Toxic interactions between dopamine, α-synuclein, monoamine oxidase, and genes in mitochondria of Parkinson's disease

Parkinson's disease is characterized by its distinct pathological features; loss of dopamine neurons in the substantia nigra pars compacta and accumulation of Lewy bodies and Lewy neurites containing modified α-synuclein. Beneficial effects of L-DOPA and dopamine replacement therapy indicate dopamine deficit as one of the main pathogenic factors. Dopamine and its oxidation products are proposed to induce selective vulnerability in dopamine neurons. However, Parkinson's disease is now considered as a generalized disease with dysfunction of several neurotransmitter systems caused by multiple genetic and environmental factors. The pathogenic factors include oxidative stress, mitochondrial dysfunction, α-synuclein accumulation, programmed cell death, impaired proteolytic systems, neuroinflammation, and decline of neurotrophic factors. This paper presents interactions among dopamine, α-synuclein, monoamine oxidase, its inhibitors, and related genes in mitochondria. α-Synuclein inhibits dopamine synthesis and function. Vice versa, dopamine oxidation by monoamine oxidase produces toxic aldehydes, reactive oxygen species, and quinones, which modify α-synuclein, and promote its fibril production and accumulation in mitochondria. Excessive dopamine in experimental models modifies proteins in the mitochondrial electron transport chain and inhibits the function. α-Synuclein and familiar Parkinson's disease-related gene products modify the expression and activity of monoamine oxidase. Type A monoamine oxidase is associated with neuroprotection by an unspecific dose of inhibitors of type B monoamine oxidase, rasagiline and selegiline. Rasagiline and selegiline prevent α-synuclein fibrillization, modulate this toxic collaboration, and exert neuroprotection in experimental studies. Complex interactions between these pathogenic factors play a decisive role in neurodegeneration in PD and should be further defined to develop new therapies for Parkinson's disease.

Neuroprotective Function of Rasagiline and Selegiline, Inhibitors of Type B Monoamine Oxidase, and Role of Monoamine Oxidases in Synucleinopathies

Synucleinopathies are a group of neurodegenerative disorders caused by the accumulation of toxic species of α-synuclein. The common clinical features are chronic progressive decline of motor, cognitive, behavioral, and autonomic functions. They include Parkinson’s disease, dementia with Lewy body, and multiple system atrophy. Their etiology has not been clarified and multiple pathogenic factors include oxidative stress, mitochondrial dysfunction, impaired protein degradation systems, and neuroinflammation. Current available therapy cannot prevent progressive neurodegeneration and “disease-modifying or neuroprotective” therapy has been proposed. This paper presents the molecular mechanisms of neuroprotection by the inhibitors of type B monoamine oxidase, rasagiline and selegiline. They prevent mitochondrial apoptosis, induce anti-apoptotic Bcl-2 protein family, and pro-survival brain- and glial cell line-derived neurotrophic factors. They also prevent toxic oligomerization and aggregation of α-synuclein. Monoamine oxidase is involved in neurodegeneration and neuroprotection, independently of the catalytic activity. Type A monoamine oxidases mediates rasagiline-activated signaling pathways to induce neuroprotective genes in neuronal cells. Multi-targeting propargylamine derivatives have been developed for therapy in various neurodegenerative diseases. Preclinical studies have presented neuroprotection of rasagiline and selegiline, but beneficial effects have been scarcely presented. Strategy to improve clinical trials is discussed to achieve disease-modification in synucleinopathies.

Disease-modifying treatment of Parkinson's disease by phytochemicals: targeting multiple pathogenic factors

Parkinson's disease is characterized by typical motor symptoms, loss of dopamine neurons in the substantia nigra, and accumulation of Lewy body composed of mutated α-synuclein. However, now it is considered as a generalized disease with multiple pathological features. Present available treatments can ameliorate symptoms at least for a while, but only a few therapies could delay progressive neurodegeneration of dopamine neurons. Lewy body accumulates in peripheral tissues many years before motor dysfunction becomes manifest, suggesting that disease-modifying therapy should start earlier during the premotor stage. Long-termed regulation of lifestyle, diet and supplement of nutraceuticals may be possible ways for the disease-modification. Diet can reduce the incidence of Parkinson's disease and phytochemicals, major bioactive ingredients of herbs and plant food, modulate multiple pathogenic factors and exert neuroprotective effects in preclinical studies. This review presents mechanisms underlying neuroprotection of phytochemicals against neuronal cell death and α-synuclein toxicity in Parkinson's disease. Phytochemicals are antioxidants, maintain mitochondrial function and homeostasis, prevent intrinsic apoptosis and neuroinflammation, activate cellular signal pathways to induce anti-apoptotic and pro-survival genes, such as Bcl-2 protein family and neurotrophic factors, and promote cleavage of damaged mitochondria and α-synuclein aggregates. Phytochemicals prevent α-synuclein oligomerization and aggregation, and dissolve preformed α-synuclein aggregates. Novel neuroprotective agents are expected to develop based on the scaffold of phytochemicals permeable across the blood-brain-barrier, to increase the bioavailability, ameliorate brain dysfunction and prevent neurodegeneration.

Neuroprotection of multifunctional phytochemicals as novel therapeutic strategy for neurodegenerative disorders: antiapoptotic and antiamyloidogenic activities by modulation of cellular signal pathways

In neurodegenerative disorders, including Alzheimer's and Parkinson's disease, neuroprotection by diet and natural bioactive compounds has been proposed to prevent the onset and progress of neurodegeneration by modification of pathogenic factors. Plant food-derived phytochemicals protect neurons via targeting oxidative stress, mitochondrial dysfunction, neurotrophic factor deficit, apoptosis and abnormal protein accumulation. This review presents the molecular mechanism of neuroprotection by phytochemicals: direct regulation of mitochondrial apoptotic machinery, modification of cellular signal pathways, induction of antiapoptotic Bcl-2 protein family and prosurvival neurotrophic factors, such as brain- and glial cell line-derived neurotrophic factor, and prevention of protein aggregation. Multitargeted neuroprotective agents are under development based on the structure of blood–brain barrier-permeable phytochemicals to ameliorate brain dysfunction and prevent neurodegeneration.

Modification of α-synuclein by lipid peroxidation products derived from polyunsaturated fatty acids promotes toxic oligomerization: its relevance to Parkinson disease

Recently, toxic α-synuclein oligomer, which can mediate cell-to-cell propagation is suggested to cause sporadic Parkinson disease. α-Synuclein interacts with membrane lipids especially polyunsaturated fatty acids to stabilize its three-dementional structure. Peroxidation of polyunsaturated fatty acids may reduce their affinity to α-synuclein and peroxidation byproducts might modify α-synuclein. 4-Hydroxy-2-nonenal derived from n-6 polyunsaturated fatty acids was reported to modify α-synuclein to produce a toxic oligomer. Moreover, the accumulation of 4-hydroxy-2-nonenal, which could induce oligomeriztion of α-synuclein, was found in parkinsonian brains. Docosahexaenoic acid, an n-3 polyunsaturated fatty acids abundant in the neuronal membrane, was also found to enhance α-synuclein oligomerization; however, the precise details of the chemical reaction involved are unclear. Propanoylated lysine, a specific indicator of docosahexaenoic acid oxidation, was increased in neuronal differentiated human neuroblastoma SH-SY5Y cells overexpressing α-synuclein. α-Synuclein might be modified by the peroxidation products and then, is degraded by the autophagy-lysosome system. In addition, in the cells overexpressing α-synuclein, the mitochondrial electrone transfer chain was found to be inhibited. Accumulation of abnormal α-synuclein modified by lipid radicals derived from polyunsaturated fatty acids may be not only an indicator of brain oxidative stress but also causative of neurodegeneration such as Parkinson disease by impairing mitochondrial function.

Type B and A monoamine oxidase and their inhibitors regulate the gene expression of Bcl-2 and neurotrophic factors in human glioblastoma U118MG cells: different signal pathways for neuroprotection by selegiline and rasagiline

Type B monoamine oxidase (MAO-B) in glial cells has been considered to be associated with neuronal death in Parkinson's disease. MAO-B inhibitors, rasagiline and selegiline [(-)deprenyl], protect neurons in animal and cellular models of neurodegeneration. However, the role of MAO-B itself in the regulation of cell death processing remains elusive, whereas type A MAO (MAO-A) mediates the induction of anti-apoptotic Bcl-2 genes by rasagiline and selegiline. In this paper, the involvement of MAOs in the induction of neuroprotective genes by MAO inhibitors was investigated in human glioblastoma U118MG cells expressing mainly MAO-B. Selegiline significantly increased Mao-B, which was suppressed by Mao-A knockdown with short interfering (si)RNA, whereas rasagiline less markedly increased Mao-B, which was not affected by Mao-A knockdown. Mao-A mRNA was also markedly increased by rasagiline and selegiline, and Mao-B knockdown significantly enhanced the induction by selegiline, but not by rasagiline. Mao-B knockdown also significantly increased mRNA levels of Bcl-2, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF). Selegiline synergistically enhanced the expression of these genes in Mao-B knockdown cells, but Mao-A knockdown suppressed the increase. Rasagiline increased BDNF and GDNF, which Mao-B and Mao-A knockdown inhibited. These results show that MAO-B might function as a repressor and MAO-A as a mediator in the constitutional expression of pro-survival genes, and that MAO-B and MAO-A might regulate different signal pathways for rasagiline and selegiline to induce neuroprotective genes. The novel role of glial MAOs in the regulation of gene expression is discussed.

Type A monoamine oxidase and serotonin are coordinately involved in depressive disorders: from neurotransmitter imbalance to impaired neurogenesis

Type A monoamine oxidase (MAOA) catabolizes monoamine transmitters, serotonin, norepinephrine and dopamine, and plays a major role in the onset, progression and therapy of neuropsychiatric disorders. In depressive disorders, increase in MAOA expression and decrease in brain levels of serotonin and norepinephrine are proposed as the major pathogenic factors. The functional polymorphism of MAOA gene and genes in serotonin signal pathway are associated with depression. This review presents recent advance in studies on the role of MAOA in major depressive disorder and related emotional disorders. MAOA and serotonin regulate the prenatal development and postnatal maintenance of brain architecture and neurocircuit, as shown by MAOA-deficient humans and MAO knockout animal models. Impaired neurogenesis in the mature hippocampus has been proposed as "adult neurogenesis" hypothesis of depression. MAOA modulates the sensitivity to stress in the stages of brain development and maturation, and the interaction of gene-environmental factors in the early stage regulates the onset of depressive behaviors in adulthood. Vice versa environmental factors affect MAOA expression by epigenetic regulation. MAO inhibitors not only restore compromised neurotransmitters, but also protect neurons from cell death in depression through induction of anti-apoptotic Bcl-2 and prosurvival neurotrophic factors, especially brain-derived neurotrophic factor, the deficiency of which is detected in depression. This review discusses novel role of MAOA and serotonin in the pathogenesis and therapy of depressive disorders.

Phytochemicals prevent mitochondrial membrane permeabilization and protect SH-SY5Y cells against apoptosis induced by PK11195, a ligand for outer membrane translocator protein

Epidemiological studies present the beneficial effects of dietary habits on prevention of aging-associated decline of brain function. Phytochemicals, the second metabolites of food, protect neuronal cells from cell death in cellular models of neurodegenerative disorders, and the neuroprotective activity has been ascribed to the anti-oxidant and anti-inflammatory functions. In this paper, the cellular mechanism of neuroprotection by phytochemicals was investigated, using the cellular model of mitochondrial apoptosis induced by PK11195, a ligand of outer membrane translocator protein, in SH-SY5Y cells. PK11195 induced mitochondrial membrane permeabilization with rapid transit production of superoxide (superoxide flashes) and calcium release from mitochondria, and activated apoptosis signal pathway. Study on the structure-activity relationship of astaxanthin, ferulic acid derivatives, and sesame lignans revealed that these phytochemicals inhibited mitochondrial membrane permeabilization and protected cells from apoptosis. Ferulic acid derivatives and sesame lignans inhibited or enhanced the mitochondrial pore formation and cell death by PK11195 according to their amphiphilic properties, not directly depending on the antioxidant activity. Regulation of pore formation at mitochondrial membrane is discussed as a novel mechanism behind neuroprotective activity of phytochemicals in aging and age-associated neurodegenerative disorders, and also behind dual functions of phytochemicals in neuronal and cancer cells.

Rasagiline prevents cyclosporine A-sensitive superoxide flashes induced by PK11195, the initial signal of mitochondrial membrane permeabilization and apoptosis

Rasagiline, a neuroprotective inhibitor of type B monoamine oxidase, prevented PK111195-induced apoptosis in SH-SY5Y cells through inhibition of mitochondrial apoptosis signaling (J Neural Transm 120:1539-1551, 2013, J Neural Transm 122:1399-1407, 2015). This paper presents that PK11195 induced superoxide flashes, the transit production burst, mediated by cyclosporine A-sensitive membrane permeability transition. Rasagiline prevented superoxide flashes, calcium efflux, and cell death by PK11195. Regulation of the initial pore formation at the inner mitochondrial membrane was confirmed as the decisive mechanism of neuroprotection by rasagiline.

Neuromelanin selectively induces apoptosis in dopaminergic SH-SY5Y cells by deglutathionylation in mitochondria: involvement of the protein and melanin component

Parkinson's disease (PD) is characterized by selective depletion of nigral dopamine (DA) neurons containing neuromelanin (NM), suggesting the involvement of NM in the pathogenesis. This study reports induction of apoptosis by NM in SH-SY5Y cells, whereas protease-K-treated NM, synthesized DA- and cysteinyl dopamine melanin showed much less cytotoxicity. Cell death was mediated by mitochondria-mediated apoptotic pathway, namely collapse of mitochondrial membrane potential, release of cytochrome c, and activation of caspase 3, but Bcl-2 over-expression did not suppress apoptosis. NM increased sulfhydryl content in mitochondria, and a major part of it was identified as GSH, whereas dopamine melanin significantly reduced sulfhydryl levels. Western blot analysis for protein-bound GSH demonstrated that only NM reduced S-glutathionylated proteins in mitochondria and dissociated macromolecular structure of complex I. Reactive oxygen and nitrogen species were required for the deglutathionylation by NM, which antioxidants reduced significantly with prevention of apoptosis. These results suggest that NM may be related to cell death of DA neurons in PD and aging through regulation of mitochondrial redox state and S-glutathionylation, for which NM-associated protein is absolutely required. The novel function of NM is discussed in relation to the pathogenesis of PD.

Glutathione redox status in mitochondria and cytoplasm differentially and sequentially activates apoptosis cascade in dopamine-melanin-treated SH-SY5Y cells

Neuromelanin (NM)-containing dopaminergic neurons in the substantia nigra are selectively vulnerable in Parkinson's disease (PD), suggesting the involvement of NM in the pathogenesis. NM is composed of protein, lipid, trace metals and melanin component, a mixture of eumelanin produced from dopamine (DA)-quinone and pheomelanin containing 5-S-cyteinyl-DA-quinone. We reported that NM induces mitochondria-mediated apoptosis in human dopaminergic SH-SY5Y cells, which was suppressed completely by Protease K-treatment, suggesting the essential requirement for the protein component. In this paper, the role of the melanin component in NM-dependent apoptosis was studied using SH-SY5Y cells and synthesized DA-melanin (DAM) and L-cysteinyl-DAM (Cys-DAM). DAM oxidatively decreased glutathione (GSH) and sulfhydryl (SH) content in mitochondria, whereas NM increased GSH by de-S-glutathionylation of complex I. DAM induced mitochondrial permeability transition (mPT), leading to membrane potential collapse and cytochrome c release, whereas Cys-DAM did not. However, the cytotoxicity of DAM itself was rather mild and thiol-targeting reducing reagents, including GSH, dithiothreitol and N-acetyl-cysteine, increased apoptosis significantly. The reducing SH reagents activated caspase 3 and induced apoptosis, but did not affect mPT. On the other hand, NM itself activated mitochondria-initiated apoptotic cascade, which GSH suppressed completely. The results indicate that DAM induces apoptosis through the sequential activation by oxidation of SH status in mitochondria and reduction in cytoplasm, in contrast to the case with NM. The regulation of apoptotic processing by SH redox state is discussed in relation to degeneration of nigra-striatal DA neurons in aging and PD, where oxidative stress is increased with impaired antioxidant capacity.

In parkinsonian substantia nigra, α-synuclein is modified by acrolein, a lipid-peroxidation product, and accumulates in the dopamine neurons with inhibition of proteasome activity

alpha-Synuclein (alphaSYN) plays a central role in the neural degeneration of Parkinson's disease (PD) through its conformational change. In PD, alphaSYN, released from the membrane, accumulates in the cytoplasm and forms Lewy body. However, the mechanism behind the translocation and conformational change of alphaSYN leading to the cell death has not been well elucidated. This paper reports that in the dopamine neurons of the substantia nigra containing neuromelanin from PD patients, alphaSYN was modified with acrolein (ACR), an aldehyde product of lipid peroxidation. Histopathological observation confirmed the co-localization of protein immunoreactive to anti-alphaSYN and ACR antibody. By Western blot analyses of samples precipitated with either anti-alphaSYN or anti-ACR antibody, increase in ACR-modified alphaSYN was confirmed in PD brain. Modification of recombinant alphaSYN by ACR enhanced its oligomerization, and at higher ACR concentrations alphaSYN was fragmented and polymerized forming a smear pattern in SDS-PAGE. ACR reduced 20S proteasome activity through the direct modification of the proteasome proteins and the production of polymerized ACR-modified proteins, which inhibited proteasome activity in vitro. These results suggest that ACR may initiate vicious cycle of modification and aggregation of proteins, including alphaSYN, and impaired proteolysis system, to cause neuronal death in PD.

Neuroprotection by propargylamines in Parkinson's disease: intracellular mechanism underlying the anti-apoptotic function and search for clinical markers

In Parkinson's and other neurodegenerative diseases, a therapeutic strategy has been proposed to halt progressive cell death. Propargylamine derivatives, rasagiline and (-)deprenyl (selegiline), have been confirmed to protect neurons against cell death induced by various insults in cellular and animal models of neurodegenerative disorders. In this paper, the mechanism and the markers of the neuroprotection are reviewed. Propargylamines prevent the mitochondrial permeabilization, membrane potential decline, cytochrome c release, caspase activation and nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase. At the same time, rasagiline induces anti-apoptotic pro-survival proteins, Bcl-2 and glial cell-line derived neurotrophic factor, which is mediated by activated ERK-NF-kappaB signal pathway. DNA array studies indicate that rasagiline increases the expression of the genes coding mitochondrial energy synthesis, inhibitors of apoptosis, transcription factors, kinases and ubiquitin-proteasome system, sequentially in a time-dependent way. Products of cell survival-related gene induced by propargylamines may be applied as markers of neuroprotection in clinical samples.

The effect of neuromelanin on the proteasome activity in human dopaminergic SH-SY5Y cells

In Parkinson's disease (PD), the selective depletion of dopamine neurons in the substantia nigra, particular those containing neuromelanin (NM), is the characteristic pathological feature. The role of NM in the cell death of dopamine neurons has been considered either to be neurotoxic or neuroprotective, but the precise mechanism has never been elucidated. In human brain, NM is synthesized by polymerization of dopamine and relating quinones, to which bind heavy metals including iron. The effects of NM prepared from human brain were examined using human dopaminergic SH-SY5Y cells. It was found that NM inhibits 26S proteasome activity through generation of reactive oxygen and nitrogen species from mitochondria. The mitochondrial dysfunction was also induced by oxidative stress mediated by iron released from NM. NM accumulated in dopamine neurons in ageing may determine the selective vulnerability of dopamine neurons in PD.

Neuromelanin induces oxidative stress in mitochondria through release of iron: mechanism behind the inhibition of 26S proteasome

Parkinson's disease is characterized by the selective depletion of dopamine neurons in the substantia nigra, particular those containing neuromelanin. Involvement of neuromelanin in the pathogenesis may be either cytotoxic or protective. Recently we found that neuromelanin reduces the activity of 26S proteasome. In this paper, the detailed mechanisms behind the reduced activity were studied using neuromelanin isolated from the human brain. Neuromelanin increased the oxidative stress, but synthetic melanin did not. Superoxide dismutase and deferoxamine completely suppressed the increase, indicating that superoxide produced by an iron-mediated reaction plays a central role. Iron was shown to reduce in situ 26S proteasome activity in SH-SY5Y cells and the reduction was protected by antioxidants. These results suggest that iron released from neuromelanin increases oxidative stress in mitochondria, and then causes mitochondrial dysfunction and reduces proteasome function. The role of neuromelanin is discussed in relation to the selective vulnerability of dopamine neurons in Parkinson's disease.

Overexpression of amyloid precursor protein induces susceptibility to oxidative stress in human neuroblastoma SH-SY5Y cells

In Alzheimer's disease amyloid beta peptide (Abeta) produced from amyloid precursor protein (APP) is considered to induce cell death. To clarify the molecular mechanism underlying Abeta neurotoxicity, we established the cell line overexpressing wild or mutant (His684Arg) APP in human SH-SY5Y cells. This paper presents that overexpression of wild-APP in the cells (SH/w-APP) increased the levels of APP and Abeta(1-40) but not Abeta(1-42), and reduced Bcl-2 level and proteasome activity with increased susceptibility to oxidative stress. The intracellular levels of reactive oxygen species in SH/w-APP increased significantly by H(2)O(2) treatment. The level of Bcl-2 protein, but not mRNA, was markedly decreased in SH/w-APP cells, which was inversely correlated with APP expression among subcloned SH/w-APP cells. These results indicate that increased expression of wild type APP renders neuronal cells more vulnerable to oxidative stress leading to cell death.

Oxidative Stress in Mitochondria: Decision to Survival and Death of Neurons in Neurodegenerative Disorders

In mitochondria, oxidative phosphorylation and enzymatic oxidation of biogenic amines by monoamine oxidase produce reactive oxygen and nitrogen species, which are proposed to cause neuronal cell death in neurodegenerative disorders, including Parkinson's and Alzheimer's disease. In these disorders, mitochondrial dysfunction, increased oxidative stress, and accumulation of oxidation-modified proteins are involved in cell death in definite neurons. The interactions among these factors were studied by use of a peroxynitrite-generating agent, N-morpholino sydnonimine (SIN-1) and an inhibitor of complex I, rotenone, in human dopaminergic SH-SY5Y cells. In control cells, peroxynitrite nitrated proteins, especially the subunits of mitochondrial complex I, as 3-nitrotyrosine, suggesting that neurons are exposed to constant oxidative stress even under physiological conditions. SIN-1 and an inhibitor of proteasome, carbobenzoxy-L-isoleucyl-gamma-t-butyl-L-alanyl-L-leucinal (PSI), increased markedly the levels of nitrated proteins with concomitant induction of apoptosis in the cells. Rotenone induced mitochondrial dysfunction and accumulation and aggregation of proteins modified with acrolein, an aldehyde product of lipid peroxidation in the cells. At the same time, the activity of the 20S beta-subunit of proteasome was reduced significantly, which degrades oxidative-modified protein. The mechanism was proved to be the result of the modification of the 20S beta-subunit with acrolein and to the binding of other acrolein-modified proteins to the 20S beta-subunit.

Mitochondrial genome variation in eastern Asia and the peopling of Japan

To construct an East Asia mitochondrial DNA (mtDNA) phylogeny, we sequenced the complete mitochondrial genomes of 672 Japanese individuals (http://www.giib.or.jp/mtsnp/index_e.html). This allowed us to perform a phylogenetic analysis with a pool of 942 Asiatic sequences. New clades and subclades emerged from the Japanese data. On the basis of this unequivocal phylogeny, we classified 4713 Asian partial mitochondrial sequences, with <10% ambiguity. Applying population and phylogeographic methods, we used these sequences to shed light on the controversial issue of the peopling of Japan. Population-based comparisons confirmed that present-day Japanese have their closest genetic affinity to northern Asian populations, especially to Koreans, which finding is congruent with the proposed Continental gene flow to Japan after the Yayoi period. This phylogeographic approach unraveled a high degree of differentiation in Paleolithic Japanese. Ancient southern and northern migrations were detected based on the existence of basic M and N lineages in Ryukyuans and Ainu. Direct connections with Tibet, parallel to those found for the Y-chromosome, were also apparent. Furthermore, the highest diversity found in Japan for some derived clades suggests that Japan could be included in an area of migratory expansion to Continental Asia. All the theories that have been proposed up to now to explain the peopling of Japan seem insufficient to accommodate fully this complex picture.

Neuromelanin inhibits enzymatic activity of 26S proteasome in human dopaminergic SH-SY5Y cells

Recently, impairment of the ubiquitin-proteasome system is suggested to be responsible for the neuronal death in ageing and Parkinson's disease. The specific degeneration of dopamine neurons containing neuromelanin (NM) suggests that NM itself may be involved in the cellular dysfunction and death, even though the direct link has never been reported. We examined the effects of NM isolated from the human substantia nigra on the proteasome activity in human dopaminergic SH-SY5Y cells. NM reduced the activities of 26S proteasome, as shown in situ using a green fluorescent protein homologue targeted to 26S proteasome and also in vitro using ubiquitinated lysozyme as a substrate. However, NM did not affect 20S proteasome activity in vitro. NM reduced the amount of PA700 regulatory subunit of 26S proteasome, but did not affect that of alpha- and beta-subunits of 20S proteasome. These results suggest that NM may inhibit the ubiquitin-26S proteasome system, and determine the selective vulnerability of dopamine neurons in ageing and related disorders.

N-Propargyl-1 (R)-aminoindan, rasagiline, increases glial cell line-derived neurotrophic factor (GDNF) in neuroblastoma SH-SY5Y cells through activation of NF-kappaB transcription factor

N-Propargyl-l(R)-aminoindan, rasagiline, an anti-Parkinson drug, was found to increase the protein and mRNA levels of glial cell line-derived neurotrophic factor (GDNF) in human neuroblastoma SH-SY5Y cells, whereas an analogue without a propargyl residue, aminoindan, did not. GDNF is known to protect dopaminergic neurons in animal and cellular models of Parkinson's disease, and the supplement has been tried for the treatment of degenerating dopamine neurons in Parkinsonian patients. In this paper, intracellular mechanism underlying the induction of GDNF was studied. Rasagiline induced phosphorylation of inhibitory subunit (IkappaB) of nuclear factor-kappaB (NF-kappaB), and translocation of active p65 subunit from cytoplasm into nuclei. Activation of NF-kappaB was also quantitatively determined by NF-kappaB p65 transcription assay. Sulfasalazine, an inhibitor of IkappaB kinase, suppressed the activation of NF-kappaB and the increase of GDNF by rasagiline simultaneously, further indicating the involvement of the IkappaB kinase-NF-kappaB pathway. The results on the activation of the transcription factor by rasagiline are discussed in relation to its possible application as a neuroprotective drug to halt declining of neurons in neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases.

An inhibitor of mitochondrial complex I, rotenone, inactivates proteasome by oxidative modification and induces aggregation of oxidized proteins in SH-SY5Y cells

In Parkinson's disease, characteristic pathological features are the cell death of nigrostriatal dopamine neurons and the formation of Lewy bodies composed of oxidized proteins. Mitochondrial dysfunction and aggregation of abnormal proteins have been proposed to cause the pathological changes. However, the relation between these two factors remains to be clarified. In this study, the effects of mitochondrial dysfunction on the oxidative modification and accumulation of proteins were analyzed using an inhibitor of mitochondrial complex I, rotenone, and antibodies against acrolein- and dityrosine-modified proteins. Under conditions inducing mainly apoptosis in neuroblastoma SH-SY5Y cells, rotenone markedly increased oxidized proteins, especially those modified with acrolein, even though the increase in intracellular reactive oxygen and nitrogen species was only transient and was not so marked. In addition, the activity of the proteasome system degrading oxidized proteins was reduced profoundly after treatment with rotenone. The 20S beta subunit of proteasome was modified with acrolein, to which other acrolein-modified proteins were found to bind, as shown by coprecipitation with the antibody against 20S beta subunit. These results suggest that mitochondrial dysfunction, especially decreased activity of complex I, may reduce proteasome activity through oxidative modification of proteasome itself and aggregation with other oxidized proteins. This mechanism might account for the accumulation of modified protein and, at least partially, for cell death of the dopamine neurons in Parkinson's disease.

Mitochondrial permeability transition mediates apoptosis induced by N-methyl(R)salsolinol, an endogenous neurotoxin, and is inhibited by Bcl-2 and rasagiline, N-propargyl-1(R)-aminoindan

The role of mitochondrial permeability transition (PT) in apoptosis induced by an endogenous neurotoxin, N-methyl(R)salsolinol [NM(R)Sal], was studied by use of dopaminergic neuroblastoma SH-SY5Y cells. NM(R)Sal reduced mitochondrial membrane potential, DeltaPsim, in the early phase of apoptosis, which was not suppressed by a pan-caspase inhibitor, but was antagonized by Bcl-2 and cyclosporin A, suggesting the involvement of the PT in NM(R)Sal-induced loss of DeltaPsim. NM(R)Sal-induced apoptosis was completely inhibited not only by Bcl-2 and a pan-caspase inhibitor, but also by cyclosporin A, suggesting the essential role of the PT in NM(R)Sal-induced apoptosis. In mitochondria isolated from rat liver, NM(R)Sal induced swelling and reduced DeltaPsim, which was inhibited by cyclosporin A and Bcl-2 overexpression. These results indicate that NM(R)Sal induced the PT by direct action on the mitochondria. Rasagiline, N-propargyl-1(R)-aminoindan, which is a now under a clinical trial for Parkinson's disease, suppressed the DeltaPsim reduction, release of cytochrome c, and apoptosis induced by NM(R)Sal in SH-SY5Y cells. Rasagiline also inhibited the NM(R)Sal-induced loss of DeltaPsim and swelling in the isolated mitochondria, proving that rasagiline directly targets the mitochondria also. Altogether, mitochondrial PT plays a key role both in NM(R)Sal-induced cell death and the neuroprotective effect of rasagiline.

An anti-Parkinson's disease drug, N-propargyl-1(R)-aminoindan (rasagiline), enhances expression of anti-apoptotic bcl-2 in human dopaminergic SH-SY5Y cells

N-Propargyl-1(R)-aminoindan (rasagiline) is now under phase III clinical trials for Parkinson's disease (PD), and it rescues dopamine neurons from cell death in animal and cellular models of PD. Recently, we proved that rasagiline protected dopaminergic SH-SY5Y cells against apoptosis induced by a dopaminergic neurotoxin, N-methyl(R)salsolinol, and the mechanism was clarified to be due to suppression of death signal transduction in mitochondria. In this paper, the effects of rasagiline on the levels of anti-apoptotic bcl-2 gene family were studied. Rasagiline increased the levels of bcl-2 and bcl-x(l) mRNA at 100-10 nM and 100-10 pM, but not the level of pro-apoptotic bax mRNA. Enhanced expression of bcl-2 family indicates the ability of rasagiline to adjust the apoptotic threshold and protect degenerating neurons in PD.

Selective nitration of mitochondrial complex I by peroxynitrite: involvement in mitochondria dysfunction and cell death of dopaminergic SH-SY5Y cells

3-Nitrotyrosine (3-NT) is a specific marker of protein nitration by peroxynitrite (ONOO-) produced from nitric oxide and superoxide. Increase in 3-NT containing protein (3-NT protein) was reported in brains from patients with some neurodegenerative disorders and aging. In this paper, intracellular localization of 3-NT protein was examined in dopaminergic SH-SY5Y cells using the selective antibody against protein-bound 3-NT. 3-NT protein was detected in plasma membrane/nucleus and mitochondria fractions, and interestingly in polypeptide composition of mitochondrial complex I. ONOO--generating SIN-1 induced apoptotic cell death with concomitant increase in 3-NT protein and reduction in mitochondrial ATP synthesis. In addition, an inhibitor of proteasomes, carbobenzoxy-L-isoleucyl-gamma-t-butyl-L-glutamyl-L-alanyl-L-leucinal, enhanced the effects of ONOO-. These results suggest that ONOO- may induce mitochondrial dysfunction and cell death in neurons through nitration of mitochondrial complex I subunits.

Glyceraldehyde-3-phospate dehydrogenase is translocated into nuclei through Golgi apparatus during apoptosis induced by 6-hydroxydopamine in human dopaminergic SH-SY5Y cells

6-Hydroxydopamine is a neurotoxin specific to dopamine neurons, and this neurotoxin at 20 muM was confirmed to induce mainly apoptosis in human dopaminergic neuroblastoma SH-SY5Y cells. During the apoptotic process, translocation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) into nuclei was observed, which is now proposed as an apoptogenic signal in various types of apoptosis. However, it remains to be clarified, how GAPDH is translocated into nuclei. In this paper, GAPDH translocation was followed by fluoro- and electron-microscopic observation using antibody against GAPDH. Before the nuclear translocation, a condensed mass of GAPDH protein was detected in the Golgi apparatus, which was identified by 6-([N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoyl)-sphingosine, a selective marker of Golgi apparatus. These results suggest that the Golgi apparatus may be involved in the compartmentalization of GAPDH from cytosol to nuclei.