XBP1 controls diverse cell type- and condition-specific transcriptional regulatory networks

Molecular Mechanisms of Cachexia: A Review

Cachexia is a condition characterized by substantial loss of body weight resulting from the depletion of skeletal muscle and adipose tissue. A considerable fraction of patients with advanced cancer, particularly those who have been diagnosed with pancreatic or gastric cancer, lung cancer, prostate cancer, colon cancer, breast cancer, or leukemias, are impacted by this condition. This syndrome manifests at all stages of cancer and is associated with an unfavorable prognosis. It heightens the susceptibility to surgical complications, chemotherapy toxicity, functional impairments, breathing difficulties, and fatigue. The early detection of patients with cancer cachexia has the potential to enhance both their quality of life and overall survival rates. Regarding this matter, blood biomarkers, although helpful, possess certain limitations and do not exhibit universal application. Additionally, the available treatment options for cachexia are currently limited, and there is a lack of comprehensive understanding of the underlying molecular pathways associated with this condition. Thus, this review aims to provide an overview of molecular mechanisms associated with cachexia and potential therapeutic targets for the development of effective treatments for this devastating condition.

Heat Shock Proteins Function as Signaling Molecules to Mediate Neuron-Glia Communication During Aging

The nervous system is primarily composed of neurons and glia, and the communication between them plays profound roles in regulating the development and function of the brain. Neuron-glia signal transduction is known to be mediated by secreted or juxtacrine signals through ligand-receptor interactions on the cell membrane. Here, we report a novel mechanism for neuron-glia signal transduction, wherein neurons transmit proteins to glia through extracellular vesicles, activating glial signaling pathways. We find that in the amphid sensory organ of Caenorhabditis elegans, different sensory neurons exhibit varying aging rates. This discrepancy in aging is governed by the crosstalk between neurons and glia. We demonstrate that early-aged neurons can transmit heat shock proteins (HSP) to glia via extracellular vesicles. These neuronal HSPs activate the IRE1-XBP1 pathway, further increasing their expression in glia, forming a positive feedback loop. Ultimately, the activation of the IRE1-XBP-1 pathway leads to the transcriptional regulation of chondroitin synthases to protect glia-embedded neurons from aging-associated functional decline. Therefore, our studies unveil a novel mechanism for neuron-glia communication in the nervous system and provide new insights into our understanding of brain aging.

Constitutive expression of spliced X-box binding protein 1 inhibits dentin formation in mice

Upon endoplasmic reticulum (ER) stress, inositol-requiring enzyme 1 (IRE1) is activated, which subsequently converts an unspliced X-box binding protein 1 (XBP1U) mRNA to a spliced mRNA that encodes a potent XBP1S transcription factor. XBP1S is essential for relieving ER stress and secretory cell differentiation. We previously established Twist2-Cre;Xbp1 CS/+ mice that constitutively expressed XBP1S in the Twist2-expressing cells as well as in the cells derived from the Twist2-expressing cells. In this study, we analyzed the dental phenotype of Twist2-Cre;Xbp1 CS/+ mice. We first generated a mutant Xbp1s minigene that corresponds to the recombinant Xbp1 Δ26 allele (the Xbp1 CS allele that has undergone Cre-mediated recombination) and confirmed that the Xbp1s minigene expressed XBP1S that does not require IRE1α activation in vitro. Consistently, immunohistochemistry showed that XBP1S was constitutively expressed in the odontoblasts and other dental pulp cells in Twist2-Cre;Xbp1 CS/+ mice. Plain X-ray radiography and µCT analysis revealed that constitutive expression of XBP1S altered the dental pulp chamber roof- and floor-dentin formation, resulting in a significant reduction in dentin/cementum formation in Twist2-Cre;Xbp1 CS/+ mice, compared to age-matched Xbp1 CS/+ control mice. However, there is no significant difference in the density of dentin/cementum between these two groups of mice. Histologically, persistent expression of XBP1S caused a morphological change in odontoblasts in Twist2-Cre;Xbp1 CS/+ mice. Nevertheless, in situ hybridization and immunohistochemistry analyses showed that continuous expression of XBP1S had no apparent effects on the expression of the Dspp and Dmp1 genes. In conclusion, these results support that sustained production of XBP1S adversely affected odontoblast function and dentin formation.

Angiotensin II type-2 receptor attenuates liver fibrosis progression by suppressing IRE1α-XBP1 pathway

The renin-angiotensin system (RAS) has been recognized as a crucial contributor to the development of liver fibrosis, and AT2R, an essential component of RAS, is involved in the progression of liver fibrosis. However, the underlying mechanisms by which AT2R modulates liver fibrosis remain elusive. Here, we report that AT2R was induced to be highly expressed during the progression of liver fibrosis, and the elevated AT2R attenuates liver fibrosis by suppressing IRE1α-XBP1 pathway. In this study, we found that AT2R is not expressed in the no cirrhotic adult liver, but is induced expression during liver fibrosis in both cirrhotic patients and fibrotic mice models. Upregulated AT2R inhibits the activation and proliferation of hepatic stellate cells (HSCs). In addition, our study showed that during liver fibrosis, AT2R deletion increased the dimerization activation of IRE1α and promoted XBP1 splicing, and the spliced XBP1s could promote their transcription by binding to the AT2R promoter and repress the IRE1α-XBP1 axis, forming an AT2R-IRE1α-XBP1 negative feedback loop. Importantly, the combination treatment of an AT2R agonist and an endoplasmic reticulum stress (ER stress) alleviator significantly attenuated liver fibrosis in a mouse model of liver fibrosis. Therefore, we conclude that the AT2R-IRE1α signaling pathway can regulate the progression of liver fibrosis, and AT2R is a new potential therapeutic target for treating liver fibrosis.

Endoplasmic reticulum stress-dependent regulation of the expression of serine hydroxymethyltransferase 2 in glioblastoma cells

Objective. Serine hydroxymethyltransferase (SHMT2) plays a multifunctional role in mitochondria (folate-dependent tRNA methylation, translation, and thymidylate synthesis). The endoplasmic reticulum stress, hypoxia, and glucose and glutamine supply are significant factors of malignant tumor growth including glioblastoma. Previous studies have shown that the knockdown of the endoplasmic reticulum to nucleus signaling 1 (ERN1) pathway of endoplasmic reticulum stress strongly suppressed glioblastoma cell proliferation and modified the sensitivity of these cells to hypoxia and glucose or glutamine deprivations. The present study aimed to investigate the regulation of the SHMT2 gene in U87MG glioblastoma cells by ERN1 knockdown, hypoxia, and glucose or glutamine deprivations with the intent to reveal the role of ERN1 signaling in sensitivity of this gene expression to hypoxia and nutrient supply. Methods. The control U87MG glioblastoma cells (transfected by an empty vector) and ERN1 knockdown cells with inhibited ERN1 endoribonuclease and protein kinase (dnERN1) or only ERN1 endoribonuclease (dnrERN1) were used. Hypoxia was introduced by dimethyloxalylglycine (500 ng/ml for 4 h). For glucose and glutamine deprivations, cells were exposed in DMEM without glucose and glutamine, respectively for 16 h. RNA was extracted from cells and reverse transcribed. The expression level of the SHMT2 gene was studied by real-time qPCR and normalized to ACTB. Results. It was found that inhibition of ERN1 endoribonuclease and protein kinase in glioblastoma cells led to a down-regulation of SHMT2 gene expression in U87MG cells. At the same time, the expression of this gene did not significantly change in cells with inhibited ERN1 endoribonuclease, but tunicamycin strongly increased its expression. Moreover, the expression of the SHMT2 gene was not affected in U87MG cells after silencing of XBP1. Hypoxia up-regulated the expression level of the SHMT2 gene in both control and ERN1 knockdown U87MG cells. The expression of this gene was significantly up-regulated in glioblastoma cells under glucose and glutamine deprivations and ERN1 knockdown significantly increased the sensitivity of the SHMT2 gene to these nutrient deprivation conditions. Conclusion. The results of the present study demonstrate that the expression of the SHMT2 gene responsible for serine metabolism and formation of folate one-carbon is controlled by ERN1 protein kinase and induced by hypoxia as well as glutamine and glucose deprivation conditions in glioblastoma cells and reflects the ERN1-mediated reprogramming of sensitivity this gene expression to nutrient deprivation.

Cardioprotective antihyperglycemic drugs ameliorate endoplasmic reticulum stress

Cellular stress, notably oxidative, inflammatory, and endoplasmic reticulum (ER) stress, is implicated in the pathogenesis of cardiovascular disease. Modifiable risk factors for cardiovascular disease such as diabetes, hypercholesterolemia, saturated fat consumption, hypertension, and cigarette smoking cause ER stress whereas currently known cardioprotective drugs with diverse pharmacodynamics share a common pleiotropic effect of reducing ER stress. Selective targeting of oxidative stress with known antioxidative vitamins has been ineffective in reducing cardiovascular risk. This "antioxidant paradox" is partially attributed to the unexpected aggravation of ER stress by the antioxidative agents used. In contrast, some of the contemporary antihyperglycemic drugs inhibit both oxidative stress and ER stress in human coronary artery endothelial cells. Unlike sulfonylureas, meglitinides, α glucosidase inhibitors, and thiazolidinediones, metformin, glucagon-like peptide 1 receptor agonists, and sodium-glucose cotransporter 2 inhibitors are the only antihyperglycemic drugs that reduce ER stress caused by pharmacological agents (tunicamycin) or hyperglycemic conditions. Clinical trials with selective ER stress modifiers are needed to test the suitability of ER stress as a therapeutic target for cardiovascular disease.

Identification of a natural PLA2 inhibitor from the marine fungus Aspergillus sp. c1 for MAFLD treatment that suppressed lipotoxicity by inhibiting the IRE-1α/XBP-1s axis and JNK signaling

Lipotoxicity is a pivotal factor that initiates and exacerbates liver injury and is involved in the development of metabolic-associated fatty liver disease (MAFLD). However, there are few reported lipotoxicity inhibitors. Here, we identified a natural anti-lipotoxicity candidate, HN-001, from the marine fungus Aspergillus sp. C1. HN-001 dose- and time- dependently reversed palmitic acid (PA)-induced hepatocyte death. This protection was associated with IRE-1α-mediated XBP-1 splicing inhibition, which resulted in suppression of XBP-1s nuclear translocation and transcriptional regulation. Knockdown of XBP-1s attenuated lipotoxicity, but no additional ameliorative effect of HN-001 on lipotoxicity was observed in XBP-1s knockdown hepatocytes. Notably, the ER stress and lipotoxicity amelioration was associated with PLA2. Both HN-001 and the PLA2 inhibitor MAFP inhibited PLA2 activity, reduced lysophosphatidylcholine (LPC) level, subsequently ameliorated lipotoxicity. In contrast, overexpression of PLA2 caused exacerbation of lipotoxicity and weakened the anti-lipotoxic effects of HN-001. Additionally, HN-001 treatment suppressed the downstream pro-apoptotic JNK pathway. In vivo, chronic administration of HN-001 (i.p.) in mice alleviated all manifestations of MAFLD, including hepatic steatosis, liver injury, inflammation, and fibrogenesis. These effects were correlated with PLA2/IRE-1α/XBP-1s axis and JNK signaling suppression. These data indicate that HN-001 has therapeutic potential for MAFLD because it suppresses lipotoxicity, and provide a natural structural basis for developing anti-MAFLD candidates.

Inhibition of signaling protein ERN1 increases the sensitivity of serine synthesis gene expressions to glucose and glutamine deprivations in U87MG glioblastoma cells

Objective. Glucose and glutamine supply as well as serine synthesis and endoplasmic reticulum (ER) stress are important factors of glioblastoma growth. Previous studies showed that the knockdown of ERN1 (ER to nucleus signaling 1) suppressed glioblastoma cell proliferation and modified the sensitivity of numerous gene expressions to nutrient deprivations. The present study is aimed to investigate the impact of glucose and glutamine deprivations on the expression of serine synthesis genes in U87MG glioblastoma cells in relation to ERN1 knockdown with the intent to reveal the role of ERN1 signaling pathway on the ER stress-dependent regulation of these gene expressions. Clarification of the regulatory mechanisms of serine synthesis is a great significance for glioblastoma therapy. Methods. The control U87MG glioblastoma cells (transfected by empty vector) and ERN1 knockdown cells (transfected by dominant-negative ERN1) were exposed under glucose and glutamine deprivation conditions for 16 h. RNA was extracted from cells and reverse transcribed. The expression level of PHGDH (phosphoglycerate dehydrogenase), PSAT1 (phosphoserine amino-transferase 1), PSPH (phosphoserine phosphatase), ATF4 (activating transcription factor 4), and SHMT1 (serine hydroxymethyltransferase 1) genes was studied by real-time qPCR and normalized to ACTB. Results. It was found that the expression level of genes responsible for serine synthesis such as PHGDH, PSAT1, PSPH, and transcription factor ATF4 was up-regulated in U87MG glioblastoma cells under glucose and glutamine deprivations. Furthermore, inhibition of ERN1 significantly enhances the impact of glucose and especially glutamine deprivations on these gene expressions. At the same time, the expression of the SHMT1 gene, which is responsible for serine conversion to glycine, was down-regulated in both nutrient deprivation conditions with more significant changes in ERN1 knockdown glioblastoma cells. Conclusion. Taken together, the results of present study indicate that the expression of genes responsible for serine synthesis is sensitive to glucose and glutamine deprivations in gene-specific manner and that suppression of ERN1 signaling significantly modifies the impact of both glucose and glutamine deprivations on PHGDH, PSAT1, PSPH, ATF4, and SHMT1 gene expressions and reflects the ERN1-mediated genome reprograming introduced by nutrient deprivation condition.

A beneficial adaptive role for CHOP in driving cell fate selection during ER stress

Cellular stresses elicit signaling cascades that are capable of either mitigating the inciting dysfunction or initiating cell death. During endoplasmic reticulum (ER) stress, the transcription factor CHOP is widely recognized to promote cell death. However, it is not clear whether CHOP also has a beneficial role during adaptation. Here, we combine a new, versatile, genetically modified Chop allele with single cell analysis and with stresses of physiological intensity, to rigorously examine the contribution of CHOP to cell fate. Paradoxically, we find that CHOP promotes death in some cells, but proliferation-and hence recovery-in others. Strikingly, this function of CHOP confers to cells a stress-specific competitive growth advantage. The dynamics of CHOP expression and UPR activation at the single cell level suggest that CHOP maximizes UPR activation, which in turn favors stress resolution, subsequent UPR deactivation, and proliferation. Taken together, these findings suggest that CHOP's function can be better described as a "stress test" that drives cells into either of two mutually exclusive fates-adaptation or death-during stresses of physiological intensity.

Endoplasmic reticulum stress-dependent regulation of carboxypeptidase E expression in glioblastoma cells

Objective. Carboxypeptidase E (CPE) plays an important role in the biosynthesis of neurotransmitters and peptide hormones including insulin. It also promotes cell proliferation, survival, and invasion of tumor cells. The endoplasmic reticulum stress, hypoxia, and nutrient supply are significant factors of malignant tumor growth including glioblastoma. There are data indicating that the knockdown of the endoplasmic reticulum to nucleus signaling 1 (ERN1) suppressed glioblastoma cell proliferation and increased invasiveness of these cells. The present study aims to investigate the regulation of the CPE gene in U87MG glioblastoma cells by ERN1 knockdown, hypoxia, and glucose or glutamine deprivations with the intent to reveal the role of ERN1 signaling in the regulation of this gene expression and function in tumorigenesis. Methods. Human glioblastoma cells U87MG (transfected by an empty vector; control) and ERN1 knockdown cells with inhibited ERN1 endoribonuclease and protein kinase (dnERN1) or only ERN1 endoribonuclease (dnrERN1) were used. Hypoxia was introduced by dimethyloxalylglycine; for glucose and glutamine deprivations, the cells were cultured in DMEM medium without glucose or glutamine for 16 h, respectively. The expression level of the CPE gene was studied by quantitative RT-PCR and normalized to ACTB. Results. It was found that inhibition of endoribonuclease and protein kinase activities of ERN1 led to a strong up-regulation of CPE gene expression in glioblastoma cells. The expression of this gene also increased in glioblastoma cells after silencing ERN1. At the same time, the expression of this gene did not significantly change in cells with inhibited ERN1 endoribonuclease only. The expression of the CPE gene was resistant to hypoxia in control U87MG cells, but increased in cells with ERN1 knockdown. The expression of this gene was up-regulated under glutamine deprivation in control glioblastoma cells, but decreased upon ERN1 knockdown. However, glucose deprivation decreased the expression of CPE gene in both types of used cells, but ERN1 inhibition enhanced this effect. Conclusion. The results of the present study demonstrate that inhibition of ERN1 strongly up-regulated the expression of pro-oncogenic CPE gene through protein kinase activity of ERN1 and that increased CPE gene expression possibly participates in ERN1 knockdown-mediated invasiveness of glioblastoma cells.

FT895 Impairs Mitochondrial Function in Malignant Peripheral Nerve Sheath Tumor Cells

Neurofibromatosis type 1 (NF1) stands as a prevalent neurocutaneous disorder. Approximately a quarter of NF1 patients experience the development of plexiform neurofibromas, potentially progressing into malignant peripheral nerve sheath tumors (MPNST). FT895, an HDAC11 inhibitor, exhibits potent anti-tumor effects on MPNST cells and enhances the cytotoxicity of cordycepin against MPNST. The study aims to investigate the molecular mechanism underlying FT895's efficacy against MPNST cells. Initially, our study unveiled that FT895 disrupts mitochondrial biogenesis and function. Post-FT895 treatment, reactive oxygen species (ROS) in MPNST notably increased, while mitochondrial DNA copy numbers decreased significantly. Seahorse analysis indicated a considerable decrease in basal, maximal, and ATP-production-coupled respiration following FT895 treatment. Immunostaining highlighted FT895's role in promoting mitochondrial aggregation without triggering mitophagy, possibly due to reduced levels of XBP1, Parkin, and PINK1 proteins. Moreover, the study using CHIP-qPCR analysis revealed a significant reduction in the copy numbers of promoters of the MPV17L2, POLG, TFAM, PINK1, and Parkin genes. The RNA-seq analysis underscored the prominent role of the HIF-1α signaling pathway post-FT895 treatment, aligning with the observed impairment in mitochondrial respiration. In summary, the study pioneers the revelation that FT895 induces mitochondrial respiratory damage in MPNST cells.

Recent advances in small molecule and peptide inhibitors of glucose-regulated protein 78 for cancer therapy

Glucose-regulated protein 78 (GRP78) is one of key endoplasmic reticulum (ER) chaperone proteins that regulates the unfolded protein response (UPR) to maintain ER homeostasis. As a core factor in the regulation of the UPR, GRP78 takes a critical part in the cellular processes required for tumorigenesis, such as proliferation, metastasis, anti-apoptosis, immune escape and chemoresistance. Overexpression of GRP78 is closely correlated with tumorigenesis and poor prognosis in various malignant tumors. Targeting GRP78 is regarded as a potentially promising therapeutic strategy for cancer therapy. Although none of the GRP78 inhibitors have been approved to date, there have been several studies of GRP78 inhibitors. Herein, we comprehensively review the structure, physiological functions of GRP78 and the recent progress of GRP78 inhibitors, and discuss the structures, in vitro and in vivo efficacies, and merits and demerits of these inhibitors to inspire further research. Additionally, the feasibility of GRP78-targeting proteolysis-targeting chimeras (PROTACs), disrupting GRP78 cochaperone interactions, or covalent inhibition are also discussed as novel strategies for drugs discovery targeting GRP78, with the hope that these strategies can provide new opportunities for targeted GRP78 antitumor therapy.

Thioredoxin regulates the redox state and the activity of the human tRNA ligase complex

The mammalian tRNA ligase complex (tRNA-LC) catalyzes the splicing of intron-containing pre-tRNAs in the nucleus and the splicing of XBP1 mRNA during the unfolded protein response (UPR) in the cytoplasm. We recently reported that the tRNA-LC coevolved with PYROXD1, an essential oxidoreductase that protects the catalytic cysteine of RTCB, the catalytic subunit of the tRNA-LC, against aerobic oxidation. In this study, we show that the oxidoreductase Thioredoxin (TRX) preserves the enzymatic activity of RTCB under otherwise inhibiting concentrations of oxidants. TRX physically interacts with oxidized RTCB, and reduces and reactivates RTCB through the action of its redox-active cysteine pair. We further show that TRX interacts with RTCB at late stages of UPR. Since the interaction requires oxidative conditions, our findings suggest that prolonged UPR generates reactive oxygen species. Thus, our results support a functional role for TRX in securing and repairing the active site of the tRNA-LC, thereby allowing pre-tRNA splicing and UPR to occur when cells encounter mild, but still inhibitory levels of reactive oxygen species.

The activation of spliced X-box binding protein 1 by isorhynchophylline therapy improves diabetic encephalopathy

The primary symptom of diabetic encephalopathy (DE), a kind of central diabetic neuropathy caused by diabetes mellitus (DM), is cognitive impairment. In addition, the tetracyclic oxindole alkaloid isorhynchophylline (IRN) helps lessen cognitive impairment. However, it is still unclear how IRN affects DM and DE and what mechanisms are involved. The effectiveness of IRN on brain insulin resistance was carefully examined in this work, both in vitro and in vivo. We found that IRN accelerates spliced form of X-box binding protein 1 (sXBP1) translocation into the nucleus under high glucose conditions in vitro. IRN also facilitates the nuclear association of pCREB with sXBP1 and the binding of regulatory subunits of phosphatidylinositol 3-kinase (PI3K) p85α or p85β with XBP1 to restore high glucose impairment. Also, IRN treatment improves high glucose-mediated impairment of insulin signaling, endoplasmic reticulum stress, and pyroptosis/apoptosis by depending on sXBP1 in vitro. In vivo studies suggested that IRN attenuates cognitive impairment, ameliorating peripheral insulin resistance, activating insulin signaling, inactivating activating transcription factor 6 (ATF6) and C/EBP homology protein (CHOP), and mitigating pyroptosis/apoptosis by stimulation of sXBP1 nuclear translocation in the brain. In summary, these data indicate that IRN contributes to maintaining insulin homeostasis by activating sXBP1 in the brain. Thus, IRN is a potent antidiabetic agent as well as an sXBP1 activator that has promising potential for the prevention or treatment of DE.

TANGO1 Dances to Export of Procollagen from the Endoplasmic Reticulum

The endoplasmic reticulum (ER) to Golgi secretory pathway is an elegantly complex process whereby protein cargoes are manufactured, folded, and distributed from the ER to the cisternal layers of the Golgi stack before they are delivered to their final destinations. The export of large bulky cargoes such as procollagen and its trafficking to the Golgi is a sophisticated mechanism requiring TANGO1 (Transport ANd Golgi Organization protein 1. It is also called MIA3 (Melanoma Inhibitory Activity protein 3). TANGO1 has two prominent isoforms, TANGO1-Long and TANGO1-Short, and each isoform has specific functions. On the luminal side, TANGO1-Long has an HSP47 recruitment domain and uses this protein to collect collagen. It can also tether its paralog isoforms cTAGE5 and TALI and along with these proteins enlarges the vesicle to accommodate procollagen. Recent studies show that TANGO1-Long combines retrograde membrane flow with anterograde cargo transport. This complex mechanism is highly activated in fibrosis and promotes the excessive deposition of collagen in the tissues. The therapeutic targeting of TANGO1 may prove successful in the control of fibrotic disorders. This review focuses on TANGO1 and its complex interaction with other procollagen export factors that modulate increased vesicle size to accommodate the export of procollagen.

SEPTIN2 suppresses an IFN-γ-independent, proinflammatory macrophage activation pathway

Interferon-gamma (IFN-γ) signaling is necessary for the proinflammatory activation of macrophages but IFN-γ-independent pathways, for which the initiating stimuli and downstream mechanisms are lesser known, also contribute. Here we identify, by high-content screening, SEPTIN2 (SEPT2) as a negative regulation of IFN-γ-independent macrophage autoactivation. Mechanistically, endoplasmic reticulum (ER) stress induces the expression of SEPT2, which balances the competition between acetylation and ubiquitination of heat shock protein 5 at position Lysine 327, thereby alleviating ER stress and constraining M1-like polarization and proinflammatory cytokine release. Disruption of this negative feedback regulation leads to the accumulation of unfolded proteins, resulting in accelerated M1-like polarization, excessive inflammation and tissue damage. Our study thus uncovers an IFN-γ-independent macrophage proinflammatory autoactivation pathway and suggests that SEPT2 may play a role in the prevention or resolution of inflammation during infection.

Changes in ADAR RNA editing patterns in CMV and ZIKV congenital infections

BACKGROUND

RNA editing is a process that increases transcriptome diversity, often through Adenosine Deaminases Acting on RNA (ADARs) that catalyze the deamination of adenosine to inosine. ADAR editing plays an important role in regulating brain function and immune activation, and is dynamically regulated during brain development. Additionally, the ADAR1 p150 isoform is induced by interferons in viral infection and plays a role in antiviral immune response. However, the question of how virus-induced ADAR expression affects host transcriptome editing remains largely unanswered. This question is particularly relevant in the context of congenital infections, given the dynamic regulation of ADAR editing during brain development, the importance of this editing for brain function, and subsequent neurological symptoms of such infections, including microcephaly, sensory issues, and other neurodevelopmental abnormalities. Here, we begin to address this question, examining ADAR expression in publicly available datasets of congenital infections of human cytomegalovirus (HCMV) microarray expression data, as well as mouse cytomegalovirus (MCMV) and mouse/ human induced pluripotent neuroprogenitor stem cell (hiNPC) Zika virus (ZIKV) RNA-seq data.

RESULTS

We found that in all three datasets, ADAR1 was overexpressed in infected samples compared to uninfected samples. In the RNA-seq datasets, editing rates were also analyzed. In all mouse infections cases, the number of editing sites was significantly increased in infected samples, albeit this was not the case for hiNPC ZIKV samples. Mouse ZIKV samples showed altered editing of well-established protein-recoding sites such as Gria3, Grik5, and Nova1, as well as editing sites that may impact miRNA binding.

CONCLUSIONS

Our findings provide evidence for changes in ADAR expression and subsequent dysregulation of ADAR editing of host transcriptomes in congenital infections. These changes in editing patterns of key neural genes have potential significance in the development of neurological symptoms, thus contributing to neurodevelopmental abnormalities. Further experiments should be performed to explore the full range of editing changes that occur in different congenital infections, and to confirm the specific functional consequences of these editing changes.

Insights into the structure and function of the RNA ligase RtcB

To be functional, some RNAs require a processing step involving splicing events. Each splicing event necessitates an RNA ligation step. RNA ligation is a process that can be achieved with various intermediaries such as self-catalysing RNAs, 5'-3' and 3'-5' RNA ligases. While several types of RNA ligation mechanisms occur in human, RtcB is the only 3'-5' RNA ligase identified in human cells to date. RtcB RNA ligation activity is well known to be essential for the splicing of XBP1, an essential transcription factor of the unfolded protein response; as well as for the maturation of specific intron-containing tRNAs. As such, RtcB is a core factor in protein synthesis and homeostasis. Taking advantage of the high homology between RtcB orthologues in archaea, bacteria and eukaryotes, this review will provide an introduction to the structure of RtcB and the mechanism of 3'-5' RNA ligation. This analysis is followed by a description of the mechanisms regulating RtcB activity and localisation, its known partners and its various functions from bacteria to human with a specific focus on human cancer.

Chemical Chaperone 4-PBA Mitigates Tumor Necrosis Factor Alpha-Induced Endoplasmic Reticulum Stress in Human Airway Smooth Muscle

Airway inflammation and pro-inflammatory cytokines such as tumor necrosis factor alpha (TNFα) underlie the pathophysiology of respiratory diseases, including asthma. Previously, we showed that TNFα activates the inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 spliced (XBP1s) endoplasmic reticulum (ER) stress pathway in human airway smooth muscle (hASM) cells. The ER stress pathway is activated by the accumulation of unfolded proteins in the ER. Accordingly, chemical chaperones such as 4-phenylbutyric acid (4-PBA) may reduce ER stress activation. In the present study, we hypothesized that chemical chaperone 4-PBA mitigates TNFα-induced ER stress in hASM cells. hASM cells were isolated from bronchiolar tissue obtained from five patients with no history of smoking or respiratory diseases. The hASM cells' phenotype was confirmed via the expression of alpha-smooth muscle actin and elongated morphology. hASM cells from the same patient sample were then separated into three 12 h treatment groups: (1) TNFα (20 ng/mL), (2) TNFα + 4-PBA (1 μM, 30 min pretreatment), and (3) untreated control. The expressions of total IRE1α and phosphorylated IRE1α (pIRE1αS724) were determined through Western blotting. The splicing of XBP1 mRNA was analyzed using RT-PCR. We found that TNFα induced an increase in pIRE1αS724 phosphorylation, which was mitigated by treatment with chemical chaperone 4-PBA. We also found that TNFα induced an increase in XBP1s mRNA, which was also mitigated by treatment with chemical chaperone 4-PBA. These results support our hypothesis and indicate that chemical chaperone 4-PBA treatment mitigates TNFα-induced ER stress in hASM cells.

Dengue virus NS1 secretion is regulated via importin-subunit β1 controlling expression of the chaperone GRp78 and targeted by the clinical drug ivermectin

IMPORTANCE

Dengue virus (DENV) is a major human pathogen that can cause hemorrhagic fever and shock syndrome. One important factor of DENV pathogenicity is non-structural protein 1 (NS1), a glycoprotein that is secreted from infected cells. Here we study the mode of action of the widely used drug ivermectin, used to treat parasitic infections and recently shown to lower NS1 blood levels in DENV-infected patients. We found that ivermectin blocks the nuclear transport of transcription factors required for the expression of chaperones that support the folding and secretion of glycoproteins, including NS1. Impairing nuclear transport of these transcription factors by ivermectin or depleting them from infected cells dampens NS1 folding and thus its secretion. These results reveal a novel mode of action of ivermectin that might apply to other flaviviruses as well.

XBP1 Modulates the Aging Cardiorenal System by Regulating Oxidative Stress

X-box binding protein 1 (XBP1) is a unique basic-region leucine zipper (bZIP) transcription factor. Over recent years, the powerful biological functions of XBP1 in oxidative stress have been gradually revealed. When the redox balance remains undisturbed, oxidative stress plays a role in physiological adaptations and signal transduction. However, during the aging process, increased cellular senescence and reduced levels of endogenous antioxidants cause an oxidative imbalance in the cardiorenal system. Recent studies from our laboratory and others have indicated that these age-related cardiorenal diseases caused by oxidative stress are guided and controlled by a versatile network composed of diversified XBP1 pathways. In this review, we describe the mechanisms that link XBP1 and oxidative stress in a range of cardiorenal disorders, including mitochondrial instability, inflammation, and alterations in neurohumoral drive. Furthermore, we propose that differing degrees of XBP1 activation may cause beneficial or harmful effects in the cardiorenal system. Gaining a comprehensive understanding of how XBP1 exerts influence on the aging cardiorenal system by regulating oxidative stress will enhance our ability to provide new directions and strategies for cardiovascular and renal safety outcomes.

The Role of XBP1 in bone metabolism

Bone is a dynamic organ that, once formed, undergoes a constant remodeling process that includes bone resorption and synthesis. Osteoclasts and osteoblasts are primarily responsible for controlling this process. X-box binding protein 1 (XBP1), a transcription factor, affects the metabolism of bones in various ways. In recent years, numerous studies have revealed that XBP1 plays a vital role in bone metabolism, including osteoclast and osteoblast development, as well as in regulating immune cell differentiation that affects the immune microenvironment of bone remodeling. In this review, we highlight the regulatory mechanisms of XBP1 on osteoclasts and osteoblasts, how XBP1 affects the immune microenvironment of bone remodeling by influencing the differentiation of immune cells, and predict the possible future research directions of XBP1 to provide new insights for the treatment of bone-related metabolic diseases.

HBV with precore and basal core promoter mutations exhibits a high replication phenotype and causes ER stress-mediated cell death in humanized liver chimeric mice

BACKGROUND AND AIMS

Mutations within the precore (PC) and basal core promoter (BCP) regions of the HBV genome are associated with fulminant hepatitis and HBV reactivation. These mutations may enhance viral replication, but little is known about whether they directly induce damage to the liver. We investigated mechanisms of direct cytopathic effects induced by the infection with PC/BCP mutants in the absence of immune response in vitro and in vivo .

APPROACH AND RESULTS

Mice with humanized livers and hepatocytes derived from humanized mice were infected with either wild-type or mutant-type PC/BCP HBV, and the HBV replication and human hepatocyte damage were evaluated. HBV proliferated vigorously in mice with PC/BCP-mutant infection, and the severe loss of human hepatocytes with a slight human ALT elevation subsequently occurred only in PC/BCP mutant mice. In PC/BCP mutant infection, the accumulation of HBsAg in humanized livers colocalized with the endoplasmic reticulum, leading to apoptosis through unfolded protein response in HBV-infected hepatocytes. RNA-sequencing revealed the molecular characteristics of the phenotype of PC/BCP mutant infection in a humanized mouse model. Reduced ALT elevation and higher HBV DNA levels in this model are consistent with characteristics of HBV reactivation, indicating that the hepatocyte damage in this model might mimic HBV reactivation followed by hepatocyte damage under immunosuppressive conditions.

CONCLUSION

PC and BCP mutations were associated with enhanced viral replication and cell death induced by ER stress using HBV infection models. These mutations might be associated with liver damage in patients with fulminant hepatitis or HBV reactivation.

ER stress decreases exosome production through adiponectin/T-cadherin-dependent and -independent pathways

Exosomes, extracellular vesicles (EVs) produced within cells, mediate both the disposal of intracellular waste and communication with distant cells, and they are involved in a variety of disease processes. Although disease modifications of exosome cargos have been well studied, it has been poorly investigated how disease processes, such as endoplasmic reticulum (ER) stress, affect EV production. We previously reported that adiponectin, an adipocyte-secreted salutary factor, increases systemic exosome levels through T-cadherin-mediated enhancement of exosome biogenesis. In the present study, we demonstrated that adiponectin/T-cadherin-dependent EV production was susceptible to ER stress and that low-dose tunicamycin significantly reduced EV production in the presence, but not in the absence, of adiponectin. Moreover, pharmacological or genetic activation of inositol-requiring enzyme 1α, a central regulator of ER stress, downregulated T-cadherin at the mRNA and protein levels as well as attenuated EV production. In addition, adiponectin/T-cadherin-independent EV production was attenuated under ER stress conditions. Repeated administration of tunicamycin to mice decreased circulating small EVs without decreasing tissue T-cadherin expression. Mechanistically, inositol-requiring enzyme 1α activation by silencing of the X-box binding protein 1 transcription factor upregulated the canonical interferon pathway and decreased EV production. The interferon pathway, when it was activated by polyinosinic-polycytidylic acid, also significantly attenuated EV production. Thus, we concluded that ER stress decreases exosome production through adiponectin/T-cadherin-dependent and -independent pathways.

High-Dose Deltamethrin Induces Developmental Toxicity in Caenorhabditis elegans via IRE-1

Deltamethrin (DM), a Type II pyrethroid, is widely used worldwide in agriculture, household applications, and medicine. Recent studies have shown that DM exerts a variety of toxic effects on organs such as the kidney, heart muscle, and nerves in animals. However, little is known about the effects of high-dose DM on growth and development, and the mechanism of toxicity remains unclear. Using the Caenorhabditis elegans model, we found that high-dose DM caused a delay in nematode development. Our results showed that high-dose DM reduced the activation of the endoplasmic reticulum unfolded protein response (UPRER). Further studies revealed that high-dose DM-induced developmental toxicity and reduced capacity for UPRER activation were associated with the IRE-1/XBP-1 pathway. Our results provide new evidence for the developmental toxicity of DM and new insights into the mechanism of DM toxicity.

RNA Crossing Membranes: Systems and Mechanisms Contextualizing Extracellular RNA and Cell Surface GlycoRNAs

The subcellular localization of a biopolymer often informs its function. RNA is traditionally confined to the cytosolic and nuclear spaces, where it plays critical and conserved roles across nearly all biochemical processes. Our recent observation of cell surface glycoRNAs may further explain the extracellular role of RNA. While cellular membranes are efficient gatekeepers of charged polymers such as RNAs, a large body of research has demonstrated the accumulation of specific RNA species outside of the cell, termed extracellular RNAs (exRNAs). Across various species and forms of life, protein pores have evolved to transport RNA across membranes, thus providing a mechanistic path for exRNAs to achieve their extracellular topology. Here, we review types of exRNAs and the pores capable of RNA transport to provide a logical and testable path toward understanding the biogenesis and regulation of cell surface glycoRNAs.

Comprehensive mRNA-sequencing-based characterization of three HEK-293 cell lines during an rAAV production process for gene therapy applications

Human embryonal kidney cells (HEK-293) are the most common host cells used for transient recombinant adeno-associated virus (rAAV) production in pharmaceutical industry. To better cover the expected gene therapy product demands in the future, different traditional strategies such as cell line sub-cloning and/or addition of chemical substances to the fermentation media have been used to maximize titers and improve product quality. A more effective and advanced approach to boost yield can be envisaged by characterizing the transcriptome of different HEK-293 cell line pedigrees with distinct rAAV productivity patterns to subsequently identify potential gene targets for cell engineering. In this work, the mRNA expression profile of three HEK-293 cell lines, resulting in various yields during a fermentation batch process for rAAV production, was investigated to gain basic insight into cell variability and eventually to identify genes that correlate with productivity. Mock runs using only transfection reagents were performed in parallel as a control. It finds significant differences in gene regulatory behaviors between the three cell lines at different growth and production stages. The evaluation of these transcriptomics profiles combined with collected in-process control parameters and titers shed some light on potential cell engineering targets to maximize transient production of rAAV in HEK-293 cells.

RSV replication modifies the XBP1s binding complex on the IRF1 upstream enhancer to potentiate the mucosal anti-viral response

Introduction

The unfolded protein response (UPR) has emerged as an important signaling pathway mediating anti-viral defenses to Respiratory Syncytial Virus (RSV) infection. Earlier we found that RSV replication predominantly activates the evolutionarily conserved Inositol Requiring Enzyme 1α (IRE1α)-X-Box Binding Protein 1 spliced (XBP1s) arm of the Unfolded Protein Response (UPR) producing inflammation, metabolic adaptation and cellular plasticity, yet the mechanisms how the UPR potentiates inflammation are not well understood.

Methods

To understand this process better, we examined the genomic response integrating RNA-seq and Cleavage Under Targets and Release Using Nuclease (CUT&RUN) analyses. These data were integrated with an RNA-seq analysis conducted on RSV-infected small airway cells ± an IRE1α RNAse inhibitor.

Results

We identified RSV induced expression changes in ~3.2K genes; of these, 279 required IRE1α and were enriched in IL-10/cytokine signaling pathways. From this data set, we identify those genes directly under XBP1s control by CUT&RUN. Although XBP1s binds to ~4.2 K high-confidence genomic binding sites, surprisingly only a small subset of IL10/cytokine signaling genes are directly bound. We further apply CUT&RUN to find that RSV infection enhances XBP1s loading on 786 genomic sites enriched in AP1/Fra-1, RELA and SP1 binding sites. These control a subset of cytokine regulatory factor genes including IFN response factor 1 (IRF1), CSF2, NFKB1A and DUSP10. Focusing on the downstream role of IRF1, selective knockdown (KD) and overexpression experiments demonstrate IRF1 induction controls type I and -III interferon (IFN) and IFN-stimulated gene (ISG) expression, demonstrating that ISG are indirectly regulated by XBP1 through IRF1 transactivation. Examining the mechanism of IRF1 activation, we observe that XBP1s directly binds a 5' enhancer sequence whose XBP1s loading is increased by RSV. The functional requirement for the enhancer is demonstrated by targeting a dCas9-KRAB silencer, reducing IRF1 activation. Chromatin immunoprecipitation shows that XBP1 is required, but not sufficient, for RSV-induced recruitment of activated phospho-Ser2 Pol II to the enhancer.

Discussion

We conclude that XBP1s is a direct activator of a core subset of IFN and cytokine regulatory genes in response to RSV. Of these IRF1 is upstream of the type III IFN and ISG response. We find that RSV modulates the XBP1s binding complex on the IRF1 5' enhancer whose activation is required for IRF1 expression. These findings provide novel insight into how the IRE1α-XBP1s pathway potentiates airway mucosal anti-viral responses.

The Unfolded Protein Response and Its Implications for Novel Therapeutic Strategies in Inflammatory Bowel Disease

The endoplasmic reticulum (ER) is a multifunctional organelle playing a vital role in maintaining cell homeostasis, and disruptions to its functions can have detrimental effects on cells. Dysregulated ER stress and the unfolded protein response (UPR) have been linked to various human diseases. For example, ER stress and the activation of the UPR signaling pathways in intestinal epithelial cells can either exacerbate or alleviate the severity of inflammatory bowel disease (IBD), contingent on the degree and conditions of activation. Our recent studies have shown that EPICERTIN, a recombinant variant of the cholera toxin B subunit containing an ER retention motif, can induce a protective UPR in colon epithelial cells, subsequently promoting epithelial restitution and mucosal healing in IBD models. These findings support the idea that compounds modulating UPR may be promising pharmaceutical candidates for the treatment of the disease. In this review, we summarize our current understanding of the ER stress and UPR in IBD, focusing on their roles in maintaining cell homeostasis, dysregulation, and disease pathogenesis. Additionally, we discuss therapeutic strategies that promote the cytoprotection of colon epithelial cells and reduce inflammation via pharmacological manipulation of the UPR.

Enhanced SARS-CoV-2 entry via UPR-dependent AMPK-related kinase NUAK2

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) remodels the endoplasmic reticulum (ER) to form replication organelles, leading to ER stress and unfolded protein response (UPR). However, the role of specific UPR pathways in infection remains unclear. Here, we found that SARS-CoV-2 infection causes marginal activation of signaling sensor IRE1α leading to its phosphorylation, clustering in the form of dense ER-membrane rearrangements with embedded membrane openings, and XBP1 splicing. By investigating the factors regulated by IRE1α-XBP1 during SARS-CoV-2 infection, we identified stress-activated kinase NUAK2 as a novel host-dependency factor for SARS-CoV-2, HCoV-229E, and MERS-CoV entry. Reducing NUAK2 abundance or kinase activity impaired SARS-CoV-2 particle binding and internalization by decreasing cell surface levels of viral receptors and viral trafficking likely by modulating the actin cytoskeleton. IRE1α-dependent NUAK2 levels were elevated in SARS-CoV-2-infected and bystander non-infected cells, promoting viral spread by maintaining ACE2 cell surface levels and facilitating virion binding to bystander cells.

The unfolded protein response transcription factor XBP1s ameliorates Alzheimer's disease by improving synaptic function and proteostasis

Alteration in the buffering capacity of the proteostasis network is an emerging feature of Alzheimer's disease (AD), highlighting the occurrence of endoplasmic reticulum (ER) stress. The unfolded protein response (UPR) is the main adaptive pathway to cope with protein folding stress at the ER. Inositol-requiring enzyme-1 (IRE1) operates as a central ER stress sensor, enabling the establishment of adaptive and repair programs through the control of the expression of the transcription factor X-box binding protein 1 (XBP1). To artificially enforce the adaptive capacity of the UPR in the AD brain, we developed strategies to express the active form of XBP1 in the brain. Overexpression of XBP1 in the nervous system using transgenic mice reduced the load of amyloid deposits and preserved synaptic and cognitive function. Moreover, local delivery of XBP1 into the hippocampus of an 5xFAD mice using adeno-associated vectors improved different AD features. XBP1 expression corrected a large proportion of the proteomic alterations observed in the AD model, restoring the levels of several synaptic proteins and factors involved in actin cytoskeleton regulation and axonal growth. Our results illustrate the therapeutic potential of targeting UPR-dependent gene expression programs as a strategy to ameliorate AD features and sustain synaptic function.

Activation of XBP1 but not ATF6α rescues heart failure induced by persistent ER stress in medaka fish

The unfolded protein response is triggered in vertebrates by ubiquitously expressed IRE1α/β (although IRE1β is gut-specific in mice), PERK, and ATF6α/β, transmembrane-type sensor proteins in the ER, to cope with ER stress, the accumulation of unfolded and misfolded proteins in the ER. Here, we burdened medaka fish, a vertebrate model organism, with ER stress persistently from fertilization by knocking out the AXER gene encoding an ATP/ADP exchanger in the ER membrane, leading to decreased ATP concentration-mediated impairment of the activity of Hsp70- and Hsp90-type molecular chaperones in the ER lumen. ER stress and apoptosis were evoked from 4 and 6 dpf, respectively, leading to the death of all AXER-KO medaka by 12 dpf because of heart failure (medaka hatch at 7 dpf). Importantly, constitutive activation of IRE1α signaling-but not ATF6α signaling-rescued this heart failure and allowed AXER-KO medaka to survive 3 d longer, likely because of XBP1-mediated transcriptional induction of ER-associated degradation components. Thus, activation of a specific pathway of the unfolded protein response can cure defects in a particular organ.

ER Stress, the Unfolded Protein Response and Osteoclastogenesis: A Review

Endoplasmic reticulum (ER) stress and its adaptive mechanism, the unfolded protein response (UPR), are triggered by the accumulation of unfolded and misfolded proteins. During osteoclastogenesis, a large number of active proteins are synthesized. When an imbalance in the protein folding process occurs, it causes osteoclasts to trigger the UPR. This close association has led to the role of the UPR in osteoclastogenesis being increasingly explored. In recent years, several studies have reported the role of ER stress and UPR in osteoclastogenesis and bone resorption. Here, we reviewed the relevant literature and discussed the UPR signaling cascade response, osteoclastogenesis-related signaling pathways, and the role of UPR in osteoclastogenesis and bone resorption in detail. It was found that the UPR signal (PERK, CHOP, and IRE1-XBP1) promoted the expression of the receptor activator of the nuclear factor-kappa B ligand (RANKL) in osteoblasts and indirectly enhanced osteoclastogenesis. IRE1 promoted osteoclastogenesis via promoting NF-κB, MAPK signaling, or the release of pro-inflammatory factors (IL-6, IL-1β, and TNFα). CREBH promoted osteoclast differentiation by promoting NFATc1 expression. The PERK signaling pathway also promoted osteoclastogenesis through NF-κB and MAPK signaling pathways, autophagy, and RANKL secretion from osteoblasts. However, salubrinal (an inhibitor of eIF2α dephosphorylation that upregulated p-eIF2α expression) directly inhibited osteoclastogenesis by suppressing NFATc1 expression and indirectly promoted osteoclastogenesis by promoting RANKL secretion from osteoblasts. Therefore, the specific effects and mechanisms of p-PERK and its downstream signaling on osteoclastogenesis still need further experiments to confirm. In addition, the exact role of ATF6 and BiP in osteoclastogenesis also required further exploration. In conclusion, our detailed and systematic review provides some references for the next step to fully elucidate the relationship between UPR and osteoclastogenesis, intending to provide new insights for the treatment of diseases caused by osteoclast over-differentiation, such as osteoporosis.

Myogenesis in C2C12 Cells Requires Phosphorylation of ATF6α by p38 MAPK

Activating transcription factor 6α (ATF6α) is an endoplasmic reticulum protein known to participate in unfolded protein response (UPR) during ER stress in mammals. Herein, we show that in mouse C2C12 myoblasts induced to differentiate, ATF6α is the only pathway of the UPR activated. ATF6α stimulation is p38 MAPK-dependent, as revealed by the use of the inhibitor SB203580, which halts myotube formation and, at the same time, impairs trafficking of ATF6α, which accumulates at the cis-Golgi without being processed in the p50 transcriptional active form. To further evaluate the role of ATF6α, we knocked out the ATF6α gene, thus inhibiting the C2C12 myoblast from undergoing myogenesis, and this occurred independently from p38 MAPK activity. The expression of exogenous ATF6α in knocked-out ATF6α cells recover myogenesis, whereas the expression of an ATF6α mutant in the p38 MAPK phosphorylation site (T166) was not able to regain myogenesis. Genetic ablation of ATF6α also prevents the exit from the cell cycle, which is essential for muscle differentiation. Furthermore, when we inhibited differentiation by the use of dexamethasone in C2C12 cells, we found inactivation of p38 MAPK and, consequently, loss of ATF6α activity. All these findings suggest that the p-p38 MAPK/ATF6α axis, in pathophysiological conditions, regulates myogenesis by promoting the exit from the cell cycle, an essential step to start myoblasts differentiation.

Mitochondria-associated endoplasmic reticulum membranes (MAMs) and their role in glaucomatous retinal ganglion cell degeneration-a mini review

Glaucoma is a leading cause of blindness worldwide, commonly associated with elevated intraocular pressure (IOP), leading to degeneration of the optic nerve and death of retinal ganglion cells, the output neurons in the eye. In recent years, many studies have implicated mitochondrial dysfunction as a crucial player in glaucomatous neurodegeneration. Mitochondrial function has been an increasingly researched topic in glaucoma, given its vital role in bioenergetics and propagation of action potentials. One of the most metabolically active tissues in the body characterized by high oxygen consumption is the retina, particularly the retinal ganglion cells (RGCs). RGCs, which have long axons that extend from the eyes to the brain, rely heavily on the energy generated by oxidative phosphorylation for signal transduction, rendering them more vulnerable to oxidative damage. In various glaucoma models, mitochondrial dysfunction and stress from protein aggregates in the endoplasmic reticulum (ER) have been observed in the RGCs. However, it has been shown that the two organelles are connected through a network called mitochondria-associated ER membranes (MAMs); hence this crosstalk in a pathophysiological condition such as glaucoma should be evaluated. Here, we review the current literature suggestive of mitochondrial and ER stress related to glaucoma, indicating potential cross-signaling and the potential roles of MAMs.

Senescence: a double-edged sword in beta-cell health and failure?

Cellular senescence is a complex process marked by permanent cell-cycle arrest in response to a variety of stressors, and acts as a safeguard against the proliferation of damaged cells. Senescence is not only a key process underlying aging and development of many diseases, but has also been shown to play a vital role in embryogenesis as well as tissue regeneration and repair. In context of the pancreatic beta-cells, that are essential for maintaining glucose homeostasis, replicative senescence is responsible for the age-related decline in regenerative capacity. Stress induced premature senescence is also a key early event underlying beta-cell failure in both type 1 and type 2 diabetes. Targeting senescence has therefore emerged as a promising therapeutic avenue for diabetes. However, the molecular mechanisms that mediate the induction of beta-cell senescence in response to various stressors remain unclear. Nor do we know if senescence plays any role during beta-cell growth and development. In this perspective, we discuss the significance of senescence in beta-cell homeostasis and pathology and highlight emerging directions in this area that warrant our attention.

Genomic targets of the IRE1-XBP1s pathway in mediating metabolic adaptation in epithelial plasticity

Epithelial mesenchymal plasticity (EMP) is a complex cellular reprogramming event that plays a major role in tissue homeostasis. Recently we observed the unfolded protein response (UPR) triggers EMP through the inositol-requiring protein 1 (IRE1α)-X-box-binding protein 1 spliced (XBP1s) axis, enhancing glucose shunting to protein N glycosylation. To better understand the genomic targets of XBP1s, we identified its genomic targets using Cleavage Under Targets and Release Using Nuclease (CUT&RUN) of a FLAG-epitope tagged XBP1s in RSV infection. CUT&RUN identified 7086 binding sites in chromatin that were enriched in AP-1 motifs and GC-sequences. Of these binding sites, XBP1s peaks mapped to 4827 genes controlling Rho-GTPase signaling, N-linked glycosylation and ER-Golgi transport. Strikingly, XBP1s peaks were within 1 kb of transcription start sites of 2119 promoters. In addition to binding core mesenchymal transcription factors SNAI1 and ZEB1, we observed that hexosamine biosynthetic pathway (HBP) enzymes were induced and contained proximal XBP1s peaks. We demonstrate that IRE1α -XBP1s signaling is necessary and sufficient to activate core enzymes by recruiting elongation-competent phospho-Ser2 CTD modified RNA Pol II. We conclude that the IRE1α-XBP1s pathway coordinately regulates mesenchymal transcription factors and hexosamine biosynthesis in EMP by a mechanism involving recruitment of activated pSer2-Pol II to GC-rich promoters.

The endoplasmic reticulum stress sensor IRE1 regulates collagen secretion through the enforcement of the proteostasis factor P4HB/PDIA1 contributing to liver damage and fibrosis

Collagen is one the most abundant proteins and the main cargo of the secretory pathway, contributing to hepatic fibrosis and cirrhosis due to excessive deposition of extracellular matrix. Here we investigated the possible contribution of the unfolded protein response, the main adaptive pathway that monitors and adjusts the protein production capacity at the endoplasmic reticulum, to collagen biogenesis and liver disease. Genetic ablation of the ER stress sensor IRE1 reduced liver damage and diminished collagen deposition in models of liver fibrosis triggered by carbon tetrachloride (CCl ₄ ) administration or by high fat diet. Proteomic and transcriptomic profiling identified the prolyl 4-hydroxylase (P4HB, also known as PDIA1), which is known to be critical for collagen maturation, as a major IRE1-induced gene. Cell culture studies demonstrated that IRE1 deficiency results in collagen retention at the ER and altered secretion, a phenotype rescued by P4HB overexpression. Taken together, our results collectively establish a role of the IRE1/P4HB axis in the regulation of collagen production and its significance in the pathogenesis of various disease states.

Bilirubin improves renal function by reversing the endoplasmic reticulum stress and inflammation in the kidneys of type 2 diabetic rats fed high-fat diet

Diabetic kidney disease (DKD), as a chronic diabetes-induced complication, is considered the most frequent leading cause of end-stage renal disease (ESRD). Regarding the observed protective effects of bilirubin, as a potential endogenous antioxidant/anti-inflammatory compound, against DKD progression, we planned to evaluate the effects of bilirubin administration on endoplasmic reticulum (ER) stress and inflammation in type 2 diabetic (T2D) rats fed high-fat diet (HFD). In this regard, thirty 8-week adult male Sprague Dawley rats were divided into five groups (n = 6). T2D and obesity were induced by streptozotocin (STZ) (35 mg/kg) and HFD (700 kcal/day), respectively. Bilirubin treatment was carried out for 6- and 14-week intervals (10 mg/kg/day), intraperitoneally. Then, the expression levels of ER stress-related genes (i.e. binding immunoglobulin protein (Bip), C/EBP homologous protein (Chop), and spliced x-box-binding protein 1 (sXbp1), as well as nuclear factor-κB (NF-κB) were analyzed using quantitative Real-time PCR experiments. Moreover, histopathological and stereological changes of kidney and its related structures were investigated for the studied rats. Bip, Chop, and NF-κB expression levels were significantly decreased under bilirubin treatment, while sXbp1 was up-regulated following the bilirubin administration. More interestingly, glomerular constructive damages seen in HFD-T2D rats, were considerably improved in the animals received bilirubin. Stereological assessments also revealed that bilirubin could desirably reverse the mitigation of kidney's total volume and its related structures, such as cortex, glomeruli, and convoluted tubules. Taken together, bilirubin has potential protective/ameliorative effects on DKD progression, especially through alleviating the renal ER stress and inflammatory responses in T2D rats with injured kidneys. In this era, clinical benefits of mild hyperbilirubinemia can be considered in human DKD.

Single-cell RNA sequencing reveals XBP1-SLC38A2 axis as a metabolic regulator in cytotoxic T lymphocytes in multiple myeloma

The mechanisms underlying the functional impairment and metabolic reprogramming of T lymphocytes in multiple myeloma (MM) have not been fully elucidated. In this study, single-cell RNA sequencing was used to compare gene expression profiles in T cells in bone marrow and peripheral blood of 10 newly diagnosed MM patients versus 3 healthy donors. Unbiased bioinformatics analysis revealed 9 cytotoxic T cell clusters. All 9 clusters in MM had higher expression of senescence markers (e.g., KLRG1 and CTSW) than the healthy control; some had higher expression of exhaustion-related markers (e.g., LAG3 and TNFRSF14). Pathway enrichment analyses showed downregulated amino acid metabolism and upregulated unfolded protein response (UPR) pathways, along with absent expression of glutamine transporter SLC38A2 and increased expression of UPR hallmark XBP1 in cytotoxic T cells in MM. In vitro studies revealed that XBP1 inhibited SLC38A2 by directly binding to its promoter, and silencing SLC38A2 resulted in decreased glutamine uptake and immune dysfunction of T cells. This study provided a landscape description of the immunosuppressive and metabolic features in T lymphocytes in MM, and suggested an important role of XBP1-SLC38A2 axis in T cell function.

Deletion of the Unfolded Protein Response Transducer IRE1α Is Detrimental to Aging Photoreceptors and to ER Stress-Mediated Retinal Degeneration

Purpose

The unfolded protein response (UPR) is triggered when the protein folding capacity of the endoplasmic reticulum (ER) is overwhelmed and misfolded proteins accumulate in the ER, a condition referred to as ER stress. IRE1α is an ER-resident protein that plays major roles in orchestrating the UPR. Several lines of evidence implicate the UPR and its transducers in neurodegenerative diseases, including retinitis pigmentosa (RP), a group of inherited diseases that cause progressive dysfunction and loss of rod and cone photoreceptors. This study evaluated the contribution of IRE1α to photoreceptor development, homeostasis, and degeneration.

Methods

We used a conditional gene targeting strategy to selectively inactivate Ire1α in mouse rod photoreceptors. We used a combination of optical coherence tomography (OCT) imaging, histology, and electroretinography (ERG) to assess longitudinally the effect of IRE1α deficiency in retinal development and function. Furthermore, we evaluated the IRE1α-deficient retina responses to tunicamycin-induced ER stress and in the context of RP caused by the rhodopsin mutation RhoP23H.

Results

OCT imaging, histology, and ERG analyses did not reveal abnormalities in IRE1α-deficient retinas up to 3 months old. However, by 6 months of age, the Ire1α mutant animals showed reduced outer nuclear layer thickness and deficits in retinal function. Furthermore, conditional inactivation of Ire1α in rod photoreceptors accelerated retinal degeneration caused by the RhoP23H mutation.

Conclusions

These data suggest that IRE1α is dispensable for photoreceptor development but important for photoreceptor homeostasis in aging retinas and for protecting against ER stress-mediated photoreceptor degeneration.

Insulin secretion deficits in a Prader-Willi syndrome β-cell model are associated with a concerted downregulation of multiple endoplasmic reticulum chaperones

Prader-Willi syndrome (PWS) is a multisystem disorder with neurobehavioral, metabolic, and hormonal phenotypes, caused by loss of expression of a paternally-expressed imprinted gene cluster. Prior evidence from a PWS mouse model identified abnormal pancreatic islet development with retention of aged insulin and deficient insulin secretion. To determine the collective roles of PWS genes in β-cell biology, we used genome-editing to generate isogenic, clonal INS-1 insulinoma lines having 3.16 Mb deletions of the silent, maternal- (control) and active, paternal-allele (PWS). PWS β-cells demonstrated a significant cell autonomous reduction in basal and glucose-stimulated insulin secretion. Further, proteomic analyses revealed reduced levels of cellular and secreted hormones, including all insulin peptides and amylin, concomitant with reduction of at least ten endoplasmic reticulum (ER) chaperones, including GRP78 and GRP94. Critically, differentially expressed genes identified by whole transcriptome studies included reductions in levels of mRNAs encoding these secreted peptides and the group of ER chaperones. In contrast to the dosage compensation previously seen for ER chaperones in Grp78 or Grp94 gene knockouts or knockdown, compensation is precluded by the stress-independent deficiency of ER chaperones in PWS β-cells. Consistent with reduced ER chaperones levels, PWS INS-1 β-cells are more sensitive to ER stress, leading to earlier activation of all three arms of the unfolded protein response. Combined, the findings suggest that a chronic shortage of ER chaperones in PWS β-cells leads to a deficiency of protein folding and/or delay in ER transit of insulin and other cargo. In summary, our results illuminate the pathophysiological basis of pancreatic β-cell hormone deficits in PWS, with evolutionary implications for the multigenic PWS-domain, and indicate that PWS-imprinted genes coordinate concerted regulation of ER chaperone biosynthesis and β-cell secretory pathway function.

RRM2 and CDC6 are novel effectors of XBP1-mediated endocrine resistance and predictive markers of tamoxifen sensitivity

BACKGROUND

Endocrine-resistant breast cancers have elevated expression of XBP1, where it drives endocrine resistance by controlling the expression of its target genes. Despite the in-depth understanding of the biological functions of XBP1 in ER-positive breast cancer, effectors of endocrine resistance downstream of XBP1 are poorly understood. The aim of this study was to identify the XBP1-regulated genes contributing to endocrine resistance in breast cancer.

METHODS

XBP1 deficient sub-clones in MCF7 cells were generated using the CRISPR-Cas9 gene knockout strategy and were validated using western blot and RT-PCR. Cell viability and cell proliferation were evaluated using the MTS assay and colony formation assay, respectively. Cell death and cell cycle analysis were determined using flow cytometry. Transcriptomic data was analysed to identify XBP1-regulated targets and differential expression of target genes was evaluated using western blot and qRT-PCR. Lentivirus and retrovirus transfection were used to generate RRM2 and CDC6 overexpressing clones, respectively. The prognostic value of the XBP1-gene signature was analysed using Kaplan-Meier survival analysis.

RESULTS

Deletion of XBP1 compromised the upregulation of UPR-target genes during conditions of endoplasmic reticulum (EnR) stress and sensitized cells to EnR stress-induced cell death. Loss of XBP1 in MCF7 cells decreased cell growth, attenuated the induction of estrogen-responsive genes and sensitized them to anti-estrogen agents. The expression of cell cycle associated genes RRM2, CDC6, and TOP2A was significantly reduced upon XBP1 deletion/inhibition in several ER-positive breast cancer cells. Expression of RRM2, CDC6, and TOP2A was increased upon estrogen stimulation and in cells harbouring point-mutants (Y537S, D538G) of ESR1 in steroid free conditions. Ectopic expression of RRM2 and CDC6 increased cell growth and reversed the hypersensitivity of XBP1 KO cells towards tamoxifen conferring endocrine resistance. Importantly, increased expression of XBP1-gene signature was associated with poor outcome and reduced efficacy of tamoxifen treatment in ER-positive breast cancer.

CONCLUSIONS

Our results suggest that RRM2 and CDC6 downstream of XBP1 contribute to endocrine resistance in ER-positive breast cancer. XBP1-gene signature is associated with poor outcome and response to tamoxifen in ER-positive breast cancer.

The endoplasmic reticulum stress and unfolded protein response in Alzheimer's disease: a calcium dyshomeostasis perspective

Protein misfolding is prominent in early cellular pathology of Alzheimer's disease (AD), implicating pathophysiological significance of endoplasmic reticulum stress/unfolded protein response (ER stress/UPR) and highlighting it as a target for drug development. Experimental data from animal AD models and observations on human specimens are, however, inconsistent. ER stress and associated UPR are readily observed in in vitro AD cellular models and in some AD model animals. In the human brain, components and markers of ER stress as well as UPR transducers are observed at Braak stages III-VI associated with severe neuropathology and neuronal death. The picture, however, is further complicated by the brain region- and cell type-specificity of the AD-related pathology. Terms 'disturbed' or 'non-canonical' ER stress/UPR were used to describe the discrepancies between experimental data and the classic ER stress/UPR cascade. Here we discuss possible 'disturbing' or 'interfering' factors which may modify ER stress/UPR in the early AD pathogenesis. We focus on the dysregulation of the ER Ca²⁺ homeostasis, store-operated Ca²⁺ entry, and the interaction between the ER and mitochondria. We suggest that a detailed study of the CNS cell type-specific alterations of Ca²⁺ homeostasis in early AD may deepen our understanding of AD-related dysproteostasis.

Sustained overexpression of spliced X-box-binding protein-1 in neurons leads to spontaneous seizures and sudden death in mice

The underlying etiologies of seizures are highly heterogeneous and remain incompletely understood. While studying the unfolded protein response (UPR) pathways in the brain, we unexpectedly discovered that transgenic mice (XBP1s-TG) expressing spliced X-box-binding protein-1 (Xbp1s), a key effector of UPR signaling, in forebrain excitatory neurons, rapidly develop neurologic deficits, most notably recurrent spontaneous seizures. This seizure phenotype begins around 8 days after Xbp1s transgene expression is induced in XBP1s-TG mice, and by approximately 14 days post induction, the seizures evolve into status epilepticus with nearly continuous seizure activity followed by sudden death. Animal death is likely due to severe seizures because the anticonvulsant valproic acid could significantly prolong the lives of XBP1s-TG mice. Mechanistically, our gene profiling analysis indicates that compared to control mice, XBP1s-TG mice exhibit 591 differentially regulated genes (mostly upregulated) in the brain, including several GABAA receptor genes that are notably downregulated. Finally, whole-cell patch clamp analysis reveals a significant reduction in both spontaneous and tonic GABAergic inhibitory responses in Xbp1s-expressing neurons. Taken together, our findings unravel a link between XBP1s signaling and seizure occurrence.

MANF regulates neuronal survival and UPR through its ER-located receptor IRE1α

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER)-located protein with cytoprotective effects in neurons and pancreatic β cells in vitro and in models of neurodegeneration and diabetes in vivo. However, the exact mode of MANF action has remained elusive. Here, we show that MANF directly interacts with the ER transmembrane unfolded protein response (UPR) sensor IRE1α, and we identify the binding interface between MANF and IRE1α. The expression of wild-type MANF, but not its IRE1α binding-deficient mutant, attenuates UPR signaling by decreasing IRE1α oligomerization; phosphorylation; splicing of Xbp1, Atf6, and Txnip levels; and protecting neurons from ER stress-induced death. MANF-IRE1α interaction and not MANF-BiP interaction is crucial for MANF pro-survival activity in neurons in vitro and is required to protect dopamine neurons in an animal model of Parkinson's disease. Our data show IRE1α as an intracellular receptor for MANF and regulator of neuronal survival.

Endoplasmic reticulum-associated protein degradation contributes to Toll innate immune defense in Drosophila melanogaster

In Drosophila, the endoplasmic reticulum-associated protein degradation (ERAD) is engaged in regulating pleiotropic biological processes, with regard to retinal degeneration, intestinal homeostasis, and organismal development. The extent to which it functions in controlling the fly innate immune defense, however, remains largely unknown. Here, we show that blockade of the ERAD in fat bodies antagonizes the Toll but not the IMD innate immune defense in Drosophila. Genetic approaches further suggest a functional role of Me31B in the ERAD-mediated fly innate immunity. Moreover, we provide evidence that silence of Xbp1 other than PERK or Atf6 partially rescues the immune defects by the dysregulated ERAD in fat bodies. Collectively, our study uncovers an essential function of the ERAD in mediating the Toll innate immune reaction in Drosophila.

Ultraviolet Light, Unfolded Protein Response and Autophagy†

The endoplasmic reticulum (ER) plays an important role in the regulation of protein synthesis. Alterations in the folding capacity of the ER induce stress, which activates three ER sensors that mediate the unfolded protein response (UPR). Components of the pathways regulated by these sensors have been shown to regulate autophagy. The last corresponds to a mechanism of self-eating and recycling important for proper cell maintenance. Ultraviolet radiation (UV) is an external damaging stimulus that is known for inducing oxidative stress, and DNA, lipid and protein damage. Many controversies exist regarding the role of UV-inducing ER stress or autophagy. However, a connection between the three of them has not been addressed. In this review, we will discuss the contradictory theories regarding the relationships between UV radiation with the induction of ER stress and autophagy, as well as hypothetic connections between UV, ER stress and autophagy.

Cortisol controls endoplasmic reticulum stress and hypoxia dependent regulation of insulin receptor and related genes expression in HEK293 cells

Objective. Glucocorticoids are important stress-responsive regulators of insulin-dependent metabolic processes realized through specific changes in genome function. The aim of this study was to investigate the impact of cortisol on insulin receptor and related genes expression in HEK293 cells upon induction the endoplasmic reticulum (ER) stress by tunicamycin and hypoxia. Methods. The human embryonic kidney cell line HEK293 was used. Cells were exposed to cortisol (10 µM) as well as inducers of hypoxia (dimethyloxalylglycine, DMOG; 0.5 mM) and ER stress (tunicamycin; 0.2 µg/ml) for 4 h. The RNA from these cells was extracted and reverse transcribed. The expression level of INSR, IRS2, and INSIG2 and some ER stress responsive genes encoding XBP1n, non-spliced variant, XBP1s, alternatively spliced variant of XBP1, and DNAJB9 proteins, was measured by quantitative polymerase chain reaction and normalized to ACTB. Results. We showed that exposure of HEK293 cells to cortisol elicited up-regulation in the expression of INSR and DNAJB9 genes and down-regulation of XBP1s, XBP1n, IRS2, and INSIG2 mRNA levels. At the same time, induction of hypoxia by DMOG led to an up-regulation of the expression level of most studied mRNAs: XBP1s and XBP1n, IRS2 and INSIG2, but did not change significantly INSR and DNAJB9 gene expression. We also showed that combined impact of cortisol and hypoxia introduced the up-regulation of INSR and suppressed XBP1n mRNA expression levels. Furthermore, the exposure of HEK293 cells to tunicamycin affected the expression of IRS2 gene and increased the level of XBP1n mRNA. At the same time, the combined treatment of these cells with cortisol and inductor of ER stress had much stronger impact on the expression of all the tested genes: strongly increased the mRNA level of ER stress dependent factors XBP1s and DNAJB9 as well as INSR and INSIG2, but down-regulated IRS2 and XBP1n. Conclusion. Taken together, the present study indicates that cortisol may interact with ER stress and hypoxia in the regulation of ER stress dependent XBP1 and DNAJB9 mRNA expression as well as INSR and its signaling and that this corticosteroid hormone modified the impact of hypoxia and especially tunicamycin on the expression of most studied genes in HEK293 cells. These data demonstrate molecular mechanisms of glucocorticoids interaction with ER stress and insulin signaling at the cellular level.

Hypoxia controls the expression of genes responsible for serine synthesis in U87MG cells on ERN1-dependent manner

Objective. Serine synthesis as well as endoplasmic reticulum stress and hypoxia are important factors of malignant tumor growth including glioblastoma. Previous studies have shown that the knockdown of ERN1 (endoplasmic reticulum to nucleus signaling) significantly suppressed the glioblastoma cell proliferation and modified the hypoxia regulation. The present study is aimed to investigate the impact of hypoxia on the expression of PHGDH (phosphoglycerate dehydrogenase), PSAT1 (phosphoserine aminotransferase 1), PSPH (phosphoserine phosphatase), ATF4 (activating transcription factor 4), and SHMT1 (serine hydroxymethyltransferase 1) in U87MG glioblastoma cells in relation to knockdown of ERN1 with the intent to reveal the role of ERN1 signaling pathway on the endoplasmic reticulum stress-dependent regulation of expression of these genes. Methods. The control U87MG glioblastoma cells (transfected by empty vector) and ERN1 knockdown cells (transfected by dominant-negative ERN1) were exposed to hypoxia introduced by dimethyloxalylglycine for 4 h. RNA was extracted from cells and reverse transcribed. The expression level of PHGDH, PSAT1, PDPH, SHMT1, and ATF4 genes was studied by real-time qPCR and normalized to ACTB. Results. It was found that hypoxia up-regulated the expression level of PHGDH, PSAT1, and ATF4 genes in control U87MG cells, but PSPH and SHMT1 genes expression was down-regulated. The expression of PHGDH, PSAT1, and ATF4 genes in glioblastoma cells with knockdown of ERN1 signaling protein was more sensitive to hypoxia, especially PSAT1 gene. At the same time, the expression of PSPH gene in ERN1 knockdown cells was resistant to hypoxia. The expression of SHMT1 gene, encoding the enzyme responsible for conversion of serine to glycine, showed similar negative sensitivity to hypoxia in both control and ERN1 knockdown glioblastoma cells. Conclusion. The results of the present study demonstrate that the expression of genes responsible for serine synthesis is sensitive to hypoxia in gene-specific manner and that ERN1 knockdown significantly modifies the impact of hypoxia on the expression of PHGDH, PSAT1, PSPH, and ATF4 genes in glioblastoma cells and reflects the ERN1-mediated reprograming of hypoxic regulation at gene expression level.