Altered localization of amyloid precursor protein under endoplasmic reticulum stress

Pathological convergence of APP and SNCA deficiency in hippocampal degeneration of young rats

The common pathogenesis of Alzheimer's disease (AD) and Parkinson's disease (PD) has been supported by biochemical, genetic and molecular evidence. Mitochondrial dysfunction is considered to be the common pathology in early AD and PD. The physiological regulation of APP and α-synuclein on mitochondria remains unclear, let alone whether they share common regulatory mechanisms affecting the development of neurodegenerative diseases. By studying gene knockout rats, the commonality of physiological APP and α-synuclein in maintaining mitochondrial function through calcium homeostasis regulation was revealed, which was critical in inhibiting hippocampal degeneration in young rats. APP and α-synuclein both control hippocampal mitochondrial calcium intake and outflow. In the mitochondrial calcium influx regulation, APP and α-synuclein are located on the mitochondrial-associated endoplasmic reticulum membrane (MAM) and converge to regulate the IP₃R1-Grp75-VDAC2 axis. Mitochondrial calcium outflow is redundantly promoted by both α-synuclein and APP. Loss of APP or SNCA leads to mitochondrial calcium overload, thus enhancing aerobic respiration and ER stress, and ultimately causing excessive apoptosis in the hippocampus and spatial memory impairment in young rats. Based on this study, we believe that the physiological function impairment of APP and SNCA is the early core pathology to induce mitochondrial dysfunction at the early stage of AD and PD, while the IP₃R1-Grp75-VDAC2 axis might be the common drug target of these two diseases.

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.

Golgi stress induces upregulation of the ER-Golgi SNARE Syntaxin-5, altered βAPP processing, and Caspase-3-dependent apoptosis in NG108-15 cells

The involvement of secretory pathways and Golgi dysfunction in neuronal cells during Alzheimer's disease progression is poorly understood. Our previous overexpression and knockdown studies revealed that the intracellular protein level of Syntaxin-5, an endoplasmic reticulum-Golgi soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE), modulates beta-amyloid precursor protein processing in neuronal cells. We recently showed that changes in endogenous Syntaxin-5 protein expression occur under stress induction. Syntaxin-5 was upregulated by endoplasmic reticulum stress but was degraded by Caspase-3 during apoptosis in neuronal cells. In addition, we showed that sustained endoplasmic reticulum stress promotes Caspase-3-dependent apoptosis during the later phase of the endoplasmic reticulum stress response in NG108-15 cells. In this study, to elucidate the consequences of secretory pathway dysfunction in beta-amyloid precursor protein processing that lead to neuronal cell death, we examined the effect of various stresses on endoplasmic reticulum-Golgi SNARE expression and beta-amyloid precursor protein processing. By using compounds to disrupt Golgi function, we show that Golgi stress promotes upregulation of the endoplasmic reticulum-Golgi SNARE Syntaxin-5, and prolonged stress causes Caspase-3-dependent apoptosis. Golgi stress induced intracellular beta-amyloid precursor protein accumulation and a concomitant decrease in total amyloid-beta production. We also examined the protective effect of the chemical chaperone 4-phenylbutylate on changes in amyloid-beta production and the activation of Caspase-3 induced by endoplasmic reticulum and Golgi stress. The compound alleviated the increase in the amyloid-beta 1-42/amyloid-beta 1-40 ratio induced by endoplasmic reticulum and Golgi stress. Furthermore, 4-phenylbutylate could rescue Caspase-3-dependent apoptosis induced by prolonged organelle stress. These results suggest that organelle stress originating from the endoplasmic reticulum and Golgi has a substantial impact on the amyloidogenic processing of beta-amyloid precursor protein and Caspase-3-dependent apoptosis, leading to neuronal cell death.

Proteasome activity modulates amyloid toxicity

Alzheimer's disease (AD) is responsible for 60%-80% of identified cases of dementia. While the generation and accumulation of amyloid precursor protein (APP) fragments is accepted as a key step in AD pathogenesis, the precise role of these fragments remains poorly understood. To overcome this deficit, we induced the expression of the soluble C-terminal fragment of APP (C99), the rate-limiting peptide for the generation of amyloid fragments, in yeast that contain thermosensitive mutations in genes encoding proteasome subunits. Our previous work with this system demonstrated that these proteasome-deficient yeast cells, expressing C99 when proteasome activity was blunted, generated amyloid fragments similar to those observed in AD patients. We now report the phenotypic repercussions of inducing C99 expression in proteasome-deficient cells. We show increased levels of protein aggregates, cellular stress and chaperone expression, electron-dense accumulations in the nuclear envelope/ER, abnormal DNA condensation, and an induction of apoptosis. Taken together, these findings suggest that the generation of C99 and its associated fragments in yeast cells with compromised proteasomal activity results in phenotypes that may be relevant to the neuropathological processes observed in AD patients. These data also suggest that this yeast model should be useful for testing therapeutics that target AD-associated amyloid, since it allows for the assessment of the reversal of the perturbed cellular physiology observed when degradation pathways are dysfunctional.

EDEM1 Regulates Amyloid Precursor Protein (APP) Metabolism and Amyloid-β Production

Endoplasmic reticulum (ER) degradation-enhancing α-mannosidase-like protein 1 (EDEM1) is a quality control factor directly involved in the endoplasmic reticulum-associated degradation (ERAD) process. It recognizes terminally misfolded proteins and directs them to retrotranslocation which is followed by proteasomal degradation in the cytosol. The amyloid-β precursor protein (APP) is synthesized and N-glycosylated in the ER and transported to the Golgi for maturation before being delivered to the cell surface. The amyloidogenic cleavage pathway of APP leads to production of amyloid-β (Aβ), deposited in the brains of Alzheimer’s disease (AD) patients. Here, using biochemical methods applied to human embryonic kidney, HEK293, and SH-SY5Y neuroblastoma cells, we show that EDEM1 is an important regulatory factor involved in APP metabolism. We find that APP cellular levels are significantly reduced after EDEM1 overproduction and are increased in cells with downregulated EDEM1. We also report on EDEM1-dependent transport of APP from the ER to the cytosol that leads to proteasomal degradation of APP. EDEM1 directly interacts with APP. Furthermore, overproduction of EDEM1 results in decreased Aβ₄₀ and Aβ₄₂ secretion. These findings indicate that EDEM1 is a novel regulator of APP metabolism through ERAD.

Structural and Functional Alterations in Mitochondria-Associated Membranes (MAMs) and in Mitochondria Activate Stress Response Mechanisms in an In Vitro Model of Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by the accumulation of extracellular plaques composed by amyloid-β (Aβ) and intracellular neurofibrillary tangles of hyperphosphorylated tau. AD-related neurodegenerative mechanisms involve early changes of mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) and impairment of cellular events modulated by these subcellular domains. In this study, we characterized the structural and functional alterations at MAM, mitochondria, and ER/microsomes in a mouse neuroblastoma cell line (N2A) overexpressing the human amyloid precursor protein (APP) with the familial Swedish mutation (APPswe). Proteins levels were determined by Western blot, ER-mitochondria contacts were quantified by transmission electron microscopy, and Ca²⁺ homeostasis and mitochondria function were analyzed using fluorescent probes and Seahorse assays. In this in vitro AD model, we found APP accumulated in MAM and mitochondria, and altered levels of proteins implicated in ER-mitochondria tethering, Ca²⁺ signaling, mitochondrial dynamics, biogenesis and protein import, as well as in the stress response. Moreover, we observed a decreased number of close ER-mitochondria contacts, activation of the ER unfolded protein response, reduced Ca²⁺ transfer from ER to mitochondria, and impaired mitochondrial function. Together, these results demonstrate that several subcellular alterations occur in AD-like neuronal cells, which supports that the defective ER-mitochondria crosstalk is an important player in AD physiopathology.

Neuroprotective effects of verbascoside against Alzheimer's disease via the relief of endoplasmic reticulum stress in Aβ-exposed U251 cells and APP/PS1 mice

BACKGROUND

Endoplasmic reticulum (ER) stress is involved in the progression of Alzheimer's disease (AD). Verbascoside (VB), an active phenylethanoid glycoside that was first isolated from Verbascum sinuatum (the wavyleaf mullein), possesses anti-inflammatory, antioxidative, and anti-apoptotic effects. The purpose of this study was to elucidate the beneficial effects of VB in amyloid β (Aβ)1-42-damaged human glioma (U251) cells and in APPswe/PSEN1dE9 transgenic (APP/PS1) mice.

METHODS

U251 cells were co-incubated with 10 μM of Aβ1-42 and treated with VB. The protective effects of VB were investigated by using 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyl tetrazolium bromide assay, flow cytometry, fluorescence staining, and transmission electron microscopy. APP/PS1 transgenic mice were treated for 6 weeks with VB. Learning and memory were evaluated using a Morris water maze test. Immunohistochemistry, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling, thioflavin-S staining, and proteomics analysis were performed to study the potential neuroprotective mechanism. Enzyme-linked immunosorbent assays and western blot were performed to analyze altered protein levels of brain lysates in APP/PS1 mice and/or Aβ1-42-damaged U251 cells.

RESULTS

In Aβ1-42-damaged U251 cells, VB significantly improved cell viability, inhibited apoptosis, reduced calcium accumulation and the intracellular concentrations of reactive oxygen species, and improved the morphology of mitochondria and ER. In APP/PS1 mice, 6-week administration of VB significantly improved memory and cognition. VB inhibited apoptosis, reduced the deposition of Aβ, reduced the formation of neurofibrillary tangles formed by hyperphosphorylated tau protein, and downregulated the expression levels of 4-hydroxynonenal and mesencephalic astrocyte-derived neurotrophic factor in the brains of APP/PS1 mice. Proteomics analysis of mouse hippocampus suggested that the neuroprotective effect of VB may be related to the reduction of ER stress. This was indicated by the fact that VB inhibited the three branches of the unfolded protein response, thereby attenuating ER stress and preventing apoptosis.

CONCLUSIONS

The results confirmed that VB possesses significant neuroprotective effects, which are related to the reduction of ER stress. These findings support the status of VB as a potentially effective treatment for AD and warrant further research.

HIV Protease Inhibitors Alter Amyloid Precursor Protein Processing via β-Site Amyloid Precursor Protein Cleaving Enzyme-1 Translational Up-Regulation

Mounting evidence implicates antiretroviral (ARV) drugs as potential contributors to the persistence and evolution of clinical and pathological presentation of HIV-associated neurocognitive disorders in the post-ARV era. Based on their ability to induce endoplasmic reticulum (ER) stress in various cell types, we hypothesized that ARV-mediated ER stress in the central nervous system resulted in chronic dysregulation of the unfolded protein response and altered amyloid precursor protein (APP) processing. We used in vitro and in vivo models to show that HIV protease inhibitor (PI) class ARVs induced neuronal damage and ER stress, leading to PKR-like ER kinase-dependent phosphorylation of the eukaryotic translation initiation factor 2α and enhanced translation of β-site APP cleaving enzyme-1 (BACE1). In addition, PIs induced β-amyloid production, indicative of increased BACE1-mediated APP processing, in rodent neuroglial cultures and human APP-expressing Chinese hamster ovary cells. Inhibition of BACE1 activity protected against neuronal damage. Finally, ARVs administered to mice and SIV-infected macaques resulted in neuronal damage and BACE1 up-regulation in the central nervous system. These findings implicate a subset of PIs as potential mediators of neurodegeneration in HIV-associated neurocognitive disorders.

GRP78 at the Centre of the Stage in Cancer and Neuroprotection

The 78-kDa glucose-regulated protein GRP78, also known as BiP and HSP5a, is a multifunctional protein with activities far beyond its well-known role in the unfolded protein response (UPR) which is activated after endoplasmic reticulum (ER) stress in the cells. Most of these newly discovered activities depend on its position within the cell. GRP78 is located mainly in the ER, but it has also been observed in the cytoplasm, the mitochondria, the nucleus, the plasma membrane, and secreted, although it is dedicated mostly to engage endogenous cytoprotective processes. Hence, GRP78 may control either UPR and macroautophagy or may activated phosphatidylinositol 3-kinase (PI3K)/AKT pro-survival pathways. GRP78 influences how tumor cells survive, proliferate, and develop chemoresistance. In neurodegeneration, endogenous mechanisms of neuroprotection are frequently insufficient or dysregulated. Lessons from tumor biology may give us clues about how boosting endogenous neuroprotective mechanisms in age-related neurodegeneration. Herein, the functions of GRP78 are revealed at the center of the stage of apparently opposite sites of the same coin regarding cytoprotection: neurodegeneration and cancer. The goal is to give a comprehensive and critical review that may serve to guide future experiments to identify interventions that will enhance neuroprotection.

Fluvoxamine alleviates paclitaxel-induced neurotoxicity

Paclitaxel (Px) is an effective chemotherapeutic agent for the treatment of various cancers. However, it is often associated with neurological side effects, including chemotherapy-associated cognitive impairment (CACI), such as "chemobrain". Previously, we reported that endoplasmic reticulum (ER) stress is involved in Px-induced neurotoxicity, and immunoglobulin heavy chain binding protein (BiP) inducer X (BIX) alleviates Px-induced neurotoxicity. However, BIX has not been used in clinical practice yet. We recently reported that fluvoxamine (Flv) alleviates ER stress via induction of sigma-1 receptor (Sig-1R). The purpose of this study was to investigate whether Flv could alleviate Px-induced neurotoxicity in vitro. SK-N-SH cells were pre-treated for 12 h with or without 10 μg/ml Flv followed by treatment with 1 μM Px with or without co-existence of 10 μg/ml Flv for 24 h. To investigate the involvement of Sig-1R in alleviation effect on Px-induced neurotoxicity,1 μM NE100, an antagonist of Sig-1R, was added for 24 h. Neurotoxicity was assessed using the MTS viability assay and ER stress-mediated neurotoxicity was assessed by evaluating the expression of C/EBP homologous protein (CHOP), cleaved caspase 4, and cleaved caspase 3. Pre-treatment with Flv significantly alleviated the induction of CHOP, cleaved caspase 4, and cleaved caspase 3 in SK-N-SH cells. At the same time, pre-treatment with Flv significantly induced Sig-1R in SK-N-SH cells. In addition, viability was significantly higher in Flv-treated cells than in untreated cells, which was reversed by treatment with NE100. Our results suggest that Flv alleviates Px-induced neurotoxicity in part through the induction of Sig-1R. Our findings should contribute to one of the novel approaches for the alleviation of Px-induced neurotoxicity, including chemobrain.

Arginine-induced insulin secretion in endoplasmic reticulum

Arginine, a semi-essential amino acid, is known as one of the most strongest insulin secretagogues in a glucose-dependent manner, but major mechanism is unknown. Arginine induced insulin secretion in mice as well as β cell line, NIT-1, in which more than 90% of intracellular insulin is prionsulin without arginine cultivation. Arginine administration reduced prionsulin amount in 30 s, then insulin is secreted from NIT1 cells. These data indicated that the target factor(s) for arginine-induced insulin secretion located in endoplasmic reticulum (ER). We established the screening system for identifying the arginine mimetics. Brazilian propolis, not Chinese propolis, induced insulin secretion. To identify target factor(s) of arginine induced insulin secretion, our previous study was that nanobeads technology facilitated us to purify chemical-target factors. This time we chose the other way, proinsulin associating factor purification and arginine-immobilized agarose. Three proinsulin associating factors and 5 arginine interacting factors were identified. Among theses factors, Calnexin (CNX) was the only one factor, which belonged to both groups, suggesting that CNX might play a key role in arginine-induced insulin secretion in ER.

Application of proteomics to identify the target molecules involved in Lonicera japonica-induced photokilling in human lung cancer CH27 cells

Background

The Lonicera japonica has been used as natural and healthy drink for its anti-inflammatory effect and pleasant odor in China and Taiwan.

Methods

2D electrophoresis was used to analyze the proteins involved in photoactivated Lonicera japonica-induced CH27 cell apoptosis. The fluorescent dyes MitoTracker Red CMXRos, calcein AM and JC-1 were used to elucidate mitochondrial function. The protein expression was performed by Western blotting. Fluorescent image of endoplasmic reticulum was accomplished by using ER-Tracker Green. This study used fluorescent dye CM-H₂DCFDA to detect intracellular generation of reactive oxygen species.

Results

The identified proteins can be classified into three major groups, which include proteins involved in mitochondrial function, cytoskeleton-related proteins and proteins associated with endoplasmic reticulum (ER) stress. Photoactivated Lonicera japonica caused a significant effect on the mitochondrial function and ER stress in CH27 cells. The reactive oxygen species producing was found to be involved in photoactivated Lonicera japonica-induced CH27 cell apoptosis.

Conclusion

Mitochondria and endoplasmic reticulum are the integral targets in photoactivated Lonicera japonica-induced CH27 cell apoptosis. We also demonstrated that ethyl acetate fraction of Lonicera japonica extracts caused photocytotoxicity in a dose-dependent manner in CH27 cells. This could explain the fact that the ethyl acetate fraction of Lonicera japonica extracts may contain compounds which exhibit the photosensitizing activity in CH27 cells.

Paclitaxel induces neurotoxicity through endoplasmic reticulum stress

Due to chemotherapy, the majority of breast cancer patients survive, but frequently complain of chemotherapy-associated cognitive impairment. This phenomenon is termed "chemobrain" or "chemofog" in the literature. However, its mechanisms are unclear. The objective of this study was to investigate the mechanisms of paclitaxel (Px)-induced neurotoxicity, with a focus on endoplasmic reticulum (ER) stress. To investigate Px-induced neurotoxicity and ER stress induction, SK-N-SH cells were treated with 1, 10, 50, and 100 μM Px for 24 h. Neurotoxicity was assessed using MTS viability assays, and ER stress was assessed by evaluating the expression of phosphorylated elF2α (phospho-eIF2α), C/EBP homologous protein (CHOP), and cleaved caspase 4 and caspase 3 (the active form of each caspase). Furthermore, to investigate whether immunoglobulin heavy-chain binding protein (BiP) inducer X (BIX), which induces the molecular chaperone BiP, could attenuate Px-induced neurotoxicity, SK-N-SH cells were pre-treated for 12 h with 3.5 μM BIX before Px treatment. Neurotoxicity was observed in SK-N-SH cells treated with Px in a dose-dependent manner compared with vehicle control. Furthermore, phospho-eIF2α, CHOP, and activated caspase 4 and caspase 3 were significantly induced in Px-treated cells. In addition, pre-treatment with BIX significantly attenuated the induction of CHOP and activated caspase 4 and caspase 3. The viability of BIX pre-treated cells prior to Px treatment was significantly increased compared with cells that were not treated with BIX. Our results suggest that Px induces neurotoxicity in part through activating the ER stress response. Our findings should contribute to novel approaches regarding the mechanism of Px-induced neurotoxicity, including chemobrain.

Mild oxidative stress induces redistribution of BACE1 in non-apoptotic conditions and promotes the amyloidogenic processing of Alzheimer's disease amyloid precursor protein

BACE1 is responsible for β-secretase cleavage of the amyloid precursor protein (APP), which represents the first step in the production of amyloid β (Aβ) peptides. Previous reports, by us and others, have indicated that the levels of BACE1 protein and activity are increased in the brain cortex of patients with Alzheimer's disease (AD). The association between oxidative stress (OS) and AD has prompted investigations that support the potentiation of BACE1 expression and enzymatic activity by OS. Here, we have established conditions to analyse the effects of mild, non-lethal OS on BACE1 in primary neuronal cultures, independently from apoptotic mechanisms that were shown to impair BACE1 turnover. Six-hour treatment of mouse primary cortical cells with 10-40 µM hydrogen peroxide did not significantly compromise cell viability but it did produce mild oxidative stress (mOS), as shown by the increased levels of reactive radical species and activation of p38 stress kinase. The endogenous levels of BACE1 mRNA and protein were not significantly altered in these conditions, whereas a toxic H2O2 concentration (100 µM) caused an increase in BACE1 protein levels. Notably, mOS conditions resulted in increased levels of the BACE1 C-terminal cleavage product of APP, β-CTF. Subcellular fractionation techniques showed that mOS caused a major rearrangement of BACE1 localization from light to denser fractions, resulting in an increased distribution of BACE1 in fractions containing APP and markers for trans-Golgi network and early endosomes. Collectively, these data demonstrate that mOS does not modify BACE1 expression but alters BACE1 subcellular compartmentalization to favour the amyloidogenic processing of APP, and thus offer new insight in the early molecular events of AD pathogenesis.

Endoplasmic reticulum dysfunction in neurological disease

Endoplasmic reticulum (ER) dysfunction might have an important part to play in a range of neurological disorders, including cerebral ischaemia, sleep apnoea, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, the prion diseases, and familial encephalopathy with neuroserpin inclusion bodies. Protein misfolding in the ER initiates the well studied unfolded protein response in energy-starved neurons during stroke, which is relevant to the toxic effects of reperfusion. The toxic peptide amyloid β induces ER stress in Alzheimer's disease, which leads to activation of similar pathways, whereas the accumulation of polymeric neuroserpin in the neuronal ER triggers a poorly understood ER-overload response. In other neurological disorders, such as Parkinson's and Huntington's diseases, ER dysfunction is well recognised but the mechanisms by which it contributes to pathogenesis remain unclear. By targeting components of these signalling responses, amelioration of their toxic effects and so the treatment of a range of neurodegenerative disorders might become possible.

Mitochondrial dysfunction and cellular metabolic deficiency in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disorder. The pathology of AD includes amyloid-β (Aβ) deposits in neuritic plaques and neurofibrillary tangles composed of hyperphosphorylated tau, as well as neuronal loss in specific brain regions. Increasing epidemiological and functional neuroimaging evidence indicates that global and regional disruptions in brain metabolism are involved in the pathogenesis of this disease. Aβ precursor protein is cleaved to produce both extracellular and intracellular Aβ, accumulation of which might interfere with the homeostasis of cellular metabolism. Mitochondria are highly dynamic organelles that not only supply the main energy to the cell but also regulate apoptosis. Mitochondrial dysfunction might contribute to Aβ neurotoxicity. In this review, we summarize the pathways of Aβ generation and its potential neurotoxic effects on cellular metabolism and mitochondrial dysfunction.

Human telomerase reverse transcriptase and glucose-regulated protein 78 increase the life span of articular chondrocytes and their repair potential

Background

Like all mammalian cells, normal adult chondrocytes have a limited replicative life span, which decreases with age. To facilitate the therapeutic use of chondrocytes from older donors, a method is needed to prolong their life span.

Methods

We transfected chondrocytes with hTERT or GRP78 and cultured them in a 3-dimensional atelocollagen honeycomb-shaped scaffold with a membrane seal. Then, we measured the amount of nuclear DNA and glycosaminoglycans (GAGs) and the expression level of type II collagen as markers of cell proliferation and extracellular matrix formation, respectively, in these cultures. In addition, we allografted this tissue-engineered cartilage into osteochondral defects in old rabbits to assess their repair activity in vivo.

Results

Our results showed different degrees of differentiation in terms of GAG content between chondrocytes from old and young rabbits. Chondrocytes that were cotransfected with hTERT and GRP78 showed higher cellular proliferation and expression of type II collagen than those of nontransfected chondrocytes, regardless of the age of the cartilage donor. In addition, the in vitro growth rates of hTERT- or GRP78-transfected chondrocytes were higher than those of nontransfected chondrocytes, regardless of donor age. In vivo, the tissue-engineered cartilage implants exhibited strong repairing activity, maintained a chondrocyte-specific phenotype, and produced extracellular matrix components.

Conclusions

Focal gene delivery to aged articular chondrocytes exhibited strong repairing activity and may be therapeutically useful for articular cartilage regeneration.

Involvement of the molecular chaperone Hspa5 in copper homeostasis in astrocytes

Copper (Cu) ion availability in tissues and cells must be closely regulated within safe limits by Cu transporters and chaperones. Astrocytes play key roles in metal homeostasis and distribution in the brain that are only partially understood. The purpose of this study was to define the role that the protein chaperone Hspa5, also known as Grp78, plays in Cu homeostasis in astrocytes. First passage cultures of primary astrocytes from neonatal rats and cultures of the C6 rat glioma cells were used as models. We found that the level of Cu accumulation in astrocyte cultures increased with Cu concentrations in the medium, and Cu treatment significantly reduced cellular levels of iron (Fe), manganese (Mn) and zinc (Zn). Cu accumulation specifically induced protein expression of Hspa5 but not metallothioneins (MTs) in astrocytes. In C6 cells, Hspa5 was identified as one component of a Cu-binding complex and shown to directly bind Cu. However, the level of Hspa5 expression was not proportional to Cu accumulation in astrocytes and C6 cells: astrocytes expressed low protein levels of Hspa5 compared to C6 cells but accumulated significantly more Cu than did C6 cells. Consistent with this finding, astrocytes expressed a lower level of the Cu-extruding protein Atp7a than did C6 cells, and depletion of Hspa5 by RNA interference resulted in significantly increased Cu accumulation and induction of MT1/2 expression. These data demonstrate that Hspa5 is involved in Cu homeostasis in astrocytes but not as a Cu storage protein.

Therapeutic targeting of the endoplasmic reticulum in Alzheimer's disease

The extensive prevalence of Alzheimer's disease (AD) places a tremendous burden physiologically, socially and economically upon those directly suffering and those caring for sufferers themselves. Considering the steady increases in numbers of patients diagnosed with Alzheimer's, the number of effective pharmacotherapeutic strategies to tackle the disease is still relatively few. As with many other neurodegenerative mechanisms, AD, is characterized by the continued presence and accumulation of cytotoxic protein aggregates, i.e. of beta-amyloid and the microtubule associated protein, tau. Therefore, one novel therapeutic avenue for the treatment of AD may be the actual targeting of factors that control protein synthesis, packaging and degradation. One of the prime cellular targets that, if effectively modulated, could accomplish this is the endoplasmic reticulum (ER). The ER can not only control cellular protein synthesis, trafficking and degradation but it is also closely associated with cytoprotective mechanisms, including calcium ion regulation and unfolded protein responses. This review will delineate some of the most important functional physiological features of the ER that, if effectively modulated, could result in beneficial amelioration or remediation of the negative cellular aspects of AD initiation and progression. While not a classical drug target, even with minimal levels of beneficial modulation, its multifactorial efficacy may amplify small effects resulting in significant therapeutic efficacy.

Intracellular trafficking and secretion of mouse mesencephalic astrocyte-derived neurotrophic factor

Recently, mesencephalic astrocyte-derived neurotrophic factor (MANF) has been reported to prevent cell death under some pathophysiological conditions. MANF, also referred to as arginine rich, mutated in early stage of tumors (Armet), was identified as an endoplasmic reticulum (ER) stress-inducible factor. Using RT-PCR, we found two variants of MANF mRNA: wild type, which contains exon 1 (wt-MANF), and one lacking exon 1, which is presumably not secreted (ΔΝ-MANF) in Neuro2a cells. The latter has a putative translational start site upstream of the second exon in the mouse MANF gene. Comparing the expression of wt-MANF with that of ΔΝ-MANF, we found that the amount of intracellular ΔΝ-MANF was much lower than that of wt-MANF. Furthermore, ΔΝ-MANF was not detected in the culture medium after its transient transfection into Neuro2a cells. Deletion of several α-helices of mouse MANF decreased its intracellular stability and secretion. Secretion of wt-MANF was almost completely inhibited by either treatment with brefeldin A (BFA), which disrupts the Golgi apparatus structure, or overexpression of a dominant negative Sar1 (Sar1[H79G]), which is reported to impair COPII-mediated transport from the ER to the Golgi apparatus. In addition, the enforced expression of glucose-regulated protein 78 kDa (GRP78) attenuated the secretion of wt-MANF and led to its intracellular accumulation. MANF lacking the four C-terminal amino acids (ΔC-MANF) accumulated at low levels in the cells, but its intracellular level was increased by GRP78 overexpression. The amount of ΔC-MANF in the culture medium was partially down-regulated after co-transfection of GRP78. Substitution of the amino acids RTDL at the C-terminus of mouse MANF with KDEL, the canonical ER localization signal in GRP78, markedly decreased MANF secretion and its secretion was further attenuated by GRP78 overexpression. Taken together, our data show that the secretion of MANF is regulated via COPII-mediated transport and that its C-terminus could be responsible for its retention in the ER through GRP78. The alternate isotype, ΔΝ-MANF, may be less stable in cells than wt-MANF and may not be secreted extracellularly.

DC2 and keratinocyte-associated protein 2 (KCP2), subunits of the oligosaccharyltransferase complex, are regulators of the gamma-secretase-directed processing of amyloid precursor protein (APP)

The oligosaccharyltransferase complex catalyzes the transfer of oligosaccharide from a dolichol pyrophosphate donor en bloc onto a free asparagine residue of a newly synthesized nascent chain during the translocation in the endoplasmic reticulum lumen. The role of the less known oligosaccharyltransferase (OST) subunits, DC2 and KCP2, recently identified still remains to be determined. Here, we have studied DC2 and KCP2, and we have established that DC2 and KCP2 are substrate-specific, affecting amyloid precursor protein (APP), indicating that they are not core components required for N-glycosylation and OST activity per se. We show for the first time that DC2 and KCP2 depletion affects APP processing, leading to an accumulation of C-terminal fragments, both C99 and C83, and a reduction in full-length mature APP. This reduction in mature APP levels was not due to a block in secretion because the levels of sAPPα secreted into the media were unaffected. We discover that DC2 and KCP2 depletion affects only the γ-secretase complex, resulting in a reduction of the PS1 active fragment blocking Aβ production. Conversely, we show that the overexpression of DC2 and KCP2 causes an increase in the active γ-secretase complex, particularly the N-terminal fragment of PS1 that is generated by endoproteolysis, leading to a stimulation of Aβ production upon overexpression of DC2 and KCP2. Our findings reveal that components of the OST complex for the first time can interact with the γ-secretase and affect the APP processing pathway.

Biosynthesis and secretion of mouse cysteine-rich with EGF-like domains 2

In this study, we found that Cysteine-rich with EGF-like domains 2 (CRELD2), a novel endoplasmic reticulum stress-inducible protein, is not only localized in the ER-Golgi apparatus but also spontaneously secreted. Deletion of four C-terminal amino acids from mouse CRELD2 or addition of tag-peptides to its C-terminus dramatically enhanced CRELD2 secretion. Intra- and extra-cellular CRELD2 is differentially glycosylated and its spontaneous secretion was significantly prevented by overexpression of a dominant negative mutant Sar1 and treatment with brefeldin A. Overexpression of wild-type GRP78 remarkably enhanced the secretion of wild-type but not mutant CRELD2. Our results demonstrate both that CRELD2 is a novel secretory glycoprotein regulated by Sar1 and GRP78 and that the C-terminal of CRELD2 plays a crucial role in its secretion.

Lactic acid induces aberrant amyloid precursor protein processing by promoting its interaction with endoplasmic reticulum chaperone proteins

Background

Lactic acid, a natural by-product of glycolysis, is produced at excess levels in response to impaired mitochondrial function, high-energy demand, and low oxygen availability. The enzyme involved in the production of β-amyloid peptide (Aβ) of Alzheimer's disease, BACE1, functions optimally at lower pH, which led us to investigate a potential role of lactic acid in the processing of amyloid precursor protein (APP).

Methodology/Principal Findings

Lactic acid increased levels of Aβ40 and 42, as measured by ELISA, in culture medium of human neuroblastoma cells (SH-SY5Y), whereas it decreased APP metabolites, such as sAPPα. In cell lysates, APP levels were increased and APP was found to interact with ER-chaperones in a perinuclear region, as determined by co-immunoprecipitation and fluorescence microscopy studies. Lactic acid had only a very modest effect on cellular pH, did increase the levels of ER chaperones Grp78 and Grp94 and led to APP aggregate formation reminiscent of aggresomes.

Conclusions/Significance

These findings suggest that sustained elevations in lactic acid levels could be a risk factor in amyloidogenesis related to Alzheimer's disease through enhanced APP interaction with ER chaperone proteins and aberrant APP processing leading to increased generation of amyloid peptides and APP aggregates.

Proteomic analysis reveals ATP-dependent steps and chaperones involvement in luteolin-induced lung cancer CH27 cell apoptosis

The present study applied 2D electrophoresis to analyze the proteins involved in luteolin (50 microM)-induced CH27 cell apoptosis. We found 7 proteins to be markedly changed. According to the data of analysis of these protein spots, we hypothesized that ATP synthetic pathway and heat shock proteins were involved in luteolin-induced CH27 cell apoptosis. In this study, luteolin induced a significant change in intracellular ATP levels and mitochondrial activity of CH27 cells. Further experiments demonstrated that pretreatment with forskolin blocked the luteolin-induced cell death. P38 and heat shock protein 27 may be important participants in the luteolin-induced changes in organization of actin microfilaments in this study. In addition, endoplasmic reticulum stress is also important in the luteolin-induced CH27 cell apoptosis. Our findings suggested that the function of mitochondria and endoplasmic reticulum is the integral factor in luteolin-induced CH27 cell apoptosis.

Histone deacetylase inhibitor valproic acid inhibits cancer cell proliferation via down-regulation of the alzheimer amyloid precursor protein

The beta-amyloid precursor protein (APP) represents a type I transmembrane glycoprotein that is ubiquitously expressed. In the brain, it is a key player in the molecular pathogenesis of Alzheimer disease. Its physiological function is however less well understood. Previous studies showed that APP is up-regulated in prostate, colon, pancreatic tumor, and oral squamous cell carcinoma. In this study, we show that APP has an essential role in growth control of pancreatic and colon cancer. Abundant APP staining was found in human pancreatic adenocarcinoma and colon cancer tissue. Interestingly, treating pancreatic and colon cancer cells with valproic acid (VPA, 2-propylpentanoic acid), a known histone deacetylase (HDAC) inhibitor, leads to up-regulation of GRP78, an endoplasmic reticulum chaperone immunoglobulin-binding protein. GRP78 is involved in APP maturation and inhibition of tumor cell growth by down-regulation of APP and secreted soluble APPalpha. Trichostatin A, a pan-HDAC inhibitor, also lowered APP and increased GRP78 levels. In contrast, treating cells with valpromide, a VPA derivative lacking HDAC inhibitory properties, had no effect on APP levels. VPA did not modify the level of epidermal growth factor receptor, another type I transmembrane protein, and APLP2, a member of the APP family, demonstrating the specificity of the VPA effect on APP. Small interfering RNA-mediated knockdown of APP also resulted in significantly decreased cell growth. Based on these observations, the data suggest that APP down-regulation via HDAC inhibition provides a novel mechanism for pancreatic and colon cancer therapy.

Phenylbutyric acid rescues endoplasmic reticulum stress-induced suppression of APP proteolysis and prevents apoptosis in neuronal cells

BACKGROUND

The familial and sporadic forms of Alzheimer's disease (AD) have an identical pathology with a severe disparity in the time of onset [1]. The pathological similarity suggests that epigenetic processes may phenocopy the Familial Alzheimer's disease (FAD) mutations within sporadic AD. Numerous groups have demonstrated that FAD mutations in presenilin result in 'loss of function' of gamma-secretase mediated APP cleavage [2], [3], [4], [5]. Accordingly, ER stress is prominent within the pathologically impacted brain regions in AD patients [6] and is reported to inhibit APP trafficking through the secretory pathway [7], [8]. As the maturation of APP and the cleaving secretases requires trafficking through the secretory pathway [9], [10], [11], we hypothesized that ER stress may block trafficking requisite for normal levels of APP cleavage and that the small molecular chaperone 4-phenylbutyrate (PBA) may rescue the proteolytic deficit.

METHODOLOGY/PRINCIPAL FINDINGS

The APP-Gal4VP16/Gal4-reporter screen was stably incorporated into neuroblastoma cells in order to assay gamma-secretase mediated APP proteolysis under normal and pharmacologically induced ER stress conditions. Three unrelated pharmacological agents (tunicamycin, thapsigargin and brefeldin A) all repressed APP proteolysis in parallel with activation of unfolded protein response (UPR) signaling-a biochemical marker of ER stress. Co-treatment of the gamma-secretase reporter cells with PBA blocked the repressive effects of tunicamycin and thapsigargin upon APP proteolysis, UPR activation, and apoptosis. In unstressed cells, PBA stimulated gamma-secretase mediated cleavage of APP by 8-10 fold, in the absence of any significant effects upon amyloid production, by promoting APP trafficking through the secretory pathway and the stimulation of the non-pathogenic alpha/gamma-cleavage.

CONCLUSIONS/SIGNIFICANCE

ER stress represses gamma-secretase mediated APP proteolysis, which replicates some of the proteolytic deficits associated with the FAD mutations. The small molecular chaperone PBA can reverse ER stress induced effects upon APP proteolysis, trafficking and cellular viability. Pharmaceutical agents, such as PBA, that stimulate alpha/gamma-cleavage of APP by modifying intracellular trafficking should be explored as AD therapeutics.

Prominent hippocampal CA3 gene expression profile in neurocognitive aging

Research in aging laboratory animals has characterized physiological and cellular alterations in medial temporal lobe structures, particularly the hippocampus, that are central to age-related memory deficits. The current study compares molecular alterations across hippocampal subregions in a rat model that closely mirrors individual differences in neurocognitive features of aging humans, including both impaired memory and preserved function. Using mRNA profiling of the CA1, CA3 and dentate gyrus subregions, we have distinguished between genes and pathways related to chronological age and those associated with impaired or preserved cognitive outcomes in healthy aged Long-Evans rats. The CA3 profile exhibited the most prominent gene expression differences related to cognitive status and of the three subregions, best distinguished preserved from impaired function among the aged animals. Within this profile differential expression of synaptic plasticity and neurodegenerative disease-related genes suggests recruitment of adaptive mechanisms to maintain function and structural integrity in aged unimpaired rats that does not occur in aged impaired animals.

HtrA2 Regulates β-Amyloid Precursor Protein (APP) Metabolism through Endoplasmic Reticulum-associated Degradation

Alzheimer disease-associated beta-amyloid peptide is generated from its precursor protein APP. By using the yeast two-hybrid assay, here we identified HtrA2/Omi, a stress-responsive chaperone-protease as a protein binding to the N-terminal cysteinerich region of APP. HtrA2 coimmunoprecipitates exclusively with immature APP from cell lysates as well as mouse brain extracts and degrades APP in vitro. A subpopulation of HtrA2 localizes to the cytosolic side of the endoplasmic reticulum (ER) membrane where it contributes to ER-associated degradation of APP together with the proteasome. Inhibition of the proteasome results in accumulation of retrotranslocated forms of APP and increased association of APP with HtrA2 and Derlin-1 in microsomal membranes. In cells lacking HtrA2, APP holoprotein is stabilized and accumulates in the early secretory pathway correlating with elevated levels of APP C-terminal fragments and increased Abeta secretion. Inhibition of ER-associated degradation (either HtrA2 or proteasome) promotes binding of APP to the COPII protein Sec23 suggesting enhanced trafficking of APP out of the ER. Based on these results we suggest a novel function for HtrA2 as a regulator of APP metabolism through ER-associated degradation.

Tyrosine 687 Phosphorylated Alzheimer’s Amyloid Precursor Protein Is Retained Intracellularly and Exhibits a Decreased Turnover Rate

Tyrosine 687 (Y687) of the Alzheimer's amyloid precursor protein (APP) was shown to be phosphorylated in the brains of Alzheimer's disease patients. This residue lies within a typical endocytosis consensus motif commonly found in molecules with receptor functions, strongly suggesting a potential role for APP in signal transduction. Consequently, the work here described addressed how phosphorylation of Y687 may be affecting APP in terms of its proteolytic cleavage and subcellular distribution. Our data show that the APP mutant mimicking constitutive dephosphorylation of Y687 had a faster turnover rate, both in terms of maturation and metabolism, when compared to Wt-APP-GFP and even more so when compared to the mutant mimicking constitutive phosphorylation. Thus, the mutant mimicking constitutively phosphorylated Y687 had a much higher t(1/2) and was significantly retained both in the ER and TGN. Additionally, this mutant was not incorporated into visible vesicular structures, with a concomitant dramatic decrease in Abeta production. Our findings point to the direct phosphorylation of APP on Y687 as an important regulatory mechanism in terms of determining the subcellular localization of APP and modulating its processing via different proteolytic pathways.

Endoplasmic reticulum chaperones inhibit the production of amyloid-beta peptides

Abeta (amyloid-beta peptides) generated by proteolysis of APP (beta-amyloid precursor protein), play an important role in the pathogenesis of AD (Alzheimer's disease). ER (endoplasmic reticulum) chaperones, such as GRP78 (glucose-regulated protein 78), make a major contribution to protein quality control in the ER. In the present study, we examined the effect of overexpression of various ER chaperones on the production of Abeta in cultured cells, which produce a mutant type of APP (APPsw). Overexpression of GRP78 or inhibition of its basal expression, decreased and increased respectively the level of Abeta40 and Abeta42 in conditioned medium. Co-expression of GRP78's co-chaperones ERdj3 or ERdj4 stimulated this inhibitory effect of GRP78. In the case of the other ER chaperones, overexpression of some (150 kDa oxygen-regulated protein and calnexin) but not others (GRP94 and calreticulin) suppressed the production of Abeta. These results indicate that certain ER chaperones are effective suppressors of Abeta production and that non-toxic inducers of ER chaperones may be therapeutically beneficial for AD treatment. GRP78 was co-immunoprecipitated with APP and overexpression of GRP78 inhibited the maturation of APP, suggesting that GRP78 binds directly to APP and inhibits its maturation, resulting in suppression of the proteolysis of APP. On the other hand, overproduction of APPsw or addition of synthetic Abeta42 caused up-regulation of the mRNA of various ER chaperones in cells. Furthermore, in the cortex and hippocampus of transgenic mice expressing APPsw, the mRNA of some ER chaperones was up-regulated in comparison with wild-type mice. We consider that this up-regulation is a cellular protective response against Abeta.