Transcriptional repression of the 5-HT1A receptor promoter by corticosterone via mineralocorticoid receptors depends on the cellular context

GSK4716 enhances 5-HT1AR expression by glucocorticoid receptor signaling in hippocampal HT22 cells

OBJECTIVES

The serotonin (5-hydroxytryptamine, 5-HT) receptor 1A (5-HT1AR) is closely associated with serotonergic neurotransmission in the brain, being the most prevalent and widely distributed receptor of its kind. The purpose of this study is to investigate the regulation mechanism of 5-HT1AR by GSK4716.

METHODS

To investigate the mechanism of GSK4716-mediated 5-HT1AR regulation, we used hippocampus-derived HT22 cells expressing 5-HT1AR. The expression level of 5-HT1AR and associated proteins, were detected by reporter gene assay and western blotting.

RESULTS

GSK4716, an estrogen-related receptor gamma agonist increased 5-HT1AR expression by interacting with the GR, a repressor of 5-HT1AR transcription. Dexamethasone, a GR agonist, decreased the GSK4716-induced increase in 5-HT1AR, which was associated with an alteration in nuclear GR. Furthermore, GR antagonist RU486 reversed the effects induced by dexamethasone, including the elevation of nuclear GR levels and the reduction of 5-HT1AR transcription and expression.

CONCLUSION

The results could provide insight into the potential applications of small molecules, such as GSK4716, in the regulation of 5-HT1AR expression, which plays a role in serotonergic neurotransmission.

In vivo serotonin 1A receptor hippocampal binding potential in depression and reported childhood adversity

BACKGROUND

Reported childhood adversity (CA) is associated with development of depression in adulthood and predicts a more severe course of illness. Although elevated serotonin 1A receptor (5-HT_1AR) binding potential, especially in the raphe nuclei, has been shown to be a trait associated with major depression, we did not replicate this finding in an independent sample using the partial agonist positron emission tomography tracer [¹¹C]CUMI-101. Evidence suggests that CA can induce long-lasting changes in expression of 5-HT_1AR, and thus, a history of CA may explain the disparate findings.

METHODS

Following up on our initial report, 28 unmedicated participants in a current depressive episode (bipolar n = 16, unipolar n = 12) and 19 non-depressed healthy volunteers (HVs) underwent [¹¹C]CUMI-101 imaging to quantify 5-HT_1AR binding potential. Participants in a depressive episode were stratified into mild/moderate and severe CA groups via the Childhood Trauma Questionnaire. We hypothesized higher hippocampal and raphe nuclei 5-HT_1AR with severe CA compared with mild/moderate CA and HVs.

RESULTS

There was a group-by-region effect (p = 0.011) when considering HV, depressive episode mild/moderate CA, and depressive episode severe CA groups, driven by significantly higher hippocampal 5-HT_1AR binding potential in participants in a depressive episode with severe CA relative to HVs (p = 0.019). Contrary to our hypothesis, no significant binding potential differences were detected in the raphe nuclei (p-values > 0.05).

CONCLUSIONS

With replication in larger samples, elevated hippocampal 5-HT_1AR binding potential may serve as a promising biomarker through which to investigate the neurobiological link between CA and depression.

Structure-function relationships of the aldosterone receptor

The cellular response to the adrenal steroid aldosterone is mediated by the mineralocorticoid receptor (MR), a member of the nuclear receptor superfamily of ligand-dependent transcription factors. The MR binds more than one physiological ligand with binding at the MR determined by pre-receptor metabolism of glucocorticoid ligands by 11β hydroxysteroid dehydrogenase type 2. The MR has a wide tissue distribution with multiple roles beyond the classical role in electrolyte homeostasis including cardiovascular function, immune cell signaling, neuronal fate and adipocyte differentiation. The MR has three principal functional domains, an N-terminal ligand domain, a central DNA binding domain and a C-terminal, ligand binding domain, with structures having been determined for the latter two domains but not for the whole receptor. MR signal-transduction can be best viewed as a series of interactions which are determined by the conformation conferred on the receptor by ligand binding. This conformation then determines subsequent intra- and inter-molecular interactions. These interactions include chromatin, coregulators and other transcription factors, and additional less well characterized cytoplasmic non-genomic effects via crosstalk with other signaling pathways. This chapter will provide a review of MR structure and function, and an analysis of the critical interactions involved in MR-mediated signal transduction, which contribute to ligand- and tissue-specificity. Understanding the relevant mechanisms for selective MR signaling in terms of these interactions opens the possibility of novel therapeutic approaches for the treatment of MR-mediated diseases.

Cellular and serotonergic correlates of habituated neuroendocrine responses in male and female rats

Male and females appear equally capable of showing habituated hypothalamic-pituitary-adrenal (HPA) axis output responses to repeated exposures of the same challenge. Whether this reflects, within males and females, common mechanisms of decreased neuronal activity within stress responding, afferents to the paraventricular hypothalamic nucleus (PVH), the final common pathway to the HPA axis, has not been examined. Here we compared in adult male and female rats the extent to which declines in HPA axis responses to repeated restraint are met by habituated cellular (Fos) responses, in addition to changes in serotonin (5-hydroxytryptamine; 5-HT) expression and signaling, which normally stimulates the HPA axis. Thus, alterations in this component of HPA axis drive could provide an underlying basis for sex differences in adaptive responses. Males and females showed reliable declines in ACTH and corticosterone responses after 10 daily episodes of repeated restraint, recapitulated, in largest part, by similar regional patterns of Fos habituation, including within the PVH, several stress sensitive cell groups of the limbic forebrain, as well as within the raphe nucleus. Serotonin staining in the dorsal raphe and terminal profiles in the forebrain continued to reflect a higher pre-synaptic capacity for the 5-HT system in females. The sexual dimorphism encountered within the lateral septum and medial preoptic area of control animals was less distinguished in the repeat condition, however, whereas 5-HT varicosities in the PVH increased after repeated restraint only in females. Relative to their singly restrained counterparts, males displayed an increase in 5-HT 1 A receptor expression in the raphe nucleus after repeated restraint, whereas females showed a decrease in 5-HT 1 A mRNA levels in the hippocampus and in the zona incerta, representing the most proximal of cell groups expressing the 5-HT 1 A receptor in the vicinity of the PVH. In conclusion, similar regional profiles of cellular habituation in males and females suggest common CNS substrates of neuroendocrine adaptation. However, this process may be met by underlying sex differences in serotonergic control, given the respective roles for pre- and postsynaptic 5-HT 1 A receptors in mediating serotonin availability and signal transfer.

Increased Levels of Cortisol in Individuals With Suicide Attempt and Its Relation With the Number of Suicide Attempts and Depression

BACKGROUND

Abnormalities in the hypothalamic-pituitary-adrenal axis (HPA) have been reported in individuals with suicide behavior. The aim of the study was to evaluate cortisol levels in peripheral plasma of individuals with more than one suicide attempt.

METHODS

Cortisol concentrations in peripheral plasma were measured using the ELISA technique. Suicide attempts were evaluated by the Columbia Suicide Severity Rating Scale, while depression was evaluated by the Hamilton Depression Rating Scale.

RESULTS

We found elevated cortisol levels in the suicide attempt group when compared with healthy controls (F = 7.26, p-value = 0.008), but no statistical differences with the psychiatric diseases group (F = 1.49, p-value = 0.22). Cortisol levels were higher in individuals with depression (F = 8.99, P = 0.004) and in individuals with two or more suicide attempts (F = 13.56, P < 0.001).

CONCLUSIONS

Cortisol levels are increased in individuals who attempt suicide and higher of cortisol concentrations in plasma regard to depression and more attempts of suicide.

Chronic sleep disruption induces depression-like behavior in adolescent male and female mice and sensitization of the hypothalamic-pituitary-adrenal axis in adolescent female mice

Depression is a prevalent mood disorder responsible for reduced quality of life for over 264 million people. Depression commonly develops during adolescence and becomes twice as prevalent in females than in males. However, the mechanisms underlying adolescent depression onset and sex differences in the prevalence rate remain unclear. Adolescent exposure to stress and subsequent sensitization of the hypothalamic-pituitary-adrenal (HPA) axis contributes to mood disorder development, and females are particularly vulnerable to HPA sensitization. Repeated exposure to stressors common to adolescent development, like sleep disruption, could partially be responsible for adolescent female susceptibility to depression. To address this possibility, 80 adolescent and adult CD-1 mice (Male, n = 40; Female, n = 40) were manually sleep disrupted for the first four hours of each rest cycle or allowed normal rest for eight consecutive days. Depression-like behavior was assessed with the forced swim test. 5-HT_1A and glucocorticoid receptor expression and concurrent cellular activation via glucocorticoid receptor/c-Fos colocalization were examined in various brain regions to assess cellular correlates of depression and HPA-axis activation. Both adolescent male and female mice displayed significantly greater depression-like behavior and prelimbic c-Fos expression after chronic sleep disruption than non-sleep disrupted adolescent and sleep disrupted adult counterparts. However, sleep disrupted adolescent females demonstrated greater dorsal raphe 5-HT_1A expression than sleep disrupted adolescent males. Adolescent females and males had decreased medial prefrontal 5-HT_1A expression after chronic sleep disruption, but only adolescent females expressed decreased hippocampal 5-HT_1A expression compared to controls. Chronic sleep disruption significantly increased corticosterone release, glucocorticoid expression in the CA1, and activation of glucocorticoid immunoreactive cells in the prelimbic cortex of adolescent females but not in adolescent males. These findings suggest that chronic sleep disruption during adolescence could give rise to depressive symptoms in male and female adolescents through differing signaling mechanisms.

Glucocorticoid and Mineralocorticoid Receptors in the Brain: A Transcriptional Perspective

Adrenal glucocorticoid hormones are crucial for maintenance of homeostasis and adaptation to stress. They act via the mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs)-members of the family of nuclear receptors. MRs and GRs can mediate distinct, sometimes opposite, effects of glucocorticoids. Both receptor types can mediate nongenomic steroid effects, but they are best understood as ligand-activated transcription factors. MR and GR protein structure is similar; the receptors can form heterodimers on the DNA at glucocorticoid response elements (GREs), and they share a number of target genes. The transcriptional basis for opposite effects on cellular physiology remains largely unknown, in particular with respect to MR-selective gene transcription. In this review, we discuss proven and potential mechanisms of transcriptional specificity for MRs and GRs. These include unique GR binding to "negative GREs," direct binding to other transcription factors, and binding to specific DNA sequences in conjunction with other transcription factors, as is the case for MRs and NeuroD proteins in the brain. MR- and GR-specific effects may also depend on specific interactions with transcriptional coregulators, downstream mediators of transcriptional receptor activity. Current data suggest that the relative importance of these mechanisms depends on the tissue and physiological context. Insight into these processes may not only allow a better understanding of homeostatic regulation but also the development of drugs that target specific aspects of disease.

Cortisol Stress Response and in Vivo PET Imaging of Human Brain Serotonin 1A Receptor Binding

BACKGROUND

Abnormalities in the hypothalamic-pituitary-adrenal axis, serotonergic system, and stress response have been linked to the pathogenesis of major depressive disorder. State-dependent hyper-reactivity of the hypothalamic-pituitary-adrenal axis is seen in major depressive disorder, and higher binding to the serotonin 1A receptor is observed as a trait in both currently depressed and remitted untreated major depressive disorder. Here, we sought to examine whether a relationship exists between cortisol secretion in response to a stressor and serotonin 1A receptor binding throughout the brain, both in healthy controls and participants with major depressive disorder.

METHODS

Research participants included 42 medication-free, depressed subjects and 31 healthy volunteers. Participants were exposed to either an acute, physical stressor (radial artery catheter insertion) or a psychological stressor (Trier Social Stress Test). Levels of serotonin 1A receptor binding on positron emission tomography with [11C]WAY-100635 were also obtained from all participants. The relationship between [11C]WAY-100635 binding and cortisol was examined using mixed linear effects models with group (major depressive disorder vs control), cortisol, brain region, and their interactions as fixed effects and subject as a random effect.

RESULTS

We found a positive correlation between post-stress cortisol measures and serotonin 1A receptor ligand binding levels across multiple cortical and subcortical regions, independent of diagnosis and with both types of stress. The relationship between [11C]WAY-100635 binding and cortisol was homogenous across all a priori brain regions. In contrast, resting cortisol levels were negatively correlated with serotonin 1A receptor ligand binding levels independently of diagnosis, except in the RN. There was no significant difference in cortisol between major depressive disorder participants and healthy volunteers with either stressor. Similarly, there was no correlation between cortisol and depression severity in either stressor group.

CONCLUSIONS

This study suggests that there may be a common underlying mechanism that links abnormalities in the serotonin system and hypothalamic-pituitary-adrenal axis hyper-reactivity to stress. Future studies need to determine how hypothalamic-pituitary-adrenal axis dysfunction affects mood to increase the risk of suicide in major depression.

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.

Corticosteroid Receptors in the Brain: Transcriptional Mechanisms for Specificity and Context-Dependent Effects

Corticosteroid hormones act in the brain to support adaptation to stress via binding to mineralocorticoid and glucocorticoid receptors (MR and GR). These receptors act in large measure as transcription factors. Corticosteroid effects can be highly divergent, depending on the receptor type, but also on brain region, cell type, and physiological context. These differences ultimately depend on differential interactions of MR and GR with other proteins, which determine ligand binding, nuclear translocation, and transcriptional activities. In this review, we discuss established and potential mechanisms that confer receptor and cell type-specific effects of the MR and GR-mediated transcriptional effects in the brain.

Aldosterone and Mineralocorticoid Receptor System in Cardiovascular Physiology and Pathophysiology

The mineralocorticoid hormone aldosterone (Aldo) has been intensively studied for its ability to influence both the physiology and pathophysiology of the cardiovascular system. Indeed, although research on Aldo actions for decades has mainly focused on its effects in the kidney, several lines of evidence have now demonstrated that this hormone exerts disparate extrarenal adverse effects, especially in the circulatory system. Accordingly, in the last lusters, a number of studies in preclinical models (in vitro and in vivo) and in humans have established that Aldo, following the interaction with its receptor-the mineralocorticoid receptor (MR)-is able to activate specific intracellular genomic and nongenomic pathways, thus regulating the homeostasis of the cardiovascular system. Importantly, through this mechanism of action, this hormone becomes a crucial regulator of the function and growth of different types of cells, including fibroblasts, cardiomyocytes, and vascular cells. For this main reason, it is plausible that when Aldo is present at high levels in the blood, it profoundly modifies the physiology of these cells, therefore being at the foundation of several cardiovascular disorders, such as heart failure (HF). On these grounds, in this review, we will provide an updated account on the current knowledge concerning Aldo activity in the cardiovascular system and the most recent preclinical studies and clinical trials designed to test better approaches able to counter the hyperactivity of the Aldo/MR signaling pathway in the setting of cardiovascular diseases.

Importance of the brain corticosteroid receptor balance in metaplasticity, cognitive performance and neuro-inflammation

Bruce McEwen's discovery of receptors for corticosterone in the rat hippocampus introduced higher brain circuits in the neuroendocrinology of stress. Subsequently, these receptors were identified as mineralocorticoid receptors (MRs) that are involved in appraisal processes, choice of coping style, encoding and retrieval. The MR-mediated actions on cognition are complemented by slower actions via glucocorticoid receptors (GRs) on contextualization, rationalization and memory storage of the experience. These sequential phases in cognitive performance depend on synaptic metaplasticity that is regulated by coordinate MR- and GR activation. The receptor activation includes recruitment of coregulators and transcription factors as determinants of context-dependent specificity in steroid action; they can be modulated by genetic variation and (early) experience. Interestingly, inflammatory responses to damage seem to be governed by a similarly balanced MR:GR-mediated action as the initiating, terminating and priming mechanisms involved in stress-adaptation. We conclude with five questions challenging the MR:GR balance hypothesis.

A Refill for the Brain Mineralocorticoid Receptor: The Benefit of Cortisol Add-On to Dexamethasone Therapy

Some serious medical conditions require life-saving treatment with high doses of synthetic glucocorticoids such as dexamethasone. A substantial number of patients subjected to this treatment develops psychosis, mood disturbances, or sleep problems. A recent clinical trial demonstrated that dexamethasone therapy for young patients with acute lymphoblastic leukemia caused severe adverse psychological effects and sleep disturbances in about 30% of these patients. These side effects were ameliorated by coadministration of a low dose of the naturally occurring glucocorticoid hormone cortisol. This paradoxical finding was predicted by the idea that the synthetic glucocorticoid targets the glucocorticoid receptor, causing suppression of cortisol secretion and, thus, depletion of the brain mineralocorticoid receptor (MR) of its endogenous ligand. The refill of the unoccupied brain MR with physiological amounts of cortisol ameliorates the dexamethasone-induced psychological side effects. In the present report, we discuss the mechanistic underpinning of the MR refill concept in glucocorticoid therapy.

Mineralocorticoid Receptor (MR) trans-Activation of Inflammatory AP-1 Signaling: DEPENDENCE ON DNA SEQUENCE, MR CONFORMATION, AND AP-1 FAMILY MEMBER EXPRESSION

Glucocorticoids are commonly used to treat inflammatory disorders. The glucocorticoid receptor (GR) can tether to inflammatory transcription factor complexes, such as NFκB and AP-1, and trans-repress the transcription of cytokines, chemokines, and adhesion molecules. In contrast, aldosterone and the mineralocorticoid receptor (MR) primarily promote cardiovascular inflammation by incompletely understood mechanisms. Although MR has been shown to weakly repress NFκB, its role in modulating AP-1 has not been established. Here, the effects of GR and MR on NFκB and AP-1 signaling were directly compared using a variety of ligands, two different AP-1 consensus sequences, GR and MR DNA-binding domain mutants, and siRNA knockdown or overexpression of core AP-1 family members. Both GR and MR repressed an NFκB reporter without influencing p65 or p50 binding to DNA. Likewise, neither GR nor MR affected AP-1 binding, but repression or activation of AP-1 reporters occurred in a ligand-, AP-1 consensus sequence-, and AP-1 family member-specific manner. Notably, aldosterone interactions with both GR and MR demonstrated a potential to activate AP-1. DNA-binding domain mutations that eliminated the ability of GR and MR to cis-activate a hormone response element-driven reporter variably affected the strength and polarity of these responses. Importantly, MR modulation of NFκB and AP-1 signaling was consistent with a trans-mechanism, and AP-1 effects were confirmed for specific gene targets in primary human cells. Steroid nuclear receptor trans-effects on inflammatory signaling are context-dependent and influenced by nuclear receptor conformation, DNA sequence, and the expression of heterologous binding partners. Aldosterone activation of AP-1 may contribute to its proinflammatory effects in the vasculature.

Novel interactions of the mineralocorticoid receptor

The mineralocorticoid receptor (MR) differs from the other steroid receptors in that it responds to two physiological ligands, aldosterone and cortisol. In epithelial tissues, aldosterone selectivity is determined by 11β-hydroxysteroid dehydrogenase type II. In other tissues cortisol is the primary ligand; in some tissues cortisol may act as an antagonist. To better target MR, an understanding of the structural determinants of tissue and ligand-specific MR activation is required. Our focus is on interactions of the ligand-binding domain (LBD) with ligand, the N-terminal domain and putative co-regulatory molecules. Molecular modelling has identified a region in the LBD of the MR and indeed other steroid receptors that critically defines ligand-specificity for aldosterone and cortisol, yet is not part of the ligand-binding pocket. An interaction between the N-terminus and LBD observed in the MR is aldosterone-dependent but is unexpectedly antagonised by cortisol. The structural basis of this interaction has been defined. We have identified proteins which interact in the presence of either aldosterone or cortisol but not both. These have been confirmed as coactivators of the full-length hMR. The structural basis of this interaction has been determined for tesmin, a ligand-discriminant coactivator of the MR. The successful identification of the structural basis of antagonism and of ligand-specific interactions of the MR may provide the basis for the development of novel MR ligands with tissue specificity.

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.

Stress-induced alterations in 5-HT1A receptor transcriptional modulators NUDR and Freud-1

The effect of stress on the mRNA and protein level of the 5-HT1A receptor and two of its key transcriptional modulators, NUDR and Freud-1, was examined in the prefrontal cortex (PFC) and hippocampus (Hp) using rodent models: olfactory bulbectomy (OB) and prenatal stress (PS) in male and female rats; chronic mild stress in male rats (CMS) and pregnancy stress. In PFC, CMS induced the most widespread changes, with significant reduction in both mRNA and protein levels of NUDR, 5-HT1A receptor and in Freud-1 mRNA; while in Hp 5-HT1A receptor and Freud-1 protein levels were also decreased. In male, but not female OB rats PFC Freud-1 and 5-HT1A receptor protein levels were reduced, while in Hp 5-HT1A receptor, Freud-1 and NUDR mRNA's but not protein were reduced. In PS rats PFC 5-HT1A receptor protein was reduced more in females than males; while in Hp Freud-1 protein was increased in females. In pregnancy stress, PFC NUDR, Freud-1 and 5-HT1A protein receptor levels were reduced, and in HP 5-HT1A receptor protein levels were also reduced; in HP only NUDR and Freud-1 mRNA levels were reduced. Overall, CMS and stress during pregnancy produced the most salient changes in 5-HT1A receptor and transcription factor expression, suggesting a primary role for altered transcription factor expression in chronic regulation of 5-HT1A receptor expression. By contrast, OB (in males) and PS (in females) produced gender-specific reductions in PFC 5-HT1A receptor protein levels, suggesting a role for post-transcriptional regulation. These and previous data suggest that chronic stress might be a key regulator of NUDR/Freud-1 gene expression.

Serotonin-prefrontal cortical circuitry in anxiety and depression phenotypes: pivotal role of pre- and post-synaptic 5-HT1A receptor expression

Decreased serotonergic activity has been implicated in anxiety and major depression, and antidepressants directly or indirectly increase the long-term activity of the serotonin system. A key component of serotonin circuitry is the 5-HT1A autoreceptor, which functions as the major somatodendritic autoreceptor to negatively regulate the "gain" of the serotonin system. In addition, 5-HT1A heteroreceptors are abundantly expressed post-synaptically in the prefrontal cortex (PFC), amygdala, and hippocampus to mediate serotonin actions on fear, anxiety, stress, and cognition. Importantly, in the PFC 5-HT1A heteroreceptors are expressed on at least two antagonist neuronal populations: excitatory pyramidal neurons and inhibitory interneurons. Rodent models implicate the 5-HT1A receptor in anxiety- and depression-like phenotypes with distinct roles for pre- and post-synaptic 5-HT1A receptors. In this review, we present a model of serotonin-PFC circuitry that integrates evidence from mouse genetic models of anxiety and depression involving knockout, suppression, over-expression, or mutation of genes of the serotonin system including 5-HT1A receptors. The model postulates that behavioral phenotype shifts as serotonin activity increases from none (depressed/aggressive not anxious) to low (anxious/depressed) to high (anxious, not depressed). We identify a set of conserved transcription factors including Deaf1, Freud-1/CC2D1A, Freud-2/CC2D1B and glucocorticoid receptors that may confer deleterious regional changes in 5-HT1A receptors in depression, and how future treatments could target these mechanisms. Further studies to specifically test the roles and regulation of pyramidal vs. interneuronal populations of 5-HT receptors are needed better understand the role of serotonin in anxiety and depression and to devise more effective targeted therapeutic approaches.

Targeting brain serotonin synthesis: insights into neurodevelopmental disorders with long-term outcomes related to negative emotionality, aggression and antisocial behaviour

Aggression, which comprises multi-faceted traits ranging from negative emotionality to antisocial behaviour, is influenced by an interaction of biological, psychological and social variables. Failure in social adjustment, aggressiveness and violence represent the most detrimental long-term outcome of neurodevelopmental disorders. With the exception of brain-specific tryptophan hydroxylase-2 (Tph2), which generates serotonin (5-HT) in raphe neurons, the contribution of gene variation to aggression-related behaviour in genetically modified mouse models has been previously appraised (Lesch 2005 Novartis Found Symp. 268, 111-140; Lesch & Merschdorf 2000 Behav. Sci. Law 18, 581-604). Genetic inactivation of Tph2 function in mice led to the identification of phenotypic changes, ranging from growth retardation and late-onset obesity, to enhanced conditioned fear response, increased aggression and depression-like behaviour. This spectrum of consequences, which are amplified by stress-related epigenetic interactions, are attributable to deficient brain 5-HT synthesis during development and adulthood. Human data relating altered TPH2 function to personality traits of negative emotionality and neurodevelopmental disorders characterized by deficits in cognitive control and emotion regulation are based on genetic association and are therefore not as robust as the experimental mouse results. Mouse models in conjunction with approaches focusing on TPH2 variants in humans provide unexpected views of 5-HT's role in brain development and in disorders related to negative emotionality, aggression and antisocial behaviour.

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.

Antidepressant-like effects of mineralocorticoid but not glucocorticoid antagonists in the lateral septum: interactions with the serotonergic system

The lateral septum (LS) is a limbic brain region that receives serotonergic projections from raphe neurons and participates in the modulation of stress responses and affective states. The present study determined whether mineralocorticoid receptors (MRs) and/or glucocorticoid receptors (GRs) located in the LS interact with the serotonergic system in the regulation of depressive-like behavior of rats subjected to the forced swimming test (FST). We also studied the effect of corticosterone release induced by the FST on MR- and GR-mRNA expression in the LS. Specifically, we studied the antidepressant-like effects of spironolactone (a MR antagonist), mifepristone (a GR antagonist), and the antidepressant clomipramine (CMI) administered directly into the LS. In addition, spironolactone and CMI actions were studied in animals with serotonergic depletion induced by dl-p-chlorophenylalanine (pCPA). Finally, adrenalectomized and Sham-operated rats were subjected to the FST to determine MR- and GR-mRNA expression in the LS at different post-FST intervals. The results showed that intraseptal injection of spironolactone, but not mifepristone induced antidepressant-like actions in the FST; this effect was blocked by pCPA treatment. CMI and spironolactone increased 5-HT concentrations in the LS of rats subjected to the FST. Increases in corticosterone release, induced by the FST, correlated with a decrease in MR-mRNA expression in the LS; no correlation was found with GR-mRNA expression. In conclusion, MRs in the lateral septum, but not GRs, participate in the regulation of depressive-like behavior of animals subjected to the FST. Both serotonin and corticosterone play an important role in MR actions in the LS.

Corticosteroid receptor signalling modes and stress adaptation in the brain

Adrenal glucocorticoid hormones modulate neuronal activity to support an adaptive response to stress. They modulate brain circuitry mediating physiological responses, emotion and cognitive processing. Chronically elevated glucocorticoid exposure is however linked to the development of mental disease. Glucocorticoid effects depend on mineralo- and glucocorticoid receptors, which are powerful transcription factors, but also can act via a diversity of non-genomic mechanisms. Here, I review generic factors that determine neuronal glucocorticoid sensitivity, in relation to brain function. First, pre-receptor mechanisms determine ligand availability. Second, there may be considerable variation in the receptor splice- and translation variants. Third, other transcription factors and many transcriptional coregulators interact with steroid receptors, determining nature and magnitude of steroid responses, in part through epigenetic regulation of DNA accessibility. Which factors underlie adaptive and pathogenic effects of stress hormones is largely unknown. Genome-wide identification of the receptor-DNA interactions in specific behavioural and physiological contexts provides a way of assessing the complete genomic range of glucocorticoid modes of action. Novel ligands that induce selective activation of particular receptor signalling modes will aid our understanding of receptor signalling and may allow selective targeting of glucocorticoid effects in emotional or cognitive domains, in research and, hopefully, in clinical settings.

The hypothalamic-pituitary-adrenal axis and serotonin abnormalities: a selective overview for the implications of suicide prevention

Suicidal behavior and mood disorders are one of the world's largest public health problems. The biological vulnerability for these problems includes genetic factors involved in the regulation of the serotonergic system and stress system. The hypothalamic-pituitary-adrenal (HPA) axis is a neuroendocrine system that regulates the body's response to stress and has complex interactions with brain serotonergic, noradrenergic and dopaminergic systems. Corticotropin-releasing hormone and vasopressin act synergistically to stimulate the secretion of ACTH that stimulates the biosynthesis of corticosteroids such as cortisol from cholesterol. Cortisol is a major stress hormone and has effects on many tissues, including on mineralocorticoid receptors and glucocorticoid receptors in the brain. Glucocorticoids produce behavioral changes, and one important target of glucocorticoids is the hypothalamus, which is a major controlling center of the HPA axis. Stress plays a major role in the various pathophysiological processes associated with mood disorders and suicidal behavior. Serotonergic dysfunction is a well-established substrate for mood disorders and suicidal behavior. Corticosteroids may play an important role in the relationship between stress, mood changes and perhaps suicidal behavior by interacting with 5-HT1A receptors. Abnormalities in the HPA axis in response to increased levels of stress are found to be associated with a dysregulation in the serotonergic system, both in subjects with mood disorders and those who engage in suicidal behavior. HPA over-activity may be a good predictor of mood disorders and perhaps suicidal behavior via abnormalities in the serotonergic system.

Early parental deprivation in the marmoset monkey produces long-term changes in hippocampal expression of genes involved in synaptic plasticity and implicated in mood disorder

In mood disorder, early stressors including parental separation are vulnerability factors, and hippocampal involvement is prominent. In common marmoset monkeys, daily parental deprivation during infancy produces a prodepressive state of increased basal activity and reactivity in stress systems and mild anhedonia that persists at least to adolescence. Here we examined the expression of eight genes, each implicated in neural plasticity and in the pathophysiology of mood disorder, in the hippocampus of these same adolescent marmosets, relative to their normally reared sibling controls. We also measured hippocampal volume. Early deprivation led to decreases in hippocampal growth-associated protein-43 (GAP-43) mRNA, serotonin 1A receptor (5-HT(1A)R) mRNA and binding ([3H]WAY100635), and to increased vesicular GABA transporter mRNA. Brain-derived neurotrophic factor (BDNF), synaptophysin, vesicular glutamate transporter 1 (VGluT1), microtubule-associated protein-2, and spinophilin transcripts were unchanged. There were some correlations with in vivo biochemical and behavioral indices, including VGluT1 mRNA with reward-seeking behavior, and serotonin 1A receptor mRNA with CSF cortisol. Early deprivation did not affect hippocampal volume. We conclude that early deprivation in a nonhuman primate, in the absence of subsequent stressors, has a long-term effect on the hippocampal expression of genes implicated in synaptic function and plasticity. The reductions in GAP-43 and serotonin 1A receptor expressions are comparable with findings in mood disorder, supporting the possibility that the latter reflect an early developmental contribution to disease vulnerability. Equally, the negative results suggest that other features of mood disorder, such as decreased hippocampal volume and BDNF expression, are related to different aspects of the pathophysiological process.

Epithelial sodium channel regulated by differential composition of a signaling complex

Hormonal control of transepithelial sodium (Na(+)) transport utilizes phosphatidylinositide 3'-kinase (PI3K) and Raf-MAPK/ERK kinase (MEK)-ERK-dependent signaling pathways, which impact numerous cell functions. How signals transmitted by these pathways are sorted and appropriately transmitted to alter Na(+) transport without altering other physiologic processes is not well understood. Here, we report the identification of a signaling complex that selectively modulates the cell surface expression of the epithelial sodium channel (ENaC), an ion channel that is essential for fluid and electrolyte balance in mammals. Raf-1 and the ubiquitin ligase, Nedd4-2, are constitutively-expressed inhibitory components of this ENaC regulatory complex, which interact with, and decrease the expression of, cell surface ENaC. The activities of Nedd4-2 and Raf-1 are inhibited cooperatively by the PI3K-dependent kinase serum- and glucocorticoid-induced kinase 1 (SGK1), and the Raf-1-interacting protein glucocorticoid-induced leucine zipper (GILZ1), which are aldosterone-stimulated components of the complex. Together, SGK1 and GILZ1 synergistically stimulate ENaC cell surface expression. Interestingly, GILZ1 and SGK1 do not have synergistic, and in fact have opposite, effects on an unrelated activity, FKHRL1-driven gene transcription. Together, these data suggest that GILZ1 and SGK1 provide a physical and functional link between the PI3K- and Raf-1-dependent signaling modules and represent a unique mechanism for specifically controlling Na(+) transport without inappropriately activating other cell functions.

Hypothalamic-pituitary-adrenal axis activity of newborn mice rapidly desensitizes to repeated maternal absence but becomes highly responsive to novelty

In CD1 mice we investigated the hypothalamic-pituitary-adrenal (HPA) axis response to maternal separation for 8 h daily from postnatal d 3 to 5. At d 3 a slow separation-induced corticosterone response developed that peaked after 8 h, and the pups became responsive to stressors. On the second and third day, the response to 8 h separation rapidly attenuated, whereas the response to novelty did not, a pattern reflected by the hypothalamic c-fos mRNA response. If maternal separation and exposure to novelty were combined, then after the third such daily exposure, the sensitivity to the stressor was further enhanced. Meanwhile, basal corticosterone and ACTH levels were persistently suppressed 16 h after pups were reunited with their mothers. To explain the HPA axis desensitization after repeated separation, we found that circulating ghrelin levels increased and glucose levels decreased after all periods of maternal separation, ruling out a role of altered metabolism. Glucocorticoid feedback was not involved either because a glucocorticoid receptor antagonist amplified the corticosterone response after the first but became ineffective after the third separation. In contrast, a mineralocorticoid receptor antagonist decreased and increased corticosterone levels after the first and third period of separation, respectively. In conclusion, the newborn's HPA axis readily desensitizes to repeated daily maternal separation, but continues to respond to novelty in a manner influenced by a central mineralocorticoid receptor- rather than glucocorticoid receptor-mediated mechanism.

Corticosteroid-serotonin interactions in the neurobiological mechanisms of stress-related disorders

Among psychiatric disorders, depression and generalized anxiety are probably the most common stress-related illnesses. These diseases are underlain, at least partly, by dysfunctions of neurotransmitters and neurohormones, especially within the serotoninergic (5-HT) system and the hypothalamo-pituitary-adrenal (HPA) axis, which are also the targets of drugs used for their treatment. This review focuses on the nature of the interactions between central 5-HT and corticotrope systems in animal models, in particular those allowing the assessment of serotoninergic function following experimental manipulation of the HPA axis. The review provides an overview of the HPA axis and the 5-HT system organization, focusing on the 5-HT(1A) receptors, which play a pivotal role in the 5-HT system regulation and its response to stress. Both molecular and functional aspects of 5-HT/HPA interactions are then analyzed in the frame of psychoaffective disorders. The review finally examines the hippocampal neurogenesis response to experimental paradigms of stress and antidepressant treatment, in which neurotrophic factors are considered to play key roles according to the current views on the pathophysiology of depressive disorders.

Selective transrepression versus transactivation mechanisms by glucocorticoid receptor modulators in stress and immune systems

Glucocorticoids control immune homeostasis and regulate stress responses in the human body to a large extent via the glucocorticoid receptor. This transcription factor can modulate gene expression either through direct DNA binding (mainly resulting in transactivation) or independent of DNA binding (in the majority of cases resulting in transrepression). The aim of this review is to discuss the mechanistic basis and applicability of different glucocorticoid receptor modulators in various affections, ranging from immune disorders to mental dysfunctions.

Central corticosteroid actions: Search for gene targets

Although many of the physiological effects of corticosteroid stress hormones on neuronal function are well recognised, the underlying genomic mechanisms are only starting to be elucidated. Linking physiology and genomics has proven to be a complicated task, despite the emergence of large-scale gene expression profiling technology in the last decade. This is in part due to the complexity of glucocorticoid-signaling, in part due to the complexity of the brain itself. The presence of a binary receptor system for glucocorticoid hormones in limbic brain structures, the coexistence of membrane and intracellular receptors and the highly contextual action of glucocorticoids contribute to this complexity. In addition, the anatomical complexity, extensive cellular heterogeneity of brain and the modest changes in gene expression (mostly in the range of 10-30%) hamper detection of responsive genes, in particular of low abundant transcripts, such as many neurotransmitter receptors and growth factors. Nonetheless, ongoing research into central targets of glucocorticoids has identified many different functional gene classes that underlie the diverse effects of glucocorticoids on brain function. These functional classes include genes involved in energy metabolism, signal transduction, neuronal structure, vesicle dynamics, neurotransmitter catabolism, cell adhesion, genes encoding neurotrophic factors and their receptors and genes involved in regulating glucocorticoid-signalling. The aim of this review is to give an overview of the current status of the field on identification of central corticosteroid targets, discuss the opportunities and pitfalls and highlight new developments in understanding central corticosteroid action.

Dex-ras1 and serum- and glucocorticoid-inducible protein kinase 1: regulation of expression by dexamethasone in HEK293 cells

The molecular and cellular basis of the psychotropic actions of adrenal corticosteroids is poorly understood. Previously, we reported that modulation of large conductance Ca2+-activated potassium channel (BK-channel) function by glucocorticoids can be recapitulated in human embryonic kidney293 (HEK293) cells (J Physiol 537:57, 2001). In the present paper, we examined the effect of dexamethasone on the expression of candidate mediator proteins of glucocorticoid action, dex-ras1 and serum and glucocorticoid inducible protein kinase 1 (SGK), in HEK293 cells. Dex-ras1 mRNA was readily detectable under basal conditions however, no changes of dex-ras1 mRNA expression occurred upon exposure to 100 nM of dexamethasone for 2 h. In contrast, a 2.5-fold increase of SGK mRNA was found under similar conditions. Total levels of cellular SGK protein were unaltered upon exposure to dexamethasone, but a marked increase of SGK in a Triton-X100 insoluble fraction was observed. BK-channel alpha-subunits could not be co-immunoprecipitated with SGK. In summary, SGK, but not dex-ras1, mRNA is rapidly induced by glucocorticoid stimulation in HEK293 cells. However, there appears to be no direct protein-protein interaction between SGK and BK-channel alpha-subunits.

Differential activities of glucocorticoid-induced leucine zipper protein isoforms

Glucocorticoid-induced leucine zipper protein (GILZ) is expressed in both epithelial and immune tissues and modulates a variety of cellular functions, including proliferation and epithelial sodium channel (ENaC) activity. A number of reports have described various GILZ activities, focusing on a single isoform with molecular mass of approximately 17 kDa, now termed GILZ1. In GILZ immunoblots using a newly developed antiserum, we detected multiple species in extracts from cultured kidney cells. Mass spectrometric analysis revealed that one of these represented a previously uncharacterized distinct isoform of GILZ, GILZ2. Rapid amplification of cDNA ends was used to clone cDNAs corresponding to four isoforms, which, in addition to GILZ1 and GILZ2, included new isoforms GILZ3 and GILZ4. Heterologous expression of these four GILZ isoforms in cultured cells revealed striking functional differences. Notably, GILZ1 was the only isoform that significantly stimulated ENaC-mediated Na+ current in a kidney collecting duct cell line, although GILZ2 and GILZ3 also stimulated ENaC surface expression in HEK 293 cells. GILZ1 and GILZ3, and to a lesser extent GILZ2, inhibited ERK phosphorylation. Interestingly, GILZ4, which had no effect on either ENaC or ERK, potently suppressed cellular proliferation, as did GILZ1, but not GILZ2 or GILZ3. Finally, rat and mouse tissues all expressed multiple GILZ species but varied in the relative abundance of each. These data suggest that multiple GILZ isoforms are expressed in most cells and tissues and that these play distinct roles in regulating key cellular functions, including proliferation and ion transport. Furthermore, GILZ inhibition of ERK appears to play an essential role in stimulation of cell surface ENaC but not in inhibition of proliferation.

Reduced serotonin-1A receptor binding in social anxiety disorder

BACKGROUND

Results from studies in serotonin-1A (5-HT1A) knockout mice and previous positron emission tomography (PET) studies in humans imply a role for 5-HT1A receptors in normal state anxiety as well as in certain anxiety disorders. The objective of this study was to investigate 5-HT1A receptor binding potential (BP) in social anxiety disorder (SAD).

METHODS

Using PET and [carbonyl-11C]WAY-100635, we compared a homogeneous group of 12 unmedicated, male SAD patients with 18 healthy control subjects (HC). A multivariate ANOVA with all regional BP values as dependent variables, age and four radiochemical variables as covariates was performed.

RESULTS

We found a significantly lower 5-HT1A BP in several limbic and paralimbic areas but not in the hippocampus (p = .234) of SAD patients. The difference in 5-HT1A binding was most significant in the amygdala (-21.4%; p = .003). There was also a more than 20% lower 5-HT(1A) BP of SAD patients in the anterior cingulate cortex (p = .004), insula (p = .003), and dorsal raphe nuclei (p = .030).

CONCLUSIONS

The lower 5-HT1A binding in the amygdala and mesiofrontal areas of SAD patients is consistent with 1) preclinical findings of elevated anxiety in 5-HT1A knockout mice, 2) a previous PET study in healthy volunteers showing an inverse correlation between 5-HT1A BP and state anxiety, and 3) another human PET study in patients with panic disorder showing reduced 5-HT1A binding, thus corroborating the potential validity of 5-HT1A receptors as targets in the treatment of human anxiety disorders.

Transcriptional regulation of intronic calcium-activated potassium channel SK2 promoters by nuclear factor-kappa B and glucocorticoids

Small-conductance Ca(2+)-activated K(+) channels (SK) of the SK2 subtype are widely expressed in the central nervous system where they contribute to the control of neuronal excitability. Two SK2 isoforms, SK2-S and SK2-L, the latter representing an N-terminally extended protein of SK2-S, are expressed in similar patterns in the brain. However, our understanding of mechanisms by which the expression of SK2 is regulated is limited. We identified one functional glucocorticoid response element (GRE) at position -2248 bp and two functional nuclear factor-kappB (NF-kappaB) response elements at positions -1652 and -1586 bp in the SK2-S promoter. An increase in SK2-S promoter activity was observed in PC12 cells transiently transfected with a wild-type SK2-S promoter-luciferase reporter gene construct and treated with aldosterone or dexamethasone. The mineralocorticoid receptor (MR) antagonist spironolactone or the glucocorticoid receptor (GR) antagonist mifepristone fully inhibited aldosterone or dexamethasone activation of the SK2-S promoter, respectively. SK2-S promoter activity was also induced by the cell-permeable ceramide analog, N-acetylsphingosine (C2-ceramide). Antisense oligonucleotides directed to NF-kappaB p65 or p50 suppressed SK2-S transcription induced by C2-ceramide. Deletion studies showed that only the -1586 bp NF-kappaB binding site was necessary for maximum C2-ceramide response. Finally, we showed that activation of GRs but not of MRs repressed the NF-kappaB-mediated induction of SK2-S transcription. These findings suggest a possible transcriptional cross talk between GRs and NF-kappaB in the intronic promoter regulation of SK2-S channel gene transcription.

Structure of the human 5-HT7 receptor gene and characterization of its promoter region

The molecular mechanism regulating serotonin 5-HT(7) receptor expression is still unclear. In this study we provide evidence that transcription of the 5-HT(7) gene is at least partly regulated by Sp1 and Sp3. We isolated and sequenced >3000 bp of the upstream sequences and identified by RACE a number of transcriptional initiation sites over a region of 300 bp upstream of the coding region. This region has a high GC content, but contains no obvious TATA or a CAAT box. Besides a Sp1/Sp3 consensus motif, regulatory elements for AP2, Egr-1 and MAZ are also present. Transient transfection assays using deletion variants indicated that the GC-rich region is essential for full promoter activity. The role of Sp1 in this was confirmed by transient overexpression of both wild type Sp1 or dominant-negative forms. By gel shift and supershift analyses, targeting the Sp1 consensus sequence and the GC-rich region just upstream of the transcription initiation sites, binding of Sp1 and Sp3 was demonstrated. Both in vitro as well as in vivo experiments, using a cell line which endogenously expresses the 5-HT(7) receptor, indicated that mithramycin A, an inhibitor of Sp1/3 transcription factor binding, was able to inhibit 5-HT(7) promoter activity. Taken together these results support the essential role of Sp factors in regulating 5-HT(7) promoter activity.

Understanding stress through the genome

Stress-induced glucocorticoid hormones support coping with and adaptation to different stressors. They act to modulate gene expression in a tissue and stressor-specific manner through activation of corticosteroid receptors, which act as transcription factors. Here, a number of recent insights in gene regulation under the influence of glucocorticoids are discussed. Emphasis is put on distinct classes of target genes that may be defined, based on categorization of (combinations of) transcription factor binding sites in responsive genes. These categories depend on insights into different mechanisms of transcriptional regulation, such as transactivation vs transrepression, and high affinity vs low affinity hormone receptor response elements. It is argued that such classes, based on mechanistic understanding of transcription regulation, in combination with the availability of complete genomic sequences and expression data from different organs, may enhance our understanding of the way in which organisms deal with different forms of stress.

Steroid receptor coregulator diversity: What can it mean for the stressed brain?

Glucocorticoid hormones modulate brain function and as such are crucial for responding and adjusting to physical and psychological stressors. Their effects are mediated via mineralo- and glucocorticoid receptors, which in large measure act as transcription factors to modulate transcription of target genes, in a receptor-, cell-, and state-specific manner. The nature and magnitude of these transcriptional effects depend on the presence and activity of downstream proteins, such as steroid receptor coactivators and corepressors (together: coregulators), many of which are expressed in the brain. We address the role of coregulators for mineralo- and glucocorticoid receptor-mediated modulation of gene transcription. We first address evidence from cell lines for the importance of coregulator stoichiometry for steroid signaling. The in vivo importance of coregulators-when possible specifically for glucocorticoid signaling in the brain-is discussed based on knockout mice, transient knockdown of steroid receptor coactivators, and distribution and regulation of coactivator expression in the brain. We conclude that for a better understanding of modulation of brain function by glucocorticoids, it is necessary to take into account the role of coregulators, and to assess their importance relative to changes in hormone levels and receptor expression.

Effect of neonatal handling on serotonin 1A sub-type receptors in the rat hippocampus

Serotonin 1A sub-type receptors play an important role in the etiopathogenesis of depression, which is known to occur more often in females than males. Early experiences can be a predisposing factor for depression; however, the underlying cellular processes remain unknown. In an effort to address such issues, we employed neonatal handling, an experimental model of early experience, which has been previously shown to render females more vulnerable to display enhanced depression-like behavior in response to chronic stress, while it increases the ability of males to cope. In rat pre-pubertal (30 days of age) and adult (90 days) hippocampus, of both males and females, the effect of neonatal handling on serotonin 1A sub-type receptor mRNA and protein levels was determined by in situ hybridization and immunohistochemistry, respectively, while the number of binding sites was determined by in vitro autoradiography using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin as the ligand. Our results revealed a significant sex difference in serotonin 1A sub-type receptor mRNA, protein and binding sites, with females having higher levels than males. Handling resulted in statistically significant decreased numbers of cells positive for serotonin 1A sub-type receptor mRNA or protein, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites in the area 4 of Ammon's horn and dentate gyrus of both pre-pubertal males and females. In adult animals the number of serotonin 1A sub-type receptor mRNA positive cells was increased as a result of handling in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of males, while it was decreased only in the area 4 of Ammon's horn of females. Furthermore, the number of serotonin sub-type 1A receptor immunopositive cells, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites was increased in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of handled males, whereas it was decreased in these same brain areas in the handled females. We can thus infer that neonatal handling results in alterations in postsynaptic serotonergic neurotransmission, which may contribute to the sex dimorphic effects of handling as to the vulnerability toward depression-like behavior in response to chronic stressful stimuli.

Analysis of the murine 5-HT receptor gene promoter in vitro and in vivo

The expression level of the 5-HT(1A) receptor gene (htr1a) in the central nervous system (CNS) is implicated in the aetiology and treatment of anxiety disorders and depression. Previous studies of the murine htr1a have revealed that its proximal promoter is GC rich and TATA-less. Several functional transcription factor binding sites, including MAZ and SP1 recognition sequences, have been identified. To further analyse the promoter of this receptor gene, additional upstream sequence information extending to -5.5 kb of the murine htr1a was generated and promoter fragments extending to -20 kb were analysed for activity in cell culture and transgenic animals. Promoter fragments greater than 4.5 kb in length were active in 5-HT(1A) receptor mRNA positive cells and inactive in 5-HT(1A) receptor mRNA negative cells. Smaller fragments were not able to confer this specificity. In agreement, using additive transgenesis to drive LacZ expression in vivo, CNS specific reporter gene expression was found with these longer constructs. Transgene expression in the 4.5- and 20-kb mouse lines resembled the endogenous htr1a expression pattern, whereas the 5.5-kb mouse lines surprisingly revealed strongly reduced expression. None of the three constructs was prone to confer ectopic expression, however, variation of expression between the transgenic lines was observed. Using colocalization studies we analysed the degree of concurrence of transgenic and endogenous htr1a expression brought about by these three different constructs. The highest degrees of colocalization were observed in mice harbouring the 20-kb construct, suggesting a large promoter fragment is required to faithfully direct transgene expression in a 5-HT(1A) receptor like pattern.

Increased mRNA expression of alpha2A-adrenoceptors, serotonin receptors and mu-opioid receptors in the brains of suicide victims

The development of new therapies for the treatment of psychiatric disorders requires an in-depth knowledge of the molecular bases underlying these pathologies, which remain largely unknown. Alterations in adrenoceptors, serotonin receptors, and other G protein-coupled receptors (GPCRs) have been associated with suicide and depression. However, to date, there is little information about mRNA expression of the GPCRs in the frontal cortex of suicide victims. Our goal was to study the expression in the brain of these receptors. For this purpose, we measured mRNA levels by RT-PCR. We found that the expressions of alpha2A-adrenoceptors, 5-HT1A, 5-HT2A serotonin receptors, and mu-opioid receptors were elevated in the post-mortem brains of these suicide victims with respect to matched controls. Moreover, in the case of alpha2A-adrenoceptors (the only for which these data were available), a significant correlation was observed between the level of mRNA and protein quantified in the brain of the same subjects, indicating that protein synthesis of this receptor was not influenced by post-translational regulatory mechanisms. In addition, the degree of adrenoceptor and 5-HT receptor expressions appeared to be correlated in the brains of suicide victims and control subjects. Alterations in the expression of adrenoceptors, serotonin, and opioid receptors indicate that these signaling proteins might be related to the etiopathology of suicidal and depressive behaviors. Alternatively, such changes may represent adaptive mechanisms to compensate for other as yet unknown alterations. The results also suggest that these receptors could share common regulatory mechanisms.

Mechanisms of mineralocorticoid action: determinants of receptor specificity and actions of regulated gene products

The mineralocorticoid receptor (MR) and its close cousin, the glucocorticoid receptor (GR), share considerable structural and functional similarity, including indistinguishable DNA binding properties, yet they mediate distinct physiological responses in some tissues. Specificity is determined by their distinct interactions with other protein factors and modification by peptides, including the small ubiquitin modifier SUMO1. Serum and glucocorticoid-regulated kinase 1 (sgk1) is one key target gene of both MR and GR, and encodes a serine-threonine kinase that stimulates the apical membrane localization of the epithelial sodium channel ENaC. Sgk1 exerts its effects, at least in part, by inhibiting an isoform of the ENaC inhibitory ubiquitin ligase Nedd4-2. This review briefly summarizes two areas of mineralocorticoid research: molecular determinants of MR specificity, and the role of Sgk1 in mediating the effects of aldosterone on epithelial Na(+) transport.

Implantation of a slow release corticosterone pellet induces long-term alterations in serotonergic neurochemistry in the rat brain

Many studies point to an involvement of deficits in the serotonergic nervous system and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis function with depression. Indeed early life stress, involving HPA axis activation, may predispose susceptible individuals to develop depression in later life. This study investigates the effects of elevating the neuroendocrine stress hormone, corticosterone, for 1 week in adolescent rats on markers of serotonergic neurone function at adulthood. Slow release corticosterone pellets were implanted for 7 days and various serotonergic parameters, as well as plasma corticosterone levels, were measured on day 7 or on day 28 (21 days following removal of the pellet). The corticosterone implant attenuated weight gain and reduced adrenal weights compared to that in control rats implanted with a cholesterol pellet. After 7 days, with the implant still in place, the diurnal variation in plasma corticosterone was reduced so that the level was approximately at that of the evening peak throughout the day. Twenty-one days after removal of the implant, the diurnal variation in plasma corticosterone returned. Corticosterone treatment decreased [3H] 8-hydroxy-2-(di-n-propylamino)tetralin binding to the 5-hydroxytryptamine1A receptor in the cortex but not in the hippocampus. Corticosterone treatment also enhanced the circadian rhythm observed in 5-hydroxyindoleacetic acid level and the ratio of 5-hydroxyindoleacetic acid to the 5-hydroxytryptamine in the frontal cortex. Despite corticosterone pellet removal 21 days earlier, there was a persistent decrease in whole body and adrenal weight, cortical 5-hydroxytryptamine1A receptor binding and an alteration in the diurnal variation in the 5-hydroxytryptamine "turnover" in the frontal cortex.

Coregulator proteins and corticosteroid action in the brain

The corticosteroid hormones cortisol and corticosterone are secreted by the adrenal gland in response to stress. They have profound effects on brain function, which are mediated by the related mineralocorticoid (MR) and glucocorticoid (GR) receptors. The MR and GR are ligand-activated transcription factors and exert different, sometimes opposing effects on the brain. The balance between these two receptor activities is considered essential for appropriate corticosteroid signalling and health. An exciting recent insight in steroid biology is that the nature and magnitude of steroid receptor-mediated responses depend not only on ligand and receptor availability, but also in a critical manner on the presence of downstream mediator proteins (coregulators), such as the steroid receptor coactivators and nuclear receptor corepressors. Members of the coregulator families differ in their specific interactions with steroid receptors, as well as in their distribution throughout the brain and pituitary. The activity of these proteins can be regulated both at the expression level, and by post-translational modifications. These characteristics make coregulator proteins of outstanding interest as determinants of receptor, cell and state-dependent effects of MR and GR signalling (and steroid receptor signalling in general) in the brain.

Alteration in 5-hydroxytryptamine agonist-induced behaviour following a corticosterone implant in adult rats

Hypercortisolism and altered serotonergic function may account for the pathological symptoms seen in depression. This study examines the impact of 4 days continuous corticosterone treatment on 5-HT agonist-induced behaviour to delineate changes in 5-HT receptor function in the adult rat. The flat body posture, reciprocal forepaw treading, elevated corticosterone, hyperglycaemia, hypothermia and reduced hippocampal 5-HT induced by the 5-HT(1A) agonist 8-OHDPAT (0.3 mg/kg ip) were all significantly attenuated by the corticosterone implant. The elevation in plasma corticosterone and back muscle contractions evoked by the 5-HT(2A) agonist DOI (1 mg/kg ip) were attenuated, whilst wet-dog shakes were enhanced by corticosterone treatment. 5-HT(2B) agonist-induced behaviour and the hypolocomotion and hypophagia induced by the 5-HT(2C) agonist m-CPP (2.5 mg/kg ip) were unaltered but the mCPP-induced elevation in corticosterone was abolished by corticosterone treatment. Hypothalamic 5-HT receptors mediating corticosterone- and 5-HT(1A) receptors, whether on serotonergic nerve terminals or postsynaptic neurones, were downregulated by corticosterone treatment. In contrast, 5-HT(2A) receptors may be up- or downregulated dependent on whether they are on supraspinal or spinal neurones, respectively. A comparison of the brain region-dependent alteration in serotonergic function produced by hypercorticosterone in the rat with that seen in depression is discussed.

Determinants of subnuclear organization of mineralocorticoid receptor characterized through analysis of wild type and mutant receptors

The mineralocorticoid receptor (MR) is a hormone-dependent regulator of gene transcription that in the absence of ligand resides both in the cytoplasm and the nucleus. Agonists but not antagonists increase the number of MRs residing in the nucleus and cause aggregation of MRs into distinct clusters. To identify the functional determinants of MR nuclear organization, we examined the localization pattern of wild type MR and a series of mutants in the presence and absence of ligands using fluorescent protein chimeras in living cells. Our data show that although MR DNA binding is not necessary to mediate nuclear localization, it is absolutely required for wild type cluster formation as is an intact N-terminal or C-terminal activation function. In contrast, destabilization of a dimerization motif within the DNA-binding domain has no effect on subnuclear receptor architecture. These data suggest that normal MR cluster formation is dependent on both DNA binding and intact transcriptional activation functions but not on DNA-dependent receptor dimerization. Because dimer mutants bind with high affinity to hormone response element DNA multimers but not to single palindromic DNA sites, we suggest that clusters represent MR aggregates bound to DNA response element multimers in the vicinity of regulated genes.