Parkin and endoplasmic reticulum stress

Interplay between α-synuclein and parkin genes: Insights of Parkinson's disease

Parkinson's disease (PD) is a complex and debilitating neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra. The pathogenesis of PD is intimately linked to the roles of two key molecular players, α-synuclein (α-syn) and Parkin. Understanding the intricate interplay between α-syn and Parkin is essential for unravelling the molecular underpinnings of PD. Their roles in synaptic function and protein quality control underscore their significance in neuronal health. Dysregulation of these processes, as seen in PD, highlights the potential for targeted therapeutic strategies aimed at restoring normal protein homeostasis and mitigating neurodegeneration. Investigating the connections between α-syn, Parkin, and various pathological mechanisms provides insights into the complex web of factors contributing to PD pathogenesis and offers hope for the development of more effective treatments for this devastating neurological disorder. The present compilation provides an overview of their structures, regional and cellular locations, associations, physiological functions, and pathological roles in the context of PD.

Melatonin Can Modulate Neurodegenerative Diseases by Regulating Endoplasmic Reticulum Stress

As people age, their risks of developing degenerative diseases such as cancer, diabetes, Parkinson's Disease (PD), Alzheimer's Disease (AD), rheumatoid arthritis, and osteoporosis are generally increasing. Millions of people worldwide suffer from these diseases as they age. In most countries, neurodegenerative diseases are generally recognized as the number one cause afflicting the elderly. Endoplasmic reticulum (ER) stress has been suggested to be associated with some human neurological diseases, such as PD and AD. Melatonin, a neuroendocrine hormone mainly synthesized in the pineal gland, is involved in pleiotropically biological functions, including the control of the circadian rhythm, immune enhancement, and antioxidant, anti-aging, and anti-tumor effects. Although there are many papers on the prevention or suppression of diseases by melatonin, there are very few papers about the effects of melatonin on ER stress in neurons and neurodegenerative diseases. This paper aims to summarize and present the effects of melatonin reported so far, focusing on its effects on neurons and neurodegenerative diseases related to ER stress. Studies have shown that the primary target molecule of ER stress for melatonin is CHOP, and PERK and GRP78/BiP are the secondary target molecules. Therefore, melatonin is crucial in protecting neurons and treating neurodegeneration against ER stress.

Hallmarks of neurodegenerative disease: A systems pharmacology perspective

Age-related central neurodegenerative diseases, such as Alzheimer's and Parkinson's disease, are a rising public health concern and have been plagued by repeated drug development failures. The complex nature and poor mechanistic understanding of the etiology of neurodegenerative diseases has hindered the discovery and development of effective disease-modifying therapeutics. Quantitative systems pharmacology models of neurodegeneration diseases may be useful tools to enhance the understanding of pharmacological intervention strategies and to reduce drug attrition rates. Due to the similarities in pathophysiological mechanisms across neurodegenerative diseases, especially at the cellular and molecular levels, we envision the possibility of structural components that are conserved across models of neurodegenerative diseases. Conserved structural submodels can be viewed as building blocks that are pieced together alongside unique disease components to construct quantitative systems pharmacology (QSP) models of neurodegenerative diseases. Model parameterization would likely be different between the different types of neurodegenerative diseases as well as individual patients. Formulating our mechanistic understanding of neurodegenerative pathophysiology as a mathematical model could aid in the identification and prioritization of drug targets and combinatorial treatment strategies, evaluate the role of patient characteristics on disease progression and therapeutic response, and serve as a central repository of knowledge. Here, we provide a background on neurodegenerative diseases, highlight hallmarks of neurodegeneration, and summarize previous QSP models of neurodegenerative diseases.

Involvement of heat shock proteins and parkin/α-synuclein axis in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurological diseases, next only to Alzheimer's disease (AD) in terms of prevalence. It afflicts about 2-3% of individuals over 65 years old. The etiology of PD is unknown and several environmental and genetic factors are involved. From a pathological point of view, PD is characterized by the loss of dopaminergic neurons in the substantia nigra, which causes the abnormal accumulation of α-synuclein (α-syn) (a component of Lewy bodies), which subsequently interact with heat shock proteins (HSPs), leading to apoptosis. Apoptosis is a vital pathway for establishing homeostasis in body tissues, which is regulated by pro-apoptotic and anti-apoptotic factors. Recent findings have shown that HSPs, especially HSP27 and HSP70, play a pivotal role in regulating apoptosis by influencing the factors involved in the apoptosis pathway. Moreover, it has been reported that the expression of these HSPs in the nervous system is high. Apart from this finding, investigations have suggested that HSP27 and HSP70 (related to parkin) show a potent protective and anti-apoptotic impact against the damaging outcomes of mutant α-syn toxicity to nerve cells. Therefore, in this study, we aimed to investigate the relationship between these HSPs and apoptosis in patients with PD.

Ubiquitin-proteasome signaling in lung injury

Cell homeostasis requires precise coordination of cellular proteins function. Ubiquitination is a post-translational modification that modulates protein half-life and function and is tightly regulated by ubiquitin E3 ligases and deubiquitinating enzymes. Lung injury can progress to acute respiratory distress syndrome that is characterized by an inflammatory response and disruption of the alveolocapillary barrier resulting in alveolar edema accumulation and hypoxemia. Ubiquitination plays an important role in the pathobiology of acute lung injury as it regulates the proteins modulating the alveolocapillary barrier and the inflammatory response. Better understanding of the signaling pathways regulated by ubiquitination may lead to novel therapeutic approaches by targeting specific elements of the ubiquitination pathways.

ER stress and the unfolded protein response in neurodegeneration

The clinical manifestation of neurodegenerative diseases is initiated by the selective alteration in the functionality of distinct neuronal populations. The pathology of many neurodegenerative diseases includes accumulation of misfolded proteins in the brain. In physiological conditions, the proteostasis network maintains normal protein folding, trafficking and degradation; alterations in this network - particularly disturbances to the function of endoplasmic reticulum (ER) - are thought to contribute to abnormal protein aggregation. ER stress triggers a signalling reaction known as the unfolded protein response (UPR), which induces adaptive programmes that improve protein folding and promote quality control mechanisms and degradative pathways or can activate apoptosis when damage is irreversible. In this Review, we discuss the latest advances in defining the functional contribution of ER stress to brain diseases, including novel evidence that relates the UPR to synaptic function, which has implications for cognition and memory. A complex concept is emerging wherein the consequences of ER stress can differ drastically depending on the disease context and the UPR signalling pathway that is altered. Strategies to target specific components of the UPR using small molecules and gene therapy are in development, and promise interesting avenues for future interventions to delay or stop neurodegeneration.

VPS35 regulates parkin substrate AIMP2 toxicity by facilitating lysosomal clearance of AIMP2

Vacuolar protein sorting-associated protein 35 (VPS35) is involved in retrograde transport of proteins from endosomes to trans-Golgi network. Gene mutations in VPS35 are linked to autosomal dominant late-onset Parkinson's disease (PD). Although the identification of VPS35 mutations has provided novel insight about its interactions with several PD-associated genes including leucine-rich repeat kinase 2 (LRRK2) and α-synuclein, little information is available about the molecular mechanisms of cell death downstream of VPS35 dysfunction. In this study, we showed that VPS35 has a role in the lysosomal degradation of parkin substrate aminoacyl tRNA synthetase complex-interacting multifunctional protein 2 (AIMP2), of which accumulation leads to poly(ADP-ribose) polymerase-1 (PARP1)-dependent cell death. VPS35 was co-immunoprecipitated with AIMP2, as well as lysosome-associated membrane protein-2a (Lamp2a). Interestingly, this association was disrupted by PD-associated VPS35 mutant D620N. VPS35 overexpression prevented AIMP2-potentiated cell death and PARP1 activation in SH-SY5Y cells. More importantly, knockdown of VPS35 led to PARP1 activation and cell death, which was AIMP2 dependent. These findings provide new mechanistic insights into the role of VPS35 in the regulation of AIMP2 levels and cell death. As AIMP2 accumulation was reported in PD patient's brains and involved in dopaminergic cell death, identification of VPS35 as a novel regulator of AIMP2 clearance via lysosomal pathway provides alternative venue to control dopaminergic cell death in PD.

De-acetylation and degradation of HSPA5 is critical for E1A metastasis suppression in breast cancer cells

Elevated expression of heat shock protein 5 (HSPA5) promotes drug resistance and metastasis and is a marker of poor prognosis in breast cancer patients. Adenovirus type 5 E1A gene therapy has demonstrated antitumor efficacy but the mechanisms of metastasis-inhibition are unclear. Here, we report that E1A interacts with p300 histone acetyltransferase (HAT) and blocks p300-mediated HSPA5 acetylation at K353, which in turn promotes HSPA5 ubiquitination by GP78 (E3 ubiquitin ligase) and subsequent proteasome-mediated degradation. Our findings point out the Ying-Yang regulation of two different post-translational modifications (ubiquitination and acetylation) of HSPA5 in tumor metastasis.

Gene expression microarray analysis of the spinal trigeminal nucleus in a rat model of migraine with aura

Cortical spreading depression can trigger migraine with aura and activate the trigeminal vascular system. To examine gene expression profiles in the spinal trigeminal nucleus in rats following cortical spreading depression-induced migraine with aura, a rat model was established by injection of 1 M potassium chloride, which induced cortical spreading depression. DNA microarray analysis revealed that, compared with the control group, the cortical spreading depression group showed seven upregulated genes–myosin heavy chain 1/2, myosin light chain 1, myosin light chain (phosphorylatable, fast skeletal muscle), actin alpha 1, homeobox B8, carbonic anhydrase 3 and an unknown gene. Two genes were downregulated–RGD1563441 and an unknown gene. Real-time quantitative reverse transcription-PCR and bioinformatics analysis indicated that these genes are involved in motility, cell migration, CO₂/nitric oxide homeostasis and signal transduction.

Potential for therapeutic manipulation of the UPR in disease

Increased endoplasmic reticulum (ER) stress and the activated unfolded protein response (UPR) signaling associated with it play key roles in physiological processes as well as under pathological conditions. The UPR normally protects cells and re-establishes cellular homeostasis, but prolonged UPR activation can lead to the development of various pathologies. These features make the UPR signaling pathway an attractive target for the treatment of diseases whose pathogenesis is characterized by chronic activation of this pathway. Here, we focus on the molecular signaling pathways of the UPR and suggest possible ways to target this response for therapeutic purposes.

ER-stress-associated functional link between Parkin and DJ-1 via a transcriptional cascade involving the tumor suppressor p53 and the spliced X-box binding protein XBP-1

Parkin and DJ-1 are two multi-functional proteins linked to autosomal recessive early-onset Parkinson's disease (PD) that have been shown to functionally interact by as-yet-unknown mechanisms. We have delineated the mechanisms by which parkin controls DJ-1. Parkin modulates DJ-1 transcription and protein levels via a signaling cascade involving p53 and the endoplasmic reticulum (ER)-stress-induced active X-box-binding protein-1S (XBP-1S). Parkin triggers the transcriptional repression of p53 while p53 downregulates DJ-1 protein and mRNA expressions. We show that parkin-mediated control of DJ-1 is fully p53-dependent. Furthermore, we establish that p53 lowers the protein and mRNA levels of XBP-1S. Accordingly, we show that parkin ultimately upregulates XBP-1 levels. Subsequently, XBP-1S physically interacts with the DJ-1 promoter, thereby enhancing its promoter trans-activation, mRNA levels and protein expression. This data was corroborated by the examination of DJ-1 in both parkin- and p53-null mice brains. This transcriptional cascade is abolished by pathogenic parkin mutations and is independent of its ubiquitin-ligase activity. Our data establish a parkin-dependent ER-stress-associated modulation of DJ-1 and identifies p53 and XBP-1 as two major actors acting downstream of parkin in this signaling cascade in cells and in vivo. This work provides a mechanistic explanation for the increase in the unfolded protein response observed in PD pathology, i.e. that it is due to a defect in parkin-associated control of DJ-1.

An involvement of oxidative stress in endoplasmic reticulum stress and its associated diseases

The endoplasmic reticulum (ER) is the major site of calcium storage and protein folding. It has a unique oxidizing-folding environment due to the predominant disulfide bond formation during the process of protein folding. Alterations in the oxidative environment of the ER and also intra-ER Ca²⁺ cause the production of ER stress-induced reactive oxygen species (ROS). Protein disulfide isomerases, endoplasmic reticulum oxidoreductin-1, reduced glutathione and mitochondrial electron transport chain proteins also play crucial roles in ER stress-induced production of ROS. In this article, we discuss ER stress-associated ROS and related diseases, and the current understanding of the signaling transduction involved in ER stress.

Parkinson's disease: from genetics to treatments

Parkinson's disease (PD) is a common neurodegenerative disease and typically presents with tremor, rigidity, bradykinesia, and postural instability. The hallmark pathological features of PD are loss of dopaminergic neurons in the substantia nigra (SN) and the presence of neuronal intracellular Lewy body (LB) inclusions. In general, PD is sporadic; however, familial PD, while uncommon, can be inherited in an autosomal dominant (AD) or autosomal recessive (AR) manner. The molecular investigations of proteins encoded by PD-linked genes have clarified that ADPD is associated with α-synuclein and LRRK2, while ARPD is linked to Parkin, PINK1, DJ1, and ATP13A2. Understanding these genes can bring insights into this disease and create possible genetic tests for early diagnosis. Long-term pharmacological treatment is so far disappointing, probably due to unwanted complications and decreasing drug efficacy. Several strategies have been proposed and tested as alternatives for PD. Cellular transplantation of dopamine-secreting stem cells opens the door to new therapeutic avenues for restoration of the functions of degenerative and/or damaged neurons in PD.

Molecular aspects of dopaminergic neurodegeneration: gene-environment interaction in parkin dysfunction

Parkinson’s disease (PD) is a common neurodegenerative movement disorder that is characterized pathologically by a progressive loss of midbrain dopaminergic neurons and by protein inclusions, designated Lewy bodies and Lewy neurites. PD is one of the most common neurodegenerative diseases, affecting almost 1% of the population over 60 years old. Although the symptoms and neuropathology of PD have been well characterized, the underlying mechanisms and causes of the disease are still not clear. Genetic mutations can provide important clues to disease mechanism, but most PD cases are sporadic rather than familial; environmental factors have long been suspected to contribute to the disease. Although more than 90% of PD cases occur sporadically and are thought to be due, in part, to oxidative stress and mitochondrial dysfunction, the study of genetic mutations has provided great insight into the molecular mechanisms of PD. Furthermore, rotenone, a widely used pesticide, and paraquat and maneb cause a syndrome in rats and mice that mimics, both behaviorally and neurologically, the symptoms of PD. In the current review, we will discuss various aspects of gene-environment interaction that lead to progressive dopaminergic neurodegenration, mainly focusing on our current finding based on stress-mediated parkin dysfunction.

A review of the mammalian unfolded protein response

Proteins requiring post-translational modifications such as N-linked glycosylation are processed in the endoplasmic reticulum (ER). A diverse array of cellular stresses can lead to dysfunction of the ER and ultimately to an imbalance between protein-folding capacity and protein-folding load. Cells monitor protein folding by an inbuilt quality control system involving both the ER and the Golgi apparatus. Unfolded or misfolded proteins are tagged for degradation via ER-associated degradation (ERAD) or sent back through the folding cycle. Continued accumulation of incorrectly folded proteins can also trigger the unfolded protein response (UPR). In mammalian cells, UPR is a complex signaling program mediated by three ER transmembrane receptors: activating transcription factor 6 (ATF6), inositol requiring kinase 1 (IRE1) and double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). UPR performs three functions, adaptation, alarm, and apoptosis. During adaptation, the UPR tries to reestablish folding homeostasis by inducing the expression of chaperones that enhance protein folding. Simultaneously, global translation is attenuated to reduce the ER folding load while the degradation rate of unfolded proteins is increased. If these steps fail, the UPR induces a cellular alarm and mitochondrial mediated apoptosis program. UPR malfunctions have been associated with a wide range of disease states including tumor progression, diabetes, as well as immune and inflammatory disorders. This review describes recent advances in understanding the molecular structure of UPR in mammalian cells, its functional role in cellular stress, and its pathophysiology.

Parkin suppresses c-Jun N-terminal kinase-induced cell death via transcriptional regulation in Drosophila

Parkin is the most prevalent genetic factor in the onset of autosomal recessive juvenile parkinsonism (AR-JP), and mutations in parkin has been reported to cause motor defects, which result from dopamine deficiency caused by dopaminergic neuronal cell death. Activation of c-Jun N-terminal kinase (JNK) has also been implicated in neuronal cell death in Parkinson's disease (PD). Moreover, Drosophila models for AR-JP, loss of function mutants of Drosophila parkin, also show dopaminergic neural degeneration associated with hyperactivation of JNK, increased apoptosis, and mitochondrial defects. However, the molecular mechanism by which Parkin protects cells from apoptosis remains unclear. In the present study, we tested whether Drosophila Parkin suppressed the JNK signaling pathway in developing tissues. Ectopically expressed parkin strongly suppressed the constitutively active form of Hemipterous (Hep(CA)), a Drosophila JNK kinase that induces an eye degeneration phenotype and apoptosis in the eye imaginal disc. Moreover, parkin also suppressed extra vein formation induced by Basket (Bsk), a Drosophila JNK. Interestingly, the bsk mRNA level was markedly reduced by parkin over-expression, suggesting that the effect of parkin on the phenotype induced by activation of JNK signaling was achieved by transcriptional regulation. Furthermore, we found that the expression level of JNK target genes was reduced by parkin over-expression. Taken together, these results suggest that Drosophila Parkin suppresses JNK signaling by reducing bsk transcription.

Effects of acute dieldrin exposure on neurotransmitters and global gene transcription in largemouth bass (Micropterus salmoides) hypothalamus

Exposure to dieldrin induces neurotoxic effects in the vertebrate CNS and disrupts reproductive processes in teleost fish. Reproductive impairment observed in fish by dieldrin is likely the result of multiple effects along the hypothalamic-pituitary-gonadal axis, but the molecular signaling cascades are not well characterized. To better elucidate the mode of action of dieldrin in the hypothalamus, this study measured neurotransmitter levels and examined the transcriptomic response in female largemouth bass (LMB) to an acute treatment of dieldrin. Male and female LMB were injected with either vehicle or 10 mg dieldrin/kg and sacrificed after 7 days. There were no significant changes in dopamine or DOPAC concentrations in the neuroendocrine brain of males and females after treatment but GABA levels in females were moderately increased 20-30% in the hypothalamus and cerebellum. In the female hypothalamus, there were 227 transcripts (p<0.001) identified as being differentially regulated by dieldrin. Functional enrichment analysis revealed transcription, DNA repair, ubiquitin-proteasome pathway, and cell communication, as biological processes over-represented in the microarray analysis. Pathway analysis identified DNA damage, inflammation, regeneration, and Alzheimer's disease as major cell processes and diseases affected by dieldrin. Using multiple bioinformatics approaches, this study demonstrates that the teleostean hypothalamus is a target for dieldrin-induced neurotoxicity and provides mechanistic evidence that dieldrin activates similar cell pathways and biological processes that are also associated with the etiology of human neurological disorders.

The calcineurin-myocyte enhancer factor 2c pathway mediates cardiac hypertrophy induced by endoplasmic reticulum stress in neonatal rat cardiomyocytes

Endoplasmic reticulum (ER) stress (ERS) is involved in various cardiovascular diseases. Our previous study verified that ERS took part in the development of cardiac hypertrophy; however, its mechanism is still unclear. This study aimed to investigate the roles of the calcineurin (CaN) signal pathway in hypertrophy induced by the ERS inductor thapsigargin (TG) in neonatal cardiomyocytes from Sprague-Dawley rats. Investigation of ER chaperone expression, ER staining, and calreticulin immunofluorescence were used to detect the ERS response. mRNA expression of atrial natriuretic peptide and brain natriuretic peptide, total protein synthesis rate, and cell surface area were used to evaluate cardiac hypertrophy induced by TG. TG induced a significant ERS response along with hypertrophy in a dose- and time-dependent manner in cardiomyocytes, which was verified by treatment with tunicamycin, another ERS inducer. Furthermore, TG induced a significant elevation of the intracellular Ca(2+) level, CaN activation, and myocyte enhancer factor 2c (MEF2c) expression in a dose- and time-dependent manner in cardiomyocytes. Cyclosporine A, a CaN inhibitor, markedly suppressed MEF2c nuclear translocation and inhibited TG-induced hypertrophy. These results demonstrate that ERS induces cardiac hypertrophy and that the CaN-MEF2c pathway is involved in ERS-induced hypertrophy in cardiomyocytes.

From genes to proteins in mendelian Parkinson's disease: an overview

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease. A progressive movement disorder typified by the production of bradykinesia, tremor, rigidity, and impairment of postural reflexes, PD is characterized by a depletion of dopamine in the striatum. For the last decade, several Mendelian forms of PD have been identified. Mutations in these genes potentially lead to autosomal dominant (alpha-synuclein and LRRK2), or autosomal recessive PD (Parkin, PINK1, DJ1, and ATP13A2). This article will spotlight these six distinct genes unambiguously associated with Mendelian PD and the function of their encoded proteins.

Cell death pathways in Parkinson's disease: proximal triggers, distal effectors, and final steps

Parkinson's disease (PD) is a common neurodegenerative disorder. Neuronal cell death in PD is still poorly understood, despite a wealth of potential pathogenic mechanisms and pathways. Defects in several cellular systems have been implicated as early triggers that start cells down the road toward neuronal death. These include abnormal protein accumulation, particularly of alpha-synuclein; altered protein degradation via multiple pathways; mitochondrial dysfunction; oxidative stress; neuroinflammation; and dysregulated kinase signaling. As dysfunction in these systems mounts, pathways that are more explicitly involved in cell death become recruited. These include JNK signaling, p53 activation, cell cycle re-activation, and signaling through bcl-2 family proteins. Eventually, neurons become overwhelmed and degenerate; however, even the mechanism of final cell death in PD is still unsettled. In this review, we will discuss cell death triggers and effectors that are relevant to PD, highlighting important unresolved issues and implications for the development of neuroprotective therapies.

An improved in vitro and in vivo Sindbis virus expression system through host and virus engineering

The Sindbis viral expression system enables the rapid production of high levels of recombinant protein in mammalian cells; however, this expression is typically limited to transient production due to the cytotoxicity of the virus. Limiting the lethality inherent in the Sindbis virus vector in order to enable long term, sustained expression of recombinant proteins may be possible. In this study, modifications to virus and host have been combined in order to reduce the cytopathic effects. Non-cytopathic replication competent viruses of two Sindbis viral strains, TE and 633, were developed using a non-structural protein (nsP) P726S point mutation in order to obtain persistent heterologous gene expression in infected Baby Hamster Kidney (BHK) cells and Chinese Hamster Ovary (CHO) cells. Cells infected with the P726S variant viruses were able to recover after infection, while cells infected with normal virus died within 3 days. The P726S mutation did not reduce the susceptibility of 5- and 14-day-old mice to 633 and TE viruses in vivo. In addition, animal survival with the P726S variant viruses was increased and GFP expression was sustained for at least 14 days while the 633 and TE infection resulted in short-term GFP expression or an earlier mortality. Modifications to the host BHK and CHO cells themselves were subsequently undertaken by including the anti-apoptotic gene Bcl-2 and a deletion mutant of Bcl-2 (Bcl-2Delta) as another method for limiting the cytopathic effects of the Sindbis virus. The inclusion of anti-apoptotic genes permitted higher production of heterologous GFP protein following Sindbis virus infection, and the combination of the TE-P726S virus and the CHO-Bcl-2Delta cell line showed the greatest improvement in cell survival. Sindbis virus infection also induced ER stress in mammalian cells as detected by increased PERK phosphorylation and ATF4 translation. Overexpression of Parkin, an E3 ubiquitin ligase that can protect cells against agents that induce ER stress, suppressed Sindbis virus-induced cell death in both BHK cells and in vivo studies in mice. Such findings show that viral and host modifications can improve cell survival and production of heterologous proteins, change viral behavior in vitro and in vivo, and assist in the development of new expression or gene delivery vehicles.

Endoplasmic reticulum stress-induced cell death in dopaminergic cells: effect of resveratrol

Resveratrol, a naturally occurring polyphenol, exhibits antioxidant, antiaging, and anticancer activity. Resveratrol has also been shown to inhibit tumor initiation, promotion, and progression in a variety of cell culture systems. Earlier, we showed that paraquat, a bipyridyl herbicide, triggers endoplasmic reticulum stress, cell dysfunction, and dopaminergic cell death. Due to its antioxidant activity, we assessed the ability of resveratrol to rescue cells from the toxic effects of paraquat. While resveratrol did not have any protective effect at low concentrations, it triggered endoplasmic reticulum (ER) stress-induced cell death at higher concentrations (50-250 microM). The present study was carried out to determine the mechanism by which resveratrol triggers ER stress and cell death in dopaminergic N27 cells. Our studies demonstrate that resveratrol triggers ER stress and cell dysfunction, caspase activation, p23 cleavage and inhibition of proteasomal activity in dopaminergic N27 cells. While over expression of uncleavable p23 was associated with decreased cell death, downregulation of p23 protein expression by siRNA resulted in enhancement of ER stress-induced cell death triggered by resveratrol indicating a protective role for the small co-chaperone p23 in dopaminergic cell death.

Cell death and endoplasmic reticulum stress: disease relevance and therapeutic opportunities

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) represents a cellular stress induced by multiple stimuli and pathological conditions. These include hypoxia, oxidative injury, high-fat diet, hypoglycaemia, protein inclusion bodies and viral infection. ER stress triggers an evolutionarily conserved series of signal-transduction events, which constitutes the unfolded protein response. These signalling events aim to ameliorate the accumulation of unfolded proteins in the ER; however, when these events are severe or protracted they can induce cell death. With the increasing recognition of an association between ER stress and human diseases, and with the improved understanding of the diverse underlying molecular mechanisms, novel targets for drug discovery and new strategies for therapeutic intervention are beginning to emerge.

Coupling endoplasmic reticulum stress to the cell death program in dopaminergic cells: effect of paraquat

Parkinson's disease (PD) features oxidative stress and accumulation of misfolded (unfolded, alternatively folded, or mutant) proteins with associated loss of dopaminergic neurons. Oxidative stress and the accumulated misfolded proteins elicit cellular responses that include an endoplasmic reticulum (ER) stress response that may protect cells against the toxic buildup of misfolded proteins. Chronic ER stress and accumulation of misfolded proteins in excessive amounts, however, overwhelm the cellular 'quality control' system and impair the protective mechanisms designed to promote correct folding and degrade faulty proteins, ultimately leading to organelle dysfunction and neuronal cell death. Paraquat belongs to a class of bipyridyl herbicides and triggers oxidative stress and dopaminergic cell death. Epidemiological studies suggest an increased risk for developing PD following chronic exposure to paraquat. The present study was carried out to determine the role of paraquat in triggering cellular stress particularly ER stress and to elucidate the pathways that couple ER stress to dopaminergic cell death. We demonstrate that paraquat triggers ER stress, cell dysfunction, and dopaminergic cell death. p23, a small co-chaperone protein, is cleaved during ER stress-induced cell death triggered by paraquat and blockage of the caspase cleavage site of p23 was associated with decreased cell death. Paraquat also inhibits proteasomal activity that may further trigger accumulation of misfolded proteins resulting in ER stress. Our results indicate a protective role for p23 in PD-related programmed cell death. The data also underscore the involvement of ER, caspases, and the proteasomal system in ER stress-induced cell death process.

An in vitro model for Lewy body-like hyaline inclusion/astrocytic hyaline inclusion: induction by ER stress with an ALS-linked SOD1 mutation

Neuronal Lewy body-like hyaline inclusions (LBHI) and astrocytic hyaline inclusions (Ast-HI) containing mutant Cu/Zn superoxide dismutase 1 (SOD1) are morphological hallmarks of familial amyotrophic lateral sclerosis (FALS) associated with mutant SOD1. However, the mechanisms by which mutant SOD1 contributes to formation of LBHI/Ast-HI in FALS remain poorly defined. Here, we report induction of LBHI/Ast-HI-like hyaline inclusions (LHIs) in vitro by ER stress in neuroblastoma cells. These LHI closely resemble LBHI/Ast-HI in patients with SOD1-linked FALS. LHI and LBHI/Ast-HI share the following features: 1) eosinophilic staining with a pale core, 2) SOD1, ubiquitin and ER resident protein (KDEL) positivity and 3) the presence of approximately 15–25 nm granule-coated fibrils, which are morphological hallmark of mutant SOD1-linked FALS. Moreover, in spinal cord neurons of L84V SOD1 transgenic mice at presymptomatic stage, we observed aberrant aggregation of ER and numerous free ribosomes associated with abnormal inclusion-like structures, presumably early stage neuronal LBHI. We conclude that the LBHI/Ast-HI seen in human patients with mutant SOD1-linked FALS may arise from ER dysfunction.

Cell type-specific upregulation of Parkin in response to ER stress

Parkin is the gene responsible for a familial form of Parkinson's disease (PD) termed autosomal recessive juvenile parkinsonism (AR-JP)/PARK2. Parkin has been shown to protect cells from endoplasmic reticulum (ER) stress and oxidative stress, presumably due to its ubiquitin ligase (E3) activity that targets proteins for proteasomal degradation. Although the authors showed that parkin is upregulated in response to ER stress, subsequent reports suggest that it does not represent a universal unfolded protein response (UPR). Here the authors report different regulation of parkin in response to ER stress in different cell lines, demonstrating upregulation of parkin as a cell type-specific response to ER stress. 2-Mercaptoethanol (2-ME) and tunicamycin increased the expression of parkin in SH-SY5Y (H) cells, Neuro2a cells, Goto-P3 cells, but not in SH-SY5Y (J) cells and IMR32 cells. In parallel with these studies, similar upregulation of the parkin coregulated gene (PACRG)/gene adjacent to parkin (Glup) was also observed by ER stress. Luciferase assays failed to detect the transcriptional activation of 500 bp parkin/Glup promoter in response to ER stress. These results indicate that induction of parkin by ER stress represents a cell type-specific response.

Proteomic changes of PC12 cells treated with proteasomal inhibitor PSI

Growing evidences suggest that the ubiquitin-proteasome system dysfunction may play an important role in the pathogenesis of Parkinson's disease (PD). In order to investigate the protein changes caused by ubiquitin-proteasome dysfunction in PD, we used a proteomic approach to determine the different protein levels in PC12 cells following proteasomal inhibitor PSI treatment. Twenty-four hour treatment of PC12 cells with PSI induced cell apoptosis and the appearance of cytoplasmic Lewy body-like eosinophilic inclusions, thus recapitulating two primary features of PD. Six protein spots whose contents were changed in response to PSI administration were unambiguously identified as: 94 kDa glucose-regulated protein (GRP94), heat shock 70 kDa protein 5 (GRP78), heat shock 27 kDa protein 1 (Hsp27), aldehyde reductase 1 (aldose reductase), p47 protein and beta-galactoside-binding lectin (galectin-1). They are mainly related with endoplasmic reticulum stress, cellular metabolism and defensive response against toxicity with the last two whose function is unknown in this model. Out of these proteins, some were described for the first time in relation to proteasomal inhibition and PD. These results may provide a valuable clue to the further exploration of the pathogenetic mechanism of PD.

Role of p53 in neurotoxicity induced by the endoplasmic reticulum stress agent tunicamycin in organotypic slice cultures of rat spinal cord

The endoplasmic reticulum (ER) is important for maintaining the quality of cellular proteins. Various stimuli can disrupt ER homeostasis and cause the accumulation of unfolded or misfolded proteins, i.e., a state of ER stress. Recently, ER stress has been reported to play an important role in the pathogenesis of neurological disorders such as cerebral ischemia and neurodegenerative diseases, but its involvement in the spinal cord diseases has not been fully discussed. We conducted this study using tunicamycin (Tm) as an ER stress inducer for rat spinal cord in organotypic slice culture, a system that we have recently established. Tm was shown to induce ER stress by increased expression of GRP78. The viability rate of spinal cord neurons decreased in a dose-dependent manner with Tm treatment, and dorsal horn interneurons were more vulnerable to Tm-induced neurotoxicity. A p53 inhibitor significantly increased the viability of dorsal horn interneurons, and immunofluorescence studies showed nuclear accumulation of p53 in the dorsal horns of Tm-treated spinal cord slices. These findings suggest that p53 plays an important role in the killing of dorsal horn interneurons by Tm. In contrast, motor neurons were not protected by the p53 inhibitor, suggesting that the role of p53 may vary between different cell types. This difference might be a clue to the mechanism of the stress-response pathway and might also contribute to the potential application of p53 inhibitors for the treatment of spinal cord diseases, including amyotrophic lateral sclerosis.

Unfolded protein response in a Drosophila model for retinal degeneration

Stress in the endoplasmic reticulum (ER stress) and its cellular response, the unfolded protein response (UPR), are implicated in a wide variety of diseases, but its significance in many disorders remains to be validated in vivo. Here, we analyzed a branch of the UPR mediated by xbp1 in Drosophila to establish its role in neurodegenerative diseases. The Drosophila xbp1 mRNA undergoes ire-1-mediated unconventional splicing in response to ER stress, and this property was used to develop a specific UPR marker, xbp1-EGFP, in which EGFP is expressed in frame only after ER stress. xbp1-EGFP responds specifically to ER stress, but not to proteins that form cytoplasmic aggregates. The ire-1/xbp1 pathway regulates heat shock cognate protein 3 (hsc3), an ER chaperone. xbp1 splicing and hsc3 induction occur in the retina of ninaE(G69D)-/+, a Drosophila model for autosomal dominant retinitis pigmentosa (ADRP), and reduction of xbp1 gene dosage accelerates retinal degeneration of these animals. These results demonstrate the role of the UPR in the Drosophila ADRP model and open new opportunities for examining the UPR in other Drosophila disease models.

Pael receptor induces death of dopaminergic neurons in the substantia nigra via endoplasmic reticulum stress and dopamine toxicity, which is enhanced under condition of parkin inactivation

Selective loss of dopaminergic neurons is the final common pathway in Parkinson's disease. Expression of Parkin associated endothelin-receptor like receptor (Pael-R) in mouse brain was achieved by injecting adenoviral vectors carrying a modified neuron-specific promoter and Cre recombinase into the striatum. Upregulation of Pael-R in the substantia nigra pars compacta of mice by retrograde infection induced endoplasmic reticulum (ER) stress leads to death of dopaminergic neurons. The role of ER stress in dopaminergic neuronal vulnerability was highlighted by their decreased survival in mice deficient in the ubiquitin-protein ligase Parkin and the ER chaperone ORP150 (150 kDa oxygen-regulated protein). Dopamine-related toxicity was also a key factor, as a dopamine synthesis inhibitor blocked neuronal death in parkin null mice. These data suggest a model in which ER- and dopamine-related stress are major contributors to decreased viability of dopaminergic neurons in a setting relevant to Parkinson's disease.

Expanding insights of mitochondrial dysfunction in Parkinson's disease

The quest to disentangle the aetiopathogenesis of Parkinson's disease has been heavily influenced by the genes associated with the disease. The alpha-synuclein-centric theory of protein aggregation with the adjunct of parkin-driven proteasome deregulation has, in recent years, been complemented by the discovery and increasing knowledge of the functions of DJ1, PINK1 and OMI/HTRA2, which are all associated with the mitochondria and have been implicated in cellular protection against oxidative damage. We critically review how these genes fit into and enhance our understanding of the role of mitochondrial dysfunction in Parkinson's disease, and consider how oxidative stress might be a potential unifying factor in the aetiopathogenesis of the disease.

The role of the ubiquitination machinery in dislocation and degradation of endoplasmic reticulum proteins

Ubiquitination is essential for the dislocation and degradation of proteins from the endoplasmic reticulum (ER). How exactly this is regulated is unknown at present. This review provides an overview of ubiquitin-conjugating enzymes (E2s) and ubiquitin ligases (E3s) with a role in the degradation of ER proteins. Their structure and functions are described, as well as their mutual interactions. Substrate specificity and functional redundancy of E3 ligases are discussed, and other components of the ER degradation machinery that may associate with the ubiquitination system are reviewed.

Endoplasmic reticulum stress: cell life and death decisions

Disturbances in the normal functions of the ER lead to an evolutionarily conserved cell stress response, the unfolded protein response, which is aimed initially at compensating for damage but can eventually trigger cell death if ER dysfunction is severe or prolonged. The mechanisms by which ER stress leads to cell death remain enigmatic, with multiple potential participants described but little clarity about which specific death effectors dominate in particular cellular contexts. Important roles for ER-initiated cell death pathways have been recognized for several diseases, including hypoxia, ischemia/reperfusion injury, neurodegeneration, heart disease, and diabetes.

Misfolded proteins, endoplasmic reticulum stress and neurodegeneration

The accumulation of misfolded proteins (e.g. mutant or damaged proteins) triggers cellular stress responses that protect cells against the toxic buildup of such proteins. However, prolonged stress due to the buildup of these toxic proteins induces specific death pathways. Dissecting these pathways should be valuable in understanding the pathogenesis of, and ultimately in designing therapy for, neurodegenerative diseases that feature misfolded proteins.

[Neurodegeneration caused by ER stress?--the pathogenetic mechanisms underlying AR-JP]

Mutations of the Parkin gene are responsible for autosomal recessive juvenile parkinsonism (AR-JP), the most common cause of early-onset familial Parkinson's disease. Parkin functions as an E3 ubiquitin ligase, thereby promoting ubiquitination and subsequent proteosomal degradation of its substrate(s). AR-JP is, therefore, thought to be caused by accumulation of an unknown toxic protein(s), which would normally be degraded by a molecular machinery involving Parkin. To date, ten different proteins are reported to be substrates of Parkin. Among these, a G protein-coupled orphan receptor called the Pael receptor (Pael-R), which is highly expressed in dopaminergic neurons, attracts particular attention. When over-expressed in cells, the Pael-R protein became improperly folded and insoluble. Excessive accumulation of insoluble Pael-R led to endoplasmic reticulum (ER) stress-induced cell death. Parkin was observed to ubiquitinate the misfolded Pael-R protein, thereby promoting its degradation and suppressing misfolded Pael-R-induced cell death. Moreover, selective dopaminergic neurodegeneration was observed when human Pael-R was ectopically expressed in Drosophila brain, further supporting the idea that Pael-R accumulation plays a major role in AR-JP. In contrast, neither dopaminergic neurodegeneration nor accumulation of any known Parkin substrates was detected in Parkin knockout mice. The role of Pael-R in AR-JP will be discussed based on recent data.

Generalized brain and skin proteasome inhibition in Huntington's disease

Mutated intracellular huntingtin is widely expressed in tissues of Huntington's disease (HD) patients. Intraneuronal nuclear protein aggregates of mutant huntingtin are present in HD brains, suggesting a dysfunction of the ubiquitin proteasome system (UPS). Because many cells and tissues can cope with the abnormal gene effects while others dysfunction and die, we determined gene-induced effects and considered the hypothesis that the gene causes multiple intracellular problems, but severe pathology is seen only in selected brain regions. In this study, we found inhibition of UPS function in both early (0-1, with no or little neuronal loss) and late (3-4, with more severe neuronal loss) stage HD patients' cerebellum, cortex, substantia nigra and caudate-putamen brain regions. Late HD stage increases in ubiquitin levels were unique to caudate-putamen. HD patients' skin fibroblasts also had UPS inhibition similar to brain despite increases in proteasome beta-subunit expression. Gene delivery and expression of proteasome activator PA28 increased UPS function in normal but not HD fibroblasts. These generalized UPS problems are associated with severe neuronal pathology only when coupled with decreases in brain-derived neurotrophic factor levels, mitochondrial complex II/III activity, and increases of ubiquitin levels particularly as seen in the caudate-putamen of HD patients.

NIRF is a ubiquitin ligase that is capable of ubiquitinating PCNP, a PEST-containing nuclear protein

We previously reported the association of a novel Np95/ICBP90-like RING finger protein (NIRF) with a novel PEST-containing nuclear protein (PCNP). NIRF is a nuclear protein with a ubiquitin-like domain, a PHD finger, a YDG/SRA domain, Rb-binding motifs and a RING finger. In this study, we showed that NIRF has auto-ubiquitination activity, the hallmark of a ubiquitin ligase. PCNP was readily ubiquitinated in 293 and COS-7 cells, and NIRF ubiquitinated PCNP in vitro as well as in vivo. Considering that NIRF is implicated in cell cycle regulation, these findings suggest that NIRF and PCNP are a ubiquitin ligase and its substrate, respectively, and may constitute a novel signaling pathway with some relation to cell proliferation.