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

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The herbicide paraquat (PQ) has increasingly been reported in epidemiological studies to enhance the risk of developing Parkinson's disease (PD). Furthermore, case-control studies report that individuals with genetic variants in the dopamine transporter (DAT, SLC6A) have a higher PD risk when exposed to PQ. However, it remains a topic of debate whether PQ can enter dopamine (DA) neurons through DAT. We report here a mechanism by which PQ is transported by DAT: In its native divalent cation state, PQ 2+ is not a substrate for DAT; however, when converted to the monovalent cation PQ + by either a reducing agent or NADPH oxidase on microglia, it becomes a substrate for DAT and is accumulated in DA neurons, where it induces oxidative stress and cytotoxicity. Impaired DAT function in cultured cells and mutant mice significantly attenuated neurotoxicity induced by PQ + . In addition to DAT, PQ + is also a substrate for the organic cation transporter 3 (Oct3, Slc22a3), which is abundantly expressed in non-DA cells in the nigrostriatal regions. In mice with Oct3 deficiency, enhanced striatal damage was detected after PQ treatment. This increased sensitivity likely results from reduced buffering capacity by non-DA cells, leading to more PQ + being available for uptake by DA neurons. This study provides a mechanism by which DAT and Oct3 modulate nigrostriatal damage induced by PQ 2+ /PQ + redox cycling.neurodegeneration | extraneuronal monoamine transporter | astrocytes | in vivo microdialysis P arkinson's disease (PD) is characterized primarily by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (1). Although in past decades discoveries of genetic mutations linked to PD have significantly impacted our current understanding of the pathogenesis of this devastating disorder, it is likely that the environment plays a critical role in the etiology of sporadic PD. Human epidemiological studies indicate that exposure to herbicides, pesticides, and heavy metals increase the risk of PD. One such environmental toxicant is paraquat (PQ 2+ , N,N′-dimethyl-4-4′-bipiridinium) (2, 3). This molecule exists natively as a divalent cation, but can undergo redox cycling with cellular diaphorases such as NADPH oxidase and nitric oxide synthase (4) (NOS) to yield the monovalent cation PQ + . From this redox cycle, superoxide is generated, leading to oxidative stress-related cytotoxicity. (For clarity and brevity, the abbreviations PQ 2+ and PQ + will be used to signify the respective cations, whereas PQ represents a general term when the valency is ambiguous.) On the basis of its structural similarity to 1-methyl-4-phenylpyridinium (MPP + ), an active metabolite of the parkinsonian agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (5), PQ 2+ has been predicted to be a potential environmental parkinsonian toxicant (6), and with subsequent recent epidemiological studies (2, 3), there has been increasing interest in this herbicide as a potential pathogenic agent in PD.When PQ 2+ is injected into mice, it induces a ...

Section: Discussionmentioning

confidence: 56%

Paraquat neurotoxicity is mediated by the dopamine transporter and organic cation transporter-3

Rappold

Cui

Chesser

et al. 2011

167

156

“…Intriguingly, CaM can be cross-linked to α-syn (60,61), suggesting that α-syn could be locally sequestering CaM to further increase cytosolic Ca 2+ flux through these channels. Moreover, SNc neurons have higher cytosolic dopamine levels than the less susceptible ventral tegmentum DA neurons (62). This neurotransmitter renders SNc neurons more susceptible to cell death and has also been linked to increased intracellular Ca 2+ levels (62).…”

Section: Discussionmentioning

confidence: 99%

“…Moreover, SNc neurons have higher cytosolic dopamine levels than the less susceptible ventral tegmentum DA neurons (62). This neurotransmitter renders SNc neurons more susceptible to cell death and has also been linked to increased intracellular Ca 2+ levels (62). The enhanced sensitivity of the SNc DA neurons to Ca 2+ stress would exacerbate the defects in vesicle trafficking (18,63,64), mitochondrial dysfunction (21,22), nitrosative stress (16,17), and metal ion homeostasis (65) caused by α-syn.…”

Section: Discussionmentioning

confidence: 99%

Calcineurin determines toxic versus beneficial responses to α-synuclein

Caraveo

Auluck

Whitesell

et al. 2014

Self Cite

104

144

Calcineurin (CN) is a highly conserved Ca 2+ -calmodulin (CaM)-dependent phosphatase that senses Ca 2+ concentrations and transduces that information into cellular responses. Ca 2+ homeostasis is disrupted by α-synuclein (α-syn), a small lipid binding protein whose misfolding and accumulation is a pathological hallmark of several neurodegenerative diseases. We report that α-syn, from yeast to neurons, leads to sustained highly elevated levels of cytoplasmic Ca 2+ , thereby activating a CaM-CN cascade that engages substrates that result in toxicity. Surprisingly, complete inhibition of CN also results in toxicity. Limiting the availability of CaM shifts CN's spectrum of substrates toward protective pathways. Modulating CN or CN's substrates with highly selective genetic and pharmacological tools (FK506) does the same. FK506 crosses the blood brain barrier, is well tolerated in humans, and is active in neurons and glia. Thus, a tunable response to CN, which has been conserved for a billion years, can be targeted to rebalance the phosphatase's activities from toxic toward beneficial substrates. These findings have immediate therapeutic implications for synucleinopathies.

“…Another distinctive hallmark of PD is the preferential destruction of dopaminergic neurons in the early stage of PD (12)(13)(14). This observation led to the conjecture that dopamine, particularly its oxidant forms generated by oxidative stress, may play an important role in PD (11,15,16). Indeed, Conway et al (17) have shown that dopamine accelerates formation of large α-Syn oligomers while suppressing fibril formation, demonstrating the connection between dopamine and large α-Syn oligomers (18)(19)(20)(21).…”

mentioning

confidence: 99%

Large α-synuclein oligomers inhibit neuronal SNARE-mediated vesicle docking

Choi

Kim

et al. 2013

238

235

Parkinson disease and dementia with Lewy bodies are featured with the formation of Lewy bodies composed mostly of α-synuclein (α-Syn) in the brain. Although evidence indicates that the large oligomeric or protofibril forms of α-Syn are neurotoxic agents, the detailed mechanisms of the toxic functions of the oligomers remain unclear. Here, we show that large α-Syn oligomers efficiently inhibit neuronal SNARE-mediated vesicle lipid mixing. Large α-Syn oligomers preferentially bind to the N-terminal domain of a vesicular SNARE protein, synaptobrevin-2, which blocks SNARE-mediated lipid mixing by preventing SNARE complex formation. In sharp contrast, the α-Syn monomer has a negligible effect on lipid mixing even with a 30-fold excess compared with the case of large α-Syn oligomers. Thus, the results suggest that large α-Syn oligomers function as inhibitors of dopamine release, which thus provides a clue, at the molecular level, to their neurotoxicity.