α-Synuclein regulates monoamine transporter capacity and amphetamine-induced efflux via modulation of transporter expression.

α-Synuclein (α-syn) is a presynaptic protein implicated in Parkinson disease and related synucleinopathies, but its physiological function remains incompletely understood. Monoamine transporters, including the dopamine transporter (DAT) and serotonin transporter (SERT), are key determinants of neurotransmitter clearance and primary targets of psychostimulant drugs. Although α-syn has been reported to interact with these transporters, its impact on transporter function remains unresolved. In the present study, we systematically manipulated α-syn expression in cellular models to determine its effects on DAT and SERT function. Overexpression of α-syn increased substrate uptake at both transporters, reflected by a selective increase in maximal transport velocity (Vmax) without significant changes in substrate affinity (KM). This effect was accompanied by elevated total DAT and SERT protein levels. Conversely, small interfering RNA-mediated knockdown or CRISPR/Cas9-mediated knockout of α-syn reduced transporter expression and decreased uptake capacity. In addition, amphetamine-induced transporter-mediated efflux was significantly attenuated in α-syn-deficient cells. These findings identify α-syn as a positive regulator of monoamine transporter expression and transport capacity. By controlling transporter abundance, α-syn modulates both substrate uptake and psychostimulant-induced reverse transport. This regulatory mechanism has important implications for monoamine homeostasis and for the pharmacological actions of transporter-targeting drugs such as D-amphetamine or 3,4-methylenedioxymethamphetamine ("ecstasy"). SIGNIFICANCE STATEMENT: The physiological role of α-synuclein (α-syn), a protein central to Parkinson disease, is not fully understood. This study demonstrates that α-syn enhances the expression and transport capacity of dopamine and serotonin transporters, thereby regulating monoamine clearance and amphetamine-induced efflux. This identifies α-syn as a key modulator of monoaminergic signaling and provides a mechanistic framework linking its normal function to disease-related alterations in neurotransmission.