Degeneration of the nigrostriatal pathway and induction of motor deficit by tetrahydrobiopterin: an in vivo model relevant to Parkinson's disease

Tetrahydrobioterin (BH4) Pathway: From Metabolism to Neuropsychiatry

Tetrahydrobipterin (BH4) is a pivotal enzymatic cofactor required for the synthesis of serotonin, dopamine and nitric oxide. BH4 is essential for numerous physiological processes at periphery and central levels, such as vascularization, inflammation, glucose homeostasis, regulation of oxidative stress and neurotransmission. BH4 de novo synthesis involves the sequential activation of three enzymes, the major controlling point being GTP cyclohydrolase I (GCH1). Complementary salvage and recycling pathways ensure that BH4 levels are tightly kept within a physiological range in the body. Even if the way of transport of BH4 and its ability to enter the brain after peripheral administration is still controversial, data showed increased levels in the brain after BH4 treatment. Available evidence shows that GCH1 expression and BH4 synthesis are stimulated by immunological factors, notably pro-inflammatory cytokines. Once produced, BH4 can act as an anti- inflammatory molecule and scavenger of free radicals protecting against oxidative stress. At the same time, BH4 is prone to autoxidation, leading to the release of superoxide radicals contributing to inflammatory processes, and to the production of BH2, an inactive form of BH4, reducing its bioavailability. Alterations in BH4 levels have been documented in many pathological situations, including Alzheimer's disease, Parkinson's disease and depression, in which increased oxidative stress, inflammation and alterations in monoaminergic function are described. This review aims at providing an update of the knowledge about metabolism and the role of BH4 in brain function, from preclinical to clinical studies, addressing some therapeutic implications.

Neural regeneration therapies for Alzheimer's and Parkinson's disease-related disorders

Neurodegenerative diseases are devastating mental illnesses without a cure. Alzheimer's disease (AD) characterized by memory loss, multiple cognitive impairments, and changes in personality and behavior. Although tremendous progress has made in understanding the basic biology in disease processes in AD and PD, we still do not have early detectable biomarkers for these diseases. Just in the United States alone, federal and nonfederal funding agencies have spent billions of dollars on clinical trials aimed at finding drugs, but we still do not have a drug or an agent that can slow the AD or PD disease process. One primary reason for this disappointing result may be that the clinical trials enroll patients with AD or PD at advances stages. Although many drugs and agents are tested preclinical and are promising, in human clinical trials, they are mostly ineffective in slowing disease progression. One therapy that has been promising is 'stem cell therapy' based on cell culture and pre-clinical studies. In the few clinical studies that have investigated therapies in clinical trials with AD and PD patients at stage I. The therapies, such as stem cell transplantation - appear to delay the symptoms in AD and PD. The purpose of this article is to describe clinical trials using 1) stem cell transplantation methods in AD and PD mouse models and 2) regenerative medicine in AD and PD mouse models, and 3) the current status of investigating preclinical stem cell transplantation in patients with AD and PD.

Effects of sulforaphane in the central nervous system

Sulforaphane (SFN) is an active component extracted from vegetables like cauliflower and broccoli. Activation of the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) signaling is a common mechanism for the anti-oxidative and anti-inflammatory activity of some herb-derived compounds, such as icariin and berberine. However, due to its peculiar ability in Nrf2 activation, SFN is recognized as an activator of Nrf2 and recommended as a supplementation for prevention and/or treatment of disorders like neoplasm and heart failure. In the central nervous system (CNS), the prophylactic and/or therapeutic effects of SFN have been revealed in recent years. For example, it has been reported to prevent the progression of Alzheimer's disease, Parkinson's disease, cerebral ischemia, Huntington's disease, multiple sclerosis, epilepsy, and psychiatric disorders via promotion of neurogenesis or inhibition of oxidative stress and neuroinflammation. SFN is also implicated in reversing cognition, learning, and memory impairment in rodents induced by scopolamine, lipopolysaccharide, okadaic acid, and diabetes. In models of neurotoxicity, SFN has been shown to suppress neurotoxicity induced by a wide range of toxic factors, such as hydrogen peroxide, prion protein, hyperammonemia, and methamphetamine. To date, no consolidated source of knowledge about the pharmacological effects of SFN in the CNS has been presented in the literature. In this review, we summarize and discuss the pharmacological effects of SFN as well as their possible mechanisms in prevention and/or therapy of disorders afflicting the CNS, aiming to get a further insight into how SFN affects the pathophysiological process of CNS disorders.

Aldose reductase deficiency leads to oxidative stress-induced dopaminergic neuronal loss and autophagic abnormality in an animal model of Parkinson's disease

Fungicide exposure causes degeneration of dopaminergic neurons and contributes to Parkinson's disease (PD). Benomyl inhibits enzymes responsible for detoxifying the reactive dopamine metabolite 3,4-dihydroxyphenylacetaldehyde. Aldose reductase (AR) is known as tetrahydrobiopterin (BH₄) reductase that generates BH₄, a cofactor for tyrosine hydroxylase (TH) involved in dopamine synthesis. AR also acts as an aldehyde reductase involved in detoxifying 3,4-dihydroxyphenylacetaldehyde. In PD patients, the level of AR is significantly lower in the cerebellum. To determine if AR deficiency contributes to PD, AR wild-type (AR^+/+) and knockout (AR^-/-) mice were administrated with 1-methyl-4-phenyl -1,2,3,6- tetrahydropyridine (MPTP). The MPTP-treated AR^-/- mice showed more severe behavioral deficits and brain damage than that of AR^+/+ mice. Contrary to expectation, under normal or MPTP-treated condition, AR^-/- mice showed a significant elevation of BH₄ and dopamine in the midbrain, suggesting that either AR does not contribute to BH₄ production, or other BH₄ synthetic pathways are induced. The AR^-/- brain showed upregulation of peroxynitrite, inducible nitric oxide synthase and downregulation of antioxidant enzymes, Cu/Zn superoxide dismutase (SOD) and peroxiredoxin 2 (Prx2), which indicate an increase in oxidative stress. In line with the animal data, pretreating the SH-SY5Y cells with AR inhibitors (Fidarestat or Epalrestat) before MPP⁺ treatment, increased severe cell death and mitochondrial fragmentation with downregulation of SOD were observed when compared to the MPP⁺ treatment alone. Cycloxygenase 2 (COX2), which can lead to the oxidation of dopamine, was upregulated in AR^-/- brains. Autophagic proteins, beclin-1 and LC3B were also downregulated. The loss of dopaminergic neurons was associated with activation of p-ERK1/2. These findings suggest that AR plays an important role in protecting dopaminergic neuron against neurotoxic metabolites in PD.

p38(MAPK)/p53 signalling axis mediates neuronal apoptosis in response to tetrahydrobiopterin-induced oxidative stress and glucose uptake inhibition: implication for neurodegeneration

BH4 (tetrahydrobiopterin) induces neuronal demise via production of ROS (reactive oxygen species). In the present study we investigated the mechanisms of its toxicity and the redox signalling events responsible for the apoptotic commitment in SH-SY5Y neuroblastoma cells and in mouse primary cortical neurons. We identified in p38(MAPK)/p53 a BH4-responsive pro-apoptotic signalling axis, as demonstrated by the recovery of neuronal viability achieved by gene silencing or pharmacological inhibition of both p38(MAPK) and p53. BH4-induced oxidative stress was characterized by a decrease in the GSH/GSSG ratio, an increase in protein carbonylation and DNA damage. BH4 toxicity and the redox-activated apoptotic pathway were counteracted by the H2O2-scavengers catalase and N-acetylcysteine and enhanced by the GSH neo-synthesis inhibitor BSO (buthionine sulfoximine). We also demonstrated that BH4 impairs glucose uptake and utilization, which was prevented by catalase administration. This effect contributes to the neuronal demise, exacerbating BH4-induced nuclear damage and the activation of the pro-apoptotic p38(MAPK)/p53 axis. Inhibition of glucose uptake was also observed upon treatment with 6-hydroxydopamine, another redox-cycling molecule, suggesting a common mechanism of action for auto-oxidizable neurotoxins.

Matrix metalloproteinase-3 contributes to vulnerability of the nigral dopaminergic neurons

Dopamine(DA)rgic neurons are particularly vulnerable due to the presence of oxidative stress-inducing molecules such as DA, tetrahydrobiopterin, iron and tyrosine hydroxylase (TH). We have recently observed that matrix metalloproteinase-3 (MMP-3) is involved in degeneration of DArgic neurons. In the present study, we sought to explore the role of MMP-3 in DArgic neurons not exposed to apparent stress conditions. In 8-week-old male mice deficient of MMP-3 gene (MMP-3 KO), the total number of DArgic neurons in the substantia nigra was considerably higher than wild type (WT). Primary cultured mesencephalic neurons from MMP-3 KO showed higher [(3)H]DA uptake capability compared to that of WT. The number of TH-immunopositive neurons and the length of average dendritic branch were also greater. This appeared to be selective for the DArgic system, because [(3)H]GABA uptake and calbindin D-28K and MAP-2 immunoreactivities were unaltered. On the other hand, no differences were noted in the levels of the striatal DA, DOPAC and BH4 and TH protein between the KO and WT. Interestingly, TH immunodensity per cell was lower in the DArgic neurons of MMP-3 KO both in primary culture and in vivo, suggesting the presence of a compensatory mechanism. These results further indicate a role of MMP-3 in the demise of DArgic neurons and suggest MMP-3 as a candidate cellular target for neuroprotective therapy.

Tetrahydrobiopterin shows chaperone activity for tyrosine hydroxylase

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamine neurotransmitters. Primary inherited defects in TH have been associated with l-DOPA responsive and non-responsive dystonia and infantile parkinsonism. In this study, we show that both the cofactor (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)) and the feedback inhibitor and catecholamine product dopamine increase the kinetic stability of human TH isoform 1 in vitro. Activity measurements and synthesis of the enzyme by in vitro transcription-translation revealed a complex regulation by the cofactor including both enzyme inactivation and conformational stabilization. Oral BH(4) supplementation to mice increased TH activity and protein levels in brain extracts, while the Th-mRNA level was not affected. All together our results indicate that the molecular mechanisms for the stabilization are a primary folding-aid effect of BH(4) and a secondary effect by increased synthesis and binding of catecholamine ligands. Our results also establish that orally administered BH(4) crosses the blood-brain barrier and therapeutic regimes based on BH(4) supplementation should thus consider the effect on TH. Furthermore, BH(4) supplementation arises as a putative therapeutic agent in the treatment of brain disorders associated with TH misfolding, such as for the human TH isoform 1 mutation L205P.

A novel intracellular role of matrix metalloproteinase-3 during apoptosis of dopaminergic cells

We have previously demonstrated that the active form of matrix metalloproteinase-3 (actMMP-3) is released from dopamine(DA)rgic neurons undergoing apoptosis. Herein, whether actMMP-3 might be generated intracellularly, and if so, whether it is involved in apoptosis of DArgic neurons itself was investigated in primary cultured DArgic neurons of wild-type, MMP-3 knockout animals, and CATH.a cells. During apoptosis, gene expression of MMP-3 is induced, specifically among the various classes of MMPs, generating the proform (55 kDa) which is subsequently cleaved to the catalytically active actMMP-3 (48 kDa) involving a serine protease. Intracellular actMMP-3 activity is directly linked to apoptotic signaling in DArgic cells: (i) Pharmacologic inhibition of enzymatic activity, repression of gene expression by siRNA, and gene deficiency all lead to protection; (ii) pharmacologic inhibition causes attenuation of DNA fragmentation and caspase 3 activation, the indices of apoptosis; and (iii) inhibition of the pro-apoptotic enzyme c-Jun N-terminal protein kinase leads to repression of MMP-3 induction. Under the cell stress condition, MMP-3 is released as actMMP-3 rather than the proform (proMMP-3), and catalytically active MMP-3 added to the medium does not cause cell death. Thus, actMMP-3 seems to have a novel intracellular role in apoptotic DArgic cells and this finding provides an insight into the pathogenesis of Parkinson's disease.

Role of cyclooxygenase-2 in tetrahydrobiopterin-induced dopamine oxidation

Dopamine is considered one of the main contributing factors in the induction of oxidative stress and selective dopaminergic neurodegeneration in Parkinson's disease. We have previously reported that tetrahydrobiopterin (BH4) leads to dopamine oxidation and renders dopamine-producing cells vulnerable. In the present study, we found that BH4 selectively upregulates cyclooxygenase-2 (COX-2) expression in dopaminergic cells. BH4 caused an induction of COX-2 mRNA, and a critical regulatory motif for BH4-induced transcriptional activation of COX-2 is CRE/AP-1. COX-2 can oxidize dopamine and cause oxidative stress, which is evidenced by the findings that significant increase in dopamine-chrome formation and protein carbonyl contents by BH4-induced COX-2 up-regulation, and the increases are abolished by COX-2 selective inhibitor meloxicam. Increased COX-2 promotes dopaminergic neurodegeneration in both SH-SY5Y cells and rat mesencephalic neurons. These data suggest that BH4-induced COX-2 expression is responsible for dopamine oxidation, leading to the preferential vulnerability of dopaminergic cells in Parkinson's disease.

Protective effect of sulforaphane against dopaminergic cell death

Parkinson's disease (PD) is a progressive neurodegenerative disorder with a selective loss of dopaminergic neurons in the substantia nigra. Evidence suggests oxidation of dopamine (DA) to DA quinone and consequent oxidative stress as a major factor contributing to this vulnerability. We have previously observed that exposure to or induction of NAD(P)H:quinone reductase (QR1), the enzyme that catalyzes the reduction of quinone, effectively protects DA cells. Sulforaphane (SF) is a drug identified as a potent inducer of QR1 in various non-neuronal cells. In the present study, we show that SF protects against compounds known to induce DA quinone production (6-hydroxydopamine and tetrahydrobiopterin) in DAergic cell lines CATH.a and SK-N-BE(2)C as well as in mesencephalic DAergic neurons. SF leads to attenuation of the increase in protein-bound quinone in tetrahydrobiopterin-treated cells, but this does not occur in cells that have been depleted of DA, suggesting involvement of DA quinone. SF pretreatment prevents membrane damage, DNA fragmentation, and accumulation of reactive oxygen species. SF causes increases in mRNA levels and enzymatic activity of QR1 in a dose-dependent manner. Taken together, these results indicate that SF causes induction of QR1 gene expression, removal of intracellular DA quinone, and protection against toxicity in DAergic cells. Thus, this major isothiocyanate found in cruciferous vegetables may serve as a potential candidate for development of treatment and/or prevention of PD.

Interaction of genetic and environmental factors in a Drosophila parkinsonism model

Catastrophic loss of dopaminergic neurons is a hallmark of Parkinson's disease. Despite the recent identification of genes associated with familial parkinsonism, the etiology of most Parkinson's disease cases is not understood. Environmental toxins, such as the herbicide paraquat, appear to be risk factors, and it has been proposed that susceptibility is influenced by genetic background. The genetic model organism Drosophila is an advantageous system for the identification of genetic susceptibility factors. Genes that affect dopamine homeostasis are candidate susceptibility factors, because dopamine itself has been implicated in neuron damage. We find that paraquat can replicate a broad spectrum of parkinsonian behavioral symptoms in Drosophila that are associated with loss of specific subsets of dopaminergic neurons. In parallel with epidemiological studies that show an increased incidence of Parkinson's disease in males, male Drosophila exhibit paraquat symptoms earlier than females. We then tested the hypothesis that variation in dopamine-regulating genes, including those that regulate tetrahydrobiopterin, a requisite cofactor in dopamine synthesis, can alter susceptibility to paraquat-induced oxidative damage. Drosophila mutant strains that have increased or decreased dopamine and tetrahydrobiopterin production exhibit variation in susceptibility to paraquat. Surprisingly, protection against the neurotoxicity of paraquat is conferred by mutations that elevate dopamine pathway function, whereas mutations that diminish dopamine pools increase susceptibility. We also find that loss-of-function mutations in a negative regulator of dopamine production, Catecholamines-up, delay the onset of neurological symptoms, dopaminergic neuron death, and morbidity during paraquat exposure but confer sensitivity to hydrogen peroxide.

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.

Proteomic analysis of spinal cord of presymptomatic amyotrophic lateral sclerosis G93A SOD1 mouse

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease, whose primary mechanisms or causes are still not defined and for which no effective treatment is available. We have recently reported that before disease onset the level of tyrosine nitrated proteins is increased in the G93A SOD1 transgenic mouse model of ALS. In the present investigation, we carried out a proteomic analysis of spinal cord extracts from G93A SOD1 mice at the presymptomatic stage of the disease to further unravel primary events in the pathogenesis and tentatively screen for potential pharmacological targets. Using a robust two-dimensional gel electrophoresis-based proteomic approach, we detected a number of proteins differentially represented in presymptomatic mice in comparison with controls. Alterations of these proteins correlate with mitochondrial dysfunction, aggregation, and stress response. Moreover, we found a variation in the isoform pattern of cyclophilin A, a molecular chaperone that protects cells from the oxidative stress.

Tetrahydrobiopterin causes mitochondrial dysfunction in dopaminergic cells: implications for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder associated with a selective loss of dopaminergic neurons in the substantia nigra. While the underlying cause of PD is not clearly understood, oxidative stress and mitochondrial dysfunction are thought to play a role. We have previously suggested tetrahydrobiopterin (BH4), an obligatory cofactor for the dopamine synthesis enzyme tyrosine hydroxylase and present selectively in monoaminergic neurons in the brain, as an endogenous molecule that contributes to the dopaminergic neurodegeneration. In the present study, we show that BH4 leads to inhibition of activities of complexes I and IV of the electron transport chain (ETC) and reduction of mitochondrial membrane potential. BH4 appears to be different from rotenone and MPP(+), the synthetic compounds used to generate Parkinson models, in its effect on complex IV. BH4 also induces the release of mitochondrial cytochrome c. Pretreatment with the sulfhydryl antioxidant N-acetylcysteine or the quinone reductase inducer dimethyl fumarate prevents the ETC inhibition and cytochrome c release following BH4 exposure, suggesting the involvement of quinone products. Together with our previous observation that BH4 leads to generation of oxidative stress and selective dopaminergic neurodegeneration both in vitro and in vivo via inducing apoptosis, the mitochondrial involvement in BH4 toxicity further suggests possible relevance of this endogenous molecule to pathogenesis of PD.

Immobilization stress causes increases in tetrahydrobiopterin, dopamine, and neuromelanin and oxidative damage in the nigrostriatal system

Oxidative stress is believed to contribute to the pathophysiology of Parkinson's disease, in which nigrostriatal dopaminergic (DA) neurons undergo degeneration. Identification of endogenous molecules that contribute to generation of oxidative stress and vulnerability of these cells is critical in understanding the etiology of this disease. Exposure to tetrahydrobiopterin (BH4), the obligatory cofactor for DA synthesis, was observed previously to cause oxidative damage in DA cells. To demonstrate the physiological relevance of this observation, we investigated whether an overproduction of BH4 and DA might actually occur in vivo, and, if it did, whether this might lead to oxidative damage to the nigrostriatal system. Immobilization stress (IMO) elevated BH4 and DA and their synthesizing enzymes, tyrosine hydroxylase and GTP cyclohydrolase I. This was accompanied by elevation of lipid peroxidation and protein-bound quinone, and activities of antioxidant enzymes. These increases in the indices of oxidative stress appeared to be due to increased BH4 synthesis because they were abolished following administration of the BH4 synthesis inhibitor, 2,4-diamino-6-hydroxy-pyrimidine. IMO also caused accumulation of neuromelanin and degeneration of the nigrostriatal system. These results demonstrate that a severe stress can increase BH4 and DA and cause oxidative damages to the DA neurons in vivo, suggesting relevance to Parkinson's disease.

The metabolic and molecular bases of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

About two-thirds of all mild phenylketonuria (PKU) patients are tetrahydrobiopterin (BH4)-responsive and thus can be potentially treated with BH4 instead of a low-phenylalanine diet. Although there has been an increase in the amount of information relating to the diagnosis and treatment of this new variant of PKU, very little is know about the mechanisms of BH4-responsiveness. This review will focus on laboratory investigations and possible molecular and structural mechanisms involved in this process.

Loss of striatal dopaminergic fibers after intraventricular injection of tetrahydrobiopterin in rat brain

We have reported previously that tetrahydrobiopterin (BH4), an obligatory cofactor for dopamine synthesis, exerts preferential toxicity on dopamine producing cells. We report in the present study that BH4 injection into the lateral ventricle leads to degeneration of the dopaminergic terminals in the striatum, evidenced by a loss of tyrosine hydroxylase (TH) immunopositive fibers, a decreased amount of TH protein, and decreased dopamine content. Thus, the results of our study further provide evidence that BH4, the molecule endogenously present in the dopaminergic neurons, may participate in the nigrostriatal degeneration as in Parkinson's disease.

JNK activation by tetrahydrobiopterin: implication for Parkinson's disease

Parkinson's disease (PD) is a progressive neurologic disease associated with selective degeneration of dopaminergic neurons in the substantia nigra. Despite extensive studies to understand the underlying cause of dopaminergic degeneration, the pathologic factors leading to this neuronal loss in PD remain obscure. We have observed previously that tetrahydrobiopterin (BH4) exerts selective toxicity and oxidative stress on dopaminergic cells, suggesting that BH4 might participate endogenously in dopaminergic neurodegeneration in PD. We investigated signaling events leading to BH4 toxicity in dopaminergic CATH.a cells. We show that c-Jun N-terminal kinase (JNK), but not extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein kinase (MAPK), is phosphorylated significantly by BH4 exposure. BH4 also leads to c-Jun phosphorylation and an increase in c-Jun protein level. The JNK inhibitor SP600125 protects cells against BH4 toxicity and inhibits cytochrome c release and apoptotic nuclear condensation induced by BH4. These data indicate that activation of the JNK pathway is important in mediating BH4-induced dopaminergic cell death.

Involvement of apoptosis and calcium mobilization in tetrahydrobiopterin-induced dopaminergic cell death

Parkinson's disease is a neurodegenerative disorder associated with selective loss of the dopaminergic neurons in the substantia nigra. We have previously shown that tetrahydrobiopterin (BH4), the obligatory cofactor for dopamine synthesis, exerts selective toxicity on dopamine-producing cells. In the present study we determined, both in vitro and in vivo, whether the cell death induced by this endogenous molecule involves apoptosis, resembling that which occurs in Parkinson's disease. Transmission electron microscopic analysis revealed that the dopamine-producing CATH.a cells underwent ultrastructural changes typical of apoptosis, such as cell shrinkage and chromatin condensation, upon exposure to BH4. The BH4 treatment also caused intranuclear DNA fragmentation as determined by TUNEL staining. A similar phenomenon also occurred in vivo, as the nigral cells became TUNEL-positive upon injection of BH4 into the substantia nigra. The BH4-induced CATH.a cell death seemed to involve macromolecule synthesis because cycloheximide and actinomycin D had protective effects. Concurrent treatment with the caspase inhibitor Z-VAD-FMK also suppressed cell death. BH4 treatment led to increases in the ratio of Bax/Bcl-x(L) mRNA and protein levels. Ca(2+) seemed to play a role in BH4-induced cell death, because BH4 caused an increase in Ca(2+) uptake and the intracellular Ca(2+) release blocker dantrolene, intracellular Ca(2+) chelator BAPTA/AM, and extracellular Ca(2+) chelator EGTA each attenuated the toxicity. These data provide evidence that the dopaminergic cell death induced by BH4 involves apoptosis and suggest relevance of this cell death to degeneration of the dopaminergic system in Parkinson's disease.