The mechanism by which dopaminergic neurons are selectively lost in Parkinson disease (PD) is unknown. Here we show that accumulation of alpha-synuclein in cultured human dopaminergic neurons results in apoptosis that requires endogenous dopamine production and is mediated by reactive oxygen species. In contrast, alpha-synuclein is not toxic in non-dopaminergic human cortical neurons, but rather exhibits neuroprotective activity. Dopamine-dependent neurotoxicity is mediated by 54 83-kD soluble protein complexes that contain alpha-synuclein and 14-3-3 protein, which are elevated selectively in the substantia nigra in PD. Thus, accumulation of soluble alpha-synuclein protein complexes can render endogenous dopamine toxic, suggesting a potential mechanism for the selectivity of neuronal loss in PD.

Dopamine D1-like receptors, composed of D1 and D5 receptors, have been documented to modulate glutamate-mediated fast excitatory synaptic neurotransmission. Here, we report that dopamine D1 receptors modulate NMDA glutamate receptor-mediated functions through direct protein-protein interactions. Two regions in the D1 receptor carboxyl tail can directly and selectively couple to NMDA glutamate receptor subunits NR1-1a and NR2A. While one interaction is involved in the inhibition of NMDA receptor-gated currents, the other is implicated in the attenuation of NMDA receptor-mediated excitotoxicity through a PI-3 kinase-dependent pathway.

Mutations in alpha-synuclein, a protein highly enriched in presynaptic terminals, have been implicated in the expression of familial forms of Parkinson's disease (PD) whereas native alpha-synuclein is a major component of intraneuronal inclusion bodies characteristic of PD and other neurodegenerative disorders. Although overexpression of human alpha-synuclein induces dopaminergic nerve terminal degeneration, the molecular mechanism by which alpha-synuclein contributes to the degeneration of these pathways remains enigmatic. We report here that alpha-synuclein complexes with the presynaptic human dopamine transporter (hDAT) in both neurons and cotransfected cells through the direct binding of the non-A beta amyloid component of alpha-synuclein to the carboxyl-terminal tail of the hDAT. alpha-Synuclein--hDAT complex formation facilitates the membrane clustering of the DAT, thereby accelerating cellular dopamine uptake and dopamine-induced cellular apoptosis. Since the selective vulnerability of dopaminergic neurons in PD has been ascribed in part to oxidative stress as a result of the cellular overaccumulation of dopamine or dopamine-like molecules by the presynaptic DAT, these data provide mechanistic insight into the mode by which the activity of these two proteins may give rise to this process.