Feedback Control of Glucocorticoid Production is Established during Fetal Development

Glucocorticoid programming of neuroimmune function

Throughout life physiological systems strive to maintain homeostasis and these systems are susceptible to exposure to maternal or environmental perturbations, particularly during embryonic development. In some cases, these perturbations may influence genetic and physiological processes that permanently alter the functioning of these physiological systems; a process known as developmental programming. In recent years, the neuroimmune system has garnered attention for its fundamental interactions with key hormonal systems, such as the hypothalamic pituitary adrenal (HPA) axis. The ultimate product of this axis, the glucocorticoid hormones, play a key role in modulating immune responses within the periphery and the CNS as part of the physiological stress response. It is well-established that elevated glucocorticoids induced by developmental stress exert profound short and long-term physiological effects, yet there is relatively little information of how these effects are manifested within the neuroimmune system. Pre and post-natal periods are prime candidates for manipulation in order to uncover the physiological mechanisms that underlie glucocorticoid programming of neuroimmune responses. Understanding the potential programming role of glucocorticoids may be key in uncovering vulnerable windows of CNS susceptibility to stressful experiences during embryonic development and improve our use of glucocorticoids as therapeutics in the treatment of neurodegenerative diseases.

A naturally hypersensitive glucocorticoid receptor elicits a compensatory reduction of hypothalamus-pituitary-adrenal axis activity early in ontogeny

We comprehensively characterized the effects of a unique natural gain-of-function mutation in the glucocorticoid receptor (GR), GRAla610Val, in domestic pigs to expand current knowledge of the phenotypic consequences of GR hypersensitivity. Cortisol levels were consistently reduced in one-week-old piglets, at weaning and in peripubertal age, probably due to a reduced adrenal capacity to produce glucocorticoids (GC), which was indicated by an adrenocortical thinning in GRAla610Val carriers. Adrenocorticotrophic hormone (ACTH) levels were significantly reduced in one-week-old piglets only. Expression analyses in peripubertal age revealed significant downregulation of hypothalamic expression of CRH and AVP, the latter only in females, and upregulation of hepatic expression of SERPINA6, by GRAla610Val Transcriptional repression of proinflammatory genes in peripheral blood mononuclear cells (PBMCs) from GRAla610Val carriers was more sensitive to dexamethasone treatment ex vivo However, no significant effects on growth, body composition, blood chemistry or cell counts were observed under baseline conditions. These results suggest that GRAla610Val-induced GR hypersensitivity elicits a compensatory reduction in endogenous, bioactive glucocorticoid levels via readjustment of the hypothalamus-pituitary-adrenal (HPA) axis early in ontogeny to maintain an adequate response, but carriers are more sensitive to exogenous GC. Therefore, GRAla610Val pigs represent a valuable animal model to explore GR-mediated mechanisms of HPA axis regulation and responses to glucocorticoid-based drugs.

Arsenic exposure during embryonic development alters the expression of the long noncoding RNA growth arrest specific-5 (Gas5) in a sex-dependent manner

Our previous studies suggest that prenatal arsenic exposure (50ppb) modifies epigenetic control of the programming of the glucocorticoid receptor (GR) signaling system in the developing mouse brain. These deficits may lead to long-lasting consequences, including deficits in learning and memory, increased depressive-like behaviors, and an altered set-point of GR feedback throughout life. To understand the arsenic-induced changes within the GR system, we assessed the impact of in utero arsenic exposure on the levels of the GR and growth arrest-specific-5 (Gas5), a noncoding RNA, across a key gestational period for GR programming (gestational days, GD 14-18) in mice. Gas5 contains a glucocorticoid response element (GRE)-like sequence that binds the GR, thereby decreasing GR-GRE-dependent gene transcription and potentially altering GR programming. Prenatal arsenic exposure resulted in sex-dependent and age-dependent shifts in the levels of GR and Gas5 expression in fetal telencephalon. Nuclear GR levels were reduced in males, but unchanged in females, at all gestational time points tested. Total cellular Gas5 levels were lower in arsenic-exposed males with no changes seen in arsenic-exposed females at GD16 and 18. An increase in total cellular Gas-5 along with increased nuclear levels in GD14 arsenic-exposed females, suggests a differential regulation of cellular compartmentalization of Gas5. RIP assays revealed reduced Gas5 associated with the GR on GD14 in the nuclear fraction prepared from arsenic-exposed males and females. This decrease in levels of GR-Gas5 binding continued only in the females at GD18. Thus, nuclear GR signaling potential is decreased in prenatal arsenic-exposed males, while it is increased or maintained at levels approaching normal in prenatal arsenic-exposed females. These findings suggest that females, but not males, exposed to arsenic are able to regulate the levels of nuclear free GR by altering Gas5 levels, thereby keeping GR nuclear signaling closer to control (unexposed) levels.

Maternal programming of sex-specific responses to predator odor stress in adult rats

Prenatal stress mediated through the mother can lead to long-term adaptations in stress-related phenotypes in offspring. This study tested the long-lasting effect of prenatal exposure to predator odor, an ethologically relevant and psychogenic stressor, in the second half of pregnancy. As adults, the offspring of predator odor-exposed mothers showed increased anxiety-like behaviors in commonly used laboratory tasks assessing novelty-induced anxiety, increased defensive behavior in males and increased ACTH stress reactivity in females in response to predator odor. Female offspring from predator odor-exposed dams showed increased transcript abundance of glucocorticoid receptor (NR3C1) on the day of birth and FK506 binding protein 5 (FKBP5) in adulthood in the amygdala. The increase in FKBP5 expression was associated with decreased DNA methylation in Fkbp5 intron V. These results indicate a sex-specific response to maternal programming by prenatal predator odor exposure and a potential epigenetic mechanism linking these responses with modifications of the stress axis in females. These results are in accordance with the mismatch hypothesis stating that an animal's response to cues within its life history reflects environmental conditions anticipated during important developmental periods and should be adaptive when these conditions are concurring.

Dissection of glucocorticoid receptor-mediated inhibition of the hypothalamic-pituitary-adrenal axis by gene targeting in mice

Negative feedback regulation of glucocorticoid (GC) synthesis and secretion occurs through the function of glucocorticoid receptor (GR) at sites in the hypothalamic-pituitary-adrenal (HPA) axis, as well as in brain regions such as the hippocampus, prefrontal cortex, and sympathetic nervous system. This function of GRs in negative feedback coordinates basal glucocorticoid secretion and stress-induced increases in secretion that integrate GC production with the magnitude and duration of the stressor. This review describes the effects of GR loss along major sites of negative feedback including the entire brain, the paraventricular nucleus of the hypothalamus (PVN), and the pituitary. In genetic mouse models, we evaluate circadian regulation of the HPA axis, stress-stimulated neuroendocrine response and behavioral activity, as well as the integrated response of organism metabolism. Our analysis provides information on contributions of region-specific GR-mediated negative feedback to provide insight in understanding HPA axis dysregulation and the pathogenesis of psychiatric and metabolic disorders.

Prenatal arsenic exposure alters the programming of the glucocorticoid signaling system during embryonic development

The glucocorticoid system, which plays a critical role in a host of cellular functions including mood disorders and learning and memory, has been reported to be disrupted by arsenic. In previous work we have developed and characterized a prenatal moderate arsenic exposure (50ppb) model and identified several deficits in learning and memory and mood disorders, as well as alterations within the glucocorticoid receptor signaling system in the adolescent mouse. In these present studies we assessed the effects of arsenic on the glucocorticoid receptor (GR) pathway in both the placenta and the fetal brain in response at two critical periods, embryonic days 14 and 18. The focus of these studies was on the 11β-hydroxysteroid dehydrogenase enzymes (11β-HSD1 and 11β-HSD2) which play a key role in glucorticoid synthesis, as well as the expression and set point of the GR negative feedback regulation. Negative feedback regulation is established early in development. At E14 we found arsenic exposure significantly decreased expression of both protein and message in brain of GR and the 11β-HSD1, while 11β-HSD2 enzyme protein levels were increased but mRNA levels were decreased in the brain. These changes in brain protein continued into the E18 time point, but mRNA levels were no longer significantly altered. Placental HSD11B2 mRNA was not altered by arsenic treatment but protein levels were elevated at E14. GR placental protein levels were decreased at E18 in the arsenic exposed condition. This suggests that arsenic exposure may alter GR expression levels as a consequence of a prolonged developmental imbalance between 11β-HSD1 and 11β-HSD2 protein expression despite decreased 11HSDB2 mRNA. The suppression of GR and the failure to turn down 11β-HSD2 protein expression during fetal development may lead to an altered set point for GR signaling throughout adulthood. To our knowledge, these studies are the first to demonstrate that gestational exposure to moderate levels of arsenic results in altered fetal programming of the glucocorticoid system.

Glucocorticoids in multiple sclerosis and experimental autoimmune encephalomyelitis

Glucocorticoids exert a variety of immunomodulatory activities. Since changes in glucocorticoid homeostasis impact on susceptibility to autoimmune diseases, and synthetic glucocorticoids are widely used in the treatment of multiple sclerosis, a detailed understanding of their mechanism of action is desirable. Experimental autoimmune encephalomyelitis is a common animal model that mirrors many hallmarks of multiple sclerosis, a chronic inflammatory disease of the CNS with presumed autoimmune origin. Experimental autoimmune encephalomyelitis has been instrumental for many years in studying multiple sclerosis, revealing the blood-brain barrier, the microglia and T-cell apoptosis as major targets of glucocorticoids in this disease. Despite the great advances in the field, the answers to many questions concerning the mechanism of glucocorticoids; for example, the contribution of nongenomic effects or the cell-type specificity of their action, remain elusive. This review will critically discuss what we have learned so far from the analysis of animal models of the molecular mode of therapeutic and endogenous glucocorticoid action in multiple sclerosis. With this knowledge in mind, we should be able to further improve the management of multiple sclerosis using this class of drugs.

Context modulates outcome of perinatal glucocorticoid action in the brain

Prematurely born infants may be at risk, because of inadequate maturation of tissues. If there are signs of preterm birth, it has become common practice therefore to treat either antenatally the mother or postnatally the infant with glucocorticoids to accelerate tissue development, particularly of the lung. However, this life-saving early glucocorticoid treatment was found to increase the risk of adverse outcome in later life. In one animal study, the authors reported a 25% shorter lifespan of rats treated as newborns with the synthetic glucocorticoid dexamethasone, but so far this finding has not been replicated. After a brief clinical introduction, we discuss studies in rodents designed to examine how perinatal glucocorticoid action affects the developing brain. It appears that the perinatal action of the glucocorticoid depends on the context and the timing as well as the type of administered steroid. The type of steroid is important because the endogenous glucocorticoids cortisol and corticosterone bind to two distinct receptor populations, i.e., mineralocorticoid and glucocorticoid receptors (GR), while synthetic glucocorticoids predominantly bind to the GR. In addition, if given antenatally hydrocortisone is inactivated in the placenta by 11β-HSD type 2, and dexamethasone is not. With respect to timing, the outcome of glucocorticoid effects is different in early vs. late phases of brain development. The context refers to the environmental input that can affect the susceptibility to glucocorticoid action in the newborn rodent brain; early handling of pups and maternal care obliterate effects of post-natal dexamethasone treatment. Context also refers to coping with environmental conditions in later life, for which the individual may have been programed epigenetically by early-life experience. This knowledge of determinants affecting the outcome of perinatal glucocorticoid exposure may have clinical implications for the treatment of prematurely born infants.

Differential subcellular localization of the glucocorticoid receptor in distinct neural stem and progenitor populations of the mouse telencephalon in vivo

Glucocorticoids are given to pregnant women at risk for premature delivery to promote lung maturation. Despite reports of detrimental effects of glucocorticoids on telencephalic neural stem/progenitor cells (NSPCs), the regional and cellular expressions of the glucocorticoid receptor (GR) in various NSPC populations in the intact brain have not been thoroughly assessed. Therefore in this study we performed a detailed analysis of GR protein expression in the developing mouse ventral and dorsal telencephalon in vivo. At embryonic day 11.5 (E11.5), the majority of Pax6-positive radial glial cells (RGCs) and Tbr2-positive intermediate progenitor cells (IPCs) expressed nuclear GR, while a small number of RGCs on the apical ventricular zone (aVZ), expressed cytoplasmic GR. However, on E13.5, the latter population of RGCs increased in size, whereas abventricular NSPCs and especially neurons of the cortical plate, expressed nuclear GR. In IPCs, GR was always nuclear. A similar expression profile was observed throughout the ventral telencephalon, hippocampus and olfactory bulb, with NSPCs of the aVZ primarily expressing cytoplasmic GR, while abventricular NSPCs and mature cells primarily expressed nuclear GR. Close to birth, nuclear GR accumulated within specific cortical areas such as layer V, the subplate and CA1 area of the hippocampus. In summary, our data show that GR protein is present in early NSPCs of the dorsal and ventral telencephalon at E11.5 and primarily occupies the nucleus. Moreover, our study suggests that the subcellular localization of the receptor may be subjected to region and neurodevelopmental stage-specific regulation.

Perinatal exposure to 50 ppb sodium arsenate induces hypothalamic-pituitary-adrenal axis dysregulation in male C57BL/6 mice

Over the past two decades, key advancements have been made in understanding the complex pathology that occurs following not only high levels of arsenic exposure (>1 ppm) but also levels previously considered to be low (<100 ppb). Past studies have characterized the deleterious effects of arsenic on the various functions of cardiovascular, pulmonary, immunological, respiratory, endocrine and neurological systems. Other research has demonstrated an elevated risk of a multitude of cancers and increased rates of psychopathology, even at very low levels of arsenic exposure. The hypothalamic-pituitary-adrenal (HPA) axis represents a multisite integration center that regulates a wide scope of biological and physiological processes: breakdown within this system can generate an array of far-reaching effects, making it an intriguing candidate for arsenic-mediated damage. Using a mouse model, we examined the effects of perinatal exposure to 50 ppb sodium arsenate on the functioning of the HPA axis through the assessment of corticotrophin-releasing factor (CRF), proopiomelanocortin (Pomc) mRNA, adrenocorticotrophin hormone (ACTH), corticosterone (CORT), 11β-hydroxysteroid dehydrogenase Type 1 (11β-HSD 1), and glucocorticoid receptor (GR) protein and mRNA. Compared to controls, we observed that the perinatal arsenic-exposed offspring exhibit an increase in hypothalamic CRF, altered CORT secretion both at baseline and in response to a stressor, decreased hippocampal 11β-HSD 1 and altered subcellular GR distribution in the hypothalamus. These data indicate significant HPA axis impairment at post-natal day 35 resulting from perinatal exposure to 50 ppb sodium arsenate. Our findings suggest that the dysregulation of this critical regulatory axis could underlie important molecular and cognitive pathology observed following exposure to arsenic.

Increased glucocorticoid receptor expression and activity mediate the LPS resistance of SPRET/EI mice

SPRET/Ei mice are extremely resistant to acute LPS-induced lethal inflammation when compared with C57BL/6. We found that in vivo SPRET/Ei mice exhibit strongly reduced expression levels of cytokines and chemokines. To investigate the role of the potent anti-inflammatory glucocorticoid receptor (GR) in the SPRET/Ei phenotype, mice were treated with the GR antagonist RU486 or bilateral adrenalectomy. Under such conditions, both C57BL/6 and SPRET/Ei mice were strongly sensitized to LPS, and the differences in LPS response between SPRET/Ei and C57BL/6 mice were completely gone. These results underscore the central role of GR in the LPS hyporesponsiveness of SPRET/Ei mice. Compared with C57BL/6, SPRET/Ei mice were found to express higher GR levels, which were reflected in increased GR transactivation. Using a backcross mapping strategy, we demonstrate that the high GR transcription levels are linked to the Nr3c1 (GR) locus on chromosome 18 itself. Unexpectedly, SPRET/Ei mice exhibit a basal overactivation of the hypothalamic-pituitary-adrenal axis, namely strongly increased corticosterone levels, ACTH levels, and adrenocortical size. As a consequence of the excess of circulating glucocorticoids (GCs), levels of hepatic gluconeogenic enzymes are increased, and insulin secretion from pancreatic β-cells is impaired, both of which result in hyperglycemia and glucose intolerance in SPRET/Ei mice. We conclude that SPRET/Ei mice are unique as they display an unusual combination of elevated GR expression and increased endogenous GC levels. Hence, these mice provide a new and powerful tool for the study of GR- and GC-mediated mechanisms, including immune repressive functions, neuroendocrine regulation, insulin secretion, and carbohydrate metabolism.

Effects of intrauterine exposure to synthetic glucocorticoids on fetal, newborn, and infant hypothalamic-pituitary-adrenal axis function in humans: a systematic review

BACKGROUND

Synthetic glucocorticoids are commonly used in reproductive medicine. Fetal organ systems are highly sensitive to changes in the intrauterine environment, including overexposure to glucocorticoids. Structural and functional alterations resulting from such changes may persist throughout life and have been associated with diverse diseases. One system that could be particularly sensitive to fetal glucocorticoid overexposure is the hypothalamic-pituitary-adrenal (hpa) axis. Many human studies have investigated this possibility, but a systematic review to identify consistent, emergent findings is lacking.

METHODS

We systematically review 49 human studies, assessing the effects of intrauterine exposure to synthetic glucocorticoids on fetal, neonate, and infant hpa function.

RESULTS

Study quality varied considerably, but the main findings held true after restricting the analyses to higher-quality studies: intrauterine exposure to synthetic glucocorticoids reduces offspring hpa activity under unstimulated conditions after pain but not pharmacological challenge. Although reduced unstimulated hpa function appears to recover within the first 2 wk postpartum, blunted hpa reactivity to pain is likely to persist throughout the first 4 months of life. There is some evidence that the magnitude of the effects is correlated with the total amount of glucocorticoids administered and varies with the time interval between glucocorticoid exposure and hpa assessment.

CONCLUSIONS

This systematic review has allowed the demonstration of the way in which intrauterine exposure to various regimens of synthetic glucocorticoids affects various forms of hpa function. As such, it guides future studies in terms of which variables need to be focused on in order to further strengthen the understanding of such therapy, whilst continuing to profit from its clinical benefits.

Development of fetal brain renin-angiotensin system and hypertension programmed in fetal origins

Since the concept of fetal origins of adult diseases was introduced in 1980s, the development of the renin-angiotensin system (RAS) in normal and abnormal patterns has attracted attention. Recent studies have shown the importance of the fetal RAS in both prenatal and postnatal development. This review focuses on the functional development of the fetal brain RAS, and ontogeny of local brain RAS components in utero. The central RAS plays an important role in the control of fetal cardiovascular responses, body fluid balance, and neuroendocrine regulation. Recent progress has been made in demonstrating that altered fetal RAS development as a consequence of environmental insults may impact on "programming" of hypertension later in life. Given that the central RAS is of equal importance to the peripheral RAS in cardiovascular regulation, studies on the fetal brain RAS development in normal and abnormal patterns could shed light on "programming" mechanisms of adult cardiovascular diseases in fetal origins.

Central cholinergic signal-mediated neuroendocrine regulation of vasopressin and oxytocin in ovine fetuses

BACKGROUND

The hypothalamic-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting arginine vasopressin (AVP) and oxytocin (OT) in response to a variety of signals, including osmotic and nonosmotic stimuli. It is well established that central cholinergic mechanisms are critical in the regulation of cardiovascular responses and maintenance of body fluid homeostasis in adults. Our recent study demonstrated that intracerebroventricular (i.c.v.) injection of carbachol elicited an increase of blood pressure in the near-term ovine fetuses. However, in utero development of brain cholinergic mechanisms in the regulation of the hypothalamic neuropeptides is largely unknown. This study investigated AVP and OT neural activation in the fetal hypothalamus induced by central carbachol.

RESULTS

Chronically prepared near-term ovine fetuses (0.9 gestation) received an i.c.v. carbachol (3 microg/kg). Fetal blood samples were collected for AVP and OT assay, and brains were used for c-fos mapping studies. I.c.v. carbachol significantly increased fetal plasma AVP and OT concentrations. Intense FOS immunoreactivity (FOS-ir) was observed in the fetal supraoptic nuclei (SON) and paraventricular nuclei (PVN) in the hypothalamus. Double labeling demonstrated that a number of AVP- and OT-containing neurons in the fetal SON and PVN were expressing c-fos in response to central carbachol.

CONCLUSION

The results indicate that the central cholinergic mechanism is established and functional in the regulation of the hypothalamic neuropeptides during the final trimester of pregnancy. This provides evidence for a functional link between the development of central cholinergic mechanisms and hypothalamic neuropeptide systems in the fetus.

Hypothalamic-pituitary-adrenal axis dysregulation and behavioral analysis of mouse mutants with altered glucocorticoid or mineralocorticoid receptor function

Corticosteroid receptors are critical for the maintenance of homeostasis after both psychological and physiological stress. To understand the different roles and interactions of the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) during stress, it is necessary to dissect the role of corticosteroid signaling at both the system and sub-system level. A variety of GR transgenic mouse lines have recently been used to characterize the role of GR in the CNS as a whole and particularly in the forebrain. We will describe both the behavioral and cellular/molecular implications of disrupting GR function in these animal models and describe the implications of this data for our understanding of normal endocrine function and stress adaptation. MRs in tight epithelia have a long established role in sodium homeostasis. Recently however, evidence has suggested that MRs in the limbic brain also play an important role in psychological stress. Just as with GR, targeted mutations in MR induce a variety of behavioral changes associated with stress adaptation. In this review, we will discuss the implications of this work on MR. Finally, we will discuss the possible interaction between MR and GR and how future work using double mutants (through conventional means or virus based gene alteration) will be needed to more fully understand how signaling through these two steroid receptors provides the adaptive mechanisms to deal with a variety of stressors.

Genetic dissection of glucocorticoid receptor function in the mouse brain

In the brain, glucocorticoids exert functions in neurogenesis, synaptic plasticity and behavioural responses, as well as in the control of hypothalamic-pituitary-adrenal axis activity. The generation of mice harbouring germline mutations that result either in loss or in gain of glucocorticoid receptor function provided a useful tool for understanding the role of glucocorticoids in the brain in vivo. The improvement of genomic technologies additionally allowed the establishment of mouse models with function-selective point mutations of the receptor as well as the generation of mice harbouring spatially and/or temporally restricted loss of glucocorticoid receptor, specifically within the brain. These models will provide the opportunity to better understand the mechanisms involved in glucocorticoid signalling within the nervous system.

Molecular physiology of pituitary development: signaling and transcriptional networks

The pituitary gland is a central endocrine organ regulating basic physiological functions, including growth, the stress response, reproduction, metabolic homeostasis, and lactation. Distinct hormone-producing cell types in the anterior pituitary arise from a common ectodermal primordium during development by extrinsic and intrinsic mechanisms, providing a powerful model system for elucidating general principles in mammalian organogenesis. The central purpose of this review is to inspect the integrated signaling and transcriptional events that affect precursor proliferation, cell lineage commitment, terminal differentiation, and physiological regulation by hypothalamic tropic factors.

The pituitary-adrenal axis of fetal rats after maternal dexamethasone treatment

Elevated glucocorticoid level in the gravid female circulation affects number of endocrine functions in fetuses and offspring. In this research female rats were injected with dexamethasone (Dx) in three consecutive daily doses of 1.0, 0.5, 0.5 mg/kg body weight, starting from day 16 of pregnancy. The influence of this treatment on the pituitary adrenocorticotrophic (ACTH) cells and adrenal glands of 19-day-old fetuses was examined immunocytochemically and by morphometric analysis. Moreover, the proliferative activity of adrenocortical cells was estimated after application of the mitotic inhibitor Oncovine. Administration of Dx to pregnant rats induced a decline of fetal ACTH cell immunopositivity and significant decreases of ACTH cell volume (23%, p < 0.05), volume density (41%, p < 0.05), and its number per unit area (17%, p < 0.05) in comparison to the control 19-day-old fetuses. Reduced proliferative activity of adrenocortical cells (31%; p < 0.05) in zona glomerulosa, as well as the volume of this zone were detected. The volume and number of fetal adrenocortical cells in the inner zone and chromoblasts were not significantly reduced after Dx treatment of pregnant rats. These results show that maternal Dx administration in the period when the fetal hypothalamo-pituitary-adrenal (PA) axis begins its function inhibited the PA axis. Reduced ACTH cell function and mitotic activity led to suppression of adrenocortical cell multiplication in zona glomerulosa, the region of the adrenal cortex where most proliferating cells were found in control 19-day-old fetuses. Thus, increased glucocorticoid levels during late pregnancy caused developmental modifications involving the fetal PA axis, which could be the basis of the altered endocrine responsiveness in adult life.

Xenobiotics and the glucocorticoid receptor: additive antagonistic effects on tyrosine aminotransferase activity in rat hepatoma cells

Methylsulfonyl-PCBs (MeSO2-PCBs) and some fungicides were studied for their functional effects on the glucocorticoid signal transduction in the Reuber rat hepatoma H-II-E-C3 cell line. 4-Substituted MeSO2-PCBs, tolylfluanid and ketoconazole displayed antagonistic effects on dexamethasone-induced tyrosine aminotransferase specific activity (IC50 ranging from 0.7-5.1 microM), but no agonist activity. These substances also had affinity to the mouse glucocorticoid receptor in competition binding studies, indicating that the inhibition of the middle cerebral artery occlusion-activity is indeed mediated by receptor binding. Thus, substances with a structural resemblance with a methyl sulfonyl group, such as the fungicide tolylfluanid, may inhibit glucocorticoid receptor-regulated gene transcription. In co-exposure experiments with three substances, multivariate modelling showed that the inhibitory effect of 4-MeSO2-2,5,6,2',4'-pentachlorobiphenyl (4-MeSO2-CB91), 4-MeSO2-2,3,6,2',4',5'-hexachlorobiphenyl (4-MeSO2-CB149) and tolylfluanid on tyrosine aminotransferase activity was close to additive. Thus, co-exposure to such different chemicals as persistent organic pollutants and pesticides may affect cells additively. Chemical interference with the glucocorticoid hormone system therefore deserves further attention in vivo.

Mutant mouse models of depression: Candidate genes and current mouse lines

Depression is a multifactorial and multigenetic disease. At present, three main theories try to conceptualize its molecular and biochemical mechanisms, namely the monoamine-, the hypothalamus-pituitary-adrenal- (HPA-) system- and the neurotrophin-hypotheses. One way to explore, validate or falsify these hypotheses is to alter the expression of genes that are involved in these systems and study their respective role in animal behavior and neuroendocrinological parameters. Following an introduction in which we briefly describe each hypothesis, we review here the different mouse lines generated to study the respective molecular pathways. Among the many mutant lines generated, only a few can be regarded as genetic depression models or as models of predisposition for a depressive syndrome after stress exposure. However, this is likely to reflect the human situation where depressive syndromes are complex, can vary to a great extent with respect to their symptomatology, and may be influenced by a variety of environmental factors. Mice with mutations of candidate genes showing depression-like features on behavioral or neurochemical levels may help to define a complex molecular framework underlying depressive syndromes. Because it is conceivable that manipulation of one single genetic function may be necessary but not sufficient to cause complex behavioral alterations, strategies for improving genetic modeling of depression-like syndromes in animals possibly require a simultaneous targeted dysregulation of several genes involved in the pathogenesis of depression. This approach would correspond to the new concept of 'endophenotypes' in human depression research trying to identify behavioral traits which are thought to be encoded by a limited set of genes.

Corticosteroid Receptor Transgenic Mice

Dysregulations and dysfunctions of corticosteroids and their receptors have been implicated in the pathogenesis of stress-related disorders, in particular in depression. It is currently under debate, however, whether corticosteroid imbalances are a cause or rather a consequence of affective disorders. Corticosteroids exert their effects mainly by two receptors: glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs). We present here analyses made on several strains of mice with targeted mutations of corticosteroid receptors. The results help to understand how corticosteroid receptors regulate the hypothalamic-pituitary-adrenal (HPA) system. Furthermore, first behavioral analyses have indicated that corticosteroid receptor mutant mice show alterations in their emotional behavior. Certain mouse strains with specific alterations of GR or MR expression may represent genetic models of depression or at least have a predisposition to develop a depressive or a depression-resistant state upon exposure to stress. The corticosteroid receptor-regulated target genes to be identified in these models may code for proteins that could represent new drug-targets for the treatment of affective disorders.

Development of the fetal intestine in mice lacking the glucocorticoid receptor (GR)

During rodent development there are two surges of circulating corticosterone: one just prior to birth and then one in the third postnatal week. Prior studies have shown that the latter controls the rate of intestinal development in the postnatal period. To date, a role for the earlier surge in the prenatal phase of intestinal development has not been investigated. We hypothesized that the late fetal surge of circulating corticosterone is involved in both morphologic and functional maturation of the intestinal epithelium, and thus that such maturation would be delayed if glucocorticoid action was abrogated. The hypothesis was tested by studying intestinal development in mice lacking a functional glucocorticoid receptor (GR). After GR+/- mice were bred onto a C57Bl/6 background, heterozygote matings yielded the expected ratios of -/-, +/-, and +/+ offspring. Analysis of GR mRNA in intestines of +/+ and -/- fetuses confirmed expression in wild-type mice but not in the GR-null mice. Intestinal histology of GR+/+ and -/- littermates at E13.5, E15.5, and E18.5 showed no effect of GR genotype on morphologic development. Further studies at E18.5 showed that GR-/- mice have normal functional maturation of the intestinal epithelium as assessed by: lactase activity in the enterocyte lineage, normal numbers of goblet and enteroendocrine cells, and normal numbers of proliferating cells in the intestinal crypts. Neither the minerolocorticoid receptor (MR) nor the pregnane X receptor (PXR) showed compensatory up-regulation in GR-/- mice. We conclude that, in contrast to our original hypothesis, the rodent intestine passes through a phase of glucocorticoid independence (late fetal) prior to becoming responsive to glucocorticoids in the postnatal period. These findings have implications for the clinical use of corticosteroids to enhance intestinal maturation in preterm infants.

Inactivation of the GR in the nervous system affects energy accumulation

The homeostatic regulation of body weight protects the organism from the negative consequences of starvation and obesity. Glucocorticoids (GCs) modulate this regulation, although the underlying mechanisms remain unclear. To address the role of central GRs in the regulation of energy balance, we studied mice in which GRs have selectively been inactivated in the nervous system. Mutant mice display marked growth retardation. During suckling age this is associated with normal fat deposition causing a 60% temporary increase of percent body fat, compared with control littermates. After weaning, fat and protein depositions are reduced so that adults are both smaller and leaner than their controls. Decreased food intake and, after weaning, reduced metabolic efficiency account for these developmental disturbances. Plasma levels of leptin and insulin, two important energy balance regulators, are elevated in young mutants but normal in adults. Leptin/body fat ratio is higher at all ages, suggesting disturbed control of circulating leptin as a consequence of chronically elevated GC levels in mutant animals. Adult mutants display increased hypothalamic CRH and NPY levels, but peptide levels of melanin concentrating hormone and Orexin A and B are unchanged. The increased levels of plasma GCs and hypothalamic CRH may act as catabolic signals most likely leading to persistently reduced energy accumulation.

Signaling and transcriptional mechanisms in pituitary development

During the development of the pituitary gland, distinct hormone-producing cell types arise from a common population of ectodermal progenitors, providing an instructive model system for elucidating the molecular mechanisms of patterning and cell type specification in mammalian organogenesis. Recent studies have established that the development of the pituitary occurs through multiple sequential steps, allowing the coordinate control of the commitment, early patterning, proliferation, and positional determination of pituitary cell lineages in response to extrinsic and intrinsic signals. The early phases of pituitary development appear to be mediated through the activities of multiple signaling gradients emanating from key organizing centers that give rise to temporally and spatially distinct patterns of transcription factor expression. The induction of these transcriptional mediators in turn acts to positionally organize specific pituitary cell lineages within an apparently uniform field of ectodermal progenitors. Ultimately, pituitary cell types have proven to be both specified and maintained through the combinatorial interactions of a series of cell-type-restricted transcription factors that dictate the cell autonomous programs of differentiation in response to the transient signaling events.

GRKO mice express an aberrant dexamethasone-binding glucocorticoid receptor, but are profoundly glucocorticoid resistant

The introduction of a targeted insertion mutation into exon 2 of the gene coding for the glucocorticoid receptor (GR) enabled production of glucocorticoid receptor knock-out (GRKO) mice. GRKO mice on a C57BL/6/129sv mixed genetic background show a variable phenotype, with 90% of -/- mice dying at birth with respiratory insufficiency but 10% of mutant mice surviving to maturity. To investigate the possibility of residual GR expression in surviving GRKO mice we have measured binding of the synthetic glucocorticoid dexamethasone in tissue extracts from adrenalectomized mice. High affinity binding of dexamethasone in protein extracts of liver, kidney, lung and brain from adult GRKO mice is found at levels 30-60% those in wild-type mice, with heterozygotes (+/-) having intermediate levels. PCR and ribonuclease protection analysis showed comparable levels of GR mRNA on the 3' side of the gene-targeted insertional mutation in exon 2 of the GR gene, with almost no GR mRNA detected from exons 1 and 2 on the 5' side of the gene-targeted insertional mutation. Western blot analysis using a C-terminal specific GR antibody detects a 39 kDa GR fragment in extracts from adult GRKO mice. Despite the evidence for expression of a ligand-binding domain fragment of the glucocorticoid receptor these mice are profoundly glucocorticoid resistant, with elevated levels of plasma ACTH and corticosterone. Thymocytes from adult and fetal GRKO mice are resistant to dexamethasone-induced apoptosis and cultured fetal hepatocytes from GRKO mice are completely refractory to glucocorticoid induction of the gluconeogenic enzyme glucose-6-phosphatase. Thus although the surviving adult homozygous GRKO mice express a dexamethasone-binding GR fragment, their classic target tissues remain profoundly glucocorticoid insensitive.

Mice with an increased glucocorticoid receptor gene dosage show enhanced resistance to stress and endotoxic shock

Targeted mutagenesis of the glucocorticoid receptor has revealed an essential function for survival and the regulation of multiple physiological processes. To investigate the effects of an increased gene dosage of the receptor, we have generated transgenic mice carrying two additional copies of the glucocorticoid receptor gene by using a yeast artificial chromosome. Interestingly, overexpression of the glucocorticoid receptor alters the basal regulation of the hypothalamo-pituitary-adrenal axis, resulting in reduced expression of corticotropin-releasing hormone and adrenocorticotrope hormone and a fourfold reduction in the level of circulating glucocorticoids. In addition, primary thymocytes obtained from transgenic mice show an enhanced sensitivity to glucocorticoid-induced apoptosis. Finally, analysis of these mice under challenge conditions revealed that expression of the glucocorticoid receptor above wild-type levels leads to a weaker response to restraint stress and a strongly increased resistance to lipopolysaccharide-induced endotoxic shock. These results underscore the importance of tight regulation of glucocorticoid receptor expression for the control of physiological and pathological processes. Furthermore, they may explain differences in the susceptibility of humans to inflammatory diseases and stress, depending on individual prenatal and postnatal experiences known to influence the expression of the glucocorticoid receptor.

Circadian rise in maternal glucocorticoid prevents pulmonary dysplasia in fetal mice with adrenal insufficiency

The hypothalamic-pituitary-adrenal (HPA) axis, including hypothalamic corticotropin-releasing hormone (CRH) and pituitary corticotropin, is one of the first endocrine systems to develop during fetal life, probably because glucocorticoid secretion is necessary for the maturation of many essential fetal organs. Consistent with this, pregnant mice with an inactivating mutation in the Crh gene deliver CRH-deficient offspring that die at birth with dysplastic lungs, which can be prevented by prenatal maternal glucocorticoid treatment. But children lacking the ability to synthesize cortisol (because of various genetic defects in adrenal gland development or steroidogenesis) are not born with respiratory insufficiency or abnormal lung development, suggesting that the transfer of maternal glucocorticoid across the placenta might promote fetal organ maturation in the absence of fetal glucocorticoid production. We used pregnant mice with a normal HPA axis carrying fetuses with CRH deficiency to characterize the relative contributions of the fetal and maternal adrenal to the activity of the fetal HPA axis, and related these findings to fetal lung development. We found that in the presence of fetal adrenal insufficiency, normal fetal lung development is maintained by the transfer of maternal glucocorticoid to the fetus, specifically during the circadian peak in maternal glucocorticoid secretion.

Ontogeny of epinephrine metabolic pathways in the rat: role of glucocorticoids

Recent studies suggest that the initial expression of adrenal phenylethanolamine N-methyltransferase (PNMT) and epinephrine (E) are dependent upon stimulation of adrenal glucocorticoid receptors. However, evidence suggests that the expression of heart and brain PNMT is independent of glucocorticoids. We measured PNMT activity and E levels in adrenal, heart and head over the latter half of gestation in rat fetuses treated chronically with glucocorticoids, and in normal controls. Chronic glucocorticoid treatment ending on embryonic day (e)12 did not affect heart, head or trunk PNMT activity or E levels. In contrast, chronic glucocorticoid exposure ending e19 or e20 resulted in marked increases in both PNMT and E in adrenal, heart and head tissues. The elevation of E in all three tissues was unaffected by maternal adrenalectomy, indicating enhanced fetal E synthesis. In the absence of exogenous glucocorticoid treatment heart PNMT activity peaked on e12, prior to the earliest reported appearance of glucocorticoid receptors. We conclude that expression of PNMT in all three tissues is glucocorticoid independent until the latter part of gestation when it is readily enhanced by glucocorticoids.

The effects of serotonin on glucocorticoid receptor binding in rat raphe nuclei and hippocampal cells in culture

The raphe-hippocampal serotonin (5-HT) system is involved in the regulation of the hypothalamus-pituitary-adrenal axis. The purpose of this study was to determine and compare the roles of 5-HT in the regulation of glucocorticoid receptor (GR) binding in the raphe nuclei and in the hippocampus. The effects of 5-HT, 5-HT agonists, and the 5-HT reuptake inhibitor citalopram on GR binding sites were studied in primary cultures of the fetal raphe nuclei and the hippocampus. Exposure of hippocampal cells to 5-HT, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI; a 5-HT2 agonist), or citalopram resulted in an increase in number of GR binding sites. The effect of DOI was blocked by ketanserin (a 5-HT2 antagonist). Specific and saturable GR binding was found in raphe cells. Exposure of raphe cells to 5-HT, (+/-)-8 hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT1A agonist), or citalopram induced a significant decrease in number of GR binding sites. The effect of 8-OH-DPAT was reversed by WAY 100135 [N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropiona mide; a 5-HT1A antagonist]. These results show that the regulation of GRs during fetal life is structure-dependent and involves different 5-HT receptor subtypes. Moreover, the regulation of hippocampal GRs by citalopram suggests an action of antidepressants independent of their effects on monoamines.

The glucocorticoid receptor is essential for maintaining basal and dexamethasone-induced repression of the murine corticosteroid-binding globulin gene

We have investigated hepatic expression and glucocorticoid regulation of the corticosteroid-binding globulin (CBG) gene in mice lacking a functional glucocorticoid receptor (GR). GR-/- mice show impaired negative feedback in the hypothalamic-pituitary-adrenal axis, resulting in elevated circulating levels of ACTH and corticosterone. This is seen in the neonatal period and continues into adulthood where ACTH and corticosterone levels are increased up to 4-5 fold. Despite high elevation of corticosterone we find no change in mean arterial blood pressure in GR-/- mice and no change in the renal activity of the glucocorticoid-metabolising enzymes 11beta-hydroxysteroid dehydrogenase type-1 (HSD1) and type-2 (HSD2). We do find markedly increased hepatic expression of CBG with a 50% increase in plasma CBG levels. Increased expression of CBG was detected in adult GR-/- mice and also at birth with a greater than 10-fold increase in CBG hepatic mRNA in day-18.5 embryonic GR-/- mice. Adult GR-/- mice were also resistant to dexamethasone-induced repression of CBG expression in the liver. These results indicate that in mice, GR is essential for maintaining the basal level of CBG gene expression in the liver, and is also required for dexamethasone-induced repression of the CBG gene in the adult.

Prenatal dexamethasone treatment does not prevent alterations of the hypothalamic pituitary adrenal axis in steroid 21-hydroxylase deficient mice

A major difficulty in the clinical management of congenital adrenal hyperplasia (CAH) is adjustment of glucocorticoid doses to suppress ACTH and androgens without causing iatrogenic hypercortisolism. The possibility that structural alterations of the adrenal or a dysfunction of the hypothalamic pituitary adrenal (HPA) axis caused by glucocorticoid deficiency during fetal life contribute to this problem was studied in 21-hydroxylase deficient mice caused by deletion of the cytochrome P-450 21-hydroxylase gene. Homozygotes showed about 200-fold elevations in plasma progesterone, hyperplastic adrenal cortices lacking zonation, and structural alterations of adrenocortical mitochondria. Histochemical studies showed increases in hypothalamic CRH messenger RNA (mRNA) and immunoreactive (ir) CRH, and pituitary POMC mRNA in homozygous mice. VP mRNA levels in PVN perikarya were normal, but irVP in parvicellular terminals of the median eminence was increased in homozygotes. Prenatal dexamethasone treatment (0.5 to 2 microg/day) prevented the increases in CRH mRNA, whereas dexamethasone only partially decreased POMC mRNA levels, and had no effect on serum progesterone levels. The data suggest that intrauterine glucocorticoid deficiency in CAH causes hyperactivity of the hypothalamic-pituitary-corticotroph axis and insensitivity to glucocorticoid feedback. These studies in 21-hydroxylase deficient mice may provide new insights on the mechanism, clinical manifestations and management of some types of human CAH.

Mutagenesis of the glucocorticoid receptor in mice

The glucocorticoid receptor is an ubiquitously expressed transcription factor involved in the regulation of many different physiological processes. Activated by glucocorticoids the receptor regulates transcription positively or negatively either by direct binding to DNA or by protein protein interactions. In order to define the role of the receptor during development and in physiology several mutations have been generated in the mouse. Mice with a disrupted glucocorticoid receptor gene die shortly after birth due to respiratory failure indicating an important role of the receptor in lung function. Transcription of genes encoding gluconeogenic enzymes in the liver is decreased, proliferation of erythroid progenitors is impaired and the HPA axis is strongly upregulated. To analyze molecular mechanisms of glucocorticoid receptor action in vivo a point mutation has been introduced into the mouse genome which allows to separate DNA-binding-dependent from DNA-binding-independent actions of the receptor. Mice homozygous for the point mutation survive indicating that DNA-binding of the receptor is not required for survival. Induction of glucoconegenic enzymes and proliferation of erythroid progenitors however is impaired. Interestingly, repression of corticotropin releasing factor (CRF) synthesis is maintained, whereas proopiomelanocortin (POMC) expression is upregulated. Since mice with a disrupted glucocorticoid receptor gene die shortly after birth attempts using the Cre/loxP-recombination system are made to bypass early lethality and to study the function of the receptor in defined cell types of adult animals.

Corticotropin-releasing hormone expression is the major target for glucocorticoid feedback-control at the hypothalamic level

Glucocorticoid production is controlled via the hypothalamo-pituitary-adrenal (HPA) axis by a negative feedback mechanism involving the glucocorticoid receptor (GR). A major site of regulation is the hypothalamus, where the GR is thought to repress the expression of genes such as corticotropin-releasing hormone (CRH) and arginine-vasopressin (AVP). To define the role of the GR in this feedback loop in more detail, the content of CRH, AVP and neurophysin in the median eminence of mice carrying a targeted disruption of the GR gene was studied using immunohistochemistry. GR-deficient mice were found to contain five times more CRH in the median eminence than wild-type littermates. In contrast, no significant change in the content of AVP was observed in the outer layer of the median eminence and neurophysin was also only moderately increased. Our studies suggest that, at the hypothalamic level, CRH synthesis is the major target for feedback control by the GR and that transcriptional control of AVP and neurophysin plays only a supportive role in this process.

Glucocorticoid signalling--multiple variations of a common theme

Recent advances in molecular genetics have brought us closer to answering the question as to which mechanisms are used by steroid hormone receptors to control transcription. While binding to specific response elements in the promoter and enhancer regions of many genes has for a long time been considered as the major mode of action, a growing number of alternative transcriptional control mechanisms have been identified in the last couple of years. With the recent finding that DNA binding of the glucocorticoid receptor is not essential for survival, mechanisms depending on cross-talk with other transcription factors through protein-protein interactions have gained attention. In this mini-review, we will discuss the roles of DNA binding-dependent and -independent transcriptional modes of action in development and physiology as exemplified by the analysis of glucocorticoid receptor function.

Genetic dissection of glucocorticoid receptor function in mice

Upon hormone binding, the activated glucocorticoid receptor (GR) functions as a transcription factor via different modes of action to control gene expression. Recent gene-targeting studies in mice provide new insight into the role of GR in vivo and are helping decipher the molecular mechanisms underlying its actions.

DNA binding of the glucocorticoid receptor is not essential for survival

Transcriptional regulation by the glucocorticoid receptor (GR) is essential for survival. Since the GR can influence transcription both through DNA-binding-dependent and -independent mechanisms, we attempted to assess their relative importance in vivo. In order to separate these modes of action, we introduced the point mutation A458T into the GR by gene targeting using the Cre/loxP system. This mutation impairs dimerization and therefore GRE-dependent transactivation while functions that require cross-talk with other transcription factors, such as transrepression of AP-1-driven genes, remain intact. In contrast to GR-/- mice, these mutants termed GRdim are viable, revealing the in vivo relevance of DNA-binding-independent activities of the GR.

Analysis of glucocorticoid signalling by gene targeting

Glucocorticoids are involved in the regulation of numerous physiological processes. The majority of these effects are thought to be mediated by the glucocorticoid receptor (GR) via activation and repression of gene expression. In most cases activation requires binding of a receptor-dimer to DNA while repression is mediated by protein-protein-interaction of GR-monomers with other transcription factors. To analyse the molecular mechanisms that underlie glucocorticoid effects, mouse mutations in the GR gene were generated and analysed. In order to address the role of glucocorticoid receptor signalling during development and in physiology, the gene was disrupted by gene targeting. Most of the mice homozygous for the mutation die shortly after birth due to severe lung atelectasis. Additional defects were found in the adrenals, liver, brain, bone marrow and thymus as well as in the feedback-regulation of the HPA-axis. To approach the question which functions of the GR are regulated by DNA-binding and which by protein-protein-interaction, a point mutation was introduced into the dimerization domain of the GR which is located in the DNA-binding domain. By homologous recombination in ES-cells using the Cre/loxP-system, mice carrying this mutation were generated [GR(dim) mice]. The mice are fully viable although they show impaired inducibility of gluconeogenetic enzymes in liver, defects in longterm renewal of erythroid progenitors and increased expression of POMC and ACTH in the pituitary. However neither in the lung nor the adrenals were any histological abnormalities found. In conclusion GR(dim)-mice represent a valuable tool to further analyse mechanisms of physiological effects of the GR.