling protein (IRE1α), has been linked to the regulation of a diverse array of cellular processes including ER-associated degradation, inflammatory signaling, cell proliferation and membrane biogenesis. Recent studies have utilized the selective, small molecule inhibitor, 4μ8c, to examine the role of IRE1α endoribonuclease (RNase) activity in various cell types including multiple myeloma, mouse embryonic fibroblasts and pancreatic beta cells [1-5]. In the present study we utilized this inhibitor to examine the role of IRE1α RNase activity in hepatoma cells (H4IIE), specifically concentrating on cell proliferation and the identification of potential off target effects under both unstressed and stressed conditions. Experiments were performed in H4IIE hepatoma cells in the absence (control conditions (LG)) or presence (LG + Thapsigargin (Thap)) of ER stress. The presence of 4μ8c decreased IRE1α RNase activity, based on reduced splicing of X-box binding protein-1 (XBP1s) and regulated IRE1α-dependent decay of mRNA in both treatments and at concentrations ranging from 10-90 μM. Cell proliferation was significantly reduced at higher concentrations (> 60 μM 4μ8c) in unstressed cells and displayed a dose-response relationship with 4μ8c in both treatments. The presence of 4μ8c did not promote cytoxicity in either of the treatment conditions but higher concentrations of the inhibitor (60 μM) were associated with apparent off-target or compensatory responses that were not observed at 10 μM. Overall, the small-molecule inhibitor, 4μ8c is an effective inhibitor of IRE1α RNase activity in H4IIE cells. Potential off-target effects associated with this inhibitor require the use of multiple inhibitor concentrations in all experiments.

Abstract The activation of Endoplasmic Reticulum (ER) stress and Unfolded Protein Response (UPR) was first observed in patients with familial interstitial pneumonia (FIP) carrying mutations in the C-terminal BRICHOS domain of surfactant protein C (SFTPC). Here, aggresome formation and severe ER stress was demonstrated in type-II alveolar epithelial cells (AECII), which specifically express this very hydrophobic surfactant protein. In subsequent studies, FIP-patients with mutations in the gene encoding surfactant protein A2 (SFTPA2) were discovered, whose overexpression in epithelial cells in vitro also resulted in significant induction of ER stress. Moreover, prominent ER stress in AECII was also observed in FIP-patients not carrying the SFTPC/SFTPA2 mutations, as well as in patients with the more common sporadic forms of IP. Additionally, cases of adult-onset FIP with mutations in Telomerase genes and other telomereassociated components were reported. These mutations were associated with telomere shortening, which is a potential cause for triggering a persistent DNA damage response and replicative senescence in affected cells. Moreover, shortened telomeres were observed directly in the AECII of FIP-patients, and even sporadic IP cases, in the absence of any gene mutations. Here, we try to figure out the possible origins of ER stress in sporadic IP cases and non-SFTPC/SFTPA2-associated FIP.

Abstract IRE-1α (inositol requiring enzyme-1α), the most evolutionarily conserved of the endoplasmic reticulum stress signaling pathways, is highly implicated in sustaining the proliferation of glioma cells and subsequent tumor growth, which is decreased by the inhibition of IRE-1α. To explore the IRE-1α mediated regulation of ubiquitin system in glioma cells, the expression of a subset of ubiquitin specific peptidases (USP) and of ubiquitin activating enzyme E1-like protein/autophagy related 7 (GSA7/ATG7) genes was studied, during hypoxic stress in wild type and U87 glioma cells with inhibited IRE-1α. Hypoxic treatment of wild type glioma cells leads to the up-regulation of USP25 and the concomitant downregulation of USP1, USP10, USP14, and GSA7 genes. USP4 and USP22 genes expression did not significantly change with hypoxic treatment. Inhibition of IRE-1α activity led to up-regulation of USP1, USP4, USP10, USP22, and USP25, while USP14 and GSA7 genes were down-regulated. Therefore, IRE-1α activity modifies substrate-targeting specificity to proteasome during hypoxic stress, which in turn can affect cell survival. Inhibition of IRE-1α correlates directly with deregulation of ubiquitin specific peptidases and GSA7 in a fashion that ultimately slows tumor growth.