Syntaxin1 Ser14 Phosphorylation is Required for Non-Vesicular Dopamine Release

Amphetamine (AMPH), a psychostimulant commonly prescribed for the treatment of neuropsychiatric and neurological disorders, has a high liability for abuse. The abuse and psychomotor stimulant properties of AMPH are primarily associated with its ability to increase dopamine (DA) neurotransmission. This increase is mediated, in large part, by non-vesicular DA release (DA efflux). DA efflux is the result of reversal of the DA transporter (DAT) promoted by AMPH. Syntaxin 1 (Stx1) is a SNARE protein that plays a pivotal role in vesicular release. Previously, we have shown that Stx1 also interacts with the distal DAT N-terminus, an event promoted by AMPH. Stx1 is phosphorylated at Ser14 by casein kinase II (CK2). Using Drosophila Melanogaster as an animal model, we show that this phosphorylation event is critical for non-vesicular DA release and regulates the expression of AMPH preference as well as the ability of AMPH to promote mating drive. We also show that reverse transport of DA mediated by DAT underlies these complex behaviors promoted by AMPH. Our molecular dynamics (MD) simulations of the phosphorylated DAT/Stx1 complex demonstrate that the phosphorylation state of these proteins plays a key role in allowing DAT to dwell in an efflux-willing state. This state also supports constitutive DA efflux (CDE), an event that occurs in the absence of AMPH. The DAT-Stx1 phosphorylated complex is characterized by the breakdown of two key salt bridges in DAT, K66-D345 and E428-R445, which are critical for the formation of the intracellular (IC) gate and for transport function. The breaking of these salt bridges leads to an opening and hydration of the DAT intracellular vestibule, allowing DA to bind from the cytosol, a mechanism that we hypothesize leads to CDE. We further determine the importance of Stx1 phosphorylation in CDE by pharmacologically inhibiting CK2 with CX-4945, a molecule currently in phase II clinical trials for cancer treatment. CX-4945 treatment prevented the expression of CDE in isolated Drosophila Melanogaster brains as well as behaviors associated with CDE. Thus, our results suggest that Stx1 phosphorylation is a possible pharmacological target for the treatment of AMPH abuse.