Scientific Understanding of Consciousness
Dopamine Transporter Antidepressant Mechanism
Nature 503, 85–90 (07 November 2013)
X-ray structure of dopamine transporter elucidates antidepressant mechanism
Vollum Institute, Oregon Health & Science University, 3181 South West Sam Jackson Park Road, Portland, Oregon 97239, USA
Aravind Penmatsa, Kevin H. Wang & Eric Gouaux
Howard Hughes Medical Institute, Oregon Health & Science University, 3181 South West Sam Jackson Park Road, Portland, Oregon 97239, USA
Antidepressants targeting Na+/Cl−-coupled neurotransmitter uptake define a key therapeutic strategy to treat clinical depression and neuropathic pain. However, identifying the molecular interactions that underlie the pharmacological activity of these transport inhibitors, and thus the mechanism by which the inhibitors lead to increased synaptic neurotransmitter levels, has proven elusive. Here we present the crystal structure of the Drosophila melanogaster dopamine transporter at 3.0 Ĺ resolution bound to the tricyclic antidepressant nortriptyline. The transporter is locked in an outward-open conformation with nortriptyline wedged between transmembrane helices 1, 3, 6 and 8, blocking the transporter from binding substrate and from isomerizing to an inward-facing conformation. Although the overall structure of the dopamine transporter is similar to that of its prokaryotic relative LeuT, there are multiple distinctions, including a kink in transmembrane helix 12 halfway across the membrane bilayer, a latch-like carboxy-terminal helix that caps the cytoplasmic gate, and a cholesterol molecule wedged within a groove formed by transmembrane helices 1a, 5 and 7. Taken together, the dopamine transporter structure reveals the molecular basis for antidepressant action on sodium-coupled neurotransmitter symporters and elucidates critical elements of eukaryotic transporter structure and modulation by lipids, thus expanding our understanding of the mechanism and regulation of neurotransmitter uptake at chemical synapses.
Chemical neurotransmission is initiated by Ca2+-induced release of neurotransmitters into the synaptic cleft. Upon release into the synaptic cleft, neurotransmitters such as glutamate, dopamine, noradrenaline, serotonin, glycine and GABA (γ-aminobutyric acid) activate G-protein-coupled receptors and ligand-gated ion channels, resulting in excitatory or inhibitory postsynaptic signalling cascades and currents. The widespread and critical roles of neurotransmitters in both central and peripheral nervous systems necessitate a requirement for strict spatiotemporal control of their levels at neural synapses. The primary mode of neurotransmitter clearance from the synaptic cleft is through secondary active transporters localized in presynaptic cells and glial cells that harness ionic gradients, across the cell membrane, to drive the uphill transport of neurotransmitters. This symport process requires both Na+ and Cl− ions, which has led to the solute carrier 6 (SLC6) family of secondary transporters being referred to as neurotransmitter sodium symporters (NSSs).
Dysregulation of NSS function is associated with several debilitating disorders that include depression, attention deficit hyperactivity disorder, orthostatic intolerance, epilepsy, Parkinson’s disease and infantile parkinsonism dystonia. NSSs are also the primary targets of antidepressants, drugs to treat neuropathic pain, attention deficit hyperactivity disorder, anxiety and of habit-forming substances of abuse such as cocaine and amphetamines. Development of antidepressants had a serendipitous beginning in the 1950s, followed by the discovery that the tricyclic antidepressant (TCA) imipramine inhibits noradrenaline reuptake in tissues. Numerous variants of imipramine, and the subsequent discovery of selective serotonin reuptake inhibitors, have revolutionized antidepressant treatment. To date, inhibition of neurotransmitter uptake remains the most widely used strategy for antidepressant therapy, despite numerous side effects.
Here we present a 3.0 Ĺ X-ray crystal structure of the Drosophila melanogaster dopamine transporter (DAT) in complex with the TCA nortriptyline. The Drosophila DAT has greater than 50% sequence identity with its mammalian counterparts and harbours a pharmacological profile that is a hybrid of the mammalian DATs, noradrenaline transporters (NETs) and serotonin transporters (SERTs), making it a powerful vehicle to study NSS pharmacology and substrate specificity.
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