RNA editing of the human serotonin 5-HT(2C) receptor delays agonist-stimulated calcium release

Conservation of A-to-I RNA editing in bowhead whale and pig

RNA editing is a post-transcriptional process in which nucleotide changes are introduced into an RNA sequence, many of which can contribute to proteomic sequence variation. The most common type of RNA editing, contributing to nearly 99% of all editing events in RNA, is A-to-I (adenosine-to-inosine) editing mediated by double-stranded RNA-specific adenosine deaminase (ADAR) enzymes. A-to-I editing at 'recoding' sites results in non-synonymous substitutions in protein-coding sequences. Here, we present studies of the conservation of A-to-I editing in selected mRNAs between pigs, bowhead whales, humans and two shark species. All examined mRNAs-NEIL1, COG3, GRIA2, FLNA, FLNB, IGFBP7, AZIN1, BLCAP, GLI1, SON, HTR2C and ADAR2 -showed conservation of A-to-I editing of recoding sites. In addition, novel editing sites were identified in NEIL1 and GLI1 in bowhead whales. The A-to-I editing site of human NEIL1 in position 242 was conserved in the bowhead and porcine homologues. A novel editing site was discovered in Tyr244. Differential editing was detected at the two adenosines in the NEIL1 242 codon in both pig and bowhead NEIL1 mRNAs in various tissues and organs. No conservation of editing of KCNB1 and EEF1A mRNAs was seen in bowhead whales. In silico analyses revealed conservation of five adenosines in ADAR2, some of which are subject to A-to-I editing in bowheads and pigs, and conservation of a regulatory sequence in GRIA2 mRNA that is responsible for recognition of the ADAR editing enzyme.

RNA editing of the 5-HT2C receptor in the central nucleus of the amygdala is involved in resilience behavior

Post-traumatic-stress-disorder (PTSD) is a stress-related condition that may develop after exposure to a severe trauma-event. One of the core brain areas that is considered to be a key regulatory region of PTSD is the amygdala. Specifically, the central amygdala (CeA) is involved in emotion processing and associative fear learning memory, two main circuits involved in PTSD. Long term dysregulation of trauma-related emotional processing may be caused by neuroadaptations that affect gene expression. The adenosine-(A) to-inosine (I) RNA editing machinery is a post-transcriptional process that converts a genomic encoded A to I and is critical for normal brain function and development. Such editing has the potential to increase the transcriptome diversity, and disruption of this process has been linked to various central nervous system disorders. Here, we employed a unique animal model to examine the possibility that the RNA editing machinery is involved in PTSD. Detection of RNA editing specifically in the CeA revealed changes in the editing pattern of the 5-HT2C serotonin receptor (5-HT2CR) transcript accompanied by dynamic changes in the expression levels of the ADAR family enzymes (ADAR and ADARb1). Deamination by ADAR and ADARb1 enzymes induces conformational changes in the 5-HT2CR that decrease the G-protein-coupling activity, agonist affinity, and thus serotonin signaling. Significantly, a single intra-CeA administration of a 5-HT2CR pharmacological antagonist produced a robust alleviation of PTSD-like behaviors (that was maintained for three weeks) as well as single systemic treatment. This work may suggest the way to a new avenue in the understanding of PTSD regulation.

Molecular, biochemical and behavioural evidence for a novel oxytocin receptor and serotonin 2C receptor heterocomplex

The complexity of oxytocin-mediated functions is strongly associated with its modulatory effects on other neurotransmission systems, including the serotonin (5-hydroxytryptamine, 5-HT) system. Signalling between oxytocin (OT) and 5-HT has been demonstrated during neurodevelopment and in the regulation of specific emotion-based behaviours. It is suggested that crosstalk between neurotransmitters is driven by interaction between their specific receptors, particularly the oxytocin receptor (OTR) and the 5-hydroxytryptamine 2C receptor (5-HTR_2C), but evidence for this and the downstream signalling consequences that follow are lacking. Considering the overlapping central expression profiles and shared involvement of OTR and 5-HTR_2C in certain endocrine functions and behaviours, including eating behaviour, social interaction and locomotor activity, we investigated the existence of functionally active OTR/5-HTR_2C heterocomplexes. Here, we demonstrate evidence for a potential physical interaction between OTR and 5-HTR_2Cin vitro in a cellular expression system using flow cytometry-based FRET (fcFRET). We could recapitulate this finding under endogenous expression levels of both receptors via in silico analysis of single cell transcriptomic data and ex vivo proximity ligation assay (PLA). Next, we show that co-expression of the OTR/5-HTR_2C pair resulted in a significant depletion of OTR-mediated Gαq-signalling and significant changes in receptor trafficking. Of note, attenuation of OTR-mediated downstream signalling was restored following pharmacological blockade of the 5-HTR_2C. Finally, we demonstrated a functional relevance of this novel heterocomplex, in vivo, as 5-HTR_2C antagonism increased OT-mediated hypoactivity in mice. Overall, we provide compelling evidence for the formation of functionally active OTR/5-HTR_2C heterocomplexes, adding another level of complexity to OTR and 5-HTR_2C signalling functionality. This article is part of the special issue on Neuropeptides.

Host Microbiota Regulates Central Nervous System Serotonin Receptor 2C Editing in Rodents

Microbial colonization of the gastrointestinal tract plays a crucial role in the development of enteric and central nervous system functionality. The serotonergic system has been heavily implicated in microbiota-gut-brain axis signaling, particularly in proof-of-principle studies in germ-free (GF) animals. One aspect of the serotonergic system that has been left unexplored in relation to the microbiota is the unique ability of the serotonin receptor 2C (5-HT_2C) to undergo post-transcriptional editing, which has been implicated in decreased receptor functionality. We investigated whether GF mice, with absent microbiota from birth, have altered 5-HT_2C receptor expression and editing in the brain, and if colonization of the microbiota is able to restore editing patterns. Next, we investigated whether microbiota depletion later in life using a chronic antibiotic treatment could affect 5-HT_2C receptor editing patterns in rats. We found that GF mice have an increased prevalence of the edited 5-HT_2C receptor isoforms in the amygdala, hypothalamus, prefrontal cortex, and striatum, which was partially normalized upon colonization post-weaning. However, no alterations were observed in the hypothalamus after microbiota depletion using an antibiotic treatment in adult rats. This suggests that alterations in the microbiome during development, but not later in life, could influence 5-HT_2C receptor editing patterns. Overall, these results demonstrate that the microbiota affects 5-HT_2C receptor editing in the brain and may inform novel therapeutic strategies in conditions in which 5-HT_2C receptor editing is altered, such as depression.

RNA Editing Deficiency in Neurodegeneration

The molecular process of RNA editing allows changes in RNA transcripts that increase genomic diversity. These highly conserved RNA editing events are catalyzed by a group of enzymes known as adenosine deaminases acting on double-stranded RNA (ADARs). ADARs are necessary for normal development, they bind to over thousands of genes, impact millions of editing sites, and target critical components of the central nervous system (CNS) such as glutamate receptors, serotonin receptors, and potassium channels. Dysfunctional ADARs are known to cause alterations in CNS protein products and therefore play a role in chronic or acute neurodegenerative and psychiatric diseases as well as CNS cancer. Here, we review how RNA editing deficiency impacts CNS function and summarize its role during disease pathogenesis.

A short history of the 5-HT2C receptor: from the choroid plexus to depression, obesity and addiction treatment

This paper is a personal account on the discovery and characterization of the 5-HT_2C receptor (first known as the 5-HT_1C receptor) over 30 years ago and how it translated into a number of unsuspected features for a G protein-coupled receptor (GPCR) and a diversity of clinical applications. The 5-HT_2C receptor is one of the most intriguing members of the GPCR superfamily. Initially referred to as 5-HT_1CR, the 5-HT_2CR was discovered while studying the pharmacological features and the distribution of [³H]mesulergine-labelled sites, primarily in the brain using radioligand binding and slice autoradiography. Mesulergine (SDZ CU-085), was, at the time, best defined as a ligand with serotonergic and dopaminergic properties. Autoradiographic studies showed remarkably strong [³H]mesulergine-labelling to the rat choroid plexus. [³H]mesulergine-labelled sites had pharmacological properties different from, at the time, known or purported 5-HT receptors. In spite of similarities with 5-HT₂ binding, the new binding site was called 5-HT_1C because of its very high affinity for 5-HT itself. Within the following 10 years, the 5-HT_1CR (later named 5-HT_2C) was extensively characterised pharmacologically, anatomically and functionally: it was one of the first 5-HT receptors to be sequenced and cloned. The 5-HT_2CR is a GPCR, with a very complex gene structure. It constitutes a rarity in the GPCR family: many 5-HT_2CR variants exist, especially in humans, due to RNA editing, in addition to a few 5-HT_2CR splice variants. Intense research led to therapeutically active 5-HT_2C receptor ligands, both antagonists (or inverse agonists) and agonists: keeping in mind that a number of antidepressants and antipsychotics are 5-HT_2CR antagonists/inverse agonists. Agomelatine, a 5-HT_2CR antagonist is registered for the treatment of major depression. The agonist Lorcaserin is registered for the treatment of aspects of obesity and has further potential in addiction, especially nicotine/ smoking. There is good evidence that the 5-HT_2CR is involved in spinal cord injury-induced spasms of the lower limbs, which can be treated with 5-HT_2CR antagonists/inverse agonists such as cyproheptadine or SB206553. The 5-HT_2CR may play a role in schizophrenia and epilepsy. Vabicaserin, a 5-HT_2CR agonist has been in development for the treatment of schizophrenia and obesity, but was stopped. As is common, there is potential for further indications for 5-HT_2CR ligands, as suggested by a number of preclinical and/or genome-wide association studies (GWAS) on depression, suicide, sexual dysfunction, addictions and obesity. The 5-HT_2CR is clearly affected by a number of established antidepressants/antipsychotics and may be one of the culprits in antipsychotic-induced weight gain.

Region-specific alterations of A-to-I RNA editing of serotonin 2c receptor in the cortex of suicides with major depression

Brain region-specific abnormalities in serotonergic transmission appear to underlie suicidal behavior. Alterations of RNA editing on the serotonin receptor 2C (HTR2C) pre-mRNA in the brain of suicides produce transcripts that attenuate 5-HT2CR signaling by impairing intracellular G-protein coupling and subsequent intracellular signal transduction. In brain, the distribution of RNA-editing enzymes catalyzing deamination (A-to-I modification) shows regional variation, including within the cerebral cortex. We tested the hypothesis that altered pre-mRNA 5-HT2CR receptor editing in suicide is region-specific. To this end, we investigated the complete 5-HT2CR mRNA-editing profile in two architectonically distinct cortical areas involved in mood regulation and decision-making in a clinically well-characterized cohort of age- and sex-matched non-psychiatric drug-free controls and depressed suicides. By using an original biochemical detection method, that is, capillary electrophoresis single-stranded conformational polymorphism (CE-SSCP), we corroborated the 5-HT2CR mRNA-editing profile previously described in the dorsolateral prefrontal cortex (Brodmann area 9 (BA9)). Editing of 5-HT2CR mRNA displayed clear regional difference when comparing dorsolateral prefrontal cortex (BA9) and anterior cingulate cortex (BA24). Compared with non-psychiatric control individuals, alterations of editing levels of 5-HT2CR mRNA were detected in both cortical areas of depressed suicides. A marked increase in editing on 5-HT2CR was especially observed in the anterior cingulate cortex in suicides, implicating this cortical area in suicide risk. The results suggest that region-specific changes in RNA editing of 5-HT2CR mRNA and deficient receptor function likely contribute to the etiology of major depressive disorder or suicide.

Serotonin receptor 2C regulates neurite growth and is necessary for normal retinal processing of visual information

Serotonin (5HT) is present in a subpopulation of amacrine cells, which form synapses with retinal ganglion cells (RGCs), but little is known about the physiological role of retinal serotonergic circuitry. We found that the 5HT receptor 2C (5HTR2C) is upregulated in RGCs after birth. Amacrine cells generate 5HT and about half of RGCs respond to 5HTR2C agonism with calcium elevation. We found that there are on average 83 5HT+ amacrine cells randomly distributed across the adult mouse retina, all negative for choline acetyltransferase and 90% positive for tyrosine hydroxylase. We also investigated whether 5HTR2C and 5HTR5A affect RGC neurite growth. We found that both suppress neurite growth, and that RGCs from the 5HTR2C knockout (KO) mice grow longer neurites. Furthermore, 5HTR2C is subject to post-transcriptional editing, and we found that only the edited isoform's suppressive effect on neurite growth could be reversed by a 5HTR2C inverse agonist. Next, we investigated the physiological role of 5HTR2C in the retina, and found that 5HTR2C KO mice showed increased amplitude on pattern electroretinogram. Finally, RGC transcriptional profiling and pathways analysis suggested partial developmental compensation for 5HTR2C absence. Taken together, our findings demonstrate that 5HTR2C regulates neurite growth and RGC activity and is necessary for normal amplitude of RGC response to physiologic stimuli, and raise the hypothesis that these functions are modulated by a subset of 5HT+/ChAT-/TH+ amacrine cells as part of retinal serotonergic circuitry. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

5-HT2C receptors in psychiatric disorders: A review

5-HT2Rs have a different genomic organization from other 5-HT2Rs. 5HT2CR undergoes post-transcriptional pre-mRNA editing generating diversity among RNA transcripts. Selective post-transcriptional editing could be involved in the pathophysiology of psychiatric disorders through impairment in G-protein interactions. Moreover, it may influence the therapeutic response to agents such as atypical antipsychotic drugs. Additionally, 5-HT2CR exhibits alternative splicing. Central serotonergic and dopaminergic systems interact to modulate normal and abnormal behaviors. Thus, 5HT2CR plays a crucial role in psychiatric disorders. 5HT2CR could be a relevant pharmacological target in the treatment of neuropsychiatric disorders. The development of drugs that specifically target 5-HT2C receptors will allow for better understanding of their involvement in the pathophysiology of psychiatric disorders including schizophrenia, anxiety, and depression. Among therapeutic means currently available, most drugs used to treat highly morbid psychiatric diseases interact at least partly with 5-HT2CRs. Pharmacologically, 5HT2CRs, have the ability to generate differentially distinct response signal transduction pathways depending on the type of 5HT2CR agonist. Although this receptor property has been clearly demonstrated, in vitro, the eventual beneficial impact of this property opens new perspectives in the development of agonists that could activate signal transduction pathways leading to better therapeutic efficiency with fewer adverse effects.

Serotonin 2C receptor as a superhero: diversities and talents in the RNA universe for editing, variant, small RNA and other expected functional RNAs

The serotonin 2C receptor subtype (5-HT2C) has a unique profession and continues to provide exciting and critical new information. The 5-HT2C is modulated at the RNA level by several mechanisms, including editing, short variant generation, and small RNAs. Recently, these phenomena, which had been demonstrated individually, were shown to be associated with each other. At present, many reports provide information about the influence of RNA regulation on receptor protein activities and expression, which was thought to be the final functional product. However, complicated behavior at the RNA stage allows us to imagine that the RNA itself has functional roles in the RNA universe. The 5-HT2C RNA may play several roles. This review will outline previous 5-HT2C studies and prospects for future studies.

5-HT2C receptors in the basolateral amygdala and dorsal striatum are a novel target for the anxiolytic and antidepressant effects of exercise

Physical activity reduces the incidence and severity of psychiatric disorders such as anxiety and depression. Similarly, voluntary wheel running produces anxiolytic- and antidepressant-like effects in rodent models. The specific neurobiological mechanisms underlying the beneficial properties of exercise, however, remain unclear. One relevant pharmacological target in the treatment of psychiatric disorders is the 5-HT(2C) receptor (5-HT(2C)R). Consistent with data demonstrating the anxiogenic consequences of 5-HT(2C)R activation in humans and rodents, we have previously reported that site-specific administration of the selective 5-HT(2C)R agonist CP-809101 in the lateral/basolateral amygdala (BLA) increases shock-elicited fear while administration of CP-809101 in the dorsal striatum (DS) interferes with shuttle box escape learning. These findings suggest that activation of 5-HT(2C)R in discrete brain regions contributes to specific anxiety- and depression-like behaviors and may indicate potential brain sites involved in the anxiolytic and antidepressant effects of exercise. The current studies tested the hypothesis that voluntary wheel running reduces the behavioral consequences of 5-HT(2C)R activation in the BLA and DS, specifically enhanced shock-elicited fear and interference with shuttle box escape learning. After 6 weeks of voluntary wheel running or sedentary conditions, the selective 5-HT(2C)R agonist CP-809101 was microinjected into either the BLA or the DS of adult Fischer 344 rats, and shock-elicited fear and shuttle box escape learning was assessed. Additionally, in-situ hybridization was used to determine if 6 weeks of voluntary exercise changed levels of 5-HT(2C)R mRNA. We found that voluntary wheel running reduced the behavioral effects of CP-809101 and reduced levels of 5-HT(2C)R mRNA in both the BLA and the DS. The current data indicate that expression of 5-HT(2C)R mRNA in discrete brain sites is sensitive to physical activity status of the organism, and implicates the 5-HT(2C)R as a target for the beneficial effects of physical activity on mental health.

Exploration of synthetic approaches and pharmacological evaluation of PNU-69176E and its stereoisomer as 5-HT2C receptor allosteric modulators

Allosteric modulators of the serotonin (5-HT) 5-HT(2C) receptor (5-HT(2C)R) present a unique drug design strategy to augment the response to endogenous 5-HT in a site- and event-specific manner with great potential as novel central nervous system probes and therapeutics. To date, PNU-69176E is the only reported selective positive allosteric modulator for the 5-HT(2C)R. For the first time, an optimized synthetic route to readily access PNU-69176E (1) and its diastereomer 2 has been established in moderate to good overall yields over 10 steps starting from commercially available picolinic acid. This synthetic approach not only enables a feasible preparation of a sufficient amount of 1 for use as a reference compound for secondary pharmacological studies, but also provides an efficient synthesis of key intermediates to develop novel and simplified 5-HT(2C)R allosteric modulators. Compound 1 and its diastereomer 2 were functionally characterized in Chinese hamster ovary (CHO) cells stably transfected with the 5-HT(2C)R using an intracellular calcium (Ca(i) (2+)) release assay. Compound 1 demonstrated efficacy and potency as an allosteric modulator for the 5-HT(2C)R with no intrinsic agonist activity. Compound 1 did not alter 5-HT-evoked Ca(i) (2+) in CHO cells stably transfected with the highly homologous 5-HT(2A)R. In contrast, the diastereomer 2 did not alter 5-HT-evoked Ca(i) (2+) release in 5-HT(2A)R-CHO or 5-HT(2C)R-CHO cells or exhibit intrinsic agonist activity.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Altered ADAR 2 equilibrium and 5HT(2C) R editing in the prefrontal cortex of ADAR 2 transgenic mice

Modulation of serotonin signaling by RNA editing of the serotonin 2C receptor (5HT(2C) R) may be relevant to affective disorder as serotonin functions regulate mood and behavior. Previously, we observed enhanced endogenous behavioral despair in ADAR2 transgenic mice. As the transcript of the 5HT(2C) R is a substrate of ADAR2, we hypothesized that perturbed ADAR2 equilibrium in the prefrontal cortex of ADAR2 transgenic mice alters the normal distribution of edited amino acid isoforms of the 5HT(2C) R and modifies the receptor function in downstream basal extracellular signal-regulated kinase (ERK) signaling. We examined groups of naive control and ADAR2 transgenic mice and found significantly increased ADAR2 expression, increased RNA editing at A, C, D and E sites and significantly altered normal distribution of edited amino acid isoforms of the 5HT(2C) R with increased proportions of valine asparagine valine, valine serine valine, valine asparagine isoleucine, isoleucine asparagine valine and decreased isoleucine asparagine isoleucine amino acid isoforms of the 5HT(2C) R in ADAR2 transgenic mice. Localized serotonin levels (5-HT) were unchanged and perturbed ADAR2 equilibrium coincides with dysregulated edited amino acid isoforms of the 5HT(2C) R and reduced basal ERK signaling. These results altogether suggest that altered 5HT(2C) R function could be contributing to enhanced depression-like behavior of ADAR2 transgenic mice and further implicate ADAR2 as a contributing factor in cases of affective disorder.

Establishment and characterization of RNA-edited serotonin 2C receptor isoform cell models and alteration of amyloid precursor protein ectodomain secretion in HEK293 APPSwe cells

RNA editing is a mechanism for generating molecular diversity by altering the genetic code at the level of RNA. The 5-HT(2C) receptor is the only G protein-coupled receptor known to be edited. It has been reported that the non-edited 5-HT(2C) receptor stimulates secretion of the APP metabolite APP ectodomain (APPs). However, it remains unknown whether RNA-edited 5-HT(2C) receptors can also affect APPs secretion. In this study, cDNAs of five non-edited or partially/fully edited 5-HT(2C) receptor isoforms (INI, VNI, VNV, VSV and VGV) were stably transfected into HEK293APPSwe cells to detect the cell proliferation and APPs secretion. The results demonstrated that the overexpression of INI and VNI caused increased proliferation of host cells while VNV, VSV and VGV caused inverse effects (P < 0.01). Compared with both control and non-edited isoform INI, APPs levels were significantly increased in the four edited 5-HT(2C) receptor isoforms, VNI (P < 0.05), VNV (P < 0.05), VSV (P < 0.05) and VGV (P < 0.01). These results suggest that the RNA editing of the 5-HT(2C) receptor may affect APPs secretion through different signaling pathways related to cell growth and protein processing, and that these cell models will provide appropriate useful information to study the association between the RNA editing of the serotonin 5-HT(2C) receptor and APP metabolism.

Altered 5-HT2C receptor agonist-induced responses and 5-HT2C receptor RNA editing in the amygdala of serotonin transporter knockout mice

Background

The serotonin 5-HT_2C receptor (5-HT_2CR) is expressed in amygdala, a region involved in anxiety and fear responses and implicated in the pathogenesis of several psychiatric disorders such as acute anxiety and post traumatic stress disorder. In humans and in rodent models, there is evidence of both anxiogenic and anxiolytic actions of 5-HT_2C ligands. In this study, we determined the responsiveness of 5-HT_2CR in serotonin transporter (SERT) knockout (-/-) mice, a model characterized by increased anxiety-like and stress-responsive behaviors.

Results

In the three-chamber social interaction test, the 5-HT_2B/2C agonist mCPP decreased sociability and sniffing in SERT wildtype (+/+) mice, both indicative of the well-documented anxiogenic effect of mCPP. This 5-HT_2C-mediated response was absent in SERT -/- mice. Likewise, in the open field test, the selective 5-HT_2C agonist RO 60-0175 induced an anxiogenic response in SERT +/+ mice, but not in SERT -/- mice. Since 5-HT_2CR pre-mRNA is adenosine-to-inosine (A-to-I) edited, we also evaluated the 5-HT_2CR RNA editing profiles of SERT +/+ and SERT -/- mice in amygdala. Compared to SERT +/+ mice, SERT-/- mice showed a decrease in less edited, highly functional 5-HT_2C isoforms, and an increase in more edited isoforms with reduced signaling efficiency.

Conclusions

These results indicate that the 5-HT_2CR in the amygdala of SERT -/- mice has increased RNA editing, which could explain, at least in part, the decreased behavioral responses to 5-HT_2C agonists in SERT -/- mice. These alterations in 5-HT_2CR in amygdala may be relevant to humans with SERT polymorphisms that alter SERT expression, function, and emotional behaviors.

Genomic features of the human dopamine transporter gene and its potential epigenetic States: implications for phenotypic diversity

Human dopamine transporter gene (DAT1 or SLC6A3) has been associated with various brain-related diseases and behavioral traits and, as such, has been investigated intensely in experimental- and clinical-settings. However, the abundance of research data has not clarified the biological mechanism of DAT regulation; similarly, studies of DAT genotype-phenotype associations yielded inconsistent results. Hence, our understanding of the control of the DAT protein product is incomplete; having this knowledge is critical, since DAT plays the major role in the brain's dopaminergic circuitry. Accordingly, we reevaluated the genomic attributes of the SLC6A3 gene that might confer sensitivity to regulation, hypothesizing that its unique genomic characteristics might facilitate highly dynamic, region-specific DAT expression, so enabling multiple regulatory modes. Our comprehensive bioinformatic analyzes revealed very distinctive genomic characteristics of the SLC6A3, including high inter-individual variability of its sequence (897 SNPs, about 90 repeats and several CNVs spell out all abbreviations in abstract) and pronounced sensitivity to regulation by epigenetic mechanisms, as evident from the GC-bias composition (0.55) of the SLC6A3, and numerous intragenic CpG islands (27 CGIs). We propose that this unique combination of the genomic features and the regulatory attributes enables the differential expression of the DAT1 gene and fulfills seemingly contradictory demands to its regulation; that is, robustness of region-specific expression and functional dynamics.

PSD-95 is essential for hallucinogen and atypical antipsychotic drug actions at serotonin receptors

Here, we report that postsynaptic density protein of 95 kDa (PSD-95), a postsynaptic density scaffolding protein, classically conceptualized as being essential for the regulation of ionotropic glutamatergic signaling at the postsynaptic membrane, plays an unanticipated and essential role in mediating the actions of hallucinogens and atypical antipsychotic drugs at 5-HT(2A) and 5-HT(2C) serotonergic G-protein-coupled receptors. We show that PSD-95 is crucial for normal 5-HT(2A) and 5-HT(2C) expression in vivo and that PSD-95 maintains normal receptor expression by promoting apical dendritic targeting and stabilizing receptor turnover in vivo. Significantly, 5-HT(2A)- and 5-HT(2C)-mediated downstream signaling is impaired in PSD-95(null) mice, and the 5-HT(2A)-mediated head-twitch response is abnormal. Furthermore, the ability of 5-HT(2A) inverse agonists to normalize behavioral changes induced by glutamate receptor antagonists is abolished in the absence of PSD-95 in vivo. These results demonstrate that PSD-95, in addition to the well known role it plays in scaffolding macromolecular glutamatergic signaling complexes, profoundly modulates metabotropic 5-HT(2A) and 5-HT(2C) receptor function.

The role of RNA editing of the serotonin 2C receptor in a rat model of oro-facial neuropathic pain

We examined whether infraorbital nerve injury affected the RNA editing efficiency of the serotonin (5HT) 2C receptor in the cervical spinal cord, in association with increased pain thresholds, and whether a 5HT reuptake inhibitor (fluvoxamine; Depromel, Meiji Seika, Tokyo, Japan) altered this editing. Accordingly, we injured rats with an infraorbital nerve loose ligation and examined the pain thresholds, mRNA and mRNA editing of the 5HT2C receptor. We evaluated changes in mRNA editing and 5HT2C mRNA expression using cloning along with sequence analysis and quantitative reverse transcription-polymerase chain reaction to compare samples taken at post-injury day 28 from spinal cord sites, including the trigeminal nucleus caudalis, in naive, sham and injured rats (groups of each type had also received fluvoxamine). 5HT2C receptor expression was maintained post-injury. The RNA editing efficiency was statistically significantly lower at molecular sites A and B in ipsilateral spinal cord samples from injured rats than in bilateral samples from naive and sham rats, and in contralateral samples from injured rats. After injury, the proportional presence of two receptor isoforms changed, i.e. statistically significantly less VNV and significantly more INV and ISV. The proportions reverted after fluvoxamine administration. The post-injury change might be evidence of a functional adaptation mechanism that increases the expression of 5HT2C mRNA isoforms that encode receptors that are more sensitive to 5HT. This would activate the brainstem-spinal descending 5HT systems and, in effect, suppress nociceptive signals from primary afferent neurons to the spinal trigeminal nucleus caudalis.

Activation of the serotonin 5-HT2C receptor is involved in the enhanced anxiety in rats after single-prolonged stress

We have recently confirmed that exposure of rats to the single-prolonged stress (SPS) paradigm induces enhanced hypothalamic-pituitary-adrenal (HPA) axis negative feedback and enhanced anxiety, and found that these changes develop time-dependently following stress exposure, suggesting that it could model the neuroendocrinological and behavioral abnormalities of the post-traumatic stress disorder (PTSD) patients. In the present study, microarray analysis was performed using RNA from the hippocampus, amygdala and anterior cingulate cortex of SPS rats and unstressed controls to unveil the molecular changes underlying SPS-induced behavioral changes. Thirty-one genes were found whose time course of expression corresponded to that of behavioral changes. One gene, 5-hydroxytryptamine2C (5-HT2C) receptor, was identified as a putative candidate. The overexpression of the gene in the amygdala of SPS rats was confirmed using real-time PCR 7 days after the SPS exposure. This molecule was then pharmacologically validated using FR260010 (N-[3-(4-methyl-1H-imidazol-1-yl)phenyl]-5,6-dihydrobenzo[h]quinazolin-4-amine dimethanesulfonate), a selective 5-HT2C receptor antagonist. FR260010 (1-10 mg/kg, s.c.) significantly inhibited the enhancement of anxiety in SPS rats. These results demonstrate for the first time that activation of the brain 5-HT2C receptor is involved in the development of behavioral abnormality in this model. This suggests that selective 5-HT2C receptor antagonists might provide novel therapeutic avenues for PTSD treatment.

Editing of the serotonin 2C receptor pre-mRNA: Effects of the Morris Water Maze

The pre-mRNA encoding the serotonin 2C receptor, HTR2C (official mouse gene symbol, Htr2c), is subject to adenosine deamination that produces inosine at five sites within the coding region. Combinations of this site-specific A-to-I editing can produce 32 different mRNA sequences encoding 24 different protein isoforms with differing biochemical and pharmacological properties. Studies in humans have reported abnormalities in patterns of HTR2C editing in psychiatric disorders, and studies in rodents show altered patterns of editing in response to drug treatments and stressful situations. To further explore the biological significance of editing of the Htr2c mRNA and its regulation, we have examined patterns of Htr2c editing in C57BL/6J mice after exposure to the hidden platform version of the Morris Water Maze, a test of spatial learning that, in mice, is also associated with stress. In brains of both swimming controls and mice trained to find the platform, subtle time dependent changes in editing patterns are seen as soon as 1 h after a probe trial and typically last less than 24 h. Changes in whole brain with cerebellum removed differ from those seen in isolated hippocampus and cortex. Unexpectedly, in hippocampi from subsets of mice, abnormally low levels of editing were seen that were not correlated with behavior or with editing levels in cortex. These data implicate responses to spatial learning and stress, in addition to stochastic processes, in the generation of subtle changes in editing patterns of Htr2c.

Molecular pathopharmacology of 5-HT2C receptors and the RNA editing in the brain

Among the 14 kinds of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes (5-HTR), 5-HT(2C) receptor (5-HT2CR) has been intensively investigated because of its physiologically and pathophysiologically important role in the brain. 5-HT2CR has been suggested to be involved in depressive disorders based on findings from pharmacological/neurochemical/behavioral studies using autopsy preparations of humans suffering from depression, animal models of depression, and animals treated with antidepressant drugs. Recently the editing of 5-HT2CR mRNA has been reported to participate in the pathogenesis of depressive disease. The RNA editing of 5-HT2CR induced by the presumable alteration of deaminase during a pathological state in depression causes changes of a base to another base (e.g., adenosine to guanosine, cytidine to uracil (thymidine)), followed by changes in amino acids constituting the second intracellular transmembrane loop that couples G proteins. Thus 5-HT2CR receptor-mediated signal transduction is changed. In the present review, the pathopharmacological significance of 5-HT2CR in special reference to RNA editing of receptors is reviewed and discussed from the aspect of development of novel therapeutics for depression.

A-to-I pre-mRNA editing of the serotonin 2C receptor: comparisons among inbred mouse strains

The serotonin receptor 5HT2CR pre-mRNA is subject to adenosine deamination (RNA editing) at five residues located within a 15 nucleotide stretch of the coding region. Such changes of adenosine to inosine (A-to-I) can produce 32 mRNA variants, encoding 24 different protein isoforms, some of which vary in biochemical and pharmacological properties. Because serotonin mediates diverse neurological processes relevant to behavior and because inbred mouse strains vary in their responses to tests of learning and behavior, we have examined the A-to-I editing patterns of the 5HT2CR mRNA in whole brains from eight mouse strains. By sequencing approximately 100 clones from individual mice, we generated detailed information on levels of editing at each site and patterns of editing that identify a total of 28 mRNA and 20 protein isoforms. Significant differences between individuals from different strains were found in total editing frequency, in the proportion of transcripts with 1 and 4 edited sites, in editing frequency at the A, B, E and D sites, in amino acid frequencies at positions 157 and 161, and in subsets of major protein isoforms. Primer extension assays were used to show that individuals within strains (six C3H.B-+rd1 and four 129SvImrJ) displayed no significant differences in any feature. These findings suggest that genetic background contributes to subtle variation in 5HT2CR mRNA editing patterns which may have consequences for pharmacological treatments and behavioral testing.

5-HT(2C) receptor RNA editing in the amygdala of C57BL/6J, DBA/2J, and BALB/cJ mice

Post-transcriptional RNA editing of the G-protein coupled 5-hydroxytryptamine-2C (5-HT(2C)) receptor predicts an array of 24 receptor isoforms, some of which are characterized by reduced constitutive activity and potency to initiate intracellular signaling. The amygdala is integral to anxiety, fear, and related psychiatric diseases. Activation of 5-HT(2C) receptors within the amygdala is anxiogenic. Here, we describe the RNA editing profiles from amygdala of two inbred mouse strains (BALB/cJ and DBA/2J) known to be more anxious than a third (C57BL/6J). We confirmed the strain anxiety differences using light<-->dark exploration, and we discovered that BALB/cJ and DBA/2J are each characterized by a higher functioning RNA editing profile than C57BL/6J. BALB/cJ and DBA/2J exhibit a roughly two-fold reduction in C site editing, and a corresponding two-fold reduction in the edited isoform VSV. C57BL/6J is characterized by a relative decrease in the unedited highly functional isoform INI. We estimated the heritability of editing at the C site to be approximately 40%. By sequencing genomic DNA, we found complete conservation between C57BL/6J, BALB/cJ, DBA/2J and 37 other inbred strains for the RNA edited region of Htr2c, suggesting Htr2c DNA sequence does not influence variation in Htr2c RNA editing between inbred strains of mice. We did, however, discover that serotonin turnover is reduced in BALB/cJ and DBA/2J, consistent with emerging evidence that synaptic serotonin levels regulate RNA editing. These results encourage further study of the causes and consequences of 5-HT(2C) receptor RNA editing in the amygdala of mice.

RNA Editing of the Human Serotonin 5-HT2CReceptor Disrupts Transactivation of the Small G-Protein RhoA

The human serotonin 5-HT2C receptor undergoes adenosineto-inosine RNA editing at five positions, generating multiple receptor isoforms with altered G-protein coupling properties. In the current study, we demonstrate that RNA editing regulates the pattern of intracellular signaling. The non-edited human 5-HT2C receptor isoform INI activates phospholipase D via the G13 heterotrimer G-protein. We present evidence that transactivation of the small G-protein RhoA is required for phospholipase D activation. In contrast, neither transactivation of RhoA nor phospholipase D activation was detected in cells expressing the fully edited VGV isoform. The ability to activate phospholipase C is also reduced in VGV-expressing cells, but not to the extent found for the phospholipase D signal. We conclude that RNA editing represents a novel mechanism for regulating 5-HT2C receptor signaling to pathways linked to actin cytoskeletal organization and regulated exocytosis.

RNA Editing and Short Variant of Serotonin 2C Receptor mRNA in Neuronally Differentiated NG108-15 Cells

Two types of serotonin 2C subtype receptor mRNA, receptor-type and short variant, has been reported. The expression of the receptor-type mRNA could be detected as well as the short variant in NG108-15 cells by using a high temperature stable reverse transcriptase and the expression of the receptor-type mRNA was enhanced in drug-induced neuronal differentiated cells. The deleted sequence of the short variant include the RNA editing site by adenosine deaminase. Analysis of the sequence at the editing site revealed that the mRNA of undifferentiated cells was highly edited at sites A and B and that cytosine deaminase activity may also be involved in neuronal differentiation.

Serotonin type II receptor activation facilitates synaptic plasticity via N-methyl-D-aspartate-mediated mechanism in the rat basolateral amygdala

The modulation of synaptic plasticity by serotonin type II (5-hydroxytryptamine type II (5-HT(2)))-receptor stimulation was explored using intracellular, field potential and Fura-2 fluorescence image recordings in a rat amygdala slice preparation. Bath application of 5HT(2) receptor agonist 1-(2,5)-dimethoxy-4-iodophen-2-aminopropane (DOI) transformed theta-burst-stimulated (TBS) synaptic plasticity from short-term potentiation to long-term potentiation. DOI enhanced N-methyl-D-aspartate (NMDA) receptor-mediated potentials and calcium influx without affecting the resting membrane potential or input resistance of the neurons. In contrast, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor-mediated excitatory synaptic responses were unaffected by DOI. The facilitating effects of DOI were blocked by the 5-HT(2) receptor antagonist, ketanserin, and by the 5-HT(2C)-receptor selective antagonist, RS102221. These results indicate that 5-HT(2)-receptor activation enhances NMDA receptor-mediated synaptic function in the basolateral amygdala (BLA).

And now, transcriptomics

In this issue of Neuron, report different patterns of 5-HT2C pre-mRNA editing in suicide victims, as compared to controls. Treatment of mice with fluoxetine alters the pattern of 5-HT2C transcript editing in the direction opposite that observed for suicide victims. The authors speculate on a possible serotonergic mechanism controlling 5-HT2C pre-mRNA editing.

RNA editing of the human serotonin 5-HT2C receptor alters receptor-mediated activation of G13 protein

The recent completion of the human genome predicted the presence of only 30,000 genes, stressing the importance of mechanisms that increase molecular diversity at the post-transcriptional level. One such post-transcriptional event is RNA editing, which generates multiple protein isoforms from a single gene, often with profound functional consequences. The human serotonin 5-HT(2C) receptor undergoes RNA editing that creates multiple receptor isoforms. One consequence of RNA editing of cell surface receptors may be to alter the pattern of activation of heterotrimeric G-proteins and thereby shift preferred intracellular signaling pathways. We examined the ability of the nonedited 5-HT(2C) receptor isoform (INI) and two extensively edited isoforms, VSV and VGV, to interact with various G-protein alpha subunits. Two functional assays were utilized: the cell-based functional assay, Receptor Selection/Amplification Technology(TM), in which the pharmacological consequences of co-expression of 5HT(2C) receptor isoforms with G-protein alpha subunits in fibroblasts were studied, and 5HT(2C) receptor-mediated rearrangements of the actin cytoskeleton in stable cell lines. These studies revealed that the nonedited 5-HT(2C) receptor functionally couples to G(q) and G(13). In contrast, coupling to G(13) was not detected for the extensively edited 5-HT(2C) receptors. Thus, RNA editing represents a novel mechanism for regulating the pattern of activation of heterotrimeric G-proteins, molecular switches that control an enormous variety of biological processes.