Control of serotonin transporter phosphorylation by conformational state.
Proceedings of the National Academy of Sciences of the United States of America – May 17, 2016
Source: PubMed
Summary
A fundamental insight reveals how the brain's serotonin transporter, crucial for mood and mental health, is precisely regulated. Its critical phosphorylation is directly controlled by the transporter's changing conformation. Researchers found that compounds like ibogaine, which stabilize an inward-open conformation, significantly boost this phosphorylation. Conversely, agents promoting an outward-open conformation reduce it. This highlights a precise mechanism where the transporter's shape dictates its activity, offering new understanding of its function.
Abstract
Serotonin transporter (SERT) is responsible for reuptake and recycling of 5-hydroxytryptamine (5-HT; serotonin) after its exocytotic release during neurotransmission. Mutations in human SERT are associated with psychiatric disorders and autism. Some of these mutations affect the regulation of SERT activity by cGMP-dependent phosphorylation. Here we provide direct evidence that this phosphorylation occurs at Thr276, predicted to lie near the cytoplasmic end of transmembrane helix 5 (TM5). Using membranes from HeLa cells expressing SERT and intact rat basophilic leukemia cells, we show that agents such as Na(+) and cocaine that stabilize outward-open conformations of SERT decreased phosphorylation and agents that stabilize inward-open conformations (e.g., 5-HT, ibogaine) increased phosphorylation. The opposing effects of the inhibitors cocaine and ibogaine were each reversed by an excess of the other inhibitor. Inhibition of phosphorylation by Na(+) and stimulation by ibogaine occurred at concentrations that induced outward opening and inward opening, respectively, as measured by the accessibility of cysteine residues in the extracellular and cytoplasmic permeation pathways, respectively. The results are consistent with a mechanism of SERT regulation that is activated by the transport of 5-HT, which increases the level of inward-open SERT and may lead to unwinding of the TM5 helix to allow phosphorylation.