alpha-Synuclein implicated in Parkinson's disease catalyses the formation of hydrogen peroxide in vitro

Several lines of antioxidant defense against oxidative stress: antioxidant enzymes, nanomaterials with multiple enzyme-mimicking activities, and low-molecular-weight antioxidants

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are well recognized for playing a dual role, since they can be either deleterious or beneficial to biological systems. An imbalance between ROS production and elimination is termed oxidative stress, a critical factor and common denominator of many chronic diseases such as cancer, cardiovascular diseases, metabolic diseases, neurological disorders (Alzheimer's and Parkinson's diseases), and other disorders. To counteract the harmful effects of ROS, organisms have evolved a complex, three-line antioxidant defense system. The first-line defense mechanism is the most efficient and involves antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). This line of defense plays an irreplaceable role in the dismutation of superoxide radicals (O2•-) and hydrogen peroxide (H2O2). The removal of superoxide radicals by SOD prevents the formation of the much more damaging peroxynitrite ONOO- (O2•-  + NO• → ONOO-) and maintains the physiologically relevant level of nitric oxide (NO•), an important molecule in neurotransmission, inflammation, and vasodilation. The second-line antioxidant defense pathway involves exogenous diet-derived small-molecule antioxidants. The third-line antioxidant defense is ensured by the repair or removal of oxidized proteins and other biomolecules by a variety of enzyme systems. This review briefly discusses the endogenous (mitochondria, NADPH, xanthine oxidase (XO), Fenton reaction) and exogenous (e.g., smoking, radiation, drugs, pollution) sources of ROS (superoxide radical, hydrogen peroxide, hydroxyl radical, peroxyl radical, hypochlorous acid, peroxynitrite). Attention has been given to the first-line antioxidant defense system provided by SOD, CAT, and GPx. The chemical and molecular mechanisms of antioxidant enzymes, enzyme-related diseases (cancer, cardiovascular, lung, metabolic, and neurological diseases), and the role of enzymes (e.g., GPx4) in cellular processes such as ferroptosis are discussed. Potential therapeutic applications of enzyme mimics and recent progress in metal-based (copper, iron, cobalt, molybdenum, cerium) and nonmetal (carbon)-based nanomaterials with enzyme-like activities (nanozymes) are also discussed. Moreover, attention has been given to the mechanisms of action of low-molecular-weight antioxidants (vitamin C (ascorbate), vitamin E (alpha-tocopherol), carotenoids (e.g., β-carotene, lycopene, lutein), flavonoids (e.g., quercetin, anthocyanins, epicatechin), and glutathione (GSH)), the activation of transcription factors such as Nrf2, and the protection against chronic diseases. Given that there is a discrepancy between preclinical and clinical studies, approaches that may result in greater pharmacological and clinical success of low-molecular-weight antioxidant therapies are also subject to discussion.

Biochemical and Molecular Pathways in Neurodegenerative Diseases: An Integrated View

Neurodegenerative diseases (NDDs) like Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) are defined by a myriad of complex aetiologies. Understanding the common biochemical molecular pathologies among NDDs gives an opportunity to decipher the overlapping and numerous cross-talk mechanisms of neurodegeneration. Numerous interrelated pathways lead to the progression of neurodegeneration. We present evidence from the past pieces of literature for the most usual global convergent hallmarks like ageing, oxidative stress, excitotoxicity-induced calcium butterfly effect, defective proteostasis including chaperones, autophagy, mitophagy, and proteosome networks, and neuroinflammation. Herein, we applied a holistic approach to identify and represent the shared mechanism across NDDs. Further, we believe that this approach could be helpful in identifying key modulators across NDDs, with a particular focus on AD, PD, and ALS. Moreover, these concepts could be applied to the development and diagnosis of novel strategies for diverse NDDs.

Characteristics of antioxidant substances and identification of antioxidant peptides in duck embryo eggs

1. Oxidative stress in duck embryos undergoes dynamic change during incubation. However, the detailed change characteristics has not been studied yet. Here, we explored the dynamic change characteristics of different antioxidant substances in duck embryo eggs during incubation.2. The following trial assayed antioxidant substances, including vitamins E (VE) and C (VC), glutathione (GSH), reduced glutathione (reduced GSH), oxidative glutathione (GSSG), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-px), and malonic dialdehyde (MDA). Five antioxidant peptides (DY1, DY2, DY3, DY4 and DY5) were screened and their protective effect on HepG2 cells and their mechanism of action determined.3. The results showed that antioxidant substances underwent a dynamic change during incubation through various mechanisms. Moreover, the T-AOC of duck embryos at different incubation times, firstly increased and then decreased, reaching the highest level on d 15 during incubation. The peptide DY2 (TVDGPSGKLWRD) exhibited high antioxidant activity in vitro, and is known to regulate the apoptosis pathway in mitochondria.4. The data indicated that duck embryos can be used for the development of related antioxidant products and purification of new bioactive peptides.

Brain regions susceptible to alpha-synuclein spreading

The spreading of misfolded alpha-synuclein (α-syn) protein has been observed in animal models of Parkinson's disease (PD) and other α-synucleinopathies that mimic human PD pathologies. In animal models, the spreading of α-syn has been associated with motor dysfunction and neuronal death. However, variability in both susceptible brain regions and cellular populations limits our understanding of the consequences of α-syn spreading and the development of associated therapies. Here, we have reviewed the physiological and pathological functions of α-syn and summarized the susceptible brain regions and cell types identified from human postmortem studies and exogenous α-syn injection-based animal models. We have reviewed the methods for inducing α-syn aggregation, the specific hosts, the inoculation sites, the routes of propagation, and other experimental settings that may affect the spreading pattern of α-syn, as reported in current studies. Understanding the spread of α-syn to produce a consistent PD animal model is vital for future drug discovery.

Parkinson's Disease Dementia: Synergistic Effects of Alpha-Synuclein, Tau, Beta-Amyloid, and Iron

Parkinson’s disease dementia (PDD) is a common complication of Parkinson’s disease that seriously affects patients’ health and quality of life. At present, the process and pathological mechanisms of PDD remain controversial, which hinders the development of treatments. An increasing number of clinical studies have shown that alpha-synuclein (α-syn), tau, beta-amyloid (Aβ), and iron are closely associated with PDD severity. Thus, we inferred the vicious cycle that causes oxidative stress (OS), due to the synergistic effects of α-syn, tau, Aβ, and, iron, and which plays a pivotal role in the mechanism underlying PDD. First, iron-mediated reactive oxygen species (ROS) production can lead to neuronal protein accumulation (e.g., α-syn andAβ) and cytotoxicity. In addition, regulation of post-translational modification of α-syn by iron affects the aggregation or oligomer formation of α-syn. Iron promotes tau aggregation and neurofibrillary tangles (NFTs) formation. High levels of iron, α-syn, Aβ, tau, and NFTs can cause severe OS and neuroinflammation, which lead to cell death. Then, the increasing formation of α-syn, Aβ, and NFTs further increase iron levels, which promotes the spread of α-syn and Aβ in the central and peripheral nervous systems. Finally, iron-induced neurotoxicity promotes the activation of glycogen synthase kinase 3β (GSK3β) related pathways in the synaptic terminals, which in turn play an important role in the pathological synergistic effects of α-syn, tau and Aβ. Thus, as the central factor regulating this vicious cycle, GSK3β is a potential target for the prevention and treatment of PDD; this is worthy of future study.

MicroRNA-7 Protects Against Neurodegeneration Induced by α-Synuclein Preformed Fibrils in the Mouse Brain

α-Synuclein is a key protein in the pathogenesis of Parkinson's disease as it accumulates in fibrillar form in affected brain regions. Misfolded α-synuclein seeds recruit monomeric α-synuclein to form aggregates, which can spread to anatomically connected brain regions, a phenomenon that correlates with clinical disease progression. Thus, downregulating α-synuclein levels could reduce seeding and inhibit aggregate formation and propagation. We previously reported that microRNA-7 (miR-7) protects neuronal cells by downregulating α-synuclein expression through its effect on the 3'-untranslated region of SNCA mRNA; however, whether miR-7 blocks α-synuclein seeding and propagation in vivo remains unknown. Here, we induced miR-7 overexpression in the mouse striatum unilaterally by infusing adeno-associated virus 1 (AAV-miR-7) followed by inoculation with recombinant α-synuclein preformed fibrils (PFF) a month later. Compared with control mice injected with non-targeting AAV-miR-NT followed by PFF, AAV-miR-7 pre-injected mice exhibited lower levels of monomeric and high-molecular-weight α-synuclein species in the striatum, and reduced amount of phosphorylated α-synuclein in the striatum and in nigral dopamine neurons. Accordingly, AAV-miR-7-injected mice had less pronounced degeneration of the nigrostriatal pathway and better behavioral performance. The neuroinflammatory reaction to α-synuclein PFF inoculation was also significantly attenuated. These data suggest that miR-7 inhibits the formation and propagation of pathological α-synuclein and protects against neurodegeneration induced by PFF. Collectively, these findings support the potential of miR-7 as a disease modifying biologic agent for Parkinson's disease and related α-synucleinopathies.

MRI quantitative susceptibility mapping of the substantia nigra as an early biomarker for Lewy body disease

BACKGROUND AND PURPOSE

Neurodegeneration of the substantia nigra in Lewy body disease is associated with iron deposition, which increases the magnetic susceptibility of the substantia nigra on MRI. Our objective was to measure iron deposition in the substantia nigra in patients with probable dementia with Lewy bodies (pDLB) and patients who are at risk for pDLB by quantitative susceptibility mapping (QSM).

METHODS

Participants included pDLB (n = 36), mild cognitive impairment with at least one core feature of DLB (MCI-LB; n = 15), idiopathic rapid eye movement sleep behavior disorder (iRBD; n = 11), and an age-and gender-matched clinically unimpaired control group (n = 102). QSM was derived from multi-echo 3D gradient recalled echo MRI at 3T, and groups were compared on mean susceptibility values of the substantia nigra and its relation to parkinsonism severity.

RESULTS

Patients with pDLB had higher susceptibility in the substantia nigra compared to controls (p< 0.001) and MCI-LB (p = 0.043). The susceptibility of substantia nigra showed an increasing trend from controls to iRBD and MCI-LB, and to pDLB (p< 0.001). Parkinsonism severity was not associated with the mean susceptibility in the substantia nigra in the patient groups.

CONCLUSIONS

Our data suggested that QSM is sensitive to the increased magnetic susceptibility due to higher iron content in the substantia nigra in pDLB. The trend of increasing susceptibility from controls to iRBD and MCI-LB, and to pDLB suggests that iron deposition in the substantia nigra starts to increase as early as the prodromal stage in DLB and continues to increase as the disease progresses, independent of parkinsonism severity.

Effects of the Cytoplasm and Mitochondrial Specific Hydroxyl Radical Scavengers TA293 and mitoTA293 in Bleomycin-Induced Pulmonary Fibrosis Model Mice

Lung fibrosis is the primary pathology in idiopathic pulmonary fibrosis and is considered to result from an increase in reactive oxygen species (ROS) levels in alveolar epithelial cells. However, the exact mechanism underlying lung fibrosis remains unclear and there is no effective therapy. The hydroxyl radical (^•OH) has the strongest oxidizing potential among ROS. Recently, ^•OH localized to the cytoplasm (cyto ^•OH) was reported to induce cellular senescence, while mitochondria-localized ^•OH (mt ^•OH) was reported to induce apoptosis. We developed the cyto ^•OH- and mt ^•OH-scavenging antioxidants TA293 and mitoTA293 to evaluate the effects of cyto ^•OH and mt ^•OH in a bleomycin (BLM)-induced pulmonary fibrosis model. Treatment of BLM-induced pulmonary fibrosis mice with TA293 suppressed the induction of cellular senescence and fibrosis, as well as inflammation in the lung, but mitoTA293 exacerbated these. Furthermore, in BLM-stimulated primary alveolar epithelial cells, TA293 suppressed the activation of the p-ATM^ser1981/p-p53^ser15/p21, p-HRI/p-eIF2^ser51/ATF4/p16, NLRP3 inflammasome/caspase-1/IL-1β/IL1R/p-p38 MAPK/p16, and p21 pathways and the induction of cellular senescence. However, mitoTA293 suppressed the induction of mitophagy, enhanced the activation of the NLRP3 inflammasome/caspase-1/IL1β/IL1R/p-p38 MAPK/p16 and p21 pathways, and exacerbated cellular senescence, inflammation, and fibrosis. Our findings may help develop new strategies to treat idiopathic pulmonary fibrosis.

Alpha-synuclein increases in rodent and human spinal cord injury and promotes inflammation and tissue loss

Synucleinopathies are neurodegenerative diseases in which α-synuclein protein accumulates in neurons and glia. In these diseases, α-synuclein forms dense intracellular aggregates that are disease hallmarks and actively contribute to tissue pathology. Interestingly, many pathological mechanisms, including iron accumulation and lipid peroxidation, are shared between classical synucleinopathies such as Alzheimer’s disease, Parkinson’s disease and traumatic spinal cord injury (SCI). However, to date, no studies have determined if α-synuclein accumulation occurs after human SCI. To examine this, cross-sections from injured and non-injured human spinal cords were immunolabeled for α-synuclein. This showed robust α-synuclein accumulation in profiles resembling axons and astrocytes in tissue surrounding the injury, revealing that α-synuclein markedly aggregates in traumatically injured human spinal cords. We also detected significant iron deposition in the injury site, a known catalyst for α-synuclein aggregation. Next a rodent SCI model mimicking the histological features of human SCI revealed aggregates and structurally altered monomers of α-synuclein are present after SCI. To determine if α-synuclein exacerbates SCI pathology, α-synuclein knockout mice were tested. Compared to wild type mice, α-synuclein knockout mice had significantly more spared axons and neurons and lower pro-inflammatory mediators, macrophage accumulation, and iron deposition in the injured spinal cord. Interestingly, locomotor analysis revealed that α-synuclein may be essential for dopamine-mediated hindlimb function after SCI. Collectively, the marked upregulation and long-lasting accumulation of α-synuclein and iron suggests that SCI may fit within the family of synucleinopathies and offer new therapeutic targets for promoting neuron preservation and improving function after spinal trauma.

Catalytic amplification based on hierarchical heterogeneity bimetal-organic nanostructures with peroxidase-like activity

In this paper, integrating heterometallic units and nanostructures into metal-organic frameworks (MOFs) were applied to improve the sensitivity of detecting hydrogen peroxide (H₂O₂) in neutral solution. The bimetal-MOFs (CuCo-BDC) and GO composite (CuCo-BDC/GO) were first synthesized via an ordinary one-step solvothermal synthesis. The CuCo-BDC/GO with admirable peroxidase-like catalytic activity could be applied to detect H₂O₂. The results have low detection limit of 69 nM (S/N = 3) and a wide linear detection range, from 100 nM to 3.5 mM. This is superior to recently published biosensors based on noble metal nanomaterials, which confirms CuCo-BDC/GO as the MOF electrocatalysts with high performance. The remarkable electroanalytical performance of CuCo-BDC/GO is due to the presence of numerous open metal active sites, the synergistic effect of Cu²⁺ and Co²⁺, hierarchical structure with high-specific surface areas and the marvelous electrochemical properties of GO. Therefore, CuCo-BDC/GO is a powerful candidate for detecting H₂O₂ in electrochemical biosensing fields. Moreover, H₂O₂ detection in real samples can be done with the CuCo-BDC/GO, including human serum samples. Therefore, the novel CuCo-BDC/GO is a promising catalyst that can be applied in biotechnological and environmental applications.

Intranasal administration of α-synuclein preformed fibrils triggers microglial iron deposition in the substantia nigra of Macaca fascicularis

Iron deposition is present in main lesion areas in the brains of patients with Parkinson's disease (PD) and an abnormal iron content may be associated with dopaminergic neuronal cytotoxicity and degeneration in the substantia nigra of the midbrain. However, the cause of iron deposition and its role in the pathological process of PD are unclear. In the present study, we investigated the effects of the nasal mucosal delivery of synthetic human α-synuclein (α-syn) preformed fibrils (PFFs) on the pathogenesis of PD in Macaca fascicularis. We detected that iron deposition was clearly increased in a time-dependent manner from 1 to 17 months in the substantia nigra and globus pallidus, highly contrasting to other brain regions after treatments with α-syn PFFs. At the cellular level, the iron deposits were specifically localized in microglia but not in dopaminergic neurons, nor in other types of glial cells in the substantia nigra, whereas the expression of transferrin (TF), TF receptor 1 (TFR1), TF receptor 2 (TFR2), and ferroportin (FPn) was increased in dopaminergic neurons. Furthermore, no clear dopaminergic neuron loss was observed in the substantia nigra, but with decreased immunoreactivity of tyrosine hydroxylase (TH) and appearance of axonal swelling in the putamen. The brain region-enriched and cell-type-dependent iron localizations indicate that the intranasal α-syn PFFs treatment-induced iron depositions in microglia in the substantia nigra may appear as an early cellular response that may initiate neuroinflammation in the dopaminergic system before cell death occurs. Our data suggest that the inhibition of iron deposition may be a potential approach for the early prevention and treatment of PD.

His18 promotes reactive oxidative stress production in copper-ion mediated human islet amyloid polypeptide aggregation

Copper ions play a critical role in human islet amyloid polypeptide (hIAPP) aggregation, which has been found in more than 90% of patients with type-2 diabetes (T2D). The role of Cu(ii) in the cell cytotoxicity with hIAPP has been explored in two aspects: inhibiting the formation of fibrillar structures and stimulating the generation of reactive oxygen species (ROS). In this work, we carried out spectroscopic studies of Cu(ii) interacting with several hIAPP fragments and their variants as well. Electron paramagnetic resonance (EPR) measurements and Amplex Red analysis showed that the amount of H₂O₂ generated in hIAPP(11-28) solution co-incubated with Cu(ii) was remarkably more than hIAPP(1-11) and hIAPP(28-37). Furthermore, the H₂O₂ level was seriously reduced when His18 of hIAPP(11-28) was replaced by Arg(R) or Ser(S), indicating that His18 is the key residue of Cu(ii) binding to hIAPP(11-28) to promote H₂O₂ generation. This is likely because the donation of electrons from the peptide to Cu(ii) ions would result in the formation of the redox-active complexes, which could stimulate the formation of H₂O₂. Overall, this study provides further insight into the molecular mechanism of Cu(ii) induced ROS generation.

His18 promotes H₂O₂ production in copper-ion mediated hIAPP aggregation.

Apoptosis signal regulating kinase 1 deletion mitigates α-synuclein pre-formed fibril propagation in mice

α-Synuclein (α-Syn) is a key pathogenic protein in α-synucleinopathies including Parkinson disease and dementia with Lewy bodies. Accumulating evidence has shown that misfolded fibrillar α-Syn is transmitted from cell-to-cell, a phenomenon that correlates with clinical progression of the disease. We previously showed that deleting the MAP3 kinase apoptosis signal-regulating kinase 1 (ASK1), which is a central player linking oxidative stress with neuroinflammation, mitigates the phenotype of α-Syn transgenic mice. However, whether ASK1 impacts pathology and disease progression induced by recombinant α-Syn pre-formed fibrils (PFF) remains unknown. Here, we compared the neuropathological and behavioral phenotype of ASK1 knock-out mice with that of wild-type mice following intrastriatal injections of α-Syn PFF. At 6 months post-injections, ASK1 null mice exhibited reduced amount of phosphorylated α-Syn aggregates in the striatum and cortex, and less pronounced degeneration of the nigrostriatal pathway. Additionally, the neuroinflammatory reaction to α-Syn PFF injection and propagation seen in wild-type mice was attenuated in ASK1 knock-out animals. These neuropathological markers were associated with better behavioral performance. These data suggest that ASK1 plays an important role in pathological α-Syn fibril transmission and, consequently, may impact disease progression. These findings collectively support inhibiting ASK1 as a disease modifying therapeutic strategy for Parkinson disease and related α-synucleinopathies.

Extracellular Alpha-Synuclein Oligomers Induce Parkin S-Nitrosylation: Relevance to Sporadic Parkinson's Disease Etiopathology

α-Synuclein (ASN) and parkin, a multifunctional E3 ubiquitin ligase, are two proteins that are associated with the pathophysiology of Parkinson’s disease (PD). Excessive release of ASN, its oligomerization, aggregation, and deposition in the cytoplasm contribute to neuronal injury and cell death through oxidative-nitrosative stress induction, mitochondrial impairment, and synaptic dysfunction. In contrast, overexpression of parkin provides protection against cellular stresses and prevents dopaminergic neural cell loss in several animal models of PD. However, the influence of ASN on the function of parkin is largely unknown. Therefore, the aim of this study was to investigate the effect of extracellular ASN oligomers on parkin expression, S-nitrosylation, as well as its activity. For these investigations, we used rat pheochromocytoma (PC12) cell line treated with exogenous oligomeric ASN as well as PC12 cells with parkin overexpression and parkin knock-down. The experiments were performed using spectrophotometric, spectrofluorometric, and immunochemical methods. We found that exogenous ASN oligomers induce oxidative/nitrosative stress leading to parkin S-nitrosylation. Moreover, this posttranslational modification induced the elevation of parkin autoubiquitination and degradation of the protein. The decreased parkin levels resulted in significant cell death, whereas parkin overexpression protected against toxicity induced by extracellular ASN oligomers. We conclude that lowering parkin levels by extracellular ASN may significantly contribute to the propagation of neurodegeneration in PD pathology through accumulation of defective proteins as a consequence of parkin degradation.

Interaction between amyloidogenic proteins and biomembranes in protein misfolding diseases: Mechanisms, contributors, and therapy

The toxic deposition of misfolded amyloidogenic proteins is associated with more than fifty protein misfolding diseases (PMDs), including Alzheimer's disease, Parkinson's disease and type 2 diabetes mellitus. Protein deposition is a multi-step process modulated by a variety of factors, in particular by membrane-protein interaction. The interaction results in permeabilization of biomembranes contributing to the cytotoxicity that leads to PMDs. Different biological and physiochemical factors, such as protein sequence, lipid composition, and chaperones, are known to affect the membrane-protein interaction. Here, we provide a comprehensive review of the mechanisms and contributing factors of the interaction between biomembranes and amyloidogenic proteins, and a summary of the therapeutic approaches to PMDs that target this interaction. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Alpha-synuclein alters differently gene expression of Sirts, PARPs and other stress response proteins: implications for neurodegenerative disorders

Alpha-synuclein (ASN) is a presynaptic protein that can easily change its conformation under different types of stress. It's assumed that ASN plays an important role in the pathogenesis of Parkinson's and Alzheimer's disease. However, the molecular mechanism of ASN toxicity has not been elucidated. This study focused on the role of extracellular ASN (eASN) in regulation of transcription of sirtuins (Sirts) and DNA-bound poly(ADP-ribose) polymerases (PARPs) - proteins crucial for cells' survival/death. Our results indicate that eASN enhanced the free radicals level, decreased mitochondria membrane potential, cells viability and activated cells' death. Concomitantly eASN activated expression of antioxidative proteins (Sod2, Gpx4, Gadd45b) and DNA-bound Parp2 and Parp3. Moreover, eASN upregulated expression of Sirt3 and Sirt5, but downregulated of Sirt1, which plays an important role in cell metabolism including Aβ precursor protein (APP) processing. eASN downregulated gene expression of APP alpha secretase (Adam10) and metalloproteinases Mmp2, Mmp10 but upregulated Mmp11. Additionally, expression and activity of pro-survival sphingosine kinase 1 (Sphk1), Akt kinase and anti-apoptotic protein Bcl2 were inhibited. Moreover, higher expression of pro-apoptotic protein Bax and enhancement of apoptotic cells' death were observed. Summarizing, eASN significantly modulates transcription of Sirts and enzymes involved in APP/Aβ metabolism and through these mechanisms eASN toxicity may be enhanced. The inhibition of Sphk1 and Akt by eASN may lead to disturbances of survival pathways. These results suggest that eASN through alteration of transcription and by inhibition of pro-survival kinases may play important pathogenic role in neurodegenerative disorders.

The impact of obesity on brain iron levels and α-synuclein expression is regionally dependent

BACKGROUND

The importance of iron homeostasis is particularly apparent in the brain, where iron deficiency results in impaired cognition and iron accumulation is associated with neurodegenerative diseases. Obesity is linked to iron deficiency systemically, but the effects of obesity on brain iron and its associated consequences, including neurodegenerative processes remain unexplored. This preliminary study examined the effect of dietary-induced obesity on brain regional iron, α-synuclein expression, and F2-isoprostane (oxidative stress marker) concentrations in selected brain regions.

OBJECTIVE

The objective of the study was to elucidate the vulnerability of selected brain regions (e.g. midbrain, hippocampus) to the possible process of neurodegeneration due to the altered iron content associated with obesity.

METHODS

Twenty-one-day-old male C57BL/6J mice were fed with a high-fat diet (60% kcal from fat) or a control-fat diet (10% kcal from fat) for 20 weeks. Brain samples were collected and dissected into hippocampus, midbrain, striatum, and thalamus regions. Iron content, ferritin H (FtH) and α-synuclein protein and mRNA expressions, and F₂-isoprostane were measured in selected regions.

RESULTS

The results indicated that obesity caused significant differences in iron levels in the midbrain and thalamus, but not in the hippocampus or striatum, compared to control mice. Furthermore, markers of neurodegeneration (α-synuclein mRNA expression and F₂-isoprostanes) were increased in the midbrain.

DISCUSSION

These results support previous findings that brain iron metabolism responds to environmental stress in a regionally distinct manner and suggests that alterations in brain iron metabolism due to obesity may be relevant in neurodegeneration.

Trehalose Inhibits A53T Mutant α-Synuclein Overexpression and Neurotoxicity in Transduced PC12 Cells

Fibrillar accumulation of A53T mutant α-synuclein (A53T-AS) in Lewy bodies is a symptom of Parkinsonism. Inhibitions of the overexpression and fibrillar aggregation of α-synuclein (AS) in vivo could be a promising strategy for treating Parkinson’s disease (PD). In this study, at concentrations lower than 1 mM, trehalose decreased the A53T-AS expression level in transduced PC12 cells. Although H₂O₂ and aluminum ions increased the expression level and neurotoxicity of A53T-AS in cells, proper trehalose concentrations inhibited the event. These studies adequately prove that trehalose at an appropriate dose would be potentially useful for PD treatment.

Interactions of iron, dopamine and neuromelanin pathways in brain aging and Parkinson's disease

There are several interrelated mechanisms involving iron, dopamine, and neuromelanin in neurons. Neuromelanin accumulates during aging and is the catecholamine-derived pigment of the dopamine neurons of the substantia nigra and norepinephrine neurons of the locus coeruleus, the two neuronal populations most targeted in Parkinson's disease. Many cellular redox reactions rely on iron, however an altered distribution of reactive iron is cytotoxic. In fact, increased levels of iron in the brain of Parkinson's disease patients are present. Dopamine accumulation can induce neuronal death; however, excess dopamine can be removed by converting it into a stable compound like neuromelanin, and this process rescues the cell. Interestingly, the main iron compound in dopamine and norepinephrine neurons is the neuromelanin-iron complex, since neuromelanin is an effective metal chelator. Neuromelanin serves to trap iron and provide neuronal protection from oxidative stress. This equilibrium between iron, dopamine, and neuromelanin is crucial for cell homeostasis and in some cellular circumstances can be disrupted. Indeed, when neuromelanin-containing organelles accumulate high load of toxins and iron during aging a neurodegenerative process can be triggered. In addition, neuromelanin released by degenerating neurons activates microglia and the latter cause neurons death with further release of neuromelanin, then starting a self-propelling mechanism of neuroinflammation and neurodegeneration. Considering the above issues, age-related accumulation of neuromelanin in dopamine neurons shows an interesting link between aging and neurodegeneration.

Metabolic Dysfunction in Parkinson's Disease: Bioenergetics, Redox Homeostasis and Central Carbon Metabolism

The loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the accumulation of protein inclusions (Lewy bodies) are the pathological hallmarks of Parkinson's disease (PD). PD is triggered by genetic alterations, environmental/occupational exposures and aging. However, the exact molecular mechanisms linking these PD risk factors to neuronal dysfunction are still unclear. Alterations in redox homeostasis and bioenergetics (energy failure) are thought to be central components of neurodegeneration that contribute to the impairment of important homeostatic processes in dopaminergic cells such as protein quality control mechanisms, neurotransmitter release/metabolism, axonal transport of vesicles and cell survival. Importantly, both bioenergetics and redox homeostasis are coupled to neuro-glial central carbon metabolism. We and others have recently established a link between the alterations in central carbon metabolism induced by PD risk factors, redox homeostasis and bioenergetics and their contribution to the survival/death of dopaminergic cells. In this review, we focus on the link between metabolic dysfunction, energy failure and redox imbalance in PD, making an emphasis in the contribution of central carbon (glucose) metabolism. The evidence summarized here strongly supports the consideration of PD as a disorder of cell metabolism.

Neurodegenerative signaling factors and mechanisms in Parkinson's pathology

Parkinson's disease (PD) is a chronic and progressive degenerative disorder of central nervous system which is mainly characterized by selective loss of dopaminergic neurons in the nigrostrial pathway. Clinical symptoms of this devastating disease comprise motor impairments such as resting tremor, bradykinesia, postural instability and rigidity. Current medications only provide symptomatic relief but fail to halt the dopaminergic neuronal death. While the etiology of dopaminergic neuronal death is not fully understood, combination of various molecular mechanisms seems to play a critical role. Studies from experimental animal models have provided crucial insights into the molecular mechanisms in disease pathogenesis and recognized possible targets for therapeutic interventions. Recent findings implicate the involvement of abnormal protein accumulation and phosphorylation, mitochondrial dysfunction, oxidative damage and deregulated kinase signaling as key molecular mechanisms affecting the normal function as well survival of dopaminergic neurons. Here we discuss the relevant findings on the PD pathology related mechanisms and recognition of the cell survival mechanisms which could be used as targets for neuroprotective strategies in preventing this devastating disorder.

Amyloid-β and α-Synuclein Decrease the Level of Metal-Catalyzed Reactive Oxygen Species by Radical Scavenging and Redox Silencing

The formation of reactive oxygen species (ROS) is linked to the pathogenesis of neurodegenerative diseases. Here we have investigated the effect of soluble and aggregated amyloid-β (Aβ) and α-synuclein (αS), associated with Alzheimer’s and Parkinson’s diseases, respectively, on the Cu²⁺-catalyzed formation of ROS in vitro in the presence of a biological reductant. We find that the levels of ROS, and the rate by which ROS is generated, are significantly reduced when Cu²⁺ is bound to Aβ or αS, particularly when they are in their oligomeric or fibrillar forms. This effect is attributed to a combination of radical scavenging and redox silencing mechanisms. Our findings suggest that the increase in ROS associated with the accumulation of aggregated Aβ or αS does not result from a particularly ROS-active form of these peptides, but rather from either a local increase of Cu²⁺ and other ROS-active metal ions in the aggregates or as a downstream consequence of the formation of the pathological amyloid structures.

Interaction of α-synuclein with biomembranes in Parkinson's disease--role of cardiolipin

One of the key molecular events underlying the pathogenesis of Parkinson's disease (PD) is the aberrant misfolding and aggregation of the α-synuclein (αS) protein into higher-order oligomers that play a key role in neuronal dysfunction and degeneration. A wealth of experimental data supports the hypothesis that the neurotoxicity of αS oligomers is intrinsically linked with their ability to interact with, and disrupt, biological membranes; especially those membranes having negatively-charged surfaces and/or lipid packing defects. Consequences of αS-lipid interaction include increased membrane tension, permeation by pore formation, membrane lysis and/or leakage due to the extraction of lipids from the bilayer. Moreover, we assert that the interaction of αS with a liquid-disordering phospholipid uniquely enriched in mitochondrial membranes, namely cardiolipin (1,3-diphosphatidyl-sn-glycerol, CL), helps target the αS oligomeric complexes intracellularly to mitochondria. Binding mediated by CL may thus represent an important pathomechanism by which cytosolic αS could physically associate with mitochondrial membranes and disrupt their integrity. Impaired mitochondrial function culminates in a cellular bioenergetic crisis and apoptotic death. To conclude, we advocate the accelerated discovery of new drugs targeting this pathway in order to restore mitochondrial function in PD.

UV-induced selective oxidation of Met5 to Met-sulfoxide leads to the formation of neurotoxic fibril-incompetent α-synuclein oligomers

Oxidative stress and the formation of cytotoxic aggregates of the presynaptic protein α-synuclein (AS) are two important events associated with the pathogenesis of Parkinson's disease (PD) and several other neurodegenerative diseases. In this context, extensive efforts have been made to elucidate the molecular basis of the cytotoxic synergy between oxidative stress and AS aggregation. In this study, we demonstrate that the exposure of AS to oxidative stress induced by UV radiation (ASUV) blocks the protein fibrillation, leading to the formation of highly toxic fibril-incompetent oligomers. In addition, ASUV exhibited stronger anti-fibrillogenic properties than H2O2-treated AS, inhibiting the fibrillation of unmodified AS at notably low concentrations. Mass spectrometry indicated that Met5 oxidation to Met-sulfoxide was the only modification promoted by UV exposure, which is reinforced by NMR data indicating that Met5 is the only residue whose amide resonance completely disappeared from the (1)H-(15)N HSQC spectrum after UV exposure. This result is supported by previous data that indicate that C-terminal Met residues (Met116 and Met127) and N-terminal Met1 are less susceptible to oxidation than Met5 because of the residual structure of the disordered AS monomer. Overall, our findings suggest that specific oxidation of Met5 might be sufficient to promote the formation of highly neurotoxic oligomers of AS.

Apoptosis signal-regulating kinase 1 modulates the phenotype of α-synuclein transgenic mice

α-Synuclein is a key pathogenic protein in α-synucleinopathies including Parkinson's disease, and its overexpression and aggregation in model systems are associated with a neuroinflammatory response and increased oxidative stress. Apoptosis signal-regulating kinase 1 (ASK1) is activated upon stress signaling events such as oxidative stress and is a central player linking oxidative stress with neuroinflammation. Here, we demonstrate that overexpression of human α-synuclein activates ASK1 in both PC12 cells and in the brains of α-synuclein transgenic mice. Deleting ASK1 in mice mitigates the neuronal damage and neuroinflammation induced by α-synuclein and improves performance of the animals on the rotarod. ASK1 deletion does not impact the aggregation profile or phosphorylation state of α-synuclein in the mouse brain. These results collectively implicate ASK1 in the cascade of events triggered by α-synuclein overexpression, likely because of the inflammatory response and oxidative stress that lead to ASK1 activation. These conclusions raise the possibility that potent antioxidants and anti-inflammatory agents may ameliorate the phenotype of α-synucleinopathies.

Antioxidant gene therapy against neuronal cell death

Oxidative stress is a common hallmark of neuronal cell death associated with neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, as well as brain stroke/ischemia and traumatic brain injury. Increased accumulation of reactive species of both oxygen (ROS) and nitrogen (RNS) has been implicated in mitochondrial dysfunction, energy impairment, alterations in metal homeostasis and accumulation of aggregated proteins observed in neurodegenerative disorders, which lead to the activation/modulation of cell death mechanisms that include apoptotic, necrotic and autophagic pathways. Thus, the design of novel antioxidant strategies to selectively target oxidative stress and redox imbalance might represent important therapeutic approaches against neurological disorders. This work reviews the evidence demonstrating the ability of genetically encoded antioxidant systems to selectively counteract neuronal cell loss in neurodegenerative diseases and ischemic brain damage. Because gene therapy approaches to treat inherited and acquired disorders offer many unique advantages over conventional therapeutic approaches, we discussed basic research/clinical evidence and the potential of virus-mediated gene delivery techniques for antioxidant gene therapy.

The role of lipid peroxidation in neurological disorders

There has been much evidence demonstrating the involvement of oxidative stress in the pathology of neurological disorders. Moreover, the vulnerability of the central nervous system to reactive oxygen species mediated injury is well established since neurons consume large amounts of oxygen, the brain has many areas containing high iron content, and neuronal mitochondria generate large amounts of hydrogen peroxide. Furthermore, neuronal membranes are rich in polyunsaturated fatty acids, which are particularly susceptible to oxidative stress. Recently, the biological roles of products produced by lipid peroxidation have received much attention, not only for their pathological mechanisms associated with neurological disorders, but also for their practical clinical applications as biomarkers. Here, we discuss the production mechanisms of reactive oxygen species in some neurological disorders, including Alzheimer's disease, Down syndrome, Parkinson's disease, and stroke. We also describe lipid peroxidation biomarkers for evaluating oxidative stress.

β-amyloid fibrils in Alzheimer disease are not inert when bound to copper ions but can degrade hydrogen peroxide and generate reactive oxygen species

According to the "amyloid cascade" hypothesis of Alzheimer disease, the formation of Aβ fibrils and senile plaques in the brain initiates a cascade of events leading to the formation of neurofibrillary tangles, neurodegeneration, and the symptom of dementia. Recently, however, emphasis has shifted away from amyloid fibrils as the predominant toxic form of Aβ toward smaller aggregates, referred to as "soluble oligomers." These oligomers have become one of the prime suspects for involvement in the early oxidative damage that is evident in this disease. This raises the question whether or not Aβ fibrils are actually "inert tombstones" present at the end of the aggregation process. Here we show that, when Aβ(1-42) aggregates, including fibrils, are bound to Cu(II) ions, they retain their redox activity and are able to degrade hydrogen peroxide (H2O2) with the formation of hydroxyl radicals and the consequent oxidation of the peptide (detected by formation of carbonyl groups). We find that this ability increases as the Cu(II):peptide ratio increases and is accompanied by changes in aggregate morphology, as determined by atomic force microscopy. When aggregates are prepared in the copresence of Cu(II) and Zn(II) ions, the ratio of Cu(II):Zn(II) becomes an important factor in the degeneration of H2O2, the formation of carbonyl groups in the peptide, and in aggregate morphology. We believe, therefore, that Aβ fibrils can destroy H2O2 and generate damaging hydroxyl radicals and, so, are not necessarily inert end points.

Nigral iron elevation is an invariable feature of Parkinson's disease and is a sufficient cause of neurodegeneration

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor deficits accompanying degeneration of substantia nigra pars compactor (SNc) neurons. Although familial forms of the disease exist, the cause of sporadic PD is unknown. Symptomatic treatments are available for PD, but there are no disease modifying therapies. While the neurodegenerative processes in PD may be multifactorial, this paper will review the evidence that prooxidant iron elevation in the SNc is an invariable feature of sporadic and familial PD forms, participates in the disease mechanism, and presents as a tractable target for a disease modifying therapy.

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.

A delicate balance: Iron metabolism and diseases of the brain

Iron is the most abundant transition metal within the brain, and is vital for a number of cellular processes including neurotransmitter synthesis, myelination of neurons, and mitochondrial function. Redox cycling between ferrous and ferric iron is utilized in biology for various electron transfer reactions essential to life, yet this same chemistry mediates deleterious reactions with oxygen that induce oxidative stress. Consequently, there is a precise and tightly controlled mechanism to regulate iron in the brain. When iron is dysregulated, both conditions of iron overload and iron deficiencies are harmful to the brain. This review focuses on how iron metabolism is maintained in the brain, and how an alteration to iron and iron metabolism adversely affects neurological function.

Applying transcriptomic and proteomic knowledge to Parkinson's disease drug discovery

It is recognised that in both genetic and sporadic cases of Parkinson's disease (PD), the basis of its etiopathology resides in the particular vulnerability of the dopaminergic neurons of the substantia nigra pars compacta (SNpc) to oxidative stress and in the failure to adequately remove abnormal proteins. These observations have been confirmed recently by microarray transcriptomic studies in human SN from PD brains and have extended understanding of the molecular pathways underlying the PD pathology. This article reviews recent gene expression profiling studies in sporadic PD postmortem SN and highlights gene candidates as putative molecular signatures for early disease diagnosis. In addition, the application of transcriptomics and proteomics in the quest for multifunctional neuroprotective-neurorescue drugs that might possess disease-modifying action is discussed.

Inhibiting toxic aggregation of amyloidogenic proteins: a therapeutic strategy for protein misfolding diseases

BACKGROUND

The deposition of self-assembled amyloidogenic proteins is associated with multiple diseases, including Alzheimer's disease, Parkinson's disease and type 2 diabetes mellitus. The toxic misfolding and self-assembling of amyloidogenic proteins are believed to underlie protein misfolding diseases. Novel drug candidates targeting self-assembled amyloidogenic proteins represent a potential therapeutic approach for protein misfolding diseases.

SCOPE OF REVIEW

In this perspective review, we provide an overview of the recent progress in identifying inhibitors that block the aggregation of amyloidogenic proteins and the clinical applications thereof.

MAJOR CONCLUSIONS

Compounds such as polyphenols, certain short peptides, and monomer- or oligomer-specific antibodies, can interfere with the self-assembly of amyloidogenic proteins, prevent the formation of oligomers, amyloid fibrils and the consequent cytotoxicity.

GENERAL SIGNIFICANCE

Some inhibitors have been tested in clinical trials for treating protein misfolding diseases. Inhibitors that target the aggregation of amyloidogenic proteins bring new hope to therapy for protein misfolding diseases.

Protein folding and aggregation into amyloid: the interference by natural phenolic compounds

Amyloid aggregation is a hallmark of several degenerative diseases affecting the brain or peripheral tissues, whose intermediates (oligomers, protofibrils) and final mature fibrils display different toxicity. Consequently, compounds counteracting amyloid aggregation have been investigated for their ability (i) to stabilize toxic amyloid precursors; (ii) to prevent the growth of toxic oligomers or speed that of fibrils; (iii) to inhibit fibril growth and deposition; (iv) to disassemble preformed fibrils; and (v) to favor amyloid clearance. Natural phenols, a wide panel of plant molecules, are one of the most actively investigated categories of potential amyloid inhibitors. They are considered responsible for the beneficial effects of several traditional diets being present in green tea, extra virgin olive oil, red wine, spices, berries and aromatic herbs. Accordingly, it has been proposed that some natural phenols could be exploited to prevent and to treat amyloid diseases, and recent studies have provided significant information on their ability to inhibit peptide/protein aggregation in various ways and to stimulate cell defenses, leading to identify shared or specific mechanisms. In the first part of this review, we will overview the significance and mechanisms of amyloid aggregation and aggregate toxicity; then, we will summarize the recent achievements on protection against amyloid diseases by many natural phenols.

TRPM2 channels are required for NMDA-induced burst firing and contribute to H(2)O(2)-dependent modulation in substantia nigra pars reticulata GABAergic neurons

Substantia nigra pars reticulata (SNr) GABAergic neurons are projection neurons that convey output from the basal ganglia to target structures. These neurons exhibit spontaneous regular firing, but also exhibit burst firing in the presence of NMDA or when excitatory glutamatergic input to the SNr is activated. Notably, an increase in burst firing is also seen in Parkinson's disease. Therefore, elucidating conductances that mediate spontaneous activity and changes of firing pattern in these neurons is essential for understanding how the basal ganglia control movement. Using ex vivo slices of guinea pig midbrain, we show that SNr GABAergic neurons express transient receptor potential melastatin 2 (TRPM2) channels that underlie NMDA-induced burst firing. Furthermore, we show that spontaneous firing rate and burst activity are modulated by the reactive oxygen species H(2)O(2) acting via TRPM2 channels. Thus, our results indicate that activation of TRPM2 channels is necessary for burst firing in SNr GABAergic neurons and their responsiveness to modulatory H(2)O(2). These findings have implications not only for normal regulation, but also for Parkinson's disease, which involves excitotoxicity and oxidative stress.

Role of α-synuclein aggregation and the nuclear factor E2-related factor 2/heme oxygenase-1 pathway in iron-induced neurotoxicity

Abnormal aggregation of α-synuclein (α-syn) plays a critical role in the pathogenesis of Parkinson's disease (PD). Iron is also believed to serve as a major contributor by inducing oxidative stress and α-syn aggregation. Here, we report that down-regulation of nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) may contribute to iron-induced α-syn aggregation. In this study, we show that ferrous iron down-regulates Nrf2 and HO-1 in a time-dependent manner in SK-N-SH neuroblastoma cells. Levels of both Nrf2 and HO-1 are decreased even more by ferrous iron in SK-N-SH cells that overexpress α-syn and results in greater cell toxicity. Consistent with these results, knockdown of α-syn expression prevents reduction of Nrf2 and HO-1 by ferrous iron, eliminates α-syn aggregates, and protects SK-N-SH cells against ferrous iron-induced cell damage. Furthermore, increased HO-1 expression exerts a protective role against ferrous iron. These results support a new hypothesis of synergistic α-syn/iron cytotoxicity, whereby ferrous iron induces α-syn aggregation and neurotoxicity by inhibiting Nrf2/HO-1. Inhibition of Nrf2/HO-1 leads to more α-syn aggregation and greater toxicity induced by iron, creating a vicious cycle of iron accumulation, α-syn aggregation and HO-1 disruption in PD.

Toxicity of extracellular secreted alpha-synuclein: Its role in nitrosative stress and neurodegeneration

It has been demonstrated that both oligomerisation and accumulation of α-synuclein (ASN) are the key molecular processes involved in the pathophysiology of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease and other synucleinopathies. Alterations of ASN expression and impairment of its degradation can lead to the formation of intracellular deposits of this protein, called Lewy bodies. Overexpressed or misfolded ASN could be secreted to the extracellular space. Today the prion-like transmission of ASN oligomers to neighbouring cells is believed to be responsible for protein modification and propagation of neurodegeneration in the brain. It was presented that oxidative/nitrosative stress may play a key role in ASN secretion and spread of ASN pathology. Moreover, ASN-evoked protein oxidation, nitration and nitrosylation lead to disturbances in synaptic transmission and cell death. The interaction of secreted ASN with other amyloidogenic proteins and its involvement in irreversible mitochondrial disturbances and oxidative stress were also described. A better understanding of the mechanisms of ASN secretion and dysfunction may help to explain the molecular mechanisms of neurodegeneration and may be the basis for the development of novel therapeutic strategies.

The role of free radicals in the aging brain and Parkinson's Disease: convergence and parallelism

Free radical production and their targeted action on biomolecules have roles in aging and age-related disorders such as Parkinson's disease (PD). There is an age-associated increase in oxidative damage to the brain, and aging is considered a risk factor for PD. Dopaminergic neurons show linear fallout of 5-10% per decade with aging; however, the rate and intensity of neuronal loss in patients with PD is more marked than that of aging. Here, we enumerate the common link between aging and PD at the cellular level with special reference to oxidative damage caused by free radicals. Oxidative damage includes mitochondrial dysfunction, dopamine auto-oxidation, α-synuclein aggregation, glial cell activation, alterations in calcium signaling, and excess free iron. Moreover, neurons encounter more oxidative stress as a counteracting mechanism with advancing age does not function properly. Alterations in transcriptional activity of various pathways, including nuclear factor erythroid 2-related factor 2, glycogen synthase kinase 3β, mitogen activated protein kinase, nuclear factor kappa B, and reduced activity of superoxide dismutase, catalase and glutathione with aging might be correlated with the increased incidence of PD.

Biology of mitochondria in neurodegenerative diseases

Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) are the most common human adult-onset neurodegenerative diseases. They are characterized by prominent age-related neurodegeneration in selectively vulnerable neural systems. Some forms of AD, PD, and ALS are inherited, and genes causing these diseases have been identified. Nevertheless, the mechanisms of the neuronal degeneration in these familial diseases, and in the more common idiopathic (sporadic) diseases, are unresolved. Genetic, biochemical, and morphological analyses of human AD, PD, and ALS, as well as their cell and animal models, reveal that mitochondria could have roles in this neurodegeneration. The varied functions and properties of mitochondria might render subsets of selectively vulnerable neurons intrinsically susceptible to cellular aging and stress and the overlying genetic variations. In AD, alterations in enzymes involved in oxidative phosphorylation, oxidative damage, and mitochondrial binding of Aβ and amyloid precursor protein have been reported. In PD, mutations in mitochondrial proteins have been identified and mitochondrial DNA mutations have been found in neurons in the substantia nigra. In ALS, changes occur in mitochondrial respiratory chain enzymes and mitochondrial programmed cell death proteins. Transgenic mouse models of human neurodegenerative disease are beginning to reveal possible principles governing the biology of selective neuronal vulnerability that implicate mitochondria and the mitochondrial permeability transition pore. This chapter reviews several aspects of mitochondrial biology and how mitochondrial pathobiology might contribute to the mechanisms of neurodegeneration in AD, PD, and ALS.

Neuromelanin enhances the toxicity of α-synuclein in SK-N-SH cells

The key pathological feature of Parkinson's disease (PD) is selective degeneration of the neuromelanin (NM)-pigmented dopaminergic neurons in the substantia nigra (SN). NM, like other risk factors, such as oxidative stress (OS) and α-synuclein (α-syn), is involved in the pathogenesis of PD. But whether or not NM synergizes with α-syn or OS in the pathogenesis of PD remains unexplored. In the present study, we examined the effects of NM on cellular viability, apoptosis and free radical production in α-syn over-expressing human neuroblastoma cell line (SK-N-SH) in the presence or absence of the oxidizer Fenton's Reagent (FR). We showed that NM synergized with FR in suppressing cell viability, and in inducing apoptosis and hydroxyl radical production in all SK-N-SH cell lines. α-Syn over-expressing cells exhibited more pronounced effect, especially the A53T mutation. Our findings suggest that NM synergizes with both OS and α-syn in conferring dopaminergic vulnerability, adding to our understanding of the pathogenesis of PD.