Paraquat and iron exposure as possible synergistic environmental risk factors in Parkinson's disease

Cogs in the autophagic machine-equipped to combat dementia-prone neurodegenerative diseases

Neurodegenerative diseases are often characterized by hydrophobic inclusion bodies, and it may be the case that the aggregate-prone proteins that comprise these inclusion bodies are in fact the cause of neurotoxicity. Indeed, the appearance of protein aggregates leads to a proteostatic imbalance that causes various interruptions in physiological cellular processes, including lysosomal and mitochondrial dysfunction, as well as break down in calcium homeostasis. Oftentimes the approach to counteract proteotoxicity is taken to merely upregulate autophagy, measured by an increase in autophagosomes, without a deeper assessment of contributors toward effective turnover through autophagy. There are various ways in which autophagy is regulated ranging from the mammalian target of rapamycin (mTOR) to acetylation status of proteins. Healthy mitochondria and the intracellular energetic charge they preserve are key for the acidification status of lysosomes and thus ensuring effective clearance of components through the autophagy pathway. Both mitochondria and lysosomes have been shown to bear functional protein complexes that aid in the regulation of autophagy. Indeed, it may be the case that minimizing the proteins associated with the respective neurodegenerative pathology may be of greater importance than addressing molecularly their resulting inclusion bodies. It is in this context that this review will dissect the autophagy signaling pathway, its control and the manner in which it is molecularly and functionally connected with the mitochondrial and lysosomal system, as well as provide a summary of the role of autophagy dysfunction in driving neurodegenerative disease as a means to better position the potential of rapamycin-mediated bioactivities to control autophagy favorably.

Phlebotomy-induced iron deficiency attenuates the pulmonary toxicity of paraquat in mice

Phlebotomy is an effective method in the prevention and treatment of some poisonings, among which iron deficiency is a well-known consequence. Given the role of iron in paraquat (PQ) toxicity, the present study investigated the effectiveness of phlebotomy in PQ pulmonary toxicity. After conducting preliminary studies, the duration time of phlebotomy was set to be seven days. Then, the mice were divided into nine separate groups. Groups 1-3 received a single dose of normal saline, and 5 and 10 mg/kg of PQ, respectively, and phlebotomy was not performed on them (NPG status). The animals in groups 4-6 first underwent phlebotomy for seven days and then received a single dose of normal saline, and 5 and 10 mg/kg of PQ (PBPT status). Groups 7-9 first received a single dose of normal saline, and 5 and 10 mg/kg of PQ and then underwent phlebotomy for seven days (PAPT status). Seven days after acute exposure to PQ, the animals were anesthetized and biochemical biomarkers as well as lung tissue changes were evaluated. The findings showed that phlebotomy before and after PQ toxicity significantly decreased serum iron compared to NPG condition. In the PBPT status, phlebotomy could prevent PQ toxicity by increasing the activity of catalase and superoxide dismutase (SOD) and decreasing the activity of myeloperoxidase (MPO), and the levels of hydroxyproline and lipid peroxidation in the lung tissue. In the PAPT status, a significant improvement was observed in SOD and MPO activities compared to the NPG status. Confirming the biochemical findings, the histological results indicated higher effectiveness of phlebotomy in preventing PQ toxicity (PBPT) compared to its therapeutic effects (PAPT). Considering the role of iron in PQ toxicity, it appears that the reduction of serum iron levels during phlebotomy can be effective in preventing lung injuries caused by PQ and improving the performance of the pulmonary antioxidant system.

Paraquat and MPTP induce alteration in the expression profile of long noncoding RNAs in the substantia nigra of mice: Role of the transcription factor Nrf2

Parkinson's disease (PD) is a common age-related degenerative disease of the central nervous system caused mainly by hereditary, pesticides, metals, and polychlorinated biphenyls. Paraquat (PQ), a widely used herbicide, causes PD. Long noncoding RNAs (lncRNAs) are nonprotein-coding transcripts, expressed in the brain and play irreplaceable roles in neurodegenerative diseases. NF-E2-related factor-2 (Nrf2) is an important genetic transcription regulator in oxidative stress. We aimed to discover novel PQ or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-Nrf2-related lncRNAs and explore their association with PD. 17157 lncRNAs and 13707 mRNAs (fold change ≥2, P < 0.05) were identified by Microarray. And the expressions of six lncRNAs were confirmed by using qRT-PCR and two by FISH. Coding-noncoding analysis and qRT-PCR were applied to discover the functions of lncRNAs and predict the targeted genes. In mice, PQ and MPTP exposure caused alteration of the lncRNA expression profile, suggesting lncRNAs may be involved in PQ- and MPTP-induced neurotoxicity. The changes in their lncRNA expression were distinct but related. PQ caused lncRNA expression profiling alteration in the substantia nigra (SN) through an interaction with Nrf2, thus changing the NR_027648/Zc3h14/Cybb and NR_030777/Zfp326/Cpne5 mRNA pathways. Similarly, MPTP caused lncRNA expression profiling alteration in SN through an interaction with Nrf2. Nrf2 may be involved in the development of neurodegeneration induced by PQ and MPTP via interaction with lncRNAs as the molecular mechanism. Our findings indicate the potential roles of lncRNAs in the development of PD by PQ or MPTP and provide positive insights into future mechanism studies.

Clathrin-Dependent Uptake of Paraquat into SH-SY5Y Cells and Its Internalization into Different Subcellular Compartments

The herbicide paraquat (PQ) is an exogenous toxin that allows the selective activation of dopaminergic neurons in the mesencephalon to induce injury and also causes its apoptosis in vitro. However, uptake mechanisms between PQ and neurons remain elusive. To address this issue, we undertook a study of PQ endocytosis in a dopaminergic SH-SY5Y cell line as well as explored the subsequent subcellular location and potential functional analysis of PQ. The PQ was found to bind the SH-SY5Y cell membrane and then became internalized via a clathrin-dependent pathway. PQ was internalized by many subcellular organelles in a time- and dose-dependent manner. Interestingly, the taken up PQ and secretogranin III (SCG3), which became dysregulated with PQ treatment that induced SH-SY5Y apoptosis in our previous study, colocalized in cytoplasmic vesicles. Taken together, our findings indicate that PQ is endocytosed by SH-SY5Y cells and that its multiple, subcellular localizations indicate PQ may potentially be involved in subcellular-level functions. More importantly, PQ distributing preferentially into SCG3-positive vesicles demonstrates its selective targeting which may affect SCG3 and cargoes carried by SCG3-positive vesicles. Therefore, it is reasonable to infer that PQ toxic insults may potentially interfere with neurotransmitter storage and transport associated with secretory granules.

Younger age at onset of sporadic Parkinson's disease among subjects occupationally exposed to metals and pesticides

An earlier age at onset of Parkinson's disease (PD) has been reported to be associated with occupational exposures to manganese and hydrocarbon solvents suggesting that exposure to neurotoxic chemicals may hasten the progression of idiopathic PD. In this study the role of occupational exposure to metals and pesticides in the progression of idiopathic PD was assessed by looking at age at disease onset. The effects of heritable genetic risk factors, which may also influence age at onset, was minimized by including only sporadic cases of PD with no family history of the disease (n=58). Independent samples Student t-test revealed that subjects with occupational exposure to metals and/or pesticides (n=36) were significantly (p=0.013) younger than unexposed controls (n=22). These subjects were then divided into three groups [high (n=18), low (n=18), and unexposed (n=22)] to ascertain if duration of exposure further influenced age at onset of PD. One-way ANOVA revealed that subjects in the high exposure group were significantly (p=0.0121) younger (mean age: 50.33 years) than unexposed subjects (mean age: 60.45 years). Subjects were also stratified by exposure type (metals vs. pesticides). These results suggest that chronic exposure to metals and pesticides is associated with a younger age at onset of PD among patients with no family history of the disease and that duration of exposure is a factor in the magnitude of this effect.

Pooled analysis of iron-related genes in Parkinson's disease: association with transferrin

Pathologic features of Parkinson's disease (PD) include death of dopaminergic neurons in the substantia nigra, presence of α-synuclein containing Lewy bodies, and iron accumulation in PD-related brain regions. The observed iron accumulation may be contributing to PD etiology but it also may be a byproduct of cell death or cellular dysfunction. To elucidate the possible role of iron accumulation in PD, we investigated genetic variation in 16 genes related to iron homeostasis in three case-control studies from the United States, Australia, and France. After screening 90 haplotype tagging single nucleotide polymorphisms (SNPs) within the genes of interest in the US study population, we investigated the five most promising gene regions in two additional independent case-control studies. For the pooled data set (1289 cases, 1391 controls) we observed a protective association (OR=0.83, 95% CI: 0.71-0.96) between PD and a haplotype composed of the A allele at rs1880669 and the T allele at rs1049296 in transferrin (TF; GeneID: 7018). Additionally, we observed a suggestive protective association (OR=0.87, 95% CI: 0.74-1.02) between PD and a haplotype composed of the G allele at rs10247962 and the A allele at rs4434553 in transferrin receptor 2 (TFR2; GeneID: 7036). We observed no associations in our pooled sample for haplotypes in SLC40A1, CYB561, or HFE. Taken together with previous findings in model systems, our results suggest that TF or a TF-TFR2 complex may have a role in the etiology of PD, possibly through iron misregulation or mitochondrial dysfunction within dopaminergic neurons.

(-)Schisandrin B ameliorates paraquat-induced oxidative stress by suppressing glutathione depletion and enhancing glutathione recovery in differentiated PC12 cells

Exposure to paraquat (PQ; N,N'-dimethyl-4-4'-bipyridium), a potent herbicide, can lead to neuronal cell death and increased risk of Parkinson's disease because of oxidative stress. In this study, we investigated the effect of (-)schisandrin B [(-)Sch B, a potent enantiomer of schisandrin B] on PQ-induced cell injury in differentiated pheochromocytoma cells (PC12). PQ treatment caused cell injury in PC12 cells, as indicated by the significant increase in lactate dehydrogenase (LDH) leakage. Pretreatment with (-)Sch B (5 μM) protected against PQ-induced toxicity in PC12 cells, as evidenced by the significant decrease in LDH leakage. (-)Sch B induced the cytochrome P-450-mediated reactive oxygen species generation in differentiated PC12 cells. The cytoprotection afforded by (-)Sch B pretreatment was associated with an increase in cellular reduced glutathione (GSH) level as well as the enhancement of γ-glutamylcysteine ligase (GCL) and glutathione reductase (GR) activity in PQ-challenged cells. Both GCL and GR inhibitors abrogated the cytoprotective effect of (-)Sch B in PQ-challenged cells. The biochemical mechanism underlying the GSH-enhancing effect of (-)Sch B was further investigated in PC12 cells subjected to an acute peroxide challenge. Although the initial GSH depletion induced by peroxide was reduced through GR-catalyzed regeneration of GSH in (-)Sch B-pretreated cells, the later enhanced GSH recovery was mainly mediated by GCL-catalyzed GSH synthesis. The results suggest that (-)Sch B treatment may increase the resistance of dopaminergic cells against PQ-induced oxidative stress through reducing the extent of oxidant-induced GSH depletion and enhancing the subsequent GSH recovery.

Environmental exposure, obesity, and Parkinson's disease: lessons from fat and old worms

BACKGROUND

A common link has been exposed, namely, that metal exposure plays a role in obesity and in Parkinson's disease (PD). This link may help to elucidate mechanisms of neurotoxicity.

OBJECTIVE

We reviewed the utility of the nematode, Caenorhabditis elegans, as a model organism to study neurodegeneration in obesity and Parkinson's disease (PD), with an emphasis on the neurotransmitter, dopamine (DA).

DATA SOURCES

A PubMed literature search was performed using the terms "obesity" and any of the following: "C. elegans," "central nervous system," "neurodegeneration," "heavy metals," "dopamine" or "Parkinson's disease." We reviewed the identified studies, including others cited therein, to summarize the current evidence of neurodegeneration in obesity and PD, with an emphasis on studies carried out in C. elegans and environmental toxins in the etiology of both diseases.

DATA EXTRACTION AND DATA SYNTHESIS

Heavy metals and DA have both been linked to diet-induced obesity, which has led to the notion that the mechanism of environmentally induced neurodegeneration in PD may also apply to obesity. C. elegans has been instrumental in expanding our mechanism-based knowledge of PD, and this species is emerging as a good model of obesity. With well-established toxicity and neurogenetic assays, it is now feasible to explore the putative link between metal- and chemical-induced neurodegeneration.

CONCLUSIONS

One side effect of an aging population is an increase in the prevalence of obesity, metabolic disorders, and neurodegenerative orders, diseases that are likely to co-occur. Environmental toxins, especially heavy metals, may prove to be a previously neglected part of the puzzle.

Molecular mechanisms of pesticide-induced neurotoxicity: Relevance to Parkinson's disease

Pesticides are widely used in agricultural and other settings, resulting in continued human exposure. Pesticide toxicity has been clearly demonstrated to alter a variety of neurological functions. Particularly, there is strong evidence suggesting that pesticide exposure predisposes to neurodegenerative diseases. Epidemiological data have suggested a relationship between pesticide exposure and brain neurodegeneration. However, an increasing debate has aroused regarding this issue. Paraquat is a highly toxic quaternary nitrogen herbicide which has been largely studied as a model for Parkinson's disease providing valuable insight into the molecular mechanisms involved in the toxic effects of pesticides and their role in the progression of neurodegenerative diseases. In this work, we review the molecular mechanisms involved in the neurotoxic action of pesticides, with emphasis on the mechanisms associated with the induction of neuronal cell death by paraquat as a model for Parkinsonian neurodegeneration.

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Iron as a cause of Parkinson disease - a myth or a well established hypothesis?

Iron is considered to be a possible trigger of oxidative stress leading to neurodegeneration. This mechanism of neuronal death is proposed as a cause of Parkinson disease. Although most of researchers agree with this, controversies remain regarding the amounts of iron needed for this process. According to non destructive methods of assessment of the concentration of the total iron in substantia nigra, there is no difference between PD and control. However there is no need for an increase of the total iron in parkinsonian SN to trigger the oxidative stress but only of the non-ferritin bound labile iron. Our recent studies suggest an increase of this iron in PD SN. This finding corresponds well to a decrease of L-ferritin concentration in parkinsonian SN and also to a difference of the size of iron core of ferritin between PD and control SN. The significance of these finding will be discussed.

Genetic mouse models of Parkinson's disease The state of the art

The identification of several mutations causing familial forms of Parkinson's disease (PD) has led to the creation of multiple lines of mice expressing similar genetic alterations. These models present a unique opportunity for understanding pathophysiological mechanisms leading to PD in a mammalian brain and provide models that are suitable for the preclinical testing of new therapies. Different lines of mice recapitulate the symptoms and pathological features of PD to various extents. This chapter examines their respective advantages and highlights some of the key findings that have already emerged from the analysis of these new models of PD.

Interferon-gamma deficiency modifies the motor and co-morbid behavioral pathology and neurochemical changes provoked by the pesticide paraquat

In addition to nigrostriatal pathology and corresponding motor disturbances, Parkinson's disease (PD) is often characterized by co-morbid neuropsychiatric symptoms, most notably anxiety and depression. Separate lines of evidence indicate that inflammatory processes associated with microglial activation and cytokine release may be fundamental to the progression of both PD and its co-morbid psychiatric pathology. Accordingly, we assessed the contribution of the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), to a range of PD-like pathology provoked by the ecologically relevant herbicide and dopamine (DA) toxin, paraquat. To this end, paraquat provoked overt motor impairment (reduced home-cage activity and impaired vertical climbing) and signs of anxiety-like behavior (reduced open field exploration) in wild-type but not IFN-gamma-deficient mice. Correspondingly, paraquat promoted somewhat divergent variations in neurochemical activity among wild-type and IFN-gamma null mice at brain sites important for both motor (striatum) and co-morbid affective pathologies (dorsal hippocampus, medial prefrontal cortex, and locus coeruleus). Specifically, the herbicide provoked a dosing regimen-dependent reduction in striatal DA levels that was prevented by IFN-gamma deficiency. In addition, the herbicide influenced serotonergic and noradrenergic activity within the dorsal hippocampus and medial prefrontal cortex; and elevated noradrenergic activity within the locus coeruleus. Although genetic ablation of IFN-gamma had relatively few effects on monoamine variations within the locus coeruleus and prefrontal cortex, loss of the pro-inflammatory cytokine did normalize the paraquat-induced noradrenergic alterations within the hippocampus. These findings further elucidate the functional implications of paraquat intoxication and suggest an important role for IFN-gamma in the striatal and motor pathology, as well as the co-morbid behavioral and hippocampal changes induced by paraquat.

Coupling endoplasmic reticulum stress to the cell death program in dopaminergic cells: effect of paraquat

Parkinson's disease (PD) features oxidative stress and accumulation of misfolded (unfolded, alternatively folded, or mutant) proteins with associated loss of dopaminergic neurons. Oxidative stress and the accumulated misfolded proteins elicit cellular responses that include an endoplasmic reticulum (ER) stress response that may protect cells against the toxic buildup of misfolded proteins. Chronic ER stress and accumulation of misfolded proteins in excessive amounts, however, overwhelm the cellular 'quality control' system and impair the protective mechanisms designed to promote correct folding and degrade faulty proteins, ultimately leading to organelle dysfunction and neuronal cell death. Paraquat belongs to a class of bipyridyl herbicides and triggers oxidative stress and dopaminergic cell death. Epidemiological studies suggest an increased risk for developing PD following chronic exposure to paraquat. The present study was carried out to determine the role of paraquat in triggering cellular stress particularly ER stress and to elucidate the pathways that couple ER stress to dopaminergic cell death. We demonstrate that paraquat triggers ER stress, cell dysfunction, and dopaminergic cell death. p23, a small co-chaperone protein, is cleaved during ER stress-induced cell death triggered by paraquat and blockage of the caspase cleavage site of p23 was associated with decreased cell death. Paraquat also inhibits proteasomal activity that may further trigger accumulation of misfolded proteins resulting in ER stress. Our results indicate a protective role for p23 in PD-related programmed cell death. The data also underscore the involvement of ER, caspases, and the proteasomal system in ER stress-induced cell death process.

Inflammatory priming of the substantia nigra influences the impact of later paraquat exposure: Neuroimmune sensitization of neurodegeneration

Activation of microglia along with the release of inflammatory cytokines and oxidative factors often accompanies toxin-induced degeneration of substantia nigra pars compacta (SNc) dopamine (DA) neurons. Multiple toxin exposure may synergistically influence microglial-dependent DA neuronal loss and, in fact, pre-treatment with one toxin may sensitize DA neurons to the impact of subsequent insults. Thus, we assessed whether priming SNc neurons with the inflammatory agent, lipopolysaccharide (LPS), influenced the impact of later exposure to the pesticide, paraquat, which has been reported to provoke DA loss. Indeed, LPS infusion into the SNc sensitized DA neurons to the neurodegenerative effects of a series of paraquat injections commencing 2 days later. In contrast, LPS pre-treatment actually protected against some of neurodegenerative effects of paraquat when the pesticide was administered 7 days after the endotoxin. These sensitization and de-sensitization effects were associated with altered expression of reactive microglia expressing inducible immunoproteasome subunits, as well as variations of fibroblast growth factor and a time-dependent infiltration of peripheral immune cells. Circulating levels of the inflammatory cytokines, interleukin (IL)-6, IL-2, tumor necrosis factor-alpha and interferon-gamma were also time-dependently elevated following intra-SNc LPS infusion. These data suggest that inflammatory priming may influence DA neuronal sensitivity to subsequent environmental toxins by modulating the state of glial and immune factors, and these findings may be important for neurodegenerative conditions, such as Parkinson's disease (PD).

Paraquat-induced apoptosis in human neuroblastoma SH-SY5Y cells: involvement of p53 and mitochondria

The herbicide paraquat is a suspected etiologic factor in the development of Parkinson's disease (PD). Paraquat was therefore used to reproduce Parkinsonian syndromes in lab animals, in which it produces dopaminergic pathogenesis. However, the factors or mechanisms by which paraquat kills dopaminergic neurons are not fully understood. Based on reported evidence that paraquat increases p53 protein levels and inhibits mitochondrial function, it was hypothesized that paraquat induces cell death in dopaminergic neurons through a mechanism in which p53 and mitochondrial apoptotic pathway are linked. To explore this possibility, dopaminergic SY5Y cells were treated with paraquat for 48 h and p53 responses were investigated, as well as biomarkers of the mitochondrial intrinsic pathway of apoptosis. Paraquat significantly increased protein levels of p53 and one of its target genes, Bax. By 24 h, paraquat decreased mitochondrial complex I activity and mitochondrial transmembrane potential and induced the release of cytochrome c from mitochondria. In addition, paraquat increased the activities of caspases 9 and 3. Finally, nuclear condensation and DNA fragmentation occurred 48 h after treatment. The decrease of mitochondrial functions, the release of cytochrome c, the increase of caspase 9 and 3 activities, and DNA damage that were produced by paraquat were inhibited by a specific p53 inhibitor, pifithrin-alpha. These findings support the conclusion that paraquat produced apoptosis in SY5Y cells through the mitochondrial intrinsic pathway associated with p53.

The bipyridyl herbicide paraquat induces proteasome dysfunction in human neuroblastoma SH-SY5Y cells

Paraquat (PQ) is suspected to be an environmental risk factor for Parkinson's disease (PD). A strong correlation between exposure to paraquat and the occurrence of PD was reported in Canada, Taiwan, and the United States. This correlation is supported by in vivo work showing that paraquat produces dopaminergic pathogenesis. In particular, paraquat forms abnormal protein aggregates in dopaminergic neurons of mice. However, it is not clear how paraquat produces this pathology. Given that proteasome dysfunction induces aberrant protein aggregation, it was hypothesized that paraquat induces proteasome dysfunction. To explore this possibility, proteasome activity and some factors possibly contributing to proteasome dysfunction were investigated in dopaminergic SY5Y cells treated with paraquat. Furthermore, levels of alpha-synuclein and ubiquitin-conjugated proteins were measured to test whether paraquat induces protein accumulation in SY5Y cells. Results showed that at a concentration of paraquat that reduced viability by about 60% at 48 h (0.5 mM) loss of proteasome activity occurred. In addition, the cells showed decreased ATP levels and reduced mitochondrial complex V activity. These changes were significant 24 h after treatment with paraquat. Furthermore, paraquat-treated cells showed decreased protein levels of proteasome 19S subunits, but not 20S alpha or beta subunits, suggesting that the effects observed were not the result of general cytotoxicity. Paraquat also increased levels of alpha-synuclein and ubiquitinated proteins, suggesting that paraquat-induced proteasome dysfunction leads to aberrant protein accumulation. Taken together, these findings support the hypothesis that paraquat impairs proteasome function in SY5Y cells.

Developmental exposure to pesticides zineb and/or endosulfan renders the nigrostriatal dopamine system more susceptible to these environmental chemicals later in life

Several epidemiological studies have suggested a role for environmental pesticide exposures in idiopathic Parkinson's disease. The purpose of this study was to test the hypothesis that exposure to pesticides such as endosulfan and/or zineb during critical periods of postnatal development could result in neuronal dysfunction and enhance the impact of these pesticides during exposure as adults. C57BL/6 mice, exposed daily to each of the pesticides or their mixtures from postnatal days 5 to 19, exhibited insignificant changes in striatal dopamine, acetylcholinesterase and alpha-synuclein levels. However, mice exposed to these pesticides as juveniles and re-exposed at 8 months of age had significantly altered striatum and brain cortex neurotransmitter levels. Thus, mice re-exposed during adulthood to zineb, endosulfan and their mixtures showed a significantly depleted striatal dopamine levels, to 22, 16 and 35% of control, respectively. Acetylcholinesterase activity in the cerebral cortex was significantly increased in all pesticide treated groups (rho< or =0.05) upon repeated exposure, and pesticide mixture treatment also significantly increased levels of normal and aggregated alpha-synuclein. Collectively, these findings support our hypothesis that exposure to pesticides such as endosulfan and zineb during critical periods of postnatal development contributes to neurotransmitter changes upon re-challenge in adulthood.

Neuroprotective effects of kobophenol A against the withdrawal of tropic support, nitrosative stress, and mitochondrial damage in SH-SY5Y neuroblastoma cells

This study examined the neuroprotective effects of kobophenol A (kob A), oligomeric stillbene, and a resveratrol tetramer. Neuronal death induced by the withdrawal of tropic support was ameliorated by kob A. The protective effect of kob A against nitrosative/oxidative or mitochondrial damages resulted in the inhibition of the ROS, intracellular calcium ion level, and mitochondrial transmembrane potential changes on SH-SY5Y cells.

Effect of Genetic Variation on Human Cytochrome P450 Reductase-Mediated Paraquat Cytotoxicity

Paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride) is a widely used herbicide and is highly toxic to human and animals. The mechanisms of paraquat toxicity involve the generation of superoxide anion through the process of redox cycling. NADPH-cytochrome P450 oxidoreductase (POR) has been reported to be a major enzyme for one-electron reduction of paraquat that initiates the redox cycling. Recently, a total of six missense variants of human POR have been identified in patients with discorded steroidogenesis. However, the effect of these genetic variations on POR-mediated paraquat toxicity is not known. Using the Flp-In Chinese hamster ovary (CHO) cells stably expressing either mouse or human POR and the cells with POR knockdown by siRNA, we confirmed that POR is responsible for paraquat-induced cytotoxicity. We further used this validated system to compare paraquat-induced toxicity among the cells that stably expressed wild-type human POR and its natural variants. While there was no difference in paraquat-induced toxicity between the cells expressing wild-type human POR and the Cys569Tyr variant, the toxicity in cells expressing all the other variants (Tyr181Asp, Ala287Pro, Arg457His, Val492Glu, and Val608Phe) was significantly decreased. Our results provide further evidence on the important role of POR in paraquat-induced toxicity and suggest that individuals carrying the functional variant POR alleles may have an altered susceptibility to paraquat exposure.

Formation of stacking complexes between caffeine (1,2,3-trimethylxanthine) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine may attenuate biological effects of this neurotoxin

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin causing symptoms that may resemble those observed in patients suffering from Parkinson's disease. Therefore, MPTP-treated laboratory animals are currently the most favored models to study therapeutic intervention strategies in this disease. It was demonstrated recently that caffeine (1,2,3-trimethylxanthine) intake decreases the risk of Parkinson's disease in various human populations and attenuates MPTP-induced neurological effects in animal models. Since the effects of caffeine on MPTP-treated animals were mimicked by several antagonists of the adenosine A(2A) receptor, it was suggested that caffeine attenuates MPTP toxicity by blocking this receptor. Here, using microcalorimetry and molecular modeling, we demonstrate that caffeine can form stacking (pi-pi) complexes with MPTP. We found that a biological activity of MPTP (induction of mutations in a microbiological mutagenicity assay), which is completely independent on the A(2A) receptor blockade, is significantly reduced by caffeine. Therefore, we suggest that caffeine may attenuate neurotoxicity of MPTP (and possibly other polycyclic aromatic toxins) and reveal its protective effects on the risk of Parkinson's disease not only by blocking the A(2A) receptor but also by sequestering neurotoxin molecules in mixed complexes, especially in stomach.

The bipyridyl herbicide paraquat produces oxidative stress-mediated toxicity in human neuroblastoma SH-SY5Y cells: relevance to the dopaminergic pathogenesis

Paraquat (PQ) is a cationic nonselective bipyridyl herbicide widely used to control weeds and grasses in agriculture. Epidemiologic studies indicate that exposure to pesticides can be a risk factor in the incidence of Parkinson's disease (PD). A strong correlation has been reported between exposure to paraquat and PD incidence in Canada, Taiwan, and the United States. This correlation is supported by animal studies showing that paraquat produces toxicity in dopaminergic neurons of the rat and mouse brain. However, it is unclear how paraquat triggers toxicity in dopaminergic neurons. Based on the prooxidant properties of paraquat, it was hypothesized that paraquat may induce oxidative stress-mediated toxicity in dopaminergic neurons. To explore this possibility, dopaminergic SH-SY5Y cells were treated with paraquat, and several biomarkers of oxidativestress were measured. First, a specific dopamine transporter inhibitor GBR12909 significantly protected SY5Y cells against the toxicity of paraquat, indicating that paraquat exerts its toxicity by a mechanism involving the dopamine transporter (DAT). Second, paraquat increased intracellular levels of reactive oxygen species (ROS), but decreased the levels of glutathione. Third, paraquat inhibited glutathione peroxidase activity, but did not affect glutathione reductase activity. On the other hand, paraquat increased GST activity by 24 h, after which GST activity returned to the control value at 48 h. Fourth, paraquat dissipated mitochondrial transmembrane potential (MTP). Fifth, paraquat produced increases of malondialdehyde (MDA) and protein carbonyls, as well as DNA fragmentation, indicating oxidative damage to major cellular components. Sixth, paraquat increased the protein level of heme oxygenase-1 (HO-1). Taken together, these findings verify our hypothesis that paraquat produces oxidative stress-mediated toxicity in SH-SY5Y cells. Thus, current findings suggest that paraquat may induce the pathogenesis of dopaminergic neurons through oxidative stress.

Monoamine transporter inhibitors and norepinephrine reduce dopamine-dependent iron toxicity in cells derived from the substantia nigra

The role of dopamine in iron uptake into catecholaminergic neurons, and dopamine oxidation to aminochrome and its one-electron reduction in iron-mediated neurotoxicity, was studied in RCSN-3 cells, which express both tyrosine hydroxylase and monoamine transporters. The mean +/- SD uptake of 100 microm 59FeCl3 in RCSN-3 cells was 25 +/- 4 pmol per min per mg, which increased to 28 +/- 8 pmol per min per mg when complexed with dopamine (Fe(III)-dopamine). This uptake was inhibited by 2 microm nomifensine (43%p < 0.05), 100 microm imipramine (62%p < 0.01), 30 microm reboxetine (71%p < 0.01) and 2 mm dopamine (84%p < 0.01). The uptake of 59Fe-dopamine complex was Na+, Cl- and temperature dependent. No toxic effects in RCSN-3 cells were observed when the cells were incubated with 100 microm FeCl3 alone or complexed with dopamine. However, 100 microm Fe(III)-dopamine in the presence of 100 microm dicoumarol, an inhibitor of DT-diaphorase, induced toxicity (44% cell death; p < 0.001), which was inhibited by 2 microm nomifensine, 30 microm reboxetine and 2 mm norepinephrine. The neuroprotective action of norepinephrine can be explained by (1) its ability to form complexes with Fe3+, (2) the uptake of Fe-norepinephrine complex via the norepinephrine transporter and (3) lack of toxicity of the Fe-norepinephrine complex even when DT-diaphorase is inhibited. These results support the proposed neuroprotective role of DT-diaphorase and norepinephrine.

Pharmacological manipulation of ataxia-telangiectasia kinase activity as a treatment for Parkinson’s disease

Parkinson's disease (PD) is a major cause of morbidity and mortality among older individuals. Although the causes of Parkinson's disease are multifactorial, considerable evidence indicates that elevated labile iron in the substantia nigra pars compacta plays an important role in producing oxyradicals which subsequently damage nigro-striatal neurons. Based on this several researchers have suggested that blood-brain barrier crossing iron chelators might have clinical efficacy in treating PD. Work demonstrating that iron chelators protect nigro-striatal neurons in the N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 6-hydroxydopamine-induced rodent PD models supports this hypothesis. Recently, we found that the ATM gene product (mutated in ataxia-telangiectasia, A-T), is required for cell survival and genomic stability maintenance following exposure to low labile iron concentrations. Iron chelators (desferal, quercetin, and apoferritin) also increase A-T cell genomic stability and viability, and activate ATM-dependent cellular events in normal cells. Additionally Atm-deficient mice exhibit a selective loss of dopaminergic nigro-striatal neurons. Based on this, we propose that iron chelators protect the substantia nigra pars compacta not only by chelating labile iron and reducing oxyradical formation, but also by inducing ATM activity, leading to increased oxidative stress resistance and DNA repair. Support for this hypothesis comes from the recent observation that the iron chelating flavonoid quercetin both directly activates ATM and protects neuronal cells from the toxic effects of the N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Therefore since; (1) ATM is required for iron toxicity resistance, (2) iron chelators such as quercetin, desferal, and apoferritin induce ATM activity and/or ATM-dependent events, and (3), Atm-deficient mice preferentially lose dopaminergic nigro-striatal neurons, we propose that ATM activity has an important function in PD. Furthermore, pharmacological manipulation of ATM activity via iron chelation might have clinical efficacy in PD treatment.

Neurotoxins and neurotoxic species implicated in neurodegeneration

Neurotoxins, in the general sense, represent novel chemical structures which when administered in vivo or in vitro, are capable of producing neuronal damage or neurodegeneration--with some degree of specificity relating to neuronal phenotype or populations of neurons with specific characteristics (i.e., receptor type, ion channel type, astrocyte-dependence, etc.). The broader term 'neurotoxin' includes this categorization but extends the term to include intra- or extracellular mediators involved in the neurodegenerative event, including necrotic and apoptotic factors. Moreover, as it is recognized that astrocytes are essential supportive satellite cells for neurons, and because damage to these cells ultimately affects neuronal function, the term 'neurotoxin' might reasonably be extended to include those chemical species which also adversely affect astrocytes. This review is intended to highlight developments that have occurred in the field of 'neurotoxins' during the past 5 years, including MPTP/MPP+, 6-hydroxydopamine (6-OHDA), methamphetamine; salsolinol; leukoaminochrome-o-semiquinone; rotenone; iron; paraquat; HPP+; veratridine; soman; glutamate; kainate; 3-nitropropionic acid; peroxynitrite anion; and metals (copper, manganese, lead, mercury). Neurotoxins represent tools to help elucidate intra- and extra-cellular processes involved in neuronal necrosis and apoptosis, so that drugs can be developed towards targets that interrupt the processes leading towards neuronal death.

Novel mechanisms and approaches in the study of neurodegeneration and neuroprotection. a review

Cellular mechanisms involved in neurodegeneration and neuroprotection are continuing to be explored, and this paper focuses on some novel discoveries that give further insight into these processes. Oligodendrocytes and activated astroglia are likely generators of the pro-inflammatory cytokines, such as the tumor necrosis factor family and interleukin family, and these glial support cells express adhesion receptors (e.g., VCAM) and release intercellular adhesion molecules (ICAM) that have a major role in neuronal apoptosis. Even brief exposure to some substances, in ontogeny and sometimes in adulthood, can have lasting effects on behaviors because of their prominent toxicity (e.g., NMDA receptor antagonists) or because they sensitize receptors (e.g., dopamine D2 agonists), possibly permanently, and thereby alter behavior for the lifespan. Cell cycle genes which may be derived from microglia, are the most-recent entry into the neuroprotection schema. Neuroprotection afforded by some common substances (e.g., melatonin) and uncommon substances [e.g., nicotine, green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG), trolox], ordinarily thought to be simple radical scavengers, now are thought to invoke previously unsuspected cellular mechanisms in the process of neuroprotection. Although Alzheimer's disease (AD) has features of a continuous spectrum of neural and functional decline, in vivo PET imaging and and functional magnetic resonance imaging, indicate that AD can be staged into an early phase treatable by inhibitors of beta and gamma secretase; and a late phase which may be more amenable to treatment by drugs that prevent or reverse tau phosphorylation. Neural transplantation, thought to be the last hope for neurally injured patients (e.g., Parkinsonians), may be displaced by non-neural tissue transplants (e.g., human umbilical cord blood; Sertoli cells) which seem to provide similar neurotrophic support and improved behavior - without posing the major ethical dilemma of removing tissue from aborted fetuses. The objective of this paper is to invite added research into the newly discovered (or postulated) novel mechanisms; and to stimulate discovery of additional mechanisms attending neurodegeneration and neuroprotection.