MPP(+) downregulates mitochondrially encoded gene transcripts and their activities in dopaminergic neuronal cells: protective role of Bcl-2

Ablation of RIP3 protects from dopaminergic neurodegeneration in experimental Parkinson's disease

Parkinson’s disease (PD) is driven by dopaminergic neurodegeneration in the substantia nigra pars compacta (SN) and striatum. Although apoptosis is considered the main neurodegenerative mechanism, other cell death pathways may be involved. In this regard, necroptosis is a regulated form of cell death dependent on receptor interacting protein 3 (RIP3), a protein also implicated in apoptosis and inflammation independently of its pro-necroptotic activity. Here, we explored the role of RIP3 genetic deletion in in vivo and in vitro PD models. Firstly, wild-type (Wt) and RIP3 knockout (RIP3ko) mice were injected intraperitoneally with MPTP (40 mg/kg, i.p.), and sacrificed after either 6 or 30 days. RIP3ko protected from dopaminergic neurodegeneration in the SN of MPTP-injected mice, but this effect was independent of necroptosis. In keeping with this, necrostatin-1s (10 mg/kg/day, i.p.) did not afford full neuroprotection. Moreover, MPTP led to DNA fragmentation, caspase-3 activation, lipid peroxidation and BAX expression in Wt mice, in the absence of caspase-8 cleavage, suggesting intrinsic apoptosis. This was mimicked in primary cortical neuronal cultures exposed to the active MPTP metabolite. RIP3 deficiency in cultured cells and in mouse brain abrogated all phenotypes. Curiously, astrogliosis was increased in the striatum of MPTP-injected Wt mice and further exacerbated in RIP3ko mice. This was accompanied by absence of microgliosis and reposition of glial cell line-derived neurotrophic factor (GDNF) levels in the striata of MPTP-injected RIP3ko mice when compared to MPTP-injected Wt mice, which in turn showed a massive GDNF decrease. RIP3ko primary mixed glial cultures also presented decreased expression of inflammation-related genes upon inflammatory stimulation. These findings hint at possible undescribed non-necroptotic roles for RIP3 in inflammation and MPTP-driven cell death, which can contribute to PD progression.

Glucose Levels in Culture Medium Determine Cell Death Mode in MPP(+)-treated Dopaminergic Neuronal Cells

We previously demonstrated that 1-methyl-4-phenylpyridinium (MPP(+)) causes caspase-independent, non-apoptotic death of dopaminergic (DA) neuronal cells. Here, we specifically examined whether change of glucose concentration in culture medium may play a role for determining cell death modes of DA neurons following MPP(+) treatment. By incubating MN9D cells in medium containing varying concentrations of glucose (5~35 mM), we found that cells underwent a distinct cell death as determined by morphological and biochemical criteria. At 5~10 mM glucose concentration (low glucose levels), MPP(+) induced typical of the apoptotic dell death accompanied with caspase activation and DNA fragmentation as well as cell shrinkage. In contrast, MN9D cells cultivated in medium containing more than 17.5 mM (high glucose levels) did not demonstrate any of these changes. Subsequently, we observed that MPP(+) at low glucose levels but not high glucose levels led to ROS generation and subsequent JNK activation. Therefore, MPP(+)-induced cell death only at low glucose levels was significantly ameliorated following co-treatment with ROS scavenger, caspase inhibitor or JNK inhibitor. We basically confirmed the quite similar pattern of cell death in primary cultures of DA neurons. Taken together, our results suggest that a biochemically distinct cell death mode is recruited by MPP(+) depending on extracellular glucose levels.

Gel-based protease proteomics for identifying the novel calpain substrates in dopaminergic neuronal cell

Calpains are a family of calcium-dependent cysteine proteases that are ubiquitously expressed in mammals and play critical roles in neuronal death by catalyzing substrate proteolysis. Here, we developed two-dimensional gel electrophoresis-based protease proteomics to identify putative calpain substrates. To accomplish this, cellular lysates from neuronal cells were first separated by pI, and the immobilized sample on a gel strip was incubated with a recombinant calpain and separated by molecular weight. Among 25 altered protein spots that were differentially expressed by at least 2-fold, we confirmed that arsenical pump-driving ATPase, optineurin, and peripherin were cleaved by calpain using in vitro and in vivo cleavage assays. Furthermore, we found that all of these substrates were cleaved in MN9D cells treated with either ionomycin or 1-methyl-4-phenylpyridinium, both of which cause a calcium-mediated calpain activation. Their cleavage was blocked by calcium chelator or calpain inhibitors. In addition, calpain-mediated cleavage of these substrates and its inhibition by calpeptin were confirmed in a middle cerebral artery occlusion model of cerebral ischemia, as well as a stereotaxic brain injection model of Parkinson disease. Transient overexpression of each protein was shown to attenuate 1-methyl-4-phenylpyridinium-induced cell death, indicating that these substrates may confer protection of varying magnitudes against dopaminergic injury. Taken together, the data indicate that our protease proteomic method has the potential to be applicable for identifying proteolytic substrates affected by diverse proteases. Moreover, the results described here will help us decipher the molecular mechanisms underlying the progression of neurodegenerative disorders where protease activation is critically involved.

Attenuation of MPTP/MPP(+) toxicity in vivo and in vitro by an 18-mer peptide derived from prosaposin

Parkinson's disease (PD) is a chronic progressive neurological disorder with an increasing incidence in the aging population. Neuroprotective and/or neuroregenerative strategies remain critical in the treatment of this increasingly prevalent disease. Prosaposin is a neurotrophic factor whose neurotrophic activity is attributed to a stretch of 12 amino acids located at the N-terminal region of saposin C. The present study was performed to investigate the protective effect and mechanism of action of a prosaposin-derived 18-mer peptide (PS18: LSELIINNATEELLIKGL) in Parkinson's disease models. We used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium ion (MPP(+))-induced dopaminergic neurotoxicity in C57BL/6J mice or SH-SY5Y cells and explored the protective effect and mechanisms of action of PS18 on dopaminergic neurons. Treatment with 2.0mg/kg PS18 significantly improved behavioral deficits, enhanced the survival of tyrosine hydroxylase-positive neurons, and decreased the activity of astrocytes in the substantia nigra and striatum in MPTP-induced PD model mice. In vitro, a Cell Counting Kit-8 assay and Hoechst 33258 staining revealed that co-treatment with 300ng/mL PS18 and 5mM MPP(+) protected against MPP(+)-induced nuclear morphological changes and attenuated cell death induced by MPP(+). We also found that PS18-FAM entered the cells, and the retention time of PS18-FAM in the cytoplasm of MPP(+)-treated cells was shorter than that of untreated cells. In addition, PS18 showed protection from MPP(+)/MPTP-induced apoptosis in the SH-SY5Y cells and dopaminergic neurons in the PD model mice via suppression of the c-Jun N-terminal kinase/c-Jun pathway; upregulation of Bcl-2; downregulation of BAX, attenuating mitochondrial damage; and inhibition of caspase-3. These findings suggest that PS18 may provide a valuable therapeutic strategy for the treatment of progressive neurodegenerative diseases such as PD.

Mechanisms to prevent caspase activation in rotenone-induced dopaminergic neurodegeneration: role of ATP depletion and procaspase-9 degradation

The evidence implicating a mode of cell death that either favors or argues against caspase-dependent apoptosis is available in studies that used experimental models of Parkinson's disease. We sought to investigate the mechanisms by which release of cytochrome c is not linked to caspase activation during rotenone-induced dopaminergic (DA) neurodegeneration. Unlike caspase activation in 6-hydroxydopamine-treated cells, both MN9D DA neuronal cells and primary cultures of mesencephalic neurons showed no obvious signs of caspase activation upon exposure to rotenone. We found that intracellular levels of ATP significantly decreased at the early phase of neurodegeneration (<~24 h) and therefore external addition of ATP to the lysates obtained at this stage reconstituted caspase-3 activity. At a later phase of cell death (>~24 h), both decreased levels of ATP and procaspase-9 contributed to the lack of caspase-3 activation. Under this condition, calpain and the proteasome system were responsible for the degradation of procaspase-9. Consequently, external addition of ATP and procaspase-9 to the lysates harvested at the later phase was required for activation of caspase-3. Similarly, caspase-3 activity was also reconstituted in the lysates harvested from cells co-treated with inhibitors of these proteases and incubated in the presence of external ATP. Taken together, our findings provided a sequential mechanism underlying how DA neurons may undergo caspase-independent cell death, even in the presence of cytoplasmic cytochrome c following inhibition of mitochondrial complex I.

Gene expression profiling of MPP+-treated MN9D cells: A mechanism of toxicity study

Parkinson's disease (PD) is a common neurodegenerative disease characterized by progressive loss of midbrain dopaminergic neurons with unknown etiology. MPP+ (1-methyl-4-phenylpyridinium) is the active metabolite of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which induces Parkinson's-like syndromes in humans and animals. MPTP/MPP+ treatment produces selective dopaminergic neuronal degeneration, therefore, these agents are commonly used to study the pathogenesis of PD. However, the mechanisms of their toxicity have not been elucidated. In order to gain insights into MPP+-induced neurotoxicity, a gene expression microarray study was performed using a midbrain-derived dopaminergic neuronal cell line, MN9D. Utilizing a two-color reference design, Agilent mouse oligonucleotide microarrays were used to examine relative gene expression changes in MN9D cells treated with 40microM MPP+ compared with controls. Bioinformatics tools were used for data evaluation. Briefly, raw data were imported into the NCTR ArrayTrack database, normalized using a Lowess method and data quality was assessed. The Student's t-test was used to determine significant changes in gene expression (set as p<0.05, fold change >1.5). Gene Ontology for Function Analysis (GOFFA) and Ingenuity Pathway Analysis were employed to analyze the functions and roles of significant genes in biological processes. Of the 51 significant genes identified, 44 were present in the GOFFA or Ingenuity database. These data indicate that multiple pathways are involved in the underlying mechanisms of MPP+-induced neurotoxicity, including apoptosis, oxidative stress, iron binding, cellular metabolism, and signal transduction. These data also indicate that MPP+-induced toxicity shares common molecular mechanisms with the pathogenesis of PD and further pathway analyses will be conducted to explore these mechanisms.

Evidence to Implicate Early Modulation of Interleukin‐1β Expression in the Neuroprotection Afforded by 17β‐Estradiol in Male Rats Undergone Transient Middle Cerebral Artery Occlusion

Neuroprotection exerted by 17beta-estradiol (17beta-E(2)) has been widely investigated in animal models of acute cerebral ischemia. Estrogens interact with intracellular receptors (ERalpha and ERbeta) to modulate the transcription of target genes, including those implicated in neuronal survival. Neuroprotection may also occur via interaction with ER-like membrane receptors mediating rapid, non-genomic, actions or via receptor-independent mechanisms. There is also evidence that blockade of inflammatory factors may represent an important mechanism involved in estrogenic neuroprotection. Here we investigate whether reduced brain damage by acute pharmacological treatment with 17beta-E(2) in male rats subjected to transient (2h) middle cerebral artery occlusion (tMCAo) involves modulation of interleukin-1beta (IL-1beta), a proinflammatory cytokine strongly implicated in the pathophysiology of ischemic stroke. Administration of 17beta-E(2) (0.2mg/kg, i.p., 1h before tMCAo) results in significant reduction of brain infarct volume, and this is reverted by the ER antagonist ICI 182,780 (0.25mg/kg, i.p.) administered 1h before 17beta-E(2). Two hours MCAo followed by 2-h reperfusion results in a significant, threefold increase of IL-1beta levels in the cortical tissue ipsilateral to the ischemic damage. Interestingly, a pretreatment with a neuroprotective dose of 17beta-E(2) attenuates the cytokine elevation and this appears to occur through ER activation. In addition, neuroprotection by 17beta-E(2) is accompanied by reduced cytochrome c translocation both in the striatum and in the cortex as revealed by Western blotting 3h after reperfusion. In conclusion, we report the original observation that neuroprotection exerted by 17beta-E(2) in a rat model of transient focal brain ischemia is accompanied by reduced cytochrome c translocation to the cytosol and involves early modulation of IL-1beta production.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

Recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain. Clinical reports have documented the effectiveness of estrogens to attenuate symptoms associated with Parkinson's disease, and to reduce the risk of Alzheimer's disease and cerebrovascular stroke. This evidence is corroborated by numerous experimental studies documenting the protective role of female sex hormones both in vitro and in vivo. Accordingly, estrogens have been shown to promote survival and differentiation of several neuronal populations maintained in culture, and to reduce cell death associated with excitotoxicity, oxidative stress, serum deprivation or exposure to beta-amyloid. The neuroprotective effects of estrogens have been widely documented in animal models of neurological disorders, such as Alzheimer's and Parkinson's diseases, as well as cerebral ischemia. Although estrogens are known to exert several direct effects on neurones, the cellular and molecular mechanisms implicated in their protective actions on the brain are not completely understood. Thus, on the basis of clinical and experimental evidence, in this review, we discuss recent findings concerning the neuronal effects of estrogens that may contribute to their neuroprotective actions. Both estrogen receptor-dependent and -independent mechanisms will be described. These include modulation of cell death regulators, such as Bcl-2, Akt and calpain, as well as interaction with growth factors, such as BDNF, NGF, IGF-I and their receptors. The anti-inflammatory effects of estrogens will also be described, namely their ability to reduce brain levels of inflammatory mediators, cytokines and chemokines. Finally, a brief overview about receptor-independent mechanisms of neuroprotection will aim at describing the antioxidant effects of estrogens, as well as their ability to modulate neurotransmission.

BimEL up‐regulation potentiates AIF translocation and cell death in response to MPTP

This study attempted to elucidate the signaling mechanism underlying dopaminergic cell death in the MPP+ model for Parkinson's disease. In neuronal-differentiated PC12 cells, through the regulation by activated JNK and c-jun, BimEL expression was markedly increased in response to MPP+ treatment, which led to the cell degeneration. In lieu of Smac translocation as seen in other paradigms, up-regulation of BimEL effected an increase in calpain I activity that, in turn, mediated AIF release from the mitochondria. In support, we found that knocking down BimEL expression resulted in a decrease in calpain I activity, as well as AIF release from the mitochondria and cell death. Finally, inhibition of calpain activity mitigated AIF release from the mitochondria and cell death. Under cell-free conditions, activated purified calpain I could induce the release of AIF from isolated mitochondria without the participation of BimEL or activated JNK, suggesting that AIF release is a direct consequence of calpain I activity. In concert, the results suggest a novel signaling pathway for dopaminergic cell degeneration, in which MPP+ induces the up-regulation of BimEL, which in turn potentiates an elevation in calpain I activity that mediates AIF release and cell death in a caspase-independent manner.

Activation of NF-κB is involved in 6-hydroxydopamine—but not MPP+-induced dopaminergic neuronal cell death: its potential role as a survival determinant

The nuclear factor-kappaB (NF-kappaB) family plays an important role in the control of the apoptotic response. Its activation has been demonstrated in both neurons and glial cells in many neurological disorders. In the present study, we specifically examined whether and to what extent NF-kappaB activation is involved in culture models of Parkinson's disease following exposure of MN9D dopaminergic neuronal cells to 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenyl-4-phenylpyridinium ion (MPP(+)). Both analysis by immunocytochemistry and of immunoblots revealed that NF-kappaB-p65 was translocated into the nuclei following 6-OHDA but not MPP(+)-treatment. A time-dependent activation of NF-kappaB induced by 6-OHDA but not MPP(+) was also demonstrated by an electrophoretic mobility shift assay. A competition assay indicated that not only NF-kappaB-p65 but also -p50 is involved in 6-OHDA-induced NF-kappaB activity. Co-treatment with an antioxidant, N-acetyl-l-cysteine, blocked 6-OHDA-induced activation of NF-kappaB signaling. In the presence of an NF-kappaB inhibitor, pyrrolidine dithiocarbamate (PDTC), 6-OHDA-induced cell death was accelerated while PDTC did not affect MPP(+)-induced cell death. Our data may point to a drug-specific activation of NF-kappaB as a survival determinant for dopaminergic neurons.

Estrogen receptor subtypes alpha and beta contribute to neuroprotection and increased Bcl-2 expression in primary hippocampal neurons

Estrogen receptor (ER) mediated neuroprotection has been demonstrated in both in vitro and in vivo model systems. However, the relative contribution by either ER subtype, ERalpha or ERbeta, to estrogen-induced neuroprotection remains unresolved. To address this question, we investigated the impact of selective ER agonists for either ERalpha, PPT, or ERbeta, DPN, to prevent neurodegeneration in cultured hippocampal neurons exposed to excitotoxic glutamate. Using three indicators of neuronal viability and survival, we demonstrated that both the ERalpha selective agonist PPT and the ERbeta selective agonist DPN protected hippocampal neurons against glutamate-induced cell death in a dose-dependent manner, with the maximal response occurring at 100 pM. Further analyses showed that both PPT and DPN enhanced Bcl-2 expression in hippocampal neurons, with an efficacy comparable to their neuroprotective capacity. Collectively, the present data indicate that activation of either ERalpha or ERbeta can promote neuroprotection in hippocampal neurons, suggesting that both receptor subtypes could be involved in estrogen neuroprotection. As ERbeta is highly expressed in the brain and has little or no expression in the breast or uterus, discovery and design of ERbeta selective molecules could provide a strategy for activating the beneficial effects of estrogen in the brain without activating untoward effects of estrogen in reproductive organs.

Nonsteroidal anti-inflammatory drugs potentiate 1-methyl-4-phenylpyridinium (MPP+)-induced cell death by promoting the intracellular accumulation of MPP+ in PC12 cells

In this study, we investigated the effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on 1-methyl-4-phenylpyridinium (MPP(+))-induced cell death in PC12 cells. Coincubation of PC12 cells with indomethacin, ibuprofen, ketoprofen, or diclofenac, but not aspirin or N-[2-(cyclohexyloxy)-4-nitrophenyl]methanosulfonamide (NS-398), significantly potentiated the MPP(+)-induced cell death. In contrast, these NSAIDs had no effect on rotenone-induced cell death. The potentiating actions of these NSAIDs were not suppressed by treatment with phenyl-N-butyl-nitrone, a radical scavenger; N-acetyl-l-cysteine, an antioxidant; Ac-DEVD-CHO, a selective caspase-3 inhibitor; or 2-chloro-5-nitro-N-phenylbenzamide (GW9662), a selective antagonist of peroxisome proliferator-activated receptor gamma. Furthermore, we observed that DNA fragmentation, which is one of the hallmarks of apoptosis, was not induced by coincubation with MPP(+) and NSAIDs. We confirmed that coincubation of PC12 cells with 30 microM MPP(+) and 100 microM indomethacin, ibuprofen, ketoprofen, or diclofenac led to a significant increase in the accumulation of intracellular MPP(+) compared with incubation with 30 microM MPP(+) alone. In addition, these NSAIDs markedly reduced the efflux of MPP(+) from PC12 cells. (3-(3-(2-(7-Chloro-2-quinolinyl) ethenyl) phenyl ((3-dimethyl amino-3oxo-propyl) thio) methyl) propanoic acid (MK 571), which is an inhibitor of multidrug resistance proteins (MRPs), mimicked the NSAIDs-induced effects, increasing cell toxicity and promoting the accumulation of MPP(+). Moreover, some types of MRPs' mRNA were detected in PC12 cells. These results suggest that some NSAIDs might cause a significant increase in the intracellular accumulation of MPP(+) via the suppression of reverse transport by the blockade of MRP, resulting in the potentiation of MPP(+)-induced cell death.

Caspase-dependent and -independent cell death pathways in primary cultures of mesencephalic dopaminergic neurons after neurotoxin treatment

Although the cause of neuronal death in Parkinson's disease (PD) is mainly unknown, growing evidence suggests that both apoptotic and non-apoptotic death may occur in PD. Using primary cultures of mesencephalic dopaminergic neurons and the MN9D dopaminergic neuronal cell line, we attempted to evaluate specifically the existence of the mitochondrial apoptotic pathway, focusing on the mitochondrial release of cytochrome c to the activation of the caspases after 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenylpyridinium (MPP+) treatment. Both immunofluorescent labeling and immunoblot analysis indicated mitochondrial release of cytochrome c into the cytosol after 6-OHDA or MPP+ treatment. However, the appearance of activated caspase-3 immunoreactivity in tyrosine hydroxylase (TH)-positive neurons was detected only after 6-OHDA. Immunoblot and biochemical analysis also confirmed that activation of both caspase-9 and caspase-3 was induced by 6-OHDA, but not by MPP+. Consequently, cotreatment with a caspase inhibitor (zVAD-fmk) or with an antioxidant (N-acetylcysteine) not only deterred 6-OHDA-induced loss of TH-positive neurons but also abolished the appearance of activated caspase-3 in TH-positive neurons. In contrast, the same treatment did not spare MPP+-treated TH-positive neurons. Interestingly, a reconstitution assay indicated that the addition of ATP to the cytosolic fraction obtained from MPP+-treated cells was sufficient to activate both caspase-9 and caspase-3. Taken together, our results indicate that distinct mechanisms underlie neurotoxin-induced cell death. They also suggest that, after mitochondrial release of cytochrome c in dopaminergic neurons after neurotoxin treatment, intracellular levels of ATP may constitute a critical factor in determining whether a neuron will die by a caspase-dependent or -independent pathway.

Alpha-synuclein protects naive but not dbcAMP-treated dopaminergic cell types from 1-methyl-4-phenylpyridinium toxicity

The pre-synaptic protein, alpha-synuclein, has been associated with the pathogenesis of Parkinson's disease. The present study indicates that alpha-synuclein, but not its mutants (A53T, A30P), can protect CNS dopaminergic cells from the parkinsonism-inducing drug 1-methyl-4-phenylpyridinium (MPP+), whereas it cannot protect from the dopaminergic toxin, 6-hydroxydopamine, hydrogen-peroxide, or the beta-amyloid peptide, A-beta. Protection from MPP+ was directly correlated with the preservation of mitochondrial function. Specifically, alpha-synuclein rescued cells from MPP+ mediated decreases in mitochondrial dehydrogenase activity and loss of ATP levels by utilizing ketosis. It also prevented toxin-induced activation of the creatine kinase/creatine phosphate system. Similarly, alpha-synuclein protected cells from the complex I inhibitor rotenone and 3-nitroproprionic acid, a complex II inhibitor. Wild-type alpha-synuclein-mediated neuroprotection and subsequent alterations in energy were not found in dbcAMP-differentiated cells. These results suggest that the normal physiological role for alpha-synuclein may change during development.

Possible Role of Interleukin-6 in PC12 Cell Death Induced by MPP+ and Tetrahydroisoquinoline

Interleukin (IL)-6 has been shown to protect neuronal cells from cell death induced by various stimulants. Although neuronal cells including PC12 cells were shown to produce IL-6, little is known about the effects of dopaminergic neurotoxins, 1,2,3,4-tetrahydroisoquinoline (TIQ) and 1-methyl-4-phenylpyridinium ion (MPP(+)), on IL-6 expression in PC12 cells. In the present study, we investigated the role of IL-6 in the TIQ- and MPP(+)-induced cell death in PC12 cells. Treatment with 3.2 mM TIQ for 24 h caused a delayed cell death (lactate dehydrogenase (LDH) leakage and nuclear DNA fragmentation) markedly 72 h after the addition. Addition of 0.4 mM MPP(+) caused LDH leakage and nuclear DNA fragmentation 24 h after the addition. The cell death induced by MPP(+) was inhibited by an inhibitor of caspases, z-Val-Ala-Asp(OMe)-fluoromethylketone. The cell death induced by TIQ or MPP(+) was inhibited by nerve growth factor and 10% serum and significantly enhanced by the treatment with anti-IL-6 antibody. Both neurotoxins decreased the IL-6 mRNA level in PC12 cells without changing the other tested mRNA levels (IL-1 alpha, beta-actin, etc.). These findings suggest that dopaminergic neurotoxins cause cell death in PC12 cells at least partially by changing IL-6 expression.