Serotonin transporter mutations associated with obsessive-compulsive disorder and phosphorylation alter binding affinity for inhibitors

Control list of high-priority chemicals based on 5-HT-RI functionality and the human health interference effects selective CNN-GRU deep learning model

The antidepressant drug known as 5-HT reuptake inhibitor (5-HT-RI) was commonly detected in biological tissues and result in significant adverse health effects. Homology modeling was used to characterize the functionalities (efficacy and resistance), and the adverse outcome pathway was used to characterize its human health interferences (olfactory toxicity, neurotoxicity, and gut microbial interference). The convolutional neural network coupled with the gated recurrent unit (CNN-GRU) deep learning method was used to construct a comprehensive model of 5-HT-RI functionality and human health interference effects selectivity with small sample data. The architecture with 2 SE, 320 neuronal nodes and 6-folds cross-validation showed the best applicability. The results showed that the confidence interval of the constructed model reached 90 % indicating that the model had reliable prediction ability and generalization ability. Based on the CNN-GRU deep learning model, seven high-priority chemicals with a weak comprehensive effect, including D-VEN, (1R,4S)-SER, S-FLX, CTP, S-CTP, NEF, and VEN, were screened. Based on the molecular three-dimensional structure information, a comprehensive-effect three-dimensional quantitative structure-activity relationship (3D-QSAR) model was constructed to confirm the reliability of the constructed control list of 5-HT-RI high-priority chemicals. Analysis with the ranking of calculated values based on the molecular dynamics method and predicted values based on the CNN-GRU deep learning model, we found that the consistency of the three methods was above 85 %. Additionally, by analyzing the sensitivity, molecular electrostatic potential, polar surface area of the comprehensive-effect CNN-GRU deep learning model, and the electrostatic field of the 3D-QSAR models, we found that the significant effects of five key characteristics (DM, Qyy, Qxz, I, and BP), molecular electronegativity, and polarity significantly affected the high-priority degree of 5-HT-RI. In this study, we provided reasonable and reliable prediction tools and discussed theoretical methods for the risk assessment of functionality and human health interference of emerging pollutants such as 5-HT-RI.

Serotonin-2B receptor antagonism increases the activity of dopamine and glutamate neurons in the presence of selective serotonin reuptake inhibition

Previous research has implicated the serotonin-2B (5-HT_2B) receptor as a possible contributor to the antidepressant-like response. Aripiprazole has been successfully used in combination with selective serotonin reuptake inhibitors (SSRIs) in treatment-resistant depression and it, among all receptors, exhibits the highest affinity for the 5-HT_2B receptor. However, the potential contribution of such an antagonistic action on 5-HT_2B receptors in the context of adjunct therapy is not known. In vivo electrophysiological recordings of ventral tegmental area (VTA) dopamine (DA) neurons, dorsal raphe nucleus (DRN) 5-HT neurons and pyramidal neurons in the medial prefrontal cortex (mPFC), and the hippocampus were conducted in anaesthetized Sprague-Dawley rats after the administration of 5-HT_2B receptor ligands alone or in combination with the SSRI escitalopram. An escitalopram-induced decrease in DA, but not 5-HT firing activity, was rescued by 2-day co-administration of the selective 5-HT_2B receptor antagonist LY266097. In the mPFC, 14-day escitalopram administration alone had no effect on pyramidal neuron firing and burst activity, whereas, aripiprazole administered alone or in combination with escitalopram for 14 days increased pyramidal neuron firing and burst activity. Likewise, the administration of LY266097 alone or its addition on the last 3 days of a 14-day escitalopram regimen increased pyramidal neuron firing and burst activity. These results indicated that 5-HT_2B receptors play, at least in part, a role in this enhancement. In the hippocampus, 5-HT_2B receptor activation by BW723c86 decreased escitalopram-induced inhibition of 5-HT reuptake, which was reversed by a 5-HT_2B receptor antagonist. Altogether, these results put into evidence the possibility that 5-HT_2B receptor blockade contributes to the therapeutic effect of aripiprazole addition to SSRIs in depression.

Genetics of obsessive–compulsive disorder: a research update

The genetic study of obsessive-compulsive disorder (OCD) has made significant gains in the past decade. However, etiological gene findings are still elusive. Epidemiological studies, including family and twin studies, strongly support a genetic component for OCD. In addition, complex segregation analyses suggest the presence of at least one major gene. The neurobiology of OCD also lends support to the notion that programmed CNS-based biological processes underlie OCD symptom expression, with mapping of brain circuits to fronto-subcortical circuits in a consistent manner. Genetic linkage studies of OCD, using families with multiple affected relatives, have generated several suggestive linkage peaks, regions that may harbor a gene or genes for OCD. However, the presence of multiple linkage peaks has added to the complexity of OCD genetics, suggesting that the exploration of gene-gene interactions and gene-environment interactions, in addition to the exploration of alternate phenotypes based on symptom expression, age at onset or comorbid conditions, may be key in locating etiologic genes. Finally, candidate gene studies, while promising, are not yet associated with linkage regions, except in the case of the glutamate transporter gene SLC1A1 in 9p24. While OCD appears to have a genetic component, additional innovative research is needed to unravel the genetic influences in the disorder.

Recent advances in the understanding of the interaction of antidepressant drugs with serotonin and norepinephrine transporters

The biogenic monoamine transporters are integral membrane proteins that perform active transport of extracellular dopamine, serotonin and norepinephrine into cells. These transporters are targets for therapeutic agents such as antidepressants, as well as addictive substances such as cocaine and amphetamine. Seminal advances in the understanding of the structure and function of this transporter family have recently been accomplished by structural studies of a bacterial transporter, as well as medicinal chemistry and pharmacological studies of mammalian transporters. This feature article focuses on antidepressant drugs that act on the serotonin and/or the norepinephrine transporters. Specifically, we focus on structure-activity relationships of these drugs with emphasis on relationships between their molecular properties and the current knowledge of transporter structure.

How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

SERT Ileu425Val in autism, Asperger syndrome and obsessive-compulsive disorder

BACKGROUND

SERT I425V, an uncommon missense single nucleotide polymorphism producing a gain-of-function of the serotonin transporter (SERT), was originally found to segregate with a primarily obsessive-compulsive disorder (OCD) but complexly comorbid phenotype in two unrelated families.

OBJECTIVE

As two individuals with SERT I425V and OCD also had Asperger syndrome (AS), an autism spectrum disorder, and as other rare SERT variants have recently shown significant associations with autism, we set out to extend our original OCD study by genotyping additional autism/AS and OCD samples.

METHODS

Case-control association study of SERT I425V in 210 AS/autism probands and 215 controls, plus 335 OCD probands and their family members.

RESULTS

SERT I425V was not found in any of the individuals with AS/autism, OCD alone or OCD comorbid with AS and other disorders, or in controls. This results in new estimates of SERT I425V having a 1.5% prevalence in 530 individuals with OCD from five unrelated families genotyped by us and by one other group and a 0.23% frequency in four control populations totaling 1300 individuals, yielding a continuing significant OCD-control difference (Fisher's exact test corrected for family coefficient of identity P=0.004, odds ratio=6.54).

CONCLUSION

As several other uncommon, less well quantitated genetic variations occur with an OCD phenotype, including chromosomal anomalies and some other rare gene variants (SGCE, GCH1 and SLITRK1), a tentative conclusion is that OCD resembles other complex disorders in being etiologically heterogeneous and in having both highly penetrant familial subtypes associated with rare alleles or chromosomal anomalies, as well as having a more common, polygenetic form that may involve polymorphisms in such genes as BDNF, COMT, GRIN2beta, TPH2, HTR2A and SLC1A1.

Control of ion selectivity in LeuT: two Na+ binding sites with two different mechanisms

The x-ray structure of LeuT, a bacterial homologue of Na(+)/Cl(-)-dependent neurotransmitter transporters, provides a great opportunity to better understand the molecular basis of monovalent cation selectivity in ion-coupled transporters. LeuT possesses two ion binding sites, NA1 and NA2, which are highly selective for Na(+). Extensive all-atom free-energy molecular dynamics simulations of LeuT embedded in an explicit membrane are performed at different temperatures and various occupancy states of the binding sites to dissect the molecular mechanism of ion selectivity. The results show that the two binding sites display robust selectivity for Na(+) over K(+) or Li(+), the competing ions of most similar radii. Of particular interest, the mechanism primarily responsible for selectivity for each of the two binding sites appears to be different. In NA1, selectivity for Na(+) over K(+) arises predominantly from the strong electrostatic field arising from the negatively charged carboxylate group of the leucine substrate coordinating the ion directly. In NA2, which comprises only neutral ligands, selectivity for Na(+) is enforced by the local structural restraints arising from the hydrogen-bonding network and the covalent connectivity of the polypeptide chain surrounding the ion according to a "snug-fit" mechanism.

Serotonin transporter phosphorylation by cGMP-dependent protein kinase is altered by a mutation associated with obsessive compulsive disorder

Human serotonin transporter (hSERT) activity expressed in HeLa cells was stimulated by agents that release nitric oxide, stimulate soluble guanylyl cyclase, or activate cGMP-dependent protein kinase (PKG). This stimulation was blocked by a PKG inhibitor. A naturally occurring mutation, I425V, associated with obsessive-compulsive disorder and other neuropsychiatric disorders, activated hSERT and eliminated stimulation via the PKG pathway. Inhibitors of soluble guanylyl cyclase or PKG decreased activity of the I425V mutant, but not wild type, indicating that both wild-type and mutant transporters could exist in both high and low activity forms. Mutation of Thr-276 in the fifth transmembrane domain (TM5) to alanine or aspartate prevented activation of wild-type hSERT through the PKG pathway and also blocked the inhibition of I425V activity by inhibitors of the pathway. The accessibility of positions in TM5 near Thr-276 was modified in T276D, but not in I425V. These results are consistent with the hypothesis that PKG phosphorylates hSERT at Thr-276 and increases its activity by modifying the substrate permeation pathway formed, in part, by TM5. The effect of the I425V mutation may shift the balance of hSERT toward the phosphorylated form, possibly by interfering with the action of a phosphatase. However, association of hSERT with protein phosphatase 2A was not decreased in the I425V mutant.

Effect of long-term fluoxetine treatment on the human serotonin transporter in Caco-2 cells

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) broadly used in the treatment of human mood disorders and gastrointestinal diseases involving the serotoninergic system. The effectiveness of this therapy depends on repeated long-term treatment. Most of the long-term studies in vivo of SSRI effects on serotoninergic activity have focused on their effects on autoreceptors or postsynaptic receptors. The chronic effect of SSRIs on the activity of the serotonin transporter (SERT) has been less studied and the results have been contradictory. The aim of this study was to determine the specific effect of long-term fluoxetine treatment on human serotonin transporter (hSERT) in vitro, by using the human enterocyte-like cell line Caco-2. Results show that fluoxetine diminished the 5-HT uptake in a concentration-dependent way and that this effect was reversible. Fluoxetine affected mainly the hSERT transport rate by reducing the availability of the transporter in the membrane with no significant alteration of either the total hSERT protein content or the hSERT mRNA level. These results suggest that the effect of fluoxetine on the expression of hSERT is post-translational and has shown itself to be independent of PKC and PKA activity. This study may be useful to clarify the effect of the long-term fluoxetine therapy in both gastrointestinal and central nervous system disorders.

Serotonin transport and serotonin transporter‐mediated antidepressant recognition are controlled by 5‐HT2Breceptor signaling in serotonergic neuronal cells

The plasma membrane 5-HT transporter (SERT) is the major protagonist in regulating extracellular 5-HT concentration and constitutes the target of drugs used to treat a host of metabolic and psychiatric disorders. The exact mechanisms sustaining SERT function still remain elusive. The present work exploits the properties of the 1C11 neuroectodermal progenitor, which acquires, upon 4 days of differentiation, a functional SERT within an integrated serotonergic phenotype to investigate regulatory mechanisms involved in SERT onset and functions. We show that poly(A) addition precedes SERT mRNA translation on day 2 of the serotonergic program. The newly translated transporter molecules immediately bind cocaine. Day 4 must be awaited to monitor antidepressant recognition and 5-HT uptake. Because external 5-HT reduces both 5-HT transport and SERT antidepressant binding, we identify 5-HT(2B) receptors as key players in controlling the overall 5-HT transport system. In the absence of external 5-HT, 5-HT(2B) receptor coupling to NO production ensures SERT phosphorylation to basal level and maximal 5-HT uptake. In the presence of 5-HT, the 5-HT(2B) receptor-PKC coupling promotes additional phosphorylations of both SERT and Na(+),K(+)-ATPase alpha-subunit, impairing the electrochemical gradient necessary to 5-HT uptake. SERT hyperphosphorylation also affects antidepressant recognition. Finally, such 5-HT(2B) receptor-mediated control of SERT activity operates in primary neurons from raphe nuclei. Altogether, our data shed new light on the 5-HT-driven post-translational modifications involved in the control of SERT activity.