Acute mitochondrial and chronic toxicological effects of 1-methyl-4-phenylpyridinium in human neuroblastoma cells

Chronic treatment with the complex I inhibitor MPP+ depletes endogenous PTEN-induced kinase 1 (PINK1) via up-regulation of Bcl-2-associated athanogene 6 (BAG6)

Mitochondrial dysfunction is implicated in sporadic and familial Parkinson's disease (PD). However, the mechanisms that impair homeostatic responses to mitochondrial dysfunction remain unclear. Previously, we found that chronic, low-dose administration of the mitochondrial complex I inhibitor 1-methyl-4-phenylpyridinium (MPP⁺) dysregulates mitochondrial fission-fusion, mitophagy, and mitochondrial biogenesis. Given that PTEN-induced kinase 1 (PINK1) regulates mitochondrial function, dynamics, and turnover, we hypothesized that alterations in endogenous PINK1 levels contribute to depletion of mitochondria during chronic complex I injury. Here we found that chronic MPP⁺ treatment of differentiated SH-SY5Y neuronal cells significantly decreases PINK1 expression prior to reductions in other mitochondrial components. Furthermore, Bcl2-associated athanogene 6 (BAG6, BAT3, or Scythe), a protein involved in protein quality control and degradation, was highly up-regulated during the chronic MPP⁺ treatment. BAG6 interacted with PINK1, and BAG6 overexpression decreased the half-life of PINK1. Conversely, siRNA-mediated BAG6 knockdown prevented chronic MPP⁺ stress-induced loss of PINK1, reversed MPP⁺-provoked mitochondrial changes, increased cell viability, and prevented MPP⁺-induced dendrite shrinkage in primary neurons. These results indicate that BAG6 up-regulation during chronic complex I inhibition contributes to mitochondrial pathology by decreasing the levels of endogenous PINK1. Given that recessive mutations in PINK1 cause familial PD, the finding of accelerated PINK1 degradation in the chronic MPP⁺ model suggests that PINK1 loss of function represents a point of convergence between the neurotoxic and genetic causes of PD.

Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

PPAR agonists as therapeutics for CNS trauma and neurological diseases

Traumatic injury or disease of the spinal cord and brain elicits multiple cellular and biochemical reactions that together cause or are associated with neuropathology. Specifically, injury or disease elicits acute infiltration and activation of immune cells, death of neurons and glia, mitochondrial dysfunction, and the secretion of substrates that inhibit axon regeneration. In some diseases, inflammation is chronic or non-resolving. Ligands that target PPARs (peroxisome proliferator-activated receptors), a group of ligand-activated transcription factors, are promising therapeutics for neurologic disease and CNS injury because their activation affects many, if not all, of these interrelated pathologic mechanisms. PPAR activation can simultaneously weaken or reprogram the immune response, stimulate metabolic and mitochondrial function, promote axon growth and induce progenitor cells to differentiate into myelinating oligodendrocytes. PPAR activation has beneficial effects in many pre-clinical models of neurodegenerative diseases and CNS injury; however, the mechanisms through which PPARs exert these effects have yet to be fully elucidated. In this review we discuss current literature supporting the role of PPAR activation as a therapeutic target for treating traumatic injury and degenerative diseases of the CNS.

Coenzyme Q10, hyperhomocysteinemia and MTHFR C677T polymorphism in levodopa-treated Parkinson's disease patients

There is evidence that increased homocysteine (Hcy) levels might accelerate dopaminergic cell death in Parkinson's disease (PD) through neurotoxic effects. Homocysteine neurotoxicity mainly relies on redox state alterations. The present work was aimed at investigating the relationships between plasma Hcy concentrations and percent content of oxidized versus total Coenzyme Q10 (%CoQ10) in 60 PD patients and 82 healthy subjects. Both groups were screened for plasma levels of Hcy, vitamin B12, folate, %CoQ10 and C677T methylenetetrahydrofolate reductase (MTHFR) gene polymorphism. The MTHFR TT677 mutated genotype was found more frequently in patients than in controls (p = 0.01). In a multivariate analysis, Hcy levels and %CoQ10 were associated with the case/control category (p < 0.0001), MTHFR genotype (p < 0.0001) and their interaction term (p = 0.0015), even after adjusting for age, sex, folate and vitamin B12. Patients carrying the TT677 genotype exhibited the highest values of Hcy and %CoQ10 (p < 0.0001). Structural equation modelling evidenced that the TT677 genotype and levodopa daily dose were independently and directly correlated with Hcy (p < 0.0001, and p = 0.003, respectively), which, in turn, showed a significant correlation (p < 0.0001) with the %CoQ10 in PD patients. Our results suggest that increased Hcy levels act as mediator of the systemic oxidative stress occurring in PD, and %CoQ10 determination might be regarded as a predictor of toxic Hcy effects.

Uncoupling of ATP-depletion and cell death in human dopaminergic neurons

The mitochondrial inhibitor 1-methyl-4-phenylpyridinium (MPP(+)) is the toxicologically relevant metabolite of 1-methyl-4-phenyltetrahydropyridine (MPTP), which causes relatively selective degeneration of dopaminergic neurons in the substantia nigra. Dopaminergic LUHMES cells were used to investigate whether ATP-depletion can be uncoupled from cell death as a downstream event in these fully post-mitotic human neurons. Biochemical assays indicated that in the homogeneously differentiated cell cultures, MPP(+) was taken up by the dopamine transporter (DAT). MPP(+) then triggered oxidative stress and caspase activation, as well as ATP-depletion followed by cell death. Enhanced survival of the neurons in the presence of agents interfering with mitochondrial pathology, such as the fission inhibitor Mdivi-1 or a Bax channel blocker suggested a pivotal role of mitochondria in this model. However, these compounds did not prevent cellular ATP-depletion. To further investigate whether cells could be rescued despite respiratory chain inhibition by MPP(+), we have chosen a diverse set of pharmacological inhibitors well-known to interfere with MPP(+) toxicity. The antioxidant ascorbate, the iron chelator desferoxamine, the stress kinase inhibitor CEP1347, and different caspase inhibitors reduced cell death, but allowed ATP-depletion in protected cells. None of these compounds interfered with MPP(+) accumulation in the cells. These findings suggest that ATP-depletion, as the initial mitochondrial effect of MPP(+), requires further downstream processes to result in neuronal death. These processes may form self-enhancing signaling loops, that aggravate an initial energetic impairment and eventually determine cell fate.

DLP1-dependent mitochondrial fragmentation mediates 1-methyl-4-phenylpyridinium toxicity in neurons: implications for Parkinson's disease

Selective degeneration of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) can be modeled by the administration of the neurotoxin 1-methyl-4-phenylpyridinium (MPP(+) ). Because abnormal mitochondrial dynamics are increasingly implicated in the pathogenesis of PD, in this study, we investigated the effect of MPP(+) on mitochondrial dynamics and assessed temporal and causal relationship with other toxic effects induced by MPP(+) in neuronal cells. In SH-SY5Y cells, MPP(+) causes a rapid increase in mitochondrial fragmentation followed by a second wave of increase in mitochondrial fragmentation, along with increased DLP1 expression and mitochondrial translocation. Genetic inactivation of DLP1 completely blocks MPP(+) -induced mitochondrial fragmentation. Notably, this approach partially rescues MPP(+) -induced decline in ATP levels and ATP/ADP ratio and increased [Ca(2+) ](i) and almost completely prevents increased reactive oxygen species production, loss of mitochondrial membrane potential, enhanced autophagy and cell death, suggesting that mitochondria fragmentation is an upstream event that mediates MPP(+) -induced toxicity. On the other hand, thiol antioxidant N-acetylcysteine or glutamate receptor antagonist D-AP5 also partially alleviates MPP(+) -induced mitochondrial fragmentation, suggesting a vicious spiral of events contributes to MPP(+) -induced toxicity. We further validated our findings in primary rat midbrain dopaminergic neurons that 0.5 μm MPP(+) induced mitochondrial fragmentation only in tyrosine hydroxylase (TH)-positive dopaminergic neurons in a similar pattern to that in SH-SY5Y cells but had no effects on these mitochondrial parameters in TH-negative neurons. Overall, these findings suggest that DLP1-dependent mitochondrial fragmentation plays a crucial role in mediating MPP(+) -induced mitochondria abnormalities and cellular dysfunction and may represent a novel therapeutic target for PD.

MPTP-induced neuroblast apoptosis in the subventricular zone is not regulated by dopamine or other monoamine transporters

For 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to exert neurotoxicity on dopaminergic neurons, 1-methyl-4-phenylpyridinium (MPP+), a metabolite of MPTP, must be taken up into the dopaminergic neuron via the dopamine transporter (DAT). Previous reports have shown that MPTP also causes neuroblast apoptosis in the subventricular zone (SVZ) of adult mice. The aim of this study is to elucidate the role of DAT and other monoamine transporters including vesicular monoamine transporter 2 (VMAT2), the serotonin transporter (SERT), and the norepinephrine transporter (NET) on the neuroblast apoptosis induced by MPTP administration. There were no DAT-positive neuroblasts in the SVZ, whereas some neuroblasts were immunopositive for VMAT2 and SERT. To examine whether these transporters are involved in MPTP-induced neuroblast apoptosis in the SVZ, terminal deoxynucleotidyl transferase-mediated dUTP endlabeling (TUNEL)-positive cells were semiquantitatively analyzed after the injection of GBR12909 (GBR), a DAT inhibitor; tetrabenazine (TBZ), a VMAT2 inhibitor; fluoxetine (FLU), a SERT inhibitor, or desipramine (DES), a NET inhibitor, prior to MPTP injection. However, the injection of these transporter inhibitors had no influence on the MPTP-induced neuroblast apoptosis in the SVZ. It is likely that neither DAT nor other monoamine transporters are involved in MPTP-induced neuroblast apoptosis. The present findings suggest that the neurotoxicity of MPTP to neuroblasts in the SVZ does not require DAT or other monoamine transporters, and the apoptosis it induces may be executed through other unknown pathways.

Disruption of dopamine transport by DDT and its metabolites

Epidemiological studies suggest a link between pesticide exposure and an increased risk of developing Parkinson's disease (PD). Although studies have been unable to clearly identify specific pesticides that contribute to PD, a few human studies have reported higher levels of the organochlorine pesticides dieldrin and DDE (a metabolite of DDT) in post-mortem PD brains. Previously, we found that exposure of mice to dieldrin caused perturbations in the nigrostriatal dopamine system consistent with those seen in PD. Given the concern over the environmental persistence and reintroduction of DDT for the control of malaria-carrying mosquitoes and other pests, we sought to determine whether DDT and its two major metabolites, DDD and DDE, could damage the dopamine system. In vitro analyses in mouse synaptosomes and vesicles demonstrated that DDT and its metabolites inhibit the plasma membrane dopamine transporter (DAT) and the vesicular monoamine transporter (VMAT2). However, exposure of mice to either DDT or DDE failed to show evidence of nigrostriatal damage or behavioral abnormalities in any of the measures examined. Thus, we report that in vitro effects of DDT and its metabolites on components of the dopamine system do not translate into neurotoxicological outcomes in orally exposed mice and DDT appears to have less dopamine toxicity when compared to dieldrin. These data suggest elevated DDE levels in PD patients may represent a measure of general pesticide exposure and that other pesticides may be responsible for the association between pesticide exposure and PD.

Role of glial cell derived neurotrophic factor in the protective effect of smilagenin on rat mesencephalic dopaminergic neurons damaged by MPP+

Tyrosine hydroxylase immunohistochemical analysis revealed that in cultured mesencephalic dopaminergic neurons smilagenin (SMI), added prior to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPP+), protected against the drop of neuron number and neurite outgrowth length caused by MPP+. Addition of anti-GDNF and/or anti-GFR alpha 1 functional antibodies to the medium prior to SMI, eliminated mostly, though incompletely, the action of SMI. The expression of glial cell derived neurotrophic factor (GDNF) mRNA, but not GDNF receptor alpha1 (GFR alpha 1) or receptor tyrosine kinase mRNA in MPP+ intoxicated neurons was markedly elevated as early as 2h after the addition of SMI with a peak at 24-48 h. Therefore, an important route of the protective action of SMI on dopaminergic neurons is to stimulate intrinsic GDNF expression.

Characterization of membrane potential-dependent uptake of the novel PET tracer 18F-fluorobenzyl triphenylphosphonium cation

PURPOSE

Mitochondrial dysfunction has been attributed a critical role in the etiology and pathogenesis of numerous diseases, and is manifested by alterations of the organelle's membrane potential (Deltapsi(m)). This suggests that Deltapsi(m) measurement can be highly useful for diagnostic purposes. In the current study, we characterized the capability of the novel PET agent (18)F-fluorobenzyl triphenylphosphonium ((18)F-FBnTP) to assess Deltapsi(m), compared with the well-established voltage sensor (3)H-tetraphenylphosphonium ((3)H-TPP).

METHODS

(18)F-FBnTP and (3)H-TPP uptake under conditions known to alter Deltapsi(m) and plasma membrane potential (Deltapsi(p)) was assayed in the H345 lung carcinoma cell line. (18)F-FBnTP biodistribution was assessed in CD1 mice using dynamic PET and ex vivo gamma well counting.

RESULTS

(18)F-FBnTP and (3)H-TPP demonstrated similar uptake kinetics and plateau concentrations in H345 cells. Stepwise membrane depolarization resulted in a linear decrease in (18)F-FBnTP cellular uptake, with a slope (-0.58+/-0.06) and correlation coefficient (0.94+/-0.07) similar (p>0.17) to those measured for (3)H-TPP (-0.63+/-0.06 and 0.96+/-0.05, respectively). Selective collapse of Deltapsi(m) caused a substantial decrease in cellular uptake for (18)F-FBnTP (81.6+/-8.1%) and (3)H-TPP (85.4+/-6.7%), compared with control. Exposure to the proapoptotic staurosporine, known to collapse Deltapsi(m), resulted in a decrease of 68.7+/-10.1% and 71.5+/-8.4% in (18)F-FBnTP and (3)H-TPP cellular uptake, respectively. (18)F-FBnTP accumulated mainly in kidney, heart and liver.

CONCLUSION

(18)F-FBnTP is a mitochondria-targeting PET radiopharmaceutical responsive to alterations in membrane potential with voltage-dependent performance similar to that of (3)H-TPP. (18)F-FBnTP is a promising new voltage sensor for detection of physiological and pathological processes associated with mitochondrial dysfunction, such as apoptosis, using PET.

Divergent Mechanisms of Paraquat, MPP+, and Rotenone Toxicity: Oxidation of Thioredoxin and Caspase-3 Activation

Paraquat, N-methyl-4-phenyl-1,2,3,6 tetrahydropyridine, and rotenone have been shown to reproduce several features of Parkinson's disease in animal and cell culture models. Although these chemicals are known to perturb dopamine homeostasis and induce dopaminergic cell death, their molecular mechanisms of action are not well defined. We have previously shown that paraquat does not require functional dopamine transporter and does not inhibit mitochondrial complex I in order to mediate its toxic action (Richardson et al., 2005). In this study, we show that paraquat specifically oxidized the cytosolic form of thioredoxin and activated Jun N-terminal kinase (JNK), followed by caspase-3 activation. Conversely, 1-methyl-4-phenylpyridinium (MPP(+)) and rotenone oxidized the mitochondrial form of thioredoxin but did not activate JNK-mitogen-activated protein kinase and caspase-3. Loading cells with exogenous dopamine did not exacerbate the toxicity of any of these compounds. These data suggest that oxidative modification of cytosolic proteins is critical to paraquat toxicity, while oxidation of mitochondrial proteins is important for MPP(+) and rotenone toxicity. In addition, intracellular dopamine does not seem to exacerbate the toxicity of these dopaminergic neurotoxicants in this model.

Potentiating effect of the ATP-sensitive potassium channel blocker glibenclamide on complex I inhibitor neurotoxicity in vitro and in vivo

Previous studies have demonstrated a deficiency in mitochondrial function in Parkinson's disease. We measured the ability of mitochondrial inhibitors of complexes I (rotenone, MPP(+), and HPP(+)), II (amdro), IV (Na cyanide), and an uncoupler (dinoseb) to release preloaded dopamine from murine striatal synaptosomes. These compounds were potent dopamine releasers, and the effect was calcium-dependent. The striatum also contains a significant density of K(ATP)(+) channels, which play a protective role during ATP decline. Blockage of these channels with glibenclamide only potentiated the dopamine release by complex I inhibitors, and a selective potentiating effect of glibenclamide on the toxicity of MPTP was also observed, in vivo, using C57BL/6 mice. Western blots of striatal dopamine transporter (DAT) and tyrosine hydroxylase (TH) proteins demonstrated that 30 mg/kg of glibenclamide alone did not affect the expression of DAT and TH after two weeks of daily treatments, but it significantly enhanced the reduction of DAT and TH by a single dose of 20 mg/kg of MPTP. Amdro or dinoseb alone, or in conjunction with glibenclamide did not alter the expression of DAT and TH. The possible mechanisms underlying dopamine release and the selectivity of glibenclamide were further evaluated, in vitro. (86)Rb efflux assay showed that glibenclamide inhibited rotenone-induced K(+) efflux, but not dinoseb-induced K(+) efflux. Analysis of ATP titers in treated synaptosomes did not support a correlation between mitochondrial inhibition and K(ATP)(+) channel activation. However, assay of reactive oxygen species (ROS) showed that greater amounts of ROS generated by complex I inhibitors was a contributory factor to K(ATP)(+) channel activation and glibenclamide potentiation. Overall, these findings suggest that co-exposure to mitochondrial complex I inhibitors and glibenclamide or a genetic defect in K(ATP)(+) channel function, may increase neurotoxicity in the striatal dopaminergic system.

Pyrethroid pesticide-induced alterations in dopamine transporter function

Parkinson's disease (PD) is a progressive neurodegenerative disease affecting the nigrostriatal dopaminergic pathway. Several epidemiological studies have demonstrated an association between pesticide exposure and the incidence of PD. Studies from our laboratory and others have demonstrated that certain pesticides increase levels of the dopamine transporter (DAT), an integral component of dopaminergic neurotransmission and a gateway for dopaminergic neurotoxins. Here, we report that repeated exposure (3 injections over 2 weeks) of mice to two commonly used pyrethroid pesticides, deltamethrin (3 mg/kg) and permethrin (0.8 mg/kg), increases DAT-mediated dopamine uptake by 31 and 28%, respectively. Using cells stably expressing DAT, we determined that exposure (10 min) to deltamethrin and permethrin (1 nM-100 microM) had no effect on DAT-mediated dopamine uptake. Extending exposures to both pesticides for 30 min (10 microM) or 24 h (1, 5, and 10 microM) resulted in significant decrease in dopamine uptake. This reduction was not the result of competitive inhibition, loss of DAT protein, or cytotoxicity. However, there was an increase in DNA fragmentation, an index of apoptosis, in cells exhibiting reduced uptake at 30 min and 24 h. These data suggest that up-regulation of DAT by in vivo pyrethroid exposure is an indirect effect and that longer-term exposure of cells results in apoptosis. Since DAT can greatly affect the vulnerability of dopamine neurons to neurotoxicants, up-regulation of DAT by deltamethrin and permethrin may increase the susceptibility of dopamine neurons to toxic insult, which may provide insight into the association between pesticide exposure and PD.

Reduced MPTP toxicity in noradrenaline transporter knockout mice

The noradrenergic neurons of the locus coeruleus (LC) are damaged in Parkinson's disease (PD). Neurotoxin ablation of the LC noradrenergic neurons has been shown to exacerbate the dopaminergic toxicity of MPTP, suggesting that the noradrenergic system protects dopamine neurons. We utilized mice that exhibit elevated synaptic noradrenaline (NA) by genetically deleting the noradrenaline transporter (NET), a key regulator of the noradrenergic system (NET KO mice). NET KO and wild-type littermates were administered MPTP and striatal dopamine terminal integrity was assessed by HPLC of monoamines, immmunoblotting for dopaminergic markers and tyrosine hydroxylase (TH) immunohistochemistry. MPTP significantly reduced striatal dopamine in wild-type mice, but not in the NET KO mice. To confirm that the protection observed in the NET KO mice was due to the lack of NET, we treated wild-type mice with the specific NET inhibitor, nisoxetine, and then challenged them with MPTP. Nisoxetine conferred protection to the dopaminergic system. These data indicate that NA can modulate MPTP toxicity and suggest that manipulation of the noradrenergic system may have therapeutic value in PD.

Isoflurane induces dopamine transporter trafficking into the cell cytoplasm

Previous investigations have shown that the binding of a selective hydrophilic positron emission tomography radiotracer for the dopamine transporter (DAT) (2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-18F-fluoroethyl)nortropane) decreased in monkey striatum during deep isoflurane anesthesia. Immunohistochemistry experiments suggested but did not prove that isoflurane induced a decrease in cell surface DAT. The present investigation was undertaken to demonstrate quantitatively the isoflurane-induced internalization of DAT using a rapid and relatively uncomplicated biochemical technique in human embryonic kidney (HEK-293) cells stably expressing the human DAT (h-DAT) protein. Biotinylation followed by Western blot analysis was used to determine the extent of change in cell surface expression of the DAT under control conditions and in the presence of a clinically relevant concentration of isoflurane. Isoflurane treatment for 30 min resulted in a highly significant decrease in the amount of h-DAT on the cell surface (21 +/- 15% of control; P < 0.01) (mean +/- SD; n = 4). These data are consistent with the hypothesis that isoflurane results in internalization of DAT from the cell membrane and further validate our qualitative results reported previously. In addition, the current results confirm the hypothesis that biotinylation can be used to quantitate the extent of disappearance of DAT from the cell surface making dose-response studies and comparisons of DAT internalization with other general anesthetics practical.

Estrogen and neuroprotection: higher constitutive expression of glutaredoxin in female mice offers protection against MPTP-mediated neurodegeneration

Incidence of Parkinson's disease is lower in women as compared with men. Although neuroprotective effect of estrogen is recognized, the underlying molecular mechanisms are unclear. MPTP (1-methyl-4-phenyl-1, 2, 3, 6, tetrahydro-pyridine), a neurotoxin that causes Parkinson's disease-like symptoms acts through inhibition of mitochondrial complex I. Administration of MPTP to male mice results in loss of dopaminergic neurons in substantia nigra, whereas female mice are unaffected. Oxidation of critical thiol groups by MPTP disrupts mitochondrial complex I, and up-regulation of glutaredoxin (a thiol disulfide oxidoreductase) is essential for recovery of complex I. Early events following MPTP exposure, such as increased AP1 transcription, loss of glutathione, and up-regulation of glutaredoxin mRNA is seen only in male mice, indicating that early response to neurotoxic insult does not occur in females. Pretreatment of female mice with ICI 182,780, estrogen receptor (ER) antagonist sensitizes them to MPTP-mediated complex I dysfunction. Constitutive expression of glutaredoxin is significantly higher in female mice as compared with males. ICI 182,780 down-regulates glutaredoxin activity in female mouse brain regions (midbrain and striatum), indicating that glutaredoxin expression is regulated through estrogen receptor signaling. Higher constitutive expression of glutaredoxin could potentially contribute to the neuroprotection seen in female mouse following exposure to neurotoxins, such as MPTP.

Dependence on electron transport chain function and intracellular signaling of genomic responses in SH-SY5Y cells to the mitochondrial neurotoxin MPP(+)

SH-SY5Y neuroblastoma cells exposed to the complex I inhibitor/parkinsonian neurotoxin methylpyridinium ion (MPP(+)) activate both survival and death-promoting signaling pathways and undergo MEK/ERK-dependent, phosphatidylinositol-3 kinase-dependent, and c-Jun kinase-dependent cell death. Because genomic responses to MPP(+) are not extensively characterized, we used nylon cDNA arrays to measure gene expression following exposure to an apoptosis-producing [MPP(+)]. Many changes occurred within 5 min, and all gene expression changes appeared before biochemical and morphological markers of apoptosis. The majority of gene expression changes in SY5Y were not found in rho(0) cells, indicating dependence of these changes on intact electron transport activity. rho(0) cells exposed to MPP(+) produced different expression profiles, indicating the potential for responses independent of complex I inhibition. MPP(+)-induced gene expression patterns in normal SY5Y cells were sensitive to inhibitors of MEK/ERK (UO 126) or phosphatidylinositol-3 kinase (LY 294002), demonstrating regulation of gene expression by these survival-promoting signaling pathways. The primary signaling molecules mediating these MPP(+)-induced gene expression changes are unknown but ultimately utilize MEK/ERK and phosphatidylinositol-3 kinase signaling. Genes suppressed by UO 126 or LY 294002 during MPP(+) exposure may mediate cell survival; those expressed in the presence of UO 126 or LY 294002 may mediate cell death in this in vitro model of Parkinson's disease.

An in vitro model of Parkinson's disease: linking mitochondrial impairment to altered alpha-synuclein metabolism and oxidative damage

Chronic systemic complex I inhibition caused by rotenone exposure induces features of Parkinson's disease (PD) in rats, including selective nigrostriatal dopaminergic degeneration and formation of ubiquitin- and alpha-synuclein-positive inclusions (Betarbet et al., 2000). To determine underlying mechanisms of rotenone-induced cell death, we developed a chronic in vitro model based on treating human neuroblastoma cells with 5 nm rotenone for 1-4 weeks. For up to 4 weeks, cells grown in the presence of rotenone had normal morphology and growth kinetics, but at this time point, approximately 5% of cells began to undergo apoptosis. Short-term rotenone treatment (1 week) elevated soluble alpha-synuclein protein levels without changing message levels, suggesting that alpha-synuclein degradation was retarded. Chronic rotenone exposure (4 weeks) increased levels of SDS-insoluble alpha-synuclein and ubiquitin. After a latency of >2 weeks, rotenone-treated cells showed evidence of oxidative stress, including loss of glutathione and increased oxidative DNA and protein damage. Chronic rotenone treatment (4 weeks) caused a slight elevation in basal apoptosis and markedly sensitized cells to further oxidative challenge. In response to H2O2, there was cytochrome c release from mitochondria, caspase-3 activation, and apoptosis, all of which occurred earlier and to a much greater extent in rotenone-treated cells; caspase inhibition provided substantial protection. These studies indicate that chronic low-grade complex I inhibition caused by rotenone exposure induces accumulation and aggregation of alpha-synuclein and ubiquitin, progressive oxidative damage, and caspase-dependent death, mechanisms that may be central to PD pathogenesis.