Interplay between Cytosolic Dopamine, Calcium, and α-Synuclein Causes Selective Death of Substantia Nigra Neurons

Department of Neurology, Columbia University Medical Center, New York, NY 10032, USA.
Neuron (Impact Factor: 15.98). 05/2009; 62(2):218-29. DOI: 10.1016/j.neuron.2009.01.033
Source: PubMed

ABSTRACT The basis for selective death of specific neuronal populations in neurodegenerative diseases remains unclear. Parkinson's disease (PD) is a synucleinopathy characterized by a preferential loss of dopaminergic neurons in the substantia nigra (SN), whereas neurons of the ventral tegmental area (VTA) are spared. Using intracellular patch electrochemistry to directly measure cytosolic dopamine (DA(cyt)) in cultured midbrain neurons, we confirm that elevated DA(cyt) and its metabolites are neurotoxic and that genetic and pharmacological interventions that decrease DA(cyt) provide neuroprotection. L-DOPA increased DA(cyt) in SN neurons to levels 2- to 3-fold higher than in VTA neurons, a response dependent on dihydropyridine-sensitive Ca2+ channels, resulting in greater susceptibility of SN neurons to L-DOPA-induced neurotoxicity. DA(cyt) was not altered by alpha-synuclein deletion, although dopaminergic neurons lacking alpha-synuclein were resistant to L-DOPA-induced cell death. Thus, an interaction between Ca2+, DA(cyt), and alpha-synuclein may underlie the susceptibility of SN neurons in PD, suggesting multiple therapeutic targets.

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Available from: Yvonne Schmitz, Mar 06, 2014
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    • "They showed that blocking dopamine degradation led to accumulation of cytosolic dopamine and caused neurotoxicity, whereas inhibiting the conversion of L-3,4-dihydroxyphenylalanine (L-DOPA) to dopamine, reduced cytosolic dopamine levels and prevented neurotoxicity (Mosharov et al., 2009). Additionally, over-expression of vesicular monoamine transporter 2 (VMAT2), a protein that sequesters intracellular dopamine into vesicles and reduces cytosolic dopamine levels, was shown to have protective effects against neuronal damage both in cultured midbrain neurons and mice (Mosharov et al., 2009; Lohr et al.,2014). Conversely, we previously reported that genetic knockdown of VMAT2 in mice produces oxidative stress and progressive degeneration of nigrostriatal neurons (Caudle et al., 2007). "
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    ABSTRACT: The dopamine transporter is a key protein responsible for regulating dopamine homeostasis. Its function is to transport dopamine from the extracellular space into the presynaptic neuron. Studies have suggested that accumulation of dopamine in the cytosol can trigger oxidative stress and neurotoxicity. Previously, ectopic expression of the dopamine transporter was shown to cause damage in non-dopaminergic neurons due to their inability to handle cytosolic dopamine. However, it is unknown whether increasing dopamine transporter activity will be detrimental to dopamine neurons that are inherently capable of storing and degrading dopamine. To address this issue, we characterized transgenic mice that over-express the dopamine transporter selectively in dopamine neurons. We report that dopamine transporter over-expressing (DAT-tg) mice display spontaneous loss of midbrain dopamine neurons that is accompanied by increases in oxidative stress markers, 5-S-cysteinyl-dopamine and 5-S-cysteinyl-DOPAC. In addition, metabolite-to-dopamine ratios are increased and VMAT2 protein expression is decreased in the striatum of these animals. Furthermore, DAT-tg mice also show fine motor deficits on challenging beam traversal that are reversed with l-DOPA treatment. Collectively, our findings demonstrate that even in neurons that routinely handle dopamine, increased uptake of this neurotransmitter through the dopamine transporter results in oxidative damage, neuronal loss and l-DOPA reversible motor deficits. In addition, DAT over-expressing animals are highly sensitive to MPTP-induced neurotoxicity. The effects of increased dopamine uptake in these transgenic mice could shed light on the unique vulnerability of dopamine neurons in Parkinson's disease. Copyright © 2014 Elsevier Inc. All rights reserved.
    Neurobiology of Disease 10/2014; 74C:66-75. DOI:10.1016/j.nbd.2014.10.016 · 5.20 Impact Factor
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    • "Recently, the direct measurement of cytosolic DA levels in cultured midbrain neurons has revealed that Ca 2+ entry through the L-type Ca 2+ channels in SNc neurons enhances DA synthesis from L-DOPA. Elevated cytosolic DA levels contribute to increase α-syn toxicity, thus suggesting that DA neurotoxicity is related to the interplay between elevated DA levels, Ca 2+ entry, and α-syn expression (Mosharov et al. 2009). The observation that animal models exposed to exogenous or endogenous toxins recapitulate the neuropathological and behavioral changes observed in PD and share common dysfunctions linked to the inhibition of respiratory chain complexes (Betarbet et al. 2000; Neafsey et al. 1989, 1995) has moved the focus of many studies to mitochondria. "
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    ABSTRACT: Calcium (Ca(2+)) is an almost universal second messenger that regulates important activities of all eukaryotic cells. It is of critical importance to neurons, which have developed extensive and intricate pathways to couple the Ca(2+) signal to their biochemical machinery. In particular, Ca(2+) participates in the transmission of the depolarizing signal and contributes to synaptic activity. During aging and in neurodegenerative disease processes, the ability of neurons to maintain an adequate energy level can be compromised, thus impacting on Ca(2+) homeostasis. In Parkinson's disease (PD), many signs of neurodegeneration result from compromised mitochondrial function attributable to specific effects of toxins on the mitochondrial respiratory chain and/or to genetic mutations. Despite these effects being present in almost all cell types, a distinguishing feature of PD is the extreme selectivity of cell loss, which is restricted to the dopaminergic neurons in the ventral portion of the substantia nigra pars compacta. Many hypotheses have been proposed to explain such selectivity, but only recently it has been convincingly shown that the innate autonomous activity of these neurons, which is sustained by their specific Cav1.3 L-type channel pore-forming subunit, is responsible for the generation of basal metabolic stress that, under physiological conditions, is compensated by mitochondrial buffering. However, when mitochondria function becomes even partially compromised (because of aging, exposure to environmental factors or genetic mutations), the metabolic stress overwhelms the protective mechanisms, and the process of neurodegeneration is engaged. The characteristics of Ca(2+) handling in neurons of the substantia nigra pars compacta and the possible involvement of PD-related proteins in the control of Ca(2+) homeostasis will be discussed in this review.
    Cell and Tissue Research 05/2014; 357(2). DOI:10.1007/s00441-014-1866-0 · 3.33 Impact Factor
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    • "The protective properties of opioids might be related to their ability to inhibit Ca 2+ currents (Waldhoer et al., 2004), as an imbalance in Ca 2+ homeostasis and regulation has been implicated in the pathogenesis of many neurodegenerative diseases, including PD, in which both striatal and DA neurons in the SN are particularly vulnerable to Ca 2+ dysregulation (Bezprozvanny & Hayden, 2004; Tang et al., 2005, 2007; Day et al., 2006; Chan et al., 2009; Dai et al., 2009; Mosharov et al., 2009). Indeed, the recent relevant studies have demonstrated that L-type Ca 2+ channels are negative regulators of synaptic function, and that their genetic deletion or their pharmacological blockade can prevent the loss of spines and synapses in striatopallidal neurons after DA depletion (Day et al., 2006; Chan et al., 2009; Mosharov et al., 2009). Accordingly, we may speculate that striatal pENK mRNA overexpression via an increased level of endogenous ENK-induced activation of striatal opioid receptors could be involved in Ca 2+ current inhibition and thus protect nigrostriatal terminals from MPTP insults. "
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    ABSTRACT: The midbrain dopamine (DA) cell death underlying Parkinson's disease (PD) is associated with upregulation of pre-enkephalin (pENK) in striatopallidal neurons. Our previous results obtained with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) parkinsonian monkeys suggest that increased striatal expression of pENK mRNA is a compensatory mechanism to alleviate PD-related motor symptoms. In this study, we tested the hypothesis that increased pENK expression in the striatum protects against the neurotoxic insults of MPTP in mice. To this end, recombinant adeno-associated virus serotype 2 also containing green fluorescent protein was used to overexpress pENK prior to DA depletion. Our results showed that overexpression of pENK in the striatum of MPTP mice induced: (i) increased levels of the opioid peptide enkephalin (ENK) in the striatum; (ii) higher densities of ENK-positive fibers in both the globus pallidus (GP) and the substantia nigra; (iii) higher locomotor activity; and (iv) a higher density of striatal tyrosine hydroxylase-positive fibers in the striatum. In addition, striatal overexpression of pENK in MPTP -treated mice led to 52 and 43% higher DA concentrations and DA turnover, respectively, in the GP compared to sham-treated MPTP mice. These observations are in agreement with the idea that increased expression of pENK at an early stage of disease can improve PD symptoms.
    European Journal of Neuroscience 04/2014; 40(2). DOI:10.1111/ejn.12596 · 3.67 Impact Factor
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