The physiological role of the unfolded protein response in the nervous system

The unfolded protein response(UPR)is a cellular stress response pathway activated when the endoplasmic reticulum,a crucial organelle for protein folding and modification,encounters an accumulation of unfolded or misfolded proteins.The UPR aims to restore endoplasmic reticulum homeostasis by enhancing protein folding capacity,reducing protein biosynthesis,and promoting protein degradation.It also plays a pivotal role in coordinating signaling cascades to determine cell fate and function in response to endoplasmic reticulum stress.Recent research has highlighted the significance of the UPR not only in maintaining endoplasmic reticulum homeostasis but also in influencing various physiological processes in the nervous system.Here,we provide an overview of recent findings that underscore the UPR’s involvement in preserving the function and viability of neuronal and myelinating cells under physiological conditions,and highlight the critical role of the UPR in brain development,memory storage,retinal cone development,myelination,and maintenance of myelin thickness.

Mesenchymal stromal cells modulate unfolded protein response and preserve β-cell mass in type 1 diabetes

Introduction:Transplantation of mesenchymal stromal cells(MSCs)is a promising therapy for type 1 diabetes(T1D).However,whether the infused MSCs affect the endoplasmic reticulum stress or subsequent unfolded protein response inβcells remains unclear.Methods:To investigate this,we induced early-onset T1D in non-obese diabetic mice using streptozotocin.Subsequently,T1D mice were randomly assigned to receive either MSCs or phosphate-buffered saline.We observed the in vivo homing of MSCs and assessed their effectiveness by analyzing blood glucose levels,body weight,histopathology,pancreatic protein expression,and serum levels of cytokines,proinsulin,and C-peptide.Results:Infused MSCs were found in the lungs,liver,spleen,and pancreas of T1D mice.They exhibited various effects,including reducing blood glucose levels,regulating immunity,inhibiting inflammation,increasingβ-cell areas,and reducing the expression of key proteins in the unfolded protein response pathway.Fasting serum proinsulin and C-peptide levels were significantly higher in the MSCs treatment group than in the T1D model group.However,there was no significant difference in the biomarker ofβ-cell endoplasmic reticulum stress,the ratio of fasting serum proinsulin to C-peptide,between the two groups.Conclusion:Ourfindings reveal that MSCs infusion does not alleviate endoplasmic reticulum stress inβcells directly but modulates the unfolded protein response pathway to preserveβ-cell mass and function in T1D mice.

Hypoxia Affects Autophagy in Human Umbilical Vein Endothelial Cells via the IRE1 Unfolded Protein Response

Objective:The purpose of this study was to investigate the role of the unfolded protein response,specifically the inositol-requiring enzyme 1(IRE1)signaling pathway,in hypoxia-induced autophagy in human umbilical venous endothelial cells(HUVECs).Methods:The expression of IRE1 and autophagy relative protein in HUVECs with hypoxia was explored by Western blotting,qRT-PCR and confocal microscopy.Further,we evaluated the biological effects of HUVECs by tube formation assay and wound healing assay in vitro.Finally,we examined the function of IRE1 in local blood vessels through animal models,Results:Hypoxia activated the IRE1 signaling pathway and induced autophagy in a time-dependent manner in HUVECs and further influenced the biological effects of HUVECs.Intraperitoneal injection of IRE1 inhibitors inhibited local vascular autophagy levels and lipid accumulation in model animals.Conclusion:Hypoxia can induce autophagy and activate the IRE1 signaling pathway in HUVECs and the IRE1 signaling pathway is involved in autophagy in hypoxic conditions.

The Role and Mechanism of Unfolded Protein Response Pathway in Tumor Drug Resistance

In the process of tumor proliferation and metastasis,tumor cells encounter hypoxia,low glucose,acidosis,and other stressful environments.These conditions prompt tumor cells to generate endoplasmic reticulum stress(ERS).As a signal mechanism that mitigates ERS in eukaryotic cells,the unfolded protein response(UPR)pathway can activate cells and tissues,regulating pathological activities in various cells,and maintaining ER homeostasis.It forms the most crucial adaptive and defensive mechanism for cells.However,under the continuous influence of chemotherapy drugs,the quantity of unfolded proteins and erroneous proteins produced by tumor cells significantly increases,surpassing the normal regulatory range of UPR.Consequently,ERS fails to function properly,fostering tumor cell proliferation and the development of drug resistance.This review delves into the study of three UPR pathways(PERK,IRE1,and ATF6),elucidating the mechanisms of drug resistance and research progress in the signal transduction pathway of UPR related to cancers.It provides a profound understanding of the role and relationship between UPR and anti-tumor drugs,offering a new direction for effective clinical treatment.

Acinar cell injury induced by inadequate unfolded protein response in acute pancreatitis

Acute pancreatitis (AP) is an inflammatory disorder of pancreatic tissue initiated in injured acinar cells. Severe AP remains a significant challenge due to the lack of effective treatment. The widely-accepted autodigestion theory of AP is now facing challenges, since inhibiting protease activation has negligible effectiveness for AP treatment despite numerous efforts. Furthermore, accumulating evidence supports a new concept that malfunction of a self-protective mechanism, the unfolded protein response(UPR), is the driving force behind the pathogenesis of AP. The UPR is induced by endoplasmic reticulum(ER) stress, a disturbance frequently found in acinar cells, to prevent the aggravation of ER stress that can otherwise lead to cell injury. In addition, the UPR's signaling pathways control NFκB activation and autophagy flux, and these dysregulations cause acinar cell inflammatory injury in AP, but with poorly understood mechanisms. We therefore summarize the protective role of the UPR in AP, propose mechanistic models of how inadequate UPR could promote NFκB's pro-inflammatory activity and impair autophagy's protective function in acinar cells, and discuss its relevance to current AP treatment. We hope that insight provided in this review will help facilitate the research and management of AP.

The unfolded protein response signaling and retinal Müller cell metabolism

The retina is one of the most energy demanding tissues in the body. Like most neurons in the central nervous system, retinal neurons consume high amounts of adenosine-5′-triphosphate（ATP） to generate visual signal and transmit the information to the brain. Disruptions in retinal metabolism can cause neuronal dysfunction and degeneration resulting in severe visual impairment and even blindness. The homeostasis of retinal metabolism is tightly controlled by multiple signaling pathways, such as the unfolded protein response（UPR）, and the close interactions between retinal neurons and other retinal cell types including vascular cells and Müller glia. The UPR is a highly conserved adaptive cellular response and can be triggered by many physiological stressors and pathophysiological conditions. Activation of the UPR leads to changes in glycolytic rate, ATP production, de novo serine synthesis, and the hexosamine biosynthetic pathway, which are considered critical components of Müller glia metabolism and provide metabolic support to surrounding neurons. When these pathways are disrupted, neurodegeneration occurs rapidly. In this review, we summarize recent advance in studies of the UPR in Müller glia and highlight the potential role of the UPR in retinal degeneration through regulation of Müller glia metabolism.

Roles of mitochondrial unfolded protein response in mammalian stem cells

The mitochondrial unfolded protein response(UPRmt)is an evolutionarily conserved adaptive mechanism for improving cell survival under mitochondrial stress.Under physiological and pathological conditions,the UPRmt is the key to maintaining intracellular homeostasis and proteostasis.Important roles of the UPRmt have been demonstrated in a variety of cell types and in cell development,metabolism,and immune processes.UPRmt dysfunction leads to a variety of pathologies,including cancer,inflammation,neurodegenerative disease,metabolic disease,and immune disease.Stem cells have a special ability to selfrenew and differentiate into a variety of somatic cells and have been shown to exist in a variety of tissues.These cells are involved in development,tissue renewal,and some disease processes.Although the roles and regulatory mechanisms of the UPRmt in somatic cells have been widely reported,the roles of the UPRmt in stem cells are not fully understood.The roles and functions of the UPRmt depend on stem cell type.Therefore,this paper summarizes the potential significance of the UPRmt in embryonic stem cells,tissue stem cells,tumor stem cells,and induced pluripotent stem cells.The purpose of this review is to provide new insights into stem cell differentiation and tumor pathogenesis.

Role of unfolded protein response in lipogenesis

The signal transduction network in regulating lipid metabolism is a hot topic of biomedical research. Recent research endeavors reveal that intracellular stress signaling from a cellular organelle called endoplasmic reticulum (ER) is critically involved in lipid homeostasis and the development of metabolic disease. The ER is a site where newly-synthesized proteins are folded and assembled into their three-dimensional structures, modified and transported to their precise cellular destinations. A wide range of biochemical, physiological and pathological stimuli can interrupt the protein folding process in the ER and cause accumulation of unfolded or misfolded proteins in the ER lumen, a condition referred to as ER stress. To cope with this stress condition, the ER has evolved highly-specifi c signaling pathways collectively termed Unfolded Protein Response (UPR) or ER stress response. The UPR regulates transcriptionaland translational programs, affecting broad aspects of cellular metabolism and cell fate. Lipogenesis, the metabolic process of de novo lipid biosynthesis, occurs primarily in the liver where metabolic signals regulate expression of key enzymes in glycolytic and lipogenic pathways. Recent studies suggest that the UPR plays crucial roles in modulating lipogenesis under metabolic conditions. Here we address some of recent representative evidence regarding the role of the UPR in lipogenesis.

3-52 Unfolded Protein Response Induced by X-rays in Breast Cancer Cells

Expand ER stress is triggered due to the loss of homeostasis in the ER which causes the accumulation ofmisfolded proteins within the ER lumen. Severe or prolonged ER stress may induce the unfolded protein response(UPR), which is an adaptive mechanism aimed at reducing levels of unfolded proteins and keeping balance in theER. CHOP, Bip, JNK, EIF2 are major elements in these pathways.In this study, we investigated the activation of CHOP, Bip, total JNK and phosphorylated JNK (P-JNK), totalEIF2 and phosphorylated EIF2 (P- EIF2) in response to X-rays in breast cancer MCF-7 and MDA-MB-231 cellsusing western blot analysis.

A vicinal oxygen chelate protein facilitates viral infection by triggering the unfolded protein response in Nicotiana benthamiana

Vicinal oxygen chelate(VOC)proteins are members of an enzyme superfamily with dioxygenase or non-dioxygenase activities.However,the biological functions of VOC proteins in plants are poorly understood.Here,we show that a VOC in Nicotiana benthamiana(NbVOC1)facilitates viral infection.NbVOC1 was significantly induced by infection by beet necrotic yellow vein virus(BNYVV).Transient overexpression of NbVOC1 or its homolog from Beta vulgaris(BvVOC1)enhanced BNYVV infection in N.benthamiana,which required the nuclear localization of VOC1.Consistent with this result,overexpressing NbVOC1 facilitated BNYVV infection,whereas,knockdown and knockout of NbVOC1 inhibited BNYVV infection in transgenic N.benthamiana plants.NbVOC1 interacts with the basic leucine zipper transcription factors bZIP17/28,which enhances their self-interaction and DNA binding to the promoters of unfolded protein response(UPR)-related genes.We propose that bZIP17/28 directly binds to the NbVOC1 promoter and induces its transcription,forming a positive feedback loop to induce the UPR and facilitating BNYVV infection.Collectively,our results demonstrate that NbVOC1 positively regulates the UPR that enhances viral infection in plants.

New insights into the unfolded protein response(UPR)-anterior gradient 2(AGR2)pathway in the regulation of intestinal barrier function in weaned piglets

Sustained dysfunction of the intestinal barrier caused by early weaning is a major factor that induces postweaning diarrhea in weaned piglets.In both healthy and diseased states,the intestinal barrier is regulated by goblet cells.Alterations in the characteristics of goblet cells are linked to intestinal barrier dysfunction and inflammatory conditions during pathogenic infections.In this review,we summarize the current understanding of the mechanisms of the unfolded protein response(UPR)and anterior gradient2(AGR2)in maintaining intestinal barrier function and how modifications to these systems affect mucus barrier characteristics and goblet cell dysregulation.We highlight a novel mechanism underlying the UPR-AGR2 pathway,which affects goblet cell differentiation and maturation and the synthesis and secretion of mucin by regulating epidermal growth factor receptor and mucin 2.This study provides a theoretical basis and new insights into the regulation of intestinal health in weaned piglets.

Mannogalactoglucan from mushrooms protects pancreatic islets via restoring UPR and promotes insulin secretion in TIDM mice

Type 1 diabetes mellitus(T1DM) lacks insulin secretion due to autoimmune deficiency of pancreaticβ-cells.Protecting pancreatic islets and enhancing insulin secretion has been therapeutic approaches.Mannogalactoglucan is the main type of polysaccharide from natural mushroom,which has potential medicinal prospects.Nevertheless,the antidiabetic property of mannogalactoglucan in T1DM has not been fully elucidated.In this study,we obtained the neutral fraction of alkali-soluble Armillaria mellea polysaccharide(AAMP-N) with the structure of mannogalactoglucan from the fruiting body of A.mellea and investigated the potential therapeutic value of AAMP-N in T1DM.We demonstrated that AAMP-N lowered blood glucose and improved diabetes symptoms in T1DM mice.AAMP-N activated unfolded protein response(UPR) signaling pathway to maintain ER protein folding homeostasis and promote insulin secretion in vivo.Besides that,AAMP-N promoted insulin synthesis via upregulating the expression of transcription factors,increased Ca^(2+) signals to stimulate intracellular insulin secretory vesicle transport via activating calcium/calmodulin-dependent kinase Ⅱ(CamkⅡ) and cAMP/PKA signals,and enhanced insulin secretory vesicle fusion with the plasma membrane via vesicle-associated membrane protein 2(VAMP2).Collectively,these studies demonstrated that the therapeutic potential of AAMP-N on pancreatic islets function,indicating that mannogalactoglucan could be natural nutraceutical used for the treatment of T1DM.

Z α-1 antitrypsin deficiency and the endoplasmic reticulum stress response

The serine proteinase inhibitor α-1 antitrypsin(AAT) is produced principally by the liver at the rate of 2 g/d.It is secreted into the circulation and provides an antiprotease protective screen throughout the body but most importantly in the lung,where it can neutralise the activity of the serine protease neutrophil elastase.Mutations leading to def iciency in AAT are associated with liver and lung disease.The most notable is the Z AAT mutation,which encodes a misfolded variant of the AAT protein in which the glutamic acid at position 342 is replaced by a lysine.More than 95% of all individuals with AAT def iciency carry at least one Z allele.ZAAT protein is not secreted effectively and accumulates intracellularly in the endoplasmic reticulum(ER) of hepatocytes and other AAT-producing cells.This results in a loss of function associated with decreased circulating and intrapulmonary levels of AAT.However,the misfolded protein acquires a toxic gain of function that impacts on the ER.A major function of the ER is to ensure correct protein folding.ZAAT interferes with this function and promotes ER stress responses and inflammation.Here the signalling pathways activated during ER stress in response to accumulation of ZAAT are described and therapeutic strategies that can potentially relieve ER stress are discussed.

UPRmt改善乙醇诱导的肝细胞坏死性凋亡的分子机制研究

背景坏死性凋亡作为一种新型程序性细胞死亡方式参与酒精性肝病的发生发展。线粒体未折叠的蛋白质反应(mitochondrial unfolded protein response,UPR^(mt))能够促进应激反应中细胞修复并改善线粒体网络的调控,探究乙醇诱导下肝细胞内UPR^(mt)调控机制可能为酒精性肝病的临床治疗提供新的潜在靶点。目的探讨UPR^(mt)对乙醇诱导下肝细胞线粒体功能和坏死性凋亡的影响,及其在肝细胞线粒体网络中的作用机制。方法利用正常小鼠肝细胞AML12构建正常对照组、乙醇组、UPR^(mt)激活对照组、UPR^(mt)激活乙醇组模型。经浓度250 mmol/L无水乙醇培养细胞构建乙醇组模型,通过造模前6 h给予10μmol/L寡霉素A激活正常小鼠肝细胞UPR^(mt)构建干预组。利用RT-PCR检测UPR^(mt)、线粒体分裂和坏死性凋亡相关基因转录水平,荧光探针观察线粒体功能,蛋白质印迹法检测自噬相关蛋白表达水平。结果乙醇诱导下UPR^(mt)相关基因mtDNAj、CHOP、ATF5和炎症因子TNF-α、IL-6、Timp1转录水平升高,坏死性凋亡关键基因RIPK3、PGAM5表达增加(P<0.05)。荧光探针观察到乙醇诱导下线粒体膜电位显著下降,线粒体ROS产生量增多(P<0.05),蛋白免疫印迹结果显示乙醇诱导下肝细胞内线粒体自噬被抑制,线粒体分裂增加。寡霉素A干预增强细胞内UPR^(mt),从而改善乙醇诱导下炎症产生和氧化应激,维持线粒体正常功能,抑制肝细胞坏死性凋亡。结论UPR^(mt)通过减少细胞氧化应激、维持线粒体正常功能,从而缓解乙醇诱导的肝细胞坏死性凋亡和炎症损伤。

Non-ionizing radiofrequency field induces unfolded protein response (UPR) in endoplasmic reticulum of mouse neuronal cells

Objective:To examine whether exposure of mouse neuronal cells to radiofrequency fields used in mobile communication devices can induce stress in endoplasmic reticulum(ER)and activate unfolded protein response(UPR).Methods:HT22 mouse hippocampus neuronal cells were exposed to continuous wave 900 MHz radiofrequency fields(RF)at 120μW/cm2 power intensity for 4 h/d for 5 consecutive days.The positive control cells were irradiated with 4 Gy of 60Coγ-rays at a dose rate of 0.5 Gy/min(GR).Twenty-four hours after the last exposure,cells were collected,and the expressions of sensor transmembrane proteins were detected using Western blot analysis.Results:The expression levels of Ire1,PERK,p-IRE1 and p-PERK,GRP78 and CHOP proteins were detected.There were no statistically significant differences in the expression levels of IRE1 and PERK proteins in control(CT),sham(SH)-,RF-and GR-exposed cells(P<0.05).The phosphorylated protein levels of p-IRE1 and p-PERK were significantly increased in cells exposed to RF and GR(P<0.05).The expression levels of GRP78 and CHOP were significantly increased in RF-and GR-exposed cells compared to CT and SH-exposed cells(P<0.05).Cells treated with 1μg/ml TM for 24 h showed significantly increased expression levels of GRP78 and CHOP proteins compared to controls(P<0.05).In the presence of 2 mmol/L PBA,TM-induced increased levels of GRP78 and CHOP proteins were reduced(P<0.05).Conclusions:The exposure of non-ionizing 900 MHz RF was able to cause stress in HT22 mouse neuronal cells and activated UPR in ER.Since UPR plays an important role in both cell survival(when UPR is mitigated)and apoptosis/death(under unresolvable stress conditions),further studies are required to determine the fate of the cells exposed to RF.

Ultrastructural variation and key ER chaperones response induced by heat stress in intestinal cells of sea cucumber Apostichopus japonicus

Abstract The unfolded protein response(UPR)is an important protective and compensatory strategy used during endoplasmic reticulum stress caused by factors including glucose starvation,low pH,or heat shock.However,there is very little information on the possible role(s)of the UPR under adverse conditions experienced by marine invertebrates.We observed that rough endoplasmic reticulum(ER)was dramatically expanded and numerous autophagosomes were accumulated in the intestinal cells of sea cucumbers,Apostichopus japonicus,under heat stress(4 h at 25°C compared with 15°C controls).Moreover,heat stress led to sharp increases in the relative transcript and protein expression levels of two primary ER chaperones:the endoplasmic reticulum resident protein 29-like(ERP29)and protein disulfi de-isomerase A6-like(PDIA6).These results suggest a potential adaptive mechanism to deal with heat-induced stress in sea cucumber intestine.

Subversion of cellular stress responses by poxviruses

Cellular stress responses are powerful mechanisms that prevent and cope with the accumulation of macromolecular damage in the cells and also boost host defenses against pathogens. Cells can initiate either protective or destructive stress responses depending, to a large extent, on the nature and duration of the stressing stimulus as well as the cell type. The productive replication of a virus within a given cell places inordinate stress on the metabolism machinery of the host and, to assure the continuity of its replication, many viruses have developed ways to modulate the cell stress responses. Poxviruses are among the viruses that have evolved a large number of strategies to manipulate host stress responses in order to control cell fate and enhance their replicative success. Remarkably, nearly every step of the stress responses that is mounted during infection can be targeted by virally encoded functions. The fine-tuned interactions between poxviruses and the host stress responses has aided virologists to understand specific aspects of viral replication; has helped cell biologists to evaluate the role of stress signaling in the uninfected cell; and has tipped immunologists on how these signals contribute to alert the cells against pathogen invasionand boost subsequent immune responses. This review discusses the diverse strategies that poxviruses use to subvert host cell stress responses.

Molecular signal networks and regulating mechanisms of the unfolded protein response

Within the cell, several mechanisms exist to maintain homeostasis of the endoplasmic reticulum （ER）. One of the primary mechanisms is the unfolded protein response （UPR）. In this review, we primarily focus on the latest signal webs and regulation mechanisms of the UPR. The relationships among ER stress, apoptosis, and cancer are also discussed. Under the normal state, binding immunoglobulin protein （BiP） interacts with the three sensors （protein kinase RNA-like ER kinase （PERK）, activating transcription factor 6 （ATF6）, and inositol-requiring enzyme la （IREla）） Under ER stress, misfolded proteins interact with BiP, resulting in the release of BiP from the sensors. Subsequently, the three sensors dimerize and autophosphorylate to promote the signal cascades of ER stress. ER stress includes a series of positive and negative feedback signals, such as those regulating the stabilization of the sensors/BiP complex, activating and inactivating the sensors by autophosphorylation and dephosphorylation, activating specific transcription factors to enable selective transcription, and augmenting the ability to refold and export. Apart from the three basic pathways, vascular endothelial growth factor （VEGF）-VEGF receptor （VEGFR）-phospholipase C-~ （PLCy）-mammalian target of rapamycin complex 1 （mTORC1） pathway, induced only in solid tumors, can also activate ATF6 and PERK signal cascades, and IREla also can be activated by activated RAC-alpha serine/threonine-protein kinase （AKT）. A moderate UPR functions as a pro-survival signal to return the cell to its state of homeostasis. However, persistent ER stress will induce cells to undergo apoptosis in response to increasing reactive oxygen species （ROS）, Ca2＋ in the cytoplasmic matrix, and other apoptosis signal cascades, such as c-Jun N-terminal kinase （JNK）, signal transducer and activator of transcription 3 （STAT3）, and P38, when cellular damage exceeds the capacity of this adaptive response.

慢性疼痛中内质网应激机制的研究进展

慢性疼痛作为公共卫生难题,其发病机制复杂,涉及脊髓神经元兴奋、胶质细胞激活及受体活化等。药物治疗虽能缓解疼痛,但不良反应限制了其应用。研究表明,内质网应激在慢性疼痛中扮演关键角色,通过影响疼痛感受器敏感性、调控伤害信号传递、触发炎症反应及神经可塑性改变,加剧疼痛并促进其发展。本文综述了内质网蛋白激酶样内切割酶(PKR-like endoplasmic reticulum kinase,PERK)、内质网应激调节因子1α (inositol-requiring enzyme 1α, IRE1α)和激活转录因子6 (activating transcription factor 6, ATF6)等通路在内质网应激与慢性疼痛中的具体机制,旨在为其深入研究和临床应用提供科学支撑,并探讨尚未解决的问题及未来发展方向。

Porcine circovirus type 2 capsid protein induces unfolded protein response with subsequent activation of apoptosis

Porcine circovirus type 2（PCV2）has recently been reported to elicit the unfolded protein response（UPR）via activation of the PERK/e IF2α（RNA-activated protein kinase-like endoplasmic reticulum（ER）kinase/eukaryotic initiation factor 2α）pathway.This study attempted to examine which viral protein might be involved in inducing UPR and whether this cellular event would lead to apoptosis of the cells expressing the viral protein.By transient expression,we found that both replicase（Rep）and capsid（Cap）proteins of PCV2 could induce ER stress as shown by increased phosphorylation of PERK with subsequent activation of the eI F2α-ATF4（activating transcription factor 4）-CHOP（CCAAT/enhancer-binding protein homologous protein）axis.Cap expression,but not Rep,significantly reduced antiapoptotic B-cell lymphoma-2（Bcl-2）and increased caspase-3 cleavage,possibly due to increased expression of CHOP.Since knockdown of PERK by RNA interference clearly reduced Cap-induced CHOP expression,caspase-3cleavage,and apoptotic cell death possibly by partially rescuing Bcl-2 expression,we propose that there is connection between Cap-induced UPR and apoptosis via the PERK/eI F2α/ATF4/CHOP/Bcl-2 pathway.This study,together with our earlier studies,provides insight into the mechanisms underlying PCV2 pathogenesis.