Toxicity of MPTP and structural analogs in clonal cell lines of neuronal origin expressing B type monoamine oxidase activity

Mild MPP+ exposure-induced glucose starvation enhances autophagosome synthesis and impairs its degradation

Parkinson's disease (PD) is a prevalent neurodegenerative disorder, mainly characterised by the progressive loss of dopaminergic neurons. MPP⁺ has been widely used as a PD-related neurotoxin, and their reports suggested the several hypotheses for neuronal cell death. However, most of these hypotheses come from the studies about the acute MPP⁺ exposure. We previously revealed that mild MPP⁺ exposure (10 and 200 μM), which induces gradual cell death, impairs autophagosome degradation at 48 h. In the present study, we further investigated the specific events of mild MPP⁺ exposure and revealed that mild MPP⁺ exposure causes the cell death through glucose starvation, but not acute toxic model (2.5 and 5 mM). At 36 h after mild MPP⁺ exposure, autophagosome synthesis was enhanced owing to glucose starvation and continued to enhance until 48 h, despite impaired autophagosome degradation. Inhibition of autophagosome synthesis reduced mild MPP⁺-induced cell death. In conclusion, we clarified that glucose starvation-enhanced autophagosome synthesis occurs at an earlier stage than impaired autophagosome degradation and is important in mild MPP⁺ toxicity.

How Parkinsonian toxins dysregulate the autophagy machinery

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

The relationship between oxidative/nitrative stress and pathological inclusions in Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's diseases (PD) are late-onset neurodegenerative diseases that have tremendous impact on the lives of affected individuals, their families, and society as a whole. Remarkable efforts are being made to elucidate the dominant factors that result in the pathogenesis of these disorders. Extensive postmortem studies suggest that oxidative/nitrative stresses are prominent features of these diseases, and several animal models support this notion. Furthermore, it is likely that protein modifications resulting from oxidative/nitrative damage contribute to the formation of intracytoplasmic inclusions characteristic of each disease. The frequent presentation of both AD and PD in individuals and the co-occurrence of inclusions characteristic of AD and PD in several other neurodegenerative diseases suggests the involvement of a common underlying aberrant process. It can be surmised that oxidative/nitrative stress, which is cooperatively influenced by environmental factors, genetic predisposition, and senescence, may be a link between these disorders.

Protection from MPTP-induced neurotoxicity in differentiating mouse N2a neuroblastoma cells

We have shown previously that subcytotoxic concentrations of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) inhibit axon outgrowth and are associated with increased neurofilament heavy chain (NF-H) phosphorylation in differentiating mouse N2a neuroblastoma cells while higher doses (> 100 microM) cause cell death. In this work we assessed the ability of potential neuroprotective agents to alleviate both MPTP-induced cell death (cytotoxicity) and MPTP-induced NF-H phosphorylation/reduction in axon outgrowth (neurotoxicity) in N2a cells induced to differentiate by dbcAMP. The neurotoxic effects of MPTP occurred in the absence of significant alterations in energy status or mitochondrial membrane potential. The hormone oestradiol (100 microM) reduced the cytotoxic effect of MPTP, but blocked di-butyryl cyclic AMP (dbcAMP)-induced differentiation, i.e. axon outgrowth. Both the cytotoxic and neurotoxic effects of MPTP were reduced by the monoamine oxidase (MAO) inhibitors deprenyl and, to a lesser extent, clorgyline. Alleviation of both neurotoxicity and cytotoxicity was also achieved by conditioned medium derived from rat C6 glioma cells. In contrast, whilst the p38 MAP kinase inhibitor, SB202190, protected cells against MPTP-induced neurotoxicity, it could not maintain cell viability at high MPTP exposures. In each case neuroprotection involved maintenance of the differentiating phenotype linked with attenuation of NF-H hyper-phosphorylation; the latter may represent a mechanism by which neuronal cells can moderate MPTP-induced neurotoxicity. The use of a simplified neuronal cell model, which expresses subtle biochemical changes following neurotoxic insult, could therefore provide a valuable tool for the identification of potential neuroprotective agents.

Neurotoxicity of 1,2,3,4-tetrahydro-2-methyl-4,6,7-isoquinolinetriol (TMIQ) and effects on catecholamine homeostasis in SH-SY5Y cells

We have investigated the potential neurotoxicity of the catecholamine depleting agent 1,2,3,4-tetrahydro-2-methyl-4,6,7-isoquinolinetriol (TMIQ) in SH-SY5Y neuroblastoma cells. TMIQ induced a time and dose related inhibition of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide; thiazoyl blue (MTT) reduction and an increase in lactate dehydrogenase release. After 72 h TMIQ (30 μM) significantly (P < 0.05) inhibited MTT reduction, and significantly increased LDH release. TMIQ cytotoxicity was not prevented by the inclusion of monoamine oxidase inhibitors (clorgyline or deprenyl), antioxidants (α-tocopherol or Trolox C) or the uptake(1) inhibitor imipramine. TMIQ also induced a dose dependent stimulation of [(3)H]noradrenaline (NA) uptake, with maximum at 100 μM and EC(50) of 8 μM. This stimulation of [(3)H]NA uptake was not prevented by the inhibition of protein kinase C, or activation of adenylate or guanylate cyclases. In addition, TMIQ significantly (P < 0.05) displaced [(3)H]nisoxetine binding from the uptake(1) recognition site with a K(i) of 71 ± 8 μM. However, as this interaction occurs at concentrations of TMIQ well above the EC(50) for [(3)H]NA uptake, it is unlikely to explain TMIQ stimulated NA uptake. Furthermore, TMIQ inhibited potassium evoked [(3)H]NA release from SH-SY5Y cells, with an IC(50) of 490 μM. Thus, TMIQ is cytotoxic to SH-SY5Y cells. However, the exact mechanism of toxicity requires further investigation, since it appears not to involve monoamine oxidase bioactivation, and is not mediated through membrane based free radical damage. Furthermore, although TMIQ inhibits mitochondrial Complex I (IC(50) = 1.5 mM) with potency apparently greater than MPTP (2.7 mM), mitochondrial respiration was unaffected. The present studies suggest that the mechanism of toxicity differs from that causing depletion of catecholamines and inhibition of tyrosine hydroxylase by TMIQ described in previous studies.

Oxidative stress in a clonal cell line of neuronal origin: effects of antioxidant enzyme modulation

The effects of intracellularly generated H2O2 on cell viability, morphology, and biochemical markers of injury have been investigated in a clonal cell line of neuronal origin (140-3, mouse neuroblastoma X rat glioma) as a cell culture model for the role of oxidative stress in the long-term loss of neurons in the brain. The H2O2 was generated from the redox cycling of menadione, or by the oxidation of serotonin catalyzed by monoamine oxidase, to simulate the effect of amine neurotransmitter turnover. Incubation with menadione at concentrations as low as 10 microM for several hours resulted in significant losses of cell viability and altered morphology. Similar effects were evident in the presence of serotonin only after incubation overnight with concentrations > 1 mM. The cytotoxicity of either agent was potentiated by preincubation with specific inhibitors of two enzymes important to cellular antioxidant defenses, 3-amino-1,2,4-triazole for catalase and 1,3-bis(chloromethyl)-1-nitrosourea for glutathione reductase. Activity of another antioxidant enzyme of particular importance to antioxidant defenses in brain, the selenoprotein glutathione peroxidase, was stimulated fourfold by growth of cultures in the presence of sodium selenite as a source of active-site Se for the enzyme. The only effect of the selenite on other functionally coupled antioxidant enzymes was a decrease in activity of superoxide dismutase at concentrations > 200 nM. The selenite substantially protected cells against oxidative stress induced by combinations of menadione, 3-amino-1,2,4-triazole, and 1,3-bis(chloromethyl)-1-nitrosourea, but was only marginally effective with serotonin as a source of oxidative stress. The monoamine oxidase inhibitor pargyline increased cell survival in the presence of serotonin, demonstrating the role of this enzyme in its cytotoxicity. DNA damage (single strand breaks), but not lipid peroxidation, correlated with the cytotoxic effects of menadione.