The endoplasmic reticulum: folding, calcium homeostasis, signaling, and redox control

ER Stress-Induced Secretion of Proteins and Their Extracellular Functions in the Heart

Endoplasmic reticulum (ER) stress is a result of conditions that imbalance protein homeostasis or proteostasis at the ER, for example ischemia, and is a common event in various human pathologies, including the diseased heart. Cardiac integrity and function depend on the active secretion of mature proteins from a variety of cell types in the heart, a process that requires an intact ER environment for efficient protein folding and trafficking to the secretory pathway. As a consequence of ER stress, most protein secretion by the ER secretory pathway is decreased. Strikingly, there is a select group of proteins that are secreted in greater quantities during ER stress. ER stress resulting from the dysregulation of ER Ca²⁺ levels, for instance, stimulates the secretion of Ca²⁺-binding ER chaperones, especially GRP78, GRP94, calreticulin, and mesencephalic astrocyte-derived neurotrophic factor (MANF), which play a multitude of roles outside the cell, strongly depending on the cell type and tissue. Here we review current insights in ER stress-induced secretion of proteins, particularly from the heart, and highlight the extracellular functions of these proteins, ranging from the augmentation of cardiac cell viability to the modulation of pro- and anti-apoptotic, oncogenic, and immune-stimulatory cell signaling, cell invasion, extracellular proteostasis, and more. Many of the roles of ER stress-induced protein secretion remain to be explored in the heart. This article is part of a special issue entitled “The Role of Proteostasis Derailment in Cardiac Diseases.”

Role of Endoplasmic Reticulum Stress in Atherosclerosis and Its Potential as a Therapeutic Target

Endoplasmic reticulum (ER) stress is closely associated with atherosclerosis and related cardiovascular diseases (CVDs). It occurs due to various pathological factors that interfere with ER homeostasis, resulting in the accumulation of unfolded or misfolded proteins in the ER lumen, thereby causing ER dysfunction. Here, we discuss the role of ER stress in different types of cells in atherosclerotic lesions. This discussion includes the activation of apoptotic and inflammatory pathways induced by prolonged ER stress, especially in advanced lesional macrophages and endothelial cells (ECs), as well as common atherosclerosis-related ER stressors in different lesional cells, which all contribute to the clinical progression of atherosclerosis. In view of the important role of ER stress and the unfolded protein response (UPR) signaling pathways in atherosclerosis and CVDs, targeting these processes to reduce ER stress may be a novel therapeutic strategy.

Methamphetamine-Induced Neuronal Damage: Neurotoxicity and Neuroinflammation

Methamphetamine (METH) is a highly addictive psychostimulant and one of the most widely abused drugs worldwide. The continuous use of METH eventually leads to drug addiction and causes serious health complications, including attention deficit, memory loss and cognitive decline. These neurological complications are strongly associated with METH-induced neurotoxicity and neuroinflammation, which leads to neuronal cell death. The current review investigates the molecular mechanisms underlying METH-mediated neuronal damages. Our analysis demonstrates that the process of neuronal impairment by METH is closely related to oxidative stress, transcription factor activation, DNA damage, excitatory toxicity and various apoptosis pathways. Thus, we reach the conclusion here that METH-induced neuronal damages are attributed to the neurotoxic and neuroinflammatory effect of the drug. This review provides an insight into the mechanisms of METH addiction and contributes to the discovery of therapeutic targets on neurological impairment by METH abuse.

Malvidin induces hepatic stellate cell apoptosis via the endoplasmic reticulum stress pathway and mitochondrial pathway

Blueberries have great beneficial effects due to high level of anthocyanins, especially malvidin. Hepatic stellate cells (HSCs) can be activated and increase excessive extracellular matrix (ECM) components, which play a central role in liver fibrogenesis. Therefore, activated HSC's apoptosis can be induced to recover liver fibrosis. Malvidin's effects on apoptosis in rat activated hepatic stellate T6 cells (HSC-T6) in vitro were investigated here. High concentration of malvidin was found to significantly induce apoptosis, activate caspase-3, increase malondialdehyde, upregulate Bax, but downregulate Bcl-2. Moreover, malvidin upregulated the protein levels of some endoplasmic reticulum stress (ERS)-typical markers, including caspase-12, glucose-regulated protein 78 (GRP78), and CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP), suggesting that malvidin induced HSC apoptosis by the ERS apoptosis pathway as well as the mitochondrial-dependent pathway. These findings indicated that blueberry anthocyanins, especially malvidin, could induce activated hepatic stellate cell apoptosis and might act as one kind of functional food ingredient or a novel nutraceutical beneficial for liver health.

Dendritic cell engineered cTXN as new vaccine prospect against L. donovani

Dendritic cells (DCs), as antigen-presenting cells, can reportedly be infected withLeishmaniaparasites and hence provide a better option to trigger T-cell primary immune responses and immunological memory. We consistently primed DCs during culture with purified recombinant cytosolic tryparedoxin (rcTXN) and then evaluated the vaccine prospect of presentation of rcTXN against VL in BALB/c mice. We reported earlier the immunogenic properties of cTXN antigen derived fromL. donovani when anti-cTXN antibody was detected in the sera of kala-azar patients. It was observed that cTXN antigen, when used as an immunogen with murine DCs acting as a vehicle, was able to induce complete protection against VL in an infected group of immunized mice. This vaccination triggered splenic macrophages to produce more IL-12 and GM-CSF, and restricted IL-10 release to a minimum in an immunized group of infected animals. Concomitant changes in T-cell responses against cTXN antigen were also noticed, which increased the release of protective cytokine-like IFN-γ under the influence of NF-κβ in the indicated vaccinated group of animals. All cTXN-DCs-vaccinated BALB/c mice survived during the experimental period of 120 days. The results obtained in our study suggest that DCs primed with cTXN can be used as a vaccine prospect for the control of visceral leishmaniasis.

Beyond the cell factory: Homeostatic regulation of and by the UPRER

The endoplasmic reticulum (ER) is commonly referred to as the factory of the cell, as it is responsible for a large amount of protein and lipid synthesis. As a membrane-bound organelle, the ER has a distinct environment that is ideal for its functions in synthesizing these primary cellular components. Many different quality control machineries exist to maintain ER stability under the stresses associated with synthesizing, folding, and modifying complex proteins and lipids. The best understood of these mechanisms is the unfolded protein response of the ER (UPR^ER), in which transmembrane proteins serve as sensors, which trigger a coordinated transcriptional response of genes dedicated for mitigating the stress. As the name suggests, the UPR^ER is most well described as a functional response to protein misfolding stress. Here, we focus on recent findings and emerging themes in additional roles of the UPR^ER outside of protein homeostasis, including lipid homeostasis, autophagy, apoptosis, and immunity.

Functional Coupling between the Unfolded Protein Response and Endoplasmic Reticulum/Golgi Ca2+-ATPases Promotes Stress Tolerance, Cell Wall Biosynthesis, and Virulence of Aspergillus fumigatus

Many species of pathogenic fungi deploy the unfolded protein response (UPR) to expand the folding capacity of the endoplasmic reticulum (ER) in proportion to the demand for virulence-related proteins that traffic through the secretory pathway. Although Ca²⁺ plays a pivotal role in ER function, the mechanism by which transcriptional upregulation of the protein folding machinery is coordinated with Ca²⁺ homeostasis is incompletely understood. In this study, we investigated the link between the UPR and genes encoding P-type Ca²⁺-ATPases in the human-pathogenic mold Aspergillus fumigatus We demonstrate that acute ER stress increases transcription of the srcA gene, encoding a member of the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) family, as well as that of pmrA, encoding a secretory pathway Ca²⁺-ATPase (SPCA) in the Golgi membrane. Loss of the UPR transcription factor HacA prevented the induction of srcA and pmrA transcription during ER stress, defining these ER/Golgi Ca²⁺ pumps as novel downstream targets of this pathway. While deletion of srcA alone caused no major deficiencies, a ΔsrcA/ΔpmrA mutant displayed a severe polarity defect, was hypersensitive to ER stress, and showed attenuated virulence. In addition, cell wall analyses revealed a striking reduction in mannose levels in the absence of both Ca²⁺ pumps. The ΔhacA mutant was hypersensitive to agents that block calcineurin-dependent signaling, consistent with a functional coupling between the UPR and Ca²⁺ homeostasis. Together, these findings demonstrate that the UPR integrates the need for increased levels of chaperone and folding enzymes with an influx of Ca²⁺ into the secretory pathway to support fungal growth, stress adaptation, and pathogenicity.IMPORTANCE The UPR is an intracellular signal transduction pathway that maintains homeostasis of the ER. The pathway is also tightly linked to the expression of virulence-related traits in diverse species of human-pathogenic and plant-pathogenic fungal species, including the predominant mold pathogen infecting humans, Aspergillus fumigatus Despite advances in the understanding of UPR signaling, the linkages and networks that are governed by this pathway are not well defined. In this study, we revealed that the UPR is a major driving force for stimulating Ca²⁺ influx at the ER and Golgi membranes and that the coupling between the UPR and Ca²⁺ import is important for virulence, cell wall biosynthesis, and resistance to antifungal compounds that inhibit Ca²⁺ signaling.

ER membranes associated with mitochondria: Possible therapeutic targets in heart-associated diseases

Cardiovascular system cell biology is tightly regulated and mitochondria play a relevant role in maintaining heart function. In recent decades, associations between such organelles and the sarco/endoplasmic reticulum (SR) have been raised great interest. Formally identified as mitochondria-associated SR membranes (MAMs), these structures regulate different cellular functions, including calcium management, lipid metabolism, autophagy, oxidative stress, and management of unfolded proteins. In this review, we highlight MAMs' alterations mainly in cardiomyocytes, linked with cardiovascular diseases, such as cardiac ischemia-reperfusion, heart failure, and dilated cardiomyopathy. We also describe proteins that are part of the MAMs' machinery, as the FUN14 domain containing 1 (FUNDC1), the sigma 1 receptor (Sig-1R) and others, which might be new molecular targets to preserve the function and structure of the heart in such diseases. Understanding the machinery of MAMs and its function demands our attention, as such knowledge might contribute to strengthen the role of these relative novel structures in heart diseases.

Role of Endoplasmic Reticulum Stress Sensor IRE1α in Cellular Physiology, Calcium, ROS Signaling, and Metaflammation

Inositol-requiring transmembrane kinase endoribonuclease-1α (IRE1α) is the most prominent and evolutionarily conserved unfolded protein response (UPR) signal transducer during endoplasmic reticulum functional upset (ER stress). A IRE1α signal pathway arbitrates yin and yang of cellular fate in objectionable conditions. It plays several roles in fundamental cellular physiology as well as in several pathological conditions such as diabetes, obesity, inflammation, cancer, neurodegeneration, and in many other diseases. Thus, further understanding of its molecular structure and mechanism of action during different cell insults helps in designing and developing better therapeutic strategies for the above-mentioned chronic diseases. In this review, recent insights into structure and mechanism of activation of IRE1α along with its complex regulating network were discussed in relation to their basic cellular physiological function. Addressing different binding partners that can modulate IRE1α function, UPRosome triggers different downstream pathways depending on the cellular backdrop. Furthermore, IRE1α are in normal cell activities outside the dominion of ER stress and activities under the weather of inflammation, diabetes, and obesity-related metaflammation. Thus, IRE1 as an ER stress sensor needs to be understood from a wider perspective for comprehensive functional meaning, which facilitates us with assembling future needs and therapeutic benefits.

Pyridine nucleotide regulation of hepatic endoplasmic reticulum calcium uptake

Pyridine nucleotides serve an array of intracellular metabolic functions such as, to name a few, shuttling electrons in enzymatic reactions, safeguarding the redox state against reactive oxygen species, cytochrome P450 (CYP) enzyme detoxification pathways and, relevant to this study, the regulation of ion fluxes. In particular, the maintenance of a steep calcium gradient between the cytosol and endoplasmic reticulum (ER), without which apoptosis ensues, is achieved by an elaborate combination of energy–requiring ER membrane pumps and efflux channels. In liver microsomes, net calcium uptake was inhibited by physiological concentrations of NADP. In the presence of 1 mmol/L NADP, calcium uptake was attenuated by nearly 80%, additionally, this inhibitory effect was blunted by concomitant addition of NADPH. No other nicotinamide containing compounds ‐save a slight inhibition by NAADP‐hindered calcium uptake; thus, only oxidized pyridine nucleotides, or related compounds with a phosphate moiety, had an imposing effect. Moreover, the NADP inhibition was evident even after selectively blocking ER calcium efflux channels. Given the fundamental role of endoplasmic calcium homeostasis, it is plausible that changes in cytosolic NADP concentration, for example, during anabolic processes, could regulate net ER calcium uptake.

Enhanced efficacy of propranolol therapy for infantile hemangiomas based on a mesoporous silica nanoplatform through mediating autophagy dysfunction

Infantile hemangioma is one of the most common vascular tumors, which might result in morbidity and mortality without timely intervention. Propranolol is currently the first-line therapy for hemangiomas, but its potential side effects and high frequency of administration make it urgent to develop a suitable drug delivery system for propranolol. In the present study, we formulated a propranolol delivery system based on mesoporous silica nanoparticles (PRN@MSN) and investigated the interplay between autophagic activities mediated by nanoparticles and improved therapeutic efficacy of PRN@MSN. The results showed that PRN@MSN nanoparticles exhibited higher cytotoxicity compared with free propranolol in vitro and in vivo, which could induce excessive autophagosome accumulation through increased autophagosome formation and impaired autophagic degradation. Inhibition of autophagy in the early stage could attenuate the cytotoxicity of PRN@MSN. ROS generation was essential for nanoparticle-mediated autophagy and cytotoxicity, and PRN@MSN-induced autophagy dysfunction could enhance endoplasmic reticulum (ER) stress in hemangioma stem cells. Our study revealed a promising PRN delivery system based on a mesoporous silica nanoplatform that could induce autophagy dysfunction with excessive autophagosome accumulation to promote the therapeutic efficacy of PRN therapy. PRN@MSN drug delivery system combined with autophagy modulation may act as a promising treatment pattern in the treatment of hemangiomas.

An activating transcription factor 6 beta (ATF6β) regulates apoptosis of hemocyte during immune response in Crassostrea gigas

The homeostasis of immune cells during immune response is vital for hosts to defend against invaders. Activating transcription factor 6 (ATF6) is an important transcription factor in the unfolded protein response (UPR) to maintaining cellular homeostasis. In the present study, one ATF6 homologue was identified from Pacific oyster Crassostrea gigas (designated as CgATF6β). The full length cDNA of CgATF6β was of 2645 bp with a 1596 bp open reading frame (ORF) encoding a polypeptide of 531 amino acids. The deduced amino acid sequence of CgATF6β was predicted to contain a transmembrane region, a conserved basic leucine zipper (bZIP) domain, a site 1 protease cleavage site, a site 2 protease cleavage site, and a Golgi localization signal. CgATF6β mRNA was constitutively expressed in hemocytes, gill, mantle, gonad, hepatopancreas and labial palp, with a slightly higher expression level in muscle (2.45-fold of that in gill, p < 0.05). After oysters were challenged with Vibrio splendidus, the mRNA expression levels of CgATF6β in hemocytes were significantly up-regulated at 3 h (2.68-fold of that in seawater group, p < 0.01) and peaked at 12 h (3.14-fold of that in seawater group, p < 0.01). The endogenic CgATF6β protein was mainly located in the cytoplasm of oyster hemocytes, and it was significantly transported into the nuclei of hemocytes at 1.5 h after the challenge with V. splendidus. After an injection with CgATF6β dsRNA, the mRNA expression of CgATF6β was knocked down to 0.26-fold of that in dsGFP group (p < 0.01). In CgATF6β dsRNA-injected oysters, the mRNA expressions of glucose-regulated protein 78 (GRP78), calnexin (CNX) and anti-apoptotic B-cell lymphoma-2 (Bcl-2) in hemocytes were significantly decreased at 12 h after V. splendidus challenge, which were 0.65-fold (p < 0.01), 0.54-fold (p < 0.01) and 0.17-fold (p < 0.01) of that in dsGFP-injected oysters, while the apoptotic rate of hemocytes was significantly up-regulated (1.97-fold of that in dsGFP group, p < 0.05). Collectively, these results suggested that CgATF6β was involved in apoptosis inhibition of oyster hemocytes upon V. splendidus challenge by regulating the expression of CgGRP78, CgCNX and CgBcl-2.

Lactobacillus paracasei-derived extracellular vesicles attenuate the intestinal inflammatory response by augmenting the endoplasmic reticulum stress pathway

Lactobacillus paracasei is a major probiotic and is well known for its anti-inflammatory properties. Thus, we investigated the effects of L. paracasei-derived extracellular vesicles (LpEVs) on LPS-induced inflammation in HT29 human colorectal cancer cells and dextran sulfate sodium (DSS)-induced colitis in C57BL/6 mice. ER stress inhibitors (salubrinal or 4-PBA) or CHOP siRNA were utilized to investigate the relationship between LpEV-induced endoplasmic reticulum (ER) stress and the inhibitory effect of LpEVs against LPS-induced inflammation. DSS (2%) was administered to male C57BL/6 mice to induce inflammatory bowel disease, and disease activity was measured by determining colon length, disease activity index, and survival ratio. In in vitro experiments, LpEVs reduced the expression of the LPS-induced pro-inflammatory cytokines IL-1α, IL-1β, IL-2, and TNFα and increased the expression of the anti-inflammatory cytokines IL-10 and TGFβ. LpEVs reduced LPS-induced inflammation in HT29 cells and decreased the activation of inflammation-associated proteins, such as COX-2, iNOS and NFκB, as well as nitric oxide. In in vivo mouse experiments, the oral administration of LpEVs also protected against DSS-induced colitis by reducing weight loss, maintaining colon length, and decreasing the disease activity index (DAI). In addition, LpEVs induced the expression of endoplasmic reticulum (ER) stress-associated proteins, while the inhibition of these proteins blocked the anti-inflammatory effects of LpEVs in LPS-treated HT29 cells, restoring the pro-inflammatory effects of LPS. This study found that LpEVs attenuate LPS-induced inflammation in the intestine through ER stress activation. Our results suggest that LpEVs have a significant effect in maintaining colorectal homeostasis in inflammation-mediated pathogenesis.

Tiny vesicles released by a bacterial species found in the human gut can reduce symptoms of inflammatory bowel disease (IBD) and prevent disease progression. People with IBD have a decreased abundance of Lactobacilli bacteria in their gut, creating an imbalance that perpetuates the disease. Replenishment of this bacteria may become a valuable therapy. Chang Mo Moon at Ewha Womans University, Yoon-Keun Kim at MD Healthcare, both in Seoul, South Korea, and co-workers demonstrated how extracellular vesicles (EVs) released by Lactobacilli paracasei can actively prevent bowel inflammation. These EVs contain a mixture of proteins, nucleic acids and other biomolecules. The team administered EV to cultured human colorectal cancer cells and to mice with induced colitis. The EVs decreased pro-inflammatory protein activity and boosted levels of protective cellular membrane proteins via augmenting ER stress pathway.

Endoplasmic reticulum stress-induced complex I defect: Central role of calcium overload

BACKGROUND

ER (endoplasmic reticulum) stress leads to decreased complex I activity in cardiac mitochondria. The aim of the current study is to explore the potential mechanisms by which ER stress leads to the complex I defect. ER stress contributes to intracellular calcium overload and oxidative stress that are two key factors to induce mitochondrial dysfunction. Since oxidative stress is often accompanied by intracellular calcium overload during ER stress in vivo, the role of oxidative stress and calcium overload in mitochondrial dysfunction was studied using in vitro models. ER stress results in intracellular calcium overload that favors activation of calcium-dependent calpains. The contribution of mitochondrial calpain activation in ER stress-mediated complex I damage was studied.

METHODS

Thapsigargin (THAP) was used to induce acute ER stress in H9c2 cells and C57BL/6 mice. Exogenous calcium (25 μM) and H₂O₂ (100 μM) were used to induce modest calcium overload and oxidative stress in isolated mitochondria. Calpain small subunit 1 (CAPNS1) is essential to maintain calpain 1 and calpain 2 (CPN1/2) activities. Deletion of CAPNS1 eliminates the activities of CPN1/2. Wild type and cardiac-specific CAPNS1 deletion mice were used to explore the role of CPN1/2 activation in calcium-induced mitochondrial damage.

RESULTS

In isolated mitochondria, exogenous calcium but not H₂O₂ treatment led to decreased oxidative phosphorylation, supporting that calcium overload contributes a key role in the mitochondrial damage. THAP treatment of H9c2 cells decreased respiration selectively with complex I substrates. THAP treatment activated cytosolic and mitochondrial CPN1/2 in C57BL/6 mice and led to degradation of complex I subunits including NDUFS7. Calcium treatment decreased NDUFS7 content in wild type but not in CAPNS1 knockout mice.

CONCLUSION

ER stress-mediated activation of mitochondria-localized CPN1/2 contributes to complex I damage by cleaving component subunits.

Exposure to the Florida red tide dinoflagellate, Karenia brevis, and its associated brevetoxins induces ecophysiological and proteomic alterations in Porites astreoides

As reef-building corals are increasingly being exposed to persistent threats that operate on both regional and global scales, there is a pressing need to better understand the complex processes that diminish coral populations. This study investigated the impacts of the Florida red tide dinoflagellate Karenia brevis and associated brevetoxins on selected facets of coral biology using Porites astreoides as a model system. When provided with choice assays, P. astreoides larvae were shown to actively avoid seawater containing red tide (5×10⁵ cells L^-1–7.6×10⁶ cells L^-1) or purified brevetoxins (0.018 μg mL^-1 brevetoxin-2 and 0.0018 μg mL^-1 brevetoxin-3). However, forced exposure to similar treatments induced time-dependent physiological and behavioral changes that were captured by PAM fluorometry and settlement and survival assays, respectively. Adult fragments of P. astreoides exposed to red tide or associated brevetoxins displayed signs of proteomic alterations that were characterized by the use of an iTRAQ-based quantitative proteomic analysis. The novel use of this technique with P. astreoides demonstrated that protein regulation was highly contingent upon biological versus chemical treatment (i.e. live K. brevis vs. solely brevetoxin exposure) and that several broad pathways associated with cell stress were affected including redox homeostasis, protein folding, energy metabolism and reactive oxygen species production. The results herein provide new insight into the ecology, behavior and sublethal stress of reef-building corals in response to K. brevis exposure and underscore the importance of recognizing the potential of red tide to act as a regional stressor to these important foundation species.

Chronic Microcystin-LR Exposure Induces Abnormal Lipid Metabolism via Endoplasmic Reticulum Stress in Male Zebrafish

In order to explore effects of low levels of continuous microcystin-LR (MC-LR) (a cyanotoxin) exposure on hepatic lipid metabolism on the basis of the endoplasmic reticulum stress (ERS) pathway, we exposed adult male zebrafish to MC-LR (0, 1, 5, and 25 μg/L) for 60 days, and hepatic histopathology as well as lipid metabolic parameters were determined with mRNA levels of ERS signal molecules and downstream factors, along with genes associated with lipid metabolism in zebrafish liver. The results revealed that prolonged exposure to MC-LR remarkably altered the levels of hepatic total cholesterol and triglyceride and led to hepatic steatosis, which was also confirmed by hepatic cytoplasmic vacuolization in Hematoxylin/eosin (H&E) stain and lipid droplet accumulation in Oil Red O stain. The severity of hepatic damage and lipidation was increased in a dose-related manner. MC-LR exposure significantly upregulated transcriptional levels of ERS markers including hspa5, mapk8, and chop, indicating the occurrence of ERS in the liver of zebrafish. Concurrently, MC-LR significantly improved mRNA expression of unfolded protein response (UPR) pathway-related genes including atf6, eif2ak3, ern1, and xbp1s, suggesting that all of the three UPR branches were activated by MC-LR. MC-LR also induced significant upregulation of downstream lipid metabolism-related factors and genes including srebf1, srebf2, fatty acid synthase (fasn), acetyl-CoA carboxylase (acaca), stearoyl-CoA desaturase (scd), HMG CoA reductase (hmgcra), and HMG CoA synthase (hmgcs1), and downregulation of genes associated with lipolysis such as triglyceride hydrolase gene (atgl), hormone-sensitive enzyme gene (hsla), and carnitine palmitoyltransferase gene (cpt1aa). Our present results indicated that the cause of hepatic lipid accumulation by MC-LR was mainly by upregulating lipogenic and cholesterol genes but downregulating the expression of lipolytic genes through the induction of srebf1 and srebf2, which were involved in the activation of ERS signal pathways.

The Endoplasmic Reticulum and Calcium Homeostasis in Pancreatic Beta Cells

The endoplasmic reticulum (ER) mediates the first steps of protein assembly within the secretory pathway and is the site where protein folding and quality control are initiated. The storage and release of Ca2+ are critical physiological functions of the ER. Disrupted ER homeostasis activates the unfolded protein response (UPR), a pathway which attempts to restore cellular equilibrium in the face of ER stress. Unremitting ER stress, and insufficient compensation for it results in beta-cell apoptosis, a process that has been linked to both type 1 diabetes (T1D) and type 2 diabetes (T2D). Both types are characterized by progressive beta-cell failure and a loss of beta-cell mass, although the underlying causes are different. The reduction of mass occurs secondary to apoptosis in the case of T2D, while beta cells undergo autoimmune destruction in T1D. In this review, we examine recent findings that link the UPR pathway and ER Ca2+ to beta cell dysfunction. We also discuss how UPR activation in beta cells favors cell survival versus apoptosis and death, and how ER protein chaperones are involved in regulating ER Ca2+ levels. Abbreviations: BiP, Binding immunoglobulin Protein ER; endoplasmic reticulum; ERAD, ER-associated protein degradation; IFN, interferon; IL, interleukin; JNK, c-Jun N-terminal kinase; KHE, proton-K+ exchanger; MODY, maturity-onset diabetes of young; PERK, PRKR-like ER kinase; SERCA, Sarco/Endoplasmic Reticulum Ca2+-ATPases; T1D, type 1 diabetes; T2D, type 2 diabetes; TNF, tumor necrosis factor; UPR, unfolded protein response; WRS, Wolcott-Rallison syndrome.

Establishment and validation of an endoplasmic reticulum stress reporter to monitor zebrafish ATF6 activity in development and disease

Induction of endoplasmic reticulum (ER) stress is associated with diverse developmental and degenerative diseases. Modified ER homeostasis causes activation of conserved stress pathways at the ER called the unfolded protein response (UPR). ATF6 is a transcription factor activated during ER stress as part of a coordinated UPR. ATF6 resides at the ER and, upon activation, is transported to the Golgi apparatus, where it is cleaved by proteases to create an amino-terminal cytoplasmic fragment (ATF6f). ATF6f translocates to the nucleus to activate transcriptional targets. Here, we describe the establishment and validation of zebrafish reporter lines for ATF6 activity. These transgenic lines are based on a defined and multimerized ATF6 consensus site, which drives either eGFP or destabilized eGFP, enabling dynamic study of ATF6 activity during development and disease. The results show that the reporter is specific for the ATF6 pathway, active during development and induced in disease models known to engage UPR. Specifically, during development, ATF6 activity is highest in the lens, skeletal muscle, fins and gills. The reporter is also activated by common chemical inducers of ER stress, including tunicamycin, thapsigargin and brefeldin A, as well as by heat shock. In models for amyotrophic lateral sclerosis and cone dystrophy, ATF6 reporter expression is induced in spinal cord interneurons or photoreceptors, respectively, suggesting a role for ATF6 response in multiple neurodegenerative diseases. Collectively our results show that these ATF6 reporters can be used to monitor ATF6 activity changes throughout development and in zebrafish models of disease.

This article has an associated First Person interview with the first author of the paper.

Summary: In this study, we validate transgenic zebrafish generated to specifically report the activity of ATF6, representing a major branch of the endoplasmic reticulum stress pathway with functions in development and disease.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Microcystin-LR exposure decreased the fetal weight of mice by disturbance of placental development and ROS-mediated endoplasmic reticulum stress in the placenta

The placenta is essential for sustaining the growth of the fetus. The aim of this study was to investigate the role of the placenta in MCLR-induced significant reduction in fetal weight, especially the changes in placental structure and function. Pregnant mice were intraperitoneally injected with MCLR (5 or 20 μg/kg) from gestational day (GD) 13 to GD17. The results showed MCLR reduced fetal weight and placenta weight. The histological specimens of the placentas were taken for light and electron microscopy studies. The internal space of blood vessels decreased obviously in the placental labyrinth layer of mice treated with MCLR. After the ultrastructural examination, the edema and intracytoplasmic vacuolization, dilation of the endoplasmic reticulum and corrugation of the nucleus were observed. In addition, maternal MCLR exposure caused a reduction of 11β-hydroxysteroid dehydrogenase type 2 (HSD11B2) expression in placentae, a critical regulator of fetal development. Several genes of placental growth factors, such as Vegfα and Pgf and several genes of nutrient transport pumps, such as Glut1 and Pcft were depressed in placentas of MCLR-treated mice, however nutrient transporters Fatp1 and Snat4 were promoted. Moreover, significant increases in malondialdehyde (MDA) revealed the occurrence of oxidative stress caused by MCLR, which was also verified by remarkable decrease in the glutathione levels, total antioxidant capacity (T-AOC) as well as the activity of antioxidant enzymes. Real-time PCR and western blot analysis revealed that GRP78, CHOP, XBP-1, peIF2α and pIRE1 were remarkable increased in placentas of MCLR-treated mice, indicating that endoplasmic reticulum (ER) stress pathway was activated by MCLR. Furthermore, oxidative stress and ER stress consequently triggered apoptosis which contributed to the impairment of placental development. Collectively, these results suggest maternal MCLR exposure results in reduced fetal body weight, which might be associated with ROS-mediated endoplasmic reticulum stress and impairment in placental structure and function.

Loss of Ca2+ entry via Orai-TRPC1 induces ER stress, initiating immune activation in macrophages

Activation of cellular stresses is associated with inflammation; however, the mechanisms are not well identified. Here, we provide evidence that loss of Ca2+ influx induces endoplasmic reticulum (ER) stress in primary macrophages and in murine macrophage cell line Raw 264.7, in which the unfolded protein response is initiated to modulate cytokine production, thereby activating the immune response. Stressors that initiate the ER stress response block store-dependent Ca2+ entry in macrophages prior to the activation of the unfolded protein response. The endogenous Ca2+ entry channel is dependent on the Orai1-TRPC1-STIM1 complex, and the presence of ER stressors decreased expression of TRPC1, Orai1 and STIM1. Additionally, blocking Ca2+ entry with SKF96365 also induced ER stress, promoted cytokine production, activation of autophagy, increased caspase activation and induced apoptosis. Furthermore, ER stress inducers inhibited cell cycle progression, promoted the inflammatory M1 phenotype, and increased phagocytosis. Mechanistically, restoration of Orai1-STIM1 expression inhibited the ER stress-mediated loss of Ca2+ entry that prevents ER stress and inhibits cytokine production, and thus induced cell survival. These results suggest an unequivocal role of Ca2+ entry in modulating ER stress and in the induction of inflammation.

Multiple system-level feedback loops control life-and-death decisions in endoplasmic reticulum stress

Scientific results have revealed that autophagy is able to promote cell survival in response to endoplasmic reticulum (ER) stress, while drastic events result in apoptotic cell death. Here, we analyse the important crosstalk of life-and-death decisions from a systems biological perspective by studying the regulatory modules of the unfolded protein response (UPR). While a double-negative loop between autophagy and apoptosis inducers is crucial for the switch-like characteristic of the stress response mechanism, a positive feedback loop between ER stress sensors is also essential. Corresponding to experimental data, here, we show the dynamical significance of Gadd34-CHOP connections inside the PERK branch of the UPR. The multiple system-level feedback loops seem to be crucial for managing a robust life-and-death decision depending on the level and durability of cellular stress.

Mechanisms of alpha-synuclein toxicity: An update and outlook

Alpha-synuclein (aSyn) was identified as the main component of inclusions that define synucleinopathies more than 20 years ago. Since then, aSyn has been extensively studied in an attempt to unravel its roles in both physiology and pathology. Today, studying the mechanisms of aSyn toxicity remains in the limelight, leading to the identification of novel pathways involved in pathogenesis. In this chapter, we address the molecular mechanisms involved in synucleinopathies, from aSyn misfolding and aggregation to the various cellular effects and pathologies associated. In particular, we review our current understanding of the mechanisms involved in the spreading of aSyn between different cells, from the periphery to the brain, and back. Finally, we also review recent studies on the contribution of inflammation and the gut microbiota to pathology in synucleinopathies. Despite significant advances in our understanding of the molecular mechanisms involved, we still lack an integrated understanding of the pathways leading to neurodegeneration in PD and other synucleinopathies, compromising our ability to develop novel therapeutic strategies.

Proteostasis In The Endoplasmic Reticulum: Road to Cure

The endoplasmic reticulum (ER) is an interconnected organelle that is responsible for the biosynthesis, folding, maturation, stabilization, and trafficking of transmembrane and secretory proteins. Therefore, cells evolve protein quality-control equipment of the ER to ensure protein homeostasis, also termed proteostasis. However, disruption in the folding capacity of the ER caused by a large variety of pathophysiological insults leads to the accumulation of unfolded or misfolded proteins in this organelle, known as ER stress. Upon ER stress, unfolded protein response (UPR) of the ER is activated, integrates ER stress signals, and transduces the integrated signals to relive ER stress, thereby leading to the re-establishment of proteostasis. Intriguingly, severe and persistent ER stress and the subsequently sustained unfolded protein response (UPR) are closely associated with tumor development, angiogenesis, aggressiveness, immunosuppression, and therapeutic response of cancer. Additionally, the UPR interconnects various processes in and around the tumor microenvironment. Therefore, it has begun to be delineated that pharmacologically and genetically manipulating strategies directed to target the UPR of the ER might exhibit positive clinical outcome in cancer. In the present review, we summarize recent advances in our understanding of the UPR of the ER and the UPR of the ER-mitochondria interconnection. We also highlight new insights into how the UPR of the ER in response to pathophysiological perturbations is implicated in the pathogenesis of cancer. We provide the concept to target the UPR of the ER, eventually discussing the potential of therapeutic interventions for targeting the UPR of the ER for cancer treatment.

Brosimone I, an isoprenoid-substituted flavonoid, induces cell cycle G1 phase arrest and apoptosis through ROS-dependent endoplasmic reticulum stress in HCT116 human colon cancer cells

Brosimone I is an isoprenoid-substituted flavonoid from Artocarpus heterophyllus. Here, we reported for the first time that brosimone I induced cell cycle G1 phase arrest and apoptosis in HCT116 human colon cancer cells. Brosimone I treatment increased the cytosolic Ca2+ level, and subsequently activated the CaMKKβ-AMPK pathway. STO-609, a CaMKKβ inhibitor, and compound C, an AMPK-specific inhibitor, attenuated brosimone I-induced loss of cell viability in HCT116 cells. Furthermore, brosimone I enhanced ER stress. Salubrinal, an ER stress inhibitor, reduced brosimone I-induced cell growth inhibition. In addition, brosimone I was found to increase ROS generation and the inhibition of ROS formation by NAC, a ROS inhibitor, attenuated brosimone I-induced cell death, cytosolic Ca2+ increase, and ER stress markers. Collectively, our findings reveal that brosimone I induces cell cycle G1 phase arrest and apoptosis via the induction of ROS-mediated increased cytosolic Ca2+, ER stress, and the activation of the CaMKKβ-AMPK signaling pathway.

Endoplasmic reticulum in viral infection

Virus exploits host cellular machinery to replicate and form new viral progeny and endoplasmic reticulum (ER) plays central role in the interplay between virus and host cell. Here I will discuss how cellular functions of ER being utilized by viruses from different families during different stages of pathogenesis. Flow of knowledge related to this area of research based on interdisciplinary approach, using biochemical and cell biological assays coupled with advanced microscopy strategies, is pushing our understanding of the virus-ER interaction during infection to the next level.

Benzalkonium chloride-induced direct and indirect toxicity on corneal epithelial and trigeminal neuronal cells: proinflammatory and apoptotic responses in vitro

Benzalkonium chloride (BAK), a quaternary ammonium compound widely used as disinfecting agent as well as preservative in eye drops is known to induce toxic effects on the ocular surface with inflammation and corneal nerve damage leading to dry eye disease (DED) in the medium-to-long term. The aim of this study was to evaluate in vitro the toxicity of a conditioned medium produced by corneal epithelial cells previously exposed to BAK (BAK-CM) on trigeminal neuronal cells. A human corneal epithelial (HCE) cell line was exposed to 5.10^-3% BAK (i.e. 0.005% BAK) for 15 min and let recover for 5 h to prepare a BAK-CM. This BAK concentration is the lowest one found in eye drops. After this recovery period, BAK effect on HCE cells displayed cytotoxicity, morphological alteration, apoptosis, oxidative stress, ATP release, CCL2 and IL6 gene induction, as well as an increase in CCL2, IL-6 and MIF release. Next, a mouse trigeminal ganglion primary culture was exposed to the BAK-CM for 2 h, 4 h or 24 h. Whereas BAK-CM did not alter neuronal cell morphology, or induced neuronal cytotoxicity or oxidative stress, BAK-CM induced gene expression of Fos (neuronal activation marker), Atf3 (neuronal injury marker), Ccl2 and Il6 (inflammatory markers). Two and 4 h BAK-CM exposure promoted a neuronal damage (ATF-3, phospho-p38 increases; phospho-Stat3 decreases) while 24 h-BAK-CM exposure initiated a prosurvival pathway activation (phospho-p44/42, phospho-Akt increases; ATF-3, GADD153, active Caspase-3 decreases). In conclusion, this in vitro model, simulating paracrine mechanisms, represents an interesting tool to highlight the indirect toxic effects of BAK or any other xenobiotic on corneal trigeminal neurons and may help to better understand the cellular mechanisms that occur during DED pathophysiology.

Galanthamine improves myocardial ischemia-reperfusion-induced cardiac dysfunction, endoplasmic reticulum stress-related apoptosis, and myocardial fibrosis by suppressing AMPK/Nrf2 pathway in rats

Background

Myocardial ischemia/reperfusion (I/R) injury is an important cause of myocardial infarction and heart failure after cardiovascular surgery. Galanthamine (Gal) is an important Amaryllidaceae alkaloid with anti-acetylcholinesterase and anti-inflammatory activity. The purpose of this study was to investigate the role of Gal in myocardial I/R injury.

Methods

In this study, an animal model of myocardial I/R injury was constructed, and the rats were divided into five groups (n=10): the sham, I/R model, I/R + Gal (1 mg/kg), I/R + Gal (3 mg/kg), and I/R + Aspirin (20 mg/kg) groups. The expression of related proteins was detected by Western blotting and Immunohistochemistry, and Histological lesion was detected by HE staining.

Results

Results showed that Gal improves I/R-induced cardiac dysfunction in rats. Moreover, Gal inhibits I/R-induced endoplasmic reticulum stress (ERS)-related apoptosis by suppressing the expression of CHOP, Cleaved caspase 12, and caspase 3, and promoting the expression of CADD34 and BiP in rats. Furthermore, Gal mitigates I/R-induced myocardial fibrosis through restraining the expression of α-SMA and Collagen I in rats. Mechanically, Gal promoted the expression of AMPKα1, Nrf2 and HO-1. However, AMPK inhibitor Compound C exhibited the opposite effects. Collectively, this finding suggests that Gal improves I/R-induced cardiac dysfunction, ERS-related apoptosis, and myocardial fibrosis by activating AMPK/Nrf2 pathway in myocardial I/R rats.

Conclusions

Given this evidence, Gal may be a potential therapeutic drug for the treatment of I/R injury.

ER Stress Activates the NLRP3 Inflammasome: A Novel Mechanism of Atherosclerosis

The endoplasmic reticulum (ER) is an important organelle that regulates several fundamental cellular processes, and ER dysfunction has implications for many intracellular events. The nucleotide-binding oligomerization domain-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome is an intracellularly produced macromolecular complex that can trigger pyroptosis and inflammation, and its activation is induced by a variety of signals. ER stress has been found to affect NLRP3 inflammasome activation through multiple effects including the unfolded protein response (UPR), calcium or lipid metabolism, and reactive oxygen species (ROS) generation. Intriguingly, the role of ER stress in inflammasome activation has not attracted a great deal of attention. In addition, increasing evidence highlights that both ER stress and NLRP3 inflammasome activation contribute to atherosclerosis (AS). AS is a common cardiovascular disease with complex pathogenesis, and the precise mechanisms behind its pathogenesis remain to be determined. Both ER stress and the NLRP3 inflammasome have emerged as critical individual contributors of AS, and owing to the multiple associations between these two events, we speculate that they contribute to the mechanisms of pathogenesis in AS. In this review, we aim to summarize the molecular mechanisms of ER stress, NLRP3 inflammasome activation, and the cross talk between these two pathways in AS in the hopes of providing new pharmacological targets for AS treatment.

NFκB/Orai1 Facilitates Endoplasmic Reticulum Stress by Oxidative Stress in the Pathogenesis of Non-alcoholic Fatty Liver Disease

Non-esterified fatty acids (NEFAs) promote de novo lipogenesis, which caused abnormal hepatic lipid accumulation, by the NFκB-Orai1 pathway. Oxidative stress and endoplasmic reticulum (ER) stress have been recognized as key mechanisms in non-alcoholic fatty liver disease (NAFLD) pathogenesis. Whether Orai1 facilitates ER stress by oxidative stress remains unknown. The rat model of NAFLD was constructed by feeding high-fat diet (HFD). BRL-3A cells were treated with NEFAs, Orai1inhibtor BTP2, NFκB inhibitor wogonin, or small interfering Orai (siOrai) 1, respectively. The content of intracellular reduced glutathione (GSH) and malondialdehyde (MDA), indicating oxidative stress, was measured by a spectrophotometer. ER stress major proteins PERK, IRE1, ATF6, CHOP, and GRP78 were quantified using Western blot and qRT-PCR analyses. For the intracellular location of reactive oxygen species (ROS) and Orai1 were measured by Western blot and immunofluorescence, and cytosolic Ca²⁺ was measured by flow cytometry. As we expected, the liver of rats with NAFLD showed lipid droplets in HE and Oil Red O. The decreased GSH and increased MDA were found in rats fed with HFD. ER stress major proteins PERK, IRE1, ATF6, GRP78, and CHOP were significantly increased in the HFD group. In BRL-3A cells, GSH content dramatically decreased from 1 h, MDA content dramatically increased from 3 h, and expression levels of ER stress significantly increased from 3 h by NEFA treatment. Furthermore, cytosolic Ca²⁺ increased from 0.5 h by NEFAs treated in BRL-3A cells. It indicated that NEFAs increased cytosolic Ca²⁺ to induce oxidative stress, thus ER stress. The content of oxidative stress and ER stress proteins showed the same trends by NEFAs treated in BRL-3A cells. These effects were reversed by the Orai1 inhibitor BTP2 and the NFκB inhibitor wogonin. Moreover, siOrai1 abrogated NEFAs' influence in BRL-3A cells. Last, ROS was found by NEFAs treated in BRL-3A cells, and NEFA treatment enhanced the nuclear localization of NF-κB p65 and ORAI1. It was considered that high NEFAs increased cytosolic Ca²⁺ and enhanced NFκB-dependent SOCE and its moiety protein Orai1 to decrease GSH and thus induced oxidative stress at earlier stages and furthermore tempted ER stress in the pathologic progress of NAFLD.

Endoplasmic Reticulum Stress-Mediated Autophagy and Apoptosis Alleviate Dietary Fat-Induced Triglyceride Accumulation in the Intestine and in Isolated Intestinal Epithelial Cells of Yellow Catfish

BACKGROUND

The intestine is the main organ for absorbing dietary fat. High dietary lipid intake leads to fat deposition in the intestine and adversely influences fat absorption and health, but the underlying mechanism is unknown.

OBJECTIVES

We used yellow catfish and their isolated intestinal epithelial cells to test the hypothesis that endoplasmic reticulum (ER) stress, autophagy, and apoptosis mediate fat-induced changes in lipid metabolism.

METHODS

Male and female yellow catfish (weight: 3.79 ± 0.16 g; age: 3 mo) were fed diets containing lipid at 6.98% (low-fat diet; LFD), 11.3% (middle-fat diet; MFD), or 15.4% (high-fat diet; HFD) (by weight) for 8 wk. Each dietary group had 3 replicates, 30 fish per replicate. Their intestinal epithelial cells were isolated and incubated for 24 h in control solution or various concentrations of fatty acids (FAs) with or without 2-h pretreatment with an inhibitor [3-methyladenine (3-MA), 4-phenyl butyric acid (4-PBA), or Ac-DVED-CHO (AC)]. Triglyceride (TG) contents, genes, and enzymes involved in lipid metabolism, ER stress, autophagy, and apoptosis were determined in intestinal tissue and cells; immunoblotting, BODIPY 493/503 staining, ultrastructural observation, and the detection of autophagic and apoptotic vesicles were performed on intestinal cells.

RESULTS

Compared with the LFD and MFD, the HFD increased intestinal TG content by 120-226%, activities of lipogenic enzymes by 19.0-245%, expression of genes related to lipogenesis (0.77-8.4-fold), lipolysis (0.36-6.0-fold), FA transport proteins (0.79-1.7-fold), ER stress (0.55-7.5-fold), autophagy (0.56-4.2-fold), and apoptosis (0.80-5.2-fold). Using isolated intestinal epithelial cells and inhibitors (4-PBA, 3-MA, and AC), we found that ER stress mediated FA-induced activation of autophagy (11.0-50.1%) and apoptosis (10.4-32.0%), and lipophagy and apoptosis mediated FA-induced lipolysis (3.40-41.6%).

CONCLUSIONS

An HFD upregulated lipogenesis, lipolysis, and FA transport, induced ER stress, and activated autophagy and apoptosis. ER stress, autophagy, and apoptosis play important regulatory roles in fat-induced changes in lipid metabolism in the intestine and intestinal epithelial cells of yellow catfish.

Endoplasmic Reticulum Stress Activates Unfolded Protein Response Signaling and Mediates Inflammation, Obesity, and Cardiac Dysfunction: Therapeutic and Molecular Approach

Obesity has been implicated as a risk factor for insulin resistance and cardiovascular diseases (CVDs). Although the association between obesity and CVD is a well-established phenomenon, the precise mechanisms remain incompletely understood. This has led to a relative paucity of therapeutic measures for the prevention and treatment of CVD and associated metabolic disorders. Recent studies have shed light on the pivotal role of prolonged endoplasmic reticulum stress (ERS)-initiated activation of the unfolded protein response (UPR), the ensuing chronic low-grade inflammation, and altered insulin signaling in promoting obesity-compromised cardiovascular system (CVS). In this aspect, potential ways of attenuating ERS-initiated UPR signaling seem a promising avenue for therapeutic interventions. We review intersecting role of obesity-induced ERS, chronic inflammation, insulin resistance, and oxidative stress in the discovery of targeted therapy. Moreover, this review highlights the current progress and strategies on therapeutics being explored in preclinical and clinical research to modulate ERS and UPR signaling.

Flaviviridae Viruses and Oxidative Stress: Implications for Viral Pathogenesis

Oxidative stress is induced once the balance of generation and neutralization of reactive oxygen species (ROS) is broken in the cell, and it plays crucial roles in a variety of natural and diseased processes. Infections of Flaviviridae viruses trigger oxidative stress, which affects both the cellular metabolism and the life cycle of the viruses. Oxidative stress associated with specific viral proteins, experimental culture systems, and patient infections, as well as its correlations with the viral pathogenesis attracts much research attention. In this review, we primarily focus on hepatitis C virus (HCV), dengue virus (DENV), Zika virus (ZIKV), Japanese encephalitis virus (JEV), West Nile virus (WNV), and tick-borne encephalitis virus (TBEV) as representatives of Flaviviridae viruses and we summarize the mechanisms involved in the relevance of oxidative stress for virus-associated pathogenesis. We discuss the current understanding of the pathogenic mechanisms of oxidative stress induced by Flaviviridae viruses and highlight the relevance of autophagy and DNA damage in the life cycle of viruses. Understanding the crosstalk between viral infection and oxidative stress-induced molecular events may offer new avenues for antiviral therapeutics.

Intermittent Hypoxia Induces Autophagy to Protect Cardiomyocytes From Endoplasmic Reticulum Stress and Apoptosis

Intermittent hypoxia (IH), characterized as cyclic episodes of short-period hypoxia followed by normoxia, occurs in many physiological and pathophysiological conditions such as pregnancy, athlete, obstructive sleep apnea, and asthma. Hypoxia can induce autophagy, which is activated in response to protein aggregates, in the proteotoxic forms of cardiac diseases. Previous studies suggested that autophagy can protect cells by avoiding accumulation of misfolded proteins, which can be generated in response to ischemia/reperfusion (I/R) injury. The objective of the present study was to determine whether IH-induced autophagy can attenuate endoplasmic reticulum (ER) stress and cell death. In this study, H9c2 cell line, rat primary cultured cardiomyocytes, and C57BL/6 male mice underwent IH with an oscillating O₂ concentration between 4 and 20% every 30 min for 1-4 days in an incubator. The levels of LC3, an autophagy indicator protein and CHOP and GRP78 (ER stress-related proteins) were measured by Western blotting analyses. Our data demonstrated that the autophagy-related proteins were upregulated in days 1-3, while the ER stress-related proteins were downregulated on the second day after IH. Treatment with H₂O₂ (100 μM) for 24 h caused ER stress and increased the level of ER stress-related proteins, and these effects were abolished by pre-treatment with IH condition. In response to the autophagy inhibitor, the level of ER stress-related proteins was upregulated again. Taken together, our data suggested that IH could increase myocardial autophagy as an adaptive response to prevent the ER stress and apoptosis.

Low expression of ryanodine receptor 2 is associated with poor prognosis in thyroid carcinoma

Genetic alterations are vital in the progression of thyroid carcinoma. Ryanodine receptor 2 (RyR2) is reported to serve an important role in several types of human carcinoma. However, the expression and effect of RyR2 in thyroid carcinoma remain unknown. Therefore, the present study analyzed the status of RyR2 in thyroid carcinoma using bioinformatics tools. The mRNA profiles of thyroid carcinoma were downloaded from The Cancer Genome Atlas. RyR2 was distinguished as a differentially expressed gene that has not been reported in thyroid carcinoma. Further analysis indicated that there was selective downregulation of RyR2 expression in thyroid carcinoma tissues compared with that in normal thyroid tissues. Survival analysis showed that RyR2 expression was associated with poorer disease-free survival (DFS) for all patients with thyroid carcinoma. Univariate analysis revealed that a low expression of RyR2 was significantly associated with lymphatic metastasis, extracapsular extension, and the Tumor-Node-Metastasis stage. Cox analysis demonstrated that RyR2 was an independent prognostic factor in thyroid carcinoma for DFS. The biological processes and signaling pathways of RyR2 were reviewed with Gene Set Enrichment Analysis. In conclusion, the present study has revealed that RyR2 is downregulated in thyroid carcinoma, and that low expression of RyR2 is associated with poor prognosis in patients with thyroid carcinoma. RyR2 may therefore serve as a promising tumor suppressor gene in thyroid carcinoma.

The roles of mitochondria-associated membranes in mitochondrial quality control under endoplasmic reticulum stress

The endoplasmic reticulum (ER) and mitochondria are two important organelles in cells. Mitochondria-associated membranes (MAMs) are lipid raft-like domains formed in the ER membranes that are in close apposition to mitochondria. They play an important role in signal transmission between these two essential organelles. When cells are exposed to internal or external stressful stimuli, the ER will activate an adaptive response called the ER stress response, which has a significant effect on mitochondrial function. Mitochondrial quality control is an important mechanism to ensure the functional integrity of mitochondria and the effect of ER stress on mitochondrial quality control through MAMs is of great significance. Therefore, in this review, we introduce ER stress and mitochondrial quality control, and discuss how ER stress signals are transmitted to mitochondria through MAMs. We then review the important roles of MAMs in mitochondrial quality control under ER stress.

Alpha-1 microglobulin as a potential therapeutic candidate for treatment of hypertension and oxidative stress in the STOX1 preeclampsia mouse model

Preeclampsia is a human placental disorder affecting 2–8% of pregnancies worldwide annually, with hypertension and proteinuria appearing after 20 weeks of gestation. The underlying cause is believed to be incomplete trophoblast invasion of the maternal spiral arteries during placentation in the first trimester, resulting in oxidative and nitrative stress as well as maternal inflammation and organ alterations. In the Storkhead box 1 (STOX1) preeclampsia mouse model, pregnant females develop severe and early onset manifestations as seen in human preeclampsia e.g. gestational hypertension, proteinuria, and organ alterations. Here we aimed to evaluate the therapeutic potential of human recombinant alpha-1 microglobulin (rA1M) to alleviate the manifestations observed. Human rA1M significantly reduced the hypertension during gestation and significantly reduced the level of hypoxia and nitrative stress in the placenta. In addition, rA1M treatment reduced cellular damage in both placenta and kidneys, thereby protecting the tissue and improving their function. This study confirms that rA1M has the potential as a therapeutic drug in preeclampsia, and likely also in other pathological conditions associated with oxidative stress, by preserving normal organ function.

Identification of five genes in endoplasmic reticulum (ER) stress-apoptosis pathways in yellow catfish Pelteobagrus fulvidraco and their transcriptional responses to dietary lipid levels

The activating transcription factor 4 (ATF4), DNA damage-inducible transcript 3 (DDIT3), growth arrest, and DNA damage-inducible protein 34 (GADD34), endoplasmic reticulum oxidoreductin 1α (ERO1α), and tumor necrosis factor receptor associated factor 2 (TRAF2) cDNAs were first characterized from yellow catfish Pelteobagrus fulvidraco. Compared to corresponding genes of mammals, all of these proteins shared similar conserved domains. Their mRNAs were widely expressed in various tissues, but at variable levels. Dietary lipid levels did not significantly influence ATF4 mRNA expression. mRNA expression of DDIT3 and GADD34 was highest for fish fed the low-lipid diets and lowest for fish fed middle-lipid diets. The mRNA levels of ERO1α and TRAF2 declined with increasing dietary lipid levels. For the first time, we characterized the full-length cDNA sequences of ATF4, DDIT3, GADD34, ERO1α, and TRAF2 and determined their tissue expression profiles and transcriptional responses to dietary lipid levels, which would contribute to our exploration into their biological functions, and providing new insights on relations between ER stress and lipid metabolism in fish.

Linking the Endoplasmic Reticulum to Parkinson's Disease and Alpha-Synucleinopathy

Accumulation of misfolded proteins is a central paradigm in neurodegeneration. Because of the key role of the endoplasmic reticulum (ER) in regulating protein homeostasis, in the last decade multiple reports implicated this organelle in the progression of Parkinson's Disease (PD) and other neurodegenerative illnesses. In PD, dopaminergic neuron loss or more broadly neurodegeneration has been improved by overexpression of genes involved in the ER stress response. In addition, toxic alpha-synuclein (αS), the main constituent of proteinaceous aggregates found in tissue samples of PD patients, has been shown to cause ER stress by altering intracellular protein traffic, synaptic vesicles transport, and Ca²⁺ homeostasis. In this review, we will be summarizing evidence correlating impaired ER functionality to PD pathogenesis, focusing our attention on how toxic, aggregated αS can promote ER stress and cell death.

KHG21834 attenuates glutamate-induced mitochondrial damage, apoptosis, and NLRP3 inflammasome activation in SH-SY5Y human neuroblastoma cells

New compounds were screened to develop effective drugs against glutamate-induced toxicity. The present study assessed the effects of the novel thiazole derivative KHG21834 against glutamate-induced toxicity in human neuroblastoma SH-SY5Y cell cultures. Treatment of SH-SY5Y cells with KHG21834 significantly protected cells against glutamate-induced toxicity in a dose-dependent manner, with an optimum concentration of 50 μM. KHG21834 protected SH-SY5Y cells against glutamate toxicity by suppressing glutamate-induced oxidative stress by 50%. KHG21834 also attenuated glutamate-induced mitochondrial membrane potential, ATP level reductions, and intracellular Ca²⁺ influx. Furthermore, KHG21834 efficiently reduced glutamate-induced ER stress and NLRP3 inflammasome activation (59% and 65% of glutamate group, respectively). In addition, KHG21834 effectively attenuated glutamate-induced levels of Bax, Bcl-2, cleaved caspase-3, p-p38, p-JNK proteins, and TUNEL positive cells. To our knowledge, this is the first study showing that KHG21834 can effectively protect SH-SY5Y cells against glutamate toxicity, suggesting that this compound may be a valuable therapeutic agent for the treatment of glutamate toxicity.

The Orai Ca2+ channel inhibitor CM4620 targets both parenchymal and immune cells to reduce inflammation in experimental acute pancreatitis

KEY POINTS

This work confirms previous reports that CM4620, a small molecule inhibitor of Ca²⁺ entry via store operated Ca²⁺ entry (SOCE) channels formed by stromal interaction molecule 1 (STIM1)/Orai complexes, attenuates acinar cell pathology and acute pancreatitis in mouse experimental models. Here we report that intravenous administration of CM4620 reduces the severity of acute pancreatitis in the rat, a hitherto untested species. Using CM4620, we probe further the mechanisms whereby SOCE via STIM1/Orai complexes contributes to the disease in pancreatic acinar cells, supporting a role for endoplasmic reticulum stress/cell death pathways in these cells. Using CM4620, we show that SOCE via STIM1/Orai complexes promotes neutrophil oxidative burst and inflammatory gene expression during acute pancreatitis, including in immune cells which may be either circulating or invading the pancreas. Using CM4620, we show that SOCE via STIM1/Orai complexes promotes activation and fibroinflammatory gene expression within pancreatic stellate cells.

ABSTRACT

Key features of acute pancreatitis include excess cellular Ca²⁺ entry driven by Ca²⁺ depletion from the endoplasmic reticulum (ER) and subsequent activation of store-operated Ca²⁺ entry (SOCE) channels in the plasma membrane. In several cell types, including pancreatic acinar, stellate cells (PaSCs) and immune cells, SOCE is mediated via channels composed primarily of Orai1 and stromal interaction molecule 1 (STIM1). CM4620, a selective Orai1 inhibitor, prevents Ca²⁺ entry in acinar cells. This study investigates the effects of CM4620 in preventing or reducing acute pancreatitis features and severity. We tested the effects of CM4620 on SOCE, trypsinogen activation, acinar cell death, activation of NFAT and NF-κB, and inflammatory responses in ex vivo and in vivo rodent models of acute pancreatitis and human pancreatic acini. We also examined whether CM4620 inhibited cytokine release in immune cells, fibro-inflammatory responses in PaSCs, and oxidative burst in neutrophils, all cell types participating in pancreatitis. CM4620 administration to rats by i.v. infusion starting 30 min after induction of pancreatitis significantly diminished pancreatitis features including pancreatic oedema, acinar cell vacuolization, intrapancreatic trypsin activity, cell death signalling and acinar cell death. CM4620 also decreased myeloperoxidase activity and inflammatory cytokine expression in pancreas and lung tissues, fMLF peptide-induced oxidative burst in human neutrophils, and cytokine production in human peripheral blood mononuclear cells (PBMCs) and rodent PaSCs, indicating that Orai1/STIM1 channels participate in the inflammatory responses of these cell types during acute pancreatitis. These findings support pathological Ca²⁺ entry-mediated cell death and proinflammatory signalling as central mechanisms in acute pancreatitis pathobiology.

Activation of TRPC6 channels contributes to (+)-conocarpan-induced apoptotic cell death in HK-2 cells

(+)-Conocarpan (CNCP), a neolignan frequently found in many medicinal and edible plants displays a broad spectrum of bioactivity. Here, we demonstrated that CNCP induced apoptotic cell death in human kidney-2 (HK-2) cells in a concentration-dependent manner (IC₅₀ = 19.3 μM) and led to the sustained elevation of intracellular Ca²⁺ ([Ca²⁺]_i). Lower extracellular Ca²⁺ concentrations from 2.3 mM to 0 mM significantly suppressed the CNCP-induced Ca²⁺ response by 69.1%. Moreover, the depletion of intracellular Ca²⁺ stores using thapsigargin normalized CNCP-induced Ca²⁺ release from intracellular Ca²⁺ stores, suggesting that the CNCP-induced Ca²⁺ response involved both extracellular Ca²⁺ influx and Ca²⁺ release from intracellular Ca²⁺ stores. SAR7334, a TRPC3/6/7 channel inhibitor, but neither Pyr3, a selective TRPC3 channel inhibitor, nor Pico145, a TRPC1/4/5 inhibitor, suppressed the CNCP-induced Ca²⁺ response by 57.2% and decreased CNCP-induced cell death by 53.4%, suggesting a critical role for TRPC6 channels in CNCP-induced Ca²⁺ influx and apoptotic cell death. Further electrophysiological recording demonstrated that CNCP directly activated TRPC6 channels by increasing channel open probability with an EC₅₀ value of 6.01 μM. Considered together, these data demonstrate that the direct activation of TRPC6 channels contributes to CNCP-induced apoptotic cell death in HK-2 cells. Our data point out the potential risk of renal toxicity from CNCP if used as a therapeutic agent.

Endoplasmic reticulum-associated degradation potentiates the infectivity of influenza A virus by regulating the host redox state

During influenza A virus (IAV) infection, significant effects of oxidative stress often emerge due to the disruption of the redox balance. Reactive oxygen species (ROS) generated during IAV infection have been known to exert various effects on both the virus and host tissue. However, the mechanisms underlying the accumulation of ROS and their physiological significance in IAV infection have been extensively studied but remain to be fully understood. Here, we show that the levels of Sp1, a key controller of Cu-Zn superoxide dismutase (SOD1) gene expression, and SOD1 are mainly dependent upon the activity of X-box-binding protein 1 (XBP1), which is a downstream factor of the endoplasmic reticulum (ER) transmembrane sensor inositol-requiring enzyme 1 (IRE1) during ER stress. In IRE1-deficient mouse embryo fibroblasts (MEFs) or A549 human lung cells treated with XBP1 siRNA, IAV-induced Sp1 loss was mitigated. However, overexpression of the spliced form of XBP1 in IRE1-deficient MEFs resulted in a further decrease in Sp1 levels, whereas the unspliced form showed no significant differences. Treatment with proteasome inhibitor MG132 markedly inhibited the IRE1/XBP1-mediated loss of Sp1 and SOD, suggesting the involvement of proteasome-dependent ER-associated degradation (ERAD). The increase in SOD1 levels with the expression of siRNA-targeting p97, a central component of the ubiquitin-proteasome system, supports the major role of the ERAD process in IAV-mediated SOD1 loss. In addition, ROS generation due to IAV infection was attenuated in cells lacking either IRE1 or JNK. These results reveal the important roles of both IRE1/XBP1-mediated ERAD and the JNK pathway in IAV infection. Interestingly, the increase in ROS due to IAV infection is correlated with the increase in the virus titer in vitro and in vivo. However, 4-phenylbutyrate (4-PBA), an inhibitor of ER stress signaling, weakened the effect of IAV infection on SOD1 loss in a dose-dependent manner. Furthermore, the treatment of mice with 4-PBA efficiently attenuated ROS generation and ER stress in lung tissue and eventually lowered the IAV titer. These results strongly suggest that the ERAD process plays a major role in IAV infection, thus making it a potential target for antiviral drug therapy.

Maize Sep15-like functions in endoplasmic reticulum and reactive oxygen species homeostasis to promote salt and osmotic stress resistance

In animals, the Sep15 protein participates in disease resistance, growth, and development, but the function of its plant homologues remains unclear. Here, the function of maize Sep15 was analysed by characterization of two independent Sep15-like loss-of-function mutants. In the absence of ZmSep15-like, seedling tolerance to both water and salinity stress was compromised. The mutants experienced a heightened level of endoplasmic reticulum stress, and over-accumulated reactive oxygen species, resulting in leaf necrosis. Characterization of Arabidopsis thaliana atsep15 mutant as well as like with ectopic expression of ZmSep15-like indicated that ZmSep15-like contributed to tolerance of both osmotic and salinity stress. ZmSep15-like interacted physically with UDP-glucose: glycoprotein glucosyltransferase1 (UGGT1). When the interaction was disrupted, the response to both osmotic and salinity stresses was impaired in maize or Arabidopsis. Co-expressing ZmUGGT1 and ZmUGGT2 enhanced the tolerance of A. thaliana to both stressors, indicating a functional interaction between them. Together, the data indicated that plants Sep15-like proteins promote osmotic and salinity stress resistance by influencing endoplasmic reticulum stress response and reactive oxygen species level.

Endoplasmic Reticulum Stress: A Critical Molecular Driver of Endothelial Dysfunction and Cardiovascular Disturbances Associated with Diabetes

Physical inactivity and sedentary lifestyle contribute to the widespread epidemic of obesity among both adults and children leading to rising cases of diabetes. Cardiovascular disease complications associated with obesity and diabetes are closely linked to insulin resistance and its complex implications on vascular cells particularly endothelial cells. Endoplasmic reticulum (ER) stress is activated following disruption in post-translational protein folding and maturation within the ER in metabolic conditions characterized by heavy demand on protein synthesis, such as obesity and diabetes. ER stress has gained much interest as a key bridging and converging molecular link between insulin resistance, oxidative stress, and endothelial cell dysfunction and, hence, represents an interesting drug target for diabetes and its cardiovascular complications. We reviewed here the role of ER stress in endothelial cell dysfunction, the primary step in the onset of atherosclerosis and cardiovascular disease. We specifically focused on the contribution of oxidative stress, insulin resistance, endothelial cell death, and cellular inflammation caused by ER stress in endothelial cell dysfunction and the process of atherogenesis.

RNA-seq reveals temporal differences in the transcriptome response to acute heat stress in the Atlantic salmon (Salmo salar)

Acute heat stress is common in aquaculture and can affect diverse physiological processes in fish; however, different species of fish have various mechanisms for heat stress adaptation. In this study, we profiled the transcriptome responses of the Atlantic salmon (Salmo salar) to heat stress at 23 °C for 6 or 24 h, compared with that of fish at a normal temperature of 13 °C. The liver was selected as the target tissue for this analysis. A total of 243 and 88 genes were differentially expressed after 6 and 24 h of heat stress, respectively. Of these, only 22 were common to both time points, and most of these common genes were molecular chaperones such as heat shock cognate 71 kDa protein and heat shock protein 90-alpha. Genes such as activating transcription factor 6, calreticulin, protein disulfide isomerase A3, and protein kinase R-like endoplasmic reticulum kinase-eukaryotic initiation factor 2-alpha were only up-regulated after 6 h of heat stress; most of these genes are involved in the endoplasmic reticulum stress pathway. Indeed, endoplasmic reticulum stress was identified at 6 h but not at 24 h, suggesting that stress response plays an important role in the adaptation of Atlantic salmon to acute heat stress. Other up-regulated genes at 6 h were related to the insulin and nucleotide oligomerization domain-like receptor signaling pathways, which directly eliminate misfolded proteins and sustain sugar and lipid homeostasis. At 24 h, heat stress influenced the expression of steroid and terpenoid backbone biosynthesis, which may influence the sexual development and differentiation of Atlantic salmon. Overall, our results elucidate the transcriptome mechanisms that contribute to short-term heat tolerance in the liver of Atlantic salmon.

Crosstalk Between Oxidative Stress and Endoplasmic Reticulum (ER) Stress in Endothelial Dysfunction and Aberrant Angiogenesis Associated With Diabetes: A Focus on the Protective Roles of Heme Oxygenase (HO)-1

Type-2 diabetes prevalence is continuing to rise worldwide due to physical inactivity and obesity epidemic. Diabetes and fluctuations of blood sugar are related to multiple micro- and macrovascular complications, that are attributed to oxidative stress, endoplasmic reticulum (ER) activation and inflammatory processes, which lead to endothelial dysfunction characterized, among other features, by reduced availability of nitric oxide (NO) and aberrant angiogenic capacity. Several enzymatic anti-oxidant and anti-inflammatory agents have been found to play protective roles against oxidative stress and its downstream signaling pathways. Of particular interest, heme oxygenase (HO) isoforms, specifically HO-1, have attracted much attention as major cytoprotective players in conditions associated with inflammation and oxidative stress. HO operates as a key rate-limiting enzyme in the process of degradation of the iron-containing molecule, heme, yielding the following byproducts: carbon monoxide (CO), iron, and biliverdin. Because HO-1 induction was linked to pro-oxidant states, it has been regarded as a marker of oxidative stress; however, accumulating evidence has established multiple cytoprotective roles of the enzyme in metabolic and cardiovascular disorders. The cytoprotective effects of HO-1 depend on several cellular mechanisms including the generation of bilirubin, an anti-oxidant molecule, from the degradation of heme; the induction of ferritin, a strong chelator of free iron; and the release of CO, that displays multiple anti-inflammatory and anti-apoptotic actions. The current review article describes the major molecular mechanisms contributing to endothelial dysfunction and altered angiogenesis in diabetes with a special focus on the interplay between oxidative stress and ER stress response. The review summarizes the key cytoprotective roles of HO-1 against hyperglycemia-induced endothelial dysfunction and aberrant angiogenesis and discusses the major underlying cellular mechanisms associated with its protective effects.