Natural and engineered coding variation in antidepressant-sensitive serotonin transporters

Abstract

The presynaptic serotonin (5-HT) transporter (SERT) is a key regulator of 5-HT signaling and is a major target for antidepressant medications and psychostimulants. In recent years, studies of natural and engineered genetic variation in SERT have provided new opportunities to understand structural dimensions of drug interactions and regulation of the transporter, to explore 5-HT contributions to antidepressant action, and to assess the impact of SERT-mediated 5-HT contributions to neuropsychiatric disorders. Here we review three examples from our recent studies where genetic changes in SERT, identified or engineered, have led to new models, findings, and theories that cast light on new dimensions of 5-HT action in the CNS and periphery. First, we review our work to identify specific residues through which SERT recognizes antagonists, and the conversion of this knowledge to the creation of mice lacking high-affinity antidepressant and cocaine sensitivity. Second, we discuss our studies of functional coding variation in SERT that exists in commonly used strains of inbred mice, and how this variation is beginning to reveal novel 5-HT-associated phenotypes. Third, we review our identification and functional characterization of multiple, hyperactive SERT coding variants in subjects with autism. Each of these activities has driven the development of new model systems that can be further exploited to understand the contribution of 5-HT signaling to risk for neuropsychiatric disorders and their treatment.

Fig. 1

Model of the 5-HT binding pocket in human SERT. (A) Hydrophobic binding pocket of LeuT_Aa-transporter side chains (gray) and L-leucine side chain (yellow) are represented by stick models surrounded by semi-transparent Van der Waals surface spheres (Figure from Yamashita et al., 2005). (B) Modeling of 5-HT within the substrate-binding site of hSERT. hSERT residues were mapped onto LeuT_Aa coordinates and probable side-chain conformations determined by SCWRL3 rotomer library analysis while maintaining the LeuT_Aa backbone. Amino acid changes in the binding pocket are represented as colored residues: Y95 (red), I172 (green), and G442 (blue). Figure reproduced from Henry et al. (2006a), by permission from Elsevier.

Fig. 2

Impact of SERT Met172 substitution on synaptosomal 5-HT transport inhibitor potency. (A) 5-HT, (B) paroxetine, (C) fluoxetine, (D) cocaine, (E) ctalopram, and (F) escitalopram were assessed for their ability to compete with [3H] 5-HT uptake. Fluoxetine, cocaine, citalopram, and escitalopram demonstrated significantly reduced potencies in SERT M172 relative to SERT I172 synaptosomes. Reprinted from Thompson et al. (2011) with permission.

Fig. 3

Sites of coding differences between DBA/2J and C57BL/6 SERT proteins. (A) Schematic illustration of the location of the amino acid changes in Slc6a4 between DBA/2J (Glu39/Arg172) and C57BL/6 (Gly39/Lys172). (B) Alignment of SERT protein sequences from human, macaque, rat, mouse, sheep, and fruit fly to illustrate relative conservation at positions analogous to amino acids 39 and 172 of mouse SERT. Reprinted from Carneiro et al. (2009) with permission.

Fig. 4

Location of autism-associated SERT coding variants. Five coding variants are overlaid on an hSERT topology model with 12 transmembrane (TM) domains. Reprinted from Prasad et al. (2009) with permission.