Disturbed activation of endoplasmic reticulum stress transducers by familial Alzheimer's disease-linked presenilin-1 mutations

Endoplasmic reticulum's role in multiple sclerosis, exploring potential biomarkers, and pioneering therapeutic strategies: a comprehensive review of literature

BACKGROUND

Multiple Sclerosis (MS) is a complex and chronic autoimmune disease that affects the central nervous system. Inflammation and demyelination characterize it, which results in a range of neurological impairments. The increasing worldwide occurrence of MS, affecting an estimated 2.8 million individuals in 2020, highlights the urgent requirement for further research to tackle the significant impact it has on individuals and healthcare systems globally.

OBJECTIVE

In this study, we wanted to explore the complex function of the endoplasmic reticulum (ER) in the origin, development, and resolution of MS, emphasizing its importance in neuroinflammatory illnesses. The ER has become a central focus in comprehending the pathogenesis of MS. Upon reviewing the literature, we observed a lack of thorough analysis that explores the involvement of endoplasmic reticulum stress in multiple sclerosis. Thus, we aimed through this research to examine the correlations between ER stress and its influence on immunological dysregulation, demyelination, and neurodegeneration in MS.

FINDINGS

Based on the latest clinical trials, we suggested theories that explore possible biomarkers linked to ER stress and the unfolded protein response. Identifying molecules that are suggestive of early stages of illness and can serve as prognostic tools for improving our understanding of the heterogeneity of MS and offering novel approaches for managing the disease. Finally, through our comprehensive search, we wanted to offer a plan for future research, suggesting new and creative methods for managing MS and encouraging the creation of specific treatments that aim to reduce the impact of MS on individuals worldwide.

Small-molecule inhibitors targeting apoptosis signal-regulated kinase 1

Apoptosis signal regulated kinase 1 (ASK1, also known as MAP3K5) is a member of the mitogen activated protein kinase kinase kinase (MAP3K) family. Since its first isolation from a human macrophage library in 1996, its research has been ongoing for over 25 years. A large number of reports have revealed that ASK1, as a key activator of the p38 mitogen-activated protein kinase and c-Jun N-terminal kinase (JNK) signaling cascade, responds to various stressors, and its inhibitors have important potential value in the treatment of diseases such as inflammation, cancer, and the nervous system and so on. This review summarizes the recent development in this field, including the structure and signaling pathways of ASK1, with a particular focus on the structure-activity relationships, and the hit-to-lead optimization strategies.

Modulating endoplasmic reticulum stress in APP/PS1 mice by Gomisin B and Osthole in Bushen-Yizhi formula: Synergistic effects and therapeutic implications for Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a prevalent neurodegenerative disorder with no effective cure. Targeting endoplasmic reticulum (ER) stress pathway may offer a novel approach to ameliorate cognitive deficits in AD. Bushen-Yizhi formula (BSYZ), a traditional Chinese medicine (TCM) prescription, has shown potential benefits for AD. To facilitate the development of new therapeutic agents for AD, it is important to identify the active components and the underlying mechanisms of BSYZ against AD.

PURPOSE

The aim of this study was to systematically screen the active components of BSYZ that could improve learning and memory impairment in AD by modulating ER stress pathway.

METHODS

A drug-target (D-T) network was constructed to analyze the herbal components of BSYZ. Network proximity method was used to identify the potential anti-AD components that targeted ER stress and evaluate their synergistic effects. The absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties and the literature evidence were considered to select promising candidates for further validation. The selected components were tested in vitro using an AD cell model (APPswe-SH-SY5Y). In vivo anti-AD effects of the components were assessed in APP/PS1 double-transgenic mice.

RESULTS

58 potential anti-AD components targeting ER stress were detected by network proximity analysis, and 13 out of them were selected based on ADMET properties and literature evidence. In vitro experiments confirmed that 5 components, namely gomisin B, β-Carotene, imperatorin, chrysophanol, and osthole (OST), exhibited anti-AD effects on the APPswe-SH-SY5Y model. Moreover, network proximity analysis suggested that OST and Gomisin B might have synergistic effects on modulating ER stress. In vivo experiments demonstrated that OST, Gomisin B, OST+Gomisin B, and BSYZ all improved learning and memory function in APP/PS1 mice. Gomisin B and OST also restored cellular morphology and tissue structure in APP/PS1 mice. Thioflavine-S (Th-S) staining revealed that they reduced amyloid plaque deposition in the brain tissue of AD model mice. The qPCR results indicated that BSYZ, OST, and Gomisin B differentially regulated IRE1α, PERK, EIF2α, DDIT3, and Caspase 12 expression levels, while the OST and Gomisin B co-administration group showed better efficacy. This trend was further confirmed by immunofluorescence experiments.

CONCLUSION

This study identified the active components of BSYZ that could ameliorate learning and memory impairment in AD by targeting ER stress pathway. OST and Gomisin B exhibited synergistic effects on modulating ER stress and reducing amyloid plaque deposition in vivo. Overall, our study elucidated the molecular mechanisms of BSYZ and its active components in attenuating AD symptoms which suggested the therapeutic potential of TCM for AD.

Endoplasmic reticulum stress in Alzheimer's disease: Molecular mechanisms and therapeutic prospects

Alzheimer's disease (AD) is a progressive neurodegenerative condition that leads to memory loss and cognitive impairment over time. It is characterized by protein misfolding as well as prolonged cellular stress, such as perturbing calcium homeostasis and redox management. Numerous investigations have proven that endoplasmic reticulum failure may exhibit exacerbation of AD pathogenesis in AD patients, in-vivo and in-vitro models. The endoplasmic reticulum (ER) participates in a variety of biological functions including folding of protein, quality control, cholesterol production, and maintenance of calcium balance. A diverse range of physiological, pathological and pharmacological substances can interfere with ER activity and thus lead to exaggeration of ER stress. The unfolded protein response (UPR), an intracellular signaling network is stimulated due to ER stress. Three stress sensors found in the endoplasmic reticulum, the PERK, ATF6, and IRE1 transducers detect protein misfolding in the ER and trigger UPR, a complex system to maintain homeostasis. ER stress is linked to many of the major pathological processes that are seen in AD, including presenilin1 and 2 (PS1 and PS2) gene mutation, tau phosphorylation and β-amyloid formation. The role of ER stress and UPR in the pathophysiology of AD implies that they can be employed as potent therapeutic target. This study shows the relationship between ER and AD and how the pathogenesis of AD is influenced by the impact of ER stress. An effective method for the prevention or treatment of AD may involve therapeutic strategies that modify ER stress pathways.

The Unfolded Protein Response: A Double-Edged Sword for Brain Health

Efficient brain function requires as much as 20% of the total oxygen intake to support normal neuronal cell function. This level of oxygen usage, however, leads to the generation of free radicals, and thus can lead to oxidative stress and potentially to age-related cognitive decay and even neurodegenerative diseases. The regulation of this system requires a complex monitoring network to maintain proper oxygen homeostasis. Furthermore, the high content of mitochondria in the brain has elevated glucose demands, and thus requires a normal redox balance. Maintaining this is mediated by adaptive stress response pathways that permit cells to survive oxidative stress and to minimize cellular damage. These stress pathways rely on the proper function of the endoplasmic reticulum (ER) and the activation of the unfolded protein response (UPR), a cellular pathway responsible for normal ER function and cell survival. Interestingly, the UPR has two opposing signaling pathways, one that promotes cell survival and one that induces apoptosis. In this narrative review, we discuss the opposing roles of the UPR signaling pathways and how a better understanding of these stress pathways could potentially allow for the development of effective strategies to prevent age-related cognitive decay as well as treat neurodegenerative diseases.

The PERK pathway: beneficial or detrimental for neurodegenerative diseases and tumor growth and cancer

Protein kinase R (PKR)-like endoplasmic reticulum (ER) kinase (PERK) is one of the three major sensors in the unfolded protein response (UPR). The UPR is involved in the modulation of protein synthesis as an adaptive response. Prolonged PERK activity correlates with the development of diseases and the attenuation of disease severity. Thus, the current debate focuses on the role of the PERK signaling pathway either in accelerating or preventing diseases such as neurodegenerative diseases, myelin disorders, and tumor growth and cancer. In this review, we examine the current findings on the PERK signaling pathway and whether it is beneficial or detrimental for the above-mentioned disorders.

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

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

Chaperone-Dependent Mechanisms as a Pharmacological Target for Neuroprotection

Modern pharmacotherapy of neurodegenerative diseases is predominantly symptomatic and does not allow vicious circles causing disease development to break. Protein misfolding is considered the most important pathogenetic factor of neurodegenerative diseases. Physiological mechanisms related to the function of chaperones, which contribute to the restoration of native conformation of functionally important proteins, evolved evolutionarily. These mechanisms can be considered promising for pharmacological regulation. Therefore, the aim of this review was to analyze the mechanisms of endoplasmic reticulum stress (ER stress) and unfolded protein response (UPR) in the pathogenesis of neurodegenerative diseases. Data on BiP and Sigma1R chaperones in clinical and experimental studies of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease are presented. The possibility of neuroprotective effect dependent on Sigma1R ligand activation in these diseases is also demonstrated. The interaction between Sigma1R and BiP-associated signaling in the neuroprotection is discussed. The performed analysis suggests the feasibility of pharmacological regulation of chaperone function, possibility of ligand activation of Sigma1R in order to achieve a neuroprotective effect, and the need for further studies of the conjugation of cellular mechanisms controlled by Sigma1R and BiP chaperones.

PS1 Affects the Pathology of Alzheimer's Disease by Regulating BACE1 Distribution in the ER and BACE1 Maturation in the Golgi Apparatus

Neuritic plaques are one of the major pathological hallmarks of Alzheimer's disease. They are formed by the aggregation of extracellular amyloid-β protein (Aβ), which is derived from the sequential cleavage of amyloid-β precursor protein (APP) by β- and γ-secretase. BACE1 is the main β-secretase in the pathogenic process of Alzheimer's disease, which is believed to be a rate-limiting step of Aβ production. Presenilin 1 (PS1) is the active center of the γ-secretase that participates in the APP hydrolysis process. Mutations in the PS1 gene (PSEN1) are the most common cause of early onset familial Alzheimer's disease (FAD). The PSEN1 mutations can alter the activity of γ-secretase on the cleavage of APP. Previous studies have shown that PSEN1 mutations increase the expression and activity of BACE1 and that BACE1 expression and activity are elevated in the brains of PSEN1 mutant knock-in mice, compared with wild-type mice, as well as in the cerebral cortex of FAD patients carrying PSEN1 mutations, compared with sporadic AD patients and controls. Here, we used a Psen1 knockout cell line and a PS1 inhibitor to show that PS1 affects the expression of BACE1 in vitro. Furthermore, we used sucrose gradient fractionation combined with western blotting to analyze the distribution of BACE1, combined with a time-lapse technique to show that PS1 upregulates the distribution and trafficking of BACE1 in the endoplasmic reticulum, Golgi, and endosomes. More importantly, we found that the PSEN1 mutant S170F increases the distribution of BACE1 in the endoplasmic reticulum and changes the ratio of mature BACE1 in the trans-Golgi network. The effect of PSEN1 mutations on BACE1 may contribute to determining the phenotype of early onset FAD.

The functions of IRE1α in neurodegenerative diseases: Beyond ER stress

Inositol-requiring enzyme 1 α (IRE1α) is a type I transmembrane protein that resides in the endoplasmic reticulum (ER). IRE1α, which is the primary sensor of ER stress, has been proven to maintain intracellular protein homeostasis by activating X-box binding protein 1 (XBP1). Further studies have revealed novel physiological functions of the IRE1α, such as its roles in mRNA and protein degradation, inflammation, immunity, cell proliferation and cell death. Therefore, the function of IRE1α is not limited to its role in ER stress; IRE1α is also important for regulating other processes related to cellular physiology. Furthermore, IRE1α plays a key role in neurodegenerative diseases that are caused by the phosphorylation of Tau protein, the accumulation of α-synuclein (α-syn) and the toxic effects of mutant Huntingtin (mHtt). Therefore, targeting IRE1α is a valuable approach for treating neurodegenerative diseases and regulating cell functions. This review discusses the role of IRE1α in different cellular processes, and emphasizes the importance of IRE1α in neurodegenerative diseases.

ER stress and UPR in Alzheimer's disease: mechanisms, pathogenesis, treatments

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by gradual loss of memory and cognitive function, which constitutes a heavy burden on the healthcare system globally. Current therapeutics to interfere with the underlying disease process in AD is still under development. Although many efforts have centered on the toxic forms of Aβ to effectively tackle AD, considering the unsatisfactory results so far it is vital to examine other targets and therapeutic approaches as well. The endoplasmic reticulum (ER) stress refers to the build-up of unfolded or misfolded proteins within the ER, thus, perturbing the ER and cellular homeostasis. Emerging evidence indicates that ER stress contributes to the onset and development of AD. A thorough elucidation of ER stress machinery in AD pathology may help to open up new therapeutic avenues in the management of this devastating condition to relieve the cognitive dementia symptoms. Herein, we aim at deciphering the unique role of ER stress in AD pathogenesis, reviewing key findings, and existing controversy in an attempt to summarize plausible therapeutic interventions in the management of AD pathophysiology.

Mechanistic Insights into Selective Autophagy Subtypes in Alzheimer's Disease

Eukaryotic cells possess a plethora of regulatory mechanisms to maintain homeostasis and ensure proper biochemical functionality. Autophagy, a central, conserved self-consuming process of the cell, ensures the timely degradation of damaged cellular components. Several studies have demonstrated the important roles of autophagy activation in mitigating neurodegenerative diseases, especially Alzheimer's disease (AD). However, surprisingly, activation of macroautophagy has not shown clinical efficacy. Hence, alternative strategies are urgently needed for AD therapy. In recent years, selective autophagy has been reported to be involved in AD pathology, and different subtypes have been identified, such as aggrephagy, mitophagy, reticulophagy, lipophagy, pexophagy, nucleophagy, lysophagy and ribophagy. By clarifying the underlying mechanisms governing these various subtypes, we may come to understand how to control autophagy to treat AD. In this review, we summarize the latest findings concerning the role of selective autophagy in the pathogenesis of AD. The evidence overwhelmingly suggests that selective autophagy is an active mechanism in AD pathology, and that regulating selective autophagy would be an effective strategy for controlling this pathogenesis.

Model Identifies Genetic Predisposition of Alzheimer's Disease as Key Decider in Cell Susceptibility to Stress

Accumulation of unfolded/misfolded proteins in neuronal cells perturbs endoplasmic reticulum homeostasis, triggering a stress cascade called unfolded protein response (UPR), markers of which are upregulated in Alzheimer’s disease (AD) brain specimens. We measured the UPR dynamic response in three human neuroblastoma cell lines overexpressing the wild-type and two familial AD (FAD)-associated mutant forms of amyloid precursor protein (APP), the Swedish and Swedish-Indiana mutations, using gene expression analysis. The results reveal a differential response to subsequent environmental stress depending on the genetic background, with cells overexpressing the Swedish variant of APP exhibiting the highest global response. We further developed a dynamic mathematical model of the UPR that describes the activation of the three branches of this stress response in response to unfolded protein accumulation. Model-based analysis of the experimental data suggests that the mutant cell lines experienced a higher protein load and subsequent magnitude of transcriptional activation compared to the cells overexpressing wild-type APP, pointing to higher susceptibility of mutation-carrying cells to stress. The model was then used to understand the effect of therapeutic agents salubrinal, lithium, and valproate on signalling through different UPR branches. This study proposes a novel integrated platform to support the development of therapeutics for AD.

Presenilin 1 Regulates Membrane Homeostatic Pathways that are Dysregulated in Alzheimer's Disease

Mutations in the PSEN1 gene, encoding presenilin 1 (PS1), are the most common cause of familial Alzheimer's disease (fAD). Since the first mutations in the PSEN1 gene were discovered more than 25 years ago, many postulated functions of PS1 have been investigated. The majority of earlier studies focused on its role as the catalytic component of the γ-secretase complex, which in concert with β site amyloid precursor protein cleaving enzyme 1 (BACE1), mediates the formation of Aβ from amyloid-β protein precursor (AβPP). Though mutant PS1 was originally considered to cause AD by promoting Aβ pathology through its protease function, it is now becoming clear that PS1 is a multifunctional protein involved in regulating membrane dynamics and protein trafficking. Therefore, through loss of these abilities, mutant PS1 has the potential to impair numerous cellular functions such as calcium flux, organization of proteins in different compartments, and protein turnover via vacuolar metabolism. Impaired calcium signaling, vacuolar dysfunction, mitochondrial dysfunction, and increased ER stress, among other related membrane-dependent disturbances, have been considered critical to the development and progression of AD. Given that PS1 plays a key regulatory role in all these processes, this review will describe the role of PS1 in different cellular compartments and provide an integrated view of how PS1 dysregulation (due to mutations or other causes) could result in impairment of various cellular processes and result in a "multi-hit", integrated pathological outcome that could contribute to the etiology of AD.

Exploring ER stress response in cellular aging and neuroinflammation in Alzheimer's disease

One evident hallmark of Alzheimer's disease (AD) is the irregular accumulation of proteins due to changes in proteostasis involving endoplasmic reticulum (ER) stress. To alleviate ER stress and reinstate proteostasis, cells undergo an integrated signaling cascade called the unfolded protein response (UPR) that reduces the number of misfolded proteins and inhibits abnormal protein accumulation. Aging is associated with changes in the expression of ER chaperones and folding enzymes, leading to the impairment of proteostasis, and accumulation of misfolded proteins. The disrupted initiation of UPR prevents the elimination of unfolded proteins, leading to ER stress. In AD, the accumulation of misfolded proteins caused by sustained cellular stress leads to neurodegeneration and neuronal death. Current research has revealed that ER stress can trigger an inflammatory response through diverse transducers of UPR. Although the involvement of a neuroinflammatory component in AD has been documented for decades, whether it is a contributing factor or part of the neurodegenerative events is so far unknown. Besides, a feedback loop occurs between neuroinflammation and ER stress, which is strongly associated with neurodegenerative processes in AD. In this review, we focus on the current research on ER stress and UPR in cellular aging and neuroinflammatory processes, leading to memory impairment and synapse dysfunction in AD.

The Role of PERK in Understanding Development of Neurodegenerative Diseases

Neurodegenerative diseases are an ever-increasing problem for the rapidly aging population. Despite this, our understanding of how these neurodegenerative diseases develop and progress, is in most cases, rudimentary. Protein kinase RNA (PKR)-like ER kinase (PERK) comprises one of three unfolded protein response pathways in which cells attempt to manage cellular stress. However, because of its role in the cellular stress response and the far-reaching implications of this pathway, error within the PERK pathway has been shown to lead to a variety of pathologies. Genetic and clinical studies show a correlation between failure of the PERK pathway in neural cells and the development of neurodegeneration, but the wide array of methodology of these studies is presenting conflicting narratives about the role of PERK in these affected systems. Because of the connection between PERK and pathology, PERK has become a high value target of study for understanding neurodegenerative diseases and potentially how to treat them. Here, we present a review of the literature indexed in PubMed of the PERK pathway and some of the complexities involved in investigating the protein's role in the development of neurodegenerative diseases as well as how it may act as a target for therapeutics.

Susceptibility to cellular stress in PS1 mutant N2a cells is associated with mitochondrial defects and altered calcium homeostasis

Presenilin 1 (PS1) mutations are the most common cause of familial Alzheimer's disease (FAD). PS1 also plays a role in cellular processes such as calcium homeostasis and autophagy. We hypothesized that mutant presenilins increase cellular vulnerability to stress. We stably expressed human PS1, mutant PS1E280A and mutant PS1Δ9 in mouse neuroblastoma N2a cells. We examined early signs of stress in different conditions: endoplasmic reticulum (ER) stress, calcium overload, oxidative stress, and Aβ 1-42 oligomers toxicity. Additionally, we induced autophagy via serum starvation. PS1 mutations did not have an effect in ER stress but PS1E280A mutation affected autophagy. PS1 overexpression influenced calcium homeostasis and generated mitochondrial calcium overload modifying mitochondrial function. However, the opening of the mitochondrial permeability transition pore (MPTP) was affected in PS1 mutants, being accelerated in PS1E280A and inhibited in PS1Δ9 cells. Altered autophagy in PS1E280A cells was neither modified by inhibition of γ-secretase, nor by ER calcium retention. MPTP opening was directly regulated by γ-secretase inhibitors independent on organelle calcium modulation, suggesting a novel direct role for PS1 and γ-secretase in mitochondrial stress. We identified intrinsic cellular vulnerability to stress in PS1 mutants associated simultaneously with both, autophagic and mitochondrial function, independent of Aβ pathology.

Critical review: involvement of endoplasmic reticulum stress in the aetiology of Alzheimer's disease

The endoplasmic reticulum (ER) stress response is regarded as an important process in the aetiology of Alzheimer's disease (AD). The accumulation of pathogenic misfolded proteins and the disruption of intracellular calcium (Ca²⁺) signalling are considered to be fundamental mechanisms that underlie the induction of ER stress, leading to neuronal cell death. Indeed, a number of studies have proposed molecular mechanisms linking ER stress to AD pathogenesis based on results from in vitro systems and AD mouse models. However, stress responsivity was largely different between each mouse model, even though all of these models display AD-related pathologies. While several reports have shown elevated ER stress responses in amyloid precursor protein (APP) and presenilin 1 (PS1) double-transgenic (Tg) AD mouse models, we and other groups, in contrast, observed no such ER stress response in APP-single-Tg or App-knockin mice. Therefore, it is debatable whether the ER stress observed in APP and PS1 double-Tg mice is due to AD pathology. From these findings, the roles of ER stress in AD pathogenesis needs to be carefully addressed in future studies. In this review, we summarize research detailing the relationship between ER stress and AD, and analyse the results in detail.

Amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD) are characterised by differential activation of ER stress pathways: focus on UPR target genes

The endoplasmic reticulum (ER) plays an important role in maintenance of proteostasis through the unfolded protein response (UPR), which is strongly activated in most neurodegenerative disorders. UPR signalling pathways mediated by IRE1α and ATF6 play a crucial role in the maintenance of ER homeostasis through the transactivation of an array of transcription factors. When activated, these transcription factors induce the expression of genes involved in protein folding and degradation with pro-survival effects. However, the specific contribution of these transcription factors to different neurodegenerative diseases remains poorly defined. Here, we characterised 44 target genes strongly influenced by XBP1 and ATF6 and quantified the expression of a subset of genes in the human post-mortem spinal cord from amyotrophic lateral sclerosis (ALS) cases and in the frontal and temporal cortex from frontotemporal lobar degeneration (FTLD) and Alzheimer's disease (AD) cases and controls. We found that IRE1α-XBP1 and ATF6 pathways were strongly activated both in ALS and AD. In ALS, XBP1 and ATF6 activation was confirmed by a substantial increase in the expression of both known and novel target genes involved particularly in co-chaperone activity and ER-associated degradation (ERAD) such as DNAJB9, SEL1L and OS9. In AD cases, a distinct pattern emerged, where targets involved in protein folding were more prominent, such as CANX, PDIA3 and PDIA6. These results reveal that both overlapping and disease-specific patterns of IRE1α-XBP1 and ATF6 target genes are activated in AD and ALS, which may be relevant to the development of new therapeutic strategies. Graphical abstract The endoplasmic reticulum (ER) plays an important role in maintenance of proteostasis through the unfolded protein response (UPR). Two major UPR signalling pathways are mediated by IRE1α and ATF6. Here, we demonstrate that these pathways activate differential gene sets in human post-mortem tissues derived from amyotrophic lateral sclerosis (ALS) compared to Alzheimer's disease (AD) cases. Our results identify IRE1α and ATF6 specific targets that can have major implications in the development of new therapeutic strategies and potential biomarkers.

Role of apoptosis in the Post-traumatic stress disorder model-single prolonged stressed rats

Post-traumatic stress disorder (PTSD) is a stress-related mental disorder which occurs following exposure to traumatic events. A number of brain neuroimaging studies have revealed that PTSD patients have reduced volume and abnormal functions in the hippocampus and the amygdala. However, the pathogenesis of abnormalities in certain brain regions, as induced by PTSD, remains unclear. Recent studies, using the single prolonged stress (SPS) model, an animal model of PTSD, have found that abnormal apoptosis in certain brain regions, including the hippocampus, the amygdala, and the medial prefrontal cortex (mPFC); these areas are closely associated with emotion and cognition. In this review, we summarize the mechanism of apoptosis in SPS rats, including the endoplasmic reticulum (ER) and the mitochondria pathways. For the ER pathway, three individual pathways: PERK, IRE1, and ATF6 showed different roles on apoptosis and neuroprotection. Three key factors are thought to be involved in the mitochondrial pathway and PTSD-induced apoptosis: corticosteroid receptors, apoptosis-related factors, and anti-apoptosis factors. We have investigated the role of these factors and have attempted to identify which factors of the pathways are more focused towards neuronal protection, and which are more direct towards apoptosis. We also discussed the role of autophagy and the specific differences between autophagy and apoptosis in SPS rats. Finally, we discussed emerging researches related to anti-apoptosis treatment, including PERK inhibitors, IRE1 inhibitors, and metformin; collectively, these were exciting, but limited, This review provides a summary of the current understanding of apoptosis in SPS rats and the potential anti-apoptosis treatment strategies for PTSD.

Modulation of AβPP and GSK3β by Endoplasmic Reticulum Stress and Involvement in Alzheimer's Disease

Alzheimer's disease (AD) is a dementia disease with neuronal loss and synaptic impairment. This impairment is caused, at least partly, by the generation of two main AD hallmarks, namely the hyperphosphorylated tau protein comprising neurofibrillary tangles and senile plaques containing amyloid-β (Aβ) peptides. The amyloid-β protein precursor (AβPP) and glycogen synthase kinase-3β (GSK3β) are two main proteins associated with AD and are closely correlated with these hallmarks. Recently, both of the proteins were reported to be modulated by endoplasmic reticulum stress (ERS) and are involved in the pathogenesis of AD. The mechanism of ERS plus the modulation of AβPP processing and GSK3β activity by ERS in AD are summarized and explored in this review.

XBP1 and PERK Have Distinct Roles in Aβ-Induced Pathology

Endoplasmic reticulum (ER) stress triggers multiple cellular signals to restore cellular function or induce proapoptosis that is altered in the brains of patients with Alzheimer's disease (AD). However, the role of ER stress in β-amyloid (Aβ)-induced AD pathology remains elusive, and data obtained from different animal models and under different experimental conditions are sometimes controversial. The current study conducted in vivo genetic experiments to systematically examine the distinct role of each ER stress effector during disease progression. Our results indicated that inositol-requiring enzyme 1 was activated before protein kinase RNA-like endoplasmic reticulum kinase (PERK) activation in Aβ42 transgenic flies. Proteasome activity played a key role in this sequential activation. Furthermore, our study separated learning deficits from early degeneration in Aβ-induced impairment by demonstrating that X-box binding protein 1 overexpression at an early stage reversed Aβ-induced early death without affecting learning performance in the Aβ42 transgenic flies. PERK activation was determined to only enhance Aβ-induced learning deficits. Moreover, proteasome overactivation was determined to delay PERK activation and improve learning deficits. Altogether, the findings of this study demonstrate the complex roles of ER stress during Aβ pathogenesis and the possibility of using different ER stress effectors as reporters to indicate the status of disease progression.

Alzheimer's disease pathology and the unfolded protein response: prospective pathways and therapeutic targets

Many vital interdependent cellular functions including proteostasis, lipogenesis and Ca homeostasis are executed by the endoplasmic reticulum (ER). Exogenous insults can impair ER performance: this must be rapidly corrected or cell death will ensue. Protective adaptations can boost the functional capacity of the ER and form the basis of the unfolded protein response (UPR). Activated in response to the accumulation of misfolded proteins, the UPR can halt protein translation while increasing protein-handling chaperones and the degradation of erroneous proteins through a conserved three-tier molecular cascade. However, prolonged activation of the UPR can result in the maladaptation of the system, resulting in the activation of inflammatory and apoptotic effectors. Recently, UPR and its involvement in neurodegenerative disease has attracted much interest and numerous potentially 'drugable' points of crosstalk are now emerging. Here, we summarize the functions of the ER and UPR, and highlight evidence for its potential role in the pathogenesis of Alzheimer's disease, before discussing several key targets with therapeutic potential.

Endoplasmic reticulum stress responses in mouse models of Alzheimer's disease: Overexpression paradigm versus knockin paradigm

Endoplasmic reticulum (ER) stress is believed to play an important role in the etiology of Alzheimer's disease (AD). The accumulation of misfolded proteins and perturbation of intracellular calcium homeostasis are thought to underlie the induction of ER stress, resulting in neuronal dysfunction and cell death. Several reports have shown an increased ER stress response in amyloid precursor protein (APP) and presenilin1 (PS1) double-transgenic (Tg) AD mouse models. However, whether the ER stress observed in these mouse models is actually caused by AD pathology remains unclear. APP and PS1 contain one and nine transmembrane domains, respectively, for which it has been postulated that overexpressed membrane proteins can become wedged in a misfolded configuration in ER membranes, thereby inducing nonspecific ER stress. Here, we used an App-knockin (KI) AD mouse model that accumulates amyloid-β (Aβ) peptide without overexpressing APP to investigate whether the ER stress response is heightened because of Aβ pathology. Thorough examinations indicated that no ER stress responses arose in App-KI or single APP-Tg mice. These results suggest that PS1 overexpression or mutation induced a nonspecific ER stress response that was independent of Aβ pathology in the double-Tg mice. Moreover, we observed no ER stress in a mouse model of tauopathy (P301S-Tau-Tg mice) at various ages, suggesting that ER stress is also not essential in tau pathology-induced neurodegeneration. We conclude that the role of ER stress in AD pathogenesis needs to be carefully addressed in future studies.

Amyloid is essential but insufficient for Alzheimer causation: addition of subcellular cofactors is required for dementia

OBJECTIVE

The aim of this study is to examine the hypotheses stating the importance of amyloid or of its oligomers in the pathogenesis of Alzheimer's disease (AD).

METHODS

Published studies were examined.

RESULTS

The importance of amyloid in the pathogenesis of AD is well established, yet accepting it as the main cause for AD is problematic, because amyloid-centric treatments have provided no clinical benefit and about one-third of cognitively normal, older persons have cerebral amyloid plaques. Also problematic is the alternative hypothesis that, instead of amyloid plaques, it is oligomers of amyloid precursor protein that cause AD.Evidence is presented suggesting amyloid/oligomers as necessary but insufficient causes of the dementia and that, for dementia to develop, requires the addition of cofactors known to be associated with AD. Those cofactors include several subcellular processes: mitochondrial impairments; the Wnt signaling system; the unfolded protein response; the ubiquitin proteasome system; the Notch signaling system; and tau, calcium, and oxidative damage.

CONCLUSIONS

A modified amyloid/oligomer hypothesis for the pathogenesis of AD is that activation of one or more of the aforementioned cofactors creates a burden of functional impairments that, in conjunction with amyloid/oligomers, now crosses a threshold of dysfunction that results in clinical dementia. Of considerable importance, several treatments that might reverse the activation of some of the subcellular processes are available, for example, lithium, pioglitazone, erythropoietin, and prazosin; they should be given in combination in a clinical trial to test their safety and efficacy. © 2017 John Wiley & Sons, Ltd.

Calcium Dysregulation in Alzheimer's Disease: A Target for New Drug Development

Alzheimer's disease (AD) is a devastating neurodegenerative disorder and the most common cause of dementia among aged people whose population is rapidly increasing. AD not only seriously affects the patient's physical health and quality of life, but also adds a heavy burden to the patient's family and society. It is urgent to understand AD pathogenesis and develop the means of prevention and treatment. AD is a chronic devastating neurodegenerative disease without effective treatment. Current approaches for management focus on helping patients relieve or delay the symptoms of cognitive dysfunction. The calcium ion (Ca²⁺) is an important second messenger in the function and structure of nerve cell circuits in the brain such as neuronal growth, exocytosis, as well as in synaptic and cognitive function. Increasing numbers of studies suggested that disruption of intracellular Ca²⁺ homeostasis, especially the abnormal and excessive Ca²⁺ release from the endoplasmic reticulum (ER) via the ryanodine receptor (RYR), plays important roles in orchestrating the dynamic of the neuropathology of AD and associated memory loss, cognitive dysfunction. Dantrolene, a known antagonist of the RYR and a clinically available drug to treat malignant hyperthermia, can ameliorate the abnormal Ca²+ release from the RYR in AD and the subsequent pathogenesis, such as increased β-secretase and γ-secretase activities, production of Amyloid-β 42 (Aβ 42) and its oligomer, impaired autophagy, synapse dysfunction, and memory loss. However, more studies are needed to confirm the efficacy and safety repurposing dantrolene as a therapeutic drug in AD.

The Role of BiP Retrieval by the KDEL Receptor in the Early Secretory Pathway and its Effect on Protein Quality Control and Neurodegeneration

Protein quality control in the early secretory pathway is a ubiquitous eukaryotic mechanism for adaptation to endoplasmic reticulum (ER) stress. An ER molecular chaperone, immunoglobulin heavy chain-binding protein (BiP), is one of the essential components in this process. BiP interacts with nascent proteins to facilitate their folding. BiP also plays an important role in preventing aggregation of misfolded proteins and regulating the ER stress response when cells suffer various injuries. BiP is a member of the 70-kDa heat shock protein (HSP70) family of molecular chaperones that resides in the ER. Interaction between BiP and unfolded proteins is mediated by a substrate-binding domain and a nucleotide-binding domain for ATPase activity, leading to protein folding and maturation. BiP also possesses a retrieval motif in its carboxyl terminal. When BiP is secreted from the ER, the Lys-Asp-Glu-Leu (KDEL) receptor in the post-ER compartments binds with the carboxyl terminal KDEL sequence of BiP and returns BiP to the ER via coat protein complex I (COPI) vesicular transport. Although yeast studies showed that BiP retrieval by the KDEL receptor is not essential in single cells, it is crucial for multicellular organisms, where some essential proteins require retrieval to facilitate folding and maturation. Experiments in knock-in mice expressing mutant BiP with the retrieval motif deleted revealed a unique role of BiP retrieval by the KDEL receptor in neuronal development and age-related neurodegeneration.

Endoplasmic reticulum stress and inflammation in the central nervous system

Persistent endoplasmic reticulum (ER) stress is thought to drive the pathology of many chronic disorders due to its potential to elicit aberrant inflammatory signaling and facilitate cell death. In neurodegenerative diseases, the accumulation of misfolded proteins and concomitant induction of ER stress in neurons contributes to neuronal dysfunction. In addition, ER stress responses induced in the surrounding neuroglia may promote disease progression by coordinating damaging inflammatory responses, which help fuel a neurotoxic milieu. Nevertheless, there still remains a gap in knowledge regarding the cell-specific mechanisms by which ER stress mediates neuroinflammation. In this review, we will discuss recently uncovered inflammatory pathways linked to the ER stress response. Moreover, we will summarize the present literature delineating how ER stress is generated in Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis, and Multiple Sclerosis, and highlight how ER stress and neuroinflammation intersect mechanistically within the central nervous system. The mechanisms by which stress-induced inflammation contributes to the pathogenesis and progression of neurodegenerative diseases remain poorly understood. Further examination of this interplay could present unappreciated insights into the development of neurodegenerative diseases, and reveal new therapeutic targets.

Concordance of Several Subcellular Interactions Initiates Alzheimer's Dementia: Their Reversal Requires Combination Treatment

The pathogenesis of Alzheimer's disease involves multiple pathways that, at the macrolevel, include decreased proliferation plus increased loss affecting neurons, astrocytes, and capillaries and, at the subcellular level, involve several elements: amyloid/amyloid precursor protein, presenilins, the unfolded protein response, the ubiquitin/proteasome system, the Wnt/catenin system, the Notch signaling system, mitochondria, mitophagy, calcium, and tau. Data presented show the intimate, anatomical interactions between neurons, astrocytes, and capillaries; the interactions between the several subcellular factors affecting those cells; and the treatments that are currently available and that might correct dysfunctions in the subcellular factors. Available treatments include lithium, valproate, pioglitazone, erythropoietin, and prazosin. Since the subcellular pathogenesis involves multiple interacting elements, combination treatment would be more effective than administration of a single drug directed at only 1 element. The overall purpose of this presentation is to describe the pathogenesis in detail and to explain the proposed treatments.

Molecular Mechanisms of IRE1α-ASK1 Pathway Reactions to Unfolded Protein Response in DRN Neurons of Post-Traumatic Stress Disorder Rats

The goal of this study was to further elucidate the molecular mechanisms of post-traumatic stress disorder (PTSD) pathogenesis and to provide experimental evidence for new drug targets for effective PTSD treatment. Expression changes of IRE1α, ASK1, and other downstream molecules of the IRE1α-ASK1 endoplasmic reticulum stress (ERS) signaling pathway were investigated. JNK, P38, CHOP, Bcl-2, and Bax were analyzed at both protein and mRNA levels of dorsal raphe nucleus (DRN) neurons of PTSD rats. The rat PTSD model was established via the single-prolonged stress (SPS) method. Animals were randomly divided into five groups: a normal control group, a 1-day SPS group, a 4-days SPS group, a 7-day SPS group, and a 14-day SPS group. Spatial memory and learning ability of rats were evaluated subsequent to SPS using the Morris water maze test. Changes of IRE1α expression in the control and SPS groups were detected via immunohistochemistry (IHC). Protein and mRNA expressions of IRE1α, ASK1, JNK, P38, CHOP, Bcl-2, and Bax in the control and SPS groups were detected via Western blot and RT-PCR, respectively. The Morris water maze test revealed significantly longer average escape latencies in all SPS groups compared to the control group. In the spatial probe test, the percentage of time spent in the target quadrant was significantly lower in the SPS groups compared to control. IHC revealed increased positive expression of IRE1α subsequent to SPS challenge, reaching maximal levels on days four and seven (P < 0.01), while significantly decreasing on day 14 (P < 0.01). Western blot and RT-PCR revealed that protein and mRNA expressions of IRE1α, ASK1, JNK, CHOP, and P38 were significantly increased compared to control, peaking on days one, four, and seven post-SPS before returning to previous levels. Compared to control, expressions of Bcl-2 and Bax presented an initial increasing tendency followed by a decrease. A peak of Bcl-2 expression appeared early on day one following SPS, then decreased to a steady level. Bax expression in the SPS groups remained constant during early stages after SPS (days one to three) compared to control; however, expression significantly increased on day four and maintained a high level. In summary, 1) SPS challenge significantly activated the IRE1α-ASK1-JNK and IRE1α-ASK1-P38 apoptosis-signaling pathways in DRN neurons of PTSD rats. This resulted in a cascade of downstream reactions and ultimately apoptosis of DRN neurons. 2) Increased expression of apoptosis-associated molecules Bcl-2 and Bax in DRN neurons following SPS challenge was revealed as a central mechanism, inducing apoptosis of DRN neurons in PTSD rats.

Interplay of endoplasmic reticulum stress and autophagy in neurodegenerative disorders

The common underlying feature of most neurodegenerative diseases such as Alzheimer disease (AD), prion diseases, Parkinson disease (PD), and amyotrophic lateral sclerosis (ALS) involves accumulation of misfolded proteins leading to initiation of endoplasmic reticulum (ER) stress and stimulation of the unfolded protein response (UPR). Additionally, ER stress more recently has been implicated in the pathogenesis of HIV-associated neurocognitive disorders (HAND). Autophagy plays an essential role in the clearance of aggregated toxic proteins and degradation of the damaged organelles. There is evidence that autophagy ameliorates ER stress by eliminating accumulated misfolded proteins. Both abnormal UPR and impaired autophagy have been implicated as a causative mechanism in the development of various neurodegenerative diseases. This review highlights recent advances in the field on the role of ER stress and autophagy in AD, prion diseases, PD, ALS and HAND with the involvement of key signaling pathways in these processes and implications for future development of therapeutic strategies.

Neuroprotection by Endoplasmic Reticulum Stress-Induced HRD1 and Chaperones: Possible Therapeutic Targets for Alzheimer's and Parkinson's Disease

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are neurodegenerative disorders with a severe medical and social impact. Further insights from clinical and scientific studies are essential to develop effective therapies. Various stresses on the endoplasmic reticulum (ER) cause unfolded/misfolded proteins to aggregate, initiating unfolded protein responses (UPR), one of which is the induction of neuronal cell death. Some of the pathogenic factors for AD and PD are associated with UPR. ER molecules such as ubiquitin ligases (E3s) and chaperones are also produced during UPR to degrade and refold aberrant proteins that accumulate in the ER. In this review, we examine the role of HMG-CoA reductase degradation protein 1 (HRD1) and the chaperone protein-disulfide isomerase (PDI), which are both produced in the ER in response to stress. We discuss the importance of HRD1 in degrading amyloid precursor protein (APP) and Parkin-associated endothelin receptor-like receptor (Pael-R) to protect against neuronal death. PDI and the chemical chaperone 4-phenyl-butyrate also exert neuroprotective effects. We discuss the pathophysiological roles of ER stress, UPR, and the induction and neuroprotective effects of HRD1 and PDI, which may represent significant targets for novel AD and PD therapies.

IRE1α-XBP1 is a novel branch in the transcriptional regulation of Ucp1 in brown adipocytes

The unfolded protein response (UPR) not only resolves endoplasmic reticulum (ER) stress, but also regulates cellular physiological functions. In this study, we first linked the UPR to the physiological roles of brown adipose tissue (BAT). BAT is one of the tissues that control energy homeostasis in the body. Brown adipocytes are able to dissipate energy in the form of heat owing to their mitochondrial protein, uncoupling protein 1 (UCP1). We found that one of the UPR branches, the IRE1α-XBP1 pathway, was activated during the transcriptional induction of Ucp1. Inhibiting the IRE1α-XBP1 pathway reduced the induction of Ucp1 expression. However, the activation of the IRE1α-XBP1 pathway by ER stress never upregulated Ucp1. On the other hand, the activation of protein kinase A (PKA) induced Ucp1 transcription through the activation of IRE1α-XBP1. The inhibition of PKA abrogated the activation of IRE1α-XBP1 pathway, while the inhibition of a p38 mitogen activated protein kinase (p38 MAPK), which is one of the downstream molecules of PKA, never suppressed the activation of IRE1α-XBP1 pathway. These data indicate that PKA-dependent IRE1α-XBP1 activation is crucial for the transcriptional induction of Ucp1 in brown adipocytes, and they demonstrate a novel, ER stress -independent role of the UPR during thermogenesis.

Role of Endoplasmic Reticulum Stress and Unfolded Protein Responses in Health and Diseases

Endoplasmic reticulum (ER) is the site of protein synthesis, protein folding, maintainance of calcium homeostasis, synthesis of lipids and sterols. Genetic or environmental insults can alter its function generating ER stress. ER senses stress mainly by three stress sensor pathways, namely protein kinase R-like endoplasmic reticulum kinase-eukaryotic translation-initiation factor 2α, inositol-requiring enzyme 1α-X-box-binding protein 1 and activating transcription factor 6-CREBH, which induce unfolded protein responses (UPR) after the recognition of stress. Recent studies have demonstrated that ER stress and UPR signaling are involved in cancer, metabolic disorders, inflammatory diseases, osteoporosis and neurodegenerative diseases. However, the precise knowledge regarding involvement of ER stress in different disease processes is still debatable. Here we discuss the possible role of ER stress in various disorders on the basis of existing literature. An attempt has also been made to highlight the present knowledge of this field which may help to elucidate and conjure basic mechanisms and novel insights into disease processes which could assist in devising better future diagnostic and therapeutic strategies.

Unfolded protein response-induced ERdj3 secretion links ER stress to extracellular proteostasis

The Unfolded Protein Response (UPR) indirectly regulates extracellular proteostasis through transcriptional remodeling of endoplasmic reticulum (ER) proteostasis pathways. This remodeling attenuates secretion of misfolded, aggregation-prone proteins during ER stress. Through these activities, the UPR has a critical role in preventing the extracellular protein aggregation associated with numerous human diseases. Here, we demonstrate that UPR activation also directly influences extracellular proteostasis through the upregulation and secretion of the ER HSP40 ERdj3/DNAJB11. Secreted ERdj3 binds misfolded proteins in the extracellular space, substoichiometrically inhibits protein aggregation, and attenuates proteotoxicity of disease-associated toxic prion protein. Moreover, ERdj3 can co-secrete with destabilized, aggregation-prone proteins in a stable complex under conditions where ER chaperoning capacity is overwhelmed, preemptively providing extracellular chaperoning of proteotoxic misfolded proteins that evade ER quality control. This regulated co-secretion of ERdj3 with misfolded clients directly links ER and extracellular proteostasis during conditions of ER stress. ERdj3 is, to our knowledge, the first metazoan chaperone whose secretion into the extracellular space is regulated by the UPR, revealing a new mechanism by which UPR activation regulates extracellular proteostasis.

Selective, potent blockade of the IRE1 and ATF6 pathways by 4-phenylbutyric acid analogues

BACKGROUND AND PURPOSE

4-Phenylbutyric acid (4-PBA) is a chemical chaperone that eliminates the accumulation of unfolded proteins in the endoplasmic reticulum (ER). However, its chaperoning ability is often weak and unable to attenuate the unfolded protein response (UPR) in vitro or in vivo. To develop more potent chemical chaperones, we synthesized six analogues of 4-PBA and evaluated their pharmacological actions on the UPR.

EXPERIMENTAL APPROACH

NRK-52E cells were treated with ER stress inducers (tunicamycin or thapsigargin) in the presence of each of the 4-PBA analogues; the suppressive effects of these analogues on the UPR were assessed using selective indicators for individual UPR pathways.

KEY RESULTS

2-POAA-OMe, 2-POAA-NO2 and 2-NOAA, but not others, suppressed the induction of ER stress markers GRP78 and CHOP. This suppressive effect was more potent than that of 4-PBA. Of the three major UPR branches, the IRE1 and ATF6 pathways were markedly blocked by these compounds, as indicated by suppression of XBP1 splicing, inhibition of UPRE and ERSE activation, and inhibition of JNK phosphorylation. Unexpectedly, however, these agents did not inhibit phosphorylation of PERK and eIF2α triggered by ER stress. These compounds dose-dependently inhibited the early activation of NF-κB in ER stress-exposed cells. 2-POAA-OMe and 2-POAA-NO2 also inhibited ER stress-induced phosphorylation of Akt.

CONCLUSION AND IMPLICATIONS

The 4-PBA analogues 2-POAA-OMe, 2-POAA-NO2 and 2-NOAA strongly inhibited activation of the IRE1 and ATF6 pathways and downstream pathogenic targets, including NF-κB and Akt, in ER stress-exposed cells. These compounds may be useful for therapeutic intervention in ER stress-related pathological conditions.

Oxidative stress protection by exogenous delivery of rhHsp70 chaperone to the retinal pigment epithelium (RPE), a possible therapeutic strategy against RPE degeneration

PURPOSE

To measure the cytoprotective effects of rhHsp70 against oxidative stress and study its cellular uptake, intracellular and intraocular distribution in the retinal pigment epithelium.

METHODS

Human retinal pigment epithelial cells (ARPE-19) were pre-treated with rhHsp70 for 24 h, 48 h, and 72 h before being exposed to 1.25 mM hydrogen peroxide. Non-treated cells served as control. We analysed interleukin 6 secretion, cell viability, and cytolysis. Uptake and intracellular distribution of fluorescently labelled rhHsp70 were investigated with flow cytometry and confocal microscopy, respectively. Ocular distribution of radioactively labelled rhHsp70 was followed ex vivo in porcine eyes by micro SPECT/CT.

RESULTS

After exposure to hydrogen peroxide, IL-6 secretion decreased by 35-39% when ARPE-19 cells were pre-treated with rhHsp70. Cell viability increased by 17-32%, and cell lysis, measured by the release of lactate dehydrogenase, decreased by 6-43%. ARPE-19 cells endocytosed rhHsp70 added to the culture medium and the protein was localized in late endosomes and lysosomes. Following intravitreal injection into isolated porcine eyes, we found 20% rhHsp70 in the RPE.

CONCLUSIONS

Recombinant hHsp70 protein offers protection against oxidative stress. RPE cells take up the exogenously delivered rhHsp70 and localize it in late endosomes and lysosomes. This work provides the basis for a therapeutic strategy to target aggregate-associated neurodegeneration in AMD.

Fluvoxamine alleviates ER stress via induction of Sigma-1 receptor

We recently demonstrated that endoplasmic reticulum (ER) stress induces sigma-1 receptor (Sig-1R) expression through the PERK pathway, which is one of the cell's responses to ER stress. In addition, it has been demonstrated that induction of Sig-1R can repress cell death signaling. Fluvoxamine (Flv) is a selective serotonin reuptake inhibitor (SSRI) with a high affinity for Sig-1R. In the present study, we show that treatment of neuroblastoma cells with Flv induces Sig-1R expression by increasing ATF4 translation directly, through its own activation, without involvement of the PERK pathway. The Flv-mediated induction of Sig-1R prevents neuronal cell death resulting from ER stress. Moreover, Flv-induced ER stress resistance reduces the infarct area in mice after focal cerebral ischemia. Thus, Flv, which is used frequently in clinical practice, can alleviate ER stress. This suggests that Flv could be a feasible therapy for cerebral diseases caused by ER stress.

Apoptosis signal regulating kinase 1 (ASK1): potential as a therapeutic target for Alzheimer's disease

Alzheimer’s disease (AD) is the most common form of dementia, characterized by a decline in memory and cognitive function. Clinical manifestations of AD are closely associated with the formation of senile plaques and neurofibrillary tangles, neuronal loss and cognitive decline. Apoptosis signal regulating kinase 1 (ASK1) is a mediator of the MAPK pathway, which regulates various cellular responses such as apoptosis, cell survival, and differentiation. Accumulating evidence indicates that ASK1 plays a key role in the pathogenesis of neurodegenerative disorders such as Huntington’s disease and AD. Of particular interest, ASK1 is associated with many signaling pathways, which include endoplasmic reticulum (ER) stress-mediated apoptosis, Aβ-induced neurotoxicity, tau protein phosphorylation, and insulin signal transduction. Here, we review experimental evidence that links ASK1 signaling and AD pathogenesis and propose that ASK1 might be a new point of therapeutic intervention to prevent or treat AD.

Endoplasmic reticulum stress and apoptosis contribute to the pathogenesis of dominantly inherited isolated GH deficiency due to GH1 gene splice site mutations

Dominantly inherited isolated GH deficiency is mainly caused by a heterozygous donor site mutation of intron 3 in the GH1 gene. An exon 3 deletion in GH (del32-71 GH) is produced from a mutant allele, whereas wild-type GH is produced from the other allele. Several studies have demonstrated a dominant negative effect of del32-71 GH on wild-type GH secretion, but the precise molecular mechanisms remain unclear. We hypothesized that unfolded del32-71 GH accumulates in the endoplasmic reticulum (ER) and causes ER stress and apoptosis in somatotrophs, promoting GH deficiency. To evaluate del32-71 GH-mediated ER stress, we established GH4C1 cell lines with doxycycline (dox)-controlled del32-71 GH expression. In 20 of 23 dox-controlled cell lines, the concentration of wild-type GH in the culture medium significantly decreased with del32-71 GH induction, demonstrating the dominant negative effect of this mutant. Cell viability, mRNA abundance of ER stress-response genes, caspase activation, and DNA fragmentation were evaluated in 5 dox-controlled cell lines selected as cellular models. In 4 of the 5 cell lines, del32-71 GH induction decreased cell viability, increased expression of 3 major ER stress response pathways (PRKR-like endoplasmic reticulum kinase [PERK], activating transcription factor-6 [ATF6], and inositol requirement 1 [IRE1]), and induced caspase-3 and caspase-7 activation. In 1 of the 4 cell lines, DNA fragmentation was demonstrated. Finally, overexpression of XBP1(S), a nuclear transcription factor downstream of IRE1, completely reversed the observed caspase activation. These data suggested that del32-71 GH-mediated ER stress and apoptosis contributed to the decrease in wild-type GH secretion observed in GH deficiency due to the GH1 gene slice-site mutations.

Role of endoplasmic reticulum stress in light-induced photoreceptor degeneration in mice

Exposure to excessive levels of light induces photoreceptor apoptosis and can be a causative factor in age-related macular degeneration (AMD). However, the cellular events that mediate this apoptotic response are poorly understood. Here, we investigated the roles of endoplasmic reticulum (ER) stress in light-induced cell death in the murine retina and murine photoreceptor cells (661W). Excessive light exposure induced retinal dysfunction, photoreceptor degeneration, and apoptosis. Furthermore, the accumulation of polyubiquitinated proteins and the transcriptional expression of ER stress-related factors, including 78-kDa glucose-regulated protein (GRP78)/immunoglobulin-binding protein (BiP) and C/EBP-homologous protein (CHOP), were increased in light-exposed retinas. Light exposure also induced both cell death and up-regulation of polyubiquitinated proteins, S-opsin aggregation, bip and chop mRNAs in 661W cells in vitro. Knock-down of chop mRNA inhibited photoreceptor cell death induced by light exposure. Furthermore, treatment with BiP inducer X (BIX), an ER stress inhibitor, induced bip mRNA and reduced both chop expression and light-induced photoreceptor cell death. These data indicate that excessive ER stress may induce photoreceptor cell death in light-exposed retinas via activation of the CHOP-dependent apoptotic pathway, suggesting that the ER stress may play a pivotal role in light exposure-induced retinal damage.