Characterization of an allosteric citalopram-binding site at the serotonin transporter

Laboratory of Molecular Neurobiology, Department of Biological Psychiatry, Aarhus Psychiatric University Hospital, Risskov, Denmark.
Journal of Neurochemistry (Impact Factor: 4.28). 02/2005; 92(1):21-8. DOI: 10.1111/j.1471-4159.2004.02835.x
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The serotonin transporter (SERT), which belongs to a family of sodium/chloride-dependent transporters, is the major pharmacological target in the treatment of several clinical disorders, including depression and anxiety. In the present study we show that the dissociation rate, of [3H]S-citalopram from human SERT, is retarded by the presence of serotonin, as well as by several antidepressants, when present in the dissociation buffer. Dissociation of [3H]S-citalopram from SERT is most potently inhibited by S-citalopram followed by R-citalopram, sertraline, serotonin and paroxetine. EC50 values for S- and R-citalopram are 3.6 +/- 0.4 microm and 19.4 +/- 2.3 microm, respectively. Fluoxetine, venlafaxine and duloxetine have no significant effect on the dissociation of [3H]S-citalopram. Allosteric modulation of dissociation is independent of temperature, or the presence of Na+ in the dissociation buffer. Dissociation of [3H]S-citalopram from a complex with the SERT double-mutant, N208Q/N217Q, which has been suggested to be unable to self-assemble into oligomeric complexes, is retarded to an extent similar to that found with the wild-type, raising the possibility that the allosteric mechanism is mediated within a single subunit. A species-scanning mutagenesis study comparing human and bovine SERT revealed that Met180, Tyr495 and Ser513 are important residues in mediating the allosteric effect, as well as contributing to high-affinity binding at the primary site.

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Available from: Connie Sánchez, Sep 28, 2014
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    • "*P <0.05 compared with Esc to counteract the action of escitalopram without causing pharmacokinetic interactions (Sanchez 2006; Mnie-Filali et al. 2007). Escitalopram interacts with both the orthosteric and allosteric sites of the SERT, whereas R-citalopram has much weaker binding to the orthosteric site, although its affinity for the allosteric site is comparable to that of escitalopram (Sanchez 2006; Chen et al. 2005). In our study, we found that compared with escitalopram treatment group, the co-culture with R-citalopram and escitalopram could partially increase the level of the phosphorylated tau at different epitopes. "
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    ABSTRACT: To investigate the effect of escitalopram (a widely used and highly efficacious antidepressant from the SSRI class) on tau hyperphosphorylation, HEK293/tau441 cells were pretreated with 4 μM of forskolin for 2 h. Then we treated the cells with different doses of escitalopram (0, 5, 10, 20, 40, 80 μM) for 22 h. We measured the phosphorylation level of tau by Western blotting. It was shown that escitalopram could protect tau from hyperphosphorylation induced by pharmacological activation of protein kinase A (PKA) at a dose of 20, 40, and 80 μM in vitro. Interestingly, the same dose of escitalopram could also increase the level of serine-9-phosphorylated GSK-3β (inactive form) and the phosphorylation level of Akt at Ser473 (active form) with no significant change in the level of total GSK-3β and Akt. Unexpectedly, 5-hydroxytryptamine 1A receptor (5-HT1A) agonist 8-OH-DPAT did not decrease forskolin-induced tau hyperphosphorylation. Our results suggest that escitalopram can ameliorate forskolin-induced tau hyperphosphorylation, which is not through the typical 5-HT1A pathway, and Akt/GSK-3β signaling pathway is involved. These findings may support an effective role of antidepressants in the prevention of dementia associated with depression in patients.
    Journal of Molecular Neuroscience 02/2015; 56(2). DOI:10.1007/s12031-015-0519-4 · 2.34 Impact Factor
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    • "The SERT has two types of binding site, the orthosteric binding site (also referred to as the primary site) to which escitalopram and other SSRIs bind, resulting in inhibition of its uptake function, and one or more allosteric sites (Chen et al., 2005a, 2005b; Sanchez, 2006). Many studies have led to the thorough characterization of the allosteric mechanism of escitalopram (Wennogle and Meyerson, 1982; Plenge and Mellerup, 1997; Chen et al., 2005a, 2005b), although other compounds have also been reported to have allosteric activities at the SERT but are less well characterized (Nandi et al., 2004; Nightingale et al., 2005; Boos et al., 2006). "
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    ABSTRACT: It is known that newer antidepressants, such as the selective serotonin reuptake inhibitors (SSRIs), provide advantages in tolerability over antidepressants such as the tricyclics. However, even within the SSRI class, differences in efficacy or tolerability exist between the individual drugs. Among the three most widely prescribed SSRIs are paroxetine, sertraline, and escitalopram. Escitalopram is commonly referred to as an SSRI, but also has well-documented allosteric properties, and thus can be further classed as an allosteric serotonin reuptake inhibitor. All three antidepressants are efficacious compared with placebo, but there is evidence that escitalopram is more effective than a range of other antidepressants. There are no direct data to regard either paroxetine or sertraline as a superior antidepressant. Escitalopram is superior compared with paroxetine, which has a less favorable tolerability profile. Paroxetine is associated with cholinergic muscarinic antagonism and potent inhibition of CYP2D6, and sertraline has moderate drug interaction issues in comparison with escitalopram. Overall, as an allosteric serotonin reuptake inhibitor that is somewhat different from classical SSRIs, escitalopram is the first choice judged by combined efficacy and tolerability, and nonclinical data have offered possible mechanisms through which escitalopram could be more efficacious, based on its interaction with orthosteric and allosteric binding sites at the serotonin transporter.
    International clinical psychopharmacology 01/2014; 29(4). DOI:10.1097/YIC.0000000000000023 · 2.46 Impact Factor
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    • "An important question is why should escitalopram have superior efficacy compared with racemic citalopram and other SSRIs? Biochemical studies have demonstrated that there are two distinct binding sites on the serotonin transporter protein, ie, a high-affinity, primary binding site that mediates the inhibition of serotonin reuptake, and a low affinity site that allosterically modulates the affinity of ligands at the primary site.9,47 Escitalopram uniquely binds to both the primary and allosteric sites,48 leading to enhanced serotonergic neurotransmission and subsequent downstream effects on synaptic plasticity and neurogenesis.49–51 "
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    ABSTRACT: Escitalopram is an allosteric selective serotonin reuptake inhibitor (SSRI) with some indication of superior efficacy in the treatment of major depressive disorder. In this systematic review, we critically evaluate the evidence for comparative efficacy and tolerability of escitalopram, focusing on pooled and meta-analysis studies. A literature search was conducted for escitalopram studies that quantitatively synthesized data from comparative randomized controlled trials in MDD. Studies were excluded if they did not focus on efficacy, involved primarily subgroups of patients, or synthesized data included in subsequent studies. Outcomes extracted from the included studies were weighted mean difference or standard mean difference, response and remission rates, and withdrawal rate owing to adverse events. The search initially identified 24 eligible studies, of which 12 (six pooled analysis and six meta-analysis studies) met the criteria for review. The pooled and meta-analysis studies with citalopram showed significant but modest differences in favor of escitalopram, with weighted mean differences ranging from 1.13 to 1.73 points on the Montgomery Asberg Depression Rating Scale, response rate differences of 7.0%-8.3%, and remission rate differences of 5.1%-17.6%. Pooled analysis studies showed efficacy differences compared with duloxetine and with serotonin noradrenaline reuptake inhibitors combined, but meta-analysis studies did not. The effect sizes of the efficacy differences increased in the severely depressed patient subgroups. Based on pooled and meta-analysis studies, escitalopram demonstrates superior efficacy compared with citalopram and with SSRIs combined. Escitalopram shows similar efficacy to serotonin noradrenaline reuptake inhibitors but the number of trials in these comparisons is limited. Efficacy differences are modest but clinically relevant, especially in more severely depressed patients.
    Neuropsychiatric Disease and Treatment 02/2011; 7(1):39-49. DOI:10.2147/NDT.S12531 · 1.74 Impact Factor
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