The mechanism of a high-affinity allosteric inhibitor of the serotonin transporter

Abstract

The serotonin transporter (SERT) terminates serotonin signaling by rapid presynaptic reuptake. SERT activity is modulated by antidepressants, e.g., S-citalopram and imipramine, to alleviate symptoms of depression and anxiety. SERT crystal structures reveal two S-citalopram binding pockets in the central binding (S1) site and the extracellular vestibule (S2 site). In this study, our combined in vitro and in silico analysis indicates that the bound S-citalopram or imipramine in S1 is allosterically coupled to the ligand binding to S2 through altering protein conformations. Remarkably, SERT inhibitor Lu AF60097, the first high-affinity S2-ligand reported and characterized here, allosterically couples the ligand binding to S1 through a similar mechanism. The SERT inhibition by Lu AF60097 is demonstrated by the potentiated imipramine binding and increased hippocampal serotonin level in rats. Together, we reveal a S1-S2 coupling mechanism that will facilitate rational design of high-affinity SERT allosteric inhibitors.

Targeting Solute Carrier Transporters through Functional Mapping

“…Such mechanisms have been described for the elevator and gated-pore transport processes [14][15][16][17] and have been suggested for the rocker switch [18]. More specifically, some allosteric inhibitors have been identified for the glutamate transporter EAAT1 (SLC1A3) [14] and the SERT (SLC6A4) [15,17]. Interestingly, in both cases, the allosteric inhibitors bind at the interface between the scaffold and mobile domains, hence preventing the conformational change.…”

“…Of particular interest for medicinal chemistry are allosteric mechanisms, by which the binding of a molecule at a site distant from the ligand (orthosteric) binding site, modulates the function of the transporter. Such mechanisms have been described for the elevator and gated-pore transport processes [14][15][16][17] and have been suggested for the rocker switch [18]. More specifically, some allosteric inhibitors have been identified for the glutamate transporter EAAT1 (SLC1A3) [14] and the SERT (SLC6A4) [15,17].…”

Allosteric Modulation of Neurotransmitter Transporters as a Therapeutic Strategy

“…The most potent S2-bound ligand was S-citalopram, but its affinity is still about 5 μM, approximately 1000-fold lower than its affinity for the S1-site (Kd ~5 nM). Recently, a ligand (Lu AF60097) was found that had an S2-site affinity of ~30 nM and an S1-site affinity of around 300 nM [ 71 ] ( Figure 3A ). These results show that it is possible to generate compounds with both high S2-site affinity and selectivity relative to the S1-site, opening the possibility of approaching therapeutic effects by targeting the S2-site.…”

“…These results show that it is possible to generate compounds with both high S2-site affinity and selectivity relative to the S1-site, opening the possibility of approaching therapeutic effects by targeting the S2-site. Moreover, the study shows that Lu AF60097 was able to potentiate the binding of imipramine to SERT, opening the possibility of decreasing its dosage and thus alleviating some of the detrimental side effects of imipramine treatment [ 71 ].…”

Toward a Systematic Structural and Functional Annotation of Solute Carriers Transporters—Example of the SLC6 and SLC7 Families

“…Thus, a functional annotation of this site could guide the design of such new modulators. In fact, a very recent study already revealed the first hSERT high affinity S2 inhibitor, demonstrating the relevance of targeting such site in drug discovery ( Plenge et al, 2020 ).…”