Attenuation of the norepinephrine transporter activity and trafficking via interactions with α-synuclein

Alpha-synuclein (alpha-Syn) has been studied in the context of Parkinson's disease, but its normative role remains elusive. We have shown that alpha-Syn regulates the homeostasis of dopaminergic and serotonergic synapses, through trafficking of the dopamine and serotonin transporter, respectively. In the present study we sought to determine if alpha-Syn could also modulate noradrenergic signaling, by studying its interactions with the norepinephrine transporter (NET). We co-transfected Ltk(-) cells with increasing amounts of alpha-Syn DNA and a constant amount of NET DNA, and observed a progressive decrease (68%) in [H-3]-NE uptake in cells co-transfected with a ratio of 3 : 1 alpha-Syn : NET DNA. The K-d of transport did not change, but increasing alpha-Syn caused a decrease in the V-max of the transporter, from 2.27 +/- 0.14 to 0.89 +/- 0.15 pmol/min/10(5) cells, with NET expression alone or 4 : 1 ratio of alpha-Syn : NET transfection, respectively. Decreases in surface biotinylation and [H-3]-nisoxetine binding kinetics in intact cells revealed that NET cell surface expression was attenuated in correlation to the amount of alpha-Syn co-transfected into cells. The interaction between NET and a-Syn occurred via the NAC domain of alpha-Syn, the region directly responsible for self-aggregation. These findings are the first to show that alpha-Syn has a central role in the homeostasis of noradrenergic neurons. Together with our previous studies on dopamine and serotonin transporters, we propose that a primary physiological role of alpha-Syn may be to regulate the homeostasis of monoamines in synapses, through modulatory interactions of the protein with monoaminergic transporters.