Cell type-dependent recruitment of trichostatin A-sensitive repression of the human 5-HT1A receptor gene

CaMKIV-Mediated Phosphorylation Inactivates Freud-1/CC2D1A Repression for Calcium-Dependent 5-HT1A Receptor Gene Induction

Calcium calmodulin-dependent protein kinase (CaMK) mediates calcium-induced neural gene activation. CaMK also inhibits the non-syndromic intellectual disability gene, Freud-1/CC2D1A, a transcriptional repressor of human serotonin-1A (5-HT1A) and dopamine-D2 receptor genes. The altered expression of these Freud-1-regulated genes is implicated in mental illnesses such as major depression and schizophrenia. We hypothesized that Freud-1 is blocked by CaMK-induced phosphorylation. The incubation of purified Freud-1 with either CaMKIIα or CaMKIV increased Freud-1 phosphorylation that was partly prevented in Freud-1-Ser644Ala and Freud-1-Thr780Ala CaMK site mutants. In human SK-N-SH neuroblastoma cells, active CaMKIV induced the serine and threonine phosphorylation of Freud-1, and specifically increased Freud-1-Thr780 phosphorylation in transfected HEK-293 cells. The activation of purified CaMKIIα or CaMKIV reduced Freud-1 binding to its DNA element on the 5-HT1A and dopamine-D2 receptor genes. In SK-N-SH cells, active CaMKIV but not CaMKIIα blocked the Freud-1 repressor activity, while Freud-1 Ser644Ala, Thr780Ala or dual mutants were resistant to inhibition by activated CaMKIV or calcium mobilization. These results indicate that the Freud-1 repressor activity is blocked by CaMKIV-induced phosphorylation at Thr780, resulting in the up-regulation of the target genes, such as the 5-HT1A receptor gene. The CaMKIV-mediated inhibition of Freud-1 provides a novel de-repression mechanism to induce 5-HT1A receptor expression for the regulation of cognitive development, behavior and antidepressant response.

Transcriptional Regulation of the Human 5-HT1A Receptor Gene by Lithium: Role of Deaf1 and GSK3β

Serotonin 1A (5-HT1A) autoreceptors located on serotonin neurons inhibit their activity, and their upregulation has been implicated in depression, suicide and resistance to antidepressant treatment. Conversely, post-synaptic 5-HT1A heteroreceptors are important for antidepressant response. The transcription factor deformed epidermal autoregulatory factor 1 (Deaf1) acts as a presynaptic repressor and postsynaptic enhancer of 5-HT1A transcription, but the mechanism is unclear. Because Deaf1 interacts with and is phosphorylated by glycogen synthase kinase 3β (GSK3β)-a constitutively active protein kinase that is inhibited by the mood stabilizer lithium at therapeutic concentrations-we investigated the role of GSK3β in Deaf1 regulation of human 5-HT1A transcription. In 5-HT1A promoter-reporter assays, human HEK293 kidney and 5-HT1A-expressing SKN-SH neuroblastoma cells, transfection of Deaf1 reduced 5-HT1A promoter activity by ~45%. To identify potential GSK3β site(s) on Deaf1, point mutations of known and predicted phosphorylation sites on Deaf1 were tested. Deaf1 repressor function was not affected by any of the mutants tested except the Y300F mutant, which augmented Deaf1 repression. Both lithium and the selective GSK3 inhibitors CHIR-99021 and AR-014418 attenuated and reversed Deaf1 repression compared to vector. This inhibition was at concentrations that maximally inhibit GSK3β activity as detected by the GSK3β-sensitive TCF/LEF reporter construct. Our results support the hypothesis that GSK3β regulates the activity of Deaf1 to repress 5-HT1A transcription and provide a potential mechanism for actions of GSK3 inhibitors on behavior.

REST Is Not Resting: REST/NRSF in Health and Disease

Chromatin modifications play a crucial role in the regulation of gene expression. The repressor element-1 (RE1) silencing transcription factor (REST), also known as neuron-restrictive silencer factor (NRSF) and X2 box repressor (XBR), was found to regulate gene transcription by binding to chromatin and recruiting chromatin-modifying enzymes. Earlier studies revealed that REST plays an important role in the development and disease of the nervous system, mainly by repressing the transcription of neuron-specific genes. Subsequently, REST was found to be critical in other tissues, such as the heart, pancreas, skin, eye, and vascular. Dysregulation of REST was also found in nervous and non-nervous system cancers. In parallel, multiple strategies to target REST have been developed. In this paper, we provide a comprehensive summary of the research progress made over the past 28 years since the discovery of REST, encompassing both physiological and pathological aspects. These insights into the effects and mechanisms of REST contribute to an in-depth understanding of the transcriptional regulatory mechanisms of genes and their roles in the development and progression of disease, with a view to discovering potential therapeutic targets and intervention strategies for various related diseases.

Possible role of transcriptional regulation of 5-HT1A receptor in the midbrain on unadaptation to stress in mice

The ability to adapt to stress is an essential defensive function of a living body, and disturbance of this ability in the brain may contribute to the development of affective illness. Previously, we reported that mice exposed to unadaptable restraint stress show emotional abnormality. Moreover, this emotional abnormality was alleviated by chronic treatment with flesinoxan, a serotonin (5-HT)_1A receptor agonist. 5-HT_1A receptor expression is regulated by several transcription factors such as nuclear deformed epidermal autoregulatory factor (NUDR/Deaf-1) and five prime repressors under dual repression binding protein 1 (Freud-1). The present study was designed to investigate the expression levels of 5-HT_1A receptor and its transcription factors in the midbrain and hippocampus of stress-adaptive and -unadaptive mice. Mice were exposed to 14 days of repeated adaptable (1 h/day) or repeated unadaptable (4 h/day) restraint stress, or were left in their home cage (non-stressed groups). In a western blot analysis, a significant increase in the expression levels of 5HT_1A receptor protein were observed in the hippocampal membrane fraction in stress-adaptive mice. In contrast, the expression levels of 5-HT_1A receptor protein in stress-unadaptive mice were significantly increased in both cytoplasmic and membrane fraction of the midbrain. Furthermore, real-time PCR analysis revealed that, in the midbrain of stress-unadaptive mice, the expression levels of 5-HT_1A receptor mRNA and Freud-1 or NUDR mRNA were significantly increased and decreased, respectively. These results suggest that increased expression of 5-HT_1A receptor due to decrease in the expression of Freud-1 and NUDR in the midbrain may play a pivotal role in the emotional abnormality of stress-unadaptive mice.

Expression of Repressor Element 1 Silencing Transcription Factor (REST) in Serotonin Neurons in the Adult Male Nile Tilapia (Oreochromis niloticus)

Repressor element-1 silencing transcription factor (REST) is highly expressed in the dorsal raphe where serotonin (5-hydroxytryptamine, 5-HT) neurons are located. REST works as a transcription factor for the 5-HT receptor and tryptophan hydroxylase two-gene expression. We hypothesized that REST is co-expressed in 5-HT neurons, which, if demonstrated, would be useful to understand the mechanism of 5-HT dysfunction-related disorders such as negative emotions and depression. Therefore, the present study was designed to examine the expression of the REST gene in the brain (forebrain, midbrain, and hindbrain) of adult male Nile tilapia (Oreochromis niloticus) using rt-PCR. Besides, using immunocytochemistry, co-localization of the REST gene was examined in 5-HT neurons and with neuronal-/glial-cell markers. We found a high expression of the REST gene in the midbrain region of the dorsal raphe, an area of 5-HT neurons. Double-label immunocytochemistry showed neuron-specific expression of REST co-localized in 5-HT neurons in the dorsal and ventral parts of the periventricular pretectal nucleus, paraventricular organ, and dorsal and medial raphe nucleus. Since midbrain 5-HT neurons express REST, we speculate that REST may control 5-HT neuronal activity related to negative emotions, including depression.

Repressor element 1 silencing transcription factor /neuron-restrictive silencing factor (REST/NRSF) in social stress and depression

Extreme stress is closely linked with symptoms of depression. Chronic social stress can cause structural and functional changes in the brain. These changes are associated with dysfunction of neuroprotective signalling that is necessary for cell survival, growth, and maturation. Reduced neuronal numbers and volume of brain regions have been found in depressed patients, which may be caused by decreased cell survival and increased cell death. Elucidating the mechanism underlying the degeneration of the neuroprotective system in social stress-induced depression is important for developing neuroprotective measures. The Repressor Element 1 Silencing Transcription Factor (REST) also known as Neuron-Restrictive Silencing Factor (NRSF) has been reported as a neuroprotective molecule in certain neurological disorders. Decreased expression levels of REST/NRSF in the nucleus can induce death-related gene expression, leading to neuronal death. Under physiological stress conditions, REST/NRSF over expression is known to activate neuronal survival in the brain. Alterations in REST/NRSF expression in the brain has been reported in stressed animal models and in the post-mortem brain of patients with depression. Here, we highlight the neuroprotective function of REST/NRSF and discuss dysregulation of REST/NRSF and neuronal damage during social stress and depression.

Developmental programming of appetite and growth in male rats increases hypothalamic serotonin (5-HT)5A receptor expression and sensitivity

BACKGROUND

Though it is well established that neonatal nutrition plays a major role in lifelong offspring health, the mechanisms underpinning this have not been well defined. Early postnatal accelerated growth resulting from maternal nutritional status is associated with increased appetite and body weight. Likewise, slow growth correlates with decreased appetite and body weight. Food consumption and food-seeking behaviour are directly modulated by central serotonergic (5-hydroxytryptamine, 5-HT) pathways. This study examined the effect of a rat maternal postnatal low protein (PLP) diet on 5-HT receptor mediated food intake in offspring.

METHODS

Microarray analyses, in situ hybridization or laser capture microdissection of the ARC followed by RT-PCR were used to identify genes up- or down-regulated in the arcuate nucleus of the hypothalamus (ARC) of 3-month-old male PLP rats. Third ventricle cannulation was used to identify altered sensitivity to serotonin receptor agonists and antagonists with respect to food intake.

RESULTS

Male PLP offspring consumed less food and had lower growth rates up to 3 months of age compared with Control offspring from dams fed a normal diet. In total, 97 genes were upregulated including the 5-HT5A receptor (5-HT5AR) and 149 downregulated genes in PLP rats compared with Controls. The former obesity medication fenfluramine and the 5-HT receptor agonist 5-Carboxamidotryptamine (5-CT) significantly suppressed food intake in both groups, but the PLP offspring were more sensitive to d-fenfluramine and 5-CT compared with Controls. The effect of 5-CT was antagonized by the 5-HT5AR antagonist SB699551. 5-CT also reduced NPY-induced hyperphagia in both Control and PLP rats but was more effective in PLP offspring.

CONCLUSIONS

Postnatal low protein programming of growth in rats enhances the central effects of serotonin on appetite by increasing hypothalamic 5-HT5AR expression and sensitivity. These findings provide insight into the possible mechanisms through which a maternal low protein diet during lactation programs reduced growth and appetite in offspring.

Transcription Factor CEBPB Inhibits the Expression of the Human HTR1A by Binding to 5' Regulatory Region in Vitro

This study identified a transcription factor that might bind to the 5′ regulatory region of the HTR1A and explored the potential effect on 5-HT1A receptor expression. Based on JASPAR predictions, the binding of the transcription factor was demonstrated using the electrophoretic mobility shift assay (EMSA). Vectors over-expressing the transcription factor were co-transfected into HEK-293 and SK-N-SH cells with the recombinant pGL3 vector, and relative fluorescence intensity was measured to determine regulatory activity. Additionally, the qRT-PCR and Western blot were also used to identify whether the transcription factor modulated the endogenous expression of 5-HT1A receptor. The results suggest that the transcription factor CCAA/T enhancer binding protein beta (CEBPB) likely binds to the −1219 to −1209 bp (ATG+1) region of the HTR1A. Two sequences located in the −722 to −372 bp and −119 to +99 bp were also identified. Although the effect of CEBPB on endogenous 5-HT1A receptor expression was not significant, it exhibited the strong inhibition on the relative fluorescence intensity and the mRNA level of HTR1A. CEBPB inhibited the human HTR1A expression by binding to the sequence −1219–−1209 bp. This is useful and informative for ascertaining the regulation of 5-HT1A receptor and mental diseases.

Genetic, epigenetic and posttranscriptional mechanisms for treatment of major depression: the 5-HT1A receptor gene as a paradigm

Major depression and anxiety are highly prevalent and involve chronic dysregulation of serotonin, but they remain poorly understood. Here, we review novel transcriptional (genetic, epigenetic) and posttranscriptional (microRNA, alternative splicing) mechanisms implicated in mental illness, focusing on a key serotonin-related regulator, the serotonin 1A (5-HT1A) receptor. Functional single-nucleotide polymorphisms and stress-induced DNA methylation of the 5-HT1A promoter converge to differentially alter pre- and postsynaptic 5-HT1A receptor expression associated with major depression and reduced therapeutic response to serotonergic antidepressants. Major depression is also associated with altered levels of splice factors and microRNA, posttranscriptional mechanisms that regulate RNA stability. The human 5-HT1A 3′-untranslated region is alternatively spliced, removing microRNA sites and increasing 5-HT1A expression, which is reduced in major depression and may be genotype-dependent. Thus, the 5-HT1A receptor gene illustrates the convergence of genetic, epigenetic and posttranscriptional mechanisms in gene expression, neurodevelopment and neuroplasticity, and major depression. Understanding gene regulatory mechanisms could enhance the detection, categorization and personalized treatment of major depression.

REST: An epigenetic regulator of neuronal stress responses in the young and ageing brain

The transcriptional repressor REST (Repressor Element-1 Silencing Transcription factor) is a key modulator of the neuronal epigenome and targets genes involved in neuronal differentiation, axonal growth, vesicular transport, ion channel conductance and synaptic plasticity. Whilst its gene expression-modifying properties have been examined extensively in neuronal development, REST's response towards stress-induced neuronal insults has only recently been explored. Overall, REST appears to be an ideal candidate to fine-tune neuronal gene expression following different forms of cellular, neuropathological, psychological and physical stressors. Upregulation of REST is reportedly protective against premature neural stem cell depletion, neuronal hyperexcitability, oxidative stress, neuroendocrine system dysfunction and neuropathology. In contrast, neuronal REST activation has also been linked to neuronal dysfunction and neurodegeneration. Here, we highlight key findings and discrepancies surrounding our current understanding of REST's function in neuronal adaptation to stress and explore its potential role in neuronal stress resilience in the young and ageing brain.

hsa-miR-3177-5p and hsa-miR-3178 Inhibit 5-HT1A Expression by Binding the 3'-UTR Region in vitro

Abnormal expression of the 5-HT1A receptor, which is encoded by the HTR1A gene, leads to susceptibilities to neuropsychiatric disorders such as depression, anxiety, and schizophrenia. miRNAs regulate gene expression by recognizing the 3'-UTR region of mRNA. This study evaluated the miRNAs that might identify and subsequently determine the regulatory mechanism of HTR1A gene. Using the HEK-293, U87, SK-N-SH and SH-SY5Y cell lines, we determined the functional sequence of the 3'-UTR region of the HTR1A gene and predicted miRNA binding. Dual luciferase reporter assay and Western Blot were used to confirm the effect of miRNA mimics and inhibitors on endogenous 5-HT1A receptors. In all cell lines, gene expression of the -17 bp to +443 bp fragment containing the complete sequence of the 3'-UTR region was significantly decreased, although mRNA quantification was not different. The +375 bp to +443 bp sequence, which exhibited the most significant change in relative chemiluminescence intensity, was recognized by hsa-miR-3177-5p and hsa-miR-3178. In HEK-293 and U87 cells, hsa-miR-3177-5p significantly inhibited the 5-HT1A receptor expression, while a hsa-miR-3178 inhibitor up-regulated HTR1A gene expression in SK-N-SH and SH-SY5Y cells. By constructing the pmirGLO-vector with the mutated HTR1A gene, we further confirmed that hsa-miR-3177-5p recognized the HTR1A gene tgtacaca at +377 bp to +384 bp, and the +392 bp to +399 bp fragment cgcgccca was identified by hsa-miR-3178. hsa-miR-3177-5p and hsa-miR-3178 had significant inhibitory effects on expression of the HTR1A gene and 5-HT1A receptor and may directly participate in the development of neuropsychiatric diseases.

Characterization and functional analyses of the human HTR1A gene: 5' regulatory region modulates gene expression in vitro

BACKGROUND

The serotonin neurotransmitter (5-HT) and its receptors have important roles in neuropsychiatric disorders such as schizophrenia. The aim of this study was to investigate the functional sequences of the 5' regulation region of the human HTR1A gene to explore the effects on the expression of the 5-HT1A receptor.

METHODS

Fourteen recombinant pGL3-basic vectors containing deletion fragments of the HTR1A gene regulatory region were transfected with HEK-293 and SK-N-SH cells. The relative chemiluminescence intensities of different length fragments were analyzed. The JASPAR software was used for the prediction of transcription factors.

RESULTS

In the HEK-293 cells, the relative chemiluminescence intensity of the - 1649 bp to - 1550 bp (ATG + 1) fragment was significantly different. Two inhibitory activity regions were found in the - 1409 bp to - 1381 bp and - 1196 bp to - 1124 bp fragments, which might be bound to the GATA or SOX10 transcription factors as predicted by the JASPAR software. In addition, the fragments located from - 1124 bp to - 1064 bp and from - 908 bp to - 722 bp up-regulated protein expression. Only the sequence from - 1550 bp to - 1409 bp demonstrated a difference in luciferase expression in the both cell lines. According to the results of the 5'-UTR truncated vectors, there was a repression region at the distal end of the 5'-UTR, an enhancer region might be present at the proximal end of the transcription start site.

CONCLUSIONS

Although the functional sequences of the HTR1A gene regulatory region were confirmed, the regulatory factors and functional components require further investigation.

LRRK2 G2019S Induces Anxiety/Depression-like Behavior before the Onset of Motor Dysfunction with 5-HT1A Receptor Upregulation in Mice

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson's disease (PD). The neuropathology of LRRK2 mutation-related PD, including increased dopaminergic neurodegeneration and Lewy bodies, is indistinguishable from that of idiopathic PD. The subtle nonmotor phenotypes of LRRK2 mutation-related PD have not been fully evaluated. In the present study, we examined anxiety/depression-like behaviors and accompanying neurochemical changes in differently aged transgenic (Tg) mice expressing human mutant LRRK2 G2019S. Through multiple behavioral tests, including light-dark test, elevated plus maze, sucrose preference test, forced swimming test, and tail-suspension test, we found that anxiety/depression-like behavior appeared in middle-aged (43-52 weeks) Tg mice before the onset of PD-like motor dysfunction. These behavioral tests were performed using both male and female mice, and there were no sex-related differences in behavioral changes in the middle-aged Tg mice. Along with behavioral changes, serotonin levels also significantly declined in the hippocampus of Tg mice. Additionally, increases in the expression of the 5-HT_1A receptor (5-HT_1AR) grew more significant with aging and were detected in the hippocampus, amygdala, and dorsal raphe nucleus. In vitro study using the serotonergic RN46A and hippocampal HT22 cells showed that 5-HT_1AR upregulation was related to enhanced expression of LRRK2 G2019S and was attenuated by the LRRK2 inhibitor LRRK2-IN-1. Wild-type LRRK2 had no significant effect on 5-HT_1AR transcription. The present study provides the first in vivo and in vitro evidence demonstrating abnormal regulation of 5-HT_1AR along with the manifestation of anxiety/depression-like, nonmotor symptom in PD related to LRRK2.SIGNIFICANCE STATEMENT Parkinson's disease (PD), the second most common neurodegenerative disorder, is clinically characterized by motor dysfunctions. In most cases, various nonmotor symptoms present several years before the onset of the classical motor features of PD and severely affect the quality of life of patients. Here, we demonstrate the causative role of leucine-rich repeat kinase 2 (LRRK2), a common PD-linked mutation, in the development of anxiety/depression-like behaviors. We found that age-dependent 5-HT_1A receptor upregulation in the hippocampus, amygdala, and dorsal raphe nucleus is accompanied by the expression of the LRRK2 mutant phenotype. Our findings demonstrating a potential mechanism for nonmotor psychiatric symptoms produced by LRRK2 mutation suggest that directly targeting the 5-HT_1A receptor can improve the therapeutic efficacy of drugs for PD-associated depression.

Abrogated Freud-1/Cc2d1a Repression of 5-HT1A Autoreceptors Induces Fluoxetine-Resistant Anxiety/Depression-Like Behavior

Freud-1/Cc2d1a represses the gene transcription of serotonin-1A (5-HT1A) autoreceptors, which negatively regulate 5-HT tone. To test the role of Freud-1 in vivo, we generated mice with adulthood conditional knock-out of Freud-1 in 5-HT neurons (cF1ko). In cF1ko mice, 5-HT1A autoreceptor protein, binding and hypothermia response were increased, with reduced 5-HT content and neuronal activity in the dorsal raphe. The cF1ko mice displayed increased anxiety- and depression-like behavior that was resistant to chronic antidepressant (fluoxetine) treatment. Using conditional Freud-1/5-HT1A double knock-out (cF1/1A dko) to disrupt both Freud-1 and 5-HT1A genes in 5-HT neurons, no increase in anxiety- or depression-like behavior was seen upon knock-out of Freud-1 on the 5-HT1A autoreceptor-negative background; rather, a reduction in depression-like behavior emerged. These studies implicate transcriptional dysregulation of 5-HT1A autoreceptors by the repressor Freud-1 in anxiety and depression and provide a clinically relevant genetic model of antidepressant resistance. Targeting specific transcription factors, such as Freud-1, to restore transcriptional balance may augment response to antidepressant treatment.SIGNIFICANCE STATEMENT Altered regulation of the 5-HT1A autoreceptor has been implicated in human anxiety, major depression, suicide, and resistance to antidepressants. This study uniquely identifies a single transcription factor, Freud-1, as crucial for 5-HT1A autoreceptor expression in vivo Disruption of Freud-1 in serotonin neurons in mice links upregulation of 5-HT1A autoreceptors to anxiety/depression-like behavior and provides a new model of antidepressant resistance. Treatment strategies to reestablish transcriptional regulation of 5-HT1A autoreceptors could provide a more robust and sustained antidepressant response.

Functional characterization of the neuron-restrictive silencer element in the human tryptophan hydroxylase 2 gene expression

Tryptophan hydroxylase 2 (TPH2) is the key enzyme in the synthesis of neuronal serotonin. Although previous studies suggest that TPH2 neuron-restrictive silencer element (NRSE) functions as a negative regulator dependent on neuron-restrictive silencer factor (NRSF) activity, the underlying mechanisms are yet to be fully elucidated. Here, we show a detailed analysis of the NRSE-mediated repression of the human TPH2 (hTPH2) promoter activity in RN46A cells, a cell line derived from rat raphe neurons. Quantitative real-time RT-PCR analysis revealed the expression of serotonergic marker genes (Mash1, Nkx2.2, Gata2, Gata3, Lmx1b, Pet-1, 5-Htt, and Vmat2) and Nrsf gene in RN46A cells. Tph1 mRNA is the prevalent form expressed in RN46A cells; Tph2 mRNA is also expressed but at a lower level. Electrophoretic mobility shift assays and reporter assays showed that hTPH2 NRSE is necessary for the efficient DNA binding of NRSF and for the NRSF-dependent repression of the hTPH2 promoter activity. The hTPH2 promoter activity was increased by knockdown of NRSF, or over-expression of the engineered NRSF (a dominant-negative mutant or a DNA-binding domain and activation domain fusion protein). MS-275, a class I histone deacetylase (HDAC) inhibitor, was found to be more potent than MC-1568, a class II HDAC inhibitor, in enhancing the hTPH2 promoter activity. Furthermore, treatment with the ubiquitin-specific protease 7 deubiquitinase inhibitors, P-22077 or HBX 41108, increased the hTPH2 promoter activity. Collectively, our data demonstrate that the hTPH2 NRSE-mediated promoter repression via NRSF involves class I HDACs and is modulated by the ubiquitin-specific protease 7-mediated deubiquitination and stabilization of NRSF.

Recruitment by the Repressor Freud-1 of Histone Deacetylase-Brg1 Chromatin Remodeling Complexes to Strengthen HTR1A Gene Repression

Five-prime repressor element under dual repression binding protein-1 (Freud-1)/CC2D1A is genetically linked to intellectual disability and implicated in neuronal development. Freud-1 represses the serotonin-1A (5-HT1A) receptor gene HTR1A by histone deacetylase (HDAC)-dependent or HDAC-independent mechanisms in 5-HT1A-negative (e.g., HEK-293) or 5-HT1A-expressing cells (SK-N-SH), respectively. To identify the underlying mechanisms, Freud-1-associated proteins were affinity-purified from HEK-293 nuclear extracts and members of the Brg1/SMARCCA chromatin remodeling and Sin3A-HDAC corepressor complexes were identified. Pull-down assays using recombinant proteins showed that Freud-1 interacts directly with the Brg1 carboxyl-terminal domain; interaction with Brg1 required the carboxyl-terminal of Freud-1. Freud-1 complexes in HEK-293 and SK-N-SH cells differed, with low levels of BAF170/SMARCC2 and BAF57/SMARCE1 in HEK-293 cells and low-undetectable BAF155/SMARCC1, Sin3A, and HDAC1/2 in SK-N-SH cells. Similarly, by quantitative chromatin immunoprecipitation, Brg1-BAF170/57 and Sin3A-HDAC complexes were observed at the HTR1A promoter in HEK-293 cells, whereas in SK-N-SH cells, Sin3A-HDAC proteins were not detected. Quantifying 5-HT1A receptor mRNA levels in cells treated with siRNA to Freud-1, Brg1, or both RNAs addressed the functional role of the Freud-1-Brg1 complex. In HEK-293 cells, 5-HT1A receptor mRNA levels were increased only when both Freud-1 and Brg1 were depleted, but in SK-N-SH cells, depletion of either protein upregulated 5-HT1A receptor RNA. Thus, recruitment by Freud-1 of Brg1, BAF155, and Sin3A-HDAC complexes appears to strengthen repression of the HTR1A gene to prevent its expression inappropriate cell types, while recruitment of the Brg1-BAF170/57 complex is permissive to 5-HT1A receptor expression. Alterations in Freud-1-Brg1 interactions in mutants associated with intellectual disability could impair gene repression leading to altered neuronal development.

Genetic Variations in the Serotonergic System Mediate a Combined, Weakened Response to SSRI Treatment: A Proposed Model

Individuals with the short (S) allele in the promoter region of the serotonin transporter gene (5-HTTLPR) show a less favorable response to selective serotonin reuptake inhibitor (SSRI) treatment than individuals with the long (L) allele. Similarly, individuals with the C(-1019)G allele for the mutation found in the promoter region of the serotonin 1A receptor gene (5-HTR1A) have shown blunted responses to SSRI treatment when compared with individuals lacking this polymorphism. While these findings have been replicated across multiple studies, only two studies to date have reported data for a gene-gene interaction associated with response to SSRI treatment. Both of these studies reported a combined effect for these genotypes, with individuals homozygous for the L allele and the C allele (5-HTT(L/L)-1A(C/C)) reporting the most favorable response to SSRI treatment, and individuals homozygous for the S allele and the G allele (5-HTT(S/S)-1A(G/G)) reporting the least favorable response to SSRI treatment. Additionally, no neural mechanisms have been proposed to explain why this gene-gene interaction has been observed. To that end, this article provides a review of the relevant literature associated with these polymorphisms and proposes a feasible model that describes a genotype-dependent modulation of postsynaptic serotonin signaling associated with the 5-HTT and 5-HTR1A genes.

Transcriptional dys-regulation in anxiety and major depression: 5-HT1A gene promoter architecture as a therapeutic opportunity

The etiology of major depression remains unclear, but reduced activity of the serotonin (5-HT) system remains implicated and treatments that increase 5-HT neurotransmission can ameliorate depressive symptoms. 5-HT1A receptors are critical regulators of the 5- HT system. They are expressed as both presynaptic autoreceptors that negatively regulate 5-HT neurons, and as post-synaptic heteroreceptors on non-serotonergic neurons in the hippocampus, cortex, and limbic system that are critical to mediate the antidepressant actions of 5-HT. Thus, 5-HT1A auto- and heteroreceptors have opposite actions on serotonergic neurotransmission. Because most 5-HT1A ligands target both auto- and heteroreceptors their efficacy has been limited, resulting in weak or unclear responses. We propose that by understanding the transcriptional regulation of the 5-HT1A receptor it may be possible to regulate its expression differentially in raphe and projection regions. Here we review the transcriptional architecture of the 5-HT1A gene (HTR1A) with a focus on specific DNA elements and transcription factors that have been shown to regulate 5-HT1A receptor expression in the brain. Association studies with the functional HTR1A promoter polymorphism rs6295 suggest a new model for the role of the 5-HT1A receptor in susceptibility to depression involving early deficits in cognitive, fear and stress reactivity as stressors that may ultimately lead to depression. We present evidence that by targeting specific transcription factors it may be possible to oppositely regulate 5-HT1A auto- and heteroreceptor expression, synergistically increasing serotonergic neurotransmission for the treatment of depression.

Transcriptional regulation of the 5-HT1A receptor: implications for mental illness

The serotonin-1A (5-HT(1A)) receptor is an abundant post-synaptic 5-HT receptor (heteroreceptor) implicated in regulation of mood, emotion and stress responses and is the major somatodendritic autoreceptor that negatively regulates 5-HT neuronal activity. Based on animal models, an integrated model for opposing roles of pre- and post-synaptic 5-HT(1A) receptors in anxiety and depression phenotypes and response to antidepressants is proposed. Understanding differential transcriptional regulation of pre- versus post-synaptic 5-HT(1A) receptors could provide better tools for their selective regulation. This review examines the transcription factors that regulate brain region-specific basal and stress-induced expression of the 5-HT(1A) receptor gene (Htr1a). A functional polymorphism, rs6295 in the Htr1a promoter region, blocks the function of specific repressors Hes1, Hes5 and Deaf1, resulting in increased 5-HT(1A) autoreceptor expression in animal models and humans. Its association with altered 5-HT(1A) expression, depression, anxiety and antidepressant response are related to genotype frequency in different populations, sample homogeneity, disease outcome measures and severity. Preliminary evidence from gene × environment studies suggests the potential for synergistic interaction of stress-mediated repression of 5-HT(1A) heteroreceptors, and rs6295-induced upregulation of 5-HT(1A) autoreceptors. Targeted therapeutics to inhibit 5-HT(1A) autoreceptor expression and induce 5-HT(1A) heteroreceptor expression may ameliorate treatment of anxiety and major depression.

Increased serotonin-1A (5-HT1A) autoreceptor expression and reduced raphe serotonin levels in deformed epidermal autoregulatory factor-1 (Deaf-1) gene knock-out mice

Altered regulation of the serotonin-1A (5-HT1A) receptor gene is implicated in major depression and mood disorders. The functional human 5-HT1A C(-1019)G promoter polymorphism (rs6295), which prevents the binding of Deaf-1/NUDR leading to dysregulation of the receptor, has been associated with major depression. In cell models Deaf-1 displays dual activity, repressing 5-HT1A autoreceptor expression in serotonergic raphe cells while enhancing postsynaptic 5-HT1A heteroreceptor expression in nonserotonergic neurons. A functional Deaf-1 binding site on the mouse 5-HT1A promoter was recognized by Deaf-1 in vitro and in vivo and mediated dual activity of Deaf-1 on 5-HT1A gene transcription. To address regulation by Deaf-1 in vivo, Deaf-1 knock-out mice bred to a C57BL/6 background were compared with wild-type siblings for changes in 5-HT1A RNA and protein by quantitative RT-PCR, in situ hybridization, and immunofluorescence. In the dorsal raphe, Deaf-1 knock-out mice displayed increased 5-HT1A mRNA, protein, and 5-HT1A-positive cell counts but reduced 5-HT levels, whereas other serotonergic markers, such as tryptophan hydroxylase (TPH)- or 5-HT-positive cells and TPH2 RNA levels, were unchanged. By contrast, 5-HT1A mRNA and 5-HT1A-positive cells were reduced in the frontal cortex of Deaf-1-null mice, with no significant change in hippocampal 5-HT1A RNA, protein, or cell counts. The region-specific alterations of brain 5-HT1A gene expression and reduced raphe 5-HT content in Deaf-1(-/-) mice indicate the importance of Deaf-1 in regulation of 5-HT1A gene expression and provide insight into the role of the 5-HT1A G(-1019) allele in reducing serotonergic neurotransmission by derepression of 5-HT1A autoreceptors.

Transcriptional dysregulation of 5-HT1A autoreceptors in mental illness

The serotonin-1A (5-HT1A) receptor is among the most abundant and widely distributed 5-HT receptors in the brain, but is also expressed on serotonin neurons as an autoreceptor where it plays a critical role in regulating the activity of the entire serotonin system. Over-expression of the 5-HT1A autoreceptor has been implicated in reducing serotonergic neurotransmission, and is associated with major depression and suicide. Extensive characterization of the transcriptional regulation of the 5-HT1A gene (HTR1A) using cell culture systems has revealed a GC-rich "housekeeping" promoter that non-selectively drives its expression; this is flanked by a series of upstream repressor elements for REST, Freud-1/CC2D1A and Freud-2/CC2D1B factors that not only restrict its expression to neurons, but may also regulate the level of expression of 5-HT1A receptors in various subsets of neurons, including serotonergic neurons. A separate set of allele-specific factors, including Deaf1, Hes1 and Hes5 repress at the HTR1A C(-1019)G (rs6295) polymorphism in serotonergic neurons in culture, as well as in vivo. Pet1, an obligatory enhancer for serotonergic differentiation, has been identified as a potent activator of 5-HT1A autoreceptor expression. Taken together, these results highlight an integrated regulation of 5-HT1A autoreceptors that differs in several aspects from regulation of post-synaptic 5-HT1A receptors, and could be selectively targeted to enhance serotonergic neurotransmission.

Region-specific regulation of 5-HT1A receptor expression by Pet-1-dependent mechanisms in vivo

Serotonin (5-hydroxytryptamine, 5-HT) neurotransmission is negatively regulated by 5-HT1A autoreceptors on raphe neurons, and is implicated in mood disorders. Pet-1/FEV is an ETS transcription factor expressed exclusively in serotonergic neurons and is essential for serotonergic differentiation, although its regulation of 5-HT receptors has not yet been studied. Here, we show by electrophoretic mobility shift assay that recombinant human Pet-1/FEV binds directly to multiple Pet-1 elements of the human 5-HT1A receptor promoter to enhance its transcriptional activity. In luciferase reporter assays, mutational analysis indicated that while several sites contribute, the Pet-1 site at -1406 bp had the greatest effect on 5-HT1A promoter activity. To address the effect of Pet-1 on 5-HT1A receptor regulation in vivo, we compared the expression of 5-HT1A receptor RNA and protein in Pet-1 null and wild-type littermate mice. In the raphe nuclei of Pet-1-/- mice tryptophan hydroxylase 2 (TPH2) RNA, and 5-HT and TPH immunostaining were greatly reduced, indicating a deficit in 5-HT production. Raphe 5-HT1A RNA and protein levels were also reduced in Pet-1-deficient mice, consistent with an absence of Pet-1-mediated transcriptional enhancement of 5-HT1A autoreceptors in serotonergic neurons. Interestingly, 5-HT1A receptor expression was up-regulated in the hippocampus, but down-regulated in the striatum and cortex. These data indicate that, in addition to transcriptional regulation by Pet-1 in raphe neurons, 5-HT1A receptor expression is regulated indirectly by alterations in 5-HT neurotransmission in a region-specific manner that together may contribute to the aggressive/anxiety phenotype observed in Pet-1 null mice.

Freud-2/CC2D1B mediates dual repression of the serotonin-1A receptor gene

The serotonin-1A (5-HT1A) receptor functions as a pre-synaptic autoreceptor in serotonin neurons that regulates their activity, and is also widely expressed on non-serotonergic neurons as a post-synaptic heteroreceptor to mediate serotonin action. The 5-HT1A receptor gene is strongly repressed by a dual repressor element (DRE), which is recognized by two proteins: Freud-1/CC2D1A and another unknown protein. Here we identify mouse Freud-2/CC2D1B as the second repressor of the 5-HT1A-DRE. Freud-2 shares 50% amino acid identity with Freud-1, and contains conserved structural domains. Mouse Freud-2 bound specifically to the rat 5-HT1A-DRE adjacent to, and partially overlapping, the Freud-1 binding site. By supershift assay using nuclear extracts from L6 myoblasts, Freud-2-DRE complexes were distinguished from Freud-1-DRE complexes. Freud-2 mRNA and protein were detected throughout mouse brain and peripheral tissues. Freud-2 repressed 5-HT1A promoter-reporter constructs in a DRE-dependent manner in non-neuronal (L6) or 5-HT1A-expressing neuronal (NG108-15, RN46A) cell models. In NG108-15 cells, knockdown of Freud-2 using a specific short-interfering RNA reduced endogenous Freud-2 protein levels and decreased Freud-2 bound to the 5-HT1A-DRE as detected by chromatin immunoprecipitation assay, but increased 5-HT1A promoter activity and 5-HT1A protein levels. Taken together, these data show that Freud-2 is the second component that, with Freud-1, mediates dual repression of the 5-HT1A receptor gene at the DRE.

Modifying 5-HT1A Receptor Gene Expression as a New Target for Antidepressant Therapy

Major depression is the most common form of mental illness, and is treated with antidepressant compounds that increase serotonin (5-HT) neurotransmission. Increased 5-HT1A autoreceptor levels in the raphe nuclei act as a “brake” to inhibit the 5-HT system, leading to depression and resistance to antidepressants. Several 5-HT1A receptor agonists (buspirone, flesinoxan, ipsapirone) that preferentially desensitize 5-HT1A autoreceptors have been tested for augmentation of antidepressant drugs with mixed results. One explanation could be the presence of the C(−1019)G 5-HT1A promoter polymorphism that prevents gene repression of the 5-HT1A autoreceptor. Furthermore, down-regulation of 5-HT1A autoreceptor expression, not simply desensitization of receptor signaling, appears to be required to enhance and accelerate antidepressant action. The current review focuses on the transcriptional regulators of 5-HT1A autoreceptor expression, their roles in permitting response to 5-HT1A-targeted treatments and their potential as targets for new antidepressant compounds for treatment-resistant depression.

Aberrant REST-mediated transcriptional regulation in major depressive disorder

There is growing evidence that aberrant transcriptional regulation is one of the key components of the pathophysiology of mood disorders. The repressor element-1 silencing transcription factor (REST) is a negative regulator of genes that contain the repressor element-1 (RE-1) binding site. REST has many target genes, including corticotropin releasing hormone (CRH), brain-derived neurotrophic factor, serotonin 1A receptor, which are suggested to be involved in the pathophysiology of depression and the action of antidepressants. However, a potential role for REST-mediated transcriptional regulation in mood disorders remains unclear. In this study, we examined the mRNA levels of REST and its known and putative target genes, using quantitative real-time PCR in peripheral blood cells of patients with major depressive and bipolar disorders in both a current depressive and a remissive state. We found reduced mRNA expression of REST and increased mRNA expression of CRH, adenylate cyclase 5, and the tumor necrosis factor superfamily, member 12-13 in patients with major depressive disorder in a current depressive state, but not in a remissive state. Altered expression of these mRNAs was not found in patients with bipolar disorder. Our results suggest that the aberrant REST-mediated transcriptional regulation of, at least, CRH, adenylate cyclase 5, and tumor necrosis factor superfamily, member 12-13, might be state-dependent and associated with the pathophysiology of major depression.

Chronic social defeat downregulates the 5-HT1A receptor but not Freud-1 or NUDR in the rat prefrontal cortex

The serotonin 1A receptor (5-HT1A) and its associated transcriptional regulators, five prime repressor element under dual repression (Freud-1) and nuclear-deformed epidermal autoregulatory factor (NUDR/Deaf-1) have been previously found to be the repressors for 5-HT1A in the serotonergic raphe neurons, and are also altered in postmortem brains of individuals with major depressive disorder (MDD) and in rats exposed to chronic restraint stress. We sought to find out if rats exposed to chronic social defeat (CSD) stress also show altered expression of these genes. Adult male Wistar rats were exposed to CSD stress for four consecutive weeks following which they were sacrificed and gene expression assessed in the prefrontal cortex (PFC) by quantitative real-time polymerase chain reaction. While CSD had no significant effects on NUDR and Freud-1 mRNA levels, 5-HT1A mRNA levels were significantly downregulated in defeated animals. The data suggest that regulatory factors other than Freud-1 and NUDR may be involved in the regulation of 5-HT1A expression in PFC during CSD stress. Furthermore, decreased levels of 5-HT1A following social defeat in the PFC are consistent with human postmortem results for this receptor in major depression and demonstrate the possibility that this receptor is involved in the pathophysiology of depression and other stress related disorders.

Transcriptional expression of serotonergic regulators in laser-captured microdissected dorsal raphe neurons of subjects with major depressive disorder: sex-specific differences

The relationship between serotonin (5-HT) and major depressive disorder (MDD) has been extensively studied but certain aspects are still ambiguous. Given the evidence that 5-HT neurotransmission is reduced in depressed subjects, it is possible that one or more of the 5-HT regulators may be altered in the dorsal raphe nucleus (DR) of depressed subjects. Candidates that regulate 5-HT synthesis and neuronal activity of 5-HT neurons include intrinsic regulators such as tryptophan hydroxylase 2, 5-HT autoreceptors, 5-HT transporter and transcription factors, as well as afferent regulators such as estrogen and brain-derived neurotrophic factor. The present study was designed to quantify mRNA concentrations of the above 5-HT regulators in an isolated population of 5-HT-containing DR neurons of MDD subjects and gender-matched psychiatrically normal control subjects. We found that mRNA concentrations of the 5-HT1D receptor and the transcription factors, NUDR and REST, were significantly increased in DR-captured neurons of female MDD subjects compared to female control subjects. No significant differences were found for the transcripts in male MDD subjects compared to male controls. This study reveals sex-specific alterations in gene expression of the pre-synaptic 5-HT1D autoreceptors and 5-HT-related transcription factors, NUDR and REST, in DR neurons of women with MDD.

Human Freud-2/CC2D1B: a novel repressor of postsynaptic serotonin-1A receptor expression

BACKGROUND

Altered expression of serotonin-1A (5-HT1A) receptors, both presynaptic in the raphe nuclei and post-synaptic in limbic and cortical target areas, has been implicated in mood disorders such as major depression and anxiety. Within the 5-HT1A receptor gene, a powerful dual repressor element (DRE) is regulated by two protein complexes: Freud-1/CC2D1A and a second, unknown repressor. Here we identify human Freud-2/CC2D1B, a Freud-1 homologue, as the second repressor.

METHODS

Freud-2 distribution was examined with Northern and Western blot, reverse transcriptase polymerase chain reaction, and immunohistochemistry/immunofluorescence; Freud-2 function was examined by electrophoretic mobility shift, reporter assay, and Western blot.

RESULTS

Freud-2 RNA was widely distributed in brain and peripheral tissues. Freud-2 protein was enriched in the nuclear fraction of human prefrontal cortex and hippocampus but was weakly expressed in the dorsal raphe nucleus. Freud-2 immunostaining was co-localized with 5-HT1A receptors, neuronal and glial markers. In prefrontal cortex, Freud-2 was expressed at similar levels in control and depressed male subjects. Recombinant hFreud-2 protein bound specifically to 5' or 3' human DRE adjacent to the Freud-1 site. Human Freud-2 showed strong repressor activity at the human 5-HT1A or heterologous promoter in human HEK-293 5-HT1A-negative cells and neuronal SK-N-SH cells, a model of postsynaptic 5-HT1A receptor-positive cells. Furthermore, small interfering RNA knockdown of endogenous hFreud-2 expression de-repressed 5-HT1A promoter activity and increased levels of 5-HT1A receptor protein in SK-N-SH cells.

CONCLUSIONS

Human Freud-2 binds to the 5-HT1A DRE and represses the human 5-HT1A receptor gene to regulate its expression in non-serotonergic cells and neurons.

RE1 silencing transcription factor is involved in regulating neuron-specific expression of alpha-internexin and neurofilament genes

Alpha-internexin and the neurofilament triplet proteins (NF-L, NF-M, and NF-H) co-assemble into intermediate filament networks in neurons. We have found that the RE1 silencing transcription factor (REST) plays a contributory role in the neuron-specific expression of the alpha-internexin, NF-H and NF-M genes. Chromatin immunoprecipitation and transient transfection experiments performed with catecholaminergic neuronal Cath a.-differentiated (CAD) cells and non-neuronal NIH3T3 cells demonstrated that REST repressed transcription of these genes in NIH3T3 cells by binding and recruiting mSin3A, CoREST, histone deacetylase (HDAC) 1 and MeCP2 to the RE1 sites in the intron-1 of alpha-internexin and the 5' flanking regions of NF-H and NF-M. No repression effect of the RE1 sites was observed in CAD cells, which express these neuronal genes but not REST. Treatment of NIH3T3 cells with trichostatin A, a HDAC inhibitor, relieved the REST-mediated repression and induced ectopic activation of alpha-internexin, NF-H and NF-M. The trichostatin A treatment did not affect the levels of REST occupancy but caused coordinated changes in acetylation and methylation of histones around the RE1 sites of these genes in NIH3T3 cells consistent with a transition from transcriptional repression to transcriptional activation. Thus, REST regulates expression of these neuronal genes, partly by a HDAC-dependent epigenetic mechanism.

The Freud-1/CC2D1A family: transcriptional regulators implicated in mental retardation

The CC2D1A gene family consists of two homologous genes, Freud-1/CC2D1A and Freud-2/CC2D1B, that share conserved domains, including several DM14 domains that are specific to this protein family, a C-terminal helix-loop-helix domain, and a C2 calcium-dependent phospholipid binding domain. Although the function of Freud-2 is unknown, Freud-1 has been shown to function as a transcriptional repressor of the serotonin-1A receptor gene that binds to a novel DNA element (FRE, 5'-repressor element). The DNA binding and repressor activities of Freud-1 are inhibited by calcium-calmodulin-dependent protein kinase. Recently, a deletion in the CC2D1A gene has been linked to nonsyndromic mental retardation. This deletion results in the truncation of the helix-loop-helix DNA binding and the C2 domains, crucial for Freud-1 repressor activity, and hence is predicted to generate an inactive or weakly dominant negative protein. The possible mechanisms by which inactivation of Freud-1 could lead to abnormal cortical development and cognitive impairment and the potential roles of Freud-1 gene targets are discussed.

The mental retardation gene CC2D1A/Freud-1 encodes a long isoform that binds conserved DNA elements to repress gene transcription

The CC2D1A/Freud-1 gene has recently been linked to non-syndromic mental retardation and a short isoform of mouse Five prime REpressor Under Dual repression binding protein 1 (Freud-1) can repress the serotonin-1A (5-HT1A) receptor gene in rodent cells. In this study, we addressed the expression, localization and regulation of the human 5-HT1A receptor gene by a long isoform of human Freud-1 protein (Freud-1L). We show that human CC2D1A/Freud-1 RNA is expressed in brain and peripheral tissues and encodes short and long isoforms, which differ by an upstream in-frame translational start site. Whereas previous studies identified the short isoform of Freud-1 as the predominant isoform in rodent cells, we demonstrate that the long isoform is more abundant in human cells, especially in the nuclear fraction. The nuclear localization of Freud-1L was enriched upon inhibition of chromosome region maintenance 1/exportin 1-dependent nuclear export, indicating a dynamic regulation of Freud-1 nuclear localization. Consistent with a functional role in the nucleus, human Freud-1L bound specifically to its dual repressor element in the 5-HT1A receptor gene in vitro and repressed transcription from these sites. Importantly, chromatin immunoprecipitation using antibodies specific for human Freud-1L demonstrated that it is bound to the dual repressor element in chromatin, indicating a functional role in regulating the basal expression of the 5-HT1A receptor gene. Taken together, these results indicate that both the short and long isoforms of Freud-1 are expressed, although Freud-1L is the major isoform that regulates the human 5-HT1A receptor gene. Disruption of transcriptional regulation by mutation of Freud-1 may play a role in abnormal brain function leading to mental retardation.

Regulation of tryptophan hydroxylase-2 gene expression by a bipartite RE-1 silencer of transcription/neuron restrictive silencing factor (REST/NRSF) binding motif

Tryptophan hydroxylase-2 (TPH2) is the rate-limiting enzyme in raphe serotonin biosynthesis, and polymorphisms of TPH2 are implicated in vulnerability to psychiatric disorders. Dynamic transcription regulation of TPH2 may underlie differences in vulnerability. We identified a transcription element in the TPH2 promoter that resembles the binding motif for RE-1 silencer of transcription (REST; also known as NRSF) transcription factor. REST limits tissue expression of non-neuronal genes through a canonical 21-bp motif called the NRSE (neuron-restrictive silencing element). The NRSE in TPH2 is a novel bipartite variant interrupted by a 6-base insertion. We confirmed that this bipartite NRSE permits transcriptional repression by REST identical to canonical NRSE in rat C6-glioma cells. Synthetic permutations of the motif revealed considerable flexibility in the juxtaposition of the two halves of bipartite NRSE. Computational analysis revealed many bipartite NRSE variants conserved between mouse and human genomes. A subgroup of these was found to bind REST by chromatin immunoprecipitation. Messenger RNAs for TPH2 and potassium channel H6, another gene with a bipartite NRSE, were up-regulated by dominant-negative REST in C6-glioma cells. These findings, which indicate that TPH2 expression is part of the developmental program regulated by REST and suggest that many previously unrecognized genes may be regulated by REST through the novel motif, have implications for the mechanism of REST action.

Differential Repression by Freud-1/CC2D1A at a Polymorphic Site in the Dopamine-D2 Receptor Gene

Freud-1/CC2D1A is a transcriptional repressor of the serotonin-1A receptor gene and was recently genetically linked to non-syndromic mental retardation. To identify new Freud-1 gene targets, data base mining for Freud-1 recognition sequences was done. A highly homologous intronic element (D2-DRE) was identified in the human dopamine-D2 receptor (DRD2) gene, and the role of Freud-1 in regulating the gene at this site was assessed. Recombinant Freud-1 bound specifically to the D2-DRE, and a major protein-D2-DRE complex was identified in nuclear extracts that was supershifted using Freud-1-specific antibodies. Endogenous Freud-1 binding to the D2-DRE in cells was detected using chromatin immunoprecipitation. The D2-DRE conferred strong repressor activity in transcriptional reporter assays that was dependent on the Freud-1 recognition sequence. In three different human cell lines, the level of Freud-1 protein was inversely related to DRD2 expression. Knockdown of endogenous Freud-1 using small interfering RNA resulted in an up-regulation of DRD2 RNA and binding sites, demonstrating a crucial role for Freud-1 in DRD2 regulation. A previously uncharacterized single nucleotide A/G polymorphism (rs2734836) was located adjacent to the D2-DRE and conferred allele-specific Freud-1 binding and repression, with the major G-allele having reduced activity. These studies demonstrate a key role for Freud-1 to regulate DRD2 expression and provide the first mechanistic insights into its transcriptional regulation. Allele-specific regulation of DRD2 expression by Freud-1 may possibly associate with psychiatric disorders or mental retardation.

Regulation of human tyrosine hydroxylase gene by neuron-restrictive silencer factor

Tyrosine hydroxylase (TH), the biosynthetic enzyme of catecholamine, is synthesized specifically in catecholaminergic neurons. Thus, it is possible that neuronal cell type-specific expression of this gene is coordinately regulated. One of the neuron-specific transcription regulators, neuron-restrictive silencer factor (NRSF)/repressor element 1 (RE1) silencing transcription factor (REST), represses the expression of neuronal genes in non-neuronal cells. To elucidate the molecular mechanisms that control catecholaminergic neuronal expression of human TH, we initially characterized the 5' regulatory region. Previous studies have shown that a 3174 bp fragment of the human TH promoter confers specific expression to the reporter gene in dopaminergic neuron-like cell lines. Within this 5' regulatory region, three putative neuron-restrictive silencer elements (NRSE)/RE1 were identified, which bound NRSF/REST in a sequence-specific manner, as confirmed using EMSA and ChIP assays. In transient transfection assays, deletion or mutation of NRSE/RE1 elements led to a 7-fold increase in activity of the 3.2 kb TH promoter in human neural stem cells (NSCs), but had no major effects on differentiated neuron-like cells. Suppression of NRSF/REST functions with either the histone deacetylase inhibitor, trichostatin, or DN-NRSF induced TH promoter activity. Our data strongly suggest that NRSF/REST functions as a repressor of TH transcription in NSCs via a mechanism dependent on the TH NRSE/RE1 sites.

Cell-specific repressor or enhancer activities of Deaf-1 at a serotonin 1A receptor gene polymorphism

The serotonin-1A (5-HT1A) receptor is the primary somatodendritic autoreceptor that inhibits the activity of serotonergic raphe neurons and is also expressed in nonserotonergic cortical and limbic neurons. Alterations in 5-HT1A receptor levels are implicated in mood disorders, and a functional C(-1019)G 5-HT1A promoter polymorphism has been associated with depression, suicide, and panic disorder. We examined the cell-specific activity of identified transcription factors, human nuclear deformed epidermal autoregulatory factor-1 (DEAF-1)-related (NUDR)/Deaf-1 and Hes5, at the 5-HT1A C(-1019) site. In serotonergic raphe RN46A cells, Deaf-1 and Hes5 repressed the 5-HT1A receptor gene at the C(-1019)-allele but not the G(-1019)-allele. However, in nonserotonergic cells that express 5-HT1A receptors (septal SN48, neuroblastoma SKN-SH, and neuroblastoma/glioma NG108-15 cells), Deaf-1 enhanced 5-HT1A promoter activity at the C(-1019)-allele but not the G-allele, whereas Hes5 repressed in all cell types. The enhancer activity of Deaf-1 was orientation independent and competed out Hes5 repression. To test whether Deaf-1 activity is intrinsic, the activity of a Gal4DBD (DNA binding domain)-Deaf-1 fusion protein at a heterologous Gal4 DNA element was examined. Gal4DBD-Deaf-1 repressed transcription in RN46A cells but enhanced transcription in SN48 cells, indicating that these opposite activities are intrinsic to Deaf-1. Repressor or enhancer activities of Deaf-1 or Gal4DBD-Deaf-1 were blocked by histone deacetylase inhibitor trichostatin A. Thus, the intrinsic activity of Deaf-1 at the 5-HT1A promoter is opposite in presynaptic versus postsynaptic neuronal cells and requires deacetylation. Cell-specific regulation by Deaf-1 could underlie region-specific alterations in 5-HT1A receptor expression in different mood disorders.

A differentially autoregulated Pet-1 enhancer region is a critical target of the transcriptional cascade that governs serotonin neuron development

The Pet-1 [pheochromocytoma 12 ETS (E26 transformation-specific)] gene plays a critical role in the development of serotonin (5-HT)-modulated behaviors via its control of embryonic 5-HT neuron differentiation. Pet-1 transcription is induced exclusively in 5-HT neuron postmitotic precursors before the appearance of transmitter, and its restricted expression is maintained in the adult. However, the mechanisms that direct Pet-1 expression to this single CNS neuronal cell type are unknown. Here, we show, using transgenic methods, that genomic sequences upstream, but not downstream or within the Pet-1-coding region, are sufficient for 5-HT neuron-specific transgene expression. Enhancer sequences within a 40 kb upstream fragment directed position-independent lacZ (beta-D-galactosidase) transgene expression to the developing hindbrain before the appearance of 5-HT. Moreover, virtually all of the 5-HT neurons in the adult were lacZ positive in all of the lines examined. Transgene expression in 5-HT neurons was maintained when the 40 kb fragment was truncated on its 5' end to either 12 or 1.8 kb, although position independence was then lost. Analysis of transgene expression in Pet-1 null mice indicated that Pet-1 was required to maintain the activity of the Pet-1 enhancer region in a subset of 5-HT neurons. These findings suggest that a conserved 1.8 kb region immediately flanking the Pet-1-coding region is a critical genomic target of the transcriptional cascade that governs 5-HT neuron development and provide additional evidence for 5-HT neuron heterogeneity at the genetic level. We discuss the potential application of the Pet-1 transgenes reported here to the selective genetic manipulation of 5-HT neurons.

5-HT1A receptors, gene repression, and depression: guilt by association

The serotonin system is implicated in major depression and suicide and is negatively regulated by somatodendritic 5-HT1A autoreceptors. Desensitization of 5-HT1A autoreceptors is implicated in the 2- to 3-week latency for antidepressant treatments. Alterations in 5-HT1A receptor levels are reported in depression and suicide, and gene knockout of the 5-HT1A receptor results in an anxiety phenotype, suggesting that abnormal transcriptional regulation of this receptor gene may underlie these disorders. The 5-HT1A receptor gene is negatively regulated in neurons by repressors including REST/NRSF, Freud-1, NUDR/Deaf-1, and Hes5. The association with major depression, suicide, and panic disorder of a new functional 5-HT1A polymorphism at C(-1019)G that selectively blocks repression of the 5-HT1A autoreceptor by NUDR further suggests a causative role for altered regulation of this receptor in predisposition to mental illness. The authors review evidence that altered transcription of the 5-HT1A receptor can affect the serotonin system and limbic and cortical areas, leading to predisposition to depression.