Development and survival of rat embryonic mesencephalic dopaminergic neurones in serum-free, antioxidant-rich primary cultures

MDMA-induced neurotoxicity of serotonin neurons involves autophagy and rilmenidine is protective against its pathobiology

Toxicity of 3,4-methylenedioxymethamphetamine (MDMA) towards biogenic amine neurons is well documented and in primate brain predominantly affects serotonin (5-HT) neurons. MDMA induces damage of 5-HT axons and nerve fibres and intracytoplasmic inclusions. Whilst its pathobiology involves mitochondrially-mediated oxidative stress, we hypothesised MDMA possessed the capacity to activate autophagy, a proteostatic mechanism for degradation of cellular debris. We established a culture of ventral pons from embryonic murine brain enriched in 5-HT neurons to explore mechanisms of MDMA neurotoxicity and recruitment of autophagy, and evaluated possible neuroprotective actions of the clinically approved agent rilmenidine. MDMA (100 μM-1 mM) reduced cell viability, like rapamycin (RM) and hydrogen peroxide (H₂O₂), in a concentration- and time-dependent manner. Immunocytochemistry revealed dieback of 5-HT arbour: MDMA-induced injury was slower than for RM and H₂O₂, neuritic blebbing occurred at 48 and 72 h and Hoechst labelling revealed nuclear fragmentation with 100 μM MDMA. MDMA effected concentration-dependent inhibition of [³H]5-HT uptake with 500 μM MDMA totally blocking transport. Western immunoblotting for microtubule associated protein light chain 3 (LC3) revealed autophagosome formation after treatment with MDMA. Confocal analyses and immunocytochemistry for 5-HT, Hoechst and LC3 confirmed MDMA induced autophagy with abundant LC3-positive puncta within 5-HT neurons. Rilmenidine (1 μM) protected against MDMA-induced injury and image analysis showed full preservation of 5-HT arbours. MDMA had no effect on GABA neurons, indicating specificity of action at 5-HT neurons. MDMA-induced neurotoxicity involves autophagy induction in 5-HT neurons, and rilmenidine via beneficial actions against toxic intracellular events represents a potential treatment for its pathobiology in sustained usage.

Perturbation of transcription factor Nur77 expression mediated by myocyte enhancer factor 2D (MEF2D) regulates dopaminergic neuron loss in response to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

We have earlier reported the critical nature of calpain-CDK5-MEF2 signaling in governing dopaminergic neuronal loss in vivo. CDK5 mediates phosphorylation of the neuronal survival factor myocyte enhancer factor 2 (MEF2) leading to its inactivation and loss. However, the downstream factors that mediate MEF2-regulated survival are unknown. Presently, we define Nur77 as one such critical downstream survival effector. Following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment in vivo, Nur77 expression in the nigrostriatal region is dramatically reduced. This loss is attenuated by expression of MEF2. Importantly, MEF2 constitutively binds to the Nur77 promoter in neurons under basal conditions. This binding is lost following 1-methyl-4-phenylpyridinium treatment. Nur77 deficiency results in significant sensitization to dopaminergic loss following 1-methyl-4-phenylpyridinium/MPTP treatment, in vitro and in vivo. Furthermore, Nur77-deficient MPTP-treated mice displayed significantly reduced levels of dopamine and 3,4-Dihydroxyphenylacetic acid in the striatum as well as elevated post synaptic FosB activity, indicative of increased nigrostriatal damage when compared with WT MPTP-treated controls. Importantly, this sensitization in Nur77-deficient mice was rescued with ectopic Nur77 expression in the nigrostriatal system. These results indicate that the inactivation of Nur77, induced by loss of MEF2 activity, plays a critical role in nigrostriatal degeneration in vivo.

Neuroprotection by a mitochondria-targeted drug in a Parkinson's disease model

The objective of this study was to assess the neuroprotective effects of a mitochondria-targeted antioxidant, Mito-Q(10), the coenzyme-Q analog attached to a triphenylphosphonium cation that targets the antioxidant to mitochondria, in experimental models of Parkinson's disease (PD). Primary mesencephalic neuronal cells and cultured dopaminergic cells were treated with 1-methyl-4-phenylpyridinium (MPP(+)), an active metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and mice were used for testing the efficacy of Mito-Q(10). MPP(+) treatment caused a dose-dependent loss of tyrosine hydroxylase and membrane potential and an increase in caspase-3 activation in dopaminergic cells, which were reversed by Mito-Q(10). MPTP treatment induced a loss of striatal dopamine and its metabolites, inactivation of mitochondrial aconitase in the substantia nigra, and a loss of locomotor activity in mice. Treatment with Mito-Q(10) significantly inhibited both MPP(+)- and MPTP-induced neurotoxicity in cell culture and mouse models. Collectively, these results indicate that mitochondrial targeting of antioxidants is a promising neuroprotective strategy in this preclinical mouse model of PD.

Oxidants induce alternative splicing of alpha-synuclein: Implications for Parkinson's disease

alpha-Synuclein (alpha-syn) is a presynaptic protein that is widely implicated in the pathophysiology of Parkinson's disease (PD). Emerging evidence indicates a strong correlation between alpha-syn aggregation and proteasomal dysfunction as one of the major pathways responsible for destruction of the dopamine neurons. Using parkinsonism mimetics (MPP(+), rotenone) and related oxidants, we have identified an oxidant-induced alternative splicing of alpha-syn mRNA, generating a shorter isoform of alpha-syn with deleted exon-5 (112-syn). This spliced isoform has an altered localization and profoundly inhibits proteasomal function. The generation of 112-syn was suppressed by constitutively active MEK-1 and enhanced by inhibition of the Erk-MAP kinase pathway. Overexpression of 112-syn exacerbated cell death in a human dopaminergic cell line compared to full-length protein. Expression of 112-syn and proteasomal dysfunction were also evident in the substantia nigra and to a lesser extent in striatum, but not in the cortex of MPTP-treated mice. We conclude that oxidant-induced alternative splicing of alpha-syn plays a crucial role in the mechanism of dopamine neuron cell death and thus contributes to PD.

Rotenone and MPP+ preferentially redistribute apoptosis-inducing factor in apoptotic dopamine neurons

Rotenone and 1-methyl-4-phenylpyridinium produce parkinsonian models and we determined whether their mitochondrially mediated actions differentially redistributed the apoptogenic proteins, apoptosis-inducing factor and cytochrome c. Cultured rat mesencephalic dopamine neurons were exposed to rotenone (30 nM) and 1-methyl-4-phenylpyridinium (300 muM, 24 and 48 h) and apoptosis and mitochondrial redistribution of cytochrome c or apoptosis-inducing factor were quantified. Tyrosine hydroxylase-positive dopamine neurons underwent apoptosis (shrinkage, less neurites) and 40% released apoptosis-inducing factor with rotenone (24 h), whereas cytochrome c release reached this value at 48 h when 70% of cells had released apoptosis-inducing factor-positive. 1-Methyl-4-phenylpyridinium produced similar redistribution patterns for both proteins. Preferential redistribution of apoptosis-inducing factor before cytochrome c in dopamine neurons indicates caspase-independent mitochondrial proapoptotic signalling predominates in these parkinsonian models.

Particular vulnerability of rat mesencephalic dopaminergic neurons to tetrahydrobiopterin: Relevance to Parkinson's disease

We determined whether tetrahydrobiopterin(BH4), an endogenous cofactor for dopamine(DA) synthesis, causes preferential damage to DArgic neurons among primary cultured rat mesencephalic neurons and whether the death mechanism has relevance to Parkinson's disease (PD). DArgic neurons were more vulnerable to BH4 than non-DArgic neurons, exhibiting sensitivity at lower concentrations, evident by morphological and neurotransmitter uptake studies. BH4-exposed DArgic neurons showed (1) increased TUNEL staining and activated caspase-3 immunoreactivity, indicative of apoptotic death; (2) mitochondrial membrane potential loss and increased cytosolic cytochrome c, suggesting mitochondrial dysfunction; (3) increased level of oxidized proteins and protection by antioxidants, indicative of oxidative stress; and (4) increased ubiquitin immunoreactivity, suggesting alteration of protein degradation pattern. Percent of cells positive for these parameters were much higher for DArgic neurons, demonstrating preferential vulnerability. Therefore, the DArgic neuronal damage induced by BH4, the molecule synthesized and readily upregulated in DArgic neurons and activated microglia, suggests physiological relevance to the pathogenesis of PD.

Differential effects of human neuromelanin and synthetic dopamine melanin on neuronal and glial cells

We investigated the effects of neuromelanin (NM) isolated from the human substantia nigra and synthetic dopamine melanin (DAM) on neuronal and glial cell lines and on primary rat mesencephalic cultures. Lactate dehydrogenase (LDH) activity and lipid peroxidation were significantly increased in SK-N-SH cells by DAM but not by NM. In contrast, iron-saturated NM significantly increased LDH activity in SK-N-SH cells, compared with 100 mg/mL ETDA-treated NM containing a low concentration of bound iron. DAM, but not NM, stimulated hydroxyl radical production and increased SK-N-SH cell death via apoptotic-like mechanisms. Neither DAM nor NM induced any changes in the glial cell line U373. 3H-dopamine uptake in primary rat mesencephalic cultures was significantly reduced in DAM-compared with NM-treated cultures, accompanied by increased cell death via an apoptosis-like mechanism. Interestingly, Fenton-induced cell death was significantly decreased in cultures treated with both Fenton reagent and NM, an effect not seen in cultures treated with Fenton reagent plus DAM. These data are suggestive of a protective role for neuromelanin under conditions of high oxidative load. Our findings provide new evidence for a physiological role for neuromelanin in vivo and highlights the caution with which data based upon model systems should be interpreted.

Newborn neurons acquire high levels of reactive oxygen species and increased mitochondrial proteins upon differentiation from progenitors

A population of embryonic rat cortical cells cultured in the presence of FGF2 and having neuronal morphology expressed higher levels of reactive oxygen species (ROS) than did progenitor cells, astrocytes, and several cell lines of neuronal and non-neuronal origin. ROS were assessed using 5-(and-6)-chlormethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester (CM-H2DCF-DA), and high levels persisted in the presence of antioxidants or lowered levels of ambient oxygen. Greater than 95% of high ROS-producing cells, isolated by fluorescence-activated cell sorting, expressed the neuronal marker beta III tubulin. These cells did not incorporate BrdU or express nestin, unlike low ROS-producing cells, 99% of which exhibited both of these characteristics. Upon growth factor removal, low ROS-expressing cells differentiated into neurons and astrocytes and these neurons expressed high levels of ROS, indicating that ROS accumulation accompanies the differentiation of progenitors into neurons. ROS levels were decreased by added superoxide dismutase and catalase, suggesting that both superoxide and hydrogen peroxide contribute to the ROS signal. High ROS-expressing cells also contained higher levels of several mitochondrial respiratory chain components. Although ROS have been associated with conditions that lead to cell death, our results and recent studies on the role of ROS as regulators of signal pathways are consistent with the possibility that ROS play a role in the development of the neuronal phenotype. Moreover, the differential production of ROS provides a useful method to isolate from mixed populations cells that are highly enriched for either progenitor cells or neurons.

Hypersensitivity of DJ-1-deficient mice to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrindine (MPTP) and oxidative stress

Mutations of the DJ-1 (PARK7) gene are linked to familial Parkinson's disease. We used gene targeting to generate DJ-1-deficient mice that were viable, fertile, and showed no gross anatomical or neuronal abnormalities. Dopaminergic neuron numbers in the substantia nigra and fiber densities and dopamine levels in the striatum were normal. However, DJ-1-/- mice showed hypolocomotion when subjected to amphetamine challenge and increased striatal denervation and dopaminergic neuron loss induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrindine. DJ-1-/-embryonic cortical neurons showed increased sensitivity to oxidative, but not nonoxidative, insults. Restoration of DJ-1 expression to DJ-1-/- mice or cells via adenoviral vector delivery mitigated all phenotypes. WT mice that received adenoviral delivery of DJ-1 resisted 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrindine-induced striatal damage, and neurons overexpressing DJ-1 were protected from oxidative stress in vitro. Thus, DJ-1 protects against neuronal oxidative stress, and loss of DJ-1 may lead to Parkinson's disease by conferring hypersensitivity to dopaminergic insults.

Dietary polyphenols protect dopamine neurons from oxidative insults and apoptosis: investigations in primary rat mesencephalic cultures

Naturally occurring polyphenols have the potential to prevent oxidative damage in various pathophysiological conditions. Various members of the flavonoid family were investigated to determine if they could protect mesencephalic dopamine (DA) neurones from injury and reduce apoptosis produced by oxidative stressors. Primary mesencephalic cultures were sensitive to oxidative insults (hydrogen peroxide, 4-hydroxynonenal, rotenone, 6-hydroxydopamine and N-methyl-4-phenyl-1,2,3,6-tetrahydropyridinium hydrochloride (MPP+)) which produced concentration-dependent decreases in cellular viability across an apoptotic-necrotic continuum of injury. Flavonoids (catechin, quercetin, chrysin, puerarin, naringenin, genestein) protected mesencephalic cultures from injury by MPP+, which was shown by DNA fragmentation studies and tyrosine hydroxylase (TH) immunocytochemistry of DA neurones to occur by apoptosis. Catechin also reduced injury produced by hydrogen peroxide, 4-hydroxynonenal, rotenone and 6-hydroxydopamine as shown by increases in cellular viability and [3H]DA uptake. When the neuroprotection of catechin against MPP+-induced injury was compared to that produced by the caspase-3 inhibitor, Z-DVED-FMK, both reduced DNA fragmentation and the injury patterns of TH-positive neurones. These data demonstrate the neuroprotective abilities of flavonoids which are able to attenuate the apoptotic injury of mesencephalic DA neurones. Since these DA neurones are under oxidative stress in Parkinsonism, our findings suggest that flavonoids could provide benefits along with other anti-oxidant therapies in Parkinson's disease.

Discrepant effects of culture conditions on survival and function of dopaminergic neurons

Primary midbrain cultures are valuable for probing the function of dopaminergic neurons and for elucidating the factors that cause their dysfunction and degeneration. To allow more effective control of the culture environment, we have characterized the survival, differentiation, and trophic factor response of dopaminergic neurons in the absence of serum. Combinations of media and supplements markedly affected all three indices measured. Combinations that produced maximal dopaminergic neuron survival are different from those that result in maximal differentiation and trophic factor response. Furthermore, antioxidant treatment was effective with only one medium/supplement combination, indicating that these neurons were not degenerating as a result of oxidative stress in the majority of culture conditions used in this study. These results demonstrate that dopaminergic neurons can be grown in serum-free conditions but that the choice of culture conditions has a marked influence on cell survival and function.

Neurospheres modified to produce glial cell line-derived neurotrophic factor increase the survival of transplanted dopamine neurons

Glial cell line-derived neurotrophic factor (GDNF) has been shown to increase the survival of dopamine neurons in a variety of in vitro and in vivo model systems. Therefore, it constitutes an important therapeutic protein with the potential to ameliorate dopamine neuronal degeneration in Parkinson's disease or to support dopamine neuronal replacement strategies. However, biophysical and practical considerations present obstacles for the direct delivery of the GDNF protein to CNS neurons. Here we show that rodent neural precursor cells isolated and expanded in culture as neurospheres (NS) can be genetically modified to express green fluorescent protein (GFP) or to release GDNF using lentiviral constructs. GDNF-NS increased the fibre outgrowth of primary embryonic dopamine neurons in cocultures, showing that the protein was released in biologically significant quantities. Furthermore, after transplantation into the 6-hydroxydopamine-lesioned rat striatum, GDNF-NS significantly increased the survival of cografted primary dopamine neurons. However, this was not reflected in behavioural recovery in these animals. We found that, by 6 weeks, few cells expressed GDNF or GFP, suggesting either that transgene expression was down-regulated over time or that the cells died. This may explain the initial effects on dopamine neuronal survival within the graft but the lack of long-term effect on subsequent fibre outgrowth and behaviour. Providing sustained levels of neural precursor-mediated transgene expression can be achieved following transplantation in the future; this approach may prove beneficial as an alternative therapeutic strategy in the cell-based management of Parkinson's disease.

Method for serum-free culture of late fetal and early postnatal rat brainstem neurons

Primary culture of postnatal brainstem neurons in defined medium has not been described in the literature. Successful primary culture of brainstem neurons is typically restricted to embryonic ages E14-E18. This study describes a method for culture of late fetal and early postnatal brainstem neurons using a serum-free culture medium. The culture system is based on Neurobasal medium supplemented with antioxidant-rich B27 (Life Technologies). Neuron survival was optimized by replacing glutamine with GlutaMaxI, by matching osmolality with neuronal age, and by using Hibernate medium to increase neuron survival during tissue dissociation. This paper describes the first reliable method for culturing brainstem neurons from late fetal and early postnatal stages of the rat for up to 6 days postpartum.

Bromocriptine protects dopaminergic neurons from levodopa-induced toxicity by stimulating D(2)receptors

Neuroprotective properties of bromocriptine, a D(2) receptor agonist, were investigated using the in vitro neurotoxicity of levodopa for dopaminergic neurons from rat embryonic ventral mesencephalon. Levodopa, when added to the culture medium, showed toxicity which was specific for dopaminergic neurons. Bromocriptine was found to protect dopaminergic neurons from levodopa toxicity. Another D(2) agonist, 2-(N-phenethyl-N-propyl-amino-5-hydroxytetralin, showed similar protective effects. The neuroprotective effect of bromocriptine was inhibited by supplementation of the culture medium with sulpiride, a D(2) antagonist, or by D(2) receptor knockdown with an antisense oligonucleotide. Dopaminergic neurons treated with levodopa showed an increase in free radicals. These data suggest that neuroprotective properties of bromocriptine seen in this cellular model of neurotoxicity are dependent on dopamine D(2) autoreceptor binding and that levodopa toxicity may be related to increased free radical generation in dopaminergic neurons.

Treatment with the spin-trap agent alpha-phenyl-N-tert-butyl nitrone does not enhance the survival of embryonic or adult dopamine neurons

Reactive oxygen species are thought to be involved in the death of dopaminergic neurons in Parkinson's disease as well as in transplanted embryonic dopaminergic neurons. The spin-trap agent alpha-phenyl-N-tert-butyl nitrone (PBN) reacts directly with radical species and may thereby prevent them from damaging important cellular molecules such as membrane lipids. We found that PBN does not increase the survival of cultured embryonic dopaminergic neurons subjected to serum deprivation, whereas the antioxidant and lipid peroxidation inhibitor lazaroid U-83836E does. Moreover, PBN does not increase the survival of grafted embryonic dopaminergic neurons or graft efficacy (monitored as changes in drug-induced motor asymmetry in hemiparkinsonian rats) when the spin-trap agent is given intraperitoneally to the graft recipient or is added to the solutions used when preparing tissue for transplantation. Another spin-trap agent, alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN) also failed to protect neurons when given to graft recipients in the same experimental paradigm. Finally, we found that adult nigral neurons subjected to a progressive retrograde 6-OHDA lesion are not protected by systemic treatment with PBN. Even though reduction of oxidative stress by overexpression of superoxide dismutase or addition of lazaroids have previously been shown to enhance the survival of cultured and grafted dopaminergic neurons, spin-trap agents PBN and POBN do not provide protection in these experimental paradigms. This may be due to antioxidants and spin-trap agents interfering in different steps of free radical-induced cell damage.