Parkinsonian mimetics induce aspects of unfolded protein response in death of dopaminergic neurons

X-box-binding protein 1-modified neural stem cells for treatment of Parkinson's disease

X-box-binding protein 1-transfected neural stem cells were transplanted into the right lateral ventricles of rats with rotenone-induced Parkinson's disease. The survival capacities and differentiation rates of cells expressing the dopaminergic marker tyrosine hydroxylase were higher in X-box-binding protein 1-transfected neural stem cells compared to non-transfected cells. Moreover, dopamine and 3,4-dihydroxyphenylacetic acid levels in the substantia nigra were significantly increased, α-synuclein expression was decreased, and neurological behaviors were significantly ameliorated in rats following transplantation of X-box-binding protein 1-transfected neural stem cells. These results indicate that transplantation of X-box-binding protein 1-transfected neural stem cells can promote stem cell survival and differentiation into dopaminergic neurons, increase dopamine and 3,4-dihydroxyphenylacetic acid levels, reduce α-synuclein aggregation in the substantia nigra, and improve the symptoms of Parkinson's disease in rats.

α-Synuclein-mediated inhibition of ATF6 processing into COPII vesicles disrupts UPR signaling in Parkinson's disease

The unfolded protein response (UPR) monitors the folding environment within the endoplasmic reticulum (ER). Accumulation of misfolded proteins within the ER activates the UPR resulting in the execution of adaptive or non-adaptive signaling pathways. α-Synuclein (α-syn) whose accumulation and aggregation define the pathobiology of Parkinson's disease (PD) has been shown to inhibit ER-Golgi transit of COPII vesicles. ATF6, a protective branch of the UPR, is processed via COPII mediated ER-Golgi transit following its activation via ER stress. Using cellular PD models together with biochemical reconstitution assays, we showed that α-syn inhibited processing of ATF6 directly through physical interactions and indirectly through restricted incorporation into COPII vesicles. Impaired ATF6 signaling was accompanied by decreased ER-associated degradation (ERAD) function and increased pro-apoptotic signaling. The mechanism by which α-syn inhibits ATF6 signaling expands our understanding of the role ER stress and the UPR play in neurodegenerative diseases such as PD.

Druggable sensors of the unfolded protein response

The inability of cells to properly fold, modify and assemble secretory and transmembrane proteins leads to accumulation of misfolded proteins in the endoplasmic reticulum (ER). Under these conditions of 'ER stress', cell survival depends on homeostatic benefits from an intracellular signaling pathway called the unfolded protein response (UPR). When activated, the UPR induces transcriptional and translational programs that restore ER homeostasis. However, under high-level or chronic ER stress, these adaptive changes ultimately become overshadowed by alternative 'terminal UPR' signals that actively commit cells to degeneration, culminating in programmed cell death. Chronic ER stress and maladaptive UPR signaling are implicated in the etiology and pathogenesis of myriad human diseases. Naturally, this has generated widespread interest in targeting key nodal components of the UPR as therapeutic strategies. Here we summarize the state of this field with emphasis placed on two of the master UPR regulators, PERK and IRE1, which are both capable of being drugged with small molecules.

RTCB-1 mediates neuroprotection via XBP-1 mRNA splicing in the unfolded protein response pathway

Parkinson's disease (PD), the second most prevalent neurodegenerative disorder, is characterized by the degeneration of dopamine (DA) neurons and age-dependent formation of protein inclusions that contain the α-synuclein (α-syn) protein. RNA interference (RNAi) screening using Caenorhabditis elegans identified RTCB-1, an uncharacterized gene product, as one of several significant modifiers of α-syn protein misfolding. RTCB-1 is the worm ortholog of the human HSPC117 protein, a component of RNA trafficking granules in mammalian neurons. Here we show that RTCB-1 protects C. elegans DA neurons from age-dependent degeneration induced by human α-syn. Moreover, neuronal-specific RNAi depletion of rtcb-1 enhanced α-syn-induced degeneration. Similar results were obtained when worms were exposed to the DA neurotoxin 6-hydroxydopamine. HSPC117 has been characterized recently as an essential subunit of the human tRNA splicing ligase complex. tRNA ligases have alternative functions in RNA repair and nonconventional mRNA splicing events. For example, in yeast, unconventional splicing of HAC1, a transcription factor that controls the unfolded protein response (UPR), is mediated by a tRNA ligase. In C. elegans, we demonstrate that RTCB-1 is necessary for xbp-1 (worm homolog of HAC1) mRNA splicing. Moreover, using a RNA ligase-dead mutant, we determine that the ligase activity of worm RTCB-1 is required for its neuroprotective role, which, in turn, is mediated through XBP-1 in the UPR pathway. Collectively, these studies highlight the mechanistic intersection of RNA processing and proteostasis in mediating neuroprotection.

Systematic comparison of the effects of alpha-synuclein mutations on its oligomerization and aggregation

Aggregation of alpha-synuclein (ASYN) in Lewy bodies and Lewy neurites is the typical pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Furthermore, mutations in the gene encoding for ASYN are associated with familial and sporadic forms of PD, suggesting this protein plays a central role in the disease. However, the precise contribution of ASYN to neuronal dysfunction and death is unclear. There is intense debate about the nature of the toxic species of ASYN and little is known about the molecular determinants of oligomerization and aggregation of ASYN in the cell. In order to clarify the effects of different mutations on the propensity of ASYN to oligomerize and aggregate, we assembled a panel of 19 ASYN variants and compared their behaviour. We found that familial mutants linked to PD (A30P, E46K, H50Q, G51D and A53T) exhibited identical propensities to oligomerize in living cells, but had distinct abilities to form inclusions. While the A30P mutant reduced the percentage of cells with inclusions, the E46K mutant had the opposite effect. Interestingly, artificial proline mutants designed to interfere with the helical structure of the N-terminal domain, showed increased propensity to form oligomeric species rather than inclusions. Moreover, lysine substitution mutants increased oligomerization and altered the pattern of aggregation. Altogether, our data shed light into the molecular effects of ASYN mutations in a cellular context, and established a common ground for the study of genetic and pharmacological modulators of the aggregation process, opening new perspectives for therapeutic intervention in PD and other synucleinopathies.

The accumulation of aggregated proteins in the brain is common across several neurodegenerative disorders. In Parkinson's disease (PD), the protein alpha-synuclein (ASYN) is the major component of aggregates known as Lewy bodies. It is currently unclear whether protein aggregates are protective or detrimental for neuronal function and survival. The present hypothesis is that smaller aggregated species, known as oligomers, might constitute the toxic forms of ASYN. Several mutations in ASYN cause familial forms of PD. In the laboratory, artificial mutations have been designed to enable the study of the aggregation process. However, different studies relied on the use of different model systems, compromising the interpretation of the effects of the mutations. Here, we addressed this by (i) assembling a panel of 19 ASYN variants and (ii) by performing a systematic comparison of the effects of the mutations in mammalian cell models. Interestingly, our study enabled us to correlate oligomerization and aggregation of ASYN in cells. Altogether, our data shed light into the molecular determinants of ASYN aggregation, opening novel avenues for the identification of modulators of ASYN aggregation, which conceal great hopes towards the development of strategies for therapeutic intervention in PD and other synucleinopathies.

The role of endoplasmic reticulum stress in human pathology

Numerous genetic and environmental insults impede the ability of cells to properly fold and posttranslationally modify secretory and transmembrane proteins in the endoplasmic reticulum (ER), leading to a buildup of misfolded proteins in this organelle--a condition called ER stress. ER-stressed cells must rapidly restore protein-folding capacity to match protein-folding demand if they are to survive. In the presence of high levels of misfolded proteins in the ER, an intracellular signaling pathway called the unfolded protein response (UPR) induces a set of transcriptional and translational events that restore ER homeostasis. However, if ER stress persists chronically at high levels, a terminal UPR program ensures that cells commit to self-destruction. Chronic ER stress and defects in UPR signaling are emerging as key contributors to a growing list of human diseases, including diabetes, neurodegeneration, and cancer. Hence, there is much interest in targeting components of the UPR as a therapeutic strategy to combat these ER stress-associated pathologies.

Silencing the gene encoding C/EBP homologous protein lessens acute brain injury following ischemia/reperfusion

C/EBP homologous protein, an important transcription factor during endoplasmic reticulum stress, participates in cell apoptosis mediated by endoplasmic reticulum stress. Previous studies have shown that C/EBP homologous protein mediates nerve injury during Alzheimer’s disease, subarachnoid hemorrhage and spinal cord trauma. In this study, we introduced C/EBP homologous protein short hairpin RNA into the brains of ischemia/reperfusion rat models via injection of lentiviral vector through the left lateral ventricle. Silencing C/EBP homologous protein gene expression significantly reduced cerebral infarction volume, decreased water content and tumor necrosis factor-α and interleukin-1β mRNA expression in brain tissues following infarction, diminished the number of TUNEL-positive cells in the infarct region, decreased caspase-3 protein content and increased Bcl-2 protein content. These results suggest that silencing C/EBP homologous protein lessens cell apoptosis and inflammatory reactions, thereby protecting nerves.

Luteolin modulates 6-hydroxydopamine-induced transcriptional changes of stress response pathways in PC12 cells

The neurotoxin 6-hydroxydopamine (6-OHDA), which causes transcriptional changes associated with oxidative and proteotoxic stress, has been widely used to generate an experimental model of Parkinson's disease. The food-derived compound luteolin has multi-target actions including antioxidant, anti-inflammatory and neurotrophic activities. The aim of this study is to investigate how luteolin affects 6-OHDA-mediated stress response pathways. The results showed that when PC12 cells were pre-treated with luteolin (20 µM) 30 min prior to 6-OHDA (100 µM) exposure, 6-OHDA-induced ROS overproduction, cytotoxicity, caspase-3 activation, and mRNA expression of BIM, TRB3 and GADD34 were significantly attenuated. Moreover, 6-OHDA-mediated cell cycle arrest and transcription of p53 target genes, p21, GADD45α and PUMA, were reduced by luteolin. Luteolin also significantly down-regulated 6-OHDA-mediated unfolded protein response (UPR), leading to decreases in phospho-eIF2α, ATF4, GRP78 and CHOP. In addition, luteolin attenuated 6-OHDA-induced Nrf2-mediated HO-1 and GCLC. Taken together, these results suggest that diminishing intracellular ROS formation and down-regulation of p53, UPR and Nrf2-ARE pathways may be involved in the neuroprotective effect of luteolin.

Transcriptional and metabolic adaptation of human neurons to the mitochondrial toxicant MPP(+)

Assessment of the network of toxicity pathways by Omics technologies and bioinformatic data processing paves the road toward a new toxicology for the twenty-first century. Especially, the upstream network of responses, taking place in toxicant-treated cells before a point of no return is reached, is still little explored. We studied the effects of the model neurotoxicant 1-methyl-4-phenylpyridinium (MPP⁺) by a combined metabolomics (mass spectrometry) and transcriptomics (microarrays and deep sequencing) approach to provide unbiased data on earliest cellular adaptations to stress. Neural precursor cells (LUHMES) were differentiated to homogeneous cultures of fully postmitotic human dopaminergic neurons, and then exposed to the mitochondrial respiratory chain inhibitor MPP⁺ (5 μM). At 18–24 h after treatment, intracellular ATP and mitochondrial integrity were still close to control levels, but pronounced transcriptome and metabolome changes were seen. Data on altered glucose flux, depletion of phosphocreatine and oxidative stress (e.g., methionine sulfoxide formation) confirmed the validity of the approach. New findings were related to nuclear paraspeckle depletion, as well as an early activation of branches of the transsulfuration pathway to increase glutathione. Bioinformatic analysis of our data identified the transcription factor ATF-4 as an upstream regulator of early responses. Findings on this signaling pathway and on adaptive increases of glutathione production were confirmed biochemically. Metabolic and transcriptional profiling contributed complementary information on multiple primary and secondary changes that contribute to the cellular response to MPP⁺. Thus, combined ‘Omics' analysis is a new unbiased approach to unravel earliest metabolic changes, whose balance decides on the final cell fate.

The Parkinsonian mimetic, 6-OHDA, impairs axonal transport in dopaminergic axons

6-hydroxydopamine (6-OHDA) is one of the most commonly used toxins for modeling degeneration of dopaminergic (DA) neurons in Parkinson's disease. 6-OHDA also causes axonal degeneration, a process that appears to precede the death of DA neurons. To understand the processes involved in 6-OHDA-mediated axonal degeneration, a microdevice designed to isolate axons fluidically from cell bodies was used in conjunction with green fluorescent protein (GFP)-labeled DA neurons. Results showed that 6-OHDA quickly induced mitochondrial transport dysfunction in both DA and non-DA axons. This appeared to be a general effect on transport function since 6-OHDA also disrupted transport of synaptophysin-tagged vesicles. The effects of 6-OHDA on mitochondrial transport were blocked by the addition of the SOD1-mimetic, Mn(III)tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP), as well as the anti-oxidant N-acetyl-cysteine (NAC) suggesting that free radical species played a role in this process. Temporally, microtubule disruption and autophagy occurred after transport dysfunction yet before DA cell death following 6-OHDA treatment. The results from the study suggest that ROS-mediated transport dysfunction occurs early and plays a significant role in inducing axonal degeneration in response to 6-OHDA treatment.

Control of dopaminergic neuron survival by the unfolded protein response transcription factor XBP1

Parkinson disease (PD) is characterized by the selective loss of dopaminergic neurons of the substantia nigra pars compacta (SNpc). Although growing evidence indicates that endoplasmic reticulum (ER) stress is a hallmark of PD, its exact contribution to the disease process is not well understood. Here we report that developmental ablation of X-Box binding protein 1 (XBP1) in the nervous system, a key regulator of the unfolded protein response (UPR), protects dopaminergic neurons against a PD-inducing neurotoxin. This survival effect was associated with a preconditioning condition that resulted from induction of an adaptive ER stress response in dopaminergic neurons of the SNpc, but not in other brain regions. In contrast, silencing XBP1 in adult animals triggered chronic ER stress and dopaminergic neuron degeneration. Supporting this finding, gene therapy to deliver an active form of XBP1 provided neuroprotection and reduced striatal denervation in animals injected with 6-hydroxydopamine. Our results reveal a physiological role of the UPR in the maintenance of protein homeostasis in dopaminergic neurons that may help explain the differential neuronal vulnerability observed in PD.

Rifampicin protects PC12 cells from rotenone-induced cytotoxicity by activating GRP78 via PERK-eIF2α-ATF4 pathway

Rifampicin has been proposed as a therapeutic candidate for Parkinson's disease (PD). We previously showed that rifampicin was neuroprotective in PD models in vivo and in vitro. However, the molecular mechanisms underlying are not fully elucidated. In this study, using the comprehensive proteomic analysis, we identified that the 78 kDa glucose-regulated protein (GRP78), a hallmark of the unfolded protein response (UPR), was upregulated in rifampicin-treated PC12 cells. Western blot analysis confirmed GRP78 activation. GRP78 functions cytoprotectively in stressed cells, therefore, we hypothesized that GRP78 mediated rifampicin-induced neuroprotection. Using RNA interference, we found that GRP78 gene knockdown significantly attenuated the neuroprotective effects of rifampicin. Next, we examined three UPR transducers, namely, protein kinase RNA-like endoplasmic reticulum kinase (PERK), inositol requiring kinase α (IREα) and activating transcription factor 6 (ATF 6), and how they regulated rifampicin-stimulated GRP78 expression. Our results showed that PERK, eukaryotic initiation factor 2α (eIF2α), and activating transcription factor 4 (ATF4) were activated in rifampicin-treated PC12 cells. Silencing the ATF4 gene using RNAi inhibited GRP78 stimulation. Interestingly, we did not detect significant IREα activation, X-box binding protein 1 mRNA splicing, or ATF6 cleavage up to 24 h after rifampicin treatment. Taken together, our data suggested that rifampicin induced GRP78 via the PERK-eIF2α-ATF4 pathway to protect neurons against rotenone-induced cell damage. Targeting molecules in this pathway could be a novel therapeutic approach for PD treatment.

Salubrinal protects against rotenone-induced SH-SY5Y cell death via ATF4-parkin pathway

Parkinson's disease (PD) is a progressive neurodegenerative disorder, for which there are no effective disease-modifying therapies. Growing evidence from studies in human PD brain, in addition to genetic and toxicological models, indicates that endoplasmic reticulum (ER) stress is a common feature of the disease and contributes to neurodegeneration. We examine whether salubrinal, a ER stress inhibitor, can protect the rotenone-induced SH-SY5Y cell death and explore the mechanisms underlying this protection. Our results demonstrated that rotenone induced a significant ER stress response and caused cell apoptosis, which was inhibited by salubrinal. Activating transcription factor 4 (ATF4), a member of the ATF/CREB family of basic leucine zipper transcription factors, has been implicated in the pathogenesis of neurodegeneration. We showed that salubrinal increased the up-regulation of ATF4 expression. An ATF4 siRNA significantly increased the rotenone cytotoxicity and decreased the salubrinal's protection. Further, we showed that ATF4 siRNA inhibited the expression of parkin, and parkin knockdown similarly aggravated the rotenone cytotoxicity and reduced the salubrinal's protection. Additionally, the protein level of parkin was declined after treatment with rotenone, whereas this reduction was rescued by salubrinal. These findings indicate ATF4-parkin pathway plays an important role in the salubrinal-mediated neuroprotection of rotenone-induced dopaminergic cell death.

Expression of the CHOP-inducible carbonic anhydrase CAVI-b is required for BDNF-mediated protection from hypoxia

Carbonic anhydrases (CAs) comprise a family of zinc-containing enzymes that catalyze the reversible hydration of carbon dioxide. CAs contribute to a myriad of physiological processes, including pH regulation, anion transport and water balance. To date, 16 known members of the mammalian alpha-CA family have been identified. Given that the catalytic family members share identical reaction chemistry, their physiologic roles are influenced greatly by their tissue and sub-cellular locations. CAVI is the lone secreted CA and exists in both saliva and the gastrointestinal mucosa. An alternative, stress-inducible isoform of CAVI (CAVI-b) has been shown to be expressed from a cryptic promoter that is activated by the CCAAT/Enhancer-Binding Protein Homologous Protein (CHOP). The CAVI-b isoform is not secreted and is currently of unknown physiological function. Here we use neuronal models, including a model derived using Car6 and CHOP gene ablations, to delineate a role for CAVI-b in ischemic protection. Our results demonstrate that CAVI-b expression, which is increased through CHOP-signaling in response to unfolded protein stress, is also increased by oxygen-glucose deprivation (OGD). While enforced expression of CAVI-b is not sufficient to protect against ischemia, CHOP regulation of CAVI-b is necessary for adaptive changes mediated by BDNF that reduce subsequent ischemic damage. These results suggest that CAVI-b comprises a necessary component of a larger adaptive signaling pathway downstream of CHOP.

How Parkinsonian toxins dysregulate the autophagy machinery

Since their discovery, Parkinsonian toxins (6-hydroxydopamine, MPP+, paraquat, and rotenone) have been widely employed as in vivo and in vitro chemical models of Parkinson's disease (PD). Alterations in mitochondrial homeostasis, protein quality control pathways, and more recently, autophagy/mitophagy have been implicated in neurotoxin models of PD. Here, we highlight the molecular mechanisms by which different PD toxins dysregulate autophagy/mitophagy and how alterations of these pathways play beneficial or detrimental roles in dopamine neurons. The convergent and divergent effects of PD toxins on mitochondrial function and autophagy/mitophagy are also discussed in this review. Furthermore, we propose new diagnostic tools and discuss how pharmacological modulators of autophagy/mitophagy can be developed as disease-modifying treatments for PD. Finally, we discuss the critical need to identify endogenous and synthetic forms of PD toxins and develop efficient health preventive programs to mitigate the risk of developing PD.

Unfolded protein response signaling by transcription factor XBP-1 regulates ADAM10 and is affected in Alzheimer's disease

In Alzheimer's disease (AD), disturbed homeostasis of the proteases competing for amyloid precursor protein processing has been reported: a disintegrin and metalloproteinase 10 (ADAM10), the physiological α-secretase, is decreased in favor of the amyloid-β-generating enzyme BACE-1. To identify transcription factors that modulate the expression of either protease, we performed a screening approach: 48 transcription factors significantly interfered with ADAM10/BACE-1-promoter activity. One selective inducer of ADAM10 gene expression is the X-box binding protein-1 (XBP-1). This protein regulates the unfolded protein-response pathway. We demonstrate that particularly the spliced XBP-1 variant dose dependently regulates ADAM10 expression, which can be synergistically enhanced by 100 nM insulin. Analysis of 2 different transgenic mouse models (APP/PS1 and 5xFAD) revealed that at early time points in pathology XBP-1 metabolism is induced. This is accompanied by a 2-fold augmented ADAM10 amount as compared with nontransgenic littermates (P=0.011). Along with aging of the mice, the system is counterregulated, and XBP-1 together with ADAM10 expression level decreased to ∼50% as compared with control animals. Analyses of expression levels in human AD brains showed that ADAM10 mRNA correlated with active XBP-1 (r=0.3120), but expression did not reach levels of healthy age-matched controls, suggesting deregulation of XBP-1 signaling. Our results demonstrate that XBP-1 is a driver of ADAM10 gene expression and that disturbance of this pathway might contribute to development or progression of AD.

Disulfide bonding in neurodegenerative misfolding diseases

In recent years an increasing number of neurodegenerative diseases has been linked to the misfolding of a specific protein and its subsequent accumulation into aggregated species, often toxic to the cell. Of all the factors that affect the behavior of these proteins, disulfide bonds are likely to be important, being very conserved in protein sequences and being the enzymes devoted to their formation among the most conserved machineries in mammals. Their crucial role in the folding and in the function of a big fraction of the human proteome is well established. The role of disulfide bonding in preventing and managing protein misfolding and aggregation is currently under investigation. New insights into their involvement in neurodegenerative diseases, their effect on the process of protein misfolding and aggregation, and into the role of the cellular machineries devoted to disulfide bond formation in neurodegenerative diseases are emerging. These studies mark a step forward in the comprehension of the biological base of neurodegenerative disorders and highlight the numerous questions that still remain open.

Bone marrow stromal cells inhibit caspase-12 expression in rats with spinal cord injury

The mechanisms underlying the potentially beneficial effect of bone marrow stem cells (BMSCs) on spinal cord injury (SCI) are unknown. Therefore, the aim of the present study was to explore the protective effect of BMSCs in rats with SCI. A total of 45 adult male Sprague-Dawley rats were randomly divided into three groups; the SCI group (n=15), the BMSC group (n=15) and the sham-operation group (n=15). In the SCI and BMSC treatment groups, a modified Allen’s weight-drop technique was used to induce SCI. The BMSC treatment group received an injection of BMSCs using a microneedle into the epicenter of the spinal cord 24 h after injury. Rats in the sham-operation group were not subjected to SCI; however, the corresponding vertebral laminae were removed. Seven days after transplantation, a rapid recovery was observed in the Basso, Beattie and Bresnahan (BBB) scores of the BMSC treatment group, whereas the BBB scores in the SCI group remained low (P<0.05). Caspase-12 expression in the SCI group was increased compared with that in the sham-operation group, whereas caspase-12 expression was attenuated 24 h after transplantation in the BMSC treatment group (P<0.05). In conclusion, the transplantation of BMSCs may improve locomotor function and attenuate caspase-12 expression following SCI. Therefore, it is likely to be an effective strategy for preventing severe injury of the spinal cord.

Endoplasmic reticulum stress and Parkinson's disease: the role of HRD1 in averting apoptosis in neurodegenerative disease

Endoplasmic reticulum (ER) stress has been known to be involved in the pathogenesis of various diseases, particularly neurodegenerative disorders such as Parkinson's disease (PD). We previously identified the human ubiquitin ligase HRD1 that is associated with protection against ER stress and its associated apoptosis. HRD1 promotes the ubiquitination and degradation of Parkin-associated endothelin receptor-like receptor (Pael-R), an ER stress inducer and causative factor of familial PD, thereby preventing Pael-R-induced neuronal cell death. Moreover, upregulation of HRD1 by the antiepileptic drug zonisamide suppresses 6-hydroxydopamine-induced neuronal cell death. We review recent progress in the studies on the mechanism of ER stress-induced neuronal death related to PD, particularly focusing on the involvement of HRD1 in the prevention of neuronal death as well as a potential therapeutic approach for PD based on the upregulation of HRD1.

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.

ATF4 protects against neuronal death in cellular Parkinson's disease models by maintaining levels of parkin

Parkinson's disease (PD) is a common neurodegenerative disorder, for which there are no effective disease-modifying therapies. The transcription factor ATF4 (activating transcription factor 4) is induced by multiple PD-relevant stressors, such as endoplasmic reticulum stress and oxidative damage. ATF4 may exert either protective or deleterious effects on cell survival, depending on the paradigm. However, the role of ATF4 in the pathogenesis of PD has not been explored. We find that ATF4 levels are increased in neuromelanin-positive neurons in the substantia nigra of a subset of PD patients relative to controls. ATF4 levels are also upregulated in neuronal PC12 cells treated with the dopaminergic neuronal toxins 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium (MPP+). To explore the role of ATF4 in cell survival in PD-relevant contexts, we either silenced or overexpressed ATF4 in cellular models of PD. In neuronal PC12 cells, silencing of ATF4 enhanced cell death in response to either 6-OHDA or MPP+. Conversely, overexpression of ATF4 reduced cell death caused by dopaminergic neuronal toxins. ATF4 was also protective against 6-OHDA-induced death of cultured mouse ventral midbrain dopaminergic neurons. We further show that parkin, a gene associated with autosomal recessive PD, plays a critical role in ATF4-mediated protection. After treatment with 6-OHDA or MPP+, parkin protein levels fall, despite an increase in mRNA levels. ATF4 silencing exacerbates the toxin-induced reduction of parkin, whereas ATF4 overexpression partially preserves parkin levels. Finally, parkin silencing blocked the protective capacity of ATF4. These results indicate that ATF4 plays a protective role in PD through the regulation of parkin.

Decreased vitamin B12 availability induces ER stress through impaired SIRT1-deacetylation of HSF1

Vitamin B12 (cobalamin) is a key determinant of S-adenosyl methionine (SAM)-dependent epigenomic cellular regulations related to methylation/acetylation and its deficiency produces neurodegenerative disorders by elusive mechanisms. Sirtuin 1 deacetylase (SIRT1) triggers cell response to nutritional stress through endoplasmic reticulum (ER) stress. Recently, we have established a N1E115 dopaminergic cell model by stable expression of a transcobalamin–oleosin chimera (TO), which impairs cellular availability of vitamin B12, decreases methionine synthase activity and SAM level, and reduces cell proliferation. In contrast, oleosin-transcobalamin chimera (OT) does not modify the phenotype of transfected cells. Presently, the impaired cellular availability of vitamin B12 in TO cells activated irreversible ER stress pathways, with increased P-eIF-2α, P-PERK, P-IRE1α, ATF6, ATF4, decreased chaperon proteins and increased pro-apoptotic markers, CHOP and cleaved caspase 3, through reduced SIRT1 expression and consequently greater acetylation of heat-shock factor protein 1 (HSF1). Adding either B12, SIRT1, or HSF1 activators as well as overexpressing SIRT1 or HSF1 dramatically reduced the activation of ER stress pathways in TO cells. Conversely, impairing SIRT1 and HSF1 by siRNA, expressing a dominant negative form of HSF1, or adding a SIRT1 inhibitor led to B12-dependent ER stress in OT cells. Addition of B12 abolished the activation of stress transducers and apoptosis, and increased the expression of protein chaperons in OT cells subjected to thapsigargin, a strong ER stress stimulator. AdoX, an inhibitor of methyltransferase activities, produced similar effects than decreased B12 availability on SIRT1 and ER stress by a mechanism related to increased expression of hypermethylated in cancer 1 (HIC1). Taken together, these data show that cellular vitamin B12 has a strong modulating influence on ER stress in N1E115 dopaminergic cells. The impaired cellular availability in vitamin B12 induces irreversible ER stress by greater acetylation of HSF1 through decreased SIRT1 expression, whereas adding vitamin B12 produces protective effects in cells subjected to ER stress stimulation.

MPP+-induces PUMA- and p53-dependent, but ATF3-independent cell death

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra (SN) and depletion of striatal dopamine (DA), leading to a range of motor symptoms, including resting tremor, rigidity, bradykinesia and postural abnormalities. The neurotoxin (MPTP) and its active metabolite, 1-methyl-4-phenylpyridinium (MPP(+)), cause dopaminergic cell loss in a variety of animal species and produce symptoms similar to those seen in PD. Our lab has shown that MPP(+) activates cell stress pathways, including the unfolded protein response (UPR) in mouse primary mesencephalic cultures. The BH3-only protein, PUMA (p53 upregulated mediator of apoptosis), has been shown to be activated in response to many cellular stresses, including endoplasmic reticulum (ER) stress and UPR, and to induce cell death. Therefore, we hypothesized that PUMA may mediate MPP(+) toxicity. To test this hypothesis, we compared the response of primary mesencephalic cultures from wild-type and PUMA deficient (-/-) mice to MPP(+). We also utilized cultures from p53 -/- and activating transcription factor 3 (ATF3) -/- mice to further elucidate the pathways involved. These studies revealed that PUMA and p53, but not ATF3, are required for MPP(+)-induced cell death, suggesting that UPR activation is parallel to the induction of MPP(+)-induced cell death.

RESP18 is involved in the cytotoxicity of dopaminergic neurotoxins in MN9D cells

RESP18 (Regulated endocrine-specific protein, 18 kDa) was first identified as a dopaminergic drugs-regulated intermediate pituitary transcript. RESP18 protein is a unique endoplasmic reticulum (ER) resident protein. Its functions in the brain especially in the nervous system disorders remain unknown. ER stress (ERS) has been proved to be one of the important pathogenesis of neurodegenerative diseases, including Parkinson's disease (PD). Here, we explored the association of RESP18 and ERS in cell models of PD. Dopaminergic neurotoxin 1-methyl-4-phenyl-pyridinium ion (MPP⁺), 6-hydroxydopamine (6-OHDA), and rotenone evoked dramatic MN9D cell death. The transcriptional expressions of RESP18 and two ERS markers--binding immunoglobulin protein/glucose-regulated protein 78 (BiP/GRP78) and CCAAT/enhancer-binding protein homologous protein (CHOP) manifested differential changes in MN9D cells treated with MPP⁺, 6-OHDA, and rotenone. The RESP18 protein levels increased in MPP⁺ and 6-OHDA-treated cells, but did not change in the cells treated with rotenone, while the protein levels of ER molecular chaperone heat shock protein 90 kDa beta member 1/glucose-regulated protein 94 (HSP90B1/GRP94) and BiP in the cells were up-regulated by MPP⁺ and 6-OHDA, respectively. Salubrinal, an ERS inhibitor, significantly reduced MPP⁺ and 6-OHDA-induced cell death. Moreover, ERS inducer--thapsigargin and tunicamycin, decreased the expression of RESP18, which is different from the changes of BiP, GRP94, and CHOP. Silencing RESP18 expression with Lenti-shRNA alleviated MPP⁺-induced cell death, while over-expression of RESP18 resulted in aggravated cell death induced by MPP⁺ and 6-OHDA. Taken together, our results suggest that RESP18 is involved in the cytotoxicity of dopaminergic neurotoxins.

The Molecular Chaperone GRP78/BiP as a Therapeutic Target for Neurodegenerative Disorders: A Mini Review

The glucose regulated protein 78 (GRP78), also known as BiP, is the endoplasmatic reticulum (ER) homologue of HSP70, which plays a dual role in the ER by controlling protein folding, in order to prevent aggregation, and by regulating the signaling of the unfolded protein response (UPR). Most neurodegenerative disorders including Parkinson's, Alzheimer's diseases and progressive retinal degeneration are characterized by activation of the UPR and modified expression of GRP78. The expression levels and activity of GRP78 are altered with age raising the question of whether the lack of GRP78 could be a predisposing factor for many neurodegenerative disorders associated with age including PD, Alzheimer and Age-related macular degeneration. Attempts to induce or upregulate GRP78 in animal models of neurodegeneration have recently been made with the help of pharmacological BiP protein Inducer X (BIX) and GRP78 cDNA delivery via adeno-associated virus (AAV) vectors. The results of these studies validate GRP78 as a new therapeutic target for treatments of forebrain ischemia, Parkinson disease and retinal degeneration. These data, together with the results from age-related studies, highlight the importance for developing drugs to induce elevation of endogenous GRP78 in order to increase cellular survival and extend functional longevity.

Functional GPR37 trafficking protects against toxicity induced by 6-OHDA, MPP+ or rotenone in a catecholaminergic cell line

G protein-coupled receptor 37 (GPR37) is suggested to be implicated in the pathogenesis of Parkinson's disease and is accumulating in Lewy bodies within afflicted brain regions. Over-expressed GPR37 is prone to misfolding and aggregation, causing cell death via endoplasmic reticulum stress. Although the cytotoxicity of misfolded GPR37 is well established, effects of the functional receptor on cell viability are still unknown. An N2a cell line stably expressing green fluorescent protein (GFP)-tagged human GPR37 was created to study its trafficking and effects on cell viability upon challenge with the toxins 1-methyl-4-phenylpyridinium (MPP+), rotenone and 6-hydroxydopamine (6-OHDA). Neuronal-like differentiation into a tyrosine hydroxylase expressing phenotype, using dibutyryl-cAMP, induced trafficking of GPR37 to the plasma membrane. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell viability and lactate dehydrogenase (LDH) cell death assays revealed that GPR37 was protective against all three toxins in differentiated cells. In undifferentiated cells, the majority of GPR37 was cytoplasmic and the protective effects were more variable: GPR37 expression protected against rotenone and MPP+ but not against 6-OHDA in MTT assays, while it protected against 6-OHDA but not against MPP+ or rotenone in lactate dehydrogenase (LDH) assays. These results suggest that GPR37 functionally trafficked to the plasma membrane protects against toxicity.

ATF6alpha promotes astroglial activation and neuronal survival in a chronic mouse model of Parkinson's disease

Accumulating evidence suggests a crucial role for the unfolded protein response (UPR) in Parkinson's disease (PD). In this study, we investigated the relevance of the UPR in a mouse model of chronic MPTP/probenecid (MPTP/P) injection, which causes severe and persistent degeneration of dopaminergic neurons. Enhanced activation of the UPR branches, including ATF6α and PERK/eIF2α/ATF4, was observed after MPTP/P injections into mice. Deletion of the ATF6α gene accelerated neuronal degeneration and ubiquitin accumulation relatively early in the MPTP/P injection course. Surprisingly, astroglial activation was strongly suppressed, and production of the brain-derived neurotrophic factor (BDNF) and anti-oxidative genes, such as heme oxygenase-1 (HO-1) and xCT, in astrocytes were reduced in ATF6α -/- mice after MPTP/P injections. Decreased BDNF expression in ATF6α -/- mice was associated with decreased expression of GRP78, an ATF6α-dependent molecular chaperone in the ER. Decreased HO-1 and xCT levels were associated with decreased expression of the ATF4-dependent pro-apoptotic gene CHOP. Consistent with these results, administration of the UPR-activating reagent tangeretin (5,6,7,8,4'-pentamethoxyflavone; IN19) into mice enhanced the expression of UPR-target genes in both dopaminergic neurons and astrocytes, and promoted neuronal survival after MPTP/P injections. These results suggest that the UPR is activated in a mouse model of chronic MPTP/P injection, and contributes to the survival of nigrostriatal dopaminergic neurons, in part, through activated astrocytes.

The neuroprotection of hydrogen sulfide against MPTP-induced dopaminergic neuron degeneration involves uncoupling protein 2 rather than ATP-sensitive potassium channels

AIMS

Hydrogen sulfide (H(2)S), a novel gaseous mediator, has been recognized to protect neurons from overexcitation by enhancing the activity of the adenosine triphosphate-sensitive potassium (K-ATP) channel. However, no direct evidence supports that the K-ATP channel contributes to the neuroprotective effect of H(2)S in neurodegeneration. Herein, wild-type and Kir6.2 knockout (Kir6.2(-/-)) mice were used to establish the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease (PD) so as to investigate the involvement of K-ATP channels in the neuroprotection of H(2)S.

RESULTS

Systemic administration of sodium hydrosulfide (NaHS) (an H(2)S donor, 5.6 mg/kg/day) for 7 days rescued MPTP-induced loss of dopaminergic (DA) neurons in substantia nigra compacta of both Kir6.2(+/+) and Kir6.2(-/-) mice. Consistently, NaHS (100 μM) protected primary mesencephalic neurons against 1-methyl-4-phenylpyridinium (MPP(+))-induced cytotoxicity in both genotypes. We further found that deficiency of mitochondrial uncoupling protein 2 (UCP2), which reduces reactive oxygen species (ROS) production and functions as upstream to the K-ATP channel in determining vulnerability of DA neurons, abolished the protective effects of H(2)S against either DA neuron degeneration in the PD mouse model or MPP(+)-induced injury in primary mesencephalic neurons. Rationally, UCP2 evokes mild uncoupling, which in turn diminishes ROS accumulation in DA neurons. Furthermore, H(2)S exerted neuroprotective effect via enhancing UCP2-mediated antioxidation and subsequently suppressing ROS-triggered endoplasmic reticulum stress as well as ultimately inhibiting caspase 12-induced neuronal apoptosis.

INNOVATION AND CONCLUSION

H(2)S protects DA neurons against degeneration in a UCP2 rather than Kir6.2/K-ATP channel-dependent mechanism, which will give us an insight into the potential of H(2)S in terms of opening up new therapeutic avenues for PD.

Sequences located within the N-terminus of the PD-linked LRRK2 lead to increased aggregation and attenuation of 6-hydroxydopamine-induced cell death

Clinical symptoms of Parkinson's disease (PD) arise from the loss of substantia nigra neurons resulting in bradykinesia, rigidity, and tremor. Intracellular protein aggregates are a pathological hallmark of PD, but whether aggregates contribute to disease progression or represent a protective mechanism remains unknown. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been linked to PD in both familial cases and idiopathic cases and aggregates of the LRRK2 protein are present in postmortem PD brain samples. To determine whether LRRK2 contains a region of protein responsible for self-aggregation, two independent, bioinformatic algorithms were used to identify an N-terminal amino acid sequence as being aggregation-prone. Cells subsequently transfected with a construct containing this domain were found to have significantly increased protein aggregation compared to wild type protein or a construct containing only the last half of the molecule. Finally, in support of the hypothesis that aggregates represent a self-protection strategy, aggregated N-terminal LRRK2 constructs significantly attenuated cell death induced by the PD-mimetic, 6-hydroxydopamine (6-OHDA).

Neuroprotective effects of longan ( Dimocarpus longan Lour.) flower water extract on MPP+-induced neurotoxicity in rat brain

In this study, the neuroprotective effect of Dimocarpus longan Lour. flower water extract (LFWE) was investigated. First, an in vitro study showed that LFWE concentration-dependently inhibited lipid peroxidation of brain homogenates incubated at 37 °C. The antioxidative activity of LFWE was more potent than that of glutathione or Trolox. Furthermore, an ex vivo study found that the basal lipid peroxidation (0 °C) and lipid peroxidation incubated at 37 °C were lower in the brain homogenates of LFWE-treated (500 mg/day) rats, indicating that the brain of LFWE-treated rats was more resistant to oxidative stress. Moreover, a Parkinsonian animal model was employed to demonstrate that oral administration of LFWE (125-500 mg/kg/day) dose-dependently attenuated 1-methyl-4-phenylpyridinium (MPP(+))-induced neurotoxicity in the nigrostriatal dopaminergic system of rat brain. In conclusion, this study suggests that LFWE is antioxidative, anti-inflammatory, and anti-apoptotic. Furthermore, oral administration of LFWE appears to be useful in preventing and/or treating central nervous system neurodegenerative diseases, including Parkinsonism.

Protein disulfide isomerases in neurodegeneration: from disease mechanisms to biomedical applications

Protein disulfide isomerases (PDIs) are a family of foldases and chaperones primarily located at the endoplasmic reticulum that catalyze the formation and isomerization of disulfide bonds thereby facilitating protein folding. PDIs also perform important physiological functions in protein quality control, cell death, and cell signaling. Protein misfolding is involved in the etiology of the most common neurodegenerative diseases, including Alzheimer, Parkinson, amyotrophic lateral sclerosis, Prion-related disorders, among others. Accumulating evidence indicate altered expression of PDIs as a prominent and common feature of these neurodegenerative conditions. Here we overview most recent advances in our understanding of the possible functional contribution of PDIs to neurodegeneration, depicting a complex and poorly understood scenario. Possible therapeutic benefits of targeting PDIs in a disease context and their use as biomarkers are discussed.

Compromising σ-1 receptors at the endoplasmic reticulum render cytotoxicity to physiologically relevant concentrations of dopamine in a nuclear factor-κB/Bcl-2-dependent mechanism: potential relevance to Parkinson's disease

The endoplasmic reticulum (ER) chaperone σ-1 receptor (Sig-1R) is cytoprotective against ER stress-induced apoptosis. The level of Sig-1Rs in the brain was reported to be lower in early parkinsonian patients. Because dopamine (DA) toxicity is well known to be involved in the etiology of Parkinson's disease, we tested in this study whether a relationship might exist between Sig-1Rs and DA-induced cytotoxicity in a cellular model by using Chinese hamster ovary (CHO) cells. DA in physiological concentrations (e.g., lower than 10 μM) does not cause apoptosis. However, the same concentrations of DA cause apoptosis in Sig-1R knockdown CHO cells. In search of a mechanistic explanation, we found that unfolded protein response is not involved. Rather, the level of protective protein Bcl-2 is critically involved in this DA/Sig-1R knockdown-induced apoptosis. Specifically, the DA/Sig-1R knockdown causes a synergistic proteasomal conversion of nuclear factor κB (NF-κB) p105 to the active form of p50, which is known to down-regulate the transcription of Bcl-2. It is noteworthy that the DA/Sig-1R knockdown-induced apoptosis is blocked by the overexpression of Bcl-2. Our results therefore indicate that DA is involved in the activation of NF-κB and suggest that endogenous Sig-1Rs are tonically inhibiting the proteasomal conversion/activation of NF-κB caused by physiologically relevant concentrations of DA that would otherwise cause apoptosis. Thus, Sig-1Rs and associated ligands may represent new therapeutic targets for the treatment of parkinsonism.

Hydrogen sulfide protects SH-SY5Y cells against 6-hydroxydopamine-induced endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress has been implicated in several neurodegenerative diseases, including Parkinson's disease. The present study attempted to investigate the effect of hydrogen sulfide (H(2)S) on 6-hydroxydopamine (6-OHDA)-induced ER stress in SH-SY5Y cells. We found in the present study that exogenous application of sodium hydrosulfide (NaHS; an H(2)S donor, 100 μM) significantly attenuated 6-OHDA (50 μM)-induced cell death. NaHS also reversed the upregulation of cleaved poly(ADP-ribose) polymerase and caspase 9 in 6-OHDA-treated cells. Consistent with its cytoprotective effects, NaHS markedly reduced 6-OHDA induced-ER stress responses, including the upregulated levels of eukaryotic initiation factor-2α phosphorylation, glucose-regulated protein 78, and C/EBP homologous protein expression. The protective effect of H(2)S on ER stress was attenuated by blockade of Akt activity with an Akt inhibitor or inhibition of heat shock protein (Hsp)90 with geldanamycin but not by suppression of ERK1/2 with PD-98059. Blockade of Akt also significantly decreased the protein abundance of Hsp90 in SH-SY5Y cells. Moreover, overexpression of cystathionine β-synthase (a main H(2)S-synthesizing enzyme in the brain) elevated the Hsp90 protein level and suppressed 6-OHDA-induced ER stress. In conclusion, the protective effect of H(2)S against 6-OHDA-induced ER stress injury in SH-SY5Y cells involves the Akt-Hsp90 pathway.

Protective effects of XBP1 against oxygen and glucose deprivation/reoxygenation injury in rat primary hippocampal neurons

The accumulation of misfolded and unfolded proteins in the endoplasmic reticulum (ER) induces ER stress, activating the unfolded protein response (UPR). One of the effectors of the UPR is XBP1, a critical transcriptional factor for genes responsible for cell survival. ER stress is also known to play a vital role in mediating ischemic reperfusion damage in the brain. In this study, we investigated the role of XBP1 in rat primary hippocampal neurons subjected to oxygen and glucose deprivation followed by reoxygenation (OGD/R) stress, an in vitro model of ischemia/reperfusion (I/R) injury. Primary neurons subjected to OGD had increased levels of spliced XBP1 (XBP1s) mRNA. Interestingly, the level of XBP1s decreased during the initial reoxygenation stress period. The combination of OGD and the subsequent 20-h reoxygenation stress period significantly increased the apoptotic death of primary cells. Overexpression of XBP1s suppressed cell death induced by OGD/R stress. These results suggest that suppression of XBP1 activation accelerates neuronal cell death after I/R and that activation of the XBP1 pathway may provide a therapeutic approach for the treatment of cerebral I/R injury.

Unfolded protein stress in the endoplasmic reticulum and mitochondria: a role in neurodegeneration

Protein-folding occurs in several intracellular locations including the endoplasmic reticulum and mitochondria. In normal conditions there is a balance between the levels of unfolded proteins and protein folding machinery. Disruption of homeostasis and an accumulation of unfolded proteins trigger stress responses, or unfolded protein responses (UPR), in these organelles. These pathways signal to increase the folding capacity, inhibit protein import or expression, increase protein degradation, and potentially trigger cell death. Many aging-related neurodegenerative diseases involve the accumulation of misfolded proteins in both the endoplasmic reticulum and mitochondria. The exact participation of the UPRs in the onset of neurodegeneration is unclear, but there is significant evidence for the alteration of these pathways in the endoplasmic reticulum and mitochondria. Here we will discuss the involvement of endoplasmic reticulum and mitochondrial stress and the possible contributions of the UPR in these organelles to the development of two neurodegenerative diseases, Parkinson's disease (PD) and Alzheimer's disease (AD).

Melatonin attenuates kainic acid-induced neurotoxicity in mouse hippocampus via inhibition of autophagy and α-synuclein aggregation

In this study, the protective effect of melatonin on kainic acid (KA)-induced neurotoxicity involving autophagy and α-synuclein aggregation was investigated in the hippocampus of C57/BL6 mice. Our data showed that intraperitoneal injection of KA (20 mg/kg) increased LC3-II levels (a hallmark protein of autophagy) and reduced mitochondrial DNA content and cytochrome c oxidase levels (a protein marker of mitochondria). Atg7 siRNA transfection prevented KA-induced LC3-II elevations and mitochondria loss. Furthermore, Atg7 siRNA attenuated KA-induced activation of caspases 3/12 (biomarkers of apoptosis) and hippocampal neuronal loss, suggesting a pro-apoptotic role of autophagy in the KA-induced neurotoxicity. Nevertheless, KA-induced α-synuclein aggregation was not affected in the Atg7 siRNA-transfected hippocampus. The neuroprotective effect of melatonin (50 mg/kg) orally administered 1 hr prior to KA injection was studied. Melatonin was found to inhibit KA-induced autophagy-lysosomal activation by reducing KA-induced increases in LC3-II, lysosomal-associated membrane protein 2 (a biomarker of lysosomes) and cathepsin B (a lysosomal cysteine protease). Subsequently, KA-induced mitochondria loss was prevented in the melatonin-treated mice. At the same time, melatonin reduced KA-increased HO-1 levels and α-synuclein aggregation. Our immunoprecipitation study showed that melatonin enhanced ubiquitination of α-synuclein monomers and aggregates. The anti-apoptotic effect of melatonin was demonstrated by attenuating KA-induced DNA fragmentation, activation of caspases 3/12, and neuronal loss. Taken together, our study suggests that KA-induced neurotoxicity may be mediated by autophagy and α-synuclein aggregation. Moreover, melatonin may exert its neuroprotection via inhibiting KA-induced autophagy and a subsequent mitochondrial loss as well as reducing α-synuclein aggregation by enhancing α-synuclein ubiquitination in the CNS.

Neurotoxin-induced ER stress in mouse dopaminergic neurons involves downregulation of TRPC1 and inhibition of AKT/mTOR signaling

Individuals with Parkinson's disease (PD) experience a progressive decline in motor function as a result of selective loss of dopaminergic (DA) neurons in the substantia nigra. The mechanism(s) underlying the loss of DA neurons is not known. Here, we show that a neurotoxin that causes a disease that mimics PD upon administration to mice, because it induces the selective loss of DA neurons in the substantia nigra, alters Ca²⁺ homeostasis and induces ER stress. In a human neuroblastoma cell line, we found that endogenous store-operated Ca²⁺ entry (SOCE), which is critical for maintaining ER Ca²⁺ levels, is dependent on transient receptor potential channel 1 (TRPC1) activity. Neurotoxin treatment decreased TRPC1 expression, TRPC1 interaction with the SOCE modulator stromal interaction molecule 1 (STIM1), and Ca²⁺ entry into the cells. Overexpression of functional TRPC1 protected against neurotoxin-induced loss of SOCE, the associated decrease in ER Ca²⁺ levels, and the resultant unfolded protein response (UPR). In contrast, silencing of TRPC1 or STIM1 increased the UPR. Furthermore, Ca²⁺ entry via TRPC1 activated the AKT pathway, which has a known role in neuroprotection. Consistent with these in vitro data, Trpc1⁻/⁻ mice had an increased UPR and a reduced number of DA neurons. Brain lysates of patients with PD also showed an increased UPR and decreased TRPC1 levels. Importantly, overexpression of TRPC1 in mice restored AKT/mTOR signaling and increased DA neuron survival following neurotoxin administration. Overall, these results suggest that TRPC1 is involved in regulating Ca²⁺ homeostasis and inhibiting the UPR and thus contributes to neuronal survival.

Pharmacological chaperoning of nicotinic acetylcholine receptors reduces the endoplasmic reticulum stress response

We report the first observation that endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) can decrease when a central nervous system drug acts as an intracellular pharmacological chaperone for its classic receptor. Transient expression of α4β2 nicotinic receptors (nAChRs) in Neuro-2a cells induced the nuclear translocation of activating transcription factor 6 (ATF6), which is part of the UPR. Cells were exposed for 48 h to the full agonist nicotine, the partial agonist cytisine, or the competitive antagonist dihydro-β-erythroidine; we also tested mutant nAChRs that readily exit the ER. Each of these four manipulations increased Sec24D-enhanced green fluorescent protein fluorescence of condensed ER exit sites and attenuated translocation of ATF6-enhanced green fluorescent protein to the nucleus. However, we found no correlation among the manipulations regarding other tested parameters [i.e., changes in nAChR stoichiometry (α4(2)β2(3) versus α4(3)β2(2)), changes in ER and trans-Golgi structures, or the degree of nAChR up-regulation at the plasma membrane]. The four manipulations activated 0 to 0.4% of nAChRs, which shows that activation of the nAChR channel did not underlie the reduced ER stress. Nicotine also attenuated endogenously expressed ATF6 translocation and phosphorylation of eukaryotic initiation factor 2α in mouse cortical neurons transfected with α4β2 nAChRs. We conclude that, when nicotine accelerates ER export of α4β2 nAChRs, this suppresses ER stress and the UPR. Suppression of a sustained UPR may explain the apparent neuroprotective effect that causes the inverse correlation between a person's history of tobacco use and susceptibility to developing Parkinson's disease. This suggests a novel mechanism for neuroprotection by nicotine.

A novel animal model of hearing loss caused by acute endoplasmic reticulum stress in the cochlea

Many stimuli such as ischemia, hypoxia, heat shock, amino acid starvation, and gene mutation, exhibit a cellular response called endoplasmic reticulum (ER) stress. ER stress induces expression of a series of genes, leading to cell survival or apoptosis. Previously, we found that in an animal model of hearing loss caused by acute mitochondrial dysfunction, several ER stress markers including C/EBP homologous protein were induced in the cochlear lateral wall. To elucidate the mechanism of hearing loss caused by ER stress, we established a novel animal model of hearing loss by perilymphatic perfusion of tunicamycin, an ER stress activator that inhibits N-acetylglucosamine transferases. Subacute and progressive hearing loss was observed at all sound frequencies studied, and stimulation of ER stress marker genes was noted in the cochlea. The outer hair cells were the most sensitive to ER stress in the cochlea. Electron microscopic analysis demonstrated degeneration of the subcellular organelles of the inner hair cells and nerve endings of the spiral ganglion cells. This newly established animal model of hearing loss from ER stress will provide additional insight into the mechanism of sensorineural hearing loss.