6-hydroxydopamine induces dopaminergic cell degeneration via a caspase-9-mediated apoptotic pathway that is attenuated by caspase-9dn expression

Neuronal degeneration and oxidative stress in the SNc of 6-OHDA intoxicated rats; improving role of silymarin long-term treatment

Progressive loss in dopaminergic neurons (DA) of substantia nigra pars compacta (SNc) leads to Parkinson's disease with a hypothesis of oxidative stress generation. The present study was conducted to determine the long-term efficacy of silymarin (SM) post-treatment on 6-OHDA-induced oxidative stress in the SNc of male rats. Male Wistar rats were received 6-OHDA (8 μg/rat) into SNc. After 3 weeks, as recovery period, the animals were treated with i.p. injection of SM at different doses of 100, 200, or 300 mg/kg for 15 days. At the end of the treatment, motor function, neuronal cell count, antioxidant enzymes, and lipid peroxidation and tyrosine hydroxylase (TH) activities were evaluated in the ventral midbrain tissue. The 6-OHDA significantly decreased (p ≤ 0.05) motor function, antioxidant enzyme activity, GSH level, and GSH/GSSG ratio and caused an augmentation in GSSG and lipid peroxidation level. The 6-OHDA also reduced the population of neurons and TH expression. The SM repaired the 6-OHDA-induced motor impairment, antioxidant enzyme suppression, and TH down-regulation. All three doses of SM could restore the MDA level to the normal range in the 6-OHDA-lesioned rats and could reversed the effect of 6-OHDA on GSH, GSSG level, and GSH/GSSG ratio. The SM treatment significantly and dose-dependently increased (p ≤ 0.001) the total number of surviving neurons in the SNc. Silymarin chronic treatment restored the brain's antioxidant capacity and salvaged neurons from oxidative stress-induced neurodegeneration. The SM could also improve motor function in parkinsonian animals by increasing TH expression. These results recommend that application of SM over initial clinical stages may depict a hopeful approach versus PD. However, more research is needed to confirm this issue.

Evaluation of the neuroprotective potential of caffeic acid phenethyl ester in a cellular model of Parkinson's disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease, and oxidative stress and mitochondrial dysfunction play a major role in the pathogenesis of PD. Since conventional therapeutics are not sufficient for the treatment of PD, the development of new agents with anti-oxidant potential is crucial. Caffeic Acid Phenethyl Ester (CAPE), a biologically active flavonoid of propolis, possesses several biological properties such as immunomodulatory, anti-inflammatory and anti-oxidative. In the present study, we investigated the neuroprotective effects of CAPE against 6-hydroxydopamine (6-OHDA)-induced SH-SY5Y cells. The neuroprotective effects were detected by using cell viability, Annexin V, Hoechst staining, total caspase activity, cell cycle, as well as western blotting. Besides, the anti-oxidative activity was measured by the production of reactive oxygen species and mitochondrial function was determined by measurement of mitochondrial membrane potential (ΔΨm). We found that CAPE significantly increased cell viability and decreased apoptotic cell death (~20%) after 150 μM 6-OHDA exposure following 24 h. 1.25 μM CAPE also prevented 6-OHDA-induced changes in condensed nuclear morphology. Furthermore, treatment with 1.25 μM CAPE increased mitochondrial membrane potential in 6-OHDA-exposed cells. CAPE inhibited 6-OHDA-induced caspase activity (~2 fold) and production of reactive oxygen species. In addition, 150 μM 6-OHDA-induced down-regulation of Bcl-2 and Akt levels and up-regulation of Bax and cleaved caspase-9/caspase-9 levels were partially restored by 1.25 μM CAPE treatment. These results revealed a neuroprotective potential of CAPE against 6-OHDA-induced apoptosis in an in vitro PD model and may be a potential therapeutic candidate for the prevention of neurodegeneration in Parkinson's Disease.

Protective effect of roscovitine against rotenone-induced parkinsonism

BACKGROUND

Protective effect of roscovitine and deregulation of the p-RB/E2F1 have not been well studied in PD models generated by repeated oral administration of rotenone.

OBJECTIVE

These experiments evaluated the effects of repeated oral gavage of rotenone on the activation of p-RB/E2F1 and the effects of roscovitine on the regulation of dopaminergic neuronal injury and the behavior of PD in mice.

METHODS

Using 2.5% carboxymethylcellulose and 1.25% chloroform as a vehicle solution, rotenone (30 mg/kg) was administered via oral gavage once daily for 30 days in C57 mice. Behavioral profiles (pole test and traction test) were assessed in these PD models, and oxidative stress levels were evaluated in the midbrain. The immunoreactivities of TH, α-synuclein (α-syn), p-RB, E2F1 and cleaved caspase-3 in the substantia nigra were examined with a laser confocal microscope. Pharmacological inhibition of cyclin-dependent kinase with roscovitine was achieved by intraperitoneal (IP) injection at a dose of 50 mg/kg daily.

RESULTS

All rotenone-administered C57 mice showed the typical behavioral features of PD: stiffness, bradykinesia, or hypokinesia. Behavioral testing with the pole test and traction test indicated that the rotenone group, but not the vehicle group, was affected. Spectrophotometric analysis demonstrated that glutathione (GSH) and superoxide dismutase (SOD) activity was decreased, and the generation of malondialdehyde (MDA) was elevated in the midbrain of the rotenone-treated group. After oral administration of rotenone, a loss of nigral tyrosine hydroxylase (TH)-positive neurons was observed. The immune response of α-syn was enhanced in the cytoplasm of dopaminergic neurons from the rotenone-induced neurotoxicity. Rb phosphorylation at serine 780, which affected Rb binding to E2F, was induced after rotenone treatment. The activation of E2F1, which is involved in the regulation of the cell cycle, was also induced from chronic exposure to rotenone. Moreover, administration of the cell cycle inhibitor roscovitine protected against rotenone-induced nigral dopaminergic neuronal injury and inhibited cleaved caspase-3 activation. Roscovitine also markedly ameliorated the behavior of PD mice.

CONCLUSIONS

Mouse models of Parkinson's disease were established by oral rotenone administration and reproduced some of the features of dopaminergic neuronal degeneration. Roscovitine protects against rotenone-induced parkinsonism.

Metabotropic glutamate receptor 5 mediates the suppressive effect of 6-OHDA-induced model of Parkinson's disease on liver cancer

Numerous epidemiological studies suggested that there is a variable cancer risk in patients with Parkinson's disease (PD). However, the underlying mechanisms remain unclear. In the present study, the role of metabotropic glutamate receptor 5 (mGluR5) has been investigated in 6-hydroxydopamine (6-OHDA)-induced PD combined with liver cancer both in vitro and in vivo. We found that PD cellular model from 6-OHDA-lesioned MN9D cells suppressed the growth, migration, and invasion of Hepa1-6 cells via down-regulation of mGluR5-mediated ERK and Akt pathway. The application of 2-methyl-6-(phenylethyl)-pyridine and knockdown of mGluR5 further decreased the effect on Hepa-1-6 cells when co-cultured with conditioned media. The effect was increased by (S)-3,5-dihydroxyphenylglycine and overexpression of mGluR5. Moreover, more release of glutamate from 6-OHDA-lesioned MN9D cells suppressed mGluR5-mediated effect of Hepa1-6 cells. Application of riluzole eliminated the increased glutamate release induced by 6-OHDA in MN9D cells and aggravated the suppressive effect on Hepa-1-6 cells. In addition, the growth of implanted liver cancer was inhibited in 6-OHDA induced PD-like rats, and was associated with increased glutamate release in the serum and down-regulation of mGluR5 in tumor tissue. Collectively, these results indicate that selective antagonism of glutamate and mGluR5 has a potentially beneficial effect in both liver cancer and PD, and thus may provide more understanding for the clinical investigation and further an additional therapeutic target for these two diseases.

Protection effect of piperine and piperlonguminine from Piper longum L. alkaloids against rotenone-induced neuronal injury

Currently available treatment approaches for Parkinson׳s disease (PD) are limited in terms of variety and efficacy. Piper longum L. (PLL; Piperaceae) is used in traditional medicine in Asia and the Pacific Islands, with demonstrated anti-inflammatory and antioxidant activities in preclinical studies, and alkaloid extracts of PLL have shown protective effects in PD models. The present study investigated the mechanistic basis for the observed protective effects of PLL. Rats treated with PLL-derived alkaloids showed improvement in rotenone-induced motor deficits, while reactive oxygen species (ROS) production was decreased, mitochondrial membrane potential was stabilized, and the opening of the mitochondrial permeability transition pore (mPTP)-which is involved in ROS production-was inhibited. In addition, rotenone-induced apoptosis was abrogated in the presence of these alkaloids, while a pretreatment stimulated autophagy, likely mitigating neuronal injury by the removal of damaged mitochondria. These findings provide novel insight into the neuroprotective function of PLL as well as evidence in favor of its use in PD treatment. This article is part of a Special Issue entitled SI: Neuroprotection.

Effects of glial cell line-derived neurotrophic factor on microRNA expression in a 6-hydroxydopamine-injured dopaminergic cell line

Parkinson's disease (PD) is the second most prevalent, progressive neurodegenerative disease and is characterized by the irreversible and selective loss of nigrostriatal dopaminergic (DA) neurons. Glial cell line-derived neurotrophic factor (GDNF), which is a potent protective factor for DA neurons, is considered a promising neuroprotective candidate for PD. microRNAs (miRNAs) have been shown to be involved in a number of neurodegenerative diseases. Both miRNAs and GDNF affect DA neuronal processes, but the molecular crosstalk between these molecules remains unclear. The present study aimed to evaluate whether GDNF modulates miRNA expression. We used microarray analysis and real-time polymerase chain reaction (RT-PCR) to investigate miRNAs expression in 6-hydroxydopamine (6-OHDA)-injured MN9D cells treated with GDNF for 30 min, 1 h, or 3 h. Our results showed that GDNF treatment led to differential expression of 143 miRNAs. To further identify mechanisms by which GDNF exerts its effects, we compared miRNAs and mRNAs microarray data at the 1-h time point. We found that various biological processes and pathways were regulated at the miRNA level following GDNF treatment. Collectively, these results provide evidence of the capacity of GDNF to influence miRNAs expression, suggesting a new mechanism of GDNF action.

Pancreatic β-cell death in response to pro-inflammatory cytokines is distinct from genuine apoptosis

A reduction in functional β-cell mass leads to both major forms of diabetes; pro-inflammatory cytokines, such as interleukin-1beta (IL-1β) and gamma-interferon (γ-IFN), activate signaling pathways that direct pancreatic β-cell death and dysfunction. However, the molecular mechanism of β-cell death in this context is not well understood. In this report, we tested the hypothesis that individual cellular death pathways display characteristic phenotypes that allow them to be distinguished by the precise biochemical and metabolic responses that occur during stimulus-specific initiation. Using 832/13 and INS-1E rat insulinoma cells and isolated rat islets, we provide evidence that apoptosis is unlikely to be the primary pathway underlying β-cell death in response to IL-1β+γ-IFN. This conclusion was reached via the experimental results of several different interdisciplinary strategies, which included: 1) tandem mass spectrometry to delineate the metabolic differences between IL-1β+γ-IFN exposure versus apoptotic induction by camptothecin and 2) pharmacological and molecular interference with either NF-κB activity or apoptosome formation. These approaches provided clear distinctions in cell death pathways initiated by pro-inflammatory cytokines and bona fide inducers of apoptosis. Collectively, the results reported herein demonstrate that pancreatic β-cells undergo apoptosis in response to camptothecin or staurosporine, but not pro-inflammatory cytokines.

Alternative splicing of caspase 9 is modulated by the phosphoinositide 3-kinase/Akt pathway via phosphorylation of SRp30a

Increasing evidence points to the functional importance of alternative splice variations in cancer pathophysiology. Two splice variants are derived from the CASP9 gene via the inclusion (Casp9a) or exclusion (Casp9b) of a four-exon cassette. Here we show that alternative splicing of Casp9 is dysregulated in non-small cell lung cancers (NSCLC) regardless of their pathologic classification. Based on these findings we hypothesized that survival pathways activated by oncogenic mutation regulated this mechanism. In contrast to K-RasV12 expression, epidermal growth factor receptor (EGFR) overexpression or mutation dramatically lowered the Casp9a/9b splice isoform ratio. Moreover, Casp9b downregulation blocked the ability of EGFR mutations to induce anchorage-independent growth. Furthermore, Casp9b expression blocked inhibition of clonogenic colony formation by erlotinib. Interrogation of oncogenic signaling pathways showed that inhibition of phosphoinositide 3-kinase or Akt dramatically increased the Casp9a/9b ratio in NSCLC cells. Finally, Akt was found to mediate exclusion of the exon 3,4,5,6 cassette of Casp9 via the phosphorylation state of the RNA splicing factor SRp30a via serines 199, 201, 227, and 234. Taken together, our findings show that oncogenic factors activating the phosphoinositide 3-kinase/Akt pathway can regulate alternative splicing of Casp9 via a coordinated mechanism involving the phosphorylation of SRp30a.

hnRNP L regulates the tumorigenic capacity of lung cancer xenografts in mice via caspase-9 pre-mRNA processing

Caspase-9 is involved in the intrinsic apoptotic pathway and suggested to play a role as a tumor suppressor. Little is known about the mechanisms governing caspase-9 expression, but post-transcriptional pre-mRNA processing generates 2 splice variants from the caspase-9 gene, pro-apoptotic caspase-9a and anti-apoptotic caspase-9b. Here we demonstrate that the ratio of caspase-9 splice variants is dysregulated in non-small cell lung cancer (NSCLC) tumors. Mechanistic analysis revealed that an exonic splicing silencer (ESS) regulated caspase-9 pre-mRNA processing in NSCLC cells. Heterogeneous nuclear ribonucleoprotein L (hnRNP L) interacted with this ESS, and downregulation of hnRNP L expression induced an increase in the caspase-9a/9b ratio. Although expression of hnRNP L lowered the caspase-9a/9b ratio in NSCLC cells, expression of hnRNP L produced the opposite effect in non-transformed cells, suggesting a post-translational modification specific for NSCLC cells. Indeed, Ser52 was identified as a critical modification regulating the caspase-9a/9b ratio. Importantly, in a mouse xenograft model, downregulation of hnRNP L in NSCLC cells induced a complete loss of tumorigenic capacity that was due to the changes in caspase-9 pre-mRNA processing. This study therefore identifies a cancer-specific mechanism of hnRNP L phosphorylation and subsequent lowering of the caspase-9a/9b ratio, which is required for the tumorigenic capacity of NSCLC cells.

Knockdown of glycogen synthase kinase 3 beta attenuates 6-hydroxydopamine-induced apoptosis in SH-SY5Y cells

Glycogen synthase kinase 3 beta (GSK3β) plays a critical role in signal transductions concerning neuronal death. In the present study, we investigated the potential role of GSK3β in 6-hydroxydopamine (6-OHDA)-induced toxicity in human neuroblastoma cell line SH-SY5Y. We assessed the apoptotic proteins and the relative levels of pGSK3β (Ser9) and pGSK3β (Tyr216) to GSK3β in 6-OHDA-treated SH-SY5Y. Furthermore, we downregulated the expression of GSK3β by short hairpin RNA (shRNA) interference and compared the cell viability and expression of apoptotic proteins in knockdown group with those in control group under the treatment of 6-OHDA. We found that 6-OHDA increased the expression of caspase-3 and caspase-9 but not caspase-8. Additionally, 6-OHDA decreased the ratio of pGSK3β (Ser9)/GSK3β and increased the ratio of pGSK3β (Tyr216)/GSK3β. Moreover, 6-OHDA induced less cell viability loss and lower expression of caspase-9 and caspase-3 in GSK3β knockdown group compared with control. The present data indicate that 6-OHDA may induce apoptosis in SH-SY5Y via the intrinsic death pathway and GSK3β knockdown can partly attenuate 6-OHDA-induced neuronal death and apoptosis, suggesting that GSK3β may have the potential to serve as a therapeutic target for PD.

Elevation of heme oxygenase-1 by proteasome inhibition affords dopaminergic neuroprotection

Postmortem studies have shown that heme oxygenase-1 (HO-1) immunoreactivity is increased in patients with Parkinson disease. HO-1 expression is highly upregulated by a variety of stress. Since the proteasome activity is decreased in patients with Parkinson disease, we investigated whether proteasome activity regulates HO-1 content. MG-132, a proteasome inhibitor, increased the amount of HO-1 protein mainly in astrocytes of primary mesencephalic cultures. Quantitative RT-PCR analysis revealed that lactacystin upregulated HO-1 mRNA expression. Proteasome inhibition with MG132 also increased the cytomegalovirus promoter-driven expression of Flag-HO-1 protein and resulted in an accumulation of ubiquitinated Flag-HO-1 in Flag-HO-1-overexpressing PC12 cells. In addition, a cycloheximide chase assay demonstrated that the degradation of Flag-HO-1 protein was slowed by MG-132. Next, the function of HO-1 which was upregulated by proteasome inhibitors was examined. Proteasome inhibitors protected dopaminergic neurons from 6-hydroxydopamine (6-OHDA)-induced toxicity and this neuroprotection was abrogated by co-treatment with zinc protoporphyrin IX, a HO-1 inhibitor. Furthermore, 6-OHDA-induced toxicity was blocked by bilirubin and carbon monoxide, products of the HO-1-catalyzed degradation of heme. These results suggest that mesencephalic HO-1 protein level is regulated by proteasome activity and the elevation by proteasome inhibition affords neuroprotection.

The unilateral nigral lesion induces dramatic bilateral modification on rat brain monoamine neurochemistry

6-Hydroxydopamine (6-OHDA) is a neurotoxic compound commonly used to induce dopamine (DA) depletion in the nigrostriatal system, mimicking Parkinson's disease (PD) in animals. The aim of the present study was to evaluate the 7-day effect of unilateral nigral lesion on rat brain monoamine neurochemistry. Five brain regions were examined: the brain stem, cerebellum, hippocampus, striatum, and cortex. 6-OHDA-unilateral lesion dramatically modified DA, serotonin (5-HT) and their metabolites contents in both sides of the different brain nuclei. Furthermore, unilateral 6-OHDA lesion reduced DA and 5-HT contents and produced a robust inversion of their turnover in the nonlesioned side compared to sham-operated rats. These data suggest that 6-OHDA unilateral nigral lesion produces bilateral monoamine level modifications, and this piece of evidence should be taken into account when one interprets data from animal models of unilateral PD.

Elevated P75NTR expression causes death of engrailed-deficient midbrain dopaminergic neurons by Erk1/2 suppression

BACKGROUND

The homeodomain transcription factors Engrailed-1 and Engrailed-2 are required for the survival of mesencephalic dopaminergic (mesDA) neurons in a cell-autonomous and gene-dose-dependent manner. Homozygote mutant mice, deficient of both genes (En1-/-;En2-/-), die at birth and exhibit a loss of all mesDA neurons by mid-gestation. In heterozygote animals (En1+/-;En2-/-), which are viable and fertile, postnatal maintenance of the nigrostriatal dopaminergic system is afflicted, leading to a progressive degeneration specific to this subpopulation and Parkinson's disease-like molecular and behavioral deficits.

RESULTS

In this work, we show that the dose of Engrailed is inversely correlated to the expression level of the pan-neurotrophin receptor gene P75NTR (Ngfr). Loss of mesDA neurons in the Engrailed-null mutant embryos is caused by elevated expression of this neurotrophin receptor: Unusually, in this case, the cell death signal of P75NTR is mediated by suppression of Erk1/2 (extracellular-signal-regulated kinase 1/2) activity. The reduction in expression of Engrailed, possibly related to the higher levels of P75NTR, also decreases mitochondrial stability. In particular, the dose of Engrailed determines the sensitivity to cell death induced by the classic Parkinson-model toxin MPTP and to inhibition of the anti-apoptotic members of the Bcl-2 family of proteins.

CONCLUSION

Our study links the survival function of the Engrailed genes in developing mesDA neurons to the regulation of P75NTR and the sensitivity of these neurons to mitochondrial insult. The similarities to the disease etiology in combination with the nigral phenotype of En1+/-;En2-/- mice suggests that haplotype variations in the Engrailed genes and/or P75NTR that alter their expression levels could, in part, determine susceptibility to Parkinson's disease.

Caspases as therapeutic targets

The development of small molecules to modulate caspase activity offers a novel therapeutic strategy in the treatment of apoptosis-related and inflammatory diseases. Caspases are key mediators of apoptosis and inflammation; deregulation of their activation or expression can lead to the development of conditions such as neurodegenerative and autoinflammatory disorders. This review details the different caspase-associated disorders while focusing on caspase-1 inhibition as a potential therapeutic strategy. Problems facing the development of effective and safe caspase therapeutics will also be addressed.

Progressive degeneration of dopamine neurons in 6-hydroxydopamine rat model of parkinson's disease does not involve activation of caspase-9 and caspase-3

6-Hydroxydopamine (6-OHDA), a neurotoxin that causes the death of dopamine (DA) neurons, is commonly used to produce experimental models of Parkinson's disease (PD) in rodents. In the rat model of PD first described by Sauer and Oertel, DA neurons progressively die over several weeks following a striatal injection of 6-OHDA. It is generally assumed that DA neurons die through apoptosis after exposure to 6-OHDA, but data supporting activation of a caspase enzymatic cascade are lacking. In this study, we sought to determine if caspases involved in the intrinsic apoptotic cascade play a role in the initial stages of 6-OHDA-induced death of DA neurons in the progressively lesioned rat model of PD. We found that injection of 6-OHDA into adult rat striatum did not activate caspase-9 or caspase-3 or increase levels of caspase-dependent cleavage products in the substantia nigra at various survival times up to 7 days after the lesion, even though this paradigm produced DA neuronal loss. These data suggest that in the adult rat brain DA neurons whose terminals are challenged with 6-OHDA do not die through a classical caspase-dependent apoptotic mechanism.

Leptin protects against 6-hydroxydopamine-induced dopaminergic cell death via mitogen-activated protein kinase signaling

The death of midbrain dopaminergic neurons in sporadic Parkinson disease is of unknown etiology but may involve altered growth factor signaling. The present study showed that leptin, a centrally acting hormone secreted by adipocytes, rescued dopaminergic neurons, reversed behavioral asymmetry, and restored striatal catecholamine levels in the unilateral 6-hydroxydopamine (6-OHDA) mouse model of dopaminergic cell death. In vitro studies using the murine dopaminergic cell line MN9D showed that leptin attenuated 6-OHDA-induced apoptotic markers, including caspase-9 and caspase-3 activation, internucleosomal DNA fragmentation, and cytochrome c release. ERK1/2 phosphorylation (pERK1/2) was found to be critical for mediating leptin-induced neuroprotection, because inhibition of the MEK pathway blocked both the pERK1/2 response and the pro-survival effect of leptin in cultures. Knockdown of the downstream messengers JAK2 or GRB2 precluded leptin-induced pERK1/2 activation and neuroprotection. Leptin/pERK1/2 signaling involved phosphorylation and nuclear localization of CREB (pCREB), a well known survival factor for dopaminergic neurons. Leptin induced a marked MEK-dependent increase in pCREB that was essential for neuroprotection following 6-OHDA toxicity. Transfection of a dominant negative MEK protein abolished leptin-enhanced pCREB formation, whereas a dominant negative CREB or decoy oligonucleotide diminished both pCREB binding to its target DNA sequence and MN9D survival against 6-OHDA toxicity. Moreover, in the substantia nigra of mice, leptin treatment increased the levels of pERK1/2, pCREB, and the downstream gene product BDNF, which were reversed by the MEK inhibitor PD98059. Collectively, these data provide evidence that leptin prevents the degeneration of dopaminergic neurons by 6-OHDA and may prove useful in the treatment of Parkinson disease.

G-CSF protects dopaminergic neurons from 6-OHDA-induced toxicity via the ERK pathway

Granulocyte colony-stimulating factor (G-CSF) is known to have various functions such as induction of survival, proliferation and differentiation of hematopoietic cells. Recently, this factor has also been shown to exhibit neuroprotective effects in rat ischemic brain. In the present study, we first demonstrated that both G-CSF and G-CSF receptor were expressed in dopaminergic neurons in the adult substantia nigra and mesencephalic cultures, suggesting that G-CSF might exert its neuroprotective effects in dopaminergic neurons. Pretreatment with G-CSF protected dopaminergic neurons from 6-hydroxydopamine (6-OHDA)-induced neurotoxicity. Investigation of the underlying mechanisms showed that the extracellular-regulated kinase (ERK), but not Janus kinase/signal transducer(s) and activator(s) of transcription (JAK/STAT), was activated following G-CSF treatment. Moreover, G-CSF also increased phosphorylation of Bad, and restored 6-OHDA-induced decrease in Bcl-xL level. The 6-OHDA-caused caspase-3 activation in dopaminergic neurons was inhibited by G-CSF. Inhibition of ERK abrogated G-CSF-mediated Bad phosphorylation, Bcl-xL expression, activated caspase-3 reduction, and the protection of dopaminergic neurons. Taken together, G-CSF prevents dopaminergic neurons from 6-OHDA-induced toxicity via ERK pathway followed by inhibiting the apoptosis-execution process. These results suggest that G-CSF might have a therapeutic potential in Parkinson's disease.

Signalling pathways underlying neural cell adhesion molecule-mediated survival of dopaminergic neurons

Stimulation of the neural cell adhesion molecule (NCAM) by homophilic interactions is known to lead to neurite outgrowth as well as to neuronal survival. Whereas a complex network of signalling molecules is known to be of importance to NCAM-mediated neurite extension, only limited information is available regarding signalling underlying NCAM-mediated neuroprotection. Here, we present data suggesting a difference in the signalling events required for survival of rat dopaminergic neurons as compared with neurite outgrowth from the same cell type. Whereas Fyn, fibroblast growth factor receptor, mitogen-activated protein and ERK kinase, protein kinase A and protein kinase C are required for both responses to NCAM-induced signalling, phospholipase C and Ca(2+)-calmodulin-dependent kinase II are only necessary for the neurite outgrowth response, but dispensable for neuroprotection.

Molecular mechanisms of 6-hydroxydopamine-induced cytotoxicity in PC12 cells: involvement of hydrogen peroxide-dependent and -independent action

The neurotoxin 6-hydroxydopamine (6-OHDA) has been widely used to generate an experimental model of Parkinson's disease. It has been reported that reactive oxygen species (ROS), such as the superoxide anion and hydrogen peroxide (H2O2), generated from 6-OHDA are involved in its cytotoxicity; however, the contribution and role of ROS in 6-OHDA-induced cell death have not been fully elucidated. In the present study using PC12 cells, we observed the generation of 50 microM H2O2 from a lethal concentration of 100 microM 6-OHDA within a few minutes, and compared the sole effect of H2O2 with 6-OHDA. Catalase, an H2O2-removing enzyme, completely abolished the cytotoxic effect of H2O2, while a significant but partial protective effect was observed against 6-OHDA. 6-OHDA induced peroxiredoxin oxidation, cytochrome c release, and caspase-3 activation. Catalase exhibited a strong inhibitory effect against the peroxiredoxin oxidation, and cytochrome c release induced by 6-OHDA; however, caspase-3 activation was not effectively inhibited by catalase. On the other hand, 6-OHDA-induced caspase-3 activation was inhibited in the presence of caspase-8, caspase-9, and calpain inhibitors. These results suggest that the H2O2 generated from 6-OHDA plays a pivotal role in 6-OHDA-induced peroxiredoxin oxidation, and cytochrome c release, while H2O2- and cytochrome c-independent caspase activation pathways are involved in 6-OHDA-induced neurotoxicity. These findings may contribute to explain the importance of generated H2O2 and secondary products as a second messenger of 6-OHDA-induced cell death signal linked to Parkinson's disease.

Differentiated dopaminergic MN9D cells only partially recapitulate the electrophysiological properties of midbrain dopaminergic neurons

The cell line MN9D, a fusion of embryonic ventral mesencephalic and neuroblastoma cells, is extensively used as a model of dopamine (DA) neurons because it expresses tyrosine hydroxylase and synthesizes and releases DA. These cells are also used to test mechanisms and potential therapeutics relevant to the loss of DA neurons in Parkinson's disease. To date, little work has been done to determine whether MN9D cells electrophysiologically resemble mature DA neurons. We examined sodium, calcium and potassium currents in undifferentiated and differentiated MN9D cells, and compared these to those found in acutely dissociated mouse substantia nigra pars compacta DA neurons. It was observed that undifferentiated MN9D cells bore no resemblance to DA neurons. Upon differentiation with butyric acid with or without a prior treatment with glial cell line-derived neurotrophic factor, differentiated MN9D cells produce an electrophysiological profile that more closely resembles substantia nigra pars compacta DA neurons even though the A-type potassium current remains noticeably absent. These observations demonstrate that undifferentiated MN9D cells are not reasonable models of DA neurons. Although differentiated MN9D cells are closer to the mature DA neuronal phenotype, they do not fully mimic DA neurons and are likely to be of questionable value as a model because of their substantive differences, including the lack of the characteristic A-type potassium current. The future use of one or a combination of growth or other factors to differentiate MN9D cells may yield a more useful model system for Parkinson's disease studies in vitro.

Oxidative stress and dopamine depletion in an intrastriatal 6-hydroxydopamine model of Parkinson's disease

Although the etiology of Parkinson's disease (PD) is unknown, a common element of most theories is the involvement of oxidative stress, either as a cause or effect of the disease. There have been relatively few studies that have characterized oxidative stress in animal models of PD. In the present study a 6-hydroxydopamine (6-OHDA) rodent model of PD was used to investigate the in vivo production of oxidative stress after administration of the neurotoxin. 6-OHDA was injected into the striatum of young adult rats and the production of protein carbonyls and 4-hydroxynonenal (HNE) was measured at 1, 3, 7, and 14 days after administration. A significant increase in both markers was found in the striatum 1 day after neurotoxin administration, and this increase declined to basal levels by day 7. There was no significant increase found in the substantia nigra at any of the time points investigated. This same lesion paradigm produced dopamine depletions of 90-95% in the striatum and 63-80% in the substantia nigra by 14-28 days post-6-OHDA. Protein carbonyl and HNE levels were also measured in middle-aged and aged animals 1 day after striatal 6-OHDA. Both protein carbonyl and HNE levels were increased in the striatum of middle-aged and aged animals treated with 6-OHDA, but the increases were not as great as those observed in the young adult animals. Similar to the young animals, there were no increases in either marker in the substantia nigra of the middle-aged and aged animals. There was a trend for an age-dependent increase in basal amounts of oxidative stress markers when comparing the non-lesioned side of the brains of the three age groups. These results support that an early event in the course of dopamine depletion following intrastriatal 6-OHDA administration is the generation of oxidative stress.

Intranigral Dopamine Toxicity and α-Synuclein Response in Rats

There is increasing evidence that, in addition to its function as the main neurotransmitter in the nigrostriatal pathway, dopamine (DA) may be neurotoxic in certain conditions. In this study, the toxicity of DA was assessed by direct injection into the substantia nigra of anaesthetised rats, and its effects were compared with those of 6-hydroxydopamine. Brains were removed 1, 2 and 3 weeks after the lesion for histological or neurochemical analysis. DA caused a significant loss of 35% of tyrosine hydroxylase-positive neurons in the pars compacta of substantia nigra and a 40% reduction of striatal DA content. Cells with signs compatible with both apoptosis and autophagy were observed. GADD153, a parameter of endoplasmic reticulum stress, was strongly induced by 6-hydroxydopamine but not by DA. DA increased the alpha-synuclein content 1 week after the lesion (but not at the later times analyzed) in tyrosine hydroxylase-positive and in non-dopaminergic fibers of pars reticulata. The alpha-synuclein increase may be a physiological temporal response to DA accumulation and/or to cell damage, but the simultaneous presence of alpha-synuclein and DA in the cell cytoplasm at concentration higher than normal is not exempt from risk. In fact, their incubation in a free cell system gives a stable dimerized form of alpha-synuclein that has been described as the critical rate-limiting step for its abnormal fibrillation.

Erythropoietin protects against 6-hydroxydopamine-induced dopaminergic cell death

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the death of midbrain dopaminergic neurons. In the present study, erythropoietin, a trophic factor that has both hematopoietic and neural protective characteristics, was investigated for its capacity to protect dopaminergic neurons in experimental Parkinson's disease. Using both the dopaminergic cell line, MN9D, and primary dopamine neurons, we show that erythropoietin (1-3 U/mL) is neuroprotective against the dopaminergic neurotoxin, 6-hydroxydopamine. Protection was mediated by the erythropoietin receptor, as neutralizing anti-erythropoietin receptor antibody abrogated the protection. Activation of Akt/protein kinase B (PKB), via the phosphoinositide 3-kinase pathway, is a critical mechanism in erythropoietin-induced protection, while activation of extracellular signal-regulated kinase (ERK)1/2 contributes only moderately. Indeed, transfection of constitutively active Akt/PKB into dopaminergic cells was sufficient to protect against cell death. Furthermore, erythropoietin diminished markers of apoptosis in MN9D cells, including caspase 9 and caspase 3 activation and internucleosomal DNA fragmentation, suggesting that erythropoietin interferes with the apoptosis-execution process. When erythropoietin was administered to mice unilaterally lesioned with 6-hydroxydopamine, it prevented the loss of nigral dopaminergic neurons and maintained striatal catecholamine levels for at least 8 weeks. Erythropoietin-treated mice also had significantly reduced behavioral asymmetries. These studies suggest that erythropoietin can be an effective neuroprotective agent for dopaminergic neurons, and may be useful in reversing behavioral deficits associated with Parkinson's disease.

6-hydroxydopamine-induced apoptosis is mediated via extracellular auto-oxidation and caspase 3-dependent activation of protein kinase Cdelta

6-Hydroxydopamine is a neurotoxin commonly used to lesion dopaminergic pathways and generate experimental models for Parkinson disease, however, the cellular mechanism of 6-hydroxydopamine-induced neurodegeneration is not well defined. In this study we have explored how 6-hydroxydopamine neurotoxicity is initiated. We have also investigated downstream signaling pathways activated in response to 6-hydroxydopamine, using a neuronal-like, catecholaminergic cell line (PC12 cells) as an in vitro model system. We have shown that 6-hydroxydopamine neurotoxicity is initiated via extracellular auto-oxidation and the induction of oxidative stress from the oxidative products generated. Neurotoxicity is completely attenuated by preincubation with catalase, suggesting that hydrogen peroxide, at least in part, evokes neuronal cell death in this model. 6-Hydroxydopamine does not initiate toxicity by dopamine transporter-mediated uptake into PC12 cells, because both GBR-12909 and nisoxetine (inhibitors of dopamine and noradrenaline transporters, respectively) failed to reduce toxicity. 6-Hydroxydopamine has previously been shown to induce both apoptotic and necrotic cell-death mechanisms. In this study oxidative stress initiated by 6-hydroxydopamine caused mitochondrial dysfunction, activation of caspases 3/7, nuclear fragmentation, and apoptosis. We have shown that, in this model, proteolytic activation of the proapoptotic protein kinase Cdelta (PKCdelta) is a key mediator of 6-hydroxydopamine-induced cell death. 6-Hydroxydopamine induces caspase 3-dependent cleavage of full-length PKCdelta (79 kDa) to yield a catalytic fragment (41 kDa). Inhibition of PKCdelta (with rottlerin or via RNA interference-mediated gene suppression) ameliorates the neurotoxicity evoked by 6-hydroxydopamine, implicating this kinase in 6-hydroxydopamine-induced neurotoxicity and Parkinsonian neurodegeneration.

Involvement of extracellular signal-regulated kinases 1/2 and (phosphoinositide 3-kinase)/Akt signal pathways in acquired resistance against neurotoxin of 6-hydroxydopamine in SH-SY5Y cells following cell-cell interaction with astrocytes

Glial cells interact with neurons and play important roles in the development, differentiation, maintenance and repair of the nervous system. Human neuroblastoma cells (SH-SY5Y) became dramatically resistant to neurotoxin 6-hydroxydopamine (6-OHDA), when co-cultured with mouse astrocytes. In order to further delineate the molecular mechanism involved in the neuroprotection in this selective cell-cell interaction, we assessed the activation of two signal pathways, namely, the MAP kinases (extracellular signal-regulated kinases, ERK1/2) and phosphoinositide 3-kinase (PI3-K)/Akt signal pathways in response to 6-OHDA insult and subsequent neuronal survival. Western blot revealed that 6-OHDA significantly increased the phosphorylation of ERK1/2 and Akt in mono-cultured SH-SY5Y cells. However, the increase in ERK1/2 in SH-SY5Y cells after co-cultured with astrocytes occurred as early as 3 h after 6-OHDA treatment in oppose to the increase after 12 h in monocultures. The phosphorylation of Akt in the co-cultured SH-SY5Y cells was much pronounced 3 h after 6-OHDA treatment compared with that in the mono-cultured cells. The anti-apoptotic protein bcl-2 was also increased in the co-cultured SH-SY5Y cells 3 h after treatment with 6-OHDA. Selective inhibitor of PI3-K/Akt signal pathway blocked the acquired resistance to 6-OHDA in SH-SY5Y cells following interaction with astrocytes. Inhibition of ERK1/2 signal pathway did not affect the cell survival. Our data suggest that PI3-K/Akt signal pathway, but not ERK1/2, is involved the acquired resistance in SH-SY5Y cells following cell-cell interaction with astrocytes against the neurotoxic 6-OHDA insult.