Endogenous catecholamine enhances the dysfunction of unfolded protein response and α-synuclein oligomerization in PC12 cells overexpressing human α-synuclein

Complex of EGCG with Cu(II) Suppresses Amyloid Aggregation and Cu(II)-Induced Cytotoxicity of α-Synuclein

Accumulation of α-synuclein (α-Syn) is a remarkable pathology for Parkinson’s disease (PD), therefore clearing it is possibly a promising strategy for treating PD. Aberrant copper (Cu(II)) homeostasis and oxidative stress play critical roles in the abnormal aggregation of α-Syn in the progress of PD. It is reported that the polyphenol (−)-epi-gallocatechin gallate (EGCG) can inhibit α-Syn fibrillation and aggregation, disaggregate α-Syn mature fibrils, as well as protect α-Syn overexpressed-PC12 cells against damage. Also, previous studies have reported that EGCG can chelate many divalent metal ions. What we investigate here is whether EGCG can interfere with the Cu(II) induced fibrillation of α-Syn and protect the cell viability. In this work, on a molecular and cellulaire basis, we demonstrated that EGCG can form a Cu(II)/EGCG complex, leading to the inhibition of Cu(II)-induced conformation transition of α-Syn from random coil to β-sheet, which is a dominant structure in α-Syn fibrils and aggregates. Moreover, we found that the mixture of Cu(II) and EGCG in a molar ratio from 0.5 to 2 can efficiently inhibit this process. Furthermore, we demonstrated that in the α-Syn transduced-PC12 cells, EGCG can inhibit the overexpression and fibrillation of α-Syn in the cells, and reduce Cu(II)-induced reactive oxygen species (ROS), protecting the cells against Cu(II)-mediated toxicity.

In vitro models of synucleinopathies: informing on molecular mechanisms and protective strategies

Alpha-synuclein (α-Syn) is a central player in Parkinson's disease (PD) and in a spectrum of neurodegenerative diseases collectively known as synucleinopathies. The protein was first associated with PD just over 20 years ago, when it was found to (i) be a major component of Lewy bodies and (ii) to be also associated with familial forms of PD. The characterization of α-Syn pathology has been achieved through postmortem studies of human brains. However, the identification of toxic mechanisms associated with α-Syn was only achieved through the use of experimental models. In vitro models are highly accessible, enable relatively rapid studies, and have been extensively employed to address α-Syn-associated neurodegeneration. Given the diversity of models used and the outcomes of the studies, a cumulative and comprehensive perspective emerges as indispensable to pave the way for further investigations. Here, we subdivided in vitro models of α-Syn pathology into three major types: (i) models simulating α-Syn fibrillization and the formation of different aggregated structures in vitro, (ii) models based on the intracellular expression of α-Syn, reporting on pathogenic conditions and cellular dysfunctions induced, and (iii) models using extracellular treatment with α-Syn aggregated species, reporting on sites of interaction and their downstream consequences. In summary, we review the underlying molecular mechanisms discovered and categorize protective strategies, in order to pave the way for future studies and the identification of effective therapeutic strategies. This article is part of the Special Issue "Synuclein".

The heart of PD: Lewy body diseases as neurocardiologic disorders

This review provides an update about cardiac sympathetic denervation in Lewy body diseases. The family of Lewy body diseases includes Parkinson's disease (PD), pure autonomic failure (PAF), and dementia with Lewy bodies (DLB). All three feature intra-neuronal cytoplasmic deposits of the protein, alpha-synuclein. Multiple system atrophy (MSA), the parkinsonian form of which can be difficult to distinguish from PD with orthostatic hypotension, involves glial cytoplasmic inclusions that contain alpha-synuclein. By now there is compelling neuroimaging, neuropathologic, and neurochemical evidence for cardiac sympathetic denervation in Lewy body diseases. In addition to denervation, there is decreased storage of catecholamines in the residual terminals. The degeneration develops in a centripetal, retrograde, "dying back" sequence. Across synucleinopathies the putamen and cardiac catecholaminergic lesions seem to occur independently of each other, whereas non-motor aspects of PD (e.g., anosmia, dementia, REM behavior disorder, OH) are associated with each other and with cardiac sympathetic denervation. Cardiac sympathetic denervation can be caused by synucleinopathy in inherited PD. According to the catecholaldehyde hypothesis, 3,4-dihydroxyphenylacetaldehyde (DOPAL), an intermediary metabolite of dopamine, causes or contributes to the death of catecholamine neurons, especially by interacting with proteins such as alpha-synuclein. DOPAL oxidizes spontaneously to DOPAL-quinone, which probably converts alpha-synuclein to its toxic oligomeric form. Decreasing DOPAL production and oxidation might slow the neurodegenerative process. Tracking cardiac sympathetic innervation over time could be the basis for a proof of principle experimental therapeutics trial targeting DOPAL.

Linking Stress, Catecholamine Autotoxicity, and Allostatic Load with Neurodegenerative Diseases: A Focused Review in Memory of Richard Kvetnansky

In this Focused Review, we provide an update about evolving concepts that may link chronic stress and catecholamine autotoxicity with neurodegenerative diseases such as Parkinson's disease. Richard Kvetnansky's contributions to the field of stress and catecholamine systems inspired some of the ideas presented here. We propose that coordination of catecholaminergic systems mediates adjustments maintaining health and that senescence-related disintegration of these systems leads to disorders of regulation and to neurodegenerative diseases such as Parkinson's disease. Chronically repeated episodes of stress-related catecholamine release and reuptake, with attendant increases in formation of the toxic dopamine metabolite 3,4-dihydroxyphenylacetaldehyde, might accelerate this process.

Structural and functional properties of prefibrillar α-synuclein oligomers

The deposition of fibrillar alpha-synuclein (α-syn) within inclusions (Lewy bodies and Lewy neurites) in neurons and glial cells is a hallmark of synucleinopathies. α-syn populates a variety of assemblies ranging from prefibrillar oligomeric species to fibrils whose specific contribution to neurodegeneration is still unclear. Here, we compare the specific structural and biological properties of distinct soluble prefibrillar α-syn oligomers formed either spontaneously or in the presence of dopamine and glutaraldehyde. We show that both on-fibrillar assembly pathway and distinct dopamine-mediated and glutaraldehyde-cross-linked α-syn oligomers are only slightly effective in perturbing cell membrane integrity and inducing cytotoxicity, while mature fibrils exhibit the highest toxicity. In contrast to low-molecular weight and unstable oligomers, large stable α-syn oligomers seed the aggregation of soluble α-syn within reporter cells although to a lesser extent than mature α-syn fibrils. These oligomers appear elongated in shape. Our findings suggest that α-syn oligomers represent a continuum of species ranging from unstable low molecular weight particles to mature fibrils via stable elongated oligomers composed of more than 15 α-syn monomers that possess seeding capacity.

The N-terminal residues 43 to 60 form the interface for dopamine mediated α-synuclein dimerisation

α-synuclein (α-syn) is a major component of the intracellular inclusions called Lewy bodies, which are a key pathological feature in the brains of Parkinson's disease patients. The neurotransmitter dopamine (DA) inhibits the fibrillisation of α-syn into amyloid, and promotes α-syn aggregation into SDS-stable soluble oligomers. While this inhibition of amyloid formation requires the oxidation of both DA and the methionines in α-syn, the molecular basis for these processes is still unclear. This study sought to define the protein sequences required for the generation of oligomers. We tested N- (α-syn residues 43-140) and C-terminally (1-95) truncated α-syn, and found that similar to full-length protein both truncated species formed soluble DA:α-syn oligomers, albeit 1-95 had a different profile. Using nuclear magnetic resonance (NMR), and the N-terminally truncated α-syn 43-140 protein, we analysed the structural characteristics of the DA:α-syn 43-140 dimer and α-syn 43-140 monomer and found the dimerisation interface encompassed residues 43 to 60. Narrowing the interface to this small region will help define the mechanism by which DA mediates the formation of SDS-stable soluble DA:α-syn oligomers.

Divalent metal ions enhance DOPAL-induced oligomerization of alpha-synuclein

Parkinson disease (PD) features profound striatal dopamine depletion and Lewy bodies containing abundant precipitated alpha-synuclein. Mechanisms linking alpha-synucleinopathy with the death of dopamine neurons remain incompletely understood. One such link may be 3,4-dihydroxyphenylacetaldehyde (DOPAL). All of the intra-neuronal metabolism of dopamine passes through DOPAL, which is toxic. DOPAL also potently oligomerizes alpha-synuclein and alpha-synuclein oligomers are thought to be pathogenic in PD. Another implicated factor in PD pathogenesis is metal ions, and alpha-synuclein contains binding sites for these ions. In this study we tested whether divalent metal ions augment DOPAL-induced oligomerization of alpha-synuclein in cell-free system and in PC12 cells conditionally over-expressing alpha-synuclein. Incubation with divalent metal ions augmented DOPAL-induced oligomerization of alpha-synuclein (Cu(2+)>Fe(2+)>Mn(2+)), whereas monovalent Cu(1+) and trivalent Fe(3+) were without effect. Other dopamine metabolites, dopamine itself, and metal ions alone or in combination with dopamine, also had no effect. Antioxidant treatment with ascorbic acid and divalent cation chelation with EDTA attenuated the augmentation by Cu(2+) of DOPAL-induced alpha-synuclein oligomerization. Incubation of PC12 cells with L-DOPA markedly increased intracellular DOPAL content and promoted alpha-synuclein dimerization. Co-incubation with Cu(2+) amplified (p=0.01), while monoamine oxidase inhibition prevented, L-DOPA-related dimerization of alpha-synuclein (p=0.01). We conclude that divalent metal ions augment DOPAL-induced oligomerization of alpha-synuclein. Drugs that interfere with this interaction might constitute a novel approach for future treatment or prevention approaches.

Mutations in PRKN and SNCA Genes Important for the Progress of Parkinson's Disease

Although Parkinson’s disease (PD) was first described almost 200 years ago, it remains an incurable disease with a cause that is not fully understood. Nowadays it is known that disturbances in the structure of pathological proteins in PD can be caused by more than environmental and genetic factors. Despite numerous debates and controversies in the literature about the role of mutations in the SNCA and PRKN genes in the pathogenesis of PD, it is evident that these genes play a key role in maintaining dopamine (DA) neuronal homeostasis and that the dysfunction of this homeostasis is relevant to both familial (FPD) and sporadic (SPD) PD with different onset. In recent years, the importance of alphasynuclein (ASN) in the process of neurodegeneration and neuroprotective function of the Parkin is becoming better understood. Moreover, there have been an increasing number of recent reports indicating the importance of the interaction between these proteins and their encoding genes. Among others interactions, it is suggested that even heterozygous substitution in the PRKN gene in the presence of the variants +2/+2 or +2/+3 of NACP-Rep1 in the SNCA promoter, may increase the risk of PD manifestation, which is probably due to ineffective elimination of over-expressed ASN by the mutated Parkin protein. Finally, it seems that genetic testing may be an important part of diagnostics in patients with PD and may improve the prognostic process in the course of PD. However, only full knowledge of the mechanism of the interaction between the genes associated with the pathogenesis of PD is likely to help explain the currently unknown pathways of selective damage to dopaminergic neurons in the course of PD.

Therapeutic approaches against common structural features of toxic oligomers shared by multiple amyloidogenic proteins

Impaired proteostasis is one of the main features of all amyloid diseases, which are associated with the formation of insoluble aggregates from amyloidogenic proteins. The aggregation process can be caused by overproduction or poor clearance of these proteins. However, numerous reports suggest that amyloid oligomers are the most toxic species, rather than insoluble fibrillar material, in Alzheimer's, Parkinson's, and Prion diseases, among others. Although the exact protein that aggregates varies between amyloid disorders, they all share common structural features that can be used as therapeutic targets. In this review, we focus on therapeutic approaches against shared features of toxic oligomeric structures and future directions.

The protective role of AMP-activated protein kinase in alpha-synuclein neurotoxicity in vitro

In the present study, we investigated the role of the main intracellular energy sensor, AMP-activated protein kinase (AMPK), in the in vitro neurotoxicity of α-synuclein (ASYN), one of the key culprits in the pathogenesis of Parkinson's disease. The loss of viability in retinoic acid-differentiated SH-SY5Y human neuroblastoma cells inducibly overexpressing wild-type ASYN was associated with the reduced activation of AMPK and its activator LKB1, as well as AMPK target Raptor. ASYN-overexpressing rat primary neurons also displayed lower activity of LKB1/AMPK/Raptor pathway. Restoration of AMPK activity by metformin or AICAR reduced the in vitro neurotoxicity of ASYN overexpression, acting independently of the prosurvival kinase Akt or the induction of autophagic response. The conditioned medium from ASYN-overexpressing cells, containing secreted ASYN, as well as dopamine-modified or nitrated recombinant ASYN oligomers, all inhibited AMPK activation in differentiated SH-SY5Y cells and reduced their viability, but not in the presence of metformin or AICAR. The RNA interference-mediated knockdown of AMPK increased the sensitivity of SH-SY5Y cells to the harmful effects of secreted ASYN. AMPK-dependent protection from extracellular ASYN was also observed in rat neuron-like pheochromocytoma cell line PC12. These data demonstrate the protective role of AMPK against the toxicity of both intracellular and extracellular ASYN, suggesting that modulation of AMPK activity may be a promising therapeutic strategy in Parkinson's disease.

The interplay between lipids and dopamine on α-synuclein oligomerization and membrane binding

The deposition of α-syn (α-synuclein) as amyloid fibrils and the selective loss of DA (dopamine) containing neurons in the substantia nigra are two key features of PD (Parkinson's disease). α-syn is a natively unfolded protein and adopts an α-helical conformation upon binding to lipid membrane. Oligomeric species of α-syn have been proposed to be the pathogenic species associated with PD because they can bind lipid membranes and disrupt membrane integrity. DA is readily oxidized to generate reactive intermediates and ROS (reactive oxygen species) and in the presence of DA, α-syn form of SDS-resistant soluble oligomers. It is postulated that the formation of the α-syn:DA oligomers involves the cross-linking of DA-melanin with α-syn, via covalent linkage, hydrogen and hydrophobic interactions. We investigate the effect of lipids on DA-induced α-syn oligomerization and studied the ability of α-syn:DA oligomers to interact with lipids vesicles. Our results show that the interaction of α-syn with lipids inhibits the formation of DA-induced α-syn oligomers. Moreover, the α-syn:DA oligomer cannot interact with lipid vesicles or cause membrane permeability. Thus, the formation of α-syn:DA oligomers may alter the actions of α-syn which require membrane association, leading to disruption of its normal cellular function.

Crosstalk between Endoplasmic Reticulum Stress and Protein Misfolding in Neurodegenerative Diseases

Under physiological conditions, the endoplasmic reticulum (ER) is a central subcellular compartment for protein quality control in the secretory pathway that prevents protein misfolding and aggregation. Instrumental in protein quality control in the ER is the unfolded protein response (UPR), which is activated upon ER stress to reestablish homeostasis through a sophisticated transcriptionally and translationally regulated signaling network. However, this response can lead to apoptosis if the stress cannot be alleviated. The presence of abnormal protein aggregates containing specific misfolded proteins is recognized as the basis of numerous human conformational disorders, including neurodegenerative diseases. Here, I will highlight the overwhelming evidence that the presence of specific aberrant proteins in Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), prion diseases, and Amyotrophic Lateral Sclerosis (ALS) is intimately associated with perturbations in the ER protein quality control machinery that become incompetent to restore protein homeostasis and shift adaptive programs toward the induction of apoptotic signaling to eliminate irreversibly damaged neurons. Increasing our understanding about the deadly crosstalk between ER dysfunction and protein misfolding in these neurodegenerative diseases may stimulate the development of novel therapeutic strategies able to support neuronal survival and ameliorate disease progression.

Dopamine-mediated oxidation of methionine 127 in α-synuclein causes cytotoxicity and oligomerization of α-synuclein

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the selective loss of dopaminergic neurons and the presence of Lewy bodies. Many recent studies focused on the interaction between α-synuclein (α-syn) and dopamine in the pathogenesis of PD, and fluorescent anisotropy suggested that the C-terminal region of α-syn may be a target for modification by dopamine. However, it is not well understood why PD-related pathogenesis occurs selectively in dopaminergic neurons. We investigated the interaction between dopamine and α-syn with regard to cytotoxicity. A soluble oligomer was formed by co-incubating α-syn and dopamine in vitro. To clarify the effect of dopamine on α-syn in cells, we generated PC12 cells expressing human α-syn, as well as the α-syn mutants, M116A, Y125D, M127A, S129A, and M116A/M127A, in a tetracycline-inducible manner (PC12-TetOFF-α-syn). Overexpression of wildtype α-syn in catecholaminergic PC12 cells decreased cell viability in long-term cultures, while a competitive inhibitor of tyrosine hydroxylase blocked this vulnerability, suggesting that α-syn-related cytotoxicity is associated with dopamine metabolism. The vulnerabilities of all mutant cell lines were lower than that of wildtype α-syn-expressing cells. Moreover, α-syn containing dopamine-mediated oxidized methionine (Met(O)) was detected in PC12-TetOFF-α-syn. Met(O) was lower in methionine mutant cells, especially in the M127A or M116A/M127A mutants, but also in the Y125D and S129A mutants. Co-incubation of dopamine and the ₁₂₅YEMPS₁₂₉ peptide enhanced the production of H₂O₂, which may oxidize methionine residues and convert them to Met(O). Y125- or S129-lacking peptides did not enhance the dopamine-related production of H₂O₂. Our results suggest that M127 is the major target for oxidative modification by dopamine, and that Y125 and S129 may act as enhancers of this modification. These results may describe a mechanism of dopaminergic neuron-specific toxicity of α-syn in the pathogenesis of PD.

Vesicular uptake blockade generates the toxic dopamine metabolite 3,4-dihydroxyphenylacetaldehyde in PC12 cells: relevance to the pathogenesis of Parkinson's disease

Parkinson's disease entails profound loss of nigrostriatal dopaminergic terminals, decreased vesicular uptake of intraneuronal catecholamines, and relatively increased putamen tissue concentrations of the toxic dopamine metabolite, 3,4-dihydroxyphenylacetaldehyde (DOPAL). The objective of this study was to test whether vesicular uptake blockade augments endogenous DOPAL production. We also examined whether intracellular DOPAL contributes to apoptosis and, as α-synuclein oligomers may be pathogenetic in Parkinson's disease, oligomerizes α-synuclein. Catechols were assayed in PC12 cells after reserpine to block vesicular uptake, with or without inhibition of enzymes metabolizing DOPAL-daidzein for aldehyde dehydrogenase and AL1576 for aldehyde reductase. Vesicular uptake was quantified by a method based on 6F- or (13) C-dopamine incubation; DOPAL toxicity by apoptosis responses to exogenous dopamine, with or without daidzein+AL1576; and DOPAL--induced synuclein oligomerization by synuclein dimer production during DOPA incubation, with or without inhibition of L-aromatic-amino-acid decarboxylase or monoamine oxidase. Reserpine inhibited vesicular uptake by 95-97% and rapidly increased cell DOPAL content (p = 0.0008). Daidzein+AL1576 augmented DOPAL responses to reserpine (p = 0.004). Intracellular DOPAL contributed to dopamine-evoked apoptosis and DOPA-evoked synuclein dimerization. The findings fit with the 'catecholaldehyde hypothesis,' according to which decreased vesicular sequestration of cytosolic catecholamines and impaired catecholaldehyde detoxification contribute to the catecholaminergic denervation that characterizes Parkinson's disease.

Cytoprotective effect of chlorogenic acid against α-synuclein-related toxicity in catecholaminergic PC12 cells

Parkinson’s disease is a major neurodegenerative disease involving the selective degeneration of dopaminergic neurons and α-synuclein containing Lewy bodies formation in the substantia nigra. Although α-synuclein is a key molecule for both dopaminergic neuron death and the formation of inclusion bodies, the mechanism of α-synuclein induction of Parkinson’s disease-related pathogenesis is not understood. In the present study, we found that the interaction between dopamine and α-synuclein requires the oxidation of dopamine. Furthermore, we examined the protective effect of chlorogenic acid, a major polyphenol contained in coffee, against α-syn and dopamine-related toxicity. Chlorogenic acid inhibits several DA/α-synuclein-related phenomenon, including the oxidation of dopamine, the interaction of oxidized dopamine with α-synuclein, and the oligomerization of α-synuclein under dopamine existing conditions in vitro. Finally, we showed that the cytoprotective effect against α-synuclein-related toxicity in PC12 cells that can be controlled by the Tet-Off system. Although the induction of α-synuclein in catecholaminergic PC12 cells causes a decrease in cell viability, chlorogenic acid rescued this cytotoxicity significantly in a dose dependent manner. These results suggest that the interaction of oxidized DA with α-synuclein may be a novel therapeutic target for Parkinson’s disease, and polyphenols, including chlorogenic acid, are candidates as protective and preventive agents for Parkinson’s disease onset.

ER stress response plays an important role in aggregation of α-synuclein

Background

Accumulation of filamentous α-synuclein as Lewy bodies is a hallmark of Parkinson's disease. To identify the mechanisms involved in α-synuclein assembly and determine whether the assemblies are cytotoxic, we developed a cell model (3D5) that inducibly expresses wild-type human α-synuclein and forms inclusions that reproduce many morphological and biochemical characteristics of Lewy bodies. In the present study, we evaluated the effects of several histone deacetylase inhibitors on α-synuclein aggregation in 3D5 cells and primary neuronal cultures. These drugs have been demonstrated to protect cells transiently overexpressing α-synuclein from its toxicity.

Results

Contrary to transient transfectants, the drug treatment did not benefit 3D5 cells and primary cultures. The treated were less viable and contained more α-synuclein oligomers, active caspases 3 and 9, as well as ER stress markers than non-treated counterparts. The drug-treated, induced-3D5 cells, or primary cultures from transgenic mice overexpressing (<2 fold) α-synuclein, displayed more α-synuclein oligomers and ER stress markers than non-induced or non-transgenic counterparts. Similar effects were demonstrated in cultures treated with tunicamycin, an ER stressor. These effects were blocked by co-treatment with salubrinal, an ER stress inhibitor. In comparison, co-treatment with a pan caspase inhibitor protected cells from demise but did not reduce α-synuclein oligomer accumulation.

Conclusions

Our results indicate that an increase of wild-type α-synuclein can elicit ER stress response and sensitize cells to further insults. Most importantly, an increase of ER stress response can promote the aggregation of wild type α-synuclein.

The structure of dopamine induced alpha-synuclein oligomers

Inclusions of aggregated alpha-synuclein (alpha-syn) in dopaminergic neurons are a characteristic histological marker of Parkinson's disease (PD). In vitro, alpha-syn in the presence of dopamine (DA) at physiological pH forms SDS-resistant non-amyloidogenic oligomers. We used a combination of biophysical techniques, including sedimentation velocity analysis, small angle X-ray scattering (SAXS) and circular dichroism spectroscopy to study the characteristics of alpha-syn oligomers formed in the presence of DA. Our SAXS data show that the trimers formed by the action of DA on alpha-syn consist of overlapping worm-like monomers, with no end-to-end associations. This lack of structure contrasts with the well-established, extensive beta-sheet structure of the amyloid fibril form of the protein and its pre-fibrillar oligomers. We propose on the basis of these and earlier data that oxidation of the four methionine residues at the C- and N-terminal ends of alpha-syn molecules prevents their end-to-end association and stabilises oligomers formed by cross linking with DA-quinone/DA-melanin, which are formed as a result of the redox process, thus inhibiting formation of the beta-sheet structure found in other pre-fibrillar forms of alpha-syn.